III. Wet and Dry Etching
Wet Dry
Ion Bombardment or Chemical Method Chemical Solutions Reactive Environment and Atmosphere, Bath Vacuum Chamber Equipment 1) Low cost, easy to implement 1) Capable of defining small Advantage 2) High etching rate feature size (< 100 nm) 3) Good selectivity for most materials 1) Inadequate for defining feature size 1) High cost, hard to < 1um implement Disadvantage 2) Potential of chemical handling 2) low throughput hazards 3) Poor selectivity 3) Wafer contamination issues 4) Potential radiation damage Isotropic Directionality (Except for etching Crystalline Anisotropic Materials)
Applied Physics 298r 1 E. Chen (4-12-2004) Pattern Generation (Transfer): Etch vs. Liftoff
Etching Liftoff Mask Mask Mask Film Film Lithography Lithography Substrate Substrate
Film Film Mask Film Deposit Mask Film Mask Etching Film Substrate Substrate
Film Film Strip Remove Mask (Resist) Substrate Mask (Resist) Substrate (Liftoff)
Applied Physics 298r 2 E. Chen (4-12-2004) Isotropic vs. Anisotropic Etching
Mask Isotropic Etching: Etching rate is the same in both horizontal and vertical direction Anisotropic Etching: Etching rate is different in
horizontal and vertical direction RL = 1 Lateral Etch Ratio: Mask
Horizontal Etch Rate (rH ) RL = Vertical Etch Rate ()rV
0 < R < 1 Isotropic Etching: RL = 1 L
Anisotropic Etching: 0 < RL < 1 Mask
Directional Etching: RL = 0
Bias: the difference in lateral dimensions between the
feature on mask and the actually etched pattern RL = 0
¨ smaller RL results in smaller bias
Applied Physics 298r 3 E. Chen (4-12-2004) “Under Cut” and “Over Etch”
Bias Bias
Mask Mask Film Film “Under Cut” Substrate Good for Lift-off Substrate
(Rl = 1, pattern dimension (Rl = 0.5, pattern dimension is poorly defined) is better defined)
Mask Mask
Substrate Substrate
Over-Etch ¬ results in more vertical profile Worse in thick film but larger bias ¬ Poor CD control in thick film using wet etch
Applied Physics 298r 4 E. Chen (4-12-2004) Mask Erosion: Film-Mask Etching Selectivity
1) film horizontal etch rate (rfh) < mask θ Mask horizontal etch rate (rmh): Film W 2 Substrate ()% = ()cotθ + Rm hf S fm
rmH W/2 (Bias) RmL = (mask lateral etch ratio) rmV Mask Film hf r (ratio of film and mask vertical S = fV 1 Substrate fm r etching rate mV – selectivity) W/2 (Bias) 2) If film horizontal etch rate (r ) > mask horizontal fh Mask etch rate (r ): mh Film W ()% = 2Rm 2 Substrate hf
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
100 100 θ = 90º 80 80 Rm = 100% y y t i it Rm = 50%
60 v 60 iv i t Rm = 10% c
ect θ = 90 l le 40 40 e e θ = 60 S S 20 20 θ = 30 Rm = 100% 0 0 0246810 0246810 W/h (%) W/h (%)
Most mask material etches isotropically ¨ 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 (100) more densely packed surface than on that of loosely packed surface
Si: {110} Diamond Lattice Structure {111}
{100} Surface Atom Density: {111} > {100} > {110}
(100) Si Wafer Etching rate: R(100) ~ 100 x R(111)
Applied Physics 298r 7 E. Chen (4-12-2004) Si Wet Etch – (100) Wafer, Mask Aligned in <110> Direction
(100)
(100)
{110} {110} {111} Mask {100}
(100) (100) 54.7º 54.7º (111) (111) (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)
(100)
{110}
Mask {110} {111} Etching on (110) Si
<100> {100} <110>
<100> <111>
(Undercut!)
Applied Physics 298r 9 E. Chen (4-12-2004) GaAs Wet Etch – (100) Wafer
(100)
{111}Ga <100> <100> As As {110} 111} 111} { { {111}Ga {100}
(100) Zincblende (1 00) Structure Etching Rate: <100> R({111}As) Mas
<100> > R({100}) k Mask > R({111}Ga)
Applied Physics 298r 10 E. Chen (4-12-2004) Typical Wet Etchants
Material Gas Etching Rate Mask Selectivity
1) KOH ~ 6 – 600 nm/min Si (a-Si) 2) HNO3 + H2O + (anisotropic) Resist > 50:1 HF ~ 100 nm/min 1) HF ~ 10 – 1000 SiO2 Resist > 50:1 2) BHF nm/min 1) HF ~ 100 nm/min Resist SI3N4 2) BHF ~ 100 nm/min > 50:1 SiO2 3) H3PO4 ~ 10 nm/min
1) H2SO4 + H2O2 GaAs +H2O ~ 10 um/min Resist > 50:1
2) Br + CH3OH 1) HCl + HNO ~ 40 nm/min Au 3 Resist > 50:1 2) KI + I2 +H2O ~ 1 um/min 1) HCl + H2O Al ~ 500 nm/min Resist > 50:1 2) NaOH
Applied Physics 298r 11 E. Chen (4-12-2004) Dry Etching + Problems with wet etching: Isotropic ¨ unable to achieve pattern size smaller than film thickness
Main Purpose of Developing Dry Etching is to
Achieve Anisotropic Etching R
Type of Dry Etching Technology Physical Sputtering - Physical bombardment • Ion Mill • Plasma sputtering + R Plasma Etching - Plasma-assisted chemical reaction Reactive Ion Etching (RIE) - Chemical reaction + ion bombardment
Applied Physics 298r 12 E. Chen (4-12-2004) Dry Etching Comparison
Chamber Beam Anisotropy Selectivity Pressure Energy Dry Etching
Plasma Etching High Low Low Very > 100 • Plasma assisted Good Mtorr chemical reaction
Low Reactive Ion Etching 10 ~ 100 Medium • Physical bombardment Medium Good mtorr + chemical reaction
Physical Sputtering Very Low (Ion Mill) < 10 High High Poor mtorr • physical bombardment
Applied Physics 298r 13 E. Chen (4-12-2004) Reactive Ion Etching (RIE)
• Etching gas is introduced into the chamber continuously • Plasma is created by RF power Gases • Reactive species (radicals and ions) are generated in the plasma ¬ radicals: chemical reaction ¬ ions: bombardment Shaw Heads • Reactive species diffused onto the sample e- e- Ar surface t Ar+ • The species are absorbed by the surface • Chemical reaction occurs, forming volatile byproduct Substrate • Byproduct is desorbed from the surface • Byproduct is exhausted from the chamber
~ Gas Selection: RF 1) React with the material to be etched 2) Result in volatile byproduct with low vapor pressure
Applied Physics 298r 14 E. Chen (4-12-2004) Typical RIE Gases
Etching Rate Material Gas Mask Selectivity (A/min) 1) CF4 Resist ~ 20:1 Si (a-Si) 2) SF6 ~ 500 Metal (Cr, Ni, Al) ~ 40:1 3) BCl2 + Cl2 1) CHF3 + O2 Resist ~ 10:1 SiO ~ 200 2 2) CF4 + H2 Metal (Cr, Ni, Al) ~ 30:1 1) CF4 + O2 (H2) Resist ~ 10:1 SI N ~ 100 3 4 2) CHF3 Metal (Cr, Ni, Al) ~ 20:1
1) Cl2 SI N ~ 10:1 GaAs ~ 200 3 4 2) Cl2 + BCl3 Metal (Cr, Ni) ~ 20:1
SI N InP 1) CH4/H2 ~ 200 3 4 ~ 40:1 Metal (Cr, Ni, Al) 1) Cl2 Resist Al ~ 300 ~ 10:1 2) BCl3 + Cl2 SI3N4 SI N Resist / Polymer 1) O2 ~ 500 3 4 ~ 50:1 Metal (Cr, Ni)
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)