RF MEMS switches fabrication by using SU-8 technology

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Microsystem TechnologiesMicro- and Nanosystems InformationStorage and Processing Systems ISSN 0946-7076 Microsyst TechnolDOI 10.1007/s00542-013-1753-8

RF MEMS switches fabrication by usingSU-8 technology

Andrea Lucibello, Emanuela Proietti,Flavio Giacomozzi, Romolo Marcelli,Giancarlo Bartolucci & Giorgio DeAngelis

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TECHNICAL PAPER

RF MEMS switches fabrication by using SU-8 technology

Andrea Lucibello • Emanuela Proietti •

Flavio Giacomozzi • Romolo Marcelli •

Giancarlo Bartolucci • Giorgio De Angelis

Received: 12 July 2012 / Accepted: 10 February 2013

� Springer-Verlag Berlin Heidelberg 2013

Abstract In this paper we present a novel process based

on SU-8 technology for the fabrication of double clamped

radio frequency (RF) micro-electro-mechanical system

(MEMS) capacitive shunt switches in coplanar configura-

tion. The key element of the exploited process is the

MicroChem SU-8 2002 negative photoresist. The poly-

meric material is widely used in MEMS device processes

because of its excellent thermal and chemical stability. In

this paper, SU-8 polymer has been utilized in a double way

to get suspended structures as double clamped beams: (1)

SU-8 for the lateral supports, and (2) as a sacrificial layer

for the release of the suspended membrane. Preliminary RF

tests on the manufactured switches have been done, and the

measured electrical performances are in good agreement

with the performed simulations.

1 Introduction

Micro-electro-mechanical system (MEMS) technology

have attracted tremendous interest across the world, and

research efforts are constantly growing for reliability and

integration purposes. The MEMS technology was born as

the fusion of the Integrated Circuits (IC) Technologies with

the most advanced micro-mechanic technologies. Using the

consolidated fabrication technique of the IC with the

opportune chemical and mechanical processes, is possible

to achieve MEMS devices perfectly integrable with IC

ones. There are three basic techniques used to fabricate

MEMS and in particular radio frequency (RF) MEMS: (1)

Bulk micromachining for manufacturing mechanical ele-

ments by starting with a silicon wafer, and then etching

away unwanted parts, and leaving the designed mechanical

devices (Kovacs et al. 1998), (2) LIGA for obtaining small,

but relatively high aspect ratio devices using X-ray

lithography (Malek and Saile 2004), and (3) Surface

Micromachining for devices made layer-by-layer starting

from the wafer surface (Bustillo et al. 1998).

Several advances have been recently performed in the

realization of RF MEMS. The integration of MEMS into

traditional RF circuits resulted in systems with superior

performances and lower manufacturing costs. The incor-

poration of MEMS based fabrication technologies into

microwave and millimetre wave systems offers viable

routes towards ICs with MEMS actuators, antennas,

switches and transmission lines (Kang et al. 2008;

Lucibello et al. 2009; Tan et al. 2003; Muldavin et al. 2006;

Lee et al. 2005; Liu et al. 2004; Di Nardo et al. 2006;

Rebeiz et al. 2002). The resulting devices operate with an

increased bandwidth and with an increased radiation effi-

ciency, and they are very promising for the implementation

of aerospace and defence systems.

The key element of the RF-MEMS technology is rep-

resented by the simple switch, also called Single Pole

Single Throw (SPST). These devices are able to process RF

signals via a Transmission (TX) Line changing the state

from ON to OFF by means of an electrostatic actuation (but

it can also be magnetic, piezoelectric, thermal and so on)

due to a voltage applied between a suspended metal

A. Lucibello � E. Proietti � R. Marcelli (&) � G. Bartolucci �G. De Angelis

CNR-IMM, Rome, Italy

e-mail: [email protected]

A. Lucibello � G. Bartolucci � G. De Angelis

Department of Electronic Engineering,

University of Roma ‘‘Tor Vergata’’, Rome, Italy

F. Giacomozzi

FBK-irst, Povo, TN, Italy

123

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DOI 10.1007/s00542-013-1753-8

Author's personal copy

membrane and an electrode. They pass from an UP state to

a DOWN state generating an open circuit or a short-circuit

on the TX line. The switches can be identified by using the

following three characteristics (see Fig. 1):

1. RF circuit configuration (ohmic series or shunt,

capacitive series or shunt);

2. mechanical structure (cantilever or air bridge);

3. kind of contact (metal-to-metal or metal-to-dielectric).

This simple device in its possible configurations, can be

the starting point for the design of more complex compo-

nents circuits and reconfigurable systems as optimized

switches with matching lines (Bartolucci et al. 2012),

digital phase shifters (Rebeiz et al. 2002; Pilz et al. 2000;

Bartolucci et al. 2007), reconfigurable filters (Rebeiz

2003), reconfigurable antennas (Brown 1998; Cetiner et al.

2003), power dividers (Rebeiz 2003), oscillators (Rebeiz

2003) and so on.

In this paper we present a novel process based on SU-8

technology for the fabrication of double-clamped RF

MEMS capacitive shunt switches in coplanar waveguide

(CPW) configuration. SU-8 negative photoresist has been

used for the lateral supports for the suspended membrane

and at the same time as sacrificial layer, greatly simplifying

the fabrication process, in fact in traditional manufacturing

processes after the deposition of the sacrificial layer a lot of

steps are required, and in particular: the alignment between

mask and wafer, the lithographic process, the development,

the rinse and dry of the wafer, the Hard Bake of the sac-

rificial layer and finally the planarization of the sacrificial

layer that in some case means the repetition of all the

points just mentioned, instead in SU-8 manufacturing

process the major part of this steps are overcome with a

unique spin coating process as will better explained in the

following paragraphs.

2 RF MEMS switches based on SU-8 technology

The CPW is a valid alternative to the microstrip trans-

mission line in RF applications. In a recent paper, also the

grounded CPW uniform line has been extensively studied

providing design rules for such a structure (De Angelis

et al. 2012).

CPW lines elevated with respect to the substrate by

SU-8 have been recently studied, leading to encouraging

results for the realization of low-loss, non-dispersive quasi-

TEM transmission lines (Figs. 2, 3) by using low resistivity

silicon wafers (Marcelli et al. 2008). It is worth noting that

the most part of the electromagnetic field is excited, for this

configuration, in the air region, thus contributing to the loss

Fig. 1 The two main

configurations in RF MEMS

micro-switches (right) clamped

beam (left) double-clamped

beam

Fig. 2 CPW lines elevated with respect to the substrate by SU-8

25 lm (Marcelli et al. 2008)

Fig. 3 Simulation of the electromagnetic field generated at the ports

of a CPW line elevated by SU-8 with respect to the wafer plane. In the

inset the magnitude of the EM field is also given (Marcelli et al. 2008)

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reduction with respect to the same CPW lines obtained

directly onto the wafer. Insertion losses lower than 0.1 dB/

mm at 20 GHz can be obtained by means of this technique

(Fig. 4).

Following this general idea for the realization of low-

loss interconnections based on SU-8, such a material was

also minded as an ideal pedestal for double-clamped RF

MEMS capacitive switches. In Fig. 5 the geometry of the

RF MEMS switch based on SU-8 technology is shown.

Actually, SU-8 mechanical properties after an optimized

series of thermal treatments allow for a strong structure that

can be used to fix the bridge sides of a double-clamped

beam.

The RF MEMS switches fabrication process was devel-

oped having in mind three main goals: (1) to use SU-8 as a

sacrificial layer, (2) to strengthen the lateral supports of the

beam, and (3) to have amore flatmetal shape of the air bridge

(Fig. 6). All of the above points have been fulfilled by a

single photolithographic step, also drastically reducing the

technological procedure respect to the conventional fabri-

cation processes (Rebeiz 2003).

The SU-8 material is a negative, epoxy-type, near-UV

photo-resist based on EPON SU-8 epoxy resin. It is a

multi-use kind of polymer, designed for micromachining

and other microelectronic applications, where a thick

chemically and thermally stable structure is desired. The

exposed and subsequently cross-linked portions of the

film become insoluble to liquid developers. SU-8 has very

high optical transparency above 360 nm, which makes it

ideally suited for realizing near vertical sidewalls also in

very thick films. In literature there is a wide set of devices

based on SU-8 technology that covers different areas

of engineering from microwave/millimetre-wave to THz

applications (Arscott et al. 1999), to be used from Bio-

sensing to TLC systems. Moreover, thanks to its own

characteristics (MEMSCYCLOPEDIA, Free MEMS

Encyclopedia 2012), SU-8 is also suitable for packaging

purposes (Reuter et al. 2005).

3 Fabrication process of RF MEMS switches based

on SU-8 technology

The RF MEMS switches as double-clamped configurations

have been manufactured on a 4-inch high-resistivity silicon

Fig. 6 RF MEMS switches based on SU-8 technology (up) SU-8

used as lateral support and sacrificial layer during the fabrication

process (down) RF MEMS switches after the release

Fig. 4 Experimental results on the CPW lines realized onto (1) low

resistivity substrate (bottom curve), and (2) LRS ? SU-8 elevation

(upper curve) (Marcelli et al. 2008)

Fig. 5 RF MEMS Switch in shunt configuration with the ground

planes elevated by 3 lm of SU-8 with respect to the wafer plane

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wafer h100i oriented, having a thickness of 400 lm. For

the fabrication of the devices, a 4-mask sequence process

has been considered. The SU8 process can be divided in

five principal Macro-steps (Fig. 7) summarized below:

1st Step—for the central conductor of the CPW lines on

bulk, that at the same time has also the function of pull-

down electrode of the switches (Fig. 7a);

2nd Step—for the Silicon Oxide (SiOx) area, to be

deposited onto the central conductor (capacitive contact)

(Fig. 7b);

3rd Step—floating metal onto the SiOx (Fig. 7c);

4th Step—SU-8 Sacrificial layer (Fig. 7d);

5th Step—Air Bridge and release (Fig. 7e).

The details of the entire fabrication process are shown

below. The first step foresees the realization of the central

conductor of the CPW (Fig. 8) by means of the following

sub-steps:

1. Wafer cleaning

2. Thermal oxidation

3. Physical vapour deposition of chrome (100 A)

4. Physical vapour deposition of gold (1,000 A)

5. Spinning of S1813 6,000 rmp for 120 s (1.1 lm)

6. Soft bake on hot plate (5 min at 90 �C)

7. UV exposure (1st Mask 8 s)

8. Photoresist development (23 s in MF 319)

9. Wet etching of Gold

10. Wet etching of chrome

11. Removal of residual photoresist (1165 Removal)

In the second step the process for obtaining the capac-

itive contact on the Silicon Oxide area between the central

conductor and the movable membrane is performed

(Fig. 9). For our purposes, in this lithography step, N1410

negative photoresist has been used. Actually, N1410 is an

excellent photoresist for lift-off process. The detail of the

process are shown below:

1. Wafer cleaning

2. Spinning of N1410 2,000 rpm 60 s. (1.1 lm)

3. Soft bake on hot plate (3 min at 160 �C)

4. UV exposure (2nd Mask 50 s)

5. Development of photoresist (2 min in Ma-D 533/S)

Fig. 7 The main steps process of the RF MEMS switch based on SU-

8 technology

Fig. 9 Silicon Oxide area for the capacitive contact between the

central conductor and the movable membrane

Fig. 8 Central conductor of the CPW line

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6. Thermal evaporation of chrome (Electron Gun 100 A)

7. Thermal evaporation of SiO2 (Electron Gun 2,500 A)

8. Lift off in acetone

The third step foresees the realization of the Floating

Metal area onto the Silicon Oxide for the uniformity of the

capacitive contact between the central conductor and the

movable membrane (Fig. 10). The fabrication process is

the same of the second step seen previously the only things

that changes are the mask and the deposited materials in this

(a bilayer of chrome and gold by PVD). Also in this case for

the lift off process is used N1410 negative photoresist. The

process details are the following:

1. Spinning of N1410 2,000 rpm 60 s. (1.1 lm)

2. Soft bake on hot plate (3 min at 160 �C)

3. UV exposure (3rd Mask 50 s)

4. Development of photoresist (2 min in Ma-D 533/S)

5. Rinse and dry

6. Thermal evaporation of chrome (Electron Gun 100 A)

7. Thermal evaporation of gold (Joule Effect 1,000 A)

8. Lift off in acetone

9. Rinse and dry

The fourth step foresees the realization of the SU-8

sacrificial layer and the lateral supports (Fig. 11). This step

is simple to be understood but, at the same time, is delicate

to be treated from a fabrication process point of view.

Actually, a thick layer of SU-8 negative photoresist is spin

coated and processed until hard bake (HB). The photoli-

thographic step does not require a mask because no

geometries have to be transferred onto the wafer but what

is done is a full exposure of the entire wafer creating in this

way at the same time the lateral supports and the sacrificial

layer. Attention has to be payed to the process of SU-8 in

order to avoid internal stress or crack of the polymer. The

details are given below:

1. Wafer cleaning

2. Spinning of SU-8 2002 800 rpm for 30 s (3 lm)

3. Ramped soft bake (from 22 to 95 �C with a raise

temperature of 2 �C/min)

4. UV exposure (Full Exposure 10 s)

5. Ramped post bake (PB) (from 22 to 95 �C with a raise

temperature of 2 �C/min)

6. Ramped HB (from 22 to 220 �C with a raise temper-

ature of 5 �C/min)

7. Hold HB at 220 �C for 30 min and cool down (SU-8

Sacrificial Layer)

The fifth and last step foresees the realization of the air

bridge (Fig. 12). In this step process after the deposition of

the seed layer composed by a three layer of metal (Cr/Au/

Cr) 1.5 lm of electroplating gold is grown onto the wafer

forming the ground planes and the air bridge. The details

are the following:

Fig. 10 Floating metal area onto the silicon oxide Fig. 11 SU-8 sacrificial layer and lateral supports

Fig. 12 RF MEMS switch based on SU-8 technology

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1. Wafer cleaning

2. PVD of chrome/gold (100 A/1,000 A)

3. Spinning of S1818 1,800 rmp for 30 s (2.8 lm)

4. SB on hot plate (5 min at 90 �C)

5. UV exposure (4th Mask 10 s)

6. Pre-development of photoresist

7. Development of photoresist (50 s MF 319)

8. Electrodeposition of gold (1.5 lm)

9. Photoresist removal (1165 Removal)

10. Wet etching of chrome seed layer

11. Wet etching of gold layer

12. Asher O2 (Release of the structure 15 min at 220 �C)

4 RF MEMS switch preliminary RF tests

Preliminary RF tests on the manufactured switches have

been performed and they are presented from Figs. 14, 15, 16.

The RF test bench is composed by the following instruments

(Fig. 13). This measurement set-up allows to measure the

S-parameters of the switches in ON and OFF state:

• Dual power supply

• 1 PC

• 1 VNA

• 1 Waveform generator

• 2 Digital multimeters

• 1 Temperature-relative humidity sensor

A Voltmeter (Hp3478 Digital Multimeter2) is in parallel

between the S pads and the GND pad to check the real

voltage and a current meter (Hp3478 Digital Multimeter1)

is in line to check the amount of current flowing in the

system. The DUT is kept under nitrogen flow and the test

conditions are:

• Temperature = 23/24 �C; RH & 30 %

• RF: SOLT Calibration from 45 MHz to 40 GHz, Power

3 dBm, # points 801

• Multi-meter resolution: Current 1 lA, Voltage 1 lV

The measured electrical performances are in good

agreement with the performed simulations. Actuation volt-

ages in the order of 20 volt have been imposed for the metal

Fig. 13 RF test set-up

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beam collapse, confirming the mechanical evaluations.

In particular, the devices have been designed to have the

best capacitance ON/OFF ratio around 10 GHz, and 0.5 dB

of insertion loss (Fig. 15), whereas 20 dB of return loss

(Fig. 14) have been obtained up to 10 GHz for the ON

(UP) state of the bridge, while an Isolation better than

25 dB has been measured at the frequency of resonance in

the OFF (DOWN) state (Fig. 16).

5 Conclusion

The realization of double-clamped RF MEMS capacitive

shunt switches in coplanar configuration by means of SU-8

technology has been proposed. In particular, cross-linked

negative photo-resist SU-8 has been used as a sacrificial

layer and, at the same time, as a pedestal to elevate the

ground planes of the CPW lines. RF MEMS switches

manufactured in this way present simplified technological

steps with respect to the usual technology based on other

kind of sacrificial materials, like SiO2 and positive photo-

resist materials. Preliminary RF tests on the manufactured

switches have been performed, and the measured electrical

performances are in good agreement with electromagnetic

simulations results.

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