Hybrid Acceleration Combining Ion Beam And Plasma Immersion Ion Implantation And Numerical Simulation Of Modulated Sheath

Ion implantation has been a very essential surface modification tool formaterials due to its outstanding advantages without change of workpiece dimension,and the problem of weak adhesion between coating and substrate. Conventional ionbeam implantation (IBI) is not suitable for treating the workpieces with complexshape owing to the limitation of light-of-sight. Plasma immersion ion implantation(PIII) process has been developed as a means without such problems and shows agreat superiority for treating complex-shaped components. Actually, in PIII process,it is difficult to ensure ion uniformity with an unconformal dynamic sheath,especially for treating the samples with concave surface like a hole. Because theshape and expansion velocity of plasma sheath are strongly dependent on manyfactors includes plasma density, duration of HV pulse, configuration of target, etc.,the sheath at concave position of sample may overlap in PIII process. This leads tothe lower incident ion dose on the surface of concave surface especially on sidewall.It urgently needs to find a new way to increase the incident dose on sidewall. In thispaper, a novel process, hybrid acceleration combining ion beam and plasmaimmersion ion implantaion has been proposed. The ion beam is induced to modulatethe plasma sheath directionally, and then the more conformal sheath may formaround concave position dut to suppression by ion beam. The incident dose on theconcave surface will be enhanced with the improved conformal plasma sheath.Firstly, the hybrid acceleration system combining ion beam and plasmaimmersion ion implantation (PIB/PIII process) has been built. A special PLC-basedhigh-voltage pulse modulator for PIII is developed, and it can deliver repetitivehigh-voltage pulses with a short rise-and fall-time. Series-stacked IGBTs as a high-voltage switch has been used to control the long tail of high-voltage pulse, which iscaused by the charges reserved in the parasitic capacitance in circuit and thecapacitance in plasma load. In this way, the adverse effect of long fall-time of high-voltage pulse can be reduced significantly in PIII process. An ion beam sourcebased on DC-bias hot filament glow discharge has been upgraded for generatingpulsed ion beam through changing the mode of power supply. An auxiliarymatching circuit has been design to accomplish the synchronization between thepulse of ion beam source and high-voltage pulse on target in PIB/PIII process.High-voltage self-excited glow discharge can be induced by pulsed-ion beam inPIB/PIII process, and its plasma density increases with increasing gas pressure andion beam energy.Particle-in-Cell/Monte Carlo Collision (PIC/MCC) mode has been used to simulate the dynamic plasma sheath in hybrid acceleration system combining ionbeam and PIII with planar target. In PIB/PIII process, the impact energy of ionsincreases because of the secondary acceleration of ions in the electric field neartarget applied to a negative high-voltage pulse. With the increase of bias on target,the impact energy of ions increases sharply. Due to the collision effects betweenions and neutral gas, the ions may lose part of carried energy and the energy lossmay grow with the working gas pressure increasing in PIB/PIII process. It isrecommended that the working gas pressure should be below0.1Pa for reducingcollision effects in PIB/PIII process, according to the numerical simulation results.The implanted area on the surface of planar target enlarges owing to the defocusingeffect of the ion beam when the diameter of target is larger than that of ion beam,but the uniformity of incident dose on the surface of planar target gets worse withincreasing target area. In order to obtain a uniform modified layer in PIB/PIIIprocess, a target with a diameter smaller than that of ion beam is preferred. It isconfirmed that the impact energy increases indeed because of secondaryacceleration effect. It is confirmed by the depth profile of N element on the surfaceof stainless steel samples treated by PIB/PIII and IBI process respectively.The dynamic plasma sheath of concave sample implanted by PIB/PIII processis also simulated. The results show that the pulsed ion beam can suppress theexpansion of plasma sheath, and then change the sheath shape at concave positionand improve the degree of similarity between plasma sheath and concave sample.Consequently, the incident dose on sidewall of concave sample is increased. Thepulsed ion beam with higher energy has stronger ability to suppress the sheathexpansion, and this is helpful for improving the incident dose on sidewall andsuitable for treating the concave samples with deeper hole. Both the incident doseand impact energy on sidewall will increase due to slight collision effect at lowerworking gas pressure, but the incident angle will enlarge. With the increase of biason target, the incident dose and impact energy on the surface of samples may alsoincrease, and the distribution range of incident angle on sidewall is wider for themore collision of ions with higher energy. The displacement of beam ions beforeimplant into the surface of sample decreases with the height of target increasing, sothat the incident dose on sidewall is improved due to stronger suppressing ability ofhigh-energy beam ions for fewer collisions. When the depth of concave holeincreases, it is difficult for ions implanting into the surface of deeper sidewall,although the pulsed ion beam can modulate the plasma sheath. XPS analysis showsthat the incident ion dose on sidewall of concave sample treated by nitrogen N ionsimplanted by PIB/PIII is enhanced significantly, but there is sputtering effect due torelatively large incident angle.Nitrogen-doped DLC films have been prepared on planar stainless steel samples using hybrid acceleration system combining ion beam implantation and PIII(PIB/PIII&D). The aim is to investigate the bombardment effect of N ions inducedby pulsed ion beam on the structure and properties of DLC films. Raman analysisshows that the amorphous structure of DLC films doped by N does not vary. XPSanalysis shows that there is no super-hard C3N4phase formed in DLC films, and theelement N exists in DLC films as the bonds of C=N and C≡N. The content of SP3bonds in DLC films decreases with the N element increasing, so that the hardness ofDLC films decreases. There is slight effect on the hardness of DLC films if a smallamount of nitrogen is included, but it can release the internal tress of DLC films.Besides, DLC films are prepared on the surface of concave sample byPIB/PIII&D process. The tribological properties and corrosion resistance of thesamples at different position (top, sidewall and bottom) are compared. The resultsshow that surface properties has beem improved and uniformity has been enhancedfor the dose enhancement, with modulated sheath and induced high-voltage self-excited glow discharge by pulsed ion beam. Moreover, the uniformity treated byPIB/PIII&D process is better with the higher bias voltage, shorter distance betweentarget and ion source, and higher energy of ion beams.