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from:
January/February 2012 - www.compoundsemiconductor.net - page 37 -
Industry - Gases
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Purified hydrogen is an essential
ingredient in the MOCVD processes used to manufacture LEDs, power devices and
photovoltaics. Moves toward larger reactors and bigger wafers are increasing the
demand for more and more ultra-pure hydrogen from increasingly reliable, compact
sources. Fulfilling this need is a novel palladium technology developed by Power
and Energy, says the company’s Stuart Bestrom.
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Sales of MOCVD tools have exploded over the last twenty
years, leading to a vast increase in the number of chips produced by this technique. But the
approach for purifying the hydrogen gas that supports this form of epitaxial growth - a
palladium membrane purification technology - has stood still.
This lack of progress on the hydrogen front is a
significant concern for LED chipmakers, who continue to migrate to more complex and demanding
processes involving ever-larger chambers and ever-bigger wafers. Sensitivity to oxygen and
carbon contamination goes up and up, hydrogen flow rates have recently tripled for the largest
MOCVD reactors, and flow changes during the process recipe runs can surge from 0 to 300 standard
litres per minute (slpm) with no transition period. The traditional palladium purifiers were
never designed to handle these new process recipes and they have a number of inherent
limitations in durability, quality and cost. This has prompted some users, particularly those in
large fabs in China and Taiwan that are particularly keen to cut costs and work with higher flow
rates, to consider alternative purification technologies. These chipmakers are finding a
solution to their needs in a palladium membrane technology developed by Power and Energy of
Ivyland, PA, in cooperation with the US Department of Defense (DoD). The DoD pursued this
development to serve its needs for pure hydrogen to power fuel cells (see box "Hydrogen fuel cells"), but the results of the project have proven
successful in semiconductor applications as well.
The resulting palladium membrane technology that we have
developed at P+E is the most significant innovation in gas purification in over 20 years. One of
the biggest breakthroughs is a doubling-to-trebling of capacity per purifier compared to
traditional palladium purifiers. This larger capacity means that a new LED fab requiring 12,000
(slpm) of hydrogen to support 50 MOCVD reactors needs only three of our gas cabinetsized
purifiers. If traditional purifiers were used, six would be needed, and each would have a much
bigger footprint.
Reducing size also has additional benefits, because it
cuts power consumption and other facility costs. What’s more, the micro-channel palladium
membrane technology at the heart of this new system has been proven to greatly improve
durability: It provides uninterrupted purification for years of continuous operation. These
advancements eliminate contamination, yield loss and downtime caused by hydrogen purity
variability in compressed, cryogenic and generator sources.
Working with
hydrogen
It is only possible to fully appreciate the benefits of
our palladium technology after understanding how a conventional hydrogen purifier works, and the
demands that are placed upon it. Conventional hydrogen purifiers have been widely used where
hydrogen of the highest purity has been demanded, in order to yield MOCVD grown wafers and crystal
ingots of the highest possible material quality.
These conventional purifiers draw on palladium’s unique
properties to act as a catalyst (see Figure 1). Hydrogen gas molecules dissociate into atoms upon contacting
the surface of the palladium membrane that is held, along with all the other parts of the
purifier, at 400 °C. At this temperature, hydrogen atoms are small enough to readily diffuse
through the membrane, driven by differential hydrogen pressure across the interface. No other
material is small enough to diffuse through palladium, so impurities such as water, oxygen,
nitrogen, carbon dioxide, carbon monoxide,
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Figure 1.
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Figure 1: Palladium membrane tubes provide the
unique ability to only allow hydrogen molecules to pass through to the pure side.
When the feed gas is brought into contact with the inner wall of the
palladium-silver membrane, molecular hydrogen (shown as blue spheres) dissociates
into atomic hydrogen and is absorbed into the metal lattice. Other molecules, such
as methane, nitrogen,water, carbon monoxide and carbon dioxide (shown as brown and
grey spheres) are too large to pass through the membrane. While diffusing through
the lattice, individual hydrogen atoms share their electron with the palladium in
the metal.
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Figure 2: A palladium purifier is challenged with up to
94 ppm of CO2 with no change in outlet purity. Outlet purity is below 50
ppt.
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hydrocarbons and rare gases remain on the inlet side of
the membrane. The solid barrier provided by palladium results in no breakthroughs. This contrasts
with catalysts and getters, which rely on chemical reactions on reactive surface areas under
controlled conditions.
Palladium technology also offers the unique ability to
remove high-ppm levels of impurities from a cylinder source gas without detriment to purifier
lifetime or outlet H2 purity (see Figure 2). Other methods of hydrogen purification, such as regenerable
catalysts and heated zirconium getters, are intended for removal of low ppm impurities, and the
purifier lifetime is directly dependent on the incoming impurity and flow rate. This robust
capability makes palladium purifiers ideal for compressed cylinder and generator sources where
the gas quality can vary significantly from day-to-day.
Hydrogen can be supplied as either a compressed gas, a
liquefied (cryogenic) gas or it can be generated on site. The purity of gas varies widely,
depending on the source and specific region. Liquid hydrogen is usually the most pure form of this
gas, and it is typically between six and seven ‘nines’ purity - in other words, total impurities
are 1,000-100 ppb.
However, liquid hydrogen is not available in Taiwan, Korea
and China, the three countries where most new high-volume LED and photovoltaic fabs are located. In
these fast-growing regions, fabs must rely on compressed and generator sources, which can include a
great deal of variability in the purity of compressed and locally generated hydrogen. Typical
sources are water, natural gas or propane, and costs associated with generating hydrogen in this
manner are high, due to substantial power requirements.
Chipmakers that use high-purity hydrogen must have a
contingency plan that can be brought into action when the primary source is unavailable due to
maintenance or unforeseen downtime. One option is to use an industrial or chemical plant in these
emergencies, but that means a compromise in gas purity. For example, a facility with onsite storage
of 99.999 percent hydrogen may be forced to use 99.99 percent backup hydrogen on rare occasions.
Purification systems have to be designed to deal with these contingencies, so that the final gas
purity is unaffected by changes in the quality of the sources. Palladium purifiers can ensure that
all impurities are removed from the hydrogen, whether typically present or the result of an unusual
event.
Innovative membranes
Through the support of a series of Navy, Army and DARPA
research contracts over a period of eight years, we have developed hydrogen purifiers based on a
unique micro-channel palladium-alloy membrane configuration. This technology is commercially
available and highly reliable, thanks to automated membrane test and inspection methods and
advanced manufacturing technologies.
Our unique membrane structure is based on an ‘insideout’
design. It features a ‘micro-channel’, in which the hydrogen enters the inside of the membrane tube
(see Figure 3). This palladium tube has an inner ‘return tube’ that is inside and
concentric with the membrane. With this design, hydrogen diffuses out into a passivated
stainless steel chamber while a small volume of hydrogen continuously sweeps all impurities to a
bleed line.
Thanks to improvements in weld and brazing methods,
quality inspection, and membrane assembly design, our micro-channel architecture can deliver a
dramatic increase in durability compared to conventional designs.
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Figure 3: Power+Energy’s
inside-out microchannel palladium purifier with vacuum brazing and laser weld
improves durability over previous designs using traditional welding techniques. The
welding of palladium to stainless steel will weaken the joint when performed with
traditional weld methods. Using laser welding improves repeatability and long-term
durability.
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Power+Energy’s hydrogen purifiers feature
a novel microc-hannel membrane an 'inside-out' flow
path.
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Stress on the membrane is reduced, eliminating the need
for a supporting spring. In addition, hydrogen recovery is enhanced with the micro-channel. This
enables efficient recovery, even with low concentrations of hydrogen from reformed fuel streams. We
have been working with our vendors to improve the alloy and tube drawing processes. These efforts,
which reduce impurities and physical defects, have yielded a more uniform alloy with fewer
micro-defects. The upshot is more reliable membranes.
The durability and lifetime of our palladium-based
hydrogen purifier has been improved through more stringent quality control that pre-emptively
screens out material defects. To realise this, we have developed a proprietary membrane inspection
station for identifying and eliminating substandard membranes at the earliest possible stage. Each
membrane is tested under extreme conditions, before being individually subjected to a helium
leak-test prior to acceptance into inventory. With our configuration, the palladium membrane tubes
undergo an advanced, computer-controlled vacuum braze process. This ensures precise, repeatable
brazing of each membrane. The membrane assembly is then laser welded into an electropolished and
passivated stainless steel manifold.
Using this design allows axial and radial expansion and
contraction without restriction, reducing stress on the membrane tubes. Each membrane is held in an
array that prevents it from ever contacting its neighbours. In contrast, traditional palladium
purifiers allow membranes to move freely against each other, thereby contributing to long-term
stress of the tubes. Preventing membranes from coming into contact also leads to a free flow of gas
and prevents tube damage.
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To demonstrate
the practical benefits of our palladium technology, we have developed automated,
accelerated-life-testing systems to replicate extreme operating conditions. Membranes were
pressure- and temperature-cycled to simulate the stresses possible from long-term operation.
The goal was to confirm a minimum cycle lifetime of 10,000 - equivalent to 10 years with 3
on/off cycles per day. This benchmark, which is well above typical operating conditions, is
always met with membranes that pass incoming inspection (see Figure 4). In fact, some membranes that failed inspection still show
excellent lifetimes. This means that our inspection methods may eliminate ‘good’ membranes.
However, they ensure that no accepted membranes fail the cycle
test.
We subjected our
sample membranes to thermal cycling to validate flow performance and stability over time. The
result: Confirmation of the effectiveness of our membrane test methods to identify and remove
membranes with micro-defects prior to
assembly.
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Figure 4: Membrane life
cycle test confirms 10,000 cycle minimum lifetime for all membranes that passed new
inspection procedures.
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Encouragingly,
our test also showed that the new microchannel membranes provide improved durability under
the most hostile operating conditions. Our 9000MZ and 9000MS high-flow purification systems
are now incorporating the new membrane. These purification systems have been manufactured
with production and quality procedures that ensure a consistent operating environment with
stable operation for many years. The compact array of membranes provides a high flow capacity
in a very compact package. A single vessel measuring 6" x 24" that previously purified up to
200 slpm can now flow 600 slpm. The compact package also reduces power consumption, and in
addition it allows savings in required floor space and HVAC sizing, important considerations
for the larger semiconductor fabrication facilities in
Asia.
Through analysis
of raw materials, assembled systems and resulting outlet gas purity, it has been possible to
improve the quality of the hydrogen supply so that impurities are consistently below 0.1 ppb
(100 ppt). This exceptional level of purity predominantly stems from proprietary
manufacturing processing for preparing and passivating stainless steel, which have reduced
sources of carbon and moisture that can contribute impurities downstream of the palladium
membranes. Thanks to this advance, we can now guarantee a start-up purity of less than 100
ppt for all impurities. In comparison, typical specifications from traditional palladium
purifiers offer impurities of less than 1 ppb, and this is only assured after a lengthy
start-up purging at a minimum 20 percent of rated
flow.
Benefits
associated with our micro-channel technology are by no means limited to LED chipmakers: They
are also a great assistance to producers of photovoltaics, polysilicon devices and fuel
cells. Improvements in the performance of all these devices are eagerly anticipated, and our
hydrogen-purifying technology should help to unlock that promise throughout the remainder of
this decade.
| Hydrogen fuel
cells |
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The Department of Defense is interested in developing
high-quality hydrogen sources for fuel cells. These devices are incredibly energy
efficient, and can generate twice as much energy per gallon of fuel as
combustion-based generators. The challenge is that military fuel supplies are high
in sulphur contamination, an impurity that drags down fuel cell performance. A
better, more durable method for separating hydrogen for fuel cells is needed to
unlock the door to auxiliary powering of vehicle, plane, ship, platoon and field
installations.
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© 2012 Angel Business
Communications.
Permission required.
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About Power+Energy
Power & Energy is headquartered in
Pennsylvania, USA. Established in 1993, the company's mission is to enable the hydrogen economy
and promote energy efficiency through the application of micro-channel technologies. The company
provides a full range of micro-channel hydrogen purifiers to ultra-high purity users across the
U.S., Asia and Europe.
Power & Energy is leading the way to the
hydrogen economy with proprietary manufacturing and patented hydrogen purification, separation
and generation technologies. Power & Energy's Micro-Channel palladium-alloy hydrogen
purifiers purify any grade of hydrogen to nine-nines purity, meeting the most stringent
requirements for semiconductor fabrication processes, including light emitting diode
manufacture, solar cell manufacture and the manufacture of other compound and silicon
semiconductors. Power & Energy's Micro-Channel hydrogen purifiers are also ideal for
metallurgy, R&D, and instrumentation applications. Power & Energy's Micro-Channel
membrane systems can also be used to efficiently separate hydrogen from reformed fuels and
synthesis gas mixtures.
Power & Energy is now applying its hydrogen expertise to the fuel cell vehicle refueling
market. Power & Energy has recognized that highly compact, low cost, and efficient hydrogen
generators that can convert existing liquid fuels to hydrogen at the refueling station will enable
the rapid development of the hydrogen refueling infrastructure that fuel-cell vehicles will
require. The company's expertise in the design and manufacture of micro-channel gas processors
provides the basis for this exciting new venture.
For further information, contact Al Stubbmann, Power & Energy Inc., 106
Railroad Avenue, Ivyland, PA 18974-1449, e-mail al.s@powerandenergy.com or call
+1-610-217-0193.
CONTACT:
Albert Stubbmann
Power & Energy, Inc.
106 Railroad Drive
Ivyland, PA 18974-1449
USA
PHONE. 215-942-4600 ext 17
FAX. 215-942-9300
EMAIL: al.s@powerandenergy.com
WEB: http://www.powerandenergy.com
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