Page 24 - Sami Franssila Introduction to Microfabrication

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Introduction

1.1 MICROFABRICATION DISCIPLINES silicon surfaces act as mirrors, or as extremely flat and
smooth supports for metallic or dielectric mirrors. Sil-
Integrated circuits industry and related industries such icon can be machined to make movable mirrors and
as microsystems/MEMS, solar cells, flat-panel dis- adaptive optical elements. Silicon dioxide and silicon
plays and optoelectronics rely on microfabrication nitride can be deposited and etched to form waveguides
technologies. Typical dimensions are around 1 µm in with graded or stepped refractive indices like optical
the plane of the wafer (the range is rather wide; fibres.
from 0.1 µm to 100 µm). Vertical dimensions range Micromechanics makes use of mechanical properties
from atomic-layer thickness (0.1 nm) to hundreds of of silicon. Silicon is extremely strong, and flexible
micrometres but thicknesses from 10 nm to 1 µm are beams and diaphragms can be made from it. Pressure
typical. sensors, resonators, gyroscopes, switches and other
The historical development of microfabrication- mechanical and electromechanical devices utilize the
related disciplines is shown below (Figure 1.1). Inven- excellent mechanical properties of silicon.
tion of the transistor in 1947 sparked a revolution. The Micromachines, as well as many microsensors and
transistor was born out of fusion of radar technology actuators, make use of active materials, for example,
(fast crystal detectors for electromagnetic radiation) and piezoelectric materials or shape memory alloys. Silicon
solid-state physics. Adoption of microfabrication meth- has the role of precise platform on which these devices
ods enabled fabrication of many transistors on a single can be built. Superconducting devices are made on
piece of semiconductor, and a few years later, the fab- silicon because silicon is compatible with a plethora of
rication of integrated circuits; that is, transistors were processing technologies.
connected with each other on the wafer rather than being Nanotechnology is an outgrowth and extension of
separated from each other and reconnected on the circuit microfabrication. Some of the tools are same, like
board. the electron-beam lithography machines, which have
Microelectronic and optoelectronic devices make use been used to draw nanometre-sized structures long
of the semiconducting properties of silicon. Doping of before the term nanotechnology was coined. Some
silicon can change its resistivity by eight orders of of the methods are based on scanning probe devices
magnitude, enabling a great number of microstructures such as the atomic force microscope (AFM), which
and devices to be made. Silicon microelectronic devices is an important instrument for microstructure char-
today are characterized by their immense complexity acterization. Thin films down to atomic-layer thick-
and miniaturization; a hundred million transistors fit on nesses have been grown and deposited in the micro-
a chip the size of a fingernail. fabrication communities for decades. Novel ways
Gallium arsenide and other III–V compound semi- of depositing films, like self-assembled monolayers
conductors are used to make light emission devices like (SAMs), have been introduced by nanotechnologists,
lasers. Silicon optoelectronic devices can be used as and some of those techniques are being investi-
light detectors, but, recently, light transmission from gated by the established microfabrication community
silicon has been demonstrated in laboratory experi- as tools for continued downscaling of microstruc-
ments. Micro-optics makes use of silicon in another way: tures.

Introduction to Microfabrication Sami Franssila
 2004 John Wiley & Sons, Ltd ISBNs: 0-470-85105-8 (HB); 0-470-85106-6 (PB)