- Open Access
Fabrication of an Au–Au/carbon nanotube-composite contacts RF-MEMS switch
© The Author(s) 2018
- Received: 13 October 2018
- Accepted: 30 October 2018
- Published: 7 November 2018
This study aims to propose a new fabrication method for an ohmic contact type RF-MEMS switch with Au–Au/carbon nanotube (CNT)-composite contacts. These contacts are proposed to improve the reliability and the high-power handling capability of the ohmic contact type RF-MEMS switch. During the fabrication, we applied Au/CNT-composite electroplated film to the contact area in the signal line, whereas Au electroplating was used to form contacts under the cantilever of the switch. We investigated the influence of the Ar ion beam etching angle on metal layers formation and optimized the Ar ion beam etching conditions to achieve a more reliable RF-MEMS switch. We designed, fabricated, and evaluated an RF-MEMS switch to demonstrate the feasibility of our new fabrication method. We successfully fabricated an Au–Au/CNT-composite contact RF-MEMS switch; it exhibited a low insertion loss of less than 0.7 dB at frequency up to 40 GHz and a cycle life 2.7 times longer than that of an Au–Au contact switch.
- RF-MEMS switch
- Au/CNT-composite layer
- Ar IBE process optimization
Radio frequency microelectromechanical system (RF-MEMS) switches are considered as promising devices for use in high frequency communication modules owing to their superior RF characteristics, which include low insertion loss and high isolation, in comparison to those of corresponding semiconductor-based devices [1–3]. Therefore, RF-MEMS switches are expected to replace conventional semiconductor switches, especially in 5G transceiver modules . Moreover, they are considered key devices in future satellite communication systems [5, 6]. However, RF-MEMS switches also have disadvantages that stem from their successive mechanical movement during ON–OFF switching. In particular, failure at contact points and self-actuation of ohmic contact type switches and capacitive type switches, respectively, are the main problems to be solved. In the case of ohmic contact type RF-MEMS switches, a failure in contacts are the most important factor limiting the devices’ life time.
Many studies have been conducted to reduce contact failure rates. These studies have revealed several failure modes and causes related to contact failures. One of them, which is the contamination by materials such as carbon, oxygen and organic polymers [7, 8], is considered a major cause of failure. Therefore, minimizing contamination during the fabrication process and developing hermetic and reliable packaging to prevent contamination from the environment when the switch is working are important objectives. Another major cause of failure in contacts is the inherent characteristics of the contact materials. Numerous materials that improve the lifetime of RF-MEMS switch contacts have been proposed. Among them, we have focused on the carbon nanotube (CNT) dispersed Au electroplated layer of contacts. The CNT is considered a suitable material for electromechnical contact. Specifically, CNT-to-CNT contact was investigated to achieve a reliable MEMS switching devices [9, 10] and applied to inertial microswitch . Moreover, the CNT has been reported to offer an advantage in terms of high-power handling capability . In previous work, however, the uniformity of CNTs in the cantilever, including that of CNTs at the contact points, has been poor and uncontrollable. In previous studies, the contacts were formed by CNT dispersed Au electroplating. In this electroplating procedure, the CNTs must be dispersed well in the electroplating solution before electroplating is performed. However, because the CNTs exhibit high cohesion in the solution, preparing an electroplating solution, where the CNTs are stably dispersed for an extended period is difficult.
Herein, we propose a new method to fabricate Au/CNT-composite electroplated film to achieve a more reliable ohmic contact type RF-MEMS switch. In the fabrication procedure, we applied CNTs to the contact metal layer and formed an Au layer on then. Moreover, to reduce fabrication failure rate, we optimized the sacrificial metal layer etching process used in the fabrication. We designed, fabricated, and evaluated an RF-MEMS switch to demonstrate the proposed new fabrication method. The performance of the Au–Au/CNT contact RF-MEMS switch was compared with that of a conventional Au–Au contact RF-MEMS switch as well.
In this study, we chose an ohmic contact type switch, which is less prone to damage from cosmic rays, because we expect the device to have aerospace applications . Electrostatic force for cantilever actuation was applied via a separated actuation electrode fabricated below the cantilever. The separated actuation electrode is represented as the bias electrode in Fig. 1. Additionally, the bias resistor was connected in series to the actuation electrode to suppress the insertion loss when the switch was ON. At the same time, the bias resistor improved isolation when the switch was OFF. In a previous study, we demonstrated the effect of a bias resistor effect on RF characteristics ; specifically, for an Au–Au ohmic contacts type RF-MEMS switch, the insertion loss improved to 58% when the DC bias line resistance was increased approximately 7.3-fold.
The Input and output ports are port1 and port2 in Fig. 1, respectively, and the pitch of the GSG probe is 150 μm. The dimensions of the cantilever were 60 μm × 150 μm, and its thickness was 5 μm. The cantilever was fabricated with an Au layer. The upper contact points were also formed with an Au layer on the cantilever. Two upper contact points were formed at the tip of the cantilever, and the contact point size was 2 μm × 2 μm. Although the upper contact points were fabricated with an Au layer owing to the ease of fabrication, the lower contact part was fabricated with the Au/CNTs-composite layer. To demonstrate the effectiveness of the Au/CNT lower contact layer, we also fabricated an RF-MEMS switch with an Au lower contact layer and compared its characteristics with those of the RF-MEMS switch with an Au/CNT lower contact layer.
To fabricate the RF-MEMS switch having Au/CNT-composite contact as the lower contact, we first attempted to obtain a CNT layer by spraying CNTs onto the substrate. We then deposited an Au layer onto the sprayed CNT layer. To deposit the Au layer, two methods were used, sputtering and electroplating. We attained a stable layer using electroplating, and therefore used this method when fabricating the switch. The RF-MEMS switch was fabricated using surface micromachining. As Ni metal layer was used as a sacrificial layer to reduce organic material contamination during the fabrication process.
Fabrication of Au/CNTs composite layer
Prior to the fabrication of the RF-MEMS switch, we conducted basic experiments to achieve a stable Au/CNT-composite layer. During the deposition of the CNT containing layer, CNTs are easily agglomerated. Since CNTs are fibrous substance, they easily entangle with each other. Once they agglomerate, they cannot be released easily, and forming a uniform layer with agglomerated CNTs is extremely difficult. Therefore, the film containing CNTs must be kept in a released state, and the formation must be carried out while they are separated from each other. To avoid fabrication problems associated with CNT cohesion, we sprayed CNTs onto the lower contact Au layer. To spray the CNTs, we used a spray coater. The CNTs used for spray coating were prepared via the super-growth method  and were dispersed in an organic solvent using a homogenizer. When the CNTs are dispersed in a solvent, they can be chemically modified to exhibit hydrophilicity. Additionally, their dispersibility may be improved by adding a surfactant. However, when the CNT containing layer is used as the contact part in the switch, the aforementioned materials and modifications may contaminate the contacts and affect the devices’ lifetime. During the fabrication process, we used an organic solvent composed mainly of N,N-dimethylformamide (DMF), which is volatile and eventually not retained. However, even with this method, maintaining the CNTs in a dispersed state for an extended period is difficult. Therefore, we repeated the process frequently using ultrasonic vibration for redispersion.
Fabrication process of the RF-MEMS switch
First, as shown in Fig. 3a, on a high resistivity Si substrate (electrical resistivity: > 1 kΩ cm; diameter: 4 in.; thickness: 500 μm) with 1 μm thick thermal oxide film, the base metal layer composed of Ti/Au was deposited. The thicknesses of the Ti layer and the Au layer were 50 nm and 300 nm, respectively. The Ti layer functions as both an adhesion layer and a resistive bias line. We then formed the Au/CNT-composite layer as described in “Fabrication of Au/CNTs composite layer” section. The photoresist in Fig. 3a was prepared for Au electroplating on the sprayed CNT layer. The photoresist in Fig. 3a was then removed, and the base metal layers of Ti/Au were patterned. The Ti/Au layers were etched with the Ar ion beam etching (Ar IBE) technique. We then obtained the Ti resistive bias line after wet etching (etchant: AURUM-304) of the Au layer on the bias line (Fig. 3b). As shown in Fig. 3c, a 1.5 μm thick Ni sacrificial layer was deposited by sputtering and then etched using the Ar IBE method. We then successively formed 0.75 μm deep dimples for the formation of contact points under the cantilever; the dimples were also fabricated by the Ar IBE method. The Au layer of the cantilever was formed by electroplating, as shown in Fig. 3d. Before electroplating, a 100 nm thick Au seed layer was sputtered and a photoresist (PMER P-LA900PM) for electroplating was patterned. The electroplated Au layer thickness was 5 μm. Finally, we removed the photoresist for electroplating and used Ar IBE to etch the part of the Au seed layer not involved in the formation of the Au electroplating layer. The Ni sacrificial layer was then removed. To prevent contamination by organic compounds, hydrochloric acid and ferric chloride, which are inorganic etchants, were used to etch the Ni layer. After the Ni sacrificial layer was etched, the solvent was replaced with isopropyl alcohol (IPA) and the CO2 supercritical drying method was used to prevent stiction of the cantilever. In Fig. 3e, the fabricated switch is described. The gap between the cantilever body and the lower contact layer was 1.5 μm; however, the distance from the contact point surface formed on the cantilever to the lower electrode was 0.75 μm as shown in magnified of Fig. 3e.
Investigation and optimization of Ar ion beam etching process
In the fabrication process, Ar IBE method was used to etch metal layers except the Au layer etching on the Ti resistive line layer. The Ar IBE method is an anisotropic etching method, which can be applicable various metals because this method is using only an Ar ion bombardment onto the material. In the case of the Ar IBE method, the Ar ion beam incident angle is the most important process parameter and should be selected to reduce re-adhesion of etched metal onto the patterned photoresist side wall. The re-adhesion of etched metal forms a thin metal wall, which is difficult to remove and causes an electrical short.
Ar ion beam incident angles investigated in Ar IBE process
Base metal layer
Herein, we proposed a new fabrication method for Au/CNT-composite contacts to improve the reliability of ohmic contact type RF-MEMS switches. In the fabrication process, to prevent the CNTs from agglomerating, we sprayed the CNTs and then electroplated Au to fix the CNTs and obtain an Au/CNT-composite layer. Moreover, the Ar ion beam incident angle during the Ar IBE process for etching metal layers was investigated and optimized. We obtained deduced that a 45° Ar ion beam incidence angle led to the best result except in the case of etching the Au seed layer. For etching the Au seed layer, we deduced that the Ar ion beam incidence angle should be 0°. By applying both fabrication processes, we fabricated an RF-MEMS switch with Au–Au/CNT-composite contacts.
We performed S-parameter measurement and preliminary life cycle tests. The isolation was sufficiently high, and the insertion loss was less than 0.7 dB at frequencies up to 40 GHz. The Au–Au/CNT-composite contact RF-MEMS switch exhibited a longer life cycle than the Au–Au contact RF-MEMS switch. We demonstrated that the Au–Au/CNT-composite contacts can be considered a solution that improves the reliability and the high-power capability of ohmic contact type RF-MEMS switches. Improving the absolute number of life cycles, demonstrating the high-power capability, comparing results with other switches with different contact materials, and measuring switching speed will be considered as our future works.
SL and TK made substantial contributions to the conception, fabrication, measurement, summary, and drafting the manuscript. KS contributed to the conception and measurements, and LZ, JL, HT contributed to device fabrication. All authors read and approved the final manuscript.
The authors would like to thank the CNT-Application Research Center, AIST, Japan and the Center for Integrated Nanotechnology Support, Tohoku University, Japan for their offer of CNTs and support for CNT coating technology, and for their support for a part of the fabrication technology, respectively.
The authors declare that they have no competing interests.
Funding information is not applicable.
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