Method of fabricating a pyroelectric infrared detector

Abstract: A pyroelectric infrared detector includes a substrate having a recess. A pyroelectric portion substantially aligns with the recess. A resin film is located between the substrate and the pyroelectric portion. The recess faces the resin film. First and second electrodes are connected to first and second surfaces of the pyroelectric portion respectively. The pyroelectric portion may include a pyroelectric film of Pb x La y Ti z Zr w O 3 where atomic fractions "w", "x", "y", and "z" satisfy one of following conditions a), b), and c): a) 0.7≦x≦1, x+y=1 , 0.925≦z≦1, w=0 b) x=1, y=0, 0.45≦z<1, z+w=1 c) 0.75≦x<1, x+y=1 , 0.5≦z<1, z+w=1

Description: BACKGROUND OF THE INVENTION Field of the Invention This invention relates to a pyroelectric infrared detector. This invention also relates to a method of fabricating a pyroelectric infrared detector. Description of the Prior Art R. Takayama et al reported high sensitive pyroelectric linear array infrared sensors fabricated by using highly c-axis-oriented and spontaneous polarized La-modified PbTiO 3 (PLT) thin films (Journal of Applied Physics 63(12), 15 June 1988, p5868-5872). The PLT thin films were deposited on (100)-orientated Pt thin film on (100)-cleaved MgO single crystals by RF magnetron sputtering. The PLT thin films had high figures of merit for voltage responsivity (Rv) and specific detectivity (D*) without a poling treatment. Since the pyroelectric infrared sensor reported by R. Takayama et al uses an MgO single crystal as a substrate, it is generally difficult to form the pyroelectric film and an external circuit on a common substrate. The MgO substrate has a high thermal conductivity. Thus, a portion of the MgO substrate which extends directly below a major part of the pyroelectric film is etched off to suppress heat transfer from the pyroelectric film to the substrate to attain a high infrared sensitivity. Japanese published unexamined patent application 4-170077 discloses a pyroelectric infrared detector fabricated as follows. A pyroelectric film was formed on a first substrate made of an MgO single crystal. The pyroelectric film and exposed portions of the first substrate were coated with an organic film. A second substrate having a central opening was bonded to the organic film by adhesive. Finally, the first substrate was etched off to complete the pyroelectric infrared detector in which the pyroelectric film was supported on the second substrate via the organic film. The pyroelectric film was located above the opening in the second substrate. In the pyroelectric infrared detector of Japanese application 4-170077, the pyroelectric film was made of Pb x La y Ti z Zr w O 3 where the atomic fractions "x", "y", "z", and "w" satisfied one of the following three different conditions (a), (b), and (c): (a) 0.7≦x≦1, 0.9≦x+y<1, 0.95≦z≦1, w=0 (b) x=1, y=0, 0.45≦z<1, z+w=1 (c) 0.83≦x≦1, x+y=1, 0.5≦z<1, 0.96≦z+w≦1 During the fabrication of the pyroelectric infrared detector of Japanese application 4-170077, the first substrate made of MgO was etched off as previously described. Generally, the removal of the MgO substrate is disadvantageous in cost and time. The opening extends through walls of the second substrate. Thus, during the etching process, etchant tended to reach the organic film and the pyroelectric film via the opening in the second substrate. In this case, the etchant adversely affected the organic film and the pyroelectric film. In some cases, the pyroelectric film was broken. Thus, it is difficult to improve the yield of detectors. On the other hand, JP-A-57-104380 discloses an infrared ray solid-state image pick-up device comprising a substrate consisting of Si-crystal layers 4, 6 of p-type which sandwich an impurity layer 5 of p + -type. This semiconductor substrate is covered with an isolation layer 8 on which a pyroelectric crystal 12 is provided. For decreasing the heat capacitance and improving the sensitivity of the sensor, Si crystal 6 under the isolation film 8 is corroded. Thus, an infrared ray solid-state image pick-up device is obtained. However, a Si semiconductor substrate is not suitable for providing excellent protection from heat transfer from the pyroelectric film to the substrate. Further, EP-A-0 417 846 relates to an infrared detector element (20) comprising a flexible continuous film (10) and a mounting arrangement for the film consisting of mutually-spaced supports (32) of a molded plastic substrate (30), thereby forming cavities (35). However, the structure is highly complicated and therefore a time-consuming and costly preparation method is inevitable. Moreover, US-A-4,532,424 discloses a thermal detector array including a Si substrate layer to which a pyroelectric layer is attached. A plurality of detector regions are defined in the pyroelectric layer by etching cavities into the surface of the pyroelectric layer. Therefore, a continuous pyroelectric layer is not provided, facilitating heat transfer to the respective pyroelectric portions. Therefore, none of the detectors of the prior art references acknowledges or even overcomes the problem of heat transfer from a pyroelectric portion thereof to the substrate. Consequently, a desired high-performance pyroelectric infrared detector could not be provided. SUMMARY OF THE INVENTION It is an object of this invention to provide an improved method of fabricating a pyroelectric infrared detector. Therefore, this invention provides a method of fabricating a pyroelectric infrared detector as defined in claim 1. Preferred embodiments thereof are defined in the dependent claims. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a sectional view of a pyroelectric infrared detector fabricated according to Example 1 of this invention. Figs. 2(a)-2(g) are sectional views of a detector substrate in various states which occur at respective steps in the fabrication of the pyroelectric infrared detector according to Example 1 of this invention. Fig. 3 is a diagram of a plasma-enhanced MO-CVD apparatus used in the fabrication of a pyroelectric infrared detector according to Example 2 of this invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS A pyroelectric infrared detector fabricated according to this invention includes a substrate having a recess; a pyroelectric portion substantially aligning with the recess: a resin film located between the substrate and the pyroelectric portion, wherein the recess faces the resin film thus forming a cavity; and first and second electrodes connected to first and second surfaces of the pyroelectric portion respectively. It is preferable that the pyroelectric portion includes a pyroelectric film of Pb x La y Ti z Zr w O 3 where atomic fractions "w", "x", "y", and "z" satisfy one of following conditions a), b), and c): a) 0.7≦x≦1, x+y=1, 0.925≦z≦1, w=0 b) x=1, y=0, 0.45≦z<1, z+w=1 c) 0.75≦x<1, x+y=1, 0.5≦z<1, z+w=1 When the conditions b) are satisfied, the crystal structure of Pb x La y Ti z Zr w O 3 is limited to a tetragonal system. In this case. better infrared detecting characteristics are attained provided that the axis of polarization in the pyroelectric crystal is set perpendicular to a major surface of the substrate. The pyroelectric portion may include a pyroelectric film oriented in a direction along an axis of polarization. According to this invention, a method of fabricating a pyroelectric infrared detector includes the steps of forming a pyroelectric portion on a given area of a surface of a first substrate; forming a resin film covering the pyroelectric portion and an exposed area of the surface of the first substrate; bonding a second substrate to the resin film, wherein the second substrate having a recess which faces the resin film and which substantially aligns with the pyroelectric portion thus forming a cavity; and etching off and removing the first substrate from the pyroelectric portion and the resin film. The first substrate may be made of a material which allows the pyroelectric portion to be oriented in a direction of polarization and which can be easily etched off. Example 1 With reference to Fig. 1, a pyroelectric infrared detector according to Example 1 includes a substrate 16 which has a central recess 16a at the upper surface thereof. A resin film 14 extending above the substrate 16 is bonded to the substrate 16 via an adhesive layer 15. The adhesive layer 15 extends outward of the recess 16a. The adhesive layer 15 is sandwiched between the substrate 16 and the resin film 14. An upper surface of the resin film 14 has a recess filled with a pyroelectric film 12. An upper surface of the pyroelectric film 12 is substantially flush with the upper surface of the resin film 14. An electrode layer 13 extends between a lower surface of the pyroelectric film 12 and the resin film 14, extending upward along a side of the pyroelectric film 12 and then extending outward along the upper surface of the resin film 14. Another electrode layer 17 extends on a given area of the upper surface of the pyroelectric film 12, and then extends outward along the upper surface of the resin film 14. In this way, the electrode layers 13 and 17 are electrically connected to the lower and upper surfaces of the pyroelectric film 12 respectively. The substrate 16 is spaced from the resin film 14 by the adhesive layer 15. The substrate 16, the resin film 14, and the adhesive layer 15 define an inner space or cavity, the lower portion of which is formed by the recess 16a in the substrate 16. The pyroelectric film 12 substantially aligns with the inner space and the recess 16a in the substrate 16. In addition, the pyroelectric film 12 is located above the inner space, and is separated therefrom by the resin film 14. Accordingly, the resin film 14 and the inner space suppress the escape of heat from the pyroelectric film 12 to the substrate 16. This design enables a high infrared sensitivity. The pyroelectric film 12 is made of Pb 0.9 La 0.1 Ti 0.975 O 3 . The electrode layers 13 and 17 are made of Ni-Cr. The resin film 14 is made of polyimide resin. The adhesive layer 15 is made of epoxy resin. The substrate 16 is made of alumina (Al 2 O 3 ). The pyroelectric infrared detector of Fig. 1 was fabricated as follows. With reference to Fig. 2(a), a first substrate 11 made of a (100) oriented MgO single crystal was prepared. As shown in Fig. 2(b), a pyroelectric film 12 made of Pb 0.9 La 0.1 Ti 0.975 O 3 was formed on the first substrate 11. The formation of the pyroelectric film 12 used an RF magnetron sputtering apparatus. Specifically, the first substrate 11 was provided with a stainless mask having a thickness of 0.2 mm and having a configuration corresponding to a desired shape of the pyroelectric film 12. The first substrate 11 with the mask was attached to a stainless holder in the RF magnetron sputtering apparatus. A mixture of PbO powder, La 2 O 3 powder, and TiO 2 powder was used as a target placed in a copper dish in the RF magnetron sputtering apparatus. The mixing ratio among the PbO powder, the La 2 O 3 powder, and the TiO 2 powder was chosen so that the amount of the PbO powder was in excess of the amount corresponding to the composition of the pyroelectric film 12 by 20 mol%. The RF magnetron sputtering apparatus was activated so that a pyroelectric film 12 was deposited on a given area (an exposed area) of the first substrate 11 by sputtering. During the sputtering, the first substrate 11 remained heated at a temperature of 600°C. Furthermore, sputtering gas composed of a mixture of argon (90%) and oxygen (10%) was used in the RF magnetron sputtering apparatus. The pressure of the sputtering gas was equal to 0.9 Pa. The RF power density in the RF magnetron sputtering apparatus was equal to 2.0 W/cm 2 . The RF power had a frequency of 13.56 MHz. The pyroelectric film 12 had a thickness of 3 µm. After the formation of the pyroelectric film 12, an Ni-Cr film having a thickness of 100 nm was formed on the pyroelectric film 12 and a given area of the first substrate 11 by sputtering. As shown in Fig. 2(c), the Ni-Cr film was made into an electrode layer 13 and extensions thereof by a patterning process using photolithography. Subsequently, as shown in Fig. 2(d), polyimide resin was applied to exposed areas of the first substrate 11, the electrode layer 13, and the pyroelectric film 12 by spin coat, being polymerized into a resin film 14. As shown in Fig. 2(e), epoxy resin forming an adhesive layer 15 was applied to given areas of the resin film 14. Then, a second substrate 16 was attached to the resin film 14 on the first substrate 11 by the adhesive layer 15. The adhesive layer 15 was hardened or polymerized, completely bonding the resin film 14 and the second substrate 16 together. The second substrate 16 had a central recess 16a facing the resin film 14. The second substrate 16 was made of alumina (Al 2 O 3 ). Subsequently, the first substrate 11 was etched off by etchant such as an aqueous solution of orthophosphoric acid. Thus, as shown in Fig. 2(f), the first substrate 11 was removed from the pyroelectric film 12, the resin film 14, and the electrode layer 13. Finally, as shown in Fig. 2(g), an Ni-Cr film having a thickness of 10 nm was formed on given regions of exposed areas of the pyroelectric film 12 and the resin film 14, being made into an electrode layer 17 and extensions thereof by a patterning process. Since the exposed areas of the pyroelectric film 12 and the resin film 14 were flat and flush with each other, the patterning of the electrode layer 17 was easy. The second substrate 16 was previously provided with a wiring pattern made of Ag-Pd conductive paste. This design enabled the pyroelectric film 12 and a peripheral circuit (an external circuit) to be mounted on a common substrate, that is, the second substrate 16. Therefore, the pyroelectric infrared detector was small in size. During the etching process to remove the first substrate 11, the recessed configuration of the second substrate 16 prevented the etchant from entering the inner space or cavity and then reaching the resin film 14 and the pyroelectric film 12. Samples of the pyroelectric infrared detector according to Example 1 were prepared. Experiments were performed on these samples which were not subjected to a poling treatment. With respect to the samples, the productive yield was equal to 92%. The sensing portions of the samples had a pyroelectric coefficient "γ" of 5.5 × 10 -8 C/cm 2 K, a relative dielectric constant εr of 180, and a volume specific heat Cv of 3.2 Jcm 3 K. Thus, the samples had a merit figure Fv (=γ/Cvεr) of 0.95 × 10 -10 Ccm/J. The merit figure Fv corresponds to a figure of merit for voltage responsivity (Rv). The relative dielectric constant of each of the samples was measured by using an LCR meter with a frequency of 1 kHz and an applied voltage of 1 V. The pyroelectric coefficient of the samples was represented by an average value in a temperature variation between 20-30°C. Samples of a reference pyroelectric infrared detector were prepared. The reference pyroelectric infrared detector is designed so that a substrate has a through hole (a through opening) instead of a recess as in the prior art pyroelectric infrared detector of Japanese application 4-170077. During an etching process in the fabrication of the reference pyroelectric infrared detector, etchant reached a resin film and a pyroelectric film via the opening in the substrate and hence adversely affected the resin film and the pyroelectric film. Experiments were performed on the samples of the reference pyroelectric infrared detector which were not subjected to a poling treatment. With respect to these samples, the productive yield was equal to 60%. The sensing portions of the samples had a pyroelectric coefficient "γ" of 4.8 × 10 -8 C/cm 2 K, a relative dielectric constant εr of 190, and a volume specific heat Cv of 3.2 Jcm 3 K. Thus, the samples had a merit figure Fv (=γ/Cvεr) of 0.79 × 10 -10 Ccm/J. The relative dielectric constant of each of the samples was measured by using an LCR meter with a frequency of 1 kHz and an applied voltage of 1 V. The pyroelectric coefficient of the samples was represented by an average value in a temperature variation between 20-30°C. Example 2 A pyroelectric infrared detector according to Example 2 is similar to the pyroelectric infrared detector of Example 1 except for design changes indicated hereinafter. In the pyroelectric infrared detector of Example 2, a pyroelectric film 12 is made of PbTiO 3 . During the fabrication of the pyroelectric infrared detector of Example 2, the pyroelectric film 12 was formed on a first substrate 11 by using a plasma-enhanced MO-CVD apparatus. Specifically, as shown in Fig. 3, a first substrate 11 was placed in a reaction chamber 21 of the plasma-enhanced MO-CVD apparatus. The first substrate 11 was previously heated at a temperature of 450°C by a heater 210. The first substrate 11 was supported on a ground electrode 22b. Lead dipivaloyl methane, Pb(C 11 H 19 O 2 ) 2 , was placed in a source vaporizer 26 and was heated by using an oil bath held at a temperature of 130°C. Lead dipivaloyl methane was vaporized as being heated. Titanium tetra-isopropoxide, Ti(i-C 3 H 7 O) 4 , was placed in a source vaporizer 27 and was heated by using an oil bath held at a temperature of 50°C. Titanium tetra-isopropoxide was vaporized as being heated. Resultant lead dipivaloyl methane vapor and titanium tetra-isopropoxide vapor were driven into the reaction chamber 21 by using carrier nitrogen gas which flowed from a carrier gas cylinder 28 at a rate of 10 ml/min. Oxygen gas serving as reactant gas flowed from a reactant gas cylinder 29 at a rate of 40 ml/min, and was driven into the reaction chamber 21 via an injection nozzle. The reaction chamber 21 was exhausted via an exhaust system 23 so that the pressure in the reaction chamber 21 was held equal to a vacuum of 3.90 Pa. Under these conditions, electromagnetic wave having a frequency of 13.56 MHz was applied from an RF power source 25 to an RF electrode 22a at a power of 400 W for 100 minutes so that plasma was generated between the RF electrode 22a and the ground electrode 22b in the reaction chamber 21 of the plasma-enhanced MO-CVD apparatus. As a result, a (001) oriented PbTiO 3 pyroelectric film 12 was deposited on a surface of the first substrate 11. The PbTiO 3 pyroelectric film 12 had a thickness of 3 µm. During the formation of the PbTiO 3 pyroelectric film 12, the first substrate 11 was rotated at 120 r.p.m. by a motor (not shown). Samples of the pyroelectric infrared detector according to Example 2 were prepared. Experiments were performed on these samples which were not subjected to a poling treatment. With respect to the samples, the productive yield was equal to 91%. The sensing portions of the samples had a pyroelectric coefficient "γ" of 2.8 × 10 -8 C/cm 2 K, a relative dielectric constant εr of 95, and a volume specific heat Cv of 3.2 Jcm 3 K. Thus, the samples had a merit figure Fv (=γ/Cvεr) of 0.92 × 10 -10 Ccm/J. The relative dielectric constant of each of the samples was measured by using an LCR meter with a frequency of 1 kHz and an applied voltage of 1 V. The pyroelectric coefficient of the samples was represented by an average value in a temperature variation between 20-30°C.

Claims: A method of fabricating a pyroelectric infrared detector, comprising the steps of: forming a pyroelectric portion (12) on a given area of a surface of a first substrate (11), forming a first electrode (13) on said pyroelectric portion (12); forming a resin film (14) covering the pyroelectric portion (12) and an exposed area of the surface of the first substrate; bonding a second substrate (16) to the resin film (14) via a polymerized adhesive layer (15), wherein the second substrate has a recess (16a) which faces the resin film (14) and which substantially aligns with the pyroelectric portion (12) thus forming a cavity; etching off and removing the first substrate (11) from the pyroelectric portion (12) and the resin film (14) wherein an etchant is prevented from entering said cavity; and forming a second electrode (17) on given regions of exposed areas of said pyroelectric portion (12). The method of claim 1, wherein the first substrate is made of a material which allows the pyroelectric portion to be oriented in a direction of polarization, and which can be easily etched off. The method of claim 1, wherein said pyroelectric portion (12) is formed to comprise a pyroelectric film of Pb x La y Ti z Zr w O 3 where atomic fractions "w", "x", "y", and "z" satisfy one of following conditions a), b), and c): a) 0.7 ≤ x ≤ 1, x + y = 1, 0.925 ≤ z ≤ 1, w = 0 b) x = 1, y = 0, 0.45 ≤ z < 1, z + w = 1 c) 0.75 ≤ x < 1, x + y = 1, 0.5 ≤ z < 1, z + w = 1 The method of claim 1, wherein said pyroelectric portion (12) is formed to comprise a pyroelectric film oriented in a direction along an axis of polarization.

Inventors: FUJII SATORU, OKANO MASAYUKI, TAKAYAMA RYOICHI, TOMITA YOSHIHIRO, TORII HIDEO, FUJII, SATORU, OKANO, MASAYUKI, TAKAYAMA, RYOICHI, TOMITA, YOSHIHIRO, TORII, HIDEO
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1993
Main classification: H01L37
IPC codes: G01J5, H01L37, G01J1