Method of and device for repetitively imaging a mask pattern on a substrate using five measuring axes

A method is described for repetitively imaging a mask pattern, on separate fields of a substrate (W), for example, for IC manufacture, which substrate fields are positioned without any field-by-field alignment so that the speed of throughput of substrates can be increased. An accurate interferometer system (50, 100, 150) having five measuring axes (MAX.sub.1, MAX.sub.2, MAX.sub.3, MAX.sub.4, MAX.sub.5) is also described, which system is intended in the first instance for use in an apparatus for performing the method, but which can also be used in a more general way in those cases where an object must be measured in five degrees of freedom.

BACKGROUND OF THE INVENTIONS 
The invention relates to a method of repetitively imaging, by means of a 
projection lens system, a mask pattern present in a mask plate each time 
on a different sub-area of an area on a substrate arranged on a substrate 
support, the mask pattern and the substrate being accurately positioned 
with respect to each other while using two mask alignment marks located in 
the mask plate outside the mask pattern and at least two substrate 
alignment marks located on the substrate outside said area, said 
positioning being realised by: 
imaging mask alignment marks and substrate alignment marks onto each 
another by means of the projection lens system, 
observing the extent of overlap between an alignment mark image and the 
alignment mark on which the image must be formed, 
displacement along a first (X) axis and a second (Y) axis of a three-axis 
system of coordinates and possibly rotation about the third (Z) axis of 
the system of coordinates of the mask pattern and the substrate relative 
to each other. 
The invention also relates to a device for performing the method. 
A method and device of this type are described in U.S. Pat. No. 4,778,275 
which relates to an apparatus for repetitive and demagnified imaging of a 
mask pattern, for example, the pattern of an integrated circuit (IC) on 
one and the same substrate, in which between two successive imaging steps 
the mask pattern and the substrate are moved with respect to each other, 
for example, along two mutually perpendicular directions in a plane 
parallel to the substrate plane and the mask plane. 
Integrated circuits are manufactured by means of diffusion and masking 
techniques. A mask having a first mask pattern is imaged on a number of, 
for example, 100 sub-areas of a substrate area. Subsequently the substrate 
is removed from the projection apparatus so as to subject it to the 
desired physical and/or chemical process steps. The substrate is then 
introduced into the same or another projection apparatus so as to expose 
the different sub-areas with a mask having a second mask pattern, and so 
forth. It must be ensured that the images of the mask patterns are 
accurately positioned with respect to the sub-areas of the substrate. 
Diffusion and masking techniques can also be used when manufacturing other 
structures having detailed dimensions of the order of micrometers. 
Structures of integrated optical systems or conductance and detection 
patterns of magnetic domain memories and a structure of liquid crystal 
picture display panels are examples. Also in the manufacture of these 
structures the images of mask patterns must be very accurately aligned 
with respect to a substrate. 
In connection with the large number of electronic components per unit of 
surface area of the substrate and the attendant small dimensions of these 
components, increasingly stricter requirements are imposed on the accuracy 
with which integrated circuits are manufactured. The positions of the 
images of the successive mask patterns on the substrate must therefore be 
fixed more and more accurately. 
To realise the desired very great positioning accuracy, for example, within 
several tenths of a micrometer, the apparatus described in U.S. Pat. No. 
4,778,275 comprises a device for aligning the substrate with respect to a 
mask pattern. With this system an alignment mark provided in the substrate 
and an alignment mark provided in the mask outside the mask pattern are 
imaged on each other and the mutual positions of the marks are determined. 
If the image of the one alignment mark coincides with the other alignment 
mark, the mask pattern is satisfactorily aligned with respect to the 
substrate at the location of the substrate alignment mark. The main 
element for imaging a substrate mark and a mask mark onto each other is 
constituted by the projection lens system with which the mask pattern is 
projected on the substrate. 
Initially, two and possibly several further substrate alignment marks 
outside the substrate area which must be repetitively illuminated with the 
mask pattern are aligned with respect to the two mask alignment marks by 
means of the alignment system. This alignment is known as the global 
alignment of the substrate. 
To enable imaging of the mask pattern on the different sub-areas, the 
substrate is displaced with respect to the mask in its own plane along two 
mutually perpendicular axes, the X and Y axes of a system of coordinates. 
The device described in U.S. Pat. No. 4,778,275 comprises a composite 
interferometer system with which the X and Y displacement and the rotation 
about the Z axis (.phi..sub.z) of the substrate can be measured. This 
interferometer system is also referred to as a three-axis interferometer 
system. The information of the interferometer system is processed together 
with that of the alignment system so that, when it is ascertained that a 
substrate alignment mark has been aligned with respect to a mask alignment 
mark, it is also known where the substrate alignment mark is located in 
the two-dimensional position matrix which is defined by the interferometer 
system. 
It is not sufficient to use said global alignment of the substrate because 
the position of each individual substrate sub-area cannot be determined 
with sufficient accuracy. Moreover, the projection lens system has a small 
depth of focus and the substrate may exhibit unevennesses so that 
defocused images which may later result in defects in the manufactured ICs 
may be produced at the areas of these unevennesses. Therefore it is 
preferred to measure, before exposure of each substrate sub-area, whether 
the substrate surface at the location of this sub-area is sufficiently 
horizontal or sufficiently parallel to the image field and a possible 
correction is performed by tilting the substrate about the X and/or Y 
axis. This is known as die-by-die levelling. By locally levelling the 
substrate, which is realised by tilting the substrate holder, and thus the 
mirrors of the interferometer attached to the substrate holder, errors may 
arise in the interferometer signals. Finally, unintentional and 
uncontrolled tilts of the substrate holder about the X and/or Y axis may 
occur, which tilts of course also affect the interferometer signals. To 
enhance the positioning accuracy of the substrate sub-areas and mitigate 
the consequences of the tilts, it has been proposed for known projection 
apparatuses, inter alia, the apparatus described in U.S. Pat. No. 
4,778,275, to add a separate alignment mark to each sub-area. In addition 
to global alignment of the substrate, there is also alignment per sub-area 
after local levelling. The latter is known as field-by-field alignment. 
However, the field-by-field alignment requires a considerable quantity of 
extra time so that the substrate throughput of the projection apparatus, 
i.e. the number of substrates which can be processed per unit of time, is 
reduced, which is an essential drawback as has been found by the 
Applicant. Moreover, the sub-area marks cover a part of the substrate 
surface to be processed so that the number of sub-areas which can be 
exposed with the mask pattern and hence the number of ICs per substrate 
will be smaller. This also holds true, though to a lesser extent, if the 
sub-area alignment marks are reduced in size. However, the mask alignment 
marks and the alignment marks located outside the substrate area to be 
illuminated must then also be reduced in size, with the result that the 
alignment is less accurate. 
SUMMARY OF THE INVENTION 
The present invention has for its object to eliminate said drawbacks. In 
accordance with a first aspect of the invention a novel method is provided 
which is characterized in that each individual substrate sub-area is 
positioned with respect to the mask pattern without any further alignment 
and by displacing only the substrate very accurately along at least one of 
said X and Y axes, in that not only the actual displacement along the X 
and Y axes and the rotation about the Z axis of the substrate but also 
tilts of the substrate about the X and Y axes are measured with respect to 
fixed references and in that all measuring results are used to realise the 
ultimate positioning of the relevant subarea in the X-Y plane. 
This method realises a breakthrough concerning the substrate throughput 
through an projection apparatus. It is based on the recognition that a 
considerable gain in time can be achieved by transferring a part of the 
task of the alignment system to the positioning of the substrate support. 
This task transfer has become possible because the Applicant has succeeded 
in enhancing the positioning accuracy of the substrate support, and 
because now the substrate tilts due to the local levelling of the 
substrate are taken into account during the X-Y positioning. 
For performing the novel method use can be made of a known three-axis 
interferometer system, such as the system described in U.S. Pat. No. 
4,655,594, which Patent only deals with the control and displacement of 
the substrate holder and does not deal with an alignment system, hence 
neither with the cooperation between this system and the interferometer 
system. 
As described in U.S. Pat. No. 4,655,594 the chief rays of the 
interferometer beams must be located in the substrate plane when using a 
three-axis interferometer system, which means that the mirrors cooperating 
with the interferometer system should project beyond the substrate plane. 
Then problems may occur in connection with the available space between the 
substrate and the projection lens system and with the sufficiently 
accurate manufacture of the relatively large mirrors. Moreover, such large 
mirrors considerably increase the weight of the element to be displaced so 
that the accuracy and the speed with which the substrate support can be 
positioned is reduced. This is also a reason why an alternative solution 
of enlarging the mirror holder in such a way that the mirrors always 
remain outside said space is not a good solution. Another reason is that 
the direction of the interferometer beams should be even more accurately 
fixed if the mirrors are further remote from the centre of the image plane 
of the projection lens system. 
An essential improvement of the above-mentioned aspects is obtained with a 
preferred embodiment of the method which is characterized in that a 
composite five-axis interferometer system is used for measuring the 
displacements, the rotation and the tilts of the substrate. 
A five-axis interferometer system comprises five measuring axes and five 
detectors whose output signals can be combined to 
an X position signal 
an Y position signal 
a signal .phi..sub.x indicating the tilt about the X axis 
a signal .phi..sub.y indicating the tilt about the Y axis 
a signal .phi..sub.z indicating the rotation about the Z axis. 
The result of the tilt measurements can be used in two ways so that there 
are two embodiments of the method. A first preferred embodiment in which a 
local levelling is performed for each substrate sub-area is characterized 
in that the result of the tilt measurements is used to correct the result 
of the displacement measurements. 
A second embodiment of the method is characterized in that the result of 
the tilt measurements is used to level the substrate. 
In addition to said methods, the invention also relates to a device for 
performing the methods, hence a device which is a part of a projection 
apparatus for repetitively imaging a mask pattern on a substrate, in which 
apparatus a so-called levelling detection device is present and in which 
the substrate is locally levelled. 
The inventive idea may, however, also be used in a similar device which is 
not combined with a levelling device and which is intended to measure and 
position an object in five degrees of freedom. Examples of similar devices 
are a device for measuring on separate substrates and masks, a device for 
positioning a mask table in an apparatus in which a pattern, for example, 
an IC pattern is written in a mask by means of a laser beam or an electron 
beam, a device for positioning a substrate table in an apparatus in which 
a mask pattern is projected on a substrate by means of X-ray radiation, 
and true measuring devices which are used at many places in industry. 
Each device according to the invention for accurately displacing and 
positioning an object, provided with an object table, an X-Y-.phi..sub.z 
drive for the object and an interferometer system for measuring 
displacements along an X axis and an Y axis and a rotation .phi..sub.z 
about the Z axis of a three-axis system of coordinates is generally 
characterized in that the interferometer system comprises five measuring 
axes for the extra measurement of tilts of the object about the X and Y 
axes and in that the interferometer mirrors are constituted by reflecting 
side faces of an object support incorporated in the object table for 
supporting the object in a fixed state. 
Since the object is coupled, as it were, optically rigidly with the 
interferometer system, this device provides the advantage that the 
movements of the object itself are measured and that the measuring signals 
are not affected by mutual movements of parts of the object table. 
The extra measuring signals in the form of tilt measuring signals are used 
in different manners in different embodiments of the device. 
A first embodiment of the device intended for use in an apparatus providing 
a local levelling facility, such as a projection apparatus for 
repetitively imaging a mask pattern on a substrate, is further 
characterized in that the interferometer measuring mirrors reach at most 
as far as the surface of an object support on which the object must be 
arranged and in that an interferometer signal processing unit is provided 
for converting all interferometer signals into control signals for the 
X-Y-.phi..sub.z drive. 
In this embodiment the tilt measuring signals are used to correct the X and 
Y displacement measuring signals and the rotation measuring signal for the 
tilts of the object, for example, a,substrate due to the local levelling 
of this substrate. 
The embodiment of the device intended for use outside an apparatus 
providing a local levelling facility is further characterized in that an 
interferometer signal processing unit is provided for converting the 
interferometer signals into control signals for the X-Y-.phi..sub.z drive 
and into control signals for actuators eliminating tilts of the object. 
An object can be very accurately manoeuvred at the desired X and Y 
positions by means of said devices without the displacement from a desired 
position to a subsequent position resulting in a tilt about the X or Y 
axis. 
With the interferometer system in which the wavelength of the 
interferometer beams is used as a standard it is possible to measure said 
displacements or tilts very accurately, for example, within 5 nm and 1/2 
microrad, respectively, provided that the optical properties of the medium 
in which the interferometer beams propagate remain constant. Due to 
variations of the ambient parameters, such as pressure, temperature, 
humidity and air composition, the refractive index of the medium may 
change so that the apparent wavelengths of the interferometer beams change 
and measuring errors may occur. 
To prevent this, the displacement and positioning device according to the 
invention is preferably further characterized in that the interferometer 
system has a sixth, reference, axis whose measuring beam cooperates with a 
stationary reflector. 
The variation of the refractive index of the medium can be measured by 
means of this extra measuring beam which is travelling in the same medium 
as the other interferometer beams. The measuring signal generated by means 
of the extra measuring beam is applied to the signal processing system so 
that the results of the X, Y, .phi..sub.x, .phi..sub.y and .phi..sub.z 
measurements can be corrected for changes of the refractive index. 
It is to be noted that it is known per se, for example, from European 
Patent Application no. 0,284,304 to use an extra beam in an interferometer 
system with which the position of a substrate table is measured so as to 
measure refractive index changes of the interferometer medium. However, 
this extra beam cooperates with a separate sub-system, namely a wavelength 
measuring system having an optical cavity which is closed at the front 
side and the rear side by a reflector and whose sides have apertures into 
and out of which the medium can flow. The extra beam is split into two 
sub-beams one of which is reflected at the front side and the other is 
reflected at the rear side. The phase difference between the reflected 
beams is a measure of the optical path length change in the cavity due to 
the refractive index change caused therein by variations in the medium. 
Very stringent requirements must then be imposed on the stability of the 
cavity. Moreover, this cavity is placed at some distance from the 
measuring beams of the interferometer system. Finally, the interferometer 
system is an X-Y system, i.e. a two-axis system. 
The article: "Ultra-precise mask metrology development and practical 
results of a new measuring machine" in SPIE vol. 1138 Optical 
Microlithography and Metrology for Microcircuit Fabrication (1989) pp. 
151-157 describes a measuring machine with an interferometer system for 
measuring the displacements of an object table in which in addition to the 
actual interferometer beams an extra beam is used for determining 
refractive index changes in the interferometer medium. The extra beam 
cooperates with a so-called etalon functioning as an extremely stable 
reference distance, similarly as the cavity described in European Patent 
Application no. 0,284,304. The interferometer system described in the SPIE 
article is also a two-axis system, not a five-axis system. Moreover the 
reflectors for the reference beams are arranged on the objective system of 
the measuring machine so that movements of this objective system with 
respect to the measuring table can be corrected. 
In order to enhance the measuring accuracy the device according to the 
invention may be further characterized in that an air shower for supplying 
a stream of air having a constant refractive index is provided above the 
space accommodating the interferometer beams. 
This does not only improve the optical quality of the interferometer 
medium, but it is also achieved that the medium has the same quality at 
the location of all measuring beams, including the possible reference-axis 
beam. The air which is blown in is preferably air of a very high degree of 
purity and of a very constant temperature. If the device is used in an 
apparatus for repetitively imaging on a substrate, this air can also be 
used to condition the space accommodating the substrate. 
It is to be noted that it is known per se, for example, from the article: 
"Wafer confinement for control of contamination in micro-electronics" in 
"Solid State Technology" August 1990, pp. S.sub.1 -S.sub.5 in the 
manufacture of integrated circuits to check the medium in the space 
accommodating the substrate accurately by rinsing this space with clean 
air of a constant temperature. However, this article does not describe an 
interferometer system for the substrate table, nor does it state anything 
about checking the interferometer medium. 
For each of the five measuring axes of the interferometer system the 
measuring beam and the reference beam should preferably be satisfactorily 
parallel to each other after they have been reflected by a mirror of the 
object table and by a reference mirror, respectively, and combined again 
by means of a beam splitter. However, due to tilts of the object table 
about the X, Y and X axes a measuring beam may acquire a direction 
different from that of the associated reference beam. Instead of one 
radiation spot, whose total intensity varies from a maximum to a minimum 
value when the object table is moved in the direction in which the 
measurement must take place, an interference pattern of light and dark 
strips is produced within the area of the spot at the location of the 
detector associated with this measuring beam, which strips are displaced 
when said object table moves. The movement of these strips and hence of 
the object table cannot be measured by the detector or can only be 
measured inaccurately. 
A preferred embodiment of the device is therefore further characterized in 
that in the path of the measuring beam of each measuring axis a 
retroreflector is arranged, which retroreflector passes said measuring 
beam, after a first reflection by a mirror of the object support, back to 
said mirror for a second reflection on said mirror. 
Due to this double reflection the original direction of the measuring beam, 
which direction is parallel to that of the associated reference beam, is 
maintained, irrespective of the tilts of the object support. 
It is to be noted that the use of a retroreflector for realising a twofold 
reflection of a measuring beam in an interferometer system is known per 
se, inter alia from the article "Linear/angular displacement 
interferometer for waferstage metrology" in: "SPIE vol. 1088 Optical/Laser 
Microlithography II" 1989, pp. 268-272. However, the interferometer system 
described in this article is not a five-axis system but a three-axis 
system. 
The device according to the invention is preferably further characterized 
in that the interferometer system comprises a first and a second 
interferometer unit, the first unit supplying the measuring beams for 
measuring along three measuring axes and the second unit supplying the 
measuring beams for measuring along two measuring axes. 
Due to this division of the interferometer system an optimum degree of 
space occupation and complexity is achieved. 
An interferometer unit is a constructive unit which comprises a plurality 
of polarization-sensitive or insensitive beam splitters, a plurality of 
polarization rotators and a plurality of detectors. 
A preferred embodiment of this device is characterized in that the 
measuring beam for the reference axis comes from the second interferometer 
unit. 
In this embodiment the required measuring beams are uniformly distributed 
over the interferometer units so that these units have the same 
construction as much as possible. 
To enhance the measuring accuracy, the device is preferably further 
characterized in that the reference mirror for the reference axis is 
fixedly connected to the second interferometer unit. 
In principle, any interferometer unit may have its own radiation source. 
However, the device is preferably further characterized in that the two 
interferometer units have a common radiation source. 
As a result, the device can be implemented in a simpler and less expensive 
manner. 
To obtain accurate measuring signals having high signal-to-noise ratios, 
the device may be further characterized in that the radiation source is a 
laser source supplying two beam comutually perpendicent frequencies and 
mutually perpendicular directions of polarization. 
A so-called heterodyne detection can then be used, which considerably 
contributes to the measuring accuracy. However, it is alternatively 
possible to use an interferometer system which utilizes the phase 
difference between a measuring beam and a reference beam, which beams are 
associated with one and the same measuring axis. 
Finally the invention relates to an apparatus for repetitively imaging a 
mask pattern on a substrate, which apparatus can work faster and more 
accurately than known apparatuses due to using the inventive concept. This 
apparatus, which comprises a mask holder, a substrate table with a 
substrate support, a projection lens system arranged between the mask 
holder and the substrate support, an alignment device for globally 
aligning the substrate with respect to the mask pattern, a levelling 
device for locally levelling the substrate and a displacement and 
positioning device for the substrate, is characterized in that the 
displacement and positioning device is a device as described hereinbefore 
which can successively be driven in a first mode, in which the substrate 
is globally positioned with respect to the mask pattern by means of the 
alignment and interferometer measuring signals, and in a second mode in 
which a sub-area of the substrate is positioned with respect to the mask 
pattern by means of the interferometer measuring signals only.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows the optical elements of an embodiment of an apparatus for 
repetitively imaging a mask pattern on a substrate. The main components of 
this apparatus are a projection column in which a mask pattern C to be 
imaged is provided and a movable substrate table WT with which the 
substrate can be positioned with respect to the mask pattern C. The 
apparatus further has an illumination system which comprises a radiation 
source LA, for example, a Krypton-Fluoride Excimer Laser, a lens system 
LS, a mirror RE and a condensor lens CO. The projection beam illuminates 
the mask pattern C which is present in the mask MA, which mask is arranged 
on a mask table MT. 
The beam PB passing through the mask pattern C traverses a projection lens 
system PL arranged in the projection column and shown only 
diagrammatically, which system forms an image of the pattern C on the 
substrate W. The projection lens system has a magnification of, for 
example, M=1/5, a numerical aperture NA=0.48 and a diffraction-limited 
image field having a diameter of 21.2 mm. The substrate is held by a 
substrate support WC which forms part of a substrate table WT which is 
only shown diagrammatically. 
The apparatus further comprises a plurality of measuring devices, namely a 
device for aligning the mask MA with respect to the substrate W in the XY 
plane, an interferometer system for determining the position and 
orientation of the substrate holder and hence of the substrate, and a 
focus error detection device for determining a deviation between the focal 
or image plane of the projection lens system PL and the surface of the 
substrate W. These measuring devices are parts of servosystems which 
comprise electronic signal processing and control circuits and drivers or 
actuators with which the position and orientation of the substrate and the 
focusing can be corrected with reference to the signals supplied by the 
measuring devices. 
The alignment device uses two alignment marks M.sub.1 and M.sub.2 in the 
mask MA, indicated in the top right corner of FIG. 1. These marks 
preferably consist of diffraction gratings, but they may be alternatively 
formed by other marks, such as squares or strips which are optically 
different from their surroundings. The alignment marks are preferably 
two-dimensional, i.e. they extend in two mutually perpendicular 
directions, the X and Y directions in FIG. 1. The substrate W, for 
example, a semiconductor substrate on which the pattern C must be imaged 
side by side several times, has a plurality of alignment marks, preferably 
also two-dimensional diffraction gratings, two of which, P.sub.1 and 
P.sub.2 are shown in FIG. 1. The marks P.sub.1 and P.sub.2 are located 
outside the areas on the substrate W where the images of the pattern C 
must be formed. The grating marks P.sub.1 and P.sub.2 are preferably phase 
gratings and the grating marks M.sub.1 and M.sub.2 are preferably 
amplitude gratings. 
FIG. 2 shows an embodiment of one of the two identical substrate phase 
gratings on a larger scale. Such a grating may comprise four sub-gratings 
P.sub.1,a, P.sub.1,b, P.sub.1,c and P.sub.1,d, two of which, P.sub.1,b and 
P.sub.1,d serve for alignment in the X direction and the two other ones, 
P.sub.1,a and P.sub.1,c serve for alignment in the Y direction. The two 
sub-gratings P.sub.1,b and have a grating period of, for example 16 .mu.m 
and the sub-gratings P.sub.1,a and P.sub.1,d have a grating period of, for 
example 17.6 .mu.m. Each sub-grating may have a dimension of, for example 
200.times.200 .mu.m. An alignment accuracy which is in principle smaller 
than 0.1 .mu.m can be achieved with this grating and a suitable optical 
system. Different grating periods have been chosen so as to enlarge the 
capture range of the alignment device. 
FIG. 1 shows a first embodiment of an alignment device, namely a double 
alignment device in which two alignment beams b and b' are used for 
aligning the substrate alignment mark P.sub.2 on the mask alignment mark 
M.sub.2, and the substrate alignment mark P.sub.1 on the mask alignment 
mark M.sub.1, respectively. The beam b is reflected by a reflecting 
element 30, for example, a mirror to the reflecting surface 27 of a prism 
26. The surface 27 reflects the beam b to the substrate alignment mark 
P.sub.2 which passes a part of the radiation as beam b.sub.1 to the 
associated mask alignment mark M.sub.2 where an image of the mark P.sub.2 
is formed. A reflecting element 11, for example, a prism is arranged above 
the mark M.sub.2, which prism directs the radiation passed by the mark 
M.sub.2 towards a radiation-sensitive detector 13. 
The second alignment beam b' is reflected by a mirror 31 to a reflector 29 
in the projection lens system PL. This reflector 29 passes the beam b' to 
a second reflecting surface 28 of the prism 26, which surface directs the 
beam b' onto the substrate alignment mark P.sub.1. This mark reflects a 
part of the radiation of the beam b' as beam b.sub.1 ' to the mask 
alignment mark M.sub.1 where an image of the mark P.sub.1 is formed. The 
radiation of the beam b.sub.1 ' passing through the mark M.sub.1 is 
directed to a radiation-sensitive detector 13' by a reflector 11'. 
The operation of the double alignment device will now be described with 
reference to FIG. 3 showing an embodiment of such a device which is 
distinguished from that shown in FIG. 1 by the different way of coupling 
the alignment beams b and b' into the projection lens system. There are 
two separate and identical alignment systems AS.sub.1 and AS.sub.2 which 
are positioned symmetrically with respect to the optical axis AA' of the 
projection lens system PL. The alignment system AS.sub.1 is associated 
with the mask alignment mark M.sub.2 and the alignment system AS.sub.2 is 
associated with the mask alignment mark M.sub.1. The corresponding 
elements of the two alignment systems are denoted by the same reference 
numerals, those of the system AS.sub.2 being distinguished from those of 
the system AS.sub.1 by their primed notation. 
The alignment system AS.sub.1 comprises a radiation source 1, for example, 
a Helium-Neon Laser which emits an alignment beam b. This beam is 
reflected towards the substrate W by a beam splitter 2. The beam splitter 
may be a partially transparent mirror or a partially transparent prism, 
but it is preferably a polarization-sensitive splitting prism followed by 
a .lambda./4 plate 3, in which .lambda. is the wavelength of the beam b. 
The projection lens system PL focuses the beam b in a small radiation spot 
V having a diameter of the order of 1 mm on the substrate W. This 
substrate reflects a part of the beam as beam b.sub.1 towards the mask M. 
The beam b.sub.1 traverses the projection lens system PL, which system 
images the radiation spot V on the mask. Before the substrate is arranged 
in the illumination apparatus, it has been prealigned in a prealignment 
station coupled to the apparatus, for example, the station described in 
European Patent Application no. 0,164,165, such that the radiation spot V 
is located on the substrate mark P2. This mark is then imaged by the beam 
b.sub.1 on the mask mark M.sub.2. The dimension of the mask mark M.sub.2 
is adapted to that of the substrate mark P.sub.2 whereby the magnification 
M of the projection lens system has been taken into account, so that the 
image of mark P.sub.2 accurately coincides with the mark M.sub.2 if the 
two marks are mutually positioned correctly. 
On its path to and from the substrate W the beams b and b.sub.1 have 
traversed the .lambda./4 plate 3 twice, whose optical axis extends at an 
angle of 45.degree. to the direction of polarization of the linearly 
polarized beam b emitted by the source 1. The beam b.sub.1 passing through 
the .lambda./4 plate then has a direction of polarization which is rotated 
90.degree. with respect to the beam b so that the beam b.sub.1 is passed 
by the polarization splitting prism 2. The use of the polarization 
splitting prism in combination with the .lambda./4 plate provides the 
advantage that coupling of the alignment beam into the radiation path of 
the alignment system is performed with minimal radiation loss. 
The beam b.sub.1 passed by the alignment mark M.sub.2 is reflected by a 
prism 11 and directed towards a radiation-sensitive detector 13 by, for 
example, a further reflecting prism 12. This detector is, for example, a 
composite photodiode having, for example, four separate 
radiation-sensitive areas in conformity with the number of sub-gratings 
according to FIG. 2. The output signals of these detectors are a measure 
of the coincidence of the mark M.sub.2 with the image of the substrate 
mark P.sub.2. These signals can be processed electronically and used for 
moving the mask with respect to the substrate by means of drive systems 
(not shown) in such a way that the image of the mark P.sub.2 coincides 
with the mark M.sub.2. An automatic alignment apparatus is thus obtained. 
A beam splitter 14 in the form of, for example, a partially transparent 
prism which splits a part of the beam b.sub.1 as beam b.sub.2 may be 
arranged between the prism 11 and the detector 13. The split beam b.sub.2 
is then incident via, for example, two lenses 15 and 16 on a television 
camera 17 which is coupled to a monitor (not shown) on which the alignment 
marks P.sub.2 and M.sub.2 are visible to an operator of the illumination 
apparatus. This operator can then ascertain whether the two marks coincide 
and move the substrate W by means of manipulators so as to cause the marks 
to coincide. 
Analogously as described hereinbefore for the marks M.sub.2 and P.sub.2, 
the marks M.sub.1 and P.sub.2 and the marks M.sub.1 and P.sub.1 can be 
aligned with respect to each other. The alignment system AS.sub.2 is used 
for the two last-mentioned alignments. 
Reference is made to U.S. Pat. No. 4,778,275 for details about the 
alignment procedure by means of the alignment systems. 
The embodiments of the alignment device according to FIGS. 1 and 3 are 
particularly suitable for an apparatus in which an illumination beam 
having a short wavelength, for example, 248 nm and an alignment beam 
having a considerably longer wavelength, for example, 633 nm are used. 
Since the projection lens system is designed for the wavelength of the 
projection beam PB, deviations occur when using this system PL for imaging 
the alignment marks P.sub.1, P.sub.2 and M.sub.1, M.sub.2 on each other by 
means of the alignment beams. The substrate alignment marks P.sub.1, 
P.sub.2 will not be imaged in the plane of the mask pattern in which the 
mask alignment marks are located but at a certain distance therefrom, 
which distance depends on the difference between the wavelengths of the 
projection beam and the alignment beams and the difference between the 
refractive indices of the material of the projection lens elements for the 
two wavelengths. If the projection beam has a wavelength of, for example 
248 nm and the alignment beam has a wavelength of 633 nm, this distance 
may be up to 2 m. Moreover, due to said wavelength difference, the 
substrate alignment mark is imaged on a mask alignment mark with a 
magnification which deviates from the desired magnification, with the 
deviation increasing with an increasing wavelength difference. 
To correct for said deviations, an extra lens, or correction lens 25 is 
incorporated in the projection column PL. The correction lens is arranged 
at such a height in the projection column that on the one hand the 
sub-beams of the different diffraction orders of the alignment beam, which 
sub-beams are formed by a substrate alignment mark, are sufficiently 
separated in the plane of the correction lens so as to be able to 
influence these sub-beams separately, and on the other hand this 
correction lens has a negligible influence on the projection beam and the 
mask image formed therewith. The correction lens is preferably arranged in 
the Fourier plane of the projection lens system. If the correction lens 25 
is arranged in a plane in which the chief rays of the alignment beams 
b.sub.1 and b.sub.1 ' intersect each other, as is shown in FIGS. 1 and 3, 
this lens can be used for correcting the two alignment beams. 
In principle, the alignment device according to FIGS. 1 and 3 operates 
satisfactorily, but under certain circumstances small alignment errors may 
still occur. The Applicant has found that these alignment errors result 
from phase differences within the selected alignment beam portions 
captured by the detector 13 or 13', which phase differences occur if the 
symmetry axis of the alignment beam portions coming from a substrate 
alignment mark is not perpendicular to the mask plate so that false 
reflections may occur within this plate. To avoid this problem, the 
Applicant has already proposed to arrange a wedge or another deflection 
element in the proximity of a mask alignment mark. 
FIG. 4 shows an embodiment of the alignment device with two such wedges 
WE.sub.1, WE.sub.2 having wedge angles .phi..sub.WE,2, .phi..sub.WE,2. 
These wedges ensure that the symmetry axes of the alignment beams b.sub.l 
', b.sub.1 are perpendicularly incident on the mask plate. 
As is shown in the Figure, the substrate alignment marks P.sub.2 and 
P.sub.1 may be illuminated by separate illumination systems IS.sub.1 and 
IS.sub.2. Each illumination system comprises a radiation source 1 (1'), 
two lenses 60, 62 (60', 62') and an adjustable plane-parallel plate 61 
(61') with which a fine adjustment of the direction of the beam b (b') can 
be realised. The lenses 60 and 62 ensure that the quality of the image of 
the source 1 (1') is maintained. Of the projection lens system only the 
lens group under the Fourier plane is diagrammatically shown in FIG. 4 by 
means of a single lens element PL.sub.1. 
The projection apparatus further comprises a focus error detection system 
for determining a deviation between the focal plane of the projection lens 
system PL and the surface of the substrate W so that this deviation can be 
corrected, for example, by moving the projection lens system along its 
axis. This system may be constituted by the elements 40, 41, 42, 43, 44, 
45 and 46 which are arranged in a holder (not shown) which is fixedly 
connected to the projection lens system. The reference numeral 40 denotes 
a radiation source, for example a diode laser emitting a focusing beam 
b.sub.3. This beam is directed at a very small angle onto the substrate by 
a reflecting prism 42. The beam reflected by the substrate is directed 
towards a retroreflector 44 by the prism 43. The element 44 reflects the 
beam in itself so that this beam (b.sub.3 ') once more traverses the same 
path via reflections on the prism 43, the substrate W and the prism 42. 
The beam b.sub.3 ' reaches a radiation-sensitive detection system 46 via a 
partially reflecting element 41 and a reflecting element 45. This 
detection system comprises, for example, a position-dependent detector or 
two separate detectors. The position of the radiation spot formed by the 
beam b.sub.3 ' on this system is dependent on the extent by which the 
focal plane of the projection lens system coincides with the plane of the 
substrate W. Reference is made to U.S. Pat. No. 4,356,392 for an extensive 
description of the focus error detection system. 
For accurately determining the X and Y positions of the substrate table WT, 
known projection apparatuses comprise a multi-axis interferometer system. 
U.S. Pat. No. 4,251,160 describes a two-axis system and U.S. Pat. No. 
4,737,283 describes a three-axis system. In FIG. 1 such an interferometer 
system is diagrammatically represented by the elements 50, 51, 52 and 53, 
the Figure showing only one measuring axis. A beam b.sub.4 emitted by a 
radiation source 50 in the form of a laser is split into a measuring beam 
b.sub.4,m and a reference beam b.sub.4,r by a beam splitter 51. The 
measuring beam reaches a reflecting side face of the substrate holder WH 
and the reflected measuring beam is combined by the beam splitter with the 
reference beam reflected by a stationary retroreflector 52, for example, a 
so-called "corner cube". The intensity of the combined beam is measured 
with the aid of a detector 53 and the displacement, in this case in the X 
direction, of the substrate support WC can be derived from the output 
signal of this detector, and also an instantaneous position of this 
support can be established. 
As is diagrammatically shown in FIG. 1, the interferometer signals 
represented by one signal S.sub.53 for the sake of simplicity, and the 
signals S.sub.13 and S.sub.13 ' of the alignment detection device are 
applied to a signal processing unit SPU, for example a microcomputer which 
processes said signals to control signals S.sub.AC for an actuator AC with 
which the substrate support is moved in the X-Y plane via the substrate 
holder WH. 
By using an X-Y interferometer system, the positions of and the mutual 
distances between the alignment marks P.sub.1 and P.sub.2 and M.sub.1 and 
M.sub.2 can be established in a system of coordinates defined by the 
stationary interferometer system during the alignment procedure. 
In accordance with a known method of repetitively imaging a mask pattern on 
a substrate, this substrate is prealigned to a certain extent, for 
example, in a prealignment station as described in European Patent 
Application no. 0,164,165. After the prealigned substrate has been 
introduced into the projection apparatus, the substrate is globally 
aligned with respect to the mask by means of the mask alignment marks 
M.sub.1, M.sub.2 and the substrate alignment marks P.sub.1, P.sub.2. 
Subsequently a substrate sub-area or field on which the mask pattern must 
be projected should be positioned under the mask pattern very accurately, 
whereafter a flash of light from the source LA transfers this pattern to 
the substrate field. Subsequently the substrate must be displaced and a 
second substrate field must be accurately positioned under the mask 
pattern whereafter a second illumination follows, and so forth until all 
substrate fields have been illuminated. 
With the currently required very small details in mask pattern image in 
view of the small depth of focus of the projection lens system and the 
possible unevenness of the substrate causing defocused images, the 
relevant substrate sub-area must first be levelled before such an imaging 
operation can take place. To this end a so-called local level sensor must 
first detect whether the relevant substrate sub-area is oblique with 
respect to the image field of the projection lens system. Such a level 
sensor is described, for example, in the article "The optical stepper with 
a high numerical aperture i-line lens and a field-by-field levelling 
system" in SPIE vol. 922, Optical/Laser Microlithography (1988), pp. 
270-276 and in U.S. Pat. No. 4,504,144. After a local oblique position has 
been found, it can be eliminated by tilting the entire substrate about the 
X and/or Y axis. However, the measuring mirrors of the interferometer 
systems are then also tilted so that the measuring signals supplied by 
this system are erroneous while the desired accurate positioning is no 
longer possible due to the so-called Abbe error. 
Moreover, unintentional and uncontrolled tilts of the substrate holder 
about the X and/or Y axis may occur, which tilts also affect the signals 
of the interferometer system. 
These difficulties could be avoided and after local levelling each 
substrate field could be positioned with the desired accuracy by providing 
each field with its own alignment mark and by aligning it separately with 
respect to a mask mark M.sub.1 or M.sub.2 after this field has been 
provided under the mask pattern, as described, inter alia in U.S. Pat. No. 
4,778,275. This so-called field-by-field alignment is, however, 
time-consuming so that the number of substrates which can be put through 
the projection apparatus per unit of time is reduced. 
Moreover, the strips between the substrate fields must then be relatively 
wide to accomodate the field alignment marks whose size must be adapted to 
that of the mask marks M.sub.1 and M.sub.2 and thus also to that of the 
substrate marks P.sub.1 and P.sub.2. As a result, the useful surface of 
the substrate, i.e. the total surface on which ICs can be formed, 
decreases. It could be considered to reduce the size of the field 
alignment marks. However, the mask alignment marks M.sub.1 and M.sub.2 and 
the substrate alignment marks must then also be reduced in size. As a 
result, the alignment accuracy would decrease, not only for the substrate 
fields but also for the total substrate. 
The present invention obviates the above-mentioned problems and provides a 
novel method of repetitively imaging and the associated positioning. In 
accordance with the novel method the substrate is only globally aligned 
along the X and Y axes and the possible rotation about the Z axis of the 
substrate is globally eliminated by means of the two substrate alignment 
marks P.sub.1 and P.sub.2 and possibly several other substrate alignment 
marks, and after levelling a substrate field, the fine positioning of this 
field is realised without any further alignment steps. The field-by-field 
positioning is now effected by very accurately measuring preferably all 
movements and positions of the substrate itself by means of the 
interferometer system. 
An essential condition and an important aspect of the present invention is 
that the substrate support is integrated with the mirror block cooperating 
with the interferometer system and that the substrate is fixed on this 
support. Then the substrate is immovable with respect to the mirror block 
and can only follow the movements of this block. This ensures that the 
measured movements and positions are absolute measures of those of the 
substrate. Thus novel use is made of the accuracy achieved nowadays with 
which the substrate holder can be driven and the movements of the mirror 
block can be measured. 
It is to be noted that in drawings associated with descriptions of known 
projection apparatuses for IC manufacture the substrate and the substrate 
support seem to be connected directly and immovably to the mirror block. 
However, this is only a diagrammatic representation so as not to 
complicate the drawings of the intricate apparatus. As already noted, the 
substrate should not only be aligned in the projection apparatuses, but it 
should also be focused globally and locally and, as already noted, it 
should be levelled locally. Focusing is understood to mean that it is 
ensured that the substrate surface coincides with the image field of the 
projection lens system. Separate actuators such as height and/or tilt 
actuators are used for these focusing and levelling operations, as 
described in SPIE vol. 922 "Optical/Laser Microlithography" (1988) pp. 
270-276: `The optical stepper with a high numerical aperture I-lens and a 
field-by-field levelling system" or in SPIE, vol. 811, "Optical 
Microlithographic Technology for Integrated Circuit Fabrication and 
Inspection", 1987, pp. 149-159, "An advanced waferstepper for sub-micron 
fabrication", or in U.S. Pat. No. 4,504,144. The height or levelling 
actuators referred to in these documents drive a so-called levelling table 
on which the substrate is arranged. Then this substrate cannot be 
connected rigidly to the mirror block. In a projection apparatus intended 
for performing the method according to the invention said actuators are 
arranged under the mirror block. 
In principle, a three-axis interferometer system can be used for measuring 
the movements along the X and Y axes and for determining the final 
positions of this table, and for determining the rotation of the table 
about the Z axis. Such a system, an embodiment of which is described in 
"SPIE, vol. 1088: Optical/Laser Microlithography, pp. 268-272, 
Linear/angular displacement interferometer for waferstage metrology", is 
diagrammatically shown in FIG. 5, together with the substrate table WT. 
The composite interferometer system comprises a Helium-Neon laser 70, two 
beam splitters 71 and 72 and three interferometer units 73, 74 and 75. A 
part of the beam b.sub.5 from the laser is reflected by the beam splitter 
71 as beam b.sub.6 to the interferometer unit 73 which cooperates with the 
mirror R.sub.1 of the substrate table WT. The beam b.sub.7 passed by the 
beam splitter 71 is split by the beam splitter 72 into a beam b.sub.8 
which is reflected to the interferometer unit 74 and into a beam b.sub.9 
which is passed to the interferometer unit 75. The interferometer unit 74 
cooperates with the mirror R.sub.1, while the interferometer unit 75 
cooperates with the mirror R.sub.2. 
FIG. 6 shows the principle of the interferometer unit 73. This unit 
comprises a beam splitter 80, for example, a partially transparent mirror 
which splits the incoming beam b.sub.6 into a measuring beam b.sub.6,m and 
a reference beam b.sub.6,r. The measuring beam is passed to the substrate 
table mirror R.sub.1 which reflects this beam to the beam splitter 80 
which in its turn reflects a part of the beam b.sub.6,m to the detector 
76. The beam b.sub.6,r reflected by the beam splitter 80 is reflected to 
the beam splitter 80 by a fixedly arranged reference mirror 81 which 
passes a part of this beam to the detector 76. When the substrate table 
mirror is moved in the X direction, constructive and destructive 
interferences alternately occur between the beams b.sub.6,m and b.sub.6,r 
incident on the detector 76, so that the output signal of this detector 
passes from a maximum value to a minimum value, and conversely, whenever 
the substrate table is displaced over a distance of .lambda./4, in which 
.lambda. is the wavelength of the beam b.sub.b. The number of maxima and 
minima of the detector signal S.sub.76 is a measure of the displacement of 
the table in the X direction. Movements of the mirrors R.sub.1 and R.sub.2 
which are much smaller than .lambda./4, for example, up to .lambda./128 or 
even .lambda./512 can be measured with the aid of known electronic 
interpolation methods. 
The interferometer units 74 and 75 have the same construction and operate 
in the same way as the interferometer unit 73. The movement of the 
substrate table in the Y direction is measured by means of the 
interferometer unit 75 and the associated detector 78. A second X 
displacement measurement is performed with the interferometer unit 74 and 
the associated detector 77. The rotation of the substrate table about the 
Z axis is computed from the signals S.sub.76 and S.sub.77. This rotation 
is given by 
##EQU1## 
in which d.sub.i is the distance between the points where the chief rays 
of the measuring beams b.sub.6,m and b.sub.8,m impinge upon the mirror 
R.sub.1. 
It is to be noted that FIG. 6 only shows the principle of an interferometer 
unit. In practice a polarization-sensitive beam splitter 80 and a number 
of .lambda./4 plates, represented by the elements 82 and 83 in FIG. 6, 
will be used for beam splitting and combination. Then the radiation loss 
is minimal, which is particularly important if only one laser 70 is to be 
used for the different interferometer units. Furthermore, retroreflectors 
as described in said article in SPIE, vol. 1088, Optical/Laser 
Microlithography II, pp. 268-272 may be incorporated in the interferometer 
units. 
To achieve the desired accuracy when using a three-axis interferometer 
system, the following two conditions should be fulfilled: 
1. The chief rays of the interferometer beams must be located in the plane 
of the substrate. 
2. The substrate support must during the displacements along the X and Y 
axes and the possible correction about the Z axis be fixed in the other 
degrees of freedom .phi..sub.x, .phi..sub.y. 
Due to the non-infinitely small cross-section of the interferometer beams 
and due to the fact that the edges of the mirrors, notably the upper edge, 
cannot be given the desired planeness (of, for example .lambda./20) so 
that the beams must be incident at a distance of, for example, at least 2 
mm from the upper edge, the first condition can only be fulfilled if the 
mirrors project beyond the surface of the substrate, as is shown in SPIE, 
vol. 1088, Optical/Laser Microlithography II, 1989, pp. 424-433: "Step and 
Scan: A systems overview of a new lithography tool". Due to the higher 
mirrors the weight of the mass to be displaced and positioned increases. 
Moreover, these mirrors require extra free space between the substrate 
surface and the lower side of the projection lens system. However, this 
space is often unavailable in practice because the interspace between the 
substrate surface and the projection lens system must be as small as 
possible so as to obtain a maximally plane image field at the area of the 
substrate surface. Moreover, this intermediate space should accommodate 
optical measuring systems for level sensing and image sensing, i.e. 
detecting deviations in the image formed by the projection light. 
It could be considered to enlarge the lateral dimensions of the block of 
which the interferometer mirrors form part, so that the mirrors projecting 
beyond the substrate surface remain sufficiently far remote from the other 
components of the projection apparatus, for example the projection lens 
system, also in the case of a lateral X-Y movement of the mirror block. 
However, this would involve an inadmissible increase of weight of the 
substrate holder and, moreover, the considerably larger mirrors cannot be 
manufactured with the desired planeness. 
Instead of a mirror block with mirrors projecting beyond the substrate 
surface, a mirror block may alternatively be used whose mirrors do not 
project beyond the substrate but extend at an angle of less than 
90.degree. to the substrate surface so that the interferometer beams are 
always perpendicularly incident on the mirror surfaces. Such a mirror 
block is shown in FIG. 7, together with an interferometer beam b.sub.6,m. 
If it is ensured that the extension of the principal axis of this beam 
impinges upon the substrate surface at the point where the optical axis 
A.sub.PL of the projection lens system intersects this surface, the exact 
position of the substrate can be derived from the interferometer signals 
and the signals supplied by the focus error detection system which is 
shown in FIG. 1 and is shown also in FIG. 7 but only by means of the beams 
b.sub.3 and b.sub.3 '. However, to ensure that the interferometer beam 
always impinges upon the mirror when the mirror block with the substrate 
table is moved over the working distance WD, this mirror must have a 
considerable height h, which means that the weight of the mirror block 
increases. 
To fulfil the above-mentioned condition, i.e. the substrate support should 
not exhibit a tilt about the X or Y axis and no dislacement along the Z 
axis, very stringent requirements must be imposed on the construction of 
the substrate table. Apart from the substrate support with the integrated 
mirror block this table comprises an X-Y-.phi..sub.z drive consisting of, 
for example, three linear motors arranged in a H configuration as 
described in U.S. Pat. No. 4,655,594, and a so-called air base AB as an 
intermediary between a base plate BP of, for example, granite and a 
substrate holder WH incorporating the X-Y-.phi..sub.z drive. When the 
substrate table is not perfectly guided, variable forces may be exercised 
on the air base when this table is moved. These forces, which are 
unpredictable in advance, result in a variable tilt of the air base, which 
results in a variable tilt of the substrate support. Furthermore, 
deviations in the planeness of the granite supporting plate may occur due 
to manufacturing tolerances or contamination. An unevenness of the plate 
will result in a position-dependent tilt of the substrate support. To 
eliminate its effect during use of the projection apparatus, a 
point-by-point calibration should have to be performed, which does not 
only complicate the total calibration of the apparatus but also renders it 
more inaccurate because the number of parameters to be calibrated 
increases. 
The stringent construction requirements could also be dropped and the then 
arising unwanted tilts about the X and Y axes and the tilts due to local 
levelling could then be measured by means of, for example, mechanical, 
ultrasonic or other non-optical sensors and the measuring results could be 
used to level out said variations or to correct the X-Y-.phi..sub.z 
measuring results. However, very stringent requirements must then be 
imposed on the accuracy of such sensors. 
In the projection apparatus according to the present invention the 
problems, which occur when maintaining the requirement that the chief rays 
of the interferometer beams must be located in the plane of the substrate, 
are avoided by dropping this requirement and by making use of a substrate 
support having an integrated mirror block whose mirrors do not project 
beyond the substrate surface. Another problem then arising, as well as the 
problems resulting from the requirement that either no tilts about the X 
and Y axes occur or that these tilts should be taken into account in the 
X-Y-.phi..sub.z adjustment, are solved by using a novel and extended 
interferometer system with which the wanted or unwanted movements can be 
accurately measured and which provides the possibility of accurate 
correction. 
Said other problem relates to the so-called Abbe error which is illustrated 
in FIG. 8. If the principal axis of an interferometer beam, for example, 
b.sub.6,m is incident on the mirror R.sub.1 at a distance a from the 
substrate surface, a tilt of the substrate surface at an angle .phi. 
generates a cross-talk signal in the X position signal supplied by the 
interferometer unit using beam b.sub.6,m. This crosstalk signal .DELTA.x 
is given by .DELTA.x.perspectiveto.a tan.phi..perspectiveto.a.phi.. As a 
result of the crosstalk signal the X servosystem will control in such a 
way that an X position error proportional to .DELTA.x is produced. For the 
tilt of the substrate surface the angle .phi. may be 1.3 m.rad and for a 
local field unevenness .phi. may be approximately 0.1 m.rad. If in the 
latter case the position error due to the Abbe arm is to be smaller than 4 
nm, which is still admissible in practice, then 
##EQU2## 
This requirement cannot be met due to the width of the beam which is, for 
example, 9 mm. It will therefore be necessary to calibrate, i.e. the X and 
Y position signals will have to be corrected with information about the 
tilt of the substrate surface. This tilt information may be obtained more 
easily and more accurately with the extensive interferometer system than 
with other means, for example, via mechanical or other non-optical 
sensors. 
FIG. 9 shows the principle of the composite interferometer system for 
measuring the five degrees of freedom X, Y, .phi..sub.X, .phi..sub.Y and 
.phi..sub.Z of the substrate support WT with an integrated mirror block. 
The system comprises, for example, two interferometer units 100 and 150 to 
which the beams b.sub.20 and b.sub.30 are supplied. These beams are 
emitted by a laser, for example a Helium-Neon laser 50. The beam b.sub.10 
coming from this laser first passes a beam-widening optical system 
diagrammatically shown by means of the lens 90 and is subsequently split 
into the two beams b.sub.20 and b.sub.30 by a beam splitter 92. The 
elements 91, 93 and 94 are mirrors which ensure that the beams are 
deflected in such a way that they are incident on the interferometer units 
100 and 150 at the correct angles. The interferometer unit 100 may be 
implemented in such a way that it emits three measuring beams towards the 
mirror R.sub.1 and receives these beams from this mirror. With these beams 
the displacement in the X direction, the tilt about the Y axis, 
.phi..sub.Y, and the rotation about the Z axis, .phi..sub.Z, of the mirror 
block and substrate support can be measured. The second interferometer 
unit 120 sends two measuring beams to and receives two measuring beams 
from the mirror R.sub.2. With these beams the displacement in the Y 
direction and the tilt about the X axis can be measured. The 
interferometer units may be implemented in various ways. FIG. 10 shows a 
first embodiment of the interferometer unit 100. It comprises a 
polarization-sensitive beam splitter 101, two .lambda./4 plates 103, 104, 
a reference mirror 105, two retroreflectors 106, 107, a composite prism 
108 and two detectors 113, 115. The detectors may be arranged on the plane 
95 of the interferometer unit 100 shown in FIG. 9. The interferometer unit 
is of the heterodyne type. The beam b.sub.20 then comes from a Helium-Neon 
laser which is implemented as a Zeeman laser. Such a laser supplies a beam 
with two mutually perpendicularly polarized components which have an 
optical phase difference of, for example 20 MHz. These two components are 
shown in FIG. 10 by means of solid lines and broken lines. 
The beam b.sub.20 entering the prism 101 is split into a measuring beam 
b.sub.20,m and a reference beam b.sub.20,r by the polarization-sensitive 
interface 102. The beam b.sub.20,m is passed to the mirror R.sub.1 of the 
substrate table and is reflected by this mirror. A .lambda./4 plate 103 
ensuring that the direction of polarization of the reflected beam, which 
has traversed the .lambda./4 plate twice, is rotated through 90.degree. 
with respect to the direction of polarization of the ongoing beam 
b.sub.20,m is arranged between the prism 101 and the mirror R.sub.1. The 
reflected beam is then reflected by the interface 102 to a retroreflector 
106, for example, in the form of a three-dimensional corner cube prism. 
The beam reflected by the prism is subsequently reflected by the interface 
102 and sent as measuring beam b'.sub.20,m to the mirror R.sub.1 again and 
reflected by this mirror to the prism again. This beam has then again 
traversed the .lambda./4 plate twice so that it is now passed by the 
interface 102. The beam b'.sub.20,m subsequently reaches a prism system 
108 and is reflected by its surface 109 to a radiation-sensitive detector 
113 via an analyser 112. 
The reference beam b.sub.20,r reflected by the interface 102 traverses the 
.lambda./4 plate 104, is reflected by the reference mirror 105 and 
traverses the .lambda./4 plate a second time. The direction of 
polarization of the beam incident on the interface 102 is rotated through 
90.degree. so that it is passed on to the retroreflector 106. The beam 
b'.sub.20,r reflected by this element is again sent as a reference beam to 
the reference mirror 105 and reflected by this mirror to the interface 
102, the direction of polarization being rotated through 90.degree. again. 
The interface 102 subsequently reflects the beam to the prism system 108 
whose face 109 sends the beam b'.sub.20,r to the detector 113. The 
direction of polarization of the analyser 112 extends at an angle of 
45.degree. to the two mutually perpendicular directions of polarization of 
the beams b'.sub.20,m and b'.sub.20,r. The components of the beams 
b'.sub.20,m and b'.sub.20,r passed by the analyser have the same direction 
of polarization and interfere with each other. The output signal S.sub.113 
of the detector 113 has an intensity modulation at a frequency which is 
equal to the Zeeman frequency difference plus or minus a frequency shift 
which is dependent on the displacement of the substrate table mirror 
R.sub.1 in the X direction. 
In principle the retroreflector 106 could also be omitted so that the 
measuring beam and the reference beam incident on the detector 113 would 
only be reflected once by the substrate table mirror R.sub.1 and the 
reference mirror 105, respectively. 
The special embodiment of the interferometer according to FIG. 10, in which 
use is made of the retroreflector 106 to reflect the measuring beam twice 
as beams b.sub.20,m and b'.sub.20,m on the substrate support mirror, has 
the great advantage that the direction of the measuring beam b'.sub.20,m 
ultimately incident on the detector 113 is independent of a tilt of the 
mirror R.sub.1 about an axis perpendicular to the X axis. As a result the 
signal S.sub.113 contains only true X displacement information. For the 
same reason, a possible tilt of the reference mirror 105 does not have any 
influence on the signal S.sub.113. 
The rotation of the substrate support about the Z axis, which is 
perpendicular to the plane of the drawing in FIG. 10, can also be measured 
by means of the interferometer unit of FIG. 10. This is effected via a 
second X measurement at a position P.sub.x,3 (P.sub.x,4) at a maximum 
possible distance from the position P.sub.x,1 (P.sub.x,2) where the first 
X measurement is performed. To this end the face 110 of the prism system 
108 is in the form of a partially transparent mirror which reflects a 
portion of the measuring beam b'.sub.20,m and the reference beam 
b'.sub.20,r as a new reference beam b.sub.21,r and a new measuring beam 
b.sub.21,m, respectively, to the beam splitter 101. The direction of 
polarization of the two beams is first rotated through 90.degree. by means 
of the .lambda./2 plate 116 so that the functions of these beams are 
interchanged. The measuring beam b.sub.21,m is passed to the substrate 
support mirror by the polarization-sensitive interface 102, while the 
reference beam b.sub.21,r is reflected to the reference mirror. The paths 
traversed by the beams b.sub.21,m and b.sub.21,r are analogous to those 
traversed by the beams b.sub.20,m and b.sub.20,r. Preferably, a second 
retroreflector 107 is provided which ensures that the measuring beam and 
the reference beam are sent a second time to the substrate table mirror 
R.sub.1 and the reference mirror as beams b'.sub.21,m and b'.sub.21,r. Via 
the beam splitter 101, the prism system 108 and a second analyser 114 the 
reflected beams b'.sub.21,m and b'.sub.21,r reach a second detector 115 
where they interfere with each other. 
The output signal S.sub.115 of this detector has an intensity modulation at 
a frequency which is equal to the Zeeman difference frequency plus or 
minus a frequency shift which, however, is now dependent on the possible 
rotation of the mirror R.sub.1 about the Z axis. In fact, if such a 
rotation occurs, the frequency shift between the measuring and reference 
beams at their first passage through the system, at which reflections 
occur at the positions P.sub.x,1 and P.sub.x,2 is different from the 
frequency shift at the second passage through the system, at which 
reflections occur at the positions P.sub.x,3 and P.sub.x,4. The frequency 
difference measured by means of the detector 115 is the difference between 
said frequency shifts. If the substrate support mirror does not have a 
rotation about the Z axis, the resultant frequency difference is equal to 
zero. 
For the way in which the signals S.sub.113 and S.sub.115 can be 
electronically processed so as to derive the X displacement and the 
rotation .phi..sub.Z about the Z axis of the substrate table from the 
frequency shifts, reference may be made by way of example to the article 
in SPIE, vol.1088 "Optical/Laser Microlithography" II, 1989, pp. 268-272. 
Instead of a beam b.sub.20 with two frequency components, a beam having 
only one frequency may be used. The displacement or rotation of the mirror 
R.sub.1 is then measured by determining the phase difference between the 
measuring and reference beams. 
According to the present invention the known interferometer unit described 
so far can be extended so that a third measurement, for example, of the 
tilt about the Y axis can be performed with this unit. To this end, for 
example, the face 109 of the prism system 108 may be implemented as a 
partially transparent mirror which passes a portion of the beams 
b'.sub.20,m and b'.sub.20,r, as is shown in FIG. 10. A reflector system 
120 is arranged in the path of the beam portions which have been passed. 
This system must reflect the beams to the beam splitter 101 and displace 
these beams parallel to themselves in the Z direction so that the beams 
will extend in a second XY plane which is located in front of or behind 
the plane of the drawing in FIG. 10. This plane is shown in FIG. 11 
together with the third measuring beam b.sub.22,m and reference beam 
b.sub.22,r. 
The path of the beams b.sub.22,m and b.sub.22,r in front of the beam 
splitter 101 incorporates a .lambda./2 plate 125 which rotates the 
direction of polarization of these beams through 90.degree. so that the 
functions of the reference beam and the measuring beams are interchanged. 
Preferably, there is a third retroreflector 128 so that the measuring beam 
is reflected twice as beams b.sub.22,m and b'.sub.22,m at the positions 
P.sub.x,5 and P.sub.x,6, respectively, by the substrate support mirror 
R.sub.1 and the reference beam is reflected twice as beams b.sub.22,r and 
b'.sub.22,r by the reference mirror. The paths traversed by the measuring 
beams and reference beams are analogous to those traversed by the 
measuring beams b.sub.20,m and b'.sub.20,m and the reference beams 
b.sub.20,r and b'.sub.20,r in FIG. 10. 
The beams b'.sub.22,m and b'.sub.22,r ultimately reach a polarization 
analyser 126 which passes the components having the same direction of 
polarization of these beams, which components interfere with each other, 
to a detector 127. The output signal S.sub.127 of this detector has an 
intensity modulation at a frequency which is equal to the Zeeman 
difference frequency plus or minus a frequency shift which is dependent on 
the possible tilt of the mirror R.sub.1 about the Y axis. In fact, if such 
a tilt occurs, the frequency shift between the measuring beam b'.sub.20,m 
and the reference beam b'.sub.20,r differs from the frequency shift 
between the measuring beam b'.sub.22,m and the reference beam b'.sub.22,r. 
The frequency difference measured by means of the detector 127 is the 
difference between these frequency shifts. If the substrate table does not 
have a tilt about the Y axis, the resultant frequency difference is equal 
to zero. This tilt can also be measured by means of a single frequency 
beam and by determining phase differences. 
FIG. 12 shows an embodiment of the reflector system 120 in detail. This 
system comprises a first reflector 121 which reflects the beams 
b'.sub.20,m and b'.sub.20,r extending parallel to the X axis towards the Z 
axis, and a second reflector 122 which reflects these beams again in a 
direction parallel to the X axis. The reflector pair 121, 122 thus 
displaces the beams parallel to themselves along the Z axis. 
In the embodiment of FIGS. 11 and 12, in which the beams b.sub.22,m and 
b'.sub.22,m are displaced in the Z direction only, the points P.sub.x,5 
and P.sub.x,6 where the chief rays of these measuring beams impinge upon 
the substrate table mirror R.sub.1 have the same X positions as the points 
P.sub.x,2 and P.sub.x,3 where the chief rays of the measuring beams 
b'.sub.20,m and b'.sub.21,m impinge upon this mirror. This is shown in 
FIG. 13 for the sake of clarity. In this Figure the points where the chief 
rays of the measuring beams b.sub.20,m, b'.sub.20,m, b.sub.21,m, 
b'.sub.21,m, b.sub.22,m and b'.sub.22,m are incident on the mirror R.sub.1 
are denoted by the circles P.sub.x,1, P.sub.x,2, P.sub.x,3, P.sub.x,4, 
P.sub.x,5 and P.sub.x,6, respectively. A so-called measuring axis is 
associated with each pair of measuring beams. These measuring axes are 
denoted by MAX.sub.1, MAX.sub.2 and MAX.sub.3 in FIGS. 10 and 11. The 
points where these measuring axes intersect the mirror R.sub.1 are denoted 
by Q.sub.1, Q.sub.2 and Q.sub.3, respectively, in FIG. 13. 
The point Q.sub.3 is preferably located in a plane through the optical axis 
A.sub.PL of the projection lens system and perpendicular to the plane of 
the drawing in FIG. 13, hence perpendicular to the mirror R.sub.1. The 
points Q.sub.1 and Q.sub.2 are preferably located symmetrically with 
respect to this plane so that the connection line 1 between the points 
Q.sub.1 and Q.sub.2 is parallel to the image plane IP of the projection 
lens system, which image plane coincides with the substrate surface WP if 
this surface is an ideal surface. 
The measuring beams and the measuring axes are further preferably parallel 
so as to prevent interference patterns instead of a single radiation spot 
to occur at the location of the detectors 113, 115 and 127. This 
parallelism, which is determined by the planeness of the surfaces of the 
beam splitter 101, of the prism system 108 and of the reflector system 
120, and by the angle between the surfaces 109 and 110 of the prism system 
108 and the angle between the surfaces 121 and 122 of the reflector 
system, can be satisfactorily realised in practice because said surfaces 
can be accurately flattened within 3 angle seconds and because said angles 
can be made accurately equal to 90.degree.. The reflector system 120 is 
preferably integrated with the prism system 108 so as to avoid alignment 
problems during assembly and to ensure stability with time. 
The distance d.sub.2 between the image plane IP and the line of connection 
1 must be as short as possible. The distance d.sub.3 between the line 1 
and the point Q.sub.3 and the distance d.sub.4 between the points Q.sub.1 
and Q.sub.2 should be as long as possible so as to be able to measure the 
tilt .phi..sub.y and the rotation .phi..sub.z as accurately as possible. 
On the other hand, these distances should remain limited so as to limit 
the dimensions and hence the weight of the mirror R.sub.1. In a realised 
embodiment of the device the distances d.sub.3 and d.sub.4 are of the 
order of 20 mm, while the distance d.sub.2 is of the order of 7 mm. 
The interferometer unit shown in FIGS. 10 and 11 has the advantage that the 
measuring beam and reference beam associated with a measuring axis are 
symmetrical with respect to the beam splitter 101 and have the same path 
lengths through this beam splitter. This substantially eliminates the risk 
of instabilities. In the device according to FIGS. 10 and 11 the 
differences between the signals associated with the measuring axes 
MAX.sub.1, MAX.sub.2 and MAX.sub.3, which differences are required for 
measuring the rotation .phi..sub.z and the tilt .phi..sub.y are determined 
optically. If the information obtained via the measuring axes is 
represented by I.sub.MAX,1, I.sub.MAX,2 and I.sub.MAX,3, the detector 
signals S.sub.113, S.sub.115 and S.sub.127 in the embodiment of FIGS. 10 
and 11 are given by 
S.sub.113 =I.sub.MAX, 1 
S.sub.115 =I.sub.MAX,1 -I.sub.MAX,2 
S.sub.127 =I.sub.MAX,1 -I.sub.MAX,3 
The measuring axis information as a function of the detector signals is: 
I.sub.MAX,1 =S.sub.113 
I.sub.MAX,2 =S.sub.113 -S.sub.115 
I.sub.MAX,3 =S.sub.113 -S.sub.127 
The signals S(X), S(.phi..sub.z) and S(.phi..sub.y) which comprise 
information about the magnitude and direction of the displacement along 
the X axis, the rotation about the Z axis and the tilt about the Y axis 
then are: 
##EQU3## 
or: Taking calibration parameters in connection with the Abbe arm into 
account, the X position, the rotation about the Z axis and the tilt about 
the Y 
##EQU4## 
axis of the substrate can be determined by means of these signals. 
Alternatively, the difference between the signals associated with the 
different measuring axes may be determined electronically instead of 
optically. Then three separate beams should be supplied to the beam 
splitter 101, as is shown in FIG. 14. 
The path of the beam b.sub.20 incorporates, in front of the 
polarization-sensitive beam splitter 101, a polarization-insensitive beam 
splitter 130 which splits the beam b.sub.20 into a first and a second beam 
b.sub.41 and b.sub.42 whose principal axes are located in a first X-Y 
plane, the plane of the drawing in FIG. 14, and a third beam b.sub.43 
whose principal axis is located in a second X-Y plane in front of or 
behind the plane of the drawing in FIG. 14. The beam splitter 130 
comprises a combination of partially or not partially transparent 
reflectors and may be implemented in various ways. For example, the 
reflectors may be faces of plane-parallel plates, so that the beams 
b.sub.41, b.sub.42, and b.sub.43 are satisfactorily parallel. Each of 
these beams is split by the interface 102 into a measuring beam and a 
reference beam b.sub.43,m and b.sub.43,r, b.sub.42,m and b.sub.42,r and 
b.sub.41,m and b.sub.41,r, respectively. For the sake of clarity only a 
part of the radiation path is shown for the reference beam b.sub.42,r. 
Preferably retroreflectors 106, 107 and 128 are arranged in the paths of 
the beams b.sub.41, b.sub.42 and b.sub.43 so that the measuring beams 
b'.sub.41,m, b'.sub.42,m and b'.sub.43,m leaving the beam splitter are 
reflected twice by the substrate support mirror R.sub.1. Each measuring 
beam, together with the associated reference beam, is incident on a 
separate detector 113, 115 or 127 via an analyser 112, 114 and 126. For 
the sake of clarity in FIG. 14 the Y positions of the points P.sub.x,5 and 
P.sub.x,6 where the chief rays of the measuring beams b.sub.43,m and 
b'.sub.43,m impinge upon the mirror R.sub.1, are different from those of 
the points P.sub.x,2 and P.sub.x,3 where the beams b'.sub.41,m and 
b.sub.42,m impinge upon the mirror. However, the Y position of P.sub.x,5 
and P.sub.x,6 preferably coincides with that of P.sub.x,2 and P.sub.x,3 
respectively so that the situation of FIG. 13 is obtained again. 
For the embodiment of FIG. 14 the relationship between the detector signals 
S.sub.113, S.sub.115 and S.sub.127 and the information obtained via the 
measuring axes is as follows: 
S.sub.113 =I.sub.MAX,1 
S.sub.115 =I.sub.MAX,2 
S.sub.127 =I.sub.MAX,3. 
The measuring signals S(X), S(.phi..sub.z) and S(.phi..sub.y) may now be: 
##EQU5## 
and, in terms of detector signals: 
##EQU6## 
The choice between a device with three independent measuring axes and a 
device with three coupled measuring axes is determined by the extent to 
which an interferometer error .DELTA. may affect the measuring signals 
S(X), S(.phi..sub.z) and S(.phi..sub.y). The interferometer error .DELTA. 
is the error caused by the interferometer itself in the detector signals 
S.sub.113, S.sub.115 and S.sub.127. When such an error .DELTA. occurs in 
each detector signal, the error in the measuring signals, in the case of 
three independent measuring axes is: 
##EQU7## 
and in the case of three coupled measuring axes: 
##EQU8## 
In order to determine the displacement of the substrate table in the Y 
direction and the tilt about the X axis, the composite interferometer 
system according to the invention includes a second interferometer unit 
which is denoted by 150 in FIG. 9. In principle, this interferometer has 
two measuring axes MAX,.sub.4 and MAX,.sub.5 and its structure is 
analogous to that of the interferometer unit 100. FIGS. 15 and 16 show the 
interferometer unit 150 in detail. 
The incoming beam b.sub.30, with two mutually perpendicularly polarized 
components having a given frequency difference, is split by the interface 
152 of a beam splitter 151 into a measuring beam b.sub.30,m and a 
reference beam b.sub.30,r. The measuring beam is reflected by the second 
substrate support mirror R.sub.2 and is reflected twice at the positions 
P.sub.y,1 and P.sub.y,2 in the presence of a retroreflector 156. The 
measuring beam b'.sub.30,m from the beam splitter 151 is combined with the 
reference beam b'.sub.30,r which is reflected twice by the reference 
mirror 155. The two .lambda./4 plates 153 and 154 ensure that the 
measuring and reference beams acquire the direction of polarization which 
is desired for the second passage through the system. The beams 
b'.sub.30,m and b'.sub.30,r are sent to a detector 160 by a reflector 158, 
passing a polarization analyser 159. The direction of polarization of this 
analyser extends at an angle of 45.degree. to the two mutually 
perpendicular directions of polarization of the beams b'.sub.30,m and 
b'.sub.30,r so that this analyser passes components of the same directions 
of polarization of the beams, which components interfere with each other. 
The output signal S.sub.160 of the detector 160 then has an intensity 
modulation at a frequency which is equal to the difference frequency of 
the beams b.sub.30,m and b.sub.30,r plus or minus a frequency shift which 
is determined by the displacement of the substrate support in the Y 
direction. 
To enable measurement of the tilt of the substrate support about the X 
axis, the reflector 158 may be implemented as a partially transparent 
mirror. The portions of the beams b'.sub.20,m and b'.sub.30,r passed by 
this element are reflected to the beam splitter 151 by a reflector system 
161. This system may comprise two mirrors 162 and 163 at an angle of 
45.degree. to the X-Y plane in FIG. 15. The mirror 162 reflects the beam 
portions in the Z direction and the mirror 163 subsequently ensures that 
the beam portions will extend in the Y direction again, but then in a 
second X-Y plane which is located in front of or behind the plane of the 
drawing in FIG. 15. This second plane is the plane of the drawing in FIG. 
16, which Figure shows how the beam portions reflected by the system 161 
traverse the interferometer unit as a new measuring beam b.sub.31,m and a 
new reference beam b.sub.31,r and how they are reflected by the substrate 
support mirror R.sub.2 and the reference mirror 155. A .lambda./2 plate 
164 is arranged in front of the beam splitter 151, which plate rotates the 
directions of polarization of the beams through 90.degree. so that the 
functions of the measuring beam and the reference beam are interchanged. 
In the presence of a retroreflector 165 the measuring beam is reflected 
twice as measuring beam b.sub.31,m and b'.sub.31,m at the positions 
P.sub.y,3 and P.sub.y,4 by the mirror R.sub.2 and the reference beam is 
reflected twice as reference beams b.sub.31,r and b'.sub.31,r by the 
reference mirror. 
The beams b'31,m and b'.sub.31,r ultimately reach a polarization analyser 
166 which passes the components having the same direction of polarization 
of these beams to a detector 167. The output signal S.sub.167 of this 
detector has an intensity modulation at a frequency which is equal to the 
difference frequency of the beams b.sub.30,m and b.sub.30,r plus or minus 
a frequency shift which is dependent on a tilt .phi..sub.x of the mirror 
R.sub.2 about the X axis. In fact, if such a tilt occurs, the frequency 
shift between the measuring beam b'.sub.30,m and the reference beam 
b'.sub.30,r differs from the frequency shift between the measuring beam 
b'.sub.31,m and the reference beam b'.sub.31,r. The frequency difference 
measured by means of the detector 167 is the difference between these 
frequency shifts. If the substrate support does not have a tilt about the 
X axis, the resultant frequency difference is equal to zero. 
The points P.sub.y,3 and P.sub.y,4 where the chief rays of the measuring 
beams b.sub.31,m and b'.sub.31,m impinge upon the mirror R.sub.2 
preferably have the same X positions as the points P.sub.y,2 and P.sub.y,1 
where the chief rays of the measuring beams b'.sub.30,m and b.sub.30,m 
impinge upon this mirror, and the measuring axes MAX.sub.4 and MAX.sub.5 
are directed perpendicularly to the Z axis. This is illustrated in FIG. 
17. This Figure not only shows these measuring axes and the points of 
intersection Q.sub.4 and Q.sub.5 of these axes with the mirror R.sub.2 of 
the substrate table WT, but also the measuring axes MAX.sub.1, MAX.sub.2 
and MAX.sub.3 of the first interferometer unit and the points of 
intersection Q.sub.1, Q.sub.2 and Q.sub.3 of these axes with the mirror 
R.sub.1 of the substrate table. 
In the interferometer unit according to FIGS. 15 and 16 the difference 
between the signals associated with the measuring axes MAX.sub.4 and 
MAX.sub.5, which difference is required for determining the tilt 
.phi..sub.x, is determined optically. If the information obtained via 
these measuring axes is represented by I.sub.MAX,4 and I.sub.MAX,5, the 
detector signals S.sub.160 and S.sub.167 in the embodiment of FIGS. 15 and 
16 are given by: 
S.sub.160 =I.sub.MAX,4 
S.sub.167 =I.sub.MAX,4 -I.sub.MAX,5 
The measuring axis information as a function of the detector signals is 
I.sub.MAX,4 =S.sub.160 
I.sub.MAX,5 =S.sub.160 S.sub.167 
The signals S(Y) and S(.phi..sub.x) representing information about the 
magnitude and direction of the displacement along the Y axis and of the 
tilt about the X axis then are: 
##EQU9## 
in which d.sub.5 is the distance between the points Q.sub.4 and Q.sub.5 in 
FIG. 17. 
Instead of using coupled measuring axes, as is shown in FIGS. 15 and 16, it 
is possible to use independent measuring axes for operating the 
interferometer unit 150 analogously as described with reference to the 
interferometer unit 100. In this case the following relationships apply 
between the measuring axis information and the detector signals: 
S.sub.160 =I.sub.MAX,4 
S.sub.167 =I.sub.MAX,5 
and for the measuring signals S(Y) and S(.phi..sub.x): 
##EQU10## 
The interferometer unit according to FIGS. 16 and 17 also provides the 
advantage that the associated measuring and reference beams traverse this 
unit symmetrically and cover the same distances through the beam splitter 
151, which is very favourable from the point of view of stability, i.e. it 
is independent of temperature, humidity, etc. 
In both interferometer units 100 and 150 it is not necessary for the 
detectors 113, 115, 127, 160 and 167 to be arranged directly behind the 
analysers 112, 114, 126, 159 and 166, but if desired, these detectors may 
be arranged at larger distances and possibly close together. Optical 
fibres can then be used to guide the beams to the detectors. Lenses for 
focusing the beams on the entrance planes of the fibres may be arranged 
between the analysers and the fibres. 
The prism retroreflectors, or three-dimensional "corner cubes", shown in 
FIGS. 10, 11, 14, 15 and 16 may also be replaced by so-called cat's eye 
retroreflectors. Such a cat's eye is constituted by a lens with a mirror 
arranged in its focal plane and it ensures that not only the principal 
axis of the reflected beam is parallel to that of the incoming beam, but 
it also ensures that these principal axes coincide. 
In view of the required accuracy of the composite interferometer system, 
changes of ambient parameters, such as temperature, pressure, humidity may 
become to play a role. These changes cause a variation of the refractive 
index of the medium in which the interferometer beams propagate. To be 
able to determine this variation, so that it can be corrected, the 
interferometer system according to the invention preferably has a sixth 
axis which is used as a reference axis along which a beam extends which 
cooperates with a fixedly arranged mirror. In FIG. 9 this mirror is 
denoted by the reference numeral 170 and the reference axis beam is 
denoted by b.sub.50. The beam b.sub.50 is preferably supplied by the 
second interferometer unit 150 and the beam coming from this unit is sent 
to the mirror 170 by a mirror 171. 
FIG. 15 shows how the beam b.sub.50 can be derived from the beam b.sub.30, 
for example, by means of a prism system 175 comprising two reflectors 176 
and 177. The first partially transparent reflector 176 reflects a part of 
the beam b.sub.30 to the second reflector 177 which sends this part as 
beam b.sub.50 to the beam splitter 151. The interface 152 splits the beam 
b.sub.50 into a measuring beam b.sub.50,m and a reference beam b.sub.50,r. 
The last-mentioned beam is reflected to the reference mirror 155, while 
the measuring beam b.sub.50,m is passed on to the mirror 171 which is 
arranged, for example, at an angle of 45.degree. to the plane of the 
drawing in FIG. 15. The measuring beam b.sub.50,m reflected by the fixedly 
arranged mirror 170 enters the beam splitter 151 via the mirror 171, in 
which beam splitter it is combined with the reference beam b.sub.50,r 
reflected by the reference mirror 155. Via a polarization analyser 179 the 
combined beam reaches a detector 180 which may be arranged behind the 
prism system if the reflector 177 is a partially transparent reflector. 
The measuring beam b.sub.50,m traverses a constant geometrical path length. 
The optical path length, which is the product of the geometrical path 
length and the refractive index of the medium, is, however, influenced by 
a variation of the refractive index, hence also by the path length 
difference between the measuring beam b.sub.50,m and the reference beam 
b.sub.50,r. The variation of the path length difference is measured by 
means of the detector 180 and the output signal S.sub.180 can be used to 
correct the information obtained via the other measuring axes for the 
refractive index variations due to variations of the ambient parameters. 
As is shown in FIG. 9, the reference mirror 170 for the reference axis is 
connected to the interferometer unit 150, preferably via a plate 190 of 
very stable material such as "zerodure" or "Invar". A very stable 
construction for the reference axis is then obtained. 
The information of the reference axis of the six-axis interferometer system 
may also be used for correcting the measuring information from other 
optical measuring systems, such as a focus error detection system and/or a 
system for detecting local levelling of the substrate surface, if the 
beams of these measuring systems traverse the same space as the 
interferometer beams. 
Due to changes of the ambient parameters such as air pressure, temperature, 
humidity, etc. the refractive index of the medium within the projection 
lens system may be affected, which may result in variations of the imaging 
quality of the projection lens system. The signal generated by means of 
the reference axis of the composite interferometer system may be used for 
correcting the imaging quality. This may be effected, for example, by 
adjusting one or more of the following parameters: 
the wavelength of the projection beam 
the gas pressure within the projection lens system 
the temperature within the projection lens system 
the composition of the medium in one or more of the compartments within the 
projection lens system 
the mutual distance between the lens elements in the projection lens 
system. 
Furthermore, also 
the zero setting of the alignment device, and 
the zero setting of the focusing device could be adjusted by means of said 
signal. 
To obtain the control signals required for this purpose, the output signal 
S.sub.180 of the detector 180 is applied to an electronic signal 
processing unit 185. As is diagrammatically shown in FIG. 15 by means of 
the signals SR.sub.1. . . SR.sub.n, various servosystems of the projection 
apparatus can be controlled from the unit 185. It is to be noted that the 
zero adjustments and the correction of the imaging quality by means of the 
reference axis signal may alternatively be realised in an apparatus 
without local levelling of the object or substrate. 
For measuring variations of the refractive index it is sufficient to use 
one measuring beam. However, if desired, a double measuring beam and a 
double reference beam may also be realised for the reference axis, as has 
been described in the foregoing for the other measuring axes. In that case 
the measuring beam b.sub.50,m and the reference beam b.sub.50,r must 
traverse the .lambda./4 plate 153 and the .lambda./4 plate 154, 
respectively, and a retroreflector at the location of the retroreflector 
156 must also be arranged for the reference axis. The measuring and 
reference beams of the reference axis than traverse the system according 
to FIG. 15 in an analogous way as the measuring beams b.sub.30 ,m, 
b'.sub.30,m and the reference beams b.sub.30,r, b'.sub.30,r of the fourth 
measuring axis. 
An even greater accuracy of the composite interferometer system can be 
obtained if it is ensured that the same circumstances prevail in the 
entire space in which the interferometer beams propagate. This can be 
realised by passing a constant, preferably laminar stream of air through 
this space. This is illustrated in FIG. 18. This Figure diagrammatically 
shows a part of the projection lens system PL and the substrate support WC 
integrated with the mirror block. The substrate support forms part of a 
substrate table WT WH which can move across a base plate BP via an air 
cushion AB under the control of a drive system in a H-configuration 
described in, for example, U.S. Pat. No. 4,665,594, whose components are 
denoted by MO.sub.1 and MO.sub.2 in FIG. 18. A mounting plate MP on which 
a holder HMS for optical measuring systems such as, for example, a focus 
error detection system and/or a system for detecting whether the substrate 
surface is locally horizontal is arranged under, and preferably connected 
to the projection lens system. The interferometer system, which is 
diagrammatically shown by IFS, is preferably also secured to the mounting 
plate. Said air stream is denoted by the arrows AF. This air stream is 
passed through an air stream conducting plate FGP. This plate may be 
dimensioned in such a way that also the space above the substrate to be 
illuminated is covered so that this substrate is present in a 
well-conditioned space. 
Both the purity and the temperature of the supplied air can be controlled. 
This air is, for example, of purity class 1 and its temperature is, for 
example, stable within 0.1.degree. C. The latter can be achieved by 
arranging a heat exchanger in the vicinity of the interferometer system 
and the substrate support. 
In addition to the embodiments shown in FIGS. 10, 11, 14, 15 and 16, 
various other embodiments of the interferometer units are possible. 
Several other possibilities will be described hereinafter. 
The interferometer unit of FIG. 19 comprises two polarization-sensitive 
beam splitters 200, 201, two retroreflectors 202, 203 and five .lambda./4 
plates 204, 205, 206, 207, 208. This interferometer unit does not comprise 
reference mirrors. The incoming beam b.sub.20 again comprises two 
components having mutually perpendicular directions of polarization and 
different frequencies .omega..sub.1 and .omega..sub.2, which components 
are denoted by solid lines and broken lines respectively. The component of 
the frequency .omega..sub.1 is passed as beam b.sub.60 to the substrate 
support mirror R.sub.10 and reflected by this mirror to the beam splitter 
200. The beam b.sub.60 reaching this beam splitter has traversed the 
.lambda./4 plate 204 twice so that its direction of polarization is 
rotated through 90.degree., so that the beam is reflected to the 
retroreflector 202. This retroreflector again reflects the beam to the 
beam splitter 200. The direction of polarization of the beam incident 
thereon is rotated through 90.degree. again so that the beam b.sub.60 is 
now passed on to the .lambda./4 plate 206. This plate converts the 
linearly polarized radiation into circularly polarized radiation, for 
example, dextrocircularly polarized radiation which is denoted by the 
circle 210. This circularly polarized light is composed of two linearly 
polarized components, one of which is passed as beam b'.sub.60 by the 
second beam splitter 201 to a polarization analyser 217 and a first 
detector 215. The beam b'.sub.60 has been reflected once by the mirror 
b.sub.10 at the position P.sub.10. 
The second component of the beam b.sub.60 is reflected as beam b.sub.62 by 
the beam splitter 201 to the mirror R.sub.10 which returns this component 
to the beam splitter 201. Due to the 90.degree. polarization rotation 
resulting from the double passage through the .lambda./4 plate 207, the 
beam splitter 201 passes the beam b.sub.62 to the retroreflector 203. On 
its way to and from this reflector the beam b.sub.62 again passes a 
.lambda./4 plate twice, namely plate 208, so that it is subsequently 
reflected as beam b'.sub.62 by the beam splitter 201 to an analyser 218 
and a second detector 216. The beam b'.sub.62 has been reflected twice by 
the mirror R.sub.10, once at the position P.sub.10 and once at the 
position P.sub.11. 
The component of the beam b.sub.20 with frequency .omega..sub.2 is 
reflected as beam b.sub.6 l by the beam splitter 200 to the .lambda./4 
plate 206, which converts the linearly polarized light into, for example, 
levocircularly polarized light, which is denoted by the circle 211. One of 
the components of this light is reflected as beam b.sub.63 to the mirror 
R.sub.10 by the beam splitter 201. After reflection by the mirror R.sub.10 
the beam b.sub.63 traverses the beam splitter 201 and the retroreflector 
203 so as to be subsequently incident on the beam splitter 201 which 
reflects the beam as beam b'.sub.63 to the detector 215. The .lambda./4 
plates 207 and 208 then again ensure the desired rotations of the 
direction of polarization. The beam b'.sub.63 has been reflected once by 
the mirror R.sub.10, at the position P.sub.12. The other component of the 
circularly polarized beam b.sub.61 is passed as beam b'.sub.61 by the beam 
splitter 201 to the detector 216. This beam has not met the mirror 
R.sub.10. 
The beam b'.sub.61 functions as a reference beam which interferes with the 
beam b'.sub.62 which has been reflected twice at positions P.sub.10 and 
P.sub.11 by the mirror R.sub.10. The detector signal S.sub.216 then 
comprises information about the displacement of the mirror R.sub.10 along 
an axis in the plane of the drawing and perpendicular to the mirror. Since 
the beam b'.sub.60 comes from the position P.sub.10, while the beam 
b'.sub.63 comes from the position P.sub.12, the signal S.sub.215 of the 
detector 215 receiving these beams comprises information about the 
difference between the displacements in the direction perpendicular to the 
mirror R.sub.10 and in the plane of the drawing of FIG. 19 of the mirror 
R.sub.10 at the area of the position P.sub.10 and at the area of the 
position P.sub.12. If such a difference occurs, the mirror rotates about 
an axis perpendicular to the plane of the drawing. 
The embodiment of FIG. 19 may be used as the interferometer unit 150 of 
FIG. 9 for measuring a displacement of the substrate table and mirror 
along the Y axis and a tilt thereof about the X axis. A system of 
coordinates as is denoted by broken lines in FIG. 19 is then associated 
with the interferometer unit, i.e. with the X axis perpendicular to the 
plane of the drawing, and the mirror R.sub.10 of FIG. 19 then is the 
mirror R.sub.2 of FIG. 9. The detector signal S.sub.215 then comprises 
information about the tilt .phi..sub.x of the mirror R.sub.2 about the X 
axis, while the detector signal S.sub.216 comprises information about the 
displacement along the Y axis. 
If extended with a third measuring axis, the embodiment of FIG. 19 may also 
be used as the interferometer unit 100 of FIG. 9 for determining a 
displacement along the X axis, a rotation about the Z axis and a tilt 
about the Y axis. Then the system of coordinates is the system shown by 
means of solid lines in FIG. 19, i.e. with the Z axis perpendicular to the 
plane of the drawing and the mirror R.sub.10 of FIG. 19 then is the mirror 
R.sub.1 of FIG. 9. 
The detector signal S.sub.215 then comprises information about the rotation 
.phi..sub.z of the mirror R.sub.1 and hence of the substrate support about 
the Z axis, while the detector signal S.sub.216 comprises information 
about the displacement along the X axis. For measuring the tilt about the 
Y axis, a polarization-insensitive beam splitter 220 which reflects a 
portion of beams with frequencies .omega..sub.1 and .omega..sub.2 in the Z 
direction and an extra reflector which subsequently deflects the beams so 
that they will extend in a second X-Y plane can be arranged between the 
.lambda./4 plate 206 and the beam splitter 201. 
The second X-Y plane is shown in FIG. 20 together with the newly formed 
beams b.sub.64 and b.sub.65 and the extra reflector 221. These beams pass 
the same system as that shown in the upper part of FIG. 19. The beam 
b.sub.64 is reflected by the mirror R.sub.1 at a position P.sub.13 and 
ultimately reaches, as beam b'.sub.64, a detector 226, together with the 
beam b'.sub.61, via an analyser 228. The last-mentioned beam, which has 
not met the mirror R.sub.1, functions as a reference beam for the beam 
b'.sub.64 which has been reflected once by the mirror R.sub.1 at the 
position P.sub.10 (FIG. 19) and once at the position P.sub.13. The output 
signal S.sub.226 then comprises information about the displacement, 
averaged over the positions P.sub.10 and P.sub.13, of the mirror R.sub.1 
in the X direction. The beam b.sub.65 is reflected by the mirror R.sub.1 
at the position P.sub.14 and reaches, as beam b'.sub.65, the detector 225 
via an analyser 227, together with the beam b'.sub.60 which has been 
reflected once by the mirror R.sub.1 at the position P.sub.10 (FIG. 19). 
The output signal S.sub.225 comprises information about the tilt of the 
mirror R.sub.1 about the Y axis. 
The elements of FIG. 20 have the same reference numerals as those of the 
upper part of FIG. 19, but they are primed so as to indicate that the 
elements of FIG. 20 may be either the same as those of FIG. 19 or separate 
and similar elements. In the first case the points P.sub.13 and P.sub.14 
have the same Y positions as the points P.sub.11 and P.sub.12, while in 
the second case the Y positions of P.sub.13 and P.sub.14 can still be 
freely chosen. 
In the interferometer unit according to FIGS. 19 and 20 (n+1) measuring 
beams are required (the information of the detector 226 is redundant 
information) for measuring along n measuring axes, while 2n measuring 
beams are required in the interferometer units according to FIGS. 10, 11, 
14, 15 and 16. In the interferometer unit according to FIGS. 19 and 20, 
however, the radiation paths are not symmetrical so that more stringent 
requirements must be imposed on the stability. 
FIGS. 21 and 22 show the principle of a further embodiment of an 
interferometer unit. In this unit a so-called cat's eye 230 is used as a 
retroreflective element. It consists of a lens 231 and a reflector 232 
arranged in its focal plane. The cat's eye reflects a beam in such a way 
that the chief ray of the reflected beam coincides with that of the 
incident beam. This provides the possibility of a compact structure. 
The component of the frequency .omega..sub.1 of the incoming beam b.sub.20 
is passed as beam b.sub.70 by the beam splitter 233 to the mirror R.sub.10 
and reflected by this mirror to the beam splitter 233. Since the beam has 
meanwhile passed the .lambda./4 plate 234 twice, its direction of 
polarization is rotated through 90.degree. so that it is now reflected to 
the cat's eye 230. The beam reflected by this element has passed the 
.lambda./4 plate 235 twice so that it is passed by the beam splitter 233. 
If a .lambda./2 plate 242 is provided, the direction of polarization of 
the beam b.sub.70 is rotated through 90.degree. again and this beam is 
passed as beam b.sub.72 to the mirror R.sub.10 by the beam splitter 236. 
The direction of polarization of the beam b.sub.72 again reaching the beam 
splitter 236 is rotated through 90.degree. due to the double passage 
through the .lambda./4 plate 237 so that it is now passed on as beam 
b'.sub.72 to an analyser 240 and a detector 241 arranged behind it. 
The component of the frequency .omega..sub.2 of the beam b.sub.20 is 
reflected as beam b.sub.71 to the beam splitter 236 by the beam splitter 
233. The .lambda./2 plate 242 rotates the direction of polarization of 
this beam through 90.degree. so that it is passed on as beam b.sub.73 to 
the mirror 239 by the beam splitter 236. The direction of polarization of 
the beam b.sub.73 reflected by this mirror is rotated through 90.degree. 
again by the .lambda./4 plate 238 so that this beam is reflected as beam 
b'.sub.73 to the detector 241 by the beam splitter 236. 
The beam b'.sub.73, which has not met the mirror R.sub.10, is a reference 
beam for the beam b'.sub.72 which has been reflected twice by the mirror 
R.sub.10 at the positions P.sub.16 and P.sub.17. The output signal 
S.sub.241 of the detector 241 then comprises information about the average 
value of the displacement at the positions P.sub.16 and P.sub.17 of the 
mirror R.sub.10 in the direction perpendicular to the mirror and in the 
plane of the drawing. 
If the .lambda./2 plate 242 is omitted, as is shown in FIG. 22, the beam 
b.sub.70 is passed as beam b.sub.74 by the beam splitter 236 to the 
reflector 239, while the beam b.sub.71 is reflected as beam b.sub.75 to 
the mirror R.sub.10. Two beams b'.sub.74 and b'.sub.75 both of which have 
been reflected once by the mirror R.sub.10 at the positions P.sub.16 and 
P.sub.17, respectively, are now incident on the detector 241. The detector 
signal S'.sub.241 then comprises information about the tilt of the mirror 
R.sub.10 about an axis perpendicular to the plane of the drawing. 
FIG. 23 shows how the possibilities shown in FIGS. 21 and 22 can in 
principle be combined in one interferometer unit. The interferometer unit 
of FIG. 23 comprises the elements of FIG. 22 in the lower and upper parts 
of FIG. 23, with which the tilt of the mirror R.sub.10 about the axis 
perpendicular to the mirror R.sub.10 and in the plane of the drawing can 
thus be determined. The interferometer of FIG. 23 comprises as additional 
components the part in the dot-and-dash line frame in which a 
polarization-neutral beam splitter 243 is present which splits off a 
certain portion of the beams b.sub.70 and b.sub.71. The split-off beam 
portions traverse, as beams b.sub.72 and b.sub.73, a system whose elements 
242, 244, 245, 246, 247, 248 and 249 have the same function as the 
elements 242, 236, 237, 240, 241, 238 and 239 of FIG. 21. The output 
signal of the detector 247 comprises information about the displacement of 
the mirror R.sub.10, averaged over the positions P.sub.16 and P.sub.18, 
along an axis perpendicular to the mirror and in the plane of the drawing, 
analogously as the detector signal S.sub.241 in FIG. 21. 
The interferometer unit of FIG. 23 can be used again as unit 150 of FIG. 9 
or, if extended by an extra measuring axis, as unit 100 of FIG. 9. The 
extra measuring axis can be obtained by splitting a part of the radiation 
of the beams b.sub.70 and b.sub.71, analogously as described with 
reference to FIGS. 19 and 20, in a direction perpendicular to the plane of 
the drawing in FIG. 23 and by a sub-system in the path of the split beam, 
analogous to the sub-system within the frame in FIG. 23. 
FIG. 24 shows another embodiment of an interferometer unit which is based 
on the same principle as that of FIGS. 21, 22 and 23. The embodiment of 
FIG. 24 comprises one beam splitter 250 having a polarization-sensitive 
separating layer 251, a polarization-neutral separating layer 252 and a 
.lambda./2 plate 253. Furthermore, only three .lambda./4 plates 254, 255 
and 258 and one reference mirror 259 are required to carry out the same 
measurements as with the unit of FIG. 23. 
The beam component of frequency .omega..sub.1 is passed as a first 
measuring beam b.sub.80 by the layer 251 to the mirror R.sub.10 on which 
the measurement must take place, by which mirror it is reflected at the 
position P.sub.19. This beam subsequently reaches the separating layer 252 
via the cat's eye 256, 257, which layer reflects a portion of the beam to 
the separating layer 251. This layer passes this beam portion as second 
measuring beam b.sub.82 to the mirror R.sub.10. The beam reflected at the 
position P.sub.20 is finally reflected by the separating layer 251 so as 
to reach, as beam b'.sub.82, a first analyser 260 and a first detector 261 
arranged behind it. The beam component of the frequency .omega..sub.2 is 
reflected as beam b.sub.81 by the layer 251 to the layer 252. This layer 
reflects a portion of the beam b.sub.81 to the separating layer 251 which 
reflects this beam portion as beam b.sub.83 to the reference mirror 259. 
The beam reflected by this mirror reaches as beam b'.sub.83 also the first 
detector 261. The beam b'.sub.83 has not met the mirror R.sub.10, while 
the beam b'.sub.82 has been reflected by the mirror R.sub.10 at the 
positions P.sub.19 and P.sub.20. The output signal S.sub.261 of the 
detector 261 therefore comprises information about the displacement, 
averaged over the positions P.sub.19 and P.sub.20 of the mirror R.sub.10, 
along the axis perpendicular to the mirror and in the plane of the 
drawing. 
The direction of polarization of the portions of the beams b.sub.80 and 
b.sub.81 passed by the polarization-neutral layer 252 is rotated through 
90.degree. by the .lambda./2 plate 253 so that the passed portion of the 
beam b.sub.81 is passed as the third measuring beam b.sub.84 by the layer 
251 to the mirror R.sub.10, reflected by this mirror at the position 
P.sub.21 and finally reflected by the layer 251 as beam b'.sub.84 to a 
second analyser 262 and a second detector 263. The portion of the beam 
b.sub.80 passed by the layer 252 is reflected, after reflection by the 
.lambda./2 plate 253, as beam b.sub.85 by the layer 251 to the reference 
mirror 259 and is finally reflected as beam b'8.sub.5 by this mirror to 
the second detector 263. The beams b'.sub.84 and b'.sub.85 both have been 
reflected once at the positions P.sub.21 and P.sub.19, respectively, so 
that the output signal S.sub.263 comprises information about a tilt of the 
mirror R.sub.10 about an axis perpendicular to the plane of the drawing. 
To clarify and complete the foregoing description, perspective elevational 
views of an embodiment of a projection apparatus and a substrate table for 
this apparatus are shown in FIGS. 25 and 26, respectively. 
In FIG. 25 the reference LA denotes a radiation source whose beam PB, 
illustrated by means of its chief ray, illuminates the mask MA via the 
mirrors RE.sub.1 and RE. The reference BE denotes an optical system which 
widens the beam and makes it uniform. The mask is supported by a mask 
manipulator or table MT. The reference PL denotes the projection lens 
system which images the mask pattern present in the mask on a sub-area of 
the substrate W which is supported by a substrate support WC integrated 
with the mirror block denoted by the mirrors R.sub.1 and R.sub.2. The 
substrate support is a component of a substrate table WT and is step-wise 
displaceable parallel to the X direction which is perpendicular to the Z 
direction and parallel to the Y direction which is perpendicular to the Z 
direction and the X direction, so that the semiconductor substrate can be 
illuminated in a large number of different illumination positions. 
The projection lens system is secured at the lower side to a mounting 
member 301 which forms part of a machine frame 300 of the apparatus. The 
mounting member 301 is a triangular plate extending in a plane 
perpendicular to the Z direction. This plane has three corner parts 302 
each resting on a frame stand 303. FIG. 25 only shows two corner parts 302 
and two frame stands 303. The frame stands are arranged on a base 304 of 
the machine frame 300 which is placed on a flat floor by means of 
adjusting members 305. 
As is shown in FIG. 26, the integrated substrate support and mirror block 
unit WC is guided by means of an air base AB provided with a static gas 
bearing along an upper face 307 of a supporting member 306 in the form of 
a rectangular granite stone. The substrate support is displaceable across 
the upper face 307 by means of a positioning device which is provided with 
the linear motors 301, 311 and 312 arranged in a H configuration as 
mentioned hereinbefore and as known from U.S. Pat. No. 4,665,594. As is 
shown in FIG. 26, the linear motors 311 and 312 are secured to a frame 315 
which is secured near its corners to the upper face 307 of the supporting 
member 306. The substrate support WC is displaceable parallel to the X 
direction by means of the linear motor 310 and parallel to the Y direction 
by means of the linear motors 311 and 312 and is rotatable through a very 
limited angle about an axis of rotation which is parallel to the Z 
direction. 
As is shown in FIGS. 25 and 26, the supporting member 306 and the 
positioning device constitute a unit 320 which is arranged on a support 
321 of the machine frame 300. The support 321 is a triangular plate 
extending perpendicularly to the Z direction, one main side 322 of which 
plate is shown in FIG. 25. Each main side of this plate extends between 
two frame stands 303. The supporting member 306 is secured to an upper 
side 323 of the support 321. This support is suspended from the lower side 
330 of the mounting member 301 by means of thin plate-shaped steel 
suspension elements only two of which, 325 and 327, are shown in FIG. 25. 
All suspension elements are formed from a plate extending in the vertical 
plane parallel to the Z direction, the vertical planes mutually enclosing 
angles of 60.degree.. 
The novel method and device have been explained hereinbefore with reference 
to their use in a projection apparatus for repetitively imaging a mask 
pattern on a substrate, with the substrate being locally levelled and the 
tilt measuring signals being used for correcting the other interferometer 
signals. Something similar may also occur in other apparatuses, for 
example in apparatuses for manufacturing patterns such as IC patterns, 
which work with a laser beam or an electron beam, so that the method and 
device can also be used in these apparatuses. 
Moreover, the device described, notably the interferometer system, can also 
be used in apparatuses in which positioning without local levelling must 
take place and in which the tilt measuring signals are used to eliminate 
the tilt by means of an actuator system. These apparatuses may also be 
pattern-generating apparatuses operating with a laser beam or an electron 
beam, but also IC projection apparatuses operating with X-ray radiation, 
or very accurate X-Y position measuring apparatuses, for example, those 
which are used for measuring masks.