Abstract:
A micro pressure sensor for inclusion within a low-pressure microelectronic device enclosure. The micro pressure sensor employs an electric field created by applying a large voltage potential difference to tiny conductive elements within the micro pressure sensor. Electrons emitted via the influence of, and accelerated by, the electric field collide with gas molecules to produce positive ions. The positive ions are then accelerated toward a conductive element coupled to a circuit. The current generated by the ions within the circuit coupled to the micro pressure sensor can be measured to determine the internal pressure within the low-pressure enclosure. The micro pressure sensor is manufactured by standard semiconductor fabrication techniques, and can be economically produced in large volumes.

Description:
TECHNICAL FIELD  
         [0001]    The present invention is related to low-pressure microenclosures for maintaining a low-pressure environment around a microchip or other microelectronic device and, in particular, to a micro high-vacuum pressure sensor included within a low-pressure microenclosure to measure internal pressures in the range of 10 −1  to 10 −8  Torr.  
         BACKGROUND OF THE INVENTION  
         [0002]    During the past forty years, extremely precise, complex, and elegant methodologies have been developed in the field of semiconductor fabrication in order to mass produce complex integrated circuits used as processor and memory components within computers. Computer control is being applied to many different types of technological areas. Microchips and other devices produced by semiconductor fabrication techniques have become common components in a wide variety of electromechanical devices and systems, including automobiles, communications systems, machine tools, and many others. More recently, semiconductor fabrication techniques have been applied to manufacture tiny electromechanical devices in an emerging technological field referred to as micro-electromechanical systems (“MEMS”).  
           [0003]    Certain MEMS devices require very low-pressure, partial-vacuum environments in which to operate. FIG. 1 illustrates one type of low-pressure MEMS device. A microfabricated MEMS device  101  is enclosed within an airtight microenclosure  103  in order to maintain an internal low-pressure environment with internal pressures below 10 −4  Torr. The microfabricated MEMS device  101  is coupled to external circuitry via internal signal lines  105  and a connector or adaptor  107 . The microfabricated MEMS device  101  may be a microchip containing many hundreds or thousands of miniature mechanical, or electromechanical, components. The airtight microenclosure  103  may have linear dimensions on the order of a few inches to factions of an inch.  
           [0004]    Although a low-pressure microenclosure can be manufactured with an internal pressure below 10 −5  Torr, the pressure within a microenclosure may gradually increase with time due to leakage, sublimation of microenclosure or microdevice materials, or vaporization of metallic layers during operation of the microfabricated MEMS device within the microenclosure. Once the internal pressure rises above a certain threshold value, the performance of the enclosed MEMS device may degrade below acceptable performance ranges or fail altogether. Once performance of the enclosed MEMS device degrades, or the enclosed MEMS device fails, a device containing the enclosed MEMS device as a subcomponent may, in turn, suddenly fail.  
           [0005]    A number of different MEMS pressure-related devices, including MEMS pressure sensors, have been developed. FIGS.  2 A-B show two parts of a typical MEMS pressure-sensing device. The pressure sensing device relates the difference in capacitance between a sensor cell, shown in FIG. 2A, and a reference cell, shown in FIG. 2B, to the pressure within an environment containing the MEMS pressure sensor. The MEMS pressure sensor device is fabricated by standard semiconductor fabrication techniques from doped silicon substrates, silicon dioxide layers, and empty cavities etched out from between silicon dioxide and doped silicon layers. The sensor cell  202  and reference cell  204  are quite similar in structure. The sensor cell  202  comprises a p-type silicon substrate  206  in which an n-well  208  is formed by standard semiconductor fabrication techniques. An empty cavity  210  lies above the surface of the n-well  208 . The walls of the cavity are formed from a field oxide layer  212 . A thin elastic diaphragm  214  comprising a poly silicon layer overlies the empty cavity. An additional silicon dioxide layer  216  lies above the elastic diaphragm  214 . The additional silicon dioxide layer  216  is etched to produce a rectangular boss  218  resting on the elastic diaphragm  214  above the empty cavity  210 . Environmental pressure pushes the boss, and diaphragm on which it rests, inward into the empty cavity  210  until the pressure within the empty cavity  210  is equal to the environmental pressure. The elastic diaphragm  214  and n-well  208  together form parallel plates of a capacitor, and the amount of charge stored within the capacitor for a given voltage differential applied to the parallel plates is inversely proportional to the distance between the plates. The reference cell  204  is nearly identical to the sensor cell, with the exception that the top silicon dioxide layer  220  of the reference cell is not etched to create a boss, and additional columns  222 - 224  are left in the field oxide layer of the reference cell  226  so that the diaphragm  228  of the reference cell remains at a constant distance from the n-well  230  of the reference cell. By measuring the difference in the charge stored within the reference cell capacitor to the charge stored within the sensor cell capacitor, the inward displacement of the boss  218  within the sensor cell relative to the distance between the diaphragm  228  and the n-well  230  of the reference cell the can be electronically measured. The measured displacement is then directly related to the environmental pressure surrounding the sensor cell.  
           [0006]    Unfortunately, diaphragm-based MEMS pressure sensing devices are not very sensitive to low-pressure differentials in the 10 −1  Torr to 10 −8  Torr range desirable for many low-pressure MEMS devices in low-pressure microenclosures, described with reference to FIG. 1. Furthermore, diaphragm-based MEMS pressure sensors can degrade in performance over time with the accumulation of debris within the empty cavity. Other types of pressure sensors are far too bulky to include within low-pressure enclosures for microfabricated MEMS devices, and reliable, sensitive, and economical techniques for externally measuring pressure within a low-pressure enclosure have not been developed. Thus, designers and manufacturers of low-pressure enclosures for microfabricated MEMS devices and other microelectronic devices and circuits have thus recognized the need for a method and system to accurately monitor the pressure in such low-pressure enclosures.  
         SUMMARY OF THE INVENTION  
         [0007]    The present invention provides a micro high-pressure pressure sensor that may be fabricated by well-known microchip fabrication techniques for inclusion within a low-pressure microchip enclosure. In a first class of embodiments, small conductive features fabricated on the surface of a dielectric layer are separated by a small distance and held at a high voltage potential differential, creating an electric field between the features. A self-sustaining discharge forms in this field. When discharged electrons collide with a gas molecule, they may provide sufficient energy to ionize the gas molecule creating another free electron. Secondary emission will supply additional electrons when the ions recombine at a cathode surface creating a detectable electric current in a circuit coupled to the cathode. This current can be detected within the circuit by well-known current measurement devices, and is related to the internal pressure within the low-pressure enclosure. In a second class of embodiments, gas molecules are ionized by electrons emitted by a field emitter or thermionic emitter, and resulting positive ions are attracted to an ion collection surface included in an circuit in which the current contributed by recombination of the ions on the ion collection surface can be detected by well-known current detection methods. As in the first class of embodiments, this current is related to the pressure within the low-pressure enclosure. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    [0008]FIG. 1 illustrates one type of low-pressure MEMS device.  
         [0009]    FIGS.  2 A-B show two parts of a typical MEMS pressure-sensing device.  
         [0010]    [0010]FIG. 3A is a first embodiment of a micro pressure sensor.  
         [0011]    [0011]FIG. 3B shows a second embodiment of the micro pressure sensor.  
         [0012]    [0012]FIG. 3C illustrates a third embodiment of the micro pressure sensor.  
         [0013]    [0013]FIG. 4A shows a fourth embodiment of the micro pressure sensor.  
         [0014]    [0014]FIG. 4B shows a fifth embodiment of the micro pressure sensor. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0015]    The present invention is related to low-pressure enclosures for microfabricated MEMS devices and other microelectronic devices that require low-pressure environments for operation. Following manufacture, the initial low-pressure environment within a low-pressure enclosure may degrade over time, leading to increased pressure and eventual failure of the enclosed microelectronic device. In order to anticipate such failures, it is desirable to measure the pressure within the low-pressure pressure enclosure continuously or at regular intervals.  
         [0016]    Because of the small size of the low-pressure enclosure, it is difficult to measure internal pressure by external methodologies. The inventors of the present invention recognized that it would be desirable to include a tiny electronic pressure sensor within the low-pressure enclosure that is electronically coupled to a circuit, so that the internal pressure within the low-pressure enclosure can be continuously monitored or monitored at regular intervals without breaching the low-pressure enclosure or requiring disassembly of an electronic device in which the low-pressure enclosure is contained.  
         [0017]    A first set of embodiments of micro pressure sensors that represent alternative embodiments of the present invention is illustrated in FIGS.  3 A-C. This first set of embodiments includes alternative embodiments in which small conductive features fabricated on the surface of a dielectric layer are separated by a small distance and are coupled to an external voltage source to induce a relatively large voltage differential between the conductive features, producing an electric field between the conductive features. A self-sustaining discharge forms in this field. Electrons drift towards the anode. When electrons collide with a molecule, they may provide sufficient energy to ionize the molecule creating another free electron. Secondary emission will supply additional electrons when the ions recombine at the cathode surface. At higher gas densities, the chances of ionizing collisions will increase, creating a higher current flow between the cathode and anode. Cosmic rays or noise initiates this self-sustaining discharge. This current can be related to the internal pressure within the low-pressure enclosure by an empirical formula, such as:  
         I=αP x    
         [0018]    where α is a constant  
         [0019]    P is the internal pressure, and  
         [0020]    x is an empirically derived value, generally between 1.1-1.4  
         [0021]    [0021]FIG. 3A is a first embodiment of a micro pressure sensor. A first conductive feature  302  and a second conductive feature  304  are embedded within a cavity  306  within a dielectric substrate  308  formed by bonding together two substrate components  310  and  312  related by mirror-plane symmetry that each includes a single conductive feature ( 302  and  304 ) along the bottom of a well-like cavity. The empty cavity  306  within the completed device communicates with the low-pressure environment within an enclosure containing the device via one or more apertures  314 .  
         [0022]    [0022]FIG. 3B shows a second embodiment of the micro pressure sensor. In the second embodiment, three parallel conductor strips  322 ,  324 , and  326  are fabricated on a non-conducting substrate  328  or a substrate with a surface dielectric layer. The outer strips  322  and  324  are electrically coupled to a conductive element  330  that may be coupled to an external circuit. The third inner strip  326 , parallel to the two outer strips  322  and  324 , is placed midway between the two outer strips. The third inner strip  326  is also coupled to the external circuit. An electric field is generated between the two outer strips  322  and  324  and the third inner strip  326 , and the device operates according to the above-described principles.  
         [0023]    [0023]FIG. 3C illustrates a third embodiment of the micro pressure sensor. In this third embodiment, annular conductive strips  332  and  334  serve the same role in the alternative pressure sensor as the outer conductive strips  322  and  324  in the second embodiment described with reference to FIG. 3B. In the third embodiment, a middle annular conductive strip  336  plays an analogous role to the inner conductive strip  326  in the second embodiment described with reference to FIG. 3B. The inner and outer annular strips  332  and  334  may be coupled to a conductive element  338  embedded within the substrate  340  on which the annular conductive strips are deposited or to which the annular conductive strips are affixed, and the middle annular conductive strip  336  may be coupled to a second embedded conductive element  342 . Operation of the third embodiment is equivalent to that of the first and second embodiments discussed with reference to FIGS.  3 A-B.  
         [0024]    A second set of embodiments of micro pressure sensors that represent alternative embodiments of the present invention is illustrated in FIGS.  4 A-B. In this second set of embodiments, a field emitter component is microfabricated on a semiconductor or dielectric substrate and held at a large voltage potential differential with respect to an anode target. The field emitter emits a beam of electrons that are accelerated in the electric field between the field emitter and the target anode. Gas molecules that stray within the beam of electrons may be ionized to generate positive gas molecule ions and additional free electrons. The ions are attracted to a third conductive surface held at a negative potential with respect to the field emitter device. When these positive gas molecule ions recombine at the third conductive surface, a small ion-induced current can be detected within a circuit coupled to the third conductive surface, and this additional ion-induced current will reflect the internal pressure within the low-pressure enclosure in which the micro pressure sensor is included. The relationship between the internal pressure within the low-pressure enclosure and the ion-induced current can be described by an empirical formula similar to the empirical formula shown above describing the ion-induced current in the embodiments of the first set of embodiments. However, for this embodiment, the value of the exponent, “x,” is close to 1.  
         [0025]    [0025]FIG. 4A shows a fourth embodiment of the micro pressure sensor. In the fourth embodiment, one or more field emitter tips  402  microfabricated on a semiconductor or dielectric substrate  404  via interleaving layers of dielectric and metallic substances  406 , and held at a large voltage potential differential with an anode target, that emits a beam of electrons that are accelerated in an electric field towards the target anode  408 . This beam of electrons ionizes gas molecules that stray into the path of the beam, generating positive ions that are attracted to a third conductive surface  410  held at negative potential with respect to the base of the field emitter tip. When these positive ions recombine at the third conductive surface  410 , a small ion current can be detected within an external circuit coupled to the third metal surface, and this current will reflect the internal pressure within the low-pressure enclosure in which the micro pressure sensor is included.  
         [0026]    [0026]FIG. 4B shows a fifth embodiment of the micro pressure sensor. This fifth embodiment is quite similar to the fourth embodiment, described above, with the exception that, in place of the field emitter tip  402 , a filament  412  suspended within a cavity  414  of a substrate  416  is used as a thermionic emission device. The filament, acting as a resistive heating element, emits electrons. As in the fourth embodiment, emitted electrons are accelerated towards a target anode  418  and ionized gas molecules are accelerated towards a conductive surface  420  held at a negative potential with respect to the field emitter filament  412 .  
         [0027]    The various embodiments of the micro pressure sensor of the present invention may be sensitive to pressure changes within a range of pressures from 10 −1  Torr to 10  −8  Torr and lower. The strength of the electric fields required to ionize gas molecules, in the first set of embodiments, or to drive the field emission devices, in the second set of embodiments, may fall within a range of 5 to 50 Volts per micron of separation between conductive elements. Voltage potentials, electric field strengths, and other parameters of the devices will vary with varying geometries, varying composition of the conductive elements, the design, size, and electrical properties of external circuits to which the devices are connected, and other such variables.  
         [0028]    Although the present invention has been described in terms of a particular embodiment, it is not intended that the invention be limited to this embodiment. Modifications within the spirit of the invention will be apparent to those skilled in the art. For example, rather than the continuous conductive strips shown in the second and third embodiments, a series of raised conductive features, or microbumps, may be employed to generate the electrical field and to serve as anodes and cathodes for electron emission and ion adsorption. In addition, a permanent magnet can be affixed above or below the plane of the micro pressure sensor to induce electrons accelerated in the electric field between conductive elements to travel in spiral paths, increasing both their time of flight and the probability of their collision with gas molecules. Micro pressure sensors of a variety of sizes using a variety of different substrate and conductor materials may be fabricated to provide desired sensitivity with respect to acceptable voltage potential differentials applied to the conductive elements of the micro pressure sensor. All five embodiments discussed above can be fabricated using any number of different well-known semiconductor microchip fabrication techniques. Additional geometries and configurations for the components of the three micro pressure sensor embodiments may be used to produce micro pressure sensors with a variety of desirable physical characteristics. Current sensing circuits integral to the pressure sensing functionality of the micro pressure sensor may be external to the low-pressure enclosure containing the micro pressure sensor, may be internally co-located with the micro pressure sensor, or may be integrated within the micro pressure sensor.  
         [0029]    The foregoing description, for purposes of explanation, used specific nomenclature to provide a thorough understanding of the invention. However, it will be apparent to one skilled in the art that the specific details are not required in order to practice the invention. The foregoing descriptions of specific embodiments of the present invention are presented for purpose of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously many modifications and variations are possible in view of the above teachings. The embodiments are shown and described in order to best explain the principles of the invention and its practical applications, to thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents: