One of the primary steps in the fabrication of modern semiconductor devices is the formation of an insulation film on a semiconductor substrate. Such insulation films are used for a variety of purposes including, among others, separating adjacent conductive layers (e.g., an intermetal dielectric (IMD) layer separates adjacent metal lines while a premetal dielectric (PMD) layer separates metal layer one from the conductive substrate) and separating adjacent active regions of the substrate (e.g., as part of a shallow trench isolation (STI) structure).
For applications such as the deposition of IMD or PMD layers in an integrated circuit or the formation of STI structures, one important physical property of the insulation film is its ability to completely fill gaps between adjacent structures without leaving voids within the gap. This property is referred to as the film's gapfill capability. Gaps that may require filling include spaces between adjacent conductive lines, spaces formed by an etched trench or the like.
As semiconductor device geometries have decreased in size over the years, the ratio of the height of such gaps to their width, the so-called “aspect ratio,” has dramatically increased. Gaps having a combination of a high aspect ratio and a small width present a challenge for semiconductor manufacturers to fill completely. In short, the challenge usually is to prevent the film from forming in a manner that closes off the gap before it is filled. Failure to fill a gap completely results in the formation of a void in the deposited layer, which may adversely affect device operation.
FIG. 1 is a simplified cross-sectional view of a partially completed integrated circuit 10 that can help illustrate the gapfill issue. Partially formed integrated circuit 10 is formed over a silicon substrate 12 that includes a plurality of shallow trench isolation structures 14. As shown in FIG. 1, integrated circuit 10 has a relatively densely packed area 16 where densely packed active devices (e.g., transistors are formed) and a relatively isolated area 18 (also referred to as an “open area”) where an active device may be separated from another active device by a distance that is an order of magnitude more than the spacing between devices in the densely packed area.
A typical shallow trench isolation structure is created by first forming a thin pad oxide layer 20 over the surface of silicon substrate 12 and then form a silicon nitride layer 22 over pad oxide layer 20. The nitride and oxide layers are then patterned using standard photolithography techniques and trenches 24 are etched through the nitride/oxide stack into silicon substrate 12. Trenches 24 are then filled with an insulating material such as silicon dioxide using a deposition process that has good gapfill properties. Prior to the gapfill process, however, an initial lining layer 26, such as an in situ steam generation (ISSG) oxide or other thermal oxide layer or a silicon nitride layer, is usually formed.
In some applications trench 24 has an aspect ratio of between about 6:1 to 8:1 and the formation of a highly conformal film such as oxide liner 26 in trench 24 may increase the aspect ratio even further to, for example 10:1 or higher. Thus, the filling of trenches 24 is typically one of the most challenging gapfill applications in the formation of the integrated circuit.
One popular method of forming a gapfill insulation layer over a substrate includes using plasma-enhanced chemical vapor deposition techniques, and in particular high density plasma chemical vapor deposition (HDP-CVD) techniques. HDP-CVD deposition forms a dense plasma at a low vacuum pressure. The combination of plasma density and pressure, as well as the application of energy to bias the plasma towards the substrate during film deposition, results in a deposition process that has simultaneous deposition and sputtering components. It was initially thought that because of their simultaneous dep/etch nature, HDP-CVD processes could fill the gaps or trenches that were created in almost any application. Semiconductor manufacturers have found out, however, that there is a practical limit to the aspect ratio of gaps that HDP-CVD films are able to fill.
A variety of techniques have been developed to extend the gapfill capabilities of HDP-CVD processes. These techniques include the use of relatively light sputtering agents in addition to or instead of argon as well as various multistep HDP-CVD deposition/etch/deposition (dep/etch/dep) processes. Despite these newly developed techniques, there are some applications that could benefit from alternative deposition approaches. Accordingly, improved and/or alternative approaches are desirable to fill gaps on semiconductor substrates with films of insulating material.