The geometric design of microbolometer elements for uncooled focal plane arrays

In this paper we discuss methods to improve the geometric design of microbolometer pixels in uncooled focal plane arrays. For cost reduction reasons, the pixel pitch of these microbolometer elements should be reduced as much as possible while keeping the same level of performance. This becomes increasingly difficult once the dimensions of the microbolometer elements reach a critical value of about 25 micrometers, mainly because the available space limits the thermal isolation and the available area for IR absorption. For these reasons it is essential to optimize not only the material properties but also the geometric aspects of the microbolometer structure to get the maximum performance for a given size of the elements. Extending the work of Liddiard [1], in the first part of this paper we discuss the design of the optical cavity, focussing mainly on the influence of the sacrificial layer thickness, which defines the properties of the resulting Fabry Perot resonator. In the second part of this paper we concentrate on the geometry of the absorbing membrane itself and give estimates for optimum film thickness and lateral dimensions.