ht. J. Hydrogen Enqy Vol. 21, No. 5, pp. 335 -i41, I996 Copyright @ 1996 International Association for Hydrogen Energy

Published bv Elsevier Science Ltd Pergamon

Printed in Great Britain. All rights reserved 036th 3199196 %lS.nO Y 0.00

ULTRA HIGH PURITY HYDROGEN GAS SUPPLY SYSTEM WITH LIQUID HYDROGEN

JUN MIYAZAKI,? TETSUO KAJIYAMA,? KAZUTO MATSUMOTO,? HIROSHI FUJIWARAt and MASARU YATABEt

lwatani International Corporation, 1-3 D-15 Nakase, Mihama-ku. Chiba. Japan tIwatani Industrial Gases Corporation, 10 Ohtakasu-cho. Amagasaki, Hyogo. Japan

(Received for publication 30 August 1995)

Abstract-The semiconductor industry has a requirement for ultra high purity gases to process wafers. Ultra high purity hydrogen is one of the gases used by this industry. Currently, the ultra high purity hydrogen used for this industry comes from one of two sources; a cryogenic adsorption system or a palladium membrance system.

The purity of cryogenic liquid hydrogen is extremely high. The extensive use of pre-liquefaction purifier technologies is essential for the safe and continued operation of a liquid hydrogen plant. The impurities would solidify in the process flow and result in a system shutdown.

This paper will describe the experimental results from a pilot plant system which maintains the high purity of the liquid hydrogen product for delivery to a semiconductor manufacturing facility. A discussion of an atmospheric pressure ionization mass spectrometer used to verify the purity of the gaseous hydrogen will highlight the capabilities of this technology and the dilution system used to calibrate this analyzer. Copyright @ 1996 International Association for Hydrogen Energy.

1. INTRODUCTION

In Japan, the bulk of the liquid hydrogen used is in the transportation restriction on delivery of liquid product. These factors combine to cause a 40-50% premium on

national aero-space development program. The high cost liquid vs gaseous hydrogen. to produce liquid hydrogen is hampering its wider use in industry. The cost to produce liquid hydrogen is driven

The purity requirements for the semiconductor indus- try may result in a much lower total cost &f&$&al

by the high power rate (ca $0.115 per kw) and the between liquid and gaseous hydrogen. The elimination

Tank Truck

Sampling

mer

Strage Tank

Fig. 1. Flow diagram of the pilot plant.

335

336 J. MIYAZAKI et al.

10' Pa 10 Pa lo+ Pa

Sample Gas - _I

1. .3 L/mi n

TMP

1 I

TMP:Turbo Molecular Pump

Fig. 2. Schematic diagram of API-MS.

of point-of-use purifier technology would reduce capital expenses and operation costs associated with the main- tenance of these systems.

The principal objective of this research was to establish a pilot plant supply system for liquid hydrogen and the analytical technologies required for the measurement of impurities in the gaseous supply.

In an Ultra high purity gases referred to herein, the total impurities measured will be 100 parts-per-billion (wb).

+I System

Capacity Max. working pressure Materials Inner tank

Outer tank Insulation method Evaporation loss

6700 1 7.0 kg/cm’ G

Stainless steel 316L (non electro-polished) Carbon steel Layer vacuum insulation 60 l/day

(2) Vaporizer (2 units alternate system):

2. EXPERIMENTAL PROCEDURE AND FACILITIES Inside volume 0.0038 m3

Capacity 20 Nm3/h In a simulation of consumption at a user’s site, we

built up a liquid hydrogen storage tank and vaporizers k;i;;ar 10 kg/cm* G

Stainless steel (electro-polished) comprising a pilot plant inside the premises of our existing liquid hydrogen plant. Liquid hydrogen was supplied to the storage tank by a self-pressurizing tank truck without use of a pump most probable to introduce contamination. A flow sheet of the pilot plant is as shown in Fig. 1.

(3) Piping:

The pilot plant was equipped with measuring instru- Material ments (thermometers, pressure gauges and liquid level

Stainless steel 316L (electro-polished)

indicators) to collect data and a sampling line was provided for analyzing.

Specifications of facilities for experiment

(1) Liquid hydrogen storage tank:

3. ANALYSIS TECHNOLOGY

Gas chromatography (GC) and/or gas chromato- graphic mass spectrometer (GC-MS) have conventionally

ULTRA HIGH PURITY HYDROGEN GAS SUPPLY 337

Pure HZ 3

ST0 GAS 3

Ippm N2iH2

I Convection Oven 0 1 ppb ~1/10000' \

---+ To API-MS

L MFC

I -

M F C :Mass Flow Controller

FM T :Mass Flow Meter

Fig. 3. High accurate dilution system.

been used to analyze hydrogen gas. However, the best detectable range with these analyzers can cover only the ppm or sub-ppm level of impurities which is unable to satisfy any requirements for analysis of ultra high purity hydrogen gas. Today, trace analysis technology is indis- pensable to analyze a marginal amount of impurities. The atmospheric pressure ionization mass spectrometer (API- MS) is now highly evaluated as an analyzer capable to meet such requirements [l].

We carried out this research with API-MS aiming to meet the requirement for trace analysis of impurities in hydrogen gas at a ppb or a sub-ppb level.

Principle qf API-MS

For higher detection sensitivity of a mass spectrometer it is necessary to increase the ionic volume of the impurities brought into the detector. Therefore it is desirable to have as many chances of reactions as possible to generate ion. Ionization under such a high pressure as the atmospheric pressure is required instead of ioniz- ation by electronic impact under vacuum.

API-MS has two types, one using radioactive isotope and the other using corona discharge for ionization.

In the case of radioactive isotope (ex.63Ni) [2], you can have a stable discharge but the absolute volume of ion generated is small. In corona discharge on the other hand, the volume of ion generated is two digits larger by dynamic range. In general, therefore, the corona discharge type of analyzer is used as an ion source in the field aiming for analysis of an ultra high purity gas. In the ion source of API-MS, each molecule causes collisions with frequency of about lo6 to 10’ times, which ionizes a lot of impurities and leads to high sensitivity.

Hydrogen gas containing a marginal amount of im- purities is brought into the ion source, and hydrogen molecule is ionized by electric energy from corona discharge (H, -+ Hl : primary ionizing reaction). H : produced here collides with hydrogen molecules to form H: (Hl H, -+ Hi + H). The H: produced collides with impurity molecules and reacts with electric charge to form impurity ion. (Proton transfer reaction: secondary ioniz- ing reaction). This reaction takes place under tbe atmos- pheric pressure in such a high probability that it lot of impurities are obtained.

Examples of ionizing processes:

H;+NZ+NZH++HL

H;+CH,+CH,H++H,

H;+COZ+COZH++H1

H;+H,O-,H,OH++HZ

Figure 2 shows the structure of API-MS. The impurity ion obtained is introduced into the

differential pumping zone through the counter electrode aperture and then into the analyzing zone. The differential pumping zone is provided to connect the ionizing zone under the atmospheric pressure to the analyzing zone under the high vacuum condition. In the analyzing zone it is mass-separated in the quadrupole mass filter and the impurity ion is detected as it is amplified as an electric current with the secondary electron multiplier. (Impuri- ties in hydrogen are detected by the mass volume at mass-charge ratio = m/e + 1.)

It should be noted, however, that some impurities (Ar, O,, He) in hydrogen have less affinity with hydrogen than H, has, and cannot be ionized for detection.

338 J. MIYAZAKI et al.

1 Calibration curve for nitrogen impurity in hydrogen gas 1

0.04 -

0.03 -

0.02 -

0.01 - A.'

l ’ .‘.

0.00 J 1 I I I 0.0 0.5 1.0 1.5 2.0 2.5

Concentration (ppb)

Fig. 4. Calibration curve for nitrogen impurity in hydrogen gas.

Absolute ion intensity and relative ion intensity

In analysis of ion with a mass spectrometer for a long time or in case of inflow of extremely impure gas, there may occur some change in the ion amount once in a while due to deterioration of SEM, stains on lenses or apertures.

By means of employment of the relative ion intensity with each impurity divided by total ion amount, therefore, you can eliminate an influence to ion amount in lapse of time.

2

1

0

-1

-2

-3

Calibration curve for API-MS

In order to make out a calibration curve for API-MS [3,4], a standard gas containing impurities at a ppb level is necessary, but is not commercially available. In general, therefore, they purchase a standard gas at a ppm level in the market and dilute it with their dilution unit to obtain such a standard gas as containing impurities at a ppb level.

The dilution unit usually consists of mass flow con- trollers, mass flow meters and valves. Efficiency of the system is determined depending upon the accuracy of the mass flow controller and the mass flow meter, and it exerts great influences on the calibration curve.

With the background as mentioned above, we have developed the dilution system which has the following features (see Fig. 3).

(1) The mass flow controllers, the mass flow meters and the valves are put into the oven at a specified constant temperature (80°C) to eliminate an influence by temperature.

(2) The mass flow controllers and the mass flow meters are designed to attain high detection sensitivity.

(3) Elaborate adjustments have been made especially for linearity of the mass flow controller and the mass flow meter in use of soap-film flow meter.

This dilution system described above has extremely high accuracy with tolerance within the range of f0.3% FS. and is fully computerized to enable stable dilution at a multiplying ratio of l/100 N l/10,000. A calibration curve

H*O.H+ 0.8 ppb -

I I I

1

I I C0.H'

! ! N2.H’ 0.1 ppb

I I I 1 I I

I ! ! . I ! I I I I -! u--- “1”“““‘I”” ~.......l.....~l.........l~“““.‘l...’.”” “J Cl 5 10 15 20 25 30 35 40 4!!i 50

MASS NUMBER (H/Z)

Fig. 5. Analysis of impurity in liquid hydrogen.

ULTRA HIGH PURITY HYDROGEN GAS SUPPLY

1 Contamination of impurities from cryogenic valves J

0.1 1 I J 0 60 120 180 240

Time (min. )

/--- + H20

.+ N2

Fig. 6. Contamination of impurities from cryogenic valves

made in use of this system shows satisfactory linearity as far as the sphere of sub-ppb [S]. (Figure 4 shows a calibration curve of N, in hydrogen).

4. SUBJECTS FOR STUDY

The pilot plant was operated on the assumption of an actual use condition at a user, and the following items were checked and studied for a stable supply system of high purity hydrogen gas. Samples were taken at the assumed point of use in the downstream of the pilot plant.

Influences to purity by the parts comprising the system

To design the ultra high purity gas supply system, much care must be taken to adopt components suitable for dead zone free, outboard leak free and high purity requirements. Fittings in use of welded and metal gaskets were employed to prevent outboard leak. As for the dead zone, much care was taken to minimize its space [6]. Taking careful note of the safety valves and the cryogenic valves of main components, we checked their influences to purity.

Purity of liquid hydrogen at point of use

The amount of liquid hydrogen used is not necessarily constant. Therefore, influences to purity were checked at various hydrogen flow rates.

Impurity concentration and liquid volume remaining in tank

This system must secure a stable purity maintained for supply, and it was studied whether the volume of liquid

hydrogen in the storage tank may exert any influences to the purity of hydrogen gas to be supplied.

5. RESULTS

The analysis of liquid hydrogen resulted in the detec- tion of only N, and Hz0 as irnpuritias, Other impurities proved to be below the detection limit (Fig 5).

(la) When sampling was impIeme&ed at the down- steam side equipped with safety valves, there occurred a change in the concentration of impurities. It was con- firmed that this phenomenon disappeared as the main valve for a safety valve was closed. It proved that the contaminants diffused in reversibly through the safety valve caused the change. Leak (5 x 10F6 atm. ems/s) was found at the seat area in the spring (metal seat) type safety valve. Generally, people are apt to think that there cannot be any impurities through the safety v&e which is Io~Ied all the time with the tank pressure, but even such a shght leak proved to become a cause of reverse diffugion of contaminants. A rupture disk was added before the safety valve to prevent leaks. The rupture disk shoaukd have such specifications as to allow the least possible leak-out (1 x 10-10 atm cm3/s is available in the market). As for the fittings it is important to pick out those in use of metal gaskets. After all these practices we made a test in the same manner as befme, and found t.hut no corm&- nation took place and hydrogen gas could be supphed with ultra high purity.

(1 b) During operation of the system there was a stable detection of impurity concentration, but when operafion was terminated there occurred increases of impurities (Fig. 6). There appeared three spikes detecting increases

340 J. MIYAZAKI et al.

Flow rate and impurity concentration

0 60 120 180 240 300 Time (min. )

Fig. 7. Flow rate and impurity concentration.

in the amount of impurities, and it was presumed that they could have been caused by the three cryogenic valves mounted on the liquid supply line, as depicted in Fig. 1, i.e. valves 1, 2, 3 and 3’. Those impurities were supposed to have accumulated adhering to the plastic parts in the dead space with the cryogenic valves and to have flown out after being gasified by heat introduced when oper- ation was terminated. To solve this problem, the dead space of the cryogenic valves was minimized, and the area apt to deposit impurities was restructured so that gas can be driven out easily. Improvements were made also in reducing a large portion of the plastic parts. As a result, there no longer appeared spikes in impurity detection when the operation was terminated at the point of use.

(2) When the storage tank was filled up, the analysis value at liquid phase was N, = 1 ppb, but when the liquid volume in tank came to nearly zero level with use of only boil-off gas, the analysis value showed N, = 2 ppb. Hydrogen gas boiled off in the tank is in a condition of being distilled and has turned out to be very pure, while at the liquid phase side the impurities are concentrated raising the value of concentration.

(3) Impurity concentration varied when the amount of use was varied (in a range between several I/mm and 20 m3/h). In case of supply from the liquid phase, when the flow rate is small, there also occurs boil-off on the surface of liquid in supply piping and turns out to be sort of destilled giving fairly high purity in tank like the gaseous phase. In case of a substantial volume, not only liquid hydrogen itself but also impurities are gasified, which gives higher concentration of impurities than the one in

case of a small volume (Fig. 7). After the improvements of the system based on the outcome of the examination, the authors have come to recognize the feasibility of a stable supply of high purity hydrogen gas (Fig. 8).

6. CONCLUSION

This experiment has proved that analysis of sub-ppb impurities in hydrogen is feasible in use of API-MS and a dilution unit, it follows that we have been able to make a great deal of progress in the research of ultra high purity gas. It has also been found with the above-mentioned analysis technologies that the purity of liquid hydrogen can adequately meet the specifications required by the semiconductor industry.

We have accomplished our development of a system to supply hydrogen to a point of use maintaining the purity of liquid hydrogen without cryogenic adsorption systems or palladium membrane systems.

Electra-polishing on the inner surface of the storage tank is going to be one of the standard specifications for the ultra high purity gas supply system in the Japanese semiconductor industry. In this research, however, it has been confirmed that ultra high purity hydrogen can be supplied even if the system does not employ electro- polishing for surface treatment.

In search for cost effective specifications of components like pipes and valves for future development, it seems to be necessary to carry out more research on feasibility of applying bright annealing instead of electro-polishing or oxidation passive state treatments.

ULTRA HIGH PURITY HYDROGEN GAS SUPPLY 341

Impurity level in high pwity liquid hydrogen system -1

0.01 ' ' 0 60 120 180

Time (min. )

Fig. 8. Impurity level in high purity liqbid hydrogen system.

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pp. 467486 ( 1992).

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