Phase unwrapping is a very important processing step in phase shift interferometry. In this work, we propose a new
method which combines the branch-cut method with error correcting. The method can avoid the propagation of the phase
errors and have higher reliability. The experiment proves the proposed method is feasible and effective.
This study proposes the auto-focusing procedure and the scan-range determining algorithm for white-light scanning
interferometry. During white-light scanning interferometry, the interference fringe must be located and to the best-focus
interferogram identified. The vertical-scan range must also be determined prior to the scanning procedure. A series of
images, either in-focus or out-of-focus, are collected in a proposed interference-fringe searching step. The contrast and
the sharpness indices of each image are calculated and applied in the auto-focusing scheme, and the vertical-scan range
is determined accordingly. Some preliminary experiments are performed to demonstrate that the best-focus
interferogram can be located precisely and the vertical-scan range can be determined.
We have demonstrated the feasibility of measuring overlay using small targets with an optical imaging tool has in
earlier papers. For 3&mgr;m or smaller targets, overlay shifts introduce asymmetry into the target image. The image
asymmetry is proportional to the overlay shift and so this effect can be used to measure the overlay.
We have used wafers built using production 45nm and 55nm processes to test these targets in production control
situations. Targets with different programmed offsets allow the necessary conversion between image asymmetry and
overlay shift to be determined empirically on the wafer under test. Measurements made using standard 25&mgr;m
bar-in-bar targets and 3&mgr;m in-chip targets agree to within 10nm (3&sgr;). By processing results from five or more fields
the agreement is improved to 5nm, a level which is limited by a mechanism other than random errors and which is
similar to differences between different styles of bar-in-bar targets.
Analysis of data from both in-chip and bar-in-bar targets shows similar patterns of overlay variation within the device
area. The pattern of overlay variation does not fit mathematical models of overlay as a function of location. The
total change of overlay within the field is 10nm, exceeds the overlay budget for critical layers at 45nm design rules.
This uncontrolled in-field variation in overlay must be reduced and ideally eliminated if process control is to be
achieved. A first step in controlling these errors is having an ability to measure them, and our data shows that this is
possible with targets no larger than 3&mgr;m in total size.
The results of combining the wrapped phase with the fringe order of this phase to increase the precision of white-light interferometry at high scanning speed are presented. Monochromatic phase data are calculated using the Fourier method and the fringe order is determined using a general coherence peak sensing method. A wide scanning interval of 5λ/8 and a narrow-band color filter with a bandwidth of 70 nm are adopted to acquire interferograms. Experiments with an rms repeatability of step height measurement of below 1 nm and a scanning speed of 40 μm/s are performed.
The feasibility of measuring overlay using small targets has been demonstrated in an earlier paper1. If the target is small ("smallness" being relative to the resolution of the imaging tool) then only the symmetry of its image changes with overlay offset. For our purposes the targets must be less than 5μm across, but ideally much smaller, so that they can be positioned within the active areas of real devices. These targets allow overlay variation to be tested in ways that are not possible using larger conventional target designs. In this paper we describe continued development of this technology.
In our previous experimental work the targets were limited to relatively large sizes (3x3μm) by the available process tools. In this paper we report experimental results from smaller targets (down to 1x1μm) fabricated using an e-beam writer.
We compare experimental results for the change of image asymmetry of these targets with overlay offset and with modeled simulations. The image of the targets depends on film properties and their design should be optimized to provide the maximum variation of image symmetry with overlay offset. Implementation of this technology on product wafers will be simplified by using an image model to optimize the target design for specific process layers. Our results show the necessary good agreement between experimental data and the model.
The determination of asymmetry from the images of targets as small as 1μm allows the measurement of overlay with total measurement uncertainty as low as 2nm.
We report a study of the effect of target size on CD measurement by angular scatterometry in two ways. One is reducing the spot size (say to 40 mm) to permit the use of a smaller target; another is to overfill the target. Starting with standard grating targets of 80 x 80 mm size, with fixed CD 400 nm and LS (Linewidth to Spacing) ratio 1:1, test gratings have been designed with X and Y dimensions varied from 80 to 10 mm in 10 mm intervals. We show how the scattering signatures are influenced by the varying target sizes and spot sizes especially when the target grating is overfilled. The errors in CD measurements caused by the target and spot size variations are also quantified. Working with an overfilled small grating target and filtering out the specular noise offers a promising way to present the scattering signal from diffraction. An empirical model to predict the scattering signatures as a function of target size is under development.
We have developed a target design for overlay measurement which is small enough (3x3μm) that it could be positioned within the active area of integrated circuit devices.
These targets have been measured using an unmodified overlay tool. The targets are too small for the image to be fully resolved using visible wavelengths, and so measurement using the normal methods based on determining the relative positions of features in the image does not produce acceptable levels of measurement uncertainty. Instead, we show that the symmetry of the image can be used to determine the overlay error.
We report initial results which show measurement uncertainty using this technique approaching the levels needed for overlay control at design rules under 100nm. These results are limited by the process used to create our test structures, and even better results may be possible with state-of-the-art lithography and processing techniques.
Meeting the stricter overlay measurement error requirements of next-generation lithography is a challenge to conventional optical metrology solution associated with bright-field microscopy. A modified thin film model was developed to simulate the optical image intensity profile from novel overlay targets with design rule features. The image is calculated based on diffraction theory, which is simpler than the rigorous application of Maxwell’s equations in three dimensions. The model is matched to the image by adding the contributions from all of the patterned regions in the target, and multiplying by a complex reflectance transfer matrix, which embodies all of the material characteristics. The overlay error in the target and the optical configuration parameters are modified to find the best fit between the image and the model. Although this method makes several assumptions about the formation of an image, very close agreement between the model and the image is obtained.
A new profiling algorithm is proposed for the scanning white-light interferometry. A series of white-light interferograms are acquired by traditional vertical scanning process. The collected intensity data of the interferograms are then Fourier-Transformed with respect to the ordinate, or the scanning axis, into the wave number domain, where two or more wave numbers are selected for further calculation. The multi-wavelength phase-unwrapping technique is then used to solve for the surface profile. Preliminary experiment has been carried out with a Mirau-type white-light interferometer on two sets of step-height standards. The proposed algorithm works as well even when the spectrum of the white-light source is not Gaussian distributed, while the conventional peak sensing algorithms do not.