Abstract:
A robust sensor that incorporates the necessary physical structure and thermal characteristics is capable of measuring fluid flow and properties under harsh environmental conditions. The sensor die is made of a material with thermal conductivity tailored to provide the thermal transmission characteristics necessary to avoid saturation of the sensor, thus enabling the measurement of high mass flux airflow and liquid properties under high pressure and often harsh environments not previously available for silicon based sensors. The robust sensor further has internal vias for back-side electrical connection, thus avoiding electrical and mechanical interference with the measurements. All of these features come together to provide a microsensor which is capable of reliable, i.e. stable, wide dynamic range and rapid-response operation under harsh environments.

Description:
This is a continuation-in-part of U.S. patent application Ser. No. 09/207,165, filed Dec. 7, 1998, entitled “Rugged Fluid Flow and Property Microsensor,” now U.S. Pat. No. 6,184,773, and U.S. patent application Ser. No. 09/368,621, filed Aug. 5, 1999 now U.S. Pat. No. 6,322,247, which is a Continuation-in-Part of U.S. patent application Ser. No. 09/239,125, filed Jan. 28, 1999 now U.S. Pat. No. 6,361,206, both entitled “Microsensor Housing”. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates generally to thermal sensors of fluids, such as fluid flow sensors implemented 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 heat transfers may be generated through forced or natural convection. The invention relates more specifically to a sensor of the Microbrick™ or microfill type having a central heating element and surrounding sensor arrays which are structurally robust and capable of operating in harsh environments. These Microbrick™ or microfill sensors include through-the-wafer interconnects thus providing very low susceptibility to environmental damage or contamination. The material of the sensor support structure is of thermal conductivity tailored to the application thus producing a more useful and versatile sensor, such as needed for high sensitivity or high mass flux fluid flow measurement or measurements in harsh environments. 
   2. Description of Related Art 
   Open microbridge structures such as detailed in U.S. Pat. No. 5,401,155, to Higashi et al., 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. 
   The typical microbridge structure has a silicon die 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 tendency to retain particles suspended in the fluid, retain liquid condensates, increase undesirable turbulence, and shift flow response. Due to its thinness, the wire is also susceptible to damage in a high mass flux environment, such as high rate liquid flow, and upon attempts to clean the sensor. 
   Membrane-based sensors overcome some of the problems of the microbridge structure because there is no opening exposed to the fluid. More specifically, there is no opening allowing the fluid to enter the underlying structure. However, because the membrane is sealed over an isolation air space and subject to differential pressure induced stress signal errors, membrane based sensors have limited application in high pressure applications. Due to the physical configuration of the membrane, it can deform or burst as pressure differences (on either side of the membrane) increase above 100 PSI (pressure levels that are very possible in high mass flux environments). The heating/sensing elements on the top surface of the membrane sensors are also typically wire bonded to other components, leaving the problems of the wire in the flow path accumulating debris and possibly breaking during cleaning attempts. 
   While many different materials may be used to make a fluid flow sensor, the choice of material can drastically affect the sensor&#39;s performance. A preferable material making up the sensor substrate would have a relatively low thermal conductivity among other characteristics. This low thermal conductivity is necessary to maintain the sensitivity for the sensor. With this relatively low thermal conductivity, all heating/cooling effects presented to the various sensing elements are caused predominatly by the fluid to be sensed. Stated alternatively, it is important to ensure that heat is not transmitted through the substrate excessively, resulting in signal shorts. 
   The micromembrane structure discussed above provides a design approach that enables accurate thermal measurements to be made in harsh environments (condensing vapors, with suspended particles, etc.). Specifically, the mass of silicon immediately below the heater/sensing elements is greatly reduced or eliminated, thus limiting potential heat losses. Even in this structure, however, the selection of materials is critical—low thermal conductivity and appropriate material strength continue to be very important. A disadvantage of this structure is its sensitivity to differential pressure (across its membrane) which induces a stress in the sensing elements and results in uncontrolled output signal changes or errors. 
   In addition to the above referenced thermal characteristics, it is highly desirable for the overall flow sensor to be chemically inert, corrosion resistant, highly temperature stable, electrically isolated, and bio-compatible. Obviously, many of these characteristics are achieved by proper selection of materials. Further, these desired characteristics are necessary in light of the sensors&#39; operating environment. The materials chosen must provide for a sensor which is capable of operating in harsh environments. 
   It would therefore be desirable to develop a flow sensor which is not susceptible to the above referenced problems. Specifically, the sensor would not be affected by vapor accumulation beneath the microbridge, and would not have exposed bonding wire near the heating and sensing elements. The desirable sensor would be structurally robust and thus capable of operating in harsh environments. Further, it would be desirable to develop a flow sensor which is not affected by signal shorts, thus capable of sensing high mass airflows and liquid flows. To accomplish this a desired flow sensor would include a robust substrate or die with relatively low thermal conductivity, high temperature stability, high electrical isolation, corrosion resistance, chemical inertness, and biocompatability. The design of such a structure would enable flow rate and thermal property sensing over wide ranges at high pressure. Further, this capability would provide trouble free operation in hostile environments at a reasonable cost. 
   SUMMARY OF THE INVENTION 
   The present invention details a microstructure flow sensor having a microsensor die with a Microbrick™ or microfill structure (each having a substantially solid structure beneath the sensing elements) and through-the-wafer electrical interconnections. Through the many benefits that are provided by this structure, a robust sensor can be created—i.e. a sensor that is operable and accurate in many different applications, including harsh environments. 
   The sensor features a flat, passivated, top surface overlying the heater and sensor elements to provide appropriate electrical isolation. Further, the die, with its through-the-wafer interconnections, eliminates the need for bonding wires with their attendant problems as discussed above. In order to withstand a wide range of pressure levels and operate in harsh environments, the die structure is configured to be very robust. The die is made up of materials that have very low thermal conductivity, thus eliminating the possibility of undesired thermal signal shorts. For example, the die may be fabricated using various glass materials, alumina, or combinations of such materials. 
   The die is attached to a substrate having a suitably matched coefficient of thermal expansion (CTE) by any number of adhesives. Electrical contact is made by thermocompression bonding, solder bumping, conductive adhesives or the like. Preferably the through-the-substrate electrical contacts terminate in the necessary electrically conductive runs for attachment to further electronics of the sensor. This allows for easy interconnection to further devices. 
   The substrate may further have a passivation layer at the mating surface with the 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. Both silicon oxide and silicon nitride layers may be 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 cleaning maintenance. 
   The ability to perform high mass flux sensing operations is largely dependent upon the physical characteristics of the sensor. Most importantly, low thermal conductivity of the die substrate is necessary in order to create a sensor capable of operating in these high mass flux sensing situations. By minimizing the thermal conductivity, interference with sensor heating/cooling effects will be minimized and the sensing capabilities are enhanced. Specifically, the characteristics of the die substrate materials will control the proper route of heat transfer, avoiding transfer through the die substrate from the heater to the sensors. Various materials can provide this characteristic. Historically, silicon nitride of a microbridge sensor chip has been used to provide certain levels of thermal conductivity, while also being easily manufactured. However, its fragility prevents is use in harsh environments. 
   A more optimum material which exhibits the desired characteristic is glass. Glass, however, has not been previously used because it has not been easily micromachined. That is, it is difficult to form the required structures using glass. Another potential substrate material is alumina, which is widely used for electronics packaging and can be machined to serve as substrate with some desirable characteristics. One undesirable feature, however, is its high thermal conductivity, which would severely reduce the sensitivity of the sensor chip. 
   Recent developments in glass materials, including photosensitive glass and pyrex, have shown that micromachining is possible and extremely effective. Consequently, this material can now provide an alternate die substrate for a micromachined flow and property sensor. The present invention exploits the characteristics of glass (photosensitive glass, fused silica, etc.) or alumina materials to produce a flow and property sensor with optimized physical characteristics. Providing a glass based sensor in a Microbrick™ or microfill structure consequently enables the fabrication of a rugged sensor for sensing liquid properties or high mass flux fluid flow, without pressure-stress-induced error signals. 
   Due to the recent developments in glass, the use of this material as a die substrate generally reduces the amount of structural machining necessary. More specifically, the substrate can now be fabricated in a Microbrick™ or microfill structure which has a substantially solid structure. In this type of sensor die, the heating and sensing elements are placed directly on the substrate and no further processing or structuring is required beneath those elements. Consequently, the substrate itself is continuous beneath the sensing elements creating a more robust sensor die. The characteristics of the glass substrate material allows this Microbrick™ structure to be effectively used in harsh environments. 
   Alternatively, the same Microbrick™ structure can be achieved utilizing a plug type configuration. In this approach, a substrate material includes a hole under the heating and sensing elements or opening extending completely therethrough. This hole is then refilled with a filler or plug of appropriate materials creating a microfill structure (i.e. a micro hole filled with solid material). The combination of this substrate and the appropriate filler or plug can effectively tailor the thermal characteristics of the microsensor die. For example, the substrate may be largely fabricated from alumina, and include a glass plug. The heating elements are then placed directly upon this plug element, thus providing the necessary thermal characteristics. 

   
     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 microsensor die showing the micromembrane heater and sensing elements; 
       FIG. 2  is a cross section of an assembled fluid flow sensor according to the present invention including a substrate structure; 
       FIG. 3  is a more detailed view of the microsensor die and a substrate; 
       FIG. 4  is a cross sectional drawing of an alternative microsensor die structure incorporating a filler portion; 
       FIG. 5  is a cross sectional drawing of yet another microsensor die structure using a plug; and 
       FIG. 6  is a schematic illustration of the backside processing required for one type of glass based sensors. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   Throughout the Description of the Preferred Embodiment, like components will be identified by like reference numerals. 
   Referencing  FIG. 1 , a fluid flow sensor die  21  includes a body  13 . Onto body  13  are deposited sensor elements  15 ,  17  surrounding a central heating element  19 ; all 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 be further elaborated on herein. Again, for convenience sake, this structure will be generally referred to as a “flow sensor,” as indicated above. 
   Referencing  FIG. 2 , a flow sensor according to the present invention may include a microsensor die  21  bonded to a substrate  23  having a suitably matched coefficient of thermal expansion (CTE). Material for substrate  23  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 microsensor die  21  in order to further planarize the top surface of the sensing apparatus and provide minimal resistance to fluid flow and minimal crevices into which particles or condensates may lodge. The top surround  25  may be implemented as a 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. 
   As shown, microsensor die  21  comprises a body  13  having through-holes serving as electrical vias, collectively  29 , filled with an electrical conductor material, preferable gold, chrome/gold alloy, or chrome/gold/palladium alloy. The use of through the way for interconnects, such as shown, provides many advantages for the flow sensor. Specifically, no wire bonds are extending upward from the upper surface of microsensor die  21 . Consequently, there are no structures which interfere with the flow being sensed. As is expected, this eliminates any turbulence, along with avoiding stresses on the particular bonding structures. 
   Again, referencing  FIG. 2 , the substrate  23  comprises a substrate body  55  comprised of alumina, mullite, or other known materials having coefficient of thermal expansion (CTE) suitably matched to the microsensor die  21 . At the top surface of the substrate structure  23  which is to be mated with the silicon microsensor die  21  there is located a thermocompression solder-bump bond  51 . 
   Silicon is often considered a very effective microsensor body material because it can be easily machined/processed using several well known silicon processing techniques. In certain applications, such as very high mass flux fluid flow sensing and high pressure applications, such silicon supported structures as microridges or mciromembranes do have certain disadvantages however. Specifically, the thermal isolation characteristics of silicon would limit structural and operational characteristics of a sensor if built directly on silicon. In order to deal with these thermal characteristics, the microsensor body of a silicon based sensor is configured in a micromembrane type structure, so as to limit the thermal mass below the heater and sensing elements. Obviously, this limits the physical strength of a silicon based sensor. In addition, this micromembrane configuration is not suitable for high mass flux sensing because its output signal saturates before reacting high flux levels. 
   In order to effectively operate in harsh environments, the flow sensor must be structurally robust. As suggested above, the membrane structure, which burst near 100 PSI, does not exhibit the structural characteristics required to create a robust sensor. What is needed is a sensor robust enough to withstand high pressures due to sources (such as high pressure pulses, ultrasonic cleaning, and water hammer). In order to sense high mass flux flow rates, it is also necessary to have a substrate material with a thermal conductivity. If it is too low (as in the case of the membrane) the output signal saturates at moderate fluxes (˜1 g/cm 2 s); but if it is too high the output signal becomes too small. Certain glass materials provide better thermal isolation characteristics (than silicon), thus increasing the sensing capabilities of the above-outlined micromachined flow and property sensor. The use of glass also allows for a more robust physical structure to be used. Additionally, the sensing elements will be protected by a passivation layer, thus reducing their sensitivity to vapors and liquids. These various characteristics result in a more versatile sensor which can be used in multiple applications. Furthermore, as outlined below, certain techniques provide for effective micromachining of glass based substrates. 
   Referring now to  FIG. 3 , there is shown a more detailed structure for a glass based air flow or fluid flow sensor. The use of glass as a microsensor body material provides multiple features which enhance the capabilities of the sensor. These features include (1) the automatic electrical insulation for through-the-wafer contacts, (2) lower thermal conductivity than silicon, (3) environmental ruggedness needed to withstand pressure pulses as for sensing liquids, and (5) the ability to use a structurally robust sensor body configuration. Furthermore, the glass based sensor meets all requirements for chemical inertness, corrosion resistance, and biocompatability. 
   As mentioned above, glass provides inherent electrical isolation between various contacts. This is compared with a silicon based sensor where electrical isolation must be achieved by incorporating silicon dioxide layers on the substrate unless more costly silicon wafers are used that a grown to be slightly insulating. Obviously, this eliminates one layer of material and one necessary processing step. This is particularly beneficial as the step of growing oxide is time consuming and done at fairly high temperatures. 
   Referring now to  FIG. 3 , there is shown a cross sectional view of the glass based sensor die  121  of the present invention. While the sensor of the present invention is generally referred to as a glass based sensor, it is understood that other materials having appropriate physical characteristics could also be used. For example, alumina could be used as the base material for forming the sensor die  121 . These other materials are intended to be within the scope and spirit of the present invention. A glass body  110  is used as the basis for forming sensor die  121 . Upon the upper surface of glass body  110  is a layer of silicon nitride (Si x N y )  112  which again serves passivation and structural functions. Upon this passivation layer  112  there is constructed the heater element  114  and sensors  116 , similar to those described above and well known by those skilled in the art. Once again, these heating and sensing elements can be fabricated from many materials, such as platinum. Covering the entire upper surface of the structure is a top layer  118  which serves as a protective passivation coating. Top layer  118  again is typically silicon nitride (Si x N y ). 
   Similar to the sensor described above, glass body  110  has a plurality of electrical vias  129  extending therethrough. These electrical vias are typically holes that are created in glass body  110  and provide innerconnection to the backside  120  thereof. Again, this allows electrical connection to further elements of the sensing system. Fabrication of these electrical vias  129  is more fully explained with reference to  FIG. 4  below. 
   Placed within electrical vias  129  is a electrically conductive connecting material  131 , which provides electrical connection to the actual heater  114  or sensor  116 . The material used for these electrical connections is chosen to closely match the thermal expansion characteristics of glass body  110 . 
   Once again, a substrate  123  is configured for attachment to the backside of microsensor die  121 . Substrate  123  includes a substrate main body  155  made up of a material chosen to closely match the thermal characteristics of glass substrate  110 . As an example, substrate  123  may be kovar-seal glass, alumina, PCB, etc. Upon the top surface of substrate body  155  is a glazing layer  160  along with a plurality of metal contacts  170 . Various through holes or vias  180  can also be provided in substrate body  155  to provide appropriate electrical connection to further components. 
   In order to provide a operational sensor, sensor die  121  is attached to substrate  123  such that all appropriate electrical connections are properly aligned. This attachment can easily be achieved through thermal compression, or other appropriate attachment mechanisms much as solder bumping or z-axis adhesives. 
   As can be seen, glass body  110  is a substantially solid block of material. That is, other than the existing electrical vias  129  that are provided for electrical interconnection to components attached to the sensor die  121 , there are no other openings or holes therein. Most significantly, the area of glass body  110  directly below heater  114  and sensing elements  116  is substantially solid. As can be expected, this provides an extremely easy structure to fabricate and minimizes the required processing steps. This type of structure can effectively be used due to the nature of the material chosen for body  110 . More specifically, by utilizing a glass based material, having low thermal conductivity, an operational fluid flow sensor can be fabricated. This type of structure, commonly referred to as a Microbrick™, provides for a very robust and environmentally sound sensor. Most importantly, this sensor is able to withstand high pressure levels without bursting. 
   As mentioned above, the use of appropriate materials for glass body  110  makes the Microbrick™ structure possible. Generally speaking, this structure does not work well when silicon is used as the substrate material, due to its high thermal conductivity. Consequently in silicon, a heat transmission path is too easily created through the substrate material itself, resulting in unusually low/signal outputs. As mentioned above, this is highly undesirable for any fluid flow sensing as it diminishes the sensitivity of sensing elements  116  relative to heater  114 . 
   Referring now to  FIG. 4 , there is shown an alternative embodiment of the present invention. In this modified-micromembrane configuration, a microsensor die  221  is again based upon a glass body  210 . As in the embodiment shown in  FIG. 3 , a passivation layer  112  is deposited immediately upon the upper surface of glass body  210 . Upon this passivation layer is fabricated a heater  114  and a pair of sensing elements  116 . Also included are top surface interconnections  119  which provide electrical interconnects between the sensing elements and all other appropriate components. Coated on top of these elements (heater  114 , sensing elements  116  and interconnections  119 ) is a protective layer  118 . 
   As can be seen, glass body  210  includes a central filler portion  212  below heater  114  and sensing elements  116 . In this embodiment, filler portion  212  further enhances the operation of microsensor die  221  by providing additional thermal isolation between heater  114  and sensing elements  116 . As mentioned above, the glass material chosen for glass body  210  provides many advantages and more optimal thermal isolation than silicon. However, glass does have some thermal conductivity characteristics, as do virtually all materials. The transit heating affects, as described above, are further reduced by utilizing a material in filler portion  212  which has thermal conductivity properties even better than glass. Consequently, the overall structure immediately adjacent heater  115  and sensing elements  115  has a very low thermal conductivity characteristic. Consequently, the sensitivity of the sensor at high mass flux fluid flow conditions is greatly enhanced. 
   Referring now to  FIG. 5 , there is shown yet another configuration for a microsensor die  321 . In this particular configuration, microsensor die  321  is based upon body  310  which is configured somewhat similarly to glass body  210  shown in  FIG. 5 . However, in this instance, body  310  may be manufactured out of other materials including both glass or silicon or alumina. In order to further tailor the thermal characteristics of microsensor die  321 , an appropriately configured plug  312  is utilized. Plug  312  extends completely or entirely through body  310  and is chosen from a material having desired thermal characteristics. As can be seen, heater  114  and sensing elements  116  are configured directly above plug  312 . For example, body  310  may be configured from alumina while plug  312  may be configured of appropriate glass material. In this respect, a solid structure is maintained beneath heater  114  and sensing elements  116 , while the thermal characteristics are again closely controlled. 
   The configuration shown in  FIG. 5  is particularly applicable when alumina or silicon is used as the body material. As is well known, alumina can be easily machined and manufactured into appropriate configurations using well known methods. Furthermore, alumina is more chemically inert than even glass or silicon. Consequently, the use of alumina alone has advantages in certain applications. Furthermore, alumina can be used in much higher temperature applications as it is more temperature resistant. As mentioned above, using an appropriate plug material, the necessary thermal conductivity can be achieved resulting in a thermal sensor having the desired operational characteristics. This plug or microfill approach can similarly be used with other materials to appropriately “tune” or tailor the characteristics of the sensor. 
   Referring now to  FIG. 6 , there is shown a block diagram of the backside processing to create the desired sensor die  121  of  FIG. 3 . More specifically,  FIG. 6  schematically outlines the process used to appropriately configure glass body  110 . Additionally, glass body  110  exists as a bare block of raw material (step  1 ). Next, in step  2 , an appropriately configured mask  180  is placed on an upper surface of glass body  110 . Mask  180  can be configured of a standard chrome material typically used with microstructure processing. 
   Next, the masked substrate is exposed to UV radiation  182 . As is well known, UV radiation will not contact the masked areas of glass body  110 , but will effect the unmasked portions. Specifically shown in  FIG. 6 , the mask is configured to have five circular openings therein. Consequently, UV radiation is allowed to impinge on glass body  110  in those circular areas. 
   Next, in step  3 , crystallization of the exposed areas is achieved. This crystallization facilitates the further processing of glass body  110 . More specifically, the glass becomes etchable in the UV exposed areas. In step  4 , this actual etching takes place wherein UV exposed areas are removed. This creates holes in glass body  110  which can then be further processed. In step  4 , the through the wafer holes are metalized to allow electrical contact between the two surfaces. At this point, the backside processing is completed and glass body  110  can be further processed to ultimately create glass based sensor die  121 . 
   Referring again to  FIG. 3 , it can be appreciated that the front side processing necessary involves the creation of heater  114  and sensors  116  and all appropriate coating and connections. More specifically, an exemplary front side manufacturing process would be as follows: (1) deposit passivation layer (silicon nitrate)  112  on the top side surface of glass body  110 ; (2) deposit platinum on passivation layer to form electrical contacts and sensor/heating element; (3) pattern the platinum coating and ion mill the platinum coating to result in the desired platinum pattern; and (4) lastly, deposit upper passivation layer  118  over entire structure. 
   As is well understood, a similar process can be used to manufacture components from Pyrex. These other processes may involve laser processing, chemical etching, or physical processing of the substrate to form the necessary holes. 
   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 presents several solutions to a myriad of problems. It will be recognized that various structures of the preferred embodiment may have counterparts substituted therefore 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.