Unit magnification optical system with improved reflective reticle

A photolithographic projection optical system comprising:

• a source of exposure energy for generating a beam of energy;
• an optical element located in the path of said beam for receiving said beam of energy and passing only a portion of said beam therethrough;
• a refractive lens group located in the path of said portion of said beam for receiving and transmitting said portion of said beam of energy;
• a reticle element located in the path of said portion of said beam, said reticle element including a transparent portion having a uniform thickness and having a pattern on one surface thereof, and an unpatterned portion adjacent thereto, said pattern being made from a material having a high degree of reflectivity with respect to optical exposure energy for reflecting said portion of said beam striking said pattern at a first intensity level and means completely covering said pattern for reflecting and changing the phase of said portion of said beam striking said covering means at a second intensity level significantly lower than said first intensity level, said reticle element being positioned for permitting said portion of said beam to pass through said thickness and for reflecting said portion of said beam through said thickness, and said lens group to said optical element;
• said optical element receiving said reflected beams and including means for reflecting a portion of said reflected beams back through said lens group and said unpatterned portion of said reticle element to a surface to be imaged.

A photolithographic projection optical system including:

• a source of exposure energy for generating a beam of energy;
• a lens assembly for receiving and transmitting energy beams from said energy source, said lens assembly having a reticle on an end thereof farthest from said energy source and located in the path of said beam of energy, said reticle having a pattern thereon on a surface farthest from said energy source and an unpatterned portion adjacent thereto, said reticle being positioned for receiving said energy beam through said lens assembly and reflecting said pattern back through said reticle and said lens assembly, said reticle element including a transparent portion having a uniform thickness and having a pattern on one surface thereof, and an unpatterned portion adjacent thereto, said pattern being made from a material having a high degree of reflectivity with respect to optical exposure energy for reflecting said beam striking said pattern at a first intensity level and means completely covering said pattern for reflecting and changing the phase of said beam striking said covering means at a second intensity level significantly lower than said level, said recticle element being positioned for permitting said beam to pass through said thickness and for reflecting said beams at said first and second intensity levels through said thickness, and said lens assembly;
• a mirror for receiving said reflected beams from said lens assembly and reflecting said beams back through said lens assembly and said unpatterned portion of said reticle onto a surface to be imaged.