Euclid E. Moon

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.

Novel on‐axis interferometric alignment method with sub‐10 nm precision

A novel on‐axis interferometric alignment scheme, especially applicable to x‐ray lithography, is described which combines the position sensitivity of interferometry and the robustness of imaging. It employs broadband illumination, and hence should be relatively immune to many of the effects that tend to corrupt alignment signals in conventional interferometric systems. In its initial demonstration a standard deviation (σ) of 6 nm was achieved in both X and Y. Ultimate limits are calculated to be below 1 nm. On an Apple Quadra 800 computer, the spatial‐phase information that measures misalignment is fully analyzed in 200 ms.

X-ray nanolithography: Extension to the limits of the lithographic process

The unique aspects of x-ray lithography that make it attractive for the sub-100nm domain include: a highly localized, sharply peaked point-spread function, leading to minimal proximity effects; absence of spurious scattering; an intrinsic resolution below 30 nm; compatibility with all pattern geometries; and parallel exposure (i.e., compatibility with volume production). The major problem areas are: the mask-sample gap (less than 5 @mm for linewidths below 70 nm), and absorber stress, which must be near zero to avoid mask distortion. Nanometer-level pattern placement and alignment are considered achievable by means of spatial-phase-locked e-beam lithography and interferometric-broad-band imaging, respectively. The efficacy of x-ray nanolithography has been demonstrated via the fabrication of a variety of sub-100 nm-featured quantum-effect devices, Si MOSFETs, and grating-based optoelectronic devices. In the event that the small gaps required of proximity x-ray nanolithography prove unacceptable in manufacturing, x-ray projection using arrays of zone plates appears to be the only approach that can employ the optimal wavelengths (i.e., ~1 nm or 4.5 nm) and achieve deep sub-100 nm resolution. A scheme is proposed that employs an array of zone plates in a pattern generator mode.

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.

Alpha-prototype system for zone-plate-array lithography

In this article, we present lithography results from a continuous-scan zone-plate-array lithography (ZPAL) system using the grating light valve (GLV) as the multiplexing element. ZPAL is an optical-maskless-lithography technique, in which an array of diffractive lenses (e.g., zone plates) focuses incident light into an array of spots on a photoresist-coated substrate. The intensity of the light incident on each lens is controlled by the GLV. By scanning the wafer and appropriately modulating the incident light, patterns are written in a “dot-matrix” fashion. We have incorporated the elements of ZPAL into an alpha-prototype system. We describe this system and characterize its lithographic performance.

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.

Nanometer-precision pattern registration for scanning-probe lithographies using interferometric-spatial-phase imaging

The authors propose a solution to drift and disturbances between a scanning-probe tip and a substrate that commonly distort scanned images and undermine effective lithographic patterning. An interferometric position detection method is employed to continuously suppress drift and control the tip-scanning trajectory with nanometer precision, relative to the substrate. An associated interferometric method is used to control tip height during approach to the substrate. Patterns with arbitrary geometries are written by means of a tap-imprint method, using probes with sub-0.7-nm tip diameters.

Fabrication and characterization of high-flatness mesa-etched silicon nitride x-ray masks

To realize a technology for x‐ray nanolithography (<100 nm features), which is compatible with manufacturing, a number of mask design requirements must be met that are unrelated to patterning, repair, and alignment. These include high‐flatness membranes and support structures so that mask‐wafer gaps less than 10 μm can be achieved without risk of damage, and a rigid mask frame to avoid problems of distortion during handling. The membrane material should be compatible with semiconductor‐processing, possess high strength, be radiation hard, and be transparent to light for alignment purposes. Details of a mask architecture that meets these requirements will be described.

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.

Scanning-spatial-phase alignment for zone-plate-array lithography

In this article, we describe a technique for level-to-level alignment in zone-plate-array lithography that does not require an external microscope, yet provides overlay superior to conventional microscopes.