In the second part, we evaluate the strength and weakness of six kinds of ML2 systems. They are zone-plate array lithography (ZPAL), optical maskless lithography (OML2), near-field lithography, dip-pen lithography, focused ion beam lithography (FIBL), and nano-imprint lithography (NIL). All of them are under developed for many years, and also has possibilities to become the mainstream technology of wafer production.
2.1. Zone-plate Array Lithography (ZPAL)
ZPAL, developed by Nanostructures Laboratory of MIT, is an optical direct write technology, which utilizes micro-scale zone-plate array and switching mirror array to generate huge numbers of focused beams and expose the whole wafer. The strength and weakness of ZPAL is summarized in Table 7. (see (7)Appendix J)
St S tr re e ng n g th t h
¾ With zone-plate technology, ZPAL can be extend to use EUV or x-rays which tend to pass through conventional lenses.
¾ Resolution enhancement techniques which are used in optical lithography can be applied to ZPAL, such as immersion technique.
W
We ea ak k ne n e ss s s
¾ The transmission of zone-plate is merely 40%.¾ In order to get high throughput and clear pattern, switching
Figure 4 Data buffer architecture for high throughput
mirrors should tilt sufficiently fast, and the tilting angles should be controlled precisely.
¾ To achieve 32nm line width, wavelength should be reduce to EUV (13.5nm). Then it will face most of the problems of EUV lithography, such as source life-time, no available resist, multi-coated mirror accuracy, and so on.
Table 7 Strength & Weakness of ZPAL
2.2. Optical Maskless Lithography (OML)
OML, developed by ASML, replaces photomasks by a dynamic pattern generating device called spatial light modulator (SLM) which consists of 1.048M micro-mirrors. With the mirrors tilt or lift, intensities and phases of small patterns can be adjusted, and the whole pattern is imaged to the substrate through high demagnification projection lens. The strength and weakness of OML is summarized in Table 8. (see Appendix K)
St S tr re e ng n g th t h
¾ Advances in conventional mask-based optical lithography can be applied to OML, such as resists and RETs.We W ea ak k ne n e ss s s
¾ It is hard to control the uniformity of 1.048M mirrors.
¾ To achieve 32nm line-width and below, wavelength should be reduce to EUV (13.5nm). It will face most of the problems of EUV lithography, such as source life-time, no available resist, multi-coated mirror accuracy, and so on.
Table 8 Strength & Weakness of optical maskless lithography
2.3. Near-field Lithography
By approaching the resist surface within the distance of one wavelength, the electromagnetic distribution would be quite different than that under far-field condition. It can be used for optical lithography to surmount diffraction limit. The strength and weakness of near-field lithography is summarized in Table 9. (see Appendix L)
St S tr re e ng n g th t h
¾ There is no more diffraction limit, so high resolutions can be obtained by using long wavelength light.
¾ Because the structure of waveguide is simple, it is easy to make a dense beamlets array.
We W ea ak k ne n e ss s s
¾ For the waveguide is extremely close to the resist, it is hard to control contaminations and damages.
¾ The energy passing through the waveguide is quite low and would reduce the exposure speed of the system.
¾ Depth of focus of near field light is short.
Table 9 Strength & Weakness of near-field lithography
2.4. Dip-pen Lithography
Dip-Pen Nanolithography (DPN) is a scanning probe nanopatterning technique in which an AFM tip is used to deliver molecules to a surface via a solvent meniscus, which naturally forms in the ambient atmosphere. The strength and weakness of dip-pen lithography is summarized in Table 10. (see Appendix M)
Strength
¾ It creates nano-structure in on step without resist.
¾ It is possible to achieve sub-10nm line-width.
¾ A in-situ position detection can be performed during lithographic process, and this ensure the alignment and leveling accuracy.
We W ea ak k ne n e ss s s
¾ The throughput is quite low compared to other lithographic technologies.
¾ The AFM tip is too close to wafer surface, and this induce the risk of unwanted contaminations and tip breakdown.
Table 10 Strength & Weakness of dip-pen lithography
2.5. Focused Ion Beam Lithography (FIBL)
FIBL is similar to EBL in applications, feature sizes and the fact that it is a serial approach, but uses a beam of ions instead of electrons. A fundamental difference is that ions are charged atomic that can interact physically and chemically with, and settle into, the exposed material, and the particles of which are many orders of magnitude more massive than electrons. The strength and weakness of FIBL is summarized in Table 11. (see Appendix N)
Strength
¾ It can generate high-aspect ratio patterns.
¾ It induces less back-scattering particles than EBL, so it suffers less proximity effect.
¾ It has the possibility of building up structures rather than just
creating structures destructively.
Weakness
¾ Ion source requirements of low energy spread and high brightness are hard to achieve.
¾ Swelling occurs when developing negative resists and as limits the resolution.
¾ Ions, with thousands times of mass compared to electrons, cause more damage to resist and lower layer structures.
¾ The ions, which lose energies and stays in the substrate, may serve as doping particles and effect the performance of circuits.
Table 11 Strength & Weakness of FIBL
2.6. Nano-imprint Lithography (NIL)
NIL makes a 1:1 pattern transfer from mold to resist. There are three types of resist to use, one can be direct printed, and another should be heated to be soft, and the other should be exposed to cure by UV light after printing. The strength and weakness of NIL is summarized in Table 12. (see Appendix O)
St S tr re e ng n g th t h
¾ It performs a fast and low-cost lithographic process.¾ It has a possibility to print a whole wafer at one time.
We W ea ak k ne n e ss s s
¾ The 1:1 mold preparation is time consuming and relies on some other high resolution, high throughput ML2 technologies.
¾ The printing uniformity is hard to control, especially in the side region of the mold.
¾ Damage and contamination happen easily in repeated printing process.
Table 12 Strength & Weakness of NIL
2.7. Conclusion
After the discussion above, we conclude that OML and ZPAL are more competitive than all the other technologies. The main reason lies in the abilities of contamination and damage control, as is important for repeated wafer manufacturing process. FIBL is a versatile technology but introduce more possibility for damage
and contamination (ex. doping particles) during the process. In near-field and dip-pen technologies, the gap between resist and writing units are so close that it requires extremely high accuracy of resist thickness control or scan leveling control.
Nano-imprint technology uses the contact mold and would suffer more contamination and damage problems.
The potentials of OML and ZPAL for 32nm node manufacturing rely on the extension to EUV. Recently, with large quantity of research and investigations, EUV technologies have many progresses, especially in source and optics. These progresses also benefit OML and ZPAL and help them become more realizable.