Scanning Electron Microscopy

Atomic Layer Deposition (ALD) is similar to CVD except that the ALD reaction occurs at lower temperature and it is broken into two half-reactions, keeping the precursor materials separated during the reaction. The separation of the precursors is important to achieve a self-limiting surface reaction to enable precise thickness control and ensure uniform coatings and compactness even in complicated 3D structures.

The present implementation of the technique is suitable for mm to cm-size samples as those typically studied by spectroscopy and microscopy.

ALD has a rich history in microelectronics. It is studied as a potential technique to deposit high-k (high permittivity) gate oxides, high-k memory capacitor dielectrics, ferroelectrics, and metals and nitrides for electrodes and interconnects.

The number of materials that can be prepared by ALD has tremendously increased in the past decade. Notably, it can deposit a wider variety of materials, including important functional oxides like Al₂O₃, MoO₃, SiO₂ and HfO₂.

PLD is a physical vapor deposition technique in which the pulsed beam of a high-power ultraviolet laser is focused inside a vacuum chamber on a target. The target is usually a sintered ceramic or a single crystal with the chemical composition of the film that is to be deposited. A significant removal of material occurs above a certain threshold energy density (depending on the material, laser wavelength and pulse duration). The ejected material forms a luminous ablation plume directed towards a substrate placed front the target at a distance of 4-8 cm, where it re-condenses to form a film. Usually the plume (composed by neutrals, ions and electrons, and more complex species) conserves the stoichiometry of the target. The deposition can occur either in ultra-high-vacuum as well as in a background gas (up to 1 mbar) such as oxygen, which is commonly used when depositing oxides.

A main characteristic of PLD is that the ablated material only arrives at the substrate during a few microseconds after each the nanosecond laser pulse. It is also relevant that atoms and ions in the plume have an energy that can be controlled by the pressure during deposition and other parameters. PLD is compatible with in-situ and-real-time characterization tools, like reflection high-energy electron diffraction. A limitation of PLD is the small deposition area, generally less than 1 cm² in standard set-ups (although a system for large area PLD up to 4 inches is available in the consortium). Another limitation is that the plume is highly directional, and thus not suitable for conformal deposition of non-flat substrates.

PLD has been used to deposit films of a variety of materials. It is extensively used to produce copper oxide superconductors, ferroelectric and ferromagnetic oxides, etc. Its use is not limited to grow complex oxides, but can be extended to other materials, including diamond-like carbon, refractory materials, 2D materials and even metals.

A typical electron beam lithography tool is a vector-scan direct-write tool with a Gaussian shaped beam. The electrons are accelerated (typically 5 keV to 30 keV), depending on the application) and the beam scanning is controlled by special beam-deflection systems. The beam is directed to a position in the main deflection field, where a pattern is written by stepping around the electron beam.  For larger patterns, the pattern is divided into main field blocks, which are completely exposed one-by-one after mechanically moving the substrate to the right position. A laser interferometer can often measure the actual stage position, and this signal is fed back to the deflection system with a resolution even below a nanometer.

Electron-beam spot sizes can be focused to sub-10 nm in diameter. At the same time, a wide range of beam currents is available (depending on the tool, typically a few pA to 10 nA). This enables high-throughput as well as high-resolution exposures with a high degree of versatility. In addition, with high accelerating voltages, thick layers of e-beam resists can be exposed, with small forward electron scattering. With electron-beam lithography, aligning multiple levels of lithography with very high overlay precision is possible, with manual or automatic detection of alignment markers. Most systems can handle full wafers, mask blanks, and custom shaped samples and provide automatic laser focusing for height measurement. Users can implement or supply their designs in most standard formats, while specialized software is usually available for pattern preparation and proximity effect correction. Typical high resolution electron-beam resists are, among others, PMMA (positive tone) and HSQ (negative tone).

AFM is a surface sensitive technique permitting to obtain a microscopic image of the topography of a material surface. Typical lateral image sizes are within a range of only a few Nanometers to several 10 Micrometers, whereas height changes of less than a Nanometer may be resolved.

A fine tip attached to a cantilever is scanned across the material surface and enables to measure height changes exploiting an optical beam deflection system (a laser that is reflected from the rear side of the cantilever onto a segmented photodiode). The position of a laser spot on the photodiode permits to track height changes as e.g. due to a nano-particle on the surface or an atomic terrace of a single crystal surface. A feedback loop controls the tip-surface distance and therefore ensures stable imaging conditions.

Different operation modes like contact or non-contact mode can be used to optimize the imaging conditions with highest lateral resolution on one hand and least sample interaction on the other hand. Measurements in different environmental conditions (liquid, gas,...) on a broad class of samples (smooth, rough, insulating, conductive, soft, stiff, wet, dry...) can be performed.

Choosing suitable tips and imaging modes, additional surface properties can be mapped together with the topography, with similar spatial resolution, like friction force, magnetization and surface potential, surface charge density and electrical resistance, as well as elastic modulus and adhesion of heterogeneous sample surfaces can be obtained. 

Optical lithography is a fast patterning technique as all patterns are transferred to the photoresist through the mask in parallel. This requires that the relevant photomasks are already available or that they also need to be fabricated usually by electron beam lithography (in the case of non-standard patterns). Standard photomasks are made of fused quartz coated with a layer of chromium. During mask fabrication Cr is etched away where the geometric patterns need to be. This way UV light has a clear path through the mask, so as to expose the photosensitive resist and transfer the patterns, while Cr acts as a light-stop layer to prevent the illumination of the rest of the photoresist. The most common UV light wavelengths used in standard photolithography are the 436 nm ("g-line"), the 405 nm ("h-line") and the 365 nm ("i-line"), all being spectral lines of an Hg lamp. The achievable resolutions at these wavelengths go down to below 1 μm. There are different modes one can perform standard photolithography either with the mask being in contact with the resist-coated sample (contact lithography) or leaving a small gap (proximity lithography) each one having its own advantages and disadvantages.

The present implementation of the technique is suitable for mm to cm-size samples as those typically studied by spectroscopy and microscopy.