1. Small Feature Reproducibility A Focus on Photolithography
UC-SMART Major Program Award
Spanos, Bokor, Neureuther
Second Annual Workshop
11/8/99
SFR Workshop - Lithography
11/8/99

2. SFR Workshop - Lithography
Agenda
8:30 – 9:00 Introductions, Overview / Spanos
9:00 – 10:15 Lithography / Spanos, Neureuther, Bokor
10:15 – 10:45 Break
10:45 – 12:00 Sensor Integration / Poolla, Smith, Solgaard, Dunn
12:00 – 1:00 lunch, poster session begins
1:00 – 2:15 Plasma, TED / Graves, Lieberman, Cheung, Aydil, Haller
2:15 – 2:45 CMP / Dornfeld
2:45 – 3:30 Education / Graves, King, Spanos
3:30 – 3:45 Break
3:45 – 5:30 Steering Committee Meeting in room 775A / Lozes
5:30 – 7:30 Reception, Dinner / Heynes rm, Men’s Faculty Club
SFR Workshop - Lithography
11/8/99

3. SFR Workshop - Lithography
Litho Milestones, Year 1
Demonstrate AFM aerial image inspection on 180nm features.
Demonstrate Specular Spectroscopic Scatterometry CD metrology for 180nm features.
Demonstrate focus classification scheme for 180nm features.
Complete 3D device simulations of mask errors and LER effects in gate-level. Threshold voltage shifts, turn-off characteristics, and saturated drain current will be evaluated.
Complete a simulation feasibility study and verification experiment on novel in-lens filtering for resolution enhancement.
Evaluate the physical basis for novel effects in interaction of light with materials and low voltage electrons with resists.
Establish web based simulation capabilities for DUV resists, mask topography effects and electron-beam lithography.
SFR Workshop - Lithography
11/8/99

4. SFR Workshop - Lithography
Litho Milestones, Year 2
Demonstrate AFM aerial image inspection on 50nm features.
Demonstrate 150nm Specular Spectroscopic Scatterometry CD metrology.
Demonstrate focus classification scheme for 150nm features.
Test NMOS devices with programmed mask errors and LER, compare measured characteristics to simulation.
Integrate scattering, imaging, resist modeling for analyzing inspection and printabilty of mask non-idealities in the context of use with OPC.
Establish a prototype system for process integration including the automatic generation of simulation-designed multi-step, short loop test structures.
Establish web based simulation capabilities for optical alignment and advanced electron-beam lithography.
SFR Workshop - Lithography
11/8/99

5. SFR Workshop - Lithography
Outline
Simulation and Metrology
Lithography Simulator Calibration
Scatterometry
Plans for Statistical Process Optimization
Line Edge Roughness
Lithography Simulation
SFR Workshop - Lithography
11/8/99

6. In-situ / On-wafer thin film Metrology
Reflectometry / Ellipsometry / Scatterometry
Thickness, n & k, chemical composition
Run-to-run and real-time monitoring
Resist surface analysis for aerial image evaluation
Thin film
Spin Coat
&
Soft Bak e
Exposure
PEB
Develop
Thickness
n and k
CD, profile and
SFR Workshop - Lithography
11/8/99

7. Motivation for Parameter Extraction
Current lithography simulators are parameter limited as opposed to model limited.
Traditional optimization techniques are unsuitable in complex, non-linear, high dimensional problems.
Importance of predictive capabilities is increasing with increasing development costs and time-to-market pressures.
SFR Workshop - Lithography
11/8/99

8. Some Critical Parameters in DUV Lithography Simulation
Amplification Rate (Pre-exp)
Amplification Rate (Activation)
Acid Loss Rate (Pre-exp)
Acid Loss Rate (Activation)
Dill’s A Parameter
Dill’s B Parameter
Dill’s C Parameter
Relative Quencher Conc.
PEB Diffusivity (Pre-exp)
PEB Diffusivity (Activation)
Maximum Develop Rate
Minimum Develop Rate
Developer Selectivity
Developer Threshold PAC
Resist Refractive Index (Real)
Resist Refractive Index (Imag.)
ARC Refractive Index (Real)
ARC Refractive Index (Imag.)
Relative Focus
Amplification Reaction Order
Exact values obtained from experiments or resist vendor
Narrow range of values available from unpatterned experiments
Wide parameter range
SFR Workshop - Lithography
11/8/99

9. Proposed DUV-SCAPE Framework
3: user specified
parameter ranges
2: global optimization
engine (SAC)
1: unpatterned
resist experiments
5: commercial
simulation program
6: simulated
profile
4: parameter interface
front end
7: global optimization
engine (SAC)
2: image processing
front end
3: experimental
profile
1: patterned resist experiments
SFR Workshop - Lithography
11/8/99

10. SFR Workshop - Lithography
Salient Features
Unpatterned Resist Models
BCAM exposure and bake models
Mack develop model
Optimization Technique
Global optimization theory (Adaptive Simulated Annealing)
Patterned Resist Model
Existing lithography simulators (e.g. SAMPLE, Prolith, Solid-C, etc.)
SFR Workshop - Lithography
11/8/99

11. Experiments - Commercial DUV Resist
Unpatterned Resist Characterization Experiments
Process 4 wafers with flood exposed sites
Measure ARC and Resist optical constants - Ellipsometry
Measure exposure and PEB parameters - FTIR/DITL
Measure develop parameters - DRM
Patterned Resist Characterization Experiments
Process 1 wafer with a focus-exposure matrix
Measure profiles for sub-quarter micron lines using AFM/cross-section CD-SEM/Specular Spectroscopic Scatterometry
SFR Workshop - Lithography
11/8/99

12. Unpatterned Experiments1 .5
135C
140C
Exposure
+ PEB
Parameters
Deprotection
120C
110C
Exposure Dose (mJ/cm2)
3000
Develop
Parameters
2000
Develop Rate in A/sec
1000
Normalized concentration of unreacted sites
SFR Workshop - Lithography
11/8/99

13. Patterned Experiments: AFM vs Simulation
mask 1
mask 2
mask 3
mask 4
mask 5
mask 6
mask 7
mask 8
mask 9
mask 10
Masks 1-10 differ in
the line-space ratios
0.25 micron process
technology
OPC assisted masks
-1 Focus +1
-1 Focus +1
SFR Workshop - Lithography
11/8/99

14. SFR Workshop - Lithography
What is Scatterometry?
Concept: Scattering (Diffraction) of light from features produces strong structure in reflected optical field.
Analyze structure to obtain topography information.
Periodic structures (gratings) can be numerically modeled “exactly”.
Incident Laser
Beam
Incident Polarized
White Light
0th order
0th order
2-q Scatterometry
Specular Spectroscopic Scatterometry
SFR Workshop - Lithography
11/8/99

15. Specular Spectroscopic Scatterometry
0th order, broadband detection
1D gratings and 2D symmetric gratings
Use spectroscopic ellipsometers
sinqm = sinqi+ml/D
|sinqm|<1 i +1 -1 D l
Cut-Off Pitch
(in nm)
SFR Workshop - Lithography
11/8/99

16. Timbre ProfilerTM Flow
Library
Generation
Electromagnetic
Simulation
Software
Generate Profile
Library
Generate Signal
Library
Typical turnaround time = 6-12 hours
Compiled Profiler
Library
Compiled Profiler
Library
Collect
Reflected Signal
Timbre
ProfilerTM
Measurement
Ellipsometer /
Reflectometer
Test Grating (Scribe Lane)
Reconstructed Profile
Total Measurement + Analysis = 5 seconds/site
Load Library
on Ellipsometer
Ellipsometry
Measurement
Analysis
SFR Workshop - Lithography
11/8/99

17. SFR Workshop - Lithography
ProfilerTM Setup
Periodic grating on mask (~ 50 mm * 50 mm area - typical spot size of production spectroscopic ellipsometers)
line/space specified
Provide optical constants for each film in the stack
Broadband ( nm)
Specify variability expected in process (in CD & thickness)
range around nominal in nm
Specify spectroscopic ellipsometer / reflectometer angle of incidence
Save broadband tan y and cos D values
Specify accuracy requirements
down to sub-nm (this automatically decides library size)
SFR Workshop - Lithography
11/8/99

18. GTK Interface at http://sfr.berkeley.edu
SFR Workshop - Lithography
11/8/99

19. Matching on tan(Y) and cos(D)
Simulated
by GTK
SFR Workshop - Lithography
11/8/99

20. Example of 0.25mm Profile Extraction
Blue is
actual (by
Veeco
AFM).
Red is
extracted
from GTK
Library.
SFR Workshop - Lithography
11/8/99

21. Case I: Resist on ARC on Si (0.18 mm technology)
Focus-Exposure Matrix
SFR Workshop - Lithography
11/8/99

22. Profile Extraction over the entire FEM
RED is
AFM.
BLUE is
extracted.
SFR Workshop - Lithography
11/8/99

23. Offset between CD-SEM and ProfilerTM as a function of Sidewall Angle
D bottom CD (CDSEM - PXM) in nm
Sidewall angle in degrees
SFR Workshop - Lithography
11/8/99

24. Case II: Resist on ARC on Metal (0.25 mm technology)
TiN Al
TiN Ti
TEOS Si
Focus-Exposure Matrix
SFR Workshop - Lithography
11/8/99

25. Profile Extraction: Resist on ARC on Metal
CD-SEM (Bottom CD)
Profiler Extraction
PXM (Bottom CD)
CD (in nm)
Correlation* = 0.93
Site Number
SFR Workshop - Lithography
11/8/99

26. SFR Workshop - Lithography
Case III: Etched Metal
TiN Al
TiN Ti
TEOS Si
Focus-Exposure Matrix
SFR Workshop - Lithography
11/8/99

27. Profile Extraction : Etched Metal
CD-SEM (Top CD)
Correlation = 0.92
CD (in nm)
Profiler Extraction
PXM (Top CD)
Site Number
SFR Workshop - Lithography
11/8/99

28. But What Is Our Real Goal? -- a good profile ? -- or high yield ?
We cannot avoid process variations
Recipe setting drift: focus ( ~0.2 m), dose, PEB temperature
Model and material parameter variation: resist n & k, developer Rmax and Rmin, acid diffusivity
System inherent variation: mask OPC feature variation
Our goal is to maximize yield for the statistical distribution of parameters and operating points.
SFR Workshop - Lithography
11/8/99

29. Parameter Variation Effect
Profile deviation from best setting
Operating Point Settings
SFR Workshop - Lithography
11/8/99

30. Parameter and Operating Point Variances Extraction
Parameter mean
+ variation
Recipe setting
+ drift
Lithography
process
Experiment
data
In-die spatial
variation
Hierarchical process disturbance extraction
SFR Workshop - Lithography
11/8/99

31. Recipe Optimization with Variations
Parameter
distributions
Spatial
variation
Simulated
Output distributions
Profiles
within spec.
Calibrated
Lithography
Simulator
Operating Point
distributions + -
Overlapping to get yield
RECIPE
OPTIMIZER
SFR Workshop - Lithography
11/8/99

32. Recipe Optimization with Multiple Feature Types
Poly layer
isolated line
periodic lines with OPC
metal layer
elbows
combination of above
Need to link recipe optimization to circuit performance!
SFR Workshop - Lithography
11/8/99

33. Parameter relationship analysis
In reality, all parameters have variations
too many dimensions for output distribution calculation
Parameter relations can be analyzed to attribute the variation of some parameters to other parameters
diffusivity  PEB temperature
developer temperature  Rmin and Rmax
What are the fundamental reasons behind the variation?
Need a comprehensive list of disturbances, linked to physical models, circuit performance.
SFR Workshop - Lithography
11/8/99

34. SFR Workshop - Lithography
Summary
Experiment
Data
Spatial
variation filter
Param. & op.
point variance
Param. mean
values
Calibrated
Sim. Eng.
Target Specs.
of features
Recipe of
max. yield
In-line sensor
measurement
Maximization of overlapping area
SFR Workshop - Lithography
11/8/99

35. SFR Workshop - Lithography
What is Next?
Extend statistical optimization to other process steps
Plasma etching
Metallization
Device level
Circuit level
Process simulator for other steps needed
Simulator for full process procedure: Avant!, Solid C
device model: BSIM3
Circuit simulator: SPICE
Study error budgets, linked to circuit performance.
SFR Workshop - Lithography
11/8/99

36. SFR Workshop - Lithography
Outline
Simulation and Metrology
Line Edge Roughness
Lithography Simulation
SFR Workshop - Lithography
11/8/99

37. Defining LER and Defect Specifications
SFR Workshop
November 08, 1999
Tho Nguyen, Shiying Xiong and J. Bokor
Berkeley, CA
The objective of this work is to understand and model the impact of lithography/etch line-edge roughness in the gate definition layer, on the electrical behavior of short channel transistors
SFR Workshop - Lithography
11/8/99

38. SFR Workshop - Lithography
Progress Since May
Hydrodynamic Model working
3D interaction of Defects
Real LER Simulation
SFR Workshop - Lithography
11/8/99

39. Effect of Gate “Errors” on Device Characteristics
Cross-section
Threshold voltage
Turn-off slope
Drive current
Device reliability n+ n+
Edge roughness
Layout views DL
Single defects DW
SFR Workshop - Lithography
11/8/99

40. SFR Workshop - Lithography
Base Design
Channel Doping Selected at 0.4 Volt
Halo Implant Incorporated to Offset Vt rolloff
Threshold Swing Vds = 2V and L = 100nm
DIBL = 70 mV/V for Vds = V
Vt RollOff Characteristics
500
450
Device Length = 200 nm
Channel Length = 100 nm
Channel Width = nm
Buried Oxide = 100 nm
Si Film Thickness = 250Å
Gate Oxide = 30 Å
400
Without Halo Implant
350
With Halo Implant
300
250
0.2
0.4
0.6
0.8 1
1.2
Channel Length (Microns)
SFR Workshop - Lithography
11/8/99

41. Real 3D LER Construction and Simulation
Real 3D LER Created by Matlab and incorporated into simulator language
LER defined by band-limited white spectrum. 2 parameters: RMS roughness, correlation length
Process simulation used for self- aligned S/D doping
Current digitized LER resolution is nm due to limited memory
160
SFR Workshop - Lithography
11/8/99

42. SFR Workshop - Lithography
Simulation Results
Hydrodynamic model has been successfully turned on in ISE simulator
With hydro on, Ion is ~ 30% higher
Simulations of “real” 3D LER has been successful W = 50nm)
I_V Curves for Different Real 3D LER
Zoom View of Leakage Current
25 % increase in Ioff for 5nm rms roughness
140% increase in Ioff for 9nm rms roughness
SFR Workshop - Lithography
11/8/99

43. SFR Workshop - Lithography
Simulation Results
Defect shows 3D interaction for channel width less than 100nm
To study LER, we have to use 3D models
Intel Work (T. Linton, et al. 1999):
Simulation of square-wave modulation of LER with Neuman boundary conditions
Shows similar 3D interaction
Leakage control by length adjustment with reasonable Ion reduction
SFR Workshop - Lithography
11/8/99

44. SFR Workshop - Lithography
Milestone Status
June 1999
Complete 3D device simulations of mask errors and LER effects in gate-level. Threshold voltage shifts, turn-off characteristics, and saturated drain current will be evaluated.
Status: Late. Student (Tho Nguyen) started Jan Second student (Shiying Xiong) started Sept Expect completion March 2000.
June 2000
Test NMOS devices with programmed mask errors as well as varied LER and compare measured characteristics with simulation results.
Status: Delayed. No company fab support. Will start Microlab run Jan if unable to arrange support from company fab.
SFR Workshop - Lithography
11/8/99

45. SFR Workshop - Lithography
Proposal for
Simulation
Effect of LER on GIDL
Effect of LER in isolation edge
Device reliability
Extend to 50 nm CD
Experiments
Complete gate roughness experiment for 100 nm CD
Isolation roughness experiment
Extend to 50 nm CD??
SFR Workshop - Lithography
11/8/99

46. SFR Workshop - Lithography
Outline
Simulation and Metrology
Line Edge Roughness
Lithography Simulation
SFR Workshop - Lithography
11/8/99

47. Konstantinos Adam Prof. Andrew Neureuther UC Berkeley
Implications of Polarization, Corner Rounding, OPC Design and OPC Fidelity on Aerial Images
Konstantinos Adam
Prof. Andrew Neureuther UC Berkeley
Use EM theory and rigorous TEMPEST simulations to investigate photomask technology issues
Current Investigations
scattering bars - polarization effects
corners - interior versus exterior
OPC features - placement and corner rounding
SFR Workshop - Lithography
11/8/99

48. Scattering Bar Simulation with TEMPEST
l=193nm
Mag=4X
CDtarget=130nm
|Ey| CD SB
x-axis
z-axis
Incident radiation
y-axis
TE : Ey polarization mm
|Ex|
TM : Ex polarization mm mm
SFR Workshop - Lithography
11/8/99

49. SFR Workshop - Lithography
SB Aerial Images
l=193nm, NA=0.7, s=0.6, Mag=4X, CDtarget=130nm
Aerial Image (Best focus)
0.1
0.2
0.3
0.4
0.5
0.6
0.8 1
1.2
1.4
SPLAT TE TM
No SB
SB size = 0.18l/NA
Normalized Intensity
(mm)
- Observe that the scatter bars (also the main feature) appear wider in TM excitation than in TE and narrower with SPLAT simulation (scalar theory)
SFR Workshop - Lithography
11/8/99

50. SFR Workshop - Lithography
SB Design Graphs
Intensity dip of SB
CD Control with SB Size Control
0.05
0.1
0.15
0.2
0.25
0.3
0.35 .2 .4 .6 .8 1
SPLAT TE TM
Perturbation model
0.1
0.2
0.3
110
120
130
140
150
160
SPLAT TE TM
Intensity
CD (nm)
Size of SB (l/NA)
Size of SB (l/NA)
SFR Workshop - Lithography
11/8/99

51. Corner Rounding (Clear Field Mask) - |Ey| Near fields
Square mm
Rounded
l=193nm
Mag=4X
CDtarget=130nm
Eincident (TE)
Eincident (TE)
x-axis
y-axis
SFR Workshop - Lithography
11/8/99

52. Corner Rounding Design Graph
0.04
0.08
0.12
0.16
0.2 2 4 6 8 10
radius of curvature in l/NA
LES increase from reference in nm
Clear field mask
Dark field mask
- LES increase versus radius of curvature is quadratic, i.e. it is proportional to the area missing from the corner due to the roundness
SFR Workshop - Lithography
11/8/99

53. External OPC – |Ey| Near Fields
Example: 0.1l/NA Square OPC and “Mouse Ear” OPC with radius=0.06l/NA
Reference
Square OPC
“Mouse Ear” OPC mm
SFR Workshop - Lithography
11/8/99

54. SFR Workshop - Lithography
OPC Design Graph
0.2
0.4
0.6
0.8 1
1.2
1.4
1.6
1.8 2 5 10 15 20 25
non-overlapping area in mm2 (1X)
LES correction in nm
x10-3
Square OPC
“Mouse ear” OPC
Data for OPC
displaced along
the diagonal
SFR Workshop - Lithography
11/8/99

55. Resist modeling, Simulation and Line-End Shortening effects
Mosong Cheng
Prof. Andrew Neureuther, UC Berkeley
Use experiment and simulation to investigate photoresist performance and provide mechanism based models, characterization methodology, accurate profile simulation and support models/fast algorithms for including resist in OPC
Current investigations
chemically amplified resist modeling - LES and SFR K2G
electric-field-enhanced post-exposure bake
fast imaging algorithm for 2-dimensional OPC
SFR Workshop - Lithography
11/8/99

56. Resist-model-based line-end shortening simulation
APEX-E , UVIIHS, K2G parameter-extraction methodology
Simulation flow
Problem: Top to Top underestimates diffusion
Problem: Micro-stepper at Berkeley has insufficient image quality
SFR Workshop - Lithography
11/8/99

57. K2G resist: DRM curves and reaction/diffusion/outgasing model
DRM curves, dissolution rate is lower at the top if no TARC.
Collaboration
with Jacek
Tyminski
Nikon
Reaction/diffusion/outgasing model
SFR Workshop - Lithography
11/8/99

58. K2G resist: modeling and simulation
Modeling methodology
Extracting dissolution parameters
Large-area exposure
Extracting reaction rate
Resist profile simulation
Extracting diffusivity
Fitting with DRM data
Fitting DRM curves
Resist profile simulation
SFR Workshop - Lithography
11/8/99

59. Electric-field-enhanced post-exposure bake
Goal: shorten PEB time, improve vertical resist profile uniformity, reduce lateral acid diffusion.
Principle: vertical electric field enhance the vertical movement of photo-acid, hence enhance the reaction cross-section. PEB time as well as lateral acid diffusion can be reduced.
Experimental Setup
Al foil
wafer
Al foil
Hotplate
Resist E
photoacid
SFR Workshop - Lithography
11/8/99

60. Electric-field-enhanced post-exposure bake: status
Experiment done in summer 1999, on UVII resist using JEOL.
RESIST
RESIST
UVII resist, 0.5µm L/S, dose 20µC/cm2, PEB with 100kHz, 3.3V AC, 140oC, 60sec.
UVII resist, 0.5µm L/S, dose 20µC/cm2, nominal PEB,140oC, 90sec.
SFR Workshop - Lithography
11/8/99

61. SFR Workshop - Lithography
Fast resist imaging algorithm for 2-dimensional OPC(submitted to SPIE’99)
Assume 2-D reaction/diffusion. Let f(x,y,t)=Cas(x,y,t), g(x,y,t)=Ca(x,y,t).
Contains Spatial Laplacian
and Uses 3rd Order Splines
Time-advancing scheme
Based on NT Aliasing and
NL Relaxation
Very Fast as only requires
repeated multiplication
with fixed coefficients
Iterative solve c2,d2, to minimize the error E.
SFR Workshop - Lithography
11/8/99

62. Fast resist imaging algorithm: simulating flow and tuning parameters
Simulating and tuning flow:
resist profile
Mask pattern
aerial image
Resist imaging
SPLAT
Resist parameter tuner
Method of Feasible
Direction
Differential
SEM picture
Extract resist parameters by tuning the image to fit with SEM picture.
SFR Workshop - Lithography
11/8/99

63. Progress on Milestones
Year 1
simulate in-lens filtering (Done)
resist exposure mechanisms (Not Started =>DARPA/SRC)
web simulation resist and mask effects (In Progress 70%)
Year 2
Integrate scattering, imaging and resist (Expanded by 3X in the number of effects characterized, In Progress 70%)
process flow generator for test structures (Not Started)
web alignment and e-beam (alignment Ongoing 30%, e-beam Not Started => DARPA/SRC)
SFR Workshop - Lithography
11/8/99

64. Future Opportunities in Lithography
Photomask EM effects (How to move faster?)
impact of non-idealities
inspection and repair
Chemically-Amplified Resists
models that work
methodology to calibrate models for production
Optical Systems
high NA
low k1
SFR Workshop - Lithography
11/8/99

65. Targeted Opportunities in Photomasks and Optics
Attenuating phase-shifting masks
high refractive index and physical height of the attenuating material adversely influences light in adjacent areas
Alternating phase-shifting masks
3D problematical structures - resonate ridges and cross-talk between features inside the photomask
Phase-shifting mask repair
guidelines for adequate repair - height, slope, river bed, stain
Optics
role of laser bandwidth in image quality
high NA thin-film polarization effects
SFR Workshop - Lithography
11/8/99

66. Targeted Opportunities in Resists and Tools
Complete comparison of Simulation and SEM's of printed features in K2G resist, quantify the accuracy of the resist model.
Complete coding of the fast but approximate image processing like algorithm and assess speed and accuracy against rigorous simulation in STORM.
Initiate tool-process-dependent line-end shortening investigation by identifying key factors contributing to line-end shortening and suggesting approaches for control and compensation tuning.
SFR Workshop - Lithography
11/8/99