In the semiconductor industry, metal silicides have typically been used to provide low resistive contacts to source/drain regions and gate electrodes in metal oxide semiconductor (MOS) and CMOS transistors. Titanium silicide (TiSi2) has traditionally been used because of its low resistivity. However, by using the conventional Ti-salicide process the sheet resistance increases for lines having a thickness of less than about 0.35 microns (μm). A high temperature anneal is needed to completely transform the silicide from the high-resistivity C49 phase to the low-resistivity C54 phase. In some lines, the transformation is not complete and the film agglomerates before complete transformation into the low-resistivity state.
TiSi2 has been replaced with CoSi2 to circumvent the problem mentioned above. However, CoSi2 consumes significant amounts of silicon during formation, which increases the difficulty of forming shallow junctions for silicon-on-insulator (SOI) substrates. Further, as the semiconductor industry approaches dimensions less than 65 nm, CoSi2 also exhibits rapid sheet resistance degradation. Specifically, the polysilicon gate sheet resistance Rs increases sharply at narrow line widths due to voids formed by vacancy coalescence at the polySi grain boundary.
Ni suicide with low resistivity, low salicidation temperature, small mechanical stress, low silicon consumption and relative insensitivity of sheet resistance to linewidth is currently being investigated to replace CoSi2. Manufacturing a functioning device requires several processing steps after contact formation where the silicide temperature typically exceeds 600° C. Under such conditions, NiSi has been reported to agglomerate and form NiSi2 which has a higher resistivity than NiSi. See, for example, J. Y. Yew, et al. “Epitaxial growth of NiSi on (111) Si inside 0.1-0.6 mm oxide openings prepared by electron beam lithography”, Appl. Phys. Lett. 69(7), (1996); B. A. Julies, “A Study of the NiSi to NiSi2 Transition in the Ni—Si Binary System”, Thin Solid Films, 347, 201, 1999; and M. C. Poon, “Thermal Stability of Cobalt and Nickel Silicides”, Microelectronics Reliability 38, 1495, 1998.
Further, when shallow junctions (on the order of about 15 nm or less) are to be formed, nickel disilicide formation is seen under the spacer resulting in shorting of the device between the gate and the source/drain regions. The thermal stability of NiSi has to be improved before it can be used as a contact in submicron microelectronic devices.
Co-pending and co-assigned U.S. patent application Ser. No. 10/334,464, filed Dec. 31, 2002, now U.S. Pat. No. 6,905,560 entitled “Retarding Agglomeration of Ni Monosilicide Using Ni Alloys” describes various metals that can be co-deposited with Ni to prevent disilicide formation. Some of these metals, such as Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, W and Re, prevent nucleation of NiSi2. Ge, Rh, Pd, Pt or mixtures thereof can be used as stabilizing agents.
The successful formation of a silicide contact through the salicide process requires deposition of a thin (less than 10 nm) Ni alloy uniformly over the substrate, heating the substrate to form the silicide over regions where silicon is present and finally selectively etching the unreacted metal without attacking the silicide. Any attack of the silicide would reduce the thickness of the line and increase the contact resistance. Further, the etchant should be capable of not only removing Ni, but all the other metals added along with it. Present etchants for removing Ni and Ni alloys in the salicide contact process are based on a mixture of hydrogen peroxide and sulfuric acid at temperatures greater than 60° C. This is disclosed for example, in U.S. Pat. No. 6,468,901 and P. S. Lee, et al. “New Salicidation Technology with Ni(Pt) Alloy for MOSFETs”, IEEE Electronic Device Letters, 22 (12) 2001. Such etchants are incapable of removing all additional elements especially noble metals such as Pt, Pd, Rh and Re used along with Ni. Presence of the unreacted metal on the spacers and trench isolation regions leads to shorting of the devices thereby preventing the manufacturing of a functioning semiconductor chip. The remaining unreacted metal is referred in the art as stringers.
In view of the above, there is a need for providing a new and improved method for forming NiSi contacts which avoids leaving unreacted metals, i.e., stringers, on the spacers and trench isolation regions.