1nm R&D plan K. Evans-Lutterodt Contributions from: C. Jacobsen, N. Bozovic, I. Bozovic, J.Maser, A.Snigirev, C. Schroer, O.Hignette, and many others

Current Trends in X-ray optics Soft x-rays are bottoming out. Hard xrays would bottom out here. Points courtesy of C. Jacobsen Bottom Line: It can be done; there is no physical reason we cannot get to 1nm However, it will take resources and a targeted effort.

Basic Issues Metrics Numerical Aperture and resolution Depth of field Efficiency Chromaticity Modulation Transfer Function If resolution is 1nm => then DOF =27nm

Implications of Goals/Source NSLS2 has a 6μm x 39μm source size in low  straight. If resolution is 1nm, and =0.1nm, => NA = 0.1 F(mm) Aperture Image Size 1 100μm 0.72x1.1nm 10 1mm 1.3x8nm 100 10mm 12x80nm Focal Length Spot Size Diffraction limit Demagnification

Paths to 1nm Optic Type Comment X 1. Single bounce Solid Metal Mirrors 2. Solid Refractive lenses 3. Binary Zone Plates 4. Multilayer Mirrors 5. Multilayer Lens 6. Kinoform

1.Single Bounce Solid Metal Mirrors Single Bounce Solid Metal Mirrors: Best case is for the most dense material Pt. Technology Theoretical limits Spot size (FWHM) Experimental spot sizes FWHM (nm) Facility Fixed profile mirrors Metal coating 25 nm 15keV SP8 (1Km BL) 68X80 nm (white) APS (BL34) Table courtesy of O.Hignette * Multiple bounce mirrors can improve the situation.

2.Binary Zone Plates The spot size is of order the smallest zone Work at harmonics, reduces efficiency As photon energy increases, the zone plate thickness T increases To get smallest spot sizes at hard x-ray energies requires Large aspect ratios that are difficult to manufacture E-beam lithography tools have tolerances of order 2nm Equation for fresnel boundaries

3.Refractive 1. Absorption Incident Beam Lens Transmission Courtesy of Schroer 1. Absorption Lens Transmission Effective Lens Aperture Solid Refractive Lens Optic Axis Incident Beam t=y2/(2F) Transmission ~ exp( -4t/) Resolution ~ 1/f To get 1nm , we can have f = 1μm, aperture 100nm ! Aperture too small!

Paths to 1nm Optic Type Comment X Pursued by ESRF,Spring8 1. Single bounce Solid Metal Mirrors X 2. Binary Zone Plates 3. Solid Refractive lenses 4. Multilayer Mirrors Pursued by ESRF,Spring8 5. Multilayer Lens Proposed NSLS-II R&D 6. Kinoform

4. Multilayer Mirror (MLM) O. Hignette Optics group- ESRF diffraction limited full width half maximum  d-spacing Wavelength f focal length L mirror length NA numerical aperture 45nm ESRF Approach being followed by ESRF and Spring8

5. Novel Approach : Multilayer Laue Lenses Crossed Linear Zone Plate : 1-D focusing optics From CNM,APS group Deposition + Sectioning + Assembling Multilayer Linear Zone Plate Varied line-spacing grating Deposit varied line-spacing grating on flat substrate (thinnest structures first!) Section to 5-20 m thickness (high aspect ratio structure) Assemble into a multilayer linear zone plate (MLZP) Assemble two MLZP’s into a single device (MLL) substrate Depth-graded multilayers on flat Si substrate

Fabrication of MLL Si substrate Growth Dicing ~ 1mm Polishing ~ 5-25 mm Si substrate Dr~58 nm Assembling by face to face lens configuration WSi2/Si, 12.4 mm Central stop Dr~10 nm Tilting

Measured Focal Spot @ 19.5 keV X-rays Sample C, Drout =10 nm Focal spot Incident beam FWHM : 72.7 nm (sample A) 57.4 nm (sample B) 30.6 nm (sample C) Incident beam size : 12.4 mm (H) X 50 mm (V) Focal spot size : ~ 30 nm (H) Latest unpublished ~ 19nm

MLL issues Need to fabricate wedged MLL to get below 5nm Wedged deposition is novel technology. Works better at higher energies > 20keV Metrology?

Why we need “wedges”

Our Proposed MLL approach 1. Use single crystal lattice matched oxides that can be grown atomically smooth.   2. High density (BaBiO3) and low density films (MgO) with z lattice spacings 0.42 and 0.43 respectively.   3. Bozovic will be a resource for the growth effort. Our proposal is to grow MLL from single xtal oxide thin films that can be grown atomically flat. I Bozovic is a world expert n such films.

Single Crystal MLL Issues Use of single crystal lattice would raise some issues. We have carried out some initial simulations to gain some insight into these: Discrete Lattice OK; like interface roughness Density contrast OK; larger contrast is better Interfacial roughness OK; reduced intensity into spot Growth rate errors Very important Apodization OK. (N.Bozovic NSLS-II)

Timelines for MLL FY07 Explore materials for single crystal MLL approach. Explore techniques to deposit multi-layers in wedged MLL geometry. Carry out coupled wave (vector) calculations of MLL to determine sensitivity to errors. Develop positioning techniques to mount and manipulate up to 4 MLL sections FY08 Develop techniques to deposit multi-layers for wedge MLL geometry Develop metrology capable of determining zone width and placement to ~2nm resolution. Develop techniques to slice an MLL section from graded multilayer FY09 Design a prototype MLL device (optics and mechanics) with 2nm limit FY10 Construct 2nm prototype device

Proposed Strategies for MLL Initially grow flat MLL, but in parallel will design chambers for wedged growth (I. Bozovic). (Possibly adapt the KB mirror fabrication techniques from APS.) Develop Metrology for layers

7.Kinoform Instead of solid refractive optic: Use a kinoform: One can view the kinoform equivalently as A blazed zone plate An array of coherently interfering micro-lenses.

Summary of the Kinoform Case Normalized focal length Intensity c) b) a) 2F F Kinoform transmission function is almost uniform as a function of lens aperture, and so => NA of the lens is not limited by absorption. 2. Kinoform does not have to be fabricated with structures as small as the resolution of the lens. 3. A compound lens gets around the small , which limits the focusing power of a single lens, and would otherwise limit the spatial resolution to 0.61/c. N.A.= Mc M lenses

Multiple kinoform lenses can go beyond critical angle limit Single refractive lens has deflection angle D= C, => resolution limit is /C We have fabricated a 4 lens compound lens with f =25mm, total aperture =0.3mm, D= 2C Imperfect lens: Actual result for lens array is 1.1 C . Need to improve fabrication.

Kinoform issues While Kinoform is further behind at present (600nm at NSLS), no new technology is needed. Investments here have been small to date. We need to improve etch quality Etch depth is 100μm; need to improve. Reduce roughness of etched sidewall Test lens designs for n>4. (n=24 needed for Si)

Timelines for Kinoform T=6m Develop Deep Vertical Si etching T=12m Optimized E-beam Si process to allow many lens writes Develop Etches for InSb, C, Si. Test Compound Si lens sub 40nm T=18m Develop E-beam for alternate materials T=24m Test Alternate materials lens in xray Test sub 20nm lens in xray T=36m Test sub 10nm lens

Other Components of Proposed R&D plan MLL  Kinoform  Measurements and metrology Simulations/theory/Computational Benefits all optics

Testing and Metrics Need to develop testing facilities. Diagnostics at NSLS Diagnostics at APS Numerical support, and investigation of new methods (Souvorov)

Computational Research Going beyond Fresnel Kirchhoff thin lens approximation Develop a simulation code that one can drop in density matrices representing real optics. Help in diagnostics simulations.

Can crossed optics get down to 1nm? Using the Fresnel Kirchhoff Integral showing the replacement of the spherical wave by a pair of orthogonal parabolic terms. Limit of the approximation: For 100 micron aperture, focal length 1cm, we can use crossed lenses down to at least 10nm, but how far can we go?

Can crossed optics get down to 1nm? Answer: Yes, but with increased background Small NA, crossed lens indistinguishable from two independent lenses Large NA ~0.1, crossed lens ok but: Central spot still sharp but weaker, More intensity outside the spot => Lower signal to noise ratio

Summary 1nm optics are possible, but will require a targeted R&D effort NSLS-II is proposing to pursue 2 of the possible paths. MLL: Develop wedged structures Kinoforms: Improve etch quality and increase lens count