EBIC

Electron-beam induced current (EBIC) characterizes electrical properties of semiconductor materials and devices at the microscopic level.

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Advantages                    Workflow

What is EBIC?

Electron-beam induced current (EBIC) technique enables the characterization of local electrical behavior of semiconductor materials and devices by measuring the electrical currents that flow when a sample or device is exposed to an electron beam.

When an electron beam strikes a semiconductor, electron-hole pairs are created. If the carriers, of said electron-hole pairs, diffuse into a region with a built-in electric field, the electrons and holes will separate and a current will flow. The EBIC technique measures that current when it flows into an external circuit. In a material with no recombination centers (locations where the free electrons and holes annihilate) the collected current would be uniform and of little interest. However, regions of the sample which cause electrons and holes to recombine, reduce the collected current to cause contrast in an EBIC map, thus reveal the flow of (minority) carriers in a semiconductor sample. 

What is EBAC?

Electron-beam absorbed current (EBAC) is a related technique where the local resistance of a sample is revealed when you measure the absorbed current that flows between the electron beam and an electrical probe. The current measured in the external circuit is proportional to the resistance of the electrical path travelled. Thus, EBAC reveals breaks in copper lines on electronic devices or, shorts and breakdown channels in capacitors.

Advantages of EBIC and EBAC

Capability Advantage
Examines p-n junction and depletion region locations Measure the effectiveness and uniformity of device fabrication
Elucidates the impact of recombination centers such as grain boundaries, dislocations, and precipitates Enables optimization of processing steps to maximize device efficiencies
Measures minority carrier diffusion lengths Allows you assess material quality with high spatial resolution
Reveal line breaks in integrated circuits Minimizes the time to diagnose fault locations

 

Workflow for EBIC

Step 1: Prepare the sample

The standard EBIC technique requires the specimen to contain an electrical junction with diode characteristics – this provides the electric field to separate electrons and holes – and ohmic contacts, to allow connection to an external circuit. Fortunately, many devices such as solar cells, laser diodes, and transistors are suitable for analysis without further preparation; the user simply needs to ensure that the region of interest is accessible to the electron beam. 

For examination of material prior to device fabrication, you can form the diode junction by depositing a thin, electron transparent, metallic Schottky barrier character on the region of interest and a second ohmic junction to allow connection to the external circuit.

Step 2: Transfer to scanning electron microscope (SEM) and verify electrical contact

In order to measure the small currents associated with EBIC or EBAC experiments, electrical contact must be made to the specimen. This is done frequently via a dedicated EBIC holder by wire bonding or the use of adjustable micromanipulators. 

A current-voltage trace is frequently acquired to analyze the diode-characteristics and determine if the specimen is suitable for EBIC. For quantitative EBIC measurements, the leakage current in reverse bias and shunt resistance should be low.

An EBIC holder and sample may be loaded onto the SEM stage through the load lock of many modern SEMs; the stage will have been modified to contain mating electrical connections that allow the EBIC or EBAC signal to be passed to the external measuring circuit. Verification of good electrical contact can again be made by acquiring an IV trace.

Step 3: Image specimen

As the electron beam is scanned across the specimen in an X,Y pattern, you can measure the collected current at each pixel. In the simplest setup, you can use an arbitrary grayscale to represent the magnitude of the current; conventionally small and larger currents are displayed in black and white, respectively. Although qualitative imaging is useful to reveal the location of electrical junctions, you can perform a more meaningful analysis through quantitative measurements. In a quantitative measurement, the grayscale levels in the image display are associated with measured currents (typically pico- to micro-A range), rather than arbitrary units. This enables meaningful comparison between samples and a true understanding of the impact of (for example) defects. 

Step 4: Analyze

You may analyze the electrical properties in absolute intensity current values. However the EBIC contrast is a more useful parameter to measure impact of defects, such as dislocations, grain boundaries, and stacking faults. This measures the current normalized by the background signal, and relates directly to the recombination strength of a defect. 

It is also possible to measure minority carrier diffusion lengths locally through analyzing the EBIC profile caused by the diffusion of minority carriers to an electrical junction.

Resources:

 

Applications

Introduction to EBIC

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