Fabrication of silicon crystals for COHERENT experiment RREPS-11 Fabrication of silicon crystals for COHERENT experiment Andrea Mazzolari University of Ferrara and INFN - Italy Egham, Sept. 15, 2011

Outlook Crystals fab Fabrication and characterization of crystals Bending by internal forces Ultra thin bent crystals Ultra thin unbent crystals We work at the Sensors and Semiconductors Laboratory and have been collaborating on channeling issues with many labs such as: IHEP, PNPI, JINR, CERN, FNAL, LNL, LNF, ESRF

Dicing of crystals Crystals were diced from a commercially available wafer using a fine-grit blade to minimize the mechanical damage during the cut. Dicing machine

Dicing of crystals Crystals were diced from a commercially available wafer using a fine-grit blade to minimize the mechanical damage during the cut. Dicing machine

Dicing of crystals Crystals were diced from a commercially available wafer using a fine-grit blade to minimize the mechanical damage during the cut.  c Mosaicity is generated at surface, which is often of the order of the critical angle of impinging particles Dicing machine

RBS spectra 4He+ Low signal of backscattered particles means good crystalline quality p Chemical etching APL 91 (2007) 061908

Crystal bending 2 Beam 0,5 A primary curvature is imparted by mechanical external forces, which result in a secondary (anticlastic) curvature.

Experiment COHERENT Multistrip detectors External line H8 of the SPS beam 1-mrad-accuracy goniometer Multistrip detectors 30 µm spatial resolution 2.5 µrad angular resolution External line H8 of the SPS 400 GeV/c protons < 8 mrad divergence RSI 79 (2008) 023303

Volume reflection Unchanneled particle Uo d Channeled particle λ Volume-reflected particle θref Volume-captured particle Volume reflection was predicted by Taratin and Vorobiov In 1988 Channeled particle

Observation of VR 6 4 5 PRL 97 (2006) 144801 PRL 98 (2007) 154801 2 3 Primary beam Channeling Dechanneling Volume reflection Volume capture PRL 97 (2006) 144801 PRL 98 (2007) 154801 Deflection occurred at about 13.5 mrad Fraction of diverted particles larger that 97% Large acceptance

VR with multistrips θref A multistrip is a technique to fabricate a crystal for multiple VR θref θref At 400 GeV it holds: θref θacc

Etch rate on different silicon planes Anisotropic etching I Anistropic etching is a feasible way to realize sub-surface damage free crystals entirely by wet chemical methods Etch rate on different silicon planes for KOH 20% at 40 °C (100) (110) (111) 7.1 m/h 10.7 m/h Negligible

Photolithography a) Starting material: (110) silicon wafer b) LPCVD deposition of silicon nitride thin layer c) Silicon nitride patterning d) Etching of Si in KOH solution, silicon nitride acts as masking layer e) Silicon strips release f) Removal of silicon nitride

Fabrication of multistrips Fabrication of either a multistrip or a batch of strips is possible through wet chemical methods

Characterization Lateral surface (AFM) High-quality surfaces achieved via ACE Entry surface (HRTEM) Sub-nm roughness was achieved Sub-nm roughness was achieved

Observation of MVR Deflection angle ~120 rad Efficiency: 93% Crystal delivered to FNAL for testing by T-980 experiment

Observation of axial channeling Deflection occurred at about 50 µrad Planar channeling into skew planes PRL 101 (2008) 234801

Recent results Observation of negative particles deflection through efficient planar channeling Observation of negative particles deflection through efficient axial channeling Observation of MVR in a single crystal Studies on channeling efficiency as function of bending radius highlighted deflection efficiency > 83% Emission of radiation from bent crystals (channeling, VR, MVR) Analysis is under way

Bending by internal forces I Counts (arbitrary units) XRD Deflection by 100 rad of a 200 m thick Si plate Curtesy of D. De Salvador and A. Carnera

Indentations An indentation “pulls up” the atomic planes resulting in a net bending which goes deep into the crystal PRL 90 (2003) 034801

Indentations Multi-indentation is a method to achieve a deflector or an undulator Curved crystalline planes in the bulk of silicon e+ photon photons

Bending by internal forces II Deposition of tensile layers on a substrate is a method to bend a sample Silicon sample Deposition at high temperature Cooling at room temperature Internal stress is generated according to Stoney’s equation NIM B 234 (2005) 40

Patterning with Si3N4 Deposition of 100 nm thin silicon nitride and patterning to lines 500 μm wide and spacing 1000 μm Perfect agreement between deformed and predicted crystal shape.

Crystalline undulator Layout of an undulator Si3N4 thickness Amplitude deformation (nm) Osculating circle radius (m) 100nm 0.7 10.3 200nm 1.5 5.2 400nm 3 2.6 L=0.5 mm Strain is more homogeneous than with the indentations and does not deteriorate the crystal APL 90 (2007) 114107

Crystalline undulator Substrate thickness (μm) Film thickness (nm) Strip width (μm) Amplitude[nm] 100 500 20.88 300 1000 9.40 200 125 0.99 250 0.83 Proper adjustment of crystalline undulalor geometrical parameters allows to easily trim oscillation amplitute

Ultra-thin crystals Crystals with thickness less than planar oscillation wavelength Crystals for 400 GeV experiment Thickness 14 µm or 28 µm Thickness uniformity better than 1 µm Lateral size 5x5 mm2 Mirror surfaces Crystals for 2 MeV experiments Thickness 90 nm Thickness uniformity better than 2 nm Lateral size 2x2 mm2 Frame 400 µm thick Membrane

Ultra-thin bent crystals Quasi mosaic crsytal Thickness as low as 45 µm Transversal size 10 mm No anticlastic bending results in wide geometrical acceptance Bend angle 30 cm Possibility to efficienty deflect low energy particle beams Not channeled beam Channeled beam Incoming beam

Conclusions Continuous improvement in crystal fabrication Observation of new physical effects Manipulation of high-energy particle beams Technical and scientific background for fabrication of miniature crystalline undulator Possibility to realize ultra-thin bent or unbent crystals.

Acknowledgments Support by INFN COHERENT, INTAS, PRIN projects Thanks to Prof. Vincenzo Guidi and all scientists with whom I have been working