Study on microwave plasma source based on parallel stripline resonator

A Dissertation for the Degree of Doctor of Philosophy Study on microwave plasma source based on parallel stripline resonator Department of Materials Science and Engineering Graduate school Chungnam National University By Tran Thanh Hai Advisor Jong-Ruyl Jeong February 2013 Study on microwave plasma source based on parallel stripline resonator Tran Thanh Hai 2013 Study on microwave plasma source based on parallel stripline resonator Advisor Jong-Ryul Jeong Submitted to the Graduate School in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy October 2012 Department of Materials Science and Engineering Graduate School Chungnam National University By Tran Thanh Hai To approve the submitted Dissertation for the Degree of Doctor of Philosophy By Tran Thanh Hai Title: Study on microwave plasma source based on parallel stripline resonator December 2012 Committee Chair Prof Soon-Ku Hong Chungnam National University Committee Dr Shin-Jae You Korea Research Institute of Standards and Science Committee Prof Jong-Ryul Jeong Chungnam National University Committee Dr Sae-Hoon Uhm Plasmas company Committee Dr Seung-Kyu Ahn Korea Institute of energy research Graduate School Chungnam National University Acknowledgment I would like to express my sincere gratitude to my advisor Dr Shine- Jae You who guided and supported me during the good and bad days of my PhD His way of thinking, like a physicist and not like an engineer, has made me look at problems with a new eye I would also like to thank him for the critical review of the publications and this dissertation I would also like to thank Prof Jong-Ruyl Jeong, my advisor in Department of Materials Science and Engineering who helped me enter the graduate school of Chungnam National University, gave me useful classes and introduced me to Dr Shine-Jae You He has always been available and this dissertation would not have been possible without his support, suggestions and enlightening discussions I am grateful to my Ph.D committee members for their valuable comments and suggestions on my thesis I would also like to thank Dr D.j.Seong, Dr J.H.Kim in the Plasma Lab of KRISS for their support and discussions My thanks also go to Dr Min Park, B.H Seo, K.H You, D.W Kim for their help in my life and my work Whenever I needed their support, they were pleased to help me Finally but not least, I would like to thank my parents and my wife They have been a great source of encouragement during these years Korea, October 2012 Tran Thanh Hai 병렬 스트림라인 공진기를 이용한 마이크로웨이브 플라즈마 소스에 관한 연구 (지도교수 정종률) 충남대학교 대학원 신소재공학과 재료공학 낮은 온도와 작은 크기를 가지는 마이크로플라즈마 소스는 바이오-메디칼, 치과 치료, 디스플레이, 광검출기 등의 다양한 분야에서 쓰일 수 있어 큰 관심을 받 고 있다 마이크로플라즈마 소스는 수 Torr 에서 대기압까지 방전이 가능하며, 일반적 으로 활용 특성상 대기압에서의 활용이 선호된다 하지만 대기압 방전시 높은 에너 지의 이온들에 의한 전극의 손상등이 문제로 제기되며, 이러한 문제를 해결하기 위한 다양한 연구가 있어 왔지만 정확한 특성연구는 부족한 실정이었다 이 논문의 주제는 마이크로웨이브 병렬 스트립라인 공진기를 이용한 마이크 로플라즈마 소스에 관한 것이다 높은 작동 주파수 (~800~MHz)에서의 공진을 이용 하여 플라즈마의 사이즈를 줄이며 기본적인 장치의 동작구조는 마이크로스트림 구조 의 소스 (J Kim, K Terashima, Applied Physics Letters 86 (2005) 191504)와 마이크로스 트렘 분할원형 공진기 (F Iza, J.A Hopwood, Plasma Sources Science and Technology 14 (2005) 397) 이다 각각의 소스의 장점을 활용하여, 본 논문에서 제안한 마이크로 플라즈마 소스는 다른 플라즈마 소스에 비하여 효과적으로 강한 전기장을 발생시켜 플라즈마를 만들다 높은 주파수에서 작동하하여 RF 용량 결합성 플라즈마 소스에 비하여 효과 적으로 전자들에 에너지를 주게 된다 마이크로플라즈마 소스의 경우 헬륨과 아르곤 가스로 대기압내에서 방전이 되며 1W 이하늬 200mW 정도에서 유지됭다 대기 방전의 경우 5W 정도에서 방전이 되며 1.4W 정도에서 플라즈마가 유지된다 본 마이크로플라즈마의 경우 낮은 압력조 건에서는 더 낮은 전압과 전력으로 구동이 된다 마이크로플라즈마 소스이 플라즈마 변수들은 광검출 스펙트로스코피를 이용 하여 측정이 되었다 플라즈마 온도의 경우 OH의 회전 스펙트로스코피 (Rotational spectroscopy) 를 이용하여 측정되었으며, 전자 온도의 경우 스타크 확산 (Stark broadening) 분석을 통하여 얻어졌다 전자 밀도의 경우 중성 헬륨의 29-level 의 충 돌성 방출 (Collisional-radiative, CR) 모델을 이용하여 얻어졌다 측정을 통하여 가스 온도, 전자 온도, 전자 밀도 각 각은 2.5W 조건에서 400~K, 1~eV, $10^{14}cm^{3}$ 에 있음이 확인되었다 이를 통하여 본 논문에서 연구한 마이크로플라즈마 소스 가 비열원 (Non-thermal)의 전자 온도가 가스 온도보다 월등히 높은 소스임이 확인 되었다 이러한 특성은 높은 효율, 작은 크기, 낮은 구동 전력 틍성을 가지는 대기압 플라즈마 소스로의 탁월함을 보여준다 ABSTRACT1 Study on microwave plasma source based on parallel stripline resonator Tran Thanh Hai Department of Materials Science and Engineering, Graduate school of Chungnam National University Daejeon, Korea (Advised by Professor Jong-Ryul Jeong) Microplasma sources, with advantages of low temperature plasmas and small-scale discharge, have received much attention for development and application in a variety of fields including bio-medical applications (treatment of living tissues, tissue sterilization and blood coagulation), dental treatment, displays, radiation sources, micro-chemical analysis systems, gas analyzers, photo-detectors A dissertation submitted to the committee of Graduate School, Chungnam National University in partial fulfillment of the requirements for the degree of Doctor of Philosophy conferred in February 2013 i Microplasmas can be generated in a wide range or pressure from a few Torrs up to a few atmospheres In normally, operation of microplasmas at atmospheric pressure is more favorite because its size can be reduced due to elimination of micro-pump However, some problems, such as the electrode erosion due to energetic ion bombardment, the difficulty of sustaining a glow discharge in the air, the higher voltages required for gas breakdown, the arcing at high pressure lead to a new set of challenges in the research field To deal with these problems, several schemes have been devised However, they not always provide perfect properties This dissertation present a microplasma source based on a microwave parallel stripline resonator (MPSR) The design for high frequency operation (840 MHz) using resonance phenomenon allows its size to minimize The basic design and performance of the device is a hybrid type of microwave discharge between microtrip structure source (MSS, J Kim, K Terashima, Applied Physics Letters 86 (2005) 191504) and microstrip split-ring resonator (MSRR, F Iza, J.A Hopwood, Plasma Sources Science and Technology 14 (2005) 397) Therefore, by virtue of combination effect of each discharge source advantage, the MPSR can concentrate a strong electric field of which direction is rather parallel to the stripline around the small gap, compared with other plasma sources Since the device is operated at high frequencies, the power is more efficiently coupled to the electrons in the plasma and better performance than in a RF capacitively coupled plasma source is achieved The increasing sheath voltages and collisionless nature of the sheath as pressure is decreased MPSR can self-ignited plasmas in helium and argon gases as input power less than 1W and maintained plasma with as little as 200mW at atmospheric pressure For air gas they are 5W for self-ignition and 1.4W for maintaining plasma This design can also be operated at low pressure with lower ignited input voltage and power The low-power requirement allows for air-cooled operation and the possibility of driving the device with low-cost off-the-self electronics currently used in telecommunication applications The parameters of a helium plasma at atmospheric pressure generated by MPSR were measured with optical emission spectroscopy method The plasma gas temperatures was estimated by OH emission rotational spectroscopy, the electron density was obtained from Stark broadening analysis of the hydrogen ii Balmer - β line, and the plasma electron temperature was determined by using 29-level collisional-radiative (CR) model for neutral helium The results show that plasma gas temperature, electron temperature, and electron density were in the order of 400 K, eV, and 1014 cm−3 as the power less than 2.5W, respectively This device is a non-thermal discharge, the electron temperature is much higher than gas temperature With the advantages of high efficient, compact size, low input power and capability of operation at atmospheric pressure, the MPSR plasma source is a suited device for portable systems iii absorbed by the load to the power supply to the resonator and is given by Pload Pin [1 − (|Γ|2 + |T |2 )]e−αl η = − (|Γ|2 + |T |2 )e−2αl η = 7.3 (7.12) Result and discussion Figure 7.4: Gap voltage versus input power of the resonator as infinite discharge gap impedance a) and 100µm discharge gap width Figure.7.4 shows the discharge gap voltage depends on the input power at various characteristic impedances in the case infinite gap impedance (without present of discharge gap) (a); and gap width 100µm (b) The results from figure.7.4.a) show that the gap voltage increases as power supply increases like √ the laws of the form V ∼ P in, and they are also show that with a given input power, the gap voltage increases with increasing of characteristic impedance Considering in the case of existing of discharge gap, the gap voltage is also increase with increasing power as shown in figure.7.4.b), however their values is smaller comparing to that in the case of infinite gap impedance as shown in figure.7.4.a) Even though a lower value of voltage obtained, however, with a small size of discharge gap in this case, high electric field strength can be achieved High electric field is necessary for generation plasma at atmospheric pressure Figure.7.5 shows the gap voltage versus load impedance a) and characteristic - 79 - impedance b) as the discharge gap 100µm and input power 1W Figure 7.5: The dependence gap width of the resonator at various characteristic impedance as input power 1W of a) discharge gap impedance and b) characteristic impedance Figure.7.6 shows the dependence on gap width at different characteristic impedance as the input power 1W and strip thickness 50µm of a) gap voltage b) and electric field in discharge gap Figure.7.6.a) shows that, in the case without present of plasma, when the gap width increases, the gap voltage increases in the first stage and then reach a saturated state The corresponding value of gap width for starting saturated state increases with the increasing of characteristic impedance, for instance, they are approximated 300µm, 650µm and 800µm corresponding to characteristic impedance 20Ω, 40Ω and 70Ω, respectively The reducing of gap voltage as discharge gap width decrease leading to the electric field in discharge gap may be not increased as gap size is reduced as shown in figure.7.6.b) The results show that when the gap width reduce from 1mm to 0.1mm, the electric field strength in discharge gap in the case characteristic impedance 20Ω is almost unchanged At higher characteristic impedance the electric field increase with the decreasing of gap size Figure.7.7 shows the efficiency of the MPSR as a function of the normalized plasma impedance (ZL /Z0 ) for resonator The plasma impedance has been considered real as this is a good approximation for atmospheric discharges - 80 - Figure 7.6: The dependence gap width of the resonator at various characteristic impedance as input power 1W of a) gap voltage and b) electric field strength The maximum efficiency is achieved when the plasma impedance is twice the characteristic impedance of the resonator (ZL = 2Z0 ) The maximum efficiency condition is very hard to be met because the plasma impedance are normally in the order of kΩ while the characteristic impedance of the resonators are typically less than 100Ω Moreover,high-impedance lines lead to reducing the width of the line increases the conductor losses as shown in figure.7.9, and the quality factor degradation does eventually negate any efficiency improvement Figure.7.8 shows the efficiency or electric field radiation of the resonator versus discharge gap width at different characteristic impedance The result shows that the efficiency reduce as gap width and characteristic impedance increase - 81 - Figure 7.7: Power efficiency of the resonator versus normalized plasma impedance - 82 - Figure 7.8: Power efficiency of MPSR versus gap width at various characteristic impedance - 83 - Figure 7.9: Quality factor versus impedance lines of the resonator - 84 - Chapter CONCLUSION A microplasma source based on a microwave parallel stripline resonator (MPSR) has been designed, analysis, fabricated and tested The design for the high frequency operation (840 MHz) using resonance phenomenon allows its size to minimize The computer field simulation for the proposed microplasma system based on microwave parallel stripline resonator (MPSR) and the comparative study with the other microwave discharge sources revealed that the MPSR is performed as a hybrid type of two micro wave discharge sources (MSS and MSRR) Therefore, by virtue of combination effect of each discharge source advantage, the MPSR can concentrate a strong electric field of which direction is rather parallel to the stripline surface around the small gap, compared with other plasma sources It can be operated readily in low power level with high plasma density because of its high strength of electric field and low plasma loss rate stemming from the parallel direction of the electric field to the stripline surface The resonance frequency and input impedance of the device were estimated with simple circuit model The capacitance components of the resonator were calculated by conformal mapping method The calculation results were in good agreement to simulation results from CST microwave studio and experiment results This device can self-ignited plasma in helium, argon gases at atmospheric pressure with power less than 1W and maintaining plasma with power as little as 200mW For air gas they are 5W for self-ignited and 1.4W for maintaining plasma This design is also capable for operating in low pressure 85 With gas temperature, electron temperature, and electron density are in a range of 400 K, eV, and 1014 cm−3 as the power less than 2.5W This microplasma source is a non-thermal plasma, and capable for low temperature applications such as bio-medical, dentist treatment Design of this microplasma source is not only allowing to make a small size one to be integrated into portable devices, but also make a large and uniform 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