Page 384 - Sami Franssila Introduction to Microfabrication

38

Moore’s Law

This chapter deals with the past, present and future was also open to new vistas. Because resistances and
of integrated circuits, concentrating on CMOS, which capacitances are not very accurate, it is useful to use
is driving scaling into smaller linewidths and higher ratios of these rather than the absolute values. Integration
device densities. Devices, fabrication processes and of analog elements on the same chip resulted in major
industrial issues are discussed with future trends, limits, improvement in ratios compared to discrete components.
opportunities and threats to continued scaling. There were many objections to ICs in the beginning
of the 1960s, as Jack Kilby reminisces:
38.1 FROM TRANSISTOR TO INTEGRATED
CIRCUIT 1. Electronics designs would become hard to change
once the circuits had been etched onto silicon.
Transistor fabrication in the 1950s was crystallography
2. Electronics engineers would be out of jobs because
and metallurgy, not microfabrication. Junction formation
all design would shift to IC manufacturers.
was an alloying process that did not share many features
3. Transistors are low-power devices that are suitable
with modern transistor fabrication. Pallets of indium, a only for some special applications.
p-type dopant, were attached to both sides of an n-type 4. ICs do not use optimum materials: NiCr resistors are
semiconductor piece, the diffusion step was performed better than silicon resistors, and Mylar capacitors are
and metal wires were attached to the two p-type and one superior to oxide capacitors.
n-type region and voil´a, the pnp-transistor was ready. 5. Yield of transistors is low, for example, 80%, and
The modern key concepts of microfabrication: dif- if, say, 20 of them are made on a single chip, the
fusion masking by an oxide layer, photolithographic combined yield will be miniscule.
patterning, wet etching of the oxide and the use of
evaporated aluminium as a conductor emerged in the
mid-1950s mostly at Bell Laboratories and at Fairchild Argument number one still holds today: especially,
Semiconductor. These techniques were put together by custom circuits take a long time to design and to
Jean Hoerni, in what is known as the planar process for fabricate, and changes are hard to make. This is,
transistor fabrication. however, a small price to pay for the enormous
The integrated circuit (IC) was invented twice, gains in speed and functionality. We now know that
simultaneously and independently. Jack Kilby of Texas argument number two was groundless as ICs propelled
Instruments demonstrated ICs in 1958 and filed for a the electronics industry into super growth. Argument
patent in early 1959. However, Kilby used germanium number three was wrong, and some people had already
transistors and gold wires bonds for connecting the seen it in the 1950s: Bob Wallace of Bell Labs stressed,
devices. Robert Noyce at Fairchild based his invention “Gentlemen, you’ve got it all wrong! The advantage
on the planar process, using evaporated aluminium for of the transistor is that it is inherently a small-size and
metallization and silicon dioxide as an insulator, and low-power device. This means that you can pack a large
created the first device that became the forefather of number of them in a small space without excessive
current ICs. heat generation and achieve low propagation delays.
Integration of transistors was only part of the story: And that’s what we need for logic applications. The
integration of analog elements, resistors and capacitors significance of the transistor is not that it can replace the

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