Abstract:
A novel reticulated array comprises islands of ceramic (e.g. BST 40) which are fabricated from novel materials using unique methods of patterning. A shallow etch stop trench (46) is first ion milled around each ceramic island on front side and then filled with an etch stop material (e.g. parylene 48). An optical coat (e.g transparent metal layer 54, transparent organic layer 56 and conductive metallic layer 58) is elevated above the etch stop material by an elevation layer (e.g. polyimide 49). For some applications, it has been experimentally verified that there is no loss, and sometimes a measured increase, in optical efficiency when the optical coating is not planar in topology. Novel fabrication methods also provide for the convenient electrical and mechanical bonding of each of the massive number of ceramic islands to a signal processor substrate (e.g. Si 86) containing a massive array of sensing circuits.

Description:
This is a continuation of application Ser. No. 08/586,358, filed Jan. 16, 1996, U.S. Pat. No. 5,679,267 which is a continuation of application Ser. No. 08/223,088, filed Apr. 4, 1994, now abandoned. 
     CROSS-REFERENCES TO RELATED APPLICATIONS 
     The following related applications were filed concurrently with the instant application: 
     
         ______________________________________                          SerialTitle                 Inventors                          Number______________________________________Dual Etching of Ceramic Materials                 Owen,    08/223,087                 BelcherEtching of Ceramic Materials with an                 Belcher, 08/664,047Elevated Thin Film    OwenDual Etching of Ceramic Materials with a Thin                 Belcher, 08/222,769Front Film            OwenAn Elevated Thin Film for Ceramic Materials                 Belcher  08/222,144Multiple Level Mask for Patterning of Ceramic                 Belcher, 08/222,146Materials             Frank,                 Jones,                 Long______________________________________ 
    
    
    
     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, March 1978; (2) U.S. Pat. No. 4,745,278, Hanson, May 1988; (3) U.S. Pat. No. 4,792,681, Hanson, December 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 absorbency, IR transparency, 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 also 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 the 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. As a consequence, a shallow isolating trench is first ion milled around each pixel location on the front side and then filled with an organic material such as parylene. For some applications, optical coat protection may be further enhanced by elevating the optical coat by use of an additional elevation layer. 
     The novel fabrication method also provides 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. In another embodiment the ceramic islands comprise a middle cross-sectional area between the optical coating layer and the electrical contacts that is greater than a first cross-sectional area proximate the optical coating layer, and is greater than a second cross-sectional area proximate the electrical contacts. The infrared sensitive ceramic islands can have reduced thermal mass and increased thermal responsivity due to the smaller first and second cross-sectional areas. 
     A method of forming an embodiment of the present invention comprises providing a ceramic substrate having a front side and a back side, and etching a first predetermined pattern partially into the front side of the substrate, thereby forming etch stop trenches having the first predetermined pattern. The method further comprises filling the etch stop trenches with an etch stop material, and forming an elevation layer having a second predetermined pattern on the front side of the substrate, wherein the elevation layer substantially aligned with the trenches and covering at least the etch stop material in the trenches. The method further comprises 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 trenches. The method further comprises etching a third predetermined pattern into the back side of the substrate, wherein the third predetermined pattern is substantially the same as the first predetermined pattern, and is substantially aligned with the etch stop trenches on the front side, thereby forming cavities extending down to and into the etch stop material, but not extending to the thin front layer. The method further comprises removing the etch stop material, and 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 processes forming the first and third predetermined patterns. 
    
    
     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 application of a continuous electrode in direct thermal contact with the substrate; 
     FIG. 2 illustrates the patterned tri-level photoresist exposing the front side of the substrate to be etched for the etch stop; 
     FIG. 3 indicates the etched trenches surrounding each pixel; 
     FIG. 4 shows the addition of an organic layer after it has been patterned by photolithography; 
     FIG. 5 illustrates (not to scale) the application of the optical coating over both the organic and continuous electrode layers; 
     FIG. 6 indicates the mounting of the front side of the substrate to a mechanical carrier; 
     FIG. 7 illustrates the composite after the deposition of the electrical contact metals to the back side; 
     FIG. 8 shows the patterned tri-level photoresist with the openings aligned with the previously made etch stops; 
     FIG. 9 depicts the ion milled isolation trenches partially completed; 
     FIG. 10 shows the completed reticulated array of IR sensing capacitors, upside down, still mounted to the carrier; 
     FIG. 11 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. 12 illustrates the etched trenches surrounding each pixel, but without an initial electrode layer; 
     FIG. 13 illustrates the deposition of an organic in the trenches and a continuous electrode formed on the surface; 
     FIG. 14 shows the addition of an organic layer after it has been patterned by photolithography; and 
     FIG. 15 depicts the ion milled isolation trenches partially completed. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Some of the preferred embodiments will be presented with reference to FIGS. 1-15 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). 
     One of the preferred embodiments uses ion milling of the BST substrate from both the front and back. The front side ion milling forms a trench etch stop around the pixel and the backside ion milling completes the thermal isolation without damage to the front side optical coating. Further protection of the optical coating is realized by an additional organic coating over the etch stop. After the removal of this additional organic coating, improved thermal isolation of the pixels results. 
     Tables 1 and 2, below, provide an overview of some embodiments and the drawings. 
     
                       TABLE 1______________________________________ID#  Description  Material (dim)                          Alternates______________________________________35   Protective metal             NiCr 1000A   500-5000Alayer                     TiW, TiW/NiCr, other                          metals and metal                          compounds                          This layer may or may                          not be present36   Substrate    BST 0.06 cm  0.01-0.25 cm37   Etch mask    Tri-level resist38   Tri-level resist top             Photoresist 1.5 μm                          0.1-2.0 μmlevel40   Tri-level resist             TiW 1000A    500-5000Amiddle layer              Other metals, SiO.sub.242   Tri-level resist             Photoresist 15 μm                          0.4-20 μmbottom layer              Polyimide, parylene,                          epoxy, spin on glass46   Trench for etch stop             Ion milled 5 μm                          4-8 μm depth             cavity48   Elevated etch stop             &#34;PIRL&#34; 12 um 0.1-20 um             above front side                          Photoresist, SiO.sub.2,                          polyimide, parylene,                          epoxy, Si.sub.3 N.sub.4                          Comprises elevation                          layer and frontside                          etch stop in a single                          layer49   Organic elevation             &#34;PIRL&#34; 12 um 0.1-20 umlayer                     Photoresist, oxide,                          parylene50   Frontside etch stop             Filler of organic or                          4-8 um             inorganic type                          Photoresist, oxide,                          polyimide, parylene,                          epoxy, nitride, &#34;PIRL&#34;52   Optical coating             3 layered    1/4 IR wavelength54   Transparent coat             NiCr 50A     25-100A                          Other metals or metal                          compounds56   1/4 wavelength             Parylene 1.4 μm                          1/4 desired IRseparator coat            wavelength                          Other organics (e.g.                          photoresist, polyimide,                          epoxy)58   Electrical con-             NiCr 1000A   500-2000Aducting coat              Other metals or metal                          compounds60   Thinned Substrate             BST 18 μm 1-200 μm62   Mechanical carrier             Glass        Silicon, quartz,                          ceramic64   Adhesive     Wax 0-10 um  Epoxy______________________________________ 
    
     
                       TABLE 2______________________________________ID#  Description  Material (dim)                          Alternates______________________________________66   Backside electrical             Bonding Alloys                          4 layer composite68   contact      In 3 um      0.15-6 um70                Au 0.1 um    0.05-0.15 μm72                NiCr 0.05 um 0.05-0.15 μm74                TiW 0.05 um  0.02-0.1 μm76   Partially etched             Airpixel isolation78   Fully etched and             Vacuum       Air, inert gascleaned pixelisolation79   Elevtaed optical             where organic hascoating      been removed80   IC Contact mesa             Polyimide 12 um                          1-30 um                          Photoresist, parylene,                          epoxy82   Ohmic connection             TiW 2000A    100-10,000A                          Other metals or                          conductors84   IC via86   IC processor Si or GaAs______________________________________ 
    
     The combined optical and electrode 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. 
     The polished BST substrate 36 is covered with a continuous thin film conducting electrode 35 as shown in FIG. 1. This film 35 and the underlying substrate 36 are patterned by tri-level resist 37 to form a trench 46 to be filled with an organic ion mill elevated etch stop 48. The tri-level resist consists of photoresist 38 which is patterned by conventional photolithography. Layer 38 is then used as an etch mask to define the underlying metal layer 40. Beneath 40 is a much thicker layer of photoresist 42. These three layers 38, 40 and 42 serve as the ion mill etch mask for the trenches 46. The details of this process are shown in FIGS. 2 and 3 and Table 1. 
     An elevated etch stop material 48 such as parylene is deposited such that it fills trenches 46 and covers the surface of layer 35. Layer 48 is then patterned by conventional photolithography yielding the structure shown in FIG. 4. In this embodiment, no planarization of the etch stop material is necessary. 
     The three layered optical coating 52 is now deposited over the entire front side as illustrated in FIG. 5. From the top toward the substrate 36 the three layers of 52 consist of a semi-transparent thin (50 A (angstroms) thick) metal 54 such as nichrome (NiCr) over a 1.4 μm thick layer of transparent organic material 56 such as parylene over a 1000 A thick layer of a conductor 58 such as NiCr. The thickness of layer 56 is designed to make a 1/4 wavelength IR filter in conjunction with the surfaces of 54 and 58. Alternate thicknesses and materials are shown in Table 1. 
     As shown in FIG. 6, the front side with the optical coating 52 is mounted to a mechanical carrier 62 with a wax adhesive 64 and the back side of the BST substrate 36 is mechanically polished to a thickness 60. Electrical coating 66 is deposited on the back side as shown in FIG. 7. While the composition of 66 is not extremely critical, 66 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 66 are shown in Table 2. From the outside toward the substrate 36 the layers are indium (In) 68, gold (Au) 70, NiCr 72 and TiW 74. 
     A tri-level resist 37 is deposited on layer 66 as shown in FIG. 8, which is then patterned using the techniques previously described. The openings in this resist are aligned with the elevated etch stop material 48 on the front side of the substrate 36. FIG. 9 shows the trenches 76 which have been ion milled down to the elevated etch stop material 48. The optical coating is protected from damage during this process by the elevated etch stop 48 which is elevating the optical coating 52. 
     FIG. 10 shows that unwanted materials have been removed to leave the thermal isolation trenches 78 surrounding the substrate pixels 36. Plasma processes may be performed to remove the remaining parylene etch stop 48. 
     As shown in FIG. 11, the same mechanical carrier 62 is now used to support the fragile IR detector assembly and align the BST 36 mesas with IC mesas 80 such that a bonding material 82 makes electrical connections between each of the pixels by the pixel metallization layer 66 and the processing IC 86 via connection 84. An additional advantage of dual side etching is that BST mesa 36 has lower thermal mass due to extra material being removed during the frontside etch which may provide improved thermal responsivity. 
     In an alternate embodiment, FIG. 12 illustrates the structure of FIG. 3 but without electrode 35. Substrate 36 is patterned by tri-level resist 37 to form a trench 46. The isolation trenches 46 are then filled with an organic frontside etch stop material 48 such as parylene, and the frontside surface of substrate 36 is planarized. Conductive electrode layer 35 is deposited over the surface of substrate 36, yielding the structure shown in FIG. 13. An organic elevation layer 49 such as Polymide Release Layer, &#34;PIRL&#34;, a trademark of Brewer Science, Inc., is applied and patterned by conventional photolithography and aligned to overlap the frontside etch stop material 48, as shown in FIG. 14. 
     The structure is then processed (optical coat deposition, mechanical carrier attachment, tri-level resist pattern formation) as described herein above. The openings in resist 37 are aligned with the frontside etch stop material 50 on the front side of the substrate 36. FIG. 15 shows the trenches 76 which have been ion milled down through frontside etch stop material 50, through electrode 35, and partially into organic elevation layer 49. The optical coating is protected from damage during this process by organic elevation layer 49 which is elevating the optical coating 52. 
     Any remaining organic etch stop layer 50 may be removed with a suitable solvent or a dry etch. Portions of electrode layer 35 remaining between pixels 36 are also etched from the structure. The optical coating is again protected during these processes by organic elevation layer 49. 
     Elevation layer 49 is finally removed to leave the thermal isolation trenches 78 surrounding the substrate pixels 36. If 49 is photoresist, conventional solvents and developers may be used. If 49 is &#34;PIRL&#34;, Brewer Science solvents may be used. Elevation layer 49 and frontside etch stop layer 50 are normally different materials, but may alternatively comprise the same material. Metal layer 35 covers the front side of pixel 36 for a full face contact as shown in FIG. 15. This full face contact can improve the electrical properties for some applications. 
     By the described novel methods is produced a unique, rugged reticulated array of IR sensing pixels. 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 etch stop material and 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. The etch stop material and the elevation layer can have the same or different compositions. Almost any metal can be used for the continuous electrode 36. Deposition and removal may vary but the functionality is the same. The removal of silicon nitride, Si 3  N 4 , could be performed by plasma etching in a Freon gas, CF 4 . Parylene may be plasma etched using a different gas mixture. &#34;PIRL&#34; or photoresist may be removed with a liquid solvent. The optical coating both in contact with and elevated above the substrate can vary drastically in thickness and geometry without materially affecting the functions performed. 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.