Static mixer for a viscous liquid

Details Static mixer for a viscous liquid Static mixer for a viscous liquid

2000-12-01

2002-03-26

Cooley, Charles E. (Department: 1723)

Agitating

Having specified feed means

Material introduced so as to cause rotary motion in mixing...

C366S174100, C366S181500, C366S339000, C366S137000, C366S165500, C366S173100

Reexamination Certificate

active

06361202

ABSTRACT:

FIELD OF THE INVENTION
The present invention generally relates to a mixing device for a viscous liquid and more particularly, relates to a static mixer for a viscous liquid that is constructed of an elongated cylindrical tank, an elongated cylindrical mixing sleeve inside the tank, a plurality of curvilinear tubes for feeding the viscous liquid, a spiral plate for guiding the liquid flow, and an outlet tube for outputting a mixture of the viscous liquid and a solvent.
BACKGROUND OF THE INVENTION
Apparatus for polishing thin, flat semi-conductor wafers is well-known in the art. Such apparatus normally includes a polishing head which carries a membrane for engaging and forcing a semi-conductor wafer against a wetted polishing surface, such as a polishing pad. Either the pad, or the polishing head is rotated and oscillates the wafer over the polishing surface. The polishing head is forced downwardly onto the polishing surface by a pressurized air system or, similar arrangement. The downward force pressing the polishing head against the polishing surface can be adjusted as desired. The polishing head is typically mounted on an elongated pivoting carrier arm, which can move the pressure head between several operative positions. In one operative position, the carrier arm positions a wafer mounted on the pressure head in contact with the polishing pad. In order to remove the wafer from contact with the polishing surface, the carrier arm is first pivoted upwardly to lift the pressure head and wafer from the polishing surface. The carrier arm is then pivoted laterally to move the pressure head and wafer carried by the pressure head to an auxiliary wafer processing station. The auxiliary processing station may include, for example, a station for cleaning the wafer and/or polishing head; a wafer unload station; or, a wafer load station.
More recently, chemical-mechanical polishing (CMP) apparatus has been employed in combination with a pneumatically actuated polishing head. CMP apparatus is used primarily for polishing the front face or device side of a semi-conductor wafer during the fabrication of semi-conductor devices on the wafer. A wafer is “planarized” or smoothed one or more times during a fabrication process in order for the top surface of the wafer to be as flat as possible. A wafer is polished by being placed on a carrier and pressed face down onto a polishing pad covered with a slurry of colloidal silica or alumina in de-ionized water.
A schematic of a typical CMP apparatus is shown in
FIGS. 1A and 1B
. The apparatus
10
for chemical mechanical polishing consists of a rotating wafer holder
14
that holds the wafer
10
, the appropriate slurry
24
, and a polishing pad
12
which is normally mounted to a rotating table
26
by adhesive means. The polishing pad
12
is applied to the wafer surface
22
at a specific pressure. The chemical mechanical polishing method can be used to provide a planar surface on dielectric layers, on deep and shallow trenches that are filled with polysilicon or oxide, and on various metal films. CMP polishing results from a combination of chemical and mechanical effects. A possible mechanism for the CMP process involves the formation of a chemically altered layer at the surface of the material being polished. The layer is mechanically removed from the underlying bulk material. An altered layer is then regrown on the surface while the process is repeated again. For instance, in metal polishing a metal oxide may be formed and removed repeatedly.
A polishing pad is typically constructed in two layers overlying a platen with the resilient layer as the outer layer of the pad. The layers are typically made of polyurethane and may include a filler for controlling the dimensional stability of the layers. The polishing pad is usually several times the diameter of a wafer and the wafer is kept off-center on the pad to prevent polishing a non-planar surface onto the wafer. The wafer is also rotated to prevent polishing a taper into the wafer. Although the axis of rotation of the wafer and the axis of rotation of the pad are not collinear, the axes must be parallel. It is known in the art that uniformity in wafer polishing is a function of pressure, velocity and the concentration of chemicals. Edge exclusion is caused, in part, by non-uniform pressure on a wafer. The problem is reduced somewhat through the use of a retaining ring which engages the polishing pad.
In the polishing operation shown in the enlarged cross-sectional view of
FIG. 1B
, the slurry solution
24
must be forced into an interface between the wafer
10
and the polishing pad
12
in order for the chemical reaction and the mechanical removal process
20
to operate efficiently. The slurry solution
24
(also shown in
FIG. 1A
) is dispensed from a dispensing nozzle (not shown) onto the polishing pad
12
. In most commercial CMP apparatus, the slurry solution
24
is stored in a reservoir and delivered to the dispensing nozzle through a conduit. The slurry solution stored in the reservoir and in the delivering conduit is not provided with a temperature control device. The slurry solution
24
is normally applied to the polishing pad
12
at the same temperature as the chamber temperature in the CMP apparatus, i.e., approximately at room temperature.
The slurry solution is normally delivered by a commercial supplier in a preset concentration which must be diluted by a solvent such as ultra-pure water (UPW) for a specific CMP process. Conventionally, the dilution of the viscous slurry solution by a solvent can be accomplished in a dynamic mixer utilizing a propeller type mixing blade for producing a turbulent flow in the solution to achieve mixing. The mixing efficiency achieved by the dynamic mixer is poor and furthermore, the dynamic mixer frequently occupies a large floor space.
More recently, static mixers have been used which utilize a static mixing mechanism without using any mechanical moving parts. In a typical static mixer, the mixer and the mixing tank assembly are combined into one unit to minimize the floor space used in a factory. Factory space utilization has become more important in the present deep-sub-micron fabrication environment, since global planarization has become an essential process for achieving high IC device density on the chips fabricated. In a global planarization process that is carried out by chemical mechanical polishing, slurries are adapted as an abrasive material for producing the planarization effect. For cost reduction purposes, it has become more effective to dilute concentrated slurry solutions with solvents, i.e. ultra-pure water. The slurry blending requirements vary from process to process. However, a slurry mixer must blend slurry in large volumes to a tool-specific blend ratio. In other words, a high mixing efficiency and a reliable viscous liquid mixer are critical requirements in the present deep-sub-micron fabrication environment. For instance, a slurry mixer may be used to blend either a two-component slurry, i.e. a tungsten slurry that has different pH values. When a mixing process is not properly conducted, a slurry of unstable quality is obtained which may lead to increased crystallization of the slurry solution and an increasing probability of pH shock.
Referring now to
FIG. 1C
, wherein a typical static mixer utilized in slurry dilution for a CMP process is shown. The static mixer system
30
is constructed of a mixing tank
32
containing a cavity
34
therein for conducting the mixing process. The mixing tank
32
is essentially empty in the cavity
34
with no mixing aid built therein. Into the cavity
34
, is fed through an to inlet conduit
36
a slurry solution from a slurry conduit
38
fed by a slurry pump
40
, an oxidizer solution
42
(used in a tungsten CMP process) fed by an oxidizer pump
44
, and ultra-pure water through a water conduit
46
fed by a water pump
48
. Simultaneously, a recirculated slurry solution is fed into the inlet conduit
36
by a recirculating pump
50
through conduit
52
for recycling the slurry

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