Probe-driving mechanism, production thereof, and apparatus and piezoelectric actuator employing the same

Abstract: A probe-driving mechanism is shown with a bimorph cantilever having a thin piezoelectric substance film (5, 7) and electrodes (4, 6, 8) holding the film therebetween. The cantilever is supported by a support (1) at one end and has a probe for information input and output at the other. The probe-driving mechanism has a first insulating layer (2) between the support and the cantilever and a second insulating layer (2) at the other side of the support. The first insulating layer has sufficient thickness to reduce parasitic capacitance induced between the electrode of the cantilever and the support. A tunnel current-detecting apparatus and an information-processing apparatus comprise the probe-driving mechanism, respectively.

Description: BACKGROUND OF THE INVENTION The present invention relates to a piezoelectric actuator, a probe-driving mechanism, methods of production thereof, and an apparatus employing the same such as a tunnel current-detecting device and an information-processing device. In recent years, application of STM technique is investigated in a variety of technical fields including observation and evaluation of semiconductors and high-molecular materials in atomic or molecular order, fine working, and recording apparatus. In particular, recording apparatuses are required to have large capacity in calculation information and image information of a computer, and so forth, and development of STM technique therefor is more and more strongly demanded. As the result of progress in semiconductor processing technique, microprocessors have come to be miniaturized and computing ability has been improved. Consequently, recording apparatuses are desired to be further miniaturized. To satisfy such requirements, a recording-reproduction apparatus is disclosed which employs a transducer having a tunnel-current-generating probe provided on a driving means capable of fine adjustment of the gap between the probe and a recording medium. This apparatus records information by applying voltage with the transducer to change the work function on the surface of the recording medium, or reads out information by detecting the change of a tunnel current caused by the change of the work function, at the minimum recording area of 10 nm square (Japanese Patent Application Laid-Open No. 63-161552). Generally, the probes need to be increased in number in order to accelerate transfer and recording of data. In the transfer or recording of data in the aforementioned apparatus, the probes run along the lines of recorded data by adjusting the space between the probe and the recording medium. Since the breadth of the lines of the recorded data is extremely small, stable recording-reproduction cannot readily be achieved owing to the drift caused by the temperature variation of the apparatus, run-out of the probes from the data lines caused by external vibration, and other influences. Accordingly, the respective probes are required to move independently in high speed in a direction perpendicular to the face of the recording medium. To satisfy such requirement, WO89/07256, for example, discloses a cantilever constituted of lamination of piezoelectric material layers 5, 7 and electrode layers 4, 6, 8 as shown in Fig. 6. The cantilever is prepared by forming an insulating layer 11 composed of silicon oxide film, patterning it, and etching anisotropically a substrate 1 to have a bimorph arm 10. In the above example of the prior art, however, the thickness of the insulating layer 11 needs to be made thin to suppress side-etching of the Si substrate 1 in the etching of the bimorph arm 10 and the insulating layer 11. Thereby, nonnegligible capacitance is induced between the lower electrode 4 and the Si substrate 1, forming an equivalent circuit by connection with another electrode through the substrate as shown in Fig. 7. Usually, the driving frequency of a cantilever is limited by the resonance frequency determined by the dimension and the layer constitution of the cantilever. In the example of the above prior art, the driving frequency is limited also by the increase of the time constant caused by the capacitance induced by the piezoelectric substance layers 5,7 and the aforementioned parasitic capacitance. Consequently, the adjustment of the spacing between a medium and the probe becomes impossible and malfunction arises in writing and reading. In a multi-probe construction, difference in lengths of wiring to the driving electrodes for respective probes causes large variation of the parasitic capacitance, which causes variation of the driving characteristics of the cantilever. If the insulating layer under the electrode is made thick to reduce the parasitic capacity for the purpose of avoiding the above problems, Si is isotropically etched, when the insulating layer 11 is etched, in the side direction in a dimension of ten times larger or more than the thickness of the insulating layer as shown in Fig. 6, which lowers the controllability of the cantilever. SUMMARY OF THE INVENTION An object of the present invention is to provide a novel piezoelectric actuator and a probe-driving mechanism, in which the parasitic capacitance induced between the cantilever and the support is reduced to improve the responsibility of the cantilever and unnecessary etching is prevented, and also to provide a method of production thereof. Another object of the present invention is to provide an information processing apparatus comprising a tunnel current-detecting device employing the above probe-driving mechanism improved in accuracy and responsibility, and means for recording, reproduction, and erasing. According to a first aspect of the present invention, there is provided a piezoelectric actuator comprising a bimorph cantilever having a layer structure comprising: two piezoelectric films layered between a plurality of electrodes, said cantilever being supported by a support at one end, the actuator being characterised in that a first insulating layer and a second insulating layer are provided between said support and said cantilever, said first insulating layer being in contact with said support, and said second insulating layer being in contact with said cantilever, the total thickness of the insulating layers reducing towards the edge of the support such that the bimorph cantilever has an inclined portion between the cantilever portion of the bimorph structure and the portion supported on the rest of the support. In one embodiment of the present invention the actuator constitutes a probe-driving mechanism, the mechanism further comprising: a probe for information input and output at the other end of the cantilever to the support. According to a second aspect of the present invention there is provided a scanning tunnelling microscope (STM) apparatus having: at least one probe-driving mechanism according to the previous paragraph; a driving means for driving the probe-driving mechanism; a control means for controlling the driving means; means for applying an electric voltage between a specimen and probe tip; means for detecting an electric tunnel current flowing between the specimen and the probe tip; and means for outputting information from a surface of the specimen according to the detected tunnel current. According to a third aspect of the present invention, there is provided a process for producing a probe-driving mechanism characterised by comprising the steps of: providing a first insulating layer of not less than 5000 Å thick on a support; patterning the first insulating layer by anisotropic etching to form an inclined portion which reduces in thickness towards the edge of the support; providing a second insulating layer on the first insulating layer; laminating a plurality of electrode layers and piezoelectric layers successively on the insulating layer and removing part of the support by etching to form a bimorph cantilever. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a sectional view of a probe-driving mechanism of the present invention. Figs. 2A to 2C illustrate a process for producing a probe-driving mechanism. Fig. 3 illustrates another process for producing a probe-driving mechanism of the present invention. Fig. 4 is a rough sketch of a multi-probe. Fig. 5 illustrates outline of an information-processing apparatus of the present invention. Fig. 6 illustrates roughly an example of prior art. Fig. 7 illustrates a circuit equivalent to the prior art of figure 6 circuit between the electrodes. Fig. 8 illustrates an electric memory effect of a recording layer employed in an information-processing apparatus of the present invention. Figs. 9A and 9B show examples of application of a piezoelectric actuator of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENT In the present invention, the rigidity of the cantilever portion is lowered and the insulating layer on the substrate is made thick, whereby the parasitic capacitance is lowered and the driving characteristics of the cantilever is improved. The insulating layer has preferably a thickness of at least 5000 Å, more preferably 1 µm or more. The present invention is described specifically by reference to examples. Example 1 Fig. 1 is a sectional view of a constitution of a cantilever and leader electrodes. The cantilever portion is constituted of a piezoelectric material layer 5, 7 and driving electrode layers 4, 6, 8. At the free end of the cantilever, a tip 9 is provided. The electrode for leading out the tunnel current is formed simultaneously with the formation of the upper electrode 8. The intermediate and upper electrodes 6, 8 are wired over a long distance practically to a bonding pad, although they are shown to be cut in the drawing. The constitution and the effect of the present invention are described by reference to the drawing of a production process in Figs. 2A to 2C. As shown in Fig. 2A, since a thick film can readily be formed on a (100) Si substrate, silicon oxide films 3 are formed thereon in a thickness of 1 µm or more in a thermal oxidation furnace. Both faces are subjected to patterning. In the patterning, the silicon oxide films on both surfaces are left remaining only on the supporting portion 13. The patterning of the silicon oxide film 3 is conducted preferably by isotropic etching with an aqueous HF solution or the like because the coverage of the lower electrode 4 to be formed later will become insufficient if the end of the edge portion 12 of the silicon oxide film is steep and the lower electrode 4 to be formed in the step of Fig. 2C is relatively thin. Thereafter, silicon nitride films 2 are formed in a thickness of from 1000 to 1500 Å by vapor growth of an SiH 2 Cl 2 gas and an NH 3 gas, and the film at the back side only is patterned. Instead of formation of the silicon nitride films, silicon oxide films may be formed again by means of a thermal oxidation furnace. Subsequently, etching is conducted by anisotropic etching of Si, for example by heating with an aqueous KOH, at the region of the cantilever portion to form an Si membrane 10 of several ten µm thick as shown in Fig. 2B. Thereafter, a lower electrode 4 of about 1000 Å thick and a piezoelectric material layer 5 are formed as shown in Fig. 2C. In the same manner, an intermediate electrode 6, another piezoelectric material layer 7, and an upper electrode 8 are laminated in this order. Further thereon, a tip 9 is formed. After the resulting matter is coated in a thickness of 10 µm or more, the Si membrane 10 and the silicon nitride film (2) are removed by anisotropic etching with an aqueous KOH solution or the like or by plasma etching with a gas such as CF 4 and SF 6 . Further the coating is removed by plasma etching with a gas such as CF 4 and SF 6 . Thus, a cantilever having constitution as shown in Fig. 1 is obtained. The advantage of the above constitution of the present invention are as below. In the traditional constitution of Fig. 6, as described above, undesired parasitic capacitances, c 1 and c 2 , are induced as shown in the equivalent circuit shown in Fig. 7, where c denotes the capacitance of the piezoelectric material; r 1 and r 2 denote resistances of wiring; and r 3 denotes the resistance of the substrate. The values of c, c 1 , and c 2 of the present invention (Fig. 1) are compared with the ones of prior art (Fig. 6) under the conditions below. Conditions Thickness of SiO 2 ,3 (Present invention) 1 µm Thickness of Si 3 N 4 ,2 (Insulating layer in prior art) 0.15 µm Thickness of piezoelectric material layer, 5, 7 0.3 µm Dimension of cantilever 300 µm × 100 µm Wiring 5 µm wide × 5 mm long Dielectric constant of Si 3 N 4 1.0 × 10 -11 (F/m) Dielectric constant of SiO 2 3.5 × 10 -11 (F/m) Dielectric constant of ZnO 1.1 × 10 -10 (F/m) Parasitic capacitance As shown above, in the cantilever of prior art, the ratio of the parasitic capacitance to the capacitance of the main body of the cantilever (c′/c) is about 40 %, while the value of c′ of the present invention is as low as 1/10 or lower thereof. In the case of a multi-probe shown in Fig. 4, the wiring length differs among the probes, which causes large differences in the parasitic capacitances of the probes and in the resistance of the substrates, thus causing variation in cantilever driving. On the contrary, in the cantilever of the present invention, the variation of the parasitic capacitance caused by the variation of length of the wiring is reduced because the parasitic capacitance is low. In other words, the present invention enables reducing greatly the variation of the characteristics among the respective cantilevers. Example 2 Another process for the production of a cantilever in described below by reference to Fig. 3. The basic constitution and effects of this Example are the same as those in Example 1. On both faces of an Si substrate 1, respectively silicon nitride films 2 are formed by reaction of SiH 2 Cl 2 and NH 3 at about 800°C in a thickness of from 100 to 1500 Å by means of an LPCVD apparatus. The silicon nitride film on the back side is subjected to patterning and etching for formation of an Si membrane region 10. Then a silicon oxide film 3 is formed only at the front face by reaction of SiH 4 and O 2 by means of a normal-pressure CVD apparatus, and the resulting film is patterned and etched to obtain constitution as shown in Fig. 3. The subsequent procedure is the same as in Example 1. Incidentally, the same result is obtained by forming the Si membrane 10 before the formation of the silicon oxide film 3. Example 3 A recording-reproducing apparatus which employs the above probe-driving mechanism is described below as an example of the information-processing apparatus of the present invention. Fig. 5 illustrates roughly a recording-reproducing apparatus of the present invention. On a silicon substrate 101, the probe-driving mechanism 102 of the present invention is provided in plural numbers. A piezoelectric element 105 for coarse driving drives the silicon substrate in the Z direction. The numeral 103 denotes a plate-shaped recording medium. Data lines 104 are formed by arranging, in lines, recording areas determined by a scannable region of the probe-driving mechanism. The recording medium 103 is moved by a movement mechanism, not shown in the drawing, in parallel in a direction indicated by the arrow mark in the drawing, and information is written into the recording areas in lines. The probe-driving mechanism 102 and the piezoelectric element 105 for Z-direction coarse-driving are constituted so as to be movable in a direction perpendicular to the data lines by means of a movement mechanism such as a linear motor not shown in the drawing. Thereby any desired data line is accessed to record and reproduce the data. The access to the desired data line is conducted by a position-detecting device such as a linear encorder. Thereafter, each of the probes of the probe-driving mechanism 102 scans the respective recording areas of the intended data lines. The recording layer 103 may be made of a material exhibiting a memory-switching phenomenon (or electric memory effect) in current-voltage characteristics, such as an organic monomolecular film or a built-up film thereof prepared by laminating on an electrode molecules comprising both a group having a π electron level and a group having only a σ electron level as described in Japanese Patent Application Laid-Open No. Sho-63-161552. The electric memory effect enables reversible transition (or switching) between two or more electroconductive states (ON state and OFF state in Fig. 8), namely a low-resistance state (ON state) and a high-resistance state (OFF state) of a thin film such as the aforementioned organic monomolecular film or a built-up film thereof placed between a pair of electrodes by application of a voltage exceeding a threshold value. The respective states can be retained (or memorized) without application of a voltage. The recording medium 103 may also be made of a material which changes the surface state to be convex or concave by local melting or local evaporation by application of a voltage higher than a threshold value, such as a metal film of Au or Pt, and the like. The method of recording and reproduction is described below. The recording is conducted by moving the Z-direction coarse-movement piezoelectric element 105 and the probe-driving mechanism 102 by a movement mechanism to a recording position and then applying a voltage exceeding a threshold value onto the recording medium 103. At that time, a bias voltage is applied to the recording medium 103 by a bias circuit 106, and the probes are held at a distance from the recording medium to allow a tunnel current to flow. The probes are brought close to the recording medium by the Z-direction coarse-movement piezoelectric element 105, and then respective probes are brought into the tunnel region by the probe driving mechanism 102. The probe is kept at a predetermined distance from the recording medium by feedback of the tunnel current, which is detected by the tunnel-current-detecting circuit 107, through the Z-direction serve circuit 110 of the respective probe-driving mechanism. In the Z-direction servo circuit 110, a low-pass-filter is provided, and the cut-off frequency is selected so as not to follow the data signal but to follow the swing of the recording medium and waves on the recording medium, thereby the average distance between the probe and the recording medium is controlled to be constant. At the time of recording, the recording signals are sent from a control circuit 112 to a pulse-applying circuit 108, thereby pulse voltage being applied to respective probes to conduct recording. In the Z direction servo circuit 110, a hold circuit is provided and driving voltage of the probe-driving mechanism 102 to which pulse voltage is applied is maintained so that the pulse application may not cause change of the distance between the probe and the recording medium. Thereby, recording bits are recorded in a matrix form in the recording area of the data lines 104. In the respective recording lines, address information is inserted, which enables discrimination of date on reproduction. The reproduction is conducted as below. In reproduction, the probe is moved to the recording area of the desired data line 104, and the difference of the tunnel current flowing between the recording medium 103 and the probe at the recorded portion from that at the non-recording portion is detected. The probe-driving mechanism 102 is controlled by an XY position-controlling circuit 109 so that the probe scans the entire recording area. The reproduction signal in one recording area is passed through a tunnel-current-detecting circuit 107 and is stored temporarily by signal processing in the control circuit 112. Therefrom, desired data only is reproduced and outputted. In such a recording-reproducing apparatus, the probe-driving mechanism having the constitution of the above Example broadens the region which is scannable with one probe, making one recording area larger, and raising the recording density as a whole. Example 4 An STM apparatus was prepared by use of a cantilever type probe of Example 1. The block diagram of the apparatus is similar to the one of Fig. 5. With this apparatus, a cleavage face of substrate of HOPG (highly oriented pyrolysis graphite) was observed at the bias current of 1 nA, and the scanning area of 100 Å × 100 Å, thereby a satisfactory atomic image being obtained. The surface of the HOPG substrate having small steps was observed in the same manner as above at a scanning area of 500 Å × 500 Å. Thereafter the probe is once removed from the substrate, and then the observation was conducted again, thereby the steps on the surface being observed at the same positions with satisfactory reproductivity. Example 5 A set of piezoelectric actuators for an ink-jet head was prepared by juxtaposing a plurality of actuators. The sectional view and the perspective view thereof are roughly illustrated in Fig. 9A and Fig. 9B, respectively. The piezoelectric actuators were prepared in the same manner as Example 1 immediately before the removal of the substrate except that the tips were not formed. Subsequently, a plurality of nozzle openings 29 were formed on a substrate 28 for nozzles. Further, spacers 27 were formed. The spacers 27 were made of an electroconductive thin plate of a noble metal or the like. The spacers connect upper electrode 8 each other and serve also as leader electrodes. Each lower electrode of the respective piezoelectric actuators was connected to a leader electrode, not shown in the drawing, to apply voltage individually. After registration, the substrate 1 having the piezoelectric actuators and the nozzle substrate 28 were glued together. Finally the substrate at the portions below the piezoelectric actuator was eliminated by anisotropic etching with an aqueous potassium hydroxide as shown in Example 1 except the one side of the end portions of the piezoelectric actuators. The ink-jet head thus prepared is capable of ejecting ink through the nozzle opening 29 by displacing vertically the free end of the piezoelectric actuator caused by the voltage applied to the lower electrode 4 and thereby increasing the pressure of the ink in the vicinity of the nozzle opening 29. By use of a plurality of actuators to which voltage is applied as above selectively and intermittently, desired printing is practiced with satisfactory responsibility to the voltage application. In the present invention, the insulating layer between the substrate and the electrode of the cantilever is left remaining only under the wiring portion on the substrate, and thereafter the electrode layer and the piezoelectric layer are laminated. Therefore, the problem of undesired side etching of Si does not arise in the etching of the insulation layer subsequent to the etching of the Si membrane at the cantilever portion. Accordingly, the insulating layer under the wiring can be made thick and thereby the parasitic capacitance between the wiring portions can be decreased, and the rigidity of the cantilever portion can be lowerd. Thus the variation in parasitic capacitances among the probes can be made low, and the values of the parasitic capacitances can be made small, whereby the controllability of the probe driving is remarkably improved. In addition, the driving characteristics of the cantilever is improved.

Claims: A piezoelectric actuator comprising a bimorph cantilever (4, 5, 6, 7, 8) having a layer structure comprising: two piezoelectric films (5, 7) layered between a plurality of electrodes (4, 6, 8), said cantilever (4, 5, 6, 7, 8) being supported by a support (1) at one end, the actuator being characterised in that a first insulating layer (3) and a second insulating layer (2) are provided between said support (1) and said cantilever (4, 5, 6, 7, 8), said first insulating layer (3) being in contact with said support (1), and said second insulating layer (2) being in contact with said cantilever, the total thickness of the insulating layers (2, 3) reducing towards the edge of the support (1) such that the bimorph cantilever has an inclined portion (12) between the cantilever portion of the bimorph structure and the portion supported on the rest of the support (1). A piezoelectric actuator according to Claim 1, wherein said first and second insulating layers (2, 3) have a total thickness of 5000 Å or more. A piezoelectric actuator according to Claim 1, wherein said first insulating layer (3) comprises a silicon oxide film. A piezoelectric actuator according to Claim 1, wherein said second insulating layer (2) comprises a silicon nitride film. A piezoelectric actuator according to Claim 1, wherein said support (1) comprises silicon. A piezoelectric actuator according to any one of the preceding claims and constituting a probe-driving mechanism, the mechanism further comprising: a probe (9) for information input and output at the end of the cantilever (4, 5, 6, 7, 8) remote from the support (1). A scanning tunnelling microscope (STM) apparatus having: at least one probe-driving mechanism according to Claim 6; a driving means for driving the probe-driving mechanism; a control means for controlling the driving means; means for applying an electric voltage between a specimen and a probe tip; means for detecting an electric tunnel current flowing between the specimen and the probe tip; and means for outputting information from a surface of the specimen according to the detected tunnel current. An information processing apparatus having: at least one probe-driving mechanism (102) according to Claim 6; a driving means (111) for driving the probe-driving mechanism (102); a control means (112) for controlling the driving means; means (106) for applying an electric voltage between a recording medium (103) and a probe tip; means (107) for detecting an electric tunnel current flowing between the recording medium (103) and the probe tip; and means for outputting information from a surface of the recording medium (103) according to the detected tunnel current. An ink-jet head comprising a piezoelectric actuator according to Claim 1 and further comprising a nozzle (29). A process for producing a probe-driving mechanism characterised by comprising the steps of: providing a first insulating layer (3) of not less than 5000 Å thick on a support (1); patterning the first insulating layer (3) by anisotropic etching to form an inclined portion (12) which reduces in thickness towards the edge of the support (1); providing a second insulating layer (2) on the first insulating layer (3); laminating a plurality of electrode layers (4, 6, 8) and piezoelectric layers (5, 7) successively on the second insulating layer (2); and removing part of the support (1) by etching to form a bimorph cantilever. A process for producing a probe-driving mechanism of Claim 10, wherein the support (1) comprises silicon. A process for producing a probe-driving mechanism of Claim 10, wherein the first insulating layer (3) comprises a silicon oxide film. A process for producing a probe-driving mechanism of Claim 10, wherein the second insulating layer (2) comprises a silicon nitride film. A process for producing a probe-driving mechanism of Claim 10, wherein the part of the support (1) is removed by anisotropic etching. A process for producing a piezoelectric actuator comprising the process of any one of Claims 10 to 14, and the additional step of forming a probe (9) for information input and output at the end of the cantilever remote from the support (1).

Inventors: HIRA YUTAKA, KASANUKI YUJI, NAKAYAMA MASARU, SHIMADA YASUHIRO, SUZUKI YOSHIO, YAGI TAKAYUKI, YAMAMOTO KEISUKE, HIRA, YUTAKA, KASANUKI, YUJI, NAKAYAMA, MASARU, SHIMADA, YASUHIRO, SUZUKI, YOSHIO, YAGI, TAKAYUKI, YAMAMOTO, KEISUKE, Hira, Yutaka, c/o Canon Kabushiki Kaisha, KASANUKI, YUJI, C/O CANON KABUSHIKI KAISHA, NAKAYAMA, MASARU, C/O CANON KABUSHIKI KAISHA, Shimada, Yasuhiro, c/o Canon Kabushiki Kaisha, Suzuki, Yoshio, c/o Canon Kabushiki Kaisha, YAGI, TAKAYUKI, C/O CANON KABUSHIKI KAISHA, YAMAMOTO, KEISUKE, C/O CANON KABUSHIKI KAISHA
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1992
Main classification: H01L41
IPC codes: G01N13, H01L41, B81B3, G01N37, G12B21, H01J37, B41J2, G11B9, G01B21