1.  Field of the Invention
The present invention relates in general to semiconductor manufacturing, and particularly to a method of forming a salicide without consuming the silicon in the substrate such that junction leakage occurs. 
2.  Description of the Related Art
In semiconductor technology, a Metal-Oxide-Semiconductor transistor comprises a gate and a source/drain.  The gate comprising a metal layer and SiO2 is deposited on a silicon-based substrate.  Most of the metal has bad adhesion with SiO2.  Thus, polysilicon with good adhesion is proposed to replace the metal layer.  However, the resistance of polysilicon interconnections is so high that polysilicon is not appropriate as a conductive layer.  One way to solve this problem is to form a metal salicide on the polysilicon to enhance the conductivity.  As methods of fabricating semiconductor integrated circuits (IC) continually improve, the number of devices that may be introduced into a single semiconductor chip has increased, while the size of each device has decreased.  Millions of devices may now be fabricated on a single chip.  Therefore, the salicide process is widely applied to ensure shallow junction or ultra shallow junction. 
FIGS.  1A-1F are schematic cross-section illustrating steps for forming a salicide of the prior art.  In FIG.  1A, a substrate 10 comprises a gate oxide layer 11, a polysilicon gate 12, a source/drain 14, a spacer 20, and a field oxide 22.  The substrate 10 is cleaned by HF solution, and a metal layer 30 is formed on the substrate 10 to cover the polysilicon gate 12, the source/drain 14, and the spacer 20, as shown in FIG.  1B. A first rapid thermal process is performed at 650-700xc2x0 C. in nitrogen atmosphere, and the metal salicide 31 is formed by reacting the portions of the metal in the metal layer 30 with the silicon in the gate 12 as well as in the source/drain 14, as shown in FIG.  1C. The portions of the metal in the metal layer 30 on the spacer 20 and the field oxide 22 remain the same without reacting.  Then, a second thermal process is performed at about 800xc2x0 C. in nitrogen atmosphere, and the phase of the metal salicide 31 is changed to a phase with lower resistance, as shown in FIG.  1E. The salicide process of prior art is accomplished. 
The silicon in the source/drain 14 consumes the metal salicide 31 during the salicide process.  For example, the reaction follows the following rule when a cobalt salicide is formed: Co+Sixe2x86x92CoSi2 The ratio of the thickness of Co:Si:CoSi2 is 1:3.6:3.5.  Silicon with about 360 xc3x85 thickness must be consumed to form a CoSi2 with about 350 xc3x85 thickness.  Therefore, 0.04 of the source/drain 14 is lost, and the problem of the junction leakage following occurs, as shown in FIG.  2. 
To solve above problem, it is an object of the present invention to provide a method of forming a salicide to excess consumption of silicon in the substrate. 
The method comprises the following steps.  First, a silicon-based substrate comprising a gate with a spacer on the side wall of the gate and a source/drain is provided.  Next, a metal layer is formed according to the topography of the substrate to cover the gate, the spacer, and the source/drain.  A first thermal treatment is performed to react the portions of the metal in the metal layer with the silicon in the gate and the source/drain to form a salicide.  Then, the unreacted metal and the spacer are removed.  An ion containing silicon is introduced into the source/drain as an extra silicon source for salicide.  Finally, a second thermal treatment is performed. 
In the preferred embodiment of the present invention, the silicon ion is introduced by ion implantation.  The first and second thermal treatment are perferably performed by rapid thermal processes between about 600xc2x0 C. and 800xc2x0 C. Besides, the unreacted metal layer and the spacer are removed by etching as is well known.