VAMMP Gives Precise, Non-Invasive Measurements

Alexander E. Braun, Associate Editor -- Semiconductor International, 10/1/1999

A precise and non-invasive real-time technology, originally developed for landmine detection, may soon find applications in semiconductor processing, particularly to monitor vacuum processes. It might even be used to detect more general contamination, such as that found in cleanroom atmospheres.

The vacuum multi-constituent monitor of plasma, or VAMMP, developed over a six-year period by Dr. Henry Blair and colleagues at Loch Harris Inc. (Austin, Texas), uses the scatter phenomenon, which is the process whereby energy is removed from a beam of electromagnetic radiation (EM) and re-emitted with a change in direction, phase or wavelength. Since all EM radiation is scattered by the medium it passes through -- whether gaseous, solid, or liquid -- this effect can be analyzed and reduced to usable data.

EM interacts primarily with the medium's electrons, not the nuclei. The scattering process in these wavelength regions consists of acceleration of the electrons by the incident beam, followed by reradiation from the induced accelerating charges. It is this reradiated energy's parameters that provide the information used by the system to characterize, measure and identify the constituents of the mixture of interest.

Fig 1 Schematic of the vacuum multi-constituent monitor of plasma. This system is capable of making real-time concentration measurements of individual chemicals in a complex mixture using a multispectral coherent-source remote sensing system. (Source: Loch Harris)

These wavelength differentials are used like a fingerprint to initially identify the material being measured. This 'spectral fingerprint' is then employed to provide the system software with first-order discrimination data enabling identification in a noisy environment. Photon density at the detector/sensor subsystem provides concentration data at a ~10 ppb level, while its tomograp hic optic furnishes 'slices' across the area of interest, generating a 3-D map of the entire chamber or 'within-wafer' area.

VAMMP can make real-time concentration measurements of individual chemicals in a complex mixture using a multispectral coherent-source remote sensing system. The chemical recognition and analysis software is composed of three parts: a multivariate analysis package for quantitative concentration and uncertain estimates; a genetic optimizer that customizes the multivariate algorithm for specific applications; and an intelligent neural net chemical filter that preselects from a chemical database to find appropriate candidate chemicals for quantitative analyses by the algorithm, as well as providing a 'quick-see' concentration estimate and consistency check. (Figure)

The system weighs 20 kg and could fit in a suitcase. It requires annual calibration, and its real-time, non-invasive in situ operation technology is transportable to any vacuum process. All it requires is a silver-dollar-sized quartz window in the chamber, to get EM energy in and out.

The VAMMP concept could go far in meeting the semiconductor industry's long-range metrology requirement, which is not to use predictive measurements, but actually to measure, monitor and then use high-level feedback loops to manage the processing environment. Presently, Loch Harris is looking for a non-financial partner to provide implementation and deployment data support, as well as an evaluation site for the prototype.