© IMEC 2011

PASSIVATION OF A METAL CONTACT WITH A TUNNEL LAYER

X. LOOZEN, J. B. LARSEN, F. DROSS, M.

ALEMAN, T. BEARDA, B. J. O’SULLIVAN,

I. GORDON AND J. POORTMANS

© IMEC 2011

Literature overview

Highest efficiency cell on Si in 1984 with front contacts

passivated by thin SiO2 (full Al-BSF cell, 18.4%)

Tunnel SiO2 under front Al contacts on a n-type emitter

(PERC cell) ISFH

Voc gain of ~35mV by use of passivated contacts on IBC

cells (best cell 721mV, 24.2%)

Voc gain of 12mV by using an ALD Al2O3 tunnel layer

(best cell 673mV, 21.7%)

2

M.A. Green et al, IEEE TED (1984)

CONTACT PASSIVATION – 3RD METALLIZATION WORKSHOP, CHARLEROI 2011 – X. LOOZEN

J. Schmidt et al, Prog. Photovolt.: Res. Appl. (2008)

K. Jaeger-Hezel et al, EU PVSEC (1995)

P.J. Cousins et al, IEEE PVSC (2010)

D. Zielke et al, Phys. Status Solidi RRL (2011)

- High potential for contact passivation

- Need for characterization

© IMEC 2011

Outline

Literature overview

AlOx candidate for contact passivation?

AlOx contact passivation seen by

photoluminescence

Contact resistance

Conclusion

CONTACT PASSIVATION – 3RD METALLIZATION WORKSHOP, CHARLEROI 2011 – X. LOOZEN 3

© IMEC 2011

Outline

Literature overview

AlOx candidate for contact passivation?

AlOx contact passivation seen by

photoluminescence

Contact resistance

Conclusion

CONTACT PASSIVATION – 3RD METALLIZATION WORKSHOP, CHARLEROI 2011 – X. LOOZEN 4

© IMEC 2011

Use of AlOx for surface passivation

CONTACT PASSIVATION – 3RD METALLIZATION WORKSHOP, CHARLEROI 2011 – X. LOOZEN 5

Effective passivation of n- & p-Si by

thick AlOx

Lifetime and Seff on bulk p- and n-Si passivated by

10nm AlOx + FGA 350ºC

2 Ωcm p-Si 2.4 Ωcm n-Si 2 Ωcm p-Si 2.4 Ωcm n-Si

© IMEC 2011

Can we use AlOx for contact passivation?

Target: Reduce recombination underneath

metal contacts via a tunneling layer

Question: Does an ultra-thin (~nm) AlOx layer

provide good surface passivation?

1) On base material?

2) On highly doped n- and p-type regions?

- Emitter

- FSF/BSF

CONTACT PASSIVATION – 3RD METALLIZATION WORKSHOP, CHARLEROI 2011 – X. LOOZEN 6

© IMEC 2011

Can we use AlOx for contact passivation?

Target: Reduce recombination underneath

metal contacts via a tunneling layer

Question: Does an ultra-thin (~nm) AlOx layer

provide good surface passivation?

1) On base material?

2) On highly doped n- and p-type regions?

- Emitter

- FSF/BSF

CONTACT PASSIVATION – 3RD METALLIZATION WORKSHOP, CHARLEROI 2011 – X. LOOZEN 7

© IMEC 2011

Passivation of base material by thin AlOx

Very low Seff already with 2nm thermal ALD AlOx

- 8 cm/s on n-Si (polished FZ)

- 20 cm/s on p-Si (polished FZ)

CONTACT PASSIVATION – 3RD METALLIZATION WORKSHOP, CHARLEROI 2011 – X. LOOZEN 8

# ALD cycles ~ # Å of AlOx

Effective passivation of n- & p-Si with

very thin AlOx

Lifetime versus AlOx thickness after FGA 350ºC

2nm

© IMEC 2011

Can we use AlOx for contact passivation?

Target: Reduce recombination underneath

metal contacts via a tunneling layer

Question: Does an ultra-thin (~nm) AlOx layer

provide good surface passivation?

1) On base material?

2) On highly doped n- and p-type regions?

- Emitter

- FSF/BSF

CONTACT PASSIVATION – 3RD METALLIZATION WORKSHOP, CHARLEROI 2011 – X. LOOZEN 9

© IMEC 2011

AlOx passivation on n+ diffused surfaces

Symmetrical deposition of thick 30nm AlOx on POCl3

diffused surfaces

CONTACT PASSIVATION – 3RD METALLIZATION WORKSHOP, CHARLEROI 2011 – X. LOOZEN 10

Lifetime of AlOx passivated n+ surfaces versus Rsheet after FGA 350ºC

As diffused

After oxidation at 1000ºC

eff limited to ~100µs for 100 Ω/ doped n+ surfaces

with AlOx passivation. Why?

© IMEC 2011

Why poorer passivation with higher doping?

CONTACT PASSIVATION – 3RD METALLIZATION WORKSHOP, CHARLEROI 2011 – X. LOOZEN 11

Base n-Si:

strong induced inversion

Medium n+ doping:

weak induced inversion

High n++ doping:

very weak or no induced

inversion

Base Si

- - - - + + +

- - - - - - - - - - - - + + + + + + + + +

+ + + + + + + + + n+

Base Si

- - - - - - - - - - - - - - - -

+ + + + + + + + +

n++

Base Si

- - - - - - - - - - - - - - - -

+ + + +

Floating junction passivation works well for base n-Si, but

not for highly doped n+ surfaces

AlOx with negative charges

© IMEC 2011

Outline

Literature overview

AlOx candidate for contact passivation?

AlOx contact passivation seen by

photoluminescence

Contact resistance

Conclusion

CONTACT PASSIVATION – 3RD METALLIZATION WORKSHOP, CHARLEROI 2011 – X. LOOZEN 12

© IMEC 2011

AlOx passivation seen by photoluminescence

Target: passivation of front contact in i-PERC cells

For PL measurement

- Suppress rear metallization

- Use tunnel candidate on front

- Compare passivated/non-passivated areas

CONTACT PASSIVATION – 3RD METALLIZATION WORKSHOP, CHARLEROI 2011 – X. LOOZEN 13

PL illumination

CZ p-Si

SiOx/SiNx passivation stack

100 Ω/ n+ emitter

SiNx ARC

Al BSF

CZ p-Si

SiOx/SiNx passivation stack

n+

SiNx ARC

© IMEC 2011

AlOx passivation seen by photoluminescence

Target: passivation of front contact in i-PERC cells

For PL measurement

- Suppress rear metallization

- Use tunnel candidate on front

- Compare passivated/non-passivated areas

CONTACT PASSIVATION – 3RD METALLIZATION WORKSHOP, CHARLEROI 2011 – X. LOOZEN 14

PL illumination

CZ p-Si

SiOx/SiNx passivation stack

100 Ω/ n+ emitter

SiNx ARC

Al BSF

CZ p-Si

SiOx/SiNx passivation stack

n+

Tunnel AlOx

© IMEC 2011

AlOx passivation seen by photoluminescence

Target: passivation of front contact in i-PERC cells

For PL measurement

- Suppress rear metallization

- Use tunnel candidate on front

- Compare passivated/non-passivated areas

CONTACT PASSIVATION – 3RD METALLIZATION WORKSHOP, CHARLEROI 2011 – X. LOOZEN 15

PL illumination

CZ p-Si

SiOx/SiNx passivation stack

100 Ω/ n+ emitter

SiNx ARC

Al BSF

CZ p-Si

SiOx/SiNx passivation stack

n+

Tunnel AlOx

© IMEC 2011

No coating

(native oxide) Ti/Pd/Ag

Tunnel

AlOx

+

Ti/Pd/Ag

Tunnel

AlOx

AlOx passivation seen by photoluminescence

Comparison passivated/non-

passivated areas

Area for QSSPC calibration

Reflection on metal no direct

comparison

CONTACT PASSIVATION – 3RD METALLIZATION WORKSHOP, CHARLEROI 2011 – X. LOOZEN 16

CZ p-Si

SiOx/SiNx passivation stack

100 Ω/ n+ emitter

SiNx ARC

Al BSF

CZ p-Si

SiOx/SiNx passivation stack

n+

Tunnel AlOx

© IMEC 2011

AlOx passivation seen by photoluminescence

CONTACT PASSIVATION – 3RD METALLIZATION WORKSHOP, CHARLEROI 2011 – X. LOOZEN 17

No coating

(native oxide) Ti/Pd/Ag

1.5 nm

Al2O3

+

Ti/Pd/Ag

1.5nm Al2O3

Lifeti

me (

µs)

Dn=5.1014cm-3 on non-metalized area

and lower on metalized areas

CZ 180µm p-Si, with 100Ω / POCl3 emitter

Low AlOx reference lifetime - Limited AlOx passivation on n+ Si

30

10

15

20

25

0

5 17.4µs

6.4µs

25µs

23µs

Significant increase of lifetime under metal with 15 AlOx

ALD cycles (~1.5nm)

© IMEC 2011 CONTACT PASSIVATION – 3RD METALLIZATION WORKSHOP, CHARLEROI 2011 – X. LOOZEN 18

Lifetime versus AlOx thickness

Ultra-thin AlOx under contact

improves passivation

Average lifetime on the Si areas covered by AlOx and/or metal

1.5-2nm

© IMEC 2011

Outline

Literature overview

AlOx candidate for contact passivation?

AlOx contact passivation seen by

photoluminescence

Contact resistance

Conclusion

CONTACT PASSIVATION – 3RD METALLIZATION WORKSHOP, CHARLEROI 2011 – X. LOOZEN 19

© IMEC 2011 CONTACT PASSIVATION – 3RD METALLIZATION WORKSHOP, CHARLEROI 2011 – X. LOOZEN

p-Si

n+, 120 Ω/

500 µm

Ti/Pd/Ag dots

V AlOx

20

Flat emitter

Departure from ohmic contact from 20 AlOx ALD cycles (~2 nm)

Textured emitter

Large current and ohmic contact through all AlOx layers, even through

20nm AlOx !!!

Influence of thickness on contact resistance

© IMEC 2011 CONTACT PASSIVATION – 3RD METALLIZATION WORKSHOP, CHARLEROI 2011 – X. LOOZEN 21

Reduced recombination under Ti/Pd/Ag contacts

with AlOx tunnel layers is observed with PL

Ohmic behavior observed on flat emitters with up

to 1.5 nm AlOx (= optimal thickness for passivation)

Contact resistance on textured emitters not clear,

but no show-stopper indication

Conclusion

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© IMEC 2011

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