Wet chemical process tank with improved fluid circulation

Details Wet chemical process tank with improved fluid circulation Wet chemical process tank with improved fluid circulation

2001-01-16

2002-12-24

Mills, Gregory (Department: 1763)

Adhesive bonding and miscellaneous chemical manufacture

Differential fluid etching apparatus

For liquid etchant

C134S001300

Reexamination Certificate

active

06497785

ABSTRACT:

FIELD OF THE INVENTION
The present invention generally relates to a wet chemical process tank that has improved fluid circulation and more particularly, relates to a wet chemical process tank that has improved liquid flow such that flakes of film residues can be effectively carried away from the etch tank.
BACKGROUND OF THE INVENTION
In semiconductor fabrication process, various techniques of etching resist-imaged photomasks, silicon wafers or other semiconductor materials have been used. A set etching technique conducted in an immersion tank is generally a practical high-throughput, flexible fabrication process. By properly selecting etchant chemicals, etch reactions with a target film are thermodynamically favored over reactions with other films. Desirable etch-rate ratios can be usually obtained.
A wet etching method is especially suitable for the blanket etching of materials such as polysilicon, oxide, nitride and metal. The method is capable of providing a desirable etch selectivity, a damage-free interface and particle-contamination-free wafers. In the more recently developed wet etching technology, automated robotic handling systems and ultra-pure chemicals are 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 techniques.
One of the key criteria in carrying out a set 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 ensure an accurate balance of the etchant components; to keep the concentration of the etchant relatively constant; and to minimize the number of times the etchant tank must be changed. An etchant bath change creates expensive down time and furthermore, the handling of highly hazardous corrosive materials creates potential safety problems.
Wet etching is a frequently used technique for stripping photoresist films from silicon wafers where the complete removal of 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, accidental etching of the metal or oxide surface should be avoided. Liquid etchant strippers should produce a 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 be reasonably short in order to permit high wafer throughput.
Wet etchants such as sulfuric acid (H
2
SO
4
) and mixtures of H
2
SO
4
with other oxidizing agents such as hydrogen peroxide (H
2
O
2
) are widely used in stripping photoresist or in cleaning a wafer surface after the photoresist has been stripped by other means. For instance, a frequently used mixture is seven parts H
2
SO
4
to three parts 30% H
2
O
2
or a mixture of 88% sulfuric acid and 12% nitric acid. Wafers to be stripped can be immersed in the mixture at a temperature between about 100° C. and about 150° C. for 5-10 minutes and then subjected to a thorough rinse of deionized water and dried in dry nitrogen. This type of inorganic resist strippers, such as the sulfuric acid mixtures, is very effective in the residual-free removal of highly postbaked resist. They are more effective than organic strippers and the longer the immersion time, the cleaner and more residue-free wafer surface can be obtained.
In a typical wet chemical process tank, in order to ensure a perfect mix of acid and other components or a good uniformity in the acid itself, constant stirring or agitation of the solution in the tank is desired. While mechanical stirring or agitation techniques have been used, the moving components of a stirrer or agitator frequently generate contaminant particles that are detrimental to the wafer surfaces. Furthermore, a mechanical stirrer or agitating device tends to work only in a localized area of the large chemical tank while leaving a large area of the tank unmixed. Since wafers are usually loaded into a wafer boat and positioned at or near the center of the tank, too vigorous stirring or agitation of the liquid in the tank may cause the wafers in the wafer boat to move and as a result, stick to each other. Such wafer sticking problems result in uneven and unsatisfactory etching on the wafer surfaces.
An improved mixing method for liquids in a wet chemical tank is to force a fluid to enter the wet chemical tank at the bottom of the tank and then forming gas bubbles to rise from the bottom to the top of the tank and thereby carrying out mixing function along the way. The fluid used is normally a substance that readily reacts with the liquid in the tank such that either a gas or a vapor is generated to form the bubbles. One of such suitable chemical reactions occurring between a liquid in a tank and a fluid pumped therein is an acid and water. For instance, when phosphoric acid (H
3
PO
4
) and ionized water are mixed in a tank, water reacts with the strong acid violently and immediately boils to form water vapor in bubbles and then the bubbles rise to the top of the liquid surface in the tank. During the process where the bubbles rise to the top of the liquid surface, the bubbles cause disturbance in the liquid and thus improve its uniformity by the agitation occurring in the liquid. A conventional system of a wet chemical process tank equipped with a fluid dispensing member, which is commonly called a spray bar, positioned at the bottom of the tank is shown in FIG.
1
.
FIG. 1
shows a wet chemical process system
10
including a process tank
12
formed by a bottom wall
14
and four sidewalls
16
which are joined perpendicular to the bottom wall
14
forming a fluid-tight enclosure. The bottom wall
14
and the sidewall
16
can be constructed of a corrosion resistant material such as panels of stainless steel or steel panels coated with a corrosion resistant material such as Teflon. Due to the extreme corrosive nature of the processing liquids that are normally used in the wet chemical processing of semiconductor wafers, a securely sealed enclosure is a critical requirement for both maintenance and safety. The size of the tank
12
should be sufficiently large such that, for instance, two wafer boats (not shown) which are capable of holding twenty-four wafers each can be placed in the center of the tank while leaving an adequate space between the wafers and the walls such that a constant movement in the processing liquid can be achieved. Into the wet chemical process tank
12
, a fluid dispensing member, or a spray bar
22
, is positioned at the bottom of the tank. A plurality of openings
24
are formed on the top surface
26
of the dispensing member
22
for allowing a fluid to enter the tank.
An enlarged, cross-sectional view of the dispensing member
22
taken at an opening
24
is shown in FIG.
1
A. An internal fluid passage
28
is provided in dispensing member
22
which is in fluid communication with conduit
32
through a connector
34
and an elbow section
36
in the dispensing members
22
. A precision volumetric pump
42
is used to pump a fluid such as deionized water from an inlet
44
through conduit
32
into the fluid passage
28
. Since the volume of the deionized water that enters the process tank for reaction with phosphoric acid must be precisely controlled such that just the right amount of bubbles are generated, the pump
42
utilized must be a high precision volumetric pump that is capable of precisely controlling the amount of liquid pumped there through. For instance, into a normal wet chemical process tank of 20-liter

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