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Etching Chapters 11 (20,21 too, but we will return to this topic)

Etching Chapters 11 (20,21 too, but we will return to this topic). sami.franssila@aalto.fi. Lithography and etching. 1) photoresist patterning 2) Etching with reactive chemicals (acids, bases, plasmas). <Si>. Etching thin film.

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Etching Chapters 11 (20,21 too, but we will return to this topic)

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  1. EtchingChapters 11 (20,21 too, but we will return to this topic) sami.franssila@aalto.fi

  2. Lithography and etching 1) photoresist patterning 2) Etching with reactive chemicals (acids, bases, plasmas) <Si> Etching thin film Same procedure applies both to etching thin films and to etching silicon wafer itself. Thicknesses vary ! Photolithography can be redone if problems detected, but after etching no repair is available. <Si> Etching bulk silicon

  3. Isotropic profile Anisotropic plasma etched profile

  4. Both are needed ! Anisotropic plasma etching is used when profile control and small linewidths are important Isotropic etching is used when we want to release structures (so that they can bend/move/vibrate)

  5. Etching basics Photoresist consumed Profile not 90o Underlying film 2 loss Photoresist Thin film 1 Thin film 2 Silicon wafer Initial state After etching

  6. Selectivity Selectivity is defined as etch rate ratio: Silicon etch rate 500 nm/min Oxide etch rate 15 nm/min Selectivity 33:1 Silicon etch rate 500 nm/min Resist etch rate 200 nm/min Selectivity 2.5:1

  7. Wet etching vs. plasma etching Wet etching: chemical reaction; simple wet bench and acids or bases needed Plasma etching: chemical and physical processes; requires RF-generator, vacuum system and gas lines

  8. Main methods of etching Wet etching solid + liquid etchant  soluble products Si (s) + 2 OH- + 2 H2O  Si(OH)2(O-)2 (aq) + 2 H2 (g) Plasma etching solid + gaseous etchant  volatile products SiO2 (s) + CF4 (g)  SiF4 (g) + CO2 (g) Ion beam etching/ ion milling solid + energetic ion  energetic atom + reflected ion Au + Ar+  Au* + Ar+

  9. Isotropic wet etching • Proceeds as a spherical wave • Undercuts the masking layer • Most wet etching processes are isotropic e.g. HF etching of oxide, H3PO4 etching of Al

  10. Typical wet etchants • SiO2 HF • <Si> KOH (10-50%) anisotropic etch • <Si> HNO3:HF:CH3COOH isotropic etch • poly-Si HNO3:HF: H2O • Al H3PO4:HNO3:H2O • W, TiW H2O2:H2O • Cu HNO3:H2O (1:1) • Ni HNO3:CH3COOH:H2SO4 (5:5:2) • Au KI:I2:H2O • Nitride H3PO4 180oC, CVD oxide mask • Pt, Au HNO3:HCl (1:3) “aqua regia”

  11. Terminology: two cases of anisotropic etching (111) (111) 54.7o (100) Anisotropy in plasma etching (and ion milling) is due to directionality of ion bombardment Wet etched anisotropic only possible in crystalline materials (silicon on this course)

  12. Plasma etcher (= RIE) • RF power applied to bottom electrode • Gas is excited and partly ionized, ion density 1010 ions/cm3 • Electric field accelerates ions towards bottom electrode and wafer • directional ion bombardment • Ions assistin etching by • -supplying energy to surface • -breaking bonds • Most etching is done by reative neutrals (their density is 1015 cm-3) Gases thru top electrode plasma Wafer on lower electrode Pumping system RIE = Reactive Ion Etching

  13. Plasma/RIE etching Three mechanisms at work simultaneously: 1) Chemical etching (reactive neutrals): spontaneous etching (thermodynamics) 2) Physical etching (ion-assisted): damage creation, broken bonds extra energy supplied (desorption) sputtering (of (CF2)n) 3) Deposition of films ( nCF2*  (CF2)n

  14. Flows and reactions in etching 1 2 3 1. etchant flow 2. ionization 3. diffusion 4. adsorption 5. reaction 6. desorption 7. diffusion 8. pump out 8 3 7 5 6 4

  15. Plasma/RIE etch gases • Silicon SF6 (or Cl2) SiF4, SiCl4 • Oxide CHF3 (or C4F8) SiF4, CO2 • Nitride SF6 (or CF4) SiF4, N2 • Aluminum Cl2 AlCl3 • Tungsten SF6 WF6 • Copper no plasma etching practical Material Etch gas Product gases

  16. Mechanism of anisotropy in RIE Fig. 11.5: All surfaces are passivated by a thin film from CF-gases, but directional ion bombardment will clear films from horizontal surface while leaving passivation film on the sidewalls, enabling etching to proceed vertically only.

  17. Aspect ratio The ratio of height to width Pillar array AR 5:1 Nanopillar 15:1 Lauri Sainiemi Nikolai Tsekurov

  18. DRIE: Deep Reactive ion etching a b c Etch pulse of SF6 Passivation pulse of C4F8 Etch pulse of SF6: remove CF-polymer from bottom, then etch silicon Sidewall is vertical, but undulating (scalloping)

  19. DRIE of silicon

  20. Photoresist as an etch mask • Most simple to use • Tolerates RIE: selectivity around 10:1 (=silicon etches 10 times faster than the resist) • Does not tolerate long RIE • Does not tolerate most wet etchants such as KOH Photomask photoresist <Si> Cleaned silicon wafer Photoresist development Lithography: Photoresist spinning Lithography: UV-exposure

  21. Hard mask Photomask photoresist SiO2 <Si> Cleaned silicon wafer Thermal oxidation @ 1100 °C Lithography: Photoresist spinning Lithography: UV-exposure Undercut Photoresist development Photoresist removal HF etching of SiO2 KOH etching

  22. DRIE thru-wafer a b c Things to consider: -mask material (hard mask needed !) -alignment of top and bottom structures -which side to etch first -what is the aspect ratio that can be etched -what wall thickness is strong enough

  23. Anisotropic wet etching of silicon • In ANISOTROPIC wet etching some atomic planes etch faster than others • (100) planes are typically fast etching planes • (111) planes are typically slow etching planes V-grooves etched on (100) silicon wafer

  24. Alkaline anisotropic etchants: some main features Etchant KOH Rate (at 80oC) 1 µm/min Selectivity (100):(111) 200:1 Selectivity Si:SiO2 200:1 Selectivity Si:Si3N4 2000:1

  25. Membrane formation Nitride membrane; no timing needed Timed silicon membrane; thickness depends on etch rate and wafer thickness control. Thin membrane thickness control bad. SOI wafer, membrane thickness determined by SOI device layer thickness

  26. Hot plate sensor Pt measurement electrodes sensor material oxide nitride membrane Pt heater Things to consider: Silicon etching in the -beginning -middle -end of the process ? Wafer becomes weak when lots of silicon removed. The thin films may not tolerate KOH/TMAH etching. Could the top side be protected by something ?

  27. Thermal pressure sensor heat sink heater resistor thermopile nitride p1 p0 p0

  28. Silicon DRIE strengths • Any shape • Any size • Any crystal orientation • High aspect ratio

  29. Silicon anisotropic wet etch strengths • Extremely simple and reliable • Smooth surfaces (RMS << 50 nm) • Exactly defined sidewall angles • P++ etch stop easy • Batch process: 25 wafers at a time

  30. RIE + isotropic+ anisotropic wet: ink jet nozzle Shin, S.J. et al: Firing frequency improvement of back shooting inkjet printhead by thermal management, Transducers ’03, p. 380

  31. Polishing (CMP for Chemical Mechanical Polishing) Smoothing Planarization Damascene SiO2 Al Cu or W Microfabrication

  32. Rotary CMP tool 60-90s per wafer Microfabrication

  33. CMP: Chemical-Mechanical Polishing Chemical Mechanical Polishing (CMP) combines (1) chemical action with (2) mechanical abrasion to achieve selective material removal. Microfabrication

  34. Polishing in action

  35. Results of CMP SiC wafer before and after CMP CMP of SiO2 Microfabrication

  36. Erosion and dishing in CMP Pattern density dependent Size dependent Microfabrication

  37. CMP tool input variables • -platen rotation 10-100 rpm • -velocity 10-100 cm/s • -applied pressure (load) 10-50 kPa • -slurry supply rate 50-500 ml/min • -slurry chemicals (pH, conc., viscosity,...) • -pad (material, porosity, hardness,...) • -abrasives (size,type, hardness, conc.,...) • -wafer (curvature, mounting) • -patterns (size, pattern density) • -films (hardness, µ-structure, stress, ...) Microfabrication

  38. CMP outputs resemble etching • -polish rate, 100-500 nm/min • -selectivity 1:1 – 100:1 (blind polishing and stopped polishing) • -overpolish time • -pattern density effects • -uniformity across wafer, 10% • -wafer-to-wafer repeatability, 10% Microfabrication

  39. Planarity Conformal deposition Surface smoothing Local: spin-on film Global: CMP Microfabrication

  40. Photonic crystals by CMP

  41. Log pile photonic crystal fabrication Poly-Si Si wafer CVD of oxide CMP of oxide CVD of poly-Si oxide

  42. Logpile (2) Poly-Si litho & etching CVD of poly-Si CVD of oxide CMP of oxide

  43. Logpile (3) • Layer 1 polysilicon etched • Deposit CVD oxide • CMP of oxide  flat surface • Continue for layers 2-6 • Etch all CVD oxide away with HF

  44. CMP vs. etching • Similar rates 100 – 1000 nm/min • Similar selectivities 1:1 to 100:1 • Both chemical and physical effects (as in RIE) • Need post-processing: Post-CMP cleaning is chanllenging particle removal, resist stripping is simpler Microfabrication

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