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Pitch, Polytron, and polyurethane: a comparison Arne Lindquist University of Rochester, Institute of Optics, New York State Center for Advanced Optical Technology, Roches- ter, New York, 14627 Received 15 August 1986. 0003-6935/86/213796-02$02.00/0. © 1986 Optical Society of America. This brief communication describes the results of an ini- tial investigation of the properties of Polytron, 1 a relatively new polishing pad material. The objective was to evaluate polishing rate and roll-off compared to pitch and polyure- thane. Surface roughness and figure were secondary consid- erations. A broad range of surface preparations were used to determine the most efficient procedure for inspection polish- ing. Fifteen substrates of BK-7, 38 mm in diameter × 15 mm thick were prepared. Fabrication started with the removal of a few tenths of a millimeter with a Yoshikawa 2 surface grinder, model YGS-16, using a 150-grit bound-diamond- abrasive wheel and water. Three samples were labeled and set aside prior to each grinding step. Twelve parts were ground with brush fed 25-μm alumina powder in water on a 203-mm diam cast-iron tool on a Standard Optical Co. 3 mod- el 210A grinding machine. About 200 μm of material was removed. At this time the cast-iron tool was replaced with a 133 -mm diam fused silica tool. A 14.5-μm alumina grind was next, removing an additional 125 μm; 75 μm was then re- moved with 9.5-μm alumina. The final three substrates were ground with 5-μm alumina to remove 50 μm of material. The substrates were grouped into three sets of five contain- ing one each of the groups of three parts as defined in the grinding stage. Hand polishing of all samples was performed on a Stras- baugh 4 model R6DE-DC-1 polishing machine. Brush fed Opaline (cerium oxide) was used for all polishing in a 10-g/ 100 cc water concentration. Spindle speeds of 50 rpm for the pitch lap and 200 rpm for the Polytron and polyurethane laps were used. The polishing laps were 152 mm in diameter. The pitch lap was Gugolz 5 73 polishing pitch. A seven- segment (six around one) honeycomb pattern was used for the Polytron lap, and a single 3-mm sheet was used for the polyurethane lap. Polishing was performed in three stages. Stage 1 consist- ed of a 1-min polish of all substrates on their respective laps. The samples were inspected and photographed on an Olym- pus 6 Vanox-T microscope with Nomarski at 150×. Stage 2 3796 APPLIED OPTICS / Vol. 25, No. 21 / 1 November 1986 Fig. 1. Time required to achieve inspection polish. consisted of a second 1-min polish. Again photomicro- graphs were taken during inspection. The substrates were inspection polished (0.02-mm diam max. dig) in stage 3, and a record was kept of the time required. A final visual inspec- tion and record were made on the microscope. At this time 633-nm interferometry was also performed using a Zygo 7 Mark II mainframe with Zygo ZAPP. The three 5-μm ground substrates were also evaluated for surface roughness on a Rank-Taylor-Hobson 8 Talystep surface profilometer using a 15-μm radius stylus. Roll-off of the polyurethane polished samples were typi- cally several waves. Both the pitch and Polytron polished substrates exhibited an edge roll-off of <0.25 wave. Polishing times are indicated in Fig. 1. Samples worked on Polytron polished out in 85% of the time required to finish parts on polyurethane. Of the pitch polished samples, only the 5-μm ground substrate was able to be finished to a nominal inspection polish in a reasonable period of time. Times indicated with an arrow in Fig. 1 represent the points at which further pitch polishing did not improve the surface. Surface figures were measured to be 0.10 and 0.18 waves for the pitch and Polytron, respectively, whereas the poly- urethane exhibited a figure of 2-5 waves exclusive of roll-off. Polishing on pitch did allow better control over figure, but that may have been as much a function of time as lap materi- al. Since the substrates were only inspection polished, there was no attempt to correct surface figure beyond the time needed to polish out the substrates. Surface roughness of the pitch polished substrate was ~30 Å rms. The Polytron
