Measuring and testing – Specific gravity or density of liquid or solid – Freely vertical reciprocable float with carried indicium
Reexamination Certificate
1999-11-29
2002-06-25
Chapman, John E. (Department: 2856)
Measuring and testing
Specific gravity or density of liquid or solid
Freely vertical reciprocable float with carried indicium
C073S445000
Reexamination Certificate
active
06408694
ABSTRACT:
FIELD OF THE INVENTION
The present invention generally relates to an apparatus and a method for on-line monitoring of a liquid density and more particularly, relates to an apparatus and a method for on-line, real-time monitoring of a liquid density by a densimeter cylinder that contains a density indicator therein for the continuous monitoring of density by flowing the liquid continuously through the densimeter cylinder.
BACKGROUND OF THE INVENTION
In the fabrication process for semiconductor devices, a pre-processed semiconductor wafer is frequently polished in order to planarize a top surface of the wafer or to remove excess materials from the surface of the wafer. While apparatus for polishing semiconductor wafers is well known in the art, a chemical-mechanical polishing method has been developed for specific applications on silicon wafers. The chemical-mechanical polishing (CMP) method is named as such because both a chemical reaction between a polishing slurry and a polished surface and a mechanical reaction for removing the debris are involved in the CMP process.
More recently, chemical-mechanical polishing apparatus has been employed in combination with a pneumatically actuated polishing head. CMP apparatus is used primarily for polishing the front surface or the device-side of a semiconductor wafer during the fabrication of semiconductor 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 cross-sectional view of a CMP apparatus is shown in
FIGS. 1A and 1B
. As shown in
FIG. 1A
, a rotating polishing head
14
which holds a wafer,
10
is pressed onto an oppositely rotating polishing pad
12
mounted on a polishing disc
26
by adhesive means. The polishing pad
12
is pressed against the wafer surface
22
at a predetermined pressure. During polishing, a slurry
24
is dispensed in droplets onto the surface of the polishing pad
12
to effectuate the chemical mechanical removal of materials from the wafer surface
22
.
An enlarged cross-sectional representation of the polishing action which results from a combination of chemical and mechanical effects is shown in FIG.
1
B. The CMP method can be used to provide a planner surface on dielectric layers, on deep and shallow trenches that are filled with polysilicon or oxide, and on various metal films. 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 outer layer is then regrown on the surface while the process is repeated again. For instance, in metal polishing, a metal oxide layer can be formed and removed repeatedly.
During a CMP process, a large volume of a slurry composition is dispensed. The slurry composition and the pressure applied between the wafer surface and the polishing pad determine the rate of polishing or material removal from the wafer surface. The chemistry of the slurry composition plays an important role in the polishing rate of the CMP process. For instance, when polishing oxide films, the rate of removal is twice as fast in a slurry that has a pH of 11 than with a slurry that has a pH of 7. The hardness of the polishing particles contained in the slurry composition should be about the same as the hardness of the film to be removed to avoid damaging the film. A slurry composition typically consists of an abrasive component, i.e., has particles and components that chemically react with the surface of the substrate. For instance, a typical oxide polishing slurry composition consists of a colloidal suspension of oxide particles with an average size of 30 nm suspended in an alkali solution at a pH larger than 10. A polishing rate of about 120 nm/min can be achieved by using this slurry composition. Other abrasive components such as ceria suspensions may also be used for glass polishing where large amounts of silicon oxide must be removed. Ceria suspensions act as both the mechanical and the chemical agent in the slurry for achieving high polishing rates, i.e., larger than 500 nm/min. While ceria particles in the slurry composition remove silicon oxide at a higher rate than do silica, silica is still preferred because smoother surfaces can be produced. Other abrasive components, such as alumina (Al
3
O
2
) may also be used in the slurry composition.
Since the concentration of particles in the slurry solution plays an important role in the CMP process, it must be carefully monitored before it is dispensed onto the surface of a wafer for performing the CMP process. Conventionally, the density, or the specific gravity of a slurry solution can be determined on a batch basis by a standard densimeter. Samples must be regularly taken from a slurry supply tank and tested for its density to insure it falls within a permissible range. This is a laborious and time consuming process, and is subjected to high probability of human errors. A conventional densimeter
30
is shown in FIG.
1
C. The densimeter is constructed by an upright, elongated cylinder
32
fabricated of a substantially transparent material. The cylinder is mounted on a base plate
34
for stability and for forming a fluid-tight container. Inside a cavity
36
of the cylinder
32
, a density indicator
40
is provided and submerged in a liquid
38
. The density indicator
40
is constructed by a floater portion
42
and a measuring stick
44
which are integrally joined together. The floater portion
42
is constructed of a material that is suitable for the density of the liquid
38
to be measured such that it suspends in the liquid as shown in FIG.
1
C. The density of the liquid
38
in the cavity
36
can be read by the position of the marker
46
on the measuring stick relative to the graduated scale
48
on the cylinder wall
50
. The process does not allow an on-line, real-time monitoring of liquid density.
Another device which utilizes ultrasonic waves sent through a liquid medium has also been used to measure density of a liquid. The device is very expensive and the technique is operator sensitive and subjected to a number of material parameters which may lead to inaccurate readings. For instance, when the liquid material contains air bubbles, the density reading obtained by the ultrasonic method may be greatly affected. Furthermore, due to the high cost of the ultrasonic equipment, the on-line density measurement technique cannot be used at all fabrication facilities. Moreover, the equipment is a complicated electronic device that requires an elaborate calibration procedure which must be flawlessly performed in order to obtain accurate results.
It is therefore an object of the present invention to provide an apparatus for on-line monitoring a liquid density that does not have the drawbacks or shortcomings of the conventional apparatus.
It is another object of the present invention to provide an apparatus for on-line monitoring of a liquid density that can be used for the continuous monitoring of a liquid density used in a chemical process.
It is a further object of the present invention to provide an apparatus for on-line monitoring of a liquid density which can be carried out on a real-time basis in a chemical process.
It is another further object of the present invention to provide an apparatus for on-line monitoring of a liquid density by utilizing a density indicator situated in a cylindrical-shaped housing for a liquid to flow therethrough continuously such that a real-time monitoring of the liquid density can be obtained.
It is still another object of the present invention to provide an apparatus for on-line monitoring of a liquid density by utilizing an upright, cylindrical housing for flowing a liquid therethrough equipped with a density indicator and positioning guides for holding the in
Kam Ching Fang
Lee Han-Chang
Lin Chun Chih
Tsai Lung-Chun
Chapman John E.
Taiwan Semiconductor Manufacturing Company , Ltd.
Tung & Associates
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