jueves, 11 de febrero de 2010

Low Cost RF MEMS Switches


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.


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].


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

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].


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


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.

No hay comentarios:

Publicar un comentario