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FLCC Seminar 10/8/2007FLCC Seminar 10/8/2007

An Optical Metrology ToolAn Optical Metrology ToolWith With Si Si Wafer Form FactorWafer Form Factor

Nathan W. CheungDept of EECS UC-Berkeley

cheung@eecs.berkeley.edu

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OUTLINEOUTLINE

• Unique features of optical metrology wafer• Wafer Fabrication• Reflectance Simulations• Applications: RIE, CMP, Wet Etch• Lateral Features Monitoring• Extension to Optical Spectroscopy

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Unique FeaturesUnique FeaturesData Transmission

Photo/RF Transmitter

Dielectric Layer as Optical Window

Si

Battery Data Acquisition Unit

500µm

■ Durable in hostile processing environments such as in plasma, wet etching, and CMP■Self-contained metrology unit with power and data acquisition■Real Time Process ControlLocation specific, real time measurements. Pinpoint failure

and cause.

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Thin LED by Laser LiftoffThin LED by Laser Liftoff1.1. Bond receptor Bond receptor

onto GaN2. Laser Liftoff (LLO)2. Laser Liftoff (LLO)

onto GaNb) thermal

detachment (40°C)

a) interfacial decomposition

receptor wafer

Al2O3

epoxyGaN

KrF pulsedexcimer laser

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Transferred LED on Plastic Transparency

Transparency

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Paper-Thin Photon Engine by Laser Liftoff(> 100 Lumens/Watt)

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High Brightness LED as Photon Source

Specular Reflection

0.0E+00

2.0E-05

4.0E-05

6.0E-05

8.0E-05

1.0E-04

1.2E-04

0 5 10 15 20 25 30

Time [s]

Cur

rent

[A] Room Light

room Light + LED

Room Light + LED +Flashlight

•Specular reflection from room lights is almost negligible

•Specular reflection from strong point sources can add significant noise to system

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Robust Optical Window Processing

80µm 700µm280µm 420µm

Optical WindowWindow Size(µm) 80 280 420 700

Differential Pressure: ± 2

ATP650nm Low Stress LPCVD Si3N4

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3X3 multi-pixel metrology wafer platform

LED

Photo Switch

Battery

Optical Metrology by ReflectanceExample: Thickness Monitoring of SiO2 Film

Calculated Reflectance

Ref

lect

ance

(R)

Thickness of Deposited SiO2 film (µm)

Dp=137.5 nm

θ=0°, λ = 525 nm

hνo

R

T

Window Si: 20 nmWindow SiO2:

2.4 µm

Deposited SiO2

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Determination of thickness change:1.Calibrate periodicity in time domain (tp) with that in spacial domain (Dp).2. Thickness change:= Dp(process time span/tp).

R

Process time

Expected Experimental data

LED Spectral Distribution EffectLED Spectral Distribution Effect

400 420 440 460 480 500 520 5400

500

1000

1500

2000

2500

3000

Ele

ctro

lum

ines

cenc

e

Wavelength (nm)

I0

I-1 I+1

Electroluminescence spectra of GaN-based LEDs

Simulation Condition:Vacuum ambient, Cu thin film, Incident angle, θ, = 40°, Si3N4 window thickness 649nm, the refractive index of Cu: n=1.16,k=2.43*.

Optical response is very sensitive to the wavelength.

LED spectral distribution will limit Cu thickness resolution to <2 nm at end-point(depending on the incident angle), assuming a signal resolution of 5% of actual signal.

0 20 40 60 80 1000.0

0.1

0.2

0.3

0.4

Effe

ctiv

e re

flect

ance

Cu thickness (nm)

I0

I-1

I+1

Itotal = I0 + I-1 + I+1

θ = 40°

I

RT

Si3N4

Cu

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Choice of Optical Window MaterialsChoice of Optical Window Materials

0 20 40 60 80 1000.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

θi = 20o

θi = 40o

θi = 60o

θi = 80o

Ref

lect

ance

Cu Thickness (nm)

Simulation Condition:Vacuum ambient, Cu thin film, window thickness 649nm, LED peak wavelength 463nm, the refractive index of Cu: n=1.16, k=2.43.

*D.L. Windt, IMD Software.

0 20 40 60 80 100

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

θi = 20o

θi = 40o

θi = 60o

θi = 80o

Ref

lect

ance

Cu Thickness (nm)

Si3N4 Optical window SiO2 Optical window

Signal sensing depends greatly on incident angle.Si3N4 optical window shows more signal variation before end-point.

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Calibration MethodologyCalibration Methodology

0 20 40 60 80 100 1200

50

100

150

200

250

PP

D re

adin

g (a

.u.)

RPD reading (a.u.)

Air (Slope=1.901+0.002) Water (Slope=1.552+0.001) P.R. (Slope=1.417+0.001)The current metrology is relied on the changes

in a reflected optical signal through an optical window

Good linearity was confirmed between PPD and RPD reading, suggesting the valid assumptions made in the current methodology.

A new function F may be used to eliminate non-measurable constants such optical component misalignment

0

0

0

0

)/()/()/(

RP

RPRP

P

PP

VVVVVV

F

RRRF

δδδδδδ −

=

−≡

Primary Photo-Detector (PPD)

LEDθi

Reference Photo-Detector (RPD)

Dielectric Window

RRRP

Film to be grown/etched

Primary Photo-Detector (PPD)

LEDθi

Reference Photo-Detector (RPD)

Dielectric Window

RRRP

Film to be grown/etched

Steve Luo, PhD Thesis, UCB

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Verification of methodology

100 150 200 250 300 350 400390

400

410

420

430

440

V r(a.u

.)

Vc (a.u.)

Air (slope=0.132+0.002) Water (slope=0.118+0.005)

Results Summary:Good linearity was confirmed between PPD reading and RPD reading. Using data fitting, incident angle was determined to be about 70 degree, which agrees well with the geometric configuration used.

Optical Window: 650nm Si3N4;Window Size: 4mm;Photodetector Active Area: 10mm2;LED peak wavelength: 463nm.

Dielectric Layer

RPD PPD LED

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Refractive Index Measurments

0 20 40 60 80 100 1200

20

40

60

80

100

120

140PP

D re

adin

g (a

.u.)

RPD reading (a.u.)

Air (slope=1.1677+0.0006) Water (slope=1.0988+0.0005) PR (slope=1.0745+0.0005)

Refractive index sensitivity better than 0.001

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Plasma Etching of Si Oxide

Plasma Etch Measurements

Metrology Wafer

HP 4145B

Wire Connection Interface

Detection Window

Measuring oxide thickness using Dektak3030 Profilometer

Plasma

PQECR, Plasma Quest

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Calibration with a Plasma Etch Process of Silicon Oxide

0 20 40 60 80 100 120 140 160 180 200

-0.8

-0.6

-0.4

-0.2

0.0

0.2

F(θ)

Oxide Thickness (nm)

Experimental Fitted

The good fit between experimental data and calculation demonstrated that the methodology worked as expected.

As expected, effective incident angle, detection window thickness and even effective incident wavelength can be determined by a calibration process.

•n.a.•(1.464,0)*•(nf, kf)•n.a.•(2.054,0)*•(nw, kw)•649•650 ±7•dw (nm)•56•n.a.•θi (°)•n.a.•463 (peak)•λ (nm)

•Extracted•Measured

*D.L. Windt, IMD Software.

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Simulation: Response Space for Copper End-Point Etch Detection

Simulation Conditions: Vacuum ambient inside the metrology wafer and water ambient outside the metrology wafer. SiO2 thickness: 500 um

I

R

T

SiO2

Cu

nm463=λ

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

-20020406080

Copper Thickness [nm]

Ref

lect

ance

[R]

15 deg30 deg45 deg60 deg75 deg

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Demo: Copper Wet Etch End-Point Detection

0

0.00005

0.0001

0.00015

0.0002

0.00025

0 5 10 15 20

Time [s]

Cur

rent

[A]

200 nm150 nm100 nm

Data sample rate: 10 Hz

Stage I Stage II

Multi-stage mechanism of Cu etching.

50 nm Cu

0 nm Cu

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DemoDemo-- Chemical Mechanical Polishing Chemical Mechanical Polishing

Wafer can distinguish various slurries with no metal

Needs optical data of slurry for model fitting

Metal layer e.g. Cu

Polishing fixture

SlurryPolishing pad

Multimeter

Metal layer e.g. Cu

Polishing fixture

SlurryPolishing pad

Multimeter

Metrology wafer

Effects of the surface condition(e.g. slurry particles and volume)

0.0 0.2 0.4 0.6 0.8 1.0

0.0

0.1

0.2

0.3

0.4

0.5

[Sig

nal (

h) -

Sign

al (h

=0)]

/ Sig

nal (

h=0)

Slurry Height, h (cm)

water

45µm-sized diamond suspension*

250nm-sized diamond suspension*

3µm-sized diamond suspension*

6µm-sized diamond suspension*

DS002-16

DS030-16

DS060-16

DS450-16

* Products are commercially available from South Bay Technology. The suspension consists of diamond particles in water with trace amount of proprietary suspending agents.

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Simulation*: Lateral Feature WettingGoal: Simulate monitoring of wetting in high aspect ratio contact

holes in dielectrics.

Effective Medium Approximation:

Effective refractive index of each layer = Volume fraction of A*(na+ika) + Volume fraction of B*(nb+ikb)

Valid as long as layer thickness and lateral dimensions of features are less than wavelength of photon.

*D. L. Windt - IMD Software

0% 25% 50% 75% 100%

Al2O3

Water

SiO2

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0.00011

0.00012

0.00013

0.00014

0.00015

0 20 40 60 80 100 120 140 160 180 200

Time [s]

Cur

rent

[A]

M1M2M3

Demo: Wetting of 200nm contact holes

Methanol observed to wet the sample.

• Reflectivity can monitor liquid wetting of 200 nm contact holes.

• Promising to monitor liner deposition in high-aspect ratio contact holes

Top SEM Side SEMTop SEM Side SEM

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IMPACT: Metrology Wafer With Spectroscopy Capability

Si3N4, SiO2 Stack

Cavity

Optical Window

Bulk Silicon WaferWhite PhosphorLED* PD Array*

Photo/RF Transmitter*

Data TransmissionMolecules

Multilayer Diffraction Stack

several mm*Internal Power Source Not Shown

500 um

• Durable in hostile processing environments such as in plasma, wet etching, and CMP*;

• Identification of chemical precursors• Temporal information for chemical kinetics

*Please see previous FLCC reports on metrology wafer construction, plasma etching results, copper end-point detection, and CMP results.

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Technical Approach: Multilayer Diffraction Stack

Light in

Mirror (Metal)Mirror (Metal)

Multilayer thin-film stack

~few um thick

1x 2x 3xPhotodiode array

Slit

Lateral length can be on the mm scale

Light out

• 10 um detector pixel gives 50 nm wavelength resolution.

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Technical Approach: Blue LED Phosphors for Broadband Spectroscopy

Excitation Emission

0102030405060708090

100

380 430 480 530 580 630 680 730 780

Wavelength (nm)

Inte

nsity

(a.u

.)

TYPE QUMK58/F-D1YTTRIUM GADOLINIUM ALUMINIUM OXIDE : CERIUM

(Y,Gd)3Al5O12 : Ce

Yellow

Excitation Emission

0

10

20

30

40

50

60

70

80

90

100

250 300 350 400 450 500 550 600 650 700 750 800

Wavelength (nm)

Inte

nsity

(a.u

.)

TYPE FL63/S-D1CALCIUM SULPHIDE : EUROPIUM CaS : Eu

Excitation Emission

0

10

20

30

40

50

60

70

80

90

100

250 300 350 400 450 500 550 600 650 700 750 800

Wavelength (nm)

Inte

nsity

(a.u

.)

TYPE HPL63/F-F1STRONTIUM THIOGALLATE : EUROPIUM SrGa2S4 : Eu

GreenRed

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Bio-Analytical Microsystem

CdS Filter

Microfluidic channel

PIN Diode

LED PixelLED exciting Fluidic Channel

0 1x10-6 2x10-6 3x10-6 4x10-6 5x10-6 6x10-6 7x10-60.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

0.35 V 0.30 V 0.25 V 0.20 V 0.15 V

Det

ecto

r Sig

nal (

mV)

Concentration (M)

Sensitivity ≤ 1×10-7 M

•Fluorescence detection limit better than 1×10-7 M.

5mm

CdS filter

CdS0.9Se0.1 filterLED

5mm

Green LED

Blue LED

5mm

CdS filter

CdS0.9Se0.1 filterLED

5mm5mm5mm

CdS filter

CdS0.9Se0.1 filterLED

5mm

Green LED

Blue LED5mm5mm5mm

Green LED

Blue LED

Cheung (EECS) Lee (BioEng)Sands (Purdue)

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Ubiquitous Blue LaserUbiquitous Blue Laser

0 50 100 150 200 250 3000

20

40

60

80

100

CW 20oC

Ligh

t Out

put

(mW

)

Current (mA)

Blue Laser Diode onCu Substrate

Laser diode on Cu ηD = 0.5 W/A

Laser Diode Array Transfer

W.S. Wong et al., Compound Semiconductor 7, 47 (2001)

Laser diode on sapphire

Improved light output due to better thermal management

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New Application AreasOptical Interference

Function (FLCC)Spectroscopy Capability

(IMPACT) Future Areas of Interest

Refractive Index Monitoring Molecular Fluorescence Spectroscopy (MFS)

Thermal/Chemical/Mechanical Induced Optical

ChangesResist Development/Stripping* Precursor Identification

ScatteringCopper End-point Etch Detection* Interface Chemical Kinetics

Phase ShiftPlasma Etch* Reaction Rate Modeling

Non-Linear Optics

CMP* Deposition Key Expected Results:• Time domain optical data• Kinetics of chemical reactions and fabprocesses.

Lateral Feature Monitoring Wet and Dry Etching

Thin Film Thickness Resist Development

*Please see previous FLCC reports on metrology wafer construction, plasma etching results, copper end-point detection, and CMP results.