JPH0376841B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a film thickness measurement method for measuring the thickness of a thin film covering a sample surface.

Conventionally, methods for measuring the thickness of an extremely thin film covering a sample surface include, for example, the first method is the microbalance method, the second is the activation analysis method, the third is the ellipsometry method, and the fourth is the multiple interference method. The fifth method is the Auger electron method, and the sixth method is the X-ray photoelectron method. However, the first, second, and fourth methods require special care in handling, and the equipment is large and expensive. However, it also has drawbacks such as requiring special pretreatment of the sample. The third method has the advantage of being able to measure a wide range of film thicknesses, but has the disadvantage that it cannot be measured if the optical properties of the base and the thin film are similar. In addition, methods 5 and 6 cannot be measured unless the sample is placed in an ultra-high vacuum, and are therefore limited by the size and shape of the sample.
Conventional film thickness measurement methods have many problems, such as expensive equipment and long measurement times.

The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide a film thickness measuring method that can easily and non-destructively measure the thickness of a thin film in a short time.

Such purpose is to provide a method for measuring the thickness of a thin film that covers the surface of a sample and has a photoelectric work function larger than the photoelectric work function of the sample. Energy less than the photoelectric work function of the thin film is applied to cause electrons to be emitted from the sample, and among the emitted electrons, only those that have passed through the thin film are detected to measure the thickness of the thin film. This can be achieved by

Here, the thin film may be a conductor, a semiconductor, or an insulator, as long as the photoelectric work function of the thin film is larger than the photoelectric work function of the sample. The photoelectric work function is the energy difference from the Fermi level to the vacuum level in metals, and the energy difference from the top of the valence band to the vacuum level in semiconductors. Further, the energy value given to the sample is greater than or equal to the photoelectric work function of the sample and less than the photoelectric work function of the thin film, and as a result, electrons are emitted only from the sample and not from the thin film. Among the electrons emitted from the sample, those with small kinetic energy lose energy in the thin film and remain in the thin film, while those with large kinetic energy pass through the thin film. As the thickness of the thin film increases, more of the kinetic energy of the electrons passing through the thin film is taken away, so the number of electrons decreases.
Next, the electrons that have passed through this thin film are captured, and their number is counted or converted into a current and detected as a current value. The atmosphere in which such measurements are performed may be various single gases, mixed gases, or the atmosphere.

Hereinafter, one embodiment of the present invention will be described based on the drawings.

FIG. 1 shows an air counter, in which numeral 1 is a case with windows 2 and 3 formed at the bottom, and this case 1 is grounded and serves as a cathode. A ring-shaped anode 4 is provided inside the case 1, and a high voltage power source 5 of, for example, 3.4 KV is connected to this anode 4. Between the anode 4 and the case 1,
For example, the first grid electrode 6 to which a voltage of 100V is applied
is installed, and a voltage of 80V, for example, is applied between this grid electrode 6 and the sample 7 placed on the bottom of the case 1.
A second grid electrode 8 to which a voltage of is applied is installed. The surface of the sample 7 is a thin film 9 whose thickness is to be measured.
covered with. 10 is a light source, and this light source 1
The light from 0 is made monochromatic by a spectroscope 11 and illuminates the sample 7 and thin film 9 through the window 2, and the reflected light exits through the window 3. Note that the intensity of the monochromatic light at this time is adjusted by two slits 12. 13 is an amplifier connected to the anode 4;
Between this amplifier 13 and the first grid electrode 6, a first
A pulse generator 14 is provided. When an electron pulse is generated at the anode 4, the first pulse generator 14 generates a pulse with a time width Te (5 m) on the first grid electrode 6, for example.
S) sends a 300V square wave pulse, and the voltage of the first grid electrode 6 is increased from 100V as shown in Figure 2b.
Increase to 400V. Further, a second pulse generator 15 is provided between the amplifier 13 and the second grid electrode 8, and when an electron pulse is generated at the anode 4, the second pulse generator 15 generates a pulse with a time width Te(5
A rectangular wave pulse of -110V is supplied to the second grid electrode 8 at a voltage of -110V at
Reduce the voltage from 80V to -30V as shown in . The first and second pulse generators 14 and 15 can arbitrarily set the time width and peak voltage of the rectangular wave pulses they generate. 16 is a counting means connected to the amplifier 13, and this counting means 16 is connected to the anode 4.
Count the electronic pulses generated. A calculating means 17 is connected to this counting means 16, and this calculating means 17 is composed of, for example, a microcomputer. Then, this calculation means 17 calculates the thickness T of the thin film 9 using the formula, log N=log N1-T, where N is the output from the counting means 16, for example, the number of counts of electron pulses per second (counting rate). /2.30λ Here, λ is the mean free path of electrons in the thin film (Angstrom), and N1 is calculated using the number of counters (counting rate) when the film thickness is zero to find the film thickness T of the thin film 9. Next, the output signal from the calculation means 17 is sent to a display means 18 such as a CRT or a printer, and the calculation result, that is, the thickness T of the thin film 9, is displayed on the display means 18.

Next, the operation of one embodiment of the present invention will be explained.

First, a sample 7 made of silicon, for example, on which a thin film 9 made of silicon oxide is attached.
Place it on the bottom of case 1. Next, as shown in FIG. 2a, from the high voltage power supply 5 to the anode 4, for example,
Apply a high voltage of 3.4KV. Next, the light from the light source 10 is converted into monochromatic light by the spectroscope 11, and its intensity is adjusted by the slit 12.
irradiate. In this example, since the sample 7 is silicon and the thin film 9 is silicon oxide, the monochromatic light that gives energy to the sample 7 has an energy greater than or equal to the photoelectric work function of the sample 7 and less than the photoelectric work function of the thin film 9. It uses ultraviolet light. After the monochromatic light irradiated on this sample 7 penetrates the thin film 9,
Light energy is applied to the surface of the sample 7 to cause the surface of the sample 7 to emit photoelectrons with low energy of 10 eV or less. At this time, since the optical energy of the monochromatic light is less than the photoelectric work function of the thin film 9, no electrons are emitted from the thin film 9. Next, some of the electrons emitted from the sample 7 pass through the thin film 9 and appear outside. That is, among the electrons emitted from the sample 7, those with small kinetic energy lose energy in the thin film 9 and remain in the thin film 9, while those with large kinetic energy remain in the thin film 9.
It goes through. Here, the number of electrons passing through the thin film 9 increases as the thickness of the thin film 9 increases, since more kinetic energy is taken from the electrons.
It gradually decreases. That is, if the energy given to the sample 7 is constant and the number of emitted electrons is constant, as the thickness of the thin film 9 increases, the number of electrons that can pass through the thin film 9 decreases, and the number of electrons that can pass through the thin film 9 decreases as the thickness of the thin film 9 increases. The relationship shown in the above equation holds true with the number of electrons. Here, since the mean free path of electrons emitted from the sample 7 in the thin film 9 is generally about several tens of angstroms, it is possible to measure a thin film 9 having an extremely thin film thickness on the order of angstroms. Next, the electrons emitted from the sample 7 and passing through the thin film 9 pass through the second grid electrode 8 and the first grid electrode 6, and are attracted to the anode 4. When the electrons reach the vicinity of the anode 4, a strong electric field is generated near the anode 4 due to the high voltage, so the electrons are accelerated by this electric field and cause a gas discharge. Due to this gas amplification effect, the potential of the voltage applied to the anode 4 decreases as shown in FIG. 2a, and an electron pulse is generated. The electron pulse generated at the anode 4 is passed through the amplifier 13 to the first pulse generator 14.
and sent to the second pulse generator 15. The first pulse generator 14 generates a rectangular wave pulse with a voltage of 300V and a time width Te and sends it to the first grid electrode 6, increasing the voltage of the first grid electrode 6 from 100V to 400V by Te time. As a result, the potential difference between the anode 4 and the first grid electrode 6 decreases by 300V, thereby
Secondary electrons generated by light and cations generated by gas amplification cannot reach the discharge voltage, and continuous discharge is prevented. On the other hand, the second pulse generator 15
sends a rectangular wave pulse with a time width of Te and a voltage of -110V to the second grid electrode 8 to increase the voltage of the second grid electrode 8.
Reduce from 80V to -30V. As a result, the cations generated by the gas amplification effect are transferred to this second
It is captured by the grid electrode 8 and neutralized. This prevents positive ions from reaching the thin film 9 and the sample 7 and affecting them. and,
After Te time elapses, the first and second grid electrodes 6,
The voltages at 8 are each restored to their original voltages, and the air counter is in a state where it can detect electrons again. The electron pulse generated at the anode 4 is simultaneously transmitted to the amplifier 13.
The counting means 16 also counts the number of electron pulses, that is, the number of electrons emitted from the sample 7 and passing through the thin film 9, and calculates the counting result, for example, the counting rate, to a calculating means. Output to 17. The calculation means 17 calculates the film thickness based on the above-mentioned formula, and sends the calculation result to the display means 18. When the calculation result is input to the display means 19, the display means 18 displays the thickness of the thin film 9.

FIG. 3 is a graph showing the results of measuring the thickness of the thin film 9, with the counting rate plotted on a logarithmic scale on the vertical axis and the film thickness plotted on an equal scale on the horizontal axis. For this measurement,
Using silicon with lattice plane (100) as sample 7,
The thin film 9 was formed by forming silicon oxide films of different thicknesses on the silicon surface, and the measurement was performed by changing the wavelength of the irradiated ultraviolet rays to 190, 200, and 210 nanometers. The line representing this measurement result has a straight line portion whose slope is (-1/2.30λ) in the above equation, indicating that the above equation is appropriate.

In the above embodiment, the air counter for detecting low-speed electrons, the counting means 16, the calculating means 17, and the display means 18 were integrated into one device, but in this invention, For example, the air counter and counting means 16, the calculation means 17 and the display means 18 may be separated, and the detection of the number of electrons and the calculation display may be performed at separate locations.

As explained above, according to the present invention, the thickness of a thin film can be measured non-destructively in the atmosphere.

[Brief explanation of drawings]

FIG. 1 is a cross-sectional view, partially shown in blocks, showing an embodiment of an apparatus for carrying out the present invention, and FIG. 2 is a graph showing voltage changes of an air counter.
FIG. 3 is a graph showing the results of measuring film thickness using the present invention. 4...Anode, 7...Sample, 9...Thin film, 10...
... light source, 16 ... counting means, 17 ... calculation means,
18...Display means.

Claims (1)

[Scope of Claims] 1. A method for measuring the thickness of a thin film that covers the surface of a sample and has a photoelectric work function larger than the photoelectric work function of the sample, the method comprising: By applying energy below the photoelectric work function of the thin film, electrons are emitted from the sample, and among the emitted electrons, only those that have passed through the thin film are detected to measure the thickness of the thin film. A film thickness measurement method characterized by the following. 2 If the number of counts (counting rate) when detecting the number of electrons passing through the thin film is N, then the thickness T of the thin film is calculated using the formula log N=log N1−T/2.30λ, where λ is the inside of the thin film. The method for measuring film thickness according to claim 1, wherein the mean free path (Angstrom) of electrons N1 is calculated using the number of counters (counting rate) when the film thickness is zero.