Seigo Kanemaru

Demonstration, analysis, and device design considerations for independent DG MOSFETs

This paper describes a comprehensive study on the threshold voltage (V/sub th/) controllability of four-terminal-driven double-gate (DG) MOSFETs (4T-XMOSFETs) with independently switched DGs. Two types of 4T-XMOSFETs (fin and vertical) are experimentally demonstrated and their V/sub th/ controllability is thoroughly investigated in relation to the initial V/sub th/ in the DG-mode based on comprehensible modeling of the devices. Based on the investigation and simulated predictions, device design guidelines for 4T-XMOSFETs are proposed. Decreasing the workfunction of the DGs and increasing the oxide thickness of the second gate (T/sub ox2/) are preferable for improving the performance of the 4T-XMOSFET. The optimum workfunction of DGs for attaining low I/sub off(stand-by)/ and high I/sub on(active)/ under the limited V/sub g2/ condition is also proposed.

Fabrication of double-gated Si field emitter arrays for focused electron beam generation

Double‐gated Si field emitter arrays (FEAs) capable of generating focused electron beams were fabricated and experimentally evaluated. The present field emitter array has a vertical triode structure consisting of a conical Si tip and two gate openings (upper and lower) surrounding the tip. The lower gate with a 2‐μm‐diam opening acts as an extraction electrode controlling the emission current, and the upper one with a 3‐μm‐diam opening acts as an electrostatic lens focusing the electron trajectories. The focusing property was evaluated by observing the spot size of a phosphor (ZnO:Zn) screen located about 20 mm apart from the field emitter array and biased to 1 kV. It was found from experimental results that decreasing the upper gate voltage (VF) down to a few volts was quite effective to generate focused electron beams. At VF of about 4 V, the electrons emitted from the tip were well collimated and a beam current of about 0.1 nA/tip was obtained.

Highly suppressed short-channel effects in ultrathin SOI n-MOSFETs

We have investigated short-channel effects of ultrathin (4-18-nm thick) silicon-on-insulator (SOI) n-channel MOSFETs in the 40-135 nm gate length regime. It is experimentally and systematically found that the threshold voltage (V/sub th/) roll-off and subthreshold slope (S-slope) are highly suppressed as the channel SOI thickness is reduced. The experimental 40-nm gate length, 4-nm thick ultrathin SOI n-MOSFET shows the S-slope of only 75 mV and the /spl Delta/V/sub th/ of only 0.07 V as compared to the value in the case of the long gate-length (135 nm) device. Based on these experimental results, the remarkable advantage of an ultrathin SOI channel in suppressing the short-channel effects is confirmed for future MOS devices.

Ultrathin channel vertical DG MOSFET fabricated by using ion-bombardment-retarded etching

A vertical ultrathin channel formation process for a vertical type double-gate (DG) MOSFET is proposed. Si wet etching using an alkaline solution has newly been found to be significantly retarded by introducing ion bombardment damage. We have also found that the ion-bombardment-retarded etching (IBRE) is independent of ion species and the implanted impurities can easily be transferred to be the dopants for source and drain regions of MOSFETs. By utilizing the IBRE, vertical type DG MOSFETs with a 12-nm-thick vertical channel were fabricated successfully. The fabricated vertical DG MOSFETs clearly exhibit the unique advantage of DG MOSFETs, i.e., high improvement of short-channel effect immunity by reducing the channel thickness. Thanks to the ultrathin channel, very low subthreshold slopes of 69.8 mV/dec. for a p-channel and 71.6 mV/dec for an n-channel vertical DG MOSFET are successfully achieved with the gate length of 100 nm.

Ultrastable emission from a metal–oxide–semiconductor field‐effect transistor‐structured Si emitter tip

A silicon field emitter tip with a dual‐gate metal–oxide–semiconductor field‐effect transistor (MOSFET) structure was fabricated and demonstrated. The present tip structure is just the same as an n‐channel MOSFET whose drain was replaced by a cone‐shaped Si tip. Two coplanar gates of 0.3‐μm‐thick Nb are made on a 0.6‐μm‐thick thermally oxidized SiO2 insulator between the source and the tip and make inversion layers in a p‐type Si substrate under each gate. One of the gates has a 1.8‐μm‐diam aperture surrounding the tip for extraction of electrons from the tip. The other is 3 μm wide and 300 μm long and is separated by 2 μm from this gate. Ultrastable emission of about 0.3 μA was demonstrated with a single tip for one day.

Fabrication and characterization of HfC coated Si field emitter arrays

We fabricated hafnium carbide (HfC) coated Si field emitter arrays (HfC FEAs) with an extraction-gate electrode to improve the emission characteristics of Si FEAs. Hafnium carbide thin film was deposited by inductively coupled plasma-assisted magnetron sputtering. The HfC film was characterized by x-ray photoelectron spectroscopy and x-ray diffraction measurement, and was found to be (111)-oriented polycrystalline film. The HfC FEAs exhibited superior performance. An emission of more than 10 mA could be obtained from the 16 000 tip array, which is 20 times higher than that for Si FEAs. The operational voltage for emission of 1 μA decreased from 61 to 45 V due to the HfC coating. The long-term emission characteristics were also measured. Si FEAs degraded rapidly even in an ultrahigh vacuum chamber. However, the emission degradation in the HfC FEAs was much slower. The number of active tips was counted using an electrostatic-lens projector, and the results revealed that the HfC FEAs had six times as many ti...

Fabrication and characterization of lateral field-emitter triodes

The author fabricated a field-emitter triode with tungsten electrodes arranged laterally on a quartz glass substrate by using the photolithography and dry etching techniques. The device consists of an array of 170 field-emitter tips with a 10- mu m pitch, a columnar gate, and an anode. The emission characteristics followed the Fowler-Nordheim tunneling theory. The mutual conductance was about 0.02 mu S at an anode voltage of 300 V. The authors improved the fabrication process to obtain an emitter with an operating voltage of about 100 V. >

Control of emission currents from silicon field emitter arrays using a built-in MOSFET

We have investigated the emission mechanism of p-type and n/p-type silicon field emitter arrays (FEAs) and have obtained the model for carrier flows, in which the emission current is limited by the supply of electrons both from the depletion layers near emitter tips and from the inversion layers under the gate electrodes. Based on this model, we have proposed and fabricated a new current-controllable silicon FEA incorporating a metal-oxide-semiconductor field-effect-transistor structure (MOSFET-structured silicon FEA). The fabricated device exhibits remarkable stability of emission currents, compared with the conventional n-type silicon FEA.

Fabrication of Silicon Field Emitter Arrays Integrated with Beam Focusing Lens

A new field emitter structure capable of generating a focused electron beam (FEB) was fabricated. The present structure is basically similar to a double-gated structure, however, the two gate openings are arranged in the same way as a confocal in-plane lens structure. The inner gate acts as an extraction gate with a 0.6-µm-diameter opening and the outer one acts as an electrostatic focusing lens with a 2.4-µm-diameter opening. The field emission characteristics were evaluated and a beam current of 70 nA/tip was obtained when a bias voltage of 70 V was applied to the two gates. The beam focusing characteristics were also evaluated and the beam could be focused by decreasing the focusing lens voltage without a significant decrease in the beam current.

Control of emission characteristics of silicon field emitter arrays by an ion implantation technique

Conical Si field emitter arrays with gate electrodes have been fabricated. Emitter tips were doped by using an ion implantation technique so as to control the emission characteristics. The doped emitters with n‐type surfaces showed typical field emission characteristics. However, the p‐type emitters did not work well with the Fowler–Nordheim theory and showed plateaus in the Fowler–Nordheim plots. It is speculated from the capacitance–voltage measurements and the experiment of photoenhanced emission that the emission current is limited due to the supply of electrons from depletion layers near the emitter tips and from inversion layers under the gate electrodes.