JPH04206198A - Plasma stabilizer - Google Patents

Abstract

PURPOSE:To generate stable plasma by using a triple probe, whereby displaying the electron temperature as well as electron density of a plasma directly through an indicating instrument, and by controlling high frequency, microwave power and gas pressure through the output. CONSTITUTION:Three probes 3 of equal surface area are inserted in a plasma 2, and after a probe P2 is amplified at an electron temperature Te measurement circuit 8, the value of Te is directly indicated to an indicator 9. The negative voltage of, for example, 10 volts is added to a probe P3, from a fixed voltage Vd3 power source 13, while the voltage reduction generated at a very small resistor 12 of one ohm of current 1 that runs from probe P3 through P1, is calculated along with the output voltage of Te by an electron density Ne measurement circuit 10, so as to determine the voltage correspondent to the electron density Ne, and the amount of electron density Ne is indicated to an indicator 11. For the output voltage of a Te measurement device 8, an insulating amplifier 151 is used as an insulating coupling device, while control voltage is added to a next step gas control circuit 16, and the pressure of sample gas is controlled by adjusting an electromagnetic valve 5.

Description

[Detailed Description of the Invention] [Industrial Field of Application] In recent years, the field of use of plasma has been expanding more and more, and a stable plasma source is particularly desired in IC manufacturing equipment.

The present invention relates to a plasma stabilization device that automatically generates stable plasma.

(Prior Art) In conventional high-frequency plasma apparatuses, high-frequency or microwave power and gas pressure were manually adjusted, so it was not possible to automatically follow fluctuations.

[Means to solve the problem]

The present invention uses a triple probe method to directly display plasma electron temperature and electron density on an indicator,
This is a plasma stabilizing device that generates stable plasma by controlling high frequency or microwave power and gas pressure individually or simultaneously using this output.

At this time, since the electron temperature and electron density measurement circuits are floating in terms of direct current, they must be output as voltages relative to ground. This output control voltage controls high frequency or microwave power and gas pressure to stabilize the plasma flame.

That is, the present invention includes a measurement circuit that measures plasma electron temperature and electron density using a triple probe, a gas pressure control circuit that controls plasma gas pressure using an electron temperature measurement output signal, and a plasma excitation power that uses an electron density determination output signal. It is composed of a power control circuit to control, an insulating coupling element that couples the measuring circuit, the gas pressure control circuit, and the power control circuit, respectively, and automatically controls at least one of plasma gas pressure and plasma excitation power. This is a plasma stabilization device that generates stable plasma. In addition, when an insulating coupling element is not used, the voltage to ground corresponding to the electron temperature measurement output signal and the electron density measurement output signal are extracted from the above measurement circuit, respectively, and the gas pressure control circuit and the electronic control circuit are controlled using them. It's okay.

The plasma characteristic detection circuit using the triple probe method is
Since the output is floating in terms of DC, in order to extract the signal to the outside, it is necessary to connect it with DC insulation. In the present invention, insulating coupling elements such as isolation amplifiers, voltage frequency conversion circuits, and optical isolators are used to
We created a system in which the control signal is transmitted to an external circuit even though it is isolated from a direct current perspective, and a system in which the voltage to ground corresponding to the output voltage is extracted using an electronic circuit. Electronic temperature Te
Control the gas pressure by guiding the output signal to the gas pressure control circuit,
Furthermore, by controlling the high frequency or microwave output power using the output signal of the electron density Ne, the plasma flame is automatically stabilized.

[Effect]

The electron density in plasma is determined by the difference between the number of electrons produced by ionization and the number of electrons annihilated by recombination within a unit time. In a general plasma region where electron energy is relatively low, an increase in electron energy, that is, an increase in applied high frequency or microwave oscillation power, increases the ionization probability, but the recombination coefficient decreases, resulting in an increase in electron density Ne. do.

While electrons gain kinetic energy through high-frequency power excitation, they lose energy through collisions with atoms or molecules, and the electron temperature Te is determined by the difference in energy gained and lost per unit time.

These electron density Ne and electron temperature Te both change depending on the applied high frequency or microwave oscillation power and gas pressure, but when we conducted an experiment while directly reading the electron density Ne and electron temperature Te using the triple probe method, we found that the excitation power Due to the increase, the indication of the electron density Ne showed a large increase, but the indication of the electron temperature Te showed only a slight increase. Furthermore, as the gas pressure increased, the indication of the electron temperature Te showed a significant decrease, and the change in the electron density Ne was minute.

Therefore, in the present invention, a voltage corresponding to the command voltage output of the electron temperature Te is guided to the gas control circuit, a solenoid valve is adjusted to control the gas pressure, and a voltage corresponding to the output voltage of the electron density Ne is guided to the power control circuit. The plasma flame was stabilized by controlling the radio frequency or microwave output power.

Although automatic control using gas pressure, excitation high frequency, or microwave power is effective when performed alone, the stabilizing effect is significantly increased when both are performed at the same time.

Regarding the measurement principle and measurement method of the triple probe method, please refer to
Rōkaku Uchida edition, Shinriki Tsutsui's book "Fundamental Plasma Engineering J168"
As detailed in pages 183 to 183, in order to directly read Ne and Te, three probes P1. Insert P2 and P3 close together, and connect a voltmeter (with high input impedance) to the load of P2 as shown in Figure 3.
When a Te measurement circuit (Te measurement circuit) is connected and the probe is placed in a floating state, the current flowing through P2 becomes zero. At this time, the electric charge of the electron is e, k is Portzmann's constant, the differential voltage generated between probe P and P2 is 4□, the constant voltage applied to probe P is Vd3, and the current I flowing from probe P3 to Pi is
If the voltage drop that occurs across the low resistance 8 is determined using an electron density Ne measurement circuit, the following relationship will be obtained.

(1-exp (-φaz) )/ (1exp (-φ
dx) l = 1/2 In the above formula, φaz-e Vaz/k Te φa3= e Vax/k Te. Therefore ■4. Assuming that is a constant value, Te can be found by measuring (6).

This formula assumes that the change in the ion current in the measurement range is small, but the ion current of a cylindrical probe in a collisionless plasma theoretically increases in proportion to the (1/2) power of the probe voltage, so the floating Ionic current at voltage V,■
, (vf) as a reference, the ion current 1, (V) at voltage ■ is expressed using a constant β as follows.

(L (V))2=flt (Vr)12 (1+
β△■) Assuming that the ion current changes like this, Vi: +=10
When Bord is constant, the electron temperature Te obtained for various β values is as shown in FIG.

According to experiments, it has been found that when β-1.05 is used, the error in Te is within several percent in general plasma measurements.

The electron density Ne is given as follows in the same book (page 172).

)"Je-(M"2/S)-1-f+ (Vaz)f
+ (L2)-1,05xlO" x (Te)1"
/(eXp(-φ6□)-1) The units used are Ne (crrr3), Broglie surface area (mm2), I (μA),
Te (eV).

M (atomic weight or molecular weight of ion), Vd2 (v)
It is. If M and S are given from the above equations, the electron density Ne can be found from the probe current ■, Te and Vd2,
This value can be read directly on the output meter.

Instruct the meter with an output voltage corresponding to the electronic temperature calculated using the above principle, and either cut off the direct current through an insulating coupling element, or use an electronic circuit to determine the output voltage equivalent to the ground point, and then output this output. The voltage is amplified by the gas control circuit and the electromagnetic valve is controlled to increase or decrease the gas pressure. That is, if Te is too high, the gas pressure is increased to lower Te; on the other hand, if Te is too low, the gas pressure is decreased to lower Te.
to rise.

On the other hand, for the meter output voltage of the electron density Ne, the output equivalent voltage with respect to the ground point is similarly determined, and this is led to the excitation power control circuit, which increases or decreases the output power of the high frequency or microwave generation source to control the excitation power of the plasma. do. That is, the electron density Ne
If is too large, the excitation power is lowered to lower the density, and vice versa, the excitation power is increased to increase the electron density. In this way, the plasma flame can be automatically kept stable.

〔Example〕

FIG. 1 shows a system diagram of an embodiment of the invention.

A sample gas is supplied to the plasma chamber 1 via an NN valve 5, and microwave power is introduced from a coupling hole 6 to generate plasma 2.

Three probes 3 having the same surface area are inserted into this plasma, and the probe P2 is amplified by the electron temperature Te measurement circuit 8, and then the indicator 9 Indicates the value of Te. A negative voltage of 10 volts is applied to probe P from a fixed constant voltage Vd3'rX13, and from probe P3 to P,
The Ne measurement circuit 10 calculates the voltage drop that occurs across the 1-ohm microresistor 12 of the electric current II flowing through the electrode, together with the output voltage of Te, to obtain the voltage corresponding to the electron density Ne.
1 shows the amount of Ne.

Since the output voltage of the Te measurement circuit 8 must be floated in direct current, an insulation amplifier 151 is used as an insulation coupling element, a control voltage is applied to the next stage gas control circuit 16, and the electromagnetic valve 5 is adjusted to adjust the sample gas. Control the pressure.

As an insulating coupling element, an insulating method using an optical isolator or a voltage frequency converter of the insulating amplifier 151, or converting a DC voltage to an alternating current and then coupling it using an isolation transformer,
A method such as returning the current to direct current can be used.

The output voltage of the Ne measuring circuit 10 is also determined by using an isolation amplifier 152 as a similar insulating coupling element, and applying the output to the excitation power control circuit 17 to adjust the power of the 2.45 GHz microwave power source 7 to generate the excitation applied to the plasma. Control the power to stabilize the plasma flame 2. The AC power necessary for these is applied via an isolation transformer 14.

As another embodiment of the present invention, a portion replacing the insulating coupling element shown in FIG. 1 is extracted and shown in FIG. The output potential of the Te measurement circuit 8 is set to V1, and the output potential of the Ne measurement circuit 10 is set to V2. V
, 3 If the ten-terminal potential of the constant voltage power supply 13 is 3, then the first
Similarly to the figure, the Te output indicator swings in proportion to the voltage difference between ■1 and ■3, and the Ne output indicator swings in proportion to the voltage difference between ■2 and ■3.

■1 potential is connected to the positive input terminal of the differential amplifier 211, and this output is amplified by the two-stage inverters 221 and 231, and the output is returned to the negative input terminal of the differential amplifier 211. Output potential ■1゛ is ■1
Equilibrium is reached when it becomes equal to .

Potentials ■2 and ■3 are also amplified by similar differential amplifiers 212 and 213 and inverters 222, 232, 223, and 233, and their outputs ■2'' and ■3'' are equal to the input potentials ■2 and ■3.
is equal to

Since the potentials Vl' and V3' are applied to the differential amplifier 241 in the next stage, its output becomes the difference voltage between 1' and V3', that is, the voltage to the ground corresponding to the Te indicated voltage, and the insulating coupling element An isolated output voltage can be obtained from the Te measurement circuit without using the Te measurement circuit. Therefore, this Te-equivalent control voltage is applied to the gas control circuit 16 to adjust the electromagnetic valve 5 to control the pressure of the sample gas and stabilize the plasma flame.

Since the potentials v2' and ■3'' are applied to the differential amplifier 242, the output becomes the difference voltage between ■2° and v3', that is, the voltage to the ground corresponding to the Ne indicated voltage, and an insulating coupling element is not used. An isolated output voltage is obtained from the Ne measuring circuit.This Ne equivalent control voltage is applied to the excitation power control circuit 17.
In addition, the output power of the microwave source is adjusted to control the excitation power of the plasma to stabilize the plasma flame.

In Figure 2, 191, 192, and 193 are all short-circuit prevention resistors, which prevent the negative effects of the drop in input load impedance from reaching the previous stage until the differential amplifier circuit reaches equilibrium. , the input load impedance is high when balanced, so the voltage drop across this resistor is not a problem. Also 201, 202 and 203
is the input terminal resistor.

〔Effect of the invention〕

There are a wide variety of plasma instability factors, and although it is not possible to eliminate all of them, they can be greatly improved by implementing the present invention. Even if the automatic control according to the present invention is applied to only the gas pressure and the high-frequency excitation power alone, a considerable effect can be obtained, but if both are performed simultaneously, the stability can be improved by an order of magnitude.

In the experiment, a sample of hydrogen gas mixed with 10% methane gas was used at an atmospheric pressure of 0.2 Torr, and a pressure of 2.45 G was used.
We attempted continuous operation for 4 hours by applying microwave oscillation power of 300 W of Hz to generate plasma, and the operation continued very stably during this period.

[Brief explanation of the drawing]

Fig. 1 is a system diagram of an embodiment of the present invention, Fig. 2 is a system diagram of another embodiment, and Fig. 3 is a diagram explaining the principle of the triple probe method.
FIG. 4 is a ■4□ voltage versus electron temperature Te characteristic diagram. 1 is a plasma chamber, 2 is a plasma flame, 3 is a probe, 4 is a gas inlet, 5 is an electromagnetic valve, 6 is an excitation power coupling hole, 7 is a high frequency or microwave power source, 8 is a Te measurement circuit, 9 is Te Output indicator, 10 is Ne measurement circuit, 1
1 is Ne output indicator, 12 is micro resistor, 13 is Vd3
Constant voltage power supply, 14 is an isolation transformer, 151 and 152 are isolation amplifiers, 16 is a gas control circuit, 17 is an excitation power control circuit, 191, 192, and 193 are short-circuit prevention resistors, 201
・202 and 203 are input terminal resistors, 211 and 212.
213 and 241/242 are differential amplifiers, 221/2
22, 223, 231, 232, and 233 are inverters.

Claims (2)

[Claims] (1) A measurement circuit that measures plasma electron temperature and electron density using a triple probe, a gas pressure control circuit that controls plasma gas pressure using an electron temperature measurement output signal, and an electric power that controls plasma excitation power using an electron density establishment output signal. It is composed of a control circuit, an insulating coupling element that couples the measurement circuit, the gas pressure control circuit, and the power control circuit, respectively, and automatically stabilizes plasma by controlling at least one of plasma gas pressure and plasma excitation power. A plasma stabilizing device that generates (2) A measurement circuit that measures the electron temperature and electron density of the plasma using a triple probe, a circuit that extracts the voltage to ground corresponding to the electron temperature measurement output signal and the electron density measurement output signal, and a plasma gas A plasma stabilizing device is comprised of a gas pressure control circuit and a power control circuit that respectively control pressure and plasma excitation power, and automatically generates stable plasma by controlling at least one of plasma gas pressure and plasma excitation power.