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Thin film accuracy

Metrology measurements play an important role in the development and manufacture of photovoltaic cells. For example, surface texturing is an important process in Silicon solar cells to improve the anti-reflective properties of the surface. In thin film CdTe photovoltatics, the surface roughness of the Transmitting Conducting Oxide (TCO) layer can be crucial to device performance. Here, Mike Conroy of Ametek Taylor Hobson and Michael Walls of Loughborough University introduce the ways in which Coherence Correlation Interferometry (CCI) can be used to obtain precision surface metrology measurements. The authors also introduce the CCI’s state of the art capability to accurately measure thin film thickness.

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Figure 1: The CCI combines rapid data collection with an ability to provide quantitative metrology data with a vertical resolution of 0.1nm over a large field of view

he quality of surfaces and thin films is important to the performance of all photovoltaic devices including crystalline and polycrystalline silicon, thin film photovoltaics (CdTe, CIGS and amorphous silicon) as well as to third generation technologies such as dye- sensitised and organic photovoltaics. A number of tools are used to measure properties such as texture, surface roughness, thin film thickness and interface roughness, these include stylus profilometry, spectroscopic ellipsometry, Atomic Force Microscopy (AFM), Scanning Electron Microscopy (SEM) etc.etc.

The CCI technique is a non-contacting surface metrology tool which takes data rapidly from a large and hence representative area from 165 um x 165 um using a 100x objective lens to 6.6 mm x 6.6 mm for a 2.5x objective lens. The technique is fast and provides a three-dimensional image of the surface together with analysis that includes root mean square roughness (Sq), maximum peak height (St). The availability of rapid metrology from the CCI technique enables fast process optimization.

Ametek Taylor Hobson and Loughborough’s Centre for Renewable Energy Systems and Technology (CREST) are collaborating to establish

the applications for CCI for the development and production of solar materials and devices.

Here we will introduce how the CCI works and then show some of the capabilities and applications of the technique.

Coherence Correlation Interferometry The CCI technique is a non-contacting surface metrology tool. Its major advantages over other metrology techniques is that it is fast, non- contacting, non-destructive and it takes its data from a relatively large and hence more representative area. The lateral resolution is ~0.5µm and is determined by the wavelength of light and the NA of the objective lens.

The CCI combines a coherence correlation algorithm with a high-resolution digital camera array to generate a three-dimensional representation of a structure by scanning the fringes through the surface and then processing the information to transform the data into a quantitative three-dimensional image with 0.01nm vertical resolution. The data can then be used to generate accurate quantitative parameters such as rms roughness Sq. Recently, the technique has been extended to measurements of thin film thickness from semi-transparent thin films. Issue X 2010

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