As the demand of the technology node shrinks from 14nm to 7nm, the reliability of tool monitoring techniques in advanced semiconductor fabs to achieve high yield and quality becomes more critical. Tool health monitoring methods involve periodic sampling of moderately processed test wafers to detect for particles, defects, and tool stability in order to ensure proper tool health. For lithography TWINSCAN scanner tools, the requirements for overlay stability and focus control are very strict. Current scanner tool health monitoring methods include running BaseLiner to ensure proper tool stability on a periodic basis. The focus measurement on YIELDSTAR by real-time or library-based reconstruction of critical dimensions (CD) and side wall angle (SWA) has been demonstrated as an accurate metrology input to the control loop. The high accuracy and repeatability of the YIELDSTAR focus measurement provides a common reference of scanner setup and user process. In order to further improve the metrology and matching performance, Diffraction Based Focus (DBF) metrology enabling accurate, fast, and non-destructive focus acquisition, has been successfully utilized for focus monitoring/control of TWINSCAN NXT immersion scanners. The optimal DBF target was determined to have minimized dose crosstalk, dynamic precision, set-get residual, and lens aberration sensitivity. By exploiting this new measurement target design, ~80% improvement in tool-to-tool matching, >16% improvement in run-to-run mean focus stability, and >32% improvement in focus uniformity have been demonstrated compared to the previous BaseLiner methodology. Matching <2.4 nm across multiple NXT immersion scanners has been achieved with the new methodology of set baseline reference. This baseline technique, with either conventional BaseLiner low numerical aperture (NA=1.20) mode or advanced illumination high NA mode (NA=1.35), has also been evaluated to have consistent performance. This enhanced methodology of focus control and monitoring on multiple illumination conditions, opens an avenue to significantly reduce Focus-Exposure Matrix (FEM) wafer exposure for new product/layer best focus (BF) setup.
With decreasing CDOF (Critical Depth Of Focus) for 20/14nm technology and beyond, focus errors are becoming increasingly critical for on-product performance. Current on product focus control techniques in high volume manufacturing are limited; It is difficult to define measurable focus error and optimize focus response on product with existing methods due to lack of credible focus measurement methodologies. Next to developments in imaging and focus control capability of scanners and general tool stability maintenance, on-product focus control improvements are also required to meet on-product imaging specifications. In this paper, we discuss focus monitoring, wafer (edge) fingerprint correction and on-product focus budget analysis through diffraction based focus (DBF) measurement methodology. Several examples will be presented showing better focus response and control on product wafers. Also, a method will be discussed for a focus interlock automation system on product for a high volume manufacturing (HVM) environment.
As leading edge lithography moves to advanced nodes, CDU requirements have relatively increased with technologies 14nm/20nm and beyond. In this paper, we want to introduce the methodology to offer an itemized CDU budget such as Intra-field, Inter-field, wafer to wafer as well as scanner contributors vs. non-scanner contributors (including detailed analysis of reticle contributors like CD, absorber thickness and SWA variation) through Top-Down CDU and Bottom-Up CDU budget breakdown and deliver sources of CD variation with measureable value so that we can estimate CDU gain from them. The test vehicle being used in this experiment is designed based on 14nm D/R basis. Measurement structures are densely located in the slit/scan direction on the reticle for the data collection plan. Hence, we can expand on this methodology to build up the tool reference fingerprint when we release new tool fleet. The final goal will be to establish a methodology for CDU budget breakdown that can be used to draw a conclusion on the root causes of the observed CDU, propose its improvement strategy and estimate the gain.
Ultrasound generation from an optical fiber, based on the photoacoustic principle, is a promising approach to many ultrasonic applications, specifically those requiring wide bandwidth and compact size in order to achieve high resolution as well as the capability of being operated in limited space. A fiber-optic ultrasound generator using gold nanopores is reported. The gold nanopores, having high absorption efficiency, were fabricated using a focused ion beam (FIB) on the fiber endface, which was excited by a nanosecond laser in order to generate ultrasound signals via the photoacoustic principle. Experimental results demonstrate that these wide bandwidth ultrasound signals can be generated by this compact fiber-optic ultrasound generator fabricated using a FIB.
Recently, many advanced ultrasound applications require wide bandwidth and compact ultrasound generators to achieve better resolution as well as the capability of being operated in a compact space. Generating ultrasound signals through photoacoustic principle is a promising way to generate wide bandwidth ultrasound signals by the optical approach. Meanwhile, optical fibers are ideal candidates for applications where compact size is required. Therefore, fiber optic photoacoustic generators, which put advantages of the photoacoustic principle and optical fibers together, lead to novel ultrasound generation devices which can meet the most advanced ultrasound applications requirements. This paper firstly reports using the gold nanocomposite to achieve the fiber optic photoacoustic ultrasound generator. The gold nanocomposite was synthesized by directly mixing the gold salt in polydimethylsiloxane. The gold nanocomposite showed high optical energy absorption capability and the high coefficient of thermal expansion. The photoacoustic generation efficiency was increased by applying such material. The synthesis protocol of the gold nanocomposite was presented in this paper. The optical fiber was coated with the gold nanocomposite to generate ultrasound signals. Experimental results have demonstrated that ultrasound signals can be generated by this approach and the fiber optic ultrasound generator can be used in ultrasound applications.
Ultrasound generation on optical fiber based on photoacoustic principle is a promising approach for many advanced
ultrasonic applications, which require wide bandwidth and compact size in order to achieve high resolution as well as the capability of being operated in limited space. This paper reports a fiber optic photoacoustic ultrasound generator using gold nanopattern. The gold nanopattern with high absorption efficiency was fabricated using focused ion beam (FIB) on the fiber endface, which was excited by a nanosecond laser to generate ultrasound signals via the photoacoustic principle. Experimental results demonstrated that ultrasound signals can be generated by this approach and the fiber optic ultrasound generator can be used in the advanced ultrasonic applications.
This paper presents a nondestructive ultrasound test method for characterizing resonant frequencies of
polydimethylsiloxane (PDMS) thin films by using a miniature fiber optic photoacoustic (PA) probe. The PA probe was
fabricated with an optical fiber and the synthesized gold nanocomposite. During the experiment, a PDMS film with
thickness of 25 μm was cured and immersed into water media within a designed holder to clamp the film. An acoustic pulse was generated from the PA probe, propagated in the water media and excited the clamped film. A fiber optic pressure sensor based on Fabry-Perot (FP) principle was applied to collect excited acoustic signals on the other side of the film. The sensed response of the acoustic pulse was used to compute the resonant frequencies of the PDMS thin film based on de-convolution method.
Recently, many studies have been exerted on developing ultrasonic transducers that can feature high frequencies for
better resolutions and compact sizes for the limit space nondestructive testing applications. Conventional ultrasonic
transducers, which are made by piezoelectric materials, suffer from issues such as low frequencies and bulky sizes due to
the difficulty of dicing piezoelectric materials into smaller pieces. On the other hand, generating ultrasonic signals by
photoacoustic principle is a promising way to generate a high frequency ultrasonic pulse. Optical fiber is a very compact
material that can carry the light energy. By combining the photoacoustic principle and the optical fiber together, a novel
ultrasonic transducer that features a high frequency and a compact size could be achieved. In this paper, an ultrasonic
transducer using gold nanoparticles as the photoacoustic generation material is described. Gold nanoparticles are
deposited on the end surface of an optical fiber acting as the ultrasonic generator. A cavity and a diaphragm are
fabricated in the center of the fiber using as the ultrasonic receiver. A phase array technique is applied to the transducer
to steer the direction of the acoustic beam. Simulation results demonstrated that the photoacoustic ultrasonic transducer
is feasible.
This paper presents a miniature fiber optic temperature sensor and its application in concrete structural health
monitoring. The temperature sensor is based on Fabry-Perot (FP) principle. The endface of the fiber was wet etched. A
piece of borosilicate glass was thermally deposited into the cavity on the etched endface to form an FP cavity.
Temperature calibration experiments were performed. A sensor with 30 μm microcavity length was demonstrated to
have a sensitivity of 0.006 nm/°C and linearity coefficient of 0.99. During the early-age of concreting, the sensor was
embedded in the concrete structure to monitor the temperature change caused by the exothermic chemical reaction
between the cement and water. The dramatically increased temperature inside the structure was directly related to its
future structural health. During the concrete hydration experiment, the measured peak temperature of concrete specimens
was 59.7 °C 12.5 hour after concrete casting.
Traumatic brain injury (TBI, also called intracranial injury) is a high potential threat to our soldiers. A helmet structural
health monitoring system can be effectively used to study the effects of ballistic/blast events on the helmet and human
skull to prevent soldiers from TBI. However, one of the biggest challenges lies in that the pressure sensor installed inside
the helmet system must be fast enough to capture the blast wave during the transient period. In this paper, an ultrafast
optical fiber sensor is presented to measure the blast signal. The sensor is based on a Fabry-Pérot (FP) interferometeric
principle. An FP cavity is built between the endface of an etched optical fiber tip and the silica thin diaphragm attached
on the end of a multimode optical fiber. The sensor is small enough to be installed in different locations of a helmet to
measure blast pressure simultaneously. Several groups of tests regarding multi-layer blast events were conducted to
evaluate the sensors' performance. The sensors were mounted in different segments of a shock tube side by side with the
reference sensors, to measure a rapidly increasing pressure. The segments of the shock tube were filled with different
media. The results demonstrated that our sensors' responses agreed well with those from the electrical reference sensors.
In addition, the home-made shock tube could provide a good resource to study the propagation of blast event in different
media.
An optical fiber biosensor featuring miniaturization, electromagnetic interference (EMI)-immunity, and flexibility is
presented. The sensor was fabricated by aligning two gold-deposited optical single-mode fiber facets inside V-grooves
on a silicon chip to form a Fabry-Perot (FP) cavity. The mirrors on the fiber facets were made of deposited gold (Au)
films, which provided a high finesse to produce a highly sensitivity. Microelectromechanical systems (MEMS)
fabrication techniques were used to precisely control the profile and angle of the V-grooves on the silicon. The biotin-terminated
thiol molecule was firstly immobilized on the gold surface. Subsequently, the molecules of Neutravidin were
specifically bound to the biotin-terminated self-assembled monolayers (SAMs). The induced changes of cavity length
and refractive index (RI) upon the gold surface lead to an optical path difference (OPD) of the FP cavity, which was
detected by demodulating the transmission spectrum phase shift. By taking advantage of MEMS techniques, multiple
biosensors can be integrated into one small silicon chip for detecting various biomolecule targets simultaneously.
Traumatic brain injury (TBI) is a great potential threat to soldiers who are exposed to explosions or athletes who receive
cranial impacts. Protecting people from TBI has recently attracted a significant amount of attention due to recent military
operations in the Middle East. Recording pressure transient data in a blast event is very critical to the understanding of the
effects of blast events on TBI. However, due to the fast change of the pressure during blast events, very few sensors have
the capability to effectively track the dynamic pressure transients. This paper reports an ultra fast, miniature and all-optical
fiber pressure sensor which could be mounted at different locations of a helmet to measure the fast changing pressure
simultaneously. The sensor is based on Fabry-Perot (FP) principle. The end face of the fiber is wet etched. A well
controlled thickness silicon dioxide diaphragm is thermal bonded on the end face to form an FP cavity. A shock tube test
was conducted at Natick Soldier Research Development and Engineering Center, where the sensors were mounted in a
shock tube side by side with a reference sensor to measure the rapidly changing pressure. The results of the test
demonstrated that the sensor developed had an improved rise time (shorter than 0.4 μs) when compared to a commercially
available reference sensor.
Traumatic brain injury (TBI) is a great potential threat to people who deal with explosive devices. Protection from TBI
has attracted more and more interest. Great efforts have been taken to the studies on the understanding of the
propagation of the blast events and its effect on TBI. However, one of the biggest challenges is that the current available
pressure sensors are not fast enough to capture the blast wave especially the transient period. This paper reports an
ultrafast pressure sensor that could be very useful for analysis of the fast changing blast signal. The sensor is based on
Fabry-Perot (FP) principle. It uses a 45º angle polished fiber sitting in a V-groove on a silicon chip. The endface of the
angle polished fiber and the diaphragm which is lifted off on the side wall of the V-groove form the FP cavity. The
sensor is very small and can be mounted on different locations of a helmet to measure blast pressure simultaneously. The
tests were conducted at Natick Soldier Research, Development, and Engineering Center (NSRDEC) in Natick, MA. The
sensors were mounted in a shock tube, side by side with the reference sensors, to measure a rapidly increased pressure.
The results demonstrated that our sensors' responses agreed well with those from the electrical reference sensors and
their response time is comparable.
The low energy-conversion efficiency in photoacoustic generation is the most critical hurdle preventing its wide
applications. In recent studies, it was found that the selection of the energy-absorbing layer material and design of the
acoustic generator structure both determine the photoacoustic conversion efficiency. The selection of the absorbing
material is based on its optical, thermal, and mechanical properties. In this research, we calculated and compared the
conversion efficiencies of six different absorbing film materials: bulk aluminum, bulk gold, graphite foil, graphite
powder-resin mixture, gold nanospheres, and gold nanorods. The calculations were carried out by a finite element
modeling (FEM) software, COMSOL Multiphysics. A 2D-axisymmetric model in COMSOL was built up to simulate a
3-layer structure: optical fiber tip, light absorbing film, and surrounding water. Three equations governed the thermo-elastic
generation of ultrasonic waves: the heat conduction, thermal expansion and acoustic wave equations. In "thick-film"
generation regime, majority of the laser energy is absorbed by the film and converted to high-frequency film
vibration, and the vibration excites the ultrasound wave in the adjacent water, while the water would not be heated
directly by the laser. From the results of this FEM simulation, the acoustic signal generated by gold nanosphere (or
nanorod) film is over two times stronger than that generated by graphite powder-resin film of the same thickness. This
simulation provides a strong support to the absorbing material selection for our proposed fiber ultrasound generator.
A 2D-axisymmetric finite element analysis (FEA) model was built to simulate an optical fiber-based photoacoustic
generator. A layer of absorbing film deposited on the tip of an optical fiber converts the pulsed-laser energy into
vibrations, which excite broadband ultrasound waves in the adjacent fluid. Instead of 1D or simplified 2D theoretical
solution, this multi-physics FEA model successfully calculates the ultrasound generated by film vibration and fluid
heating. Another advantage of this numerical calculation is that the dimensions of fiber and film can be optimized to
achieve high photoacoustic conversion efficiency. Two major conclusions were obtained from the simulation: 1) Thicker
absorbing film has higher photoacoustic conversion efficiency than thinner film; 2) Shorter laser pulse leads to higher
conversion efficiency and higher ultrasound central-frequency. The FEA results provide a practical support to the design
of this type of optical fiber photoacoustic generator, and make it possible to have a miniaturized non-destructive testing
transducer in the intravascular and intraluminal applications.
The adaptive neural network is a standard technique used in nonlinear system estimation and learning applications for
dynamic models. In this paper, we introduced an adaptive sensor fusion algorithm for a helmet structure health
monitoring system. The helmet structure health monitoring system is used to study the effects of ballistic/blast events on
the helmet and human skull. Installed inside the helmet system, there is an optical fiber pressure sensors array. After
implementing the adaptive estimation algorithm into helmet system, a dynamic model for the sensor array has been
developed. The dynamic response characteristics of the sensor network are estimated from the pressure data by applying
an adaptive control algorithm using artificial neural network. With the estimated parameters and position data from the
dynamic model, the pressure distribution of the whole helmet can be calculated following the Bazier Surface
interpolation method. The distribution pattern inside the helmet will be very helpful for improving helmet design to
provide better protection to soldiers from head injuries.
A tapered optical fiber sensor (TOFS) is a kind of optical fiber sensor that uses special geometries to measure properties
of surrounding environments or samples using evanescent waves. This paper presents a fast, highly sensitive, and
inexpensive tapered optical fiber biosensor that, using a miniature sensing structure, enables the label-free direct
detection of biomolecules. The sensor takes advantage of the interference effect between the fiber's first two modes
along the taper waist region. This effect causes some interference fringes in the transmission spectrum. Because of its
sharp spectrum fringe signals and its long biomolecule testing region, the sensor is fast and highly sensitive. To better
understand the influence of various biomolecules on the sensor, a computer simulation that varied such bio-layer
parameters as thickness and refractive index was performed. The result of 0.4 nm/nm showed that the spectrum fringe
shift of the sensor was large enough to be easily measured even when the bio-layer was nanometers thick. A tapered
optical fiber biosensor was then fabricated and evaluated with an immune globulin G (IgG) antibody-antigen pair, and
showed good performance.
A novel ultrasound generator-receiver built on a single-mode optical fiber using a layer of gold nanoparticles has been
designed. The generator takes advantage of the optical and photo-acoustic properties of gold nanoparticles. Thermal and
pressure waves are generated in the nano-particle layer when it is exposed to high intensity, short duration laser
radiation. The laser radiation is applied in an intensity range that creates an instantaneous surface heating of the layer
material that, in turn, drives a pressure wave into the layer. The pressure wave interacts with the layer-substrate interface
to create stress distributions of varying strengths and qualities, depending on the intensity and duration of the initial laser
pulse. The radiation due to laser-induced heating on the nano-particles was investigated using FEA analyses. The
maximum principal stress distribution was investigated by the FEA. Results indicate that the ultrasound generation
elements have almost zero effect on the diaphragm.
A novel ultrasound generator consisting of a single mode optical fiber with a layer of gold nanoparticles on its tip has
been designed. The generator utilizes the optical and photo-acoustic properties of gold nanoparticles. When heated by
laser pulses, a thin absorption layer made up of these nanoparticles at the cleaved surface of a single mode fiber
generates a mechanical shock wave caused by thermal expansion. Mie's theory was applied in a MATLAB simulation to
determine the relationship between the absorption efficiency and the optical resonance wavelengths of a layer of gold
nanospheres. Results showed that the absorption efficiency and related resonance wavelengths of gold nanospheres
varied based on the size of the gold nanosphere particles. In order to obtain the bandwidths associated with ultrasound,
another MATLAB simulation was run to study the relationship between the power of the laser being used, the size of the
gold nanosphere, and the energy decay time. The results of this and the previous simulation showed that the energy
decay time is picoseconds in length.
A novel Fabry-Perot interferometer pressure/acoustic sensor has been designed, fabricated, and tested. The sensor
consists of an angle-polished fiber, a V-shaped groove on a silicon substrate, and a silicon nitride diaphragm on the side
wall of the groove. The design uses MEMS technology to ensure precise cavity length control and diaphragm design
flexibility. Two shock wave tests were performed on the sensors: one where a balloon was popped near the sensors, and
another that used a shock tube to simulate a blast event. Multi-sensor assemblies, where all the sensors were calibrated to
have similar center wavelengths, were also put together. The assemblies were tested simultaneously using a single laser
source. The results of all these tests showed that the performance of the Fabry-Perot sensors closely matched that of the
reference sensors used.
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