Deposition of Polymer Films by Means of Ion Beams

The present invention is concerned with a process for depositing patterned polymer films for microlithography. In particular, it is concerned with such a process which is carried out using a beam of ions.

All ion beam lithographic processes consist of ion beam exposure of polymer films in order to reduce oxygen plasma etch rates of the films, and then dry development by oxygen plasma. In the original work of Kuwano and his coworkers, 20 KeV accelerated focused gallium ion beams were used to reduce oxygen plasma etch rates of exposed areas. See H. Kuwano, K. Yoshida and S. Yamazaki, Japanese J. Appl. Physics, 19(No. 10) L615 (1980). Wolf and his coworkers have been successful in obtaining submicron poly(methyl methacrylate) patterns using 40 KeV Si.sup.+ ions by flood exposure through a self-supporting mask. See I. Adesida, J. D. Chinn, L. Rathbun and E. D. Wolf, J. Vac. Sci. Technol., 212(2), July/Aug pp. 666-671 (1982). In these references, either specially designed ion beam guns or ion implantation systems are required. By implanting metallic ions like Si.sup.+, Ga.sup.+, the oxygen plasma etch rates of the resist films are reduced, thereby causing differentiation of the etch rates between exposed and unexposed areas, and resulting in dry development of polymer patterns.

G. Taylor and his coworkers used polymers films uniformly blended with monomer organosilicon for plasma-developed x-ray resists study. See G. N. Taylor, T. M. Wolf and J. M. Moran, J. Vac. Aci. Technol., 19(4) Nov./Dec. 872 (1981).

S. Hattori has reported post-grafting of trimethylvinylsilane onto poly(methyl methacrylate) after electron beam exposures. See S. Hattori et al, ACS Org. Coating & Plastic Chem. Report No. 33, Sept. 12-17, 1982, Kansas City meeting. Because an electron beam gun cannot operate at pressure as high as 1.times.10.sup.-4 torr, he had to transfer the exposed wafers to another chamber for the purpose of grafting.

According to the present invention, a patterned polymer film is deposited on a substrate by confining a monomer vapor in a closed chamber containing said substrate, pattern-wise exposing said vapor to a beam of ions and thereby causing deposition of a polymer film on said substrate, and then dry developing by etching said polymer film with oxygen plasma.

The present invention has a very big advantage over the prior art, in which it was not possible to carry out all the steps of the process in a single chamber. In the present invention, however, because ion beam guns can operate at a pressure as high as 1.times.10.sup.-4 to 1.times.10.sup.-3 torr, simultaneous ion beam exposure and deposition are possible.

In the present invention, any polymerizable organic vapor can be used as the monomer. The most preferred monomers are organosilicon compounds such as tetravinylsilane, and styrene or other phenyl vinyl compounds. Other useful monomers include, for example, acrylonitrile, methacrylonitrile, methyl methacrylate and phenol.

The most preferred ion beam is of hydrogen ions, i.e., protons. Other ion beams such as He.sup.30, Ar.sup.30 and others can also be used.

The polymer films may be deposited on many different substrates. For example, the substrate may be another polymer, such as poly(methyl methacrylate), or a novolac resin. Of particular interest is the use of a silicon wafer as the substrate. Resist patterns of 0.5 .mu.m thickness with sloped wall profiles (due to a slanted gun position relative to the wafer position) have been obtained directly from styrene vapor. This technique eliminates several processes required in conventional lithographic processes, like formulation of resist solutions, spin-coating, prebaking, developments, etc. Besides, no problems of adhesion, cracking, abrasion, etc. appear. The polymer films thus obtained are exceedingly CF.sub.4 plasma and reactive ion etch resistant, which is of advantage in further microcircuit fabrication processes.

Thin polymer films of organometallic compounds, including organosilicons, are deposited on polymer films like PMMA, novolac resins or any other kind of polymer films. The deposition is assisted by low energy ion beam exposure, like 5 KeV H.sup.30 beams. Ion beams are energetic enough to generate radicals from any kind of organic compounds. For lithographic applications, however, organometallic compounds like organosilicons, organotins and other volatile organometallic compounds are preferred.

Any conventional ion beam gun can be used for the process of the present invention, for example, one from Commonwealth Scientific Composition.

A poly(methyl methacrylate) substrate was placed inside a closed chamber containing tetravinyl silane at a pressure of 1.times.10.sup.-1 torr. A Magmiller ion gun from Commonwealth Scientific Corp., Alexandria, Va., 2-30 gun was used with a hydrogen pressure through the gun of 3.times.10.sup.-5 torr. The exposure was conducted at 1.times.10.sup.-4 torr, with the gun directed toward the substrate through a gas inlet tube. The ion beam exposure and simultaneous deposition were carried out for 30 minutes with the ion beam current of 10 to 50 .mu.A, amounting to ion dose of 1.times.10.sup.-4 C/cm.sup.2. This dose can be reduced significantly either by increasing the silane pressure, or by using postgrafting after the ion beam exposure.

Any kind of polymer film can be used as long as it is etched away in oxygen plasma. PMMA results in positive tone patterns in wet development after ion beam exposures. Negative tone polymer patterns are obtained in dry oxygen plasma or RIE development with our present technique; the deposited poly(tetravinylsilane) film acted as barrier layer for oxygen plasma.

Polymer patterns are developed in either oxygen plasma or oxygen RIE; in our RIE condition the oxygen pressure was 100 micron, with 4 sccm flow, with a bias potential applied to the wafer of -250 V; the etch time was 20 minutes for 2 .mu.m thickness.

In another preferred embodiment of the present invention, a silicon wafer is used as the substrate for the deposition of a film of polystyrene. The mask used was copper grid wired laid in two levels; each level was separated at least 200 .mu.m from another; by placing them at slightly off-set position fairly narrow lines could be defined.

Polymer styrene films with smooth surfaces were deposited in H.sup.+ beam assisted polymerization in well defined patterns.