Air bearing stage increases throughput

Paul Boughton

A new PlanarHD air bearing stage from Aerotech features several design enhancements for increased throughput in ultra-high-precision step-and-settle and scanning applications such as semiconductor processing and emerging MEMS/nanotechnologies.

The 500 x 500mm travel stage includes larger air bearing surfaces for improved dynamic characteristics as well as higher-power linear servo motors on both axes to deliver an impressive specification that includes a 2m/s scan velocity and peak acceleration to 5 'g' - with a positioning resolution of up to 0.25nm, repeatability to 50 nm and accuracy to +/- 300nm.

For Aerotech, this new product follows a long history of motion systems design and manufacture that addresses the high-end equipment industry's unrelenting requirements for improved production times and finished component precision.

Aerotech estimates that the PlanarHD will yield a 50 per cent increase in throughput when compared to other manufacturers' stages that have a typical scan velocity of 500mm/s and 0.5 'g' acceleration over a 300mm constant travel range. The 500mm travel range is also a future-proof investment for 450mm diameter wafer production.

The PlanarHD employs a low-mass structural bearing design with a balanced air-on-air preload system for higher lateral, vertical and angular stiffness, resulting in improved speed, acceleration and accuracy characteristics. Nominal payload is 5kg to maintain dynamic specifications with an upper limit of 30kg. The H-bridge design utilises dual air-bearings, dual linear motors and dual linear encoders on the step axis - and in conjunction with Aerotech's A3200 control system, a calibrated orthogonal/yaw control of 1arc-sec is possible for parallel scans or process swaths over the entire surface of a wafer.

A low-profile H-bridge design features the scan and step axes centrelines and all encoders at the same level, effectively co-locating the centre of mass, the force centre and the feedback centre. This ensures a high dynamic capacity that combines with the optimised air bearing preload system for a 330Hz first natural frequency and a high servo bandwidth. Furthermore, as all encoders are placed very close to the working plane, Abbe errors are minimised for improved stage precision.

Optional laser interferometer feed back is also available and the 500mm travel range may be increased up to 1.2m while still maintaining comparable performance and precision levels. As part of Aerotech's comprehensive engineered systems capability, the PlanarHD can be delivered with matching rotary and/or Z axis stages and the aluminium/granite stage construction may be replaced with more exotic materials such as ceramics, invar, stainless, and titanium for further improved performance.

The PlanarHD makes full use of Aerotech's BLM series brushless linear servomotors that feature an ironless forcer with zero cogging for very smooth motion and jitter-free in-position stability. BLM series linear motors are claimed to generate the highest force per unit volume of any competitive linear motor design and, to further improve dynamic performance, both air- and water-cooled thermal management options are available.

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