JPH0573190B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to an electronic counting device that counts the number of electrons emitted from a sample upon irradiation with light energy.

(Prior art) Conventionally, as a method for measuring the thickness of an oxide film formed on the surface of a sample such as a semiconductor in an open atmosphere, the sample surface is irradiated with light, and the irradiation of light passes through the thin film on the sample. The electrons emitted to the outside are introduced into an electron detection section, and the number of emitted electrons is counted using the gas discharge phenomenon caused by the introduced electrons.
A method of measuring film thickness based on the number of emitted electrons is known (Japanese Patent Application No. 118818/1983, etc.).

(Problem to be Solved by the Invention) However, in such a conventional electronic counting device, a high voltage is applied to the anode ring placed in the electron detection section, and the electrons emitted from the sample cause a gas discharge. The number of electrons is counted from the pulse change in the anode voltage caused by this gas discharge, but there is no clear way to determine the anode voltage. Even if the anode voltage is set to, for example, 3.4 KV, because it is open to the atmosphere, the electron counting rate will vary even for the same sample due to changes in atmospheric pressure and temperature, making stable electron counting impossible. There was a problem that I couldn't do it.

(Means for Solving the Problems) The present invention has been made in view of these conventional problems, and is an optimal anode that can always provide stable counting results of emitted electrons even when atmospheric pressure and temperature fluctuate. It is an object of the present invention to provide an electronic counting device that allows the voltage to be set.

In order to achieve this object, the present invention includes a voltage variable means for varying the voltage applied to an anode disposed in an electronic detection section, and a voltage detector for detecting a discharge starting voltage at which gas discharge is started by varying the anode voltage. means, and voltage setting means for setting the optimum anode voltage by adding to the discharge starting voltage a voltage approximately half of a predetermined plateau voltage width whose count value remains approximately constant with respect to changes in the anode voltage. It was designed to provide a.

(Function) According to the configuration of the present invention, by varying the anode voltage prior to sample measurement, the discharge start voltage, for example, the voltage at which the electron count rate exceeds a threshold based on background noise, can be adjusted. Discharge starting voltage
Vs, and the voltage (Vs+Pw/2), which is the sum of the discharge starting voltage Vs and a voltage equal to one half of the predetermined plateau voltage width Pw, is set as the optimum anode voltage.

Here, the plateau voltage width Pw means that when emitted electrons are introduced into the electron detection section and a gas discharge is caused in a high electric field by the anode, the first grid electrode is used to prevent avalanche amplification of the gas discharge. The voltage width is equal to the height of the so-called quenching voltage that increases the voltage and lowers the electric field, and in this way, the plateau voltage width Pw is uniquely defined from the height of the quenching voltage applied to the first lattice electrode. Therefore, the optimum anode voltage can be immediately set based on the detection of the discharge starting voltage.

With the optimal anode voltage set in this way, even if the characteristics of the electron count rate with respect to the anode voltage change due to changes in atmospheric pressure or temperature, the fluctuation within a range of ±Pw/2 around the optimal anode voltage will be avoided. If so, the electronic counting rate is kept substantially constant, and stable electronic counting can be performed.

(Example) FIG. 1 is an explanatory diagram showing an example of the present invention.

First, to explain the configuration, 1 is an electron detection section, and there is a detection window 2 on the side of the sample stage 8 on which the sample 10 is set.
It has a metal case 3 with an opening, and the case 3 is grounded. There is an anode ring 4 inside the case 3.
is arranged, and a high voltage power source 23 is connected to the anode ring 4.
The anode voltage can be varied within the range of . A first grid electrode 5 and a second grid electrode 6 are sequentially arranged on the detection window 2 side with respect to the anode ring 4.

On the other hand, a light source device 9 is arranged on the side of the electron detection section 1, and irradiates the surface of the sample 10 set on the sample stage 8 with single wavelength light of a predetermined wavelength from diagonally above. The light source device 9 includes a light source 11 such as a deuterium lamp.
It has a monochromator 12 for converting the light from the light source 11 into single wavelength light of a predetermined wavelength, and slits 13 and 14 are provided before and after the monochromator 12 for adjusting the light intensity. The monochromator 12 has a predetermined wavelength range, for example, 150 nm~
It has the ability to scan a single wavelength in the 600nm range.

Further, as a counting circuit for electrons emitted from the sample by the electron detection section 1, an amplifier 18 connected to the anode ring 4 via a capacitor C and an amplifier 18 for counting electrons emitted from the sample by the electrons introduced into the electron detection section 1 are used. A first pulse generator 20 and a second pulse generator 2 to which voltage pulses of the anode ring 4 are inputted
2, the output of the first pulse generator 20 is connected to the first grid electrode 5, and the output of the second pulse generator 22 is connected to the second grid electrode 6.

Here, the output waveforms of the first pulse generator 20 and the second pulse generator 22 when the voltage pulse from the anode ring 4 is outputted by the gas discharge caused by the electrons introduced from the amplifier 18 are as follows.

In the initial state, as shown in Figure 2 b and c,
The voltage of the first grid electrode 5 caused by the first pulse generator 20 is, for example, 100V, and the voltage of the second grid electrode 6 is set by the output of the second pulse generator 22, for example.
80V, and electrons emitted from the sample 10 by irradiation with light from the light source device 9 pass through the second grid electrode 6 and the first grid electrode 5 and are attracted to the anode ring 4. When the emitted electrons approach the vicinity of the anode ring 4, gas discharge development is caused by the acceleration of the electrons that receive the high electric field from the anode ring 4, and as a result, the anode voltage changes in a pulse-like falling manner as shown in Figure 2a. wake up In response to this change in anode voltage, the first pulse generator 20 increases the voltage previously applied to the first grid electrode 5 by, for example, 300V to 400V over a predetermined quenching time Te.
Therefore, the potential difference between the anode ring 4 and the first grid electrode 5 decreases by 300V, and the electric field strength near the anode ring 4 decreases, so that the light generated by the gas discharge and secondary electrons due to cations cannot reach the discharge voltage. This prevents an avalanche of discharge.

On the other hand, in synchronization with the fall of the anode voltage due to the gas discharge, the second pulse generator 22 lowers the voltage of the second grid electrode 6, which had been 80V, by 110V, for example, to -30V.
for a predetermined quenching time Te.
As the voltage of the second grid electrode 2 decreases to -30V, the cations generated by the gas discharge accompanied by the amplification effect are captured and neutralized by the second grid electrode 6, whereby the cations are transferred to the sample 10. At the same time, it blocks the introduction of electrons from the outside.

As shown in the signal waveform diagram of FIG. 2, the same operation is repeated every time an electron is emitted from the sample 10, and one voltage pulse is output from the amplifier 18 in response to one emitted electron. .

The voltage pulses outputted from the amplifier 18 in this manner are counted by the counting means 24, and a counting rate N (cps) per unit time is outputted, for example.

Following the counting means 24, a calculating means 26 is provided for setting the anode voltage to an optimum voltage.

The calculation means 26 performs the next process of setting the optimum anode voltage while a predetermined reference sample is set on the sample stage 8 and is irradiated with single wavelength light from the light source 9 prior to use of the apparatus.

(A) First, for example, set the anode voltage to the high voltage power supply 23.
Outputs a control signal to vary from 3.0KV to 4.0KV.

(B) The counting rate N obtained through the counting means 24 is monitored while the anode voltage to the high-voltage power supply 23 in (A) is varied, and the counting rate N is monitored based on the background noise when there is no electron emission from the sample 10. When a counting rate equal to or higher than a threshold value No determined by, for example, No=10 (cps) is obtained, the voltage applied to the anode at this time is detected as the discharge starting voltage Vs.

(C) A plateau voltage width corresponding to the height of the quenching voltage for the gas discharge element applied to the first grid electrode 5 shown in FIG. 2b is added to the discharge starting voltage Vs detected in (B) above. A voltage (Vs+Pw/2) obtained by adding one-half of Pw is calculated, and this voltage is set in the high-voltage power supply 23 as the optimum anode voltage Vao.

Furthermore, in addition to the above-mentioned function of setting the optimum anode voltage, the calculation means 26 calculates the counting rate N obtained from the counting means 24 when measuring the thickness of an oxide film formed on the surface of the sample 10, for example. Film thickness T based on
This film thickness calculation means is |ogN=|ogN1−T/2.30λ, where λ: electron mean free path in the thin film (Angstrom) N1: count when the film thickness is zero The film thickness T is calculated as a ratio. Of course, the electronic counting rate N obtained by the counting means 24 may be output as is. Alternatively, the wavelength may be varied, the counting rate thereof may be outputted, and the work function of metal etc. may be calculated.

The output of the calculation means 26 is given to the display means 28, and the calculated film thickness,
Electronic count rate and work function will be displayed.

Next, the operation of the embodiment shown in FIG. 1 will be explained.

FIG. 3 shows the calculation means 26 provided in the embodiment of FIG.
This is a flowchart showing the optimum anode voltage setting process according to the present invention, and the optimum anode voltage setting process is performed before using the device.

That is, when setting the optimum anode voltage, an appropriate reference sample 10 is set on the sample stage 8 as shown in the figure, and a predetermined single wavelength light is irradiated from the light source device 9 to cause the sample 10 to emit electrons. Alternatively, electrons may be emitted by placing a weak radiation source without irradiating light.

In the initial state, the calculation means 26 is connected to the high voltage power supply 23.
For example, the lowest voltage in the voltage variable range, for example 3.0KV, is set as the initial base value, and as shown in block 30, the initially set anode voltage Va is increased stepwise, and each time the anode voltage Va is increased, A discrimination block 32 determines the counting rate N obtained through the counting means 24 as a threshold No. based on background noise.
The increase in the anode voltage Va in block 30 is repeated until the counting rate N reaches the threshold value No.

When the determination block 32 determines that the counting rate N has reached the threshold No, the process proceeds to the next block 34, where the anode voltage when the counting rate N exceeds the threshold No is detected as the discharge starting voltage Vs, and this discharge starting voltage is also detected. Calculate the optimal anode voltage Vao by adding half the voltage of the predetermined plateau voltage width Pw to Vs,
The high voltage power supply 23 is controlled so that the optimum anode voltage Vao calculated in the next block 36 is achieved, and the process of setting the optimum anode voltage is completed.

Here, the counting rate N with respect to the change in anode voltage Va
The relationship has the characteristics shown in FIG.

That is, as the anode voltage Va increases, when a certain anode voltage is reached, an electron counting rate N will be obtained by gas discharge due to the introduced electrons, and the counting rate will exceed the threshold No based on background noise. The anode voltage when N is obtained is the discharge starting voltage
Let it be Vs. When the anode voltage is further increased, the counting rate N remains approximately constant until it exceeds a certain voltage, and the counting rate N that exceeds this voltage increases rapidly and becomes uncountable.

Here, the range in which the counting rate N remains approximately constant with respect to changes in the anode voltage is defined as the plateau voltage width Pw,
This plateau voltage width Pw comes to match the height of the quenching voltage applied to the first grid electrode 5 shown in FIG. 2b.

To explain specifically, for example, the first grid electrode 5
When electrons are counted at 100V and the second grid electrode 6 is set at 80V without a quenching pulse,
If an avalanche-like discharge phenomenon amplification occurs at an anode voltage of 3.4 KV, the potential difference is 3.3 KV. Now, since the height of the quenching voltage is 300V, when the potential difference between the anode ring 4 and the first grid electrode 5 is 3.0 to 3.3KV, the amplification effect of the avalanche-like discharge phenomenon can be prevented. . Therefore, during the plateau width Pw corresponding to the height of the quenching voltage shown in FIG. 4, a substantially constant electron counting rate N is obtained.

Regarding the characteristics of the counting rate N with respect to the anode voltage, in the present invention, the optimum anode voltage Vao is set to the voltage obtained by adding half the voltage of the plateau voltage width Pw to the discharge starting voltage Vs. , the optimum anode voltage Vao is set at the center of the plateau width Pw where the counting rate is approximately constant.

In this way, the anode optimum voltage Vao is the plateau width Pw
When set at the center of
The optimal anode voltage Vao is within the plateau width
Pw, the counting rate N hardly changes even if the atmospheric pressure or temperature changes, making it possible to obtain stable counting results even when the atmospheric pressure or temperature changes.

(Effects of the Invention) As explained above, according to the present invention, a discharge starting voltage is detected by varying the anode voltage by the voltage variable means, and the electron counting rate is approximately equal to the change in the anode voltage at this discharge starting voltage. Since the optimum anode voltage is set by applying a voltage approximately half of the predetermined plateau voltage width, even if the characteristics of the count rate with respect to the anode voltage vary due to changes in atmospheric pressure or temperature, A stable counting state of emitted electrons can always be obtained, and this point is important not only for changes in atmospheric pressure and temperature but also for changes in the characteristics of the counting rate N with respect to the anode voltage, which depends on changes in the distance between the electron detection unit and the sample. Similarly, stable and efficient electronic counting can be performed.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram showing an embodiment of the present invention, FIG. 2 is a signal waveform diagram showing changes in the anode voltage and the first and second grid electrode voltages in the electron detection section, and FIG.
The figure is a flowchart showing the optimum anode voltage setting process according to the present invention, and FIG. 4 is a graph showing the relationship between the electron count rate and the anode voltage. 1: Electronic detection section, 2: Detection window, 3: Case,
4: anode ring, 5: first grid electrode, 6: second grid electrode, 8: sample stage, 9: light source device, 10: sample, 11: light source, 12: monochromator, 13,
14: Slit, 18: Amplifier, 20: First pulse generator, 22: Second pulse generator, 23: High voltage power supply, 24: Counting means, 26: Arithmetic means, 28:
Display means.

Claims (1)

[Claims] 1. An electron device in which an anode ring to which a high voltage is applied is arranged in a case with a detection window on one side, and a first grid electrode and a second grid electrode are sequentially arranged on the detection window side of the anode ring. In an electronic counting device that has a detection section, introduces electrons emitted from a sample irradiated with light into the electron detection section, and counts the number of electrons based on a gas discharge generated by the introduced electrons, the anode voltage variable means for varying the applied voltage; voltage detection means for detecting a discharge starting voltage when the anode voltage is varied by the voltage variable means; An electronic counting device characterized in that it is provided with voltage setting means for setting an anode optimum voltage by applying a voltage approximately half of a predetermined plateau voltage width so that an electronic count value is approximately constant.