moisture measurement T
iger Optics reduced the lower detection limits (LDL) by a factor of ten times (10x) or more when compared to existing on-line moisture analyzer technologies such as the electrolytic process using Faraday’s Law or the quartz-crystal microbalance (QCM). Not only did the MTO-1000 offer lower detection limits; it also provided the user with an absolute measurement that did not require the use of zero or span gases. The semiconductor industry took heed. In its fabrication plants, the margin of success can hinge on the purity of required gases.
Moisture Measurement via Ring-down Spectroscopy The Tiger Optics’ core technology is based on continuous wave cavity ring-down spectroscopy (CW-CRDS). The measurement principle is shown in Figure 1. A laser beam in the near-infrared (NIR) realm is coupled to a measurement cell, with parallel, highly reflective mirrors at either end. The dielectric coating of the mirrors reflects more than 99.999 percent of the light within a specific, rather narrow, frequency band. The small amount of light that passes through the mirror at the far end of the measurement cell is captured by a detector, which measures its remaining intensity. The gas stream to be analyzed flows continuously through the measurement cell.
The measurement process starts with the continuous-wave (CW) laser energizing the cell until the light energy reaches a threshold value. The laser is then shut off for a fraction of a second. The laser beam travels back and forth between the mirrors within the measurement cell, for a total path length of close to 30 kilometers. The laser light’s intensity level follows a decreasing exponential function until the energy is exhausted (a “ring down”). The determination of the concentration of moisture is based on the time required for the light to die.
Figure 1: Tiger Optics CW-CRDS Schematic
Zero – Abklingzeit/Ring-down time,” there is no absorption of light by the moisture present in the gas stream. The associated ring-down time of 86 microseconds is solely caused by the loss of light from the measurement cell. The laser is then tuned to the wavelength of a known absorption peak of the water vapor spectrum (marked “T
CW CRDS provides an “absolute” measurement via the Beer-Lambert Law, so no zero gas is required. The zero portion of the measurement is determined by intentionally tuning the laser to a frequency at which moisture does not absorb light. Figure 2 shows the water vapor spectrum in the wavelength range of 1391 to 1393 nm. In the wide region marked “T
Peak
H2O-Band“ in Figure 2). The shorter measured ring- down time of 29 microseconds is due to moisture absorbing a portion of the light. When the T
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