Download 슬라이드 1 - Konkuk

Chapter 4.
Basics of Device Fabrication
MOSFET
The source contact is used as the voltage reference.
The conductance of the channel can be modulated by varying the gate voltage.
The source-to-drain path corresponds to two p-n junctions connected back to back.
CMOS
Invention of MOSFET
Dawon Kahng was born on May 4, 1931 in Seoul.
He received B.S. degree in Seoul National University in 1955
and Ph.D. degree from Ohio State University in 1959. He died
on May 28, 1992 in New Jersey.
In 1959, he invented the MOSFET in collaboration with
Ernesto Labate and Martin M. (John) Atalla at Bell Labs. It
was the first successful insulated-gate field-effect
transistor (FET) that overcame the "surface states" that
blocked electric fields from penetrating into the
semiconductor material. Investigating thermally grown
silicon-dioxide layers, they found these states could be
markedly reduced at the interface between the silicon and
its oxide in a sandwich structure of metal-oxide-silicon.
In 1960, he and M. Atalla fabricated the MOSFET using a gate
insulator formed from high quality SiO2 grown in-situ by a new
high-pressure steam oxidation process. It was the first
successful demonstration of MOSFET and a major milestone
in semiconductor technology.
He also invented in 1967 a field effect memory, the first
nonvolatile silicon memory (floating gate memory).
Oxidation
 Oxidation – Growth of SiO2 by reacting Si with O.
 Function of SiO2
• Insulator in a device structure
• Barrier to diffusion during device fabrication
 Dry oxidation
• Si + O2 → SiO2
• Superior Si-SiO2 interface property
 Wet oxidation
• Si + 2H2O → SiO2 + 2H2
• Faster growth rate : suitable for thick barrier oxide
Oxidation
 Oxidation – Growth of SiO2 by reacting Si with O.
 Function of SiO2
• Insulator in a device structure
• Barrier to diffusion during device fabrication
 Dry oxidation
• Si + O2 → SiO2
• Superior Si-SiO2 interface property
 Wet oxidation
• Si + 2H2O → SiO2 + 2H2
• Faster growth rate : suitable for thick barrier oxide
Oxidation System
Oxidation Rate
Diffusion
 Solid state diffusion – widely used
method for introducing dopant atoms
into a semiconductor lattice.
 Si wafer is exposed to a solid, liquid,
or gaseous source containing the
desired impurity.
 Two-step process
• Predeposition : diffusion with the
source present
• Drive-in : diffusion with the source
removed
Diffusion System
900oC ~ 1200oC
 Open tube system identical to that used for oxidation.
 N2 carrier gas is passed through a bubbler containing POCl3.
Ion Implantation
 Alternative means of introducing dopants and other atoms into the nearsurface region of a semiconductor.
 Shooting ions into the semiconductor resulting in displacement of
semiconductor atoms. ⇒ A follow-up anneal (heating) is necessary to
remove the crystal damage and “activate” implanted dopants.
 Advantages over diffusion
• Low temperature process
• Very precise control of the impurity concentration
• Very shallow concentration profile is obtainable.
 Gaussian concentration profile

(1/ 2 )[( x  R p ) / R p ]2
• Rp : projected range
N ( x) 
e
2 (R p )
• ΔRp : straggle
Ion Implantation System
Implantation Profile
Lithography
 Patterning of insulators or metals by selective removal of a thin film
at prescribed regions across the surface of a wafer
 Process flow
• Photoresist(PR) coating by spinning
• Prebake(or soft bake) : to drive solvent out of the resist
• Exposure through a mask using UV-light
• Development : removal of exposed (or unexposed) region
• Hard bake : to improve resistance to the subsequent etch
• Etch : wet etch by acid solution or dry etch by plasma
• PR Strip : using a chemical solution or an oxygen plasma
 Positive PR vs. negative PR
Lithography Process
(c) → (d)
Thin Film Deposition
 Purposes
• Metallization : connecting devices
• Deposition of intervening dielectric layers
• Diffusion barrier : preventing interdiffusion of materials
• Passivation : protecting the device or circuit from contamination.
 Physical Vapor Deposition (PVD)
• Evaporation, sputtering
 Chemical Vapor Deposition (CVD)
• LPCVD (low pressure), APCVD (atmospheric pressure), PECVD
(plasma-enhanced)
Evaporation
 Source is vaporized in a vacuum
chamber.
 Source material travels unimpeded to
the substrate and deposits as thin film.
 Hot-filament evaporation
- high levels of contamination
 Electron-beam evaporation
- X-ray ⇒ device degradation
 Not used in the production-line
fabrication of modern ICs
Sputtering
 High voltage between two plates
ionizes Ar gas at low-pressure.
 Ar+ ions accelerated by electric field
hit the source material ejecting the
source atoms.
 Ejected atoms travel to the substrate
and deposit to form the desired thin film.
 DC sputtering for metals
 RF sputtering for insulators
 Low contamination & high throughput
 Chief commercial method of depositing Al and other metals
CVD
 A gaseous compound decomposes to form the thin film or a reaction
between gas components takes place to form the film.
 CVD reactions take place preferentially on the surface of wafers.
 LPCVD - Good uniformity, less gas consumption, low contamination
 PECVD - Low deposition temperature
 CVD process produces the masking and intermetallic dielectric films.
CVD System
PECVD
LPCVD
Epitaxy
 More surface migration at higher
temperature and lower pressure
leads to epitaxy.
 Epitaxy produces a crystalline
layer that is an extension of the
underlying semiconductor lattice.
 Doping of the epi-layer is
controlled by introducing
dopant containing gas.
Epitaxy of Different Element Films
Epitaxy of Different Element Films
Process Steps
for pn Junction Diode
CPU Process Flow
CPU Process Flow