F R A U N H O F E R I N S T I T U T E F O R S O l A R E N E R g y S y S T E m S I S E

AUTOMATED WAFER INSPECTION AND QUALITY CONTROL

In order to fabricate solar cells cost-

effectively, it is important to produce high-

efficiency cells with the utmost process

stability and minimum process variation.

Not only the fabrication process is key here,

the electrical and mechanical excellence of

the wafers used is equally important. The

quality of multicrystalline wafer materials

is particularly susceptible to variation. Poor

quality wafers should be identified and

discarded at an early stage in the process to

avoid unnecessary costs.

Fraunhofer ISE uses modern, automated

inline metrology to collect and analyze

quality data for each wafer. We can test,

sort and assess the quality of wafers at

any stage of production (as-cut, diffused,

passivated, etc.) automatically and with

high throughput rates.

We offer the following services to

wafer, cell and measuring instrument

manufacturers:

n materials assessment as standard service

n customization of wafer rating models to

match client-specific cell processes

n transfer of wafer rating models to client

PL measurement systems

Automated Inspection System

An efficient tool from Hennecke Systems

is the core of Fraunhofer ISE’s Automated

Wafer Inspection System (AWIS). It com-

prises an extensive metrology portfolio for

characterizing the geometric, optical, and

electrical properties of wafers, and allows

quality data to be collected for individual

samples in large wafer batches. At the core

of our measurement system is a photolu-

minescence imaging module, which allows

optimization potential to be identified in

partially processed wafers.

1 Automated Wafer Inspection System

at Fraunhofer ISE.

2 The system is capable of sorting

wafers using predefined algorithms or

measurement results.

Fraunhofer Institute for

Solar Energy Systems ISE

Heidenhofstr. 2

79110 Freiburg

Germany

Silicon Photovoltaics –

metrology and Production Control

Dr Stefan Rein

Phone +49 761 4588-5271

Wafer Analysis

Dr Jonas Haunschild

Phone +49 761 4588-5563

sipv.metrology@ise.fraunhofer.de

www.ise.fraunhofer.de

09-750-16

1 2

Applications

n frontend inspection to assess the

quality of mono- or multicrystalline

as-cut wafers

n process control to assess the quality

of manufacturing processes and identify

potential for optimization

n device evaluation: The AWIS has empty

line positions where measuring

instruments can be temporarily installed

and tested. This gives suppliers of

metrology equipment the opportunity

to evaluate their devices in a production

environment and compare them with

high quality reference systems

Example: Wafer Rating

The quality assessment of multicrystalline

(mc-Si) and high performance multicrys-

talline (HPM) wafers in the reception in-

spection within industrial solar cell

production requires a classification scheme

that is also able to evaluate materials from

unknown manufacturers. To this end,

image processing software was developed

at Fraunhofer ISE that enables the different

defects caused by crystallization to be

recognized in photoluminescence images

(Fig. 6) and classified in quantitative terms

(Fig. 7). Here, the relevance of individual

image features is determined in relation

to their importance in predicting the open

circuit voltage (Voc) of the finished solar cell.

The resulting classification scheme was suc-

cessfully applied to 7,500 wafers in a high

efficiency PERC solar cell process, covering

virtually the entire spectrum of commercially

available material.

3 High resolution image of wafer edge with laser

markings and chipping.

4 Mapping the layer thickness of an

inhomogeneous ARC layer.

5 Photoluminescence (PL) image of a wafer from

the bottom zone of a square brick.

x (mm)

y (m

m)

edge defects (detail)

255

0

gray

sca

le v

alue

(a.u

.) 3

156

00

156x (mm)

y (m

m)

ARC thickness

90

0

ARC

thi

ckne

ss (n

m)

4

156

00

156x (mm)

y (m

m)

Photoluminescence image

1

0

PL s

igna

l (a.

u.)

5

610 620 630 640 650610

615

620

625

630

635

640

645

650

655

Measured V (mV)

Pred

icte

d V

(mV

)

MAE:2.49, RMS:3.22, Corr:0.92

mc-Si

HPM

oc

oc

7 Predicted voltage of PERC solar cells produced

by assessing the as-cut PL images for materials

from two manufacturers that were not included

in the training data set.

6 Crystal structure defects in Si wafers.

(HPM-Si above, mc-Si below). PL images of the

as-cut wafer are on the left, structural defects

extracted from these and used for the assessment

are on the right.

Furthermore, it provides a data set for

ultra-modern wafer rating algorithms which

permit the classification and sorting of mul-

ticrystalline as-cut wafer materials to a level

far surpassing current industrial standards.

metrology Tools

Alongside classic measurement assignments

with volumes of up to 10,000 wafers, we

also conduct automated wafer sorting tasks

with the AWIS with respect to different

criteria. In order to identify wafers and map

metrology data onto them, data matrix

codes on wafer surfaces and wafer-edge-

codes are read and processed. A total of

twelve measurement methods are available

that enable us to detect the following

properties and defects:

n thickness and thickness profile

n resistance profile

n wafer size

n saw marks and roughness

n edge defects

n microcracks

n reflectance spectrum

n surface contamination

n layer thickness topography

n lifetime topography

n photoluminescence imaging

Analysis Packages

We offer varying levels of data analysis

within our wafer inspection services:

n level 1: simple report with statistics and

interpretation of data

n level 2: model calibration for wafer

rating, automated image evaluation

n level 3: customization of analysis

algorithms to match wafer material or

cell process

156 mm x 156 mm

PL image HPM wafer Structural defects

PL image mc wafer Structural defects