Etching
Etching refers to the removal of bulk or coated material, typically though a masked pattern of semiconductor material, by exposing the material to a bombardment of ions that kinetically removes portions of the material from the exposed surface or immersing the substrate in wet-chemical which dissolves specific exposed materials. Typical etching processes include: reactive-ion-etching, wet metal etchants and oxygen plasma cleaning. These processes can remove layers thickness ranging anything from a few angstroms to millimetres.
Physical dry-etching
Ion Milling is a physical etching process achieved by the bombardment of ions of an inert plasma (usually Ar) which are accelerated from a wide beam ion source into the surface of a substrate in vacuum in order to etch nanometric layers of variety of materials to some is a desired. Several parameters such as the energy, angle and pressure are adjusted during the process to improve the processing time and the quality of the surface roughness. The continuous kinetic attack of the substrate by the Ar+ ions results in of heating of the substrate due to the conversion of some of the kinetic energy to heat. Therefore, the cooling of the substrate is essential to prevent surface damage. In the fields of failure analysis, device QA, and surface characterization, a defect-free substrate is vital. Thanks to the advancement in complex materials and miniaturization of assembled parts, high quality parts surface is becoming more and more relevant as the need to come up with novel and more rapid to prepare samples. Applications such as electronics, semiconductors, biomaterials, often require the use of cross-section technique and for that purpose ion milling can be used directly on the surface. When used a high angled steps are created and can reveal the cross-section of the materials in interest. The fact that this process can etch numerous materials means that even surfaces which are not possible to prepare with traditional methods can be processed by ion milling.
Chemical dry-etching
Reactive ion etching (RIE) is a plasma based material removal procedure where radiofrequency (RF) created radicals and ions species in order to etch substrate or thin films in a low-pressure reaction chamber. RIE the possible thanks to the interaction between chemically active gas and controlled energized ion kinetic bombardment. The advantage of RIE is of being faster than either noble gas physical plasma bombardment or spontaneous radical etching of reactive gases. Because ion bombardment can be directional by directing the bias, RIE has anisotropic character, with controlled ratio of lateral and vertical etch rates, which can lead to 90⁰ sidewalls or complexed shaping of pillars. RIE is essential when narrow lines or channels are needed, or when high aspect ratio structures need to be fabricated. RIE of silicon is independent of crystal planes, and therefore any shape can be fabricated, unlike anisotropic wet etching.
Deep reactive ion etching (DRIE) is a variant of the RIE process that offers significantly increased etching rates of deep 3D features. DRIE can be used for anisotropic dry-etching process that require high-aspect ratio (depth to diameter), especially when the goal is to etch a much deeper steps compared to what is possible in RIE. When etching through apertures in RIE, high aspect ratio etching causes either increasingly reduction in etch rate or oval shape cavities. The most widely used technology is the Bosh process, involving three steps that are repeated until the achievement of the desired shape of the cavity:
Deposition step: Isotropic passivation layer coating, using Octafluorocyclobutane (C4F8) that creates a Teflon-like substance on the surface of the substrate and sidewalls.
Etch A: Relatively soft etching anisotropic process, using Sulfur Hexafluoride (SF6), which perpendicularly attacking the substrate plane leaving passivation layer only along the sidewalls;
Etch B: The main etching process, again with Sulfur Hexafluoride, which only etches Si isotopically, and does not penetrate the Teflon-like coating.
The time period of each step is several seconds and the cycle is repeated for several iteration, until the required etched is reached. These three etch/deposit steps are repeated many times, resulting in etching cavities with aspect ratios of 50 to 1 and with etching rates reaching 10 times higher in comparison to wet-etching.
DRIE is frequently used in microfabrication to create channels, via holes and dies separation in different substrates.
Plasma cleaning is the process of removing all organic matter from the surface of an object through the use of a ionized gas of oxygen, argon, nitrogen and more. This process is performed in a vacuum chamber with cooled chamber for controlled process. The cleaning process is safe, environmental clean and does not require special treatment for the waste evacuation, as there are no harmful chemicals involved. Plasma can also change the surface energy and leave free radical moiety on the cleaned surface fit for chemical reactions and hydrophilic uses. Plasma cleaning can be used for activation of a metallic, ceramic or polymer surfaces in order to improve its ability to be painted or printed on. When using oxygen plasma the surface is oxidize and can attach other materials better. This process has the ability to not only clean materials, but also increase polar groups, directly improving its printability and coatability.
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Wet-chemical etching
Wet etching is a material removal process in which solvent, chemicals or etchants are used to remove materials from a wafer or create features. When specific patters are required, photoresist or metal masks can be utilized in order to defined the features. Any Materials that are not protected by this masking is dissolved away by chemical reaction. enchants can be metal based, organic, acidic, basic and buffered and can specifically etch a certain material without damaging another.
Normally enchants can remove nano-micro scale thick metal layers or resists. KOH, how ever, can remove up to 100 micrometer/hour mono crystalline Si from bulk substrate and create deep trances.