Page 18 - Sami Franssila Introduction to Microfabrication

Preface xvii

ADVICE TO STUDENTS inaccurate) as answers culled from Internet. It should be
borne in mind that even constants are often not well
This book is an introductory text. Basic university known: for instance, recent measurements of silicon
physics and chemistry suffices for background. Materials melting point have resulted in values 1408 C by one
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science and electronics courses will of course make group, 1410 C by one, 1412 C by seven groups, 1413 C
many aspects easier to understand, but the structure of by eight groups and 1416 C by three groups, and if
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the book does not necessitate them. The book contains older works are encountered, values range from 1396 C
250 homework problems, and in line with the idea to 1444 C. With thin film materials properties are
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of microfabrication as an independent discipline, they very much deposition process dependent, and different
are about fabrication processes and microstructures; not workers have measured widely different values for such
about devices. Problems fall mainly in three categories: basic properties as resistivity or thermal conductivity.
process design/analysis, simulations and back-of-the- Even larger differences will pop up, if, for instance,
envelope calculations. The problems that are designed to the phase of metal film changes from body-centered
be solved with a simulator are marked by “S”. A simple cubic to β-phase: temperature coefficient of resistivity
one-dimensional simulator will do. The “ordinary” can then be off by a factor of ten. Polymeric materials,
problems are designed to develop a feeling for orders too, exhibit large variation in properties and processing.
of magnitude in the microworld: linewidths, resistances, There are also calculations of economic aspects of
film thicknesses, deposition rates, stresses etc. It is microfabrication: wafer cost, chip size and yield. A bit
often enough to understand if a process can be done in of memory costs next to nothing, but the fabs (fab is
seconds, minutes or hours; or whether resistance range short for fabrication facility) that churn out these chip
is milliohms, ohms or kiloohms. You must learn to make are enormously expensive.
simplifying assumptions, and to live with uncertain Comments and hints to selected homework problems
data. Searching the Internet for answers is no substitute are given in Appendix A. In Appendix B you can find
to simple calculations that can be done in minutes useful physical constants, silicon material properties and
because the simple estimates are often as accurate (or unit conversion factors.