Polysilicon Gate MOSFET Research Articles

A 2Kx8-bit static MOS RAM with a new memory cell structure

A 2K/spl times/8-bit static MOS RAM with a new memory cell structure has been developed. The memory cell consists of six devices including four MOSFETs and two memory load resistors. Two load resistors are fabricated in the second-level polysilicon films over the polysilicon gate MOSFET used as the driver. Thus the memory cell area is determined only by the area of four MOSFETs. By applying the new cell structure and photolithography technology of 3 /spl mu/m dimensions, the cell area of 23/spl times/27 /spl mu/m and the chip area of 3.75/spl times/4.19 mm have been realized. The RAM is nonclocked and single 5 V operation. Access time of about 150 ns is obtained at a supply current of 120 mA.

1 µm MOSFET VLSI technology: Part VIII—Radiation effects

In this paper the effect of electron-beam radiation on polysilicon-gate MOSFET's is examined. The irradiations were performed at 25 kV in a vector scan electron-beam lithography system at dosages typical of those used to expose electron-beam resists. Two types of studies are reported. In the first type, devices fabricated with optical lithography were exposed to blanket electron-beam radiation after fabrication. In the second, discrete devices from a test chip, fabricated entirely with electron-beam lithography, were used. It is shown that in addition to the threshold voltage shift, caused by the accumulation of radiation-induced positive charge in the gate oxides, these charged centers and additional uncharged (neutral) electron traps lead to an increase in the electron trapping in irradiated oxides. Temperatures above 550°C are shown to be required to anneal both the positive and neutral traps completely from the oxide underlying polysilicon after exposure to radiation. Annealing of the radiation-induced positive charge from the oxide is shown to depend on the metallurgy overlying the gate insulator during heat treatment. Annealing treatments which remove the charged centers from aluminum-gated MOS structures are demonstrated to leave small (about 5 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> ) but significant amounts of charge in certain polysilicon-gate structures. The dependence of positive and neutral trap densities on direct electron-beam exposure was studied in the range between 10 and 200 µC/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . Studies on the electron-beam fabricated devices indicate that indirect exposure of the gate oxide by electrons scattered from the primary beam during lithography in areas away from the gate oxide is sufficient to cause appreciable damage. After postmetal annealing at 400° C for 20 min, the minimum residual charge density found in the electron-beam fabricated devices is 4 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> .

1 /spl mu/m MOSFET VLSI technology. VIII. Radiation effects

For pt.VII see ibid., vol.SC14, no.2, p.291 (1979). The effect of electron-beam radiation on polysilicon-gate MOSFETs is examined. The irradiations were performed at 25 kV in a vector scan electron-beam lithography system at dosages typical of those used to expose electron-beam resists. Two types of studies are reported. In the first type, devices fabricated with optical lithography were exposed to blanket electron-beam radiation after fabrication. In the second, discrete devices from a test chip, fabricated entirely with electron-beam lithography, were used.

Radiation Hardened N-Buried Channel CCD's Using Backside Phosphorus Gettering

This paper describes the ionizing radiation effects on 4-phase n-buried channel 128 bit CCD shift registers with a polysilicon overlapping polysilicon gate structure which were fabricated with a radiation hardened process plus backside phosphorus gettering. These CCDs have the state of the art performance with the charge transfer efficiency better than 0.99995, and 5 nA/cm2 dark current density. They are operable to doses of at least 1X106 rads without changing any biasing conditions. The threshold voltage shifts are <|-1.8|V for the buried polysilicon gate MOSFETs, and <|-0.4|V for the surface polysilicon gate MOSFETs. The CTE is better than 0.9999 after 1X106 rads. The dark current increases to 110 nA/cm2 for +10V gate bias, and 55 nA/cm2 for OV gate bias.

Design of ion-implanted MOSFET's with very small physical dimensions

This paper considers the design, fabrication, and characterization of very small Mosfet switching devices suitable for digital integrated circuits, using dimensions of the order of 1 /spl mu/. Scaling relationships are presented which show how a conventional MOSFET can be reduced in size. An improved small device structure is presented that uses ion implantation, to provide shallow source and drain regions and a nonuniform substrate doping profile. One-dimensional models are used to predict the substrate doping profile and the corresponding threshold voltage versus source voltage characteristic. A two-dimensional current transport model is used to predict the relative degree of short-channel effects for different device parameter combinations. Polysilicon-gate MOSFET's with channel lengths as short as 0.5 /spl mu/ were fabricated, and the device characteristics measured and compared with predicted values. The performance improvement expected from using these very small devices in highly miniaturized integrated circuits is projected.