RF VNA characterization with probe station

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.

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).

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. 

Electrochemical deposition or electroplating is generally used for the deposition of metallic films (Au, Ni) with various advantages. The thickness of the coating can be precisely controlled by adjusting the electrochemical parameters. In addition, compared to standard metal deposition techniques, higher deposition rates can be obtained. The electroplating technique is relatively inexpensive as there are no requirements for high vacuum or high temperature systems.  Moreover the resulting coatings are relatively uniform and compact and standard resists can be used as masks during the electroplating process. Last, electroplating is ideal for relatively thick coatings and structures with high aspect ratios.