Breakdown Voltage Characteristics of SiC Schottky Barrier Diode with Aluminum Deposition Edge Termination Structure

Transcription

1 Journal of the Korean Physical Society, Vol. 49, December 2006, pp. S768 S773 Breakdown Voltage Characteristics of SiC Schottky Barrier Diode with Aluminum Deposition Edge Termination Structure Seong-Jin Kim, Dong-Ju Oh and Soon-Jae Yu Itswell Co., Ltd, Ochang Scientific Industrial Complex, Chungbuk Yong-Deuk Woo Woosuk University, Jeonbuk (Received 23 February 2006) This paper demonstrates the fabrication of and gives a comparison of different metal edge termination structures including aluminum (Al)-deposited guard ring and Al-deposited guard ring-assisted field limiting ring (FLR) for a high-breakdown-voltage silicon carbide (SiC) Schottky-barrier diode (SBD). In order to investigate the application feasibility of the Al-deposition junction termination to a high-breakdown-voltage SiC-SBD, three types of SiC-SBDs are fabricated by using conventional photolithography, electron beam evaporation, and thermal treatment techniques without ion implantation and thermal oxidation procedures. The breakdown-voltage characteristics of the SiC- SBD are significantly improved with the Al-deposition edge termination. The SiC-SBD without the edge termination shows less than 250-V breakdown voltage, while the Al-deposited guard ring and Al-deposited guard ring-assisted FLR structures show approximately 700-V and 1200-V breakdown voltages, respectively. The pronounced improvement in the breakdown-voltage characteristics is attributed to the reduction of electric field at the Schottky contact edge well by the Al-deposition edge termination. PACS numbers: Nr, Cg Keywords: Silicon carbide, Schottky-barrier diode, Electric breakdown, Edge termination I. INTRODUCTION Si-based electronic devices have been widely utilized in high-speed, high-frequency, and high-power applications [1,2]. However, the operation switching speed and power capabilities are approaching a performance limitation due to their poor material properties. Silicon carbide (SiC) semiconductor material, which offers a wide band gap ( 3.3 ev), a high breakdown electric field ( V/cm), high thermal conductivity (3 5 W/cm C), high electron saturation velocity ( cm/s), and stable chemical bonding, has attracted considerable attention for application to high-power and high-frequency electronic devices [3,4]. The high breakdown electric field enables us to design the high-power devices with a thinner and more heavily doped voltage blocking layer [5 8]. As a result, lightweight, compact and low-cost devices that do not suffer from performance degradation can be realized. Several studies have reported high-breakdown-voltage characteristics in 4H-SiC Schottky-barrier diodes (SBD) obtained through the incorporation of an ion implan- tation field limiting ring (FLR), a candidate edge termination technique for high-breakdown-voltage devices [9 11]. However, the fabrication method of the FLR utilizing the ion implantation technique proved complicated due to formation of an ion protection mask, multi-beam ion implantation, and additional procedures including ion activation at high temperature. Especially, a unique thermal treatment is necessary in order to achieve a clean surface without any surface contamination. Therefore, a simple technology is needed for an effective cost of power devices. This paper firstly presents the breakdown-voltage characteristics of SiC-SBDs with metal edge terminations of aluminum (Al)-deposited guard ring and Aldeposited guard ring-assisted FLR. The metal edge termination structures were fabricated by using electron beam deposition and thermal treatment techniques without ion implantation. The breakdown voltage characteristics of the SiC-SBDs were substantially dependent on the metal edge termination. The Al-deposited guard ring-assisted FLR SiC-SBD showed excellent breakdownvoltage characteristics compared to those of the Aldeposited guard ring-assisted SiC-SBD. sjkim7854@naver.com -S768-

2 Breakdown Voltage Characteristics of SiC Schottky Seong-Jin Kim et al. -S769- II. DEVICE DESIGN AND FABRICATION 1. Design of SiC-SBD The lowest on-state voltage drop and the highest breakdown voltage must be achieved for a given voltage blocking layer in order to meet the full potential applications of a high breakdown-voltage SiC-SBD. The doping concentration and thickness of the voltage blocking layer affect its specific on-state resistance and blocking voltage capability, and there is a trade-off relationship between these parameters in terms of device performance. Additionally, the blocking-voltage capability is essentially affected by the species of edge termination because that changes the peak electric field at the Schottky contact edge above the parallel plane electric field. As such, the minimum thickness and maximum doping concentration of the voltage blocking layer are basically determined from a targeted breakdown voltage and an employed edge termination. Generally speaking, either lower doping concentration or an increased thickness of the voltage blocking layer improves the breakdown-voltage characteristics, while the specific on-state resistance is simultaneously increased, resulting in a higher on-state voltage drop. The specific on-state resistance (R on ) of the voltage blocking layer using an n-type SiC can be described by R on = W D (N D )/q µ n N D, (1) where W D (N D ) and N D are the thickness and doping concentration of the voltage blocking layer, respectively, q is the electric charge, and µ n is the electron mobility of bulk SiC, depending on the doping concentration. For a design of a non-punchthrough drift region, the thickness of the voltage blocking layer is equal to or greater than the plane avalanche breakdown width. In this case, the thickness of the voltage blocking layer region can be described from the basic depletion equation as W D = [ε E CR /q N D ] [(ε E CR (N D )/q N D ) 2 (2ε V BV /q N D )] 1/2 (2) where V BV is the breakdown voltage of the SiC-SBD, ε is the dielectric constant of SiC, and E CR (N D ) is the critical electric field with respect to the doping concentration of the voltage blocking layer. The critical electric field depending on the doping concentration of the voltage blocking layer can be written as [11] E CR = [ ]/[1 0.25(log(N D )]. (3) By using the above formulas, the doping concentration and thickness of the voltage blocking layer for a targetedbreakdown-voltage SiC-SBD can be determined in order to meet the lowest specific on-state resistance without degradation of breakdown-voltage characteristics. For example, in order to achieve a 1200-V-breakdown-voltage Fig. 1. (a) FLR-width and (b) FLR-spacing dependences of the breakdown-voltage characteristics for the guard ringassisted FLR SiC-SBD simulated by the numerical twodimensional SILVACO ATLAS simulator. SiC-SBD, the optimized doping concentration and thickness of the voltage blocking layer become cm 3 and 9.5 µm, respectively, and then the specific onstate resistance is 0.68 mω-cm 2. The key parameter in the design of a guard ringassisted FLR SiC-SBD is the geometric layout of the guard ring and FLR dimensions. Fig. 1 shows the theoretical breakdown-voltage characteristics of the guard ring-assisted FLR SiC-SBD as a function of FLR width and FLR spacing on the basis of a two-dimensional numerical device simulation using the SILVACO ATLAS simulator. A 4H n + -SiC substrate (carrier concentration: cm 3 ), a 12-µm-thick n -SiC voltage blocking layer (carrier concentration: cm 3 ), a 0.5-µm implantation depth, and seven FLRs with an Al-implantation concentration of cm 3 were chosen for the device simulation. The Schottky contact edge of the SiC-SBD overlaps with a 6-µm-wide guard ring and the overlap width is 3 µm. The breakdown-voltage characteristics are greatly dependent on the FLR width and FLR spacing, as shown in Fig. 1. The peak electric fields of the guard ring-assisted SiC-SBD and guard ring-assisted FLR SiC-SBD at the Schottky contact edge were, respectively, 14 % and 27 % lower than that of the normal SiC-SBD without the edge termination under 1200-V reverse-bias condition. Reduction of peak electric field at the Schottky contact edge yields an improvement in the breakdown-voltage characteristics of the SiC-SBD. An improvement in the breakdown voltage characteristics is obviously observed as the FLR width is increased, as shown in Fig. 1(a). An approximately V breakdown voltage, corresponding to 99 % of the ideal breakdown voltage, is observed for FLR dimensions of 5 µm width and 2 µm spacing. However, degradation of the breakdown-voltage characteristics is observed as the FLR spacing increases, as shown in Fig. 1(b). Furthermore, the breakdown voltage of a SiC-SBD with an FLR of 3-µm width and 2-µm spacing was also improved with an increased number of FLRs and was saturated from five FLRs. SiC-SBDs with one FLR and five FLRs, respectively, showed breakdown voltages of roughly 900

3 -S770- Journal of the Korean Physical Society, Vol. 49, December 2006 Fig. 3. (a) Plan-view SEM image, and (b) high-resolution plan-view SEM image of the type-c SiC-SBD with the Aldeposited guard ring-assisted FLR. Fig. 2. Plan views and cross-sectional schematic diagrams of SiC-SBDs with (a) the Al-deposited guard ring (type B), and (b) the Al-deposited guard ring-assisted FLR (type C). V and 1650 V. It appears that incorporating a greater number of FLRs is an efficient method of attaining a SiC-SBD having characteristics of high breakdown voltage and high reliability. 2. Fabrication of SiC-SBD Three types of SiC-SBD were fabricated by using conventional photolithography, electron beam deposition, and thermal treatment techniques in order to investigate the application feasibility of the Al-deposition junction termination to a high-breakdown-voltage SiC-SBD. The type-a device is a normal SiC-SBD without edge termination. The type-b and type-c devices have an Al-deposited guard ring and an Al-deposited guard ringassisted FLR, respectively. Fig. 2(a) shows a schematic diagram of the type-b SiC-SBD. The corresponding top contact pattern and cross-sectional schematic diagrams are also shown in Fig. 2(a). The type-b SiC-SBD consisted of a cathode, a 12-µm-thick voltage blocking layer, an anode, and an Al-deposited guard ring structure. In order to realize both a higher breakdown voltage and a lower on-state voltage drop, a 12-µm-thick voltage blocking layer with cm 3 doping concentration was chosen. The 12-µm-thick n -SiC voltage blocking layer was grown on an 8 -off 4H n + -SiC substrate by Sixon Co., Ltd. The resistivity and micropipe density of the 4H n + -SiC substrate are Ω-cm and less than 50 cm 2, respectively. The Schottky contact edge is overlapped with a 6-µm-wide Al-deposited guard ring and the overlap width is 3 µm. The corresponding top contact pattern and cross-sectional schematic diagram of the type-c SiC-SBD are shown in Fig. 2(b). The type- C SiC-SBD is the same as the type-b SiC-SBD, except for seven FLRs with 3-µm width and 1.5-µm spacing surrounding the periphery of the Schottky contact edge. All SiC-SBDs were designed with µm 2 device size and 400-A/cm 2 current density. The SiC-SBDs were fabricated by the following process sequence. After wafer cleaning using hot acetone, buffer oxide etchant (BOE), and HCl, 50-Å-thick Al metal was deposited on the top surface of the wafer by using electron beam evaporation to serve as metal edge termination. The wafer was annealed in Ar + ambient at 1000 C for 10 minutes by a rapid thermal process (RTP). Subsequently, Ni (1500 Å)/Ti (300 Å) metals were deposited onto the wafer backsurface by using electron beam evaporation to serve as ohmic contact and cathode, and the wafer was then annealed at 900 C for 90 seconds under N 2 ambient. Ti (50 Å)/Au (3000 Å) metals were deposited onto the Ni/Ti metals to prevent oxidation and contamination during device processing and the wafer was then annealed at 400 C for 90 seconds. In order to ensure a lower on-state voltage drop, it is important to decrease ohmic contact resistance formed on the backsurface of the wafer. After cleaning the surface of the wafer by using BOE, Ni (500 Å)/Al (5000 Å) metals were deposited onto the SiC voltage blocking layer by using electron beam evaporation to serve as Schottky contact and anode. The anode was not annealed, to prevent degradation of the Schottky barrier height. Recently, our study showed that a thermal treatment from 500 C to 600 C reduces the barrier height of Ni-based Schottky metal formed on a SiC voltage blocking layer [12]. A greater barrier height gives a minimal leakage current, resulting in a higher-breakdown-voltage characteristic. Fig. 3 shows a scanning electron microscope (SEM) image of the type-c SiC-SBD. The SEM image shows that the Al-deposited guard ring-assisted FLR structure at the periphery of the Schottky contact edge was successfully formed. The sample was dipped in 3M HC-40 insulating solu-

4 Breakdown Voltage Characteristics of SiC Schottky Seong-Jin Kim et al. -S771- Fig. 4. I V characteristics of SiC-SBDs with respect to different Al-deposited metal edge terminations. tion in order to measure the high current-voltage (I V ) characteristics without epoxy encapsulation. The forward and reverse I V characteristics were measured at the wafer level by using a HP-4156 semiconductor parameter analyzer and a Tektronix-310 curve tracer, respectively. III. RESULTS AND DISCUSSION The on-state voltage drop is mainly affected by the Schottky-barrier height between the semiconductor and Schottky metal at the low-voltage region and by the specific on-state resistance of the voltage blocking layer at the high-voltage region. In order to reduce the on-state voltage drop, the parasitic resistances induced from the substrate as well as the contact resistances of cathode and anode must be decreased. Partial Schottky-barrier lowering and leakage paths induced from micropipes and crystal defects in the Schottky contact area could affect the breakdown-voltage characteristics of SiC-SBDs, as the SiC-SBDs are vulnerable to leakage current. Fig. 4 shows I V characteristics of the three types of SiC-SBDs with respect to the various Al-deposited edge terminations at room temperature. All SiC-SBDs show good Schottky on-state I V characteristics. Onstate voltage characteristics of all SiC-SBDs are quite similar, irrespective of the edge termination structure. In contrast, the reverse leakage currents are significantly dependent on the edge termination structure. The turnon and on-state voltages of the type-b SiC-SBD are 1.2 V at A and 2 V at 0.1 A, respectively. The specific on-resistance of the SiC-SBD is as low as 5 mωcm 2. A Schottky-barrier height between semiconductor and Schottky metal, calculated by using the theoretically predicted value of the Richardson constant (146 A-cm 2 - K 2 ), was found to be 1.44 ev. A measured ideality factor of the SiC-SBD obtained from the slope of the forward I V plot was These results indicate that a Schottky contact was formed well on the voltage blocking epitaxial layer [5]. The breakdown electric field corresponding to the doping concentration and thickness of the studied voltage blocking layer was 2.3 MV/cm [13]. The punchthrough ideal breakdown voltage was estimated to be 2050 V from formulas (2) and (3). The normal SiC-SBD without the edge termination (type A) shows less than 250-V (only 12 % of the ideal breakdown voltage) breakdown voltage (solid line), whereas the type-b SiC-SBD with Al-deposited guard ring structure shows approximately 700-V (34 % of the ideal breakdown voltage) breakdown voltage (dashed line), as shown in Fig. 4. Furthermore, a prominent improvement in the breakdown-voltage characteristics of the type-b SiC-SBD was observed as either annealing temperature or annealing time was increased. The premature breakdown of the type-a SiC-SBD without the edge termination is possibly due to relatively large electric field crowding at the periphery of the Schottky contact edge, compared to the type-b SiC-SBD with the Al-deposited guard ring. The type-c SiC-SBD with Al-deposited guard ringassisted FLR structure also shows a typical reverse I V characteristic (dotted line), as shown in Fig. 4. The reverse leakage current of the type-c SiC-SBD was estimated to be 95 µa at 1200-V reverse voltage. This implies that the breakdown voltage of the Al-deposited guard ring-assisted FLR SiC-SBD is approximately 1200 V (59 % of the ideal breakdown voltage), which can be compared to the ion-implanted FLR-assisted SiC- SBDs reported in the previous literature [14]. In other words, the metal-deposited junction termination technique, without relatively complicated procedures such as ion implantation and the unique thermal treatment, will be an efficient means of attaining high-breakdownvoltage characteristics. There are several noteworthy mechanisms in the improvement in the breakdown voltage characteristics of the Al-deposited guard ring-assisted FLR SiC-SBD: 1) Al diffusion into the voltage blocking layer; 2) formation of Al silicide between Schottky metal and semiconductor; 3) field plate termination by guard ring metal oxidation. The Al diffusion into the voltage blocking layer was expected to generate minority carriers, resulting in a reduction of peak electric field at the Schottky contact edge. In order to investigate the Al diffusion into the voltage blocking layer, the Al-deposition edge terminations were removed from the device. The measured reverse I V characteristics of the SiC-SBD showed quite similar breakdown-voltage characteristics, compared to the normal SiC-SBD without the Al deposition junction termination. It appears that Al was not deeply

5 -S772- Journal of the Korean Physical Society, Vol. 49, December 2006 demolition of the FLR induced by electric-field crowding, which occurs at a relatively higher reverse voltage. Consequently, in order to provide a higher-breakdownvoltage SiC-SBD, crystal defects such as micropipes in the substrate and inclusions in the voltage blocking layer should be reduced, and the design of the edge termination has to be optimized. Fig. 5. SEM images of Al-deposited guard ring-assisted FLR SiC-SBDs destroyed at (a) 500-V, and (b) 1200-V, reverse voltages. diffused into the voltage blocking layer by the thermal treatment at 1000 C. In addition, the Al-based contact resistance formed on the studied voltage blocking layer was dramatically increased on increasing annealing temperature. This suggests that Al silicide would be formed at the metal-semiconductor interface. It is worth noting that the breakdown-voltage characteristics of the Al-based SiC-SBD without edge termination were almost the same as those of the Ni-based SiC-SBD without edge termination. Moreover, the Al-deposited guard ring-assisted FLR improved the breakdown-voltage characteristics more efficiently than the Al-deposited guard ring, as shown in Fig. 4. This indicates that the breakdown-voltage characteristics were not improved by the field plate effect through the Al-deposited guard ring. Therefore, we consider that the improvement in the breakdown-voltage characteristics is mainly attributed to the electric-field lowering induced from the mixed effects of the Al diffusion into the voltage blocking layer and the Al silicide formation at the semiconductor-metal interface. However, in order to confirm the mechanism of the breakdown-voltage characteristics improved by the Al-deposition edge termination, it is necessary to further systematically investigate the metal-semiconductor interface by using SIMS and the surface charge distribution depending on the annealing temperature of the metal edge termination. After the breakdown testing, the breakdown origin of the type-c SiC-SBD was investigated, as shown in Fig. 5. The SiC-SBDs destroyed at 500-V and 1200-V reverse voltages are shown in Fig. 5(a) and (b), respectively. Prior to SEM observation, a SiC-SBD was forcibly destroyed and the anode metal was then removed. The SiC-SBD destroyed at a relatively lower reverse voltage shows that there are leakage paths not in the FLR region but in the Schottky contact region, as shown in Fig. 5(a). However, for the SiC-SBD destroyed at a higher reverse voltage, the leakage passes are found only in the FLR region, as shown in Fig. 5(b). Therefore, it appears that there are mainly two origins of the breakdown phenomena in the SiC-SBD: crystal defects such as micropipes in the SiC substrate and inclusions in the voltage blocking layer, which occur at a relatively lower reverse voltage before breakdown induced by the FLR demolition, and IV. CONCLUSIONS We firstly described the breakdown-voltage characteristics of SiC-SBDs in relation to the Al deposition edge termination. The breakdown-voltage characteristics of SiC-SBDs were significantly improved by incorporating the Al-deposited guard ring and Al-deposited guard ringassisted FLR structures. The normal SiC-SBD without the edge termination showed less than 250-V (13 % of the ideal breakdown voltage) breakdown voltage, while the SiC-SBD with the Al-deposited guard ringassisted FLR exhibited over 1200-V (over 59 % of the ideal breakdown voltage) breakdown voltage. The pronounced improvement in the breakdown characteristics was attributed to the electric-field lowering induced from the Al-deposition junction termination at the Schottky contact edge. There were two origins of the breakdown phenomena in the Al-deposited guard ring-assisted FLR SiC-SBD: crystal defects such as micropipes and inclusions, which occurred at a relatively lower reverse voltage before breakdown induced from the FLR demolition, and demolition of the FLR structure induced from electricfield crowding at the Schottky contact edge. ACKNOWLEDGMENTS This work was carried out as a part of the SiC Device Development Program supported by the Ministry of Commerce, Industry and Energy of Korea. REFERENCES [1] I. J. Kwon, J. H. Gil, K. R. Lee and H. C. Shin, J. Korean Phys. Soc. 42, 251 (2003). [2] J. H. Gil, I. J. Kwon, H. C. Shin and Y. H. Cho, J. Korean Phys. Soc. 42, 241 (2003). [3] S. Sriram, R. Siergiej, R. Clarke, A. Agarwal and C. Brandt, Phys. Stat. Solidi (a) 162, 441 (1997). [4] C. Carter, J. Tsvetkov, R. Glass, D. Henshall, M. Brady, S. Muller, O. Kordina, K. Irvine, J. Edmond, H. Kong, R. Singo, S. Allen and J. Palmour, Mater. Sci. Eng. B 61, 1 (1999). [5] R. Singh, J. Cooper, M. Michael and R. Melloch, IEEE Trans. Electron Devices 49, 665 (2002).

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