Photocopying – Projection printing and copying cameras – Illumination systems or details
Reexamination Certificate
1998-03-06
2001-10-02
Adams, Russell (Department: 2851)
Photocopying
Projection printing and copying cameras
Illumination systems or details
C355S053000, C356S401000
Reexamination Certificate
active
06297876
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a lithographic projection apparatus comprising a radiation source supplying a projection beam, a mask holder, a substrate holder and a projection system arranged between the mask holder and the substrate holder, the apparatus further comprising an alignment system for ultimately aligning a substrate, provided in the substrate holder, with respect to a mask provided in the mask holder, the alignment system comprising an off-axis alignment unit for aligning an alignment mark, provided on a substrate, with respect to a reference.
Ultimate alignment is understood to mean that, although in the first instance the alignment unit is used for aligning a substrate with respect to a reference, the result of this alignment step is used in combination with other measurements for aligning the substrate with respect to the mask.
The lithographic projection apparatus is an essential component in the manufacture of integrated circuits, or ICs, by means of diffusion and masking techniques. With the aid of this apparatus, a number of masks having different mask patterns are successively imaged at the same position on a semiconductor substrate. The substrate must undergo the desired physical and chemical changes between the successive images at the same position. To this end, the substrate must be removed from the apparatus after it has been exposed with a mask pattern, and, after it has undergone the desired process steps, the substrate must be replaced at the same position again so as to expose it with a second mask pattern, and so forth, while it must be ensured that the images of the second mask pattern and the subsequent mask patterns are positioned accurately with respect to the substrate. To this end, the lithographic projection apparatus is provided with an optical alignment system with which alignment marks on the substrate are aligned with respect to alignment marks on the mask.
A lithographic apparatus may not only be used for the manufacture of ICs but also for the manufacture of other structures having detailed dimensions of the order of 1 micrometer. Examples are structures of integrated, or plenary, optical systems or guiding and detection patterns of magnetic domain memories, and structures of liquid crystal display panels. Also in the manufacture of these structures, images of mask patterns must be aligned very accurately with respect to a substrate.
The lithographic projection apparatus may be a stepping apparatus or a step-and-scan apparatus. In a stepping apparatus, the mask pattern is imaged in one run on an IC area of the substrate. Subsequently, the substrate is moved with respect to the mask in such a way that a subsequent IC area will be situated under the mask pattern and the projection lens system and the mask pattern is imaged on the subsequent IC area. This process is repeated until all IC areas of the substrate are provided with a mask pattern image In a step-and-scan apparatus, the above-mentioned stepping procedure is also followed, but the mask pattern is not imaged in one run but via scanning movement. During imaging of the mask pattern, the substrate is moved synchronously with the mask with respect to the projection system and the projection beam, taking the magnification of the projection system into account. A series of juxtaposed partial images of consecutively exposed parts of the mask pattern is imaged in an IC area. After the mask pattern has been completely imaged in an IC area, a step is made to a subsequent IC area. A possible scanning procedure is described in the article: “Sub-micron 1:1 Optical Lithography” by D. A. Markle in the magazine “Semiconductors International” of May 1986, pp. 137-142.
U.S. Pat. No. 5,243,195 discloses an optical lithographic projection apparatus provided with an alignment system and intended for the manufacture of ICs. This alignment system comprises an off-axis alignment unit for aligning a substrate alignment mark with respect to this alignment unit. In addition, this alignment system comprises a second alignment unit for aligning a substrate mark with respect to a mask mark via the projection lens (TTL). Alignment via the projection lens (on-axis alignment) is the most frequently used method in the current generation of optical lithographic projection apparatuses and provides the advantage that the substrate and the mask can be aligned directly, and thus very accurately, with respect to each other. When the off-axis alignment method is used, the baseline offset as described in U.S. Pat. No. 5,243,195 must be taken into account.
The on-axis alignment method has hitherto worked to full satisfaction, but it is to be expected that this alignment method may present problems with regard to reliability and accuracy when novel technologies are used in the IC manufacture and when the detailed sizes, or line widths, of the IC patterns decrease.
In connection with the increasing number of electronic components per unit of surface area of the substrate and the resultant smaller dimensions of these components, increasingly stricter requirements are imposed on the accuracy with which integrated circuits are made. The positions where the successive masks are imaged on the substrate must therefore be fixed more and more accurately. In the manufacture of new-generation ICs with smaller line widths, the alignment accuracy will have to be improved or, in other words, it must be possible to detect smaller deviations so that the resolving power of the alignment system must be increased. On the other hand, stricter requirements must also be imposed on the planeness of the substrate due to the required higher numerical aperture (NA) of the projection lens system in the case of decreasing line widths. The depth of focus of this system decreases as the NA increases. Since some image field curvature occurs at the desired relatively large image field of the projection lens system, there is hardly any room left for unevennesses of the substrate. To obtain the desired planeness of the substrate, it has been proposed to polish this substrate by means of the chemical mechanical polishing (CMP) process between two consecutive exposures with different mask patterns in the projection apparatus. However, this polishing process affects the accuracy of the on-axis alignment method. In this method, a grating is used as a substrate alignment mark and the sub-beams diffracted in the first order by this grating are used for imaging the substrate mark on the mask mark. In this process, it is assumed that the substrate is aligned correctly with respect to the mask when the point of gravity of the substrate grating mark is aligned with respect to the point of gravity of the mask alignment mark. In that case it has been assumed that the point of gravity for each grating mark coincides with the geometric center of the grating. However, the CMP process renders the substrate grating mark asymmetrical so that this alignment method is no longer reliable.
Furthermore, the manufacturing process for new-generation ICs is becoming more and more complicated: the number of process steps and the number of process layers on the substrate increase more and more. Some of these layers also introduce asymmetries in the substrate grating mark and hence alignment errors.
Moreover, when the known on-axis alignment method is used, strict requirements must be imposed on the depth of the grating grooves of the substrate mark which is a phase grating.
It is an object of the present invention to provide an alignment system for a lithographic projection apparatus in which the influence of said effects on the alignment signal is reduced considerably and which is more accurate and reliable than known alignment systems. To this end, the system according to the invention is characterized in that the alignment mark is a diffractive mark and in that the alignment unit is adapted to separately detect a number of at least three sub-beams diffracted by the diffractive mark in different diffraction orders which are higher than 0, each sub-beam co
Adams Russell
ASM Lithography B.V.
Kim Peter B.
Pillsbury & Winthrop LLP
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