This invention relates generally to methods and apparatus for measuring surface topographies and more particularly to methods and apparatus for measuring the surface topographies of semiconductor wafers, hard disk platters, optical blanks and other high-tolerance workpieces.
Integrated circuits are formed on semiconductor wafer substrates by a number of processing steps. These steps include deposition, etching, implantation, doping, and other semiconductor processing steps well known to those skilled in the art.
Thin films are typically formed on wafer surfaces by a deposition process. These thin films can comprise, for example, silicon dioxide, AlSi, Ti, TiN, PECVD Oxide, PECVD Oxynitride, doped glasses, silicides, etc. The thickness of such films usually ranges from about a few hundred angstroms to several micrometers. Often, three or more film layers are formed on the surface of a single semiconductor wafer.
In the art of fabricating semiconductor wafers, it is of known importance to minimize or control stresses in surface films. High surface stresses can cause, for example, silicide lifting, the formation of voids or crack and other conditions that adversely affect semiconductor devices (i.e. chips) which are fabricated on the wafers. In practice, surface stresses become more problematical as the level of circuit integration increases, and are especially troublesome when fabricating large scale integration (LSI), very large scale integration (VLSI), and ultra large scale integration (ULSI) semiconductor devices.
The stress in the surface film of a semiconductor wafer can be either compressive or tensile. A compressive stress in a surface film will cause a wafer to slightly bow in a convex direction, while a tensile stress in a surface film will cause a wafer to slightly bow in a concave direction. Therefore, both compressive and tensile stresses cause the surface of the semiconductor wafer to deviate from exact planarity. The extent of the deviation from planarity can be expressed in terms of the radius of curvature of a wafer surface. In general, the greater the magnitude of surface stress, the smaller the radius of curvature.
Because of the problems that can be caused by stresses in surface films on semiconductor wafers, it is highly desirable to be able to measure such stresses. The measurements can be used, for example, to identify wafers that are likely to provide low yields of semiconductor devices or which might produce devices prone to early failure. In normal practice, stresses in surface films are not measured directly but, instead, are inferred from measurements of the radius of curvature of the surface of interest.
A system for measuring film stress by measuring the radius of curvature of a wafer is described in an article entitled "Thermal Stresses and Cracking Resistance of Dielectric Films on Si Substrates," A. K. Sinha et. al., Journal of Applied Physics, Vol. 49, pp. 2423-2426, 1978. Other systems are described in copending patent application Ser. No. 07/822,910, filed Jan. 21, 1992 and U.S. Ser. No. 07/357,403, filed May 26, 1989. All of these systems linearly scan across a wafer to determine the curvature of the wafer along that scan line. This type of wafer scanning can be referred to as a "1-D" linear scan reflecting the fact that it is a one-dimensional scan of the wafer's surface, such as in the x direction. A 1-D scan is quite effective for wafers having fairly uniform surface topographies and uniform film layers, but may be less than adequate for more complex surface topographies or for film layers that are somewhat uneven. This is because the radius of curvature for such wafers may be significantly different when taken along different scan lines along the surface of the wafer. If the particular scan line chosen provides a radius of curvature is which far from the average radius of curvature, the film stress calculated from the radius of curvature will be incorrect.
There are other applications for a method and apparatus for measuring surface curvature besides determining the mechanical stress in films. For example, it is often desirable to know the surface curvature (i.e. the "flatness") of hard disk platters or the radius of curvature of optical elements. In the prior art, such curvature measurements were made by expensive laser interferometry equipment.