Electron density measurement and plasma process control system using a microwave oscillator locked to an open resonator containing the plasma

• US 6,741,944 B1
• Filed: 05/22/2002
• Issued: 05/25/2004
• Est. Priority Date: 07/20/1999
• Status: Expired due to Term

• Current Assignee: Tokyo Electron Limited
• Original Assignee: Tokyo Electron Limited
• Inventors: Verdeyen, Joseph T., Sirkis, Murray D., Johnson, Wayne L.
• Primary Examiner(s): Hoff, Marc S.
• Assistant Examiner(s): Tsai, Carol S. W.
• Application Number:
  US10/031,374
• Time in Patent Office: 734 Days
• Field of Search: 702/12, 702/22, 702/24, 702/31, 702/33, 702/47, 702/75, 702/76, 702/113, 702/127, 702/137, 702/189, 702/190, 702/194, 702/FOR-107, 702/FOR-115, 702/FOR-124, 702/FOR.128, 702/FOR-134, 702/FOR.143, 702/FOR.155, 702/FOR.157, 702/FOR.170, 738/619.5, 324/682, 324/636, 315/111.21, 315/111.71
• US Class Current: 702/76
• CPC Class Codes: H01J 37/32266 Means for controlling power transmitted to the plasma
• CPC Class Codes: H01J 37/32311 Circuits specially adapted for controlling the microwave discharge
• CPC Class Codes: H01J 37/32935 Monitoring and controlling tubes by information coming from the object and/or discharge
• CPC Class Codes: H01J 37/3299 Feedback systems

Abstract

A system for measuring plasma electon densities (e.g., in the range of 10¹⁰ to 10¹² cm⁻³) and for controlling a plasma generator (240). Measurement of the plasma density is essential if plasma-assisted processes, such depositions or etches, are to be adequately controlled using a feedback control. Both the plasma measurement method and system generate a control voltage that in turn controls the plasma generator (240) to maintain the plasma electron density at a pre-selected value. The system utilizes a frequency stabilization system to lock the frequency of a local oscillator (100) to the resonant frequency of an open microwave resonator (245) when the resonant frequency changes due to the introduction of a plasma within the open resonator. The amplified output voltage of a second microwave discriminator may be used to control a plasma generator (240).

Claims

1. A system for controlling a plasma generator, the system comprising:

an open microwave resonator resonating at a resonant frequency;

an oscillator with an output having a variable frequency;

a control circuit for measuring a difference between the resonant frequency and the variable frequency and for applying a correction signal to the oscillator to change the variable frequency to substantially match the resonant frequency.

2. The system as claimed in claim 1, wherein the control circuit comprises a discriminator for measuring a frequency corresponding to a plasma density in a range from 10¹⁰ to 10¹² cm⁻

3.

3. The system as claimed in claim 1, wherein the control circuit comprises a discriminator for measuring a frequency corresponding to an electron density in a range from 10¹⁰ to 10¹² cm⁻

3.

4. The system as claimed in claim 1, wherein the open resonator is immersed in a plasma.

5. The system as claimed in claim 1, wherein the open resonator further comprises a reflector for accepting input and output microwave connections.

6. The system as claimed in claim 1, wherein the control circuit comprises:

a microwave frequency discriminator; and

an amplifier connected to the microwave frequency discriminator, wherein the amplifier outputs the correction signal.

7. The system as claimed in claim 1, wherein the control circuit comprises:

a plasma generator reference resonator; and

a microwave frequency discriminator for comparing a resonant frequency of the plasma generator reference resonator to produce the correction signal.

8. The system as claimed in claim 7, wherein the control circuit further comprises a computer for processing the correction signal before applying the control signal to the plasma generator.

9. The system as claimed in claim 7, wherein the control circuit further comprises a digital signal processor for comparing an output of the microwave discriminator with input data to control the plasma generator.

10. The system as claimed in claim 1, wherein the open resonator further comprises plural sets of reflectors for accepting plural, independent controllable sets of input and output microwave connections.