Metallization

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Contents Definition Applications Typical material used in VLSI Metallization methods Metallic thin film measurement Future Trends---Dersun--2

Definition Metallization A processes that deposit metal thin film on the wafer surface.

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Applications Contact electrodes Interconnection Plug and connection Bonding pad

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Application: Contact ElectrodeTi/TiN TiN, ARC Metal 1, AlCu W STI n+ n+ BPSG P-Well USG p+ N-Well p+ TiSi2

P-epi P-wafer---Dersun--5

Application: Interconnection

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Application: Plug and ConnectionTi/TiN M1 Cu SiN FSG FSG PSG STI n+ Cu CoSi2 Ta or TaN Cu

W W

n+ USG P-Well P-Epi P-Wafer---Dersun---

p+ N-Well

p+

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Contents Definition Applications Typical material used in VLSI Metallization methods Metallic thin film measurement Future Trends---Dersun--8

Metallization material: Criteria Low resistivity Good adherence and low stress Smooth for high resolution patterning Compatible with VLSI deposit and etch process High resistance to electro-migration Low diffusivity in Si and SiO2 Stable and no degradation in VLSI lifetime---Dersun--9

Four best conducting metal The resistivities of best conducting metal Silver Copper Gold Aluminum 1.6 cm 1.7 cm 2.2 cm 2.65 cm

Aluminum was used for gate before mid-1970 Al was the most commonly used metal in VLSI---Dersun--10

Most Common Used Material in Modern VLSI Metallization Aluminum and aluminum alloys Polysilicon and silicide Titanium (Ti) and titanium alloys (TiSi) Tungsten (W) Cobalt (Co) Copper (Cu) Tantalum (Ta)---Dersun--11

Common Conducting Thin Films in VLSI Aluminum and Aluminum alloy Polysilicon and Poly-Silicides Titanium and Ti-silicide Cobalt and Co-Silicide Titanium Nitride Tungsten Copper Tantalum---Dersun--12

Some Facts About Aluminum

Name Aluminum Symbol Al Atomic number 13 Atomic weight 26.981538 Discoverer Hans Christian Oersted Discovered at Denmark Discovery date 1825 Origin of name From the Latin word "alumen" meaning "alum" Density of solid 2.70 g/cm3 Molar volume 10.00 cm3 Velocity of sound 5100 m/sec Hardness 2.75 Electrical resistivity 2.65 cm Reflectivity 71% Melting point 660 C Boiling point 2519 C Thermal conductivity 235 W m-1 K-1 Coefficient of linear thermal expansion 23.1 10-6 K-1 Etchants (wet) H3PO4, HNO4, CH3COOH Etchants (dry) Cl2, BCl3 ---Dersun--13 CVD Precursor Al(CH3)2H

Typical Aluminum Deposition EquipmentWafers Aluminum Charge

Aluminum Vapor 10-6 Torr

To Pump---Dersun---

High Current Source14

Formation of Junction Spike Al make direct contact with Si at source/drain Si dissolves in Al and Al diffuses into Si Junction spike Aluminum spikes punctuate doped junction Short source/drain with the substrate

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Junction Spike

Al p+

SiO2

Al p+

Al

n-type Silicon

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Prevention of Junction Spike Method A: ~1% of Si in Al saturates it. Method B: Thermal anneal at 400 C to form Si-Al alloy after metallization.

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Mechanism of Al Electro-migration Aluminum is a polycrystalline material Many mono-crystalline grains Current flows through an aluminum line Electrons constantly bombards the grains Smaller grains will start to move This effect is called electro-migration---Dersun--18

Electro-migration Effects Electro-migration will tear the metal line apart and make the metal line narrower. Narrow metal line cause higher current density Aggravates the electron bombardment Causes further aluminum grain migration Eventually will break of the metal line

Affect the IC chip reliability Aluminum wires: fire hazard of old houses---Dersun--19

Prevention of Electro-migration Alloy a small percent of copper with aluminum, can significantly improve electro-migration resistance of aluminum Copper serves as glue between the aluminum grains and prevent them from migrating due to the electron bombardment Al-Si-Cu alloy was used Al-Cu (0.5%) is very commonly---Dersun--20

Conducting Thin Films Aluminum and Aluminum alloy Polysilicon and Poly-Silicides Titanium and Ti-silicide Cobalt and Co-Silicide Titanium Nitride Tungsten Copper Tantalum---Dersun--21

Applications of TitaniumAl-Cu Ti

W PSG Ti

TiSi 2

n+

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W Plug and TiN/Ti Barrier/Adhesion LayerTungsten TiN/Ti

Oxide

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Contents Definition Applications Typical material used in VLSI Metallization methods Metallic thin film measurement Future Trends---Dersun--24

Metallization Methods Physical Vapor Deposition (PVD) --evaporation --sputtering Chemical Vapor Deposition (CVD) Electrochemical Plating Deposition (EPD) Electroless Chemical Plating---Dersun--25

Physical Vapor Deposition(PVD)Vaporizing solid materials Heating(evaporating) or sputtering Condensing vapor on the substrate surface Very important part of metallization

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PVD Methods Evaporation Sputtering

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Evaporation Methods Filaments Flash hot plate Electron beam

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Thermal EvaporatorWafers Aluminum Charge

Aluminum Vapor 10-6 Torr

To Pump---Dersun---

High Current Source29

Electron Beam EvaporatorWafers Aluminum Charge

Aluminum Vapor 10-6 Torr

Electron Beam

To Pump---Dersun---

Power Supply30

Sputtering Methods DC Diode simplest and cheapest DC Triode RF Diode for nonconductive film RF Triode DC Magnetron -- most commonly used RF Magnetron---Dersun--31

DC Diode Sputtering-V Target Argon Plasma Wafer Chuck Metal film---Dersun---

Wafer32

Principle of Sputtering

Ar+ Momentum transfer will dislodge surface atoms off---Dersun--33

Collimated Sputtering Used for Ti and TiN deposition Collimator allows metal atoms or molecules to move mainly in vertical direction Reach the bottom of narrow contact/via holes Improves bottom step coverage

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Collimated SputteringMagnets Target Plasma Collimator Film Via holes---Dersun--35

Inductive Coupled Sputtering Ti, TiN, Ta, and TaN deposition Ionize metal atoms through inductive coupling of RF power in the RF coil Positive metal ions impact with the negatively charged wafer surface vertically Improving bottom step coverage Reduce contact resistance---Dersun--36

Inductive Coupled SputteringV Target Inductive Coils

Ionized Metal Plasma RF Via Hole

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Cluster Tool, Aluminum CVD/PVDPre-clean Ti/TiN PVD TiN CVD Transfer Chamber Wafer Unloading Cooldown Al-Cu PVD---Dersun--38

Wafer Loading Transfer Chamber

Al CVD

Endura PVD SystemPVD Target PVD Chamber CVD Chamber---Dersun--39

Sputtering vs. EvaporatorSputtering Purer film Better uniformity Single wafer, better process control Larger size wafer---Dersun---

Evaporator More impurities Batch process Cheaper tool

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Metallization Methods Physical Vapor Deposition (PVD) --evaporation --sputtering Chemical Vapor Deposition (CVD) Electrochemical Plating Deposition (EPD) Electroless Chemical Plating---Dersun--41

Properties of CVD Metallization Good step coverage and gap fill capability Can fill tiny contact holes to make connections between metal layers. Widely used to deposit metal

Poorer quality and higher resistivity than PVD metal thin films. Used for plugs and local interconnections Not applied for global interconnections---Dersun--42

Metal CVD Process Steps Wafer into the chamber Slip valve closes Set up pressure and temperature, with secondary process gas(es) All process gases flow in, start deposition Termination of the main process gas. Secondary process gas(es) remain on Termination of all process gases Purge chamber with nitrogen Slip valve opens and robot pull wafer out---Dersun--43

CVD Metallization Methods Thermal CVD heat provides free energy needed for the chemical reaction PECVD plasma enhanced CVD process Typically used for W, WSix, Ti, and TiN RF system is used for plasma dry clean of the process chamber---Dersun--44

PECVD ChamberProcess Gases RF Power

Process Chamber

Wafer

Heated plate To pump

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PVD vs. CVD CVD: Chemical reaction on the surface PVD: No chemical reaction on the surface CVD: Better step coverage (50% to ~100%) and gap fill capability PVD: Poor step coverage (~ 15%) and gap fill capability---Dersun--46

PVD vs. CVD PVD: higher quality, purer deposited film, higher conductivity, easy to deposit alloys CVD: always has impurity in the film, lower conductivity, hard to deposit alloys

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Metallization Methods Physical Vapor Deposition (PVD) --evaporation --sputtering Chemical Vapor Deposition (CVD) Electrochemical Plating Deposition (EPD) Electroless Chemical Plating---Dersun--48

Contents Definition Applications Typical material used in VLSI Metallization methods Metallic thin film measurement Future Trends---Dersun--49

Metal Thin Film Measurements Thickness. Stress Reflectivity Sheet resistance

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Summary Mainly application: interconnection CVD (W, TiN, Ti) and PVD (Al-Cu, Ti, TiN) Al-Cu alloy is still dominant Need UHV for Al-Cu PVD W used as plug Ti used as welding layer TiN: barrier, adhesion and ARC layers The future: Cu and Ta/TaN---Dersun--51