JP4222975B2 - Thin film conductor and surface acoustic wave device using the same - Google Patents

Description

  The present invention relates to a thin film conductor formed on a substrate and a surface acoustic wave element in which an electrode is formed by the thin film conductor.

  Conventionally, in communication devices such as mobile phones, surface acoustic wave elements are used as circuit elements such as resonator filters and duplexers. For example, in the surface acoustic wave element shown in FIG. 10, interdigital transducers (52) each made of a pair of aluminum-made saddle-shaped electrodes (52a) (52a) are provided at two locations on the surface of a piezoelectric substrate (51). At the same time, reflectors (53) and (53) made of grid-like electrodes are disposed on both sides of the interdigital transducers (52) and (52). A pair of input pads (54) and (54) and a pair of output pads (55) and (55) are connected to the interdigital transducers (52) and (52), respectively.

  In recent years, with the increase in frequency of communication devices, the operating frequency of surface acoustic wave elements has been increased, and higher output has been required. In order to increase the operating frequency, it is necessary to reduce the line width of each electrode (52a). For example, when the operating frequency is a gigahertz band, the line width of the electrode (52a) is less than 1 μm. When a voltage is applied to the surface acoustic wave element having such a small line width electrode, a stress is repeatedly applied to the electrode (52a) by the surface acoustic wave generated on the surface of the piezoelectric substrate (51). If the stress exceeds the critical stress inherent to the material of the electrode (52a), stress migration occurs. In addition, electromigration occurs as the electron flow through the electrode (52a) increases in density. As a result, voids (voids) and protrusions (hillocks) are formed in the electrode (52a), and the electrode (52a) is destroyed due to deterioration of power durability, leading to an electrical short circuit and an increase in insertion loss. .

Therefore, as shown in FIG. 8, the electrode (12) formed on the piezoelectric substrate (1) has a first layer (9) made of Ti, V, Fe, Co, Ni or the like and a first layer made of Al or an Al alloy. A surface acoustic wave element having a laminated structure of two layers (10) has been proposed (Patent Document 1).
Further, as shown in FIG. 9, a covering layer (11) made of TiN, ZrN, WN, SiO _2, etc., covering the surfaces of the first layer (9) and the second layer (10) constituting the electrode (13). A surface acoustic wave device having a structure is proposed (Patent Documents 2 to 4).
JP-A-5-90268 JP 2003-92371 A JP 2001-217672 A JP-A-10-256862 JP-A-7-162256

By the way, in the surface acoustic wave element, the film thickness of the electrode is becoming smaller and smaller with the recent increase in operating frequency. For example, when the operating frequency is 1 GHz, the thickness of the Al electrode is less than 500 nm, and when the operating frequency is further increased, the thickness of the electrode is further reduced. As a result, there has been a problem that the electrical resistance of the electrode is increased, leading to an increase in insertion loss when, for example, a filter is configured.
Therefore, an object of the present invention is to reduce the loss when the thin film conductor is made thin by reducing the electrical resistance of the thin film conductor in the thin film conductor constituting the electrode of the surface acoustic wave element.

The thin film conductor according to the present invention is characterized in that an electron diffusion suppression layer (3) for forming a potential barrier at the interface with the conductive layer (2) is formed on both sides or one side of the conductive layer (2). To do.
The surface acoustic wave device according to the present invention includes an electrode serving as an interdigital transducer on a piezoelectric substrate, and the electrode is formed by the thin film conductor of the present invention.

  In the thin film conductor of the present invention, when electrons move in the conductive layer (2) by energization, the electrons are scattered and diffused inside the crystal constituting the conductive layer (2) or at the grain boundary. However, when the electrons reach the surface layer portion of the conductive layer (2), the flow of electrons in the surface layer portion deteriorates, which contributes to an increase in electrical resistance. However, in the thin film conductor of the present invention, since a potential barrier is formed at the interface between the conductive layer (2) and the electron diffusion suppressing layer (3), electrons moving in the conductive layer (2) It becomes difficult to approach toward the interface with the suppression layer (3), and most electrons flow smoothly through the central portion of the conductive layer (2). As a result, the electrical resistance is smaller than that of the conventional thin film conductor in which many electrons flow into the surface layer portion of the conductive layer (2).

In a specific configuration, the electron diffusion suppression layer (3) is formed of a p-type or n-type semiconductor, for example, Si.
When the metal composing the conductive layer (2) and the semiconductor composing the electron diffusion suppression layer (3) come into contact, depending on the type of semiconductor and the magnitude relationship between the work function φm of the metal and the work function φs of the semiconductor, When n is n-type, φm> φs is satisfied, and when the semiconductor is p-type, φm <φs is satisfied, and a Schottky junction is formed at the interface between the conductive layer (2) and the electron diffusion suppression layer (3). This forms a potential barrier at the interface.

In a specific configuration, an orientation control layer (5) is formed between the substrate (1) and the conductive layer (2).
According to this specific configuration, the formation of the orientation control layer (5) makes the crystalline structure of the conductive layer (2) dense, further reducing the electrical resistance of the conductive layer (2).

  According to the thin film conductor and the surface acoustic wave device using the thin film conductor according to the present invention, since the electric resistance of the thin film conductor (electrode) is smaller than that of the conventional case, the thin film conductor (electrode) is reduced in thickness. Also, the increase in insertion loss can be reduced.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments in which the present invention is applied to a surface acoustic wave device will be specifically described below with reference to the drawings.
As shown in FIG. 1, the surface acoustic wave device according to the present invention has an electrode (6) formed on a piezoelectric substrate (1), and the electrode (6) is formed on the surface of the piezoelectric substrate (1). The metal conductive layer (2) and a semiconductor electron diffusion suppression layer (3) laminated on the surface of the conductive layer (2).
The electron diffusion suppression layer (3) may be formed between the piezoelectric substrate (1) and the conductive layer (2).

  Further, another surface acoustic wave element according to the present invention has an electrode (7) formed on a piezoelectric substrate (1) as shown in FIG. 2, and the electrode (7) is formed on the piezoelectric substrate (1). A semiconductor electron diffusion suppression layer (4) formed on the surface, a metal conductive layer (2) formed on the surface of the electron diffusion suppression layer (4), and formed on the surface of the conductive layer (2) And a semiconductor electron diffusion suppressing layer (3).

Furthermore, as shown in FIG. 3, another surface acoustic wave device according to the present invention has an electrode (8) formed on a piezoelectric substrate (1), and the electrode (8) is a surface of the piezoelectric substrate (1). A semiconductor electron diffusion suppression layer (4) formed on the surface, a metal orientation control layer (5) formed on the surface of the electron diffusion suppression layer (4), and a surface of the orientation control layer (5). The metal conductive layer (2) is formed, and a semiconductor electron diffusion suppression layer (3) formed on the surface of the conductive layer (2).
The orientation control layer (5) may be formed between the piezoelectric substrate (1) and the electron diffusion suppression layer (4).

The surface acoustic wave elements having various laminated structures according to the present invention and the conventional surface acoustic wave elements having various laminated structures were prototyped, and the lifetimes of these surface acoustic wave elements due to the reduced power durability were reduced. Estimation and insertion loss in the filter passband were measured.
In the production of the surface acoustic wave device, a 36 ° Y-cut lithium tantalate substrate (thickness 350 nm) was adopted as the piezoelectric substrate 1 and the electrodes were processed using RIE. Then, a 1900 MHz-band ladder filter was constructed, and the insertion loss in the filter passband was measured.

Further, the p-type Si layer as the electron diffusion suppression layers (3) and (4) was formed by DC sputtering. Here, a Si target containing B as a dopant in advance was prepared, and the B doping concentration in the target was 10 ¹⁵ per cm ³ . The power was set to 1 kW and the gas pressure was set to 0.32 Pa. After sputtering, the amorphous Si layer was subjected to activation treatment at 400 ° C. for 10 hours to obtain a p-type Si layer.

FIG. 4 shows a laminated structure of various surface acoustic wave elements (a) to (f) provided with a conductive layer (2) made of Al. The thickness of each layer is as follows. In the surface acoustic wave element (c), a Ti layer (14) is formed on the surface of the conductive layer (2). In the surface acoustic wave element (d), an SiO ₂ layer (15) is formed on the surface of the conductive layer (2). ) Is formed.
(a) Al: 170 nm
(b) Al: 150 nm, Si: 23 nm
(c) Al: 150 nm, Ti: 15 nm
(d) Al: 150 nm, SiO ₂ : 20 nm
(e) Al: 150 nm, Si: 23 nm
(f) Al: 130 nm, Si: 23 nm × 2

Table 1 below shows, for each of the above six types of surface acoustic wave elements (a) to (f), the minimum value of the insertion loss in the passband (Top IL; top loss), the average insertion loss in the passband, 3 dB bandwidth, And the estimated lifetime.
Here, the minimum value (Top IL) of the insertion loss within the passband means the minimum value of the insertion loss within the passband of 1850 MHz to 1910 MHz, and the average insertion loss within the passband is within the passband of 1850 to 1910 MHz. Means the average value of the insertion loss, and the 3 dB bandwidth means a bandwidth where the insertion loss is 3 dB or less. The estimated lifetime represents a value converted to a lifetime at 0.8 W and 50 ° C. by extrapolation calculation of the Arrhenius plot obtained by the acceleration test.

  As is apparent from Table 1, the surface acoustic wave element (b) of the present invention in which an Al conductive layer (2) and a p-type Si electron diffusion suppression layer (3) are laminated on a piezoelectric substrate (1), and a piezoelectric A surface acoustic wave device (e) of the present invention in which a p-type Si electron diffusion suppressing layer (4) and an Al conductive layer (2) are laminated on a substrate (1), and p-type Si on a piezoelectric substrate (1). In the surface acoustic wave device (f) of the present invention formed by laminating the electron diffusion suppression layer (4), the Al conductive layer (2), and the p-type Si electron diffusion suppression layer (3), the minimum value of the insertion loss in the passband (Top IL), average insertion loss in the passband, 3 dB bandwidth, and estimated lifetime, characteristics superior to other conventional surface acoustic wave devices (a), (c), and (d) can be obtained. ing.

  Among these, in the surface acoustic wave element (f), since the electron diffusion suppression layers (3) and (4) are formed on both sides of the conductive layer (2), the electrons in the conductive layer (2) are both electrons. Since it is confined in the central region away from the diffusion suppression layers (3) and (4) and flows in the conductive layer (2) in an orderly manner, the best characteristics are obtained.

FIG. 5 shows a laminated structure of various surface acoustic wave elements (g) to (l) having a conductive layer (2) made of Au or W. The thickness of each layer is as follows.
(g) Au: 51 nm
(h) Au: 50 nm, Si: 20 nm
(i) Au: 49 nm, Si: 20 nm × 2
(j) W: 51 nm
(k) W: 50 nm, Si: 20 nm
(l) w: 49 nm, Si: 20 nm × 2

FIG. 6 shows a laminated structure of various surface acoustic wave elements (m) to (o) having a conductive layer (2) made of Mo. The thickness of each layer is as follows.
(m) Mo: 85 nm
(n) Mo: 84 nm, Si: 20 nm
(o) Mo: 83 nm, Si: 20 nm × 2

  Table 2 below shows the minimum value (Top IL) of the insertion loss in the passband, the average insertion loss in the passband, the 3 dB bandwidth, and the estimated lifetime for each of the nine types of surface acoustic wave elements (g) to (o). Is shown.

  As is apparent from Table 2, the surface acoustic wave element (h) of the present invention in which an Au conductive layer (2) and a p-type Si electron diffusion suppression layer (3) are laminated on a piezoelectric substrate (1), and a piezoelectric A surface acoustic wave device (i) according to the present invention in which a p-type Si electron diffusion suppression layer (4), an Au conductive layer (2), and a p-type Si electron diffusion suppression layer (3) are laminated on a substrate (1); The surface acoustic wave element (k) according to the present invention in which a W conductive layer (2) and a p-type Si electron diffusion suppression layer (3) are laminated on a piezoelectric substrate (1), and p on the piezoelectric substrate (1). A surface acoustic wave element (1) of the present invention, which is formed by laminating a type Si diffusion suppression layer (4), a W conductive layer (2), and a p type Si electron diffusion suppression layer (3), and a piezoelectric substrate (1). The surface acoustic wave element (n) of the present invention formed by laminating the Mo conductive layer (2) and the p-type Si electron diffusion suppression layer (3), and the p-type Si electron diffusion suppression layer (4) on the piezoelectric substrate (1). ), Mo conductive layer (2) and p-type Si electron diffusion suppression layer (3) In the surface acoustic wave device (o) according to the present invention, the minimum value of the insertion loss in the passband (Top IL), the average insertion loss in the passband, the 3 dB bandwidth, and the estimated lifetime, Characteristics superior to those of the conventional surface acoustic wave elements (g) (j) are obtained.

  Thus, even when Au, W or Mo is used as the material of the conductive layer (2) instead of Al, the same effect as in the case of Al is obtained. It is confirmed that the layers (3) and (4) form a potential barrier at the interface with the conductive layer (2) regardless of the material of the conductive layer (2).

FIG. 7 shows various surface acoustic wave elements having a conductive layer (2) made of Al and an orientation control layer (5) made of Ti formed between the piezoelectric substrate (1) and the conductive layer (2). The laminated structure of p)-(s) is shown, and the thickness of each layer is as follows.
(p) Al: 150 nm, Ti: 15 nm
(q) Al: 130 nm, Si: 20 nm, Ti: 15 nm
(r) Al: 110 nm, Si: 20 nm × 2, Ti: 15 nm
(s) Al: 110 nm, Si: 20 nm × 2, Ti: 15 nm

  Table 3 below shows, for each of the four types of surface acoustic wave elements (p) to (s), the minimum value of the insertion loss in the passband (Top IL; top loss), the average insertion loss in the passband, 3 dB bandwidth, And the estimated lifetime.

  As is apparent from Table 3, the elastic surface of the present invention is formed by laminating a Ti orientation control layer (5), an Al conductive layer (2), and a p-type Si electron diffusion suppressing layer (3) on a piezoelectric substrate (1). A p-type Si diffusion suppression layer (4), a Ti orientation control layer (5), an Al conductive layer (2), and a p-type Si electron diffusion suppression layer (3) are provided on the wave element (q) and the piezoelectric substrate (1). The surface acoustic wave element (r) of the present invention formed by stacking, a Ti orientation control layer (5), a p-type Si electron diffusion suppressing layer (4), an Al conductive layer (2) and p on the piezoelectric substrate (1). In the surface acoustic wave device (s) formed by laminating the type Si electron diffusion suppression layer (3), the minimum value of the insertion loss in the passband (Top IL), the average insertion loss in the passband, the 3 dB bandwidth, and the estimated lifetime In any point, characteristics superior to those of other conventional surface acoustic wave elements (p) are obtained.

Thus, even in the surface acoustic wave device in which the orientation control layer (5) is formed between the piezoelectric substrate (1) and the conductive layer (2), the electron diffusion suppression layer (on both sides or one side of the conductive layer (2)). By forming 3), a potential barrier is formed at the interface with the conductive layer (2), and the diffusion of electrons in the conductive layer (2) is suppressed.
Further, according to the comparison between the surface acoustic wave element (r) and the surface acoustic wave element (s) according to the present invention, the surface acoustic wave element (r) has the minimum insertion loss in the passband (Top IL), It can be seen that the average insertion loss in the passband is excellent, and the surface acoustic wave element (s) is superior in power durability.

  As described above, according to the surface acoustic wave device according to the present invention, since the electric resistance of the conductive layer constituting the electrode is smaller than that of the conventional one, the insertion loss is increased even when the electrode is thinned. Can be suppressed. Further, the formation of the electron diffusion suppressing layer suppresses the migration of the metal constituting the conductive layer, particularly the surface diffusion, so that high power durability can be obtained.

In addition, each part structure of this invention is not restricted to the said embodiment, A various deformation | transformation is possible within the technical scope as described in a claim. For example, the electron diffusion suppression layers (3) and (4) can be formed of a p-type semiconductor (for example, p-type GaAs) other than p-type Si or an n-type semiconductor (for example, n-type Si or n-type GaAs). . In the formation of the p-type semiconductor layer, when Si is used as the base, the dopant is not limited to B, but Al, Ga, In, Cu, etc. can be used, and when GaAs is used as the base, Mg, Si, Ge, Cd, Al, or the like can be used. In the formation of the n-type semiconductor layer, when Si is used as a base, P, Li, Sb, As, Bi, etc. can be used as a dopant. When GaAs is used as a base, as a dopant, Te, Si, Ge, Se, or the like can be used.
Furthermore, the thin film conductor of the present invention is not limited to a surface acoustic wave element, and can be implemented in various electronic components.

It is sectional drawing which shows the laminated structure of the surface acoustic wave element which concerns on this invention. It is sectional drawing which shows the other laminated structure of the surface acoustic wave element which concerns on this invention. It is sectional drawing which shows other laminated structure of the surface acoustic wave element which concerns on this invention. It is sectional drawing which shows the laminated structure and material of various surface acoustic wave elements. It is sectional drawing which shows the laminated structure and material of other various surface acoustic wave elements. It is sectional drawing which shows the laminated structure and material of other various surface acoustic wave elements. It is sectional drawing which shows the laminated structure and material of other various surface acoustic wave elements. It is sectional drawing which shows the laminated structure of the conventional surface acoustic wave element. It is sectional drawing which shows the laminated structure of the other conventional surface acoustic wave element. It is a top view which shows the electrode pattern of the conventional surface acoustic wave element.

Explanation of symbols

(1) Piezoelectric substrate
(2) Conductive layer
(3) Electron diffusion suppression layer
(4) Electron diffusion suppression layer
(5) Orientation control layer
(6) Electrode
(7) Electrode
(8) Electrode

Claims (6)

  A thin-film conductor formed on the substrate (1), on both sides or one side of the conductive layer (2), an electron diffusion suppression layer (3) that should form a potential barrier at the interface with the conductive layer (2) is formed A thin film conductor characterized by being made.   The thin film conductor according to claim 1, wherein the electron diffusion suppression layer (3) is formed of a p-type or n-type semiconductor.   The thin film conductor according to claim 1 or 2, wherein an orientation control layer (5) is formed between the substrate (1) and the conductive layer (2).   The thin film conductor according to any one of claims 1 to 3, wherein the electron diffusion suppressing layer (3) is made of Si.   In the surface acoustic wave device in which an electrode serving as an interdigital transducer is formed on a piezoelectric substrate, the electrode is an electron diffusion layer that should form a potential barrier at the interface between the conductive layer (2) and the conductive layer (2). A surface acoustic wave device having a laminated structure of a suppression layer (3).   The surface acoustic wave device according to claim 5, wherein the electron diffusion suppression layer (3) is formed of a p-type or n-type semiconductor.

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