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Enabling positioning technology

18 March 2010


One of the many varied and interesting research projects at the University of Southamptons ORC (Optoelectronics Research Centre) involves direct UV laser writing of integrated optical circuits on specialised glass substrates and on its own patented flat optical cable. With application potential from telecom network components through to lab-on-a-chip (LOC) biological and chemical sensors, the photonic circuits are produced with the help of nanometre level precision positioning systems and motion controls from Aerotech.

The types of optical components that are typically written into the substrates include curved or straight waveguides and splitters that channel light around the circuit; and Bragg gratings which can be used to measure the refractive index of fluids or other materials. These refractive index sensors are now built into complete optical microchip solutions for bio/chemical sensing by the ORC spin out company Stratophase Ltd. The basis principle involves passing a sample material over a sensing window and the wavelength of the light reflected from the Bragg grating confirms that its refractive index is within a certain band. With a typical sensitivity of one part in a million, these devices are proving an excellent solution for a wide range of commercial bio/chemical detection applications from sugar concentrations in foods, to toxin, virus and bacteria discovery for pharmaceuticals, and petro-chemical quality control processing.

When a similar UV writing process is directly applied to the special flat optical cable which is manufactured at the ORC, the potential for these types of sensors may be produced without the need to connect high loss pigtail connections between optical fibre and optical material substrates - and may be applied for multiple sensor solutions over extremely long distances for applications that could include biological and chemical monitoring in supply pipes and rivers. The work in this area is at an early stage but the potential outcome using such techniques may also lead to the holy grail of producing complete active switching circuits for optical computing.

As opposed to more traditional photolithography mask processing methods for these types of devices, the direct write approach benefits from single-step integration and is consequently much faster and very adaptable for rapid prototyping/short production runs at reasonable costs. The basics of the direct writing process involves modifying the core layer material by manipulating the substrate under the focussed UV laser, so clearly the precision and dynamic performance of the positioning system is fundamental and an intrinsic enabling technology for the success of the process.

The high power UV laser is directed through an interferometer to produce an intense dual beam interference pattern at the target area. During the writing process, its focal length is held constant and the overall positioning system flatness, derived from pitch, roll, yaw and Abbe errors from both X and Y axes, must maintain sub-micron tolerances. Furthermore, the writing process needs to be continuous as the core materials are adversely affected by heating, so the control of the UV lasers’ power and firing pattern is completely synchronised with the on-the-fly compound translation of the X and Y axes. This synchronisation is another key factor for the process and is carried out within the Aerotech controller by an advanced software feature called PSO or Position Synchronised Output.

PSO triggers the UV laser in real time during continuous motion, processing high speed encoder feedback in conjunction with XY position array information for the particular feature being written. In this way, not only is each feature held to nanometre level tolerances in all planes, but the accuracy and repeatability from feature to feature and the distance between them is also realised with fantastically high precision.

The positioning system used at the ORC is an Aerotech ABL9000 series air bearing stage complete with the A3200 Digital Automation Platform motion control system with FireWire networked Ndrive linear stage amplifiers. With a 300 mm x 300 mm travel range, this brushless linear servomotor driven, ultra-high precision stage features a balanced air-on-air preload system for maximum stiffness and glass scale encoder feedback with an encoder resolution of just one nanometre. The H-bridge design maintains both axes and their respective encoders at the same level which co-locates the centrelines of mass, force and feedback - effectively ensuring that errors are minimised for best performance straightness and flatness. With dual linear motor driven Y axes and active yaw control, the ABL9000 is able to produce perfectly parallel scans, straight lines and interpolated curves over its entire working surface. Each axis is also factory calibrated by Aerotech against a laser interferometer with the error map information added to the controller. In operation this ensures optimum translation stage accuracy.

The combination of Aerotech’s BLM series brushless linear servomotors and HLe series linear servo amplifiers provide the smoothness and stability required for nanometre resolution positioning whilst ensuring the required high bandwidth, zero crossover distortion and ultra-quiet EMC characteristics which are essential for this application.

Aerotechs A3200 Digital Automation Platform provides fully deterministic PC based motion control with advanced diagnostics, set-up and tuning features that ensure perfect results. As well as the PSO feature mentioned earlier, the A3200 includes trajectory generation with multi-block look ahead to minimise geometry errors in tight profiles by regulating the speed and/or the position on each axis. The ORC use G-code programming for their particular application but the A3200 is also equipped for programming in Aerotechs own AeroBASIC TM or in C, C++, and Visual Basic. For complete application integration the optional Ncontrol SDK software development kit may be used with Windows based ActiveX components, C++, VB and .NET class libraries. There are also a number of advanced software and hardware features available to optimise throughput performance and further improve overall machine operation.

The complete system was supplied to the ORC on a customised granite base with an additional fixed bridge arrangement that provides a stable platform for the ORC supplied interferometer optical system. The scope of supply also included direct drive rotary and vertical lift stages with comparable resolution and precision that are used together for pre-alignment of the substrate. Full cable management was included in the design which greatly assisted commissioning at the ORC.

The excellent performance of the ABL9000 series has now been enhanced with Aerotechs Planar HD air bearing stage. The new stage is aimed at high throughput scanning and ultra-high precision step-and-settle applications found in semiconductor processing and other emerging MEMS/Nano technologies. The design features larger air bearing surfaces for improved load carrying and higher dynamic characteristics. A brief performance specification includes 2 m/sec scan velocity and peak acceleration to 5 g - with a positioning resolution of up to 0.25 nanometres, repeatability to 50 nanometres and accuracy to +/- 300 nanometres.

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