WO1994020980A1

WO1994020980A1 - Cold wall reactor for heating of silicon wafers by microwave energy

Info

Publication number: WO1994020980A1
Application number: PCT/SE1994/000190
Authority: WO
Inventors: Rudolf Buchta; Jan Svennebrink
Original assignee: Stiftelsen Institutet För Mikroelektronik
Priority date: 1993-03-05
Filing date: 1994-03-04
Publication date: 1994-09-15
Also published as: SE9300742D0; SE9300742L; SE500124C2

Abstract

A cold wall reactor for heating a silicon wafer and comprising a microwave cavity to which there is connected a microwave generator for delivering microwave energy to the cold wall reactor. The invention is characterized in that the cold wall reactor (1) together with a waveguide (21) located between the microwave generator (22) and the cavity (2) is constructed to generate a rotating microwave field in the cavity (2); and in that a substrate holder (17) for supporting a wafer (18) in the reactor is positioned so that the wafer will be located at a minimum of the electric field strength in the rotating field. According to one preferred embodiment of the invention, the waveguide (21) and the microwave generator (22) are together intended to generate a circular polarized standing wave in the cavity.

Description

COLD WALL REACTOR FOR HEATING OF SILICON WAFERS BY MICROWAVE ENERGY

The present invention relates to a cold wall reactor for heating silicon wafers by microwave energy.

Swedish Patent Specification No. 8902391-5 describes a method and apparatus for heating silicon wafers in a cold wall reactor. Silicon wafers are heated in a reactor and gases are then introduced into the reactor so as to deposit different substances and/or compounds- upon the wafers. These substances or compounds are prevented from settling on the walls of the reactor, by keeping the walls cold. This is highly beneficial, since deposits on the reactor walls and other parts of the reactor interior change the process parameters and therewith prevent full control of the deposition process. Should the substances settle on the walls of the reactor and the internal reactor surfaces, it becomes necessary to clean the reactor, which can be effected by generating a plasma in the reactor.

According to the aforesaid patent specification, the reactor has the form of a microwave cavity to which microwave energy is delivered from a microwave generator, wherein a silicon wafer located in the cavity is heated by the microwave energy supplied, whereas the reactor walls and the inner reactor parts are not heated. Substantially all deposition is effected solely on the silicon wafer.

According to one preferred embodiment of the invention defined in the aforesaid patent specification, the cavity is a so-called single mode cavity, the mode preferably being a TE 112-mode.

The major difficulty in achieving a good result when heating silicon wafers by microwave heating is that it must be possible to heat the wafer to a temperature in the range

SUBSTITUTE SHEET of about 300-1200°C with temperature variations across the wafer surface of less than 1% of the temperature.

It has been found extremely difficult to achieve uniform heating in practice.

The present invention solves this problem and relates to a cold wall reactor by means of which a silicon wafer can be heated to a temperature within said temperature range and with the aforesaid tolerance across the surface of the wafer.

The present invention thus relates to a cold wall reactor which is intended for heating a silicon wafer and which is constructed as a microwave cavity and to which a microwave generator is connected to supply microwave energy thereto, wherein the cold wall reactor is characterized in that the reactor together with a waveguide between the microwave generator and the cavity is constructed to generate a rotating microwave field in the cavity; and in that a wafer substrate holder is so placed in the reactor that the wafer will be located at a minimum of the electric field strength in the rotating field.

The invention will now be described in more detail partially with reference to exemplifying embodiments thereof shown on the accompanying drawings, in which

Figure 1 is a cross-sectional view of a first embodiment of an inventive cold wall reactor; - Figure 2 illustrates a second embodiment of an inventive cold wall reactor; and

Figure 3 illustrates one principle of measuring the temperature of the wafer.

Figure 1 illustrates a cold wall reactor 1 which is constructed as a microwave cavity 2. The cavity 2 includes two mutually spaced walls 3, 4 between which a cooling water is introduced through a pipe 5. The cooling water is removed through a pipe 6. A gas inlet 7 and an associated so-called shower 8 are mounted in the upper part of the reactor. A gas suction outlet 9 is mounted in the lower part of the reactor and connected to a vacuum pump. The gas outlet suction line 9 is connected to a gas outlet suction box 10 which, in turn, communicates with the actual cavity 2 through the medium of a perforated disc 11.

Also mounted in the lower part of the reactor is an activator section 12 for delivering microwave energy to the cavity 2. The section 12 is circular in cross-section and includes two mutually spaced walls 13, 14, which define therebetween a space for accommodating cooling water. The water is delivered through a pipe 15 and removed through a pipe 16.

The cavity 2 has mounted therein a substrate holder 17 which supports a silicon wafer, one such wafer 18 being shown in broken lines in Figure 1. The substrate holder may include, for instance, three upstanding thin rods made of a material having a low loss factor. Rods made of such a material will not be heated to any appreciable extent by the microwave energy delivered to the cavity. Examples of such low loss material are highly pure quartz glass, other ceramic materials and Teflon®. The upper ends of the rods 17 are preferably pointed, so as to minimize the transmis¬ sion of heat through conduction from the wafer to the rods.

The cold wall reactor is thus constructed so that the entire inner part of the reactor will be kept well chilled. Because only a silicon wafer is intended to be heated in the reactor, the substances and compounds delivered to the reactor through the gas inlet 7 will be deposited solely on the wafer. Also mounted in the upper part of the reactor is a window 19 through which temperature is measured by means of a pyrometer which measures the infrared radiation 20 emitted by a heated wafer.

The reactor is conveniently constructed from stainless steel or aluminium. When corrosive gases are to be used, such as HC1 (hydrochloric acid) or Cl₂ (chlorine) , or gases which form corrosive products when decomposing thermally, such as TCA (trichloroethane) or TCE (trichloroethylene) , according to one preferred embodiment of the invention the inner walls of the reactor and optionally also other internal parts of the reactor, such as the pump section, are covered with a thin coating of corrosion-resistant material having a low loss factor, such as Teflon® for instance.

The gas inlet 7 can be connected in a known manner to a number of sources of different gases to be delivered to the reactor, such as deposition gases, i.e. gases used in the deposition phase and gases used for back-etching and cleaning purposes. Examples of such gases are Ar, SiH₄, WF₅, NF₃ and CF₄, where the two latter are typical etching gases.

In accordance with the invention, the cold wall reactor 1, together with a waveguide 21 located between a microwave generator 22 and the cavity 2 is constructed so as to generate a rotating field in the cavity.

According to one preferred embodiment, there is generated a circular-polarized standing wave in the cavity. The cavity is constructed as a single-mode cavity.

According to the invention, the substrate holder 17 which supports a silicon wafer 18 in the reactor is so positioned that a wafer 18 will be located at a minimum of the electric field strength of the rotating field. Because the wafer is positioned where the electric field strength has a minimum, the magnetic field strength will have a maximum at the wafer location. This means, in turn, that the resistive silicon wafer will be coupled inductively to the magnetic field and thus heated by resistive heating. Because the field rotates, perfect symmetry is achieved with regard to power distribution in the field.

According to one preferred embodiment of the invention, the microwave generator 22 is a cw-microwave generator (contin¬ uous wave) . There is preferably used a microwave generator which has a frequency of 2.45 GHz and a power of 1600 watts, for instance, when the wafer has a diameter of 100 millimeters. The frequency is lowered, suitably to 915 MHz when larger wafers that have a diameter of 200 millimeters for instance are to be processed.

According to a greatly preferred embodiment, the reactor is constructed to form a TE lln- ode in the cavity. This mode is ideally rotationally symmetrical as a consequence of the circular polarization. When this mode is used and the wafer is placed in a position where the electrical field strength has a minimum, the wafer will be heated very uniformly. In this regard, the temperature differences between different parts of the wafer will lie within 1%.

According to the invention, a circular polarized standing wave is generated in the cavity by coaction between the cavity and the waveguide 21. A first part 23 of the waveguide, which includes a rectangular waveguide intended to generate a TE 10-mode is located between the microwave generator 22 and the reactor 1. A second waveguide part 25 includes a funnel-shaped portion which is intended to adapt the first rectangular waveguide portion 23 to a third circular part 26 of the waveguide. The second waveguide part 25 is intended to transform the TE 10-mode to a linear polarized TE 11-mode.

According to one preferred embodiment of the invention, a section of the first waveguide part includes a three-port circulator and a direction coupler. The circulator and direction coupler are illustrated schematically in Figure 1 and referenced 24. The first waveguide part 23 is intended to form a TE 10-mode downstream of the direction coupler. The purpose of this construction is to enable the input energy and reflected energy to be measured.

According to a second preferred embodiment of the invention, illustrated in Figure 2, a fifth, elliptical part 37 of the waveguide is located between the third waveguide part 25 and a fourth, circular waveguide part 27. The fifth waveguide part 32 includes two circular parts 33, 34 and two funnel-shaped parts 35, 36 and the part 37 of elliptical cross-section located between the funnel-shaped parts. This elliptical waveguide part is intended to transform the linear polarized wave to a circular polarized wave, this circular polarized wave being coupled in the cavity. According to this embodiment a standing wave is thus obtained in the cavity. This embodiment of the invention also results in extremely uniform heating of the wafer with a temperature distribution within the aforesaid tolerance.

A first embodiment of the inventive reactor includes a fourth circular waveguide part 27 which has a dielectric plate 29. This plate is constructed in a known manner to transform the linear polarized wave to a circular polarized wave, this circular polarized wave being coupled in the cavity 2. Such a plate 29 is elongated in the longitudinal direction of the waveguide. The circular polarized wave is formed in certain rotational positions of the plate 29 in relation to the waveguide. When the plate is fixedly mounted, the plate will have such a rotational position.

According to one preferred embodiment of the invention which includes the plate 29, the dielectric plate 29 is carried by a rotating rod 30 which is driven by means of an electric motor 31. The rod is made of a material having a low loss factor, preferably quartz. The rod 30 extends through the waveguide to the motor 31. The rod 30 may be journalled or guided at its upper end, for instance with the aid of a number of quartz spokes which extend radially between the rod and the wall of the waveguide. The motor 31 is constructed to rotate at a speed of up to about 50 r.p.m.

The field is disturbed as the plate 29 rotates, so that a standing circular-polarized wave will only be generated in the cavity at certain rotational positions of the plate. Other field patterns are formed when the plate is located in positions other than these rotational positions. It has been found that such rotation of the plate results in uniform heating of the wafer.

Thus, in the case of this second embodiment, there is obtained either a standing circular polarized wave, i.e. when the plate does not rotate, or an occasional standing circular-polarized wave, i.e. when the plate is brought into certain rotational positions as it rotates.

It is also advantageous to provide the first embodiment which includes the elliptical waveguide part with a rotating dielectric plate 29 so as to excite other modes also in the case of the first embodiment.

It may be difficult in some instances to maintain suffi¬ cient rotational symmetry to achieve circular polarization. In these instances, it is possible to obtain a rotating field pattern with the aid of a rotating plate.

Only certain parts of the Figure 2 illustration have been identified with reference signs for the sake of clarity.

According to one preferred embodiment of the invention, microwave energy is delivered from the waveguide 21 to the cavity by means of a high density window 28 of aluminium oxide (A1₂0₃) . In the case of a cavity whose diameter is greater than 150 millimeters and with a microwave frequency of 2.45 GHz, the window has a diameter of 38 millimeters and a thickness of 6.8 millimeters.

A window of this nature has a low loss factor and is not therefore heated by microwaves transmitted through the window. Because the window is highly dense, it is also vacuum tight, which is a necessity.

Figure 2 illustrates another alternative embodiment of coupling the waveguide, in which there is located between the third waveguide part 26 and the fourth waveguide part 27 a window 38 of highly dense aluminium oxide (A1₂0₃) for coupling the circular polarized microwave energy to the fourth waveguide part 27. A circular opening 39 which is adapted to couple microwave energy to the cavity is provided between the fourth waveguide part 27 and the cavity 2. This opening is not provided with any form of window. The advantage with this embodiment compared with waveguide coupling according to the Figure 1 embodiment is that the window 38 is positioned further away from the wafer. As a result, the window is heated by the heat radiating from the silicon wafer to a lesser extent than in the case when the window is positioned in accordance with Figure 1. Both window positions can be used in both of the aforedes- cribed cases to obtain circular polarization, i.e. by means of a dielectric plate or an elliptic waveguide part.

According to another greatly preferred embodiment of the invention, a short circuiting plane is provided in the upper part of the cavity. This plane can be displaced in the axial direction of the reactor so as to change the length of the cavity and therewith also its resonance frequency.

The position of the plane is adjusted to adapt the length of the cavity to the varying impedance produced by the wafer as its temperature changes during heating of the wafer.

According to one embodiment, this plane is formed by the shower 8 of the gas inlet. The shower 8 is comprised of a hollow body provided with gas inlet holes on its bottom surface. In the case of the Figure 1 embodiment, the gas inlet pipe 7 is connected to an arm 40 which is connected, in turn, to a ball screw 41. The ball screw is driven by an electric motor 42. The arm 40 is displaced as the motor operates, therewith moving the pipe 7 and the shower 8 upwards and downwards in the direction of the arrows 43.

The directional coupler 24 is used to measure the input microwave energy and reflected microwave energy. The ratio between input energy and reflected energy is changed as a result in the change that takes place in the properties of the silicon wafer when substances are deposited thereon, for instance when depositing an electrically conductive layer on the wafer. In this regard, the short-circuiting plane is displaced so as to change the length of the cavity, therewith adapting the resonance frequency of the cavity to the impedance of the cavity including the wafer contribution. The position of the short-circuiting plane is herewith adjusted with regard to maximum input energy.

In order to optimize cavity conditions, it is essential that both the upper and the lower short-circuiting planes of the cavity are well defined.

Accordingly, a microwave trap is formed in the upper part of the cavity and in the lower part thereof for this reason, among others. The microwave traps are so formed as to obtain a well-defined short-circuiting plane in both the lower and the upper parts of the cavity. The microwave trap in the lower cavity part includes the wall portion 44 and the opening 45. From a short-circuiting aspect, the plane 46 thus extends out to the inner wall 3 of the cavity. The microwave trap in the upper part of the cavity includes the wall part 47 and the gap or slot 48. From a short-circuit¬ ing aspect, this means that the plane comprised of the bottom surface of the shower extends completely to the inner cavity wall 3.

The reactor can therefore advantageously be divided along a line A-A in Figure 1 without requiring the various parts to be in very good electric contact with one another when assembled.

It has been mentioned in the aforegoing that temperature is measured with the aid of a pyrometer. The drawback with this method, however, is that it depends on the emissivity of the surface layer and, in certain cases,' can be highly complicated by interference phenomena, for instance when depositing several thin layers on the wafer. On the other hand, when the wafer temperature is measured with the aid of a radiometer at a frequency for which the cavity is tuned but which differs from the frequency used to heat the wafer, the radiation is independent of the nature of the surface layer. An example of such a measuring frequency is 2 x 2.45 GHz (TE 114-mode) . The emissivity is always equal to 1 when this condition is fulfilled.

The reactor signal is taken out through a waveguide 40', which may be placed in the position of the window 19. Figure 3 is a simplified illustration of this principle, and shows the upper part of the reactor schematically. The signal is led from the waveguide 40' to a switch 41, by means of a coaxial cable 42. The signal is amplified in an amplifier 43 and is then detected in a detector 44, whereafter the signal is compared with a calibrated source 45 constituting a reference. The reference numeral 46 in Figure 3 identifies a driver while reference numeral 47 identifies a comparison circuit. The measurement signal is taken out through an outlet 48. The principle of measuring temperature with the aid of radiometry is well known and will not therefore be described in detail here.

Although the invention has been described above with reference to a number of exemplifying embodiments thereof, it will be understood that the reactor construction can be varied in many ways.

The invention is not therefore considered to be restricted to the aforedescribed embodiments thereof, since variations and modifications can be made within the scope of the following Claims.

Claims

1. A cold wall reactor for heating a silicon wafer and comprising a microwave cavity to which there is connected a microwave generator for delivering microwave energy to the cold wall reactor, characterized in that the reactor

(1) together with a waveguide (21) located between the microwave generator (22) and the cavity (2) is constructed to generate a rotating microwave field in the cavity (2) ; and in that a substrate holder (17) for supporting a wafer

(18) in the reactor is positioned so that the wafer will be located in the rotating field at a minimum of the electric field strength of said field.

2. A cold wall reactor according to Claim 1, character¬ ized in that the waveguide (21) and the microwave generator (22) together are constructed to form a circular polarized standing wave in the cavity.

3. A reactor according to Claim 1 or 2, characterized in that there is located between the microwave generator (22) and the reactor (1) a first waveguide part (23) which includes a rectangular waveguide which is intended to generate a TE 10-mode; in that a second waveguide part (25) which includes a funnel-shaped part is intended to adapt the first rectangular waveguide part (23) to a third, circular waveguide part (26) where said second waveguide part (25) is intended to transform the TE 10-mode to a linear polarized TE 11-mode; and in that there is located between the third waveguide part (26) and a fourth, circular waveguide part (27) a fifth, elliptical waveguide part (37) which is intended to transform the linear polarized wave to a circular polarized wave, this circular polarized wave being coupled in the cavity (2) .

4. A reactor according to Claim 1, 2 or 3, characterized in that the waveguide (21) and the microwave generator (22) together are intended to generate a rotating standing wave in the cavity by means of a rotating dielectric plate (29) which is located in a waveguide section (27) and which rotates in a linear polarized microwave field.

5. A reactor according to Claim 4, characterized in that located between the microwave generator (22) and the reactor (1) is a first waveguide part (23) which includes a rectangular waveguide which is intended to generate a TE 10-mode; in that a second waveguide part (25) which includes a funnel-shaped part is intended to adapt the first rectangular waveguide part (23) to a third, circular waveguide part (26) wherein said second waveguide part (25) is intended to transform the TE 10-mode to a linear polarized TE 11-mode; and in that a fourth, circular waveguide part (27) which includes a dielectric plate (29) is intended to transform the linear polarized wave to a circular polarized wave, this circular polarized wave being coupled in the cavity (2) .

6. A reactor according to Claim 5, characterized in that the dielectric plate (29) is carried by a rotating rod (30) driven by means of an electric motor (31) .

7. A reactor according to Claim 1, 2, 3, 4, 5 or 6, characterized in that the reactor is constructed to generate a TE lln-mode in the cavity (2) .

8. A reactor according to Claim 1, 2, 3, 4, 5, 6 or 7, characterized in that the microwave generator (22) is a cw- microwave generator.

9. A reactor according to Claim 1, 2, 3, 4, 5, 6, 7 or 8, characterized in that the upper part and the lower part of the cavity (2) include a respective microwave trap (47, 48; 44, 45) which are so constructed as to form a well-defined short-circuiting plane in both the lower and the upper parts of the cavity.

10. A reactor according to Claim 1, 2, 3, 4, 5, 6 or 7, characterized by a window (28;38) of high density aluminium oxide (A1₂0₃) which functions to couple microwave energy from the waveguide (21) to the cavity (2) .

11. A reactor according to Claim 10, characterized in that a window (38) of high-density aluminium oxide (A1₂0₃) is located between the third waveguide part (26) and the fourth waveguide part (27) for coupling the circular polarized microwave energy to the fourth waveguide part (27) ; and in that a circular opening (39) is provided between the fourth waveguide part (27) and the cavity (2) for coupling the microwave energy to the cavity (2) .

12. A reactor according to any one of the preceding Claims, characterized in that the short-circuiting plane in the upper cavity part can be displaced in the axial direction of the reactor so as to change the length of the cavity (2) and therewith its resonance frequency.

13. A reactor according to any one of Claims 3-12, characterized in that a three-port circulator and a directional coupler (24) are mounted in a section of said first waveguide part (23) which includes a rectangular waveguide.

14. A reactor according to any one of the preceding Claims, characterized in that the inner walls of the reactor and optionally also inner parts of the reactor are coated with a thin layer of corrosion-resistant material having a low loss factor.

15. A reactor according to any one of the preceding Claims, characterized in that a waveguide (40) extends through the reactor wall and functions to lead-out a frequency for which the cavity is tuned, said waveguide being connected to a coaxial cable (42) which functions to lead a waveguide signal to a measuring circuit (41, 43-48) for measuring the temperature of the wafer by radiometry.