Transcript Solar-Cell-Fundamentals-Lab-Lecture 6A

Solar Cells need a top side
conductor to collect the current
generated
They also need a conductive
film on the backside
Conductor Options
Silver is the typical choice
because it has the top
conductivity
However,
Silver is an expensive conductor
Silver is typically printed via a
screen printer to keep
manufacturing cost low
Because of equipment and
cost limitations, we will use
vacuum deposition processes
for our conductor
Thin Film Deposition
Materials 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
Mid level vacuum pumps
Roughing, fore line, mechanical
Oil pump – oil can be standard
hydrocarbon or inert synthetic
(for extreme chemical pumping)
Oil level must be monitored and
replaced periodically
Exhaust required and needs filter
to trap oil droplets
Uses no oil, usually an air or
nitrogen bearing and seal
Nitrogen and cooling
water required
Exhaust required
Ultra High vacuum Pumps
Cryogenic Pump
Turbomolecular Pump
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”
CVC 601 Sputter
down configuration
shown - the CVC
has the target on the
base and the wafers
facing down
Sputter up
configuration
On the Cameron
CVC 601
1 - 8” DC
aluminum target
2 - 3” RF
targets- multiple
materials are
available inch
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
Argon flow MFC controller
Press esc key to turn on
Chamber vacuum gauge
Must be restarted after
automatic pump down
Control rack – right side
Chamber pressure
needs to be in the
10-6 torr range
Cryo pump
temperature must
be below 20oK
Do not sputter if cooling water
alarm is sounding
Control rack – left side
RF matching
network and power
supply for 3 inch
targets
Shutters and RF
target material
tags
Argon MFC
controller
Channel 1 Argon
30sccm
Channel 4 Argon
300sccm
CVC sputter tool DC power supply
for aluminum target
DC Voltage about 4KV
DC current 0.5 to 2.0 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
Once the wafer has been coated,
the actual thickness of the metal
can be measured
The tool used to measure the thickness is
a surface profiler, in our lab it is the
Alpha Step 200
In a surface profiler a stylus is dragged
across a surface, if there is a step
present, it will measure the height of the
step (metal layer)
Alpha Step surface profiler
To create a representative step,
an evaporation mask (stencil)
can be used to create steps
Example of an Alpha Step
printout
Reference line #1
Step height
4.220KA or 422 nm
Reference line #2
Assignment
Thin film worksheet on web site
Due next lecture