| A
Sampling of Recent NIST Results of Interest
to the Semiconductor Industry
Special
for SEMICON West 2005, July 11-15, 2005
Finding
the True Measure Of Nanoscale Roughness
 |
A new NIST/SEMATECH technique should help semiconductor facilities
improve measurement of the "linewidth roughness."
Courtesy HDR Architecture, Inc./Steve Hall© Hedrich Blessing
For a high-resolution version of this photo, contact inquiries@nist.gov.
|
Straight
edges, good. Wavy edges, bad. This simple description holds true
whether you are painting the living room or manufacturing nanoscale
circuit features.
In
a technical paper* published in June 2005, researchers at the National
Institute of Standards and Technology (NIST) and SEMATECH describe
an improved method for determining nanoscale linewidth roughness,
an important quality control factor in semiconductor fabrication.
Their research shows that current industry measurement methods may
be exaggerating roughness of the smoothest circuit features by 40
percent or more above true values.
As
circuit features shrink in size to below 50 nanometers, wavy or
rough edges within semiconductor transistors may cause circuit current
losses or may prevent the devices from reliably turning on and off
with the same amount of voltage.
“With
this type of measurement,” says NIST’s John Villarrubia,
“besides the real roughness there is also a false roughness
caused by measurement noise. Our method includes a correction to
remove bias or systematic error from the measurement.”
Random
noise, by definition, causes the measured value to be sometimes
higher, sometimes lower than the true value, and can be minimized
by simply averaging an adequate number of measurements. Systematic
error, however, is consistently above or consistently below the
true value due to some quirk of the measurement method.
The
noise in nanoscale scanning electron microscope (SEM) images consistently
adds extra roughness, says Villarrubia. The NIST/SEMATECH method
involves taking two or more images at exactly the same location
on a circuit feature, comparing the values, and subtracting the
false roughness caused by measurement noise. SEM manufacturers should
be able to incorporate the new method into their proprietary software
for automated linewidth roughness measurements.
*J.S.
Villarrubia and B.D. Bunday, Unbiased Estimation of Linewidth Roughness,
Proceedings of SPIE 5752 (2005) pp. 480-488
Technical
Contact: John Villarrubia, john.villarrubia@nist.gov, (301)
975-3958
Media
Contact: Gail Porter, gail.porter@nist.gov, (301) 975-3392
Predicting
the Lifetime Of Extreme UV Optics
Extreme ultraviolet
lithography (EUVL) may be the next-generation patterning technique
used to produce smaller and faster microchips with feature sizes
of 32 nanometers and below. However, durable projection optics must
be developed before this laboratory technique can become commercially
viable. As part of its long-standing effort to develop EUVL metrology
and calibration services (summarized in a recent paper*) the National
Institute of Standards and Technology (NIST) is creating a measurement
system for accelerated lifetime testing of the mirrors used in EUVL.
The light
to be used in EUVL has a wavelength of only 13 nm. It can only be
efficiently reflected with mirrors consisting of 50 alternating
bi-layers of molybdenum and silicon, each only 7 nm thick and deposited
with near-atomic-scale precision. So although the EUVL mirrors will
be very large, up to 35 centimeter (cm) in diameter, they are actually
incredibly precise nanostructured devices. A single commercial lithography
instrument may require 6 of these mirrors at a cost of more than
$1 million each.
The mirrors
are delicate, but the EUV radiation they must reflect is intense
and damaging. The combination of this harsh radiation with the trace
levels of water vapor and hydrocarbons typically found in the vacuum
environment of EUV first-generation exposure tools can lead to rapid
corruption of the EUVL mirror surfaces. And a loss of just 1 percent
to 2 of a mirror’s reflectivity renders the optical system
useless for efficient production of nanometer-resolution circuit
features.
To help the
semiconductor industry meet its goal of EUVL production by 2010,
NIST has established a dedicated beamline at its Synchrotron Ultraviolet
Radiation Facility for durability testing of multilayer mirrors.
Initial tests established that standard mirrors topped with silicon
would have lifetimes of just minutes to hours, while ruthenium-capped
mirrors had lifetimes of a few tens of hours, still a thousand times
less than industry’s requirement.
To determine
how damage scales with various parameters, NIST researchers recently
exposed EUVL mirrors (provided by SEMATECH from work it co-funded)
to varying levels of light intensity, water, and hydrocarbon concentrations.
Contrary to
expectations, they found that increasing amounts of water vapor
caused less mirror damage, which may be due to a simultaneous increase
in the ambient hydrocarbon levels. Subsequent experiments have shown
that deliberately introducing trace amounts of a simple hydrocarbon
like methanol can mitigate significantly the water-induced damage.
NIST scientists are commissioning a new beamline devoted to accelerated
testing and will add a second branch to the existing beamline that
will provide broadband illumination (wavelengths of approximately
11 nm to 50 nm) into a single spot at approximately 100 times the
intensity of the current system.
*S. Grantham,
S.B. Hill, C. Tarrio, R.E. Vest, and T.B. Lucatorto .2005. EUV Component
and System Characterization at NIST for the Support of Extreme-Ultraviolet
Lithography. Proceedings of SPIE 5751, 1185-91.
Technical
Contact: Charles Tarrio, charles.tarrio@nist.gov, (301)
975-3737
Media
Contact: Laura Ost, laura.ost@nist.gov, (301) 975-4034
New
Infrared Tool Measures Silicon Wafer Thickness
In the last
few years, semiconductor circuit features have shrunk to sub-100
nanometer (nm) dimensions, while the size of the thin silicon wafers
that these circuits are constructed on has grown from 200 milli-meters
(mm) to 300 mm (about 12 inches). The payoff is a higher yield of
finished devices from fewer wafers.
The tough part,
however, is to make wafers substantially larger while simultaneously
meeting higher quality control specifications. The optics and materials
for “printing” nanoscale circuit lines require that
the wafers used are perfectly flat and of uniform thickness. To
help the semiconductor industry meet its 2010 quality control roadmap
goals, researchers at the National Institute of Standards and Technology
(NIST) recently developed a new instrument that accurately measures
differences in thickness across a 300 mm wafer with an excellent
repeatability of 5 nm. The researchers hope the tool, with further
refinements, will allow them to establish a new calibration service
for “master wafers” used in the industry to measure
wafer thickness.
The NIST researchers
will describe the instrument, the Improved Infrared Interferometer,
or IR3 for short, at a technical conference* in late
July. Like all interferometers, the IR3 uses intersecting waves
of light to create interference patterns, which in turn can be used
as a ruler to measure nanoscale dimensions. While most interferometers
use red laser light, the IR3 uses infrared laser light.
And unlike visible light, these much longer wavelengths pass right
through a silicon wafer. This means that IR3 can illuminate
the top and bottom on a 300 mm wafer and produce a detailed spatial
map of differences in thickness in one pass. Conventional tools
require spinning the wafer and measuring at multiple locations.
The NIST researchers
make precision measurements of the wafer’s index of refraction—the
amount that light is “bent” as it passes through the
silicon—as a critical step in correctly interpreting the interference
patterns. Increased precision in the refractive index measurement
will be necessary before “absolute” measurements of
thickness rather than relative differences will be possible with
the new instrument.
*Q. Wang, U.
Griesmann, and R. Polvani. “Interferometric Thickness Calibration
of 300 mm Silicon Wafers,” ASPE Summer Topical Meeting on
Precision Interferometric Metrology (July 20-22, 2005).
Technical
Contact: Ulf Griesmann, ulf.griesmann@nist.gov, (301) 975-4929
Media
Contact: Gail Porter, gail.porter@nist.gov, (301) 975-3392
New
Design Developed For Silicon Nanowire Transistors
 |
A schematic diagram of the NIST nanowire transistor.
Click
on image for high-resolution version.
NIST Illustration |
In an advance
for nanoscale electronics, researchers at the National Institute
of Standards and Technology (NIST) have demonstrated a new design
for silicon nanowire transistors that both simplifies processing
and allows the devices to be switched on and off more easily.
The NIST design,
described in a paper published June 29, 2005, by the journal Nanotechnology,*
uses a simplified type of contact between the nanowire channel and
the positive and negative electrodes of the transistor. The design
allows more electrical current to flow in and out of the silicon.
The researchers believe the design is the first to demonstrate a
“Schottky barrier” type contact for a nanowire transistor
built using a “top-down” approach. This barrier, an
easily formed metal contact that electrons can tunnel through, requires
much less doping with impurities than do conventional ohmic contacts,
thereby simplifying processing requirements. Schottky contacts also
offer more resistance and restrict electrical flow to one direction
when the transistor is off.
In the NIST
transistor design, the 60-nanometer-wide channels exhibit a much
greater difference in current between the on and off states than
is true for larger reference channels up to 5 micrometers wide.
This suggests that when a channel is scaled down to the nano regime,
the ultra-narrow proportions significantly reduce the current leakage
associated with defects in silicon. As a result, the transistors
are less sensitive to electronic “noise” in the channel
and can be turned on and
off more effectively, according to the paper’s lead author,
Sang-Mo Koo, a NIST guest researcher.
Silicon nanowire
devices have received considerable attention recently for possible
use in integrated nanoscale electronics as well as for studying
fundamental properties of structures and devices with very small
dimensions. The NIST work overcomes some key difficulties in making
reliable devices or test structures at nanoscale dimensions. The
results also suggest that nanowire transistors made with conventional
lithographic fabrication methods can improve performance in nanoscale
electronics, while allowing industry to retain its existing silicon
technology infrastructure.
*S.M. Koo,
M.D. Edelstein, Q.Li, C.A. Richter and E.M. Vogel. 2005. Silicon
Nanowires as Enhancement-Mode Schottky Barrier Field-Effect Transistors.
Nanotechnology 16. Posted online June 29.
Technical
Contact: Sang-Mo Koo, smkoo@nist.gov, (301) 975-8755
Media
Contact: Laura Ost, laura.ost@nist.gov, (301) 975-4034
NIST
Photon Detectors Have Record Efficiency
Sensors that
detect and count single photons, the smallest quantities of light,
with 88 percent efficiency have been demonstrated by physicists
at the National Institute of Standards and Technology (NIST). This
record efficiency is an important step toward making reliable single
photon detectors for use in practical quantum cryptography systems,
the most secure method known for ensuring the privacy of a communications
channel.
Described
in the June 2005 issue of Physical Review A, Rapid Communications,*
the NIST detectors are composed of a small square of tungsten film,
25 by 25 micrometers and 20 nanometers thick, chilled to about 110
milliKelvin, the transition temperature between normal conductivity
and superconductivity. When a fiber-optic line delivers a photon
to the tungsten film, the temperature rises and results in an increase
in electrical resistance. The change in temperature is proportional
to the photon energy, allowing the sensor to determine the number
of photons in a pulse of monochromatic light.
This type
of detector typically has limited efficiency because some photons
are reflected from the front surface and others are transmitted
all the way through the tungsten. NIST scientists more than quadrupled
the detection efficiency over the past two years by depositing the
tungsten over a metallic mirror and topping it with an anti-reflective
coating, to enable absorption of more light in the tungsten layer.
The NIST sensors
operate at the wavelength of near-infrared light used for fiber-optic
communications and produce negligible false (or dark) counts. Simulations
indicate it should be possible to increase the efficiency well above
99 percent at any wavelength in the ultra-violet to near-infrared
frequency range, by building an optical structure with more layers
and finer control over layer thickness, according to the paper.
Quantum communications
and cryptography systems use the quantum properties of photons to
represent 1s and 0s. The NIST sensors could be used as receivers
for quantum communications systems, calibration tools for single
photon sources, and evaluation tools for testing system security.
They also could be used to study the performance of ultralow light
optical systems and to test the principles of quantum physics. The
work is supported by the Director of Central Intelligence postdoctoral
program and the Advanced Research and Development Activity.
*D. Rosenberg,
A.E. Lita, Aaron J. Miller, and S.W. Nam. 2005. Noise-Free, High-Efficiency,
Photon-Number-Resolving Detectors. Physical Review A, Rapid
Communications. June.
Technical
Contact: Danna Rosenberg, rdanna@boulder.nist.gov, (303)
497-4464
Media
Contact: Laura Ost, laura.ost@nist.gov, (301) 975-4034
Shadow
Technique Improves Measurement of Micro Holes
 |
NIST
researchers and collaborators have developed a new method
for measuring the interior dimensions of small holes with
an uncertainty of only 35 nanometers. Here, a glass probe
is inserted into an optical "ferrule," a device
for connecting optical fibers used in communications systems.
NIST
Photo
Click
here for a high resolution version of this image. |
Sometimes seeing
a shadow can be as good or better than seeing the real thing. A
new measurement method* developed by researchers working at the
National Institute of Standards and Technology (NIST) is a case
in point. The method uses the shadow cast by a small glass probe
to infer the dimensions of tiny, microscale holes or other micrometer-sized
components. The technique may provide an improved quality control
method for measuring the interior dimensions of fuel nozzles, fiber
optic connectors, biomedical stents, ink jet cartridges, and other
precision-engineered products.
Designed to
be implemented with the type of coordinate measuring machine (CMM)
routinely used in precision manufacturing settings, the method uses
a flexible glass fiber with a microsphere attached on one end. The
glass probe is attached to the CMM’s positioning system, inserted
into the part to be measured, and systematically touched to the
part’s interior walls in multiple locations. A light-emitting
diode is used to illuminate the glass fiber. While the microsphere
inside the part is not visible, the shadow of the attached fiber—with
a bright band of light at its center—shows the amount of deflection
in the probe each time the part’s interior is touched. A camera
records the shadow positions. Based on prior calibration of the
force required to bend the probe a specific distance, the part’s
dimensions can be determined with an uncertainty of about 35 nanometers
(nm). The method can be used for holes as small as 100 micrometers
in diameter.
“Our
probe has a much smaller measurement uncertainty than other available
methods and it is very cost effective to make,” says Bala
Muralikrishnan, a NIST guest researcher from the University of North
Carolina at Charlotte.
The thin, glass
fiber is about 20 millimeters long and 50 micrometers in diameter,
making it especially useful for measuring relatively deep holes
not easily measured with other methods. Replacement probes cost
about $100 compared to about $1,000 for those manufactured using
silicon micromachining techniques.
*B. Muralikrishnan,
J.A. Stone, and J. R. Stoup. Measuring Internal Geometry of Fiber
Ferrules. Presented at the SME MicroManufacturing Conference, Minneapolis,
Minn., May 4-5, 2005.
Technical
Contact: Bala Muralikrishnan, bala.muralikrishnan@nist.gov,
(301) 975-3789
Media
Contact: Gail Porter, gail.porter@nist.gov, (301) 975-3392
World’s
First UV ‘Ruler’ Sizes Up Atomic World
The world’s
most accurate “ruler” made with extreme ultraviolet
light has been built and demon-strated with ultrafast laser pulses
by scientists at JILA, a joint institute of the National Institute
of Standards and Technology (NIST) and the University of Colorado
at Boulder.
The new device,
which consistently generates pulses of light lasting just femtoseconds
(quadrillionths of a second, or millionths of a billionth of a second)
in the ultraviolet region of the electromagnetic spectrum, will
be described in the May 20, 2005 issue of Physical Review Letters
*
The device
is expected to become an important tool for ultraprecise measurements
in many fields of science, including chemistry, physics, and astronomy.
A ruler made with shorter wavelengths of light makes it possible
to “see” more precise differences than ever before in
the energy levels of light emissions that identify specific atoms,
in the timing of chemical reactions, or, if additional applications
are developed, in the dimensions of certain nanometer-scale objects.
The new device also can be compared to a camera with ultrafast shutter
speeds and consistent shot-to-shot frame speed and stability, allowing
scientists to take real-time “pictures” of finer structures
and dynamics. By combining many such pictures at a high speed, scientists
can gain a more detailed understanding of many phenomena.
“This
ultraviolet light source has a spectacularly high resolution,”
says Jun Ye, a NIST Fellow who leads the JILA research group. “On
the technological side, the system we used to produce this light
is simple and low cost, without active amplifiers.”
The new laser
device generates a “frequency comb,” so-called because
the frequency spectrum—a graphical representation of the pattern
made by many successive laser pulses building on each other—looks
like the evenly spaced teeth of a hair comb. The new comb is a short-wavelength
version of the optical frequency combs that in recent years have
enabled demonstrations of optical atomic clocks, which are expected
to be as much as 100 times more accurate than today’s microwave-based
atomic clocks. A femtosecond comb, because of its high speed (or
repetition rate), has the finest teeth of any optical ruler.
For further
information, see www.nist.gov/public_affairs/newsfromnist_uvruler.htm.
*R.J. Jones,
K.D. Moll, M.J. Thorpe, and J. Ye. 2005. Phase-Coherent Frequency
Combs in the VUV via High-Harmonic Generation inside a Femtosecond
Enhancement Cavity. Physical Review Letters. May 20.
Technical
Contact: Jun Ye, ye@jila.colorado.edu, (303) 735-3171
Media
Contact: Laura Ost, laura.ost@nist.gov, (301) 975-4034
NIST
Method Improves Timing in Oscilloscopes
A new method
for correcting common timing errors in high-speed oscilloscopes
has been developed by researchers at the National Institute of Standards
and Technology (NIST). The method improves the accuracy and clarity
of measurements performed in the development and troubleshooting
of components for wireless and optical communications, military
radar, and other technologies.
Oscilloscopes
display graphical representations of electrical and optical signals
as waves, showing how the signals change over time. These instruments
often have inaccurate internal clocks that distort output patterns,
and they also can exhibit random timing errors called jitter. These
errors may lead, for example, to false detection of failure in a
communications module that is actually working, or to increased
electronic “noise” interference with measurements of
microwave signals from radar.
The NIST method,
based on an approach developed in laboratory experiments and implemented
in freely available software, constructs an alternative time base.
The software analyzes an oscilloscope’s measurements of both
a signal of interest and two reference waves that are offset from
each other. The reference waves are generated by an external device
and are synchronized in time with the signal being measured. Measurements
of the reference waves are compared with a calculation of an ideal
wave to produce an estimate of total time errors due to distortion
and jitter. These errors then can be corrected automatically for
each measurement made by the oscilloscope.
The NIST correction
method can be applied to older standard equipment, can correct time
records of almost any length, and can be applied to electromagnetic
signals of almost any frequency. It also provides the user with
an estimate of the residual timing error after the correction process
has been completed. The Timebase Correction software package is
available free of charge at www.boulder.nist.gov/div815/HSM_Project/Software.htm.
Technical
Contact: Paul Hale, hale@boulder.nist.gov, (303) 497-5367
Media
Contact: Laura Ost, laura.ost@nist.gov, (301) 975-4034
Chip-scale
Refrigerators Cool Bulk Objects
 |
| This
colorized scanning electron micrograph shows a cube of germanium
attached to a membrane. The four small light blue
rectangles at the midpoints of the membrane perimeter are
chip-scale refrigerators that cooled the cube and membrane
to only a few hundred thousandths of a degree above absolute
zero.
Click
here to download a higher resolution version of this image.
Image
credit: N. Miller, A. Clark/NIST
|
Chip-scale
refrigerators capable of reaching temperatures as low as 100 milliKelvin
have been used to cool bulk objects for the first time, researchers
at the National Institute of Standards and Technology (NIST) report.
The solid-state refrigerators have applications such as cooling
cryogenic sensors in highly sensitive instruments for semiconductor
defect analysis and astronomical research.
The work is
featured in the April 25, 2005, issue of Applied Physics Letters.*
The NIST-designed refrigerators, each 25 by 15 micrometers, are
sandwiches of a normal metal, an insulator and a superconducting
metal. When a voltage is applied across the sandwich, the hottest
electrons “tunnel” from the normal metal through the
insulator to the superconductor. The temperature in the normal metal
drops dramatically anddrains electronic and vibrational energy from
the objects being cooled.
The researchers
used four pairs of these sandwiches to cool the contents of a silicon
nitrate membrane that was 450 micrometers on a side and 0.4 micrometers
thick. A cube of germanium 250 micrometers on a side was glued on
top of the membrane. The cube is about 11,000 times larger than
the combined volume of the refrigerators. This is roughly equivalent
to having a refrigerator the size of a person cool an object the
size of the Statue of Liberty. Both objects were cooled down to
about 200 mK, and further improvements in refrigerator performance
are possible, according to the paper.
The refrigerators
are fabricated using common chip-making lithography methods, making
production and integration with other microscale devices straightforward.
The devices are much smaller and less expensive than conventional
equipment used for cooling down to 100 mK, a target temperature
for optimizing the performance of cryogenic sensors. These sensors
take advantage of unusual phenomena that occur at very low temperatures
to detect very small differences in X-rays given off by nanometer-scale
particles, enabling users such as the semiconductor industry to
identify the particles. The work was supported in part by the National
Aeronautics and Space Administration and NIST’s Office of
Microelectronics Programs.
*A.M. Clark,
N.A. Miller, A. Williams, S.T. Ruggiero, G.C. Hilton, L.R. Vale,
J.A. Beall, K.D. Irwin, and J.N. Ullom. Cooling of Bulk Material
by Electron-Tunneling Refrigerators. Applied Physics Letters.
April 25, 2005.
Technical
Contact: Joel Ullom, ullom@boulder.nist.gov, (303) 497-4408
Media
Contact: Laura Ost, laura.ost@nist.gov, (301) 975-4034
Nanomagnets
Bend the Rules
Nanocomposite
materials seem to flout conventions of physics. In the latest example
of surprising behavior, reported* April 15, 2005, by scientists
at the National Institute of Standards and Technology (NIST) and
Brookhaven National Laboratory, a class of nanostructured materials
that are key components of computer memories and other important
technologies undergo a previously unrecognized shift in the rate
at which magnetization changes at low temperatures.
The team suggests
that the apparent anomaly described as an “upturn” in
magnetization may be due to the quantum mechanical process known
as Bose-Einstein condensation. They maintain that, in nano-structured
magnets, energy waves called magnons coalesce into a common ground
state and, in effect, become one. This collective identity, the
researchers say, results in magnetic behavior seemingly at odds
with a long-standing theory.
The new finding
could prompt a reassessment of test methods used to predict technologically
important properties of “ferromagnetic” materials. The
results also could point the way to marked improvements in the performance
of microwave devices. Magnets are integral to these devices, used
in a variety of communication and defense technologies.
Ferromagnets,
including iron, cobalt, nickel, and many tailor-made materials,
become magnetic when exposed to an external magnetic field. As the
strength of the external field increases, the materials become more
magnetic, an atomic-level, temperature-influenced process called
magnetic saturation. When the external field is removed, ferromagnets
undergo an internal restructuring and the acquired magnetization
decays, or fades, very slowly at a rate that increases with temperature.
Determined
through accelerated testing methods, the temperature dependence
of magnetic saturation and the rate of magnetization decay are key
concerns in the design of permanent magnets, hard disks, and other
magnetic data storage systems.
For further
information, see www.nist.gov/public_affairs/releases/nanomagnets_bend_rules.htm.
*E. Della
Torre, L.H. Bennett, and R.E. Watson, Extension of the Bloch T3/2
Law to Magnetic Nanostructrures: Bose-Einstein Condensation. Physical
Review Letters. April 15, 2005.
Technical
Contact: Lawrence Bennett, lawrence.bennett@nist.gov, (301)
975-5966
Media
Contact: Mark
Bello, mark.bello@nist.gov, (301) 975-3776
New
Gas Sensors Patterned With Conducting Polymer
An improved
method for depositing nanoporous, conducting polymer films on miniaturized
device features has been demonstrated by researchers at the National
Institute of Standards and Technology (NIST).
Described in
the April 6, 2005, issue of the Journal of the American Chemical
Society,* the method may be useful as a general technique for
reproducibly fabricating microdevices such as sensors for detecting
toxic chemicals.
Unlike most
polymers, conducting polymers have the electrical and optical properties
of metals or semiconductors. These materials are of increasing interest
in microelectronics because they are inexpensive, flexible, and
easy to synthesize.
Polyaniline
is a particularly promising conducting polymer for microelectronics
applications, but it is difficult to process because it doesn’t
dissolve in most solvents. NIST researchers have circumvented this
problem by dispersing nanoscale particles of polyaniline into a
mild solvent.
“The
beauty of the method,” says NIST guest researcher Guofeng
Li, “is that the polyaniline chain carries a natural positive
charge.” Once the particles are formed, electrostatic repulsion
prevents them from clumping together. Moreover, the positively charged
particles then can be manipulated and patterned on complex device
structures by applying an electrical field.
The process
produces a sponge-like coating that efficiently captures gaseous
molecules. So far NIST researchers have demonstrated that such coatings
can detect the difference between methanol and water vapor. Additional
tests will be needed before the polymer devices could be used for
detecting toxic gases.
NIST holds
patents for previous work using microheaters coated with nanostructured
tin oxide films. As the microheaters cycle through a series of temperatures,
changes in electrical resistance are used to detect toxic gases
at part per billion levels. Ultimately, NIST researchers hope to
develop inexpensive arrays of microheater sensors coated with both
polymer and inorganic oxide films optimized to identify the components
of gas mixtures.
*G. Li, C.
Martinez, and S. Semancik. Controlled Electrophorectic Patterning
of Polyaniline from a Colloidal Suspension. Journal of the American
Chemical Society, April 6, 2005.
Technical
Contact: Guofeng Li, guofeng.li@nist.gov, (301) 975-4782
Media
Contact: Gail Porter, gail.porter@nist.gov, (301) 975-3392
Nano-sized
Chip Features Measured with Atom ‘Ruler’
Device features
on computer chips as small as 40 nanometers (nm) wide—less
than one-thousandth the width of a human hair—now can be measured
reliably thanks to new test structures developed by a team of physicists,
engineers, and statisticians at the National Institute of Standards
and Technology (NIST), SEMATECH, and other collaborators. The test
structures are replicated on reference materials that will allow
better calibration of tools that monitor the manufacturing of microprocessors
and similar integrated circuits.
The new test
structures are the culmination of NIST’s more than four-year
effort to provide standard “rulers” for measuring the
narrowest linear features that can be controllably etched into a
chip. The NIST rulers are precisely etched lines of crystalline
silicon ranging in width from 40 nm to 275 nm. The spacing of atoms
within the box-shaped silicon crystals is used like hash marks on
a ruler to measure the dimensions of these test structures. Industry
can use these reference materials to calibrate tools to reliably
measure microprocessor-device gates, for example, which control
the flow of electrical charges in chips.
“We have
caught up to the semiconductor industry roadmap for linewidth reference-material
dimensions with this work,” says Richard Allen, one of the
NIST researchers involved in the project. “With the semiconductor
industry, one has to run at full speed just to keep up.”
The new reference
materials, configured as a 9 millimeter (mm) by 11 mm chip embedded
in a silicon wafer, are now being evaluated by SEMATECH member companies.
Compared to a batch of prototype test structures produced by NIST
in 2001, the new reference materials offer a wider range of reference
feature sizes, including some that are much narrower, and they are
measured much more precisely (with uncertainties of less than 2
nm compared to 14 nm previously). In the absence of reference materials
such as these, companies have calibrated measurement tools using
in-house standards, which may neither be accurate nor agree with
each other.
The new materials
were unveiled publicly at a workshop co-sponsored by NIST and SEMATECH
on March 2, in conjunction with a SPIE (International Society for
Optical Engineering) meeting in San Jose, Calif.
For further
information, see www.nist.gov/public_affairs/releases/atom_rulers.htm.
Technical
Contact: Michael Cresswell, michael.cresswell@nist.gov,
(301) 975-2072
Media
Contact: Laura
Ost, laura.ost@nist.gov, (301) 975-4034
Devising
Nano Vision For an Optical Microscope
 |
|
Image credit: Beamie Young/NIST
A new
optical imaging technology under development at NIST will
use combinations of dynamically controlled light waves, optimized
for particular properties (such as polarization). How this
structured illumination field -- engineered specifically to
highlight the particular geometry of each type of specimen
-- scatters after striking the target may reveal features
smaller than 10 nanometers. |
Contrary to
conventional wisdom, technology’s advance into the vanishingly
small realm of molecules and atoms may not be out of sight for the
venerable optical microscope, after all. In fact, research at the
National Institute of Standards and Technology (NIST) suggests that
a hybrid version of the optical microscope might be able to image
and measure features smaller than 10 nanometers—a tiny fraction
of the wavelength of visible light.
In a preliminary
test of the embryonic technique, NIST scientists used violet light
with a wavelength of 436 nanometers to image features as small as
40 nanometers, about five times smaller than possible with a conventional
optical microscope.
Roughly speaking,
such a feat is akin to picking up a solitary dime with a clumsy
front-end loader. If successfully developed, the imaging technology
could be readily incorporated into chip-making and other commercial-scale
processes for making parts and products with nanometer-scale dimensions.
The wavelengths
of light in the visible part of the spectrum greatly exceed nanoscale
dimensions. Consequently, the resolution of conventional light-based
imaging methods is limited to about 200 nanometers—too large
to resolve the details of nanotechnology, which, by definition,
are no more than half that size.
However, a
newly begun, five-year research effort at NIST suggests that a novel
combination of illumination, detection, and computing technologies
can circumvent this limitation. Success would extend the technology’s
400-year-long record as an indispensable imaging and measurement
tool well into the expanding realm of nanotechnology.
Called phase-sensitive,
scatter-field optical imaging, the computer-intensive technique
under development at NIST uses a set of dynamically engineered light
waves optimized for particular properties (such as angular orientation
and polarization). How this structured illumination field—engineered
differently to highlight the particular geometry of each type of
specimen—scatters after striking the target can reveal the
tiniest of details.
“The
scattering patterns are extremely sensitive to small changes in
the shape and size of the scattering feature,” explains Rick
Silver, a physicist in NIST’s Precision Engineering Division.
Technical
Contact: Richard Silver, richard.silver@nist.gov, (301)
975-5609
Media
Contact: Mark
Bello, mark.bello@nist.gov, (301) 975-3776
Gentler
Processing May Yield Better Molecular Devices
A simple, chemical
way to attach electrical contacts to molecular-scale electronic
components has been developed by researchers at the National Institute
of Standards and Technology (NIST). The recently patented* method
attaches a layer of copper on the ends of delicate molecular components
to avoid damage to the components that commonly occurs with conventional
techniques.
Molecular
electronics—designing carbon-based molecules to act as wires,
diodes, transistors, and other microelectronic devices—is
one of the most dynamic frontiers in nanotechnology. An area equal
to the cross-section of a typical human hair might hold about a
thousand semiconductor transistors at the current state of art,
but up to 13 million molecular transistors.
A key challenge
in molecular electronics is making electrical contacts to the fragile
molecules, chemical chains that are damaged easily. Currently, this
is most often done by vaporizing a metal onto the molecules that
stand like blades of grass on a metal substrate. The vaporized metal
atoms are supposed to settle on the tops of the molecules but they
also often eat away at the delicate structures, or fall through
gaps in the “turf” and short out the device. Yields
of working devices are typically only a few percent.
NIST researchers
designed a technique in which the molecules are synthesized with
an additional chemical group attached to the top of the molecule.
The chip is immersed in a solution including copper ions, which
preferentially bind to the added group, forming a strong, chemically
bonded contact that also protects the underlying molecule during
further metallic vapor deposition steps. Tests at NIST have demonstrated
that the technique works well on surfaces patterned with microcontact
printing, producing clean, sharply defined edges, important for
the fabrication of practical devices.
*See U.S.
patent, no. 6,828,581 available here:
http://patapsco.nist.gov/TS/220/sharedpatent/pdf/6828581.pdf.
Technical
Contact: Christopher Zangmeister, christopher.zangmeister@nist.gov,
(301) 975-8709, Roger Van Zee, roger.vanzee@nist.gov, (301) 975-2363
Media
Contact: Michael Baum, michael.baum@nist.gov, (301) 975-2763
Laser
Applications Heat Up For Carbon Nanotubes
Carbon nanotubes—a
hot nanotechnology with many potential uses—may find one of
its quickest applications in the next generation of standards for
optical power measurements, which are essential for laser systems
used in manufacturing, medicine, communications, lithography, space-based
sensors, and other technologies.
As described
in a paper in Applied Optics,* scientists at the National
Institute of Standards and Technology (NIST) and the National Renewable
Energy Laboratory have made prototype pyroelectric detectors coated
with carbon nanotubes. Pyroelectric detectors and other thermal
detectors are the basis for all primary standards used to ensure
that laser power and energy measurements are traceable to fundamental
units. The coating absorbs laser light and converts it to heat,
which is conducted to a detector underneath made of pyroelectric
material. The detector’s rise in temperature generates a current,
which is measured to determine the power of the laser.
Carbon nanotubes—tiny
cylinders made of carbon atoms—conduct heat hundreds of times
better than today’s detector coating materials. Nanotubes
are also resistant to laser damage and, because of their texture
and crystal properties, absorb light efficiently. Scientists hope
that the nanotubes’ resistance to aging and hardening will
allow them to extend the range of NIST laser power standards to
ultraviolet wavelengths, which would support the development and
calibration of sensors for detecting chemical and biological weapons.
The research also may contribute to the use of carbon nanotubes
in fuel cells.
As described
in the paper, the NIST-led research team was first to demonstrate
the use of an airbrush technique to apply carbon nanotubes to a
thermal detector. The team also reported, at a workshop on carbon
nanotubes at NIST Jan. 26-28, 2005, growing multiwalled nanotubes
directly on detectors with a chemical vapor deposition process.
The team is now measuring the optical and thermal properties of
various tube compositions and topologies, using an unusual approach
that is much faster than conventional methods.
*J.H. Lehman,
C. Engtrakul, T. Gennett, and A.C. Dillon. 2005. Single-Wall Carbon
Nanotube Coating on a Pyroelectric Detector. Applied Optics,
Vol. 44. Feb. 2005, 483-488.
Technical
Contact: John Lehman, lehman@boulder.nist.gov, (303) 497-3654
Media
Contact: Laura Ost, (301) 975-4034, laura.ost@nist.gov
Tiny,
Atom-based Detector Senses Weak Magnetic Fields
A low-power,
magnetic sensor about the size of a grain of rice that can detect
magnetic field changes as small as 50 picoteslas—a million
times weaker than the Earth’s magnetic field—has been
demonstrated by researchers at the National Institute of Standards
and Technology (NIST). Described in the Dec. 27, 2004, issue of
Applied Physics Letters,* the device can be powered with
batteries and is about 100 times smaller than current atom-based
sensors with similar sensitivities, which typically weigh several
kilograms (about 6 pounds).
The new magnetic
sensor is based on the principles of a NIST chip-scale atomic clock,
announced in August 2004. Expected applications for a commercialized
version of the new sensor could include hand-held devices for sensing
unexploded ordnance, precision navigation, geophysical mapping to
locate minerals or oil, and medical instruments.
Like the NIST
chip-scale clock, the new magnetic sensor can be fabricated and
assembled on semiconductor wafers using existing techniques for
making microelectronics and microelectromechanical systems (MEMS).
This offers the potential for low-cost mass production of sensors
about the size of a computer chip. When packaged with associated
electronics, the researchers believe the mini magnetometer will
measure about 1 cubic centimeter or about the size of a sugar cube.
Magnetic fields
are produced by the motion of electrons either in the form of an
electrical current or in certain metals such as iron, cobalt, and
nickel. The NIST miniature magnetometer is sensitive enough to detect
a concealed rifle about 12 meters (40 feet) away or a six-inch-diameter
steel pipeline up to 35 meters (120 feet) underground. The sensor
works by detecting minute changes in the energy levels of electrons
in the presence of a magnetic field.
For further
information, see www.nist.gov/public_affairs/releases/CSMagnetometer.htm.
*P. Schwindt,
S. Knappe, V. Shah, L. Hollberg, J. Kitching, L. Liew, and J. Moreland.
“Chip-Scale Atomic Magnetometer.” Applied Physics
Letters. 27 Dec. 2004.
Technical
Contact: Peter Schwindt, schwindt@boulder.nist.gov, (303)
497-7969
Media
Contact: Gail Porter, gail.porter@nist.gov, (301) 975-3392
Microchip
Industry Strives To Perfect Its Timing
Time is money,
especially to the semiconductor industry. Electronics manufacturers
use extremely sophisticated equipment to churn out the latest microchips,
but they have a timing problem. It’s very difficult to get
all the fabrication tools in a manufacturing line to agree on the
time. Components within a single
tool can disagree on the time by as much as two minutes, because
of a lack of synchronization.
According
to a recently released report by the National Institute of Standards
and Technology (NIST) and International SEMATECH,* the timing deficiencies
will become important as device dimensions and tolerances continue
to shrink. In particular, timing becomes critical as firms advance
e-manufacturing concepts such as real-time automation and intelligent
control.
Tools can
be synchronized to about 100 millisecond (ms) accuracies today,
but with significant variations. The problems are myriad, according
to the report. For instance, subsystems made by suppliers may lack
the interfaces needed to synchronize their clocks with host clocks
made by original equipment manufacturers. Quality control software
that relies on time stamps to diagnose processing errors may overload
the computing resources of fabrication systems, therefore degrading
the time stamp accuracy. There also is pressure to move forward:
Methods are available to reach 1 ms accuracy in the near future,
but sub-millisecond accuracies will be required eventually.
To help achieve
that level of precision, NIST is leveraging its timekeeping expertise
to support the industry’s development of time synchronization
standards in collaboration with International SEMATECH’s e-Manufacturing
initiatives. A next-generation time synchronization protocol under
development by the Institute of Electrical and Electronics Engineers
should improve the outlook, and NIST has developed educational presentations
and white papers to summarize the key issues and potential solutions.
In addition, NIST plans to facilitate future standards development,
possibly under a new Time Synchronization Working Group, chartered
by Semiconductor Equipment Materials International.
*Y-S. Li and
B. Van Eck. 2004. Semiconductor Factory and Equipment Clock Synchronization
for e-Manufacturing. International SEMATECH Manufacturing Initiative,
NISTIR 7184.
Technical
Contact: Ya-Shian Li, ya-shian@nist.gov, (301) 975-5319
Media
Contact: Laura Ost, laura.ost@nist.gov, (301) 975-4034
New
Project Takes Measure Of Plastic Electronics
In the future,
the phrase smarty pants might be taken quite literally, referring
to trousers embedded with electronic “intelligence”
so that they change color, for example, in response to their surroundings.
The timing
of this vernacular twist will depend on when plastic “chips”
become practical—so cheap and reliable that electronic circuits
can be printed not only on clothing but also on paper, billboards
and nearly anything else. Unlike today’s largely silicon-based
technologies, organic (carbon-based) materials are flexible, can
be processed at low temperatures, and lend themselves to large-area
applications, such as wall-sized electronic murals.
Before the
emerging field of organic electronics can deliver on its commercial
promise, however, new measurements, standards, and processing capabilities
must be developed. Creating many of the requisite tools is the aim
of a new five-year research effort at the National Institute of
Standards and Technology (NIST).
“Organic
electronics is at a stage akin to the very early days of the silicon
semiconductor industry,” explains NIST polymer scientist Eric
Lin. “Lack of validated diagnostic probes and standardized
test and measurement methods is an impediment to progress.”
Unfortunately,
the job of filling this void is especially challenging. The range
of potential materials for organic electronics—from polymers
to nanocomposites—is enormous. The number of synthesis and
proc-essing methods under consideration is also daunting. Examples
include ink-jet printing, roll-to-roll printing, and various ways
to coax molecules to self-assemble into components.
Accurate,
reliable measurements will help solve current manufacturing issues
and speed widespread use of the new microchips. Ultimately, says
Lin, NIST plans to develop an “integrated measurement platform.”
The envisioned tool will allow scientists and engineers to predict
the performance of organic electronic devices based on composition,
structure, and materials properties.
Technical
Contact: Eric Lin, eric.lin@nist.gov, (301) 975-6743
Media
Contact: Mark Bello, mark.bello@nist.gov, (301) 975-3776
Better
Temperature Control Improves NIST X-ray Detector
Researchers
at the National Institute of Standards and Technology (NIST) have
developed an improved experimental X-ray detector that could pave
the way to a new generation of wide-range, high-resolution trace
chemical analysis instruments. In a recently published technical
paper*, the researchers described how they used improved temperature-sensing
and control systems to detect X-rays across a very broad range of
energies (6 keV or more), with pinpoint energy resolution (an uncertainty
of only 2 eV).
The detector’s
ability to distinguish between X-rays with very similar energies
should be especially useful to the semiconductor industry for chemical
analysis of microscopic circuit features or contaminants. Many types
of high-resolution microscopes routinely used in the industry and
throughout science produce detailed chemical maps by scanning a
surface with electrons and then analyzing the X-rays emitted, which
are characteristic of specific elements.
The NIST device,
an improved version of its previous microcalorimeter X-ray detector,
uses a quantum-level, transition edge sensor (TES). NIST has led
development of these sensors for several years. A TES works by measuring
the current across a thin metal film that is held just at the knife-edge
transition temperature between a superconducting state and normal
conductance. A single X-ray photon striking the detector raises
the temperature enough to alter the current proportional to the
energy of the photon.
TES microcalorimeters
offer an unequaled combination of high resolution with detection
of a broad energy range, allowing identification of many different
chemical elements simultaneously. The two kinds of detectors conventionally
used in X-ray microanalysis typically have a resolution of no better
than 130 eV, or have a high resolution but only for a very narrow
range of energies. TES sensors, however, must be kept at very low
temperatures (about 97 millikelvin) for hours at a stretch to collect
trace-level data. Tiny changes in temperature would cause previous
versions of the instrument to “drift” over time, requiring
constant recalibrations. The improved temperature control system
for the new detector eliminates this problem, making the system
much more practical for a broad range of applications.
*T. Jach, J.A.
Small, and D.E. Newbury. “Improving Energy stability in the
National Institute of Standards and Technology Microcalorimeter
X-Ray detector.” Powder Diffraction v. 20, No. 2,
June 2005
Technical
Contact: Terrence Jach, terrence.jach@nist.gov, (301) 975-2362
Media
Contact: Michael Baum, michael.baum@nist.gov, (301) 975-2763
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