Nanometer-level alignment to a substrate-embedded coordinate system
Schematic of one set of diffraction paths required for generation of ISPI interference fringes from a superimposed grating pair. Numerous other diffraction paths are not shown. In order for any diffracted beam to contribute to the fringe formation, it must undergo a first-order diffraction from both and gratings. The double diffraction allows cancellation of wavelength dependence, resulting in achromatic interference fringes.
Schematic of back side ISPI marks. (a) The substrate back side mark is composed of a single period in both the and directions. (b) The complementary template mark consists of a bifurcated grating that includes two periods and . (c) The grating periods are arranged such that , resulting in counterpropagating fringe motion in response to a displacement.
(a) Schematic of the absolute position detection method used on the back side mark. (b) Islands of a grid pattern are distributed throughout the checkerboard pattern. The separation of the islands increases monotonically (i.e., is chirped) in both the and directions. Each grid island backdiffracts incident illumination directly to the ISPI microscopes at the Littrow angle. The array of islands appears on an image sensor as a series of bright dots. A subset of the chirped dot array is within the field of view of the microscope. The imaged region is analyzed for its spatial frequency content, which provides unique coarse position identification. Fine position is detected by the phase disparity of interferometric fringes. The spatial frequency of the interference fringe sets is distinct from the range of chirp frequencies.
(a) ISPI image of visible-light interference fringes from front side marks. (b) ISPI image of infrared interference fringes from back side marks. (c) Plot of front side ISPI measurements in response to piezosteps. Position stability was . (d)Plot of back side ISPI measurements in response to piezosteps. Position stability was . The difference in position stability is attributed to mechanical noise.
Plots of ISPI measurements taken over a 1° range of angles by adjusting an ISPI microscope with a six-axis stage. The center of rotation is controlled to coincide with the ISPI mark. Standard ISPI marks were used on the front side of the wafer. The axis is defined as the optical axis of the microscope. (a) ISPI measurements during a rotation of . The slope of the linear fit is . (b) ISPI measurements during a rotation of . The slope of the linear fit is . (c) ISPI measurements during a rotation of . The slope of the linear fit is . The jitter in the data is primarily attributed to the stage. The nominal minimum angular step size of the stage is 0.01°.
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