Abstract:
An apparatus comprises a first substrate and a second substrate. The first substrate includes an optoelectronic device and a matching circuit. The second substrate includes a driver circuit. A frequency response of the optoelectronic device is changed by the matching circuits. The first substrate is coupled to the second substrate via respective bond pads from the first and second substrates such that the matching circuit is interposed between the optoelectronic device and the driver circuit.

Description:
BACKGROUND OF THE INVENTION 
     An optical transceiver transmits and receives light signals within a fiber optic network. The light source for the transceiver is typically an optoelectronic device, such as a Vertical Cavity Surface Emitting Laser (VCSEL), a Light Emitting Diode (LED), a laser, or other light-emitting device. The light output from the optoelectronic device is controlled by a driver circuit. 
     In digital transmissions, the driver circuit drives the optoelectronic device between high and low signal outputs to represent a digital data stream. The data is synchronized to a clock with a regular period. During a certain window of time during each clock cycle, the transmitted signal must clearly be a one or zero for the receiver to read the data correctly. Any signal transitions are done outside this window. When plotted on an oscilloscope, the transmitted signal looks like the graph in  FIG. 1 , known as an eye diagram. A transceiver with optimal performance has fast rise and fall times in its eye diagram without excessive ringing in the output. 
     The optoelectronic device and the driver circuit are typically formed as separate components on separate chips. There are several options for electrically connecting the optoelectronic device to the driver circuit.  FIG. 2A  shows one method, wherein a bond pad on the optoelectronic device  203  is connected to a bond pad on the driver circuit  205  with a conductive wire bond  207 .  FIG. 3A  shows another method, known as a flip-chip configuration: the chip with the optoelectronic device  303  is flipped onto the chip with the driver circuit  305 , and bond pads on the two chips are bonded together using solder balls  307 . There are many other configurations for bonding the two chips together. 
     Since the data rate within optical networks is constantly increasing, the transitions between high and low outputs must occur faster and faster. However, the effect of parasitic capacitances and inductances within the transceiver becomes more pronounced as the transmission frequency increases, which negatively affects the performance of the transceiver. 
     For example, the wire bond connection  207  of  FIG. 2A  introduces inductance between the optoelectronic device  103  and the driver circuit  105 , which changes the frequency response of the transceiver. Unfortunately, the size and shape of the wire bonds are not consistent: some wires may arc higher than others, and some wires may be shorter than others. Therefore, the inductance of each wire bond will vary as well. This variability lowers the manufacturing yield, because the frequency response of the transceiver may no longer meet specifications.  FIG. 2B  shows a sample eye diagram for an optical transceiver with excessive inductance in the wire bonds between the optoelectronic device and the driver circuit. The ringing effect in the transmitted signal makes it difficult for the receiver to determine whether the data is a one or a zero. 
     The flip-chip construction of  FIG. 3A  is also problematic. Since the solder balls  307  are relatively small, they introduce very little inductance into the system. However, a connection with too little inductance may also negatively affect the frequency response of the optical transceiver.  FIG. 3B  shows a sample eye diagram for an optical transceiver having too little inductance (which manifests itself in the signal as insufficient response peaking) in the wire bonds between the optoelectronic device and the driver circuit. The transmitted signal takes much longer to complete its transitions, which slows down the speed at which data can be transmitted. 
     Past attempts to compensate for these problems included redesigning the driver circuit, redesigning the bond pads, adjusting the length of the wire bond, or changing the arc of the wire. However, these methods are time-consuming, expensive, and difficult to repeat. Therefore, there remains a need for a way to electrically connect an optoelectronic device to a driver circuit to produce an optical transceiver with an optimal frequency response, and to do so with higher yield and better repeatability. 
     SUMMARY OF THE INVENTION 
     In a preferred embodiment of the present invention, the invention provides a precise and repeatable amount of inductance or capacitance to the optoelectronic device. A matching circuit is formed on the same substrate as the optoelectronic device. The matching circuit alters the frequency response of the optoelectronic device to match a driver circuit that drives the optoelectronic device. Thus, the optical transceiver can produce optimal performance within a desired frequency range. 
     The matching circuit adds a precise amount of inductance, capacitance, or a combination of both to alter the frequency response of the optoelectronic device. A flip-chip configuration is used to bond the optoelectronic device to the driver circuit, because the solder balls introduce only a negligible amount of parasitic capacitance and inductance into the system. Thus, variability in the parasitics from the bonding mechanism is eliminated. 
     The matching circuit can be a simple inductor, an inductor in series with a capacitor, an inductor in parallel with a capacitor, or any other possible combination of inductors and capacitors. Multiple stages of these inductor and capacitor combinations may also be used in the matching circuit. The inductance or capacitance in the matching circuit can be precisely calculated and designed to optimize the performance of the optical transceiver. Other components such as resistors, diodes, and stubs, may also be incorporated into the matching circuit. 
     In an alternate embodiment, the matching circuit can be altered after fabrication to change the frequency response of the optoelectronic device. The substrate of the optoelectronic device is designed with two bond pads as possible contact points to the matching circuit. Each bond pad is connected to a different amount of inductance. This configuration provides some flexibility in the amount of inductance that may be introduced into a system, since the decision as to how much inductance is introduced may be delayed until after the fabrication of the matching circuit. 
     In an alternate embodiment, the optoelectronic device is a photosensor. The performance of a photosensor is also dependent upon operating frequency, since it is also susceptible to parasitic inductances and capacitances. Therefore, a matching circuit can be used to tune the frequency response of a photosensor as well to match the frequency response of the photosensor to an amplifier. 
     Further features and advantages of the present invention, as well as the structure and operation of preferred embodiments of the present invention, are described in detail below with reference to the accompanying exemplary drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an eye diagram for an optical transceiver. 
         FIG. 2A  shows an optoelectronic device and a driver circuit connected by a wire bond. 
         FIG. 2B  shows a sample eye diagram for an optical transceiver with excessive inductance in the wire bonds between the optoelectronic device and the driver circuit. 
         FIG. 3A  shows a flip-chip configuration for bonding an optoelectronic device to a driver circuit. 
         FIG. 3B  shows a sample eye diagram for an optical transceiver having too little inductance (which manifests itself in the signal as insufficient response peaking) in the wire bonds between the optoelectronic device and the driver circuit. 
         FIG. 4  shows a high-level block diagram of a preferred embodiment made in accordance with the teachings of the present invention. 
         FIGS. 5A–5D  show possible circuit configurations for the matching circuit. 
         FIG. 6  shows a top view of a VCSEL and an inductor formed on the VCSEL substrate. 
         FIG. 7  shows an alternate embodiment for the matching circuit. 
         FIG. 8  shows a possible configuration for altering the frequency response of the optoelectronic device, after it has been fabricated. 
         FIG. 9  shows an alternate embodiment in which the matching circuit is used to tune the frequency response of a photosensor. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 4  shows a high-level block diagram of a preferred embodiment made in accordance with the teachings of the present invention. An optoelectronic device  401  is formed upon a substrate  403 . A matching circuit  405  is formed upon the same substrate  403  and coupled to the optoelectronic device  401 . A driver circuit  407  controls the output of the optoelectronic device  401 . The matching circuit  405  is designed to tune the frequency response of the optoelectronic device  401  to the driver circuit  407  for optimal performance within a desired frequency range. 
     The matching circuit  405  introduces a precise amount of inductance, capacitance, or both to alter the frequency response of the optoelectronic device  401 . The actual amount of inductance or capacitance required to produce an optimal performance may be determined empirically through circuit simulations. 
       FIGS. 5A–5D  show possible circuit configurations for the matching circuit  405 . The matching circuit  405  can be a simple inductor ( FIG. 5A ), an inductor in series with a capacitor ( FIG. 5B ), an inductor in parallel with a capacitor ( FIG. 5C ), or any other possible combination of inductors and capacitors. Multiple stages of these inductor and capacitor combinations may also be used in the matching circuit  405 . Other components such as resistors, diodes and stubs, may also be incorporated into the matching circuit  405 .  FIG. 5D , for instance, shows an inductor in series with an RC filter. 
     In a preferred embodiment of the present invention, the matching circuit is a simple inductor.  FIG. 6  shows a top view of a VCSEL  601  and an inductor  605  formed on the VCSEL substrate  603 , made in accordance with the teachings of the present invention. Although a VCSEL is shown in this particular illustration, other optoelectronic devices may also be used. The VCSEL  601  is formed on a substrate  603 . Two bond pads,  609  &amp;  611 , are provided on the substrate  603  for connecting a driver circuit  607  (not shown) to the anode and cathode of the VCSEL  601  in order to bias and control the VCSEL  601 . The inductor  605  is also formed on top of the same substrate  603 , using any conventional photolithographic process. The inductor  605  connects the VCSEL  601  to the bond pad  609 . 
     The inductor  605  can be formed with any suitable conductive material such as copper, aluminum, gold, etc. The shape, length and width of the inductor  605  may be varied to control the amount of inductance. The inductor  605  is formed over a low-loss insulating layer on the substrate  603  to prevent accidental shorting to any other circuits within the substrate  603 . Exemplary low-loss insulating materials include lightly doped silicon, polyimide, silicon dioxide, silicon nitride, and other dielectrics. 
     The optoelectronic device  601  is flipped onto the surface of the chip holding the driver circuit (not shown) and bonded in a flip-chip configuration. Since the solder balls used in the flip chip configuration introduce a minimal amount of inductance and capacitance into the system, the inductor  605  provides the frequency response adjustment for the optical transceiver. The design of the inductor  605  is flexible; the amount of inductance can be changed to match other chip bonding configurations as well. 
     In an alternate embodiment, the matching circuit can be a combination of inductors, capacitors, resistors, stubs, and other matching devices. For example,  FIG. 7  illustrates a top view of a VCSEL  701  and a matching circuit formed on a substrate  603 , made in accordance with the teachings of the present invention. The matching circuit in  FIG. 7  consists of two inductors  705  and  706  in series with a capacitor  713 . Again, although a VCSEL is shown in this particular illustration, other optoelectronic devices may also be used. A wide variety of configurations and combinations of matching devices are possible. 
     In a second alternate embodiment, the matching circuit can be altered after fabrication to change the frequency response of the optoelectronic device. In  FIG. 8 , the VCSEL  801  is designed with two possible contact points to the matching circuit-bond pads  809  and  815 . A driver circuit bonded to the first bond pad  809  will only be connected to inductor  805 . A driver circuit bonded to the second bond pad  815  will be connected to both inductors  805  and  813 . This configuration provides some flexibility in the amount of inductance that may be introduced into a system, since the decision as to how much inductance is introduced may be delayed until after the fabrication of the matching circuit. 
       FIG. 9  shows a third alternate embodiment in which the optoelectronic device is a photosensor  901 , such as a PIN (P-type Intrinsic N-type) detector or a photodiode. The performance of the photosensor  901  is also dependent upon operating frequency, since it is also susceptible to parasitic inductances and capacitances. Therefore, a matching circuit  905  can be used to tune the frequency response of a photosensor  901  as well. The matching circuit  905  is fabricated on the same substrate  903  as the photosensor  901  and matches the frequency response of the photosensor  901  to an amplifier  907 . The amplifier  907  amplifies the signals from the photosensor  901 . 
     Although the present invention has been described in detail with reference to particular preferred embodiments, persons possessing ordinary skill in the art to which this invention pertains will appreciate that various modifications and enhancements may be made without departing from the spirit and scope of the claims that follow.