Imec and KLA Tencor have
established a metrology method for optimizing the etch rate uniformity (ERU)
in a transformer coupled plasma (TCP) reactor. The proposed metrology method
makes use of PVx2 sensor wafers. For ion-assisted etch processes, the use of
this PVx2-based method for ERU tuning results in lot-turn time savings of up
to 80% compared to conventional etch rate uniformity tests.
Uniformity control in plasma etch processes and process stability requirements
are major challenges in IC high-volume production and they are becoming more
aggressive for each new technology node. Therefore, the availability of a metrology
suited for predicting and tuning the etch rate uniformity has become a key element
for effective process optimization. Imec and KLA Tencor have now demonstrated
that PlasmaVoltTM X2 (PVx2) sensor wafers, for some specific process etch chemistries,
offer a fast and reliable way of ERU tuning and can significantly improve lot-turn
Principles of plasma generation and an equivalent electrical representation of PVx2 in a TCP reactor: the transformer coupled capacitive tuning (TCCT) parameter is able to specify the portion of power applied to center vs. outer segment of the top coil. The higher the TCCT setting, the more power is directed towards the center segment of the TCP coil.
With a PVx2 sensor wafer, the RF current that is passing through each of the
surface-mounted capacitive detectors is measured and linked to the local plasma
impedance. The researchers performed their experiments in a transformer coupled
plasma reactor and used the transformer coupled capacitive tuning (TCCT) parameter
to change the etch rate uniformity distribution. Experiments to study the influence
of this parameter were carried out on blanket wafers (poly-Si and SiO2) as well
as on patterned wafers with 20nm half-pitch back-end-of-line interconnect trenches
(SiOC, BARC, and SiN etch steps). For various typical etch chemistries and different
settings of the TCCT parameter, the RF signals recorded by the PVx2 sensor wafers
were compared with conventional etch rate uniformity measurements. The latter
included thickness measurements (in case of blanket wafers) and critical dimension
(CD) uniformity measurements (in case of patterned wafers).
For blanket wafers, the wafer sensor provides an adequate tool to predict etch
rate uniformity trends in a transformer coupled plasma reactor for etching processes
that require ion assistance, such as SiO2 etching. For these processes, it has
been shown that lot-turn time savings of 80% or more can be achieved when compared
to single-use blanket wafer etch rate uniformity tests. For other types of processes
that involve e.g. spontaneous chemical reactions (like for poly-Si etching involving
fluorine-based chemistries), the method turns out to be less effective.
For the 20nm half-pitch wafers, a good correlation of the sensor wafer and
CD uniformity data was found for SiOC and BARC etching, but not for the SiN
etch step. For SiN etching, a CH3F/O2 plasma is used that is highly selective
towards the masking material, and this more chemistry-dominated mechanism most
likely explains the observed discrepancy.
These results have been presented at the 21st Annual IEEE/SEMI Advanced Semiconductor
Manufacturing Conference (ASMC 2010).