EE4645-Oct_08 1

EE 4645-Microfabrication Engineering

Dr. K. Radha KrishnanAssociate Professor

Microelectronics Division

Tel: 6790 4549Office: S1-B1C-83

Email: eradha@ntu.edu.sg

Contact HoursTuesday: 1530-1630

Wednesday: 1630-1730Thursday: 1630-1730

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Topics CoveredEPITAXIAL METHODS Si-Vapour Phase Epitaxy Advanced Epitaxial Processes

Molecular Beam Epitaxy (MBE)Metalorganic Chemical Vapour Deposition (MOCVD epitaxy)

PROCESS INTEGRATION Device Isolation

LOCOS processSWAMI processTrench Isolation

ContactsSchottky and ohmic contactsAlloyed & advanced Salicide Contacts

Multilevel Metallization Electromigration

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Topics Covered Intermetal Dielectric Layer Planarization etch back and CMP CMOS Technologies

MOSFET Device PhysicsPhysical structure (NMOS & PMOS)CMOS fabrication sequenceAdvanced CMOS processDevice Scaling & Short Channel Effects

MEMSFundamentals of Mechanics Mechanical to Electrical TransductionBulk Micromachining & Surface MicromachiningHigh-Aspect Ratio Microsystems Technology

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Epitaxy-IntroductionWhat is epitaxy? Growth of single crystal layer on a single crystal

substrate such that the deposited layer takes the single crystalline form of the underlying substrate. Similar to CZ technique, substrate serves as the seed crystal.

Homoepitaxy: Growth of single crystal layer on a single crystal substrate of the same material. Ex: Si on Si, GaAs on GaAs

Heteroepitaxy: Growth of single crystal layer on a single crystal substrate of a different material. Ex: Si on Sapphire (Al2O3), GaAs on Si, SiGe on Si, GaN on Si

Advantages of epitaxy: Control over layer quality, thickness and doping concentration Lower process temperature compared to diffusion process

-also much more abrupt doping profiles ideal for creating "artificial" semiconductor structures such as

quantum wells, quantum dots, etc

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Epitaxy-IntroductionProblems associated with heteroepitaxy: lattice mismatch formation of dislocation chemical incompatibility - alloy formation at the interface thermal expansion mismatch delamination

Properties of CZ grown and epitaxially grown Silicon:CZ grown silicon crystal: low resistivity (high impurity), low breakdown voltage, used as substrate for epilayer, MOS device substrate

Epitaxial silicon: high resistivity (low impurity), high breakdown voltage, thin

base layer in bipolar devices, high frequency applications

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Epitaxy-IntroductionSi-epitaxy for device applications

psubstratep- substrate

Well-Diode Epitaxial Diode

well controlled current flow in epitaxial diode Bipolar IC cross section

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Epitaxy-Introduction

Various Epitaxial Growth Techniques

P

r

e

s

s

u

r

e

(

T

o

r

r

)

MOCVD

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Epitaxy-Introduction

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Epitaxy-IntroductionStrain state in epitaxially grown material:

Cubic unit cell of substrate:

Cubic unit cell of layer material

Pseudomorphic case

Partially strainrelaxed

Relaxed

as

alayer

ayax

az

az ax = ay = asub

az ax = ay asub

ax ay

az

alayer

az = ax = ay = alayer

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Epitaxy-IntroductionA film with an unconstrainedlattice constant of af is depositedon a substrate with a differentlattice constant of as. It initiallygrows with the lattice constantequal to the substrate. The latticemismatch is accommodated bythe strain in the layer.

This continues until the filmthickness reaches some criticalthickness, tcFor film thickness greater thantc, the misfit is accommodated bygeneration of misfit dislocations.

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Si-Vapour Phase EpitaxySi-Vapour Phase Epitaxy Epitaxial growth is carried out in a furnace tube. For

Silicon epitaxy, SiCl4 (Silicon tetrachloride), SiHCl3SiH2Cl2, SiH3Cl or SiH4 (Silane) can be used.

Si-epitaxy by SiCl4 reduction process:SiCl4 + H2 SiCl2 + 2 HCl at T >1150oC2 SiCl2 Si + SiCl4 (surface reaction)

SiCl4 is added to H2 gas by bubbling H2 through liquidSiCl4, maintained at a constant temperature.

To grow single crystal Si film, an optimum growth rate hasto be maintained.

Too high a growth rate causes the film polycrystalline (notenough time to settle in the proper nucleation sites).

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Si-Vapour Phase Epitaxy

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Si-Vapour Phase Epitaxy Growth rate is determined by the concentration of SiCl4 in H2.

-Higher concentration results in etching instead of deposition. SiCl4 (gas) + Si (solid) 2SiCl2 (gas) etching reaction SiH2Cl2 is preferred over SiCl4 (because of higher efficiency

in depositing Si and being low temperature process)

Si-epitaxy using SiH4 SiH4 SiH2 + H2 SiH2 Si + H2 (Surface reaction at ~500oC)Advantages: no chlorine bye-products, low temperature processDisadvantages: Silane is dangerous (ignites spontaneously in air) Possibility of gas-phase nucleation leading to poor

morphology Silane is sensitive to oxidation.

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Si-Vapour Phase Epitaxy

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Si-VPE- Growth KineticsKinetics of GrowthFlux at the main gas stream F1 = hg (Cg -CS) (1)Flux at the interface: F2 = kSCS (2)In the steady state,F1 = F2 = F (3)

S

gS

S

S g g

S g

g

From eqns 1 to 3, the surface concentrationC of the reactant species:

CC (4)

1 (k h )

The growth rate, R of the semiconductok h CFR

N k h N

r layer:

(5) = = +

= +

22 3

g

s

N 5x10 atoms/cm for Si which is the no of atoms incorporated into a unit volumeof layer. h = mass transfer coefficient

k reaction rate constant

=

=

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Si-VPE- Growth Kineticst

3g t

S g t

S g

If y is the mole fraction of the reactant species, and C is the

total no of molecules per cm in the gas, C = yC

Then, the epitaxial growth rate, R is given by:k h CR y (6

k h N = +

g S

tSg S

)

Thus, the film growth rate is proportional to y.Growth rate at a given mole fraction is governed by the smallerof either h or k .

C, R k y N

If h >>k Growth is surface reaction-controlled

I

t

g gSf k >> h Growth is mass trC, R h y ansfer-co ntrolledN

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Si-VPE- Growth KineticsLow temperature region (A): Growth rate follows exponential

law, R~exp (-Ea/kT), chemical reaction at the surface depends on temperature

Surface reaction controlled growth.

High temperature region (B): Growth rate is independent of

temperature. Growth depends on the mass transport of the reactant species.

Mass transfer controlled growth.

Slight change in temperature does not alter the growth rate much.

Dopants are added using gases such as B2H6, PH3 and AsH3. Dopants with low diffusion constant are preferred because of the subsequent process.

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Si-VPE AutodopingDoping Profile in Epilayers

Autodoping causes deviation of epilayer doping profile from the ideal profile of abrupt concentration change at the substrate/epilayer interface

Autodoping:

During epitaxy, Si and dopant atoms are removed from the substrate by thermal evaporation and/or chemical reaction. These atoms mix with the incoming gas stream and modify the composition of the gas stream.

Deposited layer will show a doping profile different from the expected.

Deviation is greater near the epilayer-substrate interface.

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Si-VPE AutodopingAutodoping Effects: Case 1: Growth of undoped (intrinsic gas) Si epilayer on a doped Si substrate with a concentration, Ns. The impurity redistribution in the epilayer is: NZ = Ns e

-Z NZ is the dopant concentration at a distance Z from the interface, and is the etchback factor. = f (dopant, reactor, temperature, process) ~ 1-2x10-4 cm Case 2: Growth of doped (doped gas) Si epilayer on an undoped Si substrate. NZ = NE (1-e-Z), NE is the equilibrium dopant concentration at a distance far away from the interface for a sufficiently thick layer.

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Si-VPE AutodopingCase 3: Growth of doped Si epilayer on a doped Si substrate. For this,the impurity redistribution is the superposition of case 1 and case 2. NZ = (NS -NE) e-Z + NE Note: For dopants of opposite type impurities, change NS to -NS or NE to -NE. Case 4: Growth of n-doped epilayer on p-substrate, the junction occursin the epilayer at a distance ZJ from the substrate surface, calledJunction Lag.

NZ = (-NS -NE) e-Z + NE

Higher the etch-back factor, lower the junction lag. Hence, grow epilayer at as high a temperature as possible.

+=== ES

JZJ NN

1ln1 Z 0 N , ZAt Z

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Ideal dopant concentration profile for

(a) n-epilayer on n+ -substrate and

(b) n-epilayer on p-substrate

n+substrate

n-epilayer

n-epilayer

p-substrate

Si-VPE Autodoping

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Si-VPE Autodoping

Dopant distribution in epilayers due to autodoping:

(a) n epilayer on n+ substrate

(b) n epilayer on p-substrate

(Case 1)

(Case 2) (Case 3)

(Case 4)

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Advanced Epitaxial Processes

1. Molecular Beam Epitaxy (MBE)2. Metalorganic Chemical Vapour Deposition

(MOCVD)

MBE and MOCVD are for compound semiconductor epitaxial growth such as GaAs, GaAlAs, GaInP, InP, InGaAs, InAlAs, InGaPAs, GaN, InN, GaAlN, etc

Devices: MESFETs, High electron mobility transistors, heterojunction bipolar transistors, LDs, LEDs, etc

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Molecular Beam Epitaxy (MBE)

Molecular beams (Ga, As, Al, In, etc) produced by thermal evaporation or sublimation travel in ultra high vacuum and impinge on a hot substrate to form compound semiconductor epitaxial layer.

N-type dopant: Si; P-type dopant: Be.

Growth rate: 1 m/hr or ~ one monolayer/s MBE provides ultimate control of growth parameters (to the mono-layer level) in real time and is favourable for growing heterostructures requiring precise control of layer composition, doping profile, and sharp layer transition.

GaAs based growth: GaAs, AlGaAs, InGaAs, InGaP, etcInP based growth: InP, InGaAs, InAlAs, etcGaN based growth: GaN, AlGaN, InGaN, etc

MBE Growth

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MBE System

Vacuum pumping system: Various pumps are used to achieve a base vacuum of about 10-11 Torr.

Molecular beam sources have individual shutters or valves

Substrate heating station to supply enough thermal energy toachieve epitaxial growth

Substrate Rotation: to ensure thickness and compositionaluniformity

Reflection high-energy electron diffraction system (RHEED) forin-situ surface crystallinity characterization, growth rate,composition and roughness

Ionization gauges to measure the vacuum and the flux of molecular beam Quadrupole mass spectrometer: residual gas analysis and leak

detection.

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MBE System

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MBE System

MBE systems in EEE

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MBE System

The essential parts of a MBE growth system. Three zones where the basic processes of MBE take place.

CrucibleHeating element

Heat screen

ThermocoupleMounting flange

Single Filament Effusion Cell (In, Ga and Al)

EE4645-Oct_08 29

MBE Growth ProcessSurface processes occurring during MBE film growth

Adsorption of the constituent atoms or molecules impinging on the substrate surface

Surface migration and dissociation of the adsorbed molecules

Incorporation of the constituent atoms into the crystal lattice of the substrate or the epilayer already grown

Thermal desorption of the species not incorporated into the crystal lattice.

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MBE Growth ProcessGrowth modes:

(a).VolmerWeber mode: the atoms of the deposited constituents are more strongly bound to each other than the substrate, thus forming three-dimensional islands.(b). StranskiKrastinov mode: initial layer-by-layer growth for a few monolayers is followed by island growth.(c). Frankvan der Merwe mode: the atoms of the deposited material are more strongly bound to the substrate than to each other yielding an atomically smooth two-dimensional growth mode.Clearly, Frankvan der Merwe mode is desirable for ideal interfaces.

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MBE Growth ProcessGrowth modes:

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MBE Growth ProcessGrowth Rate using Effusion Cells

1/ 2

20 2

( , (2 ) )

2.64 10 / .

( ), ( ) , tan

Growth rate f arrival or impingement rate P mkTP molecules cm sMT

P pressure Pa m mass of a molecule kgM molecular weight k Boltzman cons tT temperature

= = =

= == ==

2

( )

, tan

Growth controlling ato

KAGrowth rate tL S

A crucible aperture area t monolayer thicknessL dis ce between the crucible aperture and the substrateS surface density of the growth controlling atoms

== ===

ms are generally the less volatile atomssuch as Ga in GaAs or InGaAs.

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MBE Growth Process2

16 2

: Effusion cell with an area of 5 is located 10 cm away from the substrate. Find the growth time required to grow 2 m of InPon InP (100) wafer, when the arrival rate of In is 5 10 / .

Ex cm

atoms cm sLat

2

16 2 2 8

2 14 2

8

constatnt of InP, a = 5.8687 A

5 10 / . , 5 , a/2 = 2.93 1010 , , 2 m2 / for InP (100) 5.82 10 /

Growth rate, R 4 10 /

ticeAGrowth rate tL S

atoms cm s A cm t cmL cm layer thickness dS a cm

cm s

Growth

== = = = == =

= 3 5 10 1.39dtime s hrs

R= = =

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MBE Growth Process

(100)

Noofatomsin(100)=4 +1=2

Atomicdensityin(100)=2/a2

1monolayer=a/2for(100)and(111)

GaAs or InPCrystal Structure

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MBE Growth Typical MBE Growth:

Before growth, GaAs substrate is heated to about 600C (or about 520Cfor InP) for several minutes to desorb and remove the thin oxide film (10 ). This is a critical operation.

Layer growth: Adjust the substrate temperature and then open the shutters to supply Ga and/or Al beams together with As beams. Growth conditions for the Growth of GaAs or AlGaAs on GaAs (100)

Optimum growth temperature Tsub = 600-640C. to get higher luminescence intensity and lower deep level concentration

Optimum growth rate: ~ 1m, to decrease the concentration of defects by giving adsorbed Ga and As atoms sufficient time to reach the appropriate lattice sites before incorporation into the growing film.

EE4645-Oct_08 36

MBE Growth ProcessGaAs growth kinetics using Ga and As4 beams:Pair-wise dissociation of As4 molecules adsorbed on adjacent Ga atoms. From any 2 As4 molecules, 4 As atoms are incorporated in GaAs lattice and the other 4 desorbed as As4 molecule. Note that interaction of As4 involves adjacent pairs of Ga atoms.

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MBE Growth-CharacterizationSurface characterization using Reflection High Energy Electron Diffraction (RHEED):An accelerated electron beam (up to 50 keV) is incident on the wafer surfacewith a glancing angle < 3 deg, and is reflected. Upon reflection, electronsdiffract, forming a diffraction pattern (streaks at the phosphor fluorescentscreen), and a specular reflection. The pattern depends on the structure andthe morphology of the film surface.

RHEED patterns from GaAs surface with the incident electron beam pointed along [011] and [011] directions.

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MBE Growth-CharacterizationRHEED Intensity OscillationsWhen MBE growth is initiated, the intensity of the RHEED features shows an oscillatory behavior which is directly related to the growth rate.

It can be routinely used to calibrate beam fluxes, monitor composition and surface roughness and determine the thickness of the layer.

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MBE Growth-CharacterizationGrowth rate: 1 cycle corresponds to one monolayer growthComposition can be determined from the frequency of oscillationIntensity relates to surface roughness

= =

=

1

E.g. If is the time required to grow one monolayer ofthe material, the Al molefraction, can be obtained for layer as:

1 1 1 1

: 1.5sec,

x x

AlGaAs

AlGaAs GaAs AlGaAs GaAs

GaAs AlGa

tx

Al Ga As

txt t t t

Ex t t = = 0.3 0.7

1.05sec 0.3The compound is Al Ga As

As x

EE4645-Oct_08 40

Metal Organic Chemical Vapour Deposition

Epitaxial growth by the pyrolitic reaction of alkyls of group III metals (liquid at RT) and hydrides of group V elements (gaseous) on the heated growth surface.

Growth pressure: atmospheric or low pressure

At the growth surface, group III alkyls dissociate, migrate to lattice sites, and deposit epitaxially by capturing the dissociated group V atom.

Growth rate is controlled by diffusion of the metal-organic species through the stagnant boundary layer.

Group III sources: Alkyls with highest vapour pressure (lowest mol wt) are preferred (stable and higher growth rate). TMGa, TMAl, TMIn, etc. Here, TM refers to trimethyl.

TMIn is solid at RT, heating is necessary between the bubbler and the reactor

Hydrogen is the carrier gas

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MOCVD System

Typical MOCVD System Configuration

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MOCVD System

MOCVD Reactor

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MOCVD GrowthGroup V sources: Requirements are purity, high vapour pressure, low temperature stability, pyrolysis above 400oC, and no parasitic reaction with group III.

Ex: AsH3, PH3, TMAs, TMP, NH3 Dopants: n-type- S, Se (H2Se), Te (DETe), Si (SiH4) p-type Zn (DEZn), Cd, Be, Mg Basic reaction: RnM + XHn MX + nRH

R is the organic radical, M is one component of the semiconductor, X is the other component, and n is an integer.

Ex: (CH3)3Ga + AsH3 GaAs + 3CH3 x(CH3)3Al + (1-x) (CH3) 3Ga + AsH3 AlxGa1-xAs + 3CH4

Alternative: RnM + RmX MX + nRH + mRH R, R are organic radicals, and M, X are semiconductor components Ex: (CH3)3Ga + (CH3)3As GaAs + 6CH4

x(CH3)3Ga + (1-x) (CH3)3In + (CH3)3Sb GaxIn1-xSb + 6CH4

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MOCVD-Chemicals

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MOCVDAdvantages:

All constituents are in vapour phase (gas flow accurately controlled) Pyrolysis reaction is relatively insensitive to growth temperature (efficient & reproducible deposition, good interfaces) Growth of complex multilayer structures is possible (computer controlled automatic gas-exchange systems) Throughput is high compared to MBE (multi-wafer capability easily achievable) Drawbacks:

Source chemicals are extremely toxic or inflammable in air Formation of adducts (Ex: (CH3)3In: PH3) in the use of hydrides Various gas flow patterns created depend on reactor geometry, temperature, gas flows, substrate rotation speed, etc Difficult to monitor growth rate exactly (no RHEED possible) Not as abrupt a process as MBE

EE4645-Oct_08 46

MOCVD GrowthGaAs growth rate Vs Temperature.

450-550oC, diffusion is good, the surface reaction dominates, and hence the growth rate is limited by the temperature

550 - 750oC, growth rate is temperature insensitive, it is limited by the diffusion of molecules

>800oC, parallel reactions lead to lower growth rate

EE4645-Oct_08 47

MBE-MOCVD ComparisonGrowth Kinetics - Comparison MBE: No chemical reaction

involved in deriving group III molecules. Growth rate depends on the arrival rate of Group III atoms.

MOCVD: As the growth pressure is high, group III alkyls diffuse through the stagnant boundary layer, get dissociated before forming the epilayer growth. Growth rate is limited by the diffusion rate of reactive species

EE 4645-Microfabrication EngineeringTopics CoveredTopics Covered Epitaxy-IntroductionEpitaxy-IntroductionEpitaxy-IntroductionEpitaxy-IntroductionEpitaxy-IntroductionEpitaxy-IntroductionEpitaxy-IntroductionSi-Vapour Phase Epitaxy Si-Vapour Phase Epitaxy Si-Vapour Phase Epitaxy Si-Vapour Phase Epitaxy Si-VPE- Growth KineticsSi-VPE- Growth KineticsSi-VPE- Growth KineticsSi-VPE AutodopingSi-VPE AutodopingSi-VPE AutodopingSi-VPE Autodoping Si-VPE AutodopingAdvanced Epitaxial Processes Molecular Beam Epitaxy (MBE)MBE System MBE SystemSlide Number 27MBE SystemMBE Growth ProcessMBE Growth ProcessMBE Growth ProcessMBE Growth ProcessMBE Growth ProcessMBE Growth ProcessMBE Growth MBE Growth ProcessMBE Growth-CharacterizationMBE Growth-CharacterizationMBE Growth-CharacterizationMetal Organic Chemical Vapour DepositionMOCVD SystemMOCVD SystemMOCVD GrowthMOCVD-ChemicalsMOCVDMOCVD GrowthMBE-MOCVD Comparison