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 a method as per the preamble of claim 1. Said method uses titanum 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 chemical 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.

0005FR-A-2 309 976 describes a method for making a network of interconnected conductors on a substrate consisting of silicon. For carrying out said method there is arranged a second polymer layer between a first polymer layer and a masking layer. While the first polymer layer consists of a polyimide, the second polymer layer consists of a polysulphone.

0006Therefore, the possible use of silicon for the masking layer cannot be compared with the use of a masking layer of silicon arranged on a layer of the copolymer according to the present invention. There is not contained in said French FR-A any indication as to the adhesion of a silicon layer to a copolymer as used in the present invention. All the expert can conclude from said FR-A is, that silicon has enough adherence to a polysulphone layer to be used for the masking layer. Additionally it is to be pointed out that said silicon is not even a preferred material for the masking layer according to the teaching of said FR-A rather a polymethylsiloxane is preferred therefor (compare page 4, lines 30 to 34 of the FR-A-2 309 976). Further it follows from the preferred use of a polymethylsiloxane as a material for the masking layer that such polymethylsiloxane must be resistant to the oxygen containing CF_4-plasma so that the copolymer used according to the invention comprises a component being etchable by the oxygen containing CF_4-plasma - namely the polyimide - and a component being resistant against being etched by said oxygen containing CF_4-plasma. Consequently it cannot have been obvious for the expert how the copolymer of both said components would behave with respect to being etched by an oxygen containing CF₄plasma.

BRIEF SUMMARY OF THE INVENTION

0007In 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 10-⁴ to 10-³ cm (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 10-⁵ cm (1000 A), 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.

0008The 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 CF₄, 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.

0009The copolymer material is chemically etched in a plasma of oxygen. Chemical etching 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

0010Figures 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

0011Briefly, 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="EP0010657B1_D0001.tif" /></chemistry>with from 5 to 50, preferably 15 to 40 mole percent and most preferably 25 to 35 mole percent intercondensed structural units of the formula: <chemistry id="chem0002" num="0002"><img file="EP0010657B1_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 to 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.

0012R is preferably an optionally substituted alkalene 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.

0013R' is 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 1 5 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.

0014Q 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.

0015n and m are integers preferably different integers from 100 to 1000.

0016The 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="EP0010657B1_D0003.tif" /></chemistry>a silicon-free diamine compound of the formula: <chemistry id="chem0004" num="0004"><img file="EP0010657B1_D0004.tif" /></chemistry>and a tetracarboxylic acid dianhydride having the formula: <chemistry id="chem0005" num="0005"><img file="EP0010657B1_D0005.tif" /></chemistry>wherein R, R', R", Q and x have the meanings given above.

0017Alternatively, 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.

0018It 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="EP0010657B1_D0006.tif" /></chemistry><chemistry id="chem0007" num="0007"><img file="EP0010657B1_D0007.tif" /></chemistry>where R, R', R", Q, x, m and n have the meanings given above.

0019The 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="EP0010657B1_D0008.tif" /></chemistry><chemistry id="chem0009" num="0009"><img file="EP0010657B1_D0009.tif" /></chemistry><chemistry id="chem0010" num="0010"><img file="EP0010657B1_D0010.tif" /></chemistry><chemistry id="chem0011" num="0011"><img file="EP0010657B1_D0011.tif" /></chemistry><chemistry id="chem0012" num="0012"><img file="EP0010657B1_D0012.tif" /></chemistry><chemistry id="chem0013" num="0013"><img file="EP0010657B1_D0013.tif" /></chemistry><chemistry id="chem0014" num="0014"><img file="EP0010657B1_D0014.tif" /></chemistry>and the like.

0020The 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="ul0001" 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;</li><li>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>2,4-bis(β-amino-t-butyl)toluene;</li><li>bis(p-β-amino-t-butylphenyl)ether;</li><li>bis(p-β-methyl-o-aminopentyl)benzene;</li><li>1,3-diamino-4-isopropylbenzene;</li><li>1,2-bis(3-aminopropoxy)ethane;</li><li>m-xylenediamine;</li><li>p-xylenediamine;</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></ul> and mixtures thereof. It should be noted that these diamines are given merely for the prupose of illustration and are not considered to be all inclusive. Other diamines not mentioned will readily be apparent to those skilled in the art.

0021The 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="EP0010657B1_D0015.tif" /></chemistry>

0022Illustrations of dianhydrides suitable for use in the present invention (with their reference acronym designated in parenthesis) include: <ul id="ul0002" list-style="none"><li>2,3,6,7-naphthalene tetracarboxylic dianhydride;</li><li>3,3',4,4'-diphenyl tetracarboxylic dianhydride;</li><li>1,2,5,6-naphthalene 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-dicarboxyphenoxy)phenyl]propane dianhydride;</li><li>benzophenone tetracarboxylic acid dianhydride (BPDA);</li><li>peryiene-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></ul> 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 polymer is not precluded.

0023Application of the random or block copolymers or blends of polymers in a suitable solvent (including, for example, N-methyl-pyrrolidone, 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°C 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°C for a sufficient time to effect the desired conversion to the polyimide structure and final cure.

0024A preferred curing cycle for materials of the above general formula is as follows: <ul id="ul0003" list-style="none"><li>(a) 15 to 30 minutes of from 135°C to 150°C in dry N_Z.</li><li>(b) from 15 to 60 minutes at about 185°C ± 10°C in dry N_z.</li><li>(c) from 1 to 3 hours at about 225°C in vacuum.</li><li>(d) from 10 to 15 minutes at about 450°C 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.

0025Should 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.

0026In order to describe the invention more distinctly, described below is the novel method of patterning the cured polymer material on the surface of a semiconductor device. In particular, with reference to Figures 1 though 6, a body 10 of semiconductor material of N-type conductivity having opposed major surfaces 12 and 14 is proposed 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.

0027The 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.

0028An 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 10-³ cm (10 microns) or more, a thickness of from 10-⁴ to 5. 10-1 cm (1 to 5 microns) is preferred.

0029A 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 5.10-⁵ cm (100 A to 2000 A) 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 10-⁵ cm (1000 A) in thickness.

0030Employing 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 90°C, approximately for a period of 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°C bake for 30 minutes in order to enhance its adhesion to the silicon layer.

0031Windows 36 and 38 now expose selective surface areas of the layer 30 of silicon therein. The structure is as shown in Figure 2.

0032The 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 of 100 watts to 200 watts of energy and from 65 to 80 Pascal (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 75°C 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 65°C to 85°C.

0033After preheating the body 10, CF₄ including 4% oxygen by volume at a gas flow rate of from 1 OOcc/minute to 1 50cc/minute is introduced and a plasma is formed at an energy level of about 100 watts. The pressure in the chamber is typically 40 to 65 Pascal (0.3 to 0.5 torr). This new plasma is employed to etch the exposed silicon layer 30 at a rate of from 5.10-6 to 10-⁵ cm (0.5 x 10³A to 1 x 10³ A) 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.

0034Upon 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 200 watts to 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 215 Pascal (1.6 torr). However, the chamber pressure may range from 105 to 265 Pascal (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 0.5.10-⁴ cm (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.

0035Undercutting 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.

0036Upon 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 is brought again to atmospheric pressure and the processed body 10 removed from the chamber. The processed body 10 is as shown in Figure 5.

0037The processed body may then be further cleaned by aqueous solutions if necessary.

0038The 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.

0039The 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.

0040To 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 device.

0041A polymer precursor solution was formed by reacting benzophenone tetracarboxylic acid dianhydride with methylene dianiline and bis(a-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°C±5°C for about 30 minutes in Nitrogen, 185°C ± 5°C in Nitrogen for about 30 minutes, at 225°C ± 5°C in a vacuum of 9.8 · 10⁴ Pascal (29" Hg) for about 2 hours and at 450°C ± 5°C in nitrogen for ten minutes. The layer of copolymer material was from 1.5 to 2.0.10-4 cm (microns) in thickness when cured.

0042A layer of silicon, about 10-⁵ cm (1 OOOA) 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 preheated in a nitrogen plasma at 100 watts and 65 Pascal (0.5 torr) for 3 minutes.

0043The exposed portions of the silicon layer in the windows of the photoresist material were etched by a plasma consisting of CF₄ with 4% oxygen (100 CCPM) at 100 watts and a total pressure of about 40 Pascal (0.3 torr). The elapsed etching time for the silicon was one minute. The plasma was turned off and the chamber was evacuated.

0044An oxygen plasma was then established at 300 watts of RF power, 300 CCPM of oxygen at a pressure of about 215 Pascal (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.

0045CF₄, 4% oxygen plasma was again established at 100 watts RF power and about 40 Pascal (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.

0046The 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.

0047It has been further discovered that the addition of from 10% by volume to 15% by volume of CF₄ to 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 the lower ambient pressures, of the order of 25 Pascal (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.