Magnetoelectric Heating In An Microwave ECR Plasma

Compared with other polishing method of ion beam, using the cyclotronmotion of ions in a strong magnetic field to achieve the polishing ofchemical vapor deposition (CVD) diamond films in a microwave electroncyclotron resonance (ECR) plasma with a magnetic mirror field have manyadvantages, such as larger polishing area and more uniform processing.However, the ion temperature in ECR plasma is low. Ions have a weaklycyclotron characteristic when they are extracted by an accelerating electrode.It causes the ions are difficult to etch diamond film. Fortunately, themagnetoelectric heating can solve this problem because the ion temperaturecan be increased by the heating method. It’s well known that higher iontemperature contributes to transfer the energy parallel to magnetic field tothat perpendicular to the magnetic filed when a gradient magnetic field isused to adjust the ion motion in ECR plasma. From that a cyclotron ionbeam with anisotropic character can be obtained and is used to etch thediamond films.In this paper, the magnetoelectric heating is studied in a self-designedmicrowave ECR plasma device and mainly works contain as below:1. Ion sensitive probe and double probe are used to measure theparameters of ion and electron respectively. The radial distribution of iontemperature and electron temperature after heating are analyzed. It is foundthat the whole heating of plasma is accomplished by the magnetoelectricheating of ions in the sheath of anode ring and the radial transport of heatedions. The temperature of ion in the axial area increase with the bias voltageof ring and their relationship is nonlinear. The heating process of electron issimilar to ions’ heating process. But the efficiency of electron heating ismuch smaller than efficiency of ion heating. The plasma density changes alittle after magnetoelectric heating. 2. The effects of magnetic field, anode ring, axial position and pressureon the ion heating are investigated. The results show that: compared thedivergence magnetic field, the mirror magnetic field is more beneficial toincrease the ion temperature during the magnetoelectric heating. Lowerpressure contributes to increase the ion temperature and the efficiency of themagnetoelectric heating. Temperature of ion increases with the decrease ofpressure when the pressures change in a range of0.02-0.8Pa. The heatingefficiency increases with the increase of microwave power when themicrowave powers change in a range of650-950W. A suitable radius ofanode ring can improve ion temperature effectively and the optimal size ofanode ring depends on the cyclotron radius of ions. The axial location ofanode ring is dependent on configuration of magnetic mirror field. It is themost advantageous to improve temperatures of ion when anode ring is set ata region where it has a weaker strength of magnetic field in the middle ofmirror magnetic field. Double anode ring can further improve the iontemperature and the ion temperature is increased twice of the single ringafter heating. Using a cylinder-electrode to heat plasma not only contributesto improve ion temperatures but also is good to the axial transport of ions.The ion temperature inside of cylinder-electrode had a big radial variationwhile it had a good radial uniform at downstream of cylinder-electrode.3. The ion beam is extracted by an electrode after heating and used to etchCVD diamond films. It is found that: ions can not be effectively extractedwhen a negative voltage is put on the accelerating electrode. But theaccelerating electrode with a positive voltage can improve ion densitydownstream of axis. Combining heating of cylinder-electrode and magneticmirror field to transfer ion beam contributes to increase ion temperature anddensity downstream of chamber. After magnetoelectric heating, the ions cantransform the sharp edges into obtuse edges when the ECR plasma is used toetch diamond films. Meanwhile, the plasma etching also can reduce effectively the mechanical strength on the surface of diamond films andimprove the efficiency of mechanical polishing.