Atomic-Scale Imaging of Semiconductor Materials Under Bias

Aggressive scaling and physical design have produced nanoscale semiconductor devices using a variety of innovative materials and structures. There is a strong need to understand the physics that control performance so that faster and more efficient devices can be commercialized.  Focused ion beam (FIB) has gained popularity as the method of choice for semiconductor device sample prep because it provides the means to study individual devices using high resolution imaging and analysis in a transmission electron microscope (TEM). However, in situ electrical characterization of these structures, which provides a wealth of information about actual device behavior, has traditionally been a challenge.

To overcome this challenge and take full advantage of the imaging and analytical power of the TEM, Protochips has developed a robust workflow that enables in situ electrical characterization of semiconductor devices. Using this workflow, researchers have linked nanoscale phenomena to their associated electrical characteristics through in situ studies. This article describes the workflow and its impact on the nanoscale analysis of semiconductor devices.

What is Fusion Select?

Protochips’ Fusion Select is the latest generation of in situ heating, electrical and electrothermal characterization system for the TEM. It is designed to maintain the full capabilities of a TEM - such as atomic scale resolution, chemical mapping, field mapping, and crystallography - while enabling precise control over thermal and electrical stimuli. This allows users to observe and understand the underlying mechanisms responsible for a device operation by correlating electrical properties with nanoscale images and analysis. Such information not only helps determine how these devices work, but also how they fail and how to prevent such failure.

FIB Sample Preparation is Essential for Semiconductor Device Analysis in the TEM

For TEM analysis, the sample must be small and thin (typically less than 100nm thick) to ensure good electron transparency. Most FIB systems have a micromanipulator used to transfer samples from the wafer or die to the TEM sample support. Once the sample region of interest is identified, extracted, and transferred to the sample support, it must thinned and electrically connected using precise FIB metal deposition (often Pt or W) to enable in situ characterization. Modern semiconductor devices have become so small and densely packed that they push even state-of-the-art FIBs to their limits. The metal deposition process used to form electrical connections in the FIB tends to leave halos of metal contamination on the device.  These often create short circuits with surrounding metal structures and prevent proper operation of the device.

The Fusion Select FIB Workflow Overcomes the Challenges of in situ Characterization of Nanoscale Devices

The Fusion Select FIB workflow features FIB-optimized sample supports, or E-chips, specifically designed to prevent short circuits during sample mounting and contact formation. In addition, the workflow is based on conventional FIB sample preparation steps for in situ TEM, which allows a fast learning curve and easy adoption of this workflow by every laboratory.

The Fusion Select system for in situ heating, electrical, and electrothermal characterization. Left: The FIB-optimized E-chip with sample attached. Right: the Fusion Select TEM holder.

The Fusion Select system for in situ heating, electrical, and electrothermal characterization. Left: The FIB-optimized E-chip with sample attached. Right: the Fusion Select TEM holder. Image Credit: Protochips

Fusion Select uses a high precision power supply, low noise cabling to achieve picoamp measurement accuracy in the TEM and the same user-friendly software suite as every Protochips product, with additional safeguards in place to prevent user error during electrical testing. The Fusion Select holder incorporates a “fixed probe design” for sample tilt in the TEM. This ensures the chip and holder maintain a clear, stable electrical signal during the sample tilting so the user can change the sample orientation safely and precisely, gathering more complete structural information at atomic resolution without any risk of disrupting a connected device under test.  These features enable safe, reliable and reproducible characterization of each sample and the acquisition of truly relevant data.

The Power of Electrical Characterization Using in Situ TEM

A promising non-volatile data storage technology called resistive RAM, or ReRAM, uses a simple resistance reading to store binary information: a 1 or a 0. If the device is conductive it is a 1, insulating it is a 0. ReRAM works by the controlled creation of a conductive path through an insulating (dielectric) material. However, the formation mechanism of this conductive filament is still not well understood. Recently, scientists have observed the live formation of such filaments in various material systems, have acquired data on its structure and chemical composition, and correlated the observations with its electrical properties.

Live formation of a conductive metal filament across an oxide layer observed during in situ electrical operation of a Resistive Memory. Chang, et al, Nano Energy, 53, 2018

Live formation of a conductive metal filament across an oxide layer observed during in situ electrical operation of a Resistive Memory. Chang, et al, Nano Energy, 53, 2018. Image Credit: Protochips

There is also strong research interest in ferroelectric materials. These materials exhibit unique electrical properties and are being evaluated for use in numerous applications including displays, electro-optics and nonvolatile memories. Scientists have recently used Fusion Select to characterize how applied voltage causes domain walls to form and migrate in a ceramic film with ferroelectric properties.

Ferroelectric material without (left) or with (middle) an applied voltage demonstrating the creation and displacement of ferroelectric domains. Domain walls can be tracked and studied with atomic resolution (right, colorized) Images courtesy Prof. Goran Drazic, Nat’l Inst of Chemistry, Slovenia.

Ferroelectric material without (left) or with (middle) an applied voltage demonstrating the creation and displacement of ferroelectric domains. Domain walls can be tracked and studied with atomic resolution (right, colorized). Images Courtesy Prof. Goran Drazic, Nat’l Inst of Chemistry, Slovenia.

Acquiring such information at high resolution and under electrical bias enables better understanding of how these devices function at the atomic scale, and how to improve this technology.

Learn More Today!

For more information regarding the Fusion Select system and its FIB workflow, please visit our product page on our website at www.protochips.com. For specific questions about your research project please email [email protected] and we will be happy to talk to you!

This information has been sourced, reviewed and adapted from materials provided by Protochips.

For more information on this source, please visit Protochips.

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