Page 59 - An Introduction to Microelectromechanical Systems Engineering

38 Processes for Micromachining

and pressure are important process variables for all types of CVD. Power and
plasma excitation RF frequency are also important for PECVD.

Deposition of Polysilicon
Chemical-vapor deposition processes allow the deposition of polysilicon as a thin
film on a silicon substrate. The film thickness can range between a few tens of
nanometers to several micrometers. Structures with several layers of polysilicon
are feasible. The ease of depositing polysilicon, a material sharing many of the
properties of bulk silicon, makes it an extremely attractive material in surface
micromachining (described later).
Polysilicon is deposited by the pyrolysis of silane (SiH ) to silicon and hydrogen
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in a LPCVD reactor. Deposition from silane in a low-temperature PECVD reactor is
also possible but results in amorphous silicon. The deposition temperature in
LPCVD, typically between 550º and 700ºC, affects the granular structure of the
film. Below about 600ºC, the thin film is completely amorphous; above about
630ºC, it exhibits a crystalline grain structure. The deposition rate varies from
approximately 6 nm/min at 620ºC up to 70 nm/min at 700ºC. Partial pressure and
flow rate of the silane gas also affect the deposition rate.
Generally speaking, LPCVD polysilicon films conform well to the underlying
topography on the wafer, showing good step coverage. In deep trenches with aspect
ratios (ratio of depth to width) in excess of 10, some thinning of the film occurs on
the sidewalls, but that has not limited using polysilicon to fill trenches as deep as
500 µm.
Polysilicon can be doped during deposition—known as in situ doping—by
introducing dopant source gases, in particular arsine or phosphine for n-type doping
and diborane for p-type doping. Arsine and phosphine greatly decrease the deposi-
tion rate (to about one third that of undoped polysilicon), whereas diborane
increases it. The dopant concentration in in-situ doped films is normally very high
(~10 cm ), but the film resistivity remains in the range of 1 to 10 mΩ•cm because
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of the low mobility of electrons or holes.
Intrinsic stresses in as-deposited doped polysilicon films can be large (>500
MPa) and either tensile or compressive, depending on the deposition temperature.
Furthermore, there is normally a stress gradient through the thickness of the film,
which results in curling of released micromechanical structures. Annealing at 900ºC
or above causes stress relaxation through structural changes in grain boundaries and
a reduction in stress to levels (<50 MPa) and stress gradient generally deemed
acceptable for micromachined structures.

Deposition of Silicon Dioxide
Silicon dioxide is deposited below 500ºC by reacting silane and oxygen in an
APCVD, LPCVD, or PECVD reactor. Due to the low temperature compared to
thermally grown oxide, this is known as low-temperature oxide (LTO). The
optional addition of phosphine or diborane dopes the silicon oxide with phosphorus
or boron, respectively. Films doped with phosphorus are often referred to as phos-
phosilicate glass (PSG); those doped with phosphorus and boron are known as
borophosphosilicate glass (BPSG). When annealed at temperatures near 1,000ºC,