by S. Wolf Chapter 15 ALUMINUM THIN-FILMS and SPUTTER-DEPOSITION 2004 by LATTICE PRESS
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 15-2
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 Steps of Physical-Vapor Deposition (PVD) 15-3
ALUMINUM THIN-FILMS Aluminum: Main IC Interconnect Metal Until 2000 Low-Resistivity Excellent Adhesion to SiO 2 Ohmic-Contacts to p + & n + Si Al-Alloys Used: Al-Si Al-Cu Deposited by Magnetron Sputtering Phase-Diagram of a Aluminum-Silicon System 15-4
ALUMINUM THIN-FILMS 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 TiSi 2 /TiN - Now Al-Cu-Alloy Suppresses: Electromigration Hillocks (a) Junction-Spiking & Silicon-Migration During Contact-Sintering (b) Pit Formation in Al Contact to Si 15-5
SPUTTER-DEPOSITION PROCESS STEPS 1. Generate Ions & Accelerate To Target 2. Ions Sputter Target-Atoms 3. Sputtered-Atoms Transported to Wafer 4. Condensation Forms Film on Wafer Billiard-Ball Model of Sputtering Noble-Gases (e.g. Ar) Used to Sputter No Chemistry Steps of Sputtering-Process (a) Billiard-Ball model of Sputtering (b) Binary Collision of Atoms A & B followed by Binary Collision of B & C (c) Energetic-Ions Strike a Sputtering-Target Surface 15-6
SPUTTER-YIELD 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 Sputtering-Yields of Noble Gases on Copper, as Function of Energy 15-7
MAGNETRON-SPUTTERING 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 (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 15-8
MAGNETRON-SPUTTERING SOURCES 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 Perspective drawing of a Planar, Circular-Magnetron Sputtering-Source 15-9
SPUTTER-DEPOSITION EQUIPMENT Sputter-Chamber Target Vacuum-Pumps Power-Supplies Sputter-Gas Distribution System Controls & Gauges Wafer-Handling System Batch: 150-mm Single-Wafer: 200-mm Schematic-Drawing of Components of a Generic Sputtering-System 15-10
COMMERCIAL SPUTTERING-TOOLS 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 Layout of ENDURA Sputtering-System 15-11
PROCESS CONSIDERATIONS OF SPUTTERING 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 Depiction of Step-Coverage showing Bottleneck due to Buildup of Material on Top Corners 15-12
ULSI SPUTTERING - 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 Schematic of a Collimator in Sputtering Tool & Photo of Collimators 15-13
ADVANCED PVD-SOURCES 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 Schematic of Magnetron Sputter-Chamber with rf-coil. Allows Ionized-PVD to occur. Better Bottom-Coverage than with Collimators Hollow-Cathode: Alternative- Source for Coating High- Aspect-Ratio Holes Hollow-Cathode-Source 15-14
METAL FILM-THICKNESS MEASUREMENT Direct-Measurement Surface Profiling Device (Profilometer) Create Step with Post- Deposition Etch-Step Indirect-Measurement Measure Sheet-Resistivity & Calculate Thickness Photo-Acoustics Schematics of Surface-Profilometers 15-15
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? 15-16