Fabrication of high-density moiré gratings

Optical metrology is widely used to characterize surface deformations due to the noncontact, full-field, and highly accurate nature of the measurements. In combination with advanced microscopes, optical metrology methods have recently been extended into the micro- and nanoscales. In measurements that use the moiré (beat pattern produced between gratings of initially equal spacing) method,^1–3 gratings act as sensors that record the original deformation of a specimen. Precise preparation of the gratings is therefore required to ensure accurate measurements.

Existing technologies for grating fabrication do not meet all the necessary requirements (i.e., production of high-frequency and large-area gratings at low cost) simultaneously. Ordinary photolithography, due to its diffraction limitation, can generally only produce gratings with hundreds of lines per millimeter. Holographic lithography can be used to make gratings with frequencies of up to 4800 lines per millimeter, but there are strict requirements on the optical system and the alignment techniques. Electron beam lithography and focused ion beam lithography can produce fine gratings with up to 10,000 lines per millimeter, but the grating area is very small (micrometer scale).^{4, 5}

We have developed a new moiré grating fabrication method that is based on the nanoimprint lithography (NIL) technique.⁶ NIL creates nanoscale patterns through mechanical deformation and has low costs, high throughput, and high resolution. This process has already been used in several microelectronic engineering, optical engineering, and bioengineering applications. Our technique can be used to produce gratings with large areas and high frequencies on different specimens, and at fast rates (see Figure 1).^7–9

Figure 1. (a) Grating fabricated on a traditional (millimeter-scale dimensions) specimen (1200 lines per millimeter, cross type).⁷ (b) Grating fabricated on a thin (<1μm) film specimen (1200 lines per millimeter, cross type).⁸

The preparation of the stamp that is used for imprinting is a key issue in our method. When imprinting is conducted at low pressure, a full-contact condition (between the specimen and stamp) is almost impossible to achieve and defects may be generated on the grating. At high pressure, however, stress concentrations can damage the brittle material from which the stamp is made. We have developed a three-layer composite structure stamp to solve this problem (see Figure 2).⁷ The top layer of the structure is a thin (0.5mm) nickel grating, which has a low thermal expansion coefficient and can easily be molded. The middle layer is made of a 3M adhesive film (heat resistant to temperatures of 250°C)⁷ that is flexible and improves the adaptive ability of the stamp. The bottom layer is made of a hard substrate—silicon (Si) or silicon dioxide (SiO₂)—that is able to support the overlying layers.

Figure 2. Nanoimprint lithography three-layer stamp design with top nickel (Ni), flexible middle, and silicon or silicon dioxide substrate bottom layers.

The grating profile (i.e., the grating pitch, opening ratio, and grating form) that we produce is determined by the profile of the NIL stamp and directly affects the measurement results. It is therefore important to optimize the stamp profile before fabrication. Our analyses show that a sine-wave grating and a square-wave grating with identical widths of black and white bars (i.e., contrast in the grating image) are the optimal grating designs for our method.⁷

We have combined an NIL grating with a moiré scanning electron microscope (SEM) to develop a 3D SEM moiré method.¹⁰ The relationship between the tilt angle and the corresponding moiré fringe styles can be analyzed to find accurate 3D height information for the tested object. We have verified the feasibility and effectiveness of this method with a set of experiments (see Figure 3). We have also shown that our new 3D SEM moiré method enhances the function of the SEM and extends the capabilities of the existing 2D scanning moiré method.

Figure 3. Measurement results for a bulged grating from (a) scanning electron microscope (SEM) scanning moiré images made at different tilt angles (0, +10, and −10°), (b) optical images, and (c) the 3D scanning moiré method.

The gratings produced with our new NIL-based fabrication technique can be used to conduct 2D and 3D optical metrology scanning measurements. Although our approach has several advantages over more standard processes, there are still some outstanding issues, e.g., distortion introduced during imprinting, that need to be resolved.¹¹ In our future work we will continue to improve the NIL grating production and investigate more applications for the gratings where they can be used to make measurements of residual stress, thermal mismatch, and local strain around stress concentrations.