Patent Publication Number: US-9425382-B2

Title: Sensor protective coating

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a divisional application of U.S. application Ser. No. 14/069,633, entitled “Sensor Protective Coating” and filed Nov. 1, 2013, now U.S. Pat. No. 9,199,840 issued Dec. 1, 2015, the entire disclosure of which is hereby incorporated by reference. 
    
    
     FIELD OF INVENTION 
     The present embodiments relate to microelectromechanical system (MEMS) sensor devices, and methods of their fabrication. 
     BACKGROUND 
     Microelectromechanical system (MEMS) sensor devices are often sealed within a cavity to protect fragile, moving structures. But some types of MEMS sensor devices are designed to be unsealed to enable the structures to interact with the surrounding environment. For example, a diaphragm of a MEMS capacitive pressure sensor device is typically exposed to the surrounding environment in the interest of developing an accurate indication of the ambient pressure level. 
     MEMS sensor devices are usually fabricated via surface micromachining procedures. The procedures are configured to produce mass quantities of the MEMS sensor devices on a wafer. The wafer is eventually sawed or diced into numerous individual die. Water is often used to cool the saw blade during dicing. Water may also be applied to the wafer to remove debris generated during dicing. 
     When MEMS sensor devices lack a protective cavity, the water used for saw blade cooling and wafer cleaning may impinge upon the fragile structures of the sensor devices. MEMS device structures, such as pressure sensor diaphragms, may be damaged as a result of such contact with the water or other debris created during the dicing process. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The components and the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. Moreover, in the figures, like reference numerals designate corresponding parts throughout the different views. 
         FIG. 1  is a schematic, simplified, cross-sectional view of a sensor device having a protective coating or layer of a polymer material in accordance with one embodiment. 
         FIG. 2  is a schematic, simplified, cross-sectional view of the sensor device of  FIG. 1  after a dicing process and removal of the polymer coating in accordance with one embodiment. 
         FIG. 3  is a flow diagram of an exemplary method of fabricating a sensor device in which a coating or layer of a polymer material is applied to reduce cracking during a sawing procedure in accordance with one embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS 
     Sensor device fabrication methods that protect fragile sensor components with a polymer coating or layer are described. Sensor devices fabricated in accordance with the fabrication methods are also described. In some embodiments, the polymer layer includes a photoresist material. The polymer coating protects the fragile sensor components during a dicing or saw process. The polymer coating may buffer the sensor components from the water used to cool the saw blade and/or clean the wafer during the dicing process, and/or buffer the components from debris created during the dicing process. The polymer coating may thus reduce or prevent cracking or fracture of sensor components either during fabrication (e.g., dicing and die assembly) or afterward (e.g., during use or handling in the field). 
     The sensor devices may lack a cap or other cavity-defining structure that would otherwise protect the components from the water. The fragile sensor components may be exposed to the environment by design. For example, the sensor device may be a MEMS pressure sensor having an exposed diaphragm to measure the ambient pressure level. 
     The polymer coating may alternatively or additionally reduce or prevent cracking from vibration. The sensor components may be released from the substrate or wafer for movement relative thereto. Once released, the sensor components are suspended or otherwise spaced from the substrate. The sensor components may be configured to deflect or bend relative to the substrate. Vibrations during the dicing process and/or other handling may cause the component to deflect or bend in an undesired manner, causing cracks to form. The polymer coating may dampen or reduce such vibrations. 
     The cracking reduced or prevented by the disclosed embodiments may be difficult to detect. The fractures may be partial or complete. Functional testing may thus not reveal the presence of the fractures. The sensor devices may not fail until the cracking propagates or otherwise worsens during use in the field or other handling. Eventually, the cracking may lead to a loss of hermeticity for the cavity between a membrane (e.g., a pressure sensor diaphragm) and a substrate. The size of the fractures may be such that the cracks can only be seen with a microscope after decoration. The disclosed embodiments may thus avoid the need for screening procedures, thereby beneficially saving time and resources during fabrication. 
     The disclosed embodiments may include patterning of the polymer layer. Photoresist or other polymer material(s) may be spun onto the wafer and subsequently processed, e.g., via photolithography, for patterning. The patterning may remove the polymer layer in selected areas, such as along scribe lines and/or bond pads. 
     The disclosed embodiments may include complete removal of the polymer layer after the dicing process. For example, the polymer layer may be removed after a die assembly process (e.g., die bonding). The removal of the polymer layer may involve, for instance, a plasma etch. In some cases, however, device structures or components may remain covered by the polymer layer throughout the remainder of the assembly and fabrication process and during use. 
     Although described below in connection with MEMS pressure sensor device examples, the disclosed embodiments may be useful in connection with fabricating various types of sensor devices. The sensor devices of the disclosed embodiments may include sensors other than pressure sensors. For example, the disclosed embodiments may be applied in the context of a MEMS strain gauge or an uncapped MEMS accelerometer. The disclosed embodiments may be useful in connection with any known or hereafter developed sensor devices having components or structures that are exposed to ambient during the dicing or sawing process, including those structures that are released prior to the dicing process. The disclosed embodiments may also be used in connection with sensors without components or structures that are exposed to ambient. 
     The sensor devices are also not limited to MEMS sensors or devices. For example, the coating of sensor structures may be applied in the context of piezoelectric or other structures. The disclosed embodiments are also not limited to sensor devices having a single sensor and/or single type of sensor. The number of sensors or sensor types within the sensor device may thus vary. 
     The disclosed embodiments are compatible with various packaging technologies and processes. For example, the sensor devices may be deployed in chip-scale packaging arrangements. The sensor devices may be subsequently attached to a circuit board using various solders, such as tin-lead (SnPb) eutectic solder and Pb-free solders. The sensors of the disclosed embodiments may be overmolded and deployed in accordance with surface mount technologies, such as quad flat no leads (QFN) packaging and land grid array (LGA) packaging. The sensors may be mounted in other types of packages. For example, the sensors may be packaged as a small outline integrated circuit (SOIC). 
       FIG. 1  is a cross-sectional view of a sensor device  10  that includes a substrate  12  and a number of sensor structures supported by the substrate  12 . The sensor device  10  is configured as a pressure sensor device. In this example, the sensor structures are configured in a multiple-cell arrangement, including a pressure sensor cell  14  and a reference pressure sensor cell  16  of the sensor device  10 . The sensor device  10  also includes a number of interconnects  18  and a number of bond pads  20  to provide, e.g., bias voltages, for the sensor device  10  and carry signals to and/or from the sensor device  10 . The interconnects  18  and the bond pads  20  are supported by the substrate  12  but otherwise shown in simplified form. The interconnects  18  and the bond pads  20  may be electrically connected with the sensor structures of the sensor device  10  at a location outside of the depicted cross-section. 
     The sensor device  10  is depicted in  FIG. 1  before a dicing process in which a die having the sensor device  10  is formed. A number of scribe lines (or lanes)  22  may be formed at the surface of the substrate  12  to facilitate the dicing process. The scribe lines  22  may form a scribe grid that defines the plurality of die to be formed from a wafer. Each scribe line  22  is shown in simplified form, and may include a groove in the substrate  12 . The groove may be deep enough to extend through an oxide or other surface layer of the substrate  12  and thereby a reach a bulk semiconductor portion of the substrate  12 . The groove may be filled, e.g., partially filled, with a material (e.g., polysilicon) for saw alignment and/or other purposes. The configuration and other characteristics of the scribe lines  22  may vary. 
     The sensor structures of the sensor cells  14 ,  16  include a membrane  24  and support structures to support the membrane  24  above the substrate  12 . In this example, the support structures include bases  26  and pedestals  28  extending from the bases  26  to support the membrane  24 . Each base  26  may be disposed on the surface of the substrate  12  to act as a footer or foundation for the membrane  24 . The sensor cells  14 ,  16  may share an inner one of the bases  26  as shown. Each pedestal  28  may extend upward to space the membrane  24  from the substrate  12 . In this example, each pedestal  28  supports a respective end of one of the membranes  24  to suspend a central segment or portion of the membrane  24  over the substrate  12 . The central segment of each membrane  24  may be bar-shaped, although other shapes may be used. The shape, arrangement, and other characteristics of the bases  26  and the pedestals  28  may thus vary from the example shown to accommodate other membrane configurations. 
     The pedestals  28  and the membrane  24  of each cell  14 ,  16  may be integrally formed. For example, one or more layers of polysilicon may be deposited to form each of the pedestals  28  and membranes  24 . In one embodiment, each pedestal  28  may be formed from a polysilicon layer having a thickness of about 2 microns, although other thicknesses, materials, and configurations may be used. 
     Each base  26  may include a dielectric material, e.g., silicon-rich silicon nitride, for isolation of the electrodes of the sensor cells  14 ,  16 . The thickness of the silicon nitride layer may be about 0.5 microns, although other thicknesses may be used. Each base  26  may also include a polysilicon or other conductive region to establish an electrical connection with the membrane  24 . 
     The membrane  24  may have a thickness of about 2 microns, but other thicknesses may be used. The shape, construction, and other characteristics of the membrane  24  may vary. For example, the membrane  24  may be a composite structure including any number of layers or materials. In some cases, the layers include both conductive and non-conductive layers. The membrane  24  may alternatively or additionally be patterned to define a conductive or electrode section, which may be disposed on an inward face of the membrane  24 . 
     Each sensor cell  14 ,  16  may include a fixed plate electrode  30  disposed on the surface of the substrate  12  beneath the membrane  24 . The fixed plate electrode  30  is spaced from the membrane  24  to define a capacitive spacing or gap that varies during operation. The fixed plate electrode  30  may include polysilicon or other conductive material. The fixed plate electrode  30  may be biased at a voltage relative to the membrane  24  via one of the interconnects  18 . The spacing between the membrane  24  and the substrate  12  may be about 0.4 microns, but other gap sizes may be used. 
     Each membrane  24  is released from the substrate  12  before sawing along the scribe lines  22 . The release process may include an etching process in which a sacrificial layer disposed between the fixed plate electrode  30  and the membrane  24  is removed. In some embodiments, the etching process is configured as a wet etch for removal of a sacrificial silicon dioxide layer. Other materials may be used for the sacrificial layer. In some cases, a dry etch process is used to remove the sacrificial layer. 
     The gap between the substrate  12  and each membrane  24  is sealed after membrane release. The gap is sealed with one or more dielectric layers. In this example, a lower dielectric layer  32  (e.g., silicon oxide) is deposited on the sensor structures, followed by an upper dielectric layer  34  (e.g., silicon nitride) on the lower dielectric layer  32 . The lower dielectric layer  32  may be formed with tetraethyl orthosilicate (TEOS). In one embodiment, the lower and upper dielectric layers  32 ,  34  have respective thicknesses of about 2.3 microns and 0.8 microns, although other thicknesses may be used. 
     The dielectric layers  32 ,  34  are patterned to completely cover and laterally surround the sensor structures of the reference sensor cell  16 . The dielectric layers  32 ,  34  cover the membrane  24  of the reference sensor cell  16  so that the ambient pressure level does not deflect the membrane  24  toward the fixed plate electrode  30 . A reference pressure level may thus be provided. 
     The dielectric layers  32 ,  34  also laterally surround the sensor structures of the sensor cell  14 . The dielectric layers  32 ,  34  are disposed along a periphery of the sensor cell  14 . In the embodiment of  FIG. 1 , the dielectric layers  32 ,  34  are also disposed along a periphery of the membrane  24  of the sensor cell  14 . The space between the membrane  24  and the substrate  12  may thus be sealed by the dielectric layers  32 ,  34 . In some embodiments, the seal may be hermetic to support the operation of the sensor device  10  as, for instance, a pressure sensor device. 
     An opening  35  is defined over the membrane  24  of the sensor cell  14 . The patterning of the dielectric layers  32 ,  34  thus allow the spacing or gap of the sensor cell  14  to be sealed without adversely affecting the operation of the sensor device  10 . 
     The sensor device  10  includes a polymer coating or layer  36  that covers the sensor structures of the sensor cells  14 ,  16 . The polymer coating  36  covers the membrane  24  of the sensor cell  14 . The polymer coating  36  may extend laterally across the sensor structures of the sensor cells  14 ,  16  to fill the opening  35  and cover the dielectric layers  32 ,  34 . In some cases, the polymer coating  36  extends slightly beyond the sensor structures of the sensor cells  14 ,  16  to ensure uniform coverage of the membrane  24 . 
     The polymer coating  36  protects the membrane  24  of the sensor cell  14 . The membrane  24  would otherwise be exposed to the ambient due to the opening in the dielectric layers  32 ,  34  and further to the absence of a cap or other enclosure of the sensor device  10 . 
     The protection is provided during fabrication of the sensor device  10 . The polymer coating  36  is useful during a dicing process during which water impinges upon sensor device  10  as a saw blade cuts through the substrate  12  along the scribe lines  22 . The polymer coating  36  may act as a buffer. The membrane  24  may also be protected by the polymer coating  36  during subsequent handling, e.g., during die assembly. As described below, the polymer coating  36  may eventually be removed to allow the membrane  24  to be directly exposed to the ambient. In other cases, the membrane  24  remains covered by the polymer coating  36  during use and operation, i.e., after the assembly and fabrication of the sensor device  10  is complete. 
     The polymer coating  36  may be configured to dampen vibrations of the membrane  24  during the dicing process. The polymer coating  36  may have a thickness over the membrane  24  of about 3 to about 4 microns, or more, although other thicknesses may be used. For example, an increased thickness may be used to further dampen the vibrations and/or provide further buffering from the water. 
     In the embodiment of  FIG. 1 , the polymer coating  36  does not cover the bond pads  20  or the scribe lines  22 . The polymer coating  36  may be patterned to avoid coverage of the bond pads  20  and/or the scribe lines  22 . 
     The patterning of the polymer coating  36  may vary to customize the lateral extent of the coverage. For example, the polymer coating  36  may be patterned to cover only the sensor cell  14 . Less coverage may be useful in cases where the polymer coating  36  is removed through a plasma etch process. In another example, the polymer coating  36  is patterned such that the scribe lines  22  remain covered. The plasma etch may be an oxygen-based (O 2 ) plasma ash etch. Other plasma etch processes may be used. 
     The polymer coating  36  may include a photoresist material. A variety of photoresist materials may be used. For example, SUMIRESIST™ positive photoresist available from Sumitomo Chemical Co., Ltd., and SU-8 negative photoresist available from MicroChem Corp. may be used as the photoresist material. Alternative or additional photoresist materials may be used. The photoresist material of the polymer coating  36  may be processed in accordance with a conventional photolithography procedure. For example, the photoresist material may be pre-baked before exposure and/or hard baked, as described below. 
     In other embodiments, the polymer coating  36  may include non-photoresist polymer materials. For example, a parylene or polyimide material may be used. In these embodiments, the polymer coating  36  may not be patterned, but instead form a universal coating over the wafer. The polymer coating  36  may be removed after dicing but before die bond. In other cases, the polymer coating  36  may be patterned in a conventional manner. In these and other cases, the polymer coating  36  may be removed either before die bond or after die bond. 
     The substrate  12  may include a semiconductor substrate. A variety of semiconductor materials may be used, including elementary semiconductors, such as silicon (Si), and compound semiconductors, such as gallium arsenide (GaAs) and gallium nitride (GaN). Other compound and non-compound semiconductor materials may be used. In the embodiment of  FIG. 1 , the substrate  12  includes (or is diced from) a silicon wafer  38 . Silicon dioxide layers  40 ,  42  are disposed on or along top and bottom sides of the silicon wafer  38 , respectively. The silicon wafer  38  and the silicon dioxide layers  40 ,  42  are not drawn to scale, as the silicon dioxide layers  40 ,  42  may be about several microns thick, while the silicon wafer  38  may be hundreds of microns thick. 
     The silicon wafer  38  may be a bulk semiconductor wafer. Other types of semiconductor substrates may be used. For example, the silicon wafer  38  or, more generally, the substrate  12 , may be configured as a semiconductor-on-insulator (SOI) substrate. In some cases, the substrate  12  is a composite substrate with any number of layers (e.g., a base layer and one or more epitaxial layers). 
     The substrate  12  may be bonded or otherwise attached to a handle or other wafer or substrate. For example, the substrate  12  may be bonded to an application-specific integrated circuit (ASIC) wafer. 
       FIG. 2  depicts the sensor device  10  after the dicing process. In this embodiment, the polymer coating  36  ( FIG. 1 ) has been removed from the opening  35  to expose the membrane  24  of the sensor cell  14  to the ambient. The polymer coating  36  is also removed from other sensor structures of the sensor device  10 , such as the structures of the reference sensor cell  16 , and the dielectric layers  32 ,  34  along the periphery of the membrane  24 . The polymer coating  36  may be removed via a plasma or other etch process, including the O 2  plasma etch described herein. 
     The sensor device  10  is shown in simplified form. In some cases, the removal of the polymer coating  36  ( FIG. 1 ) occurs after the implementation of die and other assembly procedures. However, bond wires and other assembly elements or packaging features of the sensor device  10  are not shown for ease in illustration. 
     As shown in  FIG. 1 , the substrate  12  has been sawed along the scribe line  22  ( FIG. 1 ) to define a die  50  of the sensor device  10 . The shape, size, and other characteristics of the die  50  may vary from the example shown. For example, edges of the die  50  need not be disposed adjacent the reference sensor cell  16  and the bond pad  20  as shown. 
     In other embodiments, the polymer coating  36  ( FIG. 1 ) is not removed as shown in  FIG. 2 . The sensor structures of the sensor device  10 , including the membrane  24 , may remain covered by the polymer coating  36  during operation. In another example, a plasma or other etch process to remove the polymer coating  36  is configured to only remove part of the polymer coating  36 . For example, the plasma etch process may be implemented for a shorter duration. The duration of the etch may be empirically or otherwise predetermined to achieve a desired thickness for the polymer coating  36 . The thickness of the polymer coating  36  may thus be reduced to a level suitable for operation. 
       FIG. 3  depicts a method  300  of fabricating a sensor device, such as the sensor devices described above. The fabrication method  300  includes the coverage of one or more sensor structures of the sensor device with a polymer coating to protect the structure(s) during fabrication. The method includes a sequence of acts or steps, only the salient of which are depicted for convenience in illustration. Additional, fewer, or alternative acts or steps may be implemented. For example, a gel coating act or procedure may be performed following the removal of the photoresist layer. 
     The method may begin with, or include, a number of fabrication procedures in an act  302  in which sensor structures (e.g., MEMS sensor structures) of a number of a sensor devices are formed on a wafer or substrate. The sensor structures may be configured as described above. Each sensor structure may correspond with a pressure sensor diaphragm or other membrane of the sensor device. Additional sensor structures of each sensor device, such as the structures of a reference sensor cell, may also be covered. Conventional surface micromachining procedures may be used. For example, the fabrication procedures may include an act  304  in which the sensor structure (i.e., the diaphragm or other membrane) is released. The release procedure may include a wet or dry etch to remove, for instance, silicon dioxide, disposed between the sensor structure and the substrate. Other techniques for forming a membrane or other sensor structure spaced from the substrate may be used. 
     The fabrication procedures may also include an act  306  in which the released sensor structure(s) is sealed. A number of dielectric layers may be deposited in the act  306  along the periphery of the membrane or other sensor structure to seal the spacing or gap between the released sensor structure and the substrate. A number of bond pads and other conductive (e.g., metal) structures of each sensor device may be formed for each die (e.g., deposited on the wafer) in an act  308 . 
     In an act  310 , one or more sensor structures of each sensor device are coated or otherwise covered with a polymer layer. The act  310  may be performed as the final procedure before wafer dicing. For example, the coating occurs after any release and sealing performed in the acts  304 ,  306 , as well as after formation of the bond pads and other conductive structures in the act  308 . 
     The polymer layer may include a photoresist material. In some cases, the polymer layer may thus be applied in a manner conventional to photolithography procedures. For example, the polymer layer may be applied via a spin coating procedure. The polymer layer may be pre-baked to remove any excess solvent. 
     In some embodiments, covering the sensor structures includes patterning the polymer layer in an act  312 . The polymer layer may be patterned such that the polymer layer does not cover the bond pad(s) for each die. Alternatively or additionally, the polymer layer may be patterned such that scribe lines along which the wafer is diced are not covered by the polymer layer. In some cases, coverage of the scribe lines may not be an issue if, for instance, the polymer material is sufficiently transparent or translucent. 
     The patterning may include conventional photolithography procedures. For example, the polymer layer may be subjected to a post-exposure bake before application of a developer. The portions of the polymer layer to be removed may be exposed (or not exposed) depending on whether positive or negative photoresist is used. The polymer layer may be subjected to a hard bake after exposure. The application and patterning of the polymer layer may include fewer, alternative, or additional procedures. 
     The wafer or substrate is then sawed or diced in an act  314  while the sensor structure(s) remain covered by the polymer layer. During the dicing process, water is provided to cool the saw blade in an act  316 . The water may be applied in a cooling jet or spray. In some cases, the cooling jet is provided as an atomized spray. The polymer layer may act as a buffer between the water jet impinging upon the wafer and the sensor structure(s), such as a diaphragm or membrane. Notwithstanding such buffer protection, the pressure of the water jet used to cool the sawing blade may be reduced to a minimum level. The polymer layer may also provide protection from water applied for cleaning the wafer (and/or resulting die) in an act  318 . The water used for cleaning may be provided as an atomized spray. The same and/or a different jet or spray may be used for saw cooling and wafer cleaning. 
     In an act  320 , a die assembly procedure is implemented for the die created during the dicing process. In this embodiment, the die assembly procedure is implemented before removal of the polymer layer. The die assembly procedure may include one or more die bond processes. For example, each die may be wire bonded to a lead frame. Other assembly or packaging frameworks may be used. In some cases, contactless mounting procedures may be used to reduce the propensity for damage. 
     In some embodiments, the polymer layer is removed in an act  322 . The polymer layer may be removed via a plasma etch process implemented in an act  324 . One suitable plasma etch process is an O 2  plasma ash process. Other etch processes may be used. For example, in photoresist embodiments, the polymer layer may be removed with a conventional resist remover liquid. 
     The order of the acts described above may vary. For example, the polymer layer may be removed before die assembly if, for instance, protection beyond the dicing process is not warranted or desired. 
     In one aspect, a method of fabricating a sensor device includes forming a plurality of sensor structures on a wafer, covering the plurality of sensor structures with a polymer layer, and dicing the wafer into a plurality of die while each sensor structure remains covered by the polymer layer. 
     In a second aspect, a method of fabricating a sensor device includes forming a plurality of MEMS sensors on a wafer, each MEMS sensor including a diaphragm, covering the plurality of MEMS sensors with a polymer layer, patterning the polymer layer, and dicing the wafer into a plurality of die while each diaphragm remains covered by the patterned polymer layer. 
     In a third aspect, a MEMS sensor device includes a substrate, a support structure supported by the substrate, a diaphragm supported by the support structure and spaced from the substrate, and a polymer layer covering the diaphragm. 
     The present invention is defined by the following claims and their equivalents, and nothing in this section should be taken as a limitation on those claims. Further aspects and advantages of the invention are discussed above in conjunction with the preferred embodiments and may be later claimed independently or in combination. 
     While the invention has been described above by reference to various embodiments, it should be understood that many changes and modifications may be made without departing from the scope of the invention. It is therefore intended that the foregoing detailed description be regarded as illustrative rather than limiting, and that it be understood that it is the following claims, including all equivalents, that are intended to define the spirit and scope of this invention.