Page 78 - Sami Franssila Introduction to Microfabrication

Thin-film Materials and Processes 57

Contacts to semiconductors: ohmic (metal-like) and aluminum black, act as IR absorbers. Metallic meshes
Schottky (diode-like) contacts are possible. Alu- act as IR filters.
minum, itself p-type dopant in silicon, makes good Sacrificial layers: Many devices require free-standing
ohmic contact to p-type silicon. Platinum silicide is structures. These must be fabricated on solid films,
one candidate for silicon Schottky contacts. which will subsequently be etched away. Copper
Capacitor electrodes: Capacitor electrodes need not be is often used as a sacrificial material under nickel
highly conductive. The most important capacitor or gold.
electrode, the MOSFET gate, is chosen to be Protective coatings: Sometimes the role of the topmost
polycrystalline silicon because its interface with layer is simply to protect the underlying layers from
silicon dioxide is stable, and its lithography and the ambient: from etching agents or environmental
etching properties are good. stressors. Nickel and chromium are used as masks
Plug fills: When vertical holes need to be filled with a for etching.
conducting material, CVD tungsten and electrodepo- X-ray components: Masks for X-ray lithography require
sition of copper are employed. high atomic mass materials that effectively block X-
Resistors: Doped semiconductors, metals, metal com- rays. Tungsten, gold and lead are prime candidates.
pounds and alloys can be used as resistors. Heating X-ray mirrors are made by alternating layers of heavy
resistors can be made of almost any material, but pre- (tungsten, molybdenum) and light materials (carbon
cision resistors are difficult to make. or silicon) of X-ray wavelength thicknesses.
Adhesion layers: Noble metals like gold and platinum
do not adhere well to substrates, and therefore The deposition process greatly influences the choice of
thin (10–20 nm thick) ‘glue’ layers of titanium or metals. Not all materials are amenable to all deposi-
chromium are needed. tion methods, and the resulting film properties (resis-
Barriers: Barriers are needed to prevent unwanted tivity, phase, texture, adhesion, stress, surface mor-
reactions between thin films. Amorphous metal phology) are closely connected with the details of
alloys and compounds like tungsten nitride (W:N), the deposition process, and may well be idiosyncratic
titanium–tungsten (TiW), TiN and TaN are the with the equipment. Reproducing results that have been
usual materials. obtained with another piece of equipment can be a night-
Mechanical materials: Aluminum and nickel are mate- mare.
rials for micromechanical free-standing beams and
cantilevers, in, for example, micromirrors and res-
onators. Films such as TiN can be used as mechanical 5.7.1 Properties of metallic thin films
stiffening layers to prevent mechanical changes in the
underlying softer films, like aluminum. Low resistivity is required in thin-film form. Thin-
Optical materials: Transparent conductors like indium- film resistivity is often much higher than bulk resistiv-
doped tin oxide (ITO; In x Sn y O 2 ) are needed in ity. Aluminum, copper and gold thin-film resistivities
displays and light-emitting devices. In image sensors, are close to bulk values; for most others, thin films
metals act as light shields, and in many micro-optical resistivities are factor of 2 higher. Metals of micro-
devices, as mirrors. TiN is often deposited on top of fabrication importance are listed below. Resistivities
aluminum to reduce reflectivity, because lithography are strongly deposition process–dependent as shown in
is difficult on highly reflecting surface. Table 5.2, and Table 5.4 should be used as a guideline
Magnetic materials: Nickel and nickel alloys, Ni:Fe, are only.
used in magnetic microactuators. Cores of microtrans- Alloys and compounds TiW, TiN x and TaN x have
formers are also made of these materials, which are resistivities that are even more strongly deposition pro-
usually deposited by electroplating. cess–dependent than simple metals, and the exact com-
Catalysts and chemically active layers: Chemical sen- position will also have a profound effect. Resistivities
sors often use films such as palladium and platinum of these metal compounds are usually in the range of
as catalysts. 100 to 500 µohm-cm.
Electron emitters: Vacuum microemitter tips are often Young’s moduli are the same order of magnitude
made of molybdenum because of its high melting for all metals, from 100 GPa for soft metals to
point and low work function. 600 MPa for refractory metals. Many metal properties
Infrared emitters and other IR components: Heated are related to melting point. High melting point equals
wires emit infrared, and porous metallic films, like high bond strength and stable atomic arrangement