Shielding an imaging array from x-ray noise

1. Field of the Invention

The present invention relates generally to the field of radiation-sensitive imaging devices, and more specifically to radiation-sensitive imaging devices that have improved immunity to x-ray noise.

2. Related Background Art

Increasingly, electronic imaging sensors have been replacing film-based sensors in commercial, industrial, and medical imaging applications. Examples of such electronic sensors include charge-coupled device (CCD) sensors and complementary metallic-oxide-semiconductor (CMOS) sensors, to name a few. CMOS sensors, in particular, have emerged as a preferred candidate due to advantages in manufacturing cost, integration of components, charge efficiency, and low power consumption. An excellent example of a detector system suitable for medical imaging applications that uses a CMOS active pixel sensor (APS) array is provided in U.S. Pat. No. 5,912,942 to David B. Schick et al., assigned to the assignee of the present patent application. The Schick \'942 patent is incorporated herein by reference.

In the system of the Schick \'942 patent, as in many medical imaging applications, radiation or energy from x-ray photons, commonly referred to as x-rays, is projected through a patient and must be registered or detected by a sensor. However, conventional sensors or imaging chips, which generally are fabricated from silicon, are considerably more sensitive to photon energy in the visible spectrum than to x-rays. Thus, a scintillator is disposed on the sensors to convert the energy from the x-rays to visible light. The scintillator is typically composed of gadolinium oxysulphide or cesium iodide, although other alternative materials may be used.

While it is desirous that the scintillator convert all of the x-rays to visible light, in practice only a percentage is actually converted, with the remaining x-rays passing through the scintillator and reaching the silicon. A typical gadolinium oxysulphide scintillator, for example, may have a so-called stopping efficiency of 20-30%, meaning that only 20-30% of the x-rays that impinge on the scintillator are converted to visible light, with a significant amount of the x-rays (some 70-80%) being transmitted through the scintillator into the surface of the sensor. Although the sensor may be designed to be primarily sensitive to visible light, it will nevertheless be secondarily sensitive to effects from the transmitted x-rays. As a result, the transmitted x-rays are registered as noise by the sensor, reducing the overall quality of the captured image.

To reduce such noise, conventional systems typically shield the sensor from transmitted (unconverted) x-rays. In the Schick \'942 patent, for example, such shielding is achieved by interposing a fiber-optic plate (FOP) between the scintillator and the sensor. The FOP allows light to pass, but blocks (i.e., absorbs) a large amount of the unconverted x-rays. While generally good for its intended application, this shielding approach has the drawback of adding an undesirable thickness to the sensor, which compromises patient comfort. FOPs also cause some degree of light signal loss and light spreading, each of which reduces image quality. Also, as FOP sizes increase, they become extremely expensive and fragile. Accordingly, for many reasons, it is desirable to avoid using FOPs.

Recently, advanced x-ray-sensitive photoconductive materials, such as selenium, lead iodide (PbI₂), and mercuric iodide (HgI₂), for example, have allowed designers to produce sensors that can image x-rays directly, so that a conventional scintillator is not needed. When x-rays strike the surface of such a photoconductive material, electron-hole pairs are formed. These charges are driven by an electrical potential or bias potential, which causes charges of a selected polarity to be collected by a storage element, such as a capacitor. This alternative design has the benefit of limiting light-spreading, and may achieve preferred spatial resolution as compared to typical scintillators. However, although these photoconductive materials have superior stopping or conversion efficiency, their efficiency nevertheless is imperfect. And of course, because x-rays are directly imaged, an FOP generally is not used in these arrangements. Accordingly, with these constructions as well, an improved x-ray noise immunity is desirable.

Other structures for providing radiation shielding have been proposed. For example, U.S. Pat. No. 6,690,074 B1 to Dierickx is aimed at providing a radiation-resistant semiconductor device, and is particularly concerned with reducing the leakage current between the source and drain electrodes in a MOS-type structure, resulting from an overlap between the gate electrode and the field oxide. Towards this end, the Dierickx device uses a doped guard ring interrupted by an active area. This approach, however, while perhaps adequate for its intended purpose of preventing or reducing leakage currents, which typically occur near the surface of semiconductor devices, is completely ineffective at shielding from the deleterious effects of unconverted x-rays, which usually occur well below the surface.

There is a great need, therefore, for a semiconductor x-ray imaging chip that takes an entirely fresh approach, and provides improved x-ray noise immunity, and at the same time a superior image quality.

The present invention addresses the deficiencies in the prior art by providing arrangements and designs of an electronic imaging sensor or device with an improved immunity to noise caused by unwanted x-rays. The sensor images an object by collecting charge carriers produced in the sensor when the object is exposed to radiation such as x-rays. The charge carriers may be generated indirectly by the radiation, such as with the use of a scintillator to convert x-rays into visible light, with a photocurrent produced by the visible light being used to create an image of the object. Alternatively, the charge carriers may be generated directly by the radiation, such as with the use of selenium or other types x-ray-sensitive photoconductive materials that enable a photocurrent to be generated directly from exposure to x-rays.

One or more shielding areas are formed proximate the sensor, to capture or sweep away any undesirable charge carriers generated by stray or undesired radiation. The shielding areas extend deeper beneath the surface of the sensor than the depth at which the photocurrent corresponding to the object being imaged (“the desired photocurrent”) is collected. This is because the undesirable charge carriers are formed deeper beneath the surface of the sensor. Typically, the undesired charge carriers are generated by, for example, unconverted x-rays or x-rays that are not absorbed by the x-ray-sensitive photoconductive material, which penetrate into the sensor to a greater depth than the depth at which the desired photocurrent is collected. In this way, the undesirable charge carriers are captured near the region where they are generated and before they migrate towards the surface where they can be collected and manifest as noise in the resulting image of the object.

According to an aspect of the invention, a radiation detector is provided that possesses an improved immunity to x-ray noise over conventional radiation detectors. The inventive radiation detector includes:

a scintillator that converts radiation of a first energy to radiation of a second energy;

a semiconductor substrate incorporating dopants of a first conductivity type;

a sensing region formed in the substrate and incorporating dopants of a second conductivity type, wherein a sensing junction located at an interface between the sensing region and the substrate and positioned at a first depth below a surface of the substrate collects charge carriers generated in the substrate by the radiation of the second energy; and

a shielding region formed in the substrate and incorporating dopants of the second conductivity type, wherein a shielding junction located at an interface between the shielding region and the substrate and positioned at a deeper depth below the surface of the substrate than the first depth collects charge carriers generated in the substrate by radiation of the first energy.

According to another aspect of the present invention, a radiation detector is provided that possesses an improved immunity to x-ray noise over conventional radiation detectors, and that utilizes a photocurrent generated directly from x-rays. The inventive radiation detector includes:

a photoconductive portion that generates charge carriers when irradiated with x-rays;

a semiconductor substrate incorporating dopants of a first conductivity type;

a sensing region positioned at a first depth below a surface of the substrate to collect the charge carriers generated in the photoconductive portion;