One of the primary steps in the fabrication of modern semiconductor devices is the formation of a film, such as a silicon oxide film, on a semiconductor substrate. Silicon oxide is widely used as an insulating layer in the manufacture of semiconductor devices. As is well known, a silicon oxide film can be deposited by a thermal chemical-vapor deposition (“CVD”) process or by a plasma-enhanced chemical-vapor deposition (“PECVD”) process. In a conventional thermal CVD process, reactive gases are supplied to a surface of the substrate, where heat-induced chemical reactions take place to produce a desired film. In a conventional plasma-deposition process, a controlled plasma is formed to decompose and/or energize reactive species to produce the desired film.
Semiconductor device geometries have decreased significantly in size since such devices were first introduced several decades ago, and continue to be reduced in size. This continuing reduction in the scale of device geometry has resulted in a dramatic increase in the density of circuit elements and interconnections formed in integrated circuits fabricated on a semiconductor substrate. One persistent challenge faced by semiconductor manufacturers in the design and fabrication of such densely packed integrated circuits is the desire to prevent spurious interactions between circuit elements, a goal that has required ongoing innovation as geometry scales continue to decrease.
Unwanted interactions are typically prevented by providing spaces between adjacent elements that are filled with an electrically insulative material to isolate the elements both physically and electrically. Such spaces are sometimes referred to herein as “gaps” or “trenches,” and the processes for filling such spaces are commonly referred to in the art as “gapfill” processes. The ability of a given process to produce a film that completely fills such gaps is thus often referred to as the “gapfill ability” of the process, with the film described as a “gapfill layer” or “gapfill film.” As circuit densities increase with smaller feature sizes, the widths of these gaps decrease, resulting in an increase in their aspect ratio, which is defined by the ratio of the gap's height to its depth. High-aspect-ratio gaps are difficult to fill completely using conventional CVD techniques, which tend to have relatively poor gapfill abilities. One family of electrically insulating films that is commonly used to fill gaps in intermetal dielectric (“IMD”) applications, premetal dielectric (“PMD”) applications, and shallow-trench-isolation (“STI”) applications, among others, is silicon oxide (sometimes also referred to as “silica glass” or “silicate glass”).
Some integrated circuit manufacturers have turned to the use of high-density plasma CVD (“HDP-CVD”) systems in depositing silicon oxide gapfill layers. Such systems form a plasma that has a density greater than about 1011 ions/cm3, which is about two orders of magnitude greater than the plasma density provided by a standard capacitively coupled plasma CVD system. Examples of HDP-CVD systems include inductively coupled plasma (“ICP”) systems and electron-cyclotron-resonance (“ECR”) systems, among others. One factor that allows films deposited by HDP-CVD techniques to have improved gapfill characteristics is the occurrence of sputtering simultaneous with deposition of material. Sputtering is a mechanical process by which material is ejected by impact, and is promoted by the high ionic density of the plasma in HDP-CVD processes. The sputtering component of HDP deposition thus slows deposition on certain features, such as the corners of raised surfaces, thereby contributing to the increased gapfill ability.
A variety of known techniques have often used to promote the sputtering effect, and have proved successful for gaps with relatively modest aspect ratios and widths. For example, one technique to promote sputtering is to introduce argon or a similar heavy inert gas to further promote the sputtering effect. Another technique is to create an electric field that biases the plasma towards the substrate, such as by using an electrode within a substrate support pedestal to generate the electric field. It was thus initially thought that the simultaneous deposition and sputtering characteristics of an HDP-CVD process could be used to fill the gaps that were created in almost any application. Semiconductor manufacturers discovered, however, that there is a practical limit to the aspect ratio of gaps that HDP-CVD films are able to fill. For example, one HDP-CVD process commonly used to deposit a silicon oxide gapfill film forms a plasma from a process gas that includes silane SiH4, molecular oxygen O2, and argon Ar. It has been reported that when such a process is used to fill certain narrow-width, high-aspect-ratio gaps, the sputtering caused by the argon in the process gas may hamper gapfill efforts. Specifically, it has been reported that material sputtered by argon in the process redeposits on the upper portions of the sidewalls of the gaps being filled at a rate faster than at the lower portions. This, in turn, may result in the formation of a void in the gap if the upper areas of regrowth join before the gap is completely filled.