Device for automatically determining the deviation between the structures of a pattern and those of an object compared therewith

In a device for automatically determining the deviation between the structures of a pattern and those of a reference object, the structures to be compared are imaged, in a superposed condition, on a measurement gap, a polarization characteristic being imparted to the individual image channels. By relative movement between the superposed object image and the measurement gap and the subsequent separation of the image channels in dependence upon their polarization characteristic, photometric signal curves are obtained, from which errors in the superposition of the structures are quantitatively determined. A measurement in two coordinate directions is possible by insertion of an image-rotating prism. In an observation beam path which is branched off, the polarization characteristic is converted into a color characteristic, so that an additional double-image presentation of the structure superposition is produced. The photometric scanning range can be made visible by back-illumination of the measurement gap and reflection into the observation beam path.

BACKGROUND OF THE INVENTION 
The invention relates to a device for automatically determining the 
deviation between the structures of a pattern and those of a reference 
object. 
The structures to be investigated with respect to their mutual deviation 
may be present, for example, on the masks which are customary in 
semiconductor production or also on the wafers produced by exposure 
processes. However, other microstructures can also be involved, which are 
applied to workpieces for the examination of their accuracy to gauge. 
The requirements imposed by the semiconductor industry on the accuracy of 
the measuring devices for research, development and production control 
increase to the extent that the masks and wafers become larger and their 
structures smaller. The important matter is to supervise the production 
process and to correct that process in due time, in order to achieve an 
optimal output. 
An economic process for the monitoring of the accuracy to gauge of masks 
and wafers from the current production is their comparison with a master 
mask. The mask comparison devices (e.g. LEITZ list item no. 810-109) 
developed for this purpose permit a visual observation of superposition 
errors between the structures and a visually monitored measurement of the 
superposition errors. By means of micrometer screws, the structures to be 
compared are displaced relative to one another until such time as their 
edges are disposed one above the other. This process is costly in terms of 
time, and is not free from subjective error effects caused by the person 
carrying out the measurement. 
From German Offenlegungsschrift No. 3,305,014, an arrangement is known, by 
means of which superposition errors can be determined automatically by a 
photometric measurement. As in the case of the visually monitored mask 
comparison devices, the structures to be compared are illuminated in 
complementary colors and are imaged when superposed. The superposed image 
of the structures is transferred into the plane of a measurement gap and 
moved relative to the latter. The energy of the light passing through the 
measurement gap is broken down into light-energy components of each 
respective one of the complementary colors, corresponding to the differing 
wavelengths. A signal curve is generated in each instance from the 
light-energy components, as a function of the travel coordinates of the 
image relative to the measurement gap. The edge location of the structures 
to be compared with one another is determined from these signal curves in 
accordance with processes, known per se, of photometry for the 
determination of structure widths, and the superposition error of the two 
structures relative to one another is computed therefrom by difference 
formation. This automatically operating measurement arrangement requires 
the generation of two separate image channels, which are distinguishable 
by complementary color recognition. 
While wavelength-dependent imaging differences in the two image channels 
can in part be disregarded in the visual observation of the mixed image, 
because of the limited spectral sensitivity and the resolving power of the 
eye, they are extremely important for the purposes of a photometric 
measurement, which is also intended to provide an increase in the accuracy 
of measurement. For the purposes of a separation, according to measurement 
technique, of the two image channels, the optical imaging systems must in 
the first instance be optimally corrected for the colors to be transmitted 
by them. However, this can be achieved only in the part of the imaging 
beam path in which the image channels extend separately. Accordingly, in 
the part of the imaging beam path in which the two image channels are 
transmitted while superposed it is necessary to find a compromise with 
regard to the chromatic correction. This is the more successful, the 
smaller is the wavelength difference of the two complementary colors. 
Thus, there is however, in turn, a difficulty on account of the color 
filters to be employed for the color separation. These have only a limited 
characteristic gradient, so that there is a certain spectral overlap 
region. For the purposes of the photometric measurement, this means 
cross-talk from one of the image channels into the other. 
Additional difficulties arise as a result of the differing spectral 
sensitivity of the receivers, which leads to a non-uniform signal-to-noise 
ratio in the two measurement channels. This renders the subsequent signal 
processing more difficult. 
A further serious disadvantage of the color recognition of the image 
channels is due to light-energy considerations, since the limitation of 
bandwidth necessarily restricts the utilized spectral range of the light 
source. The better is the color splitting generated by the cut-on filters, 
the greater are the light losses, which cannot be used for the 
measurement. In the case of rapid scanning of the structures, the light 
intensities remaining for the individual spectral regions are no longer 
sufficient to obtain a good signal-to-noise ratio. However, an increase in 
the total light intensity leads, in some cases, to local heating of the 
objects, which may lead to destruction of the structures. 
The contrast ratios determined by the object are particularly critical for 
the purposes of a color characteristic. Differing coloration of the 
structures, coating layers, mask materials, wafer layers etc. lead to 
differing color contrasts within the individual image channels and also in 
comparison of the two image channels with one another. 
Since, in the case of an automatic measurement, a facility for visual 
observation of the respective measurement region must also in all cases be 
possible, there are, in practice, additional restrictions with regard to 
the optimization of the individual effective parameters. Accordingly, the 
complementary colors red and green are usually selected for the color 
characteristic. 
SUMMARY OF THE INVENTION 
The object of the invention was accordingly to design a device for 
automatically determining the structure deviations in such a manner that 
it avoids the disadvantages associated with a color characteristic, and 
thus in particular permits an optimal utilization of the energy of the 
light source, creates the same imaging and photoelectric measurement 
conditions in the two image channels over the entire imaging path and 
permits a good separation of the image channels without light losses and 
the contrast properties of which are not disturbed by wavelength-selective 
properties of the objects. In addition to this, a visually perceptible 
complementary color representation of the structures should be produced, 
which has no effect on the photometric beam path, and moreover a visual 
examination of the photometric scanning region should be permitted. 
Furthermore, the device should permit a measurement in two coordinate 
directions, without the necessity to rotate the objects. 
The essential concept of the invention resides in the introduction of a 
polarization characteristic of the image channels, in place of the color 
characteristic. In this manner, it becomes possible to illuminate both 
image channels with light of the same spectral composition, so that any 
chromatic aberrations which may possibly be present act in a similar 
manner on both image channels. After splitting of the two image channels, 
a radiation of the same spectral composition is also supplied to the 
photoelectric receivers, so that receivers with the same spectral 
sensitivity can be selected. The direct optical coupling between the 
image-deflecting oscillating mirror and a position measuring system 
permits a very accurate determlnation of the relative position between 
object image and measurement gap. 
An optimal utilization of the light intensity present in the individual 
image channels is achieved if the polarization characteristic is 
introduced at the location of the combination of the image channels. 
Following the combinacion of the two image channels it is possible, by the 
insertion of a plurality of neutral divider surfaces, to produce a 
plurality of beam outputs, which can be employed for various 
representation purposes. In particular, it is possible to insert into the 
measurement beam path a prism known per se, which is intended for image 
rotation and by means of which a measurement according to two coordinate 
directions is made possible in a simple manner. Since, on rotation of the 
prism, the polarization vectors of the two image channels are no longer 
disposed perpendicular and parallel to the reflecting surfaces 
respectively, an elliptical polarization is created, which can, however, 
be compensated again by a phase plate, which is fitted at tne exit of the 
rotation prism and which is provided with an appropriate phase shift. 
It is possible to insert, in the beam outputs for the visual display, phase 
plates which generate a constant phase shift between the two polarization 
directions of the image channels. In cooperation with polarizing filters 
positioned downstream, interference effects which are known per se then 
give rise to complementary colors associated with the image channels. The 
color representation is dependent exclusively upon the phase shift 
introduced, and, in particular, has no effect whatsoever on the 
measurement beam path. 
An advantageous supplement to the device is obtained by the 
back-illumiation of the measurement gap. If the measurement gap is 
reflected via a triple prism into the beam path for visual observation, 
then the gap moves relative to the stationary structure. In this manner, 
the scanned measurement range can be monitored and possibly corrected by 
manual intervention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A mask 1 with specified structure elements or measurement marks and a mask 
copy 2 are disposed on an object stage, which is not shown in any greater 
detail and which is displaceable in the x direction and in the y 
direction. The mask and its copy are illuminated by light sources 3 and 4. 
The light sources emit white light. It is, of course, also poss ble to 
employ a single light source in place of two light sources and to direct 
its radiation onto the two objects by means of deflecting mirrors. The 
imaging of the objects takes place by means of objectives 5 and 6, as well 
as by tubular optical systems 7 and 8 which may possibly be necessary. The 
objectives 5 and 6 are connected to an autofocus device indicated by the 
arrows 9 and 10. In place of the transmitted light illumination shown, it 
is also possible, by means of the divider mirrors 11 and 12, for reflected 
light illumination to take place. 
By means of deflecting prisms 13, 14, 15, the two image channels are 
combined at the polarizing divider mirror 16. Both the glass paths and 
also the air paths in the two image channels up to combination at the 
divider mirror 16 are selected in such a manner that they correspond. The 
rays passing through the polarizing divider mirror 16 and the rays 
reflected at it are linearly polarized perpendicular to one another. The 
two image channels have thus received an identifying characteristic. 
Downstream, there are inserted into the superposed beam path two neutral 
divider mirrors 17, 18 and a totally reflecting mirror surface 19, by 
means of which three beam outputs 20, 21, 22 are created. The output 21 is 
the measurement beam path. The output 20 can be employed, for example, for 
a television representation, and the output 22 for visual observation 
through an eyepiece or in a projection on a ground glass plate. 
An Abbe-Konig prism 23 for image rotation is inserted into the measurement 
beam patn 21. At the beam output side of the prism, a phase plate 24 is 
fixedly connected to the prism 23. This may comprise, for example, a thin 
quartz plate, the optical axes of which are oriented, in the normal 
setting of the prism 23, parallel to the polarization vectors of the two 
image channels. Since a rotation of the prism 23 by 45.degree. is 
necessary in order to generate an image rotation of 90.degree., the 
reflecting surfaces of the prism in this position are no longer parallel 
and perpendicular to the polarization vectors of the two image channels 
respectively. Accordingly, both polarization vectors receive a phase 
shift, which, as has become evident in practice, corresponds to 
approximately .lambda.-4. The linear polarization has accordingly been 
converted approximately into a circular polarization. The phase plate 24, 
which is also rotated, is selected in such a manner that it cancels out 
the phase shift in the two image channels. In the continuation of the beam 
path, the two image channels then again have their original direction of 
polarization. 
The measurement beam path 21 is then conducted via an oscillating mirror 25 
to a measurement gap 26. The optical imaging system of the two image 
channels is coordinated in such a manner that a superposed image of the 
mask 1 and of the mask copy 2 is formed in the gap plane 26. By means of 
the lenses 27, 28, the measurement gap 26 is then imaged via an analyzer 
29 (polarizing divider mirror) on photoelectric detectors 30, 31. The 
analyzer 29 separates the measurement beam path according to the 
polarization directions associated with the two image channels. 
For the performance of the measurement, the mirror 25 oscillates about an 
axis which stands perpendicular to the plane of the drawing, so that the 
superposed image of the two structures is moved along periodically over 
the measurement gap 26. The electrical signal curves generated in the 
course of this procedure by the photoelectric detectors 30, 31 are passed 
to a processing circuit 32. 
A photoelectric path-measuring or angle-measuring device 33 which is known 
per se is coupled to the oscillating mirror 25. The coupling takes place 
in that the rear surface 34 of the oscillating mirror 25 is connected, in 
reflection, to the optical beam path of the measuring system 33. The 
signals of the measuring device 33 which correspond to the respective 
position of the oscillating mirror 25 are likewise passed to the 
processing circuit 32. A suitable path-measuring system 33 with a drive 
unit for the oscillating mirror 25 is described, for example, in patent 
application P No. 35 17 070.0 (our file reference A 2224/B 3972, received 
at the Patent Office on 11.05.85). 
The processing circuit 32 determines, from the signal curves associated 
with the image channels, respective values for the position of the edge of 
the measured structure. Difference formation gives the magnitude of the 
deviation between the superposed structures in one coordinate direction. 
For the examination of the scanned object region, back-illumination of the 
measurement gap 26 by means of a pilot illumination 35 and a mirror 36 
which can be swung into the measurement beam path is possible. The gap 
illumination is reflected, via the oscillating mirror 25, the rotating 
prism 23, the divider mirror 18, an auxiliary optical system 37 and a 
triple prism 38, after reflection at the divider mirror 17, for example, 
into the visual observation beam path 22. While the object image moves 
over the measurement gap in the measurement beam path, there is in the 
eyepiece observation a movement of the measurement gap over the stationary 
object image. Since, on rotation of the prism 23, the gap image is 
likewise rotated, the scanning of the object region in both coordinate 
directions can be examined. 
In order to generate a complementary color presentation, respective 
combinations of phase plate 39 and polarizing filter 40 are inserted into 
the beam paths 20 and 22. The mode of action by which this combination of 
components generates a color representation from a beam path with 
components which are linearly polarized perpendicular to one another is 
known per se. The polarization vectors of the filter 40 must be oriented 
parallel to the polarization directions of the polarizing divider mirror 
16. If, for example with the aid of the phase plate 39, for an average 
wavelength .lambda.-=550 nm from 2.lambda.- is then introduced, then, as a 
result of interferences, a red representation is obtained for one of the 
image channels and a green representation for the other image channel. 
Other combinations of complementary colors can also be selected by the 
selection of different phase shifts.