Surface cleanness of substrate transported byXHV integrated process

Masahiro Tosaa,*, Kyung Sub Leeb, Young Sung Kima,b,Akira Kasaharaa, Kazuhiro Yoshiharaa

aNational Research Institute for Metals, 1 Sengen, Tsukuba 305-0047, JapanbInstitute of Science and Technology of Sungkyunkwan University, Suwon 440-746, South Korea

Received 3 August 1999; accepted 19 November 1999

Abstract

The extreme high vacuum integrated process has been developed in order to transfer substrates among vacuum chambers

without any contamination on the ultra clean surface. The integrated process has ®ve main line chambers, six sidetrack

chambers, connector chambers and six vacuum instrument chambers for surface analyses and ®lm preparation. Magnetic

levitation transports are installed into the line chambers because they have no sliding part to generate dust particles as well as

outgassing which may much damage the ultra clean substrate surfaces and environment. The levitation transports can transfer

a substrate among connected six chambers in the pressure change of less than 10ÿ10 Pa. Auger analysis shows that surface of a

Cu coated steel substrate prepared in the ®lm preparation chamber can be kept clean without oxygen nor carbonate during the

transportation from the ®lm preparation chamber to the auger analysis chamber. # 2001 Elsevier Science B.V. All rights

reserved.

Keywords: Extreme high vacuum; Integrated process; Levitation transport; Electromagnetic; Superconducting

1. Introduction

Extreme high vacuum less than 10ÿ10 Pa (XHV)

which contains little gas molecules and atoms can

cause almost no surface contamination by the adsorp-

tion. The excellent laboratory to study and develop

advanced materials on an atomic scale is expected to

establish in XHVenvironment because it can offer and

maintain ideal ultra clean environment for long time to

arti®cially synthesize advanced materials with manip-

ulation of atoms. The study and development of

materials on an atomic scale requires such many

operations as sample cleaning, deposition, etching,

surface analysis, performance test and so on as well as

XHV environment. It is impossible to carry out all

operation in the same vacuum chamber because the

chamber becomes so large due to the installation of all

operation components and instruments into one cham-

ber. It also takes long time to acquire XHV environ-

ment once the chamber is exposed to atmosphere due

to the change of instruments for maintenance or the

version up of the components inside the instruments.

The XHV integrated process, therefore, consists of

lots of continuous operations in XHV jointed by

transfer system because it is more practical and ef®-

cient to carry out each operation allotted among

connected chambers.

Applied Surface Science 169±170 (2001) 689±694

* Corresponding author. Tel.: �81-298-59-5073;

fax: �81-298-59-5010.

E-mail address: tosa@nrim.go.jp (M. Tosa).

0169-4332/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved.

PII: S 0 1 6 9 - 4 3 3 2 ( 0 0 ) 0 0 8 1 4 - X

We have successfully developed the XHV integrated

process with two types of magnetic levitation trans-

ports using no sliding mechanism and could transfer

sample in the pressure change of less than 10ÿ10 Pa

[1,2]. The problem of the ®rst developed XHV inte-

grated process was the limitation of the number of

connecting chambers. The process cannot share more

than three connected chambers and requires large

space for the installation of all large transfer system

into the vacuum chamber for main track line.

The purpose of this work as a second step is to

downsize the XHV process to increase the number of

the connected chambers. We therefore tried to sim-

plify the electromagnetic levitation transport for main

track line and also tried to introduce the direct sample

delivery between the carrier of sidetrack line and the

sample stage of connected chambers.

2. Extreme high vacuum integrated process

The developed XHV integrated process with mag-

netic levitation transports is shown in Fig. 1 schema-

tically and Fig. 2 shows the whole photograph of the

process. The process consists of four main track

vacuum chambers connected in series, ®ve sidetrack

chambers standing in a row, ®ve coupling chambers,

connected six instruments chambers, vacuum pumps,

pressure gauges, and valves. The connected instru-

ments in the process are three ®lm preparation cham-

bers of a molecular beam epitaxy deposition system

and two sputter deposition systems and three surface

analysis chambers an auger electron spectroscopy, an

atomic force microscope and an X-ray photoelectron

spectroscopy. Generation of XHV environment

requires a very low outgassing chamber, high sensi-

tivity gauge system, high performance vacuum pumps

and so on. The surface of the inside wall of the

chambers was electrolytically polished in phospho-

ric±sulfuric acid solution and the chambers were

annealed at 823 K in a high vacuum for enough out-

gassing. Each track chamber is evacuated by an ion

pump (pump speed: 0.2 m3 sÿ1) with a titanium getter

pump (pump speed: 1.8 m3 sÿ1) after the whole sys-

tem was baked out with mantle heater system at the

temperature of more than 423 K keeping the tandem

turbo pumping system in operation. Change in the

vacuum pressure during transportation of a substrate

was measured with an extractor gauge. Surface clean-

ness of the substrate transported by the process is

evaluated with surface analysis by auger electron

spectroscopy (AES).

3. Levitation transport system

Transport system to be used in XHV should gen-

erate no particle because particles are sources of

Fig. 1. Schematic diagram of the developed XHV integrated process.

690 M. Tosa et al. / Applied Surface Science 169±170 (2001) 689±694

Fig. 2. Whole photograph of the XHV integrated process.

Fig. 3. Photo and schematic of a superconducting levitation transport used for the sidetrack.

M. Tosa et al. / Applied Surface Science 169±170 (2001) 689±694 691

outgassing as well as contamination. It is necessary to

use no sliding component to keep the process XHV. A

magnetic levitation transport system can meet the

demand, as it employs no sliding motion so that

XHV may be kept during transport. We therefore

installed two types of magnetic levitation transports

in XHV system, one is an electromagnetic levitation

transport and the other is a superconducting levitation

transport.

The electromagnetic levitation transport is intro-

duced to each main track vacuum chamber for the

advantages of stable long-distance transfer and quick

start operation. The superconducting magnetic levita-

tion transport is introduced to each sidetrack vacuum

chamber for the advantages of short-distance transfer

and tough stability against mechanical shock. The

hoist up and down motion system by air cylinder

mechanism is introduced to each coupling chamber

for the sample delivery among carriers of main track

and sidetrack.

Fig. 3 shows the photos and schematic of a super-

conducting levitation transport used for the sidetrack

because of the space saving and tough stability against

mechanical shock. The transport consists of a side-

track chamber, a cooler ®lled with helium gas coolant

cooled by a freezer at the back of the chamber and a

carrier rod with a sample holder at the head. Three

discs of high-Tc YBa2Cu3O7ÿX superconductor driven

by a rotating long bolt shaft in the cooler can be cooled

down below Tc for half a day and cause the effect of

Fig. 4. Photo and schematic of an electromagnetic transport used for the main track.

692 M. Tosa et al. / Applied Surface Science 169±170 (2001) 689±694

pinning and the diamagnetism on the three discs of

samarium cobalt attached to the bottom of the carrier.

The effect is strong enough to levitate the carrier with

a certain gap and position through the cooler wall and

to drag the carrier accurately without any stabilizer.

The carrier can transport a sample at the top speed of

3 cm sÿ1.

Fig. 4 shows the photos and schematic of an elec-

tromagnetic transport used for the main track because

of easy extension and quick startup without any

warming up. A stator on the track chamber has electro-

magnets to levitate a carrier in the chamber, a linear

synchronous motor above the stator to drive and

position sensors as well as gap sensors with electro-

magnets to stabilize levitating carrier. The electro-

magnets in the stator control the levitation gap of the

carrier about 1 mm between the carrier and chamber

wall. The running carrier can stop within the error of

0.5 mm after transporting a sample holder at the top

speed of 5 cm sÿ1.

A sample transfer direction is changed by the hoist

up and down system in the order. The sample is also

delivered from the carrier of the sidetrack to the carrier

of the main track in the same way. The sample delivery

between the carrier of the sidetrack and the sample

stage in the connected chamber is carried out with

wobble stick operation.

4. Sample transport

Operation of substrate transport from the sidetrack

for AES to the sidetrack for AFM by way of the main

track was carried out and the change of pressure less

than 2:0� 10ÿ10 Pa was observed during the levita-

tion transports of sidetrack and main track. The hoist

up and down motion caused the large pressure

increase of about 5� 10ÿ9 Pa. Less outgassing opera-

tion of the bellows wall of the hoist unit is required

because mechanical vibration may still release gas

from the wall surface. Fig. 5 shows AES spectra of a

copper-coated steel substrate after vacuum levitation

transport and AES spectra of a copper-coated sub-

strate after atmospheric transport. The copper was

coated in a chamber with radio frequency magnetron

sputter system and transported into AES chamber by

levitation transport in the XHV integrated process.

AES measurement shows that surface cleanness is

maintained only by levitation transport. A small

amount of oxygen peak observed may result from

the hoist up and down motion. Though the outgassing

from the hoist motion should be decreased by improv-

ing the system, this result indicates that the current

system is successful in the transport in an ultra high

vacuum.

5. Conclusions

We have downsized the XHV integrated process

with two types of magnetic levitation transports by

direct sample delivery to a connected chamber from a

sidetrack and could connect six vacuum instruments

with the process. The pressure change less than

2:0� 10ÿ10 Pa was obtained when a sample was

transported from one sidetrack to another sidetrack

by way of a main track. We could successfully transfer

a substrate maintaining surface cleanness by the XHV

integrated process.

Fig. 5. AES spectra of a copper-coated steel substrate after

vacuum levitation transport and AES spectra of a copper-coated

substrate after atmospheric transport.

M. Tosa et al. / Applied Surface Science 169±170 (2001) 689±694 693

Acknowledgements

We deeply appreciate the large cooperation of H.

Minami, M. Takahashi, M. Nishituji, J. Yuyama and

H. Yamakawa, staff of ULVAC Ltd., for the develop-

ment of the XHV integrated process.

References

[1] M. Tosa, A. Itakura, K. Yoshihara, Vacuum 44 (1993) 549.

[2] M. Tosa, A. Itakura, M. Harada, A. Kasahara, K. Yoshihara,

Vacuum 47 (1996) 493.

694 M. Tosa et al. / Applied Surface Science 169±170 (2001) 689±694