Atmospheric Pressure Dry Etching of Polysilicon Layers for highly Reverse Bias-stable TOPCon Solar Cells
Single-sided etching (SSE) of a-Si/poly-Si is typically considered a challenge for realizing a cost-efficient TOPCon production sequence, as there is a certain degree of unwanted wrap-around for poly-Si deposition technologies such as low pressure chemical vapor deposition, plasma-enhanced chemical vapor deposition, and atmospheric pressure chemical vapor deposition. To date, alkaline or acidic wet-chemical solutions in either inline or batch configurations are used for this purpose. Herein, an alternative SSE process is proposed using an inline dry etching tool, which applies molecular fluorine as the etching gas under atmospheric pressure conditions. The developed etching process performs complete etching of both as-deposited amorphous silicon and annealed polycrystalline silicon layers, either intrinsic or doped, and with measured etch rates of >3 μm min-1 at 10% F2 concentration allows etching of a typical layer thickness of 200 nm in just a few seconds. The etching process is also configured to perform excellent edge isolation while maintaining a low wrap-around etching (d rear < 500 μm) at the opposing-side. The etching process is successfully transferred to the industrial TOPCon solar cell architecture, yielding high parallel resistances (S shunt,avg. > 1500 kΩ cm2), low reverse current density (J rev,avg < 0.8 mA cm-2) measured at a bias voltage of -12 V, and independently certified conversion efficiencies of up to 23.3%.