Scanning Thermal Microscopy (SThM) - The Nanoscale Thermometer and Heater

Topics Covered

Introduction
Major Benefits of Scanning Thermal Microscopy (SThM)
Technology and Operating Principles of Scanning Thermal Microscopy (SThM)
Probe Types
Major Applications of Scanning Thermal Microscopy (SThM)
NT-MDT Scanning Thermal Microscopy (SThM) Equipment
     Controller and Software
     NT-MDT Scanning Thermal Microscopy (SThM) Probes

Introduction

Scanning Thermal Microscopy (SThM) is an advanced Scanning Probe Microscopy technique that is useful for obtaining nanoscale thermal properties and topographical images.

Scanning Thermal Microscopy equipment available from NT-MDT is able to record and display the temperature and thermal conductivity distribution at the surface of a sample.

Major Benefits of Scanning Thermal Microscopy (SThM)

The major benefit of Scanning Thermal Microscopy is that it displays a higher spatial resolution than Infrared Microscopy, Laser Reflectance Thermometry and Micro-Raman Thermometry.

Technology and Operating Principles of Scanning Thermal Microscopy (SThM)

The operation of a Scanning Thermal Microscope is based on Atomic Force Microscopy (AFM) Techniques.

When the atomically sharp AFM tip is placed in proximity to a sample to be studied there is a heat exchange which modifies the temperature of the tip. It is then relatively straightforward to use signals from the tip to create what is effectively a thermal map of the surface.

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The heat exchange or heat flow is affected by several factors; including the temperature of the tip, the contact pressure and the inherent material properties of the sample such as Thermal Conductivity and Specific Heat Capacity.

Although the preferred method of operation is via solid to solid direct conduction, it is possible that in the presence of aqueous species that conduction occurs within the liquid meniscus at the tip surface which can also be affected by gas conduction. For this reason the preferred method of operation is to scan surfaces under vacuum conditions.

Probe Types

Two types of probes have been used in Scanning Thermal Microscopy -

• Thermocouple type - with this arrangement the temperature is measured by a thermocouple junction at the probe tip. Typically Chromel Alumel, Au/Pd Au/Ni combinations.
• Bolometer type - in this arrangement the probe temperature is monitored by the resistance of a thin film at the probe tip. The resistor can be used to heat the probe and measure the temperature at the same time.

The signal from the Bolometer probe passes through a wheatstone bridge readout circuit which provides a fixed power source and measures the resistance of the bolometer probe.

Using this arrangement it is possible to use constant power or constant temperature. The advantage of constant temperature is the improved speed and reduced sample damage.

The fundamental limit of resolution is proportional to kT, where k is the boltzman constant and T is the temperature. At room temperature kT is ~ 10-21 J.

The key design issues relating to probes are the small size of the microfabricated probes to attain a high degree of spatial resolution coupled with good thermal isolation of a small thermal mass of probe.

Sample temperatures are typically measured on active device structures such as magnetic recording heads, laser diodes or other forms of electrical circuitry.

Conversely, thermal conductivity is more typically measured on composite samples. In such a measurement, more voltage will be applied to the probe increasing it further above room temperature. The thermal conductivity of the sample will affect the temperature of the probe by draining more or less heat away from the tip and thermal conductivity can thereby be calculated.

Major Applications of Scanning Thermal Microscopy (SThM)

Some of the major applications for SthM are for defect and hot spot detection in semiconductors, photoresist metrology and the detection of sub surface features which cannot be observed by AFM.

Typical material parameters that can be observed include the thermodynamic characterisation of material properties such as conductance, specific heat capacity, and glass transition temperatures.

The technology is also of particular relevance to pharmaceutical compounds and for the analysis of biomolecules.

NT-MDT Scanning Thermal Microscopy (SThM) Equipment

Controller and Software

The SThM system hardware includes an electronic controller, software, and probes.

The Scanning Thermal Microscopy controller (Figure 1) is connected to the main Atomic Force Microscope electronics via a standard extension socket. The system is easily adjusted through a user-friendly software interface.

Figure 1. Electronic controller

For ease of use, the SThM control program is integrated into the main NT-MDT AFM software as one of the contact mode methods.

Due to high sensitivity of the system and the low noise of the output voltage, the electronic controller provides high signal resolution.

An additional advantage is that the compact size of electronics hardware simplifies the setup and optimizes the time involved in scanning high resolution SThM images.

NT-MDT Scanning Thermal Microscopy (SThM) Probes

Scanning Thermal Microscopy mode of operation with an AFM utilizes a specialized probe with a resistor built into the cantilever (Figure 2)

Figure 2. Cantilever holder

Figure 3. AFM Head

NT-MDT's SThM module allows users to monitor the changes in resistance correlated with the temperature at the end of the probe. As a result, the system is able to monitor relative changes of sample temperature and thermal conductivity.

Figure 4. SThM Probe Setup

NT-MDT's thermal probes provide better than 100 nm lateral resolution for both topography and thermal images (Figure 5).

Figure 5. Scanning Thermal Microscopy allows one to obtain images of <100 nm lateral resolution.
Sample: Optical Fiber in Epoxy. (Left) topography image; (Right) thermal conductivity image. Scan size: 6 x 6 µm.

The specialized SThM cantilever, made of SiO₂ with a thin metal layer, is deposited on the probe in such a way that the highest resistance portion of the layer is concentrated near the tip apex.

Figure 6. SEM image of the SThM Probe

by NT-MDT, AZoNano.com