Abstract:
In a microbolometer detector the individual transducers in the focal plane array are support by leg members that are attached to the underlying readout integrated circuit (ROIC) chip at locations underneath transducers other than transducer which they support. A variety of configurations are possible. For example, the leg members may be attached to the ROIC chip at locations under adjacent transducers on opposite sides or on the same side of the supported transducer, or at locations underneath transducers that are not immediately adjacent to the supported transducer. In this way the effective length of the leg members and therefore the thermal isolation of the transducer they support can be increased.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates to microbolometer detectors and in particular to a microbolometer detector for detecting infrared radiation wherein the transducer elements are highly thermally isolated from the readout integrated circuit chip.  
         BACKGROUND OF THE INVENTION  
         [0002]    The general structure of microbolometer detectors is well known in the art. Briefly, they include a two-dimensional array of transducers, each of which is electrically connected to an underlying readout integrated circuit (ROIC) chip that contains circuitry for detecting changes in the resistance of the transducer. The array is often referred to as a “focal plane array” and the individual detectors are referred to as “pixels”. The transducers are formed of a material, such as vanadium oxide, which has a high thermal coefficient of resistance (TCR). To isolate the transducers thermally, they are typically held by a structure that supports them in a suspended position over the ROIC chip. When radiation such as infrared radiation is incident on the transducers in the focal plane array, an image of the source of the radiation is generated.  
           [0003]    Several criteria are important in determining the efficiency of a microbolometer detector. First, to increase the responsiveness and sensitivity of the individual transducers, it is important that the support structure isolate them thermally from the ROIC chip. Second, the support structure and transducer must occupy the same pixel area. Therefore, as the size of the pixels becomes smaller (e.g., to 25-50 μm) the support structure tends to take up more space of the pixel area, and a lower portion of the overall pixel area is occupied by the transducers themselves. The percentage of the pixel area that is occupied by the transducers is sometimes referred to as the “fill factor”. To maintain a high quality image it is desirable to keep the fill factor as high as possible, preferably close to 100%.  
           [0004]    U.S. Pat. No. 6,144,030 proposes one solution to these problems. Each transducer is supported by “leg members” that are located entirely beneath the transducer, and the individual leg members are formed in a “serpentine configuration” that increases the effective length of the leg members and hence the thermal isolation of the transducers. The fabrication of these serpentine structures, however, may require relatively dense photolithographic processing. This processing density may result in low yields. In addition, the effective length of the leg members, even when they are formed in a serpentine configuration, is limited by the area underneath the transducer.  
         SUMMARY OF THE INVENTION  
         [0005]    In accordance with this invention, each transducer unit is supported by at least two leg members that are attached to the underlying ROIC chip at locations beneath other transducer members. This eliminates the necessity of fabricating serpentine or other complicated configurations in order to increase the effective length of the leg members and thereby increase the thermal isolation of the transducer units. Moreover, the length of the leg members is not limited by the area underneath a single transducer unit.  
           [0006]    A variety of embodiments may be fabricated in accordance with this invention. The leg members may extend from locations underneath transducer units of opposite sides of the supported transducer unit. The leg members need not be attached to the ROIC chip at locations underneath adjacent transducer units but instead may originate at locations underneath transducer units that are further removed from the supported transducer unit. The leg members need not originate from locations on opposite sides of the supported transducer unit but may originate from locations underneath a transducer unit on the same side of the supported transducer unit, from locations underneath transducer units located at a right angle in relation to the supported transducer unit, etc. The leg members do not have to be straight but may contain transverse sections, serpentine configurations or other geometric patterns to increase the effect length of the leg members. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]    [0007]FIG. 1 is a top view of a rectangular focal plane array.  
         [0008]    [0008]FIG. 2 is a detailed view of several of the transducer units of the array shown in FIG. 1, showing an embodiment wherein each of the transducer units is supported by leg members that extend from locations below the adjacent transducer units on opposite sides of the supported transducer unit.  
         [0009]    [0009]FIG. 3 is an elevational view of two of the transducer units taken at cross-section  3 - 3  shown in FIG. 2  
         [0010]    [0010]FIG. 4 is a detailed view of several of the transducer units of the array shown in FIG. 1, showing an embodiment wherein each of the transducer units is supported by leg members which extend from locations below transducer units that are separated from the supported transducer unit by intervening transducer units.  
         [0011]    [0011]FIG. 5 is a detailed view of an embodiment that is similar to the embodiment shown in FIG. 4, except that each of the leg members contains a transverse section to avoid vertical interference with other leg members. In addition to avoiding the interference, the transverse section increases the leg length which in turn provides for increased thermal isolation.  
         [0012]    [0012]FIG. 6 is a detailed view of several of the transducer units of the array shown in FIG. 1, showing an embodiment wherein each of the transducer units is supported by leg members that extend from locations below a single adjacent transducer unit.  
         [0013]    [0013]FIGS. 7A-7N illustrate a process for fabricating the transducer and leg members shown in FIG. 3. 
     
    
     DESCRIPTION OF THE INVENTION  
       [0014]    In arrangements according to this invention, the leg members that constitute the support structure for the transducer units extend under neighboring transducer units. Many alternative embodiments are possible.  
         [0015]    [0015]FIG. 1 is a top view of a focal plane array  10  which includes transducer units  11 -  16 ,  21 - 26 ,  31 - 36 ,  41 - 46  and  51 - 56 . Focal plane array  10  is rectangular, although in other embodiments other geometries could be used. The separation between the transducer units is exaggerated for purposes of illustration. In reality, the transducer units could be closer together. Transducer unit  24  is shown as supported by a support structure that includes leg members  102  and  104 . Leg member  102  extends from a location on the ROIC chip (not shown) beneath transducer unit  23  to a point of attachment on the transducer unit  24 . Leg member  104  extends from a location on the ROIC chip beneath transducer unit  25  to a point of attachment on the transducer unit  24 . (Note: In the drawings, the dashed lines represent the leg members, the circles denote a point of attachment on the ROIC chip, and the squares denote a point of attachment to the transducer unit.)  
         [0016]    [0016]FIG. 2 shows a close-up view of the structure shown in FIG. 1, with transducer units  22 - 26  and leg members  102  and  104 . As shown, this pattern is repeated throughout array  10 , with each transducer unit being supported by leg members that extend from underneath transducer units on opposite sides of the supported transducer unit. Thus transducer unit  25 , for example, is supported by leg members  103  and  105  that extend from under transducer units  24  and  26 , respectively.  
         [0017]    [0017]FIG. 3 is an elevation view taken at cross-section  3 - 3  shown in FIG. 2, showing leg members  102  and  106  extending from an ROIC chip  12  to transducer units  24  and  25 , respectively. FIG. 3 also shows some of the details of the structures of the leg members and the transducer units. For example, leg member  102  contains an electrical path  204 , typically formed of nichrome, titanium or vanadium that extends from a contact  302  at an appropriate point on ROIC chip  12  to a contact  208  on transducer unit  24 . Electrical path  204  is encased in an insulating film  206  that is typically made of silicon dioxide or silicon nitride. Contact  208  makes electrical contact with a transducer  210  within transducer unit  24 , which is typically formed of vanadium oxide. Transducer  210  is encased in an absorber  212  that is made of silicon dioxide or silicon nitride. Leg member  106  and transducer unit  25  and the other leg members and transducer units in array  10  contain similar components. It should be understood, however, that this invention is not limited to any particular form of leg member or transducer unit. Other embodiments may include leg members and transducer units having structures different from those shown in FIG. 3.  
         [0018]    Several alternative embodiments are shown in FIGS. 4, 5 and  6 .  
         [0019]    [0019]FIG. 4 illustrates that the leg members need not originate under an adjacent transducer unit. In FIG. 4, leg member  106  extends from a location underneath transducer unit  22  to a point of attachment on the underside of transducer unit  24 ; transducer units  22  and  24  being separated by transducer unit  23 . Similarly, leg member  108  extends from a location underneath transducer unit  26  to a point of attachment on the underside of transducer unit  24 , transducer units  24  and  26  being separated by transducer unit  25 .  
         [0020]    In some cases it may be difficult to maintain vertical clearance between the leg members, particularly in embodiments such as the one shown in FIG. 4 where the leg member do not originate under an adjacent transducer unit. To overcome this problem, the leg members can be structured as shown in FIG. 5. Leg member  110 , which is functionally similar to leg member  106  and supports transducer unit  24 , contains a transverse section (e.g., section  110 A in FIG. 5) perpendicular to the main direction of the leg member to prevent interference with the leg member (leg member  112 ) that supports the adjacent transducer unit. In addition, the transverse portion of leg member  110 A provides for a longer path length, This longer path length provides for greater thermal isolation which results in a higher performance detector.  
         [0021]    In the embodiments shown in FIGS. 2, 4 and  5  the leg members that support a particular transducer unit originate from opposite sides of that transducer unit. FIG. 6 shows another alternative embodiment wherein the leg members that support each transducer unit extend from the same side of the supported transducer unit and originate under the same transducer unit. For example, leg members  116  and  118 , which support transducer unit  24 , extend from under transducer unit  23 . Leg members  120  and  122 , which support transducer unit  23 , extend from under transducer unit  22 . To avoid a vertical clearance problem the leg members can be offset with respect to each other as shown in FIG. 6.  
         [0022]    The embodiments shown in FIGS. 2, 4,  5  and  6  are not limiting. Each leg member that supports a given transducer unit may originate under any other transducer unit, whether or not the other transducer unit is in the same row or column of the array as the supported transducer unit, and whether or not the other transducer unit is adjacent to the supported transducer unit or is separated from the supported transducer unit by one or more intervening transducer units. Moreover, the leg members do not need to be straight, as shown in FIGS. 2, 4 and  6 . The leg members may contain transverse sections, as shown in FIG. 5, or other configurations such as serpentine segments.  
         [0023]    [0023]FIGS. 7A-7N illustrate a process of fabricating the microbolometer detector shown in FIG. 3. As shown in FIG. 7A, the process begins with ROIC chip  12  having metal contacts  302  and  304  that are to be connected to transducer units. As shown in FIG. 7B, a polyimide layer  306  is deposited and etched to form vias  308  which open to contacts  302  and  304 . A silicon dioxide layer  310 , typically 1,000 Å to 2,000 Å in thickness is deposited by plasma-enhanced chemical vapor deposition (PECVD) and etched to form smaller openings to contacts  302  and  304  with a film of silicon dioxide remaining around the walls of vias  308 . The resulting structure is shown in FIG. 7C.  
         [0024]    Next a nichrome layer  312 , typically less than 1,000 Å in thickness is deposited by sputtering, filling the openings to establish contact with contacts  302  and  304 , as shown in FIG. 7D, and a second silicon dioxide layer  314 , of similar thickness to the first layer  310 , is deposited over nichrome layer  312  to form the structure shown in FIG. 7E.  
         [0025]    A photoresist mask layer  315  is deposited and patterned with openings  316 , as shown in FIG. 7F, and silicon dioxide layers  310  and  314 , nichrome layer  312  and polyimide layer  306  are etched via an ion milling process through openings  316 . This essentially forms two cantilevers including nichrome layer  312  embedded in silicon dioxide. Mask layer  315  is then removed. A second polyimide layer  318  is deposited, filling openings  316  and yielding the structure shown in FIG. 7G.  
         [0026]    A second photoresist mask layer  319  is deposited and patterned to form openings  320 , as shown in FIG. 7H. Polyimide layer  318  and silicon dioxide layer  314  are etched through openings  320  exposing the nichrome layer  312 . Mask layer  319  is removed.  
         [0027]    A third silicon dioxide layer  322  is deposited, coating the openings that have been formed in silicon dioxide layer  314  and polyimide layer  318  but still leaving openings to the nichrome layer  312  and resulting in the structure shown in FIG. 71. A third photoresist mask layer  323  is deposited. Mask layer  323  is patterned to form openings in which the transducer material  324  is deposited by ion beam deposition as shown in FIG. 7J. The transducer material is vanadium oxide and typically ranges in thickness from 500 Å to 2000 Å. Subsequent to the transducer layer deposition, the photomask layer  323  is removed.  
         [0028]    A fourth photomask layer  325  is deposited and patterned to allow a second nichrome layer  326  to be deposited as shown in FIG. 7K. The nichrome deposition techniques and thickness parameters are nominally the same as the first nichrome layer  312 . The second nichrome layer  326  provides for the electrical contact between the first cantilevered nichrome layer  312  and the vanadium oxide transducer  324 . The photomask  325  is removed.  
         [0029]    A fourth silicon dioxide layer  328  is deposited resulting in the structure shown in FIG. 7L.  
         [0030]    A final photomask layer  329  is deposited and patterned to allow the delineation of the individual transducing elements. This delineation is performed by ion milling and results in the structure shown in FIG. 7M. The photomask layer  329  is removed.  
         [0031]    The final processing step involves the removal of polyimide layers  306  and  318  by dry plasma etching, yielding transducer elements  23  and  24  and leg members  102  and  106 , as shown in FIG. 7N.  
         [0032]    The embodiments described above are intended to be illustrative and not limiting. Many additional embodiments in accordance with the broad principles of this invention will be obvious to those of skill in the art from the above description.