Cleaning and liquid contact with solids – Processes – Including use of vacuum – suction – or inert atmosphere
Reexamination Certificate
2001-06-04
2003-05-13
El-Arini, Zeinab (Department: 1746)
Cleaning and liquid contact with solids
Processes
Including use of vacuum, suction, or inert atmosphere
C134S003000, C134S018000, C134S026000, C134S028000, C134S030000, C134S902000
Reexamination Certificate
active
06562144
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention.
The present invention provides a method for cleaning a surface of a silicon wafer, and more particularly, a method for cleaning the surface of a wafer with a line width below 0.15 micrometers.
2. Description of the Prior Art
In integrated circuit fabrication processes, a wafer has to repeatedly go through various chemical processes, such as chemical vapor deposition (CVD), dry etch, or chemical mechanical polishing (CMP), etc., which leave contaminating particles on a surface of the wafer. Moreover, a surface of the substrate is easily oxidized and forms a native oxide layer, which effects quality of the wafer product. For these reasons, a wafer has to be cleaned several times during processing in order to remove the contaminates and the native oxide layer, so as to maintain surface cleanliness.
In general, there are two types of cleaning methods, dry and wet. The wet cleaning method is used more frequently. The wet cleaning method employs an RCA cleaning technique, which makes use of the following solutions:
1) RCA Standard Clean 1 (SC-1), which can remove the micro-particles left on the surface of a wafer to be cleaned, and is made of one part NH
4
OH, one part H
2
O
2
, and five parts H
2
O, and in which the wafer is immersed for 5 minutes at 70° C.
2) RCA Standard Clean II (SC-2), which can remove the metallic particles left on a wafer's surface, and is made of one part HCl, one part H
2
O
2
, and six parts H
2
O, and in which the wafer is immersed for 5 to 10 minutes at 70° C.
3) Piranha (SPM), which can remove the organic particles from the surface of the wafer, is made of five parts H
2
SO
4
and one part H
2
O
2
, and in which the wafer is immersed for 10 to 15 minutes at 120° C.; and
4) Diluted hydrofluoric acid clean (DHF), which is used to remove the silicon dioxide layer from the surface of the wafer, and is made of one part HF and fifty parts H
2
O, and in which the wafer is immersed for 10 to 30 seconds at room temperature.
Recently, in order to save cleaning agents and improve cleaning quality, other cleaning agents than the above mentioned have been developed. For example, an O
3
/H
2
O solution which, being a good oxidizing agent, can remove organic particles and metallic particles at room temperature, is developed to replace SPM, which works well only at the temperature above 120° C. Moreover, surfactant and ultrasonic oscillation techniques are often used with the conventional cleaning processes, so as to achieve optimal cleaning results.
Please refer to FIG.
1
.
FIG. 1
is a flow chart of a cleaning process
10
for cleaning wafers at room temperature according to the prior art. As shown here, the cleaning process
10
comprises the following steps:
Step
12
: Immersing the wafer to be cleaned in a solution of O
3
in water (O
3
/H
2
O), as O
3
is a good oxidizing agent, to remove hydrocarbons, other organic particles, and inactive metallic ions, such as Au and Ag.
Step
14
: Immersing the wafer in an HF/H
2
O
2
solution to remove metallic ions and a native oxide layer, and, after reducing adhesiveness of the micro-particles to the surface of the wafer with a surface activator, employing the ultrasonic oscillating technique to remove the loosened particles.
Step
16
: Cleaning the surface of the wafer with ultra-pure water (UPW) to remove remaining inorganic particles left on the surface of the wafer after Step
14
.
Step
18
: Immersing the wafer in a solution of O
3
in water to remove the surface activator left on the surface of the wafer resulting from step
14
.
Step
20
: Using DHF solution to remove from the surface of the wafer the native oxide layer resulting from Step
18
.
Step
22
: Using UPW to clean the surface of the wafer.
Because of demand on the density of devices on the wafer, and with the development of photo printing techniques, the widths of contact holes, vias or trenches on the surface of a wafer are getting smaller, and the aspect ratios are getting larger. For example, a contact hole 0.15 mm in width with an aspect ratio of 10 would have a depth of 1.5 mm. For this reason, the cleaning agents can not easily reach the bottom of the contact hole under capillary attraction, resulting in a reduction in cleaning efficiency, affecting the rest of the manufacturing process, and ultimately the quality of the final product.
Please refer to FIG.
2
.
FIG. 2
is a cross-sectional view of a wafer
100
undergoing the cleaning processes. As shown, the wafer
100
is immersed in a cleaning agent
200
, and the surface of the wafer
100
comprises a contact hole
102
of width 0.15 mm, a contact hole
104
of width 0.18 mm, a contact hole
106
of width 0.3 mm, and a contact hole
108
of width 0.5 mm, wherein their aspect ratios are 6, 10, 3 and 1, respectively. Unlike the contact holes
106
and
108
, the contact holes
102
and
104
have widths less than 0.2 mm, so air chambers
202
, due to capillarity, are formed at the bottoms of the contact holes
102
and
104
, and prevent the cleaning agent
200
from reaching the bottoms of the contact holes
102
and
104
, reducing the cleaning efficiency for the wafer
100
.
SUMMARY OF THE INVENTION
The main purpose of the present invention is to provide a cleaning method for cleaning a surface of a wafer comprising contact holes, a via, or trenches less than 0.2 mm in width.
In this method, a wafer is placed in a closed cleaning chamber, and then the cleaning agent is poured into the cleaning chamber to a predetermined height, so that the wafer is completely immersed in the cleaning agent. Then, the pressure in the cleaning chamber is lowered to a sub-atmospheric state—between 0.1 and 0.9 atm. To finish, the pressure is returned to a normal value. The cleaning agent can be dilute hydrofluoric acid (DHF), ultra-pure water (UPW), RCA standard clean 1 (SC-1), RCA standard clean 2 (SC-2), a solution of O
3
in water (O
3
/H
2
O), a solution of HF in H
2
O
2
, buffered oxide etchant (BOE), or a hot phosphoric acid solution.
In the present invention, because the pressure in the cleaning chamber is lowered to a sub-atmospheric state in a few seconds, the cleaning agent can thoroughly clean the surface of a wafer that has contact holes or trenches with large aspect ratios, resulting in a good cleaning quality. Also, since the contact holes with large aspect ratios can be wetted in a very short period of time, the cleaning and etching times can be precisely controlled.
These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings.
REFERENCES:
patent: 5188136 (1993-02-01), Kumagai
patent: 6240933 (2001-06-01), Bergman
patent: 6286231 (2001-09-01), Bergman et al.
El-Arini Zeinab
Hsu Winston
Macronix International Co. Ltd.
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