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Contract No. DE-AC36-99-GO10337 

B. Nelson, S. Robbins, and P. Sheldon 

Prepared for the 31st IEEE Photovoltaics Specialists 

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February 2005 • NREL/CP-520-37458 

The NCPV Process Integration 

Project: Purpose, Status and 

Direction 
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The NCPV Process Integration Project: Purpose, Status and Direction 

Brent Nelson, Steven Robbins, and Peter Sheldon 

National Center for Photovoltaics, National Renewable Energy Laboratory 

Golden, CO 80401-3393, USA 

ABSTRACT

The primary purpose of the process integration

project of the National Center for Photovoltaics (NCPV) is

to develop an infrastructure that will allow researchers to 

gain new knowledge that is difficult—if not impossible—to 

obtain with existing equipment. This difficulty is due, in

part, to the state of our existing tool set, which lacks

sufficient in-situ or real-time measurement capabilities, or 

lacks access to analytical tools where the sample remains

in a controlled environment between deposition and 

processing or measurement. This new infrastructure will

provide flexible and robust integration of deposition, 

processing (etching, annealing, etc.), and characterization

tools via a standardized transfer interface such that

samples move between tools in a controlled ambient. The

standardization of control and data acquisition software

schemes, sample handing, and equipment components

will allow us to perform research more efficiently; 

facilitating collaborations and technique development. 

INTRODUCTION: PROCESS INTEGRATION NEEDED 

Process integration has been the key to the rapid 

advancement of the integrated circuit (IC) industry [1], 

which works with similar materials as the photovoltaics 

(PV) industry. In IC industry case, the various

manufacturers came together to develop tool standards 

and integration schemes that were shared across the

industry. In this way, they were able to drive the direction

of tool fabricators to meet the needs of the entire industry. 

This is not to say that they shared all their manufacturing

details. A useful analogy might be that they shared

kitchen designs, appliances, and ingredients, but not the 

recipes or family secrets. The integrated-circuit industry 

had two distinctive advantages in accomplishing their 

process integration: they handle only one type of 

material—silicon wafers—and they have much greater

financial resources with which to influence tool

development. One of the keys to the success of the IC

industry was the creation of mini-environments, in which

the industry significantly reduced the contamination of 

wafers between processing steps [2]. The standard 

mechanical interface (SMIF) pods are used to enclose and

transport wafers between 200-mm tools; the newer frontopening unified pods (FOUP) are used on 300-mm tools. 

Wafers are taken from one tool, loaded into these minienvironments, and transported in a clean ambient to

another tool. These mobile pods are usually filled with

clean air or inert gas. 

The PV program within the National Renewable

Energy Laboratory (NREL) has advanced through focused

efforts in materials growth and processing, development of 

novel device design, and the measurement and

characterization of materials and devices. However, the

continued success of the NREL PV program will require

the integration of these research areas similar to the steps 

taken by the IC industry. The National Center for 

Photovoltaics (NCPV) has realized this need for a long

time [3]. The III-V group at NREL has successfully used 

an integrated metal-organic chemical vapor deposition 

(MOCVD) chamber, a molecular-beam epitaxy (MBE)

chamber, and a surface science chamber for a number of 

years. By having integrated deposition and

characterization and being able to transfer samples

between chambers under ultra-high vacuum (UHV),

researchers have been able to investigate the stability of 

surfaces [4] and surface reconstruction processes [5], 

among other things. The most recent example of

integrated chambers is in the surface analysis group. The 

group has four chambers integrated via a UHV track 

transfer system, giving them X-ray photoelectron 

spectrometry (XPS), Auger electron spectrometry (AES), 

and PVD capabilities, as well as integration to a glove box

for loading samples, wet etching, and chemical bath

depositions (CBD). All in all, there are fewer than 20

chambers integrated on 8 different platforms. These early

attempts at integration make up less than 10% of the

capabilities within the NCPV. The remaining systems are 

comprised of designs that do not lend themselves to being

integrated with any substrate form factor. While most of 

these systems have the requisite instrumentation 

(measuring pressure, power, flow, temperature, etc.), 

most are manually controlled. Some are computer 

controlled, but the computer systems are dedicated to that 

tool and may use propriety software making it difficult to

get the data out of that workstation. Only a few of these

systems are “recipe driven,” following a data transfer

protocol that lends itself to full automation. 

The main design goals of the NCPV Process

Integration project are to: 

• Develop a standard sample transport between tools 

• Ensure the sample transport mechanism is robust 

• Control the ambient of that sample transfer 

• Be able to deposit uniformly and reproducibly over 

areas large enough to be meaningful to industry

• Handle a wide variety of sample substrates 

• Standardize control and data logging software

• Integrate as many techniques as practical [6].

1
APPROACH: THE PATH TO PROCESS INTEGRATION 

There are four foundations to an integrated tool set: 

(1) Hardware: being able to pass samples between tools 

(2) Software: integrated control and data acquisition 

(3) Peopleware: collaboration on common goals

(4) Facilities: providing proper space and utilities. 

Hardware Standards 

We will integrate individual deposition, processing, 

and characterization techniques via one of several

different modes. Ideally, characterization techniques will

be used for real-time analysis of materials or surfaces

during deposition or of the process itself. The next best

solution is in-situ analysis of materials or surfaces. When

neither of these integration methods is possible,

techniques will be integrated by transferring samples from 

one location to another, either via intra-tool or inter-tool

sample transport. Intra-tool transport is the movement of

samples between techniques within the same set of 

interconnected chambers, that is, a cluster tool. The

actual transfer mechanism could be robotic or a linear

track transport mechanism. Inter-tool transport is the 

movement of samples between techniques, where those 

techniques do not share direct connection. Integration

types are summarized in Table 1 and a schematic

representation is provided in Figure 1. 

Table 1: Integration Modes: Measurement Apparatus to 

either Deposition or Processing Apparatus 

Meas. 

Class 

Transport 

Ambient 

Meas. 

Local

Meas. 

Ambient 

Meas. 

Timing 

RealTime

X1 same 

chamb. fixed2 during process 

In-Situ X1 same 

chamb. fixed post or 

interrupted

IntraTool3 fixed same 

tool fixed post deposition 

InterTool4 fixed diff. 

tool fixed post 

deposition 

Mobile5 fixed mobile fixed varies w/

technique 

Ex-Situ, 

air trans. air diff. tool fixed post deposition 

Ex-Situ, 

air meas. air diff. 

tool air post 

deposition 

1 In-Situ: Latin for “in the original place.” Real-time diagnostics 

are a sub-set of in-situ. Once a sample is moved from the 

original place (chamber), it is an ex-situ measurement, even if 

it is within the same tool. 2 A fixed ambient is controlled—e.g., Ar, UHV, etc.—as opposed 

to an uncontrolled ambient such as lab air. 3 Intra-tool transport is the movement of samples between 

techniques within the same set of interconnected chambers 

(i.e., the sample transfer within a cluster tool) [7]. 4 Inter-tool transport is the movement of samples between 

chambers lacking a direct connection (i.e., independent cluster 

or stand-alone tools) [7]. 5 A mobile technique is within a chamber that can be moved 

between tools for a fixed set of experiments.

Techniques integrated via an inter-tool transport could

be in a stand-alone tool or a part of a cluster tool. The 

sample is moved from one tool into the pod, which is

sealed and disconnected from that tool before being

wheeled to another tool where the process is reversed.

While this approach is similar to that used in the IC

industry, the transfer ambient within our pod must be

either an atmosphere of ultra-high-purity inert gas or high 

vacuum. These environments are much more difficult to 

maintain than the “particulate free air” requirement of the 

SMIF pod or FOUP [2]. 

Figure 1: Schematic of inter-tool and intra-tool

integration of equipment; a stand-alone tool at the top 

and a four-chamber robotic cluster tool at the bottom. 
Design standards are necessary to build a collection

of integrated tools. Most of these hardware elements are 

represented in Figure 1 and briefly described below. 

The platen drives the requirements for the entire

design. We have set the maximum substrate size the

platen can handle to be 6.18” X 6.18” (157 mm X 157

mm). This size supports the silicon photovoltaic industry,

which has a “6-inch square” protocol in multi-crystalline 

and a “6-inch round” protocol in single crystalline. It is 

also a good size for the various thin-film technologies, as it

is large enough to demonstrate commercially viable 

processes, yet not so large as make tool and substrate

costs prohibitive. The outside dimension of 7” X 7” will

accommodate a variety of platen designs to allow for

different substrates, such as soda-lime glass, hightemperature glass (e.g., Corning 1737), crystalline wafers

(e.g., Si, Ge, GaAs), thin stainless steel, ceramic, and 

exotic materials (e.g., plastics and thin foils). Individual

researchers will be able to use platens that hold smaller 

substrates, but must be able to accept the standard size in

their tool. In specialized cases, a smaller platen will be

“piggy-backed” onto the standard platen for loading into

tools that cannot be built to accept the standard size. The 

platen itself must be able to withstand temperatures in

excess of 800oC; therefore, construction material will be 

molybdenum, Inconel, or similar material. 

In Figure 1, the chamber that moves from tool to tool

is referred to as the transport pod; it provides the inter-tool

transport between independent tools. It contains a

cassette capable of securing up to six platens for transport

between tools. A quick connection between the transport 

pod and the tool occurs in the “interstitial region,” allowing

the pod to be separated from tool and moved to another.

There is a valve on both sides of this interstitial region to 

reduce air exposure to both the pod and the tool. Once a

pod is connected to a tool, this interstitial region is

evacuated before any transfer occurs. 

The dock is the design element that houses the 

transfer mechanism for introducing platens into either a

stand-alone chamber or a transport pod. With the transfer

mechanisms housed in the dock, both the stand-alone

tools and transport pods are free of internal transfer

mechanisms. The dock contains a cassette capable of 

queuing up to six platens for use in a given tool. 

The first stand-alone tool built to these process

integration standards for the NCPV combines a PVD

chamber, a dock, and a pod. It will allow us to test heater

designs for uniformly heating the standard platen to

~800°C and be used for sputtering of metals and

transparent conducting oxides. This tool will also contain 

a flipping station to allow samples to be loaded into a 

cassette, either film-side up or film-side down for different 

applications. 

A robot moves a platen from chamber to chamber in

Figure 1, thus providing the intra-tool transport within that

cluster tool. Another way to connect independent 

chambers is to have a long chamber with a track or trolley 

system and stopping points where a separate mechanism 

moves the platen orthogonally from the track to a given

chamber. There are advantages and disadvantages to

each transfer mechanism, as described in Table 2. The

first cluster tool built to these process integration

standards for the NCPV will be a robotic cluster tool to

support silicon work (e.g., thin-silicon and silicon-nitride

growth). 

Table 2: Cluster Tool Platform Comparison 

Consideration Track

Transfer1 Robotic 

Transfer2

Transfer 

Automation 

– Difficult to 

automate 

+ Designed to

automate 

Throughput3 – Low: Limited 

automation 

+ High: Full

automation 

Robustness – Each one is a 

unique design 

+ MTBF ~80,000 

hours 

Footprint – Larger: Uses 

up lab space 

+ Smaller: Saves

lab space 

Expandability

• Add modular 

sections4 • Robot-to-robot 

handoff5

Instrumentation 

Space Available 

+ Large ch. to 

ch. distance 

– Tight chamber 

clusters 

Maintenance 

Access 

+ Easy, ch.s are 

spread out 

– Difficult, ch.s 

are packed in

Vacuum Level in 

Transfer Zone 

+ 10-10 Torr best 

– Move in min.s 

– 10-8 Torr best 

+ Move in sec.s 

1 A linear transfer along a “spine” where tools are accessed 

perpendicularly via a secondary mechanism. 2 Process chambers around a centralized chamber--with a 

circular form factor--containing a robot arm. 3 Throughput is a separate issue from automation, if the time 

between processing steps needs to be short. 4 If space is available, expandability is easy by adding a hand-off 

station to the “end of the line.” 5 Once all ports are full, a new robotic chamber needs to be 

connected via one port, losing a chamber to hand-off on the 

existing robot, but adding n-1 ports, where n = ports on new

robot. 

Software Standards 

The IC industry has learned that the integration of

control and data acquisition software is important and time

consuming [8]. Efforts to create “plug and play”

integration of manufacturing tools and data are under way 

within the IC industry [9] and are being addressed as a

part of this project. Specifically, the use of recipe/run 

control using XML-based data transfer. This is a minimal

set of CAMX protocols implementing the sending of 

recipes from a central database. The tool writes back to

the database actual readings from the process. 

Peopleware 

The Process Integration Project Engineering Team 

meets regularly with representatives from research groups

throughout the NCPV to discuss all design features. 

Typically, these meetings focus on a particular design

area, with a core group concentrating on each aspect. 

When a key decision is reached, the appropriate focus

3
group gathers consent from their existing research team

and then signs off on the appropriate aspect of the project. 

These focus groups tend to concentrate on the fine detail

of the project. 

An Umbrella Group was formed with representatives

from the core research areas within the NCPV to ensure

that the project is compatible with future needs. This 

group gives guidance and signs off on all process

integration tool purchase requests. They weigh the big

picture of the project, considering overall integration and 

functionality. 

An Advisory Group was formed with research

collaborators from industry to ensure that the project is

compatible with their future collaborative research needs.

This group gives guidance to the project and provides 

feedback to their respective organization, including project 

status and soliciting ideas as to how they can best use this

new infrastructure in future collaborations. 

Researchers within the NCPV, as well as their 

university and industrial collaborators, will have access to 

the project’s design standards. Partners can then build

their tools so that they can be integrated to other tools and

thus maximize the integrated tool collective. Eventually, 

researchers from outside NREL will be able to build

modules to the standard, bring them to NREL, and 

leverage their new experiment with the existing integrated

tool base. Such collaborations will foster student and

sabbatical placements.

Facilities 

The main thrust of this project is to give the NCPV

staff the right tools. However, NREL personnel are also 

working diligently on the construction of a new building, 

the Science and Technology Facility, which will provide

optimal facilities for an integrated set of tools. 

Construction is planned to start in February 2005, with the

building being ready for occupancy in late fall of 2006.

This facility will provide large open laboratory space to 

ease inter-tool transport. It will also provide state-of-theart utility services, such as centralized (house) hydrogen,

nitrogen, and argon. The laboratory areas are built to

international building code standards as an H-5 facility for

semiconductor fabrication and comparable research and

development using hazardous production materials. 

CONCLUSIONS 

We have achieved remarkable consent from

individuals with a diverse spectra of research interests in

the development of these process integration standards. 

In the next few years, we will have significantly improved

the tool base for collaborative research within the NCPV. 

The modular nature of chamber construction will add to

flexibility in cluster tool arrangement and foster 

collaborations. Having standard designs should reduce 

engineering and development costs of future tools. 

Having standard components should reduce spare part 

inventories, speed repairs, and increase operational

efficiency, thus allowing researchers to focus more on

science. 

The process integration concept will also require the

cooperation of experts from various material technologies

and characterization disciplines to work with each other to

answer key scientific and technological questions. 

Ultimately, this synergistic effort between NREL staff, 

universities, and the photovoltaic industry—around an

integrated tool base—will add to the PV knowledge base 

and help move PV technologies forward. 

ACKNOWLEDGEMENTS 

This work is supported by the U.S. DOE under 

Contract No. DE-AC36-99GO10337. The authors wish to 

thank the entire NCPV for their support and

encouragement in this project, the members of the 

Advisory Group for their input and direction, and especially

Susan Moon, Don Gwinner, Craig Perkins, Glenn Teeter, 

and Dean Levi for their helpful discussions about the

measurement integration terminology. 

REFERENCES 

[1] V. Comello, "Process Integration Remains Key to 

Semiconductor Wafer Profits," R&D Magazine, Vol. 36, 

pp. 18-20, 1994. 

[2] D. Calhoun, "Facing the challenges of intratool 

automation in the 300-mm era," Micro, Vol. 16, pp. 91, 

1998. 

[3] P. Sheldon, "Process integration issues in thin-film 

photovoltaics and their impact on future research 

directions," Progress in Photovoltaics, Vol. 8, pp. 77-

91, 2000. 

[4] W. E. McMahon and J. M. Olson, "A scanning 

tunneling microscopy study of As/Ge(mnn) and 

P/Ge(mnn) surfaces," Journal of Crystal Growth, Vol. 

225, pp. 410-414, 2001. 

[5] W. E. McMahon, I. G. Batyrev, J. M. Olson, and S. B. 

Zhang, "Strain-driven mesoscopic reconstruction of the 

As/Ge(111) surface," Physical Review Letters, Vol. 89, 

2002. 

[6] B. P. Nelson, S. Robbins, and P. Sheldon, "The 

National Center for Photovoltaics Process Integration 

Project," presented at 14th Workshop on Crystalline 

Silicon Solar Cells and Modules: Materials and 

Processes, Winter Park, Colorado, 2004. 

[7] SEMI, "Cluster Tool Module Interface: Mechanical 

Interface and Wafer Transport Standard," 

Semiconductor Equipment Manufacturing International, 

Vol. SEMI E21-94, 2002. 

[8] M. Pendleton, "Factory integration 'sticky points' - The 

heir apparent to SECS," Solid State Technology, Vol. 

46, pp. 92, 2003. 

[9] A. Dugenske, A. Fraser, T. Nguyen, and R. Voitus, 

"The National Electronics Manufacturing Initiative 

(NEMI) plug and play factory project," International 

Journal of Computer Integrated Manufacturing, Vol. 13, 

pp. 225-244, 2000.

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The NCPV Process Integration Project: Purpose, Status and 

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The primary purpose of the process integration project of the National Center for Photovoltaics (NCPV) is to develop an 

infrastructure that will allow researchers to gain new knowledge that is difficult—if not impossible—to obtain with existing 

equipment. This difficulty is due, in part, to the state of our existing tool set, which lacks sufficient in-situ or real-time

measurement capabilities, or lacks access to analytical tools where the sample remains in a controlled environment 

between deposition and processing or measurement. This new infrastructure will provide flexible and robust integration 

of deposition, processing (etching, annealing, etc.), and characterization tools via a standardized transfer interface such 

that samples move between tools in a controlled ambient. The standardization of control and data acquisition software 

schemes, sample handling, and equipment components will allow us to perform research more efficiently; facilitating 

collaborations and technique development. 

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PV; process integration; in-situ; real-time measurement; etching; annealing; standard transfer interface; 

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