Contacting method and apparatus in contact copying

A contacting method comprises the steps of holding a photomask and a wafer at a predetermined interval, curving at least one of the photomask and the wafer so as to form a convexity relative to the other, and moving the photomask and the wafer relative to each other to bring them into intimate contact with each other.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a contacting method in contact printing, such as, 
for example, a contacting method in which a photomask (mask) and a wafer 
are set in a predetermined relation in a step of bringing the mask and the 
wafer into intimate contact with each other and in which a pattern on the 
mask is printed on the wafer in the manufacture of semiconductor. 
2. Description of the Prior Art 
In a method of this type, the mask and wafer are held at a suitable 
interval, whereafter a vacuum is created in closed space formed between 
the mask and the wafer and atmospheric pressure is caused to act on the 
back of the mask, whereby the mask is bent toward the wafer side to 
thereby bring the mask and the wafer into intimate contact with each 
other. In this case, the initially set gap between the mask and the wafer 
has greatly affected the intimacy of the contact between the mask and the 
wafer. 
That is, when the gap amount is set to a small value, the amount of flexure 
(the amount of curvature) of the mask is small. Therefore, good intimate 
contact is provided in the marginal portion of the mask and the so-called 
pitch error is small. The pitch error is a phenomenon that the patterned 
surface of the mask becomes lengthened by curvature of the mask and the 
mask pattern is printed on the wafer at a location thereon that deviates 
from the position whereat the mask pattern should be printed. 
However, there has also been a problem that when relatively wide areas of 
the mask and wafer begin intimate contact with each other, the gas present 
near the central portion (for example, N.sub.2 gas or the like for 
protecting negative resist) is cut off from escape and shut in thereby 
causing deterioration of intimacy of the contact in the central portion. 
It is possible to wait for the escape of the gas, but this method would 
offer a problem in respect of the through-put of production. The poor 
intimacy of the contact in a minute gap generally leads to reduced 
resolving power due to diffraction of light. 
On the other hand, when the gap amount is set to a great value, the mask 
bends greatly and begins to make intimate contact with the wafer from a 
small area near the central portion thereof. Due to the increased degree 
of vacuum, this area tends to gradually widen toward the marginal portion, 
whereby the above-described phenomenon in which the gas is shut in near 
the central portions of the mask and wafer during the intimate contact 
thereof is alleviated and the intimacy of the contact in the central 
portions is improved. 
In this case, however, the mask is greatly bent over toward the wafer side 
and curved during the intimate contact thereof. Leads to a problem that 
due to the rigidity of the mask, the mask and wafer do not make intimate 
contact with each other in the marginal portions thereof. Therefore, the 
degree of intimate contact in the marginal portions is reduced to 
aggravate the quality of image and accordingly the chip collection rate in 
the marginal portions is reduced. Also, the fact that the mask is greatly 
curved toward the wafer during the intimate contact therebetween leads to 
a problem that the afore-mentioned phenomenon of pitch error becomes 
pronounced. 
SUMMARY OF THE INVENTION 
It is a first object of the present invention to provide good intimate 
contact between two bodies and enhance the degree of intimate contact 
particularly at the marginal portions thereof. 
It is a second object of the present invention to realize intimate contact 
between two bodies without shutting gas in the central portions thereof. 
It is a third object of the present invention to suppress the pitch error. 
It is a fourth object of the present invention to bring a mask provided 
with an integrated circuit pattern and a wafer into intimate contact with 
each other and to transfer a good image of the integrated circuit pattern 
onto the wafer. 
The invention will become fully apparent from the following detailed 
description thereof taken in conjunction with the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1 which shows the mask chuck and wafer chuck of a mask 
aligner and a driving device, reference numeral 1 designates a mask, 
reference numeral 2 denotes a mask holder, reference numeral 3 designates 
a wafer, reference numeral 4 denotes a wafer chuck rigidly holding the 
wafer and reference numeral 5 designates a feed screw which is threadably 
engaged with a female screw of a member 9, to be described, to constitute 
a mechanism for converting rotational movement into rectilinear movement. 
Designated by 6 is a step motor. Revolution of the step motor 6 is 
transmitted to the feed screw 5 via a gear train G1, G2. Denoted by 7 is a 
conduit for evacuating a vacuum chamber 8. The conduit 7 is connected to a 
vacuum pump, not shown. Reference numeral 9 designates a member which is 
vertically movable by rotation of the feed screw 5 while rigidly holding 
the wafer chuck 4 against rotation. Letter B denotes bearings and letters 
GA designates a guide. 
Now, the mask 1 and the wafer 3 are first set at predetermined positions by 
a well-known means. At this time, a closed space (vacuum chamber 8) is 
formed between the mask and the wafer, but a vacuum is not yet created 
therein. Designated by V is a seal plate for hermetically sealing the 
vacuum chamber. Subsequently, the mask 1 and the wafer 3 are aligned by an 
X.multidot.Y.multidot..theta. movable stage, now shown. Thereafter, the 
feed screw 5 is rotated by the step motor 6 to move the vertically movable 
member 9 upwardly (or downwardly). At the same time, the wafer chuck 4 
rigidly fixed to the vertically movable member 9 is moved upwardly (or 
downwardly) while holding the wafer 3 and finally, the wafer 3 is set at a 
position spaced apart by a predetermined gap relative to the mask 1 and is 
rigidly fixed thereat. This state is shown in FIG. 2A. In FIG. 2A, letter 
A indicates a gap amount provided between the mask and the wafer. This gap 
amount A is set to a value greater than that in the prior art. That is, 
the gap amount is greater than the added values of the degrees of flatness 
(the difference between the highest surface and the lowest surface) of the 
mask and wafer. 
The vacuum chamber 8 is then evacuated through the conduit 7, whereby 
intimate contact between the mask and the wafer begins to take place. In 
this case, the initial gap A between the mask and the wafer is great. 
Therefore, the mask is greatly curved toward the wafer side and the mask 
and wafer, even if more or less poor in their flatness, begin to make 
intimate contact with each other from a small area near the center 
thereof. This state is shown in FIG. 2B. In FIG. 2B, reference numeral 10 
designates the area of intimate contact which may be observed as a O-order 
interference fringe through a microscope, not shown. As the vacuum 
pressure is further increased, the mask 1 is attracted toward the wafer 3 
with the result that the area of intimate contact 10 is enlarged. FIG. 2C 
shows the state of intimate contact when the initial gap amount between 
the mask and the wafer is A. In the course of progress from the state of 
FIG. 2B to the state of FIG. 2C, the mask is greatly curved downwardly 
convexly and therefore, the gas shut-in phenomenon is not liable to occur. 
Accordingly, a better state of intimate contact is obtained. In this 
state, however, the degree of intimate contact in the marginal portion of 
the wafer is low due to the great initial gap amount between the mask and 
the wafer and the rigidity of the mask. For this reason, the feed screw 5 
is rotated by the step motor 6 of FIG. 1, whereby the wafer 3 held on the 
wafer chuck 4 is moved upwardly through the intermediary of the vertically 
movable member 9 until the gap amount between the mask 1 and the wafer 3 
becomes minute. As a result, the flexure of the mask can be eliminated and 
the degree of intimate contact in the marginal portion of the wafer can be 
increased. This state is shown in FIG. 2D. In FIG. 2D, the minute gap 
amount B is the difference between the initial gap amount A and the amount 
by which the wafer has been moved upwardly by the use of the step motor 6. 
It is not always when B is zero that the flexure of the mask is zero. In 
this state, the mask having a pattern thereon can eventually keep 
predetermined flatness necessary for contact printing. Thereafter, an 
exposure light is applied. Designated by 11 is a controller which actuates 
the step motor 6 when the atmospheric pressure in the vacuum chamber 8 has 
been reduced and the mask 1 has flexed by a predetermined amount. 
In the present embodiment, the wafer is moved upwardly after the vacuum 
pressure between the mask and the wafer has reached a predetermined 
pressure, but alternatively the wafer may be moved upwardly in the course 
of change of the vacuum pressure. That is, a second step may be initiated 
before a first step is terminated. Also, the speed at which the wafer is 
moved upwardly can be changed as desired if, in FIG. 1, the pulse 
frequency applied to the step motor 6 is varied. 
In the present embodiment, a motor and a mechanism for converting 
rotational movement into rectilinear movement are used to vertially move 
the wafer held on the wafer chuck. However, the present invention is not 
restricted thereto but may have a mechanism capable of changing the gap 
between the mask and the wafer as desired. 
Also, in the present embodiment, the intimate contact between the mask and 
the wafer is accomplished by creating a vacuum in the space therebetween 
to thereby attract the mask toward the wafer side. However, alternatively 
the back of the mask may be pressed to thereby curve the mask downwardly 
convexly and bring the mask into intimate contact with the wafer. 
It is also conceivable to use the present invention with other systems, for 
example, such as the system as described in Applicant's Prior Japanese 
Patent Application No. 215335/1981 wherein the degree of intimate contact 
between the mask and the wafer is enhanced by gradually increasing the 
vacuum pressure between the mask and wafer. 
FIG. 3 shows another embodiment for curving the wafer. 
In FIG. 3, reference numerals 1 to 10 designate members identical to those 
described above. Reference numerals 11 and 11' denote adsorbing grooves 
provided on the wafer chuck and disposed along the outer periphery 
thereof. Reference numerals 12 and 12' designate adsorbing grooves similar 
to 11 and 11' but disposed in the central portion of the wafer chuck. 
Designated by 13 and 14 are conduits for connecting the adsorbing grooves 
11 and 12 to the outside of the wafer chuck. The conduit 13 communicates 
with a vacuum or gas through the change-over of an electromagnetic valve 
15 under control of a control device 17. Reference numeral 18 designates a 
vacuum source and reference numeral 19 denotes a compressed gas source 
(for example, N.sub.2 gas source). The conduit 14 communicates with a 
vacuum source 20 through an electromagnetic valve 16. 
In FIG. 3, the wafer 3 has its back attracted by a vacuum-created by the 
vacuum sources (here, the central portion and marginal portion thereof 
being separated by the vacuum sources 18 and 20, respectively) while, on 
the other hand, the mask 1 is rigidly fixed with a gap amount A between 
the wafer 3 and the mask 1 which is greater than the usual gap amount, as 
in the embodiment described in connection with FIG. 2. 
Subsequently, the electromagnetic valve 15 is changed over by the 
instruction from the control device 17 so that, as shown in FIG. 4A, 
compressed gas is passed into the central adsorbing grooves 12 and 12', of 
the adsorbing grooves to which the wafer 3 is fixed. The central portion 
of the back of the wafer is curved slightly convexly with the marginal 
portion thereof remaining absorbed at grooves 11 and 11' the central 
portion is thus brought into intimate contact with the mask 1. When, as 
shown in FIG. 4B, the wafer chuck 4 is then pushed up toward the mask side 
so as to decrease the gap amount A, the area of intimate contact spreads 
to the marginal portion without trapping any gas in the central portion. 
Whether the mask 1 is pushed and curved by the wafer 3 at this time is 
determined by the rigidity of the mask and wafer and the pressure of the 
compressed gas, which can be adjusted by a compressed gas pressure 
adjusting device, not shown. 
After or in the course of said adjustment, as in the previous embodiment, a 
vacuum can be created in the space between the mask and the wafer or the 
back of the mask may be pressed to enhance the state of intimate contact. 
Further, the marginal adsorbing grooves 11 and 11' may be changed over 
from vacuum to compressed gas blow-up by change-over of an electromagnetic 
valve. 
According to the present invention, as described above, there can be simply 
obtained a good state of intimate contact without deterioration of the 
intimate contact of the marginal portion, without occurrence of any pitch 
error resulting from bending of the mask and without occurrence of the gas 
shut-in phenomenon. 
If the alignment accuracy is within the target valve, the mask pattern will 
be printed on the wafer by an exposure optical system. 
The present invention is not restricted to the above-described embodiments, 
but other embodiments in which, for example, the central portion of the 
back of the mask is pressed from the back and then the wafer is pushed up 
toward the mask side, are conceivable without departing the scope of the 
present invention. 
The present invention is useful in the field of semiconductor printing in, 
for example, a mask aligner or the like, whereas it is not restricted to 
such field but may also be used in other fields which require contact 
printing.