Abstract:
In one embodiment, a method for making an optical micro device package includes: providing a substrate wafer having a plurality of solid state light sensors integrate therein; providing a transparent cover wafer coated with a material that alters the transparency characteristics of the cover wafer; forming a layer of light sensitive, photo definable adhesive material on the substrate wafer; selectively removing part of the layer of adhesive material in a pattern for a plurality of adhesive spacers between the substrate wafer and the cover wafer with each spacer surrounding a corresponding one of the light sensors; bonding the substrate wafer and the cover wafer together at the spacers to form a wafer assembly in which each spacer surrounds and seals a corresponding one of the light sensors within a cavity bounded by a spacer and the two wafers; and singulating individual device packages from the wafer assembly.

Description:
BACKGROUND 
       [0001]    Optical micro-electro-mechanical system (MEMS) devices are often integrated into a silicon substrate using semiconductor processing techniques and then sealed under a glass cover to protect the device from environmental damage while still allowing light to reach the device. A Fabry Perot filter light receptor spectrophotometer, for example, uses solid state light sensors and Fabry Perot filters integrated into a silicon substrate. Some of the components in such spectrophotometers are very delicate, making them particularly susceptible to damage from the higher temperatures and contaminants present in conventional MEMS sealing/packaging processes. 
     
    
     
       DRAWINGS 
         [0002]      FIG. 1  is a plan view illustrating an optical micro device package according to one embodiment of the disclosure. 
           [0003]      FIG. 2  is a section view taken along the line  2 - 2  in  FIG. 1 . 
           [0004]      FIG. 3  is a plan view illustrating a micro device wafer assembly according to one embodiment of the disclosure. 
           [0005]      FIG. 4  is a detail view of a portion of the wafer assembly shown in  FIG. 3 . 
           [0006]      FIGS. 5-10  are section views illustrating one embodiment of a sequence of steps for processing a wafer assembly to form individual micro device packages such as the one shown in  FIGS. 1 and 2 . 
           [0007]      FIGS. 11-15  are section views illustrating another embodiment of a sequence of steps for processing a wafer assembly to form individual micro device packages such as the one shown in  FIGS. 1 and 2 . 
       
    
    
     DESCRIPTION 
       [0008]    Embodiments of the present invention were developed in an effort to improve MEMS packaging for Fabry Perot filter light receptor spectrophotometers. Embodiments of the invention, however, are not limited to Fabry Perot filter light receptor spectrophotometer MEMS packaging but may be used in for packaging spectrophotometers in general as well as other types of optical MEMS devices. Hence, the following description should not be construed to limit the scope of the invention, which is defined in the claims that follow the description. 
         [0009]      FIG. 1  is a plan view illustrating a micro device package  10  according to one embodiment of the disclosure.  FIG. 2  is a section view taken along the line  2 - 2  in  FIG. 1 . Referring to  FIGS. 1 and 2 , device package  10  includes a glass or other suitable transparent cover  12 , a substrate  14  and an optical micro device  16  integrated into substrate  14 . Micro device  16  represents generally one or more optical devices that include a solid state light sensor, such as a Fabry Perot filter light receptor spectrophotometer for example. Cover  12  may also include a coating  18  on one or both surfaces  20 ,  22  to filter some wavelengths, to deter reflection (an “anti-reflection” coating), and/or to otherwise alter the characteristics of transparent cover  12 . In a package  10  for Fabry Perot filter light receptor spectrophotometer device  16 , for example, cover  12  typically will include anti-reflective coatings  18 . 
         [0010]    “Transparent” means the property of transmitting electromagnetic radiation along at least that part of the spectrum that includes wavelengths of infrared, visible and/or ultra-violet light. The nature or degree of transparency for cover  12  may vary according to the characteristics of optical device  16 . For example, for an optical micro device  16  used to modulate color in a digital projector or to measure color in a Fabry Perot filter light receptor spectrophotometer, cover  12  will be transparent at least to visible light but need not be transparent to infrared and ultraviolet light. In another example, for an optical micro device  16  used to generate, modulate or detect light in the infrared range, cover  12  will be transparent at least to infrared light but need not be transparent to visible and ultraviolet light. 
         [0011]    A primary surface  20  on cover  12  is affixed to a primary surface  24  on substrate  14  by a spacer  26  that surrounds micro device  16 . Micro device  16  is enclosed within a cavity  28  defined by cover  12 , substrate  14  and spacer  26 . Electrical contact pads  30  are positioned along an exposed periphery  31  of substrate  14  for making electrical contact to micro device  16  through a circuit structure (not shown) integrated into substrate  14 . In the embodiment shown, coating  18  forms cover primary surface  20  at spacer  26  and a layer  32  forms substrate primary surface  24  at spacer  26 . Layer  32  represents generally, for example, a layer of silicon dioxide, silicon nitride, or silicon carbide, a polymeric passivation layer, or metal traces, or a combination of any such elements, that may be exposed along substrate surface  24 . 
         [0012]    As described in more detail below, spacer  26  is formed from an SU- 8  photoresist (commercially available from Microchem Corp.) or another suitable light sensitive, photo definable adhesive material that is fully curable at lower temperatures. SU-8 photoresists are epoxy based negative resists fully curable at temperatures under 300° C. that will adhere to and seal a variety of materials commonly used in micro device fabrication and packaging. Although spacer  26  is shown bonding together surface coating  18  on cover  12  and a layer  32  on substrate  14 , other configurations are possible. For example, an SU-8 or other suitable light sensitive adhesive material spacer  26  could be used to bond a glass or other transparent cover  12  directly to the surface of a silicon substrate  14 . 
         [0013]    With continued reference to  FIGS. 1 and 2 , in one example embodiment for a spectrophotometer MEMS device  16 , a gap  33  of 20 μm-50 μm should be maintained between cover  12  and device  16  for proper device performance. Thus, in this embodiment, spacer  26  should be 20 μm-50 μm thick. In addition, to facilitate the wafer scale fabrication process described below, an SU-8 spacer  26  can be comparatively narrow, as little as 50 μm for example, and still maintain adequate bonding. In the embodiment shown in  FIG. 1 , the width W x  of spacer  26  in the X direction ( FIG. 1 ) is larger where there are no contact pads and the width W y  of spacer  26  is smaller in the Y direction ( FIG. 1 ) near contact pads  30 . The width of spacer  26  for any particular application may vary from that shown depending, for example, on the bond strength needed to meet process and reliability requirements for the application, the type of light sensitive adhesive used, and any limitations in the fabrication process. SU-8 photoresists and other such photo-definable adhesives are particularly advantageous for spectrophotometer packaging because the thickness and width of spacer  26  and its alignment to the underlying structure may be precisely defined. In addition, the techniques for processing these adhesive materials is comparatively clean, thus reducing the risk that debris or other contaminants will damage the delicate components in optical device  16  or alter the transparency characteristics of cover  12 . 
         [0014]      FIG. 3  is a plan view illustrating an in-process optical micro device wafer assembly  34  containing individual in-process device packages  36 .  FIG. 4  is a detail view of a portion of the wafer assembly  34  shown in  FIG. 3 .  FIGS. 5-10  are section views illustrating one embodiment of a sequence of steps for fabricating wafer assembly  34  and singulating the individual device packages  36  from wafer assembly  34  to form packages  10  shown in  FIGS. 1 and 2 .  FIGS. 5-7 ,  9  and  10  are taken along the X-X section line shown in  FIG. 4 .  FIG. 8  is taken along the Y-Y section line shown in  FIG. 4 . Conventional techniques well known to those skilled in the art of semiconductor processing may be used to form the structures described below. Thus, the details of those techniques are not included in the description except where it may be desirable to a better understanding of the innovative aspects of an embodiment to describe a specific technique or processing parameter. 
         [0015]    Referring first to  FIG. 5 , a layer of SU-8 or other suitable light sensitive adhesive material  38  is formed on a substrate wafer  40  to the desired thickness of spacers  26 . Substrate wafer  40  represents a fully processed, or near fully processed, wafer that includes optical MEMS devices  16 , contact pads  30  and any other operational components that may be integrated into the substrate. As shown in  FIG. 6 , layer  38  is selectively removed in the desired pattern of spacers  26  surrounding devices  16 . (The pattern of spacer  26  is best seen in the plan views of  FIGS. 1 and 4 .) A glass or other suitable transparent cover wafer  42  is aligned with and bonded to substrate wafer  40  at spacers  26  as shown in  FIG. 7  using, for example, a conventional wafer bonder. Cover wafer  42  represents a fully processed, or near fully processed, wafer that includes any anti-reflective and/or filter coatings  18 . Although a coating  18  on the exposed outer surface  22  of cover wafer  42  may be formed after bonding, it is expected that any such coating  18  will usually be formed prior to alignment with and bonding to substrate wafer  40 . 
         [0016]    An SU-8 photoresist used for spacers  26 , for example, will cure fully at a temperatures in the range of 100° C.-200° C., thus avoiding the higher temperatures needed to seal the glass covers used in a conventional ceramic optical MEMS device package. The lower bonding temperature protects anti-reflective coatings  18  on cover  12 , which can delaminate at higher temperatures, and reduces the risk of damage to device  16  and other components in substrate wafer  40  from the material stresses induced by high temperature bonding. It is expected that SU-8 and other negative photoresists will be desirable for most optical MEMS packaging applications due to low curing temperatures, excellent adhesive qualities, and precise structural alignment/definition characteristics. However, other suitable light sensitive, photo definable adhesives fully curable at temperatures less than 300° C. may be used. For example, IJ5000™ (commercially available from E. I. DuPont Company) and other such polymeric adhesives used as a so-called “barrier” layer in inkjet printheads may also be suitable for spacers  26 . 
         [0017]    Referring now to the section view of  FIG. 8  (which corresponds to the Y-Y section line in  FIG. 4 ), individual device packages  36  are singulated from wafer assembly  34  by first sawing or otherwise cutting wafer assembly  34  between packages  36  in the X direction ( FIG. 4 ), as indicated by saw cut arrows  44  in  FIG. 8 . Referring to  FIG. 9 , cover wafer  42  is cut through to gap  33  in the Y direction ( FIG. 4 ) to expose contact pads  30 , as indicated by saw cut arrows  46  in  FIG. 9 . Rotating the saw blade up, away from substrate wafer  40  helps minimize the risk of damage to bond pads  30  during cutting. With an upward rotating saw blade, it is expected that a gap  33  as small as 5 μm will provide sufficient clearance to the saw blade so that pre-trenching transparent cover wafer  42  at the cut locations is not required. In  FIG. 10 , a second cut is made in the Y direction between rows of contact pads  30 , as indicated by saw cut arrows  48  in  FIG. 10 , to complete the singulation of individual packages  36 , thus forming each individual package  10  described above with reference to  FIGS. 1 and 2 . Other singulation sequences may be used. For example, it may be desirable in some applications to expose contact pads  30  first, and then cut in the X and Y directions to singulate individual die packages  36  from wafer assembly  34 . 
         [0018]    In an alternative embodiment shown in  FIGS. 11-15 , a layer of SU-8 or other suitable light sensitive adhesive material is formed on substrate wafer  40  (layer  38  in  FIG. 11 ) and on cover wafer  42  (layer  50  in  FIG. 13 ). The combined thickness of layers  38  and  50  corresponds to the desired thickness of spacers  26 . Layers  38  and  50  are selectively removed in the pattern of spacers  26  surrounding devices  16 , as shown in  FIGS. 12 and 14 , respectively. The two wafers  40  and  42  are then bonded together as shown in  FIG. 15 . Singulation may proceed as described above with reference to  FIGS. 8-10 . Each adhesive layer  38  and  50  need not be the same thickness or formed from the same adhesive material (although, of course, different adhesive materials must be compatible). For example, it may be desirable in some packaging sequences for some optical devices  16  to form only a thin film of a transparent adhesive material on cover wafer  42  and proceed with bonding under vacuum without first having to remove any of the transparent adhesive film. 
         [0019]    “A” or “an” in the claims means one or more when introducing an element of the claim. For example, “a solid state light sensor” in claim  1  means on or more solid state light sensors. “And/or” in the claims means one or the other or both. 
         [0020]    As noted at the beginning of this Description, the exemplary embodiments shown in the figures and described above illustrate but do not limit the invention. Other forms, details, and embodiments may be made and implemented. Therefore, the foregoing description should not be construed to limit the scope of the invention, which is defined in the following claims.