# Presentation: 480525

## Fabrication of crystals for channeling at INFN

- Vincenzo Guidi

*Fermilab, December 6-7, 2007*

*Sensors and Semiconductors lab University of Ferrara and INFN - Italy*

## Outlook

- Sample preparation
- Treatment of surfaces
- Surface characterization
- Participation to H8RD22 experiment(CERN, IHEP, INFN, JINR, PNPI)
- Interest in collimation experiment at FNAL

## Dicing of crystals I

- Si samples for channeling are achieved from wafers or ingots.
- Proper sizing of the crystal to the wanted application involve  dicing and other operations that partly damage the surface of the crystal

## Dicing of crystals I

- Mosaicity is being generated at surfacewhich is often of the order of the critical angle of impinging particles

- Si samples for channeling are achieved from wafers or ingots.
- Proper sizing of the crystal to the wanted application involve  dicing and other operations that partly damage the surface of the crystal

## Dicing of crystals II 

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

- Dicing machine

- Diamond blade

- Diamond grain: size = 4-6 m
- Density = 62%
- Dicing speed: 0.5 mm / min

## Isotropic chemical etching

- Planar etching erodes the surface of a crystal layer by layer.

- Planar etching erodes the surface of a Si crystal layer by layer HF, HNO3, CH3COOH (2:15:5)

-   HNO3 oxides Si to SiO2
-   HF erodes SiO2
-   CH3COOH as a solvent

## RBS - channeling mode

- Rutherford backscattering under channeling mode is a powerful method to investigate the structure of a surface

- Channeled particles penetrate deeper into the crystal and are more difficult to emerge out of the surface, resulting in a decrease of the current

## RBS spectra

- > 12m

- Chemical etching I

- APL **87** (2005) 094102

## AFM analysis

## Roughness analysis

- As cut

- CE1

- We observed roughness increase with chemical etching

## Etching vs. polishing 

- Mechanically
- polished surface

- Chemically
- etched surface

- Measurements taken in an external beam line at IHEP with 70 GeV protons

- RSI **73** (2002) 3170

## Experiment H8RD22  

- External line H8 of the SPS
-   400 GeV/c protons
-   < 8 rad divergence

- Multistrip detectors
- (10/30 m spatial resolution)
-   AGILE type
-   AMS type

- 1-rad-accuracy goniometer

- Spokesman: Dr. Walter Scandale

## Volume reflection

*U*o

*d*

- Unchanneled particle

- Channeled particle

- Channeled particle

- Volume-captured
- particle

- Volume-reflected particle

- Volume reflection was predicted
- by Taratin and Vorobiov
- In 1988

## Observation of VR 

- Primary beam
- Channeling
- Dechanneling
- Volume reflection
- Volume capture
- Primary beam

-   Deflection occurred at about 13.5 rad
-   Fraction of diverted particles larger that 97%
-   Large acceptance

- PRL **9****8** (2007) 154801

- Measurements with sample ST4 (now here at FNAL)

## Demand for lower roughness

- An application of crystals would be the collimation of beam halo in next generation of hadron machines (e.g. the LHC)

- Particles in the halo drift outwards at the rate of ~2 nm per turn. Since the tune is not integer, the particles will hit the crystal every ~10-20 turns and thereby the first impact parameter of the particles onto the crystal will be in the range of ~100 nm (curtesy of V. Previtali and R. Assmann)

- It demands a crystal with a roughness lower than 100 nm on the lateral faces of the crystal

## Rivisiting chemical etching

- Formation of a morphologically non homogeneous surface may be prevented by an unusually large concentration of HNO3

- Oxidation of Si by HNO3 preferentially begins at the defects forming nitride oxide, which locally catalyzes further the oxidation reaction in the neighboring area and finally results in formation of a crater.

- An excess of HNO3 strengthens the oxidation, which involves the whole surface nearly at uniform rate. Properly short timing for reaction leads to defect removal leaving a smooth surface

## AFM analysis

- As cut

- New CE

- Old CE

|  | New CE | Old CE |
| --- | --- | --- |
| Standard
roughness | (23±5) nm | (135±20) nm |

- Significant decrease in roughness was observed with the new CE

## RBS spectra

- Chemical etching II

- Low signal of backscattered particles means good crystalline quality

- 4He+

- p

- APL **9****1** (2007) 061908

## Structure of the surface

- The surface ***min* is defined as the ratio of the RBS yield under channelling alignment and random condition extrapolated at the surface channel. The higher the degree of crystalline order in the lattice the lower the surface ***min* due to the reduction of dechanneling from the defects in the crystal.

- Experimental ***min* for a Si [110] sample with probe particles of 2 MeV. An ideally perfect surface yields ***min* = 2%

| probe | as cut | CE1 | CE2 |
| --- | --- | --- | --- |
| 4He+ | 16% | 4.1% | 2.2% |
| p | 18% | 4% | 2% |

## Anisotropic etching

- Anisotropic etching is a feasible way to realize highly flat damage free surfaces of crystals entirely by wet chemical methods

- nanometric roughness was achieved

| (100) | (110) | (111) |
| --- | --- | --- |
| 7.1 μm/h | 10.7 μm/h | Negligible |

- Etch rate on different silicon planes
-  for KOH 20% at 40 °C

## Photolythography

- 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

## VR vs. bending radius

- Deflection angle vs. curvature radius

- Inefficiency vs. curvature radius

## Multistrip crystal

**2mm**

**2mm**

**70mm**

**Cross section**

**Thickness: 0.5mm**

**Silicon strips**

**Frame    **

## Multiple VR with multistrip

- We were able to align up to 4 strips
- With 3 strips, deflection angle is 40.5 μrad
- Reflection efficiency is 93%

## Deformation of the frame 

- Anticlastic bending in a strip

- Ideal alignment condition

- Because of the frame, global deformation of the crystal occurs

## Ultra-thin frames

- Realization of ultra-thin frames
- Verification of independence of rotation of each strip

## Interest in collimation project at FNAL 

- Provision of crystals (yes)
- Characterization of crystals (yes)
- Refurbishing of previously used crystals, e.g. O-shaped crystal (yes)
- Participation in experimental runs (yes)
- Detectors (to be determined)
- Other issues (e.g. goniometer requested)