1 Introduction

MEMS devices are fabricated utilizing microfabrication technology. These systems are usually fabricated through micromachining processes that deposit new layers on the substrate and/or selectively etch away parts of the substrate or the deposited layer to form the mechanical and electro-mechanical structures. Various physical and chemical processes can be used in order to deposit a thin layer on substrate. Famous methods for creating a thin layer on substrates are thermal evaporation, Electron Beam (E-Beam) evaporation, sputtering, spin coating, spraying, Plasma Enhanced Chemical Vapor Deposition (PECVD), Low Pressure Chemical Vapor Deposition (LPCVD) and Metal Oxide Chemical Vapor Deposition (MOCVD) (Maluf and Williams 2004). Most of the abovementioned methods need high vacuum conditions, precise and complex instrument and high quality materials. In this paper, a simple fabrication process is presented which can be used to deposit thin and thick Nickel–Phosphorus layers on substrates. Electroless of Ni–P is presented as a deposition method for metal layers deposition by a controlled chemical reaction.

Electroplating has been used to deposit some metals such as Cu, Ni, Au and Pd in MEMS fabrication processes (Al Farisi et al. 2018; Sun et al. 2017; Li et al. 2011; Sankar et al. 2011; Becker et al. 2001; Fritz et al. 2002; Chen et al. 2008). Electroless plating does not need any external electrical current, unlike electroplating method which needs a current source. Electroless plating and electroplating have been used in making interconnections between metal layers in the semiconductor industry, too. These techniques provide a simple and cheap technology with superior material properties and device performances which allows fabrication of relatively thick metallic structures using a simple process (Wang et al. 2019; Murakami et al. 1995; Lin et al. 2007).

Various MEMS devices such as pressure sensors, thermal actuators, microcoils, micromotors and microstructures based on plated metals have been fabricated in recent years (Du et al. 2016; Brewer et al. 2017; Venkateshwaran et al. 2019; Dittmer et al. 2010; Chaudhuri and Bhattacharyya 2013). RF MEMS switches are one of the MEMS devices which can be made by electroplating techniques, too. We have reported some of the RF MEMS switches in our previous work (Saffari et al. 2019, 2017a, b). A DC-contact RF MEMS switch was fabricated by Sun and et al. in order to improve the temperature stability. A thick gold layer with thickness of 7 μm was electroplated as transmission line in this switch (Sun et al. 2015). Park et al. fabricated an RF MEMS switch that has a low insertion loss of 0. 08 dB at 10 GHz, isolation of 42 dB at 5 GHz. This switch has high current carry capability due to the use of electroplated Au or Cu (Park et al. 2000). Figure 1 shows a fabricated switch by electroplating process which presented by Zavracky et al. (1999). They discussed about surface micromachined micro switches and micro relays that fabricated based on electroplated materials like Ni and Au.

Fig. 1
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SEM RF MEMS switch (Zavracky et al. 1999)

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A fabrication method was proposed for an ohmic contact type RF-MEMS switch which applied Au/CNT-composite electroplated film to the contact area in the signal line, and Au electroplating was used to form contacts under the cantilever of the switch (Kageyama et al. 2018). A new RF MEMS switch was developed by Patel which was capable of generating large forces under electrostatic actuation (Patel and Rebeiz 2010). This switch was fabricated using an 8 um -thick gold cantilever by electroplating technique. The mechanical properties of electroplated gold thin films such as Young’s modulus, residual stress, and stress gradient have been measured using micromachined beam structures by Baek et al. (2005). A set of test structures for in situ stress measurement was presented by Sharma (2011). The test structures were realized by means of surface micromachining, adopting photoresist as sacrificial layer and electroplated gold as structural layer.

Krishnan et al. (2006) reviewed many research articles devoted to the engineering aspects of the electroless Ni–P depositions and their technology. Because electroless plating is a chemical reduction process, the deposited layers are of uniform thickness all over the plated sample. The physical and mechanical properties of the deposited layers are uniform because the composition of the coating is uniform at all thicknesses. They investigated the development of electroless Ni bath, advantages and mechanisms of deposition, and applications of the Ni–P deposits. They found that adhesion of electroless nickel layers deposited on some metals are better than electrodplated nickel layers. In addition these layers have good resistance against wear which makes them proper for RF MEMS switch applications. Hur et al. (1990) have been studies the microstructures and the crystallization behavior of electroless Ni–P deposits with various phosphorus contents, in particular, near the eutectic composition by transmission electron microscopy, differential scanning calorimetry and X-ray diffractometry. They have investigated the effects of different chemical compositions on deposited layer specifications. They used also some electroless nickle solutions for memory disc underlayer.

In this paper, electroless aqua solution and proper conditions, for deposition of Ni–P layer on insulator substrates are developed. An insulator composite material which contains fiber glass and epoxy resin or poly carbonate is used as substrate. For metal electroless, usually, substrates are conductive materials which let a thin layer to be deposited. But in this research, proper electroless conditions are achieved for Ni–P deposition on insulator materials like phenol cotton paper or fiber glass. The insulator substrates are usually used in RF and electronic Printed Circuit Boards (PCBs). As the first advantage of developing this process, it would be very favorable for communication and electronics engineers to fabricate RF MEMS switches on the same substrate which other circuit parts are mounted. The second advantage is ferromagnetic property in Ni–P layer. Ni is a ferromagnetic material which can be moved by applying a magnetic field. So, magnetic actuation can be applied to MEMS structures which have movable Ni parts. It would be beneficial if applying other actuation mechanisms like electrostatic, thermal or piezo are difficult.

2 Materials and methods

Electroplating metal layers like Cu, Ni, Au, and Pd have been discussed for MEMS fabrication in some research papers, already (Costello et al. 2013; Chung et al. 2010; Nogami et al. 1999; Marauska et al. 2013; Abys and Straschil 1991; Karmalkarz and Pradeep 2004). In electroplating external electrical power is connected to metal source and substrate which are soaked in electrolyte. This process cannot be applied to insulator substrates because the electrical current loop cannot be established. In this paper, Ni–P deposition on the insulator substrate is presented based on electroless technique. Unlike electroplating, there is not any external electrical source during electroless process. All substrate surface exposed to solution will be deposited by Ni–P atoms uniformly. In most of papers which described Ni–P electroless, conductor substrates have been considered. Usually, a metal layer is sputtered on the Si substrate to make the surface conductive (Wu et al. 2019). Then, it is soaked in Ni solution. Sometimes doping P in Si will increase the conductance of substrate and lets the Ni atoms to deposit on the surface. Usually, different combinations of Ni sources like NiCl2 or NiSO4 may be used in order to provide 7.5 g/L Ni concentration (Hieu et al. 2017). NaH2PO2 is reduction agent which added to solution. For the availability of chemicals in local, CH3COONa and Na3C6H5O7 were chosen as additives. As found in the research reports, acetates and citrates are complex agents that play three roles: (1) maintaining stable pH level, (2) preventing precipitation of nickel salts, (3) reducing the concentration of free nickel ions. Table 1 shows the recommended composition of bath for Ni electroless on Al substrate. In order to deposit Ni on Al surface, it is needed to zincates Al, at first (Hieu et al. 2017).

Table 1 100 ml Ni bath composition (Hieu et al. 2017)
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In the current research, the substrate is phenolic cotton paper or fiberglass sheet which are insulators. The surface should be etched and activated by some solutions to let Ni–P atoms to be deposited on the surface. The solution and procedure reported in Table 1 define Ni electroless deposition on metal surfaces. The metal substrates are conductive and cooperate in electron transferring and metal deposition. This procedure doesn’t work well for insulator substrates like poly carbonate or fiber glass. Insulator substrates do not transfer electrons and the Ni+ ions in the solution cannot get electrons to deposit on the surface, well. In this paper, four extra processes are presented to prepare the insulator surface for Ni+ ions deposition: surface cleaning, etching, activating and accelerating.

2.1 Surface cleaning

All organic materials and oil should be removed from substrate surface. In the first step, the substrate surface should be cleaned. 50 °C hot water and detergents are used to clean the surface. Then alkaline cleaner is used to clean the surface, well. NaOH and cyanide compounds are major materials in alkaline solution. The required time is 7–10 min and allowed pH range is 11.7–12. After surface cleanliness, it should be washed by Deionized (DI) water several times.

2.2 Surface etching

Usually, the substrate surfaces are polished and adhesion force between deposited layer and substrate is weak. In order to enhance Ni layer adhesion to substrate surface, the surface should be etched locally. Regarding to substrate material, proper etchants are selected. In our case, for fiber glass PCB board a solution of 5% sulphuric acid and 5% hydrogen peroxide is applied. The substrate should be soaked in the etchant solution for 10 min in 60 °C.

2.3 Surface activating

In order to activate the surface to react with electroless solution, activation phase is necessary. This step prepares the insulator surface to react with Ni+ ions in the electroless solution. Palladium-Tin colloidal solution is the chemical used in this step. This solution will cover the surface of the substrate with evenly distributed tin-palladium colloids; each of them has an inner core of palladium surrounded by large amount of hydrous tin chlorides. The absolute charge of colloid is negative which attracts positive Ni+ ions during electroless deposition. As found by several try and error experiments, the substrate should be placed and shake in solution for 10–15 min at 45 °C. Palladium concentration is 140 ppm, tin is 5 g/Lit and HCL acid normality is 0.65. Finally, the substrate is rinsed by DI water several times and next step is started.

2.4 Accelerating

After surface activation, the substrate can be immersed in Ni–P electroless solution. In this case, deposition rate is slow and it takes a long time to deposit thin Ni–P layer. In order to increase deposition rate, acceleration is considered as one process step. During acceleration, the substrate is immersed in accelerator solution for 4 min. It consists of Flourobic Acid (HBF4) and a concentrated solution provided by Azhineh Micro System Ltd. The substrate is soaked in the solution for 2 min at 40 °C. Then it should be rinsed by DI water several times to remove extra ions and materials. Now, the substrate is ready to immerse in the electroless solution.

2.5 Ni–P electroless

In this research, nickel–phosphorus solution is used to deposit Ni–P layer on the substrate. This layer is deposited from the electrochemical reduction of nickel ions through the autocatalytic plating process in the Ni–P solution. The major industries that have benefited from using electroless Ni–P deposits include electronics, aerospace, automotive, oil and gas, power plants and some chemical industries. The crystallization and phase transformation behavior are determined by the alloy composition and thermal processing conditions such as phosphorus content, heating rate and temperature (Ashassi-Sorkhabi et al. 2004; Taheri et al. 2004). In this research, Ni–P electroless deposition has been used for metal deposition. The solution composition and plating temperature have been shown in Table 2.

Table 2 Ni–P electroless solution composition
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3 Fabrication process for a DC contact RF MEMS switch

In comparison with other deposition methods, proposed electroless method has some advantages and disadvantages. As advantages, the proposed method can be applied to both conductive and nonconductive substrates. Also, it can be scaled up easily. Many substrates can be processed at the same time in contrast with sputtering or many other deposition methods which handle only a few samples at each time. It can be very promising for mass production of MEMS devices for industries. As a disadvantage of the proposed method, the layer is not as uniform as standard PVD or CVD methods. The quality of CVD or PVD deposition layers is better than the proposed methods. Briefly, the proposed method is useful for metal layer deposition when minimum feature sizes are larger than 10 μm and layer thickness should be equal or more than 1um. A good example for such MEMS devices is DC contact RF MEMS switch. Usually, dimensions of bridge or transmission line in RF switches are about 200–800 μm and 50–300 μm, respectively (Rebeiz 2004). So, in this part, a DC contact RF MEMS switch fabrication process is described using NI–P electroless method.

Summary of fabrication process is shown in Fig. 2. Cu is sacrificial layer and Ni is body material which constructs suspended bridge. Ground and transmission lines are fabricated by Ni–P material which is deposited by proposed electroless method.

Fig. 2
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Fabrication process for DC RF MEMS switch

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Step1 shows an insulator substrate which is covered by a thick Cu layer. Usually, the thickness of Cu is about 30  μm. In proposed fabrication process, most of the Cu is etched else some parts which are considered as sacrificial parts and connection pins for M8 RF connector. Substrate is covered by photoresist and exposed to UV light through Mask1. Step2 shows the patterned resist on Cu layer. In Step3, the Cu is etched by Iron Chloride solution. Most of Cu is detached from substrate and only two rectangular parts as sacrificial layer and some circles as connecting pads are remained. Two rectangular parts can be seen in auxiliary picture ad joint to step 3 in Fig. 2. In the next steps, Ni will be electroplated on two rectangular parts to fabricate suspended bridge. After Ni electroplating, Cu will be etched to suspend the Ni bridge.

In step 4, photoresist is spun and backed on the substrate for second lithography process. UV exposure is applied by mask aligner, through Mask 2. Then the pattern is developed in KOH solution. Step 5 shows the substrate after second lithography phase. As shown in Fig. 2, copper sacrificial parts are covered by photoresist. Also, some other parts on insulator substrate are covered by photoresist.

In step 6, proposed electroless process is run to deposit a thin layer of Ni–P on the substrate. 30–60 min is enough to deposit a thin layer of Ni–P on the substrate. After Ni–P deposition, the substrate is washed by acetone to remove photoresist. Undesired Ni–P which deposited on photoresist, is removed by lift off process. Figure 2, step 6 shows the final pattern after lift off. The deposited part between two rectangular Cu is transmission line.

In step 7, the photoresist is spun and backed to do third lithography process. Step 8 shows the substrate after third lithography process. Only a small rectangular window between two rectangular Cu parts on the RF transmission line is open, as can be seen in auxiliary picture in Fig. 2 step 9, too.

At step 9, Cu is electroplated on the Ni–P which is accessible through the abovementioned window. Cu is deposited on the space between rectangular Cu parts. Other surface area is covered by photoresist as described in step 8. An aqua solution of Cu++ ions like CuSO4 solution can be used for Cu electroplating. Electroplating current and time are two major factors which define layer thickness. These factors should be adjusted to deposit Cu as much as required because layer thickness defines the gap between bridge and RF transmission line.

At step 10, the forth lithography process is run. A small rectangular window is opened in photoresist which defines the size and location of bridge as can be seen also in auxiliary picture at step 11. At step 11, Ni is electroplated to fabricate the bridge. This layer covers Cu rectangular parts, Cu electroplated part and small parts of Ni–P ground layer. Ni can be electroplated by standard Ni electroplating methods. At step 12, Cu is etched in order to release the bridge. Both Cu rectangular parts and electroplated Cu will be removed. As shown in Fig. 2, step 12 the Ni bridge is suspended over the transmission line. The gap is equal to the thickness of Cu electroplated layer.

4 Experimental results

In most of the MEMS devices, there are some suspended parts which move to sense a variable or to actuate. In RF MEMS switches, usually there is a bridge that moves vertically or laterally whenever it is actuated. By applying actuation mechanism the bridge moves and does a function like connecting or disconnecting RF signal. There are some definite methods to fabricate a suspended part. One usual fabrication process is depositing bridge material on a thin sacrificial layer. Sometimes photoresist can be used as sacrificial layer and in some other cases a thin metal layer is used.

In this paper Cu is considered as sacrificial layer and Ni as suspended bridge material. Bridge is fabricated on the Cu parts and then it will be removed by wet etching process. At first, a copper fiber glass is selected as substrate and after lithography and etching Cu, Ni–P layer is deposited and patterned on it. Cu metal parts are considered as sacrificial layer in the fabrication process. A copper fiberglass substrate consists of an insulator material which is covered by a thick Cu layer.

At first, Cu substrate is patterned by standard lithography process. Positive photoresist is spun on the substrate for 60 s at 3000 RPM by spinner equipment. Shipley 1813 positive resist is used as photoresist. At spinning phase, the speed is increased slowly during 15 s and then remains constant for 60 s. Then it slows down during 15 s to be stopped completely. In this way, a uniform thin film of photo resist is spun on the substrate surface.

After spinning, the substrate is put on the heater at 90 °C for 2 min. In this phase, the spun resist is baked to remove organic solution which is inside the resist film. Baking is a standard process to harden the resist and remove solvent fraction.

After baking, a mask is aligned with substrate by Azhineh Microsystem Mask Aligner equipment. We have used contact exposure in order to transfer mask pattern into substrate in one by one size. The first mask contains pads and connections which are needed for future connection of M8 RF ports to the switch. The substrate and mask in contact are exposed to Ultra Violet (UV) light for 50 s by Mask Aligner. Then the substrate is soaked in resist developer for 30 s to remove unwanted parts of resist from the surface.

Now we have a copper substrate with some parts covered by photoresist. All contacts and pads locations are covered by resist. Now the substrate is soaked in Iron Chloride solution which is an etchant for copper. After 10–12 min the pattern is created on the substrate. All copper area is removed else the areas covered by photo resist which are copper sacrificial parts and connection pins. The substrate should be washed by acetone and Deionized (DI) water to remove resists from the substrate completely. The resist should be removed completely to prepare surface for the next steps.

Some copper parts in the first copper lithography process are considered as sacrificial parts. These parts will be covered by Ni–P in the next steps. It is possible to remove Copper sacrificial parts to suspend Ni–P parts over them. A simple example for suspended Ni–P part is clamped–clamped suspended bridge in DC contact RF MEMS switches.

As the second step, a thin Ni–P layer should be deposited on the substrate. There is copper on some parts of the substrate but in some other parts, the copper has been removed, totally. In our case, most of the substrate is fiber glass material which is an insulator. Depositing a thin Ni–P layer on the insulator is not possible by electroplating. So Ni–P electroless process presented in Sect. 2 is applied to fabricate a thin Ni–P layer on insulator substrate.

Regarding the presented electroless process, the substrate surface is activated by activation solution. Then, it is soaked in accelerator solution to fasten surface reactions which are essential for electroless process. Finally, the substrate is placed in Ni–P electroless solution at 90 °C for more than 30 min. The temperature is controlled by an automated Proportional Integral Differential (PID) controller which defines the time duration of applying 220 V AC input voltage to the heater. The temperature is controlled with three Celsius degrees resolution which is proper for electroless bath based on data in Table 2.

Figure 3 shows a fiber glass insulator substrate which is covered by Ni–P layer. The electroless time is almost 45 min. It should be noted that by increasing the time, thinner layers can be achieved.

Fig. 3
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deposited Ni–P layer and two copper sacrificial parts on insulator substrate

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As shown in Fig. 3, there are two rectangular copper parts which are not covered by Ni–P. These copper parts are left during first lithography step. Then they are covered by photo resist to prevent Ni–P deposition on them. At the next step, Ni–P layer is deposited by electroless process as described before. The copper parts are considered as the sacrificial layer under bridge and will be etched at the final fabrication step.

Usually, in MEMS fabrication several layers are deposited and patterned over each other. Some layers may be sacrificial layer which will be etched at final fabrication process steps. In some research works, Ni has been proposed as a major material in MEMS structure and Cu as a sacrificial layer (Liu et al. 2010; Alper et al. 2006). The main structural material, Nickel, may be deposited by Physical Vapor Deposition (PVD), electroplating or electroless methods. Some papers noted that a thin Ni layer is deposited by PVD methods and then electroplating is applied. Also, Cu may be deposited by PVD or electroplating or electroless. A thin layer of Cu is deposited on the substrate, patterned in a lithography step and Ni layer is deposited on it. At last, Cu is etched by proper etchants which removes Cu while reserves Ni parts.

In current work, Cu is patterned, then Ni–P electroless process is used to deposit a thin metal layer.

Figure 4 shows a substrate which is covered by deposited and patterned Ni–P layer. The proposed electroless process was applied and a thin Ni–P layer deposited on the substrate. The resist covers some parts of the substrate and after electroless, it will be detached by acetone. So, undesired Ni–P parts are left off and second mask design will be created on the surface.

Fig. 4
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Patterned Ni–P layer on the substrate after second lithography step

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In Fig. 5 the second mask can be seen. In white parts area, the photo resist is removed and Ni–P layer will be deposited. In the black parts area, the photo resist is remained and Ni–P layer will be left off. So, black parts in second mask define spaces between conductive Ni–P parts. In this case, white area consists of the ground plane and microstrip RF transmission line in RF switch. Ni–P always covers all surface areas except some copper parts which are sacrificial layers and at next steps other Ni–P layers will be deposited on them to fabricate the suspended bridge. The mask in Fig. 5 was used to fabricate the patterned structure in Fig. 4.

Fig. 5
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Second mask which defines spaces between Ni–P deposited parts

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The Ni–P deposited line can be seen in Fig. 6. A metal line is created between two large metal areas. 90 μm space is created between the line and two large areas which are GND at both sides of RF microstrip. The width of the Ni–P line, RF microstrip, is 60 μm.

Fig. 6
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60 μm deposited Ni–P line separated by 90 μm spaces from other deposited parts

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5 Conclusions

MEMS devices are emerging in industries, medical, research, consumer electronics and many other fields. Also, an essential process in MEMS fabrication is layer deposition. In this paper an easy and cheap metal deposition process is introduced which provides Ni–P layer deposition on insulator substrates. Thin and thick metal layers can be deposited depends on process time duration. In addition, many substrates can be processed according to proposed electroless method, while in traditional deposition methods the sputtering or evaporation, a few substrates can be handled at each time. The proposed electroless process can be useful for industries and mass production concepts. Also, detail fabrication process of a DC contact RF MEMS switch is introduced which is a good example for Ni–P electroless applications.