Coating processes – Electrical product produced – Condenser or capacitor
Reexamination Certificate
1994-09-30
2001-12-18
Talbot, Brian K. (Department: 1762)
Coating processes
Electrical product produced
Condenser or capacitor
C427S080000, C427S126300, C029S025030, C029S025410
Reexamination Certificate
active
06331325
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to high dielectric constant thin films for microelectronic devices, and more particularly to improving dielectric properties of such films.
BACKGROUND OF THE INVENTION
Dynamic random-access memory (DRAM) integrated circuits, for example, are commonly used for storing data in a digital computer. Currently available DRAMs may contain over 16 million memory cells fabricated on a single crystal silicon chip, each memory cell generally comprising a single transistor connected to a tiny capacitor. In operation, each capacitor may be individually charged or discharged in order to “store” one bit on information. A DRAM is dynamic in the sense that charged memory cells must be refreshed, or recharged, periodically to maintain data integrity; otherwise, charged memory cells may quickly (generally in a fraction of a second) discharge through leakage to a point where they no longer appear to be set to the charged state.
To facilitate construction of 64 Mbit, 256 Mbit, 1 Gbit, and larger DRAMs with correspondingly smaller memory cells, capacitor structures and materials which can store the necessary charge in less chip space are needed; one of the most promising avenues of research is in the area of high dielectric constant materials (defined herein as having dielectric constants greater than 50). Lead zirconate titanate (PZT), barium titanate, strontium titanate, and barium strontium titanate are some common examples of such materials. It is desirable that such a material, if used for DRAMs and other microelectronics applications, be formable over an electrode and underlying structure (without significant harm to either), have low leakage current characteristics and long dielectric lifetime, and, for most applications, possess a high dielectric constant at frequencies of hundreds of MHz up to several GHz.
SUMMARY OF THE INVENTION
The present invention relates to a method of producing and a structure containing barium and/or strontium titanate dielectric films (hereafter referred to as BST) with improved properties. It is now believed that such a film may be fabricated using boron as an additive to effect such improvements. For example, boric acid may be added to a liquid precursor for metal-organic decomposition (MOD) to form a BST film. It has been discovered that the boric acid apparently does not negatively affect the deposition properties of such MOD BST, and that the boron remains virtually insoluble in the BST grains as they form. Generally, in the final film structure, boric oxide (B
2
O
3
) exists in a second phase, or boundary region, between the BST grains (which form the first phase).
It is believed that boric oxide residing in such a second phase impedes charge leakage (i.e. leakage current) along the BST grain boundaries when a voltage is applied across the dielectric film. This may not be the only explanation for lowered leakage current: compensation of vacancies or space charge by dopants may decrease tunneling leakage while retaining a high Schottky barrier height. Also, there is some evidence that Schottky emission may dominate leakage in the high voltage regime, e.g. above 2 V, while relaxation polarization contributes to leakage at lower voltages.
Apparently, boric oxide formation, in the precursor or during annealing, may also be beneficial in reducing temperature requirements and stress during annealing (during which time a boric oxide second phase would most likely by mobile, given that the melting point of B
2
O
3
is 450 C) of a BST film. Also, the use of boron in a semiconductor fabrication process is well understood, and boron addition appears readily adaptable to many forms of BST deposition such as: MOD using spin-on precursors, MOD using vapor phase transportation, and sol-gel techniques.
Consequently, the present invention includes a novel method of forming a barium and/or strontium titanate dielectric film on a microelectronic device. In this method, a precursor is prepared by combining compounds of the elements boron, titanium, and at least one of barium and/or strontium, preferably with the molar ratio of boron to titanium in the precursor being between 0.001 and 0.1. One or more layers of such a precursor may be deposited and densified to form a precursor film on the device. The precursor film may subsequently be annealed at a predetermined temperature (generally above 450 C, and preferably between 600 C and 800 C) in an oxygen-containing atmosphere. This annealing forms, substantially uniformly distributed, a two-phase dielectric film: the first phase contains a plurality of BST grains; the second phase principally comprises boron oxide or boron oxide and titanium oxide (the second phase is believed to be generally amorphous).
The precursor may, for example, be prepared by combining barium acetate, strontium acetate, titanium ammonium lactate, and boric acid in a common aqueous solution. Other organic solvents may be added to the precursor to, for example, adjust the viscosity. The BST grains produced may be selected over a wide range of barium-strontium combinations (with between 40% and 70% of barium-strontium lattice sites in the grains occupied by barium being preferred). The dielectric film may be purposely constructed to be titanium rich; i.e., only 93% to 99.9% of the titanium in the precursor may be required to produce stoichiometric BST; it is believed that the remainder exists in the boundary regions as oxidized titanium.
The present invention also comprises a method of forming a capacitive structure on a microelectronic device; such a capacitive structure generally having a dielectric laminate disposed between a first electrode and a second electrode. The method may comprise forming a first electrode on a substrate. The method further comprises depositing two or more films over the first electrode, with each film comprising titanium and at least one of barium and strontium. Additionally, at least one of the films further comprises boron, preferably in a ratio to titanium of at least 0.001. The method further comprises annealing the films at a temperature above the melting point of B
2
O
3
in an oxygen-containing atmosphere. This is believed to form a dielectric laminate comprising a plurality of grains with a perovskite crystal structure, each grain comprising titanium, oxygen, and at least one of barium and strontium; furthermore, essentially all of the boron exists in boundary regions between the grains as B
2
O
3
. The method may further comprise forming a second electrode over the dielectric laminate.
In accordance with the present invention, a capacitive structure on a microelectronic device may be formed. In this structure, a dielectric laminate is dispersed between first and second electrodes. Generally, this dielectric laminate comprises two or more grains (and typically a large number of grains) having a perovskite crystal structure, each grain comprising titanium, oxygen, and at least one of barium and strontium. This laminate may further comprise B
2
O
3
in boundary regions between the grains in at least a sublayer of the laminate. Preferably, such a sublayer has a ratio of boron to titanium of between 0.001 and 0.1.
The grains may further comprise dopant material (acceptor, donor, or two dopants, an acceptor and a donor) replacing at least 200 ppm of the barium, strontium, or titanium in the perovskite crystal structure. Preferably, the grains in the laminate have a median size of between 10 nm and 50 nm. Also, the structure may be made titanium-rich as previously detailed.
REFERENCES:
patent: 4379854 (1983-04-01), Soong
patent: 4408254 (1983-10-01), Chu et al.
patent: 4447549 (1984-05-01), Masujima et al.
patent: 4485094 (1984-11-01), Pebler et al.
patent: 4863883 (1989-09-01), Menashi et al.
patent: 4959089 (1990-09-01), Bhargaua et al.
patent: 4963390 (1990-10-01), Lipeles et al.
patent: 4988650 (1991-01-01), Takagi et al.
patent: 5160762 (1992-11-01), Brand et al.
patent: 5292694 (1994-03-01), Abe et al.
patent: 5368834 (1994-11-01), Kulwicki et al.
patent: 5378667 (1995-01-01)
Kulwicki Bernard M.
Tsu Robert
Brady III Wade James
Talbot Brian K.
Telecky , Jr. Frederick J.
Texas Instruments Incorporated
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