1. PROGRESS, OPPORTUNITIES AND CHALLENGES IN MODELING OF PLASMA ETCHING
Yang Yang, Mingmei Wang, Juline Shoeb and
Mark J. Kushner
Iowa State University
Ames, IA USA
May 2008
IITC_0508

2. Optical and Discharge Physics
ACKNOWLEDGEMENTS
Group Members
Ananth N. Bhoj (Novellus)
Ramesh Arakoni (Intel)
Natalie Y. Babaeva
Ankur Agarwal (Appl. Matls.)
Yang Yang
Mingmei Wang
Juline Shoeb
Luis Garcia (UG RA)
Samantha Eischeld (UG RA)
Funding Agencies
3M Corporation
Applied Materials
Micron, Inc.
Semiconductor Research Corporation
Iowa State University
Optical and Discharge Physics
IITC_0508 2

3. Optical and Discharge Physics
AGENDA
Modeling of Plasma Processing
Philosophical and Hierarchical Basis of Hybrid Modeling
Challenges and opportunities
Nanoscale processing and atomic layer control
Extremely high aspect ratio processing
Doing without data
Green processing
Concluding Remarks
Iowa State University
Optical and Discharge Physics
IITC_0508

4. PLASMAS FOR NANOSCALE FABRICATION Optical and Discharge Physics
Plasmas are and will continue to be indispensable for etching, deposition and cleaning in microelectronics fabrication.
Control of dimensions at 32 and 45 nm nodes requires resolution of a few nm or less.
Required: Unprecedented control of reactant fluxes from the plasma onto the wafer: Uniformity, Composition, Energies
Modeling is being challenged to provide added value to process and equipment development.
Mechanisms
Timeliness and Relevancy
Being ahead of the curve
Iowa State University
Optical and Discharge Physics
IITC_0508 4

5. ACTIVATION ENERGY: SUB-eV, SUB-DEGREE CONTROL
Activation energy for fabricating nanostructures is largely delivered through ion bombardment.
Distinguishing between materials will result from sub-eV and sub-degree control of ion energies.
Intel Fin-FET
Iowa State University
Optical and Discharge Physics
IITC_0508 5

6. WHY IS THIS SO DIFFICULT TO MODEL?: TYPICAL PLASMA ETCHING REACTOR
 Hitachi XT ECR
Iowa State University
Optical and Discharge Physics
IITC_0508 6

7. THE TIMESCALE CHALLENGE Optical and Discharge Physics
Technological plasmas have vastly different timescales that must be addressed in models.
Integrating timestep (required for numerical stability): t
Dynamic timescale (to resolve the evolution of phenomena): T
Plasma transport:
Dielectric relaxation t = /  1 ps – 10 ns
T = ns - ms
Surface chemistry:
t = s, T = 10 s
Iowa State University
Optical and Discharge Physics
IITC_0508

8. HYBRID MODELING
IITC_0508

9. THE FUNDAMENTAL APPROACH Optical and Discharge Physics
Virtually all processes in plasmas are interdependent.
Electron kinetics depend on circuitry, gas flow, surfaces….
Plasma harmonics depend on cable lengths….
The most fundamental and accurate modeling technique is capturing all interdependencies in a massive set of partial differential equations (or equivalent PIC).
Integrate, integrate, integrate….This is beyond the current state of the art and presents computational challenges on the par of weapons design.
Iowa State University
Optical and Discharge Physics
IITC_0508

10. Optical and Discharge Physics
HYBRID MODELING
TECHNIQUES
Hybrid models resolve multi-physics over multi-scales.
Compartmentalize physical processes into modules having minimum of overlap.
Time slice on physics times-scales.
Iowa State University
Optical and Discharge Physics
IITC_0508 10

11. Optical and Discharge Physics
MODELING PLATFORMS: HPEM, nonPDPSIM, MCFPM
HPEM (Hybrid Plasma Equipment Model)
More mature, more sophisticated physics
Lower pressures (sub-mTorr to 10s Torr)
Surface chemistry, radiation transport, circuits
Virtual sensors-actuators-real time control
Structured rectilinear meshes, generally semi-implicit
nonPDPSIM
Higher pressures (10s mTorr to multi-atmosphere)
Less mature but improving physics
Unstructured meshes, fully implicit
MCFPM (Monte Carlo Feature Profile Model)
Evolution of surface features
nm-to-atomic scale
Plasma and surface chemistry, charging
Iowa State University
Optical and Discharge Physics
IITC_0508 11

12. Optical and Discharge Physics
ELECTROMAGNETIC FIELDS AND ELECTRONS
Maxwell’s Equation:
Boltzmann’s equation for electron velocity distributions coupled with electron energy conservation equations.
S(Te) = Power deposition from electric fields
L(Te) = Electron power loss due to collisions
 = Electron flux
(Te) = Electron thermal conductivity tensor
SEB = Power source source from beam electrons
Iowa State University
Optical and Discharge Physics
IITC_0508

13. Optical and Discharge Physics
PLASMA CHEMISTRY, TRANSPORT AND ELECTROSTATICS
Continuity, momentum and energy equations are solved for each species
Implicit solution of Poisson’s equation
Iowa State University
Optical and Discharge Physics
IITC_0508

14. Optical and Discharge Physics
SURFACE MORPHOLOGY AND PROFILE EVOLUTION
Monte Carlo Feature Profile Model
Psuedoparticles representing neutrals, ions and electrons are directed towards surface.
Energy and angularly resolved fluxes from equipment scale models.
Reaction mechanism determines probability of adding, removing or changing composition of materials.
Atomic to a few nm resolution per mesh cell.
Solution of Poisson’s equation to account for charging of feature.
Iowa State University
Optical and Discharge Physics
IITC_0508

15. AFFECT PROCESS DESIGN TO ACHIEVE EXTREME SELECTIVITY
MODELING CHALLENGE:
AFFECT PROCESS DESIGN TO ACHIEVE EXTREME SELECTIVITY
IITC_0508 15

16. FLUOROCARBON PLASMA ETCHING OF Si/SiO2 Optical and Discharge Physics
In fluorocarbon plasma etching of Si/SiO2 CFx radicals produce a polymer passivation layer which regulates activation energy.
SiO2 consumes the polymer in etching; thicker layers on Si result in slower etch rates, and the ability to achieve selectivity.
Iowa State University
Optical and Discharge Physics
IITC_0508 16

17. ACHIEVING HIGH SELECTIVITY Optical and Discharge Physics
High selectivity achieved by controlling the ion energy distribution to wafer.
Sinusoidal bias: Broad IEAD does not discriminate thresholds.
Tailored bias: Produce a narrow IEAD which may discriminate between threshold energies.
Sinusoidal Bias
Ref: S.-B. Wang and A.E. Wendt, J. Vac. Sci. Technol. A, 19, 2425 (2001)
Iowa State University
Optical and Discharge Physics
IITC_0508 17

18. NON-SINUSOIDAL BIAS WAVEFORMS Optical and Discharge Physics
Ions cross the sheath quickly compared to the rf period and arrive at wafer with nearly the instantaneous sheath voltage.
By controlling the sheath potential with non-sinusoidal waveforms, one can control the IED. a
Iowa State University
Optical and Discharge Physics
IITC_0508 18

19. IEDs: DURATION OF POSITIVE SPIKE Optical and Discharge Physics
Custom bias produces constant sheath potential resulting in narrow IED.
As  increases, IEDs broaden in energy and bias becomes less positive.
15 mTorr, 500 W, 5 MHz, Vp-p, Ar/C4F8 = 75/25 
Vdc:
Iowa State University
Optical and Discharge Physics
IITC_0508 19

20. ETCH PROFILES vs VOLTAGE Optical and Discharge Physics
Non-sinusoidal bias enables etching with nearly mono-energetic ions.
Transition between “selective” and “fast” is unambiguous.
15 mTorr, 500 W, Ar/C4F8 = 75/25
Iowa State University
Optical and Discharge Physics
IITC_0508 20

21. Optical and Discharge Physics
ETCHING RECIPES
Etching usually occurs in 2 phases:
Main-etch: Fast but non-selective
Over-etch: Slow but selective
Gas mixture is often changed to distinguish main and over-etch
Slow due to gas exchange times.
Unable to resolve monolayer
Custom tailored voltage waveform
Controlling physical component
Change amplitude with ms resolution.
Iowa State University
Optical and Discharge Physics
IITC_0508 21

22. ELECTRONICALLY CONTROLLED “RECIPES” Optical and Discharge Physics
MASK
SiO2 Si
T=0.35
T=0.65
Electronically controlled recipes enable rapid etch with a “soft landing”
200 V
(Slow, selective)
1500 V
(Fast, non-selective)
1500/200 V
(Fast, selective)
Iowa State University
Optical and Discharge Physics
ANIMATION SLIDE-GIF
IITC_0508 22

23. MODELING CHALLENGE:
AFFECT EQUIPMENT DESIGN FOR ETCHING OF EXTREMELY HIGH ASPECT RATIO FEATURES
IITC_0508 23

24. HIGH ASPECT RATIO CONTACT (HARC) ETCHING Optical and Discharge Physics
Processes for HARC etching with aspect ratios > are being developed for capacitors and through wafer vias.
Very Challenging…
Twisting or curvature of features is randomly observed.
Features are so small that random fluctuations of fluxes of radicals, ions and electrons into holes produces variations.
Charging of features by electrons and ions produce random fields that deviate paths.
Iowa State University
Optical and Discharge Physics
IITC_0508 24 24

25. DUAL FREQUENCY CCP Ar/C4F8/O2 : CHARGING Optical and Discharge Physics
High energy ions penetrate deep into feature.
Electrons charge top of feature.
Internal E-fields that affect trajectories.
Randomness of charging leads to erratic paths.
10 mTorr, Ar/C4F8/O2 = 80/15/5, 10/40 MHz, 500 W.
Iowa State University
Optical and Discharge Physics
IITC_0508 25 25

26. SiO2 / Si HARC ETCH: EFFECTS OF CHARGING Optical and Discharge Physics
Etch rate higher with increasing power.
Without charging:
Generally straight profiles.
With charging:
Ion trajectories perturbed.
Overcome with voltage.
Some evidence of randomness due to small contact area.
10 mTorr, Ar/C4F8/O2 = 80/15/5, 10/ 40 MHz, 500 W.
Without Charging
With Charging
Iowa State University
Optical and Discharge Physics
IITC_0508 26 26

27. SiO2/Si HARC ETCH: RANDOMNESS OF CHARGING?
6 trenches receiving “same” fluxes.
Stochastic nature of fluxes produces random twisting.
Similar behavior observed experimentally.
Effect is amplified by finite size of particles and mesh.
10 mTorr, Ar/C4F8/O2 = 80/15/5, 300 sccm, LF 4 kW, HF 500 W.
Iowa State University
Optical and Discharge Physics
IITC_0508 27 27

28. Optical and Discharge Physics
IS THERE A FIX? DC-AUGMENTED RF
Single (or dual) frequency CCP…with external, negative dc bias on opposing electrode.
DC ion current produces dc e-beam current incident onto wafer.
dc e-beam, mono-energetic and narrow in angle, penetrates deep into feature to neutralize excess positive charge.
Iowa State University
Optical and Discharge Physics
IITC_0508

29. Optical and Discharge Physics
10 MHz LOWER, DC UPPER: [e] DENSITY, DC CURRENT
Electron density 3 x 1010 cm-3.
Dc voltage on upper electrode requires dc current path to ground.
LF electrode blocking capacitor and insulators require dc current return to side wall.
Dc current peaks near outer edge of electrode.
Ar, 40 mTorr, LF: 10 MHz, 300 W, 440V/dc=-250V
DC: 200 W, -470 V
Iowa State University
Optical and Discharge Physics
IITC_0508

30. Optical and Discharge Physics
10 MHz LOWER, DC UPPER: PLASMA POTENTIAL
LF electrode passes rf current. DC electrode passes combination of rf and dc current with small modulation of sheath potential.
Ar, 40 mTorr, LF: 10 MHz, 300 W, 440V/dc=-250V
DC: 200 W, -470 V
Iowa State University
Optical and Discharge Physics
ANIMATION SLIDE-GIF
IITC_0508

31. Optical and Discharge Physics
10 MHz LOWER, DC UPPER: [e], ION ENERGY DISTRIBUTIONS
Ion energy distribution to wafer is many degrees, 150 eV in width.
Electron energy distributions onto wafer is narrower in angle and broader in energy.
E-beam reflects instantaneous potential difference between electrodes.
Ar, 40 mTorr, LF: 10 MHz, 300 W, 440V/dc=-250V; DC: 200 W, -470 V
Iowa State University
Optical and Discharge Physics
IITC_0508

32. Optical and Discharge Physics
ETCH PROFILES: WITHOUT AND WITH E-BEAM CURRENT
E-beam current neutralizes sufficient charge to prevent major twisting.
Difference in etch depth results from randomness of fluxes.
Ar/C4F8/O2, 40 mTorr
Iowa State University
Optical and Discharge Physics
IITC_0508

33. CAN YOU BE AHEAD OF THE CURVE BY VIRTUALLY CREATING NEW PROCESSES?
MODELING CHALLENGE:
CAN YOU BE AHEAD OF THE CURVE BY VIRTUALLY CREATING NEW PROCESSES?
IITC_0508 33

34. ANOTHER HARC CHALLENGE: MICRO-TRENCHING Optical and Discharge Physics
Modeling will be most impactful by leading process development.
As AR increases, specular ion reflection off side walls leads to micro-trenching.
SiO2 remains when Si is exposed ….over-etching is required.
How do you maintain critical dimension?
Can modeling lead experiment?
Ar/C4F8 = 75/25, 100 sccm, ICP, 15 mTorr, 500 W, 100 V at 5 MHz Si
Mask
SiO2
Aspect Ratio = 1:10
Iowa State University
Optical and Discharge Physics
ANIMATION SLIDE-GIF
IITC_0508 34

35. NON-SELECTIVE SINUSOIDAL BIAS FOR OVERETCH
Over-etch is used to clear out the remaining SiO2.
Conventional etching using a sinusoidal waveform does not have enough selectivity.
Underlying Si is breached.
CD of atomic layer resolution is not maintained.
Ar/C4F8 = 75/25, 100 sccm, ICP, 15 mTorr, 500 W, 100 V at 5 MHz
Aspect Ratio = 1:10
Iowa State University
Optical and Discharge Physics
ANIMATION SLIDE-GIF
IITC_0508 35

36. PLASMA ATOMIC LAYER ETCHING Optical and Discharge Physics
Plasma Atomic Layer Etching (PALE) monolayer by monolayer removal of material in a cyclic, self-limiting process.
Step 1: Top monolayer is passivated in non-etching plasma.
Step 2: Remove top layer with lower threshold energy (self-limiting)
Extreme control of energies to prevent erosion of the under-layer.
Iowa State University
Optical and Discharge Physics
IITC_0508 36

37. Ar/c-C4F8 TAILORED BIAS PALE: IEADs Optical and Discharge Physics
PALE of SiO2 using ICP Ar/C4F8 with variable non-sinusoidal 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 in energy discriminate between threshold energies.
Ar/C4F8 = 75/25, 100 sccm, 15 mTorr, 500 W
Iowa State University
Optical and Discharge Physics
IITC_0508 37

38. TAILORED BIAS PALE: EXTENDED OVER-ETCH Optical and Discharge Physics
PALE is used for over-etch.
Extreme selectivity of PALE enables removal of SiO2 without damage to underlying Si.
15 cycles of PALE over-etch clears the feature.
More energy intensive per layer than conventional etching.
Future technology nodes will be challenged to maintain lower J/cm2 of wafer.
15 cycles of PALE
Iowa State University
Optical and Discharge Physics
ANIMATION SLIDE-GIF
IITC_0508 38

39. ADDRESSING DAY-TO-DAY ISSUES OF TOOL DESIGN AND MAINTENANCE
MODELING CHALLENGE:
ADDRESSING DAY-TO-DAY ISSUES OF TOOL DESIGN AND MAINTENANCE
IITC_0508 39

40. INTO WAFER FOCUS RING GAP Optical and Discharge Physics
PLASMA PENETRATION
INTO WAFER FOCUS RING GAP
Wafers are beveled at edge with small gap (< 500 m) between wafer and focus ring for mechanical clearance.
Penetration of plasma into gap produces particles and erodes materials.
How large can gap be without having significant plasma penetration?
PVD penetration persists after CMP
/archive/00/10/simpson.html
Iowa State University
Optical and Discharge Physics
IITC_0508 40

41. Ar/CF4 CCP FOR SiO2 Etching Optical and Discharge Physics
Dielectric etching performed in polymerizing capacitively coupled plasmas.
Typical plasma densities are cm-3.
Ar/CF4 = 97/03, 10 MHZ, 90 mTorr, 300V, 300 sccm
[CF3-]
[F-]
Iowa State University
Optical and Discharge Physics
MIN MAX
Log scale
IITC_0508 41

42. ELECTRON PENETRATION INTO GAP Optical and Discharge Physics
 1.0 mm Gap
 0.25 mm Gap
Electron penetration into gaps in anode portion of cycle is nominal due to surface charging and sheath formation.
Ar/CF4 = 97/03, 10 MHz, 90 mTorr, 300 V, 300 sccm
ANIMATION SLIDE-GIF
Iowa State University
Optical and Discharge Physics
MIN MAX
Log scale
IITC_0508 42

43. Ar+ PENETRATION INTO GAP Optical and Discharge Physics
 1.0 mm Gap
 0.25 mm Gap
Ions penetrate into larger gap throughout the rf cycle provided size is commensurate with sheath width. Smaller gap receives only nominal flux.
Ar/CF4 = 97/03, 10 MHz, 90 mTorr, 300 V, 300 sccm
ANIMATION SLIDE-GIF
Iowa State University
Optical and Discharge Physics
MIN MAX
Log scale
IITC_0508 43

44. ION PENETRATION vs GAP SIZE vs Optical and Discharge Physics
DEBYE LENGTH
Ion penetration into gap depends on size of gap relative to sheath thickness.
Gap ≥ sheath thickness allows penetration.
High plasma density tools produce smaller sheaths and more penetration.
More erosion…more cost.
Ar/CF4 = 97/03, 10 MHz, 90 mTorr, 300 V, 300 sccm
Iowa State University
Optical and Discharge Physics
IITC_0508 44

45. ION ENERGY AND ANGULAR DISTRIBUTIONS Optical and Discharge Physics
IEADs differ significantly on different surfaces in gap.
Erosion of materials depends on energy and angle of incidence.
MIN MAX
Log scale
Iowa State University
Optical and Discharge Physics
IITC_0508 45 45

46. CAPACITANCE OF FOCUS RING: IEAD Optical and Discharge Physics
Penetration of potential into low capacitance focus ring produces lateral E-fields.
IEAD on substrate is off normal… more erosion by sputtering.
Iowa State University
Optical and Discharge Physics
IITC_0508 46 46

47. DEVELOPING REACTION MECHANISMS WITHOUT FUNDAMENTAL DATA
MODELING CHALLENGE:
DEVELOPING REACTION MECHANISMS WITHOUT FUNDAMENTAL DATA
IITC_0508 47

48. HfO2 GATE STACK MODELING…WITHOUT DATA(!) Optical and Discharge Physics
High-k metal oxides are being used as SiO2 replacements to minimize gate leakage.
HfO2 has promising properties and can be integrated into current process streams.
For process integration and speed, desirable to simultaneously etch entire gate stack…Success with Ar/BCl3/Cl2 plasmas.
Challenge:
Modeling is required to speed process development and optimization.
There exists no fundamental database for process.
Develop mechanism based on experience and data from literature.
Iowa State University
Optical and Discharge Physics
IITC_0508

49. GOALS AND PREMISES FOR ETCH MECHANISM Optical and Discharge Physics
Goal: High selectivity of HfO2 over Si at low ion energies to minimize damage.
Premise 1: HfO2 etching is a multistep process requiring Hf-O bond breaking and separate removal of Hf and O.
Premise 2: Si etch rate slowed by BClx polymer. BClx first produces Si-B bonding to assist deposition of polymer.
Premise 3: TiN etching is analogous to other metals with volatile products (e.g., Al).
Use experiments from literature to build mechanism.
Material
Etch Rate
(A0 /min)
Threshold
(eV)
HfO2 90 25 Si
100 29
TiN
400 _
Material
Bond
Bond (eV)
HfO2
Hf-O
8.3 Si
Si-Si
3.4
Ref: L.Sha and J. P. Chang, J. Vac. Sci. Technol. A, v. 22, 88 (2004)
Iowa State University
Optical and Discharge Physics
IITC_0508

50. ETCHING MECHANISM IN Ar/BCl3/Cl2 PLASMA Optical and Discharge Physics
HfO2 Etching
Bond Breaking M+(g) + HfO2(s)  HfO(s) + O(s) + M(g)
M+(g) + HfO(s)  Hf(s) + O(s) + M(g)
Adsorption Cl(g) + Hf(s)  HfCl(s)
BClx(g) + O (s)  BClxO(s)
Etching M+ (g) + HFClx(s)  HfClx(g) + M(g)
M+(g) + BClxO(s)  ByOClx(g) + M(g)
Selectivity with respect to Si results from deposition of BClx polymer on Si
BClx(g) + Si(s)  SiBClx(s)
BClx(g) + SiBClx(s)  SiBClx(s) + Poly-BClx(s)
Iowa State University
Optical and Discharge Physics
IITC_0508

51. HfO2 ETCH RATE AND SELECTIVITY Optical and Discharge Physics
Photo-resist
TiN
HfO2
HfO2 etch rate increases with bias at expense of selectivity as sputtering of Poly-BClx increases.
Micro-masking from PR sputtering is problematic. Si
Ar/BCl3/Cl2 = 5/40/55, 5 mTorr, 300 W ICP
Iowa State University
Optical and Discharge Physics
IITC_0508

52. SELECTIVITY: CALIBRATION AND COLLABORATION
Complex reaction mechanism requires calibration and collabora-tion for key experimental validation.
Example:
Selectivity of HfO2 over Si requires layer of Poly-BClx, a competition between deposition and sputtering.
BClx(g) + Si(s)  SiBClx(s)
BClx(g) + SiBClx(s)  SiBClx(s) + P-BClx(s)
M+(g) + P-BClx(s)  M(g) + BClx(g)
In the absence of fundamental data, reaction mechanisms must be derived through sensitivity studies.
Iowa State University
Optical and Discharge Physics
IITC_0508

53. MODELING CHALLENGE: GREEN PROCESSING
IITC_0508 53

54. Optical and Discharge Physics
BECOMING MORE GREEN: ROLE OF MODELING
Fabs must become greener by reducing carbon footprint.
Typical energy usage: 1 kW-hr/cm2 of wafer .
About 1/3 of fab power is expended in process tools, most of them plasma tools.
Can modeling be an enabling technology to reduce Energy/cm2 and aid in fabs becoming greener?
Electricity Usage
Hu and Chuah, Energy 28, 895 (2003)
Iowa State University
Optical and Discharge Physics
IITC_0508

55. EXAMPLE: “PLASMA PHYSICS” TO MINIMIZE ENERGY USE PER WAFER
Optimize plasma etching of SiO2 vs pressure to minimize energy usage.
Change pressure while:
Constant: Gas mixture, residence time, energy deposition/molecule
Scale flow rate, total power with pressure.
Inductively Coupled Plasma
Base Case: Ar/C4F8/O2 = 80/18/2, 10 mTorr, 500 W, -200 Vdc
Iowa State University
Optical and Discharge Physics
IITC_0508

56. PRESSURE SCALING: ETCH RATES, ENERGY… Optical and Discharge Physics
Increase pressure 
Increase fluxes 
Increase etch rate
More deposition 
Thicker polymer layer 
Eventual etch stop
Energy savings (J/cm2) decreases with pressure.
Electrical load savings by optimizing process: 200 kW
Larger footprint of fab..more tools for same throughput?
Base Case: Ar/C4F8/O2 = 80/18/2, 10 mTorr, 500 W, -200 Vdc
Iowa State University
Optical and Discharge Physics
IITC_0508

57. Optical and Discharge Physics
CONCLUDING REMARKS
Modeling has enabled more rapid and economic development of plasma tools.
Value added for process development is less clear though promising.
Challenges and opportunities for modeling to lead process development:
Controlling activation energy for nanoscale resolution
Green process design
“Aging” resistant tool design.
Coordinated modeling-experiment collaboration early in development cycle can result in robust reactions mechanisms to optimize process.
Iowa State University
Optical and Discharge Physics
IITC_0508