1. 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

2. 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

3. 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

4. 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

5. 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

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

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

8. 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)

9. 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

10. 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)
PRL 98 (2007)
Deflection occurred at about 13.5 mrad
Fraction of diverted particles larger that 97%
Large acceptance

11. 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

12. 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

13. 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

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

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

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

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

18. 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

19. 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

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

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

22. 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

23. 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.

24. 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)

25. 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

26. 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

27. 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

28. 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.

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