Cavity ring-down spectrometer for semiconductor processing

An apparatus is provided for measuring a gas within a semiconductor thin film process. The apparatus includes an optical resonator disposed within an environment of the thin-film process, a tunable laser that excites the optical resonator at a characteristic frequency of the gas and a detector that detects an energy within the resonator.

FIELD OF THE INVENTION

The field of the invention relates to semiconductor processing and more particularly to apparatus for detecting gas concentrations within a semiconductor processing environment.

BACKGROUND OF THE INVENTION

Control of an environment in the processing of semiconductors is very important. Typically, semiconductors are fabricated from wafers within a processing chamber. One or more diffusion steps may be followed (or preceded) by masking steps to define semiconductor fabrication sites on a wafer. Once fabrication sites have been defined, the process may be repeated any number of times depending upon the complexity of the fabricated device.

Once semiconductor devices have been created at the defined sites, a set of connections may be formed at the defined sites. Connections may be formed by any number of masking, deposition and etching steps.

At each step of the fabrication process, different reactive materials may be used. Usually the reactive materials are in the form of gases.

In order to provide a consistent semiconductor product, the diffusion, deposition and etching processes must be reliably controlled usually through the precise control of the reactive gases. Typically, this is accomplished by controlling the flow of the reactive materials into the processing chamber.

In semiconductor thin film processing, it is very important to know the precise gas composition within the processing chamber. Conventionally, this is accomplished primarily by flow controls, but flow control alone does not identify the proportions of reactant gas present within the processing chamber especially when there are other gases present, such as carrier gases or background gases. Accordingly, a need exits for a better method of measuring gas content, especially in a process environment involving a multitude of gases.

DETAILED DESCRIPTION OF THE INVENTION

An apparatus is provided for measuring a gas within a semiconductor thin film process. The apparatus includes an optical resonator disposed within an environment of the thin-film process, a tunable laser that excites the optical resonator at a characteristic frequency of the gas, a detector that detects an energy within the resonator and a processor that detects a concentration of the gas based upon a ring down rate of the detected energy.

Turning now to the drawing,FIG. 1is a block diagram of a gas detection system10, shown in a context of use generally in accordance with an illustrated embodiment of the invention. As shown inFIG. 1, the detection system10may be used to monitor gas concentrations within a semiconductor thin film process12. While the system10is shown as being used to detect gas concentration in an outlet gas flow (stream)14, it should be understood that the system10could just as well be used to detect an inlet gas flow into the process12.

The detection system10may include an optical resonator16, an optical detector20, a tunable laser22and a central processing unit (CPU)24. The tunable laser may be tunable over some appropriate optical wavelength range (e.g., 1-12 micrometers).

The optical resonator16may be placed directly in the outlet gas flow14or offset from the gas stream within a “T” connection26as shown inFIG. 1. As would be known to those of skill in the art, the input and output gas flows of the process12may include a number of very corrosive gases. Since the optical resonator16is located in the gas stream, the optical resonator16may be fabricated of materials that are resistant to those gases. With suitable protective provision, the tunable laser22and detector20may also be located within the gas stream14or may interact with the resonator16through an optically transparent window18that functions to isolate the process environment from the instrument environment.

The window18may be fabricated of an appropriate optical glass (e.g., quartz glass). Alternatively, the window18may be fabricated from sapphire.

The optical resonator16may consist of three mirrors28,30,32in the form of a triangle. The tunable laser22may be aligned coaxially with a first leg34of the triangle. Optical energy entering the resonator16through the first mirror28propagates along the first leg34of the resonator16and is reflected by a second mirror32. Reflected energy from the second mirror32propagates along a second leg36to the third mirror30and is reflected along a third leg38to the first mirror28. The sum total of the lengths of the legs34,36,38may be an integral multiple of the resonant frequency.

At the first mirror28most of the optical energy is reflected along the first leg34and recirculates around the triangle. However, at least some of the energy propagating along the third leg38passes through the first mirror28and impinges on the detector20coaxially aligned with the third leg38.

In general, the gas detection system10functions in accordance with the principles of spectroscopy. As is known, each gas in the gas stream14absorbs optical energy at wavelengths that are characteristic for the gas. By tuning the tunable laser22to a characteristic frequency of a predetermined gas, the system10can measure the concentration of the predetermined gas independently of the presence or concentrations of other gases within the gas stream14. As is known, the greater the concentration of the predetermined gas, the greater the absorption of the characteristic wavelength of optical energy.

In order to measure gas concentrations, the tunable laser22may be tuned to a characteristic wavelength of the gas. A simple mechanical servo31may be used to adjust the sum length of the three legs34,36,38to an integral multiple of the selected wavelength.

The tunable laser22may be pulsed at some predetermined rate (e.g., 50 pulses per second) to deliver optical pulses of an appropriate duration (e.g., a few nanoseconds) to the resonator16. Pulsing may be accomplished through use of a shuttering device (e.g., an acousto-optic modulator).

The application of the pulses to the resonator16causes the optical energy to recirculate around the resonator16(i.e., the resonator16“rings”). The decay rate (i.e., the “ring down rate”) of the optical energy within the resonator16depends upon the wavelength of the optical energy, the gas within the stream14and the optical losses of the resonator16.

FIG. 2is an example of an amplitude versus time graph that shows ring down rates of the resonator16in the case of a predetermined gas and the resonator16resonating at the characteristic wavelength of the predetermined gas. As shown, the resonator16would have a first characteristic curve40in a vacuum and a second characteristic curve42in the presence of some maximum concentration of the predetermined gas. Any concentration of the predetermined gas below the maximum concentration would have its characteristic curve somewhere between curves40and42. The group of curves ofFIG. 2represents a ring down rate profile for the predetermined gas.

In order to determine a concentration of the predetermined gas in the gas stream14, the group of curves and corresponding gas concentration value ofFIG. 2may be incorporated into a ring down rate look up table44in a memory of CPU24. Within the table44, each curve40,42within the ring down rate profile may be saved as a locus of points or as an equation that characterizes the curve40,42.

In order to determine the concentration of the predetermined gas within the stream14, the CPU24may cause the tunable laser22to tune to the characteristic wavelength of the predetermined gas and impose a sequence of optical pulses on the resonator16. After each pulse, a sampling processor50may detect an amplitude of the resonating optical energy within the resonator16over a predetermined set of time intervals. The amplitude and time value at each sample point together define coordinates within a sample curve. The sampling processor50may transfer the coordinates of each sample curve to a curve matching processor48.

Within the curve matching processor48, the sample curve may be matched with at least one of the curves within the ring down profile of the predetermined gas. The curve matching processor48identifies the curve40,42with the closest match and selects the gas concentration value associated with that curve as the determined gas concentration.

Similarly, the gas detection system10may analyze and measure the concentration of any number of different gases (a set of predetermined gases) that are simultaneously present within the stream14. In this case, a ring down rate lookup table file44,46may be provided for each gas to be measured. In addition to a ring down rate profile for each gas to be measured, the file44,46may include a set of characteristic wavelengths along with an identifier of the gas.

To measure a gas concentration, the CPU24may sequentially select each gas of the set and measure a concentration as described above based upon a characteristic wavelength of the selected gas. Where the system10is used for process control, the measured values may be transferred to a process controller (not shown) as feedback control for the process.

In general, the system10may be used to monitor gas constituents of the stream14from the parts per million (ppm) to the parts per billion (ppb) range by measuring gas absorption via changes in the ring down time for unique gas absorption wavelengths and then converting this to concentration using known absorption cross sections. A one ppm atmospheric concentration measurement at one bar translates to a one milliTorr (mTorr) accuracy. Thus if processing 100 mTorr of total gases, the concentration of the known gas can be known to 0.1% accuracy. Not only is this technique good for inlet gas mixtures, but it can be used to measure the amount of 1) unreacted post-process gases or 2) other gases resulting from the reaction.