hydrogen pumping speed in cryopumps without compromising safety
Thompson and Michael Eacobacci, Helix Technology
harsh environment in which ion implant processes take place requires
consistent vacuum conditions to maximize tool availability, throughput,
and product yield. Ion implanters achieve maximum productivity only
when optimal operating conditions, such as the ion beam's interaction
with the vacuum environment, are maintained. To improve throughput and
quality, users of ion implanters—particularly in high-beam-power processes
in which high outgassing loads are generated—must increase high-vacuum
hydrogen liberated from photoresist is the dominant gas species during
the implant process and is a contributor to high outgassing pressures,
OEMs continue to focus on improving their systems' hydrogen pumping
this environment, cryopump designs must deliver the highest hydrogen
pumping speed while adhering to all safety guidelines for hydrogen and
other gas species. This article, based on work performed by Helix Technology
(Mansfield, MA) and Atmel (San Jose), addresses the potentially competing
demands for improved hydrogen pumping speeds and enhanced safety protocols
in ion implantation vacuum systems.
Implantation Processing Conditions
Composition. An understanding of implantation process gas composition
is essential for achieving the desired level of vacuum performance.
The measurements illustrated in Figure 1, which shows the results of
several residual-gas analyses (RGAs) from medium- and high-current implanters,
correlate well with earlier work.1,2 The relative composition
of the gas shows that the fraction of the gas that is hydrogen increases
with higher beam power. The percentage composition of the three main
by-product gases from the implant process is plotted as a function of
beam power. (Plotting the same data against beam energy or beam current
results in a weaker correlation.) These data indicate that lower-power
beams (mostly from medium-current implants) tend to generate a rather
evenly distributed gas mixture, with no single constituent accounting
for much more than 50% of the mixture. As the beam power grows—for
example, during high-energy and high-current implants—hydrogen quickly
begins to dominate the outgassing mixture.
1: Percent composition of photoresist outgassing for different implant
recipes. The fraction of hydrogen gas increases with higher beam
Effects of Gas Pressure. One of the primary vacuum-related
concerns associated with high-energy implanters is energy contamination.
When very-high-speed ions interact with gases in the beamline, an extra
electron may be stripped off the dopant before it passes through the
final accelerating field. This ion is then delivered to the wafer at
a higher energy than was originally intended. Uncontrolled pressure
changes can also change the implant angle or reduce uniformity.
ion beams can suffer from "blooming"—the gradual increase in width
of the beam resulting from its positive space charge. To combat blooming,
ion implant tools often add electrons to the beam space in a controlled
manner. That is most often achieved by adding an easily ionized gas
(xenon, for example) to the vacuum space. Electrons liberated from the
flood-gas molecules help keep the space charge of the beam low, thereby
helping transport the beam to the target.
their very large charge-exchange cross section, xenon atoms also interact
directly with beam ions, often causing neutralization of dopant atoms.
The photoresist-derived molecules can also cause this type of interaction.
The dopant atoms may be impacted before final optics, so that these
atoms reach the wafer at the wrong energy or do not reach it at all.
The dopant atoms can also be neutralized after the final optics, in
which case they will not be counted by the dose measurement system.
In either case, implant quality is degraded. When pressure exceeds a
given critical threshold, many implant control systems put the process
on hold to give pumps time to catch up, adversely affecting tool throughput.
issues facing medium-current implanters are similar to those that affect
high-current and high-energy ones. Furthermore, scanned- or ribbon-beam
medium-current tools use more-complicated beam optics to ensure parallel
beam incidence on the wafer. If beam ions are neutralized by the residual
gas within the electrostatic or magnetic collimating structure, incorrect
implant angles can result.
Role of Cryopumps in Ion Implant Processes
Cryopumps Work. Cryogenic high-vacuum pumps, or cryopumps,
create high vacuum by freezing molecules of air onto cryogenically cooled
surfaces inside the pump. The vacuum typically ranges from 10–4
to 10–9 Torr. Cryopumps have no moving parts that are
exposed to vacuum. Because they require no oil, they are are inherently
clean and cannot contaminate products created in the vacuum they produce.
Nothing but cryogenically cooled surfaces are exposed to the process
usually consist of two internal stages, or pump areas, which freeze
or capture specific gas species at set cryogenic temperatures. The stages
are connected to a sealed cryogenic refrigerator run by an external
compressor. The refrigerator cools the stages. In some pumps, stage
temperatures are adjustable, so that the pump can be tuned to target
specific gas species for evacuation from the process chamber.
first stage of the pump, the primary pumping surface of which is the
inlet array, is generally operated at temperatures between 65 and 100
K. Its main function is to pump or capture water vapor. The second stage
consists of a series of metal pumping surfaces, which are arranged in
patterns designed for particular applications. Generally operated at
temperatures ranging from 10 to 20 K, this stage can pump gases such
as nitrogen and argon. The metal pumping surfaces are partially covered
with charcoal granules. Gases such as hydrogen and helium, which cannot
be frozen at typical second-stage temperatures, are adsorbed by the
charcoal granules and thereby removed from the vacuum chamber. A schematic
drawing of a cryopump is pictured in Figure 2.
2: Schematic drawing of a cryopump's internal arrays.
COURTESY OF HELIX TECHNOLOGY
Implications of Increasing Cryopump Hydrogen Capacity. It is
important to differentiate between the hydrogen pumping speed required
to improve process control and the cryopump hydrogen capacity that enables
long regeneration intervals. The hydrogen speed of a cryopump is measured
using a speed dome constructed to the industry-standard specification
ISO 1608-1:1993(E). Hydrogen capacity relates to the quantity of hydrogen
the pump can "cryoadsorb" before its effective speed drops to 50% of
its peak speed.3 Capacity is dictated by the total amount
of cryoadsorbant present in the pump.
hydrogen speed and hydrogen capacity are independent functions, there
is a practical reason for the widely held perception that they are linked.
In most applications, absolute capacity, which is defined by the amount
of cryoadsorbant present in the cryopump, is never reached because vacuum
processes require a certain base pressure before production can start.
Base pressure climbs as the cryoadsorbant in a cryopump fills.
constant-flow-rate hydrogen-capacity curve in Figure 3 is based on a
standard speed test specified in ISO 1608-1:1993(E). Derived from Helix
Technology's On-Board IS 320FE cryopump, it shows pumping speed as a
function of the total amount of hydrogen pumped and illustrates the
effects of filling the capture sites of the cryoadsorbant. As the gas
accumulates and gradually saturates the cryoadsorption capacity of the
pump's arrays, pressure increases and the effective speed decreases.
3: Hydrogen capacity chart for the IS 320FE and the 10F cryopumps.
(Data from Helix Technology)
key objective when designing a cryopump is to achieve very high hydrogen
pumping speed. To achieve that objective, the overall conductance of
hydrogen to the cryoadsorbant and the cryoadsorbant's hydrogen capture
probability must be improved without increasing hydrogen capacity. While
simply adding cryoadsorbant to the vacuum space would increase the hydrogen
speed, it would also increase the absolute hydrogen capacity of the
pump, raising the hazardous-chemical potential energy stored inside.
Therefore, when developing the IS 320FE pump, engineers focused on maximizing
conductance to the cryoadsorbant, maximizing the capture probability
of the cryoadsorbant, and providing the optimal temperature for hydrogen
capture at the cryoadsorbant.
a Cryopump Design
Conductance. Maximizing the conductance of hydrogen to the
second-stage array and the cryoadsorbant there were investigated in
parallel. Careful analyses of various vacuum-space designs using Monte
Carlo calculation methods were conducted, enabling the investigators
to optimize the size of the second-stage array for a radiation shield
of a particular size. A design was developed that conducts hydrogen
to the cryopump in the most optimized fashion and that also minimizes
the probability that a hydrogen molecule will reflect back out of the
cryopump once it reaches the inside of the radiation shield.
4 shows the results of optimizing a traditional array with a standard
hydrogen capture probability of 40%. In this analysis, the diameter
of the second-stage array was varied. As the diameter of the array was
increased in size, its capture area and speed increased. At the same
time, because a larger array occupied a greater fraction of the space
available, the conductance of hydrogen to the sides and back of the
array was reduced. A balanced approach resulted in an optimum design
point, but it did not achieve the targeted gains in overall hydrogen
pumping speed. Nevertheless, significant increases in the capture probability
of the second-stage array were achieved in part by increasing hydrogen
conductance to the hydrogen-capturing cryoadsorbant in the array.
4: Second-stage array optimization for standard capture probability.
Optimum system performance is achieved with a design that balances
an increase in the diameter of the second-stage array with the necessity
of reducing conductance of hydrogen to the sides and back of the
Monte Carlo calculation in Figure 5 illustrates the thermal aspects
of an array design with a high hydrogen capture probability of >80%.
As the array increased in size, the pumping speed increased. However,
a larger array also resulted in greater heat load. In this design, the
heat load that is absorbed by the second-stage array increased dramatically
near the region where the optimum design point was reached. However,
doubling the refrigeration requirements to handle the heat load is prohibitively
expensive. System optimization must balance between increasing the capture
probability of hydrogen molecules and increasing the amount of refrigeration
required. Hence, the optimum design represents a balance between the
peak shown in Figure 4 and the practical limit dictated by the cost
of refrigeration. The correct balance was calculated with a higher first-strike
capture probability than used in Figure 4, shifting the peak pumping
speed to a higher diameter.
5: Second-stage array optimization for high capture probability.
Overall pump design optimization must balance between increasing
the capture probability for hydrogen molecules and increasing the
heat load, and therefore, the amount of refrigeration required.
the Hydrogen Capture Probability and Optimizing Temperature.
Several factors contributed to overall improved capture probability
of the cryoadsorbant:
temperature gradient between the cryogenic refrigerator second-stage
heat station and the cryoadsorbant surface was minimized. To minimize
the temperature gradient, the thermal conduction paths were sufficient
for heat from the cryoadsorbant to be transferred to the second-stage
heat station effectively. In addition, proper materials and methods
for promoting a minimal temperature gradient while providing high-quality,
durable adhesion were selected to adhere the cryoadsorbant to the arrays.
shape of the cryoadsorbant surface was microscopically rough. A rough
cryoadsorbant surface benefits hydrogen pumping speed by maximizing
the intrinsic capture probability. It also fosters secondary impacts
of hydrogen molecules not adsorbed on initial impact. A rough cryoadsorbant
surface also produces a much greater surface area for adsorption, thus
increasing the saturation threshold.
temperature of the cryoadsorbant was maintained so that there was a
balance between the capture probability at the surface and hydrogen
diffusion across the cryoadsorbant bed. While second-stage temperature
has very little effect on hydrogen capture probability in traditional
cryoadsorbants, it has a significant effect on hydrogen diffusion away
from the capture sites, as demonstrated through controlled testing.4
An optimum cryo-pump system controls second-stage refrigerator temperature
at 13.5 K ± 0.5 K while maintaining the initial pumping speed as
the pump fills with hydrogen.
Cryopump Designs. The foregoing analytical processes and design
methods have led to considerable improvements in cryo-pump hydrogen
pumping speed. For example, Figure 6, which shows hydrogen pumping speed
as a function of pressure, demonstrates that the IS 250FE cryopump can
attain a pumping speed of 7000 L/sec, an increase of 55% over the IS
250F. That increased speed occurs under true molecular-flow conditions
and is consistent across several decades of pressure below the 10–4
Torr range. The enhanced pump has the same hydrogen capacity as the
standard model, demonstrating that additional cryoadsorbant is not required
to improve hydrogen pumping speed significantly. This feature ensures
that the pump adheres to safe design practices. Also shown in Figure
6, the IS 320FE 320-mm cryopump attains a hydrogen pumping speed of
>12,000 L/sec. As shown in Figure 3, that pump holds enough cryoadsorbant
to retain approximately 32 std L of hydrogen.
6: Comparison of hydrogen pumping speed versus pressure for three
cryopumps. (Data from Helix Technology)
Loss Considerations. From a safety perspective, it is critical
to properly manage cryoadsorbed hydrogen and condensed gases during
short-term power losses. In case of a long-term power loss, the potentially
reactive gases trapped in a cryopump must be safely vented. Therefore,
cryopumps should feature normally open purge and exhaust valves.
do not want to be forced to implement long, 2–3-hour regeneration
cycles in the event of short power outages lasting as little as 30 seconds.
Significant tool throughput gains can be realized by insulating end-users
from negative effects caused by very brief power losses. Without such
a mechanism, two or three 30-second power losses per week can lead to
as many as 10 hours of unanticipated machine downtime and an attendant
loss of productivity.
cryopumps discussed here have an integrated backup power source (an
uninterruptible power supply system) that holds the vacuum-space purge
valve closed for a carefully selected 2-minute safety period. Consequently,
the system guarantees high tool uptime, even in environments where short-term
power losses are common. The time during which the vacuum-space purge
valve is closed is much longer than typical short-term power losses.
However, it is not so long that significant amounts of condensed gases
can be liberated from second-stage surfaces, as can occur during extended
Ion Implant Tool Performance
test the hydrogen pumping speed of the IS 320FE cryo-pump, the unit
was installed on an EHPi-500 ion implanter from Varian (Gloucester,
MA) and run under real-world conditions at Atmel in a semiconductor
production environment. The RGA ion current data in Figure 7 show that
for unbaked photoresist wafers, a substantial improvement in implanter
performance was achieved after substituting an IS 320FE pump for the
existing 10F model. The new-technology pump reduced hydrogen partial
pressure by 43%, which not only improves implant uniformity but also
increases the total tool throughput.
7: Comparison of hydrogen outgassing with two cryopumps. The new
pump reduced hydrogen partial pressure by 43%. The multiple dose
holds shown in the inset increase wafer process time, thus decreasing
tool throughput. (Data courtesy of Atmel)
addition to decreasing hydrogen pressure, the pump reduced the total
peak pressure of the implanter end-station by as much as 33% when unbaked
photoresist wafers were processed, as demonstrated in Figure 8. With
that reduction, multiple dose holds could be eliminated, enabling the
implanter to continue processing wafers without making disruptive stops
and starts. As shown in the figure, the 10F pump operated much closer
to the implanter's dose-hold limit than the IS 320FE.
8: Comparison of end-station peak pressure for two cryopumps. The
new pump reduced peak pressure by 33%, eliminating multiple dose
holds. (Data courtesy of Atmel)
9 compares the impact of the 10F and IS 320FE pumps on average maximum
end-station pressure. The data are based on many sets of ion-implant
runs that were performed under various process conditions using both
baked and unbaked photoresist wafers at implant energies ranging from
400 to 500 keV. In all cases, end-station pressure improved.
systems will continue to be the best means for performing high-vacuum
pumping in ion implantation systems for the foreseeable future. As companies
struggle to optimize productivity, new vacuum-system designs must take
into account all the gases produced during wafer processing, even as
processes and gas-species by-products evolve and change.
9: Maximum end-station pressure over many wafer runs and processes.
Average end-station pressure was reduced by 20%. (Data courtesy
extremely high conductance of hydrogen through the gate valve and spool-piece
assembly that is used in cryopump installations has simplified the task
of optimizing hydrogen pumping speed. Hence, the capture probability
for hydrogen has been increased to more than 30%. For atmospheric gases
that have very low vapor pressure at 13 K, the capture probability is
about 35%, and for water it is already close to unity. In contrast,
it will be very difficult to achieve further improvements in pumping
speeds for water vapor or nitrogen because of the significant conductance
limit imposed by the gate valve and spool-piece interface assembly.
incremental increases in pumping speed will require significantly new
architectures to increase the solid angle subtended by the pumping array
at the gas source. Pumps will either have to be larger or be positioned
closer to the point where the beam strikes the wafer.5
is expected that the continued extension of distributed intelligence
will drive both ongoing safety enhancements and cost-of-ownership improvements.
These trends will require closer collaboration between equipment manufacturers
and designers of vacuum-pumping subsystems, resulting in further refinements
and improvements in both process control and system uptime and productivity.
Horsky, "Photoresist Outgassing in High Energy and High Current Ion
Implantation," in Proceedings of the International Conference on
Ion Implantation Technology 1998, vol. 1 (Piscataway, NJ: IEEE,
N Tokoro et al., "Consideration of Photoresist Outgassing for MeV Ion
Implantation and Cryopump Selection," in Proceedings of the International
Conference on Ion Implantation Technology 1998, vol. 1 (Piscataway,
NJ: IEEE, 1999), 614–617.
Kimo et al., "Recommended Practices for Measuring the Performance and
Characteristics of Closed-Loop Gaseous Helium Cryopumps," Journal
of Vacuum Science and Technology 17, no. 5 (1999): 3081–3095.
Lessard, "Cryogenic Adsorption of Noncondensibles in the High Vacuum
Regime," Journal of Vacuum and Science and Technology 7, no.
3 (1989): 2373–2376.
Furuya et al., "Development of Cryopump for Ion Implantation Equipment,"
in Proceedings of the International Conference on Ion Implantation
Technology 1998, vol. 1 (Piscataway, NJ: IEEE, 1999), 396–399.
Thompson is a technical product manager for the CTI business
group at Helix Technology (Mansfield, MA). He is responsible for the
company's On-Board IS cryogenic vacuum systems used in ion implantation
processes. He received a BS in mechanical engineering and an MS in computer
science from Rensselaer Polytechnic Institute in Troy, NY. (Thompson
can be reached at 508/337-5634 or email@example.com.)
Eacobacci is a senior technologist at Helix Technology. With
28 years of experience in the vacuum industry, he holds numerous patents
in the area of cryogenic vacuum pumps. He received a BS in mechanical
engineering and an MS in materials science from Northeastern University
in Boston and received training in technology management at Babson College
in Wellesley, MA. (Eacobacci can be reached at 508/337-5221 or firstname.lastname@example.org.)