Abstract:
A thermal detector includes a fixed part, a thermal detection device, a supporting member, a cavity and a connection portion. The supporting member has a first plane and a second plane opposing to the first plane. The cavity is formed between the first plane and the fixed part. The connection portion connects the supporting member with the fixed part. The connection portion includes a curvature plane between the supporting member and the fixed part and the curvature plane facing the cavity.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a continuation application of U.S. patent application Ser. No. 13/012,937 which claims priority to Japanese Patent Application No. 2010-015123 filed on Jan. 27, 2010. The entire disclosure of Japanese Patent Application No. 2010-015123 is hereby incorporated herein by reference. 
    
    
     BACKGROUND 
     1. Technical Field 
     The present invention relates to a Method for manufacturing a MEMS device, to a method for manufacturing an infrared detector or other thermal detector, to a thermal detector and thermal detection device, and to an electronic instrument or the like. 
     2. Related Art 
     Known thermal detection devices include pyroelectric or bolometer-type infrared detection devices. An infrared detection device detects infrared rays by creating an electromotive force (pyroelectric-type) or varying a resistance value (bolometer-type) in a thermal infrared detection element on the basis of heat evolved by infrared absorption in an infrared-absorbing film. The infrared detection element is supported by a support member, and cavity for thermal separation is formed between the support member and a substrate. 
     In order to reduce dissipation of heat from the thermal infrared detection element, a support arm portion whereby the support member is linked to the substrate is preferably formed narrow and long to reduce thermal conductance and minimize the heat capacity thereof. However, forming the support arm narrow and long leads to inadequate rigidity thereof. When the rigidity of the support arm is inadequate, the support member adheres to and can no longer be separated from the bottom surface of the cavity during processing (sticking occurs), or the orientation of the detection element changes during use, and the light receiving efficiency thereof declines. 
     Techniques have therefore been proposed whereby a chamfer, shoulder, or the like for increasing the two-dimensional moment of the cross-section in terms of the longitudinal section is formed in the support arm to increase the flexural rigidity thereof (Japanese Laid-Open Patent Publication No. 2009-31197 and Japanese Laid-Open Patent Publication No. 2007-71885). 
     SUMMARY 
     However, the degree to which the flexural rigidity of the support arm can be increased is limited, regardless of the cross-sectional shape of the support arm, which is formed narrow and long in order to reduce thermal conductance. 
     Through aspects of the present invention, it is possible to provide a MEMS (Micro Electro Mechanical Systems) manufacturing method which is optimal for processing a linking structure or support structure for reducing thermal conductance, and to provide a method for manufacturing a thermal detector. 
     Through other aspects of the present invention, it is possible to provide a thermal detector and thermal detection device whereby the structure for linking or supporting an arm on a fixed part is improved, and heat dissipation from the thermal infrared detection element can be reduced. 
     Through other aspects of the present invention, it is possible to provide a thermal detector, thermal detection device, and electronic instrument whereby the structure for linking or supporting an arm on a fixed part is improved, and heat dissipation from the thermal infrared detection element can be reduced. 
     An aspect of the present invention is a method for manufacturing a MEMS device, for processing an etching layer having a first cavity, the etching layer being formed on a fixed part; and the method for manufacturing a MEMS device is characterized in comprising a first step of forming a mask layer on at least a side wall of an exposed surface of the etching layer, at which the etching layer faces the first cavity; and a second step of directing an etchant fed into the first cavity on a surface side of the mask layer to a back surface side of the mask layer, isotropically etching the etching layer, forming a second cavity communicated with the first cavity on the back surface side of the mask layer, and processing the etching layer. 
     According to an aspect of the present invention, since the side wall of the exposed surface of the etching layer, at which the etching layer faces the first cavity, is covered by the mask layer, isotropic etching of the side wall is prevented by the mask layer, and isotropic etching initially does not proceed. As isotropic etching proceeds, the etchant runs along the end part from the surface side of the mask layer and eventually feeds to the back surface side, thereby gradually enlarging the region of the second cavity. Isotropic etching of the etching layer thereby proceeds from deep to shallow in the depth direction of the first cavity, backwards in the depth direction. The amount of etching increases the greater the depth position is in the depth direction of the first cavity, and an undercut shape is formed. 
     In the method for manufacturing a MEMS device according to an aspect of the present invention, a configuration may be adopted in which the mask layer is formed also on a bottom wall of the etching layer, the bottom wall facing the cavity in the first step; and the method for manufacturing a MEMS device further comprises a step of forming an etchant feeding hole through which the etchant passes in the mask layer, the step being performed between the first step and the second step. 
     Through this configuration, the undercut shape formed in the linking part can be controlled in the desired manner by selecting the number, position, size, and other characteristics of the etchant feeding hole. 
     The method for manufacturing a thermal detector according to another aspect of the present invention is characterized in comprising the steps of forming a mask layer on at least a side wall of an exposed surface of an etching layer having a first cavity, the etching layer being formed on a fixed part, and the etching layer facing the first cavity at the side wall; forming a sacrificial layer in the first cavity; forming a support layer on the etching layer and the sacrificial layer; forming a thermal detection element on the support layer; isotropically etching the support layer to process the support layer into a support member for supporting the thermal detection element and expose the sacrificial layer; removing the sacrificial layer by isotropic etching to expose the first cavity; and directing an etchant fed into the first cavity on a surface side of the mask layer to a back surface side of the mask layer, isotropically etching the etching layer, forming a second cavity communicated with the first cavity on the back surface side of the mask layer, and forming the etching layer into a linking part for linking the support member and the fixed part. 
     According to the method for manufacturing a thermal detector according to another aspect of the present invention, an undercut shape can be formed in the linking part by using the principle of the method for manufacturing a MEMS device described above. By temporarily embedding the first cavity in the sacrificial layer, the support member or the thermal detection element can be formed above the first cavity, and after the sacrificial layer is subsequently removed, the method for manufacturing a MEMS device described above can be employed. Through this method, since the linking part can be formed having a cross-sectional area which is smaller on the side of the fixed part than on the side of the support member, the support strength of the linking part can be enhanced, and thermal conductance can be reduced, as described hereinafter. 
     In the method for manufacturing a thermal detector according to another aspect of the present invention, a configuration may also be adopted in which the mask layer is formed also on a bottom wall at which the etching area faces the cavity in the mask layer formation step, and the method for manufacturing a thermal detector further comprises a step of forming an etchant feeding hole through which the etchant passes in the mask layer, the step being performed after forming the mask layer and before the step of isotropically etching the etching layer. In this case as well, the undercut shape formed in the linking part can be controlled in the desired manner, and the support strength or thermal conductance of the linking part can be adjusted by selecting the number, position, size, and other characteristics of the etchant feeding hole. 
     The thermal detector according to another aspect of the present invention is characterized in comprising a fixed part; a thermal detection element; a support member having a first surface and a second surface which faces the first surface, wherein a cavity for thermal separation is formed between the first surface and the fixed part, and the support member supports the thermal detection element on the second surface; and a linking part for linking the support member to the fixed part; wherein the linking part is formed so that the cross-sectional area thereof is smaller on the side of the fixed part than on the side of the support member. 
     In the thermal detection element, when the support member side is designated as being in the upward direction, for example, the linking part is formed so as to have a smaller cross-sectional area on the side of the fixed part than on the side of the support member, and thereby has an undercut shape. Since this undercut shape enables a larger area to be maintained for supporting the linked end part by the support member, support strength is enhanced by distributing stress. Since the cross-sectional area is smaller on the side of the fixed part, thermal conductance can be reduced in proportion to the cross-sectional area, and it is possible to reduce the amount of heat that moves from the infrared detection element as a heat source to the side of the fixed part via the support member and the linking part. In other words, forming the linking part with a small cross-sectional area enables a thermal-resistance-increasing part to be maintained in the linking part. 
     In the thermal detector according to another aspect of the present invention, a configuration may be adopted in which the support member comprises a mounting member for mounting the thermal detection element; and at least one arm extending toward a linked end part from a proximal end connected to the mounting member; wherein the linking part comprises a first linking part for linking the fixed part and the linked end part of the at least one arm. 
     In other words, the shape of the linking part described above can be applied to the first linking end part for linking the fixed part and the linked end part of the end part of the arm to ensure support strength and reduced thermal conductance at the end part of the arm. 
     In the thermal detector according to another aspect of the present invention, a configuration may also be adopted in which the first linking part is formed so that a second edge thereof adjacent to the fixed part side is shorter than a first edge adjacent to the linked end part side in a longitudinal section in the extension direction of the at least one arm. 
     Through this configuration, in the first linking part, since the length parallel to the first and second edges varies even when the width in the direction orthogonal to the first and second edges is constant, the cross-sectional area obtained by multiplying the width by the length can be made smaller on the fixed part side than on the support member side, and a thermal-resistance-increasing part can be formed. 
     In the thermal detector according to another aspect of the present invention, a third edge linking the first edge and the second edge and facing the cavity is preferably curved in a longitudinal section of the first linking part. 
     Through this configuration, the first linking part is endowed with an arch shape, the load from the side of the support member  80  is transmitted as axial compression, and the bending moment which occurs at the top part or the side walls  102  is significantly reduced. Greater support strength can therefore be maintained. 
     In the thermal detector according to another aspect of the present invention, the linking part may further comprise a second linking part for linking the support member and the fixed part at a position other than that of the linked end part, the second linking part being formed in a columnar shape within the cavity. 
     The second linking part links the arm to the fixed part at a middle position other than that of the linked end part of the arm, and can thereby prevent the narrow, long arm from flexing due to inadequate flexural rigidity. Since the second linking part also serves as a heat transfer path, the same as the first linking part, a thermal-resistance-increasing part can also be maintained in the second linking part by forming the second linking part so that the cross-sectional area thereof is smaller on the fixed part side than on the support member side thereof. 
     In the thermal detector according to another aspect of the present invention, the shape of the longitudinal section of the second linking part may have line symmetry about the longitudinal center line of the second linking part. A line-symmetrical longitudinal sectional shape can easily be formed by causing isotropic etching to proceed from the entire periphery of the columnar portion prior to processing the second linking part. Isotropic etching is performed from the entire periphery, the cross-sectional area is reduced, and high thermal resistance can be maintained. 
     In the thermal detector according to another aspect of the present invention, a configuration may be adopted in which the thermal detector further comprises a spacer member extending in a columnar shape within the cavity toward a free end part on the side of the fixed part from a proximal end on the side of the support member, at a position other than that of the linked end part; wherein a gap communicated with the cavity is formed between the fixed part and the free end part of the spacer member. 
     Through this configuration, the interval between the support member and the fixed part can be maintained by the spacer member while the solid heat transfer path between the support member and the fixed part is completely blocked. Since the gap can be formed in the isotropic etching step, which is substantially the final step, the support member and the solid part can remain linked during the manufacturing steps prior to the isotropic etching step, and sticking can be reliably prevented. 
     The thermal detection device according to another aspect of the present invention may comprise the thermal detector described above, arranged in two dimensions along two orthogonal axes. This thermal detection device is capable of providing a light (temperature) distribution image. A high area efficiency of the thermal detector of one cell at this time makes it possible to provide an accurate image. 
     The electronic instrument according to another aspect of the present invention has the thermal detection device described above, and is most suitable in thermography for outputting a light (temperature) distribution image, in automobile navigation and surveillance cameras as well as object analysis instruments (measurement instruments) for analyzing (measuring) physical information of objects, in security instruments for detecting fire or heat, in FA (Factory Automation) instruments provided in factories or the like, and in other applications. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Referring now to the attached drawings which form a part of this original disclosure: 
         FIG. 1  is a view showing an embodiment of the method for manufacturing a MEMS device according to the present invention; 
         FIGS. 2A through 2C  are views showing the process of isotropic etching shown in  FIG. 1 ; 
         FIG. 3  is a view showing isotropic etching according to a comparative example; 
         FIGS. 4A and 4B  are a plan view and a sectional view, respectively, showing an embodiment of the thermal detector of the present invention; 
         FIGS. 5A through 5E  are sectional views showing the first half of the steps of the method for manufacturing the thermal detector shown in  FIGS. 4A and 4B ; 
         FIGS. 6A through 6E  are sectional views showing the second half of the steps of the method for manufacturing the thermal detector shown in  FIGS. 4A and 4B ; 
         FIGS. 7A through 7C  are plan views showing the steps of the method for manufacturing the thermal detector shown in  FIGS. 4A and 4B ; 
         FIGS. 8A through 8C  are plan views showing the steps subsequent to those shown in  FIGS. 7A through 7C  of the method for manufacturing the thermal detector shown in  FIGS. 4A and 4B ; 
         FIG. 9  is a view showing an embodiment in which the method of the present invention is applied to a linking part in a middle position other than that of the linked end part of the arm; 
         FIG. 10  is a view showing an embodiment in which the method of the present invention is applied to a spacer member in a middle position other than that of the linked end part of the arm; 
         FIG. 11  is a sectional view showing an example of the thermo-optical detection element; and 
         FIG. 12  is a block diagram showing an example of the configuration of the electronic instrument. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     Preferred embodiments of the present invention will be described in detail. The embodiments described below do not unduly limit the scope of the present invention as recited in the claims, and all of the configurations described in the embodiments are not necessarily essential means of achievement of the present invention. 
     1. Method for Manufacturing a MEMS Device 
     An embodiment of the method for manufacturing a MEMS device according to the invention of the present application will be described based on  FIGS. 1 and 2A  through  2 C, and in comparison with the comparative example shown in  FIG. 3 . In conventional isotropic etching, a patterned mask layer  12  is formed on the surface of an etching layer  10  as shown in  FIG. 3 , and an etchant having a high selection ratio with respect to the etching layer  10  is fed by a dry or wet process. 
     A cavity  14  is formed in the etching layer  10  by this isotropic etching. At this time, since the etching layer  10  below (on the back surface side of) the mask layer  12  is also isotropically etched, the cavity  14  is formed also below the mask layer  12 . As a result, a side wall  10 A at which the etching layer  10  faces the cavity  14  is formed below the mask layer  12 . 
     The shape of the side wall  10 A is substantially a quarter arc. The length from the left end of the etching layer  10 , for example, to the side wall  10 A is the minimum value L 21  at the top edge adjacent to the mask layer  12 , and the length L 22  further down from the top edge gradually increases above the minimum value L 21 . This tendency is the same as in FIG. 1 of Japanese Laid-Open Patent Publication No. 2009-31197, FIG. 2 of Japanese Laid-Open Patent Publication No. 2007-71885, and other publications. 
     The method for manufacturing a MEMS device according to the present embodiment, however, has a first step of forming a mask layer  24  on at least a side wall  22 A of an exposed surface of an etching layer  22  which has a first cavity  30 , the etching layer  22  at the side wall  22 A facing the first cavity  30 , and the etching layer  22  being formed on a substrate  20  which is a fixed part for functioning as an etching stop layer, for example, as shown in  FIG. 1 . In  FIG. 1 , the mask layer  24  is formed also on a top surface of the etching layer  22 , but in such cases as when other layers are formed on the top surface and the other layers have a low selection ratio with respect to the etchant, the mask layer  24  need not necessarily be formed on the top surface of the etching layer  22 . 
     A bottom wall  22 B at which the etching layer  22  faces the first cavity  30  is exposed in  FIG. 1 , but an opening (etchant feeding opening) for exposing the bottom wall  22 B on the side near the mask layer  24  may be left behind so that the bottom wall  22 B is covered by the mask layer  24 . The opening may be formed by removing a portion of the mask layer  24  by etching, for example, after the mask layer  24  is formed. 
     The etching layer  22  is formed on an etching stop layer  20  having a low selection ratio with respect to the etchant in  FIG. 1 , but this configuration is not limiting. The etching stop layer  20  may be a substrate or other base (fixed part), for example, or an etching stop layer  20  may be formed on a substrate. 
     In the present embodiment, a second step is provided whereby the etchant is fed into the first cavity  30  on the side of a surface  24 A of the mask layer  24  in  FIG. 1 , the etchant is directed toward a back surface  24 B of the mask layer  24 , and the etching layer  22  is isotropically etched. In the second step, a second cavity  32  communicated with the first cavity  30  is formed on the side of the back surface  24 B of the mask layer  24 , and the vertical side wall  22 A, for example, of the etching layer  22  is processed into the shape of a side wall  22 C. The side wall  22 A prior to processing is not necessarily vertical, and may be inclined or curved. 
     In perfect isotropic etching, the etched side wall  22 C of the etching layer  22  forms an arc having a substantially constant radius about a corner of the lower end of the mask layer  24  in terms of a longitudinal section. In other words, the etching layer  22  is processed into a so-called arch shape. 
     In other words, when the etching stop layer  20  is on the underside of the etching layer  22 , the etched side wall  22 C of the etching layer  22  is formed in an undercut shape by the second cavity  32 . The length from the left end of the etching layer  22  to the side wall  22 C, for example, is the minimum value L 11  at the top edge (first edge) adjacent to the mask layer  24 , the minimum length L 12  at the middle position downward from the top edge, and the length L 13  (L 12 &lt;L 13 &lt;L 11 ) at the bottom edge (second edge) adjacent to the etching stop layer  20  further downward. The etching layer  22  is therefore formed so that the cross-sectional area thereof is smaller at a position deeper than a position of shallow depth in the depth direction of the first cavity  30 . 
     The undercut shape essentially differs from the etched side wall  10 C in  FIG. 3 , and the side walls disclosed in FIG. 1 of Japanese Laid-Open Patent Publication No. 2009-31197, FIG. 2 of Japanese Laid-Open Patent Publication No. 2007-71885, and other publications, and the difference in the shape of the side wall is due to a difference in the direction in which isotropic etching proceeds. In  FIG. 3 , isotropic etching proceeds only in the downward and sideways directions in the region below the mask layer  12 . 
     In  FIG. 1 , however, since the side wall  22 A facing the first cavity  30  in the etching layer  22  is covered by the mask layer  24 , isotropic etching of the side wall  22 A is prevented by the mask layer  24 , and initially does not proceed. As isotropic etching proceeds in the region below the etching layer  22  covered by the side wall  22 A, isotropic etching of the side wall  22 A proceeds upward from below (from deep to shallow in the depth direction of the first cavity  30 ), as shown in  FIGS. 2A through 2C , and the shape of the etched side wall  22 C shown in  FIG. 2  is formed. In this process, the etchant runs along the end part (lower end) from the surface  24 A of the mask layer  24  and eventually feeds to the back surface  24 B, thereby gradually enlarging the region of the second cavity  32 . The amount of etching of the etching layer  22  thereby increases the greater the depth position is in the depth direction of the first cavity  30 , and an undercut shape is formed. 
     2. Thermal Detector 
     2.1 Overall Structure of Thermal Detector 
     An embodiment will be described in which the method for manufacturing a MEMS device described above is applied to a method for manufacturing a thermal detector.  FIGS. 4A and 4B  are a plan view and a sectional view, respectively, showing the basic configuration of a thermal detector  50 . 
     The thermal detector, e.g., infrared detector  50 , shown in  FIGS. 4A and 4B  represents one cell, and an infrared detection device can be formed by arranging single cells of infrared detectors in orthogonal directions in two dimensions on a substrate. 
     As shown in  FIGS. 4A and 4B , the infrared detector  50  includes a substrate  60 ; an infrared detection element (thermal detection element, broadly speaking)  70  which includes an infrared-absorbing film (light-absorbing film, broadly speaking); and a support member (membrane)  80  for supporting the infrared detection element  70 . The substrate  60  and the support member  80  are thermally separated via a cavity  90 . The infrared detection element  70  is mounted on a second surface (top surface in  FIG. 4B ) opposite a first surface (bottom surface in  FIG. 4B ) at which the support member  80  faces the cavity  90 . The infrared detection element  70  will be described in detail hereinafter. 
     In the infrared detector  50 , incident infrared rays are absorbed by the infrared-absorbing film, and heat evolved by the absorbed infrared rays causes an electromotive force to occur or changes the resistance value in the infrared detection element  70 , and the infrared rays can thereby be detected. At this time, the cavity  90  is present between the substrate  60 , which has a large heat capacity, and the support member  80  for mounting the infrared detection element  70 . The infrared detection element  70  and the substrate  60  are therefore thermally separated, and infrared rays can be detected with minimal heat loss. 
     Specifically, the support member  80  may have a mounting member  82  for mounting the infrared detection element  70 , and at least one arm  84 , e.g., first and second arms  84 , the proximal ends  84 A of which are linked to the mounting member  82 , and linked end parts  84 B of which are linked and supported on the substrate  60  side. In the present embodiment, the two arms  84  are arranged in point-symmetrical positions, for example, about the center of the mounting member  82 . Wiring layers (not shown) connected to the infrared detection element  70  may be formed in the two arms  84 . A configuration may be adopted in which a single arm  84  is provided, and the wiring layers are formed in a single arm  84 . 
     In order to thermally separate the substrate  60  and the support member  80 , two end part linking posts (connecting parts or first connecting parts, broadly speaking)  100  are provided on the surface of the substrate  60 , and the linked end parts  84 B of the two arms  84  are supported by the two posts  100 . The end part linking posts  100  are formed directly below the linked end parts  84 B of the arms  84 . A region that includes the space between the substrate  60  and the support member  80  can thus be created by the end part linking posts  100  to serve as the cavity  90 . Since the support member  80  is supported on the substrate  60  via the end part linking posts  100 , which have a small volume, the thermal conductance of the heat transfer path from the support member  80  to the substrate  60  is low, and heat dissipation from the infrared detection element  70  including the infrared-absorbing film can be reduced. The arms  84  may extend linearly in one direction as shown in  FIG. 4A , or may be formed narrow and long so as to extend along two edges of a rectangular mounting part  82 , as in  FIG. 1A  of Japanese Laid-Open Patent Publication No. 2009-31197. 
     A frame part  110  formed by the same layer as the end part linking posts  100  is provided on the border of the infrared detector  50  of one cell, but the frame part  110  is not essential. The substrate  60  corresponds to the fixed part in the present embodiment, but another layer (e.g., a gate oxide film or the like in the case of a MOS transistor being formed on the substrate) including the mask layer  24  may be present between the substrate  60  and the end part linking posts  100 , in which case the substrate  60  and the other layer form the fixed part. 
     2.2 Structure of End Part Linking Posts 
     Linking Parts or First Linking Parts 
     In the infrared detector  50  of the present embodiment, the side walls  102  of the end part linking posts  100  are formed by the method for manufacturing a MEMS device described above. The side walls  102  of the end part linking posts  100  are therefore formed in an undercut shape. In other words, the end part linking posts  100  are formed so that the cross-sectional area thereof is smaller on the side of the fixed part  60  than on the side of the support member  80 . In other words, the end part linking posts  100  are formed so that the length L 13  of the second edge adjacent to the substrate  60  as the fixed part is shorter than the length L 11  of the first edge adjacent to the linked end parts  84 B of the arms  84  in terms of the longitudinal section shown in  FIG. 4B  along the extension direction (longitudinal direction) of the arms  84 . 
     Since the undercut shape of the end part linking posts  100  enables a larger area to be maintained for supporting the linked end parts  84 B of the arms  84 , support strength is enhanced by distributing stress. Since the cross-sectional area is kept smaller on the side of the substrate  60 , which is the fixed part, thermal conductance can be reduced in proportion to the cross-sectional area, and it is possible to reduce the amount of heat that moves from the infrared detection element  70  as a heat source to the side of the substrate  60  via the support member  80  and the end part linking posts  100 . In other words, forming the undercut shape enables a thermal-resistance-increasing part to be formed in the end part linking posts  100 . 
     Since the etched side wall  10 B in  FIG. 3 , or the linking parts disclosed in FIG. 1 of Japanese Laid-Open Patent Publication No. 2009-31197, FIG. 2 of Japanese Laid-Open Patent Publication No. 2007-71885, and other publications have the opposite shape from an undercut shape, although thermal conductance may be equal, the surface area for supporting the aim end parts is small, stress is therefore concentrated, and a high support strength cannot be maintained. 
     In particular, third edges along the side walls  102  facing the cavity  90  and connecting the first edges of length L 11  and the second edges of length L 13  are curved in the longitudinal section shown in  FIG. 4B , and end part linking parts  100 B therefore have an arch shape. 
     The load from the side of the support member  80  is transmitted as axial compression by the arch-shaped end part linking posts  100 , and the bending moment which occurs at the top part or the side walls  102  is significantly reduced. Greater support strength can therefore be maintained. 
     3. Method for Manufacturing Thermal Detector 
       FIGS. 5A through 5E  are sectional views showing the first half of the steps of the method for manufacturing the thermal detector shown in  FIGS. 4A and 4B ;  FIGS. 6A through 6E  are sectional views showing the second half of the steps of the method for manufacturing the thermal detector shown in  FIGS. 4A and 4B ;  FIGS. 7A through 7C  are plan views showing the steps of the method for manufacturing the thermal detector shown in  FIGS. 4A and 4B ; and  FIGS. 8A through 8C  are plan views showing the steps subsequent to those shown in  FIGS. 7A through 7C  of the method for manufacturing the thermal detector shown in  FIGS. 4A and 4B . 
     3.1 Step of Forming Mask Layer 
     This step is a step of forming the mask layer  24  on at least side walls  100 B 2  formed by processing of the first cavity  30  in a linking layer (etching layer)  100 A formed on the substrate  60  as the fixed part. First, a linking layer  100 A of SiO 2 , for example, is formed on the substrate, e.g., a silicon substrate  60 , as shown in  FIG. 5A . An SOI (Silicon On Insulator) may be used. 
     The first cavity  30  is then formed in the linking layer  100 A, as shown in  FIG. 5B . The first cavity  30  is formed by forming a resist mask on the surface of the linking layer  100 A and applying isotropic etching, for example, to the linking layer  100 A. The depth of the first cavity  30  is controlled by controlling the etching time, for example. As shown in  FIG. 7A , the planar shape of the first cavity  30  is shown, and by isotropic etching, the linking layer  100 A is processed, and the frame part  110  and end part linking posts  100 B having vertical side walls  100 B 2  are formed. The end part linking posts  100 B have surfaces  100 B 1  which are continuous with the vertical side walls  100 B 2 , and the first cavity  30  is defined by the side walls  100 B 2  and a bottom wall  100 B 3 . 
       FIGS. 5C and 7B  show the step of forming the mask layer  24 . In the present embodiment, the mask layer  24  is formed on the entire surface of the surfaces  100 B 1  of the end part linking posts  100 B, and the side walls  100 B 2  and bottom wall  100 B 3  for defining the first cavity  30 . The mask layer  24  is formed by Al 2 O 3 , for example. Furthermore, in the present embodiment, etchant feed (openings)  26  are formed in the mask layer  24  so as to surround the side walls  100 B 2  of the end part linking posts  100 B, as shown in  FIG. 5C  and the enlarged view of portion A in  FIG. 7B . The etchant feeding holes  26  are formed in order to feed the etchant during formation of the end part linking posts  100  having the undercut side walls  102  by isotropic etching of the vertical side walls  100 B 2  of the end part linking posts  100 B in a subsequent step. 
     3.2 Step of Forming Sacrificial Layer 
     This step is a step of forming a sacrificial layer  120 , for example, in the first cavity  30 , and is shown in  FIGS. 5D and 7C . The sacrificial layer  120  is formed by polycrystalline silicon, for example, is embedded in the first cavity  30 , and the excess sacrificial layer  120  protruding from the first cavity  30  is removed by etching back. Embedding the sacrificial layer  120  in the first cavity  30  enables formation of other films on the first cavity  30 , and the sacrificial layer  120  is removed after the other films are formed. 
     3.3 Step of Forming Support Layer 
     This step is a step of forming a support layer  80 A on the sacrificial layer  120  and the end part linking posts  100 B which are etching layers. As shown in  FIG. 5E , a support layer  80 A composed of a SiN film, for example, is formed by sputtering, CVD, or another method on the entire surface of the end part linking posts  100 B and sacrificial layer  120 . 
     3.4 Step of Forming Thermal Detection Element 
     This step is a step of forming an infrared detection element film  70 A as a thermal detection element on the support layer  80 A. The infrared detection element film  70 A will be described in detail hereinafter. The infrared detection element film  70 A is patterned as shown in  FIGS. 6B and 8A , and the infrared detection element  70  is formed so that a predetermined light-receiving surface area can be maintained at the center of the one cell. 
     3.5 Step of Exposing Sacrificial Layer 
     This step is a step of isotropically etching the support layer  80 A to form the support member  80  for supporting the infrared detection element  70 , and exposing the sacrificial layer  120 . As shown in  FIGS. 6B and 8A , through isotropic etching of the support layer  80 A, the mounting member  82  for mounting the infrared detection element  70  is formed, and the two, for example, first and second arms  84  are formed extending from the mounting member  82  to the linked end parts  84 B. Through this isotropic etching, the sacrificial layer  120  as the layer under the support layer  80 A is also exposed in the region from which the support layer  80 A is removed. Examples of etchants having a high selection ratio with respect to the support layer  80 A (SiN) include etching gas in which nitrogen or chlorine is added to a gas mixture of CF 4  and oxygen, or an etching gas in which nitrogen is added to a gas mixture of fluorine and oxygen (see Japanese Laid-open Patent Publication No. 10-261616, for example). 
     3.6 Step of Removing Sacrificial Layer 
     This step is a step of removing the sacrificial layer  120  by wet or dry isotropic etching and exposing the first cavity  30 . As shown in  FIGS. 6C and 8B , the exposed sacrificial layer  120  as well as the sacrificial layer  120  hidden below the support member  80  are removed by isotropic etching. At this time, the mask layer  24  functions as an etching stop layer, and the end part linking posts  100  and the frame part  110  covered by the mask layer  24  are not etched. The linking layer (SiO 2 ) exposed via the etchant feeding holes  26 . The linking layer (SiO 2 ) exposed via the etchant feeding holes  26  has a low selection ratio with respect to the etchant, and is therefore also not etched. Fluorine-based gases such as SF 6 , CF 4 , XeF 2  can be cited as example of etchants having a high selection ratio with respect to polycrystalline silicon, for example, as the material of the sacrificial layer  120 . 
     3.7 Step of Forming Undercut Shape in Linking Parts 
     This step is a step of directing the etchant fed into the first cavity  30  on the side of the surface  24 A of the mask layer  24  to the side of the back surface  24 B of the mask layer  24  and isotropically etching the end part linking posts  100 B as etching layers. At this time, as shown in  FIGS. 6D and 8C , the second cavities  32  communicated with the first cavity  30  are formed on the back surface  24 B sides of the mask layer  24 , and the end part linking posts  100 B as etching layers are processed so as to form an undercut shape in the end part linking posts  100  (linking parts or first linking parts) for linking the support member  80  and the substrate  60  as the fixed part. Through this isotropic etching, the end part linking posts  100  are processed into arch shapes, as shown in  FIG. 6D . The isotropic etching is accomplished by the same process as shown in  FIGS. 2A through 2C . The silicon substrate  60  under the end part linking posts  100 B as etching layers functions as an etching stop layer, and the silicon substrate  60  is not etched. The etchant HF can be cited as an example of an isotropic etchant having a high selection ratio with respect to SiO 2 , for example, as the material of the end part linking posts  100 B which are etching layers. 
     3.8 Step of Removing Mask Layer 
     This step is a step of removing, as shown in  FIG. 6E , the mask layer  24  that remains in the first and second cavities  30 ,  32  as shown in  FIG. 6D , and is performed as needed. This step may be separate from the isotropic etching step of  FIG. 6D , but a configuration may be adopted in which the mask layer  24  is also removed at completion of the isotropic etching of  FIG. 6D  by adjusting the selection ratio with respect to the barrier film  24  and the linking end parts  100 B, for example. 
     4. Method for Processing Second Linking Part 
     In another embodiment of the present invention, a linking part is formed also in a middle position other than that of the linked end parts  84 B of the arms  84  shown in  FIG. 4B . As shown in  FIG. 9 , a middle linking post (second linking part)  130  formed in a columnar shape in the cavity  90  may be further included in order to link the support member  80  and the substrate  60  as the fixed part. In the present embodiment, the middle linking post  130  links a single location of a middle position  84 C between a proximal end part  84 A and the linked end part  84 B of the arm  84  with the substrate  60 . This configuration is not limiting, a plurality of middle linking posts  130  may be provided so as to link a plurality of middle positions of the arm  84 , and the mounting member  82  and the substrate  60  may be linked by the middle linking post  130 . 
     The middle linking post  130  prevents the narrow, long arm  84  from flexing during manufacturing or after finishing due to inadequate flexural rigidity by linking the arm  84  to the substrate  60  at a middle position other than that of the linked end part  84 B of the arm  84 . However, since the middle linking post  130  also becomes a heat transfer path in the same manner as the end part linking posts  100 , a thermal-resistance-increasing part must be maintained, the same as in the end part linking posts  100 . 
     An undercut shape is therefore formed in the middle linking post  130  as well, as shown in  FIG. 9 . Before the undercut shape is provided to the middle linking post  130 , a round column, square column, or other column-shaped middle linking post is formed in advance in the first cavity  30  when the etching layer  100 A is isotropically etched to form the first cavity  30  in the step shown in  FIG. 5B . An etchant feeding hole in the shape of a ring, for example, is formed in the mask layer  24  so that a bottom wall  100 B 3  on the periphery of the middle linking post is exposed. In the step shown in  FIG. 6D , by feeding the etchant via the ring-shaped etchant feeding hole, the round column, square column, or other column-shaped middle linking post can be formed into the middle linking post  130  having an undercut-shaped peripheral wall  132  shown in  FIG. 9 . 
     At this time, when the etchant is fed from the entire periphery of the middle linking post, the shape of the longitudinal section of the middle linking post  130  becomes line symmetrical about the longitudinal center line thereof. The middle linking post  130  is thus isotropically etched from the entire periphery thereof, and the cross-sectional area therefore is reduced. High thermal resistance can therefore be maintained. The shape of the middle linking post  130  can also be changed depending on the size or position of the etchant feeding hole, and the shape of the middle linking post  130  need not necessarily be formed so that the shape of the longitudinal section thereof is line symmetrical about the longitudinal center line. 
     5. Method for Processing Spacer Member 
     In another embodiment of the present invention, a spacer member  140  may be further provided extending in a columnar shape within the cavity  90  toward a free end part  140 B on the side of the substrate  60  as the fixed part from a proximal end  140 A on the side of the support member  80 , at a middle position other than that of the linked end part  84 B of the arm  84  shown in  FIG. 4B . 
     In the present embodiment, the spacer member  140  protrudes toward the substrate  60  from a single location at the middle position  84 C between the linked end part  84 B and the proximal end part  84 A of the arm  84 . This configuration is not limiting, and spacer members  140  may be provided in a plurality of middle positions of the arm  84 , or a spacer member  140  may be provided to the bottom surface of the mounting part  82 . 
     The middle linking post  130  can be considered to have a thermal-resistance-increasing part, but the solid head transfer path between the support member  80  and the substrate  60  cannot be completely blocked. A gap  92  communicated with the cavity  90  is therefore formed between the substrate  60  and the free end part  140 B of the spacer member  140 . A portion or all of the middle linking post  130  may thereby be replaced with the spacer member  140 , or the spacer member  140  may be provided in addition to the middle linking post  130 . 
     The spacer member  140  intervenes with a predetermined length between the support member  80  and the substrate  60 , and therefore has the function of maintaining the interval between the support member  80  and the substrate  60 . Since the gap  92  is formed in the step shown in  FIG. 6D , which is substantially the final step, the support member  80  and the substrate  60  can remain linked during the prior manufacturing steps, and sticking can be reliably prevented. 
     The gap  92  and an undercut-shaped peripheral wall  142  of the spacer member  140  are formed in the isotropic etching step shown in  FIG. 6D . The middle linking post  130  and the spacer member  140  can be formed simultaneously in the isotropic etching step shown in  FIG. 6D , and formation of the middle linking post  130  or the spacer member  140  can be selected by varying the size of the etchant feeding opening, for example. When the etchant feeding opening is large, isotropic etching proceeds rapidly, and the spacer member  140  can be formed. 
     6. Thermal Detection Element 
     The detailed structure of the thermal detection element is not shown in the drawings described above, but known thermo-optical absorption elements include bolometer-type and pyroelectric-type elements. In a bolometer-type element, a resistance value is varied by the heat of light, e.g., infrared rays, and the infrared rays are detected, and an infrared detection element film can be formed by a temperature-dependent resistor. A pyroelectric infrared detection element will be described below with reference to  FIG. 11 . 
     As shown in  FIG. 11 , the infrared detection element  70  includes a capacitor  200 . The capacitor  200  includes a Pt or other first electrode (lower electrode)  200 A mounted on the mounting member  82 ; a Pt or other second electrode (upper electrode)  200 B disposed facing the first electrode  200 A; and a PZT or other ferroelectric film  200 C as a pyroelectric material disposed between the first and second electrodes  200 A,  200 B. The first electrode  200 A is connected to an Al or other wiring layer  210  on one arm  84 , and the second electrode  200 B is connected to a wiring layer  212  of the other arm  84 . The capacitor  200  undergoes spontaneous polarization based on heat evolution caused by infrared rays, and the infrared rays can be detected by retrieving the charge caused by the spontaneous polarization. Examples of possible methods of infrared detection include interrupting the infrared rays by a mechanical chopper and retrieving as an alternating electrical polarization effect, or applying a polar voltage for drawing in a surface charge with respect to the orientation of the spontaneous polarization and detecting the voltage across the terminals of the capacitor, which varies according to the charge drawn after voltage application is stopped. 
     The sides and top of the capacitor  200  may be covered by an electrical insulation film  230  via a hydrogen barrier film  220  composed of Al 2 O 3  or the like for preventing degradation due to reduction of the ferroelectric film  200 C, for example. The infrared-absorbing film  240  composed of SiO 2 , SiN, SiCn, TiN, or the like, for example, is formed so as to cover the electrical insulation film  230 . A plug (electrode contact)  250  is formed embedded in a contact hole (opening) formed in the hydrogen barrier film  220  and the electrical insulation film  230 , and the second electrode  200 B and a wiring layer  260  are electrically connected via the plug  250 . 
     The surface side of the infrared detector which includes the infrared-absorbing film  240 , the support member  80 , and the wiring layers  210 ,  212  is not shown in the drawing, but may be covered by an upper protective film which functions as a mask layer during etching formation of the sacrificial layer  120  disposed in the cavity  90  in the manufacturing process. 
     7. Electronic Instrument 
       FIG. 12  shows an example of the configuration of an electronic instrument which includes the thermal detector or thermal detection device of the present embodiment. The electronic instrument includes an optical system  400 , a sensor device (thermal detection device)  410 , an image processor  420 , a processor  430 , a storage unit  440 , an operating unit  450 , and a display unit  460 . The electronic instrument of the present embodiment is not limited to the configuration shown in  FIG. 12 , and various modifications thereof are possible, such as omitting some constituent elements (e.g., the optical system, operating unit, display unit, or other components) or adding other constituent elements. 
     The optical system  400  includes one or more lenses, for example, a drive unit for driving the lenses, and other components. Such operations as forming an image of an object on the sensor device  410  are also performed. Focusing and other adjustments are also performed as needed. 
     The sensor device  410  is formed by arranging the optical detector of the present embodiment described above in two dimensions, and a plurality of row lines (word lines, scan lines) and a plurality of column lines (data lines) are provided. In addition to the optical detector arranged in two dimensions, the sensor device  410  may also include a row selection circuit (row driver), a read circuit for reading data from the optical detector via the column lines, an A/D converter, and other components. Image processing of an object image can be performed by sequentially reading data from optical detectors arranged in two dimensions. 
     The image processor  420  performs image correction processing and various other types of image processing on the basis of digital image data (pixel data) from the sensor device  410 . 
     The processor  430  controls the electronic instrument as a whole and controls each block within the electronic instrument. The processor  430  is realized by a CPU or the like, for example. The storage unit  440  stores various types of information and functions as a work area for the processor  430  or the image processor  420 , for example. The operating unit  450  serves as an interface for operation of the electronic instrument by a user, and is realized by various buttons, a GUI (graphical user interface) screen, or the like, for example. The display unit  460  displays the image acquired by the sensor device  410 , the GUI screen, and other images, for example, and is realized by a liquid crystal display, an organic EL display, or other types of display. 
     A thermal detector of one cell may thus be used as an infrared sensor or other sensor, or the thermal detector of one cell may be arranged along orthogonal axes in two dimensions to form the sensor device (thermal detection device)  410 , in which case a heat (light) distribution image can be provided. This sensor device  410  can be used to form an electronic instrument for thermography, automobile navigation, a surveillance camera, or another application. 
     As shall be apparent, one cell or a plurality of cells of thermal detectors may also be used in an object analysis instrument (measurement instrument) for analyzing (measuring) physical information of an object, in a security instrument for detecting fire or heat, in an FA (factory automation) instrument provided in a factory or the like, and in various other electronic instruments. 
     Several embodiments are described above, but it will be readily apparent to those skilled in the art that numerous modifications can be made herein without substantively departing from the new matter and effects of the present invention. All such modifications are thus included in the scope of the present invention. For example, in the specification or drawings, terms which appear at least once together with different terms that are broader or equivalent in meaning may be replaced with the different terms in any part of the specification or drawings. 
     The present invention is widely applicable to thermal detectors, and can be applied not only to pyroelectric-type thermal detectors, but to bolometer-type thermal detectors as well. The object of detection is also not limited to infrared rays, and may also be light in other wavelength regions. 
     General Interpretation of Terms 
     In understanding the scope of the present invention, the term “comprising” and its derivatives, as used herein, are intended to be open ended terms that specify the presence of the stated features, elements, components, groups, integers, and/or steps, but do not exclude the presence of other unstated features, elements, components, groups, integers and/or steps. The foregoing also applies to words having similar meanings such as the terms, “including”, “having” and their derivatives. Also, the terms “part,” “section,” “portion,” “member” or “element” when used in the singular can have the dual meaning of a single part or a plurality of parts. Finally, terms of degree such as “substantially”, “about” and “approximately” as used herein mean a reasonable amount of deviation of the modified term such that the end result is not significantly changed. For example, these terms can be construed as including a deviation of at least ±5% of the modified term if this deviation would not negate the meaning of the word it modifies. 
     While only selected embodiments have been chosen to illustrate the present invention, it will be apparent to those skilled in the art from this disclosure that various changes and modifications can be made herein without departing from the scope of the invention as defined in the appended claims. Furthermore, the foregoing descriptions of the embodiments according to the present invention are provided for illustration only, and not for the purpose of limiting the invention as defined by the appended claims and their equivalents.