Masaki Yanagisawa

Single-stripe tunable laser with Chirped Sampled Gratings fabricated by nanoimprint lithography

Fabrication of diffraction gratings of Chirped Sampled Grating Distributed Reflector (CSG-DR) laser by nanoimprint lithography (NIL) has been demonstrated. The diffraction gratings with highly uniform linewidth and period have been successfully fabricated by the combination of the reverse-tone NIL and precise etching techniques. The fabricated CSG-DR laser using NIL shows sufficiently wide tuning range of 40 nm as we designed.

High-performance GaAs MESFETs with advanced LDD structure for digital, analog, and microwave applications

GaAs MESFETs with advanced LDD structure have been developed by using a single resist-layered dummy gate (SRD) process. The advanced LDD structure suppresses the short channel effects, and reduces source resistance, while maintaining a moderate breakdown voltage. The 0.3-/spl mu/m enhancement-mode devices exhibit a transconductance of 420 mS/mm, while the breakdown voltage of the depletion-mode device (V/sub th/=-500 mV) is larger than 6 V. The standard deviation of the threshold voltage for 0.3-/spl mu/m devices is less than 30 mV across a 3-in wafer. The 0.3-/spl mu/m devices exhibit an average cutoff frequency of 47.2 GHz with a standard deviation of 1.3 GHz across a 3-in wafer. The cutoff frequency of a 0.15-/spl mu/m device is as high as 72 GHz. D-type flip-flop circuits for digital IC applications and preamplifier for analog IC applications fabricated with 0.3-/spl mu/m gate length devices operate above 10 Gb/s. In addition, the 0.3-/spl mu/m devices also show good noise performance with a noise figure of 1.1 dB with associated gain of 6.5 dB at 18 GHz. These results demonstrate that GaAs MESFETs with an advanced LDD structure are quite suitable for digital, analog, microwave, and hybrid IC applications.

A robust all-wet-etching process for mesa formation of InGaAs-InP HBT featuring high uniformity and high reproducibility

A simple InP-based heterojunction bipolar transistors (HBT) fabrication process featuring high uniformity and high reproducibility is introduced. No dry etching method was utilized for triple-mesa formation to avoid plasma damage to the device surface. An all-wet etching method was specially developed. This process is relatively simple compared to the conventional HBT fabrication process with respect to the emitter mesa formation by one-step selective etching. Uniformity of the current gain over a 3-in diameter wafer was approximately 2.9%, and the variation of the current gain of 17 wafers was 2.9 (max.-min.). The current gain cutoff frequency and the maximum oscillation frequency were 145 and 174 GHz, respectively. The mean time to failure was over 5 /spl times/ 10/sup 6/ h at 150/spl deg/ C whose criterion was over 3% changes in the current gain. This process is suitable for mass production of ultrahigh speed ICs in high yield.

Application of Nanoimprint Lithography to Fabrication of Distributed Feedback Laser Diodes

We have succeeded in employing nanoimprint lithography (NIL) to form the diffraction gratings of distributed feedback laser diodes (DFB LDs) used in optical communication. Uniform gratings and phase-shifted gratings with periods of 232 nm have been formed by using a reversal-tone NIL with a step-and-repeat imprint tool. Line edge roughness has been sufficiently low with the fabricated gratings. DFB LDs fabricated by NIL have indicated comparable characteristics with LDs fabricated by electron beam lithography. We have also demonstrated that phase-shifted DFB LDs show better uniformity in characteristics than uniform-grating DFB LDs. The results of this study indicate that NIL has high potential for the fabrication of DFB LDs.

On the frequency dependent drain conductance of ion-implanted GaAs MESFETs

The effect of MESFET structure on the frequency dispersion of drain conductance (g/sub d/) was examined, It was found that a shorter gate length, lower buried p-layer concentration, lower sheet resistance of n/sup +/ layer, and thinner active layer thickness are effective in suppressing the frequency dependent g/sub d/. These phenomena are explained by the presence of deep traps in the depletion layer between the semi-insulating substate and active layer. We also show that the cross-point change of eye-pattern for density of input signal in logic ICs is due to frequency dependent g/sub d/ The cross-point change between mark ratio of 1/8 and 7/8 shows a linear relationship with gd/sub RF//gd/sub dc/ (the ratio of the drain conductance at RF and dc input), These results indicate that an optimized device structure with g/sub d/ small frequency dispersion can be used to realize high-speed and high quality logic ICs.

Study of reactive ion etching for reverse tone nanoimprint process

We have used reverse nanoimprint for fabricating diffraction gratings of distributed feedback laser diodes. Generation of residues in the etching process of resin is a serious issue leading to poor line edge roughness of the grating patterns. We have found that the residues are composed of oxide products from Si-containing resin. We have successfully suppressed the generation of the residues by optimizing oxygen partial pressure of reactive ion etching (RIE). We have also succeeded in effectively removing the residues by utilizing sputtering effect of RIE.

High-Speed and Uniform Self-Aligned InGaAs/InP HBTs for 40 Gb/s Fiber Optic Communications Applications

We have developed the InGaAs/InP heterojunction bipolar transistor (HBT) fabrication process technology, which is capable of producing multiplexer/demultiplexer-level ICs in high yield. A simple self-aligned process utilizing an overhung profile of an InGaAs emitter contact mesa enables narrow separation between the emitter contact mesa and base electrodes. With this fabrication process, we have obtained very high uniformity of HBT characteristics. The uniformity was evaluated by measuring HBT arrays consisting of two-dimensionally arranged 26×57 (1,482) HBTs. The average DC current gain at a collector current density of 1.0×105 A/cm2 is 37.8 and the standard deviation is only 4.7%. A current gain cut-off frequency, fT, of 190 GHz and a maximum oscillation frequency, fmax, of 280 GHz at a collector current, Ic, of 17 mA were achieved with a 0.8×5.8 µm2 emitter HBT. The uniformity of RF properties was also excellent. The average fT and fmax of the HBTs with a 0.8×5.8 µm2 emitter contact mesa at Ic of 15 mA were 190 GHz and 256 GHz, and the standard deviations were 1.6% and 2.3%, respectively.

Evaluation of nanoimprint lithography as a fabrication process of phase-shifted diffraction gratings of distributed feedback laser diodes

The authors have succeeded in employing nanoimprint lithography (NIL) to form diffraction gratings of distributed feedback laser diodes (DFB LDs) used in optical communication. Uniform gratings and phase-shifted gratings with periods of 232 nm have been formed by using a reversal NIL with a step-and-repeat imprint tool. Line edge roughness has been sufficiently low with the fabricated gratings. DFB LDs fabricated by NIL have indicated comparable characteristics with LDs fabricated by electron beam lithography. LDs show high long-term stability in threshold current. The authors have also demonstrated that phase-shifted DFB LDs show better uniformity in characteristics than uniform-grating DFB LDs. The results of this study indicate that NIL has high potential for fabricating DFB LDs.

Evaluation of nanoimprint lithography as a fabrication method of distributed feedback laser diodes

We have succeeded in employing nanoimprint lithography (NIL) to form the diffraction gratings of distributed feedback laser diodes (DFB LDs) used in optical communication. Uniform gratings and phase-shifted gratings with periods of 232 nm have been formed by using a reverse NIL with a step-and-repeat imprint tool. Line edge roughness has been sufficiently low with the fabricated gratings. DFB LDs fabricated by NIL have indicated comparable characteristics with LDs fabricated by electron beam lithography. We have also demonstrated that phase-shifted DFB LDs show better uniformity in characteristics than uniform-grating DFB LDs. The results of this study indicate that NIL has high potential for the fabrication of DFB LDs.

Highly Uniform Fabrication of Diffraction Gratings for Distributed Feedback Laser Diodes by Nanoimprint Lithography

We have used a nanoimprint technique to fabricate diffraction gratings of distributed feedback laser diodes (DFB LDs) used in optical communication. We have aimed to establish the fabrication process featuring the high reproducibility of the period and linewidth of grating corrugations, which leads to an increase in the production yield of DFB LDs. The combination of the reverse tone nanoimprint and optimized etching techniques has contributed to the improvement of the reproducibility. The variation in grating period has been less than 0.2 nm and the variation in linewidth has been less than 10 nm over the six wafers. The results of this study indicate that our fabrication process for the diffraction gratings utilizing the nanoimprint technique has a high potential for the fabrication of DFB LDs.