The invention relates to an alignment device for aligning a first object, which is provided with at least a first alignment mark, with respect to a second object which is provided with at least a second alignment mark, said device comprising a radiation source for supplying at least an alignment beam, a first object holder, a second object holder, a projection system for imaging a first alignment mark and a second alignment mark onto each other, and a radiation-sensitive detection system arranged in the path of selected alignment beam portions by which these alignment marks are imaged onto each other alignment marks, the output signal of said radiation-sensitive detection system being a measure of the mutual position of these alignment marks.
The invention also relates to a lithographic apparatus in which such an alignment device is used as a precision alignment device or as a pre-alignment device. The mask used in such an apparatus is provided with at least a mask alignment mark and the substrate is provided with at least a substrate alignment mark.
The selected alignment beam portions are those portions of the alignment beam which are effectively used to image the first alignment mark on the second alignment mark. If the alignment marks are diffraction gratings, the selected alignment beam portions are the beam portions diffracted in given orders by the alignment marks.
An optical lithographic apparatus for repetitive and reduced projection of a mask pattern, for example, the pattern of an integrated circuit (IC) on one and the same substrate, is described in U.S. Pat. No. 4,778,275, in which the mask pattern and the substrate are moved with respect to each other between two successive illuminations, for example, along two mutually perpendicular directions in a plane parallel to the substrate plane and the mask plane.
Integrated circuits are manufactured by means of diffusion and masking techniques. A number of masks with different mask patterns are consecutively imaged on one and the same location on a semiconductor substrate. Between the consecutive imaging steps on the same locations, the substrate must undergo the desired physical and chemical changes. To this end, the substrate must be removed from the apparatus after it has been illuminated with a mask pattern, and, after it has undergone the desired process steps, it must be placed in the apparatus again in the same position so as to illuminate it with a second mask pattern, and so forth, while it must be ensured that the projections of the second mask pattern and of the subsequent mask patterns are positioned accurately with respect to the substrate.
Diffusion and masking techniques can also be used in the manufacture of other structures having detail dimensions of the order of micrometers, for example, structures of integrated 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, the mask patterns must be aligned very accurately with respect to a substrate.
In view of the large number of electronic components per unit of surface area of the substrate and the resultant small dimensions of these components, increasingly stricter requirements are imposed on the accuracy with which integrated circuits are manufactured. The position where the consecutive masks are projected on the substrate must therefore be established more and more accurately.
In order to be able to realize the desired, very precise positioning accuracy, within several tenths of one micrometer in the apparatus according to U.S. Pat. No. 4,778,275, of the projection of the mask pattern with respect to the substrate, this apparatus comprises a device for aligning the substrate with respect to the mask pattern with which an alignment mark provided in the substrate is imaged on an alignment mark provided in the mask. If the image of the substrate alignment mark accurately coincides with the mask alignment mark, the substrate is correctly aligned with respect to the mask pattern. The main element for imaging the substrate mark on the mask mark is constituted by the projection lens system, or imaging system with which the mask pattern is imaged on the substrate.
The alignment device described in U.S. Pat. No. 4,778,275 has hitherto worked to full satisfaction, but it is to be expected that with the use of novel technologies in IC manufacture and with decreasing detail sizes, or line widths, of the IC patterns, the alignment device may present problems relating to its reliability and accuracy.
In the manufacture of new-generation ICs with smaller line widths, stricter requirements must be imposed on the planeness of the substrate due to the required higher numerical aperture (NA) of the projection lens system upon a decreasing line width. The depth of focus of this system decreases as the NA increases. Since there will be some curvature of the image field at the desired relatively large image field of the projection lens system, there is substantially no room for unevennesses of the substrate. To obtain the desired evenness of the substrate, it may be polished, in between two illuminations, by means of the chemical mechanical polishing (CMP) process. This process is found to cause an asymmetrical distortion in a substrate alignment mark implemented as a grating, so that alignment errors may occur.
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 introduce asymmetries in a grating-shaped alignment mark and thus cause alignment errors.
Further alignment errors may be caused by the mask, or reticle. Due to false reflections by the mask and by optical elements of the alignment device, unwanted phase differences in the alignment radiation are produced. If this radiation is coherent, such as the conventional HeNe laser radiation, not only the desired image of the substrate alignment mark is produced at the location of the mask alignment mark, but also an extra, or "ghost" image is produced, while, with a varying thickness of the substrate of the mask alignment mark, the position of this ghost image with respect to the desired image is dependent on the position where the alignment radiation is incident on this substrate. This effect, which may be referred to as the RICO (Reticle Induced Coherence Offset) effect, may cause alignment errors.
If a polarization-sensitive optical system is used in the alignment device, as in the device described in U.S. Pat. No. 4,778,275, further alignment errors may be produced because the substrate of a conventional IC mask consists of quartz which has some birefringence, be it a small birefringence. This birefringence causes an offset of the image of the substrate alignment mark with respect to the alignment mark. Said birefringence is not constant throughout the mask substrate surface but has a position-dependent variation. If more alignment marks spread on the mask surface area used, as is the case, for example, in the step-and-scan apparatuses, these alignment errors can no longer be compensated by, for example, an accurate correction of the state of polarization of the alignment radiation.