Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reexamination Certificate
1999-11-01
2002-01-29
Budd, Mark O. (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
Reexamination Certificate
active
06342748
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a surface acoustic wave device, and a substrate for use in a surface acoustic wave device, and more particularly to a substrate comprising a sapphire single crystal substrate consisting of &agr;-Al
2
O
3
, a buffer layer formed on said sapphire single crystal substrate by metal organic chemical vapor deposition (MOCVD), and an aluminum nitride single crystal layer formed on the buffer layer also by MOCVD. The present invention also relates to a method of manufacturing the above mentioned substrate for use in a surface acoustic wave device.
2. Related Art Statement
Heretofore, substrates of surface acoustic wave devices have been generally made of quartz, LiNbO
3
, LiTaO
3
, Li
2
B
4
O
7
and others. These substrate materials have been utilized because they have an excellent electromechanical coupling coefficient K
2
and a temperature coefficient of delay time (TCD) which are important transmission properties of the surface acoustic wave device. Due to the wide application of surface acoustic wave devices, there has been a need for a surface acoustic wave device having a very high operation frequency. However, a propagating velocity of the surface acoustic wave in the above mentioned substrate materials is about 3000-5000 m/sec, and in order to realize a surface acoustic wave device having an operation frequency in the order of GHz, it is required to provide a substrate having a propagating velocity for the surface acoustic wave not lower than 5000-6000 m/sec.
As stated above, in order to realize a surface acoustic wave device having a very high operation frequency, it is necessary to use a substrate having a high propagating velocity. For this purpose, it has been proposed to use a substrate including aluminum nitride (AlN). An electromechanical coupling coefficient K
2
of the aluminum nitride is about 0.8% which is higher than that of quartz by about five times. A temperature coefficient of delay time TCD of the aluminum nitride is not larger than 20 ppm/° C. However, aluminum nitride has a very high melting point, and therefore it is difficult to obtain a large bulk aluminum nitride single crystal. Due to this fact, in general, an aluminum nitride single crystal layer is formed on a sapphire single crystal substrate made of &agr;-Al
2
O
3
. Such a sapphire single crystal substrate has been used because it is easily available and its lattice constant does not differ largely from that of the aluminum nitride.
As stated above, it has been proposed to use a substrate in which an aluminum nitride layer is deposited on a sapphire substrate. The inventors of the instant application have conducted various experiments, in which after performing an initial nitriding treatment by exposing an R(
1
-
102
) surface of a sapphire substrate to an atmosphere of ammonia to form a very thin aluminum nitride single crystal film, an aluminum nitride single crystal layer is deposited on the aluminum nitride single crystal film by metal organic chemical vapor deposition (MOCVD). For instance, a sapphire single crystal substrate was placed in a CVD apparatus, and then trimethylaluminum (TMA) and ammonia (NH
3
) were introduced into the CVD apparatus to deposit an aluminum nitride single crystal layer on the sapphire substrate. Therefore, it has been confirmed that the aluminum nitride single crystal layer formed by MOCVD has a good electromechanical coupling coefficient K
2
.
In the experiments, use was made of a very small sapphire substrate of a square shape having a side of 5 mm. In order to manufacture substrates for surface acoustic wave devices on a practically acceptable large scale, it is necessary to use a sapphire wafer not less than two inches (50.8 mm). To this end, it is necessary to establish a method, in which an aluminum nitride single crystal layer is formed on a surface of the two inch sapphire single crystal wafer, then a desired electrode pattern is formed on the aluminum nitride layer, and finally the sapphire single crystal wafer is divided into chips by slicing.
In one of the experiments conducted by the inventors, use was made of a two inch sapphire single crystal wafer having a thickness of 300-500 &mgr;m. A first aluminum nitride single crystal film was formed on the sapphire wafer by means of the above mentioned initial nitriding treatment. A second aluminum nitride single crystal layer having a thickness not less than 1 &mgr;m was formed on the first aluminum nitride single crystal film by MOCVD. Finally, the sapphire single crystal wafer was divided into a number of surface acoustic wave devices. It was then taken experimentally confirmed that a number of cracks were formed in the aluminum nitride single crystal layer with a mutual spacing of about 1 mm. Surface acoustic wave devices were then manufactured using the cracked substrates. It was then experimentally confirmed that the propagation loss of the surface acoustic wave devices was very large and the property of the device was deteriorated. Therefore, it was experimentally confirmed that practically usable surface acoustic wave devices could not be manufactured using the above mentioned sapphire single crystal substrate.
In a field of manufacturing light emitting semiconductor devices (LED), it has been known to use a substrate including a sapphire single crystal substrate and a III-V compound single crystal layer such as GaN and AlN single crystal layer formed on the sapphire single crystal substrate. In order to prevent cracks from being formed in the III-V compound single crystal layer, it has been proposed to form a thin buffer layer on the sapphire single crystal substrate prior to the formation of the III-V compound single crystal layer. The inventors have introduced this method in the formation of a substrate for surface acoustic wave devices. That is to say, a very thin buffer layer consisting of an aluminum nitride single crystal film having a thickness of about 5-50 nm was first formed on a sapphire single crystal substrate, followed by the formation of a thick aluminum nitride layer on the buffer layer. In this case, during the formation of the buffer layer consisting of the relatively thin first aluminum nitride single crystal layer, a surface temperature of the sapphire substrate was kept to a lower temperature such as 300-450° C. The substrate was then heated to 900-1100° C. during the formation of the relatively thick second aluminum nitride single crystal layer. Consequently, the aluminum nitride layer formed in the manner was free from cracks. However, the electromechanical coupling coefficient K
2
become substantially zero and the aluminum nitride single crystal layer lost its piezoelectric property. It is apparent that such a material could never be suitable for use as the substrate for a surface acoustic wave device. In the light emitting semiconductor device, the loss of piezoelectric property is irrelevant, but in a surface acoustic wave device, the piezoelectric property is indispensable. An explanation for the disappearance of the piezoelectric property has yet to be clarified. However, upon observing the microstructure of the surface of aluminum nitride single crystal layer, it was confirmed that many twins were formed in the surface.
The inventors have proposed, in U.S. patent application Ser. No. 08/936,614 (corresponding to EP 0833 446 A2), a substrate for a surface acoustic wave device, in which the formation of cracks can be effectively prevented through the use of a sapphire single crystal wafer having a size not smaller than two inches.
This known substrate comprises a sapphire single crystal substrate made of &agr;-Al
2
O
3
and an aluminum nitride single crystal layer formed on a surface of said sapphire single crystal substrate. The surface of the sapphire single crystal substrate is formed by an off-angled surface which is obtained by rotating an R(
1
-
102
) surface about a [
11
-
20
] axis by a given off-angle. The aluminum nitride single crystal layer is formed by a buffer lay
Nakamura Yukinori
Shibata Tomohiko
Budd Mark O.
Burr & Brown
NGK Insulators Ltd.
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