1. RECIPES FOR PLASMA ATOMIC LAYER ETCHING*
Ankur Agarwala) and Mark J. Kushnerb)
a)Department of Chemical and Biomolecular Engineering
University of Illinois, Urbana, IL 61801, USA
b)Department of Electrical and Computer Engineering
Iowa State University, Ames, IA 50011, USA
34th IEEE ICOPS, June 2007
*Work supported by the SRC and NSF

2. Optical and Discharge Physics
AGENDA
Atomic Layer Processing
Plasma Atomic Layer Etching (PALE)
Non-sinusoidal Bias Waveforms
Tailored Bias PALE Recipes
SiO2 using Ar/c-C4F8
Self-aligned contacts
Concluding Remarks
Iowa State University
Optical and Discharge Physics
ANKUR_ICOPS07_Agenda

3. ATOMIC LAYER PROCESSING Optical and Discharge Physics
Advanced microelectronics structures require extreme selectivity in etching materials with nm resolution.
Atomic layer plasma processing may allow for this level of control.
Current techniques employ specialized ion beam equipment.
The high cost of atomic layer processing challenges its use.
Plasma Atomic Layer Etching (PALE) is potentially an economic alternative.
Double Gate MOSFET
Tri-gate MOSFET
Iowa State University
Optical and Discharge Physics
Refs: AIST, Japan; Intel Corporation
ANKUR_ICOPS07_01

4. PLASMA ATOMIC LAYER ETCHING (PALE) Optical and Discharge Physics
In PALE etching proceeds monolayer by monolayer in a cyclic, self limiting process.
First step: Top monolayer is passivated in non-etching plasma.
Passivation makes top layer more easily etched compared to sub-layers.
Second step: Remove top layer (self limiting).
Exceeding threshold energy results in etching beyond top layer.
Iowa State University
Optical and Discharge Physics
ANKUR_ICOPS07_02

5. PLASMA ATOMIC LAYER ETCHING (PALE) Optical and Discharge Physics
PALE has been computationally and experimentally investigated using conventional plasma equipment.
Inductively coupled plasma (ICP)
Capacitively coupled plasma (CCP)
Since the equipment is already in fabrication facilities, no additional integration costs are incurred.
The low speed of PALE processes hinder its integration into production line.
Speed can be increased but only at the cost of losing control of CD (critical dimensions) or damaging material interfaces.
Iowa State University
Optical and Discharge Physics
ANKUR_ICOPS07_03

6. Optical and Discharge Physics
INCREASING SPEED
OF PALE … HOW?
Conventional PALE
Different gas mixtures for each step.
Although self-limiting, purge steps increase process time.
Tailored bias PALE
Create nearly mono-energetic ion distribution.
Control ion energies via changes in voltage amplitude.
Single gas mixture for both steps eliminates purge and reduces time.
Conventional PALE
Tailored Bias PALE
Iowa State University
Optical and Discharge Physics
ANKUR_ICOPS07_04

7. NON-SINUSOIDAL BIAS WAVEFORMS: IEADs Optical and Discharge Physics
Vp-p
Custom waveform produces nearly constant sheath potential resulting in narrow IEAD.
Peak energy of IEAD is controlled by amplitude.
IED broadens at higher biases due to thickening of sheath and longer transit times.
 = 10%; Vp-p = 200 V
Iowa State University
Optical and Discharge Physics
Ref: A. Agarwal and M.J. Kushner, J. Vac. Sci. Technol. A, 23, 1440 (2005)
ANKUR_ICOPS07_05

8. HYBRID PLASMA EQUIPMENT MODEL (HPEM) Optical and Discharge Physics
Electromagnetics Module: Antenna generated electric and magnetic fields
Electron Energy Transport Module: Beam and bulk generated sources and transport coefficients.
Fluid Kinetics Module: Electron and Heavy Particle Transport, Poisson’s equation
Plasma Chemistry Monte Carlo Module:
Ion and Neutral Energy and Angular Distributions
Fluxes for feature profile model
Iowa State University
Optical and Discharge Physics
ANKUR_ICOPS07_06

9. MONTE CARLO FEATURE PROFILE MODEL Optical and Discharge Physics
Monte Carlo techniques address plasma surface interactions and evolution of surface morphology and profiles.
Inputs:
Initial material mesh
Surface reaction mechanism
Ion and neutral energy and angular distributions
Fluxes at selected wafer locations.
Fluxes and distributions from equipment scale model (HPEM)
Iowa State University
Optical and Discharge Physics
ANKUR_ICOPS07_07

10. FLUOROCARBON PLASMA ETCHING OF SiO2/Si Optical and Discharge Physics
CFx radicals produce polymeric passivation layers which regulate delivery of precursors and activation energy.
Chemisorption of CFx produces a complex at the oxide-polymer interface
Low energy ion activation of the complex produces polymer.
Polymer complex sputtered by energetic ions  etching.
As SiO2 consumes the polymer, thicker layers on Si slow etch rates enabling selectivity.
Iowa State University
Optical and Discharge Physics
ANKUR_ICOPS07_08

11. VERY HIGH ASPECT RATIO FEATURES Optical and Discharge Physics
MAIN ETCH-PALE FOR
VERY HIGH ASPECT RATIO FEATURES
PALE will always be slow compared to conventional etching.
Selectivity of PALE is only needed at end of etch at material interface.
Combine:
Rapid “main etch” to reach material interface
PALE to clear feature with high selectivity.
Feature to be investigated is SiO2-over-Si trench with an aspect ratio of 1:10.
10:1 Trench
Iowa State University
Optical and Discharge Physics
ANKUR_ICOPS07_09

12. Ar/c-C4F8 ICP FOR SiO2 ETCHING Optical and Discharge Physics
Test system is inductively coupled plasma with 5 MHz biased substrate.
Ar/C4F8 = 75/25, 100 sccm, 15 mTorr, 500 W ICP
Main etch is conventional sinusoidal waveform.
PALE uses tailored bias waveform:
Passivate: 50 V (peak-to-peak)
Etch: 100 V (peak-to-peak)
Iowa State University
Optical and Discharge Physics
ANKUR_ICOPS07_10

13. MAIN ETCH OF SiO2-over-Si Optical and Discharge Physics
Mask
SiO2
Main etch performed using a sinusoidal bias waveform.
Micro-trenching at sides of feature due to specular reflection off walls.
Central SiO2 remains when underlying Si is exposed.
Significant etching into Si during over-etch to clear feature.
Ar/C4F8 = 75/25, 100 sccm, 15 mTorr, 500 W, 100 V at 5 MHz
Aspect Ratio = 1:10
Iowa State University
Optical and Discharge Physics
ANIMATION SLIDE-GIF
ANKUR_ICOPS07_11

14. Ar/c-C4F8 TAILORED BIAS PALE: IEADs Optical and Discharge Physics
PALE of SiO2 using ICP Ar/C4F8 with variable bias.
Step 1
Vp-p = 50 V
Passivate single layer with SiO2CxFy
Low ion energies to reduce etching.
Step 2
Vp-p = 100 V
Etch/Sputter SiO2CxFy layer.
Above threshold ion energies.
Narrow IEADs enable discrimination between threshold energies of undelying SiO2 and polymer complex.
Ar/C4F8 = 75/25, 100 sccm, 15 mTorr, 500 W
Iowa State University
Optical and Discharge Physics
ANKUR_ICOPS07_12

15. SiO2-over-Si: PALE vs CONVENTIONAL ETCH Optical and Discharge Physics
5 cycles of PALE
Conventional Etching
Narrow IEAD enables etching of rough initial profile at bottom.
Redeposition of etched products and polymer cover exposed Si and sidewall; avoids notching and damage.
High speeds (~ 4 ML/cycle) with high etch selectivity.
Iowa State University
Optical and Discharge Physics
 1 cell = 3 Å
ANKUR_ICOPS07_13
ANIMATION SLIDE-GIF

16. PALE: ROUGHNESS vs STEP 2 ION ENERGY Optical and Discharge Physics
110 eV
Speed of PALE can be increased via change in ion energies.
At high ion energies, distinction between threshold energies is lost.
Final etch profile is rough.
Already exposed underlying Si vulnerable at high ion energy.
Surface roughness scales linearly with ion energies.
120 eV
140 eV
Iowa State University
Optical and Discharge Physics
ANKUR_ICOPS07_14

17. PALE: ETCH RATE vs STEP 2 ION ENERGY Optical and Discharge Physics
Number of PALE cycles required to clear feature decrease with increasing ion energy.
Etch rate saturates at high ion energies due to the rough initial feature profile.
Trade-off between high etching rates and selectivity.
Etching of already exposed underlying Si leads to roughness.
Initial
Final
Iowa State University
Optical and Discharge Physics
ANKUR_ICOPS07_15

18. PALE: CONVENTIONAL vs TAILORED BIAS Optical and Discharge Physics
SiO2CxFy
Plasma
SiO2 Si
Tailored: 5 cycles
Conventional: 20 cycles
Conventional PALE scheme utilizes 20 cycles.
High speeds (~ 3-4 ML/cycle) and extreme selectivity of PALE enable fast etching of self-aligned contacts.
Final etch profile is smooth even at high etching rates.
Iowa State University
Optical and Discharge Physics
 1 cell = 3 Å
ANKUR_ICOPS07_16
ANIMATION SLIDE-GIF

19. Optical and Discharge Physics
CONCLUDING REMARKS
Atomic layer control of etch processes will be critical for 32 nm node devices.
PALE using conventional plasma equipment makes for an more economic processes.
Slow etching rates of conventional PALE need to be optimized: trade-off between high selectivity and etch rate
PALE of SiO2 in Ar/c-C4F8 plasma investigated using custom bias waveforms,
Non-sinusoidal bias waveforms enable:
Precision control of IEADs
Elimination of purge step to increase process speeds
High selectivity at high etching rates (~ 4 ML/cycle)
Iowa State University
Optical and Discharge Physics
ANKUR_ICOPS07_17