JPH0665190B2 - Negative ion sensitive probe - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a negative ion sensitive probe capable of selectively detecting negative ions in plasma and measuring the temperature, density and energy distribution thereof.

(Prior Art) With the development of plasma chemistry in recent years, surface modification such as etching, cleaning, and film formation of a solid surface using plasma has become popular. In order to control these plasma processes efficiently, it is essential to monitor the state and parameters of plasma. The plasma gas used in the plasma process is a gas such as hydrogen, oxygen, hydrocarbon, or halogen, or a gas obtained by diluting these with an inert gas. Unlike the conventional plasma, the plasma of these gases contains many negative ions.

(Problems to be solved by the invention) Therefore, the conventional Langmuir probe (single probe)
Or double probe (double probe) method
Obtaining the electron temperature and the positive ion density causes a large error. Therefore, the density of negative ions in the plasma, the temperature,
It is necessary to obtain the energy distribution independently,
There has been a problem that a suitable probe for this purpose has not existed in the past.

(Means for Solving the Problems) The above-mentioned problems are caused by a probe electrode having an orifice, a collector for collecting charged particles incident from the orifice of the electrode, and electrons of negative charged particles passing through the orifice. This is solved by a negative ion sensitive probe that is configured by a magnet that generates a magnetic field that selectively causes only negative ions to reach the collector without deflecting them to the collector.

In the present invention, it is preferable that the probe electrode is made of a highly magnetically permeable material such as Kovar or Permalloy so that the magnetic flux from the electron deflection magnet does not leak into the external plasma.

The shape of the orifice may be any of a circle, a slit, and a mesh shape.

Further, the area of the orifice is preferably small so as to prevent leakage of magnetic flux and improve the selectivity of negative ions.

(Operation) Of the negatively charged particles that have entered through the orifice of the electrode in contact with the plasma, the electron has a very small mass compared to the ion, and is therefore greatly deflected by the magnetic field.
Therefore, at a given magnetic field strength, electrons do not reach the collector, only negative ions reach the collector.

(Example) Hereinafter, the present invention will be described in detail with reference to the accompanying drawings. First
FIG. 1 is a schematic sectional view of an embodiment of the negative ion sensitive probe of the present invention. A circular orifice (hole) O is provided in the probe electrode P that contacts plasma. The probe electrode P is made of Kovar or Permalloy.
A collector C is provided behind the probe electrode P. Micro disk-shaped or plate-shaped magnets M ₁ and M ₂ are arranged between the probe electrode P and the collector C so as to face each other so that their polarities are opposite to each other. These components are glass,
It is covered with an insulating tube I of ceramic or the like. When the probe electrode P is made of Kovar or Permalloy, the magnetic field generated by the magnets M ₁ and M ₂ does not leak out of the probe and disturbs the plasma. As shown in the figure, it is preferable that the highly permeable probe electrode P extends so as to surround the magnetism M ₁ and M ₂ because the magnetic flux leakage into the plasma is further prevented. The opening diameter of the orifice O is selected so that leakage of magnetic flux therethrough can be ignored (typically, the diameter of the orifice O is 1 mm and the diameter of the probe electrode P is 1).
cm and about). FIG. 2 shows a circuit diagram for measuring the probe current i _P and the negative ion current i ₋ using the negative ion sensitive probe of the present invention. A voltage is applied to the probe electrode P by a probe voltage DC power supply V _P. A current detection operational amplifier OP is connected to the power supply V _P. This op amp
A current detection resistor R is connected between the output of OP and one input. DC power supply for collector voltage for collector C
Voltage is applied by V _C. A micro ammeter μA for detecting collector current is connected in series with the power source V _C. Due to the deflection action of the magnetic field created by the magnets M ₁ and M ₂ , only the negative ions out of the negative ions and electrons that have passed through the orifice O are collected in the collector C and measured by the ammeter μA as a function of the collector voltage Vc. It

FIG. 3 schematically shows the current i _P flowing through the probe P. In the figure, V _S is the space potential of the plasma, and V _P
To the nonlinear curve i _P −V _P
Is obtained. When V _P << V _S , the probe current mainly consists of positive ion current i ₊ , and when V _P > V _S , electron current ie and negative ion current i ₋ . In the region of V _P <V _S , the probe current mainly consists of electron repulsion current. When the velocity distribution of charged particles in plasma is Maxwell distribution, i _P is the sum of these currents and each component can be expressed by the following equation.

1) V _P << V _S i _P = i ₊₀ = n ₊ eSexp (-1/2) × (kTe / M ₊ )
^1/2 (1) 2) V _P <V _S i _P = (ie + i _- ) _- i ₊₀ (2) where ie = neeS (kTe / 2πm) ^1/2 × exp {-e (V _S -V _P )
_{/ KTe} (3) i -} = n - eS (kT - / 2πM -) 1/2 × exp {-e (V S -V P)
_{/ KT -} (4) i} +0 is given by (1).

_{3) V P> V S i} P = (ie + i -) -i + (5) where _{ie 0 = neeS (kTe / 2πm} ) 1/2 (6) i - o = n - eS (kT - / 2πM - ) ^1/2 (7) i ₊ = n ₊ eS (kT ₊ / 2πM ₊ ) ^1/2 × exp {e (V _S −V _P )
/ KT ₊ } (8) In addition, e and m are the charge and mass of the electron, k is the Boltzmann constant, M
_{+ And} M ₋ are masses of positive ions and negative ions, T ₊ and T ₋ are temperatures of electrons, positive ions and negative ions, and S is surface area of the probe.

In FIG. 6, the probe current i _P is similarly divided into electron current ie, negative ion current i ₋ , and positive ion current i ₊ .

The current passing through the orifice is obtained by multiplying the current given by the above formula by the orifice surface area / probe surface area ratio and the solid angle of the orifice sin ² (θ / 2). The minimum magnetic field for deflecting the electron current component of this current by the magnetic field created by the permanent magnet and preventing it from reaching the collector can be determined as follows.

FIG. 4 is an enlarged schematic plan view of the probe as seen from above in FIG. Here, θ is the opening angle of the orifice. Let us consider a condition in which electrons coming from the direction A grazing with the orifice are bent by the magnetic field and do not reach the collector C. When the velocity components of the incident electrons in the x and y directions are u and v, and the electric field between the probe and collector is E, the equation of motion is md ² x / dt ² = eE + eBdy / dt (9) md ² y / dt ² =- eBdx / dt (10) These equations are solved at x = 0 under the condition of dx / dt | ₀ = u, dy / dt | ₀ = v, and the condition that this electron does not reach the collector C (x =
The condition for (dx / dt) ² ≦ 0 in d is as follows.

v ≧ (2e (V _P −V _S ) / m + u ² ) / (2ωd) −ωd / 2
(11) The condition that the electrons of the initial velocities u and v determined by the above equation do not reach the collector is given in the region above curve I in FIG. On the other hand, for negative ions, the condition that can reach the collector is the inequality sign opposite to that in Eq. (11). However, for negative ions, it is necessary to replace the electron mass m with the negative ion mass M ₋ and ω with ωi (negative ion cyclotron angular frequency). This area is given below the curve II in FIG. Therefore, the electrons are deflected and do not reach the collector, but the negative ions are not significantly deflected and can reach the collector. Since the opening angle of the orifice given by the region above I and below II is 2θ, v <Ucos θ, v> -uc
The condition osθ is added to this. The area that can finally reach the collector is given by the shaded area of III. That is, the fifth
Magnetic field strength B in the figure, probe-collector distance d
It can be seen that only negative ions can be selectively detected by designing a negative ion sensitive probe so as to satisfy the region III.

Next, a method for obtaining the negative ion density n ₋ by the measuring circuit shown in FIG. 2 using the negative ion sensitive probe of the present invention will be described. In the circuit diagram of FIG. 2, an ip-Vp characteristic can be obtained by swinging Vp positively or negatively with respect to the space potential Vs (normal probe operation). Then placed by setting the Vp several fold higher potential of kT ₊ / e than V _S, sweeping electrons, V _C on it reaches conditions in the negative ion thermal velocity each other toward the probe Measure the collector current-voltage (i _C −V _C ) characteristics with. As described above, at this time, almost no electrons reach the collector. For positive ions, Vp is several kT _{+ with} respect to Vs.
The probe and orifice are barely reached because they are expensive.
FIG. 6 shows the i _C -V _C characteristics thus measured.
In the figure, the saturation value ico of i _{− when} V _C > V _C is ico ＝ n ₋ eSosin ² (θ / 2) × (kT ₋ / 2πM ₋ ) ^{1 ] 2} (1
2) given in. The characteristic when V _C <V _S is given by ic = icoexp {−e (V _S −V _P ) × (kT ₋ } (13). The solid lines in FIG. 6 are the above formulas (12) and (13). The ideal measurement result is that the measurement result obtained is a little angled as shown by the dotted line. Therefore, plotting logic-V _C at V _C <V _S shows that kT _- it can be determined, n by substituting this value into equation (12) _-. so that can determine further, ne, Te, n ₊
Can also be obtained as follows. That is, i _P −V _P
Similarly, if the characteristic is semi-logarithmic plotted, Te can be determined from the gradient. If the saturation current value i _PO of logi _P −V _P is known,
By substituting T ₋ , n ₋ , and Te into the equations (5) to (8), ne is obtained. The contribution of i ₊ at V _P <V _S is so small that it can be ignored. The above Te value is the positive ion current value at V _P << V _S ,
Substituting into equation (1), we can determine n ₊ .

Next, FIG. 7 shows a circuit for measuring the energy distribution of negative ions by applying the micro AC method. Sine wave oscillator F1, F2
Then, the signals of the frequencies f ₁ and f ₂ are sent to the carrier removal modulator MOD, and the signals of the frequencies f ₁ + f ₂ and f ₁ −f ₂ are applied to the coil T2 and superposed on the collector voltage Vc. Of the AC component of the collector current ic, the beat component 2f ₂ is amplified by the narrow band selection amplifier S and introduced into the input IN1 of the lock-in amplifier L. The signals from the sine wave oscillators F1 and F2 are sent to the mixer MX, where a signal of frequency 2f ₂ is created and introduced into the input IN2 of the lock-in amplifier L to obtain a signal ic ″ corresponding to the second derivative of ic. This allows the Drivestein method (MJDruyvestey) for the electron case.
Based on n: ZfPhysik 64 (1930) 787), the energy distribution can be obtained for negative ions as well. The circuit configuration of the method for measuring the energy distribution of electrons and negative ions by applying the above-mentioned minute AC method has already been announced (H.Amemiya: J.Phys.Soc.Japan 55 (1986) 16
9).

Another advantage of measuring the second derivative by the micro AC method is that the influence of the leakage current due to the stray capacitance C ₁ of the cable to the collector and the stray capacitance C ₂ from the power supply Vc can be eliminated. That is, if the sweep of the current Vc is fast, the displacement currents due to C ₁ and C ₂ cannot be ignored with respect to ic, and the quality of measurement is impaired. In such a case, when the above-mentioned minute AC method is applied, its output signal ic ″ is based on the non-linearity of the current-voltage, so that it is not affected by the displacement current proportional to the primary degree of Vc. 8 is a typical curve of the collector current ic and its second derivative ic ″ obtained by the measuring circuit shown in FIG.

[Brief description of drawings]

FIG. 1 is a sectional view showing the structure of the negative ion sensitive probe of the present invention, FIG. 2 is a circuit diagram for measuring the probe current i _P and the negative ion current i ₋ by the negative ion sensitive probe method of the present invention, and FIG. 3 is a Langmuir probe. FIG. 4 is a diagram schematically showing the characteristic i _P −V _P and its electron current component ie, negative ion current component i ₋ , and positive ion current component i _+, and FIG. 4 is for explaining the operation of the negative ion sensitive probe of the present invention. A schematic cross-sectional view and a diagram showing the trajectories of charged particles. Fig. 5 shows the initial velocity region where electrons cannot reach the collector (region above curve I) and the initial velocity region where negative ions can reach the collector (from curve II). Fig. 6 shows a region III where electrons are removed but negative ions are collected, taking into account the opening angle of the orifice (2θ) and the opening angle (2θ) of the orifice. (Ion current) -voltage characteristics is there. FIG. 7 is a circuit diagram showing a circuit for implementing a method of measuring the energy distribution and density of negative ions by adding a differential circuit by the AC superposition method to the negative ion sensitive probe of the present invention, and FIG. 8 is a collector current ic and its Diagram showing the relation between the second derivative ic ″ and collector voltage Vc. P: Probe electrode, I: Insulation part, C: Collector, M1, M2: Permanent magnet, O: Orifice, V _P : DC power supply for probe voltage, OP : Current detection op amp, R: resistance Vc: collector, μA: collector current detection micro ammeter 2θ: orifice opening angle, d: probe-collector distance.

Claims (2)

[Claims] 1. An electrode having an orifice, a collector for collecting charged particles incident from the orifice of the electrode, and electrons of the negative charged particles passing through the orifice not to reach the collector due to deflection, and only negative ions A negative ion sensitive probe composed of a magnet that generates a magnetic field that selectively reaches the collector. 2. The probe according to claim 1, wherein the probe electrode is made of a highly magnetically permeable material.