Selective etching of polymeric materials embodying silicones via reactor plasmas.

BACKGROUND OF THE INVENTION

1. Field of the Invention

0001This invention relates to the selective plasma chemical etching of polyimide-silicone copolymers for patterning thereof. 2. Description of the Prior Art

0002A method of making polyimide-silicone copolymers and their application to selective surface areas of electronic devices, e.g., semiconductor junctions, is known. After the copolymer material is applied to such surface areas it becomes necessary on occasion to open windows in the coating and/or to remove material from selective areas. It is desirable that the removal of the unwanted portions of the copolymer material be done as quickly and as easily as possible to facilitate semiconductor device processing. It is also most desirable to maintain the registry and dimensions of the pattern.

0003U. S. Patent 4,140, 572 describes the use of titanium as a masking material for etching with a phenol-bearing solution. However, although the process is successful, one always finds a need for a more efficient process for a particular manufacturing requirement.

0004A novel process introduced into the semiconductor art is cheniical etching by use of plasma. The plasma is an ionized gas containing a variety of highly reactive particles such, for example, as ions, free electrons and free radicals. An article entitled "A Survey of Plasma-Etching Process" appeared in Solid State Technology, May 1976, and describes the process.

BRIEF SUMMARY OF THE INVENTION

0005In accordance with the teachings of this invention there is provided a new and improved method for patterning a layer of cured silicone-polyimide copolymer material disposed on the surface of the body. Preferably, it is a method employed when the material is disposed on the surface of a body of semiconductor material. The layer is from 1 to 10 microns in thickness. The final cure is accomplished by baking at about 450 C. A layer of silicon, preferably, of the order of 1000Å, is disposed on the copolymer material layer by electron beam deposition technique. A layer of either negative or positive photoresist material is disposed on the silicon material which is to act as a mask.

0006The layer of photoresist material is processed to form one or more windows therein. Selected surface areas of the layer of silicon are exposed in the windows. The exposed silicon is chemically etched by a plasma of Freon 14, which is a tradename of E.I. DuPont de Nemours & Co., Inc. for CF , containing about 4% oxygen by volume of Freon 14 for a sufficient time to open windows in the silicon layer. The windows are aligned with those in the photoresist layer thereby enabling selected surface areas of the copolymer material to be exposed in the windows. The layer of photoresist is preferably simultaneously stripped with the etching of the polyimide-silicone copolymer material.

0007The copolymer material is chemically etched in a plasma of oxygen. Chemical etc hing is performed until windows are opened in the layer to expose surface areas of the body thereat. The processed body is then cleaned, as required, and further processed into the required semiconductor device.

DESCRIPTION OF THE DRAWINGS

0008Figures 1 through 6 are side elevation views, partly in cross- section of a body of semiconductor material processed in accordance with the teachings of this invention.

DESCRIPTION OF THE INVENTION

0009Briefly, polyimide-silicone copolymer is the reaction product of a silicon-free organic diamine, an organic tetracarboxylic dianhydride and a polysiloxane diamine which is a polymer precursor soluble in a suitable organic solvent. On curing, it yields a copolymer having recurring structural units of the formula: <chemistry id="chem0001" num="0001"><img file="EP0010657A1_D0001.tif" /></chemistry>with from 5 to 50, preferably 15 to 40 molepercent and most preferably 25 to 35 mole percent intercondensed structural units of the formula: <chemistry id="chem0002" num="0002"><img file="EP0010657A1_D0002.tif" /></chemistry>wherein R is a divalent hydrocarbon radical, R' is a monovalent hydrocarbon radical, R" is a tetravalent organic radical, Q is a divalent silicon-free organic radical which is the residue of an organic diamine, x is an integer having a value of 1-4 and m and n are different integers greater than 1, from 10 to 10,000 or more. <ul id="ul0001" list-style="none"><li>R is preferably an optionally substituted alkylene radical such as one comprising 2 to 8 carbon atoms or an optionally substituted arylene radical such as one comprising 6 - 15 carbon atoms. Most preferably R is an optionally substituted alkylene radical having 2 to 4 carbon atoms or an optionally substituted arylene radical having 6 to 9 carbon atoms.</li><li>R' in preferably an optionally substituted alkyl radical such as one comprising 1 to 8 carbon atoms or an optionally substituted aryl radical such as one comprising 6 to 15 carbon atoms. R' is most preferably an optionally substituted alkyl radical having 1 to 4 carbon atoms or an optionally substituted aryl radical having 6 to 9 carbon atoms.</li><li>Q is preferably an optionally substituted alkylene radical having 2 to 20 carbon atoms and most preferably having 6 to 18 carbon atoms or an optionally substituted arylene radical having 6 to 15 carbon atoms or an optionally substituted cycloalkyl radical having 5 to 7 carbon.</li><li>n and m are preferably different integers from 100 to 1000.</li></ul>

0010The above-mentioned random and block copolymers can be prepared by effecting reaction, in the proper molar proportions, of a mixture of ingredients comprising a diaminosiloxane of the general formula: <chemistry id="chem0003" num="0003"><img file="EP0010657A1_D0003.tif" /></chemistry>a silicon-free diamine compound of the formula: <chemistry id="chem0004" num="0004"><img file="EP0010657A1_D0004.tif" /></chemistry>and a tetracarboxylic acid dianhydride having the formula: <chemistry id="chem0005" num="0005"><img file="EP0010657A1_D0005.tif" /></chemistry>wherein R, R', R", Q and x have the meanings given above.

0011Alternatively, a polysiloxane-imide composition may be used with comparable effectiveness by blending together a polyimide composed solely of recurring structural units of Formula I with a polyimide composed solely of recurring structural units of Formula II employing the polyimide of Formula II in such a molar proportion that the structural units of the latter are within the range of from 5 to 50 mol percent of said units based on the total molar concentration of the units of Formula I.

0012It will be recognized that the ultimate polyimide siloxane composition used in the practice of this invention will consist essentially of the imido structures found in Formulas I and II. However, the actual precursor materials resulting from the reaction of the diaminosiloxane, the silicon-free organic diamine and the tetracarboxylic acid dianhydride will initially be in the form of a polyamic acid structure composed of structural units of the formulas: <chemistry id="chem0006" num="0006"><img file="EP0010657A1_D0006.tif" /></chemistry><chemistry id="chem0007" num="0007"><img file="EP0010657A1_D0007.tif" /></chemistry>where R, R', R", Q, x, m and n have the meanings given above.

0013The diamino siloxanes of Formula III which may be used in the practice of the present invention include compounds having the following formulas: <chemistry id="chem0008" num="0008"><img file="EP0010657A1_D0008.tif" /></chemistry><chemistry id="chem0009" num="0009"><img file="EP0010657A1_D0009.tif" /></chemistry><chemistry id="chem0010" num="0010"><img file="EP0010657A1_D0010.tif" /></chemistry><chemistry id="chem0011" num="0011"><img file="EP0010657A1_D0011.tif" /></chemistry><chemistry id="chem0012" num="0012"><img file="EP0010657A1_D0012.tif" /></chemistry><chemistry id="chem0013" num="0013"><img file="EP0010657A1_D0013.tif" /></chemistry><chemistry id="chem0014" num="0014"><img file="EP0010657A1_D0014.tif" /></chemistry>and the like.

0014The diamines of Formula IV above are described in the prior art and are to a large extent commercially available materials. Typical of such diamines from which the prepolymer may be prepared are the following: <ul id="ul0002" list-style="none"><li>m-phenylenediamine;</li><li>p-phenylenediamine;</li><li>4,4'-diaminodiphenylpropane;</li><li>4,4'-diaminodiphenylmethane;</li><li>4,4'-methylenedianiline; benzidine;</li><li>4,4'-diaminodiphenyl sulfide;</li><li>4,4'-diaminodiphenyl sulfone;</li><li>4,4'-diaminodiphenyl ether;</li><li>1,5-diaminophthalene;</li><li>3,3'-dimethylbenzidine;</li><li>3,3'-dimethoxybenzidine;</li><li>2,4-bis(a-amino-t-butyl)toluene;</li><li>bis(p-a-amino-t-butylphenyl)ether;</li><li>bis(p-ß-methyl-o-aminopentyl)benzene;</li><li>1,3-diamino-4-isopropylbenzene;</li><li>1,2-bis(3-aminopropoxy)ethane;</li><li>m-xylylenediamine;</li><li>p-xylylenediamine;</li><li>bis(4-aminocyclohexyl)methane;</li><li>decamethylenediamine;</li><li>3-methylheptamethylenediamine;</li><li>4,4-dimethylheptamethylenediamine;</li><li>2,11-dodecanediamine;</li><li>2,2-dimethylpropylenediamine;</li><li>actamethylenediamine;</li><li>3-methoxyhexamethylenediamine;</li><li>2,5-dimethylhexamethylenediamine;</li><li>2,5-dimethylheptamethylenediamine;</li><li>3-methylheptamethylenediamine;</li><li>5-methylnonamethylenediamine;</li><li>1,4-cyclohexanediamine;</li><li>1,12-octadecanediamine;</li><li>bis(3-aminopropyl)sulfide;</li><li>N-methyl-bis(3-aminopropyl)amine;</li><li>hexamethylenediamine;</li><li>heptamethylenediamine;</li><li>nonamethylenediamine;</li><li>and mixtures thereof. It should be noted that these diamines are given merely for the purpose of illustration and are not considered to be all inclusive. Other diamines not mentioned will readily be apparent to those skilled in the art.</li></ul>

0015The tetracarboxylic acid dianhydrides of Formula V may further be defined in that the R" is a tetravalent radical, e. g. a radical derived from or containing an aromatic group containing at least 6 carbon atoms characterized by benzenoid unsaturation, wherein each of the 4 carbonyl groups of the dianhydride are attached to a separate carbon atom in the tetravalent radical, the carbonyl groups being in pairs in which the groups in each pair are attached to adjacent carbon atoms of the R" radical or to carbon atoms in the R" radical at most one carbon atom removed, to provide a 5-membered or 6-membered ring as follows: <chemistry id="chem0015" num="0015"><img file="EP0010657A1_D0015.tif" /></chemistry>Illustrations of dianhydrides suitable for use in the present invention (with their reference acronym designated in parenthesis) include: 2, 3,6, 7-napthalene tetracarboxylic dianhydride; <ul id="ul0003" list-style="none"><li>3,3',4,4'-diphenyl tetracarboxylic dianhydride;</li><li>1, 2, 5, 6-napthalene tetracarboxylic dianhydride;</li><li>2, 2', 3, 3'-diphenyl tetracarboxylic dianhydride;</li><li>2, 2-bis(3,4-dicarboxyphenyl)propane dianhydride;</li><li>bis(3, 4-dicarboxyphenyl)sulfone dianhydride;</li><li>2, 2-bis[4-(3, 4-dicarboxyphenoxy)phenyl] propane dianhydride (BPA dianhydride);</li><li>2, 2-bis [4-*2, 3-dicarbocyphenoxy)phenyl] propane dianhydride;</li><li>benzophenone tetracarboxylic acid dianhydride (BPDA);</li><li>perylene-1, 2, 7, 8-tetracarboxylic acid dianhydride;</li><li>bis(3, 4-dicarboxyphenyl) ether dianhydride, and</li><li>bis(3,4-dicarboxyphenyl)methane dianhydride;</li><li>and aliphatic anhydrides such as cyclopentane tetracarboxylic dianhydride, cyclohexane tetracarboxylic dianhydride, butane tetracarboxylic dianhydride, etc. The incorporation of other anhydrides, such as trimellitic anhydride, to make amide-imide-siloxane polymers is not precluded.</li></ul>

0016Application of the random or block copolymers or blends of polymers in a suitable solvent (including, for example, N-methylpyrrolidone, N,N-dimethylacetamide, N, N-dimethylformamide, etc.) alone or combined with non-solvents, to the substrate material may be by conventional means such as dipping, spraying, painting, spinning, etc. The block copolymers or blends of polymers may be dried in an initial heating step at temperatures of about 75 to 125^oC for a sufficient time frequently under vacuum to remove the solvent. The polyamic acid is then converted to the corresponding polyimide-siloxane by heating at temperatures of about 150 C - 300^oC for a sufficient time to effect the desired conversion to the polyimide structure and final cure.

0017A preferred curing cycle for materials of the above general formula is as follows: <ul id="ul0004" list-style="none"><li>(a) from 15 to 30 minutes of from 135^oC to 150^oC in dry N₂.</li><li>(b) from 15 to 60 minutes at about 185⁰C + 100C in dry N₂.</li><li>(c) from I to 3 hours at about 225^oC in vacuum.</li><li>(d) from 10 to 15 minutes at about 450^oC in nitrogen.</li></ul> Alternately, it has been found that one may be able to cure the coating material in other atmospheres such, for example, as air, for ease of commercial application of this invention.

0018Should an additional layer of the material be required to achieve a desired thickness of the cured layer, it is preferably applied after the first curing step (a). More than one additional layer can be applied in this manner with a subsequent curing step (a) after each new layer of silicone-polyimide copolymer material is applied to the previous layer. The final curing steps (b), (c) and (d) are then practiced.

0019In order to describe the invention more distinctly, described below is the novel method of patterning the cured copolymer material on the surface of a semiconductor device. In particular, with reference to Figures 1 through 6, a body 10 of semiconductor material of N-type conductivity having opposed major surfaces 12 and 14 is processed to form regions 16 and 18 of P and N type conductivity respectively therein. P-N junctions 20 and 22 are formed at the abutting, contiguous surfaces of the regions 16 and the body 10 and regions 16 and 18, respectively. End portions of the P-N junctions 16 and 18 are exposed at the surface 12.

0020The material of the body 10 may be silicon, silicon carbide, germanium, gallium arsenide, a compound of a Group III element and a Group V element. In order to illustrate the novel process of this invention the material of the body is said to be of silicon.

0021An electrical contact 24 is affixed to the region 16 and is in ohmic electrical contact therewith. An ohmic electrical contact 26 is affixed to the region 18. Ohmic contacts 24 and 26 are typically made of aluminum. A layer 28 of the cured silicone-polyimide copolymer material is disposed on the surface 12 and the contacts 24 and 26. Although the thickness of the layer 28 may be of the order of 10 microns or more, a thickness of from 1 to 5 microns is preferred.

0022A layer 30 of silicon is vapor deposited upon the layer 28 of cured silicone-polyimide copolymer material preferably from a source of high purity silicon. A thickness of from 500Å to 2, 000h of the layer 30 has been found to be satisfactory as a suitable mask for selective plasma chemical etching of the material of the layer 28. Preferably, the layer 30 is of the order of 1000Å in thickness.

0023Employing photolithographical techniques, a layer 32 of a photoresist material is disposed on the layer 30 of silicon. Typically, the photoresist material is a positive material such, for example, as OFPR-2 from Tokyo Ohka America, Inc. The photoresist is pre-cured by 15 minutes of air drying at room temperature followed by baking at 90⁰C, approximately for a period of about 30 minutes in nitrogen. A suitable mask, not shown, is disposed on the photoresist layer 32, properly aligned in order that the windows to be opened in the silicone-polyimide material layer 28 are oriented properly with respect to contacts 24 and 26. The masked layer 32 of photoresist is exposed to ultraviolet radiation for the proper length of time. Developing of the exposed layer 32 removes unfixed photoresist material from the window areas 36 and 38 of the layer 32. The remainder of the material of the layer 32 is "fixed". The exposed layer 32 may be post cured by 140^oC bake for 30 minutes in order to enhance its adhesion to the silicon layer.

0024Windows 36 and 38 now expose selective surface areas of the layer 30 of silicon therein. The structure is as shown in Figure 2.

0025The processed wafer 10 is placed in a suitable plasma etching machine, not shown, such for example, as an IPC series 4000, manufactured by International Plasma Corporation. The processed wafer 10 is placed in the work chamber of the plasma etching machine and the chamber is then evacuated. The processed wafer 10 is preheated in the chamber in a nitrogen plasma formed at an RF power of a range from about 100 watts to about 200 watts of energy and from 0.5 torr to 0.6 torr for a period of about 3 minutes to bring the temperature of the processed body 10 to about 750C and the nitrogen plasma is turned off. The preheating step is practiced to bring the body 10 up to the reaction temperature required for etching in a most uniform manner. The desired reaction temperature ranges from 65⁰C to 85°C.

0026After preheating the body 10, CF₄ including 4% oxygen by volume at a gas flow rate of from about 100cc/minute to about 150cc/minute is introduced and a plasma is formed at an energy level of about 100 watts. The pressure in the chamber is typically about 0.3 to 0.5 torr. This new plasma is employed to etch the exposed silicon layer 30 at a rate of from about 0. 5 x 10³ A to about 1 x 10³ Å per minute. Plasma etching is practiced for a sufficient time to remove the exposed silicon, or for about one to two minutes, to assure complete removal of the silicon to open windows 42 and 44 (Figure 3) in the silicon to expose selected surface areas of the silicon-polyimide copolymer material therein.

0027Upon completing the silicon etch step, the previous plasma of CF₄ and oxygen is stopped. A new plasma of oxygen is initiated. The RF power is from about 200 watts to about 400 watts. About 300 watts is preferred. The gas flow rate is about 300 CCPM of oxygen to obtain a chamber pressure of preferably about 1.6 torr. However, the chamber pressure may range from about 0.8 torr to 2.0 torr. These conditions for the plasma are sufficient to etch away the exposed silicone-polyimide material exposed in the windows 42 and 44 of the silicon mask 32. The etch rate of the polyimide-silicone copolymer material is from 0.25 to 0.5 micron per minute under the stipulated conditions. Simultaneously, the oxygen plasma strips away the photoresist of the mask layer 32. Removal of the silicone-polyimide material produces windows 48 and 50 to expose the electrical contacts 26 and 24 therein as shown in Figure 4.

0028Undercutting of the silicone-polyimide material of the layer 30 is not a serious problem because of the long mean-free-path of the reactant species.

0029Upon completion of the etching of the polyimide-silicone copolymer material and the remainder of the photoresist mask, the plasma is turned off. The chamber is evacuated and a new plasma is formed, the composition being CF₄ and oxygen, the same as initially formed for etching the windows in the silicon layer 30. The remainder of the material of the silicon mask layer 30 is removed by the plasma etchant in about one minute. The plasma is turned off, the chamber brought again to atmospheric pressure and the processed body 10 removed from the chamber. The processed body 10 is as shown in Figure 5.

0030The processed body may then be further cleaned by aqueous solutions if necessary.

0031The body 10 may now be processed further as required by affixing electrical leads 54 and 56 to the respective electrical contacts 26 and 24 as illustrated in Figure 6.

0032The process as described does not attack the exposed surface area of aluminum contacts 24 and 26 at any time. However, any exposed areas of silicon on the body 10 will be etched by an amount closely related to the thickness of the masking layer 30.

0033To illustrate the teachings of this invention several wafers of silicon semiconductor material, each about 2 inches in diameter, were processed to form a plurality of devices in each wafer. Aluminum metal was deposited on predetermined surface areas of the devices.

0034A polymer precursor solution was formed by reacting benzophenone tetracarboxylic acid dianhydride with methylene dianiline and bis (α -aminopropyl)tetramethyldisiloxane, the latter two diamine materials being present in the molar ratio of 70:30. The reaction was carried out at a temperature of less than 50 C and using suitably purified and dried materials to favor a large molecular weight polymer. A solution of the polymer precursor in the form of the polyamic acid form dissolved in N-methyl-2-pyrrolidone containing 25% solids by weight was disposed on the surface of the devices. About 7 to 10 drops of the precursor solution was disposed on the surface of the wafers, and therefore the devices. The wafers were then spun at from about 2000RPM to about 3000RPM for 20 seconds. The coated wafers were cured at 150^oC + 5°C for about 30 minutes in Nitrogen, 185^oC + 5°C in Nitrogen for about 30 minutes, at 225^oC + 5⁰C in a vacuum of 29" Hg for about 2 hours and at 450 C + 5⁰C in nitrogen for ten minutes. The layer of copolymer material was from 1.5 to 2.0 microns in thickness when cured.

0035A layer of silicon, about 1000Å in thickness, was deposited from a high purity source in an electron beam evaporator. A layer of positive photoresist material was deposited on the silicon layer and processed to open windows therein. The processed wafer was placed in an IPC series 4000 plasma etching machine. The wafer was in a nitrogen plasma at 100 watts and 0.5 torr for 3 minutes.

0036The exposed portions of the silicon layer in the windows of the photoresist material were etched by a plasma consisting of CF with 4% oxygen (100 CCPM) at 100 watts and a total pressure of about 0.3 torr. The elapsed etching time for the silicon was one minute. The plasma was turned off and the chamber was evacuated.

0037An oxygen plasma was then established at 300 watts of RF power, 300 CCPM of oxygen at a pressure of about 1.6 torr. The polyimide- silicone copolymer exposed in the silicon windows was etched in 5 minutes. The remaining photoresist material was removed simultaneously during the etching of the copolymer material. After etching of the polyimide-silicone copolymer material was completed, the plasma was turned off and evacuated.

0038CF , 4% oxygen plasma was again established at 100 watts RF power and about 0. 3 torr. The new plasma was employed to etch the remaining silicon material of the mask from the copolymer material. Etching of the silicon was completed in one minute. The plasma was turned off and the chamber was brought back to atmospheric pressure. The processed body was removed and examined.

0039The exposed aluminum contact surface did not show any sign of attack by the plasma etchants. The windows were well defined and had dimensional stability. The edges of the copolymer material were not visibly undercut.

0040It has been further discovered that the addition of from about 10% by volume to about 15% by volume of CF₄to the oxygen plasma greatly increases the etch rate of the copolymer material without any significant attack of the silicon masking layer. The advantages derived from this etch rate enhancement lie in the ability to use lower RF power levels, of the order of 50 watts, and lower ambient pressures, of the order of 0.2 torr, for etching copolymer material. Additionally, larger wafer loads can be handled for each production run. Furthermore, operating at the reduced pressure and power levels lends itself to a more uniform etching of the wafer as well as improving dimensional control of the etched openings in the copolymer material.