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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.
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Solar Cells need a top side conductor to collect the current generatedThey also need a conductive film on the backside
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. • 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 CVC 601-sputter deposition CHA Mark 50 e-beam 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 Argon plasma – ionized argon in an electric field Material target 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
Varian 3125 quartz heater controller, shutter controller and planetary rotation controller Quartz heater controller E-beam shutter controller
Electron beam power supply Electron beam can be steered by magnetic fields Typically 6-8KV are required to form the electron beam
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
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 Electron beam is formed and strikes the metal crucible Molten material hot enough to vaporize (become a gas)
Varian 3125 wafer planetary Wafer planetary for Varian 3125
Varian 3125 Wafers are held down by spring clips
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
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 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 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
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)