<svg viewBox="0 0 760 470" xmlns="http://www.w3.org/2000/svg" font-family="-apple-system,Segoe UI,Roboto,Helvetica,Arial,sans-serif"><rect x="0" y="0" width="760" height="470" rx="14" fill="#0d1117"/><text x="20" y="30" fill="#e6edf3" font-size="19" font-weight="700">Etching: cut the pattern into the wafer, straight down or all around</text><text x="20" y="50" fill="#8b949e" font-size="12.5">The resist mask protects some areas; etch removes the rest &#8212; dry etch cuts vertically, wet etch soaks in</text><!-- Panel 1: dry etch --><rect x="20" y="66" width="226" height="298" rx="7" fill="#0c141d" stroke="#30363d"/><text x="32" y="88" fill="#7ee6c0" font-size="13" font-weight="700">1 &#183; Dry (plasma / RIE)</text><text x="32" y="106" fill="#8b949e" font-size="10.5">ions bombard straight down</text><rect x="42" y="118" width="182" height="150" rx="3" fill="#111a24" stroke="#30363d"/><!-- ions falling --><g stroke="#a99cf0" stroke-width="1.3"><line x1="70" y1="126" x2="70" y2="150"/><line x1="95" y1="126" x2="95" y2="150"/><line x1="120" y1="126" x2="120" y2="150"/><line x1="145" y1="126" x2="145" y2="150"/><line x1="170" y1="126" x2="170" y2="150"/><line x1="195" y1="126" x2="195" y2="150"/></g><g fill="#a99cf0"><polygon points="67,150 73,150 70,155"/><polygon points="92,150 98,150 95,155"/><polygon points="117,150 123,150 120,155"/><polygon points="142,150 148,150 145,155"/><polygon points="167,150 173,150 170,155"/><polygon points="192,150 198,150 195,155"/></g><text x="52" y="134" fill="#a99cf0" font-size="8">energetic ions (directional)</text><!-- resist mask --><rect x="60" y="160" width="34" height="14" fill="#e0b13a"/><rect x="132" y="160" width="34" height="14" fill="#e0b13a"/><text x="113" y="170" fill="#0d1117" font-size="7" text-anchor="middle">resist</text><!-- material with vertical trench --><rect x="60" y="174" width="34" height="46" fill="#38506a"/><rect x="132" y="174" width="34" height="46" fill="#38506a"/><rect x="94" y="174" width="38" height="46" fill="#111a24" stroke="#233041"/><!-- vertical walls emphasis --><line x1="94" y1="174" x2="94" y2="220" stroke="#34d399" stroke-width="1.5"/><line x1="132" y1="174" x2="132" y2="220" stroke="#34d399" stroke-width="1.5"/><text x="113" y="236" fill="#34d399" font-size="8" text-anchor="middle">vertical, anisotropic profile</text><text x="52" y="256" fill="#8b949e" font-size="7.5">reactive gas + plasma; walls stay straight</text><text x="32" y="286" fill="#adb5bd" font-size="9.5">A plasma makes reactive ions and</text><text x="32" y="301" fill="#adb5bd" font-size="9.5">radicals; a bias pulls ions straight down</text><text x="32" y="316" fill="#adb5bd" font-size="9.5">so they etch vertically, not sideways.</text><text x="32" y="331" fill="#adb5bd" font-size="9.5">That anisotropy is what lets you print</text><text x="32" y="346" fill="#adb5bd" font-size="9.5">narrow, high-aspect-ratio features.</text><!-- Panel 2: wet etch --><rect x="267" y="66" width="226" height="298" rx="7" fill="#0c141d" stroke="#30363d"/><text x="279" y="88" fill="#9fd8ef" font-size="13" font-weight="700">2 &#183; Wet (chemical bath)</text><text x="279" y="106" fill="#8b949e" font-size="10.5">acid dissolves in all directions</text><rect x="287" y="118" width="196" height="150" rx="3" fill="#111a24" stroke="#30363d"/><!-- liquid --><rect x="295" y="126" width="180" height="20" fill="#1c2733"/><text x="385" y="140" fill="#38bdf8" font-size="8" text-anchor="middle">liquid etchant (e.g. HF, KOH)</text><!-- resist --><rect x="305" y="160" width="34" height="12" fill="#e0b13a"/><rect x="431" y="160" width="34" height="12" fill="#e0b13a"/><!-- material with undercut (isotropic) --><rect x="305" y="172" width="34" height="46" fill="#38506a"/><rect x="431" y="172" width="34" height="46" fill="#38506a"/><path d="M339,172 Q356,172 356,190 Q356,210 375,210 L395,210 Q414,210 414,190 Q414,172 431,172 Z" fill="#111a24" stroke="#233041"/><!-- undercut arrows --><path d="M339,178 Q348,178 350,186" fill="none" stroke="#f87171" stroke-width="1.2"/><path d="M431,178 Q422,178 420,186" fill="none" stroke="#f87171" stroke-width="1.2"/><text x="385" y="234" fill="#f87171" font-size="8" text-anchor="middle">undercut: etches under the mask</text><text x="299" y="254" fill="#8b949e" font-size="7.5">isotropic &#8212; same rate in every direction</text><text x="279" y="286" fill="#adb5bd" font-size="9.5">Dipping the wafer in a chemical bath</text><text x="279" y="301" fill="#adb5bd" font-size="9.5">dissolves the exposed material, but the</text><text x="279" y="316" fill="#adb5bd" font-size="9.5">acid eats sideways too, rounding and</text><text x="279" y="331" fill="#adb5bd" font-size="9.5">undercutting the mask. Cheap and gentle,</text><text x="279" y="346" fill="#adb5bd" font-size="9.5">but too blurry for fine features.</text><!-- Panel 3: what matters --><rect x="514" y="66" width="226" height="298" rx="7" fill="#0c141d" stroke="#30363d"/><text x="526" y="88" fill="#c4b5fd" font-size="13" font-weight="700">3 &#183; What etch must control</text><text x="526" y="106" fill="#8b949e" font-size="10.5">the knobs that set the profile</text><circle cx="532" cy="126" r="2.4" fill="#34d399"/><text x="542" y="129" fill="#e6edf3" font-size="10" font-weight="700">Selectivity</text><text x="542" y="143" fill="#8b949e" font-size="9">etch the target fast but the mask and</text><text x="542" y="156" fill="#8b949e" font-size="9">underlying layer slowly &#8212; so you stop clean.</text><circle cx="532" cy="176" r="2.4" fill="#38bdf8"/><text x="542" y="179" fill="#e6edf3" font-size="10" font-weight="700">Anisotropy</text><text x="542" y="193" fill="#8b949e" font-size="9">vertical sidewalls hold the drawn width;</text><text x="542" y="206" fill="#8b949e" font-size="9">sideways etch blurs and shrinks features.</text><circle cx="532" cy="226" r="2.4" fill="#e0b13a"/><text x="542" y="229" fill="#e6edf3" font-size="10" font-weight="700">Endpoint &amp; uniformity</text><text x="542" y="243" fill="#8b949e" font-size="9">detect when the layer clears; etch the</text><text x="542" y="256" fill="#8b949e" font-size="9">same depth everywhere on the wafer.</text><rect x="526" y="272" width="202" height="82" rx="5" fill="#111a24" stroke="#30363d"/><text x="536" y="290" fill="#f87171" font-size="10" font-weight="700">Why dry etch dominates</text><text x="536" y="306" fill="#adb5bd" font-size="9">Fine geometry needs straight walls, so</text><text x="536" y="320" fill="#adb5bd" font-size="9">plasma etch does the critical patterning.</text><text x="536" y="334" fill="#adb5bd" font-size="9">Wet etch survives for cleaning, stripping</text><text x="536" y="348" fill="#adb5bd" font-size="9">and gentle, non-critical removal.</text><!-- bottom cards --><rect x="20" y="384" width="226" height="70" rx="7" fill="#111a24" stroke="#30363d"/><text x="32" y="406" fill="#7ee6c0" font-size="11" font-weight="700">Dry etch = vertical</text><text x="32" y="424" fill="#adb5bd" font-size="9.5">Directional ions cut straight down &#8212;</text><text x="32" y="440" fill="#adb5bd" font-size="9.5">the workhorse for fine patterning.</text><rect x="267" y="384" width="226" height="70" rx="7" fill="#111a24" stroke="#30363d"/><text x="279" y="406" fill="#9fd8ef" font-size="11" font-weight="700">Wet etch = all around</text><text x="279" y="424" fill="#adb5bd" font-size="9.5">A chemical bath dissolves evenly &#8212;</text><text x="279" y="440" fill="#adb5bd" font-size="9.5">cheap, but it undercuts the mask.</text><rect x="514" y="384" width="226" height="70" rx="7" fill="#111a24" stroke="#30363d"/><text x="526" y="406" fill="#e0b13a" font-size="11" font-weight="700">Selectivity &amp; profile</text><text x="526" y="424" fill="#adb5bd" font-size="9.5">Etch the target, spare the rest, and</text><text x="526" y="440" fill="#adb5bd" font-size="9.5">hold the sidewall the layout demands.</text></svg>

Wet Etch Processes are the liquid-chemical-based material removal techniques used throughout semiconductor manufacturing for cleaning, thin film removal, and pattern transfer — providing high selectivity, low damage, and batch processing capability, though their isotropic (non-directional) etch profile limits them to applications where dimensional control is less critical than in plasma dry etching.

Key Wet Etch Chemistries

RCA Cleaning Sequence (Industry Standard)

1. SC-1 (APM): NH4OH:H2O2:H2O (1:1:5) at 70-80°C.

• Removes: Particles, organics. Grows thin chemical oxide.

2. DHF Dip: Dilute HF (1:100) at room temp.

• Removes: Chemical oxide from SC-1. Leaves H-terminated Si surface.

3. SC-2 (HPM): HCl:H2O2:H2O (1:1:5) at 70-80°C.

• Removes: Metallic ions (Fe, Cu, Zn). Grows clean chemical oxide.

Wet Bench vs. Single-Wafer Processing

Wet Etch Characteristics

• Isotropic: Etches equally in all directions → lateral undercut equals vertical etch depth.
• Good selectivity: Chemical reactions are material-specific → stops on different films.
• No plasma damage: No ion bombardment or UV radiation.
• Batch capable: 50 wafers processed simultaneously → high throughput for non-critical steps.

Applications in Modern CMOS

• Pre-gate clean: Remove native oxide before gate dielectric deposition.
• SiGe selective etch: HCl vapor or dilute H2O2 selectively removes SiGe (nanosheet release).
• Sacrificial layer removal: Wet etch removes hard masks and spacers without damaging active structures.
• Post-etch residue removal: Fluorine-based or amine-based solutions clean etch polymer residue.

Wet etch processes are indispensable complementary techniques to dry etching — while plasma etch provides the anisotropic profiles needed for patterning, wet etch delivers the high selectivity, low damage, and cleaning capability essential for surface preparation and sacrificial layer removal throughout the CMOS integration flow.