Radiant energy – Invisible radiant energy responsive electric signalling – Semiconductor system
Reexamination Certificate
1999-07-28
2002-06-25
Hannaher, Constantine (Department: 2878)
Radiant energy
Invisible radiant energy responsive electric signalling
Semiconductor system
C250S370130
Reexamination Certificate
active
06410922
ABSTRACT:
The invention relates to methods of manufacturing radiation detectors and radiation imaging devices, radiation detectors and imaging devices manufactured by these methods and the use of such imaging devices.
A typical method of manufacturing a radiation detector for an imaging device comprises applying a layer of a metal such as aluminium to both of the main surfaces of a planar semiconductor substrate, applying a layer of pbotoresistive material to cover the semiconductor material, exposing the photoresistive material on the surface of the planar substrate with an appropriate mask pattern, removing the photoresistive material to expose a pattern of the metal to be removed, etching away the metal to be removed and then removing the remaining photoresistive material to leave a pattern of contacts on one surface of the substrate and a metallised layer on the other surface of the substrate. The contacts on the first surface of the substrate then define an arrangement of radiation detector cells.
For optical wavelengths and charged radiation (beta-rays), silicon has typically been used for the semiconductor material for the substrate. A method of the type described above has been used to good effect with this material.
In recent years, cadmium zinc telluride (CdZnTe) has increasingly been proposed as a more suitable semiconductor material for use in X-ray, gamma-ray and to a lesser extent beta-ray, radiation imaging. CdZnTe is good at absorbing X-rays and gamma-rays giving better than 90% efficiency for 100 keV X-rays and gamma-rays with a 2 mm thick detector. The leakage or dark current of these detectors can be controlled and values of the order of 10 nA/cm
2
or less at 100 Volts bias are achievable.
A small number of companies worldwide currently produce these detectors commercially in a variety of sizes and thicknesses. Usually one or both sides of the planar detectors are contacted with a continuous metal layer such as gold (Au) or platinum (Pt). As mentioned above, such detector substrates then need to be processed to produce a detector having a pattern of contacts (e.g. pixel pads) on one surface, with the opposite surface remaining uniformly metallized, in order that the detector may be position sensitive, that is in order that the detector is able to produce a detector output indicating the position at which radiation impacts the detector. A readout chip then can be ‘flip-chip’ joined to the patterned side of the CdZnTe detector (e.g., by bump bonding using balls of indium or conductive polymer material, gluing using one-way conductive materials or other conductive adhesive layer techniques) so that the position dependent electrical signals which result from incidence and absorption in the detector cells of X-rays or gamma-rays can be processed. The readout chip could be of the pulse counting type with very fast integration and processing time (typical a few microseconds or at most a few milliseconds). Alternatively, it may be one of type described in the Applicant's International Patent Application PCT/EP 95/02056 which provides for charge accumulation for individual detector cells. With art imaging device as described in PCT/EP 95/02056, integration times can be several milliseconds, or tens or hundreds of milliseconds. As the signal integration or standby/readout period increases it becomes more critical that the gold or platinum contacts on the CdZnTe surface are electrically separated to a high degree to avoid signals from neighbouring contacts (pixel pads) leaking and causing the contrast resolution to degrade.
It has been found that the traditional method of forming the contacts on the detector surface, particularly when CdZnTe is used as the semiconductor material, does not provide as high an electrical separation of the contacts as would be desired to make optimum use of the advantages which are to be derived from the imaging devices as described in the International Application PCT/EP 95/02056, which is incorporated herein by reference.
In accordance with one aspect of the invention, there is provided a method of manufacturing a radiation detector having conductive contacts on a semiconductor substrate at positions for defining radiation detector cells, wherein said method includes steps of.:
a) forming one or more layers of material on a surface of said substrate with openings to said substrate surface at said contact positions;
a(i) forming a layer of passivation material on said substrate surface;
a(ii) forming a layer of photoresistive material on said passivation layer;
a(iii) selectively exposing said photoresistive material and removing said photoresistive material from areas corresponding to said contact positions to expose said passivation layers;
a(iv) removing said passivation material from said areas exposed in step a(iii) corresponding to said contact positions to expose said substrate surfaces;
a(v) removing remaining photoresistive material of said photoresistive material layer;
a(vi) forming a further layer of photoresistive material on said exposed passivation layer and exposed substrate surfaces; and
a(vii) selectively exposing said further layer of photoresistive material and removing said further photoresistive material in a pattern corresponding to said contact positions;
b) forming a layer of conductive material over said layer(s) of material and said openings; and
c) removing conductive material overlying said layer(s) of material to separate individual contacts, including:
c(i) removing said further layer of photoresist material.
The use of an insulating layer of passivation material means that after manufacture of the detector, the passivation material remains between the contacts protecting the semiconductor surface from environmental damage in use and further enhancing the electrical separation of the contacts.
The present inventors have found that the surface resistivity of cadmium-based substrates, for example a CdZnTe semiconductor substrate is degraded when the substrate is exposed to metal etchants suitable for removing gold and/or platinum. As a result of this, the electrical separation of the individual contacts which result from the conventional method of forming such contacts is not as good as would be expected from the properties of that material before treatment. By using a lift-off method in accordance with the invention, metal etchants need not be used, thus avoiding the damage which would result if the metal etchants came into contact with the semiconductor surface. By removing a first pbotoresistive layer and subsequently applying a further photoresistive layer improved adherence to the passivation material may be obtained. Moreover, the mechanical integrity of the further layer of photoresistive material is greater than the first layer, and consequently lift-off of subsequently formed layers of material, e.g conductive material, may be achieved more reliably and has been found to provide a higher production yield for devices manufactured using this process.
Furthermore, the further layer of photoresistive material allows for the exposure of areas larger than the contact positions such that conductive material may be applied over portions of passivation material adjacent to the contact positions. Further photoresistive material may even be applied to areas or regions away from, yet operatively related to, contact positions. This is particularly advantageous and is intended for use in manufacturing high energy (1 KeV) radiation imaging devices since it allows more complex conductive material patterns and/or second conductive material layers to be formed on the passivation material. For example, for off-setting a charge collection contact of a detector cell relative to a corresponding contact of a readout substrate cell. Additionally, conductive material may extend from the contact positions over adjacent portions of the passivation material, thereby providing good mechanical contact, and reducing the possibility of gaps being formed between the conductive material and passivation material.
In a preferred embodiment
Heikkinen Hannele
Spartiotis Konstantinos Evangelos
Gabor Otilia
Hannaher Constantine
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