A Model for the Etching of Ti and Tin in SC-1 Solutions
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(A/min) PSG Thermal Si02 Silicon Si3N4 TiN TiSi2 Ti
60 5 8 2 320 11.8 300
As shown in Table I, the SC-I etch rate is highly dependent on the substrate. If thin layers are to be etched, SC-I solutions can be used for a variety of etching applications such as CVD Si0 2, poly-Silicon and thermal Si02. SC-1 solutions can even be used for etching Si 3N4 if the layer thickness to be etched is less than 50 A. For etching thick layers, the applications of SC-1 solutions are limited to Ti and TiN. 447 Mat. Res. Soc. Symp. Proc. Vol. 477 01997 Materials Research Society
In all cases, the overall etching action occurs as the result of the substrate surface being oxidized and the oxide immediately dissolving into solution. The balance of these two reactions controls the overall rate of reaction. ETCHING REACTION OF TI AND TIN Titanium readily forms orange peroxy-titanate complexes with hydrogen peroxide. These are extremely soluble in water which is the main reason why titanium dissolves so rapidly in hydrogen peroxide solutions. As schematically illustrated in Figure 1, Ti and TiN are initially oxidized by either hydrogen peroxide itself, or the peroxide anion, H02",that is present due to the ammonium hydroxide in the SC-1 solution. The dissociation of hydrogen peroxide is given by:
(1)
where, Ka = [HO2"][H+]/[H202],
H2022.HO2-+H+
TiO*+-H 202 T
H202
TiSHO2H202
"H02
Fige 1: Diffatreacfionparsfereddi
-
TiO*+.HO2-
gofTiinhydgmpeomidesokti, s.
The oxidation reactions of both metals with both oxidants are given in Table II.
Metal Ti Ti
Oxidant H 20
HO2
TiN TiN
Table II: Oxidation Reactions. Reaction Ti+2H 2 02- TiO++ +20+H 2 0 Ti+2HO0- +H2 OZTiO÷÷ +40H-
2
TiN+3H 20+ H202 TiO++ +30W +NH 4 OH TiN+ HO2- +4H 2 0 TiO+ +40H- + NH 4OH
H202 HO2"
Once TiO+ is produced, it reacts further, with either oxidant, to form peroxy-titanate complexes. For example,
TiO+÷ +
HO2" •TO
H2-
(2)
with,
K
ao,-
[TiO++-HO 2 ] TiOnd][HO2-]
-
1012.29
at 18 0C[5].
and
TiO++
+ H202-
TiO+.-
20 2
(3)
with,
448
[TiO++
H2=
1H202]
[TiO÷÷] [H202]
=
at 25 0C [6,7]
1037'
Titanium is in the 4+ oxidation state in the peroxide complexes formed in reactions (2) and (3). The relative values of the equilibrium constants, KHO - and KH202, show that the 2
peroxide complex that forms with HO2- is highly favored over that produced with H202, despite the relatively small amount of HO2. In a solution of pure hydrogen peroxide, [TiO+ •HO2"1]/ [TiO++- H202] is greater than 3x10 6. As shown below this ratio will only increase in SC-1. Solution Chemistry SC-1 solutions typically consist of mixtures of water with H202 ( 31wt0/o) and NH4 OH (29wt%/o) in ratios from 5:1:1 to 100:1:1. In order to quantify the chemistry occurring in SC-1 solutions, two dissociation reactions in addition to Equation (1) must be considered: NH 40H
+NH4 + + O-
H20 -"•H+ +OH--
where, Kb= [NH 4 ] [ OHW]/[NH 4OH] where, K,= [OH'] [H+]
(4) (5)
NH4 OH itself can be oxidized by strong oxidizing agents leading to the formation of dissolved HNO 2, N0 2 - and NO
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