Adhesive bonding and miscellaneous chemical manufacture – Differential fluid etching apparatus – For liquid etchant
Reexamination Certificate
2002-10-18
2004-06-15
Mills, Gregory (Department: 1763)
Adhesive bonding and miscellaneous chemical manufacture
Differential fluid etching apparatus
For liquid etchant
C134S113000, C422S083000, C073S863000
Reexamination Certificate
active
06749716
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention refers to an apparatus for assessing a silicon dioxide content of a phosphoric acid bath for etching a silicon nitride and to a system for etching silicon nitride.
Usually, in the manufacture of semiconductor integrated circuits, hot phosphoric acid baths are used for wet-etching of silicon nitride layers. Silicon nitride is generally used as a masking layer or as an insulator layer. When it becomes necessary to remove or to pattern the silicon nitride layer, normally, wet etching with hot phosphoric acid (HOT PHOS) is used. The chemical reaction that takes place is as follows:
Si
3
N
4
+
6
H
2
O
→
H
3
PO
4
3
SiO
2
+
4
NH
3
Since the etching rate of silicon nitride (Si
3
N
4
) is more than ten times higher than the etching rate of silicon dioxide (SiO
2
), silicon nitride is selectively etched. However, many factors influence the etching rate of both silicon nitride and silicon dioxide. They include: (i) a bath temperature which is held at a constant temperature (ii) a water content of the bath, which determines the overall etching rate of the bath, and, in particular, (iii) an SiO
2
content of the bath which strongly influences the selectivity.
Due to water evaporation and the reaction as indicated above, the concentrations of water as well as of silicon dioxide are continuously changing during the etching process. Therefore, for a well-controlled process, at least these two parameters should be controlled, too.
In particular, the SiO
2
content of a phosphoric acid etching bath should be in a certain range. More specifically, a minimum SiO
2
content is required in order to maintain the selectivity of the etching of Si
3
N
4
with respect to SiO
2
. On the other hand, there is only a limited amount of SiO
2
, which can be dissolved in the etching solution. More concretely, the higher the SiO
2
content in the solution, the higher the expected amount of non-dissolved SiO
2
. However, the particles could have a bad influence on the wafer surface or, even worse, could block filters of the etching equipment. Moreover, SiO
2
can even be existent as a colloid, which also negatively influences the characteristics of the etching bath.
However, it is difficult to measure the SiO
2
content of a phosphoric acid bath, especially with an in-situ method, which is necessary for the online control of the bath.
U.S. Pat. No. 6,207,068 discloses a silicon nitride etch bath containing a circulation line having a silicon dioxide condensing system formed of a heat exchanger and a secondary filter. More specifically, part of the phosphoric acid bath is cooled whereby the solubility of silicon dioxide is reduced and the precipitated silicon dioxide particles are extracted by the secondary filter. As a result, the overall silicon dioxide content of the phosphoric acid bath is reduced by a small amount. However, the system does not allow a measurement of the silicon dioxide content of the phosphoric acid bath. Accordingly, the silicon dioxide content is only reduced without any control thereof.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide an apparatus for assessing the silicon dioxide content of a phosphoric acid bath as well as a system for etching silicon nitride, which overcomes the above-mentioned disadvantages of the prior art devices of this general type.
With the foregoing and other objects in view there is provided, in accordance with the invention, an apparatus for assessing a silicon dioxide content of a phosphoric acid bath for etching silicon nitride. The apparatus contains a sensor for measuring a concentration of NH
3
in the phosphoric acid bath, a storage unit for storing data defining a relationship between a content of the silicon dioxide and the concentration of NH
3
, and a device for calculating the content of the silicon dioxide on a basis of the concentration of NH
3
measured and the data stored in the storage unit. The device is connected to the storage unit and coupled to the sensor.
According to the present invention, the above object is achieved by an apparatus for assessing the content of silicon dioxide of a phosphoric acid bath for etching silicon nitride. The apparatus contains a sensor for measuring the concentration of NH
3
of the phosphoric acid bath, a recording unit for storing data that define a relationship between the content of silicon dioxide and concentration of NH
3
, and a device for calculating the content of silicon dioxide on the basis of the measured concentration of NH
3
and the stored data.
Moreover, the present invention provides a system for etching silicon nitride and contains a process tank filled with a phosphoric acid bath, the apparatus for assessing the content of silicon dioxide of the phosphoric acid bath as defined above, and a monitoring system for monitoring the content of silicon dioxide.
As defined in the above equation, NH
3
is also a reaction product of the etching reaction. The influence of the NH
3
amount on the etching rate or selectivity has not yet been clarified so far. However, as the inventors of the present invention surprisingly found out, the concentration of NH
3
is directly related with the SiO
2
content of the solution. In particular, it has experimentally proven that both the SiO
2
and the NH
3
content increase proportional to the number of processed wafers, i.e. the amount of etched Si
3
N
4
.
As a consequence, it is possible to monitor the SiO
2
content by determining the NH
3
content of the solution. Since, in comparison to the detection of the silicon dioxide content, it is easier to assess the NH
3
content of a solution, for example by optical (in particular infrared) absorption measurement or titration, the SiO
2
content can easily be measured indirectly.
In particular, the apparatus for assessing the silicon dioxide content of the present invention contains a storage unit for storing data, which define a relationship between the content of silicon dioxide and concentration of NH
3
. The data can be, for example, obtained from calibration measurements, which have to be conducted for each etching system in given time intervals.
The system for etching silicon nitride of the present invention contains a monitoring system for monitoring the content of silicon dioxide. The monitoring system is specifically adapted to decide whether the content of silicon dioxide of the bath is higher than a first predetermined value and lower than a second predetermined value.
In particular, if the content of silicon dioxide of the bath is higher than the second predetermined value, SiO
2
particles are likely to precipitate. Accordingly, the content of silicon dioxide of the bath must be lowered, for example by simply exchanging the phosphoric acid bath.
On the other hand, if the content of silicon dioxide of the bath is lower than the first predetermined value, the high etching selectivity between Si
3
N
4
and SiO
2
is no longer given, and it is necessary to elevate the SiO
2
content of the bath. Since the SiO
2
content can easily be determined by the above mentioned method, the time at which the bath conditioning is to be finished can as well be easily determined.
Commonly used systems for etching silicon nitride also contain a circulation line for circulating part of the phosphoric acid bath. Accordingly, the sensor for assessing the NH
3
content can either be placed in the process tank or it can be placed in the circulation line. Alternatively, the sensor can be disposed in a bypass line connected to the circulation line.
The position of the sensor for assessing the NH
3
content can be chosen in dependence from the specific detection method used. As a suitable detection method any method as generally known in the art for assessing the NH
3
content of a liquid can be used. In particular, optical absorption using infrared light or titration can be used.
Accordingly, the present invention provides the now described advantages.
Ottow Stefan
Steuer Ulf
Greenberg Laurence A.
Locher Ralph E.
MacArthur Sylvia R.
Mills Gregory
Stemer Werner H.
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