Particles in plasma etch chambers are a major source of yield loss in semiconductor manufacturing. Particles condensing from the plasma or spalling out of films on the chamber walls are levitated in the edges of the plasma sheath for long periods, and then (too often) drop on the wafer when the plasma excitation is turned off.
Process control and tool utilization can both be improved by knowing what's happening inside the chamber while the process is going on—but how? The plasmas are usually too bright to look at, and there's only one (poor quality) window in the typical chamber, so an optical particle detector would have to work in backscatter, with a huge background.
ISICL is capable of seeing and mapping individual particles of less than 0.2 μm diameter, as they float around in the plasma, a unique capability.
An interesting combination of homodyne interferometry, laser noise cancellation, and signal processing allows reliable operation at the shot noise limit in the face of a coherent background 106 times larger than the signal and an incoherent background 108 times larger.
The techniques I developed for ISICL have proven to be very widely applicable. My collaborators David Bomse and Daniel Kane at Mesa Photonics have had a DOE project for a receiver of a similar design in a spectroscopic sensor for nuclear nonproliferation monitoring, based on heterodyne detection of sunlight.
Even more recently, with a large industrial customer, I have been working on an advanced version of ISICL for detecting and mapping submicron particles moving at extremely high velocity. Calculations show that with only a watt of laser power, we should be able to see 0.15 μm particles moving at up to 3 km/s, i.e. Mach 9. I'll post more as this exciting opportunity develops.
Detailed paper on the physics, design, and experimental results from the ISICL sensor.
(Applied Optics, 34(9), March 20, 1995)
(Philip C. D. Hobbs & Marc A. Taubenblatt)
Calculation vs experiment for the photon budget, signal processing, and detection strategy of the ISICL sensor. Shows that a photon budget is worth following, even in a complicated instrument used in hostile conditions.
(Proc. SPIE 2909 1997 P11-20)