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BACKGROUND OF THE INVENTION
(a) Field of the Invention
The present invention concerns a method of fabricating semiconductor
devices by relying on the so-called dry process which is one of the
semiconductor device fabricating methods, and more particularly it
concerns the method of applying photochemical reaction to the dry process,
and also an apparatus therefor.
(b) Description of the Prior Art
For the fabrication of various kinds of electronic devices such as
semiconductor devices represented by, for example, transistors and
integrated circuits (IC's), there are being adopted fabricating techniques
of progressively ascending levels to meet the ever-advancing requirements
for realizing higher levels of performance and greater miniaturization of
the devices. In these devices, the size of the IC-constituent devices, the
intervals between these devices and the diameters of the lead wires formed
on integrated circuits have become calibrated to measure by the order of
micrometer. Thus, the dimension of the device in the lateral direction is
presently limited for a tolerance or error of only about .+-.0.1
micrometer. With respect to the vertical direction, there prevails the
requirement for the formation of very thin films having the thickness of
about several hundred .ANG.. Depending on the cases, there is the need to
provide a multi-layer structure in which these thin films are stacked one
upon another into several laminated layers.
For the reasons mentioned above, there has been the constant requirement to
develop a very high degree of, i.e. high precision, technique or method in
the process of depositing or etching various kinds of such thin films
having different functions relative to each other.
The techniques of deposition and of etching which have recently become
practiced or have become important of late satisfying the above-mentioned
requirements are called the "dry process" in a broad sense of the words in
the field of semiconductors.
The mention of the terms "in a broad sense of the words" hereinabove is
based on the following considerations.
The technique which is called "photolithography" employed in the field of
art of semiconductors points to the art of selectively etching such a film
as SiO.sub.2 or Si.sub.3 N.sub.4 formed on the surface of the
semiconductor by the use of a photoresist and a chemical etchant
containing, for example, fluoric acid (HF). This is an important technique
which is being used currently in the process of fabricating semiconductor
devices. It has been very difficult, however, to limit the error or
tolerance of the dimension accuracy after etching to the above-mentioned
level of about .+-.0.1 .mu.m. Accordingly, as a high-precision etching
process which can substitute the abovesaid technique, there has been put
to practice the so-called sputtering process (including DC sputtering, RF
sputtering, microwave sputtering, reactive sputtering, and gas plasma
sputtering) which typically is arranged so that a substrate to be etched
is placed in a vacuum chamber, and under a gaseous atomosphere produced by
introducing an inert gas such as Argon and a reacting gas such as carbon
tetrachloride (CCl.sub.4), either a DC voltage or a radio frequency
voltage is applied across the electrodes to cause a glow discharge to
thereby etch a required site or sites of the substrate. Other than those
mentioned above, there has been started to be used also an ion etching
technique using an ion beam. The etching mechanism of this latter
technique may be regarded to be basically identical with that of
sputtering.
In the field of semiconductors, the former-mentioned etching process using
a chemical etchant is called the "wet process", in contrast to the latter
etching process using abovesaid sputtering techniques, ion beam technique
or discharge process in the field of "discharge chemistry" which is
customarily called the "dry etching process" or simply the "dry process".
This "discharge chemistry" will be briefed hereunder by taking up an
example of the abovesaid sputtering techniques. Into a vacuum chamber
containing two opposing electrodes is charged, for example, argon gas
(Ar), and a DC voltage is applied across these electrodes to produce a
glow discharge. Whereupon, Ar gas is ionized to become Ar.sup.+ which
collides against the substrate to drive out the atoms or molecules of the
substrate. This process represents the "etching". Instead of using Ar gas,
there may be charged such a gas as will cause chemical reaction with the
atoms of the substrate. By so doing, there can be performed various
processes such as deposition and etching.
However, the technical term "dry process" would be more suitable when it is
considered in a broader concept than limiting it only to the use for
specific types of etching mentioned above. The term "dry process" used in
the present invention indeed represents the abovesaid broader sense. This
is because of the consideration that the sputtering method (which, in
practice, is called either the reactive sputtering or plasma CVD
technique) is used as the art of forming, by deposition, a thin film of
such a substance as amorphous Si, polycrystalline Si, SiO.sub.2, Si.sub.3
O.sub.4 or TaN with good precision (i.e. elaborately controlling the
thickness as well as the film quality or condition). It should be noted
here that the methods of forming a thin film by the sputtering technique
or by the vacuum deposition technique are called, in general, the Physical
Vapor Deposition (PVD) in contrast to the Chemical Vapor Deposition (CVD).
It should be noted also that, for example, the vapor epitaxial growth
which is one type of the CVD technique is such that a thermal energy is
applied to a reacting gas to cause deposition by virtue of hydrogen
reduction or pyrolysis. In contrast thereto, such method as the abovesaid
plasma CVD technique is of the mechanism that the discharge energy
(electric energy) such as by glow discharge is applied to the reacting
gas, and the deposition is conducted under the conditions common to the
ordinary sputtering technique and the CVD technique. The deposition
mechanism also is not limited to one kind, but combinations of various
mechanisms would be necessary for the formation or deposition of a thin
film having a high level of functional characteristics.
Viewing this way, it will be noted that not only the ordinary CVD technique
but also the deposition process which relies on the decompressed CVD
technique intended to improve the uniformity of the thickness of the
produced film by elongating the mean free path of the reacting gas may be
likewise included in the dry process.
From the above-stated sense, the concept of "dry process" is considered to
be applicable to the whole phenomena which would arise between the
objective (article to be processed) and the gas-phase material, regardless
of whether the process is intended for deposition or etching. Currently,
however, the dry process which is capable of forming a thin film whose
thickness is controlled efficiently with good precision, or which is
capable of performing an etching which realizes the demensional precision
with an error of the micrometric order is typically represented, as the
main stream of the art, by the specific dry process arranged that a gas is
introduced into a chamber in which a glow discharge is developed to render
the gas-phase material to the activated state by the discharge energy
(electric energy) thus produced, to accelerate the progress of the growth
(deposition) of a film. In either the etching or the deposition process,
the substrate (hereinafter will be referred to as the objective) on which
these processes are conducted is placed in a sealed chamber, and the
interior thereof is evacuated, followed by the introduction of a required
gas thereinto, and electric power is applied across the electrodes housed
in the chamber to develop a glow discharge. Even if the electrodes are set
outside the chamber which is made of an insulating material, there will
inevitably exist high energy particles in the discharge space.
In such an instance, the objective is placed either on the electrode, or in
the vicinity of the electrode, or at a site relatively away from the
electrode. In other words, the objective is placed in the region wherein
there is developed an intensive glow discharge (i.e. the discharge
region), or in a region adjacent to the discharge region but no distinct
glow discharge phenomenon is produced (i.e. the non-discharge region). In
this latter instance also, the circumstance within the chamber is such
that there is hardly any difference in the gas pressure in the discharge
region as compared to the region in which the objective is placed.
Regardless of in whichever region the objective is to be placed, the
material produced in the discharge region (which material, in general,
consists of either gas-phase particles which provide the deposition layer
or gas-phase particles which serve as the material for etching the
objective) is supplied onto the objective.
When a glow discharge is developed by the introduction of a gas, those
atoms and molecules which are contained in the charged gas are subjected
to a discharge energy to be rendered to the state of having a higher
energy, i.e. the activated state. As a result, there are developed in the
gas phase not only an increase in the mere kinetic energy of atoms and
molecules, but also such complicated reactions as chemical reaction
including ionization, decomposition and synthesis, and also
polymerization. For this reason, the electricity-charged particles such as
electrons and ions are produced in considerably large amounts in the
discharge region, and they will acquire a large kinetic energy by being
subjected to an electric energy imparted by the glow discharge, i.e. they
will acquire an increased velocity. These particles which have acquired an
increased velocity will collide against neutral particles such as Ar to
ionize them or impart a kinetic energy to them. This means that not only
those particles (molecules, atoms, ions, electrons, etc.) which are
necessary for the deposition onto or the etching of the objective, but
also those particles which are not necessary for these purposes will also
be supplied to the objective in either the ionized state from the
electrical point of view or the neutral state and with a considerably high
kinetic energy. The directions in which they are supplied to the objective
are random in general. In certain cases, however, for example, in order to
enhance the deposition rate, a magnet is placed in the vessel or the
chamber, to uniformalize the orientation of supply of these particles with
the aid of the magnetic field produced by the magnet, i.e. giving
orientation of movement to the particles, in a certain type of dry
process.
Any way, when gas-phase particles having a high kinetic energy as stated
above are supplied to the objective, it often happens that the surface of
the objective is damaged due to the collision thereagainst of these
particles. This damage includes the development of such defects as lattice
dislocations, clusterings, strains, etc. in the surface of the objective,
aggravating the electric characteristics of the device or devices
contained in the objective.
As discussed above, the deposition process and the etching process which
are collectively called the dry process utilizing glow discharge is
difficult to avoid the drawback, in the conventional art, of damaging the
surface of the objective in spite of the fact that this dry process
represents a high level of technique which is intended to efficiently
control the dimension such as thickness and width of the objective with
good accuracy.
Moreover, the value of the energy which is supplied to the gas by glow
discharge is averagedly large. However, since the values of energy can
range widely, the atoms and molecules contained in the gas would be
activated in miscellaneous ways, causing various kinds of physical or
chemical reactions to take place. Thus, it will be noted that no
particular selected activation necessary for only the desired deposition
or etching purpose is carried out in the conventional art.
In case, for example, it is intended to effect the deposition of amorphous
silicon (a-Si) by relying on the plasma CVD technique using a gas
containing SiH.sub.4, the resulting a-Si film thus formed will be found to
contain not only a-Si alone, but also various types of Si.sub.x H.sub.y
substances such as Si polycrystals or SiH.sub.4. As will be appreciated
from this phenomenon also, the process concurrently has such drawback as
represented by the reactions which are not in line with the intended
purpose, or by other undesirable reactions. Also, there may occur an
instance wherein, although the dry process therein is intended only to the
etching of an objective, the performance is such that not only the etching
itself is done, but also, apart from that, irrelevant deposition would
also take place at the same time.
Thus, there may be considered a method which insures that, among various
kinds of atoms and molecules which have been imparted various types of
conditions as a result of the activation in the discharge region, only
those specfic particles which meet the intended purpose are selectively
supplied onto the objective. Such a method, however, would inevitably lead
to a very costly large-scale apparatus, and in addition, would give rise
to the difficulty to select specific kind of particles with a good
efficiency.
As the method of improving these drawbacks and problems of the prior art
mentioned above, there have been proposed methods to promote the dry
process by externally impinging light rays into the chamber or vessel in
which the dry process is to be carried out.
One of such prior methods is designed to place an objective in the chamber
in a region located adjacent to the discharge region, and to cause the
beam of light rays to impinge onto the discharge region to thereby
activate the gas which is charged in the chamber. This proposed prior art
requires that the gas pressure is set low to develop a glow discharge, but
this leads to a poor efficiency of activation of particles. Moreover, as
discussed above, in this method also, particles having a large kinetic
energy will collide against the objective, and damage the latter.
Another priorly proposed method is to place an objective in the discharge
region of the chamber in which the charged gas is activated, and a beam of
light rays are caused to impinge thereonto. It should be noted here that
this prior art technique involves the problems that the gas is activated
not only by the light rays incident thereonto, but also by the glow
discharge as well, so that there will occur not only the aimed reaction
but also those reactions which are not in line with the aimed purpose.
Moreover, the surface of the objective would become contaminated by those
products of such reactions that depart away from the aimed purpose.
Photochemical reaction process per se occurs selectively in many cases.
This means that it is possible to develop a reaction selectively.
Therefore, such a selective process acts powerfully to realize a clean
process.
SUMMARY OF THE INVENTION
It is, therefore, the object of the present invention to provide a dry
process useful for the fabrication of semiconductor devices with good
precision, which is devoid of the above-mentioned drawbacks of the prior
art.
More specifically, the object of the present invention is to provide a dry
process as referred to above, which efficiently activates gas-phase
particles such as atoms and/or molecules contained in the charged gas by a
beam of light rays, and which keeps the objective which is being processed
from being damaged by the collision of those particles having a high
kinetic energy against the surface of the objective at the time of etching
the objective or of deposition onto this surface thereof, and which,
additionally, has the feature that the reaction process is characterized
by directionality, and which, further, insures the process to be carried
out cleanly without allowing the reaction products to stay in the vicinity
of the objective, by a quick evacuation of these products therefrom.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagrammatic representation of an embodiment of the apparatus
for performing etching or deposition by relying on the dry process
according to the present invention, by irradiating a beam of light rays
onto the higher pressure gas region of the chamber based on the
photochemistry principle, while supplying onto the objective placed in the
lower pressure gas region the resulting photochemically activated
gas-phase particles through ejection nozzles or through-holes having a
small diameter provided between these two regions.
FIG. 2 is a diagrammatic representation for explaining the distribution of
the directions of flow of activated particles as they are supplied onto
the lower pressure gas region through an ejection through-hole due to the
difference in pressure of gas in these two regions.
FIG. 3 is a diagrammatic representation of an embodiment of the apparatus
intended to accomplish the present invention but having a construction
modified from that of FIG. 1.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention will hereunder be described in further detail by
referring to the illustrated embodiments of apparatus.
FIG. 1 shows a diagrammatic illustration of an apparatus structure employed
in the present invention to explain the dry process of the present
invention utilizing the photochemical technique.
Reference numeral 1 represents a light supply for exciting, by the light
ray beam emitting therefrom, the particles of atoms and/or molecules
contained in the charged gas used for the purpose of either etching or
deposition. Numeral 2 represents a chamber in which the dry process of the
present invention is carried out. The interior of this chamber 2 is
divided, by a partition wall 3, into a higher pressure gas region 4 and a
lower pressure gas region 5. These two regions 4 and 5 are connected to
independent evacuation systems 6 and 7, respectively. The higher pressure
gas region 4 is connected to a gas introducing system 8. An arrow 9
indicates the beam of light rays irradiating from the light supply 1.
Numeral 10 represents a window which transmits those light rays having a
required wavelength contained in the beam of light rays 9. The beam 9 of
light rays transmits through the window 10 to be incident onto the higher
pressure gas region 4 to activate the particles contained in this gas.
Numeral 11 represents ejection ports or through-holes formed through the
partition wall 3 for the ejection of the gas therethrough from the higher
pressure gas region 4 into the lower pressure gas region 5. Those
activated gas-phase particles which have been produced in the higher
pressure gas region 4 when subjected to chemical or physical reactions due
to their excitation by the incident light rays are allowed to eject
through the through-holes 11 into the lower pressure gas region 5 in jet
streams 12. An objective 13 for being processed is placed in the lower
pressure gas region 5 for being etched or for deposition thereon. This
objective 13 is supported on a table 14.
As will be appreciated by taking a look at the diagram of the apparatus
shown in FIG. 1 designed for carrying out the dry process, the present
invention is pointed to a dry process utilizing photochemistry, i.e.
promotion of chemical and/or physical reactions and activation of
gas-phase reactant by its exposure to the incident light rays having
wavelengths in infrared (including far-infrared), visible and ultraviolet
(including vacuum and deep ultraviolet) regions of spectrum. In many
cases, infrared light is effective to excite molecules easy to cause a
chemical reaction.
In case particles such as atoms and molecules contained in the charged gas
are excited by the exposure of this gas to a beam of such light rays as
mentioned above, the energy of photons are transmitted to the particles of
the gas due to the interaction between the photons and the particles
contained in the gas. It should be noted that, in such an instance, the
kinetic energy of photons is negligibly small. Also, when these particles
are promoted to the excited or activated state due to their absorption of
the energy of photons, this leads to an increase in the internal energy
such as an increase in the oscillation energy or rotation energy or an
increase in the electronic energy of the particles, i.e. an elevation of
the energy level of those electrons existing in the particles. Absorption
of photons having a large energy or absorption of a large number of
photons leads to ionization or decomposition of aimed particles.
Accordingly there hardly occurs an increase in the kinetic energy of the
particles themselves. Thus, the kinetic energy of the particles of the gas
after excitation will not deviate substantially from the kinetic energy
which follows Maxwell-Boltzmann distribution which, in turn, is determined
by the temperature which the gas had possessed before the excitation.
More specifically, when the supply of energy performed by irradiation of
light rays is compared with that by conventional glow discharge, the
kinetic energy of the gas-phase particles given by the former is not so
great as that imparted by the latter. Besides, the pressure of the gas
located in the region which is subjected to the irradiation of light rays
is set higher than that of the region wherein the objective is placed, so
that a high efficiency of activation can be obtained. Also, the difference
in gas pressure between the divided two regions produces a flow of gas
directed toward the objective. In such an instance, those particles
carried in the stream of gas will accomplish a collective uniform movement
along the direction of flow of gas. In case gaseous particles eject
through small-calibred through-holes from the higher pressure gas region
having a higher gas viscosity into the lower pressure gas molecular- or
atom-stream region, the streams of particles will form jet streams having
uniform directions of advancement. In such a case also, the particles are
unable to have the kinetic energy distribution deviating greatly from
Maxwell-Boltzmann distribution. Averagely, their kinetic energy is about
several 10meV at most. When, gaseous particles having a kinetic energy of
this degree collide against the objective, there hardly will occur a
damage to the surface of the objective. Moreover, because the objective is
placed in a region of a relatively low pressure of gas, the particles do
have a long mean free path in said region, and accordingly those particles
such as atoms and molecules which have been produced as a result of
completion of such reactions as etching or deposition would not stay long
at the objective, but will be removed therefrom quickly through, for
example, diffusion. Therefore, at the surface of the objective, the waste
gaseous particles are removed quickly, and in their place freshly excited
particles are supplied onto the surface. Thus, the possibility of
contamination of the surface of the objective by the products of reaction
becomes very small. In other words, the dry process according to the
method of the present invention can be termed a very clean one.
As the type of the light supply, there may be used a light emission source
having a wide emission spectra such as a mercury lamp, a xenon lamp or a
halogen lamp. It should be noted here that the light supply requires to be
such that it emits, among the light rays of the irradiating beam, those
light rays having an energy necessary for accomplishing the required
activation of particles contained in the gas. In case a light supply such
as mercury lamp, xenon lamp or halogen lamp which produces light rays
having a wide range of wavelengths, it is effective also to derive, out of
these light rays through spectroscopy, a beam of light having a desired
specific wavelength which is capable of causing an aimed reaction. The
light supply may be such that it emits selectively a beam of light rays
having, generally, necessary for being used in photochemistry, or having a
wavelength possessing an energy necessary for the particles to absorb the
light rays to elevate from their ground level up to the activated state.
There is an instance wherein those particles which have been produced
freshly due to photolysis become particles having an activated state. In
case, for example, a beam of monochromatic light rays such as laser which
meets the absorption spectrum of a specific type of particles is used, it
is possible to selectively cause an activation or its accompanying
reaction. Alternatively, by using a light supply consisting of a plurality
of kinds of monochromatic lights which are arranged to irradiate
simultaneously, it becomes possible also to excite the required
more-than-one kinds of particles.
The adjustment of pressure of the gas contained in the higher pressure gas
region is performed by using the evacuation system 6 and by using the gas
introduction system 8. In such a case, the function or the degree of
vacuum which is reached by the evacuation system 6 need not be very high.
For example, the degree of vacuum may be enough if it is of such a degree
as 10.sup.-2 Torr which can be obtained by a rotary pump. The pressure in
the higher pressure gas region is enough if it results in making the mean
free path of gaseous particles existing in the gas shorter than the
dimensional order of construction of the apparatus such as the diameter of
the ejection through-holes 11 formed through the partition wall shown in
FIG. 1. Though depending on the type of the excited particles which are to
be produced, said pressure may be the atmospheric pressure or could be a
pressure greater than that. In case the operation is performed under a gas
pressure above the atmospheric pressure, there may be an instance wherein
there is no particular need for using such evacuating means as a pump to
serve as the evacuator. It should be noted here that, as the means of
perfoming an adjustment of the internal pressure of the chamber, or as the
means of adjusting the evacuation rate, there may be employed a pump or a
needle valve or the like.
The distribution of directions of the flow of those particles into the
lower pressure gas region depends greatly upon the shape of the ejection
through-holes 11 used. Description will hereunder be made of the instance
wherein the ejection through-holes have a circular shape as an example.
FIG. 2 shows a sectional view in case the ejection through-hole is of a
circular shape, and also shows the distribution of flow of those particles
having ejected into the lower gas pressure region. The distribution of the
particles is determined by the values of the thickness L of the partition
wall and the diameter d of the through-hole, the pressure of the gas, and
the type of the gaseous particles. Generally, the larger the difference in
pressure of gas is, and the larger the value of L/d is, the more will
converge the directional distribution of particles toward the target, i.e.
in the direction Z. Thus, the distribution of the direction of flow of
particles will have to be determined by designing those conditions
mentioned above.
In case of processing, for example, a substrate (objective) having a large
surface area, it will be noted that, if the ejection through-hole for
particles is just one in number, there will develop a lateral distribution
gradient in the flow of particles within the surface area of the substrate
(objective) in accordance with the degree of the reaction conducted.
In such a case, it is only necessary to provide a plurality of ejection
through-holes to thereby uniformalize the lateral distribution of the
ejected particles which are supplied onto the surface area of the
substrate. Also, the shape of the ejection through-holes is not limited to
just a circular shape, but it may be of square, rectangular, slit-like or
any other shape, provided that the through-holes can cause oriented jet
streams of particles into the lower pressure gas region.
The embodiment of the apparatus shown in FIG. 1 is arranged so that the
higher pressure gas region is separated from the lower gas pressure region
by a partition wall, and that evacuation is effected in each region
independently from each other. In order to set these two regions whose gas
pressures are different from each other, a modified designing may be made
so as to provide a chamber having two adjacent regions having different
cross setional areas relative to each other in a direction substantially
normal to the direction of the gas flow. FIG. 3 shows a modified
embodiment of the apparatus for the explanation of such a design as stated
just above. A region having a smaller cross sectional area is connected,
for communication, with a region having a large cross sectional area. By
evacuating the region having the larger cross sectional area, the gas
pressure in the region having the smaller cross sectional area will be
made higher than the region having the larger cross sectional area. This
latter embodiment of the apparatus is intended to utilize the flow of gas
from the higher pressure gas region 4 in the region having a smaller cross
sectional area in the direction substantially normal to the direction of
tha gas flow into the lower gas pressure region 5 provided in the region
having the larger cross sectional area. A beam of light rays 9 is caused
to travel in parallel with the stream of gas flowing through the higher
pressure gas region 4. The difference in the gas pressures is determined
by such factors as the ratio of the diameters of the higher pressure gas
region 4 and the lower pressure gas region 5 in the direction normal to
the direction of the gas flow, the evacuation rate of the pump, the
pressure of the introduced gas, and the like. Also, the number of those
particles supplied onto the surface of the objective would vary depending
on the distance between the ejection through-hole 11 and the objective 13.
Since the distribution, in the direction of the flow of those particles
supplied onto the objective, also is determined by the positional
relationship between the objective and the ejection through-hole 11, there
is the necessity that, in order to obtain a uniform reaction, optimum
values be set which can be determined by the conditions such as the
dimension of the apparatus and the volume of the gas which is introduced
into the apparatus.
Apart from the technique of forming a gas flow just mentioned above,
arrangement may be made so that the passageway of gas connecting the
higher pressure gas region 4 to the lower pressure gas region 5 is
designed relatively narrow and oblong to produce a drop of pressure within
this passageway. In such an instance, an objective may be placed in the
lower gas pressure reg | | |
