Tunable Microstrip Patch Antenna - Essay Example

Critical Review of Article En d “Frequency Tunable Microstrip Patch Antenna Using RF MEMS Technology”, by Erdil et al. Published at IEEE Transactions on Antennas and Propagation, Vol. 55, 2007 In communications high frequency circuits may benefit from the arrival of the RF-MEMS technology. The RF MEMS acronym stands for radio frequency micro-electro-mechanical systems. There is a significant potential for improving inductors and tunable capacitors applying MEMS and NEMS technologies. The integration of such electrical compounds within a small MEMS device will achieve a significant reduction of size, power consumption, and cost of production of such devices. In addiction, the development of mechanical microswitches, which accomplishes this technology, is a very important component for building various microwave-controlled circuits for many important applications not only in RF MEMS devices but also for many high-throughput screening bioarrays. The demonstrated mechanical switches (microwaves) by Emre Erdil et al. have improved quality factors than pervious works, which is an important factor in building reliable RF MEMS devices. In terms of MEMS production the basic building blocks are deposition, lithography, and etching. The process of deposition is associated with making thin layers/films of certain martial. The thickness of the film can be anywhere between a few nanometer to about 100 micrometer depending on the concrete task. For MEMS deposition physical or chemical procedures can be applied. Electrodeposition or physical vapor deposition (PVD) is the physical procedures for a MEMS production. Four different chemical processing used for MEMS production are chemical vapor deposition (CVD), electrodeposition, epitaxy, and thermal oxidation. The created film by physical or chemical deposition methods is subsequently etched using lithography or etching methods to form the desired structure of patterns. The lithography in a MEMS production is usually a method for transfer of a pattern to a photosensitive material by selective exposure to a radiation source. The etching can be either wet or dry. The wet etching technology is simple and widely used technology, in which silicon wafer is exposed to potassium hydroxide (KOH) and a mask is used to restrict the etching in certain areas that are indented to stay intact. The wet etching could be either anisotropic or isotropic. Anisotropic etching in contrast to isotropic etching means different etching rates in different directions in the deposed material. There are three dry etching techniques called reactive ion etching (RIE), sputter etching, and vapor phase etching. In the paper by Erdil et al. entitled “Frequency Tunable Microstrip Patch Antenna Using RF MEMS Technology”, published at IEEE Transactions on Antennas and propagation, Vol. 55 from 2007, a RF MEMS design is employed for producing frequency tunable microstrip patch antenna using micromachining processing. The RF MEMS devices consists of a patch antenna, a microstrip connecting the antenna to a coplanar waveguide (CPW) stub via MEMS capacitors as shown in Figures 1 and 2. The application of MEMS-based capacitors is one of the main advantages in RF MEMS designs because it significantly reduces the power consumption of the whole unit compared to the conventional frequency tunable antennas. The design and the MEMS implementation of capacitors represent the main contribution made by this article. It is built bridge type MEMS capacitors. This means that the attenna’s microstrip is attached to the CPW via several micro bridges as schematically shown in Figure 1. A SEM image of the loading section of the antenna into capacitors and a magnified image of bridges are shown in Figure 3. A standard micromachining fabrication procedure is employed for the production of RF MEMS frequency tunable antennas. A monolithically technology is used for pattern formation on a 1 µm tick aluminum layer on 500 µm-thick Pyrex 7740 glass substrate. Fabrication begins with sputtering of a 100/3000 A-thick Ti/Au film that is used as a background layer for electroplating of gold layer. An electoplatting method is employed for deposition of 2 µm thick gold layer on the regions defined by the mold SPR 220-3 photoresist. The left Ti/Au film is etched using a wet technique. The next step is coating of Si3N4 layer as dc isolation layer using plasma enhanced chemical vapor deposition technique and patterned RIE technique. After spin-coating of photosensible polyimide, PI2737, an aluminum layer is sputter-deposited and patterned as a structural layer. The manufacturing process is completed by dry etching of the sacrificial layer in O2 plasma. Despite the use of several depositions and etching procedures the technology applied is relatively simple due to the use of monolithically for patterns formation. Moreover, the authors claim that they present the first RT MEMS design for a frequency tunable microstrip patch antenna operating at low dc voltages. This suggests that the manufacturing procedure is relatively simple and inexpensive. The results obtained (Figure 5) demonstrate that the antenna resonant frequency can continuously be changed from 16.05 GHz down to 15.75 GHz. At the same time, the actuation voltage is increased from 0 to 11.9 V. In addition, a light-interferometer microscope is used to view bridges of MEMS capacitors (Figure 4). Note that there is discrepancy between the amplitudes of the reflection coefficients in the simulations (2.5 x 107 s/m) and measurements (2.8 x 107 s/m). The difference between the amplitudes of the reflection coefficients in the simulations and in the measurements is explained by an effect of SMA connector used in the measurements, but not taken into account during simulations. The last results of the paper are depicted in Figures 6 and 7. Figure 6 presents simulated and measured radiation pattern of the antenna when switched at the up position while Figure 7 shown the results when switched at the down position. In both cases the simulated and measured patterns are pretty closed. A main point of this paper apart from the design of MEMS capacitors is the reconfigurability of the operating frequency of the microstrip patch antenna. This is achieved by periodically placing variable MEMS capacitors that are connected to the CWP. That means that this design can be easily adapted to different operating frequency of the antenna during the producing process. However, once produced, the operating frequency of the antenna cannot be changed on the field. There is not possibility for a reconfiguration by a replacement of the antenna. The working range of the antenna can be increased either by changing the tuning range of the MEMS capacitors or by using variable capacitors at different places during the production. This is a proof-of-principle work that demonstrates the feasibility of a RF MEMS design, in which the electrical length of the stub is adjusted as the MEMS capacitors are controlled via dc actuation voltage. This is important to note that the variable stub applied, allows frequency shift 300 MHz, while keeping the radiation pattern stable under dc bias. The size of the MEMS device can be further reduced be making smaller CWP. Here additional hole between the between CPW ground and microstrip ground has to be introduced to have a good transition. That may complicate the production of this RF MEMS device and make a bit expensive. The present design is an improvement over the RF MEMS devices etched on a silicon wafer because it possess better technical characteristics due to the new materials applied. Low resistance ohmic contacts of the basic layer used in the RF MEMS antenna are desirable because they are going to reduce the overall power consumption of the device and will make it more sensitive. In this regard, the application of Ti/Au sputtering for a creation of a basic layer is a better than directly etching on a silicon wafer. After wet etching silicon has relatively high resistance ohmic constants. Nonetheless, silicon wafers are used for implementations of first RF MEMS devices. Wet etching in silicon is a straight forward approach and still in use in many applications, in which low resistance constants are not of importance like microfluidics. However, in an engineering effort to improve RF MEMS devices new designs with better suited materials are employed as shown in the paper discussed. At the present time, R.F. sputtering of Ti/Au layer on a glass surface is a well established and wide–spread procedure. In addition, this procedure seems to be well utilized from the authors of the paper as mentioned by them. In general, a Ti/Au layer has relatively low resistance ohmic constants. However, please note that the low resistance ohmic contacts of a Ti/Au layer are achieved upon post-deposition annealing, which reduce them from 6.2 × 10-3 to 1.2 × 10-6Ωcm2. Note that the post-deposition annealing of Ti/Au layer is commonly applied when diamond surface rather than on a glass surface. It is not mentioned in the paper that post-deposition annealing process is employed. Therefore, it should be assumed that a post-deposition annealing is not employed. In order to improve the resistance constants on key spots in the RF MEMS devices a golden layer is deployed where needed by using an appropriate mask. The silicon layer, introduced after it, is needed as a dissociated layer. A dissociated layer is applied to isolate the desired part of the devices from communicating with other parts and to implement the desired logical circuit. The manufacturing process is completed by dry etching of the sacrificial layer in O2 plasma because the silicon layer left has to be preserved and an aluminum background layer is deposited after it. As the authors state this paper is a just proof-of-a-principle work as certain optimizations of the design, materials, and the manufacturing procedure may be needed before becoming a real world product. Recent studies have demonstrated that sputtering a Cr/Au instead of a Ti/Au layer yields better results in terms of resistance constants. It makes sense to investigate if a Cr/Au sputtering can be applied to simplify the production procedure by avoiding some steps like the Au coating. In addition, the glass surface used in the paper can be replaced by a silicon wafer and Cr can be directly deposit on it. However, the silicon on glass (SOG) module has been developed to integrate CMOS and high-aspect ratio MEMS sensors and actuators. Thus, if high integrated and application CMOS are needed one has to apply the glass procedure. The size of the CWP can also be reduced, which will cost of production. The antenna’s micostrip can also a subject for further optimizations in terms of its dimensions. Read More

The design and the MEMS implementation of capacitors represent the main contribution made by this article. It is built bridge type MEMS capacitors. This means that the attenna’s microstrip is attached to the CPW via several micro bridges as schematically shown in Figure 1. A SEM image of the loading section of the antenna into capacitors and a magnified image of bridges are shown in Figure 3. A standard micromachining fabrication procedure is employed for the production of RF MEMS frequency tunable antennas.

A monolithically technology is used for pattern formation on a 1 µm tick aluminum layer on 500 µm-thick Pyrex 7740 glass substrate. Fabrication begins with sputtering of a 100/3000 A-thick Ti/Au film that is used as a background layer for electroplating of gold layer. An electoplatting method is employed for deposition of 2 µm thick gold layer on the regions defined by the mold SPR 220-3 photoresist. The left Ti/Au film is etched using a wet technique. The next step is coating of Si3N4 layer as dc isolation layer using plasma enhanced chemical vapor deposition technique and patterned RIE technique.

After spin-coating of photosensible polyimide, PI2737, an aluminum layer is sputter-deposited and patterned as a structural layer. The manufacturing process is completed by dry etching of the sacrificial layer in O2 plasma. Despite the use of several depositions and etching procedures the technology applied is relatively simple due to the use of monolithically for patterns formation. Moreover, the authors claim that they present the first RT MEMS design for a frequency tunable microstrip patch antenna operating at low dc voltages.

This suggests that the manufacturing procedure is relatively simple and inexpensive. The results obtained (Figure 5) demonstrate that the antenna resonant frequency can continuously be changed from 16.05 GHz down to 15.75 GHz. At the same time, the actuation voltage is increased from 0 to 11.9 V. In addition, a light-interferometer microscope is used to view bridges of MEMS capacitors (Figure 4). Note that there is discrepancy between the amplitudes of the reflection coefficients in the simulations (2.

5 x 107 s/m) and measurements (2.8 x 107 s/m). The difference between the amplitudes of the reflection coefficients in the simulations and in the measurements is explained by an effect of SMA connector used in the measurements, but not taken into account during simulations. The last results of the paper are depicted in Figures 6 and 7. Figure 6 presents simulated and measured radiation pattern of the antenna when switched at the up position while Figure 7 shown the results when switched at the down position.

In both cases the simulated and measured patterns are pretty closed. A main point of this paper apart from the design of MEMS capacitors is the reconfigurability of the operating frequency of the microstrip patch antenna. This is achieved by periodically placing variable MEMS capacitors that are connected to the CWP. That means that this design can be easily adapted to different operating frequency of the antenna during the producing process. However, once produced, the operating frequency of the antenna cannot be changed on the field.

There is not possibility for a reconfiguration by a replacement of the antenna. The working range of the antenna can be increased either by changing the tuning range of the MEMS capacitors or by using variable capacitors at different places during the production. This is a proof-of-principle work that demonstrates the feasibility of a RF MEMS design, in which the electrical length of the stub is adjusted as the MEMS capacitors are controlled via dc actuation voltage. This is important to note that the variable stub applied, allows frequency shift 300 MHz, while keeping the radiation pattern stable under dc bias.

The size of the MEMS device can be further reduced be making smaller CWP. Here additional hole between the between CPW ground and microstrip ground has to be introduced to have a good transition.

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