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TFE4180 Semiconductor Manufacturing Technology
Etching ( Part 3 )Chapter 16: Semiconductor Manufacturing Technology by M. Quirk & J. Serda
Requirements for Successful Dry Etch1. High selectivity to avoid etching materials that are not to be
etched (primarily photoresist and underlying materials)2. Fast etch rate to achieve an acceptable throughput of wafers3. Good sidewall profile control4. Good etch uniformity across the wafer5. Low device damage6. Wide process latitude for manufacturing
• Requirements- High selectivity to the gate oxide- High degree of anisotropy
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• Forms trenches for device isolation or vertical capacitors• Requires precise dimensional control• Carbon is added to protect the walls while etching• Fluorine, adequate selectivity to the photoresist• Chlorine/Bromine, high selectivity to the oxide mask (deep
trenches)
Silicon Dry EtchSingle-Crystal Silicon Etch
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Metal Dry Etch
Major Requirements for Metal Etching1) High etch rates (>1000 nm/min)2) High selectivity to the masking layer (>4:1), interlayer dielectric (>20:1)
and to underlying layers3) High uniformity with excellent CD control and no microloading (<8% at
any location on the wafer)4) No device damage from plasma-induced electrical charging5) Low residue contamination (e.g., copper residue, developer attack and
surface defects)6) Fast resist strip, often in a dedicated cluster tool chamber, with no
residual contamination7) No corrosion
– Avoid moisture and atmospheric contamination
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Typical Steps for Etching Metal Stacks1) Breakthrough step to remove native oxide2) ARC layer etch (may be combined with above step)3) Main etch step of aluminum4) Over etch step to remove residue. It may be a continuation of the main
etch step5) Barrier layer etch6) Residue removal process to prevent corrosion7) Resist removal
TiN Al + Cu (1%)Ti
p+ Silicon substrate
p- Epitaxial layer
n-well p-well
LI Oxide
ILD-1
Metal etchPhotoresist mask
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Tungsten Etchback
Metal-2 stack
(d) Metal-2 deposition
Tungstenplug
(a) Via etch through ILD-2 (SiO2)
Metal-1 stackILD-2
ILD-1
Via SiO2
(c) Tungsten etchback
SiO2Tungstenplug
(b) Tungsten CVD via fill
Tungsten
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Wet Etch
• Advantages:– High selectivity– No plasma induced electrical charging– Simple equipment
Wet Etch ParametersParameter Explanation Difficulty to Control
ConcentrationSolution concentration(e.g., ratio of NH4F:HFfor etch an oxide).
Most difficult parameter tocontrol because the bath
concentration is continuallychanging.
TimeTime of waferimmersion in the wetchemical bath.
Relatively easy to control.
Temperature Temperature of wetchemical bath. Relatively easy to control.
Agitation Agitation of the solutionbath.
Moderate difficulty toproperly control.
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Wet Oxide Etch• Buffered oxide etch:
– Selective removal of oxide by buffered solution of HF• Etch rate depends on formation and doping• Etches equally lateral and vertically on amorphous SiO2
Table 16.81 Approximate Oxide Etch Rates in BHF Solution at 25Ca
Type of Oxide Density (g/cm3) Etch Rate (nm/s)Dry grown 2.24 – 2.27 1Wet grown 2.18 – 2.21 1.5
CVD deposited < 2.00 1.5b – 5c
Sputtered < 2.00 10 – 20a) 10 parts of 454 g NH4F in 680 ml H2O and one part 48% HFb) Annealed at approximately 1000C for 10 minutesc) Not annealed
1 B. El-Kareh, ibid, p. 277.
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Wet Chemical Strips
• Used to remove surface layers– Si3N4 etched with H3PO4 at 160°C
• Difficult to control bath• Oxynitride film is removed with HF
– Excess Ti after silicide formation
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Polysilicon Etch Technology Evolution
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Photoresist removal
• Wet removal– Photoresist stripping – Not cost effective
• Plasma Ashing– Dominant technique– Dry removal of resist with oxygen– Reacts oxygen atoms with the resist material in a plasma
environment
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Atomic Oxygen Reaction with Resist in Asher
SubstrateResist
Asher reaction chamber
2) O2 dissociates into atomic oxygen 3) Plasma
energy turns oxygen into + ions
4) Neutral O and O+ react with C and H atoms in resist
Neutral oxygen radicals 5) By-product
desorption
6) By-product removal
Exhaust
Gas delivery
Downstream Plasma
1) O2 molecules enter chamber
+
++
++
++
+
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Residue Removal
• Removal of post-etch residue– Sidewall polymers– Via veils– Elevated temperature
• Can harden residues
• Dry ashing not sufficient to remove residues
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Post Etch Via Veil Residue
Via veils
Polymer residue
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Etch Inspection
• Traditionally manual microscope inspection• Automatic inspection systems • Critical Dimension Bias• Overetching, underetching, undercutting• Metal Corrosion• Defects can not be reworked