Abstract:
A III-V compound light emitter is integrated with Si-based actuators. The proposed devices take advantage of the superior optical properties of III-V compounds and the superior mechanical properties of Si, as well as mature fabrication technologies of Si-Micro-Electro-Mechanical Systems (MEMS). The emitter can be a light emitting diode (LED), a vertical cavity surface emitting laser (VCSEL) or an edge emitting laser. Electro or magnetic based actuation from Si-based actuators provides

Description:
This patent application claims priority to U.S. Provisional Patent Application, Serial No. 60/069,569, entitled “SCANNING III-V COMPOUND LIGHT EMITTERS INTEGRATED WITH SI-BASED ACTUATORS BY WAFER BONDING” filed on Dec. 12, 1997. The present invention is drawn to a scanning III-V compound light emitter integrated with Si-based actuators. 
    
    
     Solid state semiconductor light emitters are important devices in such diverse applications such as optoelectronic communication systems and high-speed printing systems. It is well-known in the proven art of silicon to provide suspension and actuation schemes, for example, comb drives using bending springs or parallel plate actuation using torsion springs. For optical beam steering applications, these silicon steering elements are typically combined with a light source in a separate package, or even with a light source ‘glued’ or bonded onto the silicon steering chip. To make an integrated device, it is necessary to develop MEMS fabrication technology for GaAs-based materials, such techniques including deep etching techniques to make high aspect ratio structures. It is highly desirable to combine the optical characteristics of GaAs materials with the structural and electrical characteristics of silicon. 
     U.S. Pat. Nos. 5,536,988, 5,640,133, 5,629,790 and 5,025,346, “Fabrication of Submicron High-Aspect-Ratio GaAs Actuators” Zhang et al., Journal of Microelectromechanical.Systems Vol. 2, No.2, p. 66-73, June 1993, “Laterally Driven Polysilicon Resonant Microstructure” Tang et al., IEEE Micro Electro Mechanical Systems pp. 53-59, February 1989 (reprint), and “Electrostatic-comb Drive of Lateral Polysilicon Resonators” Tang et al., Transducers &#39;89, Proceedings of the 5th International Conference on Solid-State Sensors and Actuators and Eurosensors III, Vol. 2, pp. 328-331, June 1990 (reprint) show the state of the art of micro-electromechanical systems (MEMS) actuators and methods of fabricating these devices. U.S. Pat. Nos. 5,747,366 and 5,719,891, H. J. Yeh, and J. S. Smith, “Integration of GaAs VCSEL on Si by substrate removal”, Appl. Phys. Lett. Vol 64, pp. 1466-1468 (1994) and Y. H. Lo, et al. “Semiconductor lasers on Si substrates using the technology of bonding by atomic rearrangement” Appl. Phys. Lett. Vol. 62, pp. 1038-1040 (1993) show the state of the art of semiconductor light emitting assemblies. 
     U.S. patent application Ser. No. 08/761,681, entitled “Raster Output Scanner with Pivotal Mirror for Process Direction Light Spot Position Control” filed on Dec. 6, 1996 and assigned to the same assignee as the present invention teaches a MEMS torsional control device. 
     U.S. patent application Ser. No. 08/940,867, entitled “Highly compact Vertical Cavity Surface Emitting Lasers” filed on Sep. 30, 1997 and assigned to the same assignee as the present invention teaches the formation of highly compact and well-defined VCSELs. 
     U.S. patent application Ser. No. 09/173,329, entitled “Monolithic Scanning Light Emitting Devices” filed on Oct. 15, 1998 and assigned to the same assignee as the present invention teaches a micro-machined movable light emitting assembly formed on or from a III-V substrate, preferably a GaAs substrate. The movable light emitting assemblies are actuated using force generators to generate various degrees of movement depending upon the type of stage suspension and actuation mechanism used. 
     All of the above references are hereby incorporated by reference. 
     SUMMARY OF THE INVENTION 
     The present invention is drawn to integrating GaAs-based optical devices with Si-based MEMS structures. The proposed devices utilize superior optical properties of III-V compounds and superior mechanical properties of Si, as well as matured fabrication technologies of Si-MEMS. The emitter can be a light emitting diode (LED), a vertical cavity surface emitting laser (VCSEL) or an edge emitting laser. Electro or magnetic based actuation from Si-based actuator provides linear or angular scanning. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a top view of a light emitting assembly movably supported on a semiconductor substrate; 
     FIG. 2 shows the cross-section of the semiconductor substrate in FIG. 1 prior to any processing; 
     FIG. 3 shows a cross-sectional view of the light emitting assembly in FIG. 1 along line  3 — 3 ; 
     FIG. 4 shows a top view of a rotating light emitting assembly; 
     FIG. 5 shows a cross-sectional view of FIG. 4 along line  5 / 5 ; 
     FIG. 6 shows a top view of another embodiment of a rotating light emitting assembly; 
     FIG. 7 shows a top view of a dual color rotating light emitting assembly array; and 
     FIG. 8 shows a cross-sectional view of FIG. 7 along line  8 / 8 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 is a top view of a movable stage with a light emitter, actuation mechanism and suspension system integrated with a substrate. The light emitter preferably takes the form of any III-V based emitters, for example, vertical cavity surface emitting lasers (VCSELS), light emitting diodes (LEDS), and edge emitting light emitters. In the particular embodiment shown in FIG. 1, VCSEL shuttle  1  has movable VCSEL stage  10  which supports VCSEL  12 . Stage  10  has one mechanical degree of freedom (x) and is suspended with support members  16 - 19 . The III-V material remains on the shuttle structure as shown in the shaded regions  10  and  20  as well as on VCSEL stage  10 . Basically the VCSEL material is etched away except in the shaded region to expose the n-Si buffer layer to allow the fabrication of comb-drive structures on the n-Si buffer layer)Support members  16 - 19  may take the form of straight bending springs (as shown), and may also include other configurations known from the silicon art, for example, folded beams. Mechanical designs such as ‘folded spring’ suspensions, tapered or stepped comb fingers, x/y folded spring suspensions, archimedian spiral springs for a rotational degree of freedom, etc., which are well-known in silicon structures, may be implemented in the III-V structures. The in-plane shape of structures (stage, springs, combs) is relatively arbitrary within fairly wide limits. The III-V material remains on the shuttle structure as shown in the shaded regions as well as on VCSEL stage  10  and support members  16 - 19 . Movement of stage  10  is actuated with electrostatic comb drives  20  and  21 , all anchored to substrate  50 . Electrostatic comb drives may be replaced with any actuation system as is well known in the microelectromechanical systems (MEMS) art, for example, magnetic, thermal and piezoelectric systems. As shown, stage  10  is moved by applying voltage V 1  between fixed fingers  22  and movable fingers  24  of comb drive  20  at comb drive contact pad  36  and applying a voltage V 2  between fixed fingers  23  and movable fingers  25  of comb drive  21  at comb drive contact pad  37 . The fixed fingers are electrically isolated from the movable fingers by isolation grooves  70  and  71  etched down to the oxidation layer  58 . The VCSEL and the movable combs share one common ground  44  deposited top of buffer layer  60 . 
     Electrostatic forces cause movable fingers  24  and  25  of comb drives  20  and  21  to ‘pull in’ to minimize the energy stored in the system. Movable fingers  24  and  25  are attached to stage  10  with its integrated VCSEL.  12  and stage  10  is suspended with flexiblesupport members  16 - 19 . Support members  16 - 19  are anchored to substrate  50  at anchor points  30 - 33 , while fixed combs  22  and  23  are anchored to substrate  50  at anchor points  34  and  35 . In a first order approximation, the stage position is proportional to the force, which is proportional to the square of the applied voltage. Conductive heat transfer through the suspensionsupport members sinks the waste heat from the VCSEL into the substrate. The low threshold current, characteristic for VCSELs, makes it possible to keep the temperature of the stage under control. 
     An important metallization/contacting issue is how to get the electrical connections for the VCSEL(s)  12  onto the mechanically suspended stage  10 . In the embodiment shown, conductive line  40  is run from VCSEL contact pad  42  on substrate  50  to stage  10  and to VCSEL  12 . Conductive line  40  runs over the neutral fiber of suspensionsupport member  16 , and is electrically isolated from suspensionsupport member  16 . The ‘neutral fiber’ is the-stress free centerline of the flexingsupport member. The width of conductive line  40  is chosen sufficiently small relative to the width of the support member and runs along the centerline of the support member in order to minimize mechanical stress, which allows conductive line  40  to survive the mechanical scanning motion of the beam. The electrical isolation of conductive line  40  from suspensionsupport member  16  may be accomplished in many ways, for example, with a dielectric layer underneath conductive line  40  or insulating the suspension beam surface using shallow ion implantation. 
     FIG. 2 is a cross-sectional view of substrate  50  prior to any etching with substrate back side  52  and wafer front side  54 . In this embodiment, the substrate structure includes layers of doped n-Si substrate  56 , etch stop layer  58 , buffer layer n-type Si  60 , N-type quarter wave distributed Bragg reflector (DBR) mirror layers  62 , active region  64 , P-type DBR mirror  66  and P+ contact layer  68 . In a preferred embodiment layer  56  is n-Si substrate, etch stop layer  58  is SiO2, buffer layer  60  is n-Si, n-DBR layer  62  is n-Al x Ga 1−x As/Al y Ga 1−y As, active layer  64  is a quantum well layer sandwiched by AlGaAs potential confinement layers, P DBR layer is P—Al x Ga 1−x As(x˜0.05)/Al y Ga 1−y As(y˜0.95) and P+ contact layer is GaAs. 
     FIG. 3 is a cross sectional view along line  3 — 3  of FIG. 1, and shows the VCSEL structure described in FIG. 2 after the VCSEL structure has been processed or etched. In the fabrication process, the III-V material (VCSEL top layer  68  shown) is etched away from the substrate except for in the shaded regions to expose the n-Si buffer layer to allow the fabrication of comb-drive structures on the n-Si buffer layer. Etch isolation grooves  70  and  71  are formed in the fabrication process. The current of VCSEL  12  flows through the n-GaAs layer in the long suspended beam  18  to the n-Si layer  60 . Substrate  50  is etched from the substrate back  52  to provide substrate opening  80  and layers  56 ,  58 ,  60 ,  62 ,  64 ,  66  and  68  have been removed to form stage openings  82  and  84 . Implantation region  41  extends underneath conductive line  40  and is insulating so that the current can only flow into the active region of the VCSEL from VCSEL contact pad  42 . Substrate contact  43  is deposited on the corner of the substrate back side  152 . 
     In a preferred embodiment, the GaAs-based VCSEL structure is bonded on top of a commercially available Simox (Si on insulator) wafer  55  composed of layers  56 ,  58  and  60 . An inverted GaAs based VCSEL structure grown on GaAs may be bonded to Simox wafer  55 , using conventional bonding techniques, for example, wafer fusion bonding. The bonding can be done through wafer to wafer direct bonding or through an intermediate layer of metal or dielectric. In the case of metal bonding, indium or Ge—Au may be needed. For dielectric, spin-on glass is an example. In a preferred embodiment, direct wafer bonding is used. To assist good wafer bonding, an InP or InGaP intermediate layer may be used which can be grown on the Si wafer or on top of the VCSEL wafer or both. Examples of useful bonding techniques are taught in U.S. Pat. Nos. 5,728,623 and 5,493,986, which are hereby incorporated by reference. The Si/SiO 2 /Si structure will be useful for electrical isolation, which will be discussed in more detail later on. In one example, the n-Si layer  60  is 10 to 20 μm thick, which could be thicker to stand the stress in released beams and membranes after wafer bonding, SiO 2  layer  58  is 500 nm thick, and n-Si layer  56  is 400 μm thick. 
     After bonding, the GaAs substrate is removed selectively by wet etching, leaving the VCSEL epi-structure of approximately 7 μm thick on top of the Simox wafer, shaded regions, and stage support members  16 - 19 . 
     A scanning light emitting shuttle can be fabricated from substrate  50  using Si-MEMS fabrication technology. VCSEL stage  10  is suspended in the center by support members  16 - 19  fabricated from the layer  60 . Linear scanning motion of the stage  10  is realized through comb drives  20  and  21  attached to the stage and the Simox substrate  55 , which are fabricated monolithically with the VCSEL table from the Si substrate. Comb drive fingers  22 - 25  are fabricated from n-Si layer  60  as well. With a driving voltage across the comb drives, the VCSEL shuttle is scanned in a linear motion with a displacement of tens of micro-meters. 
     The VCSEL may be fabricated by oxidation techniques to oxidize an inserted Al x Ga 1−x As (x˜0.98) layer to form an oxide aperture for optical and electrical confinement. In FIGS. 1 and 3, only one VCSEL is drawn with its p-connect wire running over the center line of one suspended beam. It is possible to put two VCSELs on the shuttle side by side by utilizing another suspension beam. Comb fixed fingers  22  and comb movable fingers  24  of comb drive  20  and comb fixed fingers  23  and comb movable fingers  25  of comb drive  21  are electrically isolated from one another. Since the fingers of each comb are of opposite polarities coupled by capacitance only, electrical isolation is needed which has to stand over 100 V electrical bias without breakdown. In one example, this is realized by using etch isolation grooves  70  and  71  to etch down to etch stop layer  58  of about 500 to 1000 nm thick underneath the VCSEL structure, 
     When a bias is applied by V, between the comb drive fingers  22  and  24 , VCSEL shuttle  100  is scanned in a linear motion as indicated by the x arrow and when a bias is applied by V 2  between comb drive fingers  23  and  25 , VCSEL shuttle  1  is scanned in the opposite x direction. 
     Since the current driving the VCSEL flows through the n-DBR and then the n-Si layer, the electric transport property at the bonding interface is very important for low resistance. Lo, et al. has reported a series resistance of 100 ohms for a GaAs edge emitting laser flip bonded to a p-Si substrate. Highly doped n-GaAs and n-Si might provide much lower impedance for carrier transport at the interface. If the driving current for the VCSEL is on the order of hundreds of μA, the voltage drop across the interface would be less than 0.1 volts. 
     The VCSEL structure then undergoes ion implantation process for isolation and metallization. Holes flows through the p-DBR region confined by the ion implanted area and into that portion of the active layer which lies below the p-DBR cavity and recombine with electrons flowing upwards from the N-DBR below and producing photon emission and optical amplification. At sufficiently high current flow, this optical amplification in combination with feedback from the DBR mirrors will result in laser oscillation and emission within the VCSEL cavity. Regions formed by the ion implantation isolation process are highly resistive. 
     The VCSEL scanners can be packaged in TO-type packages (e.g. 1 VCSEL per package, or more if desirable) with built-in short focal length lens. A ‘small’ stage scan length (e.g. 5 μm) can, within limits, be considerably magnified by placing the VCSEL scanner close to a short focal length lens and using a comparatively large throw distance (e.g. 1″ optical scan length feasible for a 50 μm mechanical scan amplitude using a 10 inch or 20 inch throw distance (250×, 125× respectively). 
     FIG. 4 shows a rotating VCSEL fabricated from the bonded structure. The VCSEL is bonded to a torsional Si-based actuator, which can be rotated by bias voltages across the bottom electrodes and the Si platform, with an angular rotation range of ±40°. A micro-lens may be placed on top of the VCSEL aperture for beam collimation. The lens can be fabricated by any conventional lens fabrication process, such as photoresist reflow or pattern transferring to a SiO 2  layer. The Si platform can be rotated two dimensionally as well with another degree of rotation by electrostatic or magnetic actuation. 
     VCSEL  112  is supported on stage  110 . Semiconductor substrate  150  has VCSEL  112  on stage  110  suspended from suspensionsupport members  114  and  115 . The actuation is done with parallel plate capacitors formed between stage  110  and electrodes  116  and  117  on glass cover  120 . “Raster Output Scanner with Pivotal Mirror for Process Direction Light Spot Position Control”, U.S. patent application Ser. No. 08/761,681, filed on Dec. 6, 1996 cited earlier, discloses the manufacture and operation of a pivoting mirror, the operation of moving stage  110  being similar in operation to that of the pivoting mirror. Differential actuation of electrodes  116  and  117  produces a Θ motion; common actuation of electrodes  116  and  117  produces a z-motion. The dimensions and geometry of the suspensionsupport members can be optimized to either favor a Θ or z-degree of freedom. Conductive line  140  electrically connects VCSEL  112  with contact pad  142 . FIG. 5 is a cross-sectional view of FIG. 4, along line  5 — 5 . Glass cover  120  is supported above substrate  150  by seal  122 . 
     Another configuration for a rotating light emitting device is a VCSEL  212  on VCSEL stage  210  fabricated on a Si cantilever platform shown in FIG. 6. A micro-lens  220  may be placed on top of the VCSEL aperture for beam collimation. Microlens  220  can be fabricated by any conventional lens fabrication process, such as photoresist reflow or pattern transferring to a SiO 2  layer. The Si platform can be rotated two dimensionally by electrostatic or magnetic actuation. 
     VCSEL stage  210  with VCSEL  212  is attached to substrate  250  by cantilever beam  214 . The actuation is accomplished by actuating electrode  216  on substrate  250  by applying voltage V. Actuation of electrode  216  produces an angular motion of stage  210  in the direction of electrode  216 . Conductive line  240  electrically connects VCSEL  212  with contact pad  242 . 
     FIG. 7 shows a dual-color rotating VCSEL array. Two or more VCSEL structures of different colors are shown bonded on top of semiconductor substrate  350 . The first VCSEL  212  and supporting structure has a configuration similar to that of FIG. 6, with N-type quarter wave distributed Bragg reflector (DBR) mirror layers  262 , active region  264 , P-type DBR mirror  266 . 
     VCSEL stage  310  with VCSEL  312  and microlens  320  is attached to substrate  350  by cantilever beam  314 . The movement of stage  310  is accomplished by actuating electrode  316  on substrate  350  by applying voltage V 2 . Actuation of electrode  316  produces an angular motion of stage  310  in the direction of electrode  316 . Conductive line  340  electrically connects VCSEL  312  with contact pad  342 . FIG. 8 shows a cross-sectional view of FIG. 7 along line  8 — 8 . VCSEL  312  is a different color than VCSEL  212 . VCSEL  312  may be fabricated by first bonding one VCSEL wafer, for example a red VCSEL, with N-type quarter wave distributed Bragg reflector (DBR) mirror layers  362 , P-type DBR mirror  366  to the Simox substrate  355  and then removing the substrate of the red VCSEL, followed by bonding of another VCSEL, for example an infrared VCSEL, with N-type quarter wave distributed Bragg reflector (DBR) mirror layers  370 , active region  372 , P-type DBR mirror  374  on top of the first VCSEL wafer. For the infrared VCSEL on top, lateral contact is needed for its cathode  344  on top of the n-GaAs buffer layer. 
     As those skilled in the art will appreciate, other various modifications, extensions, and changes to the foregoing disclosed embodiments of the present invention are contemplated to be within the scope and spirit of the invention as defined in the following claims.