Use of gaseous silicon hydrides as a reducing agent to remove re-sputtered silicon oxide

This invention is related to a semiconductor article and a method for processing of semiconductor devices. In particular, the invention is directed to an improved method for cleaning unwanted sputter deposits from source/drain surfaces. Further, the invention is particularly directed to performing an in situ cleaning process to remove re-sputtered SiO x deposited on the source/drain surface in a semiconductor device.

During manufacture of a semiconductor device, it is necessary to clean selected surfaces of the device before sputter deposition of a metal, such as cobalt which is intended to form a cobalt silicide as part of the final device structure. During such a prior art process, as shown in FIG. 1, the argon ions 10 tend to sputter SiO x species 12 from the SiO 2 spacer material 14 which deposits on other surfaces, such as source/drain surfaces 16 . As a result, the contaminating SiO x species 12 on the source/drain surfaces 16 prevent deposition of cobalt onto silicon to form the desired cobalt silicide layer. The resulting CoSi layer is therefore spotty and potentially discontinuous which can result in degraded performance of a semiconductor device 18 . It is therefore important to develop a process to prevent re-sputtered SiO 2 from depositing on the source/drain surfaces 16 .

According to one form of the invention, a semiconductor device is prepared for deposition of a cobalt silicide onto a source/drain region. The device undergoes a first standard sputter pre-clean using an inert gas ion, such as argon. After this sputter pre-clean step, the device surfaces are further cleaned by use of a mixture of silicon hydride, or silanes, and hydrogen (H 2 ). In the most preferred form, H 2 is used with SiH 4 to assure no deposition of Si. These silicon hydrides can be activated by RF plasma or by an elevated wafer temperature to form SiH 2 and SiH 3 species. These silane derivatives are applied in situ to the semiconductor device, particularly to source/drain surfaces which are to be cleaned in preparation for formation of a cobalt silicide layer. The silane derivatives act to convert to elemental Si the contaminating SiO x deposits formed from the argon pre-sputter of the SiO 2 spacer material present in the semiconductor device.

These and other objects, features and advantages of the invention will be apparent from the following description of the preferred embodiments and examples, taken in conjunction with the accompanying drawings described below.

FIG. 1 shows a standard prior art method of a sputter pre-cleaning of a semiconductor device having a source/drain surface; and

FIG. 2 shows a process of the invention for further cleaning of contaminants left by the process of FIG. 1 .

FIG. 2 illustrates schematically a preferred embodiment of a method of the invention. This method is preceded by conventional sputter etch pre-cleaning of a semiconductor device 18 in the manner shown schematically in FIG. 1 . This device 18 includes a source/drain surface 16 and a SiO x spacer material 14 is impacted by an argon ion beam 10 which causes sputtering of SiO x species 20 onto the device 18 , including the source/drain surface 16 . The SiO x species 20 form at least islands of deposited material (and potentially a continuous film) and prevents deposition of cobalt, or other desired species, onto the silicon. In processing the preferred form of the semiconductor device 18 , the ultimate purpose of the cobalt deposition is to form a thin, uniform silicide layer. However, re-sputtered SiO x deposits onto the source/drain surface 16 , preventing formation of the desired cobalt silicide and can substantially impair performance of the semiconductor device 18 .

The resulting unwanted deposits of the SiO x species 20 can be removed by the process of in situ reduction of the deposited SiO x by introducing silane species, such as SiH 4 and Si 2 H 6 , for example, mixed with H 2 . The step is performed in situ causing reduction of SiO x to such gaseous species as H 2 O and SiH 4 and H 2 and leave a deposited Si on the silicon-based source/drain surface 16 . The cleaned form of the surface 16 is then subject to deposition of Co atoms 24 on the source/drain surface 16 to form a desired cobalt silicide layer 22 .

This reduction process for removal of the unwanted SiO x 12 is also performed at temperatures and pressures which are low enough to prevent chemical vapor deposition of silicon from the presence of silane species mixed with H 2 . In a most preferred embodiment, the temperature of the source/drain surface 16 is about 500°-550° C. In another form of the invention, the source/drain surface can be about 200° C., and even less, provided an RF plasma can be formed in situ to activate the silane species. The activated silane species of the desired molecular state include forming activated SiH 2 and SiH 3 species to carry out the reduction of the SiO x which have been deposited on the surface 16 .

As a result of the above-described preferred processing methods, an article of manufacture is produced which embodies a superior structure for the semiconductor device 18 . The device 18 as processed hereinbefore provides the desired substantially continuous, uniform cobalt silicide layer on the source/drain surface 16 , thereby resulting in enhanced performance and reliability of the semiconductor device 18 .

In the most preferred embodiment, the reaction chemistry is thermodynamically favorable when the sputtered contaminant is analogous to SiO. If the contaminant is SiO 2 , the thermodynamics predicts the reduction reaction by SiH 4 or H 2 will not occur. The exact chemical composition of the SiO x is likely not to be stoichiometric SiO 2 , considering that it is formed by the redeposition of sputtered Si and O atoms.

The following non-limiting example illustrates an embodiment of the method and article of manufacture of the invention.

In an intermediate step in the manufacture of integrated circuits, semiconductor wafers with exposed source/drain silicon surfaces are processed in a conventional, well-known UHV metal sputtering system with a layer of sputter-deposited cobalt. In carrying out this process step, wafers pass through a load-lock into the sputter-deposition chamber. The wafer is then sputter-etch cleaned for ˜10 sec in an argon plasma generated using 150-400 W of RF power with DC bias and an Ar flow of 5 sccm. During this step some unwanted SiO x is sputter-deposited onto the clean silicon surface. After the sputter-etch step, the wafer is heated to between 400-550 C and a mixture of SiH 4 and H 2 (1:1) gases are passed over the wafer's surface for 60 sec to reduce the SiO x to elemental silicon. Alternatively, the wafer can be heated to ˜200 C, and while the SiH 4 and H 2 mixture is present, a low power RF plasma can be produced to activate the gaseous reducing agents. After the chamber has been evacuated of reactant gases, the Co metal is sputter-deposited onto the wafer by an argon RF plasma using from between 600-100 W. The final Co thickness is between 50-250 A deposited onto a SiO x free source/drain surface. The wafer is removed from the sputter chamber for subsequent process steps.

These and other objects, features and advantages of the invention will be apparent from the following description of the preferred embodiments and examples, taken in conjunction with the accompanying drawings described below.