DW-Mai: Polysilicon metrology

02/01/1997

DW-MAI: Polysilicon metrology

Scientists at Rudolph Technologies and Bruce Technologies are developing dual wavelength multiple-angle-of-incidence (DW-MAI) ellipsometry as a reliable process tool to control the uniformity of polycrystalline silicon. Polycrystalline silicon (polySi) films are widely used in semiconductor processing for gate electrodes and interconnect materials. Controlling the uniformity of these films is critical for consistent, high yield semiconductor production.

Uniform polySi microstructure and its optical and electrical properties depend on the deposition process conditions and the optimum furnace design. To optimize and control the deposition process, it is necessary to have metrology equipment that is capable of measuring both physical properties and thickness of the film simultaneously. Neglecting even small variations in the polySi properties can cause errors in film thickness measurement. Current optical techniques for evaluating polySi films include spectroreflectometry, single-wavelength ellipsometry, spectroscopic ellipsometry, and DW-MAI.

Spectroreflectometry and related techniques assume fixed values for the optical constants. They do not take into account variations of the film`s physical properties. Standard single-wavelength ellipsometry has thickness order ambiguity problems so measurements can become unreliable. Spectroscopic ellipsometry, while a powerful research tool, requires sophisticated modeling and has problems measuring rugged polySi due to a sensitivity to surface morphology.

DW-MAI ellipsometry combines the reliability and strength of spectroscopic measurements with the simplicity of a single wavelength ellipsometer. Recently, scientists at Rudolph and Bruce characterized polysilicon deposition in a new, high capacity vertical reactor. Hot-wall reactors for polySi deposition on 200-mm wafers are typically vertical in orientation compared with historically horizontal reactors. A progression in vertical reactors has been the increase in throughput achieved by increasing the length of the flat temperature zone and the reduction in wafer spacing to enable more wafers to be processed. Flat temperature, low pressure chemical vapor deposition (LPCVD) vertical furnaces, as opposed to tilt temperature processes, enhance microstructure uniformity and minimize defect density. By employing a patented focused-beam ellipsometry system to measure both thick and thin films at dual wavelengths with multiple angles of incidence, reliable and unambiguous results on thick films could be obtained. The focused beam technique focuses a light source (generally a HeNe laser) onto a small defined spot on the wafer. The ellipsometer`s detectors then collect and record data about the phase and intensity of reflected polarized light. Based on this information, the thickness, composition, and other properties of the films can be calculated.

Studies report that the DW-MAI was able to simultaneously measure poly-thickness, poly-composition, and surface roughness of the films as well as the influence of deposition conditions on (?) within wafer f (WTW) and (?) across reactor f uniformity for standard as-deposited, rugged (HSG) and diffusion doped polySi. The results were compared with other measurement techniques and found to be consistent with those from a research spectroscopic ellipsometer, an AFM, and a resistivity probe. Questions concerning the latest developments in DW-MAI technology can be sent to Leo Asinovsky at Rudolph Research Corp., Flanders, NJ; ph 201/691-1300, or Michael Schroth at Bruce Technologies Int`l., N. Billerica, MA; ph 308/670-5501.- M.Y.M.L