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Use of diamond turned mirrors for synchrotron radiation
Nuclear Instruments and Methods 195 (1982) 251-257 North-Holland Publishing Company
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U S E OF D I A M O N D T U R N E D M I R R O R S F O R S Y N C H R O T R O N R A D I A T I O N *
Malcolm R. H O W E L L S and Peter Z . TAKACS National Synchroton Light Source, Brookhaven National Laboratory, Upton, New York 11973, U.S.A.
The diamond turning technique has great interest for users of synchrotron radiation because of its ability to produce surfaces of arbitrary shape. It also has the advantage of being well adapted to producing metal optics. These are of interest because they lend themselves to water cooling and hence represent one approach to the problem of high synchrotron radiation power loadings on optical surfaces. The optical figure produced by diamond turning is generally adequate for synchroton radiation applications. The main difficulty centers around the question of smoothness. Diamond turned surfaces must receive a final polish after machining before they are sufficiently smooth for use with ultra-violet or X-ray radiation. The manufacturing stages can be carried out by various groups in the optics industry and the National Synchrotron Light Source has procured a considerable number of mirrors and is having them polished for use on the VUV storage ring. At the time of writing one mirror has been completed and evaluated and we give the results for this and discuss the indications for the future. The important measurement of the r.m.s, height of the surface roughness has given a value of (3.4±0.9) .A using total integrated scattter of visible light at normal incidence.
1. Introduction
2. Indications for diamond turning
The problem of fabricating mirrors suitable for use with synchrotron radiation (SR) has been extensively documented [1-3]. Diamond turning [46] has been discussed as a possible approach, but has not been applied to any extent. The discussion of the merits and demerits of diamond turning has on occasion been quite heated and SR technologists have hesitated to become involved with a technology that did not seem to be fully established. However, recently, the number of operational diamond turning facilities has been increasing and so the number of operators who offer a contract machining service. During the latter part of 1979, it became clear that the National Synchrotron Light Source (NSLS) at Brookhaven would have a requirement for a considerable number of grazing incidence paraboloids for a plane grating monochromator (PGM). Three PGMs were planned so the mirror fabrication costs could be amortized over three instruments. The mirrors have now been fabricated by diamond turning and are in the process of being post-polished. The first units have already been completed and have been evaluated for figure and roughness. We report here the nature of the fabrication that was involved and the preliminary results of the evaluations.
The advantages of single point diamond machining are generally given as follows: 1) The machines are numerically controlled and thus can produce any figure with equal ease. 2) It is only slightly more difficult to manufacture several mirrors simultaneously leading to attractive cost savings for quantity production. 3) Good figure accuracy is normally produced. 4) The technique works well for metals and these lend themselves well to water or conduction cooling and to applications where high damage thresholds are important. The main disadvantage of diamond turning is that the machined surface still has the periodic groove pattern left by the cutting tool. This is of such an amplitude that the surface is not sufficiently smooth for optical work, especially at short wavelengths, and leads to considerable scattering of radiation. It thus becomes necessary to carry out the so-called "post-polishing" process. Designers are usually concerned about whether this can be done at reasonable cost and without harming the optical figure. For the NSLS PGM mirrors we found the advantages listed above extremely compelling and we are now in a position to quote our experiences with regard to the smoothness, figure quality and cost resulting from the post-polishing process.
* Work supported by the U.S. Department of Energy.
0029-554X/82/0000-0000/$02.75 © 1982 North-Holland
IV. v u v OPTICS
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M.R. Howells, P.Z. Takacs / Diamond turned mirrors
3. Description of the required mirrors Each PGM requires five different mirrors. One is a 2 ° grazing angle collimator of focal length 10 mm. Parallel light from this mirror is diffracted by the grating and then focused by a second mirror on to the exit slit. There are four focusing mirrors (see fig. 1), each with a different grazing angle corresponding to a different wavelength range. The collimator is large (300 mm long) and the focusing mirrors are much smaller (75 mm long). A more complete description of the geometry of the mirrors is given in table 1 and fig. 2. There is little difficulty in choosing a suitable supplier for these mirrors. One can see from table 1 that two of the set of five mirrors have Y0 values approaching 1 m. This coordinate is the radius of the turning circle and thus determines the size of the throat of the machine required to make the mirrors. Only one machine exists which can turn parts on a 1 m radius. This is the EXCELLO machine [7] at Union Carbide's YI2 Plant at Oak Ridge. The Y12 group was, therefore, chosen as vendor for the NSLS mirrors. Once we had received a detailed quotation, it became clear that we could buy a much larger number of mirrors than the three sets of five that we had immediate need f o r . We finally purchased twelve sets of five for a price only about 30% more than for one set. The production of fixtures, the programming and use of the machine and metrology are the primary non-recurring costs. The main Fig. 1. Views of a P4 mirror after diamond turning and before post-polishing. The tool marks are exaggerated by the lighting. The mounting pads are clearly seen.
Table 1 Description and geometry of the mirrors. Identification Incidence angle (deg) Focus/vertex distance (ram) Focal length (ram) Xo (ram) ]To (ram) Wavelength range of use (,~) Permitted r.m.s, roughness (,~) Permitted slope error (per em) Maximum depature from the ideal surface of best fit
Pl 86.0 4.866 1000.0 995.1 139.2 20-50 20 ~/5
P2 84.0 11.848 1084.4 1072.6 225.5 40-100 20 ~,/5
P3 78.0 50.594 1170.4 I119.8 476.0 70-300 30 ;k/5
P4 65.0 217.841 1219.7 1001.8 934.3 250-1500 30 ~/5
P5 88.0 12.180 10000 9987.8 697.6 20-1500 20 ~/5
X/2
X/2
~/2
0/2
~/2
M.R. Howells, P.Z. Takacs / Diamond turned mirrors LATHE SP~INDLE ETC.
/
J
to
I/
Fig. 2. Geometricallayout of the paraboloidalmirrors. A-plane alignment surface perpendicular to the x axis. B-flat reflector which can be inserted to auto-collimate light from the focus back to the focus.C-lathe fixtureto which mirrors are mounted. D-focus of parabola. E-parabola 3,2 =4ax. F-flat mounting pads at rear of mirrors.
recurring cost is electroless nickel plating. The blanks were supplied by Brookhaven. In round figures, the small mirrors were about $2k each and the large one about $4k. This leads to a total of $144k, or $48k for each of the three PGMs. Figuring the cost of a PGM, including all ancilliary equipment except the computer, the mirrors amount to about a third of the total.
4. Mechanical design questions The mechanical design of the mirrors was determined in consulation with the fabricators. The method of mounting was one which had been used successfully with much larger mirrors [8]. Some of the points at issue are as follows: 1) The mounting is by small, flat pads, captured by 1///4-20 screws tightened to 40 in. lbs. 2) Mirror thickness is very generous to avoid deformations due to centrifugal forces, mounting screws [9] or residual stresses in the material. 3) The material used for the blanks was 6061-T6 aluminium alloy. A plated surface layer (5-10 mils) of electroless nickel was applied to make the final surface. The aluminium substrate was first diamond machined, the nickel layer was then applied and it, too, was diamond machined. This procedure was intended to prevent "through-print"
253
of substrate irregularities into the final surface. The amorphous nickel layer is necessary because it is the only material which diamond turns well which will also take a good superpolish. 4) Heat treatment [10] was (a) normal stress relieving before machining the substrate; (b) 12 h at 200°C before final machining of the nickel layer. The latter was based on the idea that the mirrors should see temperatures higher than any operational temperature (e.g., 120°C bakeout) before final machining. However, it is higher by about 25°C than the recommendation of the polishers and may have caused some hardening of the nickel layer. 5) A special alignment surface was machined as shown in fig. 2. It was set to be perpendicular to the direction of the parallel light, i.e., perpendicular to the symmetry axis. The idea was to permit alignment by auto-collimation from that surface. The ease with which such a surface can be included is part of the advantage of this method of fabrication.
5. Metrology From fig. 2 one can see that by installation of a suitable flat retro-reflector one can use an equal path interferometer located at the focal point and thereby carry out metrology whilst the part is still on the machine. This procedure is used to determine errors in the optical surface so that the machine program may be adjusted to correct them. Interferograms have been supplied by the Y12 group for all of the five mirror types. In each case, two types of measurement have been made: (1) a fringe pattern taken as described above; and (2) a pattern taken using a microscope at 200 × magnification. The first picture is taken at least twice with the fringes along and across the rectangular aperture of the mirror and is interpreted using the program F R I N G E [11]. The results of this computation give a measure of the deviation of the wavefront returning back to the focus, from the proper spherical shape. This wavefront has been reflected twice from the mirror and once from the reference flat so the fringes each represent a quarter of a wavelength (1//4× 6328.A). Exactly parallel, equi-spaced fringes would indicate a perfect wavefront. Fig. 3 shows some sample interferograms. Table 2 summarized the data and IV. VUV OPTICS
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Fig. 4. Y12 interferograms of the surface of a P4 mirror. Magnification ×206, wedge factor 0.273 # m per fringe. Estimating the fringe ripples to be of amplitude 1/5 of a fringe we find a value of around 500 ,~ for the groove depth.
their reduction by means of FRINGE. The mirror contour data are derived from the wavefront data by inserting a cos 0 factor to allow for foreshortening. The second type of measurement is intended to give an idea of the depth of the tool marks. The fringes run at right angles to the tool grooves which appear as ripples on each fringe. The depth of the groove can be derived from the known wedge factor of the fringes. For the example shown in fig. 4, one derives a value of about 500 ,~ for the groove depth. The groove spacing is known from the feed rate of the machine to be 10#m.
6. Post-polishing Fig. 3. YI2 interferograms of a P4 mirror after diamond turning. (0.158/~m per fringe). The fringes are contours of equal wavefront error.
Once the mirrors had been manufactured and characterized, post-polishing could begin. We had
Table 2 Data reduction by the F R I N G E program. Mirror type
R.m.s. wavefront variations a (waves b)
Peak-to-valley wavefront variations (waves)
Peak-to-valley mirror contour errors (waves)
PI P2 P3 P4 P5
0.0052 0.015 0.018 0.056
0.029 0.080 0.10 0.27
0.42 0.76 0.48 0.64
a These are all averages over about four interferograms. b A=6328 ,~.
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Fig. 5. Interferograms taken at Brookhaven. (a) gives independent confirmation of the equivalent Y12 picture. (b) shows the same picture after post-polishing. The degradation of the surface is quite small and is limited to areas outside the required clear aperture. some advance indication that we could find willing opticians to do this, but the post-polishing of exotic d i a m o n d turned surfaces [12,13] is a sufficiently new business that we should mention that bids were received from three optical houses: Applied Optics Center [14], F a b - O p t [15] and University of Arizona, Optical Sciences Center [16]. The low bidder was Applied Optics Center and this Fig. 6. Nomarksi micrographs before and after post-polishing. The analysis of these is discussed in the text. IV. VUV OPTICS
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company is presently working on our mirrors. The price for polishing the 300 X 50 mm 2 mirrors is $690 each. The first mirror has been polished and evaluated for both figure and roughness with extremely reassuring results. Fig. 5 shows interferograms taken at Brookhaven before and after polishing. We notice that although they are different they are both indicating a good surface within about a fringe. This is particularly so considering that the size of this mirror surface is 75 X 50 mm 2 and the size of the required clear aperture is 55 X 40 mm 2. The effect of edge roll-off is, therefore, entirely outside the clear aperture. It is also gratifying to note that the Brookhaven interferograms are in good agreement with those of both the Y12 group and Applied Optics Center, thus giving an independent check on the metrology of all three optical groups. Perhaps the most important evaluation of all is the roughness measurement. This was carried out at the laboratory of Dr. J. Bennett at the Naval Weapons Center in China Lake. The method used was total integrated scatter (TIS) of visible light at normal incidence. This method as applied at China Lake is sensitive to spatial wavelengths in the range 0.6-12.0 #m, in other words, to the so-called "micro-roughness." The r.m.s, height (o) of the roughness is derived from the TIS by use of the relation: TIS=l-exp
[(4~rsin0°) ~
2]
~
(4~rsinO°) 2
The TIS was measured over a uniform mesh of 33 points covering the whole surface. The value of o in each case was derived and the average value of these 33 results was: o ----(3.4 4- 0.9) ,~. This is an excellent result. The worst of the 33 figures was less than 6.5 A. This surface was one of the best metal surfaces ever tested at China Lake.
changes in surface slope. Height changes of a few AngstrOms are said [17] to be detectable. Fig. 6a shows the surface topography of an unpolished P4 mirror as received from Oak Ridge at a magnification of X400. The regular pattern of horizontal tool marks dominates the frame. The strong contrast evident between the tool marks enhances the vertical structure of the surface profile and is a qualitative supplement to the micro-interferograms in fig. 4, from which peak-to-valley heights of about 500.A were derived. Fig. 6b is a Nomarski micrograph of a polished P4 mirror at X 160 magnification. It is essentially featureless, confirming that the polishing process indeed removed most of the tool marks. C.lose examination of the original print reveals an occasional shallow scratch mark and remnants of the bottoms of tool marks. Also evident upon close inspection are low-contrast, circular "saucer-shaped" patterns [20] scattered over the surface, each on the order of 50 # m in diameter. These are probably the "plateaux" identified by Lindsey and Franks [19], formed by through-print in the plated coating as a result of preferential growth structure on substrate topography or coating inclusions. It is known that applications such as the PGM will be sensitive to scattering by spatial wavelengths longer than those probed by TIS, i.e., ripple. This is not unusual. Ripple is significant even in many visible light instruments. The problem is that there is no good method for measuring ripple which is cheap enough to be part of the normal manufacturing and inspection process. Consequently, ripple is commonly omitted from the characterization even of quite expensive optics. We propose to address this problem in a limited way be arranging for profilometer [18] measurements of our otherwise well characterized focusing mirror. This will add a small amount of data to present knowledge of the ripple properties of the various diamond turning facilities around the country. We will report the results of the profilometer measurement in a later publication.
7. Further evaluations
In order to get an improved feeling for the effects of the tool marks and the post-polishing process, we have taken a range of before and after Nomarski micrographs. Some of these are shown in fig. 6. The Nomarski approach is largely a qualitative one but it is extremely sensitive to
8. Conclusion
We have described the fabrication of some specialized optial surfaces for use with SR. Progress so far an reasonably comprehensive evaluations seem to indicate that the optics have been pro-
M.R. Howells, P.Z. Takacs / Diamond turned mirrors
d u c e d close to specification a n d in some cases b e t t e r t h a n specification. This leads us to believe t h a t d i a m o n d t u r n i n g plus p o s t - p o l i s h i n g is a cost effective a p p r o a c h that c a n meet the special needs of S R technology. T h e a u t h o r s wish to r e c o r d their a p p r e c i a t i o n of the helpful a n d p r o f e s s i o n a l a p p r o a c h of the staff m e m b e r s of U n i o n C a r b i d e , Y12 D i a m o n d T u r n ing G r o u p , A p p l i e d O p t i c s C e n t e r a n d the O p t i c a l E v a l u a t i o n F a c i l i t y at C h i n a Lake. W e also wish to a c k n o w l e d g e a n u m b e r of useful suggestions b y Dr. R.E. Parks.
References [1] V. Rehn and V.O. Jones, Opt. Eng. 17 (1978) 504. [2] Workshop on X-ray instrumentation for SR research, Stanford (1978) SSRL Report 78/04, eds., H. Winick and G. Brown. [3] M.R. Howells, Appl. Opt. 19 (1980) 4027. [4] Precision machining of optics, Proc. SPIE (1976) 93. [5] Precision machining of optics, Proc. SPIE (1978) 159. [6] Opt. Eng. 17 6, (1978) 570, first eight papers. [7] H.L Gerth et al., Proc. SPIE 159 (1978) 93.
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[8] S.C. Robinson, H.L. Gerth and J.E. Stoneking, Optical fabrication and testing Workshop, Tucson, Arizona (1979) XIX, p. 17. [9] C. Denny, W.J. Spawr and R.L. Pierce, Proc. SPIE 181 (1979) 84. [10] W. Caithness, Proc. SPIE 65 (1975) 8. [11] J. Loomis, FRINGE 3, by University of Dayton, Ohio, Research Institute. [12] P.C. Baker and N.J. Brown, Opt. Eng. 17 6, (1978) 595. [13] S.R. Lange and R.E. Parks, Proc. SPIE 257 (1980) 169. [14] Applied Optics Center Corp., 10 'B' SR Burlington, MA 01803 (617) 273-0309. [15] Fab-Opt. 6206 Paseo Santa Cruz, Pleasanton, CA 94566 (415) 462-2857. [16] Optical Sciences Center, University of Arizona, Tucson, AR 85721 (602) 626-2749. [17] H.E. Bennett, Proc. SPIE 184 (1979) 153. [18] J.M. Bennett and J.H. Dancy, Appl. Cpt. 20 10, (1981) 1785. [19] K. Lindsey and A. Franks, Proc. SPIE 163 (1979) 46. [20] These patterns cover a very limited portion of the total area and contain spatial wavelengths within the range probed by the total integrated scatter measurement. It thus appears that at least for the present mirrors they are not affecting performance. Nonetheless, it would be desirable to eliminate them and, in the opinion of the authors, this would probably be achieved by using a purer aluminium alloy than 606 I-T6.
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U S E OF D I A M O N D T U R N E D M I R R O R S F O R S Y N C H R O T R O N R A D I A T I O N *
Malcolm R. H O W E L L S and Peter Z . TAKACS National Synchroton Light Source, Brookhaven National Laboratory, Upton, New York 11973, U.S.A.
The diamond turning technique has great interest for users of synchrotron radiation because of its ability to produce surfaces of arbitrary shape. It also has the advantage of being well adapted to producing metal optics. These are of interest because they lend themselves to water cooling and hence represent one approach to the problem of high synchrotron radiation power loadings on optical surfaces. The optical figure produced by diamond turning is generally adequate for synchroton radiation applications. The main difficulty centers around the question of smoothness. Diamond turned surfaces must receive a final polish after machining before they are sufficiently smooth for use with ultra-violet or X-ray radiation. The manufacturing stages can be carried out by various groups in the optics industry and the National Synchrotron Light Source has procured a considerable number of mirrors and is having them polished for use on the VUV storage ring. At the time of writing one mirror has been completed and evaluated and we give the results for this and discuss the indications for the future. The important measurement of the r.m.s, height of the surface roughness has given a value of (3.4±0.9) .A using total integrated scattter of visible light at normal incidence.
1. Introduction
2. Indications for diamond turning
The problem of fabricating mirrors suitable for use with synchrotron radiation (SR) has been extensively documented [1-3]. Diamond turning [46] has been discussed as a possible approach, but has not been applied to any extent. The discussion of the merits and demerits of diamond turning has on occasion been quite heated and SR technologists have hesitated to become involved with a technology that did not seem to be fully established. However, recently, the number of operational diamond turning facilities has been increasing and so the number of operators who offer a contract machining service. During the latter part of 1979, it became clear that the National Synchrotron Light Source (NSLS) at Brookhaven would have a requirement for a considerable number of grazing incidence paraboloids for a plane grating monochromator (PGM). Three PGMs were planned so the mirror fabrication costs could be amortized over three instruments. The mirrors have now been fabricated by diamond turning and are in the process of being post-polished. The first units have already been completed and have been evaluated for figure and roughness. We report here the nature of the fabrication that was involved and the preliminary results of the evaluations.
The advantages of single point diamond machining are generally given as follows: 1) The machines are numerically controlled and thus can produce any figure with equal ease. 2) It is only slightly more difficult to manufacture several mirrors simultaneously leading to attractive cost savings for quantity production. 3) Good figure accuracy is normally produced. 4) The technique works well for metals and these lend themselves well to water or conduction cooling and to applications where high damage thresholds are important. The main disadvantage of diamond turning is that the machined surface still has the periodic groove pattern left by the cutting tool. This is of such an amplitude that the surface is not sufficiently smooth for optical work, especially at short wavelengths, and leads to considerable scattering of radiation. It thus becomes necessary to carry out the so-called "post-polishing" process. Designers are usually concerned about whether this can be done at reasonable cost and without harming the optical figure. For the NSLS PGM mirrors we found the advantages listed above extremely compelling and we are now in a position to quote our experiences with regard to the smoothness, figure quality and cost resulting from the post-polishing process.
* Work supported by the U.S. Department of Energy.
0029-554X/82/0000-0000/$02.75 © 1982 North-Holland
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M.R. Howells, P.Z. Takacs / Diamond turned mirrors
3. Description of the required mirrors Each PGM requires five different mirrors. One is a 2 ° grazing angle collimator of focal length 10 mm. Parallel light from this mirror is diffracted by the grating and then focused by a second mirror on to the exit slit. There are four focusing mirrors (see fig. 1), each with a different grazing angle corresponding to a different wavelength range. The collimator is large (300 mm long) and the focusing mirrors are much smaller (75 mm long). A more complete description of the geometry of the mirrors is given in table 1 and fig. 2. There is little difficulty in choosing a suitable supplier for these mirrors. One can see from table 1 that two of the set of five mirrors have Y0 values approaching 1 m. This coordinate is the radius of the turning circle and thus determines the size of the throat of the machine required to make the mirrors. Only one machine exists which can turn parts on a 1 m radius. This is the EXCELLO machine [7] at Union Carbide's YI2 Plant at Oak Ridge. The Y12 group was, therefore, chosen as vendor for the NSLS mirrors. Once we had received a detailed quotation, it became clear that we could buy a much larger number of mirrors than the three sets of five that we had immediate need f o r . We finally purchased twelve sets of five for a price only about 30% more than for one set. The production of fixtures, the programming and use of the machine and metrology are the primary non-recurring costs. The main Fig. 1. Views of a P4 mirror after diamond turning and before post-polishing. The tool marks are exaggerated by the lighting. The mounting pads are clearly seen.
Table 1 Description and geometry of the mirrors. Identification Incidence angle (deg) Focus/vertex distance (ram) Focal length (ram) Xo (ram) ]To (ram) Wavelength range of use (,~) Permitted r.m.s, roughness (,~) Permitted slope error (per em) Maximum depature from the ideal surface of best fit
Pl 86.0 4.866 1000.0 995.1 139.2 20-50 20 ~/5
P2 84.0 11.848 1084.4 1072.6 225.5 40-100 20 ~,/5
P3 78.0 50.594 1170.4 I119.8 476.0 70-300 30 ;k/5
P4 65.0 217.841 1219.7 1001.8 934.3 250-1500 30 ~/5
P5 88.0 12.180 10000 9987.8 697.6 20-1500 20 ~/5
X/2
X/2
~/2
0/2
~/2
M.R. Howells, P.Z. Takacs / Diamond turned mirrors LATHE SP~INDLE ETC.
/
J
to
I/
Fig. 2. Geometricallayout of the paraboloidalmirrors. A-plane alignment surface perpendicular to the x axis. B-flat reflector which can be inserted to auto-collimate light from the focus back to the focus.C-lathe fixtureto which mirrors are mounted. D-focus of parabola. E-parabola 3,2 =4ax. F-flat mounting pads at rear of mirrors.
recurring cost is electroless nickel plating. The blanks were supplied by Brookhaven. In round figures, the small mirrors were about $2k each and the large one about $4k. This leads to a total of $144k, or $48k for each of the three PGMs. Figuring the cost of a PGM, including all ancilliary equipment except the computer, the mirrors amount to about a third of the total.
4. Mechanical design questions The mechanical design of the mirrors was determined in consulation with the fabricators. The method of mounting was one which had been used successfully with much larger mirrors [8]. Some of the points at issue are as follows: 1) The mounting is by small, flat pads, captured by 1///4-20 screws tightened to 40 in. lbs. 2) Mirror thickness is very generous to avoid deformations due to centrifugal forces, mounting screws [9] or residual stresses in the material. 3) The material used for the blanks was 6061-T6 aluminium alloy. A plated surface layer (5-10 mils) of electroless nickel was applied to make the final surface. The aluminium substrate was first diamond machined, the nickel layer was then applied and it, too, was diamond machined. This procedure was intended to prevent "through-print"
253
of substrate irregularities into the final surface. The amorphous nickel layer is necessary because it is the only material which diamond turns well which will also take a good superpolish. 4) Heat treatment [10] was (a) normal stress relieving before machining the substrate; (b) 12 h at 200°C before final machining of the nickel layer. The latter was based on the idea that the mirrors should see temperatures higher than any operational temperature (e.g., 120°C bakeout) before final machining. However, it is higher by about 25°C than the recommendation of the polishers and may have caused some hardening of the nickel layer. 5) A special alignment surface was machined as shown in fig. 2. It was set to be perpendicular to the direction of the parallel light, i.e., perpendicular to the symmetry axis. The idea was to permit alignment by auto-collimation from that surface. The ease with which such a surface can be included is part of the advantage of this method of fabrication.
5. Metrology From fig. 2 one can see that by installation of a suitable flat retro-reflector one can use an equal path interferometer located at the focal point and thereby carry out metrology whilst the part is still on the machine. This procedure is used to determine errors in the optical surface so that the machine program may be adjusted to correct them. Interferograms have been supplied by the Y12 group for all of the five mirror types. In each case, two types of measurement have been made: (1) a fringe pattern taken as described above; and (2) a pattern taken using a microscope at 200 × magnification. The first picture is taken at least twice with the fringes along and across the rectangular aperture of the mirror and is interpreted using the program F R I N G E [11]. The results of this computation give a measure of the deviation of the wavefront returning back to the focus, from the proper spherical shape. This wavefront has been reflected twice from the mirror and once from the reference flat so the fringes each represent a quarter of a wavelength (1//4× 6328.A). Exactly parallel, equi-spaced fringes would indicate a perfect wavefront. Fig. 3 shows some sample interferograms. Table 2 summarized the data and IV. VUV OPTICS
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Fig. 4. Y12 interferograms of the surface of a P4 mirror. Magnification ×206, wedge factor 0.273 # m per fringe. Estimating the fringe ripples to be of amplitude 1/5 of a fringe we find a value of around 500 ,~ for the groove depth.
their reduction by means of FRINGE. The mirror contour data are derived from the wavefront data by inserting a cos 0 factor to allow for foreshortening. The second type of measurement is intended to give an idea of the depth of the tool marks. The fringes run at right angles to the tool grooves which appear as ripples on each fringe. The depth of the groove can be derived from the known wedge factor of the fringes. For the example shown in fig. 4, one derives a value of about 500 ,~ for the groove depth. The groove spacing is known from the feed rate of the machine to be 10#m.
6. Post-polishing Fig. 3. YI2 interferograms of a P4 mirror after diamond turning. (0.158/~m per fringe). The fringes are contours of equal wavefront error.
Once the mirrors had been manufactured and characterized, post-polishing could begin. We had
Table 2 Data reduction by the F R I N G E program. Mirror type
R.m.s. wavefront variations a (waves b)
Peak-to-valley wavefront variations (waves)
Peak-to-valley mirror contour errors (waves)
PI P2 P3 P4 P5
0.0052 0.015 0.018 0.056
0.029 0.080 0.10 0.27
0.42 0.76 0.48 0.64
a These are all averages over about four interferograms. b A=6328 ,~.
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Fig. 5. Interferograms taken at Brookhaven. (a) gives independent confirmation of the equivalent Y12 picture. (b) shows the same picture after post-polishing. The degradation of the surface is quite small and is limited to areas outside the required clear aperture. some advance indication that we could find willing opticians to do this, but the post-polishing of exotic d i a m o n d turned surfaces [12,13] is a sufficiently new business that we should mention that bids were received from three optical houses: Applied Optics Center [14], F a b - O p t [15] and University of Arizona, Optical Sciences Center [16]. The low bidder was Applied Optics Center and this Fig. 6. Nomarksi micrographs before and after post-polishing. The analysis of these is discussed in the text. IV. VUV OPTICS
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company is presently working on our mirrors. The price for polishing the 300 X 50 mm 2 mirrors is $690 each. The first mirror has been polished and evaluated for both figure and roughness with extremely reassuring results. Fig. 5 shows interferograms taken at Brookhaven before and after polishing. We notice that although they are different they are both indicating a good surface within about a fringe. This is particularly so considering that the size of this mirror surface is 75 X 50 mm 2 and the size of the required clear aperture is 55 X 40 mm 2. The effect of edge roll-off is, therefore, entirely outside the clear aperture. It is also gratifying to note that the Brookhaven interferograms are in good agreement with those of both the Y12 group and Applied Optics Center, thus giving an independent check on the metrology of all three optical groups. Perhaps the most important evaluation of all is the roughness measurement. This was carried out at the laboratory of Dr. J. Bennett at the Naval Weapons Center in China Lake. The method used was total integrated scatter (TIS) of visible light at normal incidence. This method as applied at China Lake is sensitive to spatial wavelengths in the range 0.6-12.0 #m, in other words, to the so-called "micro-roughness." The r.m.s, height (o) of the roughness is derived from the TIS by use of the relation: TIS=l-exp
[(4~rsin0°) ~
2]
~
(4~rsinO°) 2
The TIS was measured over a uniform mesh of 33 points covering the whole surface. The value of o in each case was derived and the average value of these 33 results was: o ----(3.4 4- 0.9) ,~. This is an excellent result. The worst of the 33 figures was less than 6.5 A. This surface was one of the best metal surfaces ever tested at China Lake.
changes in surface slope. Height changes of a few AngstrOms are said [17] to be detectable. Fig. 6a shows the surface topography of an unpolished P4 mirror as received from Oak Ridge at a magnification of X400. The regular pattern of horizontal tool marks dominates the frame. The strong contrast evident between the tool marks enhances the vertical structure of the surface profile and is a qualitative supplement to the micro-interferograms in fig. 4, from which peak-to-valley heights of about 500.A were derived. Fig. 6b is a Nomarski micrograph of a polished P4 mirror at X 160 magnification. It is essentially featureless, confirming that the polishing process indeed removed most of the tool marks. C.lose examination of the original print reveals an occasional shallow scratch mark and remnants of the bottoms of tool marks. Also evident upon close inspection are low-contrast, circular "saucer-shaped" patterns [20] scattered over the surface, each on the order of 50 # m in diameter. These are probably the "plateaux" identified by Lindsey and Franks [19], formed by through-print in the plated coating as a result of preferential growth structure on substrate topography or coating inclusions. It is known that applications such as the PGM will be sensitive to scattering by spatial wavelengths longer than those probed by TIS, i.e., ripple. This is not unusual. Ripple is significant even in many visible light instruments. The problem is that there is no good method for measuring ripple which is cheap enough to be part of the normal manufacturing and inspection process. Consequently, ripple is commonly omitted from the characterization even of quite expensive optics. We propose to address this problem in a limited way be arranging for profilometer [18] measurements of our otherwise well characterized focusing mirror. This will add a small amount of data to present knowledge of the ripple properties of the various diamond turning facilities around the country. We will report the results of the profilometer measurement in a later publication.
7. Further evaluations
In order to get an improved feeling for the effects of the tool marks and the post-polishing process, we have taken a range of before and after Nomarski micrographs. Some of these are shown in fig. 6. The Nomarski approach is largely a qualitative one but it is extremely sensitive to
8. Conclusion
We have described the fabrication of some specialized optial surfaces for use with SR. Progress so far an reasonably comprehensive evaluations seem to indicate that the optics have been pro-
M.R. Howells, P.Z. Takacs / Diamond turned mirrors
d u c e d close to specification a n d in some cases b e t t e r t h a n specification. This leads us to believe t h a t d i a m o n d t u r n i n g plus p o s t - p o l i s h i n g is a cost effective a p p r o a c h that c a n meet the special needs of S R technology. T h e a u t h o r s wish to r e c o r d their a p p r e c i a t i o n of the helpful a n d p r o f e s s i o n a l a p p r o a c h of the staff m e m b e r s of U n i o n C a r b i d e , Y12 D i a m o n d T u r n ing G r o u p , A p p l i e d O p t i c s C e n t e r a n d the O p t i c a l E v a l u a t i o n F a c i l i t y at C h i n a Lake. W e also wish to a c k n o w l e d g e a n u m b e r of useful suggestions b y Dr. R.E. Parks.
References [1] V. Rehn and V.O. Jones, Opt. Eng. 17 (1978) 504. [2] Workshop on X-ray instrumentation for SR research, Stanford (1978) SSRL Report 78/04, eds., H. Winick and G. Brown. [3] M.R. Howells, Appl. Opt. 19 (1980) 4027. [4] Precision machining of optics, Proc. SPIE (1976) 93. [5] Precision machining of optics, Proc. SPIE (1978) 159. [6] Opt. Eng. 17 6, (1978) 570, first eight papers. [7] H.L Gerth et al., Proc. SPIE 159 (1978) 93.
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[8] S.C. Robinson, H.L. Gerth and J.E. Stoneking, Optical fabrication and testing Workshop, Tucson, Arizona (1979) XIX, p. 17. [9] C. Denny, W.J. Spawr and R.L. Pierce, Proc. SPIE 181 (1979) 84. [10] W. Caithness, Proc. SPIE 65 (1975) 8. [11] J. Loomis, FRINGE 3, by University of Dayton, Ohio, Research Institute. [12] P.C. Baker and N.J. Brown, Opt. Eng. 17 6, (1978) 595. [13] S.R. Lange and R.E. Parks, Proc. SPIE 257 (1980) 169. [14] Applied Optics Center Corp., 10 'B' SR Burlington, MA 01803 (617) 273-0309. [15] Fab-Opt. 6206 Paseo Santa Cruz, Pleasanton, CA 94566 (415) 462-2857. [16] Optical Sciences Center, University of Arizona, Tucson, AR 85721 (602) 626-2749. [17] H.E. Bennett, Proc. SPIE 184 (1979) 153. [18] J.M. Bennett and J.H. Dancy, Appl. Cpt. 20 10, (1981) 1785. [19] K. Lindsey and A. Franks, Proc. SPIE 163 (1979) 46. [20] These patterns cover a very limited portion of the total area and contain spatial wavelengths within the range probed by the total integrated scatter measurement. It thus appears that at least for the present mirrors they are not affecting performance. Nonetheless, it would be desirable to eliminate them and, in the opinion of the authors, this would probably be achieved by using a purer aluminium alloy than 606 I-T6.
IV. VUV OPTICS