Characterising the Temperature-Induced Topographical Evolution of Nanoscale Materials: Thermal Surface Profilometry

Optical profilometry is a rapid, non-destructive, and non-contact surface metrology technique, which is used to establish the surface morphology, step heights and surface roughness of materials. It has a wide range of applications across many fields of research, including analysing the surface texture of paints and coatings, analysing micro-cracks and scratches and creating wear profiles for structured materials including micro-electronics, and characterisation of textured or embossed nanometre-scale semiconducting components, such as silicon wafers.

Historically, it has been difficult to conduct temperature-controlled optical profilometry experiments due to imaging issues caused by changes in spherical aberration with temperature of both the front lens of the objective and the quartz window of the LTS420 stage.

To provide a solution for temperature-controlled optical profilometry Linkam has partnered with Sensofar, who specialise in the field of non-contact surface metrology, to develop a new technique for characterising the evolution of a sample’s surface topography with temperature using the S neox 3D optical profiler and Linnik interferometer coupled with Linkam’s LTS420 temperature-controlled chamber. The technique has been used to successfully map the changes in roughness and waviness of silicon wafers at temperatures up to 380°C.

Linkam LTS420 and Sensofar Linnik configuration

By using Sensofar’s new Linnik interferometer lens system with the S neox 3D optical profiler, in combination with Linkam’s LTS420 precision temperature control chamber, spherical aberration issues are resolved, enabling the accurate measurement of 3D topographic profiles of nanoscale materials at a wide range of temperatures.

For the design and construction of the Linnik objective, two Nikon 10x EPI objectives (Nikon, MUE12100) with 17.5mm working distance were used. The same configuration is available with 10xSLWD objectives (Nikon, MUE31100), providing a 37mm working distance.

This makes the thermal emissions from the camera almost imperceptible to the lens and will not affect or damage the measurement quality. The Linnik objective was mounted on the 3D optical profilometer (Sensofar, S neox), which combines 4 optical technologies in the same sensor head: Confocal, CSI, PSI and focus variation. These techniques are covered in ISO25178.

The example below shows how the 1D profilometric data can be plotted in the form of a topographic image. By stacking the 3D images as a function of temperature, it is possible to create a “4D plot”, as shown in the stacked image below.

This shows the evolution of the topographical changes at different temperatures using a colour scale to indicate height in the vertical direction. In the silicon wafers tested, it is clear that the samples bend as temperature changes. As temperature increases above room temperature towards 380°C, greater bending is experienced by the samples.

Bow evolution in (a) sample A and (b) sample B as a function of temperature. Waviness parameters Wz were extracted from the horizontal, diagonal and vertical profiles in Figure 5. Roughness parameter Sz was computed from the surface after applying an S-filter of 0.8 mm.

Stacked 4D view of the topographies extracted from (a) sample A and (b) sample B for visual comparison of the experimented bow change when samples increase from 30ºC to 380ºC.

The feasibility of the proposed configuration has been proven to carry out successful roughness and waviness measurements at different temperatures. Two different behaviours of the surface topography were observed depending on the chip design. Sample A showed an early bending behaviour when heating up the sample, whereas sample B showed the bending in a later stage.

David Páez, Sensofar Sales Support Specialist, commented: “In a recent experiment using the new technique, we were able to observe the changes in topography of silicon wafers as they evolve with temperature from 20°C up to 380°C. This is critical information for silicon wafer producers and users, so that they can optimise their process, improve semiconductor properties and wafer durability. The Linkam LTS420 chamber and T96 temperature controller are key components in our experimental set-up and enable us to ramp and control the temperature between <-195° and 420°C to a precision of 0.01°C.”

Linkam systems have provided precise temperature and environmental control to a wide range of techniques, from microscopy to X-ray analysis, for decades. This collaboration with Sensofar highlights the important role of temperature control in contributing to innovative approaches to material characterisation.

We are delighted to be able to offer a solution for temperature-controlled profilometry using Sensofar’s Linnik interferometer, and we look forward to seeing how this new technique helps scientists across many fields to advance their research.

Using the Sensofar Linnik system with the LTS420

Related Content

Liquid Crystal Sensor Offers Breakthrough for Rapid COVID-19 Diagnostic Test

The global outbreak of COVID-19 has prompted scientists all over the world to focus their research on the virus. Some concentrated on understanding the fundamental aspects of the virus and its transmission, while others have focussed on creating vaccines or therapeutics. Many began the search for rapid and robust tests that could be rolled out to a large proportion of the population to diagnose individuals as quickly as possible after infection. 

We spoke to Xiaoguang (William) Wang, PhD - recently appointed Assistant Professor in the William G. Lowrie Department of Chemical and Biomolecular Engineering at the Ohio State University (OSU), USA, to discuss his recent ground-breaking work in the field of SARS-CoV2, in particular the development of a rapid detection method for the SARS CoV-2 virus using a liquid crystal (LC) sensor.

Adil and Yang, members of Prof. Wang’s group at OSU, working with the Linkam PE120 Peltier temperature control setup in the lab.

At the onset of the pandemic, researchers rallied to develop testing protocols for this new and unknown virus. Groups from many different specialties turned their attention to SARS-CoV2 testing, and how they could adapt or pivot their research to tackle this global issue.

Tests using reverse-transcription polymerase chain reaction (RT-PCR) – the ‘gold-standard’ for molecular clinical diagnostics – became available quickly, however, these tests require long characterisation time and specialised equipment to perform. A variety of other technologies have shown promise as candidate diagnostic tools – gold nanoparticles and column agglutination test (CAT), for example – but each has significant limitations. The need arose for fast, simple, and effective techniques to detect and identify the virus.

Professor Wang and the team at OSU responded to the challenge of developing a rapid detection method for the SARS CoV-2 virus using a liquid crystal (LC) sensor. Prof. Wang comments about his decision to consider LCs as a prospective diagnostic technology: “It is commonly known that thermotropic LCs have been employed for the detection of various biomolecules, including DNA and lipids, for example. However, I know of no reports where they have been used for the detection of RNA. So, we set out to apply the technique to SARS-CoV-2 RNA.”

Using thermotropic liquid crystals for viral detection

Liquid Crystals exhibit characteristic and predictable behaviour as a result of their long-range orientational order, and the mobility of their mesogenic constituents. They have been widely utilised in fast-switching electro-optical devices, such as liquid crystal displays (LCDs). They are increasingly used for sensing applications due to their potential for tunable stimuli-responsive properties and self-assembly and ordering characteristics.

Professor Wang’s team at Ohio State university formed a collaboration with Prof. Xiaoping Bao at Purdue University for the design of biosensors for SARS-CoV-2, and Prof. Rongjun Qin at OSU for the development of a smartphone app for automatic readout of the ssRNA of SARS CoV-2.

Summarising the goals of the project, Prof. Wang notes: “LCs are ultrasensitive to small chemical modulations, and different analytes can be detected at extremely low concentrations. Therefore, taking advantage of the high-sensitivity of liquid crystals, we felt that we could develop a rapid detection method for SARS CoV-2 virus that would not need any sophisticated equipment and infrastructure. Moreover, we set out to design a portable and economical LC-based sensor detection kit for the rapid detection of ssRNA of the virus.”

Over the past decade, a series of important works have revealed the design of LC films and droplets that undergo orientational ordering transitions in response to a wide range of molecules adsorbed at an interface. For example, single-stranded DNA (ssDNA) and double-stranded DNA (dsDNA) produce different orientations of LCs at cationic surfactant-laden aqueous-LC interfaces, which leads to a change in the effect on visible light caused by the optical birefringence of the LC film.

Figure 1. Exploded diagram to show the layered make-up of the putative diagnostic device, the LC sensor is show as a dark red spot on the DMOAP glass layer

To show that ssRNA from the virus exhibited the same property, and that LC’s were suitable for COVID detection, Prof Wang’s group followed a 4-step experimental programme.  

Step 1. A cationic surfactant dodecyltrimethylammonium bromide (DTAB)-decorated interface on micrometer-thick films of a commercially available nematic liquid crystal, “E7”, was prepared. The layers were supported on a dimethyloctadecyl[3-(trimethoxysilyl) propyl]ammonium chloride (DMOAP)-functionalized glass slide, which induced a perpendicular ordering of the E7.

Step 2. A ssDNA probe, with a sequence complimentary to the defined SARS-CoV-2 ssRNA target, was adsorbed at the DTAB-decorated aqueous-E7 interface. The negatively charged ssDNA was attracted to the cationic DTAB at the aqueous-E7 interface via electrostatic interactions.

Step 3. ssRNACoV was added to the DTAB-laden E7 surface with the adsorbed ssDNAprobe. The temperature of the system was increased to 48.7°C, which is the Tm of the ssRNACoV. A Linkam PE120 Peltier hot stage was used to control the temperature of the E7 surface during these measurements.

Step 4. The optical appearance (i.e., brightness) of the RNA-adsorbed E7 films was quantified from images captured by a CCD camera on a polarized microscope.

Prof Wang summed up the success of these initial experiments, saying: “Our work has shown that LC-based sensors offer a highly sensitive, reproducible and robust method of detection of target ssRNA cov – and we could establish the detection limit of the E7 surface for the target ssRNA by varying the concentration from nanomolar to femtomolar amounts.”

Figure 2 shows a schematic illustration of the optical response of the DTAB/ssDNAprobe-decorated LC film to the adsorption of ssRNAcov.

Figure 2. Schematic showing the physical arrangement of LCs – left shows alignment pre-addition of ssRNAcov; right shows the change in alignment induced by the binding of the ssRNAcov target to the ssDNA probe. Note: the insert image captures show the visual change from occluded (left) to clear (right).

Prof. Wang characterised the Linkam PE120 stage as providing many advantages in their work to detect the ssRNAcov target: “The PE120 gave us not only the precise and reproducible temperature control we needed, but it was also very straightforward to integrate with our microscope and other equipment. The touch display on the controller made it easy and smooth to work with. Moreover, the Linkam stage is highly portable and can be transferred from one lab to another.”

Building a sensor – smartphone detection of the target ssRNA

With the principle of sensor structure and detection performance established, the group moved on to prepare a final sensor design (Fig. 2) that included polarising layers to enable detection of the confirmation change in the LCs by visible light. In addition, the sensor device was integrated with a retaining clip and an appropriate smartphone app to ensure consistent illumination of the sensor. This ensures that non-expert users can reliably read and act upon the results of a test (Fig. 4).

Figure 4. A smartphone screenshot indicating a negative result (left) and positive result (right)

The immediate next step for the COVID work is to move the test into a clinical evaluation phase – testing real patient samples in a biosafety level 3 (BSL-3) laboratory.

However, due to limited access to the labs throughout the pandemic, the group has been unable to perform those studies to date. Assuming satisfactory results of this next stage, and completion of the validation of the system in its practical application, Prof. Wang hopes to extend the collaboration to include the diagnostics industry for further studies and commercialisation, commenting: “We are optimistic that when the current phase of the pandemic is over and work in the labs is resumed normally, an LC-based test could be in the market in the relatively near future.”

In conclusion, when asked about other fields where this methodology could be used, Prof. Wang added: “I believe that this prototype of ‘LC sensor combined with smartphone app’ can be used for the detection of a wide variety of biomolecules and toxic compounds in the near future.”

The detailed experimental protocols, results and a discussion of the significance of this work has recently been published here.

Xu, Yang, et al. "Ultrasensitive and Selective Detection of SARS-CoV-2 Using Thermotropic Liquid Crystals and Image-Based Machine Learning." Cell Reports Physical Science (2020): 100276.

This article will be published in full in Biophotonics.

Related Content