**1. Introduction**

With the rapid advances in nanofabrication technology, the minimum gate length scales of transistors have been reduced to sub-10 nm [1–5], so they require precise geometric measurements, which in turn induces high demands on the accuracy of nano-measurement instruments. Therefore, it is necessary to develop nano-standards with traceability to calibrate the nano-measurement instruments to ensure the accuracy of characterization in nanofabrication, and accordingly, improve the performance of integrated circuits. In this regard, one-dimensional nano-grating standards, as one type of the important nanometric standards, are mainly used to calibrate the magnification of nano-measurement instruments. Correspondingly, a large number of research institutions and companies have developed a series of one-dimensional (1D) micro- and nano-grating standards [6–13].

**Citation:** Zhang, Y.; Wang, C.; Jing, W.; Wang, S.; Zhang, Y.; Zhang, L.; Zhang, Y.; Zhu, N.; Wang, Y.; Zhao, Y.; et al. High-Precision Regulation of Nano-Grating Linewidth Based on ALD. *Micromachines* **2022**, *13*, 995. https://doi.org/10.3390/ mi13070995

Academic Editors: Youqiang Xing, Xiuqing Hao and Duanzhi Duan

Received: 7 June 2022 Accepted: 23 June 2022 Published: 24 June 2022

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Nevertheless, most nano-grating standards have a single function which provides reliable pitch calibration values. With such standards, the controllability and uniformity of the linewidth cannot be guaranteed, hence the linewidth cannot be calibrated. In addition, a grating with a constant duty cycle cannot simultaneously match the different requirements of measurement instruments with different calibration principles, for the optimal duty cycle. That is to say, the efficiency and accuracy of the calibration can be improved by making the linewidth or duty cycle of the nano-grating standard controllable.

Micro- and nano-fabrication processes such as electron beam lithography (EBL), focused ion beam (FIB) and extreme ultra-violet (EUV) are commonly used for manufacturing micro- and nano-structures. Most of the grating standards with pitch ranging between 100 and 4000 nm and fabricated using EBL have good periodicity. However, the linewidths fabricated by the EBL process have certain randomness due to the inevitable proximity effect and the instability of the current [14–16]. Therefore, despite using the same parameters, producing the same linewidth each time cannot be guaranteed. The FIB technology is a direct patterning process without a photoresist and includes several basic principles such as milling and deposition [17]. The edge and surface roughness of grating structures obtained by milling is large due to material redeposition [18]. While the structures obtained by FIB deposition have more uniform surface topography, the thickness of the deposited metal will change the designed linewidths at the same time [19]. EUV requires a mask, so the graphic size cannot be flexibly adjusted in time according to the experimental results [20]. Furthermore, none of the above processes can repatch the linewidth again after fabrication. In summary, it is not easy to precisely regulate the linewidth or duty cycle of each grating line, and likewise, it is more challenging to ensure the linewidth accuracy of nano-grating standards.

Atomic layer deposition (ALD) can precisely grow thin films of controlled thickness (from a few to tens of nanometers) on the underlying three-dimensional (3D) structures, with high accuracy, uniformity and consistency. The technique of depositing 3D conformal films on periodic structures using ALD has also been demonstrated in several papers [21–25]. To develop multifunctional grating standards with controllable linewidth and pitch, 1D nano-grating standards with a theoretical pitch of 1000 nm were fabricated in this paper using EBL, and the linewidth of shaped nano-grating was regulated by ALD. In addition, the linewidth and pitch of the 1D grating structure were measured and evaluated by an atomic force microscope (AFM) and a scanning electron microscope (SEM), which validated the feasibility and excellent performance of precise linewidth regulation via ALD, and demonstrated the high surface quality, calibration reliability, and measurement consistency of the standards.

## **2. Design and Fabrication**

### *2.1. Structural Design*

The 1D grating standard presented in this study is a 1.5 cm × 1.5 cm chip, whose surface structure mainly includes the calibration area and the guidance area, as shown in Figure 1a,b. The calibration area is a 1000 nm pitch 1D grating structure with an overall size of 30 µm × 60 µm (Figure 1b). Notably, the size of the 1D grating structure is very small, only 0.4% of the size of whole sample. It is difficult for the users to position the grating correctly when calibrating; therefore, a guidance area is designed at the periphery of the calibration area (Figure 1a). This area consists of a multi-level marker pattern pointing towards the center of the sample, which helps the users to identify the placement orientation of the sample and locate the calibration area rapidly, hence improving calibration efficiency greatly.

**Figure 1.** (**a**) Schematic of the guidance area of 1D grating standard. (**b**) Schematic of the calibration area of 1D grating standard. (**c**) Schematic of the linewidth regulation by ALD. (**d**) Schematic of the fabrication process of 1D grating standard. **Figure 1.** (**a**) Schematic of the guidance area of 1D grating standard. (**b**) Schematic of the calibration area of 1D grating standard. (**c**) Schematic of the linewidth regulation by ALD. (**d**) Schematic of the fabrication process of 1D grating standard.

### *2.2. Materials and Fabrication 2.2. Materials and Fabrication*

The substrate material of the standard is Si(100) wafer, while the grating material is Cr and Au. The Au with well wear resistance, stability and conductivity can be used to calibrate the measurement instruments that require the conductivity of the material, such as SEM, and will have an excellent contrast with the substrate. The material for the lin‐ ewidth regulation is an amorphous Al2O3 film deposited by ALD. The film growth mode corresponds to a self‐limiting chemical reaction between the chemical vapor‐phase pre‐ cursors and the substrate surface, in the ALD process. It is worthwhile to note that the number of reacting precursors on the surface does not increase further, when the surface chemisorption reaches the saturation. As a result, ALD controls the film growth accurately by adding single atomic layers one by one until the film thickness reaches a preset value, thereby ensuring 100% uniformity and conformity of the film. Therefore, a 1D nano‐grat‐ ing standard with controlled linewidth can be produced by depositing an Al2O3 film on the surface of the grating structure with a thickness that is half the deviation of the lin‐ The substrate material of the standard is Si(100) wafer, while the grating material is Cr and Au. The Au with well wear resistance, stability and conductivity can be used to calibrate the measurement instruments that require the conductivity of the material, such as SEM, and will have an excellent contrast with the substrate. The material for the linewidth regulation is an amorphous Al2O<sup>3</sup> film deposited by ALD. The film growth mode corresponds to a self-limiting chemical reaction between the chemical vapor-phase precursors and the substrate surface, in the ALD process. It is worthwhile to note that the number of reacting precursors on the surface does not increase further, when the surface chemisorption reaches the saturation. As a result, ALD controls the film growth accurately by adding single atomic layers one by one until the film thickness reaches a preset value, thereby ensuring 100% uniformity and conformity of the film. Therefore, a 1D nano-grating standard with controlled linewidth can be produced by depositing an Al2O<sup>3</sup> film on the surface of the grating structure with a thickness that is half the deviation of the linewidth (Figure 1c).

ewidth (Figure 1c). All the experiments were performed in a class 1000 clean room with a constant tem‐ perature of (25 ± 1) °C. The patterns of sizes 10–200 μm in the guidance area were fabri‐ cated by conventional micro‐fabrication processes including ultraviolet lithography and lift‐off process [26]. The specific fabrication and regulation process of the 1D grating struc‐ ture is demonstrated in Figure 1d. The sample was cleaned sequentially in acetone, iso‐ propanol (IPA) and deionized water. After that, a layer of polymethyl methacrylate (PMMA) photoresist AR‐P 679 with a thickness of 100 nm was spin‐coated on the sub‐ strate at 2000 rpm and baked for 2 min at 150 °C on a hot plate. Then, the one‐dimensional grating structure pattern was exposed on the photoresist layer using EBL (CABL‐9000C, Crestec, Hamamatsu, Japan). After the EBL process, the sample was developed in a mix‐ ture of methyl isobutyl ketone (MIBK) and IPA (1:3) for 1 min at 25 °C. Progressively, 5 nm Cr and 25 nm Au films were evaporated (TF500, Hind High Vacuum, Crawley, United Kingdom) on the sample, followed by the removal of remaining photoresist in dioxolane solution for 10 min at 25 °C. The sample was then cleaned with acetone, IPA and deionized water for 5 min each. Next, the grating structure was measured by AFM (INNOVA, Bruker, Karlsruhe, Germany) and the deviation between the designed dimension of lin‐ ewidth and the actual fabricated dimension was calculated. Finally, the three‐dimensional amorphous Al2O3 thin film was grown on the grating surface by ALD (R‐200, Picosun, All the experiments were performed in a class 1000 clean room with a constant temperature of (25 ± 1) ◦C. The patterns of sizes 10–200 µm in the guidance area were fabricated by conventional micro-fabrication processes including ultraviolet lithography and lift-off process [26]. The specific fabrication and regulation process of the 1D grating structure is demonstrated in Figure 1d. The sample was cleaned sequentially in acetone, isopropanol (IPA) and deionized water. After that, a layer of polymethyl methacrylate (PMMA) photoresist AR-P 679 with a thickness of 100 nm was spin-coated on the substrate at 2000 rpm and baked for 2 min at 150 ◦C on a hot plate. Then, the one-dimensional grating structure pattern was exposed on the photoresist layer using EBL (CABL-9000C, Crestec, Hamamatsu, Japan). After the EBL process, the sample was developed in a mixture of methyl isobutyl ketone (MIBK) and IPA (1:3) for 1 min at 25 ◦C. Progressively, 5 nm Cr and 25 nm Au films were evaporated (TF500, Hind High Vacuum, Crawley, United Kingdom) on the sample, followed by the removal of remaining photoresist in dioxolane solution for 10 min at 25 ◦C. The sample was then cleaned with acetone, IPA and deionized water for 5 min each. Next, the grating structure was measured by AFM (INNOVA, Bruker, Karlsruhe, Germany) and the deviation between the designed dimension of linewidth and the actual fabricated dimension was calculated. Finally, the three-dimensional amorphous Al2O<sup>3</sup> thin film was grown on the grating surface by ALD (R-200, Picosun, Masala, Finland). All the specific parameters of the ALD process have been described in our previous works [26].

Masala, Finland). All the specific parameters of the ALD process have been described in our previous works [26]. To study the controllability of the modulated linewidth by ALD, three 1D grating standards with a pitch of 1000 nm, named A, B and C, were fabricated in the experiment.

By depositing Al2O<sup>3</sup> films of 5, 10, and 15 nm thickness on the surfaces of samples A, B, and C, respectively, each side of the grating lines is expected to widen by 5, 10, and 15 nm, consequently increasing the width of grating lines by 10, 20, and 30 nm.

### *2.3. Measuremnt*

The 1D nano-grating standards were measured by the AFM in the tapping mode. The scanning range was selected at the center of the grating with a size of 10 µm × 10 µm, and the number of sampling points was selected to be 256. The data measured by AFM are inevitably interspersed with some low-frequency noise signals coupled with the profile data, which can lead to bowing distortion of the measured image. Further, there exists a certain cosine error between the sample and the measurement instrument, when the sample is placed on the measurement bench. To better extract the linewidth and pitch data of the standard, linear interpolation, filtering and cosine error correction were applied to the original measurement data, to effectively reduce the tilt, bowing and other low-frequency noise, while preserving the real surface topography of the standard.

### **3. Results and Discussion**
