JPS63248100A - Negative ion sensitive probe - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a negative ion sensitive probe that makes it possible to selectively detect negative ions in plasma and measure their temperature, density, and energy distribution.

(Prior Art) In recent years, with the development of plasma chemistry, surface modification such as etching, cleaning, film formation, etc. of solid surfaces using plasma has become popular. For efficient control of these plasma processes, it is essential to monitor plasma conditions and parameters. 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 gases with an inert gas. Unlike conventional plasma, these gas plasmas contain many negative ions.

(Problem to be solved by the invention) Therefore, this is compared to the conventional Langmuir probe (single probe).
electron density,
Determining the electron temperature and positive ion density results in large errors. Therefore, the density of negative ions in the plasma, the temperature,
Although it is necessary to obtain the energy distribution independently,
There has been a problem in that no suitable probe for this purpose has conventionally existed.

(Means for Solving the Problems) The above-mentioned problems include a probe electrode having an orifice, a collector that collects charged particles incident through the orifice of the electrode, and electrons among the negatively charged particles that have passed through the orifice. The problem is solved by a negative ion sensitive probe consisting of a magnet that generates a magnetic field that selectively allows only negative ions to reach the collector without being deflected.

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

Further, the shape of the orifice may be a circle, a slit, or a mesh shape.

Additionally, the orifice volume is preferably made small to prevent magnetic flux leakage and improve negative ion selectivity.

(Function) Of the negatively charged particles that enter through the orifice of the electrode in contact with the plasma, electrons have extremely low mass compared to ions, so they are deflected to a greater extent by the magnetic field.

Therefore, at a given magnetic field strength, no electrons reach the collector, but only negative ions.

(Example) Hereinafter, the present invention will be described in detail using the accompanying drawings. 1st
The figure is a schematic cross-sectional view of one embodiment of the negative ion sensitive probe of the present invention. A circular orifice (hole) is provided in the probe electrode P that contacts the plasma. This probe electrode P is made of Kovar or Permalloy.

A collector C is provided behind this probe electrode P.

Between the probe electrode P and the collector C, minute disc-shaped or flat-plate magnets M1 and M2 are arranged facing each other so that their polarities are opposite to each other. These components are covered with an insulating tube (2) made of glass, ceramic, etc. If the probe electrode P is made of Kovar or Permalloy, the magnetic field generated by the magnets M, M2 will not leak out of the probe and will not disturb the plasma. It is preferable that the highly permeable probe electrode P extends so as to surround the magnetic M and SM2 as shown in the figure, since leakage of magnetic flux into the plasma can be further prevented. The aperture diameter of orifice 0 is selected to be small enough that leakage of magnetic flux through it is negligible (as a typical example, the diameter of orifice O is 1+nm and the diameter of probe electrode P is about 1 cm).

FIG. 2 shows a circuit diagram for measuring probe current 1p and negative ion current 1- 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 VP. A current detection operational amplifier OP is connected to this power supply V. A current detection resistor R is connected between the output and one input of the operational amplifier OP.

A voltage is applied to the collector C by a collector voltage DC power supply Vc. A microcurrent meter μA for collector current detection is connected in series with this power supply VC. Magnet M1, M
Due to the deflection effect of the magnetic field created by 2, of the negative ions and electrons that have passed through the orifice 0, only the negative ions are collected in the collector C, and the collector voltage V is measured by the ammeter μA.
It is measured as a function of c.

FIG. 3 schematically represents the current i flowing through the probe P. In the figure, ■ is the spatial potential of the plasma, and by changing ■ with respect to this, a nonlinear curve i p −V is obtained. ■.

At <<V, the probe current is mainly the positive ion current l.

, VP>Vs, the current consists of an electron current 12 and a negative ion current i-. In the region of Vp<Vs, the probe current mainly consists of electron repulsion current. When the velocity distribution of charged particles in plasma consists of a Maxwellian distribution, IP is the sum of these currents, and each component can be expressed by the following equation.

1) V, <<Vs 2) V, <V.

j, = (ie+i)−i=o (
2) Here, 1+O is given by [1).

3) v, >v.

ip= (ie −i−) −i,,
(5) Here 1eo=n, es(kTe/2πm)” (6)i
o=n, -es (kT-/2zrM-)"'
(7) Also, e and m are the charge and mass of the electron, k is Boltzmann's constant, and M. 1M- is the mass of positive ions and negative ions, T
,, T- is the temperature of electrons, positive ions, and negative ions, and S is the surface area of the probe.

In FIG. 6, the probe current i is the electron current 18,
Negative ion current me 1 Positive ion current 1. It is divided into each component and shown in the same way.

The current passing through the orifice is the current given by the above equation multiplied by the ratio of orifice surface area/probe surface serum and the solid angle of the orifice, 5 in 2 (θ/2). The minimum magnetic field for deflecting the electronic current component of this current by the magnetic field created by the permanent magnet so that it does not reach the collector can be determined as follows.

FIG. 4 is an enlarged plan view of the probe viewed from above in FIG. 1. Here, θ is the opening angle of the orifice. Let us consider a condition in which electrons coming from direction A that graze the orifice are bent by the magnetic field and do not reach collector C. The velocity components of the incident electron in the x and y directions are u,
v, and the electric field between the probe and collector is E, the equation of motion is md2x/d t2=eE+eBdy/d t
(9) md2y/d t2=-eBdx/d t
αQ These equations are d x / d t l when x=0
o=u.

Solve under dy/dtlo=v, and find the condition that this electron does not reach the collector C ((dx/dt)2 at x=d)
≦0) is determined as follows.

V≧(2e(vp Vs)/m+u2)/(2ωd)
-ωd/2αυ The conditions under which electrons with initial velocities u and v determined by the above equations do not reach the collector are given in the region above the curve ■ in Figure 5. −
Conversely, for negative ions, the condition under which they can reach the collector is the opposite inequality to the 0υ equation.

However, for negative ions, the electron mass m is expressed as negative ion XiA
It is necessary to replace M-, ω with ω1 (negative ion cyclotron angular frequency). This area is the curve in Figure 5■
given in the lower region. Therefore, electrons are deflected and do not reach the collector, but negative ions are not deflected much and can reach the collector because the opening angle of the orifice given by the region above I and below ■ is 2θ, so v The following conditions are added to this: ku cos θ, v>−ucO9θ.

The area where the collector can finally be reached is given by the diagonally shaded area. That is, it can be seen that only negative ions can be selectively detected by designing the negative ion sensitive probe so that the magnetic field strength B1 in FIG. 5, the probe-collector distance d, satisfies the region (2).

Next, a method for determining the negative ion density n- using the measuring circuit shown in FIG. 2 using the negative ion sensitive probe of the present invention will be explained. In the circuit diagram of FIG. 2, the i, -V characteristic can be obtained by making V vary positively and negatively with respect to the space potential V (normal probe operation).

Next, (2) is set to a potential several times kT,/e higher than V, and the conditions are such that electrons and negative ions reach each other at a thermal velocity toward the probe. The collector current-voltage (+c-VC) characteristic is measured by sweeping. As mentioned above, at this time, almost no electrons reach the collector.

In addition, since V of positive ions is several kT higher than V, they hardly reach the probe and orifice. FIG. 6 shows the 1c-vc characteristics measured in this manner. V in the diagram.

>Vs, the saturation value tco of i- is given by. Also Vc
The characteristics at <Vs are given by. In Figure 6, the solid line is above (
12), represents the ideal characteristics according to equations (13). The actually obtained measurement results have a slightly rounded corner as shown by the dotted line. Therefore, Iogi
When C-Vc is plotted, kT-
can be determined, and by substituting this value into the α equation, n-
can be determined. Furthermore, n, , Ta,
n.

can also be obtained as follows. That is, 1p-
If the Vp characteristic is similarly plotted semi-logarithmically, To can be determined from the slope. Also, the saturation current value of Iogi, -Vp is 1. .

Once we know, we can write T, n, −, in equations (5)-(8).
'n is obtained by substituting To. v,>V
The contribution of i in s is so small that it can be ignored. By substituting the above value of T0 into the positive ion current value at V<<Vs, equation (1), n can be determined.

Next, FIG. 7 shows a circuit for measuring the energy distribution of negative ions by applying the minute alternating current method. sine wave oscillator F1,
From F2, a signal with a frequency f,, F2 is sent to the carrier removal modulator MOD, and a signal with a frequency fI+f2+f+F2 is applied to the coil T2, and the collector voltage V is applied. superimposed on Among the AC components of the collector current ie, the beat component 2f2 is amplified by the narrow band selection amplifier S, and the lock-in amplifier L is manually powered I.
Introduced to NI.

The signals from the sine wave oscillators F1 and F2 are sent to the mixer MX, which generates a signal with a frequency of 2f2 and is sent to the lock-in amplifier L.
A signal 1c'' corresponding to the second derivative of lc is obtained by introducing it into the input IN2 of lc.This allows the Drivestein method (M.

Druyvestein, J.: 2. f, Phys.
ik 64 (1930) 787), the energy distribution for negative ions can be similarly obtained. The circuit configuration of the method for measuring the energy distribution of electrons and negative ions by applying the above-mentioned micro-current method is already published (2). Amemiya: J, Phys.
, Soc, Japan 55 (1986) 169).

Another advantage of measuring the second derivative using the minute AC method is the stray capacitance C1 of the cable to the collector and the power supply V. It is possible to eliminate the influence of leakage current due to stray capacitance C2 from. That is, if the power supply Vc sweeps quickly, these C1,
The displacement current due to C2 cannot be ignored with respect to IC, and the quality of measurement will be impaired. In such a case, if the above minute AC method is applied, the output signal 1°'' is based on the nonlinearity of current-voltage, so it is not affected by the displacement current that is linearly proportional to Vc. can be obtained by performing two integrations.

FIG. 8 shows a typical curve of the collector current IC obtained by the measuring circuit shown in FIG. 7 and its second-order differential amount 1°''.

[Brief explanation of drawings]

Figure 1 is a cross-sectional view showing the structure of the negative ion sensitive probe of the present invention, Figure 2 is a circuit diagram for measuring probe current IP and negative ion current l- by the negative ion sensitive probe method of the present invention, and Figure 3 is Langmuir probe characteristics. i, -V, and its electron current component 1. , negative ion current component l-1 positive ion current component l. 4 is a schematic cross-sectional view and a diagram showing the trajectory of charged particles to explain the operation of the negative ion sensitive probe of the present invention. FIG. 5 is an initial velocity region in which electrons cannot reach the collector. (area above curve I), initial velocity area where negative ions can reach the collector (area below curve ■), and orifice opening angle (2θ), electrons are removed but negative ions are removed. FIG. 6 is a diagram showing the collector current (negative ion current)-voltage characteristic of the negative ion sensitive probe. 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 based on the AC superposition method to the negative ion sensitive probe of the present invention, and Fig. 8 shows the collector current IC and its A diagram showing the relationship between the secondary component j) ic" and the collector voltage Vc. P nib electrode,
, M2: Permanent magnet; d: Distance between probe and collector. Figure 3 Probe voltage vp Figure 6 Collector voltage ■ Figure 8

Claims (2)

[Claims] (1) An electrode with an orifice, a collector that collects charged particles incident from the orifice of this electrode, and electrons among the negatively charged particles that have passed through the orifice are deflected to select only negative ions without allowing them to reach the collector. A negative ion sensitive probe consisting of a magnet that generates a magnetic field that causes the magnetic field to reach said collector. (2) The probe according to claim (1), wherein the probe electrode is made of a highly permeable material.