PTJMP CONTROLLER FOR PRECISION PUMPING APPARATUS
Background of the Invention
This invention relates generally to precision pumping apparatus and, more
particularly to a pump controller for accurately controlling the amount of fluid
dispensed from the precision pumping apparatus.
There are many applications where precise control over the amount and/or rate
at which a fluid is dispensed by a pumping apparatus is necessary. In semiconductor
processing, for example, it is important to control very precisely the amount and the
rate at which photochemicals, such as photoresist, are applied to a semiconductor wafer
being processed to manufacture semiconductor devices. The coatings applied to
semiconductor wafers during processing typically require a flatness across the surface
of the wafer that is measured in angstroms. Many semiconductor processes today have
requirements on the order of 30 angstroms or less. The rate at which processing
chemicals such as photoresists are applied to the wafer and spun out through
centrifugal force to the edges of the wafer has to be controlled in order to ensure that
the processing liquid is applied uniformly. It is also critical to control the rate and
volume at which photoresist chemicals are applied to the wafer in order to reduce
unnecessary waste and consumption. Many of the photochemicals used in the
semiconductor industry today are not only toxic, but they are very expensive,
frequently costing as much as $1,000 per liter. Thus, because of the cost of the
chemicals as well as the difficulties in handling toxic materials, it is necessary to
ensure that enough of the photoresist is applied to the wafer to satisfy processing
requirements while minimizing excessive consumption and waste.
Another important requirement for semiconductor processing is the ability to
repeatedly dispense a precisely controlled amount of processing chemical each time
since variations in the amount of chemicals can adversely impact consistency from
wafer to wafer. In the past, because of the unrepeatability as well as the inability to
precisely control the amount of chemical being dispensed, many pumps had to
dispense 50% to 100% more liquid than needed in order to ensure a sufficient quantity
for processing requirements. This has resulted in waste and increased processing costs.
Conventional pumping apparatus are able to accurately dispense precise
amounts of typical fluids. However, these conventional pumping apparatus cannot
accurately dispense low viscosity, low dispense rate fluids and the conventional
pumping apparatus will either cause a double dispense or a stuttered dispense of the
low viscosity fluid. In particular, at the beginning of the dispensing cycle prior to the
controlled dispensing of any fluid, a small amount of the low viscosity fluid, e.g.,
several microliters, may be undesirable ejected onto the wafer's surface resulting in an
imprecise amount of fluid being dispensed. The problems of double dispensing and
stuttered dispensing of these low viscosity, low flow rate fluids are caused by a variety
of factors which are present in a conventional pumping apparatus. For example,
pressure may be built up in the dispensing chamber of the pumping apparatus due to
the closing of a barrier valve prior to dispensing which may force some fluid into the
dispensing chamber and increases the pressure in the dispensing chamber. The extra
fluid and hence the extra pressure in the dispensing chamber may cause the small
amount of fluid to be ejected onto the wafer's surface at the start of the dispensing
cycle. In addition, the timing of the control valves operation and the dispense system
dynamics, such as tubing length, tubing diameter and nozzle size, in a conventional
pumping apparatus may also contribute to the problem of the double or stuttered
dispense of low viscosity, low dispense rate fluids.
It is desirable to provide low volume, low rate chemical dispensing pumping
apparatus capable of precise and repeatable control of the rate and volume of low
viscosity chemicals dispensed by the pumping apparatus, and it is to these ends that
the present invention is directed.
Summary of the Invention
In accordance with the invention, a low dispense rate precision dispensing
pumping apparatus and method is provided which enable precise and repeatable
control of dispense rate and volume of low viscosity fluids, and which overcomes the
foregoing and other disadvantages of conventional dispensing pumping apparatus and
method. The pumping apparatus precisely controls the dispensing amount and/or rate
of low viscosity fluids by precisely controlling the operation of several different
portions of the pumping apparatus during the dispense cycle. In particular, a pump
controller may precisely control the timing of the control valves with respect to each
other, the motion of the dispensing motor, and the timing of the control valves with
respect to the movement of the dispensing motor. The pump controller in accordance
with the invention accurately controls a pumping apparatus to avoid the double
dispense or stuttered dispense problems associated with conventional pumping
apparatus.
Brief Description of the Drawings
Figure 1 is a block diagram illustrating a pumping apparatus including a pump controller in accordance with the invention;
Figure 2 is a block diagram illustrating a two-stage pumping apparatus;
Figure 3 is a timing diagram illustrating the conventional sequence for dispensing fluids;
Figure 4 is a timing diagram illustrating a sequence for dispensing fluids in accordance with the invention; and
Figure 5 is a flowchart illustrating a method for controlling a pumping apparatus to dispense low viscosity fluids in accordance with the invention.
Detailed Description of a Preferred Embodiment
The invention is particularly applicable to a pumping apparatus which
accurately dispenses precise amounts of low viscosity fluids and it is in this context
that the invention will be described. It will be appreciated, however, that the apparatus
and method in accordance with the invention has greater utility, such as to accurately
dispensing precise amounts of other fluids which may not be low viscosity fluids.
Figure 1 is a block diagram illustrating a pumping apparatus 10 including a
pump controller in accordance with the invention. The pumping apparatus 10 may
include a two-stage pump 12, a fluid reservoir 14 and a computer 16 which operate
together to dispense a precise amount of fluid onto a wafer 18. For purposes of
illustration, a low viscosity fluid, which may have a viscosity of less than 5 centipoire
(cPs), may be dispensed at a low flow rate of about 0.5 milliliters per second, but the
invention is not limited to dispensing low viscosity fluids or low flow rate fluids. The
pump 12 is a two-stage pump since the dispensing of the fluid includes a first feed and
filtration stage and then a second separate dispensing stage as described below so that
the dispense performance does not change over the lifetime of the filter. The operation
of the various portions of the pump 12 may be controlled by a software application 20,
i.e., a computer program comprising pieces of software code which may be stored in a
memory in the computer 16 and may be executed by a processor (not shown) in the
computer. The operation of the pump may also be controlled by a software application
or pieces of software code which are being executed by a processor located inside the
pump. The location of the processor executing the instructions to control the operation
of the pump is not critical to the invention.
The software application 20 may control, for example, the opening and closing
of the various control valves in the pump and the movement of the motors or actuators
which drive the pump in order to accurately dispense a precise amount of fluid onto
the wafer 18. The method implemented by the software application for controlling the
pump 12 to dispense low viscosity, low flow rate fluids in accordance with the
invention will be described below with reference to Figure 5.
To fill itself with fluid, the pump 12 may draw fluid from the reservoir 14 into
a feed chamber as described below. The fluid may then be filtered through a filter and
fed into a separate dispensing chamber as described below. From the dispensing
chamber, the fluid may be dispensed through a filter 22 onto the wafer 18 in precise
amounts even for low viscosity, low rate fluids. The actual cycles of the pump 12 will
be described below with reference to Figures 3 and 4. Now, the details of the two-
stage pump 12 will be described in order to better understand the invention.
Figure 2 is a block diagram illustrating more details of the two-stage pump 12
with which the invention may be employed. In particular, the two-stage pump 12 may
include a feed and filtration stage 30 and a dispensing stage 32. The feed and filtration
stage 30 may include a feed chamber 34 which may draw fluid from a fluid supply
reservoir through an open inlet valve 36 as more fluid is needed. During the
dispensing stages, the inlet valve 36 is closed. To control entry of fluid into and out of
the feed chamber, a feed valve 38 controls whether a vacuum, a positive feed pressure
or the atmosphere is applied to a feed diaphragm 40 in the feed chamber. To draw
fluid into the feed chamber, a vacuum is applied to the diaphragm 40 so that the
diaphragm is pulled against a wall of the feed chamber and pulls fluid into the feed
chamber. To push the fluid out of the feed chamber, a feed pressure may be applied to
the diaphragm. To remove unwanted air bubbles, a vent valve 42 may be opened as
needed.
Once the feed chamber 34 is filled with fluid, the inlet valve 36 is shut and the
isolation valve 44 and a barrier valve 50 are opened to permit the fluid to flow through
a filter 46 into the dispensing stage 32. Once the fluid is in the dispensing stage 32 and
to isolate the feed and filtration stage from the dispensing stage, the isolation valve 44
and the barrier valve 50 may be closed. To vent unwanted air from the system or
relieve excess pressure, the filter 46 may include a vent valve 48. As the fluid is
pushed through the filter 46, unwanted impurities and the like are removed from the
fluid. The fluid then flows through a barrier valve 50 into a dispensing chamber 52 in
the second or dispensing stage of the pump, and the pump begins a dispense cycle as
will now be described.
In the dispensing cycle, once the dispensing chamber is full of fluid and the
barrier valve 50 is closed, a purge valve 54 is opened and the fluid in the dispensing
chamber 52 is pushed by a dispense diaphragm 56 to eliminate any bubbles in the fluid
in the dispensing chamber 52. To push or pull the dispense diaphragm 56, the
dispensing diaphragm may be between the dispensing chamber and a hydraulic fluid
chamber 58 filled with hydraulic fluid. The hydraulic fluid may be pressurized or de-
pressurized by a dispensing pump 60 which may include a piston 62, a lead screw 64
and a stepper motor 66. To apply pressure to the fluid in the dispensing chamber 52,
the stepper motor is engaged which engages the lead screw and pressurizes the
hydraulic fluid. The hydraulic fluid in turn pushes the dispensing diaphragm into the
dispensing chamber 52 which pressurizes the fluid in the dispensing chamber 52 or
pushes the fluid out of the dispensing chamber 52 if the purge valve 54 or an outlet
valve 68 are opened. If the outlet valve 68 is open, then an accurate amount of the
fluid is dispensed onto the wafer. Now, the typical process for dispensing fluid will be
described.
Figure 3 is a timing diagram illustrating the conventional sequence for
controlling a two-stage pump of the type shown in Figure 2 to dispense fluids. As
shown at the top of the diagram, the dispensing process may include a sequence of
stages, i.e., steps such as a ready stage 70, a dispense stage 72, a suckback stage 74, a
fill stage 76, a filter stage 78, a vent stage 80, a purge stage 82, a static purge stage 84.
The typical controlling of the motors and valves for each of these different stages will
now be described along with the result that occurs as a result of each stage. For
example, during the ready stage, the barrier and isolate valves are opened while the
outlet valve is shut to bring the system and feed chamber to an equilibrium pressure
state so that fluid may be dispensed. As the dispense stage begins, the isolate and
barrier valves close, the outlet valve is opened and the motor in the dispensing pump is
started. Due to the relative incompressibility of the fluid being dispensed and the
"stiffness" of the pump, the closing of the barrier valve pushes fluid out of the valve as
it closes which pressurizes the fluid in the dispensing chamber and may cause the
typical double dispense or stuttered dispense problem as described above since the
outlet valve is open. The closure of the barrier valve may increase the pressure in the
dispensing chamber by a predetermined amount, which may be about 2 - 3 psi. The
actual pressure increase, however, depends on the characteristics of the barrier valve
being used. In addition, since the motor is started at the same time as the outlet valve
is opened, an uneven dispensing of fluid (or stuttered dispensing) may occur since the
outlet valve takes more time to open than the starting of the motor and therefore the
motor may be initially pushing the fluid through an outlet valve which is not quite
completely open. This may cause an initial "spitting" of a small amount of fluid.
During the dispensing stage, fluid may be dispensed onto the wafer.
At the end of the dispensing stage and at the beginning of the suckback stage,
the motor is stopped and reversed or an external stop/suckback valve (not shown) may
be opened to suck any fluid remaining in the nozzle back into the dispensing chamber
to ensure that no drips occur at the end of the fluid dispensing. After the fluid has been
sucked back into the dispensing chamber, the outlet valve is closed and the motor is
stopped. Next, during the fill stage, the inlet valve is opened and a vacuum is applied
to the feed diaphragm to draw fluid into the feed chamber from the reservoir. At the
beginning of the filter stage, the inlet valve is closed, the isolate valve is opened, the
feed motor applies positive pressure to the fluid in the feed chamber, the barrier valve
is opened and the dispense motor is reversed to push fluid through the filter into the
dispense chamber. Once the fluid has exited the feed chamber, the isolate valve may
be closed.
At the beginning of the vent stage, the isolate valve is opened, the barrier valve
is closed, the vent valve is opened, the dispense motor is stopped and pressure is
applied to the feed diaphram to remove air bubbles from the filter. At the beginning of
the purge stage, the isolate valve is closed, the feed pump does not apply pressure or a
vacuum to the feed chamber, the vent valve is closed, the purge valve is opened and the
dispense pump is moved forward to remove air bubbles from the dispensing chamber.
At the beginning of the static purge stage, the dispense motor is stopped but the purge
valve remains open to continue the removal of air from the dispensing chamber. At the
beginning of the ready stage, the isolate and barrier valves are opened and the purge is
closed so that the feed pump and the system reaches ambient pressure and the pump is
ready to dispense fluid.
As described above, this conventional dispensing process suffers from double
dispense or stuttered dispense problems. In particular, the closure of the barrier valve
prior to dispensing pushes fluid out of the valve as it closes which pressurizes the fluid
in the dispensing chamber. This may cause a small amount of unwanted fluid to
dispense onto the wafer since the outlet valve is open. In addition, since the motor is
started at the same time as the outlet valve is opened, an uneven dispensing of fluid (or
stuttered dispensing) may occur since the outlet valve takes more time to open than the
starting of the motor and therefore the motor may be initially pushing the fluid through
an outlet valve which is not quite completely open. A dispensing method in
accordance with the invention which solves these problems will now be described.
Figure 4 is a timing diagram illustrating a method for dispensing fluids in
accordance with the invention. As with the conventional dispensing process described
above, the dispensing process shown in Figure 4 has the same stages, i.e., steps, 70 - 84
as the conventional process. In addition, much of the controlling of the valves and
motors is similar to the conventional method above, and only the changes in the
controlling of the valves and motors in accordance with the invention will be described
here. In particular, in order to prevent the unwanted double dispense or stuttered
dispense problems, the method changes the manner of controlling of the valves and
motors.
In particular, in accordance with invention, the barrier valve is not closed at the
beginning of the dispense stage as it done in the conventional process. Rather, the
barrier valve is closed at the beginning of the vent stage and kept closed during the
dispense stage. This avoids the sudden rise in pressure in the dispense chamber and,
therefore, fluid does not leak out of the outlet valve due to the sudden rise in pressure.
Since the barrier valve does not open and close prior to the beginning of the dispense
stage, but does close at the beginning of the vent stage, the pressure in the dispense
chamber does increase after the vent and purge states and this additional pressure must
be released. To release this pressure, during the static purge stage 84, the dispense
motor may be reversed to back out the piston 62 some predetermined distance to
compensate for any pressure increase caused by the closure of the barrier valve. As an
example, each step of the stepper motor may reduce the pressure by about 0J psi. If
the closure of the barrier valve increases the pressure by 2 psi, then the motor may be
reversed 20 steps to reduce the pressure in the dispense chamber by this amount to
compensate for the closure of the barrier valve. The actual pressure decrease, however,
depends on the characteristics of the particular stepper motor, lead screw and piston
being used. The pressure decrease caused by each step of the motor may be
determined by a pressure sensor which is located inside the dispensing chamber. In
accordance with the invention, since the outlet valve is not open when the additional
pressure is added into the dispensing chamber during the vent stage, no "spitting" of
the fluid onto the wafer may occur.
The motor may be further reversed a predetermined additional distance so that
the motor may be moved forward just prior to dispensing to adjust the dispense
pressure to zero and avoid any backlash which normally occurs when the motor is
moved backwards before the dispensing of fluid. In particular, with a piston, lead
screw and stepper motor dispense pump, the last motion prior to a dispense operation
is normally forward to avoid the fact that, as the piston changes direction, there is some
backlash. Thus, the problem of the additional pressure caused by the closure of the
barrier valve is avoided.
Next, during the beginning of the dispense stage 72, the timing of the outlet
valve and the start of the motor are changed to avoid the stuttering dispense problem.
In particular, the valve is a mechanical device that requires a finite period of time to
open. The motor, on the other hand, may start more quickly than the outlet valve may
open. Therefore, starting the motor and opening the outlet valve simultaneously will
cause a rise in pressure of the dispense fluid which in turn causes the stuttered
dispensing. To avoid this problem, the outlet valve is opened and then, some
predetermined period of time, T, later, the dispense motor is started so that the outlet
valve is completely open when the motor is started which achieves a good dispense.
The predetermined period of time depends on the characteristics of the outlet valve and
dispense motor being used, but, if the outlet valve takes approximately 50 ms to open,
then the predetermined period of time may be, for example, between 50 and 75 mS and
preferably approximately 75 mS. This predetermined period of time may also be
referred to as a delay. Thus, in accordance with the invention, the dispense motor is no
longer pushing fluid through a partially open outlet valve so that an accurate,
controlled amount of fluid may be dispensed onto the wafer. Thus, in accordance with
the invention, the problems caused by the closure of the barrier valve and the
simultaneously opening of the outlet valve and starting of the dispense motor are
avoided to provide more accurate dispensing of fluids, such as low viscosity fluids.
As described above, the valves and motors in the pumping apparatus are
controlled by a software application so that the above changes in the dispensing
process may be applied to any two-stage pumping apparatus since no hardware
changes are needed. Thus, for example, if the tubing, tubing length, nozzle height or
nozzle diameter is changed, the process in accordance with the invention may be easily
adapted. Now, the method for controlling the dispense process in accordance with the
invention will be described.
Figure 5 is a flowchart illustrating a method 100 for controlling the dispensing
of low viscosity fluids from a pumping apparatus in accordance with the invention.
At step 102, the barrier valve is closed at the end of the filtering stage which increases
the pressure in the dispense chamber. In step 104, during the static purge stage, the
dispense motor is reversed a predetermined distance to compensate for the pressure
increase caused by the closure of the barrier valve. Next, in step 106, the motor may
be reversed an additional distance so that, in step 108, when the motor is moved
forward to eliminate backlash, the pressure of the dispense chamber remains at zero.
In step 108, the pump is now ready for dispensing. In step 110, the outlet valve is
opened. Next, in step 112, the dispense motor is started some predetermined period of
time later and fluid is dispensed in step 114. The method is then completed.
While the foregoing has been with reference to a particular embodiment of the
invention, it will be appreciated by those skilled in the art that changes in this
embodiment may be made without departing from the principles and spirit of the
invention.