BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to thermal sensors of fluids, such as fluid flow or property sensors implemented on silicon in microstructure form. For convenience sake the term “flow sensor” will be used generically hereinafter for such thermal sensors. The reader will appreciate that such sensors may be utilized to measure primary properties such as temperature, thermal conductivity and specific heat; and that the flows may be generated through forced or natural convection. The invention relates more specifically to a sensor package of the microbridge or membrane type flow sensor having a central heating element and surrounding sensors which are capable of handling high pressure and have very low susceptibility to environmental damage or contamination. 
     2. Description of Related Art 
     Open microbridge structures such as detailed in U.S. Pat. No. 4,501,144, to Higashi, are well suited for measurements of clean gases, with or without large pressure fluctuations, since the microbridge structure is burst-proof. However, due to the open nature of the microbridge structure, condensates from vapor can be uncontrollably retained in the microbridge structure leading to uncontrolled changes in its thermal response, or output, making the structure susceptible to output error and poor stability. Also, in the typical microbridge structure, the silicon die is wire bonded at the top surface to a header, or substrate, carrying further electrical leads and/or electronics. Typically, such wire for the wire bonds would be a one mil gold wire. This wire has a further tendency to retain liquid condensates, increase undesirable turbulence, shift flow response. Due to its thinness, the wire is susceptible to damage in a high mass flux environment, such as liquid flow, and upon attempts to clean the sensor. 
     Membrane-based sensors such as shown in U.S. Pat. No. 5,705,745, to Treutler et al., overcome some of the problems of the microbridge structure because there is no opening between the bridge and the underlying thermal isolation cavity or air space. However, because the membrane is sealed over the isolation air space membrane based sensors have limited application in constant, near-atmospheric pressure, because the membrane can deform or burst as pressure differences increase above 100 PSI. The top surface of the membrane sensors is also typically wire bonded, leaving the problem of the wire in the flow path accumulating debris and possible breakage during cleaning attempts. 
     It would therefore be desirable to develop a flow sensor which is not susceptible to the above problems of vapor accumulation beneath the microbridge, poor ruggedness under high pressure capability of the membrane sensors, and exposed bonding wire near the heating and sensing elements. The design of such a structure would enable high pressure thermal property sensing over wide ranges at a reasonable cost and provide trouble free operation in heretofore hostile environments. 
     SUMMARY OF THE INVENTION 
     The present invention details a microstructure flow sensor having a silicon microsensor die with a micromembrane or microbridge sensing structure and through-the-wafer electrical contacts as well as a wide pressure range support structure consisting of a micromachined, back-etched honeycomb structure. A flat, passivated, top surface overlying the heater and sensor elements is featured on the silicon die. The silicon die, with its through-the-wafer electrical contacts, eliminates the need for bonding wires with their attendant problems as discussed above. 
     The die is attached to a substrate having a suitably matched coefficient of thermal expansion (CTE) by thermocompression bonding, solder bumping, adhesives or the like, preferably containing through-the-substrate electrical contacts terminating in the necessary electrically conductive runs for attachment to further electronics of the sensor. 
     The substrate may further have a glazing layer at the mating surface with the silicon die in order to provide a fluid barrier to the bottom of the die and back fill seals to prevent access to the back-side contacts and open ends of the honeycomb or micromembrane. Both silicon oxide and silicon nitride layers are used in the construction of the die. The present invention will benefit the user by trouble free and reliable service in all fluid flow applications as well as being easily fabricated and easily subjected to periodic maintenance. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be more fully and completely understood from a reading of the Description of the Preferred Embodiment in conjunction with the drawings, in which: 
     FIG. 1 is a top view of the silicon microsensor die showing the micromembrane heating and sensing elements. 
     FIG. 2 is a cross section of a fluid flow sensor according to the present invention including a substrate structure and a top surround portion further planarizing the portion of the sensor exposed to fluid flow. 
     FIG. 3 is a detailed schematic cross section of the silicon micro sensor die portion of the present invention. 
     FIG. 4 is a detail of the honeycomb structure of FIG.  3 . 
     FIG. 5 is a detailed schematic cross section of the substrate structure to which the die is attached. 
     FIG. 6 is a schematic cross section of an alternative fluid sensor to that of FIG.  1 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Throughout the Description of the Preferred Embodiment, like components will be identified by like reference numerals. 
     Referencing FIG. 1, a fluid flow sensing array  11  is implemented on a silicon substrate or body  13 . Onto the silicon substrate  13  are plated sensor elements  15 ,  17  surrounding a central heating element  19  composed of a suitable metal, such as platinum. The arrangement and theory of operation for a microstructure fluid flow sensor of this type is known to those in the art and will not further elaborated on herein. 
     Referencing in FIG. 2, a flow sensor according to the present invention may include a silicon microsensor die  21  bonded to a larger substrate  23  having a suitably matched coefficient of thermal expansion. Material for the substrate may include alumina, mullite, or known printed circuit board material having suitable CTE. A top surround body, or layer,  25  is placed on the substrate  23  to surround the silicon microsensor die  21  in order to further planarize the top surface of the sensing apparatus and prevent minimal resistance to fluid flow and minimal crevices into which particles or condensates may lodge. The top surround  25  may be implemented as an epoxy layer, a preform, or any suitably constructed and arranged deposition or structural layer serving the above noted purposes. The joints between substrate  23 , die,  21 , and top surround  25  may be further sealed or smoothed with a suitable epoxy or the like to remove potential dust and vapor traps. 
     Referencing FIG. 3, the silicon microsensor die  21  comprises a silicon body  13  having etched through-holes serving as electrical vias, collectively  29 , whose walls are plated with an electrical conductor, preferably gold  31 , overlying a layer of silicon dioxide (SiO 2 )  33 . Preferably centrally located in the die is a honeycomb structure  35  for thermal isolation and structural protection of the microbridge heater/sensor  45 . 
     A detailing of the honeycomb structure from a bottom view is provided in FIG. 4 for the edification of the reader. Both the through-the-wafer electrical vias  29  and the honeycomb structure  35  may be accomplished by deep reactive ion etching machines such as offered by Surface Technology Systems U.S.A., Incorporated, a company located in Redwood City, Calif. The silicon body  27  has a top surface  37  and a bottom surface  39 . Adhered to the top surface is a first layer  41  of silicon dioxide serving passivation and structural functions. A similar silicon dioxide layer is adhered to the bottom surface  39 . The silicon dioxide layers for the top and bottom surfaces may be formed in one pass, or serially, depending upon the capabilities of the fabrication facility. 
     Overlaying the silicon dioxide first layer  41  on the top and bottom surfaces of the silicon body  13  is a second layer  43  of silicon nitride (Si 3 N 4 ) serving passivation/structural functions. As will be recognized, the oxide and nitride layers may be used herein for passivation or structural purposes, or both. It will be noted that there is no silicon nitride layer overlaying the silicon dioxide layer in the electrical vias  29 , and that neither layer interferes with the extension of the gold electrical conductor plating  31  in the vias. It will be appreciated that other compositions of the structural/passivation layers may be suitably selected to serve the same function within the scope of the present invention. Overlaying the second layer  43  of the top surface  37  is the deposited platinum heating and sensing elements  45  including the electrical leads, or contacts  47  therefore, which in turn contact the gold plating  31  of the vias  29 . 
     A third layer  49  of silicon nitride is then preferably placed over the heater sensor and leads  45 ,  47  respectively. The third layer  49  extends to the second layer  43  of the top surface  37  in order to provide a planar, or substantially planar, top cap for the silicon microsensor structure  21 . In some instances where the liquid to be measured has observed scaling effects, such as water, a scale inhibitor layer  51  composed of polysulfone or the like may be desirably adhered to the third layer  49  to further protect the upper surface, or top cap, of the microsensor die  21 . 
     Over the second layer  43  on the bottom surface  39  of silicon body  13  is preferably placed additional thermocompression bonding material  53  such as additional gold or other suitable material for attachment of the sensor die  21  to the substrate  23 . 
     Referencing FIG. 5, the substrate  23  comprises a body  55  comprised of alumina, mullite, or other known materials having coefficient of thermal expansion (CTE) suitably matched to the microsensor structure  21 . The body  55  has a top surface  57  and a bottom surface  59 . Attached to the top surface  57  preferably, and if necessary to provide a vapor barrier between the substrate  23  and the microsensor structure  21 , is a glazing layer  61  comprised of suitable glass or other barrier material preventing the passage of vapor to the microsensor structure  21 . At the top surface of the substrate structure  23  which is to be mated with the silicon microsensor structure  21  there is located additional thermocompression bond material  67  to be mated with the thermocompression bond material  53  of the microsensor structure  21 . 
     Formed in the substrate structure body  55  and any covering layers, such as glazing  61 , are throughholes  63  which are plated with an electrically conductive material  65  such as copper or the like. As CTE considerations are not as critical in the construction of the substrate structure  23 , the throughholes  63  may be plated to the point at which they are filled. At the top surface  57  of the substrate structure  23 , and overlying the copper plated throughholes  63 , is an electrically conductive thermocompression bonding material  67  for mating with the electrically conductive thermocompression bond material  31  of the through-the-wafer electrical vias  29  of the microsensor structure  21 . This may be preferably applied as gold for all electrically conductive and thermocompression bond purposes. However, it will be appreciated that other materials may be applied to form the electrical contacts as well as the thermocompression bond between the microsensor  21  and its substrate  23 . It will be appreciated that the materials for the thermocompression bond or other means of adherement need not exclusively be electrically conductive. However, it is believed that the use of a single bonding material such as gold will aid in the ease of manufacture while providing a tight seal for the honeycomb thermoisolation structure  35  and preventing wicking into the honeycomb structure  35  which might disrupt or alter its thermoisolation capabilities. All areas of the bottom of the die  21  which will not interfere with the electrical connection between the vias  29  and the plated throughholes  65  may be covered with gold. Corresponding gold covered areas on the substrate may be used in order to provide a very strong and fluid tight physical bond between the microsensor structure and the substrate. 
     A typical fabrication scenario would be as follows. On the silicon die first grow a 0.5 micrometer (micron) oxide layer on the top and bottom surfaces. Next deposit a 0.3 micron LPCVD silicon nitride layer over the thermo-oxide layer. Next etch the oxide and nitride from the top side as necessary for deposition of the heater/sensor structure and formation of the vias thereto, as necessary. A platinum layer, actually a layered stack of 600 angstroms chrome oxide 550 angstroms of platinum and 600 angstroms chrome oxide, is then deposited A 0.2 micron LPCVD nitride deposition masking layer is then accomplished and patterned. The platinum stack is then ion milled and a 1 micron LPCVD nitride deposition is placed over the patterned and milled platinum of the heater/sensor. 
     The microsensor structure is then approached from the back side and the oxide and nitride layers on the bottom surface are etched in preparation for the deep reactive ion etching of the silicon. The silicon is then etched with the deep reactive ion etch forming the cylindrical throughholes of the vias and the honeycomb structure. This etch is stopped at the platinum stack of the heater and sensor leads  47 . The chrome oxide is then etched using a nichrome etchant. The silicon walls of the vias are then passivated via a growth of 0.1 micron thermo-oxide followed by a sputtering and plating of the via walls with one micron of gold. The gold is then masked patterned and etched for suitable electrical contact and bonding areas. The photopolymer is mask patterned so that the through-the-wafer vias are protected. A deep reactive ion etch of the silicon is then performed to form the honeycomb structure underneath the sensor element. The microsensor package is then thermocompression bonded via the application of heat and pressure to the substrate structure as detailed above. An epoxy or similar seal around the edges of the chip-substrate interface may then be placed to further seal the edges. The top cap  25  may then be applied to the surface area of the substrate  23  to surround the die  21  via the application of epoxy, epoxy preforms, or preformed substrate material or the like in order to provide a planar top surface of the sensor package with no step-up from the surface area to the die  21 . 
     The honeycomb structure will prevent the sensor membrane from excessive flexure and bursting while the planar top surface of the die prevents contamination, as does the through-the-wafer electrical contacts which lack the customary wire bond debris trap. Further, according to the preferred embodiment the use of a membrane structure provides a flat surface with no passage from the top surface to the thermo-isolation area in which condensates or debris can lodge to harm the operation of the sensor. 
     It will be appreciated by the ordinarily skilled artisan that the present invention offers many advantages and that the detailed structure of the preferred embodiment prevents several solutions to a myriad of problems. It will be recognized that various structures of the preferred embodiment may have counterparts substituted therefor when the unique advantages of that particular element are not desired for a selected sensor application. The present invention is thus only to be limited by the appended claims. 
     Having thus described the invention what is claimed is:
A thermal fluid flow or property sensor having no exposed contact wires or contact pads in the fluid flow path to obstruct measurements, flat fully passivated sensor surfaces and high corrosion resistance is implemented over a honeycombed thermal isolation chamber.