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Silicon Drift Detector (SDD) with dual Boron layer Disclosure Number: IPCOM000215120D
Publication Date: 2012-Feb-20
Document File: 4 page(s) / 49K

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Silicon Drift Detector (SDD) with dual Boron layer

Silicon drift detectors are a type of x-ray detector used in a variety of applications, including energy dispersive x-ray analysis and c-ray fluorescence analysis.

At present:

1) most SDD manufacturers have great trouble to achieve shallow implants to stretch the low energy sensitivity to a minimum

2) Entrance windows that consist of Silicon oxide have to be carefully designed, possibly at the expense of extra process steps to increase the radiation hardness of the devices.

3) There is no solution for solving field uniformity other than applying a variety of in-silicon resistors between consecutive P+ implant rings distributed from the center to the perimeter of the device. This requires a careful design and reliable and stable process conditions

     Two aspects of the silicon drift detector are mentioned for which the invention provides an improvement in performance and manufacturing possibilities

1) The entrance window:

An important aspect of the silicon drift detector is the entrance window. Because low energy x-rays have a low penetrating power it is important to have this entrance window as thin as possible. State of the art techniques involve ultra shallow p+ implants and short annealing steps.

The aim is to make this layer as thin as possible to guarantee that all generated electron hole pairs just below the surface are collected in the drift field and do not recombine in the fieldless p+ layer. Additionally the implanted region is often covered by silicon oxide and, optionally, an aluminium layer. Drawback of this process is a remaining loss in the detection of the electron hole pairs which leads to a reduced detection of the energy of the quants. Another problem is the usage of silicon oxide with potentially has problems in the area of radiation hardness. In the vicinity of the Si/SiOx interface recombination centers can be formed that trap electrons that would otherwise have drifted into the depleted Si layer.

2) The ring structure providing the radial drift field (FIG. 1)

In a silicon drift detector, incoming photons (or electrons) create electron hole pairs. The generated electrons then drift towards the central area of the detector where a collection electrode resides. This electrode can have a small capacitance, which is the main feature of the silicon drift detector. For a reliable and response it would be sufficient is the drift time of all electrons would be identical. Because of the difference in impact positions of the photons, it is unavoidable that the electrons generated along the periphery of the detector take more time to travel towards the center of the

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detector, then electrons generated by photons that impact the center of the detector. Therefore, the travel time of the outer electrons has to be limited to for instance less than 100 ns.

The radial velocity of the electrons is controlled via a radial electrostatic field. Ideally, this electrostat...