Optical-Lithography

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.

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.

The Vector Network Analyser is typically used to characterize the electrical response of a system as a function of frequency (from 10 MHz up to 40 GHz). The tool can apply a calibrated electrical stimulus of specific frequency and measure the reflected/transmitted voltage and current both in magnitude and phase. The resulting measurement presents a detailed electrical characterization of the system in terms of equivalent impedance of the network or device under test (DUT) as a function of frequency throughout the measured frequency range. Obtaining this information as a function of frequency enables detailed electrical description of complex electrical response in a simple and concise way. The technique is extremely powerful in the sense that it can provide complete electrical response information on any linear electrical system. It is commonly used to measure high frequency devices as well as measure the material properties in extended frequency ranges. The VNA can be coupled with an external dipole/quadrupole electromagnet to perform broadband FMR and spin wave spectroscopy. FMR VNA is a powerful all electric experimental technique that can be used to resolve fundamental material properties (e.g. saturation magnetization, damping coefficient, gyromagnetic ratio). The FMR VNA can sweep the frequency of the input signal and measure reflected/transmitted signal while the external magnet can sweep the magnetic field providing much bigger set of data. The quadrupole magnet can generate a vectorial magnetic field tunable in the sample plane with a maximum intensity > 0.2 T (> 0.15 T in the dipolar configuration). The sample is excited by passing the RF signals through a standing coplanar waveguide (CPW) over which the sample is positioned. The VNA setup can be used to detect spin waves propagating in the magnetic ordering of a material under study. The VNA can be connected to a pair of parallel microwave antennas through RF microprobes (e.g. coplanar waveguides and ground-signal-ground (GSG) RF probes). Each microstrip is connected to a separate port of the VNA. Spin waves are excited from the first antenna (transmitter) and detected by the second one (receiver). This analysis is most useful to understand the behavior of magnetic materials.

The principle of wet etching processes is the conversion of a solid material into liquid compounds using chemical solutions. The selectivity is usually very high, and for most solutions is greater than 100:1. Material removal rate for wet etching is usually faster than the rates for many dry etching processes and can easily be changed by varying temperature and concentration of the active species. Multiple wafers can be etched simultaneously in batch etching; in this case filters and circulating pumps prevent particles from reaching the wafers.

When a material is attacked by a liquid or vapor etchant, it can be removed with isotropic (uniformly in all directions) or with anisotropic etching. In anisotropic etching the liquid etchants etch crystalline materials at different rates depending upon which crystal face is exposed to the etchant.

Wet or dry etching processes are common steps in the technological process flow for obtaining the material under study well patterned, isolated and ready to be contacted.

In particular, the process flow usually contains the required suite of pattern transfer and/or etching processes for ancillary materials, such as silicon nitride, silicon oxide, polysilicon that can be used as etching masks, implantation and diffusion barriers, dielectric barriers, spacers or conductive layers. Those materials can be etched and patterned by chemical wet etching or by Reactive Ion Etching (RIE) using appropriate masks or making use of lift-off process. Those materials can be used themselves as etching masks (for instance in the case of Al layers for deep RIE patterning of substrates), or as adhesion or barrier layers or directly as layers for providing the required electrical contact. 

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.