JPH0779036B2 - High precision plasma energy distribution measuring device - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an apparatus for measuring energy distribution, and more particularly to an apparatus for measuring the energy distribution of electrons in a gas discharge plasma with high accuracy.

(Prior Art) It has been proved by Drivestein that the energy distribution of electrons when the electrons in plasma have an isotropic distribution is proportional to the second derivative of the probe voltage-current characteristic. The alternating current superposition method has been mainly used as a method for obtaining the same.

In this AC superposition method, an AC having a small amplitude is superposed on a probe voltage that changes in a certain range, and a frequency component proportional to the second derivative of the above characteristic is detected to obtain an energy distribution.

FIG. 2 is a block diagram of a conventional apparatus for carrying out this AC gravimetric method. A signal having a frequency f generated from the AC voltage generation source 9 is applied to the sweep voltage generated from the sweep voltage generation circuit 13 to the probe P inserted in the plasma. The selective amplifier 10 amplifies only the frequency component 2f corresponding to the energy distribution signal from the frequency components of the probe current. The output signal of the selective amplifier 10 is detected in a predetermined phase by the signal detection circuit 12 that receives the reference signal of the reference signal generation circuit 11, and the energy distribution is obtained.

(Problems to be Solved by the Invention) Generally, in the AC superposition method, it is necessary to constantly apply an AC signal to plasma to be measured through a sheath. In particular, when the noise in the plasma is large, it is necessary to naturally increase the amplitude of the AC signal in order to secure the signal-to-noise ratio of the energy distribution signal. On the other hand, since the plasma itself has instability due to ion vibration or electron vibration, the application of the AC signal disturbs the plasma state.
When a wave is excited in plasma, it leads to the result of measuring a shape different from the actual distribution function. There is a limit to the reliability of the measurement method itself of the AC superposition method.

(Means for Solving Problems) The above-mentioned problems include a staircase voltage generating circuit, a detection circuit for a probe current flowing according to the staircase output, a DC stabilized power supply for biasing the staircase output, Detection circuit of probe current flowing according to staircase wave output after bias,
A subtraction circuit of both outputs of the two current detection circuits, a frequency modulation circuit that receives the output of the subtraction circuit, generates a pulse of a frequency according to the magnitude, receives an output pulse of the frequency modulation circuit, and counts this for a predetermined time It is possible to solve the problem by a plasma energy distribution measuring device including a counting circuit for performing the calculation, and a calculation circuit for calculating the counting result of the counting circuit.

That is, the present invention changes the probe voltage stepwise,
This is to obtain the second-order difference of the probe current in each staircase, which makes the application of an AC signal unnecessary, and the smoothing action is introduced in the difference process, so that the energy distribution can be measured with high accuracy.

In addition, it is preferable that the rising speed of the staircase wave be approximately the same as the sweeping speed of the conventional method.

(Operation and Effect of the Invention) In general, since various noises are mixed in the probe signal, the noise component is emphasized when the differential operation is performed on the probe signal, and the signal-to-noise ratio is significantly lowered.

In the present invention, two probes are used to cancel the in-phase component of noise to obtain a first-order difference, and a frequency modulation circuit and a counting circuit are further used for each step of the probe voltage with respect to the first-order difference signal. After the averaging process is performed on, the secondary difference process is performed to obtain the energy distribution signal. For this reason, the signal-to-noise ratio and the resolution are improved, and therefore, it is possible to perform the energy distribution measurement with high accuracy without accompanying the unavoidable disturbance effect on the plasma in the AC superposition method.

(Embodiment) A detailed description will be given below with reference to the block diagram of FIG. 1, which is an embodiment of the present invention.

The staircase voltage generating circuit 1 generates a voltage that changes stepwise as shown as V _{2 in} FIG. DC stabilized power supply 2
Generates a voltage equal to one step change in voltage, and the voltage V ₁
Is biased to generate voltage V ₂ . The step voltages V ₁ and V ₂ are applied to the probe electrodes P ₁ and P ₂ placed in the plasma, respectively. Probe currents ip ₁ and ip ₂ as shown in FIG. 4 flow through the probe electrodes P ₁ and P ₂ depending on the applied voltage.
Each probe current is detected by the current detection circuits 3 and 4.
Each of them is detected, and the difference (ip ₁ −ip ₂ ) between these probe currents ip ₁ and ip ₂ is obtained in the subsequent subtraction circuit 5. The output amount of the subtraction circuit 5 corresponds to the first-order difference of the probe voltage-current characteristics. Counted. In this voltage-frequency conversion and counting process, a smoothing action occurs in each section. The arithmetic circuit 8 obtains the difference between the count results in the previous section T _n-1 and the next section T _n . As a result,
As shown in FIG. 5, an energy distribution signal Y proportional to the second-order difference of the probe linear pressure-current characteristic is obtained.

[Brief description of drawings]

FIG. 1 is a block diagram showing a circuit of a plasma energy distribution measuring device according to the present invention, and FIG. 2 is a block diagram of a conventional measuring circuit by an AC superposition method. FIG. 3 is a stepwise probe voltage waveform diagram applied to two probes, FIG. 4 is a Purob voltage waveform diagram obtained from two probes, and FIG. 5 is a plasma energy distribution obtained by the present invention. Fig. (Explanation of symbols) P ₁ , P ₂ ... probe electrode, Y ... energy distribution signal,
1 ... Staircase voltage generator, 2 ... DC stabilized power supply,
3, 4 ... Current detection circuit, 5 ... Subtraction circuit, 6 ... Voltage / frequency converter, 7 ... Counting circuit, 8 ... Arithmetic circuit, 9
...... AC voltage generator, 10 …… Selective amplifier, 11 …… Reference signal generator, 12 …… Signal detector, 13 …… Sweep voltage generator.

Claims (1)

[Claims] 1. A staircase voltage generating circuit, a first detection circuit of a probe current flowing according to the staircase output, a DC stabilized power supply for biasing the staircase output, and a staircase wave output after the bias. Output circuit of the frequency modulation circuit, which receives the outputs of the second detection circuit of the probe current flowing through the output circuit and the subtraction circuit of both outputs of the two current detection circuits, and generates the pulse of the frequency according to the magnitude A high-accuracy plasma energy distribution measuring device comprising a counting circuit for receiving the counting result for a predetermined time, and a computing circuit for computing the difference between the counting result before this counting circuit and the subsequent counting result.