Part 2 Current State-of-the-Art For Commercial ‘Micro’-Electronic Device Production Silicon Technology and Next Generation Lithography

Overview of Part 2 Properties of Silicon Structure Doping Overview of the Fabrication of a Silicon Based Device Photolithography Next Generation Lithographies?

Learning Objectives By the end of this part of the course you should understand (i)n- and p-type doping, (ii)The overview of photolithography, (iii)The terms positive and negative tone resists, (iv)Have an appreciation for some o f the limits of photolithography, (v)Have an appreciation for how these limits are being ‘stretched’, (vi)Moore’s Law, and (vii)That there are several competing next generation lithographic processes.

Silicon is a crystalline material, Properties of Silicon Semiconductor with a tetra- hedral unit cell. Silicon has two types of charge carriers - electrons and holes. The carrier concentration can be controlled by doping, or electrostatically. Each silicon atom has 10 core electrons (tightly bound), and 4 valence electrons (loosely bound).

Properties of Silicon Semiconductor For simplicity we can consider a flattened model structure At room temperature there are ~1 x cm -3 free carriers (out of ~2 x cm -3 ) Holes and electrons can move around the lattice, or recombine to form a complete bond. Due to thermal effects some bonds are broken, giving mobile holes and electrons. + -

Properties of Silicon Semiconductor Donor dopants increase the number of conduction electrons P+P+ A donor atom, such as phosphorus, has five valence electrons, four of which participate in bonding, leaving one extra electron that is easily released for conduction. The donor site becomes positively charged (fixed charge). Silicon doped with a donor is called n-type.

Properties of Silicon Semiconductor Acceptor dopants increase the number of holes in the lattice Silicon doped with an acceptor is called p-type. An acceptor atom, such as boron, has three valence electrons, and can therefore easily accept an electron from a neighbour, leaving a free hole. The dopant has a fixed negative charge. B-B-

Properties of Silicon Semiconductor Overall doping depends on the relative number of acceptors and donors. Silicon doped with donor and acceptor atoms is called Counter Doped, and can have multiple separate regions of n- and p-type conductivity B-B- B-B- P+P+

Properties of Silicon Semiconductor The carriers distribution is also affected by electric fields Between collisions with the lattice the carriers are accelerated in the direction of the electrostatic field. B-B- E Combining the effects of doping and fields on the carrier concentration and distribution, we can realise useful devices.

Fabrication of a Device (MOSFET) Silicon devices are built up in a series of layers, using processes such as photo-lithography, etching and doping. Oxide layer is grown on n-type silicon by heating to around 1000˚C in oxygen or steam. A light sensitive resist is coated on the Si, and exposed to a light pattern. The exposed resist is developed in a solvent The unexposed areas of resist are used as a mask to protect areas of the wafer from plasma etching, dopant implantation, or metal coating.

Fabrication of a Device (MOSFET) B+B+ B+B+ B+B+ B+B+ B+B+ B+B+ B+B+ B+B+ B+B+ B+B+ A Metal-Oxide-Semiconductor Field Effect Transistor is a fairly simple device, but still requires four separate lithography steps. The lithographic step is probably the most important in microfabrication. Modern chips frequently require 30 to 50 layers (each with multiple process steps) Gate Metal Contact Silicon Oxide Silicon Doped Silicon

Fabrication of a Device (MOSFET) Silicon devices are built up in a series of layers, using processes such as photo-lithography, etching and doping. An oxide layer is grown on n-type silicon by heating to around 1000˚C in oxygen or steam.

Fabrication of a Device (MOSFET) Light sensitive material, known as resist, is spin coated onto the surface Light is projected through a patterned mask onto the resist. A developing solvent is then used to remove the exposed resist.

Fabrication of a Device (MOSFET) The sample is etched with plasma or acids to remove the exposed oxide. A 50 nm Gate Oxide layer is thermally grown. The resist pattern is removed with chemical stripper or oxygen plasma

Fabrication of a Device (MOSFET) A 50 nm layer of poly- silicon is made by chemical vapour deposition. Another resist pattern is defined, and developed. The pattern is transferred through both the poly- silicon and the gate oxide by etching, and the resist is stripped.

Fabrication of a Device (MOSFET) Boron dopant atoms are ion implanted, B+B+ B+B+ B+B+ B+B+ B+B+ B+B+ B+B+ B+B+ B+B+ B+B+ and then driven in by heating to 950˚C in oxygen Yet another photolitho- graphy, etch and resist strip step is used, to create contact cuts in the oxide layer Contact Cuts

Fabrication of a Device (MOSFET) B+B+ B+B+ B+B+ B+B+ B+B+ B+B+ B+B+ B+B+ B+B+ B+B+ Aluminium is evaporated on to the surface. And one last patterning, and etching step to afford a MOSFET (metal-oxide- semiconductor field effect transistor) It took four separate lithography steps to realise this fairly simple device. The lithographic step is probably the most important in microfabrication. Modern chips frequently require 30 to 50 layers (each with multiple process steps)

Photolithography Energy - causes (photo)chemical reactions that modify resist dissolution rate Mask - blocks energy transmission to some areas of the resist Aligner - aligns mask to previously exposed layers of the overall design Resist - records the masked pattern of energy Energy Mask + Aligner Photoresist Wafer

The Resist The first step is to coat the Si/SiO 2 wafer with a film of a light sensitive material, called a resist. A resist must also be capable of high fidelity recording of the pattern (resolution) and durable enough to survive later process steps Solvent Evaporates RPM

The Resist Positive tone resists are generally polymers which are prone to bond breaking on irradiation h After scission of the polymer chain, the fragments have increased solubility in suitable solvents.

The Resist Negative tone resists are generally polymers which are prone to crosslinking on irradiation h After crosslinking of the polymer chain, the fragments have decreased solubility in suitable solvents

Optical Limits When light interacts with features with dimensions similar to the wavelength, diffraction effects must be allowed for.

nm radiation Gate length 65 nm So how are structure created with less than the wavelength of the radiation? Commercial State-of-the-Art For Photolithography 65 nm gate 30 nm

Immersion Photolithography Immersion lithography extends 193 nm optical lithography to even smaller resolutions, by increasing the refractive index through which the light passes.

Wave Front Engineering Correcting for and Utilising Diffraction Optical Proximity Correction: The shape of the mask is varied to take advantage of constructive interference Phase Shift Mask: The phase of the light of adjacent patterns is altered by 180˚ leading to destructive interference

Moore’s Law: Computer performance doubles every 18 months. Drivers for Miniaturization Why do we care about resolution so much?

The current 45 nm and 30 nm gate length processes are being developed Next Generation Lithography is also being developed for if/when conventional process no longer work

Next Generation Lithography In 1996, five technology options were proposed for the 130 nm gate length technology: X-ray proximity Lithography (XPL) Extreme Ultraviolet (EUV) Electron Projection Lithography (EPL) Ion Projection Lithography (IPL) Direct-write lithographies (EBDW). These options were referred to as the next generation lithographies.

Next Generation Lithography The current forerunners are Immersion Photolithography, Extreme Ultraviolet Lithography (EUV), and Electron Beam Lithography (EBL).

Immersion Photolithography Advantages of Immersion: We can keep using the 193 nm sources Disadvantages of Immersion: Contamination of the Resist by Immersion Liquid Couple of Wafer/Lens Vibrations Cavitation in the Immersion Liquid Bubble Immersion Liquid Lens Wafer Scan Direction

Energy Sources Energy Sources are required to modify the resist. An aerial image of the energy source is produced at the resist. The imaging can be done by scanning or masking the energy beam. Energy Source Wavelength (nm) UV DUV EUV13 X-ray0.5 Electrons Ions0.012 Lower the wavelength, higher the energy…

Extreme Ultraviolet (EUV) Extreme Ultraviolet refers to photons with a wavelength that is in the region of 10 nm. This is absorbed by air so the light path is kept under vacuum. Furthermore there are no known materials that transmit EUV, so reflective optics are used instead of lenses. EUV is theoretically capable of very high resolution (30 nm) given the short wavelength,

Dip Pen Lithography An ultra high resolution fountain pen!

Nanocontact printing

Atomic Manipulation Moving atoms with the STM

Conclusions Photolithography is ‘struggling’ to keep up with resolution demands There is currently no clear successor to photolithography However, it is already possible to manipulate structures on every size scale down to atomic. As the size becomes smaller though, the manufacturing speed drops dramatically.

What We Will Look at… Radiative Processes E-beam Lithography Scanning Near Field Photolithography Soft Lithography Dip-Pen Nanolithography Nano-Contact Printing

Thanks Dr Alex Robinson Nanoscale Physics Research Laboratory University of Birmingham For allowing the use of his slides.