Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
1999-12-14
2001-09-18
Baxter, Janet (Department: 1752)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
C361S212000, C361S220000
Reexamination Certificate
active
06291114
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 a mask pattern to be imaged is formed which is enclosed by an inner edge and an outer edge of a mask material, which edges are electrically separated from each other by a ring-shaped protection area.
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 has an edge of a conductive mask material, which is divided into an inner edge and an outer edge which are electrically separated from each other by a ring-shaped protection area. This ring-shaped area serves to protect the photomask against damage by electrostatic discharges (
E
lectro-
S
tatic
D
ischarge) which may occur in the mask pattern.
In operation, a photomask having a mask pattern which is enclosed by a closed edge of a conductive mask material, damage due to electrostatic discharges may occur. 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 caused by contact with clothing and other insulating materials. In order to be able to image the photomask onto the layer of photoresist, the photomask is placed in a projector on a grounded mask holder. In this manner, the closed edge of masking material present around the mask pattern is grounded. Between this edge and the mask pattern voltage differences may develop whose magnitude is such that electric discharges occur in the mask pattern leading to said damage.
In JP-A-5-100410, a description is given of a photomask of the type mentioned in the opening paragraph, in which photomask the mask material is entirely removed from the ring-shaped protection area. This empty ring-shaped area has a width ranging from 0.5 to 2 mm.
If the conductive mask material is electrically charged as described hereinabove, then only the outer edge is grounded if the photomask is placed in a projector. If the empty protection area sufficiently insulates the outer edge and the inner edge from each other, then no voltage difference between the inner edge and the mask pattern will develop. Electrostatic discharges which may damage the mask pattern seem to be precluded in this way. However, it has been found in practice that charge present on the inner edge and in the mask pattern can cause damage in spite of the presence of the empty ring-shaped protection area.
SUMMARY OF THE INVENTION
It is an object of the invention to provide, inter alia, a photomask with sufficient ESD protection. To achieve this, the photomask is characterized in accordance with the invention in that the ring-shaped protection area comprises a protection pattern formed in the layer of mask material, which protection pattern has tracks having an end situated near the inner edge or outer edge and at a distance from said inner edge or outer edge which is smaller than the smallest distance between pattern parts present in the mask pattern.
If the conductive mask material is electrically charged, and if the outer edge is subsequently grounded, then a voltage develops between the outer edge and the structures enclosed by said outer edge, i.e., viewed from said edge to the center, the protection pattern, the inner edge and the mask pattern. The protection pattern is more vulnerable than the mask pattern because the protection area comprises conductive tracks of the protection pattern which are situated very close to the inner edge and the outer edge. The distances are smaller than the distances between parts of the mask pattern; they are, for example, 0.8 &mgr;m, if the distance between the parts in the mask pattern is 1 &mgr;m. Therefore, if the voltage is high enough, an electrostatic discharge takes place in the protection pattern. Since the charge present in the photomask is at least substantially removed, damage to the mask pattern by electrostatic discharges is precluded. The protection pattern may become locally damaged, however, it has been found that in this case mask material is sputtered off, so that no short-circuit takes place between the outer edge and the inner edge. Undamaged parts of the protection pattern which are situated next to the damaged part thereof take over the protection of the mask pattern when a subsequent electrostatic discharge takes place. Only after many discharges involving damage to the protection pattern, said protection pattern would stop functioning. In practice, however, this state is not reached.
A more secure protection is obtained if the tracks have a width at the location of an end situated near the inner edge or the outer edge, which is smaller than the width of the narrowest mask tracks present in the mask pattern, the first width being, for example, 0.8 &mgr;m if the second width is 1 &mgr;m. The electric fields between the edges and the proximate ends of the tracks of the protection pattern will then, at an equal electric voltage, be greater than the electric fields between tracks in the mask pattern.
Preferably, the tracks are connected with an end situated near the inner edge or the outer edge to areas of mask material incorporated in the protection pattern. If a photomask comprising a protection pattern with areas, and a photomask comprising an identical protection pattern yet without areas are charged in a similar manner, for example in a flow of ionized air, the outer edge being grounded, then electrostatic discharge occurs sooner in the former protection pattern than in the latter protection pattern.
The mask pattern may also include relatively large areas, such as the above-mentioned bond pads. It has been found that, in an unprotected photomask, damage caused by electrostatic discharges occurs relatively easily near these relatively large areas. To make sure that the protection pattern also provides sufficient protection against said damage, the areas which are connected to the tracks having an end situated near the inner edge or the outer edge, have a surface area which is larger than that of the largest mask areas present in the mask pattern. This and the relatively small width of the tracks connected to the protection pattern are responsible for the fact that the protection pattern is more vulnerable to electrostatic discharges than the mask pattern.
Preferably, the areas which are connected to the tracks having an end situated near the inner edge or the outer edge, are connected to further tracks having an end situated near other areas incorporated in the protection pattern and at a distance therefrom which is smaller than the smallest distance between pattern parts present in the mask pattern. As a result, the discharge may take place in the protection pattern without causing serious damage to said pattern. For the reasons stated hereinabove, the width of the further tracks preferably is smaller than that of the narrowest mask tracks present in the mask pattern, and the surface area of the further areas is larger than that of the largest mask areas present in the mask pattern.
A simple pattern is obtained if in the protection pattern a
.S. Philips Corporation
Baxter Janet
Biren Steven R.
Gilmore Barbara
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