Micrometrology of difficult surfaces

Written by: Paul Fanning | Published:

Obtaining accurate 3D metrology from rough surfaces has been a problem – until now. Paul Fanning reports.

The ability to obtain accurate 3D metrology images from rough surfaces is offered by an innovative optical instrument developed by Zeta Instruments of San Jose, California, USA and could have implications in a number of industries. This delivers 3D metrology images from even the very lowest contrast samples with extreme polished surfaces.

The Zeta-20 (and Zeta-200) can handle surfaces with very high roughness and/or with high reflectivity variation. The instrument is being used to relieve metrology workloads on AFMs and SEMs at a much lower cost-of-ownership as compared with other optical and stylus profilers.

Ian Holton, scientific director of UK company Acutance, which is the UK distributor for this equipment, says of its potential applications: "It's been bought so far for surfaces that are very difficult optically, so microfluidics channels, solar cell polycrystals – metrology of those is very difficult with any other technique. Anywhere where there is micromachining for micro marks that actually need to be measured non-contact optically. This can do it where no other technique can."

Some sample types such as microfluidic channels that can be measured with a Zeta cannot be measured with any other type of instrument. Says Holton: "It's good for highly reflective surfaces with very low contrast. Likewise, the solar cell polycrystals are of extremely low reflectivity and very low contrast.

"With any other device that claims to do extended focus, the instrument tries to estimate where the height is and then decides which pixels are in focus and which are not – that's all very well if you have high sample contrast, but if there is very low sample contrast as in silicon nitride in solar cells, then the algorithm finds it very difficult to decide which is the correct height for any given pixel and puts in a number that is very likely wrong. So basically you get metrology with errors."

The Zeta equipment gets around all that simply by projecting a very fine light pattern that is in focus at the object plane. Then the software can decide unambiguously where the focus height is for each x/y pixel. Says Holton: "So what you get out are extremely high accuracy 3D maps of surfaces that you can't get in any other way."

Zeta's patented ZDot technique generates contrast from very low contrast samples such as textured solar cell wafers. This method solves inaccuracy problems experienced with instruments that merely use native sample contrast to generate high depth-of-field images, and at a fraction the cost of competing techniques such as confocal microscopy or interferometry.

Options include roughness measurement capability of better than 1nm by use of Nomarski/DIC optics, and a white light reflectometer that enables film thickness measurements. The metrology software is geared for ease-of use, providing height measurements with 70nm accuracy as well as volumetrics.

With a lateral resolution of 0.37 microns, this technique is very complementary to an SEM, allowing researchers to pre-screen samples rapidly and to select the correct sample set for advanced SEM imaging. Because no sample preparation is needed, there are no charging problems and results are delivered in the order of one minute, the Zeta profilers are being used to 'pre-filter' samples for AFM and SEM imaging in areas as diverse as optic-fibre, data-storage and compound semiconductor quality.

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