ia ic testing

7/29/2019 IA IC Testing

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UKMDr. Nowshad Amin

n-well

p-channel transistor

p-well

n-channel transistor

p+substrate

Wafer Test and Evaluation

Transistor Layers


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Final Testing Options

There are several methods used in final test to ensure thatwafers are of acceptable quality.

They test wafers ability to withstand exposure to differentenvironmental conditions while maintaining performancespecifications.

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 locatedon the wafer not individual wafer devices.

DC voltages and currents are applied while checking thecorresponding 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 continueprocessing?

Data Collection - asses process trends

Special Tests - asses specific processparameters

Wafer level reliability - as needed toassure reliability concerns of processconditions


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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 theiroperating speed performance

fab yield response- provide yield informationto asses and improve overall fab process

performance test coverage- achieve high test coverage of

internal device nodes at lowest cost


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UKMDr. Nowshad Amin

Environmental Tests Thermal Shock tests consist of placing the device into

high temperature environments and then quickly movingthem into a cold environment.

Mechanical Shock tests determine the ability of the device

to withstand physical impacts.

Pressure Cooker tests expose the device to hightemperature and higher pressures (two atmospheres) todetermine 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 theeffects of corrosion.


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Resistance and ResistivityResistivity MeasurementsResistance, Voltage, Current = Ohms Law

Four-Point ProbeFour-Point Probe OperationsProbe Blades (Contacts)Sheet Resistance

Wafer Electrical Measurements


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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 theelectrons, the greater the resistivity.


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Resistivity Measurements Relationship of

Resistance (R) to

Resistivity (p) andDimensions: R =p L/A

=p L / WxD

Units ofMeasurements: Resistance (R):

Resistivity (p ): 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 andcrystals. It can also be used to measure the resistivity of

thin layers of dopants added into the wafer surface duringthe dopant processes.

Has four thin metal probes arrayed in a line. (Two outsideprobes are connected to a power supply and two insideprobes that are connected to a voltage meter.)


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Four-Point Probe Operations

Current is passed between the twoouter probes while the voltagedrop is measured between the

inner probes.

The effects of probe-wafer contactresistance on the measurement arecanceled out.

The relationship between currentand voltage values is dependent onthe resistance of the space betweenthe probes and the resistivity ofthe material.

pp == 22 ssV/IV/I


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Probe Blades (Contacts)

Characteristics: When mounted in series, they

are very rugged

Large BTP-310E life is 8 milliontouchdowns

Used with block probe assemblyor custom burn in sockets.

All contacts obtain a long lifebecause they will not be

damaged if a block probe wereto be dropped.

Ideal for testing ASIC Chips,Hybrid Circuits (thick/thin) film.

Model BTP-310E Contact

Model BVL-310E Contact

Model MTP-33E Contact


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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 amajor influence on the electrical operation of adevice.

The dopant concentration profile is determinedoffline 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 anddopant atoms.


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Depth Profile

Test is done after dopingto prepare for beveltechnique.

Depth is measured bytwo-point probes downthe bevel.

Vertical drop of theprobes are recorded and

a resistance measure-ment is made.

Resistance value changesw/ the change in dopantsat 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 andthe 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 (Bvoxorrupture voltage.)

Bipolar transistors MOS transistors

Capacitance-voltageplotting

Contactless C/Vmeasurement

Device Failureanalysis: 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 specialdigital 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 sourcebuilds a charge directly on the oxide surface.

Thus, we can calculate the charge (drift), flat-

band voltage, surface states, and oxidethickness.


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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 detectorsand 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 variablepressure 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 kX

Accelerating Voltage = 1.91kV


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Specimen = Cross section of a semiconductor device

Original Magnification = 50.00kX

Accelerating 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

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