Experiments

Diamond based particle detectors

Diamond is an ideal material for particle detectors, mainly because of its radiation hardness, and is now assuming an important role for the upgrade of the innermost tracking layers at the Large Hadron Collider (LHC) at CERN. In this framework we developed two complementary, diamond based techniques. One is the laser, direct bonding of a silicon and a diamond wafer, which forms the so called silicon-on-diamond detector (SOD) [1,2]. This composite material is the first step toward the implementation of novel particle detectors, where minimum ionizing particles induce free charges in the diamond side, which are then processed by the front end electronics built up on the silicon side. The other technique is the implementation of 3D, conductive architectures in diamond, that are needed for collecting the radiation induced free charges [3,4]. The bonding between silicon and diamond is performed via picosecond 355 nm pulsed laser irradiation of the silicon-diamond interface, through the transparent diamond. The obtained material exhibits excellent mechanical strength and uniformity of the bonding, as shown by mechanical tests and analysis of the cross section based on scanning electron microscopy (figure 1). The bonding is ascribed to silicon carbide nanolayers at the interface which, along with amorphous silicon nanolayers, have been quantitatively detected and evaluated by means of Infrared and Raman spectroscopy measurements. A physical insight into the processes occurring at the diamond-silicon interface during the pulsed irradiation and cooling has been provided by a finite element numerical model. A rationale is then given for the observed SiC bond in terms of silicon and diamond melting and inter-diffusion at temperatures in excess of 4000 K and local pressures of 1-2 GPa, respectively. A crucial outcome of the model consists in predicting the effect of the different laser beam parameters on the bonding process, thereby allowing us to obtain a well tailored procedure. We then provide an excellent quality SOD for implementing highly integrated electronic devices for diverse application areas, ranging from pixel detectors to biosensors and prostheses for the human body. Furthermore, we also implement 3D-architectures in diamond detectors, which promises to achieve unreached performances in the radiation-harsh environment of future high-energy physics experiments. Particularly, we have recently reported on the collection efficiency under beta-irradiation of graphitic 3D-electrodes (figure 2), created in diamond wafers by laser pulses in the domains of nanoseconds (ns-made-sensors) and femtoseconds (fs-made-sensors). Full collection is achieved with the fs-made-sensors, while a loss of 25%–30% is found for the ns-made-sensors. The peculiar behavior of ns-made sensors has been explained by the presence of a nano-structured sp3-carbon layer around the graphitic electrodes, evidenced by micro-Raman imaging, by means of a numerical model of the charge transport near the electrodes. Also, a local stress is found around the graphitic electrodes, which amounts to a few GPa and is due to the embedded graphite having a lower density than the hosting diamond bulk around.

REFERENCES
[1] S. Lagomarsino, G. Parrini, S. Sciortino, M. Santoro, M. Citroni, M. Vannoni, A. Fossati, F. A. Gorelli, G. Molesini, and A. Scorzoni, Appl. Phys. Lett. 96, 031901 (2010).
[2] M. Citroni, S. Lagomarsino, G. Parrini, M. Santoro, S. Sciortino, M. Vannoni, G. Ferrari, A. Fossati, F. Gorelli, G. Molesini, G. Piani, and A. Scorzoni, Diamond and Related Materials 19, 950 (2010).
[3] S. Lagomarsino, M. Bellini, C. Corsi, F. Gorelli, G. Parrini, M. Santoro, and S. Sciortino, 3D Diamond detectors: charge collection efficiency of graphitic electrodes, Appl. Phys. Lett. 103, 233507 (2013).
[4] S. Lagomarsino, M. Bellini, C. Corsi, S. Fanetti, F. Gorelli, I. Liontos, G. Parrini, M. Santoro, S. Sciortino, Diamond Relat. Mater. 43, 23 (2014).

SEM images of a bonded silicon wafer

1×1 mm^2 3D structure consisting of 61 staggered vertical graphitic wires 500 μm long, obained in a diamond wafer and connected to interdigitated graphitic combs on the same surface of diamond.