Transcript Doping: Depositing impurities into Si in a controlled manner

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/cm3)
 Fused silica (density 2.2 g/cm3)
©Time Domain CVD
 Reaction with water
Si  O  Si  H 2 0  Si  OH  Si  OH
 Si-OH termination is stable
 structure is more porous than Si-O-Si
Thermal Oxidation
Dry oxidation
Si  O2  SiO2
 Dense oxide formed
(good quality, low diffusion)
 slow growth rate
Wet oxidation
Si  2 H 2O  SiO2  2 H 2
 NEED TO KEEP
WATER OUT OF THE
SYSTEM
 Overall reaction
 Relatively porous oxide formed
(lower quality, species diffuse faster)
 Still good quality compared to
electrochem oxidation, for
example
 faster growth rate
Dry oxide for gate ox
Wet oxide for masking
Wet Oxidation
 Proposed Mechanism
 Hydration near Silicon/ Silicon oxide interface
Si  O  Si  H 2O  SiOH  SiOH
 Oxidation of silicon
2Si  OH  Si  Si  2Si  O  Si  H 2
 Hydrogen rapidly diffuses out
 Some hydrogen may form hydroxyl group
Si  O   1 H 2  SiOH
2
Diffusivities in Oxide
Diffusivity (log scale)
 Oxygen diffuses faster (compared to water)
 Sodium and Hydrogen diffuse very fast
Hydrogen
Oxygen
Sodium
Water
1/T
Oxide Growth (Thermal)
Original Si surface
Oxide
Si
 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
Oxide Growth (Thermal)
Air (BL) Oxide
Concentration
 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
Silicon
o
i
x
Distance
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
( No  Ni )
dN
Diffusion J   D
D
dx
Reaction
Rate  k Ni
x
At steady state
No
Ni  D
kx  D
Oxidation Kinetics
 6.023x1023 molecules
DN o
 =1 mol of oxide = x g of
Flux at
J
oxide
xD
steady state
k
 = y cm3 of oxide (from
density)
dx
 Oxide Growth Rate
 2.2 x 1022 molecules/cm3
dt
 One O2 per SiO2
 Two H20 per SiO2
 = Flux/ # oxidizing species per unit volume (of SiO2)
 n = 2.2 × 1022 cm-3 for O2
 = 4.4 × 1022 cm-3 for H2O
Eqn
DN o
dx

dt x  D
J
n
Initial Condition
k
x  xi at t  0
Bruce Deal & Andy Grove
Deal-Grove Model
Solution
OR
2 DN 0
2D
x 
x
(t   )
k
n
2
2
x
x
t

B
B
 A
where
xi2
xi
 
B
B
A
 
2D
A
k
2 DN o
B
n
 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

Linear & Parabolic Regimes

x  0.5 A 


 4B
 2 
1  2 (t   )   1

 A


1
If one starts with thin oxide (or bare silicon)
 Very short Time
B
x  (t   )
A
 Longer Time
x  B(t   )
2
t  
(t   )
A2
4B
 Linear vs Parabolic Regimes
 Kinetic Controlled vs Mass Transfer Controlled
A2
4B
t  
A2
4B
Exponential Regime
If one starts with bare oxide
 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
 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
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
Solubility and Diffusion function of temp
Linear [B/A]
© May & Sze
Parabolic [B]
 Doping increases oxidation rate
 Segregation
 ratio of dopant in silicon / dopant in oxide
Effect of Doping
 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
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