Transcript Title Slide - Tufts University

Micromachined Shear Sensors for in situ
Characterization of Surface Forces during CMP
ERC Task # 425.020
A. J. Mueller, R. D. White
Dept. of Mechanical Engineering
Tufts University, Medford, MA
February 2007
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SRC/Sematech Engineering Research Center for Environmentally Benign Semiconductor Manufacturing
Project Objectives
• Fabricate and implement micromachined shear stress
sensors for characterization of surface forces during
chemical-mechanical polishing (CMP).
• Measure local, real-time shear stress at the pad-wafer
interface during CMP due to slurry and asperity
interactions with the wafer.
Polishing Pad
Asperity
Fluid Forces
Polydimethylsiloxane
(PDMS) Surface Structures
deflect to indicate shear
forces present
Asperity Forces
Substrate
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Environmental Safety and Health (ESH)
Metrics and Impacts
METRIC
IMPACT
Energy Consumption During
Process
Understanding wafer-pad
interactions during polish leads to
reduced time to polish and tool
energy consumption
DI Water Consumption During
Process
Optimized process parameters based
on in-situ characterization of contact,
and forces leads to reduced time to
polish and slurry consumption
optimization
Process Chemical Consumption
(Slurry Chemicals)
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Sensor Process & Design Overview
Si Substrate
~100 µm SU-8 Photoresist
Light
microscope
image of SU-8
mold from
design 1
Design 1
CAD
Layout
Expose/Develop =
SU-8 mold
Pour/Cure PDMS onto SU-8 mold
SEM
image of
SU-8 mold
from
design 1
Final Design
SEM
image of
PDMS
structure
from
design 1
•
Initial Design
–
–
–
30 µm  100um diameter posts
•
Final Design
–
Calibration block
85 µm high posts: successful
80, 90, 100 µm diameter: successful
10, 15, 20 µm : not fully formed; incomplete
formation of mold
–
–
Fabrication limits for final design: 30-40 µm
minimum diameter at 85-100 µm high.
Camera resolution: 5-10 µm deflection
Dyeing may improve resolution
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Mounting
CMP Axle
ABS Plastic
Acrylic ‘Windows’
Pyrex Wafer
PDMS sensor
Viewing Area
(radially symmetric)
Pressure Fit
Adhesive
Covalent Bond
through O2
plasma ashing
Sensors seen through acrylic
viewing window, Pyrex, and the
back of the PDMS wafer
Imaging
Current Optics
Rhodamine B
Without dye: 80-100
µm structures visible.
With dye: some
edges are very bright.
Expect to be able to
resolve ~5-10 µm
displacement with
existing optics.
Additional experiments
are planned to attempt
to achieve uniform
edge dyeing.
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Asperity Force Estimations
Total shear force load is COF·Downforce·Wafer Area (for 4” wafer) ≈ 0.5 (1.8 psi) (p (50 mm)2) ≈ 50 N
Option #1 : Assume 30 µm center to
center spacing on asperity tips with a
square grid.
Number of Asperities in Contact: Wafer
Area/Asperity Neighborhood Area (for 4”
wafer)
≈
(p (50 mm)2)/ ((30 µm)2)
≈
8.7 · 106 asperity contacts/wafer
Force per asperity is total force over number of asperities ≈ 50 N/(8.7·106 asperities) ≈ 6 mN
Option #2 : Determine number of
contacts based on ratio of total contact
area to individual asperity contact area.
≈ p(5 µm)2 ≈ 80 µm2
Estimate of static wafer
contact % area for
IC1000 pad at 1.8 psi
downforce is 0.7%.
~20µm
Number of Asperities in Contact:
≈
Carolina L. Elmufdi and Gregory P. Muldowney, “The Impact of
Pad Microtexture The Impact of Pad Microtexture and Material
Properties and Material Properties on Surface Contact and
Defectivity in CMP on Surface Contact and Defectivity in CMP”
(0.007·p (50 mm)2)/ 80 µm 2
6.9 · 105 asperity
contacts/wafer
≈
Force per asperity is total force over number of asperities ≈ 50 N/(6.9·105 asperities) ≈ 70 µN
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SRC/Sematech Engineering Research Center for Environmentally Benign Semiconductor Manufacturing
Expected Sensitivities and Deflections
Estimated Fluid Forces
Estimated Structure Stiffness
Estimated Sensitivities
Diameter
(mm)
30
40
50
60
70
80
90
100
L=85 µm
Compliance
Eestimated=750 kPa (mm/mN)
8.75
2.77
1.13
0.55
0.30
0.17
0.11
0.07
Compliance
(nm/Pa)
6.19
3.48
2.23
1.55
1.14
0.87
0.69
0.56
50-600
20-200
7-80
3-40
2-20
1-10
0.6-8
0.4-5
2.8
1.4
0.75
0.47
0.31
0.22
0.16
0.12
Deflection from
Single Asperity
Force (mm)
Deflection from
Fluid Forces (mm)
Deflections due to asperity forces are expected to be at least 5x to 100x larger than deflections due to fluid forces. 7
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Preliminary Sensor Calibrations
Horizontal Distance (um)
Calculated Post Deflections
@ varying locations along the post
Post Deflection (µm)
Dektak Tip Paths
Total Deflection (y) = Δ + Fi/k
Δ – Deflection due to sample tilt
PDMS Post
Downforce (µN)
Nonlinearity of post deflections
(60 µm from the post base)
Calculated stiffness at 60 µm from
the base= 3.1 N/m (σ = 0.63)
Post Deflection (µm)
Post Deflection (um)
Dektak Downforces
~10% of the value estimated from
beam theory
Non-linearity at 60 µm from the
post base with a downforce of
100 µN is around 5%
Downforce (µN)
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Future Plans
• Develop experimental apparatus for calibrating post
deflection under known:
– Fluid flow loads
– Mechanical loads
• Improve dyeing ability to improve optical post resolution.
• Integrate with CMP rig for in situ surface force
measurements.
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Industrial Collaboration/Technology Transfer
• Close collaboration with industry partners – Cabot
Microelectronics and Intel
 Monthly telecons – secure website for information exchange
 Semi-annual face-to-face meetings
 Thesis committees and joint publication authorship
 Metrology and analysis methodology technology transfer
 In-kind support – specialized supplies and equipment
 Student internships (e.g. C. Gray at Intel during Summer 2005)
 Close coordination with A. Philipossian research group at U of
Arizona
• Information and results exchange with MIT (D. Boning)
ERC project
 Monthly joint meetings of PIs and research students
 Discussion of findings with other colleagues (e.g. E. Paul – Stockton
College on leave at MIT)
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Conclusions
• The sensors developed will allow measurement of shear forces
during CMP at an estimated force resolution of 1-100 µN and
spatial resolution of 300 µm.
• Sensor fabrication feasibility has been proven and diameter
limitations have been established at 30 µm.
• Calibration and implementation of the shear sensors are ongoing.
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