Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
1999-12-14
2001-10-30
Rosasco, S. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
C428S014000
Reexamination Certificate
active
06309781
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a photomask comprising a base plate of a radiation-transmitting and electrically insulating material, a first side of which is provided with a layer of an electroconductive mask material wherein, inside a closed edge of that material, a mask pattern to be imaged is formed, the photomask being entirely enveloped in a protective layer of a radiation-transmitting, electroconductive material.
Such a photomask can particularly suitably be used in the manufacture of semiconductor devices and flat panel displays. In practice, the base plate generally is a plate of quartz glass, the layer of mask material generally contains chromium. The pattern of the mask is imaged, often on a reduced scale, on a layer of a photoresist which is provided, for example, on a layer of a metal. After exposure and development of the photoresist, the layer of metal can be etched in a pattern of conductor tracks corresponding to the mask pattern. The photomask pattern to be imaged comprises many closely spaced tracks of a mask material which are sometimes connected to larger areas of a mask material. The tracks correspond to the conductor tracks to be formed in the layer of metal, and the areas correspond to, for example, bond pads to be formed in the metal layer, which bond pads are used for contacting the devices to be manufactured.
The photomask is entirely enveloped in a protective layer of radiation-transmitting, electroconductive material. As a result, the mask pattern is incorporated in a Faraday cage which protects it against damage that might be caused by electrostatic discharges (
E
lectro-
S
tatic-
D
ischarge) in the mask pattern. When use is made of a photomask which does not have such a protective layer, in practice, the mask material present on the electrically insulating base plate may become electrically charged. This may be caused, for example, by air flowing past or by friction brought about by contact with clothing and other insulating materials. In order to be able to image the photomask onto the layer of photoresist, said photomask is placed in a projection apparatus on a grounded mask holder. In this manner, the closed edge of mask material present around the mask pattern is grounded. Voltage differences between this edge and the mask pattern may then develop, the magnitude of which is such that electric discharges occur in the mask pattern, which lead to said damage.
In JP-A-57-60335 a description is given of a photomask of the type mentioned in the opening paragraph, wherein the protective layer of radiation-transmitting, electroconductive material, in which the mask is enveloped, is a layer of indium oxide. The protective layer is deposited on the mask pattern on the first side of the mask and is situated on and between tracks of mask material.
The protective layer, which is deposited from the vapor phase, exhibits a homogeneous thickness. On the edges of the tracks of mask material extending transversely to the base plate, the thickness of the protective layer is substantially the same as on and between the mask tracks. Viewed transversely to the base plate, however, the thickness of the protective layer right next to the mask tracks is larger than centrally between the mask tracks. Consequently, right next to the mask tracks, radiation going through the mask during the projection of the mask pattern onto the layer of photoresist, passes through a much thicker layer than centrally between the mask tracks. Since such a layer has a refractive index which differs from that of air, diffraction phenomena will occur when the photomask is imaged. This may cause errors upon imaging the mask tracks onto the layer of photoresist.
SUMMARY OF THE INVENTION
It is an object of the invention to provide, inter alia, a mask having an ESD-precluding envelope which does not adversely affect imaging of the photomask onto a layer of a photoresist.
To achieve this, the photomask in accordance with the invention is characterized in that the protective layer is provided in a substantially homogeneous thickness on the first side of the base plate, at such a distance from said base plate that the protective layer remains free of the mask pattern to be imaged.
In this case, viewed transversely to the base plate, the protective layer has the same, practically homogeneous thickness right next to the mask tracks and centrally between the mask tracks. In this manner, the occurrence of diffraction phenomena, which may occur when use is made of the known photomask, is precluded. When the photomask is projected onto the layer of photoresist, the plane wherein the mask pattern is situated is sharply imaged, while the protective layer situated outside this plane is not. During imaging, the mask pattern is in focus, the protective layer is not. For this reason, small differences in thickness of the protective layer, which may still be present, do not influence the imaging operation.
In a first embodiment of the photomask, the protective layer is fixed, on the first side of the base plate, onto a ring which is provided on the edge of mask material and which encloses the pattern to be imaged. It is thus readily achieved that the protective layer is arranged at a distance from the base plate such that this protective layer remains free of the mask pattern to be imaged.
In a second embodiment of the photomask, the protective layer fixed on the ring is supported by a radiation-transmitting foil fixed on the ring. In this case, the protective layer does not have to be a continuous layer. Small openings, so-called pinholes, in the protective layer do not render the protection against electrostatic discharges less effective.
In practice it has been found that by using a protective layer of polyacetylene, damage to the mask pattern by electrostatic discharges is effectively precluded.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter.
REFERENCES:
patent: 5577610 (1996-11-01), Okuda et al.
patent: 5691088 (1997-11-01), Kubota et al.
patent: 5760335A (1982-04-01), None
patent: 05188583 (1993-07-01), None
Gemmink Jan W.
Van Hasselt Kees
Biren Steven R.
Rosasco S.
U.S. Philips Corporation
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