A national laboratory of the U.S. Department of Energy

Office of Energy Efficiency & Renewable Energy

National Renewable Energy Laboratory 

Innovation for Our Energy Future 

Subcontract Report 

NREL/SR-520-43968 

September 2008 

Initiative in Photovoltaic 

Manufacturing 

Final Subcontract Report 

2 January 2002 – 15 January 2008 

S. Danyluk 

Georgia Institute of Technology 

Atlanta, Georgia 

NREL is operated by Midwest Research Institute ● Battelle Contract No. DE-AC36-99-GO10337 
Initiative in Photovoltaic 

Manufacturing 

Final Subcontract Report 

2 January 2002 – 15 January 2008 

S. Danyluk 

Georgia Institute of Technology 

Atlanta, Georgia 

NREL Technical Monitor: Fannie Eddy 

Prepared under Subcontract No. XXL-4-44233-01 

Subcontract Report 

NREL/SR-520-43968 

September 2008 

National Renewable Energy Laboratory

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Contract No. DE-AC36-99-GO10337
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1. Introduction 

 The Manufacturing Research Center (MARC) at Georgia Tech has been addressing 

photovoltaic manufacturing under an NREL grant. The initiative led to the creation of the 

Photovoltaic Manufacturing Laboratory (PVML) to address fundamental and practical issues 

related to handling and fracture of thin polycrystalline silicon wafers used in solar cells, 

measurement of residual and applied stresses in the wafer due to processing and handling, and 

the development of factory information software tools for real-time monitoring of equipment 

used in solar cell manufacturing. The lab is researching new and improved manufacturing 

techniques to enhance solar cell quality and yield, and lower the cost of PV devices. This paper 

summarizes the major accomplishments of the lab to date in the aforementioned areas.

2. Project Objectives 

 The major objectives of the photovoltaic manufacturing initiative at Georgia Tech are to 

research and develop new and improved methods for thin crystalline silicon wafer handling, 

transport, inspection, and real-time monitoring of equipment used in solar cell/panel 

manufacturing. The advances made in this research will enhance solar cell yield and quality, and 

lower the cost of manufacturing silicon-based PV. 

3. Technical Approach 

Under this grant, the Photovoltaic Manufacturing Laboratory (PVML) was setup at 

Georgia Tech to address both fundamental and practical issues related to handling and fracture of 

thin polycrystalline silicon wafers used in solar cells, measurement of residual and applied 

stresses in wafers due to processing and handling, and the development of factory information 

software tools for real-time monitoring of equipment used in solar cell manufacturing. 

Additionally, the project surveyed industry priorities and established research collaborations with 

PV equipment and cell manufacturers. 

This report summarizes the major accomplishments of the project in each of the 

following three areas: A) Optical inspection of residual stresses in thin silicon wafers, B) Thin 

wafer handling and breakage, and C) Factory information software development. The report is 

broken into sub-sections that discuss the aforementioned accomplishments. The appendix 

contains technical papers containing details pertinent to the sub-sections authored by different 

members of the project team.

A. Residual Stress Measurement

A1. Introduction 

Our group has been working on developing a non-destructive, non-contact optical 

technique to measure the residual stress in thin silicon sheet. Figure A1 shows a schematic 

diagram of the polariscope system. Circularly polarized light, which is provided by the first 

polarizer and quarter waveplate, is used to illuminate the silicon sample. The residual stresses in 

the sample will change the polarization state of the transmitted light, and the change is extracted 

by the second waveplate and polarizer. Two partial mirrors are used to increase the sensitivity of 

the system when the residual stresses are low. A tungsten lamp with an adjustable power up to 

 1
250W is used as the light source, and the images are captured by an infrared camera. In the 

camera, a band-pass infrared filter is used to pick a near infrared wavelength center at 1150nm 

with a bandwidth of 10nm. 

Light source 

Polarizer

1/4 waveplate 

Silicon sample 

Analyzer 

ρ

1/4 waveplate 

(α,θ)

φ

β

Beam splitter

Camera 

Beam splitter

Figure A1: Setup of residual stress polariscope 

The stress optic law can be used to obtain the shear stresses in the beam if the stress optic 

coefficient is known. The stress optic law is shown in equation 1. 

( )δ

φθπ

λ

σσ

2 , 1 2 Ct =− (1) 

Where C is the stress-optic coefficient which is a function of θ (the principle stress orientation) 

and φ (the crystal grain orientation), t is the thickness of the sample, λ is the wavelength of the 

light source, and σ1, σ2 are the two principal stresses. δ is the phase retardation which can be 

measured by the six step phasing method. 

A2. Goals and Objectives 

• Calibration of anisotropic stress-optic coefficient 

• Stress separation 

• Finding the relationship between stress and dislocation density 

• Analysis of thermal effect on residual stress measurement 

A3. Summary of Approach 

• Calibration of anisotropic stress-optic coefficient. 

The stress optic coefficient, a parameter used for the polariscope, is orientation dependent, as 

can be seen from equation 1. Back Laue x-ray diffraction system is used to determine the 

crystallography and find φ. Four-point bending was used to introduce known stresses so as to 

calibrate the anisotropic stress optic coefficients. The residual stresses in the wafers were 

assumed to be negligible compared to the applied stresses. Using equation 1, stress optic 

coefficient could be obtained since stress is known. 

 2
• Stress separation 

 

Photoelasticity provides information on the principal shear stress and its orientation in the 

wafer. To obtain the full state of stress ( and ), stress separation must be applied. Fourpoint-bending together with shear difference technique is proposed to be used for stress 

separation. Four point bending can tell whether the measured shear stress is positive or negative. 

Then the shear difference technique is based on the direct integration of the equilibrium 

equations in the following finite difference format: 

σσ yx , xy τ

∑ Δ

Δ

Δ = − x

y

boundary yx

xx

τ

σσ )( and/or ∑ Δ

Δ

Δ = − y

x

boundary yx

yy

τ

σσ )( (2) 

By equation (1) and (2) and the basic stress transformation equations of mechanics, full state 

of stress ( and ) can be calculated. σσ yx , xy τ

• Finding the relationship between stress and dislocation density 

In the case of obtaining the relationship between dislocation density and residual stress, the 

experiment used an optical microscope and image analysis in addition to the polariscope that 

measured the residual stresses. EFG wafers were used for the dislocation density experiment 

since their structure is relatively constant and the surface is smooth for a clear picture of etch pits. 

A Sopori etch was done to reveal etch pits on the surface of EFG wafers. Etch pit density is taken 

to be the same as dislocation density. Wafers were also subjected to polariscopy to obtain the 

residual stress maps, and the results of etch pit density and residual stress were analyzed to find 

their relationship. 

• Analysis of thermal effect on residual stress measurement 

 

This task is designed to address measurement resolution and reliability issues associated with 

the IR-polariscope. In particular, it is designed to systematically study the role of temperature 

rise in the silicon wafer due to IR light absorption on the residual stress output by the system. 

The non-uniform energy profile of the light source can produce a thermal gradient in the Si 

sample and as a result, a thermal stress may be induced in the wafer, which will increase the 

system error. A thermal model is built from ANSYS to calculate the temperature in the silicon 

wafer. Then the temperature profile is input to a structural model to calculate the thermal stress. 

A4. Key Findings 

• The major preferential orientation in cast wafers is (111) and near (111) but other dominant 

orientations are (100), (210), (110), (144), (211), (321), and (431). The stress optic 

coefficients vary with orientation with a magnitude of 2. 

• Full components of stresses are obtained for several silicon beams. And the maximum first 

principle stress is found about 30 MPa for EFG wafers. This is a conclusion based on a small 

sample size. 

 3
• A relationship was found between dislocation density and residual stress. The graph in Figure 

A2 shows the behavior of residual stress at high values of dislocation density. Equations are 

shown in the graph to quantify the relationship. There is an uncertainty in which stress 

relaxation occurs since there are sources of residual stress other than dislocations. 

Figure A2. Relationship between dislocation density and residual stress 

• Thermal model shows the polariscopy system needs about 1 to 2 minutes to come to steady 

state. 

A5. Conclusions 

• The predominant orientations were found for cast wafers. 

• The relationship between dislocation density and residual stress is found. 

• Full stress components are obtained for several silicon beams. 

• Preliminary models are built to calculate thermal effect for the polariscopy system. 

B. Thin Wafer Handling and Breakage

B1. Introduction 

Shortage of silicon (Si) combined with the need to lower the cost of crystalline silicon based 

solar cells has contributed to the growing use of thinner and larger wafers. However, studies 

have shown that the use of thinner wafers can lead to unacceptable yields arising from wafer 

breakage during handling, transport and/or processing [3]. Consequently, it is critical to 

 4
understand the causes of wafer breakage [4]. Fundamentally, breakage of Si wafers is due to the 

propagation of cracks present in the wafer during a processing or handling step in solar cell 

manufacture. Knowledge of crack locations and sizes is therefore required to predict wafer 

breakage accurately. A crack will propagate if a sufficiently large in-plane tensile stress is 

applied orthogonal to it (assuming Mode I fracture). Thus, it is necessary to evaluate the nature, 

magnitude and distribution of stresses produced in the wafer/cell during manufacture. This 

includes residual stresses generated in a prior process step and stresses applied to the wafer/cell 

by the current processing, handling and/or transport method used in manufacture. 

B2. Goals and Objectives 

The objective of the current work was to analyze the mechanical deformation and stresses 

produced in sc-Si and mc-Si wafers and the relationship between the handling stresses and 

possible wafer breakage. In particular, we focused on one of the more commonly-used handling 

devices in the PV industry, namely the Bernoulli gripper but the approach can be used for any 

handling device. 

B3. Summary of Approach 

The experimental setup used to measure the gripper forces and wafer deformation consists of 

a 4-axis Adept SCARA robot equipped with a Bernoulli gripper used to pick up wafers using 

high–precision and controlled gripping forces. Wafer deformation profile was measured by a 

laser displacement sensor (Micro-Epsilon OptoNCDT 1700). The robot moves the wafer that is 

being handled along a specific path while the laser sensor measures the wafer deformation. The 

volumetric flow rate is kept constant during the procedure. Once the scanning is complete, the 

height between the laser and the four rubber pads on the Bernoulli gripper is measured. 

In order to predict wafer breakage it is critical to know the stress state of the wafer (applied 

stresses and residual stresses). Two approaches were developed. First, the total distribution of 

stresses produced in the silicon wafer due to handling forces exerted by the Bernoulli gripper and 

the residual stress present in the wafer were obtained by solving finite element models created 

using the ABAQUS® software using the measured deformation [5, 6]. Second, the combination 

of analytical fluid dynamics model and a finite element models created using the ABAQUS® 

allowed predicting the stress level in wafers without using experimental measurement. In this 

scenario the residual stresses need to be superposed and were not included in the model [7]. 

B4. Key findings 

• The influence of volumetric air flow rate on the maximum wafer deformation was 

investigated. It is shown that the maximum deformation increases with air flow rate. It 

also shows that an upper limit of wafer deformation exists for the thin wafers as the flow 

rate is increased. 

• From the investigation of the influence of volumetric air flow rate and wafer type on the 

wafer deformation contours, identical conclusions were also drawn. In addition, it is 

found that for all wafer types, there is a preferred orientation of the deformation as the 

volumetric flow rate is increased. Material anisotropy cannot explain this preferred 

orientation since it appears to be the more or less the same for all wafer types. 

• As shown in Table B1, the critical crack length was calculated assuming that a crack is 

present at the location of the maximum tensile stress [5]. As expected, irrespective of the 

 5
wafer type, an increase in volumetric air flow rate leads to an increase in the tensile stress 

and a smaller critical crack length for wafer breakage. 

Table B1. Max. in-plane principal tensile stress and critical crack length results 

V KIC σmax lc

(L/min) Mpa.m0.5 (Mpa) Location (mm) 

Cz wafers 

30 

0.95 

5.4 Center 10.037 

35 17.1 Edge 0.984 

40 133.2 Edge 0.016 

EFG wafers 

30 

0.9 

42.0 Center 0.146 

35 128.9 Edge 0.016 

40 226.9 Edge 0.005 

Cast wafers 

125 mm x 125 mm 

30 

0.75 

4.5 Center 8.961 

35 11.1 Center 1.456 

40 118.6 Edge 0.013 

Cast wafers 

156 mm x 156 mm 

30 

0.75 

8.0 Center 2.784 

35 49.6 Edge 0.073 

40 131.8 Edge 0.010 

Figure B1. Typical wafer deformation profile (mm) and maximum in-plane principal stress 

distribution (Pa) for Cz-wafer at both extreme volumetric flow rates (30L/min & 40 L/min) 

• Figure B1 shows the typical Cz wafer deformation profiles for the lowest and highest 

volumetric air flow rates tested. The deformation profile and the corresponding maximum 

in-plane principal tensile stress distribution are seen to change significantly with increase 

in the air flow rate. Specifically, the location of maximum tensile principal stress shifts 

from the center of the wafer to the edge. 

 6
• Therefore, for a given wafer type, breakage may initiate either in the center or at the 

edges depending on the presence, orientation and size of cracks equal to or greater than 

the critical crack length at the site of the maximum tensile stress. Similarly, if one could 

guarantee that all cracks present in the wafer are smaller than the critical crack length 

regardless of their location and orientation, wafer breakage would be avoided. 

B5. Conclusions 

This section presented the results of an experimental and modeling investigation of 

deformation and stresses produced in polycrystalline silicon wafers handled by a Bernoulli 

gripper. The experimental work showed that the air flow rate and wafer thickness have a 

significant effect on wafer deformation. For all wafer types, results showed a transition in the 

maximum in-plane principal tensile stress location from the center to the edges as the air flow 

rate was increased. The relation between the total stress and breakage was analyzed by 

calculating the critical crack length. Furthermore, with the knowledge of residual stresses, 

handling stresses (calculated from the FE model) and wafer strength, breakage can be predicted. 

C. Factory Information Systems 

C1. Introduction

 In order to accomplish the Department of Energy (DOE) goals of reducing the levelized 

cost of energy while scaling-up domestic manufacturing capacity of PV, information 

technologies must play an every increasing role in the manufacture of such devices. As such, 

innovative factory information systems were researched, developed and implemented through 

this effort. 

 DOE is targeting that the U.S. produce 10 GW of photovoltaic products each year by 

2015. This will require the sourcing, manufacture, assembly and installation of billions of parts. 

As an example, if all domestic production consisted of silicon wafer modules, the number of 

wafers would wrap the earth more than three times, more than 11,000 modules would have to be 

produced every hour of every day, which would require 100 factories running 24/7. If domestic 

PV production is to grow to this level by 2015, then commonality among systems must be 

defined so that material suppliers, equipment producers, software developers and manufactures 

can construct factories in a timely basis. A conceptual of a highly automated factory is shown in 

figure C1. 

 7
Figure C1. Highly automated photovoltaic factory of the future. 

 Factory information systems are a major and necessary component of high volume 

manufacturing. They act as conduits for information flow among equipment, software 

applications, vendors and the enterprise as a whole. They also aid in decision making from the 

factory floor to the boardroom. Without correctly architected systems, the ability to build 

factories that are capable of producing cost effective products is extremely limited. 

C2. Goals and Objectives

 The goals of this work were to determine if standards used in other industries 

(specifically electronics assembly) could be applied and extended for use with photovoltaic 

manufacturing. Specifically, the IPC computer aided manufacturing using XML (CAMX) 

standards were investigated for use in PV manufacturing. By extending an accepted technology, 

the cost and risk of implementing a common methodology is greatly reduced. The CAMX 

infrastructure located in the lab is shown in figure C2. 

 The use of CAMX standards for collecting, aggregating and presenting experimental 

photovoltaic data was also investigated by partnering with the National Renewable Energy 

Laboratory (NREL). 

 8
Wafer Inspection and Handling

CAMX Message Broker

Robot Handling 

Computer

Robot 

Controller Database 

Server

(MySQL)

CAMX API

CAMX

ODBC

Residual Stress 

Computer

CAMX API

CAMX

Application 

Server

(Apache Tomcat)

ODBC

JSP Pages Robot 

Interface Java Servlets

Java Beans

Camera

Wafer Fixture 

Optics and 

Stage

Robot

Sensors

Sensor 

Interface Video

Optics 

Controller

Controller 

Interface

Database Interface 

Application

CAMX API

CAMX

Light Source

Figure C2. The Photovoltaic CAMX infrastructure developed for the project. 

C3. Summary of Approach

 In order to determine if the CAMX standards could be used for photovoltaic 

manufacturing, a CAMX infrastructure was establish in Georgia Tech’s Photovoltaic 

Manufacturing Laboratory. A CAMX message broker that exchanges xml messages based upon 

predefined schemas was installed. Adapters were written which receive proprietary data structure 

and translate the information into CAMX standard messages were written. The adapters sent the 

xml messages to the broker, and the broker sent the messages to a application server and 

database which compiled the discrete messages into web based charts, graphs and tables for 

viewing. 

 NREL J-V experimental data was also streamed over the internet to the CAMX 

infrastructure located at Georgia Tech. Specific software adapters were developed which were 

capable of parsing the experimental data and producing aggregated xml messages based upon the 

IPC-2547 schemas. Specific reports for the CAMX monitor were developed to display the 

experimental data. 

 

C4. Key Findings

 By extending the schemas that were written for electronics assembly, the CAMX 

standards could easily be used for photovoltaic manufacturing. The IPC-2547 standards were 

especially useful for encapsulating experimental data. General manufacturing data could be 

represented through the IPC-2541 and the IPC-2546 standards. The message broker and monitor 

worked very well for photovoltaic manufacturing. 

 9
 10

C5. Conclusions

 This work demonstrated that the CAMX standards can be made use of by the 

photovoltaic industry to improve productivity and reduce cost. The barrier to applying CAMX to 

photovoltaic manufacturing is low because of the similarity between electronic assembly 

manufacturing (the industry CAMX was developed for) and photovoltaic manufacturing. The 

CAMX standards don’t fully address photovoltaic manufacturing, but could be extended to do so. 

It is recommended that a project to extend the CAMX standards be undertaken so photovoltaic 

manufacturing can receive the full benefits that this technology has demonstrated. 

References 

[1] S. He, S. Danyluk, I. Tarasov, S. Ostapenko “Residual stresses in polycrystalline silicon 

sheet and their relation to electron-hole lifetime” Applied Physics Letters (2006), 89, Issue 11, 

2006, Pages 111909-111901-3. 

[2] S. He, T. Zheng and S. Danyluk, “Analysis and Determination of Stress-Optic Coefficient Of 

Thin Single Crystal Silicon Samples” Journal of Applied Physics, Vol. 96, number 6, 15 Dec 

2004, pp3103-3109. 

[3] Wang, P. A., Industrial Challenges for Thin Wafer Manufacturing, Proceedings of the 4th 

World Conference on Photovoltaic Energy Conversion, 2006. Waikoloa, HI, pp. 1179-1182. 

[4] Sopori, B., Sheldon, P., and Rupnowski, P., Wafer Breakage Mechanism(s) and a Method for 

Screening “Problem Wafers”, Proceedings of the 16th Workshop on Crystalline Silicon Solar 

Cells and Modules, 2006, Denver, CO, pp. 129-138. 

[5] Brun, X.F. and Melkote, S.N., Analysis of Stresses During Handling and Transport, 

Proceedings of the 17th Workshop on Crystalline Silicon Solar Cells and Modules, Materials 

and Processes, 2007, Vail, CO, pp. 26-33. 

[6] Brun, X.F. and Melkote, S.N., Evaluation of Handling Stresses Applied to EFG Silicon 

Wafer Using a Bernoulli Gripper, Proceedings of the 2006 IEEE 4th World Conference on 

Photovoltaic Energy Conversion, 2006, Waikoloa, HI, USA, pp 1346-1349. 

[7] Brun, X.F. and Melkote, S.N., Experiment and Modeling of Silicon Wafer Handling Using a 

Bernoulli Gripper, Proceedings of the 2006 ISFA International Symposium on Flexible 

Automation, 2006, Osaka, Japan.
F1146-E(09/2007) 

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Initiative in Photovoltaic Manufacturing: Final Subcontract Report, 

2 January 2002 - 15 January 2008 

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14. ABSTRACT (Maximum 200 Words)

The major objectives of the photovoltaic manufacturing initiative at Georgia Institute of Technology are to research 

and develop new and improved methods for thin crystalline silicon wafer handling, transport, inspection, and real-time 

monitoring of equipment used in solar cell/panel manufacturing. This report summarizes the major accomplishments 

of the project in each of the following three areas: (1) Optical inspection of residual stresses in thin silicon wafers, 

(2) Thin wafer handling and breakage, and (3) Factory information software development. The advances made in this 

research will enhance solar cell yield and quality, and lower the cost of manufacturing silicon-based PV. 

15. SUBJECT TERMS

PV; manufacturing; solar cells; silicon wafers; real-time monitoring; stress optic law; photoelasticity; thermal effect; 

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