Patent Publication Number: US-6218716-B1

Title: Enhanced structure for salicide MOSFET

Description:
This is a division of patent application Ser. No. 09/042,366, filing date Mar. 13, 1998 now U.S. Pat. No. 6,074,922, An Enhanced Structure For Salicide Mosfet, assigned to the same assignee as the present invention. 
    
    
     BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The invention relates to the fabrication of integrated circuit devices, and more particularly, to a method of reducing resistance and increasing effective polysilicon width for a narrow polysilicon line in the fabrication of integrated circuits. 
     (2) Description of the Prior Art 
     In the fabrication of integrated circuits, a silicide is often formed on top of a polysilicon gate and overlying the source and drain regions within a substrate. Typically, a titanium layer is deposited over the wafer. The wafer is subjected to a thermal process which causes the underlying silicon to react with the titanium layer to form titanium silicide. FIG. 16 illustrates a partially completed integrated circuit device of the prior art. A gate electrode  16  has been formed on the surface of a semiconductor substrate  10 . Source and drain regions  20  have been formed within the substrate. Sidewall spacers  18  are typically composed of silicon dioxide or silicon nitride. A layer of titanium  24  is deposited over the surface of the wafer. A rapid thermal process causes the silicon atoms within the polysilicon gate and the substrate to diffuse into the titanium layer and react with the titanium to form titanium silicide  26 , as illustrated in FIG.  17 . The titanium layer  24  over the sidewall spacers  18  and the field oxide regions  12  is not reacted and can be removed easily. However, silicon from the gate and from the substrate can diffuse into the titanium layer overlying the sidewall spacers causing titanium silicide  28  to form overlying the spacers as well. This is the so-called bridging problem. The titanium silicide layer  28  over the spacers shorts the source/drain to the gate leading to malfunction of the device. 
     The salicide process is used to lower resistance and therefore improve device performance and reduce chip size. However, resistance is increased dramatically for narrow polysilicon lines, such as less than 0.25 μm. This is caused by polysilicon critical dimension variation and non-uniform salicide thickness. 
     U.S. Pat. No. 5,648,287 to Tsai et al teaches two sets of silicon oxide spacers on the sidewalls of a gate, and a nitrogen-implanted amorphous silicon layer overlying the gate and source/drain regions. The nitrogen-rich layer acts as an oxidation barrier. U.S. Pat. No. 4,912,061 to Nasr shows a salicidation process using disposable silicon nitride spacers. U.S. Pat. No. 5,091,807 to Sanchez uses polysilicon and oxide spacers before salicidation. U.S. Pat. No. 5,658,807 to Manning teaches a method of salicidation of the sidewalls of a polysilicon gate. U.S. Pat. No. 4,716,131 to Okazawa et al teaches salicidation of the polysilicon sidewalls and source and drain regions first followed by a second salicidation of the top of the polysilicon gate. 
     SUMMARY OF THE INVENTION 
     A principal object of the present invention is to provide an effective and very manufacturable method of fabricating a silicided polysilicon gate in the fabrication of an integrated circuit device. 
     Another object of the present invention is to provide an effective and very manufacturable method of fabricating a silicided polysilicon gate with reduced resistance in the fabrication of an integrated circuit device. 
     A further object of the invention is to provide a method of preventing gate to source/drain bridging in the fabrication of a silicided polysilicon gate. 
     A still further object is to provide a method of reducing resistance of a narrow polysilicon line in the fabrication of a silicided polysilicon gate. 
     Yet another object is to provide a method of increasing effective polysilicon width at a narrow polysilicon line in the fabrication of a silicided polysilicon gate. 
     Yet another object is to provide a method of increasing salicide thickness at a narrow polysilicon line in the fabrication of a silicided polysilicon gate. 
     Yet another object is to provide a method of increasing salicide thickness and effective polysilicon width at a narrow polysilicon line while reducing resistance and reducing source/drain bridging risk in the fabrication of a silicided polysilicon gate. 
     In accordance with the objects of this invention a method for increasing salicide thickness and effective polysilicon width at a narrow polysilicon line while reducing resistance and reducing source/drain bridging risk in the fabrication of a silicided polysilicon gate is achieved. A polysilicon layer is provided overlying a gate oxide layer on a semiconductor substrate. A dielectric layer, such as silicon oxide, is deposited overlying the polysilicon layer. The silicon oxide layer, polysilicon layer, and gate oxide layer are patterned to form a polysilicon gate electrode having a silicon oxide layer on top of the gate electrode. Dielectric spacers, such as silicon nitride, are formed on the sidewalls of the gate electrode and the silicon oxide layer. In an alternative, silicon spacers may be formed between the gate and the silicon nitride spacers to increase the effective width of the polysilicon line. Source and drain regions associated with the gate electrode are formed within the semiconductor substrate. The silicon oxide layer on top of gate electrode is removed whereby the silicon nitride spacers extend above the gate electrode. A metal silicide is formed on the top surface of the gate electrode and over the source and drain regions. The dielectric spacers extending higher than the gate electrode prevent source/drain bridging during silicidation. This completes the formation of the salicided polysilicon gate electrode. 
     Also in accordance with the objects of the present invention, an improved silicided polysilicon gate in an integrated circuit device is achieved. A gate dielectric layer is formed on a semiconductor substrate. A gate is formed on the gate dielectric layer. First spacers made of silicon are formed at both sides of the gate dielectric layer and the gate electrode. Second spacers made of a dielectric material are formed at the outer sides of the first spacers and extending above the gate. A metal silicide layer is formed on top of the gate and between the second spacers. Implanted source and drain regions are formed in the semiconductor substrate at the outer sides of the second spacers. Metal silicide areas are formed on the source and drain regions. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the accompanying drawings forming a material part of this description, there is shown: 
     FIGS. 1 through 9 schematically illustrate in cross-sectional representation a first preferred embodiment of the present invention. 
     FIGS. 10 through 15 schematically illustrate in cross-sectional representation a second preferred embodiment of the present invention. 
     FIGS. 16 and 17 schematically illustrate in cross-sectional representation an embodiment of the prior art. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The process of the present invention provides a method for forming a salicided polysilicon gate where resistance is not degraded, especially in the case of narrow polysilicon lines, as narrow as 0.24 microns. The process of the invention also results in reduced gate to source/drain bridging risk and improved yield. Two preferred embodiments of the present invention will be described, the first with reference to drawing FIGS. 1 through 9, and the second with reference to FIGS. 10 through 15. 
     The first embodiment of the present invention will be described with reference to FIGS. 1-9. Referring now more particularly to FIG. 1, there is shown an illustration of a portion of a partially completed integrated circuit. The semiconductor substrate  10  is preferably composed of silicon having a (100) crystallographic orientation. Isolation between active and inactive regions, such as Field Oxide regions  12 , are formed as are conventional in the art. The surface of the silicon substrate  10  is thermally oxidized to form the gate oxide layer  14 . 
     Next, a layer of polysilicon  16  is deposited, for example, by low pressure chemical vapor deposition (LPCVD), to a suitable thickness of between about 2000 and 3000 Angstroms. 
     A layer of dielectric material  30  is deposited overlying the polysilicon layer  16 . This dielectric material is used to reserve a space for salicidation, so may be any dielectric material which may be removed smoothly. For example, silicon oxide may be used. The silicon oxide layer  30 , for example, is deposited to a thickness of between about 1500 and 2000 Angstroms. 
     Referring now to FIG. 2, the silicon oxide, polysilicon and gate oxide layers are etched away where they are not covered by a mask to provide a polysilicon gate electrode. After etching, the remaining oxide thickness over the polysilicon line is about 400 to 600 Angstroms. 
     Lightly doped source and drain regions  32  are formed by ion implantation, as is conventional in the art. 
     Referring now to FIG. 3, a second dielectric layer  34  is deposited over the gate electrode and the surface of the substrate to a thickness of between about 1500 and 2000 Angstroms. This layer preferably may be silicon nitride, or some other material such as plasma-enhanced oxide (PEOX) or tetraethoxysilane (TEOS) oxide. 
     The silicon nitride layer is anisotropically etched to leave spacers  34  on the sidewalls of the gate electrode, as shown in FIG.  4 . The spacers may be between about 1000 and 1500 Angstroms wide, as is conventional in the art. However, the spacers are higher than is conventional in the art because of the presence of the silicon oxide layer  30  overlying the polysilicon gate electrode. 
     Heavily doped source and drain regions  36  are formed within the semiconductor substrate. 
     Referring now to FIG. 5, a wet dip is used to remove the silicon oxide layer  30  overlying the polysilicon gate  16 . This leaves the tall silicon nitride spacers on the sidewalls of the polysilicon gate and extending above the top of the polysilicon gate. 
     Now the salicided polysilicon gate of the present invention will be formed. A layer of titanium or other metals such as nickel, cobalt, or the like,  40  is blanket deposited over the semiconductor substrate, as shown in FIG. 6, to a thickness of between about 200 and 400 Angstroms. 
     Referring now to FIG. 7, the titanium layer  40  is transformed into titanium silicide  42  by a thermal annealing cycle. This is preferably a rapid thermal anneal (RTA) at a temperature of between about 700 to 740° C. for between about 20 to 30 seconds in a nitrogen ambient. The titanium in contact with the silicon substrate or with the polysilicon will form titanium silicide, TiSi x    42 . The tall spacers  34  provide better anti-bridging protection than conventional spacers since they extend above the surface of the gate electrode. The remaining unreacted titanium  40  can be etched away using, for example, NH 4 OH/H 2 O 2  with deionized water. The resulting structure is illustrated in FIG.  8 . 
     The process of the present invention enlarges the salicide window. Since the salicide layer  42  over the polysilicon gate electrode is thicker than conventional (because of the taller silicon nitride spacers), the salicide is more uniform than in the prior art. This solves the resistance degradation problem for narrow polysilicon lines caused by salicide non-uniformity. The bridging problem also is solved by the taller spacers. 
     The integrated circuit device is completed as is conventional in the art. For example, as illustrated in FIG. 9, an insulating layer, such as silicon oxide or borophospho-TEOS (BPTEOS)  44  is blanket deposited over the semiconductor substrate to a thickness of between about 4000 to 12,000 Angstroms and then planarized, for example, by chemical mechanical polishing (CMP). Openings are etched through the insulating layer and the titanium silicide layer to underlying source and drain regions and other areas where electrical contact is to be made. A conducting layer  48  is deposited and patterned to complete the electrical connections. A passivation layer  50  completes the fabrication of the integrated circuit device. 
     The second embodiment of the present invention now will be described with reference to FIGS. 10-15. As in the first embodiment, the polysilicon and overlying silicon oxide layers are deposited and patterned to form the gate electrode illustrated in FIG.  2  and lightly doped source and drain regions  32  are formed within the substrate. Referring now to FIG. 10, a layer of amorphous silicon or polysilicon  33  is deposited conformally over the gate electrode and the surface of the substrate to a thickness of between about 100 and 300 Angstroms. 
     Then, the silicon nitride layer  34  is deposited over the gate electrode and the surface of the substrate, as illustrated in FIG.  11 . 
     Referring now to FIG. 12, the silicon nitride layer is anisotropically etched to leave spacers  34  on the sidewalls of the gate electrode. An overetch removes the amorphous silicon or polysilicon layer  33  on top of the oxide  30  and overlying the substrate. As in the first embodiment, the presence of the silicon oxide layer  30  overlying the polysilicon gate electrode results in tall spacers  34 . 
     Heavily doped source and drain regions  36  are formed within the semiconductor substrate, as is conventional in the art. 
     Referring now to FIG. 13, a wet dip is used to remove the silicon oxide layer  30  overlying the polysilicon gate  16 . This leaves the amorphous silicon or polysilicon spacers and the tall silicon nitride spacers on the sidewalls of the polysilicon gate and extending above the top of the polysilicon gate. 
     Now the silicided polysilicon gate of the present invention will be formed. As in the first embodiment, a layer of titanium or other metal such as nickel, cobalt, or the like,  40  is blanket deposited over the semiconductor substrate, as shown in FIG. 13, to a thickness of between about 200 and 400 Angstroms. 
     The titanium layer  40  is transformed into titanium silicide  42  by a thermal annealing cycle. This is preferably a rapid thermal anneal (RTA) at a temperature of between about 700 to 740° C. for between about 20 to 30 seconds in a nitrogen ambient. The titanium in contact with the silicon substrate or with the polysilicon will form titanium silicide, TiSi x    42 . The silicon spacers  33  provide additional silicon for better silicidation which results in lower resistance. The silicon spacers  33  on the sidewalls of the polysilicon  16  also provide extra width for the salicide layer. The tall spacers  34  provide better anti-bridging protection than conventional spacers because they extend above the surface of the gate electrode. The remaining unreacted titanium  40  can be etched away using, for example, NH 4 OH/H 2 O 2  with deionized water. The resulting structure is illustrated in FIG.  14 . 
     The process of the present invention increases the effective polysilicon width at the narrow polysilicon line by the presence of the silicon spacers  33 . The salicide layer  42  over the polysilicon gate electrode is thicker than conventional (because of the taller silicon nitride spacers); therefore, the salicide is more uniform than in the prior art. This reduces the resistance increase for narrow polysilicon lines which in turn improves yield. The tall silicon nitride spacers also reduce the gate to source/drain bridging risk. 
     As in the first embodiment, the integrated circuit device is completed as is conventional in the art. For example, as illustrated in FIG. 15, an insulating layer, such as silicon oxide or borophospho-TEOS (BPTEOS)  44  is blanket deposited over the semiconductor substrate to a thickness of between about 4000 to 12,000 Angstroms and then planarized, for example, by chemical mechanical polishing (CMP). Openings are etched through the insulating layer and the titanium silicide layer to underlying source and drain regions and other areas where electrical contact is to be made. A conducting layer  48  is deposited and patterned to complete the electrical connections. A passivation layer  50  completes the fabrication of the integrated circuit device. 
     The process of the invention provides an effective method of fabricating an integrated circuit device having a silicided polysilicon gate with reduced resistance, increased effective polysilicon width, increased salicide thickness, improved yield, and reduced gate to source/drain bridging. 
     The improved silicided polysilicon gate in an integrated circuit device of the present invention will be described with reference to FIGS. 9 and 15. A gate dielectric layer  14  is formed on a semiconductor substrate. A gate  16  is formed on the gate dielectric layer. Optional first spacers  33 , shown in FIG. 15, are formed at both sides of the gate dielectric layer and the gate electrode. These spacers may be amorphous silicon or polysilicon. Second spacers  34  made of a dielectric material, such as silicon nitride, are formed on the sidewalls of the gate (as in FIG. 9) or at the outer sides of the first spacers  33  (as in FIG. 15) and extending above the gate  16 . A metal silicide layer  42  is formed on top of the gate and between the second spacers  34 . Implanted source and drain regions  36  are formed in the semiconductor substrate at the outer sides of the second spacers. Metal silicide areas  42  are formed on the source and drain regions. 
     The improved silicided polysilicon gate of the present invention has reduced resistance, increased effective polysilicon width (where silicon spacers are used as in FIG.  15 ), increased salicide thickness, improved yield, and reduced gate to source/drain bridging. 
     While the 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 the invention.