Presently known semiconductor radiation detectors, particularly those designed for the detection of gamma and x-ray photons and charged particles and typically utilized in instruments for radiation spectroscopy, medical imaging, etc., are usually of a configuration in which the bulk-detecting crystals (e.g., cadmium telluride (CdTe) and cadmium zinc telluride (CdZnTe) crystals) which form the semiconductor plate or substrate are sandwiched between spaced-apart cathode and anode electrodes. Very often, the detector is of planar configuration with opposing planar surfaces and adjacent planar side walls perpendicular to the metalized surfaces. The cathode electrode (that normally positioned towards the radiation source) of such structures may comprise a metal layer, e.g., of gold, platinum, indium, etc., formed across one of these opposing surfaces. This cathode electrode may extend to the physical edges of the detector or, in certain configurations, may extend over the planar surface onto the detector's side walls. The anode electrode (that normally positioned away from the radiation source) is also typically planar and located on the opposing surface from the cathode. This anode electrode, like the cathode, may be in the form of a single metal layer, e.g., also gold, platinum, indium, etc. and also extend across the entire surface.
For more advanced high-resolution spectroscopy or imaging applications, the anode may comprise a plurality of pixel, strip and/or grid electrodes usually arranged in patterns with open regions or spaces separating the individual elements. Additional electrodes such as guard rings or bands may form part of this construction. Either or both of the electrodes may have the faces thereof subjected to further surface treatment and processing steps, e.g., to form interfaces of desired electrical properties. Multiple metal layers, diffusion barriers or other coatings may also be applied atop the primary contact surfaces to assure enhanced properties such as better mechanical robustness, solderability and/or long-term stability. It is further known to house such detectors within suitable housings.
Depending on the detector's application (e.g., the necessary stopping power, efficiency, pixel geometry, resolution, etc.), device thicknesses typically vary from a few microns to several centimeters with total detector areas ranging from less than square millimeter to tens of square centimeters. For larger fields of view, individual detectors may be oriented in arrays. Depending on the required device geometry and the grain sizes of the raw slices, several fabrication techniques may be considered when producing detectors of this type. Basically, a larger detector of sufficient thickness may be fabricated from an individual blank of somewhat larger size than that of the final detector. This blank may be further subjected to subsequent process steps such as etching or polishing. The fabrication of detectors with side wall extended electrodes (e.g., semi-hemispherical detectors) is typically limited to this individual blank approach. Alternatively, a multitude of detectors may be fabricated by dicing out the final devices from the wafer only at some point after electrode deposition. This approach, known in the semiconductor industry, is also referred to as “post-dicing.” Dicing, also known as wafer dicing, is the process by which the individual semiconductor substrates are separated from a larger wafer of semiconductor material and may be accomplished by scribing and breaking, by mechanical sawing (normally with a machine called a dicing saw) or by laser cutting.
One technical limitation associated with the manufacture of high-resistivity semiconductor bulk detectors, including those having CdTe and CdZnTe crystals, is the difficulty of properly controlling side wall properties, especially attempting to prevent structural damage and to achieve sufficient electrical passivation. Such damage, contamination, etc. often results in excessive side surface leakage currents and/or noise generation when the final detector is exposed to high electric fields in which many such detectors are utilized. This may be a major problem with respect to detectors having a single anode electrode and/or cathode electrode deposited across the entire wafer surface. At high bulk resistivity and required high side surface resistance, certain processing issues such as the afore-described partial smearing of metal particles from the electrodes over the side walls during dicing can adversely affect sensitive device parameters. This problem cannot be readily addressed by surface passivation and can, particularly in the case of relatively very thin detectors, lead to total detector failure. Other effects from damage and contamination may of course contribute as well. One approach to hopefully prevent this is to fabricate some kind of guard electrode, so that the adverse effect on the actual read-out electrode is limited. Doing so, of course, adds to the cost and complexity of the final product.
Examples of semiconductor radiation detectors are described in the following U.S. Pat. Nos. 7,955,992; 7,816,653; 7,741,610; 7,728,304; 7,528,378; 7,391,845; 7,355,185; 7,297,955; 7,157,716; 6,333,504; 5,880,490; 5,677,539; 4,896,200; and 4,879,466. In the most recent of these, U.S. Pat. No. 7,955,992, for example, there is described a method of making a semiconductor radiation detector where the CdZnTe semiconductor substrate has opposing planar surfaces and perpendicular side walls with one planar surface having a single layer of metal, e.g., gold, as the cathode and the opposite surface populated with a pattern of metal pixels which serve as anodes. The anode pixels in turn are comprised of a gold-nickel-gold alloy and formed using photolithography processing, leaving finely defined high resistivity gaps (called inter-pixels) separating one from the other. The substrate wafer is formed, including polishing and etching, to assure its surfaces are prepared for metal deposition. In one embodiment, the metalized substrate has side walls which include an electrically insulating coating, and the formed detector is positioned within an electrically conductive housing which protects the detector from background magnetic fields while being transparent to x-ray and gamma ray radiation. In another embodiment, the side walls include passivation layers for improved product reliability. These formed oxide layers, e.g., of tellium oxide, may be formed using alkali hypo-chloride, for example. Passivation materials may also be deposited between the individual pixels, in the inter-pixel regions.
Citation of the above documents is not an admission that any are prior art to the instantly claimed invention nor is this citation an assertion that an exhaustive search has been conducted.
As defined herein, the present invention represents a new and unique method of making a semiconductor radiation detector in which deleterious effects associated with the manufacture of many such detectors are substantially eliminated. This new method is made possible using modified known processing and other techniques associated with semiconductor detector manufacture such that the final costs to the finished products are maintained relatively low. It is believed that such an invention represents a significant advancement in the art.
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