Abstract:
This invention relates to a method and apparatus for simulating a surface photo-voltage, more particularly with a photodiode on a process sized disk for calibrating surface photo-voltage measurement devices. The device for simulating a surface photo-voltage includes the photodiode, the disk, a resistor, and may further include an operational amplifier. The apparatus for simulating a surface photo-voltage of the current invention facilitates calibration of surface photo-voltage measurement devices by using a process sized disk to fit directly on a surface photo-voltage measurement chuck.

Description:
RELATED APPLICATIONS 
     This application claims priority to provisional application U.S. Ser. No. 60/115,852 filed on Jan. 13, 1999. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to a method and apparatus for diagnosing and monitoring a semiconductor based device, more particularly a surface photo-voltage in a substrate. 
     BACKGROUND OF THE INVENTION 
     Surface photo-voltage principles are an important tool used for characterizing semiconductor materials. In particular, devices applying surface photo-voltage principles are becoming one of the main technologies for non-contact diagnostics and monitoring procedures used in many manufacturing processes for silicon based devices. 
     A surface photo-voltage measurement method is disclosed in U.S. Pat. No. 4,544,887. In this method, a beam of light is directed at a region of a surface of a specimen of semiconductor material and a photo-induced change in electrical potential at the surface is measured. The wavelength of the illuminating light beam is selected to be shorter than the wavelength of light corresponding to the energy gap of the semiconductor material undergoing testing. The intensity of the light beam is modulated, with both the amplitude of the light and the frequency of modulation being selected such that the resulting AC component of the induced photo-voltage is directly proportional to the intensity of light and inversely proportional to the frequency of modulation. 
     A surface photo-voltage measuring system can be used for non-contact diagnostics and monitoring of silicon based devices. The surface photo-voltage measurement system includes a surface photo-voltage measurement probe located above a measurement chuck. A wafer to be evaluated with the surface photo-voltage measurement system is placed on the measurement chuck. 
     A critical element in the use of the surface photo-voltage measurement system is to ensure that the surface photo-voltage measurement system is properly measuring the surface photo-voltage of the wafer. In the past, reference wafers have been used to calibrate photo-voltage measurement systems. However, reference wafers do not provide consistent surface photo-voltage measurement due to the sensitivity of the reference wafers to ambient conditions. There is a need in the art for a device that can consistently calibrate surface photo-voltage measurement systems. 
     SUMMARY OF THE INVENTION 
     The present invention relates to an apparatus for simulating a surface photo-voltage in a semiconductor wafer. In one embodiment the apparatus includes a photodiode, a resistor, and a disk. The photodiode has an anode and a cathode. The resistor has a first terminal and a second terminal. The disk has a first surface and a second surface. The resistor first terminal is in electrical communication with either the photodiode anode or the photodiode cathode. The resistor second terminal is in electrical communication with the first surface of the disk and the photodiode anode if the photodiode cathode is connected to the resistor first terminal, or the photodiode cathode if the photodiode anode is connected to the resistor first terminal. 
     The disk is made from a conductive or semiconductive material. The disk can also be process sized to duplicate the normal size and weight of industry standard semiconductor wafers to evaluate a surface photo-voltage measurement system used in production. 
     In one embodiment of the present invention, the photodiode is a planar photodiode. In another embodiment of the present invention, the photodiode is a photodiode array including a plurality of individual photodiode segments. In yet another embodiment of the present invention, the individual photodiode segments are in electrical communication with one or more electrical resistors. 
     In another embodiment of the present invention, the apparatus further includes an operational amplifier having an first input terminal, a second input terminal, and an output terminal. The first input terminal is in electrical communication with either the photodiode anode or cathode, the second input terminal is in electrical communication with the other of the photodiode anode and cathode, and the output terminal is in electrical communication with the resistor second terminal. 
     The present invention also relates to an apparatus for measuring a simulated photo-voltage in a semiconductor wafer. In one embodiment the apparatus includes a photodiode, a resistor and a disk used for simulating a surface photo-voltage and further adds a measurement chuck comprising a first surface, a surface photo-voltage measurement probe, and a light source for measuring the simulated surface photo-voltage. In another embodiment the light source generates a light having amplitude modulation. 
     Another embodiment of the present invention further adds an optional optical window including an electrically conductive transparent coating. The optical window is positioned adjacent to the photodiode and between the photodiode and the light source. 
     The present invention also relates to a method for simulating a surface photo-voltage. In one embodiment the method includes providing an apparatus for generating a simulated surface photo-voltage including a simulator disk, generating a light from a light source, illuminating the simulator disk, and generating a signal in the simulator disk. In another embodiment, the method further includes providing a measurement chuck and a surface photo-voltage measurement probe, and measuring the signal with the surface photo-voltage measurement probe. In another embodiment, the method further includes providing an operational amplifier. 
     The present invention also relates to an apparatus for simulating a surface photo-voltage including a photodiode, a disk, and a resistor. The photodiode has a anode metalization and a cathode metalization. The disk has a trench. The resistor has a first and second terminal wherein the resistor first terminal is attached to the photodiode cathode metalization with a first volume of conductive epoxy and the second resistor terminal is attached to the photodiode anode metalization with a second volume of conductive epoxy. 
     The present invention also relates to an apparatus for simulating a surface photo-voltage including a photodiode, a resistor, an operational amplifier, and a disk. The photodiode has an anode and a cathode. The resistor has a first terminal and a second terminal. The operational amplifier has a first input terminal, a second input terminal, and an output terminal. The disk has a first surface. The resistor first terminal and operational amplifier first input terminal are in electrical communication with either the photodiode anode or cathode. The operational amplifier second input terminal is in electrical communication with the other of the photodiode anode or cathode, and the operation amplifier output terminal is in electrical communication with the resistor second terminal. The photodiode is mounted to the disk first surface but the photodiode is not in electrical communication with the disk. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     This invention is pointed out with particularity in the appended claims. The above and further advantages of this invention may be better understood by referring to the following description, taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a perspective view of one embodiment of a surface photo-voltage measurement system. 
     FIG. 2 is an equivalent circuit diagram for an embodiment of a surface photo-voltage detection system. 
     FIG. 3 is an equivalent circuit diagram for an embodiment of a surface photo-voltage detection system. 
     FIG. 4 is a schematic diagram of an embodiment of the simulation device. 
     FIG. 5 is a schematic diagram of another embodiment of the simulation device. 
     FIG. 6 is a schematic diagram of another embodiment of the simulation device. 
     FIG. 7 is a cross section view of an embodiment of the simulation device illustrating a photodiode, a resistor, and a disk. 
     FIG. 8 is a perspective view of an embodiment of the apparatus for measuring a simulated surface photo-voltage using a simulation device of the present invention. 
    
    
     DESCRIPTION 
     FIG. 1 is a perspective view of one embodiment of a surface photo-voltage measurement system  2  for non-contact diagnostics and monitoring of semiconductor devices. The surface photo-voltage measurement device  2  includes a surface photo-voltage measurement probe  4  located above a measurement chuck  6 . A wafer  8 , to be evaluated with the surface photo-voltage measurement system  2 , is placed on the measurement chuck  6 . The wafer  8  may be transported to and from the measurement chuck  6  by any mechanical or robotic method or apparatus known by those skilled in the art. 
     FIG. 2 illustrates an equivalent circuit  30  for an embodiment of a basic surface photo-voltage detection system. A light  32  from a light source  34  illuminates a photodiode  36 . In practice there will be leakage from the photodiode  36 . The leakage resistance can be represented in circuit  30  by a resistor  38  connected in parallel with a junction capacitor  40 . As the light  32  is illuminated on the photodiode  36  a voltage  42  is generated across the circuit  30 . In equivalent circuit  30  the entire photodiode  36  is illuminated. 
     FIG.  3 . illustrates an embodiment of an equivalent circuit  50  for an embodiment of a basic surface photo-voltage detection system where only a portion of the area of a photodiode  51  is illuminated by light  52 . Equivalent circuit  50  includes an illuminated photodiode area  60  and non-illuminated photodiode area  66 . The illuminated photodiode area  60  includes an equivalent capacitor  56  and an equivalent resistor  58 . The non-illuminated photodiode area  66  includes an equivalent capacitor  62  and equivalent resistor  64 . Equivalent circuit  50  also represents a surface photo-voltage measurement system  68  with an input impedance  70  and an impedance load  72  across the measurement system  68 . In addition, there is a voltage  74  across the measurement system. 
     FIG. 4 is a schematic diagram of an embodiment of the simulation device for simulating a surface photo-voltage in a wafer operating in a photo-voltaic mode. A simulation device  100  includes a photodiode  102 , a resistor  104  and a disk  106 . The photodiode  102  includes an anode  108  and a cathode  110 . The resistor  104  includes a resistor first terminal  112  and a resistor second terminal  114 . The resistor first terminal  112  is in electrical communication with the photodiode anode  108 . In an alternative embodiment the photodiode  102  can be inverted and the resistor first terminal  112  can be in electrical communication with the photodiode cathode  110 . The disk  106  includes a first surface  116  and a second surface  118 . The disk first surface  116  is in electrical communication with the resistor second terminal  114  and the photodiode cathode  110 . Light  122  from light source  120  illuminates the photodiode  102  to generate a surface photo-voltage. The disk second surface  118  of simulation device  100  is positioned on a measurement chuck (not shown in figure) of the surface photo-voltage measurement device  2  to generate a surface photo-voltage signal to calibrate the surface photo-voltage measurement device  2 . 
     Optionally, an optical window  126  can be placed between the light source  120  and the photodiode  102 . The optical window  126  includes a polished quartz substrate  128 , with an electrically conductive, optically transparent coating  130 . The electrically conductive optically transparent coating  130  can be Indium-Tin-Oxide. The optical window  126  can be used to filter the light  122  to the photodiode  102 . The electrically conductive optically transparent coating  130  is in electrical communication with the resistor first terminal  112 . 
     FIG. 5 is a schematic diagram of an embodiment of the simulation device for simulating a surface photo-voltage in a wafer operating in a photoconductive mode. A simulation device  150  includes a photodiode  102 ′, a resistor  104 ′, a disk  106 ′, and an operational amplifier  157 . The photodiode  102 ′ includes an anode  108 ′ and a cathode  110 ′. The resistor  104 ′ includes a resistor first terminal  112 ′ and a resistor second terminal  114 ′. The resistor first terminal  112 ′ is in electrical communication with the photodiode anode  108 ′. In an alternative embodiment, the photodiode  102 ′ can be inverted and the resistor first terminal  112 ′ can be in electrical communication with the photodiode anode  110 ′. The disk  106 ′ includes a first surface  116 ′ and a second surface  118 ′. The disk first surface  116 ′ is in electrical communication with the photodiode cathode  110 ′. The operational amplifier  157  includes an operational amplifier first input terminal  182 , an operational amplifier second input terminal  184 , and an operational amplifier output terminal  170 . The operational amplifier first input terminal  182  is in electrical communication with the photodiode anode  108 ′. The operational amplifier second input terminal  184  is in electrical communication with the the photodiode cathode  110 ′. The operational amplifier output terminal  170  is in electrical communication with the resistor second terminal  114 ′. Light  122 ′ from light source  120 ′ is illuminated on the photodiode  102 ′ to generate a surface photo-voltage. The disk second surface  118 ′ of simulation device  150  would be placed on the measurement chuck  6  of the surface photo-voltage measurement device  2  shown in FIG. 1 to calibrate the surface photo-voltage measurement device  2 . 
     Again optionally, an optical window  126 ′ can be placed between the light source  120 ′ and the photodiode  102 ′. The optical window  126 ′ includes a polished quartz substrate  128 ′ and an electrically conductive, optically transparent, coating  130 ′. For example, Indium-Tin-Oxide can be the electrically conductive optically transparent coating  130 ′. The optical window  126 ′ can be used to filter the light  122 ′ to the photodiode  102 ′. The electrically conductive, optically transparent coating  130 ′ is in electrical communication with the resistor second terminal  114 ′. 
     FIG. 6 is a schematic diagram of another embodiment of the simulation device for simulating surface photo-voltage in a wafer operating in a photoconductive mode. A simulation device  200  includes a photodiode  102 ′, a resistor  104 ′, a disk  106 ′, and an operational amplifier  157 ′. The photodiode  102 ′ includes an anode  108 ′ and a cathode  110 ′. The resistor  104 ′ includes a resistor first terminal  112 ′ and a resistor second terminal  114 ′. The resistor first terminal  112 ′ is in electrical communication with the photodiode anode  108 ′. In an alternative embodiment the photodiode  102 ′ can be inverted and the resistor first terminal  112 ′ can be in electrical communication with the photodiode cathode  110 ′. The operational amplifier  157 ′ includes an operational amplifier first input terminal  182 ′, an operational amplifier second input terminal  134 ′, and an operational amplifier output terminal  170 ′. The operational amplifier first input terminal  182 ′ is in electrical communication with the photodiode anode  108 ′. The operational amplifier second input terminal  134 ′ is in electrical communication with the photodiode cathode  110 ′. The operational amplifier output terminal  170 ′ is in electrical communication with the resistor second terminal  114 ′. Light  122 ′ from light source  120 ′ is illuminated on the photodiode  102 ′ to generate a surface photo-voltage. The disk  106 ′ includes a first surface  116 ′ and a second surface  118 ′. The disk first surface  116 ′ is used to ground the output of the operational amplifier output terminal  170 ′. A measurement chuck  238  includes a first surface  240  and a second surface  242 . An equivalent capacitor  235  is formed by the disk second surface  118 ′ and the measurement chuck first surface  240 . The measurement chuck second surface  242  is attached to a measurement chuck ground  244  that grounds the operating amplifier output terminal  170 ′ through the equivalent capacitor  235 . The grounding of the operating amplifier output terminal  170 ′ results in a voltage drop across the resistor  104 ′ and drives the input side of the circuit of FIG. 6 into common-mode. Thus, the surface photo-voltage appears as a common-mode signal on both sides of the photodiode with respect to the measurement chuck ground  244 . 
     FIG. 7 is a cross-sectional view of an embodiment of the apparatus  300  for simulating a surface photo-voltage in a semiconductor wafer illustrating a photodiode  302 , a resistor  304 , and a disk  306 . The resistor  304  includes a first terminal  320  and a second terminal  322 . In one embodiment the photodiode  302  is a planar silicon PN type VTS2080 from E.G. &amp; G. Vactec, St. Louis, Mo. 63132, in raw die form. The photodiode is attached to the disk  306  with an optical epoxy  308  which in one embodiment is an EPO-TEK 377 from Epoxy Technology Inc., Billerica, Mass. 01821. A small trench  310  is formed in the disk  306  to define an area to place the resistor first terminal  320 . The mechanical action of forming the trench  310  improves the quality of the electrical contact with the disk  306 . A conductive epoxy  312  is used for electrical connections in one embodiment. In one embodiment Epoxy Technologies, Inc. conductive silver epoxy type EPO-TEK H31D is used. The conductive epoxy  312  in the trench  310  also contacts a cathode metallization layer  314  of the photodiode  302 , and the resistor first terminal  320  completing the lower node of the electrical circuit. A separate application of silver epoxy  318  is also used to attach the resistor second terminal  322  to an anode metallization  316  on the top surface of the photodiode  302 . In one embodiment the resistor  302  is a standard surface mount 50 k ohm 0805 precision metal film such as ERA-3YE. 
     FIG. 8 is a perspective view of an embodiment of the apparatus  350  for measuring a simulated surface photo-voltage. The apparatus  350  includes a surface photo-voltage measurement probe  4 ′, and a segmented photodiode  354 . The segmented photodiode  354  is divided into a number of identical smaller segments  356 . The surface photo-voltage measurement probe  348  illuminates only a small portion  362  of the total area of the segmented photodiode  354  as indicated in FIG.  8 . The benefit of the segmented photodiode  354  is the surface photo-voltage signal generated by these illuminated segments  362  is not loaded down by the junction capacitance or leakage resistance of the non-illuminated segments  356 , owing to the fact that each individual photodiode segment is electrically isolated from all of the other segments. All of the segments expected to be illuminated are electrically shunted by external resistors  364  to tailor their simulated surface photo-voltage signals to the measurement system being calibrated. 
     Variations, modifications, and other implementations of what is described herein will occur to those of ordinary skill in the art without departing from the spirit and the scope of the invention as claimed. Accordingly, the invention is to be defined not by the preceding illustrative description, but instead by the following claims.