1999 International Symposium on Advanced Packaging Materials

Water vapor, volume % 3 .O 2.0

Abstract

Dewpoint temperature, “C +25 +I8

Sources and Control of Volatile Gases Hazardous to Hermetic Electronic Enclosures

Robert K. Lowry Senior Scientist

Harris Semiconductor Analytical Services Laboratory

Melbourne, FL 32901 Phone 407-724-7566

E-mail rlowryOl @harris.com

P.O. BOX 883, M.S. 62-07

Fax 407-729-4053

Four gases that threaten oversting reliability may be present ir, hermetic electronic enclosures. Condensates of moisture andor ammonia can cause metallization corrosion. Hydrogen is a rapid diffiser that can degrade MOS device operation. Oxygen can cause -:++ion and ensuing failure of solder attachment materials within the sealed package. Other gases, such as carbon dioxide, helium, argon, and organic volatiles are not threats to reliability, but do provide clues to package materials behavior. Knowing sealed package ambient gas composition helps improve materials and processes for hermetic sealing and awhles process control to assure reliable products. This paper describes the analysis method for hermetic microelectronics, Residual Gas Analysis (RGA), available at only a few laboratories worldwide. It discusses sealing processes and package piece part materials that are sources of volatiles hazardous to product reliability. It presents materials selection and improvement considerations to reduce and control dangerous volatiles in hermetic packages.

Introduction

In the 1970’s and early 1 9 8 0 ’ ~ ~ corrosion of metallized features on integrated circuits or other components sealed in single chip or hybrid hermetic enclosures was a leading failure mechanism 11, 2/. Corrosion failure of an aluminum metal line caused a perilous launch abort of a manned space mission. Failure analysis showed that sealed-in moisture combined with ionic contamination was root cause of the failure 131. Corrosion due to sealed- in moisture occurred often enough to lead the Air Force in conjunction with industry organizations (e.g., JEDEC) to develop methods and specifications for measuring and controlling intemal moisture in sealed packages. Measurement methods developed for water vapor include mass spectrometry 14, 51, an in-situ sensor for detecting dewpoint 16/, an in-situ sensor based on die capacitance

changes 171, an in-situ volume effect sensor 18, 91, gas chromatography, and infrared spectrophotometry 1101.

Mass spectrometric, in-situ sensor, and gas chromatographic techniques are formalized in a military t e method specification document 1111. The in-situ sensor methods provide a relatively fast measure of intemal moisture and have been applied with varying degrees of success. Chromatographic and infrared techniques have been investigated but not widely applied. Mass spectrometry developed as the most widely applied method because it gives information about water vapor and all other volatile species in a package. The method came to be known as “Residual Gas Analysis” (RGA). As packaging technologies evolved, RGA for all volatiles in a package became ar -labling technique for improving and controlling hermetic package sealing.

Gases Dangerous to Hermetically Sealed Devices

Moisture. Water vapor was the first species known to endanger hermetic part reliability. Condensed liquid water can be corrosive to the aluminum metallization used on electronic components.

2A1+ 3H20 + A120, + 3H2 (1)

Liquid water condensate forms depending on dewpoint temperature. Table 1 shows dewpoint temperatures for varying amounts of water vapor in a sealed cavity at approximately 1 atmosphere.

Table 1. Moisture levels and dewDoint temDeratures.

I 1 .o I +8 0.7 +2

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1999 International Symposium on Advanced Packaging Materials

He N,

The reaction of Eq. (1) is accelerated by any soluble ionic contaminants that may be present on package surfaces if not processed in clean environments. These would include acidic or basic substances that could make condensate pH -4 or >8 /12/ and ionic species that enhance conductivity of the condensate. The reaction is further enhanced by galvanic action.

r -

<o.o 1 v% CO.0 1 v% =99.9 e94.9

The 0” dewpoint temperature near the moisture content of 0.5 volume percent (5000 ppmv) was one of the determining factors in assigning that value as the maximum allowable water vapor in sealed packages /13/.

As the RGA method became available at several commercial laboratories, and was applied for internal water vapor reduction and control, the survey nature of the method enabled measurement of other volatile species hazardous to electronics reliability.

Oxygen. Thermal fatigue die attach failure is caused by differences in thermal expansion coefficients of package materials during thermal cycling. Where lead solders are used for die attach, packaging technologies provide for a non-oxidizing atmosphere within the enclosure, to prevent 0, contributing to oxidation and/or cracking of solder and weakening die attachment strength /14/. Accordingly, the maximum level of 0, in hermetic parts using solder die attach is specified at 2000 ppmv /15/.

Ammonia. Ammonia can outgas during cure of die adhesives with dicyandiamide [NH,C(NH)(NHCN)] curing agent. If ammonia dissolves in condensed water, a basic solution pH >8 can form which is corrosive to aluminum. Also, ammonia that condenses along with other species including water and organics contributes to electrical failure due to excessive surface leakage /16/. No specification limits are published for NH,.

Hydrogen. There are three likely sources of H, in sealed packages. In many applications, piece parts are pre-seal baked or fred and/or sealed in a reducing environment of forming gas containing 24% H, in N,. H, can outgas from grain boundaries or structural imperfections in iron-nickel alloy (Kovar, Alloy 42) lead frame materials /17/. Plated gold or nickel films can trap H, and release it after seal. H, in sealed packages can range from 1-25% or more. H, is extremely mobile. It can diffuse rapidly to Si/SiO, interfaces, where it collects to create non-uniform charge distributions leading to threshold voltage shills /18/. No specification limits are published for H,.

The benchmark for package gas analysis data is the composition of natural air, shown in Table 2. Air

composition is constant, and air serves as a calibration medium for analyses and an aid for interpreting results.

Table 2. Composition of natural air.

*depending on relative humidity

Results and Discussion

Elevated Moisture. Table 3 summarizes extent of internal water vapor noncompliances that halted shipments of a semiconductor device in a 0 . 0 2 ~ ~ metal walled package. Table 4 summarizes the typical internal gas compositions for compliant and noncompliant units from the time periods indicated.

Table 3. Internal water vapor noncompliances in a 0 . 0 2 ~ ~ metal walled IC Dackage. I Package date code r 1996 im

Total number of units tested I 369 I 250 Units compliant, <0.5v?h H,O I 313 I 24 1

I Units noncompliant, >0.5v% H,O I 56 I 9 1

Table 4. Typical internal gas compositions of 0 . 0 2 ~ ~ metal walled package (all data in volume percent). *

I Gas I Comdiant I Noncomdiant I

*Note for Tables 4-8: All RGA data in this paper are in volume percent (v”/”). TO convert to parts per million by volume (ppmv) move the decimal 4 places to the right, ( O . ~ V % = 5000 ppmv). Data are included for CH30H (methyl alcohol), CxHy (a mass value for volatile hydrocarbons, i.e., organic compounds), and FC (fluorocarbons). Entries for special discussion are shaded.

From Table 3, it was clear that product sealed in 1996 had a higher rate of noncompliance for H,O than product sealed before 1996. From Table 4, the unique gas compo-

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1999 International Symposium on Advanced Packaging Materials

sitions of compliant versus non-compliant parts are clues to root cause of elevated H,O. First, note that all units regardless of when sealed are hermetic, since no 0, or Ar is present. However, components uniquely high in the noncompliant parts include CO, and organics.

Error! Not a valid link. Figure 1. Scatter plot of CO, vs H,O content.

A scatter plot in Figure 1 of CO, vs H,O in the noncompliant units clearly shows CO, increasing as H,O increases. Unique levels of CO, and organics in the noncompliant units suggest the organic die adhesive as the source of the elevated H,O. Subsequent experimental work with the adhesive revealed that material with 1996 date code could not be cured to a completely anhydrous condition. Changing adhesive for this product to a previously qualified silver-filled adhesive enabled all product assembled thereafter to comply on H,O content.

Resolving this issue included determining functionality of noncompliant units. Two tests were conducted at +2”C, where (Table 1) any unit containing 2=7000ppmv H,O should contain condensate.

A 1996 product lot with 4 of 6 units noncompliant in H,O (7000-25,400 ppmv) was selected. From this lot, 49 units were placed on storage life test at +2”C for 2000 hours. Another 1996 product lot with 3 of 3 units noncompliant in H,O (7600 to 18,400 ppmv) was selected. From this lot, 100 units were placed on 17.7V operating bias life test at +2”C for 2000 hours.

After 2000 hours of life testing, all 149 units in the above groups were still fully functional.

Twenty-seven of the 149 units were randomly sampled at completion of the life tests for RGA. Seventeen units had H,O levels from 7000-30,000 ppmv, with one unit containing 90,000 ppmv H,O!

From these data it is certain that at least these 17 units out of the 149 total lifetested contained liquid water during the tests. If the ratio of 17/27 is extended to the whole group, more than half of the lifetested units must have contained liquid water during 2000 hours of static and bias lifetests. Yet none failed functionally.

In this particular case, despite the noncompliance to specification for intemal water vapor, product was ultimately deployed in a high reliability system. Details of this study are described elsewhere /19/.

Table 5 shows RGA data obtained on three metal lid power devices that contain die soldered to the package base, where oxygen levels can be a reliability concern. These units are sealed in a Hem, gas mix containing a few percent He, which helps confirm hermeticity of the product since He would rapidly exit the package through any leak path.

Table 5. RGA data for power device metal packages with solder die attach.

I Unit I I Unit 2 1 Unit 3 I

1 -

I <0.01 I 0.03 I 0.35 I

CO, I <0.01 <0.01 I <0.01 H, I <0.01 0.03 I <0.01

I NH.

FC I XO.01 1 <0.01 I <0.01

Unit 1 contains the typical intemal gas composition for this part type, i.e., an Hem, mix with no other gas species detected. The part is fully compliant to H,O and 0, specification requirements.

On a few occasions, however, RGA analyses fmd units that deviate from this composition. Both Units 2 and 3, while compliant for H,O, are noncompliant for 0,. Examining the data and the parts themselves revealed that units can be noncompliant for 0, for different reasons. Elevated 0, in Unit 3 is due to air, since Ar is detected. However, no Ar is present in Unit 2. Material inspections ultimately found that parts like Unit 2 contained microdefects in the hard glass seal around the package pins. Occasionally a microcrack in the glass allowed gas from these defects to enter the package cavity and be detected in the RGA analysis.

It appears that packages like Unit 3 contain air as a “contaminant” at the time of seal. Both Ar and He are present, they do not show an indication of the microdefects like those of Unit 2, and leak tests show them to be hermetic. Further, if air had leaked in after seal, He would be much reduced or absent, as it should rapidly exit the package cavity through any leak path.

Elevated ammonia.

Elevated oxygen.

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1999 International Symposium on Advanced Packaging Materials

0, Ar CO,

Table 6 shows typical RGA data reported for hybrid packages fabricated with either thin- or thick film gold on alumina substrates. The units contained various discrete components such as diodes, transistor, and CMOS chips attached to substrate with silver-filled epoxy adhesives. <o.o 1 <0.01 <0.01

<0.01 <0.01 <0.01 1.18 0.21 0.20 Table 6. Gas composition of hybrid units, some of

CH,OH

FC C*H,

which exhibited silver dendritic growth 1161-

<0.01 <0.01 <0.01 <0.01 <o.o 1 <0.01 CO.0 1 <o.o 1 <o.o 1

I

CO2 1 0.59 0.59 0.34 0.40 H, 1 0.11 0.10 0.06 0.07

Some (though not all) units with these gas compositions displayed silver migration and growth of silver dendrites. All the units are compliant for both H,O and O,, but contain highly elevated levels of NH,. All the units are hermetic as indicated by the absence of significant levels of 0, and Ar. Levels of NH, are remarkable. Further, units fiom both manufacturers show significant amounts of methyl alcohol and some signal indicative generally of volatile organics. The NH, and the organic species indicate outgassing from a die attach adhesive such as Ablefilm 550, which uses the dicyandiamide curing agent. Some units with this type of intemal gas composition grew silver dendrites. Condensates of polar species such as NH, and CH,OH on surfaces create very conductive surface leakage paths that enable electrochemical mechanisms promoting dendritic growth. Both short and long-term electrical failures occurred within these groups. Failures usually occurred during thermal cycling, with many failures appearing during the first cold cycle to -3O"C, at which point surface leakage increased dramatically. The leakage condition often disappeared when units went to the warm part of the cycle. Long term failures were due to electrical shorts and were irreversible 1161.

Elevated hydrogen.

Table 7 shows typical RGA data for braze seal semiconductor packages with relatively high intemal cavity surface areas of plated gold.

Table 7. Braze seal semiconductor packages.

CO.01 94.83 95.41 99.46

H,O 0.06

The 16 lead braze and PGA braze units are compliant for H,O and 0, but contain percentage quantities of H, that could affect stability of MOS devices. The 24 lead braze unit is not only compliant but is also free of H,. The low H, in the 24 lead unit is due to extensive pre-seal vacuum baking that removed entrapped H, from the gold. Another observation about these parts is their cleanliness with respect to organic species, due to use of eutectic die attach and the cleansing effect of H,-firing.

Other RGA data.

Table 8. RGA data for other unique situations. I Gas I WeldUnitA I WeldUnit B I Hvbrid 1

Table 8 summarizes RGA data for three additional noncompliant situations. Weld seal units A and B are noncompliant for both H,O and 0,. Unit A clearly shows evidence of non-hermeticity during leak test, as indicated by the presence of fluorocarbon. Some of the H,O, and the 0, in this unit, are from air ingress (Ar also present), while the remainder of the H,O and the CO, are from organic material outgassing. The amount of H,O in this unit is too high to be explained entirely by air ingress.

Unit B was non-hermetic at some other point in time. No fluorocarbons are present, the amount of H,O is consistent with some level of relative humidity, and the

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1999 International Symposium on Advanced Packaging Materials

0, and Ar contents roughly approximate that of air (a non-hermetic part seldom has stoichiometric air content).

The hybrid part was sealed in a He/& mixture, which completely changes the interpretation for cause of noncompliance. The unit is not a leaker since He is very high. Again, the cause is organics outgassing, indicated by the level of C,H,.

Controlling dangerous gases and making compliant product.

1. Select package piece part materials with attention to cleanliness and absence of physical defects. 2. Select die attachment and conformal coating materials that are to be inside the sealed package with a thorough understanding of their curing and outgassing behavior. 3. Cure all organic components comuletelv after assembly and before seal. Do not shortcut cure schedules or overload cure ovens with large masses of parts. 4. Do all assembly in a clean environment with attention to particle control and prevention of contamination from the environment, process equipment, and human operators 13,201. 5. Vacuum bake assembled parts thoroughly before seal. Minimum recommended conditions are 2 hours residence time at 1 5OoC and 10 millitorr. 6. Do not expose vacuum baked parts to air ambient before seal. Baked surfaces instantly re-hydrolyze. 7. Seal in equipment with ambient gas moisture monitors capable of sensing H,O at least to the 10 ppmv level. 8. Engineer robust processes validated by RGA analysis. 9. Utilize RGA analysis services to statistically monitor the process, not just to inspect sealed product for compliance prior to shipment. Establish the statistical capability of the process with respect to dangerous gases.

Conclusion.

The background of the RGA method of analysis of sealed electronic devices is presented. The harmful effects of H,O, 02, NH,, and H, on reliability of hermetic devices are described. RGA data typically obtained when these gases are in hermetic packages are presented and discussed. Some guidelines for robust hermetic sealing processes that prevent these gases from being present in the devices are specified. There is no difficulty in keeping these dangerous gases to low levels if best practices with respect to materials and processes are faithfully executed during hermetic sealing.

References ~-

1. H. Koelmls, “Metallization Corrosion in Silicon Devices by Moisture-Induced Electrolysis,” J. Electrochem. Soc., 123, January 1976, pp. 168-171. 2. Multiple authors, Proceedings of ARPA/NBS and

RLINIST Workshops on Moisture Measurement Technology for Hermetic Semiconductor Devices and Microelectronics,” NBSMIST special publications 400- 69 (March 1978), 400-72 (November 1980), 84-2852 (May 1984), 87-3588 (November 1986), 5241 (April 1993), and no number available (April 1996). 3. R.W. Thomas, D.W. Calabrese, “The Identification

and Elimination of Human Contamination in the Manufacture of IC’s,” Proc. IRPS, March 1985, pp. 228- 234. 4. R.W. Thomas, D.E. Meyer, “Moisture in

Semiconductor Packages,” Solid State Technology, 17(9), September 1974.

5. R.M. Sthac, M.J. Cohen, R.F. Wemlund, “Water Vapor Measurements in Small Volumes Using Atmospheric Pressure Chemical lonization-Mass Spectrometry,“ Proc. IRPS, March 1982, pp. 260-263. 6. R.K. Lowry, L.A. Miller, A.W. Jonas, J.M. Bird,

“Characteristics of a Surface Conductivity Moisture Monitor for Hermetic Integrated Circuit Packages,” Proc. IRPS, April 1979, pp. 97-102. 7. R.P. Merrett, “Using the Die of an Integrated Circuit

to Measure the Relative Humidity Inside Its Encapsulation,” Proc. IRPS, November 1980, pp. 17-25.

8. M.G. Kovac, D. Chleck, P. Goodman, “A New Moisture Sensor for In-Situ Monitoring of Sealed Packages,“ Proc. IRPS, April 1977, pp. 102-107. 9. J.B. Finn, V. Fong, “Recent Advances in A1,0, In-

Situ Monitoring Chips for Cerdip Package Applications,” Proc. ZRPS, April 1980, pp. 10-16. 10. P.R. Bossard, J.A. Mucha, “Dynamic Measurement of the Water Vapor Content of Integrated Circuit Packages Using Derivative Infrared Diode Laser Spectroscopy,” Proc. IRPS, March 1982, pp. 260-263. 11. MIL-STD 883, Test Method 1018.2. 12. H.H. Uhlig, “Corrosion and Corrosion Control,” Wiley, New York, NY, p. 339. 13. MIL-STD 883, Test Method 5005. 14. A.E. Roswell, G.K. Clymer, “Thermal Fatigue Lead- Soldered Semiconductor Device,” U.S. Patent No. 3,735,208, August 197 1. 15. MIL-PRF-19500KY Para. D.3.9.2.d. 16. R.C. Benson, T.E. Phillips, C.B. Bargeron, B.M. Romenesko, O.M. Uy, “Electromigration of Silver in Low-Moisture Hybrids,” Proc. 1993 Int. Symp. On Microelectronics, pp. 53 0-53 6. 17. P.W. Schuessler, S.G. Gonya, “Hydrogen Desorption from Base and Processed Packaging Alloy,” Proc.

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RUNIST Workshop on Moisture Measurement and Control for Microelectronics,” NISTDR 524 1, April 1993,

18. R.C. Sun, J.T. Clemens, J.T. Nelson, “Effects of Silicon Nitride Encapsulation on MOS Device Stability,“ Proc. IRPS, April 1980, pp. 244-25 1. 19. R.K. Lowry, “A Hermetic Package Internal Water Vapor Paradox: Nonconforming Product That Does Not Fail,” Proc. ISTFA, November 1998, pp. 175-178. 20. R.K. Lowry, J.H. Linn, G.M. Grove, CA. Vicroy, “Analysis of Human Contaminants Pinpoints Sources of IC Defects,” Semiconductor International, July 1987, pp. 73-77.

pp. 67-89.

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