Surface Characterisation of Germanium detectors

Transcription

1 Surface Characterisation of Germanium detectors T.Engert 1,2, I.Kojouharov 1, J.Gerl 1, A.Boston 2, P.Nolan 2 1 GSI, Darmstadt, Germany 2 University of Liverpool

2 Outline Background > Detector technology > Manufacturing methods Defect model Mechanical defects Ge- Crystals & Detectors > Measurement-Principle Results: Profiler & AFM > Test in a cryostat Results New treatment method > Ultrasonic Results Outlook

3 Detector technology Mechanical Mechanical processing processing: 1) Rough treatment Wet chemistry - With grinding Phase 1 Diffusion 2) Final cutting - With lapping Open surface Lapping Wet chemistry Passivation Implanting N Y O.K. Non-segmented, Non-encapsulated Detectors N N Segmenting Test 2 O.K. Y Encapsulation Test 3 O.K. Y Segmented, Non-encapsulated Detectors Phase 2 Phase 3 Critical Process Important Process Regular Process Transition Fall Back Process Transition Fall Back Process Bypass Process Termination Annealing Test 1 To the cryostat

4 Manufacturing methods Grinding process: Treatment with bonded grains Lapping process: Treatment with loose grains Lapp fluid Part grain disruption Microscopic warp area Microscopic cracks Grain Lapp disc disruption Lapp grain Surface Lapp disc Feed 1. Feed (Rotation) from abrasive 2. Abrasive grain 3. Bond 4. Material 5. Pores 6. Cut (Chip) Lapp grain Grain surface Work particles

5 Defect model 1 1. Poly crystal zone: Z Strong deeply fissured surface Particles (crystal or tool particles) Micro cracks 2. Crack zone: Many Micro cracks 3. Changing zone: Less Micro cracks Hs depth of roughness (Rt) lm 2 3 Dependent of the defect depth (Hs) of the crystal- and treatment parameters: Defect depth (Hs) grain diameter (d1) x depth of roughness (Rt) x Crack zone (Hri) > Ge; GaAs with treatment material corundum: Rt 0,2 x d1 (µm) Hri max 1,0 x d1 (µm) Hri max 5,0 x Rt (µm) 4. Elastic stress zone: In this zone are no structure defects from the mechanical treatment 4 > Ge; GaAs with treatment material Silicium carbide Rt 0,3 x d1 (µm) Hri max 1,0 x d1 (µm) Hri max 3,3 x Rt (µm)

6 Mechanical defects Mechanical defects define the voltagecurrent characteristics Current [A] Scratches Voltage [V] Edge chips or grains breaking out Depletion voltage Operational voltage Haze Plateau voltage γ-ray energy resolution Orange peel Combustion Dimples

7 Ge-Crystals & Detectors Detector Type Top area (intrinsic surface) B1 n+contact 1 x lapped, 4 x etched B2 p+contact 1 x lapped, 2 x etched B5 p+contact "old" etched area B6 p+contact 1 x lapped, 3 x etched B7 p+contact "old" etched area B11 - only lapped B12 - only grinded Running Detector Machined Germanium

8 Measurement principles A diamond needle scan across the intrinsic surface and gives the results with help of induction voltage. Photo detector Laser Diamond needle Profiler principle Ge Atomic Force Microscope (AFM) principle A diamond needle records across the intrinsic surface and gives the results with help of a Laser and a Photo detector.

9 Results: Profiler & AFM Detector B10 Detector B11 2-D Scan Mean value B10 B11 Ra [nm] 285,83 nm 660,08 nm Rq [nm] 358,88 nm 820,37 nm Rz [µm] 4,41µm m 9,82 µm Rt [µm] 3,1 µm m 8,61 µm

10 Test in a cryostat Detectors installed in a cryostat n- contact p+ contact [l] length= 8mm [b] width= 8mm [h] height= 10mm

11 Results Leakage current n- contact Current [pa] B1 B2 B5 B6 B7 p+ contact Voltage [V] [l] length= 8mm [b] width= 8mm [h] height= 10mm

12 New treatment method: ULTRASONIC and MILLING Reduced process forces > reduction of micro-cracks Excellent surface qualities

13 Result Ge-surface treated with the ultrasonic-grinding machine > micro structures with a tolerances of 2 μm > degree of roughness is a factor of 4 times better than with standard grinding techniques > and a factor of 2 times better compared to standard lapping processes.

14 Result Germanium surface machined with a different feed parameter 2mm Germanium surface machined with the best mechanical parameters Ø2mm This kind of sophisticated Ø3,5mm mechanical treatment allows: Difficult geometrical shapes!!!!! Excellent surface quality!!!!! Safe treatment of the crystals!!!!! Better cost-efficiency ratio!!!!!

15 Results *Detectors: On the basis of these mechanical and characteristic results novel types of planar Ge detectors are planned Dimensions are 32,5x32,5x12 mm 3 with rounded corners and a special guard ring *With oversize dimensions

16 Outlook > Further characterisation with the detectors With FESEM, AFM, STM and Profiler > Build the Planar-Detector To develop the other steps of detector technology

17 Thanks for your Attention!!!

18 New treatment method: ULTRASONIC and MILLING SOFT HARD ADVANCED Graphite Copper Stainless steelhardened steel Glass Carbide Ceramics Corundum Graphite Aluminium Tool steel Hardened steel Zerodur Carbide Ceramics SiC

19 Manufacturing methods HSC: Short edge contact time Lower in feed at higher feed Lower heat development Milling: Long edge contact time High in feed at lower feed Increased heat development Shearing angle (Milling) Shearing angle (HSC) Face side of the tool Cutting tool Tool flank Cut surface of the work piece Face side of the tool Cutting tool Tool flank Cut surface of the work piece -The v c is higher than the crack growth speed of the material - The material separation happens directly at the cutting edge - Sharper cutting edges are reducing the process forces - The surface roughness of the work piece directly depends on the quality of the cutting edge