IC Mask Design Training75页PPT

Czochralski Method (Contd)
- the seed crystal is slowly withdrawn - Progressive freezing at the solid-liquid interface yields a large, single crystal called Ingot - typical pull rate is a few millimeters per minute - a know amount of dopant is added to the melt to obtain the desired doping concentration - For Silicon born and phosphorus are the common dopants for p and n type materials
Material Characterization
Wafer Shaping Crystal characterization
Material Characterization (Contd)
Wafer Shaping - the two ends are removed - the surface is grinded to to give the required diameter - one or more flat regions grounded along the length of the ingot - ingots are diamond sawed to give wafers
Czochralski Method (Contd)
Crystal growing process - Polycrystalline Silicon (EGS) is placed in the crucible and heated to its melting point - A suitably oriented seed-crystal is is suspended in the crucible
Chemical-Vapor Deposition
A competing reaction also takes place
SiCl4 + Si 2SiCl2 Etching will take place if the
concentration is too high.
Chemical-Vapor Deposition
Si + 3HCl SiHCl3 + H2
TriChloroSaline (SiHCl3) is liquid at room temperature
Fractional distillation removes unwanted impurities
Czochralski Method (Contd)
Czochralski Method (Contd)
A crystal pulling mechanism – contains a seed holder and a counter-clockwise rotating mechanism
An Ambient control – a gas source (argon), a flow control and a exhaust system
Material Characterization (Contd)
Crystal characterization - Crystal defects - Material properties
Material Characterization (Contd)
Crystal Defects - Point defects - Line defects - Area defects
What we want!!!!!
Lets fabricate this first!!!!!
Steps involved in the fabrication process
Crystal Growth Epitaxial Growth Film Formation Lithography Etching Impurity Doping
Czochralski Method (Contd)
ECG is a polycrystalline material of high purity (impurity concentration is in the order of parts-per-billion) is the raw material for device-quality single crystal Silicon
Field-Oxide provides isolation from other devices
Gate-Oxide and Field-Oxide are grown using thermal oxidation
Thermal Oxidation (Contd)
Setup of Thermal Oxidation - Filtered flow of air is maintained is maintained at one end of the cylindrical tube. This minimizes dust and particulate matters in the air surrounding the wafers and minimize contamination during wafer loading - Oxidation temperature is generally 9000c – 12000c
Czochralski Method (Contd)
Crystal Puller Three main parts
A furnace – which includes a fusedsilicon (SiO2) crucible , a graphite susceptor, a rotation mechanism , a heating element and a power supply
Thermal Oxidation
Gate-oxide and Field-oxide fall are grown using this technique
Gate-Oxide is the layer below which a conducting channel is formed between source and drain
Electronic – grade Silicon (EGS) is got by hydrogen reduction of SiHCl3
SiHCl3 + H2 Si + 3HCl
This reaction takes place in a reactor with resistance-heated Silicon rod on which the deposition takes place.
S in g le C rys ta l W a fer
Single Crystal Silicon growth
Czochralski method Silicon crystal growth from the Melt > 90 % of the the semiconductor
Material Characterization (Contd)
- Slicing determines four wafer parameter
Surface orientation Thickness (0.5-0.7 mm) Taper Bow - both the sides are lapped with a mixture of Al2O3 and glycerin
industry use this option. Starting Material : Quartzite – Pure form
of Sand (SiO2)
Czochralski Method (Contd)
SiO2 is heated in a furnace along with various forms of carbon like coal, coke and wood chips
Material Characterization (Contd)
Material Properties - Resistivity - Minority carrier lifetime - Trace impurities such as oxygen and carbon - Surface flatness - Slice Taper - Slice Bow
Material Characterization (Contd)
- the damaged and contaminated regions are removed using chemical etching - polished – to provide a smooth and specular surface
Also know as Vapor-Phase epitaxy Silicon Tetrachloride (SiCl4),
Dichlorosilane (SiH2Cl2), trichlorosilane (SiHCl3) and Silane (SiH4) are used
SiCl4 + 2H2 Si + 4HCl
(Contd)
Diborane (B2H6) is used as p-type dopant
Phospine (PH3) or Arsine (AsH3) is used for n-type
Film Formation
Several different layers of thin film need to be fabricated during IC fabrication
SiC + SiO2 Si + SiO + CO
This produces a metallurgical- grade Silicon with a purity of about 98 %
Czochralski Method (Contd)
Solid Silicon is pulverized and treated with Hydrogen Chloride (HCl)
Crystal Growth
Techniques for growing single crystals of Silicon to form e
S ta rtin g M a te ria l P o lyc rysta llin e S e m ic o n d u c to r
Welcome to IC Mask Design Training
Fabrication Process
Agenda
What we want!!!! Steps involved in the fabrication process N-Well Process P-Well Process Twin Tub Process
Thin films can be classified as - Thermal Oxides - Dielectric layers - Polycrystalline Silicon - Metal Films
Chemical-Mechanical Polishing
Film Formation (Contd)
Epitaxial Growth
Growth of crystal of one mineral on another to achieve same structural orientation
Methods - Chemical-Vapor Deposition
Chemical-Vapor Deposition