COMPOSITION FOR NEGATIVE PHOTORESISTS AND USE OF THE COMPOSITION

Abstract: Composition for negative photoresist comprising a mixture of a polymeric material (A) and one or more cationic photoinitiators (B) and eventually other in photoresists commonly used components such as solvents, sensitators, etc., wherein the material A consists of a homopolymer, copolymer of a mixture of polymers where acrylamide, methacrylamide or derivatives of these is a part of at least one of the polymers present and also comprising chemical groups which can self- or cocondense with other chemical groups under acid conditions, e.g. acid catalysis, and the material B consists of one or more cationic photoinitiators which can generate acid catalysts such as Brönsteds acids or Lewis acids and use of said negative photoresist are described.

Description: This invention refers to a negative resist-photoresist and its use in a process for making a negative resist-photoresist image. The process gives in a practical way a high sensitive (∿ 10 mJ/cm²), water and/or alcohol developable resist/photoresist which gives image structures usable within the semi conductor manufacturing, such as integrated or printed circuits. The development in this field has mainly been concentrated on new types of resist materials with improved sensitivity and/or improved resolution. One interesting way of improving sensitivity is the phe­ nomenon chemical activation, which means that an initial chemical event produces a substance, e.g. a catalyst capable of trigging a series of chemical reactions giving a multiple effect. This has pro­ ved to be a good working commercial method for resist materials, which either functions by an acid catalyzed hydrolysis (positive photo resist) or by a polymerisaton mechanism (free radical polyme­ risation). One disadvantage with these methods is the low sensiti­ vity and the need for organic solvents for developing. In the Swedish patents SE 368.097 and SE 395.971 compositions for negative photo resists based on at least one polymer, at least one photopolymerisable compound and at least one photoinitiator capable to photo cure by radical polymerisation are described. Such composi­ tions, however, have a relatively low sensitivity, meaning they need high energy for starting the polymerisation reaction. As most radi­ cal polymerisation systems these have a tendency for oxygen inhibi­ tion, which deleteriously influence the reaction process. They need also, however, soluble in water, a relatively high pH (addition of alkalies) in the developing bath. In the British patent GB 2.164.347 a photopolymerisable composition for printing plates is described, where the photochemical reaction is a radical polymerisation of an acrylamide which has been conden­ sed with a melamine derivative. Such systems have, however, low sen­ sitivity and show oxygen inhibition. The present invention is a new type of chemical activation, i.e. an acid catalyzed self- and/or cocondensation of polymers. These mate­ rials generate a negative image (negative photo resist) of a given pattern, where the parts exposed to radiation undergo a chemical re­ action, in this case a crosslinking of the polymer, while the unex­ posed parts remain chemically unchanged. The polymer/polymers which are used can be crosslinked by acid catalysts. The acid is generated by exposing the applied photoresist to radiation. The polymer/poly­ mers are soluble in water and/or various alcohols. The present invention makes possible compositions with high sensiti­ vity, i.e. which can be cured by relatively low energy (radiation dose), which are insensitive to the action of air oxygen, i.e. have no tendency for oxygen inhibition, and which give a good image reso­ lution and can be developed by water or alcohol solutions without the use of hazardous solvents. By the present invention a negative image of a given pattern can be made by the following steps: 1. A substrate is coated with a film of a polymer or mixture of polymers comprising acrylamide or metacrylamide or derivatives thereof as components, and a cationic photoinitiator. Especially compositions comprising the acrylamide derivative methyl acryl­ amide glycolate methylether (MAGME) have been used. 2. The film is exposed with a pattern whereby, in exposed parts, the cationic photoinitiator is activated and generates acid. 3. The film is heated to 50-120°C. In this step the exposed parts of the film become crosslinked by the acid formed and the heat. 4. The unexposed areas of the film are washed off by dissolving in a suitable solvent, whereby a negative image has been developed. As suitable solvents water and various alcohols can be used. 5. The substrate may be of organic or inorganic origin. The process can be carried out, as an example, directly on a semi conductor substrate or a semi conductor substrate coated with an etchable layer, such as an organic polymer planarization layer. Line widths between 1 micron and 10 mm have given excellent resolu­ tions and results for semi conductor application. The resist material and the process have many pronounced advantages compared with older methods. Especially the high sensitivity (in the size of 10 mJ/cm²) and the possibility to develop the material in water and/or various alcohols (such as ethanol, isopropanol). Radia­ tion crosslinking processes using the radical process previously known are sensitive to oxygen and must therefor be carried out in vacuum or inert atmosphere in order to protect the reactive radi­ cals. In the presention invention cationic photoinitiators, insensitive to oxygen, are used. Furthermore, the polymerisation and/or the crosslinking process is not sensitive to oxygen. This means that the process can be carried out under normal conditions without restrictions. The composition of the photoresist material can be varied in numerous ways. In order for the invention to function the photo resist material shall comprise at least A) one polymer or mixture of polymers comprising chemical groups which can self- or cocondense with other chemical groups under acid conditions, e.g. acid catalysis, and B) one or more cationic photoinitiators which can generate acid catalysts such as Brönsteds acids or Lewis acids. A) is a homopolymer, copolymer or mixtures of polymers. Preferably acrylic polymers and their derivatives are used,but generally all polymers from unsaturated monomers can be used. Mixtures of polymers can be mixtures of acrylicbased polymers and other freeradical- and/or condensationpolymerisation polymers. For the invention to function it is necessary that at least one polymer contains acrylamide or metacrylamide or derivatives of these as a monomer component, which can self- or co-condense with other functional groups. Preferably polymers containing the acrylamide derivative methyl acrylamide glycolate methylether (MAGME) are used. Structure see below). Examples of produced polymers are poly(MAGME), copolymers of MAGME and methylmetacrylate, n-butylacrylate, 2-hydroxy-ethylacrylate, 2-­ hydroxypropylmetacrylate, metacrylic acid, acrylic acid, acryloni­ trile, styrene, octene, allylethers, paratrimethylsilylstyrene. Examples of produced mixtures of polymers. MAGME methylmetacrylate polymer mixed with OH-functional acrylate polymers, polyesters, epoxyesters, epoxy resins, fatty acid modified polyesters (alkyds), polyurethanes, melamine resins, Si-containing polymers. The produced MAGME-containing polymers have been 100% converted to polymers and their properties have been investigated. Molecular weight and molecular weight distribution can be adjusted by the polymerisation initiator and its concentration, by the chain trans­ mitter, by the choice of monomers, by solvents and by the process conditions. The influence of the total MAGME concentration on the material has been investigated. The B-component is the cationic photoinitiator which can be any of several known within the art. These include, but are not limited to, triaryl-sulphoniumsalts with or without sensitator, diaryliodonium salts with or without sensitator,aryldiazoniumsalts, trihalomethyl triazenes, latent UV-deblockable sulphonic acids, aryl cycloalkylferric salts with and without sensitator, antracene. The different sensitators make it possible to use different spectral areas for the photoresist applications. As stated before an acid catalysis of the MAGME-polymer is necessary for the film/photoresist to crosslink. Paratoluenesulphonic acid (PTSA) has been used for studying the selfcondensation. For films of 20 microns on glass plate, at least 120°C curing temperature is necessary. If the catalyst is changed to oniumsalts which are UV-­ radiated, both the curing temperature and the time for crosslinking will be lower. This effect is obtained even at very small doses of radiation (∿ 10 mJ/cm²). Crosslinking temperatures between 50-120°C are obtained for the system with times down to a few minutes. Example 1 Preparation of polymers Methylacrylamide glycolate methylether (MAGME) (from American Cyanamid) was hot-filtrated and recrystallized from xylene (mp 70-­ 73°C). The other monomers were purified in an aluminium oxide column. The monomer para-trimethylsilylstyrene was synthetized from para­ chlorostyrene by Grignard-reaction with chlorotrimethylsilane. Azoisobutyronitrile (AIBN) was used as free-radical-initiator in all polymerisations. Carbon tetrabromide was used as chain-transmitter. The polymerisations were carried out at 60°C in a 1:1 mixture of toluene and n-butanol under N₂-atmosphere. The reaction lasted for 3.5 h and the polymer formed was precipitated in cold diethylether. Before further use of the polymer was dissolved and precipitated once more. The polymers are described in Tab. 1. Crosslinking experiments Para-toluenesulphonic acid (PTSA) (I), UV-deblockable sulphonic acid (II) and triphenylsulphoniumhexafluoroantimonate (III) have been used as acid catalysts in the crosslinking experiments (structural formulaes are shown below). Thick films (20 microns) were applied on glass plates from a methyl­ ethylketone solution (40 weight per cent solid polymer) and thin films (1 micron) were spin-applicated on a silicon plate from a cyclohexanone solution (50 weight per cent solid polymer). Radiation by an Oriel 82410 1000 W illuminator (Xenon-lamp). In table 2 the experiment parameters are summarized. In table 3 are the solubili­ ties of the investigated polymers shown. Example 2 The copolymer of MAGME and TMSiSTY from Example 1 is used with the cationic initiator triphenylsulphoniumhexafluoroantimonate (see Table 2). The mixture is dissolved in cyclohexanone (50 weight per cent polymer). 1 micron films have been spun on silicon plates. A precuring (100°C, 15 min.), which is not necessary, is made to eliminate rests of solvents. Exposure of an image with line width of 2 microns and a line space of 4 microns is made with Oriel 82410, 1000 W illuminator with dose 10 mJ/cm². Then postcuring at 100°C, 3 min. Development is made with isopropanol for 1 min. An excellent resolution is obtained. Example 3 Dryresist sheets with composition according to Example 1 are prepa­ red of a MAGME-polymer, a MAGME-styrene-polymer and a MAGME-p-tri­ methylsilylstyrene. The preparation of a dryresist sheet is made by centrifuging the photoresist on a 30 micron sheet of polyethyleneterephtalate, giving a photoresist thickness after drying at 70°C of 11.7 microns. By a laminating procedure with an equipment from Binding Corp., USA, a 25 micron copper foil on an epoxi sheet is coated with dryresist as described above. The prepared laminate with copper foil and photoresist is exposed with 12 mJ/cm² by a Xenon-lamp from Oriel Inc., USA. The image design has line width from 5 mm to 5 microns. The exposed photoresist is developed with deionized water. The lami­ nate is dried. Resist lines with dimensions true to original are obtained down to 5 microns line width. Example 4 A copolymer of MAG (methylacrylamide glycolate) and TMSiSTY with n 4.7x10⁴ and w/n∿1.9 has been used with the cationic initiator triphenylsulphoniumhexafluoroantimonate (see Table 2). The mixture is dissolved in cyclohexanone (50 weight per cent polymer). 1 micron films are spun on silicon plates. A precuring (100°C, 15 min.), which is not necessary, is made to eliminate rests of solvents. Exposure of an image with line width of 2 microns and a line space of 4 microns is made with Oriel 82410, 1000 W illuminator with dose 10 mJ/cm². Then postcuring at 80°C, 5 min. Development is made with isopropanol/water (1:1) for 1 min. An excellent resolution is obtained. Example 5 A copolymer of BAG (butylacrylamide glycolate) and TMSiSTY with n 4.5x10⁴ and w/n∿1.9 has been used with the cationic initiator triphenylsulphoniumhexafluoroantimonate (see Table 2). The mixture is dissolved in cyclohexanone (50 weight per cent polymer). 1 micron films are spun on silicon plates. A precuring (100°C, 15 min.), which is not necessary, is made to eliminate rests of solvents. Exposure of an image with line width of 2 microns and a line space of 4 microns is made with Oriel 82410, 1000 W illuminator with dose 7 mJ/cm². The postcuring at 80°C, 5 min. Development is made with isopropanol/water (1:1) for 1 min. An excellent resolution is obtained.

Claims: 1. A composition for negative photoresist comprising a mixture of a polymeric material (A) and one or more cationic photoinitiators (B) and eventually other in photoresists commonly used components such as solvents, sensitators, etc., characterized in that the material A consists of a homopolymer, copolymer of a mixture of polymers where acrylamide, methacrylamide or derivatives of these is a part of at least one of the polymers present and also com­ prising chemical groups which can self- or cocondense with other chemical groups under acid conditions, e.g. acid catalysis, and that the material B consists of one or more cationic photo­ initiators which can generate acid catalysts such as Brönsteds acids or Lewis acids. 2. A composition of claim 1, characterized in that MAGME, methylmet­ acrylamide glycolate methylether, is a part of at least one of the polymers in material A. 3. A composition of claim 1, characterized in that MAG, methylmetacrylamide glycolate, is a part of at least one of the polymers in material A. 4. A composition of claim 1, characterized in that BAG, butylmetacrylamide glycolate, is a part of at least one of the polymers in material A. 5. Use of a negative photoresist upon an organic or inorganic sub­ strate, characterized in that a composition according to any of the claims 1-4 is applied.

Inventors: GOTHE, SVEN, HULT, ANDERS, Göthe, Sven
Applicants: AB WILH. BECKER
Application year: 1987
Main classification: G03F7
IPC codes: C08L33, G03F7