Lateral shift measurement using an optical technique

#This invention is generally in the field of manufacturing of multi-layer structures, such as semiconductor wafers and integrated circuits, and relates to an optical measuring method and system for control of layers alignment.

#Integrated circuits are multi-layer structures produced by applying a sequence of deposition-lithography-etching steps to a semiconductor wafer. In such structures layers have to be precisely aligned with each other, which is controlled by the so-called “overlay measurement”. This measurement is usually accomplished using a box-within-a-box technique consisting of the following. A rectangular frame-like structure is formed in a test site of each layer, and two adjacent layers are considered as being correctly aligned if a specific alignment between the frames on these layers is provided. Overlay defining the alignment is measured by comparing the shifts between the frames at opposite sides: determining whether the frames are precisely concentric, the smaller frame being inside the larger one (in projection).

#The above technique is carried out by an ordinary optical microscope, which is capable of measuring line width with a resolution limited by resolution of optical imaging systems, usually not less than several nanometers. The current high-performance semiconductor devices, however, have features' dimensions of 0.13 μm and less, and require measurements of overlay registration with the resolution of less than 1 nm.

#In order to understand the invention and to see how it may be carried out in practice, a preferred embodiment will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which:

#FIG. 1 is a schematic illustration of a cross-section of a site in a semiconductor wafer;

#FIGS. 2A and 2B illustrate the principles of a direct method according to the invention;

#Referring to FIG. 1, there is schematically illustrated a cross-section of a test site 10 in a semiconductor wafer suitable for use in the present invention. The site 10 contains two gratings (patterned structures) 12 and 14 of certain known periodicity located one above the other. In the present example, the gratings 12 and 14 have substantially the same period P. In the present example, the top grating 12 presents a pattern of spaced-apart photoresist-containing regions RPR, and the underneath grating 14 presents a pattern of spaced-apart aluminum-containing region RAl. In this specific example, the gratings 12 and 14 have different duty cycles defined by features critical dimensions CDPR and DCAl, respectively. It should, however, be noted that this condition is not a requirement for the technique of the present invention, but provides for a better sensitivity of the method. As further shown in the figure, the gratings 12 and 14 are shifted along the X-axis with respect to each other a distance S, which is measured as a distance between the centers of the two locally adjacent regions (lines) RPR and RAl.

#In order to find the conditions under which the method of the present invention is most effective, and show how the method is practical, the sensitivity of the method has been studied for a specific configuration of the site structure exemplified in FIG. 1. The site structure 10 is typical for the overlay problem in photoresist-on-aluminum layer structure. The pattern in the aluminum (Al) layer is typically defined by layers underneath the aluminum layer, and the photoresist (PR) layer is patterned by a lithography processing. In this specific example, the PR lines are modeled to be on top of the Al lines. It should, however, be understood, that this is not essential for the present invention, and PR lines may be located between the Al lines, as well. For simplicity, both gratings 12 and 14 have a square profile. The measurement technique used for this analysis is the normal incidence polarized spectrophotometry. According to this technique, the normal incidence reflectivity spectrum is measured with selective polarization direction of the incident light relative to the grating (TM polarization mode in this specific example). Other optical techniques, such as spectral ellipsometry, angular scatterometry, etc. may be used as well.

#The sensitivity of the spectrum (measured by the technique of the present invention, which will be described more specifically further below) to a change in the lateral shift S between the gratings 12 and 14 has been studied. The ratio between the mean change in the spectrum (defined as the root of the mean of the square differences between spectra with and without a change in the shift) caused by a change in the shift value S of 1 nm has been defined as the sensitivity test T, wherein S is the nominal shift.