CA1164713A - Method and apparatus for transferring patterns - Google Patents

Abstract

METHOD AND APPARATUS FOR TRANSFERRING PATTERNS
Abstract of the Disclosure In transferring an image of d pattern from a master to a body, for example from a reticle to a photoresist-coated semiconductor wafer, it is proposed to avoid transferring opaque blemishes by exposing the body twice by way of a master comprising two identical pattern sets. The first exposure will produce on the body a latent image of one of the two pattern sets. The second exposure will superpose a latent image of the other of the two pattern sets upon the first latent image. A latent image of a blemish produced during the first exposure will he removed during the second exposure, and vice versa, since it is extremely unlikely that blemishes in the two parts of the master will coincide. Since the two identical pattern sets are on the same master, registration of the superposed latent images can be achieved very accurately. The invention finds application in making integrated circuits or solar cells using contact printing, proximity printing, or projection apparatus, for example wafer-steppers.

Description

116 ~

This invention rela-tes to pattern -transfer and particularl~y to a method and apparatus for providing a pattern on a body or receiver by irradiating with light, x-rays, or other radiation frorn a master or oriqinal of the pattern. The invention is applicahle especiall~y, but not exclusively, to copying cameras and projection printers.
A Problem encountered in transferring patterns is that blemishes on the master are usually reproduced on the body. The problem is especially acute when high definition, complex patterns are being transferred, for example in making photomasks, semiconductor devices such as integrated circuits and solar cells, printed circuit hoards and like electrical devices. Masters for such devices often have hlemishes caused by opaque defects in tile material from ~hich the master is made, or by dirt or other contamination, such as, particles of fiber or hair. In some cases, for example where it brid~es a space between adjacent lines of a reticular pattern, the hlemishes may render the clevice unusable.
An object of the present invention is to provide method and apparatus for transferring patterns from a master to a body in

2() such a way that hlemishes in or on the master ~enerally are not transferred.
According to one aspect of the present invention a method of transferring a pattern onto a body, for example a semiconductor device, photomask or printed circuit board, includes the steps of exposing the body to radiation throucJh the interrnediary of a master having two identical sets each of at least one pattern, to produce on the body a latent imaqe of one of said sets, and 7~3 exposing the body a second time to superpose upon the latent image of said one of the sets an ima~e of the other of said sets.
Since the blemishes are usually randomly distributed, it is extremely unlikely that blemishes will occur at corresponding positions in the different pattern sets. Consequently a latent image of a blemish transferred to the body during the first exposure will be removed during the second exposure. Conversely, a blemish exposed during the second exposure will not produce a latent image because the corresponding area of the body will have been exposed during the first exposure.
Conveniently the required superposition of the second latent image upon the first latent image is achieved by translating the body and master one relative to the other between the first and second exposures. Generally, such translation will be in the direction of, and by a distance corresponding to, the spacing between corresponding points on the two sets of patterns.
In one preferred embodiment of the invention particularly suited to producing an array of superposed latent irnages, both pattern sets are exposed onto the body in each exposure 2n step, and the body and master are translated one relative to the other between such steps to produce at least one row of superposed latent images. The first and last latent images in the or each row will be of only one exposure, since each will have been exposed by only one or other of the exposure steps. If desired, the corresponding part(s) of the body can be discarded, perhaps after processing. Alternatively the first and/or last latent image or images may be arranged to fall beyond the edge of the body.

The relative movement hetween the body and master hetween exposure steps may take place boustrophedonically to produce an array of superposed latent images.
The master may comprise an array of patterns, the different sets of patterns comprising identical patterns in different lines of the array. The hody is again irradiated by way of the master twice, the second time with the image of the array offset so that latent irnages of identical patterns in one of said different lines are superposed upon latent images of patterns in another of said different lines. Where the array is generally rectilinear, each line may comprise a row or column.
In each exposure or irradiation step the master pattern set or sets may be transferred as a whole. Alternatively different parts of the pattern set may be transferred in succession, until the whole has been transferred. Such successive transfer may be effected by scanning the body, either continuously or step-by-step. The body and master mi~ht be aligne(l, before the first exposure, relative to an alignment mark or marks, and realigned hetween first and successive exposures at a position displaced from the first position 2() hy the distance required to give superposition.
Embodiments of the invention may employ size reduction, typically by a factor of 5 or 10, in transferring the pattern to the body. However, transfer may be effected without significant change in size, for example by contact exposure, by proximity exposure, or 1:1 pro~jection exposure, especially where the master pattern set itelf comprises an array of identical patterns. Further, some enlargement may be provided.

'17~3 According to a second aspect of the invention, apparatus for transferring a pattern to a body comprises a support for a mas-ter having two identical sets each of at least one pattern, a support for the body, means for irradiating the body by way of the master to produce on the body a latent image of at least one of said sets, and means operative between successive irradiation steps to configure the apparatus such that a latent image of one of said identical sets formed durinq one exposure step is superposed upon a latent image of the other of said sets produced upon the body during a preceding exposure step.
The means for configurinq the apparatus may comprise translation means for moving said body and master one relative to the other to such an extent as to provide the required superposition.
Generally, such movement will be in the direction of, and by a distance corresponding to, the spacing between corresponding points of the two identical pattern sets.
Thè translation means may be arranged to step the body relative to the master, or vice versa, repeatedly, each step equal to said distance to provide at least one row of superposed latent images 2n on the body.
The or each set of patterns may be part of an array of patterns, conveniently arranged in rows and columns. The translation means may then serve to translate the body and master one relative to the other, by one row or column, or multiple thereof, between successivè exposures.
The irradiation means may be arranged to transfer the or each entire pattern set simultaneously. Alternatively, and 7~3 especially where the set comprises an array of identical patterns, the irradiation means may serve to irradiate different areas of the body in succession to transfer a complete imaqe of the or each set onto the body. Such irradiation means may conveniently comprise scanning means, for example a shutter member, controlling scanning of said different areas, stepwise or in a continuous sweep.
More specifically, in a preferred embodirnent, the apparatus includes an imaging device, for example a lens or mirror system, between the master and the body, and a shutter between the body and radiation source. The body and master are movable together across the optical axis and relative to the imaginq device and shutter, or vice versa, to scan and expose successively the different areas of the body. The holder for the body comprises two parts, the translating means servinq to move one such part relative to the other, between successive scans, by a distance equivalent to the spacing between the sets of patterns to be superposed.
Such implementation of the invention will now be describer1 by way of example only and with reference to the accompanying drawings, in which:-Figure 1 illustrates parts of apparatus for irradiating a body comprising a semiconductor wafer or photomask;
Figure 2 illustrates the formation of latent images on the wafer or photomask during a first exposure;
Figure 3 illustrates the formation of latent images on the wafer, or photomask, during a subsequent exposure; and Figure 4 illustrates pro~jection apparatus for transferring an array of images to a semiconductor wafer or photomask.

7`~3 Fiqure 1 illustrates, schematically, parts of a "step-and-repeat" camera for exposinq semiconductor wafers or photomasks. To facilitate better understanding of the invention, the detailed construction of the camera is omitted. The reader is referred to IJ.S. patent specification No. 4,172,656, as an exemplary disclosure of cameras of this kind and their use.
Referring to Figure 1, the apparatus comprises a table 10 which supports a semiconductor wafer 12. The uppermost surface of the wafer 12 is coated with photoresist to be exposed to radiation heneath an optical column part 14. The optical column 14 includes a holder or platen 15 for a reticle 16, comprising a transparency carrying the master pattern to bè reproduced upon the wafer 12. The reticle 16 extends in a plane perpendicular to the optical axis 17 of the column 14. A lamp 18 is disposed above the reticle holder 15 so as to irradiate the reticle 16. An imaging device in the form of a lens system 20 below the reticle 16 is arranged to form an image 16' of the rnaster pattern (usually reduced by a factor of 5 or 10) in the same plane as the photoresist-coated surface of the wafer 12. The semiconductor wafer 12 and the reticle 16 are secured to the table 10 and holder 15, respectively, by vacuum chucks (not shown), or other suitable devices.
The reticle lfi is provided with two identical sets of patterns. In this case the sets comprise single patterns I and II, respectively, their longitudinal center lines 19 and 21 spaced apart by a distance D For convenience each pattern is shown as a sirnple arrow rather than the complex pattern actually used for production of an integrated circuit device and also is shown without any irnage rotation.

l3 The reticle 16 may be produced bV any of the usual processes, for example so-called rubylith cut-and-strip, hut preferably is produced wholly or in part by a pattern generator.
Such pattern generators offer accuracy and positional precision of less than 1 micron with orthogonality of about 1 are second and so are capable of positioning the two master patterns relative to each other very precisely.
In addition the reticle 16 has two fiducial marks, 22 and 24, near respective opposite edges of the reticle 16, the optical axis 17, passing through the mid point between marks 22 and 24.
Correspondinq fiducial marks 26 and 28 are provided on a reference surface 30 of the reticle holder 15 within the optical column 14.
The axis through rnarks 26 and 28 must he parallel to the transverse movement of the table 10. To position the reticle 16 relative to the optical axis 17 before use, the fiducial marks 22 and 24 are aligned with the marks 26 and 28 using a micro-manipulator assembly with screw ad,justers for lateral, longitudinal and rotational adjustments while viewing the two sets of fiducial marks by way of a split field microscope (not shown).
2n The table 10 is movable, perpendicularly to the optical axis 17 of the optical column 14, between an exposure position, wherein the semiconductor wafer 12 is beneath the optical column 14, and an alignment position wherein the wafer 12 is displaced, still in the same plane as image 16', to one side of the optical column 14.
In Figure 1, the table 10 is shown in the alignment position. There the wafer 12 is positioned beneath the objective tubes 30 and 32, respectively, of a binocular microscope mounted 7:~3 ad~jacent the optical column 14. Ali(~nment marks 34 and 3fi in the binocular tubes 30 and 32, respectively, are on an axis which is parallel to the transverse movement of the table 1~. The wafer 12 has alignment marks 38 and 40 adjacent its opposite edges, usually provided by a first processing step, which are to be aligned with the marks 34 and 36 prior to exposure. The wafer is first aligned coarsely by locating its edges against pins or other reference surfaces. If no alignment marks have previously been provided on the wafer, such coarse alignment may be used alone. To assist the reader in distinguishing marks used in positioning the reticle from those used in positioning the wafer, the marks associated wi-th the reticle 16 and its holder 15 will be referred to as "fiducial" ~arks and those associated with the wafer 12 and microscope tuhes 3n, 32 will be referred to as "alignment" marks.
Alignment of the wafer 12 is effected by minute ad~justment of the position of the table 10, which comprises three super~jacent parts, each movable relative to the others. The two lowerlnost parts provide movement laterally and longitudinally, respectively, and the uppermost part provides for rotation of the 2n wafer, in the plane of imaqe 16'. Actual movement of the table 10 is by way of laser interferometric control system 42, coupled to the parts of the table 10 as indicated by the broken lines 44. During alignment, the position of the table 10 may be controlled manually by the operator. After the wafer 12 has been aligned, further movement of the tahle 10 is controlled by a computer program,ned to move the wafer according to a predetermined sequence. Firstly, the tahle is moved in the direction of arrow A (Figure 1) to the exposure position 7~3 beneath the optical column 14, the computer taking account of the spacing between the microscope alignment marks 34 and 36, to which the wafer 12 is aligned, and the optical axis 17.
Usually an array of identical patterns, arranged in rows and columns, are required on the wafer. Accordingly the wafer 12 is positioned so that the first latent image will be formed at -the beginning of the first row. Thereafter the table 10 is stepped boustrophedonically between successive exposures to produce the required array of latent images.
The lateral distance moved by the wafer 12 in steps within each row is equal to the distance d between corresponding points on the two identical patterns at the image plane. It will be appreciated that distance d will be proportional to spacing D between the corresponding points (specifically the longitudial center lines) of the patterns I and II on the reticle 16, taking into account the reduction factor (if any) of the optical column 14. Typically the reduction will be by a factor of 5 or 10, although the invention comprehends transfer without size reduction, or with enlargment.
Figure 2 illustrates the state of the wafer 12 following the first exposure, Patterns I and II have been reproduced as latent images I' and II ', in ad~iacent areas 54 and 56 respectively, of the wafer 12. For clarity the latent images are shown full size and without rotation. In addition an opaque defect III in the part of the reticle 16 carrying pattern II, has produced a corresponding latent image III ' on the wafer 12.
After the first exposure the wafer 12 is stepped hy distance d transversely to the longitudinal center lines 19 ard 21 of the two patterns and a second exposure made. Figure 3 represents the situation after the second exposure. ~afer area 5~ has been moved outside the field of the optical column 14 and so has not been exposed a second time. Therefore it carries only a latent image I'.
Area 56 now occupies the position vacated by area 54 and has been exposed -through the part of the reticle lfi carrying master pattern I.
Therefore area 56 now has latent images II' and I' superposed.
The first half of the reticle 16 has no defect in the same position as defect III, so the corresponding region of wafer 1~ area 56 has been exposed during the second exposure step.
Consequently the latent image III' has been removed, as indicated by the broken line in area 56 in Figure 3. It should be noted that the radiation intensity and duration are se7ected in accordance with the sensitivity of the photoresist to ensure adequate exposure by only one exposure.
During the second exposure, an adjacent wafer area 58 is exposed through the second part of the reticle 16, produciny another latent ima~e II' and defect latent imaqe III'. After the second exposure, area 58 will be stepped to the position occupied by area 56 and re-exposed during the third exposure. The process is then repeated.
It will be appreciated that a defect in the reticle part carrying master pattern I will not normally produce a latent image on the wafer 12 because the corresponding area will already have been irradiated through the unblemished part of the reticle lh during the preceding exposure.

l~ti 1~3 The area 54, and a corresponding area at the opposite end of the row of latent images, receive only a single exposure and may, if desired, be discarded when the wafer is being cut up after processing. Alternatively the apparatus can be arranged so that the first and last latent images are formed outside the useful area of the wafer 12.
The invention comprehends the use of a master having more than one pattern in each of the identical sets. For example, the master could have two identical sets, each of two or more patterns, arranged in a square, or in a row, the stepping distance d being adjusted as necessary or preferred. It should be appreciated, of course, that each set might comprise a row and/or column in an array of identical patterns.
Figure 4 illustrates, schematically, an arrangernent for exposing a wafer from a master in the form of a mask comprising an array of identical patterns. The apparatus exposes different areas of the master mask in succession until the entire array has been projected onto the wafer. Usually the projection ratio is 1:1 i.e.
the image is suhstantially the same size as the master.
The apparatus comprises a support plate 6n carrying on one si~e a vacuum chuck 62 which holds a semiconductor wafer 64 so that its photoresist-coated surface is substantially perpendicular to the optical axis 68 of an imaging device 70 in the form of a lens or mirror system. The device 70 is positioned between the wafer 64 and a mask 72, carrying an array 74 of identical patterns to be reproduced upon the wafer 64. The mask 72 is held generally parallel to the wafer 64 by a vacuum chuck 76, which itself is mounted on a plate 78. The plate 78 is mounted on one side of and parallel to, a second plate, 80. A shutter plate 82 is disposed at the other side of the plate 80 and is generally parallel thereto. A lamp 84, behind the shutter plate serves to irradiate the mask 72 by way of a slit 86 in the shutter plate 82 and aligned holes 88, 90 and 92 in the chuck 76, and plates 78 and 80 respectively.
The shutter slit 86 is arcuate and generally vertical and serves to restrict light from lamp 84 so that only a narrow strip of the array 74 is illuminated at any time.
The shutter plate 82, light source 84, and imaging device 70 are fixed relative to each other. However the plates 78 and 80, and chuck 76 are movable together, in register with the wafer support plate 60 and check 62, transversely to the optical axis 68 and the length of the slit 86, so that a strip of light scans across the mask 72 and in doing so, projects the array 74 onto the wafer 64.
The scanning movement of the mask 72 and wafer 64 past the imaging device 70 may be in discrete steps or as a continuous movement. It will be appreciated that the mask and wafer could be fixed and the shutter 82, lamp 84, and imaging device 70 be scanned across them.
2n The intermediate plate 78 is rnoveable relative to the plate 80 by a drive mechanism 94 under the control of a control system 96. The control system 96 is responsively coupled to a laser interferometer device 98, which monitors displacment of the support member 78 relative to the plate 80 by reflecting a laser beam 100 off a reflective surface 102 on one side edge of the support mernber 78.
Interconnections between drive mechanisms 94, control system 96 and device 98 are indicated at 104.

s ' 12 7~

In use, a wafer h4 is loaded into the chuck 62 and the wafer fi4 and mask 72 moved to a predetermined alignment position at which they are aligned relative to each other if necessary.
Typically alignment marks on the mask 72 will be aliqned with correspondin~ alignment rnarks on the wafer 64, usually provided on the wafer 64 by a previous processing operation. Any convenient alignment system, for example split field microscope, may be employed.
The wafer 64 and mask 72 are then scanned across the axis of the optical system 70 to produce on the wafer 64 a latent image of the array 74.
The wafer 64 and mask 72 are then returned to the alignment position and the intermediate plate 7~ moved in the same direction as the scanning movement, by means of the control system 96 relative to the rearmost plate ~0 by a distance equal to the spacing hetween corresponding points in ad~jacent columns of the array 74.
The wafer 64 and rnask 72 are then scanned across the optical system again to expose the array once rnore and provide a second latent image upon the wafer 64. With the exception of possihly one pattern at 2n each end of each row, each column of the second latent image will be superposed upon a different column of the latent image previously exposed onto the wafer. Clearly the offset distance could be equal to multiples of the inter-column spacing. It is also possible to move plate 7~ relative to plate ~0, in a direction normal to that of the scannin~ movement, between scanning movements.
It should be noted that the use of a laser monitoring system to monitor the displacement of the mask between scans obviates the need for the alignment procedure to be repeated; and also allows the process to be used even where no alignment marks have previously been provided on the wafer 64. However, it would also be possible to dispense with the laser metering, and realign between scans using a set of alignment marks spaced apart from the first marks by the distance required to superpose one column upon the next. The prealignment to one set of alignment marks would then be carried out as before, followed by the first scan. The wafer 64 and mask 72 would then be returned to the alignment position, offset 1n approximately by the distance required for superposition and realigned precisely using the second set of marks before carrying out the second exposure scan.
It will be appreciated that the need to manually realign the mask and wafer before the second exposure might limit the alignment accuracy ohtainable. However, with present projection apparatus an alignment accuracy within 0.25 microns, adequate for many applications, should be possible.
The apparatus may be modified by mounting the wafer rather than the mask upon a holder having two or more parts, one part being displaced relative to the other between successive exposures.
Various other modifications are possible within the scope of the invention. For example instead of U.V. light other radiation could be used, for example visible light, ions, x-rays or electrons, the radiation-sensitive surface of the wafer or other irradiated hody bein~ chosen to suit.
Although in the described embodiments the required superposition is ohtained by translating the master and body one relative to the other, it is also envisaged that the images might be translated by altering the radiation path, for example by means of the imaging device, in which case the master and hody might be fixed relative to each other. More specifically, the master and body might lie in the same plane and the imaging device include means for directing the radiation to pass from the master to the body.
Translation of the images could then be achieved by moving the directing means relative to the body and master.
The array of images on the multiple pattern master may itself be produced by an embodirnent of the present invention, for example the boustrophedonically stepped method described with reference to Figure 1.
Although specifically described with reference to exposing wafers for integrated circuits, the invention encompasses other manufacturing processes, for example the manufacture of masks for subsequent use in exposing semiconductor wafers, of solar cells, printed circuit boards, lead frames, tape-bond tapes, and hybrid ceramics. It is further envisaged that the images might be an enlargement of the master pattern especially when making printed circuit boards.
It should be noted that the invention yields considerable savings where many pattern transfer steps, each with a different master are involved, such as in making integrated circuits and the like. As many as twelve or more different patterns might then be required. To obtain, in each transfer step, a double exposure using two separate masters, each needing to be aligned to the wafer and to the apparatus before its exposure, would require ~ 3 considerahly greater overall production time. Moreover this double alignment would increase overall alignment inaccuracy, especially since such alignment is often done manually. The corresponding sources of error in embodiments of the invention are in the positioning of the two identical patterns relative to each other on the same master and the displacement of the master relative to the wafer or other body. Both can be controlled very accurately using contemporary laser interferometric position con-trol systems, or systems of comparable accuracy.
1n Another disadvantage of double-exposure using two masters, at least in relation to semiconductor devices, arises from the usual practice of printing the alignment marks onto a batch of wafers during the first processing step. Until the wafers have been processed the alignment marks are not visible. Therefore the second of two separate masters could not be aligned to them. This disadvantage is overcome hy embodiments of the present invention in which the intermediate re-alignment is not required.
While, in the examples described, the images have been produced hy irradiation throu~h a master, that is the master is a 2n transparency for the particular radiation used, irradiation can also be by reflection, or by a radiation having as its source the master itself, this radiation being caused by irradiation of the master. In all instances, the irradiation impinges on the body, or wafer, by way of the master and can therefore be considered as being "from" the master. The term "from" is used in the claims to encompass radiation which passes through the master, is reflected from the master or ernanates from the master, howsoever caused.

Claims (20)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY
OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method of transferring an image of a pattern onto a body comprising:-exposing the body to radiation from a master having at least two identical sets each of at least one pattern to produce on the body a latent image of at least one of said sets;
exposing the body a second time to radiation from said master such that a latent image of the other of said sets is superposed upon said latent image of said one of said sets.

2. A method as claimed in claim 1, wherein:
the body and master are displaced one relative to the other between the first and second exposures so as to effect the superposition of latent images.

3. A method as defined in claim 2, wherein:
the body and master are displaced repeatedly one relative to the other so as to produce at least one line of superposed latent images.

4. A method as defined in claim 3, wherein the repeated displacement is boustrophedonic.

5. A method as defined in claim 1, 2 or 3 wherein:
each said identical set comprises at least part of a line in an array of patterns.

6. A method as defined in claim 1, wherein:
in each exposure an image of each set is transferred by exposing different parts of the set in succession.

7. A method as defined in claim 6, wherein before the first exposure the body and master are aligned at a first position and between the first exposure and said subsequent exposure displaced from the first position by a distance sufficient to cause the required image superposition.

8. A method as defined in claim 7, wherein the displacement between exposures includes aligning said one of the body and master to a different reference mark.

9. A method as defined in claim 7, wherein the displacement is effected by indexing means operative to displace the body and master by the required distance.

10. A method as defined in claim 3, wherein:
first and last latent images in each line are not superposed and are formed outside a useful area of the body.

11. A method as defined in claim 1, wherein the master comprises a transparency.

12. A method as defined in claim 1, wherein the body comprises a semiconductor material.

13. A method as defined in claim 1 wherein:
the body is exposed to radiation from both said identical sets in each exposure step.

14. Apparatus for transferring an image of a pattern onto a body, comprising:
a holder for a master having two identical sets each of at least one pattern;
a holder for the body;
means for exposing the body twice to radiation from said master so as to produce on the body during one exposure a latent image of at least one of said sets and during a subsequent exposure a latent image of at least the other of said sets superposed upon the first-mentioned latent image.

15. Apparatus as defined in claim 14, wherein:
said means for exposing comprises means for effecting relative displacement of the body and master to provide the superposition of the first and second latent images.

16. Apparatus as defined in claim 15, wherein:
at least one of the holders for the body and master, respectively, comprises at least two parts; and said means for displacing serves to displace said parts one relative to the other between said one exposure step and said subsequent exposure step.

17. Apparatus as defined in claim 151 wherein:
said means for displacing is operative to displace the body and master stepwise to produce at least one line of said superposed latent images.

18. Apparatus as defined in claim 17, wherein:
said means for displacing is operative to displace said body and master boustrophedonically.

19. Apparatus as defined in claim 14 wherein:
said means for exposing serves to expose different parts of each set in succession.

20. Apparatus as defined in claim 19, wherein:
said means for exposing comprises a shutter member between a radiation source and said body, said shutter member having a slit aperture and;
said shutter member and said body are relatively movable in a direction transverse to the length of said slit to cause radiation through said slit to scan said body.