Method of manufacturing a MOSFET with LDD regions
BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     This invention relates to a manufacturing a MOSFET, of high integrated semiconductor device and more particularly, to a method of manufacturing a MOSFET with LDD (Lightly Doped Drain) region. 
     2. Description of the Prior Art 
     In parallel with the emergence of high integrated semiconductor device, the integrity of chips increase while the channel length of transistor decreases. The decrease of channel length in transistor causes some problems associated with DIBL (drain induced barrier lowering), hot-carrier effect and short-channel effect. To overcome this, a MOSFET with LDD regions is being widely used. 
     With reference to FIG. 1 and FIG. 2A through FIG. 2D, the conventional method for forming a MOSFET with LDD will now be described. 
     FIG. 1 is a layout view illustrating aligned masks in order to manufacture a MOSFET with LDD regions , in which a gate electrode mask (60) is arranged perpendicular to an active mask (50), and the active mask (50) is overlapped with a source/drain implant mask (70) which is larger in width than the active mask (50). 
     FIG. 2A to 2D are cross-sectional views illustrated according to I--I of FIG. 1, in manufacturing a MOSFET with LDD regions fabricated by the existing technology. 
     As shown in FIG. 2A, a field oxide layer 1 providing isolation between devices is formed by LOCOS (local oxidation of silicon) process on a silicon substrate 10 having p-type impurity as the 1st conductive type which is not formed on the active region, a gate oxide layer 2 is formed on the silicon substrate of the active region, and a gate electrode 3 is formed on a predetermined region of both active region and field oxide layer 1 as is further illustrated by FIG. 2E which is a cross-sectional view corresponding to FIG. 2A but according to IV--IV of FIG. 1. 
     As shown in FIG. 2B, an oxide layer 4 is formed on the surface of the gate electrode 3, n-type impurity of low concentration as the 2nd conductive type is implanted to the silicon substrate 10 by ion implantation to form the LDD region 5, and an insulating layer 6, for example an oxide layer, is formed on the whole structure. 
     As shown in FIG. 2C, the insulating layer 6 is etched by anisotropic etch to form the spacer 7 at both side walls of the gate electrode 3. During this etching process, both gate oxide layer 2 on the silicon substrate 10 and oxide layer 4 on the gate electrode 3 are etched. An oxide layer 12 is formed on the exposed silicon substrate 10, and then photoresist is coated on the whole structure and patterned into photsoresist layer 8A using the source/drain implant mask 70. FIG. 2C also shows that the photoresist pattern 8A is overlapped on both gate electrode 3 and field oxide layer 1, and that during the anisotropic etch process, the etching causes some damages 9 to the edge of Bird&#39;s Beak of the field oxide film 1. 
     As shown in FIG. 2D, a source/drain region 11 is formed by implanting n-type impurity of high concentration as the 2nd conductive type into the LDD region 5, and the photoresist layer pattern 8A is removed. FIG. 2D also shows that during the implantation process of said impurity, the source/drain region 11 from the damaged area of the field oxide layer 1 is implanted at the outside of the LDD region 5 so that the shaping of the LDD region 5 is defined as abrupt junction. 
     The prior art has recognized disadvantages in that during the etching process to form the spacer at the side of the gate electrode, a part of Bird&#39;s Beak of the field oxide layer is etched at the overlapping site between the field oxide layer and LDD region. Therefore, during the process to form the source/drain by ion implantation of n+ or p+ impurity of high concentration hereafter, the impurity of high concentration at the etched site of Bird&#39;s Beak of the field oxide layer is implanted at the outside of the LDD region, thus the shaping of the LDD region is defined as abrupt junction. This abrupt junction, which acts electrically as an unstable factor, might weaken the breakdown voltage of a MOSFET or increase junctive leakage current. 
     SUMMARY OF THE INVENTION 
     To be free of the aforementioned defects, accordingly, it is a primary object of the present invention to provide a method of manufacturing a MOSFET with LDD regions, having; in the etching process where the spacer at the side wall of gate electrode is formed, the photoresist layer pattern for source/drain implant mask is formed on the Bird&#39;s Beak of field oxide layer having a possibility of damage, and high concentrated purity is deposited. 
     The formation of the photoresist layer pattern have the following advantages; 
     can avoid the implantation of high concentrated ion into the silicon substrate damaged by the anisotropic etch, being responsible for the formation of spacer; 
     can cope with some defects associated with abrupt junction in the shaping of the LDD; 
     can prevent either the weakening of breakdown voltage or the increase of leakage current, in that the source/drain region is completely surrounded by the LDD region. 
    
    
     BRIEF DESCRIPTION ON THE DRAWINGS 
     FIG. 1 is a lay-out view illustrating aligned masks in order to manufacture a MOSFET with LDD regions according to a prior art. 
     FIG. 2A to FIG. 2D are cross-sectional views illustrating sequential steps to manufacture a MOSFET with LDD regions according to the prior art. 
     FIG. 2E is a cross-sectional view corresponding to FIG 2A and according to IV--IV of FIG. 1. 
     FIG. 3 is a lay-out view illustrating aligned masks in order to manufacture a MOSFET with LDD regions according to the first embodiment of the present invention. 
     FIG. 4 is a lay-out view illustrating aligned masks in order to manufacture a MOSFET with LDD regions according to the second embodiment of the present invention. 
     FIG. 5A to FIG. 5D are cross-sectional views illustrating sequential steps to manufacture a MOSFET with LDD regions according to the first embodiment of the present invention. 
     FIG. 5E is a cross-sectional view cross-sectional view corresponding to FIG.5A and according to V--V of FIG. 3. 
     FIG. 6A to FIG. 6D are sectional views illustrating sequential steps to manufacture a MOSFET with LDD regions according to the second embodiment of the present invention. 
     FIG. 6E is a cross-sectional view corresponding to FIG. 6A and according to VI--VI of FIG. 4. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The novel feature of the present invention may be understood from the accompanying description when taken in conjunction with the accompanying drawings similar reference characters refer to similar parts throughout the several views of the drawings. 
     FIG. 3 is a lay-out view illustrating aligned masks in order to manufacture a MOSFET with LDD regions according to the first embodiment of the invention, in which a gate electrode mask 60 is arranged perpendicularly to an active mask 50, and a source/drain implant mask 80, being larger than the active mask 50 in width, is arranged to be overlapped with the active mask 50, but at the place where the gate electrode mask 60 and active mask 50 are overlapped, the source/drain implant mask 80, being smaller in width than the active mask 50, is arranged to be overlapped with the active mask 50 
     FIG. 4 is a layout view illustrating aligned masks in order to manufacture a MOSFET with LDD regions according to the second embodiment of the invention, in which a gate electrode mask 60 is arranged perpendicularly to an active mask 50, and a source/drain implant mask 90 having lesser width than the active mask 50 is arranged to be overlapped with the active mask 50. 
     FIG. 5A to FIG. 5D are cross-sectional views illustrated according to II--II of FIG. 3, in manufacturing a MOSFET with LDD regions fabricated by the first embodiment of the invention. 
     As shown in FIG. 5A, a field oxide layer 1 is formed by the LOCOS process on silicon substrate 10 having p-type impurity as the 1st conductive type, a gate oxide layer 2 is formed on an exposed active region in the silicon substrate 10, and a gate electrode 3 is formed on a predetermined region of both gate oxide layer 2 and field oxide layer 1 as is further illustrated by FIG. 5E which is a cross-sectional view corresponding to FIG. 5A but according to V--V of FIG. 3. 
     As shown in FIG. 5B, an oxide layer 4 is formed on the surface of the gate electrode 3, n-type impurity of low concentration as the 2nd conductive type is implanted to the silicon substrate 10 by ion implantation to form the LDD region 5, and an insulating layer 6, for example, an oxide layer, on the whole structure is formed. 
     As shown in FIG. 5C, the insulating layer 6 is etched by anisotropic etch to form the spacer 7 at the side wall of the gate electrode 3. Then, photoresist is coated on the whole structure and patterned into photoresist layer 18A using the source/drain implant mask 80, thus constituting the overlapped part of both LDD region 5 and field oxide layer 1 covered with the photoresist layer pattern 18A. During the anisotropic etch process, some damages 9 inevitably occur since the etching is extended even to the edge of Bird&#39;s Beak of the field oxide layer 1. However, the photoresist pattern 18A covers the damaged part, thereby preventing any occurrence of problem in the process of high concentrated ion implantation as a post-process. 
     As shown in FIG. 5D, the source/drain region 11 is formed by implanting n-type impurity of high concentration as the 2nd conductive type into the LDD region 5, and the photoresist pattern 8A is removed. 
     According to the first embodiment of the invention, the photoresist pattern 18A for the source/drain implant mask covers the parts damaged 9 to the edge of Bird&#39;s Beak of the field oxide layer 1 during the anisotropic etch, thus avoiding any possible defects in the next process, say, a high concentrated ion implantation process. 
     FIG. 6A to FIG. 6D are sectional views illustrated according to III--III of FIG. 4, in manufacturing a MOSFET with LDD regions fabricated by the second embodiment of the invention. 
     As shown in FIG. 6A, a field oxide layer 1 is formed on p-type silicon substrate 10 according to the LOCOS process. Then, the gate oxide layer 2 is formed on an exposed active region in the silicon substrate 10 and the gate electrode 3 is formed on a predetermined region of both gate oxide layer 2 and field oxide layer 1 as is further illustrated by FIG. 6E which is a cross-sectional view corresponding to FIG. 6A but according to VI--VI of FIG. 4. 
     As shown in FIG. 6B, an oxide layer 4 is formed on the surface of gate electrode 3, and n-type impurity of low concentration as the 2nd conductive type, is implanted to the silicon substrate 10 by ion implantation to form the LDD region 5, and an insulating layer 6 on the whole structure is formed by an oxide layer of proposed thickness. 
     Then, a photoresist layer 28 is coated and photoresist is coated on the whole structure and patterned into photoresist layer 28A by a lithographic process by using the source/drain implant mask 90. Also, FIG. 6 shows that the neighboring parts between the field oxide layer 1 and LDD region 5 is covered by the photoresist pattern 28A. 
     As shown in FIG. 6C, after the process of FIG. 6B, the exposed insulating layer 6 is etched to form the insulating pattern 6A. With the removal of the photoresist pattern 28A, the gate oxide layer 2 is etched during the etching process of the insulating layer 6, thus forming a thin oxide layer 13 on the surface of exposed LDD region 5. 
     As shown in FIG. 6D, n-type impurity of high concentration as the 2nd conductive type, is implanted to the LDD region 5 by ion implantation to form the source/drain region 11 within the LDD region 5. 
     According to the second embodiment of the invention, the insulating pattern 6A during the ion implantation of high concentrated impurity protects adequately the edge of Bird&#39;s Beak of the field oxide layer 1, so that no abrupt junction (for example, the source/drain region 11 protruded to the lowering part of LDD region) occurs. 
     As described above, though an edge part of Bird&#39;s Beak is etched during the etching process of forming the spacer, the invention prevents not only the weakening of breakdown voltage in the source/drain junction, but also the increase of junctive leakage current. 
     Although this invention has been described in its preferred form with a certain degree of particularity, it is appreciated by those skilled in the art that the present disclosure of the preferred form has been made only by way of example and that numerous changes in the details of the construction, combination, and arrangement of parts may be resorted to without departing from the spirit and scope of the invention.