Inductive Coupled Plasma (ICP) Textures as Alternative for Wet Chemical Etching in Solar Cell Fabrication

Transcription

1 Inductive Coupled Plasma (ICP) Textures as Alternative for Wet Chemical Etching in Solar Cell Fabrication 1 Motivation 2 Experimental setup 3 ICP textures as alternative technique 3.1 Surface morphology 3.2 Optical properties 3.3 Surface passivation 4 Conclusion & Outlook

2 Hemispherical Reflectance [%] 1 Motivation Standard for texturing of crystalline Si Wet chemical etching But, wet chemical textures have some drawbacks Dependence from crystal orientation Average reduction of reflection Impact of the angle of incidence Dangerous chemicals All samples without ARC Multi-Si acidic etched 20 Alternative Technique Inductively Coupled Plasma (ICP) Textures 0 Mono-Si alkaline etched Wavelength [nm]

3 2 Experimental Setup Plasma Tool: Oxford Instruments Plasmalab System 100 Capacitive (CCP) & Inductive (ICP) Source [1] Process Parameter: ICP source 5 C temperature O 2 /SF 6 Gas-Mixture Circle are with 6 cm Ø treated Advantage of ICP Process (i) No Self-Bias (ii) Low ion energy (iii) Short process times [1] Maria Gaudig, Black Silicon Prepared by Plasma Etching at Temperatures Above 0 C

4 Inductive Coupled Plasma (ICP) Textures as Alternative for Wet Chemical Etching in Solar Cell Fabrication 1 Motivation 2 Experimental setup 3 ICP textures as alternative technique 3.1 Surface morphology 3.2 Optical properties 3.3 Surface passivation 4 Conclusion & Outlook

5 3.1 Surface Morphology - Crystal Orientation Monocrystalline-Si: Anisotropic etch (pyramid like) Multicrystalline-Si: Isotropic etch (sponge like) 2 wet benches required Same ICP recipe for all samples ICP treated Multi-Si (AsCut) ICP treated FZ-Si (polished) Advantage of ICP-Textures (i) No influence by the crystal orientation (ii) Independent on surface roughness ICP treated CZ-Si (AsCut)

6 3.1 Surface Morphology Surface Structure ICP-Mono Alkaline etch 3 µm 20 µm ICP-Mono Acidic etch 2 µm 20 µm

7 3.1 Surface Morphology - Formation 0 Minutes 10 Minutes Multi-Si Mono-Si

8 3.1 Surface Morphology - Formation Average Reflectance [%] ( nm) All samples without ARC ICP-Mono ICP-Multi Minimum (7:30) 00:00 02:00 04:00 06:00 08:00 10:00 ICP Process Time [mm:ss] Same reaction speed for monocrystalline & multicrystalline silicon Minimum reflectance reached after 07:30 minutes Weak increase of reflection after 10 minutes ICP-Textures Process speed is sufficient for solar cell production

9 3.1 Surface Morphology - Formation 50 All samples without ARC AsCut Average Reflectance [%] ( nm) ICP-Mono ICP-Multi Minimum (7:30) 00:00 02:00 04:00 06:00 08:00 10:00 10 µm AsCut reference Rough surface after waver saw process ICP Process Time [mm:ss]

10 3.1 Surface Morphology - Formation 50 All samples without ARC AsCut 01:00 Average Reflectance [%] ( nm) ICP-Mono ICP-Multi Minimum (7:30) 00:00 02:00 04:00 06:00 08:00 10:00 10 µm 4 µm SF 6 saw damage removal visible Begin of the Self-Masking through SiOF ICP Process Time [mm:ss]

11 3.1 Surface Morphology - Formation 50 All samples without ARC AsCut 01:00 02:00 Average Reflectance [%] ( nm) ICP-Mono ICP-Multi Minimum (7:30) 00:00 02:00 04:00 06:00 08:00 10:00 10 µm 10 µm 1 µm Under etch of the Self-Masking First formation of nanostructures ICP Process Time [mm:ss]

12 3.1 Surface Morphology - Formation 50 All samples without ARC AsCut 01:00 02:00 05:00 Average Reflectance [%] ( nm) ICP-Mono ICP-Multi Minimum (7:30) 00:00 02:00 04:00 06:00 08:00 10:00 10 µm 10 µm 10 1 µm µm Further under etch Formation of circular surface structures with a width of 300 nm ICP Process Time [mm:ss]

13 3.1 Surface Morphology - Formation Average Reflectance [%] ( nm) All samples without ARC ICP-Mono ICP-Multi Minimum (7:30) 00:00 02:00 04:00 06:00 08:00 10:00 ICP Process Time [mm:ss] AsCut 01:00 02:00 05:00 10:00 10 µm 10 µm 10 1 µm µm 1 µm Further under etch Increased width of the structures Weak increased reflection

14 Inductive Coupled Plasma (ICP) Textures as Alternative for Wet Chemical Etching in Solar Cell Fabrication 1 Motivation 2 Experimental setup 3 ICP textures as alternative technique 3.1 Surface morphology 3.2 Optical properties 3.3 Surface passivation 4 Conclusion & Outlook

15 Hemispherical Reflectance [%] 3.2 Optical Properties Normal incidence All samples without ARC Multi-Si Acidic etched Mono-Si alkaline etched Air Si Acidic-Texture Low reduction Alkaline-Texture Average reduction ICP-Texture: Significant reduction 0 ICP-Mono/Multi Wavelength [nm] ICP-Textures Best results under normal incidence

16 3.2 Optical Properties Oblique incidence Average Reflectance + Transmission [%] ( nm) Air All samples without ARC Si Acidic etch Alkaline etch ICP-Mono/Multi Angle of Incidence [ ] Acidic-Texture No dependence Alkaline-Texture Strong dependence ICP-Texture: Weak dependence ICP-Textures Weak dependence of the angle of incidence

17 Inductive Coupled Plasma (ICP) Textures as Alternative for Wet Chemical Etching in Solar Cell Fabrication 1 Motivation 2 Experimental setup 3 ICP textures as alternative technique 3.1 Surface morphology 3.2 Optical properties 3.3 Surface passivation 4 Conclusion & Outlook

18 Effective Lifetime [µs] 3.3 Surface Passivation ALD Al 2 O Polished-Reference ALD Al 2 O 3 surface passivation Previous RCA clean 4 FZ Wafer p-doped, 2 Ω*cm 250 µm 1000 ICP-Texture Excellent passivation through ALD But, ALD requires an RCA clean and long process times QSSPC 100 1E14 1E15 1E16 Minority Carrier Density [cm -3 ] Next approach PECVD SiN Surface Passivation

19 4 Outlook & Conclusion ICP Textures are an alternative technique to wet chemical textures, because i. Independent from the crystal orientation ii. Significant reduction of the reflectance iii. Weak dependence from the angle of incidence iv. Short process times required (07:30) v. Excellent surface passivation through ALD Al 2 O 3 Outlook ICP-Texture surface passivation with SiN Performance of ICP textures on cell level Long-Term stability test Thank you for your Attention