Effect of Hydrogen on GaAs MMICs in Hermetic Packages.
D. N. Goswami
Picosecond Pulse Labs
2500 55th Street
Boulder, CO 80301, USA
Phone: (303) 209-8126, FAX: (303) 209-8203, e-mail: dgoswami@picosecond.com
Abstract
Presence of hydrogen in a hermetic package containing GaAs semiconductor devices can adversely affect
performance and reliability of the GaAs devices. The problem is believed to be caused by conversion of molecular
hydrogen to atomic hydrogen through the catalytic reaction with platinum or palladium in the gate structure of a
GaAs device, which then diffuse into the active area of the device affecting its electrical performance. The primary
source of hydrogen in the package is the trapped hydrogen in the ferrous metals used in package fabrication, and
hydrogen that gets absorbed in the nickel plating during electroplating of the package. Past studies have shown that
performance of GaAs device can degrade severely at hydrogen level as low as fraction of a percent. The device
fabrication process and gate structure/metallurgy also impact the sensitivity to the hydrogen level. This paper
presents the results of a study on the level of hydrogen in gold/nickel plated aluminum package as a function of
different high temperature bake, and presents the Residual Gas Analysis (RGA) results for hydrogen in hermetic
hybrids with GaAs MMICs that have successfully completed 1000 hour life test as a part of Telcordia Hybrid
Qualification. The results show that hydrogen content in a hermetic package can be reduced to less than 1000 ppm
by high temperature bake, and for telecommunication applications (operating ambient 60°C or less) hydrogen
poisoning of GaAs MMIC in hermetic hybrid is not a concern so long as the hydrogen level is kept below 5000 ppm.
Key Words: GaAs PHEMT, Aluminum Package, Hydrogen outgassing, Hydrogen poisoning,
1. INTRODUCTION
Use of GaAs ICs in various high frequency
applications has increased significantly over the last
few years. This has created considerable interest in
the reliability of these devices. One area of concern
has been the effect of hydrogen gas present in
hermetic packages containing GaAs devices. Early
work in this area reported that hydrogen gas in
quantities as low as 0.5% of ambient atmosphere can
cause significant performance degradation at
temperature of 125 C in a relatively short period (500
hours) [1]. The degradation has been reported for
both metal semiconductor field effect transistor
(MESFET) and pseudomorhic high electron mobility
transistor (PHEMT) devices involving amplifier
functions [2-12], but not for heterojunction bipolar
transistor [HBT], PIN diodes, and phase shifter
functions [12,13]. The problem manifests as a sudden
drop in current and transconductance, and is believed
to be caused by conversion of molecular hydrogen to
atomic hydrogen through the catalytic reaction with
platinum or palladium in the gate structure, which
then diffuse into the active area of the device
affecting its electrical performance [1,14]. No
degradation has been observed when the gate did not
contain any Platinum or palladium [8]. The primary
source of hydrogen in the package is the trapped
hydrogen in the ferrous metals used in package
fabrication, and hydrogen that gets absorbed in the
nickel plating during electroplating of the package.
Studies have shown that hydrogen content of the
package materials can be reduced, but not eliminated
by thermal annealing [2]. Since aluminum does not
absorb hydrogen, use of aluminum package is
preferred. But hydrogen can still get adsorbed in the
nickel under-layer during gold plating of the package
that can later outgas creating a hydrogen rich
environment inside the package cavity. There is also
the concern that hydrogen may react with other
residual gases and oxides inside a hermetic hybrid
creating a moisture rich environment.
Although the problem has been known for
more than a decade, and various ways to mitigate the
problem has been discussed in the literature [13,14],
there is no clear guideline or limit as to what level of
hydrogen might be considered acceptable in hermetic
hybrids. The problem is somewhat involved since the
device type, fabrication process, feature size, gate
structure/metallurgy,
and
operating
ambient
temperature, - all can impact the sensitivity to the
hydrogen level. This paper presents (1) the effect of
thermal bake on hydrogen content in Ni/Au plated
aluminum packages, and (2) 1000 hr life test data
including RGA results for hydrogen in hermetic
hybrids that used plated aluminum package with no
thermal bake.
2.
Background
Figure 1. Test Package
Several OEMs using Picosecond Pulse Labs
(PSPL) hybrid amplifier devices in the design of their
products required the hybrids to be qualified to full
Telcordia requirements before using them in
production systems. They also wanted assurance that
there would be no hydrogen related device
degradation over the operating life. Key points that
had to be addressed were:
(1) What level of hydrogen can be expected in a
gold plated aluminum package used for PSPL
hybrids, and does it pose any reliability risk?
(2) If the hydrogen level is high, what thermal
treatment may be used for the package to reduce
the hydrogen level to an acceptable level?
There was not enough time to fully address
the above issues before starting the qualification life
test if we were to meet customer’s deadline.
However, there had been no evidence of any
hydrogen related performance degradation of PSPL
devices that had been out in the field and hydrogen
outgassing did not appear to be a serious issue with
the Ni/Au plated aluminum packages used in PSPL
hybrids. Hybrid qualification testing was, therefore,
started using as-plated aluminum package, but a
parallel experiment was also carried out to study
hydrogen outgassing from Ni/Au plated aluminum
packages.
3. Hydrogen Outgassing plated Aluminum Package
Test samples consisted of rectangular
aluminum housings (1"X1"X0.375") with 0.125"
thick sidewalls (Figure 1) which were representative
of production packages used in PSPL hybrids. The
test housings were plated with 50 microinch of gold
over 100-300 microinch of nickel. An air circulating
oven was used to bake the packages. Packages were
baked, laser welded, conditioned at 125 C for 168
hour, and then sent out for RGA test.
3.1
Post Nickel Plating Bake
It seemed logical that thermal bake of the
nickel layer prior to gold plating would be the most
efficient way to get rid of the hydrogen from the
nickel plating. RGA results for hydrogen in samples
subjected to different post nickel plating bake are
shown in Table 1.
Samples 1-3 with no thermal bake showed
16500 2000 ppm of hydrogen. Such high level of
hydrogen resulting from the relatively thin (100300µ”) layer of nickel was somewhat of a surprise.
Sample 3 was plated by a different vendor who
claimed to have a nickel plating process that
minimized hydrogen absorption; however, we did not
see any evidence of it. Samples 4 and 5 with
190 C/3hr bake showed a sharp decrease in hydrogen
to less than 5000 ppm. So, when the bake
temperature/time was raised from 190 C/3 hr to
250 C/48 hr for samples 6,7, a very low level of
hydrogen was expected inside the package.
However, the samples still showed around 2000 ppm
of hydrogen. A possible explanation was absorption
of additional hydrogen by the nickel layer during
subsequent gold plating. This implied that to reduce
hydrogen level below 2000 ppm the packages would
have to be baked after gold plating.
Table 1. RGA Result for H2 (Post Ni plating Bake)
Sample
Post Ni
Post AuCavity
#
Plating bake Plating bake Content
H2 (ppm)
1,2
None
None
Empty
16,800, 14,500
3*
None
None
Empty
18,200
4,5
3 hr / 190 C
None
Empty
4,986, 4,316
6,7
48 hr / 250 C
None
Empty
2,014, 2,147
* Package plated by a different vendor from the rest.
3.2
Post gold-plating bake
Samples were prepared by performing postgold plating bake on packages with and without postnickel plating bake (see Table 2). One of the samples
(sample11) contained a Duroid substrate laminated to
Ni/Au plated Cu-W carrier which could be also a
possible sources for hydrogen, while another
(sample12) contained a RF absorbing silicone rubber
which is used under the package lid in some rf
hybrids. RGA results for hydrogen are shown in
Table 2.
Table 2. RGA Result for H2 (Post Au plating Bake)
Sample
Post Ni
Post AuCavity Content H2
#
Plating bake Plating bake
(ppm)
8
3 hr / 190 C 168 hr / 125 C
Empty
1305
9
3 hr / 190 C 130 hr / 215 C
Empty
121
Empty
721
10
11
12
None
168 hr / 125 C
48 hr / 250 C 168 hr / 125 C Duroid Subs.
Laminted to
Cu-W Carrier *
48 hr / 250 C
168 hr / 125 C
RF absorbing
Siicone rubber*
777
511
* Pre-Baked at 125 C for 168 hr
Comparing the results for samples 8, 9 with
those of samples 4, 5 in Table-1, it is clear that postgold-plating thermal bake is quite effective to reduce
the level of hydrogen. Sample 9 which was baked at
a higher temperature of 215ºC showed only 121 ppm
of hydrogen.
Sample 10 did not have any post nickel
plating bake, but it showed lower hydrogen content
than sample 8 that was baked after both nickel and
gold nickel plating. The difference is well within
statistical error, but it indicates that thermal bake of
the nickel layer prior to gold plating (which often
creates considerable logistical problem at the Plater’s
facility) is not essential. Gold Plating does not appear
to impede the escape of hydrogen from the nickel
layer and thermal bake of the package after final gold
plating is quite effective in removing hydrogen from
the nickel layer.
Sample 11 with the laminated substrate
inside the cavity did not show any significant
increase in hydrogen. 125°C/168 hr pre-bake of the
laminated substrate appeared to be quite effective in
removing any trapped hydrogen in the nickel plating
of the carrier and the substrate. Sample 12 with the
silicone rubber sheet did not show any increase in
hydrogen level either. This was not unexpected since
the silicone rubber did not contain any hydrogen.
However, the RGA results for the package showed
high level of CO2 (~9500 ppm) and methane (~2500
ppm) which may create other reliability hazards.
4.0
Life test
1000 hour Operating life test was performed
as a part of Telcordia hybrid qualification in
accordance
with
TR-NWT-000930,
Generic
Requirements for Hybrid Microcircuits Used in
Telecommunications Equipment. All qualification tests
(see Table 3) were successfully completed, but only the
operating life test and post-operating life RGA test data
are presented here.
Table 3. Telcordia Qualification Test Requirement
TEST
Electrical
Sampling Plan *
MIL-STD-883
NOTE
Method Condition LTPD SS C
30+ -
Physical
Mech Shock
2002
Vibration
Thermal Shock
(15 cycles)
Temp Cycling
(100 cycles)
2007
1011
Operating Life
1005
Low Temp.
Storage
Int Moisture
RGA
1010
-
5
-
Cond B
10
38
1
Cond B
10
10
38
38
1
1
A, B
10
38
1
B, C, D
1000 hr
10
38
1
E
-40 C/100
hr
20
11
0
Cond B,
-40 C/125 C
Cond B,
-40 C/125 C
1018
10
F
* LTPD= Lot tolerance percent defective,
SS = Sample Size, C= Max number of failures allowed.
NOTES
A. The same set of samples shall be used for all mechanical tests.
The total number of failures is counted against accept/reject
criteria. The mechanical test may be performed in any order.
B. Hermeticity tests (fine/gross leak) consistent with MIL-STD-883,
Method 1014 shall be performed in addition to electrical tests.
C. The lower temperature limit shall be -40 C as specified in
Bellcore TR-NWT-000930.
D. Samples completing Mechanical tests shall be used for this test.
E. The ambient temperature shall be chosen to maintain the
maximum IC junction temperature during Operating Life Test
in the 155 5 C range.
F. Parts completing Operating Life tests shall be used in these tests.
4.1
Test Sample
A 12.5 Gbit/Sec Modulator Driver
Amplifier, PSPL P/N 5865 (Figure 2), that is fairly
representative of the PSPL amplifier product family,
was used as the test vehicle. It contained a 3.3mmX
2.3mm GaAs PHEMT amplifier IC containing 9
cascaded pairs of dual gate FETs (0.5
m
Technology). Packages were not subject to any
thermal bake and a sample size of 38 (Telcordia
Qualification requirement) was used for the life test.
Figure 2: Test Device showing the supply
voltages used for Life test.
(No connection to pins QC, CP, and VB)
4.2
Test Details
Figure 3a. Test Chamber (left) with Power
Supply and Test Monitor (right)
An air circulating Environmental Chamber
was used for the life test (Figures 3). Life test was
performed under steady state condition using a
supply voltages of +8V for drain and –5V for the
gate (Figure 2).
No heat sink was used to cool the devices
during life test. Under the test condition, the PHEMT
IC dissipated 1.92 watt of power. Based on the
thermal resistance data provided by the IC vendor,
this corresponds to a Junction-to-Case temperature
difference of 70 C. The case temperature during life
test was maintained at 85 5 C to prevent IC junction
temperature from going much over the recommended
maximum IC junction temperature of 150 C. An
automated test program monitored the current drawn
by each device and saved the information every 60
minutes. The test circuit had a built-in safety feature
that shut down the power going to any specific device
if its current exceeded 325mA.
Electrical measurements were made at room
ambient after 0, 168, 504, and 1000 hours of life test.
Measured parameters are shown in column 1 in Table
4. A device was considered to have “failed” if it did
not meet the electrical specification for the hybrid or
if any of the following conditions occurred:
Risetime10-90% changed by more than 6 ps.
Jitter changed by more than 6 ps.
Eye amplitude (Voutp-p ) changed by more than
10% ((Input= 0.5 Vp-p).
4.3
Test Results
4.3.1
Electrical Results
A summary of electrical test results is shown
in Table 4. No evidence of any degradation of
electrical properties has been observed over the
operating life; observed changes were well within the
range of measurement error.
Figure 3b. Units under Test in the Test Chamber
Table – 4. Electrical Test Results (sample size=38)
PARAMETER
Eye Amplitudep-p
(Volt)
Risetime10-90%
(ps)
Eye Jitterp-p
(ps)
Noise Figure
(dB)
S21, S11, S22
4.3.2
0 hr 168 hr 504 hr 1000 hr
Av.
Max
Min
Av.
Max
Min
Av.
Max
Min
Av.
Max
Min
8.1
8.6
7.8
29.9
32.4
28.0
13.0
14.2
10.7
5.3
5.7
5.2
8.0
8.3
7.8
29.9
32.4
28.0
12.7
14.2
10.2
5.4
5.7
5.2
8.2
8.6
7.9
29.5
33.3
27.6
12.8
14.2
11.1
5.3
5.6
5.1
8.4
8.7
8.2
29.8
32.4
28.0
12.8
14.2
10.7
5.3
5.6
5.2
No statistically significant change.
(plots not presented here)
RGA Results
RGA testing was performed on 10 units that
completed the life test. Results are shown in Table 5.
An Ar/He (75/25) mixture was used for the welding
environment and it is reflected in the RGA data.
Hydrogen level inside the packages ranged from
1279 to 5986 ppm, while moisture content ranged
from 1375 to 2807 ppm. A fair amount of nitrogen
was also detected in the packages. This occurred due
to inadequate flushing of the nitrogen from the
welding chamber prior to welding of the packages,
and has since been corrected. Packages also showed
Argon
(%)
CO2 (%)
Moisture
(ppm)
Hydrogen
(ppm)
He (%)
Fluoro
Carbons
Others
(ppm)
K. Decker performed an extensive GaAs
hydrogen sensitivity study with PHEMT devices
(200µm X 0.25 µm test FETs) and concluded that
electrical bias did not effect the hydrogen degradation
mechanism and it was the ambient temperature (not
channel temperature) that drives the hydrogen
degradation mechanism [3]. His data fitted the model
developed by Adams, et. al. [6] that is given by the
Equation
O2 (ppm)
Table – 5. RGA results (post Operating Life)
N2 (%)
ppm level seen in the aluminum test housings. High
temperature exposure of the package during solder
lead attach and device screening (temperature
cycling, and 168 hour open burn-in) eliminated about
two-thirds of the hydrogen from
the
plated
packages.
Sample #
CO2 in the 0.67% to 1.90% range. According to the
RGA Test Lab, this is not uncommon for packages
containing FR4 and Duroid boards which the test
hybrids had. High level of fluorocarbon in one of the
units was attributed to contamination from gross leak
test performed prior to RGA.
1
2
3
4
5
6
7
8
9
10
0.68
ND
75.4
0.93
1,770
5109
22.3
ND
<100
0.79
124
75.1
0.72
2,384
5986
22.5
ND
455
0.67
ND
74.6
1.13
2807
5816
22.7
ND
158
4.70
301
74.3
1.29
2622
1279
19.3 220 ppm <100
3.44
107
65.8
1.90
2212
3706
27.2
1.78 %
<100
0.63
ND
75.2
1.23
2561
3090
22.4
ND
<100
2.49
2133
74.9
0.88
2584
3101
20.9 491 ppm <100
0.58
ND
76.0
0.67
1875
1809
22.4
ND
<100
0.72
390
75.1
0.90
2046
2709
22.7
ND
<100
0.58
ND
75,.8
0.77
1375
3603
22.4
ND
110
5.
Discussions
Computer analysis has shown that any
change in Ids of the GaAs PHEMT from possible
hydrogen poisoning will affect S21 and eye amplitude
parameters for the hybrid as shown in Figs 4 and 5
respectively. There was no evidence of any such
change during life test.
S21 (dB)
Nominal
Delta Ids = + 10%
Delta Ids = - 10%
Delta Ids = - 20%
30
25
20
t = A Pnexp(Ea/kT)
(1)
where
t = median life (hr) for 10% Idss degradation
A = a proportionality constant (5.46E-6)
P = hydrogen partial pressure (%H2 X 7.6)
n = -0.7935
Ea = activation energy (0.73 eV)
K = Boltzman Constant (8.615E-5)
T = ambient temperature (ºK)
If the model is assumed to be valid for the PHEMT
devices used
in this study, then for ambient
temperature of 85ºC and hydrogen level of 5,000
ppm, (conditions representative of the present life test
study), testing has to be continued for 36,000 hours
(4.1 yr) before the device fails (10% degradation in
Ids). It is, therefore, not surprising that we did not see
any hydrogen poisoning effect in the 1000 hr life test.
15
10
0
1
2
4
6
8
10
12
14
16
18
20
Frequency (GHz)
Chnage in Eye
Amplitude
Figure 4. Effect of change in Ids on S21
10%
Hybrids used in this study contained various
epoxy adhesives, soldered parts and duroid/FR-4
composite boards. The fact that moisture content in
the hybrids remained below 3000 ppm (allowed
maximum 5000 ppm) suggests that there is no
evidence of hydrogen reacting with outgassed
products or other oxides inside the hybrid creating
excessive moisture.
0%
-10%
-20%
-20%
-10%
0
10%
Change in Ids
Figure 5. Effect of change in Ids on Eye Amplitude
The amount of hydrogen detected inside the
hybrids after life test was much lower than the 15000
Table 5 shows the calculated level of
hydrogen from equation (1) for a median product life
of
20 years for different operating ambient
temperatures.
Table 5. H2 level for 20 year Median Life
Median
Life
T=40 C
T=50 C
T=60 C
T=70 C
20 Yr
4.96%
1.73%
0.64%
0.25%
Hydrogen Level
Since the typical operating ambient
temperature for hybrids in telecommunication
application is not expected to be more than 60°C, a
hydrogen level of 5000 ppm should be acceptable for
such application. However, to be on the safe side,
3000 ppm would be a more reasonable value, and it
can be easily achieved for aluminum package by
prebaking the package at 125°C bake for 168 hours or
a shorter bake time at a higher temperature in the
150°C to 175°C range.
6.
CONCLUSIONS:
Hydrogen desorption from Ni/Au plated
aluminum packages can result in hydrogen level
of more than 15,000 ppm of hydrogen in
hermetic packages. Hydrogen level can be
reduced to 1000 ppm level by 125°C/168hr
thermal bake of the package.
1000 hour operating life test performed during
hybrid qualification testing is not capable of
detecting potential hydrogen problem in hermetic
hybrids. Test duration is too short for the level of
hydrogen seen in typical hermetic packages.
For telecommunication equipment, a hydrogen
level of 3,000 ppm seems to be a reasonable
limit. It would meet the requirement of 20 year
life at 60°C operating ambient and can be easily
achieved for Ni/Au plated aluminum package by
thermal bake of the package for 168 hours at
125ºC or a shorter duration at a higher
temperature.
Acknowledgement
The author would like to thank Dr. James Andrews
and Clayton Smith for their support, encouragement
and helpful discussions and Dr. Jason Yoho for the
computer simulation results relating the changes in
Ids of the PHEMT die to the changes in S21 and Eye
Amplitude of the hybrid amplifier.
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