Top-Down Nanofabrication

Abstract

Starting from a bulk material, the top-down fabrication process progresses to machine, modify, and shape it into the desired shape and size. In integrated circuit manufacturing, one takes a silicon wafer and carves patterns of specified dimensions by a series of lithographic steps through aligned masking levels, performs operations such as wet and dry chemical etching, ion implantation, diffusion, oxidation, metallisation and many others until the desired device/circuit has been obtained. The key to top-down nanofabrication has been the art of lithography which has been relentlessly improved to create patterns of smaller geometries with higher resolution. The illumination/irradiation source in lithography has been changed from an intense beam of deep UV photons to extreme UV photons, and focussed electrons. Due to its extremely short wavelength, the electron beam offers a very high diffraction-limited resolution but is a comparatively slow process. Another approach followed is to make patterns by mechanical pressure, e.g., by stamping and printing using designed templates. In block copolymer lithography , the directed self-assembly of block copolymers is synergistically integrated with common lithographic techniques for practical utilization by semiconductor industry. Scanning probe lithography can manipulate individual molecules but is a low throughput technique. The vast gamut of nanolithographic tools available to a semiconductor process engineer can be leveraged for fabrication of nanostructures of wide-ranging complexities.

Review Exercises

• 23.1. Distinguish between top-down and bottom-up approaches to nanofabrication.

• 23.2. What is the source of light used for optical lithography? What are the typical wavelengths used? What wavelengths were used in older equipment? What is the photomask plate made of? What is the coating material on the mask plate? What is the photoresist polymer spin coated on the silicon wafer called? Name the three kinds of mask aligners commonly used?

• 23.3. Define the following terms for a photolithographic system: (i) Resolution of image , (ii) Depth of focus .

• 23.4. What is meant by numerical aperture of a lens ? How is it related to its angular aperture ?

• 23.5. If θ denotes the resolution of an image , λ represents the wavelength of light used in the optical system and NA stands for the numerical aperture of the lens , write the formula connecting θ with λ and NA. What is effect of decreasing the wavelength on the resolution? How does an increase in numerical aperture affect the image resolution?

• 23.6. Why is it essential to use high-planarity wafers to achieve good resolution? What is the resolution obtained for a wavelength of 193 nm and numerical aperture 0.93?

• 23.7. What is immersion lithography? How is resolution of the image increased in immersion lithography without changing the wavelength of the laser source?

• 23.8. What is EUV band of wavelengths? What value of wavelength in the EUV band is used for lithography? How does the energy of a EUV photon compare with that of a yellow light photon?

• 23.9. What is the source of light used in EUV system? Do EUV masks work in the transmission mode ? What are the main problems faced with present EUV systems?

• 23.10. Does an electron beam lithography system require masks? What is the input in this system? Which is faster lithographic technique: e-beam or optical?

• 23.11. What are the main components of an e-beam lithography system? Name a commonly used e-beam resist material.

• 23.12. Discuss proximity effect in the context of e-beam lithography? What is its basic cause? How is correction for proximity effect applied?

• 23.13. Explain the effect of substrate charging in e-beam lithography . How does it affect the image? Propose the required solutions to overcome this problem.

• 23.14. How does electron projection lithography combine the benefits of optical and e-beam lithographic techniques? Describe the operating principles of SCALPEL and PREVAIL systems for EPL.

• 23.15. What is the necessity of soft lithographic techniques ? Name three such techniques. How do these techniques differ in their way of transferring a pattern on a substrate?

• 23.16. Name a lithographic technique based on mechanical deformation of resist. What are the two common versions of this technique? How do they differ?

• 23.17. What is a block copolymer ? What property of block copolymers is utilized for nanolithography? How is a pattern transferred to a substrate in block copolymer lithography?

• 23.18. Name three scanning probe microscopy tools used for scanning probe lithography. Can SPL be used for modifying a surface chemically?