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ES 176/276 – Section # 3 – 09/26/2011. Brief Overview from Section #2 – 09/19/2011. – MEMS examples:(MEMS Airbag accelerometer, Digital Micromirror Device, Capacitive RF MEMS switch) – Planar fabrication broad overview (how ICs are made) - PowerPoint PPT Presentation
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ES 176/276 – Section # 3 – 09/26/2011
Brief Overview from Section #2 – 09/19/2011
– MEMS examples: (MEMS Airbag accelerometer, Digital Micromirror Device, Capacitive RF MEMS switch)
– Planar fabrication broad overview (how ICs are made)
– ICs versus MEMS, what are new fabrication requirements?
– MEMS fabrication broad overview
Planar Fabrication Overview
P
P Well - NMOS SubstrateN Well - PMOS Substrate
PNP+ P+ N+ N+
S G D S G D
G
D
S S
D
GSub Sub
ES 176/276 – Section # 3 – 09/26/2011
Important Process Steps/Terminology (before we begin)
Lithography: Process of transferring a pattern from a pre-made photomask into a photoresist layer
Etching: Removal of material either by a wet chemical process (wet etching) or a gaseous/plasma process (dry etching)
Deposition: Addition of material (i.e. metal, insulator, semiconductor) either by physical deposition or chemical deposition methods.
Annealing/Diffusion: High temperature process to reform a material layer
Oxidation: Growth of SiO2 by thermal annealing in an oxygen rich environment
Planarization: Polishing of a layer in order to reduce the surface features to a flat plane
Ion implantation: Exposure of a material to high energy ions which are eventually incorporated into the material lattice
ES 176/276 – Section # 3 – 09/26/2011
Integrated circuit fabrication versus MEMS fabrication
– What are the fundamental differences in the following devices?
P
P WellN Well
PNP+ P+ N+ N+
ES 176/276 – Section # 3 – 09/26/2011
ES 176/276 – Section # 2 – 09/19/2011
MEMS Surface Micro/Nanomachining
MEMS Bulk Micro/Nanomachining
ES 176/276 – Section # 3 – 09/26/2011
Today’s Plan
– Etching fundamentals
– Wet etching (chemical)
– Plasma etching (chemical & physical)
– Ion-enhanced etching (aka Reactive Ion Etching)
– Sputter etching
– Etching unique to MEMS (anisotropic wet etching, deep RIE)
ES 176/276 – Section # 3 – 09/26/2011
ES 176/276 – Section # 3 – 09/26/2011
Substrate
Film deposition Photoresist application
Deposited Film
Photoresist
Exposure
Development Etching Resist removal
Mask
Etch mask
Light
Etching, basic idea
There are two main types of etching used in IC & MEMS fabrication: wet etching and dry etching (aka plasma etching). Plasma etching dominates today.
Wet etching: uses liquid etchants – etching done exclusively by chemical process Dry etching: uses plasma - combination of chemical and physical processes
• Usually selectivity, and directionality are the first order issues.
ES 176/276 – Section # 3 – 09/26/2011
Selectivity (S) =
Etch Selectivity
etch rate of target layer
etch rate of other layers (e.g. masks, substrate)
Masks:• Soft: - e.g. photoresist • Hard- e.g. Si3N4, SiO2 (higher etch resistance)
• In general, S > 25-50 is often required.
• Selectivity comes from chemistry excellent in case of wet etching
ES 176/276 – Section # 3 – 09/26/2011
• Isotropic etching: same etch rate in all directions. Isotropic etching undercutting.
Undercutting:• Undercutting is often expressed in terms of the etch bias b.• For completely isotropic: bias= depth (i.e. b=d). • Etch anisotropy is defined as:
b
d
b
d = xf
a) b)
• Af = 0 for perfectly isotropic etching = 1 for perfectly anisotropic etching. In reality: 0 < Af < 1.
Af 1 rlat
rver1 b
d
Etch Directionality
ES 176/276 – Section # 3 – 09/26/2011
a)
b)
• Illustration of undercutting (directionality) and selectivity issues.
Usually highly anisotropic (almost vertical profiles) and highly selective etching (ratios of 25-50) are desired, but these can be difficult to achieve simultaneously.
• Physical etching more anisotropic - but less selective. • Chemical etching more selective - but isotropic.
Selectivity & Directionality
ES 176/276 – Section # 3 – 09/26/2011Example to demonstrate selectivity/directionality importance
sf sf
tf
sm x
2sf
Consider the diagram below:
A 0.5 µm thick oxide layer is etched to achieve equal structure widths and spacings (Sf). The etch process produces a degree of anisotropy of 0.8. If the distance between the mask edges (x) is 0.35 µm, what structure spacings and widths are obtained? (Neglect overetching)
ES 176/276 – Section # 3 – 09/26/2011Example to demonstrate selectivity/directionality importance
sf sf
tf
sm x
2sf
Consider the diagram below:To obtain equal widths and spacings:
Sm = Sf + 2b, Af = 1 – b/d
Sm = Sf + 2d(1 – Af)
x = 2Sf – Sm
Sf = x + 2d(1 – Af)
Sf = 0.55 µmIf our lithography is limited, what would we do if we wanted a smaller pitch?
What if we were using wet etching? What if we over etched?
What is the smallest pitch theoretically possible?
ES 176/276 – Section # 3 – 09/26/2011Wet Etching
• Due to a chemical reaction that produces H2O-soluble byproducts.• Wafers typically submerged in specific chemical baths and rinsed in DI H2O.• Processes tend to be highly selective but isotropic (except for crystallographically dependent etches).
Examples:
(i)Etching of SiO2 by aqueous HF:
(ii) Etching of Si by nitric acid (HNO3) and HF:
SiO26HF H2SiF62H2O
Si HNO3 6HF H2SiF6 HNO2 H2O H2
Buffering agents are often used to preserve etchant strength over time.e.g. :•NH4F is added to HF to prevent depletion of F ions in the oxide etch (called: BHF or BOE for buffered oxide etch)•CH3COOH is added to HNO3 +HF to limit dissociation of HNO3
(water soluble)
ES 176/276 – Section # 3 – 09/26/2011Wet Etching
In order to be able to produce structures of sizes similar to the minimum lithographic dimensions, we need to:
(i) Use very thin films not practical or(ii) Use processes with Anisotropy ~ 1 difficult to obtain via
wet etching
Because of their isotropic nature, wet chemical etches are rarely used in mainstream VLSI/ULSI fabrication
Wet etching has more mainstream use in MEMS fabrication (we will return to this)
EXCELLENT RESOURCE: Google “etch rates” and the first link is a great wet etching and general etching resource.
“Etch Rates for Micromachining Processing Part II” by Williams et al.
ES 176/276 – Section # 3 – 09/26/2011Plasma Etching
Developed and used for:1. Faster and simpler etching in a few cases.2. More directional (anisotropic) etching!!
• Both chemical (highly reactive) species and ionic (very directional) species typically play a role.
• VP is positive to equalize electron and ion fluxes.• Smaller electrode has higher fields to maintain current continuity (higher RF current density).
Electrode
GroundGas inlet( Ar, CF4, O2)
Gas outlet,pump
Matchingnetwork
RFgenerator
Plasma
RF power input
Electrode
Plasmasheaths
+
-
0Distance
Vp
0
Vol
tage
Electrode (target)
Electrode
Equal area electrodes
Unequal area electrodes(smaller electrode at left)
V2
V1
ES 176/276 – Section # 3 – 09/26/2011
Ionization: CF3+ e- CF3
+ + 2e-
Dissociative ionization: CF4 + e- CF3
+ + F + 2e-
Excitation: CF4 + e- CF4
* + e-
Recombination: CF3
+ + F + e- CF4 F + F F2
Dissociation: CF4 + e- CF3 + F + e-
• Etching gases include halide-containing species such as CF4, SiF6, Cl2, and HBr, plus additives such as O2, H2 and Ar. O2 by itself is used to etch photoresist. Pressure = 1 mtorr to 1 torr.
• Typical reactions and species present in a plasma used are shown above.
Plasma Etching
ES 176/276 – Section # 3 – 09/26/2011Plasma Etching Mechanisms
There are three principal mechanisms:
(i) chemical etching – by reactive neutral species(isotropic, selective)
(ii) physical etching – by ions (anisotropic, less selective)
(iii) ion-enhanced etching – combination of both (anisotropic, selective)
ES 176/276 – Section # 3 – 09/26/2011
Etchant (free radical) creation
Mask
Film
Etchantadsorption
Etchant transfer
Etchant/film reaction
Byproductremoval
e- +
(i) Chemical Etching
• Etching done by reactive neutral species, such as “free radicals” (e.g. F, CF3)
e CF4 CF3 Fe
4FSi SiF4
• Additives like O2 can be used which react with CF3 and reduce CF3 + F recombination. higher etch rate.
• These processes are purely chemical and are therefore isotropic and selective, like wet etching.
Mask
Film
Reactive neutral species
• Generally characterized by (n=1) arrival angle and low sticking coefficient (Sc ≈ 0.01).
cosn
ES 176/276 – Section # 3 – 09/26/2011
Mask
Film
+ +
Ionic species
++ +
+
(ii) Physical Etching
• Etching species are ions like CF3+ or Ar+ which
remove material by sputtering.
• Ion etching is much more directional (anisotropic) due to directional acceleration of ions by high E field across plasma sheath. • Depending on ion species, etching can be: purely physical (e.g. by utilizing non reactive ions like Ar+ making it more physical sputtering) ,or includes chemical component (e.g. by utilizing ions of reactive species like Cl+ or CF3
+).
• Sc ≈ 1, i.e. ions don't bounce around (or if they do, they lose their energy.) significant shadow effects • Not very selective since all materials sputter at about the same rate.
• Physical sputtering can cause damage to surface, with extent and amount of damage a direct function of ion energy (not ion density).
ES 176/276 – Section # 3 – 09/26/2011(iii) Ion Enhanced Etching
›
100 200 300 400 500 600 700 800 900
Sili
con
etc
h r
ate
(nm
min
-1)
Time (sec)
7
6
5
4
3
2
1
0
XeF2Gas Only
Ar+ Ion Beam+ XeF2 Gas
Ar+ IonBeam Only
• Etching is enhanced (in terms of net etch rate and resulting etch profile) by interaction of the chemical and physical components of the plasma.
• Etch profiles can be very anisotropic, and selectivity can be good.
• Both ions (like Ar+ ) and neutral reactive species (like XeF2) participate in the etching process.
Figure shows etch rate of silicon as XeF2 gas (not plasma) and Ar+ ions are introduced to the silicon surface. Only when both are present does appreciable etching occur.
ES 176/276 – Section # 3 – 09/26/2011
• Many different mechanisms proposed for this synergistic etching between physical and chemical components. Two mechanisms are shown above.• Ion bombardment can enhance etch process (such as by damaging the surface to increase reaction, or by removing etch byproducts), or can remove inhibitor that is an indirect byproduct of etch process (such as polymer formation from carbon in gas or from photoresist). • Whatever the exact mechanism (multiple mechanisms may occur at same time):
• need both components for etching to occur.• get anisotropic etching and little undercutting because of directed ion flux.• get selectivity due to chemical component and chemical reactions.
many applications in etching today.
Reactive neutral species
Mask
Film
Ionic species+ ++
Chemical etch enhancedby ion bombardment
PR Mask
Film
Ionic species+ ++
Inhibitor removedby ion bombardment
Inhibitor
Reactive neutral species
a) b)
ES 176/276 – Section # 3 – 09/26/2011þ
Sputter Etching and Ion Beam Milling
High Density Plasma Etching
Reactive Ion Etching
Plasma Etching
Wet Chemical Etching
Pre
ssur
e
Sele
ctiv
ity
Ani
sotr
opy
Ene
rgy
PhysicalProcesses
ChemicalProcesses
Summary
ES 176/276 – Section # 3 – 09/26/2011Summary of Key Ideas
• Etching of thin films is a key technology in modern IC manufacturing.• Photoresist is generally used as a mask, but sometimes other thin films also act as masks.• Selectivity and directionality (anisotropy) are the two most important issues. Usually good selectivity and vertical profiles (highly anisotropic) are desirable. • Other related issues include mask erosion, etch bias (undercutting), etch uniformity, residue removal and damage to underlying structures.• Dry etching is used almost exclusively today because of the control, flexibility, reproducibility and anisotropy that it provides.• Reactive neutral species (e.g. free radicals) and ionic species play roles in etching.• Generally neutral species produce isotropic etching and ionic species produce anisotropic etching.• Physical mechanisms:
• Chemical etching involving the neutral species.• Physical etching involving the ionic species.• Ion-enhanced etching involving both species acting synergistically.
ES 176/276 – Section # 3 – 09/26/2011
IC vs. MEMS Etching
Until now, all of the concepts introduced were established with IC fabrication in mind.
99% of this material transfers to the MEMS fabrication, but MEMS fabrication has additional requirements.
Wet etching is far more prevalent in MEMS fabrication
Why: - sacrificial etching requires high selectivity and complete isotropy- etching large amounts of material is cheaper/faster with wet etching- anisotropic etching of Si is a good platform for MEMS 3D geometries
Plasma etching is pushed to extremes in order to produce high aspect ratio structures (aka Deep RIE, aka Bosch Etch)
ES 176/276 – Section # 3 – 09/26/2011Silicon Wet Anisotropic Etching
ES 176/276 – Section # 3 – 09/26/2011Silicon Wet Anisotropic Etching
ES 176/276 – Section # 3 – 09/26/2011Silicon Wet Anisotropic Etching
Self-Limited Stable Profile (SLSP) . . . What is it?
ES 176/276 – Section # 3 – 09/26/2011Silicon Wet Anisotropic Etching
ES 176/276 – Section # 3 – 09/26/2011Silicon Wet Anisotropic Etching
ES 176/276 – Section # 3 – 09/26/2011Silicon Wet Anisotropic Etching
Application example: Ink Jet Nozzle
ES 176/276 – Section # 3 – 09/26/2011Silicon Deep Reactive Ion Etching (Bosch Etch Process)
The Bosch process uses a fluorine based plasma chemistry to etch the silicon, combined with a fluorocarbon plasma process to provide sidewall passivation and improved selectivity to masking materials.
A complete etch process cycles between etch and deposition steps many times to achieve deep, vertical etch profiles.
Sulphur hexafluoride (SF6) is the source gas used to provide the fluorine for silicon etching.
This molecule will readily break up in high-density plasma to release free radical fluorine.
The sidewall passivation and mask protection is provided by octofluorocyclobutane (c-C4F8), a cyclic fluorocarbon that breaks open to produce CF2 and longer chain radicals in the high-density plasma. These readily deposit as fluorocarbon polymer on the samples being etched.
ES 176/276 – Section # 3 – 09/26/2011Silicon Deep Reactive Ion Etching (Bosch Etch Process)
ES 176/276 – Section # 3 – 09/26/2011Silicon Deep Reactive Ion Etching (Bosch Etch Process)