Method for fabrication of offset gate MIS device
This is a continuation of application Ser. No. 809,106 filed June 22, 1977, now abandoned. 
    
    
     LIST OF PRIOR ART REFERENCES UNDER 37 CFR 1.56(a) 
     The following references are cited to show the state of the art; 
     (1) U.S. Pat. No. 3,787,962 to Isao Yoshida et al, Jan. 29, 1974, Class 29, subclasses 571 and 578; 
     (2) U.S. Pat. No. 3,745,425 to Julian R. A. Beale et al, July 10, 1973, subclasses 235R, 235B, and 235AM and Class 148, subclass 175; 
     (3) U.S. Pat. No. 3,679,492 to Frank F. Fang et al, July 25, 1972, Class 148-1.5; and 
     (4) C. S. Chang et al, IBM Technical Disclosure Bulletin, Vol. 16, No. 5, October, 1973, pages 1635-1636. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a method for fabrication of a MIS device and more particularly an offset gate MIS device. 
     2. Description of the Prior Art 
     According to the conventional method of fabricating a silicon gate MISFET, a gate electrode pattern is formed by photoetching on a polycrystalline silicon layer which lies on a gate insulator film, the unwanted portion of the gate insulator film is removed in a self-alignment fashion by using the gate electrode as a mask, and source and drain regions are formed through the selective diffusion of impurities. Consequently, the integration density and the characteristics, especially the operating speed, can be considerably improved in comparison with those of an aluminum gate MISFET. 
     However, according to the conventional method for fabrication f a silicon gate MISFET, the source and drain regions are formed by using the gate electrode as a mask so that the drain region of offset configuration cannot be formed. Accordingly, an inverse electric field due to the substrate-drain junction and the gate-drain field are applied near the portion of the drain region beneath the gate electrode so that a depletion or space charge layer extending from the drain toward the substrate does not expand sufficiently near the surface of the substrate. As a result, local concentrations of field are generated to be causative of avalanche breakdowns. This makes difficult the production of devices having a high breakdown voltage. This weakpoint is therefore fatal to a silicon gate MISFET having various excellent characteristics and merits. 
     There is known an aluminum gate MISFET in which source and drain regions of low impurity concentration are provided to insure good ohmic contact with source and drain electrodes, and source and drain extensions of high impurity concentration are provided to improve the source-drain breakdown voltage (see U.S. Pat. No. 3,959,025). 
     A silicon gate MISFET having a drain region of offset configuration is known in U.S. Pat. No. 3,909,306. In accordance with the fabrication method shown in this patent, a silicon oxide layer formed on a N-type silicon substrate is partially removed to diffuse P-type impurities into the exposed surface of the substrate through inpurity ion implantation and heat treatment so that a P-type drain region having a low impurity concentration and a thickness of 5-10μ is formed. During the diffusion, a silicon oxide layer is formed on the substrate surface in which the P-type drain region is formed. Next, there are removed those portions of the silicon oxide layer overlying a central surface portion of the P-type low impurity concentration drain region and that surface portion of the substrate in which a source region and a channel region are to be formed. The exposed substrate surface is thereafter oxidized to form a gate oxide film of silicon oxide. A polycrystalline silicon layer is formed on the gate oxide film and the remaining silicon oxide layer. The polycrystalline silicon layer is then partially removed so that a portion thereof on the channel region remains for forming a gate electrode. Further, the gate oxide film is partially removed so that the surface portion of the substrate for forming the source region and the central surface portion of the P-type low impurity concentration drain region are exposed. Thereafter, P-type impurities are diffused into the exposed substrate surface so that a P +  -type source region (P-type high impurity concentration source region) and a P +  -type drain region (P-type high impurity concentration drain region) in the P-type low impurity concentration drain region are formed with a thickness of about 1μ. Thus, The gate electrode is formed to bridge the source region and the drain region of low impurity concentration but to be spaced from the drain region of high impurity concentration, so that a depletion layer extends sufficiently into the drain region of the low impurity concentration to prevent electric field concentration. However, the above-explained fabrication method has the problems that the steps for fabrication are quite complicated and the advantage of self-alignment is not effectively utilized. 
     SUMMARY OF THE INVENTION 
     The object of this invention is to provide a novel method for fabricating a MIS device in an offset gate configuration. 
     According to the present method, an offset gate MIS device is fabricated by forming an insulating film with a gate insulator portion as a part thereof on the surface of a semiconductor substrate having one conductivity type, providing a gate electrode on a portion of the insulating film, using the gate electrode as a mask to apply impurities of the other conductivity type to the surface of the semiconductor substrate through the insulating film, forming a shielding film on the surface of that portion of the insulating film which is near the gate insulator portion beneath the gate electrode, using the shielding film as a mask to remove an unmasked portion of the insulating film so as to selectively expose the surface of the semiconductor substrate, applying impurities of the other conductivity type with a second impurity concentration higher than the first impurity concentration to the exposed surface of the semiconductor substrate, and heating the resultant structure to diffuse the impurities into the semiconductor substrate to form source and drain regions therein. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1 to 8 show in cross section the sequential steps of a method for fabricating an offset silicon gate MIS device as an embodiment of this invention; and 
     FIG. 9 shows in cross section an offset silicon gate MIS device as another embodiment of this invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The successive steps of the method of fabricating an offset silicon gate MIS device as an embodiment of this invention will be described below with the aid of FIGS. 1 to 8. 
     FIG. 1: A field oxide (silicon oxide) film 2 is formed in the surface of an N-type silicon substrate 1 by thermally oxidizing the surface of the substrate 1 and the part of the silicon oxide film 2 on the surface of the area of the substrate which is to serve as an active region is removed by, for example, a photoetching technique. 
     FIG. 2: The surface of the substrate area serving as the active region is thermally oxidized to form a silicon oxide film 3 having a thickness of about 1000 A a part of which is used as a gate insulator film. Then, a polycrystalline silicon film 4 having a thickness of about 5000 A a part of which is used as a gate electrode is formed on the films 2 and 3 by, for example, a CVD (chemical vapor deposition) technique. 
     FIG. 3: The polycrystalline silicon film 4 is subjected to selective etching treatment by means of, for example, an etching solution containing HF and HNO 3  to form a gate electrode 4a. Then, with the gate electrode 4a used as a mask, P-type impurities, e.g. boron, 5 are applied to the surface of the substrate 1 through the silicon oxide film 3 by an ion implantation technique. In this case, with a boron ion implanting energy of 45 KeV and implanted boron atoms of 4×10 12  /cm 2 , a boron ion implanted layer having an impurity concentration of about 10 17  atoms/cm 3  is formed. The boron impurities 5 are applied to form a drain layer in the offset portion of the element. 
     FIGS. 4, 5 and 6: The entire surface of the resultant structure is coated with a shielding film such as a photoresist film 6 as seen in FIG. 4. After the photoresist film has been removed in such a manner that the portion of the photoresist film 6 on the part of the silicon oxide film 3 as the offset portion is left behind, as shown in FIG. 5, the exposed part of the silicon oxide film 3 is removed by, for example, an etching solution containing NH 4  F, with the part of the photoresist film 6 and the gate electrode 4a used as a mask, as shown in FIG. 6. As a result, a silicon oxide film 3a of the offset portion of the element is formed. 
     FIG. 7: After the photoresist film 6 has been removed, with the gate electrode 4a and the silicon oxide film 3a used as a mask, boron 7 as impurities having a concentration higher than that of the implanted boron atoms 5 is deposited on those portions of the silicon substrate 1 which serve as source and drain layers and on the gate electrode 4a. In this case, it should be noted that the boron 7 is not deposited under the silicon oxide film 3a of the offset portion. 
     FIG. 8: The resultant structure is subjected to a heat treatment to diffuse the impurity boron 5 and 7 into the silicon substrate 1 so that P +  -type layers 7a serving respectively as source drain and layers and a P-type layer 5a as a drain layer in the offset portion are simultaneously formed. The following steps of the fabricating process are well known. Namely, a silicate glass (e.g. silicon oxide or phosphosilicate glass) film 8 as a surface protection or passivation film is formed by a CVD technique. After openings for electrodes or contacts have been provided in the film 8, a source electrode S, a drain electrode D and interconnection conductors (not shown) are formed through the vacuum deposition of aluminum and photoetching techniques. 
     The above-described method according to this invention and the silicon gate MIS device fabricated by the method have the following merits. 
     (1) The resultant MIS device is of offset gate configuration so that a depletion layer extending from the drain toward the channel expands sufficiently even near the surface of the silicon substrate 1. Consequently, no electric field concentration occurs in the region under consideration and therefore an element having a high breakdown voltage can be obtained. 
     (2) Since the offset gate structure is obtained easily and with high yield by providing the shielding film 6 such as a photoresist film on the silicon oxide film 3 of the offset portion after the gate electrode 4a has been formed, then the present method can be attained by only slightly modifying the conventional fabricating process. As a result, a silicon gate MIS device having a high breakdown voltage as well as high integration density and high speed switching operation characteristic of the silicon gate structure can be obtained. 
     This invention is by no means limited to the embodiment described above, but this invention can widely be applied to the method of fabricating a discrete element, an enhancement or depletion element or a complementary element which uses as a gate electrode material polycrystalline silicon or a refractory metal such as molybdenum, tangusten or platinum and uses various kinds of semiconductor wafers as a starting material. 
     In the above-described embodiment, the offset portion is provided only in the drain region, but offset portions may be provided for both source and drain regions, as shown in FIG. 9, by using the same method. Such a configuration is preferable especially in the case where high voltages are applied to both the source and drain regions. In the structure shown in FIG. 9, since the silicon oxide film 3 extends beyond the gate electrode 4a, the edge portions 4b of the gate electrode 4a are prevented from being subjected to damages such as cracks. In the case where a design is made such that the end of the gate electrode coincides with that of the silicon oxide film, there may be a possibility that the end portion of the silicon oxide film may be removed at the etching step, which may form a factor of crack of the edge portion of the gate electrode.