Introduction
The Hidralon project is a European collaboration of a number of institutes and companies. The aim is to develop a new generation of CMOS imagers that surpasses the current CCD imagers. The fields that are part of these developments are areas of healthcare, entertainment, and road & industrial safety.
Nikhef is involved in the characterisation of the detection materials that convert X-rays into measurable electronic signals. Especially the materials heavier than silicon such as gallium arsenide and cadmium telluride are under study because they are suited to efficiently detect X-rays with X-ray energies higher than 20keV commonly used in medical applications.
Hidralon – High Dynamic Range, Low Noise
High dynamic range and ultra-low noise are the main classifications that characterise CMOS based Single Photon Counters (SPC’s). Consequently, an SPC can image objects with a high contrast and discriminate the relevant signal from noise. As a result, a highly segmented SPC is able to do low-dose imaging with a high spatial resolution. Medipix is such a photon counting pixel detector and has therefore become very attractive for medical X-ray imaging applications.
Since a medical object usually is larger than one single Medipix chip (1.4 x 1.4 cm2), research at Nikhef is aimed at enlargement of the active surface area (current CCD imagers are typically 24 x 30 cm2) by means of tiling. In order to obtain a large seamless tessellation, two aspects are being considered:
- the amount of dead material (i.e. wire bonds and readout chip periphery) between the individual assemblies;
- the inactive sensor area along the edges.
Both must be reduced to a minimum. Efforts are being made to the former within the Relaxd project, with the development of the Through-Silicon-Via (TSV). In order to reduce the surface current at the sensor sides, a novel cutting technique called Deep Reactive Ion Etching (DRIE) is examined.
Detector Materials
For medical X-ray imaging applications, the typical energy domain is 30 – 120 keV. So, to be able to do low-dose imaging without loss of energy information, high energy absorption is needed. Compared to silicon, high-Z direct conversion materials (e.g. CdZnTe and GaAs) are better suited. Their high absorption efficiency and sensitivity are essential properties that will benefit the image capture efficiency. Again, this may lead to a reduction in equivalent dose the patient is exposed to.
Like silicon, these high-Z sensor materials also have a high intrinsic quality, thereby maintaining the diagnostic value of the images. Still, the growth of mono-crystalline high-Z materials is an immature technology.
Within the HiDRaLoN project, the influence of the various fabrication process steps on the detection properties will be studied. Specifically, the sensor edge effects resulting from different dicing techniques will be scrutinised.
