“As the semiconductor industry continues to push the limits of shrinking design dimensions, the use of extreme ultraviolet (EUV) lithography is gradually expanding into mass production environments. For advanced nodes of 7nm and below, EUV lithography is a supporting technology that simplifies the patterning process. Ultra-clean masks are essential for reliable molding at such fine dimensions.
As the semiconductor industry continues to push the limits of shrinking design dimensions, the use of extreme ultraviolet (EUV) lithography is gradually expanding into mass production environments. For advanced nodes of 7nm and below, EUV lithography is a supporting technology that simplifies the patterning process. Ultra-clean masks are essential for reliable molding at such fine dimensions.
As with all masks, masks for EUV lithography rely on mask pods for safe storage, as well as protecting them from lithography patterning, inspection, cleaning and repair. The protective reticle must last for many years without causing harmful contamination or physical damage.
A reticle box designed for 193nm immersion lithography does not provide adequate protection for EUV masks. The unique requirements of EUV lithography place additional constraints and requirements on the reticle, making the EUV reticle pod a highly specialized device with several key components.
This article describes the challenges inherent in designing reticle pods for EUV lithography and proposes solutions to enable more fabs to adopt advanced lithography nodes in their fabs.
Protect EUV mask
The finer the lithography pattern, the higher the risk of mask contamination. Potential sources of contamination include foreign particles and chemical residues. The mask coating is fragile and easily damaged. Anything that touches the mask can cause damage, whether it’s an expected process part (e.g., a robotic arm in a fab), or accidental contamination (e.g., human hair).
Immersion lithography uses a thin film as a “dust shield” to protect the mask from particle contamination during pattern exposure. The films need to be optically transparent, which means they must be transparent to light in the EUV spectrum at wavelengths around 13.5 nanometers, from an EUV lithography perspective. Most existing thin-film materials absorb EUV light, but the semiconductor industry has begun to adopt EUV-specific films (see Figure 1).
Before thin films became the standard for EUV lithography, EUV reticle pods needed to protect masks without added thin films. An NXE tool for EUV lithography requires a dual pod configuration, including an inner metal pod that is under vacuum and an outer pod that is in contact with the surrounding environment. The inner reticle box will only open when it is inside the machine equipment.
The dual reticle configuration is standard practice for EUV lithography, and such pods are commercially available. Although they are readily available, they cannot be considered a commodity for that reason. EUV pod designs (see Figure 2) are continually being improved to meet performance and lithography yield requirements.
Although the dual reticle configuration provides protection, the potential for contamination remains high. Therefore, how to reduce the risk of contamination must be considered when developing EUV pods. Especially for masks with no added films, the inner reticle serves as the primary protection, but is also a major source of potential contamination.
Design considerations for pods include the geometry of the inner and outer pods and the materials they are made of.
All surfaces that touch or surround the mask, including those of the reticle, must be kept ultra-clean to avoid introducing harmful contaminants in the form of particles or airborne chemical vapors.
Polymer outgassing can create undesirable chemical contaminants, and they can deposit on the mask surface. Therefore, pod materials should be selected with the goal of minimizing the potential for outgassing. The inner reticle box is made of metal and does not outgas. However, the outer pods are made of polymers, just like the single-mask pods used in non-UV lithography.
The practice of shrinking design guidelines makes a contamination-free environment even more important, as even small contamination particles have a high potential to cause pattern transfer errors and yield loss.
Ensuring effective mechanical protection
Reticle pods must properly protect the masks in them during intra-fab and inter-fab transportation, such as from mask fabs to integrated device manufacturers (IDMs) by air or ground. There is a microsecond balance between the need to not only protect the mask in the reticle, but also to minimize mechanical damage caused by excessive contact forces. If the resistance is too low, the mask will not be able to withstand the mechanical acceleration and vibration during transport and will be damaged.
If the resistance of the contact pad is too high, it will cause excessive contact marks on the mask. If the glass at the edge of the mask is scraped off, the glass particles can become contaminants that cause lithography defects. From the point of view of particle generation, the smaller the contact force, the better.
As long as the pod can hold the mask securely in place, the fewer contact points, the less likely the pod will cause particle contamination. The size of the touchpoint is equally important. The larger the contact surface, the lower the contact stress when the pod is closed.
The choice of pod material is also critical to minimize contact marks. The ideal pod material is resistant to wear when securing the mask and opening and closing the pod.
Clean the mask box
The outer reticle box needs to be periodically purged to remove moisture inside and maintain a dry environment for the mask. Purging gas (extremely clean dry air (XCDA) or nitrogen) enters the outer reticle box through the air inlet for purging.
While most of the gas exchange during purge occurs in the outer reticle, some gas does flow into and out of the inner pod during purge or vacuum pumping and exhaust within the NXE tool. Filters are built into the inner pod to exchange gas molecules and minimize particles entering the inner pod. When closed, the inner reticle box is also sealed so that almost all gas exchange is done through the filter rather than any leaks in the seal.
In an ideal design, the filter conductivity (a measure of the ability of gas to flow through the filter) should be significantly higher than the seal conductivity, so that at least 90% of the air entering the inner reticle is through the filter.
The filter in the inner reticle must be sufficiently permeable to allow adequate airflow in, yet strong enough to withstand the force of cleaning. To achieve the proper balance, careful selection of filter material and geometry is necessary.
If particles enter the mask through the seal between the cover plate and the substrate, the gap size between the mask and substrate should be as small as possible to ensure that the particles are at the outer edges of the mask and not transfer to areas where yield may be lost. Active area (see Figure 3).
Much of the lithography process is automated, and the robotic arm handling the reticle always needs to match the pod size. There is little room for adjustment in size, and the reticle must be compatible with standard mechanical interfaces. The pods are designed to last seven to ten years, so new equipment needs to be backward compatible with existing pods.
The long life expectancy of reticle pods means they experience thousands of on/off cycles over many years. The use of anti-abrasion materials minimizes particle contamination while extending the life of the pod.
The optical window of the reticle must be compatible with automated equipment. The camera in the lithography machine needs to be able to observe the inside of the reticle in order to detect the mask condition correctly, which imposes strict requirements on the reflectivity and flatness of the window in the reticle.
Reticle pods need to last for many years, so they must meet current and future EUV lithography requirements. Therefore, today’s pod designers should consider designing two pods, one with space to accommodate the film, and one that can be used without the addition of film. The inner pod can be modified by adding a film bag, but still meet the overall size and weight requirements of the pod.
To design a film-compatible reticle, the pod manufacturer, film supplier, and lithography machine manufacturer need to work closely together. Automated equipment assumes a small range of weight variation within the pod, which means that after a portion of the material is removed to make the film bag, a similar weight must be added elsewhere on the pod. The geometry of the film must be considered when determining the location of contact points and windows in the inner pod.
Films are extremely fragile. During lithography operations, vacuum cleaning and ventilation can cause pressure changes in the inner reticle. This differential pressure must be controlled below a certain threshold so that excessive deflection does not damage the membrane. Proper placement of windows in film-compatible inner reticle pods provides visibility so the lithography machine can detect film damage.
EUV equipment must be able to process masks with or without thin films and be able to distinguish the difference between the two types of masks. If a mask with film is accidentally placed in a reticle box without a film bag, it will cause irreparable damage to the film. The inner pod should include design features to ensure that the camera in the EUV device can scan and optically detect the pod type, reducing the risk of misidentifying pods.
The EUV reticle pod is a highly specialized device that plays a vital role in EUV lithography. During use, storage and transport, they must protect the mask while ensuring that other contaminants are not introduced or damaged. The reticle must be compatible with the lithography equipment and maintain a clean and dry environment for the mask. For masks with and without thin films, precisely designed dual pod configurations can achieve these goals, ensuring that EUV lithography is future-proof.