Why do we care?
Most silicon wafers are available with the large surfaces oriented along the (100) or (111) directions.
Silicon Prefers to Shatter along (110) planes, which intersect the surface of a (100) wafer along lines parallel and perpendicular to the "flat"
When silicon is deposited on surfaces, it takes a polycrystalline form : it is composed of silicon crystallites with dimensions from 0.1 to more than 10 microns.
Deposited silicon layers are generally used as electrical connections in microelectronic circuits.
There are several deposition techniques available (discussed later)
Polysilicon has mechanical properties similar to ordinary silicon, except that there are no crystal planes. It fractures at lower stress, and in random directions.
Oxide is a useful electronic material (low dielectric constant, very high resistance, easily formed, excellent adhesion)
Bare silicon grows a thin oxide layer in room air at room temperature with a thickness of about 30 Å. By heating to 900C, thickness as large as 1 um result after an hour.
Well-established deposition techniques also exist.
Oxide has a smaller thermal expansion coefficient than silicon, so oxide layers on silicon are usually in compression relative to the substrate.
Silicon oxide may be deposited by a chemical vapor deposition process at temperatures as low as 400C. The process involves reacting silane (SiH4) and Oxygen (O2) to make SiO2.
This temperature is low enough that other electronic materials are not destroyed. The deposition process is 'conformal', meaning that it covers all available surfaces.
LTO was developed for final layer depositions in electronics processes. After patterning, it may be re-melted ('reflow') to smooth the surface.
Because its deposition temperature is lower, its compression relative to the substrate is lower than for grown oxides.
LTO may be doped with impurities to make it conductive.
Nitride is also an excellent electronic material. It has very high resistance, low dielectric constant, and is impervious to oxygen. It resists more chemical etches than silicon. Nitride deposition processes are well-established.
Nitride may only be deposited via chemical reaction(deposition temperature 700C). The process involves reacting silane (SiH4) and ammonia (NH3)It has a larger thermal expansion coefficient than silicon.
Nitride is a better insulator than LTO, and is conformal. It does not melt until temperatures above 1100C
Nitride is very popular as an isolation layer between devices and substrate in electronics fabrication.
The chemical composition of nitride may be altered during deposition to ratios different than 3 silicon atoms and 4 nitrogen atoms.
At ratios of about 3 silicon atoms to 2 nitrogen atoms, the thermal expansion coefficient matches silicon. Therefore, these films are very nearly relaxed with respect to the substrate.
Low-stress nitride deposition is only important for micromechanical device fabrication, and was recently developed.
Low Stress nitride offers excellent mechanical characteristics, including excellent hardness, low density, and resistance to most chemicals.
Metal films are also used in electronics circuitry. Aluminum is widely used (low deposition temperature, stable properties), as is tungsten (stable at higher temperatures).
Metal films are generally deposited by evaporation or sputtering.
The mechanical properties of metal film structures are not well understood, and differ greatly from bulk properties because of the complicated grain structure, as well as the presence of surface layers.
Metal layers are generally used to make low-resistance electrical connections between semiconductor devices
Photoresist is an organic material used in lithographic pattern transfer processes. It offers the property that UV photons break chemical bonds in the molecule, allowing `exposed' regions to be etched with a 'developer'
Photoresist is deposited by spin casting (dribbling onto a spinning wafer), and baking at 100C. Typical layer thickness is 0.5 microns
Materials may be deposited on surfaces by heating to above the melting point in a vacuum system. At high temperatures, material begins to evaporate from the source, and travel by line of sight to the target.
Coating is generally by line of sight, so textured surfaces are not coated uniformly.
Materials which are evaporated include most low-melting point metals, (Al, Au, Ti, Pt, Cu, Cr,...) and some dielectrics (SiO, ...).
High vacuum is required to prevent reaction of evaporated material during flight to target.
Sputtering is a process in which a beam of atoms (Argon, generally) hits a metal target, and knocks some material off.
Sputtering is favored for high-melting point materials (W, Pt, Ceramics,...).
Sputtering generally coats surfaces conformably, because the chamber pressure is high enough (the Ar atoms...) that the mean free path is small.
Chemical Vapor Deposition (CVD) is any process in which a chemical reaction between gasses produces a deposited layer Variants include:
Low Pressure CVD (LPCVD) takes place in a quartz furnace at 100 mTorr at temperatures from 400-800C. Nitride, LTO are deposited this way. This process can coat both sides of as many as 200 wafers at a time. Uniformity is easily achieved.
Plasma Enhanced CVD (PECVD) Takes place in a metal chamber in which one of the gas constituents is excited into plasma discharge. The energy associated with the plasma excitation stimulates the chemical reaction and enables it to occur at low temperature. This process can coat 1 side of 10 wafers at a time. Uniformity can be difficult to achieve. Nitride and oxide may be deposited this way.
By using a patterned mask layer, etching allows the formation of patterned 'trenches'.
It is necessary to have an etch which is `selective' to the substrate with respect to the mask.
Silicon is etched by several different wet chemicals:
KOH solution. Potassium Hydroxide. This solution etches (100) silicon at about 1 micron/min when heated to 80C. It etches the (111) planes as much as 500 times more slowly. It also etches SiO2 300X more slowly. It does not etch nitride.
EDP solution. Ethylene Diamine Pyrocatechol etches (100) silicon at about 1 micron/min when heated to 85C. It etches (111) about 50X more slowly, depending on how much etching has already gone on. EDP etches SiO2 5000X more slowly. The fumes are toxic and flammable.
HF solution. Hydrofluoric acid etches SiO2 at about 0.05 microns/min at room temperature, etches silicon much more slowly, and doesn't etch nitride much at all. HF is extremely dangerous to humans.
When using the selective wet etches (EDP or KOH), it is important to keep the orientation selectivity in mind.
On (100) wafers, any etch pattern is eventually terminated by (111) planes which intersect the wafer surface parallel and perpendicular to the flat with a tilt of 53 degrees.
If the pattern is misoriented with respect to the wafer, a larger etch pattern would result:
Wet etching can be used to make many interesting micromechanical structures from wafers:
Wet etching can be used to make many interesting micromechanical structures from wafers:
Thin films of nitride may be released to form diaphragms and cantilevers, because the wet etches do not attack nitride.
Etch stop layers may be used to determine the final thickness of an etched structure.
For example, KOH and EDP have etch rates which can be dramatically reduced when the exposed surface is heavily doped. Wafers with buried doped layers are commercially available.
Buried Oxide layers are commercially available from wafer manufacturers. They are known as SOI (Silicon on Insulator) wafers, and offer a continuous oxide layer which is up to 5000Å thick and up to 1micron below the surface. This may be used to make thin structures of single crystal, undoped silicon.
When combined with wafer bonding to stack layers, and dopant diffusion to make strain gauges, wet etching techniques are used to create most of the micromachined silicon products available today.
Because the wafer thickness is related to the dimensions of structures which can be made, bulk micromachined devices are generally 100 microns - to - 1 mm in lateral dimension.
Chemical reactions between gas atoms and exposed regions of the substrate can be used to remove material. In most cases, fluorine-based gases (CF4, SF6) react with the exposed silicon surface, forming SiF2 or other compounds, which evaporate and are pumped away.
These reactions are generally not selective to different silicon crystal orientations, etching in equal rates in all directions.
These reactions generally require energy to run, so plasma excitation is generally required.
Dry etching is an especially good way to pattern thin films, such as oxide, poly, or nitride. This is generally referred to as Surface Micromachining.
These dry etching techniques often make regular use of deposited layers as spacers, to be removed later. These are called Sacrificial Layers.
Dry etching is more compatible with typical electronics processing than wet etching. In many cases, dry etching was developed specifically for electronics circuit applications.
One advantage of dry etching is the photoresist can be the mask material.
Dry etching generally applies only to thin films, since it is expensive to operate, and does not generally etch fast enough (0.05 microns/min) to go all the way through a wafer in a reasonable time.
The selectivity to silicon with respect to mask materials is not generally more than 10:1. This is another reason for limiting dry etching to thin films.
LIGA uses a synchrotron to expose regions in thick (>1mm) Plexiglas.
"Silicon LIGA" #1 Uses etched silicon surfaces and sacrificial oxide layer to create reusable mold for polysilicon structures.
"Silicon LIGA" #2 relies on use of a new deep dry etching process to produce narrow (10 microns) trenches all the way through a standard silicon wafer.
This takes advantage of the presence of a 'passivation layer' which forms on the sides of a trench when the substrate is cooled.
This process etches at 5 microns/minute, and will be an important way to make arbitrarily shaped 2-d structures in crystal silicon.
Analog Devices has introduced the first product based on an integrated micromechanical device with a microelectronic circuit. The ADXL50 is intended for automotive air-bag systems, but will have wider application.
This process allows co-location of decent circuitry (3 micron bipolar CMOS) with state of the art micromechanics.
This process is now open to outside designers as a foundry service.
An exciting assortment of new products should be enabled by access to this process
Available :
Bulk micromachined accelerometers
Bulk micromachined pressure sensors
Bulk micromachined valves
Surface micromachined accelerometers
Coming:
Microgyroscopes
Higher performance accelerometers and pressure sensors
Optical display devices
New Fabrication Technology
Good for making micromechanical sensors
Interesting for novel geometric structures
Expensive to have in-house
Not yet generic enough for foundry service
Mostly oversold.
Opportunities for fully integrated devices beginning to emerge.