Microwave-excited plasma processing apparatus

A microwave-excited plasma processing apparatus shows a wide pressure range and a wide applicable electric power range for normal electric discharges as a result of using slits cut through a rectangular waveguide and having a profile that allows the electric field and the magnetic field of microwave to be formed uniformly right below the microwave introducing window below an microwave antenna. The microwave-excited plasma processing apparatus is characterized by having four elliptic slits cut through the wall of the rectangular waveguide that is held in contact with the microwave introducing window of the top wall of the vacuum chamber, the four elliptic slits being arranged respectively along the four sides of a substantial square.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a plasma processing apparatus for manufacturing semiconductor devices.

2. Related Background Art

Conventional techniques of radiating a microwave from a microwave antenna and irradiating processing gas with it in order to generate plasma thereof and cause the generated plasma to act on a Si substrate for manufacturing semiconductor devices for a predetermined processing operation include one that utilizes a microwave waveguide provided with one or more than one slits and one adapted to feed a microwave to a disk provided with coaxially arranged slits (see Patent Document 1: Japanese Patent Application Laid-Open Publication No. 9-129613 and Patent Document 2: Japanese Patent Application Laid-Open Publication No. 11-111620).

FIG. 1of the accompanying drawings illustrates a conventional microwave antenna that is described in Patent Document 1. The microwave antenna comprises a waveguide A that has a bottom B and a reflection surface C. A pair of slits D are cut through the bottom B of waveguide A so as to show a profile with a width that is narrowed stepwise toward the reflection plane C of the waveguide A in order to allow the reflected wave reflected by the reflection surface C and the incident wave to produce a uniformly synthesized wave.

FIG. 2of the accompanying drawings illustrates another conventional microwave antenna that is described in Patent Document 2. It is a radial line slot antenna. The radial line slot antenna is provided with a number of coaxially arranged slots E and the circularly polarized microwave generated from the slots E is introduced into a vacuum chamber so as to generate and excite plasma by means of the electric field of the microwave.

The inventors of the present invention actually prepared antennas according to the descriptions of Patent Documents 1 and 2 and operated them to see how they perform in an experiment As a result of the experiment, it was found that the both antennas can be driven to operate only within a narrow pressure range and a narrow applicable electric power range for normal electric discharges as shown in Tables 1 and 2 below. Tables 1 and 2 summarily show the results of the experiments obtained respectively by using a microwave antenna according to Patent Document 1 and a microwave antenna according to Patent Document 2.

TABLE 1applied electricpower/pressure20 Pa50 Pa90 Pa1.0 kWXX◯1.5 kW◯◯X1.8 kW◯XX2.0 kWXXXN.B.:◯ indicates that a normal electric discharge was allowed.X indicates that a normal electric discharge was not allowed because of a large reflected wave.

TABLE 2applied electricpower/pressure20 Pa50 Pa90 Pa1.0 kWXX◯1.5 kWXX◯1.8 kWXX◯2.0 kWXX◯N.B.:◯ indicates that a normal electric discharge was allowed.X indicates that a normal electric discharge was not allowed because of a large reflected wave.

The reason for the narrow pressure range and the narrow applicable electric power range may be that a large voltage standing wave ratio (VSWR) of the reflected wave/the progressive wave arises due to an unbalanced electromagnetic field of microwave that is found right below the microwave introducing window. Therefore, the electromagnetic field that is found right below the microwave introducing window below a microwave antenna as described in each of Patent Documents 1 and 2 was determined by simulation. As a result, local concentrations and asymmetries of electromagnetic field were found as shown inFIGS. 3A,3B,3C and3D andFIGS. 4A,4B,4C and4D.

SUMMARY OF THE INVENTION

In view of the above identified circumstances, it is therefore the object of the present invention to provide a microwave-excited plasma processing apparatus showing a wide pressure range and a wide applicable electric power range for normal electric discharges by using slits cut through a rectangular waveguide and having a profile that allows the electric field and the magnetic field of microwave to be formed uniformly right below the microwave introducing window below an microwave antenna.

According to the present invention, the above object of the invention is achieved by providing a microwave-excited plasma processing apparatus comprising:a vacuum chamber having an aperture formed through a top wall thereof, a processing table arranged below said aperture to hold a substrate to be processed thereon and a microwave introducing window arranged above said aperture of the top wall so as to maintain the inside of the vacuum chamber in vacuum;a gas supply pipe for supplying processing gas into the vacuum chamber;
an exhaust system for exhausting the inside of the vacuum chamber to maintain it in vacuum; anda rectangular waveguide arranged on and in contact with the microwave introducing window, the rectangular waveguide having a terminal plane arranged perpendicularly relative to the traveling direction of the microwave in the rectangular waveguide, a microwave antenna being formed by forming four slits through the wall of the waveguide held in contact with the microwave introducing window, the four slits being arranged along the respective four sides of a quadrangle, each of the slits running either in parallel with or perpendicularly relative to the microwave progressing direction.

In a microwave-excited plasma processing apparatus according to the invention, each of the slits may be elliptic.

In a microwave-excited plasma processing apparatus according to the invention, the center of the slit located closest to the terminal plane of the rectangular waveguide may be arranged at a position defined to be equal to n/2 (n being an integer) of the wavelength (λg) of the microwave from the terminal plane in the rectangular waveguide and the center of the slit located at a position opposite to the slit located closest to the terminal plane waveguide may be arranged at a position defined to be equal to m/2 (m being an integer) of the wavelength (λg) of the microwave from the slit located closest to the terminal plane in the rectangular waveguide.

In a microwave-excited plasma processing apparatus according to the invention, the major axis of each of the slits may be defined to equal to k/2 (k being an integer) of the wavelength (λO) of the microwave in vacuum.

In a microwave-excited plasma processing apparatus according to the invention, the rectangular waveguide is provided with a terminal plane arranged in the rectangular waveguide perpendicularly relative to the progressing direction of the microwave in order to reflect a microwave and an microwave antenna is formed by forming four slits through the wall of the waveguide held in contact with the microwave introducing window, the four slits being arranged along the respective four sides of a quadrangle, each of the slits running either in parallel with or perpendicularly relative to the microwave progressing direction. With this arrangement, it is possible to reduce the VSWR and obtain a large process margin.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Now, the present invention will be described by referring to the accompanying drawings that illustrate a preferred embodiment of the invention.

FIG. 5illustrates a microwave-excited plasma processing apparatus according to an embodiment of the present invention.

Referring toFIG. 5, reference symbol1denotes a vacuum chamber, which vacuum chamber1has an aperture2formed through a top wall thereof. A processing table4is arranged in the vacuum chamber1below the aperture2at a position opposite to the aperture2in order to bear an object3to be processed thereon. The object3to be processed is a Si wafer for manufacturing semiconductor devices in this embodiment. The processing table4has a heating mechanism and an electrostatic adsorption mechanism (both being not shown) in the inside thereof so that the object3of processing can be heated to a predetermined temperature.

InFIG. 5, reference symbol5denotes a vacuum exhaust system for exhausting the inside of the vacuum chamber1to produce vacuum therein and maintaining the internal pressure thereof to a pressure level adapted to the operation of processing the object3of processing, and reference symbol6denotes a gas supply pipe for supplying processing gas into the vacuum chamber1. The vacuum exhaust system5is formed by combining a turbo molecular pump and a dry pump and a variable conductance valve is fitted thereto so as to make it possible to exhaust the vacuum chamber1at a variable exhaust rate. During a processing operation, a desired gas pressure level can be maintained between 20 and 90 Pa by controlling both the exhaust rate and the flow rate of processing gas flowing in from the gas supply pipe6.

A microwave introducing window7is arranged above the aperture2of the top wall of the vacuum chamber1with an annular seal member8disposed between them so as to maintain the inside of the vacuum chamber1in vacuum. The microwave introducing window7is made of quartz in the illustrated embodiment. InFIG. 5, reference symbol9denotes a rectangular waveguide having a closed end that operates as reflecting end10. A flange11for connecting the waveguide to some other waveguide is arranged at the end opposite to the reflecting end10of the rectangular waveguide9so that a waveguide may be connected to it in order to transmit a microwave from a microwave power source (not shown). The rectangular waveguide9is provided with a circular flange12, which circular flange12is fitted to the vacuum chamber1by way of an insulating fitting member13.

The part of the bottom of the rectangular waveguide9that is held in contact with the microwave introducing window7is provided with four elliptic slits14. In other words, the slits14are exposed to the microwave introducing window7arranged above the vacuum chamber1. As shown inFIG. 6, the four slits14are arranged along the respective sides of a rectangle in order to produce a uniform electromagnetic field by a microwave on the surface of the microwave introducing window7in the vacuum chamber1. Thus, a microwave antenna is formed in this way.

The major axis of each of the slits14is defined to be equal to ½ of the wavelength (λ0) of the microwave in vacuum. With this arrangement, the microwave emitted from the microwave antenna proceeds to the outside of the rectangular waveguide without loss. The minor axis of each of the slits14is shorter than the longer axis thereof by definition and made equal to 15 mm so as to maximize the radiation efficiency of the microwave antenna.

Each of the slits14has an elliptic contour and does not have any corner in order to eliminate any propagation loss and achieve a high machining efficiency in the process of manufacturing the microwave antenna. Of the four slits14, the one located closest to the terminal plane or the reflecting plane10of the rectangular waveguide9is arranged at a position where its central axial line is separated from the reflecting plane by a distance equal to n/2 (n being an integer) of the wavelength (λg) of the microwave in the rectangular waveguide9. With this arrangement, the position of the slit agrees with the peak position of the standing wave of the microwave magnetic field of the TE10 mode in the rectangular waveguide9so that the microwave is radiated efficiently.

The paired slits14whose major axes are perpendicular relative to the traveling direction of the microwave are separated from each other by a distance equal to m/2 (m being an integer) of the wavelength (λg) of the microwave in the rectangular waveguide9. The paired slits14whose major axes are running in parallel with the traveling direction of the microwave are arranged as a function of the size of the rectangular waveguide9.

In the instance of the embodiment illustrated inFIG. 6, the rectangular waveguide9of the microwave antenna is made to have a width of 96 mm. Thus, the major axes of the slits14are made to be 61 mm long, or equal to ½ of the wavelength (λ0) of the microwave in vacuum. The distance from the reflecting end10to the slit14located closest to the former is made to be 79 mm, or equal to ½ of the wavelength (158 mm) of the microwave in the rectangular waveguide. This slit is separated from the oppositely disposed slit also by 79 mm. The paired slits that are perpendicular relative to these slits14, or the paired slits14whose major axes are in parallel with the traveling direction of the microwave, are separated from each other in such a way the distance between the center of the slits is close to ½ of the wavelength and the outer edges of the slits are respectively in line with the corresponding inner wall surfaces of the rectangular waveguide9.

FIGS. 7A,7B,7C and7D show the electromagnetic field generated by the microwave antenna of the embodiment, whose principal part is illustrated inFIG. 6, as obtained by simulation. As seen from these drawings, both the electric field and the magnetic field are least asymmetric and show a uniform distribution. Table 3 below shows the value of the VSWR actually measured by using the microwave antenna of this embodiment in comparison with the corresponding values observed by using the prior art microwave antennas described in Patent Documents 1 and 2.

TABLE 3type of microwave antennaactually measured VSWRprior art microwave antenna of Pat. Doc. 13.9prior art microwave antenna of Pat. Doc. 28.9microwave antenna of this invention1.5

From Table 3, it is clear that the VSWR of the microwave antenna of the present invention is smallest and hence the microwave antenna efficiently radiates microwave energy.

In an experiment, a Si wafer for manufacturing semiconductor devices, or an object3of processing, was heated to 250° C. and H2O gas was made to flow at a rate of 300 sccm by way of the gas supply pipe6by means of a gas flow rate control system (not shown).

In the actual processing operation of the experiment, the Si wafer, or the object3of processing, onto which resist had been applied to a predetermined thickness, was brought into the vacuum chamber1and placed on the processing table 4. Then, processing gas was introduced from the gas supply pipe6into the vacuum chamber1and, at the same time, the internal pressure of the vacuum chamber1was held to 20 Pa by regulating the exhaust rate of the exhaust system.

Thereafter, 1.5 kW was applied from the microwave power source to the microwave antenna for about 20 seconds.

The wafer was taken out and the reduction of the resist film thickness was measured to compute the ashing rate and the distribution of the ashing rate on the Si wafer. A similar experiment was conducted by using the prior art microwave antennas described in Patent Documents 1 and 2 under the same condition and the obtained results were compared. Table 4 summarily shows the results.

From Table 4, it is clear that the microwave antenna of the present invention provides a high ashing rate and a uniform distribution of ashing rate.

The process margin was observed by means of the above described microwave antenna and by using Ar gas as processing gas, changing the pressure and the applied microwave power. Table 5 summarily shows the obtained results.

TABLE 5applied electricpower/pressure20 Pa50 Pa90 Pa1.0 kW◯◯◯1.5 kW◯◯◯1.8 kW◯◯X2.0 kWX◯XN.B.:◯ indicates that a normal electric discharge was allowed.X indicates that a normal electric discharge was not allowed because of a large reflected wave.

It is clear from Table 5 that the range of parameter value that allows a normal electric discharge is broadened to consequently broaden the process margin by using a microwave antenna according to the present invention.

While the microwave introducing window of the illustrated embodiment is made of quartz, the microwave introducing window of a microwave-excited plasma processing apparatus according to the present invention may be made of any material so long as it can transmit a microwave and maintain the inside of the vacuum chamber in vacuum.

While H2O gas and Ar gas were used in the above-described experiment, the types of gas that can be used as processing gas for a microwave-excited plasma processing apparatus according to the present invention are not limited thereto.

While slits are directly cut through the rectangular waveguide of the illustrated embodiment to produce a microwave antenna, it is also possible to cut slits through the annular flange and provide it with a U-shaped cover to make it show dimensions same as those of the rectangular waveguide.

The present invention can find applications in the field of plasma CVD system when a film forming raw material gas is used. The present invention can also find applications in the field of etcher or surface reforming mechanism when reactive gas is used. In short, a microwave-excited plasma processing apparatus according to the invention has a broad scope of application and provides a broad process margin as a result of the improvement in terms of the contour of the slits cut through the rectangular waveguide and their positioning.