GB2136987A

GB2136987A - Alignment of Two Members e.g. a Photomask and Wafer in Spaced Parallel Planes

Info

Publication number: GB2136987A
Application number: GB08405979A
Authority: GB
Inventors: Chen Hwa
Original assignee: Perkin Elmer Corp
Current assignee: Applied Biosystems Inc
Priority date: 1983-03-11
Filing date: 1984-03-07
Publication date: 1984-09-26
Also published as: GB8405979D0; GB2136987B; JPS59168445A; DE3408764A1

Abstract

In the alignment of an optical mask 11 with the pattern of the previous layer on a wafer 15 in the fabrication of an integrated circuit, the mask 11 is formed with a transparent phase reversal zone plate 10 comprising alternate zones of a height relative to the surface of the mask 11 such that their optical path difference differs from adjacent zones for normal incident light by a half- wave. A laser beam is directed perpendicularly to the planes of the mask 11 and wafer 15 and is focussed by the zone plate as a line on the surface of the wafer. The wafer is formed with a line bar 14 and the relative position of the mask and wafer is adjusted until the line is focussed on the bar 14, thus indicating exact alignment. <IMAGE>

Description

SPECIFICATION Alignment of Two Members in Spaced Parallel Planes In the fabrication of integrated circuits it is critical that mask and wafer be aligned within extremely close tolerances. As circuits are built up layer by layer on a wafer, each new layer is made by exposing a wafer to a pattern on a mask. The new pattern on the mask must overlie the pattern on the old layer on the wafer to a registration accuracy which in some cases may be as high as 0.1 microns. To obtain such overlay accuracy requires highly advanced mask-to-wafer alignment techniques.

As requirements for smaller minimum features and high overlay accuracies increase, alignment techniques have also increased in sophistication.

Two such techniques which are in the vanguard of mask and wafer alignment are described in United States Patent nos: 4,037,969 and 4,311,389.

Both of these alignment systems utilize zone plates formed on the mask.

United States Patent No. 4,037,969 describes an alignment system wherein circular zone plates are formed on both mask and wafer as alignment marks. Monochromatic light transmitted through the mask is reflected by the mask and wafer zone plates to form focused spots in a predetermined plane. Coincidence of these two spots indicates alignment of the mask and wafer.

United States Patent No. 4,311,389 discloses an alignment system which uses a Fresnel linear zone plate formed on the mask and a reflecting grating on the wafer. The mask and wafer are separated by a distance equal to the focal length (first order) of the zone plate so that monochromatic light beamed through the zone plate is focused as a light line on the surface of the wafer. Coincidence determined by the detected reflectance from the wafer of the line focus and line grating on the wafer is indicative of mask and wafer alignment.

Both of the foregoing described alignment systems are highly inefficient in use of light.

The system described in United States Patent NO.4,311,389 in particular suffers from low diffraction efficiency, low signal to noise ratio, high zero-order return, and is vulnerable in the patterning process.

The present invention overcomes these disadvantages and particularly is a significant improvement over the system described in United States Patent No. 4,311,389.

The present invention relates to the alignment of two members disposed in parallel spaced apart planes, particularly alignment of patterns on a mask with complementary patters on a wafer in integrated circuit fabrication. In carrying out the present invention a mask for use in the fabrication of integrated circuit having at least one transparent section is formed with a linear, all transparent "phase reversal" zone plate in the transparent section, for focussing monochromatic light as a line at a predetermined focal plane while a reflective line bar is formed on the wafer.

Parameters are so chosen that monochromatic light transmitted through the area of the phase reversal zone plate forms a focussed line on the surface of the wafer. The intensity of reflectance of the light from the surface of the wafer is a maximum when the line focus is positioned directly on the line bar of the wafer and is, therefore, an indication that mask and wafer and consequently the circuit patterns thereon are aligned in a given direction. The line bar may also be less reflective than its surroundings in which case the reflected light would be a minimum when the line focus and line bar are in alignment.

The all transparent phase reversal zone plate obtains focussing of light without use of the opaque areas necessary in the Fresnel zone plate and, therefore, uses light in a highly efficient manner and virtually eliminates zero-order flux.

The use of the all transparent phase reversal zone plate in the system of the present invention increases the overall first order diffraction efficiency of light by as much as sixteen times and greatly increases the signal strength and the signal to noise ratio over the system disclosed in United States patent no: 4,311,389. At the same time the virtual elimination of zero-order flux makes it possible to replace low efficiency diffraction grating patterns necessary to repress zero-order flux as the alignment mark on the wafer with a solid bar pattern.

The mask preferably comprises alternate zones of transparent material having an optical path difference relative to adjacent zones of a halfwave length, the optical path difference either being formed by built-up zones of transparent material or by zones etched in the surface of the mask.

Apparatus according to the present invention turther comprises means for detecting the light reflected from the surface of the other member back through the zone plate and means for moving the members relative to each other until the detected reflected light indicates alignment of the two members. The means for detecting the reflected light provides a signal representative of the reflected light. The signal is processed and may be fed back to move a table carrying the wafer in a direction to align the focussed line of light with the solid bar on the wafer, thus aligning mask and wafer in a given direction.

The invention will now be described in more detail, by way of example, with reference to the accompanying drawings, in which:~ Figure 1 is a sectional view of a portion of a mask having a phase reversal zone plate according to the present invention formed thereon; Figure 2 illustrates in pictorial form the phase reversal zone plate in association with a line bar formed on the wafer; Figure 3 is a schematic representation of the relationship between the alignment marks on mask and wafer with incoming monochromatic light focussed on the line bar; and Figure 4 is a block diagram showing the complete alignment system.

Referring to Figure 1, there is shown a portion of a mask 11 which has formed on a portion thereof a phase reversal zone plate 10. The mask 11 is typically transparent to light and X-ray radiation and may be composed of polyimide, SiC, BN or other suitable material. The phase reversal zone plate is formed of alternate zones 12 made of a transparent material to the wavelength of the monochromatic light source in use, e.g., photoresist such as AZ 1470 for 6328 A wavelength.The zones 12 are of a height relative to the surface of the mask 11 that their optical path difference, OPD, differs from adjacent zones 13 for normal incident light by a half-wave or A 2 The zones 12 may also be formed by etching the adjacent zones 1 3 into the surface of the mask 11 having the same OPD contrast of a 2 To satisfy the A OPD=~ 2 requirement for zones 12, height H of zones 12 above the surface of the mask 11 is given by the formula: NA H 2Am (1) where A is the wavelength of the monochromatic light source Am is the difference of the refractive indexes of the transparent material, e.g., photoresist and air, and N is any odd integer, e.g., 1, 3, 5,...

When the zone plate is made by etching zones into the mask Am in the above formula would be n~1, with n being the refractive index of the transparent section of the mask substrate.

The phase reversal zone plate is designed in accordance with the following formula:

where A the wavelength of the monochromatic light source f =zone plate focal length Xm =zone boundries measured from the axis of symmetry XO =O m =1,2,3,...

The above formula defines the widths and spacings between zones 12 and 13 to form a zone plate which focuses monochromatic light of wavelength A at a distance equal to f.

Figure 2 shows the phase reversal zone plate 10 in association with a reflective solid line bar 14 formed on the surface of wafer 1 5. This line bar pattern is formed by two surfaces, either with different reflectivities or with the height or depth of the bar satisfying the following relationship: NA H (3) 4n where A is the wavelength n is the refractive index of the medium, e.g., photoresist covering the pattern area.

In the latter two cases, the maximum phase contrast is achieved for the laser beam upon reflection.

The zones or zone lines 12 are parallel to the line bar 14. As shown in Figures 2 and 3 the distance D between the mask 11 and wafer 15 is equal to the focal length ffor the selected monochromatic or laser light to be used. Thus, as shown in Figure 3, the light transmitted through the zone plate 10 focuses as a line at the surface of the wafer 15. Further, the width W of the line bar 14 is chosen to be equal to the focal linewidth of the phase reversal zone plate defined by 2.44xAxf/#, where A is the wavelength and f/# the F number of the zone plate. Thus, when perfect alignment is achieved, the light reflected by the line bar 14 is maximum. Detection of a maximum of light reflectance thus is indicative of perfect alignment of the alignment marks.

The zone plate 10 and line mark 14 are so oriented in Figure 2, i.e., the direction of the zones 12 and line bar 14 accommodates alignment in a direction perpendicular to the zones, i.e., in the x direction. Alignment in the y direction is obtained by alignment marks oriented perpendicular to the x direction.

The first order diffraction efficiency of the all transparent phase reversal zone plate 10 of the present invention has been calculated to be approximately 40.5% versus about 10.1% for the Fresnel zone plate. Thus, more light energy goes to the signal carrying order than those orders that contribute to noise background. The signal from the diffracted beam reflected back through the phase reversal zone plate is sixteen times stronger than that from the beam through a Fresnel zone plate. The all transparent phase reversal zone plate has virtually no transmitted zero-order flux.

This can be determined by summing the power in all of the diffraction orders which turns out to be equal to the power into the phase reversal zone plate, leaving no power left over for the zero order.

It should be noted that the polarity of the phase reversal zone plate may be reversed. In this case the zones 13 would have the zones 12 and viceversa as would be the case if zones 12 were formed by etching as opposed to deposition of material. Also, the line bar 14 could be nonreflecting instead of light reflecting which would give a minimum of reflected light on alignment as opposed to a maximum. The phase reversal zone plate may also be formed on either side of the transparent section of the mask substrate as long as the phase relationship of the light going through the adjacent zones is maintained.

The forming of the phase reversal zone plates on the mask 11 whether by deposition of photoresist or etching may be accomplished quite economically. Furthermore, the mask dimension stability may be better than that with the Fresnel zone plate due to much less stress induced.

Figure 3 shows mask 11 with phase reversal zone plate 10 in operative relationship to line bar 14 formed thereon. The distance D between mask 11 and wafer 15 is equal to f the first order focal length of the zone plate 10. The distance D is chosen to satisfy systems requirements, Thus, for a given monochromatic light source, e.g., for a HeNe laser which has a wavelength of 6328 angstroms, the boundaries of a phase reversal zone plate are completely defined by the formula (2) if a suitable zone plate F number is provided.

The typical phase reversal zone plate has a dimension in the direction that the zone plate has power of approximately 50 microns, for the mask wafer gap of 50 microns, an f/l lens.

Figure 3 depicts a laser beam 16 directed perpendicular to the planes of the mask 11 and wafer 15. The laser beam 16 is focused as a light line onto the surface of wafer 1 5. As shown, the light line is focused on the line bar 14, thus indicating exact alignment of the bar. In an actual alignment operation the line bar 14 would normally be displaced from its aligned position.

During an alignment the laser beam 16 is angularly scanned at the phase reversal zone plate in a direction perpendicular to the length of line bar 14, as shown in Figure 3 by arrow 17. The reflected light from the wafer 1 5 is sensed and processed to give a signal representative of the distance of the line bar 14 from its aligned position. The signal is then used to drive a motor to move the table on which wafer 1 5 rests until line bar 14 is in alignment.

The foregoing operation which is described more fully in relation to the system of Figure 4 aligns the marks in one direction, e.g., the x direction, it being understood that alignment in the y direction would require are least one other zone plate and line bar disposed with the long dimension of the zones and line bar of each set of alignment perpendicular to each other.

With at least one pair of zone plates and line bars placed at the opposite ends of the two planes to be aligned, the in-plane rotation error is corrected through the lateral alignments performed at these sites.

At three alignment sites, maximizing the return signals simultaneously by adjusting the gap to remove the two tilt errors the parallelism and optimum gap between the planes will also be established.

Referring now more particularly to Figure 4 there is shown an optical alignment apparatus for carrying out the alignment operation of the present invention.

It comprises a laser light source 18 which provides a highly stable continuous wave, linearly polarized, monochromatic laser beam. The laser beam is directed to a scanning mirror 19 and thence through a polarized beam splitter cube 20 to the surface of the mask 11. As previously indicated, the laser beam 16 is focused as a light line onto the surface of the wafer by means of the phase reversal zone plate 10 formed on or in a transparent section of the mask 11.

The laser polarization direction is arranged so that the light transmitted through the polarized beam splitter cube 20 is maximized with minimum amount of light reflected.

A lens system comprises lenses Lr and L2 disposed between laser source 18 and scanning mirror 19. Reduction lenses are used to collimate and reduce the beam size to provide an appropriate fill factor compatible with the design parameters of the phase reversal zone plate 10.

The scanning mirror 19 is driven by driver 21 to angularly scan the laser beam 16 at the phase reversal zone plate 10 in a direction perpendicular to the long dimension of the zones 12 and 13 or in the direction parallel to the direction of alignment correction. The center of symmetry of the laser angular scan is the reference position for alignment.

Prior to reaching mask lithe laser beam 16 passes through a 1:1 telecentric imaging lens system L3 and L4 which images the beam spot at the axis of scanning mirror onto the phase reversal zone plate 10 of the mask 11.

A quarter wave plate 22 is disposed between lens L4 and mask 11 to assure that the reflected beam is appropriately polarized so that the intensity of the beam reflected by the beam splitter 20 to photodetector 24 is at a maximum.

For reasons to be discussed below, a slit type spatial filter 23 may be disposed between the beam splitter 20 and photo detector 24 for passing only desired orders of light reflected from the wafer 15.

The output voltage from photo-detector 24 is provided as an input to phase sensitive amplifier 25. The scanner driver 21 provides an input voltage to phase sensitive amplifier 25 representative of scan position. The output of phase sensitive amplifier 25 which is an error signal representative of deviation of line bar 14 from the aligned position is provided as an input to supply source 26 where it is amplified and provided as an input voltage to positioner 27. The positioner 27 is mechanically connected to table 28 and moves table 28 until the voltage error signal is reduced to zero at which time mask 11 and wafer 15 are aligned in one direction, e.g., the x direction. Alignment in the perpendicular direction, i.e., the y direction requires a similar arrangement.

In operation the laser beam 16 is angularly scanned at the phase reversal zone plate on mask 11. This sweeps the line focus of the zone plate across the surface of wafer 1 5. The light reflected back through the zone plate 10, quarter wave plate 22 and lens L4 is reflected to photo-detector 24 via beam splitter 20. The reflected light signal itself contains information of the deviation of the line bar from the aligned position. This combined with information from the scanner driver 21 which gives the refererence position provides the error signal for moving table 28 until alignment is achieved.

The quarter wave plate 22 insures that the returning light signal is appropriately polarized to provide the maximum intensity for reflection by the beam splitter 20 to photo detector 24.

Since the phase reversal zone plate 10 eliminates zero diffracted order of the light flux, a solid line bar 14 may be used.

However, in the case where the phase reversal zone plate is imperfectly made, a certain amount of zero order light will be reflected by the zone plate and collected by the lens L4. In this case an intercepted line bar or grating as opposed to a solid line bar should be used to cause the orders to be reflected at predictable angles thus permitting spatial filter 23 to pass through the desired orders, e.g., +1 orders, to the detector, and to filter out the undesired orders, e.g., the higher and zero orders. However the foregoing mentioned use of a line grating and spatial filter is mentioned for completeness only, it being pointed out that one of the novel features in the use of a phase reversal zone plate as opposed to a Fresnel zone plate is the virtual elimination of all zero order components of the light signal.

Claims

1. A mask for use in the fabrication of integrated circuits having at least one transparent section, comprises means forming a linear, all transparent "phase reversal" zone plate in the transparent section, for focussing monochromatic light as a line at a predetermined focal plane.

2. A mask according to claim 1 wherein said means comprises alternate zones of transparent material having an optical path difference relative to adjacent zones of a half-wave length.

3. A mask according to claim 2 wherein said optical path difference is formed by built-up zones of transparent material.

4. A mask according to claim 2 wherein said optical path difference is formed by zones etched in the surface of the mask.

5. A mask according to claim 3 or claim 4 wherein the difference in height between adjacent zones is given by the formula: NA 2Am where A =the wavelength Am =the difference of the refractive indexes between said transparent material and air N any odd integer 6.A method for optically aligning patterns on two members disposed in spaced apart parallel planes, one of the members including a linear, all transparent phase reversal zone plate, and the other having a line bar on its surface, by directing a beam of monochromatic light through the zone plate to focus a light line on the surface of the other member, detecting the reflection of light from the surface of the other member back through the zone plate, and moving the members relative to each other until the detected reflected light indicates that the line of light is focussed on the line bar.

7. A method according to claim 6 wherein the linear, all transparent phase reversal zone plate comprises alternate zones of the zone plate having an optical path difference relative to adjacent zones equal to half a wavelength of the monochromatic light.

8. A method according to claim 7 wherein the linear, all transparent phase reversal zone plate is produced by forming the alternate zones by depositing a transparent material on the surface of the member to a height to have an optical path difference equal to half the wavelength of the monochromatic light.

9. A method according to claim 7 wherein the linear, all transparent phase reversal zone plate is produced by forming the alternate zones by etching depressions in a transparent section of the member to a depth to have an optical path difference equal to half the wavelength of said monochromatic light.

10. A method according to any one of claims 6 to 9 wherein the line bar has different reflectivity from its surroundings in the direction of alignment correction.

11. A method according to any one of claims 6 to 9 wherein the line bar has a height or depth equal to NA/4n, where A is the wavelength n is the refractive index of the medium covering the reflecting mark N is any odd integer.

12. An apparatus for optically aligning patterns on two members disposed in spaced apart parallel planes, comprising an all transparent phase reversal zone plate formed on one of the members, a line bar formed on the surface of the other member, means for directing a beam of monochromatic light through the zone plate for focussing a light line on the surface of the other member, means for detecting the light reflected from the surface of the other member back through the zone plate, and means for moving the members relative to each other until the detected reflected light indicates alignment of the two members.

13. An apparatus according to claim 12 wherein the zone plate comprises alternate zones having an optical path difference relative to adjacent zones equal to half a wavelength of the monochromatic light.

14. An apparatus according to claim 13 wherein the zone plate further includes transparent material formed on alternate zones of a transparent section of the member to a height having an optical path difference relative to adjacent zones equal to half a wavelength of the monochromatic light.

16. An apparatus according to claim 13 wherein the zone plate comprises alternate zones etched in the surface of a transparent section of the member to a depth having an optical path difference relative to adjacent zones equal to half a wavelength of the monochromatic light.

15. An apparatus according to claim 14 wherein the transparent material is photo-resist.

17. An apparatus according to any one of claims 12 to 16 wherein the line bar is a light reflective solid line defined by the differential reflectivity between the bar pattern and its surroundings.

18. An apparatus according to any one of claims 12 to 16 wherein the line bar is a light reflective maximum-phase-contrast solid line.

19. An apparatus according to any one of claims 12 to 16 wherein the line bar is a light reflective periodically intercepted bar.