Electrical resistors – Resistance value responsive to a condition – Ambient temperature
Reexamination Certificate
2002-05-16
2004-09-21
Easthom, Karl D. (Department: 2832)
Electrical resistors
Resistance value responsive to a condition
Ambient temperature
C338S013000, C073S204250
Reexamination Certificate
active
06794981
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to thermal sensors of fluids, such as fluid flow sensors incorporated into a robust package in microstructure form. For convenience sake the term “flow sensor” will be used generically hereinafter for such thermal sensors. The reader will appreciate that such sensors may be utilized to measure primary properties such as mass flow rate, temperature, thermal conductivity and specific heat; and that the heat transfers may be generated through forced or natural convection. The invention relates specifically to a cartridge or assembly which exploits the capabilities of a microstructure flow sensor. Even more specifically, a sensor of the Microbrick™ or microfill type is utilized having a central heating element and surrounding sensor arrays which are structurally robust and capable of operating in harsh environments. These Microbrick™ or microfill sensors include through-the-wafer interconnects thus providing very low susceptibility to environmental damage or contamination. The material of the sensor support structure is of thermal conductivity tailored to the application thus producing a more useful and versatile sensor, such as needed for high sensitivity or high mass flux fluid flow measurement or measurements in harsh environments.
2. Description of Related Art
Open microbridge structures such as detailed in U.S. Pat. 5,401,155, to Higashi et al., are well suited for measurements of clean gases, with or without large pressure fluctuations, since the microbridge structure is burst-proof. However, due to the open nature of the microbridge structure, condensates from vapor can be uncontrollably retained in the microbridge structure leading to uncontrolled changes in its thermal response, or output, making the structure susceptible to output error and poor stability.
The typical microbridge structure has a silicon die wire bonded at the top surface to a header, or substrate, carrying further electrical leads and/or electronics. Typically, such wire for the wire bonds would be a one mil gold wire. This wire has a tendency to retain particles suspended in the fluid, retain liquid condensates, increase undesirable turbulence, and shift flow response. Due to its thinness, the wire is also susceptible to damage in a high mass flux environment, such as high rate liquid flow, and upon attempts to clean the sensor.
Membrane-based sensors overcome some of the problems of the microbridge structure because there is no opening exposed to the fluid. More specifically, there is no opening allowing the fluid to enter the underlying structure. However, because the membrane is sealed over an isolation air space and subject to differential pressure induced stress signal errors, membrane based sensors have limited application in high pressure applications. Due to the physical configuration of the membrane, it can deform or burst as pressure differences (on either side of the membrane) increase above 100 PSI (pressure levels that are very possible in high mass flux environments). The heating/sensing elements on the top surface of the membrane sensors are also typically wire bonded to other components, leaving the problems of the wire in the flow path accumulating debris and possibly breaking during cleaning attempts.
While many different materials may be used to make a fluid flow sensor, the choice of material can drastically affect the sensor's performance. A preferable material making up the sensor substrate would have a relatively low thermal conductivity among other characteristics. This low thermal conductivity is necessary to maintain the sensitivity for the sensor. With this relatively low thermal conductivity, all heating/cooling effects presented to the various sensing elements are caused predominatly by the fluid to be sensed. Stated alternatively, it is important to ensure that heat is not transmitted through the substrate excessively, resulting in signal shorts.
The micromembrane structure discussed above provides a design approach that enables accurate thermal measurements to be made in harsh environments (condensing vapors, with suspended particles, etc.). Specifically, the mass of silicon immediately below the heater/sensing elements is greatly reduced or eliminated, thus limiting potential heat losses. Even in this structure, however, the selection of materials is critical—low thermal conductivity and appropriate material strength continue to be very important. A disadvantage of this structure is its sensitivity to differential pressure (across its membrane) which induces a stress in the sensing elements and results in uncontrolled output signal changes or errors.
In addition to the above referenced thermal characteristics, it is highly desirable for the overall flow sensor to be chemically inert, corrosion resistant, highly temperature stable, electrically isolated, and bio-compatible. Obviously, many of these characteristics are achieved by proper selection of materials. Further, these desired characteristics are necessary in light of the sensors' operating environment. The materials chosen must provide for a sensor which is capable of operating in harsh environments.
It would therefore be desirable to develop a flow sensor which is not susceptible to the above referenced problems. Specifically, the sensor would not be affected by vapor accumulation beneath the microbridge, and would not have exposed bonding wire near the heating and sensing elements. The desirable sensor would be structurally robust and thus capable of operating in harsh environments. Further, it would be desirable to develop a flow sensor which is not affected by signal shorts, thus capable of sensing high mass airflows and liquid flows. To accomplish this a desired flow sensor would include a robust substrate or die with relatively low thermal conductivity, high temperature stability, high electrical isolation, corrosion resistance, chemical inertness, and biocompatability. The design of such a structure would enable flow rate and thermal property sensing over wide ranges at high pressure. Further, this capability would provide trouble free operation in hostile environments. The desirable flow sensor and associated housing would also minimize dead volume and promote cost-efficiency, portability, and miniaturization. The sensor would also be adaptable to monitor the flow through a predetermined flow channel attached to the sensor.
SUMMARY OF THE INVENTION
The present invention details a microstructure flow sensor having a microsensor die with a Microbrick™ or microfill structure (each having a substantially solid structure beneath the sensing elements) and through-the-wafer electrical interconnections. This structure provides a robust sensor that is operable and accurate in many different applications, including harsh environments.
Additionally, the microstructure flow sensor may be incorporated into an assembly in order to achieve a robust sensing device. The assembly is a robust package, configurable so that it may be operably integrated into a microfluidic cartridge of the type used in lab-on-a-chip systems. The flow sensor in the assembly monitors the controlled flow of fluid in the cartridge and transmits signals though flex circuits indicative of that flow. The integration of the assembly and the cartridge yields the benefits of larger instruments through a smaller device.
The sensor features a flat, passivated, top surface overlying the heater and sensor elements to provide appropriate electrical isolation. Further, a die with through-the-wafer interconnections eliminates the need for bonding wires with their attendant problems as discussed above. In order to withstand a wide range of pressure levels and operate in harsh environments, the die structure is configured to be very robust. The die is made up of materials that have very low thermal conductivity, thus eliminating the possibility of undesired thermal signal shorts. For example, the die may be fabricated using various glass materials, alumina, or
Cabuz Cleopatra
Padmanabhan Aravind
Satren Ernest
Schwichtenberg Jay G.
Easthom Karl D.
Fredrick Kris T.
Honeywell International , Inc.
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