Abstract:
Vertical cavity surface emitting laser systems and methods of making the same are described. In one aspect, a vertical cavity surface emitting laser system has a bottom side that may be flip-chip mounted to a driver substrate and a top side that is configured to transmit light through an optically transparent substrate. By this configuration, vertical cavity surface emitting laser systems may be packed together with a greater density and operated at greater speeds relative to, for example, wire bonded vertical cavity surface emitting laser systems. In addition, such systems may be flexibly tailored to produce light over a wide range of wavelengths. Such systems also may be efficiently packaged on a wafer scale.

Description:
TECHNICAL FIELD  
         [0001]    This invention relates to wafer bonded vertical cavity surface emitting laser (VCSEL) systems and methods of making the same.  
         BACKGROUND  
         [0002]    A VCSEL is a laser device formed from an optically active semiconductor layer (e.g., AlInGaAs or InGaAsP) that is sandwiched between a pair of highly reflective mirror stacks, which may be formed from layers of metallic material, dielectric material or epitaxially-grown semiconductor material. Typically, one of the mirror stacks is made less reflective than the other so that a portion of the coherent light that builds in a resonating cavity formed in the optically active semiconductor layer between the mirror stacks may be emitted from the device. Typically, a VCSEL emits laser light from the top or bottom surface of the resonating cavity with a relatively small beam divergence. VCSELs may be arranged in singlets, one-dimensional or two-dimensional arrays, tested on wafer, and incorporated easily into an optical transceiver module that may be coupled to a fiber optic cable.  
           [0003]    In general, a wafer bonding technique may be characterized as a direct wafer bonding technique or a metallic wafer bonding technique. In direct wafer bonding, two wafers are fused together by mass transport at a bonding interface. Direct wafer bonding may be performed between any combination of semiconductor, oxide, and dielectric materials. Direct wafer bonding typically is performed at high temperature and under uniaxial pressure. In metallic wafer bonding, two substrates are bonded together by a metallic layer that is melted and re-solidified at a bonding interface.  
           [0004]    Wafer bonding techniques have been used in the fabrication of optoelectronic devices. For example, U.S. Pat. No. 6,320,206 has proposed a scheme for forming optical devices having aluminum gallium indium nitride active layers and high quality mirror stacks that are wafer bonded on one or both sides of the active layers. U.S. Pat. No. 5,837,561 describes a vertical cavity surface emitting laser that is wafer bonded to a transparent substrate. A top circular metal contact is disposed on the transparent substrate and a second metal contact is disposed over the bottom mirror of the vertical cavity surface emitting laser. The transparent substrate serves as an escape medium for laser emission through the top circular metal contact. This configuration allows the heat producing active layer of the vertical cavity surface emitting laser to be mounted near a heat sink, thereby improving the performance of the device.  
         SUMMARY  
         [0005]    The invention features vertical cavity surface emitting laser systems and methods of making the same. In particular, the invention features a vertical cavity surface emitting laser system having a bottom side that may be flip-chip mounted to a driver substrate and a top side configured to transmit light through an optically transparent substrate. By this configuration, the invention enables vertical cavity surface emitting laser systems to be packed together with a greater density and operated at greater speeds relative to, for example, wire bonded vertical cavity surface emitting laser systems. In addition, such systems may be flexibly tailored to produce light over a wide range of wavelengths. Such systems also may be efficiently packaged on a wafer scale.  
           [0006]    In one aspect, the invention features a vertical cavity surface emitting laser (VCSEL) system, comprising a substrate, a vertical stack structure, and a pair of contacts. The substrate is optically transparent to light in a selected wavelength range. The vertical stack structure has a substantially planar top side, which is wafer bonded to the optically transparent substrate, and a bottom side. The vertical stack structure includes a top mirror, a bottom mirror, and a cavity region that is disposed between the top mirror and the bottom mirror and includes an active light generation region that is operable to generate light in the selected wavelength range. The vertical stack structure is constructed and arranged to direct light generated in the cavity region to the optically transparent substrate.  
           [0007]    First and second contacts are disposed over the bottom side of the vertical stack structure and are electrically connected for driving the cavity region.  
           [0008]    Embodiments in accordance with this aspect of the invention may include one or more of the following features.  
           [0009]    In some embodiments, the optically transparent substrate comprises glass (e.g., borosilicate glass). In other embodiments, the optically transparent substrate comprises gallium phosphide.  
           [0010]    The VCSEL system may further comprise a lens that is disposed on the glass substrate in alignment with the active light generation region.  
           [0011]    In some embodiments, at least one of the top mirror and the bottom mirror has a layer with a peripheral region that is oxidized into an electrical insulator as a result of exposure to an oxidizing agent. In these embodiments, the VCSEL system may further comprise two or more etched holes each extending from a substantially planar surface of the bottom mirror to the oxidized peripheral region.  
           [0012]    The top mirror and the bottom mirror preferably each comprises a system of alternating layers of different refractive index materials. For example, the top mirror and the bottom mirror each may comprise a system of alternating layers of relatively high aluminum content AlGaAs and relatively low aluminum content AlGaAs.  
           [0013]    In some embodiments, the VCSEL system further comprises an integrated circuit that is bonded to the pair of contacts and is operable to drive the cavity region.  
           [0014]    In another aspect, the invention features a method of fabricating the above described VCSEL system. In accordance with this inventive method, a sacrificial substrate is provided. A vertical stack structure having a substantially planar top side and a bottom side is formed over the vertical stack structure. The vertical stack structure includes a top mirror, a bottom mirror, and a cavity region that is disposed between the top mirror and the bottom mirror and includes an active light generation region operable to generate light in a selected wavelength range. The vertical stack structure is constructed and arranged to direct light generated in the cavity region away from the sacrificial substrate. The substantially planar top side of the vertical stack structure is wafer bonded to a substrate that is optically transparent to light in the selected wavelength range. The sacrificial substrate is removed after the optically transparent substrate has been wafer bonded to the substantially planar top side of the vertical stack structure. First and second contacts that are electrically connected for driving the cavity region are formed over the bottom side of the vertical stack structure.  
           [0015]    Other features and advantages of the invention will become apparent from the following description, including the drawings and the claims. 
       
    
    
     DESCRIPTION OF DRAWINGS  
       [0016]    [0016]FIG. 1 is a diagrammatic cross-sectional side view of a vertical cavity surface emitting laser system that is flip chip mounted to a driver substrate and an optical fiber positioned on an opposite side of a package window to receive light generated by the vertical cavity surface emitting laser system.  
         [0017]    [0017]FIG. 2 is a flow diagram of a method of making the vertical cavity surface emitting laser system of FIG. 1.  
         [0018]    [0018]FIG. 3A is a diagrammatic cross-sectional side view of a bottom mirror formed over a sacrificial substrate.  
         [0019]    [0019]FIG. 3B is a diagrammatic cross-sectional side view of a cavity region formed over the bottom mirror of FIG. 3A.  
         [0020]    [0020]FIG. 3C is a diagrammatic cross-sectional side view of a top mirror formed over the cavity region of FIG. 3B and oxidation holes formed in the resulting vertical stack structure.  
         [0021]    [0021]FIG. 3D is a diagrammatic cross-sectional side view of an optically transparent substrate wafer bonded to the top mirror of FIG. 3C.  
         [0022]    [0022]FIG. 3E is a diagrammatic cross-sectional side view of the vertical cavity surface emitting laser system of FIG. 3D after the sacrificial substrate has been removed.  
         [0023]    [0023]FIG. 3F is a diagrammatic cross-sectional side view of the vertical cavity surface emitting laser system of FIG. 3E after the bottom surface has been patterned and a pair of contacts has been formed over a bottom side of the vertical stack structure. 
     
    
     DETAILED DESCRIPTION  
       [0024]    In the following description, like reference numbers are used to identify like elements. Furthermore, the drawings are intended to illustrate major features of exemplary embodiments in a diagrammatic manner. The drawings are not intended to depict every feature of actual embodiments nor relative dimensions of the depicted elements, and are not drawn to scale.  
         [0025]    Referring to FIG. 1, in one embodiment, a vertical cavity surface emitting laser system  10  includes an optically transparent substrate  12 , a vertical stack structure  14 , and a pair of contacts  16 ,  18  that are flip chip bonded to a driver substrate  20 . In operation, a driving circuit disposed on driver substrate  20  applies a current between contacts  16 ,  18  that drives vertical stack structure  14  to generate light  22  in a selected wavelength range. Light  22  passes through optically transparent substrate  12  and is collimated by a first lens  24  that is disposed on optically transparent substrate  12 . A second lens  26 , which is disposed on an optically transparent package window  28 , focuses the collimated light  22  received from first lens  24  into an optical fiber  30 .  
         [0026]    Referring to FIGS. 2 and 3A- 3 F, and initially to FIGS. 2 and 3A, vertical cavity surface emitting laser system  10  may be fabricated as follows. Initially, a bottom mirror stack  32  is formed on a sacrificial substrate  34  (step  36 ; FIG. 2). Bottom mirror stack  32  includes a system of alternating layers of different refractive index materials that forms a distributed Bragg reflector (DBR) that is designed for a desired operating laser wavelength (e.g., a wavelength in the range of 650 nm to 1650 nm). For example, bottom mirror stack  32  may be formed of alternating layers of high aluminum content AlGaAs and low aluminum content AlGaAs. The layers of bottom mirror stack  32  preferably have an effective optical thickness (i.e., the layer thickness multiplied by the refractive index of the layer) that is about one-quarter of the operating laser wavelength. Sacrificial substrate  34  preferably is formed from a material that is lattice-matched to the layers of bottom mirror stack  32 . For example, sacrificial substrate  34  may be formed from GaAs, InP, sapphire (Al 2 O 3 ), or InGaAs and may be undoped, doped n-type (e.g., with Si) or doped p-type (e.g., with Zn). A buffer layer (not shown) may be grown on sacrificial substrate  34  before bottom mirror stack  32  is formed. As shown in FIG. 3B, a cavity region  38  is formed over bottom mirror stack  32  (step  40 ; FIG. 2). Cavity region  38  includes one or more active layers  42 ,  44  (e.g., a quantum well or one or more quantum dots). In some embodiments, active layers  42 ,  44  may be sandwiched between a pair of spacer layers (not shown). In other embodiments, active layers  42 ,  44  may be located above or below a single spacer layer. Active layers  42 ,  44  may be formed from AlInGaAs (i.e., AlInGaAs, GaAs, AlGaAs and InGaAs), InGaAsP (i.e., InGaAsP, GaAs, InGaAs, GaAsP, and GaP), GaAsSb (i.e., GaAsSb, GaAs, and GaSb), InGaAsN (i.e., InGaAsN, GaAs, InGaAs, GaAsN, and GaN), or AlInGaAsP (i.e., AlInGaAsP, AlInGaAs, AlGaAs, InGaAs, InGaAsP, GaAs, InGaAs, GaAsP, and GaP). Other quantum well layer compositions also may be used. The first and second spacer layers (if present) may be formed from materials chosen based upon the material composition of the active layers.  
         [0027]    Referring to FIG. 3C, a top mirror stack  46  is formed over cavity region  38  (step  48 ; FIG. 2). Top mirror stack  46  preferably is formed from the same material system as bottom mirror stack  32 . In the illustrated embodiment, bottom and top mirror stacks  32 ,  46  are cooperatively designed so that laser light  22  is emitted from a substantially planar top surface  50  of vertical stack structure  14  and through optically transparent substrate  12  (see FIG. 1).  
         [0028]    The layers of vertical stack structure  14  may be formed by conventional epitaxial growth processes, such as metal-organic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE). As shown, in the illustrated embodiment, vertical stack structure  14  has a planar structure that includes a number of holes  52 ,  54  that expose a number of respective side regions of the bottom and top mirror stacks  32 ,  46  to be oxidized. At least a portion of each of bottom mirror stack  32  and top mirror stack  46  is oxidized from the exposed side regions inwardly toward a centrally located aperture region  56 . In this embodiment, four holes (two of which are not shown in the drawings) are opened at locations that are equidistant from the center of aperture region  56 . The holes  52 ,  54  extend from a bottom surface of bottom mirror stack  32  to the layer (or layers) corresponding to the portion of vertical stack structure  14  to be oxidized. When the vertical stack structure  14  is exposed to heated water vapor, the heated water vapor enters the holes and oxidizes one or more layers of vertical stack structure  14  in a radial direction away from the holes. The oxidation process continues until an oxidation front from each hole merges to form the un-oxidized aperture region  56 . Other vertical stack structure embodiments may include more or fewer exposure holes or exposed regions with other shapes, such as divided arcs or rings.  
         [0029]    As shown in FIG. 3D, optically transparent substrate  12  is wafer bonded to the substantially planar top surface  50  of vertical stack structure  14  (step  58 ; FIG. 52). The optically transparent substrate  12  preferably is substantially transparent to light in a selected wavelength range. In general, the selected wavelength range encompasses the wavelength of light generated by vertical stack structure  14 . For example, in some embodiments the optically transparent substrate  12  is substantially transparent to light having a wavelength in the range of 650 nm to 1650 nm. In some embodiments, optically transparent substrate  12  is formed from glass (e.g., borosilicate glass). The use of borosilicate glass in gallium arsenide based embodiments is particularly advantageous because the thermal expansion properties of borosilicate glass and gallium arsenide are closely matched. Borosilicate glass also is advantageous because it allows low temperature processing and is transparent to visible and near-IR wavelengths of light. In other embodiments, optically transparent substrate  12  is formed from gallium phosphide. In other embodiments, optically transparent substrate  12  may be formed from other materials. In general, optically transparent substrate  12  may be attached to surface  50  by a conventional direct wafer bonding process or a conventional metallic bonding process that is tailored to the selected material systems of optically transparent substrate  12  and vertical stack structure  14 .  
         [0030]    As shown in FIG. 3E, after the optically transparent substrate  12  has been wafer bonding to vertical stack structure  14  (step  58 ; FIG. 2), sacrificial substrate  34  is removed (step  60 ; FIG. 2). In general, sacrificial substrate  34  may be removed by any one of a wide variety of conventional substrate removal processes. For example, in one embodiment, sacrificial substrate  34  may be removed by a selective wet chemical etching process. In this embodiment, an etch stop layer  62  (see FIG. 3D) preferably is formed between the sacrificial substrate  34  and bottom mirror stack  32 . In another embodiment, sacrificial substrate  34  may be removed by laser melting. In some embodiments, etch stop layer  62  also may be removed.  
         [0031]    Referring to FIG. 3F, after sacrificial substrate  34  has been removed (step  60 ; FIG. 2), vertical stack structure is patterned and etched, and contacts  16 ,  18  are formed over the bottom side of the patterned vertical stack structure (step  64 ; FIG. 2). The vertical stack structure may be formed by a conventional photolithographic patterning and etching process.  
         [0032]    Referring back to FIG. 1, in the illustrated embodiment, lens  24  is formed on the opposite surface of optically transparent substrate  12  as the vertical stack structure  14 . Lens  24  (and the corresponding lens  26  of package window  28 ) may be a replicated epoxy lens or a diffractive optical element (DOE).  
         [0033]    In the illustrated embodiment, contacts  16 ,  18  may be bonded to corresponding contacts of a suitable driving circuit disposed on driver substrate  20  using a flip-chip solder bonding process. In this embodiment, solder bumps  66 ,  68  are disposed between contacts  16 ,  18  and the corresponding metallization pattern of the driving circuit. The Z-axis dimensions of solder bumps  66 ,  68  are selected to separate the bottom side of vertical stack structure  14  from the driving circuit by an appropriate distance. During manufacture, solder bumps  66 ,  68  originally may be disposed on the metallization pattern of the driving circuit. Vertical cavity surface emitting laser system  10  is aligned with the driving circuit to within an accuracy required for solder bumps  66 ,  68  to contact the corresponding driving circuit metallization pattern. The assembly then is raised to a temperature at or above the melting point of solder bumps  66 ,  68 . Solder bumps  66 ,  68  wet the solderable contacts  66 ,  68  and surface tension forces pull vertical cavity surface emitting laser system  10  and driver substrate  20  into very precise alignment (e.g., to within +4 μm). The assembly is cooled to form a solidly bonded, accurately aligned structure.  
         [0034]    Electrical contacts  16 ,  18  enable vertical cavity surface emitting laser system  10  to be driven by the driving circuit that is disposed on driver substrate  20 . In operation, an operating voltage is applied across electrical contacts  16 ,  18  to produce a current flow in vertical stack structure  14 . In general, current flows through a central region of the vertical stack structure  14  and lasing occurs in a central portion of cavity region  38  (hereinafter the “active region”). The oxidized portions of bottom and top mirror stacks  32 ,  36  form an oxide confinement region that laterally confines carriers and photons. Carrier confinement results from the relatively high electrical resistivity of the confinement region, which causes electrical current preferentially to flow through a centrally located region of vertical stack structure  14 . Optical confinement results from a substantial reduction of the refractive index of the confinement region that creates a lateral refractive index profile that guides the photons that are generated in cavity region  38 . The carrier and optical lateral confinement increases the density of carriers and photons within the active region and, consequently, increases the efficiency with which light is generated within the active region.  
         [0035]    Other embodiments are within the scope of the claims.