Abstract:
Apparatus for detecting defects in a substrate comprises a laser for providing a laser beam, and a bi-cell photodiode comprising two cells. Circuitry coupled to the bi-cell photodiode provides a signal equal to (L−R)/(L+R), where L and R equal the signal strengths of the signals provided by the left and right photodiode cells, respectively. The photodiode is biased so that it exhibits reduced capacitance, and can provide increased output signal bandwidth.

Description:
BACKGROUND OF THE INVENTION 
     This invention pertains to a method and apparatus for inspecting substrates used during the manufacture of magnetic disks. 
     Magnetic disks are typically manufactured by the following process: 
     1. An aluminum alloy substrate is electroless plated with NiP. 
     2. The plated substrate is polished. 
     3. The polished substrate is then textured, either mechanically or using a laser. 
     4 An underlayer (e.g. Cr or NiP), a magnetic alloy (typically a Co alloy) and a protective overcoat (typically carbon, hydrogenated carbon, or zirconia) are then sputtered, in that order, onto the substrate. 
     5. A lubricant is then applied to the protective overcoat. 
     The layers formed on magnetic disks (e.g. the underlayer, magnetic layer and overcoat) are extremely thin, e.g. on the scale of several tens of nanometers. It is very important that there be no or few large defects in the substrate prior to sputtering. 
     It is known in the art to use laser scanning systems to inspect magnetic disk substrates prior to sputtering. Examples of such systems include the PMT Pit Detector, the Diskan  6000 , Diskan  9000  and Diskan  9001  systems manufactured by QC Optics of Burlington, Mass. Other prior art systems are discussed in U.S. Pat. Nos. 4,794,264; 4,794,265; and 5,389,794, each assigned to QC Optics. 
     FIG. 1 schematically illustrates a QC Optics Diskan  9001  apparatus  10  for detecting defects in a substrate, such as a substrate  12 . Referring to FIG. 1, apparatus  10  comprises HeNe lasers  14   a ,  14   b  for generating laser beams  16   a ,  16   b  respectively. Laser beam  16   a  is used to scan across and inspect one side of substrate  12 , while laser beam  16   b  is used to scan across and inspect the other side of substrate  12 . (Substrate  12  is typically rotated by a motor during this inspection, and laser beams  16   a ,  16   b  typically scan in the radial direction of the substrate.) 
     Laser beam  16   a  passes through a polarizer  18   a , ¼ waveplate  20   a , and a shutter  22   a , reflects off a mirror  23   a , passes through a lens  24   a , a beam splitter  25   a , and a lens  26   a  and reflects off of mirror  28   a . Mirror  28   a  deflects laser beam  16   a  downward to substrate  12 . Substrate  12  reflects laser beam  16   a  upwardly and back to mirror  28   a , through lens  26   a  and back to beam splitter  25   a . Beam splitter  25   a  deflects laser beam  16   b  to a photomultiplier tube  30   a . Of importance, if laser beam  16   a  strikes a defect in substrate  12  (either a pit or a bump), that defect will reflect laser beam  16   a  at an angle. The fact that laser beam  16   a  is reflected at an angle is detected by photomultiplier tube  30   a . In this way, apparatus  10  can use laser beam  16   a  to determine whether there are pits or bumps in substrate  12 . 
     The manner in which a defect deflects a laser beam can best be understood by comparing FIGS. 2A and 2B. In FIG. 2A, laser beam  16   a  strikes a portion of substrate  12  where defect  32  deflects laser beam  16   a  at an angle θ. In contrast, in FIG. 2B, laser beam  16   b  strikes a portion of substrate  12  where there are no defects. Thus, in FIG. 2B, laser beam  16   a  reflects straight back, and not at an angle. As mentioned above, photomultiplier tube  30   a  detects whether or not laser beam  16   a  is reflected at an angle by a defect on substrate  12 . 
     Referring back to FIG. 1, portions of laser beam  16   a  are also reflected past mirror  28   a , pass through spacial filter  34   a  and lens  36   a , and strike photomultiplier tube  38   a . (Spacial filter  34   a  filters out light scattering caused by the texture pattern that is formed on substrate  12 .) Of importance, photomultiplier tube  38   a  determines whether light is scattered by defects or contamination on substrate  12  at a wide angle. 
     The optical path for laser beam  16   b  is similar to the optical path of laser beam  16   a , and will not be described in detail, except to note that it includes two mirrors  28   b ′ and  28   b ″ instead of single mirror  28   a.    
     FIG. 3 is a block diagram of the circuitry coupled to photomultiplier tubes  30   a ,  30   b ,  38   a  and  38   b . As can be seen, each of photomultiplier tubes  30   a ,  30   b ,  38   a  and  38   b  is coupled to four comparators  42   a - 42   d ,  44   a - 44   d ,  46   a - 46   d  and  48   a - 48   d , respectively. Each of comparators  42   a - 42   d  compares the output signal OS 30   a  of photomultiplier tube  30   a  with an associated reference voltage RV 42   a -RV 42   d , and provides a binary output signal BOS 42   a -BOS 42   d  in response thereto. Binary output signals BOS 42   a -BOS 42   d  are stored in associated latches  52   a - 52   d , the contents of which are loaded into a memory which can then be accessed by a central processing unit CPU (not shown). Comparators  44 - 48  similarly compare the output signals from photomultiplier tubes  30   b ,  38   a  and  38   b  to reference voltage signals RV, and generate binary output signals BOS in response thereto. These binary output signals are stored in latches  54 - 58 , the contents of which can be accessed by central processing unit CPU to determine the size and character of a defect detected by the apparatus. 
     While apparatus  10  can detect some defects, it would be desirable to provide improved means for detecting such defects with greater sensitivity and accuracy. 
     SUMMARY 
     A method for inspecting a substrate in accordance with our invention comprises the step of providing a laser beam that strikes and reflects off the substrate and then strikes a bi-cell photodetector. In one embodiment, the photodetector is a photodiode. The cells of the photodetector are coupled to circuitry that generates a signal equal to (L−R), where L is the strength of the signal provided by one cell of the photodetector, and R is the strength of the signal provided by the other cell of the photodetector. The signal L−R corresponds to the difference between the amount of light striking one cell of the photodetector and the amount of light striking the other cell, which in turn depends on the extent to which the laser beam is deflected by a defect. A signal equal to L+R is also developed. Signal L+R is used to “normalize” signal L−R. In other words, signal L+R is used to compensate for sources of common mode noise, e.g. fluctuations in the intensity of the laser, variations in substrate reflectivity, etc. From these two signals, a signal proportional or equal to (L−R)/(L+R) is developed. Signal (L−R)/(L+R) is compared to a set of threshold circuits to determine the size of the defect detected. 
     In one embodiment, the bi-cell photodetector contains two photodiodes that are biased with a bias voltage so that the photodiodes exhibit reduced capacitance. Because of this, the circuit employing the bi-cell photodetector exhibits enhanced bandwidth, thereby improving the speed at which the substrate can be inspected. 
     We have found that one embodiment of apparatus in accordance with our invention is more sensitive to defects than the apparatus of FIG.  3 . For example, the apparatus of FIG. 3 was capable of detecting defects having a wall slope of about 0.05° or greater. One embodiment of our invention can detect defects having a wall slope less than 0.02°, and in one embodiment, defects having a wall slope as low as 0.005°. (A defect wall slope of 0.005° typically represents the lower limit of presently feasible substrate manufacturing processes. If one could manufacture a flatter substrate, we believe the apparatus of the present invention could detect defects having wall slopes as low as 0.003°.) 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 schematically illustrates an optical system using a laser beam to inspect a substrate for defects constructed in accordance with the prior art. 
     FIG. 2A illustrates a laser beam striking a defect on a substrate. 
     FIG. 2B illustrates a laser beam striking a portion of a substrate that does not contain a defect. 
     FIG. 3 is a block diagram of a prior art circuit for processing a signal from a set of photomultiplier tubes within apparatus  10  of FIG.  1 . 
     FIGS. 4A to  4 D are a schematic diagram of a circuit for processing a signal from a light sensing diode in accordance with our invention. 
     FIG. 5 illustrates in plan view a bi-cell photodiode used in the circuit of FIGS. 4A to  4 C. 
     FIG. 6 illustrates a module containing some of the circuitry of FIGS. 4A to  4 D. 
    
    
     DETAILED DESCRIPTION 
     One embodiment of our invention uses most of the optical elements shown in FIG.  1 . However, instead of using photomultiplier tubes  30  and the circuitry of FIG. 3, we have developed a new structure for detecting reflected laser beams  16  and generating an output signal to determine whether a defect is present on substrate  12 . Specifically, instead of using photomultiplier  30   a , a bi-cell photosensitive diode  50  is used (FIGS.  4 A and  5 ). Bi-cell photosensitive diode  50  comprises a diode  50   a  and a diode  50   b . Diodes  50   a  and  50   b  are roughly rectangular, and are formed on a common substrate  51  adjacent to one another, as shown in FIG.  5 . In one embodiment, bi-cell photosensitive diode  50  is device model number SPOT- 2 D, manufactured by UDT of Hawthorne, Calif. In another embodiment, bi-cell photosensitive diode  50  is device number SD 113-24-21-021, manufactured by Advanced Photonics of Camarillo, Calif. However, other types of photosensitive diodes, photosensitive transistors, or other devices may also be used. 
     FIGS. 4A to  4 C schematically illustrate circuitry for processing the output signal of diodes  50   a ,  50   b . This circuitry comprises numerous components, e.g. resistors, capacitors, and various integrated circuits. The value of these components, and the part number of these integrated circuits are set forth in Table I below. 
     Referring to FIG. 4A, the cathode of diodes  50   a ,  50   b  are connected to a bias circuit  52  comprising a resistor R 16  connected to a 15 volt source, and a capacitor C 16  connected to ground. The anode of diode  52   a  is coupled to a preamplifier  58   a  for generating signal R. Similarly, the anode of diode  52   b  is coupled to a preamplifier  58   b  for generating signal L. Signals L and R are voltages representing the amount of light striking diodes  50   a  and  50   b , respectively. 
     Preamplifier  58   a  comprises an operational amplifier U 7  having an inverting input lead U 7   2  coupled to diode  50   a  and a non-inverting input lead U 7   3  connected to ground. Output lead U 7   6  is coupled to input lead U 7   2  via resistor R 15  (typically 15 kΩ). Of importance, because of the manner in which diode  50   a  is biased, it exhibits a low capacitance. (All diodes exhibit a certain amount of capacitance due to their pn junctions.) The capacitance exhibited by diode  50   a  depends upon the bias voltage applied across it. By applying a relatively large voltage across photodiode  50   a , we can ensure that the capacitance of diode  50   a  is relatively low, e.g. below 13 pF. (For example, in one embodiment, the capacitance of diode  50   a  is between 0.1 and 13 pF. For example, the capacitance can be between 3 and 13 pF.) The capacitance of diode  50   a  and resistor R 15  form an RC filter. By ensuring that the capacitance of diode  50   a  is low, the time constant of this RC filter will be low, enabling preamplifier  58  to provide a signal having a bandwidth of at least 100 kHz. (The bandwidth typically exceeds 200 kHz, and can be between 500 kHz and 100 MHz. For example, in one embodiment, the bandwidth is about 10 MHz.) This bandwidth increases the speed at which the apparatus can inspect a substrate for defects. 
     Signal L is provided at input leads  60  and  61 , and signal R is provided at input leads  62  and  63  of the circuitry of FIG.  4 B. As explained below, this circuitry provides an output signal OS, indicative of defects on substrate  12 . 
     Of importance, if there is no defect present on substrate  12 , the same amount of light should strike diodes  52   a  and  52   b , and signals L and R should be equal. If there is a defect present on substrate  12 , laser beam  16   a  will be deflected in one direction or another (left or right), and one of signals L, R will be greater than the other signal R, L. As explained below, the circuitry of FIG. 4B provides a signal that is a measure of the difference between signals L and R. This signal is related to the extent to which a defect in substrate  12  deflects light to the left or right when it bounces off the defect. 
     Of importance, the amount of light provided by laser  14  (FIG. 1) can vary, thereby injecting noise into signals L and R. Such noise tends to obscure the ability to detect and measure defects in substrate  12 . Also, different substrates can exhibit different amounts of reflectivity. This reflectivity variation can also obscure the ability to detect and measure defects in substrate  12 . Accordingly, the circuit of FIG. 4B includes a sum amplifier  64  that generates a normalizing signal L+R at a lead  66  of a drive circuit  67 . Drive circuit  67  amplifies signal L+R and provides the amplified normalizing L+R signal to an integrated circuit U 2 . (Drive circuit  66  has drive characteristics that match the requirements of integrated circuit U 2 .) 
     Sum circuit  64  comprises a set of switches  65 . Switches  65  permit one to adjust a filter time constant exhibited by sum circuit [ 65 ]  64 . This permits one to either detect or ignore stain regions of varying reflectivity on substrate  12 , depending upon the setting of switches  65 . 
     Sum circuit [ 66 ]  64  also includes an amplifier U 1 D for providing an output signal on a lead [ 68 ]  69 . Buffer U 1 D provides another signal indicative of the magnitude of L+R. This signal can be used to determine when the laser beam strikes the end of substrate  12  as the laser scans across the surface. 
     Circuit U 2  receives the amplified normalizing signal L+R and signals L and R. Circuit U 2  provides a signal equal to (L−R)/(L+R) on a lead  68 . Signal (L−R)/(L+R) is a measure of the extent to which a defect deflects light to the left or to the right, corrected for any change in the total strength of signals L and R caused by laser power fluctuation or changes in disk surface reflectivity. 
     Integrated circuit U 2  also receives voltage signals ER, Y 1  and Y 2  from an amplifier circuit  70 . Signals Y 1  and Y 2  permit adjustment of an amplification constant used by integrated circuit U 2 . (This amplification is proportional to signals Y 1 −Y 2 .) Of importance, if the gain is too high, it can cause instability in circuit U 2 . 
     Signal (L−R)/(L+R) is provided to an amplifier U 3 , which provides an output signal OS at an output lead  72 . Output signal OS is coupled to a set of comparitors  70   a ,  70   b ,  70   c  and  70   d , which compare signal OS to reference voltages RVa, RVb, RVc and RVd, respectively (FIG.  4 C). If laser beam  16  is not deflected by a defect on substrate  12 , signal OS will be less than any of voltages RVa to RVd. If laser beam  16  is slightly deflected by a defect, signal OS will exceed reference voltage Rva, and comparitor  70   a  will provide an active binary output signal at an output lead OL 70   a , while concurrently, the output of comparitors  70   b - 70   d  will be inactive. If laser beam  16  is deflected to a greater extent, signal OS will exceed reference voltage RVb, causing the binary output signal of comparitor  70   b  to go active. Comparitors  70   c  and  70   d  function in a similar manner. Thus, comparitors  70   a  to  70   d  provide a measure of the extent to which laser beam  16  is deflected by defects on substrate  12 . (This, in turn, is a measure of the steepness of the defect walls, which is important because the steepness of the walls is a measure of the size of the defect.) The binary output signals on leads  0 , 70   a  to OL 70   d  are coupled to latches which can be processed by circuitry similar to that used to process signals BOS 421 -BOS 48   d , described above. 
     FIG. 4D illustrates power supply circuitry  100  used by the circuitry of FIGS. 4A and 4B. Circuitry  100  receives input voltages of 15V and −15V, and generates therefrom output voltages of 5 volts, ground and −5 volts. Circuitry for providing such output voltages are known to those skilled in the art, and thus this circuitry will not be described in further detail. 
     The bi-cell photodiode  50  and associated circuitry of FIGS. 4A to  4 D can be used to replace photomultiplier tubes  14   a ,  14   b . However, in one embodiment, photomultiplier tubes  38   a ,  38   b  are used to detect wide angle scattering of light as discussed above. 
     FIG. 6 is a cross section view of a module  200  containing a printed circuit board  201  that carries bi-cell photodiode  50  and a portion of the circuitry of FIGS. 4A to  4 D. Module  200  is mounted on a block  202  coupled to a holder  203 . Module  200  includes a first mechanism  204  for making fine position adjustments of bi-cell photodiode  50  in the direction of arrow  206 . Such adjustments are controlled by turning a first control screw  208 . Mounted on first mechanism  204  is a second mechanism  210  for making fine position adjustments of bi-cell photodiode  50  in a direction perpendicular to arrow  206 . These adjustments are controlled by turning a second control screw  214 . (Control screw  214  is perpendicular to control screw  208 .) A bock  212  is affixed to second mechanism  210 . PC board  201  is mounted within block  212 . 
     Bi-cell photodiode  50  is located in a central portion of PC board  201 . Block  212  contains a window  218  for permitting laser  16   a  to strike photodiode  50 . (As mentioned above, laser  16   a  is reflected off of the substrate being tested for defects.) Block  212  includes a first connector  220  for receiving electrical power via a wire  222  and a second  8  connector  224  for providing signal L+R. A second connector within block  212  (not shown) provides signal L−R/L+R. These signals are processed by circuitry outside of block  212  in the manner discussed above. 
     After a substrate is inspected with the apparatus and method of the present invention, the substrate is typically used to manufacture a magnetic disk. During this process, an underlayer, a magnetic layer, and a protective overcoat are deposited, e.g. by sputtering or evaporation, onto the substrate. A lubricant layer is then applied to the overcoat. An example of a process for completing the manufacture of a magnetic disk after substrate inspection is set forth in U.S. patent application Ser. No. 08/984,753, filed by Bertero, et al., assigned to the assignee of the present invention and incorporated herein by reference. 
     While the invention has been described with respect to a specific embodiment, those skilled in the art will appreciate that changes can be made in form and detail without departing from the spirit and scope of the invention. For example, different types of lasers (e.g. diode lasers or gas lasers) can be used to inspect a substrate in accordance with my invention. Further, this structure can be used to test different kinds of substrates, e.g. glass or glass ceramic substrates. Such substrates can be used to manufacture magnetic disks or other devices. Accordingly, all such changes come within the present invention. 
     
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE I 
               
               
                   
                   
               
             
             
               
                   
                 Component 
                 Value 
               
               
                   
                   
               
               
                   
                 R1 
                 3MΩ 
               
               
                   
                 R2 
                 200Ω 
               
               
                   
                 R3 
                  10Ω 
               
               
                   
                 R4 
                  82KΩ 
               
               
                   
                 R5 
                  10Ω 
               
               
                   
                 R6 
                  10KΩ 
               
               
                   
                 R7 
                  10KΩ 
               
               
                   
                 R8 
                 100KΩ variable resistor 
               
               
                   
                 R9 
                  10Ω 
               
               
                   
                 R11 
                  56KΩ 
               
               
                   
                 R12 
                  10KΩ 
               
               
                   
                 R13 
                  10KΩ 
               
               
                   
                 R14 
                  10KΩ 
               
               
                   
                 R15 
                  15KΩ 
               
               
                   
                 R16 
                  10KΩ 
               
               
                   
                 R17 
                  15KΩ 
               
               
                   
                 R18 
                  50Ω 
               
               
                   
                 R16 
                  10KΩ 
               
               
                   
                 C1 
                 0.1 μF 
               
               
                   
                 C2 
                 0.1 μF 
               
               
                   
                 C3 
                 0.1 μF 
               
               
                   
                 C4 
                 4700 pF 
               
               
                   
                 C5 
                 0.1 μF 
               
               
                   
                 C6 
                 1500 pF 
               
               
                   
                 C7 
                 0.1 μF 
               
               
                   
                 C8 
                 4.7 μF 
               
               
                   
                 C9 
                 0.1 μF 
               
               
                   
                 C10 
                 0.1 μF 
               
               
                   
                 C11 
                 4.7 μF 
               
               
                   
                 C12 
                 0.1 μF 
               
               
                   
                 C13 
                   1 pF 
               
               
                   
                 C14 
                 0.1 μF 
               
               
                   
                 C15 
                 0.1 μF 
               
               
                   
                 C16 
                 0.1 μF 
               
               
                   
                 C17 
                   1 pF 
               
               
                   
                 C18 
                 0.1 μF 
               
               
                   
                 C19 
                 0.1 μF 
               
               
                   
                   
               
             
          
           
               
                   
                 Integrated Circuit 
                 Model 
                 Manufacturer 
               
               
                   
                   
               
               
                   
                 U1A, U1B, U1C 
                 AD713R-16 
                 Analog Devices 
               
               
                   
                 U2 
                 AD734 
                 Analog Devices 
               
               
                   
                 U3 
                 LM6321M 
                 National Semiconductor 
               
               
                   
                 U5 
                 LM78L05ACZ 
                 National Semiconductor 
               
               
                   
                 U6 
                 LM79L05ACZ 
                 National Semiconductor 
               
               
                   
                 U7 
                 OPA655 
                 Burr Brown 
               
               
                   
                 U9 
                 OPA655 
                 Burr Brown