Solar Cells need a top side Solar Cells need a top side conductor to collect the conductor to collect the

current generatedcurrent generated

They also need a conductive They also need a conductive film on the backsidefilm on the backside

Conductor OptionsConductor Options

Silver is the typical choice Silver is the typical choice because it has the top because it has the top

conductivityconductivity

However,However,

Silver is an expensive Silver is an expensive conductorconductor

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

Because of equipment and Because of equipment and cost limitations, we will cost limitations, we will use vacuum deposition use vacuum deposition

processes for our processes for our conductorconductor

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.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

Ultra High vacuum PumpsUltra High vacuum Pumps

CryoCryogenic Pumpgenic Pump TurboTurbomolecular Pumpmolecular Pump

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 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

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

The tool used to measure the The tool used to measure the thickness is a surface profiler, in our thickness is a surface profiler, in our lab it is the Alpha Step 200lab it is the Alpha Step 200

In a surface profiler a stylus is In a surface profiler a stylus is dragged across a surface, if there is dragged across a surface, if there is a step present, it will measure the a step present, it will measure the height of the step (metal layer)height of the step (metal layer)

Once the wafer has been Once the wafer has been coated, the actual thickness coated, the actual thickness

of the metal can be of the metal can be measuredmeasured

Alpha Step surface profilerAlpha Step surface profiler

To create a representative To create a representative step, an evaporation mask step, an evaporation mask

(stencil) can be used to (stencil) can be used to create stepscreate steps

Example of an Alpha Step Example of an Alpha Step printoutprintout

Reference line #1

Reference line #2Step height 4.220KA or 422 nm

Assignment Assignment

Thin film worksheet on web Thin film worksheet on web sitesite

Due next lectureDue next lecture