1. Technical Field
The present invention generally relates to a semiconductor device and, more particularly, to a semiconductor device having a plug for diffusing hydrogen into a semiconductor substrate which is covered by one or more layers which substantially form a barrier to hydrogen diffusion.
2. Description of Related Art
Junction leakage is a well-known problem for semiconductor devices such as transistors and can adversely affect device operation and performance. One technique for reducing junction leakage diffuses hydrogen into the semiconductor substrate on which the semiconductor devices are fabricated. This diffusion may be accomplished by an annealing process (typically at a temperature in the range from about 400.degree. to about 600.degree. C.) in an atmosphere containing hydrogen. The source of hydrogen may be a gas which consists of H.sub.2 and N.sub.2. Pure H.sub.2 (100% H.sub.2) may also be used. The diffused hydrogen terminates the surface states and reduces junction leakage.
FIG. 1 is a sectional view of a portion of a conventional semiconductor device which includes a transistor 10 formed on a semiconductor substrate 1. Transistor 10 may, for example, be formed in a peripheral circuit region of a semiconductor memory device such as a dynamic random access memory device (DRAM) or may be part of a logic circuit. Transistor 10 includes a source/drain diffusion region 2 and a drain/source diffusion region 3 which are spaced apart by a channel region 4. A gate electrode 6 is insulatively spaced from channel region 4 by a gate insulating layer 5. Source/drain region 2 and drain/source region 3 may be formed, for example, by performing an ion implantation using gate electrode 6 as an implantation mask. An isolation region 7 of an insulator such as silicon dioxide (SiO.sub.2) electrically isolates transistor 10 from other elements formed on semiconductor substrate 1.
In high density large scale integrated circuit (LSI) devices such as 64 Mbit and 256 Mbit DRAMs, a self-aligned contact (SAC) process is often utilized for forming device contacts. Using an SAC process, the spacing between the contacts can be reduced and a high integration density can be achieved. In many SAC processes, a silicon nitride layer is formed over the LSI device area to serve as an etch-stop layer. As a result, with reference to FIG. 1, a silicon nitride layer 8 is formed over isolation region 7, source/drain region 2, drain/source region 3, and gate electrode 6. However, silicon nitride layer 8 is substantially a diffusion barrier and thus prevents the diffusion of hydrogen into semiconductor substrate 1. Thus, in high density LSIs in which a silicon nitride layer is formed during an SAC process, the diffusion of hydrogen into semiconductor substrate 1 to reduce junction leakage is prevented. This is a particular problem, for example, for transistors in the peripheral circuit region of a DRAM or for transistors in a logic circuit. While hydrogen could be diffused into the substrate before the silicon nitride layer is formed, the hydrogen can be easily removed from the surface states by high temperature LSI process steps which follow the formation of the silicon nitride layer. Thus, the hydrogen diffusion is most effectively carried out after the high temperature steps of the LSI fabrication process. However, as described above, at this time, the silicon nitride layer acts as a barrier to such diffusion.