JP2005286659A - Thin film type bulk wave device and manufacturing method thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a thin film bulk acoustic wave device that suppresses a shift in resonance frequency due to a shape change of a base of a thin film resonator as compared with the conventional one.
A thin film bulk acoustic wave device includes a thin film resonator 12 formed on a substrate 11. Further, the substrate 11 includes an amorphous film 17 that is a hardly deformable portion whose linear expansion coefficient is reduced by processing, and the thin film resonator 12 is bonded to the substrate 11 at the amorphous film 17. Thereby, the shift of the resonant frequency can be suppressed more than before.
[Selection] Figure 1

Description

The present invention relates to a thin film bulk acoustic wave device, and more particularly to a technique for suppressing a shift in resonance frequency as compared with the conventional art.

Conventionally, application of a thin film bulk acoustic wave device as a high frequency resonator or a high frequency filter has been proposed. A thin film type bulk wave element is an element that utilizes the property that a piezoelectric thin film converts an electrical signal into an elastic wave, and details of its principle and features are shown in Non-Patent Document 1.
FIG. 5 is a cross-sectional view of a conventional thin film bulk acoustic wave device.
The thin film bulk acoustic wave device includes a substrate 11 having a recess 16 on the main surface and a thin film resonator 12 formed so as to straddle the recess 16. The thin film resonator 12 includes a lower electrode 13, a piezoelectric thin film 14, and an upper electrode 15. When an AC voltage is applied between the lower electrode 13 and the upper electrode 15, the piezoelectric thin film 14 undergoes thickness vibration. Here, if the frequency of the AC voltage matches the resonance frequency of the thickness vibration that depends on the shape (particularly, the film thickness) of the thin film resonator 12, the thin film resonator 12 resonates. Using such a principle, a high-frequency resonator and a high-frequency filter can be realized.

An element for evaluating the performance of the thin film bulk acoustic wave device is the temperature stability of the resonance frequency. In other words, the closer the shift amount of the resonance frequency with respect to the change in the ambient temperature of the element is, the higher the performance is.
As described above, since the resonance frequency depends on the shape of the thin film resonator 12, in order to improve the temperature stability of the thin film bulk acoustic wave device, it is necessary to suppress the shape change with respect to the temperature change of the thin film resonator 12. It is.
Acoustic Wave Element Technology Handbook Japan Society for the Promotion of Science Elastic Wave Element Technology 150th Committee ed.

However, when a thin film bulk acoustic wave element is applied to a high frequency resonator or high frequency filter in the GHz band used for recent wireless communication, the film thickness of the thin film resonator 12 becomes as thin as about several μm. As a result, the influence from the substrate 11 cannot be ignored as a factor of the resonance frequency shift. This is because the thin film resonator 12 is deformed as the shape of the substrate 11 changes with temperature.
Accordingly, an object of the present invention is to provide a thin-film bulk acoustic wave element that suppresses a shift in resonance frequency due to a change in the shape of the base of the thin-film resonator 12 and a method for manufacturing the same.

  A thin film type bulk wave device according to the present invention is a thin film type bulk wave device including a thin film resonator formed on a substrate, and the substrate includes a hardly deformable portion whose linear expansion coefficient is reduced by processing, The thin film resonator is bonded to the substrate at the hardly deformable portion.

According to the said structure, the shape change with respect to a temperature change becomes small rather than the state with a board | substrate as it is in the difficult deformation | transformation site | part which is a part of board | substrate. Therefore, the thin-film bulk acoustic wave element can suppress the change in the shape of the base of the thin-film resonator as compared with the related art, and can accordingly suppress the shift of the resonance frequency.
Further, the hardly deformable portion may have a linear expansion coefficient reduced by using single crystal silicon as amorphous silicon.

The linear expansion coefficients of single crystal silicon and amorphous silicon are as follows.
(1) Linear expansion coefficient single crystal silicon at −30 ° C .: about 1.5 × 10 ⁻⁶ (K ⁻¹ )
Amorphous silicon: about 1.0 × 10 ⁻⁶ (K ⁻¹ )
(2) Linear expansion coefficient single crystal silicon at 120 ° C .: about 3.0 × 10 ⁻⁶ (K ⁻¹ )
Amorphous silicon: about 1.3 × 10 ⁻⁶ (K ⁻¹ )
According to this, amorphous silicon has a smaller linear expansion coefficient than single crystal silicon in both cases of −30 ° C. and 120 ° C. Moreover, it is thought that there exists a similar tendency also at temperature other than these.
As described above, since amorphous silicon has a smaller shape change with respect to a temperature change than single crystal silicon, a shift in the resonance frequency of the thin film resonator accompanying the shape change can be suppressed.

A manufacturing method according to the present invention is a method of manufacturing a thin film bulk acoustic wave device including a substrate having a concave portion on a main surface and a thin film resonator formed so as to straddle the concave portion, wherein the concave portion is formed. A modified part forming step in which a part other than the part where the concave part is to be formed on the main surface of the substrate is an altered part whose etching rate is slower than the part where the concave part is to be formed; a part of the part where the concave part is to be formed; It includes a thin film resonator forming step for forming the thin film resonator so as to cover a part or all of the thin film resonator, and a recess forming step for etching the substrate to form a recess at the recess forming portion.
In addition to the temperature stability of the resonance frequency, an element for evaluating the performance of the thin film bulk wave element is a piezoelectric coefficient. The piezoelectric coefficient represents the magnitude of strain of the piezoelectric thin film with respect to the applied electric field, and the larger the value, the better the performance of the thin film bulk wave device.
According to the above configuration, in the manufacturing method, after the thin film resonator is formed on an upper portion of the substrate main surface that is not deteriorated, the unmodified portion is removed by etching to form a recess. As described above, since the thin film resonator is formed on the upper portion of the substrate where the substrate is not deteriorated (that is, the substrate body), the crystallinity is improved, and the piezoelectric coefficient is improved accordingly.

Moreover, the said board | substrate consists of single crystal silicon, and the said quality change part formation step is good also as forming the said quality change part by implanting ion to parts other than the said recessed part formation plan part of the said board | substrate main surface.
According to the above configuration, the altered portion is formed by ion implantation. When ion implantation is performed, the linear expansion coefficient of the implantation site becomes small as the implantation site changes to amorphous.

As a result, it is possible to manufacture a thin film bulk acoustic wave element that can suppress the change in the shape of the base of the thin film resonator more than before and can suppress the shift of the resonance frequency accordingly.
The ions implanted in the altered portion forming step may be any one of gold, carbon, titanium, and platinum.
The ions listed above are known to slow the etching rate at the implantation site when implanted into a single crystal silicon substrate. Therefore, the altered portion subjected to the ion implantation has a slower etching rate than the portion where the recess is to be formed where the ion implantation is not performed. Thereby, since the recessed part formation scheduled site | part is selectively etched in a recessed part formation step, a recessed part can be formed easily.

FIG. 1 is a cross-sectional view of a thin film bulk acoustic wave device according to the present invention.
The thin film bulk acoustic wave device has a structure in which a thin film resonator 12 is bonded to a substrate 11 via an amorphous film 17. This is different from the conventional thin film bulk wave device (see FIG. 5).
The substrate 11 is made of (100) single crystal silicon and has a thickness of about 300 μm.
Each of the lower electrode 13 and the upper electrode 15 is made of molybdenum, and is formed to a thickness of about 200 nm by sputtering.
The piezoelectric thin film 14 is made of aluminum nitride and has a thickness of about 1 μm. The width of the thin film resonator 12 is about 100 μm.
The amorphous film 17 is made of amorphous silicon and is formed to a depth of about 2 μm from the surface of the substrate 11 by ion implantation. Amorphous silicon is known to have a smaller linear expansion coefficient than single crystal silicon. The numerical value of the linear expansion coefficient is as follows.
(1) Linear expansion coefficient single crystal silicon at −30 ° C .: about 1.5 × 10 ⁻⁶ (K ⁻¹ )
Amorphous silicon: about 1.0 × 10 ⁻⁶ (K ⁻¹ )
(2) Linear expansion coefficient single crystal silicon at 120 ° C .: about 3.0 × 10 ⁻⁶ (K ⁻¹ )
Amorphous silicon: about 1.3 × 10 ⁻⁶ (K ⁻¹ )
According to this, amorphous silicon has a smaller linear expansion coefficient than single crystal silicon in both cases of −30 ° C. and 120 ° C. Moreover, it is thought that there exists a similar tendency also at temperature other than these.
As described above, since amorphous silicon has a smaller shape change with respect to a temperature change than single crystal silicon, a shift in the resonance frequency of the thin film resonator accompanying the shape change can be suppressed. Specifically, in the conventional thin film type bulk wave device, the temperature coefficient of the resonance frequency is about 30 ppm / ° C., but in the thin film type bulk wave device having the above configuration, it becomes 9 ppm / ° C. and exhibits excellent temperature stability. It was confirmed.
Below, the manufacturing method of the above-mentioned thin film type bulk acoustic wave device is explained using FIG. 2, FIG. 3 and FIG.

FIG. 2 is a diagram showing a process of forming the amorphous film 17 of the thin film bulk acoustic wave device according to the present invention.
FIG. 2A is a top view of the thin film bulk acoustic wave device. A mask 24 is deposited on the portion where the recess is to be formed on the main surface of the substrate.
FIG. 2B is a cross-sectional view of the thin film bulk acoustic wave device taken along the line AA. Ions 25 are implanted into the substrate 11 from the direction perpendicular to the main surface of the substrate. As a result, ions are implanted into a portion of the substrate 11 other than the portion protected by the mask 24 to form the amorphous film 17. Here, it is assumed that about 3 × 10 ¹⁷ cm ^{−2 of} about 3 MeV gold ions are implanted at 100K.
Thereafter, the mask 24 is removed, and the process proceeds to the formation process of the thin film resonator 12.

FIG. 3 is a diagram showing a process of forming the thin film resonator 12 of the thin film bulk acoustic wave device according to the present invention.
FIG. 3A is a top view of the thin film bulk acoustic wave device. The thin film resonator 12 is formed so as to cover a part of the mask removal portion 26 from which the mask 24 has been removed and a part of the amorphous film 17.
FIG. 3B is a cross-sectional view of the thin film bulk acoustic wave device taken along the line BB. In the mask removal portion 26, the single crystal silicon of the main body of the substrate 11 is exposed on the main surface of the substrate. A thin film resonator 12 is formed so as to cover it. Therefore, the crystallinity of the piezoelectric thin film 14 is improved, and the piezoelectric coefficient can be improved accordingly. The piezoelectric coefficient represents the magnitude of strain of the piezoelectric thin film 14 with respect to the application of an electric field. The larger the value, the better the performance of the thin film bulk wave device.

Next, the process proceeds to the formation process of the recess 16.
FIG. 4 is a view showing a process of forming the recess 16 of the thin film bulk acoustic wave device according to the present invention.
FIG. 4A is a top view of the thin film bulk acoustic wave device. In order to form the recess 16, the substrate 11 is anisotropically etched with potassium hydroxide (KOH) or tetramethylammonium hydroxide (TMAH). Here, KOH has a concentration of 20 to 50 percent and TMAH has a concentration of 10 to 25 percent.

  FIG. 4B is a cross-sectional view of the thin film bulk acoustic wave device taken along the line CC. It is known that single crystal silicon has a slow etching rate when gold ions are implanted. That is, the etching rate of the amorphous film 17 is slower than that of the mask removal portion 26. Thereby, the mask removal portion 26 is selectively etched, so that the recess 16 is easily formed.

The thin film bulk acoustic wave device shown in FIG. 1 can be manufactured by the manufacturing method described above.
Note that this manufacturing method can manufacture a thin film bulk acoustic wave element having a better piezoelectric coefficient than the conventional manufacturing method. This point will be described in comparison with a conventional manufacturing method.
FIG. 6 is a diagram showing a manufacturing process of a conventional thin film bulk acoustic wave device.
First, as shown in FIG. 6A, a mask 21 is applied to the substrate 11 to form a recess 16 by anisotropic etching. Next, as shown in FIG. 6B, after a sacrificial layer 23 of phosphate silicate glass (PSG) is deposited in the recess 16, the surface of the substrate 11 is flattened by polishing. Thereafter, as shown in FIG. 6C, the thin film resonator 12 is formed on the sacrificial layer 23. Finally, as shown in FIG. 6D, the recess 16 is formed by etching the sacrificial layer 23 with hydrofluoric acid (HF + H ₂ O). Thereby, a thin film type bulk wave device is manufactured.
In the conventional manufacturing method, as shown in FIG. 6C, the thin film resonator 12 is formed on the upper surface of the PSG that is the sacrificial layer 23.
On the other hand, in the manufacturing method of the present invention, as shown in FIG. 3, the thin film resonator 12 is formed on the upper surface of the mask removal portion 26 where the single crystal silicon is exposed.
In general, the piezoelectric coefficient depends on the crystallinity of the piezoelectric thin film 14, and the crystallinity of the piezoelectric thin film 14 depends on the crystallinity of the base at the time of film formation. Since single crystal silicon has better crystallinity than PSG, the manufacturing method of the present invention can manufacture a thin film bulk wave device having a piezoelectric coefficient better than that of the conventional manufacturing method.
(Modification)
In the embodiment, the shift of the resonance frequency is suppressed by using the amorphous film 17 as the base of the thin film resonator 12, but the present invention is not limited to this. It is only necessary that the portion of the substrate 11 where the thin film resonator 12 is bonded has a smaller linear expansion coefficient than the other portions.

In the embodiment, a part of the substrate 11 is the amorphous film 17 for convenience of manufacturing, but the present invention is not limited to this. For example, the amorphous film 17 may be laminated on the upper surface of the substrate 11 and the thin film resonator 12 may be formed thereon.
In the embodiment, the etching rate is slowed down by ion implantation in parts other than the part where the recess is to be formed. However, the present invention is not limited to this, and the etching rate may be slowed down by other methods.

Note that in the embodiment, gold ions are used as ions for slowing the etching rate of the single crystal silicon substrate, but the present invention is not limited to this. For example, carbon, titanium, platinum and the like are known as ions having the same effect, and these may be used.
In the embodiment, the recess 16 is formed by etching from the surface of the substrate 11 (the surface on which the thin film resonator 12 is formed). However, the present invention is not limited to this, and the mask is removed from the back surface of the substrate 11. An embodiment in which etching is performed toward the portion 26 may be employed. In this case, in FIG. 3A, the thin film resonator 12 may be formed so as to cover not the part of the mask removal portion 26 but the whole.

  The present invention can be used for a high-frequency bandpass filter in a wireless communication terminal.

1 is a cross-sectional view of a thin film bulk acoustic wave device according to the present invention. It is a figure which shows the formation process of the amorphous film 17 of the thin film type bulk wave element concerning this invention. It is a figure which shows the formation process of the thin film resonator 12 of the thin film type bulk wave element concerning this invention. It is a figure which shows the formation process of the recessed part 16 of the thin film type bulk wave element concerning this invention. It is sectional drawing of the conventional thin film type bulk wave element. It is a figure which shows the manufacturing process of the conventional thin film type bulk wave element.

Explanation of symbols

DESCRIPTION OF SYMBOLS 11 Substrate 12 Thin film resonator 13 Lower electrode 14 Piezoelectric thin film 15 Upper electrode 16 Recess 17 Amorphous film 24 Mask 25 Ion 26 Mask removal part

Claims (5)

A thin film bulk acoustic wave device including a thin film resonator formed on a substrate,
The substrate includes a hardly deformable portion whose linear expansion coefficient is reduced by processing,
The thin film bulk acoustic wave device is characterized in that the thin film resonator is bonded to the substrate at the hardly deformable portion. 2. The thin film bulk acoustic wave device according to claim 1, wherein the hardly deformable portion has a coefficient of linear expansion reduced by using single crystal silicon as amorphous silicon. 3. A method of manufacturing a thin film bulk acoustic wave device comprising a substrate having a recess on a main surface and a thin film resonator formed so as to straddle the recess,
A modified part forming step in which a part other than the part where the concave part is to be formed on the main surface of the substrate where the concave part is not formed is an altered part whose etching rate is slower than the part where the concave part is scheduled to be formed;
A thin film resonator forming step of forming the thin film resonator so as to cover a part of the recessed portion formation scheduled portion and a part or all of the altered portion;
A method of manufacturing a thin film bulk acoustic wave device, comprising: a recess forming step of etching the substrate to form a recess in the recess formation planned portion. The substrate is made of single crystal silicon,
The altered part forming step includes:
4. The method of manufacturing a thin film bulk acoustic wave device according to claim 3, wherein the altered portion is formed by implanting ions into a portion other than the portion where the concave portion is to be formed on the main surface of the substrate. Ions implanted in the altered portion forming step are:
The method for producing a thin film bulk acoustic wave device according to claim 4, wherein the ion is any one of gold, carbon, titanium, and platinum.

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