Upside down T-shape through-wafer interconnects for fully tileable photodiode matrix for medical CT imaging

Abstract

This paper presents a technology of a fully tileable two-dimensional photodiode matrix for medical imaging, specifically X-ray Computed Tomography (CT). A key trend in the CT industry is to build imaging devices with ever larger area detectors to speed up the measurements and to avoid image blurring due to patient movement. If individual slices have been taken at different points of time, the patient may have moved. The reconstruction accuracy suffers, resulting into blurred or distorted images. This can be avoided by increasing the amount of slices recorded in single rotation, up to the length of the whole object to be imaged. In most of the current CT detector constructions the characteristic pitch of the photodiodes is ∼1mm with 16×16 to 16×32 photodiodes on one chip. A major limiting factor in providing more detector coverage is the need to read out the signals from the individual photodiode elements of the photodiode matrix through metal lines between the photodiode elements along the surface facing the radiation source, and wire bonds down to a substrate or to an electronics chip. With this method, there is a physical limitation on the size of a photodiode matrix that may be manufactured. A detector with the possibility of active area expansion is known as a ‘tileable’ detector. Various technologies have been proposed to realize a tileable detector, all having their specific limitations. An improved technology of integrating T-shaped through-wafer interconnects within traditional front illuminated photodiodes is introduced, overcoming most of the limitations of the other tileable technologies. Photocurrent signal can be read out from back side of the chip through these interconnects, giving possibility of constructing arbitrarily large area of photodiode matrix. Results of an actual CT detector photodiode manufactured with this technology are presented showing that the requirements of modern CT systems can be met.