Plasma enhanced chemical etching at atmospheric pressure for silicon wafer processing
Plasmaaktives chemisches Ätzen von Silicium-Wafer für die Solarzellenherstellung
Plasma enhanced chemical etching of silicon wafers has been successfully carried out at atmospheric pressure. The etching process was developed using a linear extended DC arc with a working width of 150 mm for plasma activation. The technology operates in a remote (downstream) mode. The experimental set-up allows continuous processing which is advantageous in terms of in-line capability and consequently reduced effort for substrate handling. Fluorine radicals generated downstream of the plasma source from CHF3, CF4, C2F6, C4F8, SF6, NF3, CHF3/O2, C2F6/O2 and SF6/O2 have been successfully utilized to etch (100)-mono-Si wafers. Static etch rates of over 11 micron/min and dynamic etch rates of 1.5 micronxm/min have been obtained with NF3. SEM images of the etched silicon wafers exhibit different surface morphologies such as pyramids of several microns size or nano structured surfaces with high aspect ratio, strongly depending on process conditions and etch precursors used. The reactive plasma was used to remove the rear side emitter and provide for edge isolation of crystalline Si solar cells. Results comparable to established wet chemical etch processes were achieved. Plasma chemical etching at atmospheric pressure can also be introduced into other steps for continuous solar wafer processing. One promising step is surface texturing in order to decrease front side silicon reflectance. Different substrate textures, i.e. pyramids of some microns and nano structures with high aspect ratio have been obtained leading to substantial decreased reflectance. Furthermore the feasibility of both saw damage etching and phosphorsilicate glass removal have been demonstrated. The PSG (Phosphorsilicate-glass) etching rate is strongly dependent on both etchant selection and plasma parameters used. The PSG rate can be factor 10 to 100 higher against the etch rate of the doped silicon wafer. The underlying plasma equipment is characterized by low footprint requirements and is well suited for in-line integration into both existing lines and newly designed lines.