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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.
Figure 1.9 and 1.10 (pp. 12 - 13)
Lithography
Oxidation
ETCHING
Wet chemical etching
For silicon dioxide HF solutionSiO2 + 6HF → H2SiF6 + 2 H2O
Si + 4HNO3 → SiO2 + 2H2O + 4NO2
For silicon HF + HNO3
Figure 5.2 (p. 87)Orientation-dependent etching. (a) Through window patterns on <100>-oriented silicon. (b) Through window patterns on <110>-oriented silicon.
KOH
Plane Etch rate
<100> 100
<110> 16
<111> 1
Figure 5.3 (p. 90)Comparison of wet chemical etching and dry etching for pattern transfer.
Anisotropy
= directionality
Af = 1- l/hf
selectivity
Problems in etching
isotropy
298.15K (25 C)
Ar (T = 298.15 K)
Ar (T = 2981.5 K)
Kinetic theory of gases
Impingement rate = Flux of molecules
Time to form a monolayer (Τm) is equal to the monolayer surface density divided by the impingement rate.
∫∞
===Φ0 22 mkT
Pm
kTndnv xx ππ
MTP201064.2 ×=Φ P (Pascals)
M – molecular mass
Mean free Pathd molecule diameter = collision distanceπd2 collision cross section n - densityl distance traveled by a molecule
2 d
Probability of a collisionP = πd2lnDistance between collisions
l= 1/ πd2nMean 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.
6.4 h500 km10-10Ultra High Vacuum
2.4 s50 m10-6High Vacuum
2.4 ms50 mm10-3Medium Vacuum
2.4 µs50 µm1Low Vacuum
3 ns66 nm760Atmospheric Pressure
TIME TO FORM A MONOLAYERτm
MEAN FREE PATHλ
PRESSUREP [Torr]
CONDITIONS
VACUUM PUMPS – LOW (FORE) VACUUM
Rotary vane pump
VACUUM PUMPS – HIGH VACUUM
Turbomolecular pumpDiffusion pump
Cryogenic pump
Plasma
1 mTorr < P < 10 TorrPlasma density ne/n 10-7 – 10-3
Neutrality ni = ne
E ≈ 0 (except near boundary)
Power supply
~500V
P = ~1Torr
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 photoresistremoval (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
redepositionof photoresist
trenching charging