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PLASMA PROCESS CONTROL

PLASMA PROCESS CONTROL 6.11

RF source. In Ref. 34, it was discovered that higher plasma densities were achieved by selecting the
appropriate modulation period. Pulsing the RF source for capacitively coupled plasma has shown to
35
alter the ion energy of the plasma. Improved etch rate and control has been obtained using pulsed
36
plasmas. Pulsing the plasma has also demonstrated the manufacturing capability of reducing lateral
etching. 37 Plasma dynamics for inductively coupled sources are described in Refs. 38–40 and for
electron cyclotron resonance plasma sources in Ref. 41.
The work to date for a control methodology of pulsing is based on either power or plasma imped-
ance monitoring. Monitoring of the plasma dynamics has been achieved through a variety of instru-
ments that include Langmuir probes, RF metrology, radiometry, inferometers, and optical emission
spectroscopy. 35,38–42 Reference 42 provides a summary that highlights the advantages of controlling
the pulsed RF based on impedance measurements. In pulsed RF plasma, a variety of processes occur
at the first stages of RF turn-on and turn-off, as illustrated in Fig. 6.10. During the transient RF turn-
on, the electron energy elevates above the steady state; electron and ion densities increase; and the
thickness of the sheaths on the electrodes varies. At the transient power turn-off, electron energy
decreases rapidly, electron and ion densities decrease, and the plasma sheaths disintegrate with the
decay of electron density. These dynamics affect the plasma impedance through the changes of the
bulk plasma resistance and the sheath capacitance and resistance. This infers that pulsed plasmas can
be characterized by an impedance measurement.

6.3.2 RF Metrology for Plasma Chambers
The following excerpt from Ref. 43 summarizes the advantages of RF metrology to control plasma
processing chambers for the manufacturing of semiconductor devices. RF sensors are important
because they are not intrusive, and they collect and report data in real time. This enables a fast
response time to any changes that occur during processing. Radio-frequency monitors also give
information about the discharge, which can be used to develop a physical understanding of the inter-
nal electrical characteristics of the plasma. At the very least, this information can be used to estab-
lish trends between the input settings and the electrical characteristics of the plasma source.
A number of sensors have been employed as RF metrology in plasma processing tools. These
include voltage probes, current loops, diplexers and directional couplers. 19,44–49 Directional couplers
typically do not have adequate directivity for effective measurement in the typical operating imped-
50
ance regimes of plasma chambers. References 51 and 52 describe a robust voltage/current sensor
design. This sensor can be configured in a coaxial line system with a fixed-characteristic impedance
or a noncoaxial line system. When configured in the noncoaxial line system, the sensor is calibrated
in a system that is a close replication of the final configuration. The construction of the probe is in
the form of a coaxial line segment. The probe is designed with an aluminum-body outer conductor

Turn-on Turn-off
Power Steady state Late afterglow

Time
Electron energy On period (t ) Off period (t )
off
on
+ ions − ions?
Charge density electrons

FIGURE 6.10 Dynamics of pulsed RF plasma.

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