Syringe injection system for measuring non-volatile residue in solvents

The concentration of non-volatile residue in a test solvent is determined by generating multiple liquid droplets from a liquid stream including the solvent and ultrapure water. The droplets are dried to form a stream of multiple particles of the non-volatile residue. A supply of ultrapure deionized water is caused to flow continuously toward a non-volatile residue monitor, at a constant fluid flow rate. Upstream of the residue monitor, a syringe is provided for intermittently injecting a test solvent into the fluid stream. In one case, the solvent is injected for several minutes at a constant flow rate substantially less than that of the ultrapure water. A mixing valve, downstream of the point of solvent introduction, causes turbulent flow to thoroughly mix the solvent and water. In an alternative approach, a syringe is used to instantaneously inject solvent in the form of bursts. In this case, flow is laminar rather than turbulent, to maintain the solvent burst separate from the water, while it flows with the water in the fluid stream. In either case, the composite of liquid and solvent is provided to the residue monitor. The monitor output is a particle count. A microprocessor receives the particle count and converts the count to derive values for non-volatile residue concentration in the solvent.

BACKGROUND OF THE INVENTION 
This invention relates to the measurement of concentrations of non-volatile 
residue in liquids, and more particularly to systems and processes for 
determining non-volatile residue concentrations in solvents. 
It is known that the fabrication of very large scale integrated (VLSI) 
circuits requires an abundance of ultrapure water. More particularly, a 
complete fabrication process may involve over fifty stages of processing 
the surface of the semiconductor wafer. A washing with the ultrapure water 
follows each stage of processing, for removal of chemicals used in that 
stage. Accordingly, thousands of liters of ultrapure water may be used in 
processing a single wafer. Any non-volatile residue present in the 
ultrapure water can remain on the surface of the wafer after the water has 
evaporated, possibly causing defects in the resulting semiconductor 
device. This gives rise to a need to monitor the ultrapure water for the 
presence of non-volatile residue, to insure that the concentration of such 
residue remains at or below an acceptable level. 
Similarly, there is a need to determine the non-volatile residue 
concentration in various solvents used in etching, deposition, cleaning 
and other stages of fabrication. Herein, "solvents" is used generically to 
include organic solvents such as isopropyl alcohol and acetone, and 
inorganic solvents such as hydrochloric acid, hydrofluoric acid, ammonium 
hydroxide, hydrogen peroxide and water. These solvents must be tested to 
determine their purity. Further, as to solvents used in cleaning, it is 
advantageous to measure residual contamination extracted from components 
that have been cleaned in the solvent, as an indication of the degree to 
which such components have been cleaned, and as an indication of whether 
the solvent remains suitable for cleaning further components. 
Systems have been developed and employed successfully in continuously 
monitoring the quality of ultrapure water. For example, U.S. Pat. No. 
5,098,657 (Blackford et al) discloses an apparatus for measuring 
non-volatile residue concentrations in ultrapure water. Fixed and 
adjustable flow restrictive elements are arranged to provide a constant, 
pressure controlled flow of the water to an atomizer. At the atomizer the 
water is formed into droplets which are later dried to provide 
non-volatile residue particles. An electrostatic aerosol detector 
determines the particle concentration, which provides an indication of the 
purity of the water. 
Solvents, however, do not lend themselves to this type of continuous flow 
system, in which the fluid flows at a rate of at least fifty milliliters 
per minute. Due in part to their volatility, and in the case of acids 
their corrosiveness, solvents give rise to safety concerns in their 
handling, release vapor emissions, and create waste disposal problems. 
Accordingly, solvents preferably are used and tested in the lowest 
workable amounts and concentrations. 
The conventional method for testing solvents for non-volatile residue is to 
evaporate a measured quantity of the solvent in a previously weighed 
container. The original volume of liquid and the weight of material 
remaining after evaporation, are used to compute residue concentration. 
Given the need to determine residue concentrations in the single part per 
billion range, a relatively high volume of the solvent (e.g. one liter) is 
required for an accurate measure of concentration. The testing procedure 
is time consuming in view of the need to completely evaporate the solvent. 
This approach is costly, yet can not provide real time residue 
concentration data. Such testing gives rise to difficulties in solvent 
handling, potentially harmful vapor emissions, and waste disposal 
problems. 
While the above discussed needs and difficulties in ascertaining solvent 
purity are perhaps particularly apparent in connection with fabrication of 
semiconductor devices, they arise in other industries, e.g. manufacture of 
disk drives and recording media, precision optics, inertial guidance and 
aerospace applications. 
Therefore it is an object of the present invention to provide a system and 
process for accurately determining levels of impurities in solvents by 
testing extremely small quantities of the solvents. 
Another object of the invention is to provide a simple and rapid means for 
obtaining real time information on the concentration of non-volatile 
residues in volatile solvents. 
A further object is to provide a low cost approach to monitoring 
contamination levels of cleaning solvents used in semiconductor wafer 
processing and other manufacturing techniques that require exceptionally 
clean parts. 
Yet another object is to provide a process for testing contamination levels 
in solvents employed during various stages of semiconductor wafer 
processing (and other processes), with equipment already utilized in 
monitoring contamination levels in ultrapure water. 
SUMMARY OF THE INVENTION 
To achieve these and other objects, there is provided an apparatus for 
measuring the concentration of non-volatile residue in a test liquid. The 
apparatus includes a droplet forming means for receiving a fluid stream 
and for using at least a portion of the fluid stream to generate multiple 
liquid droplets. A drying means is disposed downstream of the droplet 
forming means, for causing evaporation of the liquid droplets to form a 
particle stream of multiple, substantially non-volatile particles. A 
particle counting means, disposed downstream of the drying means, receives 
the particle stream. The particle counting means includes a viewing 
region, and generates a particle count of the number of the non-volatile 
residue particles passing through the viewing region. A first fluid supply 
means is coupled to the droplet forming means. The first fluid supply 
means provides a fluid stream comprised of a carrier liquid moving at a 
substantially constant first flow rate. A second fluid supply means is in 
fluid communication with the first fluid supply means. The second fluid 
supply means controllably and intermittently introduces a test liquid into 
the fluid stream at a point upstream of the droplet forming means. 
In one preferred form of the invention, the second fluid supply means is a 
motorized syringe injector that introduces the test liquid, e.g. a 
solvent, at a substantially constant flow rate less than one percent of 
the flow rate of the carrier liquid, ultrapure water. More preferably, the 
ultrapure water flow rate is at least 50 milliliters per minute, while the 
solvent flow rate is about 0.03 milliliters per minute. A mixing valve, at 
the point of solvent introduction or just downstream, causes a turbulent 
flow to insure a thorough mixing of the solvent and ultrapure water. 
In another preferred approach, the solvent is introduced to the ultrapure 
water substantially instantaneously. In contrast to the first approach, 
the solvent must be injected in a non-turbulent manner, to form plugs of 
the solvent that flow in the fluid stream with the ultrapure water, yet 
remain separate and distinct from the water. The individual plugs can be 
extremely small in volume, e.g. in the 100 microliter range or less. 
Thus, neither approach requires large amounts of the solvent being tested. 
As a result, problems associated with volatile solvents, such as 
undesirable emissions to the atmosphere, waste disposal difficulties, and 
safety concerns in handling, are kept to a minimum. In the plug injection 
approach, this is due to the small amount, per se. In the turbulent 
mixture approach, this is due to the dramatic dilution of the solvent. 
The introduction of the solvent in plugs affords several further 
advantages. First, this approach is simpler and requires less skill, since 
there is no need to maintain a constant or steady solvent injection rate. 
No mixing valve or other means to generate a turbulent flow is required, 
since there is no need to form a mixture of the solvent and the ultrapure 
water. Results are obtained more rapidly, based upon the direct response 
of the particle counter in detecting non-volatile residue particles 
corresponding to the plug of solvent. By contrast, the mixture approach 
requires more time, e.g. several minutes, to stabilize the solvent/water 
mixture and maintain its stability. 
With either approach, however, concentrations can be determined by counting 
residue particles with known and available equipment. The droplet forming 
means preferably is an atomizer, but also can be a nebulizer or a 
vibrating orifice droplet generator. The preferred counting means include 
a condensation particle counter and a diffusion filter upstream of the CPC 
for removing ultrafine particles before the particle stream reaches the 
CPC. Alternatively, a light scattering particle spectrometer, an 
aerodynamic particle sizer or an electrostatic aerosol detector may be 
employed in counting the non-volatile residue particles. 
A further aspect of the present invention is a process for determining the 
concentration of non-volatile residue in a test liquid. The process 
includes the following steps: 
moving a carrier liquid in a fluid stream at a substantially constant first 
flow rate; 
controllably and intermittently introducing a test liquid into the fluid 
stream; 
downstream of a point at which the test liquid is introduced, generating 
multiple liquid droplets comprised of at least a portion of the fluid 
stream; 
drying the liquid droplets to form a particle stream of multiple 
substantially non-volatile residue particles; 
counting the non-volatile residue particles to obtain a particle count; and 
deriving the concentration of non-volatile residue in the test liquid, 
based upon the particle count. 
The test liquid may be introduced at a constant flow rate, with the flow 
rate preferably being at most one percent, and more preferably at most 0.1 
percent, of the carrier liquid flow rate. The test liquid should be 
miscible in the carrier liquid. When this method is employed, the particle 
counting step includes obtaining a background count corresponding to a 
fluid stream including just the carrier fluid, and obtaining a composite 
count corresponding to both the test liquid and carrier liquid in the 
fluid stream. The derivation step includes subtracting the background 
count from the composite count. 
Alternatively, the test liquid may be introduced substantially 
instantaneously, in a non-turbulent manner. This forms plugs of the test 
liquid that flow in the fluid stream with the carrier liquid, yet remain 
separate and distinct. Experimentation has shown that with this approach, 
as well, there is a need to subtract a background count. 
Thus, in accordance with the present invention, a system for monitoring the 
contamination levels in ultrapure water, further is useful (with 
appropriate modifications as described) in determining the contaminate 
levels in solvents. A solvent can be introduced to form discrete plugs 
flowing in the fluid stream, or as part of a solvent/water mixture. In 
either event, the system is simple and reliable, affording highly accurate 
readings without requiring the excessive amounts of solvent used in the 
conventional approach of determining residue concentrations by 
evaporation. Tests can be performed rapidly, especially when the solvent 
is introduced as a plug, enabling repeated testing to verify results, or 
timely adjustments in response to sensing undesirably high residue 
concentrations.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Turning now to the drawings, there is shown in FIG. 1 a system 16 for 
determining the concentration of non-volatile residue in a test liquid, 
e.g. a solvent. The system includes a pump 18 for supplying ultrapure, 
deionized water through a conduit 20 to a "T" fitting 22, and then to a 
non-volatile residue monitor 24 via a conduit 26. Through proper control 
of pump 18 and further equipment not illustrated but known to those 
skilled in the art, the ultrapure water is supplied at a steady, precisely 
controlled rate, preferably in the range of about 50 to about 70 ml per 
minute. A preferred non-volatile residue monitor is available from TSI 
Incorporated of St. Paul, Minn., and sold under the brand name LIQUITRAK. 
Residue monitor 24 is used to continuously monitor the concentration of 
non-volatile residue in the ultrapure water, to insure a level of purity 
in the water sufficient for its intended use, e.g. in cleaning a wafer 
between stages of semiconductor device fabrication. A small portion (e.g. 
about one percent) of the ultrapure water is directed to an atomizer 28 of 
the monitor. Compressed air or nitrogen also is supplied to the atomizer 
at a constant flow rate, via a line 30. 
The output of atomizer 28 is a stream of droplets of the ultrapure water, 
which travel through a conduit 32 to a drying column 34. Compressed air or 
nitrogen which has been dried, filtered and heated to a temperature of 
about 120 degrees C., is supplied to the drying column through a line 36. 
The ultrapure water droplets dry rapidly and completely as they progress 
through drying column 34. Thus, the drying column output is a particle 
stream composed of multiple non-volatile residue particles. Every droplet 
provided to the drying column from the atomizer yields a residue particle. 
Cleaner ultrapure water produces smaller residue particles. 
Non-volatile residue particles leaving the drying column progress through a 
conduit 38 to a diffusion filter 40, where ultrafine particles (below a 
predetermined size) are removed from the particle stream. More 
particularly, the ultrafine particles cling to the walls of filter 40 due 
to Brownian movement. The remaining particles are provided through a 
conduit 42 to a condensation particle counter (CPC) 44, sometimes referred 
to as a condensation nucleus counter. 
In condensation particle counter 44, the stream of particles (supported by 
the air or nitrogen) travels through a chamber saturated with a vapor, 
e.g. of n-butyl alcohol, after which the stream is cooled sufficiently to 
supersaturate the vapor. The vapor condenses onto the particles forming 
aerosol droplets substantially larger than the particles themselves. After 
condensation, the aerosol droplets travel through a viewing region or 
volume 46 defined by a laser and associated optics. For further 
information on this type of device, reference is made to U.S. Pat. No. 
4,790,650 (Keady), assigned to the assignee of this application. For a 
further description of non-volatile residue monitors, reference is made to 
U.S. Pat. No. 5,098,657 (Blackford et al), also assigned to the assignee 
of this application, and incorporated by reference herein. 
Condensation particle counter 44 detects each aerosol droplet passing 
through the viewing volume, and thus generates a particle count 
corresponding to the number of non-volatile residue particles passing 
through monitor 24. The CPC output is an electrical signal, which is 
provided to a microprocessor 48. Microprocessor 48 includes an 
electronically erasable programmable read only memory (EEPROM) 50 in which 
conversion information is stored. Based on this conversion information, 
microprocessor 48 generates an output indicating the concentration of 
non-volatile residue in terms of parts per billion (PPB). The 
microprocessor output is provided to a video display terminal 52. Display 
terminal 52 provides a continuously updated record of non-volatile residue 
concentration in the ultrapure water. 
Beyond monitoring the purity of the water, it also is necessary to monitor 
the purity of various solvents employed in semiconductor device 
fabrication, e.g. for etching or depositing material during production 
stages, for cleaning semiconductor wafers between stages, and for 
measuring residual contaminates extracted from components cleaned with the 
solvent. Accordingly, a syringe injector 54, precisely controlled by a 
stepper motor (not shown) is connected to fitting 22 via a conduit or 
needle 56. To ensure against contamination, T fitting 22 includes a septum 
which operates to close the opening created by withdrawing the needle 
after injection. A preferred syringe is available from Becton-Dickinson & 
Company of Rutherford, N.J., and identified as Model No. 9663. A removable 
hypodermic needle is used in combination with the syringe. The syringe has 
a 60 ml capacity, and is controlled by the stepper motor to deliver the 
test solvent at a preferred rate of about 0.030 ml per minute. Thus, for a 
deionized ultrapure water delivery rate of 60 ml per minute, the solvent 
forms only about 0.05 percent of the composite liquid (i.e. the 
combination of solvent and water). This extreme dilution virtually 
eliminates any harmful effect the solvent might have upon the system, and 
enables substantial testing based upon minute quantities of the solvent. 
Syringe injector 54 is not operated continuously. Rather, the syringe is 
actuated intermittently, with each test lasting several minutes. When 
syringe injector 54 is not actuated, residue monitor 24 receives only the 
ultrapure water, and monitors water quality in the manner discussed above. 
When the syringe is actuated, the solvent under test is supplied at the 
preferred continuous rate, and mingles with the ultrapure water to provide 
a composite fluid flow to the residue monitor. 
At a mixing valve 58 along conduit 26, the solvent and the ultrapure water 
are thoroughly mixed, to insure that residue monitor 24 receives a 
homogeneous, uniform mixture. Mixing valve 58 includes a valve ball 60 
within an enlarged portion 62 of conduit 26. The arrangement forces both 
the water and the solvent to flow along the relatively constricted area 
between valve ball 60 and conduit 26, increasing the fluid velocity and 
causing turbulent flow just downstream of the valve, as indicated by the 
arrows to the right of the valve ball as viewed in FIG. 2. 
During testing, the residue monitor output is based on residue particles 
generated as a result of the composite flow. During "normal" operation, 
only the ultrapure water is provided to residue monitor 24, and the 
monitor output is based only upon the residue concentration in the water. 
Accordingly, the residue concentration in the solvent itself is derived in 
microprocessor 48, by subtracting a background level of residue 
concentration (in the ultrapure water, alone) from a composite level of 
concentration (based on the composite fluid including solvent and water). 
The result is a value representing concentration in the solvent alone, in 
particles per cm.sup.3. This value is converted using EEPROM 50, to yield 
a value for concentration in parts per billion. 
While one preferred rate of solvent injection is 0.03 ml per minute as 
indicated above, other injection rates may be suited for different 
solvents and applications. The chart in FIG. 3 illustrates the variance in 
concentration (PPB), responsive to changes in the rate at which the 
solvent is injected into the fluid stream. Five different rates of 
injection are graphically indicated at levels 64, 66, 68, 70 and 72. There 
is a substantially linear relationship between the injection rate and the 
resulting solvent residue concentration. 
The chart in FIG. 4 illustrates, on a log/log scale, a substantially linear 
relationship of condensation particle counter output concentration 
(particles per cm.sup.3), and concentration in parts per billion as 
measured using an atomic absorption spectrometer (AAS), in connection with 
a potassium chloride solution. The substantially linear relationship 
verifies the utility of counting particles to determine residue 
concentration. 
FIG. 5 illustrates an alternative embodiment monitoring system 74, in which 
a pump 76 supplies ultrapure deionized water to a non-volatile residue 
monitor 78 via a conduit 80, a "T" fitting 82 and a conduit 84. Residue 
monitor 78 is substantially similar to monitor 24 of system 16, and 
provides its output to a microprocessor 86 for ultimate display by a video 
display terminal 88. Microprocessor 86 and display terminal 88 are 
substantially similar to their counterparts in system 16. 
The means of injecting the solvent or other test liquid is a microliter 
syringe 90, in combination with a hypodermic needle 92. Suitable syringes 
are available from the Hamilton Company of Reno, Nev., and include a 10 
microliter capacity syringe identified as Model No. 801RN, and a 100 
microliter capacity syringe identified as Model No. 810RN. Thus, as 
compared to syringe injector 54 of system 16, syringe 90 is much smaller 
in capacity, i.e. smaller by about three orders of magnitude. 
The solvent is injected into the fluid stream via a conduit 94 to fitting 
82, where the solvent is merged into the fluid stream that also includes 
the ultrapure water. To insure against contamination, fitting 82 includes 
a septum which operates to close the opening created by withdrawal of the 
needle after injection. 
However, the introduction of the solvent in system 74 differs from that in 
system 16 in several respects. First, the amount of solvent injected, even 
at full capacity of a 100 ml syringe, is substantially less than the 
amount of solvent injected in system 16 by several orders of magnitude. 
Secondly, the solvent in syringe 90 is injected substantially 
instantaneously, each injection lasting only a fraction of a second. Thus 
there is no need to maintain a constant injection rate over time. Finally, 
system 74 does not incorporate a mixing valve, or any other structure to 
introduce turbulent flow in the fluid stream. Instead, the fluid flow is 
essentially laminar. 
As a result of the essentially laminar flow, each solvent injection forms a 
plug, as illustrated at 96 in FIG. 6. Solvent plug 96 flows in the fluid 
stream with the ultrapure water at the same linear velocity as the 
ultrapure water, yet remains separate and distinct. In practice, this 
phenomenon can be observed when conduit 84 is constructed of a transparent 
material. Plugs of solvent are visible, due to the fact that the solvent 
and water have different indices of refraction. 
The maintenance of laminar flow to preserve the integrity of the solvent 
plugs is a key factor in residue measuring efficiency. To this end, 
conduit 84 is much smaller than its counterpart in system 16. More 
particularly, the outside diameter of conduit 84 is about one sixteenth of 
an inch in diameter. Conduit 26 of system 16 has an outside diameter of 
about one quarter inch. Given the flow rate (of ultrapure water) of from 
50 to 70 ml in both systems, it is to be appreciated that linear velocity 
of the fluid, as it flows toward the atomizer, is substantially greater in 
system 74. To further insure against unwanted mixing of the solvent and 
water, the length of conduit 84, and the total flow path from fitting 82 
to the atomizer, is as short as practicable, e.g. a few inches. 
The instantaneous burst injection of the solvent, the increased linear 
fluid flow velocity, and the shorter path to the atomizer, contribute to 
substantially reducing the time involved in testing the solvent. Test 
results are displayed on terminal 88 within a matter of seconds after 
injection of a solvent burst, permitting several repetitions of the test 
to verify the accuracy of results, and providing essentially "real time" 
non-volatile residue concentration readings, substantially increasing the 
probability that timely corrective action may be taken in response to 
results that exceed an acceptable maximum concentration. 
FIG. 7 is a plot of residue concentration (PPB) over time, based upon a 
potassium chloride (KCl) solution. The solvent plugs cause sharp peaks as 
shown. Depending upon the software utilized in microprocessor 86, either 
the peak heights or (more preferably) the areas beneath the peaks can be 
used to compute non-volatile residue concentrations. The numbers shown 
directly above the peaks, ranging from 1.1 to 5.7, are the micrograms per 
injection of the KCl solution. Thus, the higher peaks correspond to 
increased levels of the solution in the corresponding injections. The 
relationship of residue concentration plotted against the amount of 
solution injected is substantially linear, as illustrated in FIG. 8. 
In practice, it has been found preferable to employ water-soluble solvents 
in a system utilizing ultrapure water as the carrier liquid. Water soluble 
solvents were found to flow smoothly through the conduit toward the 
atomizer. Water insoluble fluids, by contrast, tended to break up into 
droplets that occasionally adhered to the walls of the conduit, reducing 
the efficiency of the residue monitor. To minimize or eliminate this 
problem, liquids other than water may be employed as carrier liquids. For 
example, system 74 can be employed to detect non-volatile residue in skin 
oil, with ethanol as the carrier liquid. 
FIG. 9 illustrates part of an alternative system 98 in which ultrapure 
water is supplied to a T fitting 100 via a conduit 102, and a solvent is 
supplied to the fitting via a conduit 104. The composite liquid flows to 
an atomizer 106 through a conduit 108. The droplet output of atomizer 106 
is supplied to a light scattering particle spectrometer 110. For further 
information regarding a light scattering particle spectrometer, reference 
is made to U.S. Pat. No. 4,794,086 (Kasper et al). The output of 
spectrometer 110 is provided to a microprocessor and a video display 
terminal (not shown). 
A further alternative system 112 is shown in FIG. 10, where ultrapure water 
and a solvent are provided to a T fitting 114, and the composite liquid 
provided to a vibrating orifice droplet generator 116 via a conduit 118. 
The output of droplet generator 116 is a series of aerosol droplets of a 
precisely determined size. The droplets are provided to an aerodynamic 
particle sizer 120. The output of particle sizer 120 is provided to a 
microprocessor and a video display terminal. For further information 
regarding a vibrating orifice droplet generator and an aerodynamic 
particle sizer, reference is made to the aforementioned U.S. Pat. No. 
4,794,086. 
FIG. 11 illustrates a further alternative system 122 in which a T fitting 
124 provides a composite liquid flow of ultrapure water and a solvent to a 
nebulizer 126. The output of nebulizer 126 is provided to an electrostatic 
classifier 128, with the output of the electrostatic classifier being 
provided to a condensation particle counter 130. 
FIG. 12 illustrates yet another system 132 in which the combined 
solvent/ultrapure water output of a T fitting 134 is provided to an 
atomizer 136. The droplet output of the atomizer proceeds through a 
diffusion filter 138 to electrostatic aerosol detector 140. As indicated 
in the aforementioned U.S. Pat. No. 5,098,657, an electrostatic aerosol 
detector can be employed in lieu of a condensation particle counter. 
Thus, in accordance with the present invention, a simple, low cost and 
reliable system achieves accurate readings of non-volatile residue 
concentrations in solvents, requiring only minute quantities of the 
solvents for testing. Potential hazards from handling large amounts of the 
solvents, undesirable emissions to the atmosphere, and solvent disposal 
problems are all significantly reduced. Tests can be performed rapidly and 
repeatedly, and provide substantially real time results, particularly when 
the solvent is injected instantaneously in the form of solvent bursts. The 
testing systems can employ ultrapure water as a carrying liquid for the 
solvent. This facilitates testing of solvents, largely with equipment 
already utilized in connection with the ultrapure water systems, as 
modified to accommodate solvent injection.