Top Down Approach

FOCUSED ION BEAM

Focused ion beam, also known as FIB, is a technique used particularly in the semiconductor industry, materials science and increasingly in the biological fields for site-specific analysis, deposition, and ablation of materials.

FIB systems operate in a similar fashion to a scanning electron microscope (SEM), however, FIB systems use a finely focused beam of ions (usually gallium) that can be operated at low beam currents for imaging or high beam currents for site specific sputtering or milling.

As Figure 1 shows, the gallium primary ion beam (Ga+) hits the sample surface and sputters a small amount of material, which leaves the surface as either secondary ions (i+ or i-) or neutral atoms (n0). The primary beam also produces secondary electrons (e–). As the primary beam rasters on the sample surface, the signal from the sputtered ions or secondary electrons is collected to form an image.At low primary beam currents, very little material is sputtered and modern FIB systems can easily achieve 5 nm imaging resolution (imaging resolution with Ga ions is limited to approximately 5 nm by sputtering and detector efficiency). At higher primary currents, a great deal of material can be removed by sputtering, allowing precision milling of the specimen down to a sub micrometer or even a nanometer scale.If the sample is non-conductive, a low energy electron flood gun can be used to provide charge neutralization. In this manner, by imaging with positive secondary ions using the positive primary ion beam, even highly insulating samples may be imaged and milled without a conducting surface coating, as would be required in a SEM.

Figure 1 - Scheme illustrating the FIB operation process

 Nanospring growth: 

Photolithography

Photolithography, also termed optical lithography or UV lithography, is a process used in microfabrication to pattern parts of a thin film or the bulk of a substrate. It uses light to transfer a geometric pattern from a photomask to a light-sensitive chemical "photoresist", or simply "resist," on the substrate. A series of chemical treatments then either engraves the exposure pattern into, or enables deposition of a new material in the desired pattern upon, the material underneath the photo resist.

The main stages involved in a typical photolithography process (Figure 2) are listed below:

1)      Cleaning,

2)      Photoresist application,

3)      Exposure and developing,

4)      Etching,

5)      Deposition of thin films and

6)      Photoresist removal

Figure 2 – Scheme describing the steps of a typical photolithograpy process

Cleaning

This procedure removes organic or inorganic contaminations that are present on the wafer surface, either by wet chemical treatment or dry approaches, mostly probably using plasma.

Photoresist application

The wafer is covered with photoresist by spin coating. A viscous, liquid solution of photoresist is dispensed onto the wafer, and the wafer is spun rapidly to produce a uniformly thick layer of few micrometers. The photo resist-coated wafer is then prebaked to drive off excess photoresist solvent, typically at 90 to 100°C for 30 to 60 seconds on a hotplate.

Exposure and developing

After prebaking, the photoresist is exposed to a pattern of intense light. The exposure to light causes a chemical change that allows some of the photoresist to be removed by a special solution, called "developer" by analogy with photographic developer. Positive photoresist, the most common type, becomes soluble in the developer when exposed; with negative photoresist, unexposed regions are soluble in the developer.

A post-exposure bake (PEB) is performed before developing, typically to help reduce standing wave phenomena caused by the destructive and constructive interference patterns of the incident light.

The resulting wafer is then "hard-baked" to solidify the remaining photoresist, to make a more durable protecting layer in future ion implantation, wet chemical etching, or plasma etching.

Etching

In etching, a liquid ("wet") or plasma ("dry") chemical agent removes the uppermost layer of the substrate in the areas that are not protected by photoresist. In semiconductor fabrication, dry etching techniques are generally used, as they can be made anisotropic to avoid significant undercutting of the photoresist pattern. This is essential when the width of the features to be defined is similar to or less than the thickness of the material being etched.

Deposition of thin films

In other cases, target material (usually a thin metal layer) is deposited on the whole surface of the wafer. This layer covers the remaining resist as well as parts of the wafer that were cleaned of the resist in the previous developing step.

Photoresist removal

After a photoresist is no longer needed, it must be removed from the substrate. The photoresist is washed out together with parts of the target material covering it, and only the material that was in the "holes" remains – this is the material that has direct contact with the underlying layer. This usually requires a liquid "resist stripper", which chemically alters the resist so that it no longer adheres to the substrate.

Up until this point, the photolithography process is comparable to a high precision version of the method used to make printed circuit boards. Subsequent stages in the process have more in common with etching than with lithographic printing. It is used because it can create extremely small patterns (down to a few tens of nanometers in size), it affords exact control over the shape and size of the objects it creates, and because it can create patterns over an entire surface cost-effectively. The main disadvantages are that it requires a flat substrate to start with, it is not very effective at creating shapes that are not flat, and it can require extremely clean operating conditions.

A short video which demonstrates the photolithography process: 

Electron beam lithography (EBL)

Electron beam lithography (EBL) refers to a lithographic process that uses a focused beam of electrons to form the circuit patterns needed for material deposition on (or removal from) the wafer. This is in contrast to optical lithography, which uses light for the same purpose. Electron lithography offers higher patterning resolution than optical lithography because of the shorter wavelength possessed by the 10-50 keV electrons that it employs.Given the availability of technology that allows a small-diameter focused beam of electrons to be scanned over a surface, an EBL system no longer needs masks to perform its task (unlike optical lithography, which uses photomasks to project the patterns). An EBL system simply “draws” the pattern over the resist wafer using the electron beam as its drawing pen (Figure 3). Thus, EBL systems produce the resist pattern in a serial manner, causing it to be slower than optical systems.

Figure 3 -  Illustration of the steps involved in the EBL process. Image courtesy of LNBD, Technion.

A typical EBL system consists of the following parts: an electron gun or electron source that supplies the electrons,an electron column that shapes and focuses the electron beam, a mechanical stage that positions the wafer under the electron beam, a wafer handling system that automatically feeds wafers to the system and unloads them after processing, and a computer system that controls the equipment.The resolution of optical lithography is limited by diffraction, but this is not a problem for electron lithography. This is due to the short wavelengths (0.2-0.5 angstroms) exhibited by the electrons in the energy range used by EBL systems. However, the resolution of an electron lithography system may be constrained by other factors, such as electron scattering in the resist and by various aberrations in electron optics.Just like optical lithography, electron lithography also uses positive and negative resists. The resolution achievable with any resist is limited by two major factors: the tendency of the resist to swell in the developer solution and electron scattering within the resist.The primary advantage of electron beam lithography is that it is one of the ways to overcome the diffraction limit of light and make features in the nanometer regime. This form of maskless lithography has found wide usage in photomask-making used in photolithography, low-volume production of semiconductor components, and research & development.The key limitation of electron beam lithography is throughput - the very long time it takes to expose an entire silicon wafer or glass substrate. A long exposure time leaves the user vulnerable to beam drift or instability that may occur during the exposure. Also, the turn-around time for reworking or re-design is lengthened unnecessarily if the pattern is not being changed the second time.Figures 4 (a) and (b) demonstrate production-level high resolution electron beam lithography of lines and dots formed in resist. In this example, 20 nm sized features were delivered and have the pattern transfer capability for a wide range of materials. Nanometer level-to-level alignment accuracy is made possible by in-house software and marker design.Figure 4 (c) shows a single silicon nanowire contacted by two adjacent electrodes to enable the measurement of the silicon nanowire electrical properties.

Figure 4 - Nanoscale patterns fabricated on a silicon substrate. (a) Dot and line array patterns from a silicon mask and (b) various patterns obtained from a β-Si3N4mask (c) silicon nanowire contacted with two electrodes.