Method and apparatus for dispensing liquid

An anti-drip system for a dispenser in the form of a deformable conduit adapted to dispense a liquid, such as a photoresist, from a dispensing tip located at one end of the conduit includes a rigid housing which surrounds an intermediate portion of the conduit. The housing is closed except for a port which permits pressurized air to be supplied to and exhausted from the housing. At the commencement of a dispensing operation, the liquid is transported through the conduit as the housing is simultaneously pressurized, thereby crimping the intermediate portion of the conduit. At the conclusion of the dispensing operation, air pressure in the housing is simultaneously reduced, thereby allowing the conduit to expand and assume its natural substantially uncrimped shape. Expansion of the conduit creates a vacuum sufficient to suck back liquid which is being dispensed from the conduit. A pair of anti-drip systems may be utilized in combination with a pair of valves to actually meter liquid which is being dispensed through a deformable conduit.

FIELD OF THE INVENTION 
The present invention relates to methods and apparatus for controlling the 
dispensation of liquid through a deformable conduit, and, more 
particularly, to such methods and apparatus which inhibit the liquid from 
dripping at the conclusion of the dispensing operation. 
BACKGROUND OF THE INVENTION 
In the manufacture of semiconductors and similar devices, it is often 
necessary to repeatedly dispense metered amounts of liquid, such as a 
photoresist, onto a workpiece, such as a wafer. Heretofore, the liquid has 
been supplied to the workpiece by a dispenser which includes a flexible 
conduit and a three-way valve designed to control the flow of liquid 
through the conduit. At the conclusion of the dispensing operation, it has 
been found that liquid residue tends to collect at an outlet of the 
dispenser. The liquid, especially if it is a photoresist, may crystallize 
upon contact with air. Therefore, if a liquid residue collects at the 
outlet of the dispenser, the residue will often crystallize. Such a 
crystallized residue is undesirable, inasmuch as portions of the 
crystallized residue can be dislodged during subsequent dispensing 
operations and deposited on the workpieces. These deposited portions of 
the crystallized residue are undesirable because they can create an uneven 
or otherwise defective coating on the workpieces. 
Pinch valves have been developed which employ deformable tubes to dispense 
metered amounts of liquid in a dripless manner (see, for instance, U.S. 
Pat. Nos. 4,030,640; 3,982,724; 3,511,468 and 2,884,165). All of these 
prior devices utilize pistons, platens or similar mechanical elements as 
pinch valves to physically deform the tubes in order to control the 
dispensation of the liquid. These pinch valves cause the tubes to fatique, 
thereby limiting the operating life of the prior devices. Also, inasmuch 
as the pinch valves usually consist of seberal moving parts, the pinch 
valves are subject to frequent breakdowns, resulting in rather long 
periods of machine downtime. 
SUMMARY OF THE INVENTION 
The problems and disadvantages of the prior art device described above are 
overcome in accordance with the present invention by providing new and 
improved methods and apparatus for dispensing liquid and for inhibiting 
drippage of the liquid at the conclusion of the dispensing operation. More 
particularly, the new and improved methods and apparatus utilize a 
deformable conduit adapted to discharge or dispense liquid therefrom. In 
accordance with the present invention, the conduit passes through a 
housing which is adapted for pressurization and depressurization. During a 
dispensing operation, liquid is transported through the conduit as the 
housing is simultaneously pressurized to a pressure specifically selected 
to crimp the conduit solely by the forces applied directly to it as a 
result of the pressurization of the housing (i.e., the conduit is crimped 
without utilizing any mechanical advantage). At the conclusion of the 
dispensing operation, the housing is depressurized to an extent sufficient 
to allow the conduit to expand and assume its natural substantially 
uncrimped state. The expansion of the conduit creates a vacuum sufficient 
to suck back liquid which is being dispensed or discharged from the 
conduit, thereby inhibiting the liquid from dripping out of the conduit or 
out of a dispensing tip attached thereto. By inhibiting drippage, the 
present invention also inhibits the collection of solidified or 
crystallized liquid residue at an outlet of the conduit or at an orifice 
of the dispensing tip. Inasmuch as the conduit is not mechanically 
crimped, the present invention does not fatigue the conduit as much as the 
pinch valves employed by the prior art devices discussed above. Also, by 
eliminating the mechanical pinch valves of the prior art devices, the 
present invention is much simpler to operate and maintain than the prior 
art devices. 
By utilizing a pair of pressurized housings, the present invention can 
actually be employed to dispense metered amounts of liquid through a 
deformable conduit which passes through the housings, as well as to 
inhibit drippage of the liquid at the end of the dispensing operation. For 
instance, after filling the conduit with liquid and then closing off an 
inlet end of the conduit, both of the housings are pressurized to crimp 
the conduit and thereby discharge a predetermined amount of liquid from an 
open outlet end of the conduit. At the conclusion of the dispensing 
operation, the conduit would be closed off between the housings and its 
inlet end would be opened. Next, the housings are depressurized to permit 
upstream and downstream portions of the conduit to expand and assume their 
natural substantially uncrimped shape. The expansion of the upstream 
portion increases its volume and thereby creates a vacuum sufficient to 
fill the upstream portion with liquid sucked through the inlet end of the 
conduit. The expansion of the downstream portion increases its volume and 
thereby creates a vacuum sufficient to suck back liquid which is being 
dispensed from the outlet end of the conduit.

DESCRIPTION OF THE EXEMPLARY EMBODIMENTS 
While the present invention may be used to dispense any type of liquid, it 
is especially suitable for use in dispensing photoresist employed in 
connection with the manufacture of semiconductors and similar devices. 
Thus, the present invention will be described with particular reference to 
photoresist dispensing systems. 
With reference to FIG. 1, there is shown a photoresist dispensing system 
which includes a reservoir 10 containing photoresist 12 to be dispensed 
from an orifice 14 of a dispensing tip 16 onto a workpiece (not shown). 
The reservoir 10 is connected to the dispensing tip 16 by a deformable 
conduit 18. Both the dispensing tip 16 and the conduit 18 are made from 
semi-rigid tubing sold under the trade name Teflon TFE. The conduit 18 
may, for example, have an outside diameter of 0.250 inches and a wall 
thickness of 0.030 inches. 
A suck-back assembly 20 is positioned over an intermediate portion 22 of 
the conduit 18. The suck-back assembly 20 includes a rigid housing 24 made 
primarily from a steel pipe 26. An upstream end of the housing 24 is 
closed off by a fitting 27, which is threaded onto a corresponding end of 
the steel pipe 26. The opposite end of the steel pipe 26 is threaded into 
one leg 28 of a T-fitting 30. A second leg 32 of the T-fitting 30 
threadedly receives a composite fitting 34, which closes a downstream end 
of the housing 24. A three-way valve 36 is connected to a third leg 38 of 
the T-fitting 30 by a fitting 40. The three-way valve 36 is supplied with 
compressed air from a source 42 of compressed air through an air inlet 44. 
A regulator 46 is positioned between the air inlet 44 and the three-way 
valve 36 for regulating the pressure of the compressed air which is 
supplied to the three-way valve 36. A gauge 48 is associated with 
regulator 46 to give a visible indication of the pressure of the 
compressed air. An adjustable metering valve 50 is attached to the 
three-way valve 36 to control the flow of air discharged to the atmosphere 
from the three-way valve 36. 
Another three-way valve 52 is arranged in the conduit 18 between the 
suck-back assembly 20 and the reservoir 10. A microprocessor 54 is 
connected to the three-way valves 36, 52 in order to control their 
operation. 
In the normal initial operation of the dispensing system of FIG. 1, the 
three-way valve 52 is opened to fill the conduit 18 and the dispensing tip 
16 with the photoresist 12 contained in the reservoir 10. After the 
conduit 18 and the dispensing tip 16 have been completely filled, the 
three-way valve 52 is closed. 
In order to dispense metered amounts of the photoresist 12 from the 
dispensing tip 16, the three-way valves 36, 52 are simultaneously 
energized. When the three-way valve 52 is energized, it permits the 
constantly pressurized photoresist 12 to flow from the reservoir 10 into 
the conduit 18, thereby displacing an equal amount of the photoresist 12 
which had already been supplied to the conduit 18 and the dispensing tip 
16. Thus, the three-way valve 52 controls the amount of the photoresist 12 
which is dispensed from the dispensing tip 16. The energization of the 
three-way valve 36 permits the compressed air from the source 42 to enter 
the housing 24. Thus, during the dispensation of the photoresist 12 from 
the dispensing tip 16, the housing 24 can be pressurized to a pressure 
which crimps but does not close the intermediate portion 22 of the conduit 
18 along substantially the entire length of the housing 24. The degree to 
which the intermediate portion 22 of the conduit 18 is crimped depends 
upon the value of the pressure within the housing 24. 
After a predetermined amount of the photoresist 12 has been dispensed from 
the dispensing tip 16, the three-way valves 36, 52 are simultaneously 
de-energized. When the three-way valve 52 is de-energized, it prevents the 
photoresist 12 in the reservior 10 from entering the conduit 18. The 
de-energization of the three-way valve 36 cuts off the flow of compressed 
air into the housing 24, while opening the housing 24 to the atmosphere 
through the metering valve 50 so as to exhaust the compressed air from the 
housing 24 and thereby depressurize it. The metering valve 50 retards or 
delays the complete exhaustion of the housing 24 to insure that the 
housing 24 is not completely exhausted until the three-way valve 52 is 
completely closed. Alternatively, the de-energization of the three-way 
valve 36 can be delayed for a short period of time after the 
de-energization of the three-way valve 52, thus eliminating the need to 
provide the metering valve 50. 
Once the housing 24 is sufficiently depressurized, the natural resiliency 
of the conduit 18 causes the intermediate portion 22 to expand and assume 
its natural substantially uncrimped shape. The expansion of the 
intermediate portion 22 causes a corresponding increase in the volume of 
the conduit 18 and the consequent creation of a partial vacuum therein. As 
a result of the vacuum created in the conduit 18, the photoresist 12 in 
the vicinity of the orifice 14 of the dispensing tip 16 is sucked back 
into the dispensing tip 16, thereby inhibiting drippage of the photoresist 
12 from the dispensing tip 16 and its subsequent crystallization around 
the orifice 14. 
In order to discharge another metered amount of the photoresist 12 from the 
dispensing tip 16, the abovedescribed steps would be sequentially 
repeated. Each time these steps are repeated a metered amount of the 
photoresist 12 would be dispensed from the dispensing tip 16. 
Referring now to FIG. 2, there is shown a photoresist dispensing system 110 
which includes a non-pressurized source 112 of a photoresist to be 
dispensed, a dispensing tip 114 for dispensing the photoresist and a 
deformable conduit 116 which connects the source 112 of the photoresist to 
the dispensing tip 114. Two suck-back assemblies 118, 120 are arranged 
along the conduit 116 between the source 112 and the dispensing tip 114. 
The design and construction of the suck-back assemblies 118, 120 are 
substantially identical to the design and construction of the suck-back 
assembly 20 of FIG. 1. Two three-way valves 122, 124 are also provided 
along the conduit 116. The valve 122 is positioned between the suck-back 
assembly 118 and the source 112, while the valve 124 is positioned between 
the two suck-back assemblies 118, 120. 
In the normal initial operation of the dispensing system of FIG. 2, the 
valves 122, 124 are energized or opened so that the conduit 116 and the 
dispensing tip 114 can be filled with photoresist. After the conduit 116 
and the dispensing tip 114 have been completely filled with photoresist, 
the valve 122 is de-energized or closed, while the valve 124 remains 
energized or open. The two suck-back assemblies 118, 120 are then operated 
in the same manner as the suck-back assembly 20 of FIG. 1 to crimp the 
conduit 116 in two separate locations and thereby displace photoresist 
from the dispensing tip 114. The amount of photoresist dispensed from the 
dispensing tip 114 is determined by the amount of decrease in the volume 
of the conduit 116 as a result of its being crimped by the suck-back 
assemblies 118, 120. 
After the dispensing operation is completed, the valve 124 is de-energized 
or closed and the valve 122 is energized or opened, while the two 
suck-back assemblies 118, 120 are opened to the atmosphere to permit the 
conduit 116 to expand and assume its natural substantially uncrimped 
shape. The expansion of the conduit 116 in the vicinity of the suck-back 
assembly 118 creates a partial vacuum in an upstream portion of the 
conduit 116, resulting in photoresist being sucked into the upstream 
portion of the conduit 116 from the source 112. The expansion of the 
conduit 116 in the vicinity of the suck-back assembly 120 creates a 
partial vacuum in a downstream portion of the conduit 116, resulting in 
the suck back of photoresist into the dispensing tip 114. 
In order to discharge another metered amount of photoresist from the 
dispensing tip 114, the abovedescribed steps would be sequentially 
repeated. Each time these steps are repeated a metered amount of 
photoresist would be dispensed from the dispensing tip 114. 
It will be understood that the embodiments described herein are merely 
exemplary and that a person skilled in the art may make many variations 
and modifications without departing from the spirit and scope of the 
invention. All such modifications and variations are intended to be 
included within the scope of the invention as defined in the appended 
claims.