A Tough Coat for Silicon
March 22, 2017 | A*STAREstimated reading time: 2 minutes

A simple, green method that applies a protective coating to semiconductors could help to develop these materials for many applications, from batteries to biosensors.
Silicon forms an oxide layer on its surface when exposed to air or moisture, which can detract from its electronic properties. Adding a ‘skin’ of molecules to the silicon can provide a physical barrier that prevents oxidation, but forming these monolayers can be tricky, requiring an inert atmosphere and long processing times, or demand the use of potentially harmful organic solvents.
Sreenivasa Reddy Puniredd of the A*STAR Institute of Materials Research and Engineering and colleagues have now developed a new way to deliver the protective molecules using supercritical carbon dioxide (scCO2). Carbon dioxide is converted to scCO2 under high pressure, when it becomes a free-flowing liquid that is chemically inert, inexpensive, and more environmentally-friendly than traditional solvents.
The researchers used scCO2 to carry molecules called alkylthiols, which contain long carbon chains with a sulfur atom at one end. Sulfur forms a stable bond with silicon, while the water-repelling carbon chains make a tightly-packed skin on silicon’s surface.
To apply the coating they used alkylthiols containing between seven and 18 carbon atoms to coat silicon, germanium, and silicon nanowires. Each procedure took a few hours, and produced monolayers between 1.6 nanometers and 2.3 nanometers thick that resisted wear and repelled water. The greatest effect was seen for the longest alkylthiol chains.
The monolayers also protected the surface from oxygen for more than 50 days; those prepared using conventional solvents were typically stable for less than seven days. “The increase in stability was expected, but such long-term stability was a surprise,” says Puniredd.
Silicon nanowires are being tested for a range of biological applications, including biosensors and antibacterial surfaces. Although fragile and easily damaged by other monolayer formation methods, the silicon nanowires were undamaged by the scCO2 process, allowing the researchers to test how they interacted with human liver cells. Those protected by the 18-carbon alkylthiol significantly reduced cell growth on the nanowires, compared with unprotected nanowires or a flat silicon surface. This is probably because the cells’ proteins could not latch on to the monolayer’s long carbon chains.
“This scCO2 technology can be adopted for many kinds of inorganic surface modification,” says Puniredd. “The technology is not only scalable, but also enhances the quality and stability of the film. It can potentially replace billions of pounds of organic solvents used every year in thin-film fabrication and cleaning applications.”
Suggested Items
Knocking Down the Bone Pile: Addressing End-of-life Component Solderability Issues, Part 4
07/16/2025 | Nash Bell -- Column: Knocking Down the Bone PileIn 1983, the Department of Defense identified that over 40% of military electronic system failures in the field were electrical, with approximately 50% attributed to poor solder connections. Investigations revealed that plated finishes, typically nickel or tin, were porous and non-intermetallic.
STMicroelectronics, Metalenz Sign a New License Agreement to Accelerate Metasurface Optics Adoption
07/14/2025 | STMicroelectronicsSTMicroelectronics, a global semiconductor leader serving customers across the spectrum of electronics applications and Metalenz, the pioneer of metasurface optics, announced a new license agreement.
Digital Twin Concept in Copper Electroplating Process Performance
07/11/2025 | Aga Franczak, Robrecht Belis, Elsyca N.V.PCB manufacturing involves transforming a design into a physical board while meeting specific requirements. Understanding these design specifications is crucial, as they directly impact the PCB's fabrication process, performance, and yield rate. One key design specification is copper thieving—the addition of “dummy” pads across the surface that are plated along with the features designed on the outer layers. The purpose of the process is to provide a uniform distribution of copper across the outer layers to make the plating current density and plating in the holes more uniform.
Global PCB Connections: Embedded Components—The Future of High-performance PCB Design
06/19/2025 | Jerome Larez -- Column: Global PCB ConnectionsA promising advancement in this space is the integration of embedded components directly within the PCB substrate. Embedded components—such as resistors, capacitors, and even semiconductors—can be placed within the internal layers of the PCB rather than mounted on the surface. This enables designers to maximize available real estate and improve performance, reliability, and manufacturability.
Preventing Surface Prep Defects and Ensuring Reliability
06/10/2025 | Marcy LaRont, PCB007 MagazineIn printed circuit board (PCB) fabrication, surface preparation is a critical process that ensures strong adhesion, reliable plating, and long-term product performance. Without proper surface treatment, manufacturers may encounter defects such as delamination, poor solder mask adhesion, and plating failures. This article examines key surface preparation techniques, common defects resulting from improper processes, and real-world case studies that illustrate best practices.