Detectors and Readout
3.2 S PIDER Detector Architecture
These three approaches are detailed in [17] in the context of technology development for a space mission.
Bolometers themselves are largely incoherent; they must be coupled to incoming radia- tion by a beam-forming element. InSpider, this function is performed by dual-polarization slot antenna arrays (details, Figure 3.4). Every pixel contains one phased array of slot dipole antennas for each of the two measured orthogonal polarizations. The two polarized antenna arrays are interleaved and perfectly cocentered. Rotating one of the slot arrays 90◦ about the center of the antenna exactly reproduces the slot array for the other polarization.
The measured fractional bandwidth of the antenna is30%. Between the antenna and the bolometer, an in-line 3rd-order Chebyshev microstrip filter (Figure 3.5) crops the frac- tional bandwidth to 25%. These LC filters are constructed from coplanar waveguide (CPW) inductors and stub capacitors. The capacitance of the stubs depends on the thickness of the interlayer dielectric (SiO2 layer between the stubs and the niobium ground plane). Sig- nificant changes to a band center require scaling the antenna. Smaller changes can be made by substituting an alternate filter design or by modifying the thickness of the sputtered interlayer dielectric to alter the capacitance of the filter and shift the band.
The bolometer island is suspended on silicon nitride (Si3N4) legs. Because the niobium ground plane is cut out around the island when it is isolated, the island is not shielded from direct coupling with incident radiation. The long meandered design of the silicon nitride legs (Figure 3.6) provides a small thermal conductance within a narrow geometry, minimizing the gap between the ground plane and the long edge of the island.
After passing through the band-pass filter, the trunk of the niobium microstrip sum- ming tree from the antenna bridges on to the island where it terminates on an open-ended resistive gold meander which efficiently dissipates the electrical power on to the island as heat (Figure 3.7). The dissipated heat is measured by a thin film of titanium, a thermistor called a Transition Edge Sensor (TES). By voltage biasing the TES, the island is main- tained at a temperature in the transition between the superconducting and normal states of the titanium where it is highly sensitive to small temperature variations. The basics of bolometer operation, and of TES bolometers in particular, are reviewed in§3.4.
In designing our bolometers, we prefer to optimize their performance for the anticipated in-flight loading conditions. However, a superconducting bolometer that is optimized for flight loading conditions will saturate under room temperature loading. This would frustrate spectroscopy and beam mapping measurements. To circumvent this problem, we use a dual- TES architecture. Titanium and aluminum TES’ are wired in series on the same island.
Nb microstrip Slot antenna
Coupling capacitor Tap
Figure 3.4: SEM micrograph of a portion of a slot antenna array. Radiation excites electric fields across slots cut out of the niobium ground plane. Taps running over top of these slots couple to radiation polarized orthogonally to the long edge of the slot. Two interleaved perpendicular sets of slots comprise independent arrays. For each array, the tapped signal from all of the slots is added together by a niobium microstrip summing tree to form a single compound antenna. The summing tree is a network of tapered microstrip lines which meet at T-junctions to combine the signals from the taps at equal phase and amplitude. Future implementations of the summing tree may taper the amplitude of the excitation of slots to reduce sidelobe response. To match the (primarily real) impedance of the microstrip summing tree, a coupling capacitor at each tap compensates for the reactance induced by tapping the slots off center. The array is backside illuminated through the silicon substrate, so the summing network and other RF components are shielded from incident radiation by the backplane. The coherent interference of signals from the array of slots forms a beam with a width of approximatelyλ/dwhereλ is the wavelength in vacuum andd is the size of the antenna array. Our slot antenna arrays are square, with d≈7.2 mm for 148 GHz devices.
Coplanar waveguide (CPW) inductor Stub capacitor
Figure 3.5: The detector passband is defined by a photolithographed 3rd order Chebyshev LC filter, imaged here by a scanning electron microscope.
Figure 3.6: Long meandered silicon nitride legs suspend but thermally isolate the bolometer island. Thin rails along the outside of the meander make the structure more robust.
Si3N4
legs
Al TES
Ti TES
Hole for etch
Au meander
Microstrip from antenna TES bias
lines
Figure 3.7: Optical micrograph of a bolometer island. Titanium and aluminum TES’ on the island are wired in series. The niobium TES bias lines and antenna line run along the low conductivity silicon nitride legs that thermally isolate the island. The island is 375μm×150 μm. Holes improve the efficiency of the XeF2 etch which isolates the island during fabrication.
9ELDV>P9@
, 7(6>μ$@ 7LWUDQVLWLRQ
$OWUDQVLWLRQ
Figure 3.8: I-V curve showing both the Ti TES and Al TES transitions. The TES bias circuit is described in§3.4.
Aluminum has a much higher transition temperature (1.2 K in bulk or about 1.3–1.4 K for our films), so it is reliably deep within its superconducting region under flight conditions, and thus does not impact science mode operation. Under room temperature loading the Ti TES is normal. However, a bias current can be selected to put the aluminum TES on its transition, enabling effective bolometer operation. The Al TES’ are designed to have normal resistances more than ten times larger than the those of the Ti TES’ so that the parasitic resistance from the normal state Ti will not grossly impair their operation. The measured properties of the aluminum TES in engineering grade tiles are tabulated in Appendix B.1.