LECTURE 5

SUMMARY OF KEY IDEAS

Etching is a processing step following lithography: it transfers a circuit image from the photoresist to materials form which devices are made or to hard masking or sacrificial material, such as SiO2.

General etch requirements:

1. Obtain desired profile (sloped or vertical) 2. Minimal undercutting or bias 3. Selectivity to other exposed films and resist 4. Uniform and reproducible 5. Minimal damage to surface and circuit 6. Clean, economical, and safe

There are two main types of etching used in IC fabrication: wet chemical etching and dry or plasma etching.

Wet etching has usually excellent selectivity since chemical reactions are very selective but it is also isotropic. Selectivity comes from chemistry; directionality usually comes from physical processes, such as ion impacts in plasma etching systems.

Plasma is a gas at low pressure which contains charge particles (electrons and ions) usually created by electrical discharge. Plasma is electrically neutral, with equal number of positive and negative charges, except in the regions of sheaths (near electrodes or chamber walls) where electric fields are present. Plasma contains also excited atoms and molecules, which give it a characteristic glow, and very reactive molecular fragments (radicals).

Typically there are about 1015 cm-3 neutral species (1 to 10% of which may be free radicals) and 108-1012 cm-3 ions and electrons.

Reactors with different configurations have been developed to make use of chemical, physical or ion assisted etching mechanisms.

Parallel Plate Systems - Plasma Mode

• Only moderate sheath voltage (10-100 eV), so only moderate ionic component. • Strong chemical component. • Etching can be fairly isotropic and selective.

Parallel Plate Systems - Reactive Ion Etching ( RIE) Mode

• Higher voltage drop across sheath at wafers. (100-700 eV) and lower pressures (10-100 mtorr)are used to attain even more directional etching induced by ion bombardment. • More physical component than plasma mode for more directionality but less selectivity.

High Density Plasma ( HDP) Etch Systems

• Uses remote, non-capacitively coupled plasma source (electron cyclotron resonance - ECR, or inductively coupled plasma source - ICP). • Very high density plasmas (1011-1012 ion cm-3) can be achieved (faster etching). • Lower pressures (1-10 mtorr range), longer mean free path and more anisotropic etching. • High etch rates, reasonable selectivity, and good directionality, while keeping ion energy and damage low.

Sputter Etching

• Purely physical etching usually with non-reactive Ar+ ions: • Highly directional, with poor selectivity, can etch almost anything • Damage to wafer surface and devices can occur.

Oxidation

Lithography

ETCHING Figure 1.9 and 1.10 (pp. 12 - 13) Wet chemical etching

For silicon dioxide HF solution

SiO2 + 6HF → H2SiF6 + 2 H2O

Si + 4HNO3 → SiO2 + 2H2O + 4NO2

For silicon HF + HNO3 KOH Plane Etch rate <100> 100 <110> 16 <111> 1

Figure 5.2 (p. 87) Orientation-dependent etching. (a) Through window patterns on <100>-oriented silicon. (b) Through window patterns on <110>-oriented silicon. Anisotropy = directionality

Af = 1- l/hf

Figure 5.3 (p. 90) Comparison of wet chemical etching and dry etching for pattern transfer.

Problems in etching

isotropy

selectivity Kinetic theory of gases

298.15K (25 C)

Ar (T = 298.15 K)

Ar (T = 2981.5 K) Impingement rate = Flux of molecules

∞ kT P Φ = v dn = n π = ∫ x x 0 2 m 2πmkT

P Φ = 2.64×1020 P (Pascals) MT M – molecular mass

Time to form a monolayer (Τm) is equal to the monolayer surface density divided by the impingement rate. Mean free Path d molecule diameter = collision distance πd2 collision cross section n - density l distance traveled by a molecule Probability of a collision P = πd2ln 2 d Distance between collisions l= 1/ πd2n Mean Free path λ= 1/√2 πd2n but n = P/kT

λ= kT/√2 πd2P BASICS OF VACUUM

Examples of basic molecular parameters in different vacuum ranges

The data in the table are approximate numbers for air.

CONDITIONS PRESSURE MEAN FREE TIME TO FORM P [Torr] PATH A λ MONOLAYER

τm Atmospheric 760 66 nm 3 ns Pressure Low Vacuum 1 50 µm 2.4 µs Medium 10-3 50 mm 2.4 ms Vacuum High Vacuum 10-6 50 m 2.4 s Ultra High 10-10 500 km 6.4 h Vacuum VACUUM PUMPS – LOW (FORE) VACUUM

Rotary vane pump VACUUM PUMPS – HIGH VACUUM

Turbomolecular pump Diffusion pump

Cryogenic pump Plasma

P = ~1Torr

~500V

Power supply

1 mTorr < P < 10 Torr -7 -3 Plasma density ne/n 10 –10

Neutrality ni = ne E ≈ 0 (except near boundary) Different configurations have been developed to make use of chemical, physical or ion assisted etching mechanisms.

Barrel Etchers • Purely chemical etching. • Used for non-critical steps, such as photoresist removal (ashing). Sputter Etching • Purely physical etching: highly directional, with poor selectivity • can etch almost anything, • usually uses Ar+ ions. • Damage to wafer surface and devices can occur due to ion bombardment, including effects shown below

charging trenching redeposition of photoresist