Wet vs Dry Etching: Key Differences in Semiconductor Processing

Etching removes material from the wafer to form patterns. Wet etching dissolves the material in a liquid chemical; it is simple, fast and highly selective, but it usually etches sideways as well as down (isotropic). Dry etching uses a plasma of reactive gases in a vacuum chamber, and ions can make it strongly directional (anisotropic), which is essential for cutting narrow, vertical features in modern chips.

AspectWet etchingDry etching
MediumLiquid chemicals, such as HF for oxide, hot phosphoric acid for nitride and KOH for siliconPlasma of reactive gases in a vacuum chamber, as in RIE
DirectionalityUsually isotropic: it also etches sideways and undercuts the mask (KOH on silicon is an exception that follows crystal planes)Can be highly anisotropic, giving near-vertical sidewalls through ion bombardment and sidewall protection
SelectivityUsually very high, because the chemistry is specificLower, and tuned by gas chemistry and plasma conditions
Smallest featuresLimited by undercut; used for larger features and film removalNeeded for nanometer-scale patterns
ThroughputHigh: many wafers can be processed together in a bathLower: usually one wafer at a time
Process controlDepends on bath temperature, concentration and timePrecise, with endpoint detection from the plasma emission
DamageNo plasma damagePossible plasma damage and charging (antenna effect)
Equipment and wasteSimple equipment, but liquid chemical wasteComplex vacuum equipment and exhaust gas treatment, so higher cost
Typical usesWafer cleaning, stripping films (oxide, nitride), bulk silicon etching for MEMSPatterning gates, contacts, trenches and vias in advanced ICs

Figures are typical orders of magnitude and vary by process generation and product.

When to use which

Wet etching
When selectivity matters more than shape, or when you are removing a whole film or cleaning: stripping oxide, removing nitride, MEMS bulk etching.
Dry etching
When the pattern is fine and you need vertical walls: transistor gates, contacts, trenches and vias.
Together
A chip flow uses both. Dry etching defines the fine patterns, and wet steps clean and strip films afterwards.

FAQ

What is the difference between isotropic and anisotropic etching?

Isotropic etching removes material equally in all directions, so it undercuts the mask. Anisotropic etching removes it mainly in one direction (down), leaving vertical sidewalls. Wet etching is usually isotropic, while dry etching can be anisotropic.

Why is dry etching used for fine patterns?

Ions in the plasma give dry etching a strong direction, so narrow, vertical features can be cut. Wet etching undercuts the mask and cannot make nanometer-scale patterns.

Why is wet etching still used?

It is simple, fast, processes many wafers at once and has very high selectivity. It is used for cleaning, stripping films and bulk silicon etching in MEMS.

What is selectivity in etching?

Selectivity is how much faster the target material etches than the material you want to keep. Wet etching usually has very high selectivity, while dry etching is tuned through gas chemistry and plasma conditions.

Check your understanding

Which correctly describes anisotropic etching?

  1. AEtching proceeds mainly vertically and hardly at all laterally
  2. BEtching proceeds at the same rate in all directions
  3. COnly the mask material is selectively removed
Show answer and explanation

Answer: A. Etching proceeds mainly vertically and hardly at all laterally

In anisotropic etching, vertically accelerated ion bombardment and sidewall-protecting passivation make etching proceed preferentially downward, with almost no lateral undercut beneath the mask. Mask patterns are thus transferred without dimensional change, enabling fine vertical features such as tens of nm wide and several µm deep. Etching that proceeds equally in all directions is isotropic.

Which is a general characteristic of wet etching?

  1. AIt is highly anisotropic because it uses plasma
  2. BIt generally proceeds isotropically and readily undercuts the mask
  3. CIt requires vacuum equipment
Show answer and explanation

Answer: B. It generally proceeds isotropically and readily undercuts the mask

Wet etching dissolves films by chemical reaction in liquid, so it generally proceeds equally in all directions (isotropic) and undercuts the mask by about the film thickness. It needs simple equipment, processes many wafers at once and easily achieves high selectivity. Dry etching is used for fine patterning, but wet etching remains important for blanket film removal and cleaning.

Which correctly describes reactive ion etching (RIE)?

  1. AEtching that combines chemical reactions and ion bombardment in a plasma of reactive gases
  2. BEtching by immersing the wafer in a chemical solution
  3. CGrinding the wafer surface with a polishing pad
Show answer and explanation

Answer: A. Etching that combines chemical reactions and ion bombardment in a plasma of reactive gases

Reactive ion etching (RIE) combines chemical reactions driven by radicals from a reactive-gas plasma (fluorine- or chlorine-based, etc.) with bombardment by ions accelerated in an electric field. Ions striking perpendicular to the wafer promote reactions at the feature bottom, so etching proceeds vertically with high anisotropy. Despite the name, ions do not etch alone; the synergy with radicals is the essence.

  1. Which correctly describes one mechanism by which dry etching achieves anisotropy?
  2. During plasma etching, what is the phenomenon called in which conductors acting as antennas, such as long wires, collect charge and damage the gate oxide they connect to?
  3. Which is a characteristic of wet etching of silicon with potassium hydroxide (KOH) solution?

Practice Etching questions

Key terms explained

Wet etching
Etching that dissolves films by immersing them in chemicals. Selectivity is high and damage low, but it generally proceeds isotropically and does not suit fine patterning.
Dry etching
Removing material with plasma or gas instead of liquid chemicals. Combining ion bombardment allows anisotropic etching, which is essential for fine patterns.
Anisotropic etching
Etching that proceeds only in a particular direction (mainly vertically). Needed to pattern fine features.
Isotropic etching
Etching that proceeds at the same rate vertically and laterally. It cuts in under the mask (undercut).
RIE (reactive ion etching)
Dry etching that combines reactive species in a plasma with ions accelerated by an electric field. It enables vertical, anisotropic etching.

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