Page 61 - An Introduction to Microelectromechanical Systems Engineering

40 Processes for Micromachining

For deposition below 400ºC, nonstoichiometric silicon nitride (Si N )is
x y
obtained by reacting silane with ammonia or nitrogen in a PECVD chamber.
Hydrogen is also a byproduct of this reaction and is incorporated in elevated
concentrations (20%–25%) in the film. The refractive index is an indirect measure
of the stoichiometry of the silicon nitride film. The refractive index for stoichiomet-
ric LPCVD silicon nitride is 2.01 and ranges between 1.8 and 2.5 for PECVD films.
A high value in the range is indicative of excess silicon, and a low value generally
represents an excess of nitrogen.
One of the key advantages of PECVD nitride is the ability to control stress
during deposition. Silicon nitride deposited at a plasma excitation frequency of
13.56 MHz exhibits tensile stress of about 400 MPa, whereas a film deposited at a
frequency of 50 kHz has a compressive stress of 200 MPa. By alternating frequencies
during deposition, one may obtain lower-stress films.

Spin-On Methods
Spin-on is a process to put down layers of dielectric insulators and organic materi-
als. Unlike the methods described earlier, the equipment is simple, requiring a
variable-speed spinning table with appropriate safety screens. A nozzle dispenses
the material as a liquid solution in the center of the wafer. Spinning the substrate at
speeds of 500 to 5,000 rpm for 30 to 60 seconds spreads the material to a uniform
thickness.
Photoresists and polyimides are common organic materials that can be spun on
a wafer with thicknesses typically between 0.5 and 20 µm, though some special-
purpose resists such as epoxy-based SU-8 can exceed 200 µm. The organic polymer
is normally in suspension in a solvent solution; subsequent baking causes the solvent
to evaporate, forming a firm film.
Thick (5–100 µm) spin-on glass (SOG) has the ability to uniformly coat surfaces
and smooth out underlying topographical variations, effectively planarizing surface
features. Thin (0.1–0.5 µm) SOG was heavily investigated in the integrated circuit
industry as an interlayer dielectric between metals for high-speed electrical intercon-
nects; however, its electrical properties are considered poor compared to thermal or
CVD silicon oxides. Spin-on glass is commercially available in different forms, com-
monly siloxane- or silicate-based. The latter type allows water absorption into the
film, resulting in a higher relative dielectric constant and a tendency to crack. After
deposition, the layer is typically densified at a temperature between 300º and 500ºC.
Measured film stress is approximately 200 MPa in tension but decreases substan-
tially with increasing anneal temperatures.

Lithography
Lithography involves three sequential steps:

• Application of photoresist (or simply “resist”), which is a photosensitive
emulsion layer;
• Optical exposure to print an image of the mask onto the resist;
• Immersion in an aqueous developer solution to dissolve the exposed resist and
render visible the latent image.