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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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
7. Wafer Electrical Measurements
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. Ohm’s 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.
13. Probe Blades (Contacts) 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. Model BTP-310E Contact
Model BVL-310E Contact
Model MTP-33E Contact
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 wafer’s 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
Oxide rupture (Bvox or rupture voltage.)
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)
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 material Original Magnification = 40.00 kX Accelerating Voltage = 1.91kV
27. Specimen = Cross section of a semiconductor device Original Magnification = 50.00kX Accelerating Voltage = 0.40kV
28. Internet Links For more info about Wafer Testing, please feel free to visit these websites: