JP2007010378A - Surface acoustic wave element, its manufacturing method, surface acoustic wave sensor and surface acoustic wave sensor system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To use a structure of a diamond SAW element, which keeps a diamond layer, a piezoelectric body layer and a reed screen transducer (IDT) overlying the surface of a substrate, in the transducer of an SAW sensor as it is to enhance the frequency thereof. <P>SOLUTION: The SAW element 1 is formed by making the diamond layer 3, the piezoelectric body layer 4 of ZnO and an insulating protective film 12 of input and output IDTs 6 and 8, which comprise cross digital electrodes, and SiO<SB>2</SB>overlie the surface of an Si substrate 2. The piezoelectric body layer and insulating protective film between both IDTs are partially removed to form a response part 11 wherein the surface of the diamond layer is exposed. A receptor for recognizing a physiological or chemical substance being a measuring target is fixed to the surface of the response part to obtain the SAW sensor. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention is, for example, a sensor that detects a target biological substance or chemical substance and / or measures a physical property thereof, and is used as a transducer that converts a chemical or physical change generated at that time into an electrical signal. The present invention relates to a suitable surface acoustic wave device and a method for manufacturing the same. Furthermore, the present invention relates to a surface acoustic wave sensor and sensor system using the surface acoustic wave element.

  Recently, especially in technical fields such as biotechnology and medicine, piezoelectric elements such as quartz resonators have been used as transducers to detect chemical or physical changes in receptors that recognize biological substances or chemical substances to be measured. A piezoelectric sensor has been developed (see, for example, Non-Patent Document 1, Patent Documents 1 and 2). As the transducer, a SAW element using a surface acoustic wave (SAW) excited by an IDT (interdigital transducer) composed of interdigitated electrodes formed on the surface of the piezoelectric substrate is used as a piezoelectric element other than the crystal resonator. (For example, see Patent Documents 3 to 6).

  In addition, biosensors that analyze biological materials such as DNA have oligonucleotides that react with the measurement target when a surface treatment layer made of a carbon-based material such as diamond or diamond-like carbon (DLC) is formed on the substrate and chemically modified. It is known that a physiologically active substance can be stably and strongly fixed (see, for example, Patent Documents 2 and 7). Usually, such a biosensor forms a surface treatment layer made of a DLC film or the like on a gold electrode formed on the surface of a crystal plate of a crystal resonator, and chemically activates this to activate an oligonucleotide or the like. Create fixed.

On the other hand, recently, a so-called diamond SAW element has been developed in which an IDT is formed on a substrate on which a diamond thin film and a piezoelectric film are laminated, thereby increasing the SAW propagation speed and enabling higher frequencies (for example, non-diamond SAW elements). (See Patent Documents 2 and 3 and Patent Document 8). Furthermore, this SAW element can obtain a temperature characteristic more excellent than that of a conventional SAW element using a quartz substrate by laminating a SiO ₂ film on the uppermost layer (see, for example, Non-Patent Document 3).

Industrial Property General Information Center, “Patent Distribution Support Chart / Chemical 2 / Biosensor”, Invention Association of Japan, June 29, 2002, p. 3-5 and 16-18 Shinichi Shikata, 8 others, “Diamond SAW Device (1)”, NEW DIAMOND, Ohmsha, April 1999, Vol. 53, Vol. 15No. 2, p. 1-6 Shinichi Shikata, 8 others, “Diamond SAW Device (2)”, NEW DIAMOND, Ohmsha, July 1999, Vol. 54 Vol. 15No. 3, p. 9-16 JP 2000-65708 A JP 2003-172737 A JP-A-6-133759 JP-A-8-68781 JP 2002-48797 A Japanese Patent Laid-Open No. 9-80035 JP 2001-204463 A JP-A-7-27391

  As described above, a sensor using a piezoelectric element is expected to have a high-frequency piezoelectric element depending on the measurement object, conditions, application, and the like. In that case, it would be advantageous if a diamond SAW element could be used as the transducer of the sensor, but as far as the inventors of the present application know, there are no examples in which such a SAW sensor is actually used.

  In the conventional piezoelectric sensor, a receptor such as a cell or an adsorbent for fixing a target biological substance or chemical substance is formed on the surface of the SAW element. Similarly, when a diamond SAW element is used as a transducer, an extremely hard diamond or DLC film is formed on the surface of the SAW element in order to immobilize an oligonucleotide or a physiologically active substance that reacts with a measurement target as a receptor. There is a risk that the propagation speed of the SAW changes and the characteristics of the SAW element are deteriorated.

  The DLC film is generally formed by using a known vapor deposition method, sputtering method, ion plating method, plasma CVD method, ECRCVD method, inductively coupled plasma method, or the like. These film forming methods are often performed in a high temperature environment. Therefore, the SAW element has a problem that the Al electrode forming the IDT is damaged by high temperature when the DLC film is formed.

  Accordingly, the present invention has been made in view of the above-described conventional problems, and an object of the present invention is a diamond SAW element in which a diamond layer, a piezoelectric layer, and an IDT are laminated on the surface of a substrate, and the diamond layer It is intended to provide a novel SAW element that can fix a receptor for recognizing a biological substance or chemical substance to be measured and can be used as a transducer of a SAW sensor and a method for manufacturing the same.

  A further object of the present invention is to provide a SAW sensor and a SAW sensor system that can be used for measurement conditions and applications suitable for high-frequency SAW elements by using such SAW elements as transducers.

  According to the present invention, in order to achieve the above object, a substrate on which a diamond layer and a piezoelectric layer are laminated, an IDT composed of at least one pair of crossed finger electrodes formed on the substrate, and excited by the IDT Provided is a SAW element comprising a pair of reflectors arranged on both sides of an IDT so as to sandwich the IDT along the SAW propagation direction, and a sensitive part exposing the surface of the diamond layer along the SAW propagation direction Is done.

  By using the diamond SAW element having such a conventional structure as it is, an exposed diamond layer can be used as a sensitive part, and a receptor for recognizing a measurement object can be provided on the surface thereof. In this case, there is no need to form a separate diamond layer on the surface of the SAW element that may change the SAW propagation speed, eliminating the possibility of adversely affecting the characteristics of the SAW element or damaging the IDT Al electrode. Is done. Therefore, a high-quality and high-performance SAW sensor can be realized by using this SAW element as a transducer.

  In one embodiment, the IDT of the SAW element is composed of an excitation IDT and a reception IDT, and a sensitive unit is disposed between the excitation IDT and the reception IDT. Such a 2-port resonator type SAW element is suitable for excitation at a high frequency because the configuration of the oscillation circuit can be simplified.

  In another embodiment, the IDT of the SAW element is a one-port resonator type consisting of one IDT having a pair of cross finger electrodes, and one IDT can be omitted as compared with the two-port resonator type. It is possible to reduce the size and the manufacturing cost. In this case, the sensitive part can be disposed between the electrode fingers of the interdigitated electrodes, or can be disposed between the IDT and one reflector.

In some embodiments, an insulation protective film made of, for example, SiO ₂ is further provided so as to cover the entire IDT. As a result, a short circuit between the crossed finger electrodes can be surely prevented, and damage and corrosion of the crossed finger electrodes can be prevented regardless of the use environment and conditions, particularly when used as a transducer of the SAW sensor. Furthermore, the insulating protective film of SiO ₂ can cancel out the temperature coefficient of the piezoelectric layer and the diamond layer formed on the lower layer side, and can obtain excellent temperature characteristics.

In another embodiment, it further includes an insulating film formed on the surface of the cross finger electrode, thereby similarly preventing the short circuit between the cross finger electrodes, and at the time of use regardless of the environment and conditions. Damage and corrosion of the finger electrode can be prevented. In this case, the interdigitated electrode is formed inexpensively, for example, with Al or an alloy containing Al as a main component, and the insulating film can be easily formed with Al ₂ O ₃ by oxidizing it.

  According to another aspect of the present invention, a SAW element is formed by laminating a diamond layer, a piezoelectric layer, and an IDT composed of at least one pair of cross finger electrodes on the surface of a substrate, and using a photolithography technique. There is provided a method of manufacturing a SAW element, which includes a process of etching a surface of the SAW element and exposing a surface of the diamond layer to form a sensitive portion.

  When the piezoelectric layer is formed on the surface of the diamond layer, it can be exposed using conventionally known dry etching or wet etching. Therefore, the SAW element of the present invention suitable for the SAW sensor can be easily manufactured by using the manufacturing process of the conventional diamond SAW element as it is and adding the process of forming the sensitive part to the process.

  In one embodiment, in the process of forming the SAW element, an insulating protective film that covers the entire IDT of the SAW element is further laminated to reliably prevent a short circuit between the crossed finger electrodes and Regardless of the conditions, a SAW element capable of preventing damage and corrosion of the interdigital electrodes can be obtained.

  In another embodiment, in the process of forming the SAW element, by further forming an insulating film covering the surface of the cross finger electrode, similarly, the short circuit between the cross finger electrodes can be surely prevented and the cross finger electrodes can be used in use. A SAW element capable of preventing damage and corrosion of the substrate is obtained.

  According to another aspect of the present invention, as described above, the SAW element of the present invention suitable for high frequency and a receptor for recognizing a target substance fixed to the sensitive part of the SAW element. A SAW sensor is provided.

  Furthermore, according to the present invention, there is provided a multi-channel SAW sensor system having a plurality of such SAW sensors of the present invention and providing these SAW sensors on a single common substrate.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.
1A and 1B show a SAW element 1 having a two-port resonator type filter structure to which the present invention is applied. The SAW element 1 has a rectangular thin plate substrate 2 made of, for example, silicon. A diamond layer 3 is formed on the main surface of the substrate 2, and a piezoelectric layer 4 made of, for example, ZnO is laminated thereon. By using this diamond laminated substrate, the SAW element 1 can increase the SAW propagation speed as compared with the case of a piezoelectric substrate such as quartz crystal, thereby achieving a higher frequency.

The substrate 2 may be made of a semiconductor material other than silicon, a glass material such as Pyrex glass, a ceramic material, or a resin material such as polyimide or polycarbonate. The diamond layer 3 can be formed of a thin film of polycrystalline diamond or single crystal diamond synthesized by a conventionally known method such as a diamond-like carbon (DLC) film. The piezoelectric layer 4 can be formed of various known piezoelectric materials such as AlN (aluminum nitride), LiTaO ₃ , LiNbO ₃ , and K ₄ Nb ₆ O ₁₇ (potassium niobate) in addition to ZnO.

  The substrate 2 has an input IDT 6 composed of a pair of crossed finger electrodes 5a and 5b on the left and right in the longitudinal direction and an output composed of a pair of crossed finger electrodes 7a and 7b (received). IDT 8 is formed on the piezoelectric layer 4. A pair of reflectors 9 and 10 are arranged on the left and right sides of both IDTs 6 and 8, respectively. Further, the input and output crossing finger electrodes are connected to an external oscillation circuit via input and output connection lands (not shown), respectively.

  The crossed finger electrode and the reflector are formed of an electrode film made of Al or an alloy containing Al as a main component from the viewpoint of workability and cost, and, as in the past, for example, a desired etching process is performed by wet etching using a photolithography technique. Processed into a pattern. In another embodiment, an Al / Cu film can be used as the electrode film.

  The piezoelectric layer 4 is provided with a sensitive portion 11 in which a rectangular window is opened between the IDTs 6 and 8 along the SAW propagation direction to expose the surface of the diamond layer 3. When the SAW element 1 is used as a transducer of a SAW sensor, a receptor for recognizing a measurement target can be disposed on the sensitive unit 11. As the receptor, various conventionally known substances such as DNA, enzymes, microorganisms, antibodies, gas adsorbents, and the like can be used in accordance with the properties and characteristics of the biological substance or chemical substance to be measured. It is used in various forms known in the art, such as a membrane or a cell in which is fixed.

  As shown by an imaginary line in FIG. 1B, an insulating protective film 12 covering the IDTs 6 and 8 and the reflectors 9 and 10 can be formed on the uppermost layer of the substrate 2. When the insulating protective film 12 is used as a transducer of a SAW sensor when a foreign substance such as dust adheres to the surface of the SAW element 1 and the adjacent interdigitated electrodes are electrically short-circuited or used as a transducer of a SAW sensor, the insulating protective film 12 Damage and corrosion of the interdigital electrodes and / or reflectors can be effectively prevented.

The insulating protective film 12 is easily formed to a desired thickness by sputtering or vapor-depositing SiO ₂ , for example. In addition to SiO ₂ , various insulating materials such as oxides such as Ta ₂ O ₅ and nitrides such as Si ₃ N ₄ and TiN can be used for the insulating protective film. In particular, when an SiO ₂ film is used, the temperature coefficient of the piezoelectric layer and the diamond layer formed on the lower layer side can be canceled, and thereby excellent temperature characteristics can be obtained.

  When a predetermined high-frequency signal voltage is applied to the excitation IDT 6, SAW of the same frequency is excited in the diamond layer 3, propagates to the left and right sides of the IDT 6, and is reflected by the left and right reflectors 9 and 10. In the meantime, a SAW standing wave is generated. This SAW is received by the receiving IDT 8 and its frequency is measured. At this time, if the receptor of the sensitive unit 11 fixes the target biological substance or chemical substance and its weight changes, a change in the SAW propagation velocity is detected as a frequency change. Substance detection and / or physical property measurement can be performed with high accuracy.

Next, a process of manufacturing the SAW element 1 of FIG. 1 by the method of the present invention will be described with reference to FIGS. First, a normal diamond SAW element is prepared according to a conventional manufacturing process. That is, a diamond thin film or diamond layer 3 made of DLC and a piezoelectric layer 4 made of ZnO are laminated on the surface of a substrate 2 made of silicon. The DLC film is formed by using a known method such as an evaporation method, a sputtering method, an ion plating method, a plasma CVD method, an ECRCVD method, or an inductively coupled plasma method. The ZnO film can be formed using, for example, a vapor deposition method or a sputtering method. By forming an electrode film made of Al or an alloy containing Al as a main component on the surface of the piezoelectric layer 4 and patterning it using a photolithography technique, the IDTs 6 and 8, the connection lands, wirings connecting them and reflections A vessel 9, 10 is formed. Further in this embodiment, an insulating protective film 12 made of SiO ₂ on the entire surface of the piezoelectric layer 4 formed and IDT6,8 and the reflectors 9 and 10, for example, is formed by sputtering or vapor deposition.

A photoresist film 13 is formed on the entire surface of the insulating protective film 12 of the diamond SAW element thus formed. The photoresist film 13 is patterned using a photomask 14 disposed thereabove (FIG. 2A). Thereby, the rectangular region of the insulating protective film 12 corresponding to the sensitive portion 11 is exposed (FIG. 2B). The exposed insulating protective film 12 is removed, for example, by wet etching using buffered hydrofluoric acid to expose the surface of the piezoelectric layer 4 (FIG. 2C). In another embodiment, the exposed region of the insulating protective film 12 can be removed by dry etching such as plasma etching using CF ₄ gas.

  Next, the exposed piezoelectric layer 4 is removed, for example, by wet etching using dilute hydrochloric acid, and the rectangular window corresponding to the sensitive portion 11 is opened to expose the surface of the diamond layer 3 (FIG. 2D). ). Thereby, the rectangular sensitive part 11 which consists of a diamond layer can be provided between both IDT6,8. Finally, the remaining photoresist film 13 is removed to obtain the SAW element 1 of the present invention (FIG. 2E).

  Furthermore, the SAW sensor 1 of the present invention can be obtained by using the SAW element 1 thus obtained as a transducer and arranging a receptor for recognizing the measurement object in the sensitive portion 11. The surface of the sensitive part 11 made of a diamond layer is chemically modified in order to stably and strongly fix the oligonucleotide or the physiologically active substance in the analysis of DNA or protein, for example, as described above with reference to the prior art. Can be activated.

  At this time, the SAW element is immersed in a predetermined activation solution. However, since the IDTs 6 and 8 are completely covered with the insulating protective film 12, there is no risk of corrosion or deterioration. In addition, the insulating protective film 12 allows the SAW sensor of the present invention to be electrically short-circuited between the interdigitated electrodes even when used for a liquid sample, and to provide various environments such as a gas phase and a liquid phase. Even when used underneath, there is no risk of damaging, corroding or deteriorating the interdigitated electrodes, and high reliability can be ensured.

  3A and 3B show a modification of the SAW element of the present invention shown in FIG. In the SAW element 21 of this modification, insulating films 22 to 25 having a certain thickness are respectively formed on the surfaces of the crossed finger electrodes 5a, 5b, 7a, and 7b. Similarly, the insulating films 22 to 25 can surely prevent a short circuit between the interdigital electrodes, and can prevent the interdigital electrodes from being damaged or corroded in various environments during use. These insulating films can be easily formed at a desired thickness and at low cost by, for example, anodizing the Al forming the crossed finger electrodes.

  4A, 4B, 5A, and 5B each show another modification of the SAW element according to the present invention. Each of the SAW elements 31 and 41 of these modified examples includes one set of cross finger electrodes 32a, 32b, 42a, and 42b on the surface of a rectangular thin plate 2 on which the diamond layer 3 and the piezoelectric layer 4 are laminated. IDTs 33, 43 and reflectors 34, 35, 44, 45 on the left and right sides thereof, respectively, have a one-port resonator type configuration. In these modifications, since one IDT can be omitted as compared with the two-port resonator type of each of the above embodiments, it is possible to reduce the size and the manufacturing cost.

  In the SAW element 31 shown in FIGS. 4A and 4B, the sensitive portion 36 with the surface of the diamond layer 3 exposed forms cross finger electrodes 32a and 32b in the center of the IDT 33 along the SAW propagation direction. It is arrange | positioned in the position between an electrode finger. The sensitive part 36 can be provided with a receptor in the same manner as in the above embodiments, whereby a 1-port resonator type SAW sensor using the SAW element 31 as a transducer is obtained.

  In the SAW element 41 shown in FIGS. 5A and 5B, the sensitive portion 46 exposing the surface of the diamond layer 3 is disposed at a position between the IDT 43 and one reflector 45 along the SAW propagation direction. ing. The sensitive portion 46 can be similarly provided with a receptor, whereby a 1-port resonator type SAW sensor having the SAW element 41 as a transducer is obtained.

  Also in these 1-port resonator type SAW sensors, when a predetermined high-frequency signal voltage is applied between the interdigitated electrodes of the IDTs 33 and 43, SAW of the same frequency is excited by the diamond layer 3, and on both the left and right sides of the IDT. It propagates and is reflected by the left and right reflectors, and a SAW standing wave is generated between the two reflectors. In this state, when the receptors arranged in the sensitive units 36 and 46 fix the biological material to be detected, a change in weight due to the change becomes a change in SAW propagation speed, and is detected as a change in frequency. Thereby, the detection of the target biological substance or chemical substance and / or the measurement of physical properties can be performed with high accuracy.

Also in the SAW elements 31 and 41 of the modified example, an insulating protective film such as SiO ₂ covering the entire IDTs 33 and 43 is formed as in the embodiment of FIG. Damage and corrosion of the finger electrode can be prevented. Further, as in the embodiment of FIG. 3, an insulating film can be formed on the surfaces of the crossed finger electrodes 32a, 32b, 42a, 42b.

  The preferred embodiments of the present invention have been described in detail above. However, as will be apparent to those skilled in the art, the present invention can be carried out with various modifications and changes made to the above embodiments within the technical scope thereof. . For example, in the above embodiment, a diamond SAW element having a film layer structure in which a diamond layer and a piezoelectric layer are sequentially laminated on a substrate and having an IDT formed on the piezoelectric layer has been described. The same can be applied to various known diamond SAW elements having different film layers and IDT structures.

(A) is a plan view showing an embodiment of a SAW element according to the present invention, and (B) is a longitudinal sectional view thereof. (A)-(E) figure is a longitudinal cross-sectional view which shows the manufacturing process of the SAW element of FIG. (A) is a plan view showing a modification of the SAW element according to the present invention, and (B) is a longitudinal sectional view thereof. (A) is a plan view showing another modification of the SAW element according to the present invention, and (B) is a longitudinal sectional view thereof. (A) is a plan view showing still another modification of the SAW element according to the present invention, and (B) is a longitudinal sectional view thereof.

Explanation of symbols

1, 2, 31, 41 ... SAW element, 2 ... substrate, 3 ... diamond layer, 4 ... piezoelectric layer, 5a, 5b, 7a, 7b, 32a, 32b, 42a, 42b ... crossed finger electrodes, 6, 8, 33, 43 ... IDT, 9, 10, 34, 35, 44, 45 ... reflector, 11, 36, 46 ... sensitive part, 12 ... insulating protective film, 13 ... photoresist film, 14 ... photomask, 22-25 ... insulating film.

Claims (13)

  A substrate in which a diamond layer and a piezoelectric layer are laminated, an IDT composed of at least one pair of interdigital electrodes formed on the substrate, and the IDT sandwiched along a propagation direction of a surface acoustic wave excited by the IDT A surface acoustic wave device comprising: a pair of reflectors disposed on both sides thereof; and a sensitive portion exposing a surface of the diamond layer along a propagation direction of the surface acoustic wave.   2. The surface acoustic wave device according to claim 1, wherein the IDT includes an excitation IDT and a reception IDT, and the sensitive portion is disposed between the excitation IDT and the reception IDT. .   The IDT is composed of one IDT having a pair of cross finger electrodes, and the sensitive part is disposed between the electrode fingers of the cross finger electrodes. Surface acoustic wave device.   The elastic surface according to claim 1, wherein the IDT is composed of one IDT having a pair of cross finger electrodes, and the sensitive portion is disposed between the IDT and one of the reflectors. Wave element.   5. The surface acoustic wave device according to claim 1, further comprising an insulating protective film that covers the entire IDT. The surface acoustic wave device according to claim 5, wherein the insulating protective film is made of SiO ₂ .   5. The surface acoustic wave device according to claim 1, further comprising an insulating film formed on the surface of the interdigitated electrode. The alternate finger electrodes are formed of an alloy composed mainly of Al or Al, and a surface acoustic wave device according to claim 7, wherein the insulating film is characterized in that it is formed by Al ₂ O _3.   A process of forming a surface acoustic wave device by laminating a diamond layer, a piezoelectric layer, and an IDT comprising at least one pair of interdigitated electrodes on the surface of the substrate; and a surface of the surface acoustic wave device using a photolithography technique And a step of etching to expose the surface of the diamond layer to form a sensitive portion.   The method for manufacturing a surface acoustic wave element according to claim 9, wherein an insulating protective film that covers the entire IDT of the surface acoustic wave element is further laminated in the process of forming the surface acoustic wave element.   The method for manufacturing a surface acoustic wave element according to claim 9, further comprising forming an insulating film that covers a surface of the interdigitated electrode in the process of forming the surface acoustic wave element.   An elastic device comprising: the surface acoustic wave device according to claim 1; and a receptor fixed to the sensitive portion of the surface acoustic wave device for recognizing a target substance. Surface wave sensor.   A surface acoustic wave sensor system comprising a plurality of surface acoustic wave sensors according to claim 12, wherein the plurality of surface acoustic wave sensors are provided on a single common substrate.

Images