1SRC/Sematech Engineering Research Center for Environmentally Benign Semiconductor Manufacturing

Micromachined Shear Sensors for in situ Characterization of Surface Forces during CMP

ERC Task # 425.020A. J. Mueller, R. D. White

Dept. of Mechanical EngineeringTufts University, Medford, MA

February 2007

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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

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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

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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

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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: 80-100µm 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.

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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

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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

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Preliminary Sensor Calibrations

PDMS Post

Dektak Tip Paths

Total Deflection (y) = Δ + Fi/k

Δ – Deflection due to sample tilt

Horizontal Distance (um)

Pos

t Def

lect

ion

(um

)

Pos

t Def

lect

ion

(µm

)

Downforce (µN)

Dektak DownforcesCalculated Post Deflections @ varying locations along the post

Pos

t Def

lect

ion

(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

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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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• 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