1 Optimized cavity-enhanced compact inverse-Compton X-ray source for semiconductor metrology Jeremy Kowalczyk (BS Cornell '00 ECE) University of Hawaii at Manoa

2 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

3 Metrology technique SAXS:small angle x-ray scattering

4 What can SAXS do? Plots courtesy of Joseph Kline, NIST, Department of Commerce.

5 Ideal source for SAXS Divergence angle ~ 1 mrad Spot size < 100 micron Energy > 20 keV (low absorption) photons/sec  Conventional sources 10 6 photons/sec  Synchrotron meets spec Fits in a fab Affordable High reliability

6 SAXS needs: compact, bright X-ray source inverse-Compton E electron,i = γmc 2 E photon, i e- E electron,i = γmc 2 - (E photon,f - E photon,i ) E photon, f ≈ 4γ 2 E photon,i e- E photon, f ≈ 4γ 2 E photon,i

7 Design Philosophy Use off the shelf parts Partner with vendors  Linac, gun, undulator Minimize engineering effort Minimize cost  Ebeam = expensive  Laser = cheap

8 Design Philosophy (2) Maximize cheap laser power Sufficient expensive ebeam current Total average X-ray power  Linear in laser power, ebeam current

9 Hawaii Inverse-Compton X-ray source Image courtesy of Eric Szarmes

10 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

11 Limitation: I ave from thermionic gun Back-bombardment limits I ave < 50 μA “Laser pre-pulse” technique increases I ave by ~10X Photocathode gun solves back-bombardment but...  Only most advanced research photocathode guns can do GHz rep rates (Cornell)

12 TEMP TIME TEMP TIME RF on TEMP TIME RF off E-field e- Back-bombardment heating Short RF time Short current pulse Low I ave Cathode assembly CathodeTungsten heater

13 TEMP Laser pre-pulse cancels back-bombardment heating RF on E-field e- TEMP RF off TEMP Laser pulse TEMP Laser pulse Long RF time Long, stable pulse High I ave

14 Laser pre-pulse increases I ave ~8X increase in I ave ~26X increase in I ave Status: preliminary experiments done modest I ave increase, but temp. too high waiting long pulse (10 μs) laser

15 Expected Specs Enable SAXS

16 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

17 Thank you!! Questions/Discussion

18 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 doi: /PhysRevSTAB 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 doi: /