1SRC/Sematech Engineering Research Center for Environmentally Benign Semiconductor Manufacturing
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, MAFebruary 2007
2SRC/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.
Asperity Fluid Forces
Asperity Forces
Polishing Pad
Substrate
Polydimethylsiloxane (PDMS) Surface Structures deflect to indicate shear forces present
3SRC/Sematech Engineering Research Center for Environmentally Benign Semiconductor Manufacturing
Environmental Safety and Health (ESH)Metrics and Impacts
METRIC IMPACT
Energy Consumption During Understanding wafer-padProcess interactions during polish leads to
reduced time to polish and toolenergy consumption
DI Water Consumption During Optimized process parameters basedProcess on in-situ characterization of contact,
and forces leads to reduced time toProcess Chemical Consumption polish and slurry consumption (Slurry Chemicals) optimization
4SRC/Sematech Engineering Research Center for Environmentally Benign Semiconductor Manufacturing
Sensor Process & Design Overview
Design 1 CAD Layout
Light microscope image of SU-8 mold from design 1
SEM image of SU-8 mold from design 1
SEM image of PDMS structure from design 1
Final Design • Initial Design– 85 µm high posts: successful– 80, 90, 100 µm diameter: successful– 10, 15, 20 µm : not fully formed; incomplete
formation of mold
• Final Design– 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
30 µm 100um diameter posts
Calibration block
~100 µm SU-8 Photoresist
Si Substrate
Expose/Develop = SU-8 mold
Pour/Cure PDMS onto SU-8 mold
5SRC/Sematech Engineering Research Center for Environmentally Benign Semiconductor Manufacturing
ABS Plastic
Acrylic ‘Windows’
Pyrex Wafer
PDMS sensor
CMP Axle
Covalent Bond through O2 plasma ashing
Adhesive
Pressure Fit
Viewing Area (radially symmetric)
Mounting
Sensors seen through acrylic viewing window, Pyrex, and the back of the PDMS wafer
ImagingCurrent Optics Rhodamine B
Without dye: 80-100
µm structures visible.
Expect to be able to
resolve ~5-10 µm
displacement with
existing optics.
With dye: some
edges are very bright.
Additional experiments
are planned to attempt
to achieve uniform
edge dyeing.
6SRC/Sematech Engineering Research Center for Environmentally Benign Semiconductor Manufacturing
Total shear force load is COF·Downforce·Wafer Area (for 4” wafer) ≈ 0.5 (1.8 psi) ( (50 mm)2) ≈ 50 N
Asperity Force Estimations
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)
≈ ( (50 mm)2)/ ((30 µm)2)
≈ 8.7 · 106 asperity contacts/wafer
Option #2 : Determine number of contacts based on ratio of total contact area to individual asperity contact area.
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”
Estimate of static wafer contact % area for IC1000 pad at 1.8 psi downforce is 0.7%.
Force per asperity is total force over number of asperities ≈ 50 N/(8.7·106 asperities) ≈ 6 N
Force per asperity is total force over number of asperities ≈ 50 N/(6.9·105 asperities) ≈ 70 µN
~20µm≈ (5 µm)2 ≈ 80 µm2
Number of Asperities in Contact:
≈ (0.007· (50 mm)2)/ 80 µm 2
≈ 6.9 · 105 asperity contacts/wafer
7SRC/Sematech Engineering Research Center for Environmentally Benign Semiconductor Manufacturing
Expected Sensitivities and DeflectionsEstimated Fluid Forces Estimated Structure Stiffness
Diameter (m) 30 40 50 60 70 80 90 100
Compliance(m/N) 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
Deflection from Single Asperity Force (m)
50-600
20-200 7-80 3-40 2-20 1-10 0.6-8 0.4-5
Deflection from Fluid Forces (m) 2.8 1.4 0.75 0.47 0.31 0.22 0.16 0.12
L=85 µm Eestimated=750 kPa
Deflections due to asperity forces are expected to be at least 5x to 100x larger than deflections due to fluid forces.
Estimated Sensitivities
8SRC/Sematech Engineering Research Center for Environmentally Benign Semiconductor Manufacturing
Preliminary Sensor Calibrations
PDMS Post
Dektak Tip Paths
Total Deflection (y) = Δ + Fi/k
Δ – Deflection due to sample tilt
Horizontal Distance (um)
Po
st D
efle
ctio
n (
um
)
Po
st D
efle
ctio
n (
µm
)
Downforce (µN)
Dektak Downforces
Calculated Post Deflections @ varying locations along the post
Po
st D
efle
ctio
n (
µm
)
Downforce (µN)
Nonlinearity of post deflections (60 µm from the post base)
Non-linearity at 60 µm from the post base with a downforce of 100 µN is around 5%
Calculated stiffness at 60 µm from the base= 3.1 N/m (σ = 0.63)
~10% of the value estimated from beam theory
9SRC/Sematech Engineering Research Center for Environmentally Benign Semiconductor Manufacturing
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.
10SRC/Sematech Engineering Research Center for Environmentally Benign Semiconductor Manufacturing
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)
11SRC/Sematech Engineering Research Center for Environmentally Benign Semiconductor Manufacturing
• 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.
Conclusions
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