Page 60 - An Introduction to Microelectromechanical Systems Engineering

Basic Process Tools 39

both PSG and BPSG soften and flow to conform with the underlying surface topog-
raphy and to improve step coverage. LTO films are used for passivation coatings
over aluminum, but the deposition temperature must remain below about 400ºC to
prevent degradation of the metal.
Silicon dioxide can also be deposited at temperatures between 650º and 750ºC
in a LPCVD reactor by the pyrolysis of tetraethoxysilane [Si(OC H ) ], also known
2 4 4
as TEOS. Silicon dioxide layers deposited from a TEOS source exhibit excellent uni-
formity and step coverage, but the high temperature process precludes their use over
aluminum.
A third, but less common, method to deposit silicon dioxide involves reacting
dichlorosilane (SiCl H ) with nitrous oxide (N O) in a LPCVD reactor at tempera-
2 2 2
tures near 900ºC. Film properties and uniformity are excellent, but its use is limited
to depositing insulating layers over polysilicon.
As is the case for the LPCVD of polysilicon, deposition rates for silicon dioxide
increase with temperature. A typical LTO deposition rate at low pressure is 25
nm/min at 400ºC, rising to 150 nm/min at atmospheric pressure and 450ºC; the
deposition rate using TEOS varies from 5 nm/min at 650ºC up to 50 nm/min
at 750ºC.
Deposited silicon dioxide films are amorphous with a structure similar to fused
silica. Heat treatment (annealing) at elevated temperatures (600º–1,000ºC) results
in the outgassing of hydrogen incorporated in the film and a slight increase in den-
sity, but no change in the amorphous structure. This process is called densification.
Silicon dioxide deposited using CVD methods is very useful as a dielectric
insulator between layers of metal or as a sacrificial layer (etched using hydrofluoric
acid) in surface micromachining. However, its electric properties are inferior to
those of thermally grown silicon dioxide. For example, dielectric strength of CVD
silicon oxides can be half that of thermally grown silicon dioxide. It is no coinci-
dence that gate insulators for CMOS transistors are made of the latter type. In
general, CVD silicon oxides are under compressive stress (100–300 MPa). The
stress cannot be controlled except when PECVD is used.

Deposition of Silicon Nitrides

Silicon nitride is common in the semiconductor industry for the passivation of
electronic devices because it forms an excellent protective barrier against the diffu-
sion of water and sodium ions. In micromachining, LPCVD silicon nitride films
are effective as masks for the selective etching of silicon in alkaline solutions,
such as potassium hydroxide. Silicon nitride has also been used as a structural
material.
Stoichiometric silicon nitride (Si N ) is deposited at atmospheric pressure by
3 4
reacting silane (SiH ) and ammonia (NH ), or at low pressure by reacting dichlo-
4 3
rosilane (SiCl H ) and ammonia. The deposition temperature for either method is
2 2
between 700º and 900ºC. Both reactions generate hydrogen as a byproduct, some of
which is incorporated in the deposited film. CVD and LPCVD silicon nitride films
generally exhibit large tensile stresses approaching 1,000 MPa. However, if LPCVD
silicon nitride is deposited at 800º–850ºC and is silicon-rich (an excess of silicon in
the film) due to a greatly increased dichlorosilane flow rate, the stress can be below
100 MPa—a level acceptable for most micromachining applications.