Etching of ceramic materials with an elevated thin film
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This is a continuation of application Ser. No. 08/587,260, filed Jan. 16, 1996, which is a continuation of application Ser. No. 08/223,073, filed Apr. 4, 1994, now abandoned. 
     The following related applications were filed concurrently with the instant application: 
     
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Title                  Inventors                                          
                                Number                                    
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Dual Etching of Ceramic Materials                                         
                       Owen,    TI-17847                                  
                       Belcher                                            
Dual Etching of Ceramic Materials with an                                 
                       Belcher, TI-18727                                  
Elevated Thin Film     Owen                                               
Dual Etching of Ceramic Materials with a                                  
                       Belcher, TI-18736                                  
Thin Front Film        Owen                                               
An Elevated Thin Film for Ceramic                                         
                       Belcher  TI-18737                                  
Materials                                                                 
Multiple Level Mask for Patterning of                                     
                       Belcher, TI-19182                                  
Ceramic Materials      Frank,                                             
                       Jones,                                             
                       Long                                               
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     FIELD OF INVENTION 
     This invention generally relates to uncooled infrared (IR) detector arrays and means of fabrication. 
     BACKGROUND OF INVENTION 
     The novel IR devices and fabrication processes to be described are related to the types of IR detector arrays recorded in (1) U.S. Pat. No. 4,080,532, Hopper, 3/1978; (2) U.S. Pat. No. 4,745,278, Hanson, 5/1988; (3) U.S. Pat. No. 4,792,681, Hanson, 12/1988; and (4) &#34;LOW-COST UNCOOLED FOCAL PLANE ARRAY TECHNOLOGY&#34;, by Hanson, Beratan, Owen and Sweetser, presented Aug. 17, 1993 at the IRIS Detector Specialty Review. 
     The physical requirements of uncooled arrays and a brief description of current fabrication processes will be presented to aid in the understanding of the improvements realized by the novel methods to be described. 
     An area imager may contain several hundred to tens of thousand individual picture elements (pixels). Each of these pixels consists of a capacitor (or resistor or another type of electronic element) that has a heat (IR intensity) sensitivity. Making use of the fact that the charge stored by a capacitor is proportional to the product of its terminal voltage and its capacitance, electronic circuitry can be attached to the two terminals of the capacitor based pixel to measure the intensity of the IR impinging on a specific pixel. Obstructions in the imaging field are removed and the electronic connections to these capacitors are simplified if one of the capacitor terminals is made common to all. Hundreds to tens of thousands of connections must still be made between the other isolated terminals of the capacitors and the electronic sensing circuitry. In addition, the pixel capacitors should be thermally isolated from each other while having one terminal connected to all the other common terminals. 
     The common connection to one side of the pixel capacitors consists of a front side thin film referred to as the optical coating. This may be a composite of a plurality of thin films which have been optimized with regard to IR transparency, IR absorbency, electrical conductivity, thermal resistivity, etc. The thicker heat sensitive dielectric substrate in this case can be barium-strontium-titanate (BST) which is a ceramic perovskite material. 
     To leave the front side optical coating thin film electrically conducting while isolating the pixels thermally, one may etch deep trenches around the pixel capacitors in the BST substrate. The optical coating side of the device will be referred to as the front side. 
     SUMMARY OF THE INVENTION 
     A preferred embodiment of this invention comprises a novel reticulated array of two-terminal heat sensitive capacitors which are fabricated from novel materials using unique methods of patterning. 
     Ion milling of the BST substrate is preferred over other dry processing etching methods such as reactive ion etching, electron cyclotron resonance plasma etching and standard plasma etching because all would require undesired high temperatures to etch properly. 
     Ion milling can be used from the back side to isolate pixels. It can be difficult to control this backside ion milling process accurately enough to prevent damage to the optical coating on the front side. Elevating the optical coating on the front side allows additional material between the optical coating and the substrate above the isolation trench. Ion milling the thermal isolation trenches can now be performed from just the back side of the BST substrate without damaging the front side optical coating. 
     Novel fabrication methods also provide for the convenient electrical and mechanical bonding of each of the massive number of pixel capacitors to an integrated circuit containing a massive array of sensing circuits. 
     One embodiment of the present invention is a microelectronic structure comprising a substantially undamaged continuous optical coating layer, a reticulated array of ceramic islands disposed on the optical coating layer, and thin electrical contacts disposed on a surface of the ceramic islands opposite the optical coating layer. The optical coating layer comprises elevated portions between and partially overlapping the ceramic islands. The elevated portions of the optical coating layer can provide improved thermal efficiency for sensing impinging infrared radiation. In another embodiment, the ceramic islands comprise a first cross-sectional area proximate the optical coating layer which is larger than a second cross-sectional area proximate the electrical contacts. 
     A method of forming an embodiment of the present invention comprises providing a ceramic substrate having a front side and a back side, forming an elevation layer having a first predetermined pattern on the front side of the substrate, and depositing a thin front layer on the front side of the substrate and on the elevation layer, thereby forming elevated portions of the thin front layer above the front side. The method further comprises etching a second predetermined pattern into the back side of the substrate, wherein the second predetermined pattern substantially aligned with the elevated portions of the thin front layer on the front side, thereby forming cavities extending down to and partially into the elevation layer, but not extending to the thin front layer. The method further comprises removing the elevation layer, thereby forming isolated islands of the ceramic substrate connected by the thin front layer. The thin front layer remains substantially undamaged, having not been exposed to the etching process forming the second predetermined pattern. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself, however, as well as other features and advantages thereof, will be best understood by reference to the detailed description which follows, read in conjunction with the accompanying drawings, wherein: 
     FIG. 1 illustrates the photolithographic steps prior to etching an organic layer on the substrate; 
     FIG. 2 shows the patterned organic layer; 
     FIG. 3 illustrates (not to scale) the application of the optical coating over the organic grids; 
     FIG. 4 indicates the mounting of the front side of the substrate to a mechanical carrier, thinning the substrate and applying the metallization alloys to the backside; 
     FIG. 5 shows patterned tri-level photoresist over the metals on the back side having the openings aligned with the organic grids on the front side; 
     FIG. 6 depicts the ion milled isolation trenches etched down to the front side organic grids; 
     FIG. 7 shows the use of the carrier as a means of supporting the fragile IR sensor array while it is bonded to the planar IC containing the electronic sensing circuitry; 
     FIG. 8 displays the addition of a metal layer beneath the organic coating on the front side instead of that shown in FIG. 1; 
     FIG. 9 shows the organic patterning just as in FIG. 2; 
     FIG. 10 shows the application of the optical coating as in FIG. 3; 
     FIG. 11 illustrates the back side ion milling terminated when encountering the elevation layer; 
     FIG. 12 shows the mounting of the completed IR sensing array to the sensing IC as in FIG. 7. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Some of the preferred embodiments will be presented with reference to FIGS. 1-12 and Tables 1 and 2. It should be understood that the drawings are for illustrative purposes and are not to dimensional or relative scale. In addition, only a few detectors are illustrated in the figures, but the present invention can be applied to any number of detectors in various configurations (e.g. single line or array). 
     These preferred embodiments use ion milling of the BST substrate from the back side. Protection of the front side optical coating is realized by a patterned grid of material between the optical coating and the substrate at the points where the ion milled thermal isolation trenches will appear. After the removal of this additional material, improved thermal isolation of the pixels results. 
     The optical and electrical coatings are in direct contact with the BST pixel making in a sense a pixel heat sink. Any heat energy in the optical coating flows to the nearest pixel. Even though the optical surface is not planar, the optical imaging is actually superior to a planarized optical coating for some applications. This is due in part to the longer thermal path between pixels along the elevated optical coat. This is also due in part to the ease with which any supporting materials may be completely removed to improve thermal isolation without damage to the optical coating. 
     Tables 1 and 2, below, provide an overview of some embodiments and the drawings. 
     
                       TABLE 1                                                     
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                  Material                                                
ID#  Description  (dim)      Alternates                                   
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36   Etch mask    Photoresist                                             
                             0-5 μm                                    
     for elevation                                                        
                  1.5 μm                                               
     layer                                                                
38   Organic      &#34;PIRL&#34;     0.1-20 μm                                 
     elevation    12 μm   Photoresist, parylene,                       
     layer                   polyimide                                    
39   Protective   NiCr 1000 Å                                         
                             500-5000 Å                               
     metal                   Other metals or metal                        
     layer                   compounds                                    
40   Substrate    BST        0.01-0.25 cm (unthinned)                     
                  0.06 cm    1-200 μm (thinned)                        
                  (unthinned)                                             
                  18 μm                                                
                  (thinned)                                               
42   Optical      3 layered  1/4 IR wavelength                            
     coating                                                              
44   Transparent  NiCr 50Å                                            
                             25-100Å                                  
     coat                    Other metals or metal                        
                             compounds                                    
46   1/4 wave-    Parylene   1/4 of desired IR                            
     length       1.4 μm  wavelength                                   
     separator               other organics (e.g.                         
     coat                    photoresist, polyimide,                      
                             epoxy)                                       
48   Electrical   NiCr 1000Å                                          
                             500-2000Å                                
     conducting              other metals or metal                        
     coat                    compounds                                    
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                       TABLE 2                                                     
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ID#  Description                                                          
               Material (dim)                                             
                             Alternates                                   
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50   Backside  Bonding Alloys                                             
                             4 layer composite of                         
     electrical                                                           
52   contact   In 3 μm    0.15-6 μm                                 
54             Au 0.1 μm  0.05-0.15 μm                              
56             NiCr 0.05 μm                                            
                             0.05-0.15 μm                              
58             TiW 0.05 μm                                             
                             0.02-0.10 μm                              
60   Adhesive  Wax 0-10 μm                                             
                             Epoxy                                        
62   Mechanical                                                           
               Glass         Silicon, quartz, ceramic                     
     carrier                                                              
64   Etch mask Tri-level resist                                           
     for ion                                                              
66   milling   1.5 μm photoresist                                      
                             0.1-2 μm                                  
     isolation                                                            
68   trenches  1000Å TiW 500-1500Å-other metals,                  
                             SiO.sub.2                                    
70             15 μm photoresist                                       
                             0.4-20 μm-polyimide,                      
                             parylene, epoxy, spin on                     
                             glass                                        
72   Isolation                                                            
     trench                                                               
74   IC Contact                                                           
               Polyimide 12 μm                                         
                             1-30 μm                                   
     mesa                    Photoresist, parylene,                       
                             epoxy                                        
76   Ohmic     TiW 2000Å 100-10,000Å                              
     connection              other metals, conductors                     
78   IC via                                                               
80   IC        Si or GaAs                                                 
     processor                                                            
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     Rather than show two complete sets of drawings which in most respects serve identical purposes, one process flow will be described before introducing an optional step for another preferred embodiment. In the first process flow, the elevation layer for the optical coating is an organic layer. An alternate process flow introduces a metal layer underneath the organic layer. 
     In FIG. 1 a photoresist layer 36 has been patterned by conventional photolithography over an organic layer 38 which covers the BST substrate 40. Organic layer 38 may be any easily removed material having the desired physical characteristics such as Polymide Release Layer, &#34;PIRL&#34;, a trademark of Brewer Science, Inc., photoresist. Layer 38 is patterned into a grid over the points on the substrate 40 which will become thermal isolation trenches as shown in FIG. 2. Layer 38 may itself be photodefineable, or it may be patterned using a separate mask layer. 
     The three layered optical coating 42 is now deposited over the entire front side as illustrated in FIG. 3. From the top toward the substrate 40 the three layers of 42 consist of a semi-transparent thin (50 Å thick) metal 44 such as nichrome (NiCr) over a 1.4 μm thick layer of transparent organic material 46 such as parylene over a 1000 Å thick layer of a conductor 48 such as NiCr. The thickness of layer 46 is designed to make a 1/4 wavelength IR filter in conjunction with the surfaces of 44 and 48. Alternate thicknesses and materials are shown in Table 1. 
     As shown in FIG. 4, the front side with the optical coating 42 is mounted to a mechanical carrier 62 with a wax adhesive 60 and the back side of the BST substrate 40 is mechanically polished to the desired thickness (generally 1-200 micrometers and preferably to 18 micrometers). Electrical coating 50 is deposited on the back side. While the composition of 50 is not extremely critical, 50 serves as one side of the pixel capacitor and forms the metallic surfaces to which the complex IC sensing circuits will be mechanically and electrically bonded. The compositions and thicknesses used for 50 are shown in Table 2. From the outside toward the substrate 40 the layers are indium (In) 52, gold (Au) 54, NiCr 56 and TiW 58. 
     In FIG. 5 tri-level resist 64 is patterned such that the openings are above the organic grid 38 on the front side of 40. The tri-level resist consist of a thin layer of photoresist 66 which is used to pattern an underlying layer of TiW 68 over a much thicker photoresist layer 70. These 3 layers form the etch mask to etch the BST substrate 40. 
     FIG. 6 shows the thermal isolation trenches 72 which have been ion milled down to the organic grid 38. The grid serves as protection for the optical coating 42. The organic material used for 38 and the tri-level resist 64 are removed with suitable solvents or a dry etch which leave the elevated optical coating 42 undamaged. If 38 is photoresist, conventional solvents and developers may be used. If 49 is &#34;PIRL&#34;, proprietary Brewer solvents may be used. 
     As shown in FIG. 7, the same mechanical carrier 62 is now used to support the fragile IR detector assembly and align the BST 40 mesas with IC mesas 74 such that a bonding material 76 makes electrical connections between each of the pixels by the pixel metallization layer 50 and the processing IC 80 via connection 78. 
     By the described novel methods is produced a unique, rugged reticulated array of IR sensing pixels. Another preferred embodiment will be described with the aid of the previous presentation and FIGS. 8 to 12. FIG. 8 is in all respects identical to FIG. 1 except that a metal layer 39 such as TiW has been deposited before the organic elevation layer 38. This serves to give additional protection during the ion milling operation as well as providing additional electrical conductivity for the common conductor to all the pixel capacitors. 
     In FIG. 9 it can be seen that although the organic layer 38 is patterned into a grid, layer 39 is not. Alternatively, metal layer 39 may be patterned before, with or after organic layer 38 to facilitate backside alignment. FIG. 10 shows the optical coating 42 deposited over the ensemble as previously described except in this case 42 is in ohmic contact with 39. Metal layer 39 covers the front side of the pixel for a full face contact as shown in FIG. 14. This full face contact can improve the electrical properties for some applications. 
     The process flow is identical to that previously described until the ion milling reaches layer 38. Layers 38 and 39 provide a greater separation of the ion milling operation from the optical coating 42 as shown in FIG. 11. The ensemble is now removed from the ion milling machine and placed in a plasma etch environment to preferentially remove the remaining TiW layer 39 that has been exposed by trenches 72. Layer 39 remains over the pixel capacitor areas. The organic grid 38 is now removed as previously described. 
     FIG. 12 shows the bonding of the IR array to its corresponding sensing IC array as has been previously discussed. Metal layer 39 covers the front side of pixel 40 for a full face contact as shown in FIG. 12. This full face contact can improve the electrical properties for some applications. 
     While this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. There are many variants and combinations of the materials and dimensions shown in these embodiments partially listed in Table 1 and Table 2. The elevation layer can be many various materials such as but not limited to photoresist, oxide (e.g. SiO 2 ), polyimide, parylene, polyamide, epoxy, nitride (e.g. Si 3  N 4 ), &#34;PIRL&#34;, and combinations thereof. Almost any metal can be used for the underlying metal 39. Deposition and removal techniques may vary but the functionality is the same. In the dry removal of materials, reactive ion etching, plasma etching and electron cyclotron resonance plasma etching may often be interchanged without serious effect on the fabrication process. Various modifications and combinations of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. It is therefore intended that the appended claims encompass any such modifications or embodiments.