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Isotropic vs Anisotropic Etching: Profile and Undercut

Isotropic etching removes material at the same rate vertically and sideways, so it eats under the mask edge (undercut) and leaves a rounded profile. Anisotropic etching proceeds much faster in particular directions, and there are two different ways to get it. One is dry etching, in which ions accelerated vertically in a plasma drive the reaction at the bottom while a passivation layer protects the sidewalls, giving nearly vertical sidewalls. The other is wet etching of single-crystal silicon in alkaline solutions such as KOH or TMAH, in which the (111) planes etch very slowly, so crystal planes set the shape. Wet does not always mean isotropic and dry does not always mean anisotropic: dry etching with radicals only is isotropic. Directionality trades off against selectivity and damage, so the choice depends on the goal.

AspectIsotropic etchingAnisotropic etching (ion-assisted dry)Crystal-plane-dependent anisotropic etching (wet)
What creates directionalityNothing: the chemical reaction proceeds equally in all directionsIons accelerated vertically by the electric field promote the reaction at the bottom, while a passivation layer protects the sidewallsEtch rate differs strongly between crystal planes; silicon (111) planes etch very slowly
ProfileRounded, centered on the mask openingNearly vertical sidewalls (taper angle tuned by the conditions)About 54.7° slopes on (100) wafers; vertical walls on (110) wafers
UndercutLarge: about as much sideways as in depth, and more with overetchingSmallStops at (111) planes under mask edges aligned to <110>; convex corners and misaligned edges are undercut
Typical methodsWet etching such as HF etching of oxide, XeF2 gas, downstream plasma that delivers radicals onlyRIE, ICP etching, the Bosch processAlkaline solutions such as KOH and TMAH
SelectivityEasy to make high, since there is almost no physical removalIon bombardment also erodes masks and underlying layers, and stronger directionality tends to lower selectivityHigh: silicon nitride and oxide masks (especially with TMAH) are barely etched
DamageLowIon bombardment and charging can cause damageNo plasma damage
Shape constraintsNot suitable for features narrower than the film thicknessTransfers the mask shape almost exactly; deep holes suffer effects such as aspect-ratio-dependent rate lossWorks only on single-crystal silicon, with shapes set by crystal planes; masks must be aligned to the crystal orientation
Typical usesBlanket film removal, cleaning, sacrificial layer etching in MEMS, removing SiGe between GAA nanosheetsFine patterns such as transistor gates, contacts, trenches and vias; deep vertical holes in MEMSBulk micromachining such as MEMS diaphragms, V-grooves and pyramid-shaped pits

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

When to use which

Isotropic etching
When high selectivity or avoiding damage to the layer below matters more than dimensional accuracy, and when the undercut itself is useful, for hollowing out under a structure or removing a layer from the side.
Anisotropic etching (ion-assisted dry)
When narrow, vertical patterns must be transferred at the mask dimensions. Almost all fine patterns in advanced ICs are made this way.
Crystal-plane-dependent anisotropic etching (wet)
When precise slopes, V-grooves or thin diaphragms defined by crystal planes are needed cheaply in single-crystal silicon, mainly for MEMS bulk micromachining.

FAQ

Is wet etching always isotropic?

No. Etching amorphous or polycrystalline films in solution is generally isotropic, but etching single-crystal silicon in KOH or TMAH is anisotropic because the rate differs strongly between crystal planes. Conversely, dry etching that uses only radicals or XeF2 gas, without ions, is isotropic.

Why do ions make etching go only downward?

The negative potential that develops on the wafer electrode accelerates ions almost vertically, so they mainly hit the bottom of the feature. Ion bombardment strongly speeds up the chemical reaction with radicals, so the bottom etches much faster than the sidewalls. Polymers and other byproducts deposit on the sidewalls as a passivation layer that blocks sideways etching.

Why do directionality and selectivity trade off?

Stronger directionality comes from raising ion energy and the physical component of etching, and that component does not distinguish between materials, so it also erodes masks and underlying layers. Isotropic chemical etching has almost no physical component and easily reaches high selectivity, but it also etches sideways. Real processes balance gas composition, ion energy and passivation.

How much undercut does isotropic etching cause?

Fully isotropic etching removes about as much sideways from the mask edge as it does in depth. Etching through a 1 µm film undercuts about 1 µm, and overetching adds more. That is why features narrower than the film thickness are made with anisotropic etching.

Check your understanding

When a 1 µm film is etched just down to the underlayer by perfectly isotropic wet etching, roughly how much is etched laterally from the mask edge (undercut)?

  1. AAbout 1 µm
  2. BAbout 10 µm
  3. CAlmost 0
Show answer and explanation

Answer: A. About 1 µm

Isotropic etching proceeds equally downward and sideways, so by the time 1 µm of film clears to the underlayer, the mask edge is undercut by about 1 µm, and overetch increases this further. Dimensional changes on the order of the film thickness make it unsuitable for features narrower than the film is thick. Conversely, MEMS exploits this behavior to hollow out material beneath structures.

Which is a characteristic of downstream etching, which supplies only plasma-generated radicals to the wafer?

  1. AStrong ion bombardment makes it highly anisotropic
  2. BWith almost no ion bombardment, it etches isotropically with little damage
  3. CIt etches with chemical solutions
Show answer and explanation

Answer: B. With almost no ion bombardment, it etches isotropically with little damage

In downstream (remote-plasma) etching, the plasma is generated away from the wafer, and ions and electrons recombine en route, so only long-lived neutral radicals reach it. With no ion bombardment and plasma UV blocked, damage is minimal and etching is isotropic. It is used for resist stripping, damage-sensitive film removal and selectively removing SiGe between nanosheets.

Which gas can etch silicon isotropically without a plasma, for example to remove sacrificial layers in MEMS?

  1. AO2 (oxygen)
  2. BN2 (nitrogen)
  3. CXeF2 (xenon difluoride)
Show answer and explanation

Answer: C. XeF2 (xenon difluoride)

XeF2 sublimes near room temperature and reacts spontaneously with silicon to form SiF4 (2XeF2 + Si → 2Xe + SiF4), etching silicon isotropically without plasma. Its selectivity to oxide, nitride, resist and many metals exceeds 1000, and because no liquid is involved stiction is avoided, so it is used to release MEMS structures from silicon sacrificial layers.

  1. Exposing silicon to XeF2 gas and an argon ion beam simultaneously etches it far faster than the sum of either alone. What property of dry etching does this show?
  2. Which metric expresses how far the sidewall of an etched hole or trench tilts from vertical?
  3. Which correctly describes anisotropic etching?

Practice Etching questions

Key terms explained

Isotropic etching
Etching that proceeds at the same rate vertically and laterally. It cuts in under the mask (undercut).
Anisotropic etching
Etching that proceeds only in a particular direction (mainly vertically). Needed to pattern fine features.
Selectivity
In etching, the ratio of the etch rate of the target film to that of the mask or underlying film. Higher selectivity means less damage to the underlayer.
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.
Bosch process (deep RIE)
A dry-etching technique that alternates short etch and sidewall-passivation steps to cut deep, vertical holes and trenches in silicon. It is used for MEMS and TSVs.
Sacrificial layer etching
A typical MEMS fabrication method that etches away a temporary layer (the sacrificial layer) beneath a structure, freeing beams or membranes so they can move.
Radical
A neutral atom or molecule with an unpaired electron that is highly reactive chemically. Plasmas contain far more radicals than ions, and they drive chemical etching and deposition.
Crystal orientation
The direction of crystal planes and axes, expressed with Miller indices such as (100). The (100) surface is standard for silicon wafers used for MOSFETs.

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