JPH0519195B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a photoelectron counting device that counts the number of photoelectrons emitted from a sample.

(Prior art) Conventionally, methods for measuring the thickness of an extremely thin film covering the surface of a sample include the microbalance method, activation analysis method, empsometry method, multiple interference method, Augier electron method, and X-ray photoelectron method. It has been known.

However, these measurement methods have various problems, such as requiring special care in handling the sample and requiring large and expensive equipment.

Therefore, the inventors of the present application developed a film thickness measurement method that can easily measure the thickness of a thin film in a short time non-destructively.
Applying light energy exceeding a specified photoelectric work function to a sample whose surface is coated with a thin film causes electrons to be emitted only from the sample body located below the thin film, and the attenuation of the emitted electrons as they pass through the thin film Since the sample thickness depends on the thickness of the sample, the photoelectrons emitted to the outside through the thin film are detected using a photoelectron counting device, which uses a photoelectron detector with the function of detecting low-energy electrons present in the air as a detector. A method for measuring the thickness of a thin film covering a surface has been proposed (see Japanese Patent Application No. 118818/1983 (Japanese Patent Application No. 262005/1983).

FIG. 7 shows the device configuration of the detection section in the film thickness measurement method using this photoelectron counting device, where 1 is the photoelectron detection section that detects emitted electrons, and 2 is the thin film 2b on the surface of the sample body 2a. This is a coated sample, and the sample surface is irradiated with an ultraviolet beam 3 of a predetermined intensity from a light source, giving the sample surface light energy exceeding a specified photoelectric work function.

The photoelectron detection unit 1 includes an anode 5 made of a loop electrode,
A photoelectron detection section 1 is provided with a first grid electrode 6 and a second grid electrode 7, and is arranged so that an electron introduction port 9 opened at the tip of a case 8 is directed toward the beam irradiation position of the sample 2.

The counting of electrons emitted from the sample by the photoelectron detection unit 1 is carried out by applying a high voltage of, for example, 3.4 KV to the anode 5 and applying 100 V to the first grid electrode 6.
A voltage of about 80 V is applied to the second grid electrode 7, and in this state, low-energy photoelectrons emitted from the sample by irradiation with the ultraviolet beam 3, or anions produced when these electrons collide with gas, are transferred to the first and second grid electrodes. It is drawn to the anode 5 via two grid electrodes 6 and 7.
Since a strong electric field is generated near the anode 5 due to the high voltage, the attracted electrons are accelerated by the high electric field and cause a gas discharge phenomenon. Therefore, the anode voltage decreases and a voltage pulse for counting the number of electrons is generated.

On the other hand, in order to prevent the avalanche-like discharge amplification at the same time as the generation of the anode voltage pulse due to the gas discharge phenomenon,
For a certain period of time after the generation of the anode voltage pulse, the
The voltage of the grid electrode 6 is increased to, for example, 400V to lower the electric field strength near the anode, and the second grid electrode 7 is pulled down to -30V to prevent the introduction of electrons from the outside. prevent hitting. This operation is repeated every time a photoelectron is emitted from the sample.

(Problems to be Solved by the Invention) However, in such a conventional photoelectron counting device, when the sample surface is not a perfect mirror surface, for example, as shown in FIG. is diffusely reflected, and the reflected light 10 due to the diffused reflection enters the inside from the electron introduction port 9 of the photoelectron detection section 1, and is irradiated to the first and second grid electrodes 6 and 7, and the light is emitted from the grid electrode itself that has been irradiated with the reflected light. It begins to emit low-energy electrons that are excited by the energy.

FIG. 8 is a graph showing the counting rate with respect to the wavelength of irradiated light when the photoelectron counting device receives reflected light from a sample. For example, an aluminum plate whose surface is polished in a certain direction is used as the sample.

First, curves S1 and S2 indicate the total counting rate, and curve S1 indicates the polishing direction shown in FIG.
S2 is the counting rate when the sample is placed in the photoelectron detection section 1 in the polishing direction shown in FIG.
It shows the counting rate when placed against.

On the other hand, N1 and N2 indicate the measured values when the second grid electrode 7 of the photoelectron detection section 1 is pulled to -30V to prevent electrons from entering from the outside, and at this time the counting rate should originally be almost zero. However, when the reflected light from the sample hits the grid electrode inside the photoelectron detection section, electrons are emitted, and the irradiation wavelength
Counting rates N1 and N2 with peak values near 240nm
is measured as background noise.

When such background noise N1 and N2 occurs due to reflected light, the true counting rate can be given by subtracting the background noise from the total counting rate (S1 - N1) or (S2 - N2). However, in this case, there was a problem in that the background noise caused by the reflected light became too large and interfered with accurate counting of the number of electrons.

(Means for Solving the Problems) The present invention has been made in view of such conventional problems, and even if the light irradiated onto the sample is diffusely reflected, the background noise due to the diffused reflections can be minimized. An object of the present invention is to provide a photoelectron counting device that can accurately count photoelectrons emitted from a sample.

In order to achieve this objective, the present invention includes a light shielding plate that is placed at a position where the light reflected on the sample surface is directed toward the electron inlet to prevent the reflected light from being irradiated inside the photoelectron detection section, and furthermore, the light shielding plate is In order to prevent sample emitted electrons or anions from being obstructed by the plate,
An electric field generating voltage is applied to the light shielding plate to form a bypass path of sample-emitted electrons or anions through the light shielding plate.

(Embodiment) FIG. 1 is an explanatory diagram showing an embodiment of the present invention together with a measuring circuit.

First, to explain the configuration, reference numeral 1 denotes a photoelectron detection section, which is provided inside a case 8 with a loop-shaped anode 5 to which a high voltage is applied, a first grid electrode 6, and a second grid electrode 7, and is located opposite to the grid electrode. An electron introduction port 9 is opened at the tip of the case 8.

2 is a sample, with a thin film 2 on the surface of the sample body 2a.
The surface of the sample 2 is irradiated with an ultraviolet beam 3 of a predetermined wavelength from a light source unit 4. The light source unit 4 includes a light source 11 such as a deuterium lamp, a slit 12, and a spectroscope 13. The light from the light source 11 is narrowed down by the slit 12 and enters the spectrometer 13, which separates ultraviolet light of a predetermined wavelength. , the surface of the sample 2 is irradiated with an ultraviolet beam 3 through a slit 12.

With respect to the sample 2 which is vertically irradiated with the ultraviolet beam 3 by the light source unit 4, the photoelectron detection section 1 is arranged obliquely with the electron introduction port 9 facing the beam irradiation surface of the sample 2. Between the photoelectron detection section 1 and the sample 2 arranged in this way, a light shielding plate 14 is arranged at a position where the reflected light 10 diffusely reflected on the surface of the sample 2 is directed toward the electron introduction port 9 of the photoelectron detection section 1. By blocking the reflected light 10 caused by the electron inlet 9, the reflected light is prevented from being irradiated into the photoelectron detection section 1 from the electron introduction port 9.

For example, the light shielding plate 14 may have a semicircular cutout 14a formed at a position where the ultraviolet beam passes, as shown in FIG. The plate has holes 14b formed therein. Of course, it may be a simple plate without the semicircular notch 14a or the transparent hole 14b.

Referring again to FIG. 1, the light shielding plate 14 provided between the photoelectron detection unit 1 and the sample 2 to block reflected light is supplied with a specified direct voltage Vc from the light shielding plate applied voltage source 15. is applied. The DC voltage Vc is applied by the light-shielding plate application voltage source 15 in order to prevent low-energy electrons or anions emitted from the sample surface toward the photoelectron detection section 1 by irradiation with the ultraviolet beam 3 from being blocked by the light-shielding plate 14. , and the specified voltage by the light shielding plate applied voltage source 15 is
By applying Vc, an electric field is generated to bypass the light shielding plate 14 and form a bypass path to the electron introduction port 9 of the photoelectron detection unit 1, on the condition that the voltage of the sample 2 is lower than the specified voltage Vc. That's what I do.

That is, when the light shielding plate 14 is at the same ground potential as the sample 2, as shown in FIG ^. Since the emitted photoelectrons or anions e ^- are blocked by the light shielding plate 14 from moving towards the part 1, accurate counting of the number of electrons becomes impossible.

Therefore, as shown in FIG. 4b, a specified voltage Vc is applied to the light-shielding plate 14 to the sample 2, which is at the ground potential, and photoelectrons or anions emitted from the sample 2 are
An electric field is created to provide a detour path for e ^- to bypass the light shielding plate 14 and be attracted to the photoelectron detection section 1 .

FIG. 5 is a graph showing the relationship between the total counting rate S by the photoelectron detection section 1 and the background noise N with respect to the voltage Vc applied to the light shielding plate 14.

The total counting rate S in this graph is the anode electrode Va=
3.94V, first grid electrode voltage Vg1=100V, second
The grid electrode voltage was determined as Vg2 = 80V, and an aluminum plate was used as the sample. Also, the background noise N is determined as Vg2=-30V.

As is clear from the graph in FIG. 5, the total counting rate S also increases as the voltage Vc applied to the gobo plate increases, and the total counting rate S reaches a maximum near Vc and becomes saturated.
It has been confirmed that the emitted photoelectrons or anions are diverted by applying the voltage Vc applied to the light shielding plate.

On the other hand, the background noise N counted by drawing the second grid voltage Vg2 to -30V is suppressed to a counting rate of 10 or less regardless of the voltage Vc applied to the light shielding plate. Therefore, the voltage Vc applied to the light shielding plate may be set to an appropriate value within the range of Vc=16 to 20V.

Further, the surface of the light shielding plate 14 used in the present invention is coated with a thin coating to prevent photoelectrons emitted from the light shielding plate 14.

That is, as shown in FIG. 1, reflected light 10 due to diffuse reflection on the surface of the sample 2 hits the light shielding plate 14 placed between the sample 2 and the photoelectron detection section 1. Photoelectrons are emitted from the light shielding plate 14 itself by the irradiation of the reflected light 10, are detoured by the voltage Vc applied to the light shielding plate, are drawn toward the photoelectron detection unit 1, enter the photoelectron detection unit 1 together with the photoelectrons from the sample 2, and are counted. I end up. Therefore, applying a thin coating to the surface of the light shielding plate 14 prevents the emission of photoelectrons from the surface, weakens the amount of reflected light 10 that is irradiated to the light shielding plate 14, and directs emitted electrons excited by light energy to the surface of the light shielding plate 14. It is attenuated when passing through the paint film to prevent it from coming out.

If the thickness of the coating film formed on the surface of the light shielding plate 14 is 300 to 400 angstroms or more, no photoelectrons will be emitted from the underlying metal even if it is irradiated with the reflected light 10. Furthermore, since the light shielding plate 14 also functions as an electrode plate that generates an electric field that creates a detour path, it can sufficiently function as an electrode plate by keeping the thickness of the coating film to a few microns or less.

For the same reason, the formation of a thin coating film on the surface of the light shielding plate 14 in order to reduce background noise also applies to the first and second grid electrodes 6 and 7 provided inside the photoelectron detection section 1. It is desirable to apply a coating, which can further reduce background noise.

Next, the configuration of the measurement circuit of the photoelectronic counting device shown in the embodiment shown in FIG. 1 will be explained together with its operation.

First, the high voltage power supply 16 is connected to the anode 5 of the photoelectron detection section 1.
A specified voltage between 3 and 4 KV is applied to the The voltage applied to the anode 5 is determined by the DC cut capacitor C.
It is input-connected to the amplifier 17 via the sample 2, and amplifies and outputs a voltage pulse obtained by a gas discharge phenomenon caused by photoelectrons emitted from the sample 2. A grid voltage Vg1 of, for example, 100 V is applied to the first grid electrode 6 of the photoelectron detection section 1 from the first pulse generator 18, and a grid voltage Vg1 of about 80 V is applied from the second pulse generator 19 to the second grid electrode 7. grid voltage
Vg2 is applied.

When photoelectrons emitted from the sample 2 are attracted to the anode 5 via the first and second grid electrodes 6 and 7, a gas discharge phenomenon occurs due to acceleration due to the high electric field near the anode 5, and the anode voltage Va decreases. amplifier 1
7 outputs a voltage pulse as shown in the signal waveform diagram of FIG. 1A. In response to this voltage pulse, the first pulse generator 18 and the second pulse generator 19 change the grid voltage Vg1 of the first grid electrode 6 from the previous 100V for a predetermined period Te.
The voltage is raised to 400V, which is 300V higher, to lower the electric field around the anode 5 and prevent an avalanche-like discharge phenomenon. At the same time, the second pulse generator 19 pulls the grid voltage Vg2 applied to the second grid electrode 7 from 80V to -30V to prevent photoelectrons from entering the photoelectron detection section 1. At the same time, positive ions generated by the gas discharge are captured, and these positive ions are transferred to sample 2.
This prevents secondary electrons from being emitted by hitting the

As a result, every time the amplifier 17 detects an emitted electron from the sample 2, it outputs a voltage pulse generated due to the gas discharge phenomenon and supplies it to the counting circuit 20, which calculates the number of pulses per unit time, that is, the counting rate. N is required. The counting rate (number of counts) N by the counting circuit 20 is given to the arithmetic circuit 21, and the thickness T of the thin film formed on the surface of the sample 2 is determined based on the counting rate N.
is calculated by the following equation.

logN=logNo−T/2.3λ ……(1) However, λ is the mean free path of electrons in the thin film (Angstrom), and No is the number of counts when the film thickness is zero (counting rate), which was determined by the arithmetic circuit 21. For example, the film thickness T is
is given to a display means 22 such as a CRT or a printer,
The measured film thickness T is displayed.

FIG. 6 is a graph showing the relationship between the counting rate and the wavelength of the irradiated light when the light shielding plate 14 is provided between the sample 2 and the photoelectron detection section 1 as shown in FIG. An aluminum plate with the same surface polished in a certain direction as shown in Fig. 9 is used, and the total counting rate S1 and background noise N1 are
is the case where the sample is arranged as shown in Figure 9a,
Further, the total counting rate S2 and the background noise N2 are obtained when the sample is arranged as shown in FIG. 9b.

As is clear from the measured data in Figure 6,
By providing the light shielding plate 14 between the photoelectron detection section 1 and the sample 2, the reflected light 10 diffusely reflected on the surface of the sample 2 is blocked from entering the photoelectron detection section 1, and the first and second grids of the photoelectron detection section 1 are Since the electrodes 6, 7, etc. are irradiated with reflected light and low-energy electrons are prevented from being emitted, the counting rate of background noise N1, N2 with respect to the total counting rate S1, S2 is extremely low regardless of the wavelength of the irradiated light. Since the true counting rate is given by the difference between the total counting rate and the background noise, compared to the conventional example without a light shielding plate as shown in the graph of Figure 8,
It was confirmed that the S/N ratio of the counting rate of electrons emitted from the sample, which is used to determine the film thickness, can be significantly improved.

In this embodiment, the photoelectron counting device was explained using film thickness measurement as an example, but the present invention is not limited thereto. For example, it can be applied to a photoelectron counting device used for measuring the photoelectric work function of a sample or measuring the degree of contamination of a sample surface.

Further, in this example, a thin coating was applied to the surfaces of the light shielding plate and the first and second grid electrodes, but the present invention is not limited to a coating film, and any film that achieves the effects of this example may be used. In other words, it is sufficient to form a film with a larger work function than the energy of the reflected light. For example, it can be realized using an oxide film, a nitride film, or the like.

(Effects of the Invention) As explained above, according to the present invention, a light-shielding plate is arranged at a position where light reflected from the sample surface faces the electron inlet of the photoelectron counting device, and furthermore, the light-shielding plate is arranged at a position where the light reflected from the sample surface faces the electron inlet of the photoelectron counting device, and the light-shielding plate is further used to block the sample-emitted electrons or anions. Since the electric field generation voltage that forms the plate detour path is applied, the reflected light caused by diffuse reflection of the light irradiated on the sample is blocked by the light shielding plate and is not irradiated into the photoelectron detection section. It is possible to reliably prevent the emission of low-energy electrons from the sample itself, minimize background noise during photoelectron counting, and perform accurate counting of sample-emitted electrons.

Furthermore, by applying a thin film similar to a light shielding plate to the grid electrodes provided in the photoelectron detection section, background noise can be further reduced by preventing the emission of low-energy electrons from the grid electrodes due to disturbance light. .

[Brief explanation of the drawing]

Fig. 1 is an explanatory diagram showing an embodiment of the present invention together with a measuring circuit, Fig. 1A is a signal waveform diagram showing the time change of the electrode voltage of the photoelectronic counting device shown in Fig. 1, and Figs. Plan view of the light shielding plate used in the invention, No. 4
The figure is an explanatory diagram showing the effect of the voltage applied to the light shielding plate.
The figure is a graph showing the relationship between the total counting rate and background noise with respect to the voltage applied to the light shielding plate, Figure 6 is a graph showing the measurement results of the counting rate with respect to the wavelength of irradiated light according to the present invention, and Figure 7 is a graph showing the relationship between the total counting rate and background noise with respect to the voltage applied to the light shielding plate. Explanatory diagram showing the problem of reflected light in a photoelectron counting device, No. 8
The figure is a graph showing the relationship between the counting rate and background noise with respect to the wavelength of irradiated light in a conventional device. Figure 9 is the direction of reflected light due to sample polishing and the arrangement of the photoelectron detection unit when obtaining the measurement data of Figure 8. It is an explanatory diagram showing a relationship. 1: Photoelectron detection section, 2: Sample, 2a: Sample body, 2b: Thin film, 3: Ultraviolet beam, 4: Light source unit, 5: Anode, 6: First grid electrode, 7:
2nd grid electrode, 8: case, 9: electron introduction port, 10: reflected light, 11: light source, 12: slit, 13: spectrometer, 14: light shielding plate, 14a: notch, 14b: transmission hole, 15: light shielding plate applied voltage source,
16: High voltage power supply, 17: Amplifier, 18: First pulse generator, 19: Second pulse generator, 20: Counting circuit, 21: Arithmetic circuit, 22: Display means.

Claims (1)

[Scope of Claims] 1. In a photoelectron counting device that irradiates a sample with light, causes photoelectrons emitted from the sample to enter through an electron inlet, and counts the number of photoelectrons reflected on the surface of the sample. A light-shielding plate is disposed at a position where light is directed toward the electron introduction port, and an electric field generating voltage is applied to the light-shielding plate to form a path around the light-shielding plate for sample-emitted electrons or anions combined with the electrons. Photoelectronic counting device.