JPH0560706A - Method and device for gas concentration monitoring - Google Patents

Abstract

PURPOSE:To enable a gas concentration to be monitored accurately in a real time by reducing a temperature on a condensation surface which is exposed to a gas environ ment in a short time and then measuring time from a time when the temperatures starts to be reduced to that when a gas starts to be condensed. CONSTITUTION:A condensation surface 10 is in contact with an inner surface of a gas transportation pipe 7 and a temperature of the condensation surface 10 is cooled and heated by a Peltier element 6. The element 6 controls temperature by a program temperature control unit 8 based on a signal of a temperature sensor 11. A semiconductor laser beam from an lighting light source 4 is focused and emitted to the condensation surface 10 through a window 5 and the returning light is led to a light detector 1 which observes a reflection factor. A unit 8 operates to repeat a cycle for allowing a temperature of the condensation surface 10 to be reduced by approximately 10 deg.C in approximately 40 seconds from a reference temperature and then to be returned to the reference temperature in approximately 20 seconds. A condensation judging unit 9 obtains a gas concentration from a gas concentration-time curve which is calibrated previously according to a difference between a time when reduction in temperature of the condensation surface 10 is started and that when condensation is initiated and a return light intensity changes.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and apparatus for monitoring the concentration of raw material gas used in a CVD apparatus or the like.

[0002]

2. Description of the Related Art Conventionally, CVD has been performed using an organic metal gas or the like.
In the method and apparatus for performing the above, the concentration of the raw material gas is controlled by means such as keeping the flow rate of the carrier gas and the temperature of the raw material gas reservoir constant. However, this method has a problem that the gas concentration in the CVD atmosphere tends to fluctuate due to consumption of the raw material or the like because the concentration at the time of supplying the raw material is not directly monitored.

[0003]

Conventionally, as a means for measuring the pressure of a raw material gas such as CVD, there has been known a method of directly measuring the internal pressure of a sealed container without using a carry gas with a pressure gauge. However, in applications such as CVD where the source gas is used for film formation, it is necessary to flow the source gas using a carrier gas, and this method cannot be applied. That is, the conventional method and apparatus have no means for accurately monitoring the concentration of the raw material gas in real time. for that reason,
It is difficult to configure a highly practical device because the film forming rate during CVD is likely to change.

An object of the present invention is to provide a method and an apparatus for accurately monitoring the concentration of a raw material gas in real time, which has been a problem of the conventional method and apparatus.

[0005]

The gas concentration monitoring method of the present invention is to reduce the temperature of a condensation surface exposed to a gas atmosphere within a short time and restore it to the original temperature, and The presence or absence of gas condensation is detected, the time from the start of temperature decrease to the start of gas condensation is measured, and the gas concentration of the atmosphere is determined from that time.

In the gas concentration monitoring device of the present invention, a condensing surface is provided in a container holding a gas atmosphere, and the condensing surface is reduced by a program temperature adjusting unit capable of lowering the temperature in a short time and recovering the original temperature. A device for controlling the temperature of
Irradiating the condensed surface with illumination light from an illumination light source, from the intensity of the returned light, an optical system leading to a photodetector for observing the reflectance change of the condensed surface, and the temperature decrease start time of the condensed surface, And a condensation determination unit that determines the concentration of the gas atmosphere from the time until the intensity change occurs in the output signal of the photodetector.

[0007]

The raw material gas used in the CVD apparatus or the like is condensed into a liquid phase or a solid phase as the temperature decreases. Since the condensing temperature can be uniquely obtained from the temperature-saturated vapor pressure curve of the raw material gas, conversely, the concentration of the raw material gas can be obtained from the condensing temperature. In the present invention, the temperature of a part of the container in the atmosphere of the raw material gas is set to a few 1
By providing a structure capable of cooling at about 0 ° C, it is possible to measure the time from the start of cooling to the time when slight condensation occurs on the condensation surface, and from this time, it is possible to measure the gas concentration in the atmosphere in real time. .. Furthermore, once condensation is observed, the dead time (time that cannot be measured) caused by a large amount of condensation is minimized by returning the temperature of the condensation surface to the temperature before cooling within the same short time. To By doing so, the gas concentration can be repeatedly monitored in a short time cycle of the order of several minutes. The condensation is detected by illuminating the condensation surface with illumination light and observing the change in the reflected light intensity. Before condensation occurs, there is nothing on the condensation surface, so return light with a constant intensity is observed, but once condensation begins, the reflectance sharply decreases due to the change in surface morphology accompanying condensation, so The start of condensation can be detected.

When the cooling rate of the condensing surface is slow, the time until condensation occurs is determined by the time until the temperature of the condensing surface reaches the saturated gas concentration of the raw material gas determined by the temperature.
In that case, the gas concentration can be monitored from the temperature-saturated vapor pressure curve of the raw material gas that is generally known. Of course, from the relationship between the cooling rate and the temperature of the condensation surface, the gas concentration can be obtained from the time until the condensation starts. On the other hand, when the cooling rate is high, the condensation cannot follow the temperature decrease on the condensation surface, and the condensation may occur later than the time expected from the temperature-saturated vapor pressure curve. Also in this case, if the size of the condensing surface and the type of the raw material gas are the same, the time until the vapor condensation occurs and the raw material gas concentration show a one-to-one correspondence. By measuring the relationship between the concentrations and preparing a calibration table, the gas concentration can be obtained from the time.

The advantage of this method is that the decomposition or consumption of the raw material gas accompanying the measurement does not occur, so that there is no possibility of a change in the raw material concentration or contamination of impurities accompanying the measurement in principle. In the present invention, in order to accelerate the time response of measurement, it is effective to narrow the size of the condensing surface to a small region of about several 100 μm. By doing so, the heat capacity of the condensing surface can be reduced, and at the same time, the feed rate of the raw material from the vapor phase to the condensing region can be increased, and as a result, the response from the temperature decrease to the condensation can be accelerated. At the same time, the amount of condensation can be suppressed to a very small amount,
It is possible to reduce the rate of change in concentration of the raw material atmosphere due to condensation to a negligible level. One measurement time can be completed within a short time of about 1 minute, and by repeatedly monitoring the concentration, there is also an advantage that the gas concentration change in real time can be measured with high accuracy. ..

[0010]

[Example] Hereinafter, hydrogen gas was used as a carrier gas, and dimethyl gold acetylacetonate was used as a source gas by laser CVD.
An example in which the gas concentration monitoring method and device of the present invention are applied to the case of transportation to the device will be described in detail with reference to the drawings.

FIG. 1 is a schematic diagram showing the configuration of an embodiment of the method and apparatus of the present invention. The condensing surface 10 is in contact with a part of the inner surface of the pipe 7 that transports the raw material gas atmosphere, and the temperature of the condensing surface 10 can be cooled and heated by the Peltier element 6. The Peltier element 6 is configured to control the temperature by the program temperature adjusting unit 8 based on the signal from the temperature sensor 11 provided immediately below the condensation surface. On the other hand, a window 5 for introducing a laser beam is provided on the opposite side of the condensing surface 10, and a semiconductor laser beam having a wavelength of 0.8 μm is condensed and irradiated from the illumination light source 4 to the condensing surface 10 at the same time. An optical system including a half mirror 2 and a lens 3 is provided for guiding the return light to the photodetector 1 for observing the reflectance. The program temperature adjusting unit 8 changes the temperature of the condensing surface 10 from the reference temperature to 10 seconds in 40 seconds.
It is operated to repeat the temperature cycle of lowering the temperature by 0 ° C. and returning to the reference temperature in the next 20 seconds. The condensation determination unit 9 obtains the difference between the time when the temperature of the condensation surface starts to decrease and the time when the condensation starts and the return light intensity changes, and this time is compared with a calibration value to display the gas concentration. It is made up of structures.

FIG. 2 is a diagram showing the response of the operation of the programmed temperature control unit 8 and the change in the reflectance of the condensation surface 10 described above. FIG. 2A shows how the temperature of the condensation surface 10 changes, and FIG. 2B shows the reflectance that changes in response to the temperature change.

When the temperature of the condensing surface 10 decreases from the reference temperature T ₀ , the reflectance starts to rapidly decrease from the moment the condensation occurs on the condensing surface 10. Obtained by calibrating in advance from the difference between the time at this time and the time when the temperature starts to decrease,
The gas concentration is calculated from the gas concentration-time curve. Since the cooling rate in this case was relatively slow at 0.25 ° C./s, the gas concentration-time curve was the same as that obtained from the temperature-saturated vapor pressure curve.

The actual measurement results obtained with the above configuration will be described. The starting material, dimethyl gold acetylacetonate, is a solid at room temperature and has a vapor pressure of 8 mTorr. With the temperature of the reservoir filled with the raw material kept at 20 ° C., the change in the concentration of the raw material gas was examined when the hydrogen carrier flow rate was changed between 20 and 200 sccm. The reference temperature of the condensation surface was set to 20 ° C. The condensing surface was a polished copper substrate, and the reflectance of the returning light was 60% without condensation. The reflectance as the criterion for determining the start of condensation was 30% (reflectance of 50% reduction). When the hydrogen carrier flow rate was 20 sccm, the time when the change in the reflectance started was as short as 3 seconds after the start of cooling, and the gas concentration obtained from the calibration value was obtained as 7 mTorr. On the other hand, when the flow rate of hydrogen is 200 sccm, the time period for which the reflectance change occurs is as long as 30 seconds after the start of cooling,
A low concentration of 0.3 mTorr was required.
This indicates that even if the reservoir temperature is constant, if the flow rate of the carrier gas is large, the gas concentration gradually decreases when the source gas is transported under the same temperature condition and carrier flow rate condition. It is presumed that this is because the surface area of the solid surface of the raw material in the reservoir decreased as the raw material was consumed.

As described above, in the conventional method and apparatus, since there is no means for directly monitoring the concentration of the raw material gas, the C in the latter stage is
In VD and the like, variations in the deposition characteristics due to changes in the concentration of the raw material gas, etc. occur, whereas the use of the present invention allows the concentration of the raw material gas to be constantly monitored, so that the concentration is always a constant value. As described above, it is possible to feed back the temperature of the raw material reservoir, etc., and it is possible to significantly improve the controllability and practicality of the CVD apparatus. Further, in the present invention, since only the raw material gas is condensed, the partial pressure (concentration) of the raw material gas can be monitored without being affected by the carrier gas, so that the measurement value does not include an error due to the pressure fluctuation of the carrier gas. The point is also an advantage.

In the above-mentioned embodiments, the case where dimethyl gold acetylacetonate is used as the raw material gas has been introduced. However, in the present invention, a liquid raw material gas such as dimethyl aluminum hydride is used as the other raw material gas. It goes without saying that you can do it. In this case, since droplets are formed on the condensation surface 10 at the start of condensation, the irradiation light is scattered on the surface and the reflectance is reduced.

[0017]

As described above, according to the present invention, C
It is possible to accurately monitor the concentration of the raw material gas used for VD or the like in a short time. Further, according to the present invention, there is no problem such as consumption and decomposition of the raw material gas due to the measurement, and therefore, there is an advantage that it is excellent in practicality. Since the structure of the device is simple and can be made inexpensive, there is also an advantage that it is possible to make a highly practical device that is highly reliable and inexpensive.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram of an embodiment embodying the method and apparatus of the present invention.

FIG. 2 is a schematic diagram illustrating the principle of the present invention.

[Explanation of symbols]

 1 Photodetector 2 Half mirror 3 Lens 4 Illumination light source 5 Window 6 Peltier element 7 Piping 8 Program temperature adjustment unit 9 Condensation determination unit 10 Condensation surface 11 Temperature sensor

Claims (2)

[Claims] 1. A step of decreasing the temperature of a condensation surface exposed to a gas atmosphere and recovering it to the original temperature in a short time, and a step of detecting the presence or absence of gas condensation on the condensation surface. Measuring the time from the start of temperature decrease to the beginning of gas condensation, and determining the gas concentration of the atmosphere from the time, the gas concentration monitoring method. 2. An apparatus for controlling the temperature of the condensing surface by providing a condensing surface in a container holding a gas atmosphere, and using a program temperature adjusting unit capable of lowering the temperature in a short time and recovering the original temperature. , Irradiating the condensed surface with illumination light from an illumination light source, from the intensity of the returned light, an optical system for guiding to a photodetector for observing the reflectance change of the condensed surface, and from the start of the temperature decrease of the condensed surface. And a condensation determination unit that determines the concentration of the gas atmosphere from the time until the intensity of the output signal of the photodetector changes.