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The Process

The first layer, called the gate layer, is produced by entirely covering the glass in a conducting metal before moving the plate to the photolithography process so that the first layer can be patterned. The photolithography process consists of three parts, coating, exposure and development.  The first part of the of the photolithography process is to coat the glass with a photosensitive material, or photoresist.  It is then placed into a machine called a stepper that utilizes masks to transfer  specifically designed patterns for different parts of the plate onto the photoresist These control which part of the plate will be exposed and which will be unexposed when the light hits. The photoresist is softened everywhere it is exposed.  The photoresist that is not exposed by light remains hard and acts a protective barrier to subsequent etch processes.

Next, the plate is moved to a wet etch tool that allows for chemicals to react with the aforementioned gate layer film that is unprotected by photoresist.  All the exposed metal film is dissolved and what remains behind are the gate lines and tabs that form the foundation of the electrical switch, or thin film transistor. The same process is then repeated for all of remaining layers that form the entire thin film transistor. The next layer, after the gate layer, is called the island layer.  The same basic steps in making the gate are followed in order to make this layer.  A film is deposited on the entire glass plate and a pattern is exposed onto photoresist using the photolithography process.  However, this layer is composed of a semiconducting and insulating layer. It is called the island layer because the feature is surrounded by the gate layer tabs. It is a unique layer in that it is a self-aligned feature and does not rely completely on the stepper to align it to the previous layer.

The island layer then undergoes the aforementioned etch step process whereby the unprotected film is dissolved away and the protected film remains to form the islands on the gate tabs.  The electrical switch, or thin film transistor is now complete.

The next part of the process is to form the photodiode.  Once again, the entire plate is deposited with multiple films that form the bottom electrode of the diode as well as the photosensitive material and the top electrode. This layer is then covered with photoresist, patterned and developed, and taken to two different etch machines to remove unwanted film.  At this point, the photodiode has been created and has been connected to the thin film transistor, or electrical switch.

The next step is to create the metals lines connecting the thin film transistor and the photodiode to the outside world. An insulating layer is deposited on the entire glass plate, covering the thin film transistor and photodiode.  The insulating layer must be transparent in order to allow light to go through it and expose the diode. The plate is then coated with photoresist exposed and developed to form holes, or vias into the insulating layer.  After the vias have been etched into the insulating layer, a metal layer is then deposited onto the entire plate.  The metal layer also fills the vias.  Photoresist is used to form the metal lines that connect the thin film transistor, or the electrical switch to the outside world in order to read out the electrical charge stored by the photodiode.  The photoresist, which has been patterned in the form of metal lines leading outside of the photodiode array protects the metal to remain after the etch process.

This now results in a working plate in which a photodiode converts light into electrical charge and stores it. The thin film transistor, or switch is the way to allow the charge stored in the diode to flow to the metal lines and read out to the outside world.  When all of this is completed, a hard, protective moisture barrier is deposited onto the entire plate.  Finally, lithography is used to uncover the pads that are used to connect the outside world to the thin film transistors and photodiodes.  The plate is then tested, and if necessary, repaired for any anomalies that may have arisen at any point in the process.

Process Overview
The overall production process can be broken down into three distinct phases of production - the production of the switch, the production of the photodiode, and the production of the connectors to the outside world. The production of the gate, or the switch for the photo detector array, consists of metal, semiconducting and insulating layers. Likewise, there is a varying process of metal, semiconducting and insulating layers involved in the creation of the photodiode layer. The connectors to the outside world consist of metal lines.  Each of these three components of the photodetector array consists of multiple circuits that are created by using photolithography. This can be seen as a step analogous to the development of an old-fashioned photograph through the collection of an image onto a photosensitive material on film except that in this step of the process the photolithography allows for the transferring of an image onto a photosensitive material through a mask. The mask behaves in a manner similar to cutting designs out of a piece of paper and then shining a flashlight through it - the result being that your artful designs are projected onto a wall. Here at dpiX, instead of shining a flashlight through a paper cutout onto a wall, light is directed through a mask and onto a photosensitive material, or photoresist.  Thus, the circuitry on the mask is transferred to the photoresist.  This exact mechanism is what creates the circuitry wherein the areas of the photoresist that are exposed to the light are softened are later removed while the area that is not exposed (and in the shadow of the mask) remains hard. Finally, the plate goes through a wet and/or dry etch step that dissolves away the material that is unprotected by the photoresist and leaves the materials (insulating, semiconducting or metal layers) that is protected by the photoresist.  After testing for quality, the final step involves covering each photo detector with a protective material to help it withstand mechanical stress.​​

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