Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
2001-01-29
2003-04-08
Rosasco, S. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
Reexamination Certificate
active
06544694
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a method of manufacturing a device in at least one layer on a substrate, comprising the steps of:
imaging, by means of projection radiation having a wavelength &lgr; and a projection system having a numerical aperture NA, a specific phaseshifting mask pattern, comprising pattern features corresponding to device features to be configured in said layer, on a radiation-sensitive layer provided on said layer, and
removing material from, or adding material to, areas of said layer which are delineated by the mask pattern image, the smallest device features having a width which is smaller than &lgr;/NA.
The invention also relates to a lithographic phaseshifting mask for use in this method.
The method is used, inter alia, in the manufacture of integrated electronic circuits, or IC, devices. An IC mask pattern, present in a mask, is imaged each time on a different IC area of a substrate. This substrate, which is coated with a radiation-sensitive layer, provides space for a large number of IC areas. The lithographic method may also be used in the manufacture of other devices like, for example, integrated or planar optical systems, charge-coupled detectors (CCDs), magnetic heads or liquid crystalline display panels.
Since it is desirable to accommodate an increasing number of electronic components in an IC device, increasingly smaller features, or line widths, of IC patterns must be imaged. Thus, increasingly stricter requirements are imposed on the lithographic projection apparatus used to carry out the lithographic method. The special requirements relate especially to the imaging quality and the resolving power of the projection system, which is usually a lens system in the current lithographic apparatus. The resolving power, or resolution, which is a measure of the smallest feature which can still be imaged satisfactorily, is proportional to &lgr;/NA, in which &lgr; is the wavelength of the imaging, or projection, beam and NA is the numerical aperture of the projection system. To increase the resolution, the numerical aperture may, in principle, be increased and/or the wavelength may be reduced. In practice, an increase of the numerical aperture, which is currently already fairly large, is not very well possible because this reduces the depth of focus of the projection lens system, which is proportional to &lgr;/NA
2
and, moreover, it becomes too difficult to correct for the required image field.
The requirements to be imposed on the projection lens system may be alleviated, or the resolution may be increased, while maintaining these requirements, if a step-and-scanning lithographic apparatus is used instead of a stepping lithographic apparatus. In a stepping apparatus, a full-field illumination is used, i.e. the entire mask pattern is illuminated in one operation and imaged as a whole on an IC area of the substrate. After a first IC area has been illuminated, a step is made to a subsequent IC area, i.e. the substrate holder is moved in such a way that the next IC area is positioned under the mask pattern, whereafter this area is illuminated, and so forth until all IC areas of the substrate are provided with an image of the mask pattern. In a step-and-scanning apparatus, only a rectangular or circular segment-shaped area of the mask pattern and hence also a corresponding sub-area of the substrate IC area is each time illuminated, and the mask pattern and the substrate are synchronously moved through the illumination beam, while taking the magnification of the projection lens system into account. A subsequent sub-area of the mask pattern is then each time imaged on a corresponding sub-area of the relevant IC area of the substrate. After the entire mask pattern has been imaged on an IC substrate area in this way, the substrate holder performs a stepping movement, i.e. the beginning of the next IC area is moved into the projection beam and the mask is set, for example, in its initial position whereafter said next IC area is scan-illuminated via the mask pattern.
If even smaller features are to be imaged satisfactorily with a stepping or a step-and-scanning lithographic apparatus, use can be made of a phase-shifting mask. The technique for improving resolution in photolithography by the use of phase-shifting masks was first proposed by Levenson et al in: “Improving Resolution in Photolithography with a Phase-Shifting Mask”, IEEE Transactions on Electron Devices, Vol. ED-29, No.12, December 1982, pp 25-32. The Levenson phase-shifting mask is a conventional transmission mask which is provided with phase-shifting elements. This transmission mask comprises a transparent, for example, quartz substrate covered by an opaque, for example, chrome layer with apertures to define the desired intensity pattern, i.e. the IC pattern to be printed in a layer of the IC device. When illuminating such a conventional mask with electromagnetic radiation, the electric field of this radiation has the same phase at every aperture. However, due to diffraction at the apertures and the limited resolution of the projection lens system, the electric field patterns at the substrate level are spread. A single small mask aperture thus provides a wider intensity distribution at substrate level. Constructive interference between waves diffracted by adjacent apertures enhances the electric field between the projections of the apertures at substrate level. As the intensity pattern is proportional to the square of the electric field, this pattern of two adjacent mask apertures is spread evenly to a fairly high degree and does not show two pronounced peaks at the positions of the apertures.
In a phase-shifting mask, one of the two adjacent apertures is covered with a transparent phase-shifting layer. This layer has a thickness d=&lgr;/2(n−1), where n is the index of refraction and &lgr; is the wavelength of the radiation, such that the waves transmitted through the adjacent apertures are 180° out of phase with one another. Destructive interference now occurs between the waves diffracted by the adjacent apertures, and the electric field, and thus the intensity, between the projections of the apertures at wafer level is minimised. Any projection lens system will project the images of such a phase-shifting mask with a better resolution and a higher contrast than a corresponding mask without phase shifters.
A similar improvement can be obtained by a “chrome-less” phase shifting mask, as disclosed in EP-A 0.680.624. This mask does not comprise a pattern structure of chrome, or other opaque material, which defines the IC pattern, but this pattern is now defined by a pattern of phase transitions, for example, in the form of recesses in the quartz substrate. Such a phase transition, or pattern feature, is imaged by the projection lens system in the radiation-sensitive, or resist, layer on the wafer as a narrow line, due to the point-spread function of this lens system. The line width is typically below 100 nm and can be influenced by the numerical aperture of the projection lens system, the coherence value and the exposure dose of the lithographic apparatus. The coherence value, or &sgr; value, is the ratio of the cross-section of the projection beam in the plane of the pupil of the projection lens system and the aperture of this lens system. The &sgr; value thus indicates the degree in which the projection lens pupil is filled by the projection beam. This value is usually smaller than one. The exposure dose is the amount of projection, or exposure, radiation incident on a resist layer area during imaging of a mask feature on this area. Once the numerical aperture, the coherence value and the exposure dose are set for a lithographic apparatus, all pattern features projected in the resist layer, and later configured in the IC device layer, have the same width.
If, as is usually the case, IC features with different widths should be formed in a device layer, it is not one mask but a number of masks, corresponding to the number of different widths, that should be project
Dirksen Peter
Juffermans Casparus Anthonius Henricus
Koninklijke Philips Electronics , N.V.
Rosasco S.
Waxler Aaron
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