Page 367 - Microsensors, MEMS and Smart Devices - Gardner Varadhan and Awadelkarim

12

IDT Microsensor Fabrication

12.1 INTRODUCTION

Surface acoustic wave (SAW) devices are fabricated using processes that have been
primarily developed for integrated circuit (1C) technology in the microelectronics industry.
In this chapter, we describe all the steps required to fabricate an interdigital trans-
ducer (IDT) SAW microsensor from a stable temperature (ST) cut quartz wafer. A basic
overview of this process is given in Figure 12.1. Specifically, there are two processes that
are commonly used to define the IDTs: etching and lift-off (Hatzakis et al. 1980). Both
methods 1 are suitable for the fabrication of IDT-SAW delay-line sensors, but the ultimate
choice of either the etching or the lift-off process mainly depends on the minimum feature
size (resolution and accuracy) of the patterned structure required. Although the etching
procedure is relatively easy to realise and acceptable resolution is achievable, it is more
susceptible to electrical shorts between features than that of the lift-off process. This is a
major concern, especially for minimum feature sizes approaching 1–2 um, where the influ-
ence of contaminants, such as large dust particles, becomes more significant (Vellekoop
1994). However, for larger minimum feature sizes, of 5 um or greater, it is recognised
that the etching process is acceptable and comparable in terms of device fabrication yield
and quality to that of the lift-off process.
Section 12.2 provides full details of the steps required to make an IDT microsensor
through either an etching process or a lift-off technique. The process given here is meant
to serve as an example, and variations in the precise choice of materials and equipment
used will vary from laboratory to laboratory.
Next, the steps required to make a Rayleigh-SAW microsensor from the IDTs are
shown, together with a waveguiding layer of SiO 2 (Section 12.3) to fabricate a Love
wave microsensor.
Finally, in Section 12.4, we provide tables that summarise the etching and lift-off
processes and present their relative merits.

12.2 SAW-IDT MICROSENSOR FABRICATION

12.2.1 Mask Generation

SAW-IDT designs are written onto square, low-expansion glass plates using a process
of electron-beam (E-beam) lithography. The SAW designs are first created using a

1
Pattern transfer and etching methods were introduced in Chapter 2.