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Oxidation

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Thermal SiO2Properties

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Thermal SiO2Properties (cont.)

(7) Amorphous material3

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Oxidation Techniques

Thermal Oxidation

Rapid Thermal Oxidation

Oxidation Process

Thermal Oxidation Techniques

Wet Oxidation

Si (solid) + H2

0 SiO2

(solid) + 2H2

Dry Oxidation

Si (solid) + O2(gas) SiO2(solid)

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Conceptual Si Oxidation System

Thermal Oxidation

Heat is added to the oxidation tube during the reaction..between oxidants and silicon

- 900-1,200C temperature range- Oxide growth rate increases as a result of heat

Used to grow oxides between 60-10,000

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Dry Thermal Oxidation Process

Thin Oxide GrowthThin oxides grown (

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Wet Thermal Oxidation

Wet Thermal Oxidation Characteristics

Oxidant is water vapor

Fast oxidation rate- Oxide growth rate is 1000-1200 / hour

Preferred oxidation process for growth of

thick oxides

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The goal of oxidation is to grow a high quality oxidelayer on a silicon substrate

Goal of Oxidation Process

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Passivation

Physically protects wafers from scratches and particle..contamination

Traps mobile ions in oxide layer

The oxide layer isolates these ions from the silicon andprevents them from disrupting the performance of thedevice.

Functions of Oxide Layers (1)

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Masking: A layer of oxide grown on wafer prior to the diffusion andion implantation process steps will act as mask.

Function of Oxide Layers (2)

SiO2

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Dielectric Material Insulating material between metal layers

- Field Oxide


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Dielectric Material

Tunneling oxide- Allows electrons to pass through oxide without

resistance


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f

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Functions and Thickness of xideLayers

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Oxidation occurs in tube furnace

- Vertical Tube Furnace- Horizontal Tube Furnace

Thermal Oxidation Equipment

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Bubbler

Wet Thermal OxidationTechniques

A glass flask, referred toas a bubbler contains

deionized water and isattached to the oxidationtube.The water is heated (90-

99 C), and water vaporforms above the deionizedwater level.

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Bubbler A carrier gas, nitrogen, is bubbled through the

deionized water. As it passes through the vapor itbecomes saturated with water. The vapor travels intothe oxidation tube, where with additional heating itturns into steam and oxidation occurs.

A consistent oxide growth rate is hard to maintain withthe bubbler method because of the difficulties involvedin controlling both the amount of water vapor entering

the oxidation tube and the temperature of the water.The risk of contamination is also high.

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Flash System


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Dryox System


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Thickness of Si consumedduring oxidation

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Linear Parabolic Model

Linear (first) Stage of Oxidation- Chemical reaction between silicon and oxidants atwafer surface

- Reaction limited by number of silicon atomsavailable to react with oxidants- During the first 500 of oxide growth, the oxidegrows linearly with time

- Growth rate begins to slow down as oxide layergrows

Oxide Growth Mechanism (1)

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Parabolic Stage- Begins when 1,000 of oxide has been grown onsilicon- Silicon atoms are no longer exposed directly to

oxidants- Oxidants diffuse through oxide to reach silicon

The oxidation of silicon during this stage occurs at thesilicon/silicon dioxide interface.

As oxidation continues, the silicon dioxide layer thickens,

and the distance the oxidants must travel to reach thesilicon increases.- Reaction limited by diffusion rate of oxidant

Oxide Growth Mechanism (2)

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D l G M d l (1)

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Deal-Grove Model (1)

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Deal-Grove Model (1) F1 - Gas transport from the bulk CGO2concentration in the gas (bulk)

Cs - O2concentration in the gas-SiO2interface C0 - O2concentration in the SiO2surface (in the solid) Ci - O2concentration in the Si-SiO2interface At equillibrium we will have F1=F2= F3 Temperature Range-700-1300 celcius. Pressure- 0.2-1.0 atm Oxide Thicknes-300-20000 Angstrom. F1in the linear approximation can be assumed to be

proportional to the concentration difference between bulkgas and surface.

F2the diffussion flux through the oxide layer, will also bewritten as a difference equation.

F3 is the chemical reaction flux = (rate)X(concentration)

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Factors that Affect

Oxidation

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High Doping concentrationeffect

When high concentrations of dopants are present in siliconwafers, they tend to increase the oxide growth rate.

- During Linear Stage of oxidation N-type dopantsincrease growth rate.

For example, throughout the Linear Stage of oxidation,doped phosphorous continually moves from within thesilicon to the silicon surface of the wafer. This constantsupply of phosphorous on the silicon surface increases the

oxide growth rate.Dopants cause differential oxidation

- Results in the formation of steps- Affects etching process

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effect

Once the Parabolic Stage begins, the presence of phosphorous at thesilicon/oxide interface no longer impacts the growth rate. Instead, it is the

presence of P-type dopants in the silicon dioxide layer that influences thegrowth rate.During the Parabolic Stage of oxidation a dopant, such as boron, movesfrom the silicon into the silicon dioxide layer where it weakens the bondstructures. The weakened bonds allow oxygen and water to diffuse fasterthrough the silicon dioxide resulting in a faster oxide growth rate.Since oxide grows faster in doped regions of the wafer, the oxide will notbe of uniform thickness across the wafer.This results in differential oxidation and produces unwanted steps on thewafer as oxidation occurs at unequal rates across the wafer. As thethicker oxide layers consume more silicon, steps are formed which may

affect the operation of the device.The variation in oxide thickness across the wafer must also be taken intoconsideration during future etching. The etch process must be designedto remove the thicker oxide layers, without overetching the areas withthinner oxide layers.

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Chlorine species- Anhydrous chloride (CI2)- Anhydrous hydrogen chloride (HCI)- Trichloroethylene TCE- Trichloroethane TCA

Oxide growth rate increases

Oxide cleaner

Device performance is improved

Chlorine added with Oxidants

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Oxidation With Cl Containing Gas

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Effect of HCl on Oxidation Rate

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Oxidation Techniques and System

1. Pre-oxidation Cleaning.

2. Dry, Wet Oxidation.

3. High Pressure Oxidation.

4. Plasma Oxidation.

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Pre-oxidation Cleaning

Wafer must be cleaned to eliminate bothorganic & inorganic contamination.

Chemical cleaning involves removing organiccontaminates, followed by inorganic ion andatom removal.

Common cleaning procedure-

water+H2O2+NH4OH For removing heavy metals:-water+H2O2+HCl

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Dry, Wet Oxidation Dry Oxidation- With O2 or HCl.

Wet Oxidation- using H2 and O2 to formwater vapor and then oxidation.

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Atmospheric pressure

- Slow oxide growth rate

An increase in pressure increase oxide growthrate

Increasing pressure allows temperature to be..decreased

- Oxide growth rate remains the same

- For every 10atm of pressure the temperaturecan be reduced 30C

Effect of High Pressure Oxidation

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Hi h P O id ti

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High Pressure Oxidation

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The graphs illustrate both pressure and crystal orientation effects.The graph on the right is for a reaction time of 1 hour., the oxide

thickness increases with both pressure and temperature.

Dopant Redistribution During Thermal

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Dopant Redistribution During ThermalOxidation

Dopant concentration Dopant concentration

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Dopant Redistribution During Thermal

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Oxidation affects the electrical performance of the semiconductor device

as well.N-type dopants have a higher solubility in silicon (Si) than in silicondioxide (SiO2). Therefore as the silicon dioxide (SiO2) layer grows duringoxidation, the N-type dopants (phosphorous, arsenic, antimony) moveinto the silicon (Si) layer and away from the oxide layer. This results in a

higher concentration level of N-dopants in the silicon (Si) layer, and abuild up of N-dopants between the silicon (Si) and silicon dioxide (SiO2)layer.

On the other hand, P -type dopants (boron (B)) are drawn into the silicondioxide layer and actually deplete the silicon (Si) layer of P-dopants.

Since the location and concentrations of dopants can affect theperformance of a device, the movement of dopants during oxidation mustbe monitored.

Upon completion of the oxidation process, wafers undergo inspectiontechniques that evaluate the location and concentration of dopants toensure that the functioning of the device will not be disrupted.

Dopant Redistribution During ThermalOxidation

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Oxide inspection techniques

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Oxide inspection techniques Surface inspection:-A high-intensity ultraviolet (UV) light is used to

evaluate the surface for stains, particulates, and irregularities. Oxide thickness:-The techniques that determine oxide thickness

use scanning electron microscopes (SEMs), fringe counting,interference, stylus apparatus, ellipsometers, and colorcomparison. The wafer is also tested for the number of mobileionic contaminants (minimal number desired), dielectric strength(non- conducting property of the oxide), and refraction of the oxide(which tests for impurities). Oxide thickness is one of the most

important inspection steps in wafer fabrication. Oxide cleanliness:- Capacitance-voltage techniques are used to

detect the total number of mobile ionic contaminates present in theoxide. A low level of ionic contaminates indicates that the entireoxidation system is clean. If the level of ionic contaminates is high,then technicians must determine where the contamination is

coming from and inspect the tubes, gases, and wafers as well asthe cleaning process. Dielectric strength, and index refractionvalues both are indicators of oxide cleanliness.

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Oxides as Dopant Masks

SiO2can provide a selective mask against diffusionat high temperatures. Oxides used for masking are ~ 0.5-1 mm thick.

Dopants Diffusion Constants at 1100 oC (cm2/s)

B 3.4 10-172.0 10-14

Ga 5.3

10-11

P 2.9 10-162.0 10-13

As 1.2 10-163.5 10-15

Sb 9.9 10-1748

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Oxide Charge Locations

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Oxide Charge Definitions

1. Interface trapped charge (Qit): located atSi/SiO2interface

2. Fixed oxide charge (Qf): positive charge located

within 3nm of Si/SiO2interface3. Oxide trapped charges (Qot): associated with

defects in the SiO2

4. Mobile ionic charges (Qm): result fromcontamination from Na or other alkali ions

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M th d f i thi k f

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Methods for measuring thickness ofoxide layer

1. Profilometry.

2. Ellipsometry.

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15771 oxidation

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