Abstract:
This invention discloses the several means by which transient noise due to capacitance related displacement current can be excluded from the optical signal coming from a silicon detector used in opto-couplers. The exclusion of such noise permits a high degree of detector sensitivity which permits the use of low efficiency silicon based LEDs for opto-coupler applications.

Description:
This application claims the benefit of Provisional application Ser. No. 60/316,862, filed Sep. 4, 2001. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates to the field of semiconductor photo detectors used in opto-couplers, and particularly to reducing the disabling effect of displacement current in photo diodes used in opto couplers. 
   2. Description of the Related Art 
   Opto couplers are used to allow two electrical systems to communicate with each other while remaining electrically isolated. This communication is achieved by sending light signals using an electrically activated light emitter, typically a Light Emitting Diode (LED), to a photo detector which converts the light signals back into electrical signals. The light passes through a transparent insulator thereby electrically isolating the light emitter and its associated circuitry from the light detector and its associated circuitry. The associated circuitry of the LED side of the optocoupler can include an LED driver, amplifier, etc. The associated detector circuitry can include an amplifier, output drivers, A to D and D to A converters, etc. 
   Unfortunately, a displacement current can flow from the LED side of the opto coupler into the photo detector and cause an electrical output from the photo detector in addition to the electrical output produced by the light. The displacement current is a consequence of the unavoidable capacitance coupling between the LED side of the opto-coupler and the photo detector. This displacement current flows when the voltage applied to the coupler&#39;s LED and its surrounding electrically conductive structures changes with respect to the voltage applied to the coupler&#39;s photo detector. The magnitude of the unwanted displacement current flowing from the photo detector is dependent on the dV/dt or the rate change in the voltage between the LED side and the photo detector. The spurious detector signals produced by displacement currents can therefore be disruptive to the normal communication process of the opto-coupler. Thus, there is a strong need to keep displacement current out of the photo detector, especially if the light signal is weak as is the case with silicon based LEDs. 
   Photo detectors associated with silicon integrated circuits are typically realized using semiconductor junctions. The junction based photo detectors include PN diodes, bipolar transistors, SCRs, and Triacs. 
   In general terms, the displacement current is given by 
         I     d   ⁢           ⁢   i   ⁢           ⁢   sp   ⁢           ⁢   l   ⁢           ⁢   a   ⁢           ⁢   c   ⁢           ⁢   e   ⁢           ⁢   m   ⁢           ⁢   e   ⁢           ⁢   n   ⁢           ⁢   t       =       C     c   ⁢           ⁢   o   ⁢           ⁢   u   ⁢           ⁢   p   ⁢           ⁢   l   ⁢           ⁢   i   ⁢           ⁢   n   ⁢           ⁢   g       ⁢       ⅆ   V       ⅆ   t             
 
where I displacement  is the displacement current or current flowing into the capacitance, C coupling  is the capacitance between two electrically isolated conductors, and dV/dt is the rate change in voltage between the two isolated conductors.
 
   The relative magnitude of this undesirable current can be estimated. Assume that the transparent insulator of an opto-coupler has a thickness of 300 μm and a relative dielectric constant of 2.8. The parallel plate capacitance per unit area is 
         C     i   ⁢           ⁢   n   ⁢           ⁢   s       =       ɛ     i   ⁢           ⁢   n   ⁢           ⁢   s         t     i   ⁢           ⁢   n   ⁢           ⁢   s             
 
where C ins =capacitance per unit area, ε ins =permativity of insulator, and t ins =thickness of insulator. For t ins =0.03 cm (300 μm) and ε ins =2.8×8.854c−14 then C ins =8.26 pF/cm 2 . Since a reasonable radius for a photo detector is 150 μm assume that the detector area is equal to π×0.015 2  or 0.000707 cm 2 . Then C coupling =5.84 fF. To make the calculation worst case, assume that the fringe field coupling is 50% of the parallel plate capacitance for a total capacitance of 1.5×5.84 or 8.75 fF.
 
   Assuming a transient voltage between the chip with the LED and the light detector of 10 6  V/sec, then i coupling =8.75 nA. Some opto-coupler specifications show a “common mode” slew rate as high as 10 9  V/sec, which produces a displacement current of 8.75 μA for a coupling capacitance of only 8.75F. 
   For an opto-coupler using a silicon junction avalanche LED, a reasonable quantum efficiency is 10 −5 . Assuming a detector quantum efficiency of 0.8, then for an LED current of 10 mA the photo current would be 80 nA. Although an 80 nA data signal current by itself can be readily detected by an amplifier circuit, a superimposed spurious displacement current 100 times greater in magnitude can make data extraction difficult and error prone. 
     FIG. 1  shows an example of the cross section of a silicon based opto-coupler. Two integrated circuits,  106  and  107 , are shown separated by a transparent insulator  108 . An LED  111  is built into integrated circuit  106  and emits light through the transparent insulator  108  to a light detector  112  of the receiving integrated circuit  107 . Bond wire  104  connects package lead  102  to integrated circuit  106  and bond wire  105  connects package lead  103  to integrated circuit  107 . Also shown is the lead frame die attach plate  110  for integrated circuit  106  and the lead frame die attach plate  109  for integrated circuit  107 . Package lead  102  connects to integrated circuit  106  and establishes the base potential of integrated circuit  106  and package lead  103  connects to integrated circuit  107  and establishes the base potential of integrated circuit  107 . The base potential of each integrated circuit is established through a power supply connection to the substrate of each integrated circuit. The surface of each integrated circuit may contain areas were the voltage is different from the base or substrate potential by several volts. 
   Under normal operation, light  112  is emitted from LED  111  to light detector  112 . However, as shown in  FIG. 1 , a large electrical spike can exist between the base potentials of integrated circuits  106  and  107 . The rate change in the voltage difference produces a displacement current  113  in the insulator  108  between integrated circuits  106  and  107 . Some of the displacement current  113  flows out of the light detector  112  potentially disrupting operation. 
   As can be appreciated by one normally skilled in the art, LED  111  could also be a discrete GaAsP LED. 
   SUMMARY OF THE INVENTION 
   According to the present invention, there is provided a means by which the displacement current flowing between two electrically isolated but optically linked semiconductor devices will not cause disruption in the optical communication process between the two semiconductor devices. These methods include using two detectors for differential sensing, using a conducting transparent ground shield over the light detector, using a diffusion or an implanted top layer as a ground shield, and using a MOSFET configured as a light detector with the gate serving as a ground shield. Also, it is shown that more than one level of metal of an integrated circuit can be used to shield interconnect circuitry from displacement current. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a cross section of an opto coupler illustrating displacement current flow between the two electrically isolated semiconductor devices. 
       FIG. 2  shows a cross section of a differential junction diode photo detector used to null out displacement current. 
       FIG. 3  shows a cross section of a junction diode photo detector with a grounded transparent conductor blocking displacement current. 
       FIG. 4  shows the cross section of a photo junction diode which uses an upper grounded implant to absorb displacement current. 
       FIG. 5  shows a cross section diagram of a photo MOSFET which uses the gate electrode as a displacement current shield. 
       FIG. 6  shows a partial cross section of a photo diode electrically shielded by a transparent conductor. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 2  shows an example of a differential light detector which can be used to null out a displacement current induced by the light source elements. Placed in a P type silicon substrate  200  is an N+ implant or diffusion  201 , a second N+ implant  203 , and three P+ implants,  202 A,  202 B, and  202 C. The first N+ implant  201  forms an N+/P junction diode and the second N+ implant  203  forms a second N+/P junction diode. P+ implants  202 A,  202 B and  202 C form substrate ties or a means of connecting metal conductors to the P substrate  200 . Thus, the P substrate  200  areas near the N+ junctions  201  and  203  are robustly tied to ground  205  via the P+ ties  202 A,  202 B, and  202 C. 
   The diode junction formed with the N+ implant  201  is used as a photo detector in this example. Light  206 A coming from the LED source passes through a transparent dielectric  209  such as SiO 2  and into the diode formed by the N+ implant  201  thereby creating a photo current. The second diode formed with the N+ implant  203  is identical in size and shape to the first diode formed with the N+ implant  201  but is shielded from any light  206 B that might come in over the diode from the LED source. The second diode formed with  203  is used as a voltage reference for the photo diode formed with  201 . 
   The light shield  204  covering N+ implant  203  can be any masked, opaque insulating layer such as a dyed photo resist. The material for the light shield  204  should be thin as possible so as to minimize any difference between the displacement current  207  of the photo diode and the displacement current  208  of the reference diode  203 . Also, the light shield  204  can be made of a floating, opaque conductive material such as aluminum, copper, or a barrier metal such as tungsten. A floating, opaque conductor  204  can transmit the displacement current  208  while blocking the light  206 B. A conducting light shield  204  works the same as an insulating light shield  204  if, for the insulating light shield, the electric field is uniform over the surface of the reference diode comprising  203  and the electric field is normal or perpendicular to the insulating light shield  204 . For this case the electric potential is uniform over the surface of the insulating material and, hence, a relatively thin conducting light shield will have the same field distribution since the potential will be uniform in the shield. 
   The diodes formed by N+ implants  201  and  203  have a differential signal output  210 . Light hitting the structure of  FIG. 2  will cause the output of a photo current well above the background junction leakage current on the left output  210  lead that is connected to implant  201  whereas only junction background leakage current will appear on the right output  210  lead that is connected to implant  203 . Thus, a differential signal on  210  will result in response to illumination of the structure of FIG.  2 . 
   However, if a rate change in voltage occurs between the structure of FIG.  2  and the LED source a displacement current  207  will flow into  201  and a displacement current  208  will flow into  203 . Because of the aforementioned identical geometries of  201  and  203  displacement current  207  and the displacement current  208  will be essentially identical. Thus, a displacement current will produce a change in the common mode current of the output  210  but no change in the differential signal. Thus, light  206 A including stray light  206 B will produce a differential signal at the output  210  but displacement current  207  and  208  will not. 
   It is also noted that substrate noise is also presented as a common mode signal on the output  210  and not as a differential signal. Thus, this detector construction is also useful for rejection of substrate  200  noise. 
   As can be appreciated by one normally skilled in the art, the impurity polarities of  FIG. 2  can be reversed. That is, the P substrate can be N type, the substrate tie implants  202 A,  202 B, and  202 C can be N type, the implants  201  and  203  P type. Also, the diode structure of  FIG. 2  can be imbedded into a well which is implanted into a substrate of opposite polarity such as a the N well associated with a PFET of CMOS standard process using a P type substrate. 
   Another method of eliminating the displacement current between an LED source and the light detector is to use a transparent electrostatic shield. An Indium tin oxide electrostatic shield can be deposited over a light detector thus eliminating displacement current from appearing at the output of the detector. Another material that can be used but with less transparency is polysilicon. 
     FIG. 3  shows a cross section of a shielded photo diode detector. The shielded detector consists of a semiconductor substrate  300 , an N+ implant  301 , transparent interlevel dielectric  309  such as SiO 2 , a P+ implant  302  to connect the substrate  300  to a metal lead  312 , contact metal  308  for the N+ implant, contact metal  311  for the P+ implant  302 , a transparent conductor  303 , a ground interconnect lead  312 , and an electrical ground point  305 . 
   Light  306  from the LED propagates through the transparent conductor  303 , through the interlevel dielectric  309  such as SiO 2 , through the thin N+ implant layer  301 , and into the substrate region below the N+ implant  301  where the light is absorbed. The absorption of light in the depletion region, whose boundaries are the N+ implant  301 —P substrate  300  interface and the boundary  304 , produces an electrical current between the N+ implant  301  and the P substrate  300 . Also, photo generated carriers below but near the depletion region boundary  304  can contribute to the electrical current by diffusion. The photo generated electrical current flows out through lead  310  and is referenced to ground  305 . 
   A displacement current  307  flows through the transparent conductor  303  to ground  305  rather than through the N+ implant layer  301  to the photo current output  310 . 
   It is noted that the transparent conductor  303  adds some undesirable capacitance to ground for the output node  301  via electrostatic coupling between the N+ implant layer  301  and the transparent conductor  303 . This capacitance can be reduced by increasing the thickness of the interlevel dielectric  309 . 
   To determine the effect of the transparent conductor may have on the detector capacitance assume that the substrate doping is 1e15/cm 3  then the capacitance per unit area is 1e4 pf/cm 2  or 0.1 fF/μm 2  at zero bias. For an oxide thickness of 1 μm, the capacitance per unit area is 3453 pF/cm 2  or 0.0345 fF/μm 2 . Thus, an ITO layer placed above a diode doped 1e15/cm with an intervening SiO 2  dielectric thickness of 1 μm will add 34.5 percent more capacitance to the diode at 0 bias. A transparent conductor made of ITO can be expected to absorb approximately 10% of the light passing through it. 
   As noted earlier, a thin polysilicon layer can also be used in place of the ITO. The MOSFET gate polysilicon thickness for sub-micron processes can be as thin as 0.25 μm. To determine how much light is absorbed in a polysilicon shield it will be assumed that the absorption coefficient for polysilicon is the same as that for silicon. The light emission peak wavelength of a silicon avalanche LED is about 590 Å which is yellow. The silicon absorption coefficient for this wavelength is 2 μm. Thus, yellow light is reduced to 88% from its initial value after passing through 0.25 μm of silicon. Even thinner thicknesses for polysilicon can be used but may not be useful as the gate material of MOSFETs. Thus, polysilicon can be a useful electrostatic shield for a photo diode due to it high transmission of light if thin enough. 
   An implanted N Well photo diode with a displacement current ground shield implant can also be made. In this construction, a three terminal N Well based diode is used as shown in FIG.  4 . The N Well connects to an output lead  403  using an N+ tie implant  401  and the substrate connects to ground  407  using a P+ implant  402 . The top P+ implant layer  404  and the P substrate  400  are connected to ground  407  while the N Well  405  is used as the signal node which is output on  403 . The P+ implant layer  404  is made thin to minimize the absorption of light  406 . Thus, displacement current  407  will flow through the top P+ implant layer  404  to ground  407  and will not disturb the photo current flowing between the N Well  405  and the P+ implant layer  404  and between the N Well  405  and the P− substrate  400 . The P+ implant  404  and N Well  405  can be the same as that used to make the drain/source implant and N Well implant of a PFET, respectively, thereby allowing the structure of  FIG. 4  to be made without any additional fabrication steps over that of a standard CMOS process. 
     FIG. 5  shows the construction of a PFET  514  based photo detector. In this construction, a PFET  514  is used to create a shielded photo diode. The gate polysilicon  503  is used as the transparent electrostatic shield and is connected to ground  505 . The drain implant  504 A and the source implant  504 B of PFET  514  are connected together and are biased via node  515 . P+ implants  502  in the P substrate  500  are used to tie the substrate  500  to ground  505 . An N+ implant  501  is used to tie the N Well  516  to the detector output  508 . 
   An example of a biasing circuit for the drain  504 A and source  504 B of  514  is also shown in FIG.  5  and is comprised of a second PFET  512  and a load resistor  511  connected to the drain of PFET  512 . The source of PFET  512  is connected to ground  505 , the gate of PFET  512  is connected to the drain of PFET  512  which is connected to the load resistor  511 . The second end of the load resistor is connected to a positive supply voltage, Vdd  513 . 
   Note that the PFET  514  is biased into the inversion regime so that there is not only a photo carrier collection zone associated with the N Well  516 -Substrate  500  junction depletion region that extends from  509 A to  509 B but also with the depletion region  510  associated with the PFET inversion layer  517 . The P+ Drain  504 A/Source  504 B implants have to be biased such that the PFET  514  is in inversion and such that the N Well  516 -substrate  500  junction and the inversion layer  517 -N Well  516  junction are reversed biased. One way to generate Drain  504 A/Source  504 B bias voltage on node  515  is to use the second PFET  512  as reference voltage source as shown. With a large value of R load    511 , the output  515  of the reference PFET  512  will be slightly above its threshold voltage with respect to ground  505  and includes the body effect on threshold voltage. Assuming that the threshold voltage of  512  matches that of  514  then the voltage on  515  will cause an inversion layer  517  to form in PFET  514 . Node  508  is the photo diode output and can be biased anywhere from the reference voltage on node  515  to Vdd  513 . Note that as bias voltage on photo current output  508  is increased above the reference voltage on  515 , the depletion boundaries  509 A,  509 B, and  510  will move causing expansion of the depletion regions wherein photo generated carriers are generated and electrically collected. Thus, collection efficiency and response time suggest the bias voltage on the output node  508  be close to Vdd. 
   An alternate operating scheme for the PFET based light detector of  FIG. 5  is to use the Drain  504 A/Source  504 B implants as the signal node with the N well  516  and gate  503  tied to a quite or filtered ground. This configuration would prevent substrate  500  noise from interfering with sensing. In this mode photo carriers generated in the depletion region associated with the inversion  517  would be collected by the inversion layer  517 . The photo current would therefore flow from the N well  516  to the inversion layer  517 . The draw back of this operating configuration is the inversion layer  517  to gate  503  capacitance. 
   As can be appreciated by one normally skilled in the art, the impurity polarities and voltage polarities shown in  FIG. 5  can be reversed. 
     FIG. 6  shows a partial cross section of a photo diode electrically shielded by a transparent conductor  603  which can be made of indium tin oxide or polysilicon. The N+ implant  601  forms the cathode and collects the photo current to be output from the photo diode while the P substrate  600  forms the anode of the photo diode and is connected via lead  602  to ground  605 . The transparent conductor  603  is also tied to ground  605  so that any displacement current  604  from the LED source is shunted to ground  605 . Transparent interlevel oxide  606  such as, but not limited to SiO 2 , is used to insulate interconnect metal, polysilicon used for MOSFET gates, and the transparent conductor  603  which may also be polysilicon. Terminal  607  is a metal contact to the output interconnect metal of the photo diode. For the process used to make the photo diode of  FIG. 6  it is assumed that at least a second level of metal is available. The second level of metal is used to make an electrostatic shield  608  for the terminal  607  and any areas of the N+ implant  601  not covered by the transparent conductor  603  thereby shunting displacement current away from the photo output signal terminal  607 . This shielding method can also be applied to any interconnect metal connected to the photo detector&#39;s output. Thus, the use of a second or higher level of metal tied to ground in combination with a transparent conductor  603  can prevent to a very high degree any displacement current from flowing into the photo signal node including any interconnect metal going to a sense amplifier. It is noted that interconnect shielding method of  FIG. 6  can be applied to any of output nodes of the photo detectors described herein.