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Conductor Options

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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Conductor Options

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  1. Solar Cells need a top side conductor to collect the current generatedThey also need a conductive film on the backside

  2. Conductor Options

  3. Silver is the typical choice because it has the top conductivity However, Silver is an expensive conductor

  4. Silver is typically printed via a screen printer to keep manufacturing cost low

  5. Because of equipment and cost limitations, we will use vacuum deposition processes for our conductor

  6. 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.

  7. 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.

  8. 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

  9. Mid level vacuum pumpsRoughing, fore line, mechanical Oil pump – oil can be standard hydrocarbon or inert synthetic (for extreme chemical pumping) Uses no oil, usually an air or nitrogen bearing and seal Oil level must be monitored and replaced periodically Exhaust required and needs filter to trap oil droplets Nitrogen and cooling water required Exhaust required

  10. Ultra High vacuum Pumps Cryogenic Pump Turbomolecular Pump

  11. 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

  12. 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

  13. 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

  14. E-beam Evaporation uses a high energy electron beam to vaporize (change from a solid to vapor) materials, especially metals

  15. Overall view of the Varian 3125 vacuum chamber. This tool deposits thin films using e-beam evaporation

  16. Portion of Varian 3125 control rack

  17. Varian 3125 quartz heater controller, shutter controller and planetary rotation controller Quartz heater controller E-beam shutter controller

  18. Electron beam power supply Electron beam can be steered by magnetic fields Typically 6-8KV are required to form the electron beam

  19. Cryopump temperature-must be below 15oK

  20. Varian 3125 ion gauge controller and deposition controller Ion Gauge controller Deposition controller

  21. Varian 3125 view of open chamber Wafer planetary – can rotate or stay stationary. Can be removed for loading

  22. Varian 3125 4-pocket e-beam crucible

  23. 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)

  24. Varian 3125 wafer planetary Wafer planetary for Varian 3125

  25. Varian 3125 Wafers are held down by spring clips

  26. Varian 3126 Quartz Heaters

  27. Varian 3125 door showing glass slide holder Glass slide must be replaced before each run

  28. Overall view of the CHA Mark 50 vacuum chamber. This tool deposits thin films using e-beam evaporation

  29. Inside of CHA Mark 50 chamber showing wafer platen – can be removed from the chamber and replaced with a larger wafer platen

  30. CHA Mark 50 wafer adapter ring Adapter ring for 4”/100mm wafer Adapter rings are available for 2”, 3” and 4” wafers

  31. CHA Mark 50 4-pocket e-beam crucible Four different materials are available to do sequential evaporations

  32. 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

  33. CHA Mark 50 crystal oscillators for evaporation material thickness measurement Crystal oscillators

  34. New glass slides must be used for each evaporation

  35. CHA Mark 50 cryo-pump control Cryogenic pump temperature – should be around 20oK

  36. 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

  37. 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

  38. E-beam evaporation Crucible being heated by an electron beam

  39. Overall view of the CVC vacuum chamber. This tool deposits thin films using “sputtering”

  40. 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

  41. CVC sputter tool with chamber lid open Wafers are loaded into position

  42. Looking into the CVC sputter tool chamber, showing the 8” aluminum target Viewport – plasma can be seen here when sputtering 8 inch aluminum target

  43. 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

  44. 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

  45. Control rack – left side RF matching network and power supply for 3 inch targets Shutters and RF target material tags Channel 1 Argon 30sccm Channel 4 Argon 300sccm Argon MFC controller

  46. CVC sputter tool DC power supply for aluminum target DC Voltage about 4KV DC current 0.5 to 2.0 A

  47. CVC sputter tool view port

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