The present invention relates to a pixel structure for use in an infrared pixel array. More specifically, the structure of the present invention is used for microemitter or microbolometer applications. In the microemitter application, the pixel of the present invention is used as one portion of a pixel array to perform infrared projection. Alternatively, in the microsensor, or microbolometer applications, the pixel structure of the present invention forms one pixel of a detector array.
Microstructures generally can be used to form various types of sensors and sensor products. In certain applications, these microstructures are arranged in arrays which cause the multiple pixels to cooperate with one another. One well known application is the microemitter, where the pixel array is used to project an image. In another well known application, the pixel array is used as a microbolometer wherein the array is used to detect a two dimensional signal.
In each application, it is essential that the pixels be isolated from one another. This isolation allows each pixel to operate independently from its surroundings pixels, thus creating contrast in either the sensed or displayed image.
In the aforementioned microemitter application, each pixel is independently energized to project a signal. In the case of an IR scene projector, the pixel is energized, causing heating of the element itself resulting in the emission of IR radiation. When arranged in an array of individual pixels and appropriately addressed, this collection of individual emitters can create the desired image.
Similarly, an array of microstructure pixels can be used in the microbolometer application to detect IR radiation or signals. In this application, each individual pixel is sensitive to IR signals. When such IR signals hit the sensing surface or area of the pixel, the resistance of the pixel active element changes. This resistance change can be appropriately sensed by related circuitry and fed back to an image controller. When the signals from an array of pixels are appropriately processed, a digital picture image can be created.
For any of the microstructure devices referenced above (microbolometer or microemitter), the optimum design is influenced by many factors. Due to the practical operational requirements, these considerations are surprisingly complex. More specifically, the optimum design characteristics include large fill factors, high thermal isolation, and an appropriate thermal time constant (speed).
Generally speaking, the time constant is related to the speed of the device. As can be appreciated, in either a detection or projection mode, it is desired that the pixels are updated frequently. This requires that the pixels be able to cycle or regenerate very quickly. In practical applications, it is not unusual to have frame rates existing anywhere from 30 to 500 hertz, thus the appropriate time constant must exist. Generally speaking, the thermal time constant is equal to the thermal mass of the pixel divided by the thermal conductivity. A low thermal time is desired as this will allow for high speed operation.
As mentioned above, large fill factor is an optimum characteristic in the design of microstructure pixels. Fill factor relates to the area used by the active portion of the pixel. It is beneficial for the fill factor to be as high as possible, utilizing the largest potential area of the pixel. This is especially true for emitter applications, where it is desirable to have a large surface of the pixel emitting the desired radiation signals.
Another obvious consideration is thermal isolation. It is clearly necessary that each of the pixels be thermally isolated from one another in order to avoid any cross talk. This allows each pixel to maintain its independence and create a high contrast array.
The ability to distribute and appropriately transmit heat is of the utmost importance in designing the pixel. The ability to dissipate heat when the heated pixel is turned off clearly affects the thermal time constant and speed of the pixel. When designing the microstructure itself, heat dissipation considerations must always be taken into account.
Current microstructures for IR pixels generally can take on several configurations. Each of these configurations however has several consistent features, all related to the manufacturing process. As can be expected, the microstructures used for these infrared pixels are fabricated using thin film processes. Consequently, appropriate films and masks are used in a multi-step process to achieve the appropriate microstructure configuration.
One example of a manufacturing process for creation of these microstructures starts with integrated read out and drive circuitry, previously fabricated on a wafer. This wafer is then finished to have a planar top surface. Next a reflective layer is placed upon this finished top surface. Upon the reflective layer is deposited a sacrificial layer with connection posts at appropriate points to allow communication with the integrated circuit. Upon the sacrificial layer is deposited the actual microstructure itself which includes appropriate layers of resisted material and silicon nitride. When the sacrificial layer is then removed, the microstructure is left, appropriately coupled to the control electronics via connective posts and positioned slightly above the reflective layer. Generally speaking, the active emitter or detector material is encapsulated within an insulating film, such as silicon nitride. This coating provides for mechanical support and chemical passivation. The pixels have a uniform height, generally equal to the height of the silicon nitride layer.
The above-referenced manufacturing process creates pixels of adequate operation, however, their design is not optimized. It is desired to provide the ability to increase the speed at which the pixels can operate without compromising other performance criteria.