GB2211957A

GB2211957A - Holographic projection printing

Info

Publication number: GB2211957A
Application number: GB8725575A
Authority: GB
Inventors: John Edward Brook
Original assignee: General Electric Co PLC
Current assignee: General Electric Co PLC
Priority date: 1987-10-31
Filing date: 1987-10-31
Publication date: 1989-07-12
Anticipated expiration: 2007-10-31
Also published as: GB8725575D0; GB2211957B

Abstract

A mask 11 is holographically produced on photoresist layer 14 by passing object beam 19 through the mask and glass block 16 and reference beam 20 through the glass block. The transmission hologram 14 is then projection printed onto photoresist layer 26 by passing reference beam through glass block 21, hologram and glass block 22. Alternatively, a reflection hologram of the mask may be produced and then projection printed (Figs. 5 and 6 not shown). <IMAGE>

Description

OPTICAL PROJECTION PRINTING This invention relates to apparatus for and a method of optical projection printing, and especially to apparatus for and a method of printing micro-circuits.

In conventional systems of this kind a mask, which may be electron-beam-produced, may be used directly for contact printing onto the material from which the microcircuit is to be made eg. a silicon wafer. However, such physical contact can cause scratching of the mask from interposed dust particles as the mask is aligned with the semi-conductor. Such alignment is necessary because many masks may have to be used on the same silicon slice to build the circuit up. Also, inaccuracies can be caused by diffraction effects.

Alternatively, the mask can be used for projection printing, either 1:1 or, more usually, in a reduction.

However, such equipment is expensive and it is difficult to obtain resolution of very small features.

In fact, the minium spacing between two features for them to be resolved as two features (rather than being blured into one feature) is proportional to the wavelength of the light or other electromagnetic radiation and inversely proportional to the numerical aperture of the system and also inversely proportional to the refractive index of the medium through which the electromagnetic radiation travels (the Rayleigh Criterion). It follows that increased resolution may be obtained by using shorter wavelengths eg. X-ray's, but there are attendant disadvantages.

It has been proposed to use a hologram of a mask to project high resolution images (The Engineering Uses of Holography, published by Cambridge University Press, 1970 page 503).

However, in the arrangement shown on page 514, a simplified version of which forms Figure 1, there is a considerable separation between the mask 1 to be recorded and the recording medium 2 itself (the object beam emenating from point 3 and the reference beam being illustrated as 4), and this results in a small numerical aperture and consequent poor resolution. In an article by Karl A. Stetson in Applied Physics Letters Vol 11 no 7 dated 1st October, 1967, pages 225, 226, an arrangement is shown (a simplified version of which appears as Figure 2) in which there is a small separation between the recording medium 5 and the object 6, which would accordingly give increased resolution. The object beam 7 interferes with a reference beam 8 incident through a prism 9 which is totally internally reflected at the boundary between the recording medium 5 and air.While there is better resolution, the total inernal reflection means that there are in effect two reference beams and three holograms are formed as a result in the recording medium.

The invention provides an optical projection printing system for projecting an image of a mask onto an object, which comprises a hologram of a mask recorded through a transparent medium of refractive index greater than unity, means for producing a reconstructing beam to generate an image of the mask, and a transparent medium of refractive index greater than unity through which passes, in use, both the reconstructing beam incident on the hologram and the reconstructed beam incident on the object.

Increased resolution is produced by the transparent medium.

Advantagously, the spacing between the object and the hologram is less than half, preferably less than one fifth of the smallest dimension in the plane of the hologram. The arrangement of the invention permits a small spearation between the object and the hologram (and hence a small separation between the mask and the hologram when recording it), which provides increased numerical aperture and thus increased resolution: unlike the arrangement of Stetson, however, the hologram is recorded and played back through a medium of refractive index greater than unity, and only one hologram is formed.

The refractive indices of the transparent medium through which passes the reconstructing beam, and the reconstructed beam, are preferably substantially the same as that through which the hologram was recorded.

Advantageously, the transparent medium includes solid material, preferably glass, through which the reconstructing beam and the reconstructed beam respectively pass in use which are separated from the hologram by respective layers of liquid. The liquid masks any irregularities in the facing surfaces of the hologram and solid material and also avoids the possibility of a solid/air interface.

The refractive indices of the respective liquid layers are preferably substantially the same as that of the solid material.

The invention also provides a method of optically projecting an image of a mask onto an object, as well as a method of recording a hologram for use in an optical projection printing system.

The invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 shows the paths of light rays for recording one prior art hologram; Figure 2 shows the paths of light rays for recording another prior act hologram; Figures 3 shows the paths of light rays for recording a hologram of mask in accordance with one embodiment of the invention; Figure 4 shows the paths of light for replaying the hologram onto a silicon wafer in accordance with the first embodiment of the invention; Figure 5 shows the paths of light rays for recording a hologram of a mask in accordance with a second embodiment of the invention; and Figure 6 shows the paths of light rays for replaying the hologram onto a silicon wafer in accordance with the second embodiment of the invention.

Figures 1 and 2 have already been discussed earlier.

Referring to Figures 3 and 4, the optical system is designed for projection of an image of a mask onto a silicon wafer. The wafer is coated with photo-resist and exposed. The exposed photo-resist can then be removed, and the areas revealed can be doped with suitable n or p impurities. Complicated micro-circuits can be formed by repeated application of this technique with appropriate masks. The mask consists of a flat and parallel layer of glass bearing a layer of chromium in a desired pattern.

The pattern may be formed by selective removal of areas of the chromium by coating the chromium with a layer of electron resist and guiding an electron beam around the layer in the desired pattern.

In accordance with the invention, an image of the mask is projected onto the wafer covered with photoresist by holographic means and the beam passes through a transparent medium with an index greater than unity on its way to the mask.

Refering to Figure 3, a holographic recording is made in the following way. An electron-beam-produced mask 11 consists of a layer of chromium 12 in a desired pattern on a flat and parallel glass substrate 13. The sensitive medium 14 in which the hologram is recorded is mounted on a glass support 15. Between the mask 11 and the sensitive medium 14 is positioned a block of glass 16 and, interposed between it and the layer of chromium 12 and the sensitive medium 14 are, respectively, layers of indexmatching liquid 17 and 18 held in by surface tension and whose thickness is defined by the spacers. The thicknesses of the layers 12, 13, 17, 18, 14, 15 have all been exaggerated for clarity.

The refractive index of the glass substrate 13, the glass support 15 and the glass block 16 are all substantially the same value, and the refractive index of the liquid layers 17 and 18 is also substantially this value.

The exposure is carried out in the following way.

Collimated light beams 19 and 20 from the coherent light source in the form of an argon ion laser are incident on the sensitive medium 14. One, the object beam, passes through the mask and diverges, centered normally, onto the sensitive medium 4. The other, the reference beam, passes through the side of the block to strike the sensitive medium.

The sensitive medium is then developed in the normal way, and is then incorporated in the projection apparatus shown in Figure 4. Referring to Figure 4, in the optical projection apparatus, the hologram 14 is sandwiched between two glass blocks 21, 22 and is separated from each by a layer of liquid 23, 24 held in position by surface tension The slice of silicon 25 on which it is desired to print a micro-circuit, which is coated by photo-resist 26, is separated by a layer of liquid 27 from the surface of the block 22 remote from the hologram. The refractive indices of the block 21, 22 and the liquid layers 23, 24, 27 are all substantially the same value as that for the block 16 and the liquid layers 17, 18 used for recording the hologram.

It should be noted that, within the recording systsem of Figure 3, variations of refractive index between the various layers 13, 17, 16, and 18 of around 0.1 can be tolerated, reflections being caused at the bounderies between media of different indices. However, for playback in Figure 4, the refractive index of the glass blocks 21, 22 should duplicate that of the glass block 16 to within about 0.02, and the layers of index -matching liquid 23, 24, 27 should duplicate that of the layaers 17, 18 to within the same amount.

The silicon slice 25 is printed upon by illuminating the developed hologram 14 by a collimated beam of coherent radiation derived from a laser operating at the same frequency as that used for recording the hologram, the reconstructing beam 28 passing through the side of the block 22 and being incident on the hologram at the same angle as the reference beam was during the recording. The reconstructed beam 29 emerges normal from the hologram and projects an image of the mask onto the photo-resist 26 on the silicon slice 25.

The laser includes an etalon to suppress all but one mode of a vibration so that the laser emits a single frequency of radiation. Otherwise, the hologram would be degraded because the oblique nature of the reference beam results in there being a substantial difference in path lengths between the reference and object beams.

An advantage of recording the mask in holographic form is that it is possible to obtain better resolution than the optical system using lenses and an advantage of the interposition of a transparent medium of refractive index greater than unity is that increased resolution is achieved compared with a holographic system operating in air. Advantages of the use of liquid layers between the glass blocks and the sensitive medium and the photo-resist coated silicon wafer is that, firstly, it prevents there being an air/glass interface at the sensitive medium and at the photo-resist coated silicon wafer (which would cause deviation and total internal reflection) and, secondly, the effect of any departure from flatness in the surface of the hologram is reduced.

The optical projection system shown in Figure 4 can be used for exposing in many photo-resist coated wafers, and there is no direct solid-to-solid contact between it and the glass block, which reduces possibility of damage to the wafer. The hologram 14 of course does not come into direct contact with the glass block 21, 22 and the possibility of it being scratched is also reduced.

Similarly, when recording the hologram as shown in Figure 3, both the sensitive medium 14 and the electron-beamproduced mask 11 are protected from contact with the glass blocks fixed by the liquid, and a whole series of submaster hologram masks may be made using the apparatus.

As an example of a suitable optical configuration, the reference beam and the reconstructing beam could strike the sensitive medium at 5 degrees to the plane of the sensitive medium. Wavelengths of laser light of 458nm or 365nm from argon ion laser would be suitable. The glass blocks 16, 21, 22 could be one centimetre in thickness by ten to fifteen centrimetre side length and the sensitived medium could be 10 and the sensitive medium 10 microns in thickness, and the medium could be dichromated gelatine or another photo polymer system. The liquid layers could be 100 microns thick: if layers are substantially thicker than this then there could be convection currents in them which would cause a variation in the refractive index.A suitable refractived index would be 1.54, and the following could be used for the index matching liquids adjacent to the glass blocks: dimethyl benzene; bromonapthylene; or benzyl alcohol.

Variations in these quantities are of course possible without departing from the scope of the invention. Thus, the reference and recording beams could strike the sensitive medium at an angle of 221 degrees to 10 degrees. The glass blocks could have thicknesses within the range 0.5 to 2 centimeters and liquid layers could be less than 100 microns thick. Other materials which are transparent to the laser light frequency could be used to record and play back the hologram, both solid and liquid.

The system described with reference to Figure 3 and 4 records and plays back a transmisson hologram, that is,the object beam and reference beam strike the recording medium on the same face.

Referring to Figures 5 and 6, the invention is also applicable to a reflection hologram, that is where the object beam and the reference beam strike the recording medium on opposite faces.

The system will be described briefly because it is simply the optical inverse of the system at Figures 3 and 4, and is also shown schematically for the same reason. A layer of sensitive material 30 is exposed by an object beam 31 through an electro-beam-produced mask 32 and, on the opposite face, by a reference beam 33, in each case through glass blocks 34 and 35. Layers of indexmatching liquid (not shown) are provided at the glass/recording medium and glass/mask interfaces. The refractive indices of the glass and the liquid layers are substantially the samse as each other. Replay at the hologram 30 thus formed by reconstructing beam 36 through glass block 37 of the same index results in reconstructed beam 38 being imaged onto a chip 39 coated with photoresist. The variations described which may be made to the embodiment of figures 3 and 4 may also be made to this embodiment.

Claims

1. An optical projection printing system for projecting the image of a mask onto an object, which comprises a hologram of the mask recorded through a transparent medium of refractive index greater than unity, means for producing a reconstructing beam to generate an image of the mask, and transparent medium of refractive index greater than unity through which passes, in use, both the reconstructing beam incident on the hologram and the reconstructed beam incident on the object.

2. An optical projection system as claimed in claim 1, in which the spacing between the object and the hologram is less than half the smallest dimension in the plane of the hologram.

3. A optical projection printing system as claimed in claim 2, in which the spacing between the object and the hologram is less than one fifth of the smallest dimension in the plane of the hologram.

4. An optical projection system as claimed in any one of claims 1 to 3, in which the refractive index of the transparent medium through which the reconstructed beam passes in use is substantially the since is that through which the hologram was recorded.

5. An optical projection printing system as claimed is any one of claims 1 to 4, in which refractive index of the transport medium through which the reconstructing beam passes in use is substantially the same as that through which the hologram was recorded.

6. An optical projection printing system as claimed in in any one of claims 1 to 5 in which the transparent medium includes solid material through which the reconstructing beam passes in use which is separated from the hologram by a layer of liquid.

7. An optical projection printing system as claimed in claim 6, in which the transparent medium includes solid material through which the reconstructed beam passes in use which is separated from both the hologram and the object by respective layers of liquid.

8. An optical projection printing systsem as claimed in claim 6 or 7, in which the solid material is glass.

9. An optical projection system substantially as herein described with reference to the accompanying drawings.

10. A method of optically projecting an image of a mask onto an object, which comprises passing a reconstructing beam through a transparent medium of refractive index greater than unity onto a hologram which was recorded through a transparent medium of refractive index greater than unity, such that the reconstructed beam also passes through transparent medium of refractive index greater than unity on its way to the object.

11. A method as claimed in claim 10, in which the object is a layer of semi-conductor material coated with photo-resist.

12. A micro-circuit in which the method of claims 10 or 11 is used in its production.

13. A method of recording a hologram for use in an optical projection system for projecting an image of a mask onto an object, which comprises recording an image of the mask through a transparent medium, the refractive index of which is greater than unity.

14. A method as claimed in claim 13, in which the transparent medium consists of solid material separated from the mask and from recording medium by a layer of liquid.

15. A method as claimed in claim 14, in which the object beam passes through a face of the solid material which is parallel to the mask and to the recording medium, and the reconstructing beam passes through a face of solid material which is perpendicular to the mask and to the recording material.

16. A mothod of optically projecting an image of a mask onto an object substantially as herein described with reference to the accompanying drawings.

17. A method of recording a hologram for use in an optical projection printing system substantially as herein described with reference to the accompanying drawings.

Amendments te the claims have been filed as follows CLAIMS 1. An optical projection printing system for projecting the image of a mask onto an object, which comprises a hologram of the mask recorded through a transparent medium of refractive index greater than unity, means for producing a reconstructing beam to generate an image of the mask, and transparent medium of refractive index greater than unity arranged so that, in use, the reconstructing beam impinges on the hologram through the transparent medium, and the reconstructed beam travels between the hologram and the object through the transparent medium, the boundaries of the transparent medium including a boundary parallel to the face of the hologram and a transversly extending boundary through which transversly extending boundary the reconstructing beam passes in use.

3. An optical projection printing system as claimed in claim 2, in which the spacing between the object and the hologram is less than one fifth of the smallest dimension in the plane of the hologram.

4. An optical projection system as claimed in any one of claims 1 to 3, in which the refractive index of the transparent medium through which the reconstructed beam passes in use is substantially the same as that through which the hologram was recorded.

5. An optical projection printing system as claimed in any one of claims 1 to 4, in which refractive index of the transparent medium through which the reconstructing beam passes in use is substantially the same as that through which the hologram was recorded.

6. An optical projection printing system as claimed in in any one of claims 1 to 5, in which the transparent medium includes solid material through which the reconstructing beam passes in use which is separated from the hologram by a layer of liquid.

7. An optical projection printing system as claimed in claim 6, in which the transparent medium includes solid material through which the reconstructed beam passes in use which is separated from the hologram and the object by respective layers of liquid.

8. An optical projection system as claimed in claim 6 or 7, in which the solid material is glass.

10. A method of optically projecting an image of a mask onto an object, which comprises passing a reconstructing beam through a transparent medium of refractive index greater that unity onto a hologram of the mask which was recorded through a transparent medium of refractive index greater than unity, such that the reconstructed beam also passes through transparent medium of refractive index greater than unity on its way to the object, the reconstructing beam entering the transparent medium through a boundary that is transverse to a boundary parallel to the face of the hologram.

11. A method as claimed in claim 10, in which the object is a layer of semi-conductor material coated with photoresist.

13. A method of recording a hologram for use in an optical projection system for projecting an image of a mask onto an object, wherein the reference beam impinges on recording medium through transparent medium of refractive index greater than unity and the object beam travels between a mask and the recording medium through transparent medium of refractive index greater than unity, the reference beam entering the transparent medium through a boundary that is transverse to a boundary parallel to the face of the recording medium.

16. A method of optically projecting an image of a mask onto an object substantially as herein described with reference to the accompanying drawings.