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Sub-Diffraction Lithography

Figure 1. PInSR photochemistry. A diffraction-limited spot (a) at 473 nm creates initiating radicals via absorption by CQ. A Gauss-Laguerre "donut mode" (b) at 365 nm creates inhibiting radicals via absorption by TED. The resist polymerizes (c) only in the "donut hole" where initiation is not inhibited.
Traditional photolithography begins with single-photon absorption of patterned light by a photo-initiator to locally expose a resist, limiting the resist feature size to the smallest focal spot dimension. This optical diffraction limit constrains the size and complexity of integrated circuits and is thus the foundation of Moore’s Law However, this scale limit has recently been broken by an imaging technique called stimulated-emission depletion microscopy. Here, one laser is used to excite a fluorophore in a diffraction-limited volume while a second laser is used to suppresses this fluorescence at the periphery of the spot, confining the remaining fluorescence to a sub-diffraction region. Inspired by this idea, our lithography method uses two independent, high-throughput processes which act in opposition to create features much smaller than the optical wavelength. In photo-inhibited super-resolution (PInSR) lithography, initiating species are generated by single-photon absorption at one wavelength while inhibiting species are generated by single-photon absorption at a second color. The absorption spectra of the photo-initiator and photo-inhibitor are designed to provide independent control over the two processes as shown in Figure 1. Like a stencil used to confine spray-paint to a thin, sharp line, this inhibitory pattern acts as a remotely programmable, transient near-field mask to control the size and shape of the modified resist region.
A 473 DPSS laser is collimated and focused onto the sample to initiate the photoresist. A 365 nm argon-ion laser is collimated, shaped by a forked computer generated hologram into a donut mode and combined into the beam path with a dichroic mirror.
As the power of the Gauss-Laguerre inhibition focus is increased (red isosurfaces, i to iii), the transverse resolution of the Gaussian initiation focus (green isosurface) is progressively refined (blue isosurfaces, i to iii).
Figure 4. Cool photo of Ben Kowalski and Dr. McLeod working on the first PInSR lithography system. This image appeared in the IEEE spectrum.

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This work has been generously funded by: