Electrochemically etched pore arrays in silicon for X-ray imaging detectors

University dissertation from Stockholm : KTH

Abstract: Digital devices have now been introduced in many X-ray imaging applications, replacing slowly traditional photographic films. These devices are preferred as they offer real time imaging, easy handling and fast treatment of the images. However, the performance of the detectors still have to be improved in order to increase the image quality, and possibly reduce the X-ray dose, a vital parameter for medical use. In this thesis, three different new detector concepts have been investigated. All designs use pore arrays, which are ideal starting structures to form pixellated detectors.Electrochemical etching of n-type silicon in aqueous hydrofluoric acid solution (HF) has been studied to form these pore arrays. A broad range of pores have been fabricated with diameters varying from 200 nm to 40 µm and with depths reaching almost the wafer thickness, thus leading to very high aspect ratios. The technique was also found to be suitable for the formation of other types of structures such as pillars and tubes on the sub micrometer scale. The etching is based on the dissolution of silicon in HF under anodic bias and a supply of positive electrical carriers (holes). As holes are the minority carriers in n-type silicon, they are usually photo-generated. In this work an alternative technique, based on hole injection from a forward-biased pn junction, has been successfully pioneered.The first X-ray imaging detector concept presented in the thesis consists of a silicon charge coupled device (CCD) in proximity with a scintillating screen. The screen is made from a pore array having reflective pore walls and filled with CsI(Tl), emitting photons at a wavelength of 550 nm under X-ray exposure. The secondary emitted photons are light-guided by the pore walls and then detected by the CCD pixels. Detectors were fully fabricated and characterized. This concept provides good spatial resolution with negligible cross talk between adjacent pixels. The dependences of the detector efficiency on pore depth and on the coating of the pore walls are presented. Although most of the produced detectors had a detective quantum efficiency of about 25%, some detectors indicate that efficient scintillating screens can be achieved approaching the theoretical limit as set by poissonian statistics of the X-ray photons.The two other detector designs require the formation of vertical pn junctions, i.e. in the pore walls. In one concept the secondary emitted photons are detected by photodiodes located in the pore walls. This would lead to high charge collection efficiency as the photons do not have to be guided to one end of the pore. However, high noise due to the direct detection of X-rays in the diodes is expected. The other concept is based on generation of electron-hole pairs in a semiconductor and the ‘3D’ detector, where an array of vertical electrodes is used to separate the charges via an electric field. To uniformly dope the inside of deep pores, both boron diffusion and low-pressure chemical vapor diffusion of boron-doped poly-silicon were shown to be successful techniques. This was confirmed by SIMS profiles taken through the pore wall thickness. Finally, the possibility to form individual junction in each pore was shown. The diodes were electrically characterized, demonstrating good rectifying behavior and sensitivity to light.

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