Solar Cell
conductive grid and back
contact

Once the p-n junction has been formed, a
conductive top side grid of metal must be
created to gather the current generated
by the solar illumination
Example of top side conductor grid – multiple cells
on a single wafer
Solar cell potential conductors
Best conductor but requires
sub-layer and is expensive
Thin Film Deposition
Conductors are deposited using a vacuum
chamber
 The vacuum chamber reduces the
atmosphere to high vacuum levels (no
atmosphere)
 This reduces contaminating the films,
provides a non-contaminating
environment free of oxygen, water vapor,
etc. and allows materials to melt at lower
temperatures.

Thin Film Deposition
Thin film deposition tools are very
complex due to the need to create high
vacuum levels.
 Vacuum levels of 5x10-7 torr and better
are typical. Sea level atmospheric
pressure is about 740 torr or 7.4x102
 Because of their complexity, vacuum
chambers are very expensive.

Thin Film Deposition

To achieve high vacuum levels, several types of
vacuum pumps are used.
1.
2.
Mid level vacuum levels (2x10-3 torr) are reached with rotary
vane vacuum pumps. These pumps are also know as
mechanical or roughing vacuum pumps
High level vacuum levels are reached using



Diffusion vacuum pumps – requires liquid nitrogen to prevent oil
contamination
Turbomolecular pumps – like a small jet engine, clean and fast,
good for processes that require the introduction of a process gas.
Because of the high speed vanes, subject to catastrophic failure
Cryogenic vacuum pumps – uses low temperature (10oK) – also
clean and fast pumping but requires regeneration periodically
which is time consuming
Thin film deposition tools in the ECE
Microelectronics Clean Room
Cooke-thermal deposition
CHA Mark 50 e-beam deposition
CVC 601-sputter deposition
Varian 3125 e-beam deposition
Conductor Deposition
The Cooke thermal evaporator is not currently
used.
 The CVC sputter tool is used for aluminum
depositions. A silver/antimony and copper
targets are available.
 The Varian 3125 and CHA Mark 50 e-beam
deposition tools are used for all other
conductors, Cu, Au, Ag, Cr, Ni

– An e-beam evaporates material, it get the material so
hot it becomes a gas and evaporates. It then travels
in a straight line, because it is under vacuum, until it
condenses when it strikes a colder surface
With sputtering, an Argon plasma is formed, causing
argon ions to strike a metal target and knock loose
material. Because an electric field is created,
material is deposited on the substrate
Material
target
Substrate to
be coated
Argon plasma –
ionized argon in
an electric field
E-beam Evaporation uses a high energy electron
beam to vaporize (change from a solid to vapor)
materials, especially metals
Overall view of the Varian 3125
vacuum chamber. This tool deposits
thin films using e-beam evaporation
Portion of Varian 3125 control rack
Varian 3125 quartz heater controller, shutter
controller and planetary rotation controller
Quartz heater
controller
E-beam
shutter
controller
Electron beam power supply
Typically 6-8KV are
required to form the
electron beam
Electron beam can be
steered by magnetic
fields
Cryopump
temperature-must be
below 15oK
Varian 3125 ion gauge controller
and deposition controller
Ion Gauge controller
Deposition controller
Varian 3125 view of open chamber
Wafer planetary – can rotate
or stay stationary. Can be
removed for loading
Varian 3125 4-pocket e-beam
crucible
With an e-beam (electron beam) evaporator the
material is heated to a vapor (gas) and then
condenses on cooler surfaces
Substrates
(wafers) sit at the
top of the
chamber
Molten material hot
enough to vaporize
(become a gas)
Electron
beam is
formed and
strikes the
metal crucible
Varian 3125 wafer planetary
Wafer planetary for Varian 3125
Varian 3125
Wafers are held down
by spring clips
Varian 3126 Quartz Heaters
Varian 3125 door showing glass
slide holder
Glass slide must be
replaced before each run
Overall view of the CHA Mark 50
vacuum chamber. This tool deposits
thin films using e-beam evaporation
Inside of CHA Mark 50 chamber showing wafer
platen – can be removed from the chamber and
replaced with a larger wafer platen
CHA Mark 50 wafer adapter ring
Adapter ring for
4”/100mm wafer
Adapter rings are available for
2”, 3” and 4” wafers
CHA Mark 50 4-pocket e-beam
crucible
Four different materials are
available to do sequential
evaporations
CHA Mark 50 crucible materials and
chamber temperature monitor
Materials currently inside
the 4 pocket crucible are
shown with their pocket
number
Pocket is
chosen using
this indexer
CHA Mark 50 crystal oscillators for
evaporation material thickness
measurement
Crystal
oscillators
New glass slides must be
used for each evaporation
CHA Mark 50 cryo-pump control
Cryogenic pump temperature –
should be around 20oK
CHA Mark 50 vacuum gauge
controller
Vacuum chamber pressure. Gauge is showing a
vacuum pressure of 7.6 x 10-6 torr. E-beam
power supply is interlocked to prevent high
voltage if pressure is too high
CHA Mark 50 E-beam power supply
and controller
High
voltage
switch and
current
control
Power supply main
on/off switch
Power supply is interlocked to prevent activation if
vacuum pressure, cooling water, and zero current
conditions are not met
E-beam evaporation
Crucible being heated by an
electron beam
Overall view of the CVC vacuum
chamber. This tool deposits thin films
using “sputtering”
Sputter down
configurationshown
– the CVC inverts
this configuration
and sputters up
CVC sputter tool with chamber lid
open
Wafers are
loaded into
position
Looking into the CVC sputter tool chamber,
showing the 8” aluminum target
Viewport – plasma can
be seen here when
sputtering
8 inch aluminum target
CVC sputter tool control racks
Chamber
vacuum
gauge
Argon MFC – 30
sccm flow typical
Cryo pump
temperature –
must be below
15oK
CVC sputter tool DC power supply
for aluminum target
DC Voltage about 4KV
DC current 0.5 to 1.0 A
CVC sputter tool view port
View of argon sputter plasma in
CVC sputter tool
View of argon plasma in AJA
sputter tool
Sputter
target
Shutter
Substrate
(wafer)
stage
Wafer stage can rotate and heat
Assignment
1.
2.
3.
4.
5.
6.
7.
8.
Review the slide presentation and answer the following
electronically
What is the approximate pressure of atmosphere in
units of torr?
List one advantage and two disadvantages of using
silver as the top side conductor.
List three reasons why thin film depositions are done
under high vacuum conditions.
Name three types of high vacuum pumps.
What are the three types of thin film depositions
available in the Cameron clean room.
What is the major difference between sputtering and ebeam evaporation.
What gas is used in sputtering?