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ESD protection in deep submicron CMOS technology -- Does the


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									       ESD protection in deep submicron CMOS technology
                   -- Does the transient matter?

                     Kin P. Cheung and Avid Kamgar
   Bell Laboratories, Lucent Technologies, Murray Hill, NJ, 07974 USA

                  Abstract                          more traps in the gate-oxide than the entire
                                                    ESD event clamped at the holding voltage.
    Common ESD protection devices have a            This prediction has serious consequences in
snap-back characteristic similar to a silicon-      ESD protection design because majority of the
control-rectifier. The transient voltage            existing designs will have to be abandoned.
required to trigger these devices usually is not    However, Cheung's prediction is based on data
an important design criterion as long as it is      collected at long term stress experiments.
not too high. We show that when gate-oxide is       Whether or not the trap generation process
thin, this voltage transient creates far more       remains the same when the stress is in sub-
defects in the gate-oxide than the main ESD         nanosecond time scale is still an open
event clamped at the holding voltage. Due to        question. In this work, we experimentally
difficulty in measurement, this oxide reliability   demonstrate that the trap generation rate as a
degradation can lead to chip failure but not        function of stress voltage remains unchanged
show up in simulated ESD test.                      at very short time scale.

1. Introduction                                     2. Experimental

    ESD damages are mostly high current
related. Voltage related damage modes such as                       0.5ns/div
Gate-oxide breakdown are less often
encountered. As gate-oxide gets thinner,
however,      gate-oxide     degradation    and
breakdown becomes increasingly important.
To guard against such failure, ESD design                 1V/div
must limit the voltage experienced by the
affected circuits during an ESD event. In a
recent paper, Ameraseka et al [1] argued that
the critical voltage level that must be
controlled in a snap-back protection device is
holding voltage, not trigger voltage. Their         Fig. 1. Oscilloscope trace of the 500ps (full
explanation was that the trigger voltage,           width at half maximum) stress pulse. Scope
although higher by several volts or more, is        rise time is 300ps and pulse rise time is
                                                    <=200ps. The convoluted rise time is 360ps.
not important because the time spent at the
trigger voltage is very short (<0.5ns).
    More recently, Cheung [2], using reported          10x0.252 NMOS transistors were used in
[3] stress voltage-dependent trap-generation        the experiment. Gate-oxide thickness was 15Å
                                                    (physical, 23Å electrical). 5V, 500ps (FWHM)
rate, showed that the short transient, even
                                                    pulses were applied to the gate through a
when it is only 3 volts higher than the holding
                                                    transmission line arrangement to ensure (and
voltage, will generate orders of magnitude
to verify) that the pulse height and shape                          1.0
                                                                    0.5         Bias: negative
remain unaltered at the gate. Fig. 1 shows the
                                                                    0.0         Pulses: positive 5V
scope-limited trace of the stress pulse. The

actual rise and fall time of the pulses are                        -1.0
<=200ps. The experiment is constant voltage                        -1.5
TDDB with pulsed pre-stress. The pre-stresses                      -2.0                                      0Vbias/1kp
were 1 pulse with -2.5V bias on                                    -2.5                                      1Vbias/1kp
source/drain/well, 1000 pulses with -1.5V, -1V                     -3.0
                                                                                                             base line
and 0V bias on source/drain/well. So the actual                    -3.5
voltages across the gate-oxide are 7.5, 6.5V,                      -4.0
                                                                          0.5         1        1.5     2      2.5       3
6V and 5V. For each pulse, the                                                            log(t) [seconds]
source/drain/well bias is on for only 2ms. It is
assumed that the negative stress due to the bias       Fig. 3. TDDB distribution at 3.6V after various
during the pulse-off time has negligible effect        pre-stress conditions. The 0V bias data is
on the TDDB lifetime. As a base line, TDDB             indistinguishable from base line except the
for 100 devices were performed at 3.6V                 abrupt deviation at low percentile range. All
without pulsed pre-stress. All post-pulsing            other pre-stresses cause gradual increases in
                                                       deviation from base line as we move to lower
stresses are also performed at 3.6V. Fig. 2
shows the various stress conditions.
                   100ps                                  From the base line distribution, we
                                                       determined that the Weibull slope for the
                                                       particular oxide is 1.11 and that the median
                                                       time to breakdown is 300s. We estimated,
                                                       from data in reference 4, that the critical trap
                           5.5V                        density at 50% breakdown probability of our
                                                       devices is 2.24E16/cm3. With these quantities,
                           3.6V                        we constructed the time to breakdown
                                                       distributions for various levels of pre-existing
                                                                                 No latent traps
                                                                    1            1E13.4 latent traps
                                                                    0            1E15.3 latent traps

Fig. 2. Various pulse pre-stress conditions.                                     1E15.9 latent traps
The pulses are shifted higher by biasing the                       -1
source/drain/well of the transistor negatively.                    -2
3.6V is the post-pulse TDDB stress voltage.
5.5V is where the voltage acceleration factor
changes value according to [3].                                    -4                          Ncrit (F=0.5)=1E16.35
3. Results and discussions                                              0.5          1        1.5      2     2.5        3
                                                                                          log(t) [seconds]
   Fig. 3 shows the results of all the                 Fig. 4. Simulated TDDB distribution with
measurements. It is clear that all pre-stresses        various latent (pre-existing) traps. The no
                                                       latent trap distribution uses parameters
degraded      the     gate-oxide     breakdown
                                                       appropriate for the oxide sample in the
distribution, particularly at the low percentile       experiment.
region. To analyze the results more
quantitatively, we compare it to simulations as           Fig. 4 shows the simulated breakdown
described below.                                       distributions with pre-existing trap densities of
                                                       2.5E13, 2.0E15 and 7.94E15. These values are
chosen to simulate the observed breakdown                 devices, using scaling relations (fig. 5) and the
distributions. As we can see, if significant              base line data, to be 8.8s. Since an ESD
amount of pre-existing trap exist, the                    event typically lasts less than 0.3s, a holding
breakdown distribution will deviate from the              voltage of 3.6V is more than adequate to
virgin distribution at the low percentile range.          protect the circuit from being damaged, if no
This deviation increases gradually.                       transient exist.
    In fig. 3, breakdown distribution of the 0V               On the other hand, a single 7.5V, 500ps
bias pre-stress case follows closely the base             pulse causes about 5% of the 2.52 devices to
line distribution. The abrupt deviation at the            fail. Scaling to the 10,0002 devices, the
low end is not consistent with simulation and             failure rate would be close to 100%. We have
may be just noise due to limited samples. The             thus demonstrated, experimentally, that a
distribution for all other cases show gradual             trigger voltage of 2.9V over the 3.6V holding
increase in deviation, consistent with high               voltage will seriously damage the circuit with
level of pre-existing traps.                              15Å thick gate-oxide under protection.
    It has been reported [3] that the voltage                 If we assume that the trigger transient is
acceleration factor is about 4.7dec/V at                  1000 times shorter than the ESD event, we can
between 3 to 5.5V and about 2dec/V at 6V and              estimate that the maximum trigger voltage
above. We expect, therefore, that for our 500ps           excursion to be 0.67V (based on the 4.5dec/V
pulses, most of the traps are generated within            acceleration factor). Above that, the trigger
the ~100ps around the peak. Thus, 1000 pulses             transient will be the dominant trap generation
is equivalent to 100ns of stress. When we use             process. In other words, we determined, for
this value and the simulation to calculate the            15Å thick gate-oxide, the holding voltage
acceleration factors that will fit our data, we           could be slightly more than 3.6V while the
come away with acceleration factors of                    trigger voltage could be 4.4V. Such low
~4.5dec/V at between 3 to 5V, ~2.1dec/V at                trigger voltage is difficult to achieve in most
between 5 to 6V and 1.3dec/V at between 6                 designs.
and 7.5V. We therefore conclude that the trap                Interestingly, similar estimation suggest that
generation mechanism remains unchanged at                 even when the oxide is not so thin, a trigger
sub-nanosecond stress.                                    excursion of more than 1.5V (based on 2dec/V
                                                          acceleration factor) will dominate the trap
                         ln (1 - F1 ) æ t1 ö              generation. Note that the voltage acceleration
    Failure fraction:                =ç ÷
                         ln (1 - F2 ) ç t 2 ÷
                                                          factor levels off at voltages beyond 6V. The
                                      è ø                 1.5V maximum excursion thus applies to a
                        1       t: time to breakdown      large range of oxide thickness.
               æ ö
    Area: t1 = ç A2 ÷
                            b   F: failure fraction            It is important to recognize that the
          t2 ç A1 ÷
               è ø
                                A: area                   requirement for gate-oxide reliability is
                                b: Weibull slope
                                                          100ppm failure in 10 years of operation. This
                                                          requirement can be translated to a well-defined
Fig. 5. Scaling         relationships      of   Weibull   critical trap density, which normally builds up
statistics.                                               slowly over the oxide lifetime. If the voltage
                                                          stress during an ESD event creates a trap
   During an ESD event, a voltage pulse is                density in the oxide equal to a significant
presented to all devices connected to the I/O             fraction of the critical trap density, the time to
pad where the event occurs. The total active              build up the critical trap density during
area affected can be rather large. If a large             operation will be less than 10 years. The
circuit with 0.1cm2 total active area has 1000            circuits will no longer meet reliability
I/O pads, then it is reasonable to think that             specification.
each I/O pad may affect about 10,0002 of                    To determine if the ESD event has degraded
active area. At 3.6V stress, we calculated the            the oxide to such an extent or not, the common
time to 100ppm failure for the 10,0002
method of using simulated ESD events to test
ESD design would not work. To assure that
the failure rate is below 100ppm, the number
of tests must be more than 10,000. In addition,
most device-under-test (DUT) has too small an
active area. When area scaling is taken into
account, the number of tests required will be
much higher. The problem is worse when the
oxide is thin. When the Weibull slope is small,
the area scaling effect is larger. Using our 15Å
oxide as an example, if the DUT has an active
area of 102, one must test over 10 million
DUTs before there is enough statistic to
experimentally demonstrate that the 10,0002
devices has a failure rate of less than 100ppm.
   Since normal ESD test is done only on
limited samples with relatively small size, the
oxide degradation is not normally detectable.
As a result, one may wrongly conclude that the
ESD design is adequately protecting the gate-
oxide. To get around this test difficulty, one
can measure the voltage waveform and rely on
the known voltage acceleration factor to
calculate the level of damage created in the
oxide. Our experiment reported here
demonstrated the soundness of this approach.

4. Conclusions
   We demonstrated that the physics of gate-
oxide degradation under stress remains valid at
sub-nanosecond time scale. The prediction that
the trigger transient dominates trap generation
during an ESD event is experimentally proven.
For ultra-thin oxide, the normal ESD test
method is inadequate to determine if the gate-
oxide is degraded beyond acceptable level or
not. The actual damage can be calculated from
measured ESD waveform on the DUT.

5. References
[1] A. Amerasekera et al, IRPS-99, p159.
[2] K. P. Cheung, EOS/ESD Symp.-99, p38.
[3] J. H. Stathis et al, IEDM-98, p167.
[4] R. Degraeve et al, TED-45(4), 904(1998).

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