Semiconductor device manufacturing: process – Making field effect device having pair of active regions... – Having insulated gate
Reexamination Certificate
1999-06-21
2001-02-27
Nelms, David (Department: 2818)
Semiconductor device manufacturing: process
Making field effect device having pair of active regions...
Having insulated gate
C438S303000, C438S304000, C438S305000
Reexamination Certificate
active
06194278
ABSTRACT:
BACKGROUND
1. Technical Field
This disclosure relates to semiconductor fabrication and more particularly, to a method for improving halo implants for semiconductor devices to improve performance.
2. Description of the Related Art
Semiconductor devices include transistors formed integrally with a substrate. Since the substrate is commonly used by many transistors and for components of a same transistor, current leakage may occur.
Referring to
FIG. 1
, a cross-sectional view of a semiconductor device is shown. Semiconductor device
10
includes a substrate
12
, which is preferably a lightly doped crystalline material, such as silicon. In previous steps, which are known to those skilled in the art, a gate stack
14
is formed on substrate
12
. Gate stack
14
includes a gate oxide
16
and a conductive material for a gate conductor
18
, such as polysilicon. Gate stack
14
may include other layers as well, for example a silicide or other higher conductive material. Gate stack
14
is preferably protected from implantation by employing a nitride cap
20
and nitride spacers
22
. Diffusion regions are formed in substrate
12
on opposite sides of gate stack
14
. Dopants are implanted in theses regions by bombarding substrate
12
with the dopants. It is desirable to have the halo dopants penetrate below gate stack
14
. This may be accomplished by permitting dopants to impact a surface
24
of substrate at angle of about 10-30 degrees. This permits dopants to penetrate below gate stack
14
.
As described above, since dopant regions on both sides of the gate are relatively close, a halo implantation is performed prior to formation of sources and drains for transistors on substrate
12
. Referring to
FIG. 2
, a field effect transistor
32
is shown. Transistor
32
includes gate stack
14
between two diffusion regions. The diffusion regions which are formed by the dopant implantation described above. The diffusion regions include a source
34
and a drain
36
. Prior to formation of source
34
and drain
36
, halo implant
38
is formed to reduce current leakage from source
34
and drain
36
. Halo implants
38
include a conductivity opposite the conductivity of source
34
and drain
36
.
Referring to
FIG. 3
, a semiconductor wafer
40
is shown. Wafer
40
includes a plurality of chips
42
formed thereon. Wafer
40
includes a notch
44
which is employed to provide a reference for semiconductor fabrication processes. Lines
46
are provided to indicate angles relative to notch
44
. These angles include 0 degrees, 90 degrees, 180 degrees, and 270 degrees. Detail
4
is indicated in FIG.
3
. Detail
4
is magnified in
FIGS. 4 and 5
and illustratively shows an orientation of gate conductors
18
along a chip
42
.
As shown in
FIG. 4
, gate conductors
18
are arranged parallel to a direction of notch
44
(notch direction is illustratively indicated in FIG.
4
). For conventional halo implant processes, implantation is directed at an angle (i.e. 10 degrees-30 degrees with respect to a normal to the surface of wafer
40
) to implant under gate conductors
18
. To get under the gate the implantation tool is aimed or directed perpendicular to the direction of gate conductors
18
(i.e. along the 270 degrees direction and the 90 degrees direction). This is in addition to the angle formed with a surface normal of the substrate (as shown in FIG.
1
). This means wafer
40
(
FIG. 3
) is rotated to these positions in a processing chamber to provide the implantation under gate conductors
18
. In this way, dopants may be implanted under a portion of gate conductor
18
.
As shown in
FIG. 5
, gate conductors
18
are oriented perpendicular to notch
44
(notch direction is illustratively indicated in FIG.
5
). The perpendicular and parallel orientations shown in
FIGS. 4 and 5
represent a highly desirable arrangement for gate conductors
18
since notch
44
is used to indicate direction for the fabrication process. Since gate conductors in
FIG. 5
are rotated by 90 degrees, the implantation tool is now aimed in the 0 degrees direction and the 180 degrees.
Referring to
FIG. 6
, a top view of a gate conductor
18
with source
34
and drain
36
formed therein is shown. Arrows A are indicated only as a reference to illustratively show the direction of implantation of halo implant
38
(FIG.
2
). In a conventional device, source
34
and drain
36
are counter-doped by the halo to a concentration of about 3D where D is a halo dose of between about 1×10
12
to about 1×10
13
atoms/cm
2
. Under gate conductor
18
, a concentration of about D is provided in regions
50
and
52
. These relatively high dopant concentrations are subject to high current leakage.
The dopant concentration (D) under gate conductors may be insufficient. Since, halo implant
38
does not extend far enough below gate conductors
18
, a higher concentration of dopants (source/drain dopants) may not be sustainable without experiencing performance degradation. For example, a threshold voltage roll-off for the transistor may be increased and/or junction capacitance (between source
34
and drain
36
) may be increased.
Therefore, a need exists for a method for implanting a higher halo dose under a gate which does not degrade performance. A further need exists for a method for forming a halo implant which provides reduced junction capacitance and reduced threshold voltage roll-off to improve semiconductor device performance.
SUMMARY OF THE INVENTION
In accordance with the present invention, a method for forming a halo implant for semiconductor devices includes the steps of providing a substrate having a gate stack formed thereon. The gate stack includes a gate conductor. The gate stack extends a distance in a first direction on a surface of the substrate. Dopants of a first conductivity and dosage are provided at an acute angle relative to a normal to the surface of the substrate. The dopants are also directed at an angle of between about 30 degrees to about 60 degrees relative to the first direction such that the dopants are implanted below the gate conductor to form a halo implant for preventing current leakage for a semiconductor device.
Also, in accordance with the present invention, another method for forming a halo implant for semiconductor devices comprising the steps of providing a substrate having a gate stack formed thereon, the gate stack including a gate conductor, the gate stack extending a distance in a first direction on a surface of the substrate, providing dopants of a first conductivity and dosage at an acute angle relative to a normal to the surface of the substrate, directing the dopants at an angle of between about 30 degrees to about 60 degrees relative to the first direction such that the dopants are implanted below the gate conductor to form a halo implant, implanting dopants of a second conductivity type over the halo implant to form source and drain regions for the semiconductor device, the halo implant for preventing current leakage from the source and drain regions, and providing a portion of the dopants of the second conductivity type below the gate conductor to stabilize a threshold voltage for the semiconductor device during operation.
Yet another method for forming a halo implant for semiconductor devices includes the steps of providing a substrate having a plurality of gate stacks formed thereon. The gate stacks are disposed substantially parallel to each other and extend a distance in a first direction on a surface of the substrate. Each gate stack includes a gate conductor. Dopants are implanted into the substrate by bombarding the substrate with dopants of a first conductivity and dosage at an acute angle relative to a normal to the surface of the substrate and directing the dopants at an angle of substantially about 45 degrees relative to the first direction such that the dopants are implanted below the gate conductor to form a halo implant. The halo implant has a portion extending laterally below the gate stacks. The halo implant prevents current leakage f
Infineon Technologies North America Corp.
Le Dung A
Nelms David
Paschburg Donald B.
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