Method and apparatus for controlling plasma processing

A method and apparatus for controlling plasma generated utilizing microwaves and a magnetic field. Microwaves including right and left circularly polarized waves are generated and introduced into a processing chamber and a ratio of the right circularly polarized waves to the left circularly polarized waves is controlled to enable control of at least one of an electron temperature and a density distribution for plasma processing.

BACKGROUND OF THE INVENTION 
The present invention relates to a method and apparatus for controlling 
plasmas enabling plasma processing with high quality by controlling 
parameters for plasmas such as an electron temperature and a density 
distribution of the plasmas. 
A plasma processing apparatus for generating plasmas by introducing 
microwaves in parallel with a magnetic flux density into a processing 
chamber applied with a magnetic flux density to such an extent as to cause 
electron cyclotron resonance has been described in, for example, Japanese 
Patent Application Laid-Open No. 55-141729. 
Plasma processing apparatus using microwaves under a magnetic field have 
been used generally in recent years since they can generate plasmas at 
high density even in a high vacuum region and correspond to processing 
conditions over a wide range. The electron cyclotron resonance phenomenon 
is utilized in most of such apparatus. The electron cyclotron resonance as 
discussed herein is a resonance phenomenon that occurs upon coincidence 
between the frequency of a cyclotron movement of electrons under a static 
magnetic field and the frequency of microwaves having a wavelength of 
about 1 cm to 30 cm and it is known that the electric power of the 
microwaves can be absorbed efficiently into plasmas upon occurrence of the 
resonance. 
In a plasma processing apparatus using a static magnetic field, the density 
distribution of plasmas has been optimized by adjusting the distribution 
of the static magnetic field in order to attain uniform processing. 
However, in a plasma processing apparatus using the electron cyclotron 
resonance phenomenon, since a static magnetic field having a higher 
magnetic flux density as compared with usual plasma processing apparatus 
is used, there has been a problem that the size of an electromagnet for 
the control of distribution is also increased. 
Since the degree of integration for LSI is increasing, it is necessary to 
make the quality for the plasma processing higher. In compliance with the 
trend, a technique for controlling the characteristics of plasmas is 
necessary. The inventors have considered the case of plasma CVD. For 
instance, in a case of forming a thin Si film by using monosilane as a 
reaction gas, it has been known that SiHm radicals (m=0-3) in monosilane 
plasmas play an important role for reactions. Although it is not apparent 
which radicals, among them, are most important for the film formation, it 
is considered that if certain radicals can be excited selectively, high 
quality thin films at high purity with less hydrogen atoms liable to be 
contained as an impurity in the films can be formed. Since each of the 
radicals have inherent excitation energy respectively, it is necessary to 
control an energy given from plasmas to reaction gases in order to form 
certain radicals selectively. In view of the above, it becomes necessary 
to control the energy of electrons (electron temperature) that give an 
energy by collision to the reaction gases. However, in existent plasma CVD 
apparatus, since the plasma parameters can not directly be controlled, 
control has been conducted only by the optimization for film forming 
conditions by adjusting process conditions such as a film-forming pressure 
or a plasma-confining static magnetic field. 
The foregoing requirements are also applicable to a case of plasma etching. 
That is, for efficiently exciting active species that contribute most to 
the reactions, it is necessary to control parameters of plasmas such as 
the density and the electron temperature of plasmas. Further, in a case of 
bias sputtering film formation, it is known that the coverage ratio for 
steps and crystallinity of films vary depending on the amount and the 
energy of ions irradiated to a substrate to be processed during film 
formation. In a case of wiring films, the coverage ratio for steps and the 
crystallinity of films are important parameters that determine the life of 
wirings and it is necessary to control plasmas near the substrate to be 
processed In order to form wiring films with high quality. 
SUMMARY OF THE INVENTION 
It is, accordingly, an object of the present invention to provide a method 
and apparatus for controlling plasma processing to enable control of at 
least one of an electron temperature and a density distribution for plasma 
processing. 
It is another object of the present invention to enable control of 
parameters of plasmas such as an electron temperature and a density 
distribution of plasma by controlling the characteristics of at least two 
different polarized waves in a case of a plasma processing apparatus using 
a microwave discharge under a magnetic field. 
In accordance with the present invention, for plasma processing, right 
circularly polarized waves and left circularly polarized waves are 
introduced into a processing chamber in a direction substantially in 
parallel with a direction of a static magnetic field and the ratio between 
the right circularly polarized wave and the left circularly polarized 
waves in the microwaves is controlled so as to enable control of at least 
one of an electron temperature and a density distribution for the plasma 
processing of a substrate provided in the processing chamber. 
According to a feature of the present invention, the control of the ratio 
between the right circularly polarized waves and the left circularly 
polarized waves is achieved by controlling the angle of rotation of an 
anisotropic medium in the form of a phase shift plate mounted in a 
waveguide tube utilized for introducing the microwaves into the plasma 
processing chamber. 
In accordance with another feature of the present invention, the ratio 
between the right circularly polarized waves and the left circularly 
polarized waves is controlled by adjusting a pitch of a helical antenna 
disposed in a mode converter in the portion utilized for introducing the 
microwaves into the plasma processing chamber. 
According to a further feature of the present invention, an anisotropic 
dielectric plate utilized for converting one of left and right circularly 
polarized waves into the other of the left and right circularly polarized 
wave is provided around the substrate which is to be processed in the 
plasma processing chamber thereby enabling control of the ratio and 
control of the electron temperature and density distribution for plasma 
processing. 
These and further objects, features and advantages of the present invention 
will become more obvious from the following description when taken in 
connection with the accompanying drawings which show for purposes of 
illustration only, several embodiments in accordance with the present 
invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to the drawings, the principle of operation of the present 
invention will be described in connection with FIGS. 1 and 2. 
In accordance with the present invention, one of the plasma-controlling 
parameters that can be adjusted most easily is an amount of energy charged 
into plasmas. The electron temperature and the plasma density can be 
increased by increasing the amount of the energy charged. On the other 
hand, the parameters can be controlled also by varying a positional 
relationship between plasma generation sources and positions at which the 
parameters are intended to be controlled. Generally, the electron 
temperature and the plasma density are lowered in accordance with the 
separation distance from the plasma generation sources. The degree of 
reduction is determined depending on the states inherent to the apparatus 
and the film forming conditions such as diffusion and loss of plasmas. 
Thus, the parameters of plasmas at any desired position can be controlled 
by controlling the amount of energy charged in the plasmas and the 
positional relationship with respect to the plasma generation sources. 
For controlling the position of the plasma generation source in a case of 
using electromagnetic waves as an energy source for plasma generation and 
for increasing the density of plasmas at a certain position by the 
charging of electromagnetic waves, it is necessary to propagate the 
electric power of the electromagnetic waves to the aimed position. The 
propagation characteristics of the electromagnetic waves in plasmas 
applied with a static magnetic field in parallel with the advancing 
direction of the waves are different between right circularly polarized 
waves and left circularly polarized waves, which can be represented 
respectively by the formulae (1) and (2). 
##EQU1## 
in which c: velocity of light 
k: number of waves 
.omega.: angular frequency of waves 
.omega..sub.p : angular frequency of plasma oscillations 
.omega..sub.o : angular frequency of electron cyclotron 
In a region where the number of waves k takes an imaginary number in the 
formulae (1) and (2), waves are attenuated in the form of an exponential 
function as they advance and can not be propagated into plasmas. 
FIG. 1 shows an example of an experimental study on absorption 
characteristics of right circularly polarized waves and left circularly 
polarized waves to plasmas applied with a static magnetic field in 
parallel with the advancing direction of the waves in a gas environment of 
Ar+Hg ("Microwave Technology", by Yuichi Sakamoto, Ionics, May 1983). It 
can be seen from the figure that absorption at about 1 dB occurs near the 
electron cyclotron resonance frequency with the left circularly polarized 
waves, whereas absorption at about several tens dB occurs with right 
circularly polarized waves. The electric power of the electromagnetic 
waves is used for increasing the plasma density, the electron temperature, 
etc. of the plasmas at the absorbed positions, and it can be considered 
that the absorption position is a plasma generation source. 
Considering a case of applying electromagnetic waves causing the electron 
cyclotron resonance to semi-infinite plasmas under a substantially uniform 
static magnetic field in parallel with the static magnetic field, since 
the right circularly polarized waves undergo a great attenuation, they are 
absorbed into plasmas only near the boundary and can not penetrate deeply 
to the inside, whereas the left circularly polarized waves do not undergo 
so great attenuation. Accordingly, the left circularly polarized waves can 
penetrate to the inside of the plasmas and are absorbed over a relatively 
wide range. The energy of the absorbed electromagnetic waves contributes 
to the increase of the plasma density. 
For quantitative consideration on the control for the distribution of the 
plasma density depending on the difference of the attenuation amount, FIG. 
2 shows an example for the result of a calculation of electric power 
absorbed in each of the positions in a homogeneous medium with a constant 
attenuation amount when electromagnetic waves are irradiated to the 
medium. The abscissa indicates the distance from the end face of the 
medium. It can be seen that when the attenuation amount in the medium is 
as small as 1 dB, the absorption of the electric power is small, but it 
can be absorbed relatively effectively also at the inside of the medium. 
On the other hand, if the attenuation amount is as large as 10 dB, the 
energy of the electromagnetic waves is absorbed near the end face of the 
medium and the waves do not reach deeply to the inside of the medium and, 
accordingly, the absorption amount is abruptly lowered in accordance with 
the separation distance from the end face. 
Considering a case where the medium is plasmas, since the generation amount 
of the plasmas is substantially in proportion with the absorption amount 
of the electric power, the right circularly polarized waves with great 
attenuation amount contribute to the increase of the plasma density near 
the plasma end face. On the other hand, in the case of the left circularly 
polarized waves with low attenuation amount, the electric power of the 
microwaves can also be absorbed at a position apart from the end face of 
the plasmas. Accordingly, plasmas can be generated uniformly relative to 
the advancing direction of the microwaves as compared with the case of the 
right circularly polarized waves. 
Since the wall surface of a plasma processing chamber is usually made of a 
metal such as Al, it has high electroconductivity and microwave loss is 
small. Accordingly, even in a case of left regularly polarized waves with 
low attenuation, microwaves incident in the processing chamber repeat 
reflections on the wall surface and, finally, most of the microwave power 
is absorbed into the plasmas. 
In accordance with the present invention, the left circularly polarized 
waves with low attenuation and the right circularly polarized waves with 
high attenuation are used at the same time so that the distribution of the 
plasma generation sources in the advancing direction of the waves can be 
adjusted and the density distribution can be controlled, by controlling 
the mixing ratio of the left and right circularly polarized waves. That 
is, while it is possible to attain a density distribution increased 
locally near the boundary to which the waves are applied to the plasmas in 
a case of using 100% right circularly polarized wave as described in 
Japanese Patent Application Laid-Open No. 59-114798, which indicates that 
the etching speed is increased and etching time reduced, and while it is 
possible to attain a relatively uniform density distribution relative to 
the advancing direction of waves in a case of using 100% left circularly 
polarized waves, in accordance with the present invention, the density 
distribution can be controlled by supplying both right and left circularly 
polarized waves and continuously adjusting the mixing ratio between the 
right and the left circularly polarized waves. Furthermore, more 
complicated control is also possible by disposing a converter for the 
right and left polarized waves at the inside of the processing chamber. 
A case of a non-uniform static magnetic field can also be considered 
similarly. The right circularly polarized waves are locally adsorbed at a 
place met for the first time causing the electron cyclotron resonance 
phenomenon along with the advance of the waves and the waves are 
substantially eliminated. The left circularly polarized waves are not 
completely absorbed at a place met for the first place causing electron 
cyclotron resonance phenomenon, but are diffused in the processing chamber 
and absorbed also at other places. The density distribution can be 
controlled by utilizing the difference therebetween. 
As a method of generating circularly polarized waves there can be 
mentioned, for example, a method of disposing a phase shift plate in a 
waveguide tube (for example, reference is made to "Microwave Circuit", 
written by Ishii, Azuma, et al, published Nikkan Kogyo Shinbun Co. 
(1969)), a method of using a helical antenna (for example, reference is 
made to Radiowave Engineering" written by Enomoto and Sekiguchi in Modern 
Electric Technology Lecture, published by Ohm Co., and Japanese Patent 
Application Laid-Open No. 62-37900). In the method of disposing the phase 
shift plate, a phase shift plate of a length: .lambda./4, (.lambda.: 
wavelength in the tube) is disposed in a TE.sub.11 mode circular waveguide 
tube, being slanted 45.degree. relative to the electric field, and 
circularly polarized waves are generated by utilizing the difference of 
the phase constants between the directions vertical to and parallel with 
the phase shift plate. The same effect can also be realized by using a 
material having anisotropy. For instance, there can be mentioned a method 
of using an anisotropic dielectric material (for example, reference is 
made to "Analysis for Characteristics of Polarized Waves In a Circular 
Waveguide Tube Containing an Anisotropic Dielectric Material by a Space 
Circuit Network Method", Report Journal of Electronic Information and 
Communication Society: written by Koh, Yoshida and Fukai, C-I, Vol. 
J71-C-I, No. 8, pp. 460-472). In the method of using the helical antenna, 
circularly polarized waves are generated by propagating electromagnetic 
waves in the circumferential direction of a helix with a peripheral length 
of about 1 wavelength. 
A first embodiment of the present invention is illustrated in FIG. 3 and 
FIG. 4, wherein FIG. 3 shows a CVD apparatus in which a phase shift plate 
7 is used for controlling the characteristics of polarized waves. The same 
reference numerals are utilized in the other drawings to designated like 
parts. A processing chamber 14 is kept at a predetermined pressure by an 
introduction system for a processing gas and a vacuum evacuation system 
(not shown). A static magnetic field providing a magnetic flux density for 
generating an electron cyclotron resonance phenomenon in the processing 
chamber 14 is applied to the processing chamber 14 by an electromagnet 16. 
Microwaves are transmitted from a microwave generation source 1 by way of 
an isolator 2, a matching device 3 and a square waveguide tube 4 and then 
converted by a mode converter 5 into a TE.sub.11 mode of a circular 
waveguide tube 6. The phase shift plate 7 made of a dielectric material is 
mounted in the circular waveguide tube 6 so as to be rotatable about a 
central axis of the tube 6. The ratio between the right circularly 
polarized waves and the left circularly polarized waves in the microwaves 
is controlled by controlling the angle of rotation of the phase shift 
plate 7. Between the processing chamber 14 and the circular waveguide tube 
6, a microwave introduction window 15 made of a dielectric material with 
low microwave loss such as quartz is disposed for introducing microwaves 
while keeping the inside of the processing chamber 14 at an appropriate 
pressure for the processing. The advancing direction of the microwaves is 
substantially in parallel with the direction of the static magnetic field 
provided by the electromagnet 16. A substrate 17 which is to be processed 
is disposed opposite to the microwave introduction window 15 and high 
frequency waves are applied from a frequency power source 18 to the 
substrate 17. 
FIG. 4 shows an enlarged view of a portion of the apparatus of FIG. 3 in 
the vicinity of the circular waveguide tube 6. Support members 8 made of a 
dielectric material with low loss, for example, quartz, are disposed in 
the circular waveguide tube 6 for supporting the phase shift plate 7. A 
shaft 9 is passed through the center of the phase shift plate 7 and a disc 
10 is connected to one end of the shaft 10. A thread 11 is wound around 
the disc 10 and is led out of the circular waveguide tube 6 through a 
small aperture 12. By pulling the thread 11, the disc 10 is rotated 
thereby enabling adjustment of the angle of the phase shift plate 7 
relative to an electric field. 
When an angle of the shift plate 7 relative to the electric field is 
0.degree., linearly polarized waves are formed. When the angle is 
45.degree., circularly polarized waves are formed. Elliptically polarized 
waves are formed at an angle between 0.degree. and 45.degree.. That is, 
the ratio of the right circularly polarized waves and the left circularly 
polarized waves can be controlled by adjusting the angle of the phase 
shift plate 7 relative to the electric field within a range from 0.degree. 
to 45.degree.. 
By using the plasmas generated by the microwaves, reactions of processing 
gases can be promoted to enable film-forming processing at a high speed. 
Further, by adjusting the characteristics of the polarized waves in the 
microwaves and the electric power of the microwaves, films can be formed 
on the substrate 17 while controlling the electron temperature and the 
density distribution of the plasmas so as to be optimized for formation of 
the films. 
A second embodiment is illustrated in FIG. 5 and FIG. 6, wherein FIG. 5 
shows a CVD apparatus for practicing the present invention in which a 
helical antenna is used for controlling the characteristics of polarized 
waves. This embodiment has the same general construction as the first 
embodiment except that a portion of the microwave circuit is different. 
Microwaves are transmitted from the microwave generation source 1 by way of 
the isolator 2, the matching device 3 and the square waveguide tube 4 to a 
mode converter 19. A helical antenna 20 having a variable pitch is 
disposed in the mode converter 19, which enables generation of circularly 
polarized waves. The circularly polarized waves generated are charged by a 
circular waveguide tube 24 through the microwave introducing window 15 
into the processing chamber 14. 
FIG. 6 shows an enlarged view of a portion of FIG. 5 in the vicinity of the 
helical antenna 20. The helical antenna 20 is made of a metal material 
with high electroconductivity, for example, copper. Further, the helical 
antenna 20 is held by a columnar helix support member made of a dielectric 
material with low microwave loss, for example, quartz, and having a flange 
at the top end. The helix support member 21 is further attached with a 
ring-like members 22. It is possible to adjust the entire length of the 
helical antenna 20 and to adjust the pitch of the helical antenna 20 by 
operating a knob 23 connected to the ring-like member 22 thereby 
displacing the position of the ring-like member 22 in parallel with the 
axis of the helix support member 21 It has been known that circularly 
polarized waves can be formed when the pitch of the helical antenna 20 is 
about 1/10 to 1/2 of the wavelength (see for example, "Radiowave 
Engineering", Modern Electric Engineering Technology Lecture, written by 
Enomoto and Sekiguchi, published by Ohm Co.). As the pitch of the helical 
antenna 20 is reduced, the inter-line capacity is increased to make the 
electromagnetic coupling stronger between the lines, by which the helical 
antenna operates in a manner proximate to that of a metal column relative 
to the microwaves. Considering a case in which the helical antenna 20 is 
replaced with a metal column, the microwaves are formed as linearly 
polarized waves. In view of the above, as the pitch of the helical antenna 
20 is reduced, the microwaves are changed from the circularly polarized 
waves to the linear polarized waves, thereby enabling control of the 
characteristics of the polarized waves by operating the knob 23 so as to 
control the mixing ratio of the right and left circularly polarized waves. 
Also in this embodiment, the quality of the films formed on the substrate 
17 which is to be processed can be enhanced by controlling the electron 
temperature and the density distribution of the plasmas like that in the 
first embodiment. 
A third embodiment of the present invention is illustrated in FIG. 7 which 
shows a CVD apparatus having a construction as shown in FIG. 3, and 
additionally utilizing an anisotropic dielectric plate 24 for converting 
left circularly polarized waves into right circularly polarized waves and 
disposed around the substrate 17 which is to be processed. 
Among the microwaves introduced by way of the microwave introducing window 
15 into the processing chamber 14, right circularly polarized waves with 
high attenuation in plasmas are absorbed near the microwave introducing 
window 15 to increase the plasma density at the vicinity thereof. On the 
other hand, left circularly polarized waves with low attenuation are 
propagated to the vicinity of the anisotropic dielectric plate 24. In this 
case, the left circularly polarized waves are converted into right 
circularly polarized waves, which contribute to the increase of the plasma 
density near the substrate 17. 
By controlling the ratio of the right circularly polarized waves and the 
left circularly polarized waves in the microwaves charged into the 
processing chamber 14, the amount of plasmas generated near the microwave 
introduction window 15 and the anisotropic dielectric plate 24 can be 
controlled to thereby control the distribution of the plasma density in 
the processing chamber 14. Also in this example, the electron temperature 
and the density distribution of the plasmas can be controlled so as to be 
optimized for the formation of films in a manner similar to that in the 
first and the second embodiments. 
In addition, other conversion devices such as a helical antenna for 
conversion from the left circularly polarized waves to the right 
circularly polarized waves may be utilized instead of the anisotropic 
dielectric plate 24. 
Although the above descriptions of the first, second and third embodiments 
have been directed to utilization of the CVD apparatus as an example of 
the present invention, the present invention can also be applied to other 
plasma processing apparatus such an etching apparatus, an ashing apparatus 
and a sputtering apparatus. 
Since plasma parameters such as the electron temperature and the density 
distribution of plasmas in the plasma processing can be controlled by 
controlling the mixing ratio of right and left circularly polarized waves 
to obtain plasmas optimized for the plasma processing, plasma processing 
can be conducted with high quality. 
While we have shown and described several embodiments in accordance with 
the present invention, it is understood that the same is not limited 
thereto but is susceptible of numerous changes and modifications as known 
to those skilled in the art and we therefore do not wish to be limited to 
the details shown and described herein but intend to cover all such 
changes and modifications as are encompassed by the scope of the appended 
claims.