20.6: Design and Development of 100 GHz Folded Waveguide TWT Rajendra Kumar Sharma1, Arun Kumar Sharma1, Banshi Dhar Pant1, Sudhir M Sharma1, Suneeta Arya1, Manish Budania1, Isha Rathi1, Nagahanumaiah2, and Vishnu Srivastava1

1MWT Area, Central Electronics Engineering Research Institute (A constituent Laboratory of Council of Scientific

and Industrial Research), Pilani (Raj.)-333 031 (India). 2MicrosystemsMicro Systems Technology Laboratory, Central Mechanical Engineering Research Institute

(A Constituent Laboratory of CSIR, INDIA), Durgapur, West Bengal, INDIA. rks@ceeri.ernet.in, rksharma_830@rediffmail.com, Ph.:01596-252358, Fax: 01596-242294

Abstract: In the recent decade there has been a growing interest of the researchers all over the world in the design and development of the high frequency (in terahertz range), moderate power and efficient devices based on MEMS (semiconductor) and vacuum electronics technologies. Development of so-called vacuum microelectronic devices (VMDs) for the mm and sub-mm wave devices surmount the limitations of conventional microwave tubes (MWTs) as well as of solid-state devices. CEERI has started work towards the design and development of Folded wave guide TWT (FWTWT) initially for 100 GHz. This paper would report design and development work done for electron gun and RF structure of 100 GHz FWTWT. Keywords: THz devices, Folded Waveguide TWTs, VMDs, Electron gun, FEA. Medium power millimeter and sub millimeter wave sources find applications in high data rate communication, electronics materials, spectroscopy, medicine, biosciences, space research and remote sensing. Developments of micro electromechanical systems (MEMS) technologies have facilitated the fabrication of mechanical structures in micro-and nanometer sizes, which have motivated the researchers to investigate various vacuum microelectronics devices with very high operating frequency (> 100GHz), moderate power and efficiency. Among the various MEMS techniques, LIGA (deep etch X-ray lithography) has advantages due to its high aspect ratio, sidewall smoothness and miniature dimensions. CEERI, Pilani is an institution where both vacuum microwave power tubes and semiconductor devices technologies have existed for the last several decades. Recently, in our laboratory, the MEMS technologies have also been established for the successful development of pressure sensors, acoustic sensors, MEMs load cell, gas sensors, etc. To mention a few MEMS technologies, which we have established at CEERI, are: (a) bulk micro machining technology for single crystal silicon, (b) surface micro machining technology, (c) UV LIGA process, and (d) glass bonding. These technologies in coordination with complete CMOS fabrication line are being explored for the development of vacuum microelectronics devices.

In a TWT, SWS decides the bandwidth and the power handling capability of the device. At very high frequencies in terahertz range, planar circuit like folded waveguide type structure is preferred due to its relatively high power capability with moderate bandwidth (around 10%) and structural ruggedness along with ease of fabrications through MEMS and other micro-fabrication techniques like Micro-EDM and laser micro machining. Design of folded waveguide RF structure has been accomplished using CST-MW studio. RF structure of 100 GHz folded waveguide TWT (FWTWT) with input and output RF couplers (horn type) have been modeled in 3-D CST MW Studio (Fig. 1). The important dimensions like depth (a), width (b), height (h), period (p) and radius (r) are highlighted in Table 1. Initial estimation of the various dimensions has been obtained through synthesis. Cold test parameters have been simulated and optimized. Dispersion characteristics with and without beam hole of 800 micron are shown in Fig.2. Interaction impedance is shown in Fig.3 and transmission characteristic is shown in Fig. 4. Beam parameters of beam voltage 10kV, 50mA beam current and 200 micron beam radius have been chosen for the analysis. The designed RF structure has been fabricated through Micro-EDM and laser machining (Fig. 5).

For the development of 100GHz FWTWT, a 10kV electron gun using thermionic dispenser cathode has been designed using code EGUN and CST 3-D particle studio. CST-PS simulated electron gun is shown in Fig.6. A comparison between EGUN and CST simulated beam parameters has been given in Table 2. The gun has been developed and qualified for the temperature and emission characteristics. In addition, FEA cathode has been developed using reactive ion etching (RIE) technology. An array of silicon nano-tips (50nm) with a tip density 6.25 x 106 tips/cm2 (Figs. 7 and 8) was fabricated at CEERI using in-house facilities. Detailed simulated and experimental results for 100GHz FW-TWT will be presented in the conference.

978-1-4244-7099-0/10/$26.00 © 2010 IEEE 505

Fig.1: CST model of FWG RF structure with

Coupler (for 100 GHz)

Table 1: Optimized dimensions

For 100 GHz folded waveguide RF

structure

Dimensions in microns

Depth (a) Width (b) Height (h) Period (p) Radius (r)

1600 274

1396 528 264

9 9

1 0 0

1 0 1

1 0 2

1 0 3

1 0 4

1 0 5

1 0 6

1 1 . 2 1 . 4 1 . 6 1 . 8 2b e t a * p / p i

Freq

uenc

y (G

Hz)

W i t h B e a m H o l e

W i t h o u t B e a mH o l e

Fig. 2: Dispersion characteristics

Fig. 3: Interaction impedance

Fig. 4: Transmission characteristics.

Table 2: A comparison of EGUN and CST- particle studio (CST-PS) simulated results

Beam Parameters

EGUN Simulation

CST-PS Simulation

current 50 mA 51mA Perveance 0.05 μP 0.051μP radius 0.20 mm 0.21 mm

Fig. 5: Trial structure with micro EDM

and Micro milling

Fig. 6: 3-D simulation of electron gun using CST-

particle studio.

Fig. 7: Silicon tips FEA before oxidation sharpening.

Fig. 8: Image of Si tips after oxidation sharpening.

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