Nikon is leveraging more than thirty years of lithography equipment experience to deliver specialized low numerical aperture steppers that accommodate a range of substrate sizes and materials, as well as thick films, and extremely warped surfaces. These specially developed systems satisfy the unique resolution and depth of focus requirements for many diverse markets using high-productivity, cost-effective stepper platforms.
Thin Film Heads, TSV, C4 Packaging, and More: Low-NA SF155
There are well over 400 scan field (SF) steppers operating around the world today, with the NSR-SF155 being the latest offering in the successful equipment series. The standard NSR-SF155 has a lens numerical aperture (NA) range of 0.50 to 0.62. The SF155 is also available with a Low-NA (0.22 to 0.50) lens to image larger features. The Low-NA systems are well-suited for use in non-conventional applications such as thin film head (TFH) manufacturing, which requires larger lithography dimensions than chipmaking, as well as C4, through silicon via (TSV), and packaging processes that are approaching the limits of existing lithography equipment (Figure 1). Such applications require specialized lithography solutions like the Low-NA SF155.
TSV processes can cause substantial wafer deformation, while GaAs and GaN substrates may have significant asymmetrical deformation. To accommodate these scenarios, the Low-NA SF155 wafer holder, cool plate, and loader arm were fully redesigned to handle wafers with severe warpage, and Nikon has verified performance through extensive testing. TSV processes also necessitate that the lithography equipment can handle wafers that exceed SEMI standard thicknesses. The Low-NA SF155 wafer holder design accommodates thicker wafer handling of 200 and 300 mm substrates. Stepper frontside overlay accuracy is 25 nm. Backside alignment (BSA) may also be required, and the SF155 is compatible with an enhanced FIA system (Figure 2) that incorporates a new InGaAs infrared camera and dual HL lamp/alignment optics to enable optimal front and backside alignment.
The Low-NA SF155 systems are designed with different performance specifications to meet customers’ particular process requirements (Figure 3). The Low-NA SF155 delivers excellent CD performance with steep sidewall angles, and optimal across-field CD uniformity. In addition, it provides increased depth of focus (DOF) for thick resist applications, incorporating an optimized autofocus (AF) system that accommodates resist thicknesses on the order of tens of microns. Low-NA SF155 systems deliver resolution ≤ 450 nm, with a 1.5 μm DOF. They also have comprehensive countermeasures to mitigate against lens contamination due to resist outgassing, which may occur with very thick negative tone resist processes.
Nikon scan field steppers are being used to process more than 4000 wafers per day at manufacturing facilities around the world. The Low-NA SF155 provides a cost-effective, specialized solution that will satisfy customers’ diverse requirements for applications in TFH manufacturing, TSV, C4 packaging, inkjet applications, and more.
Meeting the Needs of MEMS, LEDs, and More: MEMS Steppers
Micro Electro Mechanical Systems (MEMS) do not have the same aggressive imaging or overlay requirements demanded by semiconductors. However, they necessitate that lithography systems are able to handle a variety of substrates - including extremely warped wafers and very thick films, and must also accommodate significant step heights. In addition, bulk MEMS applications or Taiko wafers often need precise alignment to marks located on the backside of the wafer surface.
Nikon MEMS Steppers were specially designed to satisfy the critical requirements of these dynamic markets and they provide extremely diverse processing capabilities (Figure 4). MEMS Steppers have been very successful in meeting customers’ unique specifications for not only Air Bearing Surface (ABS) fabrication and MEMS applications, but also for light emitting diodes (LEDs), discretes, and more. MEMS Steppers can also 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, MEMS Steppers’ wide DOF optimizes the insulating layers used in bumping processes to support under-bump metallization-free (UBM-free) wafer level chip scale packaging applications. Well over 150 of these specialty Nikon Steppers are in use around the world today.
MEMS Steppers use low-NA lens designs specifically optimized for MEMS-type applications, as well as shot-by-shot autofocusing. This combination enables them to deliver the necessary resolution with tremendous DOF (Figure 5). The advanced autofocus system design delivers stable AF performance even using transparent substrates. MEMS Steppers now also provide auto leveling capabilities that enable tilt compensation for each shot to maximize DOF for extremely rough surfaces. MEMS Steppers use proven Nikon IC stepper lens technology that ensures optimal CD uniformity across the wafer. In addition, their projection lens designs eliminate costly mask contamination/defectivity issues experienced with contact or proximity printing methods. 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 up to 26 μm.
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 ≤ 0.3 μm. The majority of MEMS Steppers also support critical BSA capabilities, and enable backside alignment accuracy to 0.8 μm and below to satisfy customers’ process requirements. The newly developed 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 Fixed Alignment Points (FAP) system (Figure 6). MEMS Steppers enable robust alignment for bonded wafers as well.
Nikon continues to focus on expanding our exposure system portfolio for IC and non-IC applications. We are leveraging decades of lithography experience to deliver specialty steppers that support a range of substrate sizes and materials, as well as thick films and extremely warped surfaces. These tailored systems also provide a variety of low-NA configurations to best satisfy unique imaging requirements. They do this on high productivity, cost-effective platforms to meet manufacturing objectives.
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