Experimental investigations into ion-ion plasma formation and negative ion extraction

Relatively electron-free Ion-Ion plasmas represent a novel paradigm in plasma assisted processing of materials and are promising for the impending deep sub-micron generations. They are devoid of the non-equilibrium between the positive and negative charge carrying species, that is one of the causes for plasma-induced damage in conventional electron-ion plasmas. The lack of a confining plasma potential allows negative ions to be extracted from these plasmas. We formed ion-ion plasmas in the afterglows of pulsed-power Chlorine discharges. Here electrons were lost rapidly (∼10's of μsec), after turning off the plasma excitation, to dissociative attachment with the electronegative neutral gas. We present experimental characterization and optimization of ion-ion plasmas to be used in negative ion extraction experiments. Low pressures (1 mTorr), high powers, mid-range duty ratios (50%) and low frequency (1 kHz) pulsing produced a near optimal ion-ion plasma for negative ion extraction. Ion-ion plasma decay kinetics were found to be second order due to the dominance of ion-ion recombination over diffusion in Chlorine. We measured an ion-ion recombination rate coefficient, K_rec ≈ 2.5 × 10 −7 cm3 sec−1. These optimized ion-ion plasmas were subjected to RF bias and the nature of the ions extracted was studied by time-resolved mass spectroscopy. We also present a novel biasing scheme called “Ion-Ion Synchronous” Bias. This new mode of biasing is shown to be more efficient than the state of the art ion-ion asynchronous bias techniques in extracting negative ions. We trace the reason for superior negative ion extraction with ion-ion synchronous bias to the minimal self-bias that it generates on in-line capacitive elements. We present the exciting result of temporally alternating positive and negative ions fluxes through the application of an ion-ion synchronous bias. Ion-Ion synchronous biasing was applied to Silicon processing to test its commercial viability. Through the application of ion-ion synchronous DC step-bias, we demonstrated negative-ion assisted etching of Silicon. At 50 eV energy, Cl− was found to etch 2.5 times slower than $Cl+2$ ions. The claim of reduced electron shading through synchronous bias is supported through the use of in-situ charge-monitoring circuitry. Etching blanket polysilicon wafers revealed enhanced etch selectivity control by ion-ion synchronous biasing. This was attributed to increased control over ion-energies striking the substrate. Characterization of patterned wafers etched using ion-ion synchronous bias, revealed a reduction in plasma-induced damage.