We have observed coherent synthesis of spatial profiles of THz radiation emitted from a photoconductive antenna array.
The antenna array is composed of seven independently addressable photoconductive antenna units having interdigitated
electrodes. The antennas were pumped by amplified 800 nm femtosecond optical pulses. Emitted THz radiation was
focused, and the time-resolved spatial profile of the THz radiation on the focal plane was observed using a newly developed
real-time imaging apparatus, which can correct the terahertz field images for nonuniformity in birefringence of the EO
crystal. By scanning the delay time, frequency-resolved images were also obtained, which exhibit frequency-dependent
field profiles. Field profiles observed were coherent superpositions of terahertz waves emitted from the antenna units
constituting the array. By inverting the bias voltage to the central unit of the emitter array, we observed super-resolution
beam size of terahertz waves, which are smaller than the diffraction limit.
We have developed a series of block co-polymerized polyimides for KrF lithography and EB lithography. Block co-polymerization is a synthesis method to control molecular structures. We have developed solvent soluble polyimides in powdered form. We can use these polyimides for dielectric materials in devices, and also can use as a dry etching mask in semiconductor processes. Here we report the details of dry etching properties for the block co-polymerized polyimides by reactive-ion-etching (RIE) using fluoride and chloride gases, typically used for Si etching and GaAs etching, respectively. Etching rates for EB exposed polyimide, photosensitive polyimide, novolac standard photoresist and Si at the condition of 80 W RF power and 60 sccm SF6 gas flow, were measured as < 1 nm/min, 1.7 nm/min, 34 nm/min and 31 nm/min, respectively. We also checked the etching rate for EB exposed polyimide, photosensitive polyimide, novolac standard photoresist and GaAs at the condition of 50 W RF power and 3 sccm Cl2 gas flow in the inductive coupled plasma (ICP) RIE, were measured as < 1 nm/min, 50 nm/min, 120 nm/min and 500 nm/min, respectively. Dry etching selectivity of EB exposed polyimide to Si is more than 31, and dry etching selectivity to GaAs is more than 500. These values are much higher compared to novolac standard photoresist. The high resolution EB exposed polyimide is promising for nano-patterning as stable dry etching mask. RIE resistance for polyimide photoresist is 20 times more than novolac standard photoresist for SF6 etching gas, and more than twice for Cl2 etching gas.
We have developed polyimides for optical waveguide synthesized by block-copolymerization method. We demonstrated the optical waveguide with rather low refractive indices. Lower refractive indices make larger waveguide size and easier coupling to optical fibers. We applied polyimides with lower refractive indices with a fluorinated polyimide for the clad and polyimide of dedrimer structures for core. The refractive indices are precisely controlled as 0.01 by thermal conditions. This core polyimide has patterned by i-line process and formed optical waveguide.
Polyimides are attractive candidates in microelectronics due to their high thermal and chemical stability, low dielectric constants and high dry etching resistance. However, polyimides in solution form have tendency to absorb moisture, which could lead nonreproducibility in nanometer scale patterning. Since last 10 years, we have been developing a series of solvent soluble polyimides in powdered form, by using block-copolymerization process in the presence of binary catalyst. Here, we report our developed process for customized polyimide resist formulation and on-site resist preparation prior to any lithographic applications. The results of applications of on-site mixed and prepared polyimide resists for 435 nm, 365 nm, 248 nm and e-beam lithographies are summarized in this paper. The reproducibility of high resolution nanometer scale patterns by electron beam (e-beam) could observed even after five years, using the same stock of polyimide powder using the on-site mixing process. The results are given in this paper.
We have fabricated ultrafast electrical waveguides with low-k polyimide integrated with ultrafast photoconductive switches formed by nano-anodization process for the first time. Electrical signals are affected by nonlinear capacitance of p-n junctions in this waveguides, and pass through the low-k polyimide, so the dielectric loss and the radiation loss are dramatically reduced. The electrical pulses as short as 290 fs were measured on this waveguide by an electro-optic sampling system based on a femtosecond laser.
Photosensitive polyimide is expected as a future interlayer dielectric material in LSI circuits. In this paper, we propose a new interlayer dielectric process using a positive photosensitive polyimide directly synthesized from aromatic dianhydride and aliphatic diamine by block-copolymerization. Photosensitive polyimide solution was prepared with N-methyl-2-pyrrolidone (NMP) solvent. A diazonaphthoquinone PC-5 was used as a photosensitizer. The thin film was spin-coated with changing polyimide concentration and rotation speed. The uniformity of the coated film was achieved less than +/- 0.9 % on a 3-inch wafer of silicon. A 0.5 micrometers line and space pattern was obtained by i-line lithography. The (gamma) value of the contrast was evaluated to 1.05. The dielectric constant of the base polyimide was measured for a thick film by the cavity perturbation method. The values from 2.4 to 3.0 were obtained within the frequency range from 1 GHz to 20 GHz. The break down voltage was measured to be 107 kV/mm without high-temperature heat treatment.
We have developed photosensitive polyimides synthesized by block-copolymerization for KrF lithography. The polyimides were synthesized from aliphatic tetracarboxylic dianhydrides and aliphatic diamines. Aliphatic rings have been introduced to reduce absorption at 248 nm (KrF). We have obtained line patterns of 0.17 micrometer at a dose of 170 mJ/cm2, and line and space patterns of 0.25 micrometer at a dose of 190 mJ/cm2.
We have fabricated vertical and in-plane nano-structures and measured THz signals from these structures. The vertical nano-structures have been fabricated by molecular beam epitaxy in a p-i-n diode. A superlattice structure is located in an i-region of the p-i-n diode. Carriers are photo-excited in the superlattice and accelerated by a built-in potential. The radiation was measured through a free-space electro-optic sampling, and shows rather strong radiation compared to the radiation from the bulk GaAs. The in-plane nano-structures have also fabricated as a photoconductive switch by micro-anodization process. The gap for the photoconductive switch is covered with a transparent insulator which realizes high bias voltage and ultrafast response. We also introduced current block layer under photo-absorbing layer to reduce slow component of current, which are caused by carriers excited deep inside the substrate. The ultrafast response was measured by an electro-optic sampling, and the slow component of the signal has dramatically reduced by this structure.
We have proposed and realized a structure of insulator-gap photoconductive switches by using an atomic force microscope (AFM). The insulator-gap structure prevents discharge in a photoconductive gap, to realize strong electric field in photo-absorbing region. We have made photoconductive switches with a gap of 43 nm and 100 nm. We also made multiple gap structures to reduce dark current. Ultrafast response for transmission modes have been estimated by the electro-optic (EO) sampling which can measure vector components of electric field. The radiation modes from the photoconductive switches with antenna structures have been measured by a Fourier transform polarizing interferometer.