Atomic Layer Deposition

In a scanning electron microscope (SEM), an electron beam is scanned over the sample surface in a raster pattern while signals from secondary electrons (SE) or Back-scattered electrons (BSE) are recorded by specific electron detectors. The electron beam, typically with an energy ranging from a few hundred eV up to 30 keV, is focused to a spot of about 0.4 nm to 5 nm in diameter. The latest generation of SEMs can achieve a resolution of 0.4 nm at 30 kV and 0.9 nm at 1 kV.

Beyond the ability to image a comparatively large area of the specimen, SEM can be equipped with a variety of analytical techniques for measuring the specimen's composition. Chemical composition analysis can be performed by Energy Dispersive X-ray Spectroscopy (EDS), which relies on generating an X-ray spectrum from the entire scan area of the SEM. An EDS detector mounted in the SEM chamber collects and separates the characteristic X-rays of different elements into an energy spectrum. The EDS system software then analyzes this energy spectrum to determine the abundance of specific elements. EDS can identify the chemical composition of materials down to a spot size and create elemental composition maps over a much broader raster area.

By employing a suitable polarimeter, such as a Mott analyzer, the spin polarization of secondary electrons can be detected, thus revealing the magnetization orientation in a ferromagnetic sample with high spatial resolution. This technique is known as Scanning Electron Microscopy with Polarization Analysis (SEMPA).

Scanning transmission electron microscopy (STEM) can be also performed in a SEM. It provides high-resolution images and detailed information about the composition and structure of samples, making it an essential tool in various scientific fields. STEM Images can be of different types: Bright-Field (BF) images formed by transmitted electrons that pass through the sample without significant scattering; Annular Dark-Field (ADF) images formed by high-angle scattered electrons, providing information about the atomic number distribution in the sample, and; High-Angle Annular Dark-Field (HAADF) images formed by very high-angle scattered electrons, revealing the heaviest elements in the sample with high contrast.

In front of or additionally to traditional photolithography, where the use of a photomask and a mask aligner tool or a reticle and a stepper are needed to transfer the designed layouts onto the substrates via a photosensitive resist coating, Direct Writing Lithography constitutes another photolithography technique which provides a fast and versatile technology especially suited for its use in R+D or in the prototyping stages.

In this technology, Direct laser writers are used in nsubstitution of traditional mask aligners or even steppers to directly expose the photoresist that is covering the surface of the substrates, being controlled by a software which reads the patterns in the designed layout (for instance GDS files) and guides a deflector to reproduce the patterns onto the resist.

Some tools can combine two approaches of photoresist exposition: the rasterized approach, for larger patterns, and the vectorial direct writing approach, where the radiation is focused to a narrow beam that is scanned in vector form across the resist, which allows the best resolution at the cost of lowering the exposition speed.

Noticeably, a key advantage of maskless lithography is the ability to change lithography patterns from one run to the next, avoiding incurring in most of the costs of generating a new photomask, both in economical and time terms.

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.

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. 

Dicing is typically used to separate die from a wafer to be mounted in a package or on a PC board. Some users dice a wafer into smaller pieces to process individually. It can be used to cut a wide range of materials and devices including Semiconductor devices, Ceramic Substrates, Thick-film Devices, Sensors, MEMS, Opto-electronic Components, IC Wafers.

Some tools are optimized for multi-angle dicing of thin, tight tolerance products up to 150 mm or 6" diameter and capable of more complex patterns such as multiple indexes and varying cut depths.

Wafers are typically mounted on dicing tape which has an adhesive backing that holds the wafer on a metal frame. The frame with the wafer on it is placed on the chuck of the dicing saw. The wafer is moved into an abrasive blade, usually diamond, rotating at typically 15,000 to 30,000 RPM. The abrasive chips away at the wafer as the blade rotates. Water is supplied to the blade to remove cutting residue and enhance cut quality.  

The cut depth is a function of the substrate thickness and could be optimized depending on the cut quality and blade loading. A good rule of thumb is to cut no more than about 500 µm of material per pass. Harder materials should be cut with shallower cuts and more passes to minimize blade wear. Deeper cuts require a thicker blade to minimize blade flexing or deflection.