This paper presents measurement results of the 3-D contact hole profiles using RT/CD technology for various diameter-to-space (D/S) ratios and film stacks. The key controlling parameters (hole depth, diameter, sidewall angle, and hole openness, etc.) for lithography processing of contacts and vias were studied in terms of measurement sensitivity on samples with different pitches and D/S ratios and film stacks. Good correlation (R2 ~ 0.99) between CD-SEM and RT/CD was obtained for the sample structures. The static and dynamic measurement stability of contact diameter and contact depth was better than 1 nm using simple profile modeling.
KEYWORDS: Iodine, X-rays, Angiography, X-ray sources, Signal to noise ratio, Lanthanum, Dual energy imaging, Monochromators, Optical filters, Laser systems engineering
Two types of x-ray sources for dual energy subtraction angiography (DESA), laser-based and conventional, were investigated. A Tabletop Terawatt laser was used to create x-ray source with Ba, La, Nd, Gd, and Ce targets. A theoretical model of image quality was developed. A Figure of Merit, FOM equals SNR./(integral dose)1/2, was obtained. Images of an angiographic contrast detail phantom were obtained using laser-driven x-ray source in DESA regime and a standard angiography unit in DSA regime. The log-signals due to Iodine contrast agent in the images were measured and compared with the theoretical model predictions. The integral dose was estimated. We found that the La and Ba lines extracted by a monochromator are optimal for imaging Iodine contrast with laser-based DESA. In this case, SNR exhibits three- to five-fold improvement, as compared to SNR expected for a tube-based DESA system. Consequently, dose utilization, as defined by FOM, improves by factor of two to three, depending on patient thickness and scatter conditions. When only filters are used, SNR and FOM due to laser-based system are comparable to those due to tube-based DESA. In this case, preferable target/filter combination for the laser system is Ba/I and Ce/Nd for the low- and high-beam, respectively.
Experimental and theoretical studies of image quality using iodinated contrast agent and x-ray spectra generated by laser- based x-ray source were performed. A TableTop Terawatt (T3) laser (intensity: 1017 - 1019 W/cm-2, pulse duration: 150 fs or 450 fs, with or without controlled pre-pulse) was used to crate x-ray source. Infrared and/or green beams were utilized. Ba, La, Ce, Nd, and Gd laser targets were used. For each target, a number of suitable filters was utilized to produce optimized x-ray spectra for a specific imaging task. The MTF function due to the focal spot was obtained. A simple theoretical model of x-ray detector response was developed. An index of image quality (Detective Image Quality) as well as a figure of merit for dual energy imaging FOM(DESA) were defined and optimized via x-ray spectrum manipulation. The optimum, for a specific imaging task, technique parameters such as: target/filter combination, focal spot size, laser-light wavelength and surface power density, laser pulse duration, pre-pulse delay and contrast ratio, and hot electrons temperature were obtained experimentally and confirmed theoretically. We found that an optimized laser-based x-ray source can outperform conventional x-ray tube-based source in application to vascular imaging in terms of contrast resolution and spatial resolution.
This paper presents result form a systematic study of the feasibility of triggering lightning in a controlled fashion, using ultrashort pulse lasers. We show the importance of producing a plasma with local gradients of electron density by focusing the laser beam in order to trigger streamers, which are the first condition required for the initiation of large-scale spark discharges. We present evidence of the ability of laser filaments to guide streamer discharges, which are akin to the final jump phase of a lightning discharge. We also demonstrate that the leader propagation can be considerably modified by the presence of a laser- produced plasma channel, in a rod-plane electrode geometry. Finally, we have developed numerical models for the ultrashort pulse laser beam propagation through air, plasma production and streamer inception.
A laser produced plasma (LPP) x-ray source utilizing ultra- fast laser was investigated in the context of its utility for dual energy subtraction angiography. Experiments were performed with a Table Top Terawatt (TTT) laser using BaF2 and rare-earth metal targets including La, Ce, Nd and Gd. The laser was operated in a single-pulse or in a dual pulse mode with surface power density in the 1018 - 1019 Wcm-2 range with pulse duration of 150 or 450 fs. Infrared and/or green beams were utilized. Hot electrons' temperature was in the 35 - 50 keV range. The obtained LPP x- ray spectra were comprised of a continuous bremsstrahlung component as well as discrete characteristic lines. The bremsstrahlung extended to high energies with no evident cutoff energy below 100 keV. Its shape was best described by exp(-E/kTe), where Te is the hot electron temperature. The overall efficiency was equal to approximately 9 X 10-4 for 450 fs pulse and approximately 6 X 10-4 for 150 fs pulses. The x-ray focal spot size was in the range 13 to 50 microns. We have found that the LPP x- ray source with BaF2 and rare-earth targets provide x-ray spectra that might be suitable for DESA.
Contrast resolution of angiograms created using a laser-based x-ray source in Dual Energy Subtraction Angiography (DESA) regime has been investigated. It has been compared to contrast in angiograms obtained using an x-ray tube-based clinical angiography unit in DSA mode. Contrast detail phantoms and rats with opacified vascular structures were imaged. A table top terawatt laser was used (1019 Wcm-2, 150 fs or 450 fs per pulse). For Iodine contrast agent, an Iodine filter was used with the BaF2 target to obtain images with mean x-rays energy below the Iodine K-edge. La target and La filter was used to obtain images with mean x-rays energy above the Iodine K-edge. For Ba contrast agent, a Nd filter was used with the Nd target to obtain images with mean x-rays energy below the Ba K-edge. Gd target and Nd filter was used to obtain images with mean x-rays energy above the Barium K-edge. It has been determined that the laser-based DESA with properly selected targets demonstrates better contrast than a standard x-ray tube-based DSA angiography. We conclude that laser-based x-ray source has promise for angiography in DESA regime providing that sufficient x-ray flux can be delivered by the laser.
X-ray spectra suitable for mammography, created by laser-based x-ray source at laser intensity about 1018 W/cm2, where investigated. The spectra consisted of a continuous bremsstrahlung emission and discrete K(alpha ), K(beta ) lines and have been obtained for Mo, Rh, Ag, In and Sn targets (Z equals 42, 45, 47, 49, and 50) with K(alpha ) emissions at 17.4, 20.2, 22.2, 24.2, and 25.3 keV, respectively. The continuous bremsstrahlung component extended to high energies with no cut-off energy. The shape of the continuous bremsstrahlung spectrum was described by the function E-p, where p (greater than 0) was primarily determined by the temperature of hot electrons produced in laser beam- target interaction. The absolute value of x-ray yield was proportional to the atomic number of the target Z. The intensity of characteristic x-rays was about a factor of five higher than the corresponding bremsstrahlung, and showed weak dependence (increase) when the atomic number of the target increased from Z equals 42 to Z equals 50.
We present and discuss time resolved spectra (KeV range) of solid density plasmas produced by the interaction of clean high intensity laser pulses (400 fs, 0.53 micrometers light) with solid and foil targets at intensities between 1018 W/cm2 and 1020 W/cm2. Results have been obtained with the new PX1 x-ray stream camera which has a temporal resolution of 750 fs in the KeV x-ray range.
We perform pump-probe measurements in which intense ultrashort optical pulses are the pump pulses that initiate a chemical reaction and ultrafast x-ray pulses are the probe pulses that monitor the response of the system. We present experimental results on the observation of a chemical reaction process, photoinduced dissociation of gas phase SF6 molecules, detected by ultrafast x-ray absorption spectroscopy with 3 ps time resolution near the sulfur K edge at a photon energy of 2.48 keV (4.98 A). High contrast light pulses of 400 fs duration (500 mJ energy and 0.53 micrometers wavelength) from the INRS terawatt laser were focused on high atomic number targets at an intensity of 5 X 1017 W/cm2 in order to generate an x-ray continuum around the sulfur K edge. The SF6 molecule exhibits intense near shape resonances at the sulfur K and L edges, due to the multiple scattering and interference of the emitted photoelectrons by the fluorine atoms that symmetrically surround the central sulfur atom. The shape resonance of the molecule is clearly resolved in the absence of any pump pulse, and the variation of the x-ray absorption spectrum was measured as a function of the delay between the optical pump and x-ray probe pulses. As expected from theory, the reaction process is faster than can be resolved with the 3 picosecond duration x-ray pulses used in this initial experiment. This fast response can, in principle, be used to measure the duration of ultrashort x-ray pulses.
Our goal is to watch the evolution of matter on the atomic length scale and on the time scale on which elementary chemical reactions take place. We present initial experiments made in collaboration between UCSD and the INRS laboratory in Canada, on time-resolved ultrafast, 3 ps temporal resolution, near-edge x-ray absorption of gas phase SF6 at 2.4 keV (4.89 A). We can see both the initial presence of the F atoms around the S and their absence after photodissociation produced by pumping with an intense optical pulse. Simulations of ultrafast EXAFS and diffraction experiments are presented. We are constructing an ultrahigh intensity laser to generate ultrafast x-ray pulses from laser-produced plasmas. This laser is especially designed to achieve high average power, short pulse duration and high intensity to produce very high temperature solid density plasmas and ultrahot electrons for ultrafast hard x-ray production at high x-ray photon flux, which should enable us to perform a variety of ultrafast x-ray absorption and diffraction experiments. Finally, we discuss several means to measure the duration of subpicosecond x-ray pulses.
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