Abstract:
In one embodiment, a MEMS sensor includes a mirror and an absorber spaced apart from the mirror, the absorber including a plurality of spaced apart conductive legs defining a tortuous path across an area directly above the mirror.

Description:
[0001]    This application claims the benefit of U.S. Provisional Application No. 61/695,361 filed Aug. 31, 2012, the entire contents of which is herein incorporated by reference. 
     
    
     FIELD 
       [0002]    This disclosure relates to sensor devices and methods of fabricating such devices. 
       BACKGROUND 
       [0003]    Objects at any non-zero temperature radiate electromagnetic energy which can be described either as electromagnetic waves or photons, according to the laws known as Planck&#39;s law of radiation, the Stefan-Boltzmann Law, and Wien&#39;s displacement law. Wien&#39;s displacement law states that the wavelength at which an object radiates the most (λ max ) is inversely proportional to the temperature of the object as approximated by the following relation: 
         [0000]    
       
         
           
             
               
                 λ 
                 max 
               
                
               
                 ( 
                 μm 
                 ) 
               
             
             ≈ 
             
               3000 
               
                 T 
                  
                 
                   ( 
                   K 
                   ) 
                 
               
             
           
         
       
     
         [0004]    Hence for objects having a temperature close to room temperature, most of the emitted electromagnetic radiation lies within the infrared region. Due to the presence of CO 2 , H 2 O, and other gasses and materials, the earth&#39;s atmosphere absorbs electromagnetic radiation having particular wavelengths. Measurements have shown, however, that there are “atmospheric windows” where the absorption is minimal. An example of such a “window” is the 8 μm-12 μm wavelength range. Another window occurs at the wavelength range of 3 μm-5 μm. Typically, objects having a temperature close to room temperature emit radiation close to 10 μm in wavelength. Therefore, electromagnetic radiation emitted by objects close to room temperature is only minimally absorbed by the earth&#39;s atmosphere. Accordingly, detection of the presence of objects which are either warmer or cooler than ambient room temperature is readily accomplished by using a detector capable of measuring electromagnetic radiation emitted by such objects. 
         [0005]    One commonly used application of electromagnetic radiation detectors is for automatically energizing garage door lights when a person or car approaches. Another application is thermal imaging. In thermal imaging, which may be used in night-vision systems for driver assistance, the electromagnetic radiation coming from a scene is focused onto an array of detectors. Thermal imaging is distinct from techniques which use photomultipliers to amplify any amount of existing faint visible light, or which use near infrared (˜1 μm wavelength) illumination and near-infrared cameras. 
         [0006]    Two types of electromagnetic radiation detectors are “photon detectors” and “thermal detectors”. Photon detectors detect incident photons by using the energy of said photons to excite charge carriers in a material. The excitation of the material is then detected electronically. Thermal detectors also detect photons. Thermal detectors, however, use the energy of said photons to increase the temperature of a component. By measuring the change in temperature, the intensity of the photons producing the change in temperature can be determined. 
         [0007]    In thermal detectors, the temperature change caused by incoming photons can be measured using temperature-dependant resistors (thermistors), the pyroelectric effect, the thermoelectric effect, gas expansion, and other approaches. One advantage of thermal detectors, particularly for long wavelength infrared detection, is that, unlike photon detectors, thermal detectors do not require cryogenic cooling in order to realize an acceptable level of performance. 
         [0008]    One type of thermal sensor is known as a “bolometer.” Even though the etymology of the word “bolometer” covers any device used to measure radiation, bolometers are generally understood to be to thermal detectors which rely on a thermistor to detect radiation in the long wavelength infrared window (8 μm-12 μm) or mid-wavelength infrared window (3 μm-5 μm). 
         [0009]    The sensitivity of a bolometer generally increases with better thermal isolation of the sensor from its surroundings, with a higher infrared absorption coefficient, higher temperature coefficient of resistance, higher electrical resistance, and a higher bias current. Accordingly, because bolometers must first absorb incident electromagnetic radiation to induce a change in temperature, the efficiency of the absorber in a bolometer relates to the sensitivity and accuracy of the bolometer. Ideally, absorption as close to 100% of incident electromagnetic radiation is desired. In theory, a metal film having a sheet resistance (in Ohms per square) equal to the characteristic impedance of free space, laying over a dielectric or vacuum gap of optical thickness d will have an absorption coefficient of 100% for electromagnetic radiation of wavelength  4   d . The following relation shows the expression of the characteristic impedance (Y) of free space: 
         [0000]    
       
         
           
             Y 
             = 
             
               
                 
                   μ 
                   0 
                 
                 
                   ɛ 
                   0 
                 
               
             
           
         
       
     
         [0000]    wherein ∈ 0  is the vacuum permittivity and μ 0  is the vacuum permeability. 
         [0010]    The numerical value of the characteristic impedance of free space is close to 377 Ohm. The optical length of the gap is defined as “nd”, where n is the index of refraction of the dielectric, air or vacuum. 
         [0011]    In the past, micro-electromechanical systems (MEMS) have proven to be effective solutions in various applications due to the sensitivity, spatial and temporal resolutions, and lower power requirements exhibited by MEMS devices. One such application is as a bolometer. Known bolometers use a supporting material which serves as an absorber and as a mechanical support. Typically, the support material is silicon nitride. A thermally sensitive film is formed on the absorber to be used as a thermistor. The absorber structure with the attached thermistor is anchored to a substrate through suspension legs having high thermal resistance in order for the incident electromagnetic radiation to produce a large increase of temperature on the sensor. 
         [0012]    The traditional technique used to micromachine suspended members involves the deposition of the material over a “sacrificial” layer, which is to be eventually removed and which is deposited, e.g., by spin coating or polymer coating using a photoresist. The deposition of the thin-film metal or semiconductor can be done with a variety of techniques including low-pressure chemical vapor deposition (LPCVD), epitaxial growth, thermal oxidation, plasma-enhanced chemical vapor deposition (PECVD), sputtering, and evaporation. 
         [0013]    Most of the known bolometers, however, have a generally rectangular absorber. Such absorbers exhibit reduced thermal isolation and low electrical resistance, lowering the responsivity of the device. 
         [0014]    It would be beneficial to provide an infrared sensor which exhibited increased sensitivity. A sensor which provides efficient thermal absorption is also desired. It would be beneficial for such a sensor to exhibit increased thermal isolation and high electrical resistance. A sensor with increased sensitivity which could be manufactured using known manufacturing processes would be further beneficial. 
       SUMMARY 
       [0015]    In one embodiment a MEMS sensor includes a mirror and an absorber spaced apart from the mirror, the absorber including a plurality of spaced apart conductive legs defining a tortuous path across an area directly above the mirror. 
         [0016]    In another embodiment, a method of forming a MEMS sensor includes providing an insulation layer, forming a mirror on the insulation layer, and forming an absorber spaced apart from the mirror by forming a plurality of spaced apart conductive legs defining a tortuous path across an area directly above the mirror. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  depicts a side cross-sectional view of a bolometer device taken along line A-A of  FIG. 2 , the bolometer device including an absorber that provides increased thermal isolation and higher electrical resistivity using combinations of forty-five degree angled sections of end portions to connect linear leg portions in accordance with principles of the present disclosure; 
           [0018]      FIG. 2  depicts a top plan view of the bolometer of  FIG. 1 ; 
           [0019]      FIG. 3  depicts a side cross-sectional view of a wire absorber including multiple layers of different materials to provide a rectangular cross-section; 
           [0020]      FIG. 4  depicts a side cross-sectional view of a wire absorber shaped as an I-beam to provide additional strength; 
           [0021]      FIG. 5  depicts a side cross-sectional view of a wire absorber shaped as a “U” to provide additional strength; 
           [0022]      FIG. 6  depicts a side cross-sectional view of a wire absorber with multiple materials layered and shaped as a “U” to provide additional strength and other desired characteristics; 
           [0023]      FIG. 7  depicts a side cross-sectional view taken along a line similar to the line A-A of  FIG. 2 , of a device substrate which in this embodiment is a complementary metal oxide semiconductor (CMOS), with a partial insulator layer formed on the substrate in accordance with principles of the present disclosure; 
           [0024]      FIG. 8  depicts a side cross-sectional view taken along a line similar to the line A-A of  FIG. 2 , of the device of  FIG. 7  after feed throughs and associated bond pads have been formed on the partial insulator layer; 
           [0025]      FIG. 9  depicts a side cross-sectional view taken along a line similar to the line A-A of  FIG. 2 , of the device of  FIG. 8  after the remainder of the insulator layer has been formed above the feed throughs and associated bond pads; 
           [0026]      FIG. 10  depicts a side cross-sectional view taken along a line similar to the line A-A of  FIG. 2 , of the device of  FIG. 9  with openings formed through the insulator layer to expose the bond pads and a portion of the feed throughs; 
           [0027]      FIG. 11  depicts a side cross-sectional view taken along a line similar to the line A-A of  FIG. 2 , of the device of  FIG. 10  with a mirror and a bond ring formed on top of the insulator layer; 
           [0028]      FIG. 12  depicts a side cross-sectional view taken along a line similar to the line A-A of  FIG. 2 , of the device of  FIG. 11  after a sacrificial layer has been formed over the substrate, and openings through the sacrificial layer to the feed throughs have been formed along with a trench between the openings; 
           [0029]      FIG. 13  depicts a side cross-sectional view taken along a line similar to the line A-A of  FIG. 2 , of the device of  FIG. 12  with an absorber layer formed within the openings and trench and directly above the mirror without extending into the portion of the sacrificial layer directly above the bond ring; 
           [0030]      FIG. 14  depicts a side cross-sectional view taken along a line similar to the line A-A of  FIG. 2 , of the device of  FIG. 13  with the sacrificial layer removed; 
           [0031]      FIG. 15  depicts a side cross-sectional view of a bolometer device taken along line A-A of  FIG. 16 , the bolometer device including an absorber that provides increased thermal isolation and higher electrical resistivity using combinations of two ninety degree curved pieces in an end portion to connect linear leg portions in accordance with principles of the present disclosure; 
           [0032]      FIG. 16  depicts a top plan view of the bolometer of  FIG. 15 ; 
           [0033]      FIG. 17  depicts a simplified top plan view of a bolometer device including a serpentine absorber that provides increased thermal isolation and higher electrical resistivity using combinations of ninety degree angled pieces in an end portion to connect linear leg portions; 
           [0034]      FIG. 18  depicts a simplified top plan view of a bolometer device including a complex serpentine absorber that provides increased thermal isolation and higher electrical resistivity using combinations of ninety degree angled pieces in an end portion to connect linear leg portions; 
           [0035]      FIG. 19  depicts a simplified top plan view of a bolometer device including a complex absorber that provides increased thermal isolation and higher electrical resistivity using combinations of ninety degree angled pieces in an end portion to connect linear leg portions; 
           [0036]      FIG. 20  depicts a simplified top plan view of a bolometer device including a complex serpentine absorber that provides increased thermal isolation and higher electrical resistivity using combinations of ninety degree curved pieces in an end portion to connect linear leg portions; 
           [0037]      FIG. 21  depicts a side cross-sectional view of a bolometer device taken along line A-A of  FIG. 22 , the bolometer device including an absorber that provides increased thermal isolation and higher electrical resistivity using combinations of forty-five degree angled sections of end portions to connect linear leg portions formed within an nonconductive stiffening sheet in accordance with principles of the present disclosure; and 
           [0038]      FIG. 22  depicts a top plan view of the bolometer of  FIG. 21 . 
       
    
    
     DESCRIPTION 
       [0039]    For the purposes of promoting an understanding of the principles of the disclosure, reference will now be made to the embodiments illustrated in the drawings and described in the following written specification. It is understood that no limitation to the scope of the disclosure is thereby intended. It is further understood that the present disclosure includes any alterations and modifications to the illustrated embodiments and includes further applications of the principles of the disclosure as would normally occur to one skilled in the art to which this disclosure pertains. 
         [0040]    A semiconductor sensor assembly  100  is depicted in  FIGS. 1 and 2 .  FIG. 1  depicts a side cross-sectional view of the semiconductor sensor assembly  100 , which in this embodiment is a bolometer, while  FIG. 2  is a top plan view of the sensor assembly  100 . The sensor assembly  100  in one embodiment is formed on a complementary metal oxide semiconductor (CMOS) substrate or on another type of substrate. The sensor assembly  100  includes a substrate  102 , an insulator layer  104 , a mirror  106  and an absorber  108 . The substrate  102 , which in this embodiment is a silicon wafer that may include one or more sensor assemblies  100 , includes the electronic circuitry used to access the output of the sensor assembly  100 . 
         [0041]    The insulator layer  104  in one embodiment is a deposited dielectric like, e.g., SiO 2 , which includes access openings  110  and  112  which provide access to bond pads  114  and  116 , respectively, within the insulator layer  104 . The bond pads  114  and  116  are conductively connected to respective buried feed-throughs  118  and  120  which extend within the insulator layer  104  to conductive support posts  122  and  124 . 
         [0042]    The support posts  122  and  124  extend upwardly from the buried feed throughs  118  and  120  to support the absorber  108  at a location above the upper surface of the insulator layer  104 . Each of the support posts  122 / 124  supports the absorber  108  through respective support bar  123 / 125 . 
         [0043]    The mirror  106  is located on the upper surface of the insulator layer  104  along with a bond ring  142 . The bond ring  142  extends completely about the support posts  122 / 124  and the absorber  108  but inside of the access openings  110  and  112 . The bond ring  142  is used to form a bond with a cap (not shown) thereby protecting the absorber  108  while the access openings and structures within the insulator layer  104  allow for electrical communication between the absorber  108  and external electronics. The cap further allows encapsulation of a vacuum in the space occupied by the absorber  108  to ensure proper and reliable operation of the sensor assembly  100 . More than one sensor assembly  100  can be encapsulated under the same cap structure. 
         [0044]    The mirror  106  is directly beneath the absorber  108  and may be, for example, a metal reflector or a multilayer dielectric reflector. The absorber  108  is spaced apart from the mirror  106  by a gap of about 2.0 to 3.0 μm. The gap in this embodiment is selected to optimize absorption in the long-wavelength infrared region (8-15 microns). 
         [0045]    The absorber  108 , in addition to absorbing energy from incident photons, is selected to provide a good noise-equivalent temperature difference (NETD). In order for the absorber  108  to have a good NETD, the material selected to form the absorber  108  should exhibit a high temperature coefficient of resistance while exhibiting low excess noise (1/f noise). Semiconductor materials such as vanadium oxide are common in micromachined bolometers due to their high temperature coefficient of resistance. Other materials include Si (poly/amorphous), SiGe, Ge, Pt, TiN, Ti, and combinations of the foregoing. While metals have a lower temperature coefficient of resistance than some semiconductor materials, such as vanadium oxide, metals typically have much lower excess noise than many semiconductor materials, thus offering better NETD. 
         [0046]    Accordingly, in one embodiment the absorber  108  comprises metal. Titanium and Platinum are two metals which exhibit desired characteristics. Titanium, for example, exhibits a bulk resistivity of about 7*10 −7  Ohm. Using a bulk resistivity of 7*10 −7  Ohm, the thickness of the absorber  108  needed to match the impedance of free-space (377 Ohm/square) should be about 1.9 nm. The resistivity of materials formed to a thickness less than about 50 nm, however, can be several times higher than the bulk value. Accordingly, depending on process parameters, the thickness of the absorber  108 , if made from titanium, is preferably about 10 nm. Impurities can also be introduced into the absorber  108  during formation in order to tune the resistivity if needed. 
         [0047]    Consequently, the thickness of the absorber  108  in this embodiment is about 10 nm and the length of the absorber  108  from the support post  122  to the support post  124  is typically between 15 μm and 70 μm. This configuration provides a ratio between the thickness of the absorber  108  and the length of the absorber  108  on the order of 1/1000 and the ratio of the thickness of the absorber  108  to the gap width of about 1/100. The actual distance along the absorber  108  between the posts  122 / 124  is increased, however, because the absorber  108  is serpentine. 
         [0048]    The absorber  108  is a free-standing serpentine wire structure. The free-standing serpentine wire structure provides better thermal isolation and higher electrical resistance (and therefore a higher responsivity) without an increase in size over typical absorber structures which are generally rectangular. For example, the serpentine nature of the absorber  108  increases the lineal distance along the absorber  108  between the posts  122 / 124  by a factor of over 5 compared to a rectangular absorber. 
         [0049]    The absorber  108  also includes end structures  144  which connect leg portions  146  to each other or to the support bars  123 / 125 . The end structures  144  are not configured using ninety degree angles. Rather, four forty-five degree angled sections are used to generate a 180 degree change in direction between the leg portions  146 . Accordingly, current crowding and high mechanical stress fields are avoided. 
         [0050]    In the embodiment of  FIGS. 1 and 2 , the absorber  108  is generally rectangular in cross-section (see  FIG. 1 ). Depending upon the particular embodiment, a simple rectangular cross section may not provide the desired rigidity or strength. Accordingly, in other embodiments, other cross-sectional shapes and configurations are used. By way of example,  FIG. 3  depicts an absorber section  200  that includes a base layer  202 , a middle layer  204 , and an upper layer  206 . The materials for the various layers in the absorber section  200  are selected for the desired strength, absorption, and other properties.  FIG. 4  depicts an absorber section  208  formed in the shape of an I-beam for additional strength. 
         [0051]      FIG. 5  depicts a “U-shaped” wire absorber  210 . The wire absorber  210  in some embodiments is constructed in the manner described in U.S. patent application Ser. No. 13/415,479, filed Mar. 8, 2012, the entire contents of which are herein incorporated by reference. The U-shape provides additional strength.  FIG. 6  depicts a similar absorber section  212  that includes an outer U-shaped section  214  and an inner U-shaped section  216  formed with a different material. 
         [0052]    Returning to  FIGS. 1 and 2 , the total resistance for the sensor assembly  100  measured across the support posts  122 / 124  and the absorber  108  is defined by the following equation: 
         [0000]    
       
      
       R=n*R 
       a  
      
     
         [0000]    where n is the number of linear leg portions  132  and R a  is the resistance of one of the linear leg portions, which together form the absorber  108 . The resistance of the support posts  122 / 124  is de minimis because of the relatively large bulk of material and the short length compared to the support leg portions. 
         [0053]    Upon impingement of the absorber  108  with electromagnetic radiation, the average temperature of the absorber  108  increases by ΔT. The electrical resistance of the sensor upon incident radiation changes by an amount ΔR given by: 
         [0000]      Δ R=αnR   a   ΔT  
 
         [0000]    where α is the temperature coefficient of resistance of the thin film. 
         [0054]    In one embodiment, the width of the linear leg portion is significantly smaller than the wavelength of the infrared radiation to be measured (8 μm-12 μm or 3 μm-5 μm) (also referred to as the “target” wavelength). Therefore, the assembly of linear leg portions is seen as an effective medium by the incoming infrared radiation and forms an efficient absorber. 
         [0055]    In one embodiment, the gaps between the linear leg portions is significantly smaller than the target wavelength (8 μm-12 μm or 3 μm-5 μm). Therefore, the assembly of linear leg portions as a total are seen as an effective medium by the incoming infrared radiation and forms an efficient absorber. Thus, the incoming IR radiation sees an increased effective sheet resistance and the aforementioned 377 ohm condition can be achieved with higher film thicknesses. 
         [0056]    When electromagnetic radiation (e.g. infrared light) reaches the sensor assembly  100 , the electromagnetic radiation is absorbed within the thin-film metal of the absorber  108  with an efficiency depending on the resistivity of the absorber  108 , quality of the mirror  106 , gap height between the absorber  108  and the mirror  106 , and radiation wavelength. Upon absorbing the incident radiation, the absorber  108  undergoes an increase in temperature. This temperature increase, in turn, leads to either a decrease or increase of the resistivity of the absorber  108 . The absorber  108  is then electrically probed to measure the resistivity of, and thus indirectly measure the amount of incident electromagnetic radiation on, the absorber  108 . 
         [0057]    In one embodiment, due to the typical resistivity of deposited metals and semiconductors, the suspended thin-film absorber  108  has a thickness inferior to 50 nm. Features of the deposition technique known as atomic layer deposition is preferred for this embodiment over traditional micromachining techniques, e.g. sputtering and evaporation, in forming the absorber  108 . One advantage of this device over many known devices is its simplicity of fabrication which is explained with reference to  FIGS. 7-14 . 
         [0058]    Fabrication of a sensor such as the sensor assembly  100  begins with preparation of a substrate  150  which is shown in  FIG. 7 . The substrate  150  may be a portion of a larger substrate that is used to form a number of sensors. An initial insulator layer portion  152  is formed on the upper surface of the substrate  150 . In this example, an oxide film of about 1000 A is formed on the substrate  150 . 
         [0059]    Next, feed throughs  154  and  156  along with associated bond pads  158  and  160  are formed on the upper surface of the initial insulator layer portion  152  ( FIG. 8 ). The feed throughs  154 / 156  and the bond pads  158 / 160  are formed from a conducting metal by any acceptable process such as one incorporating lithography and plasma etching. The remainder of the insulator layer  162  is then formed thereby encapsulating the feed throughs  154 / 156  and the bond pads  158 / 160  ( FIG. 9 ). The insulator layer  162  may be planarized if desired. In some embodiments, one or more of the feed throughs  154 / 156  may be connected to another portion of the CMOS device, e.g., a transistor. 
         [0060]    Referring to  FIG. 10 , portions of the feed throughs  154 / 156  and the bond pads  158 / 160  are then exposed by etching a trench completely through the insulator layer  162  to form openings  170 ,  172 ,  174 , and  176 , respectively. In  FIG. 11 , a mirror  180  and a bond ring  182  are formed on the upper layer of the insulator layer  162 . The mirror  180  and the bond ring  182  may be formed by sputtering, lithography, and etching, or any other acceptable process. The bond ring  182  and the mirror  180  may be formed simultaneously if desired. 
         [0061]    A sacrificial layer  184  is then formed over the top of the insulator layer  162 , the mirror  180 , and the bond ring  182  ( FIG. 12 ) and openings  186  and  188  are formed by etching a trench completely through the sacrificial layer  184  to expose portions of the feed throughs  154  and  156 , respectively. A serpentine trench  190  is also formed in the sacrificial layer  184  connecting the openings  186  and  188 . The openings  186 / 188  and the trench  190  may be formed using spin photoresist and lithography. 
         [0062]    In embodiments wherein electrical contact is provided using through silicon vias in the backside of the substrate  150 , the insulator layer  162  is not needed. Moreover, the mirror  180  may also be formed using the same material used to form the feed throughs  154 / 156 . Thus, the layers depicted in  FIGS. 7-12  as  162  and  184  may be formed as a single layer. 
         [0063]    An absorber layer  192  is then formed above a portion of the sacrificial layer  184 , on the exposed surface portions of the feed throughs  154 / 156 , on the sides of the openings  186  and  188 , and along the walls and bottom of the trench  190 . In some embodiments, the openings  186  and  188  are first filled, followed by forming of the trench  190  and filling of the trench  190  with an absorber layer. In different embodiments, a different material is used to fill a trench portion which becomes the support bars  123 / 125  and/or the support posts  122 / 124  as compared to the material used to form the absorber. Such modification simply requires modification of the timing of forming of the different trench/openings. 
         [0064]    The absorber layer  192  in some embodiments is formed by atomic layer deposition (ALD). ALD is used to deposit materials by exposing a substrate to several different precursors sequentially. A typical deposition cycle begins by exposing a substrate is to a precursor “A” which reacts with the substrate surface until saturation. This is referred to as a “self-terminating reaction.” Next, the substrate is exposed to a precursor “B” which reacts with the surface until saturation. The second self-terminating reaction reactivates the surface. Reactivation allows the precursor “A” to react with the surface. Typically, the precursors used in ALD include an organometallic precursor and an oxidizing agent such as water vapor or ozone. 
         [0065]    The deposition cycle results, ideally, in one atomic layer being formed. Thereafter, another layer may be formed by repeating the process. Accordingly, the final thickness of the absorber layer  192  is controlled by the number of cycles a substrate is exposed to. Moreover, deposition using an ALD process is substantially unaffected by the orientation of the particular surface upon which material is to be deposited. Accordingly, an extremely uniform thickness of material may be realized both on the horizontal surfaces (the sacrificial layer  184 , the exposed surface portions of the feed throughs  154 / 156 , and the bottom of the trench  190 ) and on the vertical surfaces (the sides of the openings  186  and  188 , the walls of the trench  190 ). Thus, in some embodiments the posts  122 / 124  are hollow, and may be further anchored as described in the &#39;479 application. 
         [0066]    In some embodiments, it may be desired to form structures using multiple layers of ALD material. For example, while the device in the present example includes a single absorber layer  192 , a stacked absorber may be useful in different embodiments. A stacked absorber or other structure may have two, three, or more layers of different or alternating materials. For example, a layer of insulating material may provide a substrate for a layer of a conducting material with yet another insulating material above the conducting material. A very thin conducting layer may thus be protected and strengthened by being sandwiched between two very thin insulating layers. Al 2 O 3  may be used as an insulating layer deposited using ALD. 
         [0067]    Once the absorber layer  192  is formed, the sacrificial layer  184  is then etched to form the final device, such as the sensor assembly  100  of  FIGS. 1 and 2  ( FIG. 14 ). 
         [0068]    In some embodiments, the wire absorber is not supported by posts such as the posts  122 / 124 . By way of example,  FIG. 15  depicts a sensor assembly  200  that includes a substrate  202 . A cavity  204  is formed in the substrate  202  and a mirror  206  is positioned at the bottom of the cavity. An absorber  208  is directly supported by the substrate  202 . Feed throughs  210 / 212  extend outwardly from the absorber  208  beneath a bond ring  214 . 
         [0069]    As shown in  FIG. 16 , the absorber  208  includes end structures  216  which connect leg portions  218  to each other. The end structures  216  are not configured using ninety degree angles. Accordingly, current crowding and high mechanical stress fields are avoided. The end structures  216  are thus similar to the end structures  144 , although more rounded. 
         [0070]    The manufacture of the sensor assembly  200  is simpler than the sensor assembly  100 . For example, after providing the substrate  202 , the cavity  204  is formed and the mirror  206  formed within the cavity  204 . A sacrificial material (not shown) is then used to fill the cavity  204  and a trench is formed in the upper surface of the substrate  202  and the sacrificial material (not shown) in the desired shape of the absorber  208  and feed throughs  210 / 212 . The trench is then filled with an absorber material, and the bond ring  214  is formed. The sacrificial material (not shown) is then etched, resulting in the configuration of  FIGS. 15-16 . 
         [0071]    While the foregoing configurations are selected to reduce current crowding and high mechanical stress fields, other shapes for the absorber  108 / 208  are possible. In some embodiments, the absorber  108 / 208  is formed with ninety degree angles. For example,  FIG. 17  is a simplified depiction of a sensor assembly  250  that is similar to the sensor assembly  100 . The sensor assembly  250  differs from the sensor assembly  100  in that the serpentine absorber  252  includes ninety degree angles. Additionally, the absorber  252  is supported by support bars  254  and  256  which are formed from a material different from the absorber material (indicated by the different hashing). 
         [0072]      FIG. 18  is a simplified depiction of another embodiment of a sensor assembly  260  that is similar to the sensor  100 . The sensor assembly  260  differs from the sensor assembly  100  in that the absorber  262  includes ninety degree angles, and is a more complex shape than the serpentine absorber  108 . 
         [0073]      FIG. 19  is a simplified depiction of another embodiment of a sensor assembly  270  that is similar to the sensor  100 . The sensor assembly  270  differs from the sensor assembly  100  in that the absorber  272  includes ninety degree angles, and is a more complex shape than the serpentine absorber  108 . 
         [0074]    In some embodiments, the ninety degree angles of the absorbers  252 ,  262 , and  272  are rounded to reduce current crowding and high mechanical stress fields. By way of example,  FIG. 20  is a simplified depiction of a sensor assembly  280  which is similar to the sensor  260  except that the absorber  262  includes rounded corners. 
         [0075]    In some embodiments, additional stiffness is provided. By way of example, a semiconductor sensor assembly  300  is depicted in  FIGS. 21 and 22 . The sensor assembly  300  is similar to the sensor assembly  100  of  FIGS. 1 and 2 , including a substrate  302 , an insulator layer  304 , a mirror  306  and an absorber  308 . 
         [0076]    The insulator layer  304  includes access openings  310  and  312  which provide access to bond pads  314  and  316 , respectively, within the insulator layer  304 . The bond pads  314  and  316  are conductively connected to respective buried feed-throughs  318  and  320  which extend within the insulator layer  304  to conductive support posts  322  and  324 . 
         [0077]    The support posts  322  and  324  extend upwardly from the buried feed throughs  318  and  320  to support the absorber  308  at a location above the upper surface of the insulator layer  304 . Each of the support posts  322 / 324  supports the absorber  308  through respective support bar  323 / 325 . 
         [0078]    The mirror  306  is located on the upper surface of the insulator layer  304  along with a bond ring  342 . The bond ring  342  extends completely about the support posts  322 / 324  and the absorber  308  but inside of the access openings  310  and  312 . The mirror  306  is directly beneath the absorber  308 . 
         [0079]    The absorber  308  is a supported serpentine structure which includes a support plate  344  in which leg portions  346  extend. The support plate  344  is a non-conductive material which provides strength/rigidity to the absorber  308  without altering the electrical characteristics of the absorber  308 . The leg portions  346  are electrically connected by end structures  348 . The support plate  344  in one embodiment is released at the same time as the absorber  308 . 
         [0080]    In some embodiments, a support plate like support plate  344  is added to the structures depicted in  FIGS. 16-20 . 
         [0081]    While the disclosure has been illustrated and described in detail in the drawings and foregoing description, the same should be considered as illustrative and not restrictive in character. It is understood that only the preferred embodiments have been presented and that all changes, modifications and further applications that come within the spirit of the disclosure are desired to be protected.