Farshid Karbassian

OxidationOxidation

• Oxide Layer Applications• Types of Oxidation

• Dry Oxidation• Wet Oxidation

• Modeling • C-V Measurement

OutlineOutline

2

Oxide Layer ApplicationsOxide Layer Applications

Name of the Oxide Thickness (Å) Application Time in

ApplicationNative 15-20 Undesirable -

Screen ~200 Implantation Mid 70s to present

Masking ~5000 Diffusion 1960s to mid 70s

Field & LOCOS 3000-5000 Isolation 1960s to 90s

Pad 100-200 1960s to present

Sacrificial <1000 1970s to present

Gate 30-120 1960s to present

Barrier 100-200 STI 1980s to present

Nitride stress buffer

Defect removal

Gate dielectric

Oxide Applications: Native Oxide

Purpose: This oxide is a contaminant and generally undesirable. Sometimes used in memory storage or film passivation.

Comments: Growth of native oxide layer at room temperature takes 3-4 hours up to about 12 Å.

p+ Silicon substrate

Silicon dioxide (oxide)

4

Oxide Applications: Gate Oxide

Purpose: Serves as a dielectric between the gate and source-drain parts of MOS transistor.

Comments: Common gate oxide film thickness range from about 30 Å to 50 Å. Dry oxidation is the preferred method.

Gate oxide

Transistor site

p+ Silicon substrate

Source Drain

Gate

5

Oxide Applications: Field Oxide

Purpose: Serves as an isolation barrier between individual transistors to isolate them from each other.

Comments: Field oxide thickness ranges from 2,500 Å to 15,000 Å. Wet oxidation is the preferred method.

Field oxide

Transistor site

p+ Silicon substrate

6

Oxide Applications: Barrier Oxide

Purpose: Protect active devices and silicon from follow-on processing.

Comments: Deposition to several hundred Angstroms thickness.

Barrier oxide

Diffused resistors

Metal

p+ Silicon substrate

7

Oxide Applications: Pad Oxide

Purpose: Provides stress reduction for Si3N4

Comments: Very thin layer of oxide is deposited.

Passivation Layer

ILD-4

ILD-5

M-3

M-4

Pad oxide

Bonding pad metal

Nitride

8

Oxide Applications: Implant Screen Oxide

Purpose: Sometimes referred to as “sacrificial oxide”, screen oxide, is used to reduce implant channeling and damage. Assists creation of shallow junctions.

Comments: Thermally grown

Ion implantation Screen oxide

High damage to upper Si surface + more channeling

Low damage to upper Si surface + less channeling

p+ Silicon substrate

9

Passivation layer

ILD-4

ILD-5

M-3

M-4

Interlayer oxide

Bonding pad metal

Oxide Applications: Insulating Layer between Metals

Purpose: Serves as protective layer between metal lines.

Comments: Deposition

10

Cross section of LOCOS field oxide(Actual growth of oxide is omnidirectional)

1. Nitride deposition

Pad oxide(initial oxide)

Silicon

2. Nitride mask & etch

Silicon

SiliconNitride

3. Local oxidation of silicon

SiO2 growth

Silicon

4. Nitride strip

Silicon

SiO2

SiO2

Nitride

Silicon

LOCOS ProcessLOCOS Process

11

Silicon oxynitride

Nitride oxidation maskBird’s beak region

Selective oxidation

Pad oxide

Silicon substrate

Silicon dioxideSilicon dioxide

Selective Oxidation and Bird’s Beak Effect

Selective Oxidation and Bird’s Beak Effect

12

Cross section of shallow trench isolation (STI)

Silicon

Trench filled with deposited oxide

Sidewall liner

1. Nitride deposition

Pad oxide(initial oxide)

Silicon

2. Trench mask and etch

Silicon

SiliconNitride

4. Oxide planarization (CMP)

Silicon

5. Nitride strip

Oxide

3. Sidewall oxidation and trench fill

Oxide over nitride

Silicon

STI IsolationSTI Isolation

13

• Crystallization of silicon dioxide is very undesirable, since it is not uniform and crystal boundaries provide easy paths for impurities and moisture.

Therefore, pre-oxidation wafer cleaning is performed to eliminate crystallization.

Pre-oxidation CleaningPre-oxidation Cleaning

14

• Pre-oxidation cleaning is performed to remove particles, organic and inorganic contaminants, native oxide and surface defects.

Pre-oxidation CleaningPre-oxidation Cleaning

15

• RCA Standard Cleaning I (SC-1)

NH4OH:H2O2:H2O 1:1:5 – 1:2:7 (70-80OC)

• DI water• RCA Standard Cleaning II (SC-2)

HCl:H2O2:H2O 1:1:6 – 1:2:8 (70-80OC)

• DI water

RCA CleaningRCA Cleaning

RCA: Radio Corporation of America Chuckto vacuum pump

SpindleWafer

• When wafers are submerged in RCA I solution, particles and organic contaminants oxidize, and their byproducts are either gaseous (e.g. CO), or soluble in the solution (e.g. H2O).

• In RCA II, H2O2 oxidizes the inorganic contaminants and HCl reacts with the oxides to form soluble chlorides, which allows desorption of contaminants from the wafer surface.

RCA CleaningRCA Cleaning

17

• Native oxide on Si is of poor quality and needs to be stripped, especially for the gate oxide which requires the highest quality.

• This is performed either in HF:H2O solution or in HF vapor etcher.

• After native oxide stripping, some F atoms bind with Si atoms and form Si-F bonds on the silicon surface.

HF etchingHF etching

18

Wet Clean• Chemicals• % solution• Temperature• Time

Oxidation Furnace• O2, H2 , N2 , Cl• Flow rate• Exhaust• Temperature• Temperature profile• Time

Inspection• Film thickness• Uniformity• Particles• Defects

Thermal Oxidation Process Flow Chart

19

• Depending on the quality and thickness which is required for the oxide layer, wet or dry oxidation may be used.

• Former is faster, but latter is cleaner and makes a better interface.

Thermal OxidationThermal Oxidation

20

Dry OxidationDry Oxidation

• In dry oxidation, pure oxygen gas (5s at least) is used. At high temp. O2 molecules diffuse across an existing oxide layer to reach the Si/SiO2

interface.

22 SiO O Si

21

Dry OxidationDry Oxidation

O2

Pur

ge N

2

HC

l

Pro

cess

N2

Control Valves

Regulator

Gas Cylinders

MFCs

coil

Process Tube

Exhaust System

Flat Zone

distance

Temp.

22

Si

SiO2

O, O2

Oxide-silicon interface

Oxygen-oxide interface

Oxygen supplied to reaction surface

Diffusion of Oxygen Through Oxide Layer

Diffusion of Oxygen Through Oxide Layer

23

TEM image of Si/SiO2

Horizontal Diffusion FurnaceHorizontal Diffusion Furnace

24

Vertical Diffusion FurnaceVertical Diffusion Furnace

25

Horizontal and Vertical FurnaceHorizontal and Vertical FurnacePerformance

FactorPerformance

ObjectiveHorizontal Furnace Vertical Furnace

Typical waferloading size

Small, for processflexibility

200 wafers/batch 100 wafers/batch

Clean roomfootprint

Small, to use lessspace

Larger, but has 4 processtubes

Smaller (single processtube)

Parallel processing

Ideal for processflexibility

Not capable Capable ofloading/unloading wafersduring process, whichincreases throughput

Gas flowdynamics (GFD)

Optimize foruniformity

Worse due to paddle andboat hardware. Bouyancyand gravity effects causenon-uniform radial gasdistribution.

Superior GFD andsymmetric/uniform gasdistribution

Boat rotation forimproved filmuniformity

Ideal condition Impossible to design Easy to include

Temperaturegradient acrosswafer

Ideally small Large, due to radiantshadow of paddle

Small

Particle controlduringloading/unloading

Minimum particles Relatively poor Improved particle controlfrom top-down loadingscheme

Quartz changeEasily done in shorttime

More involved and slow Easier and quicker, leadingto reduced downtime

Wafer loadingtechnique

Ideally automated Difficult to automate in asuccessful fashion

Easily automated withrobotics

Pre-and post-process control offurnace ambient

Control is desirable Relatively difficult tocontrol

Excellent control, withoptions of either vacuum orneutral ambient

26

Vertical Furnace Process Tube

Heater 1

Heater 2

Heater 3

Thermocouple measurements

Temperaturecontroller

Profile TCs

Control T

Cs

Overtem

perature TC

s

System controller

TC

27

Silicon

Dangling Bond

Si

Si-SiO2 Interface

Oxygen Interface State Charge (Positive)

SiO2

Si/SiO2 InterfaceSi/SiO2 Interface

28

tt 0.55t0.55t0.45t0.45t

Before oxidation After oxidation

Consumption of Silicon during Oxidation

Consumption of Silicon during Oxidation

29

• At high temp. H2O dissociates and form hydroxide, HO, which can diffuses in the SiO2 layer faster than O2 .

• A wet oxidation system may have a boiler or a bubbler or maybe it is a pyrogenic steam system, which is more common.

Wet OxidationWet Oxidation

O2H O H2 222

222 2H SiO Si OH2 30

O2

Pur

ge

N2

H2

Pro

cess

N2

Control Valves

Regulator

Gas Cylinders

MFCs

coil

Process Tube

Burn box

Exhaust

Wet Oxidation SystemWet Oxidation System

Pyrogenic Steam System

31

Heater

Water

N2 bubbles

Process tube Exhaust

Heated gas line

MFCN2 + H2O

N2

Bubbler System

Wet Oxidation SystemWet Oxidation System

32

Dry Oxidation Vs. Wet OxidationDry Oxidation Vs. Wet Oxidation

Dry oxidation Wet oxidation

33

)(22

1

02

tCDC

xD

x

01020 2/)/2( DCCDdd

B = 2 D C0 / C1A = 2 D /κ

x = [B t + 0.25 A2 + d02 + A d0]0.5 – A / 2

x2 + A x = B(t + τ) ;

τ = time for initial oxide thickness d0

Deal/Grove (Kinetic) ModelDeal/Grove (Kinetic) ModelAssumptions:

Temperature: 700 - 1300 oC Pressure: 0.2 - 1.0 atm SiO2 thickness: 0.03 - 2 μm

34

•Color chart

Oxide MeasurementOxide Measurement

35

• C-V Measurement

Metal Platform

Silicon

AluminumOxide

Heater Heater

Capacitor Meter

Large Resistor

Oxide MeasurementOxide Measurement

36

Any questions?