Integrated laser module
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The subject matter of the present Application is related to U.S. patent application Ser. No. 291,553, entitled &#34;Laser Driver ASCI Chip&#34;, bearing Kodad Docket 69,959, filed on Aug. 16, 1994. 
     FIELD OF THE INVENTION 
     This invention relates to the field of laser packaging and more particularly to a compact modular arrangement of a laser and its associated electronics. 
     BACKGROUND OF THE INVENTION 
     It is extremely desirable in an optical CD writer as well as other CD systems to place the laser electronics, generally the laser driver and HFI (High Frequency Injection) circuitry, on the optical head with the laser. This is true because transmission line effects become less critical for laser driver rise/fall times at relatively small distances from the laser, EMI from HFI signals are easier to contain from the rest of the system not associated with the head, and overall the signal-to-noise ratio is improved. On the other hand, this close coupling of the high current laser driver circuitry, with a high frequency injection circuit, for quieting the laser noise in a small package or module, creates a system&#39;s packaging paradigm of trying to manage rather stringent thermal and electrical coupling requirements. 
     The integrated laser module of the present invention is designed to optimize rise and fall times in the order of 1-2 ns (nano sec), deliver up to 400 ma (milli-amps) of combined laser drive current during a Write mode, and to minimize EMI from a 200-500 Meg Hz high frequency injection (HFI) circuit in a cost effective, light weight, small package. Since lasers typically have parameters which shift adversely as a function of temperature, it is also important to provide an extremely low thermal resistance path for the laser driver which, in the preferred embodiment of this invention, is a custom ASIC circuit. For example 400 ma of Write current can result in up to 1.6 watts of power being generated during a Write cycle which typically exists for a 50% duty cycle at 5-6 Meg Hz. These temperature dependent laser parameters include wavelength, threshold, slope efficiency, and leakage, the change of which can significantly impact mark formation during a Write mode in an optical CD system. The threshold current change and slope efficiency changes at higher temperatures require that the laser driver current be increased to maintain a constant power level out of the laser. A constant power level is required for consistent writing quality and is critical as writing speeds approach and exceed six times the read speed. This increased current further adds to the temperature rise of the junction of the laser which in turn requires more current to the laser in order to maintain constant power. The present invention addresses this problem. 
     SUMMARY OF THE INVENTION 
     In a preferred embodiment of the integrated laser module there is provided: 
     a laser responsive to a control signal for generating a radiation beam; 
     a laser driver and logic circuitry connected to said laser for providing the control signal; 
     an injection circuit connected to said laser driver for providing an injection signal component to the control signal; 
     substrate means having electrical conductors thereon for electrically coupling said laser to said laser driver, said logic circuitry, and to said injection circuit; and 
     shielding means interposed between said injection circuit and said laser for shielding the radiation from said injection circuit from said laser. 
     This integrated laser module has particular utility in &#34;compact disc&#34; systems (i.e. audio as well as optical) where compact size and low cost are driving features. The incorporated laser driver circuit includes associated logic to generate Read, Write, and Boost laser drive currents. In addition, a high frequency injection circuit is incorporated to quiet the laser and to prevent mode hopping. The integrated laser module forms a self contained environment that decouples and shields the internal components from external and internal electromagnetic interference (EMI). 
     From the foregoing it can be seen that it is a primary object of the present invention to provide a compact packaging scheme for a laser and its associated circuitry. 
     It is another object of the present invention to provide a laser packaging arrangement that minimizes temperature effects. 
     It is yet another object of the present invention to provide a compact laser module with improved EMI protection, temperature stability, and optimized signal-to-noise ratio. 
     The above and other objects of the present invention will become more apparent when taken in conjunction with the following description and drawings wherein like characters indicate like parts and which drawings form a part of the present description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates, in perspective view, the integrated laser module. 
     FIG. 2 illustrates, in exploded view, the integrated laser module of FIG. 1. 
     FIG. 3 is a top view of a substrate which supports a laser driver and associated logic circuitry. 
     FIG. 4 is a bottom view of the substrate of FIG. 3 shown supporting an HFI circuit and a laser. 
     FIG. 5A is a section view taken along the section lines 5A--5A in FIG. 3. 
     FIG. 5B is an exploded, cut away, view of a substrate 24. 
     FIG. 6 illustrates, in schematic form, the electronics used in the integrated laser module. 
     FIG. 7 illustrates, in block diagram form, an ASIC that may be used in the electronics of FIG. 6. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIGS. 1 and 2, the integrated laser module 10 is comprised, in major part, of a top can 12, lower housing 14, laser 16, a substrate assembly 42, and associated electronic components, for example 39 and 51, to be described in detail later. The lower housing 14 is formed of metal to provide the mechanical fixturing for the laser 16 and to act as the lasers main heat sink. The lower housing 14 is additionally part of an integral EMI shield, &#34;Farady cage,&#34; for the HFI circuit (to be described later). The top can 12 provides part of the totally encompassing EMI shield for the laser&#39;s electronics. Elongated alignment holes 22 and holes 20 in the lower housing 14 provide a means to mechanically secure the laser module 10 to an optical Read/Write system after optical alignment. A flex tail 18 provides the electrical input/output interface between the laser module&#39;s internal laser electronics and the electronics of the optical Read/Write system. The lower housing 14 includes a recessed area 26 that is configured to receive the substrate assembly 42. The depth of the recess 26 is slightly less than the thickness of the substrate assembly 42, so that the top cap 12 can achieve good electrical contact with a ground plane 48 of the assembly 42. Additionally, in the recessed area 26, there is provided an aperture 32 that receives the laser 16. Indexing marks 34 and 36 are used to align the laser to the lower housing. A second recessed area 30 is formed within the recessed area 26 adjacent to the aperture 32. The second recessed area provides room for receiving the electronic components extending from the bottom side of the rigid substrate 24 (shown more clearly in FIGS. 4 and 5A). In the assembled laser module 10, the rigid substrate 24 fits over the laser 16 and provides physical support to the more flexible substrate 40 and to the associated components 43 and 51. The flexible substrate 40 is provided with an opening 38 to allow an ASIC 51 to be mounted directly to the copper ground plane 48 (see FIG. 5) on the surface of the rigid substrate 24 to provide a heat sink. An integrated capacitor filter array 39 is physically mounted onto the flexible substrate 40 and is connected in circuit to each of the conductors of the flex tail 18. In the preferred embodiment of the capacitor filter array, there is one capacitor assigned to each conductor in the flex tail 18, connecting between the assigned conductor and ground. Each capacitor is sized to provide filtering for the signals carried by its assigned conductor. For final assembly, the top can 12 is electrically bonded to the lower housing 14 to EMI seal the module 10 from electromagnetic and radio energy and to provide physical protection to the internal electronic components. 
     FIGS. 3 and 4 illustrate the top side and bottom side, respectively, of the substrate assembly 42. For ease of understanding, FIGS. 3, 4 and 5A (which is a sectioned view of the substrate assembly) may be viewed together. The substrate assembly 42 is formed of two substrates, a wire bondable gold (Au) active flex 40, and the active rigid substrate 24 which is composed of FR-4 material. The two substrates can be laminated together with conductive epoxy, conductive adhesives, or traditional lead/tin (PbSb) solder. The wire bondable Au flex 40 has two levels of conductors, an upper side conductor 44 which is formed as circuit interconnects using a standard etching process, and a ground plane 46, separated by an insulating material 28. 
     The active flex 40 is designed such that it is inset from the ground plane 48 of the FR-4 substrate 24 so as to reveal a conductive border 56. After lamination, the border 56 provides a soldering surface for joining the top can 12 directly to the ground plane 48 and to the ground conductor(s) of the upper side conductor 44. In this way, the active flex 40 is not delaminated from the FR-4 substrate 24 by heat during the attachment of the top can 12 and a solid, flat laminated surface for the wirebonding of the ASIC laser driver die 51 is maintained. During the lamination process, the flex ground plane 46 is conductively attached to the ground plane 48 of the FR-4 substrate 24. In addition, tapered holes 50 in the flex 40 are filled with solder to further ensure electrical continuity between the ground conductor (s) of the upper side conductor 44, and ground planes 46 and 48. Tapering ensures that the Cu ground is exposed from any flex overcoat. As a result, there is a continuous metallic EMI shield in all dimensions for the upper side laser driver circuitry 43. The opening 38 under the ASIC die 51 is formed so that the body of the ASIC can be attached directly to the ground plane 48 of the FR-4 substrate. Since the ground plane 48 is very thick (1 oz Cu), this creates a low impedance thermal path to the top can 12 for the heat generated by the ASIC. This thermal path is independent from the thermal path between the lower housing 14 and the laser 16. 
     Referring specifically to FIG. 4, which illustrates the bottom side of the substrate 24, the positioning of the HFI circuit 47 is clearly shown. Electrical connections from the ASIC 51 to the HFI electronics is by way of bonding wires 45 passing through holes 49 in substrates 24 and 40. 
     The FR-4 substrate 24 contains three active conductor layers, a ground plane 48, an intermediate layer 58, and an HFI circuit interconnect 52 all insulated from each other by an insulating material 28. 
     The flex side ground plane 48 is a noise free ground and is part of the upper EMI shield, shielding the upper drive components 43 and 51 from the noisy HFI circuit 47. The intermediate layer 58 forms the HFI ground plane. This ground plane is part of the HFI ground shield which contains the EMI radiation from the HFI circuit 47. 
     The HFI interconnect layer 52 has a matrix of vias 54, connecting a ground ring on layer 52 to the intermediate layer 58 (HFI ground plane). This prevents EMI emissions from exiting through the sides of the FR-4 substrate 24. The lower housing 14 is soldered to the ground ring on the HFI interconnect layer 52. FIG. 5B illustrates a portion of the vias 54. The vias are of such a diameter and spacing Chat there is no clear path for EMI emissions along the planes represented by layer 58 and the interconnect layer 52. 
     The flex tail 18 contains plural conductors 60 on one side and a ground plane sheet 62, shown partially exposed in FIG. 4. The exposed portion of the ground plane contacts the lower housing 14 to eliminate EMI leakage from this area of the module. 
     In summary, the HFI EMI shield is formed by the intermediate layer 58, the matrix of vias 54, the ground ring on the bottom side interconnect layer 52, and the lower housing 14. In addition, a custom capacitor filter array 39 (see FIG. 2) is used to filter the input/output lines from EMI radiation. Additionally, the lower housing 14 provides a means to mechanically heat sink the laser 16 independently from the laser driver heat sink. 
     FIG. 6 illustrates the electronic components 43 and 51 contained on the upper side and the components 47 contained on the bottom side of the substrate 42, along with the interconnections to the ASIC 51. 
     FIG. 7, is a block diagram of an ASIC 51 formed of four logic circuits and a driver circuit that may be used with the present invention. Other chips and/or discreet components that are within the knowledge of persons of ordinary skill in the art may be substituted for the ASIC and discrete components without detracting from the present invention. 
     The preferred method of assembling the laser module is set out in the following steps, with the understanding that many variants of these steps may be used based on the teachings of the present invention. As a result, it is the intent of this disclosure to cover those variants as well. 
     Assembly Procedure 
     1) The wirebondable Au flex substrate 40 and the active FR-4 substrate 24 are laminated into one integral substrate assembly 42, using conductive epoxy, conductive acrylics, conductive adhesives, or solder process. This provides rigidity to the flex substrate 40. 
     2) The upper side electrical components 43 (laser drive circuitry) are attached to the substrate 40 via a standard surface mount solder processes, for example, using PB60/SN40 at 240 Deg. C. or by a conductive epoxy. 
     3) The bottom side electrical components 47 (HFI circuitry) are attached to the substrate 40 via a standard surface mount solder processes, for example, using PB 5/Sn95 at 222 Deg. C. or by a conductive epoxy. 
     4) The laser driver ASIC die 51 is attached to the substrate 24 via a conductive epoxy, for example, using Abelbond 84 at 150 Deg. C. or a eutectic. 
     5) The laser driver ASIC die 51 is wirebonded to the substrate 40 conductors 44 or individual upper side components 43 via a standard gold ball bond or aluminum wedge bonding techniques. 
     6) The laser driver ASIC die 51 is coated with a glob coat, for example, using Ablestick or EPO-TEK # H62 to ensure that no light impinges on the silicon of the die. 
     7) The laser diode 16 is pressed into the lower housing 14. 
     8) The substrate 42 is attached to the lower housing via a conductive epoxy, for example, using Abelbond 84 or a low temperature solder. 
     9) The laser diode&#39;s leads are soldered through hole leads in substrates 24 and 40 to appropriate circuit conductors 44, for example, using Sn 60, Pb 40 at 188 Deg. C. 
     10) The module is tested for functionality. 
     11) The top can 12 is attached to the FR-4 ground plane 48 at the border 56 via a conductive epoxy, for example, using EPO-TEK H20E at 100 Deg. C. or a low temperature solder. 
     12) The completed module is tested with functional and parametric tests. 
     In conclusion, the present invention provides an integrated laser module that incorporates laser drive circuitry, a laser, and HFI circuitry in a compact size. The module meets thermal and HFI requirements, and additionally, the module incorporates a single substrate that utilizes the electrical component attachment technologies of wire bonding, dual sided surface mounting, soldered through hole techniques, as well as an integral HFI shield and an EMI containment shield resulting in a cost effective, lightweight module. 
     While there has been shown what is considered to be the preferred embodiment of the invention, it will be manifest that many changes and modifications may be made therein without departing from the essential spirit of the invention. It is intended, therefore, in the annexed claims, to cover all such changes and modifications as may fall within the true scope of the invention. 
     Parts List 
     10 Integrated laser module 
     12 Top can 
     14 Lower housing 
     16 Laser 
     18 Flex tail 
     20 Holes 
     22 Elongated alignment holes 
     24 Rigid FR-4 substrate 
     26 Recessed area 
     28 Insulating material 
     30 2nd recessed area 
     32 Aperture 
     34 Indexing mark 
     36 Indexing mark 
     38 Hole 
     39 Custom capacitor filter array 
     40 Flexible substrate 
     42 Substrate assembly 
     43 Upper side laser driver circuitry 
     44 Upper side conductors 
     45 Bonding wires 
     46 Ground plane 
     47 HFI circuit 
     48 Ground plane 
     49 Holes 
     50 Tapered holes 
     51 Custom laser driver ASIC die 
     52 HFI interconnect layer 
     54 Vias matrix surrounding the HFI circuit 
     56 Border 
     58 Intermediate layer 
     60 Conductors 
     62 Ground plane sheet