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Electron Beam Field

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ZeroResistance:
While reading on Electron Beam Lithography on wiki, https://www.wikipedia.com/en/Electron-beam_lithography
I came accross the following quote:


--- Quote --- Because of the inaccuracy and because of the finite number of steps in the exposure grid the writing field is of the order of 100 micrometre – 1 mm
--- End quote ---

I find this quote a bit perplexing, especially regarding the writing field limited to between 100um and 1mm , given that Electron beams are used in TV sets, CRO's etc. where they have a large field upto a few feet in some cases, I mean the tip of the beam in a TV set would sweep left and right upto a few feet.

Why then is the writing field limited to 1mm in lithography applications?

ejeffrey:
Because the electron beam in a TV set isn't trying to make 10 nm steps, nor focus the beam down to a 10 nm spot size.  The CRT uses a long lever arm to get that huge beam deflection, but sacrifices resolution and spot size.  It basically the same reason a photo enlarger can easily make 8x10" prints but a UV stepper can only expose dies that are 20-30 mm. 

Also, take a look at your old TV.  The glass is curved, either spherically for a conventional CRT or cylindrically for a trinitron.  This is because it is hard to keep the image field flat for a large field of view (also it is easier to keep a curved tube strong enough to withstand air pressure). 

Large field imaging systems, whether optical or e-beam get harder and harder to design as you get further off axis especially if you want good resolution in the peripheral.

ZeroResistance:

--- Quote from: ejeffrey on July 29, 2019, 07:35:48 pm ---Because the electron beam in a TV set isn't trying to make 10 nm steps, nor focus the beam down to a 10 nm spot size.  The CRT uses a long lever arm to get that huge beam deflection, but sacrifices resolution and spot size.  It basically the same reason a photo enlarger can easily make 8x10" prints but a UV stepper can only expose dies that are 20-30 mm. 

Also, take a look at your old TV.  The glass is curved, either spherically for a conventional CRT or cylindrically for a trinitron.  This is because it is hard to keep the image field flat for a large field of view (also it is easier to keep a curved tube strong enough to withstand air pressure). 

Large field imaging systems, whether optical or e-beam get harder and harder to design as you get further off axis especially if you want good resolution in the peripheral.

--- End quote ---

Profound words indeed!
I read that 3 to 4 times over. I sort of understood the curved nature of deflection that you speak of.
But I didn't get he resolution part and to some extent the spot size.
Resolution of the beam would be dependent on the deflection system correct? I mean how finely do you control the electromagnetic coils. So what kind of problems do you foresee here?
Also regarding the spot size once you have focused the beam now to say 10nm if might change to oval shape at the left and right extremes of the scan, is it that what you are referring to?

SilverSolder:
The resolution of a TV picture is not super high -  e.g. vertically there are only 525/625 lines, depending on what part of the world you are (were) in.  Assuming a best case PAL standard TV, the image has a resolution comparable to 704×576 pixels, while NTSC has less resolution (but higher frame rate, in fairness).

CRTs did not need to be super high precision with small dot sizes to make a decent representation of the signal.  The magnetic deflection in a TV did not need to be high precision engineering either, since the eye is quite tolerant of geometric distortion as long as it is smooth. As you allude to, the focus is not constant across the screen width and is also imperfect on any real TV set with a CRT.

The best CRT computer monitors were capable of resolutions up to 2560x1600, far better than any TV tube, requiring high precision manufacturing of the tube as well as extensive amounts of control and correction of the deflection currents (as well as dynamic focus control) - these monitors were probably the peak of the CRT art.  If you think in terms of this number of pixels of resolution, on a 20" monitor, you are probably close to what is realistically possible in terms of dot size and placement.

T3sl4co1l:
Trinitron monitors were probably pushing the limits of e-beam accuracy in a production context.  Consider mine, which does 1600x1200 resolution; for a pixel to be where it ought to be, that's about a 0.1% error, not bad at all.  It's not that good in practice though.  If I display a 50% grating pattern, an interference pattern versus the aperture grille / phosphors is visible.  This has about 20 fringes across the width of the screen, varying by region.  The distortion is imperceptible to the eye (probably, the slight cylindrical face dominates that), but would be measurable with simple tools.

I would hope / expect that e-beam litho machines have far greater distortion-free dynamic range; but we're still talking about "pixels" of ~10nm, which would cover a ~20um area for optics and controls comparable to the Trinitron; to get up to ~2mm (equivalent to a 200,000 pixel wide CRT!) is heroic.

Corrections could be made -- the repeatable distortion could be mapped and corrected as a preprocess step.  What's really important is that tiling can be done correctly, i.e., periodic boundary conditions must be satisfied, to full accuracy.  This could be tuned or corrected for, even at the expense of distortion within the field.  I'm guessing, what's left is an uncorrectable random error that just can't be nailed down and so, for example, what should be straight lines in your raster end up wobbly and spotty, and some lines overlap and some ends don't match up, and... yeah, a big mess if too much area is done all at once.

In short, it's a dynamic range problem, just with signals in different units (distance rather than voltage).

Tim

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