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A Mixed GaAs Modulator and HEMT MMIC Process Line On 150mm Wafers
A Mixed GaAs Modulator and HEMT MMIC Process Line
On 150mm Wafers
J. Thompson, D.J. Warner, C.L. Sansom, P.A. Claxton and D. Parker
Marconi Caswell Limited, Caswell, Towcester NN12 8EQ, United Kingdom
Phone: +44 1327 350581, e-mail: dave.warner@marconi.com © 2001 MANTECH
Abstract
This paper describes the establishment of a 150mm wafer
fab to process both GaAs optical modulators and
pseudomorphic HEMT MMICs in the same facility.
I
NTRODUCTION
The recent huge demand for systems for Internet access
and other high data density telecommunications traffic is
now fuelling an expansion in optoelectronic component
production volumes as well as increasing the number of
MMICs required. Typical optoelectronic devices needed for
these systems are semiconductor lasers (both gallium
arsenide- and indium phosphide-based), photodiodes and
GaAs or lithium niobate modulators. The work reported here
describes the setting up of the worlds first 150mm GaAs
line to process GaAs modulators for telecoms applications as
well as electron beam-written 0.2
µ
m HEMT MMICs.
Gallium arsenide wafers have been processed for many
years into monolithic microwave integrated circuits
(MMICs) for telecommunications and radar systems, with
volumes of devices relatively small compared with those of
silicon wafer fabs. Recently, however, the demand for high
electron mobility transistor (HEMT) MMICs for low noise,
high power telecoms applications has expanded dramatically,
and this has resulted in the establishment of several GaAs
process lines running 6 (150mm) diameter wafers instead of
3 or 4 substrates. The setting up of these lines, and
sometimes of whole new wafer fabs, has been reported
previously at Mantech conferences. [1-5]
W
AFER FAB CONSTRAINTS
Gallium arsenide wafers have been processed at Caswell
since the 1970s, with the present facility used for making
MMICs on 3-inch diameter wafers since 1992. The present
cleanroom suite was designed in the early 1980s for 4 and
5 silicon processing in class 100 conditions. The
fabrication area, of 1000m
2
, is modular in design with long
narrow clean areas interdigitated with service chase areas
housing the equipment utilities. The return air flow for the
cleanrooms is through the hollow walls. MMICs are made in
prototype and small scale production volumes as a foundry
activity, as well as for standard product MESFET- and
HEMT-based integrated circuits. Caswell has been involved
in the development of optoelectronic devices for two
decades, and since 1998 the wafer fab facility has also
accommodated the equipment necessary to process GaAs
Mach-Zehnder modulators, GaAs lasers, plus InP lasers and
photodiodes. At the start of this work there was therefore a
variety of equipments in the facility, from manual photoresist
spinners and simple hotplates to fully automated and PC-
controlled sputtering systems.
In order to process sufficient optoelectronic and
microwave devices to service customer requirements over
the coming years it was decided to upgrade the wafer fab
facility, starting in mid-2000. The decision was made to
convert the modulator and MMIC processing directly from
3 to 150mm (6) diameter GaAs wafers, missing out the
transition through 4 wafers already undertaken by many
other GaAs fabs. This decision was dominated by two
factors: the need for bigger wafers due to the dimensions of
modulator devices (about 30mm long); and the recognition
that well-established silicon processing tools and techniques
could be accessed at 150mm, reducing the technological risk
and ensuring a swifter conversion process. The target was to
generate sufficient capacity to multiply the supply of
working dice more than twenty-fold. However, the task was
complicated by the need to retain an ongoing, and increasing
volume, production capability to process 2 indium
phosphide and 3 GaAs laser, modulator and MMIC wafers
within the same cleanroom facility for the duration of the
conversion to 150mm. It was also necessary to retain a
research and development capability at all three wafer
diameters to bring the next generation of products to the
marketplace.
These simultaneous requirements posed particular
difficulties in terms of floor space and access to essential
services such as process gases, electricity and ultrapure
water. The initial planning for the upgraded facility
concentrated on identifying surplus equipments that could be
immediately removed to start to create space for new tools.
Duplicate equipments from the existing 3 line also had to be
removed at this early stage, from which stemmed the need to
improve the planned maintenance of the remaining tools in
order to minimise equipment downtime and to maintain an
increased level of production. The opportunity was taken at
this stage to re-site certain equipments to improve the
process flow within the wafer fab and to reduce the amount
of walking between cleanroom modules for the process
technicians. Detailed planning then concentrated on siting
probable new tools for 150mm processing in between
Copyright © 2001, GaAs MANTECH, Inc. existing equipments that had to be left in place for the
duration. After tool selection and order placement, detailed
facilities sheets and dimensional drawings were obtained
from suppliers and used to refine the cleanroom floor plan,
and to enable the installation of the required services to each
tool. Difficulties arose on more than half of the equipment
installations, where the information provided by the supplier
did not match what was required by the tool when it was
delivered. Ninety percent of tools were also delivered
significantly later than the agreed delivery date, with the
recent upsurge in demand for semiconductor processing
equipment being the most frequently quoted reason. As a
consequence the new equipment installations (Figures 1 and
2) ran three months later than planned, necessitating very
rapid commissioning and new process development in order
to recover the demanding programme schedule. Many
suppliers were however extremely cooperative in assisting
with training and with the development of the required
processes on their tools.
S
TAFF AND PROCESSING LOGISTICS
The wafer fabrication and epitaxial growth activities need
to reflect the flexible manufacturing system. The requirement
to house R&D and production processing in the same
cleanroom suite, and to process 2" InP , 3" GaAs, and
150mm GaAs-based wafers within the same environment
stimulates the need for a system with an unusual range of
processes and production disciplines in a single operating
environment. It was necessary to find optimum solutions to
such organizational issues if we were to realize our
competitive advantage, and exercise our capability to grow
and process both MMIC and photonics based wafers in a
single facility.
The expanding market for broadband communications
systems has presented Marconi with an excellent opportunity
to exploit the photonics materials research and
manufacturing skills of Caswell Technology. The rapid
ramp-up in demand dictated that the wafer fabrication and
growth upgrades at Caswell must take place in parallel with
an increased throughput from the existing lines. This
challenge is perhaps the hardest of all to accommodate, since
process engineering skills are needed in three areas
simultaneously. These areas are:
(i)

to sustain and improve the current process lines;
(ii)

to train new staff as the current lines expand;
(iii)

to engineer new processes for the 150mm line.
Thus the first phase of the line upgrades involved a vigorous
recruitment campaign to establish an enlarged process
engineering group, and additional metal-organic vapour
phase epitaxy (MOVPE) growth engineers. The phased
recruitment of wafer fabrication and MOVPE growth
technicians occurred later. It became clear as the expansion
gathered pace, that the training of technician staff was the
limiting factor in determining capacity. This resulted in
novel approaches to training being sought, such as sub-
contracting initial practical skills training and training in
group sessions rather than one-to-one (Figure 3).
Although the original intention was to opt for maximum
flexibility, with experience the wafer fabrication activities
were split into two organizations. The low-volume 2" InP
based process line was clearly demanding different skills and
disciplines compared with the high-volume 3" and 150mm
GaAs based lines. Hence this operation was "ring-fenced"
with its own manufacturing system re-defined. Dedicated
staff (both process engineers and technicians) now operate
this line.
Whilst the InP line is run on a small batch basis, the high-
volume lines employ a flexib