EBB 323 Semiconductor Fabrication Technology Oxidation Dr Khairunisak Abdul Razak Room 2.03 School of Material and Mineral Resources Engineering Universiti Sains Malaysia khairunisak@eng.usm.my

Outcomes By the end of this topic, students should be able to: List principle uses of silicon dioxide (SiO2) layer in silicon devices Describe the mechanism of thermal oxidation Draw a flow diagram of a typical oxidation process Describe the relationship of process time, pressure, and temperature on the thickness of a thermally grown SiO2 layer Explain the kinetics of oxidation process Describe the principle uses of rapid thermal, high pressure and anodic oxidation

Uses of dielectric films in Semiconductor technology

Principle uses of Si dioxide (SiO2) layer in Si wafer devices What is oxidation? Formation of oxide layer on wafer High temperature O2 environment Principle uses of Si dioxide (SiO2) layer in Si wafer devices Surface passivation Doping barrier Surface dielectric Device dielectric

1. Surface passivation SiO2 layer protect semiconductor devices from contamination: Physical protection of the sample and underlying devices Dense and hard SiO2 layer act as contamination barrier Hardness of the SiO2 layer protect the surface from scratches during fabrication process SiO2 passivation layer Si Si

Cont.. ii. Chemical in nature Avoid contamination from electrically active contaminants (mobile ionic contaminants) of the electrically active surface e.g. early days, MOS device fabrication was performed on oxidised Si remove oxide layer to get rid of the unwanted ionic contamination surface before further processing

2. Doping barrier In doping  need to create holes in a surface layer in which specific dopants are introduced into the exposed wafer surface through diffusion or ion implantation SiO2 on Si wafer block the dopants from reaching Si surface All dopants have slower rate of movement in SiO2 compared to Si Relatively thin layer of SiO2 is required to block the dopants from reaching SiO2

Cont.. SiO2 possesses a similar thermal expansion coefficient with Si At high temperature oxidation process, diffusion doping etc, wafer expands and contracts when it is heated and cooled  close thermal expansion coefficient, wafer does not warp Dopants Si SiO2 layer as dopant barrier

3. Surface dielectric SiO2 is a dielectric  does not conduct electricity under normal circumstances SiO2 layer prevents shorting of metal layer to underlying metal Oxide layer MUST BE continuous; no holes or voids Thick enough to prevent induction If too thin SiO2 layer, electrical charge in metal layer cause a build-up charge in the wafer surface  cause shorting!! Thick enough oxide layer to avoid induction called ‘field oxide’

Metal layer Oxide layer Wafer Dielectric use of SiO2 layer source Drain S D MOS gate Field oxide

4. Device dielectric In MOS application Grow thin layer SiO2 in the gate region Oxide function as dielectric in which the thickness is chosen specifically to allow induction of a charge in the gate region under the oxide Thermally grown oxides is also used as dielectric layer in capacitors Between Si wafer and conduction layer

Types of oxidation Thermal oxidation High pressure oxidation Anodic oxidation

Device oxide thicknesses Most applications of semiconductor are dependent on their oxide thicknesses Silicon dioxide thickness, Å Applications 60-100 Tunneling gates 150-500 Gates oxides, capacitor dielectrics 200-500 LOCOS pad oxide 2000-5000 Masking oxides, surface passivation 3000-10000 Field oxides

Thermal oxidation mechanisms Chemical reaction of thermal oxide growth Si (solid) + O2 (gas)  SiO2 (solid)  Oxidation temperature 900-1200C Oxidation: Si wafer  placed in a heated chamber  exposed to oxygen gas

SiO2 growth stages Si wafer Initial In a furnace with O2 gas environment Oxygen atoms combine readily with Si atoms Linear- oxide grows in equal amounts for each time Around 500Å thick Si wafer Linear Above 500Å, in order for oxide layer to keep growing, oxygen and Si atoms must be in contact SiO2 layer separate the oxygen in the chamber from the wafer surface Si must migrate through the grown oxide layer to the oxygen in the vapor oxygen must migrate to the wafer surface Si wafer Parabolic

Three dimension view of SiO2 growth by thermal oxidation SiO2 surface Original SiO2 surface SiO2 Si substrate

Parabolic relationship of SiO2 growth parameters Linear oxidation Parabolic oxidation of silicon where X = oxide thickness, B = parabolic rate constant, B/A = linear rate constant, t = oxidation time Parabolic relationship of SiO2 growth parameters where R = SiO2 growth rate, X = oxide thickness, t = oxidation time

Cont.. Implication of parabolic relationship: Thicker oxides need longer time to grow than thinner oxides 2000Å, 1200C in dry O2 = 6 minutes 4000Å, 1200C in dry O2 = 220 minutes (36 times longer) Long oxidation time required: Dry O2 Low temperature

Dependence of silicon oxidation rate constants on temperature

Oxide thickness vs oxidation time for silicon oxidation in dry oxygen at various temperatures

Oxide thickness vs oxidation time for silicon oxidation in pyrogenic steam (~ 640 Torr) at various temperatures

Kinetics of growth Si is oxidised by oxygen or steam at high temperature according to the following chemical reactions: Si (solid) + O2 (gas)  SiO2 (solid) (dry oxidation) Or Si (solid) + 2H2O (gas)  SiO2 (solid) + 2H2(gas) (wet oxidation) Also called steam oxidation, wet oxidation, pyrogenic steam Faster oxidation – OH- hydroxyl ions diffuses faster in oxide layer than dry oxygen 2H2 on the right side of the equation shows H2 are trapped in SiO2 layer Layer less dense than oxide layer in dry oxidation Can be eliminated by heat treatment in an inert atmosphere e.g. N2

2 mechanisms influence the growth rate of the oxide Actual chemical reaction rate between Si and O2 Diffusion rate of the oxidising species through an already grown oxide layer No or little oxide on Si the oxidising agent easily reach the Si surface Factor that determine the growth rate is kinetics of the silicon-oxide chemical reaction Si is already covered by a sufficiently thick layer of oxide Oxidation process is mass-transport limited Diffusion rate of O2 and H2O through the oxide limit the growth rate is diffusion of O2 and H2O through the oxide A steam ambient is preferred for the growth of thick oxides:H2O molecules smaller than O2 thus, easier diffuse through SiO2 that cause high oxidation rates

Si oxidation Oxygen concentration profile during oxidation

Mass transport of O2 molecules from gas ambient towards the Si through a layer of already grown oxide Flux of O2 molecules is proportional to the differential in O2 concentration between the ambient (C*) and oxide surface (C0) Where h is the mass transport coefficient for O2 in the gas phase Diffusion of O2 through the oxide is proportional to the difference of oxygen concentration between the oxide surface and the Si/SiO2 interface. The flux of oxygen through the oxide, F2 becomes Where, Ci = oxygen concentration at theSi/SiO2 interface D = diffusion coefficient of O2 or H2O in oxide tox = oxide thickness

Kinetics of the chemical reaction between silicon and oxygen is characterised by reaction constant, k: In steady state, all flux terms are equal: F1 = F2 = F3. Eliminating C0 from the flux equations, we can obtain:

If N0x is a constant representing the number of oxidising gas molecules necessary to grow a unit thickness of oxide, one can write: The solution to this differential equation is:

If tox=0 when t=0, th eintegration yields: Or Defining new constant A and B in terms of D, ks, Nox and C*: We can obtain: From which we find tox :

Which is linear with time.  is introduced to take into account the possible presence of an oxide layer on the Si before thermal oxide growth being carry out Oxide layer can be a native oxide layer due to oxidation of bare Si by ambient air or thermally grown oxide produced during a prior oxidation step =0 if the thickness of the initial oxide is equal to zero When thin oxides are formed the growth rate is limited by the kinetics of chemical reaction between Si and O2. Eq. 5.12 becomes: Which is linear with time. The ratio is called “linear growth coefficient”, and is dependent on crystal orientation of Si

The parabolic growth coefficient can be increased: When thick oxides are formed, the growth rate is limited by the diffusion rate of oxygen through the oxide. Eq 5.12 becomes: The coefficient B is called “parabolic growth coefficient” and is independent on crystal orientation of Si. The parabolic growth coefficient can be increased: Increase the pressure of the ambient oxygen up to 10-20 atm (high pressure oxidation) The linear growth coefficient can be increased: Si consists of high concentration of impurities e.g. phosphorous: increase point defects in the crystal which increase the oxidation reaction rate at the Si/SiO2 interface Oxidation process also generate point defects in Si which enhance diffusion of dopants. Some dopants diffuse faster when annealed in oxidising ambient than in neutral gas such at N2

Oxidation rate Controlled by: Wafer orientation Wafer dopant Impurities Oxidation of polysilicon layers Large no of atoms allows faster oxide growth <111> plane have more Si atoms than <100> plane Faster oxide growth in <111> Si More obvious in linear growth stage and at low temperature

Crystal structure of silicon <100> plane <111> plane

Dependence of oxidation linear rate constant and oxide fixed charge density on silicon orientation

Wafer dopant(s) distribution Oxidised Si surface always has dopants; N-type or P-type Dopant may also present on the Si surface from diffusion or ion implantation Oxidation growth rate is influenced by dopant element used and their concentration e.g. Phosphorus-doped oxide: less dense and etch faster Higher doped region oxidise faster than lesser doped region e.g. high P doping can oxidise 2-5 times the undoped oxidation region Doping induced oxidation effects are more obvious in the linear stage oxidation

Schematic illustration of dopant distribution as a function of position is the SiO2/Si structure indicating the redistribution and segregation of dopants during silicon thermal oxidation

Distribution of dopant atoms in Si after oxidation is completed During thermal oxidation, oxide layer grows down into Si wafer- behavior depends on conductivity type of dopant N-type: higher solubility in Si than SiO2, move down to wafer. Interface consists of high concentration N-type doping P-type: opposite effect occurs e.g Boron doping in Si move to SiO2 surface causes B pile up in SiO2 layer and depletion in Si wafer  change electrical properties

Oxide impurities Certain impurities may influence oxidation rate e.g. chlorine from HCl from oxidation atmosphere  increase growth rate 1-5%

Oxidation of polysilicon Oxidation of polysilicon is essential for polysilicon conductors and gates in MOS devices and circuits Oxidation of polysilicon is dependent on Polisilicon deposition method Deposition temperature Deposition pressure The type and concentration of doping Grain structure of polysilicon

Thermal oxidation method Thermal oxidation  energy is supplied by heating a wafer SiO2 layer are grown: Atmospheric pressure oxidation  oxidation without intentional pressure control (auto-generated pressure); also called atmospheric technique High pressure oxidation  high pressure is applied during oxidation 2 atmospheric techniques Tube furnace Rapid thermal system

Oxidation methods Thermal oxidation Atmospheric pressure Tube furnace Dry oxygen Wet oxygen Rapid thermal High pressure Dry or wet oxygen Chemical oxidation Anodic oxidation Electrolytic cell Chemical

Horizontal tube furnace Quartz reaction tube – reaction chamber for oxidation Muffle – heat sink, more even heat distributing along quartz tube Thermocouple – placed close to quartz tube. Send temp to band controller Controller – send power to coil to heat the reaction tube by radiation/conduction Source zone- heating zone Place the sample

Horizontal tube furnace Integrated system of a tube furnace consists of several sections: Reaction chamber Temperature control system Furnace section Source cabinet Wafer cleaning station Wafer load station Process automation

Vertical tube furnaces Small footprint Maybe placed outside the cleanroom with only a load station door opening into the cleanroom Disadvantage: expensive

Rapid Thermal Processing Based on radiation principle heating Useful for thin oxides used in MOS gates Trend in device miniaturisation requires reduction in thickness of thermally grown gate oxides < 100Å thin gate oxide Hard to control thin film in conventional tube furnace Problem: quick supply and remove O2 from the system

RTP system: able to heat and cool the wafer temperature VERY rapidly RTP used for oxidation is known as Rapid Thermal Oxidation (RTO) Have O2 atmosphere Other processes use RTP system: Wet oxide (steam) growth Localised oxide growth Source/ drain activation after ion implantation LPCVD polysilicon, amorphous silicon, tungsten, silicide contacts LPCVD nitrides LPCVD oxides

RTP design e.g. RTP time/temperature curve

High Pressure Oxidation Problems in high temperature oxidation Growth of dislocations in the bulk of the wafer  dislocations cause device performance problems Growth of hydrogen-induced dislocations along the edge of opening  surface dislocations cause electrical leakage along the surface or the degradation of silicon layers grown on the wafer for bipolar circuits Solve: low temperature oxidation BUT require a longer oxidation time

High pressure oxidation results in faster oxidation rate High pressure system  similar to conventional horizontal tube furnace with several features: Sealed tube Oxidant is pumped into the tube at pressure 10-25 atm The use of a high pressure requires encasing the quartz tube in a stainless steel jacket to prevent it from cracking High pressure oxidation results in faster oxidation rate Rule of thumb: 1 atm causes temperature drop of 30C In high pressure system, temperature drop of 300-750C  This reduction is sufficient to minimise the growth of dislocations in and on the wafers

Advantage of high pressure oxidation Drop the oxidation temperature Reduce oxidation time Thin oxide produced using high pressure oxidation  higher dielectric strength than oxides grown at atmospheric pressure High pressure oxidation

Oxidant sources Dry oxygen Water vapor sources Bubblers/ flash Dry oxidation Chlorine added oxidation

1. Dry oxygen Oxygen gas must dry  not contaminated by water vapor If water present in the oxygen: Increase oxidation rate Oxide layer out of specification Dry oxygen is preferred for growing very thin gate oxides ~ 1000Å

2a. Bubblers Bubbler liquid – DI water heated close to boiling point 98-99C create a water vapor in the space above liquid When carrier gas is bubbled through the water and passes through the vapor  saturated with water Influence of elevated temp inside tube  water vapor becomes steam and results in oxidation of Si surface Problem: contamination of tube and oxide layer from dirty water and flask

2b. Dry oxidation (dryox) O2 and H2 are introduced directly into oxidation tube  mixes High temperature in tube forms steam  wet oxidation in steam Advantage: Controllable: gas flow can be controlled by flow controllers Clean: can purchase gases in a very clean and dry state Disadvantage: explosive property of H2  overcome by flow in excess O2

2c. Chlorine added oxidation Chlorine addition: Reduce mobile ionic charges in the oxide layer Reduce structural defects in oxide and Si surface Reduce charges at Si-SiO2 interface Chlorine sources: Gas: anhydrous chlorine (Cl2), anhydrous hydrogen chloride Liquid: trichloroethylene (TCE), trichloroethane (TCA) TCA is preferred source for safety and ease of delivery

Post-oxidation evaluation Surface inspection quick check of the wafer surface using UV light (surface particulates, irregularities, stains) Oxide thickness several techniques such as color comparison, fringe counting, interference, ellipsometers, stylus apparatus, scanning electron microscope Oxide and furnace cleanliness Ensure oxide consists of minimum number of mobile ionic contaminants. Use capacitance/voltage (C/V) evaluation: detect total number of mobile ionic contaminants NOT the origin of contaminants

Thermal nitridation < 100Å SiO2 film possesses poor quality and difficult to control Silicon nitride (Si3N4) Denser than SiO2  less pin holes in thin film ranges Good diffusion barrier Growth control of thin film is enhanced by a flat growth mechanism (after an initial rapid growth)

Nitridation of <100> silicon