Abstract:
Provided is a microcomponent holder for retaining a micro-scale component. The microcomponent holder includes at least one aperture for receiving a micro-scale component therein. At least one loop-shaped support member is disposed about the aperture for contacting the micro-scale component to retain the micro-scale component within the microcomponent holder. The invention allows for retaining of microscale objects, such as lenses or other parts of micro-optical, micro-mechanical, micro-electromechanical, and other micro-scale systems.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit of priority under 35 U.S.C. §119(e) to U.S. Provisional Application No. 60/640,741, filed Dec. 30, 2004, the entire contents of which are incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     The present invention relates to microscale apparatus for retaining microscale objects, such as lenses or other parts of micro-optical, micro-mechanical, micro-electromechanical, and other micro-scale systems. The present invention also relates to methods for making such apparatus.  
         [0003]     Conventional optical systems typically comprise various discrete components such as light sources, detectors, lenses, gratings, mirrors, beam splitters, modulators, filters, polarizers, collimators, shutters, choppers, and so forth. Such conventional components usually have dimensions on the order of a centimeter or larger. Thus, in conventional systems, discrete components are positioned and aligned on optical benches using precisely machined centimeter-scale parts such as fixtures or grips that serve as holders, mounts, adjustable positioners, and alignment tools for the various optical components. However, new generations of optical systems have been drastically miniaturized and utilize optical components with dimensions on the scale of tens of microns or smaller, which are not compatible with conventional fixtures or grips. The manipulation of such micro-optical components, specifically with regard to their relative positioning, alignment and orientation, is often problematic. In addition, many emerging applications impose challenging and increasingly demanding requirements on such microcomponent holders, especially with regard to their compatibility with the variable shapes and small dimensions of the components being manipulated, as well as with respect to the precision and accuracy of their manipulation. Furthermore, additional considerations for such microcomponent holders include their ease of use, manufacturability, reproducibility, and cost. Thus, there is a need for better apparatus for handling and mounting such microcomponents. These needed apparatus, include, for example, gripping tools for holding and manipulating components. Specifically, there is a need for apparatus into which microcomponents can be loaded for either releasably mounting or permanent or semi-permanent mounting. The apparatus may themselves be incorporated along with their loaded components into a system or instrument as part of its manufacture or assembly.  
         [0004]     For example, an important and representative application in which microcomponent holders are needed involves photodiodes and laser diodes. These semiconductor devices have die dimensions ranging from tens to several hundred microns, and often have photosensitive areas or emissive surfaces of only a few microns in extent. The detectors and laser diodes are typically coupled to optical systems by positioning a microlens of comparable dimensions in close proximity to the laser or detector. Hence, for effective optical coupling, the placement and alignment within sub-micron tolerances of such components is desired with regard to accuracy, consistency, and reproducibility. Further, it is often desired that such devices be capable of mass production with high precision at low-cost. Currently, expensive and tedious micromanipulation systems employing scanning, real-time optical measurements and feedback are used to align semiconductor devices and other microscale optical components. Thus, there is a need for microcomponent holders that permit low cost and accurate mounting of a microcomponent within the holder.  
       SUMMARY OF THE INVENTION  
       [0005]     In accordance with a first aspect of the invention, provided is a microcomponent holder for retaining a micro-scale component. The microcomponent holder includes at least one aperture for receiving a micro-scale component therein. At least one loop-shaped support member is disposed about the aperture for contacting the micro-scale component to retain the micro-scale component within the microcomponent holder.  
         [0006]     In accordance with a further aspect of the invention, provided is a microcomponent holder for retaining a micro-scale component. The microcomponent holder includes at least one aperture for receiving a micro-scale component therein. At least one support member is disposed at the aperture for contacting the micro-scale component to retain the micro-scale component within the microcomponent holder. A reference surface is disposed at a selected location relative to the aperture to facilitate positioning the micro-scale component at a selected location in an assembly.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]     The foregoing summary and the following detailed description of the preferred embodiments of the present invention will be best understood when read in conjunction with the appended drawings, in which:  
         [0008]      FIG. 1  schematically illustrates a microcomponent holder in accordance with the present invention having a flat mounting surface and having three inwardly-directed, loop-shaped support members for retaining a microcomponent within the holder;  
         [0009]      FIG. 2  schematically illustrates the annular lens holder of  FIG. 1  but with a microlens disposed therein;  
         [0010]      FIG. 3  schematically illustrates another microcomponent holder in accordance with the present invention having several peripheral flat mounting surfaces, and having three support members in the shape of a spiraling arms for retaining a microcomponent within the holder;  
         [0011]      FIG. 4  schematically illustrates an oblique view of a lens holder with several sockets that are positioned in opposition to similar shaped sockets on a platform, and with balls seated in corresponding sockets of the lens holder and platform that function as self-aligning standoffs for the lens holder;  
         [0012]      FIG. 5  schematically illustrates a top perspective view of the lens holder shown in  FIG. 4 ;  
         [0013]      FIG. 6  schematically illustrates a tiered arrangement of several lens holders stacked in parallel configuration to position and align several optical components; and  
         [0014]      FIG. 7   a - h  shows a process flow description for microfabrication of a microlens holder using lithography and reactive ion dry etching; 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0015]     Referring now to the figures, wherein like elements are numbered alike throughout, and in particular in  FIGS. 1 and 2 , a microcomponent holder  100  in accordance with the present invention is shown. The holder  100  may have a generally annular shape and may include annular outer segments  103  disposed about the periphery of the holder  100 . The microcomponent holder  100  may also include one or more support members  104  which extend inwardly from the outer annular segments  103  of the holder  100  for retaining a microcomponent within the holder  100 . The support members  104  may be configured to provide an aperture  102  therebetween into which a microcomponent  110 , such as a microlens, may be positioned and retained by the holder  100 . As illustrated in  FIG. 1 , the aperture  102  may have a generally circular or other suitable shape.  
         [0016]     For example, the support members  104  may be loop-shaped and have two ends  111  that are monolithically joined to the outer annular segments  103  at fulcrums  105 . Alternatively, as illustrated in  FIG. 3 , a microcomponent holder  300  may include cantilevered support members  304  that have the shape of a spiral arm with a single point of monolithic joinder to the periphery of the holder  300  at a fulcrum  305 .  
         [0017]     Returning to the configuration of  FIG. 1 , the support members  104  may include a central loop portion  107  which includes a mounting surface  109  that may have a complementary shape to that of the microcomponent to be retained in the holder  100 . For example, as illustrated in  FIG. 2 , the central loop portions  107  may have an arcuate shape suitable for engagement with a circular microcomponent, such as microlens  110 . Alternatively, the support members  104  may have other shapes suited to holding particularly shaped microcomponents. The support members  104  may further include a recessed area, such as a groove, or may include a protrusion, such as a tab  106 , to assist in retaining the microcomponent in the holder  100 . In the particular configuration illustrated, the three support members  104  have tabs  106  that contact the sides of the microcomponent  110  at three equiangular positions around the periphery of the microcomponent  110 .  
         [0018]     The support members  104  may deform, for example in the radial direction, upon placement of the microcomponent  110  within the aperture  102  so that the support members  104  conform to the shape of the microcomponent  110  to thereby assist in securely retaining the microcomponent  110  within the holder  100 . The support members  104  may be designed to deform by virtue of their shape, size, and material of construction. In particular, the support members  104  may elastically or inelastically deform. For instance, the support members  104  may resiliently deform to function like a spring and provide a force against the microcomponent  110  to retain a microcomponent within the holder  100 . For example, for an annular holder such as holder  100 , the support members  104  may be configured to provide a radially inward forced directed towards the center of the aperture  102 . In particular, it may be desirable to configure the support members  104  to permit self-centering of the microcomponent  110  within the holder  100 . For example, as shown in  FIGS. 2 and 3 , the support members  104 ,  304  may be symmetrically disposed about the center of the aperture so that a circular microcomponent  110 ,  310  is self-centered within the aperture.  
         [0019]     The holder  100  may also desirably include one or more reference surfaces  108 , such as an edge, disposed at a fixed, known location relative to the center of the aperture  102  so that the retained microcomponent  110  may be placed at a known location relative to other components in a system based on the position of the reference surface  108  within the system. In this regard, the holder  100  may desirably comprise a monolithic part that includes the annular segments  103 , support members  104 , and reference surface  108 . Provision of a monolithic part promotes accurate location between the reference mounting surface  108  and support members  104  thereby avoiding potential alignment errors associated with assembling discrete components. A monolithic holder may be fabricated from materials amenable to micromachining and/or photlithographic processes, such as a silicon wafers.  
         [0020]     The reference surface  108  may be located on the periphery of the holder  100 , or interior to the periphery and may have any shape suited to providing a reference on the holder  100  from which the location of a desired feature of the microcomponent  110  can be determined. For example, the reference surface  108  may include a flat surface disposed at a known position relative to the center of the aperture  102 , as shown in  FIGS. 1-3 . The reference surface  108  may desirably include a structure that is not rotationally symmetric about the center of the aperture  102 , such as a flat surface, so that a unique angular orientation of the holder  100  may be determined. Such a configuration may be particularly desirable for use with microcomponents that have rotationally asymmetric properties, such as a linear polarizer. The reference surface  108  may also provide a mounting surface, and in this regard may be seated in a slot to aid proper positioning. Further, the microcomponent holder  100  may be translated and aligned on the surface of a baseplate, by seating the reference surface  108  in a groove that allows motion in one dimension.  
         [0021]     Alternatively, as shown in  FIGS. 4 and 5 , one or more reference surfaces may be provided interior to the microcomponent holder  400  in the form of sockets  406 , for example. The sockets  406  may comprise cylindrical holes through the microcomponent holder  400  suitable for receiving registration elements such as microspheres  412 . The microcomponent holder  400  can be aligned with respect to a baseplate  408  which has depressions  410  configured to receive the microspheres  412 . The depressions  410  are located in the baseplate  408  in a corresponding arrangement to the sockets  406  of the microcomponent holder  400  so that each microsphere  412  may be seated within a holder socket  406  and an opposing baseplate depression  410 . With each of the microspheres  412  engaged between an opposing socket  406 /depression  410  pair, the microcomponent holder  400  will resist further translational or rotational motion relative to the baseplate  408 , thus effecting alignment of the microcomponent holder  400  with the baseplate  408 . Thus, a microcomponent, such as a lens, retained within an aperture  402  of the holder  400  can be optimally positioned and optically coupled to a microcomponent  414 , such as a detector or laser diode, mounted on the baseplate  408 .  
         [0022]     In yet a further aspect of the present invention,  FIG. 6  shows a tiered arrangement  600  of microcomponent holders, with the use of multiple microcomponent holder plates  602 ,  604 ,  606 , each comprising one or more microcomponent holders  620 , and each realized as annular-shaped apertures with structural elements that abut and hold a microcomponent  610 ,  612 ,  614 , similar or analogous to that as described with respect to  FIGS. 1-3 . The holder plates  602 ,  604 ,  606  are connected to one another using mountings elements  608 , such as rods, that function as alignment pins and/or support columns which may be retained within respective microcomponent holders  620  of the holder plates  602 ,  604 ,  606  to offset the holder plates  602 ,  604 ,  606  to form an evenly spaced tier of plates. In addition, one or more of the microcomponent holders  620  of each holder plate  602 ,  604 ,  606  may be loaded with microcomponents  610 ,  612 ,  614 . Such an assembly of stacked microcomponent holders  620  permits multiple microcomponents to be accurately aligned along a common optical axis, for example. For instance, a microcomponent, such as a die  610 , may be bonded to a selected holder plate  602  and aligned with microcomponents  612 ,  614 , e.g., lenses, disposed on other holder plates  604 ,  606 . The die  610  may include may include, for example, a detector, laser diode, or light-emitting diode. The spacing between plates may be adjusted by applying a force parallel to the alignment rods to slide the holder plates along the rods. Optionally, microcomponent holders  620  of the same configuration may be used to retain mounting elements  608  as well as the microcomponents  612 ,  614 . That is, the microcomponent holders  620  may be configured to accept either a microcomponent  612 ,  614 , such as a microlens, or an alignment rod  608 , as depicted in  FIG. 6 .  
         [0023]     The monolithic, microcomponents described above may be made by microfabriaction technologies including lithography and dry etching, for example, reactive-ion-etching, of polished silicon wafers, such as those used to make semiconductor devices. In the present invention, the etching may be used to selectively and completely remove portions of silicon unprotected by photoresist. As such, designs for microcomponent holders, and in particular for mounting elements, may desirably rely on the continuity of all geometric features, and in particular, the mounting elements may be contiguous with the annular portions of the device. In this regard, the shape and dimensions of the mounting element, along with the thickness of the wafer and the intrinsic mechanical properties determine the elastic behavior and utility of the mounting element in clamping objects of variable shape and size.  
         [0024]     The microcomponent holder can be made by a number of techniques. The following examples offer three approaches that are representative of feasible methods to realize microcomponent holders. However, it will be understood to those skilled in the art that there are numerous modifications of these processes, as well as alternative processes and techniques, that would also be workable.  
       EXAMPLE 1  
     Fabrication of Metal Microcomponent Holder by Electroforming  
       [0025]     A microcomponent holder can be made by electroforming techniques, whereby a metal holder structure is formed by electroplating metal onto a template that is lithographically-defined in a photoresist coating deposited on a supporting substrate. The shape of the electroplated metal part conforms to the topography of the patterned photoresist layer. The electroformed part is then separated from the photoresist-coated substrate, somewhat akin to releasing a cast workpiece from a mold.  
         [0026]     To make the template used to electroform the microcomponent holder, photoresist is applied to an electrically-conductive substrate, such as a doped silicon wafer. Typically, the silicon wafer is first coated with a thin (e.g., 50-200 Å) conductive metal layer (e.g., Ti, Cr) deposited by evaporative coating or sputtering. This first-deposited metal layer is then coated with a second conductive layer, such as a film of gold, platinum, copper, or nickel, with a thickness typically ranging from 150-400 Å. The second layer functions as a ‘priming’ or ‘seeding’ layer that promotes adhesion to the substrate and provides an electrically conductive path for the plating process. Next, the conductive substrate is coated with a thick (e.g., 30-400 microns) photoresist, that serves as a mask in a subsequent electroplating step. The photoresist is patterned by photolithography techniques. The patterning produces openings in the photoresist mask that expose selected areas of the substrate to an electrolytic plating solution upon immersion of the substrate in a plating bath. To plate the metal, the substrate is immersed in an electroplating bath containing a solution with metal component(s). The backside of the substrate (opposite the side of the substrate bearing the photoresist layer) is connected to an electric power supply so that the substrate functions as an electrode in an electrolytic plating reaction. A second electrode is also immersed in the plating bath. The polarity and magnitude of a voltage difference imposed between the substrate and second electrode is such that the substrate functions as a cathode and the second electrode functions as an anode in an electrolytic plating reaction wherein metal constituents of the plating bath solution are deposited on the cathodic substrate. Because the photoresist layer is electrically insulating, there is virtually no electro-deposition of metal on the photoresist layer itself. On the other hand, parts of the photoresist layer that have been removed in the patterning process and where, as a result of said patterning, areas of the underlying substrate are exposed to the plating solution will witness preferential electroplating of metal. Typically, nickel is used as the electroplated metal although other plated metals such as gold, copper, nickel-iron, nickel-cobalt and other alloys may be used.  
         [0027]     To increase the thickness of the electroformed part, several plating and photolithography patterning steps may be performed in succession. The final step is to release the microcomponent holder from the substrate wafer, as for example by dissolving the photoresist layer. In practice, the photoresist pattern will define a multitude of microcomponent holders that will be electroformed simultaneously on the same wafer in the plating process, and further several or more wafers can be processed in a batch operation. Thus, a large number of microcomponent holders can be manufactured in a cost effective manner with a high degree of reproducibility and consistency between individual components.  
       EXAMPLE 2  
     Fabrication of Silicon Microcomponent Holder by Etching  
       [0028]     The following process, shown schematically in  FIG. 7  where side-view cross-sections of the workpiece in successive stages of fabrication are depicted, can be used to shape a microcomponent holder from a silicon wafer according to the designs disclosed herein.  
         [0029]     As in  FIG. 7   a , the starting material is a silicon wafer  702 . The thickness of the wafer depends on the targeted ultimate thickness of microcomponent part and its desired mechanical properties, and typically will be in the range of 400 to 1000 microns. Suitable silicon wafers are commonly used and commercially available in the microelectronics industry for fabrication of silicon semiconductor devices and integrated circuits.  
         [0030]     The wafer is cleaned, for example, with an oxygen plasma. In the next step ( FIG. 7   b ), 200 nm thick aluminum masking layers  704 ,  706  are deposited on both sides of the wafer  702  using sputter deposition, or thermal or electron-beam evaporation. The aluminum layer  704  formed on one side of the wafer, referred to as the backside of the wafer  702 , acts as an etch-stop layer, i.e., a protective barrier coating that is particularly resistant to subsequent silicon etching processes. The side of the wafer  702  opposite the backside is referred to as the frontside of the wafer. An aluminum masking layer  706  on the frontside of the wafer provides a similar etching barrier function to be patterned to serve as a stencil during etching of the silicon substrate. Other metals such as chrome, or dielectric layers including silicon dioxide formed by thermal oxidation of the silicon surface, can also be used as etch masks, for example. Silicon-on-insulator (SOI) wafers which contain an embedded oxide layer below one surface of the silicon wafer, and wafers with resist masks can also be used. The best thickness of metal or dielectric mask layers is highly dependent on the characteristics of the silicon etching step used to shape the microcomponent holder.  
         [0031]     In the next step ( FIG. 7   c ), a photoresist layer  708  is applied to the frontside of the wafer  702  and exposed with ultraviolet light through a photolithography mask in a mask aligner system. The photoresist layer  708  is developed with a commercial developer solution and baked. As shown in  FIG. 7   d , the process steps of exposure and development selectively remove areas to provide openings  710  of the photoresist layer  708  according to a pattern defined by the photolithography mask through which the workpiece is exposed. The aluminum areas  714  unprotected by the photoresist layer  708  are then etched from the silicon wafer  702  with a phosphoric acid solution, leaving exposed silicon surfaces  716  according to the photoresist mask openings  710 , as indicated in  FIG. 7   e . At this stage of the process corresponding to  FIG. 7   f , the photoresist has served its purpose and can be stripped from the workpiece using solvents recommended by the photoresist manufacturer, leaving a silicon wafer  702  protected on the back side by continuous metal film  704  and on the opposing frontside by a metal mask stencil  718 , realized by lithographic patterning of metal film  706 .  
         [0032]     As indicated in  FIG. 7   g , the silicon wafer  702  with the patterned aluminum mask  718  defining the microcomponent holder shape is then subjected to a ‘dry’ reactive ion etch (RIE) process which removes silicon in areas  716  not protected by the aluminum mask. The reactive ion etch may be performed according to the Bosch process (F. Laermer and A. Schilp, “Method of Anisotropically Etching Silicon”, Robert Bosch GmbH: U.S. Pat. No. 5,501,893, which is incorporated herein by reference.). The reactive ion etching shapes the silicon wafer  702  according to the pattern defined by the photolithography mask  718 . The silicon etching sculpts the wafer  702  completely etching through the wafer  702  in selected areas  720 .  
         [0033]     After reactive-ion etching, and as shown in  FIG. 7   h , the metal masks  704 ,  706  from both sides of the silicon wafer  702  are then stripped from silicon wafer  702  using, for example in the case of aluminum masks, a hydrochloric acid etchant. According to the designs, the silicon remaining after the etch step is a single continuous piece  722  with sufficient structural stability. Commonly, a portion of the periphery of the silicon wafer is protected and preserved in the etching to facilitate handling. Typically, a photolithography mask with multiple, repeated patterns for plural microcomponent holders is used so that many holders can be fabricated simultaneously. The individual lens (or other microcomponent) holders are then singulated from the silicon workpiece by cleaving or dicing the silicon wafer  702 .  
       EXAMPLE 3  
     Fabrication of Polymer Microcomponent Holder  
       [0034]     As a third example of a method for fabricating microcomponent holders, a structural photoresist is patterned in the shape of the component. In this case, the photoresist is not merely a means by which the microcomponent holder member is shaped, but rather the component is comprised of the photoresist material itself. That is, the photoresist is non-sacrificial. A thick layer of photoresist is applied to a substrate. The photoresist is patterned, developed, and hardened. The shaped photoresist is then separated from its substrate, yielding a free-standing microcomponent holder. Structural photoresists such as SU-8 (Microchem, Inc., Newton, Mass.) are well-suited for this task. SU-8 is an epoxy-type, near-UV sensitive photoresist that can be processed in thicknesses in excess of 1 millimeter, while achieving sufficient aspect ratios.  
         [0035]     It will be recognized by those skilled in the art that changes or modifications may be made to the above-described invention without departing from the broad inventive concepts of this invention. It is understood, therefore, that the invention is not limited to the particular embodiments disclosed herein, but is intended to cover all modifications and changes which are within the scope of the invention as defined in the appended claims.