JPH11160057A - Piezoelectric crystal oscillation type film thickness gauge - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a piezoelectric crystal oscillation type film thickness gauge capable of reducing the exchange frequency of a piezoelectric crystal and prolonging the service life than before. SOLUTION: The natural frequency of the piezoelectric crystal 3 is made lower than 5 MHz, influence by the stress of a film deposited on a surface 3a is reduced and the fundamental vibration of the piezoelectric crystal is maintained. Also, electrode films 10a and 10b formed on the front and back surfaces 3a and 3b of the piezoelectric crystal 3 are respectively formed in an island shape, the formation areas of the electrode films 10a and 10b are prevented from being overlapped on the front surface 3a and the back surface 3b at the peripheral edge part of the piezoelectric crystal 3, parasitic vibration appearing at the peripheral edge part of the piezoelectric crystal 3 is suppressed and the fundamental vibration of the piezoelectric crystal 3 is maintained. Thus, the service life of the piezoelectric crystal 3 is prolonged substantially longer than before.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a piezoelectric device for measuring the thickness of a film deposited on a film-forming object (substrate) by measuring the film deposition on a piezoelectric crystal installed in a vacuum chamber. It relates to a crystal oscillation type film thickness meter.

[0002]

2. Description of the Related Art Conventionally, in vacuum deposition or sputtering, a technique called a quartz oscillator method has been used to measure the film thickness and film formation rate of a film to be formed. The principle of this method is based on the fact that the natural vibration of a crystal resonator, which is a piezoelectric crystal, changes due to a change in its mass. In other words, when a thin film is deposited on the crystal unit,
If the mass of the thin film is sufficiently small compared to the mass of the crystal unit, the same effect as simply increasing the mass or thickness of the crystal unit will occur, and the natural frequency will change in proportion to the mass change. It is a thing using. By measuring the change in the natural frequency of the crystal unit at this time, the film thickness and the film formation rate can be measured.

In this type of conventional piezoelectric crystal oscillation type film thickness meter, a device using a quartz oscillator having a natural frequency of 5 MHz (megahertz) or 6 MHz as a detecting element is used from the viewpoint of ease of use. Also, for the calibration of the crystal oscillator method,
Microbalance technology is used (JSPS Thin Film 131st Committee, Thin Film Handbook I)
Chapter 2, p. 146 (1983)).

[0004]

However, this technique is easy to handle and high in accuracy, but the piezoelectric crystal has a long life when a thick film is deposited or even a thin film depending on the substance to be deposited. There is a problem that needs to be replaced. In addition, when the film stress is large, such as an optical film, even if the film is thin, it is not possible to vibrate in the basic vibration, and the measurement shifts to another vibration mode (spurious vibration mode), and measurement becomes impossible. . In other words, the use limit of the vibrator is reached and the life is reached. This is because, for a transducer of finite size, the amplitude is maximum near the center and decreases toward the edge (omitted), and the appearance of higher-order vibration of thickness shear vibration and higher-order vibration of contour vibration, or It has the potential to shift to a combined form of the thin film "(JSPS Thin Film 131st Committee, Thin Film Handbook, I, Chapter 2, p.1)
39 (1983)), such a phenomenon becomes more remarkable for a vibrator having a higher natural frequency, that is, a vibrator having a smaller thickness.

The present invention has been made in view of the above problems, and has as its object to provide a piezoelectric crystal oscillation type film thickness meter that can reduce the frequency of replacement of piezoelectric crystals and extend the life.

[0006]

The above object is achieved by depositing a film forming material on a piezoelectric crystal installed in a vacuum chamber,
Based on a change in the natural frequency of the piezoelectric crystal due to the deposition of the film forming material, in a piezoelectric crystal oscillation type film thickness meter that measures the film thickness of the film forming material deposited on the substrate to be processed, The piezoelectric crystal oscillation type film thickness meter is characterized in that the fundamental frequency of the piezoelectric crystal is maintained during film formation by lowering the natural frequency of the piezoelectric crystal to less than 5 MHz. .

In a conventional thickness gauge using a piezoelectric crystal as a detecting element, a quartz oscillator having a natural frequency of 6 MHz or 5 MHz has been used. Generally, a relationship of f = N / d is established between the natural frequency f of the crystal and its thickness d, where N is a frequency constant of the crystal and 1670 kHz for an AT-cut crystal.
Mm. So, at 5MHz, 0.33mm, 6M
It is 0.28 mm in Hz. As described above, the natural frequency and the thickness of the piezoelectric crystal are inversely proportional, and when the natural frequency of the piezoelectric crystal is increased in order to increase the response frequency,
Conversely, a piezoelectric crystal of 5 MHz or 6 MHz has been more often used than before, because it is thinner and easily cracked. Therefore, the present invention reduces the natural frequency of the piezoelectric crystal from 6 MHz (megahertz) or 5 MHz conventionally used,
By reducing the sensitivity of the piezoelectric crystal to the film thickness, the film is made more resistant to the stress of the film or the absorption of vibration by the film, and the fundamental vibration is maintained to extend the life of the piezoelectric crystal.

[0008]

Embodiments of the present invention will be described below with reference to the drawings.

FIGS. 1 to 3 show a piezoelectric crystal used in a piezoelectric crystal oscillation type film thickness meter according to an embodiment of the present invention. The piezoelectric crystal 3 is made of a single crystal quartz resonator cut in a direction called AT cut having a good temperature characteristic (smallest thermal expansion coefficient), and has a natural frequency of 3.25 MHz.
z (thickness 0.51 mm). In addition, as shown in FIG. 2, the front surface 3a serving as the vapor deposition surface of the crystal unit 3 is flat, while the back surface 3b is a slightly convex curved surface. This is a conventional technique for suppressing instability such as shifting to a form of higher-order vibration of thickness-shear vibration or higher-order vibration of contour vibration, or a combination thereof.

Electrode films 10a and 10b on which gold (Au) or silver (Ag) is deposited are formed on the front surface 3a and the back surface 3b of the crystal unit 3, and in the present embodiment, these electrode films 10a, 10b is formed in an island shape as shown by hatching in the figure, and the electrode film 1 on the front surface 3a and the back surface 3b in the peripheral portion of the crystal unit 3 is formed.
The formation areas of 0a and 10b do not overlap.
Thereby, the parasitic vibration at the peripheral portion of the crystal unit 3 is suppressed, and the life of the crystal unit 3 is improved.

FIG. 4 schematically shows the structure of a piezoelectric crystal oscillation type film thickness meter, wherein 1 is a vacuum chamber, 2 is an electron beam evaporation source, 4 is a sensor having a quartz oscillator 3, and 6 is a sensor. A heating power supply 7 is a film thickness monitoring device 7. The sensor 4 supplies the natural frequency of the crystal resonator 3 to the film thickness monitoring device 7. The substrate to be processed is located at the back or front of the sensor 4 even though not shown, and is disposed at the same distance from the sensor 4 as viewed from the evaporation source 2. A change in the natural frequency of the quartz oscillator 3 is measured by the film thickness monitoring device 7, and the output of the heating power supply 6 is adjusted based on the change to control the film forming speed.

FIG. 5B shows 3.25 according to the present embodiment.
The graph shows the life when MgO (magnesium oxide) is deposited on the quartz crystal resonator 3 at a deposition rate of 40 ° / s and the fluctuation of the deposition rate on a monitor with an increase in the thickness. For comparison, FIG. 5A shows a case of a conventionally used 5 MHz quartz oscillator. Here, the fluctuation of the film forming speed on the monitor means a fluctuation with respect to the average film forming speed, and in this experiment of forming a film at 40 ° / s,
A change of 100% means that the change is in the range of 0 to 80 ° / s (not a change in the actual deposition rate but on the monitor display, and a change in the natural frequency of the crystal unit). Means the fluctuation of the film forming speed calculated based on the above.)

In the conventional 5 MHz quartz oscillator shown in FIG. 5A, the film forming speed starts to change at a film thickness of about 5 μm. This indicates that the fundamental vibration of the crystal resonator has begun to be destroyed due to the stress of the film. Also,
The film forming speed fluctuates greatly, and the quartz oscillator stops functioning before the film thickness reaches 20 μm. That is, the vibrator has reached the end of its life.

On the other hand, 3.2 according to the present embodiment.
In the case of the 5 MHz crystal resonator 3, the film formation rate hardly changed until the film thickness reached 20 μm, and it was clearly shown that stable fundamental vibration was maintained.
When the thickness exceeds 0 μm, a change in the film forming speed starts to appear, but the change is smaller than that in the conventional 5 MHz quartz oscillator. This is nothing but an increase in the resistance to film stress due to the lowering of the natural frequency of the crystal resonator 3. Further, it can be seen that the fundamental vibration is maintained even when the film thickness reaches 40 μm. From the above, 3.25 MHz
In the crystal resonator of No. 5, the life was extended by about three times as compared with the conventional 5 MHz crystal resonator, and the fluctuation of the film forming speed was reduced.

As described above, according to the present embodiment, the natural frequency of the crystal unit is made lower than before, the thickness of the crystal unit is increased, and the fundamental vibration of the crystal 3 is maintained. As a result, the life of the crystal 3 can be extended. In addition, the vibrator always has a sub-resonance other than the main resonance controlling the fundamental vibration (due to the parasitic vibration of the peripheral portion). If the vibration level of the sub-resonance is large, the main resonance cannot be maintained and the life is extended. In the present embodiment, the electrode film 10
The shapes of the electrode films 10a and 10b are not overlapped between the front surface 3a and the back surface 3b of the peripheral portion of the crystal unit 3 by making the shapes of the a and 10b islands as described above. The level of resonance has been lowered.
Thereby, the fundamental vibration of the crystal resonator 3 is more stably maintained.

Although the resolution of the quartz oscillator 3 is reduced as the natural frequency is reduced, the frequency resolution of the film thickness monitor itself (that is, the film thickness monitoring device 7 in FIG. 4) is improved at present. So it doesn't matter. For example, according to the circuit example described in Japanese Patent Application No. 6-217664, the resolution is 100 times higher than that of the conventional film thickness monitor, and it can sufficiently cope with a film formation rate of several Å / s. Therefore, also in the present embodiment, it is possible to control the film forming speed based on the change in the natural frequency of the crystal resonator 3.

Although the embodiments of the present invention have been described above, the present invention is, of course, not limited thereto, and various modifications can be made based on the technical concept of the present invention.

For example, in the above embodiment, the natural frequency of the quartz oscillator 3 is set to 3.25 MHz. However, the present invention is not limited to this, and the present invention is applicable to a piezoelectric crystal of 3 MHz, 2 MHz, and 1 MHz or less. It is. That is, the lower the natural frequency, the longer the life of the crystal. In addition, although quartz is used as the piezoelectric crystal, the present invention is not limited to this, and a Rochelle salt or a piezoelectric ceramic such as barium titanate or lead zirconate can be used. In addition, although MgO was used as a film forming material, of course, it is not limited to this.

In the above embodiment, the electrode films 10a and 10b are formed so that the formation regions of the electrode films 10a and 10b do not overlap between the front surface 3a and the back surface 3b in the peripheral portion of the crystal resonator 3. Needless to say, the formation patterns of the electrode films 10a and 10b can be appropriately changed.
Even when the overlap of b is small, the same effect as in the above embodiment can be obtained.

[0020]

As described above, according to the piezoelectric crystal oscillation type film thickness meter of the present invention, the fundamental vibration of the piezoelectric crystal is maintained by lowering the fundamental vibration of the piezoelectric crystal as compared with the prior art. Therefore, the life of the piezoelectric crystal can be improved, and the frequency of replacement can be reduced.

[Brief description of the drawings]

FIG. 1 is a plan view of a crystal resonator applied to a piezoelectric crystal oscillation type film thickness meter according to an embodiment of the present invention.

FIG. 2 is a side view of the same.

FIG. 3 is a back view of the same.

FIG. 4 is a schematic diagram showing a configuration example of a piezoelectric crystal oscillation type film thickness meter according to the present embodiment.

FIG. 5 is an experimental result showing a change in a film forming rate and a lifetime.
A indicates a conventional 5 MHz piezoelectric crystal, and B indicates 3.25M.
3 shows a piezoelectric crystal at Hz.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Vacuum chamber 2 Evaporation source 3 Quartz crystal oscillator 3a Front surface 3b Back surface 4 Sensor 6 Heating power supply 7 Film thickness monitor 10a (front surface) electrode film 10b (back surface) electrode film

Continued on the front page (72) Inventor Toshiharu Kurauchi 5-9-7 Tokodai, Tsukuba City, Ibaraki Pref.

Claims (4)

[Claims] A film forming material is deposited on a piezoelectric crystal provided in a vacuum chamber, and is deposited on a substrate to be processed based on a change in a natural frequency of the piezoelectric crystal due to the deposition of the film forming material. In the piezoelectric crystal oscillation type film thickness meter configured to measure the film thickness of the film forming material, the fundamental frequency of the piezoelectric crystal during film formation is reduced by making the natural frequency of the piezoelectric crystal lower than 5 MHz. The piezoelectric crystal oscillation type film thickness meter characterized by maintaining. 2. An electrode film formed on the front surface and the back surface of the piezoelectric crystal is formed in an island shape, and the overlap of the electrode film formation region on the front surface and the back surface at the peripheral portion of the piezoelectric crystal is reduced. 2. The piezoelectric crystal oscillation type film thickness meter according to claim 1, wherein the thickness is eliminated. 3. The piezoelectric crystal oscillation type film thickness meter according to claim 1, wherein a film forming speed of the film forming material can be controlled based on a change in a natural frequency of the piezoelectric crystal. 4. The piezoelectric crystal oscillation type thickness meter according to claim 1, wherein the piezoelectric crystal is a quartz oscillator.