4. Thermal Oxidationa) Equipment – Atmospheric Furnace

Oxidation requires precise control of:

• temperature, T

• ambient gas, G

• time spent at any given T & G, t

Vito Logiudice 34

4. Thermal Oxidationb) Mechanism

Reaction of silicon and oxygen to form silicon dioxide:

Vito Logiudice 35

Dry oxidation:

Wet oxidation:

• For every angstrom of oxide grown, 0.45 angstrom of silicon is consumed.

• This reaction occurs even at room temperature, yielding approximately 20 to 30Ang of “dirty oxide”.

• Elevated growth temperatures are needed to create higher quality, thicker oxides.

4. Thermal Oxidation

• When oxygen atoms initially come into contact with a bare Si surface, the Si & O2 atoms can react readily.

• Thus initially, oxide growth occurs in a linear fashion and the oxidation mechanism is said to be reaction rate limited.

• Linear oxidation occurs to an SiO2 thickness of approximately 500 Ang.

• Oxide growth occurs at the silicon to oxide interface, thus oxygen atoms must diffuse through the growing oxide down to the Si/SiO2 interface before being able to react with the underlying silicon.

• Past ~500Ang of oxide, the reaction rate becomes limited by the diffusion rate of the oxygen atoms through the growing oxide layer and at this point, the growth mechanism is said to be diffusion rate limited.

Vito Logiudice 36

4. Thermal Oxidation• Oxide growth therefore

goes through an initial linear oxidation to about 500Ang, followed by a parabolic growth stage:

• The differing thick vs. thin oxide growth rates should be taken into consideration when oxidizing, etching, and reoxidizing silicon substrates successively.

Vito Logiudice 37

4. Thermal Oxidation

Vito Logiudice 38

• Due to this parabolic growth, thick oxides take very long to grow. In order to reduce these times, a combination of higher temperatures and wet ambients (ie.,steam) rather than dry (ie., straight oxygen) can be used during oxidation.

• An additional consideration when designing process flows: Silicon wafer orientation affects the oxidation rate.

Dry oxidation

Wet oxidation

4. Thermal Oxidationc) Wet Oxidation Recipe – example (~1000Ang SiO2)

STEP DESCRIPTION DUR. TEMP. N2 O2 H2

hh:mm:ss Celsius sccm sccm sccm

00 STANDBY n/a 625 4000 250 0

01 PUSH-IN (2inch/min) 625 4000 250 0

02 STABILIZATION 00:10:00 625 4000 250 0

03 TEMPERATURE RAMP UP (10C/min) 00:30:00 625 ->

925 4000 250 0

06 STABILIZATION 00 :10 :00 925 4000 250 0

06 WET OXIDATION 00:29:18 925 0 1750 3000

07 TEMPERATURE RAMP DOWN (10C/min) 00:30:00 925->

625 4000 0 0

08 PULL OUT (2inch/min) 625 4000 0 0

09 COOL DOWN 00:10:00 625 4000 0 0

Vito Logiudice 39

5. Photolithography

Negative Resist Chemistry

Positive Resist Chemistry

Vito Logiudice 40

a) Photoresist – Fundamental Properties

PR’s are designed to be sensitive to different exposure wavelengths

5. Photolithographya) Positive & Negative Photoresists

Vito Logiudice 41

Negative Resists:

• Very sensitive, therefore short exposure times• High etch resistance• Adhesion to substrate may be an issue• Swelling during development (low resolution)• Not expensive

Positive Resists:

• Less sensistive, therefore longer exposure times• High etch resistance• Excellent adhesion possible via adhesion promoters• No swelling during development = better resolution• Expensive

5. Photolithography

Vito Logiudice 42

b) Photoresist Application

Static Dispense ProcessCritical process parameters (repeatability): • dispense quantity• spin acceleration• spin speed (target)• spin duration• pre-exposure bake• post-exposure “hard” bake (PEB)

Critical environmental parameters:• temperature• humidity• stability of these over time

Potential Issues

5. Photolithography

Vito Logiudice 43

OR

Newer coat techniques aimed at improving uniformity

Apply PR while chuck being spun at low RPM’s (Dynamic Dispense)

Apply PR across wafer prior to spin, rather than applying in center only (Moving Arm Dispense)

5. Photolithographyc) Exposure – Contact / Proximity Alignment

Alignment (theta, x, y) Contact prior to exposure

NOTE: Some aligners allow for “soft” (ie., limited) physical contact between mask and wafer in the case of Proximity aligners.

Contact aligners; issues with: • Defect densities• Mask damage• Mask cost• Resolution accuracyacross wafer

Proximity alignment addresses defect and damage issues, but resolution problems remain and may in fact be greater due to light scattering between mask and wafer during exposure.

Vito Logiudice 44

5. Photolithographyd) EVG 620 Front to Back Aligner (McGill Fab)

Vito Logiudice 45

5. Photolithographye) Exposure – Projection Alignment / Steppers

Key features: • Cheaper Mask/Reticle cost since can have 5 to 10 times reduction of minimum feature size via use of reduction lenses.• Excellent resolution across wafer• Production systems developed with automatic alignment capabilities• Excellent (ie., low) defect densites

Vito Logiudice 46

5. Photolithographyf) Canon Mark III G-Line Stepper (McGill Fab)

Vito Logiudice 47

6. Wet Etchinga) Chemistry

– Etch processes rely on:• The transport of reactants to the surface• The surface reaction itself• The transport of reaction by-products from the surface

– Key ingredients:• Oxidizer to oxidize the surface being etched (eg.: H2O2, HNO3)• Acid or base to dissolve the resulting oxidized surface (eg.: H2SO4, NH4OH)• Transport media for reactants & by-products (eg.: H2O, CH3COOH)

– Process parameters affecting etch results:• Solution:

• temperature• filtering• agitation/stirring• strength

• Time to rinse & rinse efficiency• Surface wetting & bubbles

Vito Logiudice 48

6. Wet Etchingb) Isotropic Etching

• Vertical and lateral etch rates are identical.

• Undercutting should be taken into consideration.

• Orientation of the mask edge to the crystal planes does not affect etch rates

• Example: HF-based etching of silicon dioxide

Wet Etching - Isotropic

Vito Logiudice 49

• Etch rates depend on crystal orientation.

• The orientation of a given mask edge to the crystal axes will determine whether the lateral or vertical etch rate is dominant.

• The orientation of a mask edge and the mask pattern itself will ultimately determine the shape of the etched structure.

• Example: silicon etching with KOH or EDP solutions.

•The creation of very complex shapes is possible –refer to Madou – Chp.4 for more insights.

6. Wet Etchingc) Anisotropic Etching

Top view Side view along

indicated section

Recall - different crystal planes etch at different rates:

(100) fastest

(110)

(111) slowest

Vito Logiudice 50

6. Wet Etchingc) Anisotropic Etching (KOH based)

M.J.Madou

Si (100) etch rate (line) & nonuniformity (column)

1hr etch time (thin line) & for 60um etch depth (thick line)

Vito Logiudice 51

6. Wet Etchingc) Anisotropic Etching (solutions, selectivity)

M.J.Madou

Vito Logiudice 52