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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to a method for generating excited
neutral particles for etching and deposition processes in semiconductor
technology on the basis of a plasma discharge fed by microwave energy,
whereby microwave energy having a specific frequency is generated, is
coupled into a waveguide system and, as a standing transversal electrical
wave, is concentrated therein at predetermined locations, and whereby
process gases intended for excitation are conducted through the waveguide
system with a plasma discharge tube aligned in the direction of the
electrical field of the wave, whereby a plasma is ignited and excited
particles are generated. The invention is also directed to an apparatus
for the implementation of this method.
2. Description of the Related Art
A method for generated excited neutral particles of said species is known
from T. Sugano, "Applications of Plasma Processes to VLSI Technology",
Wiley-Interscience, New York, 1985, Sections 2.2 and 2.3 (particularly see
2.2.2).
In addition to lithography and doping techniques, etching and deposition
techniques are fundamental processes that are employed again and again in
the process sequence for manufacturing LSI circuits of silicon substrates
(see, in general, "Technologie hochintegrierter Schaltungen", D. Widmann,
H. Mader, H. Friedrich, Springerverlag 1988, particularly Sections 3.1.1
and 5.2..2-4). For example, an important method is vapor phase deposition,
also called CVD, whereby it is presently often standard to undertake the
excitation of the initial reaction gases to form dissociated, reactable
constituents and the initiation of the deposition reaction primarily not
on the basis of an elevation of the temperature (of the silicon wafer) to
approximately 1000.degree. C. but to undertake this by a plasma or by a
high-energy radiation. Dry etching processes, for which the formation of a
gaseous, volatile reaction product is a prerequisite, also usually only
proceed spontaneously, i.e. without 10 the application of external energy
when the gases are already present in atomic form.
It is obviously a critical concern for the successful implementation of
such etching and deposition processes to generate high-energy and,
therefore, reactable neutral particles, particularly radicals, with an
adequately high efficiency. The technical solution of this demand is
increasingly being striven for simultaneously with satisfaction of the
farther-reaching demands for a prevention of the influence of electrical
fields and charged particles on the substrate to be processed and for an
optimally broad range of working pressure for the etching and deposition
processes.
In order to protect the substrates against undesired electrostatic fields
and against ions, which are always co-produced in addition to the neutral
particles in the standard dissociation of process gases in a plasma gas
discharge, it is known to spatially separate the generation of excited
neutral particles from their employment in an etching or deposition
process occurring in a reaction chamber (downstream process). Due to the
short life span of the charged particles, their concentration decreases
greatly immediately after the excitation zone in the downstream method,
whereas the excited neutral particles--as a consequence of their
substantially longer life span-reach the reaction chamber in what is a
suitable concentration for many applications via a suitable feeder.
Magnetron generators having a working frequency of a number of GHz are
often utilized as energy sources for the radio-frequency plasma discharge
in order to generate corresponding microwaves. This energy is coupled into
a cavity resonator or, respectively, into a waveguide system and--on the
basis of suitable dimensioning and tuning--is concentrated therein at
specific locations in the form of a standing wave. A plasma discharge tube
is then usually conducted through the waveguide system at one of these
locations, i.e. exactly at a location at which the energy of the standing
wave concentrates. In this way, radicals having a long life span can be
generated in process gases that are supplied to the plasma discharge tube
and can be subsequently conveyed to the reaction chamber with a feeder.
The localization of the energy of the standing wave at the correct
location is basically unproblematical; however, a substantial part of the
energy is not converted to excitation but is reflected untuned and must be
absorbed in the waveguide, usually in a water load, so that the magnetron
is not damaged (see Sugano, Opp. Cit., Section 2.2.2).
The described, only partial conversion of the available microwave energy
proves problematical particularly in the light of the afore-mentioned
demand for a broad range of working pressures insofar as it is precisely
the low pressure range below approximately 13, particularly below 1.3 Pa,
that is interesting and advantageous for semiconductor technology.
Low pressures are of significance, for example, for surface-controlled CVD
processes in order to avoid depositions having undesired layer properties.
In etching processes, too, a high etching rate and the prevention of
microload effects, i.e. a local etching rate dependent on the environment,
can often only be realized at extremely low pressures. However, ignition
difficulties in the plasma discharge already begin to arise in the
pressure range below 13 Pa, since the excitation density and, thus, the
efficiency of the generating decrease too greatly.
It is in fact known (see Sugano, Op. Cit., Section 2.3.2) to also stabilize
the plasma in the pressure range below 13.times.10.sup.-2 Pa with the
assistance of being enclosed in a magnetic field whose cyclotron frequency
is in resonance with the frequency of the microwaves (ECR method). As
proceeds, for example, from the article "Downstream Plasma Etching and
Stripping", by J. M. Cook, Solid State Technology, Apr. 1987, particularly
150, excited neutral particles cannot be made available in adequate
numbers and with adequate density overall with such methods, i.e.
particularly at the wafer itself. This is not surprising in view of the
fact that only at most 30% of the microwave energy in the discharge is
converted even given improved ECI methods.
Taking a method that can be manipulated into consideration, moreover, it is
also not possible to substantially increase the infed microwave energy
itself, usually approximately 1 kW, in order to enhance the efficiency of
the generating.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a method of the species
initially cited and to provide an apparatus for the implementation of this
method that, in particular, has an adequately high efficiency in the
pressure range below approximately 13 Pa.
In a method of the species initially cited, this object is achieved in that
a plasma discharge tube having a diameter that corresponds to a quarter
wavelength of the standing wave is selected and the waveguide system is
dimensioned and tuned such that the standing wave forms a first voltage
maximum at a first side of the plasma discharge tube and the standing wave
is also supplied reflected, so that a second, anti-phase voltage maximum
forms at a second side of the plasma discharge tube that is opposite the
first side and faces toward an end termination of the waveguide system.
The method of the invention is preferably developed such that, given a
working pressure of process gases below approximately 13, particularly
below 1.3 Pa, electrons generated in the plasma are forced onto helical
orbits with an applied, particularly controlled magnetic field, whereby,
in particular, the magnetic field that is optimum for the respective
working pressure is identified with a sensor means and is set such that
the maximum of particle generation is achieved.
In conformity with the stated object, the following are provided for the
implementation of the method of the invention:
An apparatus having a microwave generator that makes microwave energy
available in a waveguide system, the microwave energy concentrating as a
standing transversal electrical wave of a defined frequency at
predetermined locations, whereby the waveguide system preferably
comprising a rectangular cross section comprises leadthroughs for a plasma
discharge tube that is aligned in the direction of the electrical field of
the wave and is conducted through opposite walls of the waveguide system,
whereby, given an ignited plasma discharge, a continuous generation of
excited neutral particles from supplied process gases is present in the
plasma discharge tube, and whereby the plasma discharge tube comprises a
diameter corresponding to a quarter wavelength of the standing wave, and
the waveguide system is dimensioned such and provided with tuning means
such that the end termination of the waveguide system represents a
reflection face and that wall of the plasma discharge tube facing toward
it comes to lie in the voltage maximum of the standing wave supplied
reflected, whereas that wall of the plasma discharge tube facing toward
the microwave generator comes to lie in a voltage maximum of the standing
wave, so that two anti-phase voltage maximums are available at two
opposite sides of the plasma discharge tube for igniting and maintaining
the plasma discharge.
The invention is based on the perception that only the energy of a single
half-wave voltage maximum, which should come to lie in the middle of the
plasma discharge tube insofar as possible on the basis of appropriate
tuning, is converted for the plasma discharge in the known method. As a
result of the measures of the invention, by contrast, two anti-phase
voltage maximums at opposite sides of the plasma discharge tube are made
available for the plasma discharge, whereby the appertaining maximum of
current comes to lie in the middle of the plasma discharge tube because of
its spacing of a quarter wavelength established by the diameter. The
greatest possible voltage advantage, which has been doubled in comparison
to the previous method, is thereby exploited, so that a power conversion
that is four times higher results.
It is also advantageous that it is not simply two half-wave maximums of the
standing wave that are exploited.
On the contrary, for example with the assistance of a reflection face at
the end termination of the waveguide system as well as with the assistance
of the tuning of the reflected part of the standing wave with a phase
shifter, the standing wave is also supplied reflected and, thus a second
voltage maximum at the spacing of only a quarter wavelength from a voltage
maximum of the standing wave is exploited. In that the energy is utilized
concentrated onto the smallest space as a result of the measures of the
invention, a high-energy plasma with a high plasma temperature forms, this
also remaining stable at low pressures. High etching or, respectively,
deposition rates can be achieved due to the high efficiency with which the
highly excited particles are generated.
Overall, nearly all of the energy generated by the microwave generator is
employed for the plasma discharge or, respectively, for generating
radicals, so that the excitation density of the particles is so high that
it is comparable in terms of effect with a thermal activation. More than
50% of all existing gas molecules are excited even in the low pressure
range down to approximately 5.times.10.sup.-2 Pa.
Further advantages of the invention are achieved in a method characterized
in that, given a working pressure of the process gases below approximately
13, particularly below 1.3 Pa, electrons generated in the plasma are
forced onto spiral orbits with an applied magnetic field, particularly a
controlled magnetic field. This method is further characterized in that
the magnetic field that is optimum for the respective working pressure is
identified with a sensor means and is set such that the maximum of the
particle generating is achieved. A particular embodiment of the method is
characterized in that a pressure gauge means is employed as sensor means,
whereby the increase in pressure corresponding to an increasing particle
generation is identified and is employed for setting the optimum magnetic
field.
In the method, the brightness of the plasma discharge is employed for
tracking the magnetic field. The maximum of occurring, specific
wavelengths of the excited gas is identified by emission spectroscopy and
is employed for setting the magnetic field.
According to the invention, the excited neutral particles are supplied with
a feeder from the plasma discharge tube to a reaction chamber for etching
and deposition processes which is separated therefrom.
The invention also provides an apparatus for the implementation of the
method, comprising a microwave generator that makes microwave energy
available in a waveguide system, said microwave energy concentrating at
predetermined locations as a standing transversal electrical wave having a
specific frequency, whereby the waveguide system preferably comprising a
rectangular cross section comprises leadthroughs for a plasma discharge
tube that is aligned in the direction of the electrical field of the wave
and is conducted through opposite walls of the waveguide system, whereby,
given an ignited plasma discharge, a continuous generation of excited
neutral particles from supplied process gases is present in the plasma
discharge tube, characterized by the features:
the plasma discharge tube comprises a diameter corresponding to a quarter
wavelength of the standing wave;
the waveguide system is dimensioned such and is provided with tuning
devices such that the end termination of the waveguide system represents a
reflection face and the wall of the plasma discharge tube facing toward it
comes to lie in the voltage maximum of the standing wave supplied
reflected, whereas the wall of the plasma discharge tube facing toward the
microwave generator comes to lie in a voltage maximum of the standing
wave,
so that two anti-phase voltage maximums at two opposite sides of the plasma
discharge tube are available for igniting and maintaining the plasma
discharge.
In the apparatus for the implementation of the method, means are present
for generating a magnetic field that increases the excitation density of
the particles and can be controlled on the basis of a sensor means,
whereby the sensor means, which preferably comprises a pressure gauge
means, is designed for identifying the optimum magnetic field strength.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention shall be set forth in yet greater detail on the basis of an
exemplary embodiment and with reference to the attached drawing. Shown
are:
FIG. 1 a schematic of a known, commercially available downstream etching
system with microwave excitation taken from the sales brochure "Model
CDE-VIII Microwave Downstream Etching System", Specification #850008, 1
April 1986, Revision 2 of the company Tylan/Tokuda, USA; and
FIG. 2 a part of an apparatus for the implementation of a method of the
invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 shows a microwave generator 1 that generates microwaves that are
coupled into a waveguide system 2. With the assistance of a tuning unit 4
and due to the dimensioning of the waveguide system 2, a standing wave
forms with which the microwave energy is concentrated at predetermined
locations of the waveguide system. The energy that is reflected untuned
and is not converted must be absorbed somewhere in the waveguide system 2,
for example in the T piece 3 or at the end of the waveguide 2, this
usually occurring with a water load. For generating radicals on the basis
of microwave energy, a plasma discharge tube 5 that is aligned in the
direction of the electrical field of the standing wave is conducted
through the waveguide system 2. When suitable process gases are supplied
to the input 6 of the plasma discharge tube 5 and the plasma is ignited,
then, among other things, excited neutral particles also arise. These are
subsequently conveyed to the etching reaction chamber 8 with a feeder 7
that is approximately 1 meter long. Excited neutral particles thus proceed
onto the surface of substrate wafers 10 secured on a turntable 11 where
they trigger desired etching reactions. The reaction chamber 8 can be
evacuated with a pump 9 and the volatile reaction products can be
extracted.
The method of the invention, which requires only a slight modification of
the known etching system for being implemented, enables the conversion of
the microwave energy available with the highest possible efficiency in a
working range from barely 300 Pa to below 13.times.10.sup.-2 Pa. This is
achieved, on the one hand, in that the plasma discharge tube 5 has a
diameter of 30 mm, corresponding to a quarter wavelength, given a working
frequency of 2.45 GHz, which corresponds to a wavelength of 12 cm.
FIG. 2 shows a plasma discharge tube 5' that is mounted such in the
waveguide system that the side facing toward the magnetron 1' comes to lie
in the voltage maximum of the standing wave, whereby the wall of the
plasma discharge tube 5' (just as at the opposite side) can be intersected
by the voltage maximum; however, the voltage maximum can also be
advantageously located at the inside of the wall. The plasma discharge
tube 5' itself is preferably fabricated of an insulating material such as
quartz or aluminum oxide. A reflection face is provided as an end
termination 12 of the waveguide systems 2'. The reflected energy is tuned
such with a tuning means that the standing wave returned reflected reaches
a voltage maximum at that side of the plasma discharge tube 5' facing
toward the reflection face. A direct, mechanical tuning with tuning pins
13 and 14 in a known way can ensue on the basis of a phase shift. As a
surface wave, the standing wave simultaneously propagates at the top and
bottom in the waveguide system 2' preferably having a rectangular cross
section (approximately 80.times.40 mm) close to the approximately 80 mm
wide limiting surfaces, so that a transverse discharge and a further,
intervening discharge arise. Deriving overall is that energy is coupled
into the plasma discharge tube 5' with extremely high efficiency with this
method, so that, for example, an etching rate that is approximately 3
through 5 times higher than in known methods results due to the high
density of neutral particles. Finally, sleeve dipoles 15 and 16 that damp
the emission as mechanical throttles, a sensor means 18 as well as
magnetic field windings 17 are also shown in FIG. 2.
In order to be able to generate radicals with full efficiency in the
downstream method even in the low pressure range below 13, particularly
below 1.3 Pa, the excitation density can be increased by applying a
magnetic field, particularly a controlled magnetic field. The fact that,
given gas discharge paths, the effective ionization can be increased with
a magnetic field in that electrons must librate in a helical motion and,
thus, have a lengthened ionization path is already known in and of itself
as the Penning effect.
What is critical, however, is that the magnetic field only acts in the
plasma discharge zone and is not tuned to a fixed value of the
electron-cyclotron resonant field strength (circular paths) as in likewise
known ECR sources, but that the magnetic field that is optimum for the
respective working pressure is identified with a sensor means 18 and is
set such that the maximum of the particle generation is achieved.
Implemented tests show that there is in fact an appertaining magnetic
field maximum increasing with decreasing pressure per working pressure as
optimum in view of the generating. This is presumably because the spiral
orbits of the electrons in the plasma become too tight and, thus,
ineffective given too high a magnetic field.
An increase in volume occurs due to the dissociation of the process gases
in the plasma discharge tube. A pressure gauge means can therefore be
advantageously employed as sensor means, whereby the increase in pressure
corresponding to an increasing generation of particles is evaluated and is
employed for tracking or, respectively, for setting the optimum magnetic
field.
As a further sensor method, the brightness of the plasma discharge can be
employed for tracking the magnetic field, whereby the maximum of the
brightness thereby also corresponds to the maximum of the dissociation.
The maximum of occurring, specific wavelengths of the excited gas can also
be identified with emission spectroscopy and can be employed for setting
the magnetic field.
Although other modifications and changes may be suggested by those skilled
in the art, it is the intention of the inventors to embody within the
patent warranted hereon all changes and modifications as reasonably and
properly come within the scope of their contribution to the art.
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