1. Jake McCoy
University of Iowa
SPIEAstronomicalTelescopes + Instrumentation
June 30th, 2016
Edinburgh, UK

2.  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:
▪ JamesTutt 9905-253
▪ Casey DeRoo 9905-256

3.  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

4.  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

5.  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

6.  Search for imprinting procedure compatible with soft
resist structures made viaTASTE
 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)

7. 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

8. 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
resist
Si3N4
Si substrate
= = = = =
1) Spin resist onto nitride coated Si wafer
7) Master grating template in etched Si
EBL, RIE,
KOH etch,
etc.
Imprint
solution

9.  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

10. This work was supported by a NASA SpaceTechnology Research
Fellowship.