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.