Es. Aydil et al., MULTIPLE STEADY-STATES IN ELECTRON-CYCLOTRON-RESONANCE PLASMA REACTORS, Journal of vacuum science & technology. A. Vacuum, surfaces, and films, 11(6), 1993, pp. 2883-2892
Although electron cyclotron resonance (ECR) plasma reactors are being
used in the microelectronics industry for etching and deposition of th
in films they are prone to instabilities that can wreak havoc with man
ufacturing processes. Plasma conditions are often unstable because of
nonlinear coupling between wave propagation, power absorption, neutral
density, and charge density profiles. We report on hysteresis, multip
le steady states and abrupt transitions in an ECR plasma reactor that
can alter the plasma properties drastically. Substrate platen floating
potential and Langmuir probe measurements are used to identify severa
l abrupt transitions and regions in the operating parameter space wher
e the plasma can exist in either one of two different states under ide
ntical operating conditions (microwave power, pressure, flow rate, mag
netic field, rf bias etc.) Depending on how the plasma is started eith
er branch can be obtained. Abrupt transitions and hysteresis in plasma
properties are observed both with and without rf bias under typical c
onditions used in plasma processes. Such transitions can cause drastic
deviations in plasma parameters-density, current, uniformity-and may
result in degraded process characteristics-uniformity, rate, selectivi
ty, linewidth control. Statistical techniques used to design and devel
op processes cannot properly account for discontinuous changes in the
state of the plasma and can produce brittle processes that can suddenl
y lead to catastrophic failure. Care must be taken to map out the regi
ons of bistability for the purpose of avoiding them or controlling the
plasma in these regions. While the mechanism responsible for the mult
iple steady states and abrupt transitions reported here is not well un
derstood, it is clear that these phenomena depend critically on the ne
utral density.