UV (Mercury Vapor Lamp)
Plasma etching operates on both high kinetic energy and chemical reactions between neutrals (N) and the substrate materials.
Commercially used silicon wafers are typically 525 μm thick.
PMMA (Polymethyl methacrylate) is the most popular photoresist material for the LIGA process.
Substrates that can be etched include Single crystal Silicon, Quartz, Ge, SiC, GaAs, and special polymers.
Dry etching involves the removal of substrate materials by gaseous etchants. It is more a physical than chemical process.
The inner surfaces of the photoresist mold produced by X-ray lithography need to be plated with thin metal layers for securing permanent microstructure geometry. Metals available for the plating are: Ni, Cu, Au, NiFe, and NiW.
Silicon has a diamond cubic crystal structure.
Isotropic etching is used for shaping 3D structures, removing surface damages, and cleaning surfaces.
A common source of energy for generating plasma in plasma etching is the radio frequency (RF) source.
The (110) planes offer the cleanest surfaces in microfabrications.
The Young's Modulus (E) for the <111> orientation in silicon is 186.5 GPa.
HNA is used for isotropic etching at room temperature.
The typical etching rate of silicon in <100> orientation using KOH is 0.25-1.4 μm/min.
Most etchants are used with a 1:1 by weight mixture with water.
Optical components are microfabricated parts used in optical systems, often created using the LIGA process.
The Young's modulus of silicon is approximately 2x10^5 MPa, which is about the same as that of steel.
Etching, which can be either dry or wet etching, is the principal technique used in Bulk Micromanufacturing.
Silver Nitride
The LIGA process was developed during the 1980s at the Research Center in Karlsruhe, Germany.
Single silicon crystals are basically of 'face-cubic-center' (FCC) structure.
The rate of conventional dry etching by ions is slow, at about 0.1 μm/min.
The (111) plane contains the shortest bonds between atoms, making it the strongest plane and the toughest to work with.
The lateral etch rate of Phosphosilicate glass (PSG) in the given etching process is 4.1670 μm/min.
Anisotropic etching is a process where etching is faster in one direction than in others.
Cancer biomarking involves the use of microfabricated devices to detect biomarkers associated with cancer.
The thermal expansion coefficient of silicon is about 8 times smaller than that of steel and more than 10 times smaller than that of aluminum.
The DRIE process provides thin films of a few microns protective coatings on the sidewalls during the etching process. It involves the use of a high-density plasma source and allows alternating processes of plasma (ion) etching of the substrate material and the deposition of etching-protective material on the sidewalls. Special polymers are frequently used for side-wall protective films.
Dry etching uses plasma to remove material at the desired locations on a substrate.
Tapered trenches are not desirable in applications such as resonators that involve pairs of 'centipedes-like' micro devices with overlapped 'fingers'.
Anisotropic etching is a type of etching that is easier to control but has a slower rate of etching (< 1 μm/minute) and is temperature-sensitive, requiring temperature-resistive mask materials for best performance at elevated temperatures.
A single silicon crystal has a total of 8 atoms: 8(1/8) from the corners, 6(1/2) from the faces, and 4 extra atoms in the interior.
(h k m) = designation of a 'face', or a plane; <h k m> = designation of perpendicular direction to the (hkm) plane; {h k m} = designation of family of planes.
Mask 1, made of silicon nitride (Si3N4), is used to cover the Phosphosilicate glass (PSG) layer and define the areas where the PSG will be etched away to create the beam’s support area.
The <100> direction is the easiest for etching.
(100)-orientated wafers form square-based pyramids with {111} surfaces, such as those used in solar cells.
Popular anisotropic etchants include alkaline aqueous solutions such as NaOH, LiOH, CsOH, and NH4OH.
X-Ray
The ions (+) attack the normal surface only, causing the etching front to advance much faster in depth than on the sides.
Photoresist materials in the LIGA process must be sensitive to x-ray radiation, have high resolution and resistance to dry and wet etching, have thermal stability up to 140°C, be absolutely insoluble during development, and have good adhesion to the substrate during electroplating.
The added material layers in surface micromachining can be 2-5 μm thick each, or as high as 5-20 μm thick each.
The unsymmetrical distribution of atoms within the crystal makes pure silicon anisotropic in its mechanical properties.
Intercepts: 1 ∞ ∞; Family: {100}
Polysilicon is deposited over the masked region using Chemical Vapor Deposition (CVD) to form the structural layer of the cantilever beam.
Three popular anisotropic etchants are KOH (potassium hydroxide), EDP (ethylene-diamine and pyrocatecol), and TMAH (tetramethyl ammonium hydroxide).
Micromotors are tiny motors used in various applications, often fabricated using microfabrication techniques like LIGA.
The LIGA process is a microfabrication technique that involves deep x-ray lithography, electroforming, and molding, known for its high initial costs and ability to create microstructures with virtually unlimited aspect ratios.
Isotropic etching is a type of etching that is orientation independent.
Substrates in the LIGA process must be electrically conductive to facilitate subsequent electroplating over the photoresist mold.
Surface micromachining is a process that builds up devices layer by layer using films deposited on a wafer. It creates 3-D microstructures by adding material to the substrate, which may not be the same as the substrate material.
Some disadvantages of surface micromachining include the need for multiple masks, which is expensive and time-consuming, and the requirement for sacrificial layers, which can be wasteful and present technical problems.
Silicon’s orientation in machining is often specified using Miller indices, e.g., Silicon (1 0 0).
A sacrificial layer is a material layer that is deposited and later removed to create a free-standing structure, such as a cantilever beam, in the surface micromachining process.
The etching process for removing sacrificial layers typically involves a mixture of 1:1 HF:H2O and 1:1 HCl:H2O, followed by rinsing with deionized water and drying under an infrared lamp.
The (111) plane makes an angle of 54.74 degrees with the (100) plane.
The selectivity ratio of silicon nitride to EDP is 10^4.
A capacitive accelerometer is a device that measures acceleration using changes in capacitance, often fabricated using microfabrication techniques.
The density of silicon is about 2.3 g/cm^3, which makes it as light as aluminum.
Bulk Micromanufacturing involves creating 3-D components by removing materials from thick substrates (silicon or other materials) using primarily etching methods.
PMMA
Interfacial stresses in micromachining are caused by a mismatch of the Coefficient of Thermal Expansion (CTE) between different layers.
The reactive neutrals (N) attack both the normal surface and the side walls of the substrate material.
Metals such as steel, copper plates, titanium, and nickel, or silicon with a thin titanium or silver/chrome top layer, and glass with thin metal layers can be used as substrates in the LIGA process.
The major steps in the LIGA process are X-ray irradiation, resist development, electroforming, and resist removal.
The three main techniques of dry etching are ion etching, plasma etching, and reactive ion etching.
The equation that defines a point on a plane is hx + ky + mz = 1.
The Young's Modulus (E) for the <100> orientation in silicon is 129.5 GPa.
Silicon compounds are much stronger etching resistive materials than silicon, making them suitable as masks for etching silicon substrates.
A microspectrometer is a miniaturized spectrometer used for analyzing the spectral composition of light, often fabricated using microfabrication techniques like LIGA.
Bulk micromanufacturing is a process that involves removing material to create microstructures, which is less expensive but results in high material loss and is suitable for simple geometries with low aspect ratios.
There is greater flexibility in design and manufacture with silicon because treatments and fabrication processes for silicon substrates are well established and documented.
Timing, agitated flow patterns, and the endurance of the masks can affect the geometry of an etched substrate during wet etching.
An etch stop is a method to stop etching, which can be controlled by doping or by an electrochemical etch stop. Doped silicon dissolves faster in etchants than pure silicon, and etching stops at the interface of p and n-type doped silicon.
Stiction is the collapsing of layers supported by sacrificial layers once they are removed by etching. It occurs due to Van der Waals and chemical forces between surfaces with narrow gaps, and is a serious technical problem in surface micromachining.
The interfaces of layers are vulnerable areas for structural failures due to the quality of adhesion of layers and interfacial stresses caused by a mismatch of the Coefficient of Thermal Expansion (CTE).
Anisotropic etching is a type of etching that is orientation dependent.
Wet bulk micromachining involves the use of chemical solvents, called etchants, to remove material.
DRIE is a plasma etching process that can produce deep trenches with nearly vertical sidewalls (θ ≈ 0), unlike the tapered angles produced by standard plasma etching.
A major advantage of the LIGA process is that it can produce microstructures with a high aspect ratio.
A major disadvantage of the LIGA process is the requirement of a special facility, specifically a synchrotron radiation (X-ray) source, which is very expensive.
The sacrificial layer in surface micromachining supports the structural layer until it is etched away to create cavities.
Examples of isotropic etchants include acidic etchants such as HF, HNO3, and HNA.
A typical FCC crystal structure has a total of 14 atoms: 8 at the corners and 6 at the faces.
In plasma etching, a chemical reactive gas, such as Dichlorodifluoromethane (R12), is mixed with plasma to aid in the etching process.
Plasma etching can increase the rate of dry etching to 2 μm/min, and it can be stretched to 5 μm/min, making it much faster and cleaner than wet etching.
Intercepts: 1 1 ∞; Family: {110}
Commonly used sacrificial layer materials in surface micromachining include Phosphosilicate glass (PSG), Silicon dioxide (SiO2), and Boronphosphosilicate glass (BPSG).
Pure silicon crystals are not isotropic in their properties due to the non-uniform distribution of atoms in their interior.
The <111> direction is the hardest for etching.
Alkaline chemicals with pH >12 are used for anisotropic etching.
The etching rate of silicon dioxide using EDP is 12 nm/hr.
Anisotropic etching was developed at Bell Laboratories.
Mechanical gear systems are microfabricated components used in various mechanical applications, created using the LIGA process.
A distance sensor is a device that measures the distance to an object, often fabricated using microfabrication techniques like LIGA.
Dry etching is good for most materials, has good directionality, low environmental impact, and good critical dimensional control but is expensive and has a variable etch rate. Wet etching is only suitable for single crystal minerals, has poor directionality, high environmental impact, and less critical dimensional control but is less expensive and has a fast etch rate.
LIGA stands for Lithographie (Lithography), Galvanoformung (Electroforming), and Abformung (Molding).
Nickel is a common material used in LIGA products.
Plasma etching involves the use of plasma, a neutral ionized gas carrying a large number of free electrons and positively charged ions, to remove substrate materials.
The (100) planes contain the least number of atoms, making it the weakest plane and the easiest to work with.
The Shear Modulus (G) for the <110> orientation in silicon is 61.7 GPa.
(110)-orientated wafers form perpendicular trenches with {111} side-walls, such as microchannels in micromechanics and microfluidics.
The selectivity ratio of silicon dioxide to KOH is 10^3 - 10^4.
Fluid control valves are microfabricated components used to control the flow of fluids, created using the LIGA process.
Single crystal silicon is popular for MEMS and microsystems because it is mechanically stable, can be integrated with electronics, has a high Young's modulus similar to steel, is lightweight like aluminum, has a high melting point of 1400°C, a low thermal expansion coefficient, shows virtually no mechanical hysteresis, and offers great flexibility in design and manufacture.
Miller indices are commonly used to describe the faces of crystalline materials.
Intercepts: 1 1 1; Family: {111}
The three distinct directions in which etching takes place are <100>, <110>, and <111>.
The selectivity ratio is the measure of resistivity to etchants, defined as the etching rate of silicon divided by the etching rate of the material using the same etchant.
A polarization sensor is a device that detects the polarization state of light, often fabricated using microfabrication techniques.
Silicon is considered an ideal candidate material for sensors and actuators because it shows virtually no mechanical hysteresis.
A microgyroscope is a small-scale gyroscope used for measuring or maintaining orientation, typically fabricated using microfabrication techniques.
Surface micromachining is a process that involves building layers of materials over a substrate, requiring complex masking and etching of sacrificial layers, and is suitable for creating complex geometries.
Drug delivery systems are microfabricated devices designed to deliver drugs in a controlled manner, often created using the LIGA process.
Silicon wafers are extremely flat, which is beneficial for coatings and additional thin film layers that can be integral structural parts or perform precise electromechanical functions.
The melting point of silicon is 1400°C, which is about twice as high as that of aluminum. This high melting point makes silicon dimensionally stable even at elevated temperatures.
