Abstract:
A metalization process forms metal contacts having defined profiles for contact between microelectromechanical (MEMS) devices or chemical sensors with semiconductor devices. Gold contacts may be used for connecting the MEMS devices or chemical sensors to integrated CMOS devices. Gold contacts are deposited over a photoresist via having sidewalls for forming upwardly extending flanges. The metal contacts to the underlying semiconductor device, are formed using a polymethylmethacrylate (PMMA) etch back process for exposing and dissolving the gold metalization layer save the metal contact under a surviving portion of the etched back PMMA layer in a dimple of the gold layer over the photoresist via. The photoresist layer serves to form deep well gold contacts having upwardly extending flanges for connection to the MEMS devices or chemical sensors and to the integrated semiconductor devices.

Description:
FIELD OF THE INVENTION 
     The invention relates to the field of semiconductor processing. More particularly the present invention relates to etch back methods for forming sensor contacts during thin film semiconductor processing. 
     BACKGROUND OF THE INVENTION 
     The microelectromechanical systems (MEMS) are being manufactured using process steps often found in traditional semiconductor processes. MEMS fabrication services are becoming widely used in many desirable developmental variations. The use of MEMS technology often presents difficult challenges when integrating MEMS devices into and with compatible semiconductor devices and processes. The semiconductor processes cover many types of devices and materials. One such semiconductor device and process is complementary metal oxide silicon (CMOS) technology. The CMOS process has been traditionally used for fabricating fast low power digital devices. Most MEMS devices are analog type devices. Complete system designs often require the speed and accuracy of modern digital computer processing systems that are coupled to the real world using analog input and output devices. Complete system designs lead to the integration of digital devices and analog devices on a chip with the advantage of an economy of scale. However, such integration of different devices and the corresponding different process steps must be accomplished with inherent compatibility. Many analog devices operate using gold connector contacts because gold is a good electrical conductor that is also non-corrosive and durable. Aluminum is a good conductor, but is highly corrosive, and not desirable for use as an exposed conducting contact. Gold is a large atom, and gold atomic migration through the lattice structures of semiconductor devices often leads to a decrease in the mean time between failure as gold atoms function as an impurity when migrating from an original deposition site. Though highly conductive, gold and silver impurities near the gate junctions of metal oxide silicon (MOS) transistors can lead to premature failures. Hence, in CMOS semiconductor circuits, often polysilicon, aluminum, and tungsten are used as conductors to avoid the migration problem when using gold or silver. 
     A sensor contact metal, such as gold or tungsten, can be deposited in a contact via well leading to a semiconductor device in a preexisting semiconductor circuit. For example, a sensor contact metal can be deposited over the contact via well with a potentially corrosive analog sensor then being deposited onto the sensor contact metal. The sensor contact metal can then be covered by a deposited protection layer to protect the sensor and contact metal from corrosion when the sensor is exposed to the environment. As a preexisting example, a silicon substrate may have an aluminum conducting etch run that is covered by an insulation layer such as silicon dioxide. During photoresist application, mask exposure and development, a contact via is formed through the photoresist. Photoresist is usually applied by spinning a coating onto a silicon wafer. The silicon dioxide layer is then etched in the location of the photoresist via to form the contact via through the silicon dioxide layer. The photoresist layer is then removed leaving the silicon dioxide layer over the aluminum conductor excepting for the contact via through the silicon dioxide layer. The formation of the contact via through the silicon dioxide layer to the conductor etch run of the semiconductor is an initial starting process point for depositing sensor contact metal upon a buried conductor etch run, prior to then depositing the sensor on the contact metal. The metal sensor contact is deposited as a layer and then patterned. The metal sensor contact should have profile that mates to the profile of the contact via well and extends up and over the insulation layer for contact with the sensor. Often, the metal contact will have a dimple over the contact via well as the metal is deposited evenly over the contour of the contact via well. Various processes have been used to accurately form the profile of the metal sensor contact during well filing. 
     The tape liftoff process applies an adhesive tape to the deposited sensor contact metal layer. The adhesive tape makes adhesive contact with the sensor contact metal except over the contact via where the dimple is created in the surface of the metal sensor contact layer. As the contact metal being is deposited down into the contact via well, a surface dimple is created. As the adhesive tape is pulled away from the metal sensor contact layer, the contact layer is removed, except where the dimples are located. Hence, the metal sensor contact survives and remains in the contact via wells. The tape liftoff process is imprecise in forming a metal sensor contact profile and creates ragged edges and stresses in the metal contact, leading to separation failures. Liftoff patterning processes require stepped slopes in the contact wells and constrain the metalization layer to small thicknesses. The Liftoff processes are incompatible with good step coverage and deposition techniques, such as sputtering. 
     The subtractive process also first deposits a metal sensor contact layer. Patterned photoresist portions are formed over the contact wells, exposing the metal contact layer but not over the contact well. The metal sensor contact layer is removed by a dissolving solution. The metal sensor contact layer is dissolved save the protected metal sensor contacts under the patterned photoresist portions. Then, the pattern photoresist portions are removed exposing the metal sensor contacts that have upwardly extending flanges created on the side walls of the metal contact via and lying upon the insulating layer. The problem with subtractive process is that during the metal sensor contact layer removal step, the metal sensor contacts are undercut under the edges of the pattern photoresist portions leading to imprecise metal sensor contact profiles and flange formation. The metal sensor contacts may also fail to sufficiently adhere to the subsequently deposited sensor. 
     The chlorobenzene liftoff process uses a single photoresist layer to create large sized sensor contact profiles, the flanges of which can be large. The chlorobenzene liftoff process creates a lip in the photoresist layer that can be damaged during sputtering or heated depositions leading to imprecise formation of the sensor contact profiles. Chemical hazards are disadvantageously created when using exotic and unfamiliar chemicals, such as chlorobenzene, to modify the photoresist. 
     The multiple layer photoresist process uses multiple layers of photoresist that when respectively repeatedly applied, exposed and then developed, create a thick photoresist via through which the metal sensor contact is deposited to create a unique gold contact profile. The multiple layer photoresist process suffers from the repeated photoresist steps and requires very accurate process controls. 
     As such, conventional techniques for contact formation disadvantageously suffer from imprecise formations leading undesirable profiles of the metal sensor contact. Often, the metalization layer, including the metal contact can have undesirable contours, such as the metal contact dimples. Conventional etch back methods have been used to remove undesirable surface contours of previously patterned layers. The etch back methods are used for ensuring continuous step coverage and for reflattening the surface for further high resolution photolithography. That is, the etch back method is applied to previously patterned layers. In the case of the CMOS planar etch back method, a metal contact layer, such as tungsten, is deposited over the contact well creating a dimple in the metal layer over the contact well. Because further processes may require substantially flat surfaces, the dimple is removed by a planar etch back process. An insulating layer, such as glass, is reflowed by heat, onto a metal layer. Phosphosilicate glass is applied by chemical vapor deposition and can be reflowed at high temperatures of about 700C. The reflowed glass layer is then etched back to expose the metal layer having surviving portions of the reflowed glass in the dimples. Next, the metal layer is etched back down to the insulation layer where the contact well is then filled with the metal and the surface is then substantially flat. The tungsten layer is effectively patterned into the wells solely by the preexisting lithography. Next, the metal layer is again deposited on a flat surface forming a metalization layer with a flat surface. The flat metalization surface can then be etched to pattern the metal layer without having the contact well dimples. This CMOS planar etch back process provides a metal contact profile that has no dimples. However, the CMOS planar etch back process disadvantageously required two metal deposition processes and two metal etching processes, and results in flat metal sensor flanges that may be unsuitable for connection to MEMS sensors. These and other disadvantages are solved or reduced using the invention. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to provide a method for forming a metal contact in a contact well. 
     Another object of the invention is to provide a method for forming in a contact well, a metal contact having upwardly extending metal contact flanges. 
     Yet another object of the invention is to provide a method for forming in a contact well, a metal contact having a metal contact dimple. 
     Still another object of the invention is to provide a method for forming in a contact well, a metal contact having a metal contact dimple, and an upwardly extending metal contact flange. 
     Still a further object of the invention is to provide a method for forming in a contact well, a metal contact having a metal contact dimple, and an upwardly extending metal contact flange for connecting to a sensor. 
     A further object of the invention is to provide a method for forming in a contact well, a metal contact having a metal contact dimple, and an upwardly extending metal contact flange using polymethylmethacrylate (PMMA) etch back. 
     Yet a further object of the invention is to provide a method for forming in a contact well, a gold contact dimple, and an upwardly extending metal contact flange using PMMA etch back. 
     The method is directed to the fabrication of metalized well contacts, such as gold well contacts, for electrical connection between semiconductor microcircuits and microelectromechanical systems (MEMS) devices and sensors, using standard metalization and etch processes with a minimum of subsequent photolithographic processing tools and steps. The method can be performed on variously sized substrates. The method can be used in a variety of fabrication processes for integrating MEMS devices and sensors with semiconductor devices, and is particularly well suited for integrating chemical sensors with conventional metal oxide silicon (MOS) semiconductor processes, such as complementary metal oxide silicon (CMOS) processes. The method can be applied to MEMS devices integrated with conventional semiconductor processes, such as CMOS processes, that can not tolerate a heavy metal, such as a gold metal that acts as an impurity and leads to failure of many silicon devices. 
     In the preferred form, a complementary metal oxide silicon (CMOS) semiconductor process device, such as a CMOS amplifier having etch run connection under a metal contact well in an insulating layer, is connected to an organic sensor applied to a metal sensor contact in the metal contact well. In the preferred form, the contact metal is gold for electrochemical stability in the presence of a chemical sensor. The use of gold offers good electrical conductivity and high non-corrosiveness. The semiconductor device can be made on large diameter wafers and hence the method offers the potential of an economy of scale when integrating semiconductor processes with inherently incompatible MEMS devices and chemical sensors. 
     The method is particularly adapted to forming a metal contact with a desirable profile for secure contact with a corrosive chemical sensor. Particularly, a patterning layer, such as a photoresist (PR) layer, is deposited for forming a larger sized patterned contact via over the insulating layer via, for creating upwardly extending metal contact flanges. The patterned contact via effectively increases the well depth and width, while the sidewalls of the patterned contact via provide a bottom surface from which the flanges upwardly extend. The metalization contact layer, such as a gold layer, is deposited over the patterned contact via and insulation contact via in the insulation layer then forming the metal contact with the upwardly extending flanges and with a dimple in the metal contact layer over the contact via using a single metalization deposition step. After metalization, a thick planarization layer, such as a thick layer of PMMA is deposited for filling in the dimple. The planarization layer is then etched back exposing the metalization layer while the dimple and the upwardly extending flanges remain covered with the PPMA. The metalization layer is then removed save the metal contact protected by the PMMA within the contact dimple. The photoresist layer and the remaining PMMA in the contact dimple are then removed to expose the metal contact including the upwardly extending flanges. The flanges extend upward about the height of the PR layer. Hence, the PR layer is used to form the profile of the upwardly extending flanges of the metal contact. With the metal flanges extending upwardly, a chemical sensor or MEMS device can be deposited onto or connected to the upwardly extending flanges of the metal contact. The upwardly extending flange portion of the metal contact and the overall size and shape of the metal contact profile can be precisely formed. The metal contact is suitable for electrical contact between chemical sensors and the underlying semiconductor devices using a minimum number of process steps compatible with existing semiconductor processes. These and other advantages will become more apparent from the following detailed description of the preferred embodiment. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is diagram of layers after process deposition. 
     FIG. 1B is a diagram depicting the layers after polymethylmethacrylate (PMMA) etch back. 
     FIG. 1C is a diagram depicting the layers after gold pattern etch. 
     FIG. 1D is a diagram depicting the layers after sensor deposition. 
     FIG. 2 is a flow diagram of the gold contact PMMA etch back process. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     An embodiment of the invention is described with reference to the figures using reference designations as shown in the figures. Referring to both of the figures, a preexisting substrate  10 , such as a silicon substrate may be supporting a semiconductor device, such as a CMOS device, comprising a conducting portion, such as aluminum layer  12 , deposited during step  30 . An insulating layer  14  is deposited during step  32 . The insulating layer  14  is preferably a glass layer made of silicon dioxide. During step  32 , a contact via  15  is formed as a passageway through the insulating layer  10  to the semiconductor device conducting layer  12 . A via layer  16  is deposited over the insulation layer  14  during step  34 . The via layer  16  is a patternable and removable layer, such as a preferred patterned photoresist (PR) layer. The via layer  16  is processed to form a flange via  17  that is larger than and centered over the contact via  15 . After applying the via layer  16  to a desired thickness, exposing the via layer  16  for patterning, and developing for via layer  16  for flange via removal, the PR via layer  16  includes the flange via  17  that is substantially larger than the contact via  15  in the insulation layer  14 . Then, the metalization layer  18 , such as a sputtered gold (Au) layer, is deposited during step  36  over the via layer  16 , into the flange via  17 , into the contact via  15 , and onto the conducting layer  12 . As may be apparent, the via layer  16  serves to create a flange step in the dimpled portion of the metalization conduction layer  18  over the flange via  17 . Next, an etch back planarization layer  20 , such as a polymethylmethacrylate (PMMA) etch back layer  20 , is deposited typically by flooding during step  38 , over the conducting layer  18  and into the stepped dimple of the conducting metalization layer  18 . The PMMA, planarization layer  20  is then etched back during step  40 , to expose the conducting layer  18 , but not to expose the stepped dimple of the conducting layer  18  then forming a surviving PMMA dimple portion  22  of the PMMA layer  20  that remains in the stepped dimple portion  24  of the conducting layer  18 . The exposed portion of the conducting layer  18  is then removed during step  42 , typically by chemical etching, for exposing surviving portions of the via layer  16 , with the contact dimple portion, that is, the metal contact  24  surviving under the surviving PMMA dimple portion  22  of the PMMA planarization layer  20 . The surviving portion  22  of the planarization layer and the PR layer  16  are removed during step  44  to fully expose the metal contact  24 . As may now be apparent, the metal contact  24  has upwardly extending flanges, shown on the left side and right side of the profile of the metal contact  24 . Finally, a MEMS device, such as a chemical sensor  26 , can be connected to or patterned onto the substrate  10  by direct deposition onto the semiconductor device insulation layer  14  in electrical contact with the metal contact  24 . The metal contact  24  provide an electrical connection between the conduction layer  12  of a semiconductor device and the MEMS sensor device  26 . An upwardly extending flange of the metal contact  24  is in electrical contact with and can penetrate into the sensor device  26  so that the metal contact  24  makes good electrical connection between the sensor device  26  and the semiconductor device conductor  12 . 
     The via layer  16  has the flange via  17  having a sloped side wall that particularly serves to form a sloping surviving portion of the conducting layer  18  that becomes the upwardly extending flange of the metal contact  24  after removing the exposed portions of the conducting layer  18  and the patterning via layer  16 . Using PR for the via layer  16 , the PR will not be under cut over the conducting layer  18 . Modest proximal migration of the gold atoms about the contact  24  over the conduction layer  12  should be distal to locations where gold impurity might cause failure in a distal semiconductor device, not shown, connected to the MEMS device  26  through the conducting layer  12 . In this manner, MEMS devices or chemical sensor devices can be integrated with semiconductor devices on the same semiconductor substrate  10  without detrimental atomic migration effects. 
     The PMMA etch back method can use industry standard photoresist patterning processes on the wafer to define the pattern of the subsequently applied metalization layer  18 . The method takes advantage of high resolution photolithography, high reliability, and large wafer processes that can be performed in large volumes to offset the cost of expensive equipment without conflict to process compatibility. The method enables arbitrary metalization or conducting etch run patterning between the MEMS devices  26  and the semiconductor devices. The metalization or conducting run patterns are derived from high resolution photolithographic processes thereby conforming to and integrating with the underlying small scale semiconductor devices, such as CMOS semiconductor devices. 
     In the preferred form, the gold metalization layer  18  is applied over the wafer including the substrate  10  and over the PR layer  16  covering the whole wafer. The metalization layer can be deposited by vacuum evaporation or sputtering. A flowable thermoplastic planarization layer  20 , such as the PMMA layer  20 , is applied to the wafer and over the metalization layer  18 . Surface tension of the thermoplastic planarization layer  20  tends to form a nearly planar top surface over underlying contours including the underlying contact dimples. Typically, the PMMA is applied during spin coating as a solution of the thermoplastic material in a volatile solvent. The planarization layer  20  may also be applied by reflow during heating and melting of the thermoplastic material. Flowable inorganic substances may be used for the planarization layer  20  as well. The planarization layer  20  provides a thick flat surface that can be uniformly etched and removed down to the conduction layer  18  so as to cover the dimple portion  22  and protect the metal contact  24  including the upwardly extending flanges from being removed during chemical removal of the metalization layer  18 . The etch back is a planar surface removal process typically performed by etching or lapping at a controlled rate and uniformly across the wafer, such as by oxygen plasma processing, until the underlying metalization layer  18  is exposed at high points but not at low points of the underlying contours of the top surface of the metalization layer  18 . When PR is used as the via layer  16 , the photoresist tends to dominate the contour of the top surface of the conduction layer  18 . The exposed portion of the metalization layer  18  is etched selectively with conventional techniques that erode completely through the metal layer  18 . Typically, metalization often consists of two or three functional layers, for various functions including adhesion, diffusion barrier, and electrical conductivity. Sufficient metalization etching would then be needed to remove all of the functional layers of the metalization layer  18  in the exposed areas so as to expose the via layer  16  but not the metal contact  24 . 
     For isotropic etchants such as wet chemicals, some recession of the metal edge occurs as an over etch margin. The nearly vertical portions of the metalization, covering the slopes of the resist, tend to provide some sacrificial etch distance, allowing over etching to occur without degrading the lateral dimensions of the metalization pattern. The method is well suited for wet chemical etching. The flowable plastic of the planarization layer and the PR of the via layer should be removed by processes that do not attack the underlying wafer or the applied metalization layer  18 . For example, organic solvents or oxygen plasma may be used. 
     In a broad sense, the etch back method is positively used to create profile features of a metal contact, that is, the upwardly extending flanges of metal contact  24 . The etch back method is used to define the actual shape of the metalization layer  18  disposed on the via layer  16  and under the planarization layer  20 . The PR is used to not only pattern the metalization layer  18  into the contact via  15 , but also to form vertical aspects of the profile of the metal contact. The method is applicable to a variety of semiconductor materials and metals, such as polysilicon or tungsten. The use of a via layer  16 , such as the relatively vulnerable photoresist, under the conducting material layer  18 , offers an ability to uniquely form the conducting layer  18  in the vertical dimension, in combination with a subsequent etch back layer  22 , that is preferably a plastic coating planarization layer  20 . In the broad aspect, the method provides a process sequence for forming desired three dimensional conductive metal contact structures. In the preferred form, the contract  24  is used to electrically connect an underlying semiconductor device with an overlying MEMS device integrated on the same substrate, but is applied generally for forming the profile of the metal contact  24 . The method offers a standard semiconductor integration process for MEMS devices or chemical sensor devices with standardized semiconductor devices using a variety of conventional materials. Those skilled in the art can make enhancements, improvements, and modifications to the invention, and these enhancements, improvements, and modifications may nonetheless fall within the spirit and scope of the following claims.