This presentation is a part of course on Microelectronics Fabrication Process Flow. It discusses a technique for deposition

1. Low Pressure Chemical Vapour
Deposition (LPCVD)
Sudhanshu Janwadkar
TA, ECED, SVNIT

2.  Chemical vapor deposition (CVD) is a process in
semiconductor fabrication that deposits thin
films on semiconductors
 It is a process where a gaseous species reacts
on a solid surface or wafer and the reaction
that occurs produces a solid phase material
CVD- Chemical Vapour Deposition

3. CVD- Chemical Vapour Deposition - steps
1. The reacting gaseous species are
transported to the surface of the wafer
2. The gaseous species adsorb into the
surface of the wafer
3. The heterogeneous surface reaction
produces reaction products, which get
deposited on wafer
4. The residue gaseous reactants are
removed from the chamber containing
wafer

4. LPCVD- Low Pressure CVD
 LPCVD is a CVD process carried out at around 10-
1000 Pa while standard atmospheric pressure is
101,325 Pa.
 It can be used for the deposition of thin films on
semiconductors ranging from a few nanometers to
micrometers.
 LPCVD is used to deposit a wide range of possible
film compositions such as polysilicon for gate
contacts, thick oxides used for isolation, doped
oxides for global planarization, nitrides and other
dielectrics.

5. LPCVD- Low Pressure CVD
Why LPCVD?
 CVD is affected by two factors-
 Mass transport velocity (depends on reactant
concentration, rate of diffusion of reactants etc)
 Velocity of reaction on the surface
 At atmospheric pressure CVD, the two velocities are
almost same
 The Velocity of reaction is LOW and hence the
critical factor
 Desirable: Mass Transport velocity be reduced so
that it becomes much lesser than velocity of reaction

6. LPCVD- Low Pressure CVD
Why LPCVD?
 If the pressure is lowered from atmospheric
pressure to about 100 Pa, the rate of diffusion will
decrease by almost 1000 times. This means that the
velocity of mass transport will decrease meaning the
reactants can approach the surface of substrate
more closely and the deposited films show better
uniformity and homogeneity.

7. LPCVD- Low Pressure CVD

8. LPCVD- Low Pressure CVD
1. A quartz tube is placed in a spiral heater that starts
with tube pressure at very low pressure around 0.1 Pa.
2. The tube is then heated to the desired temperature
and the working gas is inserted into the tube at the
pressure between 10-1000 Pa.
3. The working gas consists of dilution gas and the
reactive gas that will react with the substrate and
create a solid phase material on the substrate.
4. This working gas reacts with the substrates and forms
the solid phase material and the excess material is
pumped out of the tube.

9. LPCVD- Low Pressure CVD
Advantages of LPCVD
 Excellent uniformity of thickness and purity
 High reliability
 Better homogeneity of deposited layers
 High reproducibility

10. LPCVD- Low Pressure CVD
Disadvantages of LPCVD
 Lower deposition rates than APCVD
 Higher temperatures needed for the process.
 The excess material after the reaction has taken
place is more volatile. It can be harmful & needs to
be handled properly.

11. LPCVD- Low Pressure CVD
Applications
1. LPCVD is most successfully applied in deposition of
polysilicon thin films. These films are used for gate
contact and short interconnect lines. This is done using
compounds like SiH4 in the temperature range 600-660 ºC.
2. LPCVD can also be used for deposition of nitrides that are
used for encapsulation. This is done by adding compounds
like Silane at 700-900 °C.