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# OCD measurement and modeling of large µlens objects

Justine Grasland, Delphine Le Cunff, Maxime Besacier, Jean-Herve Tortai

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Justine Grasland, Delphine Le Cunff, Maxime Besacier, Jean-Herve Tortai. OCD measurement and modeling of large µlens objects. EMLC 2024 - The 39th European Mask and Lithography Conference, Uwe Behringer, Jun 2024, Grenoble, France. hal-04766873

HAL Id: hal-04766873

https://hal.science/hal-04766873v1

Submitted on 14 Mar 2025

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# OCD measurement and modeling of large µlens objects

J. Grasland¹,², D. Le Cunff¹, M Besacier² and JH Tortai²

¹ STMicroelectronics, 850 rue Jean Monnet, 38926 Crolles, France

² Univ. Grenoble Alpes, CNRS, CEA-LETI Minatec, LTM, 38000 Grenoble, France

As the technology for CMOS Image Sensors evolves, more and more printed patterns exhibit 3D shapes. Among these specific shapes, microscopic lenses are used in imagers to focus the light on the photodiode. Today, optical lenses are measured routinely in manufacturing line by SEM for CD (Critical Dimension) and AFM for height and curvature. Still, the control of the 3D dimensions poses challenges from metrology perspective: spacing between lenses is getting too narrow to be measured correctly by AFM and contrast between jointed lenses is getting too low for CD-SEM measurements. Moreover, AFM and CD-SEM have their own intrinsic limitations: AFM is slow consequently it is applied only for small amount of µlens, CD-SEM is fast but only provides local images in 2D top view mode. OCD is an attractive alternative as it is a fast optical technique, and it allows better statistical information compared to local CD-SEM and AFM measurements. However, OCD does not measure dimensions directly but optical properties of a diffracting grating: an accurate geometrical modeling and robust inverse problem solving is needed to estimate the shape. For the specific case of 3D µlens, the process can be very time consuming. In this study, we present results related to the evaluation of OCD technique to measure the shape of joined 3D microlenses.

Experiments and acquisitions were carried out on three successive steps of the 1µm pitch microlenses fabrication on 300mm wafers. First, rounded vertical pillars were shaped by lithography, then sacrificial µlenses were obtained by melting the resist pillar and finally µlenses were patterned by plasma etching to the underlayer (Fig.1). Conventional AFM and CD-SEM measurements were performed at three locations of the wafer: center/ left edge / right edge to assess for process variability. OCD spectra were obtained by spectroscopy ellipsometry (SE) measurement using industrial inline equipment. For illustration, the experimental output data from AFM, CD-SEM and OCD are displayed in Table 1 for each of the three fabrication steps.

For OCD model development phase, AFM measurements were used to generate an initial pattern shape. The shape was then optimized to get as close as possible to the experimental signal (Fig.2). The optical properties of the grating were calculated with RCWA model-based code. The model was readjusted by modifying input parameters for AFM and CD SEM such as underlayer thicknesses, refractive index of materials and pattern dimension. Good fitting quality was obtained, and Figure 3 presents the comparison of the µlens heights between OCD after optimization and AFM for the different locations of the wafer. The same trend is observed which gives positive sign that OCD might be a good candidate to measure these 3D shapes.

<figure><img src="image_2.png" /><figcaption>Figure 1: Illustration of the different microfabrication steps of ulens from resins deposition to etched ulens</figcaption></figure>
<table><caption>Table 1: Measurement technique for the three different process steps</caption><thead><tr><th>Measurement</th><th>Lithography step</th><th>Melt step</th><th>Etched step</th></tr></thead><tbody><tr><td>AFM<br>H<sub>pattern</sub></td><td></td><td></td><td></td></tr><tr><td>SEM<br>Gap</td><td></td><td></td><td></td></tr><tr><td>OCD<br>H<sub>pattern</sub><br>Gap<br>H<sub>underlayer</sub></td><td></td><td></td><td></td></tr></tbody></table>

<figure><img src="image_3.png" /><figcaption>Figure 2: (1) AFM acquisition (2) Illustration of the OCD model profile with 3D RCWA algorithms and (3) inverse problem spectra for etched µlens.</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>Figure 3: (1) Mapping of the different measurement sites (2) Height determination by OCD compared with AFM for the 9 sites.</figcaption></figure>