Patrick N. Everett

Sub-100 nm metrology using interferometrically produced fiducials

Pattern-placement metrology plays a critical role in nanofabrication. Not far in the future, metrology standards approaching 0.2 nm in accuracy will be required to facilitate the production of 25 nm semiconductor devices. They will also be needed to support the manufacturing of high-density wavelength-division-multiplexed integrated optoelectronic devices. We are developing a new approach to metrology in the sub-100 nm domain that is based on using phase-coherent fiducial gratings and grids patterned by interference lithography. This approach is complementary to the traditional mark-detection, or “market plot” pattern-placement metrology. In this article we explore the limitations of laser-interferometer-based mark-detection metrology, and contrast this with ways that fiducial grids could be used to solve a variety of metrology problems. These include measuring process-induced distortions in substrates; measuring patterning distortions in pattern-mastering systems, such as laser and e-beam writers; and me...

Interferometric-spatial-phase imaging for six-axis mask control

We describe a unified approach to measuring alignment and gap with nanometer detectivity between two planar objects (e.g., a mask and a substrate) in close proximity. The method encodes lateral position in the phase of interference fringes, formed by diffraction from grating and checkerboard alignment marks, designed to enable a wide acquisition range. For gapping, the method incorporates, in the same mark, coarse-gap detection (30–300 μm) and absolute-gap detection at sub-30 μm using a chromatic Fabry–Perot scheme. Fine detection of sub-30 μm gaps is inferred from the frequency and phase of fringes, calibrated using the chromatic Fabry–Perot. Illumination with a variable-bandwidth source enables either “achromatic” aligning or “chromatic” gapping. Sub-nanometer detection and feedback control of mask position is demonstrated in X, Y, and θ. Overlay of exposed patterns is demonstrated to be <3 nm.

Application of interferometric broadband imaging alignment on an experimental x-ray stepper

A series of experiments were performed with an interferometric-broadband imaging (IBBI) alignment system on an experimental x-ray lithography stepper. These experiments demonstrated sub-1 nm consistency of independent IBBI measurements and the ability to feedback lock the mask relative to a wafer to within a mean of 0.0 nm and a standard deviation of 1.4 nm. Comparisons of displacement measurements made with IBBI and closed-loop piezo drives confirmed scale consistency to within about 1.5%. In the absence of feedback control, spurious relative displacements were observed, due to temperature gradients and the nonrigid mechanics. This argues in favor of feedback control during exposure. The robustness of IBBI in allowing nanometer-level alignment measurements during x-ray exposure, with remotely located long-working-distance, low-cost microscopes should enable it to be used as a stand-alone alignment system or as an adjunct to global alignment.

Novel mask-wafer gap measurement scheme with nanometer-level detectivity

We describe a means of measuring the gap between mask and substrate in an x-ray lithography system. The method does not require that the gap be scanned. The method encodes the gap in the spatial phase, spatial frequency, and separation of sets of interference fringes. The fringes result from the diffraction from a checkerboard on the mask, with constant period in one direction and varying period in the transverse direction. The separation of fringe sets gives an unambiguous measure of gap when the mask is approaching the substrate, from 400 to 30 μm. At the smaller gaps used for exposure, checkerboards with different chirp periods are utilized to indicate the gap without ambiguity. The phases of the fringes as a function of gap were calibrated with a Fabry-Perot interferometer. The repeatability of the phases between consecutive scans of gap was found to have a 5 nm standard deviation. This method of measuring gap may prove useful in a variety of applications that require a controlled gap between two plates.

Dynamic alignment control for fluid-immersion lithographies using interferometric-spatial-phase imaging

We demonstrate the application of a high-sensitivity alignment method called interferometric-spatial-phase imaging (ISPI) to a nanometer-level overlay in fluid-immersion lithography, using step-and-flash imprint lithography as the test vehicle. As a stringent test we used alignment marks that consist of pure phase gratings in a fused silica template, immersed in a fluid of similar refractive index, resulting in a low-contrast alignment signal. Feedback control of alignment is demonstrated with mean=0.0nm and σ=0.1nm using an immersed template. Overlay results, with UV-exposed imprint fluid, were limited to ∼4nm, due to a mechanical disturbance. Because ISPI enables continuous monitoring of the alignment signal, we were able to identify the origin of the mechanical disturbance and can eliminate it in future experiments. In addition, we demonstrate the ability to actively reduce misalignment during the progression of crosslinking in the imprint fluid.

Immunity to signal degradation by overlayers using a novel spatial‐phase‐matching alignment system

We describe improvements to the interferometric broad‐band imaging alignment scheme introduced in 1993. Alignment is signified by matching, across the midline of a charge‐coupled device, the spatial phase of interference fringes formed by diffraction from complementary marks on mask and substrate. Image contrast is enhanced by back diffraction from hatched alignment marks on the substrate. Overlayers of resist, polysilicon, and aluminum have negligible effect on interferometric broad‐band imaging alignment; they alter image contrast but not spatial phase. Novel alignment marks that incorporate four gratings increase the capture range to several tens of micrometers. By spatial filtering in the back focal plane of the alignment microscope, mask‐sample gap may be determined from the resulting spatial phase shift. An alignment system for x‐ray nanolithography (XLS‐4) that incorporates the interferometric broad‐band imaging scheme has been constructed.

Nanometer gap measurement and verification via the chirped-Talbot effect

We describe a noncontact, optical method of measuring, with nanometer-level sensitivity, the gap between two planar objects in close proximity, such as a substrate and either a proximity-lithography mask or an imprint template. Interference fringes from a chirped-checkerboard mark on one object are observed using a nonexposing wavelength with long-working-distance, oblique-incidence microscopes. The gap is determined from the spatial frequency and phase of the fringes. We verify the gap measurement using a variation of the Talbot effect with the chirped-checkerboard mark. The two forms of gap measurement are complementary since one is suited to measuring and setting gap prior to exposure, and the other is ideal for confirmation of the gap that existed during exposure.

Simultaneous measurement of gap and superposition in a precision aligner for x‐ray nanolithography

Previously we described an x‐ray mask alignment system, capable of nanometer‐level superposition precision, whose alignment signal did not appear to be adversely affected by overlayers of resist, polysilicon, or metal [E. E. Moon, P. N. Everett, and H. I. Smith, J. Vac. Sci. Technol. B 13, 2648 (1995)]. The system, called interferometric broad‐band imaging (IBBI), employs grating and grid type alignment marks on mask and substrate, respectively. These are viewed at an inclined angle, through the mask, using f/10 optics with a working distance of 110 mm. The inclined angle and long working distance avoid interruption of the x‐ray beam. Using a charge‐coupled‐device camera, misalignment is measured from two identical sets of interference fringes (∼50 μm period) that move in opposite directions as the mask is moved relative to the substrate. Alignment corresponds to matching the spatial phases of the two sets of fringes. Here we demonstrate that the same alignment optics and grating type alignment mark on th...

Dynamic Three-Dimensional Mask-Wafer Positioning with Nanometer Exposure Overlay

We describe a means of dynamically controlling the relative mask-wafer position in X, Y, and Z during proximity X-ray exposures. The axis of a point X-ray source is detected by a direct X-ray obscuration scheme. Point-source-induced exposure runout is controlled to <1 nm along the source axis. Nanometer exposure overlay is measured by the spatial phase of fringes in resist, observed with a separate, normal-incidence microscope. These aligning and gapping techniques, in combination with the method of aligning the axis of a point X-ray source to a fiducial point on the mask, result in a minimum controlled exposure overlay of 2.5 nm.

Aligning lithography on opposite surfaces of a substrate

Equipment has been developed for aligning lithographic features between opposite surfaces of substrates to within 1 microm. It will work with opaque substrates and allows registration to existing features on the other surface.