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Controlled removal of a material by a chemical, plasma or physical process. etch selectivity compares removal rates for different materials; directionality compares removal rates along different geometric directions. High selectivity does not imply a vertical profile, and high directionality does not guarantee an undamaged surface.

A durable patterned layer used to protect underlying material during an etch. Its thickness and etch selectivity must survive the intended depth; erosion and lateral bias can change final feature dimensions even when the starting pattern is correct.

A sacrificial layer temporarily supports the intended moving structure and determines its clearance above the substrate. One workable surface micromachining sequence deposits silicon dioxide on silicon, opens anchor windows in that oxide, deposits a structural polysilicon film and patterns the beam and release openings. The structural film contacts the substrate at the anchors. Selective removal of the oxide beneath the beam leaves the beam suspended while retaining its attachment. An access opening or exposed perimeter is essential; an etchant cannot instantly reach sealed material.

The release needs substantial lateral removal, so an isotropic etch with high etch selectivity is useful. Hydrofluoric-acid oxide release is compatible with a suitable polysilicon structure; a directional reactive-ion etching step alone would mainly clear exposed vertical paths and leave material beneath the beam. Choose the structural material, anchor design, sacrificial material and etchant together, and allow for transport-limited undercut and structural residual stress. Wet release followed by ordinary evaporation can pull a compliant structure onto the substrate through capillary force and cause stiction. critical point drying or a compatible vapor-phase release etch avoids the receding liquid interface. Neither method removes the need to check residues, selectivity and anchor survival. Pattern the support and structure first, then selectively undercut the support while preserving the anchors.^[ref]

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 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. 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.

Near-normal ion milling preferentially removes exposed horizontal silicon and gives a more directional trench with much less lateral undercut. Energetic argon ions eject surface atoms by momentum transfer. The resist blocks direct trajectories to the protected regions.

After about $200\mathrm{nm}$ removal of exposed silicon, expect an approximately flat recessed floor and steep transitions near the mask edges. The mask is also sputtered: finite etch selectivity, angular yield variations, mask-edge faceting and redeposition can produce sloped walls, fences or residue. No resist erosion depth can be deduced without its sputter rate. Thus a perfectly unchanged rectangular mask is only the ideal sketch.

Choose process B, the citric-acid/peroxide wet etch. Its GaAs-to-AlGaAs etch selectivity is $200/1=200$, giving strong protection of the underlying barrier and a controllable removal rate for the thin cap.

For comparison, A has selectivity one and therefore no useful etch stop. It is also extremely fast: a nominal $20\mathrm{nm}$ GaAs layer would be removed in only $0.048\mathrm{s}$. C has selectivity $1000/20=50$, useful but inferior to B, and clears the same cap in about $1.2\mathrm{s}$. D has selectivity four, clears the cap in about $0.060\mathrm{s}$ and additionally risks plasma-induced surface damage or altered interface properties.

B clears the nominal cap in about $6\mathrm{s}$. Its slower underlying etch permits a more forgiving endpoint or modest overetch, although actual rates must be checked for the relevant composition and conditions. Wet-etch lateral undercut and contact-metal compatibility still matter; selectivity alone does not make every mask geometry acceptable.