Ion Enhanced Etching
Mechanism of ion enhanced etching
(4) Importance of radicals:
Radical－atomic or molecular specie which is neutral but incompletely bonded, very reactive Example: Etching SiO2 by CF4 plasma e + CF4 → CF3 + F + e F, CF3－radicals
Ion induced etching:
Ion bombardment initiates chemical etching No etching without ions
Sidewall passivation: Stable films formed on sidewall due to lack of bombardment
2) SiO2 etching:
By F-based etchants (etching product---SiF4): CHF3 , CF4 + H2 , C2F6, C3F8，mixtures To get good selectivity over Si, need F-deficient plasma CF4 + H2 → F/C ratio↓ → Si etch rate ↓
Chapter 12 Dry Etching by Plasma Processing
Outline
I. Dry etching---one of the key technologies for VLSI/ULSI II. Plasma etching mechanisms III. Various plasma-etching reactors IV. Etching Anisotropy V. Dry etching for different materials VI. Contamination during Plasma Etching VII. Radiation Damage VIII.Endpoint Detection
V Dry etching for different materials
1) General etching process requirements: Feature profile Selectivity (SAB = EA/EB) Uniformity and Reproducibility Throughout Low contamination and damage
Patterning feature profile control－ Vital importance for VLSI/ULSI The degree of anisotropy of an etch process: RL A =1− RV
RL: lateral etch rate RV: vertical etch rate.
Sidewall film prevents etching by other isotropic etchant Sidewall film deposition: *By deposition of polymeric material from C-containing plasmas: －CF2－polymers
(3) Possible mechanisms of ion bombardment in etching process
Ions create damage/defects which catalyses chemical reaction
(See Fig.(a) below)
Ion bombardment directly dissociates reactant molecules Ion bombardment removes involatile residues that could inhibit etching (See Fig.(b) below)
2. Complex physical and chemical process during plasma/RIE 4 fundamentally different etching mechanisms:
Sputtering Chemical etching via plasma species →Isotropic etching Chemical/physical etching via reactive ion bombardment → Anisotropic etching Ion assisted or ion induced: Ion bombardment + chemical specie → Effective etching Usually no reaction of the etching gas or gas mixture without the presence of the plasma
Various reactors different in configurations, etching modes and power sources:
Barrel vs. Parallel Plate vs. Hexode Batch vs. Single Wafer Plasma vs. RIE Direct vs. Downstream RF vs. Microwave •High density plasma etcher--by Inductive coupled plasma (ICP)
3) Si3N4 etching:
Similar etchants with SiO2 +NF3 －better selectivity over SiO2 ( 9-10)
4) Si etching
Various etchants: (etching products:SiCl2 ,SiCl4, SiF2 , ) Cl2, CCl4, BCl3+Cl2, CFCl3,CF4+O2, SF6, NF3 or other Cl-containing mixture High selectivity required (for poly-Si gate): Cl-based plasma－very selective High anisotropy required: for trench isolation, DRAM capacitors
Components of plasma etching systems:
Etching chamber with electrodes Pumping system for establishing & maintaining processing gas pressure Pressure gauges to monitor pressure in the chamber Variable conductance between the pump and chamber for independent controlling pressure and flow rate Gas handling capability to test and control the flow of reactant gas Power source Process monitor and endpoint detector
IV Etching Anisotropy
Pure chemical reaction: Isotropic etching Ionic bombardment: Anisotropic etching
RIE and Plasma etching
Plasma etch: Wafer on grounded electrode ---less anisotropic RIE: Wafer on powered electrode ---more anisotropic
Perfect anisotropic etching not always desired (because metal continuity failure at steps, contacts, non-removal of residues…)
Non-vertical etch profiபைடு நூலகம்es: by using more isotropic etch (P↑→V↓) …
I Dry etching---one of the key technologies for VLSI/ULSI
Predominant dry etching technology—Plasma and Reaction Ion Etching (RIE) Etching process in gas plasma discharge (at 10-3-100 torr) Gaseous reactants Gaseous products Advantages: Anisotropic etching Formation of small-geometry device structure High density of reactive species available Environmentally less hazardous Other dry etching techniques: Ion beam enhanced etching Electron－beam enhanced etching Photon assisted etching Gas－phase etching: Anhydrous HF vapor
e + CF4 → CF3+ + F + 2e