OXIDATION- Overview  Process Types  Details of Thermal Oxidation  Models  Relevant Issues

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)

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

Structure  Tetrahedral Structure  each Si to four O  each O to two Si  Single crystal quartz (density 2.6 g/cm 3 )  Fused silica (density 2.2 g/cm 3 )  Reaction with water ©Time Domain CVD  Si-OH termination is stable  structure is more porous than Si-O-Si

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

Wet Oxidation  Proposed Mechanism  Hydration near Silicon/ Silicon oxide interface  Oxidation of silicon  Hydrogen rapidly diffuses out  Some hydrogen may form hydroxyl group

Diffusivities in Oxide  Oxygen diffuses faster (compared to water)  Sodium and Hydrogen diffuse very fast Water Oxygen Hydrogen Sodium 1/T Diffusivity (log scale)

Oxide Growth (Thermal) SiOxide Original Si surface  To obtain 1 unit of oxide, almost half unit of silicon is consumed (0.44)  Oxidation occurs at the Si/SiO 2 interface  i.e. Oxidizing species has to diffuse through ‘already existing’ silicon oxide

Oxide Growth (Thermal) SiliconOxideAir (BL)  At any point of time, amount of oxide is variable ‘x’  Usually, concentration of oxidizing species (H 2 O or O 2 ) is sufficiently high in gas phase  ==> Saturated in the oxide interface x Distance Concentration o i

Oxidation Kinetics  At steady state  diffusion through oxide = reaction rate at the Si/SiO 2 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

Oxidation Kinetics  Oxide Growth Rate Flux at steady state  = Flux/ # oxidizing species per unit volume (of SiO 2 )  n = 2.2 × cm -3 for O 2  = 4.4 × cm -3 for H 2 O Eqn Initial Condition  6.023x10 23 molecules  =1 mol of oxide = x g of oxide  = y cm3 of oxide (from density)  2.2 x molecules/cm 3  One O 2 per SiO 2  Two H 2 0 per SiO 2

Deal-Grove Model Solution where OR  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

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)

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  is not zero in the model fit  A  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

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

Temp Variation of Linear/Parabolic Coeff Linear [B/A] Parabolic [B] Solubility and Diffusion function of temp © May & Sze

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

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

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