Wafer Test and Evaluation

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Chapter 14 Wafer Test and Evaluation
Transistor Layers
p-well
n-well
n-channel transistor
p-channel transistor
p substrate
by Roy Stamps
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Final Testing Options

• There are several methods used in final test to
  ensure that wafers are of acceptable quality.
• They test wafers ability to withstand exposure to
  different environmental conditions while
  maintaining performance specifications.
• Several of the tests used are listed and
  explained below.

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Inline Parametric Tests

• Also known as WET (wafer electrical test) or DC
  test
• Electrical test performed on test pattern
  structures located on the wafer not individual
  wafer devices.
• DC voltages and currents are applied while
  checking the corresponding voltages on the output
  pins.
• Typically done after 1st metal layer deposited
  and etched
• Allows probes to make electrical contact with
  special test structure pads.

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Objectives

• Identify process problems - save
• Pass/fail criteria - should wafers continue
  processing?
• Data Collection - asses process trends
• Special Tests - asses specific process parameters
• Wafer level reliability - as needed to assure
  reliability concerns of process conditions

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Wafer Sort

• Objectives of Wafer Sort
• chip functionality - verify operation of all chip
  functions
• chip sorting - sort good chips based on their
  operating speed performance
• fab yield response - provide yield information to
  asses and improve overall fab process performance
• test coverage - achieve high test coverage of
  internal device nodes at lowest cost

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• Environmental Tests
• Thermal Shock tests consist of placing the
  device into high temperature environments and
  then quickly moving them into a cold
  environment.
• Mechanical Shock tests determine the ability
  of the device to withstand physical impacts.
• Pressure Cooker tests expose the device to
  high temperature and higher pressures (two
  atmospheres) to determine the ability to
  withstand galvanic corrosion due to the
  encapsulating materials.
• Humidity tests expose the device to higher
  temperature and humidity when powered. This test
  measures the effects of corrosion.

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Wafer Electrical Measurements
Resistance and Resistivity Resistivity
Measurements Resistance, Voltage, Current Ohms
Law Four-Point Probe Four-Point Probe
Operations Probe Blades (Contacts) Sheet
Resistance
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What is Resistivity?

• A measure of the resistance to current flow in a
  material. A function of the attraction between
  the outer electrons and inner protons of a
  material. The more tightly bound the electrons,
  the greater the resistivity.

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Resistivity Measurements

• Relationship of Resistance (R) to Resistivity
  (p) and Dimensions
• R p L/A
• p L / WxD

• Units of Measurements
• Resistance (R) O
• Resistivity (p ) O x cm

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Ohms Law for Resistance

• R V/I
• (p ) L/A (p ) L/(W x D)
• R resistance
• V voltage
• I current
• p resistivity of sample
• L length of sample
• A cross-sectional area of sample
• W width of sample
• D depth of sample

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Four-Point Probe

• An instrument used to measure resistivity on
  wafers and crystals. It can also be used to
  measure the resistivity of thin layers of dopants
  added into the wafer surface during the dopant
  processes.
• Has four thin metal probes arrayed in a line.
  (Two outside probes are connected to a power
  supply and two inside probes that are connected
  to a voltage meter.)

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Four-Point Probe Operations

• Current is passed between the two outer probes
  while the voltage drop is measured between the
  inner probes.
• The effects of probe-wafer contact resistance on
  the measurement are canceled out.
• The relationship between current and voltage
  values is dependent on the resistance of the
  space between the probes and the resistivity of
  the material.

p 2? s V/I
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Probe Blades (Contacts)

• Model BTP-310E Contact
• Model BVL-310E Contact
• Model MTP-33E Contact

• Characteristics
• When mounted in series, they are very rugged
• Large BTP-310E life is 8 million touchdowns
• Used with block probe assembly or custom burn in
  sockets.
• All contacts obtain a long life because they will
  not be damaged if a block probe were to be
  dropped.
• Ideal for testing ASIC Chips, Hybrid Circuits
  (thick/thin) film.

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What is Sheet Resistance?

• Sheet Resistance Rs
• The electrical quantity measured on a thin layer.
• Units ? / square
• Rs 4.53 V/I
• 4.53 is a constant that arises from the probe
  spacing.

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Concentration Depth Profile

• Distribution of dopant atoms in the wafer is a
  major influence on the electrical operation of a
  device.
• The dopant concentration profile is determined
  offline by two techniques
• (SIMS) Secondary Ion Mass Spectrometry
• (DHE) Differential Hall Effect

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SIMS

• What is it? (secondary ion mass spectrometry)
• A combination of ion milling and secondary ion
  detection methods.
• Ions are directed at the sample surface.,
  removing a thin layer.
• Secondary ions are generated from the removed
  material, which contains the wafers material and
  dopant atoms.

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Depth Profile

• Test is done after doping to prepare for bevel
  technique.
• Depth is measured by two-point probes down the
  bevel.
• Vertical drop of the probes are recorded and a
  resistance measure-ment is made.
• Resistance value changes w/ the change in dopants
  at each level.

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(DHE) Differential Hall Effect

• Requires sequential removal of the doped layer
  down to the junction.
• As each layer is removed the resistivity and the
  Hall coefficient are measured.
• The Hall coefficient related to carrier mobility
• Dopant concentration is calculated from the two
  measured parameters

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Device Electrical Measurements

• Equipment
• Resistors
• Diodes
• Oxide rupture (Bvox or rupture voltage.)
• Bipolar transistors
• MOS transistors

• Capacitance-voltage plotting
• Contactless C/V measurement
• Device Failure analysis Emission microscopy

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Test Equipment

• Probe Machine
• Has the capability of positioning needlelike
  probes on the devices
• Switch box (to apply the correct voltage)
• current
• polarities

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Equipment Probe Cards

• Probe cards are used when there are to be many
  devices measured.
• Also used in the wafer sort process.
• Uses curve tracer or a special digital voltmeter
  as a display.

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Equipment Probe Stations

• Air Lift Station Step Repeat Station

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Contact-less C/V Measurements

• COS
• Result is similar to a MOS transistor
• C stands for corona source. This source builds
  a charge directly on the oxide surface.
• Thus, we can calculate the charge (drift),
  flat-band voltage, surface states, and oxide
  thickness.

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Device Failure Analysis

• Emission Microscopy
• Used to locate and produce an image of trouble
  spots on a semiconductor device.
• Microscopes are fitted with sensitive detectors
  and charged-coupled imaging devices.
• Very useful for electrical measurements.

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Scanning Electron Microscopes - LEO 1455VP

• The LEO 1455VP is a research grade variable
  pressure SEM, designed to accommodate and fully
  maneuver large and awkwardly shaped specimens,
  for non-destructive imaging and analysis.

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Specimen Via hole in semiconductor
materialOriginal Magnification 40.00
kXAccelerating Voltage 1.91kV
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Specimen Cross section of a semiconductor
deviceOriginal Magnification
50.00kXAccelerating Voltage 0.40kV
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Internet Links

• For more info about Wafer Testing, please feel
  free to visit these websites
• http//www.icprobotics.com
• http//www.jemam.com
• http//www.jeol.com
• http//entcweb.tamu.edu/zoghi/semiprog/linker.htm
• http//www.cea.com/cai/simstheo/caistheo/htm