In recent years for the fabrication of millimetre wave circuits, the removal of the substrate has been proposed as a
solution for the reduction of losses, especially for silicon substrates. However, the micromachining of GaAs is an
exciting less explored alternative for manufacturing high performance communication systems. GaAs micromachining is
very interesting for the millimeter and submillimeter wave applications, due to the potential for easy monolithic
integration of passive circuit elements with active devices manufactured on the same chip.
This paper presents the monolithic integration of a two-director membrane supported Yagi-Uda antenna with a Schottky
diode, both having as support a 2 μm thick GaAs membrane. The design was based on the full-wave electromagnetic simulation software Zeland-IE3D. The following Molecular Beam Epitaxy (MBE) structure was grown on a
semiinsulating GaAs wafer: 0.2 μm thin AlxGa1-x As layer with x > 0.55 (the etch-stop layer) followed by a 2 μm Low
Temperature (LT) GaAs layer ("the membrane layer") and then by a 0.3 μm thin GaAs, (1x1018 cm-3-"ohmic layer").
Finally a 0.3 μm thin GaAs (1x1017 cm-3-"Schottky layer") was grown. An eight-mask process was developed for the
receiver manufacturing. The process includes some difficult steps regarding the integration of a very small Schottky
diode (with a diameter of about 3 μm) with the antenna with dimensions of a few millimeters, the polyimide-bridge
manufacturing, and the membrane formation using Reactive Ion Etching (RIE). The receiver characterization, including
the isotropic voltage sensitivity, was performed using "on wafer" measurements and has shown a good agreement with
the simulated results. High performance receiver circuits for operating frequency of 45 GHz have been demonstrated.
The technology developed can be used for applications up to THz.
GaN exhibits unique physical properties, which make this material very attractive for wide range of applications and
among them ultraviolet detection.
For the first time a MSM type UV photodetector structure was manufactured on a 2.2 μm. thick GaN membrane obtained
using micromachining techniques. The low unintentionally doped GaN layer structure was grown by MOCVD on high
resistivity (ρ<10kΩcm) <111< oriented silicon wafers, 500μm thick. The epitaxially grown layers include a thin AlN
layer in order to reduce the stress in the GaN layer and avoid cracking. Conventional contact lithography, e-gun Ni/Au
(10nm /200nm) evaporation and lift-off techniques were used to define the interdigitated Schottky metalization on the
top of the wafer. Ten digits with a width of 1μm and a length of 100μm were defined for each electrode. The distance
between the digits was also 1μm. After the backside lapping of the wafer to a thickness of approximately 150μm, a
400nm thick Al layer was patterned and deposited on the backside, to be used as mask for the selective reactive ion
etching of silicon. The backside mask, for the membrane formation, was patterned using double side alignment
techniques and silicon was etched down to the 2.2μm thin GaN layer using SF6 plasma. A very low dark current (30ρA
at 3V) was obtained. Optical responsivity measurements were performed at 1.5V. A maximum responsivity of 18mA/W
was obtained at a wavelength of 370nm. This value is very good and can be further improved using transparent contacts
for the interdigitated structure.
George Konstantinidis, Alexandru Mueller, George Deligiorgis, Ioana Petrini, Dan Vasilache, Dan Neculoiu, Michalis Lagadas, Cristina Buiculescu, Viorel Avramescu, Sergiu Iordanescu, Pierre Blondy
This paper presents the fabrication processes for micromachined millimeter wave devices on micromachined GaAs substrate. For the first time, a 2.2 micrometers thin GaAs/AlGaAs membrane, obtained by MBE growth and micromachining of semiinsulating <100> GaAs, is used as support for millimeter wave filter structures. Cascaded coplanar waveguide open-end series stubs filter type structures, with central frequency of 38 respectively 77 GHz were designed and manufactured on GaAs micromachined substrate. `On wafer' measurements for the filter structures were performed. Losses less than 1.5 dB at 38 GHz and less than 2 dB at 77 GHz have been obtained for both the silicon as well as for the GaAs based micromachined filters.
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