Semiconductor device having a radiation resistance and method for manufacturing same

A semiconductor device having a field insulating film which comprises a semiconductor substrate having an active region and a field region, a first oxide film formed on a surface of the substrate within the field region and etched on an upper surface of the first oxide film, an amorphous silicon layer formed on the surface of the first oxide film by ion implantation, and a second oxide film formed on the amorphous silicon layer whereby a field insulating film has a three-layered structure consisting of the first oxide film, the amorphous silicon layer and the second oxide film. A method for manufacturing the semiconductor device is also described.

REFERENCE TO RELATED APPLICATION 
This application claims the right of priority under 35 U.S.C. 119 of 
Japanese Application Serial No. 107351/1990, filed on Apr. 25, 1990, the 
entire disclosure of which is incorporated herein by reference. 
BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a semiconductor device having a high radiation 
resistance and a method for fabricating the same and more particularly, to 
such a semiconductor device and its manufacturing method wherein an 
insulating film in a field region is improved. 
2. Description of the Related Art 
In recent years, there is an increasing tendency toward the use of 
semiconductor integrated circuits in space and around nuclear reactors. 
The semiconductor integrated circuits placed under such severe conditions 
usually undergo various radiation damage by exposure to ionizing radiation 
such as X-rays, .gamma.-rays and the like, with the possibility of causing 
malfunction and breakage of the circuits, thus leading to a lowering of 
the function of the system. In particular, the characteristic degradation 
is considered to result from positive charges generated in a field oxide 
film. In order to solve this degradation problem, three semiconductor 
devices have been heretofore proposed. 
One of the semiconductor devices includes, as shown in FIG. 2, a thick 
oxide film 1 formed in a field region so as to separate the respective 
elements from one another, a highly concentrated impurity region 2 formed 
beneath the oxide film 1, a polycrystalline semiconductor layer 3 covered 
with the thick oxide layer at side and bottom faces thereof, and an 
electrode 4 electrically connected to the polycrystalline semiconductor 
layer 3. When a negative potential is applied to the electrode 4, the 
degradation by exposure to radiation can be reduced. 
As another example, there has been proposed a semiconductor device having a 
field insulating film incorporated with a conductor such as of Al. In this 
semiconductor device, a negative potential is applied to the conductor as 
in the first device, whereby the degradation by exposure to radiation is 
reduced. 
A semiconductor device of a further example is one which includes, as shown 
in FIG. 3 , a conductive impurity-doped field insulating film 111 and a 
conductive layer 112 formed on the film 111. In this instance, a negative 
potential is applied to the conductive layer 112, as in the 
above-described devices, thereby reducing the degradation owing to damages 
caused by radiation. By this measure, the generation of a threshold or 
leakage voltage can be suppressed. 
However, these semiconductor devices have, respectively the problem that it 
is very difficult to fabricate such devices in view of the actuality of 
the fabrication methods. 
SUMMARY OF THE INVENTION 
It is accordingly an object of the invention to provide a semiconductor 
device which is resistant to radiation and can be manufactured relatively 
simply. 
It is another object of the invention to provide a method for manufacturing 
a semiconductor device whose field region has a good radiation resistance. 
According to the invention, there is provided a semiconductor device having 
a field insulating film which comprises: 
(a) a semiconductor substrate having an active region and a field region; 
(b) a first oxide film formed on a surface of the substrate within the 
field region and etched on an upper surface of the first oxide film; 
(c) an amorphous silicon layer formed on the surface of the first oxide 
film by ion implantation; and 
(d) a second oxide film formed on the amorphous silicon layer thereby 
forming a field insulating film having a three-layered structure 
consisting of the first oxide film, the amorphous silicon layer and the 
second oxide film. 
There is also provided a method for manufacturing a semiconductor device 
according to the invention, which comprises the steps of; 
(a) selectively forming a field oxide film on a surface of a semiconductor 
substrate and partly etching the field oxide film on the surface thereof; 
(b) subjecting the etched surface of the field oxide film to ion 
implantation of silicon to form an amorphous silicon layer; and 
(c) forming an additional field oxide film on the amorphous silicon layer, 
to thereby form a field region having a three-layered structure of the 
oxide film, the amorphous silicon layer and the additional oxide film. 
The additional oxide film may be formed by oxidation of the amorphous 
silicon layer or by a deposition process. 
According to the semiconductor device of the invention, the amorphous 
silicon layer of the three-layered structure in the field region catches 
up holes generated by irradiation of radiation to extinguish them by 
re-combination with electrons. Further, the holes generated by exposure to 
radiation is taken up with a crystal structure which has been formed, 
owing to the etching damage, on the etched surface of the field oxide film 
by the partial etching of the field oxide film surface. As a result, the 
holes are re-combined with electrons and become extinguished. Thus, the 
semiconductor device of the invention makes it possible to reduce positive 
charges accumulated in the field region, so that a satisfactory radiation 
resistance in the field is ensured. 
According to the manufacturing method of the invention, after the formation 
of the field oxide film, the etching of the film surface, the ion 
implantation of physically, reliably stable oxidizable silicon and 
reoxidation are peformed. Thus, a semiconductor device having a good 
radiation-resistant effect can be manufactured by a relatively simple 
procedure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to the drawings, a method of manufacturing semiconductor devices 
according to the present invention is explained with reference to FIGS. 
1(a) to 1(g). 
Each of the measurements, shapes and arrangements of the preferred 
embodiment described herein are illustrative and not restrictive, the 
scope of the invention being indicated by the appended claims and all 
versions which come within the meaning of the claims are intended to be 
embraced therein. 
Referring now to FIGS. 1(a)-1(g) of the drawings, wherein like reference 
characters designate like or corresponding parts throughout the several 
views, there is shown an embodiment of the invention. 
In this embodiment, a well layer 12 is formed on a silicon substrate 11 in 
a usual manner as shown in FIG. 1(a). Thereafter, according to a selective 
oxidation technique (LOCOS method) using a nitride film mask, a field 
oxide film 13 is selectively formed on the surface of the substrate 11 in 
a thickness of about 6000 to 8000 angstroms. 
Then, as shown in FIG. 1(b), a resist or LP-SiO.sub.2 film 14, which serves 
as a buffer layer when the field oxide film 13 is etched back to some 
extent, is coated or formed entirely on the substrate 11 by a LPCVD method 
in a thickness of 1000 to 1500 angstroms. 
Thereafter, the resist or LP-SiO.sub.2 film 14 is etched back by ordinary 
dry etching. At the same time, the surface of the field oxide 13 which is 
exposed during the course of the dry etching is etched back to 1000 to 
2000 angstroms as shown in FIG. 1(c). 
Subsequently, a resist pattern 15 is formed on the well layer 12 as shown 
in FIG. 1(d). This resist pattern 15 should have a thickness such that 
subsequent ion implantation of silicon does not influence the well layer 
12 (active region). 
Next, the ion implantation of silicon is effected on the surface of the 
field oxide film 13 using the resist pattern 15 as a mask. This is 
particularly shown in FIG. 1(e). As a result, an amorphous silicon layer 
16 is formed. At the time, care should be taken so that the ion 
implantation is carried out at an energy as low as possible (e.g. at an 
acceleration voltage of not higher than 200 KeV) in order that the silicon 
is not implanted to a depth greater than necessary. The dose of the 
silicon is in the range of from 1.times.10.sup.10 to 1.times.10.sup.20 
ions/cm.sup.2. 
The resist pattern 15 is removed and, after sufficient washing, a gate 
oxide film 17 is formed on the surface of the well layer 12 in a thickness 
of from 100 to 200 angstroms by thermal oxidation (in an atmosphere which 
may be either wet or dry) at temperatures of not higher than 900.degree. 
C. At the same time, part of the amorphous silicon layer 16 is also 
oxidized, by which the field oxide film 13a is additionally formed. As a 
result, the field oxide 13 has a three-layered structure which consists of 
the oxide film 13a, the amorphous silicon layer 16 and the oxide film 13b. 
Then, as shown in FIG. 1(g), a gate electrode 18 of polysilicon or polycide 
is formed on a gate oxide film 17. Moreover, a mask oxide film 19 is 
formed on the gate electrode 18 and also on the surface of the well layer 
12 at opposite sides of the gate electrode 18 by annealing at not higher 
than 900.degree. C. in an O.sub.2 gas. Source/drain regions 20 are formed 
within the well layer 12 at opposite sides of the gate electrode 18, 
followed by coverage of the entire surface with an intermediate insulating 
film 21. The film 21 is provided with contact holes 22 for the 
source/drain region 20 and Al wirings or contact 23 are formed as shown. 
Finally, a passivation film 24 is formed over the entire surface to 
complete a semiconductor device. 
It will be noted that the oxide film 13a formed on the amorphous silicon 16 
may be formed by a step different from the formation of the gate oxide 
film 17. In the case, the oxide film 13a may be formed by a chemical vapor 
deposition (CVD) process. When the oxide film 13a is formed by a different 
process step, the thickness of the oxide film 13a can conveniently be 
selected arbitrarily irrespectively of the thickness of gate oxide film 
17. 
In the manufacturing method of the invention, after the etching back of the 
original field oxide film 13 and the ion implantation of silicon, a number 
of thermal treatment procedures are necessary (e.g. thermal oxidation 
carried out to form the oxide films 17, 19, flow and contact flow of the 
intermediate insulating film 21, and the like). These procedures should 
all be effected at temperatures of not higher than 900.degree. C. so that 
the radiation resistance-improving effect is not impeded. If the thermal 
treatments are performed over 900.degree. C., the etching damage is 
restored with a loss of the hole catch-up effect. In addition, the silicon 
is diffused, so that the effect of extinguishing holes by re-combination 
in the amorphous silicon layer 16 is lowered. Thus, the radiation 
resistance is not improved significantly. 
As described in detail, the semiconductor device of the invention can 
enhance the radiation resistance in the field region with the possibility 
of manufacturing a highly reliable device. Moreover, according to the 
manufacturing method of the invention, the semiconductor device can be 
made very simply by formation of a field oxide film, etching of the film 
surface, ion implantation of physically, reliably stable, oxidizable 
silicon and re-oxidation. Accordingly, the manufacture process of the 
invention can be readily incorporated in an existing manufacture line.