Abstract:
The present invention provides a wafer level chip size package having cavities within which micro-machined parts are free to move, allowing access to electrical contacts, and optimized for device performance. Also a method for fabricating a wafer level chip size package for MEMS devices is disclosed. This packaging method provides a well packed device with the size much closely to the original one, making it possible to package the whole wafer at the same time and therefore, saves the cost and cycle time.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present invention claims priority from Chinese Patent Application Ser. No. 200710131491.3 filed Aug. 31, 2007, entitled “Wafer Level Chip Size Package For MEMS Devices And Method For Fabricating The Same” by Wang et al., which is incorporated herein by reference for all purposes. 
     FIELD OF THE INVENTION 
     The present invention relates generally to wafer level chip size package, and more particularly to wafer level chip size package for micro-electromechanical system (MEMS) devices and methods for fabricating the same. 
     BACKGROUND OF THE INVENTION 
     With the miniaturization of electronic devices and increase of circuit density in semiconductor industry, technology of chip size package (CSP) is now under great development, of which the package size is similar to the semiconductor chip encased therein. The ratio of a typical CSP to the edge dimension is no larger than 120% as defined by Joint Electron Device Engineering Council (JEDEC). Compared with the conventional packaging technology, such as wire bonding, tape automatic bonding (TAB) and flip chip, CSP has following advantages: ultra-small package reaching the original size, full protection on the bare die, good electrical and heat performances, convenient for tests, easy to be welded, assembled and replaced. CSP is manufactured either in the form of individual chips diced from a wafer, or in a wafer form and then individual chip size packages are singulated from the wafer. The latter is referred to as a wafer level chip size package (WLCSP). 
     For WLCSP, generally a plurality of compatible pads formed in a peripheral arrayed type on semiconductor chips are redistributed through conventional redistribution processes involving a redistribution layer into a plurality of metal pads, sometimes called solder bumps, in an area array type. Since packaging and testing are performed at wafer level and then the wafer is diced, WLCSP has significant advantages as follows: Firstly, the process is optimized to perform packaging directly at wafer level, whereas, for prior art methods, the wafer has to be diced and classified before being packaged; secondly, packaging, marking and testing are all performed at wafer level; therefore the production cycle and cost are significantly reduced. 
     Shellcase Co. Israel developed its unique and advanced WLCSP technologies as ShellOP, ShellOC, and ShellUT to package image sensors, as disclosed in U.S. Pat. Nos. 6,646,289, 6,777,767 and 6,972,480. Unlike many packaging methods, the Shellcase process requires no lead frames or wire bonding. Briefly, ShellOP utilizes a glass/silicon/glass sandwiched structure to enable image-sensing capabilities through the actual packaging structure so as to protect the sensors from being contaminated by external environment. ShellOC adopts the same sandwich structure, but extra cavities are configured on a first glass by spinning photo-imageable epoxy on the glass to form a pattern with lithography technique, for accommodating the above imagers. Also, cavities enable the use of micro-lenses for enhancing image quality. ShellOC is thus a packaging solution to choice of image sensors with micro-lenses. In the ShellUT package, cavities are still kept but a second glass is removed so that the related package height is reduced. 
       FIG. 1  is a typical cross-section view of a packaged chip device by prior art ShellOC, wherein a first/top glass with cavity walls thereon covers compatible pads furnished silicon chip. An epoxy is used to bond a second/bottom glass to the chip on which a portion of compatible pads have been exposed before using photolithography and plasma etching techniques. After a barrier solder mask is coated on the second glass, notching is performed so that inverted leads, via sputtering deposition, connect electrically to the compatible pads in form of a so-called T-shape junction. The leads are coated with a protective solder mask thereon. The solder mask is a dielectric material that electrically isolates the leads from external contacts, and protects the lead surface against corrosion. Solder bumps are attached to the bottom end of leads, and are suitable for printed circuit board (PCB) mounting by known methods. Solder bumps may be formed by known methods like screen printing.  FIG. 2  is a typical cross-section view of a packaged chip device by prior art ShellOP.  FIG. 3  is a typical cross-section of a packaged chip device by prior art ShellUT. 
     MEMS is a kind of tiny mechanical device that is built onto semiconductor chips and is measured in micrometers. It makes use of silicon micromachining technology, LIGA (Lithograpie Galvanoformung Abformung) and precision mechanical engineering technology to fabricate microscopic mechanisms on the surface of a semiconductor wafer. MEMS has a wide range of applications, such as accelerometers, pressure sensors, actuators and many others. In any case above, MEMS devices are usually adapted to employ spatially active elements (gears, hinges, slides, etc) which must be typically free to move and therefore require a cavity surrounding. 
     Good packaging techniques are relatively more important for the successful performance of the components. The conventional packaging techniques for MEMS devices are ceramic package and metal package, both of them can provide a strong and hermetic enough component. However, both of them are ordinarily very expensive, for many conventionally produced MEMS components, the processes of packaging may take up about 70-85% from the total fabrication cost. Moreover, the packages in most cases are not small enough for handhold applications like cell phones. It is well-known that price and size are the two factors which are most sensitive in customer electronics that ought to be the biggest market of MEMS devices. 
     Although the Shellcase technique is quite suitable for optical and image sensors, e.g., charge-coupled devices (CCD) and/or CMOS imagers integrated on a silicon wafer, it is actually not fully satisfiable for MEMS devices. 
     First, in most cases, MEMS package has to provide inert gas filling in or high vacuum so as to protect the fragile movable parts, which requires high hermetic packaging, but the epoxy made cavity wall by conventional Shellcase technique is impossible to handle this. Secondly, many MEMS devices make use of inner gas as a medium. For example, a kind of MEMS accelerometer places sensors at certain position to detect the heat distribution of the inner gas so that it can monitor the movement of the gas and accordingly convert to the value of acceleration. In this case, the more gas filled in, the more sensitive this device would be. In order to get a better performance, it is necessary to make cavities on the cap substrate. But these cavities will make it difficult to form cavity wall, it is easy to understand that spin coating can only be performed on flat surface unless the uniformity of coating is out of concern. Thirdly, because of its complex structure, substrate wafer can not be thinned as much as in general IC wafer, with a typical thickness of about 400-500 um rather than 100 um. Under this circumstance, traditional RIE process can hardly ensure well enough groove shape and well enough uniformity. 
     Therefore, there is a need of wafer level chip size package for MEMS devices which would reduce both of the packaged size and cost. This package technique must provide a well controlled cavity within which micro-machined parts are free to move, allowing access to electrical contacts, and optimized for device performance. 
     SUMMARY OF THE INVENTION 
     The present invention intends to solve the problems existing in prior art WLCSP, and to provide an improved wafer level chip size package satisfied for MEMS devices and methods for fabricating the same. 
     According to the present invention, a wafer level chip size package for MEMS devices is provided, comprising: 
     a substrate wafer having formed thereon MEMS dies, with a plurality of compatible pads disposed at the periphery of each of the MEMS dies on the substrate wafer; 
     a cap substrate with cavities, the cap substrate includes cavity walls by which the cap substrate is combined with the substrate wafer, each of the MEMS dies being encased within a corresponding cavity; 
     a first insulating layer over a non-active surface of the substrate wafer, while leaving portions of the compatible pads exposed; 
     a layer of redistribution leads over portions of the first insulating layer and the exposed compatible pads; 
     a second insulating layer on the redistribution leads, while leaving portions of the redistribution leads exposed; 
     a plurality of solder bumps formed on exposed portions of the redistribution leads, each solder bump corresponding to a compatible pad. 
     Wherein, the position of each of the cavities is defined by the cavity walls. 
     Wherein, the depth of the cavities is no less than 350 um. 
     Wherein, the cavities are filled with inert gas. 
     Wherein, the thickness of the cavity walls is within the range of 10-40 um. 
     Wherein, the cavity walls are made of BCB (DOW chemical) or glass frit. 
     Wherein, the cap substrate is made of silicon or glass. 
     Wherein, the redistribution leads are made of Al/Ni alloys. 
     Wherein, the second insulating layer is made of photo sensitive thermoplastic polymer. 
     Wherein, the solder bumps are made of lead-free solder paste. 
     According to the present invention, a method for fabricating a wafer level chip size package for MEMS devices is also provided, comprising following steps: 
     providing a substrate wafer having formed thereon MEMS dies, with a plurality of compatible pads disposed at the periphery of each of said MEMS dies on the substrate wafer; 
     providing a cap substrate, spin coating photo sensitive BCB on said cap substrate, then form a plurality of cavity walls by lithography, the position of each cavity wall is corresponding to that of the MEMS die; 
     aligning and bonding the substrate wafer with the cap substrate; 
     thinning the substrate wafer by backside grinding, notching to remove most of the substrate wafer material at scribe lines; 
     plasma etching at the backside of the substrate wafer to remove the rest of the substrate wafer material and expose part of the compatible pads, trenches being formed; 
     filling the trenches with insulating materials to cover the exposed compatible pads, spin coating this insulating material on the backside of the substrate wafer as a first insulating layer; 
     notching again to expose flanks of the compatible pads; 
     depositing a metal layer on the backside of the substrate wafer by sputtering, forming redistribution leads by lithography and plating; 
     spin coating a photo sensitive second insulating layer to cover the redistribution leads, making openings by lithography for solder bump forming; 
     forming solder bumps at the openings of the second insulating layer by screen printing, each solder bump corresponding to a compatible pad; 
     dicing the substrate wafer along the scribe lines to singulate the MEMS dies therefrom. 
     Wherein, the method further comprises following steps prior to the step of aligning and bonding the substrate wafer with the cap substrate: 
     forming cavities on the cap substrate by wet etching or plasma etching, utilizing the cavity walls as a mask so that the position of each cavity corresponds to that of the MEMS die. 
     Alternatively, a method for fabricating a wafer level chip size package for MEMS devices, comprising following steps: 
     providing a substrate wafer having formed thereon MEMS dies, with a plurality of compatible pads disposed at the periphery of each of said MEMS dies on said wafer; 
     providing a cap substrate; 
     screen printing glass frit on the cap substrate to form cavity walls; 
     aligning and bonding the substrate wafer with the cap substrate; 
     thinning the substrate wafer by backside grinding, notching to remove most of the substrate wafer material at scribe lines; 
     plasma etching at the backside of the substrate wafer to remove the rest of the substrate wafer material and expose part of the compatible pads, trenches being formed; 
     filling the trenches with insulating materials to cover the exposed compatible pads, spin coating this insulating material on the backside of the substrate wafer as a first insulating layer; 
     notching again to expose flanks of the compatible pads; 
     depositing a metal layer on the backside of the substrate wafer by sputtering, forming redistribution leads by lithography and UBM technique; 
     spin coating a photo sensitive second insulating layer to cover the redistribution leads, making openings by lithography for solder bump forming; 
     forming solder bumps at the openings of the second insulating layer by screen printing, each solder bump corresponding to a compatible pad; 
     dicing the substrate wafer along the scribe lines to singulate the MEMS dies therefrom. 
     Wherein, the method further comprises following steps prior to the step of screen printing glass frit on the cap substrate to form cavity walls: 
     depositing a passivation layer on frontside of the cap substrate by PECVD, spin coating photo resist on the passivation layer, making pattern by lithography, the area which need not to be etched should be covered by photo resist; 
     plasma etching at frontside of the cap substrate, after etching, stripping the photo resist, the pattern of the passivation layer being the same as that of the photo resist; 
     making cavities on the cap substrate by wet etching or dry etching, utilizing the passivation layer as a mask so that the position of each cavity corresponds to that of the MEMS die. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Hereinafter, the present invention will be further described in details in combination with the accompanying drawings and the preferred embodiments. 
         FIG. 1  illustrates a schematic cross-sectional view of a packaged chip device by prior art ShellOC; 
         FIG. 2  illustrates a schematic cross-sectional view of a packaged chip device by prior art ShellOP; 
         FIG. 3  illustrates a schematic cross-sectional view of a packaged chip device by prior art ShellUT; 
         FIG. 4˜FIG .  14  illustrate a schematic process flow according to embodiment 1 of the present invention; 
         FIG. 15˜FIG .  18  illustrate a preceding stage of the whole schematic process flow according to embodiment 2 of the present invention. 
     
    
    
     In the figures, the reference numeral  5  denotes a cap substrate, the numeral  10  denotes a cavity wall, the numeral  15  denotes a compatible pad, the numeral  20  denotes a substrate wafer, the numeral  25  denotes a cushion layer, the numeral  30  denotes a redistribution lead, the numeral  35  denotes a solder mask, the numeral  40  denotes a solder bump, the numeral  45  denotes a passivation layer, the numeral  50  denotes a photo resist. 
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     As shown in  FIG. 14 , according to embodiment 1 of the present invention, a wafer level chip size package for MEMS devices includes a substrate wafer  20 , a cap substrate  5 , a cushion layer  25 , a layer of redistribution leads  30 , solder mask  35  and a plurality of solder bumps  40 . 
     The substrate wafer  20  has formed thereon MEMS dies formed with certain MEMS structures, such as gears, hinges, slides, etc. There are also electrical terminals, preferably in the form of so called compatible pads  15  disposed at the periphery of each of the MEMS dies on the substrate wafer  20 . 
     The cap substrate  5  has cavities which can be filled in inert gas with a certain pressure. The cap substrate  5  includes cavity walls  10  by which the cap substrate  5  is combined with the substrate wafer  20 , each of the MEMS dies being encased within a corresponding cavity. Preferably though not necessarily, the depth of cavities is within the approximate range of 300-400 um, the thickness of cavity walls  10  is within the approximate range of 10-40 um. 
     The cushion layer  25  is disposed over a non-active surface of the substrate wafer  20 , while leaving portions of the compatible pads  15  exposed. 
     The layer of redistribution leads is disposed over portions of the cushion layer  25  and the exposed compatible pads  15 . 
     The solder mask  35  is disposed on the redistribution leads, while leaving portions of the redistribution leads exposed. Preferably though not necessarily, the solder mask  35  is formed of photo sensitive thermoplastic polymer such as BCB, polyimide and epoxy etc. 
     The solder bumps  40  are formed on exposed portions of the redistribution leads, each solder bump  40  corresponding to a compatible pad  15 . Preferably though not necessarily, the solder bumps  40  are formed of lead-free solder paste. 
     There are two optional methods to form cavity walls  10 . One is to spin coat photosensitive BCB on flat cap substrate so as to form cavity walls by lithography. BCB is capable to endure KOH, so in this case, BCB is utilized as a mask for wet etching when forming the cavity. The other one is to etch the cap substrate to form cavities first, then screen print glass frit on etched cap substrate to form cavity walls. 
     According to embodiment 1 of the present invention, an exemplary process flow of forming the wafer level chip size package for MEMS devices is illustrated from  FIG. 4  to  FIG. 14 . 
     As illustrated in  FIG. 4 , spin coat BCB on a cap substrate  5  at first, then form cavity walls  10  by lithography, the position of each cavity wall  10  is corresponding to that of the substrate wafer  20 . The cap substrate  5  is formed of materials with a proper value of CTE and should be easy to be etched, such as silicon or glass. 
     As illustrated in  FIG. 5 , form cavities on the cap substrate  5  by wet etching or plasma etching; utilize the cavity walls  10  as a mask so that the position of each cavity corresponds to that of the substrate wafer  20 . Particularly, if gas filling is not required, this step can be skipped. 
     As illustrated in  FIG. 6 , align the substrate wafer  20  with the cap substrate  5  and apply pressure to achieve bonding. Each cavity wall  10  will be pressed onto the compatible pads  15  and the MEMS structure is inside of the cavity. During bonding, air can be pumped out to get high vacuum and inertia gas can be filled in for certain applications. 
     As illustrated in  FIG. 7 , thin the substrate wafer  20  by backside grinding, notch it by special blade with certain angle to remove most of the silicon at the scribe lines. Preferably though not necessarily, the thickness of remaining silicon is around 10 um. 
     As illustrated in  FIG. 8 , plasma etch at the backside of the substrate wafer  20  to remove the rest silicon and expose part of the compatible pads  15 . 
     As illustrated in  FIG. 9 , fill the trench with insulating materials to cover the silicon and the exposed compatible pads  15 . Spin coat this insulating material on the backside of the substrate wafer as a cushion layer  25 . 
     As illustrated in  FIG. 10 , notch again to expose the flank of the compatible pads  15 . Particularly, if the cap substrate  5  is made of electric material such as silicon, the blade cannot cut into the cavity wall  10  otherwise the substrate wafer will be connected electrically to the cap substrate  5 . 
     As illustrated in  FIG. 11 , deposit a metal layer on the backside of the substrate wafer  20  by sputtering. Preferably though not necessarily, this metal layer is Al. 
     As illustrated in  FIG. 12 , form redistribution leads  30  by lithography and well known UBM (under bump metallization) technique. 
     As illustrated in  FIG. 13 , spin coat a photo sensitive solder mask  35  to cover the redistribution leads  30 , make openings by lithography for solder bump planting. 
     As illustrated in  FIG. 14 , plant solder bumps  40  at the openings of the solder mask  35  by screen printing, each solder bump  40  corresponding to a compatible pad  15 . 
     As an end of the process, after all the above steps, dice the substrate wafer  20  along the scribe lines to singulate the MEMS dies therefrom. 
     Preferably though not necessarily, this process flow may also include the following steps: before cavity formation, make one pair of alignment marks at the backside of the cap substrate  5  by lithography, the alignment of cavity formation (expose) and bonding will be according with these marks; before the first notch, spin coat photo resist on the non-active surface of the substrate wafer  20  and make photo resist openings at the positions corresponding to those of the compatible pads  15  by backside align system or IR align system. The first notch will then be aligned according to these openings and the photo resist will be utilized as a mask for etching which is the following step of the first notch. Preferably though not necessarily, the type of photo resist for these two steps is positive, the gases for etching are fluorocarbon or sulfur-fluoride. 
     According to embodiment 2 of the present invention, instead of the steps illustrated in  FIG. 4  and  FIG. 5 , an alternative method of forming the cavity walls  10  is illustrated from  FIG. 15  to  FIG. 18 . 
     As illustrated in  FIG. 15 , deposit a passivation layer  45  on the front side of the cap substrate  5  by PECVD (plasma enhanced chemical vapor deposition), spin coat photo resist  50  on the passivation layer  45 , make pattern by lithography, the area which need not to be etched should be covered by photo resist  50 . 
     As illustrated in  FIG. 16 , plasma etch at the front side of the cap substrate, the photo resist  50  will protect the passivation layer  45  from being etched while the exposed passivation layer  45  will be etched. After etching, strip the photo resist  50 , the pattern of the passivation layer  45  is the same as that of the photo resist  50 . 
     As illustrated in  FIG. 17 , make cavities on the cap substrate  5  by wet etching or dry etching, utilize the passivation layer  45  as a mask so that the position of each cavity corresponds to that of the MEMS die. 
     As illustrated in  FIG. 18 , screen print glass frit on the cap substrate  5  as cavity walls  10 . 
     The subsequent steps are the same as those in embodiment 1 which are illustrated from  FIG. 6  to  FIG. 14 . 
     Particularly, if gas filling is not required, the steps described from  FIG. 15  to  FIG. 17  can be skipped. Preferably though not necessarily, this process flow may also include the following steps: before cavity formation, make one pair of alignment marks at the backside of the cap substrate  5  by lithography, the alignment of cavity formation (expose) and bonding will be according to these marks; with reference to the marks at the backside, making another pair of alignment marks at frontside of the cap substrate  5  by lithography, so that the screen print will be aligned according to these marks; before the first notch, spin coat photo resist on the backside of the substrate wafer  20  and make photo resist openings at the positions corresponding to those of the compatible pads  15  by backside align system or IR align system. The first notch will then be aligned according to these openings and the photo resist will be utilized as a mask for etching which is the following step of the first notch. Preferably though not necessarily, the type of photo resist for these two steps is positive, the gases for etching are fluorocarbon or sulfur-fluoride. 
     In summary, an improved wafer level chip size package for MEMS devices and method for fabricating the same has been provided. The first process utilize photo sensitive BCB to form cavity walls which will protect the cap substrate  5  from being etched when forming cavities and the BCB bonding results in a hermetic enclosure. Alternatively, screen print glass frit on etched substrate and utilize it as cavity walls. The glass frit bonding also realizes a hermetic enclosure. The innovative solution of removing most of the silicon by the first notch before plasma etching effectively solves the difficulty of etching which is induced by the thick silicon. This packaging method provides a well packed device with the size much closely to the original one, making it possible to package the whole wafer at the same time and therefore, saves the cost and cycle time. 
     Although this invention has been described in connection with specific forms and embodiments thereof, it will be appreciated that various modification may be made from the specific details described here in without departing form the spirit or scope of the invention as set forth in the appended claims.