Online proceedings - EDA Publishing Association

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11-13 May 2011, Aix-en-Provence, France
Characterisation and Comparison of Water and
Alcohol as Catalysts in Vapour Phase HF Etching of
Silicon Oxide Films
D. Drysdale 1 , T. O’Hara 2 , C. H. Wang 1
1
School of Engineering & Physical Sciences, Heriot-Watt University, Edinburgh, EH14 4AS, UK
3
memsstar Ltd, Scottish Microelectronics Centre, University of Edinburgh, Edinburgh, EH9 3JF, UK
Abstract- The comparison of etch rates and selectivities for thin
films of silicon dioxide and silicon nitride with respect to water
and alcohol based (ethanol in this case) catalysts in a vapour phase
HF etching process is discussed. Observation of etch rates for both
PECVD Oxide and Nitride films are used to describe the
behaviour of silicon dioxide etching. These behaviour
characteristics can also be used to develop selectivity behaviours
between the two films based on each of the catalysts. A number of
factors are considered in the vapour phase etching process: the
total gas flow for the etching process, process temperature and the
etching pressure. The paper discusses the differences between
both water and ethanol as process catalysts for the improvement
of silicon dioxide etching selectivity with respect to silicon nitride.
Results show that using water as a catalyst, a selectivity of up to
40:1 can be achieved while with a direct comparison of the same
etch process with ethanol, the highest achievable selectivity is 15:1.
On the other hand, with comparable etch rates to that of the
water catalyst process, the highest selectivity achieved was 10:1.
I. INTRODUCTION
The use of anhydrous HF vapour as an etchant has become
commonplace within microelectromechanical systems
(MEMS). It is typically used in the production of free-standing
structures for a range of MEMS devices such as RF switches,
accelerometers and microphones. From the initial etching
technique using acid baths as described by Holmes and Snell
[1] with further study by G. Van Barel et al. [2],[3] in
understanding behaviour within wet processing to the recent
work of vapour phase HF etching by Witvrouw et al. [4]. The
use of HF for etching silicon dioxide has become a standard
process technique and as its impact becomes more prominent
with the growth of the MEMS industry in fabricating many
devices. A critical process in MEMS production is the
integration of the release process to the existing semiconductor
fabrication processes.
Many large-scale fabrication facilities still work solely with
CMOS processes and materials thus developing new etching
options that integrate well with these standardised methods
reduces the difficulty of developing next generation MEMS
devices. Many of today’s modern MEMS devices typically
require one or more of five common CMOS materials:
aluminium, silicon, polysilicon, silicon dioxide and silicon
nitride. While HF etching is not a problem in terms of
selectivity to the first three materials, problems arise in using
an HF etch which is selective to silicon dioxide with respect to
silicon nitride. The use of silicon dioxide as the sacrificial
material of choice is for many reasons; typically used as
passivation and insulating layers as well as dielectric layers for
a device depending on the thickness of the film.
The main focus of silicon dioxide in this study is its role as a
sacrificial layer for the fabrication of MEMS devices such as
microphones and RF MEMS switches. It is however, common
to have silicon nitride layers and silicon dioxide layers stacked
on top of each other with the nitride layer forming part of the
structural or functional part of the device while the oxide acts
as a sacrificial layer to be removed to realise the final freestanding
structure.
While a wet process can typically be used, this generates a
widely experienced phenomenon called stiction [5],[6]. Stiction
is the near permanent adhesion of two surfaces commonly due
to electrostatic forces, hydrogen bonding and Van der Waals
forces. Should the restoration force of a movable structure be
less than the adhesion force being applied to it by an external
source (such as a bead of moisture), it will adhere reducing
yield of functional devices and causing high levels of device
failures. Stiction commonly occurs due to moisture present in
micron scale devices as scaling laws suggest that even a single
droplet of moisture applies a strong enough force to hold a
released structure. This is often seen as the other key reason in
the push to vapour phase processing from wet processing
commonly employed the world over. By etching in a vapour
phase, moisture generation is reduced and stiction is therefore
reduced which in turn creates a higher yield for devices. The
equation for etching silicon dioxide is defined as:
catalyst catalyst
SiO2 + 4HF ! SiF4 + 2H2O (1)
This equation requires a catalyst for the reaction to begin and
is typically considered to be either water or alcohol. As can be
seen by the reaction, water is generated as a by-product and
while the presence of too much water or any moisture within
the reaction chamber can damage product wafers, its presence
is needed not only to initiate the reaction, but to maintain it. It
is the ability not only to remove the excess water generated but
to control the other process factors of the etch to keep a high
and repeatable etch process that is the key to its success. By
studying the behaviour of these two key materials, it is hoped
that a better understanding of the etching behaviour can be
achieved thus helping future designs for MEMS devices.
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