九州大学大学院総合理工学府報告第 42 巻 第 2 号 12-17 頁 令和 3 年 2 月

Engineering Sciences Reports, Kyushu University
Vol. 42, No. 2, pp. 12-17, FEB, 2021

Studies on Sn Thin Film Decomposition Using VHF
hydrogen plasmas
Mengran JI*1,† Ryo NAGATA*1 and Kiichiro UCHINO*2
E-mail of corresponding author: ji.mengran.266@s.kyushu-u.ac.jp†(Received January 7, 2021, accepted January 14, 2021)
One of the serious problems of EUV lithography is the accumulation of tin debris on the surfaceof the condenser mirror, which reduces the reflectance. This study investigated the possibility ofusing VHF hydrogen plasmas to decompose attached Sn debris. First, a simple VHF hydrogenplasma was generated, and the efficiency and characteristics of Sn removal with respect to hydrogengas pressure, gas flow rate, and sample temperature were investigated. Regarding the gas pressure
dependence of Sn removal, a Sn etching rate of 4.8 nm/min was obtained at 35 Pa. This was morethan twice as large as the value previously reported using a RF hydrogen plasma. Furthermore, as aresult of combining the hollow cathode discharge, which has an electron confinement effect, with the
VHF discharge, the electron density was more than doubled. For this case, the maximum Sn etchingrate of 8.3 nm/min was obtained.
Key words: EUV lithography, Sn debris removal, VHF hydrogen plasma, Hollow-cathode discharge

1. Introduction
Semiconductor devices are becoming more
and more highly integrated. Therefore, a 7 nm
node semiconductor process using EUV(Extreme Ultra-Violet) lithography has been
started.1) The EUV lithography light source
uses the resonance line (wavelength 13.5 nm)of tin polyvalent ions from high-temperatureand high-density tin plasmas obtained byirradiating tin droplets with a CO2 laser. At
that time, tin debris is generated from the tin
droplets and adheres to the EUV condenser
mirror, which causes a problem of lowering thereflectance of the mirror. Although 250 W EUV
lightsourceshavealreadybeencommercialized,2,3) it is necessary to further
increase the output in the future, and the
problem of debris removal will become evenmore important.
As a countermeasure, it has been proposed
to react tin debris with hydrogen atoms togenerate volatile gas SnH4 and remove it fromthe mirror surface.4) In an actual EUV lightsource device, the chamber is filled withhydrogen gas of about 10 Pa, and hydrogenmolecules are dissociated and ionized by EUV
Department of Applied Science for Electronics andMaterials, Graduate student*2 Department of Advanced Energy Science andEngineering*1light (photon energy 92 eV). Finally, hydrogenatoms are reacted with tin atoms to generate
SnH4, and SnH4 is exhausted together withhydrogen gas.5,6) As a study of the tin removal
process, Ugur et al. investigated thedecomposition of tin through a chemical
reaction using a hydrogen atom flux generated
by dissociating hydrogen molecules with a hot
filament.7,8) As a result, it was reported that
one tin atom was removed for every 100,000
hydrogen atoms.
As another method, it was considered togenerate a hydrogen plasma and efficiently
remove tin by RIE (Reactive Ion Etching)using the energy of hydrogen ions. In thatmethod, Elg et al. reported that SnH4
generation was promoted by breaking the bond
between metal tin atoms with the energy ofhydrogen ions.9,10) In that study, RF (radio
frequency: 13.56 MHz) power was supplied tothe powered electrode to irradiate the tin thinfilm on the electrode surface with hydrogenions. Hydrogen ions were accelerated throughthe cathode sheath by the negative bias
voltage (-300 V) appearing on the electrodesurface. As a result, hydrogen ions obtainedenergy of approximately 300 eV. Elg et al. haveachieved a tin etching rate of 1.7 nm/min.
However, Elg et al. concluded that tin etching
was independent of gas flow rate and surface
temperature, and readhesion was negligible.

令和 2 年度

九州大学大学院総合理工学報告

Furthermore, the use of ions of several
hundred eV may cause damage due tosputtering on the mirror surface.
The authors have already investigated thedependence of tin removal on hydrogen ion
energy.11) Bias voltages from -50 V to +7.5 V
were applied to a Si substrate on which a thinfilm of tin is deposited (hereinafter, Sn sample)
exposed to hydrogen plasma to change thehydrogen ion energy. As a result, it has beenclarified that the number of tin atoms removed
per hydrogen ion is maximized with hydrogenion energy of approximately 10 eV.
As for the hydrogen plasma generation
method, a higher density plasma can beobtained by using a VHF (very high frequency,
30-300 MHz) power supply. In this study, thepossibility of using VHF plasma for tin
removal was investigated. By placing a Sn
sample on the ground electrode, the voltage ofthe ion sheath becomes about 10 V, and it isexpected that RIE will proceed with maximum
efficiency as described above. In this study, we
first clarified the basic tin removal
characteristics by VHF plasma using threevariable parameters: hydrogen gas pressure,hydrogen gas flow rate, and Sn sample
temperature. Furthermore, we investigatedthe possibility of realizing efficient tin removalusing RIE by combining VHF discharge withthe hollow cathode effect to further increase
the electron density.12, 13)

2. Experimental Setup
A schematic diagram of the experimental
equipment is shown in Fig. 1 (a) and 1 (b). In
Fig. 1 (a), VHF plasma was generated in theentire chamber, and the tin removal
characteristics of the basic VHF plasma were
investigated in this arrangement. A 60 MHz
VHF power supply was connected to the top
electrode to power the device. The innerdiameter of the quartz glass tube in the
discharge space was 62 mm, and a Sn sample
for tin removal experiments was installed at
the bottom of the chamber (earth electrode)and fixed with a stainless-steel holding plate(thickness 0.1 mm) and screws. The Sn sample
is a 100 nm thick tin film deposited on thesurface of a silicon substrate with an area of 15mm × 15 mm and a thickness of 0.625 mm. The
tin film has the same potential as the groundelectrode through the holding plate. The port
for introducing hydrogen gas and the port forexhausting hydrogen gas were arranged in astraight line, most of the hydrogen gas flowed

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13

(a)

(b)
Fig. 1 Schematic of experiment system.

above the sample and the generated SnH4
molecules were carried away by this flow. The
vacuum vessel was evacuated to a pressure of
7 × 10-2 Pa using a turbo molecular pump.
Constant temperature water was flowed inside
the ground electrode to control thetemperature. The Sn sample was brought into
close contact with the ground electrode, andthe temperature of the Sn sample was also
controlled. The power supply was used in the
output range of 20 W to 30 W.
As shown in Fig. 1 (b), the vacuum vessel
was divided into an upper space and a lower
space by a partition metal plate which was at a
ground voltage, and a structure having ahollow cathode effect was created in the centerof this partition metal plate. The side wall of
the upper space was a quartz glass tube, andthe height of the space was 30 mm. The VHF
plasma was generated in this upper space. The
lower space had an inner diameter of 56 mm
and a depth of 16 mm and was surrounded bystainless-steel surfaces and a partition metal
plate. That is, the entire surrounding surface
was at the ground voltage. A metal cylinder
with an inner diameter of 16 mm and a height

14

Studies on Sn Thin Film Decomposition Using VHF hydrogen plasmasof 19 mm was attached to the center of thepartition metal plate. The Sn sample wasplaced on the bottom surface just below thecylinder. In addition, 20 or more holes with adiameter of 3 mm were made in the partition
metal plate around this cylinder so thathydrogen gas introduced from the side surfaceof the lower space could be supplied to theupper space as well. The hydrogen plasma
generated in the upper space was ejected into
the lower space through the cylinder in thecenter of the partition metal plate. The insideof this cylinder caused the hollow cathodeeffect, and the Sn sample was irradiated withhydrogen plasma having a higher electron
density than that of VHF plasma alone. Hollow
cathode discharge did not occur in the holes
with a diameter of 3 mm.
XRF (X-Ray Fluorescence) and SEM(Scanning Electron Microscope) were used forthe evaluation of tin removal. The device used
for XRF analysis was Shimadzu EDX800. The
actual film thicknesses of the Sn samples were
measured by the crystal oscillator method.
They were used for calibration during XRF
analysis. Before and after the experiment, theamount of tin in the Sn sample was detected asa SnLα signal and converted to film thickness.
At the center of the sample, changes in thestate of the tin film surface were observed by
SEM.

(a)

(b)

3. Results and Discussion
Figure 2 shows sample-surface observation
images by SEM. In these images, themagnification is 5000 times. Figure 2 (a) is animage of the surface of the sample not exposed
to plasma. Gray tin is evenly distributed withalmost no gaps. Figure 2 (b) is an image when
the plasma irradiation time is 10 min. Abouthalf of the tin film (thickness 100 nm) has been
removed, and the film appears to be uniformly
etched. Figure 2 (c) is an image with a plasma
irradiation time of 20 min. The part that looks
black is the silicon base. Tin remains likeislands. In this case, it is considered that therelationship between the amount of Sn
removed and the time deviates from thelinearity. The etching rate should bedetermined before the time when half of thetin is removed.
Figure 3 shows the pressure dependence oftin removal. The hydrogen gas pressure waschanged to 15, 35, 75 and 150 Pa. The power
was fixed at 20 W, the hydrogen gas flow rate
was 20 sccm, and the ground electrode(c)Fig. 2 SEM images of Sn samples for different plasma
exposure conditions. (a) no plasma exposure, (b) 10
min plasma exposure and (c) 20 min plasma exposure.

temperature (that is, Sn sample temperature)
was fixed at 20 °C. The exposure time of thesample to plasma was 10 min. The tin removal
amount is the highest at 35 Pa, which isequivalent to 48 nm thickness. That is, theetching rate is 4.8 nm/min. It is probable thatthe tin removal amount at the pressure of 35
Pa increased as compared with the case of thepressure of 15 Pa because the electron density
became higher and the hydrogen ion flux
irradiated to the Sn sample increased. It canbe believed that the decrease in tin removal atpressures of 75 and 150 Pa above 35 Pa is dueto the fact that SnH4 exiting the tin surface
collides with hydrogen molecules in a shortmean free path, increasing the probability of令和 2 年度

九州大学大学院総合理工学報告第 42 巻第2号

15

Tamaru,14) the reaction rate of SnH4
dissociation increases by about an order ofmagnitude with a temperature rise of 60 °C.
The results of this experiment make us realize
how easily readhesion occurs when thetemperature of the tin surface is high.

Fig. 3 Dependence of Sn removal on hydrogen gas
pressure.

returning to the tin film surface and causing
readhesion.
Figure 4 shows the dependence of tin
removal on the hydrogen gas flow rate. The
power was fixed at 20 W, the hydrogen gas
pressure was fixed at 50 Pa, and the Sn sample
temperature was fixed at 20 °C. The plasma
exposure time was 30 min. As the flow rateincreased, the amount of tin removal clearly
increased. That is, in tin removal usinghydrogen plasma, it is important to quickly
carry the generated SnH4 away from the tin
surface by the flow of hydrogen gas.

Fig. 5 Dependence of Sn removal on Sn sample
temperature.

In the pressure dependence of tin removal,
the highest etching rate of 4.8 nm/min was
obtained at a gas pressure of 35 Pa. In thiscase, we evaluate the Sn yield which is thenumber of tin atoms removed per hydrogen ion.
Under the conditions of this experiment, thehydrogen ion was H3+.15) Since the density ofmetallic tin is 7 × 103 kg/m3 and the mass of atin atom is 2.0 × 10-25 kg, the number of tin
atoms removed in 10 minutes per unit area oftin film is evaluated as 1.7 × 10 21 m-2. On theother hand, the dose amount of hydrogen ions
irradiated to the tin film is required. An ion
sheath is formed on the surface of the Sn
sample at the ground potential exposed to VHF
hydrogen plasma, and the Bohm flux ofhydrogen ions passing through the sheath end
is expressed by the following equation.16)

i  0.61ne
Fig. 4 Dependence of Sn removal on hydrogen gas flow
rate.

Figure 5 shows the Sn sample temperaturedependence of tin removal. The power was
fixed at 20 W, the hydrogen gas pressure was
fixed at 50 Pa, and the gas flow rate was fixed
at 80 sccm. The plasma exposure time was 30
min. According to the experimental results,
almost all tin with a thickness of 100 nm wasremoved when the Sn sample temperature was
20 °C, but the amount removed was reduced to
one-fourth at 60 °C. According to the report of

k BTe
mi

(1)

Here, kB is the Boltzmann constant, mi is thehydrogen ion mass, ne is the electron density,
and Te is the electron temperature. In order tomeasure the electron density and electron
temperature of the VHF plasma, the probe
measurement was performed at a height ofabout 8 mm from the center bottom of thechamber. The measured values at a pressure of
35 Pa were an electron density of 2.8 × 1015 m-3
and an electron temperature of 2.6 eV. From
this, the dose amount of hydrogen ions per unit
area for 10 minutes is 8.5 × 1021 m-2. That is,

16Studies on Sn Thin Film Decomposition Using VHF hydrogen plasmas

the Sn yield is 0.20. In other words, fivehydrogen ions remove one tin atom.
In order to investigate the promotion of tin
removal by adding the hollow cathode effect to
VHF hydrogen plasma, a tin removal
experiment was conducted using the device
shown in Fig. 1 (b). From the experimental
results of the basic tin removal characteristics
in VHF plasma, the optimum conditions werea gas pressure of 35 Pa and a sample
temperature of 20 °C, so these conditions were
fixed. The power was 30 W. The plasma
exposure time was set to 6 min so that theamount of tin film removed did not exceed 50
nm in thickness. In order to investigate thedependence of tin removal on the hydrogen gas
flow rate, the change in tin film thickness wasmeasured by changing the hydrogen gas flowrate to 20, 40, and 60 sccm. The results areshown in Table 1. As can be seen from the table,a peak etching rate of 8.3 nm/min was
obtained with a hydrogen gas flow rate of 20
sccm. As a result of probe measurement under
the cylinder provided in the center of thepartition metal plate, an electron density of 6.6
× 1015 m-3 and an electron temperature of 3 eVwere obtained. When the efficiency as RIE isevaluated based on these data, the Sn yield is0.12. Taking the reciprocal, this corresponds to
the removal of one tin atom by 8.1 hydrogenions. The efficiency of this tin removal is lower
than the above-mentioned maximum value
when VHF plasma is used.
As can be seen from Table 1, the results ofthis experiment incorporating the hollow
cathode discharge clearly contradict theresults of Fig. 4 in that the amount of Sn
removed decreases with the hydrogen gas flow
rate. This experimental result suggests that it
is out of the optimum conditions. In thisarrangement, the flow of hydrogen gas isorthogonal to the plasma ejected from thehollow cathode discharge, and the higher thehydrogen gas flow rate, the more likely it isthat the irradiation of the plasma on the Sn
sample surface is hindered. This means that,in the current equipment configuration, therelationship of Sn removal with respect to thehydrogen gas flow rate cannot be optimized,
and it is necessary to improve the equipment
in order to obtain the optimum etching rate.

4. Conclusion
In this study, in order to clarify thepossibility of efficient Sn debris removal by
VHF hydrogen plasma, we first investigated

Table 1 Results of Sn removal by hollow-cathodedischarge

Gas flow rate

Decomposed thickness

Etching rate

[sccm]of Sn [nm][nm/min]

20

49.7

8.3

40

48.8

8.1

60

34.7

5.8

the efficiency and characteristics of Sn removalwith respect to hydrogen gas pressure,hydrogen gas flow rate, and Sn sample
temperature. As a result, an etching rate of 4.8
nm/min
was achieved.
The
following
conclusions were obtained from these basic
data. (i) The higher the electron density, thebetter to promote RIE and increase the etchingrate. (ii) If the gas pressure is high and SnH4 isnot sufficiently separated from the Sn sample
surface, readhesion is likely to occur. (iii) To
prevent readhesion, it is effective to increase
the gas flow rate and lower the Sn sample
temperature. In order to increase electron
density, hollow cathode discharge wascombined with VHF discharge. As a result, theelectron density was more than doubled, andthe maximum etching rate of 8.3 nm/min was
obtained. However, the Sn yield value for
evaluating the efficiency of RIE was reduced toabout 60% in the case of VHF discharge alone.
In a device in which the hollow cathodeeffect is superimposed on the VHF discharge,
the etching rate is expected to be further
improved by optimizing the gas flow so as topromote the separation of SnH4 from the tin
surface.
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