Etching Chapters 11, 20, 21 (we will return to this topic in MEMS) sami.franssila@aalto.fi

Lithography and etching <Si> 1) photoresist patterning 2) Etching with reactive chemicals (acids, bases, plasmas) 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

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+

Isotropic etching Proceeds as a spherical wave Undercuts structures (proceeds under mask) Most wet etching processes are isotropic e.g. HF etching of oxide, H3PO4 etching of Al

Isotropy: good and bad If you want closely spaced lines (as in comb-drive capacitor), undercutting is bad, but if...

Undercutting in action: dome resonator RIE etching a small hole in polysilicon Isotropic HF wet etching of oxide under polysilicon  Membrane can move H. G. Craighead

Ion beam etching Ar+ ions will hit the wafer, and sputter away surface atoms. Applicable to all materials. Very slow process. Poor selectivity. https://cmi.epfl.ch/etch/IBE350.php

Plasma etcher (= RIE) RIE = Reactive Ion Etching 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 assist in 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

Plasma etched profiles Franssila: Microfabrication

Anisotropic etching of silicon (100) (111) Anisotropic wet etched profiles in <100> wafer. The sloped sidewalls are the slow etching (111) planes; the horizontal planes are (100). Etching will terminate if the slow etching (111) planes meet.

Anisotropic wet etched profiles http://greman.univ-tours.fr/axis-3/development-of-anisotropic-structures-by-electrochemical-etching-for-3d-devices-276485.kjsp https://www.mems-exchange.org/MEMS/fabrication.html

Terminology: two cases of anisotropic etching Anisotropy in plasma etching (and ion milling) is due to directionality of ion bombardment In anisotropic wet etching crystal planes etch at different rates 54.7o (100) (111) Franssila: Microfabrication

Etch mask Protective layer that is very slowly attacked by etchant Resist is the simplest etch mask to use  always consider resist mask first Aggressive etchants will prevent use of resist  hard mask required Quiz: what is a photomask ? How does it relate to etch mask ?

Photoresist as an etch mask Most simple to use Tolerates RIE: selectivity 10:1 at best Does not tolerate long RIE Does not tolerate most wet etchants such as KOH photomask photoresist Lithography: Photoresist spinning Lithography: UV-exposure Photoresist development Etching with resist mask

Selectivity Selectivity is defined as etch rate ratio: S= rate 1/rate 2 Silicon etch rate 500 nm/min Oxide etch rate 15 nm/min Selectivity 33:1 Resist etch rate 200 nm/min Selectivity 2.5:1 S ≠ ∞ There is always some unintentional loss of material

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

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

Etching two-layer films Ideal case Real RIE profile Isotropic, example <Si> If two layers are perfectly aligned, they were made in the same etch step. Otherwise alignment error would be visible.

Etching two-layer films, problems <Si> One film can be etched anisotropically but the other is etched isotropically (there is no guarantee that plasma etch profile will be anisotropic) Both layers isotropic, but different rates

Equipment: 1- or 2-sided processing Beam processes 1-sided Immersion processes 2-sided -photon beams (=lithography) -liquids (=wet etching) -atom beams (=evaporation) -liquids (=cleaning) -ion beams (=implantation) -gases (= oxidation, diffusion) -mixture of these (=plasmas) -gases (=CVD)

Wet etch vs. plasma etch Oxide wet etch in HF Undercut, isotropic Oxide plasma etch in CHF3 Vertical walls, no undercut Film removed from backside Film remains on backside

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 Au KI:I2:H2O Pt, Au HNO3:HCl (1:3) “aqua regia”

Plasma/RIE etching Three mechanisms at work simultaneously: Chemical etching: spontaneous etching (thermodynamics) Deposition of films ( nCF2*  (CF2)n Physical etching: damage creation, broken bonds extra energy supplied (desorption) sputtering (of (CF2)n) Vertical walls possible, but not guaranteed

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

Plasma/RIE etch gases Silicon SF6 (or Cl2) SiF4, SiCl4 Material Etch gas Product 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

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.

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

DRIE: Deep Reactive ion etching 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)

DRIE of silicon

DRIE thru-wafer 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 a b c

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

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

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

Hot plate sensor 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 ? Pt heater nitride membrane Pt measurement electrodes sensor material oxide

Silicon anisotropic wet etch strengths Extremely simple and reliable Batch process: 25 wafers at a time Smooth surfaces (RMS << 50 nm) Exactly defined sidewall angles Franssila: Microfabrication

Silicon DRIE strengths Any shape Any size Any crystal orientation High aspect ratio

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

Nanofluidic filter