Polymers in Sensors - American Chemical Society

Chapter 16

Downloaded by KTH ROYAL INST OF TECHNOLOGY on August 25, 2015 | http://pubs.acs.org Publication Date: April 17, 1998 | doi: 10.1021/bk-1998-0690.ch016

Trace-Moisture Sensor Based on the Electrolytic Technique 1

Naim Akmal and Yining Zhang Teledyne Electronic Technologies-Analytical Instruments, 16830 Chestnut Street, City of Industry,CA91749

This paper describes the development of an electrolytic moisture sensor that can quickly detect trace levels of moisture in different gases such as N , Ar, He, H , O , and compressed air. This new moisture sensor uses a novel nanocompsite film as an electrolyte which is cast on an integrated array of electrodes made up of different metallic films as cathode and anode electrodes. A DC potential of 5 to 20 volts was applied across the electrodes. During the sensing process, moisture diffuses through the solid nanocomposite film and is electrolyzed into hydrogen and oxygen on the electrode surface. The produced current was measured and was used to determine the concentration of moisture in the sample gas. The response time for low ppb levels of moisture was found to be less than 30 min and the detection limit was 0.5 ppb . The sensor was found to be linear in the range of low ppb to 100 ppm levels of moisture. Also, the sensor was modified to work in hydrogen background avoiding any recombination phenomenon. 2

2

2

v

v

v

v

The measurement of moisture in gases is critical in chemical process industries, because of the awareness and control of water content in the feed streams. Reaction mixtures and product streams often determine the efficiency of a manufacturing process, quality of the product, and overall success of the process. The production of semiconductors and integrated circuits requires that the amount of moisture in the gases utilized during the fabrication process be quickly detected and measured to achieve better yields and reduce production costs. Presence of moisture in the semiconductor industry can cause particle formation and may result in electrical 1

Current Address: Union Carbide Technical Center, 3200 Kanawha Turnpike, South Charleston, WV 25303.

174

©1998 American Chemical Society In Polymers in Sensors; Akmal, N., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1998.

175 property defect, that could adversely affect devices yield. In the polymer processing industry detection of moisture content in the monomer is very important to keep the reactors running at their maximum efficiency and control the desired molecular weight of the polymer . In corrosive and reactive gases, moisture contamination can severely damage gas lines, valves, pressure regulators, and mass-flow controllers. The moisture content and its measurement depends on the type of and nature of the process involved. Moisture levels below 100 ppb in inert gases are now commonplace, and several state-of-the-art facilities are operating below 10 ppb . Systems operating below 1 ppb have been reported (1). Measurement of moisture below 1 ppb level requires careful handling of the delivery system and an instrument that has a high signal to noise ratio. Several techniques are available for detecting moisture in process gases (2-4). Such as, chilled mirror hygrometers, quartz crystal oscillators, capacitive sensors, and the highly accurate and expensive atmospheric ionization mass spectroscopy (APIMS) techniques. The application of the technique varies depending upon the process. Chilled Mirror Hygrometers. Chilled mirror hygrometers measure the temperature at which moisture changes its phase on the surface of a cooled mirror (from gas to liquid or from gas to solid). At the frost point, ice on the mirror of the hygrometer is at equilibrium with water vapor in the gas. The detection is performed by scattering of light by the frost film. The light is generated by a L E D source, and its intensity is measured by a detector. A change in the moisture content of the gas has the potential to change the thickness of the film that would in turn change the lightscattering intensity. The temperature of the mirror is varied to maintain a constant frost film and light-scattering intensity. The response time of such a device is very slow since it requires the moisture in the gas and the solid phase to be at equilibrium. In addition, this system is very bulky and involves a high maintenance cost. In order to observe a 10 ppb moisture in a process gas, the mirror is required to be chilled down to -104°C. In addition it can take up to 12-16 h to attain equilibrium at this low levels ( 0 + H + 2e" 2

2

+

2 H + 2e' —> H

2

2 H 0 —> 2 H + 0 2

2

2

(anode)

(2)

(cathode)

(3)

(4)

2

In general, moisture diffuses through the moisture-sensitive layer to reach the electrode, and hence the determination of the concentration of moisture in the sample gas is dependent upon the partial pressure of water and not the flow rate. In commercially available electrolytic hygrometers the electrolysis of water produces oxygen and hydrogen. These 0 and H products spontaneously recombine with the sample gas, i f it is hydrogen, oxygen or a compressed air forming additional water molecules, due to the catalytic effect of the sensor electrodes. This recombination phenomenon results in a higher moisture reading than the actual moisture levels in the sample gas. The distortion of the moisture level due to this recombination effect becomes significant when the moisture level is H (from sample gas) -> H 0 (produced by recombination reaction) (6) This additional amount of moisture produces a false reading of moisture level in the sample gas. Response in Nitrogen, Argon and Helium Gases. Figure 4 shows the response of the sensor for 250 ppb of moisture in a nitrogen gas. The moisture was generated from a moisture generation system and heat traced lines. The 90% response of the sensor for 250 ppb of moisture is less than 30 min. It takes approximately 2 h to attain equilibrium. This is due to the complete breakthrough of the moisture in the sample system. The recovery time has been found to be a little slower than the response time. This may be due to the surface condition of the sample system. Figures 5 and 6 show the response of 250 ppb of moisture in argon and helium gases respectively. The 90% response time in helium for 250 ppb of moisture is relatively slow. This is due to the difference in the molecular size of the sample gas and moisture. Response in Hydrogen Gas. In the past, some success has been obtained in eliminating the recombination phenomenon by using two gold electrodes. To test this theory we used a pair of gold electrodes on an alumina substrate. Initial evaluation was successful, but after a few hours of usage the output dropped and finally there was no substantial change in the output of the sensor by varying the concentration of moisture in the sample gas. Migration of metal ions from one electrode to the other causes the output to continuously increase, until it reaches a point where the cell becomes electrically shorted. During this process the metal particles go into an oxidation process, i.e., they seems to be dissolved. This phenomenon is represented in Eqn. 7. 2

2

2

v


v

v

v

+

M

> M n + ne" (at anode)

Mn

+

Mn

+

migmlion

+ H

2

+

> M n (to cathode) > M (at cathode) + H+

(7) (8) (9)

The ions migrate within the solid state electrolyte by the applied electrical field (Eqn. 8), and are then reduced by the hydrogen produced during the electrolysis process, as shown in Eqn. 9. This step occurs extremely fast in a hydrogen background. Finally, this metallic deposition (migration) fills the gap between the anode and cathode causing the electrode array to be electrically shorted. The migration effect depends upon the electrochemical oxidation potential of the anode materials and the value of the potential applied across the electrodes. The value of the applied potential across the electrodes cannot be reduced below a certain value, in order to keep a continuous dissociation of water. Different electrode materials have different catalytic effects on the recombination phenomena and have different migration effects upon the application of an applied potential.

In Polymers in Sensors; Akmal, N., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1998.

rsVI^i


Ciisln

Moisture Scrubber

V2V

Moistlire Gcneilator

V4

V5

-X-

Moisture Bypass

•

-CX

MFC Moisture SenSor

V6

Sample Vent

Figure 3. Flow schematic of the sensor system.

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100

---

/

80 ο Q. Μ

—

60

ω

-- . . . — —

40

—

—

-

—

—

—

20

-- 0

2

4

6

—

.....

8 10- 12 Time (Hours)

14

16

18

20

Figure 4. Response of the sensor for 250 ppb of moisture in nitrogen gas. v


182

120


100 $ 80 c ο S" 60

—

CD

a: 35

40

\

20 0

0

2

4

6

8

10

12

14

16

Time (Hours)

Figure 5. Response of the sensor for 250 ppb of moisture in argon gas. v

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^

40

CD

/ J

80

20 00

\ \

V

J 2

4

6

8

10 12 14 16 18 20

Time (Hours)

Figure 6. Response of the sensor for 250 ppb of moisture in helium gas. v


183 It has been found that a pair of platinum electrodes show the highest recombination effect in a hydrogen and oxygen background, but no migration effect was noted; whereas gold electrodes show the least recombination effect and maximum migration effect. Rhodium electrodes show very little recombination effect and virtually no migration effect. Therefore, it is reasonable to select gold as a cathode and rhodium as an anode for an electrolytic sensor to detect moisture in hydrogen and oxygen gases. Figure 7 shows a response of 250 ppb of moisture in hydrogen gas. It is obvious that no recombination of moisture is taking place during the measurement. The recombination phenomenon has been eliminated by making one of the electrodes of rhodium and the other of gold. Linearity. The electrolytic sensor has been found to be linear in all gases from ppb range to ppm,, ranges of moisture. Figure 8 and Figure 9 show the linearity of the moisture sensor in hydrogen, and helium gases respectively. Moisture for the linearity test was generated using a permeation device and maintaining it at a constant temperature and a flow rate. The sensor has been found to be linear for most of the ranges except around 400 ppb levels of moisture . The slight non-linear behavior at this level may be due to the physical constraints of the sample system or due to the position of the sensor inside the sensor block. A few changes in the position of the sensor inside the block have produced better results. Comparison with Available Techniques. Table I provides a list of various moisture detection systems in terms of cost and performance, and their comparison with the new polymer based electrolytic sensor. Most of the techniques cannot detect moisture below 10 ppb level and their response and recovery times for low levels of moisture are in hours. The polymer film based electrolytic sensor can successfully detect moisture in single digit ppb levels with a response time of a few minutes.


v

v

v

v

v

0

2

4

6

8

10

12

14

16

Time (Hours)

Figure 7. Response of the sensor for 250 ppb of moisture in hydrogen gas. v


In Polymers in Sensors; Akmal, N., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1998. v

v

Figure 8. Linearity test of the sensor in nitrogen gas for 20 ppb to 560 ppb of moisture.


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185 600.00

3

500.00

400.00

ET ΕΣ:

EE g> 300.00


32 200.00

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-•offset

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rnndiiuj

0.00 0.00

100.00

200.00

300.00

400.00

500.00

600.00

true H20, ppb

Figure 9. Linearity test of the sensor in hydrogen gas for 20 ppb to 560 ppb of moisture. v

v

Table I Performance and Cost Comparison of Various Moisture Detection Systems Charact eristic

Chilled Mirror

Quartz Crystal

Capacitive

Electrolytic

APIMS

Modified Electrolytic

Response 2-3 h for Time ppb levels of moisture

80% of value in 10 m in at 10ppb

2-3 h at ppb levels

60-90 min at ppb levels

Immediate

80% of value at 5 min at 10 PPb

Recover y Time

10-15 min, when exposed to double digit ppb levels

2-3 h at ppb level

Single-digit PPb

Double digit ppb

Double digit PPb

Sub-ppb

Sub-ppb

100ppb percent levels

ppb to low ppm levels

ppb level

Single digit ppb to 100 ppm levels

v

v

v

v

3-4 h when exposed to double digit moisture

v

v

90-120 min

Immediate

at ppb levels v

v

v

Limit of Detectio η

Single-digit ppb level

Workin g Range

0-1000 ppb levels

v

v

v

v

1-1000 ppb levels

v

v

v

v

v

v

10ppb

20 ppb

10ppb

Approxi mate Cost

$70,000

$39,000

$5,000

$23,000

v

v

v

1 ppb

v

v

v

Accurac y

v

Less than 15 min when exposed to double digit ppb levels

v

lOpptv

0.5ppb

$220,000

$28,000

to $500,000


v

186 Response to 0.50 ppb of Moisture in Nitrogen Gas. Figure 10 shows the response of the sensor to a 0.5 ppb moisture challenge. The peak-to-peak noise is less than 80 ppty, and the signal-to-noise ratio is approximately 9:1. This indicates that the sensitivity of the sensor is in the double digit ppt^ region. The slow response time is due to the slow breakthrough of the moisture in the sample system. The system and the sample line were completely dried before introducing the 0.5 ppb of moisture. Long Term Stability. The output of the sensor registered a drop of approximately twenty percent during the first few weeks and therefore required frequent calibrations. After four weeks of continuous operation the output was stabilized, and the calibration was required only once a week. The initial twenty percent drift was due to the loss of water from the membrane and the substrate and is considered to be normal. The continued calibration that is required on a less frequent basis (weekly) is due to the conditioning of the membrane and the loss of conductivity. The effective life time of the sensor has been found to be roughly around 18-24 months. After this time period, the membrane may develop cracks leading to a loss in conductivity and eventually its moisture sensitivity. A n added advantage of this sensor is that its exposure to high levels of humidity does not affect its performance, provided it is conditioned for a few days before being put into use for low levels of moisture (ppb ) detection. v

v


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0.8 zero gas in

500 pl>t in

•

0.7 0.6 0.5 0.4

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Time (HR.-MIN) Figure 10. Response of the sensor to 500 ppt,, of moisture in nitrogen gas.


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187


Response in Carbon Dioxide and Corrosive Gases. A n attempt was made to detect moisture in carbon dioxide gas. Initially the output was stable, but after a few hours of continuous operation it started to drop and finally no response was obtained. A careful observation of the sensor revealed that it had lost its conductivity in the carbon dioxide background. A n attempt was also made to detect moisture in the corrosive gases such as chlorine, hydrochloric acid, etc. The sensor failed to respond. This is due to the nature of the corrosive gases which tend to destroy the polymer film, that is responsible for scavenging moisture from the sample gas.

Conclusion In this paper a new generation of moisture sensors and their capabilities have been discussed. The new electrolytic based sensor, for the rapid detection of moisture in inert gases, will open the door to learn more about the process and requirements to control the moisture content, specially in the semi-conductor industry. The sensor has been successfully demonstrated to perform in hydrogen and oxygen gases. A high signal to noise ratio allows us to successfully see a response to ppty levels of moisture in inert gases. Additional work is in progress to improve the response time of the sensor and its ability to detect moisture in corrosive gases. Effects of film thickness on the performance of the sensor are also being studied.

References 1. 2. 3. 4. 5. 6.

Yabumoto, N. et al. Ultra-Clean Technology 1990, 1 (1), 13. McAndrew, J. J.; Boucheron D. Solid State Technology 1992, 35, 55. Bhadha, P. M. Welding Journal 1994, May, 57. Carroll, D. I.; Dzidic I.; Horning, E. C. Applied Spectroscopy Reviews 1981 17(3), 337. Hasegawa,S.Int'lSypm. on Moisture and Humidity, ISA 1985, Washington DC., 15. Stallard, B. R.; Espinoza, L. H.; Niemczyk T. M. Proceedings of the Institute of Environmental Sciences,41 Annual Technical Meeting 1995, 41, 1. Wei, J.; Pillion, J., E.; King, S. M.; and Verlinden, M. Micro 1987, 15 (2), 31. Keidel, F. A.Anal.Chem. 1959, 31, 2043. Goldsmith, P.; Cox, L. C. Sci. Instrum. 1967, 44, 29. st

7. 8. 9.


Polymers in Sensors - American Chemical Society

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