Renaud Jourdain

Plasma surface figuring of large optical components

Fast figuring of large optical components is well known as a highly challenging manufacturing issue. Different manufacturing technologies including: magnetorheological finishing, loose abrasive polishing, ion beam figuring are presently employed. Yet, these technologies are slow and lead to expensive optics. This explains why plasma-based processes operating at atmospheric pressure have been researched as a cost effective means for figure correction of metre scale optical surfaces. In this paper, fast figure correction of a large optical surface is reported using the Reactive Atom Plasma (RAP) process. Achievements are shown following the scaling-up of the RAP figuring process to a 400 mm diameter area of a substrate made of Corning ULE®. The pre-processing spherical surface is characterized by a 3 metres radius of curvature, 2.3 μm PVr (373nm RMS), and 1.2 nm Sq nanometre roughness. The nanometre scale correction figuring system used for this research work is named the HELIOS 1200, and it is equipped with a unique plasma torch which is driven by a dedicated tool path algorithm. Topography map measurements were carried out using a vertical work station instrumented by a Zygo DynaFiz interferometer. Figuring results, together with the processing times, convergence levels and number of iterations, are reported. The results illustrate the significant potential and advantage of plasma processing for figuring correction of large silicon based optical components.

Reactive Atom Plasma for Rapid Figure Correction of Optical Surfaces

Nanometre-scale figuring technique at atmospheric pressure for large optical surfaces is a high profile research topic which attracts numerous competing state-of-the-art technologies. In this context, a dry chemical process, called Reactive Atom Plasma (RAP), was developed as a prospectively ideal alternative to CNC polishing or Ion Beam Figuring. The RAP process combines high material removal rates, nanometre level repeatability and absence of subsurface damage. A RAP figuring facility with metre-scale processing capability, Helios 1200, was then established in the Precision Engineering Centre at Cranfield University. The work presented in this paper is carried out using Helios 1200 and demonstrates the rapid figuring capability of the RAP process. First experimental tests of figure correction are performed on fused silica substrates over 100 mm diameter areas. A 500 nm deep spherical hollow shape is etched onto the central region of 200x200 mm polished surfaces. The test is carried out twice for reproducibility purposes. After two iterative steps, a residual figure error of ~16 nm rms is achieved. Subsequently, the process is scaled up to 140 mm diameter areas and two tests are carried out. First, the developed algorithm for 500 nm deep spherical hollow test is confirmed. Residual deviation over processed area is ~18 nm rms after three iterations. Finally, a surface characterised by random topography (79 nm rms initial figure error) is smoothed down to ~ 16 nm rms within three iteration steps. All results presented in this paper are achieved by means of an in-house developed tool-path algorithm. This can be described as a staggered meander-type tool motion path specifically designed to reduce heat transfer and consequently temperature gradient on the surface. Contiguously, classical de-convolution methods are adapted to non-linear etching rates for the derivation of the surface scanning speed maps. The figuring procedure is carried out iteratively. It is noteworthy that iteration steps never exceed ~7 minutes mean processing time.

Fast figuring of large optics by reactive atom plasma

The next generation of ground-based astronomical observatories will require fabrication and maintenance of extremely large segmented mirrors tens of meters in diameter. At present, the large production of segments required by projects like E-ELT and TMT poses time frames and costs feasibility questions. This is principally due to a bottleneck stage in the optical fabrication chain: the final figuring step. State-of-the-art figure correction techniques, so far, have failed to meet the needs of the astronomical community for mass production of large, ultra-precise optical surfaces. In this context, Reactive Atom Plasma (RAP) is proposed as a candidate figuring process that combines nanometer level accuracy with high material removal rates. RAP is a form of plasma enhanced chemical etching at atmospheric pressure based on Inductively Coupled Plasma technology. The rapid figuring capability of the RAP process has already been proven on medium sized optical surfaces made of silicon based materials. In this paper, the figure correction of a 3 meters radius of curvature, 400 mm diameter spherical ULE® mirror is presented. This work demonstrates the large scale figuring capability of the Reactive Atom Plasma process. The figuring is carried out by applying an in-house developed procedure that promotes rapid convergence. A 2.3 μm p-v initial figure error is removed within three iterations, for a total processing time of 2.5 hours. The same surface is then re-polished and the residual error corrected again down to λ/20 nm rms. These results highlight the possibility of figuring a metre-class mirror in about ten hours.

Initial Strategies for 3D RAP Processing of Optical Surfaces Based on a Temperature Adaptation Approach

The new Reactive Atom Plasma (RAP) facility Helios 1200 at Cranfield Precision Engineering Centre (UK) has a unique material removal rate and proven nanometre level repeatability. Thus, it incorporates high potential capability for ultra-precise, cost-effective figuring of large specular surfaces. In this paper, experimental results concerning substrate temperature and time dependence of the removal rate are presented, these constituting a fundamental part for an in-process temperature adaptation required to “steer” material etching. In particular, the etching process is believed to follow an Arrhenius’ type law and the removal rate is assessed in the range of 20-105 °C. The plume footprint is characterised to allow classical de-convolution methods (Lucy-Richardson, Van Cittert) to be later investigated to compute initial dwell-time maps and tool-path algorithms for free form figuring. Attempts to implement the process through thermal effects compensation techniques are presented. Those experimental results are analyzed and discussed. Characterisation of pre- and post-processed substrates is performed using phase-shift interferometry for shape assessment and white light interferometry for surface topography measurement. rms-Roughness of 3-4 nm results after neutral removal rasterings on synthetic fused silica substrates over a 70x200 mm area, with depths ranging from 100 to 200 nm.

Pre-Stressed Piezoelectric Bimorph Micro-Actuators Based on Machined 40-Micron PZT Ceramic Thick Films—Batch Scale Fabrication and Integration With MEMS

The projected force-displacement capability of piezoelectric ceramic films in the 20–50μm thickness range suggests that they are well suited to many micro-fluidic and micro-pneumatic applications. Furthermore when they are configured as bending actuators and operated at ∼1V/μm they do not necessarily conform to the high-voltage, very low-displacement piezoelectric stereotype. Even so they are rarely found today in commercial micro-electromechanical devices, such as micro-pumps and micro-valves, and the main barriers to making them much more widely availability would appear to be processing incompatibilities rather than commercial desirability. In particular, the issues associated with integration of these devices into MEMS at the production level are highly significant and they have perhaps received less attention in the mainstream than they deserve. This paper describes a fabrication route based on ultra-precision ceramic machining and full-wafer bonding for cost-effective batch-scale production of thick film PZT bimorph micro-actuators and their integration with MEMS. The resulting actuators are pre-stressed (ceramic in compression) which gives them added performance, they are true bimorphs with bi-directional capability and they exhibit full bulk piezoelectric ceramic properties. The devices are designed to integrate with ancillary systems components using transfer bonding techniques. The work forms part of the European Framework 6 Project ‘Q2M - Quality to Micro’.Copyright

UK Developments Towards Rapid Process Chains for Metre Scale Optics

The demand for large scale optics is increasing. The need has been driven by major science programmes, next generation lithography systems, space based earth orbiters and solar energy generating systems. A previous low requirement for precision optics of over 350mm size meant their effective mass production was neither emphasized nor established, consequently prices remained unduly high. This paper introduces some UK developments in support of establishing an effective production line for fabricating optics in the metre scale range. A target production brief was to produce 1 metre sized optics of 10nm RMS form accuracy with 3 discrete process stages each taking less than 10 hours. This paper reports progress against this goal.