［ Vacuum Equipment ］

Large-Current Variable Vacuum
Capacitor ( VVC)
Keywords

Variable vacuum capacitor, Large current, Low torque, Long life

Abstract

The Variable Vacuum Capacitor (VVC) is used for the impedance matchingin Radio-Frequency (RF) matching network, indispensable for thin-ﬁlm vacuum
coating processes where RF plasma is used. Electronic devices, mobile devices(smartphones or tablet PCs), semiconductor elements, and ﬂat panel displays,
for example, are essential to modern society. For manufacturing processes ofsuch items, they need to go through thin-ﬁlm vacuum coating processes. For
these processes, RF matching network (mainly consisting of load and tune capacitors as well as an inductive coil) are being used. Along with increase in silicon wafer size and expansion of Flat Panel Displays (FPDs), the RF matching
network is required to generate higher output power. The VVCs to be used insuch network is required to carry more current and withstand more voltage. We
are working on the development and commercialization of VVC products to meetthe higher needs of our customers (semiconductor manufacturing equipmentsuppliers)

1 PrefaceFig. 1 shows a construction of the VP Series.
The Variable Vacuum Capacitor (VVC) is composedof an insulation envelope to insulate the gap
between electrodes, a pair of opposed electrodes toScrew actuator shaft

Bellows (for vacuum sealing)

Bellows (for current carrying)

Ceramic envelope
ElectrodeFig. 1

Toshinori Tatsumi,
Daizo Takahashi,
Shoya Murayama

Construction of the VP Series

Construction of double bellows is shown: one for hermeticallysealed bellows to retain vacuum and the other one to carrycurrent.

create static capacitance, hermetically sealed bellows used to set up electrode allocations in the vacuum chamber, and a shaft for static capacitanceregulation by which the opposing area of electrodes
can be changed. The VP Series of the large-current
VVC employs our unique double bellows. Apartfrom the bellows intended for vacuum hermetic
sealing, another large-sized bellows with low resistance is allocated to carry line current. Until now, wehave been developing large-current type VVCs.
Until 2016, we had developed six product series
including a product series with a permissible currentof 400Arms (at 13.56MHz air-cooled.) This paper
introduces our programs to produce higher currentVVCs.

2 Background of Demand for LargeCurrent VVC
In the recent Flat Panel Displays (FPDs) market, the FPD panel suppliers in China, Korea, andTaiwan are making a huge investment for the nextgeneration glass substrates for displays.
In 2016, there were large investments into theFPD manufacturing plant for 10th – generation glass
substrates for displays (panel size: 2880 × 3130mm.)
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In this connection, there were sizable demands forlarge over-40kW Radio-Frequency (RF) power supplies and matching units. There is a similar trend for
VVCs. The market calls for larger currents and higher withstand voltage. In 2016, we released the large
current VVCs – VP200 Series carrying large currents of 400Arms.
The FPD manufacturing equipment suppliers
and high frequency power sources suppliers aredeveloping for the 11th generation glass substrates
for displays (panel size: 3000 × 3320mm.) We areworking on the development of our VVCs and areproposing our products to meet the higher current
VVC needs of our customers (above-mentionedsuppliers.) Through such efforts, we would like toincrease our market share of VVCs for such applications.

3 Our Programs for Higher Current
Capacity VVCs
Fig. 2 shows Frequency – Max. Allowable
Current Characteristics. The three domains of themaximum allowable current for the Meiden VVCs
are as itemized below.
( 1) The limit domain due to RF allowable voltage:I ＝ 2πfCVrms (C: Static capacitance)log( 2 ) The limit domain due to maximum allowabletemperature( 3 ) The limit domain due to skin effect: I ＝ IRF (FRF /f )1/4
Regarding the development of our largecurrent VVC, our major challenge lies in the current-carrying bellows. Within the limit domains of( 2 ) and ( 3 ) above, the maximum allowable current
can be increased further by suppressing the bellows loss and heat generation. The amount of heatgeneration while a heavy current is carried in theVVC is obtainable from the expression below.
Ploss ＝ (ESRf ＋ Rc) ( IRF)2Given IRF Arms is a current carried in the VVCand Rc is a current carrying conductor connection
resistance, heat generation in the VVC while current
is carried is expressed mainly by the resistance loss
Ploss W in the Equivalent Series Resistor (ESR).
Since the surface temperature on the VVC main
body while a maximum current is carried is speciﬁed at 125℃ (Δ100℃ when room temperature is25℃), the bellows is required to assure low heatgeneration and low loss when the attainment oflarge current is plotted.
In the case of VVC, resistance due to the effectof “skin effect” on the surface of the bellows is themajor factor of ESR, and this ESR is dependent onfrequency. Our VVCs are mainly used in the RFmatching network and the typical frequency is13.56MHz. Fig. 3 shows an example of ESR – frequency characteristics. Compared with the UW

1000

VM & UWSeries
100
(1)

(2)

(3)

ESR (mΩ)Max. allowable current (Arms)

VP Series

UW Series

10Frequency (MHz)Fig. 2log

The positive line to the central top of the maximum allowablecurrent for the VP Series shows the domain where the current islimited by the maximum RF allowable voltage. The negative linefrom the central top belongs to the domain where the current islimited by the productʼs permissible temperature. For the UWSeries, the range limited by permissible temperature is wide.

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VP Series

Frequency – Max. Allowable Current
Characteristics

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1Fig. 31

10100
Frequency (MHz)

1000

Example of ESR – Frequency Characteristics

Compared with the ESR characteristics of the UW Series, thoseof the VP Series are almost half, i.e. 3〜4mΩ (at 13.56MHz).

Series, measured values of ESR are extremely low
for the VP Series where a double bellows construction is adopted. Such a feature results from theenlarged surface area thanks to the larger diameterof current-carrying bellows and the reduction of
conductor resistance by the copper materials
that are excellent in electrical characteristics.
Nevertheless, the level of requirements for the mostadvanced equipment in the market is much higher.
We have been working on the R&D on new technologies such as low-loss bellows. Performance characteristics requested to the bellows are not only low
loss but other factors as well. These are, material
shape and strength that can withstand repeatedexpansion and contraction, and the acquisition of alow spring constant that does not affect the rotating
torque performance of shaft for the static capacitance adjustment. Development of bellows is an
important factor for the competitive advantage ofour products against other competitors.

4 Postscript
The VVC Series using our unique double bellows offers the external diameter range: from 65mm
to 200mm. The maximum allowable current ranges
from 130 to 400Arms. The rotating torque of theshaft for static capacitance regulation is 0.18N・mthat is common to 65mm to 110mm in diameter. Fordiameters 150mm and 200mm, 0.6N・m is applicable. These ﬁgures are the smallest values in thisindustry. Water-cooled ﬂanges and ball screws areavailable as options.
Going forward, we expect there will be ademand for large-current VVC models with a maximum allowable current of 500Arms or above in thefuture. We will continue to develop new VVCs withlarger allowable currents. In doing so, we would liketo meet the higher expectation of our customers(the FPD manufacturing equipment suppliers andhigh frequency power sources suppliers.)
・All product and company names mentioned in this paper are
the trademarks and/or service marks of their respective owners.

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