Abstract:
A method of making a plurality of flow sensors is provided, each flow sensor having a substrate with a sensing element and flow channel aligned over the sensing element. The sensing element senses at least one property of a fluid. The flow channel is aligned by one or more guide elements formed in an alignment layer. The flow channel across the sensing area is accurately and precisely aligned due to the guide elements provided at the wafer-level, facilitating reliable, low-cost, and consistent results among multiple flow sensors. The flow sensor is adapted for use in harsh environments.

Description:
This is a divisional of U.S. patent application Ser. No. 10/930,546, filed Aug. 31, 2004, entitled “Flow Sensor With Self-Aligned Flow Channel,” which is a continuation-in-part of U.S. patent application Ser. No. 10/128,775, filed Apr. 22, 2002, entitled “Sensor Package for Harsh Environments”, now U.S. Pat. No. 6,911,894, which is a continuation-in-part of U.S. patent application Ser. No. 09/656,694, filed Sep. 7, 2000, entitled “Robust Fluid Flow and Property Microsensor Made of Optimal Material,” now U.S. Pat. No. 7,109,842, which 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, which is a Continuation-in-Part of U.S. patent application Ser. No. 09/239,125, filed Jan. 28, 1999, both entitled “Microsensor Housing,” now U.S. Pat. Nos. 6,322,247 and 6,361,206 respectively. The content of the foregoing patent applications and patents are hereby incorporated by reference. 

   FIELD OF THE INVENTION 
   The present invention relates generally to sensors utilized to detect the quality and movement of fluids, in either gaseous or liquid form. The present invention relates more particularly to thermal sensors of such fluids, such as fluid flow or property sensors implemented on silicon, glass, quartz, or other substrates in microstructure form. The present invention relates to sensor packages for harsh environments. The invention relates to the alignment of a flow path over a sensing area. 
   BACKGROUND 
   Flow sensors are utilized in a variety of fluid-sensing applications for detecting the movement of fluids, which may be in gaseous of liquid form. One type of flow measurement, for example, is based on thermal sensors, which can be utilized to detect the properties of a fluid. Thermal sensors may be implemented, for example, over a silicon substrate in microstructure form. For convenience sake, and without limitation, the term “flow sensor” can be utilized to refer to such thermal sensors. (See e.g. U.S. Pat. No. 6,322,247 FIGS. 10 a - f , and U.S. Pat. No. 6,184,773, which are both incorporated herein by reference.). The reader will appreciate that such sensors may also be utilized to measure intrinsic fluid properties such as thermal conductivity, specific heat (e.g. U.S. Pat. Nos. 5,237,523 and 5,311,447, which are both incorporated herein by reference.), non-intrinsic properties such as temperature, flow velocity, flow rate, and pressure, and other properties; and that the flows may be generated through forced or natural convection. 
   A thermal-type flow sensor can be formed from a substrate that includes a heating element and one or more heat-receiving, or sensing, elements. If two such sensing elements are utilized, they can be positioned at the upstream and downstream sides of the heating element relative to the direction of the fluid (liquid or gas) flow to be measured. When fluid flows along the substrate, it is heated by the heating element at the upstream side and the heat is then transferred non-symmetrically to the heat-receiving elements on either side of the heating element. Since the level of non-symmetry depends on the rate of fluid flow, and that non-symmetry can be sensed electronically, such a flow sensor can be used to determine the rate and the cumulative amount of the fluid flow. 
   Such flow sensors generally face potential degradation problems when exposed to harsh (e.g., contaminated, dirty, condensing, etc.) fluids, including gases or liquids that can “stress” the sensor via corrosion, radioactive or bacterial contamination, overheating, deposits or freeze-ups. The sensitive measurement of the flow, or pressure (differential or absolute) of “harsh” gases or liquids that can stress, corrode, freeze-up, or overheat the sensing elements is a challenge that is either unmet or met at great expense. Among the solutions proposed previously are passivation with the associated desensitization of the sensor, heaters to raise the temperature of gaseous fluids to be measured to avoid condensation or freeze-ups (or coolers to prevent overheating) at the expense of sensor signal degradation, cost increase and possible fluid degradation, or filters to remove objectionable particulate matter. Frequent cleaning or replacement and recalibration of the sensors are additional, but costly, solutions. Sensitive, membrane-based differential pressure sensors can be protected against contamination because no flow is involved, but they are less sensitive, typically cover a smaller flow range and are more expensive than thermal microsensors, in addition to not being overpressure proof. 
   The measurement of liquid flow via thermal microsensors, especially of electrically conductive fluids, thus presents challenging problems in terms of electrical insulation, flow noise, chip corrosion, potential for leaks or structural integrity of the flow channel, and thermal measurement. The electrical contacts to the sensor chip generally should be insulated from each other so the resistance to electrical leakage is above approximately 20 MΩ to avoid interference with the sensing function. Some Si 3 N 4  passivation films, for example, have pinholes; spin-on coatings of compounds that form glass or Teflon® films upon curing have not shown insulation beyond a few days of contact with salt water. (Note that Teflon® is a registered trademark of the E.I. Du Pont De Nemours &amp; Company Corporation of 101 West 101 West 10 th St., Wilmington, Del. 19898.) Even potting the wire-bonds in highly cross-linked epoxy led to either resistances dropping to, for example, 30MΩ and/or bond breakage if the epoxy became too brittle due to excessive cross-linking and/or thermal cycling. Additionally, an odd shape of the flow channel above the chip causes extra turbulence and corresponding signal noise. Another approach to providing electrical insulation for the electrical contacts and leadout wires is to move them out of the fluid-flow channel and contact area; however, such sidewise displacement adds real estate to the chip size and therefore to its cost. 
   Regarding structural integrity, a sensitive 1 μm-thick flow sensing membrane can easily break as a result of the stronger viscous and inertial forces that a liquid can exert on it. Such breakage has even been observed in cases of sharp gaseous pressure or flow pulses. Finally, with respect to thermal measurement issues, the heater temperature rise typically permissible in liquids (e.g., ≦20° C.) is much smaller than the one typically utilized in gases (e.g., 100-160° C.). The resulting, relatively small signal causes more significant increases in the effect of composition-, sensor-material- and temperature-dependent offsets, which can cause significant errors in the sensor flow readouts. 
   Based on the foregoing, the present inventors have concluded that a solution to the aforementioned problems lies appropriately in the “smart” application onto the sensing chip of a film that is strong enough to function as a protective barrier to the transfer of electrical charges and of molecular mass transfer but can be thin enough to enable transfer of heat to allow thermal measurements. The films may be fashioned of materials composed of inorganic compounds (even metals) or of hydrophobic or hydrophilic polymeric materials, as explained in further detail herein, which can result in operational flow sensors of high reliability, no electrical leakage, no fluid leakage by virtue of the non-intrusive character of the flow measurement, no corrosion, no fluid contamination, reduced flow noise and significantly reduced offset and drift problems. 
   Another challenge in the design and manufacture of flow sensors is the alignment of the fluid flow path across the sensing element. Precise and accurate alignment is necessary to achieve optimal performance of the sensor. Such precise alignment of sensors generally requires components of each sensor to be individually aligned, which is labor intensive and expensive. Time and cost in manufacturing flow sensors is greatly reduced when more of the production steps are completed while the sensors are at the wafer level. The present invention provides a solution to aligning the flow path precisely when the microsensors are at the wafer level. 
   SUMMARY OF THE INVENTION 
   The present invention provides a thermal sensor utilized in the detection of the quality or properties of fluids, including gas and liquid. The thermal sensor can be implemented on silicon, glass, quartz, or other substrates in microstructure form. 
   In one embodiment, the flow sensor has a substrate with a sensing element, one or more guide elements, and a flow channel; wherein the guide elements align the flow channel over the sensing element. The sensing element senses at least one property of a fluid. In a further embodiment, first and second guide elements define the flow channel. The present flow sensor provides a sensor in which the flow path across the sensing area is accurately and precisely aligned, facilitating reliable and consistent results among multiple flow sensors. 
   In another embodiment of the invention, a molded element defining one or more flow channel extensions is positioned over the guide elements, with the flow channel extensions in fluid communication with the flow channel. The combination of the flow channel and flow channel extensions define a fluid flow path over the sensing element. The molded element can form the top of the fluid flow path, or a cap can be attached to the molded element to form the top of the fluid flow path. In another embodiment of the invention, the flow sensor includes a substrate with a sensing element, an alignment layer deposited on the substrate and defining a location channel aligned over the sensing element, and a flow tube positioned within the location channel. 
   A method is provided for making a plurality of flow sensors each having a flow channel aligned with a sensing element. The method involves providing a substrate with a plurality of sensing elements aligned in a pattern, depositing a polymer layer onto the substrate and forming a plurality of guide elements in the polymer layer, with the guide elements positioned to align flow channels over a sensing element. In some embodiments, the guide elements form the flow channels. The substrate is then cut or diced into a plurality of pieces or chips, with each piece having a flow channel precisely aligned over a sensing element. 
   In yet another embodiment of the present invention, an apparatus is disclosed herein for detecting liquid flow in what may generically be referred to as a “harsh environment”, in which toxic or corrosive fluids are analyzed. This embodiment can also be used for sensing pure or super-clean fluids, such that their contact with the sensor does not result in any detectable contamination of the fluid or adverse effects to the sensor. This improvement results from the sensor being isolated from the fluid flow path. 
   A sensor can be configured to generally include a flow channel block having a flow channel formed therein. The sensor additionally includes a substrate fastened to a sensor chip and contacted by at least one bonding element and a molded core tube inserted into the flow channel of the flow channel block, which thereby reduces flow noise and potential corrosion, improves electrical insulation, structural integrity and thermal measurements thereof derived from the sensor chip. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying figures, like reference numerals refer to identical or functionally similar elements throughout the separate views. 
       FIG. 1  illustrates a prior art cross-sectional view of a flow channel block; 
       FIG. 2  depicts a cross sectional view of an isolated flow channel block with an inserted core tube; 
       FIG. 3  illustrates a side sectional view of an improved flow channel block with an inserted core; 
       FIG. 4  depicts a graph illustrating the performance of thermal flow sensors with salt water at ambient temperature relative to a flow sensor without a core tube; 
       FIG. 5  illustrates a front view of a flow sensor in accordance with an embodiment of the present invention; 
       FIG. 6  depicts a cross-sectional perspective view of a temperature field generated by a flow sensor heater, in accordance with an embodiment of the present invention; 
       FIG. 7  illustrates a cross-sectional end view of a flow sensor assembly with a glass chip under a Teflon® tube in an epoxy matrix, in accordance with an embodiment of the present invention; 
       FIG. 8  depicts a graphical diagram illustrating a sensor package for harsh environments applied to large flow channels, or to property measurements, which may be implemented in accordance with an embodiment of the present invention; 
       FIG. 9  illustrates a sectional top view and a bottom view of a flow sensor assembly with a small core tube located within walls of a flow channel block thereof, in accordance with an embodiment of the present invention; 
       FIG. 10  depicts sectional views of an assembly of a flow channel block and a core tube, in accordance with an embodiment of the present invention; 
       FIGS. 11A and 11B  are front and top views, respectively, of a microsensor assembly with an alignment layer according to the invention; 
       FIG. 12  is a perspective view of another embodiment of microsensor according to the invention; 
       FIGS. 13A and 13B  are front and top views, respectively, of a further embodiment of microsensor according to the invention; 
       FIGS. 14-17  are front cross-section views of other embodiments of the invention in which guide elements provide alignment for a flow channel or flow tube; 
       FIG. 18  is a graph illustrating the performance of thermal flow sensors made of different materials and having different wall thicknesses (WT), with salt water at ambient temperature relative to a flow sensor; and 
       FIG. 19  is a graph illustrating the performance of a stainless steel flow tube with and without oil added to the junction between the flow tube and microbrick. 
   

   DETAILED DESCRIPTION 
   The particular values and configurations discussed in these non-limiting examples can be varied and are cited merely to illustrate various embodiments of the present invention and are not intended to limit the scope of the invention. 
   One aspect of the present invention is related to the design and fabrication of the electrical insulation for electrical contacts to sensor chips using either front-wire-bond (FWB) or through-the-wafer (TTW) contacts of certain thermal flow microsensors or of environmental sensors in general. The present inventors previously insulated Au-wires and Au-pads of FWB sensor chips via materials, such as, for example, dip-coatings, dip-coatings with or without alumina thin-film undercoating, Si 3 N 4 , flowable sealants, solvent-resistant sealant with fluoro-silicon, and epoxies. Insulation based on such materials has been attempted as defined generally by the resistances between the sensing elements and the liquid (e.g., salt water) in a flow tube. Such resistances, however, are unacceptable if ≦20 MΩ. The invention described herein thus introduces a unique solution for solving such problems. 
   As will be explained in further detail herein, by potting insulating material (e.g., epoxy) around a core-mold of Teflon® wire or pipe of 0.010 to 0.060″ OD, which may or may not be removed after curing, and using for example, a robust microbrick or an epoxy-back-filled microbridge, the aforementioned problems can be essentially eliminated. The increased thickness of the insulating “layer”, relative to a dip-coat for example, causes the intrusion of fluids (e.g., water) and other conductive materials, such that their contribution to electrical conduction in the polymer becomes negligible. A straight and smooth flow channel, which can reduce turbulence and flow noise, thus replaces the old flow channel spaces located above previously utilized sensor chips. 
   Replacing an unprotected microbridge by a microbrick chip can eliminate breakage due to fluid-generated forces. Note that the utilization of a microbrick chip or other such devices are not considered limiting features of the present invention but are mentioned herein for illustrative and general edification purposes only. The increased insulation thickness enables the application of larger voltages to the sensor heating elements, which raises the heater temperature (which may or may not be in direct contact with the liquid) and leads to larger output signals. As a result, heater resistance drift, and temperature-, fluid-type-, sensor-asymmetry-, and electronics-dependent offsets are less prominent. 
   In one embodiment, a flow sensor includes a flow channel block defining a flow channel, a molded core tube positioned within the flow channel, a substrate, a bonding element, and a sensor element or chip. The bonding element can be configured to comprise one or more front wire bonds (FWBs) and/or through-the-wafer (TTW) contacts. 
   As used herein, the term “tube” means a conduit or channel of any shape through which a fluid flows. The cross section of the tube can be cylindrical, polygonal, elliptical, etc. The molded core tube can be formed from a polymeric material, such as Teflon®, or other materials, such as glass, quartz, sapphire and/or metal, such as, for example, stainless steel. The tube can be made of a mixture of different plastics or polymers. The molded core tube generally comprises a wall thickness that removes a surface of the sensor chip from direct contact with a fluid flowing through the molded core tube by a distance corresponding to the wall thickness, thereby desensitizing the sensor to fluid flow variations. Additionally, this tube wall thickness in contact with the sensor chip combines a high dielectric strength and chemical inertness with properties such as hydrophobic, hydrophilic and lipophilic as needed. Such properties may be realized with inorganic or organic materials. Note that as utilized herein the term fluid can be meant generally to refer to a gas or liquid. Thus, sensor packages disclosed herein can be utilized to measure the quality or property of a gas or a liquid. 
   The film can be enlarged to comprise a potting or molding compound associated with the bonding elements, whereby the molded core tube generally shapes the potting compound. The film itself may be formed from a material such as, for example, an epoxy material. Also, the molded core flow channel can be configured to include a constriction in a cross section of the molded core tube at the sensor chip to optimize performance thereof. The molded core flow channel and the substrate can be replaced by a flat film, which can be wrapped or shrunk about a header and sealed by an O-ring to provide sensor capabilities thereof. The flow tube is generally configured from a flow channel block and can be a disposable flow tube. Additionally, the sensor can be associated and/or integrated with a heat sink mechanism for heat sinking a reference resistance and/or temperature sensor associated with above flow sensor so that the flow sensor does not increase in temperature and drive an associated heater temperature to a point where a fluid flowing through the flow channel boils. 
   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 can be fabricated using various glass materials, alumina, or combinations of such materials. 
   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 that 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 structure 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 structure 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 allow this microbrick structure to be effectively used in harsh environments. 
   Flow sensors are either non-isolated, in which the fluid flows directly over the sensing element, or isolated, in which the fluid flow is separated from the sensing element.  FIG. 1  illustrates a prior art cross-sectional view  100  of a plastic non-isolated flow channel block  104 .  FIG. 1  further illustrates a sensor chip  106  which is fastened or in communication with a substrate  102 . The substrate  102  can support electrical I/O lead-outs, which may in turn be connected or bonded to various elements on chip  106  via “front wire bonds” (FWBs)  107  or “through-the-wafer” (TTW) contacts (not shown). A top flow channel  111  with an appropriate opening for the chip can then be fastened over the sensing chip  106 . Ideally, care should be exercised so as not to spill excess adhesive into the path intended for the fluid. Thus, view  100  represents a drawing of a microsensor, prior to the introduction of the “core mold” concept of the present invention, as explained in further detail herein. 
     FIG. 2  depicts a cross sectional view  110  of an isolated flow channel block  104  with an inserted core tube  118 .  FIG. 2  additionally illustrates a sensor chip  116  and a substrate  112 . Flow channel block  114 , which is analogous to flow channel block  104  of  FIG. 1 , now possesses an inserted core tube  118 . Substrate  112  may be composed of, for example, alumina, PCB, glass, quartz, or other substrate-type materials. Substrate  112  of  FIG. 2  is generally analogous to substrate  102  of  FIG. 1 . Note that the term “substrate” as utilized herein can refer to a “substrate” or a “substrate board.” The composition of the substrate is discussed further below. The flow channel block  114  is also generally analogous to flow channel block  104 , with the exception that core tube  118  has been added to block  114 . This facilitates the process of fastening flow channel block  114  to an “alumina” substrate  112 . 
   The inserted core tube  118  is not pulled out but is maintained in place to provide the above-discussed advantages. Note that the wall thickness of inserted core tube  118  removes the surface of sensing chip  116  from direct contact with the fluid by a distance corresponding to that thickness, thus desensitizing the sensor to flow changes, which is the price paid for the other benefits mentioned above. Additionally, it is important to note that flow channel block  114  may be configured in the shape of a tube, thereby functioning as a flow tube. Flow channel block  114  thus may form a flow tube. 
   It can be appreciated by those skilled in the art, however, that flow channel block  114  may be configured in the form of other shapes, such as for example, a triangular-, square-, rectangular-shaped flow channel block, half-circles, or various other geometric shapes. Thus, the shape of flow channel block  114  can be an arbitrary design choice and is not considered a limiting feature of the present invention. Additionally, it can be appreciated that flow channel block  114  can be formed from a variety of materials, including, but not necessarily only, plastic. 
   In one embodiment, flow channel block  114  is a polymer alignment layer defining a location channel into which core tube  118  is inserted. The alignment layer provides a location channel precisely aligned over sensor chip  116  and allows core tube  118  to be precisely aligned over the sensor chip  116 . The composition of an alignment layer is discussed further below. 
   Substrate  112  can support electrical I/O lead-outs, which may in turn be connected to various elements on sensor chip  116  via “front wire bonds” (FWBs)  127  and  129  illustrated in  FIG. 2 . Similarly, FWBs  107  and  109  are depicted in  FIG. 1 . Additionally, bonding elements can be configured as through-the-wafer (TTW) contacts, which are not illustrated in  FIGS. 1 and 2 . Flow channel block  114  can then be fastened over sensing chip  116  and substrate  112 . Ideally, care should be exercised so as not to spill excess adhesive into the path intended for the fluid in  FIG. 1 . In  FIG. 2 , core tube  118  can prevent such spills and generally surrounds channel  121  through which a fluid may flow. Note that if core tube  118  is removed from flow channel block  114 , channel  121  can be left in place after core tube  118  is removed from molding surrounding core tube  118 . In this sense, core tube  118  may also be referred to as a “molded core tube.” 
   The use of such a core tube can thus reduce flow noise, sensitivity, and the risk of contamination of super-clean fluids, fluid leakage, chip corrosion and leakage potential, while improving electrical insulation, structural integrity and thermal measurements thereof derived from an associated sensor chip (e.g., sensor chip  116 ). Such a core tube can also be used to shape and mold an inner flow channel, which can be removed after curing of the molding compound. The flow sensor can then regain flow sensitivity and maintain low “flow noise” but may lose some chip corrosion protection, fluid and electrical leakage prevention, fluid contamination, non-intrusiveness and structural integrity. 
   Again, referencing  FIG. 2 , the substrate  112  can be comprised of alumina, mullite, quartz, or other known materials having coefficient of thermal expansion (CTE) suitably matched to the microsensor die. 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 micromembranes 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. 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 important to have a substrate material with an appropriately low thermal conductivity (≦1.5 W/(mK)). 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. 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. 
   The use of glass as a microsensor body material provides multiple features that 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 (4) the ability to use a structurally robust sensor body configuration. Furthermore, the glass-based sensor meets all requirements for chemical inertness, corrosion resistance, and biocompatibility. 
   As mentioned above, glass provides inherent electrical isolation between various contacts. This is compared with a silicon based sensor where electrical isolation is achieved by incorporating silicon dioxide layers on the substrate unless more costly silicon wafers are used that are 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. 
   While the sensor of the present invention can be implemented as glass-based sensor, it is understood that other materials having appropriate physical characteristics could also be used. For example, the substrate can be manufactured out of other materials including glass, quartz, silicon, alumina, or ceramic. 
     FIG. 3  illustrates a side cross-sectional view  131  of an improved flow channel block with an inserted core, in accordance with an embodiment of the present invention. The left side of view  131  further illustrates a side-sectional view of the prior art configuration illustrated in  FIG. 1 , while the right side illustrates the position of the core tube  118 . Note that in  FIGS. 1 to 3  analogous or like parts are generally indicated by identical reference numerals. For example, flow channel block  104  of  FIG. 1  is analogous to flow channel block  114  of  FIG. 2 . Thus, as indicated in view  131  of  FIG. 3 , walls  122  and  124  form walls of flow channel blocks  104  and  114 . 
     FIG. 3  is included herein primarily to highlight the differences between the prior art configuration depicted in  FIG. 1  and the improved flow channel block design illustrated in  FIG. 2 . A dashed line  140  in  FIG. 3  indicates a separation point between the prior art design of  FIG. 1  and the improved design of  FIG. 2 . Thus, half of sensor chips  106  and  116  are illustrated in  FIG. 3 , along with half of substrates  102  and  112 . A chip top view  142  is also indicated, showing respective halves of sensor chips  106  and  116 . As indicated above, walls  122  and  124  form walls of flow channel blocks  104  and  114 . Both flow channel blocks  104  and  114  include walls  122  and  124 . Walls  122  and  124  are indicated on both sides of dashed line  140 . An arrow  144  indicates a flow of fluid through channel  111  and  121 . Walls  130  and  132  of inserted core tube  118  of  FIG. 2  are also depicted in  FIG. 3 . 
   As explained previously, the wall thickness of the tube removes the sensing chip surface from direct contact with the fluid by a distance corresponding to that thickness, thus desensitizing the sensor to flow changes. This effect can be minimized and possibly balanced by increasing the temperature of the heater temperature above an ambient level, and additionally by designing the wall thickness at the sensor chip contact surface to be as small as possible. Note that even with the use of TTW contacts, the suggested use of a core pipe, whether left in place or not after bonding the “clear plastic” part with the “alumina”, reduces flow noise and the risk of leakage or corrosion and enables the application of higher heater temperatures, which also leads to higher sensor temperatures and reduced offsets. Note that as utilized herein, the term “bonding” generally connotes electrical contacting with the wire bonds (e.g., FWB), while the term “fastening” generally connotes mechanical securing elements and techniques thereof. 
   In prior art devices and systems, companies such as for example, Unit Instruments, Emerson Inc. and others, have marketed mass flow controllers based on thermal flow sensors with macroscopic core tubes of stainless steel for decades. Such devices typically feature the heater and sensing elements in the form of wire windings around the core metal tube. This fabrication approach, however, can result in large, slow-responding and costly sensors and is generally an ineffective solution. 
   Flow sensors, including the overall structures depicted in  FIGS. 2 and 3  can be thus designed, especially as the diameter of the core tube decreases, thereby resulting in more favorable surface-to-volume tube ratios. In the embodiment illustrated in  FIG. 2 , for example, an approximately 0.061″ OD Teflon® tubing (i.e., normally used as wire-insulation) can be threaded through the “clear plastic” flow channel  121  cross-sectionally at the sensor chip  116 . Either epoxy or RTV can then be injected via a syringe hole towards the chip area until excess spills out, while the unsealed alumina substrate to flow channel block interface remains under vacuum to minimize trapped air bubbles. 
   Another fabrication technique can also be implemented, in accordance with an another embodiment of the present invention, in which excess adhesive is generally applied to the individual parts prior to joining, evacuating and thereafter bringing the parts together, while squeezing excess adhesive from the bonding surfaces. After curing of the adhesive, the Teflon® core tube can be easily removed, if desired. Measurement of the electrical resistance between the sensing elements and the introduced conductive aqueous salt solution indicates resistances between an initial ≧200 MΩ and subsequently after several days, ≧30MΩ, with the Teflon® tube removed. No degradation or electrical leakage may be measured if the tube can be left in place. 
     FIG. 4  depicts a graph  200  illustrating the performance of flow sensors with salt water at ambient temperature, in accordance with an embodiment of the present invention.  FIG. 4  is presented for illustrative and edification purposes only and is thus not considered a limiting feature of the present invention. Graph  200  indicates that measured flow sensor output versus flow for several flow channel configurations and heater temperature values can be obtained. As illustrated in graph  200 , flows that occur below 0.5 nL/s are measurable for a smaller core tube of only 150 μm internal diameter. In such instances, noise levels may be approximately in the 1 mV range, for which no compensation for fluctuations in ambient temperatures may be in place. Those skilled in the art can thus appreciate that graph  200  illustrates a range of data collected over time regarding nulled-output versus flow rate. Graph  200  thus generally illustrates the beneficial influence of lower wall thickness (WT) and higher thermal conductivity materials for the core tube, which increases sensitivity and flow ranges. An example of a higher thermal conductivity material, which may be utilized in association with an embodiment of the present invention, is Pyrex®. (Note that Pyrex® is a registered trademark of the Corning Glass Works Corporation of Corning, N.Y. 14831.) A further explanation of  FIG. 4  is thus not necessary. 
     FIG. 5  illustrates a front view of a flow sensor  300  that can be implemented in accordance with another embodiment of the present invention. Flow sensor  300  includes an outer surface  310  and an inner surface  312  of a core tube  308  located above a sensing chip  302  with FWBs  306  and  304 . 
     FIG. 6  also depicts a cross-sectional side view of a temperature field  400  generated by a flow sensor heater, in accordance with an embodiment of the present invention, whereby the heater can be raised to an exemplary 100° C. above ambient in a plane just 25 μm off the center with no flow present.  FIG. 6  generally illustrates the results of a finite-element computation of the temperature profile of a temperature field near the sensor chip (e.g., sensing chip  302  of  FIG. 5  or chip  116  of  FIG. 2 ), thus indicating that even the ΔT h =6.5° C. isotherm barely penetrates the water and accounts for the loss in sensitivity if the thickness of the flow channel block can be chosen to be as large as, for example, 250 μm. The use of thin-wall tubes, made of materials of higher thermal conductivity (e.g., approximately 1 W/(mK)) has been demonstrated as a valid approach to minimizing the sensitivity loss. 
   It can be appreciated that modifications to the aforementioned improved sensor configuration (i.e., sensor package) can be made in accordance with the present invention. For example, heat sinking a reference resistance, R r , which is not shown in  FIG. 6 , to achieve proper control of an associated heater, can be implemented by the skilled when familiar with the SoA. Choosing a thin but strong core tube, made of material with intermediate thermal conductivities is another technique that can be utilized, as described above, in accordance with the apparatus of the present invention. Other variations and alternative embodiments are further described below. 
     FIG. 7  illustrates a cross-sectional end view  500  of a flow sensor assembly  505  with a glass chip  510  under a Teflon® tube  516  in a flow channel body or block  520  (e.g., of about 0.25×0.25″ in cross section), which may be implemented in accordance with a further embodiment of the present invention, and which can be sized to fit into a Honeywell flow channel housing AWM720. As illustrated in  FIG. 7 , chip  510  can be located above a substrate  508 , which may be composed of alumina, glass, or other substrate material. Chip  510  can be configured to include FWB contacts to substrate  508  via wires  522  and  524 . Core tube  516  can be 0.060″ in diameter. Additionally, a 0.002″ wall thickness can be utilized to sense water flows between &lt;10 to &gt;1000 μL/min. The smaller core tube  514  can be inserted into a groove in rod  512 , in place of core tube  516  and may be utilized to sense the flows illustrated in  FIG. 4  in a range of, for example, 0.03 μL/min to 3 μL/min. Note that rod  512  is positioned generally above block  521  in  FIG. 7 . Block  521  is in turn positioned above substrate  508 . 
     FIG. 8  illustrates a sensor package for harsh environments applied to large flow channels, which may be implemented in accordance with an embodiment of the present invention. The sensor chip  616  can be fastened onto a header (e.g., #T 08   614  or #T 05   612 ) and electrically bonded to one or more associated posts via wires  617  and  619 . Instead of exposing the chip surface to the fluid flowing in a channel as large as 0.5″ or more, the sensor chip can be protected by film  610 , which may be composed of any number of single or laminated, organic or inorganic materials. The thin film  610  is applied to the sensing element, wherein said thin film is applied thinly, thereby enabling reliable, sensitive, low-noise, non-intrusive, non-contaminating, and flow-channel-disposable measurements thereof. In one embodiment, for example, the thin film is applied to the sensing element at a thickness in an inclusive range from about 0.001-in to about 0.010-in. 
   Note that a virtual channel  606  is depicted in dashed lines in  FIG. 8 . Such a virtual channel  606  may be, for example, approximately 0.060-in in diameter. As illustrated in  FIG. 8 , voids  602  may be filled with adhesive such as epoxy. An underside  604  of the polymer film  610  may be “etched” to promote adhesion. An O-ring  618  can be placed around the base of the header to enable sealing against the fluid in the large channel. The header can be fastened by known fastening techniques against the large flow channel block  608 , of which only a corner is illustrated in  FIG. 8 . 
     FIG. 9  illustrates a sectional top view  901  and a bottom view  900  of a flow sensor assembly with a small core tube  918  (e.g., of ˜0.014″ outer diameter and 0.006″ inner diameter) located within walls  906  and  908  of a flow channel block thereof, in accordance with an embodiment of the present invention. Note that top and bottom views  901  and  900  are separated from one another in  FIG. 9  by a dashed line  903 . During assembly of the structure illustrated in  FIG. 9 , an epoxy can be utilized to fill all voids except the inner diameter of core tube  918 . Pusher element  910  can be utilized to press core tube walls  912  and  914  onto sensor chip  920  to minimize any void between sensor chip  920  and tube wall  914 . This design simplifies for some applications the assembly of small core tubes as explained herein (e.g., the small core tube of  FIG. 7 ). Thus, a simplified yet efficient core tube structure for use with sensor packages for harsh environments can be readily constructed, particularly in view of commercially available parts (e.g., block walls  906 ,  908 , and sensor chip  920 , wherein wall  906  can also comprise the circuit bearing substrate). 
     FIG. 10  depicts sectional views  1000  and  1001  of an assembly of a flow channel block  1119  and a core tube  1120 , in accordance with an embodiment of the present invention. View  1000  illustrates an epoxy adhesive  1009  located beneath a film  1008  (e.g., Teflon® tape), which can encase one or more FWBs. During curing and after the insertion of epoxies  1009  and  1121  and assembly thereof, a core tube  1120  can be placed above film  1008  and pressed via a pusher element  1002  onto a sensor chip  1130  and a substrate  1006 . Note that pusher element  1002  of  FIG. 10  is similar to pusher element  910  of  FIG. 9 . 
   By making certain that film  1008  does not adhere to substrate  1006  and sensor chip  1130 , nor to epoxy  1121  and flow channel block  1118 , one can take the top and bottom halves apart after the epoxy has been cured and remove the film as well. Film  1008  can be formed from a material such as, for example, a Teflon® fluoropolymer or Aclar®. (Note that Aclar® is a registered trademark of the Allied Chemical Corporation of Morris Township, N.J.) The structure indicated in  FIG. 10  can thus be fabricated, thereby permitting the perfectly mated top and bottom halves to be reassembled, such that the surface of core tube  1120  contacts the sensing surfaces of sensor chip  1130 . After completion of the measurements, the top half of the assembly illustrated in view  1001  of  FIG. 10  can be discarded (e.g., it may contain a blood or other biological fluid), without having contaminated the non-disposable and generally more costly part holding the sensor chip  1130  and its calibrated circuit on substrate  1006 . 
   Based on the foregoing it can be appreciated that a number of alternative sensor configurations can be implemented in accordance with the present invention to achieve electrical insulation for liquid or “harsh environment” sensor chips. For example, covering a “to-be-sealed” sensor chip to sense liquid flow or liquid properties with a film that combines high dielectric strength and chemical inertness with hydrophobic properties, whether inorganic or not, may be utilized to achieve such electrical insulation. 
   Another technique for achieving electrical insulation for liquid or “harsh environment” sensor chips, in accordance with the present invention disclosed herein, involves enlarging and shaping the film as a potting compound of the wire-bonds around the chip, whereby the potting-sealant-adhesive (e.g., epoxy, RTV, etc.) can be shaped by a removable mold core (such as thin tubing or film of fluoropolymer, glass or metal) to reliably provide minimum insulation, while maximizing sensing performance (e.g., higher signal reliability/accuracy due to reduced offsets, lower-noise, longer service life, etc). In such an instance, the tubular mold core tube may be left in place as insulation after potting. The flow sensor itself, according to the present invention disclosed herein, thus can be exposed to the fluid, because the core tube (i.e., core flow tube) can be removed after using it to mold the flow channel. Alternatively, the flow sensor may also be exposed to the fluid if the core tube is left in place. The core tube thus may comprise a disposable flow tube. 
   In addition, smartly performing the potting enables the fabrication of disposable flow tubes (e.g., for blood or chemical analysis) without disposing of the calibrated sensor and its electronics. Additionally, a constriction in the cross section of the core tube can be provided at the site of the sensor chip (e.g., see  FIG. 1 ) to optimize performance at the location of the highest flow velocity (and signal) and governing pressure drop (i.e., to minimize overall Δp). 
   Furthermore, the tube and the flat substrate can be replaced by a flat film (e.g. 20-100 μum thick Teflon®) wrapped or shrunk around a header such as, for example, a TO 5  or TO 18 , and sealed by an O-ring  618  as shown in  FIG. 8 . Finally, as indicated previously, heat sinking the reference resistance, R r , so that it does not heat up and accidentally drive the heater temperature too high and boil the liquid can be utilized to achieve electrical insulation for liquid or “harsh environment” sensor chips. For example, a small metallic thermal conductor may be utilized, which can be epoxied onto the R r  and increase its heat exchange surface in a direction away from heater resistance, R h . Another aspect of the present invention is related to the alignment of flow channels over a sensor chip. The use of an alignment layer creating a location channel provides another technique for achieving electrical insulation for “harsh environment” sensor chips. 
   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 suitable material for the sensor substrate would have a relatively low thermal conductivity. A low thermal conductivity is important to maintain the sensitivity for the sensor. With a relatively low thermal conductivity, all heating/cooling effects presented to the various sensing elements are caused predominantly 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. 
   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 biocompatible. The sensor features a flat top surface overlying the heater and sensor elements to provide appropriate electrical isolation. The top surface of the sensor can be passivated. The heater and sensor elements are embedded in or attached to a substrate, or die. The sensor can be configured to include one or more front wire bonds and/or through-the-wafer contacts. Through-the-wafer interconnections eliminate the need for bonding wires. The substrate is made up of materials chosen to have a relatively low thermal conductivity, thus eliminating the possibility of undesired thermal signal shorts. For example, the substrate may be fabricated using various glass materials, silicon, alumina, quartz, ceramic, polymers, metal, or combinations of such materials. 
   As shown in  FIGS. 11A and 11B , the sensor  2010  includes substrate  2020 , sensing element  2070 , guide elements  2030  formed in an alignment layer, a flow channel  2040  defined by the guide elements  2030 , and molded element  2050  defining flow channel extensions  2080 ,  2081 . Substrate  2020  has notches  2060  cut into the top surface to accommodate the molded element  2050 . Flow channel  2040  is aligned over sensing element  2070 , which includes a heater  2090  and thermal sensors  20100 ,  20101 . The substrate  2020  can be any conventional material used for microsensors, including silicon, ceramic, metal, glass, such as Pyrex®, or quartz. 
   The fluid flow path, indicated by the arrow in  FIG. 11B , is precisely aligned over the sensing element  2070  in the sensor  2010  shown in  FIGS. 11A and 11B  through the use of guide elements  2031  defining flow channel  2040 . The alignment layer is a polymer material deposited over an entire wafer containing multiple sensing elements  2070  arranged in a pattern. In one embodiment, the polymer is a positive resist such as poly(methyl methacrylate) (PMMA). In another embodiment, the polymer is an epoxy based negative resist. One such resist is SU-8, which is sensitive to UV radiation, and is thermally and chemically stable after development. The alignment layer is masked, and guide elements  2030  are formed by photolithography. 
   The guide elements  2030  are positioned adjacent the sensing elements and serve to guide or align a flow channel over each sensing element. In one embodiment, shown in  FIG. 17 , a single guide element  20630  is positioned adjacent the sensing element  20670 , and a molded element  20650  having a flow channel  20680  is positioned over the guide element  20630  to align the flow channel  20680  over the sensing element  20670 . In other embodiments, the alignment layer is masked such that the guide elements create channels with vertical walls aligned over the sensing elements. In one embodiment, shown in  FIGS. 11A and 11B , the alignment layer is masked to create guide elements  2030  on either side of the sensing element  2070 . In this embodiment, the guide elements  2030  form a flow channel  2040  that serves as the flow path. In other embodiments, shown in  FIGS. 13-16 , the guide elements form a location channel  20240 ,  20340 ,  20440 ,  20540  into which is placed a flow tube  20285  ( FIG. 13A ), molded element  20350 ,  20450  ( FIGS. 14 ,  15 ), or both a molded element  20550  and flow tube  20585  ( FIG. 16 ). The flow tube or molded element in these embodiments functions as the flow path. 
   In another embodiment, the substrate is a silicon wafer and the guide elements are etched using a procedure such as deep reactive ion etching (DRIE). In a further embodiment, the guide element is a V-groove formed by an anisotropic etch of KOH and water. A truncated V-groove having a flat bottom of etch resistant boron doped silicon formed initially beneath a layer of epitaxially grown silicon can also be used. 
   Forming the guide elements precisely aligned adjacent to the sensing elements while processing is still at the wafer level allows for multiple sensors to be manufactured with identically aligned flow paths. The wafer is diced into individual sensors, and molded elements or flow tubes can be positioned over the guide elements to provide a fluid flow channel precisely aligned over the sensing element. 
   In the sensor shown in  FIGS. 11A and 11B , guide elements  2030  define the flow channel  2040 , which is also the flow path. The substrate  2020  forms the bottom of the flow path, the guide elements  2030  form the walls of the flow path, and molded element  2050  forms the top of the flow path. Molded element  2050  includes flow channel extensions  2080 ,  2081  that connect to the ends of the flow channel  2040  to provide an interface with a fluid system. Molded element  2050  can be attached to the substrate  2020  with an adhesive. The guide elements  2030  provide a barrier to prevent migration of the adhesive into the flow path or onto the sensing element. In this embodiment, multiple sensors with identical, precisely aligned flow paths can be produced with minimal post-wafer processing. 
   Another embodiment of sensor is shown in  FIG. 12 . Substrate  20120  has a sensing element  20170 . A single oval guide element  20130  is formed in the alignment layer and defines a flow channel  20140  aligned over sensing element  20170 . Molded element  20150  is configured to fit over guide element  20130  and form the top of the flow channel  20140 . Molded element  20150  includes flow channel extensions  20180 ,  20181  that connect with the ends of the flow channel  20140  at an angle and provide an interface with a fluid system. In some embodiments, flow channel extensions  20180 ,  20181  have the same dimensions. In other embodiments, the two flow channel extensions  20180 ,  20181  are differently sized. For example, the flow channel extensions may have different inner diameters to accommodate the tubing or flow paths in the larger fluid flow system in which the flow sensor is placed. In sensors having one or more angles in the flow path, the sensing element  20170  is positioned at a distance from the last upstream angle that is greater than or equal to ten times the diameter of the flow channel. This spacing allows turbulence in the fluid caused by the angle to dissipate before the fluid passes over the sensing element. 
   The flow sensor embodiments shown in  FIGS. 11A ,  11 B, and  12  are non-isolated sensors. The fluid flow path is directly over the sensing element  2070 ,  20170  and is bound by the substrate  2020 ,  20120  on the bottom, the guide elements  2030 ,  20130  on the sides, and the molded element  2050 ,  20150  on the top. The flow sensors shown in  FIGS. 13A ,  13 B, and  14 - 16  are isolated sensors. 
   The flow sensor  20210  shown in  FIGS. 13A and 13B  includes a substrate  20220 , an alignment layer defining guide elements  20230  that form a location channel  20240  over a sensing element  20270 , a molded element  20250  including flow channel extensions  20280 ,  20281 , a flow tube  20285 , and a cap  20200 . The guide elements  20230  are precisely aligned to form the location channel  20240  over the sensing element  20270  and serve to align the molded element  20250  and flow tube  20285  over the sensing element  20270 . The molded element  20250  provides additional support for the flow tube  20285 . Cap  20200  serves to hold the flow tube  20285  in contact with the sensing element  20270 . The flow tube  20285  provides an isolated flow path over the sensing element  20270 . 
   The flow tube  20285  has a wall thickness that removes a surface of the sensor from direct contact with a fluid flowing through the flow tube by a distance corresponding to the wall thickness, thereby desensitizing the sensor to fluid flow variations and protecting the sensor from what may generically be referred to as a “harsh environment.” A harsh environment may include fluids that are contaminated, dirty, condensing, corrosive, radioactive, etc. Also included are fluids that may overheat, leave deposits, or freeze up the device. The cross section of the flow tube can be cylindrical, polygonal, elliptical, etc. In some embodiments, the flow tube  20285  is disposable, providing a flow sensor that is reusable for multiple contaminated fluid samples, such as blood. To change the flow tube  20285 , the cap  20200  is removed, the used flow tube is replaced with a new flow tube and the cap is replaced. Additionally, this tube wall thickness in contact with the sensor combines a high dielectric strength and chemical inertness with properties such as hydrophobic, hydrophilic and lipophilic as needed. Such properties may be realized with inorganic or organic materials. 
   In some embodiments, cap  20200  includes a protrusion  20205  sized to extend downward to hold smaller flow tubes  20285  in contact with the sensing element  20270 . The sizes of the molded element  20250  and cap  20200  can be selected to provide stability for various sizes of flow tubes  20285 . In this way, multiple sensors cut from a single wafer, each with identical sized location channels  20240 , can be used with different sizes of flow tubes  20285 . Additionally, the molded element  20250  can extend into the location channel  20240  to provide a narrower channel for receiving small diameter flow tubes  20285 . The molded element  20250  can be attached to the substrate  20220  and cap  20200  using adhesive. The guide elements  20230  provide a barrier to prevent migration of the adhesive into the flow path or onto the sensing element. 
   The tube  20285 , molded element  20250 , and cap  20200  can be made of glass such as Pyrex®, fused silica, quartz, sapphire, ceramic, epoxy, one or more polymers such as PEEK (polyetheretherketone), PTFE (polytetrafluoroethylene), or Teflon®, or metal such as stainless steel. Mixtures of different types of glass or mixtures of different polymers can also be used to manufacture the tube  20285 , molded element  20250 , and cap  20200 . A stainless steel flow tube  20285  can be attached to the device with heat transfer paste or fluid. Oil can be added to the joint between the tube  20285  and molded element  20250  and/or substrate  20220  to enhance heat transfer. 
   Another embodiment of isolated flow sensor is shown in  FIG. 14 . The location channel  20340  formed between guide elements  20330  is wider than the sensing element  20370  on the substrate  20320 . A molded element  20350  extends over the guide elements  20330  and forms the bottom and sides of a flow channel  20380  that fits within the location channel  20340 . A cap  20300  forms the top of the flow channel  20380 . In one embodiment, the molded element  20350  has a flow channel bottom  20352  with a thin region  20355  that contacts the sensing element  20370 . The remaining flow channel bottom  20352  is spaced apart from the substrate  20320 , forming air pockets to reduce loss of the thermal signal. Sensors with different sized flow channels  20380  can be made from the same wafer by using molded elements  20350  with different sized flow channels  20380 . In some embodiments, the molded element  20350  is disposable and replaceable. The cap  20300  can also be disposable. Additionally, the cap  20300  can have a protrusion  20305  extending into the flow channel  20380  to alter the dimensions of the flow channel  20380 . As shown in  FIG. 15 , a cap  20400  with a protrusion  20405  extending across the flow channel  20480  reduces the height and overall dimensions of the flow channel  20480 . The interface between the exterior of the molded element  20450  and the interior of the location channel  20440  provides accurate and precise alignment of the flow path over the sensing element, regardless of the interior size of flow channel  20480 . 
   In a further embodiment, shown in  FIG. 16 , a flow tube  20585  is positioned over a thin region  20555  in the flow channel bottom  20552  of molded element  20550 . The flow tube  20585  forms the flow channel  20580 . The molded element  20550  is aligned over the sensing element  20570  by the guide elements  20530 . Cap  20500  contacts the top of the flow tube  20585  and maintains the flow tube  20585  in position. In some embodiments, adhesive is also used to maintain flow tube  20585  in position. In one embodiment, the flow tube  20585  is disposable and replaceable. This embodiment is particularly suited for analyzing fluids that are toxic, corrosive, hazardous, contaminated, etc. 
   Another embodiment of isolated flow sensor is shown in  FIG. 17 . An alignment layer is deposited onto a substrate  20620  and a single guide element  20630  is formed in the alignment layer. The guide element  20630  is adjacent the sensing element  20670  on the substrate  20620 . A molded element  20650  having a first side  20652  a second side  20654  and a flow channel  20680  therebetween is positioned such that the first side  20652  extends over the guide element  20630  to align the flow channel  20680  over the sensing element  20670 . A cap  20600  forms the top of the flow channel  20680 . In one embodiment, the molded element  20650  has a flow channel bottom  20652  with a thin region  20655  that contacts the sensing element  20670 . In some embodiments, the remaining flow channel bottom  20652  is spaced apart from the substrate  20620 , forming air pockets  20661  to reduce loss of the thermal signal. 
     FIG. 18  depicts a graph illustrating the performance of flow sensors with salt water at ambient temperature, in accordance with an embodiment of the present invention.  FIG. 18  indicates that measured flow sensor output versus flow for several flow channel configurations and heater temperature values can be obtained. As illustrated in the graph, flows that occur below 0.5μL/min are measurable for a smaller core tube of only 150μm internal diameter. In such instances, noise levels may be approximately in the 1mV range, for which no compensation for fluctuations in ambient temperatures may be in place. Those skilled in the art can thus appreciate that the graph illustrates a range of data collected over time regarding nulled-output versus flow rate.  FIG. 18  thus generally illustrates the beneficial influence of lower wall thickness (WT) and higher thermal conductivity materials for the core tube, which increases sensitivity and flow ranges. An example of a higher thermal conductivity material, which may be utilized in association with an embodiment of the present invention, is Pyrex®. (Note that Pyrex® is a registered trademark of the Corning Glass Works Corporation of Corning, N.Y. 14831.) 
     FIG. 19  depicts a graph illustrating the performance of flow sensors with stainless steel flow tubes on a Pyrex® microbrick with and without oil. The stainless steel flow tube had an inner diameter of 0.004inches and an outer diameter of 0.008inches. Water was used as the fluid. As show in the graph, a drop of oil added to the joint between the flow tube and substrate enhanced heat transfer and increased the signal approximately two-fold. 
   The flow sensor package disclosed herein offers several advantages over prior art liquid flow sensor packaging approaches. For example, the application of reliably controlling the thickness of the insulating layer, molded element, or flow tube can eliminate electrical leakages and the risk of electrical shorts. This controlled thickness also enables the application of larger voltages to the sensor heating elements, thus higher heater temperatures, and thus leads to larger output signals, reduced effect of sensor and electronic offsets and without boiling the liquid. An isolated flow channel located above the chip cuts down on flow noise while providing the aforementioned benefits, including eliminating the risk of fluid leakage or corrosion and, additionally, providing electrical insulation of the chip contacts. In addition, the isolated flow channel can provide a “clean”, contaminant-free environment for preserving the maximum fluid cleanliness. 
   Thus, according to the invention described herein, a sensor can be configured to generally include a flow path formed over a sensor chip for sensing fluid flow, wherein a fluid in the flow path surrounds the sensor chip. Alternatively, the sensor chip can be isolated from the flow path by a flow tube or molded element, which provides electrical insulation and corrosion protection to the sensor chip, reduces flow noise, essentially eliminates the risk of fluid leakage, and maintains the fluid super-clean and contamination-free while improving structural integrity for the thermal measurements derived from the sensor chip. The use of such an isolated configuration also can protect the sensor from corrosion, radioactive or bacterial contamination, deposits, overheating, or freeze-ups. Such an isolated configuration also enables the flow tube and/or molded element to be detachable and disposable, without requiring the replacement of the more costly sensor chip and its associated electronics. 
   The flow path is precisely aligned over the sensor chip by an alignment layer that forms a location channel. The location channels can be formed on a substrate at the wafer level, providing an inexpensive, efficient means of producing multiple sensors with identically aligned flow paths. 
   The present invention can be used in glucose monitoring, laboratory on a chip, drug delivery, cytometer, fluid flow, dialysis, infusion, and other applications. Further, the present invention is applicable to microfluidics and flow sensing applications that need to measure liquids, condensing air or contaminated air. 
   The embodiments and examples set forth herein are presented to best explain the present invention and its practical application and to thereby enable those skilled in the art to make and utilize the invention. Those skilled in the art, however, will recognize that the foregoing description and examples have been presented for the purpose of illustration and example only. Other variations and modifications of the present invention will be apparent to those of skill in the art, and it is the intent of the appended claims that such variations and modifications be covered. The description as set forth is not intended to be exhaustive or to limit the scope of the invention. Many modifications and variations are possible in light of the above teaching without departing from the scope of the following claims. It is contemplated that the use of the present invention can involve components having different characteristics. It is intended that the scope of the present invention be defined by the claims appended hereto, giving full cognizance to equivalents in all respects.