In addition to supporting IC and thin film head applications, Nikon provides specially-developed lithography systems, known as MEMS Steppers, to deliver functional and affordable litho solutions for leading-edge MEMS, LED, and advanced packaging applications. The lithography tools for these markets must provide sufficient resolution to satisfy imaging requirements, but it is imperative that they also deliver large depth of focus (DoF) and field-by-field focusing capabilities to achieve the desired CD fidelity. Backside alignment capabilities and robust handling of warped substrates are also imperative.
There has been rapid growth and tremendous changes in MEMS, LED, and packaging processes. Nikon continues to expand the MEMS Stepper product line to meet the highly dynamic requirements of these specialty markets, by offering more versatile solutions featuring continuous enhancements to the steppers’ projection lenses, optics, alignment systems, and autofocus. They also provide optimized wafer handling capabilities, and support custom 300 mm (round) or 250 mm x 200 mm panels, and other substrate materials as well.
Nikon offers a broad product portfolio to satisfy customers’ specific production objectives (Figure 1A). MEMS steppers maximize depth of focus with low, variable lens numerical apertures, enhance imaging with ghi/i-line capabilities, and support a wide range of field sizes also. In addition, newly added shot magnification control optimizes contrast, depth of focus, and profile control (Figure 1B), while the large working distance between the substrate surface and the bottom lens minimizes resist outgassing effects. A multitude of configurations/specifications are offered to cover all aspects of customers’ needs.
Deformed and warped wafers are a well-known challenge in MEMS manufacturing, and Nikon MEMS Steppers provide a number of functions to mitigate these situations. At the recent LithoVision technical symposium, our Nikon Engineering team reported significant improvements in grid alignment on deformed wafers using optional ERA (Each Row Alignment) software compared to traditional linear EGA corrections (Figure 2A). Nikon MEMS Steppers support Through the Lens (TTL) reticle alignment capabilities as well. Using this flexible reticle alignment system, TTL is able to align to any reticle marks including competitors’ marks. It also verifies projection lens (PL) magnification for shot mag control (feedback to Reticle Z position). As with traditional IC steppers, focus calibration is also available with the TTL function to enhance focus control and stability (Figure 2B).
Nikon recognizes that a range of field-size requirements must be met for these applications, and the newly released NES2-i10 system enables a wide 44 mm x 44 mm square standard field size, but the field size is variable for mix-and-match with 22 mm stepper fields, or double scan fields, as well as for 60 mm round rotary encoder applications (Figure 3A). Customers around the world are meeting their unique manufacturing goals using MEMS Steppers to successfully process extremely fragile surfaces, and highly deformed substrates of different sizes and shapes.
Many exposure tools used in packaging applications require the reticle to be in close proximity to the wafer. Proximity effects not only reduce the usable lifetime of the reticle, but also can contaminate the optics due to resist outgassing. Nikon Steppers employ a large working distance between the substrate surface and the bottom lens to minimize outgassing effects (Figure 3B).
In addition to stepper innovations, resist processes for these technologies continue to advance as well. Nippon Kayaku/MicroChem provide cutting-edge resist processes to meet the needs of many of these complex applications. They offer the newly developed SU-8 3000CF DFR, a photo definable epoxy-based dry film resist, which uses low halogen levels (total chlorine < 900 ppm) and is antimony free. It enables first-rate resolution and high aspect ratio with a vertical profile greater than 3 using a 20 μm film thickness. In addition to providing outstanding thermal stability, it cures at a low temperature and absorbs little water as well. Coupling the NES2W-i10 stepper’s large depth of focus with ultra-thick SU-8 DFR and SU-8 developer has enabled exceptional patterning of 30 µm diameter micro stone columns (Figure 4A). Nippon Kayaku/MicroChem also offers the KMRD-015A resist, which delivers excellent vertical sidewall performance. When combined with the low numerical aperture and sigma control of the NES2W-i06 stepper illumination enhancement technology, the exposure dose can be optimized to pattern a feature with a very large taper angle (Figure 4B).
There is substantial market demand for these unique systems, and in recent months, Nikon has showcased our leading-edge technology at events such as MEMS Manufacturing 2017, LithoVision 2018, and iMAPS 2018. Nikon MEMS stepper technology will also be featured in our booth at the upcoming SEMICON West 2018 show in San Francisco, CA in July.
We are proudly building on a long history of opto-electronic and precision technology engineering to deliver optimal lithography cost of ownership and enhanced overlay performance. We continue to offer customers a superior alternative to mask aligners or outdated steppers that have historically been used in many non-IC applications. Well over 150 MEMS Steppers are in customer use around the world today, and our expanded portfolio provides functional and affordable lithography solutions for MEMS, LED, and packaging applications.