Thin Film Silicon Dioxide(primarily oxidation of silicon)

ECE 4752

Thanks to Prof. Brand and Bakir for some of the included slides.

Common Methodologies for Thin Film Silicon Dioxide

• Thermal Silicon Dioxide• Chemical Vapor Deposited SiO2

• Physical Vapor Deposited SiO2

• Sol Gel Deposited SiO2

• Electro-chemical (anodization) SiO2

• Ion Implantation of O2 in Silicon (SIMOX Process)

Oxidation of Si? Thermal Oxidation• Silicon wafer is oxidized in an O2

or water vapor atmosphere at temperatures ranging from 900-1200˚C

• Ability to grow a high quality thermal oxide has propelled Si into the forefront of all semiconductor technologies

• Thermal oxidation consumes silicon: for 1 µm grown oxide, approximately 440 nm of silicon are consumed

• Thermal oxidation generally results in the best quality oxide

Sze, Fig. 11.3

Application of Thermal Oxidation• Thermal oxidation is used for

– Growth of field oxide separating neighboring transistors

– Growth of high quality gate oxide of MOS structure

– Growth of diffusion mask (even though ion implantation is mostly used for pre-deposition today)

• When to use it and when not to use it

– Use when highest quality silicon dioxide layers are required

– Do not use when process temperature is restricted (e.g. after aluminum metallization)

Structure of Thermal SiO2• The form of thermally grown SiO2 is

fused silica, which is an amorphous solid• SiO2 is a glass (similar to window glass)

and can be reflown if heated above its glass transition temperature

• Fused silica consists of silicon atoms sitting at the center of oxygen polyhedra; the polyhedra are randomly connected at bridging oxygen sites, i.e. O atoms that bond to two silicon atoms

• Dry oxides have a larger ratio of bridging to non-bridging sites compared to wet oxides, and are thus “more stable”

• Doping SiO2 with boron or phosphorus (so called network modifiers) reduces the bridging-to-nonbridging ratio, which allows the glass to reflow at lower temperatures, smoothing surface topographies (known as phosphosilicate glass, PSG, and borophosphosilicate glass, BPSG)

Campbell, Fig. 4.7

Structure of Thermal SiO2

(a) Polyhedra (Tetrahedron) Basic Structure of SiO2 (b) Quartz crystal (c) Amorphous SiO2

Horizontal (Oxidation) Furnace• Horizontal furnace is most common furnace type; it comprises four

components: (1) the gas cabinet, (2) the furnace cabinet, (3) the load station and (4) the computer controller

• Resistive furnace heating elements are wound around fused silica tube; wafers are placed vertically in fused quartz boat and inserted into heated furnace tube using cantilevered load system

Sze, Fig. 11.2 http://www.thermcosystems.eu

Vertical (Oxidation) Furnace• Vertical furnace are similar to a horizontal furnace turned on

end; the wafers are pushed from below the furnace up into the tube

• Advantages of vertical furnaces:– Simplified wafer handling by pushing carrier up instead of

using long (transversally loaded) cantilevers; support system can remain in tube during thermal process (long cantilevers tend to warp)

– Uniform spacing between wafers improves process uniformity

– Smaller cleanroom footprint compared to horizontal furnace• As a result, vertical furnaces have become the standard,

especially for large diameter wafer processing (200 and 300mm wafers)

Figure 3.11 Furnaces used for oxidation and diffusion(a) A three-tube horizontal furnace with multizone temperature control(b) Vertical furnace (Courtesy of Crystec, Inc.)

(a)

(b)

R. C. Jaeger 2002

3.1 Dry/Wet Thermal Oxidation

Reaction Chemistry

• Dry Oxidation

• Wet Oxidation

Si(solid) O2 (gas) SiO2

Si(solid) 2H2O (gas) SiO2 2H2

Wet/Dry Oxidation Charts• Easiest way to determine

oxidation parameters (wet/dry, temperature) for a desired oxide thickness

• If pre-oxidized wafer is further oxidized at temperature T, the final oxide thickness is found by– Finding first the time τ

required to grow the initial oxide at the planned T

– Add τ to the intended oxidation time and find resulting overall thickness

Sze, Fig. 11.8

Dry Thermal Oxidation

See also http://www.lelandstanfordjunior.com for an on-line calculator

Sze, Fig. 11.8

Wet Thermal Oxidation

Sze, Fig. 11.8

A <100> silicon wafer has a 2000-Å oxide on its surface

(a) How long did it take to grow this oxide at 1100o C in dry oxygen?

(b)The wafer is put back in the furnace in wet oxygen at 1000o C. How long will it take to grow an additional 3000 Å of oxide?

(a) According to Figure, it would take 2.8 hr to grow 0.2 m oxide in dry oxygen at 1100o C.

(b) The total oxide thickness at the end of the oxidation would be 0.5 m which would require 1.5 hr to grow if there was no oxide on the surface to begin with. However, the wafer “thinks” it has already been in the furnace 0.4 hr. Thus the additional time needed to grow the 0.3 m oxide is 1.5-0.4 = 1.1 hr.

4.2 Deal-Grove Model for Oxidation• Diffusivity of Si in SiO2 is several orders of magnitude smaller than

that of O2; thus, O2 diffuses through the already existing oxide and reacts with Si at the Si-SiO2 interface, further growing the oxide layer

• As a result, the interface at which the SiO2 grows is not exposed to the environment and thus free of impurities

• At room temperature, a native oxide layer with a thickness of ≈ 25 Å grows on top of silicon

• Deal-Grove model is suitable for predicting thermal oxide thicknesses > 30 nm

• Assuming a dry oxidation, the Deal-Grove model considers transport of O2 through the stagnant gas layer to the SiO2 surface, diffusion of O2 through the SiO2 layer and reaction of the O2 with Si at the SiO2-Si interface

Deal-Grove Oxidation Model• Oxygen flux by diffusion of O2

through the stagnant gas layer

or more general

with thickness of stagnant gas layer tsl and mass transport coefficient hg

• Cg is related to the partial pressure of O2 in the furnace via the ideal gas law

Cg oxygen conc. in gas far from waferCs oxygen conc. in gas at wafer surfaceCo oxygen conc. in oxide at wafer surfaceCi oxygen conc. at Si-SiO2 interface

Campbell, Fig. 4.1

J1 DO2,gas

Cg Cs

tsl

J1 hg (Cg Cs)

Cg nV

pg

kT

Deal-Grove Oxidation Model (cont.)• Oxygen flux by diffusion of O2 through the oxide layer

• Oxygen flux associated with oxygen reacting with Si at Si-SiO2 interface

with chemical rate constant ks

• In equilibrium, the three fluxes must be equal, J1 = J2 = J3, resulting in two equations with three unknowns Cs, Co, and Ci

• In addition, Henry’s law states that the concentration of an adsorbed species at a surface of a solid is proportional to the partial pressure of that species in the gas above the solid

with Henry’s gas constant H (not really a constant, but depending on species, surface, T, etc.) as the proportionality constant

J2 DO2,SiO2Co Ci

tox

J3 ks Ci

Co Hpg HkTCs

Deal-Grove Oxidation Model (cont.)• The resulting three equations with three unknowns can be

solved, yielding

• The oxide growth rate is given by the ratio of the oxygen flux J [s-1cm-2] and the number N1 of oxygen molecules per unit volume of SiO2 [cm-3]:

with D = DO2,SiO2; dry oxidation: N1 = 2.2 1022 cm-3 (because O2 diffuses); wet oxidation: N1 = 4.4 1022 cm-3 (because O diffuses)

• The above rate equation is a differential equation for tox; it can be solved assuming the boundary condition

Ci H pg

1ks

hg / HkT

kstox

D

Ci 0 for D 0

Ci Hpg

1ks

hg /HkT

for D

tox(t 0) t0

R dtox

dt

JN1

H ks pg

N1 1ks

hg / HkT

kstox

D

Diffusion limited

Reaction limited

Deal-Grove Oxidation Model (cont.)• The solution of the DE is

• Typically, the coefficients A and B are known for a variety of process conditions; for oxidation at atmospheric pressure, we have ks << h = hg/HkT and the growth rate becomes independent of gas phase mass transport and thus reactor geometry; A and B are proportional to D and thus will follow an Arrhenius function

• The time τ is the time needed to grow the initial oxide thickness t0; thus, the oxidation actually starts at time –τ and at t=0, the oxide thickness is t0

tox2 A tox B(t )

A 2D1ks

1h

B 2DHpg

N1

t0

2 A t0

B

tox A A2 4B(t )

2

Deal-Grove Oxidation Model (cont.)• Thin Oxides: For sufficiently thin oxides, we can neglect the

quadratic term and the oxide thickness becomes

with the linear rate coefficient B/A [µm/hr]• Thick Oxides: For sufficiently thick oxides, we can neglect the

linear term and the oxide thickness becomes

with the parabolic rate coefficient B [µm2/hr]

tox BA

(t )

tox2 B(t )

Campbell, Tab. 4.1

Linear/Parabolic Rate Coefficient

B B/A

Campbell, Figs. 4.2/4.3

Deal Grove Model Rate Constants

Impurity Segregation Coefficient

Masking Diffusions using SiO2

Determining the Thickness of SiO2

1. Color Charts2. Ellipsometry3. Controlled Etching4. Optical Interference

1. based on reflected light intensity spectra

= real part of the oxide refractive index

= difference in the wavelength between the minand max intensity in the fringe pattern spectra

Determining the Thickness of SiO2

Practical Considerations• Metallic Impurity Gettering: Halogen species (Cl, F, etc.) are often

introduced to getter metallic impurities from the tube during an oxidation; this also tends to increase the oxidation rate for thin oxides (linear term). HCl is the safest to use (bubbled into the furnace as described in the diffusion discussion) but is highly corrosive to the gas tubing; trichloroethylene (TCE) and trichloroethane (TCA) are less corrosive but can be toxic. TCA can form phosgene (COCl2) at high temperatures.

• Thick Oxides: The growth of thick oxides is proportional to B and thus to the oxygen partial pressure in the gas pg; thus, the growth rate can be increased by growing the oxide at high pressure.

• Thin Oxides: Most thin (tox ≤ few hundred angstroms) dry oxides grow at a rate faster than Deal-Grove predicts. Corrections can be made to work down to ~ 300 Å, but modern MOS gate oxides are < 100 Å.

• Orientation Dependence: Oxidation rate depends on orientation. MOS uses <100> oriented wafers because it has the fewest atoms per cm2, results in lower interface state density.

•Isolation technology in MOS processes

•Provides isolation between nearby devices

•Fully recessed process attempts to minimize bird’s beak

Local Oxidation of Silicon(LOCOS)

R. C. Jaeger 2002

Local Oxidation of Silicon(LOCOS)