［ Pulse Generator ］

Variable-Width Pulse Generator

Keywords

Eiji Sasamoto,Toshihiro Osada,
Ikuo Azuma

Pulsed power, High-voltage switching technology, SiC-MOSFET, EUV, Film-forming

Abstract

The pulse generator is equipment made to generate momentary high power.
It is being used especially for plasma-applied ﬁelds (ﬁlm-forming and environmental (air-cleaning and water-processing) ﬁelds for example) by variable-width
pulse generator. We focused on high-speed switching and low loss generation
characteristics of Silicon Carbide (SiC) – Metal-Oxide-Semiconductor FieldEffect Transistor (MOSFET) which are recently actively being developed. We established our own approaches for the parallel connection of semiconductor devices, and realized outstanding characteristics of variable pulse width
performance (pulse width: 0.1～2μs), high-frequency operation (repetitive frequency: 120kHz or below), high power (output voltage: Max. 6kV), and highspeed operation (rising and falling time: 30ns or less). This unique product is apulse generator that cannot be found in our former product lineups. As an example of its application, it is now being used in the ﬁelds of Pockels cell drivers andplasma applications. We veriﬁed the presence of pulse voltage output in negative polarity or the operation of high-frequency at 400kHz by adjusting the equipment to meet requirements.

1 Preface
The pulse generator is an equipment to generate a high pulsed power to the unit for an extremely
short amount of time of microseconds or nanoseconds. Such a pulse generator is used in the technical ﬁeld of Pulsed Power (1) and its application ispositively promoted. Formerly, the magnetic pulse
compression method (2) was widely adopted for pulse
generating circuits to deal with laser loads. This system, however, could not change the pulse width
easily. For this reason, in many cases it was difﬁcultfor the system to be applied to the plasma-appliedﬁelds (such as ﬁlm-forming and environmental “aircleaning and water-processing” ﬁelds). In order toapply this technology easily to plasma-applied ﬁelds,
we have recently developed a variable-width pulse
generator that can generate high-speed and highvoltage power with variable pulse width capability.
For the purpose of high-voltage switching, we used
Silicon Carbide (SiC) Metal-Oxide-Semiconductor
Field-Effect Transistors (MOSFETs) for this product.
Recently, the SiC MOSFETs are actively being
developed recently worldwide. By the parallel con-nection of semiconductor devices, this product realized high dielectric strength. This paper introduces
the operational principle and circuits of the variable-width pulse generator and some applications.

2 Operational Principle of theVariable-Width Pulse Generator
Fig. 1 shows an external appearance of thevariable-width pulse generator and Fig. 2 shows

Fig. 1

Variable-Width Pulse Generator

An external appearance of the variable-width pulse generator isshown.

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Variable-width pulse generator
R1

SW1

CDC
source

①

L
R2

SW2

②

Load

C: Capacitor
R1, R2: Resistors
L: Stray inductance

Circuit Configuration of the Variable-Width Pulse
Generator

Fig. 2A circuit conﬁguration of the variable-width pulse generator isshown.

ONSW1
OFFTimeON
SW2
OFFLoad voltage

TimeTiming Chart of Variable-Width Pulse Generator

The timing chart of variable-width pulse generator operation isshown.

the circuit conﬁguration. The variable-width pulse
generator comes with two main switches (SW1 andSW2). Through the ON/OFF control of each switch,
pulse-state voltages are applied to a load. The operating power is obtained from a DC power sourcethat is supplied externally. A capacitor C is inserted
where a voltage drop is anticipated due to a
response characteristic of the DC power. Resistors
R1 and R2 are inserted for the prevention of ringing
phenomena that may be caused by the inductance
L between the source and the load or by the load
itself. In the case of a resistive load, R2 and SW2
need not be installed. In the case of a capacitive
load, however, R2 and SW2 are required.
Fig. 3 shows a timing chart when a capacitive
load is installed. When the load voltage is required
to rise, SW1 is turned ON and SW2 is turned OFF.

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3 Conﬁguration of the Switching Circuit
For the pulse generator used in plasmaapplied ﬁelds, high-speed and high-voltage pulse
output characteristics must be secured in addition
to the capability of changing the pulse width. High
speed in this contexture means the shortness of rising and falling time (in several tens of nanoseconds).
High voltage means the height of a voltage peak(several to tens of kilovolts). When generating apulse output in a circuiting system as shown in
Fig. 1, the pulse output characteristics are greatly
inﬂuenced by the characteristics of the switchingcircuit itself (of SW1 and SW2). A conﬁgurationof the switching circuit is introduced below for
obtaining high-speed and high-voltage pulse outputs.

3.1 Selection of Power Devices
TimeFig. 3

At that time, the current from the DC source runs
toward the load along the route indicated by the dotted line ① in Fig. 2 so that the load voltage can rise.
When the load voltage must fall, SW1 is turned OFF
and SW2 is turned ON. In this case, a current ﬂows
along the route indicated by the dotted line ② inFig. 2 and the load voltage falls as a result.

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As a power device for generating a pulsedpower, the thyristor, Insulated Gate Bipolar Transistor(IGBT) and MOSFET (3) - (5) are generally adopted.
There is, however, no such switch that functions inseveral tens of nanoseconds and at several tens ofkilovolts coming into a single device. When one
wants to establish a high dielectric strength by connecting devices in series, it is inevitably necessary
to choose power devices that work at some tensof nanoseconds. Table 1 shows the characteristiccomparison table of the power devices. Even theSilicon (Si)-MOSFET makes it possible to perform
high-speed operation, but in this case, the ONresistance is high and the resultant loss is substantial. Although the SiC-MOSFET by each suppliercan perform high-speed operation, we adopted theSiC-MOSFET produced by Company “C” because
its speciﬁcations on a catalog are outstanding.

3.2 Conﬁguration of the Switching Circuit
BlockFig. 4 shows a conﬁguration of the switchingcircuit block. For our switching circuit developed forthis time, we established a high dielectric strengthTable 1

Characteristic Comparison Table of the PowerDevice

The table below shows comparison of characteristics of thepower device that can be applied to the variable-width pulse
generator.
Device material

Si-MOSFET

SiC-MOSFET

Table 2Major Specifications of the Pockels Cell Driverfor EUV

Major speciﬁcations of the Pockels cell driver for EUV are shown.
Output voltage

0〜6kV (variable)

Repetitive frequency

0〜120kHz (variable)

Maker nameCompany “A” Company “B” Company “C”

Load capacitance

10〜40pF

Rated current(peak current) (A)

12 (48)14 (35)

12.5 (40)Output pulse width

0.1〜2.0μs (variable)

Rising/falling time

30ns or lessRising time (ns)111911Jitter

Not more than 4ns

Falling time (ns)18.5

2910Cooling system

Water-cooled

ON resistance (Ω)

620

280

160

Mass

16kg Max.

Dimensions

W270 × H290 × D210mm or less

HV
SW10SW9SW10V

Fig. 4

Configuration of the Switching Circuit Block

A conﬁguration of the switching circuit block is shown. The block
is composed of 10 switching devices connected in series. A
pulse transformer system is adopted for the gate driving circuit.
In consideration of voltage sharing, each device is connectedwith a resistor and a capacitor.

by connecting ten devices of 1200V rating each in aseries. When creating a high dielectric strength withdevices in a series connection, a variety of device
driving systems are available. Unless all the devices
are switched simultaneously, there will be collapseof voltage sharing balance and devices may be broken. For a reasonable solution, we have adopted asystem where devices are driven by a pulse transformer. In consideration of voltage sharing on eachdevice in regular mode and transient mode, resistors and capacitors are connected in parallel toeach device.

4 Examples of Applications
4.1 Pockels Cell Driver for Extreme Ultraviolet(EUV) Light Source
As the most powerful light source for photo
lithography to be used for the next generation versions, there is an EUV equipment rated at the wave-

length of 13.5nm. R&D activities on this equipment
are promoted at Gigaphoton Inc. – an EUV lightsource equipment supplier in Japan. Our pulse generator is used as a Pockels cell driver at this company where the EUV light source is controlled. Table 2
shows the major speciﬁcations of the Pockels celldriver for EUV. This pulse generator is used in positive polarity. It can frequently perform repeated generation of high-speed and high-voltage pulse output
in pulse width variable mode. In addition, Jitter(deviation width in time-axial direction) is extremely
small. Compared with conventional equipment madeby other suppliers, power consumption in Pockels
cell driver itself has been reduced by approximately
30%.

4.2 Pulse Generator for Plasma-Applied
FieldsThe variable-width pulse generator is an equipment to generate a pulse-state voltage output
by ON/OFF control of the main switches. If theswitching technology with the use of main switches
is established, it is possible to set up the frequency,
polarity, output voltage, and other factors according
to the userʼs demands. For example, the descriptions below are related to the modiﬁcation of powersource according to the user requirements.
We once developed a pulse generator of themagnetic pulse compression circuiting system to be
used for empirical Diamond-Like Carbon (DLC)ﬁlm-forming(4). At this time, this power source has
been modiﬁed according the userʼs request sothat the variable-width pulse generator can be usedin negative polarity for DLC ﬁlm-forming experiments. Fig. 5 shows the circuit conﬁguration of thevariable-width pulse generator in negative-polarity.
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Variable-width pulse generator
R1

SW1

C
DC
sourceFig. 5①

L
R2

SW2

②

LoadCircuit Configuration of the Variable-Width Pulse
Generator in Negative-Polarity

A circuit conﬁguration of the variable-width pulse generator in
negative-polarity is shown. Compared with Fig. 1, the circuit
conﬁguration may be slightly modiﬁed to enable voltage pulse
output generation in negative polarity.

2.05 Postscript

Output voltage (kV)

Pulse width: 100ns0－2.0

Voltage: Approx.
－5.0kV

－4.0
－6.0
－8.0

Fig. 6

Time: 50ns/div

Example of Output Voltage Waveform for the
Variable-Width Pulse Generator in NegativePolarity

An example of an output voltage waveform for the variable-width
pulse generator in negative-polarity is shown. The output voltage is approximately −5.0kV and the pulse width is 100ns.

Output voltage (kV)

3.01.0－1.0Fig. 7・All product and company names mentioned in this paper are
the trademarks and/or service marks of their respective owners.

( 1 ) Hara and Akiyama: High-Voltage Pulsed Power Engineering,Morikita Publishing Co., Ltd., 1991 (in Japanese)( 2 ) Sasamoto: “Pulsed Power Applied Technologies,” Meiden Jiho
Vol.323, 2009/No.2 (in Japanese)( 3 ) Ishii: “Future Prospect of Semiconductor Pulse Generator,”

5.0

Voltage: Approx. 6.0kV

This paper introduced the operational principleof our variable-width pulse generator and someexamples of its applications. The pulse generator isused in a variety of industrial ﬁelds such as air puriﬁcation and water processing in addition to applications introduced in this paper. Its application range
is considered to expand further in the future. Goingforward, we will work on improving our technologies
so that we can expand our product line-ups andincrease the ﬁelds of applications.

《References》

Period: 2.5μs
7.0

Pulse width: 1.0μs

Time: 1μs/div

Example of Output Voltage Waveform Observed
during Operation at 400kHz

An example of output voltage waveform is shown, observed
when the variable-width pulse generator is operated at 400kHz.
The output voltage is approximately 6.0kV.

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Compared with the positive-polarity circuit in Fig. 1,a DC power source in negative polarity can be provided separately making it possible to make a minimal wiring change inside the unit. Fig. 6 shows anexample of an observed output voltage waveform.
We conﬁrmed that a pulsed voltage output in negative polarity can be generated.
There is demand for more highly repetitive
operation in regard to the variable-width pulse generator. For this time, we adjusted parts and circuitsof the control circuit and conﬁrmed that operation
at a high frequency of 400kHz is possible. Fig. 7
shows an example of output voltage waveform
observed.

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Academic Journal on Plasma and Nuclear Fusion Engineering,
Vol.81, 2005/No.5, p.375 (in Japanese)( 4 ) Sakugawa, Namihira, Katsuki, Akiyama, Nagata, and Koganezawa:“Application of Semiconductor Pulse Generator to Plasma-AppliedEquipment,” Academic Journal on Plasma and Nuclear Fusion
Engineering, Vol.81, 2005/No.5, pp.350 〜354 (in Japanese)( 5 ) Koh and Takagi: “Roles of Power Semiconductor Devices,”
Academic Journal on Plasma and Nuclear Fusion Engineering,
Vol.87, 2011/No.2, pp.106〜111 (in Japanese)( 6 ) Sasamoto: “Pulsed Generator for DLC Film-Forming,” Meiden
Review Vol.161, 2014/No.2