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Description  |
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The invention relates to an opto-lithographic device comprising a lens
system which is arranged between a mask support and a substrate table and
which is telecentric on the side of the substrate, in which device the
distance between the lens system and the substrate table is adjustable by
means of an actuator cooperating with a circular-cylindrical holder which
is coupled to the lens system and is suspended in a fixedly arranged frame
by elastically deformable coupling members, the extent of deformation of
said coupling members being determined by the force exerted on the holder
by means of the actuator.
The invention further relates to a method of controlling the imaging
properties of an optical lens system in an opto-lithographic device.
In a known opto-lithographic device of the kind mentioned in the opening
paragraph (see "Philips Technical Review" 41, p. 268-278, 1983/84, No. 9),
the lens system comprises a fixedly arranged collimator lens and an
objective lens displaceable with respect to it along the optical axis of
the lens system. The distance between the objective lens and the substrate
table is adjustable by means of the actuator, which is provided with an
eccentric which is driven by a motor and is in engagement with the
circular-cylindrical holder of the lens system. The holder is enclosed by
a fixedly arranged sleeve to which the electric motor is secured. The
holder is provided with a number of cuts extending in the circumferential
direction, as a result of which elastically deformable coupling members
integrated in the holder are formed. Such coupling members are located on
either side of the area (viewed in the longitudinal direction of the
holder) at which the eccentric engages the wall. This means that upon
rotation of the eccentric a relative displacement occurs between the
fixedly arranged collimator lens and the objective lens with a
simultaneous elastic deformation of the coupling members. By variation of
the distance between the collimator lens and the objective lens, the focal
plane of the lens system is displaced and the image distance is adjusted
to the desired value. Since the lens system is telecentric on the side
facing the substrate table, with a constant distance between the mask and
the lens system there can be focusing without the enlargement being
varied. It should be noted that the lens system in the known
opto-lithographic device is also telecentric on the side facing the mask
support. This affords the advantage that the enlargement is independent of
the distance (object distance) between the mask and the lens system so
that no enlargement correction is required for, for example, thickness
variations and unevennesses in the mask. The enlargement is determined in
this case by a distance once adjusted during mounting between elements of
the lens system. This distance determined by grinding of engaging faces is
not adjustable during operation. It appears from a publication of F.
Sporon-Fiedler and J. Williams in "SPIE Proceedings", Vol. 538, No. 11,
1985, p. 86-90, as to "Optical Microlithography IV" that atmosphere
pressure variations result on considerable errors in the enlargement. It
is suggested to make the distance between mask and lens system adjustable.
A practical construction in which pressure variations are corrected in
this manner is not described in the article. However, it has been pointed
out that care should be paid to prevent the mask support from tilting
and/or rotating. Furthermore, a method and a construction are described,
by which the enlargement can be corrected by a combined defocusing and
subsequent displacement of the light sourc with respect to the lens
system. This method can be carried out only with comparatively great
difficulty and the extent of the correction is determined by the
admissible variation in depth of focus. It has moreover been found that
atmospheric pressure variations also render it necessary that the focusing
be corrected. In the two known devices no solution is provided for a
general correction both of the enlargement and of the focusing. In
general, it should be possible to correct the focusing and/or the
enlargement for deviations due to ambient influences, such as temperature
and atmospheric pressure, and for deviations due to manufacturing and
mounting tolerances. The deviations may relate both to the device itself
and to the mask and the substrate. For example, it should be possible to
correct for deformations resulting from the load connected with the
support of mask and substrate. Furthermore, there should be the freedom to
use for the various processing steps of the same substrate several
opto-lithographic devices or to switch without further expedients to
another opto-lithographic device in the case of disturbances or
maintenance. In such a case, the imaging properties of the various devices
should be accurately tuned to each other. There should further be a
possibility of focusing in a surface of the substrate located below the
top surface.
The invention has for its object to provide an opto-lithographic device, in
which an incidental and/or continuous adjustment or correction both of the
enlargement and of the focusing is possible by means of a construction
which is rigid against tilting and minimizes relative displacement of mask
and lens system in planes at right angles to the optical axis of the lens
system.
For this purpose, the invention is characterized in that the device
comprises a second circular-cylindrical coaxial holder displaceable with
respect to and together with the first circular-cylindrical holder and a
fixedly arranged first actuator cooperating with one of the holders and a
second actuator secured to one of the holders and co-operating with the
other holder, one holder being secured to the other holder by means of a
first elastic coupling member and one of the holders being secured to the
frame by means of a second elastic coupling member, while the distance
between the lens system secured to one of the holders and the substrate
table is adjustable with a constant distance between the mask support
coupled to the other holder and the lens system by means of the first
fixedly arranged actuator by elastic deformation of the second elastic
coupling member and the distance between the lens system and the mask
support is adjustable with a constant distance between the substrate table
and the lens system by means of the second actuator by elastic deformation
of the first coupling member. Due to the fact that both the distance
between the substrate table and the lens system and the distance between
the mask support and the lens system are adjustable in a simple periodical
or continuous manner, whilst the variation of the first-mentioned distance
does not lead to a simultaneous variation of the last-mentioned distance,
and conversely, an opto-lithographic device is obtained having a high
flexibility in compensation of influences causing imaging errors, such as
atmospheric pressure variations, manufacturing tolerances, mounting
tolerances and temperature variations.
A particular embodiment of the opto-lithographic device which is relatively
rigid against tilting and dynamically stable, is further characterized in
that the lens system is coupled to the first holder, which is surrounded
at least in part by the second holder and is secured by means of a third
elastic coupling member to the frame, while the second holder is secured
by means of a fourth elastic coupling member to the first holder.
A further embodiment of the opto-lithographic device with elastically
deformable coupling members that can be manufactured in a comparatively
simple and inexpensive manner, is characterized in that the elastic
coupling members are constituted by elastically deformable metal rings
provided with an even number of pairs of elastic pivots between which are
situated comparatively long sections of the ring, while comparatively
short sections of the ring are situated between the elastic pivots of a
pair, whereby the comparatively short sections of the first coupling
member are secured alternately to the first holder and the second holder
and the comparatively short sections of the second coupling member are
secured alternately to the frame and to one of the holders.
A still further embodiment of the opto-lithographic device, in which with
unilaterally driven holders a very high resistance is nevertheless offered
against relative tilting of the holders, is characterized in that at least
one pre-stressed first compensation spring is arranged between the
holders, while at least one prestressed second compensation spring is
arranged between the frame and one of the holders.
A still further embodiment of the opto-lith ographic device comprising
simply constructed clearance-and hysteresis-free actuators is
characterized in that the first and the second actuator are constituted by
a first and a second eccentric which are in engagement with one and the
other holder, respectively, and are driven by a first and a second direct
current motor, the first motor being secured to the frame, while the
second motor is secured to one of the holders.
A further embodiment of the opto-lithographic device which is particularly
suitable for automatic control by a processor is characterized in that the
device is provided with servo control means comprising a first and a
second control loop, while in the first control loop an optical focusing
detector secured to the holder with the lens system supplies a first
control signal to a focusing motor coupled to the first actuator and in
the second control loop an optical enlargement detector supplies a second
control signal to an enlargement motor coupled to the second actuator.
The invention has further for its object to provide a method of controlling
the imaging properties of an optical lens system in an opto-lithographic
device. in which arbitrary corrections with respect to the focusing and
enlargement, respectively, of the lens system can be carried out manually,
semi-automatically or fully automatically.
A method according to the invention is for this purpose characterized in
that in a first control loop of a servo control means a first control
signal determined by a focusing detector is supplied to a focus motor,
which is coupled to a first actuator, by which the desired distance
between the lens system and a substrate is adjusted, while in a second
control loop of the servo control means a second control signal determined
by an enlargement detector is supplied to an enlargement motor which is
coupled to a second actuator, which adjust the desired distance between
the mask and the lens system.
A further embodiment of a method according to the invention, which provides
a simple correction possibility for deviations from the focusing and the
enlargement of the lens system caused by varying atmospheric pressure, is
characterized in that the distance between the lens system and the
substrate on which the mask is imaged is measured by means of the optical
focus detector, while the first control signal supplied by the focus
detector is correlated to the desired distance between the lens system and
the substrate and to the atmospheric pressure, after which the desired
distance between the lens system and the substrate is adjusted by
supplying a first control signal to the focus motor of the first actuator,
after which, whilst the first control loop remains activated, the second
control loop is activated and the second control signal determined by the
enlargement detector and correlated to the atmospheric pressure is
supplied to the enlargement motor of the second actuator for adjusting the
desired enlargement, which second control signal is replaced after
adjustment of the desired enlargement by a third control signal obtained
by means of a relative position sensor and calibrated by the second
control signal, after which the activation both of the first and of the
second control loop is maintained during a number of successive exposures
of the substrate.
The invention will be described more fully with reference to the drawing,
in which:
FIG. 1 shows diagrammatically an opto-lithographic device according to the
invention,
FIG. 2 is a perspective view of a part of the device shown in FIG. 1,
FIG. 3 is a sectional view of the means for securing the first and the
second holder as used in the device shown in FIGS. 1 and 2,
FIG. 4 shows in elevation and in diametrical sectional view an elastic
coupling member,
FIG. 5 shows an optical focus detector as used in the device shown in FIGS.
1 and 2,
FIG. 6 shows an optical enlargement detector as used in the device shown in
FIGS. 1 and 2,
FIG. 7 shows a circuit diagram of servo control means as used in the device
shown in FIGS. 1 and 2.
The opto-lithographic device shown in FIG. 1 (a so-called "wafer stepper")
comprises a fixedly arranged frame 1, in which a rectangular granite plate
3 is situated, which serves as a support for four vertical columns 5
secured thereto. The columns 5, of which only two are shown in FIG. 1, are
arranged near the edges of the plate 3 in a rectangular pattern. Three
plates 7, 9 and 11 further form part of the frame 1 and these plates are
secured to the columns 5. The metal plate 7, the metal plate 9 and the
glass plate 11 extend in horizontal planes and are parallel to the granite
plate 3. The opto-lithographic device includes an optical lens system 13
comprising a number of lens elements, such as the elements 15 and 17, of
which an optical axis 19 coincides with a Z axis of an orthogonal system
of axes X, Y, Z shown in FIG. 1. The lens system 13 is secured in a first
circularcylindrical metal (lens) holder 21 surrounded partly by a coaxial
second circular-cylindrical metal (mask) holder 23. The centre lines of
the lens holder 21 and of the mask holder 23 coincide with the optical
axis 19 and the Z axis. On the mask holder 23 is disposed a support 25
shown further in FIG. 2 with engagement pads 27 for a mask 29 (FIG. 3), of
which the pattern has to be imaged by the lens system 13 on a reduced
scale (10:1) on a semiconductor substrate 31 disposed on a table 33. The
table 33 is guided by means of an aerostatically supported foot 35 over
the granite plate 3 and can be displaced paralel to the X axis and/or Y
axis and can be rotated about an axis parallel to the Z axis. This table
33 displaceable by means of three linear electric motors (cf. FIG. 2) is
known and described by R.H. Munnig Schmidt and A. G. Bouwar in the
magazine "De Constructeur" of October 1983, No. 10. As shown in FIG. 2,
the drive of the table 33 comprises a linear motor having an X-stator 37
and an X-translator 39 secured to the table 33 for the translation
parallel to the X-axis and two linear motors for the translation parallel
to the Y-axis and the rotation about an axis parallel to the Z-axis,
respectively. Of the two last-mentioned motors, one has a Y.sub.1 -stator
41 and a Y.sub.1 - translator 43, while the other has a Y.sub.2
-translator 45 and a Y.sub.2 -translator 47. The table 33 can perform very
accurate continuous, stepwise or oscillating movements due to a measuring
system on the basis of laser interferometry known in principle from U.S.
Pat. No. 4,251,160. A laser beam 53 originating, for example, from a
helium neon laser 49 is passed by a prism 55 to two semi-transparent
prisms 57, 59 and a prism 61 so that three subbeams 63, 65, 67 are formed.
The subbeams 63, 65 and 67 reflected on reflecting side edges of the table
33 are joined in interferometers 69, 71 and 73 with reference beams (not
visible) to form interference beams 75, 77 and 79, of which the intensity
is measured by photo-cells in receivers 81, 83 and 85. The substrate 31 is
exposed in a number of different positions of the table 33 with respect to
the lens system 13 by means of a light source 87 (FIG. 1), of which the
light is reflected by a parabolic mirror 89. The light is passed to the
mask 29 by a mirror 91, a shutter 93, a diaphragm 95, a mirror 97 and a
condenser lens 99. By means of a particular coupling to be described more
fully hereinafter with reference to FIGS. 1, 2, 3 and 4 of the lens holder
21 and the mask holder 23 to each other and to the frame 1, respectively,
the distance between the lens system 13 and the substrate 31 can be
adjusted for the focusing and the distance between the mask 29 and the
lens system 13 can be adjusted for the enlargement.
As appears from FIGS. 1, 2 and 3, the lens holder 21 (first holder) is
coupled to the mask holder 23 (second holder) by means of a first elastic
coupling member 101 in the form of an elastically deformable metal ring,
while the mask holder 23 is coupled by means of a second elastic coupling
member 103 in the form of an elastically deformable metal ring to the
frame 1 by means of the plate 7. The lens holder 21 is further secured by
means of a third elastic coupling member 105 to the frame 1 by the glass
plate 11, while the mask holder 23 is secured by means of a fourth elastic
coupling member 107 to the lens holder 21. The third and fourth coupling
members 105 and 107, respectively, are also in the form of an elastically
deformable ring. The rings 101, 103, 105 and 107 are in principle
identical and of the kind shown in FIG. 4, although their dimensions are
different. As is shown in FIG. 4, each ring 101, 103, 105 and 107 has an
even number of pairs of elastic segments of reduced thickness called
pivots 109 and 111 with comparatively long sections 113 between the pairs
and comparatively short sections 115 between the two pivots 109 and 111 of
each pair. Each short section 115 is provided with a threaded hole 117. As
appears from FIG. 3, the lens holder 21 has an annular flange 119, which
is arranged opposite to an annular shoulder 121 of the mask holder 23. The
short sections 115 of the ring 101 are secured by means of bolts (not
shown in FIG. 3) alternately to the flange 119 of the lens holder 21 and
to the shoulder 121 of the mask holder 23. Spacer plates 123 and 125 are
clamped between the short sections 115 and the flange 119 and the shoulder
121, respectively, and these spacer plates ensure that the long sections
113 are arranged so as to be free from the flange 119 and the shoulder
121. The mask holder 23 further has an annular flange 127, which is
arranged opposite to a ring 129 secured in the plate 7. The short sections
115 of the ring 103 are secured by means of bolts 131 alternately to the
flange 127 and to the ring 129. Spacer plates 133 and 135 are clamped
between the short sections 115 of the ring 103 and the flange 127 and the
ring 129, respectively, and these spacer plates ensure that the long
sections 113 are arranged so as to be free from the flange 127 and the
ring 129. The lens holder 21 has an annular shoulder 137, against which is
screwed a rigid flat ring 139 by means of bolts 141 (in FIG. 3 only one
bolt 141 is shown). The ring 139 extends below the mask holder 23 and
serves to secure the ring 107 to the lens holder 21 by means of bolts 143
which are passed through the holes 117 of the short sections 115 of the
ring 107 and are secured alternately to the mask holder 23 and the ring
139. Spacer plates 145 and 147 are clamped between the short sections of
the ring 107 and the mask holder 23 and the ring 139, respectively, and
these spacer plates ensure that the long sections 113 of the ring 107 are
arranged so as to be free from the mask holder 23 and the ring 139. The
lens system 13 is supported on a flat ring 149, which is disposed on a
shoulder 151 of the lens holder 21. The ring 149 and the lens system 13
are secured in the lens holder 21 by means of bolts 153. The lens holder
21 is further provided with an annular flange 155, which serves to secure
the ring 105 to the lens holder. Below the ring 105, four rings 157, 159,
161 and 163 are disposed. By means of bolts 165, which are passed through
the holes 117 of the short sections 115 of the ring 105, the ring is
secured alternately to the flange 155 and to the ring 157. Spacer plates
158 and 160 are clamped between the short sections 115 of the ring 105 and
the flange 155 and the ring 157, respectively, and these spacer plates
ensure that the long sections 113 of the ring 105 are arranged so as to be
free from the flange 155 and the ring 157. The ring 157 is secured by
means of bolts 167 to the ring 159, while the ring 161 is secured by means
of bolts 169 to the ring 163 mounted in the glass plate 11. The rings 161
and 163 together constitute a U-shaped chamber in which the ring 159 is
enclosed. The ring 163 is provided with a threaded hole 171 for an
adjustment bolt by means of which the radial position of the lens holder
21 with respect to the glass plate 11 can be adjusted. For this purpose, a
radial clearance 173 and a radial clearance 175 are provided between the
rings 157, 161 and the rings 159, 163, respectively. After the lens holder
21 has been correctly adjusted, (this step will be described hereinafter),
the ring 159 is clamped in a manner not shown further for the sake of
simplicity between the rings 161 and 163, while the ring 159 is relieved
in radial direction by turning the adjustment bolt backwards in the
threaded hole 171. Thus, the roundness of the ring 159 is guaranteed.
On the metal plate 9 is mounted a socle 177, in which a bearing bush 179 is
provided for a driving shaft 181 which is coupled to the outgoing shaft of
a direct current motor 183, which will be designated hereinafter as focus
motor 183. The driving shaft 181 is rotatably journalled in the bearing
bush 179 by means of a ball bearing 185. The end of the driving shaft 181
has secured to it an eccentric sleeve 187, on which a ball bearing 189 is
mounted, of which an outer race 191 engages the ring 139 arranged above it
and secured to the lens holder 21 and to the ring 107. The centre line of
the bush 179 lies eccentrically with respect to the centre line of the
driving shaft 181. Thus, a coarse adjustment of the holders 21 and 23 is
obtained that can be carried out manually. The focus motor 183, the
driving shaft 181, the eccentric sleeve 187 and the ball bearing 189
together constitute the said fixedly arranged first actuator for focusing
the lens system 13. The wall of the mask holder 23 is provided with a
circular hole 193, in which a rotatable bearing bush 195 is mounted. The
said bearing bush 195 has secured to it the housing of a direct current
motor 197, whose driving shaft is rotatably journalled in the bearing bush
195. An eccentric 199 engaging the lower edge of a hole 201 in the wall of
the lens holder 21 is secured on the driving shaft of the motor 197, which
will be designated hereinafter as the enlargement motor 197. The bearing
bush 195 can be clamped in the hole 193 by means of a bracket 203. The
centre line of the circular eccentric 199 has shifted parallel with
respect to the centre line of the driving shaft of the enlargement motor
197, while the centre line of the bearing bush 195 has shifted parallel
with respect to the centre line of the circular-cylindrical bearing bush
195 rotatable in the hole 193. Thus, the possibility is provided of
obtaining first a coarse and then a fine adjustment of the relative axial
position (parallel to the Z-axis) of the lens holder 21 and the mask
holder 23, as will appear more clearly from the following description. The
enlargement motor 197 with its driving shaft, the bearing bush 195 and the
eccentric 199 together constitute the said displaceable second actuator
for adjusting the enlargement of the lens system 13.
The overall weight of the mask holder 23, the lens holder 21 and all the
parts secured to it and displaceable parallel to the Z-axis acts, viewed
in the circumferential direction, is supported in three points, which are
located substantially on a circle at right angles to the Z-axis,
distributed over sections of 120.degree.. The sectioal view of the pin 205
in FIG. 3 is rotated through 60.degree. for the sake of simplicity. One of
the said three points is the contact point between the ball bearing 189
and the ring 139, while the remaining two points are constituted by two
pins engaging the lower side of the ring 139. Only one pin 205 of these
two pins is shown in FIG. 3. The pin 205 is pressed against the ring 139
by a pre-stressed helical spring 207 (second compensation spring), which
is arranged in a holder 209. The hold der 209 is threaded and can be
adjusted by means of a threaded sleeve 211, which is secured in the metal
plate 9. The wall of the lens holder 21 is provided by means of a bolt 213
with a supporting sleeve 215 for a pin 217, which is screwed on the upper
side into a circular-cylindrical support 219 and on the lower side into
the supporting sleeve 215. The pin 221 is pressed against a sleeve 225
secured by a bolt 223 to the wall of the mask holder 23 by a pre-stressed
helical spring 227 (first compensation spring). viewed in the
circumferential direction, two pins 221 are present, which are located
together with the contact point of the eccentric 199 with the lens holder
21 substantially on a circle at right angles to the Z-axis, distributed
over sections of 120.degree.. The sectional view of the pin 221 in FIG. 3
is rotated through 60.degree. for the sake of simplicity. Because the
rings 101, 103, 105 and 107 are clamped by means of spacer plates at the
area of the short sections 115, the long sections 113 are arranged so as
to be free and can consequently be subjected to a comparatively small
elastic deformation in two directions by bending forces and torsional
forces. The largest contribution to the elastic deformation is provided by
the bending forces. The elastic deformation of the rings 101, 103, 105 and
107 is obtained as follows (cf. more particularly FIGS. 2 and 3).
If with a stationary eccentric 199 the eccentric sleeve 187 is rotated by
energization of the focus motor 183 and as a result the ball-bearing 189
is moved up and down in vertical direction, the rings 103 and 105 are
elastically deformed, while the rings 101 and 107 are not deformed. It
should be noted that the elastic deformation due to the eccentric sleeve
187 is meant here and not the static deformation to which the rings 101
and 107 have been subjected by the weight of the mask holder 23, the lens
holder 21 and the parts displaceable simultaneously therewith. The manner
in which the eccentric 199 is stopped, will be explained more fully
hereinafter with reference to FIG. 7. Due to the vertical displacement of
the ball bearing 189, the holders 21 and 23 are displaced together
parallel to the fixed Z-axis so that an increase or a decrease of the
distance between the lens system 13 and the substrate 31 is obtained.
Consequently, the displacement described can be used for focusing the lens
system 13 because the image distance can be controlled by a simultaneous
displacement of both holders. The object distance between the mask 29 and
the lens system 13 then consequently remains unchanged. During focusing,
the enlargement remains unchanged because the lens system 13 is
telecentric on the side of the substrate. It should be noted that a lens
system is telecentric on a given side (in the present case therefore the
picture side) if the ray (main ray) of the light beam passing through the
centre of the so-called pupil is always at right angles to the image plane
or object plane corresponding to that side. The lens system 13 in the
opto-lithographic device according to the invention is intentionally
constructed so as not to be telecentric on the object side in order that
the enlargement can be controlled in a simple and practical manner. It
will be described more fully hereinafter, what influences make such a
control of the enlargement desirable.
If with a stationary eccentric sleeve 187 the eccentric 199 is rotated by
energization of the enlargement motor 197 and as a result with a
stationary lens holder 21 the mask holder 23 is moved up and down in
vertical direction, the rings 101, 103 and 107 are elastically deformed,
while the ring 105 is not deformed. Also in this case, the dynamic elastic
deformation due to solely the rotation of the eccentric is meant and not
the static deformation to which the rings 101, 103 and 107 have been
subjected by the weight of the mask holder 23 and the parts connected
thereto and displaceable with suspect to the lens holder 21. The manner in
which the eccentric sleeve 187 is stopped, will be described more fully
hereinafter with reference to FIG. 7. Due to the relative vertical
displacement of the mask holder 23 with respect to the lens holder 21, the
distance between the lens system 13 and the mask 29 is increased or
decreased. The displacement described is utilized for controlling the
object distance with a constant image distance so that the enlargement of
the lens system can thus be controlled or adjusted.
It should be noted that the pins 205 and 221, which are pressed by the
pre-stress of the compensation springs 207 and 227 against the ring 139
and the sleeve 225, respectively, fulfil a double function. The pins 205
and 221 ensure that the mechanical load of the two holders, the elastic
coupling members and the frame is distributed as uniformly as possible,
viewed in the circumferential direction of the holders. The presence of
the pins 205 and 221 insures that the ball bearing 189 and the eccentric
199 are not loaded too heavily. The pin 205 and the ball bearing 189 carry
the weight of the two holders 21, 23 and the parts connected thereto and
displaceable simultaneously. The pin 205 prevents the holders 21, 23 from
being tilted due to the unilateral force exerted on the ring 139 by the
ball bearing 189. If the pre-stress of the compensation spring 207 is
defined as the spring tension associated with the so-called central
position of the ball bearing 189, the compensation springs are loaded
during focusing by the pre-tension force plus or minus a third of the
force required to displace the two holders 21, 23 during focusing. In an
analogous manner, the pin 221 and the compensation spring 227,
respectively, and the eccentric 199 are loaded during adjustment of the
enlargement by the pre-tension force plus or minus a third of the force
required to displace the mask holder 23 with respect to the stationary
lens holder 21. The pin 221 prevents the mask holder 23 from being tilted
during adjustment of the enlargement by the unilateral engagement of the
eccentric 199. The pins 205 and 221 also prevent the holders 21, 23 from
being tilted in the static position, that is to say when no focusing or
enlargement movements take place.
The focusing and the adjustment of the enlargement are effected by means of
sensors, namely, a focus detector 229 shown in FIG. 5 and an enlargement
detector 231 shown in FIG. 6. The focus detector 229 and the enlargement
detector 231 are optical detectors supplying a focus error signal and an
enlargement error signal, respectively, in a servo control system 233,
which is shown in FIG. 7. The focus detector 229 described in principle in
U.S. Pat. No. 4,356,392, of which a particular embodiment with optical
zero point adjustment is suggested and described in Netherlands patent
application No. 8600253, comprises a radiation source 235, such as, for
example, a diode laser, a polarization splitting cube 237, a number of
prisms 239, 241, 243, a mirror 245 and two radiation-sensitive detectors
247 and 249. A lens 251 produces a radiation spot S.sub.1 on the substrate
31, while a lens 253 images together with a lens 255 the radiation spot
S.sub.1 as a radiation spot S.sub.2 on the detectors 247 and 249. The
mirror 245 lies in the focal plane of the lens 253. The mirror 245 and the
lens 253 together constitute a so-called retro-reflector, by which a light
beam arriving at the mirror 245 is reflected in itself and is focused as a
mirror image in the radiation spot S.sub.1 so that a symmetrical radiation
spot is obtained. Local reflection differences of the substrate 31
consequently will not influence the intensity distribution within the
radiation spot S.sub.2 formed on the detectors 247 and 249. The parts of
the focus detector 229 denoted by reference numerals 235, 237, 239, 247,
249 255 and 257 are fixedly arranged, while the parts of the focus
dectector 229 denoted by reference numerals 241, 243, 245, 251, 253 and
259 are connected to the lens holder 21. With a variation of the distance
between the lens system 13 and the substrate 31 due to, for example,
thickness differences of the substrate 31, the radiation spot S.sub.2 is
displaced with respect to the detectors 247 and 249 so that one detector
receives a smaller or larger radiation intensity than the other detector.
The detectors 247 and 249 thus produce signals of different value. The
output signal of a differential amplifier 257 connected to the detectors
247 and 249 forms a focus error signal .DELTA. F which is used in the
servo-control system 233. (cf. FIG. 7). If atmospheric pressure variations
are found to exert too large an influence on the focusing, this can be
corrected by means of a rotatable plane-parallel glass plate 259, by which
the optical path length is adjustable. Since the focus detector 229 does
not detect a variation of the focusing due to a variation of the barometer
pressure, a point adjustment or a so-called "off-set" and a correction of
the focus error signal .DELTA.F can be obtained by means of an adaptation
of the optical path length with the aid of the rotatable plate 259. The
plate 259 may be adjusted both by hand and by a motor. By means of, for
example, a table or a graph in which the influence of the atmospheric
pressure on the focusing is determined by means of measurements, the
extent of the manual rotation of the plate 259 can be determined. Since
barometer variations are generally comparatively slow variations, for
example a period of one day or a few days may be chosen between successive
corrections. Corrections may also be carried out continuously and
automatically by using the measurement signal of a barometer in a separate
servo system in which a motor drive for the plate 259 is included. Such a
continuous focus correction as such does not influence the enlargement
because the lens system 13 is telecentric on the substrate side. The
manual adjustment of the plate 259 may be used for experimental images on
a test substrate and for focusing on a plane which does not coincide with
the top surface of the substrate 31 but is located in the substrate 31.
It should be noted that positioning of the elastic coupling members in the
opto-lithographic device shown diagrammatically in FIG. 5 differs from
that of the device shown in FIGS. 1, 2 and 3. The elastic coupling member
105 (ring 105) in the last-mentioned figures is replaced by an elastic
coupling member 261 (third coupling member) in the form of a spring 261
between the mask holder 23 and the fixedly arranged frame not shown. Such
an embodiment is an alternative to the construction shown in FIGS. 1, 2
and 3 and also lies within the scope of the invention. The characteristic
difference is that the mask holder 23 is directly connected at two points
(instead of at one point) by elastic coupling members to the frame, while
the lens holder 21 is not at all connected by elastic coupling members
directly to the frame, but is connected at two points to the mask holder
23.
The enlargement detector 231 already suggested and described in Netherlands
patent application No. 8601278 which is the priority application for U.S.
patent application Ser. No. 918,758 filed on Oct. 14, 1986 which is
assigned to the assignee of this application is constituted by a system,
in which a test mask or circuit mask 29 is provided with two amplitude
rasters 263, 265, which are imaged on two corresponding amplitude rasters
267, 269 in the table 33. The lens system 13 indicated diagrammatically in
FIG. 6 by two lenses 271, 273 images the raster 263 on the raster 269 and
images the raster 265 on the raster 267. Of a light beam 275 imaging the
raster 263 on the raster 269 only a main ray 277 is indicated, while of a
light beam 279 imaging the raster 265 on the raster 267, besides a main
ray 281 also edge rays 283 and 285 are shown. The beams 275 and 279 may
form part of a single wider light beam 287, which is preferably the same
beam by which later the circuit masks 29 are imaged on the substrate 31.
Small deviations in the raster images then cannot be obtained due to a
difference in wavelength because the lens system 13 is corrected for
aberrations at the wavelength used for the later repeated exposures.
Radiation detectors 289 and 291 are arranged in the light path of each of
the light beams 279 and 277, respectively, passing through the rasters 267
and 269, respectively. The detectors 289 and 291 are situated in the table
33. In the case in which the rasters 263 and 265 are imaged with the
correct enlargement on the rasters 269 and 267, the frequencies of the
corresponding raster images are equal to the frequencies of the rasters
269 and 267. The quantity of radiation received by the detectors 289 and
291 from the rasters 265, 267 and 263, 269, respectively, then accurately
aligned with respect to each other is equal. With an enlargement error,
the imaged raster 263 and the raster 269 and the imaged raster 265 and the
raster 267 do not fit to each other and a Moir pattern is obtained, whose
period is a measure for the enlargement error. In order that this
enlargement error can be converted into an enlargement signal, the mask
rasters 263, 265 and the table rasters 267, 269 are periodically displaced
with respect to each other parallel to the X-axis by means of a drive 293,
which is coupled to the table 33. The drive 293 may be constituted by the
linear electric motor having the X-stator 37 and the X-translator 39
already present in the opto-lithographic device (cf. FIG. 2). The
interferometer system (63, 81) already described may be used for
controlling the periodical displacement of the table 33 on behalf of the
measurement of the enlargement. Due to the periodical movement of the mask
rasters with respect to the table rasters, a phase difference occurs
between the substantially sinusoidal output signals 295 and 297 of the
detectors 289 and 291 in the case of an enlargement error (and hence a
Moir pattern with a finite period). The signals 295 and 297 are supplied
to a phase comparator 299, which supplies an enlargement error signal
.DELTA. M.sub.1, which is used in the servo control system 233 shown in
FIG. 7. Since the enlargement detector 231 also detects the enlargement
errors due to atmospheric pressure variations, the enlargement signal
.DELTA. M.sub.1 can also be used to correct for pressure variations by
means of the servo control system 233. A zero point adjustment or a
so-called "off-set" as used in the focus detector 229 is therefore not
necessary with pressure variations for the enlargement detector 231. This
means that the enlargement correction desired because of pressure
variations is directly discounted in the enlargement error signal .DELTA.
M.sub.1 so that the latter need not necessarily be derived from tables or
graphs based on measurements and calculations. The risk of making errors
during the enlargement correction is thus considerably reduced due to the
fact that reading errors and interpolation errors are avoided.
The servo control system 233 shown in FIG. 7 comprises a first control loop
301 for adjusting the focusing and a second control loop 303 for adjusting
the enlargement. As appears from FIG. 3, a sleeve 305 passed with
clearance through the mask holder 23 and secured to the lens holder 21
projects beyond the contour of the mask holder 23 and a relative
displacement of the holders 21 and 23 parallel to the Z-axis is converted
into a translatory movement of a feeler 307, which is slidably journalled
in a block 309 connected to the mask holder 23. The sleeve 305, the feeler
307 and the block 309 together constitute a relative position sensor,
which is designated by reference numeral 311 in FIG. 7. The first control
loop 301 can be activated from a control circuit 313 by a switch 315. The
control loop 301 further includes a proportional integrating controller
317, the focus motor 183 and the focus detector 229 supplying the focus
error signal .DELTA. F. The second control loop 303 can be activated from
the control circuit 313 by a switch 319. The control loop 303 includes
further a proportional integrating controller 321, the enlargement motor
197 and, depending upon the position | | |
