1

Background

• In Situ Process Control Is Critical for theFuture Factory.

• Wafer State Monitors (CD, Film Thicknessand Profile) Are Important for the AdvancedSemiconductor Process Control.

• Physical Metrology Is Challenged by theShrinking Device Dimensions.

2

Overview• Specularly Reflected Light Measurements

(Ellipsometry, Reflectometry) Have ProvenAccuracy in Monitoring Blanket Thin FilmThicknesses.– Limited applications to patterned wafers

• Non-specular Scattering (Diffraction)Measurements Can Yield Detailed TopographyInformation From Patterned Wafers.– Scatterometry , Fourier Imaging– Both require multiple-angle view of wafer (hard to

implement for in situ monitoring)

3

Goals of Research

• Quantitative Analysis of Ellipsometry DataFrom Patterned Wafers Extract PatternTopography Information (Especially Depth)– Process control using product wafers

• Reduction/elimination of test wafers• Reduced offline metrology

– Increased knowledge of processes at real timelevel

4

Ellipsometry

Eip Erp

Eis Ers

Polarizer Analyser

LightSource Detector

Sample

)exp()tan(//

∆⋅=== iEEEE

RR

isrs

iprp

s

p ψρ

• Tan(Ψ ) and Cos(∆) are Measured by Ellipsometer.— Functions of λ

5

Scope of SE Applications

λ

4Smooth Surface (d ~ 0)– Thin film characterization d: feature size of uneveness

4Rough Surface (d << λ)– Surface/interface characterization

s Patterned Surface (d ~ λ)– Pattern profile characterization

This Work

6

Models For This Work

• Three Approaches for ModelingSpecular Reflection FromPatterned Structures– Scalar approach of Heimann

– Surface integral equation (SIE)

– Rigorous coupled-wave analysis(RCWA)In

crea

sing

Acc

urac

y an

dC

ompu

tatio

n Ti

me

SolvingMaxwell’s EquBoundary ValueProblem

7

Sample: Si Relief Grating

•Use Simple Structure to Minimize the Complexity in Initial Efforts•The Lineshapes can be Modeled as Trapezoidal.

— Described by 4 parameters : period, top linewidth, depth, wall angle•We Measure the Structure from SEM as: period=3.96 µm, top linewidth =2.2 µm, depth = 0.52 µm, and wall angle = 73.9º.

8

Configurations of SE Measurement

75º

•Align The Grooves Normal to the Plane of Incidence•Two Kinds of Measurements Conducted— Near normal (6º) incidence— Oblique (75º) incidence

9

region 1 region 2

2pR2sR,

1pR1sR,

Description of the Scalar Model

∆Ψ==ρ

+=+=

je)tan( RR

.Raf .Raf R .Raf .Raf R

s

p

s2s1sp2p1p 2121

af1 , af2 : Area Fractions Rp , Rs : Reflectances in pp and ss polarizations

• Nominally, af1 + af2 = 1. But, af1 and af2 may be free parameters to consider unmodeled effects, e.g., Reflection from side walls. • To obtain the reflectances, we add the corresponding complex fields, not their intensities.

10

4000 5000 6000 7000 80000

0.05

0.1

0.15

0.2

0.25

λ (Å)

|Rp|2|Rp|2

4000 5000 6000 7000 80000

0.05

0.1

0.15

0.2

0.25

|Rs|2|Rs|2

λ (Å)

Scalar Model Applied to the SiRelief Grating

• Region 1: 57.83% (4797.1 Å Si / Si )• Region 2: 25.11% (Si )

• Region 1: 57.83% (4797.1 Å Si / Si )• Region 2: 25.11% (Si )

MeasurementModel

11

SIE Model of Vector Diffraction

Patterned Wafer

Interference fromSecondary SourceIncident Wave

Secondary Source

• Based on the Surface Equivalence Theorem (Generalization of Huygens’ Principle )

12

SIE Simulation of Near-normalEllipsometry

Measured and SIE Fitted Ellipsometry Data at 6º for the Nominal 500 nmDeep, 4 µm Period Structure

0.6

0.8

1

1.2

1.4

0.4 0.5 0.6 0.7 0.8

tan(psi) Measured tan(psi) SIE

wavelength ( m)µ

Tan(

ψ)

13

Rigorous Coupled-Wave Analysis (RCWA)

• Numerical Eigen-matrix Solution for Maxwell’s Equation• Groove Is Sliced Into a Number of Thin Layers• Amplitudes of Different Diffraction Orders Are Obtained by Solving Coupled-wave Equations

ε 1

ε2

0-1 1

2-2Transmitted Waves

0

1-1Reflected Waves

14

RCWA Simulation of 75ºSpectroscopic Ellipsometry

Measured SE Data From the 500 nm Depth, 4 µm Period Sample. TheRCWA Simulation Yielded a Period of 4.0 µm, a Top Linewidth of 2.2µm, a Sidewall Angle of 72.95º, and a Depth of 480 nm.

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

-2

-1.5

-1

-0.5

0

0.5

1

0.4 0.45 0.5 0.55 0.6 0.65 0.7 0.75 0.8

TAN(psi)-RCWATAN(psi)-measured

COS(delta)-RCWACOS(delta)-measurede

Wavelength µ( m)

Tan(

ψ)C

os(∆)

15

Grating Analysis Approach• Successively Better Approximations

– Estimate Etch Depth From Near-normal IncidenceSpectral Reflectometry in p-polarized Mode (Fast)

– Extract the Grating Period From the DiffractionExperiment

– Refine Depth Estimate and Estimate Period,Linewidth, and Wall Angle Using SIE Analysis of s-and p-polarized Reflectances (~1 min/λ)

– Refine Topography Esitmates Using RCWA onSpectroscopic Ellipsometry Data (~5 min/λ)

16

Si Grating Etched to Different Depths

Si Relief Gratings with Nominal 2µm Lines and Spaces Etched to Depths ofApproximately 100, 200, 300, 400, and 600 nm.

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

0.4 0.5 0.6 0.7 0.8

tan(psi) etch1tan(psi) etch2tan(psi) etch3tan(psi) etch4tan(psi) etch5

Wavelength( m)µ

Tan(

ψ)

17

Simulation of Grating Etch

Approximate Simulation of Si Relief Grating Etch. Note That the Trends ofthe Spectroscopic Ellipsometry Data Are Captured.

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

0.4 0.5 0.6 0.7 0.8

tan(psi) simulation1tan(psi) simulation2tan(psi) simulation3tan(psi) simulation4tan(psi) simulation5

Wavelength( m)µ

Tan(

ψ)

18

Comparison With SEM

The Depths Extracted From the Simulation Are in Very Good AgreementWith Those Measured From SEM. The Non-uniformity Among Different DieMay Be Responsible for the Small Difference.

0

100

200

300

400

500

600

0 20 40 60 80 100

depth (simulation)depth (SEM)

Etching Time (sec)

Etc

hing

Dep

th (n

m)

19

Real Time Data Collection

• Part of SE Data During the Etch of Patterned PR/4800ÅPoly/300Å Sio2/si (6 sec in Total With 1 sec Intervals)

• The RTSE Possesses a Data Collection Capability of <0.5 secWith a Whole Visible Light Spectral Range.

0.2

0.3

0.4

0.5

0.6

0.7

1.2 1.6 2 2.4 2.8 3.2 3.6 4

Tan(psi)-40sTan(psi)-41sTan(psi)-42sTan(psi)-43sTan(psi)-44sTan(psi)-45sTan(psi)-46s

Energy (ev)

Tan(

ψ)

20

Deep Sub-micrometer Regime

• Simulations Shows That for 100nm Features, the Near-normalSE Curves Still Exhibits Strong Structures, As Opposed to EvenSmaller Structures and Blanket Wafers.

• The 100nm and 110nm Curves IS Different.• Conclusion: Can “See” 100nm and Resolute 10nm.

0.2

0.4

0.6

0.8

1

1.2

1.4

0.4 0.5 0.6 0.7 0.8

Tan(psi)-BareTan(psi)-100nmTan(psi)-110nm

Wavelength ( m)µ

Tan(

ψ)

21

Conclusions

• Spectroscopic Ellipsometry Can Give AccurateDepth and Topography Information FromPatterned Structures.

• Quantitative Analysis of Diffraction Effects IsComputationally Time-intensive.

• In Situ Rapid Data Acquisition Is Possible WithRTSE.

22

Future Efforts

• Improved Algorithms for Higher Speed VectorDiffraction Analysis

• Quantitative Analysis of Topography ParameterSensitivities and Optimal MeasurementConditions

• Non-linear Regression Method for TopographyParameter Extraction

23

Technology Transfer Possibilities

• Working With National SemiconductorMentors on Applications to Gate LinewidthControl

• Analysis Methods Easily Transferable/noNew Instrumentation Required– Exist SE’s for Blanket Measurements Can Be

Used for Patterned Wafer Metrology

24

Acknowledgements• This Work Was Supported in Part by

– Semiconductor Research Corporation (Contract 97-FC085)

– AFOSR/DARPA MURI Center for IntelligentElectronics Manufacturing (AFOSR F49620-95-1-0524)

– State of Michigan Center for Display Technology andManufacturing

• The Authors Would Also Like to Thank Dr. D. S.Grimard and Ms. M. Gulari for Assistance WithSample Fabrication