The Evolution of Mercury Lamps in Photolithography
By J. Jason Petty | President, Specialty Optical Systems
This resource is written for engineers, cleanroom professionals, operations teams, and procurement leaders who support semiconductor manufacturing environments where g-line and i-line lithography still play a role today. Many of these processes are inherited rather than newly developed, and the reasons behind long-standing practices are not always well documented. Understanding how mercury lamps evolved—and why they were trusted for so long—helps explain how modern fabs learned to operate with stability, predictability, and scale.
Photolithography has always been a discipline defined by control. While the industry today often focuses on advanced nodes, EUV, and next-generation materials, much of the semiconductor manufacturing foundation was built on technologies that proved reliable, predictable, and scalable over long periods of time. Among the most important of those technologies was the short-arc, high-pressure mercury lamp used in g-line and i-line photolithography.
For decades, mercury lamps were not simply a component inside a stepper or aligner. They were a critical enabler of production stability. The industry standardized around specific wavelengths—most notably 436 nm and 365 nm—because they could be generated consistently and paired with photoresists and optical systems that delivered repeatable results. Once that ecosystem formed, mercury-based illumination became the backbone of mature photolithography.
As lithography tools evolved, expectations placed on the lamp evolved as well. Early on, the emphasis was largely on power and throughput. Fabs needed enough light to expose wafers quickly and consistently, and lamp development followed that demand. Over time, however, fabs and photo engineers recognized that brightness alone was not sufficient. What mattered just as much was how a lamp behaved over weeks and months of continuous operation.
This realization marked an important phase in the evolution of short-arc mercury lamps. Design and manufacturing improvements increasingly focused on stability over life, usable operating windows, and consistency from lamp to lamp. These changes had a direct impact on production. As lamps became more predictable, fabs were able to extend preventative maintenance intervals, reduce unplanned downtime, and lower overall cost of ownership. The value was not in simply making lamps last longer, but in making them easier to plan around.
Throughout this period, mercury lamps were used across a wide range of lithography platforms and applications. Whether supporting ASML PAS systems, Nikon and Canon i-line steppers, Ultratech packaging tools, or Karl Suss aligners, the relationship between lamp behavior and process control remained consistent. When illumination was stable, critical dimensions were easier to manage. When it was not, engineers often spent valuable time troubleshooting symptoms elsewhere in the process before identifying illumination as the root cause.
One common production scenario illustrates this well. A tool would continue to meet exposure dose targets, yet engineers would notice increasing calibration frequency or subtle shifts in CD uniformity across the wafer. In many cases, the lamp had not failed and was still within rated life, but its behavior was no longer predictable enough to support stable operation. Over time, fabs learned to treat these signs as early indicators and plan relamps proactively rather than reactively. Much of this practical knowledge was developed by photo engineers, equipment technicians, and operations teams solving real production problems on the fab floor.
My own career has followed this evolution closely. I began working with Ushio mercury lamps as a distributor, supporting high-volume semiconductor manufacturers where exposure tool uptime was non-negotiable. One of the most instructive experiences during that time was supporting Texas Instruments through Supplier Managed Inventory programs. In that role, lamp availability, forecasting, and responsiveness were as critical as the product itself. Managing inventory around tool usage and maintenance schedules helped eliminate lamp-related downtime and ensured continuity of production.
That operational focus resulted in Specialty Optical Systems receiving eleven Texas Instruments Supplier Excellence Awards, recognition tied directly to performance, reliability, and sustained uptime. Those experiences reinforced an important lesson: in a fab environment, reliability and predictability matter as much as optical specifications.
Today, my role has shifted, but the focus remains the same. I currently support Ushio customers across the entire Eastern United States, while our Vice President of Sales, Terry Nelson, covers Texas. Texas is undergoing a significant transformation, with large-scale semiconductor fabs being built and expanded across the state. This growth positions Texas as one of the most important semiconductor manufacturing regions in the world and represents a major step forward for domestic chip production.
Even as lithography technology advances and leading-edge processes move to laser-based and EUV light sources, mercury-lamp-based photolithography remains firmly in place for many applications. G-line and i-line processes continue to support analog devices, power electronics, MEMS, sensors, and advanced packaging—environments where reliability, cost control, and throughput are critical. The operational discipline developed around these systems, including how illumination is monitored, maintained, and planned for, continues to inform best practices today.
At the same time, the semiconductor market is entering an exciting new chapter. While mercury lamps may no longer be part of the equation at the leading edge, the momentum around fab construction, supply chain investment, and U.S. manufacturing capability is stronger than it has been in decades. The industry’s ability to scale to this point was built on technologies that emphasized control, repeatability, and long-term thinking.
SOSCleanroom maintains technical resources like this not to promote specific products, but to help customers understand the practices and decisions that shaped modern semiconductor manufacturing. Preserving the history of mercury lamps in photolithography is not about looking backward. It is about understanding how the industry learned to operate reliably at scale—and why those lessons still matter as the technology continues to evolve.
Photolithography Glossary for Non-Photo Engineers
Practical definitions used in cleanroom operations and semiconductor manufacturing support.
Photolithography terms are often used together because they are connected. Changes in illumination can affect dose, which influences critical dimension (CD), which can impact uniformity and yield. Those outcomes often drive preventative maintenance (PM) and relamp decisions.
Critical Dimension (CD)
Critical Dimension, commonly referred to as CD, describes the measured width or size of a patterned feature on a wafer that is essential to device performance. In practical terms, it is the dimension that must stay within a tight tolerance for the device to function correctly. If CDs become too large, too small, or inconsistent across the wafer, electrical performance and yield suffer. Because photolithography defines these features, CD control is one of the primary measures of process health in a fab.
Dose
Dose is the amount of light energy delivered to the photoresist during exposure. It is typically controlled by adjusting lamp power and exposure time. From a production perspective, dose must be consistent and repeatable. Too little dose can result in incomplete pattern formation, while too much dose can cause features to shrink or distort. Dose drift over time is one of the earliest indicators that an exposure system—or its light source—is no longer behaving predictably.
Uniformity
Uniformity describes how evenly exposure energy is delivered across the exposure field and across the entire wafer. Good uniformity means every die sees essentially the same exposure conditions. Poor uniformity leads to variations in CD from the center to the edge of a wafer or from die to die. In practice, uniformity problems often show up as yield loss even when average CD measurements appear acceptable.
Preventative Maintenance (PM)
Preventative Maintenance, or PM, refers to planned maintenance activities performed to keep equipment operating within specification and to avoid unplanned downtime. In lithography, PMs often include lamp replacement, system calibration, optical inspection, and verification runs. PM schedules are influenced by tool manufacturer recommendations, lamp performance characteristics, and fab experience. Well-planned PMs help protect throughput, CD control, and yield.
Relamp
Relamping is the process of replacing the exposure lamp in a lithography tool. Unlike routine maintenance, relamping is treated as a controlled process event because it can affect exposure behavior, CD control, and uniformity. Relamps are typically scheduled based on lamp behavior trends rather than failure, allowing fabs to minimize disruption and maintain stable operation.
Throughput
Throughput refers to how many wafers a tool or process can complete in a given period of time. In lithography, throughput is influenced by exposure time, lamp intensity, calibration requirements, and maintenance frequency. Stable illumination helps tools operate at higher throughput without sacrificing control.
Process Window
The process window describes the range of operating conditions—such as dose and focus—where acceptable CDs can be reliably produced. A wide process window provides manufacturing flexibility, while a narrow window increases sensitivity to small variations. Stable exposure sources help maintain a usable process window over time.
Educational use notice
This article is provided as an educational resource. Practices and requirements may vary by tool platform, manufacturer, and facility. Always follow applicable OEM guidance and site-specific procedures.
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