Silicone grease for semiconductors

Abstract

A neutral, dielectric, strong desiccant, free of material which might change the semiconductor junction doping, is added to a silicone grease. The grease is used to surround semiconductor junctions as a heat transfer media. The presence of the desiccant in the grease reduces the noise background which is generated in semiconductors in the presence of small amounts of water.

Parent Case Text

This application is a continuation of application Ser. No. 160,163, filed Jul. 6, 1971 and now abandoned which was a continuation-in-part of copending application Ser. No. 764,999, filed Oct. 4, 1968 and now abandoned.

Description

This invention relates to improved organopoly-siloxane grease compositions. More particularly, the present invention is concerned with organopolysiloxane grease compositions which provide a much lower noise level when used as heat transfer media surrounding the junction of a semiconductor.

Organopolysiloxane greases and grease compositions are well known in the art and have been used as lubricants, dielectric compounds, sealing compounds, high vacuum greases, and as heat transfer media in the manufacture of semiconductors. These organopolysiloxane greases have been particularly valuable in the manufacture of semiconductors because of their high degree of heat stability, their water repellency, their high-low temperature viscosity flow characteristics, dielectric properties, heat transfer properties, and their ability to surround a semiconductor junction without chemically contaminating the semiconductor junction. While the greases themselves do not cause background noise when used in the fabrication of semiconductors, it has been found that when the greases employed contain trace amounts of water, that a background noise develops due to this water contamination.

In an attempt to solve the problem of background noise in semiconductors due to water present in the silicone grease, various desiccants were added to the grease to absorb the water present. While many of the desiccants served their purpose as far as water pickup was concerned, they produced fatal side affects. Some would absorb water, become dissolved in the water, and become conductive, destroying the semiconductor operation. Others, such as apparently neutral clays apparently contained internally trapped acids or bases, which degraded the silicone grease, causing separation of the components and a partial destruction of the heat transfer capabilities of the grease. Boron oxide caused a hardening of the grease upon water pickup resulting in a destruction of the flow properties of the grease which decreased the heat transfer capabilities.

It was found in order to be effective, the desiccant must be non-conductive, neutral, non-abrasive, non-catalytic, non-reactive with the semiconductor junction and free of small amounts of impurities which destroy its capabilities. The abrasive character of the desiccant will not interfere with the semiconductor operation but does damage the milling equipment used to manufacture the grease.

It is an objective of the present invention to provide improved organopolysiloxane grease compositions which retain all the highly beneficial properties of heretofore known organopolysiloxane grease compositions used in the manufacture of semiconductors and which, in addition, reduce the noise level produced by a semiconductor. The grease compositions of the present invention contain a neutral, dielectric, strong desiccant free of materials which might change the semiconductor junction doping. The presence of the desiccant provides lowered noise levels when the grease composition is used in the manufacture of semiconductors.

The grease compositions of the present invention comprise on a weight basis:

1. from 15 to 97.9 percent of a polysiloxane fluid containing silicon-bonded organo groups selected from the class consisting of alkyl radicals having from one to 22 carbon atoms, lower alkenyl radicals having from two to eight carbon atoms, cycloalkyl radicals having from five to seven carbon atoms in the ring, mononuclear and binuclear aryl radicals, mononuclear aryl lower alkyl radicals, cyano lower alkyl radicals and up to 50 percent of the R radicals can also be hydrogen;

2. from 0.1 to 33 percent and preferably from 1 to 10 percent of a neutral, dielectric desiccant selected from the class consisting of anhydrous calcium sulfate and synthetic zeolites;

3. from 2 to 84.9 percent of a grease thickening and thermal conducting agent; and

4. optionally from 0.01 to 5 percent of a grease stabilizer such as the polyether and boron compounds disclosed in U.S. Pat. Nos. 3,037,933 and 3,103,491 of Wright.

The fluid organopolysiloxanes employed in the practice of the present invention are known in the art. The organopolysiloxane fluids may be characterized as organopolysiloxane fluids in which all of the valences of silicon other than those satisfied by oxygen atoms are satisfied by monovalent organic radicals attached to silicon through a silicon carbon linkage. This type of organopolysiloxane can be characterized as having the average formula:

R.sub.n SiO.sub.4.sub.-n/2 ( 1)

where R represents a member selected from the class consisting of alkyl radicals having from one to 22 carbon atoms, cycloalkyl radicals having from 5 to 7 carbon atoms in the ring, mononuclear and binuclear aryl radicals, mononuclear aryl lower alkyl radicals, cyano lower alkyl radicals and lower alkenyl radicals having form two to eight carbon atoms and n has a value of from 2.002 to 3.0. Up to 50 percent of the R radical can also be hydrogen in which case the formula also embraces organohydrogenpolysiloxanes. For the purposes of the present application, these materials too will be referred to as organopolysiloxanes. It is preferred, however, that R be an organo group as above defined as the reactivity of the silanic hydrogen may interfere with the simiconductor operation.

Among the specific radicals represented by R in formula (1) are alkyl radicals having from one to 22 carbon atoms, e.g., methyl, ethyl, propyl, octyl, decyl, dodecyl, tetradecyl, etc., radicals; cycloalkyl radicals having five to one carbon atoms in the ring, e.g., cyclopentyl, cyclohexyl, cycloheptyl, etc.; mononuclear and binuclear aryl radicals, e.g., phenyl, naphthyl, biphenyl, tolyl, xylyl, etc. radicals; mononuclear aralkyl radicals having up to 16 carbon atoms in the alkyl group, e.g., benzyl, phenylethyl, phenyloctyl, etc. radicals; lower alkenyl radicals having from two to eight carbon atoms, e.g., vinyl, allyl, pentenyl, etc. radicals; and cyano lower alkyl radicals having two to five carbon atoms, e.g., cyanomethyl, alpha-cyanopropyl, etc., radicals.

The preparation of methyl alkyl polysiloxanes within the scope of formula (1) involves an SiH-olefin addition reaction. This reaction simply involves the addition of an alpha-olefin having from two to 22 carbon atoms to some type of methylhydrogenpolysiloxane. For example, the preparation of a trimethylsilyl chain-stopped methyl higher alkylpolysiloxanes of formula (1) involves the reaction between a methylhydrogenpolysiloxane having the formula:

(CH.sub.3).sub.n-p (H).sub.p (R").sub.q SiO.sub.4.sub.-n/2

where n is as above defined and p has a value of from 0.01 to 1 with an alpha-olefin. The reaction of the alpha-olefin and the methylhydrogen polysiloxane can take place in the presence of one of the elemental platinum or platinum compound catalysts. The platinum compound catalyst can be selected from that group of platinum compound catalysts which are operative to catalyze the addition of silicon-hydrogen bonds across olefinic bonds.

Among the may useful catalysts for this addition reaction are chloroplatinic acid as described in U.S. Pat. No. 2,823,218 -- Speier et al., the reaction product of chloroplatinic acid with either an alcohol, an ether or an aldehyde as described in U.S. Pat. No. 3,220,972 -- Lamoreaux, trimethylplatinum iodide and hexamethyldiplatinum as described in U.S. Pat. No. 3,313,713 -- Lamoreaux, the platinum olefin complex catalysts as described in U.S. Pat. No. 3,159,601 of Ashby and the platinum cyclopropane complex catalyst as described in U.S. Pat. No. 3,159,662 of Ashby.

The SiH-olefin addition reaction may be run at room temperature or at temperatures up to 200.degree.C, depending upon catalyst concentration. The catalyst concentration can vary from 10.sup..sup.-7 to 10.sup..sup.-3 and preferably 10.sup..sup.-5 to 10.sup..sup.-4 moles of platinum as metal per mole of olefin containing molecules present. Generally, the methylhydrogenpoly-siloxane is mixed with a portion of the alpha-olefin, all of the catalyst is added, and then the remaining alphaolefin is added at a rate sufficient to maintain the reaction temperature in the neighborhood of from about 50.degree. to 120.degree.C and, at the end of the addition of the alpha-olefin, the reaction is completed.

The addition reaction is effected by adding to the methylhydrogenpolysiloxane a platinum catalyst of one of the types previously described. The appropriate amount of alpha-olefin is then added and reacted via the aforedescribed SiH-olefin addition reaction.

When preparing a linear copolymer of the type described in formula (1), the general procedure as described earlier is followed. The methylhydrogenpolysiloxane is reacted with the appropriate amount of alpha-olefin.

It should be understood that the viscosity of the organopolysiloxane fluid within the scope of formula (1) will vary with the molecular weight of the fluid and with the nature of the silicon-bonded organic groups in the fluid. Although any organic polysiloxane fluid within the scope of formula (1) is applicable, in the practice of the present invention, it is preferred that the fluid have a viscosity of from about 10 centistokes to 100,000 centistokes when measured at 25.degree.C.

It should also be understood that the organopolysiloxane fluids of formula (1) can include siloxane units of varied types and formulations such as triorganosiloxane units and diorganosiloxane units alone or in combination with monoorganosiloxane units. The only requirement is that the ratio of the various siloxane units employed be selected so that the average composition of the copolymeric fluid is within the scope of formula (1). These various siloxane units may contain the same or different siliconbonded organic radicals. For example, the siloxane units employed in preparing the fluid for formula (1) can contain trimethylsiloxane units, methylphenylsiloxane units, diphenylsiloxane units, triphenylsiloxane units, methyl-beta-cyanoethylsiloxane units, methylsiloxane units, methyloctylsiloxane units, phenylsiloxane units, beta-cyanoethylsiloxane units, etc.

The dielectric desiccants employed in the practice of the present invention are selected from the class consisting of anhydrous calcium sulfate and synthetic zeolites otherwise known as molecular sieves. The molecular sieves that can be used in this invention are the zeolites encompassed by the average formula:

0.95.+-.0.25M.sub.2/m O:A1.sub. 2 O.sub.3 :XSiO.sub.2 :YH.sub.2 O

where M is a cation having a valence of not more than 3 (such as calcium, strontium, barium, sodium, potassium and lithium, but preferably sodium), m is the valence of cation M and has a value of 1 to 3, X has a value of 1.35 to about 9 and preferably a value of 2.175.+-.0.825 and y has a value of from 0 to about 6, preferably from 0 to 3. The molecular sieves and processes for their production are disclosed in numerous publications and patents including U.S. Pat. Nos. 2,882,243 and 2,882,244 -- Milton, U.S. Pat. No. 3,130,006 -- Rabo and U.S. Pat. No. 3,130,007 -- Breck. In U.S. Pat. No. 2,882,243 of Milton, the cation M of the zeolites is defined as monovalent and divalent actions, such as lithium and magnesium; metal ions in group I of the periodic table such as potassium and silver; group II metal ions such as calcium, and strontium; metal ions of the transition metals such as nickel; and other ions, for example, hydrogen and ammonium - - - . The transition metals are those whose atomic numbers are from 21 to 28, from 39 to 46 and from 72 to 78 inclusive, namely, scandium, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, yttrium, zirconium, niobium, molybdenum, ruthium, rhodium, palladium, halfium, tantalum, tungsten, rhenium, osmium, iridium and platinum. In U.S. Pat. No. 2,882,244 the zeolites are defined by the formula

0.9.+-.0.2M'.sub.2/m.sub.' 0:Al.sub.2 O.sub.3 :2.5.+-.0.5SiO.sub.2 Y'H.sub.2 O

wherein M' represents a cation having a valence of not more than 3, m' represents the valence of M', and Y' may be any value up to 8. The molecular sieves that have been found particularly useful in this invention are the sodium forms of the molecular sieves and the calcium forms of the molecular sieves which are encompassed by the following formulas:

0.9.+-.0.2Na.sub.2 O:Al.sub.2 O.sub.3 : 2.5.+-.0.5SiO.sub.2 : YH.sub.2 O

and

0.9.+-.0.2CaO:Al.sub.2 O.sub.3 :2.5.+-.0.5SiO.sub.2 :YH.sub.2 O

wherein y has the above defined meaning. These molecular sieves have openings capable of absorbing materials having molecular diameters in the range of 5A or less. The preferred particle size of the molecular sieves is in the neighborhood of a 5 micron diameter assuming the molecular sieve particles are generally spherical.

The preferred form of calcium sulfate to be used in the practice of the present invention is an anhydrous calcium sulfate formed by the dehydration of CaSO.sub.4.2H.sub.2 O by heat. In the preferred form of my invention these calcium sulfate particles are also in the neighborhood of a 5 micron particle size.

The dielectric desiccants employed in the practice of the present invention should be relatively pure materials. They should not contain impurities such as acids or bases since acids and bases affect the molecular structure of silicone molecules employed in the grease formulation. A water slurry of the dielectric desiccants employed should have a pH of between 5 and 10. The dielectric desiccants in addition should contain no foreign materials which would contaminate the semiconductor. They should also not contain any materials that are conductive as this would result in current flow in the semiconductor which is the reverse of the design current flow path.

The third component of the grease compositions of the present invention are the grease thickening agents which are well known in the art. This invention contemplates the use of many of these well known thickening agents to form a grease composition of the desired consistency. Stability is important in the manufacture of the present greases in that the thickening agent must remain suspended in the grease composition. If the thickening agent were to settle out after application, this would result in a change in the electrical characteristics of the semiconductor.

In many applications heat transfer is also important such as in power transistors. The filler in such cases must be one which will allow the grease to conduct the heat away from the transistor junction fairly rapidly. Excessive heat build up will destroy the transistor.

Examples of suitable thickening agents include finely divided inert oxides of metallic and quasi-metallic materials such as silica, alumina, iron oxide, titania, zinc oxides, glass fibers and clays. Silica, when used as a thickening agent, is preferably employed as an aerogel but may also be employed as fumed silica, precipitated silica or natural deposits such as diatomaceous earth. Other thickening agents include polyethylene, polypropylene, polymethylmethacrylate and other inert organic thickeners. Zinc oxide is the preferred thickener. One of the primary reasons for this preference is its excellent heat transfer properties.

The term "grease" as employed in the present application is intended to refer to grease-like materials which may have consistencies varying from readily flowable materials to materials which exhibit almost no flow. The consistency of the greases of the present invention depend on the amount of thickening agent employed, the type of thickening agent employed and the particular polysiloxane fluids in the grease.

As it is the water in the grease compositions used in semiconductor manufacture that causes increased noise levels, it is, of course, understood that the thickening agents used in the manufacture of such greases be substantially free from such interferring water. While the amounts of thickening agent employed in the grease compositions of the present invention are not critical and may vary within wide limits depending on the particular consistency desired in the final product, it has been found that the amount of thickening agent usually varies from about 2 to about 80 percent and preferably from about 5 to 65 percent by weight based on the weight of the grease composition.

The grease compositions of the present invention can be prepared in conventional fashion by merely mixing the polysiloxane fluid as a thickening agent and the dielectric desiccant in conventional mixing and/or milling equipment. Especially satisfactory results have been obtained when the compositions have been mixed on a conventional paint mill or on a conventional colloid mill.

The following examples are illustrative of my invention and are not intended for purposes of limitation. All parts are by weight unless otherwise indicated.

EXAMPLE 1 1

To 87 parts of a trimethylsilyl end-stopped dimethylsilicone fluid having a viscosity of 350 centistokes were added 25 parts of molecular sieves having a pore diameter of 5A and a particle size of 5 microns, 9 parts of finely divided precipitated silica (dry) and 1 part of trimethoxyboroxine. The composition was mixed in a grease kettle and then milled three passes on a three roll mill. The resulting grease had a penetration of 269 and a water content of less than 50 PPM. A comparable grease made without the molecular sieves had a penetration of 274 and a water content of over 280 PPM.

EXAMPLE 2

To 29 parts of a trimethylsilyl end-stopped dimethylsilicone fluid having a viscosity of 350 centistokes were added 1 part of molecular sieves having a pore diameter of 5A and a particle size of 5 microns, and 70 parts of finely divided zinc oxide. The composition was mixed in a grease kettle and then milled three passes on a three roll mill. The same composition was prepared without the addition of the molecular sieves and used as control. Semiconductors made using the grease composition containing the molecular sieves to surround the semiconductor junction and to fill the semiconductor case for heat transfer purposes had a very low noise level as compared with control.

EXAMPLE 3

To 27 parts of a trimethylsilyl end-stopped dimethylsilicone fluid having a viscosity of 1,000 centistokes was added 5 parts of finely divided anhydrous calcium sulfate and 68 parts of zinc oxide. The composition was mixed in a grease kettle and then milled three passes on a three roll mill. The same composition was made without employing the calcium sulfate and used as control. A semiconductor made utilizing the grease containing the calcium sulfate had a much lower noise level than a semiconductor used without employing anhydrous calcium sulfate. The grease containing the calcium sulfate had a water content of less than 50 PPM and the grease prepared without the calcium sulfate had a water content of over 200 PPM.

While the foregoing examples and discussions have illustrated many of the variations and compositions possible within the scope of the present invention, it should be understood that this invention relates broadly to a silicone grease to be used in a semiconductor manufacture which contains a dielectric desiccant selected from the class consisting of synthetic molecular sieves and anhydrous calcium sulfate.

Claims

What I claim is:

1. A silicone grease composition consisting essentially of on a weight basis:

1. from 15 to 97.9 percent of a polysiloxane fluid of the formula:

(R).sub.n SiO.sub.4.sub.-n/2

where

R represents a member selected from the class consisting of alkyl radicals having from one to 22 carbon atoms, lower alkenyl radicals having two to eight carbon atoms, cycloalkyl radicals having from five to seven carbon atoms; mononuclear and binuclear aryl radicals; mononuclear aryl lower alkyl radicals; cyano lower alkyl radicals and up to 50 percent of the R radicals can also be hydrogen; and n has a value of from 0.002 to 3.0,

2. from 0.1 percent of a neutral, dielectric desiccant being at least one selected from the class consisting of anhydrous calcium sulfate and synthetic zeolites being at least one selected from the class consisting of:

0.95+0.25M.sub.2/m O.Al.sub.2 O.sub.3 :2.175+0.825SiO.sub.2 :YH.sub.2 O

wherein

m represents at least one of the materials in the groups consisting of hydrogen, ammonium, metals in Groups I and II of the periodic table and the transition metals of the periodic table, m represents the valence of M, and Y may be any value up to about 6, and

0.9+0.2M'.sub.2/m O:Al.sub.2 O.sub.3 ;2.5+0.5SiO.sub.2 Y'H.sub.2 O

wherein

M' represents at least one cation having a valence of not more than 3, m' represents the valence of M', and Y' may be any value up to about 8, and

3. from 2 percent to 84.9 percent of a grease thickening agent being at least one selected from the class consisting of finely divided, non-conducting inert oxides of metallic and glass-metallic materials and inert organic thickening materials and mixtures thereof substantially free of interfering water.

2. A composition of claim 1 wherein the neutral, dielectric desiccant is anhydrous calcium sulfate.

3. The composition of claim 1 wherein the dielectric desiccant is a synthetic zeolite within the scope of the formula:

0.95+0.25M.sub.2/m O.Al.sub. 2 O.sub.3 :2.075+0.825SiO.sub.2 :YH.sub.2 O

wherein

M represents at least one of the materials in the groups consisting of metals in Groups I and II of the periodic table.

4. The composition of claim 1 wherein the dielectric desiccant is anhydrous calcium sulfate made by the dehydration of hydrated calcium sulfate.

5. A composition within the scope of claim 1, consisting essentially of on a weight basis:

1. from 15 to 97.9 percent of a polysiloxane fluid of the formula

(CH.sub.3).sub.n SiO.sub.4.sub.-n/2

2. from 0.1 percent to 35 percent of a neutral dielectric desiccant selected from the class consisting of:

0.9+0.2Na.sub.2 O:Al.sub.2 O.sub.3 :2.5+0.5SiO.sub.2 :YH.sub.2 O

and

0.9+0.2CaO:Al.sub.2 O.sub.3 :2.5+0.5SiO.sub.2 :YH.sub.2 O

3. from 2 percent to 84.9 percent of zinc oxide.

6. The composition of claim 5 wherein the dielectric desiccant is:

0.9+0.2Na.sub.2 O:Al.sub.2 O.sub.3 :2.5+0.5SiO.sub.2 :YH.sub.2 O

7. The composition of claim 5 wherein the dielectric desiccant is:

0.9+2CaO:Al.sub.2 O.sub.3 :2.5+0.5SiO.sub.2 :YH.sub.2 O

8. A composition within the scope of claim 1 wherein R represents a member selected from the class consisting of alkyl radicals having from one to 22 carbon atoms and mononuclear aryl radicals.

9. The composition of claim 1 wherein the grease thickening agent is zinc oxide.