Chemistry: electrical and wave energy – Processes and products – Coating – forming or etching by sputtering
Reexamination Certificate
2001-05-31
2003-12-30
VerSteeg, Steven H. (Department: 1753)
Chemistry: electrical and wave energy
Processes and products
Coating, forming or etching by sputtering
C427S350000, C427S331000, C427S444000, C427S553000, C427S569000, C427S573000
Reexamination Certificate
active
06669825
ABSTRACT:
BACKGROUND
The inventions described herein generally relate to semiconductor devices and a fabrication processes thereof. More particularly, the inventions relate to a forming an insulation film and fabrication process of a non-volatile semiconductor memory device capable of rewriting information electrically, including a flash memory device.
There are various volatile memory devices such as DRAMs and SRAMs. Further, there are non-volatile memory devices such as a mask ROM, PROM, EPROM, EEPROM, and the like. Particularly, a flash memory device is an EEPROM having a single transistor for one memory cell and has an advantageous feature of small cell size, large storage capacity and low power consumption. Thus, intensive efforts are being made on the improvement of flash memory devices. In order that a flash memory device can be used stably over a long interval of time with low voltage, it is essential to use a uniform insulation film having high film quality.
Further, high-quality insulation film, characterized by uniform film quality and low leakage current, is important not only in a flash memory device but also in other various semiconductor devices that uses a capacitor, including a ferroelectric semiconductor memory device that uses a ferroelectric film. Further, a high dielectric film of uniform film quality characterized by small leakage current is important as a gate insulation film of a high-speed semiconductor device having a gate length of 0.1 &mgr;m or less.
First, the construction of a conventional flash memory device will be explained with reference to
FIG. 1
showing the concept of a generally used flash memory device having a so-called stacked gate structure.
Referring to
FIG. 1
, the flash memory device is constructed on a silicon substrate
1700
and includes a source region
1701
and a drain region
1702
formed in the silicon substrate
1700
, a tunneling gate oxide film
1703
formed on the silicon substrate
1700
between the source region
1701
and the drain region
1702
, and a floating gate
1704
formed on the tunneling gate oxide film
1703
, wherein there is formed a consecutive stacking of a silicon oxide film
1705
, a silicon nitride film
1706
and a silicon oxide film
1707
on the floating gate
1704
, and a control gate
1708
is formed further on the silicon oxide film
1707
. Thus, the flash memory of such a stacked structure includes a stacked structure in which the floating gate
1704
and the control gate
1708
sandwich an insulating structure formed of the insulation films
1705
,
1706
and
1707
therebetween.
The insulating structure provided between the floating gate
1704
and the control gate
1705
is generally formed to have a so-called ONO structure in which the nitride film
1706
is sandwiched by the oxide films
1705
and
1707
for suppressing the leakage current between the floating gate
1704
and the control gate
1705
. In an ordinary flash memory device, the tunneling gate oxide film
1703
and the silicon oxide film
1705
are formed by a thermal oxidation process, while the silicon nitride film
1706
and the silicon oxide film
1707
are formed by a CVD process. The silicon oxide film
1705
may be formed by a CVD process. The tunneling gate oxide film
1703
has a thickness of about 8 nm, while the insulation films
1705
,
1706
and
1707
are formed to have a total thickness of about 15 nm in terms of oxide equivalent thickness. Further, a low-voltage transistor having a gate oxide film of 3-7 nm in thickness and a high-voltage transistor having a gate oxide film of 15-30 nm in thickness are formed on the same silicon in addition to the foregoing memory cell.
In the flash memory cell having such a stacked structure, a voltage of about 5-7V is applied for example to the drain
1702
when writing information together with a high voltage larger than 12V applied to the control gate
1708
. By doing so, the channel hot electrons formed in the vicinity of the drain region
1702
are accumulated in the floating gate via the tunneling insulation film
1703
. When erasing the electrons thus accumulated, the drain region
1702
is made floating and the control gate
1708
is grounded. Further, a high voltage larger than 12 V is applied to the source region
1701
for pulling out the electrons accumulated in the floating gate
1704
to the source region
1701
.
Such a conventional flash memory device, on the other hand, requires a high voltage at the time of writing or erasing of information, while the use of such a high voltage tends to cause a large substrate current. The large substrate current, in turn, causes the problem of deterioration of the tunneling insulation film and hence the degradation of device performance. Further, the use of such a high voltage limits the number of times rewriting of information can be made in a flash memory device and also causes the problem of erroneous erasing.
The reason a high voltage has been needed in conventional flash memory devices is that the ONO film, formed of the insulation films
1705
,
1706
and
1707
, has a large thickness.
In the conventional art of film formation, there has been a problem, when a high-temperature process such as thermal oxidation process is used in the process of forming an oxide film such as the insulation film
1705
on the floating gate
1704
, in that the quality of the interface between the polysilicon gate
1704
and the oxide film tends to become poor due to the thermal budget effect, etc. In order to avoid this problem, one may use a low temperature process such as CVD process for forming the oxide film. However, it has been difficult to form a high-quality oxide film according to such a low-temperature process. Because of this reason, conventional flash memory devices had to use a large thickness for the insulation films
1705
,
1706
and
1707
so as to suppress the leakage current.
However, the use of large thickness for the insulation films
1705
,
1706
and
1707
in these conventional flash memory devices has caused the problem in that it is necessary to use a large writing voltage and also a large erasing voltage. As a result of using large writing voltage and large erasing voltage, it has been necessary to form the tunneling gate insulation film
1703
with large thickness so as to endure the large voltage used.
Thus, there is a need of a high-quality insulation film that provides small leakage current even in the case the insulation film has a small thickness, wherein the need of such a high-quality insulation film is not limited to flash memory devices but also in other various semiconductor devices.
SUMMARY
In general the inventions provide novel and useful arrangements and methods of forming a dielectric film wherein the foregoing problems are eliminated.
More specifically, the inventions provide a method of forming a high-quality oxide film, nitride film or oxynitride film in which reduction of film thickness is possible without causing substantial leakage current.
Also, the inventions provide a method of forming an oxide film characterized by the steps of:
forming an oxide film on a substrate; and
modifying a film quality of said oxide film formed on said substrate by exposing said oxide film to atomic state oxygen O*.
According to the inventions, the atomic state oxygen O* penetrate easily into the oxide film formed on the substrate and terminate dangling bonds or weak bonds in the oxide film. As a result, an SiO
2
film formed by a process such as a CVD process can have a quality similar to that of a thermal oxide film as a result of exposure to the atomic state oxygen O*. Thus, the oxide film formed according to the present invention has various advantageous features such as small number of surface states, having a composition substantially identical with a stoichiometric composition, small leakage current, and the like. It should be noted that the atomic state oxygen O* can be formed efficiently by microwave excitation of a mixed gas of Kr and oxygen.
The inventions also provide a method of forming a nitride film that en
Ohmi Tadahiro
Sugawa Shigetoshi
Ohmi Tadahiro
Pillsbury & Winthrop LLP
VerSteeg Steven H.
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