Method of making barium strontium titanate (BST) thin film by erbium donor doping

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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 miniature 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 traction 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. Although BST materials have been manufactured in bulk form previously, the properties of such materials are not yet well understood when formed as a thin film (generally less than 5 .mu.m), i.e., on a semiconducting device. It is known that the dielectric constant of undoped bulk BST is maximized for median grain sizes roughly between 0.7 .mu.m and 1.0 .mu.m, and that for smaller grain sizes, dielectric constant fails off rapidly (thus BST with extremely small grains is usually undesirable). Unfortunately, BST applications in submicron microcircuits (e.g. DRAM capacitors) may place particular constraints on BST grain size. First, annealing temperature for BST thin films must generally be kept far below temperatures commonly used for sintering bulk BST ceramics (generally less than 700 C. vs typically greater than 1100 C. for bulk BST) to avoid damage to underlying device structure, thus limiting grain nucleation and growth kinetics. Second, desired film thickness may be much less than 5 .mu.m (preferably between 0.05 .mu.m and 0.1 .mu.m), and it has been found that median grains sizes generally less than half the BST film thickness are required to control dielectric uniformity, e.g., across a large number of capacitors, and avoid shorted capacitors.

A BST grain in the 0.7 .mu.m to 1.0 .mu.m size range constitutes at least several billion unit cells connected in a perovskite crystal structure; a small BST grain of 0.01 .mu.m size may only contain a few thousand. The larger grains typically have less than 0.5% of their unit cells lying on the outer shell of the grain, while the smaller grains may have 25% or more of their unit cells on the outer shell. It is believed that small grains consequently suffer from much larger lattice distortion and sensitivity to grain boundary composition. Additionally, the degree of polarization depends on BST grain sizes, crystallinity, and composition and the effects of adding BST dopants is difficult to predict for such grain sizes as it may not follow what is known for bulk ceramics.

It has now been found that erbium may be useful as a dopant for thin films comprised of small (10 nm to 50 nm median size) grains. We now believe that the solubility of erbium in thin films annealed at temperatures generally less than 700 C. may be much higher than for bulk ceramics. Several attributes of such films differ from ceramics: grain size is much smaller and grain surface area much larger; films form in a brief time, at low temperature, such that ionic diffusion is limited and thermal equilibrium may not be established; and high stresses occur due to mismatch with the substrate. Surprisingly, several percent erbium dopant has now been found to advantageously lower dielectric leakage for such a small-grained BST film. For example, erbium acetate may be added to a liquid precursor for metal-organic decomposition (MOD) to form a BST film. This technique appears readily adaptable to many forms of BST deposition such as: MOD using spin-on precursors, MOD using vapor phase transportation, MOD chemical vapor deposition, sol-gel, and sputtering.

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 erbium, titanium, and at least one of barium and/or strontium, preferably with the molar ratio of erbiu