Silicon photovoltaic – Wafer check after diamond wire sawing

Diamond wire sawing (DWS) is an established technology for wafering semiconductor ingots, as it has many advantages over other technologies, such as of slurry cutting. Some of these advantages include faster cutting speeds with higher cutting efficiency, production of thinner wafers with improved thickness uniformity, easier way to filter silicon debris for slurry recycling (if desired), and use of cutting wire for more than one cut.

Diamond wire sawing uses a long (hundreds of kilometres) wire, impregnated with diamond flakes (grit) as a cutting medium. The cutting wire is made up of a stainless-steel core (80– 120 µm in diameter) that is coated with diamond flakes (8–25 µm in size) which are then bonded to the wire by a layer of electroplated Ni or a layer of a resin material.

The as-sawn DWS wafer might look perfect to the naked eye, but the diamond wire and the strategy of moving the diamond wire through the ingot (speed and reciprocation), has a high impact on the quality of the as-sawn wafers as well as subsequent processes such as lapping, grinding or etching. For PV wafers in particular, there can be a huge difference in the sub surface damage over the as-sawn square wafer and this needs to be accounted for in the damage etch and texturing etch process step that proceeds the DWS process.

SPV spectroscopy using the HR-SPS tool provides direct information about the quality of the as-sawn wafers – it can be used to find areas of wire snap-off, wire reciprocation and also provide a map with relation to subsurface damage depth. It is contactless and fast, allowing for an integration of the tool into a process line QC control of as-sawn wafers. Below is a shown an example for a 156 x 156 mm² pseudo square DWS PV wafer (n-type monocrystalline, 1-3 Ohm-cm).

Figure 1 shows maps of the SPV height across the n-type PV wafer by illuminated with three different photons energies – the penetration depths of the photons into the wafer is approximately 0.1, 10 and 100 mm, respectively. Also shown are the relaxation time constant across the wafer – defined as the logarithmic SPV signal transient decay time after turning off the light. It is clear from the SPV maps (left side) that the wafer has varying degrees of subsurface damage across the wafer; the top part has more subsurface damage than the bottom part and the periphery also has more damage. This is to be expected because of the force of the wire is higher at the periphery of the ingot to be sawn. In the relaxation maps (right side), there is a distinct zone below the centre of the wafer towards the bottom. This distinct centre has a lower than expected relation time (-10%) and this probably marks the reciprocation process start. The SPV and relaxation maps complement each other and give a fingerprint of the state of the as-sawn PV wafer.