Integrated metrology for microlithographic objective reducing lens

Metrology for a microlithographic objective reducing lens 10 is integrated into a metrology block 20 secured to a lower region of the barrel 15 for lens 10. Block 20 has a central opening 27 around a lowermost element 11 of lens 10; and block 20 mounts distance detectors 28 around central opening 27, a pair of microscope objectives 30 and 31, and a pair of X and Y mirrors 40 and 41.

BACKGROUND 
Microlithographic objective reducing lenses are used for reducing and 
imaging patterns for electronic microcircuits on semi-conductor wafers. 
The microlithography industry seeks ever-greater accuracy in the 
registration and imaging to facilitate miniaturizing the circuitry and to 
preserve accuracy and reliability in the final product. Hence, any 
substantial increase in accuracy for microlithographic objective reducing 
lenses is a welcome advance in this art. 
Metrology for microlithographic objective reducing lenses is necessary for 
referencing the wafer relative to the lens to ensure that the desired 
interrelationship between the lens and the wafer is maintained as 
accurately as possible. Six degrees of freedom of relative motion are 
possible and include X and Y registration in the plane of the wafer 
surface, Z axis distance between the lens and the wafer, tip and tilt 
angles between the lens and wafer, and the rotational angle of the wafer 
around the Z axis of the lens. Computers are programmed to control the 
movements involved, and detectors or sensors for gathering information on 
the position of the wafer relative to the lens are included within the 
metrology components. These have previously been mounted on a frame or 
housing surrounding the barrel of the lens, and they have included such 
elements as microscope objectives for registering with location marks on 
the wafer surface, mirrors for reflecting the beams of fringe-counting 
interferometers, and a grazing incidence interferometer for detecting the 
Z axis distance between the wafer surface and the lens. 
We have discovered a better and more accurate way of integrating the 
metrology components with a microlithographic objective reducing lens. Our 
metrology arrangement improves the attainable stability and accuracy, 
which we accomplish partly by integrating the metrology components with 
the barrel of the lens to eliminate instabilities caused by the previous 
metrology frames or housings around the lens barrels. We have also 
selected and arranged the metrology components that we integrate into the 
lens barrel to minimize distances between the lens and its metrology 
components and to ensure that everything cooperates for enhancing 
accuracy. 
SUMMARY OF THE INVENTION 
Our metrology for a microlithographic objective reducing lens is integrated 
into a block secured to a lower region of the barrel for the lens. The 
block has a central opening around a lowermost element of the lens, and 
the block is otherwise securely mounted on the lens barrel to ensure an 
accurate relation between the block and the lowermost lens element. The 
metrology components integrated into the block include distance detectors 
arranged on the block to face downward around the central opening, a pair 
of microscope objectives mounted on the block outside the central opening, 
and a pair of X and Y mirrors also mounted on the block. All these 
components are then spaced only a short distance from the lowermost lens 
element in positions that are stabilized by the block to improve the 
accuracy of the metrology system based on these components.

DETAILED DESCRIPTION 
Microlithographic objective reducing lens 10 is shown partially cut away 
and emptied of all but the lowermost element 11 of its many lens elements, 
which are carefully assembled with the highest possible accuracy into 
generally cylindrical lens barrel 15. Some parts have been eliminated for 
simplicity, and the controlled environment in which lens 10 operates is 
also not illustrated. 
The metrology used to control the positional interrelationships, between 
wafer 13 and lens 10 enjoys better accuracy, we find, when it is closely 
and accurately associated with lowermost surface 12 of lens element 11. 
To accomplish this, we have devised metrology block 20 to contain and mount 
metrology components and to fasten directly and securely to a lower region 
of lens barrel 15. The metrology components that we prefer include a 
capacitive distance detector 25 having four elements 28, a pair of 
microscope objectives 30 and 31, and X and Y reference mirrors 40 and 41. 
We prefer that all these metrology components be mounted directly on 
metrology block 20, as explained more fully below, so that they are all 
within the outside diameter of lens barrel 15 where they are closely and 
accurately associated with lowermost element surface 12. Although we 
prefer capacitive distance detector 25 for metrology block 20, alternative 
distance detectors such as pneumatic gauging or optical transducers could 
also be used for measurements in the Z axis direction. Also, it is 
desirable, but not required, that metrology components on block 20 be 
mounted within the outside diameter of lens barrel 15. 
We prefer forming metrology block 20 in two main parts that include a 
generally circular ring 21 fastened to a lower region of lens barrel 15 
and an annular sensor plate 22 fastened to ring 21. Three screws 23 secure 
ring 21 to lens barrel 15, and ring 21 has small locator feet 24 
surrounding each of the screws 23 for accurately locating the plane of 
ring 21 relative to lens barrel 15. Three screws 26 fasten sensor plate 22 
to ring 21 and locate the plane of plate 22 relative to ring 21 in a 
similar way. Ring 21 and plate 22 can also be machined as a single unit. 
We also prefer that metrology block 20 be formed of a dimensionally stable 
material, such as a ceramic or an Invar alloy of nickel and iron. The 
material of block 20 should be dimensionally stable within one millionth 
of an inch during an indefinite lifetime in a controlled environment. 
Dimensional stability to maintain the positional relationships of the 
metrology components to lowermost lens element 11 is more important than 
any specific shape or dimensions. 
Plate 22 has a central opening 27 around lowermost element surface 12, and 
capacitive distance detector 25, with its elements 28, is preferably 
arranged on a downward facing surface of plate 22 around central opening 
27 and within guard ring 19. This disposes distance detector 25 in as 
close a relationship as possible with lowermost element surface 12 for 
measuring the distance to the upper surface of wafer 13. Since room is 
available, some of the electronics 29 for capacitive distance detector 25 
are mounted on sensor plate 22 as schematically shown. 
Microscope objectives 30 and 31 are threaded into mounts 32 that are 
arranged in holes in sensor plate 22. Each mount 32 is held in place by 
collars 33 on screws 34 so that objectives 30 and 31, and their 
beam-reflecting prisms 35, are rotatable relative to plate 22 for 
alignment purposes. Objectives 31 and 32 are also vertically adjustable 
relative to plate 22 by means of shims or spacers 38. Bore holes 36 and 37 
through ring 21 afford passageways through which beams from the microscope 
objectives are transmitted outward. We direct these beams to video 
cameras, either through a conventional optical transmission path outside 
of lens barrel 15 or via fiber optic cables. Microscope objectives 30 and 
31 are preferably arranged within the inside diameter of lens barrel 15 
and outside of central opening 27. We prefer using two microscope 
objectives spaced diametrically on opposite sides of capacitive distance 
detector 25, but microscope objectives can also be arranged elsewhere. In 
our positioning of objectives 30 and 31, they are spaced only an inch and 
a half from the lens axis and only three inches apart. This improves on 
the stability attainable with microscope objectives positioned outside of 
lens barrel 15 at a greater distance from the lens axis. 
We prefer mounting X and Y mirrors 40 and 41 on respective flat surfaces 42 
and 43 formed on ring 21 perpendicular to each other. Each mirror 40 and 
41 mounts on a three-point stand 45, two points of which are adjustment 
screws 46 surrounded by spring washers 47 and threaded into ring 21. The 
other point for each stand 45 is a ball bearing 48 held between ring 21 
and stand 45 by a spring clip 49. Reference mirrors 40 and 41 can be 
accurately aligned relative to the beams of fringe-counting 
interferometers by adjusting screws 46. 
The preferred metrology components, as arranged in metrology block 20, 
provide information for all six degrees of freedom of relative motion 
between lens 10 and wafer 13. Microscope objectives 30 and 31 can be 
registered with location marks made on an upper surface of wafer 13, and 
we prefer registering one microscope objective with its respective 
location mark and then rotating the wafer relative to the lens to register 
the other mark with the other microscope objective. This establishes an X, 
Y reference that is also correct for rotational angle. The microscope 
objectives preferably have a field of view larger than the location marks 
to facilitate this registration, which is preferably made visible on a 
video monitor. From the reference position, the wafer is movable under the 
lens in the X and Y directions under control of fringe-counting 
interferometers that use mirrors 40 and 41 as reference mirrors and use 
similar mirrors (not shown) on the wafer-moving stage as test mirrors. By 
this means, each increment of motion of wafer 13 relative to lens 10 is 
measured in both X and Y directions. 
Capacitive distance detector 25 is highly accurate in measuring the 
distance to the upper surface of wafer 13. Since distance detector 25 has 
at least three and preferably four elements 28, these can detect tip and 
tilt angles between lens 10 and the top surface of wafer 13. It is even 
possible for our metrology to scan and map the topography of the top 
surface of wafer 13 before imaging the wafer so as to make adjustments 
based on knowledge of the topography during the imaging process. We prefer 
that the plane of distance detector 25 be within about one-half a 
millimeter of wafer 13 for a working range at which a capacitive distance 
detector is highly accurate. Other distances may apply for distance 
detectors operating pneumatically or with optical transducers. 
The close association of distance detector 25 with lowermost element 
surface 12 assures that distance and tip and tilt angle measurements are 
made more accurately than with existing methods. The positioning of 
microscope objectives 30 and 31 close to element 11, and preferably within 
the inside diameter of lens barrel 15, improves their stability and 
long-term accuracy relative to prior art locations spaced outside of lens 
barrel 15. Locating reference mirrors 40 and 41 directly on the machined 
rim of metrology block 20, and preferably within the outside diameter of 
lens barrel 15, also assures a simple, direct, and highly accurate 
relationship between these mirrors and lens element 11. The dimensionally 
stable support that metrology block 20 gives to all these components helps 
maintain the accuracy they can attain. 
By virtue of these measures, our metrology arrangement substantially 
improves the accuracy of the positioning of our microlithographic 
objective reducing lens relative to the wafer, compared to the previously 
attainable accuracy. Our accuracy improvement is by an order of magnitude; 
and at the same time, our metrology arrangement is simpler and less 
expensive than prior art metrology systems arranged outside of lens barrel 
15.