Low temperature CVD processes for preparing ferroelectric...

Details Low temperature CVD processes for preparing ferroelectric... Low temperature CVD processes for preparing ferroelectric...

1998-12-09

2002-12-31

Chen, Bret (Department: 1762)

Coating processes

Coating by vapor, gas, or smoke

Mixture of vapors or gases utilized

C427S255320, C427S314000, C427S372200

Reexamination Certificate

active

06500489

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates to chemical vapor deposition methods for providing a Bi oxide-containing film on a surface of a substrate by decomposing a precursor of Bi oxide.
Interest in ferroelectric has increased in recent years, due to the utility of these materials in applications such as non-volatile memories. Information in these memories is stored by the polarization of a thin ferroelectric film which is placed between the two plates of a capacitor. The capacitor is connected to a transistor to form a storage cell, which controls the access of read-out electronics to the capacitor.
The information stored in the cell can be changed by applying an electric field to the thin ferroelectric film and flipping the polarization. Ferroelectric random access memories (FERAMs), unlike DRAMs (dynamic random access memories), retain the stored information if the power supply is turned off. In addition, they do not require refresh cycles. Desirable electrical properties for ferroelectrics used in memory applications include: (a) a low coercive field, which makes the use of as low a voltage supply as possible; (b) a high remanent polarization, which is needed for high reliability of information storage; (c) minimal fatigue, which is required for a long life-time; and (d) no imprint, as an imprint would alter the stored information.
Strontium bismuth tantalate (SrBi
2
Ta
2
O
9
) (SBT) is a ferroelectric material that meets all of these requirements. Significant efforts are therefore being made to integrate this material into memory devices. Capacitors in which SBT is incorporated using a sol-gel method have good electrical properties. The sol-gel method provides only a low integration density of SBT, however. To achieve a higher integration density of SBT, an alternative method, such as chemical vapor deposition (CVD), must be used.
SUMMARY OF THE INVENTION
In one aspect, the invention features a method of forming a Bi-containing metal oxide film on a substrate, by dissolving a precursor of Bi oxide in a solution, decomposing the precursor to form Bi oxide, and depositing the Bi oxide on the substrate at a temperature lower than 450° C. Bi complexes which include at least one alkoxide group are used as the precursor of Bi oxide.
Embodiments of this aspect of the invention may include one or more of the following features.
The deposition temperature may be lower than 400° C. The Bi oxide-containing film may also be provided by adding the steps of decomposing a precursor of Sr oxide, and a precursor of Ta oxide to form Sr oxide and Ta oxide, respectively, and depositing the Bi oxide, the Sr oxide and the Ta oxide on the substrate.
The film of Bi, Sr, and Ta oxides may be deposited as a ferroelectric film or can be converted into a ferroelectric film by an annealing process.
In accordance with the invention, the ferroelectric layer is formed from an amorphous as-deposited layer or film. The amorphous film is annealed, transforming it into the ferroelectric layer. We have discovered that by forming the ferroelectric layer from an amorphous layer, a lower thermal budget is consumed by the ferro-anneal as compared to that of ferroelectric layer formed from conventional techniques. The lower thermal budget avoids or reduces excessive diffusion of one or more of the constituents of the ferroelectric layer and oxidation of the contacts.
The amorphous layer is processed to produce a ferroelectric layer in accordance with the invention. The amorphous layer comprises materials that can be transformed into a ferroelectric layer. In one embodiment, the amorphous layer comprises a Bi-based oxide ceramic. The Bi-based oxide ceramic comprises, for example, strontium bismuth tantalate (SBT) or a material derived from SBT (SBT derivative). The amorphous layer is annealed under appropriate conditions transforming it into a ferroelectric layer.
The amorphous film comprises a material which can be processed, for example by a ferro-anneal, to form a ferroelectric film. In one embodiment, the amorphous layer comprises a metal oxide ceramic material. Preferably, the amorphous layer comprises a Bi-based oxide ceramic material. More preferably, the amorphous layer comprises a Bi-based oxide ceramic material that can be processed to transform it into a ferroelectric.
The Bi-containing metal oxide film is formed by placing the substrate in a CVD chamber, heating the substrate to a deposition temperature lower than 450° C., introducing vapors of the precursors of Bi, Sr, and Ta oxides to the CVD chamber, decomposing the precursors of Bi, Sr, and Ta oxides, and depositing the oxides on the substrate. Precursors of Bi, Sr, and Ta oxides may be decomposed in the presence of an oxidizer by oxidative decomposition, where examples of the oxidizers include O
2
, singlet O
2
, O
3
, H
2
O
2
, N
2
O, NO
x
(1≦x≦3), and downstream oxygen plasma, and where the concentration of the oxidizer is between 5% and 95% of the total gas and vapor flow into the CVD chamber. At least one of O
2
and N
2
O may be used as the oxidizer. The oxidizer may be formed in the CVD chamber by converting an oxidizer molecule into an active oxidizer by applying to the CVD chamber plasma, UV light, heat, a sensitizer, or ion beams.
The precursor of Bi oxide may have the formula Bi(OR)
3
, Bi(OR)
2
(OR′), or Bi(OR)(OR′)(OR″), where each of R, R′, and R″ is, independently, an alkyl, aryl, or silyl group. For example, R may be
t
pentyl, pentyl,
t
Bu, Bu,
i
Pr, Pr, Et, Me, Ph, aryl, or SiR′″
3
, and R′″ may be
t
Bu, Bu,
i
Pr, Pr, Et, or Me. Examples of precursors of Bi oxide further include Bi(O
t
Bu)
3
and Bi(OCMe
2
Et)
3
. The precursor of Bi oxide may also include an alkoxy group, a phenoxy group, or a donor atom such as N, O, or S. For example, the precursor may include the group —CH
2
CH
2
—N(CH
3
)
2
.
The Bi-containing metal oxide deposited on the substrate may have the formula (Bi
2
O
2
)
2+
(A
m−1
B
m
O
3m+1
)
2−
, where A is Bi
3+
, L
3+
, L
2+
, Ca
2+
, Sr
2+
, Ba
2+
, Pb
2+
, or Na
+
, B is Fe
3+
, Al
3+
, Sc
3+
, Y
3+
, L
3+
, L
4+
, Ti
4+
, Nb
5+
, Ta
5+
, W
6+
, or Mo
6+
, and L is Ce
4+
, La
3+
, Pr
3+
, Ho
3+
, Eu
2+
, or Yb
2+
, and where 1≦m≦5. The Bi-containing metal oxide may also have the formula Bi
2
WO
6
; BiMO
3
, where M is Fe or Mn; Ba
2
BiMO
6
, where M is V, Nb or Ta; Pb
2
BiMO
6
, where M is V, Nb or Ta; Ba
3
Bi
2
MO
9
, where M is Mo or W; Pb
3
Bi
2
MO
9
, where M is Mo or W; Ba
6
BiMO
18
, where M is Mo or W; Pb
6
BiMO
18
, where M is Mo or W; KBiTi
2
O
6
; or K
2
BiNb
5
O
15
. These metal oxides can be obtained by decomposing precursors which contain the above-described metals.
The Bi-containing metal oxide film can also be a SBT derivative. Examples of such derivatives include SrBi
2
Ta
2
O
9
; SrBi
2
Ta
2−x
Nb
x
O
9
, where 0≦x≦2; SrBi
2
Nb
2
O
9
; Sr
1−x
Ba
x
Bi
2
Ta
2−y
Nb
y
O
9
, where 0≦x≦1 and 0≦y≦2; Sr
1−x
Ca
x
Bi
2
Ta
2−y
Nb
y
O
9
where 0≦x≦1 and 0≦y≦2; Sr
1−x
Pb
x
Bi
2
Ta
2−y
Nb
y
O
9
, where 0≦x≦1 and 0≦y≦2; and Sr
1−x−y−z
Ba
x
Ca
y
Pb
z
Bi
2
Ta
2−p
Nb
p
O
9
, where 0≦x≦1, 0≦y≦1, 0≦z≦1, and 0≦p≦2. An element of the metal oxide may be substituted by a metal such as Ce, La, Pr, Ho, Eu, and Yb.
The precursor of Sr oxide generally has the formula Sr(thd)
2
or Sr(thd)
2
adduct, and may include a polyether or a polyamine. The polyether has the formula R—O—(CH
2
CH
2
O)
n
—R′, where 2≦n≦6, and where each of R and R′ may be, independently, an alkyl group, an aryl group, or hydrogen. The polyamine has the formula R—NR″-(CH
2
CH
2
NR″)
n
-R′, where 2≦n≦6, where each of R and R′ may be, independently, an alkyl group, an aryl group, or hydrogen, and where R″ is H, Me, Et, or Pr. The precursor of Sr o

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