Circulation system of slurry

A slurry filtration system for supplying a slurry to a polisher to perform a chemical mechanical polishing process. The system comprises a first three-way valve, to receive the slurry supplied from an external system; a slurry pump, to control the slurry flowing from the first three-way valve, and to maintain a circulating state of the slurry within the circulating system after the chemical mechanical polishing process comes to a stop; a slurry filter, to filter a plurality of large size particles in the slurry pumped from the slurry pump; a second three-way valve, to supply the slurry flowing from the filter to the polisher; and a transportation pipe, connecting between the first and the second three-way valves to transport the slurry from the second three-way valve back to the first three-way valve when the polishing process has stopped.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates in general to a filtration circulation system of a 
chemical mechanical polisher, and more particularly, to a circulation 
system which maintains a constant flow of slurry in a polisher to prevent 
the deposit of particles. 
2. Description of the Related Art 
Chemical mechanical polishing (CMP) is the only technique that provides a 
global planarization of devices for very large scaled integration (VLSI), 
or even ultra-large scaled integration (ULSI). In chemical mechanical 
polishing, a polishing apparatus similar to a grinder is adapted with an 
appropriate chemical agent to planarize uneven profiles of chips. A slurry 
is commonly used as a chemical agent in chemical mechanical polishing. The 
slurry comprises a colloidal silicon dioxide or a dispersoid of alumina 
mixed with the solution of potassium hydroxide (KOH) or ammonium 
hydroxide. The very highly abrasive particles in such a slurry and some 
added oxidant and organic solvent therein are used to facilitate the 
polishing process. 
Therefore, the factors which determine the resultant planarization from 
chemical mechanical polishing include the components of slurry 
distribution and size of slurry particles, slurry supplying speed, and pH 
value of the slurry. In chemical mechanical polishing systems, a filter is 
installed for filtering slurry particles in a large size in order to 
reduce the scratch on the surface during polishing. However, while the 
polishing process stops or after a duration of operation, these large size 
particles deposit. In addition, while the polishing is in process, small 
size particles easily agglomerate into large size particles. The scratch 
on surfaces of chips during polishing is inevitable. Moreover, the deposit 
of large size particles blocks the filter and affects the slurry flow. 
Once the slurry flow is affected, scratches or damage to the chips become 
even more severe. 
FIG. 1 shows a filtration system in a conventional chemical mechanical 
polisher. As shown in the figure, a slurry is controlled by a switch 10 to 
flow through a slurry pump 12. The slurry is then pumped by the slurry 
pump 12 to a slurry filter 14. After the particles of large size have been 
filtered by the filter 14, the flow of slurry is controlled by another 
switch 16 to flow to a polisher for performing a chemical mechanical 
polishing. While the polisher stops working, the switches 10 and 16 are 
shut, so that the slurry in the slurry filtration system 18 can not 
maintain a flowing and circulating state. The particles in the slurry 
therefore agglomerate into large particles. To avoid the deposit or 
agglomeration of particles, a deionized water (DI water) is used in the 
filtration system 18 to clean the residue slurry for polishing. Therefore, 
while performing chemical mechanical polishing, it is difficult to supply 
slurry into the filtration system 18 in a timely fashion. For example, the 
concentration of the abrasive particles is insufficient to performing 
polishing at the beginning of the process. A long duration is required to 
supply slurry with a sufficient concentration of small size particles. The 
stability and performance of chemical mechanical polishing processes are 
thus affected. 
SUMMARY OF THE INVENTION 
It is therefore an object of the invention to provide a slurry filtration 
system. While the chemical mechanical polishing process comes to a stop, 
the particles in a slurry do not agglomerate or deposit. The slurry is 
thus supplied fluently to maintain an effective chemical mechanical 
process. 
To achieve these objects and advantages, and in accordance with the purpose 
of the invention, as embodied and broadly described herein, the invention 
is directed towards a slurry filtration system for supplying a slurry to a 
polisher to perform a chemical mechanical polishing process. The system 
comprises a first three-way valve, to receive the slurry supplied from an 
external system; a slurry pump, to control the slurry flowing from the 
first three-way valve, and to maintain a circulating state of the slurry 
within the circulating system after the chemical mechanical polishing 
process comes to a stop; a slurry filter, to filter a plurality of large 
size particles in the slurry pumped from the slurry pump; a second 
three-way valve, to supply the slurry flowing from the filter to the 
polisher; and a transportation pipe, connecting between the first and the 
second three-way valves to transport the slurry from the second three-way 
valve back to the first three-way valve. 
It is to be understood that both the foregoing general description and the 
following detailed description are exemplary and explanatory only and are 
not restrictive of the invention, as claimed.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 2 shows a slurry filtration circulation system in a preferred 
embodiment according to the invention. As shown in the figure, while the 
system is performing chemical mechanical polishing, a slurry is supplied 
from an external system 20 to a three-way valve V1 in a circulation system 
30. Through the three-way valve V1 which can be controlled by a software, 
the slurry flows to a slurry pump 22. The slurry is then pumped into a 
filter 24. After the particles of a large size in the slurry have been 
filtered by the filter 24, the slurry flows through a second three-way 
valve V2. Similarly to the first three-way valve V1, the second three-way 
valve can be controlled by a software. After the second three-way valve, 
the slurry flows into a polisher 26 for performing chemical mechanical 
polishing. 
When the chemical mechanical polishing process comes to a stop, the first 
three-way valve V1 is automatically switched to avoid supplying additional 
slurry into the circulation system 30. The second three-way valve V2 is 
shut to stop supplying the slurry to the polisher 26. The slurry remaining 
in the circulation system 30 is still circulating by the polishing pump 
22. That is, the slurry is pumped by the polishing pump 22 so as to flow 
back to the first three-way valve V1 through a transportation pipe 28. The 
slurry is thus circulating through the polishing pump 22, the polishing 
filter 24, the second three-way valve V1, the transportation pipe 28 and 
the first three-way valve V2 within the circulating system 30, even when 
the chemical mechanical polishing process stops. Since the slurry within 
the circulating system 30 is kept circulating, the problem of agglomerated 
particles or deposit of particles is avoided. The chemical mechanical 
polishing is performed without being degraded. 
In addition, a flow meter M is installed on the transportation pipe 28 to 
monitor the operation status of the slurry filter 24. When the chemical 
mechanical polishing comes to a stop, the circulating status of slurry is 
detected by the flow meter M. In case that the slurry filter 24 is blocked 
by too many large size particles or a deposit of particles, the 
information is obtained from reading the flow meter M. A treatment of the 
slurry is required to prevent from damaging the chips to be polished. 
One of the advantage of the invention is that the slurry is kept 
circulating within the circulation system when the polishing comes to a 
stop. Therefore, the agglomeration of particles and deposit of particles 
can be avoided, so that the slurry supply is not blocked. 
Another advantage of the invention is that without the formation of 
agglomerated particles, the chips are protected from being scratched, or 
even damaged by the slurry during polishing. 
The other advantage of the invention is that by installing a flowing meter 
on the transportation pipe, the slurry flow within the circulation system 
is being detected. In case that any problem of slurry supply occurs, a 
treatment of the slurry is given to prevent damaging the chip to be 
polished. 
Other embodiment of the invention will appear to those skilled in the art 
from consideration of the specification and practice of the invention 
disclosed herein. It is intended that the specification and examples to be 
considered as exemplary only, with a true scope and spirit of the 
invention being indicated by the following claims.