Patent Application
20060289777
The invention relates generally to the field of non-invasive imaging and more particularly to the use of detectors composed of photodiode arrays in such imaging techniques.
The field of non-invasive imaging has wide ranging applications in the areas of medical, industrial, and security imaging. For example, in modern healthcare facilities, medical diagnostic and imaging systems are invaluable for diagnosing and treating physical conditions and disorders inside the human body. Similarly, in industrial applications, non-invasive imaging is a valuable tool for scanning various objects for quality control and defect recognition. Likewise, in security applications, non-invasive imaging allows package, baggage, and even passenger screening to be performed in a non-invasive, unobtrusive, and rapid manner.
For example, one class of non-invasive imaging techniques that may be used in these various fields is based on the differential transmission of X-rays through a patient or object. In the medical context, a simple X-ray imaging technique may involve generating X-rays using an X-ray tube or other source and directing the X-rays through an imaging volume in which the part of the patient to be imaged is located. As the X-rays pass through the patient, the X-rays are attenuated based on the composition of the tissue they pass through. The attenuated X-rays then impact a detector that converts the X-rays into signals that can be processed to generate an image of the part of the patient through which the X-rays passed based on the attenuation of the X-rays. Typically the X-ray detection process utilizes a scintillator, which generates optical photons when impacted by X-rays, and an array of photosensor elements, which generate electrical signals based on the number of optical photons detected. Typically the array of photosensor elements is an array of photodiodes where each photodiode is equated to a picture element, or pixel, in images generated using the detector.
One problem that may arise in the detection process occurs when one of the photodiodes used to detect optical photons is open, i.e., does not form a closed circuit. During imaging operations such an open photodiode may continue to accumulate charge, which is eventually injected to neighboring photodiodes as a bipolar diffusion current. Such diffusion currents interfere with the operation of the neighboring pixels. As a result, an open photodiode in the middle of an array of photodiodes may interfere with the operation of nine pixels, i.e., the open photodiode itself as well as the eight adjacent photodiodes.
For a single bad pixel, interpolation based on nearby good pixels may be used to provide some degree of correction. However, where an entire 3×3 array of pixels is impacted by the effect of an open photodiode, it may be difficult or impossible to obtain the desired degree of correction based solely on interpolation. Similarly, calibration may be helpful in mitigating the effects of an open photodiode to some extent. However, due to the dependence of the injected diffusion current on varying environmental factors such as signal level and temperature, calibration may be insufficient for satisfactory correction.
The problems created by open photodiodes are undesirable in larger, higher resolution detector arrays comprising more and smaller photodiodes assemblies. For example, in multi-slice computed tomography (CT) systems, where the CT scanner is able to acquire more than one image slice simultaneously, the detector size is large and is of high complexity. As the complexity of the photodiode array increases in such detectors, it becomes increasingly difficult to properly connect every photodiode in the arrays. As a result, the increasing size and complexity of multi-slice CT arrays may result in open photodiodes that degrade image quality around them. Similarly, detectors in other X-ray imaging modalities, such as radiography, mammography, tomography, and so forth may suffer from similar detector quality problems as detector size and/or complexity increases.
Thus, there is a need for a technique that prevents open photodiodes from contaminating neighboring photodiodes in a detector array.
In accordance with an implementation of the present technique, a detector is disclosed. The detector includes a photodetector array. The photodetector array includes a plurality of photodiodes and an electrically isolating structure separating each photodiode of the plurality of photodiodes.
In accordance with another implementation of the present technique, a method of manufacturing a detector is disclosed. The method involves providing a photodetector array that includes a plurality of photodiodes. The method also involves separating each photodiode of the plurality of photodiodes with an electrically isolating structure.
In accordance with yet another implementation of the present technique, an imaging system is disclosed. The imaging system includes a radiation source configured to emit radiation and a detector that is configured to generate a plurality of signals in response to the emitted radiation. The detector in the imaging system further includes a photodetector array having a plurality of photodiodes and an electrically isolating structure separating each photodiode of the plurality of photodiodes.
In accordance with yet another implementation of the present technique, a method for electrically connecting a photodiode array is disclosed. The method involves providing a plurality of vias through the photodiode array. Further, the method involves electrically connecting each via to a respective photodiode of the plurality of photodiodes and electrically connecting each via to readout circuitry configured to acquire signals from at least one of the photodiodes at a time.
These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:
Turning now to the drawings and referring first to
The imaging system 10 includes a source 12 adapted to emit X-rays through a target 16 disposed within an imaging volume. After passing through the target 16 the X-rays impact a detector 18 which generates signals in response to the incident X-rays. The signals may be acquired by detector acquisition circuitry 20 and processed by image processing circuitry 22 to generate one or more images for display on an operator workstation 24 or an image display workstation 26. The imaging system 10 also includes a system controller 28 adapted to communicate with at least the X-ray source 12, the detector 18 and the operator workstation 24. In certain implementations of the present technique, the imaging system 10 may also include a motion subsystem 30 that is controlled by the system controller 28 and adapted to control the motion of at least the X-ray source 12 and/or the detector 18. Alternatively, in other embodiments, the motion subsystem 30 may be adapted to control the motion of the target 16 (or a support on which the target 16 rests) in addition to or instead of the motion of the X-ray source 12 and/or the detector 18. As will be appreciated by those of ordinary skill in the art, the operator workstation 24 may be configured to communicate with the system controller 28 to control the operation of source 12, the detector acquisition circuitry 20, and/or the motion subsytem 30, if present.
In the present exemplary embodiment, the source 12 is configured to emit X-rays. However, in other embodiments, the source may be configured to generate electromagnetic energy at wavelengths outside what is considered to be the X-ray energy spectrum (such as gamma ray, visible or near visible) known in the art at an appropriate wavelength and intensity such that the electromagnetic energy may be used for imaging (such as by transmission or reflection) in conjunction with a suitable detector 18. Furthermore, in certain other implementations, it should be noted that the functions of some or all of the detector acquisition circuitry 20, the image processing circuitry 22, the operator workstation 24, the image display workstation 26, the system controller 28 as well as the motion subsystem 30 may be grouped into a single unit or various sub-units and that such modifications should be construed as being within the scope of the present technique. For example, in one embodiment the functions of the system controller 28, image processing circuitry 22, and the operator workstation 24 may be performed by a processor-based system, such as a general or special purpose computer system or workstation. In another embodiment, the functions of the operator workstation 24 and the image display workstation 26 may be performed by such a processor-based system or computer workstation
In an exemplary embodiment, the detector 18 is adapted to generate electrical signals in response to radiation, such as X-rays, incident upon the detector 18. In certain implementations, the detector 18 may be configured to generate electrical signals in direct response to the incident radiation. However, in other implementations, the detector 18 may be configured to generate the electrical signals in response to an intermediary signal generated in response to the incident radiation. For example, in one embodiment, the detector 18 includes of a scintillator array and a photodetector array. The radiation impacting the detector 18 strikes the scintillator array, which generates optical photons in response to the radiation. In such embodiments, the photodetector array may either be a front-lit photodetector or a back-lit photodetector. A front-lit photodetector is one in which the radiation first encounters the surface of the photodetector array containing the photodiodes, i.e., the “front” of the photodetector. Conversely, in back-lit photodetectors, radiation first encounters the surface of the photodetector array opposite the photodiodes, i.e., the “back side” of the photodetector. Each photodiode is configured to generate an electrical signal or charge in response to optical photons striking the photodiode. Each charge on each photodiode may then be read out to determine the incidence of optical photons, and therefore radiation, at the photodiode location. This charge information for some or all of the array may, therefore, be aggregated and processed to generate an image describing the radiation incidence on the detector at a given time. In accordance with the present technique, each photodiode of the detector 18 is electrically isolated from neighboring photodiodes such that crosstalk and diffusion currents are reduced or eliminated, thereby preventing the accumulation of charge from an open photodiode from interfering with the signal acquired from neighboring photodiodes. Different embodiments of the photodetector array in detector 18 in accordance with the present technique are discussed below.
While the depicted embodiment of
In the presently illustrated embodiment, each photodiode is electrically isolated from adjacent photodiodes via an N+ diffusion region 56, such as the depicted diffusion grid, which reaches down to the underlying substrate 50 to produce the electrical isolation. For example, the N+ diffusion region 56 is formed around each photodiode such that any diffusion current from an open photodiode does not flow to any neighboring photodiodes. It may be appreciated by those skilled in the art that the diffusion region may be of other doping types as well.
In the depicted embodiment, each of the photodiodes 72 is electrically isolated from adjacent photodiodes via a trench 74 formed between each photodiode down to the substrate 50. As will be appreciated by a person skilled in the art, the trench 74 may be formed in the photodiode layer 48 by chemical or mechanical techniques such as precision mechanical sawing, etching, and so forth. Etching may be performed by chemical etching techniques, reactive ion etching, or by other etching techniques known in the art. The formation of trenches between each photodiode electrically isolates each photodiode.
In one embodiment, the sides of each trench are passivated and protected, by known methods including a thermal oxide, other deposited film or a N+ doped layer. Such passivation techniques reduce the recombination or loss of signal carriers at these surfaces and reduce the generation of leakage current in the photo diode.
Similarly,
Though the preceding discussion discusses exemplary embodiments in which each photodiode of an array of photodiodes is electrically isolated from one another, other embodiments are also possible. For example, instead of isolating each photodiode, it may instead be desirable to isolate rows or columns of photodiodes, i.e., it may be desirable to provide electrical isolation in only one-dimension, as opposed to the two-dimensional isolation schemes discussed above. As will be appreciated by those of ordinary skill in the art, in such implementations the techniques discussed herein may be employed, however, instead of a full grid of diffusion regions or trenches, the diffusion regions or trenches may be provided as parallel strips separating the photodiodes into the desired rows or columns. In this manner, each row or column of photodiodes may be electrically isolated from other respective rows or columns, but photodiodes within a respective row or column would not be electrically isolated from one another. In addition, as one of ordinary skill in the art will appreciate, the preceding examples, for simplicity, each describe only one technique for electrically isolating photodiodes. However, in other embodiments combinations of trenches and diffusion grids may be employed. For example, electrical isolation in one dimension may be accomplished via parallel strips of diffusion regions while electrical isolation in a second dimension may be accomplished via parallel trenches. Likewise, different techniques, i.e., diffusion regions and/or trenches, may be concurrently employed to achieve electrical isolation in one-dimension if so desired.
While the preceding discussion provides various techniques for electrically isolating photodiodes of a detector array, it may also be desirable to provide interconnect structures on the backside of the diodes for connecting photodiodes, electrically isolated as described herein to suitable readout circuitry, such as to the detector acquisition circuitry of
As will be appreciated by those of ordinary skill in the art, the electrical isolation techniques described herein, as well as the interconnect techniques may be used in conjunction with a detector or detector array as discussed with regard to
While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.
