NOVELTY - An apparatus to form deposition medium on substrate, comprises deposition chamber with plasma activation region (34) and collision region (65); first conduit (24) to introduce energy-transferring gas into the plasma region, to form plasma; a mechanism for directing ions, ion-radicals and neutral radicals of plasma, to collision region; second conduit (60) to introduce precursor gas into collision region, to form pre-deposition medium; and electrically-biased screen (70) for excluding ions and ion-radicals from pre-deposition medium to form deposition medium enriched with neutral radicals.    USE - As an apparatus to form a deposition medium for deposition on a substrate such as continuous web substrate (claimed) made of steel or plastic; useful for forming photovoltaic materials (e.g. amorphous, nanocrystalline, microcrystalline and polycrystalline forms of silicon, germanium, silicon-germanium alloys, and their hydrogenated and fluorinated forms) for deposition on a substrate, to manufacture photovoltaic devices e.g. triple-junction solar cell.    ADVANTAGE - The apparatus enables in-situ generation of neutral-enriched deposition medium conducive to the formation of thin film materials having low intrinsic defect concentration, at any speed on the substrate. Thus, the apparatus provides high rate deposition of thin film materials on stationary/continuous substrate, without sacrificing the quality of the deposited thin film layer on the substrate. The apparatus has control over the entire manufacturing process, as the diagnostic unit and feedback control element permit process-programming to achieve and maintain optimal distribution of preferred species throughout the deposition process within the apparatus.    DETAILED DESCRIPTION - An apparatus to form a deposition medium for deposition on a substrate, comprises a deposition chamber including a plasma activation region (34) and a collision region (65); a first conduit (24) for introducing an energy-transferring gas into the plasma activation region, to form a plasma containing ions, ion-radicals and neutral radicals, from the energy transferring gas, in plasma activation region; a mechanism for directing the ions, ion-radicals and neutral radicals of the plasma, to the collision region, via pressure differential between the plasma activation region and collision region; a second conduit (60) for introducing a precursor gas into the collision region, so that the ions, ion-radicals and neutral radicals of the plasma interact with the precursor gas in the collision region, to form a pre-deposition medium containing a mixture of ions, ion-radicals and neutral radicals in a non-plasma state; and at least one electrically-biased separation screen (70), for selectively excluding the ions and ion-radicals from the pre-deposition medium, to form a deposition medium enriched with neutral radicals, for deposition on a substrate. The deposition medium forms a thin film material on the substrate. The apparatus further comprises the substrate; and a process control system including a diagnostic unit and a feedback control element. The diagnostic unit monitors conditions in the interior of the apparatus. The feedback control element receives information from the diagnostic unit, and regulates the conditions in the apparatus in response to the information received from diagnostic unit. The diagnostic unit senses composition of the energy transferring gas, precursor gas, plasma, pre-deposition medium or deposition medium; and senses concentration of defects in thin film material. The feedback control element controls energy/frequency of the plasma; controls flow rate of the energy transferring gas or precursor gas; modifies the pressure differential between the plasma activation region and the separation screen; and controls electrical bias of the separation screen. The feedback control element preferably stores data corresponding to optimized conditions in the apparatus, to be compared with the information received from the diagnostic unit, for adjusting the conditions in the apparatus.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic view of a continuous web deposition apparatus.    Conduit for introducing energy-transferring gas (24)    Plasma activation region (34)    Conduit for introducing precursor gas (60)    Collision region (65)    Region for forming pre-deposition medium (67)    Electrically-biased separation screen (70)    Region for forming deposition medium (72)    Roller for dispensing continuous web substrate (75)    Gas gates (80)    Roller for collecting continuous web substrate (85)