Page 150 - Sami Franssila Introduction to Microfabrication

Etching 129

for example. However, both gases etch silicon too, and formation. Cu(hfac) 2 (hfac – hexafluoroacetylacetonate)
they are suitable for non-selective etching only. etching reaction proceeds according to
CHF 3 is used as oxide etch gas when selectivity
against silicon is required. It provides fluorine and CuO + 2Hhfac −→ Cu(hfac) 2 + H 2 O
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carbon for etching (SiF 4 , CO 2 etch products), and CF 2
radicals, which are polymer precursors. Polymerization The reaction products must be stable enough so that they
takes place on silicon surfaces, whereas on oxide surface can be transported away. Decomposition would result in
(CF 2 ) n polymerization does not take place due to oxygen redeposition residues and non-uniform etching.
supply: ion bombardment–induced reactions on oxide If aluminium is alloyed with copper (to improve
result in CO 2 formation. electromigration resistance), aluminium etching will be
difficult for the same reason. Al-0.5%Cu is still fairly
easy to etch but Al-4%Cu leaves residues of copper
11.6.3 Silicon nitride chlorides, which are difficult to remove.
Nitride etching has aspects of both silicon and oxide
etching. SF 6 - and CF 4 -based processes etch nitride 11.6.6 Refractory metals and silicides
fast, but isotropically and without selectivity against
silicon. They are, however, selective against oxide Tungsten etching is similar to silicon in many respects.
with selectivities of ca. 2:1. CHF 3 -based processes, In fluorine plasmas, the reaction product is WF 6 ;
on the other hand, etch nitride and provide selectivity in oxygen–halogen plasmas, it is WOF 4 or WOCl 4 .
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against silicon. In fact, CHF 3 -oxide etch processes Tungsten hexafluoride has a boiling point of 17 C and
usually perform well as nitride etch processes, and isotropic etching profile easily results. Oxyfluorides and
oxychlorides are less volatile and ion bombardment is
result in anisotropic profiles unlike SF 6 - and CF 4 -based
needed to remove them completely, which translates to
processes.
better anisotropy. Molybdenum, too, is etched by both
chlorine and fluorine plasmas, with or without oxygen.
11.6.4 Aluminum For titanium etching, chlorine etching is preferred, but
fluorine etching is possible; and for TiW (30 at %
Aluminum has native oxide, Al 2 O 3 , which is very Ti), SF 6 is a typical choice. Tantalum and niobium
difficult to etch. Chlorine (Cl 2 ) and chlorine-containing are etched similarly. Silicides WSi 2 , MoSi 2 and TaSi 2
gases are used, with AlCl 3 as the main etch product. are etched in processes that resemble silicon and/or
Multi-step etching is needed to etch aluminium: in respective metal etching.
the first 10 s, high power is used to sputter native
Al 2 O 3 away, power is then reduced to etch the bulk
of aluminium. Aluminum is spontaneously etched in 11.7 ETCH TIME AND SPACERS
Cl 2 , and a polymerizing agent is needed to passivate Etch time seems like a simple concept: film thickness
sidewalls for anisotropic profile; CHCl 3 and CH 4 are
divided by etch rate. A slight overetch is required
often used. In some low-pressure reactors, Cl 2 /BCl 3 because there are uncertainties in both etch rate and
gases without polymer-forming gases will result in in film thickness, which typically vary by, say 5%.
clean, anisotropic profiles. Nitrogen or argon is often However, when the films to be etched run over
added to stabilize the plasma and to improve photoresist topography, the situation changes dramatically.
selectivity.
If film deposition is conformal, film thickness at the
edge of a step will be the sum of the film thickness
and step height. If anisotropic etching is stopped at
11.6.5 Copper
the end point calculated from planar film thickness, a
Copper is not plasma-etched in current microfabrication residue equal to original step height remains at the edge
processes. It is a difficult material to etch because neither (Figure 11.12).
fluorides (CuF 2 ), nor chlorides (CuCl 2 ), are volatile Long overetch will eventually remove this residue
at room temperature. Increased temperature will help, but this makes high demands on etch selectivity
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but even at 100 to 200 C, the rate is low and the between the two materials. Sometimes it is desirable
photoresist is severely attacked. Organic etch gases have to leave this residue in place, and utilize it in the
been tried with modest success. The first step is the fabrication process. It is then termed spacer. Spacers
oxidation of copper, followed by volatile compound have various applications, which will be discussed in