Active solid-state devices (e.g. – transistors – solid-state diode – Non-single crystal – or recrystallized – semiconductor... – Field effect device in non-single crystal – or...
Reexamination Certificate
2003-02-23
2004-09-21
Smith, Matthew (Department: 2825)
Active solid-state devices (e.g., transistors, solid-state diode
Non-single crystal, or recrystallized, semiconductor...
Field effect device in non-single crystal, or...
C310S31300R
Reexamination Certificate
active
06794683
ABSTRACT:
TECHNICAL FIELD
The present invention relates to diamond substrates having piezoelectric thin films, and to methods of their manufacture, utilized for surface-acoustic wave devices and optical-device materials employable in high-frequency bands.
BACKGROUND ART
Surface-acoustic wave devices exploiting surface-acoustic waves, (abbreviated SAWs hereinafter), which propagate over solid-body surfaces, are miniature, lightweight and highly temperature-stable, and have various features such as being superior in phase characteristics. Accompanying multi-channel/high-frequency transformations in mobile communications and satellite communications fields in recent years have been calls for the development of surface-acoustic wave devices employable in high-frequency bands; methods that employ materials whose surface-acoustic-wave propagation speed is fast, or methods that narrow electrode pitch have been considered. Among these, electrode pitch, being dependent on semiconductor device manufacturing technology, has limitations
Meanwhile, as a material whose surface-acoustic-wave propagation speed is fast, diamond is the fastest. In this respect, a laminated structure as is in Japanese Pub. Pat. App. No. H07-321596, in which a LiNbO
3
layer is arranged on top of diamond, has been proposed. According to this literature, c-axis oriented LiNbO
3
is formed onto a diamond (111) surface by an RF-magnetron sputtering technique; but with the a value expressing the c-axis orientation being below 5°, (under 4°, furthermore), the c-axis orientation cannot be said to be all that satisfactory. Likewise, Japanese Pub. Pat. App. No. H05-78200 discloses a method of synthesizing a Li(Nb
x
Ta
1−x
)O
3
(0≦x≦1) thin film onto a sapphire substrate. Although the obtaining of a single-phase Li(Nb
x
Ta
1−x
)O
3
(0≦x≦1) thin film is disclosed in this literature, nothing is disclosed as to the quality of the thin film produced. Furthermore, in Japanese Pub. Pat. App. No. H05-319993 a surface-acoustic wave device in which a LiNbO
3
layer is formed onto sapphire is proposed. Nonetheless, although the LiNbO
3
layer's orientation is satisfactory, with propagation speed being some 4500 m/s because the acoustic velocity of sapphire compared with diamond is slower by some 30%, performance is unsatisfactory compared with devices employing diamond. Given this situation, being able to form a piezoelectrically satisfactory Li(Nb
x
Ta
1−x
)O
3
(wherein 0≦x<1) thin film onto diamond should lead to improved device performance.
In that regard, LiNbO
3
single crystals or LiTaO
3
single crystals are grown industrially by the Czochralski method (CZ method). This method is carried out by selecting a congruent-melting (congruent) composition such that as the crystal proceeds to harden from the melt, the composition does not change. In LiNbO
3
as a congruent composition, the atomic proportion of Li to Nb in the composition—the ratio of their atomic percentages—is 0.942, which is off-balanced toward Nb from the Li/Nb=1.0 that is its stoichiometric composition. The percentages of Li and Nb affect the characteristics of LiNbO
3
monocrystals. In a situation in which, for example, they are off-balanced toward Nb as in the congruent composition just noted, if strong light such as a laser beam is shone on LiNbO
3
, a so-called optical damage phenomenon in which the refractive index alters will be apparent A need therefore arises to sacrifice the optical characteristics of the LiNbO
3
itself and enhance the optical-damage resistance by the addition of MgO or the like.
With Li(Nb
x
Ta
1−x
)O
3
(0≦x≦1) thin films as well, the percentages of Li and Nb
x
Ta
1−x
are crucial, and in order to bring out favorable piezoelectric, electrochemical, and nonlinear-optical characteristics, the atomic constituent proportion of Li to Nb
x
Ta
1−x
(also referred to simply as “Li/Nb
x
Ta
1−x
ratio” hereinafter) must be controlled to be a value of over 0.80 and under 1.10. Wherein LiNb
x
Ta
1−x
O
3
thin films are fabricated by laser ablation as aforementioned, a tendency for Li to decrease from the Li and Nb
x
Ta
1−x
percentages of the raw-material targets is apparent.
Therefore, in order to suppress eduction of LiNbO
8
, etc. and obtain single-phase LiNbO
3
when forming LiNbO
3
thin films by laser ablation, making the Li constituent of the raw-material targets excessive is known. Japanese Pub. Pat. App. No. H05-78200, for instance, devises making the target Li to Nb proportion over 1.5 and under 3.5 when forming a LiNbO
3
thin film onto a sapphire substrate. It has been found, however, that if by this method LiNbO
3
is formed onto a diamond substrate, the Li/Nb proportion in the thin film at substrate temperatures of under 1000° C. turns out to be over 1.10. In then measuring by X-ray diffraction, Li
3
NbO
4
has appeared; and even with the single-phase LiNbO3, the optical characteristics under optical-damage phenomena have differed from those in which Li/Nb is from 0.80 to 1.10.
Likewise, with Li(Nb
x
Ta
1−x
)O
3
, wherein it has plural polarization regions, because domains whose piezoelectric constants differ cancel each other out and the overall piezoelectric characteristics do not hold, carrying out a polarization process, either when forming a Li(Nb
x
Ta
1−x
)O
3
thin film over diamond or after it is formed, is necessary. Nonetheless, post-crystallization polarizing of Li(Nb
x
Ta
1−x
)O
3
is known to be problematic. In general, the polarization of a ferroelectric substance can be aligned if after heating it beyond the Curie temperature, an electric field greater than the coercive electric field is applied to the crystal. In the case of LiNbO
3
, however, in addition to its coercive electric field being 21 kV/mm, the 1210° C. that is the Curie temperature is close to the 1250° C. that is the melting point of LiNbO
3
. Therefore, heating LiNbO
3
so that it does not melt to above the Curie temperature and applying to the crystal an electric field greater than the coercive electric field is difficult.
Likewise, because at room temperature the polarization-reversing electric field strength is close to the avalanche-current-generating electric field strength, there is a danger that avalanche current will be generated during polarization, destroying the crystal. A method of carrying out polarization in bulk LiNbO
3
crystals without insulator breakdown occurring despite avalanche current being generated to a certain extent, by applying a pulsed electric field to electrodes that have been furnished on either end face of the crystal, has been devised. Nevertheless, in a composite substrate of an LiNbO
3
thin film and polycrystalline diamond, which is an insulating substance, when avalanche current is generated, electrical load concentrates at the interface of the LiNbO
3
thin film and the diamond, and polarization cannot be carried out efficiently and moreover insulator breakdown ends up occurring. Given these circumstances, in composite substrates in which a LiNbO
3
thin film is formed onto a diamond substrate, carrying out polarization simultaneously during deposition to build the LiNbO
3
thin film is desirable.
Furthermore, although throughout the past betterments have been rendered in the c-axis orientation of formative Li(Nb
x
Ta
1−x
)O
3
thin films, even with better c-axis orientation, the piezoelectric characteristics of the formed thin film are not always satisfactory. Li(Nb
x
Ta
1−x
)O
3
thin films have polarization along the c-axis direction. In some c-axis oriented thin films, the +/− of the polar directions is mixed depending on the domain. In such cases, piezoelectric characteristics are manifested locally; but if the thin film is examined as a whole, the piezoelectric characteristics turn out to be nil, and the characteristics if it is made into a circuit element turn out to be unobtainable.
DISCLOSURE OF INVENTION
The present invention was devised in order to overcome the foregoing problemati
Imai Takahiro
Tatsumi Natsuo
Judge James
Smith Matthew
Sumitomo Electric Industries Ltd.
Yevsikov V
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