VIEWS: 7 PAGES: 32 POSTED ON: 3/27/2011
ME 463 Micro – nanotechnology Lecture 3 Etching IOANA VOICULESCU Etching Subtractive Process - removing materials Wet etching - liquid etchants: Acids Hydroxides Dry etching - etching gases / plasma: Physical etching (impact of atoms / ions) Reactive chemicals / ions Enhanced by RF energy (instead of temperature) Bulk Micromachining Types of bulk etching Anisotropic and Isotropic etching Bulk Micromachining examples Isotropic Vs. Anisotropic Etching Isotropic etching – Same etch rate in all directions – Lateral etch rate is about the same as vertical etch rate – Etch rate does not depend upon the orientation of the mask edge Anisotropic etching Etch rate depends upon orientation to crystalline planes – Lateral etch rate can be much larger or smaller than vertical etch rate, depending upon orientation of mask edge to crystalline axes – Orientation of mask edge and the details of the mask pattern determine the final etched shape • Can be very useful for making complex shapes • Can be very surprising if not carefully thought out • Only certain “standard” shapes are routinely used Isotropic Vs. Anisotropic Etching Isotropic – same etch rate in all directions Etch depth / surface uniformity problems Diffusion dependent Etch silicon wafer and deposited films Anisotropic – crystal plane dependent Etch ratios more than 100:1 silicon wafers only <111> Crystal planes: 54.7º <100> Si Wet Vs. Dry Etching Dry etch Reactive ion etching (RIE) High – cost Wet etch Very “directional” Chemical reaction “Unlimited” 2D geometries Low – cost 0.1 – 10 microns/min Hard to control direction Limited 2D geometries 1 – 10s of microns/min Wet etching set-up Experimental set-up for anisotropic wet etching. Anisotropic Etching Typically, etch rates are: (110) > (100) > (111). The (111) family of crystallographic planes are normally the “stop” planes for anisotropic etching. There are 8 (111) planes along the ± x ± y ± z unit vectors. Intersections of these planes with planar bottoms produce the standard anisotropic etching structures for (100) Si wafers: V-grooves pyramidal pits pyramidal cavities Anisotropic Etching <100> <111> 54.7 Silicon Substrate Anisotropic Etch Examples Typical etch pit on (100) wafer Etching Topologies Anisotropic wet etching: (100) Isotropic wet etching: Agitation Anisotropic wet etching: (110) Isotropic wet etching: No Agitation Concave & Convex Corners Anisotropic wet etching Convex corners are undercut Concave corners stop at (111) intersections. Anisotropic Etchants Alkali hydroxides (KOH, NaOH, etc.): very smooth walls, can use isopropyl alcohol to increase selectivity (111) vs. (100), attacks aluminium, oxide etches somewhat, nitride good mask. Ethylene Diamine Pyrochatechol (EDP): similar to KOH but much more toxic, does not attack metals (even Al in some cases) nor oxide. Tetramethyl Ammonium Hydroxide (TMAH): similar to EDP but safer, in some cases will not attack Al, can be masked with oxide. Under cutting Isotropic Etching of Silicon HNA=Hydrofluoric acid (HF)+ Nitric acid (HNO3)+ Acetic acid (CH3COOH) or water (used for isotropic etching) HNO3 oxidizes Si to SiO2 HF converts SiO2 to soluble H2SiF6 Acetic acid prevents dissolution of HNO3 Etch masks SiO2 etched at 30-80 nm/min Not etching Au or Si3N4 Etch Stop Factors that determines etch rate: -temperature, -concentration of etchant -stirring, -light, -wafer irregularity, -surface preparation. Dopant Etch Stop Layer Controlling the absolute depth of an etch is often difficult, particularly if the etch is going most of the way through a wafer. Etch stop layers can be used to drastically slow the etch rate, providing a stopping point of high absolute accuracy. Boron doping is most commonly used for silicon etching. Etch Stop layer Boron etch stop (p++) used for wet -etching Dry Etching Overview There are 3 types of etching: Physical etching Sputter etch Chemical etching Plasma etch Combination of physical and chemical etching Reactive ion etching (RIE) Examples of applications Integration of CMOS and MEMS Deep reactive ion etching (DRIE) Bosch Process - etches deep tranches Plasma Basics What is plasma? Plasma is an ionized gas, in which a certain proportion of electrons are free rather than being bound to an atom or molecule. The ability of the positive and negative charges to move somewhat independently makes the plasma electrically conductive so that it responds strongly to electromagnetic fields. Most of the gas is made up of neutral particles but plasma is quasi-neutral Number of negatively charged particles = number of positively charged Physical Etching (Sputter) The etching is produced by ions accelerated under the effect of a magnetic field towards substrate wafer In the sputtering process the substrate is bombarded by ions Substrate atoms are dislodged during collisions Ion energy > Bonding energy Highly anisotropic Low selectivity Etches all surfaces equally Back-scatter can form particulates, that will forms impurities on the MEMS structures Chemical Etching Plasma creates chemically reactive species Diffuse to wafer surface Adsorb onto surface Chemically react with the wafer surface Reaction byproducts desorb and diffuse away Isotropic High selectivity Note: Gaseous byproducts can be harmful Chemical Vs. Physical Etching Wide arrival angle Narrow, vertical arrival High selectivity Low selectivity Low sticking coefficient Anisotropic etch profile Isotropic etch profile Reactive Ion Etching (RIE) RIE= Chemical etching combined with physical ion bombardment Reactive ions damage wafer surface that is transformed in chemical reaction site Increases etch rate Anisotropic High selectivity Etch parameters (gas pressure, concentration, RF power, ...) control etch rate, anisotropy, selectivity RIE Advantages & Issues Advantages Control of selectivity/anisotropy, etc. by adjusting etch process parameters Minimizes shortcomings of pure sputter and chemical dry etch Etchant chemicals and byproducts are collected in confined systems Issues High ion energies can cause device damage Chemicals/byproducts environmentally harmful Dry Etching of Silicon Dry etching Plasma phase Vapor phase Parameters Gas and species generated ~ ions, radical, photons RF frequency 13.56 MHz RF power 10’s – 1000’s W Pressure m Torr - >100 Torr Wet Vs. Dry Etch Profiles in Si Vapor phase etching Vapor phase etching is another dry etching method, which can be done with simpler equipment than what RIE requires. In this process the wafer to be etched is placed inside a chamber, in which one or more gases are introduced. The material to be etched is dissolved at the surface in a chemical reaction with the gas molecules. XeF2 High pressure, chemical only No plasma (just pump) 10 µm/min No damage Isotropic Very selective (Si over Al, photoresist, oxide and nitride) CMOS compatible Bosch Process Steps Si Etch Step SF6 / Ar Anisotropic Polymer Deposition CHF3 / Ar Thin on horizontal Thick on vertical Bosch Process Steps Etch again Polymer protects sidewalls Trench deepens Continue alternate etch and deposition to desired trench depth Bosch Process Summary Alternate deposition/ etch processes HDP systems required Very high aspect ratio Up to 30:1 claimed Highly anisotropic Scalloped sidewalls Etch rate fast ~30x RIE etch rate
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