Seog-woo Hong

Frequency shifts in two-level ultra-deep reactive ion etched slow-wave structures for 0.1 THz backward-wave oscillations

We present microfabricated slow-wave structures for millimeter- or terahertz-wave vacuum electronic sources. A two-level ultra-deep reactive ion etching (u-DRIE) on highly doped silicon wafers has been employed and allowed for complicated 3-dimensional structures with high aspect ratio. The measured spectra of return loss, however, show 1.2% and 6.8% upshifts in both cutoff and resonant frequencies, respectively. We found the suppression of two-level u-DRIE at the narrow channel between resonant cavities has caused the change of aspect ratios, i.e., saddle-shaped bottom surfaces, which is proved to be associated with the difference in frequency shifts as well as RF attenuation by comparison with theoretical prediction.

Return loss measurement of a microfabricated slow-wave structure for backward-wave oscillation

Precise measurement on the RF return loss was performed for a microfabricated slow-wave structure. The interaction circuit was designed to operate at 100 GHz of W-band frequency. A deep reactive ion etching (DRIE) showed a good side-wall profile but inaccurately curved bottom surface. The result represents that the etch rate was strongly dependent on the mask-opening area, which caused a frequency shift of about 5%.

Experimental observation of sub-terahertz backward-wave amplification in a multi-level microfabricated slow-wave circuit

In our earlier paper dealing with dispersion retrieval from ultra-deep, reactive-ion-etched, slow-wave circuits on silicon substrates, it was proposed that splitting high-aspect-ratio circuits into multilevels enabled precise characterization in sub-terahertz frequency regime. This achievement prompted us to investigate beam-wave interaction through a vacuum-sealed integration with a 15-kV, 85-mA, thermionic, electron gun. Our experimental study demonstrates sub-terahertz, backward-wave amplification driven by an external oscillator. The measured output shows a frequency downshift, as well as power amplification, from beam loading even with low beam perveance. This offers a promising opportunity for the development of terahertz radiation sources, based on silicon technologies.

Dispersion retrieval from multi-level ultra-deep reactive-ion-etched microstructures for terahertz slow-wave circuits

A multi-level microstructure is proposed for terahertz slow-wave circuits, with dispersion relation retrieved by scattering parameter measurements. The measured return loss shows strong resonances above the cutoff with negligible phase shifts compared with finite element analysis. Splitting the circuit into multi levels enables a low aspect ratio configuration that alleviates the loading effect of deep-reactive-ion etching on silicon wafers. This makes it easier to achieve flat-etched bottom and smooth sidewall profiles. The dispersion retrieved from the measurement, therefore, corresponds well to the theoretical estimation. The result provides a straightforward way to the precise determination of dispersions in terahertz vacuum electronics.

MEMS BASED BULK ACOUSTIC WAVE RESONATORS FOR MOBILE APPLICATIONS

ABSTRACT A silicon based film bulk acoustic wave resonator (FBAR) composed with a filter and a duplexer is fabricated using the bulk micro-machining process. It has a simple MIM (metal-insulator-metal) membrane structure using molybdenum (Mo) for the top and bottom electrodes. The bulk acoustic wave resonances are generated by the piezoelectricity of aluminum nitride (AlN) with an air gap cavity fabricated below the membrane by silicon deep-etch process to reduce acoustic loss of FBAR. The fabricated FBAR is measured with HP 8510C vector network analyzer in wide (0.5∼10.5 GHz) and narrow (1.7∼2.1 GHz) frequency range. The measured series and parallel resonance frequencies are 1856 MHz and 1907 MHz, respectively. The minimum insertion losses are less than 0.07 dB at the series resonance frequency. With the increase of the membrane area, insertion loss decreases and effective electromechanical coefficient increases. The measured effective electromechanical coefficients are higher than 6.4%. The circuit modeling of FBAR is preformed based on the MBVD (modified Butterworth Van-Dyke) model. The above results demonstrate that the fabricated FBAR has sufficient performance to be the building block of RF filters for mobile applications.

Experimental measurement of W-band backward-wave amplification driven by external pulsed signals

The experimental implementation of W-band backward-wave oscillator is achieved by using a multilevel microfabrication of interaction circuit including beam tunnel, slow-wave structure, and output transition, on deep reactive ion etched (DRIE) and metal deposited silicon wafers. The interaction circuit shows precise accuracy in full 3 dimensions, and the return loss measurement agrees well with HFSS simulation. Here we describe the experimental observation of W-band backward-wave oscillation and amplification after successful vacuum sealed integration of interaction circuit, electron gun, beam collector, and output window.

MEMS applied backward-wave oscillator for 0.1 THz

The microfabrication of a backward-wave oscillator is presented for a compact, high-power source in the terahertz range of electromagnetic spectrum. One of Micro-Electro-Mechanical Systems (MEMS) technologies, a deep reactive ion etching (DRIE), was employed and achieved the fully 3-dimensional accuracy of coupled cavities operating at 0.1 THz. The prediction and measurement on the scattering parameter of the interaction circuit showed a good agreement.

Sub-terahertz slow-wave circuits for coherent radiation sources

In this letter, we propose sub-terahertz (sub-THz) slow-wave circuits for coherent radiation sources through beam-wave interaction mechanism. The circuits are prepared using microfabrication in advanced silicon (Si) technologies. Our approach is to split the circuit into multi levels allowing a low aspect ratio configuration and alleviating the loading effect of deep-reactive-ion etching on silicon wafers. This makes it easier to achieve flat-etched bottom and smooth sidewall profiles in nanoscale accuracy for high frequency operation. The dispersion relation retrieved from the measurement, therefore, corresponds well to the theoretical estimation. In particular, the sub-THz radiation is successfully measured in pulsed operation through the vacuum-sealed integration of the slow-wave circuit with a 15-kV, 90-mA thermionic electron gun. This observation offers a promising opportunity for the development of terahertz radiation sources based on silicon micro- and nanofabrication technologies.

Enhanced RF performance in multi-tunnel backward-wave oscillators

We propose an efficient beam-wave interaction circuit employing a multi-tunnel, slow-wave structure for W-band backward-wave oscillators. The tunnel is disposed one of above and below the beam tunnel, which enhances RF characteristics. The interaction circuit is prepared using a deep-reactive ion etched (DRIE), multi-level microfabrication on silicon wafers. The return loss shows strong resonances predicted by finite-element method (FEM) simulations. The multi-tunnel interaction circuit demonstrates almost similar aspect in return loss to the circuit without beam tunnel. A 1.6 times increase in RF output power is estimated from the particle-in-cell calculation, when compared to the case without multi-tunnel structure. Therefore, we conclude that the multi-tunnel, slow-wave structure successfully improves RF performance.

Development of W-band backward-wave oscillator

The precise patterning of periodic slow-wave structures can be successfully accomplished by modern photolithography technology on flat substrates in high frequency regime (>;100 GHz). When the aspect ratio of the structure between in-plane and out-of-plane dimensions becomes higher than unity, however, controlled MEMS (micro-electromechanical systems) technologies are strongly required to achieve accurate depth profiles of slow-wave beam-wave interaction circuits. Here we report a W-band backward-wave oscillator using microfabrication technologies by which a fully 3-dimensional slow-wave interaction circuit is successfully employed on multi-bonded silicon wafers. The return loss measurement of the circuit appears to be very similar to the simulation, which indicates not only the dimensions but also the surface roughness is under control. A more detailed discussion on the design, fabrication, and RF test result will also be included.