Page 230 - Electrical Properties of Materials

212 Principles of semiconductor devices

A survey of published results on plasma etching shows that r.f.- excited
plasmas, usually of argon and various reactive gases, operate at total gas pres-
sures of 9–90 Pa and generate plasma ion densities of between 2.1 × 10 12 and
–3
2 × 10 16 m . With an applied magnetic flux for electron cyclotron reson-
ance (about 4.8 × 10 –4 T for the standard frequency), the operating pressure
16
range is down to 0.01–3 Pa but the ion density is up to 7 × 10 –9 × 10 17
–3
m . This trend to lower pressure and higher density has been carried on using
magnetron-type discharges and helicon waves. All of this is described in the
book by Sugawara mentioned above.
The basic procedure with the equipment of Fig. 9.56 is to use the diffusion
pump to evacuate the chamber to about 10 –6 Torr, baking the whole to outgas
air, water vapour, and residual chemicals, and then to throttle down the pump-
ing line and open needle valves to admit gases so that the required gas pressure
is attained in equilibrium; there is provision to remove volatile products when
the r.f. power and the etching process are started.
Another method of getting high ionization is to use microwaves fed through
a waveguide into the plasma vessel. Frequencies similar to that used for mi-
crowave cooking have been used: 2.45 GHz. This corresponds to a wavelength
of 12.2 cm so the reaction vessel can be made a resonant cavity, with a high-
field region in the vicinity of the Si slice. Again electron cyclotron resonance
can be utilized; this requires a magnetic flux density of about 8.7 × 10 –2 T,
which greatly increases the cost and bulk of the equipment, and so is not very
popular.
Now we must consider plasma chemistry. The ion must be chemically re-
active with the SiO 2 surface. By analogy with the wet processes which use HF,
a fluorine ion is needed. Carbon tetrafluoride (CF 4 ), a gas which is chemically
a close relation of the well-known degreasing fluid carbon tetrachloride, has
been used, as well as other fluorides of carbon. It is usually mixed with one
of the inert gases argon or xenon to enhance gas breakdown. A further use of
dry processing is to get rid of the photoresist coating. This can be burnt off
in an ashing process using a gas mixture including oxygen. The slice can then
be taken out of the plasma reactor, recoated, and masked again for the next
process, until the final contact layer is made.

9.24 Recent techniques for overcoming limitations
The minimum feature size depends on resolution, and the resolution achievable
at a particular wavelength λ has been known for well over a century to be equal
to λ/(2n sin θ), where n is the index of refraction and θ is the cone angle of the
beam.
• Immersion lithography. This technique uses a higher-index material than air.
The simplest one that can be used is water, which can improve resolution by
about 35%. The disadvantage is the impact of water on the photoresist. This
technique has been used to reduce a to 65 nm and then to 45 nm, and it is
likely to be used for reaching the next target of 32 nm.
• Double patterning. This increases the number of fabrication steps. In double
exposure, a photoresist layer is exposed twice with alternating phase-shift
masks. The principle can be appreciated by considering a masking pattern