Page 345 - Sami Franssila Introduction to Microfabrication

324 Introduction to Microfabrication

If transfer and process chambers take only one wafer at a Plasmas used in microfabrication are low-temperature,
10
time, the volume to be pumped can be made very small. low-density plasmas (ca. 10 cm −3 ion density), com-
In a batch deposition system, the vacuum vessel volume pared to, for example, welding or fusion plasmas. In
is easily 100 L, and the corresponding pumpdown time microfabrication, high-density plasma (HDP) means ion
−3
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is of the order of an hour, or hours, and somewhat less density in excess of 10 cm . The degree of ionization
with a loadlock. is still fairly low: at 1 mtorr pressure, it is only a fraction
Loadlocks come in two varieties: single loadlocks, or of a percent.
separate entry and exit loadlocks. The former loadlocks Plasma etching has a very high number of param-
are used when the process time is long compared to eters that need to be controlled (Figure 32.2). This
transfer time. Load locks serve many purposes: they makes plasma etching difficult, both experimentally and
protect the main chamber from clean room air, and the simulation-wise. Furthermore, the machine parameters
clean room air from harmful or toxic gases that have been affect plasma parameters, which, together with surface
used in the process. They can also protect the wafers from reactions, determine the final outcome: rate, selectivity
the atmosphere: for instance, after aluminium plasma and other process responses of interest.
etching, chlorine residues remain on the wafer (in the
resist and on aluminum surfaces), and if the wafer is 32.3.1 Direct plasmas
taken into cleanroom air with 45% humidity, the chlorine
will react with water vapour, and HCl is formed: Plasma etch reactors can be classified in various ways,
and the following is just one. A parallel-plate diode
2AlCl 3 + 3H 2 O −→ Al 2 O 3 + 6HCl (32.14) reactor with two electrodes, one powered and one
grounded, is a basic construction for an etcher (see
Hydrogen chloride will etch aluminium locally. This is Figure 11.9). It is called RIE when the wafer(s) is
termed corrosion. Exit loadlock can be used to strip the (are) on the biased electrode, or PE when the wafer(s)
photoresist in oxygen plasma, and to passivate aluminum is (are) on the grounded electrode. Wafers are placed
surfaces to Al 2 O 3 . on electrodes that produce the plasma; plasma density,
In an evaporator, there is just residual gas to be sheath voltage and ion bombardment that hit the
pumped out; but in sputtering and UHV-CVD systems,
wafers are thus dependent on each other, and cannot
we feed in process gases intentionally, and must be able be controlled independently. Despite this seemingly
to pump them out. Despite similar base vacuum, the inconvenient state of affairs, this arrangement is very
process vacuum in sputtering and UHV-CVD is 1 to widely used because of its simplicity. 13.56 MHz RF
10 mtorr, 3 orders of magnitude higher than the base generators are used to create plasmas of typical density
vacuum, and 10 to 100 Pa-L/s pumps can be used. 10 −3
10 cm .
32.3 PLASMA ETCHING 32.3.2 Remote plasmas
Plasma generation has a major role in etching, sputter- In remote plasmas, plasma generation takes place in
ing, ion implantation, photoresist stripping and PECVD. a region outside the wafers, and the wafers see a

Plasma parameters
Reactor -electron density and energy
parameters -ion density and energy Etch responses
-radical density
-power -fluxes -rate
-frequency -selectivity
-pressure Surface reaction -anisotropy
-flow rate parameters -uniformity
-temperature -loading effects
-pattern size effects
-temperature -damage
-sticking coefficient
-reaction probability
Figure 32.2 Plasma etching parameters and process responses