JPH02291711A - Surface acoustic wave element - Google Patents

Abstract

PURPOSE:To satisfactorily increase the feed-through suppression despite the large capacity of an IDT (transducer) by forming a 1st high density impurity dispersion area in an area held between the IDTs on the surface of a high resistance silicon single crystal substrate and also forming a 2nd high density density impurity dispersion area under each IDT. CONSTITUTION:A 1st high density impurity dispersion layer 8 is formed in an area held between the IDTs 4 on the surface of a high resistance single crystal substrate 7. At the same time, the 2nd high density impurity dispersion areas 6 and 6 are formed under the IDTs 4 respectively. In such a constitution, the effective resistance is increased between both IDTs and the impedance is equivalently increased between the IDTs. Then the field-through suppression is satisfactorily increased despite the large capacity of the IDT.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an improvement in a surface acoustic wave element in which a piezoelectric thin film is formed on a non-piezoelectric substrate. [Summary of the Invention] The present invention provides a surface acoustic wave device that uses direct waves of electromagnetic waves that affect device characteristics.
・Feed-through) [Prior Art] Conventionally, there are surface acoustic wave (SAW) elements shown in FIGS. 4 and 5 in which a piezoelectric thin film is formed on a non-piezoelectric substrate. The surface acoustic wave device shown in FIG. 4 has a silicon oxide film 2 formed on a low resistance silicon single crystal substrate 1, and a ZnO
A piezoelectric thin film 3 is formed, and 4 is a metal electrode for transmitting and receiving SAW provided on the ZnO piezoelectric thin film 3. The surface acoustic wave device shown in FIG. 5 has an epitaxial Si layer 5 between a low resistance silicon single crystal substrate 1 and a silicon oxide film 2 similar to those described above, and further improves the excitation efficiency of the IDT (}-transducer). A high concentration impurity diffusion region 6 is provided in the epitaxial Si layer S under the metal electrode 4 in order to increase the concentration. SA with ZnO/Si structure as above
Various types of W elements are known, but the structure of an element using Sezawa waves with a particularly large electromechanical coupling coefficient is the one shown in FIGS. 4 and 5. Si of the above element
Using a low resistance board as the board. A-E (acousto-elec) between SAW and carrier in semiconductor
tric) This is to minimize energy loss due to interaction, and the configuration shown in Figure 4 is a structure that mainly realizes a SAW filter. Furthermore, in order to realize a functional element that actively utilizes the A-E interaction, such as a convolver, the configuration shown in FIG. 5 in which a relatively thin epitaxial Si layer 5 is provided is adopted. Of course, in order to provide the desired function to the above structure, a gate electrode, a pn diode, etc. Various additional structures such as FETs and gratings are provided to realize actual devices. [Problem M to be solved by the invention] However, in the above element structure, when the number of logarithms of the IDT becomes very large or the capacity of the IDT becomes large, direct waves of electromagnetic waves (feed through
h) may have a large effect on device characteristics. The causes of this feed through are (1) inductive coupling that occurs in matching coils, etc. (2) Capacitive coupling that occurs on the substrate surface of the transmitting and receiving transducer; (3) Capacitive coupling that occurs through the bulk substrate of the transmitting and receiving transducer. In the above element structure, balanced drive has conventionally been adopted to suppress feed-through and also to improve the effective coupling coefficient. When the capacitance of the transducer is small, sufficient suppression is achieved, but when the capacitance becomes large, sufficient suppression cannot be achieved with balanced drive alone. In order to suppress the coupling between the coils, measures such as shielding or providing a shield electrode between IDTs are usually used, but it is difficult to suppress the capacitive coupling due to cause (3) of feed-through. In other words, the element configuration shown in Figures 4 and 5 is. ID
It can be said that it has the disadvantage that it is difficult to suppress feed-through when the capacity of T is large. [Object of the Invention] The present invention is a surface acoustic wave element structure having a piezoelectric thin film formed on a non-piezoelectric substrate, and provides a basic structure in which feed-through is sufficiently suppressed even when the capacitance of an IDT becomes large. It is intended to provide. [Means for Solving the Problems] The first invention includes a laminate including at least a high-resistance silicon single crystal substrate and a piezoelectric thin film, a pair of transducers formed at a predetermined interval on the piezoelectric thin film, A first high-concentration impurity diffusion region formed on the surface portion of the high-resistance silicon single crystal substrate between the transducers, and a first high-concentration impurity diffusion region formed on the surface portion of the high-resistance silicon single crystal substrate below each transducer. The gist of this is a configuration including a second high concentration impurity diffusion region. Furthermore, the second invention includes a laminate including at least a high-resistance silicon single crystal substrate, an epitaxial SL layer, and a piezoelectric thin film, a pair of transducers formed at a predetermined interval on the piezoelectric thin film, and the high-resistance silicon single crystal substrate. A first high-concentration impurity diffusion region formed in a region between the transducers on the surface of the resistive silicon single crystal substrate, and a second high-concentration impurity diffused region formed in the surface of the epitaxial SL layer below each transducer. The gist of this is a configuration that includes a diffusion region. [Function] In the surface acoustic wave device having the above configuration, the resistance between the IDTs becomes effectively large, the impedance between the IDTs becomes equivalently high, and even if the capacitance of the IDTs becomes large, feed-through can be sufficiently suppressed. It is possible to make it larger. [Example] Before starting the example, Fig. 1 shows a feed-through bus of the conventional surface acoustic wave element structure shown in Figs. 4 and 5 described above. As can be seen from Figure 1, the element structure can be regarded as an equivalent circuit consisting of a capacitor with a large capacitance value and a resistor with low resistance connected in series between the input and output ports. From this figure, we can see that to suppress feed-through, the resistance should be increased, ideally to infinity. FIG. 2 shows an embodiment of the present invention in which the above idea is applied to the element structure.

In the figure, 7 is a high resistance silicon single crystal substrate, 2 is a silicon oxide film, 3 is a ZnO piezoelectric thin film, and 4 is a transducer. In the sandwiched area, the first
A high concentration impurity diffusion region 8 is formed, and second high concentration impurity diffusion regions 6, 6 are formed under each transducer. The reason for using the high resistance silicon single crystal substrate 7 is to increase the series resistance in the bulk single crystal substrate. It is difficult to realize a Si substrate with perfect insulation properties. Therefore, when the SAW propagates within the high-resistance silicon single-crystal substrate 7, there is energy loss due to the A-E interaction. A high concentration impurity diffusion region 8 is provided. What is shown in FIG. 3 is another embodiment of the present invention, which includes:
7 is a high resistance silicon single crystal substrate, 5 is an epitaxial S
The i layer, 2 is a silicon oxide film, 3 is a piezoelectric thin film, and in the epitaxial Si layer 5 there are high concentration impurity diffusion regions 6,
6 is formed, and a high concentration impurity diffusion region 8 is formed in the high resistance silicon single crystal substrate 7. In addition, when it is necessary to apply an electric field in the vertical direction of the element, such as in a convolver. If a gate electrode is provided on the ZnO piezoelectric thin film, and impurities are implanted at a high concentration into the epitaxial Si layer 5 outside the interaction region, and ohmic contact is established over a large area, the high concentration impurity can be diffused between the gate and the high concentration impurity without increasing the series resistance. An electric field can be applied between the region 8 and the region 8. In each of the above embodiments, ZnO is used for the piezoelectric thin film, but the same effect as described above can be obtained by using A EI N , Ta, O, or the like. [Effects of the Invention] As described above, according to the surface acoustic wave element configuration of the present invention, the resistance between the IDTs becomes effectively large, and the impedance between the IDTs becomes equivalently high. Even if the IDT capacity increases, feed-through suppression can be sufficiently increased.

[Brief explanation of drawings]

FIG. 1 is a configuration diagram of a surface acoustic wave device explaining the principle of the present invention, FIGS. 2 and 3 are side views of the surface acoustic wave device showing embodiments of the present invention, and FIGS. 4 and 5 are FIG. 2 is a side view of a conventional surface acoustic wave device. 1...Low resistance Si single crystal substrate, 2...
・・・・・・Silicon oxide film, 3・・・・・・・・・Z
nO piezoelectric thin film, 4...Transducer, 5...Epitaxial Si layer, 6...
...High concentration impurity diffusion region, 7...
...High resistance silicon single crystal substrate, 8...
...High concentration impurity diffusion region. Patent Applicant Clarion Co., Ltd. Agent Patent Attorney Takesaburo Nagata Figure 4 Figure 5 Figure I

Claims (2)

[Claims] (1) A laminate including at least a high-resistance silicon single crystal substrate and a piezoelectric thin film, a pair of transducers formed on the piezoelectric thin film at a predetermined interval, and the transducer on the surface of the high-resistance silicon single crystal substrate. A first high concentration impurity diffusion region formed in the sandwiched area, and a second high concentration impurity diffusion region formed on the surface portion of the high resistance silicon single crystal substrate under each transducer. surface acoustic wave device. (2) A laminate including at least a high-resistance silicon single crystal substrate, an epitaxial Si layer, and a piezoelectric thin film, a pair of transducers formed at a predetermined distance on the piezoelectric thin film, and a surface portion of the high-resistance silicon single crystal substrate. a first high-concentration impurity diffusion region formed in an area sandwiched between the transducers; and a second high-concentration impurity diffusion region formed in a surface portion of the epitaxial Si layer under each transducer. Characteristic surface acoustic wave device.