Process for drawing patterns with extremely fine features in the production of VLSI, LSI and IC systems

In the patterning process in the fabrication of VLSI, LSI and IC systems, the electron beam is used to write a pattern over a resist layer on a wafer, but the resist layer is exposed by X-rays. More particularly, a finely focused beam of electrons writes a pattern on a thin metal film formed over a resin layer which in turn is formed over a wafer and the secondary X-rays; that is, the characteristics X-rays (such as K.alpha.) emitted from the thin metal film when the electron beam strikes it, expose the resist layer which is sensitive to the X-rays, whereby a high degree of resolution can be obtained.

BACKGROUND OF THE INVENTION 
The present invention relates to a patterning process in the fabrication of 
VLSI, LSI and IC systems and more particularly a process capable of 
transferring the pattern with extremely fine features to the wafer 
surface. 
The recent trend of semiconductor devices is toward increasingly higher 
densities. With conventional photolithography with ultraviolet light (of 
the wavelength of 300-400 nm), the highest obtainable resolution is 
limited to one micrometer and consequently the feature size is limited to 
2-3 micrometers. In order to attain a resolution higher than can be 
achieved by photolithography, extensive research and development has been 
carried out in order to perfect the X-ray and electron beam lithography 
techniques, which use shorter wavelengths than photolithography. 
Electron beam lithography uses a beam of charged electrons, so that it has 
an advantage of being capable of easily controlling the electron beam 
electrically. Therefore, a digital process can be used under the control 
of a computer to draw a pattern on a wafer. However, the emitted electrons 
are electrically charged so that when they strike a resist layer, they 
interact with molecules in the resist layer and consequently are scattered 
until they lose their energy and finally are trapped somewhere. Because of 
this scattering effect together with the backscattering effect, the 
highest obtainable resolution is 0.1 micrometer. 
In the case of X-ray lithography, X-rays are not electrically charged, so 
that the pattern features are distorted only by the secondary electrons 
emitted and scattered as a result of bombardment of nuclei in a resist 
material by the X-rays. X-ray lithography can, therefore, attain a high 
resolution of 0.01 micrometer. However, as described above, the X-rays are 
not electrically charged so that they cannot be electrically controlled 
and consequently the scanning exposure process used in electron beam 
lithography cannot be used for X-rays. It follows, therefore, that X-ray 
lithography requires specially designed masks. Masks for the X-ray 
lithography are prepared by forming a pattern in the form of an extremely 
thin film of gold or the like over the surface of a substrate of silicon 
or other high-molecular weight material. 
As shown in FIG. 1, the thus prepared mask 1 is placed in an X-ray 
lithography device or machine comprising an exposure chamber 24 and an 
X-ray source chamber 23 with a target 21 and an electron gun 22. A wafer 2 
which is supported on a wafer holder is spaced apart from the mask 1 by S 
(which is typically from 5 to 25 micrometers). The wafer 2 is exposed 
through the mask 1 with soft X-rays (0.2-1.5 nanometers in wavelength) 
emitted from the target 21. However, factors such as the diameter d of the 
target 21 or the initial diameter of the emitted beam of X-rays, the 
radiation angle .theta. and the spacing S between the wafer 2 and the mask 
1 combine to give rise to undesired exposures; that is, distortions or 
blur .delta. to the edge profile of the beam. 
SUMMARY OF THE INVENTION 
The present invention was made to overcome the above and other problems 
encountered in electron beam and X-ray lithography and has for its object 
to provide a patterning process which combines (i) the advantage of 
electron beam lithography that a finely focused beam of electrons can be 
scanned over the wafer surface with (ii) the advantage of X-ray 
lithography that no scattering, which adversely affects the dimensional 
accuracy of pattern features, will occur as a result of bombardment of the 
wafer surface by the X-rays, whereby extremely fine pattern features can 
be well defined. 
According to the present invention, a finely focused beam of electrons 
strike or scan a thin metal film formed over a resist layer which in turn 
is formed over the surface of a wafer so that the characteristic X-rays 
(for instance K.alpha.) inherent to the thin metal film and emitted as a 
result of bombardment of the thin metal film by the electron beam expose 
the resist layer whereby a desired pattern can be transferred into the 
resist layer. Therefore, according to the present invention, the 
fabrication of high-density VLSI, LSI and IC systems can be much 
facilitated.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In FIG. 2 is shown in section an electron beam exposure device which is 
used in the present invention. A DC voltage is applied between a wafer 32 
securely held on a wafer holder 31 and an electron beam source 33. An 
electron beam 36 passes through a blanking unit 34 and a scanning coil 35 
so that the accelerated electron beam 36 scans the top surface of the 
wafer 32 to write a pattern. 
Next, referring to FIG. 3, a process for writing a pattern with extremely 
fine features in accordance with the present invention will be described 
in detail. The top surface of the wafer 32 is coated with a film of X-ray 
sensitive resist 12 of 0.5-1 micrometer in thickness by means of a spinner 
or the like. The X-ray sensitive resist 12 may be a positive resist such 
as polymethyl methacrylate or a negative resist such as polybutadiene, 
polyvinyl ferrocene, polydiaryl-O-phthalate. 
Thereafter, a thin metal film 13 of 500-1000 angstrom in thickness is grown 
over the surface of the resist 12 by a sputtering or electron-beam vacuum 
evaporation process (see FIG. 3A). The thin metal film 13 may be Au, Ag or 
Al. 
The next step is the scanning over the surface of the thin metal film 13 
with a spot of accelerated electron beam 14 so that secondary X-rays 15 
are emitted from local spots (see FIG. 3B). To this end, a DC bias is 
applied between the electron beam source 33 and the surface of the wafer 
32 so that the electron beam 14 is accelerated. For instance, in the case 
of a thin gold film, the electron beam 14 is accelerated at 10-20 KeV so 
that characteristic X-rays 15 L.alpha..sub.1 (128 .ANG.) and 
M.alpha..sub.1 (584 .ANG.) of gold are locally emitted and consequently 
expose the X-ray sensitive resist 12 underlying the thin metal film 13. 
Thereafter, the thin metal film 13 is removed by etching and then the X-ray 
sensitive resist 12 is developed with a suitable developing solution so 
that a desired resist pattern 16 is obtained (see FIG. 3C). 
For instance, when the electron beam of 10.sup.-6 A is converged at the 
acceleration voltage of 20 KeV into a beam spot of 0.5 micrometers in 
diameter (with the area of about 0.2 square micrometers) as shown in FIG. 
4, the output of the characteristic X-rays emitted due to the striking of 
the beam spot against the thin metal film 13 is of the order of 
2.times.10.sup.-4 W/.mu.m.sup.2. In the case of an X-ray sensitive resist 
film of epoxidated polybutadiene (EPB) of the thickness of 0.5 micrometers 
which is overcoated with a thin gold film 800 angstrom in thickness, the 
sensitivity of EPB is 10.sup.-12 J/.mu.m.sup.3, so that the exposure time 
is 1.25.times.10.sup.-8 sec/.mu.m.sup.2. More particularly, in the case of 
a four-inch-diameter wafer (with the area of about 80 cm.sup.2), the 
scanning exposure time is about 100 seconds. With a negative resist, the 
area which must be exposed is less than 10% of the whole area of the wafer 
so that the exposure time becomes less than 10 seconds. This exposure time 
is a satisfactory factor which gives a desired throughput in the IC wafer 
process. 
In the case of the beam spot 0.5 micrometers in diameter as shown in FIG. 
4, the emitted secondary X-rays are scattered about 0.5 micrometers beyond 
the edge of the beam spot, but the size of features in the pattern can be 
made of the order of 0.6-0.7 micrometers by suitably controlling the 
developing conditions. When the electron beam e strikes the thin metal 
film 13, the secondary X-rays are emitted and scattered as shown by the 
arrows so that the area or portion 12a remains as an element of a resist 
pattern. In this case, the electrons which strike the thin metal film 13 
flows along it so that they are prevented from penetrating into the X-ray 
sensitive resist film 12. The secondary electrons given off when the 
electrons e strike the thin metal film 13 are substantially entrapped in 
the film 13 so that, unlike the conventional electron-beam lithography 
system, the distortions of the pattern may be avoided. 
According to the conventional electron beam lithography, blur or 
distortions of the transferred pattern image occur, but according to the 
present invention such defects can be substantially eliminated. In 
addition, the electron beam or charged electrons can be converged into a 
spot and a desired pattern can be transferred by the beam spot scanning. 
Therefore, the process of the present invention is especially adapted for 
use in a system for drawing a pattern with a finely focused beam of 
electrons under the control of a computer in an X-ray sensitive resist 
over a wafer. 
In the conventional electron beam lithography, a finely focused beam of 
electrons directly strikes a resist layer to draw a pattern, but according 
to the present invention, the beam spot strikes a thin metal film 
overlying an X-ray sensitive resist layer so that electrons flow through 
the thin metal film and consequently "charge-up" can be avoided. 
Therefore, a pattern with extremely fine features can be drawn. 
Furthermore, according to the present invention, the use of a mask for 
exposing an X-ray sensitive resist can be eliminated and since the X-ray 
sensitive resist is exposed with the X-rays, a high degree of resolution 
can be attained. 
In order to grow a thin metal film, Ag (K.alpha.=0.56 .ANG.), Cu 
(K.alpha.=1.54 .ANG.), Al (K.alpha.=8.34 .ANG.), Cr (K.alpha.=2.29 .ANG.) 
and Mo (K.alpha.=0.71 .ANG.) may be used. A high degree of efficiency can 
be attained if the acceleration voltage is selected two or three times the 
energies of K.alpha., L.alpha..sub.1 and M.alpha..sub.1. 
As described above, the present invention combines the advantageous 
features of the X-ray and electron beam lithography systems so that the 
process of the present invention is especially advantageous in the 
fabrication of semiconductor ICs and especially high-density LSIs.