1. Field of the Invention
The present invention relates, in general, to chemical vapor deposition (CVD) apparatus and processes and, more particularly, to a method and apparatus for depositing highly conformal CVD films.
2. Statement of the Problem
Integrated circuit technology has advanced through continuing improvements in photolithographic processing so that smaller and smaller features can be patterned onto the surface of a substrate. Spaces or gaps exist between these patterned features. Integrated circuit surfaces also contain trench or "via" structures protruding down into the surface. The lateral dimension of such structures is hereafter referred to as the width of the gap, trench, or via; the vertical dimension of such structures is referred to as the depth. The aspect ratio is the ratio of depth to width.
The smaller features, with smaller spaces between features, result in gaps, trenches, and vias with high aspect ratios. These high aspect ratio structures must be filled with an appropriate material before continued processing. This problem is acute in the case of multiple layers metal (MLM) designs where dielectric must be deposited after each metal layer is formed and patterned before a subsequent metal layer can be formed and patterned.
When a deposited film is used to completely fill the high aspect ratio structure, three different results can occur. In one case, the deposited material fills the trench without leaving a seam or void. In a second case, a seam arises from the point where the sidewall layers merge during deposition. In a third case, a void arises if the deposition produces re-entrant profiles at earlier stages of the filling process. The first case creates integrated circuits with the highest reliability. The seams and voids are undesirable, since chemicals or materials may be present in the seam or void to corrode or degrade the structure. Moreover, voids are rarely hermetically sealed, so subsequent exposure to chemicals or materials deposition can alter the material structure substantially.
Deposition onto trench and via structures is commonly practiced at several stages in the fabrication of semiconductor devices and interconnections. Most often the objective is to provide highly conformal films or void-free (and preferably seam-free) filling. The problem and challenge presented is that of trench structures with increasing aspect ratios, consistent with the trend toward higher lateral device density.
Low-pressure chemical vapor deposition (LPCVD) and plasma-enhanced chemical vapor deposition (PECVD) are widely used to provide conformal deposition of thin films over topography. Physical vapor deposition techniques (evaporation, sputter-deposition) are typically limited to structures with low aspect ratios. LPCVD processes offer better conformality and filling properties.
A number of chemical vapor deposited (CVD) films are currently used at various steps of integrated circuit manufacturing processes. Typically, sidewall coverage is not uniform along the height of a trench or via. A tapered shape has thicker sidewall coverage toward the bottom of the sidewall than toward the top, while the situation is reversed for a re-entrant shape. Generally speaking, the tapered shape is more desirable than the re-entrant shape, because in the latter the overhang of deposited material near the top of the trench shadows the region below, and the consequences of subsequent deposition can be ill-defined.
CVD processes operate by confining one or more semiconductor wafers in a chamber. The chamber is filled with one or more reactant gases that surround the wafer. Energy is supplied within the chamber and particularly to the reactant gases near the wafer surface. The energy activates the reactant gas chemistry to deposit a film from the gas onto the heated substrate. Such chemical vapor deposition of a solid onto a surface involves a heterogeneous surface reaction of the gaseous species that adsorb onto the surface. The rate of film growth and the quality of the film depend on the wafer surface temperature and on the gas species available.
More recently, low-temperature plasma-enhanced deposition and etching techniques have been used to form diverse materials, including metals such as aluminum and tungsten, dielectric films such as silicon nitride and silicon dioxide, and semiconductor films such as silicon. The plasma used in the PECVD is a low-pressure reactant gas that is developed in a radio frequency (RF) field. The RF plasma results in a very high electron temperature, making possible the deposition of dense, good quality films at lower temperatures and faster deposition rates than are typically possible using purely thermally activated CVD processes.
Current CVD processes have important limitations. With higher integration levels or higher performance structures, higher aspect ratios are required, stretching the ability of known CVD processes. Re-entrant profiles, seams, and voids all endanger the manufacturability of the semiconductor product due to yield and reliability problems. When higher growth temperatures improve conformality or profiles, other properties of the three-dimensional structure may be degraded (e.g., abrupt doping profiles due to diffusion). Alternatively, lower reaction probabilities ("reactive sticking coefficient") for well-chosen CVD chemistries can yield higher conformality, but throughput is degraded, making the approach less competitive. Thus, conventional CVD processes may not be capable of achieving the three-dimensional profiles and filling characteristics needed for next-generation technologies.
Step coverage for CVD films is a continuing problem in the integrated circuit manufacturing industry. A method and apparatus are needed for highly conformal CVD deposition, particularly deposition of oxide films.
3. Solution to the Problem
The present invention is directed to providing a CVD process offering controlled deposited layer thickness over three-dimensional patterned features and trench and via structures. The present invention provides the ability to control how the thickness of the deposited layer varies along bottom, sidewall, and top surfaces of high aspect ratio features patterned on an integrated circuit. The invention permits controlled shaping of thin film layers including, for example, (1) tapered rather than re-entrant shapes (i.e., thicker at the bottom rather than at the top), (2) enhanced sidewall and/or bottom coverage of trench structures, (3) voidless, seamless filling even at high aspect ratios, and (4) asymmetric sidewall coverage.