Abstract:
A method of fabricating an integrated optoelectronic circuit. The method includes positioning a microchip on a first flexible dielectric substrate. A polymer electro-optic waveguide is positioned on or within the first flexible dielectric substrate. A ground electrode is positioned along the electro-optic waveguide. A signal electrode is positioned along the electro-optic waveguide opposite the ground electrode. A first patterned metallization layer is applied to the first flexible dielectric substrate. A second flexible dielectric substrate is positioned along the first flexible dielectric substrate. A plurality of via openings are provided in the first and second flexible dielectric substrates. A second patterned metallization layer is applied to the second flexible dielectric substrate.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is a divisional of U.S. patent application Ser. No. 10/248,148, filed Dec. 20, 2002, the entire contents of which are incorporated herein by reference. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    This disclosure relates to an optoelectronic circuit and more specifically to a combination of microwave and photonic components for a compact, self contained Mach-Zehnder interferometer (MZI) modulator.  
           [0003]    Modulation of an optical signal at microwave frequencies, typically above 10 GHz, requires external modulation of a laser source to prevent unintentional modulation of the laser frequency (e.g. chirping). Towards this end, a Mach-Zehnder interferometer structure is often employed to create an optical phase and/or amplitude modulator. One or both arms of the Mach-Zehnder interferometer contains electrodes to permit phase modulation of an optical signal via the electro-optic effect. These electrodes require a drive amplifier to supply adequate electric field to produce the electro-optic effect. The amplifier requires sufficient bandwidth and output capability to drive the reactive load presented by the Mach-Zehnder electrodes.  
           [0004]    Early electro-optic (EO) modulators required a large external power amplifier to provide hundreds of volts to produce the electro-optic effect. Recent devices have the modest drive requirement of 8-12 volts, but still require an external RF power amplifier to operate. Advances in polymer technology have allowed for the development of materials with large EO figures of merit, resulting in low V π  numbers.  
         BRIEF DESCRIPTION OF THE INVENTION  
         [0005]    An embodiment of the invention is a method of fabricating an integrated optoelectronic circuit. The method includes positioning a microchip on a first flexible dielectric substrate. A polymer electro-optic waveguide is positioned on or within the first flexible dielectric substrate. A ground electrode is positioned along the electro-optic waveguide. A signal electrode is positioned along the electro-optic waveguide opposite the ground electrode. A first patterned metallization layer is applied to the first flexible dielectric substrate. A second flexible dielectric substrate is positioned along the first flexible dielectric substrate. A plurality of via openings are provided in the first and second flexible dielectric substrates. A second patterned metallization layer is applied to the second flexible dielectric substrate. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]    [0006]FIG. 1 is a schematic diagram of an integrated optoelectronic circuit for modulating an optical signal;  
         [0007]    [0007]FIG. 2 is a schematic diagram of an RF power amplifier and phase control circuit for the integrated optoelectronic circuit of FIG. 1;  
         [0008]    [0008]FIG. 3 is a schematic diagram of an electric circuit for the RF power amplifier and phase control circuit of FIG. 2;  
         [0009]    [0009]FIG. 4 is a sectional side view of a portion of an integrated optoelectronic circuit including a monolithic microwave integrated circuit (MMIC) and a Mach-Zehnder interferometer (MZI) on a flexible dielectric substrate;  
         [0010]    [0010]FIG. 5 is a sectional side view of the integrated optoelectronic circuit of FIG. 4 including a microwave absorber and heat exchanger;  
         [0011]    [0011]FIG. 6 is a sectional side view of the integrated optoelectronic circuit of FIGS. 4 and 5 including a flexible inter-electrode dielectric substrate and showing electrical interconnections for the modulating signal between the MMIC and the MZI;  
         [0012]    [0012]FIG. 7 is a sectional side view of the integrated optoelectronic circuit of FIGS. 4 and 5 including a flexible inter-electrode dielectric substrate and showing electrical interconnections for the ground signal return between the MMIC and the MZI;  
         [0013]    [0013]FIG. 8 is a sectional end view of the integrated optoelectronic circuit of FIGS. 6 and 7;  
         [0014]    [0014]FIG. 9 is a plan view of the integrated optoelectronic circuit of FIGS. 6 and 7;  
         [0015]    [0015]FIGS. 10A through 10E comprise a diagram of a method of fabricating a MMIC die for use in the optoelectronic circuit of FIGS.  4 - 8 ;  
         [0016]    [0016]FIGS. 11A through 11D comprise a diagram of a method of fabrication of the optoelectronic circuit of FIGS.  4 - 8 ;  
         [0017]    [0017]FIG. 12 is a plan view of a second embodiment of the integrated optoelectronic circuit of FIGS. 6 and 7; and  
         [0018]    [0018]FIGS. 13A through 13E comprising a diagram of a method of fabrication of the integrated optoelectronic circuit of FIG. 12. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0019]    Referring to FIG. 1, an integrated optoelectronic circuit is shown generally at  100 . The optoelectronic circuit  100  comprises an electro-optic device, such as a Mach-Zehnder (MZI)  104  or an electro-absorptive modulator receptive of an optical signal  120 . A monolithic microwave integrated circuit (MMIC)  102  is coupled to a polymer based MZI  104  by way of a pair of electrodes  128 ,  130  and an output transmission line  124  and a ground signal return transmission line  126 . These transmission lines are in the nature of microstrip transmission lines. Electrode  130  is a ground electrode and electrode  128  is a radio frequency (RF), or signal, electrode (e.g., a microstrip transmission line). The MMIC  102  is receptive of a radio frequency modulating signal  132  for modulating the optical signal  120  in the MZI  104  providing thereby as output a modulated optical signal  122 . Examples of suitable organic materials in the MZI include poly(acrylates); poly(alkyl methacrylates), for example poly(methyl methacrylate) (PMMA); poly(tetrafluoroethylene) (PTFE); silicones; and mixtures comprising at least one of the foregoing organic materials, wherein the alkyl groups have from one to about twelve carbon atoms.  
         [0020]    The MZI  104  comprises an input channel  108  receptive of the optical signal  120 . A beam splitter  114  splits the optical signal  120  into two beams  154 ,  156  and directs them separately along a first branch  110  and a second branch  112 . In the embodiment of FIG. 1, the electrodes  128 ,  130  are positioned diametrically opposed to one another across one of the branches of the MZI  104 . Alternatively, a plurality of ground electrodes  130  may be separately positioned along the first branch  110  and second branch  112  with electrode  128  positioned between and along the first branch  110  and the second branch  112  (FIGS.  4 - 8 ).  
         [0021]    Polymers are usually centrosymmetric in nature and thus do not display the electro-optic effect. However, polymers may be made to display the electro-optic effect by poling of highly optically nonlinear chromophores/molecules, which can be incorporated into a polymer host. Thus, the optical signals  154 ,  156  in a polymer based MZI  104  are modulated by the RF modulating signal  132 ,  152  by way of the MMIC  102 , the transmission lines  124 ,  126  and electrodes  128 ,  130 . The optical signals  154 ,  156  are combined at a beam combiner  116 , thus providing as output a modulated output signal  122  at exit channel  118 .  
         [0022]    In FIG. 2, the MMIC  102  comprises an RF power amplifier  202  receptive of the RF modulating signal  132 . The MMIC  102  is coupled to the electrodes  128 ,  130  by way of the transmission lines  124 ,  126 . A phase offset circuit  204  receptive of a phase offset signal  232  is connected to the MZI  104  at  234  and provides control of the static phase offset of the optical signals  154 ,  156  in the MZI  104 .  
         [0023]    In FIG. 3, the RF power amplifier  202  comprises a power divider  206  receptive of the modulating signal  132  at a first input thereto. A pair of amplifiers  208  are receptive of the power divided signal and are impedance matched  212 ,  214  to a power combiner  216 . The power combiner  216  provides an amplified modulating signal  152  to the electrodes  128 ,  130  by way of transmission line  124 . Continuing in FIG. 3, the phase offset circuit  204  comprises an opamp  224  circuit in noninverting configuration receptive of a DC bias  232  for setting the phase offset of the MZI  104 .  
         [0024]    Referring to FIG. 4, a cross section of an embodiment of the optoelectronic circuit  100  is shown. The MMIC  102  is positioned on a first side of a flexible dielectric substrate  144 . The flexible dielectric substrate  144  is approximately 1 to 2 mils thick and may comprise for example a polyamide polymer such as KAPTON®. The MMIC  102  is mounted directly to the flexible dielectric substrate  144  using a die mount carrier ( 306  in FIG. 1E). The MZI  104  is positioned on a second side of the flexible dielectric substrate  144  opposite the side of the MMIC  102  or may be directly embedded in the flexible dielectric substrate  144 . Transmission line  126 , carrying ground return signal  150  from the MZI  104 , is positioned on the flexible dielectric substrate  144  and connected to ground electrode  130  opposite the MMIC  102 . Circuit connection (via hole)  142  is made between the MMIC  102  and ground electrode  130  of the MZI  104  by way of signal transmission line  126 .  
         [0025]    A poling electrode  140  is positioned on the flexible dielectric substrate  144  opposite the MZI  104 . Poling electrode  140 , is positioned on the same side of the flexible dielectric substrate  144  as the MMIC  102  and is substantially removed from the area near and around the MMIC  102  to prevent stray microwave signals from coupling to the poling electrode  140 . The die mount  306  and the bonding layer  304  may be removed and the MZI  104  and the MMIC  102  prepared for further processing. The die mount  306  and the bonding layer  304  are removed if the finished module is to be attached to another circuit assembly, which may then provide the same function as the die mount  306 . Otherwise the heat exchanger is required. Alternatively, the die mount  306  and the bonding layer  304  may be retained and used as a heat exchanger, such as a thermoelectric cooler, to control the temperature of the MMIC  102 . In FIG. 5, the MMIC  102  may also be encased or encapsulated within a microwave absorbing plastic  146  such as a ferrite doped plastic or paint to reduce interference effects from extraneous microwave signals.  
         [0026]    Referring to FIG. 6, a second flexible dielectric substrate  148  is positioned above the first flexible dielectric substrate  144  to serve as an inter-electrode dielectric for the transmission lines  124 ,  126 . The interconnection for DC bias  234  (FIG. 9) and landing pads  248  (FIG. 9) for passive devices  240  (FIG. 9) are made. The input transmission line  136  and the output transmission line  124  in FIGS.  4 - 8 , which couple the RF modulating signal  132  to the MMIC  102 , and from the MMIC  102  to the MZI  104 , are fabricated with specific geometries so as to achieve an appropriate characteristic impedance, Z o . The dimensions h and t in FIGS.  4 - 7  are approximately 38 micrometers and the width of transmission lines  124 ,  126 ,  128   170  are adjusted to provide a 50 ohm transmission line. The width to height (h, t) is based upon the value of 3.4 for the relative dielectric constant of KAPTON®. The characteristic impedances, Z o , and therefore the exact geometries of transmission lines  124 ,  126 ,  128 ,  136  are matched (i.e., impedance matched) to the MZI  104  by mathematical modeling, computer simulation and empirical data to optimize the arrangement of the transmission lines  124 ,  126 ,  128 ,  136  and performance of the optelectronic circuit over the operating frequency of the MMIC  102  (1 MHz-50 GHz). One set of performance measurements analyzed is the microwave scattering parameters s 11 , s 12 , s 21 , s 22 . Transmission line interconnects are directly fabricated on the flexible dielectric substrates  144 ,  148  to provide exact impedances and thus a MMIC  102  impedance matched to the MZI  104 .  
         [0027]    [0027]FIG. 9 is a plan view providing greater detail of the integrated optoelectronic circuit of FIGS. 6 and 7. The DC bias network  234 , which controls the static phase offset for the MZI  104 , is also fabricated directly onto the flexible dielectric substrate assembly  144 ,  148 . Passive components such as resistors, capacitors and inductors  240 , for the amplifier MMIC  102  and the MZI  104  are mounted directly onto the second flexible dielectric substrate  148 . A bias tee structure  242  providing power to the MMIC  102 , is fabricated directly into metallization on the second flexible dielectric substrate  148 . The bias tee  242  comprises integrated passive inductors and resistances fabricated directly into metallization on the second flexible dielectric substrate  148 . The geometries of the passive resistors, capacitors and inductors, and the layout thereof, are also based on mathematical modeling, computer simulation and empirical data for the fabrication process. The assembled flexible dielectric substrate module  144 ,  148  can then be packaged further or combined with other devices in a multi-unit module.  
         [0028]    FIGS.  10 A- 10 E depict additional details on the preparation of the MMIC amplifier  102  for integration with the MZI  104 . In FIG. 10B, an adhesive  304  is applied to a bare die  302  of FIG. 10A. In FIG. 10C, a die mount  306  is attached to the bare die  302  via the adhesive  304 . In FIG. 10D, the die is mounted to a fixture  308  and ground to a thickness as needed. In FIG. 10E, the die assembly  102  is prepared for mounting to the flexible dielectric substrate  144 .  
         [0029]    A second optoelectronic circuit for modulating an optical signal is depicted in cross section in FIGS. 11A through 11D. In FIG. 11A, microstrip waveguides  124  are applied to a first flexible dielectric substrate  144 . A MMIC  102  is mounted to the first flexible dielectric substrate  144  and electrical connections made to the microstrip waveguides  126 . In FIG. 11B, bonding layer  304  and die mount  306  are removed. Metallization is added for grounds and backside connections. In FIG. 11C, a second flexible substrate  148 , which acts as a signal layer in the optoelectronic circuit  100 , is laminated (e.g., adhesively bonded at  156 ) over the first flexible dielectric substrate  144 . The polymer based MZI  104  is positioned within cavity  180  and RF  128  and ground  130  connections are made thereto. For a single MMIC drive device  102 , connections are made from MMIC to MZI using tuned transmission lines  124 ,  126 . For a dual drive optoelectronic circuit, where the two arms  110 ,  112  of the MZI  104  are driven in a push-pull arrangement, a third layer  158  of flexible dielectric substrate is added (FIG. 11D). All of the key advantages of the preferred embodiment would apply. This approach may have advantages in terms of ease of fabrication, and reduced size of the final component. Also, this embodiment may be more compatible with other device construction techniques already in use, and therefore provide for a higher level of integration.  
         [0030]    Referring to FIG. 12, a plan view of a dual drive optoelectronic circuit is shown. The dual drive optoelectronic circuit comprises a pair of preamplifiers  140   a ,  140   b , each receptive of an RF modulating signal  132   a ,  132   b  over transmission line  136   a ,  136   b . The preamplifiers  140   a ,  140   b  amplify the RF modulating signals  132   a ,  132   b . The amplified RF modulating signal  152   a ,  152   b  are alternately (in a fashion similar to time division multiplexing) guided along microstrip transmission lines  124   a ,  124   b  to a tandem pair of traveling wave amplifiers  160   a ,  160   b ,  162   a ,  162   b . The traveling wave amplifiers  160   a ,  160   b ,  162   a ,  162   b  are positioned on opposing sides of the RF electrode  128  for modulating the optical signals  154 ,  156  in the polymer based MZI  104 . Traveling wave amplifiers  162   a ,  162   b  are in a flip chip configuration, e.g., the direct electrical connection of facedown electronic components onto flexible dielectric substrates by means of conductive bumps on bond pads.  
         [0031]    A third optoelectronic circuit for modulating an optical signal is depicted in cross section in FIGS. 13A through 13E. In FIG. 13A, microstrip waveguides  124  are applied to flexible dielectric substrate  144  in which is embedded a polymer based MZI  104  (only one branch of which is shown at  110 ,  112 ,  106 ). RF electrode  128  and ground electrode  130  are positioned on opposing sides of the MZI branch. In FIG. 13B, traveling wave amplifier  160   a , fixed to the die mount  168 , is connected to the RF electrode  128  and the appropriate via connection  170 . Also in FIG. 13C, the die mount  168  and adhesive layer (not shown) are removed.  
         [0032]    In FIG. 13D, the assembly of FIGS.  13 A- 13 C is adhesively joined with a second flexible dielectric substrate  176 . The second flexible dielectric substrate  176  includes traveling wave amplifier  162   a  fabricated therein whereby traveling wave amplifiers  160   a ,  162   a  are positioned on opposing sides of the MZI. In FIG. 13D, the assembly of FIGS.  13 A- 13 C is also encapsulated with an encapsulating material  174  and brought into contact with a heat exchanger  168  for cooling the  160   a . Continuing in FIG. 13D, a second flexible dielectric substrate  174 , including traveling wave amplifier  162   a , is adhesively bonded to flexible dielectric substrate  144 , thus providing the arrangement of FIG. 12.  
         [0033]    In FIG. 13E, a second embodiment of the arrangement of traveling wave amplifiers  160   a ,  162   a  is shown in cross section. Flexible dielectric substrate  144 , containing the MZI  104 , is adhesively joined with a flexible dielectric substrate  178  containing traveling wave amplifier  160   a  and brought into communication with heat exchanger  168 . RF electrode  128  of the MZI  104  in flexible dielectric substrate  144  is electrically connected to traveling wave amplifier  160   a  by way of bump mount  172 . Also in FIG. 13E, flexible dielectric substrate  180 , having traveling wave amplifier  162   a  positioned thereon, is adhesively joined to the flexible dielectric substrate  144 . Traveling wave amplifier  162   a  positioned on flexible dielectric substrate  178  and is connected to the MZI  104 .  
         [0034]    Any reference to first, second, etc., or front or back, right or left, top or bottom, upper or lower, horizontal or vertical, or any other phrase indicating the relative position of one object, quantity or variable with respect to another is, unless noted otherwise, intended for the convenience of description, and does not limit the present invention or its components to any one positional, spatial or temporal orientation. All dimensions of the components in the attached Figures can vary with a potential design and the intended use of an embodiment without departing from the scope of the invention.  
         [0035]    While the invention has been described with reference to several embodiments thereof, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiments disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.