Preparation of crystals Preparation of crystals for channelingfor channeling

University of Ferrara

V. Guidi Department of Physics and INFN, Via Paradiso 12, I-44100 Ferrara, Italy

A. Vomiero INFN Laboratori Nazionali di Legnaro, Viale Università 2, I-35020 Legnaro (PD), Italy

• To remove organic and metallic impurities, the wafers are To remove organic and metallic impurities, the wafers are degreased in trichloroethylene, acetone, and isopropanol.degreased in trichloroethylene, acetone, and isopropanol.

• Wafers are cleaned in a solution of water, hydrogen peroxide, and Wafers are cleaned in a solution of water, hydrogen peroxide, and ammonium hydroxide (5:1:1) at 75 ammonium hydroxide (5:1:1) at 75 °°C for 10 min.C for 10 min.

• A short dipping in diluted hydrofluoric acid (10% in weight).A short dipping in diluted hydrofluoric acid (10% in weight).

• The wafers are washed in water ,hydrogen peroxide and The wafers are washed in water ,hydrogen peroxide and

hydrochloric acid (4:1:1) at 75 hydrochloric acid (4:1:1) at 75 °°C for 10 min.C for 10 min.

Preparation of the samples-1Preparation of the samples-1

We attempt the removal of such layer by a We attempt the removal of such layer by a wet planar wet planar etching (HF,HNOetching (HF,HNO33,CH,CH33COOH).COOH).

• Dicing of the samples by a diamond-blade saw avoiding alignment Dicing of the samples by a diamond-blade saw avoiding alignment with major crystalline axes.with major crystalline axes.

• Defects are induced by the dicing saw (a surface layer estimated to Defects are induced by the dicing saw (a surface layer estimated to be as thick as 30be as thick as 30m is rich in stratches, dislocations, line defects m is rich in stratches, dislocations, line defects and anomalies).and anomalies).

Preparation of the samples-2Preparation of the samples-2

70 GeV

Extraction efficiency for 70-GeV protons. Recent results: (*, strips, 1.8, 2.0, and 4 mm long), results of 1999-2000; (, “O-shaped” crystals 3 and 5 mm), and of 1997 (� , strip 7mm). Also shown (o) is the Monte Carlo prediction for a perfect

crystal with 0.9 mrad bending.

Results-1Results-1

Example of the bent crystal plate

Results-2Results-2

Surface treatment proved to be useful to improve the quality of Surface treatment proved to be useful to improve the quality of the extracted beamthe extracted beam

Images of the 70 GeV protons deflected through mechanically treated Images of the 70 GeV protons deflected through mechanically treated and chemically polished crystalsand chemically polished crystals

Mechanically polished Chemically polished

Study of crystal surfaceStudy of crystal surface

In order to study the optimal procedure to remove In order to study the optimal procedure to remove defects, we have prepared several Si-samples.defects, we have prepared several Si-samples.

• some samples have been chemically polished by wet some samples have been chemically polished by wet planar etching.planar etching.

AFMAFM measurementsmeasurements

AFMAFM images images of the of the surface of an as-diced Si surface of an as-diced Si crystal (crystal (leftleft up) and up) and after after 40 min chemical etching 40 min chemical etching (left down). Chemical (left down). Chemical polishing enhances the polishing enhances the standard roughness (standard roughness (RRaa) )

(right up), which tends to (right up), which tends to decrease for longer decrease for longer etching times (right etching times (right down).down).

30mAs-cut sample• A layer as thick as 30 m is rich in defects

2.0 MeV RBS-channeling particles spectra

Chemically polished• HF 49%, HNO3 70%, CH3COOH 100% (2:15:5)

• Damage stop

2.0 MeV RBS-channeling particles spectra

2.0 MeV RBS-channeling particles spectra

The enhanced yield of the mechanically cut sample (0.5M) with respect to the layer The enhanced yield of the mechanically cut sample (0.5M) with respect to the layer after 30 min of etching (0.5/30C) is attributed to the presence of a more disordered after 30 min of etching (0.5/30C) is attributed to the presence of a more disordered surface structure.surface structure.

2.0 MeV RBS-channeling proton spectra

• A lower dicing speed (0.5 mm/min) seems to create a smaller dead layer.A lower dicing speed (0.5 mm/min) seems to create a smaller dead layer.

• The etched samples exhibit enhanced channeling indication of removal of The etched samples exhibit enhanced channeling indication of removal of the amorphized layer.the amorphized layer.

• We understood the physical reasons for such a behavior through We understood the physical reasons for such a behavior through characterization of the crystal surfaces.characterization of the crystal surfaces.

• Chemical etching improves channeling efficiency and leads to better Chemical etching improves channeling efficiency and leads to better performance than conventional mechanically treated samples.performance than conventional mechanically treated samples.

• Extraction efficiency of the order of 85% was repeatedly obtained at Extraction efficiency of the order of 85% was repeatedly obtained at IHEP Protvino for an impinging intensity as high as 10IHEP Protvino for an impinging intensity as high as 101212 protons. protons.

ConclusionsConclusions