GB2036367A - Masks for the Projection Exposure of Semiconductor Substrates - Google Patents
Masks for the Projection Exposure of Semiconductor Substrates Download PDFInfo
- Publication number
- GB2036367A GB2036367A GB7935831A GB7935831A GB2036367A GB 2036367 A GB2036367 A GB 2036367A GB 7935831 A GB7935831 A GB 7935831A GB 7935831 A GB7935831 A GB 7935831A GB 2036367 A GB2036367 A GB 2036367A
- Authority
- GB
- United Kingdom
- Prior art keywords
- glass
- mask
- mask layer
- projection lens
- mask according
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Withdrawn
Links
- 239000000758 substrate Substances 0.000 title claims abstract description 21
- 239000004065 semiconductor Substances 0.000 title claims abstract description 11
- 239000011521 glass Substances 0.000 claims abstract description 98
- 239000000463 material Substances 0.000 claims description 6
- 244000283070 Abies balsamea Species 0.000 claims description 4
- 235000007173 Abies balsamea Nutrition 0.000 claims description 4
- 239000004858 Canada balsam Substances 0.000 claims description 4
- 230000005855 radiation Effects 0.000 claims description 3
- 239000002904 solvent Substances 0.000 claims description 3
- 239000004922 lacquer Substances 0.000 claims description 2
- 230000001681 protective effect Effects 0.000 claims 1
- 239000000428 dust Substances 0.000 abstract description 9
- 239000002245 particle Substances 0.000 abstract description 8
- 239000010410 layer Substances 0.000 description 36
- 239000012535 impurity Substances 0.000 description 11
- 230000003287 optical effect Effects 0.000 description 11
- 230000000694 effects Effects 0.000 description 6
- 238000000034 method Methods 0.000 description 6
- 230000002411 adverse Effects 0.000 description 5
- 230000008901 benefit Effects 0.000 description 4
- 238000000576 coating method Methods 0.000 description 4
- 230000004075 alteration Effects 0.000 description 3
- 239000011248 coating agent Substances 0.000 description 3
- 238000004519 manufacturing process Methods 0.000 description 3
- 238000007789 sealing Methods 0.000 description 3
- UQSXHKLRYXJYBZ-UHFFFAOYSA-N Iron oxide Chemical compound [Fe]=O UQSXHKLRYXJYBZ-UHFFFAOYSA-N 0.000 description 2
- 239000012790 adhesive layer Substances 0.000 description 2
- 239000005388 borosilicate glass Substances 0.000 description 2
- 238000004140 cleaning Methods 0.000 description 2
- 238000006073 displacement reaction Methods 0.000 description 2
- 238000003384 imaging method Methods 0.000 description 2
- VYZAMTAEIAYCRO-UHFFFAOYSA-N Chromium Chemical compound [Cr] VYZAMTAEIAYCRO-UHFFFAOYSA-N 0.000 description 1
- 230000015572 biosynthetic process Effects 0.000 description 1
- 230000008859 change Effects 0.000 description 1
- 238000001311 chemical methods and process Methods 0.000 description 1
- 229910052804 chromium Inorganic materials 0.000 description 1
- 239000011651 chromium Substances 0.000 description 1
- 230000008021 deposition Effects 0.000 description 1
- 230000008030 elimination Effects 0.000 description 1
- 238000003379 elimination reaction Methods 0.000 description 1
- 239000000839 emulsion Substances 0.000 description 1
- 239000005357 flat glass Substances 0.000 description 1
- 238000010438 heat treatment Methods 0.000 description 1
- 231100000989 no adverse effect Toxicity 0.000 description 1
- 230000002265 prevention Effects 0.000 description 1
- 230000008569 process Effects 0.000 description 1
- 210000001747 pupil Anatomy 0.000 description 1
- 230000009467 reduction Effects 0.000 description 1
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 1
Classifications
-
- G—PHYSICS
- G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
- G03F—PHOTOMECHANICAL PRODUCTION OF TEXTURED OR PATTERNED SURFACES, e.g. FOR PRINTING, FOR PROCESSING OF SEMICONDUCTOR DEVICES; MATERIALS THEREFOR; ORIGINALS THEREFOR; APPARATUS SPECIALLY ADAPTED THEREFOR
- G03F1/00—Originals for photomechanical production of textured or patterned surfaces, e.g., masks, photo-masks, reticles; Mask blanks or pellicles therefor; Containers specially adapted therefor; Preparation thereof
- G03F1/38—Masks having auxiliary features, e.g. special coatings or marks for alignment or testing; Preparation thereof
- G03F1/48—Protective coatings
Landscapes
- Physics & Mathematics (AREA)
- General Physics & Mathematics (AREA)
- Exposure And Positioning Against Photoresist Photosensitive Materials (AREA)
Abstract
A mask for a device for the projection exposure of a semi- conductor substrate (5) comprises a mask layer (2) having a pattern of permeable and impermeable surfaces, a first glass (1) disposed on the mask layer (2), and a second glass (3) disposed on the mask layer (2) opposite the first glass (1), the second glass being directed towards a projection lens (4) and having a thickness exceeding the Rayleigh depth of the projection lens (4) in the region overlapping with the mask layer (2). The glass plates protect the mask and prevent dust particles on the mask being sharply imaged on the substrate. <IMAGE>
Description
SPECIFICATION
Masks for Devices for the Projection Exposure of Semiconductor Substrates
This invention relates to masks for devices for the projection exposure of semiconductor substrates.
In the production of integrated circuits a number of masks of different patterns are imaged on a semiconductor substrate, a surface of the substrate being coated with a photosensitive resist. Between consecutive imaging operations the substrate is subjected to physical and chemical processes, e.g. the exposed and unexposed portions of the photosensitive resist applied to the subsrate and the underlying layers are etched. In the past, the mask with the pattern to be imaged was brought into direct contact with the substrate during exposure. Due to the increasing demands of accuracy, this method has been changed lately in that the mask with the pattern to be reproduced is imaged on the substrate by positioning a lens between the mask and the substrate.There has also been a tendency to image one and the same pattern of one mask by means of the step and repeat method, instead of producing a great number of identical patterns, which are to be imaged on the substrate or wafer, by means of one mask having a corresponding number of patterns. Masks embodying the present invention are of particular advantage when used in connection with the step and repeat method, without being limited to use in such a method.
The masks (also called photomasks) usually comprise a square carrier glass, the standardised lengths of its edges being in the range of approximately 62.5 mm (2.46 inches) to 152.4 mm (6 inches), and its thickness being between 1.5 mm (0.06 inches) and 6.3 mm (0.25 inches).
Either a photographic emulsion layer (fine-grained film material) or a so-called 'hard film', i.e. an evaporated coating or approximately 1 ym thickness of chromium, iron oxide or other materials, is applied to the carrier glass. The actual mask pattern is produced in this layer by means of photographic or photolithographic processes. These masks are then put into or stored in the projection exposure device, the side having the layer being directed towards the projection lens. Dust particles and other impurities have direct access to the actual mask layer, which is particularly disadvantageous when using a step and repeat exposure method since all exposure fields on the wafer are systematically affected by mask irregularities. For this reason, each mask must be thoroughly checked and cleaned, i.e. dust must at least be 'blown off' before each use.When cleaning and handling the mask there is always a great danger of damaging it. There are no means at all of preventing the deposition of dust on the layer during operation.
Masks embodying the present invention and described hereinbelow are intended to be insensitive or at least less sensitive to dust and impurities. More specifically, such masks are intended to be capable of imaging the mask pattern without impairing the optical properties of the associated optical system and in a manner to reduce the influence of dust and impurities on the mask.
According to the present invention there is provided a mask for a device for the projection exposure of a semiconductor substrate coated with a photosensitive resist, in which the mask is imaged on the semiconductor substrate by means of a projection lens, the mask comprising a mask layer having a pattern of permeable and impermeable surfaces for exposure radiation, a first glass disposed on the mask layer, and a second glass disposed on the mask layer opposite said first glass, the second glass being directed towards the projection lens and having a thickness exceeding the Rayleigh depth of the projection lens in the region superimposed on the mask layer.
The two glasses protect the mask layer against damage. They also facilitate the prevention of impurities coming into contact with the mask layer as well as facilitating the handling and storing of the mask without involving any risks. It is by no means obvious that the technically simple measure of using the first and second glasses eliminates or at least reduces the adverse effect of small impurities, particularly small particles, being deposited on the surface during operation without substantially disturbing at the same time the optical system comprising the mask, projection lens and substrate.
The elimination or reduction of the adverse effects of small impurities on the mask is based on the fact that, when using the usual projection lens and the second glass having a particular thickness, the glass surface, on which impurities are deposited, will necessarily be outside the area which is sharply imaged onto the substrate by the projection lens. This fact can precisely be defined by the Rayleigh depth. On the one hand, Gaussian optics, which neglect diffraction phenomena, teach that object points outside their geometrical optical focal plane are images as circles of confusion. On the other hand, due to the wave character of light, which is neglected in geometrical optics, light spots are not images as image points by real optical devices but as diffraction discs, since the light wave front is always limited by an entrance pupil, thereby creating diffraction.The so-called Rayleigh criterion for the limit of resolution teaches that said limit has been reached when the centre distance of the two diffraction discs is equal to the radius of the bright centre of the diffraction disc, the so-called Airy disc. In a similar manner, the limited area in the object space whose points are imaged in the image space as circles of confusion, the diameters of said circles of confusion being smaller than the diameters of the respective Airy discs, is called the Rayleigh depth.
In order to get a sharp image it is necessary, in view of the above, for the object to be positioned at a distance deviating from the ideal distance by not more than the Rayleigh depth, preferably by only 2/3 of said depth. In the present case, however, in which the reproduction of impurities should be as unsharp as possible, the thickness of the second glass has to be greater, preferably three times greater, than the Rayleigh depth of the projection lens in the area of the mask layer.
This ensures that particles, which are invisible in flare light, have no adverse effect, and greater particles can easily and entirely be removed from the second glass. It is surprising, therefore, that the positioning of a second glass of sufficient thickness to eliminate the adverse effects of impurities imaged on the wafer has substantially no disadvantageous effect in the beam path of the projection lens, although the usual projection lenses are characterised by a great numerical aperture in the cases of a correspondingly small depth of focus. The range of thickness of the second glass is very wide. On the one hand, impurities on the second glass are unsharply imaged within this range, and on the other hand, the distortion being caused by the second glass is smaller than the distortion being caused by the projection lens itself.This is a great advantage as, when using thicker second glasses, which would actually be possible, the lens would have to be adjusted accordingly, said adjustment exluding the use of conventional masks without second glasses in the same device. As will be described in more detail below, a slight displacement of the mask in the direction of the optical axis is the only change to be made in the optical arrangement, when inserting or operating without the second glass, provided that the thickness of the second glass does not exceed a certain dimension.
Otherwise, as already mentioned, a slight correction has to be made to the lens.
The invention will now be further described, by way of illustrative and non-limiting example, with reference to the accompanying drawings, in which:
Figure 1 is a schematic view of an arrangement in which a mask embodying the present invention is used;
Figure 2 shows a beam path in the area of the mask of Figure 1;
Figure 3 is a side view of another embodiment of the mask;
Figure 4 is a vertical section of an enlarged portion of a further embodiment;
Figure 5 is a vertical section of yet another embodiment;
Figure 6 is a sectional view taken along line VI-VI in Figure 5; and
Figure 7 is a vertical sectional view of a further embodiment.
Figure 1 shows a projection exposure device comprising a mask including a first glass 1 and a mask layer 2, as known in the art, and a projection lens 4. When exposing the mask layer 2, the projection lens 4 forms an image of the mask layer 2 on the surface of a semiconductor substrate 5 coated with a photosensitive resist.
The device also includes means (not shown) for exposing the mask.
An important feature of the present arrangement is that the mask also comprises a planar second glass 3 arranged opposite the first glass 1. As is also known in the art, it is possible to use the first glass 1 as a carrier glass for said mask layer 2. The second glass 3 is directed towards the projection lens 4 and can also be used as a carrier glass for the mask layer 2. The thickness of the second glass 3 exceeds the
Rayleigh depth of the lens 4 in the region overlapping or superimposed on the mask layer 2.
The second glass 3 is a plane-parallel plate and is positioned in the optical path between the mask layer 2 and the lens 4. The plane-parallel plate glass 3 induces aberrations which are of no importance if the plate is sufficiently thin. Such aberrations are reduced by the projection or the image-forming lens 4. In this case, distortion is the main form of aberration.
In an embodiment having an object-to-image distance 00' of approximately 1 m (3.28 feet), a focal length of the lens 4 of 66.5 mm (2.62 inches) and an image scale of 10:1 , the distortion is +0.05 ym (+0.00197 mil) in an image field having a diameter of 14.48 mm (0.57 inches), when the second glass 3 has a thickness of 2.0 mm (0.0787 inches). The distortion, however, due only to the lens is +0.3 ym (+0.0118 mil).
Thicker second glasses can also be used, in principle, in which case the lens 4 must be accordingly adjusted.
When using the second glass 3, the distance 00' must be increased in any case in order to maintain the image scale. This can be seen in
Figure 2. It is assumed that AA is the optical axis.
A beam issues from a detail P' on the mask layer 2 and impinges at an angle E' at R on the refracting boundary surface of the second glass 3, which is of thickness d. (A gap between the mask layer 2 and the second glass 3 is neglected.) The beam leaves the second glass 3 at an angle E and intersects the optical axis AA at F.
It should be pointed out that without using the second glass 3 the detail P' would have to be moved to the point P so that P includes the same angle E with the optical axis as P' when using a second glass in order to maintain the image scale.
The distance X by which the plane of the mask layer 2 must be shifted (i.e. the difference between the object Os with the second glass 3 and the object distance Oc without the second glass) is calculated from the thickness d and the refractive indices n and n' of the surrounding medium and the second glass 3, respectively by means of the law of refraction and trigonometric formulae.
The present invention is not restricted to the use of particular projection exposure device with masks embodying the invention or to the use of masks in which second glass is of a particular thickness. The masks of a set serving for the exposure of the same substrate are preferably provided with second glasses of the same thickness in order to avoid axial adjustment of the positioning of the mask between individual exposure cycles.
The mask illustrated in Figure 3 comprises the first glass 1, which acts as a carrier glass for the mask layer 2, and the second glass 3, which protects the mask layer 2, such members being held together by clamps 9. The first glass 1 is in the present case a square plate of borosilicate glass having a thickness of approximately 6 mm (0.236 inches). Borosilicate glass is used because of its small thermal expansion coefficient. The supporting material only slightly deflects in the case of mechanical loads, because of its relatively great thickness. The entirely planar first glass 1 carries at its lower side, as shown in Figure 3, the pattern-forming mask layer 2, the thickness of the mask layer 2 being exaggerated in the drawing.
The second glass 3 rests directly against the mask layer 2, the second glass 3 preferably being made of the same material as the first glass 1 in order to avoid displacement between the members 1 and 3 in the case of temperature changes. The first glass 1 and the second glass 3 are planar to such an extent that the space between them in which the mask layer is disposed is sealed dust-tight.
Instead of using the clamps 9, the first glass 1 and the second glass 3 can be held together by an adhesive layer 10, as shown in Figure 4. The layer 10 must naturally be transparent and may be of
Canada balsam. Canada balsam is widely used in optics as it has the same refractive index as glass.
in order to prevent the adhesive layer 10 from being damaged by solvents used for cleaning the mask, the joint between the first glass 1 and the second glass 3 is, as shown in Figure 4, covered by a solvent-resistant lacquer coat 6 constituting a sealing strip. The low thermal resistance of
Canada balsam is of great advantage as it allows an easy disassembly of the arrangement by means of heating.
As illustrated in Figures 5 and 6, there can be a small space or gap between the mask layer 2 and the second glass 3. In this case, a marginal strip 7 forms a sealing surface for the second glass 3, the marginal strip being made during the production of the mask layer 2 as a closed frame surrounding the working pattern and alignment targets. The marginal sealing strip 7 can have labyrinthshaped channels 8 permeable to air and water molecules but not to dust particles. By means of the channels 8, balancing of air pressure and humidity with respect to the outside atmosphere can take place. Such pressure compensation is of particular importance when the enclosed volume is relatively large, as differences in pressure can cause a buckling of the second glass 3 and thus create adverse effects on the optics.In contrast to the embodiment of Figure 4, it is of particular importance, when the second glass 3 is spaced from the mask layer 2, to provide an antireflection-coating on both sides of the second glass 3, particularly at the exposure wavelength.
Such a coating, applied for example by means of an evaporated film, prevents or reduces the formation of adverse interference effects, which make free apertures in the mask appear darker.
An optimum result is obtained when not only the second glass 3, but also the first glass 1, is provided with anti-reflection-coatings on both sides.
If dust particles are on the second glass 3 of the mask according to Figures 3 to 6, images thereof are out of focus. In the case of a numerical aperture of 0.35, the Rayleigh depth of the projection lens 4 according to Figure 1 is only 360 Mm (14.17 mil) so that the distance between the impurities and the mask layer 2 is approximately-six times the Rayleigh depth when the second glass has a thickness of 2 mm (0.0787 inches).
Figure 7 shows an embodiment in which the second glass 3 is directed, in use, towards the projection lens 4 and acts as a carrier glass for the mask layer 2. The second glass 3 has a thickness of 2.3 mm (0.09 inches). Due to this thickness, dust particles are not sharply imaged without any correction of the lens 4 being necessary. A further advantage of the use of the second glass 3 for carrying the mask layer 2 is the small mass of the second glass being of importance for the printing or production of the mask pattern. The required stability of the mask is obtained by the first glass 1 having a thickness of, for example, 6.35 mm (0.25 inches).
Claims (13)
1. A mask for a device for the projection exposure of a semiconductor substrate coated with a photosenstivie resist, in which the mask is imaged on the semiconductor substrate by means of a projection lens, the mask comprising a mask layer having a pattern of permeable and impermeable surfaces for exposure radiation, a first glass disposed on the mask layer, and a second glass disposed on the mask layer opposite said first glass, the second glass being directed towards the projection lens and having a thickness exceeding Rayleigh depth of the projection lens in the region superimposed on the mask layer.
2. A mask according to claim 1, wherein the second glass acts as a carrier glass for the mask layer and the first glass acts as a protective glass for the mask layer.
3. A mask according to claim 1 or claim 2, wherein the thickness of the second glass is greater than three times said Rayleigh depth.
4. A mask according to claim 1 or claim 2, wherein the maximum thickness of the second glass is that in which distortion caused by the second glass is equal to distortion caused by the projection lens.
5. A mask according to any one of the preceding claims, wherein the second glass is
coated on both sides with an anti-reflectioncoating for the wavelength of the exposure
radiation.
6. A mask according to any one of claims 1 to
5, wherein the first glass and the second glass are in direct contact with the mask layer, the mask comprising clamp means securing together the mask layer, the first glass and the second glass.
7. A mask according to any one of claims 1 to 6, wherein one of the first and second glasses is glued to the mask layer by Canada balsam.
8. A mask according to claim 7, wherein a joint is formed between said first and said second glasses and a solvent-resistant lacquer coat covers the joint.
9. A mask according to any one of claims 1 to 6, wherein a dust-tight marginal strip surrounding said mask layer is provided between the first glass and the second glass.
10. A mask according to claim 9, wherein the marginal strip is made of the same material as the mask layer.
1 A mask according to claim 9 or claim 10, wherein the marginal strip has a labyrinth-shaped profile.
12. A mask according to any one of the preceding claims, wherein the first and the second glass are of the same material.
13. A mask for a device for the projection exposure of a semiconductor substrate, the mask being substantially as herein described with reference to Figures 1 and 2, Figure 3, Figure 4,
Figures 5 and 6 or Figure 7 of the accompanying drawings.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| DE19782845147 DE2845147B1 (en) | 1978-10-17 | 1978-10-17 | Projection mask |
| US239479A | 1979-01-10 | 1979-01-10 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| GB2036367A true GB2036367A (en) | 1980-06-25 |
Family
ID=25776129
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| GB7935831A Withdrawn GB2036367A (en) | 1978-10-17 | 1979-10-16 | Masks for the Projection Exposure of Semiconductor Substrates |
Country Status (3)
| Country | Link |
|---|---|
| FR (1) | FR2439418A1 (en) |
| GB (1) | GB2036367A (en) |
| NL (1) | NL7907643A (en) |
Cited By (7)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB2128365A (en) * | 1982-09-27 | 1984-04-26 | Western Electric Co | Photomask |
| US4499162A (en) * | 1983-06-24 | 1985-02-12 | At&T Technologies, Inc. | Photomask and method of using same |
| US4529299A (en) * | 1984-05-03 | 1985-07-16 | At&T Technologies, Inc. | Interposer element for photomasks in projection printer |
| US4537498A (en) * | 1984-05-03 | 1985-08-27 | At&T Technologies, Inc. | Focal plane adjusted photomask and methods of projecting images onto photosensitized workpiece surfaces |
| EP0851294A4 (en) * | 1996-07-17 | 2000-04-05 | Mitsui Chemicals Inc | Mask protecting device |
| US6103427A (en) * | 1991-05-17 | 2000-08-15 | Dupont Photomasks, Inc. | Pressure relieving pellicle |
| EP1310827A3 (en) * | 2001-11-08 | 2004-07-28 | Dai Nippon Printing Co., Ltd. | Photomask with dust-proofing device and exposure method using the same |
-
1979
- 1979-10-16 FR FR7925689A patent/FR2439418A1/en not_active Withdrawn
- 1979-10-16 GB GB7935831A patent/GB2036367A/en not_active Withdrawn
- 1979-10-16 NL NL7907643A patent/NL7907643A/en not_active Application Discontinuation
Cited By (8)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB2128365A (en) * | 1982-09-27 | 1984-04-26 | Western Electric Co | Photomask |
| US4499162A (en) * | 1983-06-24 | 1985-02-12 | At&T Technologies, Inc. | Photomask and method of using same |
| US4529299A (en) * | 1984-05-03 | 1985-07-16 | At&T Technologies, Inc. | Interposer element for photomasks in projection printer |
| US4537498A (en) * | 1984-05-03 | 1985-08-27 | At&T Technologies, Inc. | Focal plane adjusted photomask and methods of projecting images onto photosensitized workpiece surfaces |
| US6103427A (en) * | 1991-05-17 | 2000-08-15 | Dupont Photomasks, Inc. | Pressure relieving pellicle |
| EP0851294A4 (en) * | 1996-07-17 | 2000-04-05 | Mitsui Chemicals Inc | Mask protecting device |
| EP1310827A3 (en) * | 2001-11-08 | 2004-07-28 | Dai Nippon Printing Co., Ltd. | Photomask with dust-proofing device and exposure method using the same |
| US6933082B2 (en) | 2001-11-08 | 2005-08-23 | Dai Nippon Printing Co., Ltd. | Photomask with dust-proofing device and exposure method using the same |
Also Published As
| Publication number | Publication date |
|---|---|
| NL7907643A (en) | 1980-04-21 |
| FR2439418A1 (en) | 1980-05-16 |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| WAP | Application withdrawn, taken to be withdrawn or refused ** after publication under section 16(1) |