Abstract:
A large diameter glass wafer is pattern-etched to provide a plurality of elongated lens elements arranged side-by-side, the etching leaving small rods in place to keep the lens elements connected to the wafer during mirror processing. The etching provides curved surfaces for lenses and flat surfaces for mirrors. The mirrors are formed by selectively depositing reflective material on the flat surfaces. The reflective material may comprise an oxide, nitride, sulfide, or fluoride of a transition metal. The flat surfaces that define the mirrors are disposed at angles to the longitudinal dimension of each lens element. In use in an optical disc system, light from a laser diode is reflected by the mirrors and directed at an optical disc through a first lens. Light returns from the disc on a parallel path through a second lens, passes through the lens element, and is directed at a photodetector. The system may include an elongated base element attached to each lens element. Pattern-etching of a second glass wafer provides multiple base elements per wafer. Each base element may include an angled surface on which a reflective material is deposited to form a mirror for reflecting laser light during use in the system.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application is a divisional of application Ser. No. 10/025,601, filed Dec. 18, 2001, now U.S. Pat. No. 7,006,426 from which priority is claimed. 

   BACKGROUND OF THE INVENTION 
   The present invention generally relates to optical elements for use in transmitting laser light to and from an optical disc, and more particularly to techniques for miniaturizing such optical elements. 
   SUMMARY OF THE INVENTION 
   A principle object of the present invention is an improvement in the manufacture of glass components and related structures used in optical disc systems. The present invention has advantageous application with miniature optical discs. 
   Other objects and advantages of the invention will become apparent from the following description of the invention, with reference to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a plan view of a glass wafer that has been etched to provide multiple lens elements; 
       FIG. 2  is a plan view of a glass wafer that has been etched to provide multiple base elements; 
       FIG. 3  is a perspective view of a lens element after it has been separated from the wafer of  FIG. 1 , the view looking at the back side of the lens element; and 
       FIG. 4  is a side view of an assembled optical unit and supporting semiconductor chip that provides an electro-optical interface, the optical unit including a lens element mounted on a base element. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring specifically to  FIG. 1 , a clear glass wafer  10  is shown with multiple lens elements  12  therein, the lens elements having been defined by etching away portions of the wafer. Before the lens elements  12  are etched to arrive at the configuration shown in  FIG. 1 , certain surface features are formed, as described below in connection with  FIG. 3 . In particular, each element  12  has a slot  14  that preferably extends halfway down through the element to enable formation of a reflective surface therein. The elements  12  are supported as part of the etched wafer  10  by vertical glass rods  16  that remain after portions of the wafer have been etched out. 
   Starting wafers  10  can be cut from larger sheets of optical-grade glass in circular diameters that correspond to the size of conventional silicon wafers used in semiconductor manufacturing. Such wafers are typically sized in two-inch increments, such as six-inch, eight-inch, or ten-inch wafers. A V-groove  18  can be cut at one point in the circumference of the wafer  10  for automatic alignment purposes with handling equipment. Alternatively, as with some conventional silicon wafers, a flat edge extending several centimeters along one edge can be used instead of the V-groove  18  specifically shown. Such flat edges are typically used with six-inch silicon wafers. A preferred thickness for a six-inch glass wafer that is suitable for the present invention is about 0.75 mm. 
   The etching used to produce the structure shown in  FIG. 1  can be accomplished using techniques that are well known in the semiconductor industry. A conventional photoresist is applied to the starting wafer. Then, an image of the pattern of material to be etched out is exposed on the photoresist using conventional semiconductor photolithography. The exposed photoresist is then dissolved and removed from the wafer. Next, the wafer is subjected to a plasma etching technique that cuts straight through the glass wafer in the areas that are not covered by the photoresist. Such plasma etching techniques are conventional and are used for etching silicon dioxide and other glass-like materials in semiconductor processing. The remaining photoresist is then removed from the wafer. 
   Now referring to  FIG. 2 , a glass wafer  20  is shown after it has been etched using a different pattern to produce multiple base elements  22 , which are interconnected with each other and the wafer  20  by glass rods  24  in a manner similar to that previously described in connection with the wafer  10  of  FIG. 1 . After formation of the wafers  10  and  20  in the configurations shown in  FIGS. 1 and 2 , the lens elements  12  and base elements  22  are separated from the wafers by cutting through the rods  16  and  24 . The cutting operation can be performed using a conventional diamond saw process. 
     FIG. 3  shows additional details of one of the lens elements  12  that has been separated from its starting wafer. The slot  14  can be seen extending from a front surface  26  to a depth of almost exactly one-half the thickness of the element  12 . A reflective film is formed on the walls within the slot  14  to provide a mirror  28 . A front lens  30  is formed at one end of the element  12  extending inward from the front surface  26 . A back lens  32  is juxtaposed next to the front lens  30  and extends inward from a back surface  34  of the lens element  12 , each lens occupying almost exactly one-half the thickness of the element  12 . It will be noted that each of the lenses  30  and  32  has a curved surface that retains the same curvature in cross-sectional planes parallel to the opposed front and back planar surfaces  26  and  34 . This is caused by plasma etching straight down into the wafer. The two lenses  30  and  32  permit laser light to enter and exit the lens element  12  along different parallel paths, as explained below. Partially reflective mirrors  36  and  38  at opposite ends of the element  12  are used to reflect light beams traveling through the lens element. 
   Various reflective thin films formed on the glass surface of the elements  12  and  22  are suitable for the present invention. The reflective thin films preferably include a transition metal with a high reflection value in a form that is suitable for conventional deposition techniques. Certain oxides, nitrides, sulfides, and fluorides of such metals are stable, highly reflective and suitable for deposition on glass surfaces. Of these materials, our experiments show that titanium nitride meets these criteria and can be deposited in different thicknesses to achieve partial reflectivity of a desired value. 
   Mirrors  36  and  38  are constructed to be about 62 percent reflective. This is preferably accomplished during processing by depositing a thin titanium nitride film on the angled surfaces at the opposite ends of the lens element  12  and on the walls within the slot  14 . The thickness of the titanium nitride film is controlled to achieve the preferred 62 percent reflectivity. Mirror  28  can be made with a like thickness of titanium nitride, or can be provided with a much thicker titanium deposition to make it much more reflective, preferably at least 90 percent reflective. The mirrors  28 ,  36 , and  38  preferably comprise multiple film layers including a protective outer coating. 
   Referring to  FIG. 4 , a side view of an assembled optical unit  40  is shown supported by a semiconductor chip  42 , which functions as an electro-optical interface for the optical unit  40 . The optical unit  40  and chip  42  together define an electro-optical device for reading an optical disc (not shown). The optical unit  40  includes a lens element  12  mounted on a base element  22  in the manner indicated. In particular, extensions  44  and  46  of the rods  16  of lens element  12  fit into corresponding recesses  48  and  50  in the top surface  52  of the base element  22 . Extensions  54  and  56  of the rods  24  of the base element  22  rest against the bottom surface  58  of the lens element  12 . The mating of the rod extensions  44  and  46  in the respective recesses  48  and  50  achieves precise optical alignment of the lens element  12  with the base element  22 . Similar recesses can be provided in the top surface of the semiconductor chip  42  for receiving the ends of the rods  24  on the bottom of the base element  22 . Such recesses are preferred but are optional. External fixtures can be used as an alternative assembly technique and adhesive can be applied to the mating surfaces.  FIG. 2  shows the base elements  22  without recesses for use in such an alternative assembly technique. 
   Photodetectors  60  and  62  are provided at the upper surface  64  of the semiconductor chip  42 . The photodetectors  60  and  62  are aligned with respective light beams  66  and  68  that originate from a light beam  70  returning from an optical disc (not shown) entering the lens element  12  through the back lens  32 . The split-beam approach using a pair of photodetectors  60  and  62  as depicted in  FIG. 4  compares corresponding electrical signals within the chip  42  to eliminate unwanted signal noise. Band-pass filters (not shown) can also be added on surface portions  72  and  74  of the upper surface  52  of the base element  22  in the path of light beams  66  and  68  to filter out unwanted noise. Alternatively, the entire base element  22  can be clad with a thin film that filters out unwanted light noise. 
   Mounted above the semiconductor chip  42  is a laser diode  76  that emits a laser light beam  78 , which is reflected upward by a mirror  80  on a 45° angled surface of the base element  22 . Mirror  80  is constructed to be nearly 100 percent reflective. The laser diode  76  is supported by a mounting block  81  that enables precise alignment of the light beam  78  with the mirror  80 . The light beam  78 , after reflection by the mirror  80 , passes through the base element  22  and enters the lens element  12  just beneath the slot  14 . The light beam  78  is substantially reflected by the mirror  28  in the slot  14  back to the right in the view of  FIG. 4  horizontally at the mirror  36 . Mirror  28  is oriented at a 45° angle to reflect the upwardly directed beam horizontally (or longitudinally with respect to the lens element  12 ) at the mirror  36 . 
   Because the mirror  36  is constructed to be about 62 percent reflective, about 62 percent of the light beam  78  is reflected up and passes out through the lens  32  as focused laser light beam  82 . Mirror  36  may be oriented at 45° to the longitudinal direction of the lens element  12  or may have a slightly different angle to accommodate the orientation of the lenses  30  and  32 . The light beam  82  is directed at an optical disc (not shown), which partially reflects back the information-containing beam  70 . The entering beam  70  returns on a path that is spaced from and generally parallel to the out-going beam  82 , as directed by optics (not shown) between the lens element  12  and the optical disc that is being read by the system. 
   Referring to  FIGS. 3 and 4  together, the information-containing light beam  70  enters the lens element  12  through the back lens  32 , which directs the beam downward at the mirror  36 . The light beam  70  is split by the mirror  36  into horizontal and downward components, the downward component corresponding to the beam  68  that impinges on photodetector  62 , as previously discussed. The horizontal component  84  of the beam  70  that is reflected by the mirror  36  passes through the back half of the lens element  12 , thus avoiding the mirror  28  in the slot  14 , and continues on horizontally striking the mirror  38 . The mirror  38 , which is oriented at 45° to the horizontal direction, reflects most of the horizontal component  84  of the information-containing beam  70  downward as beam  66 , which impinges on the photodetector  60 . The photodetector  60  is in electrical communication with analog-to-digital (A/D) converter circuitry  86  within the semiconductor chip  42 . Likewise, the photodetector  62  is in electrical communication with A/D converter circuitry  88  within the chip  42 . The A/D converters  86  and  88  are connected to logic circuitry within the chip  42  that performs data and control functions and communicates with other system components (not shown). 
   In the previous description, the lens element  12  and base element  22  are cut from separate wafers  10  and  20 , shown in  FIGS. 1 and 2 . This is useful to permit different processes to be performed on the separate parts. For example, the mirror  80  may be formed in a different way from mirrors  28 ,  36  and  38 . Since mirror  36  needs to be partially reflective to split the returning beam  70  into two components, the mirrors  28  and  38  can be made partially reflective for simplification of the process. However, it will be appreciated that mirrors  28  and  80  can be made fully reflective or almost fully reflective, consistent with process design. If the base element  22  is processed separately from the lens element  12 , it is convenient to use a different material to make mirror  80  fully reflective. However, if a more complex process is used to permit forming both partially reflective mirror surfaces and fully reflective mirror surfaces during the process sequence, then the lens element  12  and base element  22  can be cut from a single glass wafer. 
   Another consideration is the desirability of using essentially identical photodetectors  60  and  62 . Therefore, if mirrors  36  and  38  are each made to be about 62 percent reflective, then about 38 percent of the returning beam  70  will fall on each of the photodetectors  60  and  62 . The mirror  28  can be made in a similar way to mirrors  36  and  38  to be partially reflective, or can be made to be essentially fully reflective. It will also be appreciated that if mirror  38  is made to be almost fully reflective, mirror  36  can be made to be about 50 percent reflective to have the same light intensity impinging on the photodetectors  60  and  62 . However, this will reduce the intensity of the out-going laser light beam  82 . 
   According to the preferred process, and with reference to  FIGS. 1 and 3 , multiple lens elements  12  are made from a glass wafer, such as the wafer  10 , as follows. A first photoresist pattern is formed with openings that define the slots  14  for each of the lens elements  12 . Then, a conventional fluoride-based plasma etch step is performed for a time duration that etches openings corresponding to the slots  14  to a depth almost exactly halfway through the wafer  10 . Then, a conformal titanium nitride deposition step is performed to coat the walls within the openings with a thin film of titanium nitride, one surface of which later defines mirror  28 . The thickness of the titanium nitride deposition is preferably controlled to make the mirror  28  more than 90 percent reflective. The photoresist is then stripped and a second photoresist pattern is formed that has openings that define the angled surfaces at the opposite ends of each lens element  12  where the mirrors  36  and  38  are to be formed. A second plasma etch is performed to cut a second set of openings vertically down into the wafer  10 , the openings having walls that include the surfaces on which the mirrors  36  and  38  will be formed. Then, a second conformal titanium nitride deposition step is performed to coat the walls with the second set of openings with titanium nitride, the walls including surfaces that define the position of the mirrors  36  and  38  on the finished lens elements  12 . The second photoresist pattern is then stripped off the wafer  10 . The thickness of the second titanium nitride deposition is preferably controlled so that the mirrors  36  and  38  are very nearly 62 percent reflective. 
   The process continues with formation of a third photoresist pattern on the wafer  10  with openings for defining the front lens  30  of each element  12 . A plasma etch cuts down into the wafer  10  from the front surface  26  to a depth that is almost precisely halfway through the thickness of the wafer, which is achieved by controlling the etch duration. When the lens elements  12  are later separated from the wafer  10 , the front lens  30  of each element  12  will be provided by this patterning and etching sequence. 
   Next, the wafer  10  is turned upside down and a fourth photoresist pattern is formed on the back side of the wafer  10  to define the surfaces that will become the back lens  32  of each lens element  12 . Then, a plasma etch is performed to etch openings in the wafer  10  through corresponding openings in the photoresist pattern to define surfaces that include the surfaces of each back lens  32  of each lens element  12 . The fourth photoresist pattern is then stripped away. As seen in  FIG. 3 , the result of the etching is that back lens  32  is defined by a surface cut straight down from the back surface  34  halfway through the lens element  12 . 
   A fifth photoresist pattern is formed to define the outlines of the lens elements  12 , as shown in  FIG. 1 , with their connecting rods  16 . This photoresist pattern can be formed on the front side of the wafer  10 , or a mirror-image pattern can be formed on the back side. A plasma etch then is performed to cut through the wafer  10  to provide the structures shown in  FIG. 1 . Individual lens elements  12  are then provided by cutting the rods  16  as previously described. 
   A similar but less complicated sequence of steps can be used to form the base elements  22  within the wafer  20  of  FIG. 2 . Since it is desirable for mirror  80  to be fully or almost fully reflective, a relatively thick titanium nitride deposition can be used. Alternatively, an aluminum deposition can be performed. In either case, the deposition step proceeds after appropriate windows are etched through a photoresist pattern. If aluminum is used for the mirrors  80  of each base element  22 , a nearly 100 percent reflective surface can be achieved. To keep the mirrors  80  from oxidizing and adversely affecting their reflectivity, a thin glass layer is deposited on the aluminum immediately after the metal deposition step. 
   Although preferred embodiments of the invention have been described, it will be appreciated that various changes can be made without departing from the spirit and scope of the invention as defined by the appended claims.