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

DETAILED DESCRIPTION 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