Deep reactive ion etching Deep silicon etch Consumer Electronics • MEMS is a solution where there is a need for a device or sensor to be miniaturised • Trends towards increased integration and complexity Micro-Electro-Mechanical Systems Smart Devices Automotive BioMEMS Why DSE?  Complex structure pattern can be made by DSE  Higher etching rates, aspect ratio  Compatibility with photoresist masks  The ability to produce vertical sidewalls on silicon substrates of any crystal orientation g) /s) (g) 4(( olmalcK534+FSiF4+Si  • Silicon can be etched by any halogen atom (Cl, Br, F, I) • Only Fluorine reacts with silicon spontaneously • silicon etches isotropically (in all directions) • need passivation to achieve a vertical (anisotropic) profile • the reaction releases heat Silicon ICP Etch Using Fluorine Si(s) + 4F(g)  SiF4(g) + 435 Kcal/mol Isotropic Etch Anisotropic Etch 6m 100m deM x • Bosch DSE is typically used for features >1µm and depths >10µm • Cryogenic DSE is typically used for smooth sidewalls and/or nano-etching • Mixed processes are an option for shallow, low aspect features Microneedles (Bosch) Micro-mould (Cryo) Bosch Depth/m 0.1 1 10 100 1000 Feature Size / m 0.1 1 10 100 1000 (>10m/min) (2m/min) (<1m/min) Cryo Mixed 20m 20m Fluorine-based silicon etch choices Bosch DSE and isotropic etch conditions Alternating sequence of deposition and etch • Enabling: • High etch rate • High selectivity • Highly anisotropic (vertical) profile • 3 step Bosch cycle: • Deposition / Breakthrough / Etch • allows optimised breakthrough Isotropic etching Isotropic etching Resulting profile Deposition Removal of bottom passivation layer by ion bombardment Cryogenic DSE • Low temperature process ( -110°C) • Particularly suited to photonic, moulding and nano applications • Fluorine radicals etch the silicon • Sidewalls are continuously passivated (SiOxFy) • Ions keep the etch front passivation-free • Moderate etch rates (>2µm/min) • High selectivity (100:1 to PR) • Smooth sidewalls (<10nm) Page Cryo Etch Process – Profile Control • Main control parameters are: • Temperature • O2 flow • Bias -130°C, 24% O2 -130°C, 12% O2-100°C, 24% O2 Calculation of etching parameters Selectivity Aspect ratio (AR) Parameter Bosch Cryo Mixed Gas Rate (m/min) High (< 20) Moderate (< 4) Low (< 0.5) Selectivity to PR Very High (< 250) High (< 100) Low (< 10) Profile Veritical Veritical or Sloped Veritical or Sloped Aspect Ratio Very High (> 70) High (> 30) Low (< 10) Sidewalls Scallops (< 15nm) Smooth Smooth Cooling Chiller or Cryo Cryo only Chiller or Cryo Mask PR / SiO2 PR (thin) / SiO2 PR / SiO2 Min. feature /nm  300  10  30 DSE Process Comparison cmiaerCPC g I npimuP letns IaG gnpimaCl erafW li gnoocmueliH edoectrEl toraerneG asBi toraerneG PC i ort I PsysalnA • High density ICP plasma • Separate control of ion density and energy • Fast gas / pressure control • Low chamber volume with good throughput • Efficient cooling • He-assisted cooling • Cryo electrode DSE Equipment Requirements Analysis Port ICP Generator Bias Generator Electrode Helium cooling Wafer Clamping Gas Inlet Pumping ICP Ceramic DSI Process Chamber Schematic Helium Backside Cooling • Helium between wafer and chuck for good thermal coupling • Small leakage into process chamber (< 1 sccm) • Pressure < 50 mbar to avoid decrease of thermal conductivity due to collisions of He atoms Thermal conductivity of different gases in W/(m · K) Hydrogen 0.186 Helium 0.157 Argon 0.018 Water cooled chuck wafer Helium Helium O-ring Pressure Gauge Carrier CEITEC DRIE • PlasmaPro 100 from Oxford Instruments • Etching Si, SiO2, SiN • Bosch or cryo process • RF ICP max. power 3000 W • RF substrate biasing • Substrate temperature -150 – 800C • wafer max. 6’’, mechanical clamping • Loadlock • Metal free reactor • Gases: SF6,CHF3,C4F8,Ar, O2 Vědci a využití Sadílek Jakub, Gablech Imrich, Zahradníček Radim, Svatoš, Vojtěch Feng, Jianguo Pekárek, Jan Fecko, Peter Kurdík, Stanislav • 275 hodin • školení 9 hodin ( 1 %) • Údržba 54 hodin (21 %) • vědci 212 hodin (75 %)