Effects of Electron Beam Lithography Process Parameters on the Structure of Nanoscale Devices Across Three Substrate Materials

Abstract

Electron beam lithography (EBL) is a pivotal technology in the fabrication of nanoscale devices, renowned for its high precision and resolution capabilities. This paper explores the effect of EBL process parameters on various substrate materials, including silicon dioxide, silicon-on-insulator (SOI), and silicon nitride. We specifically investigate the impact of the charging effect and reveal the narrow exposure dose windows necessary to achieve optimal pattern fidelity. Based on the measurement results of linewidth, the relationship between exposure dose and the width of the structure pattern after development was analyzed. The optimum exposure dose window for each substrate is identified. Furthermore, through simulations of the charge effect, we demonstrate strategies for mitigating this effect on different substrates, even in complex structural configurations. Our findings contribute to enhancing the capabilities of EBL in semiconductor and insulator manufacturing and research.

1. Introduction

As Moore’s Law nears its physical limits, the semiconductor manufacturing industry is facing unprecedented challenges [1,2,3]. Electron beam lithography (EBL) has emerged as a pivotal technology capable of producing the highly precise masks necessary for today’s increasingly complex chip patterns [4,5,6,7]. Despite its relatively slow processing speed, EBL has carved out a niche for itself in high-precision mask making, small-batch customization of devices, and prototype development of micro/nano devices [8,9,10]. It is particularly well-suited and widely used in optoelectronic fields, such as for the sampling and small-scale production of electronic and optoelectronic chips, as well as in small-batch manufacturing for the grating industry, binary optics, micro/nano-optics, and meta-surfaces. EBL’s customizable processing capabilities have also contributed to a range of cutting-edge research areas, including new materials, advanced physics, photonics, biology, microelectronics, and micro-electro-mechanical systems (MEMSs). Therefore, EBL machines, along with etching and thin-film deposition equipment, which are related to key steps in semiconductor manufacturing, play a crucial role in advancing research on essential semiconductor manufacturing processes.

Therefore, EBL is employed for the fabrication of sub-micrometer and nanoscale structures on various substrates [11,12]. This method utilizes an electron-sensitive chemical, known as an electron resist or photoresist, that is exposed to a finely focused beam of electrons. Upon exposure, the chemical properties of the resist in the irradiated areas undergo a change. Through precise computer control of the electron beam’s movement and position, intricate patterns can be scanned onto the surface of the resist, enabling the direct transfer of these patterns to the underlying substrate. However, when the electron beam is working, the semiconductor [13,14] and insulator substrates [15,16,17] can become charged, and the charged area will generate an electric field that subsequently affects the electron beam. When EBL is used on semiconductor and insulator substrates, it is often affected by charging effects, which can lead to issues such as distortion and displacement in the lithography results. The impact of charging effects on pattern fidelity is multifaceted. In addition to beam deflection, charging can cause variations in exposure dose, leading to inconsistent feature sizes and edge roughness. These issues are particularly problematic for applications requiring high precision [18,19], such as the fabrication of photonic crystals, nanoscale sensors, or quantum devices [20,21]. To mitigate these effects, adjustments such as altering the electron acceleration voltage, increasing the resist thickness, or using more conductive substrate materials can be employed.

Optimizing the EBL process is crucial for improving efficiency and quality [22]. The exposure dose is a pivotal parameter influenced by several factors, including the type of EBL resist, the developer solution, the baking temperature, and the specifications of the electron beam exposure system. A variety of materials, such as PMMA [23,24,25], EBR-9 [26,27], PBS [28], and ZEP [29,30,31], are available, each possessing unique sensitivity and resolution properties suited to particular applications. Furthermore, different substrates display varying levels of charging effects, which can affect the choice and regulation of the exposure dose.

In this paper, we investigate the impact of exposure dose on the developed resist structure profile in EBL, as well as the influence of charging effects on different substrates. Simultaneously, based on simulations of the charging effect, we demonstrate methods for reducing the impact of charge on various substrates, including those with complex structures. The statistical results provide significant reference points for subsequent EBL parameter settings for the same substrate materials. EBL holds great promise for advancing nanotechnology and semiconductor manufacturing, making it an indispensable tool for researchers and manufacturers in these fields.

2. Experimental Equipment

The experiment utilized substrate (size: 2 cm × 2 cm) materials of silicon oxide, silicon-on-insulator, and silicon nitride. The processing was carried out using the EBPG 5150 (Raith BV, Best, The Netherlands) with acceleration voltage up to 100 kV and the PIONEER Two (Raith, Dortmund, Germany) with acceleration voltage up to 30 kV systems. The specific processing steps include sample preparation (cleaning and dehydration), photoresist spin coating, exposure, development, and testing. The EBL equipment used can precisely determine the exposure position and employs auxiliary software, eliminating the need for lithography masks. It can prepare line width patterns of 10 nm or less. Poly-methyl methacrylate (PMMA, 679.03), 6200.09 positive electron beam photoresist, and 7520.17 negative electron beam photoresist were used due to their high resolution and contrast, low sensitivity, higher acceleration exposure voltage, minimal swelling during development, and ease of stripping. The spin coating speed was 4000 r/min for 60 s. Subsequently, the samples were placed on a hot plate for pre-bake. The pre-bake conditions and resulting thicknesses for each substrate and photoresist combination were as follows: the 679.03 layer on silicon oxide was pre-baked at 150 °C for 60 s (thickness: 150 nm); the 6200.09 layer on silicon oxide was pre-baked at 150 °C for 120 s (thickness: 400 nm); and the 7520.17 layer on silicon oxide was pre-baked at 80 °C for 120 s (thickness: 800 nm). After the EBL process, an immersion development step is required, conducted at room temperature (21 °C). For the 679.03 resist, a developer (600-56) was applied for 30 s, followed by a stopper (600-60) for another 30 s. For the 6200.09 positive electron beam photoresist, the developer (600-546) was used for 45 s, followed by the stopper (600-60) for 45 s. For the development of the 7520.17 negative electron beam photoresist, a developer (300-47) was employed for 4 min, followed by a rinse with DI water as the stopper for 4 min. Finally, an optical microscope (Nikon LV150N, Tokyo, Japan) and scanning electron microscope (Zeiss Gemini 560, Oberkochen, Germany) were employed to measure the size of the resist profile after lithography.

3. Results

In EBL, the electron beam serves as the energy source. Substrate conductivity is crucial, as charging issues can arise, leading to quality problems like broadened or uneven exposure patterns. Charging effects occur due to several factors: insulating substrates accumulate charge upon exposure, thicker resist layers hinder charge conduction, and insufficient accelerating voltage reduces beam penetration, increasing charge accumulation. This paper analyzes how substrate materials, resist properties, and structural design influence the lithography process using three example substrates. Our insights aim to guide process design and material selection, helping researchers and manufacturers optimize EBL for higher quality and precision in nanostructures.

3.1. Analysis of Process Issues on Silicon Dioxide Substrates

To address the charging issues in EBL on insulating silicon oxide substrates, we conducted a comprehensive process analysis. Our experiments were conducted using a Raith5150 electron beam lithography system with a high acceleration voltage of 100 kV, capable of delivering high-speed electron beams to ensure precise exposure through photoresist layers of varying thicknesses. We selected 679.03 as the photoresist and applied it using a standard spin-coating technique, achieving a final thickness of 160 nm. Subsequently, we carried out exposure experiments on silicon oxide substrates (glass with a thickness of 160 μm) and examined the results using an optical microscope after development.

Figure 1 presents the structural appearances at various exposure doses: 200 μC/cm², 300 μC/cm², 400 μC/cm², and 500 μC/cm². Our observations revealed that at doses below 300 μC/cm², the structural manifestations exhibited a relatively light color. As the exposure dose increased, the color of these manifestations intensified and ultimately stabilized. The results reveal the existence of a specific dose threshold necessary for complete photoresist exposure. Only when the exposure reaches or exceeds this threshold, can the photoresist be fully exposed. Based on this crucial finding, we can now initially assess the sufficiency of the exposure by observing the intensity of the structural manifestations before proceeding with the subsequent steps. To boost characterization efficiency and rapidly identify the optimal exposure dose, we prioritized electron microscope measurements and detailed characterizations of structures exposed to a range of suitable doses. This targeted approach not only improves experimental efficiency but also allows us to rapidly identify the ideal exposure dose, providing strong support for optimizing the electron beam lithography process.

Initial observations from the optical microscope revealed that the photoresist was not fully exposed at an exposure dose of 200 μC/cm². Consequently, structures exposed at doses of 300 μC/cm² and above were further analyzed using a scanning electron microscope (SEM). The linewidth after exposure at four different doses was measured, with SEM images presented in Figure 2 and detailed data on linewidth and exposure dose on the silicon oxide substrate provided in

Table 1. Line width and exposure dose after development process on a silicon oxide substrate.

No.	Exposure Dose (μC/cm2)	Developing Line Width (nm)
1	200	183
2	250	182
3	300	185
4	350	191
5	400	204
6	450	202
7	500	207
8	550	218
9	600	223
10	650	225

. Analysis of the SEM images showed that at an exposure dose of 300 μC/cm², the developed structures were close to the designed target size of 200 nm, with an actual measurement of 185 nm. As the exposure dose increased to 400 μC/cm² and 500 μC/cm², the structure sizes more closely approximated the design value, measuring 204 nm and 207 nm, respectively. However, at an exposure dose of 600 μC/cm², the structures exhibited excessive widening, with a measurement of 223 nm. We have included an error margin of ±2 nm for all line width measurements reported in Table 1, which was determined by repeated measurements. These findings provide valuable experimental data for understanding the effect of exposure dose on structure size in the EBL process and strongly support subsequent optimization efforts.

Incorporating the data from Table 1, it is evident that a dose window of 350 to 400 μC/cm² results in structure sizes that align with the design specifications. Below this range, the developed structure sizes are smaller than the designed dimensions, while doses exceeding this range produce larger structures. To achieve the exact designed structure size more precisely, additional exposure and characterization can be conducted within this dose window to determine the precise exposure dose. In practice, it is observed that the silicon oxide substrate exhibits a strong charging effect during exposure, subjecting the electron beam to complex electric field influences and narrowing the effective dose window. This charging effect is evident in the field emission SEM images shown in Figure 2, where numerous bright regions indicate charge accumulation. Consequently, materials with pronounced substrate charging effects often present challenges in identifying an appropriate exposure dose during EBL process development. Furthermore, changes in the structure design and the type of photoresist used can significantly impact the charging effect’s intensity, thereby influencing the optimal exposure process parameters.

3.2. Analysis of Process Issues on Silicon-on-Insulator (SOI) Substrates

Silicon-on-insulator (SOI) substrates are equally prone to charging effects during electron beam lithography. The SOI substrates used in our study feature a top layer of 220 nm silicon and an insulator layer of 2 μm silicon oxide on a silicon substrate. This structural configuration is significant because when the electron beam strikes the photoresist, it causes localized charge accumulation. Although the surface silicon layer can conduct away some of the charge, the intervening insulating layer hinders the prompt dissipation of charge, leading to a potential difference between the photoresist and the substrate. This potential difference can disrupt the normal exposure path of the electron beam, resulting in distortions of the exposed structures. A set of line structures was designed for exposure on an SOI substrate to investigate and analyze the associated process issues. For this task, we used a system with a 100 kV acceleration voltage, which provides a high-velocity electron beam capable of penetrating various thicknesses of photoresist. The selected photoresist, 6200.09, was spin-coated to a thickness of 200 nm. Exposures were then carried out on the SOI substrate under the specified conditions. The developed structures were subsequently characterized using a field emission microscope.

Figure 3 displays the structures exposed at various doses: 200 μC/cm², 220 μC/cm², 240 μC/cm², 260 μC/cm², 280 μC/cm², and 300 μC/cm². As mentioned earlier, there is a threshold for fully exposing the photoresist, which can be discerned through intensity differences in optical microscope images. When examined under a field emission SEM, the morphology of underexposed photoresist becomes even more evident, as depicted in Figure 3a–c. These images reveal that underexposed structures retain photoresist residues in a net-like form at the exposure sites. As the exposure dose increases, the density of this net-like photoresist diminishes, indicating the gradual breakdown of cross-links within the photoresist.

Furthermore, as shown in Figure 3d–f, although the photoresist network in the exposed structures largely disappears, significant photoresist protrusions remain at the edges of the structures, resulting in irregular edges that do not meet the requirements for lithography. This phenomenon can be simply explained by the fact that the edges of the lithography pattern often require a higher exposure dose than the central areas to achieve full exposure. This is primarily due to the secondary electrons generated when the lithography electron beam strikes the material surface, which also contribute to the exposure of the photoresist. Consequently, the central areas of the exposure pattern are more likely to be affected by a larger number of secondary electrons, while the edges are less so. As a result, when the same dose is applied, the edges are often underdosed. It is necessary to adjust the dose to find the optimal dose for the entire structure.

Figure 4 presents the structures that were exposed to higher doses: 320 μC/cm², 340 μC/cm², 360 μC/cm², 380 μC/cm², and 400 μC/cm². These structures were characterized using field SEM. The results show that there is almost no photoresist residue in the center of the structures, indicating that the photoresist was completely exposed at these doses.

In conjunction with

Table 2. Line width and exposure dose after development process on an SOI substrate.

No.	Exposure Dose (μC/cm2)	Developing Line Width (nm)
		230	190	100
1	260	229	175	57
2	280	227	179	59
3	300	228	181	93
4	320	224	183	94
5	340	224	188	94
6	360	228	185	94
7	380	230	189	100
8	400	228	186	101

, it is evident that a dose of approximately 380 μC/cm² produces structures that align closely with the design specifications for all three line widths. The effective dose windows vary among the lines: 300 to 380 μC/cm² for the 230 nm line, 340 to 380 μC/cm² for the 190 nm line, and 380 to 400 μC/cm² for the 100 nm line. These differences in effective dose windows are attributed to the influence of secondary electrons during exposure. Specifically, the smaller 100 nm line, with its shorter exposure time and smaller area, experiences less impact from secondary electrons, thus requiring a higher exposure dose compared to the 230 nm and 190 nm lines.

Based on the above results, we can summarize the characteristics of SOI substrate exposure as follows: the presence of an insulating layer within the SOI substrate leads to persistent substrate charging during exposure, which influences the exposure process parameters. Consequently, methods effective in mitigating charging in silicon oxide substrates are also applicable to SOI substrates. Moreover, our structural design results indicate that different structure sizes may necessitate varying exposure doses to achieve optimal outcomes. Particularly when substrate charging is more pronounced, fine-tuning the dose for each structure size may be essential. Additionally, since different structure sizes are grouped together and can influence each other via secondary electrons, it is vital to maintain the original design when testing a set of structures of varying sizes. This approach ensures that the mutual influence between structures is taken into account, enabling the accurate determination of appropriate exposure process parameters.

3.3. Analysis of Process Issues on Silicon Nitride Substrates

Silicon nitride substrates are widely employed in integrated laser beam combiners (ILBCs) due to their extensive range of transparent wavelengths, low nonlinearity, high optical power handling capability without degradation, and exceptional thermal and mechanical stability. In research contexts, these devices often undergo processing via EBL. However, the unique properties of silicon nitride present inherent challenges in the processing technique. Specifically, the selection of a photoresist that is compatible with the silicon nitride substrate is critical. Various photoresists may exhibit differential adhesion, etch resistance, and development efficiency when applied to silicon nitride. Incompatibility between the photoresist and substrate can result in issues such as pattern delamination, distortion, or reduced resolution during the lithography process. Furthermore, achieving superior lithographic morphological characteristics on the sidewalls of silicon nitride materials, which are intended to function as waveguides post-lithography and etching, is vital for ensuring the optimal performance and long-term reliability of ILBCs in practical applications.

To investigate and analyze the processing issues, a line structure was designed for exposure on a silicon nitride substrate, which consist of a top layer of 700 nm silicon nitride on silicon substrate. An electron beam with an acceleration voltage of 100 kV was utilized to generate a high-speed electron beam for lithography, enabling penetration through various thicknesses of photoresist during exposure. Given that silicon nitride is commonly used in research for the fabrication of waveguide structures, it was essential to retain the photoresist as a mask in the exposed areas while etching the unexposed regions. Therefore, negative photoresist 7520.17 was chosen for the exposure process. Following the standard spin-coating procedure, a photoresist thickness of 800 nm was achieved. Subsequently, the silicon nitride substrate was exposed, and after development, select structures were characterized using an optical microscope, as depicted in Figure 5. The exposure doses for the structures ranged from 500 μC/cm² to 850 μC/cm² in 50 μC/cm² increments. Compared with Figure 1 through intensity differentiation, these eight structures, exposed to different doses, exhibited a uniform color, indicating that they have been sufficiently exposed. Moreover, Figure 5 presents a wider range of exposure doses (500 μC/cm² to 850 μC/cm²) than those used in Figure 1 and Figure 2 on the silicon oxide substrate.

Further characterization was performed on structures exposed to intermediate doses, as shown in Figure 6, which displays side-view images of the exposure structures on a silicon nitride substrate using negative photoresist. The exposure doses ranged from 600 μC/cm² to 800 μC/cm² in 50 μC/cm² increments. Both optical microscope images and surface dimension measurements of the exposed structures indicate that the structural sizes are near the designed 250 nm. However, the steepness of the sidewalls of the lithographic structures is a crucial performance indicator, as these structures will undergo etching to form the silicon nitride structures.

Upon examining the side-view images of the lithographic structures, especially the morphological changes near the substrate, it is clear that at an exposure dose of 800 μC/cm², the structural dimensions remain consistent from top to bottom, with the sidewall steepness approaching 90°. This dose not only ensures that the exposed structural dimensions meet the requirements but also delivers the best sidewall quality. Therefore, in conjunction with the data in

Table 3. Line width and exposure dose after development process on a silicon nitride substrate.

No.	Exposure Dose (μC/cm2)	Developing Line Width (nm)
1	500	238
2	550	237
3	600	242
4	650	245
5	700	249
6	750	248
7	800	251
8	850	253

, it can be concluded that, although the developed structural dimensions are close to the designed dimensions within the dose range of 700 μC/cm² to 850 μC/cm², the optimal process exposure dose, based on sidewall characterization, is 800 μC/cm².

Compare the dose windows of the three materials mentioned above. Insulating materials (e.g., glass, SOI) showed narrow dose windows that demand precise dose control to mitigate edge roughness caused by charge accumulation. Semiconducting materials (e.g., SiN) exhibited wider dose windows that enable the use of a broader range of doses to enhance resolution. In addition, insulating materials (e.g., sapphire, quartz) displayed similar narrow dose windows (e.g., 20–25 μC/cm²) [32] and conductive/semiconducting materials (e.g., Si, GaN) showed wider dose windows (e.g., 20–45 μC/cm²) that allow for higher doses [33]. These findings directly reduce trial-and-error costs in manufacturing processes, improving yield rates for photonic crystals or quantum dot devices. Furthermore, dose sensitivity disparities among materials are useful for the material selection and device design. In the future, leveraging substrate conductivity-dose window correlations will enable real-time feedback systems. Therefore, testing and finding the dose windows are helpful for optimizing EBL processes.

4. Conclusions

In this paper, we explore the influence of EBL process parameters on the structure of nanoscale devices across different substrate materials. The study covers fundamental principles, practical experiences, and common challenges in EBL fabrication, with a focus on proper photoresist handling, mitigation of charging effects, and precise exposure dose control. Experimental analysis reveals narrow exposure dose windows essential for achieving high-quality material patterning. Charge effects in certain materials, such as SOI substrates, can affect the accuracy of the exposed structures, necessitating meticulous control and optimization strategies. Beyond semiconductor processing, electron beam lithography shows potential for applications in biomedicine, optics, sensors, and beyond. By refining production processes, reducing costs, and enhancing efficiency, electron beam lithography can significantly contribute to the advancement of semiconductors and related industries. The findings offer insights for optimizing EBL processes to improve the precision and quality of nanostructure fabrication.