Abstract:
Photonic integrated circuits (PIC) semiconductor chips are provided with thermal isolation and/or heat dissipation structures between integrated optical components in the PIC chip, particularly integrated active optical components. These structures may also serve as a ground path for electrical circuitry on the PIC chip. An important function is the enhanced thermal isolation from, or dissipation of heat from, between adjacent or neighboring optical components in the PIC so that required spacing between adjacent optical components can be made even less than the thickness of the substrate thereby realizing a more compact optical component array on the monolithic PIC chips.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]    This application claims priority of U.S. provisional application, Ser. No. 60/350,218, filed Nov. 2, 2001, which application is incorporated herein by its reference. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    This invention relates generally to photonic integrated circuit (PIC) chips and more particularly to thermal isolation and heat dissipation of integrated optical components formed in a PIC chip or in a wafer comprising PIC chip die.  
           [0004]    2. Description of the Related Art  
           [0005]    In a photonic integrated circuit (PIC), many active and passive optical components are integrated on the same semiconductor substrate. Some active photonic devices or components require large amounts of input power and, as a result, generate a substantial amount of heat in the chip. For example, arrays of laser diodes (LDs), electro-optic modulators and/or semiconductor optical amplifiers (SOAs) on a single PIC chip may require large amounts of drive current and/or bias. If many of these optical components are to coexist monolithically on the same semiconductor support or substrate, it is important to insure that they are sufficiently thermally isolated or that heat generated by them can be efficiently dissipated away from adjoining components and off the PIC chip.  
           [0006]    There are a number of methods that can be employed for thermal isolation in photonic devices or components integrated in PIC chips. Components are often separated physically, and the host wafer is often thinned. Typically, the rule of thumb is that, if the separation between optical components in the chip is much less than the thickness of the wafer, then the on-chip integrated optical or photonic components will need to be thermally isolated from one another.  
           [0007]    Another method that is employed relates to the use of flip chip techniques for placement of chips onto a heatsink. If the heatsink has a higher thermal conductivity than the semiconductor substrate, the heat sink will serve to thermally isolate optical chip components that are adjacent to each other.  
           [0008]    Both of these techniques have some problems. PIC devices are typically made on compound semiconductor wafers, e.g., fabricated on InP wafers, which are much more fragile than silicon wafers. As a result, although thinning a wafer helps with thermal isolation, it also results in increased wafer breakage and lowers chip yields. Flip-chip is often used for small photonic devices. For larger devices, the difference in the thermal expansion coefficient, the solder and the heat sink results in excess stress which, in turn, results in poor reliability. For a large photonic integrated circuit, flip chip is rendered impractical by the size of the optical components and the resultant PIC chip.  
           [0009]    For high speed electronic circuits, holes or vias are often used to transfer the ground plane at the bottom of the semiconductor to the top surface. A via is created by etching a hole through the semiconductor and then filling the hole with metal, typically by means of electroplating.  
         OBJECTS OF THE INVENTION  
         [0010]    It is an object of the present invention to overcome the aforementioned problems by providing improvements in optical component thermal isolation and/or heat dissipation.  
         SUMMARY OF THE INVENTION  
         [0011]    According to this invention, a thermal isolation or dissipation structure is employed in PIC geometry comprising openings or vias, such as, for example, or holes or troughs or trenches, in a semiconductor chip or photonic integrated circuit (PIC) to provide a thermal path for on-chip integrated optical components to thermally isolate adjacently spaced optical components or provide a high thermally conductive path from between adjacent optical components. If need be, the formed conductive paths can also serve as an on-chip electrical conduit. Rather than using small vias purely for electrical connection, much larger sized vias are employed so that the thermal volume of the metal is substantial. Thus, the openings or vias may, respectively, extend partially into or through the body of the wafer or chip. The openings may extend into the top or bottom surface and preferably extend through openings into the bottom of the PIC substrate so that when the openings are filled with a thermally conductive material, the material will be in direct thermal contact with an underlying heatsink or other thermal submount for the PIC chip or chips. The heatsinks that may be employed, for example, are AlN, BeO, copper sandwiched BeO or CVD diamond. The important factor is that the coefficient of thermal expansion of the heatsink should substantially match that of the PIC chip. This becomes more critical with the development of a structure providing for thermal isolation and/or thermal dissipation paths built into the PIC chip thereby changing its coefficient of thermal expansion properties.  
           [0012]    An important result of the deployment of this invention is that the distance between the large metal plated vias and the optical active integrated components can be made to be less than the substrate thickness thereby realizing a more compact optical component array on a PIC chip than previously thought possible. Thus, the primary path of heat flow would be laterally to the metal vias, rather than vertically through the substrate. Therefore, it is possible to use substrates that have not been thinned and do not have to be made thinner to meet the above mentioned rule of thumb. Neighboring optical components are now thermally isolated because the high thermal conductivity of the metal means that the vias will be maintained at a nearly constant temperature as the on-chip optical components heat up due to an applied bias or current. The metal used for the vias in any of the embodiments set forth in this description may be, for example, gold or aluminum.  
           [0013]    The vias forming the structure may be left unfilled, i.e., empty (thermal isolation) as well as being filled with a thermally conductive material, such as a metal (thermal dissipation). Unfilled structures serve a similar purpose to isolate the optical components as in the case of filled vias where the heat is efficiently carried away. In either case of filled or unfilled, the openings or vias comprising the structure, being positioned between adjacent optical components integrated in the PIC chip, greatly reduces any lateral thermal conductivity, so that on-chip optical components will not longer experience thermal association with neighboring or adjacent optical components.  
           [0014]    Thus, by laterally separating optical components by very low thermal conductivity material or by high conductivity material, both serve to thermally isolate them from neighboring optical components, provided, however, that in the case of the high conductivity material, the material should be contiguous with a heatsink submount for the PIC chip.  
           [0015]    Other objects and attainments together with a fuller understanding of the invention will become apparent and appreciated by referring to the following description and claims taken in conjunction with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    In the drawings wherein like reference symbols refer to like parts  
         [0017]    [0017]FIG. 1 is a schematic plan view of a photonic integrated circuit (PIC) chip illustrating a first exemplary embodiment of a structure according to this invention.  
         [0018]    [0018]FIG. 2 is an enlarged view of a portion of the PIC chip shown in FIG. 1.  
         [0019]    [0019]FIG. 3 is an enlarge view of a portion of a PIC chip, similar to FIG. 2, illustrating a second exemplary embodiment of a structure according to this invention.  
         [0020]    [0020]FIG. 4 is an enlarge view of a portion of a PIC chip, similar to FIG. 2, illustrating a third exemplary embodiment of a structure according to this invention.  
         [0021]    [0021]FIG. 5 is a schematic cross-sectional view of a fourth exemplary embodiment shown in FIGS.  2 - 4 .  
         [0022]    [0022]FIG. 6 is a schematic cross-sectional view of a fifth exemplary embodiment of a structure according to this invention.  
         [0023]    [0023]FIG. 7 is a schematic cross-sectional view of a sixth exemplary embodiment of a structure according to this invention.  
         [0024]    [0024]FIG. 8 is a schematic cross-sectional view of a seventh exemplary embodiment of a structure according to this invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0025]    Reference is now made to FIG. 1 which illustrates one example of a PIC chip  10 . The example shown in FIG. I is a transmitter photonic integrated circuit (TxPIC) chip, but it should be understood that in the employment of the thermal isolation/dissipation structure according to this invention, any PIC chip requiring thermal separation of neighboring optical components, whether active or passive types of such optical components, can be employed. While the invention herein may have its best functioning relative to thermal isolation or heat dissipation from between active optical components, i.e., components requiring an applied bias or current or other applied energy in order to perform a photonic function, it should be understood that the utility of this invention also extends to passive optical components, i.e., components that require no applied bias or current or other energy source to perform a photonic function, such as in cases, for example, where the passive optical component optical properties may be affected by changes in temperature, such as change sin refractive index.  
         [0026]    TxPIC chip  10  is shown in more detail in earlier filed U.S. patent applications, Ser. No. 10/267,331 and U.S. Ser. No. 10/267,346 both of which were filed on Oct. 8, 2002 and are incorporated herein by their reference. PIC chip  10  is deployed in an optical transmitter (Tx) module such as disclosed in U.S. patent application, Ser. No. 10/267,212, filed Oct. 8, 2002, which application is incorporated herein by its reference.  
         [0027]    Chip  10  comprises an array of DFB lasers  12  optically coupled to an array of electrooptic modulators (EOMs)  14 , which are optically coupled, via waveguides  18 , to an optical combiner  16 , shown here as an arrayed waveguide grating (AWG)  16 . As, an example, TxPIC chip  10  may have eight signal channels with different channel wavelengths of λ 1  to λ 8  forming a first wavelength grid approximating that of a standardized wavelength grid. However, the number of channels may be greater than eight channels, the latter depending upon the ability to spatially integrate an array of semiconductor modulator/laser (SML) sets  15  on a chip while providing minimal cross-talk levels.  
         [0028]    Other types of optical combiners may be utilized instead of an AWG  16  shown in FIG. 1. For example, optical combiner  16  may be a power coupler, a star coupler, a MMI coupler or an Echelle grating. AWG combiner  16  is an optical combiner of choice because of its capability of providing narrow passbands for the respective channel signals with low insertion loss. AWG combiner  16 , as known in the art, comprises an input slab or free space region  20 , a plurality of grating arms  22  of predetermined increasing length (Al), and an output slab or free space region  24 . The multiplexed channel signal output from slab  24  is provided to an output waveguide  26  for exit from chip  10 , such as optical coupling the multiplexed channel signal output to an optical fiber (not shown).  
         [0029]    As explained in the above incorporated applications, chip  10  may be comprise of Group III-V compound semiconductors, in particular, for example, InGaAsP/InP or AlInGaAs/InP alloys. In these chips, a coupling core waveguide or a continuous core waveguide is formed along each optical path comprising a respective SML set  15  to a respective input of optical combiner  16  and then to output waveguide  26  of chip  10 . The core waveguide is, for example, InGaAsP or AlInGaAs. As shown in both FIGS. 1 and 2, between each of the SML sets  15  is formed a structure  30  providing thermal isolation or for heat dissipation, or both, between adjacent SML sets  15  each comprising optically coupled  12  and  14 . Structure  50  is shown for isolating entire components of sets  15 . However, thermal isolator/dissipater structure  50  can also be limited to be between any spatially neighboring components, e.g., between adjacent or neighboring DFB lasers  12 .  
         [0030]    As will be seen, structure  50  may be an opening or via, or may be an opening or via filled with a thermal conductive material, such as a metal. However, the purposes of FIG. 1, along with FIGS.  2 - 4 , are to illustrate a few examples of the geometry for such thermal isolating and heat dissipating structures. Other examples will be readily come to mind by those familiar with this art upon seeing these exemplary geometries. In this regard, the geometry shown in FIG. 2 comprises an opening or via, such as a rectangle formed between adjacent SML sets  15  in chip  10 . As shown in FIG. 3, the geometry need not be continuous between sets  15 , but rather can be a plurality of spatially separated openings, such as circles  32 , or squares, triangles, polygons, or other geometrical shapes. As shown in FIG. 4, the geometry can be “wavy” or with uneven edges, e.g., a scrolling kind of geometry shown at  34 . These openings or vias  30 ,  32  and  34  can extend into the chip from the bottom or from the top of chip  10 . Also, it is within the scope of this invention that combinations of such geometries may be employed between adjacent optical components  12  or  14  in chip  10 .  
         [0031]    Reference now is made to FIG. 5 which illustrates, in cross-section, chip  10  through either the region of either DFB lasers  12  or EOMs  14 , representing the multiple SML sets  15 . In FIG. 5, the core waveguide  17  is illustrated for each SML set  15 , comprising just one of a plurality of layers formed on an InP substrate  38 , such as by means of MOCVD or MBE. Adjacent or neighboring SML sets  15  are thermally isolated from one another by means of an opening  40  such as a trough or groove. In this manner, the heat generated from a neighboring SML set  15  or its components cannot directly effect the operation of another adjacent SML set  15 . In FIG. 5, grooves  40  can have any desirable geometrical shape such as discussed above in connection with the geometries of FIGS.  2 - 4 .  
         [0032]    Reference is now made to FIG. 6 which illustrates, in cross-section, another chip  10  having a plurality of structures  44  formed in grooves or openings  42  formed through the bottom of InP substrate  38  in combination with openings  40 . Openings  42  can have any desirable geometrical shape such as discussed above in connection with the geometries of FIGS.  2 - 4 . Openings  42  are then filled with a thermally conductive material, such as gold or aluminum. The bottom surfaces  25  of the formed thermal dissipaters  44  are place in thermal contact with a heatsink or other such submount (not shown) such as AlN, BeO, copper sandwiched BeO or CVD diamond. In this manner, the heat generated from SML sets  15  is thermally conducted through channels  44  directly to the heatsink rather than laterally to neighboring SML sets  15 . The use of metal filled structures  44  in conjunction with heatsink also applies to any of the other embodiments in this disclosure.  
         [0033]    Reference is now made to FIG. 7 which illustrates another embodiment for thermal isolation or heat dissipation comprising vias  46  formed through the semiconductor bulk such as by means of selective etching between adjacent SML sets  15  or their individual optical components  12  or  14 . As shown in FIG. 7, a selective etch is performed on one side of chip  10  forming a set of larger openings  48 A and, then, formed from the other side of chip  10  forming a set of smaller openings  48 B. Etching is completed when through-vias  48  are formed. The geometric shape of vias  48  can have any desirable geometrical shape such as discussed above in connection with the geometries of FIGS.  2 - 4 . The vias  48  can then be maintained, as is, to provide for thermal isolation between the SML sets  15  or, alternatively, filed with a thermally conductive material  50 , as shown in FIG. 7.  
         [0034]    It should be noted that an additional advantage of the embodiment of FIG. 7 is that the on-chip, metal-filled vias  48  provide an electrical conduit for current supply paths between the top and bottom surface of the chip which can facilitate the wire bonding when packaging chip  10  in a module package. For example, the metal-filed vias can function as electrical ground paths for active components or can function, in part, as microwave shielding for adjacently disposed electro-optic modulators  14  to reduce RF cross-talk between the modulators.  
         [0035]    Reference is now made to the embodiment shown in FIG. 8, which is the same as that shown in FIG. 7 except that only the larger portion  48 A of vias  48  are filed with a thermally conductive material while the smaller portion  48 B of vias  48  are left as open space. Thus, in this embodiment, thermal isolation is provided between neighboring SML sets  15  is provided by means of openings  48 B, while accompanying heat dissipation is provided from between SML sets  15  by means of openings  48 A filled with thermal conductive material  50 .  
         [0036]    While the invention has been described in conjunction with several specific embodiments, it is evident to those skilled in the art that many further alternatives, modifications and variations will be apparent in light of the foregoing description. Thus, the invention described herein is intended to embrace all such alternatives, modifications, applications and variations as may fall within the spirit and scope of the appended claims.