Abstract:
One embodiment of the present invention provides a system for detecting light which is incident to a first semiconductor die. During operation, the system receives light at a photo-detector on the first semiconductor die, wherein associated circuitry converts the received light into a current. In doing so, the associated circuitry biases a gate voltage of an integrating transistor to be close to a threshold voltage of the integrating transistor, and applies the current from the photo-detector to the gate of the integrating transistor so that the current causes a charge to collect at the gate of the integrating transistor. This charge builds up and causes the integrating transistor to switch, thereby indicating that light has been received by the photo-detector.

Description:
GOVERNMENT LICENSE RIGHTS 
   This invention was made with United States Government support under Contract No. NBCH020055 awarded by the Defense Advanced Research Projects Administration. The United States Government has certain rights in the invention. 

   BACKGROUND 
   1. Field of the Invention 
   The present invention relates to integrated circuit devices. More specifically, the present invention relates to a method and an apparatus for detecting position using beams of light between semiconductor dies. 
   2. Related Art 
   Advances in semiconductor technology presently make it possible to integrate large-scale systems, including tens of millions of transistors, into a single semiconductor chip. Integrating such large-scale systems onto a single semiconductor chip increases the speed at which such systems can operate, because signals between system components do not have to cross chip boundaries, and are not subject to lengthy chip-to-chip propagation delays. Moreover, integrating large-scale systems onto a single semiconductor chip significantly reduces production costs, because fewer semiconductor chips are required to perform a given computational task. 
   Unfortunately, these advances in semiconductor technology have not been matched by corresponding advances inter-chip communication technology. Semiconductor chips are typically integrated onto a printed circuit board that contains multiple layers of signal lines for inter-chip communication. However, signal lines on a semiconductor chip are about 100 times more densely packed than signal lines on a printed circuit board. Consequently, only a tiny fraction of the signal lines on a semiconductor chip can be routed across the printed circuit board to other chips. This problem has created a bottleneck that continues to grow as semiconductor integration densities continue to increase. 
   Researchers have begun to investigate alternative techniques for communicating between semiconductor chips. Proximity Communication is one such alternative technique that communicates using the coupling capacitances between face-to-face chips. The Proximity Communication technique requires good mechanical alignment between chips, and hence requires a method to measure the position between chips. One promising technique involves using light energy to convey position between semiconductor chips. However, it is not a simple matter to build a photo-detector on a semiconductor die with sufficient sensitivity to detect the position of an incident beam of light. 
   Hence, what is needed is a method and an apparatus for building a photo-detector on a semiconductor die with sufficient sensitivity to detect the position of an incident beam of light. 
   SUMMARY 
   One embodiment of the present invention provides a system for detecting light which is incident to a first semiconductor die. During operation, the system receives light at a photo-detector on the first semiconductor die, wherein associated circuitry converts the received light into a current. In doing so, the associated circuitry biases a gate voltage of an integrating transistor to be close to a threshold voltage of the integrating transistor, and applies the current from the photo-detector to the gate of the integrating transistor so that the current causes a charge to collect at the gate of the integrating transistor. This charge builds up and causes the integrating transistor to switch, thereby indicating that light has been received by the photo-detector. 
   In a variation on this embodiment, the system receives light at a second photo-detector on the first semiconductor die, wherein the second photo-detector is part of an array of photo-detectors on the first semiconductor die. The system then applies the current from the second photo-detector to a corresponding integrating transistor in a corresponding array of integrating transistors. 
   In a further variation, the integrating transistor causes a detector output to switch. Furthermore, the system stores the detector outputs from the array of integrating transistors into an output register, wherein the output register indicates which photo-detectors in the array of photo-detectors received light. 
   In a further variation, the system generates the light from a light source on a second semiconductor die which is located in close proximity to the first semiconductor die, so that the light is received at the photo-detector on the first semiconductor die. 
   In a further variation, the system determines an alignment of the first semiconductor die relative to the second semiconductor die based upon which photo-detectors in the array of photo-detectors received the light. 
   In a variation on this embodiment, the photo-detector can include a photo-diode or a P/N-junction photo-detector. 
   In a variation on this embodiment, the light is generated by one of a Zener diode, a light emitting diode (LED), a vertical cavity surface emitting laser (VCSEL), and an avalanche breakdown P/N diode. 

   
     BRIEF DESCRIPTION OF THE FIGURES 
       FIG. 1  presents a circuit diagram illustrating a light detection system in accordance with an embodiment of the present invention. 
       FIG. 2  illustrates a detector array in accordance with an embodiment of the present invention. 
       FIG. 3  illustrates a photo-diode and associated circuitry in accordance with an embodiment of the present invention. 
       FIG. 4  presents a circuit diagram illustrating an integrating amplifier in accordance with an embodiment of the present invention. 
       FIG. 5  presents a flowchart illustrating the process of detecting light in accordance with an embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
   The following description is presented to enable any person skilled in the art to make and use the invention, and is provided in the context of a particular application and its requirements. Various modifications to the disclosed embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the present invention. Thus, the present invention is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed herein. 
   Background 
   Using light to align two objects is not a new concept. Well-recognized methods include aligning two objects using photo-resistive changes or photo-generated current. However, using photo-alignment to align two silicon chips is new. Given advancements in microprocessor design technology, one embodiment of the present invention uses a standard CMOS process to produce a light emitting and light detection system to align multiple semiconductor dies. In doing so, the challenge is to produce a light emitter and a light receiver within the constraints of a standard CMOS process flow. 
   In one embodiment of the present invention, a light emitter is constructed out of a P+/N+ junction. Applying a positive voltage to the cathode breaks down the junction and creates light. The forced reverse bias current transfers energy to the electrons of the silicon atoms. The electrons emit light when returning to lower energy states. This light can then be collimated; although the beam narrowness is limited by diffraction. Light generated by a semiconductor laser improves this but violates the process simplicity goal. Forming the emitter junction is problematic because CMOS active areas are areas shorted by the metal silicide process. Adjacent P/N areas without deletion of silicide will short the P/N junction. 
   Receivers for typical data transmission are well known. The receivers, or photo detectors, detect changes in the power of the light beam. The goal of photo-detectors when used for communications is high speed data transmission. However, currently for chip to chip alignment, a human is part of the action between movement and measurement. Human reaction times are in the hundreds of milliseconds while speeds in silicon are in nanoseconds. Thus, major parts of the detector can be very slow. 
   The integration of electrons released by photons is proportional to the light intensity. If required, a low rate of photons releasing electrons can be integrated over milliseconds. An integrating detector can be made easily in CMOS by using the transistor gate as a charge storing node. 
   In one embodiment of the present invention, a light emitting diode (LED) can be used as a transmitter. A Zener diode emitter connected to an N-well series resistor can form an emitter circuit. This N-well resistor prevents current hogging between two or more emitter circuit connected in parallel. At least two emitter networks can be arranged perpendicular to each other forming the X-Y axis and are connected to a common raw pad. The raw pad carries a voltage of six volts or more to turn on the Zener diode emitters. 
   In one embodiment of the present invention, the receivers are arrayed PMOS transistors whose elongated drains act as reversed biased P/N junctions. These junctions generate electrons by the well-known photo-voltaic process. Each PMOS transistor is at a measured distance from one another. The current flows at each PMOS junction but is highest at the junction with the highest light intensity. This current is integrated at the gate of a displaced charge pre-conditioned NMOS transistor. 
   To distinguish which PMOS drain junction has the highest light intensity the generated current is integrated and amplified to a logic level by an NMOS amplifier. A large OR gate signals when at least one amplifier has reached a predetermined limit. At this time all the amplified logic levels are latched. After latching, the charge on all the amplifiers is discharged momentarily and the process is repeated. The latched data then can be scanned out digitally or converted to an analog signal and measured. 
   Light Detection System 
     FIG. 1  presents a circuit diagram illustrating a light detection asynchronous state machine in accordance with an embodiment of the present invention. The light detection asynchronous state machine has two controls: the “flite”  100 , input at inverter  102 , and the “stop”  101 , input at inverter  104 . (Flite stands for “false light.”) When flite  100  is asserted, all the detectors in detector array  106  will output PMOS IDs current as if they are receiving strong light. Hence, the flite  100  input can be used to test the overall circuit. When flite  100  is asserted, the circuit oscillates through the asynchronous sequence of discharge, detect and update states. The D[ 0 : 7 ] word stored in register  110  will vary because of time variant transistor mismatch and ambient light variations. When flite  100  is deasserted, the detectors in detector array  106  will output PMOS IDs current when they receive light from an external source. Once a detector is receiving light, T-back circuit  108  causes the light detection asynchronous state machine to update register  110  with the detector that received light, and then resets the detectors in detector array  106 . When the stop input  101  is asserted, this halts the feedback signal from continuously discharging the integrating amplifier input node. 
   Photo Detector Array 
     FIG. 2  illustrates detector array  106  in accordance with an embodiment of the present invention. As is illustrated in  FIG. 2 , detector array  106  has eight photo-detector integrating amplifiers  201 – 208 . Each photo-detector integrating amplifier  201 – 208  has a corresponding integrating transistor and a photo-diode  231 – 238  for receiving light. 
   Photo-Diode 
     FIG. 3  illustrates a photo-diode  304  and associated circuitry in accordance with an embodiment of the present invention. In this layout, the pitch is the minimum allowed by fabrication technology. In the actual layout, the drain of the PMOS transistor  302  is elongated and acts as the P/N-junction photo-voltaic generator. 
   Integrating Amplifier 
     FIG. 4  presents a circuit diagram illustrating an integrating amplifier  201  in accordance with an embodiment of the present invention. Asserting signal B biases the amplifier near the triode/saturation transition. Bringing signal B LO displaces charge on the gate of the primary NMOS transistor  402 , thereby setting the amplifier to be incrementally de-biased. At this point the amplifier is in the integrating mode. The final output signal  404  is either logic LO or HI depending on the voltage at node D 1 . 
   Semiconductor Die Alignment 
   In one embodiment of the present invention, the light emitters are Zener diodes. To measure two axes of alignment, a minimum set of two is required, one for X and one for Y. They are connected in parallel and require one raw pin. The test pin exhibits a reverse breakdown curve of a P/N-junction. The break down is in the range of six volts. The actual tester electronics have a provision for a series resistor external to the chips to limit current. Maximum current is limited to 20 milliamperes by using a 210 ohm resister in series with the pin and an external applied voltage of 10 volts. 
   Initially, the scan path sets flite  100  LO and stop  101  HI to reset the D[ 0 : 7 ] signal. Asserting stop signal  101  LO begins the detection process. After a time of less than 1 millisecond, D[ 0 : 7 ] will be stable and will register one or two bits LO in a field of HI data. These data bits can be then scanned out after stop signal  101  is asserted HI. 
   Note that a potential for metastability exists because the data can be in transition at assertion “or-out” or the stop signal  101 . Noise causes some uncertainty. Hence, the data detection may require several trials to obtain unambiguous data because of metastability and noise. 
   A test mode begins in the same manner as above with the exception that the flite signal  100  is asserted HI after stop  101  is asserted LO. The array of PMOS transistors that make up the detector array provide current that mimics strong light. 
   Process of Detecting Light 
     FIG. 5  presents a flowchart illustrating the process of detecting light in accordance with an embodiment of the present invention. The system starts when flite signal  100  and stop signal  101  are de-asserted (step  502 ). Next, as light illuminates one or more photo-detectors in detector array  106 , corresponding integrating transistors in integrating amplifiers  201 – 208  pass through the threshold state (step  504 ). At some periodic interval, the system pulses T to latch D[ 0 : 7 ] in register  110  (step  506 ). Finally, the system pulses B to clear the photo-detectors in detector array  106  and returns to step  504  to repeat the process (step  508 ). 
   The foregoing descriptions of embodiments of the present invention have been presented for purposes of illustration and description only. They are not intended to be exhaustive or to limit the present invention to the forms disclosed. Accordingly, many modifications and variations will be apparent to practitioners skilled in the art. Additionally, the above disclosure is not intended to limit the present invention. The scope of the present invention is defined by the appended claims.