a conceptual understanding of the principles of the inventive alignment technique can be obtained from the apparatus shown in fig1 . typically , the inventive apparatus is part of a step and repeat photolithographic system . turning to fig1 semiconductor wafer 11 is located on movable table 12 . fresnel zone plate alignment mark 13 is located on a portion of wafer surface 14 which has a local tilt θ . illustratively , θ is on the order of 10 - 4 radians . radiation source 20 and mirror 21 serve as a means for directing incident radiation 22 onto the zone plate alignment mark . illustratively , incident radiation 22 is collimated parallel to optical axis 9 . radiation 22 is focused by the fresnel zone plate alignment mark into a plurality of focused real images located at prescribed distances q = f , f / 3 , f / 5 . . . in front of the wafer surface and a plurality of focused virtual images located at prescribed distances q =- f , - f / 3 , - f / 5 . . . behind the wafer surface . in fig1 distances in front of the wafer surface are taken as positive and distances behind the wafer surface are taken as negative . distances in front of the wafer and behind the wafer are measured from plane 30 which represents the average wafer surface . for a typical zone plate alignment mark , f is on the order of 300μ . two of the focused images formed by the zone plate alignment mark are shown in fig1 . the first is the real image , r , associated with the + f focal length and located a first prescribed distance q =+ f in front of the wafer surface . the second is the virtual image , v , associated with the - f focal length and located a second prescribed distance q =- f behind the wafer surface , r and v are equally and oppositely displaced from optic axis 9 . for purposes of alignment , it is advantageous to use these two images because they are brighter than the higher order real and virtual images which are not shown in fig1 . bifocal lens 25 is a means for projecting images r and v onto single photodetector means 26 to form first projected image r &# 39 ; and second projected image v &# 39 ;. it should be noted that while r &# 39 ; and v &# 39 ; are shown as points in fig1 they do have finite spatial extent and illustratively are gaussian spots . a bifocal lens is used to project r and v onto detector means 26 because r and v are not equidistant from surface 27 of detector means 26 . the bifocal lens reimages orthogonal polarization components of r and v onto surface 27 to form r &# 39 ; and v &# 39 ;, respectively . the use of orthogonal polarization components to form r &# 39 ; and v &# 39 ; is advantageous in that it prevents interference effects near surface 27 . ( see a . d . white u . s . pat . no . 3 , 990 , 798 , issued on nov . 9 , 1976 , for a discussion of a bifocal lens element .) it should be noted , however , that under appropriate circumstances means other than a bifocal lens can be used to project r and v onto the detector surface . photodetector means 26 , along with comparator circuit 28 , which is responsive to signals generated by detector means 26 , form a position sensing means adapted to determine when first projected image r &# 39 ; and second projected image v &# 39 ; are displaced from positions preselected for each of the projected images by amounts substantially proportional to the first prescribed distance q = f ( distance from r to wafer surface ) and the second prescribed distance q =- f ( distance from v to wafer surface ), respectively . in the illustrative example of fig1 the same location p is preselected for each of the two projected images . thus , alignment is achieved in accordance with the principles of the present invention for the illustrative case shown in fig1 when v &# 39 ; and r &# 39 ; are substantially equally and oppositely displaced from preselected location p . this is because focused images r and v are located equal and opposite prescribed distances from the wafer surface . this alignment condition is valid independent of the amount of local wafer tilt . it should be noted that the preselected location p is chosen so that in the absence of local wafer tilt the two projected images r &# 39 ; and v &# 39 ; would coincide with p . if the above - mentioned alignment condition is not satisfied , micropositioner 13 and movable table 12 form a moving means , responsive to output from comparator circuit 28 , for moving the wafer until the alignment condition is fulfilled . advantageously , photodetector means 26 is a four - quadrant photodetector arrangement . generally , the preselected position p is at the origin of the four - quadrant arrangement . if r &# 39 ; and v &# 39 ; are gaussian spots , they are substantially equally and oppositely displaced from the preselected position p if the radiation intensities in the first and third quadrants are substantially equal and if the radiation intensities in the second and fourth quadrants are substantially equal . standard comparator circuit 28 is adapted to determine when this condition is satisfied . it should be noted that the arrangement of fig1 is merely illustrative of the principles of the present invention and other arrangements may be used to implement those principles . for example , instead of using the real image associated with the + f focal length and the virtual image associated with the - f focal length , other pairs of images formed by the fresnel zone plate alignment mark can be used to carry out the alignment process . in addition , instead of projecting the first and second focused images onto a single photodetector means , they can be projected onto first and second separate photodetector means , respectively , to form the first and second projected images . in this case a position on the first photodetector means is preselected for the first projected image and a position on the second photodetector means is preselected for the second projected image . an alternative system which can be used to practice the principles of the present invention is shown in fig2 . numerical parameters stated in connection with the embodiment of the invention shown in fig2 are intended to be illustrative only and are not intended to limit the scope of the claimed subject matter . turning to fig2 fresnel zone plate alignment mark 31 is located on surface 32 of semiconductor wafer 33 . the wafer is located on movable table 500 . typically , the apparatus of fig2 is part of a step and repeat photolithographic system and the mask whose pattern is to be transferred onto the wafer surface lies in reticle plane 100 . in fig2 radiation is directed at the zone plate alignment mark as follows . light from monochromatic source 301 is focused by lens 302 at location a . the radiation focused at a is directed by lens 304 into optical means 305 . optical lens 305 is adapted to separate the radiation directed therein into first and second radiation components having first and second orthogonal polarization states , respectively , and to introduce a path length difference in the optical paths traveled by the first and second radiation components , respectively , whereby the first radiation component is focused at location b and the second radiation component is focused at location c . the optical path of the first radiation component ( perpendicularly polarized ) is shown by lines 310 and the optical path of the second radiation component ( parallel polarized ) is shown by lines 320 . in the illustrative embodiment of the invention shown in fig2 optical lines 305 comprises polarizing beam splitters 400 and 410 and prism 420 . beam splitter 400 comprises right angle prisms 401 and 402 which meet to form polarizing interface 403 . similarly , beam splitter 410 comprises right angle prisms 411 and 412 which meet to form polarizing interface 413 . radiation incident on means 305 , which is polarized parallel to plane 450 ( plane of fig2 ), is transmitted by interfaces 403 and 413 . the parallel polarized radiation ( radiation 320 ) is reflected by mirror 470 and brought to a focus at point c . radiation incident on means 305 , which is polarized perpendicular to plane 450 ( radiation 310 ), is reflected at interface 403 , directed through prism 420 , and reflected again at interface 413 . illustratively , the perpendicularly polarized radiation is brought to a focus at b . it should be noted that the difference in optical path lengths traveled by the two orthogonal radiation components can be adjusted by changing the relative positions of beam splitters 400 and 410 and prism 420 . illustratively , locations b and c are chosen so that photolithographic lens 306 , which has a magnification m , is able to bring radiation from location b to a real focus at point d a distance q = 2f above the wafer surface and bring radiation from location c to a virtual focus at point e a distance q =- 2f below the wafer surface . typically , the difference in optical path lengths traveled by the two orthogonal radiation components is on the order of 12 cm , photolithographic lens 306 has a magnification , m , of about 10 , and fresnel zone plate 11 has a focal length f of about 300μ . in the absence of wafer tilt and in the absence of a nonuniform resist thickness , the zone plate then forms real and virtual images , r and v , which are almost coincident with d and e , respectively . ( this is easily understood by an application of the lens equation to the present geometry in which the objects are respectively a distance + 2f and - 2 f from the alignment mark which has focal lengths of + f and - f . higher order real and virtual images formed by the alignment mark from the incident radiation are not utilized in the illustrative embodiment of the invention shown in fig2 .) actually , because a knife edge is used to separate the incident and reflected radiation in the illustrative embodiment of the invention shown in fig2 locations d and e are both displaced slightly above ( below ) the plane of fig2 and r and v are both displaced slightly below ( above ) the plane of fig2 . after passing through lens 306 and optical means 305 , knife edge 312 and lens 313 are used to project the radiation from r and v onto surface 311 of detector 310 to form a pair of projected images r &# 39 ; and v &# 39 ;. in the case of no local wafer tilt and nonuniform resist thickness , r &# 39 ; and v &# 39 ; coincide . the reason for this is that r and v are coaxial and have no lateral displacement in the absence of local wafer tilt and nonuniform resist thickness . alignment is achieved when r &# 39 ; and v &# 39 ; coincide with position p which is preselected for each of the projected images . in the presence of local wafer tilt and / or nonuniform photoresist thickness , the real and virtual images , r 1 and v 1 , formed by alignment mark 31 of fig2 are equally and oppositely displaced from the positions occupied by r and v in the absence of local wafer tilt and nonuniform resist thickness . in this case , projections of r 1 and v 1 on surface 311 are located at r 1 &# 39 ; and v 1 &# 39 ;, respectively . alignment is achieved when r 1 &# 39 ; and v 1 &# 39 ; are equally and oppositely displaced from preselected position p . if the alignment condition is not satisfied , signals from comparator circuit 510 cause micropositioner 520 to move table 500 , on which the semiconductor wafer lies , until the wafer is positioned so that the alignment condition is satisfied . it should be noted that in the embodiment of the invention shown in fig2 r 1 &# 39 ; and v 1 &# 39 ; are formed from radiation having orthogonal polarization states to prevent interference effects near the detector surface . alternatively , interference effects may also be prevented by using radiation of two different frequencies to form real image r 1 and virtual image v 1 . finally , it is to be understood that the above - described arrangements and procedures are merely illustrative of the many possible specific embodiments which can be devised to practice the principles of the present invention . numerous and varied other arrangements can be devised in accordance with these principles by those skilled in the art without departing from the spirit and scope of the following claims .