Low Cost RF MEMS Switches

RF MEMS

Fabricated on Microwave Laminate

Printed Circuit Boards

Abstract—A novel fabrication process for directly constructing

RF MEMS capacitive switches has been developed on microwave

laminate printed circuit boards (PCBs). The integrated process

uses metal wet etching to form the metal lines for coplanar waveguides,

compressive molding planarization (COMP) to planarize uneven

surface heights for switch membrane formation, and high density

inductively coupled plasma chemical vapor deposition (HDICP-CVD)

for low temperature silicon nitride deposition. This technology will also

allow further monolithic integration with antennas on the same PCBs to

form reconfigurable antennas without the concerns of mismatching

among components.

Index Terms—RF MEMS switches, compressive molding planarization,

high density inductively coupled plasma chemical vapor deposition,

printed circuit board, reconfigurable antennas.

 INTRODUCTION

F MEMS is an emerging technology that promises the potential of

revolutionizing RF and microwave system implementation for the next

generation of telecommunication applications. Its low power, better

RF performance, large tuning range, and integration capability are

the key characteristics enabling system implementation with

potential improvements in size, cost, and increased functionality.

An RF MEMS switch is one of the basic building blocks in the

RF communication system. Switches operating at frequencies

up to 40 GHz with very low insertion loss and high isolation have

been successfully demonstrated [1-2]. In addition, they consume

very little energy and exhibit very linear characteristics with

extremely low signal distortion, making them ideally suited for

modern radar and communications applications [3]. Using

RF MEMS switches, various RF circuits such as variable

capacitors, tunable filters, on-chip inductors, and phase

shifters have been demonstrated with superior performance

over conventional semiconductor devices [4-7].

Typically, RF MEMS switches are classified into two types:

resistive series and capacitive shunt switches. They are

normally built on high-resistivity silicon wafers, GaAs wafers,

and quartz substrates using semiconductor microfabrication

technology with a typical four to six mask level processing.

[8-14]. Transfer process technique to integrate MEMS onto

RF compatible substrates on which direct MEMS fabrication

is not feasible has also been proposed [15-17]. While

RF MEMS switches have been successfully demonstrated

to have outstanding RF performance as discrete components

, their use in communication system is still limited due to the

high cost of packaging them and the high cost of RF matching

requirements in module board implementation.

Furthermore, the fabrication of most RF MEMS switches

requires thickness control and surface planarization of wide

metal lines prior to deposition of a metal membrane bridge,

which poses a major challenge to manufacturability. To ease

the fabrication of RF MEMS switches and to facilitate their

integration with other RF components such as antennas, phase

delay lines, tunable filters, it is imperative to develop a

manufacturable RF MEMS switch technology on a common

substrate housing all essential RF components. In this paper,

a novel fabrication process is proposed to construct RF MEMS

switches directly on printed circuit boards (PCBs) for ready

integration with all other RF components [18]. Since RF

switches are fabricated directly on PCB, the need for RF wire

bonding and impedance matching to external components is

eliminated and packaging is simplified, thus yielding a low

cost, fully integrated RF system technology [19].

 FABRICATION PROCESS

The process starts with RO4003-FR4 substrate with

copper layers of 16.5 μm and dielectric thickness of 1.57 mm.

The RO4003-FR4 is a high performance laminate (εr=3.38,

tanδ=0.002) chosen as a preferred PCB substrate for RF switches

because of its relatively low cost and capability to handle

high frequencies [20].

Fig.1 shows the fabrication sequence. At the first step, the

top copper layer is wet etched to form a coplanar waveguide

(CPW) structure. For a 50 Ω CPW line, the width of the center

signal line and that of the gap between signal and ground lines

are 180 μm and 30 μm respectively. A thin layer of dielectric

film is then deposited on the signal line to provide signal

isolation from DC at the "down" position of the capacitive

switch.

Due to the maximum allowable processing temperature of

many PCBs generally being 175 oC, depending on the time

of explosive, the current commonly used PECVD silicon nitride

deposition process at 250-400 oC cannot be employed here. A

novel, low temperature (below 100 oC) high density inductively

coupled plasma chemical vapor deposition (HDICP-CVD) process

has recently been developed for low hydrogen content and high

density silicon nitride deposition [21]. Films deposited by the

HDICP-CVD have excellent properties, such as higher breakdown

voltage, lower pinhole density, and better uniformity than

traditional PECVD SiNx.

Prior to the formation of the bridge membrane on the CPW lines,

a sacrificial photoresist layer is needed to planarize the highly

topographic surface with 16.5 μm step height and thus to ensure

mechanical integrity of deposited membrane. To do so, a novet

echnique, compressive molding planarization (COMP) has been

developed [22].

The photoresist is first applied by spin coating to create a much

thicker film than the desired one. It is then partially soft baked to

drive off the solvent inside. Next, a mold master with the spacer

defining the desired thickness for the sacrificial layer is applied

on top of the photoresist film for molding. The thickness of this

sacrificial layer determines the height of the membrane over the

CPW line with 6 μm being chosen here.

Note that the height control of the spacing between signal line

and bridge membrane can be achieved with the spacer in the

COMP process. The mold material is treated to prevent the

photoresist film from sticking to it during the process. During

the molding process, heat is also applied to the platen to soften

and mold the photoresist layer. After the molding process, the

substrate is cooled down and released from the mold. Fig.2 shows

the profiles of the sacrificial photoresist layer obtained by using a

Tencor Alpha Step profilometer with and without the COMP

process. It is evident that the COMP process is capable of

providing a very flat surface with a precise control of film

thickness. Following the COMP process, the sacrificial

photoresist layer is lithographically defined into the required

membrane pattern and then the silicon nitride layer is stripped

away by dry etching.

An aluminum layer is deposited on the surface and then selectively

wet etched to form the membrane. The final step of membrane release

is to soak the wafer in acetone for removing the sacrificial photoresist

and then to rinse it in boiling methanol for reducing liquid surface

tension of the membranes [23].

 RESULTS

The RF characterization of the MEMS switches was done using a

HP 8510C network analyzer connected to a Cascade probe station.

The system was calibrated using standard thrureflection- line (TRL)

on-wafer standards. S-parameter data was recorded over the frequency

range of 1-18 GHz, according to the frequency specifications given

for RO4003-FR4 [15].

The measured insertion loss and return loss of the switch in the up

state is illustrated in Fig.4a. The switch has approximately 0.1 dB

loss at 10 GHz and 0.3 dB at 18 GHz.

To carry out the down position RF measurements, we used external

bias-tee circuits and the bias voltage was set at 35 V, which is found

to be the typical pull down voltage for these switches. The results are

shown in Fig.4b. The isolation is about 34 dB at 12.8 GHz which is the

resonant frequency for the down position.

Although our CPW lines are formed by a simple wet etching of the

copper layer with non-vertical sidewall profile, its characteristic

impedance is not affected by the sidewall

CONCLUSIONS

A novel fabrication process, utilizing, COMP and HDICPCVD, for

constructing RF MEMS capacitive switches directly on microwave

laminate printed circuit boards (PCBs) has been developed and

demonstrated. The switches show excellent RF performance of low

loss (0.3 dB at 18 GHz) and high isolation (34 dB at 12.8 GHz) in the

frequency range of 1-18 GHz. The low temperature nature of the

developed process allows the choice of any PCB substrate with the

desired electrical properties for the intended applications. In addition,

this technology also allows further monolithic integration with other

RF components on the same PCBs to form complex, programmable

communication systems without the concerns of impedance

mismatching among the components.

Página: www. memtronics.com

Realizado: Franco A Rivera C.
CRF