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MICROCHIP MANUFACTURING by S. Wolf

Chapter 15

ALUMINUM THIN-FILMS andSPUTTER-DEPOSITION

© 2004 by LATTICE PRESS

© 2004 by LATTICE PRESS Sunset Beach CAMICROCHIP MANUFACTURING 15-2

CHAPTER 15 - CONTENTS• Aluminum Thin-Films

• Sputter-Deposition Process Steps

• Physics of Sputter-Deposition

• Magnetron-Sputtering

• Sputter-Deposition Equipment

• Sputter-Process Considerations

• Step-Coverage & Via/Contact-Hole Filling by Sputtering

• Metal Film-Thickness Measurements

PHYSICAL VAPOR DEPOSITION (PVD)


Steps of Physical-Vapor Deposition (PVD)

• PVD Thin-Films Formed without Chemical-Reactions• Evaporation - Earliest PVD Process

• Sputtering - Today’s Dominant PVD Process

• Three-Steps

• Vaporize Solid Source-Material

• Transport Vapor Thru Vacuum

• Vapor Condenses on Substrate to Form Solid-Film

ALUMINUM THIN-FILMS


Phase-Diagram of a Aluminum-Silicon System

• Aluminum: Main IC Interconnect Metal Until 2000

• Low-Resistivity

• Excellent Adhesion to SiO2

• Ohmic-Contacts to p+ & n+ Si

• Al-Alloys Used:• Al-Si• Al-Cu

• Deposited by Magnetron Sputtering

ALUMINUM THIN-FILMS


(a) Junction-Spiking & Silicon-Migration DuringContact-Sintering (b) Pit Formation in Al Contact to Si

• Pure-Al Films: Junction-Spiking Destroys Devices

• Al-Si-Alloy Helps

• Shallow-Junctions Need Even More Protection

• Barrier-Metal Films Between Al & Si

• PtSi/TiW - Early

• TiSi2/TiN - Now

• Al-Cu-Alloy Suppresses:• Electromigration

• Hillocks

SPUTTER-DEPOSITIONPROCESS STEPS


Steps of Sputtering-Process

(a) Billiard-Ball model of Sputtering (b) BinaryCollision of Atoms A & B followed by BinaryCollision of B & C (c) Energetic-Ions Strike aSputtering-Target Surface

1. Generate Ions & Accelerate To Target2. Ions Sputter Target-Atoms 3. Sputtered-Atoms Transported to Wafer4. Condensation Forms Film on Wafer

• Billiard-Ball Model of Sputtering• Noble-Gases (e.g. Ar) Used to Sputter No Chemistry

SPUTTER-YIELD


Sputtering-Yields of Noble Gases on Copper, as Function of Energy

• Energy of Bombarding Ions:

• Dislodges Target-Atoms (Sputtering)• But Most Heats Target

• Sputter-Yield: Number of Ejected Target- Atoms Per Incoming-Ion

• In Sputter Processes: Ejected-Atoms/Ion ~2-3

• Off-Normal Bombardment Increases Sputter-Yield

• Faceting

MAGNETRON-SPUTTERING


(a) Motion of electron ejected from a surface with velocity v into a region having a magnetic-field B parallel to the surface, with no electric-field (b) With a linearly-decreasing electric-field

• Secondary-Electrons (SE) Needed to Sustain Discharge

• DC-Diode-Source Produces Few SE Low Sputter-Rates

• Magnetron-Source Increases SE Emission

• Magnetic-Field Keeps SE Near Target

• Higher Sputter-Rates

• Now Dominant Sputtering-Source

MAGNETRON-SPUTTERING SOURCES


Perspective drawing of a Planar, Circular-Magnetron Sputtering-Source

• Fixed-Magnets Behind Target• Target Erosion Pattern: Racetrack• Poor Target-Utilization

• Moveable Magnet Behind Target• Full-Face Erosion

• Planar, Circular Targets Now Most Popular

• Water-Cooled

SPUTTER-DEPOSITION EQUIPMENT


Schematic-Drawing of Components of a GenericSputtering-System

• Sputter-Chamber• Target• Vacuum-Pumps

• Power-Supplies• Sputter-Gas Distribution System

• Controls & Gauges

• Wafer-Handling System

• Batch: ≤≤≤≤ 150-mm• Single-Wafer: ≥≥≥≥ 200-mm

COMMERCIAL SPUTTERING-TOOLS


Layout of ENDURA Sputtering-System

• Dominant 200-mm System: Applied Materials ENDURA

• Single-Wafer, Cluster Tool

• Multi, Loadlocked Sputter-Chambers

• Ultra-High-Vacuum in Standby-Mode

• Transfer-Chamber Robotics

• Pre-Bake & Sputter-Clean Chambers

• High-Reliability & MTBF

PROCESS CONSIDERATIONS OF SPUTTERING


Depiction of Step-Coverage showing Bottleneck due to Buildup of Material on Top Corners

• Deposition-Rate & Sputtering-Tool Throughput

• Sputter-Deposition of Alloy-Films

• Sputtered-Film Adhesion

• Film Step-Coverage

• Heat Wafer During Sputtering

• Substrate-Surface Cleanliness

• Reactive-Sputtering• TiN• TaN

ULSI SPUTTERING - COLLIMATORS


Schematic of a Collimator in Sputter-ing Tool & Photo of Collimators

• As Aspect-Ratio of Holes Increases: Harder to Fill

• In PVD: Collimators Help • Plate with Honeycomb of Holes:

• Inserted Between Target & Wafer

• Intercepts Atoms Ejected from Targets at High-Angles

• Atoms Arrive at Wafer Only at Near-Normal Directions

• Improves Hole Bottom-Coverage

• Collimator Pros & Cons

ADVANCED PVD-SOURCES


Schematic of Magnetron Sputter-Chamberwith rf-Coil. Allows Ionized-PVD to occur.

Hollow-Cathode-Source

• For ≤≤≤≤ 0.25-µm: Collimator Inadequate• Ionized-PVD Steps-Up

• Sputtered Atoms Ionized During Transport to Wafer• Accelerated by dc-Bias on Chuck toward Wafer

• Add rf-Coil to Sputter-Chamber

• Better Bottom-Coverage than with Collimators

• Hollow-Cathode: Alternative- Source for Coating High- Aspect-Ratio Holes

METAL FILM-THICKNESS MEASUREMENT


Schematics of Surface-Profilometers

• Direct-Measurement• Surface Profiling Device (Profilometer) • Create Step with Post- Deposition Etch-Step

• Indirect-Measurement• Measure Sheet-Resistivity & Calculate Thickness

• Photo-Acoustics

SUMMARY OF KEY CONCEPTS


• Sputtering: Chief Method for Depositing Variety of ULSI Metal-Films

• Al-Alloys• Cu-Seed-Layer (See Chap. 24)• TiN Barrier-Layer for Al & W-Plugs• Ta & TaN Barrier-Layers for Cu

• Advantages over CVD• Magnetron-Sputtering: Predominant Process

• Other Advanced Techniques• Collimator-Assisted• Ionized-PVD• Hollow-Cathode-Source

• AMAT Endura: 80% Market-Share for 200-mm Tools

• Main 300-mm Tools May Have Different Configuration?

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