Abstract:
PCT No. PCT/JP78/00022 Sec. 371 Date July 7, 1979 Sec. 102(e) Date July 2, 1979 PCT Filed Nov. 6, 1978 PCT Pub. No. WO79/00284 PCT Pub. Date May 31, 1979 
     A cross-linkable positive-working ionizing radiation-resist or ultraviolet ray- resist polymer composition including: in polymerized form, 
     (a) units derived from a methacrylic acid ester of the formula: 
     
       CH.sub.2 ═C(CH.sub.3).COOR 
     
      where R is an alkyl or haloalkyl group having from 1 to 6 carbon atoms, benzyl or cyclohexyl, 
     (b) units derived from a monoolefinically unsaturated carboxylic acid from 3 to 12 carbon atoms and having from 1 to 3 carboxyl groups, and 
     (c) units derived from methacrylic acid chloride. 
     The amount of the units (c) in the polymer composition is such that the number of moles of the units (b), multiplied by the number of the carboxyl group or groups in each of the units (b) ranges from about 1 to about 20% based on the total number of moles of the units (a), (b) and (c); the amount of the units (c) ranges from about 0.05 to about 3.0% by mole based on the total number of moles of the units (a), (b) and (c); and the molar ratio of the units (b), multiplied by the number of the carboxyl groups in each of the units (b), to the units (c) is greater than 2/1 but less than 250/1.

Description:
DESCRIPTION 
     BACKGROUND OF THE INVENTION 
     This invention relates to a cross-linkable positive-working ionizing radiation-resist or ultraviolet ray-resist polymer composition and to a method of forming a positive resist pattern on a substrate using the resist polymer composition. 
     Polymethyl methacrylates have heretofore been widely used as positive-working resists in electron or X-ray lithography. Although the polymethyl methacrylate resists exhibit a satisfactory resolution, their sensitivity to radiation is poor. In other words, there is only a slight difference in solubility between irradiated regions and non-irradiated regions, particularly at low ionizing radiation exposures. Accordingly, it has eagerly been desired to provide polymeric resist materials exhibiting a resolution approximately similar to and a sensitivity far greater than those of the conventional polymethyl methacrylate resists. 
     In order to provide methyl methacrylate polymer resists of improved sensitivity, it has been proposed to copolymerize methyl methacrylate with a sensitivity-enhancing acrylic monomer such as hexafluorobutyl methacylate. However, such a methyl methacrylate copolymer resist is still unsatisfactory in that it has an undesirably low softening point and a poor thermal resistance compared with conventional polymethyl methacrylate resists. 
     In U.S. Pat. No. 3,981,985, a mixture comprised of (a) a copolymer of a monoolefinically unsaturated carboxylic acid, such as methacrylic acid, and monoolefinically unsaturated compound, such as methyl methacrylate, and (b) a copolymer of a monoolefinically unsaturated carboxylic acid chloride, such as methacrylic acid chloride, and a monoolefinically unsaturated compound was also proposed for use as a polymer resist. The proportions of the two copolymers, (a) and (b), in this mixture are such that the carboxylic acid and the carboxylic acid chloride are essentially stoichiometric. When a resist coating of this mixture is heated, carboxylic acid anhydride cross-links are formed, and thus the resist coating becomes thermally resistant. However, the sensitivity of this resist coating is still not completely satisfactory. 
     SUMMARY OF THE INVENTION 
     The main object of the present invention is to provide positive-working ionizing radiation-resist or ultraviolet ray-resist polymeric materials which exhibit enhanced sensitivity as well as good thermal resistance, contrast and resolution. 
     The other objects and advantages of the present invention will be apparent from the following description. 
     One aspect of the present invention provides a cross-linkable positive-working ionizing radiation-resist or ultraviolet ray-resist polymer composition, comprising, in polymerized form, 
     (a) units derived from a methacrylic acid ester of the formula: 
     
         CH.sub.2 =C(CH.sub.3).COOR 
    
      wherein R is an alkyl or haloalkyl group having from 1 to 6 carbon atoms, a benzyl group or a cyclohexyl group, 
     (b) units derived from a monoolefinically unsaturated carboxylic acid having from 3 to 12 carbon atoms and having from 1 to 3 carboxyl groups, and 
     (c) units derived from methacrylic acid chloride; 
     the amount of the units (b) being such that the number of moles of the units (b), multiplied by the number of the carboxyl groups in each of the units (b) range from about 1 to about 20%, based on the total number of moles of the units (a), (b) and (c); the amount of the units (c) ranging from about 0.05 to about 3.0% by mole based on the total number of moles of the units (a), (b) and (c); and the molar ratio of the units (b), multiplied by the number of the carboxyl groups in each of the units (b), to the units (c) being greater than 2/1 but less than 250/1. 
     The main feature of the cross-linkable positive resist polymer composition of the present invention resides in the fact that the proportion of the units (b) (derived from the monoolefinically unsaturated carboxylic acid) to the units (c) (derived from methacrylic acid chloride) differs widely from the stoichiometric proportion, i.e., the molar ratio of the units (b), multiplied by the number of the carboxyl groups in each of the units (b), to the units (c), is larger than 2/1. The unexpected advantage of the present invention resides primarily in the fact that its sensitivity is far more enhanced than the sensitivity of the positive resist polymer composition described in U.S. Pat. No. 3,981,985, which contains substantially stoichiometric amounts of monoolefinically unsaturated carboxylic acid units and monoolefinically unsaturated carboxylic acid chloride units. 
    
    
     BRIEF DESCRIPTION OF THE FIGURE 
     The drawing is a graph showing the relationships between the % weight loss of different polymer resists and the heating (i.e., prebaking) temperature thereof (°C.). 
    
    
     DETAILED DESCRIPTION 
     The positive resist polymer composition of the present invention is in the form of either a terpolymer comprised of the methacrylic acid ester units (a), the unsaturated carboxylic acid units (b) and the methacrylic acid chloride units (c), or a blend of at least two polymers each of which is comprised of at least one of the units (a), (b) and (c). Preferably, the polymer composition is either the terpolymer of the units (a), (b) and (c), or a blend of a copolymer comprised of the units (a) and (b) with a copolymer comprised of the units (a) and (c). 
     When a coating of either of the above mentioned polymer composition is applied to a substrate and heated, dehydrochlorination occurs between the carboxyl groups and the acid chloride groups, and a three-dimensional network of carboxylic acid anhydride cross-links is formed. This polymer network is insoluble in a solvent used as a developer. It is presumed that when the polymer network is irradiated with ionizing radiation or untraviolet rays, both the main chains and the cross-links of the polymer network are destroyed at the irradiated regions and the polymers are degraded into lower molecular weight polymers. These degraded lower molecular weight polymers are soluble in a developer solvent, and, when an irradiated pattern in the polymer resist is developed by using a solvent, the polymers in the irradiated regions are selectively removed so as to leave the positive resist pattern on the substrate. 
     The methacrylic acid esters used for the preparation of the positive resist polymer composition are esters of an alkyl or haloalkyl group having from 1 to 6 carbon atoms, a benzyl group and a cyclohexyl group. The methacrylic acid esters include, for example, methyl methacrylate, tert.butyl methacrylate, iso-propyl methacrylate, hexafluorobutyl methacrylate, hexafluoroisopropyl methacrylate, cyclohexyl methacrylate and benzyl methacrylate. These methacrylic acid esters may be used either alone or in combination. 
     The monoolefinically unsaturated carboxylic acids used are those which possess from 3 to 12 carbon atoms and from 1 to 3 carboxyl groups. They include, for example, methacrylic acid, acrylic acid, itaconic acid and cinnamic acid. These monoolefinically unsaturated carboxylic acids may be used either alone or in combination. The amounts of the monoolefinically unsaturated carboxylic acids are such that the number of moles of the carboxylic acids, multiplied by the number of the carboxyl groups in each carboxylic acid molecule, range from about 1 to about 20%, preferably from about 1 to about 10%, based on the total number of moles of all monomers used for the preparation of the polymer composition. When the amount of the carboxylic acids is too small, the positive resists have, when heated, an undesirably low degree of cross-linking and are thus relatively soluble in a solvent developer and poor with respect to thermal resistance. 
     The amount of methacrylic acid chloride may range from about 0.05 to about 3.0% by mole, preferably from about 0.05 to about 1.0% by mole, based on the total number of moles of the monomers used for the preparation of the polymer composition, but it should be such that the molar ratio of the above-mentioned unsaturated carboxylic acid, multiplied by the number of the carboxyl groups in the carboxylic acid molecule, to the methacrylic acid chloride is greater than 2/1 and less than 250/1, preferably ranging from about 5/1 to about 15/1. 
     When the amount of methacrylic acid chloride is too small, the positive resist exhibits, when heated, an undesirably low degree of cross-linking. When the above-mentioned molar ratio of the carboxylic acid to the methacrylic acid chloride is equal to or less than 2/1, the sensitivity of the positive resist is unsatisfactory. When this molar ratio is far less than 2/1, the sensitivity becomes lower than that of a conventional polymethyl methacrylate resist. In contrast, when this molar ratio exceeds 250/1, the positive resist exhibits, when heated, an undesirably low degree of cross-linking and hence is relatively soluble in a developer and poor with respect to thermal resistance. 
     In addition to the above-mentioned units (a), (b) and (c), the polymer composition of the present invention may contain, although unsually in amount of less yhan 50% based on the total weight of the polymer composition, units derived from other monoolefinically unsaturated monomers, provided that the polymer resists are not harmfully influenced. 
     The polymer composition of the present invention, which is in the form of either a terpolymer of the above-mentioned units (a), (b) and (c), or a blend of polymers, each containing at least one of the units (a), (b) and (c), may be prepared in a conventional manner. The molecular weight of the polymer composition may range from about 30,000 to about 1,000,000, preferably from about 30,000 to about 400,000, as determined by a gel permeation chromatography procedure. The ratio of the weight average molecular weight to the number average molecular weight may range from 1/1 to 4/1, preferably from 1/1 to 3/1. 
     A positive resist pattern may be produced on a substrate as follows. A solution of the polymer composition in a solvent such as, for example, 2-ethoxyethyl acetete, 2-methoxyethyl acetate or cyclohexanone, is coated on a substrate by using, for example, a spinner. Then the coating of the polymer composition is usually heated to a temperature of 140° to 220° C. for a period of 5 to 30 minutes. The optimum temperature and time period are approximately 200° C. and approximately 15 minutes, respectively. The polymer network resist so formed is irradiated with ionizing radiation such as electron ray, and X-rays or ultraviolet rays in accordance with a desired pattern until the acid-anhydride cross-links and the main chains are broken at the irradiated regions. The irradiated resist is developed by applying thereto a developer solvent, such as methyl isobutyl ketone, ethyl acetate or acetone so as to leave the positive resist pattern on the substrate. 
     The invention will be explained in more detail by the following illustrative examples, in which some of the characteristics of the positive resists were determined as explained. Irradiation was carried out in accordance with a vector scanning procedure by using an electron exposing apparatus (Cambridge Instrument EBMF-1). 
     Contrast &#34;γ&#34; was determined in accordance with the equation: 
     
         γ=0.5/[log(D.sub.0 /D.sub.0.5)], 
    
     wherein D 0  is the electron dose in C/cm 2  required to reduce the initial resist thickness to zero and D 0 .5 is the electron dose in C/cm 2  required to reduce the initial resist thickness to a half thereof. The initial resist thickness was 0.5 micron as measured after the coated resist was baked in order to form cross-links. 
     Sensitivity was expressed in terms of the electron dose (C/cm 2 ) required to reduce the polymer resist thickness from 0.5 microns to zero, at at least one part thereof, when the irradiated polymer resist was dipped in methyl isobutyl ketone or another liquid developer at a temperature of 20° C. for one minute. The smaller the electron dose, the greater the sensitivity. 
     Resolution was evaluated by determining the minimum possible size of each line and each space during the production of a parallel line resist pattern having lines and spaces of the same size, and further by determining the maximum possible height to width ratio of each linear ridge of the line pattern. 
     Thermal stability was evaluated in terms of the softening temperature, determined by using a scanning type electron microseope to observe the shape of the resist of a parallel line pattern while the resist was gradually heated in a nitrogen atmosphere. The softening temperature is defined as the lowest temperature at which the polymer resist loses its predetermined shape and starts to flow. Furthermore, thermal stability was evaluated in terms of the thermal decomposition temperature, determined according to a thermogravimetric analysis wherein a polymer resist specimen is heated in a nitrogen atmosphere at a rate of 10° C./min. The thermal decomposition temperature is defined as the temperature at which the weight of the specimen starts to be reduced. 
     EXAMPLE 1 
     94.5% by mole of methyl methacrylate (MMA), 5.0% by mole of methacrylic acid (MA) and 0.5% by mole of mechacrylic acid chloride (CIMA) were copolymerized by using a conventional solution polymerization procedure. The terpolymer obtained (M.W.=300,000) was dissolved in 2-ethoxyethyl acetate to obtain a 9% by weight solution. This solution was coated on a silicon substrate using a spinner rotating at 4,500 rpm. The thickness of this coating when dry was 0.6 microns. The coated substrate was next heated at a temperature of 200° C. for a period of 15 minutes. Then, the coated substrate was irradiated with electron rays by using an electron beam accelerator at an accelerating voltage of 30 kV. The irradiated substrate was dipped in n-propyl acetate at a temperature of 20° C. for one minute to obtain a resist of the line pattern (invention specimen 1). 
     For comparison purposes, similar positive resists were prepared from a terpolymer (M.W.=250,000, comparative specimen 1) made from a mixture of 99% by mole of methyl methacrylate (MMA), 0.5% by mole of methacrylic acid (MA) and 0.5% by mole of methacrylic acid chloride (ClMA); a terpolymer (M.W.=250,000, comparative specimen 2) made from a mixture of 97% by mole of methyl methacrylate (MMA). 1.5% by mole of methacrylic acid (MA) and 1.5% by mole of methacrylic acid chloride (ClMA); a terpolymer (M.W.=250,000, invention specimen 2) made from a mixture of 94.95% by mole of methyl methacrylate (MMA), 5% by mole of methacrylic acid (MA) and 0.05% by mole of methacrylic acid chloride (ClMA); and a conventional polymethyl methacrylate (PMMA, M.W.=300,000, comparative specimen 3). The procedures used in the preparation of these positive resists were similar to those mentioned above with respect to the specimen of the present invention, except that the polymethyl methacrylate resist (comparatve specimen 3) was developed at a temperature of 20° C. for one minute using a methyl isobutyl ketone/isopropyl alcohol mixture having a volume ratio of 1/3. 
     The characteristics of the positive polymer resists are shown in Table I, below. 
     
                                           TABLE I__________________________________________________________________________Specimen     Invention 1              Com. 1                   Com. 2                         Invention 2                               Com. 3  MMA   94.5  99.0 97.0  94.95 100Compostion  MA    5.0   1.0  1.5   5.0   0(mole %)  CIMA  0.5   1.0  1.5   0.05  0__________________________________________________________________________Contrast (γ)        6.4   5.5  5.5   5.0   3.1Sensitivity (C/cm.sup.2)        8 × 10.sup.-6              2 × 10.sup.-5                   3.5 × 10.sup.-5                         3 × 10.sup.-6                               1.6 × 10.sup.-Resolutionmin. size of line and        0.2   0.2  0.2   0.2   0.2space (μ)max. ratio of H/W        6     6    6     6     2.5Thermal resistanceSoftening temp. (°C.)        140   120  120   140   110Decomposition temp. (°C.)        330   300  300   330   250__________________________________________________________________________ 
    
     The polymer resists of the invention, and comparative specimens 1 and 2 were tested for their degrees of cross-linking as follows. Each polymer resist, coated on a substrate but not yet heated, was heated (i.e., prebaked) at various temperatures for 15 minutes in a nitrogen atmosphere, and then dipped in acetone at a temperature of 20° C. for two minutes. The weight loss of the polymer resist resulting from its immersion in the acetone was measured. The relationships between the % weight loss and the heating (prebaking) temperature (°C.) are shown in the accompanying drawing. In the drawing, curves a, b and c correspond to comparative specimen 1, invention specimen 1 and comparative specimen 2, respectively. 
     EXAMPLe 2 
     Following a procedure similar to that mentioned in SExample 1, a polymer resist pattern was produced wherein the following copolymer blend was used instead of the MMA/MA/ClMA terpolymer. The copolymer blend used was comprised of 50% by weight of a copolymer (M.W.=300,000) of 90.5% by mole of methyl methacrylate (MMA) and 9.5% by mole of methacrylic acid (MA) and 50% by weight of a copolymer (M.W.=250,000) of 98.0% by mole of methyl methacrylate (MM) and 2.0% by mole of methacrylic acid chloride (ClMA). 
     The characteristics of the polymer resist pattern are shown in Table II, below. 
     EXAMPLE 3 
     Following a procedure similar to that mentioned in Example 1, a polymer resist pattern was produced wherein the following terpolymer was used instead of the MMA/MA/ClMA terpolymer. The terpolymer used was prepared using a conventional solution polymerization procedure and was comprised of 91.4% by mole of benzyl methacrylate (BzMA), 8.1% by mole of methacrylic acid (MA) and 0.5% by mole of methacrylic acid chloride (ClMA). The terpolymer had a M.W. of approximately 250,000. 
     The characteristics of the polymer resist pattern are shown in Table II, below. 
     EXAMPLE 4 
     Following the procedure similar to that mentioned in Example 1, a polymer resist pattern was produced wherein the following terpolymer was used instead of the MMA/MA/ClMA terpolymer. The terpolymer was prepared using a conventional solution polymerization procedure and was comprised of 94.0% by mole of methyl methacrylate (MMA), 5.0% by mole of itaconic acid (IA) and 1.0% by mole of methacrylic acid chloride (ClMA). The terpolymer had a M.W. of approximately 200,000. 
     The characteristics of the polymer resist pattern are shown in Table II, below. 
     
                       TABLE II______________________________________Specimen    Ex. 2      Ex. 3      Ex. 4       MMA     45.2       MA      4.8    BzMA  91.4 MMA   94.0Composition MMA     49.0   MA    8.1  IA    5.0(mole %)    ClMA    1.0    ClMA  0.5  ClMA  1.0______________________________________Contrast (γ)       5.5        4.0        4.5Sensitivity (C/cm.sup.2)       1 × 10.sup.-5                  8 × 10.sup.-6                             1 × 10.sup.-5Resolutionmin. size of line and       0.2        0.3        0.2space (μ)max. ratio of H/W       6          5          6Thermal stabilitySoftening temp.(°C.)       140        130        140Decompositiontemp. (°C.)       330        320        335______________________________________