Page 346 - Sami Franssila Introduction to Microfabrication

Vacuum and Plasmas 325

controlled flux by, for example, a separate bias power 10 −7 to 10 −9 mtorr because high purity argon sputtering
source. Alternatively, the wafers may be shielded from gas (99.9999%) contributes less than background gases.
ions completely by a Faraday cage. Because of this Sputtering systems have, in addition to plasma
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decoupling, high-density plasmas (10 –10 cm ) can generation and vacuum subsystems, many other features:
be achieved, without high sheath voltages or severe ion the wafers can be heated, they can be biased and they
bombardment on the wafer. Since a high density of can be shielded from the plasma by shutters, as shown
ions and radicals means a high concentration of active in Figure 32.3.
species, high-density plasmas (HDP) offer higher etch
and deposition rates. DRIE reactors use ICP (inductively
32.4.1 Reactive sputtering
coupled plasma) and employ 2 to 5 kW power sources
for plasma generation. Sputtering in a reactive atmosphere, in argon/nitrogen
Higher etch rate, lower damage, easier photoresist or argon/oxygen mixtures, results in nitride or oxide
removal and higher selectivity favour HDP reactors. films, or stuffed films with small amounts of reactive
Remote plasma reactors are often difficult to scale impurities at grain boundaries. Typical applications of
to large diameters because of the physical separation reactive sputtering are TiN, Ta 2 O 5 , ZnO, AlN, TiW:N
between plasma and wafer, whereas in parallel-plate and WO 3 . Often, reactively sputtered films are not
reactors, the plasma is naturally ‘aligned’ to the wafer. stoichiometric, and a (reactive) annealing step (e.g., in
But larger wafer sizes make direct plasma reactors less oxygen) is needed to improve film quality.
attractive: in order to maintain the same power density, Introduction of small amounts of nitrogen or oxygen
the absolute size of the RF-generator may grow far into argon plasma does not appreciably change the
too big. properties of the discharge or of the growing film, but
after a critical partial pressure is reached, the target
surface transforms into nitride or oxide, and the plasma
32.4 SPUTTERING discharge is established at another equilibrium. If the
reactive gas flow is then reduced, the target remains
The oldest and simplest of sputter deposition systems nitrided/oxidized, and return to initial conditions takes
is the DC-diode system, which consists of a negatively place at much lower partial pressures, that is, reactive
biased plate (target cathode), which is bombarded by sputtering exhibits hysteresis.
argon ions at ca. 100 mtorr pressure (see Figure 5.4). In
order to get high deposition rate, high sputtering power
32.4.2 Sputter etching and bias sputtering
has to be used, which leads to high voltage operation.
This is undesirable because of damage to thin oxides. If the voltages in a sputtering system are switched,
In order to improve DC diodes, RF diode systems and power is applied to the wafer electrode instead
were introduced. RF sputtering systems usually work of the target, the wafers will experience argon ion
at 13.56 MHz. They can be used to deposit dielectrics, bombardment. This is called sputter etching. (Sputtering
something that is not possible with DC systems because systems can be turned into true plasma etch systems by
of charging. Electrons oscillating in an RF field couple introducing reactive gases instead of argon. The term
energy more efficiently to the plasma, and higher RSE, for reactive sputter etching, was used in the early
deposition rates are possible in RF than in DC, at the days of plasma etching.)
same power levels. However, a very high voltage of If the wafer electrode is biased during sputtering (by
2000 V is used. a separate power supply), the wafer will experience
Magnetron sputtering has emerged as the main con- simultaneous deposition and etching. This will generally
figuration. A magnet behind the target creates a field that densify the film because ion bombardment kicks off
confines electron movement, and therefore, ionization is loosely bound film atoms, and it also affects film
much more efficient, leading to high deposition rates stresses. Geometry of structures is important because
at low power (5–20 kW are used, depending on target argon etching depends on the angle of incidence:
size). Voltages in magnetron systems are, for example, convex corners are etched faster, and faceting occurs.
500 V (and argon ion energies are 500 eV), clearly lower This is pictured in Figure 32.4 (PECVD oxide has
than in RF diodes. Magnetron sputtering systems work been etched in argon). Smoothing of sharp corners
at ca. mtorr pressures (0.1–10 mtorr), with argon flows is beneficial for step coverage in the next deposition
of 10 to 100 sccm. Impurity-wise, however, sputtering step, but such dep-etch (deposition-etch) processes are
systems are described by their base pressures, which are understandably slow.