MAGNETICALLY ENHANCED PLASMA DEPOSITION OF INTRINSIC A-SI:H LAYERS IN ROLL-TO-ROLL SYSTEMS

IF THIN FILM A-SI SOLAR CELLS ARE EVER TO MEET THEIR PROMISE OF LARGE AREA MANUFACTURE AT LOW COST, CERTAIN PROBLEMS MUST BE SOLVED. UNIFORM HIGH RATE DEPOSITION OF HIGH QUALITY, PINHOLE-FREE INTRINSIC LAYERS OVER LARGE AREAS IS ONE OF THOSE PROBLEMS. R&D ON IMPROVED REACTION CHAMBERS IS NEEDED FOR CONTROLLED FABRICATION OF LARGE AREA A-SI LAYERS. FOR THE STANDARD PLASMA ENHANCED CHEMICALVAPOR DEPOSITION (PECVD) PROCESS, THE BEST FILMS ARE GROWN AT VERY LOW RATES. IF RF POWER IS INCREASED, THE DEPOSITION RATE INCREASES, BUT EFFICIENCY AND STABILITY OF THE SOLAR CELLS DECREASE. AT THE SLOW DEPOSITION RATES, THE CAPITAL COSTS OF MANUFACTURE ARE EXCESSIVE. WE PROPOSE A NEW TYPE OF REACTION CHAMBER USING MAGNETIC ENHANCEMENT OFTHE PLASMA DEPOSITION IN COMBINATION WITH A COST EFFECTIVE ROLL-TO-ROLL MANUFACTURING PROCESS. ADDING A MAGNETIC FIELDCAUSES ELECTRONS IN THE PLASMA TO MOVE IN PATHS THAT REDUCE THE RATE AT WHICH THEY ESCAPE THE PLASMA OR COLLIDE WITH THE SUBSTRATE. THIS RESULTS IN A HIGHER DENSITY PLASMA AND LOWER PLASMA SHEATH POTENTIAL. THE HIGHER PLASMA DENSITY MEANS MORE CHARGED PARTICLES GENERATED WITH LOWER ENERGY PER PARTICLE, AND THE LOWER SHEATH POTENTIAL MEANS LESS VOLTAGE ACCELERATING POSITIVELY CHARGED PARTICLES TOWARD THE SUBSTRATE. THE COMBINATION OF THESE GIVES A HIGHER DEPOSITION RATE WITH LESS BOMBARDMENT DAMAGE TO THE GROWING FILM. THE REDUCED ENERGETICS OF THE GAS PHASE COLLISIONS IS ALSO EXPECTED TO REDUCE GAS PHASE NUCLEATION AND PINHOLE CAUSING DUST FORMATION. ATTAINING UNIFORM FILM THICKNESS USING MAGNETIC FIELD ENHANCEMENT WOULD BE VERY DIFFICULT FOR STANDARD, FIXED SUBSTRATE, DEPOSITION SYSTEMS BECAUSE OF THE DIFFICULTY IN ATTAINING A UNIFORM MAGNETIC FIELD OVER LARGE AREAS IN TWO DIMENSIONS. BECAUSE OUR CONTINUOUS ROLL-TO-ROLL PROCESS HAS THE SUBSTRATE ALWAYS MOVING, DEPOSITION RATE VARIATIONS AVERAGE OUT ALONG ONE AXIS SO THAT THE MAGNETIC FIELD ONLY NEED BE UNIFORM OVER ONE DIMENSION. IMPROVEMENTS IN DEPOSITION USING MAGNETIC ENHANCEMENT FOR THE I LAYER IN A-SI SOLAR CELLS WILL IMPROVE PRODUCT PERFORMANCE AND YIELD, THUS REDUCING MODULE COST.