Lecture 17: Sensors and Metrology I

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Sensors and Metrology

A Survey

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Outline

• General Issues & the SIA Roadmap

• Post-Process Sensing (SEM/AFM, placement)

• In-Process (or potential in-process) Sensors

– temperature (pyrometry, thermocouples, acoustic waves)

– pressure and flow (manometers, momentum devices)

– composition (OES, LIF, RGA, mass Spectroscopy Actinometry)

– thickness (reflectometry, ellipsometry, scatterometry)

– smart-dummy wafers and smart substrates

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Introduction

• Sensors (and actuators) are key limiting factors in

application of control techniques to semiconductor
manufacturing

• sources of difficulty

– implementation environment (vacuum, clean facilities, etc.)

– perception that in-situ sensors affect process

– ex-situ sensors can reduce throughput

– cost of ownership

– traditional resistance in industry

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General Remarks on Sensors

• modeling is often key part of sensing

– physical quantity of interest may not be directly measured (ex:

OES indirectly contains info about etch process state)

– thus, sensors are based on a model of the underlying physical

process

sensors = data + model

• signal processing

– needed to reduce noise, improve bandwidth

– difference between data and information

• problems

– sensors require calibration

– must account for drift

• other issues

-

sensor fusion

- data compression

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Key Issues in Sensors

• some key tradeoffs

– non-invasive vs. invasive

– non-destructive vs. destructive

– in-situ vs. ex-situ

– speed vs. accuracy

– noise

• bias (accuracy) vs. precision (repeatability + reproducibility)

– a sensor could be inaccurate, (thermocouple readings are off by 4

o

K)

– but the sensor might have good precision, (it is consistently off)

– precision is often more important for process control

• modern filtering and estimation methods can be of great use

in improved sensing software.

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Accuracy vs. Precision

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SIA Road Map Challenges above 45nm (through 2009)

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SIA Road Map Challenges below 45nm (beyond 2009)

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The 2004 update Metrology Road Map – near term

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The 2004 update Metrology Road Map – long term

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The 2004 update Lithography Metrology Road Map –near term

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The 2004 update Lithography Metrology Road Map – long term

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CD Metrology

• CD-SEM is today’s pre-eminent technique…

• Electron yield in interaction volume is a function of

surface topography (Secant effect) and atomic number.

• Extracting CD is not so simple...

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Various Edge Detection Algorithms are in use...

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CD-SEM is often calibrated with AFM

Atomic force Microscopy uses either
repulsive forces (sub nm proximity), or
weaker attractive forces (a few nm
away).

Tip tracks surfaces using feedback
control.

Shape and size of tip is the critical
source of errors.

Standard features are use to calibrate
and “de-convolve” the tip profile from the
measurements.

AFM is 100-1000 times slower than a
CD-SEM.

AFM is sensitive to line-edge roughness.

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CD-SEM vs. AFM standards

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CD-SEM cannot “see” the actual profile, so AFM is

used for this purpose...

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AFM/SEM reading of Dense vs. Isolated Lines

dense

iso

AFM

SEM

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Comparison for Contact holes

AFM

SEM

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More CD-SEM vs. AFM comparisons

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Optical CD Measurement

I

• Not very repeatable

• Limited spatial resolution

• Relatively inexpensive

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Electrical CD Measurement

Measurement is very repeatable and fast.
Can only be used in conductive layers.
Need at least one conductive layer and one insulator.
Can be extended for misalignment measurements.

SEM

Lines

ID: XXN

Grating

Horz Iso

W/S= 180/

1000

Grating

Vert Iso

W/S= 180/

1000

Grating

Horz Iso

W/S= 180/

360

Grating

Vert Iso

W/S= 180/

360

Grating

Horz

Medium

W/S = 180/

240

Grating

Vert Medium

W/S = 180/

240

Grating

Horz Dense

W/S = 180/

180

Grating

Vert Dense
W/S = 180/

180

DUT1

Vert

DUT2

Vert

DUT3

Horz

DUT4

Horz

VDP

DUT5

Horz

DUT6

Horz

DUT7

Vert

DUT8

Vert

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SEM
Lines

ID: XXO

DUT1

Vert

DUT2

Vert

DUT3

Horz

DUT4

Horz

VDP

DUT5

Horz

DUT6

Horz

DUT7

Vert

DUT8

Vert

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SEM

Lines

CF_LENS coma/flare

CD_LIN linearity

MEFH

MEF

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Basic SEM Structure

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The Many Modes of SEM

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The Issue of Spatial Resolution

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Chrome on Silicon Example

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Signal Depends Strongly on Material

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Voltage Contrast SEM

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CD-SEM Resolution

Scanning Resolution shown to 1-5nm.

CD metrology on resist has 5-10nm precision.

Other solutions:

ATF, has (theoretically) atomic resolution.

Problem:

What is CD??

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CD-SEM Conclusions

• Accuracy is several (many) nm

• Precision (1-

σ repeatability + reproducibility) is

1-2nm today

• CD-SEM is stand-alone (i.e. expensive)

• CD-SEM measurements are available only after

patterning, and data integration with control
systems is difficult at best.

• AFM-based calibration will not be possible for

trenches less than ~100nm wide.

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State of the Art CD/Imaging-SEM