Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Having insulated electrode
Reexamination Certificate
1999-12-22
2002-02-19
Lee, Eddie (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Having insulated electrode
C257S310000, C438S003000, C427S126300
Reexamination Certificate
active
06348705
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to formation of high dielectric constant capacitor structures comprising amorphously frustrated ferroelectric materials deposited by metalorganic chemical vapor deposition (MOCVD).
2. Description of the Related Art
In recent years, ferroelectric materials such as Pb(Zr,Ti)O
3
and SrBi
2
Ta
2
O
9
have been the focus of widespread interest as components of non-volatile memory devices. Non-volatile ferroelectric memory devices effect information storage by polarization of a thin ferroelectric material layer disposed between two plates of a capacitor structure. Each such ferroelectric capacitor is connected to a transistor to form a storage cell, in which the transistor controls the access of the read-out electronics to the capacitor. The transistor therefore is connected to bit-line and word-line elements, to constitute the storage cell.
The ferroelectric material may be utilized in a stacked capacitor structure that overlies the top of the transistor. The transistor drain (e.g., of a MOSFET structure) is connected to the bottom electrode of the capacitor by a plug formed of a suitable material such as polysilicon or tungsten.
Information subsequently can be changed in the ferroelectric memory cell by applying an electric field to the thin ferroelectric material layer to reverse (“flip”) the polarization characteristic of the ferroelectric material. Ferroelectric memories (FRAMs), in contrast to dynamic random access memories (DRAMs), have the advantages of retaining stored information in the event of termination of the power supply, and do not require refresh cycles.
In such memory applications, ferroelectric materials desirably have the following electrical properties: (1) a low coercive field characteristic, facilitating use of a low voltage power supply; (2) a high remanent polarization characteristic, ensuring highly reliable information storage; (3) absence of significant fatigue or life-time deterioration characteristics; (4) absence of any imprint which would alter the stored information (e.g., leading to a preference of a certain polarization such as a logical “1” over a logical “0” character) or otherwise impair the ability to “read” the stored information; and (5) extended retention time, for reliable data storage over an extended period of time.
The foregoing electrical property criteria are satisfied in the layered pseudo-perovskite or “Aurivillius” phase of materials such as strontium bismuth tantalate, SrBi
2
Ta
2
O
9
, sometimes hereinafter referred to as “SBT.” As a result of these favorable characteristics, significant efforts have been initiated to integrate SBT in memory devices.
Ferroelectric capacitor structures utilizing SBT as the ferroelectric material have been made in the prior art by sol-gel techniques and demonstrate superior electrical properties. Nonetheless, the sol-gel methodology of forming ferroelectric thin films for this application permits only a low integration density to be achieved. Some improvement in the sol-gel methodology may be gained by mist or electrospray methods, permitting fabrication of memories up to 4 megabit in capacity. To achieve higher integration density of ferroelectric thin films of materials such as Pb(Zr,Ti)O
3
(PZT) and SrBi
2
Ta
2
O
9
(SBT) with smaller structure sizes (e.g., having a minimal feature size below about 0.7 micron), it is necessary to utilize chemical vapor deposition (CVD) processes, since CVD affords better conformality and step coverage than layers produced by any other deposition method. Further, the CVD process yields deposited films having high film uniformity and high film density, with the capability to grow very thin films at high throughput and low cost.
The art continues to seek improvement in high dielectric constant materials for thin film capacitor structures and ferroelectric memory applications, and in improved processes for the formation of such materials, as for example CVD processes for the deposition of bismuth oxide materials (e.g., SrBi
2
Ta
2
O
9
, Bi
4
Ti
3
O
12
, etc).
Among dielectric materials, thin film “frustrated ferroelectrics” or “superparaelectrics” such as (Ba,Sr)TiO
3
(BST) have extremely high dielectric constants. Such materials are ferroelectric in bulk form, but are paraelectric in thin film form. The mechanism of “frustrating” or suppressing the ferroelectric switching in these films is a field of current research, however, the resulting capacitor structures have high capacitance, large changes in capacitance with bias voltage, and low loss. These materials continue to be developed for high-density DRAM devices and high frequency tunable devices.
The aforementioned paraelectric thin film materials are deposited by MOCVD at temperatures above 500° C., which however is too high for many backend processes used in semiconductor processing. These paraelectric thin films also exhibit large changes in dielectric constant with bias voltage, a property quite useful for making tunable devices, but not for other applications where stable capacitance is desired. The dielectric constant also changes significantly with changes in temperature.
Taken against the background of the above-discussed state of the art, there is a continued need for improved low temperature, high dielectric constant capacitors.
SUMMARY OF THE INVENTION
The present invention relates in one aspect to an amorphous metal oxide thin film material of a composition that is ferroelectric in both crystalline thin film and crystalline bulk forms. The material comprises the elements of the corresponding ferroelectric composition, and has a same stoichiometry as the ferroelectric material.
The invention also contemplates amorphous metal oxide thin film materials compositionally related to such ferroelectric compositions, but which are “away from” the ferroelectric material, i.e., off-stoichiometric in relation thereto. Such “away from” ferroelectric compositions may for example, in the case of SBT, have metal site ratios of from about 0 to 20% Sr, 5 to 70% Bi and 10 to 95% tantalum or 0 to 10% strontium, 5 to 55% bismuth and 35 to 95% tantalum or 0 to 5% strontium, 15 to 50% bismuth and 45 to 85% tantalum.
The optimum composition for such amorphous dielectric film contains the elements of the bulk ferroelectric with a stoichiometry that is significantly away from the optimum ferroelectric stoichiometry. This same behavior has been observed in the case of Ba—Sr—Ti—O as described in U.S. Pat. No. 5,932,905, wherein compositions from 65-90% Ti on the metal site produced the best combination of capacitance, leakage, and linearity. Such phenomena is general and for each ferroelectric composition herein aforementioned, a highly favorable amorphous dielectric composition is located nearby.
Thin film amorphous materials of the invention have a voltage independent capacitance, a capacitance density in the range of from about 1000 to about 10000 nF/cm
2
, and a current leakage of <10
−7
A/cm
2
.
Another aspect of the invention relates to a thin film capacitor microelectronic device structure, comprising:
a first electrode;
a dielectric material selected from the group consisting of amorphous metal oxide thin film materials of a composition that is ferroelectric in both crystalline thin film and crystalline bulk forms, and amorphous metal oxide thin film materials related to ferroelectric compositions but deviating from the stoichiometry of the ferroelectric material, wherein the dielectric material in thin film amorphous form has a voltage independent capacitance, a capacitance density in the range of from about 1000 to about 10000 nF/cm
2
, and a current leakage of <10
−7
A/cm
2
; and
a second electrode.
A still further aspect of the invention relates to a method of forming a thin film capacitor structure on a substrate, comprising:
forming a first electrode on the substrate;
depositing on the first electrode a dielectric material selected from the group consisting of amorphous metal oxide thin film materials of a composit
Advanced Technology & Materials Inc.
Eckert II George C.
Fuierer Marianne
Lee Eddie
McLauchlan Robert
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