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Doping Depositing impurities into Si in a controlled manner PowerPoint Presentation

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On : Jan 08, 2015

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  • Slide 1 - OXIDATION- Overview Process Types Details of Thermal Oxidation Models Relevant Issues
  • Slide 2 - Uses As a part of a structure e.g. Gate Oxide For hard masks e.g. In Nitride Etch, implant mask ... Protecting the silicon surface (Passivation ) Insulator (ILD/IMD) As part of ‘mild etch’ (oxidation / removal cycles) Whether useful or not, automatically forms in ambient Native Oxide ( ~ 20 A thick) except H-terminated Si (111)
  • Slide 3 - Processes Thermal Oxidation (Heating) Dry vs Wet Electrochemical Oxidation (Anodization) Oxide (and nitride) adhere well to the silicon good insulator Breakdown voltage 10 MV/cm ==> Can make a very thin gate
  • Slide 4 - Structure Tetrahedral Structure each Si to four O each O to two Si Single crystal quartz (density 2.6 g/cm3) Fused silica (density 2.2 g/cm3) Reaction with water ©Time Domain CVD Si-OH termination is stable structure is more porous than Si-O-Si
  • Slide 5 - Thermal Oxidation Dry oxidation Dense oxide formed (good quality, low diffusion) slow growth rate NEED TO KEEP WATER OUT OF THE SYSTEM Wet oxidation Overall reaction Relatively porous oxide formed (lower quality, species diffuse faster) Still good quality compared to electrochem oxidation, for example faster growth rate Wet oxide for masking Dry oxide for gate ox
  • Slide 6 - Wet Oxidation Proposed Mechanism Hydration near Silicon/ Silicon oxide interface Oxidation of silicon Hydrogen rapidly diffuses out Some hydrogen may form hydroxyl group
  • Slide 7 - Diffusivities in Oxide Oxygen diffuses faster (compared to water) Sodium and Hydrogen diffuse very fast Water Oxygen Hydrogen Sodium 1/T Diffusivity (log scale)
  • Slide 8 - Oxide Growth (Thermal) Si Oxide Original Si surface To obtain 1 unit of oxide, almost half unit of silicon is consumed (0.44) Oxidation occurs at the Si/SiO2 interface i.e. Oxidizing species has to diffuse through ‘already existing’ silicon oxide
  • Slide 9 - Oxide Growth (Thermal) Silicon Oxide Air (BL) At any point of time, amount of oxide is variable ‘x’ Usually, concentration of oxidizing species (H2O or O2) is sufficiently high in gas phase ==> Saturated in the oxide interface x Distance Concentration o i
  • Slide 10 - Oxidation Kinetics At steady state diffusion through oxide = reaction rate at the Si/SiO2 interface Oxygen diffuses faster than Water However, water solubility is very high (1000 times) ==> Effectively water concentration at the interface is higher ==> wet oxidation faster At steady state Diffusion Reaction
  • Slide 11 - Oxidation Kinetics Oxide Growth Rate Flux at steady state = Flux/ # oxidizing species per unit volume (of SiO2) n = 2.2 × 1022 cm-3 for O2 = 4.4 × 1022 cm-3 for H2O Eqn Initial Condition 6.023x1023 molecules =1 mol of oxide = x g of oxide = y cm3 of oxide (from density) 2.2 x 1022 molecules/cm3 One O2 per SiO2 Two H20 per SiO2
  • Slide 12 - Deal-Grove Model Solution where OR t is the time needed to grow the ‘initial’ oxide A and B depend on diffusivity “D”, solubility and # oxidizing species per unit volume “n” A and B will be different for Dry and Wet oxidation Bruce Deal & Andy Grove
  • Slide 13 - Linear & Parabolic Regimes Linear vs Parabolic Regimes Kinetic Controlled vs Mass Transfer Controlled Initially faster growth rate, then slower growth rate Very short Time Longer Time If one starts with thin oxide (or bare silicon)
  • Slide 14 - Exponential Regime Hypothesis 1 Charged species forms holes diffuse faster / set up electrical field diffusion + drift ==> effective diffusivity high space charge regime controls length = 15 nm for oxygen, 0.5 nm for water ==> wet oxidation not affected For dry oxidation, one finds that t is not zero in the model fit A t corresponding to an initial thickness of 25 nm provides good fit Initial growth at very high rate Approximated by exponential curve If one starts with bare oxide
  • Slide 15 - Exponential Regime Hypothesis 2 In dry oxidation, many ‘open’ areas exist oxygen diffuses fast in silicon hence more initial growth rate once covered by silicon di oxide, slow diffusion Hypothesis 3 Even before reaction (at high temp), oxygen dissolved in silicon (reasonable diffusion) once temp is increased, 5 nm quick oxide formation
  • Slide 16 - Temp Variation of Linear/Parabolic Coeff Linear [B/A] Parabolic [B] Solubility and Diffusion function of temp © May & Sze
  • Slide 17 - Effect of Doping Doping increases oxidation rate Segregation ratio of dopant in silicon / dopant in oxide e.g. Boron incorporated in oxide; more porous oxide more diffusion parabolic rate constant is higher P not incorporated in oxide no significant change in parabolic rate constant © May & Sze
  • Slide 18 - Issues Na diffuses fast in oxide Use Cl during oxidation helps trap Na helps create volatile compounds of heavy metals (contaminant from furnace etc) use 3% HCl or Tri chloro ethylene (TCE) Ref: VLSI Fabrication Principles by S.K. Ghandhi
  • Slide 19 - Electrochemical Use neutral solution and apply potential Pt as counter electrode (Hydrogen evolution) Use Ammonium hydrogen Phosphate or Phosphoric acid or ammonia solution Silicon diffuses out and forms oxide Increase in oxide thickness ==> increase in potential needed self limiting Oxide quality poor Used to oxidize controlled amount and strip for diagnosis

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