Implantation profile control with surface sputtering

An ion implantation process for producing a buried insulating layer of silicon dioxide in a silicon substrate which takes advantage of the effects of surface erosion and sputtering inherent to the ion implantation process. The process allows the production of an insulating layer buried within a silicon semiconductor wherein the width of the insulating layer can be contoured by controlling the beam energy during implantation.

FIELD OF THE INVENTION 
This invention relates to a process used in semiconductor production and, 
more particularly, relates to a process for contouring an insulating layer 
formed in a semiconductor substrate by ion implantation. 
BACKGROUND OF THE INVENTION 
Ion implantation is a process in which atomic particles are introduced into 
a substrate for the purpose of changing the electrical or chemical 
properties of the substrate. This process uses high energies to accelerate 
ions which enter the surface and are slowed down by electronic or nuclear 
collisions with the substrate atoms, and come to rest some small distance 
below the surface. Modern semiconductor technology is one field in which 
ion implantation is particularly useful and wherein implanted ions are 
used to alter the conductivity of the base material as well as to form 
buried insulating layers. One object, in the case of buried insulating 
layers, is the minimization of the volume of electrically active 
semiconductor material to reduce parasitic effects such as 
device-to-device effects (known as latch-up), leakage capacitance, 
resistance, etc. and to minimize sensitivity to radiation. 
In the early 1980's, a process known as separation by implanted oxygen or 
SIMOX was developed in which a high-dose of oxygen ions are implanted into 
a solid monocrystalline silicon substrate, making it possible to form a 
buried layer of silicon dioxide SiO.sub.2). The resultant layer 
dielectrically isolates circuit elements, enabling the fabrication of 
smaller, closer and faster circuits which are immune to the noted 
parasitic and radiation effects which cause latch-up and add to circuit 
capacitance. 
Using the SIMOX process, oxygen ions are implanted into silicon at a 
constant beam energy between 150 and 200 keV at a dose of approximately 
1.6 .times. 10.sup.18 ions/cm.sup.2. After implantation, the material is 
annealed to form the chemically bonded silicon dioxide. A typical anneal 
cycle involves heating the substrate to approximately 1300 degrees for six 
hours. This annealing phase redistributes the oxygen ions which are 
implanted in a roughly Gaussian profile with respect to the most probable 
depth (range) such that the silicon/silicon dioxide boundary on either 
side of the silicon dioxide layer becomes markedly more abrupt, thus 
forming a sharp and well-defined region centered at the most probable 
depth. 
During implantation, incoming high-speed oxygen ions sputter silicon ions 
from the surface resulting in surface erosion. Due to the effect of this 
surface erosion, the original surface of the silicon layer is displaced, 
causing ions implanted toward the end of the implant cycle to come to rest 
at a depth deeper than ions implanted at the start of the implant cycle. 
In practice, this erosion effect is magnified when a surface layer of 
silicon dioxide is provided to protect surface features because silicon 
dioxide erodes more rapidly than silicon during implantation. As a result 
of the variation in implantation depth of oxygen ions, a broader band of 
silicon oxide layer is created. This, in turn, increases the minimal 
acceptable dose of oxygen ions required to create the silicon dioxide 
layer, thereby causing even greater surface damage and longer processing 
times. 
SUMMARY OF THE INVENTION 
The present invention relates to a process for producing a buried 
insulating layer, typically of silicon dioxide (SiO.sub.2) or silicon 
nitride (Si.sub.3 N.sub.4), within a silicon substrate in a manner which 
advantageously regulates the implantation energies in response to the 
surface erosion to produce a thinner, low dosage insulating layer suitable 
for typical low voltage use, or a thicker, more even layer in a 
semiconductor substrate wherein the thickness of the buried insulative 
layer is determined by controlling the beam energy used to implant ions 
into the silicon layer as a function of surface erosion. The silicon layer 
erodes at a rate which is dependent on the density of the ions being 
implanted into the silicon. The peak ion distribution is maintained at a 
constant position by reducing the beam energy and corresponding 
penetration by an amount corresponding to the depth of erosion. This more 
rapidly achieves the ion density required to fully form an oxide barrier 
with less exposure of the substrate to ion implantation effects. The oxide 
barrier thus formed, while thinner, provides protection for most 
semiconductor applications which are typically low voltage applications. 
Conversely, by increasing the beam energy over the implantation period, the 
peak of the ion implantation distribution occurs at progressively deeper 
depths, advantageously benefited by the surface erosion which allows the 
distribution to shift naturally. In this case, the oxide barrier is formed 
over a greater depth to provide additional insulative protection in high 
voltage applications. 
The process for producing a buried insulating layer according to the 
present invention is advantageous for producing an active component of an 
integrated circuit positioned on crystalline silicon material above an 
insulating layer. This technique produces a radiation hardened material 
and also dielectrically isolates circuit elements enabling smaller, closer 
and faster circuits to be fabricated with a marked reduction in stray 
capacitance and an increase in the operating speed of the circuits. 
Additionally, the material shows great promise for mixed application such 
as BI-CMOS circuits which combine power and logic on the same chip.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention contemplates energy control during oxygen or nitrogen 
ion implantation to produce a narrowed or a widened insulating layer in a 
silicon substrate. According to the prior art, as shown in FIG. 1A, an 
initially uniform silicon layer 10 is bombarded with oxygen ions generated 
by a beam source 16 wherein the oxygen ions travel through the silicon 
crystal structure 10 and come to rest in the crystal matrix of silicon 
atoms. Since the purpose of the implantation process is to create a buried 
layer of silicon dioxide (SiO.sub.2), two oxygen atoms are implanted for 
each silicon atom to achieve the appropriate chemical structure. The 
oxygen ions come to rest as a broadened band 14 buried in the silicon in a 
roughly Gaussian distribution with respect to depth where the depth is 
dependent upon the ion beam energy used to project the oxygen atoms into 
the silicon layer. Annealing subsequently forms a silicon dioxide layer 
having sharpened boundaries. 
FIG. 1B shows the surface erosion resulting from the oxygen implantation on 
the silicon layer 10. The dotted line 18 represents the original top face 
of the silicon layer before ion implantation. As ions are bombarded 
against the surface of the silicon, the top face 18 of the silicon layer 
12 is eroded or sputtered such that the top of the silicon layer 
progressively retreats toward a lower level 20. The effect of the erosion 
is that ions implanted in the course of an implant cycle come to rest at 
progressively deeper depths within the silicon layer and farther from the 
original top face 18 than the ions implanted at the start of the implant 
cycle. 
FIG. 1B also shows an additional characteristic of the prior art ion 
implantation process in that small islands 22 of silicon dioxide may be 
formed in layer 14 and just above due to ions which are deposited in the 
tail of the Gaussian distribution to form a chemical bond outside of the 
sharpened Gaussian distribution formed during annealing. This is an 
undesirable phenomenon which alters the electrical conductivity 
characteristics of the silicon semiconductor layer. 
FIG. 1C shows the broadened Gaussian distribution of oxygen ions that would 
result from an ion implantation using constant. Under this prior art 
arrangement, it can be seen that the ion concentration is distributed over 
a broad Gaussian depth distribution 30. 
In summary, the net effect of this surface erosion is that the distribution 
of ions is spread over a wide range which causes the buried silicon 
dioxide layer to be thicker and also creates islands of silicon dioxide 
deposits outside of the insulating layer to be formed. Additionally, the 
minimum acceptable dose of ions necessary to create a quality insulating 
layer is high since the chemical bonding of the insulating layer must be 
achieved over a broader range. 
The present invention is adapted to controllably reduce or increase the 
beam energy during ion implantation. Such controlled variation in beam 
energy permits specific contouring of the width of the buried insulator 
layer band. Moreover, by compensating for the known effects of surface 
erosion, greater regulation of the implantation process is achieved. 
FIG. 2A is a diagramatic sectional view of a first embodiment of the 
present invention wherein the applied beam energy from source 16 is 
controllably decreased by an energy control 17 to decrease the 
implantation depth such that the peak of the ion distribution is 
maintained at the same depth despite the existence of surface erosion. 
Thus, after annealing, a buried 9 silicon dioxide layer 24 forms as a thin 
strata of insulating material which dielectrically separates the former 
silicon semiconductor 10 into two newly formed layers 28 and 26. The 
energy decrease with time and ion sputtering is shown in FIG. 2B. 
FIG. 3A shows the effect of reducing beam energy with time, as surface 18 
is eroded to a surface 20'. In FIG. 3B a three dimensional graph shows the 
roughly Gaussian distribution profiles 34a, b, c, d which represent the 
ion concentration at different times as the beam energy is decreased 
during the implantation cycle. By using ion beam energy control to perform 
this energy reduction, it can be seen that the peak ion concentration can 
be limited to a very narrow range 32 within the silicon substrate as the 
surface 18 gradually recedes due to erosion thereof. 
As an illustration of the first embodiment of the present invention, a beam 
energy of 150 keV will erode silicon at a rate of approximately 
34/10.sup.17 ions/cm.sup.2. Further, it is inherent in this ion 
implantation process that ions are implanted at a peak depth of 
approximately 19 A/keV. Thus, the depth of the peak of the ion 
distribution can be maintained at a constant level by reducing the implant 
voltage by approximately 1.8 keV/10.sup.17 ions/cm.sup.2 applied. For 
example, if a normal dose of ions is approximately 1.6 .times.10.sup.18 
ions/cm.sup.2, a controlled reduction of approximately 28.8 keV over the 
period of implantation will result in a constant peak depth distribution 
at a level approximately 2850 A below the original top face of the silicon 
layer. This embodiment also allows processing time to be reduced from 
approximately five hours to approximately four hours. 
In a second embodiment of the present invention, as shown in FIG. 4A, and 
using the same relationships regarding erosion rate and peak depth, the 
process described above is altered to form a thicker silicon dioxide band 
40 by controllably increasing the beam energy used to implant ions. An 
increase in the beam energy, as illustrated in FIG. 4B, advantageously 
positions the peak distribution of ions at increasingly greater depths due 
to the combined effects of the increase in beam energy plus the surface 
erosion. For example, by increasing the beam energy from 150 keV to 170 
keV over the course of an implantation cycle, the peak distribution depth 
of the implanted ions is increased from a level 2850 A below the original 
top face of the silicon layer to a level 3230.degree. A below the eroded 
top face of the silicon layer. At a dose of 1.6 .times.10.sup.18 
ions/cm.sup.2, this eroded top face sits at a level 544 A lower than the 
original top face. Thus, the peak distribution of ions lies in a band 
ranging from 2850 A to 3774 A below the original top face of the silicon 
layer. 
This second embodiment of the invention is further illustrated in FIG. 5 
which shows the progressively deeper distribution curves in which the ions 
are implanted. This creates a far wider range 36 of distribution and a far 
wider insulating layer. This wide insulating layer supports a higher 
voltage for high voltage applications. 
While the invention has been illustrated for oxygen implantation, it is to 
be understood that nitrogen or other implantable ions may be substituted. 
It will further be appreciated that the embodiments described are 
illustrative only and are not to limit the invention, the scope of which 
is defined only in the following claims.