Abstract:
A flip-chip flow sensor has electrical components, such as temperature sensors and a heater, on the top of a substrate and has a channel formed in the bottom of the substrate. The channel is separated from the substrate&#39;s top by a membrane of substrate material. A fluid flowing through the channel is separated from a heater, upstream temperature sensor, downstream temperature sensor, bond pads, and wire bonds by the membrane. Heat flows through the membrane easily because the membrane is thin. As such, the electrical elements of the flow sensor, the bond pads and the wires are physically separated from a fluid flowing through the channel but can function properly because they are not thermally isolated.

Description:
TECHNICAL FIELD 
   Embodiments relate to flow sensors and sensor systems. Embodiments also relate to using temperature sensors on either side of a heater to measure fluid flows. 
   BACKGROUND OF THE INVENTION 
   The measurement of fluid flows is important in manufacturing, medicine, environmental monitoring, and other areas. One type of flow sensor measures the rise in temperature of a fluid flowing past a heater. Smaller temperature rises correspond to faster flow. Gases and liquids are both fluids and their flow rates can be measured by flow sensors that include temperature sensors and heaters. 
   Current technology provides systems and methods for producing temperature sensors and heaters on substrates using well-established semiconductor processing techniques. Those skilled in the art of semiconductor manufacturing are familiar with these system and methods such as photolithography, deposition, vapor deposition, etch, wet etch, plasma etch, reactive ion etch, as well as numerous other processes. 
   Many measurable fluids and gases react chemically with fluid flow sensors and bond pads because some of the sensor materials and the fluid come into contact and because they are reactive. Additionally, wire bonds made on the surface exposed to the gas or liquid being measured may interact with the flow or may be corroded by the flow material. Aspects of the embodiments directly address the shortcoming of current technology by preventing the fluid from contacting the reactive sensor materials. 
   BRIEF SUMMARY 
   The following summary is provided to facilitate an understanding of some of the innovative features unique to the embodiments and is not intended to be a full description. A full appreciation of the various aspects of the embodiments can be gained by taking the entire specification, claims, drawings, and abstract as a whole. 
   It is therefore an aspect of the embodiments to provide a substrate such as a wafer typically used in semiconductor processing. The substrate can be a silicon wafer, a glass wafer, a quartz wafer, a silicon on insulator (SOI) wafer, or another type a wafer. The substrate has a top and a bottom. 
   Semiconductor processing techniques can be used to form an upstream temperature sensor, downstream temperature sensor, and heater. A fluid flowing through a channel formed in the bottom of the substrate has a flow direction that takes the fluid from an inlet to an outlet. As such, the fluid flows into the inlet, past the upstream temperature sensor first, then the heater, then the downstream temperature sensor, and finally out the outlet. A flow sensor can have an inlet temperature sensor that measures the fluid temperature at the inlet. A flow sensor can also have a substrate temperature sensor that is positioned away from the channel. The temperature sensors and the heater are on the top of the substrate and the channel is formed into the bottom of the substrate. 
   Being formed into the substrate bottom, the channel top is separated from the substrate top by a membrane of substrate material. A fluid flowing through the channel cannot contact the upstream sensor, heater, downstream sensor, bond pads, and wire bonds because they are on the other side of the membrane. 
   The substrate material can have high thermal conductance. Ideally, heat can flow through the membrane easily. Heat flowing through the rest of the substrate, however, can reduce a flow sensor&#39;s sensitivity, accuracy, and precision. A thermal dam around the membrane can protect the sensor from spurious heat flows. 

   
     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 aspects of the embodiments and, together with the background, brief summary, and detailed description serve to explain the principles of the embodiments. 
       FIG. 1  illustrates a side view of a substrate in accordance with aspects of the embodiments; 
       FIG. 2  illustrates a side view of a flow sensor in accordance with aspects of the embodiments; 
       FIG. 3  illustrates a top view of a flow sensor in accordance with aspects of the embodiments; 
       FIG. 4  illustrates a side view of a silicon on insulator substrate in accordance with aspects of some embodiments; 
       FIG. 5  illustrates a side view of a flow sensor on a silicon on insulator substrate in accordance with aspects of some embodiments; 
       FIG. 6  illustrates a side view of a flow sensor having a thermal dam in accordance with aspects of the embodiments; 
       FIG. 7  illustrates a top view of a flow sensor having a thermal dam in accordance with aspects of certain embodiments; 
       FIG. 8  illustrates a top view of a substrate having a trapezoidal channel in accordance with aspects of certain embodiments; 
       FIG. 9  illustrates a cut view of a substrate having a trapezoidal channel in accordance with aspects of certain embodiments; 
       FIG. 10  illustrates a top view of a flow sensor having a thermal dam enclosing the membrane in accordance with aspects of certain embodiments; and 
       FIG. 11  illustrates a high level flow diagram of producing a flow sensor in accordance with aspects of the embodiments. 
   

   DETAILED DESCRIPTION 
   The particular values and configurations discussed in these non-limiting examples can be varied and are cited merely to illustrate at least one embodiment and are not intended to limit the scope thereof. In general, the figures are not to scale. 
     FIG. 1  illustrates a side view of a substrate  101  in accordance with aspects of the embodiments. The substrate can be a wafer of silicon, glass, quartz, or another material as are commonly used in semiconductor manufacturing. 
     FIG. 2  illustrates a side view of a flow sensor in accordance with aspects of the embodiments. A channel  201  in the bottom of the substrate  101  is separated from the substrate top by a membrane  205 . A substrate temperature sensor  202  is positioned away from the channel  201 . Other sensor elements  203  are on top of the membrane  205 . An inset distance  204  separates the sensor elements  203  from the edge of the channel  201 . 
   The membrane must be thick enough to be physically robust and thin enough to allow heat to easily pass between the channel the substrate top. Experimental results have revealed that silicon and quartz membranes can be used with membrane thickness ranging from 0.02 mm to 0.08 mm. Thinner and thicker membranes can be used to meet other sensor requirements such as increased robustness. 
   When using silicon based processing, an anisotropic crystallographic orientation dependent etching can be used to produce sloped channel sidewalls. Typically, a wet etch process, such as KOH processes, TMAH processes, EDA processes, or EDP processes, is used. Deep reactive ion etching can be used to produce sidewalls that are nearly perpendicular to the substrate surface. KOH is potassium hydroxide, TMAH is trimethylammonium hydroxide, EDA is ethylenediamine, and EDP is ethylenediamine pyrocatechol. 
   Plasma etching can be used to form a channel in silicon dioxide based glass substrates. Monocrystalline quartz can be anisotropically etched with hydrofluoric acid and anhydrous HF. HF is hydrogen fluoride. 
     FIG. 3  illustrates a top view of a flow sensor in accordance with aspects of the embodiments. Fluid can flow into an inlet  301 , through the channel, and out of an outlet  302 . The flow direction  307  is the direction fluid flows. An inlet temperature sensor  303  can be used to measure the fluid temperature as it enters the channel. An upstream temperature sensor  304  is used to measure the fluid temperature immediately before the fluid reaches a heater  306 . A downstream temperature sensor  305  is used to measure the fluid temperature immediately after the fluid passes the heater  306 . The substrate temperature sensor  202  is positioned away from the channel.  FIG. 3  illustrates a view of the system of  FIG. 2  from above. 
   The heater can be a resistive element that heats up when an electric current passes through it. Heaters can comprise materials such as, platinum, permalloy (NiFe), chrome silicon (CrSi), doped silicon thin film resistors or other types of silicon-based resistors, nickel chrome (NiCr), tantalum, and nickel. 
   A material that repeatedly and predictably changes resistance in a repeatable and predictable manner when heated or cooled can be used as a temperature sensor. Temperature sensors can comprise materials such as, platinum, permalloy (NiFe), chrome silicon (CrSi), doped silicon thin film resistors or other types of silicon-based resistors, nickel chrome (NiCr), tantalum, and nickel. 
     FIG. 4  illustrates a side view of a silicon on insulator (SOI) substrate in accordance with aspects of some embodiments. An SOI substrate can comprise a handler substrate  401 , an insulating layer  402 , and an upper layer  403 . For example, silicon wafers can be used as handler substrates  401 , the insulating layer can be a thermal insulator  402  such as silicon dioxide or silicon nitride, and the upper layer  403  can be a silicon layer. A silicon nitride insulating layer can have a silicon oxide upper layer. A major advantage of using an SOI substrate is that the thickness of the insulating layer and the upper layer can be accurately controlled across the entire substrate. Either of those layers can be used as an etch stop layer resulting uniform and repeatable membrane thickness. Those practiced in the art of semiconductor manufacturing know of many varieties of SOI substrates. 
     FIG. 5  illustrates a side view of a flow sensor on a silicon on insulator substrate in accordance with aspects of some embodiments.  FIG. 5  is similar to  FIG. 2  with a few exceptions. The membrane  501  consists of material from the upper layer  403 . Furthermore, sensor elements  203  and the substrate temperature sensor  202  are illustrated as being positioned on the upper layer  403 . The substrate temperature sensor  202  can also be positioned on the handler substrate  401  by etching a hole through the upper layer  403  and the insulting layer  402  and positioning a substrate temperature sensor within the hole. 
     FIG. 6  illustrates a side view of a flow sensor having thermal dams  601  in accordance with aspects of the embodiments. A thermal dam  601  can be produced using lithography and oxidation processes. A thermal dam  601  can also be produced using lithography, etch, and fill processes. When etch process are used to produce the thermal dam, the channel should be produced after the thermal dam  601  because otherwise the membrane can be freed from the substrate  101  during processing. The thermal dam can comprise silicon dioxide, silicon nitride, silicon oxynitride, or a polymeric material such as polyimide. 
     FIG. 7  illustrates a top view of a flow sensor having thermal dams  601  in accordance with aspects of certain embodiments.  FIG. 7  is similar to  FIG. 3  with the addition of the thermal dams  601 .  FIG. 7  illustrates a view of the system of  FIG. 6  from above. 
     FIG. 8  illustrates a bottom view of a substrate  101  having a trapezoidal channel  804  in accordance with aspects of certain embodiments. The channel  804  has a sloped sidewall at the inlet  801  and at the outlet  802 . Trapezoidal channels leave thicker sections of substrate at the inlet  801  and outlet  802 . Flow sensors with trapezoidal channels are more robust than those with straight channels, such as those illustrated in  FIGS. 2-3 . A more robust sensor can better survive handling and dicing operations. A common semiconductor manufacturing operation, dicing is the process of cutting individual units from a substrate that can have thousands of units. A cut line  803  is illustrated to facilitate understanding of the cut view of  FIG. 9 . 
     FIG. 9  illustrates a cut view of a substrate  101  having a trapezoidal channel  804  in accordance with aspects of certain embodiments. The cut view is edge on along the cut line  803  of  FIG. 8 . The thicker material and sloping sidewalls of the channel  804  at the inlet  801  and outlet  802  can be seen. 
     FIG. 10  illustrates a top view of a flow sensor having a thermal dam  1001  enclosing the membrane in accordance with aspects of certain embodiments. Thicker material at the inlet and the outlet, as illustrated in  FIGS. 9-10 , can increase a flow sensor&#39;s sensitivity to spurious heat flows conducted by the substrate. Introducing thermal dams at the inlet and the outlet can mitigate this sensitivity. As such, a continuous thermal dam  1001  encloses the area of the membrane containing the upstream temperature sensor, the heater, and the downstream temperature sensor. 
     FIG. 11  illustrates a high level flow diagram of producing a flow sensor in accordance with aspects of the embodiments. After the start  1110  a substrate is obtained  1102 . A heater and temperature sensors, such as upstream temperature sensors, downstream temperature sensors, inlet temperature sensors, and substrate temperature sensors are produced  1103  on the top of the substrate. Next, a channel is formed  1104  on the bottom of the substrate and the process is done  1105 . Further processing, such as dicing, may be necessary. Furthermore, electrical connections to the heater and temperature sensors can be established. Those skilled in the art of semiconductor processing know of many methods, such as producing contact pads and wire bonding, for establishing electrical contact to components on a substrate. 
   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.