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

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

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

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.

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 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 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

Cryopump temperature-must be below 15oK

Varian 3125 ion gauge controller and 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 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

Wafers are held down by spring clips 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 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

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

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

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

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

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

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

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

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

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?