Abstract:
High-density multilayered integrated circuits are fabricated by first generating all photomasks for the respective layers concurrently during the same exposure operation. Since ambient conditions are identical during generation of these related masks, mask-to-mask misregistration is virtually eliminated. While photosensitive substrates are supported on a rotating turntable, a laser beam is incremented radially toward the turntable axis and modulated to expose selected portions of each of a series of contiguous concentric bands on each substrate sequentially. Thus, in generating a four-layered circuit, the four different photomasks for the respective layers would be generated by selective exposure of each mask in sequence within the same concentric band; then, after the beam is incremented radially, each mask would again be selectively exposed in sequence within a contiguous concentric band, etc. Alternatively, the laser beam is replaced by an electron beam; and a deflection yoke electrostatically deflects the electron beam to provide a series of chord-like contiguous scans as each substrate passes through the beam path. This provides rectilinear patterns on the masks without need for the programming necessary with a laser beam to convert X-Y coordinate data into polar coordinate data. The turntable is dynamically balanced in a unique manner; and clocking is precision monitored to enable precise registration of the masks capable, when photoreduced, to provide patterns with image elements as fine as five microinches.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to methods and apparatus for producing patterns on photosensitive substrates, and relates more particularly to methods and apparatus for generating the different photomasking patterns required for all respective layers of a particular high-density multilayered integrated circuit concurrently under identical environmental conditions to minimize misregistration error. 
     Mask pattern generating machines currently commerically available employ X-Y stepping of a substrate requiring over thirty hours to complete the exposure of a single mask pattern; and only one pattern can be made at a time. As circuit density increases, this time is expected to increase to over 120 hours per mask, further increasing the likelihood of misregistration errors that can result from changes in ambient conditions occurring from the time the mask for the first layer is started until that for the last layer is completed. 
     The most pertinent known prior art is U.S. Pat. No. 3,622,742. This patent shows and describes an apparatus in which a modulated laser beam machines, by vaporization, thin films on a plurality of substrates mounted about the periphery of a rotating drum. The beam passes through a lens that is oscillated to change the position of the beam focus to compensate for change in depth of field as the flat-coated surface of the substrate rotates past the beam. The beam is also stepped in an axial direction to impinge successively on different parts of the substrate during successive revolutions of the drum. Code plates are fixed to the periphery of the drum. Each plate has a photosensed timing track and is precisely located by index pins relative to its respective substrate. Clocking pulses for each substrate are thus discontinuous; i.e., they are generated only while a laser beam is passing through slots in a code plate, and hence cannot provide the clock continuity necessary for a high-frequency clocking system required for high-density, fine-resolution circuits. 
     This cited patent suggests, however, that if desired, the substrates may be mounted on the surface of a disk, rather than the periphery of a drum, and notes that coordinate transformation (such as employed in the present invention) would then be necessary to produce rectilinear patterns. The patent further states that: &#34;Normally all of the circuits to be machined are identical, and if this be the case, the modulation of the beam is identical through each successive substrate that intercepts the beam during one drum rotation.&#34; Even if this is construed to imply that the substrates may differ, there clearly is no teaching or even remote suggestion that the substrates would be so related as to fabricate in the same exposing operation all of the photomasks necessary for making a particular multilayered circuit. Moreover, no prior art is known which suggests making all such related masks concurrently in the same exposing operation. Nor is there any teaching of a continuous clock arrangement to insure, with a permissible limited degree of self-regulation, precision registration of patterns on the respective masks. 
     SUMMARY OF THE INVENTION 
     The principal object of this invention is to provide an improved method and apparatus for fabricating photomasks for high-density, multilayered integrated circuits in a minimum amount of time and with minimal likelihood of misregistration error. 
     A plurality of substantially identical, flat substrates having a photosensitive coating are supported on a flat turntable at precise locations uniformly spaced apart both radially and circumferentially. The turntable is rotated at a substantially constant preselected angular velocity. A laser beam or electron beam is incremented radially inward toward the turntable axis once per revolution of the turntable to scan contiguous narrow bands on each substrate sequentially. 
     A magnetic disk file, tape or other source of digital information provides a series of electrical pulses or bits, each representing successive spots at which exposure of the photosensitive coating is desired according to the particular pattern proposed on each respective layer of the integrated circuit. The light beam is modulated according to said series of bits to cause selective exposure of portions of the incremental bands of the photosensitive coating on each respective substrate sequentially to provide an image on each substrate corresponding to its unique layer pattern, so that at the conclusion of the series of contiguous scans, the masking patterns for all requisite layers will have been completely imaged in parallel under identical environmental conditions. 
     Another object is to provide a continuous clocking arrangement embodying means for correcting the beam modulation rate, and hence &#34;printing&#34; rate, should slight variations in turntable velocity occur between consecutive rotations. According to this feature, two concentric magnetic clocking tracks are provided. One track provides a base clock (e.g., 5000 bits per inch) which at the preselected velocity generates a string of pulses that are increased tenfold by an oscillator in a phase lock loop circuit to provide continuous high frequency system clocking pulses (e.g., at a rate of 32MHz). The other track provides one index pulse precisely the instant each revolution of the turntable is completed. The phase lock loop circuit and another oscillator that controls turntable velocity cooperate mechanically and electronically to maintain synchronization between the base magnetic clock pulses, the high frequency system clocking pulses and the index pulse. The phase lock loop corrects beam modulation rate if the turntable velocity varies slightly from one revolution to the next. Circuitry hereinafter described initiates a shutdown signal unless a precise preselected number of clocking pulses is generated between each successive index pulse; failure to generate said preselected number of clocking pulses indicates that there is a timing error, as a result of which image spots generated in the scan just completed failed to register with the predetermined precision relative to those in the preceding scan. 
     Still another object is to provide an improved and simplified means for accurately and quickly achieving dynamic balance of the turntable, which is critical to maintain interlayer registration within very close tolerance (e.g., five microinches). 
     These and other objects, features and advantages will become apparent from the following more particular description of preferred embodiments thereof as illustrated in the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a photomask generating apparatus embodying the invention; 
     FIG. 2 is a schematic view, including certain control circuitry, of the apparatus shown in FIG. 1; 
     FIG. 3 is a perspective view of a lens holder assembly shown in FIGS. 1 and 2; 
     FIG. 4 is a plan view of a dynamic balancing mechanism forming part of the apparatus embodying the invention. 
     FIG. 5 is a perspective view of a photomask generating apparatus constructed according to a variation of the invention; and 
     FIG. 6 is a schematic representation of the manner in which the electron beam used in the apparatus of FIG. 5 is electrostatically deflected. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     As illustrated in FIG. 1, the apparatus embodying the invention comprises a heavy granite base 10 supported on an essentially vibration-free control table 11. Table 11 supports a hysteresis synchronous motor 12 that by a suitable damped coupling, such as a magnetic coupling 13, rotates a spindle 14 (FIG. 2) that supports and drives a coaxially connected turntable 15. Turntable 15 is preferably heavy enough, such as 150 lbs., to provide a flywheel effect to minimize changes in rotational velocity from a nominal value, such as 600 revolutions per minute. 
     A plurality (e.g., eight) substrate holders 16 are precisely affixed to turntable 15 equidistant circumferentially from each other and at equal radial distances from the rotational axis. Each holder registers in a precise position a substrate 17 overcoated with a photosensitive coating of essentially uniform thickness. The coating may be directly on a substrate of transparent glass or on a chrome layer that is coated over transparent glass. 
     Coaxially supported just above turntable 15, inboard of the substrate holders 16, is a clocking disk 18 having two concentric clocking tracks 19, 20. Outer track 19 is a clocking track having permanently encoded thereon a series of uniformly spaced magnetized bits at a preselected density (e.g., 5000 bits per inch); these bits are sensed by a magnetic head 21. Inner track 20 is an index track having a single bit encoded thereon that is sensed by a magnetic head 22, so as to provide a signal only once every revolution of turntable 15. These magnetic heads 21, 22 are carried by a stationary support block 23 in closely spaced relation above the respective tracks 19, 20 on clocking disk 18. Disk 18 is secured so as to rotate in unison with turntable 15. 
     A suitable source, such as a laser 25, directs a beam of collimated, monochromatic, coherent light to a mirror 26 which, as illustrated, reflects it into a beam compressor 27, thence through a beam modulator 28 and a beam expander 29 to a mirror 30, and thence into a lens holder assembly 31. 
     As best shown in FIG. 3, the beam is reflected from mirror 30 to a mirror 32 and then vertically downward through an objective lens 33 via an aperture 34 that is smaller than the beam diameter to allow for alignment and spot energy distribution losses about the periphery of the beam for providing a very fine resolution spot. Lens holder assmebly 31 carries mirror 32 and lens 33. A precision beam 38 anchored to granite base 10 provides horizontal and vertical guide surfaces 38a, 38b that extend in a direction parallel to an imaginary radial line extending from the axis of turntable 15. 
     Laterally affixed to the side of assembly 31 is a bracket 39 having a central vertical leg 40 at the ends of which are two integrally formed legs 41, 42 that extend horizontally in opposite directions. As assembly 31 is stepped radially inward, a capacitance probe 43 depending from upper leg 41 senses vertical alignment of the laser beam dynamically, and a capacitance probe 44 projecting horizontally from central leg 40 senses radial alignment of the beam. A capacitance probe 45 depending from lower horizontal leg 42 senses the vertical position of the substrate 17 and holder 16 prior to pattern generation. 
     Affixed to surface X of holder assembly 31 is a piezoelectric block 48. An angle member 46 is affixed to the lower surface of block 48, and piezoelectric block 47 is affixed to the vertical surface Y of angle 46. Objective lens 33 and aperture 34 are supported by a retaining member 35 which is affixed to vertical surface Z of block 47. The foregoing arrangement enables precise vertical and radial alignment of the exposure beam. Each probe 43, 44 is set for a preselected gap (e.g., 0.022 inch). Piezoelectric crystal block 47 will expand and contract horizontally under control of probe 44 as necessary to keep lens 33 and aperture 34 laterally aligned on a true radius of turntable 15. Crystal block 48 will expand and contract vertically under control of probe 43 to maintain lens 33 and aperture 34 at a constant preselected height above the substrate 17 to maintain spot size constant. 
     As shown schematically in FIG. 2, the photomask generator apparatus embodying the invention comprises a crystal controlled oscillator 50 for providing pulses that are amplified by an amplifier 51 for controlling the hysteresis synchronous motor 12. It further comprises an input device, such as a reel 52 of magnetic tape, having magnetically encoded bit patterns thereon that correspond to the different photomask patterns desired on the respective masks during each successive incremental scan by the laser beam. Said bit patterns operatively control the modulator 28 to interrupt the light beam from laser 25 intermittently. This provides a series of fine resolution light pulses that are so timed, in relation to the rotation of turntable 15, as to cause light to selectively expose the photomask coating at desired image areas on the various substrates. 
     More specifically, the magnetic indicia on reel 52 is suitably processed in a conventional manner, as by a minicomputer 53. The data is gated out from computer 53 under control of suitable computer clock signals. The data preferably is in compressed form, requiring decompression by decompression logic circuitry 54 before transmission to control logic circuitry 55. 
     Circuitry 55 controls operation of modulator 28; it also controls incremental movement of lens holder assembly 31 in a radial direction relative to the axis of turntable 15, to thereby cause the modulated laser beam to scan successive contiguous incremental concentric bands of the photosensitive coated substrates 17 as each is rotated under the modulated beam. This incremental stepping of lens holder assembly 31 is controlled by a closed loop servo laser interferometer system (not shown) of the two-frequency beam type commercially available from Hewlett-Packard. Since the data is in the form of bits representing X-Y coordinate positions at which a spot is to be exposed on each substrate, it must be converted into polar coordinate (R,θ) data by suitable programming, in a manner familiar to those skilled in the programming art. The raster width of the laser beam scan is about an order of magnitude smaller than the thinnest spot or line of the desired mask pattern, for thereby enabling all curved lines to be straightened with satisfactory resolution. 
     The electronic circuitry further comprises a servosystem including a phase lock loop circuit 56, a &#34;divide by n&#34; counter 57 and a compare circuit 58. The phase lock loop circuit 56 is of conventional type. It comprises a phase detector (not shown) to which the magnetic clock pulses sensed by read head 21 are supplied at a preselected nominal frequency (e.g., 3.2MHz) as determined by the density of the uniformly spaced magnetized bits on clock track 19 and the preselected angular velocity of turntable 15. The pulses from the phase detector are supplied via a low pass filter (not shown) to a voltage controlled oscillator set to provide a series of high frequency (e.g., 32MHz) system clocking pulses in a line 59. Circuit 56 includes a &#34;divide by n&#34; counter (not shown) set to divide by 10, interposed in a feedback loop (not shown) connecting a branch of line 59 with the phase detector. If the frequency of magnetic clock pulses sensed from clock track 19 is higher than the frequency of the pulses fed back by the &#34;divide by 10&#34; counter, the low pass filter will go plus and cause the voltage controlled oscillator to speed up to bring the fed back pulse frequency closer to that of the magnetic clock pulses; and vice versa. In this manner, the beam modulation rate, and hence the &#34;printing&#34; rate, will modify to compensate for slight variations in turntable velocity between consecutive rotations. 
     Counter 57 is interposed between a branch of line 59 and the compare circuit 58. It thus counts the system clocking pulses and divides that count by a preset number corresponding to the number of system clocking pulses that should occur per rotation of the turntable. Counter 57 will thus provide a signal to compare circuit 58 at that instant when said preset number of system clocking pulses have been counted. This signal should coincide, within a selected &#34;window&#34;, with the signal as sensed by index track read head 22 and also fed to compare circuit 58. 
     If this coincidence occurs, it verifies that the image spots generated in the scan just completed register with the requisite precision relative to the image spots in the preceding scan. If these signals do not coincide, however, a timing error is indicated; i.e., the scan just completed has image spots unacceptably out of registry with those that preceded it, rendering further operation of the pattern generator impractical as the photomasks will be out of matching tolerance. In this event an error signal is generated by circuit 58 in a line 60, branches (not shown) of which are connected to computer 53, control logic circuitry 55, drive motor 12, amplifier 51 and laser 25. This error signal will deactivate computer 53, control logic circuitry 55, laser 25, and amplifier 51 and also actuate a direct-current braking system (not shown) for stopping the motor 12 and hence turntable 15. If no such error signal occurs, the control logic circuitry 55 will operate, after a certain angular displacement past the index bit on track 20, to incrementally step lens holder assembly 31 radially inward a preselected slight degree in readiness for the next contiguous scan. After a full scan is completed, the lens holder assmebly 31 will be incremented radially inward, and the apparatus is programmed to defer exposure during this time interval. 
     As best illustrated in FIGS. 1 and 4, the apparatus embodying the invention also comprises a simple dynamic balancing mechanism. Supported on base 10 above turntable 15 is a weighing scale 65 (FIG. 1) having a balance beam 66 calibrated in terms of deviation of the substrate weight from a preselected value; this scale may either be in terms of increasing values from a known minimum value or in terms of a plus or minus deviation from a nominal value. In either event, the photosensitive-coated substrate 17 is placed on a platform 67, balanced, and the scale reading noted. 
     As shown in FIG. 4, slug 68, housed in holder 16, has a calibrated scale inscribed thereon which corresponds to the deviation terms inscribed on balance beam 66 of scale 65. Slug 68 is radially positioned at a precise location at which a stationary pointer 70 is opposite and points to the number (deviation term) that had been noted on balance beam 66. Static balance of turntable 15 and holder 16 are corrected for, since the mass of turntable 15 and holder 16 has been compensated for. All slugs 68 and associated mechanism are equidistant radially with all slugs at the same deviation number (e.g., 4). Hence, after noting the static balance of each substrate 17 as indicated on the balance beam 66, the operator merely rotates a respective thumb wheel 69 as necessary to position each slug 68 at the same deviation term as noted on balance beam 66. 
     To insure that the operator places a substrate 17 in each holder 16 and/or that he has adjusted each thumb wheel 69 to correct for variation in substrate weight, a runout probe 71 is provided to sense dynamic runout. This probe 71 is associated with circuitry (not shown) to supply to line 60, via suitable OR circuitry, an error signal, similar to the type heretofore described, for stopping motor 12 and turntable 15. The apparatus will remain shut down until the deficiency has been rectified. This feature is essential to minimize runout to insure interlayer mask integrity will be preserved. 
     After all substrates 17 are inserted in their respective holders 16, there will be a slight static imbalance--especially if there is a maximum variation in substrate weights. However, this static imbalance is minimal. It is compensated to zero as the turntable is rotated to its preselected velocity. These slight differences in weight are minimal from a static standpoint, although at the high preselected velocity of the turntable they are significant from a dynamic standpoint. 
     The invention as thus far described has been concerned with exposure of photosensitive coatings by a laser beam. It should be understood, however, that if desired the method and apparatus heretofore described may be modified slightly as indicated in FIGS. 5 and 6 to employ an electron beam (instead of a laser beam) as the exposure energy source. 
     In the modified apparatus illustrated in FIGS. 5 and 6, structure identical with that heretofore described in connection with the laser embodiment of FIGS. 1-3 will be designated by like reference numerals. The modified apparatus differs from that of FIGS. 1-3 in that laser 25 and the other parts 26-33 associated therewith are eliminated; and lens holder assembly 31 is replaced by a source 100 of electron beam energy with which is associated a deflection yoke 101 of the type conventionally used for television tubes for deflecting the electron beam a limited degree. Also, since electron beam energy must be employed in an evacuated atmosphere, the apparatus must be suitably sealed by a cover 102 over source 100 and a cover 103 for turntable 15; the area enclosed by said covers being connected to a suitable vacuum source (not shown). The source 100 (such as an electron beam gun) is mounted on a Y-slide 104 and incremented radially in substantially the same manner as described in connection with the laser embodiment of FIGS. 1-3. 
     The electron beam embodiment has certain advantages. The deflection yoke 101 is controlled in conventional manner by suitable hard wired logic circuitry (not shown) so as to deflect the electron beam electrostatically to provide a chord-like scan 105 (FIG. 6) as each substrate 17 passes thereunder, returning to a normal curved path (i.e., a concentric scan) between substrates, as indicated at 106. This deflection yoke control desirably eliminates the special programming needed in the embodiment of FIGS. 1-3 to effect the polar coordinate rectification to convert X-Y data into polar coordinate (R,θ) data. It also eliminates the need for maintaining a lens, like 33, constantly perpendicular to the substrate and in critical focus, such as is required when using a laser beam because of its limited focal range. 
     The term &#34;fine resolution energy medium,&#34; as used in the claims, is therefore intended generically to cover both a laser beam and an electron beam, inasmuch as the invention, as broadly defined herein, may be implemented with either. 
     It should be understood that this apparatus and method may be used to make either positive or negative patterns. To make a positive pattern, as heretofore assumed, the X-Y digital data bits normally would be a &#34;0&#34; but be a &#34;1&#34; whenever the laser or electron beam is to be turned on to expose the photosensitive coating. To make a negative pattern, the X-Y digital data bits normally would be a &#34;1&#34; but be a &#34;0&#34; whenever the laser or electron beam is to be turned on to expose the coating. 
     While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and detail, including but not limited to those above suggested, may be made therein without departing from the spirit, scope and teaching of the invention. Accordingly, the apparatus and method herein disclosed are to be considered merely as illustrative and the invention is to be limited only as specified in the claims.