JPS63167255A - Electron counting device - Google Patents

Abstract

PURPOSE:To obtain the invariably stable counting state of emitted electrons even if the characteristics of a counting rate to an anode voltage varies owing to variation in atmospheric pressure or temperature by adding a voltage which is nearly a half as wide as a Plateau voltage to a discharge start voltage and obtaining the best anode voltage. CONSTITUTION:A voltage pulse outputted every time an electron is emitted by a sample 10 is counted by a counting means 24 through an amplifier 18 to obtain an output indicating a counting rate N, and the output is supplied to an arithmetic means 26. The arithmetic means 26 sets the best anode voltage while single-wavelength light is projected by a light source 9 with a specific reference sample set on a sample table 8 prior to the use of the device. The counting rate N is monitored first while the anode voltage to a high-voltage power source 23 is variable to detect the discharge start voltage. The voltage which is a half as wide as the Plateau voltage corresponding to the height of a quenching voltage for a gas discharging element to be applied to a 1st grating electrode 5 is added to the discharge start voltage and the resulting voltage is set as the best anode voltage for a high-voltage power source 23.

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 light passes through the thin film on the sample. A known method is to introduce electrons emitted to the outside into an electron detection section, count the number of emitted electrons using the gas discharge phenomenon caused by the introduced electrons, and measure the film thickness based on the number of emitted electrons. (Japanese Patent Application No. 118818/1984, 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 disposed in the electron detection section, and the electrons emitted from the sample are generated in a gas discharge. The number of electrons is counted from the pulse change in the anode voltage caused by this gas discharge, but it is not clear how to determine the anode voltage, and it is difficult to determine the appropriate anode voltage that causes the gas discharge when emitted electrons are introduced. For example, even if the voltage is set to 3.4 KV, since 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 it difficult to stably count electrons. The problem was 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 the anode disposed in the electron detection unit, and a voltage detection unit for detecting the discharge starting voltage at which gas discharge is started by varying the anode voltage. means, and voltage setting means for setting the discharge starting voltage as an optimum anode voltage by adding a voltage approximately half of a predetermined plateau voltage width such that the count value remains approximately constant with respect to changes in the anode voltage. It is 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 back grout noise, can be adjusted. The discharge starting voltage ys is detected, and a voltage (Vs + PW/2>) obtained by adding 1/2 of the predetermined plateau voltage width Pw to the discharge starting voltage Vs 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 Fenching pressure that increases the voltage and lowers the electric field, and the plateau voltage pw is uniquely determined from the height of the Fenching pressure applied to the first grid 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, ±Pw/2 around the optimal anode voltage
If the fluctuation is within the range of , the electronic counting rate is kept approximately constant,
Stable electronic counting can be performed.

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

First, to explain the configuration, 1 is an electron detection section, and sample 1
It has a metal case 3 with a detection window 2 opened on the side of the sample stage 8 where 0 is set, and the case 3 is grounded. An anode ring 4 is disposed inside the case 3, and a high voltage power supply 23 is connected to the anode ring 4.
．． The anode voltage can be varied in the range of 0 to 4.0. 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,
The surface of a sample 10 set on a sample stage 8 is irradiated with single wavelength light of a predetermined wavelength obliquely from above. The light source device 9 has a light source 11 such as a deuterium lamp, a monochromator 12 for converting the light from the light source 11 into single wavelength light of a predetermined wavelength, and further has a monochromator 12 for adjusting the light intensity before and after the monochromator 12. slits 13 and 14 are provided. The monochromator 12 has a predetermined wavelength range, for example, 150nm to 600nm.
It has the function of scanning a single wavelength in a range of m.

Furthermore, 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. Anode ring 4
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 first grid electrode 5. The output 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 a voltage pulse is output from the anode ring 4 due to the gas discharge caused by the electrons introduced from the amplifier 18 are as follows.

In the initial state, as shown in FIGS. 2(b) and 2(c), the voltage of the first grid electrode 5 generated by the first pulse generator 20 is, for example, 100V, and the voltage of the second grid electrode 6 is set to 100V when the second pulse is generated. The output of the device 22 is, for example, 80 V, and the electrons emitted from the sample 10 by the light irradiation 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, a gas discharge phenomenon is caused by the acceleration of the electrons under the high electric field of the anode ring 4, and as a result, the anode voltage rises in a pulsed manner as shown in Fig. 2(a). It causes a downward change. In response to this change in anode voltage, the first pulse generator 20 changes the voltage applied to the first grid electrode 5 up to that point,
For example, the voltage is increased by 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, to, for example, 110V lower to -30V over a predetermined quenching time Te. . This second grid electrode 2
- As the voltage decreases to 30 V, 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 reach the sample 10 and affect the emission of photoelectrons. 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 output, 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 in the test unit 8 and single wavelength light is irradiated from the light source 9 prior to use of the apparatus.

(A> First, set the anode voltage to the high voltage power supply 23, for example, 3.
Outputs a control signal to vary from 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 of (A> is varied), and is based on the background noise when there is no electron emission from the sample 10. Threshold value NO1 determined by, for example, N
When the counting rate of o-10 (cps) or more is 19, the voltage applied to the anode at this time is detected as the discharge starting voltage vS.

(C) A plateau voltage corresponding to the height of the fencing pressure for the gas discharge element applied to the first grid electrode 5 shown in FIG. 2(b) in the discharge starting voltage VS detected in (B) above. The voltage (Vs+P
w/2> 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 setting of the optimum anode voltage, the calculating means 26 calculates the counting rate obtained by calculating 1q from the counting means 24 when measuring the thickness of an oxide film formed on the surface of the sample 10, for example. A means for calculating the film thickness T based on N is required, and this film thickness calculation means calculates I ogN=I ogNl−T/
2.30λ However, λ: Electron mean free path in the thin film (Angstrom) N1: The film thickness T is calculated as the counting rate when the film thickness is zero. Of course, the electron h1 number 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,
The calculated film thickness, electron count rate, and work function are displayed on the display means 28.

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

FIG. 3 is a flowchart showing the optimum anode voltage setting process by the calculation means 26 provided in the embodiment of FIG.
Before using the device, the optimum anode voltage is set.

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 sets the lowest voltage in the voltage variable range for the high voltage power supply 23 as an initial value, for example 3.0 KV, and increases the initially set anode voltage Va in steps, for example, as shown in block 30. and anode voltage■
Each time a increases, the counting rate N obtained through the counting means 24 is compared with a threshold value NO based on background noise in a discrimination block 32, and the anode voltage Va of the block 30 is increased until the counting rate N reaches the threshold value No. repeat.

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

Here, the relationship between the counting rate N and the change in the anode voltage Va 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 N will exceed the threshold No based on background noise. The anode voltage obtained is defined as the discharge starting voltage VS. When the anode voltage is further increased, the counting rate N remains approximately constant until a certain voltage is exceeded, and when this voltage is exceeded, the counting rate N rapidly increases and becomes impossible to count.

Here, the range in which the counting rate N is approximately constant with respect to changes in the anode voltage is defined as the plateau voltage width Pw, and this plateau voltage width Pw is the same as that of the 11j8 child electrode 5 shown in FIG. 2(b).
It will match the height of the pressure applied to the pressure.

To explain specifically, for example, the first grid electrode 5 has a voltage of 100V.
, when the second grid electrode 6 is set to 80 V and electrons are counted without a quenching pulse, the anode voltage is 3.4
If an amplification effect of an avalanche-like discharge phenomenon occurs at KV, the potential difference is 3°3KV. Now, since the height of the pressure is 300V, the anode ring 4
When the potential difference between the lattice voltage and the first grid voltage (Φ5) is 3.0 to 3.3 KV, the amplification effect of the avalanche-like discharge phenomenon can be prevented. Therefore, it corresponds to the height of the fenching pressure shown in Figure 4. During the plateau width pw, 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 plateau voltage width P is set to the discharge starting voltage vS.
Since the optimum anode voltage vaO is set as the voltage obtained by adding half the voltage of w, the optimum anode voltage Vao is set at the center of the plateau width Pw where the counting rate is approximately constant.

When the anode optimum voltage yao is set at the center of the plateau width Pw in this way, the characteristic curve shown in Figure 4 shifts as shown by the broken line due to changes in atmospheric pressure and temperature, but this fluctuation range is As long as it is within ±PW/2, the optimal anode voltage yao is within the plateau width pw, and the counting rate N hardly changes even if the atmospheric pressure or temperature changes, so it is stable against changes in the atmospheric pressure or temperature. h1 number results can be obtained.

(Effects of the Invention) As described above, according to the present invention, the anode voltage is varied by the voltage variable means to detect the discharge starting voltage, and the electronic count value is added to the discharge starting voltage with respect to the change in the anode voltage. Since the optimum anode voltage is set by applying a voltage that is approximately half of the predetermined plateau voltage width, which is approximately constant, even if the characteristics of the count rate with respect to the anode voltage vary due to changes in atmospheric pressure or temperature J3i, , it is possible to always obtain a stable counting state of emitted electrons, and this point is due to changes in the characteristics of the counting rate N with respect to the anode voltage, which depend not only on changes in atmospheric pressure and temperature, but also on changes in the distance from the electron detection unit to the sample. In the same way, 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 anode voltage and first and second grid electrode voltages in the electron detection section, and Fig. 3 is an explanatory diagram showing an embodiment of the present invention. FIG. 4 is a flowchart showing the optimum anode voltage setting process according to the method, and FIG. 4 is a graph showing the relationship between the electron count rate and the anode voltage. 1: Electron detection unit 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 Nislit 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] Electron detection 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 which has a part and which introduces electrons emitted from a sample irradiated with light into the electron detection part and counts the number of electrons based on a gas discharge generated by the introduced electrons, a voltage variable means for varying the applied voltage; a voltage detection means for detecting a discharge starting voltage when the anode voltage is varied by the voltage variable means; 1. An electronic counting device comprising 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.