POLYMERIC BORON-NITROGEN DOPANT

Abstract: A polymeric borazole is produced by reacting BCl₃ and an amine at temperatures between -60 and -5°C in an inert solvent to form an intermediate triaminoborane, and then further polymerizing the intermediate after removal of the amine hydrochloride salt via thermal condensation at 410°C. This polymer, in the form of a glassy resin, and other borazole polymers derived by the polymerization of monom­ ers of the formula (XBNY)₃ wherein X and Y are used to designate the indicated position of H, OH, OR, halogen, amino and R, substituents on the polymer and wherein R is selected from the group consisting of alkyl radicals of 1-4C atoms, cyclohexyl, phenyl and methyl and ethyl substituted phenyl radicals is dissolved in toluene or xylene at the 1 to 20% (wt/wt) level. The polymeric solution may be employed in the manufacture of semicon­ ductor components and, as such, the solution may be spun-on silicon wafers to function as a boron impurity source (dopant) in thermal diffusions. Such doped wafers are then used in integrated circuit manufacture.

Description: DESCRIPTION POLYMERIC BORON NITROGEN DOPANT Technical Field This invention relates to the use of boron-nitrogen polymers as dopant materials used in semiconductor manufacture and more particularly to dopant compositions adapted to be spun on to a semiconductor substrate and used in integrated circuit manufacture. The dopant comprises a polymer, at least a substantial part of the backbone of which contains boron-nitrogen borazole (borazine) monomer units, in the form of a glassy resin dissolved in an organic solvent. BACKGROUND OF THE INVENTION Since the diffusion of boron impurity atoms into silicon is necessary for semiconductor device fabrication, a number of boron containing chemicals and processes to "dope" the boron into the silicon have been proposed. Some of the known prior art products (and processes) are the following: 1) boron nitride wafers are oxidized in a diffusion furnace to B203 or HBO2, and the oxide is transferred in the vapor phase to the silicon wafer surface or boric oxide wafers can be used directly; 2) boron tribromide liquid is vaporized and oxi dized in the diffusion furnace to B203 which coats the wafers from the vapor phase; 3) boron trichloride is used as a source of boron ions for use with an ion implanter. Less commonly used sources of boron include the boron spin-on solutions, (which typically are B203 or a borosilicate glass precursor dissolved in a solvent) borosilicate films deposited by chemical vapor deposition, or diborane (B2H6) gas, which is used in a procedure similar to that used for BBr3. While the above are effective boron sources, all have disadvantages. Specifically, ion implantation involves high capital costs, low throughput, and damage to the silicon crystal structure, while boron oxide and related sources are unusually sensitive to moisture, create damaged or stained silicon surfaces, and/or involve costly and complicated processes to achieve quality doping. Advantageously, the polymer to be useful must be homogeneous and soluble so that it is easily distributed uniformly on the surface, such as by the spin-on method; it must be relatively non volatile so that when the solvent is removed and the material heated to diffusion temperatures the desired film will be substantially intact; it must be capable of being rendered insoluble i.e., cross linked thermally or otherwise; and it must be characterized as glassy rather than crystalline. It is thus apparent that a need exsists for a more economical and efficiently applied spin-on boron dopant system. SUMMARY OF THE INVENTION It is an object of the invention to provide a novel polymeric borazole spin-on boron dopant that offers potentially significant advantages over the prior art. More particularly, it is an object of the invention to provide a boron-nitrogen species for semiconductor dopant compositions that, as a diffusion source, is less sensitive to moisture and thereby affords greater processing latitude, and is less prone to create damage to and/or staining of the silicon crystal substrate. The polymeric borazoles employed in the invention are polymers derived from the class of materials known as borazines (or borazoles). These borazines are heterocyclic hexatomic ring compounds having the general formula (XBNY)3 with the substituents X and Y on the ring designated with "B" and "N" prefixes, respectively, to indicate the positions. The polymers derived from the borazines are of two general types: 1) polymers with borazine rings linked directly between a boron in one ring and a nitrogen atom in the adjacent ring. and EMI3.1 2) polymers, at least a substantial portion of whose backbone is borazine rings linked via other (different) groups Z, i.e.: EMI3.2 wherein Z is selected from the group consisting of -N(R)-, -O-, -N(R)-R'-N(R)-, and -R-, and wherein R and R' denote organic radicals of 1 to 10 carbon atoms and X and Y are radicals selected from the group consisting of hydrogen, alkyl radicals of 1-8 carbon atoms, unsubstituted aromatic, alkyl-, halogen-, hydroxy-, and aminosubstituted aromatic radicals, heterocyclic aromatic, 16 carbon atom alkyl substituted heterocyclic aromatic, amino, hydroxy, alkoxy and halogen radicals. These borazine polymers can be produced as homo polymers from a single monomeric borazine or as copolymers from two different borazines. DESCRIPTION OF THE PREFERRED EMBODIMENTS In preparing the polymeric dopant composition in accordance with the invention, a polymeric borazole is produced by first reacting BC13 and an amine at temperatures between -6O0C and -50C in an inert solvent to form an intermediate triaminoborane. After removal of the amine hydrochloride salt, the intermediate is polymerized via thermal condensation at a temperature between 380-4200C. The polymer, in the form of a glassy resin, is then dissolved in an organic solvent such as toluene or xylene at a 1 to 20% (wt/wt) level and spun-on silicon wafers to function as a boron impurity source (dopant) in thermal diffusions. The doped wafers thus prepared are then used in integrated circuit manufacture. In accordance with the invention, the polymers derived from the borazines are of two general types: 1) polymers with borazine rings linked directly between a boron in one of the six-membered boron/nitrogen ring and a nitrogen atom in the adjacent six-membered boron/nitrogen ring. and EMI4.1 2) polymers, at least a substantial portion of whose backbone is borazine rings linked via other (different) groups Z, i.e.. EMI5.1 wherein Z is selected from the group consisting of -N(R)-, -O-, -N(R)-R'-N(R)-, and -R-, and wherein R and R' denote organic radicals of 1 to 10 carbon atoms and X and Y are radicals selected from the group consisting of hydrogen, alkyl radicals of 1-8 carbon atoms, unsubstituted aromatic, alkyl-, halogen-, hydroxy-, and aminosubstituted aromatic radicals, heterocyclic aromatic, 16 carbon atom alkyl substituted heterocyclic aromatic, amino, hydroxy, alkoxy and halogen radicals. These borazine polymers can be produced as homopolymers from a single monomeric borazine or as copolymers from two different borazines utilizing a variety of synthetic routes known to those skilled in the art of which the English, et al. U.S. Pat. 3,166,520 and Aubrey, et al., JCS (1959) pp.2927-31 are illustrative references. Thus, homopolymers can be produced from the thermal polymerization of B-amino borazines (two or three -NHR- or -NR2 groups) or thermal polymerization of borazines with X and Y being selected from the group above defined. Similarly, copolymers can be made by reacting two different borazines wherein X and Y are chosen as above defined. Copolymers can also be produced by reacting B-amino or B-halogen borazines with a wide variety of aliphatic, aromatic, and heterocyclic diamines. A borozine copolymer can be produced from the reaction of a cyclic methyl-amino-borane trimer with a diene or polyene. B-halogen borazines can be copolymerized with N-lithiated borazines, and borazine copolymers with rings linked via oxygen can be made by the controlled hydrolysis of B-halogen borazines. It will be apparent that the foregoing are not all inclusive and that other borazine polymers within the above definition are intended. In employing the borazine polymers of the invention, the traditional advantages of a spin-on dopant are maintained i.e., greater uniformity, less capital cost, higher throughput, more control, and less cleaning of dittusior. tubes and boats. The invention will be further described by the following specific examples, the details of which, it will be understood, are given primarily for purpose of illustration and not for the limitation of the invention in its broader aspect. EXAMPLE 1 In accordance with the invention and as shown by the reaction below, boron trichloride (so13) (359, 0.3 moles) gas is added to a -50C to -600C solution of cyclohexyl amine (1959, 2.0 moles) dissolved in 2 liters of petroleum ether (bp 30-600C). The reaction results in the formation of a triaminoborane, B(NHCy)3(where Cy = C6Hll, cyclohexyl), and the cyclohexylamine hydrochloride salt. After the salt is removed by filtration, the triaminoborane is heated under a nitrogen atmosphere to 3000C for 1-2 hours to form a monomeric intermediate and thereafter heated to 380-4200C over a 2-4 hour period, during which time a condensation polymerization occurs and cyclohexylamine is evolved. A glassy resin (string temperature 140 30 C) is formed upon cooling; this amber solid is dissolved in toluene (xylene may also be used) at the 1 to 20% (wt/wt) level. The resulting product is a dark amber to almost black solution. (a) BC13 + 6NH2Cy > B(NHCy)3 + 3NlI3Cy C1 EMI7.1 [ B3(NCy)4NHCy ] x + xNH2Cy fcondensatioril L polymer EXAMPLE 2 The procedure of Example 1 is repeated except that 186g (2.0 moles) of aniline is used in place of cyclohexylamine. The aniline reacts to form the triaminoborane B(NHPh)3, (Ph =-C6H5) which is polymerized under the same conditions as in Example 1 to yield [B3L(NPh)4 (NHPh) ] X EXAMPLE 3 The procedure of Example 1 is repeated except that 909 (2.0 moles) of ethylamine is used in place of cyclohexylamine. The ethylamine reacts to form B(NEEt)3 which is polymerized at a temperature between 3200C and 3600C to produce [ B3(NEt)4(NHEt) ] x wherein Et = -C2Hs. EXAMPLE 4 In applying the polymeric composition to substrates, 2-6 ml of the solutions, prepared in accordance to Example 1 (and Examples 2 and 3), are spun-on by dispensing the polymeric solution onto the surface of a silicon wafer and the wafer rotated at 2,000-10,000 rpm, which results in thin films (100-7000A) of the polymer. The treated wafers, which may be held for up to 24 hours, are placed in a diffusion furnace and the wafer diffused according to one of the following diffusion procedures. Film Diffusion It will be understood by those skilled in the art that diffusion times, temperatures, furnace ambients, and exact processes may vary. Described below are: two typical bipolar base diffusion processes (a) and (c); a bipolar isolation diffusion (b); and a metal oxide silicon (MOS) source/drain diffusion (d). (a) A conventional two-step diffusion process involves a predeposition step under nitrogen at temperatures between 8500C and 10000C for diffusion times up to 1 hour to achieve sheet resistivities from 15-275 ohms per square. This predeposition step is followed by a deglazing step in concentrated aqueous HF, and a drive step at 10500C - 1150 CC for 1-2 hours under an atmosphere selected from oxygen, wet oxygen and nitrogen and mixtures thereof. (b) A less conventional one-step diffusion process involves a predeposition stage that begins under a nitrogen atmosphere at 8500C-10000C for 5-60 minutes and is immediately followed by a wet oxygen stage at a slightly lower temperature for 5-45 minutes. The final stage of this one step (single diffusion tube) process involves raising the temperature to 10500C-11500C and diffusing the film for 1-2 hours under an atmosphere of oxygen, wet oxygen and nitrogen ambients and mixtures thereof. This one step process eliminates the HF deglazing step as well as the utilization of two (or more) diffusion tubes that would be required in procedure (a). (c) A bipolar isolation diffusion is effected as in procedure (a), but higher predeposition temperatures (11000C-12000C) are employed to yield sheet resistivities of 1-4 ohms per square. (d) A complementary CMOS or positive MOS (PMOS) source/drain diffusion can also be carried out as in procedure (a) with the drive step omitted. In a typical run, at a temperature of 9350C for 1 hour under a N2 atmosphere, sheet resistivities of 40 ohms per square at a junction depth of 0.4 microns can be achieved. Industrial Applicability The boron-nitrogen product prepared in accordance with the invention may be applied effectively as a boron source film for both base and isolation diffusion applications for bipolar integrated circuits and evaluated for absolute sheet resistivities. The resulting products are characterized as having single crystal acrossthe-wafer and wafer-to-wafer resistivity uniformity as well as batch-to-batch resistivity uniformity, minimal wafer damage and staining, ease of processing, and compatibility with existing processes. In each of these areas the polymeric source film forming compositions prepared in accordance with the invention performs effectively. This product may be used on a wide range of devices for the semiconductor fabrication industry, e.g., bipolar and MOS integrated circuits, discrete devices, photovoltaic (solar) cells, and the like. Of particular applicability is usage, of the borazine ring containing polymer composition of the invention, by manufacturers of bipolar integrated circuits for base and isolation diffusions in a manner typically described by Example 4. It will be apparent that variations in the details presented herein may be made by those skilled in the art. Therefore, the invention is not to be limited except as set forth in the claims which follow.

Claims: What is claimed is: 1. A conductivity inducing polymeric source film forming composition containing at least 1.0 percent by weight in a volatile solvent of a polymer at least a substantial part of the backbone of which is selected from: (a) polymers with borazine rings linked directly between a boron in one ring and a nitrogen atom in the adjacent ring EMI10.1 and (b) polymers with borazine rings linked via t-;- groups Z. EMI10.2 wherein X and Y locate the substituents on the ring boron and nitrogen atoms and Z is selected from the group consisting of -N(R)-, -O-, -N(R)-R'-N(R)-, or -R-. (R and R' denote organic radicals of 1 to 10 carbon atoms) and X and Y are radicals selected from the group consisting of hydrogen, alkyl radicals of 1-8 carbon atoms, unsubstituted aromatic, alkyl, halogen, hydroxy and amino substituted aromatics, heterocyclic aromatics, 1-6 carbon alkyl substituted heterocyclic aromatic amino, hydroxy, alkoxy and halogen radicals. 2. A conductivity inducing polymeric source film forming composition containing at least 1.0% by weight in an organic solvent of a polymer derived from the homopolymerization of monomers of the formula B(NHR)3 wherein R is selected from the group consisting of alkyl radicals of 1-4 carbon atoms, cyclohexyl-, phenyl-, and methyl- and ethyl-substituted phenyl radicals, said resin being relatively less volatile than the solvent and being further polymerizable, upon deposition on a substrate and volatilization of the solvent, to yield an insoluble glassy film. 3. The composition of claim 1 wherein R is a cyclohexyl substituent. 4. The composition of claim 1 wherein R is a phenyl substituent. 5. The composition of claim 1 wherein R is an ethyl substituent. 6. The composition of any of claims 1 through 5 wherein the solvent is selected from xylene or toluene. 7. A method of applying the polymeric composition of claim 1 to semiconductor substrates comprising dispensing the polymer in solution onto the surface of a silicon wafer, rotating the wafer at 2,000-10,000 rpm to produce a thin film of the order of 100-7000A of the polymer on the substrate, heating the treated wafer in a diffusion furnace for 0.5-2.0 hours at 8500C-11000C in a nitrogen ambient, deglazing the heated substrate in HF and heating the substitute at elevated temperature of 1050-12000C for 1-2 hours under an atmosphere selected from oxygen, wet oxygen and nitrogen ambients and combinations thereof. 8. A method of doping semiconductor material comprising: (a) spinning a semiconductor substrate (b) simultaneously depositing on said substrate a solution of at least 1.0% by weight of a resin derived from the polymerization of a monomer of the formula B(NHR)3 wherein R is selected from the group consisting of alkyl radicals of 1-4 C atoms, cyclohexyl, phenyl, and methyl- and ethyl-substituted phenyl, radicals. (c) heating said deposition until the solvent present in said solution is substantially volatilized; and (d) and polymerizing said resin deposition to a glassy insoluble state. 9. The method of claim 8 wherein R is cyclohexyl. 10. A method of doping and thereafter diffusing a semiconductor substrate wherein the diffusion of the applied dopant film effected in a one-tube diffusion comprising: a) Placing the coated wafers in diffusion furance for about 1 hour under nitrogen at a temperature between 850 and l2000C; b) Replacing the nitrogen ambient with wet oxygen after decreasing the temperature to between 7000C and 8000C for 15-30 minutes; and c) Increasing the temperature to between 900 and l2500C under a nitrogen, oxygen, and/or steam ambient, or any combination thereof, for the time needed to achieve the required junction depth.

Inventors: KIRTLEY, STEPHEN WAKEFIELD, WOOSTER, GEORGE SIDNEY
Applicants: ALLIED CORPORATION
Application year: 1983
Main classification: H01L21
IPC codes: C08L85, H01L21, C08G79, C30B31