WO1993002980A1

WO1993002980A1 - Low temperature lead vanadium sealing glass compositions

Info

Publication number: WO1993002980A1
Application number: PCT/US1992/006240
Authority: WO
Inventors: Maurice E. Dumesnil; Leo Finkelstein
Original assignee: Vlsi Packaging Materials, Inc.
Priority date: 1991-08-07
Filing date: 1992-07-29
Publication date: 1993-02-18

Abstract

Low temperature lead vanadium sealing glass compositions that contain 10 to 30 % by weight TeO2 and 0.1 to 15 % by weight ZnO, CdO, and/or ZrO.

Description

LOW TEMPERATURE LEAD VANADIUM

SEALING GLASS COMPOSITIONS

Technical Field

This invention relates to novel low temperature metal oxide glasses useful as semiconductor packaging materials. It is particularly concerned with low melting sealing glasses with or without compatible low expansion refractory fillers capable of hermetically sealing ceramic semiconductor packages at about 350°C. It is also concerned with silver/glass die attach pastes for bonding semiconductor integrated circuit devices to a ceramic substrate such as alumina in the 300 to 400°C range.

The present invention describes highly stable and chemically resistant low temperature glasses derived from the lead vanadium oxide binary system by the

addition of specific metal oxides . These glasses can be admixed in powder form with refractory fillers or with noble metals preferably silver metal powder.

Background

This invention addresses the problem of sealing semiconductor devices in hermetic ceramic packages with a low temperature sealing glass. Concurrently this invention addresses the problem of bonding (die attach) to a ceramic surface certain types of temperature sensitive semiconductor devices at the lowest possible temperature.

Since the onset of integrated circuits fabricated on silicon single crystal wafers around 1964, very fast semiconductor devices have been designed by a process known as bipolar technology which relies on deep diffusion silicon structures. These devices being somewhat temperature and surface insensitive were readily alloyed, die attached, and hermetically sealed in alumina ceramic packages at 450-500°C.

A rapidly growing competing design technology based not on pn junction high current injection but on surface capacitive channel switching called CMOS

(complementary metal silicon oxide semiconductor)

requires much less power to operate. Since the speed of CMOS designs is increasing so quickly they will soon outstrip almost all competing semiconductor technologies with a concurrent increasing impact in portable, work station and mainframe computers. This increased

operational speed in CMOS is critically dependent on submicron scale masking technology.

Very large scale integrated semiconductor devices (VLSI) such as large 300 to 600 mil square CMOS and BiCMOS silicon chips are quite sensitive to the thermal processes required during their last fabrication steps. These include metal contact alloying, die attach and final seal. These are presently performed in the 400 to 450°C range for a few minutes. The thermal

sensitivity of CMOS semiconductor devices arises due to the presence of extremely dense, compact, ultrafine metallization lines reaching a fraction of a micron line widths combined with ultrathin dielectric films reaching the 100 angstrom range thickness. These three

dimensional surface interconnection patterns are prone to immediate or longer term failure modes such as metal diffusion, alloying and dielectric punch through.

Industry consensus indicates that these fabrication steps should be made below 400°C and preferably close to 350°C to insure greater fabrication yields, throughput and long term reliability. Glass sealing and silver/glass die attach processes are critically dependent on the available materials which today are derived from the lead borate oxide glass system. Commercial lead borate glasses used for semiconductor packaging applications are characterized with glass transition temperatures (Tg) in the region of 325°C and softening points in the region of 375°C. Present package sealing and silver/glass die attach materials require a processing temperature of 430 to 450°C.

The key factor in potentially achieving lower processing temperature would be the availability of the right glass material. While some very low temperature metal oxide glasses are known to exist with glass transition temperatures in the 250 to 350°C level, most are not useful for semiconductor application. The limiting factors are thermal instability (the glass recrystallizes too early), mechanical instability (the glass recrystallizes when ground to a fine powder), poor moisture resistance (many metal oxide glasses dissolve in hot water) and the presence in the glass formulation of alkali metals and halides, components known to affect deleteriously the performance of most semiconductor devices.

To date, serious attempts to design a practical and reliable lower temperature (300-400°C) sealing glass have met with formidable technical barriers, the search being hampered by the fact that in the course of new material evolution the design of glasses remains a largely empirical science.

The requirements for a semiconductor ceramic package sealing glass are numerous and demanding.

Somehow these must be met with one single chemical formulation preferably produced as a glass melt rapidly quenched to room temperature. The basic material and processing requirements for a commercially practical sealing glass can be listed as:

1. formation of a true solution (homogeneous melt) of the metal oxide mixture;

2. glass formation during rapid cooling of the melt (solidified liquid);

3. low glass viscosity at seal temperature (350°C);

4. no tendency to crystallize (glass stability) during seal formation and completion;

5. a reasonably low linear thermal expansion (50 to 110 x 10^-7/°C);

6. ease of linear thermal expansion adjustment by the addition of a lower expansion coefficient filler;

7. glass chemical stability (insoluble in water, resistant to acids, alkalies and hot water);

8. good wetting and high bonding strength to alumina ceramic surfaces;

9. no presence in the formula of alkali or other fast-migrating ions (electronic applications) or volatile components that create serious health hazards (such as arsenic oxide, thallium oxide, etc.);

10. capacity of producing a strong, tight and hermetic seal to a glass, metal and ceramic surface and capability of surviving several hundred cycles of thermal shocks, liquid to liquid, condition C (MIL-STD-883); and

11. ease of commercial processing.

More recently lead vanadium oxide based glasses characterized with a transition temperature (Tg) in the 250°C range, very low softening points, eminently

suitable for sealing ceramic lids and for die attach application were described in commonly owned U.S. Patents 4,743,302 and 5,013,360. Extensive reliability testing data on these new lead vanadium sealing glasses has been reported in "New Technology in Electronic Packaging, " by R. Tetschlag ASM INTL. 1991, San Francisco, pages 31 to 41.

The present invention provides further refinements and improvements in some characteristics of the lead vanadium glasses described in those patents, namely enhanced flexibility of processing. This is achieved by the addition of selected amounts of tellurium oxide and zinc oxide, cadmium oxide and/or zirconium oxide to those lead vanadium oxide glasses.

In this regard several prior patents describe lead vanadium glasses containing tellurium oxide.

Commonly owned U.S. 4,743,302 describes lead vanadium glass composed of five oxide components, three of which are lead oxide, vanadium oxide and zinc oxide. Column 5 of the patent states that its glass may also contain up to 5% by weight of common glass additives, with TeO₂ being listed among such additives. The glass of the present invention is distinct from these patented glasses in that they contain significantly greater amounts of TeO₂ than the patented glasses.

U.S. 4,945,071 describes glasses comprised primarily of TeO₂ (35-75% by weight) and vanadium oxide (12-40% by weight) stabilized with lead oxide and silver oxide. The patent further indicates the glasses may include minor amounts of other oxides, one being ZnO (0- 5% by weight). The presently claimed glasses are

distinct from these patented glasses in that the present glasses contain lower amounts of TeO₂ and do not contain silver oxide which has a destabilizing effect on the present glasses.

Finally, U.S. 5,013,697 describes lead vanadium glasses that may contain 1-15% by weight TeO₂, although the only TeO₂ containing glass exemplified contains 1.38% TeO₂. The patented glass also contains P₂O₅ and does not contain any ZnO, CdO and/or ZrO as required by the present invention.

Other patents of interest relating to lead vanadium glasses include U.S. Pats. Nos. 3,408,212 and 3,454,408.

It is an object of this invention to present a series of stable lead vanadium glasses, with or without compatible refractory fillers, suitable for sealing ceramic packages at around 350°C and suitable as well for use in silver/glass die attach systems in the 300 to 400°C range.

Summary of the Invention

The glasses of the invention are derived from the lead tellurium vanadium oxide ternary with the addition of other specifically selected metal oxides.

These additives are required since the basic lead

tellurium vanadium oxide ternary does not produce in powder form stable glasses. The resulting glasses are characterized by low transition temperatures (many equal to 250°C), a marked resistance to recrystallization even in a fine powder form, and good durability in the

presence of moisture.

The glasses of this invention comprise 35 to 55 weight percent lead oxide (PbO), 35 to 55 weight percent vanadium pentoxide (V₂O₅), 10 to 30 weight percent tellurium oxide (TeO₂), and 0.1 to 15 weight percent zinc oxide, cadmium oxide and/or zirconium dioxide.

Another aspect of the invention is the above- described glass mixed with 1 to 50% by weight, based on the mixture, of a refractory filler powder having a low coefficient of linear expansion.

Still another aspect of the invention is a die attach paste comprising the above-described glass blended with silver, gold or platinum powder and dispersed in a nonvolatile organic liquid.

A further aspect of the invention is an article of manufacture for use in sealing an electronic part comprising a metal, glass, or ceramic body coated with a pattern of the above-described glass.

Detailed Description of the Invention

The novel low melting glass compositions of this invention comprise (in weight percent calculated on an oxide basis):

(a) PbO: 30 to 55%

(b) V₂O₅: 30 to 55%

(c) TeO₂: 10 to 30%

(d) ZnO, CdO and/or ZrO₂: 0.1 to 15% where the percentages add to 100. The compositions may also contain up to 10% by weight singly or in combination of:

Cu₂O, BaO, CaO, SrO, Bi₂O₃,

and optionally up to 5% by weight each of:

TiO₂, SnO₂, WO₃, MoO₃, P₂O₅, Nb₂O₅,

F (fluorine), B₂O₃.

Preferred glass compositions of this

invention consist essentially of (in weight percent calculated on an oxide basis):

(a) PbO: 35 to 45%;

(b) V₂O₅: 35 to 45%;

(C) TeO₂: 12 to 24%;

(d) ZnO, CdO and/or ZrO₂: 0.5 to 8%; (e) BaO: 0 to 5%; and

(f) Nb₂O₅: 0 to 5%.

where the percentages add to 100.

These glasses can also contain minor amounts up to a total of 5 percent by weight of one or more oxides found in commercial glasses such as antimony, arsenic, thallium, selenium, cobalt and other transitional metals, and rare earths. The precise amount of each compound will of course depend on the particular application and its solubility in the glass composition.

Glasses of this invention are characterized by

Tg's as low as 250°C and softening points in the 300 to 325°C range. They exhibit good resistance to

recrystallization even as fine powders and excellent durability to moisture.

The invention glasses can be utilized with or without the addition in powder form of compatible

refractory ceramic filler to adjust the net coefficient of expansion of the resulting glass/filler mixture.

These glasses can be used without filler in applications requiring thermal expansion coefficients in the 130 to 150 x 10^-7/°C range. If lower thermal expansion

coefficients are desired, these glasses can be combined in powder form with about 1% to 50% by weight based on the combination of expansion modifying refractory filler. Increased amounts of a low thermal expansion refractory filler will correspondingly decrease the linear expansion of the sealing glass, the decrease being practically a linear function of the glass/filler volume ratio. Such fillers are commonly used to make solder glass suitable for sealing to lower expansion ceramics, glasses, or metals. Close matching of thermal expansion of the sealing glass to the ceramic parts (e.g., alumina, berylia, or steatite parts) to be joined is critical to maintain zero stress in the seal joint. This insures strength and hermeticity under extreme conditions of thermal cycling and thermal shock.

It is also known that the presence in a glass of a crystalline second phase is beneficial in

strengthening a glass seal. The addition of a particulate filler will minimize crack propagation throughout the glass.

These refractory fillers include the conventional low-expansion crystalline ceramic materials found useful in the technology of lead solder glasses: beta eucryptite, spodumene, cordierite, zirconium

silicate, zinc silicate (willemite), and titanates such as lead titanate and lead calcium titanate. Also

included are refractory fillers made from Group V metal oxides in the periodic table (P, As, Sb, V, Nb, and Ta), as listed in Table 1 such as refractory zirconium

phosphate, titanium phosphate, calcium zirconium

phosphate, calcium titanium phosphate, niobium phosphate, tin phosphate, niobium pentoxide, and its derivatives such as aluminum niobate, niobium titanate, and niobium zirconate.

Table 1 below lists examples of this class of refractory fillers, together with linear thermal

expansion values, where known.

It should be obvious to one skilled in the arts that the choice and quantity of filler to admix with a particular glass from this invention is a function of compatibility, thermal expansion differential, particle size distribution, and the type of ceramic parts to be bonded, or metal or glass substrate. The maximum ratio of filler to glass powder is limited by the onset of lack of flow of the sealing glass when melted.

The mixtures are prepared by introducing the glass and refractory powder into a ball mill and milling in a conventional manner to reduce the bulk components to finely divided particles that are uniformly mixed.

Alternatively the glass can be ground separately,

screened, and then combined with the filler powder in a V blender.

The resulting glass refractory mixtures may be applied to the work piece as such, or they may be mixed with an organic vehicle to form a paste to coat the work piece which is thereafter heated to melt the glass and produce the seal coating. The organic vehicles are synthetic solvents boiling preferably in the range of 150-220°C, such as butyl carbitol , carbitol acetate , or similar solvents.

A metal powder filler such as silver or gold or platinum, preferably silver, may be mixed with the glass powder of the invention in amounts of 50 to 95% by weight, usually 70-80%, based on the mixture, for die-attach applications in semiconductor chip packaging (e.g., to bond semiconductor devices to substrates such as alumina). The metal powder/glass die-attach

compositions of this invention are intimate mixtures of powdered silver metal and powdered glass. The powdered silver may be spherical or flake powder or mixtures of the two, preferably having surface areas from about 0.3 to 1.3 square meters per gram and tap densities from about 2.4 to 3.4 gram per cubic centimeter.

The die attach adhesives of this invention are an admixture of flaked and/or dendritic silver metal (or gold, or platinum) and the multicomponent glass of this invention in fine powder form in a ratio of 2:1 to 100:1 by weight, preferably 3:1 to 20:1 along with a high boiling point solvent such as aliphatic hydrocarbon boiling between about 180° and 230°C, e.g., dodecane, and optionally a polymeric binder such as acrylic binder. The preferred metal for use is silver. Because of the very low melting point of the glasses, a lower

temperature is required to achieve a good bond than conventional die attach silver/glass pastes.

Although the prime objective in the use of these glasses and glass-filler mixtures of this invention is a low sealing temperature in the 300°C range, it should be understood that there may be special

applications requiring a higher temperature. Thus, no upper limit in temperature is inherent in the application of the glass materials of this invention.

It will be readily understood by those of skill in the glass-making art that litharge (Pb₃O₄), lead dioxide (PbO₂), or any chemical precursors to the oxides of the compositions described in this application can be used to formulate the glasses.

The sealing glasses of the invention are coated onto metal, glass, or ceramic parts at thicknesses in the range of about 100-700 microns. These metal, glass, or ceramic parts are usually produced in the form of square or rectangular bodies in sizes ranging from about 6-25 mm per side and 200-2500 microns thick, flat or with a recess. The sealing glass pattern (coating) over the entire surface or around the edges are formed by printing and glazing. These parts are sealed at low temperature on ceramic electronic packages known commercially as side-brazed packages, chip carriers, and pin grid arrays, as well as metal packages.

The following examples describe the preparation and composition of the sealing glasses of the invention. These examples are not intended to limit the invention in any manner.

Example 1

A glass was prepared by mixing 40 grams of lead oxide, 40 grams of vanadium pentoxide, 16 grams of tellurium oxide, 1.5 grams of zinc oxide and 1.5 grams of barium carbonate. After heating the mixture in a ceramic crucible at 750°C for one hour the very fluid melt was poured through revolving stainless steel rollers to quench the liquid melt to produce highly stressed glass flakes. The resulting glass has a composition in weight percent as follows:

PbO 40.9% by weight

V₂O₅ 40.9

TeO₂ 16.4

ZnO 1

BaO 0.8

This glass has a linear thermal expansion coefficient (25-200°C) equal to 136 x 10^-7/°C and a glass transition temperature (Tg) of 260°C.

The glass flakes were ground in a ceramic ball mill and the resulting powder screened through a 100 mesh screen. The resulting glass powder was spread on an alumina plate to measure the thermal stability of the glass in powder form as a function of temperature. The present glass powder melted and bonded to alumina at 325°C. The temperature was increased to 350 then to

375°C. The glass from this example remained quite stable and did not show any sign of instability such as

recrystallization. Example 2

A glass was prepared by mixing 300 grams of lead oxide, 300 grams of vanadium pentoxide, 120 grams of tellurium oxide, 30 grams of niobium pentoxide, (also known as columbium pentoxide), 15 grams zinc oxide,

15 grams barium carbonate and 15 grams zirconium oxide.

After heating the mixture in a ceramic crucible at 750°C for one hour the very fluid melt was poured through revolving stainless steel rollers to quench the liquid melt to produce highly stressed glass flakes. The flakes were ground in a ceramic ball mill to 200 mesh. The resulting glass powder has a composition in weight percent as follows:

PbO 37.9 by weight

V₂O₅ 37.9

TeO₂ 15.2

Nb₂O₅ 3.8

ZnO 1.9

BaO 1.4

ZrO₂ 1.9

This glass has a linear thermal expansion coefficient (25 to 200°C) equal to 138 x 10-7/°C and a glass transition temperature (Tg) of 280°C.

Additional examples of low melting glasses of this invention were prepared following the procedure described in examples 1 and 2. These additional examples 3 to 27 are listed in tables 3, 4, and 5 below. When heated in powder form all of these glass remained stable with no sign of recrystallization in the temperature range of 300 to 420°C or above. The powders were tested through a belt furnace with a peak temperature of 10 to 15 minutes.

Table 2 lists glasses comprising lead oxide, tellurium oxide and vanadium oxide only. In powder form these glass materials are unstable and devitrify rapidly above 300°C.

Example 28

The glass powder prepared according to Example 1 was passed through a 325 mesh screen and blended with 85 percent by weight silver metal powder. About 12 percent of a low vapor pressure solvent with a boiling point of around 200°C was added to the powder mixture to form a silicon die attach paste. After roll milling the blend to produce a well dispersed flowing paste, a small quantity of the silver glass paste was deposited on a ceramic surface. A set of 500 square mil silicon semiconductor chips were imbedded in the paste dots, the wet paste thickness being controlled to 8 mils. The parts were placed wet on a moving belt furnace and heated to 350°C for about 10 minutes at peak temperature.

Stud pull test performed on a Sebastien 111 tensile test analyzer indicated adhesion in the 90 to 100 kilo range at which point the die broke with cohesive failure in the silver/glass layer. It can be seen that the strength of this low temperature silver/glass bond is extremely high and comparative to commercial silver/glass materials fired at 430°C.

Example 29

The glass powder prepared according to Example 27 was passed through a 400 mesh screen and blended with 85 percent by weight silver metal powder, the silver powder itself being a mixture of parts by weight silver 80 flakes, 15 parts dendritic silver powder (i.e., precipitated irregularly shaped silver powder) and 5% silver oxide.

About 12 percent of a low vapor pressure solvent with a boiling point in the 200°C range was added to the powders to form a silicon die attach paste. After roll milling the blend to produce a well dispersed flowing paste the resulting dotting paste was tested according to example 28 with similar excellent adherence results.

Example 30

The glass flakes prepared according to

Example 2 were ground in a ball mill and the resulting powder passed through a 200 mesh screen. The fine glass powder was mixed with 35 percent by weight, based on the mixture, niobium pentoxide (Nb₂O₅), a refractory metal oxide with an expansion of -5 x 10-7/°C, and formed into a paste with 12% butyl carbitol acetate.

The resulting paste was screen printed on alumina parts (brown 92% alumina, balance SiO₂, MhO, FeO, NiO), dried and heated to about 360°C to melt the sealing glass material. The thickness of the fused glass layer was of the order of 200 microns.

The glazed alumina parts were inverted and held in position by pressure exerted by a metal clip to a conventional gold plated ceramic microelectronic base. The structure was heated at a rate of 75°C per minute to a peak of 370°C for ten minutes, then cooled to room temperature to produce a tight, strong vitreous seal.

The structure was tested for gross and fine leak

(5 x 10-9cc/sec He). The parts were then subjected to 100 cycles of liquid-to-liquid thermal shock, condition C (MIL-STD-883), demonstrating the unusually strong nature of the sealing glass of this invention even to a gold ring surface. Example 31

Similarly to Example 30 a sealing glass material was prepared by mixing the glass powder from Example 2 with 36 percent by weight cordierite powder (aluminum magnesium silicate with a linear thermal expansion coefficient close to zero) with sealing test results very similar to those in Example 30.

Example 32

The glass flakes prepared according to

Example 2 were ground in a ball mill and the resulting powder passed through a 140 mesh screen. The fine glass powder was mixed with 44 percent by weight, based on the mixture, niobium pentoxide powder and formed into a printing paste with about 12 percent butyl carbitol acetate. The resulting paste was screen printed on alumina Cerdips (ceramic dual in line packages), dried and heated to about 360°C to melt the glass powder. The thickness of the fired glass layer was of the order of 15 to 25 microinches. A metal lead frame made of alloy 42 was inserted into one of the Cerdip parts (base). The two ceramic parts with the glass side face to face were bonded by heating the structure at a rate of 75°C per minute in air for about 10-12 minutes at peak (375°C) without any external pressure then cooled to room

temperature to produce a tight, strong, vitreous seal.

The structure was tested for gross and fine leak (5 x 10-9 cc/sec He). The parts were then subjected to 100 cycles of liquid-to-liquid thermal shock,

condition C (MIL-STD-883) with no hermetic failure demonstrating the unusually strong nature of the sealing glass of this invention.

Example 33

The glass powders prepared according to

Examples 1-26 are mixed in varying amounts with different types of low expansion refractory powder including those listed in Table 1.

Modifications of the above described modes of carrying out the invention that are obvious to those in the fields of glass manufacture, semiconductor or other electronic part packaging and related fields are intended to be within the scope of the following claims.

Claims

1. A low melting glass composition comprising in weight percent calculated on an oxide basis:

(a) PbO: 30 to 55%;

(b) V₂O₅: 30 to 55%;

(c) TeO₂: 10 to 30%; and

(d) ZnO, CdO and/or ZrO₂: 0.1 to 15%.

where all percentages add to 100.

2. The glass composition of claim 1 wherein the composition includes up to 10% by weight singly or in combination of:

Cu₂O, BaO, CaO, SrO or Bi₂O₃.

3. The glass composition of claim 1 wherein the composition contains up to 5% by weight each of:

TiO₂, SnO₂, WO₃, MoO₃, P₂O₅, Nb₂O₅, F, or B₂O₃.

4. The glass composition of claim 2 wherein the composition contains up to 5% by weight each of:

TiO₂, SnO₂, WO₃, MoO₃, P₂O₅, Nb₂O₅, F, or B₂O₃.

5. A low melting glass composition consisting essentially of in weight percent calculated on an oxide basis:

(a) PbO: 35 to 45%;

(b) V₂O₅: 35 to 45%;

(c) TeO₂: 12 to 24%

(d) ZnO, CdO and/or ZrO₂: 0.5 to 8%;

(e) BaO: 0 to 5%; and

(f) Nb₂O₅ : 0 to 5%;

wherein all percentages add to 100.

6. The glass composition of claim 2 mixed with about 1 to 50% by weight, based on the mixture, of a refractory filler powder having a low coefficient of linear expansion.

7. The glass composition of claim 6 wherein the refractory filler is a metal niobate, phosphate, titanate, silicate or zirconate.

8. The glass composition of claim 6 wherein the filler is a Group V metal oxide.

9. The glass composition of claim 6 wherein the refractory filler is niobium pentoxide.

10. The glass composition of claim 3 mixed with about 1 to 50% by weight, based on the mixture, of a refractory filler powder having a low coefficient of linear expansion.

11. The glass composition of claim 4 mixed with about 1 to 50% by weight, based on the mixture, of a refractory filler powder having a low coefficient of linear expansion.

12. The glass composition of claim 5 mixed with about 1 to 50% by weight, based on the mixture, of a refractory filler powder having a low coefficient of linear expansion.

13. The glass composition of claim 12 wherein the refractory filler is niobium pentoxide.

14. The glass composition of claim 1 mixed with 50% to 95% by weight, based on the mixture, of silver, gold or platinum powder.

15. A die attach composition consisting essentially of an admixture of:

(a) silver metal powder;

(b) the glass composition from claim 1 in powder form; and

(c) a high boiling point solvent.

16. The die attach adhesive of claim 15 wherein the silver metal powder contains silver oxide powder.

17. The die attach adhesive of claim 15 wherein the silver metal powder is selected from the group consisting of flaked metal, powdered precipitated metal, and mixtures of flaked and powdered silver metal.

18. The glass composition of claim 14 blended with an organic liquid to form a printing ink or a dotting paste.

19. The glass composition of claim 11 blended with an organic liquid to form a printing ink.

20. An article of manufacture for use in sealing an electronic part comprising a metal, glass, or ceramic body coated with a pattern of the glass

composition of claim 11.