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?