520/530/580.495 Microfabrication Laboratory and 520.
773 Advanced Topics in Fabrication and Microengineering
Wet Etching
520/530/580.495 Fall 2004 A.G. Andreou and J. Wang
Reading for this lecture: (1) May, Chapter 5.1 (2) Madou p163-p177 (website) (3) Williams paper (website)
HW #5: Due Oct. 14
520/530/580.495 Fall 2004 A.G. Andreou and J. Wang
Etch Parameters
Etch Rate:
- rate of material removal (m/min) - function of concentration, agitation, temperature, density and porosity of the thin film or substrate,
Etch Selectivity:
- relative (ratio) of the etch rate of the thin film to the mask, substrate, or another film
Etch Geometry:
- sidewall slope (degree of anisotropy)
520/530/580.495 Fall 2004 A.G. Andreou and J. Wang
Types of Etching Processes
Anisotropic
Isotropic
Anisotropic:
- best for making small gaps and vertical sidewalls - typically more costly
Isotropic :
-quick, easy, cheap -best to use with large geometries, when sidewall slope does not matter, undercut/release -rounding of sharp anisotropic corners to avoid stress concentration
520/530/580.495 Fall 2004 A.G. Andreou and J. Wang
Wet Etching
Mixtures of acids, bases, and water
- HF, H3PO4, H2SO4, KOH, H2O2, HCl, .. Can be used to etch many materials - Si, SiO2, Si3N4, PR, Al, Au, Cu,
Etch Rate:
- wide range
Etch Selectivity
- typically quite high - sensitive to contamination
Etch Geometry: - typically isotropic, some special cases are anistrpoic
520/530/580.495 Fall 2004 A.G. Andreou and J. Wang
Hydrofluoric Acid (for SiO2)
Selective (room temperature) - etches SiO2 and not Si - will also attack Al, Si3N4,.. Rate depends strongly on concentration - maximum: 49% HF (concentrated) ~ >2 m/min - controlled: 5 to 50:1 (timed) ~ <0.1 m/min Dangerous ! - not a strong acid - deceptive (looks just like water) - penetrate skin (adsorption) and attacks slowly - will target bones Etch Geometry - completely isotropic (used to undercut/release)
Reactions: SiO2 + 6HF H2SiF6(aq) + 2H2O
520/530/580.495 Fall 2004 A.G. Andreou and J. Wang
Buffered HF (for SiO2)
Buffered HF (BHF), also called Buffered oxide etch (BOE) addition of NH4F to HF solution - control the pH value - replenish the depletion of the fluoride ions to maintain stable etching performance
SiO2 + 4HF + 2NH4F (NH4)2SiF6 + 2H2O SiO2 + 3HF2- + H+ SiF62+ + 2H2O
520/530/580.495 Fall 2004 A.G. Andreou and J. Wang
Phosphoric Acid (for SixNy)
Selectively (high-temperature)
- etches SixNy and not Si or SiO2 - etches Al and other metals much faster
Rate:
- Slow ! R ~ >0.0050 m/min for H3PO4 at 160 0C
Tough Masking Materials Needed
- PR will not survive - Oxide is typically used
Etch Geometry
- completely isotropic
520/530/580.495 Fall 2004 A.G. Andreou and J. Wang
HNA (for Silicon)
-mixture of nitric (HNO3), hydrofluoric (HF) and acetic (CH3COOH) acids -HNO3 oxides Si, HF removes SiO2, repeat
Si + 4HNO3 SiO2 + 2H2O + 4NO2 SIO2 + 6HF H2SiF6 + 2H2O
-high HNO3:HF ratio (etch limited by oxide removal) -low HNO3:HF ratio (etch limited by oxide formation) -dilute with water or acetic acid (CH3COOH) Iso-etch Curve (From Robbins. et al) -acetic acid is preferred because it prevents HNO3 dissociation
520/530/580.495 Fall 2004 A.G. Andreou and J. Wang
Orientation-Dependent Etching
KOH - Si +OH- + 2H2O SiO2(OH)22- + 2H2(g) - Etch rate : {110} > {100} >> {111} - Used at the elevated temperature (~80 0C) - Resist will not survive, oxide is attacked slowly - Nitride is not attacked (best masking material)
Neuron well
520/530/580.495 Fall 2004 A.G. Andreou and J. Wang
Optical DNA Sensor
Miller Indices
Miller Indices of a plane is determined by - Take the intercepts of the plane along the crystallographic axes
eg. 2, 1, 3
The reciprocal of the three integers are taken and are multiplied by the smallest common denominator
eg. 1/2, 1/1, 1/3 (multiple by 6) (3, 6, 2)
If a plane is parallel to an axis, its intercept is at infinity and its Miller index is zero Important plans in Si lattice
{1,0,0} : (1,0,0), (01,0), (0, 0,1), (1,0,0), (0,1, 0),.. {1,1,0} {1,1,1}
520/530/580.495 Fall 2004 A.G. Andreou and J. Wang
Etch rate of Si in KOH Depends on Crystallographic Plane
520/530/580.495 Fall 2004 A.G. Andreou and J. Wang
Etch rate of Si in KOH Depends on Temperature
520/530/580.495 Fall 2004 A.G. Andreou and J. Wang
Etch Rate of Oxide in KOH
520/530/580.495 Fall 2004 A.G. Andreou and J. Wang
Anisotropic Etching : Si in KOH
{100}
cross-section
SiO2 on top of Si wafer {110}
Wb = W0 2 l cot 54.7 0
Wb = W0 2 l
520/530/580.495 Fall 2004 A.G. Andreou and J. Wang
Bulk Micromachining
520/530/580.495 Fall 2004 A.G. Andreou and J. Wang
Bulk Micromachining
520/530/580.495 Fall 2004 A.G. Andreou and J. Wang
