Method for rapidly determining an impurity level in a gas source or a gas distribution system

Provided is a novel method for rapidly determining an impurity level in a gas source. A gas source and a measurement tool are provided for measuring an impurity level in a gas flowing from the gas source. The measurement tool is in communication with the gas source through a sampling line. The sampling line has a gas inlet disposed upstream from a gas outlet. The sampling line is baked according to a baking strategy, such that when baking is terminated, a concentration profile of the impurity in the sampling line contains a first region and a second region. In the first region, extending from the gas inlet to a point downstream from the inlet, the vapor phase concentration of the impurity is less than the vapor phase concentration of the impurity in the gas entering the sampling line. In the second region, located downstream from the first region and extending to the gas outlet, the vapor phase concentration of the impurity is greater than the vapor phase concentration of the impurity in the gas entering the sampling line. A method for rapidly determining an impurity level in a gas distribution system which delivers gas to a point of use is also provided. Particular applicability is found in the semiconductor processing industry to measure impurities in gases delivered to processing tools.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a novel method for rapidly determining an 
impurity level in a gas source. The present invention also relates to a 
method for rapidly determining an impurity level in a gas distribution 
system. 
2. Description of the Related Art 
The manufacture of integrated circuits (IC's) involves processes in which 
semiconductor wafers are contacted with various gases. Such processes 
include, for example, chemical vapor deposition (CVD), diffusion, 
oxidation, sputtering, rapid thermal processing, etching and ion 
implantation processes. The gases can be stored in, for example, gas 
cylinders, bulk storage systems or supplied by an onsight separation 
plant. 
Because of the high sensitivity of IC devices to impurities, the impurity 
level in the manufacturing environment plays a crucial role in the yield, 
and hence, in the profitability of a wafer fabrication facility (wafer 
fab). Microelectronics manufacturers require extremely high purity gases, 
for example ultra-high purity (UHP) gases, delivered to the point of use 
(i.e., to the processing tool). Gas source suppliers and gas distribution 
system installers are therefore required to certify performance of a gas 
source or a gas distribution system in terms of impurity levels in the gas 
source or gas distribution system. 
For microelectronics applications, impurity measurements in the parts per 
billion (ppb) to sub ppb range are generally required. Thus, the use of 
sophisticated analytical instrumentation, such as atmospheric pressure 
ionization mass spectrometers (APIMS), is required. Moreover, in making 
the requested measurements, great care in the sampling procedure is 
necessary. As a result, the costs associated with the analytical equipment 
and the manpower necessary to perform the certification measurements 
represent a significant investment. Given the industrial trend towards 
decreasing the construction time of wafer fabs, reductions in time and 
resources necessary for certification of a gas source or gas distribution 
system have substantial value. 
Moisture level measurement in industrial scale systems is typically carried 
out with the use of a sampling line connected between the gas source or 
gas distribution system and a measurement tool. The sampling line takes a 
gas sample from the gas source or from a given point in the gas 
distribution system and delivers it to the measurement tool, where the 
impurity level is measured. 
The sampling line itself contributes to the measured impurity level, and 
hence, the gas must be purged through the sampling line until the impurity 
value measured by the measurement tool represents the actual value of the 
gas source or the gas distribution system. This is especially true for 
impurities which have extremely slow response times (e.g., moisture) due 
to the strong interaction of the impurity with the inner surfaces of the 
gas distribution system and sampling line. 
The interaction of the impurity to be measured with the surfaces of the gas 
distribution system can affect both the length of time it takes to dry 
down a gas distribution system as well as the sensitivity of the 
measurement tool that samples the gas through the sampling line. This 
effect has been recognized in a publication by McAndrew et al (Using 
Simulation to Optimize Gas Distribution System Cost and Performance, 
Journal of the IES, September/October 1994, pp. 30-39), which describes 
how the time delay for a moisture upset to travel through a sampling line 
affects the size and duration of upset that can be detected. 
Various procedures have been used in the past to decrease the time required 
to measure the value of an impurity in a gas distribution system via a 
sampling line. These procedures include, for example, use of a sampling 
line constructed from a material that interacts less strongly with the 
impurity of interest than the distribution system itself (see, Venet et 
al, from fall 1991 AICHE conference (1992), pp. 26-28), and baking of the 
system at an elevated temperature to drive off adsorbed impurities from 
the surfaces making up the sampling line (see, Jurcik et al, The Effect of 
Baking on the Dry Down of UHP Distribution Systems: From Laboratory to 
Industrial Scale via Numerical Simulation, from proceedings of the 1995 
IES meeting (1995)). 
However, the above procedures are ineffective to minimize the time required 
to bring the concentration of the impurity measured at the outlet of the 
sampling line to the same level as in the gas introduced through the 
sampling line inlet. For example, while construction of the sampling line 
out of a material that interacts less strongly with the impurity of 
interest may decrease the time required to measure the actual value of 
that impurity, currently available tubing materials and surface finishes 
exhibit strong interactions with moisture. Thus, measurement time is 
substantial. 
Furthermore, although baking of the sampling line can be effective in 
decreasing measurement time, artificially low values may be observed for 
extended periods of time if care is not taken in the selection of an 
appropriate baking strategy. In such a case, the true impurity level in 
the gas source or gas distribution system is not measured, and the gas 
source or gas distribution system is certified at a lower than actual 
value. 
To meet the requirements of the semiconductor processing industry, and to 
overcome the disadvantages of the related art, it is an object of the 
present invention to provide a novel method for determining impurity 
levels in a gas source or a gas distribution system. The inventive method 
allows for accurate and rapid measurements of impurity concentration. 
Thus, certification of a gas source or a gas distribution system can be 
performed with a greater degree of accuracy and more quickly than was 
previously possible. Substantial cost savings in the set-up of wafer fabs 
can be realized. 
SUMMARY OF THE INVENTION 
The foregoing objectives are met by the methods of the present invention. 
According to a first aspect of the invention, a novel method for rapidly 
determining an impurity level in a gas source is provided. The method 
includes providing a gas source and a measurement tool for measuring an 
impurity level in a gas flowing from the gas source. The measurement tool 
is in communication with the gas source through a sampling line. The 
sampling line has a gas inlet disposed upstream from a gas outlet. The 
sampling line is baked according to a baking strategy, such that when 
baking is terminated, a concentration profile of the impurity in the 
sampling line contains a first region and a second region. In the first 
region, extending from the gas inlet to a point downstream from the inlet, 
the vapor phase concentration of the impurity is less than the vapor phase 
concentration of the impurity in the gas entering the sampling line. In 
the second region, located downstream from the first region and extending 
to the gas outlet, the vapor phase concentration of the impurity is 
greater than the vapor phase concentration of the impurity in the gas 
entering the sampling line. 
According to a second aspect of the invention, a method for rapidly 
determining an impurity level in a gas distribution system is provided. 
This second aspect is similar to the above-described method, except a gas 
distribution system for delivering a gas to a point of use is connected to 
the gas source. The measurement tool is in communication with the gas 
distribution system through the sampling line, such that gas flows from 
the gas source through the gas distribution system and the sampling line 
to the measurement tool.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION 
The invention provides an effective method for rapidly and accurately 
determining an impurity level in a gas source or a gas distribution 
system. Applicants have surprisingly and unexpectedly determined that 
baking of a sampling line can be very effective in decreasing the time 
required to measure the actual concentration of the gas source or gas 
distribution system if the duration and intensity of the baking are 
carefully controlled. 
The invention has two substantial benefits over known methods for detecting 
an impurity in a gas source or gas distribution system. First, more rapid 
determination of the impurity level in a gas source or a gas distribution 
system is possible, and second, improper certification at artificially low 
levels can be prevented. 
As used herein, the term "gas source" refers to any gas stored in a 
cylinder or in a bulk storage system, either in a gaseous or liquified 
state. That term also refers to gas produced in a gas manufacturing plant. 
Also as used herein, "gas distribution system" refers to the gas piping 
connecting the gas source with a point of use, such as a semiconductor 
processing tool. The gas distribution system also encompasses the 
components between the gas system and the point of use, such as 
regulators, valves, flowmeters, etc. 
Also as used herein, the term "baking strategy" refers to the controlled 
temperature versus time profile of the sampling line. 
Also, the terms "moisture" and "water vapor" herein have been used 
interchangeably. 
The gas source is preferably an ultra high purity (UHP) gas source which 
can be used in the manufacture of semiconductor devices. Thus, the gas 
distribution system must be compatible with the purity of gas introduced 
therein. Preferable gases used in connection with the inventive method 
include nitrogen (N.sub.2), oxygen (O.sub.2), argon (Ar) and helium (He). 
This list, however, is in no way limitative. 
The impurity to be measured can be, for example, water vapor, a hydrocarbon 
(e.g., CH.sub.4), a metal, NO, CO or CO.sub.2. The impurity to be measured 
is preferably water vapor. 
In order to measure impurities in a gas obtained from the gas source or gas 
distribution system, the gas source or gas distribution system is placed 
in communication with a measurement tool. Any fast response, trace level 
measuring instrument sensitive to the impurity of interest can be used. 
Suitable measurement tools are known in the art, and include, for example, 
atmospheric pressure ionization mass spectrometers (APIMS) and tunable 
diode laser absorption spectrometers (TDLAS). 
A gas sample from the gas source or gas distribution system is delivered to 
the measurement tool through a gas sampling line. The gas sampling line is 
preferably constructed from 316L electropolished stainless steel (EP SS), 
in a diameter which is selected dependent upon the specific measurement 
tool being used. Flowrate and pressure of the gas sample introduced into 
the sampling line should be selected based upon the specifications and 
requirements of the measurement tool. For example, in the case of an APIMS 
measuring moisture, the gas sample pressure is preferably in the range of 
from about 1 to 12 Bar, and the sample flowrate is preferably in the range 
of from about 1 to 20 slm. 
In order to thermally regulate the sampling line, one or more heaters for 
baking the sampling line and a controller for controlling the temperature 
of the sampling line according to a desired baking strategy are provided. 
The heater should have the capability of baking the sampling line up to a 
temperature of about 100.degree. C. with an accuracy of about 
.+-.2.degree. C. Any type of heater which can control the temperature of 
the sampling line according to the desired baking strategy can be used. 
Examples of suitable heaters include, for example, resistance-type heaters 
and heat lamps. A resistance-type heater which surrounds the sampling line 
(e.g., heating tape) is preferred. 
In measuring an impurity level in a gas source, a gas sample is taken from 
the gas source through the sampling line. When the gas source is in a gas 
cylinder or a bulk storage device, the gas sample is typically withdrawn 
directly through the gas cylinder valve or bulk storage device valve, or 
immediately downstream therefrom, for example through a "T-type" fitting. 
In the case of a gas manufacturing plant, the gas sampled can be withdrawn 
from a final product storage tank or a final product outlet line. 
In the case of a gas distribution system, sampling can be performed at 
various points along the distribution system between the gas source and 
the point of use. 
FIG. 1 illustrates the results of a simulation example in which an 
artificially low certification would be expected. The simulated drydown of 
a 50 foot, 1/4 inch (O.D.) 316L electropolished stainless steel (EP SS) 
sampling line which is baked at 100.degree. C. is shown. The source gas in 
this simulation contains 0.25 parts per billion (ppb) water vapor. For the 
conditions for which the simulation was performed, the baking is turned 
off after about 45 hours. At that time, the measured water vapor 
concentration is substantially reduced. The impurity level remains well 
below 0.1 ppb for an extended period of time, for example, greater than 
one week. Behavior of this nature has been observed both in simulation and 
in actual measurements. As a result of these drydown characteristics, 
certification of a gas source and/or a gas distribution system at a level 
lower than the actual level would be expected. 
FIG. 2 illustrates an example in which the sampling line baking is 
terminated after about 500 minutes, which prompts a substantial decrease 
in the measured moisture level. The actual source gas concentration is 
about 0.15 ppb. As can be seen from this figure, the measured level is 
well below the actual impurity value for an extended period of time. 
In FIG. 3, both experimental and simulation results are shown for three 
sampling line baking strategies. The three baking strategies include a 
continuous 28.degree. C. pulse (no baking), a continuous 100.degree. C. 
pulse and a variable 100.degree. C./28.degree. C. pulse. In the variable 
pulse baking strategy, the sampling line temperature was switched from 
100.degree. C. to 28.degree. C. after a measured water vapor concentration 
of 11 ppb was attained. For the simulations, it was assumed that the 
temperature of the sampling line was uniform (although the temperature 
varied with time). 
The water vapor concentration of the source gas entering the sampling line 
was 0.25 ppb. To obtain a proper comparison of the experimental and 
simulation data, the sampling line drydown was simulated as the response 
after contamination of the sampling line with 50 ppb water vapor, followed 
by purging with the 0.25 ppb source gas. It can be clearly seen that the 
variable pulse baking method achieved this level more rapidly than either 
the continuous 28.degree. C. pulse (no baking case) or the continuous 
100.degree. C. pulse approaches. 
Furthermore, as can be seen from FIG. 3, the simulation results track the 
actual results extremely closely. This indicates that the modeling 
approach can accurately predict the effects of a baking strategy on 
measured concentration in the sampling line. 
The baking strategy should be designed to bring the measured impurity level 
to that of the gas entering the sampling line as quickly as possible 
without an artificially low level being measured. Such a baking strategy 
is called the "optimum baking strategy." The use of computer simulation to 
predict the moisture response is useful in determining the optimum baking 
strategy, thereby preventing the measurement of artificially low values. 
The physical phenomena that is being taken advantage of in the invention is 
the variation of the impurity adsorption isotherm on the inner surface of 
the sampling line as a function of temperature. At temperatures greater 
than room temperature, the capacity of a metal surface to hold moisture 
(e.g., in units of number of molecules/cm.sup.2) is reduced compared to 
the capacity at room temperature. As a result, when baking temperature is 
reduced, for example to room temperature, the effective adsorption 
isotherm at the lower temperature becomes effective. Because of the lower 
temperature adsorption isotherm, the metal surface acts as a moisture 
getterer. Consequently, the measured moisture level in the vapor phase 
becomes decreased compared to the measured level at the higher baking 
temperature. 
According to the inventive method, after the sampling line baking is 
concluded, the impurity concentration profile of the line contains a first 
region in which the vapor phase impurity concentration is less than the 
vapor phase impurity concentration of the gas entering the sample line, 
and a second region in which the vapor phase impurity concentration is 
greater than the vapor phase impurity concentration of the gas entering 
the sampling line. 
The effectiveness of the baking strategy can be understood by a comparison 
of concentration profiles in a sampling line at various times during 
drydown of the line. Estimated impurity concentration profiles can be 
determined from numerical simulation and require the knowledge of the 
adsorption isotherms for the impurity to be measured. Methods for 
measuring the adsorption isotherm as well as methods of simulating baking 
strategies to determine corresponding concentration profiles in gas lines 
are known, and are described and validated in the literature. 
FIGS. 4-6 illustrate simulated concentration profiles along the length of a 
50 foot sampling line for various baking strategies. The gas sample flow 
rate was assumed to be 10 slm at a pressure of 7 Bar. For the simulations, 
it was assumed that the sampling line was initially equilibrated at 50 ppb 
moisture, and that the purity of the gas entering the sampling line was 
0.05 ppb. 
FIG. 4 shows the evolution of moisture concentration profiles along the 
length of the sampling line over a period of 12 hours wherein the sampling 
line is purged with the gas and is not baked. The sampling line inlet is 
located at the origin of the x-axis, and the gas outlet is located at the 
50 foot point on that axis. As the gas is purged through the sampling 
line, the vapor phase impurity concentration in the sampling line in the 
vicinity of the gas inlet first approaches the actual vapor phase 
concentration of the impurity in the gas being introduced into the 
sampling line. With an increase in purging time, that region of the 
sampling line at the same concentration as the gas entering the sample 
line (i.e., the purging wave) extends downstream from the gas inlet. After 
12 hours of purging, the purging wave has not yet reached the gas outlet. 
That is, the concentration of water vapor at the sampling line outlet 
remains elevated relative to the concentration of the water vapor in the 
gas entering the sampling line. 
FIG. 5 illustrates the effects of baking strategy on the evolution of water 
vapor concentration profiles in a sampling line. The sampling line is 
baked at 50.degree. C. for 4 hours, and then the baking is terminated. The 
evolution of the sampling line concentration profiles is qualitatively 
similar to the profiles shown in FIG. 4 (purging without baking) for the 
first 4 hours. However, when the temperature of the line is decreased upon 
termination of sampling tube baking, the concentration profile changes. 
When the baking is terminated (see, profile after baking turned off, 4 
hr), there is a first region extending from the gas inlet to a distance of 
about 34 feet along the sampling line in which the measured vapor phase 
concentration is below that of the source gas introduced into the sampling 
line (i.e., 0.05 ppb). This first region effectively acts as a moisture 
getterer. That is, there is a net adsorption of water vapor on the inner 
surface of the sampling line in the first region. 
In a second region of the sampling line, located downstream from the first 
region and extending to the gas outlet, the water vapor concentration is 
elevated with respect to the concentration of the source gas being 
introduced into the sampling line. This second region is considered to be 
in a purging state as a result of its elevated concentration relative to 
the source gas concentration. 
The first region of the line is effectively being "contaminated" with gas 
having the same purity as the gas source or gas distribution system, while 
the second region of the gas sampling line continues to be purged. Since 
the length of the second region immediately after termination of baking 
(i.e., 16 feet) is effectively shorter than the length of the entire 
sampling line (i.e., 50 feet), the dry down time is effectively decreased 
compared to the situation in which baking is not used. 
At 7 hours into the baking strategy, the measured vapor phase impurity 
concentration at the gas outlet end of the sampling line approaches that 
of the gas being introduced into the sampling line. When compared with the 
concentration profiles illustrated in FIG. 4, it is clear that an 
improvement of about 5 hours is realized over the situation in which 
baking is not used. 
FIG. 6 illustrates the effect of overbaking the sampling line. The baking 
strategy in this example includes baking the gas sampling line to 
100.degree. C. for a period of 6 hours, and then terminating the baking. 
Upon conclusion of baking, the concentration profile along the entire 
length of the sampling line decreases to less than the impurity 
concentration in the gas introduced into the sampling line. Because the 
capacity of the sampling line internal surfaces to adsorb water vapor 
(i.e., the gettering capacity) is very high along the entire length of the 
sampling line, the rate of propagation of the source gas concentration 
level (i.e., the contamination wave) through the sampling line is very 
slow. Consequently, the moisture concentration measured at the gas outlet 
of the sampling line is lower than the impurity concentration of the gas 
introduced into the sampling line for an extended period of time. If care 
is not taken, the gas source or gas distribution system can easily be 
certified at a lower than actual impurity level. 
A comparison of the impurity concentration profiles shown in FIGS. 4-6 
makes clear that in order to decrease measurement time for a gas source or 
a gas distribution system, a controlled sampling line baking strategy 
should be implemented. The baking strategy should be designed such that, 
subsequent to the termination of baking, there are two regions in the 
sampling line. In a first region, extending from the sampling tube gas 
inlet to a point downstream from the inlet, the impurity concentration is 
less than the impurity concentration of the gas introduced into the 
sampling line. In the second region, extending from a point downstream 
from the first region to the gas outlet of the sampling line, the impurity 
concentration is greater than the impurity concentration of the gas 
introduced into the sampling line. 
Furthermore, the baking strategy should be designed such that the time 
required for the first region of the sampling line to be brought to the 
impurity level of the gas introduced into the sampling line is less than 
the purging time required to lower the impurity concentration in the 
second region to that impurity level. In other words, the baking strategy 
should be designed such that the rate of propagation of the source gas 
contamination wave is sufficiently fast that it reaches the purging wave 
before the purging wave exits the gas sampling line. In order to minimize 
the impurity measurement time, a baking strategy should be selected such 
that the point at which the contamination wave reaches the normal purging 
wave is located as closely as possible to the gas outlet of the sampling 
line. 
The impurity measurement is performed until the impurity level becomes 
stabilized. That is, the true impurity level is reached when there is no 
further decrease in the measured level. To ensure that the impurity level 
is stabilized, the flowrate through the sampling line can be measured at a 
plurality of gas flow rates. For example, measurements can be made at the 
sampling flowrate, one half the sampling flowrate, and double the sampling 
flowrate. If the measured level is the same for the three measurements, 
the impurity level can be assumed to be stabilized. 
The use of a simulation program is very effective in determining the 
optimum baking strategy. A precise knowledge of the impurity concentration 
in the gas entering the sampling tube is not required. However, a liberal 
estimate should be used to determine the baking strategy. This estimate 
can be based on, for example, past experience or the impurity level for 
which the plant and/or distribution system is designed for. Alternatively, 
information from other measurements taken in the distribution system can 
be used. The specific conditions governing the sampling (e.g., flowrate, 
pressure, length of gas sampling tube, gas sampling line surface 
properties, gas sampling line diameter, baking capabilities and ambient 
temperature) can be used in the simulation program to devise an effective 
or optimum baking strategy. As demonstrated by the above examples, a time 
savings of greater than 50% compared to prior methods can be achieved by 
means of the inventive method. 
While the invention has been described in detail with reference to specific 
embodiments thereof, it will be apparent to one skilled in the art that 
various changes and modifications can be made, and equivalents employed, 
without departing from the scope of the appended claims.