Zirconium-doped BST materials and MOCVD process forming same

Details Zirconium-doped BST materials and MOCVD process forming same Zirconium-doped BST materials and MOCVD process forming same

2000-11-29

2003-07-29

Kopec, Mark (Department: 1752)

Compositions

Electrically conductive or emissive compositions

Metal compound containing

C501S138000, C501S139000

Reexamination Certificate

active

06599447

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to metalorganic chemical vapor deposition (MOCVD) formation of (Ba,Sr)(Zr,Ti)O
3
perovskite crystal thin films of high breakdown strength, low leakage, low loss tangent, high permittivity, and substantial tunability with application of electric fields. Such (Ba,Sr)(Zr,Ti)O
3
thin films can be used to manufacture dielectric capacitors or other related microelectronic devices of significantly improved performance useful in many applications, especially for use under elevated temperature and/or high frequency.
2. Description of the Related Art
A wide variety of semiconductor materials is used in integrated circuitry for electronic devices. Increasingly more compact integrated circuits with greater capacities are required for new devices, as well as existing device applications. This in turn necessitates the use of materials with higher specific capacitance in order to further reduce the size of individual transistors and capacitors in such integrated circuitry.
Ferroelectric materials typically have high specific capacitance due to their high permittivity ∈, which usually ranges from about 200 to about 500, making these materials correspondingly attractive as dielectric materials for capacitors. Conventional ferroelectric materials used in integrated circuit applications include ferroelectric dielectric compounds of perovskite crystalline structure, such as Pb(Zr,Ti)O
3
(PZT), BaTiO
3
(BT), and (Ba,Sr)TiO
3
(BST).
Despite their advantages and favorable characteristics, these conventional ferroelectric materials have an associated disadvantage of relatively low breakdown strength. As a result, dielectric devices made from these materials can fail catastrophically when applied voltage rises above a specific level and causes strong short-circuit currents. In order to avoid system failure incident to such short-circuiting, applied voltage on the dielectric devices made from these materials has to be controlled carefully to keep the applied voltage below the breakdown limit. Since energy storage capacity of a capacitor positively correlates with the square of applied voltage on such capacitor, limitations on applied voltage correspondingly limit the electrical energy storage characteristics of the capacitor.
Further, conventional ferroelectric materials exhibit high current leakage under elevated voltage conditions. This in turn leads to high power dissipation as evidenced by high loss tangent &dgr; of the material. The high power dissipation rate greatly reduces the energy storage efficiency of corresponding dielectric devices fabricated from such conventional ferroelectric materials.
In contrast, many linear dielectric materials, such as SiO
2
and Ta
2
O
5
, exhibit high breakdown strength. Unfortunately, the permittivity of these materials is very low, usually in the range from 3 to 20. Such low permittivity results in unsatisfactorily low specific capacitance of these dielectric materials, rendering them unsuitable for energy storage applications in integrated circuit devices.
Accordingly, there exists a compelling need for improved dielectric materials with both high specific capacitance and high breakdown strength, as well as low current leakage and low loss tangent. Dielectric materials having such combination of properties would enable manufacture of micro-capacitors or other micro-electronic devices with substantially improved energy storage and operating characteristics, relative to currently used materials.
SUMMARY OF THE INVENTION
The present invention relates in one aspect to an improved dielectric thin film comprising modified (Ba,Sr)TiO
3
(BST) perovskite crystal material doped with zirconium, and to devices comprising same.
As used herein, the term “thin film” refers to a film having a thickness of less than 20,000 Å.
Devices utilizing dielectric Zr-doped BST perovskite crystal material within the broad scope of the present invention include, but are not limited to: electroluminescent displays (ELDs); pulse discharge capacitors; high frequency devices operated under a frequency of at least 5 MHz, more preferably in a range from 10 MHz to 40 GHz; dynamic random access memory cells (DRAMs) and ferroelectric random access memory cells (FeRAMs); microwave phase shifting and tunable varactors (variable capacitors); piezoelectric actuating elements; passive as well as active microelectromechanical system (MEMS) devices; optical devices, including both geometric and spectral- or interference-based devices, such as movable microlens arrays or movable micromirror arrays; spectral devices to alter a resonant cavity in an etalon structure to detune the reflectance of the device; micropumps and microvalves based on piezoelectric film cantilever structures, e.g., for applications such as medication dose delivery systems, or operation of hydraulic or fluidic systems in a MEMS apparatus; ultrasonic transducers, e.g., for high frequency flaw detection systems; vibration control devices; dfribillators; gate dielectrics; uncooled infrared radiation pyroelectric detectors; EEPROM and flash memory replacements; etc.
In a specific aspect, the present invention relates to a Zr-doped BST perovskite crystal material thin film formed by a MOCVD process, having high permittivity, high breakdown strength, low leakage, high-energy storage density, and high dielectric constant tunability. Such Zr-doped BST perovskite crystal material thin film in one particular aspect is characterized by at least one of the following improved dielectric properties:
a breakdown strength of at least 1.3 MV/cm, more preferably at least 1.5 MV/cm;
a leakage current of not more than 1×10
−3
A/cm
2
under applied voltage of about ±3V or above and at temperature of about 100° C. or above, more preferably not more than 1×10
−4
A/cm
2
, and most preferably not more than 1×10
−5
A/cm
2
, under the same applied voltage and temperature conditions; and
an energy storage density, based on volume of dielectric, of at least 15 J/cc, more preferably at least 20 J/cc, and most preferably at least 25 J/cc.
Another specific compositional aspect of the present invention relates to Zr-doped BST perovskite crystal material thin film comprising 0.5% to 50% Zr by total weight of such perovskite crystal material, preferably 2% to 15%, more preferably 4% to 14%, and most preferably 5% to 12%.
High quality thin films with low defect density tend to exhibit superior breakdown strength compared to analogous bulk material. The Zr-doped BST perovskite crystal material thin film of the present invention in a preferred embodiment has a thickness in one of the following ranges: from 150 Å to 10,000 Å; from 150 Å to 5000 Å; from 150 Å to 2500 Å; or from 150 Å to 1000 Å. More preferably such Zr-doped BST perovskite crystal material thin film is about 300 Å to about 700 Å thick, e.g., about 500 Å thick.
Growth temperature of a perovskite crystal material thin film deposited by MOCVD process has a significant impact upon crystalline structure of the thin film that is deposited, which consequently affects dielectric properties of such thin film. Carrying out the MOCVD deposition process under lower growth temperature (e.g., in a range of from about 540° C. to about 560° C.) tends to form films of amorphous or microcrystalline structure, which have relatively lower permittivity and energy storage capacity.
By contrast, carrying out the deposition process under higher growth temperatures (e.g., 600° C.) significantly enhances crystal grain growth, resulting in larger crystal grains with generally longer range order and better aligned crystal lattice structure and correspondingly higher permittivity and energy storage capacity. When the growth temperature is about 660° C., the deposited thin film becomes primarily <110> oriented with the most preferred crystalline structure and dielectric properties.
Thus, a preferred embodiment of the pre

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