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Monday, 2 December 2019

Micromachined Antennas

Micromachine is a technique used in an antenna or a system for improving the performance of the system. The performance improvement includes increasing the gain, bandwidth, efficiency etc.  In other words, we can say that by using a micromachine technique, we can improve the radiation intensity of the antenna. We know that the surface waves will affect the radiation (fringing waves) of an antenna (since it will affect the major and minor lobes). By using micromachine technique, we can reduce the surface waves. Thus the major lobe and minor lobe levels can be increased.   

Micromachine

First we are considering a substrate over which we are creating a cavity. i.e., the removal of some portion of substrate for  providing the patch over the substrate. Micromachine are of two types.

(i) Surface micromachining

Surface micromachining means removal of portion (small) of substrate from the surface of the substrate for producing the pitch.

(ii) Bulk micromachining

Bulk micromachining means bulk amount of substrate can be removed from the substrate. In order to remove the substrate portion, we use the etching process. etching process is of two types.


Dry etching:

In dry etching, an evacuated chamber is used. In the evacuated chamber, the plasma is generated and this plasma is used to remove the silicon substrate.

Wet etching:

In wet etching, a durable mask is used. The region of the substrate which need to be etched is unmasked and all other portions are masked, where we have to undergo chemical reactions.


GaAs ( Gallium Arsenic) Substrate:
ɛr = 12.9

Thickness = 350 μm
Patch length = 1.21 mm
Patch width = 1.91 mm

The permittivity of the substrate determine the performance. The synthesised permitivity can be obtained as


Where,  ɛr_ syntheff is the effective synthesized permittivity and can be obtained as:
Where, K0 is the free space wave number and β is the propagation constant.


The effective synthesized permittivity can be obtained as:



From the graph, it is clear that as the microstrip width increases, the permittivity decreases. As permittivity increases, the performance also increases. Hence, in order to improve the permittivity, we must select the width of the microstrip as very small.

Micromachined slotline Antenna:

The micromachined slotline antenna can be easily integrated with 2 or 3 terminal active devices.
There are mainly 2 types of slot ring antenna.

(i) With trenches
(ii) Without trenches

GaAs FET activated by DC bias.

For fixed resonant frequency, a trenched one is having larger size than the untrenched. So there will be a higher radiation efficiency for the trenched one. E - plane and H - plane radiation power is about 1 to 2 dB higher for trenched.

Gate is used for control of flow of charge carriers from source to drain (S to D). The amount of fringing can be calculated by the applied voltage. Depending upon our requirement, we can increase the cavity by changing the DC applied voltage.




By applying the voltage, the capacitor can be trenched. For backside trenching, anisotropic etchant (KOH) is used. By doing like this, 550 tapered edges are produced. For slot line, isotropic etchant CP4 is used (mixture of nitric acid, hydrofluoric acid and water).

Microelectro mechanical system antenna:

The micro electro mechanical system antenna is used when a single antenna is used for a range of frequencies. This can be applied in telecommunication systems, radars etc. Since a single antenna is used for the entire system, we can reduce the size and cost of the system. This can be achieved by using micro electro mechanical switches or capacitors in antenna. The capacitors used here are cantilever type capacitors and fixed beam type capacitors.



The substrate used is glass type (ɛr = 400). The sputtering is done at 100/3000 A0. The seed layer is Ti/Au.



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