Micro-Technology
Technology & Applications
19 August 2002. Ole Hansen MIC Topics in Microelectronics
Outline
• Applications - motivation • Micro-Technology – Silicon Starting Material – Microlithography – Pattern Transfer – Material Deposition – Material Modification
19 August 2002. Ole Hansen MIC Topics in Microelectronics
1 Applications of Micro-Technology
• Electronics • Sensors & Actuators – IC’s: CMOS, Bipolar – Pressure sensors – Discrete devices – Accelerometers – Power Devices – Gyroscopes – Micro-probes (AFM) • Optoelectronics • Bio/chemical m-systems – Imaging: CCD, CMOS – mTAS – Lasers & LED’s – Cell-sorting – Photodetectors – DNA-chips – Solar Cells – PCR-reactors – Silica Wave-guides on – Katalytic reactors Si
19 August 2002. Ole Hansen MIC Topics in Microelectronics
The start of the Silicon Age
December 1947: Invention of the Bipolar Transistor Bardeen, Brattain & Shockley 19 August 2002. Ole Hansen MIC Topics in Microelectronics
2 IC’s from 1960 to 2002
Fairchild, Gordon Moore, 1960 First planar IC. 19 August 2002. Ole Hansen MIC Topics in Microelectronics
IC Technology History
Moore’s Law
19 August 2002. Ole Hansen MIC Topics in Microelectronics
3 Where is the Limit?
IBM 1990: STM image STM modified surface, Xe on Silicon
19 August 2002. Ole Hansen MIC Topics in Microelectronics
Expected future Si technology
SIA roadmap 103 103 ] M [ p ] i h m c / n s [ r s o
s 2 2 t e
10 10 s i n s k n c a i r h t t , ] r t i o
Lithography b h GateLithography G t 1 1 [
d 10 GateOxide Thick. 10 i Memory Size e z w i e Transistors/chip S
n Clockfreq. GHz i y r L o m e
100 100 M 1995 2000 2005 2010 2015 Year 19 August 2002. Ole Hansen MIC Topics in Microelectronics
4 MEMS Non-electronic Applications
Comb drive resonator Fold-up mirror Microcantilevers in flowchannel Selective coating Piezoresistive strain gauge
Mirror-array for image projection 19 August 2002. Ole Hansen MIC AFM-probes- Topics in Microelectronics Sharp tips on microcantilevers
Why Silicon?
Technological properties Physical properties • Abundant element ~28% • Appropriate bandgap ~1.1eV • Simple purification – High breakdown voltage • Good crystal quality – No deep traps • Strong & hard material, 7GPa – Indirect-high carrier lifetime • Controllable doping • Reasonable mobility • Good oxide SiO2 – Electrons 1400cm2/Vs – Diffusion mask 2 – Dense pin-hole free dielectric – Holes 500cm /Vs – Good adhesion – Surface passivation
19 August 2002. Ole Hansen MIC Topics in Microelectronics
5 Resistivity vs. doping density An ”Extrinsic” material Dopants: Donors: P, As, Sb Acceptors: B, Al, Ga 1 r = qmnn + qm p p 1 rn » qmn N D 1 r p » qm p N A
19 August 2002. Ole Hansen MIC Topics in Microelectronics
Czochralsky Crystal Growth
• Low cost • Large diameter crystals ~300mm • Low defect density • Low dislocation density <100/cm2
• High contamination from crucible – O~25ppm, C~5ppm • Heavy metal ~ 1ppb • Moderate resistivity <50Wcm • Moderate carrier lifetime 0.3ms
19 August 2002. Ole Hansen MIC Topics in Microelectronics
6 Heat Poly-Silicon Lumps – Melt it
19 August 2002. Ole Hansen MIC Topics in Microelectronics
Molten Silicon
19 August 2002. Ole Hansen MIC Topics in Microelectronics
7 Dip Seed Crystal in the Melt
19 August 2002. Ole Hansen MIC Topics in Microelectronics
Pull Seed Rapidly Necking – Eliminates Defects
19 August 2002. Ole Hansen MIC Topics in Microelectronics
8 Slow Pull to Final Diametre
19 August 2002. Ole Hansen MIC Topics in Microelectronics
Control Pull Speed Keep Constant Diametre
19 August 2002. Ole Hansen MIC Topics in Microelectronics
9 Almost Finished Crystal Hanging in the Seed Crystal
19 August 2002. Ole Hansen MIC Topics in Microelectronics
Chemical Mechanical Polishing CMP
Slurry: KOH/H2O with 10nm Silica Particles
(KFe(CN)/H2O with Alumina Particles)
Semirigid Polishing pad: Global Planarization Etchrate: R=K*p*v 19 August 2002. Ole Hansen MIC Topics in Microelectronics
10 Basic Micro-Technology
• Material Deposition
• Photolithography
• Pattern Transfer
• Material Modification – Doping & anneal
19 August 2002. Ole Hansen MIC Topics in Microelectronics
Photolithography & Pattern Transfer The Key Technology
19 August 2002. Ole Hansen MIC Topics in Microelectronics
11 Basic Optical Aligners Photoresist Exposure Systems
l W » 0.6 Resolution :W » l·G NA l DOF » 2 19 August 2002. Ole Hansen MIC 2(NA ) Topics in Microelectronics
Standing Wave Interference Monochromatic Illumination
19 August 2002. Ole Hansen MIC Topics in Microelectronics
12 Photoresists & Photosensitive Materials
Photoresists Photosensitive structure removed after pattern transfer materials • Positive interlayer dielectrics & • Negative mechanical structures • Electrodeposited resists • Polyimides • SU-8 Epoxy Spin-on deposition • BCB Cyclotene •Simple process •Good thickness control •Planarising process
19 August 2002. Ole Hansen MIC Topics in Microelectronics
Pattern Transfer Etching:
Film Deposition Lithography Etch Film Strip Resist MOULD Filling:
Lithography Selective Deposition Strip Resist
Lift-Off:
Lithography Film Deposition Lift-Off
CMP:
Lithography&Etching Film Deposition Polish (CMP) 19 August 2002. Ole Hansen MIC Topics in Microelectronics
13 Pattern Transfer - Etching Important Parameters Anisotropy
Perfect Anisotropy Anisotropic Perfect Isotropy
Selectivity Mask Smask = Rfilm/Rmask Film Ssubstrate= Rfilm/Rsubstrate Substrate
Selectivity & Anisotropy interaction
Conformal Film / Step After Nominal Etch After 100% Over Etch
19 August 2002. Ole Hansen MIC Topics in Microelectronics
Dry Etching Reactive Ion Etching- RIE
Gas inlet, e.g SF6
-50V 0V F F Plasma Glow RF F F + F F F + F F Cathode Darkspace
To Pump
Pressure: 0.01-1Torr Etchant :e.g. Atomic Flourine Enhanced by ion bombardment Anisotropy : Sidewall passivation Selectivity: good to poor
Gas additives:
O:2 Increase F and Silicon etch rate Sidewall passivation - Anisotropy
H:2 Decrease F and Silicon etch rate Selective oxide etch SF6+O2 Silicon etch 19 August 2002. Ole Hansen MIC Oxide mask Topics in Microelectronics
14 Wet Silicon Etching in Alcaline Solutions
Crystallographic Anisotropy R111<
Boron Etch Stop
p-type n-type (Positive Bias) Outer corners are attacked by the etch Electrochemical pn Etch Stop 19 August 2002. Ole Hansen MIC Topics in Microelectronics
Thin Film Deposition Methods Native Films Physical Vapour deposition - PVD • Thermal Oxidation / Nitridation • Vacuum Evaporation Chemical Vapour Deposition - CVD • Molecular Beam Epitaxy – MBE • Vapour phase epitaxy - VPE • Sputtering – Reactive sputtering • Metals & Semiconductors III-V • Atmospheric pressure – APCVD Electrochemical deposition • Low pressure CVD – LPCVD • Electroplating, Electroless plating • Plasma enhanced CVD – PECVD • Metals • Semiconductors Spin-on deposition • Dielectrics • Dielectrics (Doped glasses) • Metals • Polymers (Photoresist) Wafer Bonding Liquid Phase Epitaxy – LPE • Anodic bonding (Silicon/PYREX) • Semiconductors III-V • Fusion bonding (Silicon/Silicon)
19 August 2002. Ole Hansen MIC Topics in Microelectronics
15 Thin Film Deposition Important Issues Step Coverage
Perfect, Conformal Poor Step Coverage No Step Coverage (LPCVD PECVD PVD-evaporation)
Trench Filling
Perfect Fill Partial Fill Poor Fill
Additionally: Growth temperature. Uniformity < 5%. Adhesion. Morphology, stoichiometry & density. Pinhole density < 1/cm2. Stress – built-in and thermal mismatch. 19 August 2002. Ole Hansen MIC Topics in Microelectronics
Oxidation Furnaces
O2 or H2O
Diffusion SiO2 Reaction 800-1200oC Si+O2 ®SiO2 Si+ 2HO®SiO +2H N , O , H , H O, HCl 2 2 2 2 2 2 2 Silicon 2x4 Dummy-wafers 3 Test-wafers 15-190 Production-wafers Typical19 August 2002boat. -load Ole Hansen MIC Topics in Microelectronics
16 LOCOS: Local Oxidation
• Silicon nitride mask Silicon Nitride • Low diffusivity of O2, H2O in Si3N4 • Slow oxidation of Si3N4 Oxide LOCal Oxidation of Silicon Silicon LOCOS • Reduced surface topography • Self aligned diffusions • Used in most modern processes • Needs a stress relieve oxide - Pad oxide – Eliminates slip lines in Si
19 August 2002. Ole Hansen MIC Topics in Microelectronics
Epitaxy
• Single Crystal Films on Single Crystal Substrates • Homo Epitaxy: Substrate & Film Identical – Si/Si, GaAs/GaAs etc. • Hetero Epitaxy: Substrate & Film Different – GaAlAs/GaAs, SiGe/Si
Single Crystal Film
Single Crystal Substrate
19 August 2002. Ole Hansen MIC Topics in Microelectronics
17 Vapour Phase Epitaxy – VPE The Horizontal Reactor
•Indirectly Heated Cold Wall Reactor •Growth Temperature: 1000-1300oC •Pressure ~1Bar or less •Very high gasflow rates Other Si-Sources: •Low Si-compound Molar Fraction Silane: SiH 4 •Quite Low Wafer Throughput Chlorosilane: SiHxCly 19 August 2002. •HClOle Hansen used MIC for pre-epi etch/clean Topics in Microelectronics
Elemental Steps in VPE/CVD
1. Convective Gas Phase Transport
2. Gas Phase Reactions: SiCl4+H2 « SiCl2+2HCl 3. Mass Transport to Surface (Diffusion) 4. Adsorption to surface (Desorption from Surface) 5. Chemical reaction on Surface 6. Diffusion on Surface 7. Lattice Incorporation 8. Product Desorption 9. Mass Transport from surface 1 2 9 8 5 3 4 5 6 7
19 August 2002. Ole Hansen MIC Topics in Microelectronics
18 Step Coverage &Trench Filling Important Parametres
Source Material: • Flux intensity • Angular Distribution Source/sample: • Sticking coefficient Improved Coverage/Filling: • Surface Diffusivity High surface diffusivity • Surface Reaction Rate Low sticking coefficient • Product Desorbtion Wide angular distribution (SC) Sample: Low flux intensity Low reaction rate • Angle of view High angle of view
19 August 2002. Ole Hansen MIC Topics in Microelectronics
Low Pressure CVD - LPCVD
Low pressure => Improved Hot wall reactor Masstransport=> High productivity High productivity P~0.01-1Torr o 19T~400 August 2002-900. C Ole Hansen MIC Topics in Microelectronics
19 APCVD – LPCVD Growth Rate Temperature Dependence
APCVD/LPCVD Growth Rates Compared. Source gas concentration assumed constant. Surface reaction control maintained at higher rates & temperatures. 19 August 2002. Ole Hansen MIC Topics in Microelectronics
LPCVD TEOS Oxide
Si(OC2H5)4
Good Step Coverage & Trench Filling – Almost Conformal Due to high mobility of TEOS on the surface 19 August 2002. Ole Hansen MIC Topics in Microelectronics
20 Problems in APCVD - LPCVD
Low temperature deposition impractical: • Very low rates at low temperature
– ~exp(-Ea/kT) • Low film quality – Porous due to low surface diffusivity – Poor step coverage/ trench filling • High sticking coefficient • Low surface diffusivity • Solution: PECVD – Plasma Enhanced CVD
19 August 2002. Ole Hansen MIC Topics in Microelectronics
Plasma Enhanced CVD PECVD Gas inlet, e.g SiH+NO42
Cathode Darkspace 200-400Co RF Plasma Glow 0.1-10Torr
Heated Electrode
To Pump
The gas discharge creates:
Reactive, Energetic Molecular Fragments – Increases ks Energetic Ions – Ion bombardment densify the film
19Result: August 2002High. deposition ratesOle Hansen & dense MIC films at low temperatures. Topics in Microelectronics
21 PECVD Oxide Film o SiH4+N2O at 300 C
Poor trench filling, voids & cracks due to low surface mobility Step coverage and trench filling is a general PECVD problem. 19 August 2002. Ole Hansen MIC Topics in Microelectronics
PVD - Evaporation
Rate monitor Cold surface Condensation
Evaporation Hot surface -5 p < 10 Torr Shutter l > 5m Basic principle Vacuum chamber •Very flexible tool •Wide range of pure materials •”No” gas-phase collisions •Line-of-sight deposition Vacuum •High purity possible pump •UHV, p < 10-9 Torr 19 Augustsystem 2002. Ole Hansen MIC Topics in Microelectronics •Pure source & e-beam
22 Vapour Pressure
Clausius- Clapeyron (Carnotengine) : dp DH = dT T(vg - vl )
Ideal gas: pvg = RT dp DH pDH = » dT æ RT ö RT 2 Useful Rates Tç -vl ÷ è p ø æ DH ö DH p = C expç- ÷ , or ln p = - + A è RT ø RT Real materials: DH = f (T ) DH ln p = - + A+ BlnT + DT + + RT Parameters, see CRC handbook. Evaporation: Melt®Gas Melting Point Sublimation: Solid®Gas
19 August 2002. Ole Hansen MIC Useful rates: p >10mTorr Topics in Microelectronics
PVD – Sputtering: Basic Principle Energetic ions, usually Ar+, knock Wafer out source atoms. These atoms travel and deposit on the substrate. e- e- e- + + Gas phase collisions occur before e- e- deposition, p~10-100mTorr, l<5mm. - + + e + - Result: Deposited atoms arrive from + + e a wide space angle. Improved step coverage.
Sputter yields (atoms/ion) rather insensitive to material: Alloy deposition possible Target: source material 19 August 2002. Ole Hansen MIC Topics in Microelectronics
23 DC - Sputtering
Plasma sustaining reaction: Impact ionisation Ar + e- ® Ar+ + 2e- Improbable at low pressure, Basic two-electrode DC sputter system since l ³ L,
p~10-100mTorr, VDC~0.5-5kV. Improbable at high pressure, since E ~ lV/L < E 19 August 2002. Ole Hansen MIC ionization Topics in Microelectronics
Electroplating
Reduction of Cu Low cost technology Cu++ + 2e- ® Cu Easily scaled to industrial scale Current density ~1-5A/dm2 Plastic Wafer Holder Temperature 20-70oC
Stress & morphology affected by Cathode ++ ++ •Current density Cu Cu •Temperature •Bath composition CuSO +H OCu++ 4 2 Cu++ •Bath additives
Cu Anode Materials: Cu, Ni, Au, NiFe, CoNiFe, Sn Oxidation & dissolution of Cu ++ - Cu ® Cu + 2e Problems: Purity, uniformity, 19 August 2002. Ole Hansen MIC composition & stress control Topics in Microelectronics
24 Electroplated Ni Structures
Cu Si
Ni Cu Si
Ni
Si
Cu sacrificial layer
19 August 2002. Ole Hansen MIC Topics in Microelectronics
Wafer Bonding Anodic & Fusion Bonding
Anodic Bonding Fusion Bonding
PYREX - Silicon 400V Hotplate 300-400Co Room Temperature High Temperature + Pre-Bonding Anneal Needs:Thermal Matched Pyrex Needs: Particle Free Surfaces Medium Ionic (Na)+ Conductivity Atomically Smooth Surfaces Low Particle Density
19 August 2002. Ole Hansen MIC Topics in Microelectronics
25 Modern Metallizations A Paradigm Shift
Cu conductors: lower resistivity & electromigration TiSi2/Si contacts: improved stability & lower contact resistance TiN barrier: protects Si against ”unfriendly” Cu Fabrication using CMP – Chemical mechanical polishing
19 August 2002. Ole Hansen MIC Topics in Microelectronics
Selfaligned TiSi2/Si Contacts TiN Local Interconnects Sputterdeposit Titanium
Gate Oxide Silicide formation: 1. Diffusion of Si in Silicide Silicon Substrate 2. Reaction Ti+Si ® TiSi2 3. Selective, only in contacts 4. Simultaneous thermal nitridation possible Form TiSi22 & TiN ( N @ 600C, 1 min.) Selective masked TiN etch 5. Oxygen free atmosphere Gate Oxide Stable contacts TiSi2/Si Local TiN interconnects (10W) Silicon Substrate Integrated TiN diffusionbarrier Local interconnect ”price” 1 mask-etch step 19 August 2002. Ole Hansen MIC Topics in Microelectronics
26 Single Damascene Metallisation
(PE)CVD Oxide CMP Planarize Etch Contacts (RIE)
TiN & LPCVD W CMP (Remove W) Sputter/Pattern Al/TiN
Quite planar structures Multilayer by repetition
19 August 2002. Ole Hansen MIC (PE)CVD SiO2 CMPTopics Planarizein Microelectronics
Conventional & Damascene Metal
19 August 2002. Ole Hansen MIC Topics in Microelectronics
27 Double Damascene
(PE)CVD Oxide CMP Planarize LPCVD SiN342 & SiO
RIE-etch Metal-trench RIE-etch Contacts Sputter TiN or TaSiN Deposit Cu or Al
Perfectly planar Multilayer by repetition Metal etching eliminated Cu can be used 19 August 2002. Ole Hansen MIC No separate via plug CMP (Remove Metal) (PE)CVDTopics in Microelectronics Insulator
Double Damascene Cu Multilayer Metallization
19 August 2002. Ole Hansen MIC Topics in Microelectronics
28 Material Modification Ion-implant Doping
Disadvantages: •Crystal damage •Anomalous TED •transient diffusion •Insulator charging
Preferred doping method: •Accurate dose control •High purity •Buried profiles •Easy masking
19 August 2002. Ole Hansen MIC Topics in Microelectronics
Composite Masking & Self-Registration Implant B++ or As Resist Gate Oxide
Silicon Substrate
Gate Oxide
Silicon Substrate
Final Structure: FieldMaskGateMas**kImplantMask Perfect registration : SourceDrain Diffusion-,Gate SourceDrain Diffusion-FieldOxide 19 August 2002. Ole Hansen MIC Topics in Microelectronics
29 Sub-Lithographic Features - Sidewall Oxide Deposit Conformal CVD Oxide
Gate Oxide
Silicon Substrate
Anisotropic RIE Oxide Etch & Implant Gate Oxide
Silicon Substrate
Lightly Doped Drain with offset Heavily Doped Drain-Contact 19 August 2002. Ole Hansen MIC Topics in Microelectronics
Device Isolation LOCOS or Trench/CMP
Form SiN/SiO/Si342 Field Pattern Implant Channel Stop
Thermal Oxidation
CVD Oxide fill
Local Oxidation
Chemical Mechanical Polish
LOCOS Isolation CMP Isolation Planar & Compact 19 August 2002. Ole Hansen MIC Topics in Microelectronics
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