Abstract:
A method of fabricating read only memory, (ROM), devices has been developed. This process is accomplished using self-alignment of buried N+ bit lines. Thick field oxides are used for isolation purposes. The programmable cell is obtained by growing a gate oxide in a region in which the thick field oxide has been removed. The non-programmable cells contain thick gate oxides. Polysilicon gate structures are processed to function as the word lines.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a simplied process for fabricating, read only memory, (ROM), devices, using recessed gates, that are manufactured with the use of self-aligned source and drain bit lines and recessed oxide technology. 
     DESCRIPTION OF PRIOR ART 
     The major objective of the semiconductor industry is to continually improve chip performance, while still being able to continue cost reduction. The ability to place more chips, on a specific sized wafer, does help in reducing the cost per chip. This is accomplished by reducing key chip features, allowing denser semiconductor devices, that is a greater number of circuits per chip to be realized, and thus resulting in many more smaller and faster, chips per wafer. In addition performance enhancements are also achieved by reducing chip size. For example shorter channel lengths and shorter wiring runs have allowed metal oxide semiconductor field effect transistor, (MOSFET), technologies to become competitive with bipolar technologies. 
     Efforts to increase device density, by reducing specific device dimensions, were initially accomplished by the rapid evolution of the photolithograhy discipline. Improved optics and the development of very sensitive organic photoresist materials, have resulted in sub-micrometer images to be routinely obtained. Other semiconductor fabrication processes, such as reactive ion etching, (RIE), and ion implantation, (I/I), have also contributed to the achievement of reduced chip dimensions. In addition, recent semiconductor processing techniques, have also been a major factor in microminiturazation. For example in U.S. Pat. No. 4,648,937, Ogura, et al, describe a &#34;spacer&#34; or sidewall process, using a combination of chemical vapor deposition, (CVD), and RIE, that allows smaller dimensions to be achieved. In U.S. Pat. No. 5,108,937, Tsai et al, illustrate a process of removing a recessed oxide and using the exposed silicon for subsequent devices. This process also allows reductions in chip size to occur. 
     The combination of using advanced tools, photolithoghy, RIE, etc, and the new process sequences, sidewall spacers, etc, can be effectively used to build high density read only memory, (ROM), structures. The buried bit line process, usually accomplished by a heavy N+ I/I step, has been used extensively in ROM products. The programmable cell in turn, is usually achieved by a heavy I/I of P type dopant, in an area between two adjacent N+ bit lines. As the dimension of the programmable cell, or the space between the adjacent bit lines decrease, the counterdoping of the P type cell, form the adjacent N+ bit lines, becomes more significant, and adversly influences the performance of the device. Hong, in U.S. Pat. No. 5,308,777, describes a process that reduces the counterdoping effect. However the process adds more complexity, and thus cost to the final product. 
     The invention now described will show a simplified process for fabricating ROM devices, eliminating the need for a P type I/I, programmable cell, and using the recessed oxide technology as a major component of this ROM process. 
     SUMMARY OF THE INVENTION 
     It is an object of this invention to provide a method of fabricating an improved ROM device with a minimum of process steps. 
     It is another object of this invention to use self-alignment techniques to achieve the source and drain regions. 
     It is still another object of this invention to use an area of removed recessed oxide, for the programmable cell. 
     It is still yet another object of this invention to use a thin gate oxide as part of the programmable cell. 
     In accordance with the present invention a method is described for fabricating ROM devices in which recessed or field oxide, (FOX), regions are grown, followed by a self-aligned N+ I/I, into the spaces between the FOX regions, for purposes of creating the buried bit lines of the ROM device. The specific cells, needed for the programmable cell, are next subjected to a FOX removal process. Thin gate oxides are then regrown in the programmable cell channel region followed by polysilicon deposition, doping and pattterning to form the gate structure. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The object and other advantages of this invention are best explained in the preferred embodiment reference to the attached drawing that include: 
     FIGS. 1-2, which schematically illustrate a fabrication sequence, used to obtain a specific N+ bit line dimension. 
     FIGS. 3-6, which schematically represent, in cross-sectional form, the key fabrication stages of this invention. 
     FIG. 7, a top view of the ROM cell, fabricated using this invention. 
     FIG. 8, which schematically illustrates a cross-section of a metallized ROM cell. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The method of forming the ROM cell, without the use of a code ion implantation step, will now be described in detail. This structure can be used as a part of MOSFET chips that are now being manufactured in industry, therefore only the specific areas unique to understanding this invention will be described in detail. 
     Referring now to FIG. 1, a substrate, 1, composed of P type single crystal silicon, with a &lt;100&gt; orientation, is used. A thin pad oxide, 2, between about 200-1000 Angstroms in thickness, is thermally grown, followed by a low pressure chemical vapor deposition, (LPCVD), of silicon nitride, 3, to a thickness of between about 500-1500 Angstroms. Standard photolithogaphy processing is then used to create an image in the photoresist, and this photoresist shape is in turn transferred to the underlying silicon nitride--silicon dioxide layers via reactive ion etching, (RIE). After photoresist removal, followed by wet organic and inorganic cleans, a thick field oxide, 4, (FOX), is thermally grown at a temperature of between about 800° to 950° C., to a thickness of between about 4000 to 6000 Angstroms. At this stage the the oxidation masking silicon nitride, 3, and the pad oxide, 2, layers are removed via use of a hot phosphoric acid treatment, followed by a buffered hydrofluoric acid process. This is shown schematically in FIG. 2. The region of exposed silicon, 5, located between FOX regions, will be subsequently used, in a self-alignment scheme, to produce the buried N+ bit lines. 
     A self-aligned, buried N+ bit line process is now performed, using ion implantation of arsenic at an energy of between about 50 to 100 Kev., at a dose of between about 1E14 to 1E16 atoms/cm2, and phosphorous at an energy of between about 20 to 60 Kev., at a dose of between about 1E14 to 1E16 atoms/cm2. The N+ bit lines, 6, are shown schematically in FIG. 3. Again it should be noted that this step is performed using a self-alignment scheme. That is the width of the N+ bit lines were not directly defined using photolithography, but were created by ion implantation of the exposed silicon regions, while the remaining silicon areas are masked by FOX regions, 4. Next an oxidation step is performed at a temperature of between about 900° to 1000° C., in order to grow a thickness of between about 500 to 1000 Angstroms of silicon dioxide, 7, on the N+ bit lines, 6. This is shown in FIG. 3. 
     The process to create the programmable cell now is now initiated. Photolitography, 8, is used to expose region 9, followed by removal of the FOX insulator using a buffered hydrofluoric acid solution. This is shown in FIG. 4. It can also be seen that the exposed region of oxide layer, 7, ovelying the N+ bit lines, is also removed. However subsequent oxidation processes will result in a regrowth of this passivation film. After removal of photoresist, and careful wet cleans, a gate oxide, 10, is grown at a temperature between about 800° to 1000° C., to a thickness between about 100 to 300 Angstroms. It should also be noted that the portion of oxide, 7, removed in the previous buffered hydrofluoric acid step, is now regrown during the gate oxidation process. This can be seen in FIG. 5. 
     Next the word line structure is fabricated by first depositing polysilicon, using LPCVD techniques, to a thickness of between about 1000 to 3000 Angstroms. Ion implantation of phoshorous, at an energy between about 30 to 80 Kev., at a dose between about 1E14 to 1E16 atoms/cm2, is used to dope the polysilicon, and photohithography and RIE processing are used to create the polysilicon word line structure, 11, shown schematically in FIG. 6. It can be seen that the programmable cell, 12, is differentiated from the non-programmable counterparts, by a significant difference in gate oxide thickness, thus significant differnces in turn on voltages exist. FIG. 7 illustrates a top view of this structure. 
     Finally a silicon oxide layer, 13, is deposited to a thickness of between about 5000 to 8000 Angstroms, followed by photolithography and RIE processing, to create via, 14, to the polysilicon word line, 11, and to the N+ bit line, 6, (not shown). Contact metallurgy is achieved via metal sputtering of aluminum to a thickness of between about 6000 to 12000 Angstroms, followed by conventional photolithography and RIE processing to produce metal contact, 15, shown schematically in FIG. 8. 
     The process used to create the ROM device, witout a code ion implant step, can be applied to P type, (PMOSFET), as well as N type, (NMOSFET), read only memory chips. This procedure can also be applied to complimentary metal oxide silicon, (CMOS), as well as to bipolar-CMOS, (BiCMOS), devices. 
     While this invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of this invention.