Temperature controller for Woollam ellipsometer

In the era of thin film technology, material and film properties have to be seen as temperature-dependent. Especially, since many materials show significant changes in the optical constants as function of temperature. This could be phase transitions in semiconductors or the transition from semiconducting to metallic phase as in vanadium oxide (VO2), the latter resulting in drastic changes in the optical and electronic properties.

Ellipsometric measurement
Fig. 1a: Ellipsometric measurements of polystyrene on silicon over several temperature cycles between 50 °C and 140 °C
Ellipsometric measurement
Fig. 1b: Change in film thickness plotted against the temperature change to determine T<sub>g</sub>
Temperature controller
Fig. 2: Linkam temperature controller on M-2000/RC2 mounting adapter with control unit, N<sup>2</sup> dewar and accessories case
Fig. 3: Linkam temperature controller integrated in automated M-2000

The glass transition temperature Tg is one of the main characteristics of amorphous and semi-crystalline polymers, where abrupt changes may occur in the physical properties. Ellipsometry allows the measurement of Tg of very thin films. Changes in the film thickness are determined throughout several temperature ramps. Figures 1a and b show the results for polystyrene.

Linkam’s temperature controller allows temperature-dependent ellipsometry measurements over a wide range from -70 °C to 600 °C with all Woollam systems like M-2000, RC2 and VASE. The heating stage is a self-contained system with optical windows for an angle of incidence of 70°. It may also be operated as open system with other angles of incidence. Gas purging connectors are attached.

An N2 dewar and pump are included for actively cooling the system with liquid nitrogen. This allows the  automatic running of heating and cooling cycles with predefined recipes. Desired temperature profiles can be generated easily. Controlling of the system is completely integrated into the ellipsometer software. So the respec­tive temperature is saved with each measurement point/measurement spectrum and can be used in the model as set point of the corresponding optical constant.

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