Page 16 - Sami Franssila Introduction to Microfabrication

Preface

Microfabrication is generic: its applications include gate oxides today are thinner than the ultimate limits
integrated circuits, MEMS, microfluidics, micro-optics, conceived in the 1970s. And it is pedagogically better
nanotechnology and countless others. Microfabrication to show applications of CVD films before plunging into
is encountered in slightly different guises in all of these pressure dependence of deposition rate, and to discuss
applications: electroplating is essential for deep sub- metal film functionalities before embracing sputtering
micron IC metallization and for LIGA-microstructures; yield models.
deep-RIE is a key technology in trench DRAMs and in In this book, another major emphasis is on materials.
MEMS; imprint lithography is utilized in microfluidics Materials are universal, and not outdated rapidly. New
where typical dimensions are 100 µm, as well as in materials are, of course, being introduced all the
nanotechnology, where feature sizes are down to 10 nm. time, but the basic materials properties like resistivity,
This book is unique because it treats microfabrication in dielectric constant, coefficient of thermal expansion
its own right, independent of applications, and therefore and Young’s modulus must always be considered
it can be used in electrical engineering, materials for low-k and high-k dielectrics, SnO 2 sensor films,
science, physics and chemistry classes alike. diamond coatings and 100 µm-thick photoresists alike.
Instead of looking at devices, I have chosen to Silicon, silicon dioxide, silicon nitride, aluminium,
concentrate on microstructures on the wafer: lines tungsten, copper and photoresist will be met again
and trenches, membranes and cantilevers, cavities and in various applications: nitride is used not only in
nozzles, diffusions and epilayers. Lines are sometimes LOCOS isolation, but also in MEMS thermal isolation;
isolated and sometimes in dense arrays, irrespective of aluminium not only serves as a conductor in ICs
linewidths; membranes can be made by timed etching but also as a mirror in MOEMS; copper is used for
or by etch stop; source/drain diffusions can be aligned
to the gate in a mask aligner or made in a self- IC metallization and also as a sacrificial layer under
aligned fashion; oxidation on a planar surface is easy, nickel in metal MEMS; photoresist acts not only as
but the oxidation of topographic features is tricky. The a photoactive material but also as an adhesive in
microstructure-view of microfabrication is a solution wafer bonding.
against outdating: alignment must be considered for Devices are, of course, discussed but from the
both 100 µm fluidic channels and 100 nm CMOS gates, fabrication viewpoint, without thorough device physics.
etch undercutting target may be 10 nm or 10 µm, but it The unifying idea is to discuss the commonalities
is there; dopants will diffuse during high temperature and generic features of the fabrication processes.
anneals, but the junction depth target may be tens of Resistors and capacitors serve to exemplify concepts
nanometres or tens of micrometres. like alignment sequence and design rules, or interface
A common feature of older textbooks is concen- stability. After basic processes and concepts have
tration on physics and chemistry: plasma potentials, been introduced, process integration examples show
boundary layers, diffusion mechanisms, Rayleigh res- a wide spectrum of full process flows: for example,
olution, thermodynamic stability and the like. This is solar cell, piezoresistive pressure sensor, CMOS, AFM
certainly a guarantee against outdating in rapidly evolv- cantilever tip, microfluidic out-of-plane needle and
ing technologies, but microfabrication is an engineering super-self-aligned bipolar transistor. Small process-
discipline, not physics and chemistry. CMOS scaling sequence examples include, similarly, a variety of
trends have in fact been more reliable than basic physics structures: replacement gate, cavity sealing, self-aligned
and chemistry in the past 40 years: optical lithography rotors and dual damascene-low-k options are among the
was predicted to be unable to print submicron lines and others.