JP7337339B2 - Film thickness sensor element - Google Patents

Description

The present invention relates to a film thickness sensor element, and more particularly to a film thickness sensor element using a piezoelectric vibrator.

In the manufacture of semiconductor devices, semiconductor layers, insulating layers, and the like are formed by various vapor deposition methods. For example, there is an atomic layer deposition (ALD) method as a vapor deposition method having various characteristics such as the ability to form a high-quality thin film in a more uniform state at a temperature of about 200 to 300°C. Atomic layer deposition is a technique for forming a thin film in units of atomic layers by alternately supplying raw materials for each element constituting a film to be formed to a substrate. In the atomic layer deposition method, only one layer or n layers (n is an integer of 2 or more) are adsorbed on the surface while supplying the source material of each element, and the excess source material is prevented from contributing to the growth. The atomic layer deposition method has both high shape adaptability and film thickness controllability like general chemical vapor deposition (CVD) methods.

In order to improve the degree of integration, semiconductor devices with elements having a three-dimensional structure have been developed, and devices with complex structures such as MEMS (Micro Electro Mechanical Systems) have also been developed. In the manufacture of these devices, it is necessary to uniformly form a thin film in a complicated shape, and the atomic layer deposition method having the above-mentioned features is being used frequently.

On the other hand, in an apparatus for forming a thin film by the vapor phase growth method, it is important to measure the thickness of the formed thin film because the thin film is formed with a thickness of nm units. In this type of film thickness measurement, a film thickness monitor using a film thickness sensor composed of a crystal oscillator is used. This film thickness monitor measures the thickness of the deposited thin film by utilizing the fact that the natural frequency of the crystal oscillator that constitutes the film thickness sensor decreases as the mass of the thin film deposited on the film thickness sensor increases. (see Patent Document 1).

JP-A-04-223211

However, when the temperature of the measurement environment reaches a temperature of 300°C or higher, the piezoelectric performance deteriorates due to the influence of the phase transition of the crystal, and the film thickness sensor made of the crystal oscillator cannot measure the film thickness accurately. There is In addition, a film thickness sensor made of a crystal oscillator has a problem that when the temperature of the measurement environment changes, the resonance frequency changes and the accuracy of film thickness measurement decreases. In addition, a film thickness sensor composed of a crystal oscillator has a problem in linearity between changes in film thickness and changes in natural frequency.

SUMMARY OF THE INVENTION An object of the present invention is to solve the problems described above, and to enable more accurate measurement of film thickness even at high temperatures of 300° C. or higher.

A film thickness sensor element according to the present invention includes a vibrator, and a first electrode and a second electrode formed facing each other with the vibrator interposed in the center of the vibrator. A film thickness sensor element for measuring thickness, wherein the oscillator is Ca ₃ Ta _1+a (Ga _1-d Al _d ) _3+b Si _2+c O ₁₄ (a=1.03, b= -0.05, c = 0.08), and the single crystal has the X axis as the rotation axis and the rotation angle from the XZ plane as θ, and the temperature of the atmosphere for measuring the thickness of the thin film is T , 0.00008T ² +0.00855T + 20.97064 = θ .

In one configuration example of the film thickness sensor element, the first electrode and the second electrode are elliptical in plan view, with the long axis direction along the X-axis of the single crystal.

One structural example of the film thickness sensor element further includes a protective film made of aluminum oxide and formed to cover the surface of the vibrator.

In one configuration example of the film thickness sensor element, the plate thickness of the vibrator is thicker at the central portion than at the peripheral portion.

In one configuration example of the film thickness sensor element, the plate thickness of the vibrator gradually increases from the peripheral portion to the central portion.

In one configuration example of the film thickness sensor element, the vibrator has a plate shape that is circular in plan view.

As described above, according to the present invention, the film thickness can be measured more accurately even at a high temperature of 300° C. or higher.

FIG. 1A is a plan view showing the configuration of a film thickness sensor element according to an embodiment of the present invention; FIG. FIG. 1B is a plan view showing the configuration of the film thickness sensor element according to the embodiment of the invention. FIG. 1C is a cross-sectional view showing the configuration of the film thickness sensor element according to the embodiment of the present invention. FIG. 1D is a plan view showing the configuration of another film thickness sensor element according to the embodiment of the present invention. FIG. 2 is a perspective view showing the configuration of vibrator 101 of the film thickness sensor element according to the embodiment of the present invention. FIG. 3 is a characteristic diagram showing the relationship between the cut angle of the vibrator 101 and the vertex temperature. FIG. 4 is a characteristic diagram showing changes in the resonance frequency of the film thickness sensor element arranged in the film formation chamber in the film formation of the aluminum oxide film by the atomic layer deposition method. FIG. 5 is a characteristic diagram showing a change in resonance frequency of a film thickness sensor element having a protective film formed thereon and arranged in a film forming chamber during the formation of an aluminum oxide film by atomic layer deposition. FIG. 6 shows the frequency-temperature dependence (solid line) of the film thickness sensor element composed of quartz manufactured with a cut angle at which the vertex temperature is 285° C., and the frequency-temperature dependence ( dotted line) is a characteristic diagram showing the result of comparison.

A film thickness sensor element according to an embodiment of the present invention will be described below with reference to FIGS. 1A, 1B, 1C, 1D and 2. FIG. Note that FIG. 1C shows a cross section taken along line aa' in FIG. 1A.

This film thickness sensor element includes a vibrator 101, and a first electrode 102 and a second electrode 103 formed facing each other with the vibrator 101 interposed in the center of the vibrator 101. FIG. This film thickness sensor element is used to measure the thickness of the thin film deposited on the vibrator 101 .

The oscillator 101 is composed of Ca ₃ Ta _1+a (Ga _1-d Al _d ) _3+b Si _2+c O ₁₄ (−0.5<a≦0 or 0<a<0.5, −0.5 <b≦0 or 0<b<0.5, −0.5<c≦0 or 0<c<0.5, 0≦d<1, except when a=b=c=0) It is composed of a single crystal. Further, the vibrator 101 has, for example, a plate shape that is circular in plan view.

Further, as shown in FIG. 2, the single crystal constituting the vibrator 101 is rotated about the X axis, and when the angle of rotation from the XZ plane is θ and the temperature of the atmosphere in which the thickness of the thin film is measured is T, , 0.00008T ² +0.00855T+20.97064 =θ . Note that the XY plane is the (001) plane of the Ca ₃ Ta _1+a (Ga _1-d Al _d ) _3+b Si _2+c O ₁₄ single crystal, and the YZ plane is the Ca ₃ Ta _1+a (Ga _1-d Al _d ) _3+b Si _2+c O ₁₄ single crystal (110) plane and XZ plane are Ca ₃ Ta _1+a (Ga _1-d Al _d ) _3+b Si _2+c O ₁₄ single crystal (0 It is the (100) plane of <x≦1). θ is defined by IEC (International Electrotechnical Commission). In this definition, for example, the crystal AT-cut angle θ is 31°15′.

The vibrator 101 is cut into a plate shape with a wire saw from a Ca ₃ Ta _1+a (Ga _1-d Al _d ) _3+b Si _2+c O ₁₄ single crystal ingot obtained by the Czochralski method, for example. , obtained by grinding both sides to the desired thickness.

A first wiring 104 is connected to the first electrode 102 . A first terminal 105 is connected to the first electrode 102 via a first wiring 104 . A second wiring 106 is connected to the second electrode 103 . A second terminal 107 is connected to the second electrode 103 via a second wiring 106 . Wires and terminals other than the first electrode 102 and the second electrode 103 are arranged so as not to face each other with the vibrator 101 interposed therebetween. For example, as shown in FIG. 1D, the second wiring 106a having a wider area can be formed in a range that does not overlap the first terminal 105 and the first wiring 104 in plan view.

This film thickness sensor element measures the thickness of the deposited thin film by means of a circuit that applies a signal (excitation signal) to each of the first terminal 105 and the second terminal 107 and changes in the resonance frequency of the vibrator 101. Used by connecting a measuring circuit, etc.

The first electrode 102, the first wiring 104, and the first terminal 105 are formed by vapor-depositing a predetermined metal material on one surface of the vibrator 101 to form a metal film. It can be formed by pattern processing. Also, the first electrode 102, the first wiring 104, and the first terminal 105 can be formed by a known lift-off method. The metal film can consist, for example, of a multi-layer structure of gold and chromium layers. A chrome layer is placed on the vibrator 101 side. The metal film can also be composed of a multilayer structure of platinum layers and titanium layers, or a multilayer structure of platinum layers and tungsten layers. A titanium layer or a tungsten layer is placed on the vibrator 101 side. The second electrode 103, the second wiring 106, and the second terminal 107 are also the same.

The film thickness sensor element according to the embodiment is used by being housed in a predetermined holder, for example. The holder is provided with connection terminals, and the connection terminals and the first terminal 105 and the second terminal 107 are electrically connected by mechanical contact such as a wire or a spring terminal. Also, the holder storing the film thickness sensor element is installed at a predetermined location in the processing chamber of the film forming apparatus. An opening is formed in the holder. Via this opening, the raw material gas of the thin film formed by the film forming apparatus can come into contact with one of the surfaces of the vibrator 101 . A thin film formed by a film forming apparatus is formed on the vibrator 101 in contact with the raw material gas supplied into the processing chamber under predetermined film forming conditions.

On the other hand, when an excitation signal is applied to each of the first electrode 102 and the second electrode 103 via the first terminal 105, the second terminal 107, the first wiring 104, and the second wiring 106, the thickness of the vibrator 101 mainly changes. Resonance occurs due to the natural frequency determined by the height. As described above, when a thin film is formed on the vibrator 101, the resonance frequency of the vibrator 101 changes due to the increase in mass caused by the formation of the thin film. The film thickness can be measured by capturing the change in this resonance frequency.

In general, film formation by vapor phase epitaxy is performed by controlling the temperature in the film formation chamber to a predetermined set condition. , for example, changes due to the introduction of the raw material gas. Since the resonance frequency of the vibrator 101 also changes due to such a temperature change, it is desirable to keep the temperature of the atmosphere of the vibrator 101 constant under the set conditions. For this reason, for example, a temperature measuring device and a heater are provided inside the holder, and heating by the heater is controlled so that the temperature measured by the temperature measuring device becomes the temperature set in the film forming conditions. It is also possible to suppress changes in temperature inside the .

Here, as described above, in this type of film formation, the temperature in the film formation chamber is controlled to the film formation temperature. On the other hand, the resonance frequency of the vibrator 101 also changes with temperature. This change is generally along a quadratic curve. In order to suppress the change in the resonance frequency due to temperature, the apex of the quadratic curve at which the temperature change in the resonance frequency of the vibrator 101 is minimized should coincide with the film formation temperature, that is, the temperature of the atmosphere in which the thickness of the thin film is measured. Next, the rotation angle (cutting angle) θ from the XZ plane with the X axis of the vibrator 101 as the rotation axis is adjusted.

A more detailed description will be given using examples. First, Ca ₃ Ta _1+a Ga _3+b Si _2+c O ₁₄ (a=1.03, b=−0.05, c=0.08) was measured using the Czochralski method, ie, the rotary pull-up method. A single crystal of Next, seven conditions were set for the cut angle θ, and vibrating reeds were formed under the conditions of each cut angle. Further, each of the seven vibrating bars thus formed was polished and processed to have a plate thickness such that the resonance frequency was approximately 6 MHz. Further, each vibrating bar was subjected to outer shape processing to produce a disk-shaped vibrator having a diameter of 14 mm in plan view. In addition, a first electrode, a second electrode, a first wiring, a second wiring, a first terminal, and a second terminal having a multi-layer structure composed of a chromium layer and a gold layer were formed for each vibrator. The chromium layer and metal layer were formed by a known vapor deposition method. The chromium layer was 0.01 μm thick, and the gold layer was 0.3 μm thick.

By the way, under the initial conditions, when the first electrode and the second electrode were formed in a circular shape in plan view and the frequency characteristics were measured, many spurious emissions due to unnecessary secondary vibration were generated near the resonance frequency. Therefore, the optimum electrode shape was calculated using simulation. As a result of this simulation, it was found that the first electrode and the second electrode should preferably be elliptical in plan view, with the long axis along the X-axis of the single crystal. Further, in the film thickness sensor element formed with the above dimensions, if the first electrode and the second electrode are elliptical with a ratio of the major axis to the minor axis of 1:0.56+0.1/-0.2, It turned out good. Based on the results of these studies, the first electrode and the second electrode were formed in an elliptical shape with a major axis of 4 mm and a minor axis of 2.8 mm.

The relationship between the cut angle and the peak temperature shown in FIG. 3 was obtained by measuring the frequency temperature characteristic for each set cut angle condition.

Next, an aluminum oxide film was formed by an atomic layer deposition method under a film forming temperature condition of 365°C. The film formation temperature condition of 365° C. is a condition in which it is difficult to measure the film thickness with a film thickness sensor element using crystal. A predetermined substrate is brought into the film forming chamber, the film forming chamber is sealed, and the inside of the film forming chamber is heated until the temperature in the film forming chamber stabilizes at 365° C., and then an aluminum oxide film is formed on the substrate. did.

First, a raw material gas made of TMA (trimethylaluminum) is introduced into the film forming chamber, and the raw material gas is supplied onto the substrate. As a result, an adsorption layer is formed on the substrate in which TMA molecules as a raw material are adsorbed.

Next, the introduction of the raw material gas into the film formation chamber is stopped, and the raw material gas is discharged from the film formation chamber. Next, an oxidizing gas such as ozone gas is introduced into the film forming chamber, and the ozone gas is supplied to the surface of the adsorption layer. The ozone gas oxidizes the adsorption layer adsorbed on the surface of the substrate, and an aluminum oxide layer corresponding to one atomic layer of aluminum is formed on the surface of the substrate. Next, the introduction of the ozone gas into the film formation chamber is stopped, and the ozone gas is discharged from the film formation chamber.

Next, a source gas is supplied onto the substrate in the same manner as described above to form a new adsorption layer on the aluminum oxide layer. Next, the introduction of the raw material gas into the film formation chamber is stopped, and the raw material gas is discharged from the film formation chamber. Next, ozone gas is introduced into the film forming chamber in the same manner as described above and supplied to the surface of the TMA molecular layer (adsorption layer). As a result, the adsorption layer adsorbed on the surface of the already formed aluminum oxide layer is oxidized, and a new aluminum oxide layer corresponding to one atomic layer of aluminum is formed on the surface of the aluminum oxide layer.

As described above, an aluminum oxide layer is formed on the substrate by a series of basic steps of adsorption→discharge of raw material gas→oxidation→discharge of oxidizing gas. The step of forming an aluminum oxide layer (aluminum oxide film) by such atomic layer deposition is repeated to form an aluminum oxide film having a desired thickness on the substrate.

In the film formation of the aluminum oxide film described above, the resonance frequency of the film thickness sensor element arranged in the film formation chamber corresponds to an increase in atomic layer by one atomic layer up to 25 atomic layers, as shown in FIG. did not show a linear response. On the other hand, after 30 atomic layers, it was found that the resonance frequency changed linearly as the number of atomic layers increased. The amount of change was 3 Hz/atomic layer. The initial non-linear behavior described above is considered to be caused by changes in the surface state due to oxygen defects of the vibrator in this type of film formation processing atmosphere.

Based on the above findings, a film thickness sensor element was produced in which a protective film made of aluminum oxide was formed to cover the surface of the vibrator. A protective film was formed by forming an aluminum oxide film having a thickness of 30 atomic layers or more on the surface of the vibrator by the atomic layer deposition method described above. When the same film thickness measurement as described above was performed using the film thickness sensor element of the embodiment using the vibrator having the protective film formed on the surface, as shown in FIG. Even when the aluminum film was formed, a linear resonance frequency response was obtained corresponding to an increase in atomic layer by atomic layer.

Next, the frequency temperature dependence (solid line) of the film thickness sensor element composed of crystal manufactured with a cut angle at which the vertex temperature is 285° C. and the frequency temperature dependence (dotted line) of the film thickness sensor element of the above-described embodiment ) are shown in FIG. As shown in FIG. 6, the film thickness sensor element of the embodiment indicated by the dotted line has less frequency change with temperature change than the film thickness sensor element made of crystal. In other words, it can be seen that the film thickness sensor element of the embodiment is less affected by disturbances such as temperature changes in the film forming chamber, and thus can perform highly accurate film thickness measurement. In the film thickness sensor element of the embodiment, even at 365.degree. In atomic layer deposition, source gases are adsorbed and desorbed, so the resonance frequency of the vibrator varies up and down according to each state.

The embodiment described above is merely an example, and the resonance frequency of the vibrator is not limited to 6 MHz. Also, the vibrator can be formed in a rectangular shape in a plan view. Moreover, the first electrode and the second electrode are not limited to having an elliptical shape in a plan view, and may have other shapes. The protective film is not limited to an aluminum oxide film, and may be composed of films such as TiO ₂ , TiN, AlN, and HfO ₂ .

Also, the thickness of the vibrator can be gradually increased from the periphery to the center. For example, one surface or both surfaces of a plate-like vibrator can be formed in a convex shape. By configuring in this way, the secondary vibration of the vibrator can be further suppressed, which is preferable. Further, by beveling the ends of the vibrator to form an inclination in a cross-sectional view, the same effect as described above can be obtained.

As described above, in the present invention, the oscillator is Ca ₃ Ta _1+a (Ga _1-d Al _d ) _3+b Si _2+c O ₁₄ (a=1.03, b=-0. 05, c = 0.08), this single crystal has a rotation axis of the X axis, a rotation angle from the XZ plane is θ, and the temperature of the atmosphere for measuring the thickness of the thin film is T. 0.00008T ² +0.00855T+20.97064 =θ . As a result, according to the present invention, the film thickness can be measured more accurately even at a high temperature of 300° C. or higher.

It should be noted that the present invention is not limited to the embodiments described above, and many modifications and combinations can be implemented by those skilled in the art within the technical concept of the present invention. It is clear.

Reference numerals 101: vibrator, 102: first electrode, 103: second electrode, 104: first wiring, 105: first terminal, 106: second wiring, 107: second terminal.

Claims (6)

A film for measuring the thickness of a thin film deposited on the vibrator, comprising a vibrator, and a first electrode and a second electrode formed facing each other with the vibrator interposed in the center of the vibrator. A thickness sensor element,
The oscillator is composed of a single crystal of Ca ₃ Ta _1+a Ga _3+b Si _2+c O ₁₄ (a=1.03, b=−0.05, c=0.08),
The single crystal is 0.00008T ² +0.00855T+20.97064 = θ, where the X axis is the rotation axis, the rotation angle from the XZ plane is θ, and the temperature of the atmosphere for measuring the thickness of the thin film is T. A film thickness sensor element characterized by: The film thickness sensor element according to claim 1,
The film thickness sensor element, wherein the first electrode and the second electrode are elliptical in plan view, with the long axis extending along the X-axis of the single crystal. The film thickness sensor element according to claim 1 or 2,
A film thickness sensor element, further comprising a protective film made of aluminum oxide and formed to cover the surface of the vibrator. In the film thickness sensor element according to any one of claims 1 to 3,
A film thickness sensor element, wherein the plate thickness of the vibrator is thicker in the central portion than in the peripheral portion. In the film thickness sensor element according to claim 4,
The film thickness sensor element, wherein the plate thickness of the vibrator gradually increases from the peripheral portion to the central portion. In the film thickness sensor element according to any one of claims 1 to 5,
The film thickness sensor element, wherein the vibrator has a plate-like shape that is circular in plan view.

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