Radiant energy – Irradiation of objects or material – Irradiation of semiconductor devices
Reexamination Certificate
2001-06-21
2003-12-02
Anderson, Bruce (Department: 2881)
Radiant energy
Irradiation of objects or material
Irradiation of semiconductor devices
C378S035000
Reexamination Certificate
active
06657208
ABSTRACT:
The invention relates to a method of forming a desired optical image having a given minimum feature size on a selected surface of material sensitive to optical radiation, the method comprising the steps of:
providing a source of optical radiation having at least one predetermined wavelength &lgr;;
providing a layer of material, sensitive to optical radiation having the wavelength &lgr;, with a selected surface to receive an optical image thereon;
positioning a mask, comprising image information and being at least partly transparent to the source radiation, between the radiation source and the layer of sensitive material, so that source radiation is transmitted through the mask towards the sensitive layer, and
illuminating the selected surface with source radiation received by transmission through the mask to produce the desired optical image.
The invention also relates to a mask for use with this method, a method of manufacturing a device using this method and an apparatus for carrying out this method.
This method and apparatus, referred to as proximity printing method and apparatus, are used, inter alia, in the manufacture of liquid crystalline display (LCD) panels, customer ICs (integrated circuits) and PCBs (printed circuit board). Proximity printing is a fast and cheap method of forming an image, comprising features corresponding to the device features to be configured in a layer of a substrate, in a radiation-sensitive layer on the substrate. Use is made of a large photo mask that is arranged at a short distance, called the proximity gap, from the substrate and the substrate is illuminated via the photo mask by, for example, ultraviolet (UV) radiation. An important advantage of the method is the large image field, so that large device patterns can be imaged in one image step. The pattern of a conventional photo mask for proximity printing is a true, one-to-one copy, of the image required on the substrate, i.e. each picture element (pixel) of this image is identical to the corresponding pixel in the mask pattern.
Proximity printing has a limited resolution, i.e. the ability to reproduce the points, lines etc., in general the features, in the mask pattern as separate entities in the sensitive layer on the substrate. This is due to the diffractive effects which occur when the dimensions of the features decrease with respect to the wavelength of the radiation used for imaging. For example, for a proximity gap of 100 &mgr;m, the resolution is 10 &mgr;m, which means that pattern features at a mutual distance of 10 &mgr;m can be imaged as separate elements. To increase the resolution in optical lithography, real projection apparatus, i.e. apparatus having a real projection system like a lens projection system or a mirror projection system, are used. Examples of such apparatus are wafer steppers or wafer step-and scanners. In a wafer stepper, a mask pattern, for example an IC pattern is imaged in one step by a projection lens system on a first IC area of the substrate. Then the mask and substrate are moved relative to each other until a second IC area is positioned below the projection lens. The mask pattern is then imaged on the second IC area. These steps are repeated until all IC areas of the substrate are provided with an image of the mask pattern. This is a time consuming process, due to the sub-steps of moving, aligning and illumination. In a step-and-scanner, only a small portion of the mask pattern is illuminated and, during illumination, the mask and the substrate are synchronously moved with respect to the illumination beam until the whole mask pattern has been illuminated and a complete image of this pattern has been formed on an IC area of the substrate. Then the mask and substrate are moved relative to each other until the next IC area is positioned under the projection lens and the mask pattern is scan-illuminated, so that a complete image of the mask pattern is formed on the next IC area. These steps are repeated until all IC areas of the substrate are provided with a complete image of the mask pattern. The step-and-scanning process is even more time consuming than the stepping process.
If a 1:1 stepper, i.e. a stepper with a magnification of one, is used to print an LCD pattern, a resolution of 3 &mgr;m can be obtained, however at the expense of much time for imaging. Moreover, as the image field is divided into sub-fields, stitching problems may occur, which means that neighbouring sub-fields do not fit exactly together.
It is an object of the present invention to provide a proximity printing method and a mask for use therewith which allow increase of the resolution of the order of that obtainable with an optical lithographic projection apparatus for the same purpose, and, moreover, opens the way to new possibilities which cannot be realized by either conventional proximity printing methods or steppers or step-and-scanners. The method is characterized in that use is made of a mask in the form of a diffractive element wherein the image information is encoded in a two-dimensional array of image cells having dimensions which are smaller than the minimum feature size, each image cell having one out of at least two specific transmission levels and one out of at least three phase levels.
The amplitude level and the phase level of an image cell are measures of the degree to which the amplitude and the phase, respectively, of a beam portion incident on this image element are changed by the image cell.
The method of the invention makes effective use of the diffraction effect, which is the resolution limiting factor in the conventional method. The mask pattern is no longer identical to the required image on a pixel by pixel basis, but is encoded in small image cells. Such an image cell is not identical to a given pixel in the required image, but changes, in co-operation with a number of neighbouring image cells, both the amplitude and phase of the portion of the illumination beam passing through the area of these image cells, such that a portion of the required image is formed in the sensitive layer on the substrate. The method uses two independent coding parameters: the amplitude and the phase of an image cell, which enlarges the mask design possibilities. Each of these parameters has one out of a number of discrete values, or levels. The number of amplitude levels is two or more and the number of phase levels is three or more. The mask design is the result of a computing technique similar to the technique of computer generated holograms. The resulting amplitude level for each image element may be implemented in the mask by a coat layer, having a specific transmission, on the image information carrying surface of the transparent mask substrate and the phase level by adapting the thickness of this substrate.
It is noted that the article: “X-ray holography for VLSI using synthetic bilevel holograms” in SPIE, Vol. 3183, 1997, pages 2-13 describes a specific X-ray proximity printing method which uses synchroton radiation of very short wavelength, of the order of 1 nm, synchrotron radiation for the manufacture of VLSI electronic circuits having smallest feature sizes of the order of 100 nm. To increase the gap width from the unpractical value of 4 &mgr;m to the more practical value of 10 &mgr;m or larger, the conventional X-ray mask, having a pattern configuration identical to the configuration of the image to be formed, is replaced by a so-called bilevel computed hologram. The known method differs from that of the present invention not only in that the ratios of the smallest feature size and the wavelength used and the gap width, respectively are totally different, but also in that the mask elements are several hundred X-ray wavelengths thick, so that waveguide effects play a role. Moreover, the hologram used in the X-ray method has only two phase levels, which introduce a phase shift of zero and &pgr; radians, respectively.
Also U.S. Pat. No. 5,612,986 describes a method for imaging an IC mask pattern on a substrate via a mask and by means of X-ray radiation. The convention
Anderson Bruce
Waxler Aaron
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