2. 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

3. Example of top side conductor grid – multiple cells on a single wafer

4. Solar cell potential conductors
Best conductor but requires sub-layer and is expensive

5. 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.

6. 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.

7. 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

8. 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

9. 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

11. 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

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

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

15. Portion of Varian 3125 control rack

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

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

18. Cryopump temperature-must be below 15oK

19. Varian 3125 ion gauge controller and deposition controller

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

21. Varian 3125 4-pocket e-beam crucible

22. 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)

23. Varian 3125 wafer planetary
Wafer planetary for Varian 3125

24. Wafers are held down by spring clips
Varian 3125
Wafers are held down by spring clips

25. Varian 3126 Quartz Heaters

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

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

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

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

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

31. 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

32. CHA Mark 50 crystal oscillators for evaporation material thickness measurement

33. New glass slides must be used for each evaporation

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

35. 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

36. 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

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

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

39. Sputter down configurationshown – the CVC inverts this configuration and sputters up

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

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

42. CVC sputter tool control racks
Chamber vacuum gauge

43. Argon MFC – 30 sccm flow typical
Cryo pump temperature – must be below 15oK

44. CVC sputter tool DC power supply for aluminum target
DC Voltage about 4KV
DC current 0.5 to 1.0 A

45. CVC sputter tool view port

46. View of argon sputter plasma in CVC sputter tool

47. View of argon plasma in AJA sputter tool
Sputter target
Shutter
Substrate (wafer) stage
Wafer stage can rotate and heat

48. Assignment
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 e-beam evaporation.
What gas is used in sputtering?