21. LECTURE 21: INTRODUCTION TO DIELECTRIC BREAKDOWN

Transcription

1 LECTURE 21: INTRODUCTION TO DIELECTRIC BREAKDOWN 21.1 Review/Background This class is an introduction to Time Dependent Dielectric Breakdown (TDDB). In the following 9 chapters, we will discuss the theory of TDDB in terms of thin and thick dielectrics and statistics of it. Dielectric breakdown has been with us for a long time. Corona discharge, arc discharge, discharge in gas tube are examples of dielectric breakdown which are reversible, that is, the dielectric is restored to its original state once the stress voltage is removed and no permanent damage is expected (except perhaps at the electrodes). Nowadays, these processes arewidely used in plasma display, fluorescent lamps and so on which depend on the reversible breakdown gases and vapor for their operation. Since people have used electricity as power sources about one hundred years ago, dielectric breakdown in solid state material has been observed which is an irreversible process. Breakdown phenomenon of metal wires and connector coated with insulation materials has become issues of long haul power lines. Problem of dielectric breakdown process occurs in MOSFETs. Although Negative Bias Temperature Instability (NBTI) and Hot Carrier Injection (HCI) involve in interface between semiconductors and insulator film, TDDB involves breaking Si-O bonds in a bulk insulator which does not relax and hence defects continue to build up accumulatively throughout the lifetime of the operation of the device. After the invention of the MOSFETs, historically, TDDB has not been seriously concerned compared to other reliability issues like NBTI and HCI because relatively thicker dielectric and low applied voltage; however, the continued scaling of MOSFETs 98

2 99 dimension causes TDDB to be a governing issues for developing highly scaled MOSFETs. Aggressive scaling of dielectric in MOSFETs for higher electrical performance leads to new insight into the phenomena when it was observed the breakdown mechanism behind of TDDB of thin and thick dielectric are very different and as we will discuss later this difference is a reason how industry can survive. Briefly, the reason is thick dielectric breakdown is correlated but thin dielectric breakdown is uncorrelated and random. Therefore the TDDB in thin dielectric needs statistical approach because its thin layer consists of a few atoms. The different TDDB mechanism allows thin dielectric film to have much longer life time and better survivability. It was not understood well in 1990s and has become clear only in mid 2000s. We think TDDB is catastrophic degradation because it happens accidentally during normal operation; however, NBTI and HCI is parametric degradation because it shows gradual degradation and it is finally broken Basic features of gate dielectric breakdown Feature: Breakdown can correlated or uncorrelated There are two kinds of the breakdown mechanism of the defect formation. Briefly, the thick dielectric is the correlated breakdown and the thin dielectric is the uncorrelated breakdown. First of all, TDDB in thick dielectrics were first noticed in 1950s and 1960s in power lines, where the dielectrics were seen to break in a characteristic pattern, known as the water tree and thus named because it was thought that the cause of the breakdown was water seepage and because the breakdown looked like a tree (thick dielectric correlated breakdown). The dielectric in this case looks like as in Error! Reference source not found. 99

3 100 Fig Tree and Bush type electric discharge structures This was a very important reliability issue, especially in long haul undersea cables. Lightning is also a dielectric breakdown phenomenon, where the clouds and the ground act like the plate of a capacitor and the air in between being the dielectric. The physics of the breakdown is same as in thick dielectrics (same fractal dimension) Fig An example of thick dielectric breakdown 100

4 101 On top of that, We observe TDDB of thin dielectrics in modern CMOS inverter because of the oxide thickness being a couple of nanometers. The nature of breakdown is very different in thin dielectrics compared to thick dielectrics. It is uncorrelated and random in nature. For thin dielectrics the transport of the electrons through it is ballistic in nature, therefore carriers don t lose energy in the oxide. This kind of transport is also completely dominated by the physics of the contacts (gate and the substrate) and we shall study it in details in the following lecture. Fig Regions of applied voltage stress and defect mechanism observed in each region It can be seen from Fig that TDDB occurs at a constant stress. At transition, we have HCI. The difference between NBTI and TDDB is one of the voltage regime and the defect generation region (in the oxide). NBTI is a low voltage phenomenon that occurs at the Si-SiO2 interface, TDDB occurs in the bulk of the oxide. Historically scaling of PMOS was less problematic from the point of view of TDDB. However, as we shall see in later lectures, it became a much more dominant problem than TDDB in NMOS scaling. Around early 2000 s or so, TDDB became a major issue due to low oxide thickness and reduction of HCI due to the use of Lightly Doped Drain (LDD). 101

5 Features: TDDB voltage (not field)-accelerated TDDB is a voltage accelerated process not field accelerated process which is a minor role in TDDB. To estimate the life time of a device, we can do a voltage accelerated testing as shown inerror! Reference source not found.. However a big problem with such a test is that of the extrapolation of the results for a few samples at high voltage and small breakdown time to low voltage and long times because of the inherent non-linear nature of the phenomena (as we will see in later lectures) due to Fig Nonlinear projection of TDDB presence of a threshold. Extrapolation for such processes with a threshold is non-linear in nature, which was not understood for a long time (till early 2000s). It is this precise non-linear projection that allowed the scaling efforts to continue by providing much more lifetime for the devices than is predicted by a linear model Features: Failure times are Weibull distributed The process of breakdown in MOSFETs is statistically distributed due to vertical stacks of the defect. Therefore in a collection of devices the time of failure of each device is different. 102

6 103 Fig Statistical distribution of failure times Each device in a circuit may have different breakdown times (Error! Reference source not found.). The net reliability of the circuit is then dependent on the reliability of the least reliable device. The range of distribution(weibull distribution as we shall see in later lectures) of the failure time is very important because the upper values maybe orders of magnitude different from the lower values and for reliable operation the lower tail should be high. It is the lower tail that has to be certified. The average value has no meaning in TDDB. 103

7 104 Fig 21.6 Weibull distribution of failure rates vs. time for failure Features: NMOS vs. PMOS Reliability In 1990 s and early 2000 s there was stalemate in the industry because the reliability models prevalent at that time predicted severely degraded reliability performance for the ITRS predicted scaling for the oxide. 104

8 105 Fig ITRS (dotted) v. Reliability predictions (red) for oxide scaling In Error! Reference source not found., we can see the maximum voltage that is allowable for safe operation vs. the oxide thickness. The black dotted line shows the industry consensus about what should be the voltage scaling that would maintain the performance. However, the predicted models showed that at about year 2000, this threshold would be crossed and if the industry roadmaps would be followed, severe reliability penalties would be incurred. The other alternative would be to follow the red curve of reliability prediction for safe voltages, but this would incur performance penalties due to lower operating voltage. This issue was resolved by understanding the fundamental non-linear physics of the dielectric breakdown phenomena. As we can see from Error! Reference source not found., the NMOS safe operating voltages continued to be higher by almost 1V over the ITRS voltage requirements, but PMOS reliability degraded, especially below 2 nm. This is due to the phenomena of minority ionization in case of PMOS (absent in NMOS); 105

9 however, soft breakdown mechanism makes surviving PMOS technology even below 2nm that we shall study in following lectures. 106 Fig NMOS v PMOS reliability. PMOS is the problem Features: Soft vs. Hard breakdown The breakdown need not be catastrophic in nature (molten poly silicon); it can be soft (without melting the poly silicon, only small pinholes) and cause the device to degrade at a much slower rate. This can raise the lifetime of the device by orders of magnitude and is a major reason why devices continue to perform reliably for a long time. 106

10 107 Fig Soft vs. Hard breakdown In Error! Reference source not found., we can see such a distinction between the hard breakdown and soft breakdown. It can be seen that for the hard breakdown, if a constant current is desired, after breakdown the voltage that should be applied on the gate should be very small, meaning that for same amount of voltage as before the breakdown, we would expect a lot more current. However, we can see that for soft breakdown this problem is not so drastic and the device can maintain the constant current characteristics even after the breakdown without much difference in the gate voltage as shown in Error! Reference source not found. Therefore the current will not increase much under the constant voltage regime. 107

11 108 Fig Increase in leakage current induced by soft breakdown 21.3 Physical characterization of breakdown spot TDDB can cause significant damage to the device. It can melt the silicon and make it go through the oxide into the poly silicon gate and eventually it reaches to source drain contact. Following figures describes various breakdown spots. 108

12 109 Fig Contact burnt out by heat dissipation of dielectric breakdown Serious heat dissipation from breakdown melts entire things such as dielectrics, poly-si gate and even source and drain contact as shown in Error! Reference source not found. Fig An example of Dielectric Breakdown Induced Epitaxy (DBIE) It is very difficult to take this image that Dielectric Breakdown Induced Epitaxy (DBIE). It looks like broken dam representing oxide layer as shown Error! Reference source not found.. Fig An example of Dielectric Breakdown Induced Migration (DBIM) Sometimes, engineer used silicide materials for gate contact to reduce resistivity of the gate contact. In this case, the serious heat from the dielectric breakdown makes 109

13 silicide to be spread out which is called as Dielectric Breakdown Induced Migration (DBIM) as shown Error! Reference source not found Fig An example of Dielectric Breakdown Induced Epitaxy (DBIE) of MOSFETs before breakdown In this case, DBIE is still in progress. Thus, the dielectric becomes short which means the defects are forming by the breakdown process. Here, the device may still be functional as shown Error! Reference source not found.. 110

14 111 Fig An example of Dielectric Breakdown Induced Epitaxy (DBIE) of MOSFETs after breakdown and re-crystallized In this stage, oxide layer is just broken up and then, poly gate region is recrystallized by the heat from the breakdown process. As we saw above figures, the dielectric breakdown process causes several negative effects on the devices and the process damages the device entirely as shown Error! Reference source not found Time-dependent defect generation A very important consideration in all our reliability issues till now has been the time exponent for the defect generation. We need to understand how it changes with change in bias and temperature. Also, the Si-O bonds are distributed in energy. In HCI, they do not all break at the same time. In TDDB due to the huge amount of energy involved in the process of defect generation, all the bonds can break (we will see the mechanism in details in later lectures). Thus the universality of the time exponent is expected to be different Conclusions We saw that the dielectric breakdown has a long history with broad implications for technology. We also saw that the physics of the breakdown of thin and thick dielectrics in very different. In thin dielectrics, understanding of the statistical distribution of the failure times is very essential. Also, measurement of TDDB and extrapolation is difficult due to non-linear nature of the phenomenon; therefore a theory of dielectric breakdown is essential along with accurate measurement techniques. 111

15 112 REFERENCES [21.1] V. Lopatin, M.D. Noskov, R. Badent, K. Kist, A.J. Swab, Positive Discharge Development in Insulating Oil: Optical Observation and Simulation IEEE Trans. On Dielec and Elec. Insulation [21.2] Alam M A, Bude J D and Ghetti A Field acceleration for oxide breakdown-can an accurate anode hole injection model resolve the E vs. 1/E controversy? 2000 IRPS Proc [21.3] Pey et al, Physical analysis of Ti-migration in 33 Å gate oxide breakdown, 2002 IRPS Proc [21.4] D. R. Volters and J. F. Verwey, Breakdown and Wearout Phenomena in SiO2 Films, Chap. 6 p.329, in Instabilities in Silicon Devices. G. M. Barbottin and A. Vapaille Eds., 1986 Elsevier Science Publishers. Stathis, IBM J. Res/Dev, 46, [21.5] Weir et al, SOFT BREAKDOWN IN ULTRA-THIN OXIDES, MRS 1999 [21.6] KL Pey, Tutorial,11th Workshop on Gate Oxide Technology, [21.7] LJ Tang et al., IEEE TDMR., 4(1), 2004, p. 38. [21.8] J. H. Stathis and D. J. DiMaria, IEDM Technical Digest, p.167, QUESTIONS 1. Mention a few differences between thick and thin oxide breakdown. 2. For thin oxides, is PMOS or NMOS more of a concern? 3. What is a Water-tree? Does it arise in thick or thin oxides? 4. What was the controversy in 1990s? And what was the resolution? 5. What are three characteristics in thin oxides that we should think about? 6. In what ways is TDDB comparable with NBTI and HCI time-degradation? Which one does it compare well to and why? 7. Why do you suspect that hard breakdown destroys the oxide while in thin oxides breakdown can be soft? 112