Drying and gas or vapor contact with solids – Process – Gas or vapor contact with treated material
Reexamination Certificate
2001-03-28
2002-06-18
Lararus, Ira S. (Department: 3749)
Drying and gas or vapor contact with solids
Process
Gas or vapor contact with treated material
C034S072000, C034S443000, C034S428000, C134S061000, C134S902000, C134S104100
Reexamination Certificate
active
06405452
ABSTRACT:
FIELD OF THE INVENTION
The present invention generally relates to a method and an apparatus for drying wafers after a wet bench process and more particularly, relates to a method and an apparatus for drying wafers by alcohol vapor after a wet bench process is conducted by incorporating a DI water rinse step, an inert gas blown dry step and a step of reducing the alcohol vapor flow rate into the drying tank.
BACKGROUND OF THE INVENTION
In the fabrication of semiconductor devices, a large quantity of deionized (DI) water is frequently used to clean wafers in a wet bench process. For instance, when residual chemical must be removed from the surface of a wafer, DI water rinse is used in the wet bench process to perform major wafer cleaning operations such as quick-dump-rinse and cascade overflow rinse. It is desirable that the surface of the wafer be cleaned by DI water after a chemical or polishing process has been conducted on the wafer, i.e. oxide or nitride deposition, etching or chemical mechanical polishing process. The wet bench wafer cleaning step can be accomplished by equipment that is installed either in-line or in a batch-type process.
A typical automated wafer scrubber combines brush and solution scrubbing by DI water. The scrubber utilizes a hyperbolic high-pressure spray of DI water with a retractable cleaning brush. A typical wafer scrubbing process consists of a DI water spray step followed by a spin dry and nitrogen gas blow dry step. More recently, the solvent drying technology such as the use of isopropyl alcohol (IPA) has been developed to further improve the drying technology.
In a solvent drying technology, such as one that utilizes IPA shown in
FIG. 1
, the drying process is conducted in a static manner or with the wafer positioned statically without movement. The wafer drying
10
is constructed of a drying tank
12
constructed with a wafer receptacle
14
, a chiller
16
, a sidewall heater
18
and a bottom heater
20
. A cleaned and wet wafer is transported into the drying tank
12
, or the vapor chamber. A vapor of IPA is transported into the chamber cavity
22
by a carrier gas such as a high purity nitrogen, or any other high purity inert gas. The vapor enters into cavity
22
is then heated by the bottom heater
20
such that IPA is further vaporized and rises into the cavity
22
. The wafer
24
is surrounded by the IPA vapor and, due to the high volatility of IPA, water on the wafer surface can be evaporated away without leaving any water mark, contaminating particles or metal particles. The vapor pressure of IPA can be suitably adjusted such that there is a steady flow of IPA vapor in the cavity
22
fed from the IPA reservoir tank
26
.
In the conventional IPA drying tank
10
shown in
FIG. 1
, the only moving part for transferring wafers into and out of the chamber cavity is a robot arm. There are no other moving parts which can produce contaminating particles. The IPA drying chamber can thus be kept in an extremely clean condition to avoid any contamination of the wafer surface. To further maintain the cleanliness of the chamber cavity
22
, an air filter
28
is utilized for filtering incoming air into the cavity
22
and for providing a suitable flow rate of the IPA vapor. After the cleaning process is completed, the water-containing IPA vapor is condensed by the chiller
16
into IPA liquid and is collected at the bottom of the drying chamber
12
for recycling and reuse by the process. The IPA vapor drying process is normally controlled by three major elements, i.e. the purity and the water content of IPA; the flow rate and flow speed of the IPA vapor; and the cleanliness of the IPA vapor.
An improved solvent drying technique has been proposed in recent years which is similar in principal to that described above. In a Maragoni dryer, the drying principal is based on the different surface tensions of IPA and DI water. The different surface tensions cause the ejection of water molecules from the wafer surface which are then collected by a reservoir in the drying apparatus. The Maragoni drying process is carried out by slowly withdrawing a wafer from a DI water tank immersed in DI water. At the same time, IPA vapor carried by N
2
carrier gas is flown onto the wet wafer surface such that IPA is saturated on the exposed wafer surface above the water level. Since the concentration of IPA on the surface of the exposed wafer is larger than the concentration of DI water, the surface tension of IPA is smaller than the surface tension of water in the water tank. This causes the water molecules on the surface of the exposed wafer to be retracted into the water tank and thus achieving the drying purpose.
In the Maragoni drying process, the alcohol vapor frequently cause other processing difficulties. For instance, when residual alcohol is left in the drying tank by depositing on the tank walls, alcohol becomes a source of contamination for the bare silicon surface and causes silicon hole defects. Since alcohol is a necessary element in the Maragoni drying process, it is difficult to completely eliminate residual alcohol on the chamber walls after each batch of wafers is cleaned.
TABLE I
Defect Count
Test
Slot
Slot
Slot
No.
Condition
POD
1
24
25
1
All DIW bath (DIW
1
5
7
6
filling) + no
MG/D
2
Empty DIW bath
1
162
11
91
(no water) + MG/D
2 times
3
Empty DIW bath
1
62
11
161
(no water) + MG/D
1 time
4
Empty DIW bath
1
13
9
18
(no water) + no
MG/D
In a normal Maragoni drying process, it has been discovered that the wafer positioned in slot
25
suffers the most severe effect caused by any contaminating conditions in the process. As shown in Table I, Test No. 1 was conducted with the bath filled with DI water, however, no Maragoni drying process was conducted, i.e. the wafers were not exposed to alcohol vapor. It was shown that the defect count is very low on wafers selected from the three different slots of
1
,
24
and
25
. When the Maragoni drying process was conducted two times, as in Test No. 2, where the bath was empty with no water, the defect counts increases drastically from 6 to 91. A similar Maragoni drying test was conducted in Test No. 3 wherein the Maragoni drying process was carried out only once, the defect count is still very high when compared to data in Test No. 1. Test No. 4 was conducted under conditions similar to that used in Test No. 1, i.e. with no Maragoni drying process being conducted. It is seen that the defect count is very low due to the lack of exposure of the bare wafer surfaces to alcohol, or to the organic vapor of alcohol.
The data shown in Table I suggests that the residual alcohol, or the organic residue after a Maragoni drying process, is the major cause of the high defect count on the wafers. The defects are frequently shown as silicon holes on the bare silicon surface. When the defect count is higher than about 75, the entire wafer is considered as unacceptable for quality reasons. When the defect count is much higher than 75, the wafer may be scrapped. Ideally, the defect count on the bare silicon wafer should be less than 40 for the wafer to pass quality control tests.
It is therefore an object of the present invention to provide a method for drying wafers after wet bench that does not have the drawbacks or shortcomings of the conventional Maragoni drying process.
It is another object of the present invention to provide a method for drying wafers after wet bench that minimizes the amount of organic residue left in the drying apparatus.
It is a further object of the present invention to provide a method for drying wafers after wet bench by reducing the flow rate of alcohol vapor into the drying tank.
It is another further object of the present invention to provide a method for drying wafers after wet bench by reducing the flow rate of alcohol vapor into the drying tank to less than 20 liter/min.
It is still another object of the present invention to provide a method for drying wafers after wet bench by providing an additional fluid conduit into the dryin
Cheng Chia-Chun
Guo Ming-Dar
Hsiao Yu-Chien
Twu Jih-Churng
Lararus Ira S.
Rinehart K. B.
Taiwan Semiconductor Manufacturing Co. Ltd
Tung & Associates
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