1.

Introduction

The controlled fabrication of surface topography—with nanoscale patterns more intricate than square-edged channels or gratings—has seen a recent rise in interest.¹ In particular, controlled patterning of surfaces with limited, 2.5-dimensional (2.5-D) (for patterns with periodicity in the illumination plane) or complete [fully three-dimensional (3-D)] pattern variation normal to the wafer surface has seen increased demand.² These structures find application in propagating plasmon-mode biosensors^3–6 and assays that can control protein⁷ or block copolymer⁸ absorption.

However, no general and parallel fabrication technique has emerged to fill this need. Focused ion beam sputtering is well known;⁹ however, it presents clear integration difficulties since it is a serial, scanned-beam process. Conformal phase masks have been used to fabricate micron-thick layers with intricate 3-D structure,^10,11 yet are not capable of patterning a surface layer without first patterning the mask itself, and without taking advantage of an optical reduction factor. Interference lithography¹² limits the feature size to between $\lambda /2$ and $3\lambda /2$, placing severe constraints on final dimensions of devices made with conventional tools. Recent developments in spatially controlled deposition kinetics¹³ are promising, but limit materials and geometry. Grayscale¹⁴ and multilayer techniques¹⁵ limit feature sizes as well, and require expensive single mask or multiple mask sets.

In this work, a fabrication approach that does not suffer from these difficulties is presented. By controlling not just the exposure dose delivered, but also the illumination profile in the photoresist (the aerial image), periodic structures with a wide variety of profiles are fabricated. These structures are created using off-the-shelf hardware, operating with well-understood $i$-line illumination ($\lambda =365\mathrm{nm}$) and with commercially available photoresists, and modeling the exposure with commercially supported codes. Examples of using this technique to fabricate structures with variation in size and curvature are demonstrated. Moreover, the approach is generalized not only to the fabrication of structures in photoresist, but also the etch transfer of those structures to an underlayer. This is accomplished by control of the aerial image; by modulating the exposure dose delivered, the image focus, and the postexposure conditions:¹⁶ the postexposure bake, spray development, and hard bake. The modified mask feature spacing controls the interaction between the focal plane and the aerial image; the dose and focus modulations change the absorbed axial image, and the development parameters control the kinetics of the development. Each of these can be simulated numerically to sufficient precision to reliably produce the desired 2.5-D pattern.

2.

Experiments and Methods

To develop an appropriate test structure, full-field finite-difference time domain (FDTD) simulations were performed in Lumerical FDTD. The desired image plane intensity was simulated with the PROLITH lithography simulation package, by KLA-Tencor. The design space under consideration consists of devices with sinusoidal or smoothly curved sidewall profiles, as depicted in Fig. 1. With a specific design in mind, PROLITH was used to calculate the desired photoresist image, taking into account the photoresist and anti-reflection layers, the substrate materials and variation in sidewall profile. The material parameters used came exclusively from the PROLITH database, with the exception of the data for the SPR-220 resist, which was supplied by the manufacturer. Having established a materials stack, iterations were performed over physically controllable inputs—the focus plane and optical dose delivered—to determine the experimental parameters.

Fig. 1

Overlays of the developed results against the PROLITH simulation target for a number of designs. From left to right, the dose increases from 50 to $80\mathrm{mJ}{\mathrm{cm}}^{-2}$; from bottom to top, the focus offset (measured toward the optical system) increases from $-1.2$ to 0.3 μm. The PROLITH simulation yields the expected sinusoidal pattern with a reasonable fabrication window; perturbations around the best achievable value degrade the aerial image variation.

To verify these results experimentally, the appropriate photoresist thickness for the design feature size and depth was calculated. Typically, the simulation optical stack consists of a constant 165 nm layer of Brewer Science XHRi-16c anti-reflective coating (ARC) beneath the Megaposit SPR-220 photoresist, with a thickness of $\approx 2\mu \mathrm{m}$. A line-space grating of 400 nm lines with 500 nm spaces was used, operating at the linear resolution limit of $i$-line lithography. This mask was used to fabricate metal-dielectric structures in which plasmon resonances, discussed below, were observed.

Bare silicon wafers (with Miller indices $\langle 100\rangle$) were coated with the photoresist stack. Manufacturer suggested solvent bake times for each layer were used; in the case of SPR-220: 90°C for 60 s. The wafers were exposed using a GCA Autostep 200 $i$-line wafer stepper, using a numerical aperture of 0.45 ($\sigma =0.5$). The tool dose was verified before each batch: values of $\approx 10\mathrm{mJ}{\mathrm{cm}}^{-2}$ are typical, corresponding to exposure times of $\approx 0.04\mathrm{s}$. A combined focus-exposure matrix, with focus offset values suggested by simulation, was developed to optimize exposure; each die was exposed once. Depending on desired geometry, focus offset values were between 300  nm and 1.5 μm. Wafers were then postbaked and developed in accordance with simulated parameters, with typical values near the manufacturer suggestions for postbake ($\approx 110°\mathrm{C}$ for 60 s on hot-plate) and spray developed for 60 s in AZ 726 MIF.

Having developed the optical process, an etch process was developed¹⁷ to transfer the resultant sinusoidal geometry into the underlayer: in this case, the bulk silicon wafer. A reactive ion etch process was used, with ${\mathrm{CF}}_4$ as the etchant and ${\mathrm{CHF}}_3$ to increase selectivity to photoresist, in a $5:1$ flow ratio. The forward power was 150 W and the chamber pressure was 40 mtorr.

The reflectivity, at variable angle of incidence, was characterized by an optical setup contained a tungsten–halogen source, Horiba iHR550 infrared monochromator, collimating silver mirror, polarizing prism, focusing silver mirror, and silicon photodetector. The measurements were performed using a linearly polarized collimated beam with a spectral range spanning 400 to 1100 nm. Collected data was normalized against the reflection of a flat silver mirror.

3.

Results and Discussion

3.2.

Photoresist Patterning, Etch Transfer

Patterning photoresist using this technique yields a variety of controlled cross sectional profiles. Figure 2 shows a scanning electron microscopy (SEM) cross section of photoresist, exposed and developed according to a simulation tailored toward a sinusoidal image. The subfigures demonstrate other geometries that can be created with the above technique, similarly in photoresist. An asymmetrical grating is shown with a curved base and sharper tops. Moreover, a more extreme example is developed: an array of sharply tipped lines.

Fig. 2

A scanning electron microscopy (SEM) image of patterned photoresist using the controlled defocus lithography technique emphasizing the variety of producible patterns. An extreme example of aerial image modulation is visible in inset (a). Inset (b) is an edge-on sine wave pattern. Finally, we see an interim example of the possible gradation in inset (c).

Multiple iterations of this process yield reliable pattern generation and transfer, as demonstrated in Fig. 1. Using a dose–focus matrix for optimization, patterns that compare closely to those generated in simulations are produced.

While creating patterns in photoresist is useful, far greater utility lies in transferring these to underlayers and enabling deposition of a coating. In Fig. 3, the transfer of the photoresist pattern into the silicon substrate is demonstrated, along with a gold coating. Using the etch parameters described above, etch selectivity within 7% of unity is achieved, yielding minimal change in the photoresist aspect ratio.

Fig. 3

An edge-on ($\theta ={10}^{°}$) SEM image of a periodic pattern transferred into the substrate, and coated in 25 nm gold. The photoresist pattern, here, is taken from the sample shown in Fig. 2(c).

3.3.

Narrowband Plasmon Resonance

We used this approach to construct a surface plasmon-based asymmetrical grating, with a decreased density of plasmon states near the grating edge, yielding particularly narrowband performance. Reflection spectra calculated by FDTD simulation are compared to experimental measurements. The reflectivity of the metal-dielectric sample in Fig. 3 was measured using normal incident angle of transverse magnetic polarized probing light, and is presented in Fig. 4.

Fig. 4

The measured $p$-polarized spectral reflectance (points), with the simulated spectral reluctance (solid curve), at normal incidence. The measured reflection minimum is at 911 nm, whereas the calculated reflection minimum is at 909 nm. This resonance is due to optical excitation of propagating plasmon modes along with the asymmetric curved surface. Below $\approx 800\mathrm{nm}$, diffraction losses decrease directional reflectivity.

The peaks, each indicating the incident illumination coupling into a surface mode, overlap to within $\mathrm{\Delta }\lambda =2\mathrm{nm}$. The sample demonstrated a narrow-band reflectivity minimum for the $p$-polarized probe field at the wavelength of 911 nm (Fig. 4). The minimum wavelength was blue-shifted with increasing angle of incidence. In contrast, the reflection of $s$-polarized light did not demonstrate any minimum of reflectivity at 911 nm or any other wavelength within the measurement range. The resonance is narrow, with a full-width at half-maximum of 19 nm, indicating a high-quality periodic structure.

4.

Conclusion

Above, the fabrication of 2.5-D periodic structures using a one-step optical lithography process compatible with conventional $i$-line tools is demonstrated. Etch transfer of the shape, with high fidelity, into the underlying silicon is performed.

Limitations of this technique became evident during the development of this process. Image fidelity is affected by focus control. Moreover, technology transfer to industrial processes, using 200 mm substrates and with decreased tolerances for resist uniformity and wafer bow, may be challenging.

While this approach has been implemented before using electron-beam lithography,¹⁸ an approach using photolithography enables integration with a parallel workflow, and a reduction in fabrication cost. In the realm of all-optical techniques, the aerial image modulation approach enables topography patterning of feature sizes too large for holographic lithography and too small for grayscale approaches.