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PLASMA ETCHING

12.6 WAFER PROCESSING

is enhanced and etching is suppressed. The addition of H to mixed halocarbon-based plasma such
2
43
as CClF has similar effects as observed with CF plasma. Such effects can also be achieved by
3 4
adding CH , C H , CHF or alternatively C F , C F , and so forth to CF discharge. The underlying
4 2 4 3 2 6 3 8 4
principle of manipulating the balance (or imbalance) between polymer formation and etching is to
influence the F/C ratio in the plasma. 44
Noble gases such as Ar and He are often added to stabilize plasma. Ar addition is known to cause
inert ion bombardment of surfaces, and results in anisotropic etching in reactive ion etching (RIE)
of silicon in Cl plasma. Some believe that the addition of the chemically inert gas may significantly
2
change the electron energy distribution in plasma and alter the reactive species population. For exam-
ple, the addition of He, Ar, and Kr in BCl plasma was found to enhance the dissociation of BCl . 45
3 3
The addition of inert gas also dilutes etchant concentration in plasma.
Etch Selectivity. Etch selectivity defines the relative etch rates of materials in a plasma. Etch selec-
tivity of Si is required with respect to the etch mask, due to the implications in the choice of mask
materials and pattern transfer fidelity, or to the underlayer materials if overetch needs to involved.
Three mechanisms can be responsible for the attainment of etch selectivity:
1. Selective formation of etch inhibitor layer on one material. Take fluorocarbon-based plasma
etching of Si and SiO , for example. Both F-induced etching and film deposition take place in
2
CHF plasma. If the process conditions are chosen so that etching and deposition are nearly bal-
3
anced on SiO , this balance may be tilted to deposition on Si. This is possible because the gas-
2
phase precursor for passivating film has higher sticking coefficients and different reactivity with
the Si surface.
2. Nonreactivity of a material in plasma. Ashing of photoresist on a SiO film in O plasma is an
2 2
example. The photoresist layer volatizes by forming C-O, H-O, and other related species, but the
SiO layer is not attacked by the O plasma.
2 2
3. Nonvolatility of reaction product. An example is ashing of photoresist on the Si surface in O
2
plasma. Involatile silicon dioxide will be formed on the Si surface but the photoresist layer vola-
tizes by forming C-O, H-O, and other related species.
The most often used mask materials on silicon are photoresists and SiO . A photoresist is attacked
2
in O plasma and chlorine plasma, so selectivity is low. The presence of energetic ions in the plasma
2
also reduces the selectivity of resist masks. The oxide mask is frequently used in etching Si trenches,
patterning of polysilicon gate contacts over gate oxide, and a great number of other steps. Normally,
high Si/SiO etch selectivity of ≥30:1 is possible using F- and Cl-based plasmas. But due to high
2
Si/SiO etch selectivity, a thin native oxide layer on Si can completely prevent etching of Si or, if
2
nonuniformly etched, cause grass formation on the Si surface.
When etching SiO with Si as an underlayer, it is usually difficult to obtain good etch selectivity
2
in CF or C F plasma. However, adding 30 to 60 percent of H to the CF plasma makes it possible
4 4 8 2 4
to minimize the etching of Si as compared to the etching of SiO . 46 Si etch rate monotonically
2
decreases as the percentage of H is raised and eventually Si etch stops, while only a small decrease
2
in SiO etch rate is observed. Addition of CHF to CF plasma can also achieve similar results. 47
2 3 4
Etch Uniformity. Etch uniformity refers to two things—the evenness of etch rate across a wafer
and the degree to which etch rates are maintained from wafer to wafer in the same etcher. Uniformity
is more dependent on hardware configurations. So this is the area where plasma etch equipment
makers have invested great efforts in innovation, as evident by the presence of vast amounts of
patents. Nevertheless, there are several common factors that influence etch uniformity:
• Uniformity of etchant species, which is controlled by gas flow, chamber configuration (for exam-
ple, spacer and confinement ring), and plasma parameters such as pressure.
• Uniformity of ion density in plasma. The nonuniformity of ion density can be affected by chamber
configuration such as spacer and confinement ring. Figure 12.4 gives an example in an ICP chamber.
The nonuniformity affects ion-enhanced etch processes prominently.
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