10. Plasma Enhanced Chemical Vapor Deposition 10.1 Introduction to PECVD Chemical Vapor Deposition (CVD) thermally driven chemical deposition from gas phase: 1. transport of reactants to the deposition space q | p | i 2. diffusion of reactants to the substrate I 1 I I surface 3. adsorption of reactants 9as phas 4. phys.-chem. processes ^ film growth and by-products q i R i i 5. desorption of by-products i 7 i i 6. diffusion of by-products in gas flow 7. transport of by-products from deposition space Low Pressure CVD (LPCVD) is often used in microelectronics or in applications requiring excellent control over impurities Plasma Enhanced (or Assisted) CVD (PECVD or PACVD) CVD method in which discharge is ignited in the gas mixture: ^ collisions of energetic electrons with heavy gas particles ^ production of highy reactive species ^ more competing processes take place, deposition can be generally divided into thermal and plasma branches gas phase solid £ surface 4 PECVD x CVD reaction branch: 700-900°C 3SiH4 + 4NH3 -» Si3N4 + 12H2 plasma SiH 4 + NH 3 -» SiNH + 3H2, plasma reaction branch at PECVD is much more importan1^Ba>agaS53 because: ^^^\ /\ ^ sticking coefficient is much higher for reactive radicals V and activated surface ^ activation energies of chemical reactions are lower for excited reactants PECVD - lower deposition temperature, novel reaction schemes leading to new materials, replacement of toxic and dangerous reactants but high complexity of chemical reactions and processes, worse selectivity and reaction control, possibility of damages by energetic ions, UV radiation or electrostaticaly (charge accumulation) 10.2 PECVD of Silicon-Based Thin Films O dielectric films for microelectronics silicon nitride: SiH4+NH (final protective T~250-4 passivation for integrated circuit) silicon oxide: (insulating film - el. separation) SiH4+N20/NO/C02/02 T=200-400 °C S1H4+NH3 or SiH4+N2 T=250-400 °C Si(OC2H5)4+02 \ tetraetoxysilane (TEOS) PECVD of materials with silicon O more dielectric films for microelectronics low-k dielektrics: organosilicons + 02/... + ... (el. separation for ULSI) £^ organosilicon glass O semiconducting films for microelectronics (^G) epitaxial silicon: SiH4+H2 T=800 °C polycrystalline silicon: SiH4/SiH2Cl2+H2/Ar T=450-700 °C (gate electrode, connections in MOS i.e., solar energy pannels) O SiOx and SiOxCyHz for many other applications scratch resistant films for plastics, anticorrosion films for metals, barrier films for packaging and pharmacy, biocompatible films mixtures with organosilicons (TEOS, HAADSO, HAADSZ) PECVD of films using HMDSO (hexamethyldisiloxane) CH3 CH3 source of Si-O-Si bonds CH3-Si— —Si-CH3 CH3 CH3 source of CH3 groups SiOo-like films SiOxCyHz plasma polymers • concentration of HMDSO in the gas feed, especially oxygen • power • bias voltage / ion energy • pressure pulsing matching box \ blocking capacitor Irf power supply \ T f rf driven self-biased electrode! sheath plasma reactor grounded CCP: > o ' ^hmdso > pressur > rf powe > dc self- o2/CH4 SOURCE Modulator/polarizer ELLIPSOMETER Matching box Power supply 13.56 MHz External Antenna OPTICAL FIBER/ LANGMUIR PROBE helical > pres > rf po > subs 0*1 :Hmi| Variation of film composition 0 o c CO -Q O (/) -Q CD O CCP 40 Pa _i_I_i_ Chi CCP 2.5 Pa _i_I_I_i_ 1000 2000 wavenumber [cm" ] 3000 -1- cd ü c co _q o c/) £1 co o c 4000 5 % HMDSO in a CCP 2.5Pa, 0ms CCP 2.5Pa, 15ms CCP 40Pa, 0ms CCP 40Pa, 15ms 800 1000 1200 wavenumber [cm1] ^> 0.4 Pa: Si02 structure, almost no impurities ^> 2.5 Pa: Si02 structure, OH groups and H20 ^> 40 Pa: organosilicon films Domains of stresses without treatment 500 400 > 300 two different coatings choosen for treatment testing: ■ P = 100 W, Q02=45 sccm> d = 0.5 um ■ P = 400W, QO2=10sccm, d = 1.2 um 200 - 100 - O ' o I I high , compressive stress ideposition conditions ' suitable for pQparatioji ' of intermediate layers N _L_i J-V 0 10 ^02 ^ ^HMDSO 15 20 1HMDSO Film microstructure for CCP 40 Pa 10.3 PECVD of Carbon-Based Thin Films Diamond, graphite and much more Besides well known materials such as crystalline diamond or graphite carbon can form many other interesting nanomaterials such as fullerenes, carbon nanotubes, graphene. s O crystalline diamond films PECVD of carbon based materials 0.1 - 5% CH4/C2H2/... in H2 T=700-1000°C RF plasma p=0.01-4kPa, Tgas=1000-1500°C, P=0.5-3kW MW plasma p=2-10kPa, Tgas=2000-2500°C, P=0.5-2kW O amorphous diamond like carbon (DLC) films H !! ion bombardment CH4/C2H2/... + (Ar/H2), T < 300 °C cc.V Spot Magn Det WD Exp 6% of C O polymer hydrogenated carbon films (a-C:H) 3i ■I ~ -^Vvi,. Classification of carbon films » classification of carbon films by Fraunhofer Institute for Surface Engineering and Thin Films (1ST) 2009 > activities on international standardization, e.g. workshop at 12th International Conference on Plasma Surface Engineering (PSE) in 2010 Carbon (Arm Designation l ptvqejgaj farm 2 Amorphous carbon f 1ms (diamond-Hoe-carbon fUms / DLC) 3 CrystaKne carbon f Arm Diamond Urns Graprtte tarns Thin r*n/ thick fám Thnilrn ThnfArn TWnihi Trick »Im (free starting) Thntim Doping. additional dements hydrogsvtree hydrogouesd undoped doped indoped doped 111 tr\i marttod mailed vwth metal •ATI nan-metal Crystal sue an tie growth side J. (amaphjous) lb 500nm. 1 1 It ayitaltine 05 b 10 pm. mikrc-crystalline Olio 5 vim (5 pm to) SO to 500 pm 80 to 500 pm Prcdomina ling C C bond type sp2« 3 SP . bond sp2 sp2 ™3 sp ** «3 sp m3 sp ™3 3*1 SP3 «3 sp SP2 Fám No. 1 21 12 2 3 24 2-5 26 2 1 i 1 J2 i 'S 34 J *j 16 Designation fAm Hydrogen free amorphous carbon ram TetrahedraJ hydrogen-free amorphous carbon Htm Metat containing hydrogen-free amorphous ejjbM Idm H iKfrogenated amorphous carbon (Am Tetrahedrat hydrogenated amorphous carbon (Am Metat containing hydrogenated amorphous carbon ftm Modified hydrogenated amorphous carbon fAm runo* crystaffcne CVD diamond fAm m*cro oryst aline CVD diamond fAm doped CVD diamond fAm CVD damond doped CVD diamond graphite fAm Reeonv mended abbreviation 1 aC a-CMe a CM taCH aCJHMe better mechanical properties, optical gap 1-2 eV. * hydrogenated tetrahedral amorphous carbon fims (ta-CW): *icreased C-C sp3 content whist keeping a H content low (25-30 at. %} => higher density (up to 24 gem3) and Young's modulus (up to 300 GPa) » graphite-lite a-C:H (GLCH): tow H content (< 20at.%}: high sp2 content and sp2 clustering => gap under 1 eV C. Casiragh. A C Ferrari, and J. Robertson. Phys. Rev. B 72(8) :1-14.2005.