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1 July 2002

Semiconductor gases exact max

BY JOHN DUDEK, KEVIN LEHMANN, PAUL RABINOWITZ, AND WEN-BIN LAN

This system measures moisture below 500 parts per trillion.

Losing a batch of silicon wafers costs several hundred thousand dollars. Semiconductor manufacturers and ultrahigh purity gas suppliers focus great attention and resources on monitoring trace moisture in semiconductor gases.

The laser infrared (IR) technique is one of the only methods available that has the sensitivity for moisture contamination monitoring in corrosive gases. By adsorptive forces, moisture strongly adheres to any surface with which it comes in contact. This is especially troubling in ultrahigh purity piping installations.

In a typical semiconductor wafer manufacturing process, it takes dozens of gases to produce a single microchip. These gases flow through stainless steel piping and through tools in the semiconductor wafer production line. In the presence of gases such as chlorine, moisture can cause pitting and accelerated corrosion damage on the steel components.

Moisture can also create defects on the silicon wafer, which negatively impacts yields. As wafer features approach 0.10 micrometers, there is an ongoing need to monitor and control moisture. Trace moisture in ambient bulk gases must be measured currently to less than 1,000 parts per trillion (ppt) and by 2008 to less than 100 ppt.

For moisture in corrosive gases, less than 500 parts per billion (ppb) is required today, a level that is difficult to detect using existing analytical technologies. Thus, trace moisture analyzers must have ultralow sensitivity and be extremely responsive to avoid losses of silicon wafer batches in semiconductor processing.

This analyzer operates on the fundamental first principle of the Beer-Lambert Law for absorption spectroscopy, which precludes the need for calibration. The technology, cavity ring-down spectroscopy (CRDS), employs a continuous wave diode laser operated at a narrow bandwidth line.

The concentration for a trace gas impurity derives directly from the ring-down measurement. While a gas mixture standard is not necessary for calibration, one can use a standard to verify performance. Measurement accuracy is traceable to the nationally accepted high-resolution transmission spectroscopic database.

The spectrum of a trace gas species consists of many sharp rotational lines that provide high selectivity. The spectrum provides a molecular fingerprint specific to the absorbing trace impurity and can contain many spectral lines throughout the absorption band.

Water vapor is particularly favorable because its spectrum extends into the near IR (NIR). The NIR region extends from 750 nanometers (nm) to 3,000 nm. NIR also detects carbon dioxide; carbon monoxide; nitric oxide; nitrogen dioxide; methane; hydrogens fluoride, chloride, and bromide; ammonia; and phosphine, all of which are important to semiconductor process gas monitoring.

CRDS measures the time it takes for the light intensity to decrease, or ring down, in a sample cell cavity. In a cavity ring-down measurement, a fraction of the light radiation, I_o, from a laser is sent through the cell cavity for a time in microseconds, then abruptly turned off.

Mirrors reflect the light inside the cavity many times, leaking out a tiny amount upon each reflection. The leaking light intensity out of the cavity is the ring-down signal. For a given ring-down signal tau (), the ring-down time is determined from a simple first order exponential decay.

The CRDS measurement involves first taking a measurement of the ring-down time while the cell is empty, _empty, by charging the cell with the laser radiation, diffracting the light quickly away from the cell, and then determining the empty cell background ring-down time, _empty.

where d is the cell length, c is the speed of light, and R (unitless) is the reflectivity of the mirror.

The ring-down time for the sample absorption, (v), is continuously measured at the maximum peak frequency of the moisture absorption line. The sample gas flows into the cell, and the ring-down time, (v), is calculated.

where

is the absorption cross section, and

is the molecular density.

To calculate the sample concentration (N) in ppb or other concentration units, one subtracts the empty cell ring down from the sample absorption ring-down time. Then, multiply the difference by the spectroscopic constant at that given line frequency.

MAXIMIZE FIXED PATH LENGTH

In absorption measurements, the longest path length of the light intensity through the sample cell gives the highest sensitivity, according to Beer's Law. Some path lengths of conventional IR analyzers could extend more than several meters for a single pass through of light to improve sensitivity.

To maximize the fixed path length in the CRDS analyzer, reflective mirrors anchor on each end of the sample cell cavity and reflect the light path many, many times. The path length for a 1.3-meter cell is 90 kilometers.

This represents an effective path length of greater than 100,000 times the size of the optical bench in the analyzer. This feature significantly enhances the accuracy and sensitivity of a trace gas measurement.

CRDS is not affected by laser noise because it measures the time variable for light decay. In conventional IR absorption measurements, the operator has to carefully monitor the light intensity because the measurement area is large, due to the inherent power variability of the pulsed laser. Limitations to measuring low ppt concentrations are due to the amplitude noise of the laser source and the detector.

In a cavity ring-down cell, even when amplitudes are different, the ring-down time is the same. The amplitude fluctuation does not affect the measurement of time. Unlike conventional IR spectroscopic measurements, there are no amplitude noise limitations with CRDS, as it measures only the time it takes for the light signal to ring down, or decrease in amplitude.

The best coherent light source is a laser beam that provides IR light with little or no scatter to the sample cell. A continuous wave diode laser works in the trace gas analyzer. The advantages to the continuous wave diode laser are ease of operation, small size, low power requirements, a lifetime in thousands of hours, simple construction, and moderate cost.

These lasers operate in the visible and IR regions of the electromagnetic spectrum. The diode lasers are very reliable and exist in many common, everyday uses such as scanning bar codes.

CUT ABOVE AND BEYOND SENSITIVE

The cavity ring-down spectroscopic, IR-based analyzer performs trace moisture measurements to less than 500 ppt with high sensitivity. It's highly selective and specific, as each trace gas impurity has its own measurable IR fingerprint or set of spectral wavelength lines.

The analyzer operates by charging a sample cell cavity with IR light from an inexpensive diode laser. The spectroscopic measurement signal is the time (or ? constant) for the light to leak out from the cell.

CRDS is fast. The ring-down time measures in microseconds. The major advantages for high sensitivity of the CRDS analyzer, compared with conventional IR analyzers, is the elimination of light background noise and an effective path length of the analyzer that is many times the physical length of the apparatus (1 meter to 90 kilometers). IT

Behind the byline

Kevin Lehmann is a professor of chemistry at Princeton University. John Dudek has a Ph.D. in chemistry and is doing postdoctoral work at Harvard University. Wen-Bin Yan is the director of laser analysis at Tiger Optics. Paul Rabinowitz is a visiting scientist at Princeton University.

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