XPost: sci.electronics.design   
   From: pcdhSpamMeSenseless@electrooptical.net   
      
   Rhydian wrote:   
   > On Tue, 18 May 2021 21:13:25 -0400, Phil Hobbs wrote:   
   >   
   >> So I have this project looking to measure babies' blood oxygenation   
   >> noninvasively, i.e. using an optical sensor looking through the mom's   
   >> abdomen.   
   >>   
   >> 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   
   >> advertised use.   
   >>   
   >> Turns out that they have about 1.5_MICROFARAD_ shunt capacitance.   
   >> Where's Radio Shack when you need them?   
      
   > So 10 uF per square metre.   
      
   I misspoke--mine are 30x50 _millimetres_.  So it's more like 1   
   millifarad per square metre, or 100 nf/cm**2.  Really good PIN   
   photodiodes run about 40-100 pf/cm**2 when fully depleted, about 5-7x   
   that at zero bias.   
      
   I'm out of the lab today, but I'll try resonating the capacitance and   
   see what kind of Q I get.  I need to work around 220 Hz, which is far   
   enough from harmonics of both 50 and 60 Hz for my purposes--at 1.5 uF,   
   that needs a 300-mH inductor.   
      
   There's such a thing as a parametric gyrator, so it might even be   
   possible to use a Y5V cap and some magic to make a sufficiently-quiet   
   simulated inductor.   
      
   (I'll get a nanoamp of photocurrent if I'm lucky, so the Q has to be   
   high or I'm better off with a smaller detector.)   
      
   I don't really think that the nasty $3 AliExpress gizmo is the right   
   answer for the actual measurement, but it's worth checking out the   
   parameter space.   
      
   That sounds like a lot, until I worked it out   
   > for the Osram SFH2700FA I'm using for a new design.  0.59 x 0.59 mm and   
   > 4.6 pF, which works out to 13 uF per square metre.  It's a PIN device,   
   > which is supposed to reduce the capacitance.  Or am I missing something?   
   >   
   > I like the Osram parts so far, but it's not a very demanding application   
   > bandwidth wise.  It _is_ very space constrained, hence the small area.   
   > It's the best of a similar bunch I tested for sensitivity.   
      
   Cheers   
      
   Phil Hobbs   
      
      
   --   
   Dr Philip C D Hobbs   
   Principal Consultant   
   ElectroOptical Innovations LLC / Hobbs ElectroOptics   
   Optics, Electro-optics, Photonics, Analog Electronics   
   Briarcliff Manor NY 10510   
      
   http://electrooptical.net   
   http://hobbs-eo.com   
      
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