So I have this project looking to measure babies' blood oxygenation
noninvasively, i.e. using an optical sensor looking through the mom's
The idea is to make the business end cheap--ideally disposable. Come
with me, if you will, on a trip down memory alley.
Circa 1992, my friend and colleague Ted van Kessel and I did an
interesting semiconductor process control instrument for DRAM fab at
IBM, Burlington VT. (This was back in the 0.5-micron days, when optical
inspection was competitive.)
At the time, photoresist was generally acid catalyzed, i.e. it developed
something like photographic film. The litho tool (wafer stepper)
exposed the resist, liberating a bit of acid. Then the wafer went onto
a hot plate so that the acid could act like developer, breaking a bunch
more bonds and rendering the image developable.
The resulting line width depended on both the exposure dose and the
temperature/duration of the bake step. So Ted and I built this gizmo to
look at the diffraction pattern of the latent image as it developed on
the hot plate, and lift the wafer off it when the diffracted beam
strength was just right. That way we had a closed-loop method for
controlling line width in litho, shazam. (Turned out the fab folks
didn't want it, but I digress.)
IBM's DRAM cells were arranged in a hexagonal pattern, so when you
shined a LED vertically down on the wafer, you got a
hexagonally-symmetric optical diffraction pattern from the latent image
in the resist, with some contribution from the lower layers (previously
fabricated). Ordinarily you'd only need one diffracted order for a
measurement like that, but to correct for diffraction from the
underlying structure we needed clean +-1 orders in at least one of the
three symmetry axes of the hexagonal pattern. Unfortunately, there was
no way to control the orientation of the wafer on the hot plate, because
previously there was no reason to care about it, so the diffraction
orders could be anywhere in azimuth.
We wound up with seven 1x3-inch solar cells arranged like a 360-degree
poker hand around the vertical axis (i.e. with a bit of a taper in the
direction away from the wafer). With sevenfold symmetry, regardless of
how the wafer was oriented, we got clean measurements of at least one
+-1 order pair.
Those cells worked fine up to about 20 kHz, running into a
common-emitter stage followed by a regular op amp TIA. All the cathodes
were connected to the summing junction, and the anodes were multiplexed
to ground using open-drain outputs of a zero-power PAL (PALCE16V8Z).
(Zero-power PALs didn't push power supply noise out their outputs when
in open-drain mode.) So probably 20 nF or so.
Coming back to the fetal pulse ox gizmo, I thought it would be fun to
see how fast a modern amorphous cell could go. I got some 30x50 cm ones
from AliExpress, which looked OK, and in fact they work fine for their
Turns out that they have about 1.5_MICROFARAD_ shunt capacitance.
Where's Radio Shack when you need them?
Dr Philip C D Hobbs
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510