104 / 390
with inductively coupled plasma). Both steps bear an enormous potential to lose
boron and phosphorus which are known to form several volatile compounds.
How can these losses be explained? According to the model of silicon dissolution
in HF/HNO
3
mixtures by Robbins and Schwarz the reaction products are only H
2
SiF
6
,
NO and H
2
O [2,3]. However, this model turned out to be insufficient: The reduction
of nitric acid yields to various nitrous oxides, such as NO
2
[4], NO, N
2
O [7] and N
2
O
3
[4,5] and proceeds finally to the ammonium ion [6]. Besides the nitric acid other
reactive species have been identified that are involved in the reaction again [4]. Kooij
et al. detected hydrogen as reaction product [7], which was later quantified as function
of the etching mixture composition and temperature [8]. The material balance by
Acker et al. reports the formation up to 0.33 mol H
2
per mol of dissolved silicon in a
mixture of 90% HF / 10% HNO
3
(v/v) [9]. So it is reasonable to assume that the
massive liberation of nitrogen oxides and hydrogen favours losses of volatile boron
and phosphorus compounds throughout the dissolution of silicon.
At first the volatile phosphorus compounds should be discussed. It is known that
phosphine is formed from phosphidic intermetallic compounds by hydrolysis in water.
During the dissolution of highly doped silicon in alkaline medium already the
formation of gaseous PH
3
is indicated by its characteristic smell [10]. Once formed,
PH
3
is lost for the chemical analysis since it practically not hydrolyzes in aqueous
solution. [11] Furthermore, it is reasonable to assume, that PF
3
and PF
5
are formed in
the dissolution of Si. PF
3
hydrolyzes very slowly in aquerous solution to phosphorous
acid and HF (Eq. 1) [11].
(
)
3) + 2 +32 2+ +)
⎯⎯→
(1)
H
3
PO
3
is a strong reducing agent that is oxidized [11] to phosphoric acid (Eq. 2).
(
)
(
)
⎯⎯→
+
←⎯
+32 2+ + 2 32 2+ + H
(2)
In aqueous solution PF
5
hydrolyses slowly and stepwise via phosphoryl fluoride to
phosphoric acid [11] according to Eq. 3.
(
)
(
)
(
)
+ 2
+ 2
+ 2
+ 2
+)
+)
+)
+)
3)
32)
32 2+ )
32 2+ )
32 2+
+
+
+
+
−
−
−
−
⎯⎯⎯→ ⎯⎯⎯→
⎯⎯⎯→
⎯⎯⎯→
←⎯⎯⎯ ←⎯⎯⎯
←⎯⎯⎯
←⎯⎯⎯
(3)
Whichever gaseous phosphorus compound is formed, extensive studies by ICP-
OES and ion chromatography proved that the dissolved phosphorus is entirely present
as phosphate. This implies that the major loss of phosphorus primarily occurs during
Si dissolution [10].
BF
3
was identified as the major volatile boron compound in the HF/HNO
3
dissolution of silicon [8]. It was furthermore shown, that boron losses up to 30% can
be achieved if silicon is dissolved under harsh conditions and high dissoluion rates
[10]. Under moderate conditions most of the BF
3
seems to react with water to give
boric and hydrofluoric acid (Eq. 4).
(
)
⎯⎯→
%) + 2 % 2+ +)
(4)
BF
3
reacts with hydrofluoric acid subsequently to tetrafluoroboric acid (Eq. 5) [11]:
⎯⎯→
%) +)
+%)
(5)
HBF
4
and boric acid are in equilibrium to each other and linked by several
intermediates, which is determined by HF/F
-
concentration and pH [13]:
(
)
(
)
(
)
(
)
±
±
±
±
⎯⎯⎯→
⎯⎯⎯→
←⎯⎯⎯
←⎯⎯⎯
⎯⎯⎯→
⎯⎯⎯→
←⎯⎯⎯
←⎯⎯⎯
B
B
B
B
+ 2
+ 2
+)
+)
+ 2
+ 2
+)
+)
+%)
+%) 2+
+%) 2+
+%) 2+
+% 2+
(6)
In contrast to phosphorus, boron has the potential to evaporate during the
evaporation of the silicon and acid matrix. The volatility of BO
x
species from HF-free
96