Patent Background

Flat panel image sensors are commercially successful products that are able to effectively detect electromagnetic radiation in and near the visible light spectrum. Flat panel image sensors are fabricated by depositing and patterning various metals, insulators, and semiconductors on glass substrates as is done in flat panel displays. Such sensors commonly employ photosensitive elements, such as amorphous silicon (a-Si) PIN diodes. The photosensitive element is coupled to a readout switch, such as thin film transistor (TFT) that provides data indicative of received light.

A common use for flat panel image sensors is for medical and industrial applications to detect x-rays. The image sensor includes a phosphorescent screen that overlays an array of image sensing elements. The phosphorescent screen converts received x-rays to visible light. The array receives the visible light and generates a photocurrent responsive to the light. The photocurrent is read out as data indicative of the sensed light.

Other applications disclose detecting light reflected through a transparent sensor element incorporating a TFT. J. H. Kim et al., "Fingerprint Scanner Using a-Si:H TFT array," SID '00 Digest, Long Beach, Calif., USA, pp. 353-355 discloses a contact image sensor that requires a transparent sensor element to pass reflected light.

Conventional image sensing applications have not considered the use of TFTs to detect relatively weak x-ray emissions. In order to detect x-ray emissions, the sensitivity of the imaging sensing TFT is a primary concern. Conventional devices are unable to provide adequate light detection for x-ray applications. Transparent pixels, in particular, do not provide sufficient sensor element density to detect light resulting from x-ray emissions. Thus, it would be an advancement in the art to provide a TFT image sensor with enhanced light detection capability and suitable for x-ray applications. Such a device is disclosed in this patent.

Summary of the Invention

An image sensor array includes photo sensors disposed on a substrate and arranged to receive and sense an image. Each photo sensor represents a pixel for a received image. The photo sensors each include a photo TFT that generates a photocurrent in response to the image. The photo TFT may be manufactured using common processes for TFTs in flat panel applications. The photo TFT has a gate electrode which is shorted to its source electrode to obtain a photocurrent that is substantially independent of source-drain bias. The photo TFT may also be configured with interdigitated source and drain electrodes to increase the photosensitivity. Each photo TFT is coupled to a bias line to enable operation and a storage capacitor to store a charge and discharge upon generation of a photocurrent.

In an alternative embodiment, a photo TFT has its source electrode and gate electrode coupled to independent bias lines to reduce the effect of dark current. The corresponding storage capacitor is coupled to the source and drain electrodes.

In yet another alternative embodiment, a photo TFT has its gate electrode coupled to a bias line and its source electrode coupled to a select line of an adjacent sensor element. The corresponding storage capacitor is coupled to the gate and drain electrodes.

In a further alternative embodiment, the photo TFT may be disposed above the storage capacitor in stacked configuration. In this implementation, the surface area of the photo TFT may be increased to yield greater light detection capability. The storage capacitor may also be enlarged for greater capacitance.

The storage capacitor is preferably coupled to a readout TFT that is also manufactured using known processes. The readout TFT passes a charge from the storage capacitor to a data line. Operation of the readout TFT is enabled by a select line which is coupled to the gate electrode of the readout TFT. A light shield may be disposed over the channel of the readout TFT to prevent a charge leaking through the readout TFT.

Each photo TFT may be coupled to a reference TFT. A reference TFT is similar in structure to a corresponding photo TFT and provides an equivalent dark current to compensate for dark current in the photo TFT. The reference TFT includes a light shield so as to not generate a photocurrent in response to received light.

The photo TFTs can provide an effective and economical alternative to conventional photodiodes. Photo TFTs may be manufactured concurrently with the manufacture of corresponding readout TFTs using conventional methods, thereby reducing mask counts and costs. Photo TFTs may further yield photocurrents greater than that of photodiodes. Furthermore, photo TFTs have a lower fill factor and provide less spillover to adjacent pixels.

The three Oregon based engineers that invented this process are Willem den Boer, Tin Nguyen and Patrick Green. The granted patent is number 7,773,139. Patent FIG. 20 presented as our cover graphic is a cross-sectional view of an embodiment of a sensor element.

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