NEED: Plasma resistivity (××××10)

NEED: Resonance frequency (MHz)

20 nm a-Si-i as deposited after 70 s H2 plasma etching after 110 s H2 plasma etching

www.pvcomb.de

INTRODUCTION

PECVD Process MonitoringPECVD Process Monitoring

in Thin Film Silicon Solar Cell Manufacturingin Thin Film Silicon Solar Cell Manufacturing

Onno Gabriel1, Ivo Erkens

2, Matthias Zelt

1, Björn Rau

1, Bernd Stannowski

1, Rutger Schlatmann

1

1 PVcomB/Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Berlin, Germany

2 Department of Applied Physics, Eindhoven University of Technology, Eindhoven, The Netherlands

Email: onno.gabriel@helmholtz-berlin.de

ACKNOWLEDGEMENTS

PLASMA DIAGNOSTICS: A-SI/µC-SI PHASE TRANSITION

Competence Center Thin-Film- and Nanotechnology for Photovoltaics Berlin

Figure: OES measurement of the

SiH and H radical densities (top),

RGA measurement of H2 and SiH4

densities (middle) and plasma re-

sistivity and resonance frequency

(bottom) during H2 plasma etch.

PECVD SYSTEM & IN SITU PLASMA DIAGNOSTIC TOOLS

Figure left: NEED raw data example

showing higher harmonics in the rf

current signal.

PLASMA MONITORING: PROCESS STABILITY

Amorphous and microcrystalline layers for a-Si:H/µc-Si:H thin film silicon tandem cells are de-

posited at PVcomB in an fully automated Applied Materials AKT1600A PECVD system. Sub-

strates are 30x30 cm² glass panels coated with SnO2 or ZnO front contact TCO.

The PECVD chambers are equipped with several in situ diagnostics tools with the aim of

1. Plasma monitoring to control process stability and the determination of long term drifts,

2. Plasma diagnostics to measure plasma parameters for a better understanding of the

PECVD processes, i.e. the plasma chemistry and the plasma/surface interaction and film

growth.

AMAT

cluster tool

SEERS

SEERS

OES

OES

OES

spectra

Logfiles, Recipes

Mass

spectraChA

ChB

RGA

RGA

SEERS

data

Sun

Workstation

Input

folder

PVcomB Data Server:

Scripts

MySQL

data base

Web

Server

Desktops,

Laptops

Desktops,

Laptops

Desktops,

Laptops

200 300 400 500 600 700 8000

100

200

300

400

500

600

700

800

µc-Si:H BCi deposition

a-Si:H BCn deposition

Si

Si

Si

SiH

Hβ

Inte

nsity (

a.u

.)

Wavelength (nm)

H2 fulcher

Hα

400 500 600 700 800 900 10000

500

1000

1500

2000

F

F

F

N2

N2

Process start

Process end

F

N2

N2

NF

N2

Inte

nsity (

a.u

.)

Wavelength (nm)

NF3 plasma clean Species Transition Wavelength

Si 3s23p

2 – s

23p4s 288.3 nm

SiH X2Π – A

2Δ 409 – 422 nm

Hα n=3 – n=2 656.3 nm

Hβ n=4 – n=2 486.1 nm

H2 2s3Σ

+g – 3p

3Π

−u 570 – 640 nm

F 2s22p

4(

3P)3s - 2s

22p

4(

3P)3p 703.7 nm

F 2s22p

4(

3P)3s - 2s

22p

4(

3P)3p 624.0 nm

Ar 3s23p

4(

3P)4s - 3s

23p

4(

3P)4p 496.5 nm

SiHx

a-Si-H BCn Deposition

Si2Hx

SiF3

Hx

44CO2

48SiHF

49SiH2F16O

14N

18H2O

34PH3

89PHF3?

19F20HF

47SiF

Figure: Equivalent circuit of

the rf circuit.

NONLINEAR EXTENDED ELECTRON DYNAMICS (NEED)

RESIDUAL GAS ANALYSIS (RGA)

OPTICAL EMISSION SPECTROSCOPY (OES)

PEAVI: PECVD ON AKT VISUALIZATION

Figure: Scheme of an AKT1600 process chamber with in-

stalled in situ plasma diagnostic tools OES, NEED and RGA.

SUMMARY

The PECVD chambers for thin film silicon deposition at PVcomB are equipped with in situ

plasma diagnostic tools (OES, RGA and NEED), which deliver several complementary plasma pa-

rameters. These parameters give a better insight into the plasma chemistry and plasma/surface

interaction. Moreover, some of them are identified to be useful for plasma monitoring: they are

reproducibly indicators of the process stability, revealing short and long term drifts of the

PECVD processes in thin film solar cell production.

-0.05

-0.04

-0.03

-0.02

-0.01

0

0.01

0.02

0.03

0.04

0.05

0 36.87 73.74 110.61 147.48

Time (ns)

Current @ sensor (a.u.)

Voltage @ generator (a.u.)

A Hercules N250 NEED system (Plasmetrex GmbH) delivers

absolute and reproducible parameters that characterize the

rf circuit. They depend strongly on the chamber history and

give valuable information about the process stability.

Table right: Overview about

plasma parameters derived from

NEED measurements.

Densities of various atoms and molecules within the plasma gas phase are measured by OES.

Since the emission lines originate from electronically excited species, the densities are relative

and depend not only on the ground state densities, but also on the excitation process (electron

collisions). Species detected: Ar, F, NF, N2, Si, SiH, H, H2. The gas temperature is derived, too.

A MKS Vision 2000C residual gas analyzer (mass

spectrometer) is installed close to the pump exit

at the chamber. It measures stable atom and

molecule densities (see example spectrum).

All measured data in combina-

tion with AKT process data

(gas flow rates, chamber pres-

sure etc.) is stored in a data

base and analyzed in PEAVi, a

an data analysis software de-

veloped at PVcomB.

PLASMA DIAGNOSTICS: NF3 CHAMBER CLEAN

[1] G. Dingemans, M. N. van den Donker, A. Gordijn, W. M. M.

Kessels, M. C. M. van de Sanden, Applied Physics Letters 91

(2007) 161902

Pressure (Torr)

Hα @ 656 nm (thick)

Hβ @ 486 nm (thin)

Ar @ 696 nm

F @ 703 nm

2 31

NEED: Resonance frequency (MHz)

NEED: Plasma resistivity (××××10)

The PECVD chambers are cleaned by an active NF3/Ar

etch plasma after each deposition (Fig. right).

The plasma chemistry during the clean is complex

and heavily influenced by the surface conditions.

Even a very thin a-Si:H film on the glass substrate can

strongly change the plasma (Fig. bottom).

To determine the a-Si:H/µc-Si:H phase transition, films grown under various SiH4 concentrations

are etched by a H2 plasma. It has been shown that the SiH radical density in this H2 etch plasma

depends on the degree of crystallinity of the previous grown film due to different etch rates [1].

We found, that several other plasma parameters are influenced by the film surface, too:

The a-Si:H/µc-Si:H phase

transition can be determined

fast and in situ by the usage

of plasma diagnostics for

various different plasma con-

ditions.

Figure right: Plasma resistivity (top) and resonance

frequency (bottom) measured by NEED during 9 dif-

ferent chamber cleans. The black curve is the first

clean of the day (different chamber condition).

01/10/2012 01/18/2012 01/24/2012

NEED is an excellent plasma monitoring tool

that delivers absolute and comparable pa-

rameters characterizing the process stability.

The slight drifts in NEED parameters over a day

are not fully understood. They are probably due

to temperature effects. However, parameters

during the first deposition of a day are very re-

producible.

Figure: Pressure, OES signals and NEED data dur-

ing Ar/NF3 etch: 1) - 3): electrode, susceptor and

remaining chamber walls completely cleaned.

Figure left: Plasma resistivity (top) and resonance fre-

quency (bottom) measured on three different days during i

-layer deposition.

Plasma resistivity = νeff ωgen

ωe (ne)2(basically the collision rate

over electron density)

Parameters derived from NEED measurements:

Resonance frequency of the total system

depending on

- bulk inductance Lp

- the sheath capacitances Cs

- the inductance of the electrode system/chamber LC. (all directly depending on electron density, chamber design and

gap between electrode and substrate)

1

2

3

4

Uniformity Edge = ratio of geometric dimension to skin

depth (RE/2.405∙δskin)

Nonlinearity= ability of the plasma to generate higher

harmonics (here: fixed to 0.125)

5 RF Fundamental = RF current at generator frequency

6 V1rms_mV = sensor signal amplitude of the first

harmonics (in mV)

This work was supported by the Federal Ministry of Education (BMBF) and the state

government of Berlin (SENBWF) in the framework of the program “Spitzenforschung

und Innovation in den Neuen Ländern” (grant no. 03IS2151) and by the BMBF and the

Federal Ministry for Environment, Nature Conservation and Nuclear Safety (BMU) in

the framework of the program “Innovationsallianz Fotovoltaik” (grant no. 0325317C).