Process for positioning a mask relative to a workpiece

To increase operating efficiency and prevent operating errors, such as adjustment errors and the like, by automatic computation of the distance between the workpiece alignment marks, according to the invention, workpiece alignment marks located at two locations on a workpiece are subjected to image recording by alignment units, their positions are stored as first positions, then by rotation of the workpiece by a preset very small angle, the workpiece alignment marks located at two locations on the workpiece are determined again, and their positions are stored as second positions. Based on the data of the first and second positions, the distance between the workpiece alignment marks is determined. Then, based on the distance data, the angular offset of the workpiece is determined. After correction of this angular offset, the mask and/or the workpiece is/are moved such that the images of the mask alignment marks and the workpiece alignment marks come to rest on top of one another.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to a process for automatic positioning of a mask 
relative to a workpiece and a device for executing the process in an 
exposure device which is used for producing a semiconductor device, a 
printed board, and a LCD (liquid crystal display) and for similar 
purposes. The invention relates especially to a process for positioning a 
mask relative to a workpiece in which the distance between the two 
alignment marks can be automatically computed; the marks can be used for 
positioning a mask relative to a workpiece and they are recorded on the 
workpiece. The invention furthermore relates to a device for executing 
this process. 
2. Description of Related Art 
Production of electrical and electronic components and parts of various 
types in which processing of structures in the micron range is necessary 
encompasses an exposure process. These electronic parts are semiconductor 
components, liquid crystal displays, printer heads of the inkjet type, 
multichip modules in which a plurality of different electronic components 
are produced on a substrate and thus a module is formed, and the like. In 
this exposure process, a mask is used in which metal, such as chromium or 
the like, is vacuum-evaporated and etched on a transparent substrate, such 
as glass or the like, and a pattern formed. Through this mask, ultraviolet 
rays are emitted onto the workpiece and the mask pattern is transferred to 
the photoresist which has been applied to the workpiece. 
Exposure systems are divided into the projection printing type, the contact 
printing type, and the proximity printing system type. In the projection 
printing type, a mask image is imaged onto the workpiece by a projection 
lens. In the contact printing system, parallel light is emitted in a state 
in which the mask and the workpiece are arranged directly abutting one 
another. In the proximity printing system, parallel light is emitted in a 
state in which a small intermediate space is formed between the mask and 
the workpiece. 
In this exposure process, it is important in the case of transfer of the 
mask pattern onto the workpiece, that a pattern to be transferred 
subsequently is exactly positioned relative to a pattern formed 
beforehand. The above described positioning is ordinarily done such that 
the alignment marks of the mask and the workpiece come to rest on top of 
one another. 
FIG. 1 schematically shows the arrangement of a projection exposure device 
in which the invention can be used. First of all conventional positioning 
of the mask relative to the workpiece is described below using FIG. 1: 
In the figure, an exposure light irradiation device (or a nonexposure light 
irradiation device) 1, which has a lamp 1a which emits exposure light, for 
example a high-pressure mercury lamp or the like, a focussing mirror 1b, a 
shutter 1c, an optical filter 1d which is used when nonexposure light is 
emitted, and a condenser lens 1e, is shown. 
Furthermore, a mask carrier 2 is shown which is driven by means of a drive 
device (not shown) in the X-Y-Z-.THETA. directions (X-axis, Y-axis: 
orthogonal axes on a plane parallel to one workpiece carrier surface, 
Z-axis: an axis perpendicular to the workpiece carrier surface, 
.THETA.-axis: an axis of rotation around the Z-axis). 
Reference letter M designates a mask on which a mask pattern and mask 
alignment marks MAM1, MAM2 are recorded for purposes of positioning. 
Reference number 3 indicates a projection lens and reference letter W a 
workpiece. Workpiece alignment marks WAM1, WAM2 are recorded on workpiece 
W for purposes of positioning by a workpiece carrier which is driven by a 
drive (not shown) in the X-Y-Z-.THETA. directions, 
Reference number WA1 indicates a workpiece alignment mark partial 
illumination system. The nonexposure light emitted from a light source 
(not shown) is incident via optical fibers 6a, furthermore via lens 6b and 
mirror 6c, on a half mirror 5e of alignment unit 5, and via a lens 5b and 
a half mirror 5c, irradiates workpiece alignment mark WAM on workpiece W. 
An alignment unit 5 consists of a lens 5a, an objective lens 5b, half 
mirrors 5c and 5e, and an image sensor 5d which has a CCD camera. The mask 
alignment mark MAM projected onto the workpiece W and the workpiece 
alignment mark WAM irradiated by workpiece alignment mark partial 
illumination system WA1 are recorded via half mirror 5c, objective lens 
5b, half mirror 5e and lens 5a by means of image sensor 5d. 
On each of mask M and workpiece W are several mask alignment marks MAM1, 
MAM2 and several workpiece alignment marks WAM1, WAM2 (each at two 
locations in this case), and alignment unit 5 and workpiece alignment mark 
partial illumination system WA1 are assigned accordingly to the respective 
alignment mark. 
In the figure, mask M is positioned relative to the workpiece W in the 
following manner: 
(1) Workpiece W on which workpiece alignment marks WAM1, WAM2 are recorded 
is subjected to prealignment and placed on workpiece carrier 4. 
(2) Nonexposure light (or exposure light) is emitted from exposure light 
irradiation device (or a nonexposure light irradiation device) 1 onto mask 
alignment marks MAM1, MAM2 of mask M. 
(3) The images of the above described mask alignment marks, which are 
imaged on workpiece W, are recorded by image sensors 5d of alignment units 
5 and their positions are stored by an image processing device (not 
shown). Here, it is assumed that the line to the X-axis or Y-axis formed 
between the above described mask alignment marks MAM1 and MAM2 is set 
parallel and represents the direction of motion of the workpiece carrier 
and/or mask carrier. 
(4) Nonexposure light (or exposure light) is emitted from workpiece 
alignment mark partial illumination systems WA1 onto workpiece alignment 
marks WAM1, WAM2 of workpiece W and workpiece alignment marks WAM1, WAM2 
on workpiece W are determined by means of alignment units 5. 
(5) Based on the positions of the images of the above described mask 
alignment marks and the positions of the images of the workpiece alignment 
marks which were determined by alignment units 5, a measure of the 
position deviation of mask M from workpiece W is computed. Based on this 
value, mask carrier 2 and/or workpiece carrier 4 is/are moved and 
positioning of the mask M relative to the workpiece W is performed. 
Computation of the above described measure of the position deviation and 
the positioning of the mask to the workpiece accomplished thereby are 
performed in the following manner: 
(1) How many degrees workpiece alignment marks WAM1, WAM2 are angularly 
offset with reference to mask alignment marks MAM1, MAM2 is computed. This 
means that how many degrees the segment formed between workpiece alignment 
marks WAM1 and WAM2 is angularly offset with reference to the segment 
formed between mask alignment marks MAM1 and MAM2 is computed. (This 
angular offset is hereinafter called ".DELTA..THETA."). 
FIG. 8 schematically represents the process for computing the above 
described measuring of the position deviation. In the figure, the images 
of the mask alignment marks MAM1, MAM2 and the images of workpiece 
alignment marks WAM1, WAM2, which were recorded by means of alignment 
units 5, are shown. In the figure, A and B each identify the images 
recorded by alignment units 5 located at two positions. 
In FIG. 8, the following formula (1) applies when the above described 
angular offset is are represented as .DELTA..THETA., the respective 
position coordinates of the workpiece alignment marks WAM1, WAM2 are 
represented as (x1, y1), (x2, y2) and the distance between them is 
represented by L: 
EQU .DELTA..THETA.=sin.sup.-1 ((y2-y1) (1) 
The angular offset .DELTA..THETA. can be determined using the above 
described equation (1) when the position coordinates of the workpiece 
alignment marks WAM1, WAM2 are found, since the distance L is stipulated. 
(2) If the angular offset Ae has been determined in this way, workpiece 
carrier 4 (or mask carrier 2) is rotated according to angular offset 
.DELTA..THETA.. 
After rotation, again, mask alignment marks MAM1, MAM2 and workpiece 
alignment marks WAM1, WAM2 are determined by means of alignment units 5. 
Workpiece carrier 4 and/or mask to carrier 2 is/are moved in the X-axis 
direction and/or Y-axis direction such that two alignment marks MAM1, WAM1 
and MAM2, WAM2 come to rest on top of one another. 
After positioning of mask M to workpiece W in the above described manner, 
exposure light is emitted from exposure light irradiation device (or 
nonexposure light irradiation device) 1, the mask pattern is projected on 
workpiece W, and exposure is accomplished. 
The distance L between workpiece alignment marks WAM1 and WAM2 on the 
workpiece does change according to the size of workpiece W. But, it is a 
value that is specific to the manufacturer. Distance L was therefore 
conventionally integrated in the computer run in the process of producing 
the exposure device. 
Furthermore, under certain circumstances there was additionally a means for 
distinguishing the size of workpiece W, different values of L were stored 
in the device and a value of L which corresponds to the size of workpiece 
W was called up. 
However, workpiece alignment marks WAM are often located in the strips 
between the circuit patterns and in the peripheral area of the workpiece 
in which a circuit pattern cannot be formed. Recently, therefore, there 
have been workpieces with different distances L between workpiece 
alignment marks WAM1 and WAM2 as a result of the different types (shape, 
size) of circuit patterns produced in the workpiece, even if they 
originate from the same manufacturer and have the same size. It has, 
therefore, become more and more difficult to integrate the value of L 
beforehand in the exposure device as a constant. 
To handle products with different distances L between workpiece alignment 
marks WAM1 and WAM2, therefore, the value of L must be manually set; this 
causes more working steps. Furthermore, here, there is the danger that 
adjustment errors are caused when the value of the above described 
distance L is set. 
SUMMARY OF THE INVENTION 
The invention was devised on the basis of the above described facts. 
Therefore, a primary object of the invention is to make the input of the 
distance between the workpiece alignment marks superfluous, to increase 
operating efficiency and to prevent operating errors, such as adjustment 
errors and the like, by automatic computation of the distance between the 
workpiece alignment marks. 
This object is achieved according to the invention as follows: 
(1) When a mask is positioned relative to a workpiece, by irradiation of 
the mask alignment marks with exposure light or nonexposure light from a 
light irradiation part, positioning is performed by determining the images 
of the mask alignment marks which are imaged on a workpiece, and 
determining of workpiece alignment marks of the workpiece, and by moving 
and offsetting the mask and/or the workpiece in two orthogonally 
intersecting directions parallel to the workpiece surface and rotating the 
mask and/or workpiece around an axis of rotation which is perpendicular to 
a plane which contains these two directions, so that the two alignment 
marks come to rest on top of one another, the workpiece alignment marks 
located at two locations on the workpiece are each determined and their 
positions are stored as the first positions. Afterwards the workpiece is 
rotated by a stipulated first angle. After this rotation, the above 
described alignment marks located at two locations on the workpiece are 
each determined again. Their positions are stored as second positions. 
Based on the data of the above described first and second positions, and 
as a result of the above described first angle, the distance between the 
workpiece alignment marks located on the workpiece is determined at two 
locations. Then, based on the data of the distance between the above 
described workpiece alignment marks, a second angle is determined which is 
formed by a line which passes through the workpiece alignment marks at the 
two locations on the above described workpiece, and by a line which passes 
through the mask alignment marks at the two locations. According to the 
above described second angle, the mask or the workpiece is rotated around 
the above described axis of rotation. After this rotation, the images of 
the mask alignment marks imaged on the workpiece and the workpiece 
alignment marks are determined, and the mask and/or workpiece moved such 
that the two alignment marks come to rest on top of one another. 
(2) If the distance between the workpiece alignment marks on the workpiece 
determined above in (1) is outside a stipulated range, further processes 
are stopped and an error messages sent. 
In accordance with the invention, by determining the distance between 
workpiece alignment marks in the manner described above in (1) and by 
positioning of the mask relative to the workpiece, in spite of the 
different distances between the workpiece alignment marks, positioning of 
the mask relative to the workpiece can be achieved without inputting of 
the distance between the workpiece alignment marks, if the two workpiece 
alignment marks are located within the visual fields of the alignment 
units. In this way, operating errors, such as adjustment errors of the 
value of distance L and the like, can be prevented. 
Furthermore, with respect to workpieces with different distances between 
the workpiece alignment marks, only by adjusting the positions of the 
alignment units such that the workpiece alignment marks extend into the 
visual fields of the alignment units can flexible measures for assistance 
be taken. 
In addition, by the measure by which the distance between the workpiece 
alignment marks is measured each time, it can be immediately checked 
whether major changes of the distance between the workpiece alignment 
marks have occurred or not. This occurs when, as a result of the different 
distances between the workpiece alignment marks, problems arise with which 
positioning of the mask relative to the workpiece cannot be performed. 
With the invention, products can be protected from problems by the measure 
described above in (2) by which further processes are stopped and error 
messages are sent when the distance between the workpiece alignment marks 
are outside of a stipulated area. 
These and further objects, features and advantages of the present invention 
will become apparent from the following description when taken in 
connection with the accompanying drawings which, for purposes of 
illustration only, show several embodiments in accordance with the present 
invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
In the following, one embodiment of the invention is described using the 
exposure device of the projection printing type shown in FIG. 1. However, 
the invention can be used not only for the above described exposure device 
of the projection printing type, but also for the above described exposure 
devices of the contact printing type and proximity printing type. 
FIG. 2 is a block diagram of the arrangement of a system for controlling 
the projection exposure device shown above in FIG. 1. In the figure, are 
shown a control console 11, an arithmetic-logic unit 12 for controlling 
the projection exposure device shown in FIG. 1, and a carrier control 
element 13 which drives a .THETA. carrier, a Y-axis carrier, a X-axis 
carrier and a Z-axis carrier and which moves mask carrier 2 and the 
workpiece carrier 4 (shown in FIG. 1) in the X-Y-Z-.THETA. directions. 
Also depicted is an image processing part 14 which recognizes mask 
alignment marks MAM1, MAM2 and workpiece alignment marks WAM1, WAM2 
recorded by means of the image sensors 5d of alignment units 5 and 
determines the position coordinates hereof, as is described below. 
Furthermore, arithmetic-logic unit 12 determines the deviations of the 
positions of above described mask alignment marks MAM1, MAM2 and workpiece 
alignment marks WAM1, WAM2 which were determined in image processing part 
14. Carrier control element 13 moves mask carrier 2 and/or workpiece 
carrier 4 such that the two agree with one another. Mask alignment marks 
MAM and workpiece alignment marks WAM recorded by alignment units 5 are 
displayed on monitor 15. 
Here, the center of rotation of workpiece W on workpiece carrier 4 is 
generally located between workpiece alignment marks WAM1 and WAM2, as is 
illustrated in FIG. 3(a). In this embodiment, therefore, mainly the case 
of FIG. 3(a) is described. However, it can also be applied to the case 
shown in FIG. 3(b) in which the center of rotation of workpiece W is 
outside of workpiece alignment marks WAM1, WAM2, as is described below. 
In the following, the positioning of the mask relative to the workpiece 
according to the embodiment of the invention is described. Following steps 
(1) to (4) are identical to the conventional example described above. 
(1) Workpiece W on which workpiece alignment marks WAM1, WAM2 are recorded 
is subjected to prealignment and placed on workpiece carrier 4. 
(2) Nonexposure light (or exposure light) is emitted from exposure light 
irradiation device (or nonexposure light irradiation device) 1 onto mask 
alignment marks MAM1, MAM2 of mask M. 
(3) The images of the above described mask alignment marks which are imaged 
on workpiece W are recorded by means of image sensors 5d of alignment 
units 5, and their positions are stored by means of an image processing 
device which is not shown in the drawings. 
(4) Nonexposure light (or exposure light) is emitted from workpiece 
alignment mark partial illumination systems WA1 onto workpiece alignment 
marks WAM1, WAM2 of workpiece W, and workpiece alignment marks WAM1, WAM2 
are recorded on workpiece W by means of image sensors 5d of alignment 
units 5. The images recorded by image sensors 5d are sent to image 
processing part 14 shown in FIG. 2 which determines and stores the 
position coordinates of workpiece alignment marks WAM1, WAM2 based on the 
images. FIG. 4(a) is a schematic of mask alignment marks MAM1, MAM2 and 
workpiece alignment marks WAM1, WAM2 recorded by alignment units 5. In the 
figure, reference letters A and B respectively label the images which were 
recorded by alignment units 5 located at two positions. Here, angular 
offset .DELTA..THETA. is .alpha., the position coordinates of the 
workpiece alignment mark WAM1 are (x1, y1) and the position coordinates of 
the workpiece alignment mark WAM2 are (x2, y2). 
(5) Carrier control element 13 shown in FIG. 2 drives the .THETA. carrier 
of workpiece carrier 4 and rotates it around the center of workpiece 
carrier 4 only by a very small angle .gamma. which is predetermined in the 
.THETA. direction (for example, by roughly 0.1 degree) to the extent that 
the workpiece alignment marks WAM1, WAM2 do not emerge from the visual 
fields of two alignment units 5. 
(6) Workpiece alignment marks WAM1, WAM2 are again recorded by means of 
image sensors 5d of alignment units 5 and their position coordinates are 
determined and stored in image processing part 14. FIG. 4(b) is a 
schematic of mask alignment marks MAM1, MAM2 and workpiece alignment marks 
WAM1, WAM2 recorded by alignment units 5 recorded the second time. Here, 
angular offset .DELTA..THETA. is .beta., the position coordinates of 
workpiece alignment mark WAM 1 are (x3, y3) and the position coordinates 
of workpiece alignment mark WAM2 are (x4, y4). 
(7) Arithmetic-logic unit 12 in FIG. 3, on the basis of the position 
coordinates of workpiece alignment marks WAM1, WAM2 which were determined 
above in steps (4) and (6), determines the position deviation as a result 
of precision rotation in the .THETA. direction in (5). Based on this 
position deviation, distance L between the workpiece alignment marks WAM1 
and WAM2 is computed at two locations in the following manner: Here, it is 
assumed that workpiece W has undergone prealignment and that the 
positioning of the deviation in the .THETA. direction was produced such 
that the workpiece alignment marks WAM1, WAM2 extend into the visual 
fields of alignment units 5. Conventionally, an arrangement is used in 
which the distance L between the workpiece alignment marks WAM1 and WAM2 
is 50 mm to 150 mm and the visual fields of alignment units 5 are roughly 
1.5 mm. The deviation in the .THETA. direction is computed approximately 
in the manner described below: 
tan.sup.-1 (1.5 mm/50 mm).about.1.7.degree.-0.03 radian 
1) According to FIG. 4(a), the following equation (2) applies: 
EQU sin .alpha.=(y2-y1)/L (2) 
Furthermore, according to FIG. 4(b) the following equation applies: 
EQU sin .beta.=(y4-y3)/L (3) 
Here, the following approximation applies since the deviation in the 
.THETA. direction from the above described assumed condition is relatively 
small: 
EQU sin .alpha.=.alpha. and sin .beta.=.beta. 
2) FIG. 4(c) shows a schematic in which triangles C and D in FIGS. 4(a) & 
4(b) come to rest on top of one another. As is apparent from the drawings, 
following equation (4) applies: 
EQU .beta.=.gamma.=.alpha. 
That is, 
EQU .gamma.=.alpha.-.beta. (4) 
The value of L can therefore be computed using following equation (5): 
EQU sin .alpha.-sin .beta.=(y2-y1)/L-(y4-y3)/L=.alpha.-.beta.=.gamma.(5) 
EQU .gamma.=((y2-y1)-(y4-y3))/L 
EQU L=((y2-y1)-(y4-y3))/.gamma. 
(the unit of .gamma. is radian) 
(8) Arithmetic-logic unit 12 computes distance L on the basis of above 
described formula (5) and stores it. As was described above, by 
automatically determining distance L between workpiece alignment marks 
WAM1 and WAM2 of respective workpiece W and by a comparison of the value 
of this distance L with the upper and lower boundary values established 
beforehand, it can be checked whether the distance between the workpiece 
alignment marks has changed to a great degree or not. If major changes 
occur with respect to the distance between the above described workpiece 
alignment marks, error messages can be produced and this reported to the 
operator or further steps halted and the occurrence of problems prevented. 
Furthermore, because distance L between the workpiece alignment marks can 
be measured and stored each time, it can be immediately checked whether, 
as a result of changes of the exposure accuracy for the arrangement of 
workpiece alignment marks in the prior process, major changes of the 
distance between the workpiece alignment marks have occurred or not. This 
happens when problems occur with which positioning of the mask to the 
workpiece cannot be performed. 
In addition, by storing distance L between the workpiece alignment marks of 
the respective workpiece W, data on the distance between the workpiece 
alignment marks can be collected. This makes it possible to determine 
changes of the exposure accuracy in the arrangement of the workpiece 
alignment marks in the process before positioning, and changes such as 
deterioration of the efficiency of the exposure device which fixes the 
workpiece alignment marks on workpiece W, and the like. Therefore, a time 
scale for adjusting the exposure device and the like can be obtained 
before problems occur in products. 
(9) On the basis of the value of above described distance L 
arithmetic-logic unit 12 computes the deviations of workpiece alignment 
marks WAM1, WAM2 in the .THETA. direction with reference to mask alignment 
marks MAM1, MAM2, i.e., angle .DELTA..THETA.. 
.DELTA..THETA. is determined by the above described and the following 
formula (1) because the value of L was determined by above described 
formula (7): 
EQU .DELTA..THETA.=sin.sup.-1 ((y2-y1)/L) (1) 
After angular offset .DELTA..THETA. was determined, as was described above, 
carrier control element 13 rotates workpiece carrier 4 (or mask carrier 2) 
by the drive of the .THETA.-carrier of the workpiece carrier 4 through the 
angular offset .DELTA..THETA., as in the above described conventional 
example. After rotation, again mask alignment marks MAM1, MAM2 and 
workpiece alignment marks WAM1, WAM2 are determined by means of alignment 
units 5. The X-axis carrier and Y-axis carrier of workpiece carrier 4 are 
driven by carrier control element 13, and workpiece carrier 4 is moved 
such that two alignment marks MAM1, WAM1 and MAM2, WAM2 come to rest on 
top of one another. 
In the following, it is described how the value of L can be computed 
according to above described equation (4) even in the case in which the 
center of rotation of workpiece W is outside of workpiece alignment marks 
WAM1, WAM2, as is shown in FIG. 3(b). 
FIGS. 5(a) & (b), 6(a) & (b) and 7 are schematics of computations of the 
amount of position deviations in the case in which the center of rotation 
of workpiece W is outside of workpiece alignment marks WAM1, WAM2. FIG. 
5(a) corresponds to the above described FIG. 4(a) and FIG. 5(b) 
corresponds to FIG. 4(b). 
As FIGS. 5(a) and (b) show, in this case equations (2) and (3) given below 
and also the approximation of sin .alpha.=.alpha. and sin .beta.=.beta. 
also apply: 
EQU sin .alpha.=(y2-y1)/L (2) 
EQU sin .beta.=(y4-y3)/L (3) 
Here, it is assumed that, for rotary motion with angle .gamma. around 
center of rotation O, the workpiece alignment mark WAM1 is moved from P to 
P' and workpiece alignment mark WAM2 is moved from Q to Q', as is 
illustrated in FIG. 6(a). As becomes apparent from FIG. 6 (a), angle P'Q'O 
and angle PQO are the same. 
Therefore, if segment QP and segment Q'P' are lengthened, and if the 
intersection thereof is labelled O', the angle between segment QO' and 
segment Q'O' and angle .gamma. are the same, as is illustrated in FIG. 
6(b). Therefore, the following applies: 
EQU .gamma.=.alpha.-.beta. 
Here, above described equation (4) applies, and distance L can be 
determined by above described equation (5). 
A case of positioning of mask M relative to workpiece W by the drive of 
workpiece carrier 4 was described above. However, mask carrier 2 or 
workpiece carrier 4 and mask carrier 2 can be driven and positioning of 
mask M to workpiece W can be performed. 
In the above described embodiment, when mask alignment marks MAM1, MAM2 are 
projected onto workpiece W from exposure light irradiation device (or 
nonexposure light irradiation device) 1, nonexposure light is emitted. 
But, mask alignment mark partial illumination systems can be placed in 
addition; they partially illuminate the peripheral areas of mask alignment 
marks MAM1, MAM2, and mask alignment marks MAM1, MAM2 can be projected 
onto workpiece W by these partial illumination systems. Different 
modifications can be effected as means for determining mask alignment 
marks MAM1, MAM2 and workpiece alignment marks WAM1, WAM2. 
Action of the Invention 
As was described above, the following effects can be obtained according to 
the invention: 
(1) By automatically determining the distance between the workpiece 
alignment marks and by positioning the mask relative to the workpiece, in 
spite of the different distances between the workpiece alignment marks, 
the mask can be positioned relative to the workpiece without input of the 
distance between the workpiece alignment marks beforehand, if the two 
workpiece alignment marks are located within the visual fields of the 
alignment units. In this way, operating errors, such as misadjustment of 
the value of distance L and the like, can be prevented. Furthermore, with 
respect to workpieces with different distances between the workpiece 
alignment marks, only by adjusting the positions of the alignment units 
such that the workpiece alignment marks extend into the visual fields of 
the alignment units can flexible measures be taken. 
(2) Because distance L between the workpiece alignment marks can be 
measured and stored each time, it can be immediately checked whether major 
changes of the distance between the workpiece alignment marks have 
occurred or not as a result of changes of exposure accuracy in the 
arrangement of the workpiece alignment marks in the previous process. This 
occurs when problems arise with which positioning of the mask to the 
workpiece cannot be performed. 
(3) By storing distance L between the workpiece alignment marks of the 
respective workpiece W, data on the distance between the workpiece 
alignment marks can be collected. This makes it possible to determine 
changes of the exposure accuracy in the arrangement of the workpiece 
alignment marks in the process before positioning, and changes such as 
deterioration of the efficiency of the exposure device which places the 
workpiece alignment marks on workpiece W, and the like. Therefore, a time 
scale for adjusting the exposure device and the like can be obtained 
before problems occur in products. 
(4) By a comparison of the value of this distance L with the upper and 
lower boundary values established beforehand and by stopping further 
processes and by sending error messages when the distance between the 
workpiece alignment marks is outside the stipulated range, the occurrence 
of problems in products can be prevented. It is to be understood that 
although preferred embodiments of the invention have been described, 
various other embodiments and variations may occur to those skilled in the 
art. Any such other embodiments and variations which fall within the scope 
and spirit of the present invention are intended to be covered by the 
following claims.