Method of and device for measuring water content

This invention consists in a device consisting of a vessel (1) containing chemical solution soluble in water, a sample room or sample itself (13), and electric conduction meter (3); and a method in which the chemical solution is brought into contact with the sample so as to absorb water adsorbed to the surface of the sample and then electric conductivity of the chemical solution is measured. By means of such a structure as above, water content having been adsorbed by the sample can be measured with high precision in a short, period of time.

TECHNICAL FIELD 
The present invention relates to a method and device for measuring water 
content. For example, the present invention is preferably employed in the 
determination of the water content adsorbed to the surface of various 
materials, such as the determination of the water content adsorbed to a 
thin film surface such as a silicon film or a silicon oxide film or the 
like formed on the inner surface of a highly clean electropolished pipe, 
the determination of the water content adsorbed to a metal surface, or the 
like; furthermore, the present invention relates to a method and device 
for measuring water content which is capable of determining the water 
content adsorbed to various types of wafer surfaces in production 
processes of semiconductors and water content adsorbed to inner surfaces 
of semiconductor manufacturing apparatuses. 
BACKGROUND ART 
In order to achieve a shift to ultra LSI and to attain higher performance, 
an atmosphere having greater and greater cleanliness is required for the 
formation and processing of the elements, and technology for the 
production of ultrahigh vacuums, ultraclean low pressure atmospheres, 
ultrahigh purity gas atmospheres, and supply systems has become more 
important. 
Such atmospheres are contaminated by leaks from the outside of the 
apparatus or the gas piping system, or by desorption of impurities 
adsorbed to the inner surface thereof. Among these impurities, water 
molecules adsorbed by the inner surface of the apparatus or the gas piping 
system, in particular, desorb during manufacturing processes such as, for 
example, thin film formation or processing, and the contamination of the 
atmosphere as a result of the desorption of these adsorbed molecules 
creates a problem in, that it tends to cause a worsening of the 
characteristics of the elements or of the precision of the processing. 
Accordingly, it is necessary to construct such semiconductor manufacturing 
apparatuses using material having a small adsorbed water content, and from 
which the adsorbed water desorbs easily and within a short period of time; 
for this reason, surface treatment, such as planarization treatment, 
post-oxidation passivation treatment, fluoridation passivation treatment, 
and the like, is carried out on the surface of the structural material. 
In order to produce a highly clean atmosphere containing no moisture, it is 
necessary to develop materials having little adsorbed water and to develop 
materials from which adsorbed water desorbs quickly, and methods of 
evaluation by which the amount of water adsorbed by a wafer or the like 
can be accurately measured after various types of manufacturing processes. 
Conventionally, in the case in which amounts of water adsorbed by piping or 
the like were measured, methods were employed in which a gas of high 
purity (for example, Ar gas having a water content of less than 50 ppt) 
was caused to flow through piping as a carrier gas while subjecting the 
piping to baking, and the desorbing water content was analyzed by means of 
an atmospheric pressure ionization mass spectrometer (APIMS) or by the 
Karl Fisher method. 
However, in the method in which an APIMS was employed, a number of hours 
were required for the water molecules adsorbed by the surface of the 
sample to completely desorb and for the water content concentration in the 
carrier gas to return to its original value, so that high speed 
measurement was not possible, and furthermore, there was an upper limit to 
the measurement of high moisture concentrations. Furthermore, in the Karl 
Fisher method, it was unclear whether the water molecules contained in the 
carrier gas were completely absorbed into the solvent, and furthermore, 
there was a lower limit to the measurement of low moisture concentrations, 
so that a method having a high degree of reliability was not available. In 
this situation, there was a strong demand for a method for measuring 
adsorbed water content which had high reliability and which was capable of 
rapid measurement. 
The above discussion centered on the field of semiconductor manufacturing 
technology; however, this is not limited to the semiconductor field, but 
rather, in the manufacture of magnetic discs, laser discs, and micro 
devices such as liquid crystals and EL flat plate displays, and the like, 
as well, in order to attain high performance manufacturing processes, a 
method for the measurement of water content adsorbed by solid surfaces is 
very important, as remaining adsorbed water content presents the greatest 
obstacle. 
The present invention has as an object thereof to provide a measurement 
method and a measuring device for measuring water content, which is 
capable of measuring water content adsorbed by various samples, with high 
precision and in a short period of time. 
DISCLOSURE OF THE INVENTION 
A first feature of the present invention resides in a measurement method 
for water content, characterized in that a chemical solution possessing 
solubility in water, i.e. a hydrophilic solution, is brought into contact 
with a sample, and thereby the water content adsorbed by the surface of 
the sample is absorbed into the chemical solution, and subsequently, the 
electric conductivity of the chemical solution is measured. 
A second feature resides in a device for measuring water content, 
characterized in that a storage vessel for storing a chemical solution 
possessing solubility in water, a sample room or a sample itself, and an 
electric conduction meter are provided, and furthermore, a mechanism for 
supplying chemical solution from the storage vessel to the sample room or 
the sample itself while controlling the flow rate of the chemical solution 
within the storage vessel, and a mechanism for sending chemical solution 
from the sample room or the sample itself to the electric conduction 
meter, are provided. 
FUNCTION 
When a chemical solution having a large mutual interaction with water, such 
as anhydrous hydrogen fluoride, is brought into contact with a solid 
sample, the water molecules adsorbed by the solid surface are quickly 
absorbed into the chemical solution. The water molecules absorbed into the 
chemical solution dissociate into ions in the chemical solution, and-as a 
result, the electric conductivity varies in accordance with the amount of 
water content. Accordingly, by measuring the electric conductivity of the 
chemical solution, it is possible to measure the water content adsorbed by 
the solid surface. 
Furthermore, by using a chemical solution having a large mutual interaction 
with water, such as anhydrous hydrogen fluoride, the water is easily 
dissolved in the chemical solution irrespective of the state of the 
surface adsorbing the water, so that it is possible to conduct the 
measurement of the adsorbed water content in a short period of time. 
EMBODIMENT EXAMPLES 
A structural example of the present invention is shown in FIG. 1; using 
this FIGURE, the embodiment examples of the present invention will be 
explained. 
In the FIGURE, reference 1 indicates a chemical solution storage vessel, 
reference 2 indicates a chemical solution flow rate control mechanism, 
reference 3 indicates a leak-tight electric conduction meter, and 
reference 12 indicates a reference pipe for maintaining the interior 
portion of the sensor cell of the electric conduction meter in an 
ultraclean state. Reference 13 indicates a sample pipe which is produced 
with a variety of interior surfaces and which adsorbs water. 
Valves 4 and 5 are opened, a highly pure inert gas (for example, N.sub.2, 
Ar, or the like ) is introduced into chemical solution storage vessel 1, 
pressure is applied, and chemical solution is supplied from vessel 1 to 
the piping side. The flow rate of the chemical solution flowing through 
the piping is controlled at a fixed flow rate by the flow rate control 
mechanism 2. First, valves 8 and 9 are opened, and valves 10 and 11 are 
closed, and thereby, the chemical solution is sent via reference pipe 12 
to electric conductivity meter 3, and here, the electric conductivity of 
the chemical solution is measured. When the electric conductivity reaches 
a constant value valves 8 and 9 are closed, and valves 10 and 11 are 
opened, and chemical solution is introduced into sample pipe 13 and the 
adsorbed water content of the sample pipe is dissolved, and the chemical 
solution is then sent to electric conduction meter 3. 
The relationship between the elapsed time from the introduction of chemical 
solution into sample pipe 13 and the electric conductivity of the chemical 
solution is shown in FIG. 2 as a graph. 
In other words, when the chemical solution which has dissolved the water 
content adsorbed by the surface of the sample pipe reaches conduction 
meter 3, the measured value of the electric conduction meter rises, and 
after this, declines until it reaches the electric conductivity of the 
original chemical solution. In the case in which anhydrous hydrogen 
fluoride is used as the chemical solution, the water content dissolved in 
the anhydrous hydrogen fluoride dissociates completely in the anhydrous 
hydrogen fluoride, so that the relationship between the electric 
conductivity as measured by the electric conduction meter and the water 
content is linear, as shown in FIG. 3. FIG. 3 shows the values when the 
temperature of the hydrogen fluoride is 0.degree. C. Integrating the peak 
of FIG. 2, it is possible to obtain the number of adsorbed water molecules 
per unit surface adsorbed by the pipe from the relationship between the 
electric conductivity and water content of FIG. 3. The chemical solution 
possessing solubility with respect to water which is used in the present 
invention has high mutual interaction with water and mixes with water in 
any proportion; for example, anhydrous hydrogen fluoride is preferably 
used. It is possible to use industrial anhydrous hydrogen fluoride of 
approximately 6N as this anhydrous hydrogen fluoride; however, anhydrous 
hydrogen fluoride having a purity of 9N or more which is obtainable by the 
repeated refining, such as distillation or the like, of industrial 
anhydrous hydrogen fluoride, which has a water content of less than 40 
ppb, and an electric conductivity of 1.0.times.10.sup.-6 S/cm or less, is 
preferable for use in the measurement of very small amounts of water. By 
using anhydrous hydrogen fluoride of this purity, measurement of water 
content over a wide range from low concentrations to high concentrations 
becomes possible. 
It is preferable that a stainless steel mass flow controller for liquids 
which is capable of the precise control of flow rates be used as the 
mechanism 2 for controlling the flow rate of the chemical solution. 
Furthermore, it is preferable that electric conduction meter 3 be capable 
of measuring electric conductivity within a range of 10.sup.-7 -10.sup.-2 
S/cm, and in particular, a sealed-type inline-type electric conductivity 
meter which is capable of attachment to the piping is preferable. 
The mechanism for supplying anhydrous hydrogen fluoride to the sample 
comprises a piping system connecting the anhydrous hydrogen fluoride 
storage vessel and the sample, and furthermore, the mechanism for sending 
anhydrous hydrogen fluoride from the sample to the electric conduction 
meter comprises a piping system connecting the sample and the electric 
conduction meter. These piping systems may employ various metals or 
alloys, since anhydrous hydrogen fluoride does not corrode metal. It is 
also possible to use Teflon or plastic; however, it is preferable that 
stainless steel be used which adsorbs little impurity gas, has high heat 
resistance, and an inner surface of which has been subjected to 
passivation treatment after being subjected to electrolytic polishing. 
In the case in which anhydrous hydrogen fluoride is used as the chemical 
solution, because the boiling point of the anhydrous hydrogen fluoride is 
19.5.degree. C., it is desirable that the piping system, through which the 
hydrogen fluoride solution normally flows as a liquid, the flow rate 
control mechanism, and the electric conduction meter sensor cell be 
maintained at a temperature of 19.5.degree. C. or less, and it is further 
desirable that these elements be maintained at a temperature of, for 
example, 0.degree. C. Furthermore, the hydrogen fluoride which is 
discharged from the electric conduction meter 3 is recycled to a sealed 
vessel which is cooled to a temperature within a range of, for example, 
-10.degree. C..about.-30.degree. C. 
In the above, a measurement method for water content adsorbed by piping was 
discussed; however, by providing a sample room in place of the sample pipe 
of FIG. 1, and placing a sample within this room, it is possible to 
measure water content adsorbed not merely by piping, but by samples in 
various forms; for example, in the form of a powder or the like.

Explanation of the References 
1 chemical solution storage vessel, 2 flow rate control mechanism, 3 
electric conduction meter, 4, 5, 6, 7, 8, 9, 10, 11 valves, 12 reference 
line, 13 sample line. 
BEST MODE FOR THE EXECUTION OF THE INVENTION 
Hereinbelow, the present invention will be explained based on embodiments. 
Embodiment 1 
Ar gas having the water content concentrations shown in Table 1 was 
introduced at a temperature of 25.degree. C. into stainless steel pipes 
having a diameter of 1/4 inches and a length of 4m, which had been 
subjected to various surface treatments, and after an equilibrium 
adsorption state was reached, the water content adsorbed by the inner 
surface of the stainless steel pipe was measured. In these measurements, 
anhydrous hydrogen fluoride having an electric conductivity of 18 micro 
S/cm was used as the chemical solution. An example of measurement results 
obtained with respect to the change over time in electric conductivity of 
the anhydrous hydrogen fluoride is shown in FIG. 4, and furthermore, the 
adsorbed water contents obtained with respect to various pipes are shown 
in Table 1. 
TABLE 1 
______________________________________ 
Water Content 
Concentration 
Adsorbed Water Content 
in the Ar Gas 
(molecule/cm.sup.2) 
(ppb) BA EP OP 
______________________________________ 
140 9 .times. 10.sup.13 
7 .times. 10.sup.13 
20 .times. 10.sup.13 
1000 15 .times. 10.sup.13 
15 .times. 10.sup.13 
36 .times. 10.sup.13 
______________________________________ 
BA: pipe subjected to brightening annealing, EP: pipe subjected to 
electrolytic polishing, OP: pipe subjected to oxide passivation 
processing. 
The above results Showed good agreement with results measured by means of 
the APIMS method. 
Embodiment 2 
Sample stainless steel tubes which had been subjected to electrolytic 
polishing treatment were produced having formed, on the inner surface 
thereof, a Si film, a SiO.sub.2 film, a film (Si--H) formed by the 
termination of dangling bonds by means of hydrogen after Si film 
formation, and a film (Si--F) formed by the termination of dangling bonds 
by means of fluorine after Si film formation, and Ar gas having a water 
content of 100 ppb was introduced into the sample pipes at a temperature 
of 25.degree. C., and after the achievement of and equilibrium adsorption 
state, the water content adsorbed by the inner surface was measured in a 
manner identical to that of embodiment 1. An example of the measurement 
results obtained with respect to the change over time in the electric 
conductivity of the anhydrous hydrogen fluoride is shown in FIG. 5, and 
furthermore, the adsorbed water content obtained with respect to various 
pipes is shown in Table 2. 
TABLE 2 
______________________________________ 
Type of Membrane Formed on 
Adsorbed Water Content 
the Inner Surface of the Pipe 
(molecules/cm.sup.2) 
______________________________________ 
Si 3 .times. 10.sup.13 
SiO.sub.2 20 .times. 10.sup.13 
Si--H 1.5 .times. 10.sup.13 
Si--F 32 .times. 10.sup.13 
______________________________________ 
The results of Table 2 showed good agreement with the measurement results 
obtained by means of the APIMS method. 
The measurement of the adsorbed water content in the above embodiments was 
completed in less than 15 minutes per sample, so that; the measurement 
period, which conventionally required a number of hours, was greatly 
reduced. 
INDUSTRIAL APPLICABILITY 
By means of the present invention, it is possible to provide a method and a 
device for measuring water content which is capable of measuring water 
content adsorbed by various samples with a high degree of precision and in 
a short period of time.