THz sheet beam traveling wave tube amplifier for microwave power module (MPM) application: MEMS-fabrications and characteristic analysis

Summary form only given. Terahertz vacuum electronic circuits require submillimeter features of a few microns to a few hundred microns, which pose significant challenges for conventional machining techniques due to tool size and fabrication tolerance requirements. Recently, MEMS techniques of nano CNC milling, lithography, etching, and deposition developed by the semiconductor industry have been applied to microfabrication and integration of vacuum electronic devices. Nano CNC Milling: The Mori Seiki NN1000 (developed by DTL, Davis CA) is a 5-axis CNC mill with nanometer command resolution. We have successfully used the NN1000 to make broadband vacuum electronic devices that transmit frequencies at 220 GHz with 60 to 80 GHz bandwidth and low attenuation (~ 6 dB for 40 mm long circuits). The design of our circuits has features as small as 115μm, and depths of 345 μm. After accounting for tool deflection, tool wear, and spindle vibration, we were able to hold a tolerance of ±0.5 μm, with surface finishes of 40 nm Ra. LIGA: The LIGA process is comprised of photolithography, electrodeposition, mold removal, and surface planarization. The lithography-patterned KMPR mold and the LIGA fabricated circuit on the OFHC substrate have undercuts of <; 10 μm at the corners of the vane array, a sidewall slope of >; 89°, and circuit dimensional accuracy of 3 5 μm. DRIE: The metalized silicon structure can also be formed using deep reactive ion etching technique. Photoresist is spun onto the silicon substrate at a thickness of 16 μm. This pattern is then etched through the full thickness of the 770 μm thick silicon wafer (full circuit height). This "through wafer" etch is accomplished using a deep reactive ion etcher with a repeating etch (SF6) and passivation (C4Fg) cycle, commonly referred to as the Bosch process. For quantitative analysis of the applicability of various micro processing techniques to- terahertz device applications, we measured the surface roughness on the sidewall of the processed samples using AFM. For example, the surface roughness of the LIGA-fabricated sample is 18 nm (rms) and 30 nm (max.) over the 50 X 50 μm2area. The circuit exhibited transmission in the frequency range 214 GHz 266 GHz with insertion loss fluctuating between 5 to 10 dB in general. Return Loss was a little high and fluctuated + 3 dB around 7.5 dB.