Silicon Nitride Etch
of Silicon Nitride (Si3N4)
with Hot Phosphoric Acid (H3PO4).
By Scott Clark, MSCE
Silicon nitride (Si3N4)
films have several important applications in advanced semiconductor
devices. Included among the integrated circuit (IC) manufacturing
applications of silicon nitride films are: as an alkali, water,
and oxygen diffusion barrier;1,2 as masking layers
for local oxidation of silicon (LOCOS);3 as final
mechanical protection layers for ICs;4 and as electrical
insulators which have a comparatively high dielectric constant.5
A number of techniques have been used to form silicon nitride
films. A summary of the various techniques used to prepare silicon
nitride films is presented in Table 1.
Table 1. Summary of Preparation Techniques
adapted from .
Silicon nitride films can be etched by hydrofluoric
acid (HF), buffered HF, and phosphoric acid (H3PO4)
solutions. The rate of etching silicon nitride films depends
on the exact stoichiometry of the films, the substrate temperature
during deposition, and the density of the films.1,2,5
The difference in etch rates based upon the difference in film
preparation is shown in Table 2.
Table 2. Etch Rates of Silicon Nitride, adapted
The focus of this paper will be on the etching
of silicon nitride with silicon dioxide used as a mask in hot
phosphoric acid. It has been reported that silicon nitride is
etched at a significantly higher rate than silicon dioxide when
hot phosphoric acid (H3PO4)
is used as an ethcant.2 The results of the work done
by van Gelder and Hauser2 are presented in Table
Table 3. Etch Rates of Si3N4
and SiO2 in H3PO4
adapted from .
The reaction equation for silicon nitride etching in phosphoric
acid can be summarized as follows:
From Equation 1 it can be seen that water is hydrolyzing the
silicon nitride to form hydrous silica and ammonia, the ammonia
remains in the solution in the form of ammonium phosphate. This
stoichiometry suggests that water is an integral part of the
chemistry involved in the etching of silicon nitride. It can
also be seen from Equation 1 that as the nitride layer is etched
hydrated silicon dioxide (H2OSiO2)
is formed which in turn inhibits the etching of SiO2
from the surface of the wafer. This is the result of the Le
Châtelier's principle; that is, a system in equilibrium
will react in such a way as to maintain equilibrium. Hence,
silicon dioxide may initially be etched from the wafer surface,
but as the SiO2 concentration
continues to increase over time in the etching solution the
rate at which SiO2 is etched
from the wafer surface decreases.
Chemistry and Application
When using phosphoric acid as an etchant it is important to
consider the effect that temperature has on acid concentration
and, as a result, the etch rate and selectivity. The relationship
between the boiling point temperature and phosphoric acid concentration
is shown in Figure 1.
Figure 1. Boiling Point Temperature vs.
Concentration, adapted from .
Within the semiconductor industry the typical operating temperatures
for phosphoric acid range from 150 to 180C. Thus, at these boiling
point temperatures the concentration of H3PO4
ranges from ca. 85 to 92 weight percent (wt%). The difficulty
with using phosphoric acid at these elevated temperatures is
that maximum concentration at which phosphoric acid is commercially
available is 85 wt% which has a boiling point of ca. 154C. Since
it not possible to heat a solution beyond its boiling point,
it is necessary that some water be removed so that the solution
becomes more concentrated thereby making it possible to heat
the solution to higher temperatures. Operating at elevated temperatures
is further complicated by the requirement that the acid concentration
must be strictly maintained to regulate the temperature of the
This requirement is the result of the Gibb's phase rule. The
number of independent variables that must be arbitrarily fixed
to establish the intensive state of a system, that is, the degrees
of freedom of the system, is given by the Gibb's phase rule:
Where F = degrees freedom, =
number of phases, and N = number of chemical species. Considering
the water-phosphoric acid system for the two phases liquid and
gas in equilibrium, one finds that there are two degrees of
freedom. Thus, of the three variables pressure, temperature,
and concentration one can only choose two; the third is fixed
when the choice is made. Therefore, when operating at ambient
pressure only one degree of freedom remains to be selected.
Hence, for a given operating temperature the concentration is
fixed. As the temperature is elevated water is evolved which
in turn causes the acid concentration to increase; the temperature
of the bath will increase to the boiling point of the resulting
Therefore, a constant temperature can be maintained by regulating
the acid concentration. This can be accomplished by using a
reflux system or by replacing the water at the same rate at
which it is evolved. The former method is difficult to maintain
in a production environment. Thus, the latter method is commonly
used in the semiconductor industry. This method provides stable
temperature and acid concentration; two important considerations
It is of paramount importance that the temperature be controlled
by the acid concentration and not by proportioning the heat
source. If the temperature is controlled by proportioning the
heat source water could be evolved, yet the temperature would
not increase due to the proportioning of the heaters. This would
result in an ever increasing acid concentration which would
be detrimental to the consistency of the etch rate from one
lot to the next. The effects of varying acid concentration on
the etch rate of silicon nitride are illustrated in Figure 2.
Figure 2. Etch Rates of Silicon, Silicon
Dioxide, and Silicon Nitride in Hot
Phosphoric Acid, adapted from *.
*In the work conducted by van
Gelder and Hauser the Si3N4
film was prepared by the use of a pyrolytic process with SiH4
and NH3 at 880C, the SiO2
film was prepared by reacting SiCl4
with O2 and H2
Here the solid line labeled Si3N4
represents the etch rate at the boiling point of the solution,
whereas the dashed line represents the etch rate of concentrated
phosphoric acid at a given temperature. The results presented
in Figure 2 show that within the temperature range of 150 to
180C the silicon nitride etch rate of concentrated H3PO4
is lower than the etch rate of a more dilute acid concentration..
For example, a solution which boils at 160C (ca. 87 wt% H3PO4--see
Figure 1) produces a nitride etch rate of ca. 55 Å/min,
at the same temperature a 94.5 wt% phosphoric acid etches nitride
at a rate of ca. 25 Å/min. This implies that for a given
temperature a solution which has a higher water content will
result in a higher nitride etch rate.
However, it is important to remember that while a more dilute
acid concentration results in a higher nitride etch rate there
is a limit as to how much water can be added at a given temperature.
That is, since it is not possible to heat a solution above its
boiling point one is limited to the boiling point concentration
for that specific temperature. Thus, at 160C the maximum sustainable
water concentration is 13 wt%, which corresponds to a H3PO4
concentration of 87 wt%. If the solution is diluted such that
concentration is lowered to 86 wt% the maximum temperature of
the solution would be ca. 156C which would result in a nitride
etch rate of ca. 50 Å/min. This etch rate is 9 % lower
than the etch rate of the more concentrated 87 wt% solution
at the higher temperature of 160C.
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