Page 147 - Sami Franssila Introduction to Microfabrication

126 Introduction to Microfabrication

Table 11.4 Typical etch gases
Fluorine Chlorine Bromine Stabilizers Scavengers/
others

CF 4 Cl 2 HBr He O 2
SF 6 BCl 3 Ar
CHF 3 SiCl 4 N 2
NF 3 CHCl 3
C 2 F 6
C 4 F 8
XeF 2

Plasma etching is based on reaction product volatility.
Silicon is easily etched by halogens (Table 11.4): both
Figure 11.9 Plasma etching system (RIE, Reactive Ion
Etcher): gases are introduced through the top electrode, fluorides (SiF 4 ), chlorides (SiCl 4 ), and bromides (SiBr 4 )
wafers are on the powered bottom electrode of silicon are volatile at room temperature, at millitorr
pressures. No ion bombardment is needed for etching if
the reactions are thermodynamically favoured and the
tools could be used to increase device-packing density. role of ion bombardment is to induce directionality.
Plasma etching has been an indispensable tool since the Silicon nitride (Si 3 N 4 ) is etched by fluorine, producing
early 1980s, and it has always been able to etch, with SiF 4 and NF 3 . Aluminum is spontaneously etched by
high precision, those structures that lithography has been Cl 2 , but the surface of aluminium is always protected
able to print in photoresist. by native aluminum oxide, and aluminium etching can
Plasma etching is done in a vacuum chamber by only commence after this oxide has been removed. Ion
reactive gases excited by RF-fields (Figure 11.9). Both bombardment is essential for native oxide removal.
the excited and ionized species are important for plasma
etching. Excited molecules like CF are very reactive,
∗
4
and ionic species like CF + are accelerated by the RF 11.4.2 Plasma etch mechanisms
3
field, and they impart energy directionally to the surface. Chemical bonds need to be broken for etching to
Plasma etching is thus a combination of chemical take place. Bond energies, therefore, give indications
(reactive) and physical (bombardment) processes. of possible etching reactions (Table 11.5). Reactions
that lead to bonds stronger than the Si–Si bond
will etch silicon; and if the products have stronger
11.4.1 Plasma etch chemistries
bonds than Si–O, silicon dioxide will be etched.
These simple predictions are experimentally confirmed:
In a plasma discharge, a number of different mecha- fluorine, chlorine and bromium will etch silicon because
nisms for gas-phase reactions are operative. Discharge silicon–halogen bonds are stronger than silicon–silicon
generates both ions and excited neutrals, and both are bonds. Only Si–F bond is stronger than Si–O bond
important for etching.
and therefore only fluorine is predicted to etch oxide.
However, because of ion bombardment, oxide is slightly
−
Ionization e + Ar −→ Ar + 2e −
+
etched in chlorine and bromine plasmas also, but to a
∗
Excitation e + O 2 −→ O 2 + e − much lesser extent than in fluorine plasmas.
−
In practice, the volatility of reaction products (i.e.,
−
∗
Dissociation e + SF 6 −→ e + SF 5 + F ∗
−
high vapour pressure) is used as a criterion for
etchant selection. Boiling points of reaction products
The most abundant species in the plasma reactor is the
source gas. Etch reaction products are the next most
abundant, and they may represent a few or 10% of Table 11.5 Bond energies (kJ/mol)
all moieties. Excited neutrals may be present at a few C–O 1080 Si–F 550
percent, but ions are just a very minor component, Si–O 470 Si–Cl 403
1 in 100 000. They are, however, often important for Si–Si 227 Si–Br 370
the mechanism.