Lecture 3 Fundamentals of Multiscale Fabrication Multiscale fabrication II: Silicon bulk micromachining Paper reading: “Bulk Micromachining of Silicon”, Proceedings of The IEEE, GREGORY T. A. KOVACS, NADIM I. MALUF, AND KURT E. PETERSEN, Vol 86, No. 8, pp 1536-1551, 1998. Kahp-Yang Suh Associate Professor SNU MAE sky4u@snu.ac.kr

Further readings (ISI web search) Title: Fabrication techniques of convex corners in a (100)-silicon wafer using bulk micromachining: a review Author(s): Pal P, Sato K, Chandra S Source: JOURNAL OF MICROMECHANICS AND MICROENGINEERING   Volume: 17   Issue: 10   Pages: R111-R133   Published: OCT 2007 Times Cited: 7 Title: Micromachining for optical and optoelectronic systems Author(s): Wu MC Source: PROCEEDINGS OF THE IEEE   Volume: 85   Issue: 11   Pages: 1833-1856   Published: NOV 1997 Times Cited: 181 Title: Silicon microstructuring technology Author(s): Lang W Source: MATERIALS SCIENCE & ENGINEERING R-REPORTS   Volume: 17   Issue: 1   Pages: 1-55   Published: SEP 1996 Times Cited: 94 Title: Development of surface micromachining techniques compatible with on-chip electronics Author(s): French PJ Source: JOURNAL OF MICROMECHANICS AND MICROENGINEERING   Volume: 6   Issue: 2   Pages: 197-211   Published: JUN 1996 Times Cited: 25

Micromachining

Si Micromachining Methods SFB: Silicon Fusion Bonding

Bulk Micromachining

Benefits of Si bulk micromachining

Etching Process Students need to notify the process and related terms. Shapes of cut originated from Si characteristics. http://www.saabmicrotech.se/node4084.asp

Bulk Micromachining The purpose of bulk micromachining Selectively remove significant amounts of silicon from a substrate Broadly applied in the fabrication of micromachined sensors, actuators, and structures Fabrication method: dry/wet etching Undercut structures that are required to physically move Form membranes on one side of a wafer Make a variety of trenches, holes, or other structures Bulk micromachining is used for mass removal of substrate. Usually wet etching is used but some are performed by gas.

Bulk Micromachining Isotropic etching performed by acid, and anisotropic etching is performed by alkali on the other hand. Etched sides are form a trapezoid or a rectangle. Boron layer stops etching. (used for etch stop layer) Figure 1. Bulk silicon micromachining: (a) isotropic etching; (b) anisotropic etching; (c) Anisotropic etching with buried etch-stop layer; (d) dielectric membrane released by back-side bulk etching; (e) dopant dependent wet etching; (f) anisotropic dry etching

Production-Line Automation Bulk Micromachining Terminology: Etch Rate – How fast material is removed Selectivity – Ratio of etch rates of materials exposed to etching Anisotropy – Degree of lateral etch to vertical etch Wet Etching – chemical bath Dry Etching – chemical / physical material removal using gas / vapor Dry Etching Wet Etching Production-Line Automation Good Poor Cost chemicals Low High Selectivity Can be very good Sub-micron features Applicable Not Applicable Etch Rate Slow (0.1um/min) Fast (1 um/min) (Madou)

Silicon Crystallography (1)

Silicon Crystallography (2) Refer to open courseware -> readings -> etc (see MEMS Open Courseware: Readings_Etc)

Isotropic Wet Etching Isotropic wet etching Etching with chemical reaction  etching in all directions a substrate Induces etch reaction with silicon through the opening region  agitation required: stirring, ultrasonic Considering opening area to size of reaction bubble Bubble disturbs exchange of etchant Slow down etch rate The most common isotropic wet silicon etchant: HNA HNA = HF + HNO3 + CH3COOH Reaction: 18HF + 4HNO3 + 3Si  2H2SiF6 + 4NO(g) + 8H2O

Anisotropic Wet Etching Anisotropic etchants etch much faster in on direction than in another  exposing the slowest etching crystal planes over time  (111) planes have the slowest etch rate Several solutions: Alkalic OH (KOH, NaOH) Tetramethylammonium hydroxide (THAH, (CH3)4NOH) Ethylenediamine pyrocatechol (EDP, NH2 (CH2)2 NH2+C6H4(OH)2) Etching at concave corners on (100), stop at (111) intersections. Convex corners are under cut

Example of Anisotropic Wet Etching

Anisotropic Etching Mechanism

Dry Etching Xenon Difluoride Etching (XeF2) Non-plasma, isotropic dry etch process Very high selectivity for Al, SiO2, Si3N4, PR Reaction: 2XeF2 + Si  2Xe + SiF4 Etch rate: 1~ 3 m/min Drawback: Rough surface reaction with water (HF) silicon fluoride polymer Use of BF3 ~ much smooth surface

Example of isotropic Dry Etching

Dry Etching Plasma/Reactive Ion Etching (RIE) Anisotropic dry etch process Process in which chemical etching is accompanied by ion bombardment Combination of physical and chemical etching

RIE principles

Mask materials for RIE Etch mask: PR (Photo Resist), Hard mask (SiO2, Al) Selectivity = etch rate of etching material / etch rate of mask Usually, standard PR (for CMOS) is not adequate for O2 plasma etch, for which hard mask is required Selectivity of silicon: AZ1512 Cl based etch (physical etch): <2 F based etch (chemical etch): <10 (if O2 gas is inserted into the chamber, the selectivity would be smaller than 10)

Reaction in RIE process (1) Physical reaction is superior for Cl-based process. It has high selectivity!!! Chemical reaction is superior for F-based process. It has low selectivity!!!

Reaction in RIE process (2)

Example using BCl3

Reaction in RIE process (3)

Example using SF6

Deep reactive ion etching (DRIE)

Wet etching vs. Dry etching In wet etchants, the etch reactants come from a liquid source In dry etchants, the etch reactants come from a gas or vapor phase source and are typically ionized Atom or ions from the gas are the reactive species that etch the exposed film Selectivity: in general, dry etching has less selectivity than wet etching Anisotropy: in general, dry etching has higher degree of anisotropy than wet etching Etch rate: in general, dry etching has lower etch rate than wet etching Etch control: dry etching is much easier to start and stop than wet etching Dry etching is easily automated. If selectivity is high, we could etch deep and long.

MEMS: Deposition + Lithography + Sacrificial Etching