Patent Document

STATEMENT OF RIGHTS OF INVENTION 
     The United States Government has rights in this invention pursuant to Contract No. W-7405-ENG-48 between the United States Department of Energy and the University of California. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to apparatus and methods for grinding and polishing optical elements, such as lenses, prisms, windows, mirrors and similar optical systems. This invention could also be used to polish ceramics and semi-conductor surfaces. 
     2. Description of the Related Art 
     A recirculating slurry system  10  of the prior art for grinding and polishing of optical parts is shown in FIG. 1. A slurry tank  12  holds a slurry  14 , which usually consists of deionized water and grinding or polishing particles of fairly uniform size, and a pump  16 . The slurry  14  is pumped from the tank  12  through pipe  18  to either a delivery pipe  24  or a bypass pipe  26 . If valve  20  is open, the slurry  14  is returned to the slurry tank  12 , which helps keep the grinding particles of the slurry  14  in suspension by agitation. 
     Valve  20  is usually kept open during the grinding and polishing phases, which include grinding away the surface roughness (stock removal) along with the subsurface damaged material (usually  7  times the depth of the surface roughness (peak-to-valley) is removed). When valve  22  is opened, the slurry  14  is pumped to the grinder/polisher  30 , which grinds or polishes (depending on the size of the particles used) the surface of a part (not shown). For example, a loose abrasive grinding could be performed by using a grinder with a hard lap (metal or glass). The polishing could be performed by using a polisher with a resilient material such as pitch or polyurethane. The part being ground and polished could be an optical lens or a semiconductor blank inserted in the grinder/polisher  30 . During the final polishing stage, the valve  20  is usually shut or partially closed to allow the slurry to begin to settle. The slurry in the grinder/polisher  30  is returned to the slurry tank  12  via return pipe  28 . It is desirable to shut valve  22  after polishing the part is complete so that slurry will circulate through bypass pipe  26  in order to avoid the settling and caking of the polishing compound. 
     The loose abrasive grinding used in the slurry is typically an aluminum oxide or silicon carbide with mean grit sizes between 9-30 μm. Polishing slurry usually uses cerium oxide or zirconium oxide with a mean grit sizes between 1-3.5 μm. For polishing slurries, the range of polishing powders on the market is usually between 0.4 to 3.7 μm APS (average particle size). However, a compound with a specific particle size will have a range of different sized particles based on standard distribution curves. For example, a 2.5 μm APS compound has particles ranging from 0.5 to 8.0 μm in size. Although the larger particles may be a weak agglomeration of smaller particles, the surface of the polished object may be scratched if the larger particle do not break apart. In another example, a 12.5 μm APS compound was tested to find that the size of the particles ranged from 7 to 25 μm and some particles were as large as 40 μm. Although these very large particles will most likely create scratches, these particles fortunately either settle down quickly or are too large to penetrate between the lap and the part being polished. 
     In order to avoid some of these problems, manufacturers perform two polishing operations: the so called “pre-polishing” process and the “final polishing” process. This is time consuming method of polishing parts because it requires each part to be removed, washed, and transferred into the final polishing machine. 
     FIG. 2 shows another recirculating slurry system  40  of the prior art where the pump  46  is located outside of the slurry tank  42 . The slurry  44  held in the slurry tank  42  is removed via pipe  48  and valve  50 . The slurry is pumped via pipe  56  to either delivery pipe  60  via valve  58  or bypass pipe  52  via valve  54 . The grinder/polisher  62  receives the slurry  44  from delivery pipe  60  and returns the slurry to the slurry tank via return pipe  64 . 
     The recirculating slurry systems shown in FIGS. 1 and 2 will pump relatively rough grit into the attached polisher. While this is advantageous during most of the polishing or grinding process, it is objectionable during the final stages of the polishing because the surface finish of substrates depends on the grit size being used. Instead of removing the partially polished part to insert into a separate “final polishing” machine, the lap is flushed with deionized water to perform a “water polishing” step. It is commonly believed that the remaining polishing particles will embed themselves in the lap material, thus exposing just the tips of the grains, which are obviously smaller than the whole slurry grains. Thus, a finer polished surface will result. However, the pH of the deionized water (pH of 7) is usually different than the optimal value for the slurry. Therefore, the slurry compound will begin to aggregate and the continued polishing of the part will create scratches. It has been demonstrated that just 15 minutes of “water polishing” deteriorates the part&#39;s surface and scratches may appear after 30 minutes. In addition, the unnecessary water added to the slurry tank will change the density of slurry and affect the polishing of subsequent parts. 
     The recirculating slurry systems shown in FIGS. 1 and 2 can produce an optical part with a smoothness of only 4-5 Å RMS. 
     SUMMARY OF THE INVENTION 
     The present invention discloses a slurry system that draws slurry from a slurry tank via one of several intake pipes. Each intake pipe has an intake opening at a different depth in the slurry tank. The process begins by continuously removing slurry from the intake pipe with an intake opening at the deepest level. The slurry is returned to the slurry tank via a bypass pipe in order to continue the agitation of the slurry. The slurry is then diverted to a delivery pipe, which supplies slurry to the polisher. When the surface roughness is smoothed out, the flow of slurry in the bypass pipe is stopped in order for the slurry in the slurry tank to begin to settle. As the polishing continues, slurry is removed from different pipes so that the final pipe is at the shallowest depth such that the finest grit is pulled from the slurry. When the polishing is complete, the flow of the slurry in the bypass pipe is resumed to start agitating the slurry and the intake pipe with the intake opening at the deepest level is used again. 
     The present invention also discloses a slurry system that draws slurry from a slurry tank via an adjustable intake pipe. The process begins by continuously removing slurry from the intake pipe at a predetermined level. The slurry is returned to the slurry tank via a bypass pipe in order to continue the agitation of the slurry. The slurry is then diverted to a delivery pipe, which supplies slurry to the polisher. The flow of slurry in the bypass pipe is stopped in order for the slurry in the slurry tank to begin to settle. As the polishing continues, the adjustable intake pipe is adjusted such that slurry is removed from predetermined levels where each level is shallower than the previous level. When the polishing is complete, the flow of slurry in the bypass pipe is resumed to start agitating the slurry and the intake pipe is returned to its original position. 
     An object of the invention is to provide progressively finer polishing grit to a polisher. 
     Another object of the present invention is to avoid using water as a final polishing step when polishing surfaces of optical ceramic and semiconductor elements. 
     Other objects and advantages of the present invention will become apparent when the apparatus of the present invention is considered in conjunction with the accompanying drawings, specification, and claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a better understanding of the invention and further features thereof, reference is made to the following detailed description of the invention to be read in connection with the accompanying drawings, wherein: 
     FIG. 1 depicts a prior art recirculating slurry system; 
     FIG. 2 depicts another recirculating slurry system known in the prior art; 
     FIG. 3 shows a recirculating slurry system of the present invention described as the first preferred embodiment; 
     FIG. 4 shows an alternate method of extracting the slurry from the tank in the first preferred embodiment; and 
     FIG. 5 shows a recirculating slurry system of the present invention described as the second preferred embodiment. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     While this invention is described in some detail herein, with specific reference to illustrated embodiments, it is to be understood that there is no intent to be limited to these embodiments. On the contrary, the aim is to cover all modifications, alternatives and equivalents falling within the spirit and scope of the invention as defined by the claims. For example, three pipes are used to pull slurry from the slurry tank at different depths. However, any number of pipes could be used. As another example, the preferred embodiments will be used for polishing the parts; however, the preferred embodiments can also be used for grinding a part by adjusting the characteristics of the slurry. 
     Referring to FIG. 3, a multi-stage recirculating slurry system  70  of the first preferred embodiment of the present invention is shown. The slurry tank  72  holds slurry  74 , which consists of grit of different sized particles. Pump  76  pumps slurry  74  from the tank  72  via one of several pipes or pipes. In this first preferred embodiment, there are three pipes  80 ,  82 ,  84 , which each have an intake opening that is at a different depth in the slurry  74 . As grit settles down when undisturbed, larger particles settle faster than smaller ones. The settling time depends on Stoke&#39;s law, that is, the settling time is directly proportional to the liquid viscosity and inversely proportional to the square of the grain diameter. Therefore, the ideal depths of these intake openings can be determined based on the amount and rate of slurry material to be removed. Thus, it can be predetermined that pipe  84  will pull larger grit than pipe  82 , and pipe  80  will pull the finest grit. The valves  86 ,  88 ,  90  control the flow of slurry in corresponding pipes  80 ,  82 ,  84 . Only one valve is open at a time providing grit of a specific mean size to pipe  96 . If valve  94  is open, then the grit is recirculated via bypass pipe  92  in order to agitate the slurry. If valve  98  is open, then the grit will be pumped to the polisher  102  via pipe  100 . The grit is returned to the slurry tank  72  via return pipe  104 . However, valve  98  is closed during non-operational periods to continue slurry agitation in order to prevent caking of the slurry compounds in the tank. 
     To polish the surface of an optical part or semiconductor substrate, the following operation of the multi-stage recirculating slurry system  70  will occur. During the rough polishing process, where most of the subsurface damage is removed, the bypass pipe valve  94 , valve  98  and the valve  90  of the deepest pipe  84  are open. Thus the grit is being agitated in the slurry tank  72  and pumped to the polisher  102 . After the polishing has achieved a certain level of smoothness, the bypass pipe valve  94  is closed, thus allowing the grit to start settling down in the slurry tank  72 . The bypass pipe valve  94  could be partially closed to slow the settling of the grit by allowing some slurry agitation to occur. After a specific time interval, which will be established by the requirements of the polisher  102 , valve  90  is closed and the next valve  88  will be opened to allow grit from the depth of pipe  82  to be drawn. The slurry continues to settle during this polishing process. After another time interval, valve  88  is closed and valve  86  is opened to allow slurry to be drawn via the shallowest pipe  80 . Because the slurry has settled and the heavy particles have drifted toward the bottom of the slurry tank  72 , only very fine particles will be pumped into the polisher  102 . When the polishing is complete, the polished part can be removed from the polisher. Usually, valve  98  remains open during operation in order to allow the lap to remain at a constant temperature in order to avoid the lap from warping. This is especially important with pitch laps, where the temperature affects the viscosity of the lap, and ultimately affects the geometric accuracy of the substrates and their smoothness. Bypass valve  94  is opened to recirculate the slurry and hence agitate the slurry to mix the particles. A new part can be inserted in the polisher to start the process again. 
     During the polishing process, the pH of the slurry should be carefully monitored. For example, the slurry could be monitored with a pH meter  106  or by installing a pH compensator into the tank. Although suspension agents are added to the slurries to retard settling and to prevent caking of settled polishing compounds, the changing pH of the slurry can have a significant impact on the characteristics of the slurry. For example, some polishing compounds totally lose their suspension properties and settle hard when the proper pH is not maintained. Another problem is when some types of glass may be etched or leached during polishing. Most glass by its composition is alkaline. Therefore, the polishing slurry becomes progressively more alkaline as the part&#39;s surface residue (swarf) is mixed into the slurry. In contrast, both heat-absorbing glass and the continuously abraded lap pitch are acidic. Thus, the pH of the slurry changes as the percentage of these by-products of the polishing process increase. Another concern is that some polishing compounds have a higher stock removal in certain pH ranges. So by adjusting the pH level, there will be an increase or decrease in the amount of removal of the part. 
     For example, the proper pH for fused silica is 4.0. When performing polishing of fused silica substrates, the removal rate was the highest at this pH value and so was the resulting surface finish. It was also discovered that when the pH was at a different value, there was a significant amount of undesirable redeposition of material on the substrate surface. 
     In addition to adjusting the pH of the slurry to keep it from aggregating, a perforated pipe (not shown) located at the bottom periphery of the tank could be used to blow air bubbles into the slurry to continuously agitate it. Another method to assist in agitating the slurry is to use a conical-shaped tank (not shown) with a tapered angle of 60 degrees with the pump located in the center of the cone. 
     Each of the valves described in the first preferred embodiment could be manually operated. However, a controller  108  as shown in FIG. 3 could be used to automatically control each of the valves in the correct sequence. Also, a gauge  107  could be used in the polisher  102  that measures the amount of removed stock. Thus, the controller  108  would wait until the lens or substrate reached a predetermined specification before opening and closing the appropriate valves. An alternative to using a gauge is to measure a “monitor,” which is a part that is being worked along with the real parts. The monitor is removed from the machine for measurement purposes. 
     The first preferred embodiment eliminates the necessity of having “pre-polishing” process and the “final polishing” process. The entire polishing process is done continuously to achieve a part surface smoothness in the 2 Å RMS level. This is remarkably smooth for a 48″ diameter CP machine. However, the process should be able to achieve 1 Å RMS smoothness or better, in elements with the proper monitoring of pH, slurry composition and other factors described above. 
     A slightly modified design for extracting the slurry from the tank is shown in FIG. 4 as the second preferred embodiment. The intake pipes are attached to the wall of the tank  72 . Each intake pipe is positioned at the appropriate depth for removing the different sized grit. Similar to the previous description for the first preferred embodiment, valve  90  would be open when recirculating the slurry. After the bypass valve  94  (see FIG. 3) is closed, the grit in the slurry  74  begins to settle. Valve  90  will then be closed and valve  88  is opened allow grit at the higher level to be extracted from the tank  72 . After another time interval, valve  88  is closed and valve  86  is opened to allow slurry to be extracted via the highest level of the tank. The appropriate valves are adjusted for recirculating the slurry after the polishing is complete. 
     An alternate design of the first preferred embodiment could be to position the intake pipes at the bottom of the tank. Therefore, the intake would be at the same height as in the first preferred embodiment, but the slurry would be drawn under the tank and sent to the polisher. 
     Referring to FIG. 5, multi-stage recirculating slurry system  110  of the third preferred embodiment of the present invention is shown. The slurry tank  112  holds slurry  114 , which consists of grit of different size particles. Pump  116  pulls slurry  114  from the tank  112  via pipe  11   8 . In this third preferred embodiment, the depth of this pipe  118  is adjusted via a motor  120 . Therefore, grit can be removed at any level within the slurry. When valve  126  is open, the slurry  114  flows through pipe  118 , a flexible hose  122  and pipe  124  to the pump  116 . If valve  132  is open, then the grit is recirculated via bypass pipe  130  and the slurry will be continuously agitated. If valve  134  is open, then the grit will be pumped to the polisher  140  via pipe  136 . The grit is returned to the slurry tank  112  via return pipe  142 . 
     Similar to the first preferred embodiment, both the bypass pipe valve  132  and the valve  126  are open to keep the slurry  114  in an agitated state. The motor  120  moves the adjustable pipe  118  toward the bottom of the slurry tank  112 . Valve  134  is opened to begin sending particles of different sizes to the polisher  140 . As the polishing continues, valve  132  is closed so that the slurry begins to settle. The motor  120  can either move the adjustable pipe  118  to specific levels or continuously move the pipe upwards at a predetermined rate. As the pipe  118  is moved to a shallower depth, finer particles are pumped to the polisher  140 . The larger particles in the polisher  140  are returned to the slurry tank  112  via pipe  142 . When the polishing is complete, the bypass valve  132  is opened to recirculate the slurry and hence agitate the slurry to mix the particles of grit. 
     Similar to the first preferred embodiment, each of the valves could be manually operated. Instead of a motor  120  to adjust the adjustable pipe  118 , a clamp could be used. However, it is preferable to use a controller  146  that automatically controls each of the valves in the correct sequence. The controller  146  would also operate the motor  120  to adjust the pipe  118  to the appropriate level. Also, a gauge  148  could be used in the polisher  140  that measures the amount of stock that has been removed. Thus, the controller  146  would wait until the lens or substrate reached a predetermined specification before opening and closing the appropriate valves and adjusting the depth of the pipe  118 . Also, the multi-stage recirculating slurry system  110  could be mounted on a rolling stand  144  to assist in moving the system. 
     Although the foregoing invention has been described in some detail by way of illustration for purposes of clarity of understanding, it will be readily apparent to those of ordinary skill in the art in light of the teachings of this invention that certain changes and modifications may be made thereto without departing from the spirit or scope of the appended claims.

Technology Category: 7