JP2002300001A - Substrate for surface acoustic wave device, the surface acoustic wave device employing it, and its manufacturing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a substrate for a surface acoustic wave device made of a lithium tantalate single crystal with excellent uniformity and to provide the surface acoustic wave device employing it and its manufacturing method. SOLUTION: This invention provides the substrate for a surface acoustic wave device that is made of a lithium tantalate single crystal the abnormal optical refractive index of which is 2.1767-2.1795 and the double refraction value of which is 0.0004-0.0032, which are respectively measured by using a He-Ne laser at a temperature of 20-30 deg.C.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a surface acoustic wave device substrate made of lithium tantalate single crystal, a surface acoustic wave device using the same, and a method of manufacturing the same.

[0002]

2. Description of the Related Art Lithium tantalate single crystal (hereinafter referred to as LT)
Is widely used as a substrate material for electronic components and optical functional devices, and in recent years, has been particularly suitably used as a substrate for surface acoustic wave devices used in communication equipment and information equipment typified by mobile phones and the like. I have.

A wafer made of a lithium tantalate single crystal is manufactured in substantially the same manner as a manufacturing process of a semiconductor single crystal wafer represented by a silicon single crystal. That is, after growing the crystal by the melt pulling method, the outer diameter of the crystal is rounded, cut to obtain the necessary crystal orientation of the wafer, and sequentially subjected to the steps of rough polishing, chamfering, and mirror polishing. Produced (for example, Showa 5
(See Toshiba Review, Vol. 33, No. 9, pp. 761-763).

[0004] On the other hand, the surface acoustic wave device using an LT wafer has a finer structure as the frequency becomes higher, and the production yield is reduced. Further, as the cost of devices decreases, it is desired to manufacture devices with a high yield using large-diameter wafers. In particular, the uniformity of the surface acoustic wave velocity of the wafer sensitively affects the yield of the surface acoustic wave device, but conventionally the surface acoustic wave is measured by measuring the Curie temperature, which is correlated with the surface acoustic wave velocity, by thermal analysis or the like. It is common to control the uniformity of speed (Japanese Patent Application Laid-Open No. Sho 61
(See JP-A-127219)

[0005]

The surface wave velocity uniformity in the conventional wafer surface has a correlation with the crystal composition according to, for example, Japanese Patent Application Laid-Open No. 61-127219, but the precision of quantitative analysis of the composition is low. It has been proposed to control by the Curie temperature correlated with the composition.

However, thermal analysis used for Curie temperature measurement cannot be said to be highly accurate because a measurement error is expected to be about ± 1.5 ° C. In particular, the frequency of surface acoustic wave filters used in mobile phones in recent years has increased to the giga-Hz band, and the minimum electrode size of the device has been reduced to one-fifteenth and has reached the order of submicron.
Variations in surface acoustic wave velocities are directly linked to yield reduction, and the management of surface acoustic wave velocities has become even more important. Further, in the thermal analysis, it takes several hours for the measurement, so that the efficiency of the wafer sorting is low.

Accordingly, the present invention has been proposed in view of the above-mentioned problems, and is provided with a substrate for a surface acoustic wave device made of lithium tantalate single crystal having excellent uniformity, a surface acoustic wave device using the same, and a surface acoustic wave device using the same. It is intended to provide a manufacturing method.

[0008]

In order to solve the above problems, a substrate for a surface acoustic wave device according to the present invention has a temperature of 20-30.
It consists of a lithium tantalate single crystal having an extraordinary refractive index of 2.167 to 2.1795 or a birefringence value of 0.0004 to 0.0032, measured using a He-Ne laser at ° C.

In addition, the fluctuation of the extraordinary light refractive index or the birefringence value is particularly ± 0.0003.

Further, the main surface of the substrate rotates by 33 ° to 46 ° Y
It is a cut surface.

The surface acoustic wave device according to the present invention is characterized in that a surface acoustic wave excitation electrode is formed on a surface acoustic wave device substrate.

The method of manufacturing a surface acoustic wave device according to the present invention is characterized in that a substrate made of a single crystal of lithium tantalate is subjected to an extraordinary light refractive index or a birefringence value using a He—Ne laser at a temperature of 20 to 30 ° C. And selecting a substrate of lithium tantalate single crystal having an extraordinary refractive index of 2.167 to 2.1795 or a birefringence of 0.0004 to 0.0032, and a selected substrate. Forming a surface acoustic wave excitation electrode thereon.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A method for manufacturing a lithium tantalate single crystal wafer suitably used as, for example, a piezoelectric substrate of a surface acoustic wave filter of a cellular phone, and in particular, a method for managing uniform surface acoustic waves will be described below. This will be described in detail with reference to the drawings shown in FIG.

Since the velocity of a surface acoustic wave, which is closely related to the frequency of a surface acoustic wave device, is closely related to the refractive index, which is an optical property, it is widely used for mobile phones used in high frequency bands. The orientation of the wafer substrate was examined in detail.

As a result, by selecting the refractive index or the birefringence value of the extraordinary ray so as to be within a specific range, it becomes possible to manufacture a surface acoustic wave device having a constant characteristic and a high yield. all right.

More specifically, a wafer whose one side is mirror-polished is manufactured from a pulled and grown LT single crystal through a wafer processing process. Thereafter, the extraordinary light refractive index within the wafer and between the wafers is measured using a He-Ne laser (wavelength 632.8 nm) in an environment of a temperature of 20 to 30 ° C., and the extraordinary light refractive index is 2.167 to 2 .1795, and its in-plane variation is ±
The wafer was selected so as to be 0.0003 and used in the device process, or under the same measurement conditions, the birefringence value of the lithium tantalate single crystal wafer was 0.0004 to 0.0
Only the wafers whose fluctuations are selected within the range of ± 0.0003 in 032 are used in the device process. As a result, the surface acoustic wave device to be manufactured can produce a high-yield surface acoustic wave device having a small frequency variation and constant characteristics.

The device process is manufactured using expensive equipment as the electrode structure of the surface acoustic wave excitation electrode formed on the substrate becomes finer, and equipment investment accounts for the majority of the cost of the surface acoustic wave device. Therefore, the improvement of the yield in the device process was the most important.

As described above, the method of manufacturing a surface acoustic wave device according to the present invention is characterized in that a substrate made of a single crystal of lithium tantalate is treated with a He—Ne laser at a temperature of 20 to 30 ° C. using an extraordinary light refractive index or multiple light. Measuring the refraction value;
A step of selecting a substrate of lithium tantalate single crystal having an extraordinary refractive index of 2.167 to 2.1795 or a birefringence of 0.0004 to 0.0032, and a surface acoustic wave excitation electrode on the selected substrate And forming a.

In particular, based on the right-handed rectangular coordinate system shown in FIG. 4, a lithium tantalate single crystal wafer having a rotation angle θ of 33 ° to 46 ° is widely used in a high-frequency surface acoustic wave device, and has a submicron fine electrode. Due to the structure, securing the yield is an important issue, and the effect is enormous.

In the drawing, 9 indicates a Y-cut plane, 8 indicates a θ-rotation Y-cut plane, and θ indicates a rotation angle (°). FIG. 1 shows the rate of change of the surface acoustic wave velocity with respect to the extraordinary light refractive index and the birefringence value of the wafer. The characteristics of the 33 ° rotated Y cut substrate are 2a and 2b, the characteristics of the 36 ° rotated Y cut substrate are 3a and 3b, and the characteristics of the 42 ° rotated Y cut substrate are 4
The characteristics of the Y-cut substrates a and 4b and 46 ° rotation are 5a and 5b.

The grounds for limiting the values of the extraordinary light refractive index and the birefringence value are as follows. From the relationship between the extraordinary light refractive index and the birefringence value and the surface acoustic wave velocity variation rate of the characteristic shown in FIG. The lower limit was set to 0.15%, which is large and lowers the device yield.

The refractive index was measured by the prism coupling method shown in FIG. The extraordinary refractive index ne of the wafer in the optical path 1 that reflects the laser beam at the critical angle of reflection α at the contact surface with the prism np having the refractive index np in contact with the surface of the wafer S made of lithium tantalate single crystal
And the birefringence value Δn can be obtained by the following equation using np and α.

Ne = np sin θ (provided that the optical axis of the LT coincides with the polarization direction of the laser, whereas the refractive index of ordinary light whose polarization direction is rotated by 90 ° is no.
Δn = | no-ne | The surface acoustic wave velocity change rate is obtained from the measurement system shown in FIG. That is, the electrical characteristics obtained by applying a high-frequency voltage to a comb-like electrode (IDT electrode) 7, which is a surface acoustic wave excitation electrode, provided on a wafer made of a single crystal of lithium tantalate are analyzed by a network analyzer M.
, And the surface acoustic wave velocity is obtained from the product of the resonance frequency fr and the electrode period 6. The rate of change in the surface acoustic wave velocity is represented by a ratio of the amount of change in the surface acoustic wave velocity within the wafer surface or between the wafers to the reference surface acoustic wave velocity.

[0024]

EXAMPLES Next, more specific examples of the present invention will be described. Example 1 A method for producing a 42 ° rotated Y-cut lithium tantalate single crystal wafer will be described. A seed crystal having an orientation perpendicular to the wafer cut was brought into contact with an LT raw material melt having a harmony composition and gradually cooled and pulled up while rotating to obtain a single crystal having a diameter of about 100 mm. Both ends of the crystal were cut along a 42 ° rotated Y-cut plane using an inner peripheral blade cutter and an X-ray diffractometer.

Next, a silver paste is applied to both cut surfaces, and 1.5 to 1.5 ° C.
A single domaining process was performed while applying a voltage of 5 V / cm. The crystal ingot subjected to the single-domain treatment was subjected to cylindrical grinding, slicing, rough polishing, chamfering, and mirror polishing to produce a wafer having one surface mirror-polished.

[0026]

[Table 1]

Table 1 shows the wafer having different in-plane extraordinary light refractive indices and birefringence values, the rate of change of the surface acoustic wave velocity, and the frequency ± 0.
The result which compared the yield within 15% is shown. It was found that the rate of change of the surface acoustic wave velocity increased and decreased in accordance with the amount of change in the extraordinary light refractive index and the birefringence, and the frequency yield also increased and decreased accordingly. FIG. 1 is a graph showing the relationship including the data on the Y-cut substrate rotated by 33 ° to 46 °, and FIG. 1 shows that the relationship is proportional.

In particular, when the rate of change of the surface acoustic wave velocity exceeds 0.15% in the device manufacturing process, the yield of the surface acoustic wave device is greatly reduced. It was measured using a He-Ne laser (wavelength 632.8 nm) in an environment, and 2.176
7 to 2.1795 and its in-plane variation is ± 0.0003. Or it turned out that it can be achieved by using a wafer selected under the same measurement conditions as the lithium tantalate single crystal wafer, in which the birefringence value of the wafer is 0.0004 to 0.0032 and the variation is in the range of ± 0.0003.

More specifically, FIG. 1 shows that the rate of change of the surface acoustic wave velocity is within 0.15% when the extraordinary light refractive index and the birefringence value are 33.degree.
2.773 and 0.0004 to 0.001; for the 36 ° rotation Y cut, 2.1772 to 2.1778 and 0.0
009 to 0.0015, 2.1 for 42 degree rotation Y cut
782-2.1788 and 0.0019-0.002
2.1789 to 2.179 for 5, 46 ° rotation Y-cut
5 and 0.0026 to 0.0032 are effective ranges.

The measuring method will be described. The speed of the surface acoustic wave was measured by fabricating a comb-shaped resonator schematically shown in FIG. 3 by a device process, and using a prober (40A manufactured by GGB).
-GSG-250-DB) and a network analyzer (8753D, manufactured by Hewlett-Packard Company), and the resonance frequency fr was measured and calculated from the product of fr and the wavelength λ. The extraordinary light refractive index ne and the birefringence value Δn were calculated by the prism coupling method shown in FIG. 2 using the refractive index np of the prism and the critical incident angle θ of total reflection of the laser from the following equation. In particular, the measured values were calibrated using LaSF35 high refractive index glass (n632.8 = 2.01493) manufactured by SCHOTT.

As a result, as shown in Table 1 and FIG. 1, there is a correlation between the surface acoustic wave velocity, the extraordinary light refractive index and the birefringence value, and the extraordinary light refractive index is 2.767 and up to 2.1795. Fluctuation ± 0.0003 or birefringence 0.000
By selecting the variation of the surface acoustic wave velocity within the range of ± 0.0003 in the range of 4 to 0.0032, the variation rate of the surface acoustic wave velocity can be selected in advance within 0.15% which is the limit of the frequency yield reduction of the surface acoustic wave device. It becomes possible. This is widely used as a high frequency surface acoustic wave device in the giga-Hz band.
It has been found to be particularly effective in the rotation Y-cut azimuth of between .degree.

As a result, a wafer for a surface acoustic wave device can be easily and accurately selected in a short time and with high accuracy as compared with a conventional method of selecting a wafer based on the Curie temperature, which has required a long time for measurement and has a problem in measurement accuracy.

[0033]

As described above, according to the present invention, the temperature is 20 to 30.
Surface acoustic wave composed of a lithium tantalate single crystal having an extraordinary refractive index of 2.167 to 2.7995 or a birefringence of 0.0004 to 0.0032, measured using a He-Ne laser at ℃. An apparatus substrate is used. Further, the fluctuation of the extraordinary light refractive index or the birefringence value is ± 0.0
The surface acoustic wave device substrate 003 is used. further,
A substrate for a surface acoustic wave device in which the main surface of the substrate is a Y-cut surface rotated by 33 ° to 46 ° is used. Furthermore, a step of measuring an extraordinary light refractive index or a birefringence value using a He—Ne laser at a temperature of 20 to 30 ° C. on the substrate made of a lithium tantalate single crystal;
1767 to 2.1795, or the birefringence value is 0.0
The production including the step of selecting a substrate of 004 to 0.0032 of lithium tantalate single crystal and the step of forming a surface acoustic wave excitation electrode on the selected substrate is performed.

As a result, wafer sorting can be performed more easily and in a shorter time than in the conventional method. Furthermore, the surface acoustic wave device manufactured using the selected wafer has good characteristics,
Since a uniform substrate can be manufactured at a high yield, the manufacturing cost can be reduced, and in addition, the number of defects can be reduced, so that waste can be reduced and the effect of environmental protection can be expected.

[Brief description of the drawings]

1A is a graph showing a relationship between a surface acoustic wave velocity change rate and an extraordinary light refractive index, and FIG. 1B is a graph showing a relationship between a surface acoustic wave velocity change rate and a birefringence value.

FIG. 2 is a cross-sectional view schematically illustrating a prism coupling method.

FIG. 3 is a perspective view schematically illustrating a measurement system for measuring the velocity of a surface acoustic wave.

FIG. 4 is a coordinate diagram illustrating a wafer cut direction.

[Explanation of symbols]

 1: Optical path of laser light 2a, 2b: Characteristics of 33 ° rotated Y-cut surface 3a, 3b: Characteristics of 36 ° rotated Y-cut surface 4a, 4b: Characteristics of 42 ° rotated Y-cut surface 5a, 5b: 46 ° rotated Y-cut surface Characteristics of cut surface 6: Electrode period (wavelength) 7: Electrode S: Lithium tantalate single crystal wafer np: Prism with refractive index np α: Critical reflection angle of laser M: Network analyzer 8: θ ° rotation Y cut surface 9: Y cut plane θ: Rotation angle of Y cut plane around X axis

Claims (5)

[Claims] 1. At a temperature of 20 to 30 ° C., He-Ne
The extraordinary light refractive index measured using a laser is 2.176.
7 to 2.1795, or a birefringence value of 0.0004 to
0.0032, a substrate for a surface acoustic wave device comprising a lithium tantalate single crystal. 2. The method according to claim 1, wherein the fluctuation of the extraordinary light refractive index or the birefringence value is ± 0.0003.
4. The substrate for a surface acoustic wave device according to 1. 3. The surface acoustic wave device substrate according to claim 1, wherein the main surface of the substrate is a Y-cut surface rotated by 33 ° to 46 °. 4. A surface acoustic wave device having a surface acoustic wave device excitation electrode formed on the surface acoustic wave device substrate according to claim 1. 5. A step of measuring an extraordinary light refractive index or a birefringence value of a substrate made of a single crystal of lithium tantalate at a temperature of 20 to 30 ° C. using a He—Ne laser; 2.767 to 2.1795,
Alternatively, the method includes a step of selecting a substrate of lithium tantalate single crystal having a birefringence of 0.0004 to 0.0032, and a step of forming a surface acoustic wave excitation electrode on the selected substrate. A method for manufacturing a surface acoustic wave device.