= Solution
Treat the letter-shaped regions as openings to be etched, using positive-tone resist; the clear mask areas therefore correspond to those openings. Label the two L features $L_1,L_2$ from left to right. An alternating assignment is \b[mask A: H, $L_1$, O; mask B: E, $L_2$]. Each full letter remains on one mask, avoiding unnecessary stitch boundaries across its stems. Nearest-neighbor interletter gaps now occur between different exposures, rather than having to be resolved within one exposure. Verify that each individual letter's internal spaces are still printable; without dimensions, this partition is a process proposal rather than proof of a particular numerical minimum linewidth.
A robust sequence is <litho-etch-litho-etch double patterning> into a durable <hard mask>. Deposit an oxide <hard mask> on <silicon>; expose and develop mask A and transfer those openings through the <hard mask>. Strip resist, apply fresh resist, align and expose mask B, then transfer its openings into the same <hard mask>, the second resist covering mask-A openings during that second oxide transfer. Remove resist and perform one final deep <silicon> etch through the union of the openings. An inductively coupled <deep reactive-ion etching> process with alternating fluorine <etching> and sidewall <passivation> gives much more vertical, high-aspect-ratio features than an <isotropic etch>. Mask thickness and <etch selectivity> must survive the required depth. Control sidewall scalloping, aspect-ratio-dependent etch rate, charging, microloading and the combined overlay/etch-bias budget.
A spacer-based process can produce very fine regular pitches, but arbitrary letter junctions and terminations require additional cut/block patterning; it is not automatically a two-mask solution for this target. \b[Use two independently transferred sparse masks, then a selective directional deep etch.] The schematic gives the assignment and a generic corner-correction example without reproducing the supplied pattern.
\Image[/past-exam-of-the-mphil-in-micro-and-nanotechnology-enterprise-course-of-the-university-of-cambridge/2017/paper-1-double-patterning.png]
{title=Alternating mask assignment and original L-corner <optical proximity correction>}
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