This invention relates to an apparatus for supplying a polishing solution
for use in polishing, for example, semiconductor substrate, and relates in particular
to an apparatus for steadily supplying a polishing solution having a constant dispersion
of abrading particles in the liquid, as per the preamble of claim 1 or claim 9.
Such an apparatus is disclosed, for example, by WO 96 02319 A.
Recent advances in circuit integration in semiconductor devices have
produced micro-sized circuit patterns with narrow line widths. As a result, circuit
pattern printing by optical lithography requires extremely shallow depth of focus,
so that the substrate surface needs to be precisely flat in the focal plane of the
A method of obtaining a flat surface on a semiconductor substrate
is to polish the wafer using a polishing tool (for example, polishing table with
a polishing cloth), and a wafer holding member for holding and pressing the surface
to be polished of the wafer against the polishing table, and moving the surface
to be polished relative to the polishing tool while supplying a polishing solution
at the contact interface. Such a polishing apparatus can perform not only mechanical
polishing using a polishing solution containing abrasive particles, but can also
perform chemical polishing using an alkaline or acidic polishing solution. For example,
a slurry for polishing oxidized surface of the wafer is based on a KOH or NH4OH
solution with a dispersion of silica particles.
To produce a good substrate using such a polishing apparatus, it is
required that the polishing solution of a constant concentration be steadily supplied
at a constant rate. A system for supplying a polishing solution has an undiluted
solution tank to store mixed solution of KOH, NH4OH and silica powder;
a dilution tank to dilute the undiluted solution with pure water and others; and
supply piping to deliver the solution from the dilution tank to the nozzle of the
However, to meet the demand of cost reduction for equipment and operation,
it is desired to supply the polishing solution from one tank to a plurality of polishing
apparatuses, so that there is a tendency for long lengths of delivery piping. A
result is that the polishing solution becomes stagnant inside the pipe, and tends
to cause aggregation of abrasive particles so that abrading particles tends to cluster,
causing damage (scratch) to the substrate surface or changing the amount of polishing
as a result of changes in solution concentration, or plugging in the line.
Disclosure of Invention
This apparatus is presented in view of the problems outlined above,
and it is an object of the present invention to provide a polishing apparatus as
claimed in claim 1 or claim 9.
Brief Description of Drawings
Best Mode for Carrying Out the Invention
- Figure 1 is a diagram showing the overall configuration of the polishing solution
supply apparatus; Figures 2A∼2C are graphs showing the effects of ultrasonic
processing; Figures 3A∼3C are similar graphs showing the effects of ultrasonic
processing; Figure 4 is also a graph showing the effects of ultrasonic processing;
Figure 5 shows another embodiment of the polishing solution supply apparatus; Figures
6A∼6C are various views of the structures of the ultrasonic vibration device
shown in Figure 5.
In the following, a first embodiment will be presented with reference
to Figure 1. This apparatus for delivering a polishing solution comprises: two stock
tanks 10 for storing an undiluted solution; a dilution tank 12 for delivering a
dilution solution to dilute the undiluted solution to a given concentration; a mixing
section 18 for mixing the solutions supplied from the tanks through pipes 14, 16
to produce a polishing solution of a given concentration; a circulation passage
20 for circulating the polishing solution; and a delivery pipe 24 to supply the
polishing solution from the circulation passage 20 to the polishing apparatus 22.
The stock tank 10 has a stirrer 70 inside, and a ultrasonic vibrator 72 is attached
to the bottom section. And, each stock tank 10 has a liquid level sensor 73, a temperature
sensor 75 and others.
There are two stock tanks 10, and when one tank becomes empty, a valve
11 is opened to switch to the undiluted solution supply line 14. Each of the supply
line 14 and the dilution liquid supply line 16 is connected to a buffer tube 18,
which is a mixing section, through respective shutoff valve 26 and flow adjusting
valve 28, thereby producing a polishing solution of a given ratio inside the buffer
The buffer tube 18 acting as the mixing section, in this embodiment,
is disposed in a path of the circulation pipe 20 that supplies a polishing solution
to a plurality of polishing apparatuses 22. The buffer tube 18 is a cylindrical
container 30 of a diameter larger than that for the circulation pipe 20, and is
disposed vertically, and has a discharge opening 32 at the bottom section, and the
top section is covered by a lid 36 with an O-ring 34. A return pipe for the circulation
pipe 20 and supply pipes 14, 16 for the undiluted solution and the dilution solution
are connected to the buffer tube 18 at its top.
The container 30 is provided with liquid level sensors 40a, 40b and
40c for detecting the upper, lower and lowermost levels, for example, and output
respective signals to a controller (not shown). The controller outputs control signals
to a shutoff valve 26 and a flow adjusting valve 28, so that the undiluted solution
and the dilution solution will be supplied when the liquid level drops or the supply
will be stopped when the liquid level reaches the upper level. If the liquid level
should reach the lowermost level, the controller generates a warning signal or a
stop signal for the polishing unit 22.
Circulation pipe 20 is constructed such that the solution exits from
the discharge opening 32 at the bottom of the buffer tube 18, and circulates near
one or more polishing unit 22 for supplying polishing solution and return to the
buffer tube 18 through the return pipe. Circulation pipe 20 is provided with a circulation
pump 46 for circulating the polishing solution, a one-way valve(check valve) 48
for preventing a reverse flow, and a pressure sensor 50 and the like. Output signal
from the pressure sensor is input in the controller, and the controller controls
the operation of the circulation pump 46 according to the output signals of the
pressure sensor so as to maintain the internal pressure in the circulation pipe
20 at a constant value. Circulation pipe 20 is branched into delivery pipes 24 in
a proximity of each polishing unit 22 to deliver the polishing solution, and each
delivery pipe 24 is connected, through a shutoff valve 52 and an adjustable flow
pump 54, to a spray nozzle 56 directed at a certain location of each polishing unit
Accordingly, by circulating the polishing solution at all times inside
the piping to guide the solution to the neighborhood of the polishing unit 22, changes
in solution concentration and line plugging caused by precipitated solid clusters
from a stagnating polishing solution can be eliminated. Also, because the arrangement
of the supply device permits the use of a long length of circulation piping, one
supply source (mixing section) 18 can be used to supply a polishing solution, in
a stable condition, and the cost of the overall facility can be reduced. Because
each polishing unit 22 has its own working schedule, the polishing solution may
become stagnant in some delivery pipes 24 in which the flow is stopped, but any
adverse effects of stagnation can be eliminated by flowing a sufficient quantity
of polishing solution to replace the stagnant liquid in the delivery pipes at the
beginning of each operation.
Next, the effect of ultrasonic vibration applied to the solution on
the abrading particles or polishing qualities will be described with reference to
Figures 2A through 4.
Figures 2A through 2C show an example of changes in the particle size
distribution when vibrations are applied over a period of time. The stirrer 70 was
operated for 30 minutes to produce a distribution of average particle size 51.7
µm, and a standard deviation 49.7 µm, as shown in Figure 2A. After 10 minutes of
ultrasonic vibration applied to the solution, average particle size 0.29 µm and
a standard deviation 2.73 µm were obtained, as shown in Figure 2B. After processing
of ultrasonic vibration applied to the solution for 60 minutes, average particle
size 0.15 µm and a standard deviation 0.029 µm were obtained, as shown in Figure
2C. When vibration was applied longer than 60 minutes, further changes beyond those
shown in Figure 2C were not observed.
Figures 3A through 3C show changes in a particle size distribution
observed when the vibrated solution was left standing. Figure 3A shows the change
after 120 minutes of standing, Figure 3B shows the change after one day of standing,
and Figure 3C shows the change after six days of standing. The results indicated
that the solution retains a fine particle size distribution for a considerable length
of time after ultrasonic vibration is applied.
Figure 4 shows a comparison of polishing performance of the solutions
treated without ultrasonic vibrations and with ultrasonic vibrations, and a comparison
with commercial polishing solution containing silica powder. The results show that
polishing rate is increased when ultrasonic vibrations are applied because the particles
become finely dispersed. The results also show that the polishing rates of a test
slurry subjected to vibrations are about the same for commercial polishing slurry.
The results observed in Figures 2A through 4 regarding the effects of ultrasonic
vibration treatment on the particle size distribution and polishing capability,
were applied to the polishing solution supply apparatus in this embodiment.
The operation of the polishing solution supply apparatus will be explained
below. The stock tank 10 is opened by lifting the lid, and a silica powder and given
quantities of polishing liquids such as KOH, NH4OH are added and stirred
with the stirrer 70 to disperse the abrading (silica) particles. Concurrently with
stirring or after stirring for a given time, the ultrasonic vibrator 72 is operated
for a given interval. This step disperses clustered powder particles that exhibited
a relatively wide range of particle sizes, and produces a particle size distribution
centered about a narrow range of fine particle sizes. The processing interval and
frequency of application of ultrasonic vibration are governed by the scale of the
tanks. For example, ultrasonic vibration may be carried out in a regular pattern,
for example, for two minutes continuously over a period of sixty minutes or five
minutes continuously over a period of thirty minutes.
Next, by operating the undiluted solution supply pump 28 and dilution
pump 28 are operated to produce a polishing solution of a given mixture ratio. The
control device controls the circulation pump 46 so that the downstream pressure
is maintained above a certain value, and generate a steady circulating flow of polishing
solution in the circulation passage 20.
When the individual polishing apparatuses 22 are operated, a portion
of the polishing solution is delivered through the respective delivery pipes 24
into the nozzles 56 of the respective polishing apparatuses 22. When the solution
level inside the buffer tube 18 becomes lower than the lower limit, the level sensor
40b sends a signal to the control device to open the valve 26, thereby the undiluted
solution and pure water, whose flow rates are controlled by the flow control valves
26, are supplied to the buffer tube 18 at a constant mixing ratio, until the liquid
level reaches the upper limit. In this step, because the undiluted solution has
been treated by ultrasonic vibration for a given length of time in the stock tank
10, silica is less likely to aggregate.
Figure 5 shows another embodiment, in which the ultrasonic vibrators
are provided at various locations in the supply passage. For example, vibrators
72a, 72b, 72c, 72d of suitable sizes and shapes are applied at one or more locations
including the mixing section (buffer tube) 18 for the undiluted solution and dilution
solution, circulation pipe 20, near the nozzle 56, and on the turntable 23.
Figures 6A through 6C show details of attaching the vibrators 72a,
72b, 72c, 72d. As shown in each diagram, the vibrators 72a through 72d comprise
ultrasonic elements 74a through 74d and ultrasonic oscillators 76a through 76d.
Figure 6A shows an installation of the vibrators 72a on the bottom section of the
buffer tube 18. Vibrator 72b is similarly disposed about the circulation pipe 20.
Figure 6B shows the vibrator 72c installed near the tip of the nozzle 56 which directs
polishing solution onto the turntable 23. Vibrators 72a through 72c can be installed
in any suitable place on the buffer tube 18 and each piping.
Figure 6C shows a cross sectional view of the ultrasonic vibrator
72d imbedded in the turntable 23. The vibrator 72d is imbedded near the center of
the abrading surface of the turntable underneath the polishing pad 78. In this embodiment,
the vibrator is imbedded near the center, but the location of the vibrator 72d may
be underneath and off-center near the location of supply of solution on the turntable,
or near the pressing point for polishing the wafer.
In these embodiments, the solution can be supplied on the apparatus
22 in a well dispersed state, because the point of solution delivery is a downstream
location of the solution flow, or close to the location where the solution is actually
being applied to the wafer. Also, even when the polishing apparatuses 22 are stopped
and the solution flow rate drops or the solution becomes stagnant, particle clustering
is less likely to occur. In this embodiment, additional ultrasonic vibrations are
applied to locations other than the stock tank, so that, compared with the case
of applying the ultrasonic vibrations only at the stock tank, clustering can be
prevented even if the size of the apparatus for supplying the polishing solution
As explained above, a polishing solution having a constant distribution
of polishing particle size can be delivered to polishing apparatuses by dispersing
the agglomerated powder particles by subjecting the solution to ultrasonic vibration.
It follows that polishing can be performed in a stable manner by preventing surface
scratches caused by aggregated power particles, or changes of polishing rate caused
by changes in the particle concentration.