Solar Cells need a top side Solar Cells need a top side conductor to collect the conductor to collect the
current generatedcurrent generated
They also need a conductive They also need a conductive film on the backsidefilm on the backside
Conductor OptionsConductor Options
Silver is the typical choice Silver is the typical choice because it has the top because it has the top
conductivityconductivity
However,However,
Silver is an expensive Silver is an expensive conductorconductor
Silver is typically printed via Silver is typically printed via a screen printer to keep a screen printer to keep manufacturing cost lowmanufacturing cost low
Because of equipment and Because of equipment and cost limitations, we will cost limitations, we will use vacuum deposition use vacuum deposition
processes for our processes for our conductorconductor
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. 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
2. 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
Ultra High vacuum PumpsUltra High vacuum Pumps
CryoCryogenic Pumpgenic Pump TurboTurbomolecular Pumpmolecular Pump
Thin film deposition tools in the ECE Microelectronics Clean Room
CVC 601-sputter deposition
Varian 3125 e-beam deposition
CHA Mark 50 e-beam deposition
Cooke-thermal 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, With sputtering, an Argon plasma is formed, causing argon ions to strike a metal target causing argon ions to strike a metal target
and knock loose material. Because an and knock loose material. Because an electric field is created, material is deposited electric field is created, material is deposited
on the substrateon the substrate
Material target
Argon plasma – ionized argon in an electric field
Substrate to be coated
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
Molten material hot enough to vaporize (become a gas)
Electron beam is
formed and strikes the
metal crucible
Substrates (wafers) sit at the top of the
chamber
Varian 3125 wafer planetaryVarian 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 rings are available for 2”, 3” and 4” wafers
Adapter ring for 4”/100mm
wafer
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
Power supply is interlocked to prevent activation if vacuum pressure, cooling
water, and zero current conditions are not met
Power supply main on/off switch
High voltage switch
and current control
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 configuration shown – 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
8 inch aluminum target
Viewport – plasma can be seen here when
sputtering
CVC sputter tool control racks
Chamber vacuum gauge
Cryo pump temperature – must be below 15oK
Argon MFC – 30 sccm flow typical
CVC sputter tool DC power supply for aluminum target
DC Voltage about 4KVDC 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
The tool used to measure the The tool used to measure the thickness is a surface profiler, in our thickness is a surface profiler, in our lab it is the Alpha Step 200lab it is the Alpha Step 200
In a surface profiler a stylus is In a surface profiler a stylus is dragged across a surface, if there is dragged across a surface, if there is a step present, it will measure the a step present, it will measure the height of the step (metal layer)height of the step (metal layer)
Once the wafer has been Once the wafer has been coated, the actual thickness coated, the actual thickness
of the metal can be of the metal can be measuredmeasured
Alpha Step surface profilerAlpha Step surface profiler
To create a representative To create a representative step, an evaporation mask step, an evaporation mask
(stencil) can be used to (stencil) can be used to create stepscreate steps
Example of an Alpha Step Example of an Alpha Step printoutprintout
Reference line #1
Reference line #2Step height 4.220KA or 422 nm
Assignment Assignment
Thin film worksheet on web Thin film worksheet on web sitesite
Due next lectureDue next lecture
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