Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Having insulated electrode
Reexamination Certificate
2001-02-13
2003-12-09
Prenty, Mark V. (Department: 2822)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Having insulated electrode
C257S347000, C257S510000
Reexamination Certificate
active
06661065
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to semiconductor devices and SOI substrates, and particularly to a semiconductor device and an SOI substrate having improved insulating film and improved buried insulating film forming semiconductor elements.
2. Description of the Background Art
With miniaturization of MOSFETs (Metal Oxide Semiconductor Field Effect Transistors), attempts are being made to reduce the film thickness of gate insulating films for the purposes of improving the current driving capability and alleviating the roll-off of the threshold voltage (the amount of variation of the threshold voltage caused as the gate length and gate width vary).
There are two reasons behind this:
(1) Improving the current driving capability speeds up operation of the circuitry, thus increasing the operating frequency of the semiconductor chip.
(2) Alleviating the threshold voltage roll-off reduces variations of transistors' threshold voltages caused as the gate length and gate width vary during the process of transfer etc., thus facilitating the mass production.
Thinning a gate insulating film made of silicon oxide (SiO
2
) to a thickness of 3 nm or less causes serious gate leakage current which is due to direct tunneling from the silicon substrate to the gate electrode. Therefore the film thickness of about 3 nm is the limit of the silicon oxide gate insulating film. However, there are demands for gate insulating films having film thicknesses of 3 nm or less, calculated in terms of silicon oxide film (referred to as reduced film thickness thereinafter), in order to improve the current driving capability.
Further, when a silicon oxide gate insulating film is formed in contact with a polysilicon film which heavily contains boron (which is used as a gate electrode in a surface-channel P-type MOSFET), boron in the polysilicon film thermally diffuses into the gate insulating film during thermal processing and reaches the channel to cause the threshold voltage to vary.
As a method for solving this problem, a MOSFET
90
as shown in
FIG. 43
is used in the generation of gate length of 0.12 &mgr;m or less, for example.
In
FIG. 43
, the MOSFET
90
has a gate insulating film composed of a two-layer film of a silicon oxide film
11
and a silicon nitride film
12
formed in order on the silicon substrate
1
, and a gate electrode composed of a three-layer film of a doped polysilicon film
13
, a barrier metal layer
14
(WNx, TiNx, Ta, TaN, etc.) and a metal film
15
formed in order on the silicon nitride film
12
. The gate insulating film formed of a silicon oxide film and a silicon nitride film is referred to as ON (Oxide-Nitride) film hereinafter.
The MOSFET
90
has a coating insulating film
16
covering the gate insulating film and gate electrode, a sidewall insulating film
17
covering at least the sides of the coating insulating film
16
, a channel layer
7
provided in the surface of the silicon substrate
1
under the gate electrode, a pair of extension layers
6
facing each other through the channel layer
7
, pocket layers
5
provided in the pair of extension layers
6
, and a pair of main source/drain layers
4
adjacent to the pair of extension layers
6
. Although the extension layers
6
should be referred to as source/drain extension layers
6
since they have the same conductivity type as the main source/drain layers
4
and function as source/drain layers, they are called extension layers
6
for convenience.
The active region of the MOSFET
90
is defined by an STI (Shallow Trench Isolation) film
3
, a kind of element isolation insulating film. A channel stopper layer
2
is provided in the silicon substrate
1
and a first interlayer insulating film
21
, an insulating film
22
, a second interlayer insulating film
23
and a third interlayer insulating film
24
are deposited over the MOSFET
90
.
FIG. 43
shows a structure including contacts
31
passing through the first interlayer insulating film
21
and the insulating film
22
to reach the pair of main source/drain layers
4
, a first interconnection layer
32
connected to one of the contacts
31
, a contact
33
passing through the second interlayer insulating film
23
to reach the other contact
31
, and a second interconnection layer
34
connected to the contact
33
. This structure is just an example and other structures are also possible.
FIG. 44
shows, for reference, dopants used in individual layers of MOSFETs.
FIG. 44
classifies N-type MOSFET and P-type MOSFET each into surface channel type and buried channel type and lists dopants which can be used in the channel layer, channel stopper layer, main source/drain layers, extension layers, pocket layers and doped polysilicon layer.
Next, advantages of the above-described ON film are described. The ON film has the following two advantages:
(1) Under the condition that the gate current due to direct tunneling hardly flows, the reduced film thickness can be made thinner than 3 nm.
(2) It is free from variation of the threshold voltage caused by thermal diffusion of dopant in polysilicon: the dopant in polysilicon does not thermally diffuse in the gate insulating film to reach the channel since the dopant diffusion coefficient in the silicon nitride is much smaller than that in silicon oxide.
While attempts have been made to form a silicon nitride film as the gate insulating film on the silicon substrate, this strategy has not been put in practice since the interface state density increases at the silicon nitride/silicon substrate interface. When the interface state density increases, the mobility and effective carrier density decrease as carriers moving in MOSFET repeat trap/de-trap, which reduces the drain current. This in turn reduces the operating speed of the semiconductor integrated circuit formed of the MOSFETs.
While the ON film has many advantages as described above, it has some problem with hot carrier resistance.
FIGS. 45
to
47
are schematic diagrams used to explain the mechanism of hot-carrier-induced deterioration of an ON film formed on a silicon substrate. Hydrogen atoms are introduced into the ON film during formation of the silicon oxide film or during subsequent processing (hydrogen sintering etc.) and they combine with part of silicon atoms in the silicon oxide film of the ON film as shown in FIG.
45
.
FIG. 45
shows bonded structures of a silicon atom (Si) and a hydroxyl group (OH). Three atoms shown by R are bonded to a silicon atom by single bond. This shows that three atoms of oxygen (O), hydrogen (H), silicon, etc. are bonded through single bond. This expression is used also in
FIGS. 47 and 48
.
Hydrogen atoms are introduced also into the silicon nitride film during formation of the film or subsequent processing. The hydrogen atoms introduced in the process of hydrogen sintering etc. join and terminate dangling bonds of silicon atoms at the SiO
2
/Si interface.
When stress voltage is applied to the MOSFET (for example, with an N-type MOSFET, power-supply voltage VDD to the drain and gate and 0 V or base power-supply voltage VBB=−1 V to the source), hot carriers HOT in the silicon substrate, which have been accelerated by the internal electric field and gained energy larger than the barrier energy at the SiO
2
/Si interface, pass through the interface into SiO
2
as shown in FIG.
45
.
Due to the energy of the hot carriers HOT, the bonds of hydrogen atoms of hydroxyl groups bonded to silicon atoms are cut and the dangling bonds of oxygen function as fixed charges.
As shown in
FIG. 46
, the hydrogen atoms freed from the bonds reach the SiO
2
/Si interface because of drift caused by the electric field in the gate insulating film or thermal diffusion. The hydrogen atoms which have arrived at the interface react with the combined structure of Si atoms and hydrogen atoms at the interface to form hydrogen molecules.
These hydrogen molecules volatilize as gas and as shown in
FIG. 47
the dangling bonds of silicon atoms at the SiO
2
/Si interface function as interfa
Mitsubishi Denki & Kabushiki Kaisha
Prenty Mark V.
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