US3704219A

US3704219A - Impedance matching network for use with sputtering apparatus

Info

Publication number: US3704219A
Application number: US131903A
Authority: US
Inventors: Robert Bruce Mcdowell
Original assignee: Mcdowell Electronics Inc
Current assignee: Mcdowell Electronics Inc
Priority date: 1971-04-07
Filing date: 1971-04-07
Publication date: 1972-11-28
Anticipated expiration: 1989-11-28

Abstract

AN IMPEDANCE MATCHING NETWORK FOR USE WITH A RADIO FREQUENCY BIAS SPUTTERING SYSTEM WHERE THE SYSTEM IS IMPEDANCE MATCHED TO A RADIO FREQUENCY POWER GENERATOR CONNECTED VIA AN IMPUT LINE TO THE IMPEDANCE MATCHING NETWORK. THE MATCHING NETWORK PERMITS NEAR 0 TO 100% BIAS SPUTTERING IN EITHER DIRECTION.

Description

' Nov. 28, 1972 R. B. MCDOWELL 3,704,219 IMPEDANCE MATCHING NETWORK FOR USE WITH SPUTTERING APPARATUS Filed April 7, 1971 INVENTOR ROBERT BRUCE MCDOWELL ATTORNEYS.

mozmmzmo m 1 United States Patent Office Patented Nov. 28, 1972 3,704,219 IMPEDANCE MATCHING NETWORK FOR USE WITH SPUTTERING APPARATUS Robert Bruce McDowell, Metuchen, N.J., assignor to McDowell Electronics, Inc., Metucben, NJ. Filed Apr. 7, 1971, Ser. No. 131,903 Int. Cl. C23c 15/00 US. Cl. 204-192 13 Claims ABSTRACT OF THE DISCLOSURE An impedance matching network for use with a radio frequency bias sputtering system where the system is impedance matched to a radio frequency power generator connected via an input line to the impedance matching network. The matching network permits near to 100% bias sputtering in either direction.

BACKGROUND OF THE INVENTION This invention relates to sputtering systems and, in particular, to impedance matching networks for use therewith.

The purpose of radio frequency bias sputtering is to support a certain amount of back sputtering during the forward sputtering process. This small amount of back sputtering, or small percentage of RF bias sputtering insures a cleaner sputtered material since all loosely bonded particles are sputtered back off the substrate. The radio frequency energy from the stabilized radio frequency power supply must be applied to the sputtering electrode assembly including an anode and cathode through an impedance matching network. Conventional bias sputtering systems include two separate impedance matching networks, one for the anode and the second for the cathode. However, there is much difiiculty in tuning this type of system since the tuning of each network detunes the other. Thus, many dials have to be tuned and retuned many times for each level of radio frequency biasthat is, percentage of radio frequency bias, and for each change in electrode spacing or sputtering chamber pressure.

Further, conventional metering used with bias sputtering systems is that used for measuring radio frequency power and voltage or current. With this type of metering, it becomes necessary to calculate the actual sheath voltage diiferential, which in reality is a negative direct current potential.

SUMMARY OF THE INVENTION A primary object of this invention is to provide a unique single impedance matching network for matching a radio frequency power supply to a sputtering electrode system.

A further object of the invention is to provide a unique impedance matching network of the above type having direct filtered D.C. metering of each sheath voltage.

It is a further object of the invention to provide an impedance matching network of the above type which is easily tuned to effect the desired impedance match between the radio frequency power source and the sputtering electrode system.

It is a further object of this invention to provide a unique impedance matching network of the above type having an input circuit portion which can be either series or shunt tuned to an input line to the network of any fixed characteristic impedance.

It is a further object of the invention to provide a unique impedance matching network of the above type having an output portion which can be series or shunt tuned to the impedance of individual electrodes of the sputtering electrode system with respect to each other or with respect to ground.

It is a further object of this invention to provide a unique impedance matching network of the above type which enables the placement of positive DJC. sheath voltages on at least one of the electrodes of the sputtering electrode system whereby a plasma anodization type of sputtering technique is available.

It is a further object of this invention to provide a unique radio frequency bias sputtering system utilizing at least one positive D.C. sheath voltage for implementing a plasma anodizing kind of sputtering technique.

It is a further object of this invention to provide a unique impedance matching circuit of the above type wherein a circuit may be readily modified to implement radio frequency diode sputtering.

Other objects and advantages of this invention will become apparent upon reading the appended claims in conjunction with the following detailed description and the attached drawing.

BRIEF DESCRIPTION OF THE DRAWING The figure of the drawing is a schematic diagram of illustrative circuitry constituting the unique impedance matching network of this invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION Referring to the drawing there is shown an illustrative impedance matching network in accordance with the invention. This network may be connected directly to a Radio Frequency generator 10 or connected to the generator 10 via a coaxial cable 12. The characteristic impedance of cable or line 12 is typically 50 ohms but may be of any fixed characteristic impedance. Connected to cable 12 is Radio Frequency watt meter 14, which may represent one or more watt meters to measure forward and reflected power on cable 12.

The input terminal 16 of the impedance matching network is connected to an inductive voltage divider network generally indicated at 18 comprising two parallel connected variable inductors 20- and 22 which are respectively connected to the inner conductor 24 of the coaxial cable and the outer conductor 26 which is grounded as indicated at 28.

The inductors 20 and 22 are tuned to the input line 12 by means of a shunt input tuning capacitor 30. Broadly speaking, inductors 20' and 22 and tuning capacitor 30 may be termed the input portion of the impedance matching circuit.

Adjustable taps 32 and 34 for variable inductors 20 and 22 are ganged so that when the voltage from one of the inductors is at a maximum the voltage from the other is at a minimum and vice versa with intermediate settings of one inductor increasing whenever the intermediate settings of the other decrease or vice versa. The taps 32 and 34 are respectively connected to

series tuning capacitors

36 and 38 which are respectively connected to the anode 40 and the cathode 42 of the sputtering electrode assembly diagrammatically indicated by the dotted line 44. A shutter 46 is also illustrated and is connected to ground via conductor 48. The variable capacitors 36, which will be brought out in more detail hereinafter, allow differential sheath voltage adjustment in addition to that obtainable from the variable inductors 20 and 22 whereby any desired amount of radio frequency bias sputtering in either direction can be obtained. Broadly speaking, the

capacitors

36 and 38 may be termed the output portion of the impedance matching network.

Each of the sheath voltages at the anode 40 and catho'de 42 are respectively measured by direct

current meters

50 and 52. These meters are isolated by Radio Frequency 3

chokes

54 and 56 respectively from the anode 40 and cathode 42. Appropriate meter loading is elfected by switches 58 and 60. Switch 58 selects the appropriate one of resistors 62, 64 and 66 while switch 60 selects the appropriate one of resistors '68, 70 and 72 to effect the appropriate loading of the

meters

50 and 52.

In operation, the radio frequency power generator is properly tuned to the characteristic impedance of coaxial or shielded line 12, as indicated by directional Radio Frequency watt meter 14. After accomplishing this, the inductive voltage divider 18 is set at the approximate position required for the desired amount of radio frequency bias and the desired sputtering direction. Capacitor 30 is then adjusted until the input portion of the impedance matching network is tuned to the characteristic input impedance of line 12 as indicated on the directional watt meter 14 so that there is maximum forward power and minimum reflected power in line 12. Although the tuning is shown as shunt in the figure, it is to be understood that this tuning could also be series tuning whereby a pair of variable capacitors would be respectively connected in the lines including variable inductors 20 and 22.

The desired sheath voltage of the Radio Frequency high electrode, as predetermined by the setting of the taps of the inductive voltage divider 18, is tuned in by one of the

series capacitors

36 or 38. Assuming that the anode 40 is the high voltage electrode, the variable capacitor 36 would be tuned until the desired sheath voltage is obtained for the anode. This would be indicated by the reading on meter 50. The tuning adjustment of the high voltage electrode is critical and its setting may require the returning of the input tuning capacitor 30, if an increase in reflected line power is indicated in line 12.

The final adjustment is that of the low voltage electrode. Assuming that this electrode is the cathode 42, this is accomplished by adjusting tuning capacitor 38 to the desired sheath voltage as indicated by meter 5-2. Thus, the desired percentage of radio frequency bias is obtained. Hence, if the sheath voltage of the anode were --1500 volts and that of the cathode were 300 volts, the percentage of radio frequency bias would be 20%.

The adjustment of the low voltage electrode 42 is not critical and in most cases will not necessitate the retuning of the input circuit portion. Assuming that capacitor 38 is employed to effect the tuning of the low voltage electrode, it is possible with this capacitor to obtain an extremely wide sheath voltage range even in the positive direction, the ramifications of which will be described in more detail hereinafter.

Although tuning of the anode 40 and cathode 42 has been described in terms of the

series capacitors

36 and 38, the tuning of these electrodes can also be effected by shunt tuning whereby first and second shunt capacitors would be respectively connected from the lines connected to taps 32 and 34 respectively to ground.

Since it is clear either the anode or cathode may be selected as the high voltage electrode, sputtering may be effected in either direction. Further, since the sheath voltage dilferential can be roughly established by the setting of taps 32 and 34 and very precisely established by

variable capacitors

36 and 38, the percentage of radio frequency bias can also be accurately established and in fact, near 0 to 100% bias sputtering can be effected in either direction without physically or electrically changing wires or connections.

Further, this is accomplished through only four tuning dials (not shown), one of which would be connected to the ganged taps 32 and 34, the second of which would be connected to capacitor 30, the third of which would be connected to capacitor 36 and the fourth of which would be connected to capacitor 38.

A further aspect of the invention is that by a simple modification of the circuitry of the figure, the circuitry can be converted from one implementing radio frequency bias sputtering to one that implements conventional radio frequency diode sputtering. This is done by merely switching switch 74 from the position shown in the figure to terminal 75 thereby grounding the anode 40 and disconnecting the variable capacitor 36 from the circuit. The tuning procedure for radio frequency diode sputtering would be the same as that for radio frequency bias sputtering, as described hereinbefore, with the single exception that the variable capacitor 36 would not be tuned since it would, of course, not be a part of the circuit.

As stated hereinbefore, the tuning range of the capacitor associated with the low voltage electrode may be extremely wide and thus positive sheath voltages are available at this electrode. Thus, if capacitor 38 is varied to establish the sheath voltage at cathode 42, it being assumed that the cathode is the radio frequency voltage electrode, the capacitor 38 may be so varied such as to establish a positive sheath voltage at the cathode 42. Present anodizing processes are generally effected electrolytically in a liquid bath across which a DC. potential has been applied to produce free oxygen, the part to be oxidized or substrate being connected to the positive terminal. Negative oxygen ions form on the substrate thereby oxidizing it.

With positive RF bias or DC sheath voltage on the cathode 42, the cathode or substrate electrode 42 will be positive with respect to the plasma. Hence, anodization of both electrically conductive and insulative materials is possible. Further, with positive radio frequency bias voltage or DC. sheath voltage, insulating substrates can be anodized by the radio frequency induced anodizing voltage with no forms of conducting clips, as required in conventional methods of anodizing.

A further advantage in positive RF bias sputtering or positive D.C. sheath voltages appears to be present in the process of sputtering tantalum to insulating materials such as quartz or ceramics. The advantage would be better adhesion. During the cycle of positive sheath voltage, tantalum oxide would be sputtered. When the substrate electrode is returned to establish zero sheath voltage or negative sheath voltage, tantalum would then be sputtered, rather than tantalum oxide.

Numerous modifications of the invention will become apparent to one of ordinary skill in the art upon reading the foregoing disclosure. During such a reading it will be evident that this invention provides a unique impedance matching network for accomplishing the objects and advantages herein stated.

What is claimed is:

1. An impedance matching network for use in a radio frequency bias sputtering system including first and second electrodes, said system being connected to a radio frequency power generator, said network comprising:

(a) an input circuit including first and second variable inductors connected in parallel across the output of said generator where first and second taps are respectively connected to said first and second variable inductors to derive the respective variable voltages therefrom and where said first and second taps are ganged together so that when the voltage output from said first variable inductor changes in magnitude in one direction, the voltage output from said second variable inductor changes in magnitude in the other direction and at least one variable capacitor for tuning said input circuit to the output impedance of said generator;

(b) means connected to the variable output voltage terminal of said first variable inductor and said first electrode for further varying the voltage from said first variable inductor to thereby establish the sheath voltage at said first electrode; and

(c) means connected to the variable output voltage terminal of said second variable inductor and said second electrode for further varying the voltage from said second variable inductor to establish the sheath voltage at said second electrode;

whereby near to 100% bias sputtering can be effected in either direction between said first and second electrodes.

2. A network as in claim 1 where said first and second tuning means each includes a variable capacitor connected in series between its associated inductor and electrode.

3. A network as in claim 2 where said one variable capacitor of said input circuit is connected across the said output of said generator.

4. An impedance matching network as in claim 1 including switching means for connecting said first electrode to (1) said means for establishing a sheath voltage at said first electrode or (2) a reference potential whereby said impedance matching circuitry is switched to (l) a radio frequency diode sputtering mode of operation when said first electrode is connected to said reference potential and (2) a radio frequency bias sputtering mode of operation when said first electrode is connected to said means for establishing the sheath voltage at said first electrode.

5. An impedance matching network for use in a radio frequency bias sputtering system including first and second electrodes, said system being connected to a radio frequency power generator by an input line, said network comprising:

(a) an input circuit including first and second variable inductors connected in parallel across said input line where first and second taps are respectively connected to said first and second variable inductors to derive the respective variable voltages therefrom and where said first and second taps are ganged together so that when the output voltage from said first variable inductor changes in magnitude in one direction, the output voltage from said second variable inductor changes in magnitude in the other direction and at least one variable capacitor for tuning said input circuit to the characteristic impedance of said input line;

(b) means connected to the variable output voltage terminal of said first variable inductor and said first electrode for further varying the output voltage from said first variable inductor to thereby establish the sheath voltage at said first electrode; and

(c) means connected to the variable output voltage terminal of said second variable inductor and said second electrode for further varying the voltage from said second variable inductor to establish the sheath voltage at said second electrode whereby near 0 to 100% bias sputtering can be effected in either direction between said first and second electrodes.

6. A network as in claim 5 where said first and second tuning means each includes a variable capacitor connected in series between its associated inductor and electrode.

7. A network as in claim 6 where said one variable capacitor of said input circuit is connected across said input line.

8. A network as in claim 5 where said first and second electrodes are an anode and a cathode respectively.

9. A network as in claim 8 including a first measuring circuit connected between said anode and a reference potential for measuring the anode sheath voltage and a second measuring circuit connected between the cathode and reference potential for measuring the cathode sheath voltage.

10. A network as in claim 9 where each said measuring circuit includes a voltage meter in series with a radio frequency choke.

11. A network as in claim 5 where said input line is a coaxial cable and where the outer conductor thereof is grounded.

12. A method of operating the network of claim 5 including the steps of:

(a) adjusting the position of the ganged taps of said first and second variable inductors until the desired sputtering direction and amount of radio frequency bias is obtained;

(b) adjusting said one variable capacitor until said input circuit is impedance matched to said input line;

(c) adjusting said means for establishing the sheath voltage at said first electrode until said last-mentioned sheath voltage is established; and

(d) adjusting said means for establishing the sheath voltage at said second electrode until said last-mentioned sheath voltage is established.

13. The method as in claim 12 where said means for establishing the sheath voltage at said second electrode is adjusted until a positive bias voltage with respect to ground is established at said second electrode.

References Cited UNITED STATES PATENTS 3,471,396 10/1969 Davidse 204298 3,525,680 8/1970 Davidse et al. 204298 3,616,405 10/1971 Beaudry 204298 JOHN H. MACK, Primary Examiner S. S. KANTER, Assistant Examiner US. Cl. X.R. 204298