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Controlling the Threshold Voltage of MOSFET Transistors

IP.com Disclosure Number: IPCOM000039280D
Original Publication Date: 1987-May-01
Included in the Prior Art Database: 2005-Feb-01
Document File: 2 page(s) / 29K

Publishing Venue

IBM

Related People

Calise, JA: AUTHOR [+3]

Abstract

A technique is described whereby the threshold voltage of MOSFET transistors is controlled through the use of fluorine-containing ions implanted into the gate insulator of the MOSFET transistor, so as to produce a positive flatband voltage shift. This then enables device performance to be enhanced, by providing an optimal choice of substrate doping parameters. In prior art, controlling the threshold voltage of MOSFET transistors was to adjust the surface doping density by ion implantation. However, limitations were evident particularly in scaled down devices. The surface doping density required by the threshold voltage could be sufficiently high to cause degradation of channel carrier mobility, reduction of the sub-threshold slope and increase in the body effect of the transistor.

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Controlling the Threshold Voltage of MOSFET Transistors

A technique is described whereby the threshold voltage of MOSFET transistors is controlled through the use of fluorine-containing ions implanted into the gate insulator of the MOSFET transistor, so as to produce a positive flatband voltage shift. This then enables device performance to be enhanced, by providing an optimal choice of substrate doping parameters. In prior art, controlling the threshold voltage of MOSFET transistors was to adjust the surface doping density by ion implantation. However, limitations were evident particularly in scaled down devices. The surface doping density required by the threshold voltage could be sufficiently high to cause degradation of channel carrier mobility, reduction of the sub-threshold slope and increase in the body effect of the transistor. In CMOS circuits, using only n-type polysilicon as gate material, an implant buried channel was necessary to obtain a low threshold voltage for PMOS transistors. This buried channel increased the short channel effects, resulting in poor turn-off characteristics. Although new gate materials could be used, the technique described herein utilizes the concept of implanting a charged species, that remain in charged state, into the gate insulator of the transistor, thereby causing a flatband voltage shift. It also provides the possibility of continuously tuning the threshold voltage, overcoming the limitations of the prior method. By using fluorine-containing ions, such as BF12, a positive flatband voltage shift is attainable. A typical implantation dosage of BF12 into 12.5...