POLY(HALOSTYRENE) FOR NEGATIVE RESISTS

Abstract: This invention relates to a poly (halostyrene) composi­ tion useful as a negative lithographic resist mask. The poly (halostyrene) has a molecular weight in the range of from 1 × 10⁵ to 1 × 10⁶ and a dispersivity from 1.5 to 2.5.

Description: POLY (HALOSTYRENE) FOR NEGATIVE RESISTS This invention relates generally to the preparation of negative electron beam resists for photomask fabrication and/or semiconductor device fabrication and more particularly to poly(halostyrene), useful in the preparation of said negative resists. Briefly, a resist is an adhering layer of a material with patterned openings on a substrate which is used as a mask for etching, either partially or completely through, whereby the substrate is being exposed by the openings in the resist. Generally, resists are categorized as positive or negative. The formation of positive resist masks from layers of radiation degradable polymers is described, for example, in U.S. Patent No. 3,535,137. In this process, a radiation degradable polymer layer is coated on a substrate and is subjected to patternwise exposure to high energy radiation such as, for example, X-rays, deep W , nuclear radiation or electrons. The irradiated regions of the polymer suffer a decrease in molecular weight and thereby become more rapidly soluble. A developer is then used to preferentially remove the irradiated portions of the layer. The substrate is then subjected to an additive or substractive process such as metallization or etching with the remaining portions of the resist layer acting to protect the substrate from the processing. Alternatively, a negative resist mask is formed by coating a crosslinkable polymer on a substrate. The polymer is generally applied as a solution in a suitable solvent to the substrate and allowed to dry as a thin film. High energy radiation, such as an electron beam, short wave-length UV light, or X-rays, is permitted to irradiate the surface of the polymer in the desired pattern to form a negative resist by imparting sufficient energy to cross-link the polymer. Cross-linking causes the irradiated portion to be insoluble in certain solvents. After irradiation is completed, the resist is subjected to a solvent and the non-irradiated portion of the resist is removed, thereby leaving openings in the resist that correspond to the desired pattern. The present invention relates to negative resists. Prior art negative resist materials have several shortcomings, among which is the difficulty in obtaining sharp images of high resolution. Moreover, deep Uv lithography using the wavelength range from 2000 to 3500 A is receiving considerable attention as a possible economic mass production method of manufacturing ultra fine patterns (about lssm) for making VLSI. As these deep Uv tools become available, much emphasis is being placed on finding compatible negative resist-materials with high resolution, high contrast and plasma resistance. Recently, polystyrenes have been receiving renewed attention as electron and X-ray resists, primarily since it has been shown by others that very high contrast can be obtained with high molecular weight polystyrene exposed to electron beams at doses in the range of 10 coul/cm2 (100 ij 2 coul/cm ). It has also been shown that the sensitivity of polystyrenes can be considerably improved by physical doping with cross-linking agents and sensitizers or by incorporating such elements as chlorine, bromine or iodine in the polymer chain. In this regard, see, for example J.C. Jagt and A.P.G. Sevriens, Photopolymers Principles Processes and Materials, SPE, Ellenville, NY, p. 152 (1979); T. Tsunoda, et al., Phot. Sci. & Engr., 17(4), 390 (1973); N.A. Weir and T. H. Milkie, J. Poly. Sci. (Chem), 17, 3735 (1979); W. Burlant, et al., J. Poly Sci., 58, 491 (1962); E. D. Reit, Photopolymers Principles - Processes and Materials, SPE, Ellenville, NY, p. 91 (1979); and H. Shiraishi, et al., Photopolymers Principles - Processes and Materials, SPE, Ellenville, NY, p. 56 (1979). However, this literature illustrates that even though the sensitivity of the polystyrenes was increased in 2 the 10 coul/cm range, residues and swelling of the exposed pattern are always experienced when trying to develop a high resolution pattern in a relatively thick resist layer ( lem). The invention provides a new class of negative deep Uv resist materials which are highly sensitive and plasma resistant. The negative resist materials are highly sensitive in the deep nv region, particularly between 2000 and 2600A, forming high contrast negative images that are reactive ion etch (RIE) resistant. The negative resists of the present invention allow high contrast and resolution to be obtained on electron beam exposure leaving no residue in the unexposed region and minimum swelling of the exposed region. According to the invention there is provided a poly(halostyrene) having the general formula EMI3.1 wherein X is chlorine, bromine or iodine, n represents a number in the range of from about 750 to about 7,500, the molecular weight (Mw) of the polymers being in the range of from 1 x 10 to 1 x 106 and the dispersivity EMI3.2 of the polymers being in the range of from 1.5 to 2.5. The invention will now be described by way of example with reference to the accompanying drawings in which: Fig. 1 is a graphical illustration of the optical absorption of a poly(4-chlorostyrene) resist (lWm thick) of the present invention at varied wavelengths; Fig. 2.1 illustrates the contrast curves for poly(4-chlorostyrene) resists of about MW 700,000 (curve A) and about MW 250,000 (curve B), useful in the present invention; Fig. 2.2 shows scanning electron micrographs of electron beam irradiated poly(4-chlorostyrene) resists in accordance with the present invention; Fig. 3 shows scanning electron micrographs illustrating the thermal stability of electron beam exposed poly(4-chlorostyrene) resists of the present invention and the almost complete elimination of resist residue by postbaking; ; Fig. 4 is a series of scanning electron micrographs illustrating the excellent resolution obtained with electron beam exposed poly(4-chlorostyrene) resists of the present invention; minimum line = 0.4 pm and minimum space = 0.4 ssm; Fig 5.1 shows scanning electron micrographs illustrating the sharp line profiles obtained on deep Uv exposure of poly(4-chlorostyrene) resists of the present invention; and Fig. 5.2 shows scanning electron micrographs illustrating the narrow patterns obtained on X-ray exposure of poly(4-chlorostyrene) resists of the present invention. The present invention involves the use of a new class of poly (halogenatqd styrene), as negative electron beam, X-ray or U.V. resist materials. This new class of poly(halo-styrenes) allows high contrast and high resolution images to be obtained at electron beam exposures of, -6 2 for example, 1-5 x 10 cou V cm , depending on the molecular weight and molecular weight distribution of the polymer, leaving almost no residue in the unexposed region and minimum swelling of the exposed image even at thicknesses above lssm. More specifically, the polymeric compositions which are employed in preparing the resists of the present invention comprise poly(halogenated styrenes) having the following general formula: EMI5.1 wherein n is a number in the range of from about 750 to about 7,500, X is halogen, such as Cl, Br or I, and wherein the molecular weight distribution of the polymers in the composition results in a dispersivity EMI5.2 of from 1.5 to 2. 5. The poly(halogenated styrene) compositions of the present invention generally have a molecular weight (bow) of from about 1 x 10 to about 1 x 106. Representative halogenated polystyrenes included in the above formula, are, for example, poly(4-chlorostyrene), poly(4-bromostyrene) and poly(4-iodostyrene). Of these, the most preferred for the purposes of this invention is poly (4-chlorostyrene). Preferably, the polymers of the present invention have a molecular weight in the range of from about 250,000 to about 700,000 and a dispersivity in the range of from about 1.7 to about 2.2 and most preferably about 2. In accordance with the present invention, it has been discovered that the hereinbefore-identified poly(halogenated styrenes) must be prepared by free radical solution polymerization of the monomer in order to achieve the negative resist characteristics desired herein. In contrast to preparing the halogenated polystyrene polymers by bulk polymerization, solution polymerization is believed to result in forming less branching. Branching is deleterious since it may cause swelling of the cross-linked portions on development. Moreover, in the preparation of the poly(halostyrenes) herein, it has been found advantageous to carry out the solution polymerization at a slow rate and at a low temperature. These conditions are believed to result in less head-to-head linkages, which are weak polymer links and which would readily break down on irradiation. More particularly, using para-chlorostyrene monomer for exemplification purposes, the preparation of the new class of negative resist polymers herein involves initially purifying the chlorostyrene monomer, e.g. by fractional distillation, to remove deleterious impurities. Since it is usually customary to add polymerization inhibitors, such as 3,5-di-tertbutyl catechol, to monomeric materials to prevent polymerization during storage, these inhibitors must be removed. Column chromatography, for example, using activated alumina, is one method which can be used for this purpose. Degassing of the monomer, to remove dissolved air/02, for example, by freeze-thawing under vacuum in liquid nitrogen has also been found advantageous since the presence of small amounts of oxygen in the monomer may result in oxygen being present in the backbone of the polymer, which also causes undesirable weak linkages in the chain. The monomer is then ready to be slowly solution polymerized in accordance with the present invention. Specifically, the monomer is diluted with Åa low chain transfer coefficient solvent, such as for example, cholorbenzene, flurobenzene, perflurobenzene and the like and permitted to polymerize at, for example, a temperature of 600C - 100 C, for a time of, for example, from about two hours to about one month. The time for polymerization is dependent upon several factors including temperature and/or the presence of a polymerization initiator. For instance, to obtain satisfactory polymer for the purposes of this invention, in the absence of an initiator and at temperatures of about 65 C, the polymerization is permitted to proceed for about one month. On the other hand, higher temperatures e.g. 100 C and the presence of initiator, e.g. azo compounds, such as l,l1-bis aziosobutyronitrile, the polymerization may be terminated in about two hours. Other initiators, which may be utilized in the solution polymerization contemplated herein, include, for example, organic peroxides, such as benzoyl peroxide, tert-butyl-hydroperoxide, hydrogen perioxide, and peracetic acid. Generally, if used, the initiator is employed at levels of from about 0.05 to about 0.20 mole %. After the polymerization reaction is terminated, the reaction solution is fractionated, via differential solubility of molecular weights in various solvent mixtures, to remove undesired low molecular weight polymer and to isolate the polymeric fraction having a molecular weight of from about 1 x 10 to 1 x 106 and a dispersivity of from about 1.5 to about 2.5. Solvents useful for fractionation of this sort include, for example, methyl ethyl ketone and methanol. In forming the negative resist, the halogenated polystyrene is generally applied as a solution, such as, for example, in cyclohexanone, to a substrate, such as, for example, a silicon wafer and spin coated. To obtain the desired thickness on the support, the spin coating is usually carried out at from about 1,000 RPM to about 10,000 RPM. The coated substrate is prebaked for example at 160 C i 50C for about 15 minutes. Then an electron beam or other source of irradiation is caused to sweep or flood the surface of the polymer on the substrate in the desired pattern to form a negative resist by imparting sufficient energy to cross-link the polymer, thereby making the irradiated portions insoluble in certain solvents. Exposure dosages for the purposes of this invention are generally in the range of from about 1 to about 5 ijc/ctr2 with electron beams, depending on the molecular weight and MWD of the polymer. With ultraviolet light (us), the dosage is in the range of 2 about 20-60 mJ/cm at wavelengths from 200-260 nm. Also, with X-ray irradiation at 8-12 angstroms wavelength, a dose of 20-60 mJ/cm2 is required. After the electron beam, UV or X-ray exposure is completed, the resist is subjected to a solvent, such as methylisobutyl ketone, 2-ethoxyethanol and the like and mixtures thereof and the non-irradiated portion of the resist is removed, thereby leaving openings in the resist that correspond to the desired pattern. If this pattern is to be RIE etched, the resist shows superior resistance due to the fact that it etches at 508 of the rate of PMMA in CF4, H2 2 and CF4/10% H2 RIE plasmas. In order that those skilled in the art may better understand how to practice the present invention, the following examples are given by way of illustration and not by way of limitation. EXAMPLE 1 Inhibitor (3,5-di-tert-butyl-cathecol) is removed from 4-chlorostyrene by adsorption on an activated alumina column. To a round bottom flask is added 192 ml of monomer and 350 ml of chlorobenzene (about 1/2 molar) and 0.22g of benzoyl peroxide (0.1 wt%). The solution is degassed by bubbling N2 with stirring for two days. The flask is then heated to 650C (f 0.5 C) with stirring, under N2 for 6 days (144 hrs). After precipitation, polymer yield is 140g (%60%), Mn = 115,000, Mw = 430,000, D = 3.75. Fractionation is effected as follows: Add 100g of the poly-4chlorostyrene to 1000 ml methyl ethyl ketone. After dissolution, add dropwise (w/vigorous stirring) 250 ml methanol. Stir for 2 hours after addition. Separate and precipitate the resultant two layers. Low fraction: Mn = 46,000, Mw = 102,000, D = 2.2 High fraction: Mn = 138,000, Mw = 275,000, D = 2.0 The high fraction is especially useful as a negative resist in accordance with the present invention. EXAMPLE 2 Procedure is similar to Example 1 with the following variations. The flask contains 110 ml of 4-chlorostyrene, 200 ml chlorobenzene, and no initiator. The temperature and degas conditions are the same. Time for polymerization is 26 days. Polymer yield is 80g (X65%), Mn = 208,000, Mw = 500,000, D = 2.4. Two fractionations are performed (as per Example 1) the highest fraction is saved. Poly-4-chlorostyrene is maintained as a 10% solution in MEK, with the following MeOH/MEK ratios: fraction #1 = 0.25; fraction #2 = 0.22. The highest fraction (#2), Mn = 390,000R, Mw = 680,000K, D = 1.75 is especially useful as a negative resist in accordance with the present invention. EXAMPLE 3 Preparation of Negative E-beam (UV/X-Ray Resist) Poly(4-chlorostyrene), (high fraction), prepared in accordance with Example 1, is dissolved in cyclohexanone to produce an 11% solution. A silicon wafer (treated with a drop of hexamethyldichlorosilane) is spin coated (at about 5000 RPM) with the 118 polymer solution to obtain about o 7000A thick film after bake. The coated silicon support is prebaked at 1600C + 50C for 15 minutes and exposed to an electron beam at a base 2 dose of 5 > coul/cm . The resist is developed by dipping the irradiated coated support in a solution of 4 parts methylisobutyl ketone (MIBK) to 3 parts 2-ethoxyethanol for at least 3 minutes, then dipping in 2-ethoxyethanol, without allowing to dry for 30 seconds and then spraying with 2-ethoxyethanol for 5 seconds. The so treated wafer is then blow dried with nitrogen gas, and the resist pattern is ready for further processing. If hardening is desired, the wafer is exposed to a deep Uv source (low pressure mercury lamp), prior to the next processing step, for 3-5 minutes and is posthaked at 160-200 C.

Claims: 1. A poly(halostyrene) having the general formula EMI10.1 wherein X is chlorine, bromine or iodine, n represents a number in the range of from about 750 to about 7,500, the molecular weight (Mw) of the polymers being in the range of from 1 x 10 to 1 x 106 and the dispersivity EMI10.2 of the polymers being in the range of from 1.5 to 2.5. 2. A poly(halostyrene) as claimed in claim 1, in which X is chlorine, Mw is from 250,000 to 700,000 and said dispersivity is from 1.7 to 2.2. 3. A method of preparing poly(halostyrene) as claimed in claim 1, which comprises polymerizing halostyrene monomer in a low chain transfer coefficient solvent at a temperature in the range of from 600C to 100 C for a time of from 2 hours to 1 month and subsequently isolating the polymeric fraction having a molecular weight (Mw) in the range of from 1 x 10 to 1 x 106 and a dispersivity EMI10.3 in the range of from 1.5 to 2.5. 4. A method as claimed in claim 3, in which said halostyrene monomer is 4-chlorostyrene. 5. A method as claimed in claim 3 or 4, in which said halostyrene monomer is degassed prior to polymerization. 6. A method as claimed in any one of claims 3 to 5, in which said low chain transfer coefficient solvent is selected from the group consisting of chlorobenzene, fluorobenzene and perfluorobenzene. 7. A method of forming a negative resist mask comprising the steps of coating a film of a poly(halostyrene) as claimed in claim 1 or 2, on a substrate, exposing said film to electron beam, X-ray or UV radiation in a predetermined pattern to cause the irradiated portion of the poly((halostyrene) to cross-link and dissolving the uncross-linked portion of said poly(halostyrene) with a solvent which dissolves and removes the uncross-linked polymer, thereby leaving said cross-linked portion of said poly(halostyrene) on said support with openings in a desired pattern. 8. A method as claimed in claim 7, in which the coated substrate is heated at a temperature in the range of from 120 CC to 2000C for 15 to 60 minutes prior to exposure. 9. A method as claimed in claim 7 or S, in which said solvent is a mixture of methylisobutyl ketone and 2-ethoxyethanol.

Inventors: HATZAKIS, MICHAEL, LIUTKUS, JOHN JOSEPH, PARASZCZAK, JURIJ ROSTYSLAV, SHAW, JANE MARGARET
Applicants: INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1982
Main classification: C08F12
IPC codes: G03F7, H01L21, C08F12