Method of manufacturing semiconductor devices in which oxide regions are formed by an oxidation mask disposed directly on a substrate damaged by ion implantation

A method of manufacturing semiconductor devices of the type wherein regions of oxide such as silicon oxide recessed or inset in a silicon substrate are formed by oxidation of the silicon with the use of a masking layer protecting locally against the oxidation. In order to prevent the formation of a projecting oxide beak under the masking layer a nitride oxidation mask is applied directly to the substrate which has been previously ion-implanted to a controlled depth and then annealed to generate a dense dislocation network array on the substrate surface to prevent mechanical stress defects which normally would occur when a nitride mask is applied directly to a substrate.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to the fabrication of semiconductor devices 
with recessed oxide regions and more particularly to a method for applying 
a nitride mask directly on a substrate previously damaged by ion 
implantation and then annealed. 
2. Description of the Prior Art 
It is well known in the fabrication of semiconductors having recessed oxide 
regions which are delineated by masks composed for example of silicon 
nitride, that on growing the thick oxide region a thin protruding oxide 
also grows underneath the silicon nitride oxidation mask. This thin 
protrusion in what will be the gate region is termed "bird's beak" due to 
its beak-like cross-sectional profile and its presence is ascribed to 
lateral diffusion of oxygen underneath the thin silicon dioxide pad which 
is disposed between and separates the silicon nitride mask from the 
surface of the silicon substrate. The thin silicon dioxide pad is used 
because disposing the silicon nitride mask directly on the silicon 
substrate produces stress induced defects and dislocations in the silicon 
substrate which deleteriously affect device performance. Thus, the use of 
the silicon dioxide pad to prevent stress defects results in the undesired 
bird's beak condition. 
Heretofore, attempts have been made to improve the fabrication process by 
continuing to use a separation pad and to minimize the resultant bird's 
beak condition. 
For example, U.S. Pat. No. 3,900,350 issued Aug. 18, 1975 to Appels et al. 
and assigned to U.S. Philips Corporation teaches an approach to reducing 
the bird's beak condition by using a polycrystalline silicon pad under the 
oxidation mask instead of the usual silicon oxide. This patent also 
provides a substantial teaching of the stress defects that will occur when 
the silicon nitride oxidation mask is disposed directly on the silicon 
substrate. 
U.S. Pat. No. 3,961,999 issued June 8, 1976 to Antipov and assigned to IBM 
Corporation also describes a method for minimizing the bird's beak 
problem. In this patent the usual silicon dioxide pad is located between 
the silicon substrate and the silicon nitride layer. The technique taught 
in this patent involves etching holes through the silicon dioxide pad, the 
holes correspond to the openings in the nitride mask to enable the 
undercutting and exposure of the underside of the silicon nitride layer at 
the periphery of the silicon dioxide layer openings. 
The two aforementioned patents are typical of the prior art in that the 
approaches continue to employ a pad between the nitride mask and the 
silicon substrate and then attempt to minimize the resultant bird's beak 
problem that is caused thereby. 
The method of the present invention is unique in that it eliminates the 
need for on the intermediate pad between the mask and the substrate which 
gave rise to the bird's beak problem in the first place. The method of the 
present invention permits the nitride mask to be disposed directly on the 
silicon substrate and eliminates the stress defects which have heretofore 
been caused by this arrangement. The method of its present invention 
includes the steps of initially damaging the surface of the silicon 
substrare by ion implantation to a controlled depth and then annealing to 
generate a dense dislocation network array which prevents the stress 
induced defect propagation from the masking layer. 
It is known that ion implantation of silicon as employed in the present 
invention can be used to harden silicon. The publication of S. M. Hu in 
the IBM Technical Disclosure Bulletin, Vol. 19, No. 2, July 1976 entitled 
"Hardening Silicon Wafers by Ion Implantation" uses such techniques to 
reduce dislocations in thermally stressed silicon wafers. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide an improved method for 
fabricating semiconductor devices wherein the problem of the formation of 
a projecting oxide beak under an oxidation masking layer is eliminated. 
Another object of the present invention is to provide an improved method 
for fabricating semiconductor devices wherein an oxidation masking layer 
is disposed directly on a silicon substrate without resultant stress 
induced defects. 
A futher object of the present invention is to provide an improved method 
for fabricating semiconductor devices wherein a silicon substrate is ion 
implanted and annealed to produce a dense dislocation network array on the 
surface thereof to allow an oxidation mask to be disposed on the substrate 
surface without resultant stress induced defects being produced.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIGS. 1A, 1B and 1C illustrate a cross-section of a silicon substrate, a 
silicon dioxide pad and a silicon nitride mask combination and the manner 
in which a bird's beak occurs. In FIG. 1A the silicon substrate 10 is 
shown having a silicon nitride 14 mask over the desired region. The 
silicon nitride mask 14 is separated from substrate 10 by a silicon 
dioxide pad 12 because the combination of the nitride mask 14 directly on 
substrate 10 will produce stress deformations in the silicon substrate 
which will cause poor device performance. 
In FIG. 1B the recessed oxide 16 is grown in the ion mask region and a 
projecting oxide spur 16a occurs under the mask 14. After oxidation the 
mask 14 is removed as shown in FIG. 1C. Due to the comparatively wide spur 
of silicon oxide 16a along the recessed oxide pattern, a part of the 
beak-like spur of silicon oxide 16a remains upon removing the nitride mask 
14 and the underlying thin oxide layer 12 by the etching process. Such 
remaining part of the spur on bird's beak produces an undesired masking 
effect upon subsequent semiconductor diffusion processes, and may possibly 
even determine the lateral boundary of the diffused zone, in which case 
the semiconductor p-n junction of the zone with the remaining region of 
the originally present material may have curved edges. In later 
semiconductor steps in forming the diffused zone it is even possible that 
the p-n junction could become exposed. 
As previously stated with reference to U.S. Pat. No. 3,900,350, a pad 
composed of polycrystalline silicon instead of silicon dioxide may be 
used. The polycrystalline silicon pad on the monocrystalline silicon 
substrate reduces the stresses caused by the nitride mask and at the same 
time minimizes the bird's beak. This technique still employs the use of a 
relatively thick intermediate pad which must be removed by etching or 
conversion to oxide followed by etching. 
Referring to FIG. 2A, a cross-section of a silicon substrate 10 is shown 
which is ion implanted by beam 18 to a controlled depth d. The ion 
implantation produces a heavily damaged and even amorphous layer 20 on the 
surface 10. Substrate 10 is then annealed and the heavily damaged layer 20 
generates a very dense dislocation network array, the microstructure of 
which depends on the ion energy, ion dose and ion species employed in the 
implantation 
The dense dislocation network array produced by the implantation and 
annealing protects the underlying single crystal silicon from stress 
induced defects, and permits a nitride oxidation mask 14 to be disposed 
directly on substrate 10 as illustrated in FIG. 2B. 
The implanted and annealed surface of substrate 10 is also protected 
against oxidation so that bird's beaking, which results from the lateral 
spur of silicon oxide which is formed by oxidation of the silicon present 
below the mask by lateral diffusion of oxide via the prior art thin 
silicon oxide pad, is not formed. Thus, FIG. 2C illustrates the 
cross-sections of the structure after oxidation, and FIG. 2D illustrates 
the structure after the removal of the nitride oxidation mask and, if 
necessary, the dense dislocation network 20 wherein the bird's beak is not 
present. 
A specific example of the fabrication of a particular embodiment is 
provided as follows: Step (1) Implantation of Ar into the bare-silicon 
substrate 10 with an implantation energy of 20KeV at a total dose of 
approximately 10.sup.15 cm.sup.-2 to form layer 20 which then may be 
annealed by conventional techniques; Step (2) Low temperature deposition 
of Si.sub.3 N.sub.4 to a thickness in the range of 300 to 1000 Angstroms 
to form layer 14; Step (3) Patterning of the Si.sub.3 N.sub.4 oxidation 
mask by conventional (i.e. photolithographic) procedures; Step (4) 
Isolation oxidation, for example, 6500 Angstroms dry-wet-dry; Step (5) 
Removal of the Si.sub.3 N.sub.4 oxidation mask by conventional process 
such as buffered HF + hot H.sub.3 PO.sub.4 ; Step (6) Removal of the 
damaged Si layer, if necessary, by either etching of the Si (about 500 
Angstroms) or by oxidation of the damaged region (growth of about 1000 
Angstroms SiO.sub.2) followed by etching off the oxide. 
It is possible to perform Step (1) after Step (2) provided that the ion 
energy is increased sufficiently to carry out implantation through the 
nitride layer. The choice of the ion species is determined mainly by the 
fact that it should, in nearly all cases, not be electrically active in 
silicon. For example, Si, Ge, Ar, Ne and O are possible candidates. 
The ion energy controls the depth of the damaged region. The ion dose 
should be close to the critical dose for the formation of a continuous 
amorphous layer in silicon, for example, 5 .times. 10.sup.14 to 10 .times. 
10.sup.14 ions per cm.sup.-2 for Ar in Si. Also, Step (6) may not be 
required in fabricating bipolar devices but is necessary in MOSFET 
construction. 
What has been described is an improved method for fabricating semiconductor 
devices wherein the condition known as "bird beaking" is eliminated by 
treating the semiconductor substrate by ion implantation in order to 
generate a dislocation network, a nitride mask may be directly applied to 
the resultant surface of the semiconductor substrate without the need of 
an intermediate silicon dioxide pad. 
While the invention has been particularly shown and described with 
reference to preferred embodiments thereof, it will be understood by those 
skilled in the art that the foregoing and other changes in form and 
details may be made therein without departing from the spirit and scope of 
the invention.