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Sensing & Measurement

Optical fiber interferometer for potentially fast on-line surface measurement

Fast surface-profile measurements have been conducted with an uncertainty of just 4.1nm and no active noise compensation.
4 May 2007, SPIE Newsroom. DOI: 10.1117/2.1200704.0625
The broad use of micro-electro-mechanical systems (MEMS) has highlighted one of the most critical problems of micro- and nanoscale manufacturing: a need for fast surface measurement for quality control on the production line. It has been reported that currently the quality of fabrication depends largely on the experience of the engineer through an expensive trial-and-error approach (hit and miss). As a result, most of the items manufactured suffer from high scrap rates (low yields).
Optical on-line and in-process surface measurement has been continuously reported by many research groups around the world over the last thirty years. Most of these are based on light-scattering techniques1–4 that can provide a qualitative evaluation of surface texture or detect defects with micro-level features. Their main drawback is that they cannot be used to quantitatively assess the sub-micron features of surface texture with any sort of traceability. To get around these problems, we previously developed a multiplexed fiber interferometer for on-line surface metrology.5
More recently, to further eliminate the phase noise in the original fiber interferometer, we proposed an optical fiber interferometry system with an improved configuration. The basic idea of the proposed system is to replace traditional mechanical stylus scanning with optical beam scanning to realize fast and long-range profile measurements. The system composed of an on-machine optical dispersive probe and a remote opto-electro system as illustrated in Figure 1. Light from a tuneable laser is coupled into an optical circulator and is then collimated by a graded-index lens. The light is passed through a phase grating in the probe and is projected onto the surface to be measured: it is then reflected and collected by the optical probe. The measurement signal of this optical interferometer is obtained by interference between the zeroth-order and first-order diffracted beams. Wavelength changes in the tunable laser induces first-order diffraction angle changes on the phase grating, and produces the spatial scanning, whilst the zeroth-order diffracted beam remains at the same position.

Figure 1. A schematic diagram of the proposed surface measurement system: the interference (which produces the measured signal) occurs between the zeroth and first-order diffracted beams.
In order to ascertain the accuracy of the setup, a piezoelectric transducer was used to displace the reference mirror in 50nm steps over a 250 nm range. The profile of the mirror surface was measured after each step. The measurement beam was scanned over the surface of a flat mirror by sweeping the laser wavelength between 1560nm and 1595nm, which translates to a lateral scan width of 1.89mm across the mirror. The difference between each consecutive step was then taken and was plotted in Figure 2(a).

Figure 2. Results of the repeatability experiment. The surface was measured five times by sweeping the laser wavelengths between 1560nm and 1595nm, which corresponds to 1.89mm scan range across the surface: (a) raw data and (b) data with DC offset removed.
Figure 2 (b) shows the average displacements of each consecutive step and the linear fit for the data: the standard deviation is 4.1nm. It is assumed that the variation between the results is due to temperature drifts, as there was an approximately 15s time lapse between each measurement.
In conclusion, a surface measurement method with a near-common-path configuration has been introduced. The beauty of the measurement system lies in the implementation of the zeroth order diffracted beam as the reference for the interferometer, so that it remains still when the measuring beam scans the surface. Most of the environmental noise in the fiber is eliminated without any servo control, as there is no beam splitting across the fiber.
Current research is focused on designing a complete optical-electronic system to further reduce the environmental noise, providing dynamical compensation for the optical system, and trying to provide a new measurement instrument for fast on-line micro/nano-scale surface measurement.

Xiangqian Jiang
Centre for Precision Technologies,
University of Huddersfield
Huddersfield, UK
Dr. Xiangqian Jiang is a professor of Precision Metrology. She has been involved in optical instrumentation for more than fifteen years, doing work that has included phase-grating, optical-fiber, and optical-chip interferometry for micro- and nanoscale surface measurement.