Fabrication of field-effect transistor for alleviating...

Details Fabrication of field-effect transistor for alleviating... Fabrication of field-effect transistor for alleviating...

2001-09-04

2003-07-29

Niebling, John F. (Department: 2812)

Semiconductor device manufacturing: process

Making field effect device having pair of active regions...

Having insulated gate

C438S302000, C438S279000

Reexamination Certificate

active

06599804

ABSTRACT:

FIELD OF USE
This invention relates to semiconductor technology and, in particular, to field-effect transistors (“FETs”) of the insulated-gate type. All of the insulated-gate FETs (“IGFETs”) described below are surface-channel enhancement-mode IGFETs except as otherwise indicated.
BACKGROUND
An IGFET is a semiconductor device in which a gate dielectric layer electrically insulates a gate electrode from a channel zone that extends between a source and a drain in a semiconductor body. The channel zone in an enhancement-mode IGFET is part of a body region, commonly termed the substrate or substrate region, that forms respective pn junctions with the source and drain. In an enhancement-mode IGFET, the channel zone consists of all the semiconductor body material situated between the source and drain. During operation of an enhancement-mode IGFET, charge carriers move from the source to the drain through a channel induced in the channel zone along the upper semiconductor surface. The channel length is the distance between the source and drain along the upper semiconductor surface.
Over the last forty years, the minimum value of IGFET channel length has decreased generally in the manner prescribed by Moore, “Progress in Digital Integrated Electronics,”
Tech. Dig
., 1975 Int'l Elec. Devs. Meeting, Dec. 1-3 1975, pages 11-13. Per Moore's “law”, the minimum channel length decreases roughly in proportion to a factor of 1/{square root over (2)} (approximately 0.7) every three years. IGFETs employed in state-of-the-art integrated circuits (“ICs”) manufactured at volume-production quantities today have minimum channel lengths considerably less than 1 &mgr;m, typically 0.25 &mgr;m and moving towards 0.18 &mgr;m. The minimum channel length for volume-production ICs is expected to be roughly 0.1 &mgr;m in eight to ten years.
An IGFET that behaves generally in the way prescribed by the classical model for an IGFET is often characterized as a “long-channel” device. An IGFET is described as a “short-channel” device when the channel length is shortened to such an extent that the IGFET's behavior deviates significantly from the behavior of the classical IGFET model. Both short-channel and long-channel IGFETs are variously employed in ICs. Because drive current generally increases with decreasing channel length, the great majority of IGFETs used in very large scale integration applications are laid out to have as small a channel length as can be reliably produced with the available lithographic technology.
One short-channel effect is roll-off of the threshold voltage. See (a) Yau, “A Simple Theory to Predict the Threshold Voltage of Short-Channel IGFET's”,
Solid
-
State Electronics
, October 1974, pages 1059-1069, and (b) Liu et al, “Threshold Voltage Model for Deep-Submicrometer MOSFET's”,
IEEE Trans. Elec. Devs
., Vol. 40, No. 1, January 1993, pages 86-95. The threshold voltage is the value of gate-to-source voltage at which an IGFET switches between its on and off states for given definitions of the on and off states.
FIG. 1
illustrates a typical example of how threshold voltage V
T
rolls off for a conventional n-channel enhancement-mode IGFET whose parameters, other than channel length L, are fixed. As
FIG. 1
indicates, threshold voltage V
T
has relatively little variation in the long-channel regime where channel length L is greater than transition value L
X
approximately equal to 0.4 &mgr;m here. When channel length L drops below L
X
, the IGFET enters the short-channel regime in which threshold voltage V
T
rolls off sharply to zero.
In designing IGFETs with increasingly reduced channel length, an important trade-off is between drive current and leakage current. The drive current, preferably high, is the current that flows between the source and drain when the IGFET is turned fully on. The leakage current, preferably low, is the current that flows between the source and drain when the IGFET is turned off with the gate electrode electrically shorted to the source. Decreasing the channel length typically leads to an increase in the drive current. However, the leakage current also typically increases when the channel length is reduced.
Due to the foregoing trade-off, a short-channel IGFET is typically designed so that channel length L is of a value close to where threshold voltage V
T
starts to roll off sharply to zero. An L value of 0.25 &mgr;m satisfies this requirement in FIG.
1
. The resulting V
T
value of slightly more than 0.5 V is sufficiently high to enable a 0.25 &mgr;m n-channel IGFET to switch reliably between its on and off states. However, threshold voltage V
T
for an n-channel IGFET having an L value of 0.18 &mgr;m, as occurs in the next generation of IGFETS, is approximately 0.2 V. This is too low to be able to reliably turn such a 0.18 &mgr;m IGFET off at zero gate-to-source voltage, especially in light of typical manufacturing variations.
The scaling principles developed by Dennard et al, “Design of Ion-Implanted MOSFET's with Very Small Physical Dimensions”
IEEE J. Solid
-
State Circs
., Vol. SC-9, No. 5 Octubre 1974, pages 256-268, have been utilized in downsizing IGFETs. In brief, Dennard et al specifies that IGFET dimensions are to be reduced approximately in proportion to a given scaling factor as the average net dopant concentration in the channel zone, i.e., the semiconductor body material situated between the source and drain in an enhancement-mode IGFET, is increased by the scaling factor. The voltages across various parts of the reduced-dimension IGFET are also generally to be reduced in proportion to the scaling factor.
The scaling theory of Dennard et al functions relatively well down to channel length in the vicinity of 1 &mgr;m. Unfortunately, certain scaling limitations are encountered when the channel length is reduced significantly below 1 &mgr;m. For example, electron tunneling effects preclude reducing the gate dielectric thickness to the value prescribed by the scaling theory.
Also, when the threshold voltage is to be adjusted by simply implanting the channel zone with ions of the same conductivity type as the channel zone, it is typically preferable that the threshold adjust implant be distinguishable from the vertical dopant profile in the bulk of the channel zone. In scaling an IGFET to channel length significantly less than 1 &mgr;m according to the theory of Dennard et al, the threshold adjust implant merges inseparably into the vertical dopant. profile in the bulk of the channel zone, thereby simply raising the average net dopant concentration in the channel zone by an approximately fixed amount that is largely independent of channel length. Attempting to extend the scaling theory to channel length significantly less than 1 &mgr;m does not work well.
Various techniques have been utilized to improve the performance of IGFETs, including those operating in the short-channel regime, as IGFET dimensions are reduced. One performance-improvement technique involves providing an IGFET with a two-part drain for reducing hot carrier injection.
FIG. 2
illustrates such a conventional n-channel enhancement-mode IGFET
10
created from a monocrystalline silicon semiconductor body having region
12
of lightly doped p-type body material. IGFET
10
has n-type source
14
, n-type drain
16
, intervening p-type channel zone
18
, gate electrode
20
, gate dielectric layer
22
, and gate sidewall spacers
24
and
26
. Drain
16
consists of heavily doped main portion
16
M and more lightly doped extension
16
E. Source
14
similarly consists of heavily doped main portion
14
M and more lightly doped extension
14
E. When IGFET
10
is turned on, electrons travel from source
14
to drain
16
by way of a thin channel induced in channel zone
18
along the upper semiconductor surface.
A pair of depletion regions extend respectively along the drain/body and source/body junctions. Under certain conditions, especially when the channel length is small, the drain depletion region can extend laterally to the source depletion region

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