Focal Plane Array Concept

The proposed array will utilize a array of fixed-backshort, waveguide mixers. An assembly diagram of the mixer array is shown below. The ``Horn Block'' consists of four, corrugated feedhorn subarrays. Each subarray is formed by bonding two gold plated, silicon wafers in which mirror images of the corrugated feedhorns have been laser micromachined. Alignment between the two halves of a horn subarray is insured by posts and corresponding holes wet-etched in each half before laser machining.

Each horn is micromachined with a reduced-height waveguide transition. The reduced-height waveguide provides an excellent match to the real part of the bolometer impedance, permitting the use of only a single, fixed backshort. Electromagnetic finite element analysis software (HFSS) has been used in conjunction with 5 GHz scaled models to determine the embedding impedance offered by the waveguide mount to the bolometer.

At high frequencies ( 650 GHz) it becomes nearly impossible to manually mount detectors across small waveguide structures. An alternative approach is to fabricate the mixing elements on  1 m thick silicon nitride membranes which subsequently are integrated with the horn structure. Co-I Lichtenberger (in association with Qing at MIT) has experience with the fabrication of superconductive mixers on thin SiN membranes in the developement of micromachined mixer imaging arrays. To date, this approach has been used succesfully with SIS junctions up to frequencies of  850 GHz (Kooi et al. 1996). As in this successful design, we will fabricate the bolometers on membranes several times larger than the waveguide aperture. These windows are formed by etching a pyramidal hole from the backside of the silicon wafer supporting the membrane.

As with the horn block, all exposed surfaces of silicon are gold plated to provide conduction. The bolometers are fabricated on rf probes in the center of the waveguide. Low pass filtering is provided by a suspended stripline circuit extending across the length of the membrane. Once onto the silicon substrate, coplanar waveguide is used to carry the IF signals to wire bonding pads on the edge of the Bolometer Array Block. Each IF output of the array is wire bonded to a microstrip matching network located just outside the periphery of the Bolometer Array Block. Bolometer bias is provided through the matching network.

Unlike the earlier work at JPL, which utilizes an ebeam liftoff process along with several self registered etching steps to fabricate the HEB elements, these HEB bolometers will be fabricated at the University of Virginia utilizing a state of the art foccused ion beam system (FIB) to carve each 50 - 100 nm sized Nb microbridge in a single step. Our FIB tool has a 3nm size beam which can be utilized to etch anisotropically through the thick Au and thin Nb layers with the required precision to carve these structures. It also has sensitive imaging and material endpoint detection capabilities to aid in the sculpting of these structures. Though conventional microstrip circuitry will be defined in subsequent lithographic and reactive ion etching steps, the FIB process will not result in the production of thousands of elements per wafer as would a standard lithographic process. However, there are two important reasons why this is not a drawback: (1) the geometry and layout of the mixer array is such that there are not nearly as many HEB mixers on a wafer as there might be for a conventional single element waveguide mixer design and (2) we plan to utilize a development strategy where wafers are frequently fabricated with processing modifications in response to feedback from RF evaluation. The FIB process is ideally suited to such a quick but precise fabrication strategy with the added flexible benefit that each HEB element can be tailored and varied as desired since they are not processed simultaneously.

The fixed rectangular backshort is laser micromachined on a pyramidal structure designed to fit the cavity behind the membrane. The pyramidal structures can be readily made by wet-etching silicon through an SiO mask evaporated on the wafer (Rebeiz et al. 1987). Once etched, the Backshort Wafer is gold plated.

We have optimized the design of the waveguide structures and IF circuitry using Hewlett Packard's High Frequency Structure Simulator (HFSS) and Microwave Design System (MDS). The performance of the array will be optimized in the lab by trying different backshort lengths.