Process of forming ultrafine pattern

A process for forming an ultrafine pattern on a surface of a substrate, which includes steps of irradiating the substrate surface with radiation modulated according to information to be patterned, subjecting the substrate surface to deposition with a material reactive or not with the substrate, and subjecting the substrate surface to etching if a substrate-reactive material is used for deposition. By this process, an ultrafine pattern can easily be formed with a high accuracy.

BACKGROUND OF THE INVENTION 
The present invention relates to a process for forming an ultrafine pattern 
by a combination of selective irradiation, deposition and etching. More 
particularly, the invention relates to a process for forming an ultrafine 
pattern suitable for the production of high-density integrated circuits. 
Photolithography has commonly been used for formation of a patterned thin 
layer on the surface of a substrate. Photolithography for the formation of 
a pattern composed of, for example, a silicon dioxide film on a substrate 
in the form of, for example, a silicon wafer ordinarily includes steps of: 
(a) coating a photoresist film on the substrate, 
(b) superposing a mask on the photoresist film and exposing the photoresist 
film through the mask to light emitted from an ultra-high pressure mercury 
lamp, etc. to print the pattern of the mask on the photoresist film, 
(c) removing the uncured photoresist film on the unexposed areas, 
(d) etching the silicon dioxide film, and 
(e) removing the photoresist film. 
Apart from the above-described steps, photolithography involves steps of 
preparing a mask to be used in the step (b) above. The step (b) is 
frequently carried out by irradiation with X-rays, etc. The 
above-described conventional processes are, however, accompanied by the 
disadvantages that the pattern-formation processes require many steps, 
including the preparation of a mask, and much difficulty is encountered in 
forming an ultrafine pattern of submicron dimensions. Moreover, although 
extensive development efforts regarding the formation of fine patterns by 
photolithographic techniques have hitherto been made, the patterns 
obtained by the conventional processes have a lower dimensional limit of 
about 0.1 .mu.m. 
In order to form a finer pattern, there has recently been proposed a 
process in which radiation condensed into a narrow beam is selectively 
irradiated onto the substrate, and then another material is deposited on 
the substrate to form a fine pattern. This process is based on the 
principle that stable absorption sites for the material to be deposited 
are formed by the radiation and that the deposited and heated atoms move 
on the surface of the substrate to the stable adsorption sites. As a 
result, this process cannot be applied to cases where the materials to be 
deposited are reactive with the substrate. 
SUMMARY OF THE INVENTION 
An object of this invention is to provide a process for easily forming a 
fine pattern of the submicron order. 
In one embodiment of this invention, a pattern is first formed by 
irradiating a substrate with radiation modulated according to the 
information to be patterned, after which the substrate surface is 
subjected to a deposition treatment to form deposited areas and 
non-deposited areas in correspondence with the non-irradiated areas and 
irradiated areas, respectively. Finally, a film provided on the substrate 
is subjected to an etching treatment to thereby form a pattern on the 
substrate surface. 
In another embodiment of this invention, an ultrafine pattern is formed by 
irradiating a substrate with radiation and depositing a material having 
reactivity to the substrate material onto the substrate.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The process according to the present invention is illustrated below with 
reference to FIGS. 1A and 1B. 
In FIG. 1A, an input signal 1, indicative of information to be recorded, is 
applied to a radiation generator 3 via a modulator 2. Radiation 4, 
modulated according to the signal 1 to be recorded, is directed onto the 
surface of the substrate 6 while the latter is being moved in the 
direction of an arrow 5, thereby forming an irradiated area 7 and a 
non-irradiated area 8. The substrate is then subjected to a deposition 
treatment whereby deposition is not effected in the irradiated area 7. 
In the first embodiment of this invention, the substrate, having formed 
thereon deposited areas, is then etched using any known technique, such as 
chemical etching, electrolytic etching, gaseous phase etching, and the 
like. Upon etching under proper conditions, the substrate is eroded deeply 
in the non-deposited areas and shallowly in the deposited areas to thereby 
form a pattern corresponding to that of the radiation. 
The radiation which can be used in the present invention includes 
.alpha.-rays, .beta.-rays, .gamma.-rays, electron rays, laser beams, 
X-rays, ion particle beams, neutron beams, and proton beams. Of these, 
electron beams are particularly preferred. 
The substrate used in the practice of the invention can be selected from 
among semiconductors such as Si, Ge, GaAs, etc.; metals, e.g., Cu, Al, Fe, 
Ni, Co, Au, Sn, Pb, Mo, W, V, etc., and alloys of these metals; and 
inorganic materials, e.g., glass, polymers, etc. The surface of the 
substrate may be coated with a thin film. 
The material to be deposited on the substrate in accordance with the first 
embodiment of the invention may be a metal such as Cu, Al, Fe, Ni, Co, Au, 
Sn, Pb, Mo, W, V, etc., and alloys of these metals. 
The material to be deposited on the substrate in accordance with a second 
embodiment of the invention should be reactive with the substrate 
material. The substrate material and the material reactive therewith can 
be selected from a wide variety of combinations. It is preferable in some 
cases, depending on the combination of the substrate and the reactive 
material to be deposited thereon, that the substrate be heated during the 
deposition treatment. 
For the purpose of ensuring accuracy in the formation of a fine pattern, 
the substrate after deposition is heated if necessary. The heating 
temperature should be chosen in accordance with the type of material of 
the substrate and the deposited material. For example, in a case where the 
reaction between the substrate and the material to be deposited is carried 
out at low temperatures, the heating temperature may also be low. 
The mechanism which prevents deposition from being effected in the areas 
where radiation has been applied is not clear, but it is considered that 
irradiation of electron beams, etc., on the substrate surface may render 
the irradiated area unstable for the formation of a deposited film. 
Particularly in the case of using electron beams, a clear distinction 
between deposited areas and non-deposited areas can be ensured by the 
presence of an oil in a vacuum pump emloyed for maintaining the substrate 
in a vacuum atmosphere. The preferred oils used as the vacuum pump oil are 
hydrocarbons, such as alkylnaphthalenes, eicosylnaphthalene, etc.; 
silicone oils, such as phenylmethyl polysiloxane, pentaphenyltrimethyl 
trisiloxane, pentaphenyltrimethyl trisiloxane; etc.; and ester-type oils, 
such as di-2-ethylhexyl sebacate, di-2-ethylhexyl phthalate, etc. 
In addition to the use of the above-described vacuum pump oil, it is also 
preferable to perform irradiation of the energy beam, such as an electron 
beam, in a vacuum atmosphere containing a trace amount of an organic gas, 
e.g., styrene. 
The present invention will now be illustrated in greater detail with 
reference to the following examples, but it should be noted that these 
examples are not limitive of the present invention. 
EXAMPLE 1 
A substrate composed of a silicon wafer having coated thereon a silicon 
dioxide film was irradiated with a modulated electron beam under 
conditions of an accelerating voltage of 25 KV, a beam current of 50 pA 
and a beam diameter of 200 A using a scanning-type electron microscope in 
a vacuum atmosphere created by evacuation with a cryopump. Thereafter, 
chromium was deposited on the substrate to a thickness of 80 A, and the 
substrate was heated to 100.degree. C. The substrate thus treated was 
etched in a plasma etching apparatus by introducing carbon tetrafluoride 
gas to cause electric discharge. By this etching, the silicon dioxide film 
on the areas that were not coated with a chromium film was removed. The 
etching conditions and the chromium film thickness were selected so that 
the chromium film on the SiO.sub.2 -coated substrate could also be removed 
by the etching. 
It was confirmed with a scanning-type electron microscope that the 
substrate had formed thereon the same pattern as the electron beam 
pattern. 
EXAMPLE 2 
A substrate composed of a silicon wafer was irradiated with an electron 
beam under conditions of an accelerating voltage of 30 KV, an electron 
beam current of 10 pA and a beam diameter of about 50 A. The irradiation 
was carried out by scanning in a square area of 5 .mu.m.times.5 .mu.m for 
5 minutes in such a manner that the scan lines formed a striped pattern 
with an interval of 150 A between adjacent lines within the area. During 
the scanning, the beam was shielded each time the beam was shifted to 
another scan line. The substrate was deposited with nickel to a thickness 
of 60 A at a substrate temperature of 30.degree. C., and the substrate was 
then heated while observing the surface thereof through a scanning-type 
electron microscope. These operations were carried out in a vacuum 
atmosphere obtained by an evacuation system using a combination of a 
rotary pump and a diffusion pump. Through electron-microscopic 
observation, a striped pattern made of nickel lines having a width of 
about 80 A was noted in the non-irradiated part of the square area when 
the heating temperature reached 250.degree. C. 
As the heating temperature was further elevated, the width of the Ni lines 
gradually became narrower. When the heating temperature reached 
650.degree. C., nickel was not observed at all and, instead, an ultrafine 
pattern made of a newly produced reaction product was observed. 
Thus, the present invention makes it possible to easily form a fine pattern 
with a high accuracy. 
While the invention has been described in detail and with reference to 
specific embodiments thereof, it will be apparent to one skilled in the 
art that various changes and modifications can be made therein without 
departing from the spirit and scope thereof.