GB2201258A

GB2201258A - A mask holding device

Info

Publication number: GB2201258A
Application number: GB08802217A
Authority: GB
Inventors: Hidehiko Fujioka
Original assignee: Canon Inc
Current assignee: Canon Inc
Priority date: 1987-02-09
Filing date: 1988-02-02
Publication date: 1988-08-24
Anticipated expiration: 2008-02-02
Also published as: JPH07101663B2; DE3803738A1; FR2610740B1; DE3803738C2; JPS63194331A; GB2201258B; GB8802217D0; FR2610740A1

Description

-I- 1.

22.' 0 12 5 G A MASK HOLDING DEVICE FIELD OF THE INVENTION AND RELATED ART

This -invention relates to a mask holding device usable in a lithographic process for the. manufacture of semiconductor microcircuits and, more particularly, to arranged to hold In the devices are used transferred onto wafer. Usually, attract and hold magnetic force.

1 such a mask holding device which is a mask'by use of a magnetic force. lithographic process, mask holding to hold a mask having a pattern to be a workpiece such as a semiconductor the mask holding device is arranged to the mask by use of vacuum suct7ion or a In the latter case, the attracting magnetic force is provided by an electromagnet, such as disclosed in Japanese Laid-Open Patent Application, Laid-Open No. Sho 60-77424. Alternatively, the attracting magnetic force may be provided by a permanent magnet, such as disclosed in Japanese Laid- Open Patent Application,-Laid-Open No. 61-294441.

The flatness of a membrane of a mask is determined by total precision which is determined bythe flatness of a holding pUrface of the mask holding device to.which the mask is attracted, the flatness of a mask frame-(lip surface and/or a surface to be attracted) to which the membrane is attached, the -flexure of the mask as a whole due to the gravityj etc.

1. t --:'2- once the mask is attracted to and held by the mask holding device, a certain flatness is determined.

Conventional mask holding devices include no specific means for controlling or correcting the flatness of the mask membrane. For example, in the mask holding device disclosed in the aforementioned Japanese Laid-Open Patent Application, Laid-Open No. Sho 61-294441, an atracting means comprises a ring- shaped permanent magnet. Therefore, it is difficult to control the flatness of the membrane. If the membrane of the mask held by the mask holding device is not disposed exactly in a plane which is predetermined, there occurs uneven pattern transfer (e.g. blurring of the transferred pattern) as a result of the exposure of a semiconductor wafer to the pattern of the ma.sk. This is undesirable.

SUMMARY OF THE INVENTION

Accordingly, it is an obj ' ect of the present invention to provide a mask holding device and method by which the flatness of a membrane of a mask can be corrected so as to assure desired flatness.

in accordance with one aspect of the present invention, there is provided a mask holding device having a mask holder and a'plurality of electromagnets op e rable to attract and hold a frame ofa mask, wherein control led electric currents are supplied to solenoid 1 -1 1 coils of the electromagnets so as to assure desired flatness.

In another aspect, the mask holding device of the present invention includes a mask holder to which a mask frame is detachably attracted, a plurality of solenoid coils embedded in a circumferentiai portion of the mask holder, permanent magnet means effectiv e to attract the mask frame to the mask holder, and control means operable to supply controlled and different electric currents to the solenoid coils, respectively. Permanent magnets may be disposed and used as cores of the solenoid coils of the mask holder, respectively. Also, at least a portion of the mask frame may be made of a magnetic material so that the mask frame can be attracted by the permanent magnets Alternatively, each solenoid coil of the mask holder may be provided with a core or yoke m ade of a Magnetic material and a permanent magnet may be provided on or in the mask frame at a position opposed to the magnetic core or yoke. In any case, the permanent magnets used function to attract the mask frame to the mask holder, and controlled electric currents are supplied to the solenoid coils so as to improve the flatness of the membrane of the mask attracted to and held by the mask.

holder. gamely, by adjusting the attracting force,-the flatness of the mask membrane is controlled.

These and other objects, features and k 1 advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction.with-the accompanying 5 drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a schematic plan view of a mask holding device according to a first embodiment of the present invention.

Figure 2 is a schematic section of the mask holding device according to the first embodiment, taken on a line A-A in Figure 1.

Figure 3 is a schematic section showing a modified form of the mask holding device of the first embodiment.

Figure 4 is a fragmentary perspective view showing one of magnetically attracting units used in the mask holding device of the first embodiment.

Figure 5 is a schematic and diagrammatic view of an exposure system with a-flatness correcting function, into which the mask holding device of the Figure 1 embodiment is incorporated.

Figure 6 is a flow chartshowing the sequence for obtaining data necessary for the flatness correction, made in a flatness measuring portion included in the exposure system shown in Figure 5.

1 4 Figure 7 is,a flow chart showing the sequence of flatness correction made in-an exposure portion included in the exposure system shown in Figure 5.

Figure 8 is a 'schematic and diagrammatic view showing another exposure, system with flatness correcting-function, -into which the mask holding device of the Figure 1 embodiment is incorporated.

Figure 9 is a fragmentary perspective view showing a modified form of a mask holding surface of the mask holding device of the Figure 1 embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring first to Figures 1 and 2, there is shown a mask holding device according to a first embodiment of the present invention. In this. embodiment, the present invention is applied to a mask holding device which is suitably usable as a mask chuck in an X-ray exposure apparatus..

Denoted at 1 in Figure 2 is a mask membrane and denoted at 2 is a mask frame which is made of a magnetic material. A mask holder 3 has four permanent magnets 4a, 4b, 4c and 4d which are fixedly mounted-to the mask holder 3, for attracting the mask frame 2 to the mask holder 3. Sol enoid coils 5a, 5b, 5c and 5d are embedded in a circumferential portion of the mask holder 3. These solenoid coils are adapted to produce magnetic forces which are-effective to variably control 1 the attracting force to be applied to the mask frame 2.

In this-embodiment, the permanent magnets 4a 4d are used as cores of the slenoid coils Sa - 5d, respectively. This is made so that the solenoid coils. Lead wires 6a - 6d function to supply electric currents to the solenoid coils 5a - 5d, respectively.

In the present embodiment, the permanent magnets 4a - 4d and the solenoid coils 6a - 6d are used in four combinations to provide four magnetically attracting units, one of which is illustrated in Figure 4. As best seen in Figure 1, these four units are disposed in the mask holder 3 in an equiangular relation with respect to the circumferential direction of the mask holder. Namely, they are disposed in a right-angle relation. Further, in this embodiment, these magneticaliy attracting units is accommodated within a housing defined by the mask holder 3. This is made in consideration of the effect of heat generation by the solenoid coils to the outside of the mask holder. The four magnetically attracting units are operable to control or correct the flatness of the mask membrane, as will be described later. However, if it is desired to achieve the fl atness control more precisely, a larger number of solenoid coils and permanent magnets (i.e. magnetically attracting units) may be provided. For example, eight units may be disposed circumferentially in a 45-degree-relation. In i any case, it is preferable that the magnetically attracting units are disposed symmetrically with respect to the center of the mask holder 3, as seen in Figure 1.

Description will now be made of the manner of correcting the flatness of the mask membrane 1 by use of the mask holding device of the first embodiment.

When, in operation, the mask frame'- 2 having a ring-like shape and being moved toward the mask holder 3 by a mask transporting hand (not shown) comes close to the mask holder 3, it is attracted to the-mask holder 3 by the magnetic forces of the permanent magnets 4a - 4d which are disposed circumferentially, as best seen in Figure 1. After the attraction, the mask membrane 1 has a certain flatness (e.g. the surface irregularity, i nclibation) which is determined, as described hereinbefore, the flatness of the mask holding surface of the mask holding device, the flatness of the mask frame (a lip surface and/or a surface to be attracted to the mask holder) to which the mask membrane 1 is attached, the flexure of the mask due to the gravity, etc. In the state as assumed at the time when the mask frame 2 is completely attracted to and held by the mask holder 3, the flatness of the mask membrane 1 is measured by use of an.examining apparatus such as a flatness measuring interferometer which is well in-the art. Namely, the state of the. membrane 1 1 10 1 surface with respect to a predetermined reference plane is observed by use of.a suitable observation device such as the flatness measuring interferometer. By this, any surface irregularity of the membrane 1 and any inclination of the membrane 1 surface with respect can be detected in Subsequently, in accordance with the thus obtained surface information concerning the irregularity and/or the inclination of the membrane 1, the magnetic force (magnetically attracting force) of one or more magnetically attrac ting units which correspond to such portion or portions of the mask membrane 1 that should be "corrected" is controlled. More specifically, controlled electric currents are supplied to appropriate solenoid coils of the magnetically attracting units to control the magnetic forces of" the corresponding permanent magnets to thereby suitably change the magnetically attracting force of the mask holder 3. This is made to adjust the interval between such a portion of the mask membrane 1 (to which the correction should be made) and a plane including theholding surface of the mask holder 3, so that the surface of the mask membrane 1 is placed exactly in the to a predetermined reference plane the form of "surface information".

reference pla.n.e or positioned within a range of tolerance set with respect to the reference plane. In summary,.by suitably changing the distribution of i; 41 I" magnetic forces, distributed circumferentially, of the -magnetically attracting means of the mask holder 3, the flatness of the mask membrane is corrected as a whole.

In order to enlarge the amount of flatness correction, the mask holder 3 may be provided with a plurality of surface protrusions such as illustrated in Figure 9. The-provision of such protrusions is effective to allow a larger amount of deformation of the mask frame 2. Of course, such protrusions may be jo provided on the mask frame 2.

Referring now to Figure 5, there is shown an exposure system arranged in accordance with an embodiment of the present invention. In this embodiment, as will be described later, two mask holding devices each being such as described with reference to Figures 1 and 2 are incorporated.

The exposure system of the Figure 5 embodiment includes a flatness measuring portion which is denoted generally at A and an exposure portion which is denoted generally at B. The measuring portion A includes a flatness measuring interferometer 10 which is supported by a surface plate. There are provided' temperature sensors (one of which is diagrammatically illustrated at 13a) and a pressure sensor which is also diagrammaticall y illustrated at 13b. One (denoted at 13a) of the temperature sensors is provided to sense.any temperature vari-aton in.an ambience surrounding 1 r, the measuring station, while the other temperature sensor is provided to sense any temperature variation in a mask holder 3. Driving means 14 drives a plurality of magnetically-attracting units, independently of each other, which units are provided on the mask holder 3. The'driving means 14 is controlled by a computer 12.

The exposure portion B includes a mask holder 15 which is separate from the mask holder 3 of the measuring portion A. The exposure portion B further includes a wafer stage 16 which is movable along a guide bar 17. A wafer 18 having a surface layer (resist layer) is placed on a wafer holder 19 which is supported by the wafer stage 16. Driving means 20 drives a plurality of magnetically attracting.units, independently of each other, provided on the mask holder 15. The driving means 20 is controlled by the computer 12. There are provided temperature sensors (one of which is diagrammatically illustrated at 21) and' a pressure sensor which is also diagrammatically illustrated at 21b. One (denoted at 21a) of the temperature sensors is provided to sense any temperature variation in an ambience surrounding the -exposure station (more particularly, a mask held by the_ mask holder 15), while the other temperature sensor is provided to sense any temperature variation in the mask holder 15 itself. Denoted at 24 in Figure 5 is a 1 stationary supporting frame on which the mask holder 15 is fixedly secured.

Referring now to the flow charts of Figures 6 and 7. the operation of the exposure system of the Figure 5 embodiment will be described.

In the exposure system of the Figure 5 embodiment, each of the mask holder 3 included in the measuring portion A and the mask holder 15 included-in the exposure portion B has the same structure as of the lo mask holder described with reference to Figures 1 and 2. Namely, the mask holders 3 and 15 have the same configuration and their holding surfaces have been formed substantially with the same machining accuracy. Thus, the holding surfaces of these mask holders have substantially the same flatness. In other words, two mask holders having substantially the same mask holding characteristics are used in this embodiment. In the measuring portion A, a mask having a membrane 1 is attracted to and held by the mask holder 3. In the state in which the mask is held by the holder 3, the flatness measuring interferometer 10 is used to observe the state of the surface of the membrane 1, to measure the flatness of the. membrane 1. The surface information concerning th e state of the surface of the membrane 1, obtainedby the interferometer 10, is supplied to the computer 12.. The computer 12 operates, on the basis of the supplied.surface information,. to 1 control the driving means 14 to cause it to supply controlled electric currents to appropriate one or ones of the solenoid coils of the magnetically attracting units (only two of which are illustrated in Figure 5). As a result, the magnetically attracting force of the mask holder 3 provided by the permanent magnets included in the magnetically attracting units is controlled to adjust the interval between such portion of the mask membrane 1 to which.the correction should be made and a plane cofitaining the holding surface of the mask holder 3, so that the mask membrane 1 as a whole is placed exactly in a predetermined reference plane or positioned within a range of tolerance set with respect to the reference plane.

As described hereinbefore, the driving means 14 supplies individually controlled electric currents to the magnetically attracting units, such that the magnetically attracting units are controlled independently of each other for the correction of the surface state of the mask membrane 1. At this time, in consideration of the resiliency of the mask frame 2 or otherwise, the computer 12 controls the driving means 14 so as to avoid that a large difference, greater than a predetermined, is produced between the holding forces_ of the attracting units (i.e. the magnetically attracting forces of them). ThiSAs made to assure smooth correction; Further, in order to avoid Z 1 r i accidental damage of the mask membrane 1, the computer 12 controls the driving means 14 so that gradually increasing electric currents are supplied to the magnetically attracting units.

After the correction, the measuring inter ferometer 10 is used again to observe the state of the mask membrane 1 and the flatness (surface irregulari ties, inclination and the like) of the mask membrane 1 is measured. If it is detectedthat the membrane 1 surface is in a satisfactory state as determined by the aforementioned tolerance, the values-concerning the electric currents which are supplied at that time to the magnetically attracting units as well as the values sensed at that time by the pressure sensor and the temperature sensors are stored into a suitable memory included in the computer 12. If, on the other hand, the state of the membrane 1 surface is not yet satisfactory, the above-described surface correction is repeated.

After the re-measurement, the mask is detached from the mask holder 3 and is transported to the mask holder 15 of the exposure portion B. The mask is placed on the mask holder '15, and it is attracted to.

and held by the mask holder 15. A suitable sensing means is used to check whether or not the mask is held by the mask holder 15. When the holding is discriminated, the computer 12 operates to correct the data concerning the electric currents and having been stored therein as a result of the measurement operation made in the measuring portion A. The data correction is made on the basis of any difference in the character- istics of corresponding magnetically-attracting units of the mask holders 3-and 1.5 (the information concerning such difference has been preparatorily stored in the computer 12) and the difference in the environmental conditions as sensed by the four temperature sensors and the two pressure sensors, in this embodiment. The computation for calculating the correction is made in accordance with a preset program. If it is desired to use different masks, the control data related to all the masks may be preparatorily stored, and in accordance with a selected mask, a corresponding portion of the data may be accessed.

After the data correction, the computer 12 supplies a control signal to the driving means 20, in response to which the driving means 20 drives the magnetical ly attracting units of the mask holder 15, independently of each other. As a result, the desired surface state is reproduced within the exposure port:ion B. Although two mask holders-3 and 15 having substantially the same mask holding characteristics are used in the present embodiment, only one mask holder may be used'if such a minute difference in the mask holding characteristics, Which may be caused by a-slight difference in the machining accuracy or otherwise, is considered undesirable. Namely, the mask holder 15 of the exposure po rtion B may be omitted and, in place thereof, the mask holder 3 of the measuring portion A may be modified so that it can be demounted from the measuring portion A and conveyed to the exposure portion B. 51 Referring now to Figure 8, there is shown anexposure system with.flatness correcting function, 10_ arranged in accordance with another embodiment of the present invention. The same reference numerals as of the foregoing embodiment are assigned to corresponding or similar elements.

Wafer stage 16 is movable along a guide bar 22 which is fixedly supported by a frame 23. Mask holder 3 is fixedly secured to a holder frame 24. As in the foregoing embodiments, the exposure system of the Figure 8 embodiment includes temperature sensors one of which is diagrammatically illustrated at 25a and a pressure sensor which is also diagrammatically illustrated at 25b. One (denoted at 25a) of the temperature sensors is provided to sense any temperature variation in an ambience surrounding the exposure station (pattern transfer station), while the oth er temperature sensor is provided -to sense the temperature of the mask holder 3. One.of distinctive features of the present embodiment over the Figure 5 embodiment lies in that the measuring portion A and the exposure portion B of the Figure 5 embodiment are combined as a unit so that, in the present embodiment, the flatness measuring is.attainable at the exposure station. The manner of measurement and correction of the surface state of the mask membrane is essentially the same as that made in Figure 5 embodiment. It should be noted that, i n this embodiment, only one and the same mask holder such as at.3 is used at the exposure station both for the measurement and the correction, such that there is no necessity of the data correction in relatio-n to the difference in the mask holding characteristics as has been described with reference to the Figure 5 embodiment.

In the embodiments described hereinbefore, the magnetically attracting units are provided on the mask holder 3 side. However, the present invention is not limited.to such structure. Namely, as illustrated in Figure 3, permanent magnets (only two of which are shown at 4a and 4c) may be provided on a mask frame 2 while, on the other hand, solenoid coils (only two of which are shown at 5a and 5c) may be embedded in a peripheral portion of a mask holder 3. In this example, as illustrated, separators (only two of which. are shown at 7a and 7c) and cores made of a magnetic material (only two of which are shoqn at 8a and 8c) are also provided on the-mask. holder 3 side. It'will be 1 0 f 1 readily understood that the mask holding device of this example is capable of variably controlling the attracting force of the permanent magnets, by supplying controlled electric currents.to the solenoid coils. Additionally, in this example, it is possible to control the solenoid coil to cause it to produce a magnetic field opposing to or repelling the magnetic field of the permanent magnet. Such repelling force_ may be utilized to precisely adjust the clearance between the mask'and the holding surface of the mask holder, for correction of the surface state of the mask membrane 1.

While the invention has been described with reference to the structures disclosed herein, it is not confined to the details set forth and this app.lication is intended to cover such modifications or changes as may come within the purposes of the improvements or the scope of the following claims.

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Claims

WHAT IS CLAIMED IS:

1. A mask holding device usable in an exposure apparatus for exposing a wafer having a radiationsen.sitive surface layer to a pattern of a mask having a frame to which a membrane haVing the pattern is attached, said device comprising:

mask holder; pluraliy of attracting means provided on said mask holder, for attracting the mask frame to said mask holder by use of inagnetic force; and means for changing distribution of the magnetic force which attracts the mask frame to said mask holder.

2. A device according to Claim 1, whefein the magnetic force i. distributed circumferentially.

3. A device according to Claim 1, wherein said mask holder is provided with a plurality of solenoid coils embedded in a peripheral portion of said mask holder, wherein said attracting means includes permanent magnet means comprising a plurality of permanent magnets each being used as a core of a corresponding solenoid coil of said mask holder and wherein at least a portion of the mask frame is made of a magnetic material.

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4. A device according to Claim 1, wherein said mask holder is provided with a plurality Of solenoid coils embedded in a peripheral portion of said mask.holder and wherein said attracting means includes permanent magnet means which comprises a plurality of magnetic material members, each being used as a core of a corresponding solenoid coil of said mask- holder, and permanent magnets provided on the mask frame so as to be opposed to said cores.

5. A device according to Claim-1, wherein said plural attracting means,are disposed in a housing defined in said mask holder.

6. A device according to Claim 1, wheriEin said plural attracting means are controlled independently of each other.

7. A device accordingto Claim 1, wherein said plural attracting means are disposed symmetrically with respect to a center of the mask frame.

8. A device according to Claim 1, wherein said mask holder has a plurality of protrusions-tormed on its surface that holds the mask frame.

9.

An exposure apparatus for exposing a r, workpiece having a radiation-sensitive surface layer to a pattern of a mask, said apparatus-comprising: first holding means disposed at a first location and being operable to detachably hold the mask at said first location; second holding means disposed at a second location, separate from said first location, and being operable to detachably hold the mask at said second location; sensing means for sensing ambient conditions of said first and second holding means, respectively; and control means operable, in accordance with the sensing by said sensing means, so that said first and second holding means show substantially the same mask holding characteristics.

10. An apparatus according to Claim 9, wherein each of said first and second holding means includes a permanent magnet and a solenoid coil.

11. An apparatus according to Claim 9, wherein said sensing means is adapted.to sense at least one of temperature and pressure.,

12. A flatness correcting device, usable with an exposure apparatus for exposing a wafer having a.

1 Z 1 h 1 lk i 1 radiation-sensitive surface layer to a pattern of a mask having a frame to which a membrane having.the pattern is attached, for correcting'the flatness of -the mask membrane, said device comprising: a mask holder; attracting means having a plurality of electromagnets and being operable to attract the mask frame to said mask holder so that the mask frame is held by said mask holder; 10. examining means operable to examine the state of the surface of the mask membrane when the mask frame is attracted to said mask h61der by said attracting meanst said examining means producing surface information concerning the.state of the surface of the mask membrane, in accordance with the examination; and control means operable to control electric currents to be supplied to said electromagnet, independently of each other and in accordance with the surface information produces by said examining means, to change the attracting force of said attracting means to thereby correct the flatness of the mask membrane.

13. A device according to Claim 12, wherein each of said electromagnets includes a sol,enoid coil and a permanent magnet.

1

14. A mask holding-device comprising a mask holder, means for magnetically attracting a frame of a mask to the mask holder, and means for adjusting the distribution of-the magnetic 5 force of attraction.

1 1 Published 198,8 at The Patent Office, State House, 6BP71 High Holborn, London WCIR 4TP. Further copies may lye obtained from The Patent Office, Sales Branch, St Mary Cray, Orpington, Kent BR5 3RD. Printed by Multiplex techniques Itd, St MarY CraY, Kent. COM 1187-