Temperature-controlled ellipsometry: how a collaboration between Linkam and Woollam provides a novel solution

In addition to being a very effective standalone tool for thermomicrometric sample analysis, Linkam stages are often used to add temperature and environmental control to a range of complimentary techniques, from fluorescence microscopy to Raman spectroscopy, and from X-ray diffraction to ellipsometry. Each of these techniques requires careful experimental planning to ensure that the sample can be analysed correctly with the two devices working in unison, and at Linkam we regularly work with other instrument manufacturers to provide the market-leading temperature control solution across all of these techniques. Here, we take a deeper look at one technique in particular – ellipsometry – and how our collaboration with ellipsometry experts J.A. Woollam enables precision temperature and environmental control with a range of Woollam ellipsometers.

Ellipsometry measures the change in polarisation state of a light beam reflected or transmitted from a material structure. This measurement is used to characterise the thin film and bulk material properties, with layer thickness and refractive index being the most common application. Ellipsometry is also applied to characterise composition, crystallinity, roughness, doping concentration, and other material properties associated with a change in optical response.

The application that put ellipsometry “on the map” was its ability to measure nanometre-scale layers used in microelectronics, but the use of this technology has since expanded to research in physical sciences, semiconductor and data storage solutions, flat panel display, communication, biosensor and optical coating industries.

(a) Illustration of elliptically polarised light as used by an ellipsometer and (b) output from a temperature-controlled ellipsometry experiment, showing the coefficient ‘k’ as a function of temperature

Phase transitions and transition temperatures can also be observed by monitoring temperature-dependent optical properties. J. A. Woollam customises its ellipsometers according to the needs of individual users, for example by integrating temperature-controlled modules. This is particularly important during the R&D process so researchers can better understand how their materials behave as a function of temperature. Jeremy Van Derslice, of Woollam’s Applications Group, describes the considerations this requires: “When our customers are using ellipsometry for R&D, they themselves are usually working on something brand new, often requiring uniquely configured instrumentation. We do our best to accommodate customisation requests from our customers and have built a business around that concept.”

J. A. Woollam and Linkam began discussing possible projects in 2013, before entering into a full collaboration in 2014. One of the strengths of combining the expertise from each company is providing thin film researchers with personalised temperature-controlled ellipsometers. Jeremy comments on this value: “Based on the modular nature of our product line and requests from our customers, we often ask Linkam to modify a configuration.”

Linkam HFSEL600 temperature control stage mounted on the Woollam M-2000

The modular design of J. A. Woollam ellipsometers facilitates the delivery of these novel configurations, and its collaboration with companies such as Linkam allows thin film researchers to test their materials in new ways. Linkam’s HFSEL600 temperature control stage is based on our standard ellipsometry stage, the THSMEL600. The HFSEL600 allows temperature-dependent ellipsometry measurements with all J. A. Woollam systems, such as the M-2000, RC2 and IR-VASE instruments. The heating stage is a self-contained system with optical windows for ellipsometry measurements at an angle of incidence of 70°. It may also be operated as an open system with other angles of incidence. Jeremy comments on the integration of these systems:

“There are many applications where it becomes critically necessary to measure optical properties at different temperatures in real time. For example, materials used on satellites launched into space where they are expected to function over a wide range of temperatures, or in silicon photovoltaic panels where the quantum efficiency is temperature dependent. The HFSEL600 temperature-controlled ellipsometer stage from Linkam provides the ideal solution.”

The HFSEL600 is completely integrated into the J. A. Woollam CompleteEASE ellipsometry software package, allowing the user to save the respective temperature with each ellipsometry measurement, and can be used in the model as a set point for the corresponding optical constants. “Linkam has continued to work with us and our customers in adapting their temperature-controlled stages to suit the J. A. Woollam ellipsometers – we ask a lot of them in terms of modifying their solutions, and they do an excellent job of accommodating our requests. Linkam’s products are highly reliable, which is something we were seeking in order to maintain the integrity of our overall package.”

Combining ellipsometers with Linkam’s HFSEL600 heating stage for highly precise and accurate temperature control, provides the ideal instrumentation to characterise these versatile materials. Duncan Stacey, Linkam’s Sales and Marketing Director, comments: “As with many of our collaborations, the initial discussion with Woollam was around our standard product. It soon became clear that due to the flexible nature of J. A. Woollam’s ellipsometry systems, we could offer a much better solution for the customer by developing a customised solution. We brought our R&D teams together, and even though working on different sides of the Atlantic, we were quickly able to develop a product that fully met their needs. Both companies pride themselves on providing high performance and quality solutions to their customers. This has made the collaboration particularly easy and fruitful.”

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