Optical and Interferometric Lithography - Nanotechnology Enablers

Interferometric lithography (IL), the interference of a small number of coherent optical beams, is a powerful technique for the fabrication of a wide array of samples of interest for nanoscience and nanotechnology. The techniques and limits of IL are discussed with particular attention to the smallest scales achievable. With immersion techniques, the smallest pattern size for a single exposure is a half-pitch of /spl lambda//4n where /spl lambda/ is the optical wavelength and n is the refractive index of the immersion material. Currently with a 193-nm excimer laser source and H/sub 2/O immersion, this limiting dimension is /spl sim/34 nm. With nonlinear spatial frequency multiplication techniques, this limit is extended by factors of 1/2, 1/3, etc.-extending well into the nanoscale regime. IL provides an inexpensive, large-area capability as a result of its parallelism. Multiple exposures, multiple beams, and mix-and-match with other lithographies extend the range of applicability. Imaging IL provides an approach to arbitrary structures with comparable resolution. Numerous application areas, including nanoscale epitaxial growth for semiconductor heterostructures; nanofluidics for biological separations; nanomagnetics for increased storage density; nanophotonics including distributed feedback and distributed Bragg reflectors, two- and three-dimensional photonic crystals, metamaterials, and negative refractive index materials for enhanced optical interactions are briefly reviewed.     

Wavelength-division demultiplexing using graded-index planar structures

In this paper, the authors propose a novel technique for wavelength-division demultiplexing using a graded-index planar structure. The device consists of three layers of the same bulk material: The two outer layers are homogeneous media with refractive indices n/sub 1/ and n/sub 2/, while the inner layer is an inhomogeneous medium where its refractive index is graded according to a certain profile. The proposed technique exploits the spatial shift that results from the material dispersion found in dispersive media such as silicon dioxide (silica). It is found that the graded-index structure produces a spatial shift that is much higher than that encountered in conventional prisms, provided a certain refractive index profile is chosen. Unlike graded-index fibers, it is found that the value of /spl alpha/ of the refractive index profile (/spl alpha/-profile) for the proposed device must be < 1 to get large spatial dispersion. A mathematical expression for the spatial shift between adjacent wavelengths is found by determining the path profiles followed by the different wavelengths as they propagate through the graded-index layer. Theoretically, it is found that any spatial shift can be obtained by either reducing the value of /spl alpha/ far below 1 for a fixed size of the structure or increasing the size of the structure for a fixed value of /spl alpha/.     

Fabrication of Germanosilicate glass optical fibers containing Tm/sup 2+/ ions and their nonlinear optical properties

Germanosilicate glass optical fibers incorporated with the Tm/sup 2+/ ions were fabricated to enhance optical nonlinearity by providing a strong reduction environment based on the solution doping technique in the modified chemical vapor deposition (MCVD) process. The incorporation of the Tm/sup 2+/ ions into the fiber core was identified by the electron paramagnetic resonance (EPR) spectrum in the fiber preform, and the absorption and emission properties between 350 and 1600 nm of the Tm/sup 2+/ ions in optical fibers and the fiber preform. A strong broad absorption band due to the Tm/sup 2+/ ions appeared from 350 to /spl sim/900 nm, and a broad emission from /spl sim/600 to /spl sim/1050 nm and the other emission from /spl sim/1050 to /spl sim/1300 nm, which were not shown in the Tm/sup 3+/ ions, were found upon Ar-ion laser pumping at 515 nm. Both absorption and emission results confirm that the Tm/sup 2+/ ions in the germanosilicate glass have the 4f-5d energy band from 350 to /spl sim/900 nm and the 4f-4f energy level at /spl sim/1115 nm. Also, the resonant nonlinearity at /spl sim/1310 and /spl sim/1530 nm due to the Tm/sup 2+/ ions in the fiber was measured upon the 515 nm optical pumping by using a long-period fiber grating (LPG) pair method. The nonlinear refractive index n/sub 2/ at /spl sim/1310 and /spl sim/1530 nm was found to be /spl sim/4/spl times/10/sup -15/ m/sup 2//W, where 70% and 30% of the n/sub 2/ are attributed to the nonradiative transitions and the radiative transitions, respectively.     

Investigation of a periodically segmented waveguide Fabry-Pe/spl acute/rot interferometer for use as a chemical/biosensor

We present a periodically segmented waveguide Fabry-Pe/spl acute/rot interferometer (PSW-FPI) intended to be used for tagless real-time chemical/ biological sensing through bulk-material interaction. The differential sensor detects changes in the refractive index (RI) of a sample regardless of its absolute RI value. Experiments with a series of sucrose solutions of various concentrations are compared with theoretical results, and a very good match is found between the two. The theoretical sensitivity limit for a 50-dB-signal-to-noise-ratio (SNR) measurement system is estimated as /spl delta/n=3/spl middot/10/sup -7/. For a 29-dB-SNR measurement system, a measured sensitivity limit of /spl delta/n=4/spl middot/10/sup -5/ is comparable with the previously reported sensitivities of label-free real-time optical biosensors (2/spl middot/10/sup -5/-5/spl middot/10/sup -5/). However, the total required sensing length (720 /spl mu/m) of our PSW-FPI sensor is much shorter than that of the previously reported devices (9-20 mm).     

A new integrated optical-mode stripper configuration: Numerical analysis and design

A novel integrated-optical mode stripper configuration is proposed and analyzed numerically by the BPM. Its operation is based on the coupling of all higher order modes into regions with higher refractive index adjacent to the optical waveguide. It is shown that a high transmission of the fundamental mode and a high suppression of higher order modes can be realized at /spl lambda/=1.3 /spl mu/m and /spl lambda/=1.5 /spl mu/m.     

Electron beam direct-write tunable polymeric waveguide grating filter

We report a tunable electron beam direct-write polymeric waveguide Bragg grating filter based on a negative tone epoxy, The waveguide filter, with a 5-mm-long first-order grating, exhibits a transmission peak of -27 dB and a 3-dB bandwidth of /spl sim/0.8 nm, and there is an excellent agreement between experimental data and simulation results. The temperature response of the filter is also characterized. The rate of change of refractive index dn/dT is /spl sim/ -1.8 /spl times/ 10/sup -4///spl deg/1C at 1550-nm wavelength for both transverse electric and transverse magnetic polarizations, and the rate of change of peak wavelength d/spl lambda//dT is /spl sim/ -0.14 nm//spl deg/C. The tuning performance is comparable to other grating devices fabricated using multiple processing steps.     

Dramatic reduction of optical crosstalk in deep-submicrometer CMOS imager with air gap guard ring

A dielectric structure, air gap guard ring, has been successfully developed to reduce optical crosstalk thus improving pixel sensitivity of CMOS image sensor with 0.18-/spl mu/m technology. Based on refraction index (RI) differences between dielectric films (RI = 1.4 /spl sim/ 1.6) and air gap (RI = 1), total internal reflection occurred at dielectric-film/air-gap interface, thus the incident light is concentrated in selected pixel. Excellent optical performances have been demonstrated in 3.0 /spl times/ 3.0 /spl mu/m pixel. Optical spatial crosstalk achieves 80% reduction at 20/spl deg/ incidence angle and significantly alleviates the pixel sensitivity degradation with larger angle of incident light.     

High responsivity InP-InGaAs quantum-well infrared photodetectors: characteristics and focal plane array performance

We report the detailed characteristics of long-wavelength infrared InP-In/sub 0.53/Ga/sub 0.47/As quantum-well infrared photodetectors (QWIPs) and 640/spl times/512 focal plane array (FPA) grown by molecular beam epitaxy. For reliable assessment of the detector performance, characterization was performed on test detectors of the same size and structure with the FPA pixels. Al/sub 0.27/Ga/sub 0.73/As-GaAs QWIPs with similar spectral response (/spl lambda//sub p/=/spl sim/7.8 /spl mu/m) were also fabricated and characterized for comparison. InP-InGaAs QWIPs (20-period) yielded quantum efficiency-gain product as high as 0.46 under -3-V bias with a 77-K peak detectivity above 1/spl times/10/sup 10/ cm/spl middot/Hz/sup 1/2//W. At 70 K, the detector performance is background limited with f/2 aperture up to /spl sim/ 3-V bias where the peak responsivity (2.9 A/W) is an order of magnitude higher than that of the AlGaAs-GaAs QWIP. The results show that impact ionization in similar InP-InGaAs QWIPs does not start until the average electric-field reaches /spl sim/25 kV/cm, and the detectivity remains high under moderately large bias, which yields high responsivity due to large photoconductive gain. The InP-InGaAs QWIP FPA offers reasonably low noise equivalent temperature difference (NETD) even with very short integration times (/spl tau/).70 K NETD values of the FPA with f/1.5 optics are 36 and 64 mK under bias voltages of -0.5 V (/spl tau/=11 ms) and -2 V (/spl tau/=650 /spl mu/s), respectively. The results clearly show the potential of InP-InGaAs QWIPs for thermal imaging applications requiring high responsivity and short integration times.     

定量相位显微中分辨率增强技术综述

定量相位显微（Quantitative Phase Microscopy,QPM）将相位成像和光学显微技术相结合,为微观物体的三维形貌、透明物体的厚度/折射率分布提供了一种快速、无损、高分辨率测量手段。然而,传统QPM成像系统依然是一个衍射受限系统,高分辨率与大视场难以同时兼顾。因此,如何在保持大视场的前提下提高成像空间分辨率是QPM亟需解决的问题之一。近年来,国内外学者采用离轴照明、散斑照明、结构照明、以及亚像元技术形成合成数值孔径,实现了QPM的大视场、高分辨成像。文中对以上QPM的分辨率增强技术进行了综述,并对不同方法的优缺点进行了分析。     

640 /spl times/ 512 pixel long-wavelength infrared narrowband, multiband, and broadband QWIP focal plane arrays

A 640 /spl times/ 512 pixel, long-wavelength cutoff, narrowband (/spl Delta//spl lambda///spl lambda//spl sim/10%) quantum-well infrared photodetector (QWIP) focal plane array (FPA), a four-band QWIP FPA in the 4-15 /spl mu/m spectral region, and a broadband (/spl Delta//spl lambda///spl lambda/ /spl sim/ 42%) QWIP FPA having a 15.4 /spl mu/m cutoff have been demonstrated. In this paper, we discuss the electrical and optical characterization of these FPAs, and their performance. In addition, we discuss the development of a very sensitive (NEDT /spl sim/ 10.6 mK) 640 /spl times/ 512 pixel thermal imaging camera having a 9 /spl mu/m cutoff.     

Measurement of gain, group index, group velocity dispersion, and linewidth enhancement factor of an InGaN multiple quantum-well laser diode

Gain, group index, group velocity dispersion (GVD), temperature variation of refractive index, and linewidth enhancement factor of an In/sub 0.15/Ga/sub 0.85/N/In/sub 0.02/Ga/sub 0.98/N multiple quantum-well blue laser diode was measured using the Fourier transform method as a function of wavelength from 400 to 410 nm. At the lasing wavelength (403.5 nm), the group index is 3.4, the GVD (dn/sub g//d/spl lambda/) is -37 /spl mu/m/sup -1/, the temperature variation of refractive index dn/dT is 1.3/spl times/10/sup -4/ K/sup -1/, and the linewidth enhancement factor is 5.6.     

A novel bidirectional optical coupling module for subscribers

To overcome drawbacks and limitations of planar lightwave circuit based modules for bidirectional communications, such as the demand for several chips and in consequence more packaging efforts, we have recently developed a novel optical coupling technique using our unique 155 Mbps bidirectional laser chip. Since the chip is structured with a pin-photodiode monolithically integrated on a laser diode's waveguide, the optical coupling requires only the alignment of the chip with a fiber. To optically couple the laser diode and photodiode simultaneously with a single fiber, we have designed an unusual coupling structure using a fiber having a cleaved surface whose normal is 35/spl deg/ angled to the fiber core axis, and using an index-controlling medium with a refractive index of /spl sim/1.3. The bidirectional chip is flip-chip bonded and the fiber is passively aligned using a V-groove on the same substrate of 2.5/spl times/1.3 mm/sup 2/ in size. Even with this extremely small and simple scheme for bidirectional optical coupling, we could obtain an optical output power of -7/spl sim/-10 dBm and a responsivity of <-30 dBm, which are satisfactory to the STM-1 level telecommunications specifications.     

Micro-Raman temperature measurements for electric field assessment in active AlGaN-GaN HFETs

Temperature profiles in the source/drain (S/D) opening of a single finger AlGaN-GaN heterostructure field-effect transistor were studied at increasing S/D voltages by micro-Raman spectroscopy with <1 /spl mu/m spatial resolution. These profiles imply high field regions near the gate edge of length /spl sim/0.4 /spl mu/m for S/D voltages between 45 and 75 V. Electric field strengths of /spl sim/1.2 and /spl sim/1.9 MV/cm are estimated for 45 and 75 V S/D voltage. The experimental results are in excellent agreement with 2-D Monte Carlo simulations.     

Monolithically integrated InGaAs-AlGaInAs Mach-Zehnder Interferometer optical switch using quantum-well intermixing

A highly efficient monolithically integrated Mach-Zehnder interferometer (MZI) optical switch based on a 1.5-/spl mu/m InGaAs-AlGaInAs multiple-quantum-well structure is reported. The device was fabricated by integrating phase shifter sections with bandgap-shifted low-loss waveguides obtained by a single step sputtered SiO/sub 2/-annealing quantum-well intermixing technique. Evaluation of the refractive index change induced by current injection (plasma, band-filling, band-shrinkage effects) and applied electric field (quantum confined Stark effect) based on the MZI was investigated. A device with an active length of 400 /spl mu/m in one of the MZI arms had an extinction ratio /spl ges/20 dB with a half-wavelength current (I/sub /spl pi//) and half-wavelength voltage V/sub /spl pi// as low as 3.2 mA and 3.5 V (1.9 V in push-pull configuration), respectively.     

Microfluidic integration of porous photonic crystal nanolasers for chemical sensing

We have recently developed planar photonic crystal nanolasers based on porous cavity designs. High-quality factor cavities confine light within the pores of the photonic crystal and, thus, our lasers are ideally suited for the investigation of nanoscale interactions between light and matter. We have demonstrated the operation of photonic crystal lasers within different chemical solutions, embedded them into silicone microfluidic flow channels, and were able to detect refractive index changes as small as /spl Delta/n=0.005. We predict that our porous nanolasers can detect refractive index changes as small as /spl Delta/n=8.23/spl middot/10/sup -4/.     

Trimming phase and birefringence errors in planar lightwave circuits with deep ultraviolet femtosecond laser

A deep ultraviolet femtosecond laser was employed to trim phase and birefringence errors in silica planar lightwave circuits. A permanent refractive index change of /spl sim/3.8/spl times/10/sup -4/ and a birefringence change of 1.0/spl times/10/sup -4/ were induced in hydrogen-free Mach-Zehnder planar waveguide circuits. The ultrafast laser enhances the ultraviolet photosensitivity response in silica waveguides by two orders of magnitude greater than that of a nanosecond 248 nm KrF excimer laser.     

A bandpass mismatch noise-shaping technique for /spl Sigma/-/spl Delta/ Modulators

A simple bandpass mismatch noise-shaping technique for /spl Sigma/-/spl Delta/ modulators is presented. This technique uses the data-directed scrambler structure and achieves /spl sim/20 dB improvement in signal-to-noise ratio (SNR) for an oversampling ratio (OSR) of 32. Compared to the technique of bandpass mismatch noise shaping of Lin and Schreier, here, the SNR is improved by /spl sim/ 7dB resulting in a 1-bit improvement in resolution. The selection logic is half the size of the logic in Lin and Schreier and is fully compatible with the low-pass selection logic of Kwan et al.. This compatibility allows implementation of bandpass/low-pass modulator as a single product.     

Analysis of bulk material sensing using a periodically segmented waveguide Mach-Zehnder interferometer for biosensing

Analysis of a periodically segmented Mach-Zehnder interferometer (MZI), intended for use as a biosensor, was performed using a "modular block" algorithm. The theoretical sensitivity limit increases with the number of cycles but is limited by cumulative attenuation. The periodically segmented MZI was found to exhibit better sensitivity than alternative methods under the same working conditions. However, devices with a high number of cycles (>350) were found to be impractical due to high attenuation losses. A 48-cycle periodically segmented waveguide MZI, with a predicted sensitivity limit of /spl delta/n/sub min//spl sim/3.96*10/sup -5/, requires a shorter sensing length (0.24 mm) than that of alternative devices with similar sensitivity.     

High-performance fully depleted silicon nanowire (diameter /spl les/ 5 nm) gate-all-around CMOS devices

This paper demonstrates gate-all-around (GAA) n- and p-FETs on a silicon-on-insulator with /spl les/ 5-nm-diameter laterally formed Si nanowire channel. Alternating phase shift mask lithography and self-limiting oxidation techniques were utilized to form 140- to 1000-nm-long nanowires, followed by FET fabrication. The devices exhibit excellent electrostatic control, e.g., near ideal subthreshold slope (/spl sim/ 63 mV/dec), low drain-induced barrier lowering (/spl sim/ 10 mV/V), and with I/sub ON//I/sub OFF/ ratio of /spl sim/10/sup 6/. High drive currents of /spl sim/ 1.5 and /spl sim/1.0 mA//spl mu/m were achieved for 180-nm-long nand p-FETs, respectively. It is verified that the threshold voltage of GAA FETs is independent of substrate bias due to the complete electrostatic shielding of the channel body.     

Enhanced negative substrate bias degradation in nMOSFETs with ultrathin plasma nitrided oxide

The degradation induced by substrate hot electron (SHE) injection in 0.13-/spl mu/m nMOSFETs with ultrathin (/spl sim/2.0 nm) plasma nitrided gate dielectric was studied. Compared to the conventional thermal oxide, the ultrathin nitrided gate dielectric is found to be more vulnerable to SHE stress, resulting in enhanced threshold voltage (V/sub t/) shift and transconductance (G/sub m/) reduction. The severity of the enhanced degradation increases with increasing nitrogen content in gate dielectric with prolonged nitridation time. While the SHE-induced degradation is found to be strongly related to the injected electron energy for both conventional oxide , and plasma-nitrided oxide, dramatic degradation in threshold voltage shift for nitrided oxide is found to occur at a lower substrate bias magnitude (/spl sim/-1 V), compared to thermal oxide (/spl sim/-1.5 V). This enhanced degradation by negative substrate bias in nMOSFETs with plasma-nitrided gate dielectric is attributed to a higher concentration of paramagnetic electron trap precursors introduced during plasma nitridation.