Hello Everyone,

Regarding coherence:

I digged a bit deeper into the topic. Unfortunately, it turns out that coherence and monochromaticity indeed matter.

Light source in I-line and G-line steppers is usually mercury vapor discharge tube, that is polychromatic and incoherent. However, polychromaticity results in dispersion in lens material and chromatic aberration, which reduces modulation index and usable resolution significantly. Also, incoherent illumination (i.e. light source with nonzero size) results in the existence of plane wave components in pre-reticle illumination propagating with an offset angle to the optical axis. After diffraction on the reticle, higher order diffraction modes of these off-axis components would not be captured by the projection optics, resulting in incomplete image reconstruction (reduced spatial bandwidth, "blurring") at the wafer level, further reducing modulation and resolution. To counter this, illumination systems in steppers contain a narrow-band filter (Bragg-filter or similar) to filter out exactly one narrow spectral line (hence I-line and G-line), and additional optics to eliminate off-axis components and increase coherence (therefore, what reaches the reticle is more or less "laser-like", highly coherent and monochromatic radiation with very narrow FWHM). Also, the projection optics in the stepper is usually a "narrow-band" system in terms of wavelength due to dispersion, and fine-tuned to work only at the wavelength of the line it is specified for.

The micro-LEDs in the JBD display, as are all other LEDs, are polychromatic (the FWHM of the one I considered in the calculation is 15nm), and it also can be assumed that coherence is not too good, too (as high coherence would mean near-zero viewing angle, which is a problem for a display). In addition, the center wavelength of the display falls half-way between I-line and G-line.

In my opinion, coherence may be improved by using micro-optics at each micro-LED that produce highly collimated beams (JBD patents 10304375 and 9977152 describe microlenses and microreflectors, I don't know if they are really capable of and committed to doing this or it is just "shelling-patenting"). The wavelength of the emitters can be fine-tuned to match the nominal wavelength of the projection optics (GaAs/InGaAs LEDs are tunable to some degree by changing the Ga:In ratio). However, the effects of residual incoherence and of the 15nm polychromaticity need to be evaluated further, which is beyond my level of competence, and possibly requires the detailed knowledge of the actual projection system and/or fiddling around with one.

Regards,

Ferenc