Cleaning and liquid contact with solids – Processes – For metallic – siliceous – or calcareous basework – including...
Reexamination Certificate
1999-03-31
2001-04-10
Gulakowski, Randy (Department: 1746)
Cleaning and liquid contact with solids
Processes
For metallic, siliceous, or calcareous basework, including...
C134S001300, C134S026000, C134S030000, C134S105000, C134S108000, C134S902000
Reexamination Certificate
active
06214128
ABSTRACT:
FIELD OF THE INVENTION
The present invention generally relates to a method and an apparatus for wet cleaning an electronic substrate and more particularly, relates to a method and an apparatus for preventing silicon hole defect formation after wet cleaning caused by vaporized ammonia in the environment of the wet cleaning system.
BACKGROUND OF THE INVENTION
Various techniques of etching resist-imaged photomasks, silicon wafers or other semiconductor materials have been used in semiconductor fabrication processes. A wet etching technique conducted in an immersion tank is a practical high-throughput, flexible fabrication process. By properly selecting etchant chemicals, etch reactions with the target film are thermodynamically favored over reactions with other films. Desirable etch-rate ratios can usually be obtained.
A wet etching method is especially suitable for the blanket etching of polysilicon, oxide, nitride and metal. The method is capable of providing the necessary etch selectivity, a damage-free interface and particle-contamination-free wafers. In more recently developed wet etching technology, automated robotic handling systems and ultra-pure chemicals have been used to further improve particle control and process consistency. A well-controlled wet etching technique is therefore the choice of etching process in VLSI and ULSI fabrication processes.
One of the key criteria in carrying out a wet etching process is that the etch products must be soluble in the etchant solution and therefore, no contaminating particles are generated. In an immersion etching process, the volume of the etching tank should be large enough to create enough pressure on the wafer surface in order to dislodge hydrogen gas bubbles evolved during etching reactions; to ensure an accurate balance of the etchant components; to keep the concentration of the etchant relatively constant; and to reduce the number of times the etchant tank must be changed in a production environment. An etchant bath change creates expensive down time, and furthermore, the handling of highly hazardous corrosive materials should be minimized from a safety standpoint.
Wet etching is a frequently used technique for stripping photoresist films from silicon wafers where a complete removal of the resist images without adversely affecting the wafer surface is desired. The resist layer or images should be completely removed without leaving any residues, including contaminant particles that may have been present in the resist. The underlying surface of the photoresist layer should not be adversely affected, for instance, undesirable etching of the metal or oxide surface should be avoided. Liquid etchant strippers should produce reasonable bath yield in order to prevent redeposition of dissolved resist on the wafers. The etchant should completely dissolve the photoresist layer in a chemical reaction, and not just lifting or peeling so as to prevent redeposition. It is also desirable that the etching or stripping time should be reasonably short in order to permit a high wafer throughput.
Wet chemical cleaning process is also a form of wet etching process for electronic substrates. A most commonly used wet chemical cleaning technique is based on hot alkaline or acidic hydrogen peroxide (H
2
O
2
) solutions. The mixture is commonly used to remove chemically bonded films from the surface of an electronic substrate prior to critical processing steps. A frequently used cleaning process is known as the RCA cleaning technique which is based on a two-step process of a Standard Clean-1 (or SC-1) followed by Standard Clean-2 (or SC-2). Both the SC-1 and the SC-2 cleaning solutions incorporate the strong oxidizing capability of hydrogen peroxide. Specifically, SC-1 is an aqueous alkaline solution that readily removes organic films. For instance, SC-1 etches thermally grown oxide at a rate of about 0.1 nm/min. SC-1 is typically a 1:1:5 solution mixture of ammonium hydroxide (NH
4
OH, at 27% concentration), unstabilized H
2
O
2
(at 30% concentration) and DI water. The SC-1 cleaning solution is very effective in removing organic contaminants since NH
4
OH readily dissolves organic films.
A typical wet chemical cleaning system
10
is shown in FIG.
1
. The conventional system has a wet chemical cleaning tank
12
which includes an inner tank
14
and an outer tank
16
. The outer tank
16
is positioned outside the inner tank
14
for providing overflow protection. This allows an overflow protection where the inner tank
14
is usually filled with a cleaning solution through inlet
18
. The wet cleaning tank
12
is further positioned inside a safety containment tank
20
for preventing accidental spill of the caustic or acidic solution that are normally used in the cleaning tank
14
. The safety containment tank
20
further provides containment to rinse tanks
22
and
24
. The rinse tanks
22
,
24
are normally filled with DI water or any other suitable rinse solution for pre-rinse and post-rinse, respectively before and after the cleaning process in tank
14
. The wet cleaning tank
12
may further include a filter system and a heater system which are not shown in FIG.
1
.
A major component of the SC-1 cleaning solution, i.e., an aqueous solution of ammonia, or ammonium hydroxide of 29% concentration is stored in a supply tank
30
. The aqueous solution of ammonia
28
is pumped by a recirculating pump
34
into the inner tank
14
through outlet
18
. The flow of the aqueous solution of ammonia
28
is controlled by a flow control valve
36
provided in the flow conduit
32
. A safety overflow conduit
38
is also provided in fluid communication with the supply tank
30
to prevent the supply tank
30
from being over-pressured.
In operation, an electronic substrate such as a silicon wafer is first pre-rinsed in the rinse tank
22
, then cleaned by soaking in the wet cleaning tank
14
for a predetermined period of time. The electronic substrate is then removed from tank
14
and dipped into the post-rinse tank
24
filled with DI water for rinsing off the cleaning solution. In the conventional set-up shown in
FIG. 1
, the safety containment tank
20
which surrounds the rinse tanks
22
,
24
and cleaning tank
12
is substantially isolated from the atmosphere for safety reasons. This presents a problem in that through the outlet
42
of the safety overflow conduit
38
, vaporized ammonia frequently escapes to enter the safety containment tank
20
. The ammonia vapor
40
fills the chamber interior
26
of the safety containment tank
20
.
Since, aqueous solution of ammonia has a relatively low boiling temperature, i.e., at approximately 32° C., aqueous ammonia vaporizes easily at temperatures higher than 32° C. The aqueous solution of ammonia
28
in the supply tank
30
is normally kept at an elevated temperature of 70° C. to facilitate mixing with the other two components of the SC-1 cleaning solution, i.e., the H
2
O
2
and DI water. The mini-environment
26
that is present in the safety containment tank
20
is normally maintained at a temperature around 55° C. The surrounding temperature where the aqueous solution of ammonia
28
is kept is therefore substantially higher than 32° C. which causes the aqueous ammonia to vaporize and to escape from outlet
42
of the safety overflow conduit
38
to fill the interior chamber
26
of the safety containment tank
20
.
When the chamber interior
26
is filled with ammonia vapor, at the instant that a silicon wafer is being removed from the SC-1 tank
12
into the chamber interior
26
, ammonia vapor
40
attaches itself onto the surfaces of the silicon wafer (not shown). When the wafer is again dipped into the post-rinse tank
24
that is filled with DI water, the ammonia vapor attached on the wafer surface reacts with DI water thus forming aqueous solution of ammonia. When the aqueous solution of ammonia is concentrated at localized spots on the wafer surface, a defect of “silicon hole” occurs on the surface of the wafer. The silicon hole is caused by the removal, or the wet etching, of
Chu Hui-Ming
Hsieh Bor-Hann
Lin Jeng-Lian
Shen Fu-Hsing
Gulakowski Randy
Taiwan Semiconductor Manufacturing Company , Ltd.
Tung & Associates
Wilkins Yolanda
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