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.
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.
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 For example, some AFMs still use outdated piezo tube scanner technology.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.
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.
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.
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
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 .