ME 141B: The MEMS Class Introduction to MEMS and MEMS Design Sumita Pennathur UCSBOutline today • Introduction to thin films • CVD • Epitaxy • Electrodeposition 10/21/10 2/45Taxonomy of deposition techniques • Chemical  Thermal Oxidation  Chemical Vapor Deposition (CVD) • Low Pressure (LPCVD), Atomspheric pressure (APCVD), Plasma Enhanced (PECVD), Ultra High Vaccum CVD (UHCVD)  Epitaxy  Electrodeposition (Electroplating) • Physical  Physical Vapor Deposition (PVD) • Evaporation • Sputtering  Spin-casting 10/21/10 3/45CVD • CVD is a chemical process used to produce high-purity, high-performance solid materials • Typical CVD process  Wafer exposed to one or more volatile precursers  These react and/or decompose on surface  This produces desired deposit • Can deposit in various forms  Monocrystaline  Polycrystalline  Amorphous  Epitaxial • Materials include silicon, carbon fiber, carbon nanofibers, filaments, carbon nanotubes, SiO2, silicon-germanium, tungsten, silicon carbide, silicon nitride, etc… 10/21/10 4/45Chemical Vapor Deposition (CVD) • How CVD works:  Gaseous reactants, often at low pressure  Long mean free path; reactants reach substrate  Reactants react and deposit products on the substrate  Unlike Oxidation, does not consume substrate material • Energy sources facilitate CVD reactions:  High temperature, plasma, laser • Processing temperatures vary widely • Commonly deposited films: Oxide, silicon nitride, polysilicon • CVD results depend on pressures, gas flows, temperature  Film composition, uniformity, deposition rate, and electrical and mechanical characteristics can vary 10/21/10 5/45Types of CVD • Atmospheric pressure CVD (SPCVD) – CVD processes at atmospheric pressure • Low Pressure CVD (LPCVD)- CVD processes at subatmospheric pressures.  Reduced pressures tend to reduce unwanted gas-phase reaction and improve uniformity across the wafer  Most modern CVD processes are LPCVD or UHCVD • Ultrahigh vacuum CVD (UHCVD) – CVD process at a very low pressure, ~10-8 torr • Plasma-enhanced CVD (PECVD) – CVD process that utilize a plasma to enhance chemical reaction rates of the precursors  Allows low temperatures • Other types, MPCVD, ALCVD, MOCVD 10/21/10 6/45Some reasons to use CVD • Oxide formation:  To get a thicker layer than thermal oxidation can provide  To create oxide on a wafer that can’t withstand high temperatures (for example because of metal features)  To create oxide on top of a material that is not silicon • For film formation in general  To tailor the film properties (like form stress) by adjusting pressures, flow rates, external energy supply, ratios of different precursor gases (to adjust proportions of different materials in the final product)  Conformailty : (more of less) even coating on all surfaces • Drawbacks:  Films deposited at low temperature are often lower quality than high temp versions, and have less predictable properties  Flammable, toxic or corrosive source gases 10/21/10 7/45Thick Film Formation • CVD is a common MEMS tool for creating thick films on the wafer surface  In practice, film stress limits thickness (film delamination or cracking, or curvature of underlying structures)  Can deposit thick oxides; nitrides are still typically submicron  Must anneal deposited oxides for some applications – lose low stress property on anneal 10/21/10 8/45Commonly Deposited Substances • Polysilicon  Deposited from silane (SH4) (SiH4 Si + 2H2)  Usually preformed in LPCVD systems  Growth rate 10-20 nm per minute • Silicon dioxide  Source gases include silane and oxygen, dichlorosilane, nitrous oxide, or TEOS (tetraethlyorthosilicate)  Choice of source depends on thermal stability of substrates • ie. aluminum is sensitive to high temperature  TEOS is the best, but needs 650-700C, silane is lower quality. Thermal oxidation is best  Ozone may deposit TEOS at lower temperatures – being explored 10/21/10 9/45Commonly Deposited Substrates • Silicon Nitride  LPCVD generally used here • Metals  Molybdenum, tatalum, titatnium, nickel and tungsten  Deposited by LPCVD 10/21/10 10/45CVD enables conformal coating 10/21/10 11/45LPCVD Polysilicon • Amorphous at lower deposition temperatures and high deposition rates  Typical temperature: ~590 C • Polycrystalline at higher deposition temperatures and lower deposition rates  Typical temperature: ~625 C • Grain size and structure depend on detailed deposition conditions  E.g. thicker films  larger grains • Structure, electrical properties, and mechanical properties also vary with post-deposition thermal processing  Grain growth  Dpoant activation or diffusion 10/21/10 12/45CVD Machine 10/21/10 13/45Epitaxy • CVD deposition process in which atoms move to lattice sites, continuing the substrate’s crystal structure • Deposits monocrystalline film on a monocyrstalline substrate  Deposited film = epitaxial film or epitaxial layer • Epi = “above” , “taxis” = “in ordered manner” • May be grown from gaseous or liquid precursoers.  Since it acts on a seed crystal, film takes on lattice structure and orientation identical to those on substrate  Different from CVD or thermal oxide!! 10/21/10 14/45Types of Epitaxy • Homoepitaxy: only one material, i.e. Si on Si  A crystalline film is grown on the a substrate or film of the same material  Used for growing a more purified film than the substrate  Also known as “epi” • Heteroepitaxy: different materials, i.e. AlGaAs, on GaAs  Cystalline film grows on a substrate or film of another material  Often applied growing films of materials of which single crystals cannot be obtained • Heterotoepitaxy – similar to Heteroepitaxy but the thin film growth is not limited to two dimensional growth 10/21/10 15/45Epitaxy applications • Used in nanotechnology and semiconductor fabrication • Only affordable method of high crystalline quality growth for silicon-germanium, gallium nitride,