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Wafer Test and Evaluation

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There are several methods used in final test to ensure that wafers ... Uses curve tracer or a special digital voltmeter as a display. Equipment: Probe Stations ... – PowerPoint PPT presentation

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Title: Wafer Test and Evaluation


1
Chapter 14 Wafer Test and Evaluation
Transistor Layers
p-well
n-well
n-channel transistor
p-channel transistor
p substrate
by Roy Stamps
2
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.

3
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.

4
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

5
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

6
  • 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.

7
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
8
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.

9
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

10
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

11
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.)

12
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
13
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.

14
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.

15
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

16
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.

17
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.

18
(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

19
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

20
Test Equipment
  • Probe Machine
  • Has the capability of positioning needlelike
    probes on the devices
  • Switch box (to apply the correct voltage)
  • current
  • polarities

21
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.

22
Equipment Probe Stations
  • Air Lift Station Step Repeat Station

23
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.

24
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.

25
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.

26
Specimen Via hole in semiconductor
materialOriginal Magnification 40.00
kXAccelerating Voltage 1.91kV
27
Specimen Cross section of a semiconductor
deviceOriginal Magnification
50.00kXAccelerating Voltage 0.40kV
28
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
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