Design of semiconductor detectors for digital mammography

1. IWORID 2002 Amsterdam, 8-12 September Design of semiconductor detectors for digital mammography S.R.Amendoliaa, M.Boscardind, M.G.Bisognib, G.F. Dalla Bettad, P.Delogub, M.E.Fantaccib, M.Novellib, M. Quattrocchic, V.Rossob, A.Stefaninib, S.Zuccab a: Istituto di Matematica e Fisica dell’Universita’ di Sassari e INFN, Sezione di Pisa, Italy b: Dipartimento di Fisica, Universita’ di Pisa and Sezione INFN Pisa, Italy c: Dipartimento di Fisica, Universita’ di Napoli and Sezione INFN Pisa, Italy d: ITC-irst, Divisione Microsistemi, 38050 Povo (TN), Italy • marzia.novelli@pi.infn.it

2. Outline Study on Gallium Arsenide and Silicon detectors to realize a digital imaging mammographic system • Characterization of GaAs detectors to find a material with good c.c.e. and detection efficiency properties • Simulation of Silicon detectors to choose a structure to avoid the risk of electrical discharge between detector and electronic chip

3. Introduction Digital imaging mammographic system based on a semiconductor pixel detector Detector • Detector • semiconductor : • Si thickness 300 mm • GaAs thickness 200 mm • pixel 170 x 170 mm2 • Schottky 150x150 mm2 • chanel 64 x 64 • area 1.2 cm2 25 mm Electronic chip Images of the RMI 156 mammographic phantom film 12 bit 100mm Si 300 mm (scansion 6x6) Source=mammographic tube (Mo target) Dose = 4 mGy 6 cm

4. GaAs pixel detectors It’ s important to define the optimal reverse voltage for the detector because : Operating bias (Vbias ) influencesimage quality Increasing Vbiasleakage current and noiseincrease breakdown limit in order to have response uniformity, a good c.c.e and detection efficiency the detector must be overdepleted Vbias = 300 Volt • Conditions to have a good detector: • Breakdown voltage > 500 Volt • Current Density < 50 nA/mm2 (@ 300 Volt) • Charge Collection Efficiency > 75 % (@ 300 Volt) We have observed that it’s important to have a well defined concentration of Carbon in the bulk of GaAs so we have studied the behaviour of single diodes, some with a dopant concentration known and some unknown

5. GaAs detectors Single diode : diameter 2 mm and thickness200 mm • Schottky Contact: multilayer Ti/Pt/Au • Ohmic Contact: non alloyed • (AMS in Rome, Italy) Contacts : Dopants concentrations MCP (LEC) [C] 1015 cm-3 AMXC 2065 (VGF) [C] 1015 cm-3 FRE (LEC) [C] < 1015 cm-3 ACR 68 (LEC) [C] = 1.3 1015 cm-3 ACR 13 (LEC) [C] = 5 1015 cm-3 ACR 79 (LEC) [C] = 1 1016cm-3 SMT1CR (LEC) [C] < 1015 cm-3 ,[Cr] = 1017 cm-3

6. Electrical Characterization Current densities as a function of reverse voltage have shown the behavior typical of a diode under reverse voltage. Current density decreases when Carbon concentration become higher.

7. Spectroscopic Characterization ACR 013 [C] = 5 1015 cm-3 ACR 068 [C] = 1.3 1015 cm-3 300 Volt 500 Volt ACR 079 [C] = 1 1016 cm-3 Irradiation with 231Am Source (59,54 KeV) Comparison of spectra acquired using diodes with the different Carbon concentrations 400 Volt

8. Spectroscopic Characterization AMXC 2065 [C] 1 1015 cm-3 MCP [C] 1 1015 cm-3 300 Volt 200 Volt SMT 1 CR [C] 1 1015 cm-3 [Cr] = 1017 cm-3 FRE [C] < 1 1015 cm-3 200 Volt 300 Volt

9. Spectroscopic Characterization ACR 068 [C] = 1.3 1015 cm-3 500 Volt ACR 068 is the material which shows the best compromise between electrical and spectroscopic characteristic

10. Spectroscopic Characterization Systematic study on the material ACR 068 with the characterization of 10 single diodes [C] = 1.3 1015 cm-3

11. Simulation of Silicon detectors: ISE-TCAD Creation of a geometry Creation of dopant distribution Creation of elementary domain Iterative Calculus Physical Equation y electrostatic potential e electric permittivity q elementary charge n, p electron and hole densities ND, NA donors and acceptors Jn, Jp current densities R recombination rate m mobility F quasi-Fermi potential Poisson Equation Continuity Equation

12. Electric Potential at the cutting edge Electric Potential p+ junction NB~ 2 1019 cm-3 n+ contact NP~ 1 1020 cm-3 Bulk NP~ 5 1011 cm-3 Scribe Line NP~ 1 1017 cm-3 Oxide fixed charge ~ 4 1011 e- cm-2 p+ guardring p+ oxide scribe line 150 20 300 300 300 p+ Junction at ground Guardring at ground n+ Contact at 100 Volt 1070 n+

13. New structure: Electric Potential p+ junction NB~ 2 1019 cm-3 n+ contact NP~ 1 1020 cm-3 Bulk NP~ 5 1011 cm-3 scribe Line NP~ 1 1017 cm-3 Oxide fixed charge ~ 4 1011 e- cm-2 Multiguardring (450 mm) 3 x (oxide 15 mm + p+ 15 mm) 3 x (oxide 20 mm + p+ 15 mm)3 x (oxide 25 mm + p+ 15 mm)3 x (oxide 30 mm + p+ 15 mm) p+ guardring p+ multiguardring oxide scribe line 150 450 200 20 300 150 n+

14. Electric Potential and Electric Field width

15. Wafer layout at ITC-IRST 12 wafers 4 inches in diameter Thickness 300 mm and 500 mm Type of production : p+/n 18 detectors for Medipix 1 6 detectors for Medipix 2 (a matrix of 256x256 square pixels of 55 mm in side) Several test structures These detectors are ready and they are in phase of bump-bonding to the electronic chip at VTT

16. Photos of some details multiguardrings Pixel 150 mm x 150 mm Medipix 1 guardring multiguardrings Pixel 45 mm x 45 mm guardring Medipix 2 Snake pads multiguardrings

17. Test structure in the wafer To check the properties of the wafers we have tested the electrical characteristic of some single diodes (3 diodes for each wafer always in the same position) Central diode: Diameter = 2 mm Gap = 8 mm First guardring thickness = 300 mm Gap = 30 mm Second guardring thickness = 150 mm

18. Electrical Characterization Thickness 300 mm Current in the central diode with guardring at ground T= 21.5 °C U=65 % Wafer 15 Wafer 9 Wafer 7 Wafer 5

19. Electrical Characterization Thickness 500 mm T= 21.5 °C U=65 % Wafer 4 Current in the central diode with guardring at ground Wafer 5

20. Detector of area larger than the electronic chip Another approach for the solution of the HV at the cutting edge Wafer 4 inches in diameter, type of production p+/n, thickness 300 – 500 mm flip-chip bonding I/O pads flip-chip bonding PCC - electronic chip semiconductor detector multiguardring around the matrix and the lines for the fanout guardring around the active matrix of pixels Series of p+ implantation to isolate the active region from the lines These detectors will be ready at the end of the year

21. 14200 Chip dimension Active matrix (64 x 64) Main guard-ring 18600 Termination structure (12 rings) Resistors to isolate the active region from the I/O pads Bus line for the fanout

22. Conclusions • Research of the optimal concentration of dopants in GaAs detectors • Realization of the optimal geometry of Silicon detector with simulation and characterization of some test structures to check the properties of the new wafers • Status in the production of the detectors • The GaAs detectors with the optimal Carbon concentration for imaging • applications are ready and they are in phase of bump bonding by AMS. • The Si dectors are in phase of bump bonding by VTT. • Future works: • When the assemblies will be ready we want to check their performance and to compare the results with those obtained in the past years.