Chemistry Reference
In-Depth Information
tridymite, moganite, coesite, stishovite, elchatelierite, opal, and inorganic amorphous
silica.
469,899
In particular, quartz, which makes up 12% of the minerals in the Earth's
crust, is the pure oxide representative of the silicates which comprise 90% of the min-
erals in the crust. The rates of dissolution that are of interest in the chemical etching
of silicon oxides in the electronic industry are usually larger than whereas
those in the geological field are generally smaller than This difference thus
dictates the methods for determination of silica dissolution rate in these two fields. The
dissolution rates are generally determined as weight loss or change of thickness of the
silica substrate in the field of materials science. In geological studies, on the other hand,
they are mainly determined as change of concentration of the dissolved silicates in the
solutions using silica particulate because it would take too long to cause a measurable
difference in weight or thickness in the solutions that are involved in geological
systems. It would take about half a day to dissolve a sand of 1 mm in diameter at a rate
but it would take about 10,000 years at The fields of
interest regarding dissolution of silicon oxides, in terms of the range of dissolution rates,
are illustrated in Fig. 4.1.
The etching of silicon oxides has been most extensively investigated in HF-based
solutions. Various acids, salts, and other substances may be added to the HF solution
in order to obtain better control of etch rate, sensitivity, uniformity, and stability of solu-
tion composition. The number of etching systems in terms of the possible combinations
of solutions and oxides is limitless. As indicated in Table 4.1, the numerous etching
solutions offer a wide range of etch rates for each specific oxide type. Also, silicon
oxides produced by different processes may have vastly different etch rates in a given
etch solution. For each particular solution, a given oxide has a specific etch rate and
different oxides may etch very differently in different etching solutions.
Given the large number of oxide/etchant systems, it is difficult to generalize the
etching characteristics of silicon oxides. However, an overall comparison can be
obtained by grouping the etch rate data for the different unalloyed silicon oxides in HF
solutions. Figure 4.2, which is a plot of the data listed in Table 4.1, shows the etch rates
of four silicon oxides: quartz (single crystalline), thermal oxide, chemical vapor
deposited (CVD) oxide, and anodic oxide. Several general remarks can be made: (1)
The etch rates of all the oxides increase with HF concentration; they can be roughly
described by an equation of the form:
on the order of
and the reaction order
n
is significantly
