JPS5811595B2 - High-speed plasma energy distribution measuring device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an apparatus for measuring energy spectra, and more particularly to an apparatus for rapidly determining the energy spectrum of electrons in a gas discharge plasma.

Determining the energy spectrum of electrons is fundamental to understanding various mechanisms in gas discharge plasma, and the alternating current superposition method, which is based on the Trivestein method, has been used as a method for this purpose.

Generally, electrons in a plasma having a low energy of about 0 to 50 eV are inserted into the plasma and detected by a probe to which a voltage is applied.

The voltage-current characteristics of the probe are expressed by the nonlinear curve shown in FIG. 1, and the probe current 1P (Vp) is given by the following equation.

Here, e and m are the charge and mass of the electron, s is the probe surface area, and f(E) is the energy spectrum function to be sought.
E is the energy and Vp is the probe voltage (takes a positive value in the direction lower than the plasma potential with the plasma space potential as a reference).

If we take the second derivative of equation (1) with respect to Vp, we get 1p
From the second derivative of (Vp), the energy spectrum function f(
It can be seen that E) can be obtained (Trivestein method).

The relationship between the probe current ip and the two-fold differential energy spectrum function f(B) is shown in FIG.

As is clear from this figure, shows the same change as f(B).

In the AC superposition method using this Trivestein method, an AC voltage v(t) of minute amplitude is superimposed on the probe voltage Vp which changes within a certain range, and the distorted wave generated due to the 1p-Vp convex curve nonlinearity is proportional to the second derivative. The energy spectrum f(E) is obtained by detecting the frequency component (for example, the 2ω component when v(t)=asinωt).

FIG. 2 is a block diagram of a conventional device that implements this AC superimposition method.

P is a probe inserted into the plasma, 1 is a probe voltage generator that generates a staircase voltage, and 2 is an AC voltage source that generates an AC voltage with a minute amplitude, and each step of the staircase voltage has a minute amplitude. superimposes an alternating current voltage.

3 is an amplifier that selectively amplifies only a specific frequency component representing f(E) of the probe current 1p(t) flowing through the probe circuit, and 4 is a reference signal generation circuit, which includes a nonlinear circuit and a selection amplifier. , a phase shifter and a waveform shaping circuit.

Reference numeral 5 denotes a sampler that extracts and holds the output of the selection amplifier in response to the sampling pulse from the reference signal generation circuit 4.

Conventionally, the output signal of the sampler 5 was smoothed by an integrating circuit to obtain a measurement signal.

However, there is an inherent time delay in using such an integrating circuit.

Generally, the time constant of the integrating circuit needs to be selected to be several tens of times or more the period of the signal to be integrated from the viewpoint of S/N ratio.

On the other hand, the delay time of the integrating circuit is of the same order as the time constant.

Therefore, in a device that employs an integrating circuit, the upper limit of high speed is limited by the time constant of the integrating circuit.

That is, when this time is shortened in an attempt to realize high-speed measurement, there arises the problem that the signal-to-noise ratio is significantly reduced.

An object of the present invention is to provide an apparatus that improves the signal-to-noise ratio and can measure the energy distribution of electrons in a plasma at high speed.

That is, in the device of the present invention, the signal-to-noise ratio is improved by extracting the detected wave signal at the maximum amplitude time of each period using a sampling pulse generated from the reference frequency signal, and the extracted signal is converted into a digital quantity. For each step of the staircase wave of the probe voltage, the average value of the number of AC signals superimposed on this step is obtained, thereby eliminating the conventional integration circuit and enabling high-speed measurement. It is.

Embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

FIG. 3 shows an embodiment of the present invention.

In this figure, P, 1, 2, 3, and 5 indicate the same circuits as in FIG. 1, that is, a probe, a staircase voltage generator, an AC voltage source, a selective amplifier, and a sample-and-hold circuit, respectively.

6 is a frequency multiplier circuit (or minute amplitude AC voltage Vt=a
In the case of siω1t+bsinω2t, it is a mixed circuit), 7
1 is a shaping delay circuit, 8 is an analog-to-digital converter, 9 is an accumulator, and 10 is a control circuit for controlling the operation of each circuit of 1.8°9.

The operation of the measuring device according to the present invention will be explained with reference to FIG.

First, a minute AC voltage from a minute amplitude AC voltage source 2 (Figure 4B) is superimposed on the output voltage of the staircase wave generating circuit 1 (Figure 4A), and this is applied to the probe P1.

On the other hand, the AC signal from the AC power source 2 is converted into a double frequency signal (reference signal) by the circuits 6 and 7 (4th d),
After converting this into a rectangular wave (Fig. 4 e), taking into consideration the delay time of the selection amplifier 3, the waveform is further reshaped by a time τ as shown in Fig. 4 f, and a narrow sampling pulse is generated. It is said that

(Fig. 4 g) The above sampling is performed at the same time as the maximum amplitude of the output signal of the circuit 3 (Fig. 4 h) that selectively amplifies only a specific frequency component from the distorted wave of the probe current caused by the nonlinear characteristics of the probe. A pulse is generated, and the sample and hold circuit 5 responds to this sampling pulse to extract and hold the maximum value of the output signal waveform from the selective amplification circuit 3 (FIG. 4i).

The output of this sample and hold circuit is converted into a digital quantity by a circuit 8 and then averaged by an accumulator 9 (FIG. 4j).

Next, when the staircase wave goes up one step, the above operation is repeated.

As a result, each time the staircase wave rises one step at a time, the accumulator 9 sequentially obtains additional values corresponding to the number of AC signals superimposed on one step for each step, and the average of these values is obtained as the measurement signal x.

FIG. 5 shows, as an example of measurement results, a case using the high-speed plasma energy distribution measuring device of the present invention and a case using a conventional device in comparison.

In the conventional device, when the probe voltage is swept in both positive and negative directions due to the delay caused by the integral operation, the measured energy spectrum will be out of alignment.On the other hand, with the device of the present invention, there is no delay due to the integral operation, so the probe voltage can be swept in both the positive and negative directions. Even if the spectrum is swept, the same spectrum can be obtained.

As already mentioned, from the viewpoint of the signal-to-noise ratio, the time constant of the integrating circuit needs to be several tens of times or more the period of the signal to be integrated.

This determines the upper limit of the measurement speed for high-speed measurements, and higher speeds could not be expected as long as an integrating circuit was used. However, the present invention eliminates the use of an integrating circuit, and thereby increases the speed. The upper limit has been significantly raised.

[Brief explanation of the drawing]

Figure 1 shows the current-voltage characteristics of the probe and the probe current
The relationship between the order differential d2ip/dV2p and the energy spectrum is shown. FIG. 2 shows a conventional apparatus for measuring the energy spectrum of electrons in plasma. FIG. 3 shows an apparatus for measuring the energy spectrum of electrons in plasma according to the present invention. FIG. 4 shows signal waveforms at various parts of the measuring device shown in FIG. FIG. 5 shows a comparison of energy spectra measured by the conventional device and the device of the present invention. P... Probe, 1... Staircase wave generation circuit, 2...
minute amplitude AC voltage source, 3... selection amplifier, 4... reference signal generation circuit, 5... sample hold circuit, 6.
... Frequency multiplier circuit, 7... Shaping delay circuit, 8...
・Analog-digital converter, 9...accumulator,
10... Control circuit.

Claims (1)

[Claims] 1. A probe connected to a staircase wave generation circuit and a minute alternating current voltage source, a circuit connected to this probe to selectively amplify a specific frequency component of the probe current, and a circuit connected to the minute alternating voltage source to generate the minute alternating voltage from the minute alternating voltage. a circuit that creates a reference signal of a specific frequency component and creates a sample signal from this signal that is synchronized with the maximum amplitude of the output signal of the selective amplification circuit;
A sample and hold circuit connected to this circuit and the selective amplification circuit, an analog-to-digital conversion circuit connected to this sample and hold circuit, and a selective amplification circuit connected to this conversion circuit for each step of the staircase wave. 1. A high-speed plasma energy distribution measuring device characterized by comprising a digital accumulator that adds and averages amplitudes of specific frequency components obtained from a predetermined frequency.