In addition to delivering industry-leading immersion scanners for cutting-edge semiconductor applications, Nikon also offers a suite of specialty steppers to address the unique lithography requirements of a multitude of other markets. Widely known as “MEMS Steppers,” these systems have been very successful in meeting customers’ specifications for not only Micro Electro Mechanical Systems (MEMS) applications, but also for back-end processes, Air Bearing Surface (ABS) fabrication, light emitting diodes (LEDs), discretes, and more.
Fan-out wafer level chip scale packaging (WLCSP) applications require large field sizes with tremendous depth of focus (DOF) and large substrate handling capabilities. MEMS sensors for the internet of things (IOT) need enhanced resolution, very high dose capabilities, and autofocus systems that are compatible with transparent substrates. In contrast, power devices must accommodate SiC and GaN substrates and TAIKO® wafers, while providing backside alignment for dual-side patterning (Figure 1A).
Nikon leverages 100 years of opto-electronic and precision technology experience to deliver high productivity, cost-effective MEMS Steppers to satisfy the unique requirements of these and other diverse markets. These lithography systems are available with low numerical apertures (NA) and varied field sizes, and are compatible with a range of substrates and materials, as well as thick films, and extremely warped surfaces. MEMS Steppers can be used to pattern Cu pillar insulating layers as the steppers’ large depth of focus contributes to outstanding pillar resolution and sharp profiles. In addition, the large DOF optimizes the insulating layers used in bumping processes to support under-bump metallization-free wafer level chip scale packaging applications. It is imperative to consider the optimal balance between resolution and DOF for interconnections and the rewiring process in next-generation fan out wafer level packaging (FO-WLP). Figure 1B shows a 10 µm open via area with 20 µm thick photoresist that takes advantage of a DOF greater than 20 um, which is well-suited for the current design rules of back-end processes used in FO-WLP. Even using 7 µm thick photoresist, a 2 µm L/S is well resolved, which will meet coming requirements for advanced back-end processes. When considering patterning at the top layer of a multilayered pattern area, it is again important to have sufficient resolution for patterning at the thin layered area as well as the thicker area, with large depth of focus while not leaving any unexposed photosensitive insulator.
At the recent LithoVision technical symposium Yutaka Hasegawa, Nikon Engineering Account Manager, introduced the latest generation NES2W-i10 stepper, and highlighted its key components for MEMS, LED, and packaging applications (Figure 2A). The newly developed i10 lens enables a wide 44 mm x 44 mm square field size and 74 µm DOF (3.7 µm L/S) coupled with a flexible alignment system, and advanced substrate holding/handling capabilities. The handling system’s edge grip design works well with TAIKO® and other thin wafers, and the verneuil chuck utilizes non-contact delivery. The chuck design also enables dual-sided non-contact exposure. MEMS Steppers such as the NES2W-i10 enable ultra-precise machining for a wide range of materials’ handling, and customers are successfully processing different substrate shapes, with varied deformity levels, bonded substrates, and extremely fragile surfaces.
In MEMS processing, bonded wafers are now often used, and Nikon steppers provide robust alignment for these with pre-alignment performed on the Carrier Wafer to achieve proper orientation of the Target Wafer for precise alignment. MEMS Steppers also effectively handle warped non-silicon wafers. The Zoned Vacuum System accommodates deformed wafers by using a gradual increase of the vacuum contact area to flatten the surface of the wafer. Evaluations have shown the Zoned System reduced loaded wafer warpage from more than 250 µm to below 3.1 µm (Figure 2B).
Low-NA lens designs specifically optimized for MEMS-type applications, as well as shot-by-shot autofocusing are also employed. This combination enables MEMS steppers to deliver the necessary resolution with tremendous DOF (Figure 3A). The advanced autofocus (AF) system design delivers stable AF performance even using transparent substrates, and MEMS Steppers also provide auto leveling capabilities that enable tilt compensation for each shot to maximize DOF for extremely rough surfaces. The MEMS Stepper product portfolio includes a range of exposure field size/NA/resolution capabilities to meet specific manufacturing requirements. Ghi, i-line wavelength systems are offered, and systems are available with resolution down to 1.0 μm or DOF capabilities to 74 μm and beyond (Figure 3B).
Nikon MEMS Steppers also provide a high degree of alignment flexibility. They utilize proven Enhanced Global Alignment (EGA) technology with FIA alignment capabilities, and enable overlay accuracy to 0.3 μm (m+3σ). The majority of MEMS Steppers also support critical backside alignment (BSA) capabilities, and enable BSA accuracy to 0.8 μm and below to satisfy customers’ process requirements. The hybrid backside alignment system uses direct backside alignment (D-BSA) coupled with infrared backside alignment (IR-BSA) to optimize accuracy, and is well suited for power device applications like insulated gate bipolar transistors (IGBT). Backside alignment performance can be further enhanced to below 0.5 μm with the Nikon Fixed Alignment Points (FAP) system.
There are a multitude of configurations and add-on functions available for MEMS Steppers (Figure 4), and Nikon is eager to work with customers to develop customized, fab-specific solutions as well.
Well over 150 MEMS Steppers are in customer use around the world today, and Nikon continues to focus on expanding our exposure system portfolio for IC and non-IC applications. We are proudly building on a long history of opto-electronic and precision technology engineering to meet the diverse performance and budgetary objectives of our customers. Newly developed MEMS Steppers enhance imaging with ghi/i-line capabilities, provide up to 44 x 44 mm field sizes, and support substrates up to 200 mm. Nikon MEMS Steppers also deliver optimal cost of ownership and enhanced overlay performance to offer customers a superior alternative to mask aligners, which have historically been used in many non-IC applications.
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