Abstract:
A method for forming a micro-optical switch component includes providing a semiconductor substrate having a surface. An opto-electronic device is integrated into the semiconductor substrate at a site. A pedestal of microlens material is formed on the semiconductor substrate surface at the site of the opto-electronic device. The pedestal extends from the semiconductor substrate surface and has a top surface spaced apart from the semiconductor substrate surface. A print head is provided and contains an optical fluid which is hardenable and capable of serving as a micro-optical element. The printhead includes an orifice from which micro-droplets of the optical fluid are ejected in response to control signals. Optical fluid is deposited onto the top surface of the pedestal to thereby form a micro-optical element on the pedestal.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention relates to methods for forming micro-optical switch components, and more particularly to an integrated fiber optic switch. 
     BACKGROUND OF THE INVENTION 
     The demand for high-speed data transmission has accelerated the development of optical networks. For a local area network and very short reach data links, high efficiency and low equipment/operational cost have become central issues for meeting market needs. Local networks have evolved to include the use of vertical-cavity surface-emitting lasers (VCSEL) and PIN photodetector (PD) as light transmitters and receivers, respectively, and use multimode fiber (MMF) as signal transport media. A VCSEL is a diode laser where the laser oscillation and output occur normal to the PIN junction plane. Such lasers are formed in a structure of semiconductor layers deposited on a semiconductor substrate, and emit light from a port in the surface of the structure. A VCSEL generates a much more symmetrical light beam than an edge-emitting laser. As a result, the light from the VCSEL can be coupled into the optical system of a laser printer or optical communication link more efficiently than the light from an edge-emitting laser. Low divergent circular output, single longitudinal mode operation, and high two-dimensional packaging density for arrays, make VCSELs attractive for applications such as optical recording, communications, and computing. 
     Parallel technology has been applied to VCSEL arrays, PD arrays and fiber ribbons. Specific electronic circuits for driving VCSELs, processing PDs output signals, as well as for implementing small factor connectors have gradually standardized transceivers for short range communications. However, deficiencies exist in dealing with giga-bit-per second level of transmission over reasonable distance with a single channel. These limitations occur from the integration of optoelectronic parts with electronic circuits. Current VCSEL-microlens array integration schemes utilize wire connectors in which the parasitic capacitance of the wire connection limits the data processing rate of the unit. As a result, size-sensitive applications, and chip level integration have been a focus in the development of VCSELs for telecommunication and data communication applications. 
     Application technologies for VCSELs, PDs and complimentary-metal-oxide-semiconductor (CMOS) electronic circuits are well known. However, the interconnections, both electrical and optical, are difficult due to the small physical size, specific geometry, and materials employed. The simultaneous achievement of both interconnect types creates additional difficulties due to interactions. 
     Existing approaches for integrated switches utilize flip-chip bonding to attach the VCSEL and detector array to a silicon chip. Light passes through vias etched on the silicon chip and is coupled to the fibers by a reflection mirror. Additionally, a co-planar design is utilized in which the n-contact of the VCSEL must be removed after bonding to leave a path for the top emitting laser. It is also possible to bond the n-contact to the substrate and then remove the substrate beyond the n-contact, resulting in bottom emitting. For other structures, many processing steps are required for electrical connection, etching, metal deposition, reflow and rinsing. Additional processing steps are necessary for the alignment and coupling of light into and out of fiber array. Existing designs have severe cost disadvantages. Therefore, a need has arisen for new optical switches with new fabrication methods to meet existing market needs. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a method for forming a small, low cost, integrated fiber optic switch is provided. The switch component is based upon VCSEL arrays and PD arrays, both coupled directly to an integrated circuit. The use of ink jet dispensing of polymers and solders create high quality optical and electrical interconnects to the active elements. Collimating and focusing polymer microlenses are printed directly onto the VCSEL arrays with photolithographic accuracy so that the light emitting from the VCSELs will directly couple into arrays of optical fibers. Collimating and focusing polymer microlenses are also utilized for coupling light from optical fibers into detector arrays. Ink jet dispensing of solders is utilized to electrically interconnect the active optical elements to the integrated circuit with minimal interconnect distance. 
     In accordance with the present invention, a method for forming a micro-optical switch component includes providing a semiconductor substrate having a surface. An opto-electronic device is integrated into the semiconductor substrate at a site. A pedestal of microlens material is formed on the semiconductor substrate surface at the site of the opto-electronic device. The pedestal extends from the semiconductor substrate surface and has a top surface spaced apart from the semiconductor substrate surface. A print head is provided and contains an optical fluid which is hardenable and capable of serving as a micro-optical element. The print head includes an orifice from which micro-droplets of the optical fluid are ejected in response to control signals. Optical fluid is deposited onto the top surface of the pedestal to thereby form a micro-optical element on the pedestal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention and for further advantages thereof, reference is now made to the following Description of the Preferred Embodiments taken in conjunction with the accompanying Drawings in which: 
     FIGS. 1 a  and  1   b  are schematic diagrams illustrating the formation of an opto-electronic component of an optical switch in accordance with the present invention; 
     FIGS. 2 a ,  2   b ,  2   c , and  2   d  are schematic diagrams illustrating the formation of a pedestal used with the present opto-electronic components; 
     FIG. 3 is a schematic diagram illustrating the formation of a pedestal array on a semiconductor substrate; 
     FIG. 4 is a schematic diagram illustrating the formation of a microlens on a pedestal; 
     FIG. 5 is a block diagram illustrating the components of a semiconductor substrate for use with the present method for forming an optical switch; 
     FIG. 6 is a schematic block diagram illustrating the formation of a VCSEL array and PD-array formed on the semiconductor substrate of FIG. 5; 
     FIG. 7 is an enlarged perspective view of an electrical connection between a VCSEL and semiconductor substrate of the optical switch of FIG. 6; and 
     FIG. 8 is a schematic diagram illustrating an ejection head for dispensing solder for forming an electrical connection between an optoelectronic component and the semiconductor substrate of FIG.  5 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIGS. 1 a  and  1   b , schematic diagrams illustrating the formation of an optical switch component in the form of a VCSEL array is illustrated. A VCSEL  20  is formed in a semiconductor substrate  22  using techniques well known to those skilled in the art. Typical spacing between VCSELs in an array is about 250 microns, center-to-center. In accordance with the present method, a pedestal  24  is fabricated directly on substrate  22  at the site of VCSEL  20 . A microlens  26  is formed directly on pedestal  24 . Light emission of VCSEL  20  propagates through pedestal  24  and is coupled to an optical fiber  28  by microlens  26 . FIG. 1 a  illustrates light being collimated when coupled to optical fiber  28 , while FIG. 1 b  illustrates light being focused on to optical fiber  28 . 
     Referring now to FIG. 2, the formation of pedestal  24  in accordance with the present invention is described. FIG. 2 a  illustrates the formation of a thick film  36  of photosensitive polymer with acceptable transmission properties at the VCSEL wavelength either positive or negative which is coated on substrate  22  by spin coating or other known methods. The thickness of film  36  is in the range of about 50 to about 250 microns. The thickness of film  36  corresponds to the height of pedestals  24 . 
     FIGS. 2 b  and  2   c  illustrate ultraviolet exposure of thick film  36  through a photomask containing a pattern of pedestals  24 . FIG. 2 b  illustrates thick film  36  as a positive photosensitive polymer. A photomask  38  has mask sites located above VCSELs  20  for masking ultraviolet radiation  40  from the site of VCSELs  20  fabricated within substrate  22 . FIG. 2 c  illustrates thick film  36  as a negative photosensitive polymer having a photomask  42  having mask sites for allowing ultraviolet radiation  40  to expose the surface of substrate  22  at the site of VCSEL  20 . Photomasks  38  and  42  contain the pedestal  24  pattern which is aligned concentrically with the VCSEL  20  pattern on substrate  22 . Photomasks  38  and  42  with reversed colors are used for positive and negative photomasks, respectively. The pre-exposure and post-exposure bake are performed before and after the exposure. 
     FIG. 2 d  illustrates the formation of pedestals  24  which are formed after pattern developing. The diameter of pedestals  24  is in the range of about 50 to about 225 microns. FIG. 2 illustrates one technique for forming pedestals  24 , it being understood that other techniques can be used, and the present invention is not limited to the technique described. For example, a mask can be formed on the surface of a non-photosensitive film  36  and a dry etch technique used to form pedestals  24 . 
     FIG. 3 is a schematic diagram illustrating the formation of an array of pedestals  24  fabricated directly on semiconductor substrate  22  at the site of VCSELs  20 . The array of pedestals  24  is aligned with the array of VCSELs  20 . Although pedestals  24  are shown having a circular configuration, other shapes, such as, for example, elliptical can be used depending on the characteristics of light being emitted from VCSELs  20  and the coupling desired. 
     Referring now to FIG. 4, in accordance with the present invention, microlenses  26  are printed directly on the top surface  48  (FIG. 3) of pedestals  24 . A fluidic optical polymer is printed directly on top surface  48  of pedestals  24  to form a microlens  26  upon curing. Each microlens  26  is self-centered to the underlying pedestal  24  by surface tension of the dispensed fluid. The height of microlens  26  is in the range of about 25 to about 120 microns, depending upon the design of optical coupling to optical fiber  28 . 
     Microlens  26  printing is performed using a digitally-driven printhead  50  depositing a predetermined size and number of micro-droplets  52  of optical fluid onto surface  48  of pedestals  24  to form microlenses  26 . Methods of operating printhead  50  to deposit optical polymeric material in a fluid state are disclosed in U.S. Pat. Nos. 5,498,444 and 5,707,684 both entitled “Method for Producing Micro-Optical Components” by the assignee hereof, the disclosures of which are incorporated herein by reference. Printhead  50  ejects micro-droplets  52  of optical fluid through an orifice  54 . The diameter of orifice  54  is preferably between about 20 microns to about 120 microns. Printhead  50  includes a piezoelectric device operable in a drop-on-demand mode and is heatable to control the viscosity of the optical fluid. The movement of printhead  50  and substrate  22  relative to each other is computer-controlled. Substrate  22  is positioned on a computer-controlled stage moveable in the x-y plane. The computer moves the stage so that a pedestal  24  is positioned to receive optical fluid micro-droplets  52  deposited by the digitally-driven printhead  50 . Ejection of micro-droplets  52  by printhead  50  is preferably controlled by the same computer. After printing a microlens  26 , the computer moves substrate  22  to position the next pedestal  24  under the ejection orifice  54  and then activates printhead  50  to eject the micro-droplets  52  onto the next pedestal  24 . The height of microlens  26  is determined by the number and size of micro-droplets  52  deposited on top surface  48  of pedestals  24 . 
     The optical fluid utilized by printhead  50  can be any material, or combination of materials, capable of forming a relatively transparent micro-optical element after hardening. Optical epoxies are an example. Commercial materials which are suitable for forming micro-optical lenses  26  include Summers Optical SK9 (Refractive Index 1.49) by Summers Optical, Inc., P.O. Box 162, Fort Washington, Pa. 19034; Norland No. NOA-73 (Refractive Index 1.56) by Norland Products, Inc., P.O. Box 7149, New Brunswick, N.J. 08902; and Epotek No. OG-146 (Refractive Index 1.48) by Epoxy Technology, Inc., 14 Fortune Drive, Billerica, Mass. 01821. In a preferred embodiment of the present method, an ultraviolet (UV) light-curable epoxy is utilized to form microlenses  26 . When used, the diameter of the epoxy micro-droplets  52  is in the range of about 8 microns to about 300 microns. Typically, micro-droplets  52  would be in the range of about 50 microns. 
     Referring now simultaneously to FIGS. 5 and 6, a fiber optic switch generally identified by the numeral  60 , is illustrated as produced by the method of the present invention. Switch  60  is formed on a semiconductor substrate  62  such as, for example, a silicon integrated circuit. Semiconductor substrate  62  includes three subsystems, a laser driver array  64  for driving VCSELs  20  formed in a VCSEL array  66 , a transimpedance amplifier array  68  coupled to a photodetector (PD) array  70  and an electronic switching subsystem  72 . Photodetector array  70  is composed of multiple PIN photodetectors formed in a substrate  72  and includes pedestals  24  and microlenses  26  fabricated in a manner similar to the fabrication of pedestals  24  and  26  previously described with respect to VCSELs  20 . VCSELs  20  and PIN diodes of photodetector array  70  are formed utilizing methods described in, for example, U.S. Pat. Nos. 5,285,466; 5,577,064; 5,812,582, and 5,835,514, whose descriptions are hereby incorporated by reference. 
     Laser driver array  64 , transimpedance amplifier  68  and electronic switching subsystem  72  are integrated into semiconductor substrate  62 . Laser driver array  64  functions to provide appropriate currents for bias and modulation of the VCSELs  20 . Laser drivers and driver arrays are manufactured and sold by AMCC and Maxim Integrated Products such as, for example, MAX3273 which has a programmable bias current range of about 1 mA to about 100 mA and a programmable modulation range of about 5 mA to about 60 mA and includes a power control circuit. Transimpedance amplifier array  68  includes front-end amplifiers to amplify the current from photodetector array  70  to produce a differential output voltage. Transimpedance amplifier array  68  may include, for example, an AMCC transimpedance amplifier S3090 which detects signals down to 19 μA(peak) with a signal-to-noise ratio of 21.5 dB. The outputs of this device are buffered and voltage limited to 1.4 v. Switching subsystem  72  receives an amplified signal from one of the PIN detectors of photodetector array  70  and routes the signal to a laser driver within laser driver array  64  which converts the digital signal to the analog signal that drives a VCSEL  20 . 
     As illustrated in FIG. 6, photodetector array  70  and VCSEL array  66  are electrically bonded at right angles to semiconductor substrate  62 . The size of a single VCSEL is approximately 250 microns on each edge and results in an approximate square cross-section. Photodiode array  70  may include, for example, PIN photodetectors having four element AlGaAs arrays on 250 micron centers. 
     Microlenses  26  associated with VCSEL array  66  function to couple light from VCSELs  20  to the optical fibers  28 . Microlenses  26  associated with photodetector array  70  couple the fiber output light to the PIN detector surface. The parameters of pedestal  24  and microlens  26  (height, radius of curvature, index of refraction and diameter) are selected to maximize the coupling efficiency from VCSELs  20  to optical fibers  28  and from optical fibers  28  to photodetectors. 
     VCSELs  20  and PIN photodetectors formed in accordance with the present invention can also be utilized in optical transceivers in which a VCSEL  20  and phodetector are typically located on the same side of the integrated circuit. Additionally an array of alternating VCSELs  20  and photodetectors can be formed using the present optical components. 
     Referring now to FIGS. 7 and 8, the present method utilizes the print head  80  for dispensing microdroplets of solder  82  for electrically interconnecting VCSEL array  66  and photodetector array  70  to semiconductor substrate  62 . The solder interconnect electrically connects a metal pad  84  on semiconductor substrate  62  to a metal pad  84  on VCSEL array  22  and a metal pad  88  on photodetector array  70  (FIG.  8 ). Print head  80  accurately places a molten drop of solder  82  at the location where the two pads meet. The solder will then flow and wet the pads and form a mechanical and electrical joint. The second electrode on the back of VCSEL array  66  and photodetector array  70  is connected using the same process. Methods of operating an ink jet print head to deposit solder are disclosed in U.S. Pat. Nos. 5,229,016 and 5,377,902 by the assignee hereof, the disclosures of which are incorporated by reference. As illustrated in FIG. 8, the bonding of pads  88  to pads  84  creates an approximate 90° solder joint and the jetting angle of print head  80  is at approximately 45° from normal to semiconductor substrate  62 . One of the main advantages of using ink jet deposition of solder is that it has been shown to be a fluxless process which is critical to keeping optical surfaces clean. 
     It therefore can be seen that the present method provides for the formation of an integrated fiber optic switch based upon VCSEL arrays and PD arrays, both coupled directly to an integrated circuit. Ink jet dispensing of polymers and solders is used to create high quality optical and electrical interconnects to the active elements. Collimating and focusing polymer microlenses are printed directly on the VCSEL arrays and PD arrays with photolithographic techniques so that light emitting from the VCSELs will directly couple into arrays of optical fibers and light emitting from optical fibers will be directly coupled to detector arrays. The optical fibers may be multimode or single mode. 
     Whereas the present invention has been described with respect to specific embodiments thereof, it will be understood that various changes and modifications will be suggested to one skilled in the art and it is intended to encompass such changes and modifications as fall within the scope of the appended claims.