Cold trap equipped with curvilinear cooling plate

Details Cold trap equipped with curvilinear cooling plate Cold trap equipped with curvilinear cooling plate

1999-08-02

2001-06-05

Smith, Duane (Department: 1724)

Gas separation

Deflector

With heating or cooling means

C055SDIG001, C062S055500

Reexamination Certificate

active

06241793

ABSTRACT:

FIELD OF THE INVENTION
The present invention generally relates to a cold trap used in a semiconductor manufacturing process for trapping reaction byproducts in a furnace exhaust, and more particularly, relates to a cold trap that is equipped with a curvilinear cooling plate with cooling fins for trapping reaction byproducts and unreacted gases in a furnace exhaust during a silicon nitride deposition process.
BACKGROUND OF THE INVENTION
Silicon nitride has been an important material in various semiconductor applications. For instance, silicon nitride has been used as a mask against oxygen diffusion during a local oxidation (LOCOS) process; as a passivation layer for its superior barrier property to contaminants; as a gate dielectric layer in memory devices; and as an interlevel dielectric layer in an oxide-nitride-oxide (ONO) stacked-gate structure. Silicon nitride also has superior barrier properties against metal ions and moisture.
Silicon nitride has been widely used as a passivation layer for protecting a semiconductor component. Silicon nitride can be formed by either a LPCVD or PECVD technique. The LPCVD technique, where dichlorosilane is used as the reactant gas, can be carried out in a hot-wall LPCVD system, such as in a vertical furnace. The chemical reaction can be described as follows:
 3SiH
2
Cl+10NH
3
→Si
3
N
4
+6NH
4
Cl+6H
2
The hot-wall LPCVD system is normally carried out at a temperature between about 750°~800° C. and the chamber pressure is kept at several hundred m Torr. A layer of stoichiometric silicon nitride can thus be deposited on a wafer surface. A typical deposition equipment utilizing a vertical furnace is shown in FIG.
1
.
During a vertical furnace silicon nitride deposition process, as described by the above mechanism for the chemical reaction, a reaction by-product such as ammonium chloride (NH
4
Cl) in the form of a fine powder can easily deposit on any cold surface in the furnace or in the ducting system for the furnace. The ammonium chloride powder must be captured by a cold trap such that it does not form on the inner walls of the ducting system or in the furnace presenting a serious contamination source. For instance, fine powder in the ducts may be syphoned back into the furnace during a deposition process if the pressure in the furnace is not carefully controlled. The capture efficiency of the cold trap for the ammonium chloride fine powder is therefore an important factor in the successful deposition of silicon nitride films in a furnace technique.
As shown in
FIG. 1
, a vertical furnace unit
12
is the heart of a silicon nitride deposition system
10
. During the deposition of a silicon nitride film on a plurality of wafers positioned in the vertical furnace, the furnace exhaust gas
14
which contains unreacted reactant gases such as dichlorsilane, ammonium and reaction byproduct ammonium chloride powder is sent through a cold trap
20
before it enters into a gas treatment unit
18
and be released into a factory exhaust system
22
. The capture of substantially all the ammonium chloride fine powder in a cold trap
20
is therefore an important step in a successful exhaust gas treatment process for depositing silicon nitride.
A cross-sectional view of a conventional cold trap
20
complete with an inlet
24
and an outlet
26
is shown in FIG.
2
. The cold trap
20
is normally constructed of a curvilinear housing
28
which supports a cooling plate
30
therein. The cooling plate
30
has a flat platen structure with a cavity contained therein for allowing a cooling medium to pass therethrough. A suitable cooling medium can be chilled water, i.e., chilled deionized water or city water at a temperature of about 15° C. The cooling plate
30
is normally constructed in a rectangular shape, i.e., having a dimension of about 4 inch×6 inch and is equipped with a plurality of cooling fins
32
extending from a front surface
36
of the cooling plate
30
. The cooling fins
32
are provided to facilitate the evaporation of heat absorbed by the cooling medium in the cavity and furthermore, to provide a cold surface for the deposition of ammonium chloride powder. The cooling plate
30
is further provided with a cooling medium inlet and a cooling medium outlet (both not shown) for the input and output of the cooling medium into and from the cavity.
In the configuration of the cooling plate
30
shown in
FIG. 2
, the cooling plate and the spherical housing
28
must be frequently cleaned, i.e., by a preventive maintenance cleaning procedure in about every two weeks. Fine particles
38
of ammonium chloride tend to clog the cooling fins
32
and thus blocking the inlet
24
for the exhaust gas. The requirement for frequent cleaning of the cold trap
20
therefore presents a problem in the silicon nitride furnace system in causing down time which reduces the fabrication yield.
It is therefore an object of the present invention to provide a cold trap that can be efficiently used in a semiconductor fabrication process for collecting unwanted particles that does not have the drawbacks or shortcomings of the conventional cold traps.
It is another object of the present invention to provide a cold trap that can be used effectively in a semiconductor material deposition system such that the cleaning frequence required for the cold trap can be reduced.
It is a further object of the present invention to provide a cold trap for use in a semiconductor fabrication process which does not need frequent cleaning.
It is another further object of the present invention to provide a cold trap for use in a semiconductor fabrication process that is equipped with a curvilinear cooling plate.
It is still another object of the present invention to provide a cold trap for use in a semiconductor film deposition system that is equipped with a convex cooling plate equipped with cooling fins extending from the convex surface.
It is yet another object of the present invention to provide a cold trap for use in a vertical furnace silicon nitride film deposition system which is equipped with a curvilinear cooling plate.
It is still another further object of the present invention to provide a cold trap for use in a silicon nitride furnace deposition process that is equipped with a convex cooling plate and cooling fins in the convex surface.
It is yet another further object of the present invention to provide a cold trap for use in a vertical furnace for depositing silicon nitride films wherein the trap has greatly improved efficiency for trapping ammonium chloride fine powder.
SUMMARY OF THE INVENTION
In accordance with the present invention, a cold trap that is equipped with a curvilinear cooling plate having greatly improved trapping efficiency for fine powder is provided.
In a preferred embodiment, a cold trap for collecting unwanted reactants and particles in an exhaust gas is provided which includes a housing of generally curvilinear shape, a gas inlet at a first end of the housing, a gas outlet at a second end of the housing opposite to and in fluid communication with the gas inlet, and a cooling plate equipped with cooling fins in a convex surface of the plate facing the gas inlet, the cooling plate is cooled by a heat-transfer fluid.
In the cold trap for collecting unwanted reactants and particles, the housing may be formed in a spherical shape. The cooling plate may be cooled by a heat-transfer fluid flown through a cavity contained in the cooling plate. The cooling plate may further include a cavity contained therein, a cooling fluid inlet and a cooling fluid outlet. The cooling plate may have a rectangular shape.
In the cold trap for collecting unwanted reactants and particles, the cooling plate may further include a plurality of cooling fins positioned between about 0.2 cm and about 2 cm apart, preferably between about 0.5 cm and about 1.5 cm apart, and more preferably between about 1 cm apart. The cooling plate may have a curvature that has a diameter of at least 12.5 cm. The cooling plate may have a curvature that has a maximum dep

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