Lecture 7 and 8: Etching 1 and 2 ME 645: MEMS: Design Fabrication Design, Fabrication and Characterization P.S. Gandhi Mechanical Engineering IIT Bombay PRASANNA S GANDHI gandhi@me.iitb.ac.in Recap: Last Class � E-beam lithography � Direct laser writer � X-ray lithography � Ion beam lithography � Photoresist fundamentals � Soft lithography PRASANNA S GANDHI gandhi@me.iitb.ac.in 1
Today’s Class � Clean room fundamentals � Si wafer preparation � Chemical etching process � Anisotropic Etching � Silicon micromachining � Silicon micromachining � Surface micromachining � Bulk micromachining � How to produce devices PRASANNA S GANDHI gandhi@me.iitb.ac.in Clean Room Fundamentals � Need Need � Class of a clean environment � Class X clean room � not more than X particles (of size 0.5 μ m or larger) per cubic foot of air; ex class 10 100 1000 cubic foot of air; ex class 10, 100, 1000 � How this cleanliness is produced and maintained?? PRASANNA S GANDHI gandhi@me.iitb.ac.in 2
Clean Room Fundamentals � Air conditioning plant Air conditioning plant � Air handling unit: Closed loop for air through clean room � HEPA (High Efficiency Particle Arresting) filters filters � Air recirculation through these filters multiple times per hour PRASANNA S GANDHI gandhi@me.iitb.ac.in HEPA filter � Air conditioning plant Ai conditioning plant � Air handling unit: Closed loop for air through clean room � HEPA (High Efficiency Particle Arresting) filters filters � Air recirculation through these filters Laminar flow benches multiple times per hour Clean Room: Suman Mashruwala PRASANNA S GANDHI gandhi@me.iitb.ac.in Advanced Microengineering Lab 3
Si-wafer preparation � Czochralski process Pull Seed Pull crystal Silicon Ingot Silicon Molten Chunks Silicon � Cutting � Chemical Mechanical Polishing PRASANNA S GANDHI gandhi@me.iitb.ac.in � Cleaning (100) n-type Primary (100) p-type Primary flat flat { 110} direction Secondary flat Secondary flat Secondary flat (111) n-type Primary Primary (111) p-type flat flat Secondary flat (45 o ) Primary and Secondary wafer flats are used to identify orientation and type. PRASANNA S GANDHI gandhi@me.iitb.ac.in 4
Chemical Etching � Isotropic etching Isotropic etching Without agitation (5) – Etchant: HNA mixture. – HNA can dissolve 550 μ m thick silicon wafer in about 20 min. With With agitation (20) it ti (20) – HNA mixture removes silicon equally in all directions. � SiO2 etch: 10-30nm/min PRASANNA S GANDHI gandhi@me.iitb.ac.in Chemical Etching � Isotropic etching Isot opic etching Without agitation (5) � Undercut � Etch bias � Materials & etchants Materials & etchants Refer to http://www.bsac.EECS.Berkeley.EDU/db for all details of etch rates and other data PRASANNA S GANDHI gandhi@me.iitb.ac.in 5
Chemical Etching Choice of etchant: Choice of etchant: � Etch rate � Topology of the surface to be etched � Etch selectivity of mask material and other materials other materials � Toxicity � Ease of handling PRASANNA S GANDHI gandhi@me.iitb.ac.in Etch Stop Mechanisms � Time etch stop: self explainatory Time etch stop self e plainato � Dopant B+ (heavy dope) as etch stop � Thin films � Electrochemical etch stop � Anisotropic Etching planes PRASANNA S GANDHI gandhi@me.iitb.ac.in 6
Etch Stop Mechanisms � Dopant B+ (heavy � Dopant B+ (heavy Dopant dose dope) as etch stop � Disadv.: creating electronic piezoresistive sensors not easily possible because of possible because of high dose Etching stops at point when it encounters high dopant dose region PRASANNA S GANDHI gandhi@me.iitb.ac.in Etch Stop Mechanisms � Thin film as etch stop � Thin film as etch stop � Excellent control over Thin film deposition thickness of say diaphragm or so. � Disadvantage: the material may not be material may not be very appropriate for the application!!! Etching stops at point when it encounters thin film having relatively PRASANNA S GANDHI gandhi@me.iitb.ac.in low etch rate 7
Etch Stop Mechanisms Electrochemical etch stop Electrochemical etch stop Diffused n-type region Diffused n type region � Principle: When si is biased + with sufficient anodic potential relative to etchant V it tends to oxidize - Electrolyte (electrochemical passivation) � Create a reverse biased pn p junction to implement this principle to achieve etch stop As soon as the n-type region is hit electrochemical reaction starts the electrochemical passivation. PRASANNA S GANDHI gandhi@me.iitb.ac.in < 100> < 001> Chemical Etching < 111> < 010> � Anisotropic bulk etching � Anisotropic bulk etching < 100> surface wafer – Etchant: KOH, EDP (ethylen diamine pyrocatechol), TMAH (Tetra methyl ammonium hydroxide) – < 111> direction has lower < 110> surface wafer 110 f f etching rates than < 100> � Can produce grooves, slanted/vertical walls PRASANNA S GANDHI gandhi@me.iitb.ac.in 8
< 100> < 001> Chemical Etching < 111> < 010> � Silicon crystal geometry � Silicon crystal geometry � Examples of use of the crystal geometry in etching � Fundes regarding etch shapes under different conditions* PRASANNA S GANDHI gandhi@me.iitb.ac.in < 100> Anisotropic < 001> < 111> Etching < 010> KOH, EDP and TMAH KOH EDP and TMAH � EDP etches oxide 100 times slower than KOH, � KOH, TMAH dangerous to eye � KOH less dangerous than EDP & TMAH � Etch curves (etch rate vs concentration) 5hrs to etch 300 μ m thick wafer etch 300 μ m thick wafer � H2 bubbles during KOH etching of Si � EDP ages quickly in contact with oxygen producing red brown color, vapor is harmful � HF dip is necessary for EDP: native oxide problem PRASANNA S GANDHI gandhi@me.iitb.ac.in 9
A [110] [110] 54.7 o Cross Section A-A’ Top View A’ A square < 110> -oriented mask feature results in a pyramidal pit. PRASANNA S GANDHI gandhi@me.iitb.ac.in [110] A Masking Layer [100] [ 100] Silicon A’ Cross Section A-A’ A A Convex corners are rapidly undercut A’ [100] A 54.7 o Cross Section A-A’ A’ Convex corners where { 111} planes meet are not stable. They are PRASANNA S GANDHI gandhi@me.iitb.ac.in rapidly undercut. This permits creation of suspended structures. 10
Boundary of A rectangular pit [ 110] [100] Masking Layer Undercut regions Cross Section A-A’ A A’ Any mask-layer feature, if etched long enough, will result in a rectangular V-groove pit beneath a rectangular that is tangent to the mask features, with edges oriented along < 110> directions. PRASANNA S GANDHI gandhi@me.iitb.ac.in Boundary of rectangular pit [ 110] 5 o misalignment The effect of misalignment is to enlarge the etched region. This figure shows the effect of a 5 o misalignment for a rectangular feature. PRASANNA S GANDHI gandhi@me.iitb.ac.in 11
Re-entrant resist profile Wafer with photoresist Directional evaporation (e-beam) Strip resist and lift off metal PRASANNA S GANDHI gandhi@me.iitb.ac.in Illustrating the lift-off method for patterning evaporated metals Wafer with double-layer undercut masking Directional evaporation evaporation gradually closes opening At closure, a sharp tip is formed After lift-off PRASANNA S GANDHI gandhi@me.iitb.ac.in The use of a modified lift-off process to create sharp tips. 12
Conclusions � Chemical etching � Isotropic � Anisotropic � Bulk and Surface micromachining � Etch stop mechanisms � Liftoff process PRASANNA S GANDHI gandhi@me.iitb.ac.in Thank You PRASANNA S GANDHI gandhi@me.iitb.ac.in 13
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