MANUFACTURINGMETROLOGY


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Figure 4: A laser ablated trench in a CdTe thin film device. The trench is made precisely to the TCO layer (courtesy of M-solv Ltd)


A key process in the manufacture of crystalline or polycrystalline silicon is the introduction of surface texture to provide anti- reflective properties to the surface. Texture is introduced by etching the surface to form pyramid structures which reflect light back into the surface


cells and connect them in series to optimize the Voltage of a solar module. The CCI image provides a 3D and quantitative analysis of the width and depth of the laser groove.


Thin film Thickness


Figure 5: A CCI image of a 103nm anti- reflective thin film of Silicon Nitride deposited on a silicon substrate with an rms surface roughness of 1.4 nm


The capability of the CCI has been extended recently to the measurement of semi-transparent thin and thick film thickness using a new and patented algorithm. Thin film thickness is an important parameter in many solar applications. For example, thin film thickness of a TCO in thin film photovoltaics determines its sheet resistance and transmission. The thickness of a silicon nitride layer is critical for its anti-reflective properties since a quarter wavelength is optimal. For the algorithm to be effective it is necessary to either measure or assume a value for refractive index. Figure 5 shows a CCI image of a silicon nitride anti-reflective layer


deposited on silicon. The analysis reveals that the thickness of the Silicon Nitride film is 103.2nm with surface rms roughness of 1.4nm. This analysis shows the power of the technique allowing thin film thickness and surface roughness measurements to be obtained from a single analysis.


Discussion


Metrology plays an vital role in the development of all photovoltaic devices. CCI provides a three dimensional surface analysis which has the advantage that it is non-contacting and thereby non-destructive. The vertical resolution of the technique is at the atomic scale. The lateral resolution is governed by the wavelength of light (~0.5µm) and the NA of the objective lens and this will limit its suitability for some applications. However, the data is collected from an area which is more much larger than that obtained from techniques such as AFM and hence the analysis will tend to be more representative. The CCI technique provides a rapid means of characterizing a number of important parameters including surface roughness, step height, surface form, surface texture and its capability has now been extended to include thin film thickness. The Photovoltaic Laboratory at CREST at Loughborough University is now working with Ametek Taylor Hobson to establish areas of application where the technique can be used in quality control on silicon PV, thin film PV and third generation PV devices.


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www.solar-pv-management.com Issue X 2010


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