III. Wet and Dry Etching
`
`Wet
`
`Dry
`
`Method
`
`Chemical Solutions
`
`Ion Bombardment or Chemical
`Reactive
`
`Atmosphere, Bath
`
`Vacuum Chamber
`
`1)
`
`1)
`
`2)
`3)
`4)
`
`Capable of defining small
`feature size (< 100 nm)
`
`High cost, hard to
`implement
`low throughput
`Poor selectivity
`Potential radiation damage
`
`Anisotropic
`
`NICHIA EXHIBIT 2014
`Vizio, Inc. v. Nichia Corporation
`Case IPR2018-00386
`E. Chen (4-12-2004)
`
`Environment and
`Equipment
`
`Advantage
`
`Disadvantage
`
`Directionality
`
`1) Low cost, easy to implement
`2) High etching rate
`3) Good selectivity for most materials
`1) Inadequate for defining feature size
`< 1um
`2) Potential of chemical handling
`hazards
`3) Wafer contamination issues
`Isotropic
`(Except for etching Crystalline
`Materials)
`
`Applied Physics 298r
`
`1
`
`
`
`Wet
`
`Dry
`
`Method
`
`Chemical Solutions
`
`Ion Bombardment or Chemical
`Reactive
`
`Atmosphere, Bath
`
`Vacuum Chamber
`
`1)
`
`1)
`
`2)
`3)
`4)
`
`Capable of defining small
`feature size (< 100 nm)
`
`High cost, hard to
`implement
`low throughput
`Poor selectivity
`Potential radiation damage
`
`Anisotropic
`
`NICHIA EXHIBIT 2014
`Vizio, Inc. v. Nichia Corporation
`Case IPR2018-00386
`E. Chen (4-12-2004)
`
`Environment and
`Equipment
`
`Advantage
`
`Disadvantage
`
`Directionality
`
`1) Low cost, easy to implement
`2) High etching rate
`3) Good selectivity for most materials
`1) Inadequate for defining feature size
`< 1um
`2) Potential of chemical handling
`hazards
`3) Wafer contamination issues
`Isotropic
`(Except for etching Crystalline
`Materials)
`
`Applied Physics 298r
`
`1
`
`
`
`Pattern Generation (Transfer): Etch vs. Liftoff
`
`Etching
`
`Lithography
`
`Etching
`
`Strip
`Mask (Resist)
`
`Mask
`Film
`
`Substrate
`
`Mask
`Film
`
`Substrate
`
`Film
`
`Substrate
`
`Liftoff
`
`Lithography
`
`Mask
`
`Mask
`
`Film
`
`Substrate
`
`Film
`
`Mask
`
`Deposit
`Film
`
`Remove
`Mask (Resist)
`(Liftoff)
`
`Film
`
`Mask
`
`Film
`
`Substrate
`
`Film
`
`Substrate
`
`Applied Physics 298r
`
`2
`
`E. Chen (4-12-2004)
`
`
`
`Isotropic vs. Anisotropic Etching
`
`Isotropic Etching:
`
`Anisotropic Etching:
`
`Lateral Etch Ratio:
`
`Etching rate is the same in both
`horizontal and vertical direction
`Etching rate is different in
`horizontal and vertical direction
`
`R =
`L
`
`Isotropic Etching:
`Anisotropic Etching:
`Directional Etching:
`
`)
`
`(
`Horizontal
`Rate
`r
`Etch
`H
`)V
`(
`r
`Rate
`Etch
`Vertical
`RL = 1
`0 < RL < 1
`RL = 0
`
`Bias: the difference in lateral dimensions between the
`feature on mask and the actually etched pattern
`(cid:168) smaller RL results in smaller bias
`
`Mask
`
`Mask
`
`Mask
`
`RL = 1
`
`0 < RL < 1
`
`RL = 0
`
`Applied Physics 298r
`
`3
`
`E. Chen (4-12-2004)
`
`
`
`“Under Cut” and “Over Etch”
`
`Bias
`
`Bias
`
`Mask
`Film
`
`Mask
`Film
`
`Substrate
`
`“Under Cut”
`Good for Lift-off
`
`Substrate
`
`(Rl = 1, pattern dimension
`is poorly defined)
`
`(Rl = 0.5, pattern dimension
`is better defined)
`
`Mask
`
`Substrate
`
`Over-Etch
`(cid:172) results in more vertical profile
`but larger bias
`
`Mask
`
`Substrate
`
`Worse in thick film
`(cid:172) Poor CD control in
`thick film using wet etch
`
`Applied Physics 298r
`
`4
`
`E. Chen (4-12-2004)
`
`
`
`Mask Erosion: Film-Mask Etching Selectivity
`
`θ
`
`Mask
`
`Film
`
`Substrate
`
`W/2 (Bias)
`Mask
`Film
`
`Substrate
`
`W/2 (Bias)
`Mask
`Film
`
`Substrate
`
`hf
`
`1
`
`2
`
`1)
`
`film horizontal etch rate (rfh) < mask
`horizontal etch rate (rmh):
`
`(
`θcot
`+
`
`)m
`R
`
`(
`%
`
`)
`
`=
`
`2
`S
`
`fm
`
`hW
`
`f
`
`(mask lateral etch ratio)
`
`(ratio of film and mask vertical
`etching rate
`– selectivity)
`
`rr
`
`mH
`
`mV
`
`rr
`
`fV
`
`mV
`
`R =
`mL
`
`S =
`fm
`
`2)
`
`If film horizontal etch rate (rfh) > mask horizontal
`etch rate (rmh):
`
`(
`)
`2% =
`
`R
`m
`
`hW
`
`f
`
`Applied Physics 298r
`
`5
`
`E. Chen (4-12-2004)
`
`
`
`Film-Mask Selectivity (Sfm) vs. Etching Bias
`
`Selectivity vs Mask Lateral Etch Ratio Rm
`
`Selectivity vs Mask Wall angle θ
`
`Rm = 100%
`
`6
`4
`W/h (%)
`
`8
`
`10
`
`θ = 90
`θ = 60
`θ = 30
`
`100
`
`80
`
`60
`
`40
`
`20
`
`0
`
`Selectivity
`
`θ = 90º
`
`Rm = 100%
`Rm = 50%
`Rm = 10%
`
`100
`
`80
`
`60
`
`40
`
`20
`
`0
`
`Selectivity
`
`0
`
`2
`
`6
`4
`W/h (%)
`
`8
`
`10
`
`0
`
`2
`
`Most mask material etches isotropically
`(cid:168) Selectivity > 20:1
`
`Applied Physics 298r
`
`6
`
`E. Chen (4-12-2004)
`
`
`
`Wet Etch Crystalline Materials
`
`• Typically, wet etching is isotropic
`• However on crystalline materials,
`etching rate is typically lower on the
`more densely packed surface than on
`that of loosely packed surface
`
`Si:
`Diamond Lattice Structure
`
`Surface Atom Density:
`{111} > {100} > {110}
`
`Etching rate:
`R(100) ~ 100 x R(111)
`
`(100)
`
`{110}
`
`{111}
`
`{100}
`
`(100) Si Wafer
`
`Applied Physics 298r
`
`7
`
`E. Chen (4-12-2004)
`
`
`
`Si Wet Etch – (100) Wafer, Mask Aligned in <110> Direction
`
`(100)
`
`{110}
`
`Mask
`
`(100)
`
`{110}
`
`{111}
`
`54.7º
`
`(100)
`
`54.7º
`
`(111)
`
`(111)
`
`{100}
`
`(100)
`
`(200-nm size pyramidal
`pit on (100) Si substrate)
`
`Applied Physics 298r
`
`8
`
`E. Chen (4-12-2004)
`
`
`
`Si Wet Etch – (100) Wafer, Mask Aligned in <100> Direction
`
`(100)
`
`{110}
`
`Mask
`
`<100>
`
`(Undercut!)
`
`(100)
`
`{110}
`
`<100>
`
`{111}
`
`{100}
`
`Etching on (110) Si
`
`<110>
`
`<111>
`
`Applied Physics 298r
`
`9
`
`E. Chen (4-12-2004)
`
`
`
`<100>
`
`(1
`
`00)
`
`Mask
`<100>
`
`GaAs Wet Etch – (100) Wafer
`
`(100)
`
`{111}Ga
`
`{111}As
`
`{100}
`
`{111}Ga
`
`{111}As
`
`{110}
`
`Zincblende
`Structure
`Etching Rate:
`R({111}As)
`> R({100})
`> R({111}Ga)
`
`<100>
`
`(100)
`
`<100>
`
`Mask
`
`Applied Physics 298r
`
`10
`
`E. Chen (4-12-2004)
`
`
`
`Typical Wet Etchants
`
`Material
`
`Si (a-Si)
`
`SiO2
`
`SI3N4
`
`GaAs
`
`Au
`
`Al
`
`Gas
`
`Etching Rate
`
`1) KOH
`2) HNO3 + H2O +
`HF
`1) HF
`2) BHF
`1) HF
`2) BHF
`3) H3PO4
`1) H2SO4 + H2O2
`+H2O
`2) Br + CH3OH
`1) HCl + HNO3
`2) KI + I2 +H2O
`1) HCl + H2O
`2) NaOH
`
`~ 6 – 600 nm/min
`(anisotropic)
`~ 100 nm/min
`~ 10 – 1000
`nm/min
`~ 100 nm/min
`~ 100 nm/min
`~ 10 nm/min
`
`~ 10 um/min
`
`~ 40 nm/min
`~ 1 um/min
`
`~ 500 nm/min
`
`Mask
`
`Resist
`
`Resist
`
`Resist
`SiO2
`
`Resist
`
`Resist
`
`Resist
`
`Selectivity
`
`> 50:1
`
`> 50:1
`
`> 50:1
`
`> 50:1
`
`> 50:1
`
`> 50:1
`
`Applied Physics 298r
`
`11
`
`E. Chen (4-12-2004)
`
`
`
`Dry Etching
`
`Problems with wet etching:
`Isotropic (cid:168) unable to achieve pattern size
`smaller than film thickness
`
`Main Purpose of Developing Dry Etching is to
`Achieve Anisotropic Etching
`
`Type of Dry Etching Technology
`Physical Sputtering
`- Physical bombardment
`• Ion Mill
`• Plasma sputtering
`Plasma Etching
`- Plasma-assisted chemical reaction
`Reactive Ion Etching (RIE)
`- Chemical reaction + ion bombardment
`
`+
`
`R
`
`+ R
`
`Applied Physics 298r
`
`12
`
`E. Chen (4-12-2004)
`
`
`
`Dry Etching Comparison
`
`Chamber
`Pressure
`
`Beam
`Energy
`
`High
`> 100
`Mtorr
`
`Low
`
`Dry Etching
`
`Plasma Etching
`• Plasma assisted
`chemical reaction
`
`Anisotropy
`
`Selectivity
`
`Low
`
`Very
`Good
`
`Low
`10 ~ 100
`mtorr
`
`Medium
`
`Reactive Ion Etching
`• Physical bombardment
`+ chemical reaction
`
`Medium
`
`Good
`
`Very Low
`< 10
`mtorr
`
`High
`
`Physical Sputtering
`(Ion Mill)
`• physical bombardment
`
`High
`
`Poor
`
`Applied Physics 298r
`
`13
`
`E. Chen (4-12-2004)
`
`
`
`•
`
`•
`•
`
`•
`
`•
`•
`
`•
`•
`
`Gases
`
`Shaw Heads
`
`t
`
`e-
`
`e-
`
`Ar
`
`Ar+
`
`Reactive Ion Etching (RIE)
`Etching gas is introduced into the chamber
`continuously
`Plasma is created by RF power
`Reactive species (radicals and ions) are
`generated in the plasma
`(cid:172) radicals: chemical reaction
`(cid:172) ions: bombardment
`Reactive species diffused onto the sample
`surface
`The species are absorbed by the surface
`Chemical reaction occurs, forming volatile
`byproduct
`Byproduct is desorbed from the surface
`Byproduct is exhausted from the chamber
`
`Substrate
`
`Gas Selection:
`1) React with the material to be etched
`2) Result in volatile byproduct with low vapor pressure
`
`~
`
`RF
`
`Applied Physics 298r
`
`14
`
`E. Chen (4-12-2004)
`
`
`
`Typical RIE Gases
`
`Material
`
`Si (a-Si)
`
`SiO2
`
`SI3N4
`
`GaAs
`
`InP
`
`Al
`
`Gas
`
`1) CF4
`2) SF6
`3) BCl2 + Cl2
`1) CHF3 + O2
`2) CF4 + H2
`1) CF4 + O2 (H2)
`2) CHF3
`
`1) Cl2
`2) Cl2 + BCl3
`
`1) CH4/H2
`
`1) Cl2
`2) BCl3 + Cl2
`
`Resist / Polymer
`
`1) O2
`
`Etching Rate
`(A/min)
`
`Mask
`
`Selectivity
`
`~ 500
`
`~ 200
`
`~ 100
`
`~ 200
`
`~ 200
`
`~ 300
`
`~ 500
`
`Resist
`Metal (Cr, Ni, Al)
`
`Resist
`Metal (Cr, Ni, Al)
`Resist
`Metal (Cr, Ni, Al)
`
`SI3N4
`Metal (Cr, Ni)
`
`SI3N4
`Metal (Cr, Ni, Al)
`Resist
`SI3N4
`SI3N4
`Metal (Cr, Ni)
`
`~ 20:1
`~ 40:1
`
`~ 10:1
`~ 30:1
`~ 10:1
`~ 20:1
`
`~ 10:1
`~ 20:1
`
`~ 40:1
`
`~ 10:1
`
`~ 50:1
`
`Applied Physics 298r
`
`15
`
`E. Chen (4-12-2004)
`
`
`
`Arts of Nanofabrication: Nano-Dots, -Holes and -Rings
`
`Applied Physics 298r
`
`16
`
`E. Chen (4-12-2004)
`
`
`
`Arts of Nanofabrication : Nano-Gratings
`
`Applied Physics 298r
`
`17
`
`E. Chen (4-12-2004)
`
`
`
`Arts of Nanofabrication : Nano-Fluidic Channels
`
`Applied Physics 298r
`
`18
`
`E. Chen (4-12-2004)
`
`
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