Gas Concentration MeasurementThe invention relates to an apparatus and to a method for detecting, and for measuring the concentration of, a gas or vapour. The measured gas may be a component of a mixture of gases or vapours. In this specification the term gas is to be interpreted as including vapour.
According to the present invention there is provided an apparatus for detecting, or measuring the concentration of, a gas, the apparatus comprising means for generating a beam of radiation having a range of wavelengths, means for causing the beam to pass through a region in which the gas is to be detected, and means to detect the beam after its passage through the region, and also comprising a narrow bandwidth filter onto which the beam is incident, and means to cause oscillation of the angle of incidence of the beam on the filter and so an oscillatory variation in the wavelength of radiation received by the detector, the variation in the wavelength occurring over a range of wavelengths within which the gas varies in its absorption, and means responsive to signals from the detector to detect or to measure the concentration of the gas.
The radiation might be infrared, visible, or ultraviolet, but the preferred radiation is nearinfrared. For sample the gas might be an organometallic compound having one or more C-H bonds. Such bonds have a vibration mode with a first overtone at a wavenumber of about 6000 cm-1 (i.e. wavelength of about 1.67 rum), which is in the near-infrared region of the electromagnetic spectrum.
The filter may be a reflecting filter, but is preferably a transmission filter as this enables simpler optical paths to be used. It is preferably a multi-layer filter. The bandwidth (measured at half the peak transmission) is desirably no more than 1/20 the wavelength of peak transmission, more preferably less than 1/50, for example 1/150, of the said wavelength.
The filter may allow radiation over a narrow bandwidth to be propagated while preventing propagation of other wavelengths, or may prevent propagation of radiation over a narrow bandwidth while allowing propagation of radiation of other wavelengths, the latter being known as a notch filter.
The angle of incidence of the beam onto the filter may be varied by varying the direction of the beam while leaving the filter fixed, but in the preferred apparatus the beam direction is fixed while the filter is oscillated, as this provides a simpler apparatus. The wavelength of the radiation passed by the filter varies with the angle of incidence (being approximately proportional to cos of the angle of incidence); the filter might for example be oscillated through an angle of no more than 600, preferably no more than 300, for example 200, and the oscillation might be either side of the plane perpendicular to the incident beam direction, or might be primarily or entirely to one side of that plane.
The filter may be oscillated at less than 10 Hz, for example 1 or 2 Hz. The detector preferably includes means sensitive to signal oscillations at an integral multiple of the filter oscillation frequency, preferably one, two, or four times the filter oscillation frequency.
Absorption by the gas will cause signal oscillations at such frequencies and so these signals enable the gas to be detected. By being sensitive only to such frequencies the signal to noise ratio for the apparatus is enhanced, because background noise is substantially eliminated.
The invention will now be further described by way of example only and with reference to the accompanying drawings in which:Figure 1 shows a diagrammatic representation of agas measuring apparatus; andFigure 2 shows the near-infrared absorptionspectrum of an organic vapour.
Referring to Figure 1 there is shown a gas measuring apparatus 10 for measuring the concentration of tetraethoxysilane (TEOS) vapour in a transparent-walled container 12. The apparatus 10 includes a light-emitting diode 14 which emits a beam 15 of radiation in the nearinfrared part of the spectrum over the wavelength range about 1.5 to 2.1 Wm. The beam 15 passes through a filter 16, through the gas container 12, and is then incident upon a lead sulphide detector 18 whose optimum sensitivity is in the wavelength range 1 to 3 Wm. The filter 16 is a glass plate of diameter 20 mm and with a multi-layer coating such that it transmits a narrow band of wavelengths about 1.70 Clam. It is supported on a motorized turntable 20 so the angle of incidence, i, of the beam 15 to the filter 16 can be varied.
The apparatus 10 also includes a computer 22. The computer 22 controls the light-emitting diode 14 via a drive circuit 24, which supplies a 200 mA peak, 1 kHz square wave current. The computer 22 also controls the rotary turntable 20 via a driver circuit 26; the turntable 20 is driven to turn to and fro through an angle of 190 at a frequency of 1 Hz, so the angle of incidence i varies between 90 and 280. Signals from the detector 18 are supplied via a driver and pre-amplifier circuit 30 to a lock-in amplifier 28. The lock-in amplifier 28 selectively amplifies any signals varying at a frequency of 1 Hz (i.e. the same frequency as the driver circuit 26). The output from the lock-in amplifier 28 is supplied to the computer 22.
Referring now to Figure 2 there is shown part of the infrared absorption spectrum of TEOS vapour, as a graph showing the variation of absorption (A) with wavelength t It is apparent that there are peaks of absorption at wavelengths in the vicinity of 1.677 ,um and 1.726 pm.
The filter 16 has a central wavelength of about 1.70 pm for radiation incident normally; as it oscillates between i = 90 and i = 280 the peak wavelength varies between about 1.699 Rm and 1.657 Rm (the filter's bandwidth is about .02 um at i = 0, and this also varies with the angle of incidence, as does its peak transmission).
Consequently as the filter 16 is oscillated, the wavelength of the radiation passing through the gas container 12 varies over a range of wavelengths for which the TEOS vapour absorption varies significantly. Hence the amplitude of the signal from the detector 18 varying at 1 Hz may be used to measure the concentration of TEOS vapour in the container 12. There will also be a variation at 1 Hz due solely to the changes in the transmission of the filter 16 with angle, even in the absence of TEOS, and this must be determined separately, to be subtracted from the 1 Hz signal obtained when TEOS is present. The apparatus 10 is sensitive down to about 200 ppm of TEOS at a pressure of 1 atmosphere.
It will be appreciated that the apparatus 10 may be modified in various ways while remaining within the scope of the present invention. For example the lock-in amplifier 28 might be arranged to selectively amplify signals at twice or four-times the frequency of the driver circuit 26. Detecting signals at twice the driver circuit frequency has the advantage that at that frequency no signal is expected in the absence of TEOS.
Clearly the source of radiation 14, the filter 16, and the detector 18 must be chosen in accordance with the nature of the gas to be detected, so that a range of wavelengths are observed over which the gas varies in its absorption. It may also be beneficial to provide means to maintain the temperature of the source of radiation 14 at a desired value during operation, for example to keep it at room temperature by means of a Peltier cooler. The turntable 20 might be replaced by a different oscillatory support, for example a pivoted support and an oscillatory drive linkage; the multi-layer filter 16 might be replaced by a different type of filter for which the wavelength varies with the angle of incidence, for example a holographic filter.
Although the apparatus 10 has been described in relation to the detection and measurement of TEOS it might also be used to measure the concentrations of other materials, in particular other organo-metallic compounds (e.g. organic compounds of gallium, aluminium, silicon, or phosphorus) because the wavelength range around 1.7 ,um corresponds to the frequencies at which vibration overtones of C-H bonds occur, so that all these compounds can be expected to have similar absorption spectra in this range. It will also be appreciated that the apparatus 10 may be used to provide feedback in equipment supplying TEOS vapour, for example for chemical vapour deposition plant, so ensuring more accurate control of the TEOS concentration during operation of the plant.