Abstract:
This invention discloses the basic chip architecture and packing configuration required to build an all silicon opto-coupler in which a forward biased silicon PN junction diode is used as the LED. Construction of the LED and the detector are disclosed as well as the package chip configuration. Methods for isolating circuit structures from the LED are also disclosed so that CMOS and bipolar circuits can freely added to the transmitting chip as well as the receiving chip. Bi-directional data transmission and multi-channel operation is also shown.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of provisional application 60/316,863 filed Sep. 4, 2001. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable 
     REFERENCE TO A MICROFICHE APPENDIX 
     1. Field of the Invention 
     This invention relates to opto-couplers used to electrically isolate signals. 
     2. Prior Art 
     Traditional opto-couplers are made using a discrete GaAs based LED and a silicon detector. In the simplest opto-couplers the detector is a single device such as a PN junction diode, a bipolar transistor, an SCR, or a Triac. Detector chips may also include circuits such as amplifiers and various types of output buffer/drivers. Moreover, an additional silicon chip can be added such as an input buffer/driver for the LED. The input signal may be, for example, a TTL type which can not directly drive the LED. Linear opto couplers can also been made which can transmit a voltage or a current level to an output from an isolated input. 
     The GaAs based LEDs used in opto couplers typically emit light in the deep red region of the visible spectrum where silicon PN junction diodes are efficient at converting the LED light into an electrical signal. 
     SUMMARY OF THE INSTANT INVENTION 
     This invention relates to opto-couplers which electrically isolate signals. It is the objective to show how to make an all silicon opto coupler using a forward biased silicon PN junction diode as the LED. With lattice damage added by, for example, not annealing an implant used to make the diode, the optical efficiency can be improved to make practical all silicon opto-couplers. Thus, the traditional discrete GaAs based LED used in existing opto-couplers is replaced with a silicon forward biased PN junction diode which has a lower “on” voltage (1.6V versus 0.65V for silicon). Furthermore, the forward biased silicon PN junction LED can be easily and cheaply integrated into a silicon integrated circuit using standard silicon processing techniques. 
     It is another object of this invention to show how a light detector capable of responding to the light produced by a forward biased silicon PN diode can integrated into silicon using standard silicon processing methods. Specifically, the light produced by a forward biased silicon PN junction diode produces light at a peak wavelength of about 1.15 μm which is poorly absorbed by silicon. Silicon has a light absorption coefficient of only 0.025% per micron at 1.15 μm. Thus, silicon PN junction diodes can not be used to detect the light emitted by a forward biased silicon PN junction diode. However, Schottky barrier diodes can detect 1.15 μm with a quantum efficiency of better than 10% and can be integrated into silicon using standard silicon integrated circuit processing. Furthermore, the methods describe herein permit multiple, bi-directional optical channels to be realized. Also, an [the] assembly technique makes use of the essentially transparent transmission of 1.15 μm light through the silicon substrate of the integrated circuit. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A Cross Section of a All Silicon Opto-Coupler in which the LED is a Forward Biased PN Junction Diode located on the bottom substrate and the light detector on the top substrate. 
     FIG. 1B Cross Section of a All Silicon Opto-Coupler in which the LED is a Forward Biased PN Junction Diode located on the top substrate and the light detector on the bottom substrate. 
     FIG. 2 shows a cross section and top view of the opto-coupler package 
     FIG.  3 . Top View silicon PN junction LED using separately driven cathode implant stripes. 
     FIG.  4 . Top View of All Silicon Opto-Coupler as in FIG. 3 but with finer driver drive segmentation. 
     FIG.  5 . Cross section of a bi-directional coupler chip showing light isolation techniques. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1A shows the cross section of an all silicon opto-coupler using a forward biased silicon PN junction LED. The opto-coupler consists of two silicon integrated circuits formed on silicon substrates  105  and  108 . Substrate  108  has a built in silicon based LED  117  along with functional circuitry  114 . For the sake of illustration, an NFET and PFET are shown in the circuit  114 . A second silicon integrated circuit substrate  105  has a functional circuit  116  and a built in light or photo detector  118 . Separating the two substrates  105  and  108  is a transparent insulator  107 . Thus, for the opto coupler shown in FIG. 1A silicon substrate  108  has the integrated light transmitting LED  117  and silicon substrate  105  has the integrated light detector  118  with a transparent insulator  107  in between. Bond pad  113  and bond wire  112  show an example of a connection between the integrated circuit of  108  and a package pin not shown. Correspondingly, bond pad  125  and bond wire  106  show an example of a connection between the integrated circuit of  105  and another package pin. Note that a protruding substrate ledge  120  is required for the bond pad  113  of substrate  108 . Also note that LOCOS oxide isolation  104  is shown for both silicon integrated circuits  105  and  108 . However, as can be appreciated by one normally skill in the art, trench isolation could have also be used. Deposited oxide  100  on substrate  105  and deposited oxide  127  on substrate  108  is also shown and is typical for standard integrated circuit processing. 
     The LED is formed using a forward biased PN junction diode with un-annealed implant dislocations used to enhance light emission (see W. Ng, M. Lourenco, R. Gwilliam, S. Ledain, G. Shao, and K. Homewood, “An efficient room-temperature silicon based light emitting diode”, Nature, Vol. 410, pp. 192-194, Mar. 8, 2001). The LED diode is comprised of an N+ implant region  110  in a P type substrate  108 , and a P+ implant  109  that is used to make a good electrical connection between the substrate and the anode metal terminals  115  and  116 . A connection is made to the LED&#39;s N+ cathode  110  using the metal terminal  111 . Because of resistance based de-biasing in the substrate, there is very little current injection  119  under the metal cathode connection  111 . Thus, the light produced by the forward biasing of the N+  110 , P  108  junction is not obscured by the cathode metal  111 . Most of the injection, in fact, occurs at and in the vicinity of the ends of the N+ implant  110  facing the P+ implant  109 . 
     A forward biased N+  110 , P  108  junction emits infrared light  121  with a peak wave length of 1.15 μm. At this wavelength, the optical absorption coefficient for silicon is about 2.5 cm −1 . That is, for 10 μm of silicon, only about 0.25% of light at 1.15 μm is absorbed. Thus, silicon is not a practical detector of light with a wavelength of 1.15 μm and, in fact, can be considered transparent at 1.15 μm. To detect the light generated by the forward biased silicon junction diode  117 , a Schottky barrier diode  118  is used. A Schottky barrier diode is a metal-semiconductor diode. In FIG. 1 the metal part of the diode is layer  102  and the semiconductor part is the substrate  105 . Using platinum silicide (PtSi) as the metal  102 , quantum efficiencies of 10% or better can be achieved. According to the Ng paper, an LED quantum efficiency of 10 −3  can be achieved for a forward biased silicon diode with implant damage. Thus, an LED to detector quantum efficiency on the order of 10 −4  is possible. That is, for every 1 mA input to the LED 0.1 μA is output from the detector. 
     The Schottky barrier metal  120  is generally quite thin. For PtSi this layer thickness can be about 20A (see W. Kosonocky, F. Shallcross, T. Villani and J. Groppe,: 160×244 Element PtSi Schottky-Barrier IR-CCD Image Sensor”, IEEE Trans. Electron Devices, Vol. ED-32, No. 8, pp. 1564-1573, August, 1985). Thin layers of barrier metal are used since detection quantum efficiency is improved. 
     An N+ band  103  around the periphery of the Schottky barrier metal  102  is used to reduce leakage current.  101  is standard interconnect layer of metal used to connect the cathode Schottky metal  102  with circuits such as an amplifier. The N+ band  103  also forms a contact pad under the metal  101  thus preventing metal  101  from punching through the thin Schottky barrier metal  102  and shorting to the substrate  105 . 
     Not all of the light striking the Schottky barrier is absorbed. To improve quantum detection efficiency, an optional metal layer  122  can be added to reflect light not absorbed by the Schotty barrier diode  118 . Thus, the light not absorbed by the first pass through the Schottky barrier will be redirected back through the barrier for a second pass via  122 .  122  can be realized using an upper level of metal that is commonly found in modern processes which have many levels of metal. 
     Light  124  generated by the LED  117  that propagates laterally and into the active circuit area  114  should not cause any significant interference because of the poor absorption of the 1.15 μm light by the silicon providing the silicon junction areas of active devices are kept small. 
     For bonding convenience, note that detector integrated circuit chip  105  sits on top of the integrated circuit chip  108 . This assembly configuration allows bond wires from both chips such as  106  and  112  to come in from the top. The light  121  generated by LED  117  then passes through transparent insulator  107  and through an optional anti-reflecting layer  123  before entering the bottom of the silicon substrate  105 . Since silicon is a poor absorber of light with a peak wavelength of 1.15 μm, most of the light passing through the silicon substrate  105  will reach the Schottky light detecting diode  118 . Also, the substrate  105  can be thinned to further reduce absorption. For a substrate thickness of 100 μm for  105  the light attenuation is 2.5%. Thinning of silicon substrates is common in the integrated circuit industry because of the need to reduce package height. 
     FIG. 1B shows a cross section of an all silicon opto-coupler in which the LED  168  is located on the top substrate  155  and the light detector  170  is located on the bottom substrate  158 . Support circuitry for the opto coupler function is assumed to exist in the area  177  on substrate  158  and in the area  164  on substrate  155 . Separating the two substrates  155  and  158  is a transparent insulator  160 . Bonding pad  178  and bond wire  179  are an example of a connection from the circuit  177  of substrate  158  to a package not shown and bonding pad  157  and bond wire  156  are an example of a connection from the circuit  164  of substrate  155  to the package. The isolation shown for both substrates is achieved with LOCOS  154  although shallow or deep trench isolation could also be used. A deposited dielectric  150  on substrate  155  and a deposited dielectric  172  on substrate  158  are used for isolation of metal layers. 
     The LED  168  is formed as before using a forward biased PN junction diode. The LED  168  is comprised of an N+ implant region  166  in a P substrate  155 , and P+ implants  159  and  167  are used to efficiently connect the substrate  155  to anode metal terminals  151  and  162  which are connected together. A connection is made to the LED&#39;s N+ cathode  166  using the metal terminal  161 . Note that light is generated not only in the direction of the light detector  170  but also in a direction away from the light detector  170 . A light reflector  163  is used to reflect the upward propagating light  165  downward  164  toward the light detector  170 . The light reflector  163  can be made using an upper level of metal commonly found in silicon processes. 
     The light detector  170  is a Schottky barrier diode comprising a barrier metal  174  in contact with a semiconductor  158 , and an N+ implant buffer  173  and  153  placed on the perimeter of the barrier metal  152 . Connection to the Schottky diode&#39;s cathode  174  is made with a metal interconnect  171 . The anode of the Schottky diode  170  is the P substrate  158  that is in contact with the Schottky barrier metal  152 . Connection to the anode of the Schottky barrier diode is made with a P+ implant  176  and metal interconnect  175 . For a good quantum efficiency the Schottky barrier metal must be thin such as 20 Å. 
     The operation of the coupler of FIG. 1B begins with signals coming in via bond wires such as  156  which then interact with circuitry  164 . After processing by the circuitry  164 , the signal causes the silicon diode LED  168  to be biased in the forward direction which then causes the LED  168  to emit light. The light is emitted both upward  165  and downward  169  into the bulk of substrate  155 . The light emitted upward  165  is reflected downward  164  by the optional metal layer  163 . The light then propagates through the anti-reflective coating  173 , the transparent insulator  160 , the deposited oxide layer  172 , and finally to the light detector  170 . The light detector  170  then outputs a signal in response to the light from LED  168 . The output node  171  from the detector  170  feeds processing circuit  177  also located on substrate  158 . The processing circuit  177  may include an amplifier for magnifying the weak signal coming from the detector  170 . The processed signal from detector  170  can then be sent out of the integrated circuit of substrate  158  using one or more bond wires such as  179 . Thus, isolation is achieved between the signals coming into the integrated circuit of substrate  155  and going out of integrated circuit of substrate  158 . Note that by combing the elements of FIG. 1A with the elements of FIG. 1B a bi-directional opto-coupler can be realized. That is, signals can be transmitted back and forth between the electrically isolated integrated circuits. 
     As can be appreciated by one normally skilled in the art, the doping polarities shown in the previous example can be reversed. For example, a Schottky barrier diode can also be realized using a metal to N type semiconductor junction. Also, the LED can be made with a P+ implant in an N doped region such as the N well associated with the construction of a PFET in a CMOS process. 
     FIG. 2A shows a top view of the package  200  encapsulating the all silicon based opto-coupler and FIG. 2B shows a cross section of the opto-coupler containing two optically coupled silicon integrated circuits,  206  and  208 . Package pins  211  are connected to integrated circuit  206  and package pins  212  are connected to integrated circuit  208 . A transparent insulator  204  is used to separate the two integrated circuits  206  and  208 . Metal plate  209  is the floor plate of the package lead frame and is used to secure the lower integrated circuit die  208  to the package lead frame.  202  is a cross section example of a package lead connected to integrated circuit  206  via bond wire  215  and  203  is an example of a package lead connected to integrated circuit  208  via bond wire  205 . LED  210  and light detector  213  are associated with integrated circuit  206  and LED  214  and light detector  207  are associated with integrated circuit  208 . Note that in this example there is bi-directional optical link between integrated circuits  206  and  208  with LED  210  and light detector  207  forming a first optical signal path and with LED  214  and light detector  213  forming a second, reverse optical signal path. 
     FIG. 3 shows a top view of the layout of a silicon LED consisting of N+  300 ,  302 A,  302 B, and P+  301 ,  303 A,  303 B stripes in a P substrate  108 . As noted earlier, due to de-biasing, most of the carrier injection and subsequent light emission occurs at the junction edges such as boundary region  310  between the N+ implant  300  and the substrate  309  facing implant  301 A. Metal lines such as  305  for the N+ implant and  306  for the P+ implant are used to connect the diode to the power terminals. Because of de-baising, little light emission occurs under the metal interconnect lines such as  305 . 
     For the sake of illustration, the P+ implants  301 A,  301 B,  303 A, and  303 B connecting to the LED anode are grounded and the N+ implants  300 ,  302 A, and  302 B which form the diode cathode are connected to current sources. Note that each N+ implant is connected to an individual current source such as  307 A,  307 B, and  307 C so that carrier injection and subsequent light generation is more uniform. It is well known that diode conduction has both a steep I-V slope (60 millivolts per decade change in current at 25C) and a negative temperature coefficient. These two factors tend to cause non-uniform conduction. Thus, providing current sources for each individual N+ junction helps reduce the possibility of non-uniform conduction and corresponding non-uniform light emission. 
     Note that end junctions  308 A and  308 B are made smaller than the center junction  300 . This is done to make a more rounded light emission pattern. As can be appreciated by one normally skilled in the art, with more junction stripes a more circular pattern can be created by appropriate tapering of the junction stripe lengths. 
     FIG. 4 shows a layout variation of the LED shown in FIG.  3 . In this case the implanted regions of FIG. 3 are split in two implants such as the N+ implant  300  being split into  401  and  402 . This is done to further segment the implants for the sake of better current uniformity and correspondingly, better light uniformity. For the case shown in FIG. 4 three additional current drivers are introduced,  307 D,  307 E, and  307 F. The penalty for this  2  fold increase in segmentation is slightly lower emission area due to a gap  400  between implants. 
     FIG. 5 shows the cross section of a bi-directional opto-coupler integrated circuit wherein a means for isolating the light emitted by the LED  500  from other circuitry is demonstrated. In particular, if a Schottky barrier light detector  501  is on the same chip as the LED for the purpose of bi-directional communication then it may be necessary to isolate the light detector and possibly other circuits on the substrate  510  from the light emitted by the LED  500  into the substrate  510 . 
     Two light components that are emitted from LED  500  into the substrate  510  are shown. One is a component that is emitted latterly  507  and the other is a component that is emitted into the bulk of the substrate  508 . To suppress the hole-electron pairs generated in the bulk a heavily doped substrate  510  is used with a lighter doped epitaxial layer  511 . The heavy doping of the epitaxial layer will facilitate the recombination of hole-electrons produced by silicon absorption of the light. 
     The lateral light component  507  can cause hole-electron pairs in such critical regions as the depletion zone of the light detector diode  501 . To block such light emissions, a deep trench  504  is made through the epitaxial layer  511  and down to the substrate  510 . A thermal oxide  506  is then made on the surface of the silicon. Sloped trench walls can be optionally made to enhance reflection of the light  507  off of the silicon-oxide interface of the wall. Also, metal  505  can be deposited into the trench to further enhance reflection of the lateral light  507 . Thus, the trench serves to reflect the lateral light  507  into the bulk of the substrate  510  and away from active surface devices such as the light detector  501 .