Transcript Lithography - Chemical Engineering IIT Madras

Dry Etch: Index
 Basics. Plasma, RIE
 Operation
 Oxide, Nitride, Al etches
 Issues: De veil, ESD, Faceting, Trenching
 Summary
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Dry Etch: Overview
 Similar to Sputtering in some respects
 Plasma used
 Etch can be started or stopped very quickly
 Some sputtering occurs (mechanical removal)
 Directional removal (anisotropic)
 Loss of selectivity
 Need to control with sputtering energy
 control of directional removal
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Plasma Etch Overall Mechanism
 Etching species (radicals) generated in Plasma
 Diffusion to the surface
 Adsorption
 Reaction
 Desorption (product has to be volatile)
 Diffusion to gas phase
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Plasma Etch Basics
 Usually forms a ‘veil’
 Helps in directional etch
 Creates problem in removing! (De veil)
 Wet de-veil is better than dry de-veil (selectivity and
cleaning are very good)
 Cu cannot be etched
 Just plasma etching is chemical (like wet etch)
 Just sputtering is physical (wafer is like sputter target)
 Reactive Ion Etching is physical + chemical (RIE)
 Reasonable selectivity and anisotropic
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Dry Etch: Plasma: Operation
 Similar to RF sputter (but not the same)
 esp. In plasma etching, no sputtering
 plasma is used to produce highly reactive radicals and ions
 input and output gas flow pattern determines etch
uniformity
 Loading (micro loading), PPC
 Rise in temperature (exothermic reactions, heat generated by
plasma, low specific heat capacity of gases)
 variation in etch rate (temp changes ==> reaction rate
change)
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Dry etch: RIE: Operation
 RIE closer to RF sputter (but not the same)
 plasma is used to produce highly reactive radicals and ions
 some sputtering also occurs,
 more directional etch
 loss of selectivity (photo resist may erode faster!)
 photo resist ‘hardening’ with plasma (before etch), may
help (low power, short time plasma hardening)
 lower pressure than plasma etch ==> directional path for
ions
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Dry etch: RIE: Operation
 However, RIE is NOT the same as RF sputter
 Instead of Ar ions, other chemical plasma is used
 primarily a chemical etching. Some sputtering to enhance
directional etch
 mainly ions participate in the reactions
 ions come with lot of energy. Substrate temp variation,
impurity concentration (PSG), crystalline nature are not that
important.
 Loading effects are less (since ions are accelerated)
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Dry etch: RIE: Issues
 Increase in wafer temp
 slower etch rate (lower pressure ==> lower concentration)
 lower selectivity
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Oxide Etch
 Fluorine based
CF4 , CHF3 , C2 F6
 Forms SiF4
(volatile)
 Many more
species have been
proposed / observed
 Intermediate species/ reactions
e  CF4  CF3  F  e
 CF2  F  F

 CF  3  F  2e
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 CF3  F

 CF 3  F
 CF4 (on its own)
will not etch Si, but F
species (in plasma)
will etch
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Oxide Etch
 Add Oxygen to increase etch rate by increasing F concentration
 When oxygen
concentration is very
high, dilution of F
decreases rate
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Oxide Etch
 Need to be selective with respect to silicon (FEOL)
 Add hydrogen and reduce
F conc.
 H+ + e- + F => HF
 Si etch rate decreases a
lot, SiO2 rate decreases a
little
 ==> controlled selectivity
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Oxide Etch: Control
 More C ==> polymerization
 More F ==> etching
 Add H to increase C/F ratio ==> polymerization
 Add O to decrease C/F ratio ==> more etching
 Use C2F4 instead of CF4 (for example) to change C/F ratio
 OR C2F6 or C4F10 ...
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Nitride etch
 Fluorine based / Oxygen added
 Intermediates includes O+, O-, O2+, O2-, COF, OH, O, OF
 Oxygen helps in reducing carbon contamination, and
increasing F
 In oxide etch, some oxygen is provided by oxide; in
nitride etch, it is supplied
 For example....
SiO2  CF4  SiF4  CO2
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Si3 N 4  3CF4  3SiF4  2 N 2  3C ???
Si3 N 4  3CF4  3O2  3SiF4  2 N 2  3CO2
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Al etch
 AlF3 is not volatile ==> No fluorine based compound
 Use chlorine based compounds
 Al has some Si and Cu added
 SiCl4 and AlCl3 are volatile (relatively)
 Cu or CuCl2 are not volatile
 sputtered during RIE
 CuCl2 washed in DI water rinse
 Pure CCl4, BCl3, additional Cl2
 CCl4 ==> chloro carbon polymer
 BCl3 ==> potential contamination from Boron
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Al etch
 Residual chlorine (absorbing moisture) will etch Al and
Cu
 Expose to Fluorine plasma (replace Chlorine with
Fluorine)
 Fluorine compounds less likely to absorb moisture
 eg. HF does not completely dissociate in water.
HF  H   F 
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Photo resist etch
 Plasma etcher
 Oxygen based
 Forms CO, CO2, H2O (gaseous)
Sample image
Image of a plasma etcher

RIE
Image of a RIE etcher
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Dry Etch: Veil
 Polymers
(carbon/fluorine
based) form on the
side wall
 will not dissolve
in plasma chemistry
 will be removed
by ion collision
(sputtering)
 usually glancing
angle sputter for side
wall ==> no removal
 Anisotripic etch
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Dry Etch: De-Veil
 After dry etch (via etch for example), veil has to be removed
 De-veil
 Wet clean (de veil)
 dry clean : Ashing(etch tool can be used)
 Wet clean is better
(selective dissolution)
 Sometimes ashing + wet clean
ULVAC dry etch tool
 De veil should NOT attack other materials!
Image of a Dry
Etch tool
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Dry Etch: De-Veil
©EKC
 Effect of de-
veil
 Remember:
Marketing!
SEM image of a
structure before
and after de veil
IPA = Iso
Propyl Alcohol
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Dry Etch: De-Veil
©EKC
 TiN is liner
 TEOS based
insulator
SEM image of a
structure before
and after de veil
(oxide/dielectric /
IMD/ ILD)
 AlSiCu = Cu and
Si doped Al
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Dry Etch: Extra
 Confinement of Plasma: ESC-ElectroStatic Chuck
 ©Lam Research Inc
Schematic showing confined and unconfined plasma
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ESD Issues
 Plasma ==> electrical charge
 large metal area getting reduced to small metal lines
==> charge accumulation and potential discharge
 ==> Gate oxide integrity
 ==> Need protective diodes *
 Especially for aluminum metal lines (many parallel lines)
 DR limits the maximum length of lines (or the area to be
precise)
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Other Issues
 Faceting
 What you want
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 What you get
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Other Issues
Glancing angles
sputter

Trenching
 What you want
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 What you get
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Dry Etch Summary
 Better control for anisotripic etch
 Problems like photoresist adhesion etc do not arise
 suitable for sub-micron
 Contamination control is easier (no metal contaminant)
 Running cost lower
 Easier to start and stop etch
 Results are more reproducible (wafer to wafer)
 Inline monitoring can be done (spectroscopy, interferometry)
 need little over etch
 ==> lower defectivity because of over etch
 Not the complete story: center/edge difference exists
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Dry Etch Summary
 Lower selectivity (especially for photo resist)
 Higher investment for the equipment
 Usually single water process (lower throughput)
 De-veil is an added step
 Microloading is present (as in wet etch)
 need PPC
 Not all materials can be etched: Cu is an important example
 ESD
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