Method and apparatus for inspecting substrates

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.

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°.)

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.

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.