SOI transistor threshold optimization by use of gate oxide having positive charge

Threshold optimization for SOI transistors is achieved through the formation of a layer of positive charge within the gate to correspond to the positive polarity formed in the substrate by ion implantation for threshold voltage control. A positive charge layer is formed by furnishing sulfur ions on the substrate before growth of an oxide to form a portion of the gate oxide. The sulfur will form a charge layer on the surface of the oxide, and an additional oxide is then deposited on the same to form the gate oxide as a sandwich with the positive charge layer in the same.

BACKGROUND OF THE INVENTION 
The present invention relates to integrated circuitry and, more 
particularly, to affecting the threshold voltage of integrated circuit 
devices. 
Silicon-on-insulator (SOI) technology has been developed for forming 
transistors and other electrical devices in integrated circuitry. An SOI 
differs from more common bulk semiconductors in that the active region 
(the region in which the semiconductor electrical devices are formed) is a 
semiconductor layer separated from a semiconductor base by a dielectric 
region. If the semiconductor is silicon as is common, the dielectric 
typically is silicon dioxide. The active region of the SOI then becomes 
the layer of silicon which exists between the dielectric (or, in other 
words, the insulator) and the surface. 
Use of SOI technology has the advantage of simplifying isolation of various 
electrical devices. Moreover, SOI transistors have low parasitic 
capacitance, low leakage currents, soft error immunity, etc. However, to 
obtain the full advantage of SOI, it is desirable that if the device is a 
transistor, it be fully depleting, i.e., that surface depletion extend 
through the active region to the dielectric region. Full depletion has 
many advantages, including providing higher mobility for the passage of 
charge, and it provides integrated circuitry having continuous 
current/voltage characteristics. 
One problem with reducing the thickness of the active region in line with 
integrating semiconductor devices, is that the doping concentration 
necessary for threshold control of transistors formed on the active region 
may prevent full depletion. In this connection, the factor that governs 
the extent to which the surface depletion level extends into the active 
region is the concentration of dopants in such region. The high 
concentration which has been believed to be necessary to provide a desired 
threshold voltage prevents full depletion when the transistor gate oxide 
is thin. For example, if the oxide has a thickness of 50 .ANG. units and 
it is desired that the threshold voltage be maintained at 0.5 V, the 
doping concentration for such threshold voltage is about 3.times.10.sup.17 
/cm.sup.3. With this thickness, a fully depleted device requires for the 
transistor to be fully depleting an active region thickness of the order 
of 500 .ANG.. Such a thickness might not be compatible with the 
optimization of devices as the function of the devices would have a 
thickness of 500 .ANG. and their resistance will be high. If thick active 
regions are used, many of the advantages of silicon-on-insulator (SOI) 
transistors are lost. 
SUMMARY OF THE INVENTION 
The present invention reduces the threshold voltage implantation which must 
be provided in an active region to control the operation of a device 
requiring a positive threshold voltage. It does this by incorporating a 
positive charge within the gate oxide of the device. Thus, from the broad 
standpoint, the method of the invention includes the step of intentionally 
providing a positive charge within a gate oxide of an integrated circuit 
device. It is particularly applicable to silicon-on-insulator (SOI) 
devices. As mentioned previously, to obtain the full advantage of SOI it 
is necessary that the active region be fully depleting. The incorporation 
within the gate oxide of a positive charge when the threshold voltage 
implant polarity is positive, alleviates the necessity of incorporating a 
large concentration of positive charge within the active region. The 
invention includes steps that have been discovered to enable a positive 
charge to be incorporated easily within the gate oxide. The invention also 
includes the product resulting from the method. 
Other features and advantages of the invention either will become apparent 
or will be described in connection with the following, more detailed 
description of preferred embodiments of the invention.

DETAILED DESCRIPTION OF THE INVENTION 
The following relatively detailed description is provided to satisfy the 
patent statutes. However, it will be appreciated by those skilled in the 
art that various changes and modifications can be made without departing 
from the invention. 
A portion of a silicon-on-insulator (SOI) transistor is generally referred 
to in FIG. 1 by the reference numeral 11. Within this portion a positive 
charge layer of the invention is formed within the gate oxide. With 
reference to such figure, a silicon-on-insulator substrate 12 is partly 
illustrated having both a gate oxide 14 and a polysilicon gate 16 of a 
transistor thereon. While the SOI substrate can be formed by any 
convenient mode utilized in the industry, in submicron technology such a 
substrate often is formed by ion implantation of an oxygen species, 
followed by thermal annealing to produce a buried oxide layer represented 
at 17. In this particular embodiment the silicon wafer is of p-type 
silicon as is illustrated. 
As mentioned previously, although silicon-on-insulator transistors have 
been developed in the past, it is desirable that the SOI material in the 
transistor active region be fully depleted so that one can obtain the full 
advantages of SOI technology. Means also typically are provided to permit 
a positive threshold voltage to be established. The common approach is to 
use ion implantation to provide ions of the appropriate charge at the 
surface of a substrate. The difficulty with this approach with 
silicon-on-insulator technology is that as the thickness of the gate oxide 
is significantly reduced, the implantation dosage required to permit a 
typical positive threshold voltage to be attained, is quite high. If such 
a high dosage implantation is used, the SOI devices will be prevented from 
becoming fully depleted. 
The present invention tackles the above problem by incorporating a positive 
charge in the gate oxide of a transistor to reduce (or in the extreme case 
entirely eliminate) the ion implantation dosage required for the 
establishment of a threshold voltage. To this end, the gate oxide is 
provided as two layers 18 and 19 between which a positively charged layer 
21 (see FIG. 2) is provided. This construction results in a positive 
charge being not only inside the gate oxide but also close to the 
substrate-gate oxide interface. That is, it is spaced sufficiently far 
from the gate 16 to act to provide the capacitance necessary for threshold 
voltage. 
It is to be noted that it is not intended that the positive charge layer 
provide all of the charge necessary for establishing a threshold voltage. 
Rather, standard ion implantation in the substrate is combined with the 
effect of the positive charge layer to provide threshold voltage. This ion 
implantation for threshold voltage purposes is indicated in FIGS. 1 and 2 
by dots 22. 
The required dosage concentration for the ion implantation to achieve 
threshold control is significantly reduced in view of the provision of the 
positive charge layer. For example, when the charge layer is provided with 
a concentration of positive charge equal to about 1.5.times.10.sup.12 
atoms/cm.sup.2 and the doping concentration in the silicon need only be 
about 5.times.10.sup.16 atoms/cm.sup.3 for a threshold voltage of 0.5 
volts. This enables the thickness of the silicon to be reduced to 0.1 
.mu.m and yet be fully depleting when the gate oxide is about 65 .ANG. 
thick. 
Positive charge layer 21 can be formed in various ways. It can be formed by 
sulfur ions. They can be introduced into the gate oxide by treating the 
substrate with an aqueous H.sub.2 SO.sub.4 /H.sub.2 O.sub.2 solution 
following the traditional pre-oxidation cleaning procedures. An oxide 
layer 19 is thermally grown in a conventional manner. The sulfur and, 
hence, the positive charge will be concentrated at its surface. The oxide 
layer 19 then can be formed via conventional deposition. 
The positive charge is quite stable and does not degrade over time. In 
addition, subsequent high temperature oxidations and/or anneals as the 
integrated circuitry is being formed do not reduce the extent of the 
positive charge. The inclusion of the positive charge reduces the ion 
implantation concentration necessary to enable the desired threshold 
voltage to be established. 
As mentioned at the beginning of the detailed description, applicants are 
not limited to the specific embodiment described above. Various changes 
and modifications can be made. The claims, their equivalents and their 
equivalent language define the scope of protection.