THREE-DIMENSIONAL QUASI DIRECT CURRENT PLASMA IMMERSION ION IMPLANTATION INTO BIOMEDICAL NICKEL-TITANIUM SHAPE MEMORY ALLOY ROD

In plasma immersion ion implantation, the sample is immersed in a plasma and negative high voltage pulses are applied to the sample. When the sample is negatively biased, an ion sheath is established and ions are implanted into the sample. In conventional PHI, in order to avoid sample overheating, the experiments are typically conducted using a low pulsing frequency such as 50 Hz and a low duty cycle of 0.02. In order to achieve a conformal plasma sheath and maintain good implant fluence uniformity over the entire sample surface, a pulse duration of a few tens of microseconds is usually required. The ratio between the plasma treatment time (pulse-off period) and ion implantation time (pulse-on period) can be readily adjusted in the quasi- DC (direct current) PHI technique to process flat samples such as silicon wafers previously reported by our research group. In this low pressure, steady-state DC mode, a grounded conducting grid divides the chamber into two parts. In the lower part, a strong electric field is formed between the negatively biased wafer stage and the boundaries created by the grid and the lower part of the chamber walls. The upper part confines the plasma since the grounded grid stops the expansion of the ion sheath toward the lower part. In this way, a continuous low-pressure discharge can be maintained in the volume above the grid. Positive ions from the plasma diffuse into the lower part through the grid and are implanted into the sample. Since the ion sheath is stopped by the grounded conducting grid, the pulse duration can be increased to over lOOIcircfrac14s without experiencing the adverse effects encountered in conventional PHI such as extensive sample heating. The processing time is thus greatly reduced. This concept can be extended to three dimensions samples. In this work, the object is shielded by a grounded cylindrical cage made of stainless steel mesh. S-shape biomedical NiTi bar used for scoliosis correction is...