Abstract:
A flow sensor system and a method for fabricating the same. A substrate is provided, comprising a detector wafer upon which a flow sensor is formed. One or more shells can then be configured upon the substrate whose walls form a flow channel. The flow channel is fabricated directly upon the substrate in a manner that allows the flow channel to couple heat transfer directly to the flow sensor in order to eliminate the need for two or more different types of sacrificial layers during the fabrication of the flow sensor upon the substrate and in which the shell(s) is coupled with fluidic measurement to provide for the flow sensor.

Description:
TECHNICAL FIELD 
   Embodiments are generally related to the detection of fluids. Embodiments are additionally related to liquid flow sensors. Embodiments are also related to techniques for fabricating liquid flow sensors. 
   BACKGROUND OF THE INVENTION 
   Sensors have been used to measure flow rates in various medical, process, and industrial applications, ranging from portable ventilators supplying anesthetizing agents to large-scale processing plants in a chemical plant. In these applications, flow control is an inherent aspect of proper operation, which is achieved in part by using flow sensors to measure the flow rate of a fluid within the flow system. In many flow systems, e.g., fuel cell flow systems containing a binary mixture of methanol and water, the chemical composition of the fluid may change frequently. 
   A flow system is often required to flow more than one fluid having different chemical and thermo physical properties. For example, in a semiconductor processing system that passes a nitrogen-based gas, the nitrogen-based gas may at times be replaced by a hydrogen-based or helium-based gas, depending on the needs of the process; or in a natural gas metering system, the composition of the natural gas may change due to non-uniform concentration profiles of the gas. 
   Fluid flow sensors are thus important in a variety of applications. It is often necessary to determine the composition of a fluid utilizing a liquid or fluid flow sensor. One method for determining the composition of the fluid is to measure its thermal conductivity and compare the resulting value to a standard value. Measurements can be obtained by measuring power transferred from a heater to the fluid. 
   Current approaches for fabricating liquid flow sensors for very low flow applications are constrained by methods of coupling flow to the flow sensor while maintaining a minimal system volume. Coupling between the sensing element and an isolated flow channel has proved difficult to produce. Earlier processes involved the use of a nickel sacrificial layer, which often oxidized, resulting in an undependable final release. It is believed that such problems can be overcome through the introduction of a flow sensor structure and fabrication technique in which the flow channel is built directly on the detector wafer and thereby couples the heat transfer directly to the liquid flow sensor. Such improvements are discussed in, greater detail herein. 
   BRIEF SUMMARY OF THE INVENTION 
   The following summary of the invention is provided to facilitate an understanding of some of the innovative features unique to the present invention and is not intended to be a full description. A full appreciation of the various aspects of the invention can be gained by taking the entire specification, claims, drawings, and abstract as a whole. 
   It is, therefore, one aspect of the present invention to provide for an improved flow sensor. 
   It is another aspect of the present invention to provide for an improved method for fabricating a flow sensor. 
   It is yet a further aspect of the present invention to provide for a flow sensor that can be fabricated with a flow channel formed directly on a detector wafer to thereby couple heat transfer directly to the sensor. 
   It is still a further aspect to provide for an improved flow sensor fabrication process that eliminates the need for two different types of sacrificial layers. 
   The aforementioned aspects of the invention and other objectives and advantages can now be achieved as described herein. A flow sensor system and a method for fabricating the same are disclosed. In general, a substrate is provided, comprising a detector wafer upon which a flow sensor is formed. One or more shells can then be configured upon the substrate whose walls form a flow channel. The flow channel is fabricated directly upon the substrate in a manner that allows the flow channel to couple heat transfer directly to the flow sensor in order to eliminate the need for two or more different types of sacrificial layers during the fabrication of the flow sensor upon the substrate and in which the shell(s) is coupled with fluidic measurement to provide for the flow sensor. 
   The “shell” flow sensor described herein can be fabricated utilizing a standard microbridge process with the exception that an additional low stress dielectric can be provided to increase the pressure range of the completed device. A thin (e.g., ˜1-2 kA) layer of polyimide can be capped with SiO 2  and patterned to form what will be slots at the edge of the flow channel. Next, a thick layer of polyimide can be deposited and patterned with a slope to form what will become the flow channel. A thick layer of SiO 2  can be conformably deposited over the polyimide (e.g., TEOS) and then patterned to expose the thin polyimide slots at the edge of the flow channel. 
   The polyimide can be then removed utilizing an oxygen plasma etch, thereby releasing the shells forming the flow channels. The small polyimide slots are then filled in with a final TEOS growth that will seal the flow channels. The oxide is removed from the bond pads. Three holes can also be patterned on the back of each die and DRIE etched through the wafer from the back. Two of the holes can be utilized to attach capillaries, which can be formed from materials such as glass coated with polyimide, steel, etc. to couple the fluid flow into and out of the chip (i.e., flow sensor system) and the third hole will act to remove the silicon from below the flow sensor providing thermal isolation for the sensor device/system. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying figures, in which like reference numerals refer to identical or functionally-similar elements throughout the separate views and which are incorporated in and form a part of the specification, further illustrate the present invention and, together with the detailed description of the invention, serve to explain the principles of the present invention. 
       FIG. 1  illustrates the first step of a process for fabricating a shell flow sensor, in accordance with a preferred embodiment; 
       FIG. 2  illustrates the second step of a process for fabricating a shell flow sensor, in accordance with a preferred embodiment; 
       FIG. 3  illustrates the third step of a process for fabricating a shell flow sensor, in accordance with a preferred embodiment; 
       FIG. 4  illustrates the fourth step of a process for fabricating a shell flow sensor, in accordance with a preferred embodiment; 
       FIG. 5  illustrates the fifth step of a process for fabricating a shell flow sensor, in accordance with a preferred embodiment; 
       FIG. 6  illustrates the sixth step of a process for fabricating a shell flow sensor, in accordance with a preferred embodiment; 
       FIG. 7  illustrates the seventh step of a process for fabricating a shell flow sensor, in accordance with a preferred embodiment; 
       FIG. 8  illustrates the eighth step of a process for fabricating a shell flow sensor, in accordance with a preferred embodiment; 
       FIG. 9  illustrates the ninth step of a process for fabricating a shell flow sensor, in accordance with a preferred embodiment; 
       FIG. 10  illustrates the tenth step of a process for fabricating a shell flow sensor, in accordance with a preferred embodiment; 
       FIG. 11  illustrates the eleventh step of a process for fabricating a shell flow sensor, in accordance with a preferred embodiment; 
       FIG. 12  illustrates the twelfth step of a process for fabricating a shell flow sensor, in accordance with a preferred embodiment; 
       FIG. 13  illustrates the thirteenth step of a process for fabricating a shell flow sensor, in accordance with a preferred embodiment; 
       FIG. 14  illustrates the fourteenth step of a process for fabricating a shell flow sensor, in accordance with a preferred embodiment; 
       FIG. 15  illustrates the fifteenth step of a process for fabricating a shell flow sensor, in accordance with a preferred embodiment; 
       FIG. 16  illustrates the sixteenth step of a process for fabricating a shell flow sensor, in accordance with a preferred embodiment; 
       FIG. 17  illustrates a close-up photographic view of a released and sealed SiO 2  shell, which can be implemented in accordance with a preferred embodiment; 
       FIG. 18  illustrates a close-up photographic view of a sealed etch vent port, which can be implemented in accordance with a preferred embodiment; 
       FIG. 19  illustrates a cross-sectional view of a shell flow channel sensing system, which can be implemented in accordance with an alternative embodiment; 
       FIG. 20  illustrates a close-up photographic view of a cleaved shell structure, which can be implemented in accordance with an alternative embodiment; and 
       FIGS. 21-22  illustrate alternative top views of the shell flow channel sensing system of  FIG. 19 , in accordance with an alternative embodiments. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The particular values and configurations discussed in these non-limiting examples can be varied and are cited merely to illustrate at least one embodiment of the present invention and are not intended to limit the scope of the invention. 
     FIG. 1  illustrates the first step  1  of a process for fabricating a shell flow sensor, in accordance with a preferred embodiment. Note that in  FIGS. 1-16 , identical or similar parts or elements are generally indicated by identical reference numerals.  FIGS. 1-16  thus illustrate together an overall fabrication technique for creating shell flow channels in the context of a phased-type gas analyzer IC chip that does not require a second wafer for the configuration of a flow channel. In each of  FIGS. 1-16 , side and plan views of the same device or structure are provided to provide for an enhanced view of the particular fabrication step. As indicated in the side view of  FIG. 1 , a double polished silicon wafer substrate  102  can be provided with polished layers  104  and  106  that are located adjacent substrate  102 . The layer  104  is shown in the plan view of  FIG. 1 . 
     FIG. 2  illustrates the second step  2  of a process for fabricating a shell flow sensor, in accordance with a preferred embodiment. As depicted in  FIG. 2 , a bottom bridge nitride (e.g., approximately  5000 A) layer  108  can be low stress sputtered with respect to layer  104 . Heater components  110  and  111  (e.g., CrOx/Pt/CrOx) can be deposited and patterned above substrate  102  as indicated by the side and plan views illustrated in  FIG. 2 . 
     FIG. 3  illustrates the third step  3  of a process for fabricating a shell flow sensor, in accordance with a preferred embodiment. As indicated by side and plan view shown in  FIG. 3 , a layer  112  can be deposited above layer  108 , and another layer  114  formed above layer  112 . Layer  112  constitutes a nitride layer and layer  114  comprises a nickel layer. Step  3  depicted in  FIG. 3  thus includes cap Pt features with a top nitride (e.g., approximately 8000 A) layer  112 , which is then covered with the thin nickel etch stop layer  114  for later etches to protect surface materials. 
     FIG. 4  illustrates the fourth step  4  of a process for fabricating a shell flow sensor, in accordance with a preferred embodiment. Follow processing of the operation depicted in  FIG. 3 , an operation can be performed in which a thin polyimide (e.g., approximately 4000 A) layer  116  is deposited and capped by a thin SiO 2  layer  118  (e.g., approximately 1000 A PECVD). 
     FIG. 5  illustrates the fifth step  5  of a process for fabricating a shell flow sensor, in accordance with a preferred embodiment.  FIG. 5  indicates that the SiO 2  layer  118  and the thin polyimide layer  116  can be patterned for what will later be etch vents to remove the thicker polyimide. Patterning of layer  118  results in SiO 2  structures  128 ,  130 ,  132 , and  134 , which are depicted in the plan view shown in  FIG. 5 . Patterning of the polyimide layer  116  results in polyimide structures  120  and  122 , which are illustrated in the side view shown in  FIG. 5 . 
     FIG. 6  illustrates the sixth step  6  of a process for fabricating a shell flow sensor, in accordance with a preferred embodiment. In the operation depicted in  FIG. 6 , a dielectric layer can be cut to expose electrical contacts and silicon where a gas inlet and outlet holes will be located. This access feature can also allow for plasma etching of polyimide in the vicinity of the resulting inlet/outlet port. A gap  121 , for example, is shown in the side view in  FIG. 6 . Such a gap  121  is located above the heater component  110 , which in turn sits above layer  108 . 
     FIG. 7  illustrates the seventh step  7  of a process for fabricating a shell flow sensor, in accordance with a preferred embodiment. The operation illustrated in the side and plane views of  FIG. 7 , involve the deposition of a thick polyimide layer  140 . Depending upon the operation involved, it is possible that two to four sequential coats with hard brakes therebetween may be necessary. A very thick resist can then be applied over the polyimide and patterned where gas channels are to be located, thereby allowing for shrinkage of any photoresist features which automatically create sloped Pi sides. Slopes should preferably land on vent tabs. 
     FIG. 8  illustrates the eighth step  8  of a process for fabricating a shell flow sensor, in accordance with a preferred embodiment. In this step, the main shell can be created by depositing a layer  142  of approximately 1-2 μm of TEOS SiO 2  using a 300 C formulation. Note that the term “TEOS” as utilized herein refers generally to tetraethyl orthosilicate, which is a major chemical compound with the formula Si(OC 2 H 5 ) 4 . Often abbreviated TEOS, this molecule generally is composed of four ethyl groups attached to the SiO 4   4−  ion, which is called orthosilicate. As an ion in solution, orthosilicate does not exist. Alternatively TEOS can be considered to be the ethyl ester of orthosilicic acid, Si(OH) 4 . 
     FIG. 9  illustrates the ninth step  9  of a process for fabricating a shell flow sensor, in accordance with a preferred embodiment. In step  9 , the shell oxide can be patterned and LAM-etched in order to expose the ends of vent holes, and also expose bond pads, while leaving the oxide for the shell. 
     FIG. 10  illustrates the tenth step  10  of a process for fabricating a shell flow sensor, in accordance with a preferred embodiment. Using the same mask as used in step  9 , an ion mill field etch stop metal operation can be performed.  FIG. 11  illustrates the eleventh step  11  of a process for fabricating a shell flow sensor, in accordance with a preferred embodiment. In the operation depicted in  FIG. 11 , a layer  144  can be deposited and etched as reinforcing SU-8 blocks to strengthen capillary insertion points as well as ridges that can support the wafer or substrate  102  when the entire device is located upside down so that it does not hit the shell. 
     FIG. 12  illustrates the twelfth step  12  of a process for fabricating a shell flow sensor, in accordance with a preferred embodiment. In the operation depicted in  FIG. 12 , a photoresist coat can be sprayed onto the back of the wafer substrate  102  and a LAM etch SiO2 operation can be performed for thermal isolation areas and gas inlet/outlet features. Layer  146  constitutes such an SiO2 layer. A gap  147  can form part of a gas inlet/outlet feature. 
     FIG. 13  illustrates the thirteenth step  13  of a process for fabricating a shell flow sensor, in accordance with a preferred embodiment. In the operation depicted in  FIG. 13 , silicon can be DRIE etched to form inlet/outlet holes and thermal isolation features. 
     FIG. 14  illustrates the fourteenth step  14  of a process for fabricating a shell flow sensor, in accordance with a preferred embodiment. Step  14  depicted in  FIG. 14  involves an O 2  plasma etch of the polyimide in order to strip the resist  140  back from the wafer/substrate  102 . Once the resist  140  is pulled back a gap is located below the layer  142 . Layer  144  is maintained above layer  142 . 
     FIG. 15  illustrates the fifteenth step  15  of a process for fabricating a shell flow sensor, in accordance with a preferred embodiment. In the operation of step  15 , a layer  150  is configured above layer  146  and  142 . Layer  150  can constitute a component to seal the resulting shell. Layer  150  may be, for example, a layer of approximately 5000 A TEOS SiO 2  (e.g., an approximately 200 C formulation). 
     FIG. 16  illustrates the sixteenth step  16  of a process for fabricating a shell flow sensor, in accordance with a preferred embodiment. In the operation depicted in  FIG. 16 , a spray coat and a pattern seal oxide can be provide to expose the bond pads  128 ,  130 . An oxygen plasma etch can also be provide to remove any resist.  FIGS. 1-16  thus generally illustrate one possible shell fabrication process for configuring and providing a shell flow sensor. It is possible, for example, to implement such a process without the use of bottom dielectrics and with DRIE etched access holes. 
     FIG. 17  illustrates a close-up photographic view of a released and sealed SiO 2  shell  170 , which can be implemented in accordance with a preferred embodiment.  FIG. 18  illustrates a close-up photographic view of a sealed etch vent port  180 , which can be implemented in accordance with a preferred embodiment. Such shells can be utilized to create enclosed flow channels of the order of 25 μm high by 100-200 μm wide. The SEM&#39;s can be of channels 25 μm high by 110 μm wide. Higher flow channels for lower pressure drop can also be implemented in accordance with the embodiments described herein. Possible applications of the on chip fluidic channels are PHASED pre-concentration and separation devices, minimal volume thermal flow sensor for liquids or gases, optical analysis of liquid or gas stream (e.g., channels are transparent and DRIE etch could provide fiber alignment to channel). 
   Providing well controlled flow conditions for a microbridge flow sensor is a challenging issue. In particular, attempting to use the sensor for liquid flow sensing can be challenging. The disclosed concept results in the construction of not only a flow sensor but walls defining the flow profile on one chip. The use of “shell” processing concepts can permit construction of a glass shell flow containment above a microbridge like device (e.g., diaphragm version) to maximally couple the fluidic heat transfer between the fluid flow defined by the shell and the heater/sensor configuration built into the bottom diaphragm. 
     FIG. 19  illustrates a cross-sectional view of a shell flow channel sensing system  190 , which can be implemented in accordance with such shell processing techniques and in accordance with possible alternative embodiment. System  190  generally includes a layer  191 , which can constitute a thick TEOS oxide shell enclosing the flow channel  206  therein. Arrow  201  generally indicates a direction of fluid flow within flow channel  206 . A layer  192  is located below the flow channel  206  and can be configured as a thick TEOS base oxide to strengthen the diaphragm A layer  193  is located below layer  192 . One or more sensing elements  241  are also illustrated in  FIG. 19 . Layer  193  generally constitutes a DRIE thermal isolation layer. 
   Layers  192  and  193  together can constitute a diaphragm thermal isolation for flow sensor elements  241 . The substrate  199  is located below the layer  193 . A capillary inlet  197 , a capillary outlet  198  and a thermal isolation opening can also be formed partially within layer  199 . The direction of flow with respect to capillary inlet  197  is indicated by arrow  194 . Likewise, the direction of flow with respect to capillary outlet  198  is indicated by arrow  195 . Capillary inlet  196  is surrounded partially by an epoxy attachment  196 , and capillary outlet  198  is surrounded partially by an epoxy attachment  207 . The nature of the sensor is inherently bidirectional and so inlet and outlet aspects can be interchangeable. Note that in some embodiments, if the oxide layers are about 2 microns thick, and the diameter of the shell is 100 microns, then the burst pressure is about 140 PSI (assuming simple pipe model, and SiO2 tensile strength of ˜7000 PSI) An SEM of a cleaved shell test structure is shown in the next illustration depicted in  FIG. 20 . 
     FIG. 20  illustrates a close-up photographic view of a cleaved shell structure  200 , which can be implemented in accordance with an alternative embodiment. In generally, the fabrication process described herein can begin with sensors fabricated utilizing a standard microbridge process with the exception of having additional low stress dielectric (e.g., TEOS) to increase the pressure range of the completed device. A thin (˜1-2 kA) layer of polyimide capped with SiO 2  can be patterned to form what will be slots at the edge of the flow channel  206  illustrated in  FIG. 19 . Next, a thick layer of polyimide can be deposited and patterned with a slope to form what will be the flow channel  206 . A thick layer of SiO 2  can be conformably deposited over the polyimide (TEOS) and then patterned to expose the thin polyimide slots at the edge of the flow channel  206 . The polyimide is then removed using an oxygen plasma etch, releasing the shells forming the flow channels. The small polyimide slots are then filled in with a final TEOS growth that will seal the flow channels. The oxide is removed from the bond pads. 
   Three holes can be patterned on the back of each die and DRIE etched through the wafer from the back. Two of the holes will be approximately  380  microns in diameter and be used to attach glass capillaries  197 ,  198  via respective epoxy attachments  196   207  to couple the fluid flow into and out of the chip (i.e., see arrows  194 ,  195 ) and the third hole will remove the silicon from below the flow sensor system  190  providing thermal isolation for the sensor or system  190 . 
     FIGS. 21-22  illustrate alternative top views of the shell flow channel sensing system  190  of  FIG. 19 , in accordance with an alternative embodiment. Note that in FIGS.  19  and  21 - 22 , identical or similar parts or elements are generally indicated by identical reference numerals. Thus, layer  199  and layer  192  are also shown in  FIG. 21-22  with respect to up/down sensor pads  220 ,  222 ,  224  and flow channel  206 . Additionally, heater pads  226 ,  228  are shown with respect to the flow channel  206 , which is surrounded by a glass shell channel  214 . Inlet and outlet channels  197  and  198  are also shown in  FIG. 21-22  along with a plurality of thin slots  212  for the removal of polyimide.  FIGS. 21-22  represent options available for shell flow sensors (i.e., system  190 ) including the ability to adjust the thermal conduction of the supports. In applications for gas flow sensing, it is preferred that a minimum thermal conduction be present with respect to the supports for maximum sensitivity. This can be tailored by the size of the opening beneath the diaphragm as illustrated in  FIG. 21 , or by adding perforations  251  between the shell flow channel  206  and the silicon support. 
   Note that the thermal isolation can be adjusted by design. In the case of air or other gases, as much thermal isolation as possible is preferred, and can be enhanced by placing cuts through the diaphragm outside of the shell (so it does not leak) to reduce thermal conduction ( FIG. 22 ) or the diaphragm can be made larger ( FIG. 21 ). There are trade-offs in device ruggedness and response time for this performance improvement. In the case of liquids, an enhanced thermal isolation may not be desirable, in which case the diaphragm can be configured smaller, or possibly not etched all the way through the silicon leaving a thin (e.g., ˜1-10 um) layer of silicon under the shell. 
   It will be appreciated that variations of the above-disclosed and other features and functions, or alternatives thereof, may be desirably combined into many other different systems or applications. Also that various presently unforeseen or unanticipated alternatives, modifications, variations or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims. 
   The embodiments of the invention in which an exclusive property or right is claimed are defined as follows.