Explanation of Stepper Lenses and Aberrations in Semiconductor Lithography

In today’s era of advancing semiconductor miniaturization, the performance of exposure equipment has become increasingly critical. Among these, the projection lenses installed in stepper systems play a key role in determining the quality of pattern transfer. Achieving high resolution requires not only refined optical system design and structure but also thorough management of factors such as optical aberrations.

This article explains the fundamentals of steppers and projection lenses, which form the basis of the semiconductor exposure process, and examines in detail how aberrations affect exposure accuracy and the techniques used to minimize them. By understanding the mechanisms that cause aberrations, one can gain a clearer insight into the sophistication of optical design and the cutting-edge technology behind fine pattern formation.

Furthermore, we introduce the latest trends in stepper lens technology developed to meet diverse needs, such as processing small substrates and power devices. This includes how high-speed handling of small substrates and enhanced lens durability are achieved, providing a perspective on the future direction of semiconductor manufacturing.

Fundamentals of Steppers and Projection Lenses

Understanding the fundamental concepts of steppers and projection lenses, which are widely used in semiconductor manufacturing, is essential for gaining an overview of the exposure process.

A stepper is an exposure system used in photolithography to accurately transfer mask patterns onto a wafer. This process, which prints the circuit patterns that form the outline of semiconductor devices, is crucial in the production of increasingly high-precision components. To achieve precise pattern reduction while meeting the demands of mass production, both the reliability and processing speed of the stepper are critical.

Steppers incorporate numerous optical elements, with the projection lens at the core of the system. The performance of the projection lens directly affects pattern resolution, positional accuracy, and shape distortion, necessitating highly sophisticated design. Inside the lens barrel, multiple lens elements are precisely arranged to optimize the light path and minimize aberrations.

In recent years, the use of different light source wavelengths and high numerical apertures (NA) has enabled even finer pattern formation. At the same time, as device orientations become increasingly diverse, design considerations such as the variability and long-term stability of projection lenses have become indispensable.

The History and Function of Steppers in Semiconductor Manufacturing

Steppers began to gain rapid adoption in the late 1970s and have evolved alongside photolithography technology. In the early stages, they had low resolution and could only create relatively large patterns. However, advances in high-precision lens development and shorter-wavelength light sources have enabled higher speed and finer patterning, contributing significantly to the increased capacity of semiconductor chips.

The defining feature of steppers lies in their sophisticated mechanism for accurately transferring photomask patterns onto wafers. With the advancement of mask-to-wafer alignment technology, extremely small pattern dimensions can now be formed with minimal misalignment, enabling the production of highly integrated devices.

Additionally, because the exposure process requires processing large numbers of wafers in a short time, stepper productivity is a critical concern. The adoption of high-speed wafer stage control and automation systems has improved throughput while ensuring the stable formation of fine patterns.

Overview of Projection Lens Types and Their Features

Projection lenses can be broadly classified into two types: reduction projection lenses and one-to-one projection lenses. The mainstream today is reduction projection lenses, which can project large mask patterns onto a wafer at a reduced scale, enabling advanced miniaturization. One-to-one projection lenses, on the other hand, are typically used in specialized applications or research and development where extremely high precision is required.

In achieving higher resolution, increasing the numerical aperture (NA) is crucial. However, simply enlarging the NA can lead to increased optical aberrations, such as spherical aberration. To mitigate this, multi-element lens designs and specialized coatings are employed, combining multiple lenses and advanced optical technologies to reduce aberrations.

Recently, equipment using deep ultraviolet (DUV) or extreme ultraviolet (EUV) light sources has emerged to achieve even shorter wavelengths. In these systems, lens materials and configurations differ significantly from traditional designs, and the types and amounts of aberrations that must be controlled also change. As a result, at the forefront of projection lens development, design, manufacturing, and metrology are closely integrated, driving continuous advancement.

What Are Aberrations? Optical Factors Affecting Exposure Accuracy

Aberrations are closely related to the performance of projection lenses, which are critical for the precision of steppers. Controlling aberrations from the design stage is essential for achieving high-accuracy exposure.

Aberrations refer to the general deviations between the image that would be formed under ideal optical transmission and the actual image produced. They arise from a combination of factors such as lens shape and arrangement, as well as the wavelength of the light source, and can result in blurring or distortion of patterns. In semiconductor lithography, where fine circuit patterns are formed, even slight aberrations can directly affect device quality, making careful control imperative.

The primary types of aberrations include spherical aberration, coma, and astigmatism, each affecting image formation differently. Additionally, when aiming for uniform pattern transfer across the entire projection field, the distribution of aberrations must be carefully managed to avoid biases between the center and peripheral regions. Correction becomes increasingly complex as the numerical aperture (NA) of the lens increases.

Thorough management of aberrations is also important for minimizing variations between exposure tools, ensuring smooth wafer production across multiple machines. Even if minor pattern misalignments occur due to slight lens distortions, calibration and alignment techniques can correct them, allowing for consistent product quality in factories using multiple exposure systems.

Main Types of Aberrations and Their Mechanisms

Spherical aberration occurs when light passing through the outer regions of a lens focuses at a different point than light passing through the center, causing image blurring. Coma arises when off-axis point sources are distorted into comet-like shapes, with the effect becoming more critical as the lens numerical aperture increases. Astigmatism is a phenomenon where the focal points differ in the vertical and horizontal directions, making it difficult to transfer patterns uniformly.

These aberrations result from a variety of factors, including lens shape, material homogeneity, the spectral characteristics of the light source, and alignment with the back-end system. In modern steppers, intricate designs are employed to optimize overall performance while intentionally leaving minimal aberrations in certain cases, balancing fine patterning with productivity.

Additionally, aberrations in the illumination optics and deviations in wafer flatness can compound these effects. Therefore, comprehensive optical system adjustment across the entire exposure tool is essential, and the ability to suppress aberrations effectively is a key competitive advantage in semiconductor lithography.

Design and Correction Techniques for Reducing Aberrations

A major contributor to aberration reduction is the multi-element lens design, in which multiple lens groups are optimally arranged. By combining not only spherical lenses but also aspherical lenses and materials with different refractive indices, multiple types of aberrations can be corrected simultaneously. In addition, specialized coatings are applied to lens surfaces to control transmittance for different wavelengths, further suppressing aberrations.

Beyond the lens design itself, dynamic correction technologies integrated across the entire stepper have advanced. Alignment systems linked to the wafer stage and automatic calibration functions enable real-time distortion adjustments, minimizing the impact of aberrations.

These technological innovations have significantly improved exposure accuracy and contributed to productivity gains, such as increased wafer throughput per hour. Consequently, even when using the same model of exposure equipment, differences in pattern transfer quality can arise depending on the precision of lens design, alignment, and aberration correction techniques.

Key Points of Stepper Lenses in Semiconductor Lithography

Stepper lenses play a crucial role not only in achieving high resolution for accurate pattern transfer but also in controlling defect suppression and alignment precision.

A key aspect of managing stepper lenses is ensuring high-resolution reduction projection while preventing defects caused by foreign particles or dust. Microscopic particles on lens surfaces, as well as deterioration of coatings designed for short-wavelength light sources, can significantly affect pattern quality.

In addition, the exposure process involves stacking multiple layers, and even slight misalignment between layers can negatively impact overall yield. By implementing advanced coordinated control between the stepper lens and wafer stage, inter-layer positional deviations can be minimized, enabling the mass production of high-quality devices.

To manage these factors comprehensively, fabs commonly track the characteristics of each stepper and share and manage correction data. Regular measurements of lens aberration characteristics and distortion distributions across tools, followed by appropriate calibration, allow for stable manufacturing over extended periods.

High Resolution via Reduction Projection and Suppression of Defective Exposure

The reduction projection method transfers patterns on a mask to the wafer at a reduced scale, such as one-quarter or one-fifth, enabling extremely high resolution down to very fine line widths. However, achieving higher resolution requires increasing the numerical aperture (NA), and as NA increases, correcting aberrations in the lens system becomes more challenging, making highly precise optical design essential. As a result, not only the line width but also the clarity of pattern edges is critically important.

To ensure high productivity, tuning must be applied not only to the lenses and exposure light source but also across the entire process. For instance, in high-utilization equipment, the risk of lens contamination increases, potentially causing defective exposures. Therefore, operational management such as lens surface protection and regular cleaning is indispensable for maintaining high precision.

Through these combined design and operational efforts, defective exposures of fine patterns are effectively suppressed. Improving yield requires coordination across the entire production line, with aberration management of stepper lenses playing a central role.

Alignment Technology and Wafer Stage Control

To maximize the performance of stepper lenses, high-precision control of the wafer stage is indispensable. Even micrometer-scale deviations in the stage that moves the wafer can result in significant errors in the projected image, potentially causing circuit defects. Therefore, precision positioning systems using laser interferometers and similar technologies are implemented.

In addition, real-time alignment systems coordinated with the stepper lens detect and correct minute shifts caused by vibrations or temperature variations during exposure. This enables stable pattern transfer even during continuous wafer processing. Corrections are sometimes performed at the scale of a few nanometers, supported by advanced control technologies.

In recent years, higher throughput demands have further emphasized the challenge of balancing increased stage speed with control precision. Even slight stage vibrations or particle intrusion can affect quality, making it essential to maintain vacuum environments and clean processing conditions alongside advanced stage control.

Support for Small Substrates and Power Devices, and Newly Developed Lenses

In the increasingly diversified semiconductor market, demand is growing for smaller substrates and highly durable power devices, driving continuous evolution in lens design. Recently, stepper lenses capable of exposing small-diameter wafers and glass substrates have attracted attention, requiring stable, high-speed pattern transfer while maintaining high reduction ratios and minimizing aberrations. In the field of power devices, flexible handling of substrates made from materials such as SiC and GaN, across a wide range of thicknesses and sizes, is also crucial.

These newly developed projection lenses achieve both high transmittance and durability, contributing to improved productivity through shorter exposure times and reduced aberrations. Compared to conventional models, they significantly reduce lens aberrations and accommodate a broader range of substrate sizes.

Moreover, the new lens designs and process technologies form a foundational support for innovative fields such as power devices and green devices. Going forward, numerical apertures, materials, and optical coatings will continue to be optimized to meet diverse device needs, further enhancing precision and productivity.

Summary

Stepper lenses and aberration management technologies are critical factors that directly impact the quality and yield of semiconductor manufacturing.

High-precision lens design not only enables the formation of high-resolution patterns but also contributes significantly to defect reduction and cost efficiency. In addition, advancements in alignment technology and wafer stage control help minimize variations in ultra-fine regions, thereby enhancing the efficiency of mass-production lines.

The development of next-generation steppers is expected to combine innovations in lens design with systems capable of real-time aberration correction. In particular, addressing the expanding market for smaller substrates and power devices requires overcoming technical challenges related to design diversification and enhanced lens durability.

To support increasingly complex semiconductor devices, further refinement of stepper lenses and advanced aberration control will be indispensable. These technologies will continue to play a central role in underpinning the growth of the semiconductor industry and will remain a focal point at the forefront of research and development.