PROCESS DESCRIPTION
P
ROCESS
M
ANUAL
4.1-1
4.1 Nitride from NH
3
and SiH
2
Cl
2
4.1.1
Basic configuration
NH
3
, SiH
2
Cl
2
(DCS)
Temperature ramp at 700-800
o
C
4.1.2
Description
4.1.2.1 Purpose
Nitride is used as an insulating and masking layer in electrical applications, and as an anti-
reflecting coating in optical applications.
4.1.2.2 Chemicals
SiH
2
Cl
2
, also known as DCS (DiChloroSilane), is a liquid at room temperature with a vapor
pressure of 16 psi. Due to this relatively low vapor pressure it easily condensates at cold spots
and care should be taken to prevent these.
The residue of the reaction of DCS and NH
3
is NH
4
Cl.
Some properties of NH
4
Cl:
Sublimation at 320
o
C @ 1 atm.
Sublimation at 120
o
C @ 100 mtor. See also its vapor pressure curve in the appendix.
Two different forms are likely to occur:
A white powdery form and a glassy solid form. The glassy form is what occurs as the gasses
cool down abruptly on a cold surface. This is the form that is wanted on the coldtrap. The
white form occurs from condensation in the gas phase, and will not be trapped by the
coldtrap. Instead, it will be found at the inlet filter of the dry pump and in the exhaust of the
same pump, or in the oil of a wet pump.
4.1.2.3 Process
The chemical reaction is as follows assuming a complete reaction:
3SiH
2
Cl
2
+ 7NH
3
Æ
Si
3
N
4
+ 6H
2
+ 3HCl + 3NH
4
Cl
The thickness uniformity decreases along the load due to depletion of DCS, much like
depletion of SiH
4
in the poly-Si process.
A temperature ramp as high as of + and – 30
o
C around the center temperature is sufficient to
overcome this problem. Cross-wafer uniformity is usually very good, since the deposition
process is limited by the surface reaction part. A decreasing process pressure does improve
the uniformity, mainly the cross load. If relatively small wafers are used compared to the tube
diameter an increased gas flow improves cross-wafer and cross-load uniformity dramatically.
