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Transcript of McCoy SPIE 2016 Edinburgh
New lithographic techniques for X-ray spectroscopy
Jake McCoyUniversity of Iowa
SPIE Astronomical Telescopes + InstrumentationJune 30th, 2016Edinburgh, UK
Fabricating off-plane gratings Astronomical soft X-ray spectrographs
Resolving power ~ few thousand Effective area ~several hundred cm2
I. High groove density 5,000 grooves/mm Sub-200 nm scale structures
II. Radial groove profile Off-plane variable line space
III. Blazed sawtooth facets Sub-nm RMS roughness
IV. Suitable for alignment Large area format Replication procedure
References Cash (1983, 1991) McEntaffer et al. (2013) Tonight’s poster session:
▪ James Tutt 9905-253▪ Casey DeRoo 9905-256
Current recipe: anisotropic etching Electron-beam
lithography High groove density Approximately radially
ruled grooves Pattern on 6” wafers
Etching processes Blazed groove facets
in crystalline Si UV-nanoimprint
lithography Replicas on flat fused
silica References
Chang et al. 2003 Voronov et al. 2011 McEntaffer et al. 2013
resist
Si3N4
Si substrate
1) Spin resist onto nitride coated Si wafer
2) Define grating profile in resist
3) Descum, transfer pattern into nitride with RIE
Native SiO2
4) Piranha etch to remove remaining resist
5) Remove native SiO2 with HF dip <111>
6) KOH etch to achieve blaze
7) Nitride mask removal with HF
See Fabrication and efficiency testing of a new generation of off-plane gratings, Casey DeRoo 9905-256
UV-NIL resist
Fused silica
substrate
8) UV-NIL to replicate master
Deposited Cr/Au
9) Coat for reflectivity
Demonstrated 35% efficiency in working orders
Grayscale electron-beam lithography (EBL)
EBL is typically used to pattern bi-level features in resist Dose D0 maintains spin coat thickness Dose Dclear etches down to substrate Create laminar grating by alternating D0 and Dclear
Resist contrast maps dose to resist thickness Access doses D0<D<Dclear
Carefully controlling D allows for 3D nm-scale patterning in resist
Electron backscatter through substrate doses nearby regions (up to ~microns) Proximity effect correction (PEC) algorithms with
GenISys BEAMER software package
Fig: Dr. Gerald Lopez, GenISys
Schleunitz & Schift 2010; PMMA 950k, IPA:MIBK 1:1, 60s
Thermally Activated Selective Topography Equilibration (TASTE)
Grayscale EBL to pattern approximately blazed grooves (staircase structures) Dose reduces molecular weight of remaining resist in
exposed steps▪ Glass-liquid transition temperature▪ Viscous flow regime
Thermally treat sample for exposed steps to flow selectively Surface tension dominates over gravity on small scales
▪ Triangular sawtooth structures ▪ Up to 45o incline achieved
Achieved on micron scales▪ PMMA (Schleunitz et al. 2014)▪ ZEP 520A (Kirchner et al. 2016)
Roughness of ~few nm RMS reported Improvement needed
Kirchner et al. 2016; ZEP 520A, scale bar = 200 nm
Schleunitz & Schift 2010
Substrate Conformal Imprint Lithography (SCIL)
Search for imprinting procedure compatible with soft resist structures made via TASTE UV-NIL requires a hard stamp (e.g. etched Si)
Soft lithography Form flexible rubber stamp from master Fabricate replicas in silica based material Compatible with soft resist structures
Sequential imprinting Wafer-scale conformal contact reduces defects due to trapped
air Local conformation around particle contaminants reduces
damage during imprinting process Alternative to UV-NIL
Available commercially
Reduced particulate damage; Verschuuren, Ph.D. thesis, 2010
SUSS MicroTec (suss.com/scil)
Proposed recipe: TASTE + SCIL
4) Sequentially separate to leave replica in silica based material
1) Spin resist onto bare wafer
2) Pattern approximately blazed grooves via grayscale EBL
3) Thermally treat sample for exposed resist to flow into smooth surfaces
1) Form flexible rubber stamp from inverse of master
2) Sequentially imprint stamp into liquid resist on fused silica
3) Cure resist via sol-gel process
resist
substrate
Fused silica
substrate
Imprint solution
Modified PDMS
Reflective layer
Rubber stamp
SCILTASTE
Flexible glass
Anisotropic etching + SCIL
4) Sequentially separate to leave replica in silica based material
1) Form flexible rubber stamp from inverse of master
2) Sequentially imprint stamp into liquid resist on fused silica
3) Cure resist via sol-gel process
Fused silica
substrate
Modified PDMS
Reflective layer
Flexible glass
Rubber stamp
SCILEBL+ anisotropic etching
resistSi3N4
Si substrate1) Spin resist onto nitride coated
Si wafer
7) Master grating template in etched Si
EBL, RIE, KOH etch, etc.
Imprint solution
Remarks TASTE fabrication approach
Offers benefits…▪ Flexible groove shape for moderate blaze angles ▪ Fabrication recipe with fewer steps▪ Freedom from cubic structure of Si
… but comes with challenges▪ Fine PEC to achieve smaller scales▪ Improving facet roughness ▪ Producing large area gratings
UV-NIL Limited area conformal contact Defects due to trapped air, particulates
SCIL Defects are greatly reduced
▪ Wafer scale sequential imprinting Replica features produced in silica
▪ Expected to be suitable for flight
Vistec EBPG5200 EBL tool at PSU
Thank youThis work was supported by a NASA Space Technology Research Fellowship.