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Microelectronics Processing
Plasma Etching E. Finkman – Microelectronics Processing
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Dry Etch Chemical reaction or physical etch between gas etchants and surface material on wafer: Dry etching methods Glow discharge methods Dry physical etching (Sputter etching) Plasma assisted etching Dry chemical etching (Plasma etching) Reactive ion etching (RIE) Ion beam methods Ion mlling Reactive ion beam etching Chemical assisted ion milling Common materials to dry etch Si, SiO2, Si3N4, Al, W, Ti, TiN, TiSi2, Photoresist Difficult materials to dry etch Fe, Ni, Co, Cu, Al2O3, LiNbO3, etc. E. Finkman – Microelectronics Processing
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Dry Etch Process Characteristics of Dry Etch Process Highly selective
Anisotropic etch For use of features smaller than 3 microns Expensive equipment Limited human exposure to hazardous chemicals RF Power safety risks E. Finkman – Microelectronics Processing
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Dry Etch Techniques Physical etch Reactive Ion etch
Chemical (plasma) etch Dry E. Finkman – Microelectronics Processing
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Gas (plasma) etching advantages
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Plasma: the 4th state of the matter
On earth we live upon an island of "ordinary" matter. The different states of matter generally found on earth are solid, liquid, and gas. We have learned to work, play, and rest using these familiar states of matter. Sir William Crookes, an English physicist, identified a fourth state of matter, now called plasma, in Plasma is by far the most common form of matter. Plasma in the stars and in the tenuous space between them makes up over 99% of the visible universe and perhaps most of that which is not visible. Natural plasma Man-made plasma Sources: E. Finkman – Microelectronics Processing
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Basic properties of plasma
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Ionization and recombination
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Screening potential near a point charge
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Plasma-solid interface
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Homogeneous and heterogeneous processes in plasma processing
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Dry Etch Process Physical Etch
Also referred to as ion beam etching, sputtering, ion .milling Ions bombard wafer surface causing molecules to .sputter off the surface - Argon introduced into RF Power Chamber Advantages - Low level of undercutting - Anisotropic etch Disadvantages - Low selectivity rate - Requires high level of RF Power E. Finkman – Microelectronics Processing
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Sputter etching In this process all of the electrical energy, usually RF, is applied to the substrate. Physically bombard the films to be etched with energized chemically inert ions or atoms. Material is removed by ion bombardment of the substrate. This process is most often used to pre-clean substrates prior to deposition. Glow discharge is used to energize chemically inert ions or atoms (e.g., Ar) Highly anisotropic etching Damage to underlying material => may change device properties Rarely used in VLSI E. Finkman – Microelectronics Processing
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Dry Etch Process Ion Beam Etcher
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Dry chemical etching (Plasma etching)
Purely chemical etching. RF energy is applied to a separate electrode with the substrates grounded. Chemical reaction between etchant gas and surface layer of wafer etches the wafer. Glow discharge is used to produce chemically reactive species (atoms, radicals, or ions). Advantage - High selectivity rate Disadvantage - Isotropic etch E. Finkman – Microelectronics Processing
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Plasma assisted etching
Plasma assisted etching sequence Take a molecular gas CF4 Establish a glow discharge CF4+e CF3 + F + e Radicals react with solid films to form volatile product Si + 4F SiF4 Pump away volatile product (SiF4 ) E. Finkman – Microelectronics Processing
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Etchants and etch products (different sources)
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RF-powered Plasma Etch System
Steady-state voltage distribution in RF-powered plasma etch system E. Finkman – Microelectronics Processing
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Plasma Etching Mechanisms
Chemical etching Due to their incomplete bonding, free radicals are highly reactive chemical species. Various reactions and species present in a plasma E. Finkman – Microelectronics Processing
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Reactive Ion Etcing (RIE)
Combines both physical and chemical etching techniques If RF energy is applied to the substrates in a low pressure halogen-rich environment, material can be removed by both chemical means and ion bombardment of the substrate surface. Greater control over line widths and edge profiles is possible with oxides, nitrides, polysilicon and aluminum. Accomplished by replacing the neutral gas in a r.f. sputtering system by one or more chemical species Glow discharge is used to produce chemically reactive species (atoms, radicals, or ions) and chemically inert ions Widely used in VLSI fabrication Advantages - High selectivity rate - Anisotropic E. Finkman – Microelectronics Processing
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Dry Etch Process : RIE E. Finkman – Microelectronics Processing
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Ion Energy vs. Pressure for a Plasma
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Chemical vs. chemical/physical etching
(using reactive neutral species and ionic species) Anisotropic etching Purely chemical etching (using only reactive neutral species) Isotropic etching E. Finkman – Microelectronics Processing
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Effect of the inhibitor
w/o inhibitor => Isotropic w/ inhibitor => Anisotropic fast inhibitor deposition => Bevelled walls E. Finkman – Microelectronics Processing
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Importance of RIE Importance of reactive ion etching
Highly anisotropic etching Precise pattern transfer High resolution Less consumption of chemicals Cost effective Environmentally benign Clean process Vacuum Ease of automation E. Finkman – Microelectronics Processing
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Barrel plasma system Quartz tube
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High density plasma (HDP) system
Electron Cyclotron Resonance (ECR) Inductively coupled plasma (ICP) ECR vs ICP ECR was introduced at OPT in 1985. ICP was introduced much later ( ) for plasma processing. most important with both: independent control of ion energy and ion current density lower (substrate) electrode grounded, RF driving opt. E. Finkman – Microelectronics Processing
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Sputter etch and ion milling system
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RIE: Examples E. Finkman – Microelectronics Processing
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How to Control Anisotropy?
Ionic bombardment damage exposed surfaces. Sidewall coating by inhibitor prevents sidewall etching. E. Finkman – Microelectronics Processing
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How to Control Selectivity?
Example 1: E. Finkman – Microelectronics Processing
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How to Control Selectivity?
Example 2: Si etching in CF4 + O2 mixture. E. Finkman – Microelectronics Processing
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How to Control Smoothness?
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Temperature Dependence of Selectivity
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In General: Etching Process Includes Several Sequential Steps
Example 1: RIE of Al lines. E. Finkman – Microelectronics Processing
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Sequential Steps in Etching process
Example 2: Etching of deep trenches. E. Finkman – Microelectronics Processing
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Sequential Steps in Etching deep trenches
Solution: Multiple step RIE sequence. E. Finkman – Microelectronics Processing
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Summary of Plasma Systems & Mechanisms
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Dry Etch Process Summary
Factors Influencing Dry Etch Process Etch rate - RF Power level - Gas formula - Etch Temperature Pressure - Extremely high pressure results in an isotropic etch - Low pressure with high energy can damage wafer E. Finkman – Microelectronics Processing
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Dry Etch Process Summary
Factors Influencing Dry Etch Process Micro-loading - Different etch rates across wafer surface - Ashing can occur Post-etch corrosion - Due to residual etchant left on wafer after final rinse - Using a none Chlorine based etchant like Fluorine …..eliminates the problem. E. Finkman – Microelectronics Processing
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Plasma-Etching Gases Used in VLSI Fabrication
PSG BPSG E. Finkman – Microelectronics Processing
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Wafer Cleaning Resist Stripping
Removes residual resist after etch process Wet stripping - For use on wafers that have not been plasma …. …..etched - For non-metallic surfaces an acid solvent is used - For metallic surfaces an organic solvent is used Plasma Stripping - For use on wafers that have been etched by plasma - Uses oxygen as stripping plasma to remove …..photoresist E. Finkman – Microelectronics Processing
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