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Dry Etching Etching Issues Etching Issues - Anisotropy • Isotropic etchants etch at the same rate in Dry Etching every direction mask An-isotropic Dr. Bruce K. Gale Fundamentals of Micromachining BIOEN 6421 EL EN 5221 and 6221 Isotropic ME EN 5960 and 6960 Etching Issues - Selectivity Dry Etching Overview • Selectivity is the ratio of the etch rate of the • What is dry etching? target material being etched to the etch rate – Material removal reactions occur in the gas phase. of other materials • Types of dry etching • Chemical etches are generally more – Non-plasma based dry etching selective than plasma etches – Plasma based dry etching • Selectivity to masking material and to etch- • Why dry etching? stop is important Mask • Development of dry etching target • Plasma parameters/influences Etch stop Dry Etching Advantages Dry Etching • Eliminates handling of dangerous acids and solvents • Disadvantages: • Uses small amounts of chemicals – Some gases are quite toxic and corrosive • Isotropic or anisotropic etch profiles – Re-deposition of non-volatile compounds • Directional etching without using the crystal orientation of Si • Faithfully transfer lithographically defined photoresist patterns – Need for specialized (expensive) equipment into underlying layers •Types: • High resolution and cleanliness – Non-plasma based = uses spontaneous reaction • Less undercutting of appropriate reactive gas mixture • No unintentional prolongation of etching – Plasma based = uses radio frequency (RF) • Better process control power to drive chemical reaction • Ease of automation (e.g., cassette loading) Non-plasma Based Dry Etching Xenon Difluoride (XeF2) Etching • Isotropic etching of Si • Isotropic etching of Si • Typically fluorine-containing gases • High selectivity for Al, SiO2, Si3N4, PR, (fluorides or interhalogens) that readily etch PSG Si •2XeF2 +Si 2Xe + SiF4 µ • High selectivity to masking layers • Typical etch rates of 1 to 3 m/min • No need for plasma processing equipment • Heat is generated during exothermic reaction • Highly controllable via temperature and •XeF2 reacts with water (or vapor) to form partial pressure of reactants HF Interhalogen (BrF3 & ClF3) Etching Plasma Based Dry Etching • Nearly isotropic profile • RF power is used to drive chemical reactions • Gases react with Si to form SiF4 • Surface roughness: ~40 to 150 nm • Plasma takes place of elevated temperatures or very reactive chemicals •Masks: SiO2, Si3N4, PR, Al, Cu, Au, and Ni •Types: – Physical etching – Chemical etching – Reactive ion etching (RIE) – Deep reactive ion etching (DRIE) Plasma Plasma Formation •Plasma= partially ionized gas consisting of equal numbers of “+” (ions) and “-“ (electrons) charges and • Chamber is evacuated a different number of neutral (un-ionized) molecules • Chamber is filled with gas(es) • An ion-electron pair is continuously created by • RF energy is applied to a pair of electrodes ionization and destroyed by recombination • Applied energy accelerates electrons • Typical kinetic energy (KE) of an electron in plasma is increasing kinetic energy 2-8 eV •KE = ½ mV2 = 3/ kT • Electrons collide with neutral gas 2 2 eV electron has molecules, forming ions and more electrons – m = particle mass T ≈ 15,000 K – V = particle mean velocity • Steady state is reached (plasma); ionization V ≈ 6 x 107 cm/s – k = Boltzmann constant = 1,342,16176 mph = recombination – T = temperature (K) Plasma Formation Plasma Parameters • Plasma discharge is characterized by central glow • Temperature •Power or bulk region and dark or sheath regions near – Etching rate – Ion density electrodes – Spontaneous chemical – Ion kinetic energy • Bulk region = semi-neutral (nearly equal number reaction of electrons and ions) – Etching directivity • Other variables • Sheath regions = nearly all of the potential drop; –Gas flow rate accelerates “+” ions from bulk region which • Pressure – Reactor materials bombard the substrate – Reactor cleanliness – Ion density • Maintained at 1 Pa (75 mtorr) to 750 Pa (56 torr) – Loading with gas density of 27 x 1014 to 2 x 1017 – Ion directivity (microloading) molecules/cm3 – Mask materials Physical Etching (Sputter Etching) Two Basic Plasma Systems • Based on physical bombardment with ions or atoms • Plasma is used to energize a chemically inert projectile so that it moves at high velocity when it strikes the substrate • Momentum is transferred during the collision • Substrate atoms are dislodged if projectile energy exceeds bonding energy • Very similar to ion implantation, but low-energy ions are used to avoid implantation damage • Highly anisotropic • Etch rates for most materials are comparable (ie, no masking) • Argon is the most commonly used ion source • May result in redeposition Plasma Etchers Chemical (Plasma) Etching: • Plasma is used to produce chemically reactive species (atoms, radicals, and ions) from inert molecular gas • Six major steps: – Generation of reactive species (eg, free radicals) – Diffusion to surface – Adsorption on surface – Chemical reaction – Desorption of by-products – Diffusion into bulk gas • Production of gaseous by-products is extremely important Plasma Etching Steps Plasma Etching Systems • Plasma Etching (PE) • Barrel, barrel with downstream and symmetrical parallel plate system • Pure chemical etching • Isotropic etching Reactive Ion Etching (RIE) RIE System • RIE = process in which chemical etching is • Reactive Ion Etching (RIE) accompanied by ionic bombardment (ie ion- • Asymmetrical parallel plate system assisted etching) • Bombardment opens areas for reactions • Plasma, sheath and • Ionic bombardment: boundary layer • Combination of physical – No undercutting since side-walls are not and chemical etching exposed • Anisotropic etching – Greatly increased etch rate – Structural degradation – Lower selectivity Disadvantages of RIE Deep Reactive Ion Etching (DRIE) • Conflict between etching rate and anisotropic • Uses electron cyclotron resonance (ECR) profile source to supplement RIE system → – Etching rate (+) Reactive species • Microwave power at 245 GHz is coupled → → concentration (+) Gas pressure (+) into ECR Collision (+) → Anisotropic (-) • Magnetic field is used to enhance transfer of • Conflict between damage of high etching microwave energy to resonating electrons rate and anisotropic profile • DRIE uses lower energy ions less → → – KE (+) Etching rate (+) damage (+) damage and higher selectivity • Plasma maintained at 0.5 to 3 mtorr ECR Systems ICP System (DRIE) • Inductively Coupled • Electron Cyclotron Resonance Plasma (ICP) (ECR) • Simple system • Higher plasma density at lower • Almost same process pressure result as that from the • Control the density of the ECR system reactive ions and their kinetic • Two RF power energy separately generators to control • Downstream of plasma further ion energy and ion limits the exposure to reduce density separately damage Deep Reactive Ion Etch Deep Reactive Ion Etching BOSCH Patent STS, Alcatel, Trion, Oxford Instruments … • high density ICP plasma Unconstrained geometry • high aspect ratio Si structures Uses high density plasma to alternatively 90° side walls • cost: $500K etch silicon and deposit a etch-resistant ☺ High aspect ratio 1:30 • vendors: STS, Alcatel, PlasmaTherm polymer on side walls Easily masked (PR, SiO2) Source: LucasNova Process recipe depends on Polymer geometry Polymer deposition Silicon etch using SF chemistry Source: STS Source: AMMI 6 Source: STS Scalloping and Footing Issues of DRIE Top wafer surface cathode Top wafer surface Scalloped sidewall Top wafer surface anode Tip precursors microgrid <100 nm silicon nanowire • Increased capacitance for over >10 micron gap DRIE Structures actuation and sensing • Low-stress structures Milanovic et al, IEEE TED, Jan. 2001. – single-crystal Si only structural material 1 • Highly stiff in vertical µm Footing at the bottom of direction device layer – isolation of motion to wafer Thermal Actuator plane – flat, robust structures • Organic FilmsEtch Chemistries – Oxygen plasma is required Comb-drive Actuator – By-products: CO, CO –Masks:Si, SiO – Addition of fluorine containing gases significantly increases2 , Al, etch or 2Ti ,rate H but decreases 2DoF Electrostatic actuator 2 O selectivity (due to HF formation) • Oxide and NitrideEtch Films Chemistries – Fluorine plasma is required (eg, CF –Mask: PR – Addition of O • Increases etch rate • Silicon• Adjusts PR : oxide2 and PR : nitride selectivity 4 ) – Fluorine plasma (CF – Chlorine plasma (Cl – Mixed (fluorine and chlorine) plasma (Cl 4 or SF 2 ) 6 ) 2 + SF 6 ).
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