IMEC, LEUVEN, BELGIUM, 2 KU LEUVEN, BELGIUM, 3 U HASSELT, BELGIUM

Transcription

1 INVESTIGATION OF RADIATION DAMAGE OF CU PLATED IBC CELLS CAUSED BY SPUTTERING OF SEED LAYER SUKHVINDER SINGH 1, BARRY O SULLIVAN 1, SHRUTI JAMBALDINNI 1, MAARTEN DEBUCQUOY 1 AND JEF POORTMANS 1,2,3 1 IMEC, LEUVEN, BELGIUM, 2 KU LEUVEN, BELGIUM, 3 U HASSELT, BELGIUM

2 OUTLINE Motivation and background Process flow for sputtering damage investigation Damage evaluation on wafers with and without diffusion Sputter damage as a function of passivating oxide thickness Bulk lifetime effect Conclusions 2

3 MOTIVATION o A seed layer and Cu-plating process for large area IBC cells has been developed at imec o It resulted in high cell efficiencies up to 21.9 %, measured over the full area of the 6 inch semi-square cell o Seed layers requirements (< 500nm) for Cu plated IBC cells o Plating on both n+ & p+ doped regions o Three layer stack o o o First layer: AlSi for good contact, rear reflection Second layer: Barrier against Cu diffusion Third layer: Cu for further plating o Sputtering of seed layer stack Can cause damage to the Si/SiO 2 interface and Si bulk 3 * S. Singh et al., 31st EUPVSEC (2015)

4 SPUTTER DAMAGE o Sputter damage: caused by sputtering of seed layer [1-3] o Caused by x-rays generated during sputtering o Depends on thickness and density of passivating oxide [1] : o Thinner oxide: deeper penetration of x-rays into silicon surface o Less penetration (damage) for Si 3 N 4 than SiO 2 Can the damage be avoided or recovered effectively by FGA treatment? 4 1. K. Kathouda et al. 27 th EUPVSEC, Frankfurt, Germany (2012). 2. F. Volpi et al. J. Appl. Phys. 95 (2004) D. Reinwand et al., 24 th EUPVSEC, Hamburg, Germany (2009).

5 OUTLINE Motivation and background Process flow for sputtering damage investigation Damage evaluation on wafers with and without diffusion Sputter damage as a function of passivating oxide thickness Bulk lifetime effect Conclusions 5

6 PROCESS FLOW Saw Damage Removal No diffusion / POCl 3 / BBr 3 diffusion Thermal oxide + FGA Photoluminescence (PL) and QSSPC Laser ablation / No laser ablation Metal sputtering (AlSi or AlSi/Barrier or AlSi/Barrier/Cu) FGA Etch metal Photoluminescence (PL) and QSSPC 6

7 PROCESS FLOW Saw Damage Removal No diffusion / POCl 3 / BBr 3 diffusion Thermal oxide + FGA Photoluminescence (PL) and QSSPC Laser ablation / No laser ablation Metal sputtering (AlSi or AlSi/Barrier or AlSi/Barrier/Cu) FGA Etch metal Photoluminescence (PL) and QSSPC 7

8 PROCESS FLOW Saw Damage Removal No diffusion / POCl 3 / BBr 3 diffusion Thermal oxide + FGA Photoluminescence (PL) and QSSPC Laser ablation / No laser ablation Metal sputtering (AlSi or AlSi/Barrier or AlSi/Barrier/Cu) FGA Etch metal Photoluminescence (PL) and QSSPC 8

9 OUTLINE Motivation and background Process flow for sputtering damage investigation Damage evaluation on wafers with and without diffusion Sputter damage as a function of passivating oxide thickness Bulk lifetime effect Conclusions 9

10 EFFECTIVE LIFETIME EFFECT OF SEED LAYER BARRIER: WAFERS WITHOUT DIFFUSION Effective lifetime at 1e15/cm 3 (µs) AlSi Laser ablated samples AlSi/B1 AlSi/B1/Cu AlSi/B2 AlSi/B2/Cu Metal stack o Both barrier layers -> lower lifetime than AlSi only o Cu does not cause additional sputtering damage o Barrier B2 was chosen for better lifetime in low injection regime 10

11 EFFECTIVE LIFETIME EFFECT OF SEED LAYER BARRIER: WAFERS WITH BBr 3 AND POCl 3 DIFFUSION Effective lifetime at 1e15 /cm 3 (µs) Laser ablated samples AlSi Metal stack AlSi/B2/Cu POCl 3 BBr 3 On diffused samples Similar lifetimes and J o for AlSi and for AlSi/B2/Cu Sputtering damage is masked by diffused layers Sputtering damage can be recoved by FGA on chosen seed layer stack 11

12 CELL RESULTS DEVELOPED SEED LAYER IMPLEMENTATION Metallization Area [cm 2 ] J sc [ma/cm 2 ] V oc [mv] FF [%] Eta [%] Sputtered AlSi (3µm) Sputtered Cu seed + plated Cu (8µm) Resistive losses in metal strongly reduced by Cu plating Efficiency up to 21.9 % was achieved 12 * S. Singh et al., 31st EUPVSEC (2015)

13 V OC VARIATION OVER SEVERAL RUNS WITH CU PLATED METALLIZATION Best cell (V oc ) 688 Run 1 Run2 Run3 Run4 13 Al PVD metalization Cu plated metalization Un-intended V oc variation has been observed Does sputtering damage vary over different runs? Lowest V oc for run # 3 Sputtered AlSi 3um Sputtered seed + plated Cu Could it be related to thickness of thermal oxide passivation? As sputtering damage is reported to be depend on the passivation layer thickness

14 OUTLINE Motivation and background Process flow for sputtering damage investigation Damage evaluation on wafers with and without diffusion Sputter damage as a function of passivating oxide thickness Bulk lifetime effect Conclusions 14

15 MEASURED OXIDE THICKNESS (JUST BEFORE METALLIZATION) Oxide Thickness (nm) Emitter Run # 4 Run # Position on wafer (mm) Run # Oxide thickness emitter (nm) Oxide thickness BSF (nm) Start End Start End Run Run

16 MEASURED OXIDE THICKNESS (JUST BEFORE METALLIZATION) Oxide Thickness (nm) Emitter Run # 4 Run # Position on wafer (mm) Oxide Thickness (nm) BSF Run # 4 Run # Position on wafer (mm) Run # Oxide thickness emitter (nm) Oxide thickness BSF (nm) Start End Start End Run Run

17 PROCESS FLOW SPUTTERING DAMAGE: EFFECT OF OXIDE THICKNESS Saw Damage Removal Diffusion BBr 3 (emitter) / POCl 3 (BSF) WO+DO (emitter) / DO (BSF) Check oxide thickness Etch oxide to 25, 50, 75, 125 and 185 nm (emitter) / to 25 and 50 nm (BSF) No metal or AlSi-2um or Cu seed Dice ¼ Process flow mostly similar to the one described earlier, except Thermal oxide etched intentionally before metal sputtering Emitter oxide nm BSF oxide 50 & 25 nm FGA Etch metal PL/QSSPC 17

18 LIFETIME/J o EMITTER vs OXIDE THICKNESS AlSi-2um Cu seed AlSi-2um Cu-seed J o (fa/cm 2 ) Oxide thickness (nm) 1e15 cm -3 (us) Oxide thickness (nm) No significant difference in J o and effective 1e15 cm -3 Also for BSF samples Focused mainly on emitter samples as they represent 80 % of cell area 18

19 LOW INJECTION LIFETIME CHECK Effective lifetime (ms) E13 1E14 1E15 1E16 n (cm 3 ) 19 Non Diced Diced After dicing, edge recombination decreases measured lifetime in low injection Compromises measurements in low injection Prepared new non-diced wafers with 185nm and 70 nm oxide thickness

20 LIFETIME/J o EMITTER (NON-DICED WAFERS) 185 NM AND 70 NM OXIDE THICKNESS No metal AlSi-2um Cu seed No metal AlSi-2um Cu seed J o (fa/cm 2 ) Oxide thickness (nm) 1e15 cm -3 (us) Oxide thickness (nm) o Non-significant difference in J o o Lower lifetime for Cu seed, especially for lower oxide thickness 20

21 LIFETIME/J o EMITTER (NON-DICED WAFERS) 185 NM AND 70 NM OXIDE THICKNESS J o (fa/cm 2 ) No metal AlSi-2um Cu seed Oxide thickness (nm) V oc (mv) 687,0 686,5 686,0 685,5 685,0 684,5 684,0 0.6 mv Simulated J o (fa/cm 2 ) o 3 fa/cm 2 increase in J o 0.6 mv drop in V oc o Not enough to explain large variation in V oc 21

22 LOW INJECTION LIFETIME 185 NM AND 70 NM OXIDE THICKNESS Effective lifetime (ms) nm oxide 1E14 1E15 1E16 n (cm -3 ) No Metal AlSi-2µm AlSi-2µm Cu seed Cu seed Effective lifetime (ms) nm oxide No metal AlSi-2µm AlSi-2µm Cu seed Cu seed 1E14 1E15 1E16 n (cm -3 ) o Clear indication of lower lifetime in low injection with decreasing oxide thickness o Non significant difference in high injection lifetime (and J o ) V oc difference not expected o Difference in low injection lifetime could effect pff of the cells.

23 OUTLINE Motivation and background Process flow for sputtering damage investigation Damage evaluation on wafers with and without diffusion Sputter damage as a function of passivating oxide thickness Bulk lifetime effect Conclusions 23

24 OTHER POSSIBLE REASONS FOR V OC VARIATION CHECK BULK LIFETIME OF DEVICES Etch metal + oxide + doped regions on finished devices Followed by i/n + a-si:h passivation on both sides Effective lifetime (ms) Run 23 1 Run Run Run 4 0 1,0E+14 1,0E+15 1,0E+16 n (cm -3 ) Significant difference in bulk lifetime of wafers Run 3: 2.75 ms vs. Run 4: 6.0 ms 24

25 QUOKKA SIMULATION : BULK LIFETIME V oc (mv) mv FF (%) 81,2 81,0 80,8 80,6 0.4% J sc (ma/cm 2 ) 41,55 41,50 41,45 41,40 41, ma/cm Bulk lifetime (ms) 80, Bulk lifetime (ms) 41, Bulk lifetime (ms) o For lifetime drop from 6 to 2.75 ms: o Drop in V oc by 2 mv expected o Should also result in lower FF by 0.4% o Also drop in J sc by 0.1 ma/cm 2 expected 25

26 DEVICE RESULTS Run Area [cm2] J sc (ma/cm2) V oc (mv) FF (%) pff (%) Eta (%) Run Run * Delta Trends: as expected from Quokka simulation Lower V oc could be partly related to bulk lifetime difference 26 *B O Sullivan IEEE J Photovoltaics (accepted)

27 CONCLUSIONS Radiation damage is not the cause of major V oc variations Decrease in oxide thickness shows small effect on J o Difference in J o too small to explain ~5 mv differences in V oc However for Cu seed, lower oxide thicknesses leads to lower lifetime in low injection regime Could effect pff of the cells V oc variations partly caused by Bulk lifetime reduction by 3 ms (6 to 2.75 ms) 2 mv lower V oc Also leads to lower FF (pff) and Jsc of cells 27

28 Thank You!