JP3432887B2 - Ultrasonic transducer - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an ultrasonic transducer formed on a silicon substrate. [0002] Ultrasonic waves have a slower propagation speed than light.
Since information such as distance, position, and object shape can be accurately measured in a non-contact manner and has good dispersibility, it is also suitable for sensing over a wide spatial area. Taking advantage of these features, it has been applied to a wide range of fields such as biological tomography diagnostic devices, fish finder, material damage, thickness gauges, level gauges, ultrasonic microscopes, monitoring and alarm devices, and remote switches. In such a sensing technology, technical demands for high-speed, high-precision measurement, and point measurement to surface measurement and further to three-dimensional measurement are increasing. For this reason, there has been proposed an ultrasonic transducer having a silicon monolithic structure in which a detection section and a signal processing section are integrated (for example, Japanese Patent Application Laid-Open No. 61-220596).
(See H04R 17/00). The element structure of a silicon monolithic ultrasonic transducer has a length of several tens to several hundreds of micrometers, a width of several tens of micrometers,
Thousands of angstroms of silicon dioxide (SiO
₂ ) A cantilever composed of ₂ ) is formed on a silicon (Si) substrate by anisotropic etching. On this cantilever, a lower electrode such as Pt, AlN, PbTiO ₃ , Zn
O, PZT (lead zirconate titanate), PLZT ((P
_{b 1-x, La x)} (Zr y, Ti 1-y) 1-x / 3 O 3) such as a piezoelectric film is formed by sequentially stacking an upper electrode made of Al or the like. When an ultrasonic wave is irradiated from above,
The cantilever made of the composite membrane vibrates. The vibration is very large at some mechanical resonance frequencies determined by the structure of the cantilever. Due to this vibration, a stress is generated inside the piezoelectric thin film, and a voltage is generated between the lower electrode and the upper electrode by the piezoelectric effect, thereby detecting an ultrasonic wave. [0006] When a piezoelectric film is formed, the temperature of the substrate is often increased.
Therefore, in order to avoid a reaction with the base, the lower electrode serving as the base is made of Pt which is a metal having low reactivity. In addition, the piezoelectric film is required to have a crystalline orientation in terms of characteristics. On the other hand, when AlN is used as the piezoelectric film, AlN cannot be formed on Pt with good adhesion, and there has been a problem that fine processing such as patterning cannot be performed. For this reason, considering only the adhesion to AlN, it is conceivable to use Al as the lower electrode.
When Al is formed on the SiO ₂ film, the orientation of Al is not good, and the AlN formed thereon also has a problem that the orientation is deteriorated, which is not practical. Further, Al is A
Since it is etched at the same time as the patterning of the 1N film, there is also a problem that it is difficult to use it as a lower extraction electrode. SUMMARY OF THE INVENTION It is an object of the present invention to provide an ultrasonic transducer having a piezoelectric film with good orientation and good characteristics, which has been made to solve the above-mentioned conventional problems. I do. [0010] The present invention provides a silicon dioxide thin film supported on a silicon substrate in a cantilever manner,
An ultrasonic transducer comprising a piezoelectric thin film laminated on the silicon dioxide thin film via a lower electrode, and an upper electrode laminated on the piezoelectric thin film, wherein the ultrasonic transducer is oriented on the silicon dioxide thin film. Platinum (Pt), palladium (Pd), iridium (I
r) or a lower electrode made of a metal selected from rhodium (Rh), on which a metal layer selected from aluminum (Al), zinc (Zn) or copper (Cu) has an orientation of the lower electrode. A piezoelectric thin film having an orientation is formed on the metal layer reflecting the orientation of the metal layer. According to the present invention, Pt,
Pd, Ir and Rh can be formed with good orientation. Then, Al, Zn, and Cu can be formed on the lower electrode made of these metals by reflecting the orientation of the lower electrode.
By forming a piezoelectric thin film on this Al, Zn, Cu,
The piezoelectric thin film can obtain good orientation, and can be made of Al, Z
Good adhesion to n and Cu. Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a plan view showing an embodiment of the present invention, and FIG. 2 is a longitudinal sectional view of the embodiment. An SiO ₂ film 20 having a thickness of 0.15 to 1.2 μm, in this embodiment, 0.5 μm, is provided on a (100) Si single crystal substrate 1 by a thermal oxidation method. This S
The iO ₂ film 20 is formed by photolithography using <110>
An opening in a concave shape is formed in the direction, and the SiO ₂ film 20 is used as a mask to form a piece on the Si substrate 1 by anisotropic etching using an etching solution with an alkaline solution, for example, a mixed solution of ethylenediamine, pyrocatechol and water. A cantilever 2 made of a SiO ₂ film supported in a cantilever form is formed. On the SiO ₂ film 20, platinum (Pt), palladium (Pd), iridium (Ir) or rhodium (R) which is easily oriented and has low reactivity to become the lower electrode 3 is formed.
h) is provided. In this embodiment, a lower electrode 3 made of Pt is provided. This Pt is a SiO ₂ film 2
0 can be formed with good orientation. On the lower electrode 3, Al, Zn
Alternatively, a metal layer 4 made of Cu and having a thickness of 50 to 300 Å is formed reflecting the orientation of the lower electrode.
In this embodiment, a metal layer 4 made of Al is formed. On this metal layer 4, AlN, PbTiO ₃ ,
A piezoelectric film 5 made of ZnO, PZT, and PLZT is provided. The piezoelectric body 5 is formed with good orientation reflecting the orientation of the metal layer 4. In this example,
A piezoelectric film 5 made of AlN is formed. And
An upper electrode 7 made of Al is provided via a SiO ₂ film 6 to prevent a short circuit between the lower electrode 3 and the extraction portion 3a. Thus, the SiO ₂ film 20 is supported in a cantilever manner on the Si substrate 1, and the lower electrode 3, the metal layer 4, the piezoelectric film 5, and the upper electrode 6 are stacked thereon to form a vibrator. The vibrating body vibrates up and down in the etching groove provided in the Si substrate 1 to transmit and receive ultrasonic waves. As described above, P on the SiO ₂ film 20
A lower electrode 3 made of t, Pd, Ir, and Rh is provided. These lower electrodes 3 can be formed on the SiO ₂ film 20 with good orientation. On these lower electrodes 3, Al, Zn, C
u can be formed reflecting the orientation of the lower electrode 3. By forming the piezoelectric thin film 5 on this Al, Zn, or Cu, the piezoelectric thin film 5 can have a good orientation and good adhesion to Al, Zn, and Cu. Next, a second embodiment of the present invention will be described with reference to FIGS. FIG. 3 is a plan view showing a second embodiment of the present invention, and FIG. 4 is a longitudinal sectional view of the second embodiment. In the second embodiment, Pt is used as an extraction electrode for the lower electrode 3 in the first embodiment, whereas the lower electrode 3 is extracted using Al of the metal layer 4. I have. That is, a window is opened in a part of the SiO ₂ film 20, and an extraction electrode portion 4c made of Al having good crystallinity is provided on the upper portion of the window. The other configuration is the same as that of the first embodiment, and the description is omitted here to avoid duplication of the description. Next, the ultrasonic transducer of the present invention will be described together with a production example. 5 and 6 are cross-sectional views showing a method of manufacturing the ultrasonic transducer according to the present invention step by step. First, as shown in FIG.
00) The single-crystal substrate 1 is thermally oxidized to a thickness of 0.15 to
1.2 μm, in this embodiment, 0.5 μm SiO ₂ film 2
0 is formed. This SiO ₂ film 20 may be formed by an RF sputtering method, a CVD method or the like other than the thermal oxidation method. The lower electrode 3 is provided on the SiO ₂ film 20. 0.1 to 0.4 μm of Pt as the lower electrode 3, R
It is formed by an F sputtering method or an ion beam sputtering method. The lower electrode 3 may be made of Pd, Ir, Pd or the like other than Pt. Thereafter, the lower electrode 3 made of Pt is patterned by photolithography. The lower electrode 3 is patterned by ion beam milling using a mask having a thickness of about 0.8 μm. The conditions of the ion beam milling are such that the Ar gas pressure is 2.8 × 10 ⁻⁴ To
rr, acceleration voltage 500 V, ion current 0.3 mA /
cm ² , milling speed 2000 angstrom / 1
0 minutes. [0025] After the ion milling is finished, the resist O ₂
Removal is performed by plasma or an organic solvent such as acetone or methyl ethyl ketone, or a solution of sulfuric acid and hydrogen peroxide. After that, the metal layer 4 has a thickness of 50 to 300.
Angstrom Al is formed by ion beam sputtering. The conditions for forming the Al film by the ion beam sputtering method are as follows: the target is 99.999% aluminum, the substrate temperature is kept at room temperature to 300 ° C., the Ar gas pressure is 2.8 × 10 ^-4 Torr, The acceleration voltage was 800 V, the ion current was 0.76 mA / cm ² , and the Al film formation rate was 35 Å / min. Subsequently, as shown in FIG.
The piezoelectric film 5 having a thickness of 0.1 to 20 μm made of
Subsequently, it is formed by ion beam sputtering. In this example, 0.7 μm AlN was laminated. When the ion beam sputtering method is used, the target for forming the AlN film is 99.999.
% Aluminum, the substrate temperature is from room temperature to 800 ° C.,
In this embodiment, the temperature is maintained at 300 ° C. and the nitrogen gas flow rate is set to 8 CC
M, Ar flow rate 4CCM, Kauffman ion gun 80
0 eV, 0.76 mA / cm ² , nitrogen ion beam energy is 72 to 200 eV, and in this embodiment, 100 e
V, a nitrogen ion beam current of 0.20 to 0.74 mA /
cm ² , in this example 0.32 mA / cm ² , EC
The R macrowave power is 200 to 500 W, in this embodiment, 300 W, the ultimate pressure is up to 2 × 10 ⁻⁷ Torr, the pressure during film formation is 1.4 × 10 ⁻⁴ Torr, and the film formation rate of AlN is 1
0-70 angstroms / min, in this example 30
Angstrom / min. The metal layer 4 made of Al and the piezoelectric film 5 made of AlN are patterned by wet etching or ion milling. Al and AlN films are not etched by most acids at a low temperature near room temperature, and need to use an alkaline solution. The conditions for patterning the Al and AlN films by wet etching are as follows: after forming a mask using a resist, an alkaline solution such as 1.0 N-KOH is heated to 40 to 70 ° C. for etching. In the case of an Al or AlN film formed by an ion beam sputtering method,
The etching rate by such wet etching is 70
Angstroms / minute. The conditions for patterning the Al and AlN films by ion milling are as follows: masking with a resist, Ar gas pressure of 2.8 × 10 ⁻⁴ Torr, acceleration voltage of 500 V,
The ion current is 0.3 mA / cm ² . The milling speed was set to Al, Al formed by ion beam sputtering.
In the case of the N film, it is 40 Å / min. Then, the resist used at the time of etching is removed by O ₂ plasma, an organic solvent such as acetone or methyl ethyl ketone, or a solution of sulfuric acid and hydrogen peroxide. Thereafter, as shown in FIG. 5C, an SiO ₂ film 6 is formed as an interlayer insulating film of the extraction electrode, and then, as shown in FIG. 1 to 0.8 μm, in this example 0.3 μm Al
Is selectively formed by ion beam sputtering. The conditions for forming the Al film by the ion beam sputtering method are as follows. The target is 99.999% aluminum, the substrate temperature is kept at room temperature to 300 ° C., the Ar gas pressure is 2.8 × 10 ^-4 Torr, The accelerating voltage is 800 V, the ion current is 0.76 mA / cm ² , and the deposition rate of Al is 3
5 Å / min. The upper electrode 7 may be made of a metal film such as Au or Pt, or a conductive oxide such as InO or SnO ₂ ITO other than Al. Further, when a passivation film is provided, a passivation film such as a SiO ₂ or polyimide film may be selectively formed. When a plurality of chips are formed on the same substrate, dicing may be performed on each chip at this point or half-cut dicing may be performed to facilitate division. After that, Si etching is performed to form a SiO ₂ cantilever. However, since the etching solution used for this Si etching is an alkaline solution, Al and AlN are not damaged.
Use r. As shown in FIG. 6B, a Cr film 8 is formed by ion beam sputtering or RF sputtering.
Is formed, and the portion of the Cr film 8 where the Si etching is to be performed is removed by photolithography, and then the SiO ₂ film 20 where the Cr film 8 has been removed is etched with buffered hydrofluoric acid. When the Cr film 8 is formed by, for example, ion beam sputtering, the film forming conditions are as follows: a target is 99.999% Cr;
The temperature was kept at 00 ° C., and the Ar gas pressure was 2.8 × 10 ⁻⁴ Torr,
Accelerating voltage 800V, ion current 0.76mA / cm
^2. The deposition rate of Al was 35 Å / min. The etching of the Cr film 8 was performed by using 25 g of ceric ammonium nitrate as an etching solution.
A mixture of 6.5 ml of 0% perchloric acid and 150 ml of water is used. This etching solution selectively etches the Cr film 8 and hardly damages Al, Pt, AlN, SiO _{2 and the} like. The etching rate with this etching solution is 1300 Å / min. Subsequently, as shown in FIG. 6C, the Si substrate 1 is etched using the Cr film 8 as a mask,
O ₂ cantilever 2 is created. Etching is terminated when a desired cantilever is obtained. The conditions for this Si etching are as follows: a mixture of 75 ml of ethylenediamine, 12 ml of pyrocatechol and 24 ml of water (EPW) is heated to 116 ° C. as an etching solution. Width 40
An SiO ₂ cantilever 2 having a thickness of 200 μm and a length of 200 μm can be formed by etching Si for about 2 hours.
Other than the above, KOH, NaO
An etching solution such as H, hydrazine, and NH ₄ OH may be used. These etching solutions consist of Si (10
The etching rate for the (0) plane is much higher than that for the (111) plane. For example, in the case of EPW, anisotropic etching can be performed about 40 times or more. Although the SiO ₂ film is slightly etched by this etching solution,
Is very slight in comparison with the etching rate of When deep Si etching is performed on the Si substrate 1 having the (100) plane through the opening, (10)
On the (0) plane, a concave portion 11 surrounded by four sides arranged in the <110> direction is formed. As described above, (11
1) Although the etching of the surface and the SiO ₂ film 20 is slow,
The etching rates of the (00) and (110) planes are high, and by this etching, the Si substrates 1 below the cantilevers 2 are arranged in the <110> direction and are etched away. Finally, as shown in FIG. 6 (d), the Cr film 8 is removed, and the chip is divided and assembled by bonding, etc., to obtain an ultrasonic transducer according to the present invention. In the above-described embodiment, the AlN film is used as the piezoelectric film 5, but in addition to the AlN film, PbTi
O ₃ , ZnO, PZT, PLZT can also be used,
Even when the piezoelectric film 5 of these materials is used, the metal layer 4
The adhesion is improved and the characteristics are improved as compared with the case where no steel is interposed. When Al is used as the metal layer 4, the piezoelectric film 5 to be laminated thereon is preferably formed at a temperature of 400 ° C. or less in consideration of the diffusion of Al and the like. When the film forming temperature is 400 ° C. or higher, the metal layer 4 may be C
By using u, diffusion does not occur very much, and deterioration of the characteristics of the piezoelectric film 5 laminated thereon can be suppressed. Further, when ZnO is used for the piezoelectric film 5, when Zn is used for the metal layer 4,
However, even if is diffused during the formation of the piezoelectric film 5, deterioration of the characteristics can be prevented. As described above, according to the present invention, Pt, Pd, Ir, and Rh are deposited on a silicon dioxide thin film.
Forming a lower electrode composed of these metals, forming Al, Zn, and Cu on the lower electrode composed of these metals by reflecting the orientation of the lower electrode, and forming a piezoelectric thin film on the Al, Zn, and Cu. Thus, the piezoelectric thin film can have good orientation, and can be laminated with good adhesion to Al, Zn, and Cu, and an ultrasonic transducer having good characteristics can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a plan view showing a first embodiment of the present invention. FIG. 2 is a longitudinal sectional view showing the first embodiment of the present invention. FIG. 3 is a plan view showing a second embodiment of the present invention. FIG. 4 is a longitudinal sectional view showing a second embodiment of the present invention. FIG. 5 is a sectional view showing an example of manufacturing the ultrasonic transducer of the present invention for each process. FIG. 6 is a sectional view showing an example of manufacturing the ultrasonic transducer of the present invention for each process. [Description of Signs] 1 Si substrate 2 Cantilever 3 Lower electrode 4 Metal layer 5 Piezoelectric film 7 Upper electrode 20 SiO ₂ film

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-5-260596 (JP, A) JP-A-6-132579 (JP, A) JP-A-61-220596 (JP, A) 167497 (JP, U) (58) Fields investigated (Int. Cl. ⁷ , DB name) H04R 17/00 330

Claims (1)

(57) [Claim 1] A silicon dioxide thin film supported on a silicon substrate in a cantilever form, a piezoelectric thin film laminated on the silicon dioxide thin film via a lower electrode, and a piezoelectric thin film An upper electrode laminated on the thin film, wherein the lower electrode made of a metal that is easily oriented and has low reactivity is provided on the silicon dioxide thin film, and the lower electrode is provided on the lower electrode. A metal layer selected from aluminum, zinc or copper is formed reflecting the orientation of the lower electrode, and a piezoelectric thin film having an orientation is formed on the metal layer reflecting the orientation of the metal layer. Ultrasonic transducer.