Thin films and engineered surfaces are used in a myriad of applications including semiconductor electronics, data storage, and precision optics. In many cases, surface roughness and related texture of these device components directly impact their overall quality and performance. Measurements of surface roughness and morphology can be essential, whether to validate an individual processing step or to obtain a quality control metric for the final product.

Table 1. Comparison of tools for surface characterization. Specifications given for non-AFM tools are typical ranges for commercial instruments. *Both AFMs and scanning electron microscopes can provide information about materials properties beyond topography. For example, AFMs can measure a host of electrical (e.g., conductivity, permittivity), mechanical (e.g., elastic modulus), and functional properties (e.g., piezoelectric response).

The choice of instrument best suited for surface roughness measurements depends strongly on the specific material being measured as well as on the expected size and shape of its surface features. As device dimensions in many industries continue to shrink, it is becoming increasingly important to characterize surface roughness at the scale of nanometers and even lower. Tools commonly used for this purpose include interferometric optical profilometers, scanning electron microscopes, and atomic force microscopes (AFMs). Among these, the AFM is the only technique offering unparalleled three-dimensional spatial resolution and the ability to measure most types of materials ( Table 1). AFM provides complete 3D surface quantification by imaging topography (height), as shown in Figure 1.

Figure 1. Topography image of a silicon carbide wafer [6H SiC (0001)]. The graph shows the average profile between the white lines in the image and gives a step height of 3.03 ± 0.08 Å between adjacent terraces.

Atomic Force Microscopes

AFM topography images give information on overall surface morphology, can reveal defects, and distinguish amorphous and crystalline phases, and identify nucleation and growth modes. Furthermore, images can be analyzed to calculate areal surface roughness parameters such as Sa (3D roughness average) and Sq (root mean square roughness) or statistics on grain and domain size. In addition, individual line sections can be used to determine step heights, film thickness, and even lattice spacings.

While AFMs provide many advantages, it is important to note that not all AFMs offer equivalent capability, performance, and ease of use. For instance, some models offer very high performance but can only hold one small sample (typically 12 cm) at a time. The Jupiter XR is a high-performance AFM that can accommodate large samples such as 200 mm wafers or an array of several small samples. Furthermore, commercial AFMs are still evolving rapidly, so newer models can differ significantly from those only a few years old. For example, some AFMs still use outdated piezo tube scanner technology. Such scanner designs are limited to much slower image acquisition times (5-10 minutes per site or more), are subject to many more imaging artifacts, and have extremely delicate, easy-to-break parts making them much less robust and reliable.

Oxford Instruments recently introduced the Asylum Research Jupiter XR AFM. Jupiter leverages Asylum’s experience in developing high-performance AFMs to provide an all-new AFM with a large inspectable area, that delivers high resolution images, with reliable sub-nanometer roughness measurements coupled with high throughput and high measurement confidence. These advantages are discussed in more detail and illustrated by the case studies presented in Figures 3-6.

Figure 3. Topography images of glossy printer paper demonstrate that height parameters in addition to Sa can more fully describe surfaces. Skewness (Ssk) is a measure of the distribution of features relative to the mean, where Ssk > 1 indicates a surface dominated by peaks and Ssk < 0 indicates a surface with pits or valleys. Kurtosis (Sku) is sensitive to the sharpness of the peaks and/or pits: Sku > 3 indicates especially sharp features, Sku = 3 indicates a normally distributed surface, and Sku < 3 indicates a more gradually varying surface. (z) Over a very wide scan size (100 μm, 195 nm pixel size), the coating exhibits longer-scale waviness (Ssk=-0.49, Sku=0.58, Sa=235.8 Å). (bottom) However, at a smaller scale (5 Zμm scan size, 10 nm pixel size), it is evident that the coating also contains much finer grain structure with interspersed pits (Ssk= 1.49, Sku=3.l5, Sa=23l.l Å). Though the apparent roughness is very similar at both length scales, the skewness and kurtosis parameters accurately describe the gradual, rolling waviness at longer scales and the pitted surface at smaller scales.

Jupiter XR AFM offers an 8 inch, fully addressable inspectable sample stage in its standard configuration, meaning it can accommodate large samples up to 210 mm in diameter and 35 mm in thickness. Alternatively, multiple smaller samples (e.g., those mounted on ~10 mm sample disks) can be magnetically mounted on the sample stage for successive inspection. The fully addressable motorized sample stage allows images to be acquired anywhere in a 200 × 200 mm2 range without the need for sample rotation. The full wafer accessibility allows for quicker wafer inspection, resulting in higher throughput and productivity.

Case Study #1: Surface Characterization of Glossy Paper Coating Process

The formulation, morphology, and surface roughness of paper coatings directly impact print quality and appearance. These properties are carefully tailored to different printing processes, inks and intended end uses. Though large-scale coating uniformity is obviously important, the surface roughness of the coating at the nanoscale has a large influence on how ink interacts with the paper. AFM is the tool of choice for such characterization.

Figure 3 shows examples of a consumer-grade, inkjet photo paper imaged with the Jupiter XR AFM. The images demonstrate surface characterization benefits at very different length scales. The full scan range 100 μm image shows the waviness of the paper (darker areas are lower and brighter areas are higher) and some scratch marks (indicated with arrows). Such AFM data provides information about the product quality and could influence the handling procedures so as to prevent scratching and bending of the paper. The adjacent 5 μm size image provides insight into the uniformity of the finer grain structure of this paper sample and can be used as a reference for future process optimization.

Asylum has applied its core of advanced AFM technology to ensure that Jupiter can measure surface roughness lower than the detection threshold of other characterization techniques. Moreover, Asylum Research AFMs can measure roughness 2-4× lower than most other AFMs. The ultra-low noise floor—the minimum resolvable height determined by mechanical and electronic instrument noise—of 0.25 Å allows the accurate measurement of angstrom-scale surface roughness.

Figure 4. Non-uniform grain size is often observed in epitaxial silicon layers, especially near the wafer edge where greater variations in process conditions may occur. As shown in the diagram, a series of locations on a 150-mm (6-in) wafer were pre-defined in software and used in an automated routine to acquire topography images. (A) Image acquired 200 μm from the wafer edge with Sa=0.785 Å. (B) Image at 1.6 mm from the edge with Sa=0.833 Å. (C) Image acquired 62.4 mm from the edge (12.8 mm from the center) with Sa=0.902 Å.

High throughput and increased productivity are also important factors to consider. Jupiter can image 5-20× faster than most AFMs, such that single images can routinely be acquired in less than 2 minutes and as little as 15 seconds (depending on scan range and overall roughness). In addition, its high-speed motorized sample stage moves between sites in 5 seconds or less with micrometer precision. High-speed scanning and positioning are especially valuable when combined with automatic image acquisition as described in case study #2 (see Figure 4).

Case Study #2: Automated Inspection of Epitaxial Silicon Wafer Roughness

Epitaxial layers are commonplace in modern semiconductor processing. Silicon epitaxy processes allow precise layers with different dopant types and concentrations, while heteroepitaxial layers of III-V compounds and other materials enable even more options. An additional benefit of epitaxial layers is their extremely low surface roughness compared to substrates prepared by chemical-mechanical polishing. Figure 4 shows an example of roughness measurements on a wafer with an epitaxial silicon layer. It demonstrates Jupiter’s ability to make angstrom-level roughness measurements as well as automating measurements at different sites on the wafer.

Figure 5. These topography images of a glass disk media substrate are the first and last in a sequence of 1000 acquired over 15h of unattended operation (~54 seconds per image). In each image, the inset is a digitally zoomed magnification of the lower left region to help show the finer structure. The graph shows that the measured roughness Sa remains constant within 1% over the entire period, demonstrating the remarkable stability enabled by blueDrive tapping mode imaging.

Exclusive blueDrive™ cantilever excitation enables high measurement confidence and reliability. Surface roughness measurements often influence critical business decisions, so it is vital that measurements are trustworthy and the instrument is reliable. Asylum’s exclusive blueDrive tapping mode technology improves imaging stability compared to conventional AFMs that use piezo excitation. This extends tip lifetime and, in turn, makes roughness results more consistent and repeatable (see Case study #3, Figure 5).

Case Study #3: Quality Control of Disk Drive Media

Magnetic hard disk drives continue to dominate over solid state drives in applications that require massive amounts of inexpensive data storage. This advantage has been maintained by a continued increase in the data storage density of magnetic disk media. Achieving these high densities has required a corresponding decrease in media roughness. The example in Figure 5 shows roughness measurements on the substrate of a modern lubricated disk media. With 1000 images acquired unattended over 15 hours, it demonstrates extreme measurement fidelity as well as high measurement throughput.

Case Study #4: Quality Assurance of Chemically Strengthened Display Glass

Figure 6. Many applications for display glass demand both low surface roughness and high abrasion resistance. (left) Example of routine roughness measurements made in 52 seconds per site. With a 1 μm scan size, the image gives Sa=9.70 Å. (right) A glass sample was subjected to intentional scratching with metal tweezers, and a large-area image (30 μm scan size) was completed to characterize the dimensions of the scratch. The profile corresponding to the white line in the image shows the damage actually consists of a series of fine parallel scratches. In real QA measurements, such testing could be done using a calibrated load to initiate the scratch (e.g., via a tribometer).

Thanks to chemically strengthened display glass, today’s mobile devices are more resilient to bumps, drops, and scratches than ever before. These special glass formulations undergo an ion exchange process wherein sodium ions near the surface are replaced by potassium ions, creating a compressive surface stress that dramatically strengthens the glass sheet. Figure 6 shows measurements on chemically strengthened display glass acquired at different length scales. The images demonstrate how a flexible scan range and superior resolution can be used to monitor both overall roughness and scratch resistance.

This article was written by Ted Limpoco, PhD, Applications Scientist, Oxford Instruments Asylum Research, (Concord, MA). For more information, contact Oxford Instruments here  or visit here .

Photonics & Imaging Technology Magazine

This article first appeared in the September, 2020 issue of Photonics & Imaging Technology Magazine.

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