sign in sign up
<<
Home , Silicon dioxide, Thermal oxidation

EE-527:

Oxidation of

R. B. Darling / EE-527 / Winter 2013 (SiO2)

• The single thermodynamically stable of silicon. • Essential to the fabrication of MOS devices. – Creates extremely high electronic quality gate . • Essential to the patterning of silicon for high temperature processing. – Photoresist cannot handle temperatures much above 150C.

– Patterned SiO2 can be used for masking diffusions, etches, and other processes up to temperatures of >1400C.

• The extreme purity and perfection of the Si/SiO2 interface is the ultimate reason why silicon has been the #1 for microelectronics. – And is likely to remain so…

R. B. Darling / EE-527 / Winter 2013 Oxidation of Silicon • Carried out at temperatures of 900 – 1200C.

• Dry oxidation: N2 carrier gas + O2

–O2 must diffuse through the growing SiO2 layer.

• Wet oxidation: N2 carrier gas + O2 + H2O (sat. vapor)

–H2O must diffuse through the growing SiO2 layer.

– Diffusion of H2O is much faster than O2; wet oxides grow faster.

–H2 must diffuse back out; usually very rapid and not a limiter.

•SiO2 grows from the SiO2/Si interface:

– Oldest SiO2 remains on the surface.

– Youngest SiO2 finishes at the SiO2/Si interface.

O2, H2O H 2 SiO2 Silicon

R. B. Darling / EE-527 / Winter 2013 Deal-Grove Model of Oxidation - 1

• B. E. Deal and A. S. Grove, J. Appl. Phys. 36, 3770 (1965). • A fairly simple and very descriptive model of silicon oxidation from a

gaseous source (O2, H2O), modeled after that for the oxidation of surfaces. • Based upon the assumption of gaseous diffusion of the reacting species through a stagnant film adjacent to the growing oxide. • Exhibits both transport-limited and reaction-rate-limited regimes.

R. B. Darling / EE-527 / Winter 2013 Deal-Grove Model of Oxidation - 2

Stagnant Film Model for Dry Oxidation with O2: concentration in gas

O2 concentration concentration in

Cg

C* Cs

Co

F3 F1

F2 Ci

supply gas stagnant gas film growing SiO2 Si

position, x

xsf xox

R. B. Darling / EE-527 / Winter 2013 Deal-Grove Model of Oxidation - 3 D Diffusion of O2 through the stagnant film: F1  (Cg  Cs )  hg (Cg  Cs ) xsf D Diffusion of O2 through the growing oxide: F2  (C0  Ci ) xox Surface reaction of O with the silicon: 2 F3  ksCi

In steady-state, the fluxes must be equal: F1  F2  F3

Henry’s Law relates the concentration at the Co  HPs  HkTCs hg surface to the concentration in the vapor above it h  (H = Henry’s constant): C*  HPg  HkTCg HkT

Flux of O in the stagnant film can be expressed 2 F  h (C C )  h(C *C ) in terms of equivalent concentrations in the oxide: 1 g g s 0

ks xox C * 1 Algebraic solution of the above produces: C  D i Co  C * ks ks xox k k x 1  1 s  s ox h D h D R. B. Darling / EE-527 / Winter 2013 Deal-Grove Model of Oxidation - 4

The flux of O2 to a growth of the SiO2 layer and a consumption of the Si surface: dx dx k C * F  N Si  N ox  s Si SiO2 k k x dt dt 1 s  s ox h D

This is a nonlinear differential equation for xox(t) which can be solved by separation of variables: 2 xox  Axox  B(t  )   2 1 1 2DC * xi  Axi x (t  0)  x A  2D   B    ox i  ks h  NSiO2 B The oxide thickness as a function of time becomes: B short time limit: xox (t)  (t  ) A  4B  A x (t)   1 t  1 ox  2  2  A  long time limit: xox (t)  B(t  ) x is the initial oxide thickness i R. B. Darling / EE-527 / Winter 2013 Deal-Grove Model of Oxidation - 5

surface reaction rate limited: B linear regime x (t)  (t  ) ox A

102 diffusion transport limited: parabolic regime

x xox(t)  B(t  ) ox 101 A/ 2 general case

0 A 4B  10 x (t)   1 t  1 ox  2  2  A 

10-1 10-1 100 101 102 103 t  A2 / 4B R. B. Darling / EE-527 / Winter 2013 Deal-Grove Model of Oxidation - 6

The diffusive transport rate constant B shows an Arrhenius temperature dependence with an activation energy EAt:

EAt / kT EAt = 1.24 eV for dry oxidation with O2 B  B0e EAt = 0.74 eV for wet oxidation with H2O The reaction rate constant B/A also shows an Arrhenius temperature dependence with an

activation energy EAr: B  B EAr = 2.00 eV for dry oxidation with O2  eEAr / kT   E = 1.96 eV for wet oxidation with H O A  A0 Ar 2

R. B. Darling / EE-527 / Winter 2013 Deal-Grove Oxidation Parameters

For wet oxidation starting from bare silicon, use  = 0. For dry oxidation starting from bare silicon, note that  > 0 must be used. Note that wet oxidation rates are significantly faster than dry rates.

Type / Temperature A (m) B (m2/hr) B/A (m/hr)  (hr) Wet @ 1200C 0.05 0.720 14.40 0 Wet @ 1100C 0.11 0.510 4.64 0 Wet @ 1000C 0.226 0.287 1.27 0 Wet @ 920C 0.50 0.203 0.406 0 Dry @ 1200C 0.040 0.045 1.12 0.027 Dry @ 1100C 0.090 0.027 0.30 0.067 Dry @ 1000C 0.165 0.0117 0.071 0.37 Dry @ 920C 0.235 0.0049 0.0208 1.40 Dry @ 800C 0.370 0.0011 0.0030 9.0

Data from Wolf and Tauber, Vol. 1, 2nd Ed., p. 277. R. B. Darling / EE-527 / Winter 2013 Thin Oxides - 1

• For dry oxides less than ~35 nm, the rate of oxide growth is much faster than that predicted by the Deal-Grove model. • Proper fit to the Deal-Grove characteristics require the use of a

fictitious initial thickness of ~20 nm = xi. ( > 0)

oxide thickness linear regime

~ 20 nm

actual oxidation characteristics

oxidation time

R. B. Darling / EE-527 / Winter 2013 Thin Oxides - 2 • Native oxides formed from air exposure: – Grow in a step-wise manner. – Usually saturate at about 0.8 nm for lightly doped silicon, or about 1.3 nm for heavily doped silicon. • Boiling DI water can be used to create very thin oxides. – Can be used to strip away a few 10s of nm of Si for surface prep. • Thin dry oxides are used for MOSFET gates. – Usually the best quality oxide that can be produced. – Empirical growth rate model: • First term with A,B is the standard Deal-Grove term. • Second and third terms correct for faster initial growth rate.

dxox B xox / L1 xox / L2   C1e  C2e dt 2xox  A

R. B. Darling / EE-527 / Winter 2013 Thin Oxides - 3

• Modern require gate oxides with thicknesses of only 2-5 nm! (usually called ultra-thin oxides) • Controllable growth of oxides this thin requires: – Reduced temperature growth – Reduced pressure growth – Rapid (RTO)

• Pure O2 at 1 atm • ~1050C for 40 seconds, via lamps

R. B. Darling / EE-527 / Winter 2013 Use of

• Increases the oxidation rate. • Improves the oxide quality: – Reduced mobile ionic charge (Na+ gettering) – Increased minority carrier lifetime in underlying silicon – Increased oxide breakdown voltage – Reduced interface and fixed charge – Reduced oxidation-induced stacking faults • Chlorine sources:

– Chlorine gas, Cl2 – chloride, HCl

– Trichloroethylene, TCE, Cl3CCH

– 1,1,1-Trichloroethane, TCA, Cl3CCH3

– Trans-1,2-Dichloroethane, DCE, Cl2CCH2

– Oxalyl Chloride, OC, Cl2C2O2

R. B. Darling / EE-527 / Winter 2013 High Pressure Oxidation (HIPOX)

• Approximately linear dependence of Deal-Grove B and B/A rate coefficients with oxygen pressure.

• HIPOX systems usually operate at PO2 ~ 10-25 atm. • Approximately, each atm of pressure is equivalent to 30C in temperature to keep the oxide growth rate constant. • Usually needed to grow oxides thicker than 1.5-2.0 microns in a reasonable of time. – Thick field oxides • Gasonics is one well-known vendor for HIPOX systems.

R. B. Darling / EE-527 / Winter 2013 Consumption of Silicon by Oxidation • Atomic & molecular weights:

– Si: 28.09 g/mole = mSi xox – O: 16.00 g/mole = mOx original surface –SiO: 60.09 g/mole = m xSi = 0.462 xox 2 SiO2 new surface • : 3 – Si = 2.328 g/cm 3 – SiO2 = 2.30 g/cm silicon wafer • Atomic densities: 22 -3 –NSi = NASi/mSi = 4.992 x 10 cm 22 -3 –NSiO2 = NASiO2/mSiO2 = 2.305 x 10 cm • Oxide growth:

–xSiNSi = xoxNSiO2

–xSi = 0.462 xox

•SiO2 grows 46% downward and 54% upward from the original surface.

•SiO2 is under compressive strain of ~ 300 MPa = 43,500 psi. R. B. Darling / EE-527 / Winter 2013 Local Oxidation of Silicon (LOCOS) - 1

• A fundamental method for isolating MOSFETs on an IC. – Used for most MOS/CMOS processes down to ~0.5 m geometries. – Used to define the active region of a MOSFET from the isolation areas. • Active region: thin , ~2-200 nm, depending upon technology node • Isolation region: thick field oxide, ~0.5-2.0 m, depending upon tech. node – Limited by bird’s beak encroachment, stresses, and dopant redistribution. • Typically, bird’s beak encroachment is comparable to field oxide thickness. W

ACTIVE layout layer

Source POLY layout layer

Gate L FIELD OXIDE is where ACTIVE is not. MOSFET channel is defined by the overlap Drain of ACTIVE and POLY layout layers

R. B. Darling / EE-527 / Winter 2013 Local Oxidation of Silicon (LOCOS) - 2

• LOCOS process steps: 150 nm Si3N4 oxidation mask – 50 nm pad oxide 50 nm SiO2 pad oxide – 150 nm CVD nitride layer – Pattern and etch nitride – Channel stop implant channel stop implant (B) – Wet oxidation of field oxide silicon wafer (p) • Typ. 1000C for 4-10 hours. • HIPOX often used for this. – Strip nitride "bird's beak" – Strip pad oxide locally oxidized silicon ~1-2 m field oxide

silicon wafer

R. B. Darling / EE-527 / Winter 2013 Shallow Trench Isolation (STI) - 1

• Key features: – Silicon trench is etched around active areas for MOSFETs, – Deposited dielectric is backfilled into trenches, and – CMP is used to planarize the result. • Used for nearly all submicron MOS processes (L < 0.35 m) – Eliminates the bird’s beak encroachment of LOCOS. – Supports the use of implants for retrograde wells. – Channel stop implants not needed for sufficiently deep trenches. – Reduced mechanical stress from LOCOS nitride bending. – Reduced channel stop dopant redistribution.

R. B. Darling / EE-527 / Winter 2013 Shallow Trench Isolation (STI) - 2

150 nm Si N trench mask • STI process steps: 3 4 50 nm SiO2 pad oxide – 50 nm pad oxide CVD trench refill – 150 nm LPCVD nitride layer – Pattern resist layer – Etch nitride and pad oxide – 400-500 nm deep anisotropic thermal trench oxide trench etch, 70-80 sidewalls silicon wafer – Strip resist – 50 nm thermal oxidation of trench side walls – CVD insulator to refill trench after CMP planarization: – CMP planarization down to nitride – Dielectric densification ~900C – Strip nitride silicon wafer

R. B. Darling / EE-527 / Winter 2013 Crystallographic Orientation Effects

• Most oxide growth rates are quoted for the (100) planes of Si, which is most common for microelectronics processing. • Surface reaction rate depends upon the density of Si atoms on a given surface and the orientation of the bond angles to that surface. • (111)Si has a greater density of atoms than (100)Si. – This is a subtle point, often argued, and ends up depending upon which plane is chosen to cut through the . – Note that the average surface density of Si atoms is the same for all orientations. • (111)Si has perpendicular surface bonds, while (100)Si has surface bond angles that are oblique to the surface (~54 from normal). • Experimentally, the B/A rate constant is found to be a factor of 1.65 – 1.75 larger for the (111) planes over the (100) planes. • This is responsible for the “corner-pinch” effect observed for where (111) planes meet (100) planes in an etched pit. R. B. Darling / EE-527 / Winter 2013 Oxidative Sharpening • Compressive stress and transport limited oxidation rate cause the oxide to grow less on both internal and external corners:

SiO2

SiO2

Silicon Silicon

interior corner exterior corner • Tip radii < 10 nm can be achieved, which are useful for AFM and LVFE tips:

R. B. Darling / EE-527 / Winter 2013 Oxidation of Doped Silicon • Oxidation of doped silicon (B, P, As): – The dopant becomes incorporated into the oxide. – The dopant enhances the growth rate of the oxide when present in high concentrations (~ > 1020 cm-3). • Uptake of the dopant by the oxide is controlled by the segregation coefficient m: [N] m  Si [N] SiO2 – If m < 1, the oxide will absorb the dopant during oxidation. – If m > 1, the oxide will expel the dopant during oxidation. – The segregation coefficient becomes an equilibrium matching condition

for the Si/SiO2 interface. • Diffusion of the dopant within the oxide can be either faster than or slower than in the silicon.

– This determines the ability of SiO2 to serve as a mask for dopant diffusion. R. B. Darling / EE-527 / Winter 2013 Dopant Redistribution During Oxidation

[N] [N]

SiO2 Si SiO2 Si

Example: Example: : m ~ 10-2 Phosphorous: m ~ 102 oxide growth absorbs boron; oxide growth expels phosphorous; boron is depleted at the Si surface. phosphorous piles up at the Si surface.

R. B. Darling / EE-527 / Winter 2013 Structure of SiO2

•SiO2, silica, is normally a :

– Short-range order due to SiO4 tetrahedral structure – Limited long-range order due to amorphous, random

interconnections of the SiO4 tetrahedrons

Silica Tetrahedron Quartz Crystal (simplified) Silica Glass O-O: 2.27 Angstroms  = 2.65 g/cm3, n = 1.55, H = 7  = 2.15 – 2.25 g/cm3 Si-O: 1.62 Angstroms from Wolf & Tauber, p.267 R. B. Darling / EE-527 / Winter 2013 Silica Glass

4- • Unit building block is the silica tetrahedron: [SiO4] • Two silica tetrahedrons are connected by sharing an apical oxygen that forms a bridge between the two tetrahedra. • If each tetrahedron is bridged to 4 other tetrahedrons, the system becomes charge neutral, and the overall chemical

formula becomes SiO2. • Non-bridging (NBOs) carry a −1 charge and will collect around unbound cation .

R. B. Darling / EE-527 / Winter 2013 Silica Glass Structural Defects

• Network formers: also form , e.g. B2O3, P2O5 – substitute for Si in the tetrahedra: • Boron, B3+, eliminates one bridging oxygen, weakens the network • Phosphorous, P5+, creates one bridging oxygen, strengthens the network • Network modifiers: do not form glasses by themselves – interstitial impurities: Na, K, Pb, Ba • Network terminators: hydroxyl groups: OH−

from Revesz (1965) R. B. Darling / EE-527 / Winter 2013 Quartz

• Quartz is crystalline SiO2 –2nd most common on (after feldspar). – Rhombohedral : a = b = c,  =  =  = 60. – Silica tetrahedra are organized into rows and planes with 3-fold rotational symmetry for the  phase. – Note: fused silica is often called “quartz” or “quartz-ware,” but it is really pure silica glass.

Low Quartz ( phase) High Quartz ( phase) 573C from Hurlbut & Klein, p.152 R. B. Darling / EE-527 / Winter 2013 Low Temperature Glasses

• Examples: – LTO = Low Temperature Oxide – PSG = Phospho- Glass – BSG = Boro-Silicate Glass – BPSG = Boro-Phospho-Silicate Glass • All are deposited by CVD or LPCVD processes. • Are used extensively for inter-level dielectrics in back-end processing. • Low temperature refers to a low temperature which allows the deposited glass to be reflowed and densified at temperatures which are compatible with the interconnect , usually < 400C.

R. B. Darling / EE-527 / Winter 2013 Spin-On Glasses (SOG) • Used for inter-level dielectrics in back-end processing. • Usually have viscosities and characteristics which allow them to fill in recesses and flow away from high spots, providing a degree of planarization for the next metal layer. • Based upon organic- compounds: – Methyl siloxane polymers – Ethyl siloxane polymers • Can be easily applied by spin-coating to thicknesses in the range of 50 to 300 nm. Thicker films can be obtained through multiple coats.

• Resulting SiO2 layer after curing is of lower quality than that of a thermally grown oxide: – More porous, less dense, lower . – Higher etch rates in HF and BOE. – Too many surface states and trapped charge to be useful as a MOS insulator.

– Lower breakdown voltage. R. B. Darling / EE-527 / Winter 2013 Spin-On Glass Curing Schedule • The concept is to spin cast the film, evaporate off any coating solvent, polymerize the , and then decompose and evaporate the carrier organics. • Typical curing schedule: –80C for 1 minute on a hotplate. – 250C for 1 minute on a hotplate.

– 425C for 2 hours in a box furnace with 5-10 SCFH of N2 cover gas. • While the electrical characteristics of a SOG film are not the best, the ease of application makes it attractive for many instances where a insulator is required. • SOG films are also particularly useful as dopant sources. – Boron and phosphorous SOG sources are available. – They offer much simpler and safer alternatives to gas source or solid source dopants. – Process recipes usually have to be adjusted to accommodate them.

R. B. Darling / EE-527 / Winter 2013 Oxides of Other

•

–GeO, GeO2: • Unstable at high temperatures • Water soluble! • Arsenide

–GaO, Ga2O3, GaO2, As2O3,AsO2: • A mixture of several oxides with differing properties • Unstable at high temperatures • Oxide grows backwards:

– As and Ga are more volatile than O2, so they diffuse out through the growing oxide, rather than O2 diffusing in. – Oxide is loosely attached to the GaAs surface; it can often be shaken or rubbed off.

• Si and SiO2 are very unique and ideal for microelectronics!

R. B. Darling / EE-527 / Winter 2013 Oxides of Metal Surfaces • Oxidation of some metal surfaces also follows the Deal-Grove model: •Alumina:

– 4Al + 3O2 → 2Al2O3 Because the oxidation process • Titania: involves the diffusion of oxygen –Ti + O→ TiO through the growing oxide layer, 2 2 the process becomes self-limiting • (III) oxide (ferric): (hematite) and the oxidation passivates the – 4Fe + 3O2 → 2Fe2O3 metal surface. • Iron (II) oxide (ferrous): The red, gelatinous rust of iron or –2Fe + O→ 2FeO 2 steel is a hydrated form of ferric • Iron (II,III) oxide: (magnetite) oxide, Fe2O3·H2O. – 3Fe + 2O2 → Fe3O4 • (II) oxide (cupric) (black)

–2Cu + O2 → 2CuO2 • Copper (I) oxide (cuprous) (red) (a semiconductor!) –4Cu + O→ 2Cu O 2 2 R. B. Darling / EE-527 / Winter 2013