JPH05327398A - Piezoelectric thin film - Google Patents

Abstract

PURPOSE:To select any arbitrary central frequency and temperature characteristic by setting the density of oxigen contained in an AlN thin film on a sapphire board and the crystalization of the thin film lower than specified value. CONSTITUTION:The density of oxigen contained in the AlN thin film on the sapphire board is made less than 1%, and the crystalization of the thin film is set less than 0.50 deg.. Thus, the temperature characteristic of the propagating velocity of surface acoustic waves(SAW) on the thin film is accurately controlled, and the piezoelectric thin film for a SAW device is obtained to enable to select any arbitrary central frequency and temperature characteristic. In this case, it is desirable that the sapphire board is a sapphire (110) board, the surface of the AlN thin film parallel to the board is a (001) surface or sapphire (110) or (001) and the surface of the AlN thin film is the (001).

Description

Detailed Description of the Invention

[0001]

The present invention relates to a SAW filter, SA
The present invention relates to a piezoelectric thin film used in SAW devices such as W resonators and SAW sensors.

[0002]

2. Description of the Related Art Aluminum nitride (AlN) is a piezoelectric film having a high insulating property (bandgap is 6.2 eV). Utilizing the high insulation property, application of a III-V group compound semiconductor as a passivation film is considered. In addition, since the propagation velocity (hereinafter abbreviated as Vs) of a surface acoustic wave (hereinafter abbreviated as SAW) is the largest in the piezoelectric body, it is applied as a SAW device represented by a SAW filter in a high frequency region. Is expected.

The center frequency, insertion loss, temperature characteristics and other filter characteristics of the SAW filter are the propagation characteristics of the SAW and the comb-shaped electrode pattern (Inter
Digital Transduser, hereinafter abbreviated as IDT). The thickness of the AlN thin film standardized by the velocity of the surface acoustic wave and the wavelength of the surface acoustic wave [kH, where k = 2π / λ
It is conventionally known that the relationship shown in FIG. 5 or FIG. 6 is established between (λ: wavelength of surface acoustic wave) and H represents the film thickness of the AlN thin film.

Since the center frequency F ₀ of the SAW filter is determined by the equation (1), F ₀ = Vs / λ (1) In order to obtain an arbitrary F ₀ , the determination should be made according to FIGS. 5 and 6. .. However, of the characteristics of the SAW device, the temperature characteristics of the center frequency have not been reproducible so far, and there has been no control method. For the relationship between temperature characteristics and kH, for example, 1975 IEEE Ultraso
n. Symp. , P .; 234 (1975), 1978I
EEE Ultrason. Symp. , P .; 598
(1978) and other reports, Mikoshiba et al. MOCV
IEEE Trans. onSonics
and Ultrason. , Su-32P. 634
The report in (1985) shows that the zero temperature coefficient can be realized by limiting the oxygen concentration in the AlN thin film. However, the findings from these reports were poorly reproducible and did not reach a clear tendency.

Therefore, at present, there has been no method for controlling the temperature characteristics by kH, which has been a major obstacle in using the AlN thin film as a substrate for SAW devices.

[0006]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a piezoelectric thin film for SAW devices, which is made of AlN and can be selected with arbitrary center frequency and temperature characteristics.

[0007]

The present invention is as follows. 1. In a laminated substrate composed of a sapphire substrate and an AlN thin film on the sapphire substrate, the oxygen concentration contained in the thin film is 1% or less, and the crystallinity of the thin film is 0.50 °.
The following is a piezoelectric thin film. 2. 2. The piezoelectric thin film according to claim 1, wherein the sapphire substrate is a sapphire (012) substrate, and the surface of the AlN thin film parallel to the substrate is the (110) plane. 3. 2. The piezoelectric thin film according to claim 1, wherein the sapphire substrate is a sapphire (001) substrate, and the surface of the AlN thin film parallel to the substrate is the (001) plane. 4. The piezoelectric thin film according to claim 1, wherein the sapphire substrate is a sapphire (110) substrate, and the surface of the AlN thin film parallel to the substrate is the (001) plane.

As a result of intensive studies by the inventors, the reason why the temperature characteristics of the center frequency of the filter cannot be reproducible is that the oxygen concentration in the AlN thin film is not constant and the crystallinity of the AlN thin film is lacking. It became clear.
There are three possible causes of oxygen contamination in the AlN thin film,
One is the presence of oxygen impurities contained in the film forming raw material,
One is that O ₂ and H ₂ O contained in the film forming atmosphere are directly incorporated into the AlN thin film, and the other is that O ₂ and H ₂ O contained in the film forming atmosphere react with the film forming raw material. It is taken into the AlN thin film.

Especially in the case of the MOCVD method, the third cause is considered to be serious. Generally, when synthesizing an AlN thin film by the MOCVD method or the like, the film formation chamber is sufficiently evacuated to remove the O ₂ and H ₂ O adsorbed in the chamber and the pipe, and the reaction is started. Since substances such as aluminum have extremely high reactivity with O ₂ and H ₂ O, even if they are present in a very small amount, oxygen will be mixed into the AlN thin film, and O ₂ contained in the film-forming atmosphere will be included. , A slight variation in the amount of H ₂ O led to a variation in the amount of oxygen mixed in the AlN thin film.

There are two possible causes that the crystallinity of the AlN single crystal thin film becomes non-uniform. One is that impurities such as oxygen are A
It is a solid solution in the 1N film, and the other is a physical mismatch with the substrate single crystal, that is, a lattice strain and a stress strain caused by a difference in lattice constant and a difference in thermal expansion coefficient. In particular, the latter cause is a general problem of heteroepitaxy, and it is important to solve this problem in order to put the AlN single crystal thin film into practical use as a SAW device.

To solve this problem, Japanese Patent Laid-Open No. 2-14149
No. 5, JP-A-2-153896, JP-A-2-
In Japanese Patent No. 153897, by interposing a buffer layer at the interface between the sapphire substrate and the AlN thin film, A
A method for improving the crystallinity of an 1N thin film is disclosed.
As a method for synthesizing the AlN thin film for the present invention, a vapor phase method is preferable. In particular, Japanese Patent Application No. 2-276538
In order to optimize the temperature gradient in the vicinity of the sapphire substrate during the vapor phase synthesis method as disclosed in U.S. Pat. By a gas phase synthesis method in which two or more kinds of carrier gases are introduced into the reaction furnace in order to optimize the temperature gradient from the substrate to the gas phase and to vaporize the addition product. It is preferable to use a halide-based aluminum as a film forming raw material in order to solve the above-mentioned problem of oxygen contamination. Combining such a vapor phase synthesis method with the method of interposing a buffer layer at the interface between the sapphire substrate and the AlN thin film described above is a preferable mode for carrying out the present invention. in this case,
As shown in FIG. 7, it is preferable that the crystallinity of the AlN thin film has a correlation with the film thickness of the AlN thin film. Figure 7
Such a correlation can be obtained by optimizing the substrate temperature, reaction pressure, reaction atmosphere and the like in the vapor phase synthesis method disclosed in Japanese Patent Application No. 2-276538. Particularly, it is essential that the oxygen concentration in the AlN thin film as defined below is 1% or less and the crystallinity is 0.50 ° or less. More preferably, the crystallinity is 0.40 ° or less. Here, the oxygen concentration in the AlN thin film is measured by depth direction analysis using Auger electron spectroscopy. The quantification method will be described below. The relative sensitivity coefficient of O to Al is obtained from the measurement of the standard substance (sapphire substrate). The depth profile of the AlN thin film sample at high sensitivity and the relative sensitivity coefficient obtained in
And the ratio of Al. The concentration of Al in the AlN thin film sample was 50 at. %, The absolute concentration of O in the AlN thin film sample is calculated from the value of. Here, the measuring device and the measuring conditions used are shown.

Device: JAMP-7100 [JEOL Ltd.]
Manufacturing] Acceleration voltage: 3 kV Irradiation current: 30 nA (normal measurement), 150 nA
(High-sensitivity measurement) Sample inclination: 75 ° Analysis area: 300 μm × 60 μm Integration time: 256 ms (normal measurement), 1024
ms (high-sensitivity measurement) Ar ⁺ ion acceleration voltage: 3 kV Further, the crystallinity of the AlN thin film was evaluated with the half width of the X-ray rocking curve. The divergence slit and the scattering slit are 1/6 °, and the light receiving slit is 0.3 mm. The measurement was performed using RAD-IIIA manufactured by Rigaku Denki Co., Ltd.

Τ _v (ppm / ° C.) is calculated by the equation (1). _{τ v = {(V T -V} 25) / V 25 (T-25)} × 10 6 (1) except, V _T is T ° C., V ₂₅ is the propagation velocity of the surface acoustic wave at 25 ℃ (m / s ).

[0014]

EXAMPLES Next, the present invention will be described in more detail by way of examples. FIG. 8 shows an outline of an apparatus suitable for carrying out the present invention. The thickness of the AlN thin film is measured optically.
multichannel analyzer (E
G & G PRINCETON APPLIED RES
It was performed by a multi-wavelength spectroscopic light flux interference photometry method using EARCH.

[0015]

Example 1 The present invention is carried out by the CVD method using the apparatus shown in FIG. AlC as evaporation source of aluminum metal
using l _3, to obtain a vapor pressure heated at a predetermined temperature placed in a constant temperature bath after filling into the stainless steel container. As the nitrogen source, 99.99995 vol% ammonia gas is used.
99.99995 vol% N ₂ and 9 as carrier gas
A mixed gas of 9.99999 vol% H ₂ is used, a graphite disk is used as a substrate heating support, and the substrate is locally heated by high frequency induction wave heating. The conditions used for implementation are listed below.

Single crystal substrate Sapphire R surface AlCl ₃ retention temperature 140 ° C. AlCl ₃ supply gas amount 20 sccm Ammonia supply amount 5 sccm Carrier gas amount N ₂ 420 sccm H ₂ 55 sccm Reaction pressure 60 torr Pre-reaction pressure 1 × 10 ⁻² torr AlN by using different substrate temperature and reaction time
The thin film was synthesized and the thickness of the obtained AlN thin film and AlN
Table 1 shows the oxygen concentration in the thin film and the crystallinity of the AlN thin film.
Summarized in.

After these films were synthesized, diamond pace
Surface is mirror-polished with a photolithography process.
Interdigital transducer (hereinafter referred to as I
(Abbreviated as DT). Vs temperature characteristic τ_vSeeking
There are 3 types of IDTs for all
The number of fingers is 64 for both input and output, and the wavelength (λ) is
There are three types, 6 μm, 8 μm, and 10 μm. These I
SAW filter characteristics of the samples in Table 1 were evaluated by DT.
Value, Vs temperature characteristic τ_vI asked. The results are shown in Table 2.
Summarized. For the measurement of SAW filter characteristics, Yokogawa
Hewlett Packard Co., Ltd. network analyst
Using Iser HP 8510B, Cascade Micro
It was performed using a probe manufactured by Tech. In addition, elastic surface
The wave propagation direction is parallel to the c-axis projection line of the sapphire R surface.
To form the IDT. These results are shown in FIG.
I put it. τ_vIs expressed by equation (2), and_v= -84.61 (kHz)^-0.2539+36.27 (2) The maximum deviation from this curve is 0.21 ppm / ° C.
According to equation (2) _vShows that is controlled
It

[0018]

COMPARATIVE EXAMPLE 1 Using the same apparatus as that used in Example 1, a CVD method was carried out under the conditions shown below. In addition, No. 1 of Comparative Example 1. 1, No. 9 as a nitrogen source in 2
9.99vol% ammonia gas, 9 for carrier gas
9.9995 vol% nitrogen gas was used. N of Comparative Example 1
o. In No. 3, 99.9995 vol% ammonia gas and 99.99995 vol% nitrogen gas were used. In addition, in Comparative Example 1, No. The pre-reaction pressure in Example 3 is the same as in Example 1.
It is 1 × 10 ^-2 torr as in the above.

Single-crystal substrate Sapphire R surface AlCl ₃ holding temperature 150 ° C. AlCl ₃ supply gas amount 25 sccm Ammonia supply amount 5 sccm Carrier gas amount 470 sccm Reaction pressure 100 torr Pre-reaction pressure 1 × 10 ⁻¹ torr Using the above conditions, the substrate temperature and AlN by changing the reaction time
The thin film was synthesized, and the film thickness and crystallinity of the obtained AlN thin film and the oxygen concentration in the AlN thin film are summarized in Table 3.

These films were subjected to the same polishing treatment as in Example 1, the SAW filter characteristics were evaluated by the same IDT as in Example 1, and τ _v was determined. The results are summarized in Table 4 and plotted in FIG. As is clear from Table 4 and FIG. 9, τ _v on the AlN thin film obtained by the comparative example has a large deviation from the curve represented by the formula (2) in the present invention, and exceeds 1.00 ppm / ° C. at least. There is. From this result, the oxygen concentration in the AlN thin film is 1% or less, and
It is shown that when the crystallinity of the N thin film is not satisfied at 0.50 ° or less, it does not always fit the correlation between kH and τ _v shown in the equation (2).

[0021]

Example 2 An AlN thin film was synthesized by the same method as in Example 1 except that a sapphire C plane was used for the single crystal substrate.
The conditions used for implementation are listed below. Single crystal substrate Sapphire C surface AlCl ₃ retention temperature 140 ° C. AlCl ₃ supply gas amount 20 sccm ammonia supply amount 5 sccm carrier gas amount N ₂ 420 sccm H ₂ 55 sccm reaction pressure 55 torr pre-reaction pressure 1 × 10 ^-2 torr AlN by changing temperature and reaction time
The thin film was synthesized, and the film thickness and crystallinity of the obtained AlN thin film and the oxygen concentration in the AlN thin film are summarized in Table 5.

After synthesizing these films, the same film as in Example 1 was used.
The SAW filter characteristics are evaluated by the
Characteristic τ_vWas calculated and the results are summarized in Table 6. Incidentally, elasticity
The propagation direction of surface waves is parallel to the a-axis of the sapphire C plane
To form the IDT. The results of Table 6 are plotted in Figure 2.
did. τ_vIs expressed by equation (3), and_v= 7.42 (kHz) -70.70 (3) The maximum deviation from this line is 0.62 ppm / ° C.
According to equation (3) _vShows that is controlled
It

[0023]

[Comparative Example 2] The same apparatus as that used in Example 2 was used, and CVD was performed under the following conditions. In addition, No. 2 of Comparative Example 2. 1, No. 9 as a nitrogen source in 2
9.99vol% ammonia gas, 9 for carrier gas
9.9995 vol% nitrogen gas was used. N of Comparative Example 2
o. In No. 3, 99.9995 vol% ammonia gas and 99.99995 vol% nitrogen gas were used. In addition, No. The pressure before the reaction in Example 3 is the same as in Example 2.
It is 1 × 10 ^-2 torr as in the above.

Single crystal substrate Sapphire C surface AlCl ₃ retention temperature 150 ° C. AlCl ₃ supply gas amount 25 sccm Ammonia supply amount 5 sccm Carrier gas amount 470 sccm Reaction pressure 95 torr Pre-reaction pressure 1 × 10 ⁻¹ torr Using the above conditions, the substrate temperature and AlN by changing the reaction time
The thin film was synthesized, and the film thickness and crystallinity of the obtained AlN thin film and the oxygen concentration in the AlN thin film are summarized in Table 7.

These films were subjected to the same polishing treatment as in Example 2, and SAW filter characteristics were evaluated by the same IDT as in Example 2 to obtain τ _v . The results are summarized in Table 8 and plotted in FIG. As is clear from Table 8 and FIG. 10, τ _v on the AlN thin film obtained in Comparative Example 2 has a large deviation from the straight line represented by the formula (3) and exceeds at least 1.00 ppm / ° C. From this result, Al
When the oxygen concentration in the N thin film is 1% or less and the crystallinity of the AlN thin film is 0.50 ° or less,
It is shown that the correlation between kH and τ _v shown in the equation (3) does not necessarily fit.

[0026]

Example 3 An AlN thin film was synthesized by the same method as in Example 1 except that the sapphire A surface was used as the single crystal substrate.
The conditions used for implementation are listed below. Single crystal substrate Sapphire A surface AlCl ₃ retention temperature 140 ° C. AlCl ₃ supply gas amount 20 sccm Ammonia supply amount 5 sccm Carrier gas amount N ₂ 425 sccm H ₂ 50 sccm Reaction pressure 50 torr Pre-reaction pressure 1 × 10 ^-2 torr AlN by changing temperature and reaction time
The thin film was synthesized, and the film thickness and crystallinity of the obtained AlN thin film and the oxygen concentration in the AlN thin film are summarized in Table 9.

After synthesizing these films, the SAW filter characteristics were evaluated by the same method as in Example 1, and the SAW propagation velocity Vs at 25 ° C. and the temperature characteristic τ _{v of} Vs were measured by SA.
The propagation directions of W are <1, -1,0> AlN and <1,1,0
It was determined for two cases of> AlN. The result <
For 1, -1,0> AlN, Table 1 shows <1,1,
0> AlN is summarized in Table 11.

The result of SAW propagation velocity Vs is <1,-
For 1,0> AlN, <1,1,0> A in FIG.
InN is plotted in FIG. The results of the temperature characteristic τ _v are shown in FIG. 3 for 1, -1, 0> AlN as <1,
For 1,0> AlN, it is plotted in FIG. <1,
In the case of −1,0> AlN, τ _v is represented by the equation (4), and τ _v = −57.05 (kH) ^−0.4618 +7.109 (4) The deviation from this line is 0.9 ppm / maximum. C, and shows that τ _v is controlled by the equation (4).

In the case of <1,1,0> AlN, τ _v is represented by the equation (5), and τ _v = −48.44 (kH) ^−0.5003 +0.4210 (5) The deviation from this straight line is the maximum. However, it was 0.7 ppm / ° C., indicating that τ _v was controlled by the equation (5).

[0030]

COMPARATIVE EXAMPLE 3 The same apparatus as that used in Example 3 is used and the CVD method is performed under the following conditions. In addition, No. 3 of Comparative Example 3. 1, No. 2 as a nitrogen source.
99 vol% ammonia gas, 99.
9995 vol% nitrogen gas was used. Comparative example 3 No.
In the case of 3, 99.9995 vol% ammonia gas,
99.99995 vol% nitrogen gas was used. Further, in Comparative Example 3, No. The pre-reaction pressure in 3 is 1 × 10 ^-2 torr as in Example 3.

Single crystal substrate Sapphire C surface AlCl ₃ retention temperature 150 ° C. AlCl ₃ supply gas amount 25 sccm Ammonia supply amount 5 sccm Carrier gas amount 470 sccm Reaction pressure 100 torr Pre-reaction pressure 1 × 10 ⁻¹ torr Using the above conditions, the substrate temperature and AlN by changing the reaction time
The thin film was synthesized, and the film thickness and crystallinity of the obtained AlN thin film and the oxygen concentration in the AlN thin film are summarized in Table 12.

These films were subjected to the same polishing treatment as in Example 3.
The SAW fill is performed by the IDT similar to that of the third embodiment.
The characteristics of the_vI asked. The result is <1,-
Table 1 shows the case of 1,0> AlN and <1,1,0> Al.
The case of N is summarized in Table 14, and is shown in FIG. 13 and FIG. 14, respectively.
Plotted. In the case of <1, -1, 0> AlN, Table 1
3, as shown in FIG. 13, obtained by Comparative Example 3.
On the deposited AlN thin film_vFrom the curve shown by equation (4)
Deviation is large, exceeding at least 1.00 ppm / ℃
ing. From this result, the oxygen concentration in the AlN thin film is 1% or more.
Below and the crystallinity of the AlN thin film is 0.50 ° or less.
If and are not satisfied, kH and τ shown in equation (4)_v
It is shown that it does not necessarily fit the correlation of. <
Even in the case of 1,1,0> AlN, Table 14 and FIG.
As is clearer, the AlN thin film obtained in Comparative Example 3
Τ on the membrane_vIs a large deviation from the curve shown in equation (5)
At least 1.00 ppm / ° C. this
From the results, the oxygen concentration of the AlN thin film is 1% or less, and
Does not satisfy that the crystallinity of the thin film is less than 0.50 °
In this case, kH and τ shown in equation (5) _vBe sure to correlate
It is shown that it does not fit.

[0033]

[Table 1]

[0034]

[Table 2]

[0035]

[Table 3]

[0036]

[Table 4]

[0037]

[Table 5]

[0038]

[Table 6]

[0039]

[Table 7]

[0040]

[Table 8]

[0041]

[Table 9]

[0042]

[Table 10]

[0043]

[Table 11]

[0044]

[Table 12]

[0045]

[Table 13]

[0046]

[Table 14]

[0047]

The AlN on the sapphire substrate according to the present invention
By using a thin film, it is possible to obtain a SAW device in which the temperature characteristics of the propagation velocity of surface acoustic waves on the thin film are accurately controlled.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between the SAW velocity temperature characteristic τ _v and kH on the sapphire R-face AlN thin film of Example 1.

FIG. 2 is a graph showing the relationship between the SAW velocity temperature characteristic τ _v and kH on the AlN thin film on the sapphire C surface of Example 2.

FIG. 3 is a graph showing the relationship between the temperature characteristic τ _v of the SAW velocity in the <1, −1, 0> AlN direction and the kH on the AlN thin film on the sapphire A surface of Example 3.

FIG. 4 is a graph showing a relationship between temperature characteristics τ _v and kH of a SAW velocity in a <1,1,0> AlN direction on an AlN thin film on a sapphire A surface in Example 3.

FIG. 5 is a graph showing a relationship between SAW velocity on an AlN thin film on a sapphire R surface and kH.

FIG. 6 SAW velocity and k on AlN thin film on sapphire C
It is a graph which shows the relationship of H.

FIG. 7: Crystallinity and Al of AlN thin film on R surface of sapphire
It is a graph which shows the relationship of N film thickness.

FIG. 8 is a schematic view of a synthesizer used in the present invention.

9 is a graph showing the relationship between the SAW velocity temperature characteristic τ _v and kH on the sapphire R-face AlN thin film of Comparative Example 1. FIG.

FIG. 10 is a graph showing the relationship between the SAW velocity temperature characteristic τ _v and kH on the sapphire C-face AlN thin film of Comparative Example 2.

FIG. 11 is a graph showing the relationship between the temperature characteristic τ _v of the SAW velocity in the <1, −1, 0> AlN direction and the kH on the AlN thin film on the sapphire A surface of Example 3.

FIG. 12 is a graph showing a relationship between temperature characteristics τ _v and kH of the SAW velocity in the <1,1,0> AlN direction on the AlN thin film on the sapphire A surface of Example 3.

FIG. 13 is a graph showing the relationship between the temperature characteristic τ _v of the SAW velocity in the <1, −1, 0> AlN direction and the kH on the AlN thin film on the sapphire A surface of Comparative Example 3.

FIG. 14 is a graph showing the relationship between the temperature characteristics τ _v and kH of the SAW velocity in the <1,1,0> AlN direction on the AlN thin film on the sapphire A surface of Comparative Example 3.

[Explanation of symbols]

1. AlCl ₃ vaporization container 2. Ammonia gas inlet 3. Susceptor 4,5. Diluent gas inlet 6. High frequency induction heating coil 7. Cooling water 8. Heater 9. Sapphire substrate 10. exhaust port

Claims (4)

[Claims] 1. A sapphire substrate and A on the sapphire substrate
A piezoelectric thin film, comprising: a laminated substrate comprising an 1N thin film, wherein the oxygen concentration contained in the thin film is 1% or less, and the crystallinity of the thin film is 0.50 ° or less. 2. The sapphire substrate is a sapphire (012) substrate, and the surface of the AlN thin film parallel to the substrate is (110).
The piezoelectric thin film according to claim 1, which is a surface. 3. The sapphire substrate is a sapphire (001) substrate, and the surface of the AlN thin film parallel to the sapphire substrate is (001).
The piezoelectric thin film according to claim 1, which is a surface. 4. The sapphire substrate is a sapphire (110) substrate, and the surface of the AlN thin film parallel to the substrate is (001).
The piezoelectric thin film according to claim 1, which is a surface.