Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Having insulated electrode
Reexamination Certificate
2000-06-30
2002-09-03
Chaudhuri, Olik (Department: 2814)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Having insulated electrode
C257S368000, C257S406000, C257S410000
Reexamination Certificate
active
06445033
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a gate-insulating film of an MIS-type field-effect transistor or the like and its forming method, particularly to a gate-insulating film having a polycrystalline film made of a metal oxide and its forming method.
2. Description of the Prior Art
In development of a semiconductor device, it is one of the most important problems how to form a gate-insulating film of an MIS-type field-effect transistor at a high reliability and a high controllability. In case of a recent logic-based MIS(metal-insulating-film-semiconductor)-type device, the thickness of a gate-insulating film of a transistor rapidly decreases and an oxide film having a thickness of 3.0 nm or less is used. A silicon oxide film has been used up to now because of a relatively simple process as compared with other processes that the film can be formed by heat-treating a silicon substrate in an oxygen atmosphere in addition to its preferable insulating characteristic and interfacial characteristic. However, when the thickness of the film decreases from 5.0 nm to 4.0 nm or less, a phenomenon having been latent so far is actualized and becomes an obstacle for obtaining a device characteristic same as ever. For example, the phenomenon appears when leak current of a gate-insulating film increases. In case of a silicon oxide film having a low relative permittivity, it is impossible to control tunneling electrons at the time of reducing the thickness of the film.
To reduce the leak current, a method is known which uses a material having a higher relative permittivity as a gate-insulating film instead of a silicon oxide film. For example, Japanese Patent Laid-Open No. 7-231088 discloses an art of using a composite laminate film comprising one of materials having a high permittivity such as Ta
2
O
5
, (Ba
1−x
Sr
x
)TiO
3
, and PbZr
1−x
TiO
3
and a silicon oxide film having a thickness of 10 nm or less as an insulating film of a MIS-type field-effect transistor.
At the time of using one of these films having a high permittivity as a gate-insulating film, it is possible to increase the physical thickness of the gate-insulating film by a value equivalent to the high permittivity when equalizing the thickness of a silicon oxide film with the electrical film thickness of a dielectric, that is a film thickness converted into silicon-oxide-film thickness. Thus, an effect is obtained that the tunnel distance of an electron increases and a direct tunnel current, that is, a gate leak current does not easily flow.
However, to use a material having a high relative permittivity as a gate-insulating film, many problems must be solved. It is one of the big problems that these high-permittivity insulating films do not have a very large relative permittivity because they are amorphous when deposited and a process for performing a high-temperature treatment in an oxygen atmosphere after deposited is necessary. The influence of a high-temperature treatment on a high-permittivity film in an oxygen atmosphere is, of course, preferable. However, a silicon oxide film is formed on the lower side of a high-permittivity film, that is, on the silicon interface due to the high-temperature heat treatment in an oxygen atmosphere and makes it difficult to decrease the whole electrical thickness (converted into silicon-oxide-film thickness) of a gate-insulating film.
FIG. 7
roughly shows the sequence of a process for forming a high-permittivity gate-insulating film in accordance with a conventional method. In this case, a tantalum oxide film is used as a high-permittivity film. Surface purification (pretreatment
16
) for forming a tantalum oxide film on a silicon substrate is performed and then the tantalum oxide film is deposited up to a desired thickness (metal oxide film deposition
26
) through CVD. Then, a comparatively-low-temperature heat treatment (low-temperature oxidation treatment
36
) is performed in an oxidizing atmosphere in order to introduce oxygen for reinforcement and finally, heat treatment is performed in an oxidizing atmosphere at a high temperature (high-temperature oxidation treatment
46
). The final high-temperature oxidation treatment
46
is performed to make the tantalum oxide film polycrystalline and perform reinforcing oxidation. This polycrystallization makes it possible to reduce a gate leak current and improve a relative permittivity.
FIGS. 8A and 8B
schematically show sectional views of the tantalum oxide film thus obtained on a silicon substrate
11
.
FIG. 8A
shows a state immediately before the high-temperature oxidation treatment
46
, in which a silicon oxide film
12
having a thickness of approx. 0.5 nm formed when an amorphous tantalum oxide film
13
is deposited on the silicon substrate
11
, the amorphous tantalum oxide film
13
, and an oxygen-rich amorphous tantalum oxide film
14
into which oxygen is additionally introduced through the low-temperature oxidation treatment
36
such as UV/O
3
treatment are superposed.
Then, by performing the high-temperature oxidation treatment
46
, the tantalum oxide film is crystallized and a polycrystalline tantalum oxide film
15
is formed as shown in FIG.
8
B. When a high-permittivity film is crystallized, it easily becomes columnar grains and a crystal is formed which becomes columnar in the film-thickness direction, that is, in which grain boundaries
16
are directly connected each other from the surface up to the silicon substrate.
A problem in this case is that the thickness of the silicon oxide film
12
increases and the thick silicon oxide film
12
is formed on the lower side of the high-permittivity film, that is, on the interface of the silicon substrate. Though crystallization occurs immediately when a high temperature is applied, a certain time is necessary for reinforcement oxidation of the tantalum oxide film. In this case, oxidation species in the atmosphere are diffused by passing through grain boundaries of the crystallized tantalum oxide film to reach the silicon substrate and directly oxidize the silicon substrate. For example, at the time of depositing a tantalum oxide film having a thickness of 8 nm, performing UV/O
3
treatment at 500° C. for 10 min and moreover performing heat treatment in a dry oxygen atmosphere at 800° C., the thickness of a silicon oxide film formed on the interface of the silicon substrate reaches approx. 3.5 nm.
The thickness of a high-permittivity film when used as a gate-insulating film instead of a silicon oxide film is just kept in a range of 3.0 nm or less. Therefore, a requested film thickness cannot be realized due to the 3.5-nm silicon oxide film unavoidably formed.
Of course, it is possible to control the thickness of the silicon oxide film at the interface by lowering the heat-treatment temperature. However, a relative permittivity is not increased as expected but a gate leak current increases. Moreover, grain boundaries directly extending in the film-thickness direction cause the gate leak current to increase.
BRIEF SUMMARY OF THE INVENTION
Objects of the Invention
It is an object of the present invention to provide a gate-insulating film comprising a polycrystalline film made of a metal oxide which has a high relative permittivity, controls a gate leak current to a completely small value, and has a small film thickness converted into silicon-oxide-film thickness (electrical film thickness) of the whole gate-insulating film because formation of a silicon oxide film is controlled at the interface of a silicon substrate and a gate-insulating film forming method.
SUMMARY OF THE INVENTION
A gate-insulating film of the present invention is a gate-insulating film having a polycrystalline film made of a metal oxide, wherein a grain boundary plane extending in parallel with a plane of the polycrystalline film is present at the position of a predetermined thickness of the polycrystalline film and grain boundaries extending in the film-thickness direction of polycrystals configuring the polycrystalline film are discontin
Chaudhuri Olik
McGinn & Gibb PLLC
NEC Corporation
Pham Hoai
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