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Dicing of Gallium Arsenide (GaAs) Wafers with the Laser
MicroJet
Challenges, Improvements and Safety Issues
Natalia M. Dushkina Laboratory of Laser Applications, The Gem
City Engineering Co.
Dayton, Ohio 45404, USA
Bernold Richerzhagen SYNOVA SA
Lausanne, CH 1015, Switzerland
GaAs wafers are fragile and brittle and, therefore, the
well-developed dicing saw technique, which is widely used in the
silicon industry, faces serious problems when used for dicing GaAs
wafers. GaAs wafers are very sensitive to changes in the dicing
tools and to drifts in the dicing machinery, which makes the dicing
difficult and causes some throughput issues. At the moment, the
most commonly used dicing saw process is dicing with a 30 m thick
resinoid blade. Although, these blades provide high-quality kerf,
they are also very fragile and, thus, have a very short life.
Dicing of GaAs wafers with the Synova Laser MicroJet, which
implements a YAG-Nd laser beam confined in a water jet, gives
exciting and promising results for a general solution of the
problem. By means of the Laser MicroJet, wafers as thin as 25 m can
be diced in streets of 30 50 m width, providing kerf quality
comparable to the dicing saw cut and in some cases even better than
the dicing saw. The Synova MicroJet increases the wafer throughput
and under certain conditions yields 100% throughput. As far as we
are aware, the presented results are the first for laser dicing of
GaAs wafers, and, therefore, provoke a detailed discussion about
the safety of the new technique. In this paper, we address the
advantages and optimization, as well as safety issues of the laser
water-jet dicing process for GaAs wafers. Key words: laser cutting,
water jet, Q-switched Nd:YAG laser, GaAs wafers
I. Introduction Gallium Arsenide accounts for almost three
quarters of the total production of compound semiconductors for the
last few years, according to a study of Kline&Company, Inc., a
leading business consulting firm serving the electronics, chemical
and material industries worldwide.1 Compound semiconductors based
on non-silicon wafers have rapidly invaded the semiconductor market
in the last decade, which was dominated for more than twenty years
by silicon due to its outstanding industrial mastery and low price.
GaAs and GaAs-on-Si have significant market potential, both as a
substitution technology branch for manufacturing traditional GaAs
devices and as a new technology for monolithic integration of GaAs
devices and silicon integrated circuits. While logic and memory
devices still rely on the well-established silicon technology, the
compound semiconductors made a strong impact on the fast-growing
market of various communications and photonics devices. As the
device and system industries are rapidly
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becoming a commercially active branch, the substrate production
and wafer processing remains mired in the research and development
stage. Compound semiconductors are more expensive: they are more
fragile and brittle than silicon, their melting points are lower,
which compromise fabrication, and the basic boules, from which the
wafers are cut, are much smaller in diameter. Therefore, only $3
billion, of the $119 billion of all produced integrated circuits in
2001, accounted for the compound semiconductors, while the much
bigger part still belongs to the silicon production. However, the
market for compound semiconductors expands rapidly due to the
incessantly increasing demands for higher speed of the wireless and
broadband communication industry. The growing impact of GaAs in the
fields of fast telecommunications and photonics requires
sophisticated and less expensive methods for wafer preparation and
processing. A particular challenge is the precise and fast dicing
of the fairly brittle GaAs wafers. Common saw methods using
resinoid blades are close to their limits and it is doubtful if
these methods will meet the future demands of flexibility, high
cutting speed, production rate and yield. The employment of the
Synova Laser MicroJet, on the other hand, increases appreciably the
flexibility of GaAs wafer processing. Moreover, it allows arbitrary
shape cutting, which is not possible with the conventional saw
techniques. However, manufacturing and processing of compound
semiconductors, and especially of GaAs, reveals serious industrial
hygiene concerns due to hazardous chemical compounds and/or
byproducts found in certain processing equipment and environment.
In this paper we describe the advantages of the laser water-jet
dicing process of GaAs wafers, and discuss optimized cutting
parameters, as well as safety issues of dicing with the laser
MicroJet.
II. The water jet guided laser technology The water jet guided
laser technology was invented and developed by one of the authors
(B.R.), who called it Laser MicroJet.2,3 The new technology has a
huge market potential for processing of semiconductor wafers and
other materials with subtle thermal effect, high speed and high
kerf quality. In 1998, the concept was implemented in sophisticated
laser cutting and dicing machines produced by Synova SA in
Switzerland. The water-jet guided laser technology provides
low-temperature laser dicing since the laser beam is coupled in a
fine, stable water-jet and conducted to the sample by means of
total internal reflection like in a glass fiber (Fig. 1). Thus, the
water jet can be referred to as a fluid optical waveguide of
variable length. This feature allows a working distance of 2-3
inches and eliminates the problems connected with focusing on the
sample when using conventional lasers. The diameter of the water
jet is determined by the nozzle size, which might be 30, 50, 75,
100 and 150 m. The high laminarity of the water jet provides kerf
width of the same size as the water jet diameter. The relatively
low pressure (10 - 30 MPa) of the tiny water jet results in a
negligible force on the sample; thus, there is no mechanical stress
during cutting. This is of great importance when dicing thin and
brittle GaAs wafers, which might be as thin as 25 m. The water jet
itself does not cut the sample, but plays a trifold function in the
cutting process: 1) it guides the laser beam to the sample; 2)
immediately cools the area of interaction of the laser light with
the matter, and 3) simultaneously cleans the residues from the
kerf.
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Fig. 1. The mechanism of wafer-dicing process. The water jet
guides the laser beam as an
optical fiber. Synova MicroJet machines use YAG:Nd lasers in
pulsed and Q-switch working regimes with wavelength at 1064 nm.
Thus, the water jet guided laser is suited for processing of any
material that absorbs at this wavelength. Almost any metal,
semiconductor and some ceramics are suitable for the MicroJet
cutting, the only limitation being the thickness. Unfortunately,
materials that are transparent for the YAG:Nd laser wavelength,
like glass and oxide layers, cannot be processed with high quality.
To broaden the range of materials that might be cut with the
MicroJet, the team of Synova is developing a shorter wavelength
laser system using the frequency-doubled YAG laser with a
wavelength of 532 nm.
III. Properties of GaAs important for the processing with
MicroJet
The spectral transmission of GaAs is shown in Fig. 2. GaAs
absorbs strongly the 1064 nm wavelength of the YAG:Nd laser light,
and is therefore an appropriate material for the MicroJet
applications. At temperatures higher than 250oC it starts to show
the phenomenon of thermal runaway: that is, the hotter it gets, the
more the absorption increases. Its thermal conductivity of
Fig. 2. Transmission spectrum of GaAs.4
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5x101 W/(m K) is more than 2 times that of ZnSe and its density
of 5.31 kg/m3 is twice that of silicon.4 The melting temperature of
GaAs is 1238oC. If heated above 480 oC, it decomposes to evolve
arsenic vapor, which pressure reaches 1 atmosphere at the melting
point.
IV. Challenges of dicing GaAs wafers In a drive towards higher
production volumes and lower costs, all major players of the GaAs
industry have moved or are moving to 6-inch manufacturing
capability. Yield improvement is one of the key performance
indicators depending strongly on the quality of the wafer post-fab
processing since at that stage the wafer has the highest value.
Dicing of GaAs wafers is not a trivial process, because GaAs wafers
are fragile and brittle. The most commonly dicing techniques at the
moment are the saw and scribe/break processes. The saw process
involves dicing wafers with 30 micron-thick resinoid blades.
Although the resinoid blades provide high-quality kerf, they also
raise a lot of handling problems: because of the thickness of the
blade, they are very fragile and can easily break; they have very
short life (
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Table 1. Typical Laser Parameters Parameter YAG:Nd laser
Pulse energy Energy reproducibility Average power Pulse length
Repetition rate
2.5 mJ
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Fig. 5. Comparison of the front side quality with the saw. The
wafers are GaAs/Ge with thickness of 178 m and different top
surfaces: a) saw cut with speed 1.8 mm/s, magnification 50 times;
b) and c) laser MicroJet kerf with speed of 15 mm/s, magnification
400 times. The wafers were not cleaned after cutting. No chipping
or edge cracks are seen on b) and c). The speed factor vs. saw is
8.3. Customers require speed factor more than 4 to consider
replacement of the existing saw equipment with the Laser
MicroJet.
No backside metalization With backside metalization
Fig. 6. Backside quality of GaAs/Ge wafer cut with the laser
MicroJet: wafer thickness 0.007 (178 m), 75 m nozzle, water jet
pressure 200 bar, speed 15 mm/s, magnification 400 times. Careful
optimization of the dicing parameters is necessary in the case of
backside metalization in order to avoid chipping and usually
requires lower speed.
Fig. 7. Some problems: a) If the starting point of the cut is on
the wafer, the wafer might crack following the crystalline
directions. This problem can be avoided by starting the cut outside
of the wafer; b) and c) The cross section quality might be improved
by reducing the pulse energy, increasing the speed and the number
of passes.
a)
a) b) c)
b) c)
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VI. GaAs and Safety VI.1. GaAs test trial run Pure compound GaAs
contains 51.8 wt% arsenic, and is, therefore, considered a
hazardous material according to the standards of the Occupational
Health & Safety Administration (OSHA). GaAs is described in the
Material Safety Data Sheet (MSDS) as toxic by inhalation and a
possible human carcinogen. These facts raise a lot of concerns from
an environmental, health and safety standpoint in the GaAs
industry. As a supplier of the new technology and machines to the
American market, we have to provide also information about the
potential hazards when working with the laser MicroJet, as well as
of the measures that should be observed in order to satisfy the
high safety requests of OSHA and the Environmental Protection
Agency (EPA). Such information was not available due to the
innovation of the technique and the lack of experience in
processing hazardous materials with the laser MicroJet. The scanty
information that we were able to get from some GaAs manufacturers
could not be applied directly due to the different mechanism
between the saw and the MicroJet cutting process.
Therefore, we performed a six-hour trial run of non-stop dicing
of GaAs wafers. The goal of the test was to clarify qualitatively
and quantitatively the potential hazards in real working time.
Preparing the test, we considered eventual formation of ai-born
arsenic and arsine gas, which is acute poison, heavy contamination
with arsenic of the wastewater and cutting chamber, as well as some
contamination of the working area around the machine, the level of
which we could not predict, and, therefore, we took the highest
precaution measures for the safety of the operator - bunny-suit,
rubber gloves, a respirator with HEPA filter P100 and a PentAir
adjustable flow airline hood supplying fresh air from a breather
box - air filtration box with carbon monoxide monitor. During the
test, the wastewater was entirely collected in a barrel that was
afterwards disposed as a hazardous material; the exhaust port of
the machine was equipped with high efficiency particulate air
filter (HEPA); the ventilation system of the laser room was shut
down and all supply and exhaust openings in the room were sealed;
access to the room was restricted. All materials used during the
test, as well as those for cleaning afterwards, were gathered in a
specially provided drum and disposed as solid hazardous. A
representative of Ohio Bureau of Workers Compensation, Division of
Safety & Hygiene, surveyed the preparation and the trial run
itself. VI.2. Sampling and Analysis Summary Here we will discuss
only briefly the GaAs trial run and the results of it, since the
details will be published soon in a separate paper.5 During the
test we monitored: 1) the presence of arsine gas by three digital
arsine gas detectors of electrochemical type (model SEC 1500,
manufactured by Sensor Electronics Corporation, Minneapolis, MN,
with sensitivity from 0 to 1000 PPB), set to three levels of arsine
concentration - 10 PPB (low); 30 PPB (middle) and 40 PPB (high)
with alarm warning for immediate danger (2001 TLV=50 PPB; TLV
stands for total lethal value).
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2) the air-born arsenic was monitored by five detectors using
pre-weighed 37-millimeter mixed cellulose ester filters in-line
with calibrated SKC Airchek 224-PCXR-4 high-flow air pumps (method
reference #NIOSH 7900). The samples were collected within the
operators breathing zone and in areas of worse case exposure:
inside the exhaust chamber, on the top of the machine and around
it. 3) the contamination of the room by As and GaAs particles, as
seven wipe samples were taken from the cutting chamber, table next
to the operator, and room floor and walls immediately after the
trial run was completed. Additional samples were taken after
cleaning of the equipment and room. The samples were analyzed at
NATLSCO Laboratory. 4) the contamination of the wastewater, as a
sample was taken every 15 minutes directly from the tank of the
machine. The samples were later analyzed by TestAmerica, Inc.,
using methods EPA 200.8 and EPA 200.2. 5) personal safety -
according to the MSDS, acute poisoning from GaAs is unlikely (NIOSH
#LW8800000), but high atmospheric concentrations may lead to
systematic toxic effects of arsenic poisoning. Therefore, the
operator had a medical check for arsenic the day before and after
the GaAs trial run. The results from the GaAs trial run, as well
the Permissible Exposure Level (PEL) and recommended precaution
measures, are summarized in Table 2. As we expected, the
contamination of highest degree was of the wastewater, where the
concentration of arsenic was about 1000 times higher than the EPAs
current maximum allowable amount. The highest concentration of
air-born arsenic and particles deposition, which was 13 and 30
times higher than the OSHA cancer hazard, respectively, was
measured inside the cutting chamber, while outside the machine the
contamination level was lower than the OSHA arsenic standard. The
fact that Arsine gas was not detected was not surprising for us.
Since the laser beam is coupled in a water jet and the laser pulses
are very short (around 450 ns), the time for interaction of the
laser light with matter is very short, and immediately followed by
the cooling effect of the water. Therefore, one could suppose that
the average temperature in the cutting spot is not high. This
hypothesis was proved by an experiment performed a week before the
GaAs test. For its purpose an infrared camera and a frame-grabbing
system was used to monitor the cutting process.5,7 The recorded
temperature at any working conditions did not exceed 160 degrees
Celsius. The simultaneous action of laser beam and water jet keep
the average temperature in the cutting point far below the level of
decomposition of the material and generation of arsine gas in
dangerous concentrations. The concentration of inorganic arsenic in
the human body, determined after the test, half of the limit
considered as carcinogenic level. In summary:
1. No arsine gas was detected inside and outside the cutting
chamber; 2. The main concerns should be the wastewater - severe
measures for filtering using arsenic
filter and proper recycling should be taken under consideration.
3. An exhaust system with high efficiency dust/mist filtration,
arsenic filter or closed
recycling of the waste water, and wet post-cleaning of equipment
and facility, are highly recommended, as well as
4. Full personal protection rubber or plastic gloves, HEPA
respiratory filter, glasses or protective shield. The personnel
working full 8-hour shifts on a daily basis should
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perform a medical check and urine test every six months and keep
tracking records of the results.
Table 2. Data for the Arsenic Concentration During the GaAs
Test.
PEL Detected during the test Recommendations Arsine gas,
ppm TLV=0.05 Not detected
Air concentration of Arsenic,
g/m3
10 (OSHA cancer
hazard)6
130 (in the cutting chamber)
4 (outside the machine)
Exhaust system with a particulate filter
Water concentration of
Arsenic, g /L
BEI1=35 50 (EPA)2
62700 (in the waste water without filtering)
Closed recycling or Arsenic filter
Presence of Arsenic in the human body,
g/L
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The authors thank the Management of The Gem City Engineering Co.
for their support of the Synova project and Prof. Rado Kovacevic
from SMU for organizing the temperature monitoring experiment!
Special thanks are addressed to the customers, who provided the
samples and additional information!
References
1. Kline&Company, Inc., Little Falls, N.J., PRNewswire via
Comptex, May 7, 2002. 2. Richerzhagen, B., B. Richerzhagen,
"Development of a System for Transmission of Laser
Energy," Ph. D. Thesis work, EPFL, Switzerland, 1994. 3. B.
Richerzhagen, G. Delacrtaz, R.P. Salath, "Complete Model to
Simulate the Thermal
Defocusing of a Laser Beam Focused in Water," Optical
Engineering, vol. 35, No. 7, 1996, 2058 2066.
4. Ready, J.F., (ed.), LIA Handbook of Laser Materials
Processing, (1st ed.), Laser Institute of America, Magnolia
Publishing, Inc., 2001, p. 136.
5. Dushkina, N.M., Safety Concerns in Dicing of GaAs Wafers with
Synova Laser MicroJet, to be published.
6. Clansky, K.B., Ed. Suspect Chemical Sourcebook: A Guide to
Industrial Chemicals Covered Under Major Federal Regulatory and
Advisory Programs. Roytech Publications, Inc. Burlingame, CA. 1990.
Update, p. xlvii; section 3, pp.86, 112-113.
7. The experiment was performed by M. Valant in the frame of a
collaboration work with Prof. Radovan Kovacevic, Director of SMU
Research Center for Advanced Manufacturing, Southern Methodist
University, Richardson, Texas.
Meet the Authors Natalia Dushkina is Ph.D. in Physics, author of
more than 35 scientific papers and presentations at international
conferences in the areas of optical properties of semiconductors,
optical methods and laser applications. After five years research
in Japan, Dr. Dushkina moved to Bowling Green State University,
Ohio. She is the Manager of the Laboratory of Laser Applications at
The Gem City Engineering Co., Dayton, OH, since August 2001.
Bernold Richerzhagen (born 1964 in Cologne, Germany) received his
MSc in mechanics from the Technical University of Aachen, Germany,
and his PhD in micro-technology from the Swiss Institute of
Technology, Lausanne, Switzerland. He is the inventor of the water
jet guided laser technology. Since this invention in 1994, he has
published a great number of articles on combining laser and water
jet for which he has received several awards. He is the CEO of
SYNOVA SA, Lausanne, an incorporated company manufacturing high
precision laser machines, which he has founded in 1997.