JP3119665B2 - Flow control device using piezo valve - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a technique for controlling a flow rate using a piezo valve, and more particularly to a technique for manufacturing a thin film semiconductor using a reactive gas (including an ionized gas).
Alternatively, when producing oxide superconductors using MOCVD or ozone, or when studying catalytic reactions or surface adsorption / desorption reactions, quantitatively determine how many molecules of gas are introduced into the reaction vessel per second. The present invention relates to a control device suitable for controlling the flow rate of a small amount of gas, for example, for controlling the physical properties of a film formed by measurement.

[0002]

2. Description of the Related Art As an example of gas flow rate control, a valve which is conventionally called a needle valve is mainly used, and a small tapered needle valve is provided so as to be able to be inserted into and removed from a small hole to close the small hole. In many cases, a method of controlling the flow rate by controlling the gap between the needle and the small hole by feeding back manually or using a step motor or the like so that the pressure becomes constant is widely used. There is also known a gas introduction system in which a piezo element that generates distortion by applying a voltage is driven by a DC voltage, and a flow rate is controlled by adjusting a degree of opening of a small hole. Also,
As a gas flow measurement technique, a part of the tube is bypassed to a capillary tube, the capillary part is heated by a heater, and the flow rate is determined by measuring how much the temperature distribution of the capillary part changes according to the gas flow rate. It has been known.

[0003]

However, in the flow control method using the needle valve, the position of the handle for adjusting the gap between the needle and the small hole is only a rough guide, so that the accuracy is low and fine adjustment is impossible. There is a problem that the change with time is large and the film is weak against mechanical vibration. Further, in the case where the piezo element is driven by a DC voltage to adjust the valve opening, the driving voltage has a threshold value (30 V) due to the non-linearity of the piezo element and the valve mechanism itself, and is not exactly proportional to the flow rate. There is a problem that the relationship between the flow rate and the voltage has a history (hysteresis). Further, the flow rate measuring method for heating the heater has a maximum sensitivity of 0.1 sccm (4.
5 × 10 ¹⁶ molecules / sec), and it takes a long time for the temperature distribution to be constant, so that the response speed is slow. Even if it is fast, it takes 0.5 seconds or more, and the sensitivity varies depending on the type of gas. . Furthermore, in the production of semiconductor thin films, when a raw material gas is supplied from a cylinder to a vacuum device, a relatively thick film of several μm or more has conventionally been produced in many cases, and precise measurement control of the gas flow rate has not been performed. In recent years, attempts to utilize the quantum effect of electrons by growing a thin film in units of one atomic layer have become active, and an apparatus for supplying a small amount of gas with high accuracy has been demanded.

The present invention has been made in view of such circumstances, and an object of the present invention is to provide a flow control device using a piezo valve which can greatly improve reproducibility, accuracy, sensitivity, and response speed. I do.

[0005]

According to the present invention, a gas containing gas molecules consumed by a reaction is supplied through a piezo valve which is opened and closed by a piezo element driven by a pulse, and is continuously supplied to a reaction vessel. A pulse width modulation means for converting a flow rate control signal into a pulse width, controlling opening and closing of a piezo valve with a pulse width modulated drive pulse, and monitoring a supply flow rate with a pulse width modulated signal. It is characterized by being made possible. Further, the present invention includes two buffer tanks to which gas is supplied from a gas supply source, and a means for measuring a pressure difference between the pressures in both buffer tanks, and supplies gas to the piezo valve through one of the buffer tanks. It is characterized by the following.

[0006]

According to the present invention, the flow rate control signal is pulse width modulated to control the opening / closing of the piezo valve, the flow rate is determined by the length of time the piezo valve is open, and the non-linear region of the piezo element is not used. By using only the open state and the closed state, the reproducibility and accuracy are greatly improved, the maximum flow rate sensitivity is improved by about three orders of magnitude to 2.5 × 10 ¹³ molecules / sec, and the response speed This can be improved by about two digits to several milliseconds. In particular, it becomes possible to control a small amount of gas flow for growing a thin film in units of one atomic layer.

[0007]

Embodiments of the present invention will be described below. FIG. 1 is a view showing one embodiment of a flow control device of the present invention, FIG. 2 is a block diagram for explaining the configuration of a pulse width modulation valve controller in FIG. 1, and FIG. 3 is a diagram showing the structure of a piezo valve used in the present invention. FIG. 4 is a diagram showing an example, and FIG. 4 is a diagram for explaining a control signal subjected to pulse width modulation. In the figure, 1 is a reaction vessel, 2
Is a pressure gauge, 3 is a converter, 4 is a pulse width modulation valve controller, 5, 6 are piezo valves (PV), 7, 8 are buffer tanks, 9, 10 are pressure gauges, 11 is a hydrogen gas cylinder, 12 is a helium gas cylinder, 13 is a methane gas cylinder, 14 is a rotary pump, 15-1 to 15-3 are pressure reducing valves, 16-1 to 16-10 are open / close valves, 20 is a differential amplifier, 21 is an integrator, 22 is a buffer amplifier, and 23 is A reference frequency oscillator, 24 is a triangular wave generator, 25 is a voltage comparator, 26 is a pulse height adjuster, and 27 is a high voltage amplifier.

In FIG. 1, a cleaning gas such as hydrogen gas, helium gas, methane gas or the like and a raw material gas are supplied from a cylinder 11 to 13 into a reaction vessel 1 for forming a thin film.
5-3, open / close valves 16-1 to 16-10, buffer tanks 7, 8 and the like, are supplied through piezo valves 5, 6 to be described later, and are continuously exhausted. The system is evacuated by a rotary pump 14.

As shown in FIG. 3, the piezo valve is provided with an inlet communicating with the flow passage L1 by an inlet mounting nut 34 in the housing 31 and an outlet 35, and the piezo valve has an inlet at the center of the inside thereof. A flow path forming member 36 is provided at right angles to a center line connecting to the outlet, and the lower part is covered with the adjusting housing 32 and the upper part of the housing 31 is covered with the cover 33. Flow path forming member 36
Is formed with a flow path L2 open at the upper end and a flow path L3 communicating with the outlet 35. The vertical position is adjusted by an adjusting screw 38 against the spring 37.
9, a space S is provided below the piezo element 39, and the flow paths L1 and L2 communicate with each other through the space S. A piezo element 39 to which a voltage is applied through a wire 46 is pressed downward by a Teflon ball 45 attached to a preload spring fixed to a holding member 44 with a screw 42. A filter 49 pressed against the O-ring by a spring 50 is provided on the inlet side.

In the piezo valve 30 having such a structure, when a pulse-width-modulated voltage is applied through the wire 46 as described later, the piezo element 39 is displaced to move the piezo element 39 between the seat 41 and the upper end of the flow path forming member 36. Opens and closes, and when opened, the upper end of the flow path L2 opens into the space S, and as a result, the inflow port →
Filter 49 → flow path L1 → space S → flow path L2 → flow path L3
→ A flow path is formed as in the outlet 35, and gas flows. If no voltage is applied, the seat 41 and the upper end of the flow path forming member 36 are in contact with each other, and the flow path L2 is closed and no gas flows. Thus, the flow rate is controlled by ON / OFF of the voltage application to the piezo element.

The control of the piezo valves 5 and 6 is as follows.
As shown in FIG. 1, the pressure in the reaction vessel 1 detected by the pressure gauge 2 is converted into an electric signal by the converter 3, and the pulse width modulation valve controller 4 performs the operation based on the difference between the detected pressure and the set pressure.

The controller 4 has a configuration as shown in FIG. That is, a difference signal between the pressure detection value and the set value is obtained by the differential amplifier 20, and this difference signal is
While being integrated by 1, the voltage level is converted for modulation by the buffer amplifier 22 and output as an analog monitor for monitoring. On the other hand, the reference frequency transmitter (10
0) is applied to a triangular wave generator 24, and the shaped triangular wave is compared with the output of the buffer amplifier by a voltage comparator 25, and a pulse having a width corresponding to the magnitude of the output of the buffer amplifier is output. Is obtained, and pulse width modulation is performed. Then, the pulse is adjusted by the pulse height controller 26 so as to have a constant peak value, amplified, and applied to the piezo element.

Thus, the pressure in the reaction vessel 1 is small,
When the pressure difference from the set value is large, the pulse having the width as shown in FIG. 4 (a) becomes wider as shown in FIG. 4 (b) and the time during which the piezo valve is open becomes longer, thereby increasing the flow rate. I do. As a result, the supply flow rate increases and the reaction vessel 1
Is increased, the difference from the set value decreases, and the input to the integrator 21 becomes 0 in a state where the pressure becomes equal to the set value. Therefore, the output signal voltage of the integrator is maintained, and a predetermined amount of flow rate is maintained. Supplied constantly. In this case, the supply amount of the buffer tanks 7 and 8 in FIG. 1 is set to, for example, 1000 Torr, and when the gas is supplied through the valve 16-9, the pressure in the buffer tank 7 is reduced (for example, 1 Torr). If the detection is made in step 9, the capacity of the buffer tank 7 is determined (V1 in the figure), so that the total number of gas molecules supplied at this time can be obtained. Therefore, calibration can be performed using the differential pressure between the buffer tanks 7 and 8, and the flow rate can be quantified.

FIG. 5 uses a pulse of 100 Hz and 55 V, a modulation voltage is plotted on the horizontal axis, and a flow rate (mbar · l / s) is plotted on the vertical axis.
ec) shows the experimental results. As can be seen from the figure, the linearity is extremely good and the gas supply amount can be accurately set by the modulation voltage. Therefore, since the supply flow rate can be known by monitoring the modulation voltage of the piezo valve, the piezo valve itself has the function of detecting the flow rate, and there is no need to provide a separate flow rate sensor.

When the reaction is started while the gas is supplied at a constant flow rate, gas molecules are consumed, so that the modulation voltage automatically changes to maintain a constant pressure. As described above, the reaction rate can be estimated by monitoring the modulation voltage or the duty ratio that is proportional to the flow rate.

In the above embodiment, the case of supplying a small amount of gas in forming a thin film or the like has been described. However, the present invention is not limited to the control of gas flow rate.
The present invention is also applicable to liquid flow rate control in a system for monitoring H, a system for separating and analyzing a trace amount of biological material in a biological fluid such as blood, urine, and cerebrospinal fluid.

[0017]

As described above, according to the present invention, the following effects can be obtained. Since the supply flow rate can be known by monitoring the modulation voltage of the piezo valve, the piezo valve itself has the function of detecting the flow rate, and there is no need to provide a separate flow rate sensor. The linearity can be improved by using only the fully opened state and the closed state without using the nonlinear region of the piezo element. Since the piezo valve itself has a response of about 2 msec, the response can be drastically increased if the time constant of other devices in the system can be shortened. In a conventional quartz crystal film thickness meter that checks the deposited film thickness from the change in the oscillation frequency due to the deposition of molecules, the change in the surface coverage is 0.1% / sec, that is, about 10 ^-1 atomic layer / sec. while it can only be detected up to about
In the present invention, a sensitivity of about 10 ⁻⁶ mbar · l / sec is obtained, which corresponds to 2.5 × 10 ¹³ molecules / sec.
2.5 × 1 when consumed by ² substrates or targets
0 ¹¹ molecules / sec · cm ² , and one atomic layer is about 10 ¹⁵
Since it is atoms / cm ² , it corresponds to a reaction rate of 10 ⁻⁴ atomic layers / sec. Therefore, a change in surface coverage can be detected up to 0.01% / sec, and the sensitivity is dramatically improved. Can be.

Since the modulation voltage changes when gas molecules are used by the reaction in a state where the gas is supplied at a constant flow rate in the production of the thin film, the reaction rate is estimated by monitoring the modulation voltage or the duty ratio. Can be.

[Brief description of the drawings]

FIG. 1 is a diagram showing one embodiment of a flow control device of the present invention.

FIG. 2 is a diagram for explaining a configuration of a pulse width modulation valve controller.

FIG. 3 is a diagram illustrating the structure of a piezo valve.

FIG. 4 is a diagram illustrating flow control according to the present invention.

FIG. 5 is a diagram showing a relationship between a modulation voltage and a flow rate.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Reaction container, 2 ... Pressure gauge, 3 ... Transducer, 4 ... Pulse width modulation valve controller, 5, 6 ... Piezo valve (P
V), 7, 8: buffer tank, 9, 10, pressure gauge, 11: hydrogen gas cylinder, 12: helium gas cylinder, 13: methane gas cylinder, 20: differential amplifier, 21
... Integrator, 22 ... Buffer amplifier, 23 ... Reference frequency oscillator, 24 ... Triangle wave generator, 25 ... Voltage comparator, 26 ... Pulse height adjuster, 27 ... High voltage amplifier.

Claims (2)

(57) [Claims] An apparatus for supplying a gas containing gas molecules consumed by a reaction through a piezo valve controlled to be opened and closed by a pulse driven piezo element and for controlling a gas flow rate continuously supplied to a reaction vessel. So,
A piezo valve comprising pulse width modulation means for converting a flow control signal into a pulse width, opening and closing control of the piezo valve by a pulse width modulated drive pulse, and monitoring a supply flow rate by a pulse width modulation signal. Flow control device used. 2. The apparatus according to claim 1, further comprising two buffer tanks to which gas is supplied from a gas supply source, and a means for measuring a pressure difference between the two buffer tanks, wherein A flow control device using a piezo valve, wherein gas is supplied to the piezo valve.