1.  Field of the Invention
The present invention relates to a heater for a microreaction chamber and in particular, to use of a thin-film transistor as the heating element in the microreaction chamber and method of making the same. 
2.  Description of the Related Art
The treatment of some fluids, whether liquid or gas, involves an increasingly precise temperature regulation.  It is frequently necessary to have very small amounts of fluid be rapidly elevated to a particular temperature under precise controls. 
Current inkjet technology relies on placing a small amount of ink within an ink chamber, rapidly heating the ink and ejecting it to provide an ink drop at a selected location on an adjacent surface, such as a sheet of paper.  Traditionally, ohmic resistors which heat up rapidly when current is passed therethrough have been used to provide the necessary temperature increase of the ink.  See, for example, a detailed discussion of ink ejection in an article titled xe2x80x9cThermodynamics and Hydrodynamics of Thermal Ink Jets,xe2x80x9d by Allen et al.,  Hewlett-Packard Journal, May 1985, pp.  20-27, incorporated herein by reference. 
DNA amplification processes also rely on precise temperature control in the various phases.  During various phases of the DNA amplification process, the fluid is required to undergo a number of thermal cycles.  As the DNA based fluid undergoes a number of cycles of being repeatedly heated and cooled within a selected temperature range, certain thermally activated biological/chemical processes are carried out. 
Microheaters are also used for optical switching based on a vapor bubble formation to deflect a light beam, optical switching of liquid crystals, and repeated heating of a biological fluid for decomposition detection of bioorganic compounds. 
In each of the above instances in use to date, an ohmic resistor through which a current is passed is used as the heating element. 
These devices comprise a semiconductor material body accommodating buried channels that are connected, via an input trench and an output trench, to an input reservoir and an output reservoir, respectively, to which the fluid to be processed is supplied, and from which the fluid is collected at the end of the reaction.  Above the buried channels, heating elements and thermal sensors are provided to control the thermal conditions of the reaction (which generally requires different temperature cycles, with accurate control of the latter), and, in the output reservoir, detection electrodes are provided for examining the reacted fluid.  The heat is generated by supplying electric current to a metal heating element formed on a wafer comprising a semiconductor body with contact regions in electrical contact with the two opposite ends of the heating element and connected to a drive transistor, typically a MOSFET formed on the same wafer. 
Microchips are highly suited for miniaturized heater applications.  Generally, present techniques for generating local heating in a microchip are based on ohmic resistors made of metal alloys, such as TaAl, HfB, ternary alloys, etc.,  or polycrystalline semiconductors.  The heating resistor is driven by external circuitry or an integrated power MOSFET.  In existing applications, such as thermal ink-jet printers, the heating resistor value is preferably higher than the MOSFET channel resistance (RON or RDS) to minimize the parasitic effects and dissipate power in the heating resistor only.  Normally, each power MOSFET occupies a large percentage of the chip area to minimize its RON.  
At present, various techniques allow thermal control of chemical or biochemical reagents.  In particular, from the end of the 1980s, miniaturized devices were developed, and thus had a reduced thermal mass, which could reduce the times necessary to complete the DNA amplification process.  Recently, monolithic integrated devices of semiconductor material have been proposed, able to process small fluid quantities with a controlled reaction, and at a low cost (see, for example, U.S. patent applications Ser.  No.  09/779,980, filed on Feb. 8, 2001; Ser.  No.  09/874,382 filed on Jun. 4, 2001; and Ser.  No.  09/965,128, filed Sep. 26, 2001; all assigned to STMicroelectronics, S. r. l. and incorporated herein by reference). 
One drawback with this arrangement is that the resistance of such ohmic resistors is fixed and cannot be modulated, thus limiting their flexibility.  Other drawbacks are that ohmic resistors are subject to material degradation (such as oxidation, segregation, etc.),  and electromigration, especially at high temperatures.  These phenomena limit their lifetime and are a concern for the reliability of devices that incorporate them into their design. 
Yet another drawback is power control.  Ohmic resistors, which are either current or voltage driven, dissipate a power that is a quadratic function of the parameters.  This results in poor control over their output, as small variations in current or voltage can cause significant fluctuations in power and temperature output. 
The present invention provides a miniaturized heater that provides the desired characteristics for many microfluidic and micromechanical applications, while overcoming the drawbacks noted above, while providing other related advantages. 
According to principles of the present invention, a thin-film transistor is used as the heating element for a microreaction chamber.  The channel of the thin-film transistor is used as the heat element.  The current passing through the channel raises the temperature of the channel itself to an elevated level.  The channel material as well as its size and properties are selected so that it has a known and desired temperature response for to current passing therethrough.  The gate voltage, and thus the amount of current is selected to provide a desired heat response to elevate the temperature of the channel region itself and thus heat any adjacent structures, such as a microreaction chamber. 
The thin-film transistor includes a channel region which is formed above a semiconductor substrate and separated therefrom by a dielectric layer.  The source and drain are contiguous with and directly connected to the channel layer and are also positioned above the semiconductor substrate.  The gate electrode is positioned within the semiconductor substrate and adjacent the channel region.  Placing a selected voltage on the gate electrode causes current to run through the channel region of a desired amount, providing a desired temperature increase.  A microreaction chamber is positioned adjacent the channel region so as the channel region temperature increases, the microreaction chamber is heated. 
According to one embodiment, an electrically insulating layer is positioned over the channel region to electrically separate the channel region from the microreaction chamber.  In an alternative embodiment, the dielectric layer is not needed and the channel region itself is directly exposed within the microreaction chamber. 
The microreaction chamber is formed from a layer of material which can easily be etched or micromachined.  For example, the microreaction chamber may be comprised of an organic polymer material.  In one embodiment, the microreaction chamber has a lid on the top thereof so as to provide an enclosure for the heat responsive reaction chamber.