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.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

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 configurationshown – 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

1.1. Review the slide presentation and answer the Review the slide presentation and answer the following electronicallyfollowing electronically

2.2. What is the approximate pressure of atmosphere What is the approximate pressure of atmosphere in units of torr?in units of torr?

3.3. List one advantage and two disadvantages of List one advantage and two disadvantages of using silver as the top side conductor.using silver as the top side conductor.

4.4. List three reasons why thin film depositions are List three reasons why thin film depositions are done under high vacuum conditions.done under high vacuum conditions.

5.5. Name three types of high vacuum pumps.Name three types of high vacuum pumps.

6.6. What are the three types of thin film depositions What are the three types of thin film depositions available in the Cameron clean room.available in the Cameron clean room.

7.7. What is the major difference between sputtering What is the major difference between sputtering and e-beam evaporation.and e-beam evaporation.

8.8. What gas is used in sputtering?What gas is used in sputtering?

AssignmentAssignment