Page 231 - Electrical Properties of Materials

Building in the third dimension 213

consisting of an opaque array of bars interspersed with transparent regions.
The shadow of a bar will be made fuzzy by diffraction. Now, if altern-
ate ‘windows’ are ‘glazed’ with a half-wavelength phase shifter, the light
through adjacent windows is out of phase, so the light diffracting around the
bar will interfere destructively, reducing the fuzziness and making the bar’s
shadow sharp, as if it were a larger object, away from the diffraction limit.
• High-k gate dielectrics. This is the jargon used. For some reason semicon-
ductor technologists denote the relative dielectric constant by k, instead of
the ε r that is used by practically everybody else. The problem is with the
insulator between the metal electrode and the channel in a MOSFET. As a
result of scaling down all the dimensions, this insulating layer went down
to 1.2 nm, containing just five atoms. That led to some leakage current by
tunnelling when the transistor was off. Leakage current means more heat
and shorter battery life. Thus something had to be done when the next scal-
ing down, to 45 nm, was considered. A thicker insulator was needed but
without an increase of the voltage drop across it. The solution was to replace
silica (ε r = 4.1) by an insulator which had a higher dielectric constant. The ∗ It follows from the continuity of D,
∗
two insulators considered by the various companies were hafnium dioxide, the electric flux density, that the electric
field is lower in a material with a higher
with a dielectric constant around 20, and zirconium dioxide, with a slightly
dielectric constant.
higher dielectric constant of 25. It turned out that for technological reasons
†
it was also necessary to return to a metal gate electrode. The metal used † In fact, metal electrodes were replaced
was molybdenum, a high-melting-point material. by polysilicon ones in the late 1960s
• Extreme ultraviolet (known also as soft X-ray) lithography. The shorter the but the acronym MOSFET, referring to
‘metal–oxide–semiconductor’, survived.
wavelength, the higher is the resolution. On that basis, all we need is to find
Concerning the principles, it made no
some sources at the right wavelength. If we want a minimum feature size of difference so there was no reason to
13 nm (some optimists believe that that is possible) then we need sources at mention this in Section 9.22. Lately,
about the same wavelength. As it happens, there are methods to create such however, polysilicon has been aban-
doned and metals have regained their
radiation. The favourite scheme is to have a powerful pulsed laser, direct it rightful place.
on a piece of metal that has lots of electrons (tin, molybdenum, and silver
have been favourite candidates), and turn the metal into a plasma, which will
convert the incident radiation with reasonable efficiency (a figure of a few
per cent has been quoted) into the desired extreme UV wavelength. This is
not easy to do, but if one succeeds, that is only the beginning of the prob-
lems. At this wavelength there are no lenses and mirrors of the kind that
exist at longer wavelengths. Mirrors have been produced from multilayers
(see Section 10.5) but they are very lossy. In fact all materials are lossy at
these wavelengths. So no proper projecting optics exist, nor are there ap-
propriate masks or photoresists. Efforts are of course being made in this
direction and one day they may succeed, but it is difficult to see when.

9.25 Building in the third dimension
All the microelectronic circuits we have been talking about were built in two
dimensions; that’s why the technique is often called the planar technique. It is ‡ Flatland: A romance of many dimen-
invariably a plane surface that is manipulated for producing a host of devices. sions is a short novel by Edwin Abbot,
‡
It is like Flatland, a world of two dimensions. Can we get out of Flatland and published in London in 1884. It is about
life that is lived in two dimensions and
start building circuits in the third dimension? A start has already been made. It how the inhabitants can deduce evidence
is believed that the present arrangement of the source, drain, and gate (in spite about the existence of a third dimension.