MICROCHIP MANUFACTURING by S. Wolf

Transcription

1 MICROCHIP MANUFACTURING by S. Wolf Chapter 22 DRY-ETCHING for ULSI APPLICATIONS 2004 by LATTICE PRESS

2 CHAPTER 22 - CONTENTS Types of Dry-Etching Processes The Physics & Chemistry of Plasma-Etching Etching Silicon & Silicon Dioxide in Fluorocarbon Plasmas Anisotropic Etch-Mechanisms Dry-Etching of Various Types of Materials in ULSI Processing Process Monitoring: Endpoint Detection Batch Dry-Etch-Equipment Configurations Single-Wafer Etchers High-Density Plasma Sources MICROCHIP MANUFACTURING 2004 by LATTICE PRESS Sunset Beach CA 22-2

3 DRY-ETCHING FOR ULSI Feature-Sizes smaller-than 1.0-µm cannot be defined with Isotropic Wet-Etching Processes Anisotropic-Etching is necessary to form Submicron-sizes Dry-Etching is Anisotropic! It also eliminates Handling, Consumption, & Disposal of large quantities of dangerous Acids & Solvents Dry-Etching Steps: Feed-Gases Enter Etch-Chamber Plasma Creates Reactive- Species (Etchants) Etchants Diffuse to Wafer-Surface Etchants Adsorb on Surface Etch-Reaction Occurs Etch-Products Diffuse away from Wafer-Surface Etch-Products Pumped Out MICROCHIP MANUFACTURING Microscopic Processes that occur during Dry-Etching of Silicon Wafers 2004 by LATTICE PRESS Sunset Beach CA 22-3

4 Lower-Figure shows Various Etching-Mechanisms Physical Basis (Sputter Etching or Ion Milling) Highly Anisotropic Non-Selective Chemical Basis (Plasma Etching & Wet-Etching) Highly-Selective Isotropic Combination of the Two (Reactive-Ion Etching - RIE) Controlled Anisotropy Adequate Selectivity PROBLEMS OF DRY-ETCHING Mask Selectivity - Line Width Control Substrate Selectivity - Device Damage Plasma-Etch Damage - Device Damage Corrosion Removal of Stringers Removal of Post-Etch Residue End-Point Detection High-Cost & Low-Throughput The Etching Spectrum 2004 by LATTICE PRESS 22-4

5 PHYSICS & CHEMISTRY OF PLASMA-ETCHING Chemically-Reactive Species are created by an rf-glow-discharge from a Relatively-Inert Gas Reactive-Species Do the Etching Sequence of Steps is Shown on 22-1 Slowest-Step sets Etch-Rate Reactive-Species Created by rf-glow-discharge: Radicals - Higher-Concentration - Responsible for Etching Ions - Lower-Concentration - Create Surface-Damage & Enhance Etch-Rate Plasma-Potential produces Ion- Bombardment Parameters Impacting Plasma-Etch rf Power & Frequency Gas-Types & Flow Rates Surface-Conditions & Temperature Electrical-Potential of Surface RF-glow-discharge including reactions & species present in a dry-etching plasma MICROCHIP MANUFACTURING 2004 by LATTICE PRESS 22-5

6 DRY-ETCHING SILICON & SILICON-OXIDE IN FLUOROCARBON PLASMAS Important Etch-Processes for IC Manufacturing Rf-Glow-Discharge is needed to Create Etchants: CF 4 does not Etch Si or SiO 2 But F-atoms do! Flow CF 4 into Etch Chamber Rf-Glow-Discharge Dissociates & Ionizes CF 4 into Various Fragments CF 4 + e CF F + 2e where: CF 3 + are Ions & F are Radicals F-atoms Etch Si by forming SiF 4 SiF 4 breaks away from Si-Surface and diffuses away from it Proposed mechanisms for F-atom reaction with Si-film leading to products SiF 2 & SiF 4 MICROCHIP MANUFACTURING 2004 by LATTICE PRESS 22-6

7 DRY-ETCHING SILICON & SILICON-OXIDE IN FLUOROCARBON PLASMAS Adding O 2 or H 2 to CF 4 changes Plasma-Etching of Si & SiO 2 Adding O 2 Increases Si Etch-Rate O 2 combines with C and CF x to produce CO 2 Increases Concentration of Free-F-atoms Si Etch-Rate is Increased But, too much O 2 dilutes CF 4 Si Etch-Rate Slows Adding H 2 Decreases Si Etch-Rate, But Does Not Slow-Down SiO 2 Etch-Rate H 2 Combines with Free-F-atoms to form HF Fewer F-atoms available to Etch Si But, Etching SiO 2 releases O The Released-O Combines with H 2 to form H 2 O Etch-Rate of SiO 2 Remains Unchanged Adding H 2 to CF 4 is Effective to Increase SiO 2 -to-si Selectivity - Needed for Etching Contact-Holes in SiO 2 Etch-Rate of Si & SiO 2 versus O 2 - Concentration in CF 4 -O 2 Plasma Etch-Rate of Si, Resist, & SiO 2 as Function of Concentration of H 2 in CF 4 -H 2 Etch-Gas MICROCHIP MANUFACTURING 2004 by LATTICE PRESS Sunset Beach CA 22-7

8 ANISOTROPIC DRY-ETCHING MECHANISMS Etching of Si with F-atoms is a Purely Chemical Process & Proceeds in all Directions at Same Rate - Isotropic To Etch Small-Features: Anisotropy is Needed Ion-Bombardment must be Exploited Winters-Kay Experiment (see Figure) XeF 2 -Gas Alone: Etches Si Very Slowly Ar-Ion-Bombardment Alone: Etches-Si Very-Slowly Combine XeF 2 + Ar Ions: Etches Si 10X Faster Cooperative-Effect of Both Enhances Etch-Rate Use this Result to Seek Anisotropic-Etch Ion-assisted gas-surface chemistry in etching of Si with XeF 2 MICROCHIP MANUFACTURING 2004 by LATTICE PRESS 22-8

9 ANISOTROPIC DRY-ETCHING MECHANISMS TWO MODELS ARE PROPOSED FOR HOW ENERGETIC-IONS ENHANCE ETCH-RATES & PRODUCE ANISOTROPIC-PROFILES High-Energy Ions (> 50-eV) produce Damage where they Strike. Damage Extends Several-Monolayers Beneath Surface & Chemical Reaction-Rates are Enhanced. Feature-Sidewalls get no Bombardment & Etching proceeds more slowly. Lower-Energy Ions (< 50-eV) provide enough energy to Desorb Non-Volatile Polymer-Layers that Deposit & are Physisorbed on Surfaces being Etched (Very-Little Energy is needed to Dislodge them). Surfaces not struck by such Ions retain these Surface-Inhibiting Layers and are not Etched (a) Surface-Damage Mechanism (b) Surface-Inhibitor mechanism for Ion-Assisted Anisotropic Etching MICROCHIP MANUFACTURING 2004 by LATTICE PRESS Sunset Beach CA 22-9

10 ANISOTROPIC DRY-ETCHING PROBLEMS Completely-Anisotropic Etching Also Introduces New Difficulties Since Etching Proceeds Vertically, Residue-Material is Left at Steps: STRINGERS, PICKET-FENCES If Conductive, Will Short-Out Adjacent Conductor Lines Must be Removed by Overetching But, Nonconductive Residues are used as MOSFET- Spacers Variations in Via-Depth From Anisotropic- Etching: Deposition Problems (a) Residue at base of steps after anisotropic-etch process (b-d) Stringers arising after anisotropic-etch process MICROCHIP MANUFACTURING Via-depths vary in doublelevel metal process if fullplanarization exists 2004 by LATTICE PRESS 22-10

11 DRY-ETCHING VIAS IN SILICON-OXIDE Contact-Hole & Via Etching are Critical Etch-Steps in IC Fabrication Every Opening Must be Completely Etched or Circuit will not Function Contact/Via-Etching Involves Etching SiO 2 Key Issues: Sidewall-Profile Control Selectivity of Etch to Underlying Si or Poly Selectivity to PR-MAsk Etch-Rate Uniformity (Microloading) Etch-Residue Removal from Contacts Wine-Glass Shape Uses Two-Step Etch Isotropic Anisotropic For 0.35-µm & Smaller Vertical-Profiles are Needed (a) & (b) Wineglass Via-Etch Profile (c) Drawing of Vertical & Tapered-Etch Vias (d) SEM of Vertical-Etched Vias MICROCHIP MANUFACTURING 2004 by LATTICE PRESS Sunset Beach CA 22-11

12 DRY-ETCHING POLYSILICON Etching Polysilicon is another Key Etch-Step, because it establishes the Critical-Dimension (CD) of MOSFET Gates Gate-Poly CD should be ± 5% of the Design Specification High-Degree of Anisotropy Also Required (see Figure) High-Degree of Selectivity with respect to SiO 2 Thin-Gate Oxide Under Poly Overetch Needed to Remove Stringers Since F-atoms Attack Si Isotropically, F-based Gases Not Used Cl & Br-Based Etch-Gas- Mixtures are Common Multi-Step Process Poly-Oxide Breakthrough Step Polycide Etch Step Main Polysilicon Etch Step Overetch Step to Remove Stringers Polysilicon Etching MICROCHIP MANUFACTURING 2004 by LATTICE PRESS Sunset Beach CA 22-12

13 DRY-ETCHING ALUMINUM Aluminum-Films are Etched to form Interconnect Lines of ICs Anisotropic Dry-Etching Needed for Submicron-Wide Al-Lines F-Based Gases cannot be Used - AlF 3 is NonVolatile Cl-Based Gases Etch Al - AlCl 3 is Sufficiently Volatile Problem Arises due to Al 2 O 3 -Skin on Al-surfaces Cl does not Etch Al 2 O 3 Must use BCl 3 to Remove Al 2 O 3 Etch-Gas: Mixture Cl 2 + BCl 3 Loadlocked-Chamber to Keep H 2 O Out Loadlocking Difficult in Batch Etchers Easier in Single-Wafer Etchers Anisotropic-Etching of Al achieved by Blocking Mechanism - Inhibitor- Layer formed from Resist + Feed- Gas Chlorocarbons (CCl x ) Vapor-Pressure of AlF 3, AlCl 3, & AlBr 3 as a function of temperature MICROCHIP MANUFACTURING 2004 by LATTICE PRESS Sunset Beach CA 22-13

14 DRY-ETCHING ALUMINUM Copper is Added to Al-Films in Small Quantities (1-2%) to Improve Electro- Migration Behavior Creates Al-Etch Difficulties Vapor-Pressure of CuCl too Low CuCl Must be Etched by Ion- Bombardment - Higher-Energy RIE Al Post-Etch Corrosion Al 2 Cl 3 reacts with H 2 O to form HCl HCl Attacks Al After Al-Dry-Etch H 2 O in Air causes Corrosion in this Way Worse with Cu in Al-Film Post-Etch Treatment Needed In-Situ Resist Strip in O 2 CF 4 -Plasma Immediate UPW-Rinse Vapor pressure of CuCl as function of temperature Corrosion of etched Al lines MICROCHIP MANUFACTURING 2004 by LATTICE PRESS Sunset Beach CA 22-14

15 ENDPOINT-DETECTION Etch Processes Must be Well- Controlled for Reproducibility Etch should proceed to End, But Not Beyond! - Selectivity Since so many Factors Impact Etch-Rate, Can t Use Timed-Etch Need End-Point Detection Two Methods to Detect End-Point Optical-Reflection Transparent-Films - Interference Laser-Light Reflected from Film Needs Special Structure on Wafer Emission-Spectroscopy Light from Plasma Glow- Discharge is Monitored Different Species in Plasma Emit Light at Different Wavelengths Look for Change in Emitted- Light Intensity Apparatus for Optical-Reflection Method of End-Point Detection Apparatus for using Emission-Spectroscopy as an End-Point Detector MICROCHIP MANUFACTURING 2004 by LATTICE PRESS 22-15

16 DRY-ETCH EQUIPMENT CONFIGURATIONS Batch Reactors PLASMA-ETCH SYSTEMS CONSIST OF THE FOLLOWING SUBSYSTEMS Etch Chamber Pumping-System Pressure-Gauges Power-Supplies Gas-Handling System Electrodes BARREL ETCHER First Dry-Etcher Isotropic Etcher Used to Etch Si 3 N 4 PARALLEL-ELECTRODE Reinberg Design (1980s) Anisotropic-Etcher Barrel Plasma-Etching System Parallel-Electrode (Planar) Type Dry-Etcher MICROCHIP MANUFACTURING 2004 by LATTICE PRESS 22-16

17 DRY-ETCH EQUIPMENT CONFIGURATIONS: Batch Reactors Most Widely-Used Batch-Etcher: Cylindrical (Hexode) Tool Invented at ATT-Bell Labs Commercialized by Applied Materials Inner-Electrode is a Hexode - Each Face Holds Multiple-Wafers Chamber-Wall is Outer-Electrode Highly-Asymmetrical Parallel-Electrode Configuration Allows Energetic- Ion-Bombardment of Wafers Only Built for Wafers Up to 150-mm in Size Single-Wafer-Etchers were used for larger Wafer Sizes Single-Wafer-Etchers Pioneered by LAM Hexode Batch-Etcher MICROCHIP MANUFACTURING 2004 by LATTICE PRESS Sunset Beach CA 22-17

18 Single-Wafer & High-Density-Plasma (HDP) Reactors Single-Wafer-Etchers replaced Batch-Mode when Wafers reached 200-mm Parallel Plate Downstream Medium-Density - MERIE (Magnetically-Enhanced) High-Density-Plasma (HDP) Sources Produce 10X-100X Higher Ion-Densities More Ion-Flux to Wafer Less-Energetic Ions Less Plasma Damage Plasma Gen by Magnetic & Electric Fields MERIE-Reactor with permanent magnets Four-HDP Etcher Types Electron-Cyclotron Resonance (ECR) Helicon Helical Resonator Inductive Types of high-density plasma (HDP) sources: (a) Electron cyclotron resonance (ECR) (b) Helicon (c) Helical resonator (d) Inductive MICROCHIP MANUFACTURING 2004 by LATTICE PRESS 22-18

19 ELECTROSTATIC CHUCKS Became Standard Chucks for holding Wafers in many Process Tools in 1990s Eliminate need for Front-Side Ring-Clamps or Clips used Earlier to hold Wafers Wafers also Pulled-Flat to Chuck Enhances Cooling-Efficiency & Reduces Particle-Generation Static-Charge Generated by applying Voltage to Surface- Electrode (Insulated from Wafer-Backside by Polyimide-Film) Electrical Attraction between Wafer & Chuck Clamps Wafer to it MICROCHIP MANUFACTURING Electrostatic Chuck 2004 by LATTICE PRESS Sunset Beach CA 22-19

20 SUMMARY OF KEY CONCEPTS Etching of Thin-Films is a Key Technology in Modern Microchip Manufacturing Dry-Etching is used almost exclusively today because of the Control, Flexibility, Reproducibility, and Anisotropy it provides Challenges include: Selectivity, Etch Damage, & Residues Reactive Neutral-Species (e.g., Free Radicals) & Ionic-Species play roles in Etching Generally, Neutral-Species produce Isotropic-Etching & Ionic-Species produce Directional-Etching Modern Dry-Etch Processes combine Chemical & Physical Etch-Phenomena to Achieve Needed Anisotropy & Selectivity There are 2 Anisotropic Etch-Mechanisms: Damage & Blocking Most Thin-Films used in ULSI can be Dry-Etched (not Copper) MICROCHIP MANUFACTURING 2004 by LATTICE PRESS Sunset Beach CA 22-20