Active solid-state devices (e.g. – transistors – solid-state diode – Polysilicon containing oxygen – nitrogen – or carbon
Reexamination Certificate
1998-11-20
2002-06-25
Whitehead, Jr., Carl (Department: 2822)
Active solid-state devices (e.g., transistors, solid-state diode
Polysilicon containing oxygen, nitrogen, or carbon
C257S392000, C257S607000, C438S520000, C438S528000, C438S423000
Reexamination Certificate
active
06410991
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to semiconductor devices and methods of manufacturing the same, and in particular to a semiconductor device wherein one device has a plurality of gate oxide films with different film thicknesses, and a method of manufacturing the same.
2. Description of the Background Art
In recent years, with integration of semiconductor devices, a device (dual gate oxide device) having gate oxide films with different thicknesses in one chip has been increasingly used. The dual gate oxide device has been significantly increasingly used particularly for a device in which a memory device including a dynamic random access memory (DRAM) and a logic device are mounted in a mixed manner.
A method of manufacturing a semiconductor device having a conventional dual gate oxide will now be described.
FIGS. 57-62
are schematic cross sectional views illustrating steps of a method of manufacturing a semiconductor device having a conventional dual gate oxide. Referring first to
FIG. 57
, a field oxide film
2
is formed at a surface of a silicon substrate
1
and thermal oxidation is then applied.
Referring to
FIG. 58
, the thermal oxidation allows a first gate oxide film
6
a
to be formed on a surface of silicon substrate
1
. A normal photolithography technique is employed to form a resist pattern
61
a
on a predetermined region. The first gate oxide film
6
a
that is not covered by resist pattern
61
a
is removed through e.g. wet-etching.
Referring to
FIG. 59
, the wet-etching causes a surface of silicon substrate
1
to be exposed at the portion from which silicon oxide film
6
a
is removed. After resist pattern
61
a
is removed, thermal oxidation is again applied.
Referring to
FIG. 60
, the thermal oxidation allows a second gate oxide film
6
b
to be formed on an exposed surface of silicon substrate
1
and the first gate oxide film
6
a
to be increased in thickness. Thus the film thickness of the first gate oxide film
6
a
is greater than that of the second gate oxide film
6
b
to form a dual gate oxide.
Referring to
FIG. 61
, a conductive layer
7
for a gate is formed on the entire surface. A normal photolithography technique is employed to form a resist pattern
61
b
on a predetermined region of conductive layer
7
for a gate. Resist pattern
61
b
is used as a mask to etch conductive layer
7
. Then resist pattern
61
b
is removed.
Referring to
FIG. 62
, this etching allows conductive layer
7
for a gate to be patterned to form a gate electrode layer
7
. Gate electrode layer
7
, field oxide film
2
and the like are used as a mask for injection of an impurity to form source/drain region
8
a
,
8
b
at a surface of silicon substrate
1
. Thus a metal oxide semiconductor (MOS) transistor having relatively thick gate oxide film
6
a
and a MOS transistor having relatively thin gate oxide film
6
b
are obtained.
While the method described above allows formation of dual gate oxide, it requires different thermal oxidation steps for forming gate oxide films having different thicknesses and this results in a cumbersome manufacturing process. Methods of formation of dual gate oxide in a simpler process have been disclosed in e.g. Japanese Patent Laying-Open Nos. 7-297298, 9-92729 and 63-205944. A method disclosed in Japanese Patent Laying-Open No. 7-297298 will now be exemplarily described.
FIGS. 63-65
are schematic cross sectional views illustrating steps of the method of manufacturing a semiconductor device having a dual gate oxide that is disclosed in Japanese Patent Laying-Open No. 7-297298. Referring first to
FIG. 63
, a field oxide film
2
is formed at a surface of a silicon substrate
1
.
Referring to
FIG. 64
, a normal photolithography technique is employed to form a resist pattern
71
on silicon substrate
1
at a predetermined region. An oxidation promoting substance, such as F or Cl, as a substance from the halogen group is ion-injected into a surface of silicon substrate
1
that is not covered by resist pattern
71
. Then resist pattern
71
is removed.
Referring to
FIG. 65
, an oxidation step is provided to form a gate oxide film. In this oxidation step, the substance from the halogen group acts to promote oxidation. Thus gate oxide film
6
a
formed at a region which is ion-injected with an oxidation promoting substance is formed thicker than gate oxide film
6
b
formed at a region which is not ion-injected with the oxidation promoting substance. Thus a dual gate oxide is formed.
The method shown in
FIGS. 63-65
allows dual gate oxide to be formed with one oxidation step and can thus simplify the process.
Japanese Patent Laying-Open No. 7-297298 also discloses a method of forming a dual gate oxide with one oxidation step by ion-implanting nitrogen (N) as an oxidation restraining substance rather than an oxidation promoting substance.
For this ion injection, it is difficult to draw ions with an acceleration energy of less than 1 keV. Thus, ion-injecting an oxide promoting substance or an oxide restraining substance requires an injection energy of at least 1 keV. However, if ions are injected with the injection energy of at least 1 keV, the oxidation promoting substance or oxidation restraining substance will be distributed to a location more than 2 nm deeper than the surface of silicon substrate.
According to the method described above, an oxidation promoting substance or an oxidation restraining substance is introduced into silicon substrate
1
through ion injection. The ion injection is a technique of physically injecting ions into silicon substrate
1
and the injection energy is relatively large. Thus, ion injection of an oxidation promoting substance and the like results in a large number of lattice defects at a surface of silicon substrate
1
and thus significantly damages the surface of silicon substrate
1
. In order to repair the significant damage, a thermal process (annealing) step is additionally required and thus renders the manufacturing process cumbersome.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a semiconductor device having a dual gate oxide with the substrate less damaged in a simplified process, and a method of manufacturing the same.
A semiconductor device according to the present invention includes a semiconductor substrate, first and second gate oxide films, and an oxidation rate adjusting substance. The semiconductor substrate has first and second regions. The first gate oxide film is formed in the first region such that the first gate oxide film is in contact with a main surface of the semiconductor substrate. The second gate oxide film is formed in the second region such that the second gate oxide film is in contact with a main surface of the semiconductor substrate. The second gate oxide film is different in thickness from the first gate oxide film. The oxidation rate adjusting substance is only added within a depth range of no more than 2 nm from the main surface of the semiconductor substrate in the first region.
The semiconductor device according to the present invention has an oxidation rate adjusting substance distributed only within a depth range of no more than 2 nm from a main surface of the semiconductor substrate, i.e. to a location shallower more than conventional. Thus the energy required in adding the oxidation rate adjusting substance can be greatly reduced as compared with that required for conventional ion injection. Thus a semiconductor device can be obtained which is less damaged by lattice defect and the like.
In the above semiconductor device, preferably the oxidation rate adjusting substance is an oxidation promoting substance and the first gate oxide film is greater in thickness than the second gate oxide film.
Thus, when gate oxidation is applied to the first and second regions, simultaneously the first region with the oxidation promoting substance added thereto can be greater in gate oxide film thickness than the second region without the oxidation promoting substance added thereto.
For the ab
Kawai Kenji
Yonekura Kazumasa
Jr. Carl Whitehead
Mitchell James
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