Method for Manufacturing Semiconductor Devices Having Oxide Films and the Semiconductor Devices Manufactured Thereby

This invention relates to a method for manufacturing a semiconductor device having an oxide film on the surface of a semiconductor body, and to the semiconductor device manufactured thereby.

In general, silicon crystal bodies having major surfaces lying parallel to a {111} crystal plane have been widely used in manufacturing semiconductor devices in which the major surfaces of the silicon bodies are covered with oxide films of insulator material which is moisture-resistant and is chemically stable such as, for example, of silicon dioxide SiO.sub.2. A planar transistor in which diffused regions are formed in the major surface of the body lying parallel to a {111} crystal plane covered with a passivating film is an example of such semiconductor devices.

In the case where the above-mentioned silicon dioxide film is formed on the surface of a semiconductor substrate, a donor-type surface charge appears in the surface of the semiconductor substrate immediately below the silicon dioxide film, irrespective of the conductivity type of the substrate. This phenomenon is sometimes called as the channel effect. Accordingly, it has been proposed to produce field effect transistors by utilizing the channel effect.

A large amount of the donor-type induced surface charge, however, usually gives disadvantages to semiconductor devices, such as planar transistors and MOS field effect transistors. For example, the N-type channel or inversion layer appeared in a surface of a P-type silicon substrate due to the induced surface charge gives rise to adverse results such as increase in the collecor cut-off current I.sub.co of the planar transistor. Further, due to the large amount of the induced surface charge, an MOS field effect transistor having a high mutual conductance g.sub.m can hardly be obtained. Furthermore, the large amount of the donor-type surface charge within the channel layer means that in a field effect transistor the drain current at the time of zero gate voltage has a certain large value.

For reducing the amount of the induced surface charge, a heat treatment combined with application of voltage accross the oxide film can be applied to such semiconductor devices. However, even by this heat treatment, there has been a limit to the amount of controllable surface charge, that is, a limit below which the amount of the donor-type surface charge cannot be reduced.

It is therefore an object of this invention to provide semiconductor devices in which the amount of the donor-type surface charge induced in the semiconductor surfaces by the existence of the oxide films is desirably reduced, and to provide the method for manufacturing the same.

It is another object of this invention to provide a method for producing a planar transistor having a low value of the collector cutoff current I.sub.co.

It is a further object of this invention to provide a method for manufacturing an MOS field effect transistor having the high value of the mutual conductance.

It is a still further object of this invention to provide a method for manufacturing an MOS field effect transistor having high amplifying gain.

It is a still further object of this invention to provide a method for manufacturing an MOS field effect transistor having the steep rising slopes of the current-voltage curves in the low drain voltage range, that is, for manufacturing an MOS field effect transistor of high sensitivity for small signals.

According to one embodiment of the present invention an MOS-type field effect transistor is manufactured by forming an oxide film such as of silicon oxide on the surface of a silicon semiconductor crystal body, which is oriented to a crystal plane lying substantially parallel to a {100} or {110} plane and forming a gate electrode covering the oxide film. The combination may be subjected to heat treatment in the state of application of a voltage between the semiconductor crystal body and the gate electrode on the oxide film.

The nature, principle, and details of the invention will be more clearly apparent from the following detailed description when read in conjunction with the accompanying drawings in which like parts are designated by like reference characters.

FIG. 1 is a perspective view including a part in section for a description of the principle of the invention;

FIGS. 2a, 2b and 2c are graphical representations indicating conductance vs. gate voltage characteristics of MOS field effect transistors manufactured according to the present invention and according to the prior art.

FIGS. 3 and 4 are graphical representations indicating the drain current vs. drain voltage characteristics of MOS field effect transistors manufactured in accordance with the prior art method any by the method according to the invention, respectively.

FIG. 5 is a sectional view of a planar transistor manufactured by the method according to this invention.

The principle of the invention will be explained with reference to FIGS. 1 and 2a to 2c.

A plurality of MOS field effect transistors are manufactured by using three sorts of silicon crystalline semiconductor bodies having major surfaces lying parallel to a {100}, a a {110} and a {111} plane, respectively. Each sample transistor has, as shown in FIG. 1, a P conductivity-type silicon crystalline substrate 1 of 100 ohm cm, a silicon dioxide film 2 approximately 1,500 angstroms thick grown on the substrate 1, and a gate electrode 13 of aluminum deposited on the film 2 by a conventional evaporation method. A channel layer 4 of a donor-type surface charge or of surface donors is induced in the surface of the semiconductor substrate 1. In addition, N-type regions 5 of 1,600 micron length and approximately 10 micron depth are formed with a spacing therebetween of 7 microns in the surface of the crystal substrate 1 and are provided respectively with a source electrode 6 and a drain electrode 7. The N-type regions 5 are formed for providing ohmic contacts to the channel 4.

Then, measurements of the electric characteristics such as the density N.sub.DS of surface donors and the conductance G are effected with the sample devices having such structures. The conductance G is measured between the terminals 6 and 7. This conductance may be, in general, expressed by the following equation. ##EQU1## where

q is the electric charge which an electron has;

N.sub.DS is the density of the induced surface donors;

Q is the charge applied to electrode 13 by the outside biasing means; and

.mu.d is the surface electron mobility.

When Q = 0, the above equation becomes ##EQU2## the conductance G is proportional to the density N.sub.DS of the surface donors.

Where a voltage V.sub.G having such a voltage value V.sub.GO as to induce the charge

is applied to the gate electrode 13, the conductance G will become equal to zero. On the other hand,

where C.sub.G is the capacitance of the gate input.

From Equation (3), it is possible to determine the value of Q. Now considering the case of Q = q.sup.. N.sub.DS and Q = V.sub.GO.sup.. C.sub.G , the density N.sub.DS of the induced surface donors can be expressed by the following equation.

Using the relation shown in the equation (4), the density of the induced surface donors can be calculated in accordance with the results obtained by measuring the applied gate voltage V.sub.GO and the capacitance C.sub.G.

Before effecting measurements, each sample device is subjected to the above-mentioned heat treatment in order to decrease deviations in measured values among the same type of samples. A DC voltage of 5 volts is applied between the gate electrode 13 and electrode 6 (or the drain electrode 7) with the positive polarity applied to the source electrode 6 (or electrode 7). Then, as this voltage is applied, each sample transistor is heat-treated at 350.degree. C for 2 hours, whereupon the density of induced surface donors in the channel layer 4 is decreased remarkably relative to that prior to treatment and reached a minimum density value of surface donors.

The relationship between the gate voltage V.sub.G (V) and the conductance G (m ) measured between the source and the drain electrode of each sample field effect transistor after treatment in the above-described manner are graphically indicated in FIGS. 2a to 2c which show characteristic curves for transistors in which the {100}, {110}, and {111} crystal planes are used, respectively. The results, including the relationship in connection with surface electron mobility, may be represented as shown in the accompanying Table 1.

As is apparent from Table 1 the values of gate voltage V.sub.GO corresponding to the case of G = 0 become smaller in the order of {111}, {110}, and {100} planes. Since this voltage V.sub.GO is proportional to the density N.sub.DS of surface donors as can be observed from Equation (4), a small value of V.sub.GO means a small value of N.sub.DS. Therefore, it is apparent that the density N.sub.DS of surface donors in the channel layers 4 becomes smaller in the order of the above-mentioned three kinds of crystal planes.

Furthermore, a high electron mobility .mu.d means a large conductance variation with respect to gate voltage variation, that is, a high sensitivity of voltage. The high value of election mobility is advantageous particularly for MOS-type field effect transistors.

The present invention, which is based on the above considerations, is principally characterized in that a silicon crystalline substrate having a major surface lying parallel to a crystal plane other than a {111} plane, particularly to a {100} or a {110} crystal plane, is prepared and that an oxide film is formed on the major surface, the {111} plane having been widely used in the art.

In order to indicate more fully the nature of the invention, the following typical examples are set forth. It should be understood, however, that these examples are presented as illustrative only, and that it is not intended to limit the scope of the invention.

A silicon crystalline substrate 1 of P type 4 ohm cm resistivity having a major surface lying parallel to a {100} plane is prepared. The crystalline substrate 1 is heat-treated for 20 minutes in an oxidizing atmosphere containing steam at approximately 1,000.degree. C to form thereon a silicon dioxide film 2 of approximately 1,500 angstrom thickness as shown in FIG. 1. As a result, a channel layer 4 of donor-type surface charge may be induced immediately below the silicon dioxide film 2. Aluminum is deposited by a conventional evaporation method on the silicon dioxide film 2 to form a gate electrode 13 having the length L of 5 microns and the width W of 1,600 microns as shown in FIG. 1. In addition, N-type regions 5 of 1,600 micron length, 10 micron depth, and a resistivity of approximately 0.5 ohm cm are formed in the substrate 1 as shown with a spacing of 7 microns therebetween by a conventional selective diffusion method, and a source electrode 6 and a drain electrode 7 are connected thereto, respectively.

In order to obtain a better result, the field effect transistor may be subjected to the above-mentioned heat treatment so as to reduce the density of surface donors. That is, a DC voltage of 5 volts is applied between the source electrode 6 (or drain electrode 7) and the gate electrode 13 of the transistor fabricated by the above-described processes, the voltage being applied with positive polarity to the electrode 6 (or 7). Then, as the voltage is so applied, the transistor is heat-treated at a temperature of 350.degree. C for a period of time of one hour or longer which is sufficient to decrease the density of surface donors in the channel layer 4 to a minimum value.

The impressed DC voltage, the heating temperature, and the period of time for heat treatment set forth above are merely illustrative, and a shorter period of time for heat treatment suffices when the impressed DC voltage is raised. The heating temperature should be at least 75.degree. C in the case of a silicon substrate. Otherwise, the density of surface donors cannot be reduced to the minimum value. The only requirement is that the combination of the above mentioned three factors of this treatment be such that the density of surface donors in the channel layer 4 is decreased.

The density N.sub.DS of surface donors induced in the major surface lying parallel to the {100} plane of the MOS field effect transistor according to this invention is 2 .times. 10.sup.11 /cm.sup.2. In contrast, the density of surface donors in a major surface lying parallel to a {111} plane of a like conventional MOS field effect transistor fabricated by a like method is 5 .times. 10.sup.11 /cm.sup.2. Thus, the value of the density of surface donors in the transistor according to the present invention is 0.4 time of that of a conventional device.

Drain current vs. drain voltage characteristics of MOS type field effect transistors fabricated by the prior art method and by the method according to the present invention are shown in FIGS. 3 and 4, respectively. The spaces between the curves for different gate voltage V.sub.G become much wider in FIG. 4, which shows characteristics for the transistor according to this invention, than in FIG. 3 for a conventioned transistor. This indicates that the mutual conductance g.sub.m in the device of the instant invention is higher than that of the known device, whereby a device of high gain can be obtained.

Furthermore, the rising slopes of the current-voltage curves in the low drain voltage range are steeper than those of the known device, whereby it is evident that a device of high sensitivity can be obtained.

Another embodiment will be explained with reference to FIG. 5. A P-type silicon crystalline substrate 1 having a major surface lying substantially parallel to a {100} or a {110} plane is prepared. An N-type base region 8 and a P-type emitter region 9 are formed in the major surface of the substrate 1 by a conventional selective diffusion method. The major surface of the substrate 1 is covered with a silicon oxide film 2. Then, electrodes 10 and 11 are connected to the base and the emitter region through holes formed in the oxide film 2, respectively.

In the transistor, the density N.sub.DS of surface donors of an N-type inversion layer 3 induced in and spreading over the major surface of the semiconductor 1 immediately below the film 2 is made much smaller than that in a like PNP-type planar transistor but having major surface of the semiconductor substrate lying parallel to a {111} plane. Accordingly, the value of the collector cut-off current I.sub.co can be substantially decreased, and a planar transistor having highly desirable characteristics can be obtained.

It will also be obvious that the present invention can be applied to MOS type diodes.

It should be understood, therefore, that the foregoing disclosure relates to only an illustrative embodiment of the invention and that it is intended to cover all changes and modifications of the example of the invention herein chosen for the purposes of the disclosure, which do not constitute departures from the spirit and scope of the invention.