Abstract:
The use of robust substrates on the surface micro-machined structures combines (1) the use of micro-machining technology; (2) the use of electronic packaging technologies; and (3) the use of conventional machining techniques to create a new class of micro-machined structures. A particular robust substrate-based micro-machine structure is a capacitive pressure sensor that includes a pressure sensitive diaphragm and an electrode.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This is a Division of application Ser. No. 09/326,272, filed Jun. 4, 1999. 
     The present application claims priority to and the benefit of the filing date of Provisional Patent Application Ser. No. 60/088,063 filed Jun. 5, 1998, entitled “Robust-Substrate-Based Micro-machining Techniques and Their Application to Micro-machined Sensors And Actuators,” which, in its entirety, is hereby incopororated by reference in this application. 
    
    
     FIELD OF THE INVENTION 
     The present invention generally relates to the field of integrated circuits, and in particular to robust substrate-based micromachined devices. 
     BACKGROUND OF THE INVENTION 
     As known in the art, micro-fabrication processes are utilized to construct miniature devices that can be batch fabricated at a relatively low cost. In this regard, multiple devices are typically manufactured on a single wafer during micro-fabrication. Well known micro-fabrication techniques are used to form similar components of the multiple devices during the same manufacturing steps. Once the multiple devices have been formed, they can be separated into individual devices. Examples of micro-fabrication techniques that allow the batch fabrication of multiple devices include, but are not limited to, sputtering, evaporation, etching, electroforming (e.g., electroplating, electrowinning, electrodeposition, etc.), packaging techniques (e.g., lamination, screen printing, etc.), photolithography, and thick or thin film fabrication techniques. Since a large number of devices can be formed by the same micro-fabrication steps, the cost of producing a large number of devices through microfabrication techniques is less than the cost of serially producing the devices through other conventional techniques. It is therefore desirable, in many applications, to fabricate devices through micro-fabrication techniques. 
     Micro-machining is a new technology used to realize microstructures by exploiting common micro-fabrication techniques, e.g., integrated circuit processing technologies. Various microstructures (in particular, miniaturized sensors and actuators) have been realized using this technology; for example, accelerometers, pressure sensors, air flow sensors, micromotors, and micropumps. Since the root of micro-machining technology is in integrated circuit processing technology, micro-machined devices have been primarily realized using silicon substrates. In many applications, the use of traditional silicon-substrate in micro-machined devices may be limited. One limitation for example, is the lack of ability of a surrounding silicon substrate to absorb large mechanical shocks and forces in a harsh environment. Another limitation is the difficulty is interfacing silicon electronics to micro-machined devices using packaging techniques. Thus, heretofore unaddressed needs exists in the industry to address the aforementioned deficiencies and inadequacies. 
     SUMMARY OF THE INVENTION 
     As will be explained more fully hereinafter, the present invention is the application of robust substrates for both bulk and surface micromachined structures, as well as forming essential structural components of the device package on the robust substrates. 
     In particular, the use of robust substrates in the surface micro-machined structures described herein is unique in that it combines (1) the use of micro-machining technology (such as film deposition and etching, substrate etching, electroplating, lithography, etc.); (2) the use of electronic packaging technologies (such as screen printing, lamination, chip assembly, etc.); and (3) the use of conventional machining techniques (e.g., drilling, polishing, etc.) to create a new class of micro-machined structures. 
     These new structures have numerous advantages including low cost, large area fabrication, co-fabrication of micro-machined devices and their packages, and incorporation of new materials into micro-machined systems. Furthermore, structures that are impractical to fabricate using solely traditional micro-machining techniques are now possible. 
     To maintain robustness, in a preferred embodiment of the invention, metal micro-machined devices use metal shim stock and electroplating technology to fabricate the devices. Many metal shim stocks with appropriate thickness can be good candidates for robust-substrate-based micro-machining. 
     As mentioned above, one of the advantages of the use of a robust substrate is the possibility of the co-fabrication of a package and micro-machined device using the robust substrate as both a substrate and package. 
     Another advantage is the robustness of the robust substrate based micro-machined devices. These robust micro-machined devices have the potential to be used in mechanically harsh environments. 
     In an alternative embodiment, a sensor is fabricated using the robust substrate. The robust substrate based pressure sensor is fabricated using micro-machining technology in combination with conventional machining. In particular, a robust substrate based pressure sensor is fabricated using stainless steel as a substrate, Kapton™ polyimide film as a pressure-sensitive diaphragm, and electroplated nickel as a back electrode. 
     Other features and advantages of the present invention will become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional features and advantages be included herein within the scope of the present invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A &amp; 1B are cross-sectional views of the formation of electronics on a flat substrate and a machined substrate, which have microstructures, and bonding of the two substrates to form a micro-machined device. 
     FIGS. 2A-2D are cross-sectional views of the formation of a micro-machined device on a robust substrate using a robust-substrate-based surface micro-machining technique. 
     FIG. 3 is a schematic diagram of the cross-sectional view of the micro-machined capacitive pressure sensor on a robust substrate. 
     FIGS. 4A-4C are schematic diagrams of a fabrication sequence for a robust capacitive pressure sensor array. 
     FIG. 5 is a plan-view of the electrical corrections for the micro-machined capacitive pressure sensor array fabricated in FIG.  4 A. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference now to the drawings wherein like reference numerals designate corresponding parts throughout the several views, a preferred embodiment of the present invention is a robust substrate micro-machined device fabricated using a combination of micro-machining technology, electronic packaging, and conventional fabrication technology. The robust substrate micro-machined devices include for example, accelerometers, pressure sensors, air flow sensors, actuators, micromotors, and micropumps 
     Many types of metal can be formed into shim stocks having an appropriate thickness for use in robust-substrate-based micro-machining. They can be used for both bulk and surface micro-machining. In traditional silicon micro-machining, bulk micro-machining is a process technique to sculpt microstructures using wet (potassium hydroxide [KOH], ethylene-diamine and pyrocatecol [EDP], and hydrazine) and dry (reactive ion etch) etching techniques in conjunction with etch masks and etch stops. Surface micro-machining is a technique useful when building free standing microstructures using sacrificial layers (PSG-polysilicate glass, polyimide, photoresists), which are dissolved away by a dry or wet chemical etchant that does not attack the structural parts. The building of free standing micro-structures is further described in U.S. Pat. No. 5,847,631, entitled “MAGNETIC RELAY SYSTEM AND METHOD CAPABLE OF MICROFABRICATION PRODUCTION” which is herein incorporated by reference. 
     Illustrated in FIGS.  1 (A &amp;B), are cross-sectional views depicting the robust-substrate-based bulk micro-machining technique of the preferred embodiment. In the first step, electronics packages  12  and  15  are framed on a flat substrate  11  and machined substrate  13  respectively, which have microstructures. In the bulk micro-machining using robust substrates, conventional machining tools can be used to create holes and pits to sculpt microstructures, such as pit  14  in the machined substrate  13 . Possible candidates for conventional machining tools include milling machines and electro-discharge machining (ESD). Wet chemical etchants are also possible candidates for creating bulk micro-machined microstructures. 
     As shown in FIG. 1B, the two substrates  11  and  13  are bonded together to form micro-machined devices. Subsequent bonding techniques, such as lamination, can create a robust bulk micro-machined device which may not require external packaging. 
     Illustrated in FIGS.  2 (A-D), are block diagrams illustrating the building of surface micro-machined devices using robust substrates. First, a robust substrate  21  is obtained. Next is the formation of bottom microstructures  22  and  23  as shown in FIG.  2 B. The deposition of sacrificial layer(s)  24  is shown in FIG.  2 C. The formation of top microstructures  22 A and  23 A is accomplished with the removal of the sacrificial layer(s)  24 . Since the robust substrates are used as substrates, wet and dry etchants and other chemicals used for subsequent processing should be selected so as not to attack robust substrates or the substrates should be protected with suitable materials. 
     Among many possible micro-machined accelerometers, pressure sensors, air flow sensors, micromotors, and micropumps and/or actuators fabricatable using robust substrate-based micro-machining, a surface micro-machined capacitive pressure sensor has been chosen for the illustration of these principles. 
     Illustrated in FIG. 3, is a schematic diagram of the cross-sectional view of a capacitive pressure sensor  30 . The capacitive pressure sensor  30  is based on the pressure-induced deflection of a metallized diaphragm  35  and the subsequent measurement of the capacitance between this deflecting diaphragm  35  and a fixed surface micro-machined backplate  38  suspended over the diaphragm  35 . The capacitive pressure sensor  30 , has an initial gap distance  39  between the micro-machined backplate  38  and the deflecting diaphragm  35 . The pressure is determined from the change in the capacitance of the capacitive pressure sensor  30 . The change in the capacitance is caused by the deflection of the diaphragm  35  at the center from the point of origin  33 . This deflection is caused by the applied pressure on the diaphragm  35  through the inlet hole  32 . The thickness and material of the deflecting diaphragm  35  determines the minimum and maximum pressure range of the capacitive pressure sensor  30 . 
     In another preferred embodiment, a capacitive pressure sensor  30  is fabricated using stainless steel as a substrate  31 (A &amp; B), Kapton™ polyimide film (Dupont HN 200) as a pressure-sensitive diaphragm  35 , and electroplated nickel as a back electrode  38 . The fabrication techniques for the capacitive pressure sensor  30  are especially suited for circuit integration, since the electronic packaging techniques used in its fabrication also allow definition and integration of externally fabricated silicon chips using various known chip-attach techniques. 
     Illustrated in FIGS.  4 (A-C), is a fabrication sequence of the robust capacitive pressure sensors  40 (A &amp; B). The process begins with milling an array of pressure inlet holes  46  in a stainless steel shim stock  41 . Lamination of a film  42  on the stainless steel stock  41  is performed. The regions of the film  42  over the pressure inlet holes  46 (A &amp; B) will form the pressure-sensitive diaphragms  35  (FIG.  3 ). A metallic layer  43  is deposited on the surface of the film  42  and patterned to form bottom electrodes, bonding pads, and electroplating seed layers  43 (A-F) for backplate posts 
     Multiple layers of a polylmide  44  are coated on the patterned metallic layer  43 (A-F), hard cured and dry etched to form plating molds. A metal layer is applied to form backplate posts  47 (A-D). Another metallic seed layer is deposited and patterned as electroplating molds for the fixed backplates  44 (A &amp; B). Then the backplates  44 (A &amp; B) are formed and the molds and metallic seed layers are removed. 
     Finally, a dry etch process is performed to remove the polylmlde molds for backplate posts  47 (A-D) and polyimide sacrificial layers to create air cavities  45 (A &amp; B) between the fixed backplates  44 (A &amp; B) and the pressure sensitive diaphragms. 
     In another preferred embodiment, the milling comprises an 8×8 array of pressure inlet holes  46  that are approximately 2 mm in diameter. It is preferred that the stainless steel shim stock is approximately 0.5 mm thick, and approximately 2¼ inch on a side. The lamination of the film on the stainless steel can be a Dupont Kapton™ HN200 (50 microns thick) polylmide film that is applied using a hot press. The metallic layer  43  of Ti/Cu/Ti is deposited on the surface of the Kapton™ film and patterned using a lift-off technique to form bottom electrodes  43 B and  43 E, bonding pads  43 (A, C, D &amp; F), and electroplating seed layers for backplate posts  47 (A-D). 
     The multiple layers coating the patterned metallic layer are preferably Dupont polylmide P12611. The multiple layer coating is hard cured (resulting in a final thickness of about 36 microns), and is anisotropically dry etched using reactive ion etching to form plating molds. The metal layer applied to form backplate posts is preferably nickel that is electroplated through the polyimide molds to form the backplate posts. The other metallic seed layer molds for the fixed backplates are preferably another Ti/Cu/Ti metallic layer that is deposited and thick photoresist (Shipley PR 5740) used for a pattern for the fixed backplates. The backplates are preferably electroplated nickel that is approximately 15 microns thick. The dry etch process is an isotropic dry etch process performed using a barrel plasma etcher (a gas mixture of O2 and CF4). 
     Illustrated in FIG. 5, is a top view of FIG. 4A illustrating the electrical connections to array  50  including a multiplicity of robust capacitive pressure sensors  30 . The pressure sensitive diaphragms  51  and the foundation for capacitive backplate posts  47 (A&amp;D) (FIG. 4B) are connected to the electrodes  56 (A-D) and  57 (A-D). These electrodes allow the robust capacitive pressure sensors  30  to conduct electrical signals to meters that can measure the capacitance of the device to determine the amount of supplied pressure. Alternatively, these electrodes can conduct electrical signals to pads suitable for surface mounting for flip-chip integration of electronic circuitry. 
     While specific apparatus arrangements for implementing a robust capacitive pressure sensor in accordance with the present invention are described, it should be understood that alternative apparatus arrangements are anticipated. Furthermore, it will be obvious to those skilled in the art that many variations and modifications may be made to the preferred embodiments as described above without substantially departing from the spirit and scope of the present invention. It is intended that all such variations and modifications be included within the scope of the present invention, as set forth in the following claims.