Design of semiconductor detectors for digital mammography IWORID 2002 Amsterdam, 8-12 September + S.R.Amendolia a, M.Boscardin d, M.G.Bisogni b, G.F. Dalla Betta d, P.Delogu b, M.E.Fantacci b, M.Novelli b, M. Quattrocchi c, V.Rosso b, A.Stefanini b, S.Zucca b 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, Povo (TN), Italy

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

Detector semiconductor : Si thickness 300  m GaAs thickness 200  m pixel 170 x 170  m 2 Schottky 150x150  m 2 chanel 64 x 64 area 1.2 cm 2 Introduction Images of the RMI 156 mammographic phantom Digital imaging mammographic system based on a semiconductor pixel detector 25  m Detector Electronic chip film 12 bit 100  m Si 300  m (scansion 6x6) Source=mammographic tube (Mo target) Dose = 4 mGy 6 cm

It’ s important to define the optimal reverse voltage for the detector because : Operating bias (V bias ) influences image quality Increasing V bias leakage current and noise increase breakdown limit in order to have response uniformity, a good c.c.e and detection efficiency the detector must be overdepleted V bias = 300 Volt Conditions to have a good detector:  Breakdown voltage > 500 Volt  Current Density < 50 nA/mm Volt)  Charge Collection Efficiency > 75 % 300 Volt) GaAs pixel detectors 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

GaAs detectors Single diode : diameter 2 mm and thickness 200  m MCP (LEC) [C] cm -3 AMXC 2065 (VGF) [C] cm -3 FRE (LEC) [C] < cm -3 ACR 68 (LEC) [C] = cm -3 ACR 13 (LEC) [C] = cm -3 ACR 79 (LEC) [C] = cm -3 SMT1CR (LEC) [C] < cm -3, [Cr] = cm -3 Schottky Contact: multilayer Ti/Pt/Au Ohmic Contact: non alloyed (AMS in Rome, Italy) Contacts : Dopants concentrations

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

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

MCP [C] cm Volt AMXC 2065 [C] cm Volt FRE [C] < cm -3 SMT 1 CR [C] cm -3 [Cr] = cm Volt Spectroscopic Characterization

ACR 068 [C] = cm Volt Spectroscopic Characterization ACR 068 is the material which shows the best compromise between electrical and spectroscopic characteristic

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

Creation of a geometry Iterative Calculus Creation of dopant distribution Creation of elementary domain Physical Equation  electrostatic potential  electric permittivity q elementary charge n, p electron and hole densities N D, N A donors and acceptors J n, J p current densities R recombination rate  mobility  quasi-Fermi potential Poisson Equation Continuity Equation Simulation of Silicon detectors: ISE-TCAD

p+guardring p n oxide scribe line 300 p+ Junction at ground Guardring at ground n+ Contact at 100 Volt Electric Potential Electric Potential at the cutting edge p+ junction N B ~ cm -3 n+ contact N P ~ cm -3 Bulk N P ~ cm-3 Scribe Line N P ~ cm -3 Oxide fixed charge ~ e - cm -2

p guardring p+multiguardring scribe line oxide 150 Multiguardring (450  m) 3 x (oxide 15  m + p + 15  m) 3 x (oxide 20  m + p + 15  m) 3 x (oxide 25  m + p + 15  m) 3 x (oxide 30  m + p + 15  m) New structure: Electric Potential p+ junction N B ~ cm -3 n+ contact N P ~ cm -3 Bulk N P ~ cm-3 scribe Line N P ~ cm -3 Oxide fixed charge ~ e - cm -2 n+

width Electric Potential and Electric Field

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 12 wafers 4 inches in diameter Thickness 300  m and 500  m Type of production : p+/n Wafer layout at ITC-IRST

Photos of some details Medipix 1 Medipix 2 Pixel 150  m x 150  m Pixel 45  m x 45  m guardring multiguardrings Snake pads

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  m First guardring thickness = 300  m Gap = 30  m Second guardring thickness = 150  m

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

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

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

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

o Research of the optimal concentration of dopants in GaAs detectors o 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 o The GaAs detectors with the optimal Carbon concentration for imaging applications are ready and they are in phase of bump bonding by AMS. o The Si dectors are in phase of bump bonding by VTT. Future works: oWhen the assemblies will be ready we want to check their performance and to compare the results with those obtained in the past years.Conclusions