Awhile back, I wrote an article describing the concept of technology transfer, explaining that when a fab gets ready to start manufacturing a new technology, it must work closely with the technology developer to create the perfect recipe – including the materials, process, tools and equipment, and other variables.
In this article, I would like to focus specifically on the materials piece of that recipe. I’ll explain what ‘materials’ are in the context of semiconductor manufacturing and explore what we mean at Weebit Nano when we talk about our use of ‘fab-friendly materials’ as an advantage of Weebit ReRAM.
Let’s define ‘materials’
‘Materials’, according to the Merrian Webster dictionary, are “the elements, constituents, or substances of which something is composed or can be made.”
Manufacturing most goods today includes myriad materials that are used in a complicated chains of processing steps. Take leather goods for instance. There is a broad set of materials needed to process the finished leather, before it becomes ready to be made into end products like shoes and bags. Materials include those that comprise the end products, such as the hides themselves, as well as dyes, waxes and oils. There is an even broader mix of materials that are used in the fabrication process and then thrown away, such as cleaning agents, preservation and tanning materials and finishing agents.
Similarly, when we talk about semiconductors, we are talking not only about the materials that comprise the actual semiconductors themselves (in the stack and interfaces), but also about the substances that are used in the fabrication/manufacturing of the wafers and then disposed of.
Some materials are uniformly applied, while others are applied in patterns. In leather goods, this is done through techniques such as embossing and engraving using predefined texture plates. In semiconductors, many materials are applied using photomasks through which the materials are deposited on specific areas of the wafer.
Materials used in semiconductor manufacturing
Some materials like copper and aluminum are very good conductors of electricity, and others like rubber and wood have insulating properties and therefore prevent conductivity. Semiconductor materials have properties of both conductors and insulators, enabling conductivity under certain controllable conditions like temperature, pressure, and other process parameters. These materials are critical to electronic devices, enabling control of the electricity flow.
According to the IEEE, the most widely used semiconductor materials are silicon (Si), germanium (Ge), and gallium arsenide (GaAs). Germanium was used early on and is still used for some applications, but silicon, which is the second most abundant element in the earth’s crust, has been used extensively in semiconductors since the 1950s.
Silicon is efficient and economical to extract, purify and crystallize, and it’s straightforward to mass-produce. Within standard CMOS (Complementary Metal-Oxide-Semiconductor) manufacturing flows, silicon or compounds of silicon are the most widely used. Since pure silicon is more of an insulator than a conductor, a process called ‘doping’ is used to add tiny impurities (atoms of other materials) to the silicon to make it more conductive.
There are also materials used in manufacturing that are built into the stack for metal layers, interconnects, conductive barriers, and so on. Then there are the materials that are disposed of after use. Like the cleaning, curing and tanning agents we need to process leather, these materials – including gases, solvents, polymers, and others – are not actually part of the end products, but they are needed for processing.
Above: an example CMOS process courtesy of https://www.scl.gov.in/cmos.html
“Friendly” versus “unfriendly” materials
Choices of materials are often based on their inherent electrical properties, changes under different environmental conditions, interactions with other materials, and their compatibility with existing CMOS technologies.
This latter consideration is something we refer to at Weebit as being ‘fab friendly’ and it’s a differentiator for our ReRAM (RRAM).
An article written awhile back by my colleague, Eran Briman details an environmental initiative we completed with our R&D partner CEA-Leti which clearly shows the difference between friendly and unfriendly materials. The initiative compared Weebit ReRAM to MRAM technology, and results showed that the environmental impact of ReRAM is much lower than that of MRAM. Read Eran’s article to learn more about the criticality of various materials used in semiconductor manufacturing, and why the materials used in Weebit ReRAM are much friendlier than those used in MRAM.
Unfriendly materials
Critical and rare earth materials are very expensive often due to the cost of their extraction. Their extraction is also often very unfriendly to the environment, and their use can be complicated by political factors. Rare earth materials can add huge costs and complexities to the manufacturing process, so minimizing their use is very important.
Some materials aren’t rare earth materials, but they still aren’t very friendly – whether to the environment, or to adoption by a fab. Using more exotic materials can require additional dedicated clean room space, vacuum technology, extra tooling, standalone etch and deposition tools, special cleaning protocols, different wafer handling processes, and myriad other considerations. There can also be added issues with equipment degradation, sewage and recycling requirements.
Materials like iron (Fe) and Nickel (Ni) are considered unfriendly to CMOS fabrication, so strict protocols are followed to prevent contamination from these metals. Even trace amounts can lead to significant reductions in performance, stability, and reliability, so their use requires specialized equipment and cleanroom conditions to maintain the purity and performance of the devices.
Another example is PZT (a combination of lead zirconate (PbZrO₃) and lead titanate (PbTiO₃), which is used in medical imaging equipment, sensors and actuators, energy harvesting systems and FeRAM (Ferroelectric Random Access Memory). But in a CMOS fab, PZT is unwanted because of the lead contamination risk, material compatibility issues, additional process complexity, and reliability concerns associated with ferroelectric materials. In modern CMOS fabs which aim to minimize contamination risks and simplify the manufacturing process, PZT is typically avoided in favor of more compatible and environmentally friendly alternatives for applications requiring piezoelectric or ferroelectric properties.
Fab-friendly ReRAM
The materials used for creating our ReRAM are already used as standard in CMOS flows. Such materials compatibility makes it easier for companies to adopt our ReRAM, as they already know how to deal with these materials, and normally already have the tools needed to work with them. This enables them to avoid high cost and potential issues with reliability or cross-contamination with other materials in their flow.
Since fabs are such delicate environments with extremely sensitive (and very expensive) tools, any change can be very costly and have significant impacts. Introducing “unfriendly” materials which are not commonly used in the fab (or not used at all), requires a deep understanding of them and how they impact the environment in the fab. It often requires new processing tools t and new manufacturing procedures. But most of all, these materials have the potential to contaminate the environment. All these reasons make such materials difficult to adopt and become a major hurdle for new technologies using them. This is a key reason for the failure of some of the competing ReRAM technologies.
The ‘Secret Sauce’ of Materials
The many components and pieces of IP within a semiconductor chip can be comprised of different materials.
For ReRAM, different types of metal oxides can be used as resistive materials, with choices made based on electrical performance and compatibility with existing CMOS technology. There are also inert metals added on the top and bottom electrode layers which are chosen for various technical reasons.
As a developer of ReRAM IP, there are many materials that we can select from that provide the performance needed within a standard CMOS flow. We’ve chosen those that are the fab-friendliest. And it’s not just about the specific materials themselves; we are able to combine materials under different conditions to achieve the best possible performance. This is enabled by the expertise of our Device and Process teams, including more than a dozen PhDs in Physics and Chemistry.
It’s also about the process itself. Manufacturing Weebit ReRAM adds two masks to the manufacturing flow, versus up to 10 added masks for flash. Choosing ReRAM translates to fewer layers, less waste, and a more environmentally friendly process. On top of this inherent advantage, the Weebit Process team focuses on how to manufacture the cell efficiently in different fabs and different process nodes.
As customers look to choose the right embedded NVM for their SoC, there are many factors that can affect their decision. When it comes to materials, choosing Weebit ReRAM is the friendliest option – from a cost, complexity and environmental perspective. When combined with our performance and reliability advantages, the decision is clear!
