State of the art after-develop (ADI) overlay is measured with multi-wavelength micro diffraction-based overlay techniques. A micro diffraction-based overlay target consists of two pairs of gratings, with the same pitch in the top and bottom layer. The gratings in the top layer have a bias offset with respect to the bottom layer in the positive or negative direction. When illuminated, +1st and -1st order light is diffracted. The asymmetry in the intensity of these signals contains the overlay information. In this paper, ADI overlay is measured with a new dark-field target design for ADI overlay. Like a micro diffraction-based overlay target, it consists of pairs of gratings in the top and bottom layer. Instead of a bias offset between top and bottom gratings, different pitches are used resulting in a continuous-bias throughout the grating pair. When illuminated the diffracted light contains moiré fringes, in which the overlay is stored in the phases. This technique has improved accuracy and robustness by design, because it is immune to symmetrical process changes like stack height variations and grating imbalance. Additionally, it shows more stable behavior through wavelength, both in signal strength and overlay. These characteristics make it possible, with a single wavelength, to achieve similar or better performance than micro diffraction-based overlay using a multi-wavelength solution, resulting in higher throughput. This is demonstrated on Samsung’s latest memory node where on average an 21% reduction is achieved in the 3sigma of the mis reading correction with a single-wavelength phase-based overlay measurements, compared to multi-wavelength micro diffraction-based overlay measurements.
The current state of the art ADI overlay metrology relies on multi-wavelength uDBO techniques. Combining the wavelengths results in better robustness against process effects like process induced grating asymmetries. Overlay information is extracted in the image plane by determining the intensity asymmetry in the 1st order diffraction signals of two grating pairs with an intentional shift (bias). In this paper we discuss a next evolution in DBO targets where a target is created with multiple biases. These so called cDBO (continuous bias DBO) targets have a slightly different pitch between top and bottom grating, which has the effect of having a different bias values along the grating length and are complimentary to the uDBO technology. Where for the uDBO target, the diffraction results in a uniform Intensity pattern that carries the Overlay signal, for cDBO, an oscillating intensity pattern occurs, and the Overlay information is now captured in the phase of that pattern. Phase-based Overlay has an improved, intrinsic robustness over intensity-based overlay and can reduce the need for multi-wavelength techniques in several cases. Results on memory technology wafers confirm that the swing-curve (through-wavelength) behavior is indeed more stable for phase-based DBO target and that for accurate Overlay, this target can be qualified with a single wavelength recipe (compared to the uDBO dual wavelength recipe). In this paper, both initial results on a Micron feasibility wafer will be shown as well as demonstrated capability in a production environment.
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