The generation of a wide-range THz wave is investigated from a photoconductive antenna excited using a chaotic oscillation multimode semiconductor laser with optical delayed feedback by an external mirror. The stable THz wave is obtained from the multimode-laser diode excited photoconductive antenna by using a laser chaos. For a high sensitive detection, a metal V-grooved waveguide (MVG) is also used. About 10 times high amplitude signal is obtained using laser chaos. The signal is also increased about 1.6 time using MVG compared to Si lens. As the MVG gap is narrower from 200 to 20[μｍ], the detected signal is increased about twice.
Liquid crystals (LCs) have very attractive properties that the response can be controlled by the external stimulus such as electric field and temperature. Terahertz (THz) devices based on LC have wide applications. But the development of high-performance LC THz functional devices is still in its infancy. We firstly develop a large birefringence LC material in THz range. Then broadband tunable transmissive driven with porous graphene and reflective THz waveplates based on this LC are proposed. Furthermore, graphene-assisted high efficiency tunable THz metamaterial absorber is demonstrated. A magnetically and electrically polarization-tunable THz emitter that integrates a ferromagnetic heterostructure and the large-birefringence LC is also demonstrated. Last but not least, we use a temperaturesupersensitive cholesteric liquid crystal (CLC) to not only measure the beam profiles but also detect the powers of THz waves generated from a nonlinear crystal pumped by a table-top laser, which is visible, cost-effective, portable and robust at room temperature. These LC based THz devices can be used in applications such as THz imaging, biological sensing, and inspection.
The terahertz emissions from cobalt ferromagnetic heterostructures are demonstrated. The performances of heterostructures Fe/Au and Co/Pt have been compared. The amplitude of the terahertz emission sensitively depends on the non-magnetic materials. The terahertz emission is experimentally proved to be related to the spin currents by checking the terahertz emission using reversed magnetization and sample orientation. We further studied the Pt-layerthickness dependence of the amplitude of terahertz emission. A theoretical model is developed based on a comprehensive analysis of the spin transport. The efficiency of the spin injection from the ferromagnetic to nonmagnetic layer is evaluated by taking into account the interfacial spin loss. This work reveals that interfacial spin loss needs to be carefully addressed for the design of efficient terahertz emitter based on the ferromagnetic/nonmagnetic heterostructure.
Terahertz pulse generation from metallic nanostructures irradiated by femtosecond laser pulses is of interest because the conversion efficiency from laser pulses to terahertz waves is increased by the local field enhancement resulting from the plasmon oscillation. In this talk we present our recent study on terahertz generation from metal nanoparticle ink. We baked a silver nanoparticle ink spin-coated onto a glass coverslip in various temperatures. On the surface of the baked ink, bumpy nanostructures are spontaneously formed, and the average size of bumps depends on the baking temperature. These structures are expected to lead to local field enhancement and then large nonlinear polarizations on the surface. The baked ink was irradiated by the output of regeneratively amplified Ti:sapphire femtosecond laser at an incidence angle of 45°. Waveforms of generated terahertz pulses are detected by electro-optical sampling. The generation efficiency was high when the average diameter of bumps was around 100 nm, which is realized when the ink is baked in 205 to 235°C in our setup. One of our next research targets is terahertz wave generation from micro-patterned metallic nanoparticle ink. It is an advantage of the metal nanoparticle ink that by using inkjet printers one can fabricate various patterns with micrometer scales, in which terahertz waves have a resonance. Combination of microstructures made by a printer and nanostructure spontaneously formed in the baking process will provide us terahertz emitters with unique frequency characteristics.
LiteBIRD is a next generation satellite aiming for the detection of the Cosmic Microwave Background (CMB) B-mode polarization imprinted by the primordial gravitational waves generated in the era of the inflationary universe. The science goal of LiteBIRD is to measure the tensor-to-scaler ratio r with a precision of δr < 10<sup>-3</sup>♦, oﬀering us a crucial test of the major large-single-field slow-roll inflation models. LiteBIRD is planned to conduct an all sky survey at the sun-earth second Lagrange point (L2) with an angular resolution of about 0.5 degrees to cover the multipole moment range of 2 ≤ ℓ ≤ 200. We use focal plane detector arrays consisting of 2276 superconducting detectors to measure the frequency range from 40 to 400 GHz with the sensitivity of
3.2 μK·arcmin. including the ongoing studies.
We investigated the ultrafast terahertz response to the photoexcitation for vanadium dioxide single
crystals and thin films using the optical-pump terahertz-probe technique at room temperature. The optical
excitation at 800 nm induced an ultrafast decrease of the transmittance of the terahertz pulse within 0.7 ps,
and then the transmittance decreases gradually up to 100 ps. The decrease of the transmittance is assigned
to the appearance of the high electric conductivity due to metallic state. The conductivity increases more
than ten times in the picoseconds time range after photoexcitation and it is concluded that the metallic
electronic states appear. The rapid and gradual changes of the electric conductivity are very similar to the
previous reports of the time resolved X-ray and electron diffractions. This fact indicates that the increase
of the electric conductivity and the change of the lattice structure proceed in parallel. It is suggested that
the photo-induced insulator-metal phase transition is of the Peierls type.
In order to achieve miniaturization of the device, and still following device design rules, the photo-resist film thickness has decreased. The thinner photo-resist thickness will improve the resolution limit and prevent the pattern collapse issue. In order to solve these problems a multilayer process is used that has several advantages over previous process designs: reflectivity control in hyper-NA lithography process, decreasing LWR, and the viewpoint of lithographic process margin. The multilayer process consists of three layers: layer one is patterned photo-resist, the second layer is Si-ARC (Si contented Anti Reflective Coatings), and the third layer is SOC (Spin on Carbon) also known as underlayer. There are two processes to deposit Si-ARC and SOC, the first is by spin coating with either a track or spin coater, the second is with a Chemical Vapor Deposition (CVD). From a cost of ownership standpoint the spin on process is better. In the development of spin on Si-ARC and SOC materials it is important to consider the resist profile and the shelf life stabilities. Another important attribute to consider is the etching characteristics of the material. For the Si-ARC the main attribute when determining etch rate is the Si content and for the SOC material the main attribute is the C content in the material. One problem with the spin on multilayer process is resist profile and this paper will examine this problem along with the characteristics of developed material is described.
The pattern shrinkage of semiconductor devices has been achieved by moving to shorter and shorter wavelengths in the
optical lithography technologies. According to the ITRS, it is estimated that this trend will be continued through
advanced lithography techniques such as Hyper NA immersion lithography, double patterning technique and EUV
In the future, photo-resist film thickness requirements will approach 100 nm or less to achieve suitable aspect ratios.
Therefore, organic bottom anti-reflective coating (BARC) film thicknesses must also be reduced from the viewpoint of
the etching process. Due to these design changes, the performance of BARCs, especially photo-resist profile control and
maintaining enough of a lithography process margin at the critical CD has become more crucial. Problem of photo-resist
profiles, such as missing holes or scumming for contact holes (C/H) and footing in line-space (L/S) patterns by
contamination from the substrate are known as resist poisoning. In order to prevent this issue, BARC films need to have
not only reflection control properties but they also need to capable of contamination or poison blocking. Therefore,
barrier properties to prevent contamination or poisoning should be included in the design of these new BARC materials.
For developing these BARC that are designed to have both barrier properties and reflection control at around 30 nm
thickness, we investigated their performance by evaluating both the chemical and physical property of BARC film. The
design of these barrier films and details of evaluation experiments are discussed in this paper.
Organic Bottom Anti-Reflective Coatings (BARCs) has been used in the lithography process. BARCs may play an important
role to control reflections and improve swing ratios, CD variations, reflective notching, and standing waves.
In 32-45nm node, application of the immersion lithography technique is not avoided to obtain the high resolution. To obtain
the high resolution, numerical aperture (NA) of the optical system needs the Hyper-NA lens of 1.0 or more but come up to the
problem of affections the polarized light in the Hyper-NA lens. The substrate of reflection control also will become more
difficult by using single BARCs system and the thin film resist becomes the necessity and indispensable at Hyper-NA
lithography. To achieve an appropriate reflection control, to suppress the CD difference to the minimum, and to prevent the
pattern collapse, hard mask with the spin coating film and antireflection characteristic is needed. In order to solve these issues,
we designed and developed new materials with the suitable optical parameter, square resist shape and large dry etching
selectivity. These Multi-layer materials of each process are spin-coated by using the current system and conventional ArF
photo resist or immersion resist is available in this process. This paper presents the detail of our newest materials for Hyper NA
Integrated circuit manufacturers are consistently seeking to minimize device feature dimensions in order to reduce chip size and increase integration level. Feature sizes on chips are achieved sub 65nm with the advanced 193nm microlithography process. R&D activities of 45nm process have been started so far, and 193nm lithography is used for this technology. The key parameters for this lithography process are NA of exposure tool, resolution capability of resist, and reflectivity control with bottom anti-reflective coating (BARC). In the point of etching process, single-layer resist process can't be applied because resist thickness is too thin for getting suitable aspect ratio. Therefore, it is necessary to design novel BARC system and develop hard mask materials having high etching selectivity. This system and these materials can be used for 45nm generation lithography. Nissan Chemical Industries, Ltd. and Brewer Science, Inc. have been designed and developed the advanced BARCs for the above propose. In order to satisfy our target, we have developed novel BARC and hard mask materials. We investigated the multi-layer resist process stacked 4 layers (resist / thin BARC / silicon-contained BARC (Si-ARC) / spin on carbon hard mask (SOC)) (4 layers process). 4 layers process showed the excellent lithographic performance and pattern transfer performance. In this paper, we will discuss the detail of our approach and materials for 4 layers process.
We investigated the radiation mechanisms of THz radiation from semiconductor surfaces at high-density excitation under a magnetic field. Excitation density dependences of radiation intensity and the waveforms of the terahertz radiations from InAs and semi-insulating InP surfaces were investigated with and without magnetic fields (0, 2T, and - 2T). Substantial changes of the intensity and the waveforms including a polarity reversal were observed by changing the excitation densities. In InAs, the enhancement of the radiated energy is observed under a magnetic field of ±2 T and the radiated energy increases quadratically with increasing the excitation density below 0.1 μJ/cm<sup>2</sup>. The behavior of the dependence for ±2 T changes clearly above 1 μJ/cm<sup>2</sup>. The drastic change of the wave forms was observed at high density excitation and was explained by the polarity reversal of the THz wave induced by the magnetic field. The reversal originates from the crossover of the radiation mechanism of the magnetic induced component from the electrons in the accumulation layer to the diffusion current by the photogenerated electrons at high-density excitation under a magnetic field. In InP, the characteristic behavior including the polarity reversal of the angle independent component was observed in the crystal orientation angle dependence by changing the excitation density. These facts indicate that three different radiation mechanisms co-exist and that the dominant radiation mechanism changes with increasing the excitation density from the drift current for low-excitation density to the diffusion current and the optical rectification for high-excitation density.