How come really smart people are so dumb sometimes?


Just had an ambush call.  (that is when Sales invites you on a call, but “forgets” to clue you in on what is to be discussed). Needless to say, some pretty deep hip waders were needed.

The whole premise was that we failed to accomplish some performance goals in a demo.  We make high end, scientific instruments.  Part of the analyses we do, required you to “find” a region of interest.  We, our competitors, and indeed all products on the market like ours uses a similar design. A video camera, a microscope objective/or telescope, and a real time window on the UI to see the sample/instrument.  

The problem is that they are looking at SRAM cells, that are 45nm in dimension.  And they kept harping on the “magnification”. If only we made the image look bigger, we would see the features.

Uh, no.  The classic difference between resolution and magnification.  A rule of thumb is that the limit of resolution is proportional to wavelength/(2 * NA).  There is a constant, but it really can be ignored or assumed to be 1.  If you have broad spectrum white light, your central wavelength is 540nm (a “green” color), and you use that to calculate resolution.

For a super high resolution system your NA can be as high as 0.95 in air (> 1 if you can do oil immersion).  But since we have lots of hardware in the way, we need a much longer working distance.  50 or so millimeters of WD.  The best commercially available optics at this range will give a NA of 0.15 or so.  Thus, we become diffraction limited at about 1.8um.  Since they want to find features that are 0.045um in dimension, the resolution limit is going to be equivalent to 40 cells.  That means that there is no hope to see the features they want.

Of course, they didn’t understand this, and kept repeating the “more magnification” mantra. FML.


*NA = Numerical Aperture – a measure of the light gathering capability of an optical system.

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By geoffand


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November 2012

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