Patent No. 6861844 Electron density measurement and plasma process control system using changes in the resonant frequency of an open resonator containing the plasma

 

Patent No. 6861844

Electron density measurement and plasma process control system using changes in the resonant frequency of an open resonator containing the plasma (Verdeyen, et al., Mar 1, 2005)

Abstract

A system for measuring plasma electron densities (e.g., in the range of 1010 to 1012 cm-3) and for controlling a plasma generator. Measurement of the plasma electron density is used as part of a feedback control in plasma-assisted processes, such as depositions or etches. Both the plasma measurement method and system generate a control voltage that in turn controls the plasma generator. A programmable frequency source sequentially excites a number of the resonant modes of an open resonator placed within the plasma processing apparatus. The resonant frequencies of the resonant modes depend on the plasma electron density in the space between the reflectors of the open resonator. The apparatus automatically determines the increase in the resonant frequency of an arbitrarily chosen resonant mode of the open resonator due to the introduction of a plasma and compares that measured frequency to data previously entered. The comparison is by any one of (1) dedicated circuitry, (2) a digital signal processor, and (3) a specially programmed general purpose computer. The comparator calculates a control signal which is used to modify the power output of the plasma generator as necessary to achieve the desired plasma electron density.

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CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is related to co-pending applications entitled "ELECTRON DENSITY MEASUREMENT AND PLASMA PROCESS CONTROL SYSTEM USING A MICROWAVE OSCILLATOR LOCKED TO AN OPEN RESONATOR CONTAINING THE PLASMA," Ser. No. 60/144,878 and "ELECTRON DENSITY MEASUREMENT AND PLASMA PROCESS CONTROL SYSTEM USING A MICROWAVE OSCILLATOR LOCKED TO AN OPEN RESONATOR CONTAINING THE PLASMA," Ser. No. 60/144,880 both of which have been filed concurrently herewith. Both of those applications are herein incorporated by reference in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention provides a method and system for measuring and controlling electron densities in a plasma processing system, such as is used in semiconductor processing systems.

2. Description of the Background

Known microwave-based techniques for determining plasma electron densities include: (1) microwave interferometry, (2) measurement of reflection and absorption, and (3) perturbation of cavity resonant frequencies. Microwave interferometry involves the determination of the phase difference between two microwave beams. The first beam provides a reference signal, and the second beam passes through a reactive environment and undergoes a phase shift relative to the first beam. The index of refraction is calculated from the measured change in the phase difference between the two beams. The interferometric technique has been document by Professor L. Goldstein of the University of Illinois at Urbana. Interferometry is described in the following U.S. Pat. Nos.: 2,971,153; 3,265,967; 3,388,327; 3,416,077; 3,439,266; 3,474,336; 3,490,037; 3,509,452; and 3,956,695, each of which is incorporated herein by reference. Examples of other non-patent literature describing interferometry techniques include: (1) "A Microwave Interferometer for Density Measurement Stabilization in Process Plasmas," by Pearson et al., Materials Research Society Symposium Proceedings, Vol. 117 (Eds. Hays et al.), 1988, pgs. 311-317, and (2) "1-millimeter wave interferometer for the measurement of line integral electron density on TFTR," by Efthimion et al., Rev. Sci. Instrum. 56 (5), May 1985, pgs. 908-910. Some plasma properties may be indirectly determined from measurements of the absorption of a microwave beam as it traverses a region in which a plasma is present. Signal reflections in plasmas are described in U.S. Pat. Nos. 3,599,089 and 3,383,509.

Plasma electron densities have also been measured using a technique which measures the perturbations of cavity resonant frequencies. The presence of a plasma within a resonator affects the frequency of each resonant mode because the plasma has an effective dielectric constant that depends on plasma electron density. This technique has been documented by Professor S. C. Brown of the Massachusetts Institute of Technology. Portions of this technique are described in U.S. Pat. No. 3,952,246 and in the following non-patent articles: (1) Haverlag, M., et al., J. Appl Phys 70 (7) 3472-80 (1991): Measurements of negative ion densities in 13.56 MHZ RF plasma of CF.sub.4, C.sub.2 F.sub.6. CHF.sub.3, and C.sub.3 F.sub.8 using microwave resonance and the photodetachment effect; and (2) Haverlag, M., et al., Materials Science Forum, vol. 140-142, 235-54 (1993): Negatively charged particles in fluorocarbon RF etch plasma: Density measurements using microwave resonance and the photodetachment effect.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a more accurate plasma measuring system than the prior art.

It is a further object of the present invention to provide an improved plasma measuring system using plasma induced changes in the frequencies of an open resonator.

These and other objects of the present invention are achieved using a voltage-controlled programmable frequency source that sequentially excites a number of the resonant modes of an open resonator placed within the plasma processing apparatus. The resonant frequencies of the resonant modes depend on the plasma electron density in the space between the reflectors of the open resonator. The apparatus automatically determines the increase in the resonant frequency of an arbitrarily chosen resonant mode of the open resonator due to the introduction of a plasma and compares that measured frequency to data previously entered. The comparison is by any one of (1) dedicated circuitry, (2) a digital signal processor, and (3) a specially programmed general purpose computer. The comparator calculates a control signal which is used to modify the power output of the plasma generator as necessary to achieve the desired plasma electron density.