IMPACT OF STRAIN ON MEMORY AND LATERAL POWER MOSFETS

Transcription

1 IMPACT OF STRAIN ON MEMORY AND LATERAL POWER MOSFETS By UMAMAHESWARI AGHORAM A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA

2 Umamaheswari Aghoram 2

3 To my family 3

4 ACKNOWLEDGMENTS I would like to express my sincere gratitude to my advisor Dr. Scott E. Thompson for his guidance and support over the years. I thank my co-chair Dr. Toshikazu Nishida for his encouragement and helpful discussions. I would also like to thank Dr. Ant Ural and Dr. Franky So for serving on my committee. Special thanks go to the industry liaisons from Texas Instruments, Dr Sridhar Seetharaman, Dr Marie Denison Dr Rick Wise, and Dr Sameer Pendharkar for their help and guidance on the SRC project. I am grateful to SRC and TI for the opportunity to work on their project. My heart-felt thanks to all the current and past members of our research group: Andy, Amit, Guangyu, Hyunwoo, Jingjing, Ji-Song, Kehuey, Lu, Mehmet, Min, Nidhi, Sagar, Sri, Tony, Ukjin, Xiaodong, Yongke, Younsung for their assistance, support and friendship. I would like to thank Saurabh, Daniel and David for helping me with TCAD simulations. My sincere gratitude goes to Dr. Toshinori Numata for his invaluable help and mentorship. Last but not the least; I would like to thank my colleagues at TI, friends, faculty and staff and everyone who helped me during my graduate studies here at UF. A special thanks to my good friends Min, Krishna, Manasa, and Saranya for their encouragement and support. I dedicate my dissertation to my family for their unwavering love, encouragement, and support. 4

5 TABLE OF CONTENTS page ACKNOWLEDGMENTS... 4 LIST OF TABLES... 7 LIST OF FIGURES... 8 CHAPTER 1. INTRODUCTION Strained Silicon Technology Memory Overview Lateral Power MOSFET Organization EFFECT OF MECHANICAL STRESS ON MEMORY DEVICES Introduction DRAM Retention Experiment Results and Discussion Conclusion Flash Memory Retention Experiment Results and Discussions Summary EFFECT OF STRAIN ON LATERAL POWER MOSFETS Introduction LOCOS vs. STI technology Introduction Experimental Approach Results and Discussion On-resistance Breakdown voltage Orientation Dependence Experiment Results and Discussions High Stress Device Voltage Rating Summary STRESS SIMULATION OF STRAINED MEMORY AND LDMOSFET

6 Introduction Overview of Simulation tool Simulation Setup Strain in DRAM Transistor Strained Flash Memory Device Strained LDMOSFET Nitride capping STI induced stress Results and Discussion Strain in DRAM Transistor Strained Flash Memory Strained LDMOS Nitride capping with dummy gates STI induced stress Summary SUMMARY AND RECOMMENDATIONS FOR FUTURE WORK Summary Recommendations for Future Work LIST OF REFERENCES BIOGRAPHICAL SKETCH

7 LIST OF TABLES Table page 1-1. Comparison between DRAM and Flash memory Expected and measured longitudinal coefficients of <110> NLDMOS and DEPMOS Comparison of carrier mobility and coefficient in logic MOSFET and bulk along different orientations. [8, 9, 46] Expected and measured coefficients for <100> and <110> N-LDMOS and DEPMOS with the measurement uncertainty in brackets Material Constants used in simulation Beneficial stress for N-LDMOSFET

8 LIST OF FIGURES Figure page 1-1. Strained silicon technology nodes of Intel nano transistors [5] Process induced uniaxial and biaxial stress [6] Layout of mixed signal integrated chip Four point wafer bending jig Schematic for applying uniaxial and biaxial stress [8] Classification of semiconductor memory World wide memory market in year 2000 [10] Evolution of LDMOS from NMOS System rating of power devices [15] DRAM leakage components Band diagram Typical currents-gate voltage (Vg) characteristics of NMOSFET Setup to measure leakage in MOSFET Shift in substrate current of n-mosfet with SiO 2 dielectric under the tensile stress GIDL shift under mechanical stress for n-mosfet with SiO 2 and high-κ gate dielectric at high (filled marks) and low (open marks) electric fields Band diagram of NVM under retention Experiment setup for data retention bake under stress of NVM cell NVM cell retention after baking at 190C for 24h Cross section of lateral power MOSFET with LOCOS and STI Drain current enhancement under low and high gate bias Polar pi plot of n-bulk under longitudinal tensile stress along <110> direction Shift in breakdown voltage of N-LDMOS under tensile stress

9 3-5. Mobility enhancement factors for (001)-surface n-mosfets under 100 longitudinal, 110 -longitudinal, and biaxial tensile stress [5] Channel orientation on (100) wafer Linear Drain current enhancement versus stress for NLDMOS Linear Drain current enhancement versus stress for DEPMOSFET N-bulk piezoresistance vs. angle of spread in drift region under stress P-Bulk piezoresistance vs. angle of spread in drift region On-Resistance distribution of N-LDMOS at rated gate bias Linear drain current enhancement under high stress Linear drain current enhancement versus stress with increasing drift region length Applied Materials scheme for strained NAND Flash. (Courtesy AMAT) Multi-gate simulation structure with HARP STI and PMD N-LDMOSFET with dummy gates and nitride capping Longitudinal channel stress vs. gate length in logic MOSFET with tensile capping layer [59] D capping layer on N-LDMOSFET simulation structure DIELER structure Principle of Dielectric RESURF [60] STI induced stress simulation structure for 2-D and 3-D A process simulation of built-in stress in the middle and the gate edge of the channel in a strained Si MOSFET with stressed silicon nitride CESL Stress in NAND flash memory due to STI and PMD Stress in nested gate structure. A) Expected simulation result of LDMOS with tensile capping layer B) Actual simulation cross section from [60] Stress in device center as a function of gate to gate separation STI induced stress in active region [59]

10 D stress contours of STI induced stress in active drain extension fingers Transverse compression along the fin Out of plane tension along the fin Longitudinal tension along fin

11 Abstract of Dissertation Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy IMPACT OF STRAIN ON MEMORY AND LATERAL POWER MOSFETS By Umamaheswari Aghoram May 2010 Chair: Scott Thompson Cochair: Toshikazu Nishida Major: Electrical and Computer Engineering To circumvent the limitations of conventional scaling, the semiconductor industry incorporated strained silicon technology to boost the performance of digital logic devices. Since strain alters several semiconductor properties, its effect on all device parameters needs to be investigated. This work focuses on the effect of mechanical stress on memory and power devices. Growth of digital electronics is largely attributed to the success of CMOS memory such as DRAM and Flash. The most significant device characteristic for memory is the duration of time for which the memory cell is capable of storing the data with integrity or retention time. Using four-point wafer bending apparatus to apply mechanical stress the dependence of memory retention time on strain is studied. From measurements it was observed that while DRAM retention degenerates with mechanical stress, NVM improved with tensile stress. Power MOSFETs are used as high current and voltage drivers in automotive, telecommunication and power industries. The two main figures of merit of power devices are their on-resistance and breakdown voltage. The design of these devices is complicated by the tradeoff between the requirements for minimum on-resistance and 11

12 maximum breakdown voltage. This work focuses on the application of mechanical stress to improve the performance of Lateral Diffusion MOSFET. The device behavior was analyzed by measuring and extracting piezoresistance coefficients of these devices and by monitoring avalanche breakdown with mechanical stress. It was found that the on-resistance reduced with stress, while breakdown voltage remained a constant thus making strain a viable performance booster in these devices. With the understanding of device behavior with strain, the application of stress via process was simulated with FLOOPS and Sentaurus process. The amount/ type of stress present in device gives insight into strained device structure and performance. 12

13 CHAPTER 1 INTRODUCTION Strained Silicon Technology The revolutionary growth of the semiconductor industry can be attributed to scaling of devices. Every two years the chip density has doubled in accordance to Moore s law [1]. However improving the performance of transistors by scaling is progressively becoming difficult and this has spurred the industry to look for other mechanisms to boost the performance of devices. In early 90 s strained silicon on Si-Ge was investigated as a means to improve logic performance [2]. In 2003, Intel introduced strained silicon technology in its 90nm node [3]. Since then fourth generation strained silicon has been incorporated into the 32nm technology [4]. International Technology Roadmap for Semiconductors (ITRS 09) named strained silicon as one of the potential solutions to improve the performance of devices in 22nm node (Figure 1-1). Not only is strain intentionally introduced to improve performance (Figure 1-2), but strain is inherently present due to device fabrication process such as isolation, oxidation, silicidation, implant and packaging. Since strain is known to alter several semiconductor properties such as: Band structure, carrier mass, scattering and trap properties, it is important to study its effect on device properties, so that we can engineer strain to improve the device. 13

14 90nm Node nm Node st Generation strained Silicon 2 nd Generation strained Silicon 45nm Node nm Node nm Node rd Generation strained Silicon 4 th Generation strained Silicon Strained Silicon? Fig 4. Technology node for Intel nano-transistors Figure 1-1. Strained silicon technology nodes of Intel nano transistors [5] Uniaxial Stress Biaxial Stress Figure 1-2. Process induced uniaxial and biaxial stress [6] The layout of a standard mixed signal integrated chip is shown in Figure 1-3. It consists of a digital core with semiconductor memory and the interface to the real world is provided by the surrounding analog shell [7]. The analog shell protects the core from external electric stresses, provides power management and acts as the communication link between the control logic circuitry and the load. This technology is gaining more importance with the increasing interest in System-On-Chip for consumer electronics, telecommunications, automobile and power management for portable equipments. With 14

15 the scaling of the digital core to increase functionality there is an increased pressure on the analog shell to scale as well. Traditionally, scaling in semiconductor industry has been predominantly digital centric, i.e. focused on logic Metal Oxide Semiconductor Field Effect Transistors (MOSFET). Scaling of the analog shell lags several years behind digital logic due to cost considerations and stress from increased power density. Since the main performance metric for memory and power devices are not similar to that of logic MOSFETs, the effect of strain on these devices needs to be studied. Also, it is of interest to see if strain can improve the performance in these devices. V DD V DD Analogue shell Inputs Digital Core Memory loads Power Circuitry Figure 1-3. Layout of mixed signal integrated chip Experiment Setup: The four point wafer bending apparatus is used to apply mechanical stress to device wafer. This apparatus, shown in Figure 1-4, has been calibrated using wafer curvature and strain gage measurements. Both uniaixal and biaxial stresses can be applied by using the setup shown in Figure 1-5. The distance of separation between the rods determines the amount of stress per graduation and can be calculated using the simple formula (Equation 1-1). The jig is easy and reliable to 15

16 use and is especially valuable in taking repeated device measurements under low stress levels. Eyt σ = 2a L 2 2a 3 (1-1) Where E-Young s Modulus, y- vertical displacement, t- wafer thickness, L- distance between outer two rods, a- Distance between outer rod and inner rod. t a L y Figure 1-4. Four point wafer bending jig transverse longitudinal schematic for applying uniaxial stress schematic for applying biaxial stress Figure 1-5. Schematic for applying uniaxial and biaxial stress [8] The device electrical characteristics are monitored using Keithley 4200 semiconductor characterization system. Historically the strain induced change in device 16

17 drive current is quantized by extracting the piezoresistance coefficient ( -coefficient). - coefficient is defined as the normalized change in resistance per unit applied stress. I I D (1-2) 0 0 D0 Where -semiconductor piezoresistance -applied mechanical stress, -semiconductor resistivity, m-carrier mobility, and I D -Drain current First reported by Smith [9] in 1954, the piezoresistance of silicon has been the basis by which industry predicts strained device behavior. Memory Overview The growth of digital electronics can be largely attributed to the success of semiconductor memory. It is found in most consumer electronics such as cell phones, computers, digital cameras, global positioning systems, etc. CMOS memory can be divided into two main categories: Volatile memory (Random access memory or RAM) and non volatile memory (NVM) or read only memory (ROM) [10]. Volatile memory loses the stored information once the power supply is turned off. NVM on the other hand retains the data. Figure 1-6 shows the various types of semiconductor memory devices. Semiconductor Memory Volatile Non Volatile SRAM DRAM EEPROM FLASH EPROM ROM Figure 1-6. Classification of semiconductor memory 17

18 Figure 1-7. World wide memory market in year 2000 [10] Dynamic Random Access Memory (DRAM) and Flash memory are billion dollar industry and are the most popular CMOS memory types (Figure 1-7). Commercially introduced by Intel in 1971, DRAM consists of one transistor and one capacitor (1T-1C) [11]. It is a volatile memory that requires the stored information to be refreshed after every read cycle. It has fast read/write times. With the advent of trench capacitors the density of DRAM has increased dramatically. Floating gate flash memory [12] was produced by Toshiba in It consists of only one transistor with a polysilicon floating gate acting as the storage element. Channel hot electron (CHE) injection is used to store electrons in the floating gate, thus altering the threshold voltage of the device (programmed state). Data is erased by removing the electrons from the floating gate by Fowler Nordheim tunneling (F-N tunneling). In 2001, AMD introduced the first commercial Mirror bit flash [13] which was capable of storing two independent bits of information. The electrons are locally trapped in two separate locations on the silicon nitride trapping layer. A comparison between DRAM and Flash memory is shown in Table

19 Bit Line Bit Line Bit Line Table 1-1. Comparison between DRAM and Flash memory DRAM DRAM Floating Floating Gate Gate FLASH FLASH Mirror Mirroe Bit Bit FLASH Bit FLASH FLASH Structure Structure Structure Word Word Line Line Capacitor Capacitor Control Control Gate Control Gate Control Gate Control Control Gate Control Control Gate Gate Gate Gate Gate Blocking Blocking Oxide Blocking Oxide Blocking Oxide Blocking Oxide Blocking Blocking Oxide Oxide Oxide Oxide Oxide Floating Floating 0 0Trap layer layer1 Floating Gate Gate 00 Gate 0Trap Trap layer layer layer Tunneling Tunneling oxide Oxide Oxide Tunneling oxide oxide oxide oxide n + n+ n+ + n + + n+ n+ n + + n+ n + n + n + n + + n + n + n + n + n + p-si p-si p-si p-si p-si p-si p-si p-si p-si p-si Bits of of information Bits Bits of information of 1 bit1 1 bit Bit bit 1 1 bit bit bit Bit 2 2 independent 2 Bitsbitsbits bits Cell Area Cell Area Cell Area Cell Area (6-8)F F (6-8) (6-8) F 2 F 2 2 (2-4)F (2-4)F (2-4)F (2-4)F 2 2 Read Destructive Yes Non-Destructive Non-Destructive Volatile Read/Erase/Program Volatile Yes Yes No No No No Volatile speed Refresh nsneeded (10 s-ms/ms/ns yrs retention) (10 yrs s-ms/ms/ns retention) Refresh Refresh needed needed (10 (10 yrs yrs retention) (10 yrs yrs retention) Program program Cap charging CHE CHE CHE program program Cap charging Cap charging CHE CHE CHE CHE Erase Cap Discharging F-N tunneling BTB hot hole tunneling Erase Cap Discharging F-N tunneling BTB hot hole tunneling RelativeErase Cost/Bit Erase Cap Discharging Cap Discharging Low F-N F-N tunneling Medium tunneling BTB hot hot Medium hole hole tunneling tunneling Scaling Challenge Capacitor Voltage, retention L Scaling Challenge Tunnel oxide, High CH, Voltage, retention Scaling Challenge Capacitor Voltage, retention CH Tunnel Voltage, oxide, retention High Scaling Challenge Capacitor Voltage, retention L Voltage CH, Voltage, retention Voltage Lateral Power MOSFET Power MOSFETs are solid state switches with high power handling capability that evolved from CMOS technology. The process of increasing the blocking voltage capability of lateral MOSFET structure led to the development of Diffusion MOSFET (DMOS) [14]. In NLDMOS structure the source and drain n+ regions are self aligned with the poly gate. The p-base region is then driven in deeper than the source [15]. This is called Double-diffusion process. The difference in lateral diffusion determines the channel length of the device. Thus, these devices have very small channel lengths and hence low on-resistance. These devices have an n-type drift region which increases the breakdown voltage. Also the presence of field plate and RESURF [16] reduces the electric field at the surface. Figure 1-8 shows the evolution of LDMOS from NMOSFET. 19

20 Low Voltage logic NMOS n + Gate p-si n + logic LDD NMOS Gate n + n - n + p-si DENMOS Gate n + n n + p-si n + p-body Gate STI n-region p-si n + n + p-body Gate LOCOS n-region p-si n + n + p-body Gate p-si n-region n + N-LDMOS (STI) N-LDMOS (LOCOS) N-LDMOS Figure 1-8. Evolution of LDMOS from NMOS Figure 1-9 shows the system rating of power devices. The lateral power MOSFET has high input impedance, operation frequency, current and voltage handling capability and ease of integration. They find wide application in automotive, telecommunication and power industries. 20

21 Current Rating (Amperes) POWER SUPPLIES AUTOMOTIVE ELECTRONICS ROBOTICS LAMP BALLAST TELECOM DISPLAY DRIVES HVDC TRANSMISSIONS ELECTRIC TRAINS MOTOR DRIVES Voltage rating (Volts) Figure 1-9. System rating of power devices [15] Organization The effect of mechanical stress on memory retention and power MOSFET device characteristics is presented in this work. Chapter 2 discusses the change in GIDL leakage current with mechanical stress and its effect on DRAM retention. Chapter 3 reports the piezoresistance coefficient of n-ldmos and DEPMOS devices. The experimental result of strain induced change in breakdown voltage is also shown. Chapter 4 deals with simulation of strained memory and power MOSFET structures. Summary and recommendations for future work are provided in chapter 5. 21

22 CHAPTER 2 EFFECT OF MECHANICAL STRESS ON MEMORY DEVICES Introduction Mechanical stress alters several properties of semiconductors such as band structure, carrier mass, scattering and trap properties. Thus several characteristics of memory devices may be altered by strain. The most important properties in memory devices are [17]: Endurance: Read/Write cycles the device can endure before failure Data Retention: The value of time for which a memory cell retains data Memory Disturb: Loss or gain of charge in a memory cell Program/Erase time: Time required for programming/erasing memory cell Among all these properties, retention time is the most important parameter. The industry generally requires Non Volatile Memory (NVM) to have 10 years retention time. In order to predict the memory retention time, accelerated testing methods such as Data Retention Bake (DRB) is used. In DRB programmed memory devices are baked at high temperatures such as 250C for hours. The difference in device characteristics before and after bake, determines the retention. DRAM Retention Scaling of memory devices is ultimately determined by the data retention capability of the designed memory cell. In DRAM, leakage occurs from the capacitor through the MOSFET during retention period. The main leakage mechanisms in DRAM (Figure 2-1) are Subthreshold leakage (I SUBVt ): This leakage refers to the flow of charge carriers from source to drain even when the device is in the off condition. 22

23 This leakage current is especially large in short channel MOSFETs due to the Drain induced barrier lowering (DIBL). Gate induced drain leakage, GIDL (I GIDL ): This leakage refers to band to band leakage between the reversed biased gated diode of drain-substrate region. For instance in an NMOSFET, high positive bias on the drain results in the drain- substrate junction being reverse biased. However when there is a negative bias on gate resulting in an accumulation region in the channel, the depletion region width of the reverse bias diode reduces at the surface below the gate. This causes an increased electric field and a subsequent increase in band to band tunneling current (Figure 2-2A). Junction leakage (I JXN ): This merely refers to the reverse biased junction leakage of the drain-substrate diode. With the scaling of the transistor in DRAM, the increase in subthreshold leakage is countered by applying a negative gate bias during retention. This has resulted in the increase of GIDL leakage. Thus GIDL current has become the most dominant leakage mechanism in DRAM. 23

24 Poly Si Poly Si BL I SUBVt I GIDL I JXN WL n + n + GND I GIDL I JXN p-si insulator Figure 2-1. DRAM leakage components GIDL current is dominated by band-to-band tunneling (BTBT) leakage current at high normal electric fields and by interface-trap-assisted tunneling (TAT) leakage current at low electric fields [18-20]. Mechanical stress in DRAM may be present due to the technology node or as a result of processing and packaging. Several studies have been conducted on the effect of Shallow Trench Isolation (STI) and process induced strain on GIDL [21-23] in MOSFETs with oxide and high- dielectrics. They report an increase in GIDL current under compressive stress due to band-gap narrowing and increase in intrinsic carrier density. The reduction in silicon band-gap due to strain induced band splitting is reported in reference [24]. Since band-to-band tunneling is exponentially dependent on bandgap (E G ), mechanical stress can be very detrimental to DRAM retention. b b 1 exp 1/ 2 AE E g 3 2 g J (2-1) Where J b-b -Band-to-band tunneling leakage current, E g -Band gap and A-Constant 24

25 Tensile stress 6 unstrained (compressive) 4 ( 2 ) 2 ( 4 ) E g (0) E g ( ) A B VB HH (LH) LH (HH) Figure 2-2. Band diagram A) GIDL B) Strain induced Si band-gap narrowing Experiment The devices used in this study were Silicon (Si) n-channel MOSFETs with oxide (SiO 2 ) and high- gate dielectrics on (001) wafer. The SiO 2 gate dielectric MOSFETs had n + poly Si gate and 1.4-nm SiO 2 gate dielectrics. The high- gate devices were formed with 10-nm Titanium Nitride (TiN) gate and 2-nm Hafnium silicate (HfSiO) gate dielectric. The oxide interfacial layers were created by the gate stack formation. The MOSFETs with compressive built-in stress were also formed by the process modulated silicon nitride contact etch stop layers (CESL). The typical currents and gate voltage (V g ) characteristic of a long channel n- MOSFET is shown in Figure 2-3. In thin gate dielectric devices, the minimum drain current is dominated by the gate leakage. Thus to monitor GIDL, the substrate current (I sub ) was measured. Figure 2-4 shows the measurement setup used to distinguish between the various I sub components. I sub is composed of the GIDL current at negative V g and the impact ionization current at positive V g, both of which were higher than the corresponding p-n junction leakage between drain and substrate. 25

26 Current, I [A] Tensile and compressive mechanical stresses were applied longitudinally along the <110>-channel by using the four-point bending jig. 1.E-05 V D = 1.2 V 1.E I D 1.E-07 I G 1.E-08 1.E I GIDL SUB_GIDL Impact I SUB_I/I Ionization 1.E-10 I SUB 1.E-11 Junction I SUB_PN leakage 10 1.E Gate voltage, Vg [V] Figure 2-3. Typical currents-gate voltage (Vg) characteristics of NMOSFET. V G V G I G V D I G V D I G V D I D I SUB_PN I SUB_GIDL I SUB_P-N I SUB_I/I I SUB_GIDL I SUB_P-N A pn Junction leakage A GIDL pn Junction leakage A GIDL Impact ionization pn Junction leakage Figure 2-4. Setup to measure leakage in MOSFET Results and Discussion Figure 2-5 shows the I sub - V g and the shift in I sub for tensile stress of 150MPa on long channel MOSFET with the SiO 2 gate dielectric with zero built-in stress. On the 26

27 application of tensile stress, the I sub shift shows a constant enhancement at high negative gate voltages. On the other hand at low V g, I sub decreases. It is thus found that the GIDL current increases in the high electric field and decreases in the low electric field for the tensile stress. In the impact ionization region (high positive V g ), I sub shows a constant enhancement. Figure 2-6 shows the GIDL current shift with stress at high and low electric fields. It is seen that at high electric field, GIDL current increase for both tensile and compressive stresses. In this electric field region, GIDL current is generally dominated by BTBT, which is sensitive to the variation in band gap. The Si band gap decreases for both tensile and compressive stress, although the sub-bands shifts for 2 and 4 valleys and heavy and light holes are opposite for the tensile and compressive stresses [6]. On the other hand, the GIDL current in low electric field is due to trapassisted generation of electron hole pairs, which can be described by using Shockley- Read-Hall model [20]. It is reported that the intrinsic carrier density is the main stressdependant parameter in the bulk generation-recombination current in p-n junction [25]. A theoretical calculation shows that, for the wafer orientation and stress range considered in this paper, the intrinsic carrier density decreases with tensile and increases with compressive stress [26, 27]. This is similar to the strain-altered GIDL shifts observed at low electric field. However, for a more accurate analysis several other stress-altered parameters such as the surface current, trap energy and new generationrecombination centers need to be considered. Figure 2-6 shows the relative change in GIDL current of high- gate devices under applied stress. This trend was observed in the entire range of electric field up to the break down gate voltage. In zero-built-in stress device, the GIDL current increases with 27

28 an increase in the compressive stress and gently decreases with tensile stress. These trends are similar but larger than those obtained at low electric fields in the SiO 2 gate dielectric devices. In high- gate dielectric devices, an additional GIDL component is introduced by trap-assisted tunneling from the remote traps located at the interface of the high- dielectric and oxide in the gate stack [28]. The larger change in GIDL current measured in these devices seems to suggest that the change in trap energy with strain further enhances the shift in generation-recombination current. Figure 2-6 also shows the tensile strain altered GIDL current shift of devices with compressive built-in stress. The slope of curve changes beyond a certain stress value (flex point). The rate of the GIDL current shift beyond the flex point is identical to that of the device with zero built in stress. The flex points of the process-stressed devices changes with gate length. This type of trend strongly suggests that the channel region, where GIDL current is generated, has a non zero-built-in stress modulated by externally applied mechanical stress. Therefore the stress value corresponding to the flex points give an estimate of the original built-in stress in the channel near the gate edge where the GIDL current is generated. These flex points also shows that the built-in stress still remains in the long channel devices with stressed CESL. 28

29 Isub shift, Isub [%] Isub [na] MPa tensile stress 3.E-09 4 Isub 2 2.E E E-09-8 Isub 5.E Gate voltage, Vg [V] 0 0.E+00 Figure 2-5. Shift in substrate current of n-mosfet with SiO 2 dielectric under the tensile stress. 29

30 GIDL current shifts [%] 5 SiO Zero-built-in stress -5 High compressive built-in stress Lg=0.5 m Lg=0.1 m Stress [MPa] Figure 2-6. GIDL shift under mechanical stress for n-mosfet with SiO 2 and high-κ gate dielectric at high (filled marks) and low (open marks) electric fields. Conclusion The GIDL current dominated by BTBT leakage in high electric field increases for both tensile and compressive stress and the stressed enhancement of Si MOSFETs is about 2-3 % per 100MPa in this measurement. Even for strained long channel devices, the stress in the gate drain overlap region is not negligible. This has a serious impact on DRAM retention and off-state leakage of devices using narrow band gap materials [24, 29]. In n-mosfets with high- dielectric, compressive stress reduces GIDL. Thus strain on DRAM memory with high- MOSFETs shows potential in increasing retention time. 30

31 Flash Memory Retention Mirror bit Flash or SONOS (silicon-oxide-nitride-oxide-silicon) memory consists of a trapping nitride layer, instead of the floating gate as the charge storage layer. In retention mode electrons trapped in the nitride layer leaks back into the silicon substrate through several mechanisms illustrated in Figure 2-7 [30]. Retention in these devices is monitored by data retention bake followed by measuring the shift in threshold voltage. At high temperature and in the absence of external electric field, thermal emission followed by drift of electrons due to the internal field is the main leakage mechanism [31]. Electron drift _ Thermal excitation Trap to band tunneling Silicon + Band to trap tunneling Oxide _ E T E T _ trap to trap tunneling Nitride trap layer Figure 2-7. Band diagram of NVM under retention Experiment A long strip of device wafer is cleaved and all the isolated devices are programmed by applying high gate and drain bias for a few microseconds at room 31

32 temperature. Under these bias conditions, channel hot electrons are captured into the nitride layer. This results in a shift in threshold voltage (V T ) of the Flash cell. Next, mechanical stress is applied on devices on one die of the sample by using four-point bending jig. The sample along with the jig is then baked at 190 C for 24 hours. The advantage of this experiment setup is that both stressed and unstressed samples are baked at simultaneously. The V T before and after baking is monitored since the delta in V T is directly proportional to the loss of electrons from the trapping layer. Figure 2-8. Experiment setup for data retention bake under stress of NVM cell Results and Discussions Shift in threshold voltage before and after the baking for stressed and unstressed devices is shown in Figure 2-9. As can be seen from Figure 2-9a, even for small values of stress as those applied in this experiment, all devices under tensile stress show improved retention (smaller V T shift) than unstressed devices. On the other hand compressive stress is observed to deteriorate retention (Figure 2-9b). The leakage current density due to thermal emission is given by equation similar to Arrhenius relationship. The shift in threshold voltage is proportional to the integral of 32

33 current density over time. Thus the threshold voltage shift is exponentially proportional to the inverse of trap activation energy. V T J AT e E TA kbt 2 (2-2) From the threshold voltage shift data of device with W/L=0.16/1 at two different temperatures at zero stress, we extracted the trap activation energy to be 1.54eV. This value lies within the range reported in literature for electron traps in nitride layer [30]. This gives us confidence that thermal emission is in fact the dominant leakage mechanism. V V T T ( T1 ) ( T ) T T e E 1.54eV TA e E k TA b ETA T1 T (2-3) Strain is known to alter trap activation energy (E TA ) [32]. From the relationship between V T shift and activation energy of traps, an increase in E TA under tensile stress and a decrease in E TA of nitride traps under compressive stress may explain the observed trend. 33

34 A Figure 2-9 NVM cell retention after baking at 190C for 24h. A) Under Tensile stress and B) Under Compressive stress [33] Summary Effect of mechanical stress on retention time of DRAM and flash memory was investigated. Stress deteriorated DRAM retention while tensile stress improved charge trapping memory. Careful consideration needs to be given to process induced stress while manufacturing memory devices to maintain device reliability. B 34

35 CHAPTER 3 EFFECT OF STRAIN ON LATERAL POWER MOSFETS Introduction In 1970 s power MOSFETs replaced power Bipolar MOSFETs for high speed, medium power applications. Their high input impedance, operating frequency and excellent safe operating area make them ideal candidates for power, automotive and telecommunications industry. These devices form the analogue shell that surrounds the digital core and provides the link between the IC and real world. The main requirement in these devices is high drive current and ability to withstand large voltages. Since the total current and power dissipation in these devices is limited by the on-resistance and the maximum voltage rating by the breakdown voltage, minimizing on-resistance and maximizing breakdown voltage is the key to designing a high performance power MOSFET. However the conflicting requirements needed to satisfy both these conditions complicates the design of these devices. Hence scaling of these devices lags behind the current CMOS technology. Despite having lower power rating than vertical power MOSFETs, lateral power MOSFETs are widely used because of their ease of integration. Following the conventional way of reducing on-resistance by reducing dimensions has resulted in the need for innovative techniques such as REduced SURface Field (RESURF) [16, 34], field plating [15], and Superjunctions [35] to improve device breakdown. Strained silicon technology is a novel technique that can enhance power MOSFET performance by reducing the on-resistance (R ON ) without affecting the breakdown voltage. Kondo et al [36] investigated biaxially stressed lateral double diffused MOSFETs (LDMOSFETs) formed on strained silicon grown over relaxed Si Ge

36 layer % lower on resistance of was reported. Recently, Moens et al [37] reported a strained trench power MOSFET. Tensile stress of the order of 200MPa was produced in the drift region by trench oxide on either side. MicroRamanSpectroscopy was used to verify the stress magnitude and direction. Strain induced mobility enhancement reduced the on-resistance in the device by 10%. However no report on the effect of industry preferred uniaxial stress on power MOSFET performance has been made till date. This chapter deals with the effect of strain on n and p type LDMOSFETs, investigating the dependence on type of device isolation, voltage rating, channel orientation, and high mechanical stress. A simple analytical model using channel and bulk pi coefficients is used to explain the 4-point probe bending data. Based on the experimental result conclusions regarding the best option for strain enhanced LDMOSFET is reached. LOCOS vs. STI technology Introduction The presence of isolation oxide in drift region reduces electric field and thus plays a critical role in improving the breakdown voltage in power MOSFET devices. In older analog technologies, LOCal Oxidation of Silicon (LOCOS) was used. Current technologies use the Shallow Trench Isolation process for isolation. The cross-sections of the devices using LOCOS and STI isolations are illustrated in Figure 3-1. The effect of strain on the isolation technology is studied by monitoring its effect on On-resistance and breakdown voltage of the device. 36

37 Experimental Approach <110>-oriented N-LDMOS and DEPMOS on (100) substrate (Figure 3-1) with shallow trench isolation and LOCal Oxidation of Silicon isolation were used in this study. External mechanical stress was applied using four point wafer bending apparatus and the I-V characterization was done using the Keithley 4200 SCS. Linear device characteristics were measured by applying a constant DC bias of 0.1V at the drain and sweeping the gate bias from 0 to rated gate bias, while keeping source and substrate grounded. Breakdown characteristics were obtained by sweeping the drain bias till avalanche breakdown occurred and the drain current was 10nA, while maintaining the gate, source and substrate at 0V. In case of STI isolated devices, measuring the breakdown voltage is complicated by the excessive charging of the STI oxide. Source P+ N+ P-BODY Gate POLY POLY FOX N+ N-WELL Drain Source P+ N+ P-BODY Gate POLY STI Drain N+ N-WELL P-SUBSTRATE P-SUBSTRATE A Back Gate Source Drain Drain Gate Source Back Gate n+ FOX p+ POLY1 p+ p+ POLY1 p+ STI n+ N-WELL P-WELL P-WELL N-WELL NBL NBL B Figure 3-1. Cross section of lateral power MOSFET with LOCOS and STI. A) N-LDMOS and B) DEPMOS 37

38 Results and Discussion On-resistance The percentage gain in linear drain current (I DLIN ) with longitudinal tensile stress on N-LDMOSFET with STI and LOCOS isolation at low and high gate bias (V G ) is shown in Figure 3-2a and Figure 3-2b shows a similar plot for DEPMOSFET. At low V G, the piezoresistance coefficient ( - coefficient) of LDMOS with STI and LOCOS is similar to logic MOSFET ( NLDMOS =-25 and NMOS =-31; DEPMOS =55 and PMOS =71). As V G increases, the gain in I DLIN of N-LDMOS decreases. Also at high V G, the gain in I DLIN was higher in LOCOS device (1.2% for 100 MPa) than STI device (0.9% per 100 MPa). No V G or device isolation dependence was observed in the DEPMOS devices. Figure 3-2. Drain current enhancement under low and high gate bias. A) NLDMOS B) DEPMOS A 38

39 I DLIN Gain (%) I DLIN Gain (%) 12 Vg-Vt=0.3V 12 Vg=Rated Voltage 90nm Logic PMOS Smith 8 8 LOCOS LOCOS 4 STI 4 STI Longitudinal Compressive stress (MPa) Longitudinal Compressive stress (MPa) B Figure 3-2. Continued The above results can be understood by examining the effect of strain on the individual components of R ON of the power MOSFET. The -coefficient of a laterally diffused power MOSFET is estimated based on its device model which consists of an enhancement mode MOSFET in series with a resistor [38-40]. At low V G (weak inversion), the channel resistance (R CH ) dominates the on-resistance (R ON ) and is given by R CH WC ox L eff ( V V ) n2 D G t (3-1) Where, L eff - effective channel length, W- channel width, C ox - gate capacitance/unit area, μn2-d- electron inversion layer mobility, V G - applied gate bias, and V T - Threshold voltage of enhancement mode MOSFET. Thus the -coefficient of the power device is predicted to be similar to the logic MOSFET. At high V G, R ON is dominated by the drift resistance (R D ). 39

40 R D LD (3-2) q Q W nbulk d Where, L D -drift region length, μ n-bulk - bulk electron mobility, W- channel width, and Q d - drift region charge. Hence, the -coefficient is expected to be identical to bulk silicon (Si) -coefficient. Based on this concept, the expected values for the -coefficient of power MOSFET are listed in Table 3-1. The values for the logic MOSFET were taken from [8] and Smith s values were used for bulk [9]. From table 3-1 it can be observed that at low V G, the strained lateral power device follows expectation. However at high V G the values of the N-LDMOS differs significantly from bulk Si. On the other hand no such degradation of enhancement is observed in DEPMOSFETs. This discrepancy can be understood by considering the strain induced change in mobility of carriers that spread vertically into the drift region. Table 3-1. Expected and measured longitudinal coefficients of <110> NLDMOS and DEPMOS Longitudinal (x10-11 Pa -1 ) LOCOS STI Low V G High V G Low V G High V G NLDMOS Expectation DEPMOS Expectation For the LDMOSFET, R ON is the sum of channel resistance (R CH ), accumulation resistance (R ACC ), spread resistance (R S ), and drift resistance (R D ) [41]. The - coefficient of R CH is the same as logic MOSFET and the value of R D and R S is that of 40

41 Coefficient -Coefficient of N-Bulk along that direction. The -coefficient of R S ( S ) is a function of the angle of spread of carriers into the bulk of the drift region. For materials with cubic symmetry such as Si, the -coefficient along any direction can be determined from the piezoresistance tensor and direction cosines [42, 43]. The dependence of S on spread angle for <110> n-channel device under longitudinal tensile stress shown in Figure 3-3 is derived as 2 Cos 2 s Sin (3-3) 2 Where 11, 12 and 44 are the three basic -coefficients, and represents the spread angle Coefficient º negative 20º 30º positive 40º 50º 60º 70º 90º 80º <100> <110> Channel direction Substrate stress gate S D N-well Figure 3-3. Polar pi plot of n-bulk under longitudinal tensile stress along <110> direction 41

42 From the polar plot for N-Bulk (Figure 3-3), it is easy to see that as electrons spread more vertically into the substrate, the spread resistance increases with the applied mechanical stress. At high V G where R D and R S dominate R ON, the increase in R S with stress results in the observed degradation of the coefficient of N-LDMOS. In the case of low breakdown DEPMOS devices, there is no isolation present in the drift region. However, even in the presence of isolation, the insignificant coefficient of the spreading resistance results in DEPMOS I DLIN enhancement to be independent of V G. Breakdown voltage From Figure 3-4 it is seen that the shift in breakdown voltage for lateral power MOSFETs is only ~80 mv for 60 MPa, comparable to the amount of shift caused by charging of the oxide from repeated measurements. Assuming a linear trend with stress, this translates to ~1 V for 1 GPa stress. This shift is rather insignificant when dealing with breakdown voltages of magnitude ~35 V. 42

43 I D (A) E E E E-09 V BD at I D =10nA 0MPa V BD ~80mV 60MPa 1E V D (V) Figure 3-4. Shift in breakdown voltage of N-LDMOS under tensile stress From the measurement of piezoresistance coefficients of <110>-lateral power devices, it is observed that the coefficients of power devices are not similar to logic MOSFETs. This is due to the vertical spread of carriers in the drift region. The enhancement for N-LDMOS device is not as high as expected. For 20% reduction in R ON the required amount of stress exceeds 1.5 GPa. This large value of stress makes application of strained silicon technology not a very attractive option for <110> NLDMOS devices. Orientation dependence Work from Kanda [36] reports that strain induced enhancement in a semiconductor is dependent on the direction of current flow. For n-type logic devices, determination of the best strained orientation was conducted by Uchida. From his plot on mobility enhancement with stress, it is easy to see that <100> is the best orientation for NMOSFET at low values of stress and <110> is the best orientation at high stresses. 43

44 However from the previous discussion, it is easy to see that <110> is not the best orientation for strained N-LDMOSFET. <110> Uniaxial <100> Uniaxial <110> Biaixial Figure 3-5. Mobility enhancement factors for (001)-surface n-mosfets under 100 longitudinal, 110 -longitudinal, and biaxial tensile stress [5] Use of alternate orientations and hybrid wafer substrates in the manufacture of power MOSFETs has been widely investigated especially for p-type power devices[44, 45]. From the comparison of hole mobility for different substrate and channel orientations (Table 3-2), <110> direction on (110) substrate is the ideal orientation for p- type power MOSFETs. The piezoresistance of strained power LDMOSFET is a function of both MOSFET and bulk value. A comparison of carrier mobility and coefficient of MOSFET and bulk silicon along different orientations (Table 3-2) is useful in determining the best orientation for strained lateral power MOSFET. 44

45 Table 3-2. Comparison of carrier mobility and coefficient in logic MOSFET and bulk along different orientations. [8, 9, 46] Wafer/Channel Orientation Channel Mobility Channel coefficient Bulk coefficient n p n p n P (100) [110] n p (100) [100] ~ n 1 08 p (110) [110] 0.4 n 2.6 p (110) [110] 0.6 n 1.7 p From this table it is easy to see that <100> channel and <110> channel on (100) wafer is the best orientation for strained N-LDMOSFET and DEPMOSFET respectively. By simply rotating the wafer notch by 45 degrees, the same processing for the devices with standard orientation will yield devices with channel along the <100> direction (Figure 3-6). This enables the simultaneous production of both n- and p-type strained power MOSFETs on the same wafer. 45 o [010] [100] [01 1] Figure 3-6. Channel orientation on (100) wafer Experiment The current-voltage characteristics of <100>-oriented N-LDMOSFET and DEPMOSFET with STI on (100) wafer under mechanical stress was measured and 45

46 compared with the control <110>-oriented device. -coefficients were extracted at low and high gate bias. coefficients of on-chip resistances were also measured to accurately determine bulk values. From the measurement, an analytical model for the piezoresistance of lateral power MOSFET was developed and the lowest value of stress required for 20% reduction in on-resistance was calculated. Results and Discussions The linear drain current enhancement with applied mechanical stress for <100> and <110> oriented n-type lateral power MOSFETs is shown in Figure 3-7 and the extracted -coefficients is listed in Table 3-3. Both <110> and <100> channel device showed lower enhancements of I DLIN at high V G ( <110> = -9 x10-11 Pa -1 and <100> = - 20x10-11 Pa -1 ) than at low V G ( <110> = -25 x10-11pa-1 and <100> =-36x10-11 Pa -1 ). At high V G, longitudinal tensile stress on <100> channel N-LDMOS shows the largest improvement of ~2% per 100 MPa. Also, transverse stress on N-LDMOS is not as beneficial as longitudinal tensile stress. 46

47 I DLIN Enhancement (%) I DLIN Enhancement (%) 5 Low gate overdrive V GS -V t =0.3V <100> Longitudinal <110> 2.5 <110> Transverse <100> Tensile Stress (MPa) High gate overdrive V GS = rated voltage Longitudinal Transverse <100> <110> <110> <100> Tensile Stress (MPa) A B Figure 3-7. Linear Drain current enhancement versus stress for NLDMOS. A) <100>oriented device B) <110> oriented device 47

48 I DLIN Enhancement (%) In case of strained DEPMOSFET on a (100) wafer, at high V G the standard orientation namely <110>, is the preferred channel direction. Nearly a 5% enhancement is observed from Figure 3-8 for every 100 MPa of longitudinal compressive stress. For the <100> channel DEPMOS, transverse compression is more beneficial than longitudinal stress. 8 V GS -V t =0.3V Low gate overdrive <110> 4 Longitudinal <100> 0 Transverse <100> <110> Compressive Stress (MPa) A Figure 3-8. Linear Drain current enhancement versus stress for DEPMOSFET. A) <100> oriented device B) <110> oriented device 48

49 I DLIN Enhancement (%) High gate overdrive V GS = rated voltage Longitudinal Transverse <110> <100> <100> <110> Compressive Stress (MPa) B Figure 3-8. Continued Table 3-3. Expected and measured coefficients for <100> and <110> N-LDMOS and DEPMOS with the measurement uncertainty in brackets. N-LDMOS Longitudinal Transverse coefficient (x10-11 Pa -1 ) Low E OX High E OX Low E OX High E OX (100) <110> expected (100) <110> measured -25 (2) -9 (5) -19 (8) -6 (6) (100) <100> expected (100) <100> measured -36 (7) -20 (10) -15(8) 9 (11) DEPMOS Longitudinal Transverse coefficient (x10-11 Pa -1 ) Low E OX High E OX Low E OX High E OX (100) <110> expected (100) <110> measured 52 (22) 51(24) -32 (7) -32 (6) (100) <100> expected (100) <100> measured 7 (2) 11 (4) 26 (8) 19 (8) Modeling: The -coefficient of a laterally diffused power MOSFET is estimated based on the device model which consists of an enhancement mode MOSFET in series 49

50 with a resistor [38-40]. At low V G (weak inversion), the channel resistance (R CH ) dominates the on-resistance (R ON ). Thus the -coefficient of the power device is predicted to be similar to the logic MOSFET. At high V G, R ON is dominated by the drift resistance (R D ). Hence, the -coefficient is expected to be identical to bulk silicon (Si) coefficient. Based on this concept, the expected values for the -coefficient of power MOSFET are listed in Table 3-3. The values for the logic MOSFET were taken from [8]. Since the bulk -coefficient is dependent on the doping concentration [43], measured values of n-well resistor on the wafer were used and these values were found to be similar to Matsuda s experimental values for n-bulk [47] and Smith s values were used for p-bulk [9]. As mentioned earlier, in order to accurately estimate the -coefficient of the power MOSFET along different orientations, it is important to understand the effect of strain on all components of R ON. For the LDMOSFET, R ON is the sum of channel resistance (R CH ), accumulation resistance (R ACC ), spread resistance (R S ), and drift resistance (R D ) [41]. The -coefficient of R S ( S ) is a function of the angle of spread of carriers into the bulk of the drift region. For materials with cubic symmetry such as Si, the -coefficient along any direction can be determined from the piezoresistance tensor and direction cosines [42, 43]. The dependence of S on spread angle for <110> n-channel device under longitudinal tensile stress shown in Figure 3-9 is derived as shown in Equation 3-3. Similar expressions can be derived for all stress type and directions and the result in the form of polar plots are shown in Figure 3-9,

51 coefficient coefficient coefficient coefficient coefficient [110] coefficient [100] 10 negative 10 o 20 o 20 negative 10 o 20 o positive 60 o 50o 40 o 70 o 40 [00-1] <110> channel 30 o spreading angle positive 60 o 50o 40 o 70 o o 80o <100> channel [00-1] 30 o spreading angle A coefficient [110] coefficient [100] 10 negative 10 o 20 o o 20 o 30 o 20 positive 40 o 50o o 70 o 90 o 80o 40 <110> channel [00-1] positive 60 o 50o [00-1] 80o <100> channel 70 o 40 o 30 o spreading angle B Figure 3-9. N-bulk piezoresistance vs. angle of spread in drift region under stress. A) Longitudinal B) Transverse 51

52 coefficient coefficient coefficient coefficient coefficient [110] coefficient [100] o 1 10 o o o 80o [00-1] positive 60 o 50o <110> channel 40 o 30 o spreading angle 20 o negative positive 20 o 30 o 40 o 50o 60 6 o 70 o 90 7 o 80o <100> channel [00-1] A coefficient [110] coefficient [100] o 10 o negative 40 o 50 50o 60 o o 90 o 80o 70 <110> channel [00-1] 30 o spreading angle 20 o negative 60 o 50o 40 o 70 o o 80o <100> channel [00-1] 30 o spreading angle 20 o B Figure P-Bulk piezoresistance vs. angle of spread in drift region. A) Longitudinal stress B) Transverse stress At low V G, R ON is almost completely dominated by R CH. At high V G the percentage contribution of each component to R ON for N-LDMOS is shown in Figure Since carriers are confined to the surface both in inversion (R CH ) and accumulation (R ACC ), the -coefficient of these resistances is similar to that of the logic MOSFET ( MOSFET ). On the other hand the drift resistance is similar to a bulk Si resistor ( Bulk ). Utilizing the - 52

53 Percent (%) coefficient of bulk Si and logic MOSFETs from Table 3-3, and associating a weight to each of the contributing resistance from the resistance distribution, it is possible to calculate the -coefficient of lateral power MOSFET for a given V G and channel orientation. In case of N-LDMOSFET, for rated V G, LDMOS.44 x x MOSFET S Bulk 0 (3-4) Where, x is a fitting parameter representing the percentage contribution of the R S. For the <110> channel N-LDMOS, LDMOS 9 MOSFET = 32 and Bulk = 32. Since the geometry of the STI causes the electrons to spread at an angle of ~80 o, S from Figure 3-10 at this spreading angle is 34. Substituting these values into Equation 3-4 the value of x is determined to be With this value of x in Equation 3-4, the calculated transverse -coefficient for <110> channel is -2 as opposed to the measured value of 6. Similarly using the polar plots in Figure 3-10 to determine S and using x=0.21, the calculated longitudinal and transverse -coefficients for <100> channel N-LDMOS are - 26 and 10 respectively. These calculated values are very close to the measured values R DRIFT +R SPREAD R ACCUMULATION R CHANNEL 0 Rtotal Figure On-Resistance distribution of N-LDMOS at rated gate bias Unlike the LDMOSFETs, DEMOSFETs have long channel lengths. Thus, in these devices R CH remains the dominant contributor of R ON even at high V G. This results in the 53

54 I DLIN Enhancement (%) high V G -coefficient of DEPMOS to be similar to that at low V G, as observed from Table 3-3. High Stress Experimentally measured linear drain current enhancement with stress on <100>= oriented NLDMOS and <110>-oriented DEPMOS is shown in Figure No saturation effects or non-linearity was observed in lateral power MOSFETs even at stresses as large as 500MPa. Thus performance predictions can be made using a linear relationship between the -coefficient and applied stresses up to half a GPa. <110> DEPMOS 16 <100> <110> N-LDMOS 12 8 V GS -V t =0.3V 4 V GS =rated voltage Longitudinal Stress (MPa) Figure Linear drain current enhancement under high stress Device Voltage Rating In order to increase the amount of voltage the device can withstand the length of the drain extension or drift region is increased. This causes the drift resistance of the device to become the dominating component of R ON as device rating increases. An increased R DRIFT means that the -coefficient of the power MOSFET becomes closer to the bulk value. 54

55 I DLIN enhancement (%) Figure 3-13 shows the measured strain-enhanced drain current of <100> NLDMOSFET and <110> DEPMOSFET with increasing drift length. As observed, I DLIN of <100> channel N-LDMOS is seen to increase as the drift region length (L Drift ) of the device increases. With increase in L Drift the contribution of the R S to the total R ON reduces resulting in the -coefficient of the N-LDMOS to approach that of n-bulk. For DEPMOSFETs, the low voltage rated device has no STI in the drain extension region. Thus, the lack of R S results in of low breakdown device being similar to the bulk value. As L Drift increases to increase breakdown voltage, a thick STI oxide is introduced in the drift region to reduce surface field. Despite the increase in spread resistance due to the STI geometry, no dependence with L Drift is seen from Figure 3-13 for the <110> channel DEPMOS. This is because longitudinal compressive stress does not alter the spread resistance significantly as shown in Figure DEPMOS N-LDMOS 4 Increasing L Drift Longitudinal Stress (MPa) Figure Linear drain current enhancement versus stress with increasing drift region length. 55

56 Summary Under 1GPa mechanical stress, the maximum expected I DLIN enhancement is ~20% for n-type and ~50% for p-type power MOSFET with the breakdown voltage shifting < 1 V. The strain induced on-resistance improvement was largest for the <100> N-LDMOS channel making it the preferred orientation for the strained n-type power device. For DEPMOS devices on (100) wafer the large -coefficient along the <110> direction makes it the preferred channel orientation. Modeling showed the contribution of the spread resistance is significant and needs to be factored when predicting the behavior of the strained lateral power device. With application of strain technology and careful device design to minimize spreading, it is possible to significantly reduce onresistance for a given breakdown voltage. 56

57 CHAPTER 4 SIMULATION OF STRAINED MEMORY AND LDMOSFET Introduction From the previous chapters, it is clear that strain improves data retention in Flash memory and reduces on-resistance in N-LDMOSFETs. It is also shown to deteriorate retention in Dynamic Random Access Memory (DRAM). This chapter investigates the incorporation of stress via process in the Flash and power MOSFET devices and identifies the amount of process-induced stress in DRAM structures. Current CMOS processing techniques include several methods to incorporate strain into device structure. Uniaxial stress is introduced via capping layers and embedded Source/Drain regions. Channel stress as high as 1GPa has been reported [48, 49]. Biaxial stress on the other hand is produced by growing a silicon layer over relaxed Si-Ge. The lattice mismatch introduces biaxial stress in the Si layer. The industry has adopted uniaxial stress over biaxial stress because of the larger performance improvement [50]. Although there has been significant discussion on the effect of STI induced compressive stress on device behavior [51-54], logic technology rarely uses STI induced mechanical stress for performance enhancement. This is due to the irregular patterning of STI. The structure and dimensions of Flash and power devices are very different from logic devices. In general, flash has thick gate stack layer to store data and power devices have large drift regions in order to withstand high voltages. Thus applying processing techniques of logic devices to strain these devices may not be efficient. Simulation is an effective and cost-saving method to investigate the various structures to incorporate stress in these devices. 57

58 Overview of Simulation tool S-Process (Sentaurus process) and ISE FLOOPS (Florida Object Oriented Process Simulator) are the two primary tools used to investigate the stress in device structures. The main goal of the simulation is to determine the best method to effectively strain the device structures that benefit from stress. For this reason simple structures without the complication of actual processing conditions are used. FLOOPS is a front end process modeling tool that is extremely useful in determining process induced stress in strained silicon devices. It is based on C++ and uses ALAGATOR as the scripting language. Stress in the device is determined by the stress solve command. Elastic model is used, i.e. all the stress induced displacements are within elastic limit. The boundary conditions are reflecting since the device stress is assumed to be unaffected by the boundary. Stress and strain in a complex device structure is obtained by solving basic balance of force equation and Hooke s Law using finite element method. In 2-D the simulated structure is divided into triangles of area Δ, the material properties being defined by matrix D and the node coordinates in matrix B. Hooke s law is solved at each node to obtain the displacement with the stiffness matrix given as equation 4-1. From the displacement strain and stress at each node is obtained as below. k t B DB (4-1) Bx (4-2) EBx (4-3) S-Process is a multi-dimensional process simulation tool based on FLOOPS developed by Synopsys. The scripting language is Alagator. Three dimensional 58

59 structures were simulated using this tool. Stress is determined by solving force equilibrium equations at all nodes of the mesh consisting of tetrahedrons. Various mechanics models are available for solving stress in materials such as: Viscous, Viscoelastic, Elastic, etc. Elastic model is used in our simulations along with the default Dirichlet boundary conditions which initializes the velocity normal to the plane as zero. This boundary condition is applied to left, right, front and back surfaces. The strain tensor in S-Process like FLOOPS, consists of two parts: Deviatoric- which describes the material behavior under arbitrary deformation without change in volume and Dilatational- which describes the behavior when there is a pure volume change. Built-in stress in materials can easily be defined as well as stress caused by thermal and lattice mismatch. Simulation Setup 2D simulation using FLOOPS and 3D simulations using S-Process were executed. Mesh was chosen such that the area of interest in the device had closely spaced nodes (5-100nm) resulting in a tight mesh. Mechanical constants were initialized to the values shown in Table 4-1. The stress in straining layer (oxide/capping layer) was incorporated as an intrinsic isotropic stress. Stress in the device structures were analyzed using Stress solve/ Mechdata command and stress contour plots were generated at various device cross-sections. Since the magnitude of stress changes with the material constants, amount of intrinsic stress, layer thicknesses and presence of structures such as spacer, the numerical results from these simulations may not be an exact reflection of that present in the actual device. However, the trends obtained, gives insight into strained devices. 59

60 Table 4-1. Material Constants used in simulation Modulus (GPa) Material Bulk Shear Young s Poisson s ratio <100> Silicon <110> Silicon Nitride Oxide Strain in DRAM Transistor One of the conclusions in Chapter 2 was that GIDL current in transistors increased nearly 2-3% in for 100MPa of mechanical stress. This is a real concern in DRAM. Historically the industry was focused on reducing the mechanical stress inherent to fabrication to improve DRAM retention [55, 56]. However with the current technology nodes incorporating strained silicon to boost device performance, the amount of stress in the lowly doped drain (LDD) where GIDL takes place, becomes even more significant. The 2-D simulation structure involves a MOSFET with tensile CESL with isotropic stress of 1.2GPa. Channel lengths ranging from 45nm-10um were studied. The mesh was chosen so as to be fine near in the channel region and coarser deeper into the substrate. The stress values were taken 50A below the surface of silicon at the channel center. 60

61 Strained Flash Memory Device Both floating gate and mirror bit flash memory retention shows improvement under the application of tensile stress [33]. Currently, the structure shown in Figure 4-1 is commercially available from Applied Materials. Unlike logic, Flash memory has repeating patterns of STI. This makes STI stress an excellent option to strain these devices. Normally STI induced stress is compressive in nature. However by using High Aspect Ratio Process (HARP) [57], it is possible to generate tensile stress using STI. This method along with pre-metal dielectric stress is used in Figure 4-1. Figure 4-1. Applied Materials scheme for strained NAND Flash. (Courtesy AMAT) In this work, the effectiveness of HARP STI along with PMD on a multi-gate structure shown in Figure 4-2 was simulated using 2D Floops. Tensile stress of 1GPa magnitude was the intrinsic stress present in PMD and oxide layers. The stress in the memory was monitored in the center of the array at 100A below the silicon/oxide interface. 61

62 PMD Silicon HARP STI Silicon Thickness=5 m Poly Gate Length=65nm Thickness=100nm L diff =200nm Oxide Thickness=27A STI L, W=0.5 m Stress xx,yy =1GPa Pre Metal Dielectric Thickness =80nm Stress xx,yy =1GPa Figure 4-2. Multi-gate simulation structure with HARP STI and PMD Strained LDMOSFET From the measured -coefficient it is clear that the application of certain types of mechanical stress reduces power MOSFET on-resistance without affecting its breakdown voltage. In case of <100> N-LDMOSFET, longitudinal tension or transverse compression can reduce device on-resistance. However, an important consideration in moving to a newer technology is that the industry expects at least a 20% improvement in device performance to justify the increase in manufacturing cost. It is evident from table 4-2 that nearly 1 GPa stress is needed for the required 20% reduction in onresistance. 62

63 Table 4-2. Beneficial stress for N-LDMOSFET Stress Type Strained Region Determination Reduction in R ON / GPa stress Longitudinal Tension Transverse Compression Entire Device <100> N-LDMOS 20% Drift region <100> N-Bulk resistor 17% Measured bulk -coefficients indicate that for <100> N-LDMOSFET, transverse compression on the bulk drift region can also significantly reduce R ON (~1.7% for 100MPa). This is because nearly 50% of R ON in LDMOSFET is contributed by drift region. Further in <100> oriented device, even the carriers spreading into the bulk of the drift region will experience transverse compression if the stress is along <110>. Thus all carriers are benefitted by the applied transverse compression resulting in a reduction in spreading resistance. Nitride capping Nitride capping layers can apply either compressive or tensile stress on the entire device. However capping layers are most effective for short channel devices. Thus in order to use this method to strain power devices which have large drift lengths, dummy gates with short gate lengths are utilized throughout the drift region. This structure was proposed in reference [58] and is shown in Figure

64 Nitride Capping p+ n+ STI Poly Si P-well N - -well n+ Figure 4-3. N-LDMOSFET with dummy gates and nitride capping The simulation nested gate structure (Figure 4-4) is similar to Figure 4-3 and the dimensions are based on results from reference [59]. Channel stress dependence on gate length illustrated in Figure 4-5 shows that above 50nm of gate length the stress in the channel is virtually negligible. Also the channel stress increases with decreasing gate length. Thus the dimension of the dummy gates is chosen to be 45nm. Since nearly a Giga-Pascal of longitudinal tensile stress is required for 20% device performance improvement, an in-built isotropic stress of 1GPa is present in the nitride capping layer. The simulation is used to determine the stress contours along the drift region. 64

65 X Z Poly gate YY Gate to gate distance Direction of increasing substrate depth Silicon substrate Tensile Nitride capping Si substrate: Thickness = 10 m Nitride Capping Layer Thickness = varying Default = 80nm Built-in stress = xx =1GPa, yy =1GPa Poly Si Gate Gate length = 45nm Thickness=140nm Gate to gate distance = varying Figure D capping layer on N-LDMOSFET simulation structure Figure 4-5. Longitudinal channel stress vs. gate length in logic MOSFET with tensile capping layer [59] 65

66 STI induced stress DIELEC RESURF [60] is a structure that is currently being investigated, since it has the potential to increase the breakdown voltage of the lateral power MOSFETs. This structure shown in Figure 4-6 also has regular patterns of STI interleaving the drift region. Presence of the STI increases the depletion width across the reverse-biased p-n junction thus decreasing the electric field (Figure 4-7). The regular patterns of STI in this structure can be utilized to strain the drift region of the power MOSFET. Since drift region resistance (R D ) accounts for more than half of the on-resistance (R ON ) of the device, stressing the drift region so as to reduce its resistance is also an effective method to reduce R ON. source gate N+ STI Gate STI N+ N-well STI STI STI STI P-Sub drain STI STI STI STI Figure 4-6. DIELER structure 66

67 p-si p-si W d Wd n-si n-si oxide V BD =21.5V V BD =22.5V Figure 4-7. Principle of Dielectric RESURF [60] 2D structure was simulated using FLOOPS and the three dimensional structure shown in Figure 4-8 was simulated using Sprocess. Dependence on active area (moat width) and STI dimensions was investigated along with the uniformity of stress along the STI depth. Z Moat/STI depth X Y STI Moat/STI width Active spacing oxide Silicon Silicon Substrate: Depth = 10 m STI Width=1 m Depth=1 m Built in stress xx = yy =1GPA Active region Width = 0.24 m oxide N-Silicon Silicon Substrate: Depth = 5 m Thickness = 1 m STI Width=1 m Depth=1 m Built-in stress xx = yy = zz =1GPa Active region Width = 0.24 m Figure 4-8. STI induced stress simulation structure for 2-D and 3-D 67

68 Built-in Stress [MPa] A 17% reduction in on-resistance is estimated by 1GPa transverse compression on the <100> oriented drift region. Even carriers that spread vertically into the substrate experience the transverse compressive stress, thus there is no detrimental effect due to the spreading of carriers. Results and Discussion Strain in DRAM Transistor The magnitude of stress at the center and gate-edge in devices of different channel length is shown in Figure 4-9. As is seen from figure, even long channel devices suffer from nearly 100MPa of stress at gate edge even though the channel stress is negligible. This means that the GIDL is 2-3% larger than a device without any capping layer. Thus effect of stress needs to be considered when considering strained transistors for DRAM Middle of channel Channel under gate edge Gate length, Lg [um] -100 Figure 4-9. A process simulation of built-in stress in the middle and the gate edge of the channel in a strained Si MOSFET with stressed silicon nitride CESL 68

69 Tensile Stress (MPa) Strained Flash Memory The simulation results (Figure 4-10) prove that as the distance between the STI increases (number of bits increases) the stress starts to decrease. Also the stress from the STI is maximum at its edges and progressively reduces as we move away from the STI. This means that as the number of gates in the multigated flash memory increases, the uniformity in the magnitude of the stress induced by the STI reduces. However this is overcome by the tensile PMD. The simulation result shows that STI or PMD alone is not as effective in straining the NAND flash as is the combination of the two. From the measurement on Flash devices, it was seen that even a small amount of tensile stress was able to make a measurable difference in retention. So straining the flash memory using PMD and STI can successfully enhance performance in these devices. Stress monitor device # of bits # of Bits Figure Stress in NAND flash memory due to STI and PMD 69

70 Strained LDMOS Nitride capping with dummy gates As seen from Figure 4-5, even at 45nm gate length the channel stress is ~400MPa. So the stress in device through this method is small. The dummy gate structure results in tensile stress beneath the gate but along the distance between the two gates, the stress becomes compressive (Figure 4-11). Thus stress uniformity is a concern in this structure especially since longitudinal compressive stress is detrimental to device performance. Figure Stress in nested gate structure. A) Expected simulation result of LDMOS with tensile capping layer B) Actual simulation cross section from [60] A 70

71 B Figure Continued Gate to gate distance can be reduced to decrease the non uniformity of stress type along the device. However this comes with the price of reducing the magnitude of stress. Beyond pinch off (Figure 4-12), we can expect the stress to increase, however this method is strictly limited by processing considerations. Increasing the thickness of nitride layer might improve the stress magnitude as shown in inset. However as shown in Figure 4-11, the stress magnitude over the entire device is insufficient to produce any considerable gain in device performance. 71

72 Figure Stress in device center as a function of gate to gate separation. The inset [59] shows the change in stress in device center as a function of capping layer thickness Another issue in this method is that the magnitude of stress reduces drastically as we go deeper into the substrate. This method is effective for producing stress on the surface; however electrons spread vertically into the substrate in LDMOSFETs. So the success of this method to effectively improve power MOSFET device performance is questionable. STI induced stres From results reported in reference [59] (Figure 4-13), STI is an excellent method to incorporate stress in devices. The type and magnitude of stress will depend on processing conditions of the STI oxide and dimensions of active and STI regions respectively. Stress produced via the STI interleaves is analogous to the channel stress 72

73 induced by the Si-Ge source drain in logic PMOSFETs. Thus significantly large value of transverse compressive stress is expected. Figure STI induced stress in active region [59] From literature the stress is expected to be uniform along the depth of the STI region [61, 62] and thus in LDMOSFETs even the carriers that spread into the bulk of the drift region can be strained by choosing the depth of STI taking the current flow pattern into consideration. 2D floops simulation confirms the uniformity of STI induced compressive stress along the depth direction. Also magnitudes of stress are ~650MPa. Deeper trenches are more useful in straining LDMOSFETs since the current spreads up to a micron depth in these devices. 73

74 Y Y 0 28 m X 1 m X Figure D stress contours of STI induced stress in active drain extension fingers S-process simulation of 3 D structure showed some interesting results. Not only was there large magnitudes of transverse compression, but there was also equally large out of plane tensile stresses present. Figures 4-15 through 17 show the stress contours in the silicon fin across device cross-section taken in the center of the silicon fin of the 3-D simulation structure. 74

75 yy Z Y Figure Transverse compression along the fin Z zz Y Figure Out of plane tension along the fin 75