Enhancing LED yield via Comprehensive Surface Metrology webinar

November 12, 2010 Slide 1Bruker NSB Stylus and Optical Unit R&D Status Update 08OCT10 V1.4

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Bruker Nano Surface Business-- Stylus and Optical Metrology Unit

Enhancing LED yield via Comprehensive Surface Metrology

Dr. Erik Novak

Director of Technology Development

Bruker Nano Surfaces




Outline

• Scanning white light interferometry

• Wafer Bow and Flatness Control

• Surface Roughness Measurements

• Patterned Sapphire Substrate

• Automation

• Measurement in production environment

• Conclusion




White Light Interferometric Profilers Provide Fast, Accurate 3D Measurements

� A standard microscope is modified to include:

�Specialized objective that splits and recombines light

�A precision scanner such that the sample is scanned through focus




Interference lines are similar to contour map lines

• On a map each dark line represents a fixed elevation, determined by the mapmaker. Typically the spacing is 100 feet.

• On an interferogram each line also represents a fixed elevation, where the spacing between dark lines is ¼ the wavelength of the light. Typically the spacing is 300nm.

100’

300nm




Phase Shifting Mode is the Fastest and Most Precise Measurement

� High precision and high resolution

� Vertical Resolution <0.01nm

� Measurement Time < 0.5 seconds

� Well suited for measuring smooth substrates – bare or EPI-coated

� Cannot measure surfaces with Ra >20nm

� Collect multiple frames

� Calculate phase

� Phase unwrapping




Vertical Scanning Provides the Most Versatility for Samples

∑

∑

=

==

N

n

N

n

nG

nnG

z

1

1

0

)(

)(

PSIVSI

� Vertical Resolution about 3 nm

� Measurement time about 5 seconds

� Can measure virtually any sample or step

� Perfect for phosphor roughness, rough

wafers, textured substrates

Compare resolution between VSI and PSI




Wafer Bow and Thickness




Wafer Bow Affects Wavelength Uniformity

• LED wavelength uniformity is a function of

– MQW InGaN Thickness (growth rate)

– MQW InGaN Composition

• Thickness is affected by flow conditions

• Composition is greatly affected by thermal uniformity across wafer

• Bow affects both flow conditions and thermal conductivity of the wafer in its carrier

• Convex, Concave, and randomly warped wafers each have different thermal signatures




Wafer Curvature During Growth

-250

-200

-150

-100

-50

0

50

5000 10000 15000 20000

Heating upannealing GaN QW

6" 1300um

4" 950um

2" 430um

Time, sec.

Wa

fer

Bo

w,

µm

h

Curvature “c”

Concave profile

Convex profile

• Ideal situation if wafer bow is matched to wafer carrier.




Important to Ensure Metrology Capability

• Easiest test is to rotate wafer 90 degrees and remeasure

• Significant features should all rotate with the wafer as in the images here

• For 10 micron wafer bow spec, system should have repeatability and reproducibility well below 1 micron




Wafer Shape Can Vary Greatly even for ‘Good’ Wafers




Radius of Curvature is a Good Metric of Shape

Rad Crv

0

500000

1000000

1500000

2000000

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35

Wafer #

Ra

d C

rv (

mm

)

• Wafers 1-25 from Supplier 1

• Wafers 26-35 from Supplier 2

• Supplier 2 has less bow, but may be worse because it is less consistent




Roughness can be measured using vertical scanning interferometry

• This can be on the unpolished back side to check wafer suitability for processing

• This can be on a specially textured substrate prior to deposition

• Roughness can also be measured on partially processed layers to examine voids and density




Polished Surface Roughness Should be Checked Regularly

• Sapphire Roughness can vary from 0.1nm Ra to 0.5nm Ra even within one lot of wafers

• High magnification (20X or higher) should be used to capture roughness as opposed to shape

• Tracking several wafers through the process can identify what is critical in terms of roughness, scratches, and other defects




There is Correlation Between Back Side Roughness and Bow

• Examined Average, Center, and Edge Roughness

• Correlatation to ROC to roughly a 70% R2 level

• On very flat samples, we do not see such correlation




Wafer Thickness Variation Affects Wavelength Uniformity and PSS

• Thickness Variation Most Commonly Measured as Follows:

– Use a very flat wafer chuck, or one with known shape

– Apply vacuum to sapphire substrate and measure it’s shape

– Subtract out chuck shape if necessary

– Remaining shape is the thickness variation

• Without autofocus, etch heights will vary with wafer thickness

• Thickness variation will also directly affect thermal conductivity of the wafer

• Both have affects on final LED performance




Example of Wafer with Taller PSS Structures where Wafer is Thinner

•Wafer thickness varies by about 1.5 microns•PSS height varies by about 150nm




Rapid, Precision PSS Measurements




Patterned Sapphire Substrates Enhance LED Performance

Structure of LED device Structure of LED device with PSS

� Enhance light emission efficiency

� Reduce dislocation defect between Sapphire and Nitride interface




PSS Structures Distort the Interference Signals

• Diffraction and steep angles cause changes in the received signal from the PSS structures

• This leads to distorted heights using normal analyses

Strong, localized

signal from substrate

Weak, broadened PSS

signal




Advanced Algorithms and Optics Used for Proper PSS Measurement

� Specialized 115X objective collects more light from the steep slopes

� PSS algorithm combines phase, contrast, intensity, and focus for accurate measurement

HDVSI

algorithm




PSS Uniformity is Critical for LED Performance

• Wavelength uniformity, efficiency, and lifetime all are affected by PSS structures

• Height variations across wafers can be >10% in production

• Defects are common, especially near the edges of substrates

• Rapid process control feedback improves yield greatly




Interferometry Results Compare Closely to AFM But Are Much Faster

2.85um

1.15um

1.14um

2.83um

Statistics over field of view




Automation Enables Rapid Sampling

• Extensive automation for tilt, focus, stage positioning• Vibrationally stable design for production environments• Designed for reliability, with 100,000 MTBF light source• 2” to 8” sapphire substrates measured on one instrument• Fast: 13 sites measured in under 2 minutes




Site Statistics Reported for Each Measurement as They Are Taken




Methodology Must Be Verified as Robust and High Throughput for Production Suitability

• Static repeatability

– Same site 30 measurements

– Short term and long term

• Dynamic repeatability, single wafer

– 13 sites, one wafer, 20 runs

• Throughput

– Test measurement time including loading for automated, multi-site meausremnets




Short Term Repeatability < 5nm 3 sigma for PSS height

Short Term Repeatability < 5nm 3 sigma for PSS height

PSS Height Repeatability

1000

1040

1080

1120

1160

1200

0 20 40 60 80 100

Measurements

PS

S H

eig

ht

(nm

)

PSS Width Repeatability

2.7

2.75

2.8

2.85

2.9

2.95

3

0 20 40 60 80 100

Measurements

PSS W

idth

(um

)

� 1 site, 30 measurements, 3 cycles, 4 hours cycle

PSS Pitch Repeatability

3.47

3.49

3.51

3.53

3.55

0 20 40 60 80 100

Measurements

PSS P

itch (um

)




Long Term Repeatability < 15 nm 3 sigma for height

Long Term Repeatability < 15 nm 3 sigma for height

PSS Height Long Term Repeatability

10001020

10401060

108011001120

11401160

11801200

0 100 200 300 400 500

Measurements

PS

S H

eig

ht

(nm

)

PSS Width Long Term Repeatability

2.7

2.75

2.8

2.85

2.9

2.95

3

0 100 200 300 400 500

Measurements

PS

S W

idth

(u

m)

PSS Pitch Long Term Repeatability

3.47

3.48

3.49

3.5

3.51

3.52

3.53

3.54

3.55

0 100 200 300 400 500

Measurements

PS

S P

itch

(u

m)

� 15 runs over 5 days, 30 measurements per run, 4 to 8 hours between runs




Throughput < 2 min. per waferThroughput < 2 min. per wafer

1

• 13 sites 20 times:

1:41 per run

• Test included autofocus, analysis, and data logging




13-Site Reproducibility < 5 nm 3

sigma

PSS Height Plots

900

950

1000

1050

1100

1150

1200

0 2 4 6 8 10 12 14

PS

S H

eig

ht

(nm

)

PSS Width Plots

2.2

2.3

2.4

2.5

2.6

2.7

2.8

2.9

0 2 4 6 8 10 12 14

PS

S W

idth

(u

m)

Site PSS Height Reproducibility

1

1.4

1.8

2.2

0 2 4 6 8 10 12 14

Measurement Sites

Sig

ma

(n

m)

Site PSS Width Reproducibility

3.54.5

5.56.5

7.5

0 2 4 6 8 10 12 14

Measurement Sites

Sig

ma

(n

m)

Height reproducibility

Average of 13 sites: 3σ: 4.98nm

Width reproducibility

Average of 13 sites: 3σ: 17.42 nm

� 13 site, 20 cycles




Film Thickness Can Also be Measured Using WLI Systems

• Each interface provides a signal to the instrument

• The separation between the signal peaks determines film thickness

• Can be used on films from 1.5 µm thick to 600 µm

– Upper limit allow sapphire thickness to be measured

• Repeatability <10nm 1σσσσ

White light interferogram for 6 micron thick film

as seen by single pixel and by a row of pixels.

Measured optical path




Top Surface, Bottom Surface, and Film Thickness All Reported




ConclusionsConclusions

• Many factors affect final device performance for LED’s

• Measurements early in the process prevent value from being added to defective material

• Wafer Bow, Thickness, and Roughness affect deposition – Can reduce wavelength uniformity

– Can cause device defects

• Proper PSS structures are critical for LED device performance

• White light optical profilers are well suited for detailed wafermeasurements and production PSS measuerments– Fast

– Accurate

– Repeatable

– Stable

– Automated




Thank you!

Questions?

My contact information:

[email protected]

(520) 741-1044

www.bruker-axs.com

Documents

Enhancing LED yield via Comprehensive Surface Metrology webinar