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NANOFABRICATION ‐1 CRYSTAL GROWTH EEE5425 Introduction to Nanotechnology
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Crystal Growth Bulk crystal growth Growth of semiconductors crystals (e.g. Si, GaAs, Ge) as standalone pieces. •Czochralski method •Bridgman method •Various floating zone methods Epitaxial crystal growth Growth of a thin crystal layer on a wafer of a compatible crystal. •Molecular beam Epitaxy (MBE) •Metal organic chemical vapor deposition (MOCVD) © Nezih Pala
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Purity Purity is very important for semiconductor crystals to fabricate high quality electronic devices. Today's Si wafers have the impurity level of parts per billion or ppb. 1ppb = 5 x 1013 cm‐3. What is ppb? Imagine the world population is 6 billion. And there are 6 aliens. They would feel very lonely…
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Defects
a) Interstitial impurity atom; b) Edge dislocation; c) Self interstitial atom; d) Vacancy;
e) Precipitate of impurity atoms; f) Vacancy type dislocation loop; g) Interstitial type dislocation loop; h) Substitutional impurity atom
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Crystal Growth (Si) Starting Material Silica
Very impure Silicon
SiCl4 (liquid)
Ultrapure SiCl4
Ultrapure polycrystalline Si
Ultrapure Polycrystalline Si Czochralski method Single Crystal Growth Grinding Polished Wafer
Sawing
Chemical mechanical Polishing (CMP)
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Purification Low-grade (or metallurgical grade MGS) Si (or ferrosilicon) with a purity of about 95 ~ 98% is firstly produced by placing silica (SiO2, sand, quartzite) in a furnace (at about 1,800 oC) with various forms of carbon to get rid of oxygen. SiO2 (s) + SiC (s) → Si (s) + SiO (g) + CO (g) or SiO2 (s) + 2C (s) → Si (s) + 2CO (g)
The ferrosilicon is chlorinated to yield SiHCl3 (or SiCl4), both of which are liquids at room temperature (BP ≈ 32 oC).
or
Si (s) + 3HCl → SiHCl3 (l) + H2 (g) Si (s) + 4HCl → SiCl4 (l) + 2H2 (g)
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Purification Multiple distillation of the liquids removes the unwanted impurities (99.9999% pure). The purified SiHCl3 (or SiCl4) is then used in a hydrogen reduction reaction to prepare the electronic-grade Si (EGS).
or
SiHCl3 (g) + H2 (g) → Si (s) + 3HCl (g) SiCl4 (g) ( )+2 2H2 (g) ( ) → Si ((s)) + 4HCl Cl ((g))
The EGS, a polycrystalline material of high purity, is the raw material used to prepare device-quality, single-crystal Si. Pure EGS generally has impurity concentrations in the parts-per-billion range.
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Czochralski Method Jan Czochralski (cho-HRAL-skee) (1885 1953) was a Polish chemist who invented the Czochralski process, which is used to grow single crystals and is used in the production of semiconductor wafers. He discovered the Czochralski method in 1916 1916, when he accidentally dipped his pen into a crucible of molten tin rather than his inkwell. He immediately pulled his pen out to discover that a thin thread of solidified metal was hanging from the nib. The nib was replaced by a capillary, and Czochralski verified that the crystallized metal was a single crystal.
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Czochralski Method The ultrapure poly-Si is placed in a quartz crucible and heated in an inert atmosphere to form a melt (1412 oC). A small single crystal (or Si seed crystal), with the normal to its bottom face carefully aligned along a predetermined direction (typically a or direction), is then lamped to a metal rod and dipped into the melt. The molten Si may be doped n-type or p-type in the melt to produce a doped Si substrate. As the seed crystal is slowly pulled out, the Si in the melt near the seed cools off and crystallizes onto the seed, extending the crystal downward and outward. The diameter (or radius r ) of the cylindrically shaped single crystal of Si (ingot) increases, depending on the rate at which the seed pulls (typically (typically, the growth rate ~1/√r ). ) Recently, 300-/400-mm (12-/16-inch) wafers are in production.
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Czochralski Method
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Czochralski Method
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Bridgman Growth Method •Mainly used for GaAs growth and other new crystals (eg. Bi2TeO5 , LiB4O7, CdTe) for research. •Typical wafer diameter is 2”. •Growth of larger crystals requires very accurate control of the stoichiometry of axial and radial temperature gradients to control dislocation density. •Allows very small thermal gradients and, therefore, low dislocation densities.
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Float‐Zone Crystal Growth •Used to form single crystal semiconductor substrates as an alternative to Czochralski growth process; •Polycrystalline material (typically in the form of a circular rod) is converted into single-crystal by zone heating (zone melting) starting at the plane g crystal y seed is contacting g where single polycrystalline material; •Used to grow Si wafers with very high purity (i.e. very high resistivity) single crystal Si; •Does not allow as large Si wafers as CZ does (200 mm and 300 mm) and radial distribution of dopant in FZ wafer not as uniform as in CZ wafer;
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Sawing and Polishing Grinding → X-ray crystallography → Sawing →Lapping → Chamfering → Polishing
Sawing
Chamfering
Chemical mechanical polishing Using slurry of fine SiO2 particles in a basic NaOH solution.
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Economical Value From sand
$35 / ton
to
ultrapure Si single crystal wafers
$200 / 300mm wafer
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CPUs
$300 / CPU 15
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Doping doping : process of intentionally introducing impurities into an extremely pure (also referred to as intrinsic) semiconductor in order to change its electrical properties.
For any material, there is a different affinity for impurities for the liquid phase and the solid phase. This characteristic is described by th distribution the di t ib ti coefficient ffi i t kd. kd = CS/CL CS = impurity concentration in the solid CL = impurity concentration in the liquid kd = f( material, impurity, temperature) Example: if kd=1/2, there are twice the impurities in the liquid as in the solid. © Nezih Pala
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Distribution Coefficient Example Find the concentration of phosphorous (P) atoms in the melt to obtain Si doped with 1016 atoms/cm3 (Czochralski growth) kd = 0.35 for P in Si.
kd
1016 atoms / cm 3 cS cL 2.86 1016 atoms / cm 3 0.35 cL
How many grams of P should be added if the initial load in the crucible is 5 kg of Si? (density of Si = 2.33g/cm3 )
v
m 5000g 5000 g 2145.9cm3 d 2.3g / cm3
In the total melt volume, we want 2.86X1016 atoms/cm3. The number of atoms is:
2.86 1016 atoms / cm 3 VSi VP P atoms = ZP=31g
but VP VSi, so VC Vsi
2.86 10 atoms / cm 2145.0cm 6.135 1019 16
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6.135 1019 atoms 31g / mole 3.159 103 g 3.16mg 6.023 1023 atoms / mole
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Epitaxial Growth Epitaxial growth technique, a process whereby a thin, single crystal layer of material is grown (or deposited) on the substrate of a single-crystal substrate, which is used extensively in a device and integrate circuit fabrication. The single-crystal substrate acts as the seed. Epitaxial growth can be performed at temperatures considerably below the melting point of the substrate crystal. •Homoepitaxy: an epitaxial layer grown on the same substrate material (e.g., Si/Si-sub) •Heteroepitaxy: an epitaxial layer grown on the different substrate material (e.g., AlGaAs/GaAs-sub)
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Lattice Matching x=0.53
In creating heterojunctions, one must start with an available substrate material that is lattice matched (i.e., the lattice constants of the two materials must be as nearly equal as possible) Defects will occur or there is strain on the lattice near the junction unless the lattice constants of the two materials are very well matched. © Nezih Pala
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Vegard's Law a(AxB1-x )= x.a(A)+ (1-x).a(B) aInxGa1-xAs= x.aInAs+ (1-x) aGaAs aGaAs=5.65Å
aInAs=6.06Å
aInP=5.87Å 5.87= x6.06+ (1-x) 5.65 5.87=5.65 +0.41x 0.22=0.41x x=0.53 © Nezih Pala
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Vegard's Law – Example aAlN=3.112Å
aInN=3.533Å
aGaN=3.189 Å AlxIn11-xxN on GaN x=??? aAlxIn1-xN= x.aAlN+ (1-x) aInN 3.189 = 3.112 x + (1-x) 3.533 3.189 = 3.533 – 0.421x x = 0.82 © Nezih Pala
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Liquid‐Phase Epitaxy (LPE) •One of the earliest way to grow epitaxial layers (primarily for compound semiconductors) •The wafers are on a tray that slides Advantages: Near-equilibrium growth, excellent crystal quality Inexpensive; Fast
Disadvantages: Difficult to scale up for production Dimensional control poor Structure complexity limited
Current status: Used for LEDs and laser diodes in well established processes. Rarely used in new installations. © Nezih Pala
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Liquid‐Phase Epitaxy (LPE)
Koyo Thermo Systems Co., Ltd © Nezih Pala
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Molecular Beam Epitaxy (MBE)
• A highly versatile technique to put down monolayers for extremely precise control of material growth. • Sort of a solid-phase epitaxy (SPE). • The individual elements (and dopants) are heated in their separate crucibles under high vacuum. The gates to the individual crucibles can be opened and closed to vary composition of the layers.
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Molecular Beam Epitaxy (MBE)
Single filament effusion cell
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Molecular Beam Epitaxy (MBE) Advantages: Extremely flexible, simple chemistry Insitu monitoring; Atomic layer control Non-equilibrium technique Disadvantages: No in situ cleaning g or purifying p y g reactions Expensive (to assemble and operate) Non-equilibrium technique Current status: A research workhorse; heavily used in production
SEM of GaAs (dark) AlGaAs (light) layers. Each layer is 4 monolayers (11.3 A) thick.
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Chemical Vapor Deposition (CVD) or Vapor‐Phase Epitaxy (VPE) A process whereby an epitaxial layer is formed by a chemical reaction between gaseous compounds: SiCl4 (g) + 2H2 (g) → Si (s) + 4HCl (g) •Performed at atmospheric pressure (APCVD) or at low pressure (LPCVD) •A technique commonly used to Si layers on Si •Also used grow III‐V compounds (e.g., a GaAs substrate is exposed to an atmosphere of AsH3 + PH3 + GaCl to obtain GaAsP film
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Metal‐Organic Chemical Vapor Deposition (MOCVD) In obtaining the compounds containing Al, VPE does not work ideally because the Al does not diffuse well on the surface and because of its high activity. •In MOCVD, the Ga and Al metals are introduced in organic compounds such as Ga(CH3)3 and Al(CH3)3. •MOCVD is capable of growing monolayers (layers one atom thick), which makes possible abrupt changes in composition and highly precise control.
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Metal‐Organic Chemical Vapor Deposition (MOCVD) Advantages: All sources gaseous Precise composition and dimension control Disadvantages: Involves complex chemistry U Uses ttoxic i gases (A (AsH H3, PH3) Current status: Viewed as the standard production process for many epitaxial heterostructures. Especially in GaN based HEMT and LED wafer production.
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