Final Report
Hydrogen Effects on GaAs Microwave
Semiconductors
Prepared for:
California Institute of Technology
Jet Propulsion Laboratory
Pasadena, California
Report Number: SMC97-0701
Purchase Order Number: 000732172
July 1997
Shason Microwave Corporation
1120 NASA Road One, Suite 106
Houston, TX 77058
Phone: 281.333.1950 Fax: 281.333.1954
TABLE OF CONTENTS
SECTION
PAGE
Introduction
1
I. History
1
II. Failure Effects
5
a. MESFETs
7
b. PHEMTs
10
c. Indium Phosphide (InP) HEMT
18
III. Hydrogen Sources
19
IV. Possible Solutions
21
a. Thermal Treatment of Packages
22
b. Package Materials with Low H2 Absorption
23
c. Gate Barrier Metals
24
d. Non-hermetic Packages
25
e. Use of Hydrogen Getters
26
V. Recommendations of High Reliability Applications
28
a. Existing Designs
29
b. Designs in Development
30
Appendix 1.
34
LIST OF ILLUSTRATIONS
Illustration
Page
Figure 1. Schematic Diagram of MESFET
7
Figure 2. Comparison of MESFET Idss in Hydrogen and Nitrogen
9
Figure 3. Median Life of GaAs MESFETs in 10% Hydrogen
10
Figure 4. Schematic Diagram of a typical PHEMT
11
Figure 5. Change in transconductance and Idss versus time in 4% hydrogen
12
Figure 6. Median life versus ambient temperature for GaAs PHEMTs
14
Appendix 1. A partial list of GaAs Device failure data
35
LIST OF TABLES
Table
Page
Table 1. Comparison of median life for PHEMTs and MESFETs from the same
Process line, using calculated and measured data
17
Table 2. A listing of parameter shifts seen for exposure to hydrogen
19
Table 3. Hydrogen contributions from typical microwave module components
21
HYDROGEN EFFECTS ON GaAs MICROWAVE
SEMICONDUCTORS
INTRODUCTION
Hydrogen gas present in hermetic packages containing gallium arsenide (GaAs) fieldeffect-transistors (FETs) and microwave monolithic integrated circuits (MMICs) has a
deleterious effect on their performance and lifetime.
This effect was first reported in
approximately 1989, and other documents have been published since that first report. In recent
years, work has been performed to characterize the problem and efforts have been directed
toward solving it.
Initial work in GaAs development and reliability investigations failed to identify the
hydrogen problem, as most work was performed in nitrogen or other inert environments. A
study of the problem has been performed and this report details the history and effects of the
hydrogen failure mechanism, the attempts to solve the problem and recommendations for using
GaAs devices in high reliability applications.
I. History
Small, lightweight, high performance microwave amplifiers and other circuit functions
have been made possible by the use if GaAs or other compound semiconductor devices.
Compact size and high-efficiency performance of the circuits make them ideal for use in
applications having limited space and power availability.
Reliability investigations, beginning in the early 1980’s, studied failure modes and
mechanisms and began developing reliability enhancements needed to meet stringent reliability
requirements of military systems. Failures from these tests reported activation energies ranging
from 0.4 to 2.5 electron volts (eV), and median lives ranging from 2E5 hours (22.8 years) to
1.6E9 hours (1.8E5 years) at 150°C [1].
It is generally accepted that current MESFET
technology, from a stable process, will yield activation energies of 1.5 to 2.0eV with lifetimes of
greater that 1E6 hours at 150°C [26, 27, 28, 29]. A table giving partial listing of this early
reliability work is included as Appendix 1.
Most of the early reliability work, however, was performed in laboratory conditions and
in inert environments, to insure studies resulted in evaluation of devices and not their
environment. In 1989, degradation of GaAs FETs and MMICs caused by hydrogen gas trapped
inside the devices’ hermetic packages was reported.
It was shown that hydrogen gas in
quantities as low as 0.5% of ambient atmosphere can cause significant degradation at elevated
temperatures (125°C), in a relatively short period of time (168 hours). It was proposed in this
work that the mechanism was due to catalytic conversion of molecular hydrogen to atomic
hydrogen by platinum in the gate.
It was thought the atomic hydrogen diffused into the
semiconductor and compensated the silicon dopant (donors), thus causing a reduction in current
and gain of the device. A secondary mechanism observed was reduction in the Schottky gate
barrier height, thought to be due to modification of the gate-semiconductor interfacial layer [2].
Investigations to determine sources of hydrogen in hermetic packages, and efforts to
eliminate the sources were reported in 1991. The primary source was found to be outgassing
from ferrous metals used in package fabrication, and electroplating was also found to be a
contributor. Various studies to characterize the hydrogen content of the package material found
that hydrogen levels could be reduced, but not eliminated, and that one possible solution was to
use hydrogen free alloy or to passivate package surfaces [3].
Studies in 1993 concluded that hydrogen desorption from ferrous metals was a function
of metal thickness, with the greatest percentage being observed in thicker samples (0.01, 0.04
and0.06 inch test coupons). It was also found that moisture would increase in hermetic packages
as oxides were reduced by desorbed hydrogen. Amounts of desorbed hydrogen in this study was
dependent on processes used in package preparation (sealing technique, anneal, etc.), with one
package vendor using an anneal cycle that reduced desorbed hydrogen to undetectable levels.
Plated samples gave consistent results [4]. Work in this area has been continued by LockheedMartin.
Reports of hydrogen work increased in 1994 with six papers on device results and one,
hydrogen related, workshop held. The increased 1994 activity signaled acceptance that hydrogen
was an industry wide problem.
The Hydrogen Effects on GaAs Devices workshop, sponsored by IEEE, 1994
International Reliability Physics Symposium (IRPS), was attended by 21 organizations, and was
conducted as an open forum, with three industry representatives presenting test data on the
problem. Results indicated that all field-effect devices tested, exhibited a problem and that
degradation could occur with fractions of a percent of hydrogen [5].
Three papers on metal-semiconductor-field-effect-transistors (MESFETs) and three on
pseudomorphic HEMTs (PHEMT) were published in 1994. Tests of 0.25 micron (uM) titaniumplatinum-gold (Ti/Pt/Au) gate MESFETs exhibited a sudden reduction of transconductance and
an increase in operating for 1.0% and 0.1% hydrogen. This data indicated an activation energy
of 0.4eV, and approximately linear dependence on hydrogen concentration [6].
Another
investigation was on Ti/Pt/Au and titanium-palladium-gold (Ti/Pd/Au) gate MESFETs of
different sizes.
This report indicated increases in channel current and pinch-off, with
degradation times dependent on FET size and construction, hydrogen concentration and whether
the gate had platinum or palladium. Platinum was found to produce larger changes, at earlier
times, when compared to palladium [7]. MESFET tests at two temperatures in 10% hydrogen
yielded an activation energy of 1.0eV, with degradation being a sudden decrease in current and
transconductance [8].
Hydrogen tests on Indium-Phosphide HEMTs and GaAs PHEMTs with Ti/Pt/Au gates
showed that InP HEMTs degraded more rapidly and that channel current increased as opposed to
decreasing in the GaAs PHEMT. Source resistance for both were found to remain unchanged
and the amount of degradation was found to depend on platinum thickness, no degradation was
observed when platinum was reduced to 50 angstroms. Also, samples constructed with Ti/Al
gates did not exhibit degradation [9]. Limiter amplifiers, constructed with 0.25uM, Ti/Pt/Au
gate GaAs PHEMTs were tested under five different hydrogen percentages and seven
temperatures. Work from these tests yielded a model, which accounts for activation energy and
pressure acceleration terms when calculating lifetimes. Observed failure was a decrease in
operating current [10]. Tests on 0.25 uM, Ti/Pt/Au gate GaAs PHEMTs under four hydrogen
concentrations and three temperatures showed a dependence on temperature and hydrogen
concentration.
Changes observed were a sudden decrease in channel current, decrease
transconductance, an increase in low-field channel resistance and a 30millivolt increase in
Schottky barrier height. Activation energy was measured at 0.34eV at low concentration and
1.73eV in nitrogen. Package bake (350°C, 7 hours), prior to seal was found to double median
life for the samples tested [11].
Investigations continued in 1995, as manufacturers addressed the extent of the problem
and began to look for solutions to the problem. GaAs MESFETs exposed to hydrogen and
deuterium degraded in each case. Data from this experiment indicates that hydrogen diffusion
occurs at the platinum sidewalls, and not at the gold surface of Ti/Pt/Au gates [12]. Experiments
conducted on low noise PHEMT amplifiers in two package types (aluminum-silicon (A40 and
Kovar) indicated that post-plating package bakes of 250°C for 168 hours reduces hydrogen
sufficiently to meet a twenty year mission life in spacecraft environments [13]. There was test
data reported on 0.15uM gate PHEMTs exposed to various concentrations, with and without
bias, and at two temperatures. Failure time was dependent on concentrations for amounts above
25%, but appeared less dependent on concentrations below that amount. As in other work,
failure was indicated by a sudden drop in channel current [14]. An industry survey established
the problem as industry wide, and that potential solutions included thermal treatment of
packages, use of hydrogen getters, or the use of new barrier metals in gates (replacing platinum
or palladium) [15].
In 1996, results of tests on two hundred 0.25uM Ti/Pt/Au gate GaAs PHEMTs was
reported. This experiment was performed to verify the model reported in [10], to determine if
hydrogen degradation was bias dependent, and to determine which temperature (ambient or
junction) established failure rates [16]. A report describing failure mechanisms, observed effects
on device parameters, and an outline of possible solutions to the problem was published [17].
Initial work on development of a physical model of the failure mechanism, accurately
duplicating experimentally observed results was reported [18]. Experiments were performed
using hydrogen plasma in an attempt to passivate the surface of GaAs PHEMTs, thus stabilizing
breakdown voltage and improving power performance of these devices. Large decreases in
current and mobility were observed much like tests of devices subjected to gas flows [19].
After a slow start, significant effort has been expended on this problem. As the above
history indicates, it was five years between discovery of the problem and reported device work
from others.
This indicated a reluctance to accept the problem as real, and a delay in
accumulating data from testing. It is now realized the problem exists industry wide and work is
continuing to determine the best solution to the problem. Much more must be completed before
the problem is no longer a factor.
II. Failure Effects
The hydrogen problem, as described earlier, has been observed in MESFETs, PHEMTs
and InP HEMTs, and the effects have been different for each of these technologies. However,
one general statement can be made about the problem; devices subjected to hydrogen
atmospheres will change operating performance, with the time to change being a function of
temperature and hydrogen concentration. Reports of degradation have been limited to amplifier
functions.
No observations of the problem have been reported for heterojunction bipolar
transistors (HBT), PIN diodes, switch and phase shifter functions. Table 2, at the end of this
section, is a summary of observed changes.
When FETs or MMICs are tested in hydrogen with RF applied, gain degradation is
observed concurrently with the change in operating current a transconductance.
Since gain
degrades concurrently with reduced Idss and gm, ivestigators have performed degradation studies
utilizing cost effective storage tests; failure criteria used has been either a 10% or 20% change in
DC parameters. It should be noted that changes are not catastrophic, as seen in typical wear-out,
therefore some system applications may function adequately even with the observed degradation.
The exact failure mechanism is not known, but has been under investigation for several
years. One popular theory assumes that molecular hydrogen is converted to atomic hydrogen
through a catalytic reaction with platinum or palladium in the gate structure and diffuses into the
Schottky metal and channel, resulting in: 1) compensation of donors by monatomic hydrogen, 2)
a shift in barrier height and 3) a neutralization of impurities near the Schottky interface causing a
barrier height shift.
Tests have shown barrier height shifts when GaAs Schottky diodes (gates) are subjected
to hydrogen [11, 20, 21, 22, 23]. It has been reported that atomic hydrogen generated by plasma
results in changes in the barrier height of Ti Schottky diodes when compared to control samples
and that the hydrogen-impurity complexes near the interface may be the cause of this shift [22,
25]. Others report reduced barrier height and a neutralization of shallow dopants [11, 22].
Thermal recovery of these changes indicated an activation energy of 0.6eV. Others have shown
[24, 25] that silicon-hydrogen complexes do exist in Si doped GaAs when subjected to hydrogen
gas, supporting the compensation theory. Some have observed increases in current, which does
not support the compensation of donors, indicating there is, in some cases, another mechanism at
work. P.C. Chao, et. al. [9] proposed that hydrogen affects on pinning levels at the gate
metal/semiconductor
interface may result in a decreased built-in potential and, therefore,
increased current in InP HEMTs subjected to hydrogen.
Additional study is needed to
understand this phenomenon.
II.a. MESFETs
The degradation seen in MESFETs, shown in the schematic diagram of Figure 1, is
generally seen as a sudden and rapid change in transconductance (gm) and saturated drain current
(Idss), followed by partial recovery as time continues. The two top plots of Figure 2 demonstrate
the change in Idss when MESFETs are subjected to 10% hydrogen at 140°C and 180°C. Figure
5 shows changes in Idss and gm when devices are subjected to alternating exposures of 4%
hydrogen and 100% nitrogen, at 270°C. Other parameters change, although not always as severe
as these two parameters.
Nitride
Gate
Drain
Source
Silicon Doped GaAs
(Active Channel)
a)
Semi-insulating GaAs Substrate
Gold
Platinum
Titanium
b)
GaAs
Figure 1. Schematic diagram of a typical MESFET. a) View showing the active
channel, gate, source, drain and nitride overlay. b) Enlarged view of the gatesemiconductor interface
W.O. Camp, et. al. [2] observed MMIC gain reduction (from 0.0 to 5.0dB per 30dB of
gain) when subjected to 125C for 168 hours in sealed Kovar packages. Residual gas analysis
(RGA) showed a correlation of gain reduction to packages containing hydrogen. As little as
0.5% hydrogen content led to degradation. A series of tests in 100% hydrogen, at 150C for 4
hours, determined that hydrogen and platinum in the gates was responsible for the change, and
that reduced current and transconductance accompanied the gain changes. However, devices
from some manufacturers did not show change, leading the author, and manufacturer, to believe
those devices were immune to the problem. Longer exposure times would have shown change in
all devices with Ti/Pt/Au or Ti/Pd/Au gates. This data was not conducted to establish median
life and activation energy, but it did identify the problem and was the foundation for all other
work conducted in this field. It was the conclusion from the authors that donor compensations
were the cause of failure.
Delaney, et. al. [6], reported that 0.25uM, Ti/Pt/Au gate MESFETs were tested in 1.0%
hydrogen at 175°C and 200°C and 0.1% at 200°C. Transconductance degraded sharply and
current at fixed bias increased. Data analysis yielded a 0.4eV activation energy with a 100°C
median life (10% gm decrease) of approximately 1200 hours; time to 10% current change was
approximately 1E4 hours.
The author noted degradation was approximately linear with
hydrogen concentration.
W. Roesch [7] indicated a decrease in gm and an increase in current, along with a large
percentage increase in pinchoff voltage. One significant difference in this data is the long time
to “onset” of changes. Also, the increase in current in this and [6] is not supported by the donor
compensation theory. This author reported no change in 1000 hours at 185°C and a need to raise
temperature to 250°C for reasonable test times. In addition, this author noted several items not
previously reported; 1) devices with Pt degraded faster than those with Pd, 2) large FETs
degraded faster than small ones and 3) changes caused by hydrogen were opposite those from
normal wear out, causing a possible enhancement to lifetimes in the presence of hydrogen. The
data observed in [6] and [7] are an indication a mechanism other than donor compensation is
probable in these processes.
The magnitude of change in 10% hydrogen, as reported by Decker [8], reaches 25%
before beginning recovery. Concurrently, abrupt increases in breakdown voltage and decrease in
pinchoff is observed, which is consistent with donor compensation. Some recovery is seen as
devices are maintained in the hydrogen environment. Time to 10% Idss degradation was 32
hours at 180°C and 395 hours at 140°C. Figure 1 shows the device parameters when exposed to
hydrogen and nitrogen environments at these two temperatures.
Figure 2. Comparison of MESFET Idss in Hydrogen and Nitrogen Environments
Reproduced from Ref. [8]
This data resulted in an activation energy of approximately 1.0eV and a 100°C median life of
800 hours. Figure 3 is a plot of median life for hydrogen and nitrogen testing for MESFETs
from the same process.
Figure 3. Median Life of GaAs MESFETs in 10% Hydrogen and 100% Nitrogen
Reproduced from Ref.[8]
Losses in gm and Idss were observed when commercially available MESFETs were
tested at 250°C in 5% hydrogen [12]. Experiments were performed with different gate metals
(Ti/Pt/Au, Ti/Pt, Ti) and the devices with Pt exposed had a much higher incorporation of
hydrogen (21 times) in the gate metal film for the Ti/Pt/Au gate after on hour at 250°C in 5%
hydrogen. This implies hydrogen diffuses at the Pt edge, and could lead to a gate process which
would not be effected by hydrogen.
II.b. PHEMTs
In general, PHEMTs, shown schematically in Figure 4, degrade in the same manner as
MESFETs, with at least one report [5] that PHEMT degradation occurs faster. A discussion of
hydrogen exposure work for this technology follows.
Source
Gate
Drain
nGaAs
n AlGaAs
Silicon Planer Doped Layer
AlGaAs Spacer
Undoped InGaAs
GaAs Buffer
Semi-insulating GaAs Substrate
Figure 4. Schematic diagram of a typical PHEMT. Reproduced from [27]
NOTE: Drawing is not to scale.
P.C. Chao, et. al. [9] reported that PHEMTs with Ti/Pt/Au gates exposed to 4% hydrogen
at 270C exhibited changes similar to those seen in MESFETs. Transconductance and Idss
decreased, pinchoff increased, and there was no change in source resistance. Figure 5 shows a
plot of Idss and gm during alternate exposures to hydrogen and nitrogen. Note that the current
recovers when baked in nitrogen, however it does not recover to the original value. The author
reported similar recovery after four days in nitrogen at room temperature. Increased pinchoff
and no change in source resistance indicates the change is dominated by changes at the
metal/semiconductor interface. To further understand the degradation effects, device current and
transconductance versus gate bias was measured and compared before and after a 10 minute
exposure. Comparison of the data shows the curves are shifted by the amount of pinchoff
change, which would indicate the change is due primarily to increased built-in potential and not
to donor compensation.
Devices fabricated with Ti/Al gates were subjected to the same
conditions and performance did not shift, indicating the change due to hydrogen environments is
associated with Pt in the gate. Gates with varying thickness of Ti and Pt were exposed for 30
minutes. It was found that current changes are strongly sensitive to Pt thickness and relatively
insensitive to Ti thickness; the current was significantly less sensitive when Pt was reduced to 50
angstroms.
Figure 5. Change in transconductance (gm) and Idss versus time in 4% hydrogen and
100% nitrogen, at 270°° C. Reproduced from Ref. [9].
Data analysis from exposures to multiple temperatures and hydrogen concentrations
produced mathematical models, which account for the temperature and H2 interdependence on
lifetime [10]. Hydrogen concentrations from 0.0005 to 0.5% and temperatures ranging from 100
to 250°C were sued and 20% Idss degradation was used as the failure criteria. Adams, et. al.
[10] developed the following expression:
t = A Pnexp(E a/kTj)
(1)
where:
t = median life (hours)
A = a proportionality constant
P = hydrogen partial pressure
Ea = activation energy
K = Boltzmann’s constant
Tj = junction temperature (°K)
Adams, et. al. [10] proposed a second model which replaces Ea in (1) with:
Ea = Ea – B ln P
Where B ln P is an interaction term, which in interpreted as the Freundlich potential, an
expression useful for describing absorption rates of gases at surfaces. This is the first work
reported on a model which allows the calculation of lifetimes with known values of hydrogen.
Hu, et. al. [11], reported on more testing of power and low noise PHEMTs with four
partial pressures and three temperatures which verified that time to failure was hydrogen partial
pressure dependent. PHEMTs sealed in packages made of Ni/Au plated Tungsten, with Ni/Au
plated Kovar lids were used for this test. Ten packages were vacuum baked at 350°C for seven
hours prior to assembly and five were left untreated; this allowed test to determine the effect of
temperature treatments on degradation. Observed failures in both groups were large, sudden
decrease in Idss (as much as 40%), with partial recovery as test time continued. Median time to
failure of the untreated packages was 800 hours at 125°C, and approximately 1600 hours for the
treated group. Although improvement was seen for the baked samples, the conditions used were
not sufficient to solve the problem. Additional testing in the four concentrations yielded data
which indicated the failures were dependent on both temperature and partial pressure. Failure
analysis indicated degradation was loclaized under the gate due to modification of the effective
gate voltage. Some compensation of donors were thought to occur in the later stages of a 9000
hour life test.
Saito, et. al. [13] reported on test performed to determine the maximum amount of
allowable hydrogen for spacecraft use of PHEMTs, assuming a twenty year mission life. In
addition, package bake experiments were performed to determine bake conditions necessary to
insure hydrogen levels in the package remained below the maximum allowable levels. Low
noise PHEMTs with 0.25uM Ti/Pt/Au gates were tested in 1% and 3% hydrogen at 125°C and
200°C. Median life at 125°C and 3% hydrogen was found to be 2200 hours, with an activation
energy of 0.52eV; degradation observed was a decrease in Idss and gain. From their work, the
authors concluded maximum allowable hydrogen to be 0.04% at 125°C and 0.46% at 70°C to
meet a 20 year mission life.
Package bake experiments determined a 250°C bake for 168 hours would reduce
hydrogen content from plated Kovar housings to 0.04% and plated Al/Si (A40) 0.004%. The
authors concluded most of the hydrogen evolved from plated nickel in the housings, and could
not be sufficiently baked to yield reliable assemblies. Some of these findings contradict some of
the results reported in [3], [4] and [11].
K. Decker reported [16] on testing of 200 GaAs PHEMT transistors to answer two
questions not answered in previous reports. The specific questions the tests were designed to
answer were:
•
Does ambient or channel temperature drive this mechanism?
•
Does electrical bias effect FET hydrogen degradation?
Tests of unbiased samples in nitrogen, 0.01, 0.1 and 1.0% hydrogen at four temperatures. Tests
were conducted for 2650 hours and the observed degradation in hydrogen was reduced Idss and
transconductance; in contrast, no degradation was observed in nitrogen.
Figure 6 is the
Arrhenius plot for the hydrogen data and reference nitrogen life test data for a PHEMT amplifier
manufactured with the same process.
Figure 6. Median Life versus ambient temperature for GaAs PHEMTs.
Reproduced from Ref. [16].
Note that in the hydrogen test the unbiased samples (baked) and biased samples fit the same plot
only if they are plotted using ambient temperature for both. This indicates that the temperature
driving the failure mechanism is ambient temperature instead of junction temperature.
Activation energy was found to be 0.73eV. Data analysis found the data to fit the model
developed by Adams, et. al. [10] and described in (1):
t = A Pnexp(E a/kT)
(1)
where:
t = the time to 10% Idss degradation
T = the ambient temperature in Kelvin
P = the hydrogen partial pressure in torr (%H2 x 7.6)
K = Boltzmann’s constant (8.615E-5)
Using the measured activation energy of 0.73eV and solving for n and A, then
Ea = 0.73eV
A = 5.46E-6
N= -0.7935
An extremely important finding from this work was that the ambient temperature (Ta), not
junction temperature (T j), drives the hydrogen degradation mechanism. Examination of (1), the
time to failure expression, shows temperature to be a critical variable effecting time to failure; A
and n are assumed to be determined by the process and P is a function of the assembly. Time to
fail is exponential in 1/T, resulting in decreased life as temperature increases. Wear-out failures
are a function of junction temperature and, prior to the results of [16], it was assumed junction
temperature would apply to hydrogen degradation.
Bias applied to transistors causes heat to be generated, resulting in increased junction
temperatures and decreased lifetimes for wear-out failures.
Common bias conditions for
PHEMT power devices can result in junction temperatures being 30 to 40C above ambient
temperatures. The 300uM gate-width devices tested in [16] were biased at 2.0V and 0.05A with
a resulting junction temperature rise of 24C.
Using data from Figure6 for PHEMTs tested in 1.0% hydrogen, and assuming a junction
temperature rise of 40C, a comparison of time to 10% Idss degradation has been made for Ta =
60C (Tj = 100C).
Ta = 60C; median life = 1.0E5 hours (11.4 years)
Tj = 100C; median life = 9.0E3 hours (1.03 years)
This is an increase of 11 times when based on ambient temperature, and demonstrates the
importance of ambient temperature determining lifetime of devices subjected to hydrogen.
Establishing that bias does not effect degradation also confirms that tests performed
without bias are valid and allows much simpler and cost effective tests to continue the study of
this problem.
In related work at University of California, Santa Barbara, S.S. Shi, et. al. [19], reported
on efforts being made to stabilize the surface of PHEMT power devices in order to make
breakdown voltages more reproducible and enhance power performance. Earlier work had
shown surface treatment with hydrogen ions to stabilize the surface through removal of excess
arsenic in the material. Excess As can form As Ga antisite defects which may be the cause for
non-reproducible breakdowns and degradation such as power slump. During surface treatment,
device parameters were found to degrade, and degradation was in prportion to the hydrogen ion
dose. When treated with the optimum dose for stabilization, maximum drain current dropped
10.8%, transconductance decreased 3.2%, and gain-to-date leakage current at 15 volts reverse
bias decreased 77%. Further exposures on van der Pauw structures resulted in decreased
mobility and donor concentration, with the most severe decreases at higher hydrogen ion doses.
This data shows a donor neutralization in support of [2]. Further, the authors observed recovery
of donor concentration and mobility after a 5 minute, 40°0C anneal, which explains recovery of
device parameters after exposure to higher temperatures.
To compare MESFET and PHEMT hydrogen sensitivities of devices manufactured on the same
process line, MESFET data from [8] has been compared to calculations for PHEMTs tested in
[16]. Reported hydrogen exposures of PHEMTs did not include enough data to compare
lifetimes of PHEMTs and MESFETs from the same process line and under the same exposure
conditions.
However, models developed in [10] and verified in [16] allow calculation of
expected failure times for PHEMTs at various hydrogen partial pressures. Calculations for
median time to failure (MTTF) for PHEMTs in 10% hydrogen have been performed for devices
in [16], utilizing the expressions and parameters measured in that work. The calculated MTFF
for PHEMTs is then compared to data obtained from Figure 3 for MESFETs, from the same
manufacturer, measured in 10% hydrogen. The failure criteria for both was selected as a 10%
degradation of Idss. Results are shown in Table 1.
Table 1. Comparison of median life for PHEMTs and MESFETs from the same process
line, using calculated and measured data.
DEVICE
PHEMT
MESFET
150oC, 10% H2 Median Life
96 hours, calculated from [16]
200 hours, from [8] data
This comparison supports, as reported in [5], that PHEMTs are more sensitive to hydrogen than
MESFETs manufactured on the same process line.
In summary, every reported PHEMT test resulted in decreases in transconductance and
Idss. However, it is still not known what mechanism is responsible for the decreases. Changes
in barrier height were observed, and some believe the Idss shift to be due to donor compensation,
while others believe the shift is due to changes in material near the gate region. As can be seen
in the above summaries of work performed to date, and the fact that a consensus on the cause is
lacking, more work is needed to fully understand the failure mechanisms.
II.c. Indium Phosphide (InP) HEMT
Only one report relative to hydrogen effects on InP HEMT is available for review [9].
InP HEMT devices provide higher gain, mobility and cutoff frequencies, and have the high
breakdown characteristics common to the HEMT family of devices. Due to these characteristics,
they are favored in millimeter wave applications.
During investigation of susceptibility to
hydrogen, observed failures were not the same as generally observed in other device
technologies. P.C. Chao, et. al. [9], reported that exposure to 4% hydrogen at 270°C resulted in
decreased gm, but Idss increased instead of decreasing. As in PHEMTs, some recovery toward
pre-exposure values was observed and room temperature exposure to nitrogen for four days
resulted in recovery. InP HEMTs and GaAs PHEMTs from [9], above, were processed on the
same device line and the result is a direct comparison of these two technologies. Retest was
performed on nine wafer lots, processed over a 18 month period. Eight of the lots exhibited the
same changes, while Idss degraded on devices from the ninth lot. Samples from the eight lots
had a mix of devices with and without nitride, thus ruling out nitride effects as a failure cause.
The increase in current was not consistent with compensated donors, but the cause of observed
changes was not understood and must be studied further. Test temperatures of 270°C may have
introduced extraneous effects. However, unpublished preliminary data, at lower temperatures,
confirms the observed trend of increasing current for InP HEMTs under hydrogen exposure.
In summary, all MESFETm PHEMT and Inp HEMT devices changed characteristics
when tested in hydrogen environments. Not all devices changed in the same manner, the cause
of which is not understood. It is possible that surface conditions (traps, defects, etc), stress from
gate metal or differences in process history cause these effects to be different. The problem is
still under investigation, and much work remains before the effect is fully understood. In Table
2, a summary of changes observed to date is given.
Table 2. A listing of some parameter shifts seen for exposure to hydrogen. When available,
the median life and activation energies are given.
Device
Type
MESFET
MESFET
MESFET
MESFET
MESFET
PHEMT
PHEMT
PHEMT
PHEMT
PHEMT
Idss
gm
Gain
decrease
increase
increase
decrease
decrease
decrease
decrease
decrease
decrease
decrease
decrease
decrease
decrease
decrease
decrease
decrease
decrease
decrease
-decrease
decrease
--decrease
----decrease
--
PHEMT
InP HEMT
decrease
increase
decrease
decrease
---
H2
(%)
100
0.1, 1.0
multi
10
5
4
multi
multi
1, 3
0.01, 0.1,
1
plasma
4
Temp.
(oC)
150
175, 200
250
140, 180
250
270
100 - 250
60, 175, 200
125, 200
146, 170,
186, 210
25
270
Ea
(eV)
-0.4
-1
---0.3 - 1.0
0.53
0.73
Ref.
---
[19]
[9]
[2]
[6]
[7]
[8]
[12]
[9]
[10]
[11]
[13]
[16]
III. Hydrogen Sources
Several sources of hydrogen in hermetic packages have been identified, with desorption
from ferrous metal package materials being the primary source [3, 4].
Hydrogen can be
“trapped” in the metal at structural imperfections; grain boundaries, precipitate interfaces,
dislocation cores, etc. These hydrogen trap sites will increase the metal hydrogen solubility by
orders of magnitude, and this trapped hydrogen can then be desorbed from the metal during
heating, such as that seen during burn-in. Some sources of the hydrogen [4] are reaction of H2
or H2O to free interstitial H at the metal surface; melt and casting processes; hydrogen
annealing, brazing or stress relief; air annealing in humid atmospheres; acid cleaning or
corrosion; and electroplating or electrocleaning. Gold and nickel plating used on packages is
permeable to hydrogen diffusion and may also be a source of hydrogen [4, 13]. During burn-ins
and steady state lifetests, untreated hermetic packages will desorb trapped hydrogen and levels of
1 – 2% will accumulate in the package [17]. As seen in previous sections, degradation will occur
at much lower levels.
Microwave absorber, sometimes used in package cavities to suppress oscillations and
interactions between microwave functions within the package, is a source for hydrogen [13].
The absorber is constructed of powdered iron fillings suspended in a carrier such as silicone
rubber or other plastic, and outgassing of hydrogen from this source may be dependent on
thermal treatments received by the absorber prior to package sealing. Y. Saito. Et. al. [13],
reported that outgassing from microwave absorber accounted for 16.3% of the total hydrogen
from sealed housing samples baked for 336 hours at 150°C; the samples had not been heat
treated prior to sealing. The author stated that hydrogen from this material would be much less
at 125°C due to a 1.6eV activation energy, measured during unpublished work at TRW.
Other metals used in microwave modules are also known to outgas hydrogen, but studies
of amounts and rates of outgassing are incomplete. One early indication is that cold rolled steel
is much than Kovar; Invar is also known to outgas significant hydrogen. It is generally found
that each metal tested has its own characteristics and that each candidate for use must be
characterized before use.
It is possible other materials utilized in module assemblies may be sources of hydrogen.
Epoxy is suspected by some to be a source; tests have shown that epoxies used in assemblies will
outgas hydrogen, but not to the same extent as Kovar and other packaging materials [13].
Subassemblies and circuit functions in complex microwave modules can be made up of many
components. Items such as circulators have metal housings, ferrite pucks made of iron powder,
circuit substrates and metal films and may be sources for hydrogen.
Capacitors, resistors,
interconnects and substrate materials within the package should be characterized for possible
hydrogen outgassing. One method for determining hydrogen outgassing utilizes baking of test
coupons in sealed ampules, followed by analysis of the ampule contents with residual gas
analysis (RGA) after baking is complete. Another technique involves total melting of test
coupons and performing gas chromotography on the gasses formed during melting.
This
technique must be used with caution, however, as hydrogen containing compounds can be
broken down and give erroneous indications of hydrogen.
Many potential sources of hydrogen exist in complex microwave modules and
evaluations assessing the risk or outgassing, utilizing some of the techniques described above,
should be performed before a design is considered complete. Table 3 is a listing of module
components and their role in contributing to the hydrogen problem.
Table 3. Hydrogen contributions from typical microwave module components.
Component
Ferrous package materials
Electroplating
Ceramic substrates
Epoxy
Isolator/Circulators
Interconnects
Microwave absorber
Hydrogen contributor?
Yes
Yes
No
Yes
Probable
Unknown
Yes
IV. Possible Solutions
Several potential solutions have been suggested to eliminate the hydrogen degradation
problem. Suggestions include pre-seal package thermal treatment [4, 13], the use of package
materials with low hydrogen absorption [4], a change of barrier metals in gates [6, 15], the use of
non-hermetic packages [6, 15] and the use of hydrogen getter materials in the package [6, 15].
Each of these possibilities will be discussed in the following paragraphs.
IV. a. Thermal Treatment of Packages
Investigations have shown that thermal treatment of packages prior to sealing can
significantly reduce absorbed hydrogen [4, 13].
Monatomic hydrogen is the only form of
hydrogen capable of diffusing through metals as an interstitial solute, but hydrogen can be
“trapped” in the metal at structural imperfections and incoherent boundaries such as grain
boundaries, dislocations, vacancies, micropores, precipitate interfaces, inclusions and particle
boundaries [4]. Absorbed hydrogen in excess of the lattice solubility will segregate to trap sites
within the metal and increase total hydrogen solubility by orders of magnitude. This hydrogen
trap site can be desorbed from the metal by thermal treatment. Each trap site has an associated
activation energy for out diffusion, and results in strong and weak trap sites. Studies using
thermal treatments of 150°C for 200 hours showed that thicker samples of Kovar desorbed
significantly larger amounts of hydrogen than thin samples. As an example, 0.06, 0.04 and 0.10
inch thick samples desorbed 4.0%, 2.0% and <0.02% hydrogen, respectively. The exact cause of
the differences are not known, but it is expected that it is due to differences in grain structure and
manufacturing processes.
Samples showed increasing inclusion density with decreasing
thickness; it is assumed the finer grain structure of thicker samples will have strong trap sites and
the course structure of thin samples contain more weak traps. The weaker traps would be more
inclined to release trapped hydrogen during manufacturing processes (annealing, etc.) and thus
have less hydrogen to desorb during subsequent bakes.
Microwave module housings are
generally made from sheet stock much thicker than 0.06 inches, and will have more available
hydrogen in strong trap sites and therefore may require thermal treatments to desorb the
hydrogen. Thermal treatments have shown that hydrogen can be baked out to acceptable levels
[4, 13]. One manufacturer has used this technique to produce modules acceptable for use in
spacecraft applications [13]. Proper bake procedures must be established to prevent bondability
problems with the post-bake housings. Bake temperatures of 350°C will successfully evolve the
hydrogen, but bondability begins to degrade. At elevated temperatures, Ni can diffuse through
gold and Ni oxides can form on the surface and degrade bondability. Y. Saito, et. al. [13],
reported that optimization of bake conditions for Au/Ni plated Kovar housings were established
to be 250°C for 168 hours in one. Post plating bake reduced evloved hydrogen from 0.6% to
0.0033% and bonding tests showed bond strength to be >7grams. A bake of 250°C for 168 hours
is severe and may damage the package finish. Studies of gold-over-nickel plating have shown
that nickel can diffuse through the gold and form a nickel oxide of the gold surface [4], causing
bondability problems. Schuessler, et. al. [4], reported most hydrogen could be desorbed from
Kovar samples with a 200 hour bake at 150°C, which is much less severe than the conditions
reported in [13]. More investigations are required before a treatment condition can be adopted.
IV.b. Package Materials with Low H2 Absorption
Materials with low hydrogen absorption have been studied for use as packaging
materials, with at least one material type used to solve the problem. An aluminum-silicon
(Al/Si) based alloy (A40) is a potential material for this use, and has been used by one
manufacturer for high reliability applications [13]. As in the Kovar case, post plate baking
reduced hydrogen levels to 0.002% after a 250°C, 168 hour bake. Aluminum does not absorb
hydrogen, and various alloys have been investigated for use in lightweight module applications
[31]. Alloys are necessary to gain the required rigidity needed in thin-wall applications such as
those needed in space applications.
A possible choice for packages with low hydrogen content is to manufacture them from
Kovar which has had hydrogen desorbed from it. Schuessler, et. al. [4], reported that one
package manufacturer was able to supply heat treated Kovar which did not desorb detectable
levels of hydrogen in subsequent bake experiments.
The author stated the treatment was
performed at high temperatures and must be performed prior to plating.
When Kovar or nickel-plated Kovar are subjected to heat treatment in air, an oxide layer
forms and blocks hydrogen desorption [3]. Bare Kovar was treated for 100 hours at 320C and
nickel plated Kovar was treated for 24 hours at 320C.
Subsequent desorption bakes
demonstrated that hydrogen levels had been reduced from 0.7 to 0.9% hydrogen to nondetectable levels. Passivation of this type would be required prior to plating. An approach such
as this could be used if acceptable plating and module seal techniques could be developed for
oxidized metal.
IV.c. Gate Barrier Metals
The hydrogen degradation problem is caused when available hydrogen reacts with
platinum or palladium in the device gate structure. This catalytic reaction produces mono atomic
hydrogen which then diffuses through the gate metals and reacts with silicon dopants within the
material, thus causing degradation. A possible solution to this problem is to replace the gate
barrier metal (Pt, Pd) with a barrier metal which does not react with hydrogen. Amorphous thin
films of the Ta-Si-N type have shown good barrier properties and are thermally stable [30].
Tests have been performed on these materials with no observed hydrogen degradation [15].
However, additional tests are needed to determine if this material, which is applied via a
sputtering process, is adaptable to high production processes. Some areas of concern are sputter
damage and gate size repeatability.
Another approach would be to replace Pt and Pd with barrier materials such as tungsten
(W), molybdenum (Mo) or other suitable barrier to gold in the gate structure. Unpublished data
on materials such as these have shown excellent results, but again these materials must be
studied for adaptability to production processes. These materials are applied via evaporation, but
the temperatures at which they melt are much higher than that for Pt, Pd, Ti and Au. It remains
to be seen if normal gate lithography processes can be repeatable and adaptable to a high rate of
production. Unfortunately, work of this nature is considered proprietary by investigators, and
data is not yet available on them.
Work conducted at Texas Instruments (TI) has produced alternate gate metals with
dramatically improved performance in hydrogen environments.
Activation energies have
increased from 1.0eV to 1.4eV, and time to failure in 10% hydrgogen at 150°C has increased
from 200 hours (1.2 weeks) for Ti/Pt/Au gates to 2.0E5 hours (22.8 years) for alternate gate
metals. This is three orders of magnitude improvement in MTTF, and gives a MTTF of 1.0E7
hours (1141 years) at 100°C, which is certainly sufficient for most applications. Qualification
and producability test have begun, and a form of this gate structure is planned for production
January, 1998 [31]. Metals used in this alternate metal approach are proprietary and cannot be
given.
IV. d. Hon-hermetic Packages
In typical hermetically sealed modules, desorbed hydrogen is trapped inside module
housings and cannot readily escape. Us of non-hermetic packages would allow the hydrogen a
path to escape to surrounding environment and thus would not be available to degrade devices.
Several considerations must be taken into account if this approach is to be used. However, the
use of a non- hermetic module should be considered very carefully. The use of non-hermetic
packages would allow ambient atmospheres to be present in the package. The presence of
moisture and ionic contamination may result in performance degradation or failure [32]; all
components within the assembly would need to be impervious to such moisture effects before
this approach would be viable. To date, there isn’t appreciable data available on susceptability to
moisture of GaAs and InP based devices. Studies pertaining to long term performance in this
condition would be required before this approach could be deemed acceptable.
Other
contaminants (particles, chemicals, etc) would possibly be available for package ingress.
Strenuous pre-use environmental controls to prevent possible contamination would be required if
this approach were to be considered.
IV. e. Use of Hydrogen Getters
Hydrogen getters installed within the hermetic package could be a solution to the
degradation problem. Before the getter could be used, some considerations must be addressed:
(1) Getter capacity – The getter must have the capacity to react with and retain the total
quantity of hydrogen expected to be available through outgassing during operating
life of the module assembly. Information on total hydrogen to be desorbed and the
getter capacity with respect to that quantity would be required.
(2) Getter pumping/reaction rate – Desorption of hydrogen will occur over time at a rate
determined by package materials and thermal history.
Investigations on this
desorption rate would be required and compared to the gettering rate of any material
used.
(3) Getter useable life – Data pertaining to the life of the getter, taking into account the
thermal cycles, reaction rates and total reaction would be needed.
Most high
reliability applications of GaAs-based modules require long lifetimes and will subject
the assemblies to many thermal cycles. The getter material would be required to
remain effective during the mission life.
(4) Physical stability – Reactions with hydrogen may generate new complexes within the
getter or module cavity.
This reaction must be able to occur without creating
particles or other contaminants within the sealed assemblies.
Hydrogen getters are commercially available from Allied Signal Aerospace (HMC
Getter, patent pending) [36]. This product shows gettering capability sufficient to be used in
microwave packages containing GaAs devices. The capabiltiies are listed as being able to
remove hydrogen over a temperature range of –55 to 150°C, and some other general data from
the getter data sheet are:
•
Hydrogen reaction is irreversible
•
Zero vapor pressure
•
Maintains hydrogen level to less than 1 part per million (ppm)
•
Maintains dew point to less than –100F
•
Is bondable
•
Mold to size is available
Test data shows, when the getter is used with Emerson & Cuming Eccosorb MFS-124 RF
absorber, with a volume ratio of Getter/Absorber = 1:2, that hydrogen outgassing from the
absorber is dramatically reduced. Control samples of the absorber produced 18,080ppm (1.8%)
hydrogen when baked at 125°C for 3792 hours, while the 1:2 mixture produced only 0.19ppm
(0.000019%) hydrogen during the same bake. This is a reduction of five orders of magnitude
and appears to be more than sufficient to solve hydrogen problems in microwave modules. The
getter system is available in several forms.
A getter of the type described above could be the solution to hydrogen degradation in
hermetic packages. Texas Instruments has utilized getters similar to the one described above
[33]. Engineers at TI have developed a getter design methodology which utilizes data on module
component desorption rates, getter pumping rates, total hydrogen available, mission thermal
profile and maximum allowable hydrogen within the module to design getters for application in
microwave modules using GaAs PHEMTs. The design is performed and then the getter is sized
to have capacity in excess of the design. Most of the work in this area is considered confidential
and cannot be given here.
Another possible getter could be designed containing titanium and platinum or palladium
[34]. The getter could be fabricated with alternating contiguous layers to form a sandwich-like
structure, using sputtering or evaporation. In a hermetic package, the getter would be secured to
the package or might be a coating applied to the lid used to seal the package. The amount of
titanium used would be designed to absorb all the hydrogen expected to be desorbed from the
package.
Titanium will absorb up to 67 atomic percent hydrogen [35], making it an excellent
choice for capturing and holding hydrogen. Either a palladium or platinum coating of over the
titanium serves two purposes; 1) it prevents oxidation or titanium, which would block hydrogen
flow into the titanium, and 2) the palladium would catalytically convert ambient hydrogen,
allowing it to diffuse into and be absorbed by the titanium.
V. Recommendations for High Reliability Applications
Hydrogen degradation of GaAs and InP based microwave semiconductor devices has
been identified as a serious problem, especially for high reliability applications where this noncatastrophic degradation may reduce system performance to below acceptable levels. Due to
recent developments, however, acceptable solutions to the problem make the devices a viable
technology for use in space and other high reliability applications.
Solutions to the hydrogen problem will involve two basic scenarios and may involve
either short term or long term fixes for the problem. In the first case, existing designs utilizing
GaAs MMICs or discrete devices will have been committed to applications and a risk assessment
pertaining to potential hydrogen problems will be required. In the second case, designs under
development will require a hydrogen susceptability assessment and, if required, a solution put in
place to eliminate the risk of hydrogen degradation.
Short term fixes to the hydrogen problem could include making sealed packages nonhermetic or designing and installing a hydrogen getter in the package. Getter use would require
packages capable of unsealing and resealing after installation of the getter material. If either of
these approaches are to be used the precautions noted in section IV should be followed.
Longer term fixes could include use of hydrogen getters, use of low hydrogen absorption
material for packages, using thermally treated packages and designing with devices which have
new gate barrier metals.
Each of these have shown the potential for solving hydrogen
degradation problems. However, all except getters require more time to implement and would
best be used with new designs.
For each of the cases to be discussed in the following paragraphs, the user’s system
designers will be required to define and supply allowable failure rates for the planned mission
life, minimum acceptable performance for each functional block containing the devices and an
estimate of the mission temperature profile. These parameters, in conjunction with data for
device hydrogen degradation rates will be used to determine any actions necessary to meet or
exceed the system mission life.
IV. a. Existing designs
Existing designs will require an immediate risk assessment to determine if a solution to
the hydrogen degradation problem is required. The need for implementing a solution should be
based on the comparison of device degradation data, for each microwave function, to minimum
performance requirements established by the system engineers.
Acceptable data to be used in the risk assessment can be generated by exposing devices
to hydrogen, be obtained from prior test of devices, be deduced from prior hydrogen degradation
studies of the process, or by examination of changes in steady state lifetests during qualification.
In most cases, steady state lifetests are performed during qualification and the pass-fail
criteria will be based on the end of test performance (gain, etc. are required to meet
specifications).
Typical test steady state lifetests are conducted for 1000 hours at 125°C with
DC bias. If hydrogen degradation is a problem, many of the designs will show gain and current
decreases under these conditions, but will still meet gain specifications after the test.
Examination of the before and after data would reveal the shift. To insure that data shifts are due
to hydrogen, unsealing samples and subjecting them to bakes in a nitrogen atmosphere will be
necessary. Partial recovery will indicate a hydrogen degradation problem and a solution to the
problem will be required if the data indicates the test time does not accurately simulate the
required mission life. If degradation is not evident, or if recovery is not achieved with a post-test
bake, the degradation will not be due to hydrogen.
IV. b. Designs in Development
Designs being developed for use in future applications should be characterized for
hydrogen degradation and, if needed, a solution which will eliminate the problem should be
included in the module design. The chosen solution will be vendor dependent, since not all
vendors will have the capability to implement all of the potential solutions. However, data
should be acquired which will allow a demonstration that the mission life can be met.
When the proposed solution is to replace the gate materials with one not affected by
hydrogen, it is recommended that qualification data should be available which shows the devices
are capable of passing, as a minimum:
•
multiple temperature lifetests
•
thermal cycle
•
thermal shock
•
susceptability to hydrogen
•
susceptabiltity to moisture
The data should include hydrogen exposure test data that includes time to failure, pressure
dependence and the activation energy for hydrogen degradation.
When heat-treated packages or low moisture rate package materials are propsed,
analytical data showing that the amount of hydrogen to be expected during the mission life will
not degrade device performance to below acceptable levels.
Use of getters will require prediction equations which clearly show the getter is capable
of maintaining hydrogen levels to a value below that which would cause device degradation, and
that the getter will not degrade during the intended mission.
Implementing solutions to hydrogen degradation problems in designs utilizing GaAs of
InP based microwave devices will make these devices an excellent choice for use in applications
requiring lightweight and high efficiency performance.
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[2]
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[7]
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[8]
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[15]
Sammy Kayali, “Hydrogen Effects on GaAs, Status and Progress”, GaAs Reliability
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[16]
K. Decker, “GaAs PHEMT Hydrogen Sensitivity Study”, GaAs Reliability Workshop
Digest, 1996
[17]
Sammy Kayali, “Hydrogen Effects of GaAs Device Reliabilty”, Gallium Arsenide
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[18]
David P. Rancour, Sammy A. Kayali, “modeling of Hydrogen Effects in GaAs FETs”,
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[19]
Song S. Shi, Ying-Ian Chang, Evelyn L. Hu and Julia J. Brown, “Surface Passivation of
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[31]
Private conversations with Ken Decker, Texas Instruments, Inc.; unpublished data
[32]
S. Kayali, G. Ponchak, R. Shaw, “GaAs MMIC Reliability Assurance Guidelines for
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[33]
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[35]
Max Hansen, “Constitution of Binary Alloys”, McGraw-Hill, 1958
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Appendix 1
A partial listing of results from early GaAs device reliability tests
FAILURE MECHANISM
DESCRIPTION
FETs
Sinking Gate
Sinking Gate
JC-14.7 Failure Mechanism/Acceleration Factor List
ACCELERATION FACTOR
ACCELERATING
MEDIAN LIFE
(Activation Energy, Current CONDITIONS AT FAILURE
Density Exponent, Etc.)
SITE: (Temp., Voltage, Etc.)
Hours/Temp.
>1.6E9/150oC
4,000 hour test
245, 260, 275, 290, 310 oC
DC bias @ Vds=8V, 1/2 Idss,
Tj=225, 245, 260m 290oC
<150, <190, <225oC
Tch = 140, 150oC
Tch = 58, 190, 225oC
Tch = 83oC
INTERDIFFUSION
Interconnect
Airbridge
Nichrome Thin Film
Activation Energy=2.4eV
Activation Energy=0.43eV
Activation Energy=1.03eV
250, 275, 300oC
200, 225, 250oC
125, 175, 200oC
>8E7/150oC
>2E5/150oC
6E5/150 oC
ELECTROMIGRATION
Ohmic
Ohmic Metal
Interconnect
Airbridge
Nichrome Thin Film
Gate Metal Voiding
Gate Metal Voiding
Interconnect
N-Factor = 3.5 (203oC)
Activation Energy=1.5eV
N-Factor = 1.5 (300oC)
N-Factor = 4 to 5 (250oC)
N-Factor = 3.0 (200oC)
Activation Energy=1.5eV
Activation Energy=1.65eV
N-Factor = 1.5, Ea = 0.7eV
0.455, 0.91 mA/cm2
<180, <240, <270oC
0.455, 0.91, 1.365 mA/cm2
1, 2, 4 mA/cm2
2.5, 5, 4.5 mA/cm2
Tch = 150, 190, 225 oC
Tch = 180, 240, 270 oC
150, 175, 200oC
03.9, 1.0, 1.6 mA/cm2
INTEGRATED CIRCUITS
Microwave Amplifier
Digital Counter
MMIC Switch (sinking gate)
Activation Energy=1.75eV
Activation Energy=1.65eV
Activation Energy=1.34eV
225 & 240 oC
260 & 275 oC
HTRB @ 225, 250, 260 oC
RF biased @ 200oC
Tch = 185, 210, 235 oC
Burn-out (Breakdown)
Au-Ga Intermetallic
Schottky Degradation
W-Ni gate contamination
Preamp
MMIC Amplifier
Activation Energy=2.5eV
Activation Energy=1.3eV
Unknown (early failure)
24,000 hour life test
>8E7/150oC
>2E5/150oC
6E5/150 oC
>1.2E6/150oC
J = 4E5
8.4E6/150oC
3.5E6/150oC
REPORTED BY:
(Author, Etc.)
SOURCE
(Publication, Etc.)
DATE
Roesch, et. al.
M/A COM Internal
Ersland, et. al.
Russel, et. al.
Postal, et. al.
Riley
Postal, et. al.
Man-Tech
Unreported
IRPS
IRPS
GRW
IRPS
1988
1987 1988
1986
1983
1987
1983
Roesch, et. al TQS
Roesch, et. al TQS
Roesch, et. al TQS
Man-Tech
Man-Tech
Man-Tech
1988
1988
1988
Roesch, et. al. TQS
Riley, et. al.
Roesch, et. al. TQS
Roesch, et. al. TQS
Roesch, et. al. TQS
Russell, et. al.
Riley, et. al.
Thompson
AT&T Bell Labs
Man-Tech
GRW
Man-Tech
Man-Tech
Man-Tech
IRPS
GRW
Internal
1988
1987
1988
1988
1988
1986
1987
1990
Data Sheet
Data Sheet
GaAs IC
Symposium
GaAs IC
Symposium
IRPS
1989
1989
1988
Rubalcava, et. al TQS
Ingle, et. al. TQS
Ersland & Lanterni
M/A Com
Activation Energy=1.3eV
2.5E5/150oC
Spector & Dodson
AT&T Bell Labs
o
o
Activation Energy=0.64eV
Tch = 225, 255, 275 C
2E5/125 C
Christianson, NRL
A partial list of GaAs Device failure data - Reproduced from
EIA/JEDEC JC-14.7, Committee on GaAs Reliability, Minutes
1987
1992
SESSION III
HYDROGEN EFFECTS
October 12, 1997
Anaheim, California
Sponsored by JEDEC Committee on GaAs
In cooperation with the Electron Society of the Institute of
Electrical and Electronics Engineers, Inc.
Hydrogen Evolution of Packaging Materials
Test Plan
A. Identify the baseline hydrogen content of populated modules with no absorber or hydrogen getter.
Evaluate (6) samples. The modules will be processed in the following manner:
• standard seam seal process (16 hour, 150°C vacuum bake)
• condition at 125°C for 168 hours
• measure hydrogen content inside the module (RGA)
B. Identify the baseline hydrogen content of populated modules fabricated with absorber. Evaluate (3)
samples with Emerson & Cuming Eccosorb CRS-124-RF absorber. The modules will be processed
in the following manner:
• standard seam seal process (16 hour, 150C vacuum bake)
• condition at 125C for 168 hours
• measure hydrogen content inside the module (RGA)
C. Identify the hydrogen content of populated modules fabricated with absorber. Evaluate (3) samples
with Emerson & Cuming Eccosorb CRS-124-RF absorber attached to a cover. The absorber was
applied to the cover with Dow Corning Sylgard 577. The cover and absorber were vacuum baked at
165°C and minus one atmosphere for 72 hours, prior to sealing. The modules will be processed in the
following manner:
• standard seam seal process (16 hour, 150°C vacuum bake)
• condition at 125°C for 168 hours
• measure hydrogen content inside the module (RGA)
D. Indentify the hydrogen content of populated modules fabricated with getter. Evaluate (2) samples
with Allied Signal getter compound, lot 118 attached to a cover. The compound had a tacky surface
and was attached directly to the cover. The getter was 0.023 inches thick with a surface area of 1.20
square inches. The modules will be processed in the following manner:
• standard seam seal process (16 hour, 150°C vacuum bake)
• condition at 125°C for 168 hours
• measure hydrogen content inside the module (RGA)
E. Indentify the hydrogen content of populated modules fabricated with absorber and getter. Evaluate
(2) samples with Emerson & Cuming Eccosorb CRS-124 RF absorber attached to a cover and Allied
Signal compound, lot 118, loose in the module. The absorber was applied to the cover with Dow
Corning Sylgard 577. The cover and absorber were vacuum baked at 165°C and minus one
atmosphere for 72 hours, prior to sealing. The getter was previously used in another experiment
(populated module no absorber). The getter was 0.023 inches thick with a surface area of 1.08 square
inches. The modules will be processed in the following manner:
• standard seam seal process (16 hour, 150°C vacuum bake)
• condition at 125°C for 168 hours
• measure hydrogen content inside the module (RGA)
F. Indentify the hydrogen content of modules fabricated with absorber and getter. Evaluate (1) sample
with Emerson & Cuming Eccosorb CRS-124 RF absorber attached to the cover and Allied Signal
getter compound, lot 118, loose in the module. The absorber was applied to the cover with Dow
Corning Sylgard 577. The getter was previously used in another experiment (populated module no
absorber). The getter was 0.023 inches thick with a surface area of 1.08 square inches. The modules
will processed in the following manner:
• standard seam seal process (16 hour, 150°C vacuum bake)
• condition at 125°C for 168 hours
• measure hydrogen content inside the module (RGA)
G. Identify the hydrogen content of populated modules fabricated with absorber and getter. Evaluate (2)
samples with Emerson & Cuming Eccosorb CRS-124 RF absorber attached to a cover and Allied
Signal getter compound, lot 198HC, loose in the module. The absorber was applied to the cover with
Dow Corning Sylgard 577. The cover and absorber were vacuum baked at 165°C and minus one
atmosphere for 72 hours, prior to sealing. The getter was 0.017 inches thick with a surface area of
1.08 square inches. The modules will be processed in the following manner:
• standard seam seal process (16 hour, 150°C vacuum bake)
• condition at 125°C for 168 hours
• measure hydrogen content inside the module (RGA)
H. Indentify the hydrogen content of populated modules fabricated with absorber and getter. Evaluate
(1) sample with Emerson & Cuming Eccosorb CRS-124 RF absorber attached to a cover and Allied
Signal getter compound, lot 198HC, loose in the module. The absorber was applied to the cover with
Dow Corning Sylgard 577. The getter was 0.017 inches thick with a surface area of 1.08 square
inches. The modules will be processed in the following manner:
• standard seam seal process (16 hour, 150°C vacuum bake)
• condition at 125°C for 168 hours
• measure hydrogen content inside the module (RGA)
Summary of Gas Analysis
Sample
Identification
Sample
Conditioning
A
populated module
no absorber
post seal temperature conditioning
168 hours at 125C
360
343
494
511
407
121
range = 121 – 511
average = 373
B
populated module
CRS-124 absorber
post seal temperature conditioning
168 hours at 125C
46100 (4.61%)
46500 (4.65%)
47200 (4.72%)
range = 4.61 – 4.72%
average = 4.6%
C
populated module
vacuum baked CRS-124 absorber,
72 hours at 165C, -1 atmosphere
post seal temperature conditioning
168 hours at 125C
526
618
548
range = 526 – 618
average = 564
D
populated module
Allied Signal H2 getter, lot 118
(0.023” thick)
post seal temperature conditioning
168 hours at 125C
none detected
none detected
E
populated module
vacuum baked CR124 absorber,
72 hours at 165C, -1 atmosphere
Allied getter (0.024” thick) loose, lot 118
post seal temperature conditioning
168 hours at 125C
none detected
none detected
F
populated module
unbaked CRS-124 absorber,
Allied getter (0.016” thick) loose, lot 118
post seal temperature conditioning
168 hours at 125C
none detected
Hydrogen Content (ppm)
G
populated module
vacuum baked CRS-124 absorber,
72 hours at 165C, -1 atmosphere
Allied getter (0.024” thick) loose, lot 198
post seal temperature conditioning
168 hours at 125C
none detected
none detected
H
populated module
unbaked CRS-124 absorber,
Allied getter (0.016” thick) loose, lot 198
post seal temperature conditioning
168 hours at 125C
none detected