Heterodyning optical phase measuring device for diffraction based overlay
Methods and systems are provided for diffraction-based overlay (DBO) metrology of a multilayered sample. In one example, a method may include generating spatially structured light via a light source and an optical modulator, transmitting the spatially structured light onto the multilayered sample, detecting diffracted spatially structured light at one or more of a plurality of sensors, and estimating an overlay error of the multilayered sample based on the diffracted spatially structured light detected at the one or more of the plurality of sensors.
1 . A method, comprising:
generating spatially structured light from a light source by projecting light emitted from the light source through an optical modulator, the optical modulator imparting a selectable spatial period to the structured light;
transmitting the spatially structured light onto a multilayered periodic sample;
detecting diffracted spatially structured light from the multilayered periodic sample at at least one sensor communicatively coupled to a computing system; and
estimating an overlay error of the multilayered periodic sample based on a phase shift induced by heterodyning between the spatial period of the structured light and the periodicity of the sample, as determined from signals received at the computing system.
2 . The method of claim 1 , wherein the optical modulator generates spatially structured light with a given spatial period.
3 . The method of claim 2 , wherein the spatial period of the spatially structured light can be modulated through amplitude modulation, frequency modulation, or a combination of both.
4 . The method of claim 3 , wherein estimating the overlay error includes regressing over at least one of the spatial period of the spatially structured light or a wavelength of the light source.
5 . The method of claim 4 , further comprising a plurality of lens elements, with a first lens element of the plurality of lens elements placed in between the optical modulator and the multilayered periodic sample, and each lens element of a remainder of the plurality of lens elements placed in between the multilayered periodic sample and the at least one sensor, and wherein the diffracted spatially structured light detected at the at least one sensor is compared to a reference signal.
6 . The method of claim 5 , wherein estimating the overlay error is based on estimating an asymmetry in an electric field of the diffracted spatially structured light as a function of time.
7 . The method of claim 5 , wherein estimating the overlay error includes estimating a phase difference between the diffracted spatially structured light and the reference signal via lock-in amplification of the diffracted spatially structured light with respect to the reference signal.
8 . The method of claim 4 , further comprising an objective lens placed between the multilayered periodic sample and the at least one sensor, the objective lens projecting a spatial Fourier-transform of the diffracted spatially structured light into a pupil plane of the objective lens to be detected at the at least one sensor, and wherein estimating the overlay error includes applying an inverse Fourier transform to the spatial Fourier-transform of the diffracted spatially structured light detected at the at least one sensor.
9 . The method of claim 4 , wherein the spatially structured light transmitted from the optical modulator includes a two-dimensional (2D) spatial variation projected onto a plane of the multilayered periodic sample, and wherein estimating the overlay error includes estimating a vector displacement of the overlay error based on the 2D spatial variation of the diffracted spatially structured light detected at the at least one sensor.
10 . The method of claim 5 , wherein estimating the overlay error includes estimating a phase difference of the diffracted spatially structured light with respect to the reference signal, the reference signal generated by the computing system.
11 . An optical system for diffraction based overlay (DBO) metrology of a multilayered periodic sample, the optical system comprising:
an axis;
an optical modulator coupled with the axis;
a lens element;
a light source configured to shine light through the optical modulator to project spatially structured light along the axis onto a multilayered periodic sample through the lens element;
a first sensor configured to detect a 0 th -order diffracted component of the spatially structured light from the multilayered periodic sample;
a second sensor configured to detect a 1st-order diffracted component of the spatially structured light from the multilayered periodic sample; and
a computing system including:
a synchronization module configured to apply lock-in amplification of the detected diffracted structured light at each of the first and second sensors with respect to a reference signal; and
an analysis module configured to compute a phase difference between the reference signal and each of the diffracted structured light components detected at the first and second sensors, the phase difference being a function of the spatial period of the structured light and optionally a wavelength of the light source.
12 . The method of claim 1 , further comprising using a staging apparatus to position the multilayered periodic sample before and/or during a measurement.
13 . The optical system of claim 11 , further comprising a staging apparatus configured to position the multilayered periodic sample before and/or during a measurement.