Nanolithography Introduction. PDF

 1. Introduction to Nanolithography (Completed) 1. Introduction to Nanolith...

 1. Introduction to Nanolithography (Completed) 1. Introduction to Nanolithography Up until now, we have looked at how we can use di;erent probes (light, electrons, tips) to capture images of the structure and sometimes chemical composition of a sample. In this Anal topic we will see that we can use the principles of scanning probe microscopy to e;ectively write on a surface, known as scanning probe lithography (SPL). Before we get to that, we need to consider what lithography is, and what other lithographic techniques exist at the nanoscale. Lithography (Completed) Lithography The name lithography comes from Greek, with lithos meaning "stone" and graphein meaning "to write". Essentially, it is an old method of printing in which the fact that oil and water don't mix is exploited to create a template that can be used to transfer an image to a substrate. It is the basis of most modern o;set printing used to produce newspapers, magazines, books, etc. Below is an example of a limestone lithography stone (left) used to transfer the inverted image of a map of Munich to paper (right), which also mirrors the image to be the correct orientation. The stone was prepared by Arst having the map drawn onto it with a fatty/oily medium (e.g. a wax crayon). The whole surface was then covered with a weak nitric acid solution, in which the exposed limestone reacted with the solution to form a hydrophilic layer of calcium nitrate: CaCO3 + 2 HNO3 ⟶ Ca(NO3)2 + CO2 + H2O The surface was then washed with turpentine to remove any excess greasy material, however, a thin molecular hydrophobic layer remains attached to the surface where it had been originally drawn on. The stone would then be kept wet when printing, which would saturate all the areas in which no grease had been applied. By using an oily ink only those hydrophobic (drawn on) areas will accept the ink, allowing for a neat transfer of the ink from the stone to a sheet of paper which is placed on top of it. Source: Wikimedia There is incomplete content above. You must complete this before you can proceed through the course. Nanolithography (Completed) Nanolithography We can take many of the principles from lithography and apply them to techniques which produce patterns at the nanoscale (features in the range of 1 − 1000 nm). These can be roughly categorized as follows: Top-down (subtractive) Techniques that remove material to arrive at the desired structure Bottom-up (additive) Use of atoms and molecules to build up the desired structure (e.g. via self-assembly), i.e. by doing some chemistry. Developments into the latter were in particular spurred on by the now famous talk by Richard Feynmann There's Plenty of Room at the Bottom in 1959. While there are many di;erent nanolithographic techniques, we will be focusing on just a few important ones, most of which will use many of the concepts we have previously covered in this section of the course: Photolithography Electron beam lithography (EBL) Nanoimprint lithography (NIL) Scanning probe lithography (SPL) There is incomplete content above. You must complete this before you can proceed through the course. Photolithography (Completed) Photolithography Photolithography uses light to produce a pattern, by selectively chemically modifying di;erent parts on a surface. Through a series of exposure, development, and etching steps quite complex structures can be created. In fact, it is the primary method used to produce the majority of integrated circuits used in modern technology (e.g. processors and memory found on smartphones and laptops). As one would expect when using light we are di;raction limited, but by using some very clever tricks (beyond the scope of this topic) are used to achieve impressive feature sizes (5 nm being mass produced since 2020). Masks (expensive!), photoresists, and UV light are used to produce nanometer sized structures. Each step is very fast, as the entire surface can be patterned at once. It also requires a vacuum for high resolutions and a clean room to ensure no particles in the air contaminate the sample. Since material is being etched away, it is a top-down approach. Source: Stanford There is incomplete content above. You must complete this before you can proceed through the course. So how does it work? Let's use the diagram below to walk through the most common sequence of steps in a photolithographic process, starting from the top left and following the arrows. 1. The substrate is coated in a thin Alm of material which we wish to selectively etch (e.g. a metal). 2. A thin layer of photoresist is coated on top of the bottom Alm, which will react with the light in the next step. 3. A mask is brought close to the surface, which is similar to the limestone template shown earlier, and exposed to UV light. The mask will ensure only the desired areas of the surface/photoresist will react with the light. 4. The sample is then washed, which removes the photoresist only in the areas which had been exposed to the UV light. 5. The unprotected parts of the original thin Alm are then carefully etched away, e.g. down to the substrate. 6. The photoresist is stripped away, and the sample is ready to be coated with another thin Alm and the whole process repeated again with a di;erent mask. By repeating this many times and layering on di;erent materials throughout, an intricate structure can be created. Thin film Photoresist Coating Mask align Substrate (1) Exposure (2) Development Stripping Etching There is incomplete content above. You must complete this before you can proceed through the course. Electron Beam Lithography (EBL) (Completed) Electron Beam Lithography (EBL) Electron beam lithography uses the same principle as photolithography, but uses electrons instead of photons to pattern a resist. As the electron beam can be controllably scanned across the surface, a mask is not required. The resist used will also be special in that it is electron-sensitive (e.g. PMMA), and will be used to deprotect the pattern to be etched. Feature sizes down to 2 nm have been achieved, but 20 nm is more typical. Unlike photolithography, EBL is slow as the electron beam has to move across the entire surface rather than exposing the surface completely in one go. A vacuum is also required, for reasons seen previously about the mean free path of an electron through various pressures. Again, this is a top-down approach. Source: NFFA There is incomplete content above. You must complete this before you can proceed through the course. Nanoimprint Lithography (NIL) (Completed) Nanoimprint Lithography (NIL) Much like in traditional lithography, nanoimprint lithography (NIL) uses a mold or stamp to transfer a pattern to a surface in one of two ways: Can use a thermoplastic resist that will All the gaps of the mold when this is pressed down, and, upon curing (hardening) with heat, will leave behind the desired pattern. This is a top-down approach and can be seen on the left. Dipping the mold in a solution containing self-assembling molecules (“ink”) allows it to be used as a “stamp” on the surface, leaving behind the desired pattern. Referred to as a template-assisted bottom-up technique, shown on the right. Low-cost and high throughput, with up to 10 nm resolution. However, the original mold does need to be produced using a di;erent lithographic technique (can self-replicate after this!). Mold Stamp Resist Monolayer "ink " Substrate Substrate Pressure + Time Heat There is incomplete content above. You must complete this before you can proceed through the course. Recap: Nanolithography Recap: Nanolithography    0:00 / 11:40  1x   There is incomplete content above. You must complete this before you can proceed through the course.   2. Scanning Probe Lithography (SPL) (Completed) 2. Scanning Probe Lithography (SPL) We saw in the last topic that we can measure many di>erent tip-sample interactions; why not exploit these to create patterns on a surface? To achieve this, we use a scanning probe (tip) to interact with the surface and modify it, by putting energy into the sample, rather than just measuring the interaction. As before, this can be done in any environment (air, liquid, vacuum), and is the convergence of top- down and bottom-up nanofabrication. As with imaging, there are many di>erent types of stimuli we can use to produce a pattern: Surface tension Mechanical Optical Thermal Oxidative Electrical Magnetic Here we will be focusing on those in bold writing. There is incomplete content above. You must complete this before you can proceed through the course. Self-Assembled Monolayers (SAMs) (Completed) Self-Assembled Monolayers (SAMs) Key to many of the following techniques are self-assembled monolayers (SAMs). These are organic assemblies that are formed spontaneously by the adsorption of molecular constituents from solution or gas phase onto a substrate with a speciOc aPnity of its headgroup. By changing the functional group on the end of the tail (very easy) one can a>ect the surface properties. In the diagram below we can see how a molecule such as dodecylphosphonic acid (DDPA) may attach to a titanium oxide surface. The −PO3H end of the molecule will bind to the oxide, while the long alkyl chain will extend at an angle from the surface. At the other end of the molecule is a methyl group, so if we cover the entire surface in a monolayer of DDPA then the whole surface will be non-polar (and hydrophobic). By changing the methyl group at the end of the alkyl chain (tail) to something like an amino (−NH2), we could make the surface reactive, and if we do this selectively, then only the parts that we want to be reactive will be. This will make more sense once we go through a couple of examples later. Since we are forming structures from constituent molecules, this is a bottom-up approach to nanofabrication. There is incomplete content above. You must complete this before you can proceed through the course. Below is a table of the adsorbates (headgroups) which attach to particular substrates (surfaces). For example, thiols (−SH) will bind very well to precious metals (gold, silver, platinum), other metals (palladium, copper, mercury), and semiconductors. A carboxylic acid (−COOH) headgroup will bind very well to aluminum and titanium oxides, and a silane (−Si(x)3) binds strongly (permanently) to silicon dioxide. While you do not need to memorize the entire table, the above examples are worth remembering. Dip-Pen Nanolithography (DPN) (Completed) Dip-Pen Nanolithography (DPN) The Orst scanning probe technique we will be looking at is dip-pen nanolithography (DPN), which uses the concept of surface tension and SAMs to e>ectively allow one to "write" on a surface much like one would with a pen (hence the name). DPN uses the meniscus of water that forms between an AFM tip and the surface to deliver self-assembling molecules onto the surface via capillary forces. A reservoir of this molecular "ink" ensures there is a fresh supply as the tip moves across the surface. Since we have full control over the position of the tip, we can use the scanner to “draw” SAMs onto the surface, creating the desired pattern. AFM tip SAM Meniscus Substrate On the left we can see a SEM image of a DPN tip in contact with a surface, including the small meniscus at the end of it. On the right we can see a small pattern created by attaching some 16-mercaptohexadecanoic acid molecules onto a gold surface. Only in the regions over which the AFM tip moved over can we see the "raised" parts of the image. While this speciOc example will have no practical use, the same principle can be used to create more complex structures (e.g. sensors). Original paper available here. Recap: SAMs & DPN (Completed) Recap: SAMs & DPN    0:00 / 4:54  1x   Example: Selective Polymer Growth (Completed) Example: Selective Polymer Growth In this example, DPN was used to selectively attach some molecules that would serve as a polymerization site. The rest of the surface was then coated in decanethiol to make it inactive, and a catalyst was used to activate the polymerization sites. Finally, by exposing the surface to a monomer solution, polymerization occurs at the originally designated locations, resulting in polymer brushes. This general sequence of steps is summarized to the left. On the right is an image showing the polymer brush arrays which the researchers created through their initial patterning using DPN. Below each image is a "line proOle" of a marked part of the image, showing to what extent the height changes between the bulk of the surface and the brushes (≈ 7 nm). Original paper can be found here. Near-Oeld Scanning Optical Microscope (NSOM/SNOM) (Completed) Near-Oeld Scanning Optical Microscope (NSOM/SNOM) The second stimulus we can use to create patterns on a surface is light, which is how the near-?eld scanning optical microscope (NSOM/SNOM) works. You might recall from the topic on super-resolution that we brie6y touched upon "true" sub-di>raction techniques. SNOM is one of these, as it is not bound by the di>raction limit. Below is a schematic diagram of the main parts of a SNOM, of which you should recognize the majority. The rest of the parts will be related to taking the excitation light and ensuring it is being placed just a few nanometers from the sample surface, as well as detecting it after going through the sample. You might be wondering, how does the SNOM get past the di=raction limit? By using "near-Oeld" evanescent waves. excitationlight fibercoupler fibercoupler fiberprobe shear-forcecontrol Xhighvoltage piezotubescanner Yhighvoltage Zhighvoltage sample scanningand obiective feedbackcontrol filter computer PMT Evanescent Waves (Completed) Evanescent Waves Evanescent waves are a result of the requirement for the boundary continuity of Maxwell's equations of electromagnetic radiation. Should you wish to learn more, this should allow you to Ond the relevant information, but you do not need to know the details for this module. In practice, when light is passed through an aperture that is smaller than its wavelength, light will come out of it parallel to the aperture and without being di=racted. However, this light decays exponentially with distance (within 100 nm or so) and thus it is known as an evanescent wave and occurs in the near-?eld.Further out, in what is known as the far-?eld, light becomes di>racted limited again. If we are able to keep an aperture (e.g. at the end of an optical Obre) close to the surface, we have a way to only probe a tiny portion of the sample with light at any given time, and below the di>raction limit. Evanescent waves are also exploited by surface plasmon resonance (SPR) techniques which you may encounter later in your studies. In the diagram below we can see how the blue light comes out of the small aperture and begin by having a similar width to the aperture (this is the near-Oeld region). By keeping the sample within this near-Oeld, it is possible to capture images using light at resolutions beyond the di>raction limit (and without any clever tricks as we saw in the topic on super-resolution). Depending on how the di>erent crystal regions of the sample interact with these evanescent waves, the contrast of the image will change. Recap: Evanescent Waves & SNOM (Completed) Recap: Evanescent Waves & SNOM    0:00 / 6:26  1x   NSOM/SNOM Tip (Completed) NSOM/SNOM Tip The aperture of the tip used will determine the resolution capable by a SNOM. Below is an electron micrograph of the aperture of a typical SNOM probe. Tips for SNOM are normally formed by etching a glass Obre in HF, coating the end in a recective metal, and cutting o> the end. As in other scanning probe techniques, the tip is then brought close enough to the surface so that the evanescent waves interact with the surface. Example: SNOM Imaging (DNA) (Completed) Example: SNOM Imaging (DNA) Before we move onto using SNOM for patterning, below is an image of DNA captured with the instrument. The thicker strands are double helix DNA, while the thinner ones are just a single helix. Z-range:1.485nm Z-range:591.2pm Y-range:3um -1.50 1.50 1.50 1.50 X-range:3um X-range:3um Source: École polytechnique fédérale de Lausanne Example: SNOM Protein Patterning (Completed) Example: SNOM Protein Patterning In this example, a SNOM was used to selectively pattern a surface using green cuorescent protein (GFP). First, a SAM of photoactive molecules was attached to a surface, which when exposed to UV light would cleave in half leaving behind a reactive functional group (shown in red in the diagram on the left). By exposing the surface to a solution containing GFP, the proteins would then attach to the desired locations. By using a SNOM, researchers were able to expose only parts of the surface to UV light, ultimately allowing for nanoscale structures to be created. In the image to the right, (a) shows a complex structure, (b) demonstrates that proteins are speciOcally attaching to the patterned regions, and (c-d) show that features below 100 nm are possible using this technique. Original paper available here. It is also possible to repeat the UV exposure and protein adsorption steps multiple times with a di>erent cuorescent protein each time. This can be seen below where an additional pattern has been added on top of the previously patterned and imaged sample (except on the right, where a di>erent sequence was patterned). These were captured using a confocal microscope, and therefore the colors seen are real. Nanoshaving/scratching (Completed) Nanoshaving/scratching The Onal SPL technique we will be looking at is known as nanoshaving or nanoscratching, which quite simply uses an AFM tip pressed hard into the sample and moved across to physically remove any attached molecules, as the name would imply. The tip would normally need to be quite sharp in order for the pressure involved to be great enough to physically cleave chemical bonds. Below is a schematic of what is happening in practice. Example: Nanoshaving (Completed) Example: Nanoshaving Below are a couple of patterns that were created using an AFM to "shave o>" molecules from a SAM, imaged using the very same instrument (with a fresh tip, as the one used for shaving would become blunt). Advantages & Disadvantages of SPL (Completed) Advantages & Disadvantages of SPL There are a number of reasons why one might wish to use SPL for creating nanostructures, as well as a major disadvantage. These will be mostly the same as for SPM techniques. Advantages High resolution Sub 10 nm is very achievable Versatile Can choose a tip-surface interaction that suits your system Single-step process Direct-write, unlike some other nanolithographic processes (e.g. photolithography) Not all SPL techniques require a vacuum Many work even in liquid Generally simple sample preparation Self-assembled monolayers are cheap and easy to produce Uses the same equipment as for imaging Some additional parts may be required, however Disadvantage Speed Have to move a tip across the surface sequentially, which takes time (limited throughput) Parallel SPL (Completed) Parallel SPL We can overcome the serial nature of SPL by using many probes in parallel. By having an array of thousands of cantilevers, can e>ectively be considered to be equivalent to other parallel processes. On the left is an electron micrograph of IBM's millipede memory device, which used heat at each of its thousands of cantilevers to store and read data. Meant to be a competitor to traditional hard drive storage, advances in cash memory meant it never made it to market. On the right is a similar design, but this time being a modiOcation of a SNOM to write in parallel and dubbed the snomipede (after the millipede).   3. Atomic Manipulation & Conclusions (Completed) 3. Atomic Manipulation & Conclusions Atomic Manipulation (Completed) Atomic Manipulation One 5nal way to manipulate matter at the nanoscale is by using the scanning tunneling microscope. If using enough bias voltage, it is possible to pick up individual atoms (or molecules) and move them anywhere on the surface. This was 5rst shown by IBM in 1989 by writing the company name using 35 xenon atoms (left image). On the right, by arranging iron atoms into a circular "corral" on a copper surface, it is possible to see the eLect of surface wavefunctions as a series of ripples on the surface. Remember, the tunneling of electrons is dependent on the wavefunction in both the tip and surface. Recap: Atomic Manipulation (Completed) Recap: Atomic Manipulation    0:00 / 2:05  1x   IBM researchers got pretty good at manipulating matter in this way as can be seen in the image below, in which carbon monoxide molecules have been arranged on a copper surface. A Boy and His Atom (Completed) A Boy and His Atom They even went so far as to create a small stop-motion animation (called "A Boy and His Atom") using a few dozen carbon monoxide molecules on a copper surface. As before, the ripples seen are due to surface wavefunctions.    0:00 / 1:05  1x   Conclusion (Completed) Conclusion In this part of the module, we have seen a great number of techniques, and how they allow us to gather valuable information about the sample we are investigating. At 5rst, we saw how we can use light as a probe to determine how diLerent parts of a sample interact with the light. By using Suorescent tags, we are also able to be more selective as to what parts of a sample we wish to see. However, these techniques were limited by diLraction, which super-resolution methods aim to overcome. Mainly, we looked at how the eLective spot size can be reduced by forcing Suorophores to emit light at diLerent wavelengths, or by using time to diLerentiate between spots that are close together. We then saw how we can use the wave-particle duality to use electrons to image a sample, either by measuring secondary electrons or those transmitted through a thin sample. Additional chemical information could also be collected by either measuring emitted X-rays or the change in kinetic energy of the measured electrons. Finally, in the last two topics we saw how we can use a physical probe that is moved close to a surface to gather an image, depending on what the interaction between tip and sample is. For example, we might want to measure a current between tip and sample (STM), or measure the force experienced by a cantilever deSecting (AFM). By collecting this data at every point horizontally and vertically, an image is able to be collected. We can also use these same instruments to apply the principles of lithography, creating nanoscale features and structures on a surface to be used in the fabrication of devices. There is incomplete content above. You must complete this before you can proceed through the course.