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21. 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

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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.

21.2 Basic features of gate dielectric breakdown

21.2.1 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.

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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. 21.1. Tree and Bush type electric discharge structures

Fig. 21.2. An example of thick dielectric breakdown

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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.

It can be seen from Fig. 21.3 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).

Fig. 21.3. Regions of applied voltage stress and defect mechanism observed in each region

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21.2.2 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

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.

21.2.3 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.

Fig. 21.4. Nonlinear projection of TDDB

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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.

Fig. 21.5 Statistical distribution of failure times

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21.2.4 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.

Fig 21.6 Weibull distribution of failure rates vs. time for failure

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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);

Fig. 21.7. ITRS (dotted) v. Reliability predictions (red) for oxide scaling

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however, soft breakdown mechanism makes surviving PMOS technology even below

2nm that we shall study in following lectures.

21.2.5 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.

Fig. 21.8. NMOS v PMOS reliability. PMOS is the problem

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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.

Fig. 21.9. Soft vs. Hard breakdown

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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.

Fig. 21.10. Increase in leakage current induced by soft breakdown

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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.

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..

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

Fig. 21.11. Contact burnt out by heat dissipation of dielectric breakdown

Fig. 21.12. An example of Dielectric Breakdown Induced Epitaxy (DBIE)

Fig. 21.13. An example of Dielectric Breakdown Induced Migration (DBIM)

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silicide to be spread out which is called as Dielectric Breakdown Induced Migration

(DBIM) as shown Error! Reference source not found..

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..

Fig. 21.14. An example of Dielectric Breakdown Induced Epitaxy (DBIE) of

MOSFETs before breakdown

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In this stage, oxide layer is just broken up and then, poly gate region is re-

crystallized 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..

21.4 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.

21.5 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.

Fig. 21.15. An example of Dielectric Breakdown Induced Epitaxy (DBIE) of

MOSFETs after breakdown and re-crystallized

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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,

2002.

[21.5] Weir et al, “SOFT BREAKDOWN IN ULTRA-THIN OXIDES” , MRS 1999

[21.6] KL Pey, Tutorial,11th Workshop on Gate Oxide Technology, 2006.

[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, 1998.

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?