D3 – Photo detectors
for spectral CT
Demonstrator leader: Johan Klootwijk (Philips Research)
 
Introduction
Spectral CT allows for tissue differentiation. It is regarded as a break-through in CT technology and high on the agenda of leading CT manufactures. The first generation of Spectral CT detectors consists of two stacked and pixelated scintillator materials, each one with a different spectral response, which is read out by an array of vertically mounted photo diodes. This arrangement has been proven to be very successful for the first generation of spectral CT scanners. The success of these scanners will open the development route towards new architectures based on direct conversion thereby allowing to disperse X-rays
Vertical architecture for dual layer detector: Two layers of scintillators are superimposed. The generated light is detected through a vertically mounted photodiode array, Horizontal architecture for dual layer detector: Two layers of scintillators are superimposed. The generated light is detected through two horizontally placed photodiode arrays. In this project two different routes will be followed to come to the next generation of spectral detectors:
  1. Rationalizing the present vertical detector architecture by using advanced die-to-board connectors.
  2. Utilizing horizontal stacked detector configuration requiring:
    1. Special diode fabrication to eliminate direct conversion
    2. Development of an assembly flow to allow for four sided butting of detector tiles
 
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Left: vertical architecture for dual layer detector: Two layers of scintillators are superimposed. The generated light is detected through a vertically mounted photodiode array. Right: Horizontal architecture for dual layer detector: Two layers of scintillators are superimposed. The generated light is detected through two horizontally placed photodiode.

 
Industrial challenges
In the present vertical detector architecture individual photo-diode tiles are soldered vertically to a ceramic interposer. This is a sequential process requiring dedicated equipment while the replacement of faulty photo-diodes is not possible. This is a severe cost-adder. For the widespread acceptance of Spectral CT a cost down is therefore necessary. For the present architecture it is necessary to develop a reversible assembly method. Alternatively, other architectures, such as the horizontal architecture, in which a photodiode is sandwiched between the top and bottom scintillator, will be developed.
In the horizontal detector architecture, the high energy X-rays which have not been absorbed by the top low-energy scintillator pass though the top photo-detector. Without special measures this will cause unwanted direct conversion, which will be added to the detection signal. The contribution of the unwanted conversion may be suppressed by: reducing the effective thickness of the optically sensitive part of the sensor substrate.
Another issue is the interconnection of each individual sensor pixel to the edge of the detector. Solutions have to be developed in close cooperation with the developments for the sensor.
 
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Vertical photodiodes are inserted into a connector which is provided with a ball grid array structure at the bottom.
 
 
Approach
The vertical detector is available. What needs to be developed is a 16 fold vertical connector structure with 576 connections at pitch 0.5 mm in a plane of 18 mm * 22 mm. The connector has a grid array interconnect at the bottom for interconnect to the signal processing section and further front-end electronics. The optimal bond pad metallization and connector design will be evaluated. The final connector will be demonstrator on a single detector tile.
For the horizontal architecture the contribution of the unwanted conversion may be suppressed by: reducing the effective thickness of the optically sensitive substrate, using SOI or by using optimized junction isolation schemes. In this project the different approaches will be evaluated both theoretically as well as experimentally on single pixel devices.
Four sided butting is the most versatile and low cost technique to make variable large continuous detector areas out of smaller units. Three different approaches will be followed to allow for four sided butting of individual detector tiles into the 2D matrix structure required to build a complete CT detector.
 
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Innovation pipeline
 
 
Partner Role
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