The international congress on optical science and engineering

P#t¢|$ Conference Reports 19-23 September 1988, Hamburg, FRG

The International Congress on Optical Science and Engineering Sponsored by the European Physical Society (EPS), the European Federation for Applied Optics (Europtica) and SPIE, the International Congress on Optical Science and Engineering comprised five programmes: • • • • •

Optical Sensing and Metrology, Optical Systems and Components, Optical Materials, Lasers and Laser Processing, Coherence and Imaging.

Within these programmes 21 topics were covered by nearly 700 papers. The most interesting sessions for Precision Engineering were 'Surface Measurement and Characterization' and 'Micromachining Optical Components and Precision Engineering'. Although, as in all these types of meetings, it was impossible to see every presentation, this conference report should give an overview of some of the sessions. The first paper in the Surface Measurement and Characterization session was presented by Jean M Bennett (Naval Weapons Center). It was a good general introduction giving an overview of recent developments and she covered six topics: • • • • • •

optical profiling, statistical analysis and comparison, optical scattering, defect detection, other optical methods and non-optical methods.

She also detailed the reasons why surface characterization is necessary: • • • • •

for reducing scatter, for inspection of surface cleanliness, for research, for quality control and for investigating whether the surface meets specifications.

However, despite her comprehensive presentation there was obviously not enough time to cover everything and this prompted her into saying that we had 'just scratched the surface' of surface measurement characterization. Ring laser gyroscopes (RLGs) are responsible for a great deal of work in the surface characterization fields as their mirrors have to be smooth. Mike Downs (NPL) described interferometric profile measurement of

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'supersmooth" surfaces: he had been asked to measure the surface roughness to 1 nm on ring laser gyro mirrors. To do this, he used his common path interferometer which he briefly described. It has a sensitivity to surface height of better than 0.01 nm and a surface wavelength range of 0.5 to 15/~m. He was also looking at non-resolvable features but through geometrical shading effects. Questions following this presentation dealt with the potential difficulties of using a confocal system. Mike Downs explained that it was not that difficult. Also, for lateral resolution a HeNe laser should be used. Tsuguo Kohno (Tokyo Metropolitan Institute of Technology) described a few functions of the HIPOSS (High Precision Optical Surface Sensor) and its applications. The principle of the HIPOSS is the focus detection of the critical angle. The system has two sensing ranges the precision range and the wide range. The precision range has a sensitivity of better than 0.2 nm rms with about 2 iLm measuring range. On the other hand with the wide range a relatively large measuring range of around 20/~m with 1 nm resolution can be achieved. A new three-dimensional non-contact laser interferometer microscope which uses computerized phase measuring interferometry to achieve subnanometer vertical resolution was described by Bob Smythe (Zygo). Laser illumination coupled with selectable incoherent and coherent illumination allows variable interference fringe ranges from a few micrometres to millimetres in length. He described the coated reference surface to eliminate multiple reflections, then detailed the Fizeau and Mireau arrangements and when to use them: Mireau with transparent objects when the back focus is unnecessary and Fizeau when great fringe depth is required. The applications for the system include diamond turned surfaces, thin transparent substrates and fibre optics. Finally, he proved that whatever man can do nature can do better, as he showed slides of the superb optics in a dragonfly's eye. The creature had accidently flown into his lab and unfortunately, died. The slides showing the quality of the optics in its eye were a remarkable conclusion to an excellent presentation, although the dragonfly was probably less than happy about itW A microprofiler for measurements of the surface features over long lengths should have been described by Tom Bristow, but instead was presented by Jean Bennett. She described the

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PR/¢|S instrument and presented a few profiles. It is based on the Nomarski principle. A few profiles were presented with lengths of 2 mm, 10 mm and 28 mm.

The enhancement of optical and non-contact profilers was covered by Jim Wyant (WYKO) who detailed Fizeau and Mireau interferometry and gave the advantages of white light over laser light: lower noise, few spurious fringes from optics, and the focus is easy to determine. Also, for steps and steep slopes where there is fringe ambiguity if multiple wavelengths are used then the colours of the fringes can define which fringes belong together. The distortions which practical measurement techniques introduce into finished measurements was described by Eugene Church (Brookhaven National Laboratory) while the limits of surface measurements were covered in two papers by Margaret Stedman (NPL). First she dealt with stylus instruments. She compared stylus and optical instruments and claimed that the stylus type were making a comeback. The history of the stylus instrument and its operation was traced up to the Talystep. Then she moved on to the scanning tunnelling microscope, which is certainly a stylus and almost a contact instrument. Finally she described and showed examples from the Nanosurf, a customized Talystep head with most of the rest of the system made from Zerodur. After coffee, Margaret Stedman gave her second presentation which complemented the first. It was entitled 'The limits of surface measurement by optical probes'. She reviewed the subject generally and discussed some particular optical instruments, covered their limits of dimension, slope, curvature, statistical limits and interpretation. The instruments she covered included the polarizing interferometer, the HIPOSS instrument, the Ennos-Virdee profilometer, a photographic sciences profiler, the Wyko-Topo and Zygo 5500 profiler. A comparison was made in a presentation by J R McNeil (University of New Mexico) of optical scatterometer and profilometer techniques for characterizing smooth surfaces. A number of samples having different microstructure properties were characterized using these techniques and results were discussed. In a good talk, R Brodmann (Rodenstock) compared light scattering from rough surfaces with optical and mechanical profilometers and explained how optical instruments can work in manufacturing processes. He went through the theory of scattered light interpretation. In comparison with the traditional stylus methods the main disadvantage of the light scattering technique is a lack of easy understanding and the more complicated interpretation of its results. He had with him a handpolished model of a space shuttle which was for use in a wind tunnel so that its roughness was proportional for its size.

PRECISION ENGINEERING

John Guerra (Polaroid) described the construction and components of a scatterometer. It is a very low cost system with off-the-shelf components and yet still offers excellent results. Large area silicon photo-voltaic cells are used for detection of the scatter and specular light from the sample. The thin cell profile permits placement of the scatter detector closer to the sample for minimal scatter obscuration. The large cell area accepts the blur circle from an inexpensive moulded acrylic dome for scatter collection. A dedicated pocket computer and printer calculates, displays and prints sample roughness average, and standard deviation for multiple measurements; it also controls laser user-access, prints a tutorial and identifies a sample, operator and date/time. The laser is a 2 mW HeNe (633 nm). The characterization of mirror surfaces, again for laser gyro applications, was described by Dirk-Roger Schmitt (DFVLR, Braunschweig). He presented some excellent Normarski differential interference contrast illustrations. It was demonstrated that the system described resolves an rms roughness of lower than 0.1 nrn. The total integrated scattering of the bare substrates was measured. For this purpose an Ulbricht integrating sphere with a specially designed diaphragm to block unwanted light scatter by the rear side of the transparent substrate was used. Investigations of different surfaces gave results for the total integrated scattering from 10 x 10 -6 to 3850 x 10 -6. The corresponding rms roughnesses are 0.19 nm and 3.61 nm respectively. He also had an excellent visual representation of a laser gyro, which used polarizers on the overhead projector. Following, Lars Mattsson considered the characterization of supersmooth surfaces by light scattering techniques. He pointed out that today even window panes are down to 0.2 to 0.3 nm rms roughness. This shows how well things can be polished. He went through the new F1048 ASTM standard test method for measuring the effective surface roughness of optical components based on total integrated scattering (TIS). The amount of scattering caused by the surface roughness is of primary interest for optical applications while the roughness itself is of great concern in the fields of micro-electronics and magnetic memory storage. One should, he said, watch out when doing conversions from rms roughness to expected TIS. The physical meaning of sub-angstrom roughness measurement by TIS was covered. On optical components after deposition with aluminium film roughness can increase. The amount of oxygen in the evaporating atmosphere is important: roughness increases with oxygen partial pressure. The best way, it was said, to clean samples is, in some cases, not to clean them. Examples were given of the strip coat test and ultrasonic cleaning. The discussion following the presentation dealt with the best way to clean silicon.

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On the second evening a special reception for all participants was held at the Hamburg City Hall, the Rathaus. This was hosted by the City of Hamburg, a representative of which gave a brief history of the city and the distinct impression that Hamburg is facing financial difficulties. Hence, he said, that evening's one free drink each was in no way a reflection of the regard the city felt towards the participants of the meeting. It is to be hoped that these participants helped Hamburg in some way while staying in the city. The next day, the specifications necessary for on-machine measurement of surface texture parameters and other factors affecting texture were covered by Lionel Baker (Sira). He described the various methods of measurement and then went on to describe a simple comparator and the type of surface measurements that can be made with the system. A very good, interesting presentation by John Guerra (Polaroid) detailed the theory and practice of a photon tunnelling microscope. This is a combination of frustrated total internal reflection microscopy with real-time scanned scene densitometry and restored 3D display. The light being refracted in a prism placed on a dielectric is affected by the surface of the dielectric sample. Where the gap between the prism and dielectric is small, photons can tunnel through. However, where the gap increases there will be no tunnelling and total internal reflection will take place. A resolution of approximately 0.4/~m has been obtained. The manufacturing and measuring of aspheres integrated in one ultraprecision machine was outlined by P H Knol (University of Bremen). He described the super Mikroturn CNC 0.1 /~m machine and gave its specifications. Keith Carlisle (Cranfield Precision Engineering) described the work of the ultraprecision machining laboratory ( U P M L ) and then went on to deal with ultraprecision air bearing spindles for grinding of ceramics. He detailed methods to eliminate, as much as possible, the thermal growth of the spindles and outlined other spindle errors. The design of a variety of ultraprecision air bearing workhead spindles was outlined and the efficient high speed metrological techniques for test and calibration of such spindles down to 1 nm resolution was described. In addition, a diamond turning machine being built at CUPE and a machine for off-axis grinding of large mirrors were also described. The various influences which can affect workpiece accuracy were described by T Bispink ( I P T / T H Aachen). Process, environmental and machine tool influences were all outlined. He compared the surface shapes and microstructure of produced workpieces. A correlation between the measurement results of the machine and the analysed workpiece surfaces was presented. Corrections of finish errors in diamond turned

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surfaces were detailed by Robert Parks (Optical Sciences Center, University of Arizona). He showed examples of surfaces before and after polishing and described a method of polishing which is in use now and which surprisingly was applied to the polishing of aluminium ten years ago. S G McCandlish (University of Exeter) went through the method of using an interferometer to look at a rotating surface. This was to monitor the spindle and workpiece on an ultraprecision single point diamond facing machine. It was not only looking at the surface, but also at axial motion a n d / o r tilt. In his paper entitled "Moir6 deflectometry - a ray tracing alternative to interferometry' E Keren (Rotlex Optics), instead of pointing out the benefits of his system, began by detailing the deficiencies of Fizeau interferometers. This did not seem to go down to well with those w h o use such instruments. However, the instrument he described was quite neat. In effect, it is two Ronchi rulings placed a distance apart. As the shadow of one falls on the other, fringes are formed and these are highly sensitive and can be tuned from the millimetric to the interferometric range. R Kriiger (PTB, FRG) described a problem at his laboratory. Measuring lasers and the lasers used as standards are kept in different places. It was therefore decided to connect them to one another rather than to carry them from building to building for frequency comparison and calibration. Therefore, the transmission line between the lasers must have a high efficiency, little feedback to the laser and it must preserve the polarization state of the input. He then went on to describe the design and realization of an optimized coupler for these gas lasers and the polarization preserving fibres. The principle of single-mode fibre connection was outlined as were methods of reducing reflected light from the fibre end faces. At the lab they managed to achieve a backscattered light of less than 10 -4 and an uncertainty of frequency comparison of ~ 10 -8. It was an interesting problem and a good talk. Super polishing of A-cut sapphire was detailed by O Weis (University at UIm, FRG) in a method to achieve atomically smooth surfaces with no damage layer below the surface. The method is relatively fast and is suitable for curved surfaces. The flatness is determined by pre-polishing but sapphire crystals were, it is claimed, polished with a residual surface roughness of less than 0.3 nm Rz measured with a talystep. Calibration of optical and mechanical sensors by x-ray interferometry was the subject of a talk by P Seyfried (PTB, FRG). He was using crystal lattices and x-rays as an interferometer. When the crystals are moved the lattice planes become misaligned and variations in intensity of the interfering x-rays occur. The fringes are proportional to the lattice planes. This method can be used for

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t#=¢15 calibration, as lattice planes have fixed known distances. A Modjarrad (Renishaw Metrology) detailed a laser scanning probe for coordinate measuring machines (CMMs) for scanning profiles at high speed. The system has a stand-off distance of 20 mm, a spot size of 25 #m, 1 /~m resolution and 10/~m resolution accuracy. It has a 4 mm measuring range and 50 readings s -1. The system uses the optical triangulation method and deals with speckle in a way which, unfortunately, he said he would not describe. However, it cannot cope with extremely reflecting or extremely absorbing surfaces. Applications for the device can be found in the automobile and aerospace industries while future

developments include an automatic probe changer with optical and trigger probes. The International Congress on Optical Science and Engineering was a good meeting with enough interesting papers in its programmes to satisfy most. Often, however, the problem was that by sitting in one session one was missing at least one other session that could have been interesting: a common problem with parallel session meetings. However, at least that is better than having nothing of interest whatsoever. There was no danger of that at this meeting. The next meeting will be 2 - 6 April 1990, again in Hamburg, FRG. PRW

2 4 - 2 7 October 1988, Atlanta, Georgia, USA

Third annual meeting of the American Society for Precision Engineering (ASPE)

Preceding the American Society for Precision Engineering (ASPE) Third Annual Meeting were eight precision engineering tutorials. Unfortunately, having had an extremely delayed arrival (14 hours late thanks to a certain airline), I was unable to attend any of them, but they were reportedly highly successful, useful and extremely popular, so much so that ASPE will be running similar tutorials at the combined ASPE/IPES5 meeting in September this year. The tutorials were: 1. Computer interfacing for precision engineering Robert J. Fornaro (NCSU) 2. Control of dynamic systems Thomas A. Dow (NCSU) 3. Basic diamond turning techniques Richard L. Rhorer (LANL) and Phillip R. Hannah (Moore Special Tool) 4. Surface finish metrology Theodore Vorburger (NIST) and Jay Raja (Michigan Technological University) 5. Air bearing design and applications Kenneth J. Stout (Coventry Polytechnic) 6. Principles of design for precision engineering E. Clayton Teague and Christopher Evans (NIST) 7. Vibration isolation Daniel DeBra (Stanford University) 8. Distance measuring interferometry David Parker ( Hewlett- Packard) Following these tutorials, Ray McClure presented

PRECISION ENGINEERING

the ASPE distinguished lecturer award to Milton Shaw, who then gave a lecture, imparting some of his experiences and wisdom to those involved in precision engineering. He spoke on 'Metal cutting principles pertinent to precision engineering' and explained the generation and measurement of surfaces of precise geometry and integrity. For those new to the field it was an excellent introduction covering conventional cutting, grinding, chemical action, and friction sawing. In the now traditional ASPE manner there were a few problems with the equipment--a difficulty which is not unheard of in precision engineering circles. At the second ASPE conference it has been the radio microphone. This time it was a personal microphone and its leads. The conference sessions started the following day with the session on dimensional metrology chaired by Clayton Teague. Although there is not room to cover all the papers with the depth deserved the following should give a flavour of what was a highly enjoyable, informative meeting. Norman Bobroff ( I B M ) gave the first paper on a problem which limits the accuracy of heterodyne interferometers: that is, non-linearities in the relation between the phase of the beat signal and the actual displacement of the measurement arm mirror. There are two methods for measuring the non-linearities. The first involves comparison with two independent interferometers while the second, an in situ technique, involves using the relationship between amplitude modulation and phase error to measure the non-linearity. Using active correction the

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