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Optimized cavity-enhanced compact inverse-Compton X-ray source for

semiconductor metrology

Jeremy Kowalczyk(BS Cornell '00 ECE)

(jeremymk@hawaii.edu)University of Hawaii at Manoa

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The need for compact X-ray sources Shorter wavelength extreme UV light sources not

good enough for next generation semiconductors [1].

Instead industry using multi-patterning approach [2] Same feature size as previous generation Stack multiple layers of circuits to increase density

New manufacturing challenges Mis-shapen vertical structures Misalignment of layers

Need in-line metrology to detect and correct

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Metrology technique SAXS:small angle x-ray scattering

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What can SAXS do?

Plots courtesy of Joseph Kline, NIST, Department of Commerce.

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Ideal source for SAXS

Divergence angle ~ 1 mrad Spot size < 100 micron Energy > 20 keV (low absorption) 1010 photons/sec

Conventional sources 106 photons/sec Synchrotron meets spec

Fits in a fab Affordable High reliability

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SAXS needs:compact, bright X-ray source

inverse-Compton

Eelectron,i

= γmc2 Ephoton,i

e-

Eelectron,i

= γmc2 - (Ephoton,f

-Ephoton,i

) Ephoton, f

≈ 4γ2Ephoton,i

e-

Ephoton, f

≈ 4γ2Ephoton,i

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Design Philosophy

Use off the shelf parts Partner with vendors

Linac, gun, undulator Minimize engineering effort Minimize cost

Ebeam = expensive Laser = cheap

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Design Philosophy (2)

Maximize cheap laser power Sufficient expensive ebeam current Total average X-ray power

Linear in laser power, ebeam current

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HawaiiInverse-Compton X-ray source

Image courtesy of Eric Szarmes

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Why GHz rep rate?

Allows inexpensive optical storage cavity Short length

GHz rate allows stacking on every pass

Small mirrors Near confocal cavity Very tight tolerance Large mirrors with tight tolerance = $$$$

Keeps thermal load manageable CW laser is a non-starter mirror distortion at high average power

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Limitation: Iave

from thermionic gun

Back-bombardment limits Iave

< 50 μA

“Laser pre-pulse” technique increases Iave

by ~10X

Photocathode gun solves back-bombardment but... Only most advanced research photocathode guns

can do GHz rep rates (Cornell)

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TE

MP

TIME

TE

MP

TIME

RF on

TE

MP

TIME

RF off

E-field

e-

e-

Back-bombardment heating

Short RF timeShort current pulseLow I

ave

Cathode assembly

CathodeTungsten heater

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TE

MP

Laser pre-pulse cancels back-bombardment heating

RF on

E-field

e-

e-

TE

MP

RF offTE

MP

Laser pulse TE

MP

Laser pulse

Long RF timeLong, stable pulseHigh I

ave

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Laser pre-pulse increases Iave

~8X increase in Iave

~26X increase in Iave

Status: preliminary experiments donemodest I

ave increase, but temp. too high

waiting long pulse (10 μs) laser

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Expected Specs Enable SAXS

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UH source more cost effective Lyncean Technologies

Similar specifications Focus on

ebeam hardware, small storage ring Low rep rate

~$10 millon UH source

Focus on inexpensive laser hardware high rep rate

Partners: KLA-Tencor, Boeing, Wenbing Yun (Xradia) ~$2 to 3 million

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Thank you!!

Questions/Discussion

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References

J. M. D. Kowalczyk and J. M. J. Madey. Back-bombardment compensation in microwave thermionic electron guns.

Physical Review Special Topics - Accelerators and Beams, 17(12):120402, Dec. 2014. doi:10.1103/PhysRevSTAB.17.120402.

J. M. J. Madey, E. B. Szarmes, M. R. Hadmack, B. T. Jacobson, J. M. D. Kowalczyk, and P. Niknejadi. Optimized cavity-enhanced x-ray sources for x-ray microscopy.

In Proc. SPIE 8851, X-Ray Nanoimaging: Instruments and Methods, pages 88510W–1 – 88510W–9, Sept. 2013. doi:10.1117/12.2027193.