High rate resist polymerization method

An improvement in the method of forming polymerization resists by directing high energy particles such as electron beams along a path across a vacuum chamber and onto polymerizable molecular species at a substrate surface with sufficient energy to polymerize the polymerizable molecular species in situ is provided, comprising maintaining a chamber-isolated relatively higher pressure layer of polymerizable molecular species vapor locally at the substrate surface during, e.g. electron beam exposure to form the resist while maintaining the beam path free of polymerizable molecular species during beam traverse of the chamber. Polymerization resist generation apparatus is also provided comprising a high energy particle, e.g. electron beam source including an electron beam gun and a vacuum chamber therebeyond, means adapted to support a substrate having a surface on which a resist is to be generated in electron beam exposed relation, means defining a closed volume between the supported substrate and the electron beam source, and means to introduce polymerizable molecular species vapor into the closed volume for electron beam exposure and polymerization in situ on the substrate surface.

TECHNICAL FIELD 
This invention has to do with the manufacture of integrated circuits, 
particularly very large scale integrated circuits, and more particularly 
with improvements in the generation of resists during integrated circuit 
manufacture, for high precision resolution of circuit patterns. The 
invention is particularly concerned with resist formation by 
polymerization techniques, and with improvements in known resist formation 
techniques which enable enhanced speed, accuracy of placement and 
minimization of linewidth. 
Background Art 
The delineation of circuit patterns by the formation and selective removal 
of resist masks is well known and widely practiced. One form of resist is 
the polymerization resist in which a polymerizable molecular species is 
selectively irradiated at a surface to form a patterned thin film. It is 
known to form polymerization resists by electron beam or other high energy 
particle exposure of the polymerizable molecular species in situ on the 
substrate to be masked. The presence of polymerizable molecular species 
accidently in irradiation zones has been known to cause the inadvertent 
formation of so-called "contamination" resists. See, for example, 
"Electrode Contamination in Electron Optical Systems", K. M. Poole, 1953 
pp. 542-547; "Direct Measurement of Contamination and Etching Rates in an 
Electron Beam", R. F. Egerton, et al, 1976, J. Phys. D: Appl. Phys. Vol. 
9, pp. 659-663. The deliberate formation of such resists has been 
practiced by applying a coating of the polymerizable molecular species 
onto the substrate surface, and by exposing the coated substrate to 
electron beam energy. Additionally, use has been made of the deliberate 
introduction of increased concentrations of volatile polymerizable species 
into the vacuum ambient required for electron or ion beam resist exposure. 
See, for example: "Formation of Thin Polymer Films by Electron 
Bombardment" 1960 J. Appl. Phys. Vol. 31 No. 9, pp. 1680-1683; and 
"Electron-beam Fabrication of 80A Metal Structures" A. N. Broers et al 
1976 Applied Physics Letters, Vol. 29, No. 9, pp. 596-598. 
A difficulty with the use of the vapor method has been the slowness of rate 
of film formation owing, at least in part, to the absence of adequate 
concentrations of vapor; where increased concentrations of vapor are used 
to increase rate, there is, however, erratic, unacceptable distortion of 
the electron beam as it passes through the vapor. 
A further difficulty with known polymerization resist techniques for 
pattern delineation purposes lies in the scattering of the high energy 
particle beam by the vapor, which precludes high rate polymerization of 
vapor by electron beam polymerization in situ on the substrate surface 
without unacceptable sacrifice of pattern resolution resultant from beam 
distortion by the vapor being polymerized. 
DESCRIPTION OF THE INVENTION 
It is, therefore, an object of the present invention to provide 
improvements in the generation of polymerization resists with high energy 
particles, such as electron beams and ion beams, both in rate of film 
formation for more economical processing and manufacture, and in precision 
of pattern and minimum linewidths through minimum distortion of the high 
energy particle beam. It is a further object to provide method and 
apparatus for polymerization resist generation in which the concentration 
of vapor is increased selectively in the vicinity of the substrate 
surface, while maintaining the beam path free of polymerizable molecular 
species vapor causing distortion of the beam. It is another object to 
increase the partial pressure of polymerizable molecular species vapor at 
the substrate surface locally by confining the vapor there beyond the 
vacuum chamber in which the ion or electron beam is generated and directed 
and at a pressure conducive to rapid film formation and higher than the 
pressure elsewhere within the vacuum chamber. It is a highly particular 
object to provide method and apparatus in which a membrane substantially 
transparent to electron or ion beams confines the polymerizable vapor 
adjacent the substrate surface enabling rapid polymerization while 
preserving the integrity of the beam path chamber, and in a manner that 
the beam is not distorted in the chamber or in the substrate 
surface-adjacent layer of polymerizable molecular species. 
These and other objects of the invention to become apparent thereinafter, 
are realized in accordance therewith in the method of forming 
polymerization resists, which includes directing high energy particles 
along a path across a vacuum chamber and onto polymerizable molecular 
species at a substrate surface with sufficient energy to polymerize the 
polymerizable molecular species in situ, through the improvement 
comprising maintaining a chamber-isolated relatively higher pressure layer 
of polymerizable molecular species vapor locally at said substrate surface 
during high energy particle exposure to form the resist while maintaining 
said particle path substantially free of polymerizable molecular species 
during beam traverse of said chamber. 
Typically, the polymerizable molecular species vapor layer is at a vapor 
pressure which at a given thickness is normally disruptive of electron or 
ion beam travel, and the method therefore includes maintaining the 
polymerizable molecular species layer at a lesser thickness than the given 
thickness; the polymerizable molecular species vapor layer is at a 
pressure not less than about 10.sup.-4 Torr. during electron beam 
exposure; the polymerizable molecular species vapor layer is selectively 
communicated with a polymerizable vapor supply beyond the chamber; and an 
electron or ion beam is generated as the source of the high energy 
particles. 
In preferred embodiments, the method includes partitioning the chamber into 
two zones including a relatively lower pressure chamber first zone which 
is maintained at a pressure less than 10.sup.-4 Torr., and a relatively 
higher pressure chamber second zone containing the polymerizable vapor in 
chamber first zone isolated relation; defining a wall of the chamber 
second zone with the substrate surface portion; extending an electron or 
ion beam transparent, polymerizable molecular species impermeable, 
self-supporting membrane across the chamber parallel to the substrate 
surface and spaced therefrom a distance to define a second wall of the 
chamber second zone in chamber partitioning relation; generating an 
electron or ion beam as the source of the high energy particles; and 
effecting the mentioned partitioning of the chamber with an electron or 
ion beam transparent membrane extended across the chamber above the 
substrate. 
As in other embodiments, where the polymerizable molecular species layer is 
at a vapor pressure of about 10.sup.-4 to 1 Torr. which at a given 
thickness for the polymerizable molecular species is normally disruptive 
of electron or ion beam travel, the method includes maintaining the 
polymerizable molecular species layer at a lesser thickness than the given 
thickness, use of a volatile polymerizable molecular species which is an 
electron or ion beam cross-linkable polymer precursor, selectively 
communicating the chamber second zone with a polymerizable vapor supply 
beyond the chamber first and second zones, defining the lower wall of the 
chamber second zone with the substrate surface, and extending the membrane 
across the chamber parallel to the substrate surface and spaced therefrom 
to define the upper wall of the chamber second zone in chamber 
partitioning relation. 
The invention contemplates apparatus for carrying out the method in the 
form of a high energy particle polymerization resist generation apparatus, 
comprising a high energy particle source and a vacuum chamber therebeyond, 
and within said vacuum chamber: means adapted to support a substrate 
having a surface on which a resist is to be generated in high energy 
particle exposed relation, means defining a closed volume between the 
supported substrate and the source, and means to introduce polymerizable 
molecular species vapor into the closed volume for high energy particle 
exposure and polymerization in situ on the substrate surface. 
In preferred embodiments, the apparatus includes also an electron beam gun 
source of electrons as the high energy particles; the closed volume 
defining means comprises an electron beam transparent, polymerizable vapor 
impermeable membrane and support structure therefor relative to the 
substrate whereby a closed volume is maintained between the membrane and 
the substrate; and the membrane comprises a thin film of synthetic organic 
polymer, metal, semiconductor, or inorganic dielectric which is 
self-supporting above the substrate surface exposed to the electron beam.

Preferred Mode 
Bombardment of organic polymerizable molecular species by an electron beam, 
an ion beam, or other high energy particles results in polymerization, 
probably by a free radical polymerization mechanism, of ambient species 
present at or on the surface intended to be bombarded by the beam. 
Advantage can be taken of this phenomenon to form pattern resists by 
selectively polymerizing a polymerizable molecular species at a surface on 
which a pattern is desired. To do this a vapor of polymerizable molecular 
species is provided in the exposure zone and upon irradiation the 
polymerizable molecular species polymerizes upon the substrate in the 
pattern determined by the beam direction and travel. It is evident that 
the rate of resist formation by this technique is severly limited by the 
rate at which the polymerizable molecular species arrives at the surface 
of the substrate as well as by the kinetics of polymerization of the 
particular polymerizable molecular species by the particular beam, which 
is a function of the sensitivity of the polymerizable molecular species 
and the current density of the beam. 
The present method and apparatus focus on increasing relatively the partial 
pressure of the polymerizable molecular species vapor at the substrate 
surface while keeping the vapor species from the vacuum chamber which the 
beam traverses, for the dual purpose of avoiding distortion of the beam 
resultant from collision with the large constituent molecules, and of 
maximizing polymerization rate through concentration of polymerizable 
species at the surface on which the resist pattern is to be formed. 
Rate of polymerization, assuming a given beam current density and a 
particular polymerizable molecular species, varies with molecular arrival 
rate at the substrate which in turn is dependent on the polymerizable 
molecular species partial pressure. If, for a given molecular species 
being bombarded at a surface in an electron beam system at pressures of 
about 10.sup.-6 Torr., one monolayer/second of resist is formed, 
increasing the partial pressure of the polymerizable molecular species to 
1 Torr. may increase the rate to 10.sup.4 monolayers/second, assuming 
sufficient beam current to maintain such a rate. Thus, an exposure 
requiring one hour at 10.sup.-6 Torr. could be accomplished in a fraction 
of a second at 1 Torr. 
It is not desirable to use pressures much greater than about 10.sup.-6 in 
an electron beam vacuum chamber since at higher pressures, operation is 
erratic and unreliable. Thus polymerization resist formation in previously 
known apparatus has not been practical at rates greater than the 
relatively slow rate noted. 
Through the use of a partitioned apparatus, however, as taught herein, the 
vacuum chamber is maintained generally free of polymerizable molecular 
species vapor, but the region immediately adjacent the substrate surface 
where the polymerization is desired is rich with such polymerizable 
molecular species vapor. A membranous film impermeable to the 
polymerizable molecular species vapor, and impervious to chemical attack 
thereby, and which is both transparent to the high energy particles, e.g. 
the beam electrons or ions, and self-supporting by itself or in 
conjunction with another suitable material connected to it, is used to 
partition the chamber. Among suitable polymerizable molecular species are 
low molecular weight organic compounds containing aliphatic or aromatic 
unsaturation, and polymers thereof, e.g. polyparaxylylene (Trademark 
Paralene) silicones such as methylphenylpolysiloxane, hydrocarbon and 
silicone lubricating oils, polyimides and the like. Typically useful 
materials are referred to in the articles identified above which are 
incorporated herein by this reference. 
The polymerizable molecular species preferably have a vapor pressure in the 
10.sup.-4 to 1 Torr. range at room temperature and are polymerization 
responsive to high energy particle exposure, i.e. exhibit a higher rate of 
polymerization than scission at exposure levels, and which particularly 
include negative resist materials and silicone oils. 
An electron beam is the preferred source of high energy particles. Other 
suitable high energy particles include protons and other ions. Current 
density levels are chosen to match the rate of arrival of the 
polymerizable molecular species at the substrate target, as a function of 
the particular species selected, the specie's vapor pressure, the 
temperature of the substrate, electron energy levels, and other factors, 
including the desirability of minimizing damage to the substrate. In 
general, higher substrate temperatures increase species mobility on the 
substrate surface, increasing the rate of polymerization. 
With reference now to the FIGURE, a conventional electron beam gun (not 
shown on this FIGURE) directs a beam 12 along a predetermined path 
indicated by the several dotted lines across a vacuum chamber 14 in a 
pattern desired for formation of the polymerization resist and at an 
adequate current density for the purpose. The beam 12 is directed at 
substrate 16 which is typically a silicon semiconductor at a particular 
stage of processing where it is desired to mask off one or more areas of 
the substrate, or of a predeposited coating on the substrate, for purposes 
of selective etching or deposit of additional material in a predetermined 
pattern on the substrate. The substrate is supported at the lower end of 
the vacuum chamber 14 by a frame 18 having base 20 and sidewall 22, 
provided with opposed annular grooves 24, 26 into which O-ring seals 28, 
30 are snugly fitted. Substrate 16 is held between the base 20 and 
sidewall 22 by means not shown in sealed relation by virtue of the O-rings 
28, 20. Frame wall 22 has an interior shoulder 32 which overlies the 
periphery 34 of the substrate 16. A vapor impermeable, self supporting 
film 36 of polyimide, 0.5 micron thick is secured adhesively or by 
fasteners not shown to the frame sidewall shoulder 32 to extend across the 
beam target area 38 between the beam source, and the substrate 16. The 
film 36 lies about 10 to 50 microns above the substrate forming a small 
zone 40 thereby, below and separated from the larger zone 42 defined by 
the vacuum chamber 14 by the film 36. The polymerizable species, e.g. 
silicone oil, is supplied from supply 44 through inlet line 46 at a rate 
sufficient to maintain sufficient quantity of polymerizable molecular 
species vapor in the zone 40 during beam exposure for adequate resist 
formation. As the beam impinges on the species, it polymerizes on the 
substrate 16 in areas corresponding to and defined by the beam impingement 
pattern. 
The pressure within zone 40 is in the range of 10.sup.-4 to 1 Torr. which 
is optimum for resist polymerization. The pressure in the vacuum chamber, 
on the other hand is less than 10.sup.-4 Torr. which is optimum for 
untrammeled beam propagation. 
The thickness of the film 36, and the transverse extent of the zone 40 are 
each selected such that the distortion of the beam passing them is 
minimized, contributing maximum precision in forming the desired pattern, 
despite the presence of vapor of the polymerizable molecular species in 
the zone 40. The thickness of the polymerizable molecular species vapor 
layer, delimited by the height of the zone 40, is insufficient to cause 
troublesome distortion of the beam. 
Thus the objectives first set out are realized. Beam traversal of the 
vacuum chamber is free of interference because the vapor is not in the 
major portion of the chamber. The thickness of vapor at the substrate is 
selected to be insufficient to interfere with the beam there. The amount 
of vapor used is concentrated at the surface where it is to polymerize, 
resulting in large efficiencies. The film membrane which divides the 
chamber into vapor free and vapor rich zones, to the extent the nature of 
the film barrier and sealing will allow, is itself non-interfering with 
the charged particle beam precision. The result is a more precise, and 
more efficient, in terms of production of parts and in terms of 
consumption of materials, process, and in terms of better resolution of 
intended pattern as well.