Abstract:
Formation of a structure with through-holes includes attaching two sub-structures to one another. The resulting structure may be used in a sub-assembly for various types of micro components and may serve as a lid or base of a housing that encapsulates one or more micro components. The techniques may provide greater flexibility in the shape of the through-holes and may reduce costs compared with other known techniques.

Description:
BACKGROUND 
     This disclosure relates to techniques for providing a structure with through-holes that may be used, for example, in a sub-assembly for micro components. 
     Sub-assemblies for micro components such as optoelectronic devices or optoelectronic hybrid integrated circuits may include a semiconductor structure formed as a lid, which may be sealed to a base. One or more active and passive micro components may be encapsulated within the sub-assembly. Typically, electrical or optical connections may need to be provided to the components mounted within the assembly. 
     The assignee of the present application previously has disclosed, in U.S. Published application Ser. No. 2003/0071283, a double-sided etching technique for providing a semiconductor structure with one or more through-holes, which may be sealed hermetically, for example, by a feed-through metallization process. The structure in which the through-holes are formed may include a silicon-on-insulator (SOI) structure. Electrical contacts to an optoelectronic or other component housed within the assembly may be provided via the feed-through metallization. 
     The present invention discloses alternative techniques for providing similar structures. 
     SUMMARY 
     The present disclosure relates to the formation of a structure with through-holes. The structure may be used, for example, in a sub-assembly for various types of micro components and may serve as a lid or base of a housing that encapsulates one or more micro components. As used in this disclosure, the phrase “micro components” means electronic devices, optical devices, electromagnetic devices, chemical devices, micro-mechanical devices, optoelectronic devices, micro-electromechanical system (MEMS) devices, micro-optoelectromechanical system (MOEMS) devices and other such devices that contain tiny, micron and sub-micron-sized elements. 
     According to one aspect, a method of forming an assembly to house one or more micro components includes providing a first sub-structure having a first through-hole; and attaching the first sub-structure to a second sub-structure having a group of one or more through-holes each of which is smaller than the first through-hole in the first sub-structure. The group of smaller through-holes in the second sub-structure is positioned opposite the through-hole in the first sub-structure. 
     In another aspect, a method of forming an assembly to house one or more micro components includes providing a first substantially planar sub-structure having a first through-hole and attaching the first sub-structure to a second substantially planar sub-structure. Subsequently a group of one or more through-holes is formed in the second sub-structure. Each through-hole in the second sub-structure is smaller than the first through-hole in the first sub-structure, and the group of smaller through-holes in the second sub-structure is positioned opposite the through-hole in the first sub-structure. 
     In various implementations, one or more of the following features may be present. For example, the structure comprising the first and second sub-structures may be used as a lid of a housing that encapsulates the one or more micro components. Alternatively, the structure comprising the first and second sub-structures may be used as a base of a housing that encapsulates the one or more micro components. 
     The method may include providing feed-through metallization in at least one of the through-holes in the second sub-structure. The second sub-structure may be at least several times thinner than the first sub-structure. 
     The first sub-structure may be etched or molded to form the first through-hole therein. Various etching processes, including dry etching, isotropic wet etching, or anisotropic wet etching may be used. Depending on the technique used to form the through-holes, they may have various shapes including, for example, concave sidewalls or slanted sidewalls. 
     Forming the through-holes in the second sub-structure may include partially etching the second sub-structure to form grooves in a surface on a first side of the second sub-structure and thinning the second sub-structure from a side opposite from the first side. The thinning process may include, for example, a chemical-mechanical polishing process. To facilitate the thinning process, the second sub-structure may be attached to a carrier substrate before performing the thinning of the second sub-structure. After the first sub-structure is attached to the second sub-structure, the carrier substrate from the second sub-structure may be removed. The first sub-structure may be attached to the second sub-structure after partially etching the second sub-structure to form the grooves, but before performing the thinning. 
     Various implementations may include one or more of the following advantages. The sub-structures may be formed, for example, of silicon or glass, which may be less costly than using a SOI structure as disclosed in the published application identified above. Furthermore, the techniques described in the present application may provide greater flexibility in the shape of the through-holes. 
     Other features and advantages may be apparent from the following detailed description, the accompanying drawings and the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1 . is a cross-section of an example of an assembly that houses a MEMS device according to the invention. 
         FIGS. 2 and 3  illustrate formation of a lid for the assembly using pre-processed sub-structures. 
         FIGS. 4 through 7  illustrate various shapes of through-holes in the lower sub-structure for the lid. 
         FIGS. 8A and 8B  illustrate formation of the upper sub-structure for the lid according to one implementation. 
         FIGS. 9A and 9B  illustrate formation of the upper sub-structure for the lid according to another implementation. 
         FIGS. 10A through 10H  illustrate examples of lid structures with various combinations of through-hole shapes according to the invention. 
         FIGS. 11A through 11D  illustrate additional examples of lid structures with various combinations of through-hole shapes according to the invention. 
         FIG. 12  illustrates part of a molding tool that may be used to form the lid. 
         FIGS. 13A through 13D  illustrate an alternative technique for formation of the lid with feed-through metallization. 
         FIG. 14  is a cross-section of an example of an assembly that houses an optoelectronic device according to the invention. 
         FIGS. 15A and 15B  illustrate an example of a base for the assembly according to another implementation. 
         FIGS. 16A and 16B  illustrate another example of a lid structure using rhomboid-shaped holes. 
     
    
    
     DETAILED DESCRIPTION 
     As shown in  FIG. 1 , an assembly  20  for a MEMS component  24  includes two structures  22 ,  25  that are soldered together to provide a hermetically sealed package that encapsulates the MEMS device. The bottom structure  22  includes a semiconductor or other substrate that serves as a base on which the MEMS device  24  is mounted or in which the MEMS device is formed. Metal leads  28  may be provided on the surface of the base and are electrically connected to the MEMS device  24 . The upper structure  25  serves as a lid and includes feed-through metallization  27  to permit electrical connections from the exterior of the package to the MEMS device inside the package. A recessed cavity  40  is provided in the lower section of the lid  25  to allow room for the MEMS device. 
     The feed-through metallization  27  extends through one or more micro-vias (i.e., through-holes) in the upper section of the lid  25 . The feed-through metallization  27  extends along the outer surface of the lid  25  and is electrically connected to solder bumps  32  for printed board assembly. The feed-through metallization  27  also extends outside the recessed cavity  40  along the cavity-side surface of the lid structure  25 . Solder  30  is deposited onto the feed-through metallization outside the recessed cavity  40  to provide solder bumps for electrical connection to the metal leads  28 . Additional solder  30  is deposited on the cavity-side of the lid structure  25  to provide a solder seal ring. 
     Other active and passive micro components may be mounted to the base  22  and encapsulated within the package. Although in  FIG. 1  the feed-through metallization  27  is shown extending through only two through-holes in the upper section of the lid  25 , feed-through metallization may be provided through additional through-holes for electrical contact to the same or other micro components in the package. 
       FIGS. 2 and 3  illustrate formation of the lid  25  using two pre-processed substantially planar sub-structures  44 ,  46 . The first, lower sub-structure  46 , which may comprise, for example, silicon or glass, includes a relatively large through-hole  48  that serves as the recessed cavity  40  when the two sub-structures are attached together. The second, upper sub-structure  44 , which also may comprise, for example, silicon or glass, includes one or more through-holes  42 . The through-holes  42  in the upper sub-structure  44  may be many times smaller than the through-hole  48  in the lower sub-structure  46 . For example, in many applications, the through-holes  42  may have diameters less than ten microns, and for some applications, diameters on the order of several microns (e.g., 3-5 μm) may be preferred. The upper sub-structure  44  may be many times thinner than the second, lower sub-structure  46 . 
     The lid  25  may be formed by attaching the two pre-processed sub-structures  44 ,  46  so that the smaller through-holes  42  in the second sub-structure are located opposite the larger through-hole  48  in the first sub-structure as shown in  FIG. 3 . The particular technique used to attach the sub-structures to another may vary depending on the materials involved. Examples of such techniques include silicon-direct-bonding, anodic bonding, glass frit bonding, glass soldering and reflow (eutectic) soldering. In some implementations, some of the through-holes  42 ,  48  may be formed (or completed) after the two sub-structures  44 ,  46  are attached to one another. 
     The lower sub-structure  46  may include multiple large through-holes, each of which is similar to the through-hole  48 . Similarly, the upper sub-structure  44  may include multiple groups of smaller through-holes  42 , with each group of smaller through-holes located opposite a respective one of the larger through-holes in the lower sub-structure. 
     Formation of the through-hole(s)  48  in the lower sub-structure  46  may be achieved, for example, by etching a plain silicon or other semiconductor wafer. Known photolithographic or other techniques may be used to define the areas of the wafer in which the through-holes are to be opened. The through-holes  48  may have various shapes depending on the particular etch process used. For example, a dry etching or laser drilling technique may result in through-holes with steep sidewalls ( FIG. 4 ), whereas an isotropic wet etching technique may result in through-holes with concave sidewalls ( FIG. 5 ). An anisotropic wet etching technique may result in slanted sidewalls ( FIG. 6 ). For normal cut silicon wafers with (100) surface crystal orientation, the angle between the sidewall and the wafer surface is about 54.74°. The angle can be tuned by cutting the wafers off-angle from the ingot. For some applications, an angle of about 45° is preferred ( FIG. 7 ). 
     In some implementations, the sub-structure  46  may be formed from etched or molded glass. 
     Formation of the through-holes  42  in the upper sub-structure  44  may include, for example, partially etching a plain silicon or other semiconductor wafer  50  as shown in  FIGS. 8A and 9A . The etch depth should exceed the thickness of the final upper sub-structure. Known photolithographic or other techniques may be used to define the areas of the wafer in which the holes are to be opened. An oxide or other layer may be used as a mask. The shape of the etched holes  54  may vary depending on the type of etch employed. Dry etching or laser drilling may result in holes with steep sidewalls ( FIG. 8A ) whereas anisotropic wet etching may result in etched grooves with slanted sidewalls ( FIG. 9A ). If a plain silicon wafer with a (100) surface crystal orientation is used, the angle between the sidewalls and the wafer surface is about 54.74°. A combination of dry etching followed by anisotropic wet etching (e.g., KOH etching) may be used to provide rhomboid-shaped holes  54  ( FIG. 16A ). 
     To complete formation of the through-holes in the upper sub-structure  44 , the wafer  50  can be thinned, for example, by chemical-mechanical polishing (CMP). To ensure proper handling of the wafer  50  for the thin upper sub-structure  44 , it may temporally be attached to a carrier structure  52  as in  FIGS. 8B  or  9 B. The carrier structure  52  may include another wafer or some other substrate. The wafer  50  is thinned until enough material has been removed from the side of the wafer opposite the holes  54  to result in through-holes extending through the wafer. 
     The upper and lower sub-structures  44 ,  46  may be attached to one another as discussed above, and the carrier structure  52  then may be removed from the upper sub-structure. Various lid structures are shown in  FIGS. 10A through 10H . As can be seen from the examples, a wide range of combination of shapes for the through-holes  42  and  48  is possible. 
     In some implementations, use of the separate wafer  52  as the carrier structure during the CMP thinning process may be avoided. For example, after formation of the partially etched holes  54  in the wafer  50  (see, e.g.,  FIGS. 8A ,  9 A and  16 A), and prior to the thinning process, the wafer  50  for the upper sub-structure may be attached to the pre-processed wafer for the lower sub-structure  46 . In that case, the side of the wafer  50  in which the holes  54  were etched is placed face down against the wafer for the lower sub-structure  46 . The CMP process then may be performed to thin the wafer  50 , with the lower sub-structure  46  supporting the wafer  50  as it is thinned to complete the through-holes  42  in the upper sub-structure  44 . Examples of lid structures that result when anisotropic wet etching is used to form the holes  54  ( FIGS. 8A and 9A ) are shown in  FIGS. 11A through 11D . An example of a lid structure that results from a rhomboid-shaped hole ( FIG. 16A ) is shown in  FIG. 16B . Rhomboid-shaped holes may be advantageous in situations where the through-hole is completed after bonding the sub-structures  44 ,  46 . Such techniques can allow the final size of the opening for the through-hole to be controlled more accurately. 
     In yet other implementations, the pre-processed lower sub-structure  46  (e.g.,  FIGS. 4 ,  5 ,  6  or  7 ) may be attached to a plain wafer that is to serve as the upper sub-structure. The wafer for the upper sub-structure then may be thinned, for example, by a CMP process. Following the thinning process, the through-holes  42  may be etched into the wafer for the upper sub-structure. 
     In an alternative implementation, the lid structure  25  may be formed from a single piece of molded glass. As illustrated in  FIG. 12 , a molding tool  60  may include a large island  62  for formation of the large recessed cavity  48  and smaller protrusions  64  corresponding to the through-holes  42 . The molded glass may be polished from the side opposite the molded surface until the indentations formed by the smaller protrusions  64  become through-holes. 
     After formation of the lid structure  25 , hermetically sealed feed-through connections  27  may be provided for electrical contacts from the exterior of the lid to its underside. The through-holes  42  may be hermetically sealed, for example, using an electro-plated feed-through metallization process as the base for the through-hole connections. The feed-through metallization also may include a diffusion barrier, and the sealing material may include, for example, a non-noble metal. Further details of a feed-through metallization process that may be used are disclosed in applicant&#39;s published U.S. application identified above. 
     In the foregoing examples, it was assumed that the feed-through metallization is provided after completion of the through-holes  42  (i.e., after the CMP thinning process). However, in other implementations, the feed-through metallization may be provided before the CMP thinning process. As shown for example, in  FIGS. 13A and 13B , the wafer for the upper sub-structure  44 , with partially etched vias  66  for the through-holes  42 , may be attached to the pre-processed wafer for the lower sub-structure  46 . Feed-through metallization  27  then may be provided in the cavity  40  and in the vias  66  (see  FIG. 13C ). After formation of the feed-through metallization, the upper sub-structure  44  may be thinned, for example, by a CMP process to expose the feed-through metallization (see  FIG. 13D ). 
     Once the lid structure  25  is completed, it may be attached to the base  22  on which the MEMS device  24  is mounted to form a hermetically sealed assembly. 
     The foregoing techniques for forming a structure with through-holes also may be used in connection with assemblies for optoelectronic or other devices. Furthermore, instead of a lid, the structure with the through-holes may serve as the base on (or in) which the micro components are mounted. 
     As shown, for example, in  FIG. 14 , the assembly  120  includes two structures  122 ,  125  that are soldered together to provide a hermetically sealed package that encapsulates an optoelectronic device  124 , which may be, for example, a photodiode. The upper structure  122  serves as a lid and should be transparent to the wavelength(s) that are to be sensed or emitted by the optoelectronic device  124 . 
     In the illustrated example, the optoelectronic device  124  is mounted on a integrated circuit chip  126 , which, in turn, is mounted on the lower structure  125  which serves as a base. If the optoelectronic device  124  is a photodiode, the integrated circuit chip  126  could be, for example, a trans-impedance amplifier. As shown in  FIG. 14 , the optoelectronic device is mounted within a recessed cavity  140  formed in the same structure that includes the feed-through metallization  127 . Other circuitry as well as passive components may be mounted in the recessed cavity and encapsulated within the package. The feed-through metallization  127  extends through one or more micro-vias (i.e., through-holes) in the lower section of the base  125 . The feed-through metallization  27  extends along the outer surface of the base  125  and is electrically connected to solder bumps  132  for printed board assembly. The feed-through metallization  127  also extends outside the recessed cavity  140  along the cavity-side surface of the base structure  125 . Wire bonds  128  may provide the electrical connections from the feed-through metallization  127  to the optoelectronic device  124  and the integrated circuit chip  126 . 
     A solder seal ring  130  may be provided on the cavity-side surface of the base structure  125  for hermetically attaching the lid  122  to the base  125 . 
     The various techniques discussed above in connection with  FIGS. 2 through 11  may be used to form the through-hole structure  125 . For example, if the through-hole structure includes silicon or other semiconductor substrates, dry etching or wet etching techniques may be used to form the through-holes. In one particular implementation, where an edge emitting optoelectronic device is to be mounted in the recessed cavity  140  of the base  125 , an anisotropic wet etching technique may result in slanted sidewalls (see, e.g.,  FIG. 6 ) to provide a reflective surface that re-directs light from the device through the lid  122  and out of the assembly  120 . 
     In some implementations, the sub-structure  125 A (see  FIG. 14 ) may be formed from etched silicon, or etched or molded glass. Such techniques may facilitate the formation of additional, smaller recesses  142  in the larger recess  140 , as shown, for example, in  FIG. 15A . The smaller recesses may define areas for mounting various micro components such as a laser  144 , a monitor diode  146 , and an optical lens  148  (see  FIG. 15B ), while allowing the use of a flat lid to cover the components as in  FIG. 14 . Etching techniques may require the use of multiple photolithographic steps to define the separate smaller recesses  142 . 
     The foregoing techniques and structures may be used in connection with assemblies for a wide range of micro components, including, but not limited to, electronic devices, optical devices, electromagnetic devices, chemical devices, micro-mechanical devices, optoelectronic devices, MEMS devices, MOEMS devices and other such devices that contain tiny, micron and sub-micron-sized elements. 
     Other implementations are within the scope of the following claims.