Radiant energy – Invisible radiant energy responsive electric signalling – Semiconductor system
Reexamination Certificate
1999-10-19
2001-04-10
Hannaher, Constantine (Department: 2878)
Radiant energy
Invisible radiant energy responsive electric signalling
Semiconductor system
C250S370090
Reexamination Certificate
active
06215123
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to methods for manufacturing radiation detectors and radiation imaging devices, radiation detectors and imaging devices manufactured by such methods, and the use of such radiation detectors and imaging devices.
2. Description of the Prior Art
A typical method of manufacturing a radiation detector for an imaging device comprises applying a layer of a metal such as aluminum to both of the main surfaces of a planar semiconductor substrate, applying a layer of photoresistive 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 metallized 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 on 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 (typically a few microseconds, or at most a few milliseconds). Alternatively, it may be one of a type described in the Applicants's International patent application PCT EP95/02056 which provides for charge accumulation for individual detector cells, the disclosure of which is expressly incorporated herein by reference. With an imaging device as described in PCT EP95/02056, 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 neighboring contacts (pixel pads) leaking and causing the contrast resolution to degrade.
It has been found that traditional methods of forming contacts on a detector surface, particularly when CdZnTe is used as the semiconductor material, do not provide as high an electrical separation of the contacts as is desired to make optimum use of the advantages which are to be derived from the imaging devices as described in the International patent application PCT EP95/02056.
SUMMARY OF THE INVENTION
According to an embodiment of the present invention, a method for forming metal structures (e.g., metal contacts) on a semiconductor substrate at spaced positions (e.g., for defining radiation detector cells) includes the steps of forming one or more layers of material on a surface of the substrate with openings to the substrate surface at the contact positions; forming a layer of metal over the layer(s) of material and the openings; and removing metal overlying the layer(s) of material to separate individual contacts.
The present inventors have found that the surface resistivity of a CdZnTe semiconductor substrate is degraded when the substrate is exposed to metal etchants suitable for removing gold and/or platinum. As a result, 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 method in accordance with the present invention, the surface of the semiconductor substrate between the contacts can be isolated from the metal etchants, thus preventing the damage which would result if the metal etchants came into contact with the semiconductor surface.
Thus, in this first embodiment, the step of forming one or more layers of material on a surface of the substrate with openings to the substrate surface at the contact positions may include the sub-steps of forming a layer of photoresistive material on the substrate surface; and selectively exposing the photoresistive material, and removing the photoresistive material from areas corresponding to the contact positions to expose the substrate surface.
Alternatively, this step may include the sub-steps of forming a layer of passivation material on the substrate surface; forming a layer of photoresistive material on the passivation layer; selectively exposing the photoresistive material, and removing the photoresistive material from areas corresponding to the contact positions to expose the passivation material layer; and removing the passivation material from the exposed areas corresponding to the contact positions to expose the substrate surface. 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. To protect the other main surface and the sides (edges) of the semiconductor substrate, photoresistive material can additionally be applied to all exposed surfaces prior to the step of removing the passivation material from the exposed areas corresponding to the contact positions.
Again with reference to the above-referenced first embodiment, the step of removing metal overlying the layers of material to separate individual contacts may include the steps of forming a further layer of photoresistive material on at least the metal layer; selectively exposing the photoresistive material of the further layer, and removing the photoresistive material of the further layer apart from areas corresponding generally to the openings; and removing metal not covered by the photoresistive material of the further layer. In addition, any remaining photoresistive material may be removed. In a variation on this embodiment, the areas corresponding generally to the openings are larger than the corresponding openings, so that after removal of the metal not covered by the photoresistive material of the further layer, the contacts cover the opening and also extend up and laterally beyond the opening. In this way the ingress of metal etchant around the photoresistive material, whereby the metal etchant might reach the semiconductor surface, can be avoided.
The present invention is particularly useful with substrates formed of cadmium zinc telluride (CdZ
Jalas Panu Y.
Orava Risto O.
Pyyhtia Jouni I.
Sarakinos Miltiadis E.
Schulman Tom G.
Gagliardi Albert
Hannaher Constantine
Kenyon & Kenyon
Simage Oy
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