The overlay accuracy for the proximity aligner and the mirror projection aligner is poor in comparison to that of the contact aligner. The contact aligner has the highest throughput of all four methods. Both the contact aligner and the proximity aligner have the same advantages of low maintenance requirements and short learning curves for personnel. Throughput on the stepper is comparable to that of the mirror projection printer. The main disadvantages of the contact aligner are high defect density, low die yield, and short mask lifetime.
Small scratches left by the wafer during exposure cause a short mask lifetime.
Another major drawback is dust particles. A dust particle or a speck of silicon dust on the wafer can embed themselves into the wafer when the mask makes contact with the wafer. The imbedded particle causes permanent damage to the mask and results in defects with each succeeding wafer. The proximity aligner has a lower defect density than that of the contact printer, but higher than the other methods. Edge diffraction and surface reflection are also disadvantages of proximity printing causing distortion in the die pattern that is projected onto the wafer.