Etching of Oxides - Electrochemistry of Silicon and Its Oxide - page 164

dissolve faster in HF solution than in BHF solution is because is hygroscopic and
more soluble in water than is . 425 The etching process of doped glasses may thus
be seen as a combined result of the dissolution of the by fluoride species and that
of the dopant oxide by water. This hypothesis is supported by the observations that (1)
there is an initial increase in the etching rate of heavily doped glasses with increasing
water content and (2) in BHF the etch rate maximum shifts to more dilute solutions
with increasing dopant concentration. The relative contribution of the dissolution of
by water and the dissolution of by HF determines the overall etching rate
of glasses. The difference in the etch rates of the glasses with different
dopants is then related to the relative solubilities of the dopant oxides.
Monk et al . 451 attributed the role of doping elements in the oxide to the electronic
characteristics of the elements. Whether the doped atom is electrophilic or nucleophilic
toward the Si atom governs how easily is coordinated with the Si atom. If the oxide
film has an electrophilic dopant, the etch rate increases, and if it is nucleophilic, the
etch rate decreases. In a PSG film the doped P atom, which has a valence of 5, has one
valence electron more than Si. This means that a doped oxide can provide the electron
to oxygen more easily than can a nondoped one. Therefore, the silicon-oxygen bond
in P-doped oxide can be broken more easily.
The presence of hydroxyl in silicon oxide causes a high etch rate since hydroxyl
groups are nonbridging oxygen in the oxide network and a high concentration of
hydroxyl groups means looser structure. If a thermal oxide is hydrolyzed, the etch rate
becomes much higher (Fig. 4.12). 427 Also, the relatively high etch rates of anodic oxides
(Fig. 4.2) can be attributed to their loose structure due to the high concentration of water
and hydroxyl groups in the oxides.
Nielsen and Hackleman 380 found that the dissolution of thermal in BHF solu-
tions is impeded by the application of a cathodic potential of 2-4V across the oxide
film and postulated that a layer of partially reduced oxide is formed on the surface and