1. Yeong-Shin Park, Yuna Lee, Kyoung-Jae Chung and Y. S. Hwang
Study on Characteristics of Plasma Ion Source utilizing Anode Spot with Positively Biased Electrode for Stable and High Current Ion Beam Extraction
Yeong-Shin Park, Yuna Lee, Kyoung-Jae Chung and Y. S. Hwang
Dept. of Nuclear Eng., Seoul National Univ., 599 Gwanak-ro, Gwanak-gu, Seoul , Korea,
Motivation to Study Plasma Ion Source with Positively Biased Electrode
Plasma Ion Source utilizing Localized Plasma near Extraction Aperture
Higher density plasma is more preferable to enhance the ion beam current in plasma ion source.
What plays the more important role to affect ion beam current is the plasma density near extracting hole rather than the density of bulk plasma.
If a dense and small plasma confined near the extraction hole is utilized, the plasma ion sources have a better performance rather than conventional plasma ion source.
Advantages:
High power efficiency: smallness of the localized plasma
High current density ion beam: high density of the localized plasma.
Constricted DC Plasma Ion Source [1,2]
Key Feature: A kind of dc plasma ion source having a small anode compared to the cathode
A small and dense plasma ball attached to the anode is the key to achieve high current ion beam with high power efficiency.
Disadvantages:
High operating pressure (0.3~ 1 Torr): needs more differential pumping.
High discharge voltage (0.4~1 kV): cause damages on the electrodes due to sputtering and thermal load.
Plasma Ion Source with Positively Biased Electrode[3]
Key Feature: Installation of a positively biased electrode to rf plasma ion source.
Characteristics:
Ion beam currents are enhanced by 10~100 times as the bias electrode is biased positively with about 30~40 V, accompanying current jump at bias electrode.
The ion beam currents depend on the biased current regardless of power for ambient plasma.
The reason was considered that
an additional ionizations occurs near extraction aperture and the discharge is similar to the constricted dc plasma ion source.
Advantages over the constricted dc plasma ion source:
The plasma ion source can be operated at lower bias voltage and lower operating pressure.
However, the operating conditions of the plasma ion source have not investigated because of the lack of the knowledge about the additional ionization.
Outline of the Research
Characterization of the additional ionization in detail.
Investigation of the extracted beam characteristics for the optimum operating voltage to achieve maximum ion beam current while the additional ionization is sustaining stably.
Study on the effect of the operating conditions of ambient plasma on the performance of the plasma ion source.
[1] A. Anders and M. Kühn, Rev. Sci. Instrum. 69, 1340 (1998) [2] Y.S Park and Y.S. Hwang, Rev. Sci. Instrum. 81, 02B309 (2010)
[3] Y.J. Kim et al, Rev. Sci. Instrum. 77, 03B507 (2006)
Anode Spot in front of Positively Biased Electrode
Ion Beam Extraction from the Anode Spot
Advantages of Anode Spot as a Source for Ion Beam
Gas
Pressure
RF frequency
RF power
He, Ne, Ar, Kr
10~150 mTorr
13.56 MHz
100 ~ 400 W
Operating conditions of the plasma ion source
RF antenna
Bias voltage
Quartz I.D.
Quartz height
2-turn
< 150 V
54 mm
80 mm
Power + 10 %
As the anode spot is utilized in plasma ion source, ion beam current density and power efficiency can be enhanced drastically since the anode spot have a advantageous properties as a ion source as following.
High Density: enhances beam current density.
Smallness: enhances power efficiency.
Ion beam current increases by 10~100 times with small additional dc power for anode spot, which is about 10 % of rf power for ambient plasma.
200 W
200 W
+20 W
Specification of Bias Electrode
has an extraction aperture of 0.1~1 mm in diameter.
is electrically isolated from plasma electrode by insulator.
exposed area of the bias electrode: 3 mm in diameter
is positively biased by 150 V with respect to the plasma electrode.
Bias Voltage and Bias Current
The voltage and current between bias and plasma electrodes.
15 μ A
600 μ A
Current X 40
◈ Schematic diagram of ion beam extraction without/with the anode spot at 150 mTorr of Helium plasma with 200 W of rf power.
Ion Beam Extraction from Anode Spot at different Bias Voltages
Current and energy of ion beam from anode spot are measured as varying bias voltage. The anode spot is operated in ‘E’~’C’ regime.
◈ Schematic diagram of the inductively coupled plasma ion source with a bias electrode.
Bias Voltage < 100 V
Beam Current increases with Bias Current
Ion beam current are linearly increased with bias current.
The bias current is known to be proportional to the plasma density of anode spot.
The enhancement of the ion beam current is caused by the increase of the plasma density of the anode spot with the bias voltage.
Bias Voltage > 100 V
Beam current decreases with Bias Voltage
Over 100 V, however, the ion beam currents are observed to decrease with the bias voltage although the bias current increase.
The Additional Plasma
= Anode Spot
In terms of shape and I-V characteristics, the additional plasma is identical to the anode spot.
≠ constricted dc plasma
since this additional glow is ignited only when the ambient plasmas are turned on.
Anode Spot[4-6]
Anode spot has been known as localized discharge on positively biased electrode.
The anode spot is known to be generated by ionization collisions between neutrals and accelerated electrons from ambient plasma toward the positively biased electrode.
The fact that the anode spot has a higher plasma density than the ambient plasma is the most important feature in terms of plasma ion source.
I-V Curve at the Bias Electrode(a)
Bias currents are measured as varying bias voltage.
‘O’~‘A’: similar to that of Langmuir probe.
‘A’ : sudden bias current jump occurs.
‘A’~‘B’: steep increases of bias current with voltage.
‘B’~‘C’: bias current tends to be saturated.
‘C’~‘D’: bias current moderately decrease with bias voltage.
‘D’~‘E’: bias current decrease steeply.
‘E’: bias current decrease before current jump.
The additional discharge is confirmed in the I-V curves.(a)
The bias current is sustained at lower bias voltage when the bias voltage is decreased than when the bias voltage is increased.
This hysteresis indicates that the self-sustaining discharge occurs near the bias electrode.
Observation of the Additional Plasma by photographs(b-c)
In ‘O’~’A’ and ‘E’~’O’, bias electrode is observed clearly without the additional discharge.
However, a small and bright ball generates in front of the bias electrode in the regime ‘A’~’E’.
◈ (a) I-V curves of bias electrode and photographs of (b) bias electrode and (c) the additional plasma.
Anode Spot
(+) Bias Electrode
Ambient Plasma
◈ Ion beam current and bias current with bias voltage at 100 W of rf power and 150 mTorr of helium.
Measurement of Potential of Anode Spot
Ion beam energies (measured by retarding field analyzer) indicates the space potential of anode spot in which the ions are generated, since the ion thermal energies is negligibly small (< 0.1 eV).
Potential Barrier increases with Bias Voltage
The average potential of the anode spot is lower than the bias voltage.
The positive potential difference plays a role as a potential barrier which obstruct ion beam extraction from the anode spot.
The potential barrier increases monotonically with the bias voltage.
In high bias voltage regime, ion beam extraction is disturbed due to the high potential barrier between anode spot and extraction hole.
◈ Schematic diagram and photograph of the anode spot
Characteristics of the Additional Plasma compared to the ambient plasma
Smallness: diameter of the ball is an order of the bias electrode, 3 mm.
High density: brightness of the ball is higher than the that of the ambient plasma.
Both the brightness and the size of the ball increases with the bias voltage.
◈ Potential of the anode spot and potential difference between the anode spot and bias electrode as a function of bias voltage at 100 W of rf power and 150 mTorr of helium.
Optimum Operating Condition of the Anode Spot for High Current Ion Beam
Optimum bias voltage is found to be near the ‘knee’ of the characteristic I-V curve of anode spot.
before the knee:
Increase of anode spot density > increase of potential barrier
after the knee
Increase of anode spot density < increase of potential barrier
Optimum bias current, corresponding bias current to the optimum bias voltage, is proportional to the maximum extractable current from the anode spot.
[4] B. Song, et al., J. Phys. D: Appl. Phys. 24, 1789 (1991). [5] R. L. Stenzel et al., Plasma Sources Sci. Technol. 17, (2008). [6] S. D. Baalrud et al., Plasma Sources Sci. Technol. 18, (2009).
Effect of the Ambient Plasma on the Anode Spot
Pressure Effect on Anode Spot
Power for Ambient Plasma
Gas Effect
Conclusion
The anode spot, a dense and small plasma formed near the extraction aperture, is found to be a key to enhance the performance of the plasma ion source with a positively biased electrode.
The anode spot is desirable to operate with the optimum bias voltage and current which are found near the ‘knee’ of the characteristic current-voltage curves in order to maximize ion beam current.
Plasma density of the anode spot increases with bias current.
Potential barrier to disturb ion extraction from the anode spot increases as the bias voltages are increased.
The performance of the plasma ion source utilizing the anode spot can be easily controlled by external control knobs.
Stable operation: the optimum bias voltage can be reduced by increasing the pressure.
High beam current operation: the optimum bias current increases with plasma density of ambient plasma.
◈ I-V curve of anode spot at different pressure.
◈ I-V curve at different rf power for ICP.
As increasing the pressure, the optimum bias voltages at the knee decrease while bias currents are not varied.
Pressure affects the operating voltage of the anode spot because the probability of the ionization collision between electron & neutral increases with the population of neutrals.
Plasma density of anode spot less depends on the pressure.
Stable Operation of the Anode Spot
The plasma ion source with the anode spot can be operated more stably at lower bias voltage, avoiding damages on electrode, by increasing operating pressure.
As increasing the power for ambient plasma, the optimum bias currents increases while bias voltages are not varied.
The higher number of electrons in ambient plasma with higher input power causes more frequent ionization-collisions with neutrals in the anode spot, increasing the plasma density of the anode spot. For the reason, the bias current increases with density of ambient plasma.
High Current Operation
Ion beam current can be more increased at fixed bias voltage by increasing plasma density of the ambient plasma.
◈ Optimum bias voltage/current with different noble gases.
Optimum bias voltages are decreased as the gas species having lower ionization potential.
Ionization potential: Kr < Ar < Ne < He
Optimum bias current is higher as the ionization cross-section of the gas is higher.
Ionization cross-section: Kr > Ar > Ne > He
Inert gas which have higher atomic number is preferable for high current and stable ion beam extraction from the anode spot.
The present study is expected to be useful in determining the operating conditions of the plasma ion source utilizing the anode spot in order to achieve stable, high current ion beams.
5. The plasma ion source utilizing the anode spot is being investigated to enhance the brightness of plasma ion source for focused ion beam(FIB) and nano medium energy ion scattering(Nano-MEIS).
◈ FIB using plasma ion source utilizing anode spot and acquired image of micro-size pattern
C09. Brightness Enhancement of Plasma Ion Source utilizing Anode Spot for Nano Applications
14th International Conference on Ion Sources (ICIS) on September , Giardini Naxos, Italy