Single-crystal – oriented-crystal – and epitaxy growth processes; – Forming from vapor or gaseous state – With decomposition of a precursor
Reexamination Certificate
2000-11-13
2002-11-26
Hiteshew, Felisa (Department: 1765)
Single-crystal, oriented-crystal, and epitaxy growth processes;
Forming from vapor or gaseous state
With decomposition of a precursor
C117S084000, C117S093000, C117S102000, C117S105000, C427S255320, C427S255360, C427S255370
Reexamination Certificate
active
06485564
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a thin film forming method and, for example, to (Ba, Sr)Ti
3
(barium strontium titanate: to be referred to as BST hereinafter) used in, e.g., a semiconductor memory device.
As the semiconductor integrating technique develops rapidly, downsizing and increases in speed and integration degree have been achieved in various types of components constituting a semiconductor integrated device. For example, in the field of semiconductor memory devices, in addition to the issues described above, an increase in capacity has been required, and development in technique is very fast. This will be described using an example. In a DRAM (Dynamic Random Access Memory) as one of typical semiconductor memory devices, downsizing and an increase in capacitance per unit area are sought for in a capacitor as one of the main constituent elements. As a dielectric material generally used to form the capacitor of a DRAM, silicon dioxide is generally used from the viewpoint of the semiconductor process. Since silicon dioxide has a dielectric constant of 10 or less and a capacitance per unit area of 4 fF/&mgr;m
2
, it cannot cope with a requirement for an increase in capacitance per unit area. For this reason, recently, BST and the like with a dielectric constant higher than that of silicon dioxide have received a great deal of attention.
The arrangement of the DRAM described above will be briefly described.
FIG. 13
shows some DRAM memory cells formed on an Si substrate
1301
. Each DRAM memory cell is constituted by a transistor
1302
and capacitor
1310
. The transistor
1302
is connected to the capacitor
1310
through a plug
1303
connected to the drain terminal. The plug
1303
is formed in a contact hole formed in an interlevel insulating film
1304
made of an insulator such as silicon dioxide.
The plug
1303
is made of polysilicon which is doped with an impurity so as to be conductive, and is connected to a storage node
1305
serving as one electrode of the capacitor
1310
. The storage node
1305
is formed on the flat interlevel insulating film
1304
and is made of, e.g., a platinum, ruthenium, or ruthenium oxide film. The storage node
1305
is connected to the plug
1303
through a barrier film
1303
a
made of, e.g., TiN.
A capacitor film
1306
is formed on the interlevel insulating film
1304
including the storage node
1305
. A cell plate
1307
is formed on the capacitor film
1306
, and forms the capacitor
1310
together with the storage node
1305
and capacitor film
1306
. An upper interlevel insulating film
1308
is formed on the capacitor
1310
. Although not shown, a word line to be connected to the gate of the transistor
1302
, a bit line to be connected to the source terminal, and the like are arranged on the upper interlevel insulating film
1308
. As described above, the capacitor film is formed to cover one electrode formed on the transistor through the interlevel insulating film
1304
and serving as a capacitor.
As the requirements needed for this capacitor film serving as a capacitor, first, the capacitor film must have a high dielectric constant. Examples of the material with such a high dielectric constant are compounds containing Ba, Sr, Ti, Pb, Zn, Bi, Ta, or the like. Second, the capacitor film must have a small leakage current.
Even if a material with a high dielectric constant is used, a DRAM of 1 Gbit or more cannot be implemented by a planar capacitor, and it is indispensable to employ a stereoscopic structure, as described above. To form a stereoscopic capacitor, a thin film forming method is required which can form a film with a uniform thickness, composition, and characteristics on the flat portion and side wall of the storage node of the stereoscopic structure and which has a good coverage on a complicated stepped shape.
As a thin film forming technique with the good step coverage, chemical vapor deposition (CVD) is available. With CVD, the source of a material for forming a thin film to be formed must be transported onto a substrate in the form of a gas. The most preferable CVD source is the one that takes the form of a gas in room temperature. When a gas is used as the source, it can be controlled well because its supply amount is determined by only the gas flow rate. However, of the elements described above that form a high dielectric or ferroelectric, no one forms a compound which is gaseous at room temperature. Accordingly, when a material with a high dielectric constant is to be formed by CVD, the source is supplied by means of bubbling. More strictly, if the source is a solid source, it is supplied by sublimation.
When the source is to be supplied by bubbling, if the source gas is liquid, supply amount control can be performed more stably. The requirement for the source gas is a high vapor pressure, that is, if possible, it must have a sufficiently high vapor pressure at room temperature or less. However, almost none of the elements described above forms a compound with a sufficiently high vapor pressure, and most of compounds that have some vapor pressure are organometallic compounds.
From the above viewpoint, barium titanate (BaTiO
3
: BT), strontium titanate (SrTiO
3
: ST), and BST as a solid solution of BT and ST have received attention as the material of the capacitor film of a DRAM that can be formed with CVD described above.
More specifically, BT and ST are high dielectric constant materials with a high dielectric constant of about 100, and BST is a high dielectric constant material with a high dielectric constant of about 200 to 300, thus satisfying one performance described above which is required for the capacitor film. Barium, strontium, and titanium can form organic compounds, and a BT, ST, or BST thin film can be formed by MO (Metal organic) CVD. This thin film forming method is thermal CVD which forms a metal or compound film at a comparatively low temperature (400° C. to 500° C.) by using an organometallic compound (MO). In this manner, a BT, ST, or BST thin film can be formed with CVD that provides a good step coverage, and satisfies the second performance described above which is required for the capacitor film.
When forming a BST film by MOCVD, generally, a substrate as a thin film forming target is heated and, e.g., evaporated Ba(thd)
2
, Sr(thd)
2
, and Ti(O—iPr)
2
(thd)
2
are supplied onto the substrate, thus forming a BST film on the substrate. Ba(thd)
2
is a barium source, Sr(thd)
2
is a strontium source, and Ti(O—iPr)
2
(thd)
2
is a titanium source.
If the substrate is heated to an excessively high temperature during formation of the BST film described above, the step coverage suffers. When the substrate has a step with an aspect ratio of 3 or more, it is heated to a temperature of 450° C. or less in order to set the step coverage to 90% or more. When the temperature is decreased, the film formation rate becomes low. Hence, when forming a BST film, the substrate is heated to about 450° C.
When a BST film is to be formed by MOCVD, unless the temperature of the substrate as the thin film forming target becomes 500° C. or more, the formed BST film cannot be set in a crystalline state wherein the formed BST film has a high dielectric constant enough use it as a capacitor film. More specifically, when a thin film is formed at a substrate temperature of about 450° C. described above, the obtained BST film is in the amorphous state and is thus not appropriate as a capacitor film. Therefore, conventionally, a BST film in a crystalline state, which can be used as a capacitor film, is formed by three-step thin film formation to have good step coverage (reference: Japanese Patent Laid-Open No. 8-176826).
First, a thin strontium titanate film (ST film) is formed by using a predetermined MOCVD apparatus at a substrate temperature where desired step coverage can be obtained. Since crystal growth of the ST film occurs on a heterogeneous substrate more easily than a BST film, a comparatively thin initial film in a perovskite state with good crystallizability can be forme
Liu Yijun
Magara Takashi
Shinriki Hiroshi
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Hiteshew Felisa
Tokyo Electron Limited
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