Etching ( Part 3 ) - NTNUfolk.ntnu.no/jonathrg/fag/TFE4180/slides/Ch16 Etching (Part 3).pdf ·...
Transcript of Etching ( Part 3 ) - NTNUfolk.ntnu.no/jonathrg/fag/TFE4180/slides/Ch16 Etching (Part 3).pdf ·...
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TFE4180 Semiconductor Manufacturing Technology
Etching ( Part 3 )Chapter 16: Semiconductor Manufacturing Technology by M. Quirk & J. Serda
Saroj Kumar PatraTFE4180 Semiconductor Manufacturing Technology
Norwegian University of Science and Technology ( NTNU )
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Objectives
• Give an application example for dielectric, siliconand metal dry etch
• Discuss wet etch and its applications• Explain how photoresist is removed• Discuss etch inspection
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TFE4180 Semiconductor Manufacturing Technology
Table of Contents• Dry Etch Applications
– Dielectric Dry Etch– Silicon Dry Etch– Metal Dry Etch
• Wet Etch– Types of Wet Etch
• Polysilicon Etch Technology Evolution• Photoresist Removal
– Plasma Ashing• Etch Inspection
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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
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Dry Etch Critical Parameters
Equipment Parameters:• Equipment design• Source power• Source frequency• Pressure• Temperature• Gas-flow rate• Vacuum conditions• Process recipe
Other Contributing Factors:• Cleanroom protocol• Operating procedures• Maintenance procedures• Preventive maintenance schedule
Process Parameters:• Plasma-surface interaction:
- Surface material- Material stack of different layers- Surface temperature- Surface charge- Surface topography
• Chemical and physical requirements• Time
• Quality Measures:• Etch rate• Selectivity• Uniformity• Feature profile• Critical dimensions• Residue
Plasma-etchinga wafer
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Dielectric Dry Etch of Oxide
• The most complicated etch process• Etch to form contact holes and vias
– Selectivity and high-aspect ratio important• Plasma etching process based on fluorocarbon
chemistry– CF4, CHF3, C3F8, C4F8, NF3, SiF3– May add Ar and/or He (buffer gas)
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Oxide Etch Reactor
CF4
C3F8
C4F8
CHF3
NF3
SiF4
Ar
Wafer
Electrostatic chuck
Plasma
Selection of fluorocarbon and hydrocarbon chemicals
HF CF2
F
CHF
CF4
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Dielectric Dry Etch of Oxide
• Underlying material selectivity– Add O2 <20% => better selectivity between Si and
oxide– Add H2 <40% => reduces etch rate for Si~0– Hard ”etch stop”– Polymer formation
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Etch Stop Hard Mask Layer
n-well p-well
LI Oxide
p+ Silicon Substrate
p- Epitaxial Layer
2 Doped oxide CVD
Nitride etch
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Oxide CMP 3 4 Oxide etch1 Nitride CVD
Example: Silicon nitride, Si3N4, serves as etch-stop during LI oxide etchNote: The numbers show the order of the five operations
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Dielectric Dry Etch of Oxide
• Photoresist selectivity• Contact etching to varying
depths• Sidewall profileContact holes
S DG
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Dielectric Dry Etch of Silicon Nitride
• Gas-chemistry‒ CF4 mixed with N2 and O2
• LPCVD‒ High density
• PECVD‒ Low density higher etch rate
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Silicon Dry Etch
• Mainly used for- Polysilicon gate formation- Single-crystal silicon trench creation
• Gases containing chlorine/bromine are preferred for etching- High selectivity to oxide- Etches anisotropically
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Silicon Dry Etch
• Polysilicon Gate Etch Process Steps1) Breakthrough step2) Main-etch step3) Overetch step (avoid microtrenching)
Polysilicon gate formation
• 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
• Disadvantages:– Isotropic– Safety risk– High disposal costs– Photoresist lift-off– Bubble formation
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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
• Surface particle contamination can be cleaned
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TFE4180 Semiconductor Manufacturing Technology