Optics: measuring and testing – For light transmission or absorption
Reexamination Certificate
2000-10-13
2003-04-22
Rosenberger, Richard A. (Department: 2877)
Optics: measuring and testing
For light transmission or absorption
C356S432000
Reexamination Certificate
active
06552792
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention (Technical Field)
The present invention relates generally to the high-sensitivity detection of contaminants in gases by optical techniques generally termed photoacoustic and optoacoustic spectroscopy.
2. Background Art
Trace impurities in semiconductor process gases are among the most significant limits to product yield. Contaminants at the part-per-billion level can be problematic. In many cases, the unwanted compounds are ubiquitous in air—water vapor and oxygen are common examples—and can enter process tools along a variety of paths. Gas suppliers and end users face two problems, guaranteeing gas purity prior to shipment and maintaining purity during distribution within semiconductor fabrication facilities. Thus, there is a need for relatively inexpensive sensors for continuous, real time measurement of gas purity. Ideal sensors would be sufficiently cost effective that one could be installed in line at each process tool. The present invention's improvements to photoacoustic spectroscopy (PAS) and wavelength modulated photoacoustic spectroscopy (WM-PAS) provide these significant advantages for trace gas detection.
Optical spectroscopy is an effective, non-contact method for trace species detection and is well suited to continuous monitoring in process control systems. When wavelength-tunable diode lasers are used as light sources, their monochromatic output makes possible an exceptional combination of detection sensitivity and selectivity. Selectivity refers to the ability to detect the target species even in the presence of a huge excess of other compounds. Two types of techniques have been developed for achieving highly sensitive gas detection using linear optical absorption spectroscopy with diode lasers. In one case, wavelength modulation techniques (similar to frequency modulation) shift the detection bandwidth from DC, where the lasers are most noisy, to higher frequencies where laser excess noise (1/f noise) is unimportant. The other approach, called the noise canceler, uses a fast, simple transistor circuit to subtract the common mode noise in the measurements of the power exiting the laser and the power after the light beam has passed through the sample. When commercially available near-infrared diode lasers are used, both approaches have theoretical minimum detectable absorbances in the 10
−8
range for a 1 Hz bandwidth. Here, absorbance is the fractional change in laser power due to molecular absorption. In practice, however, optical artifacts in the form of unwanted interference fringes (etalons) usually limit absorbance sensitivities to ~1×10
−5
.
Previous work by other researchers shows that absorbances in the 10
−8
range and smaller can be detected using a simple, short (~10 cm), single pass, optical cell using photoacoustic detection. Relatively inexpensive, compact instruments for continuous monitoring of trace impurities in semiconductor process gas are possible. An improvement to PAS, called wavelength modulated photoacoustic spectroscopy (WM-PAS), eliminates a major noise source associated with traditional implementations of PAS.
WM-PAS has been practiced in the prior art. An early description of the technique was provided by C. F. Dewey, Optoacoustic Spectroscopy and Detection (Y-H Pao, ed., Academic Press, New York, 1977), pp. 62-64. Others have since practiced the technique including M. Feher, et al., Applied Optics 33, 1655 (1994); A. Miklos, et al., Applied Physics B 58, 483 (1994); and B. E. R. Olsson, et al., Applied Spectroscopy 49, 1103 (1995). All use sinusoidal wavelength modulation waveforms and do not simultaneously provide for locking the optical source wavelength to the peak of the gaseous absorption feature as with the present invention.
To reiterate, photoacoustic spectroscopy is a well-known method pioneered by Bell for measuring weak optical absorbances indirectly. Optical absorption by the target compound heats the sample. The small temperature rise creates a change in pressure that is detected with a microphone. The magnitude of the pressure change depends in part on the product of the sample absorbance and the light source intensity. Usually, the light is chopped at an audio frequency, and the photoacoustic signal is detected using a lock-in amplifier synchronized to the chopping frequency. Photoacoustic detection is useful because modern microphones have low background noise and good linearity.
Wavelength modulated photoacoustic spectroscopy eliminates a major source of noise in photoacoustic spectroscopy and provides high sensitivity detection using modest power (few milliwatt) diode lasers. Also, the use of wavelength modulation with photoacoustic detection removes the main impediment to wavelength modulated optical absorption spectroscopy, optical interference fringes. The combined techniques provide a superior method for trace gas detection.
Photoacoustic measurements are often limited by noise due to weak absorption at the cell windows. This background signal is synchronous with the chopped or pulsed laser beam and can overwhelm signals due to absorbance by the target gas. Researchers have implemented a number of approaches to avoiding window noise, such as using acoustic baffles between the windows and the microphone or trying to time-resolve the “true” signal that originates closer to the microphone, but window effects remain a significant problem for photoacoustic detection.
WM-PAS avoids window noise by modulating the laser wavelength instead of the laser intensity. Optical absorption at the windows will still occur, but does not generate a synchronous, interfering signal. The basic principle of wavelength modulated photoacoustic detection is shown in FIG.
1
. The laser wavelength is modulated sinusoidally across the absorption line. This wavelength modulation induces synchronous absorption which generates synchronous pressure waves at frequency f and its integer harmonics.
FIG. 1
shows a photoacoustic wave whose primary frequency component is 2f. In this case, where the average (i.e., unmodulated wavelength) is coincident with the absorption line center, the 2f frequency component dominates because the laser wavelength samples the absorption line peak twice during each modulation period. These pressure waves—the photoacoustic signal—are detected using a microphone connected to a lock-in amplifier.
The laser wavelength is modulated by a small amount: only ~0.1 cm
−1
for a gas at atmospheric pressure. Absorption bands due to windows are orders of magnitude broader, so that the window absorption cross section is virtually constant across the wavelength modulation range. As a result, absorption due to the window does not introduce a synchronous acoustic signal. Wavelength modulation is ideally suited to diode lasers because laser wavelengths tune linearly with changing current. It is straightforward to add a small AC component to the laser drive current in order to effect the wavelength modulation.
The method of the present invention improves on traditional sinusoidal modulation of the wavelength in WM-PAS. It is known that the modified square wave (MSQ),
FIG. 6
b,
modulation waveform can provide increased signal levels for 2f detection in wavelength modulated absorption experiments. T. Iguchi, Journal of the Optics Society of America B 3, 419 (1986). However, the MSQ waveform also amplifies etalon signal amplitudes in the same way as the gaseous absorption signal amplitude. Thus, in traditional WMS absorption experiments there is usually no advantage to the MSQ waveform. However, the limiting noise source in the WM-PAS technique is not usually etalons. Therefore, the MSQ waveform can provide increased PAS signals without concurrently increasing the limiting noise. The application of the MSQ to traditional WM-PAS is shown in FIG.
2
. Raw photoacoustic signal is shown in
FIG. 3
, which has been obtained under identical conditions save for the type of wavelength modulation waveform. The MSQ waveform provides increased signal compa
Bomse David S.
Pilgrim Jeffrey S.
Myers Jeffrey D.
Rosenberger Richard A.
Southwest Sciences Incorporated
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