Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reexamination Certificate
2000-08-14
2003-06-17
Budd, Mark O. (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
C310S360000
Reexamination Certificate
active
06580196
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to piezoelectric single crystal wafer, particularly to piezoelectric single crystal wafer used for fabrication of surface acoustic wave or leaky surface acoustic wave device.
2. Description of the Related Art
Surface acoustic wave device or leaky surface acoustic wave device is a circuit element that convert electric signal respectively to surface acoustic wave or leaky surface acoustic wave to conduct signal processing. They have been conventionally used for a filter, a resonator, a delay line or the like.
Such a surface acoustic wave device or leaky surface acoustic wave device is generally produced by forming electrodes for transmit-receive of surface acoustic wave or leaky surface acoustic wave on a piezoelectric single crystal wafer and cutting it in a form of a chip.
The above-mentioned piezoelectric single crystal wafer itself is generally produced as follows. First, piezoelectric single crystal having a piezoelectricity is grown by an adequate method for growing a single crystal. For example, a crystal ingot made of lithium tantalate is grown by Czochralski method (CZ method). Then, the single crystal ingot is subjected to a cylindrical grinding so that a section vertical to an axis thereof may have a round shape. The resultant cylindrical single crystal ingot is sliced to provide a wafer having a constant crystal surface orientation. Then one side or both sides of the wafer produced thereby was subjected to lapping, mirror polishing in sequence. Thereby, the piezoelectric single crystal wafer can be produced.
On the surface subjected to a mirror polishing of the piezoelectric single crystal wafer thus produced, an electrode mainly made of Al is formed in a regular direction (hereunder the surface on which the electrode is formed is occasionally called “an electrode forming surface” or “a front surface”) and cut in a shape of a chip. Thereby, the surface acoustic wave or leaky surface acoustic wave device can be fabricated.
Performance of the surface acoustic wave or leaky surface acoustic wave device produced as above depends on various factors such as material to be used, crystal orientation, design of electrodes, conditions for fabrication or the like. The factor for performance of the surface acoustic wave or leaky surface acoustic wave device that is particularly to be noted is a surface work damage layer existing at a depth of tens &mgr;m or less from the surface of the wafer.
The relation between surface acoustic wave and a surface work damage layer, and a method of polishing a single crystal wafer for a surface acoustic wave considering a surface work damage layer have been reported as follows.
Kimura et al. (Kimura et al., Shingaku Gihou, US 75-56, 17-23, 1975) reported that a relation between the surface work damage layer and Q factor of surface acoustic wave was investigated using quartz, and it was found that when a significant surface work damage layer such as micro crack was present to a depth of a half of wavelength, it was confirmed as difference of the above-mentioned Q factor of surface acoustic wave.
Kimura et al. (T. Kimura et al., J. Appl. Phys., 50 (7), 4767-4772, (1979)) reported that a relation between a surface work damage layer and propagation loss was investigated and analyzed using quartz, and reported a relation between a depth and an amount of micro cracks and propagation loss.
As for a method of polishing a lithium tantalate single crystal wafer for surface acoustic wave, there have been reported as a similar method used for processing of a silicon single crystal wafer, a polishing method wherein SiO
2
colloidal polishing agent is used after a lapping process (Nihon Gakujutushinkoukai Danseihyoumenhasoshigijutu, 150th committee, acoustic wave element handbook, 296-298 (1991)). It has been reported that it is important in a polishing step to obtain a state of a mirror surface and completely remove a mechanical damage layer generated in a lapping step, and that polishing with a stock removal of 10 &mgr;m or less is sufficient, since abrasive grains having a diameter of 15 &mgr;m or less is generally used for lapping, and thus a mechanical damage layer is generally formed at a depth of 10 &mgr;m or less.
As described above, there have been previously reported a relation between a surface acoustic wave and a surface work damage layer, or a method of polishing a single crystal wafer for surface acoustic wave considering a surface work damage layer. However, there have not been reported a relation between leaky surface acoustic wave and a surface work damage layer, or a method of polishing a single crystal wafer for leaky surface acoustic wave.
There is a significantly increased demand for surface acoustic wave or leaky surface acoustic wave device as a filter for mobile communication that has been diffused explosively in these years. There is also a demand for improvement in quality thereof, especially a demand for improvement in uniformity of surface acoustic wave velocity or leaky surface acoustic wave velocity on the surface of the piezoelectric single crystal wafer used for a device, and the uniformity among wafers.
The filter for mobile communication is required to be improved not only in quality, but also in cost performance with increase of a demand. The yield in production of the filter have much influence on production cost.
The relation between surface acoustic wave velocity or uniformity thereof and composition, cutting method or propagation direction has been reported (Nihon Gakujutushinkoukai Danseihyoumenhasoshigijutu, 150th committee, surface acoustic wave handbook, 289-302, (1991), and Aikawa et al., Denshijouhoutsushingakkai shuukitaikai, 19 (1994)).
However, there comes to be a problem of a deviation of surface acoustic wave velocity or leaky surface acoustic wave velocity on the surface of the piezoelectric single crystal wafer, and among the wafers, which is considered unable to be explained only with composition, a cutting method, or propagation direction.
There are various factors for failure in production of the filter. Many of them are caused by fine contaminations (particles) adhered on the surface of the wafer or due to breakage of the wafer. The recent filter for mobile communication is often used in a range of frequency of 0.1 to 3 GHz, especially 1 to 2.5 GHz. Accordingly, an electrode having a line width of 1 &mgr;m or less has been requested. Therefore, the fine contaminations having a size of several &mgr;m that did not matter previously are getting a big problem. For example, if an electrode is formed on the wafer on which contaminations having a size of several &mgr;m are adhered, the electrode cannot be formed at a part on which the contaminations are adhered, and thus desired propagation properties cannot be achieved. As a result, yield is lowered, and production cost is increased.
The present invention has been accomplished to solve the above-mentioned problems, and the first object of the present invention is to provide a piezoelectric single crystal wafer for surface acoustic wave or leaky surface acoustic wave device wherein a deviation of surface acoustic wave velocity or leaky surface acoustic wave velocity is small, namely the velocity uniformity is excellent. The second object of the present invention is to provide a piezoelectric single crystal wafer wherein fine contaminations adhered on the surface on which an electrode is formed and breakage of the wafer are significantly reduced, and to provide a piezoelectric single crystal wafer that can provide a filter or the like having an excellent property in high production yield.
The inventors noted and studied from the following two reasons a relation between a deviation of surface acoustic wave velocity or leaky surface acoustic wave velocity and a surface work damage layer.
One of the reasons is that although a relation between Q factor or propagation loss of surface acoustic wave and a surface work damage layer has been reported as described above, a relation betwe
Kuwabara Yoshinori
Ryuo Toshihiko
Shiono Yoshiyuki
Budd Mark O.
Oliff and Berrides, PLC
Shin-Etsu Chemical Co. , Ltd.
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