Graphene Enables Broad Spectrum Sensor Development

Team successfully marries a CMOS IC with graphene, resulting in a camera able to image visible and infrared light simultaneously.

Graphene Enables Broad Spectrum Sensor Development

By Wisse Hettinga

Researchers at ICFO—the Institute of Photonic Sciences, located in Catalonia, Spain—have developed a broad-spectrum sensor by depositing graphene with colloidal quantum dots onto a standard, off-the-shelf read-out integrated circuit. It is the first-time scientists and engineers were able to integrate a CMOS circuit with graphene to create a camera capable of imaging visible and infrared light at the same time. Circuit Cellar visited ICFO

Stijn Goossens is a Research Engineer at ICFO- the Institute of Photonic Sciences.

Stijn Goossens is a Research Engineer at ICFO- the Institute of Photonic Sciences.

and talked with Stijn Goossens, one of the lead researchers of the study.

HETTINGA: What is ICFO?

GOOSSENS: ICFO is a research institute devoted to the science and technologies of light. We carry out frontier research in fundamental science in optics and photonics as well as applied research with the objective of developing products that can be brought to market. The institute is based in Castelldefels, in the metropolitan area of Barcelona (Catalonia region of Spain).

HETTINGA: Over the last 3 to 4 years, you did research on how to combine graphene and CMOS. What is the outcome?

GOOSSENS: We’ve been able to create a sensor that is capable of imaging both visible and infrared light at the same time. A sensor like this can be very useful for many applications—automotive solutions and food inspection, to name a few. Moreover, being able to image infrared light can enable night vision features in a smartphone.

HETTINGA: For your research, you are using a standard off-the-shelf CMOS read-out circuit correct?

GOOSSENS: Indeed. We’re using a standard CMOS circuit. These circuits have all the electronics available to read the charges induced in the graphene, the rows and columns selects and the drivers to make the signal available for further processing by a computer or smartphone. For us, it’s a very easy platform to work on as a starting point. We can deposit the graphene and quantum dot layer on top of the CMOS sensor (Photo 1).

PHOTO 1 The CMOS image sensor serves as the base for the graphene layer.

PHOTO 1
The CMOS image sensor serves as the base for the graphene layer.

HETTINGA: What is the shortcoming of normal sensors that can be overcome by using graphene?

GOOSSENS: Normal CMOS imaging sensors only work with visible light. Our solution can image visible and infrared light. We use the CMOS circuit for reading the signal from the graphene and quantum dot sensors. Tt acts more like an ‘infrastructure’ solution. Graphene is a 2D material with very special specifications: it is strong, flexible, almost 100 percent transparent and is a very good conductor.

HETTINGA: How does the graphene sensor work?

GOOSSENS: There are different layers (Figure 1). There’s a layer of colloidal quantum dots. A quantum dot is a nano-sized semiconductor. Due to its small size, the optical and electronic properties differ from larger size particles. The quantum dots turn the photons they receive into an electric charge. This electric charge is then transferred to the graphene layer that acts like a highly sensitive charge sensor. With the CMOS circuit, we then read the change in resistance of the graphene and multiplex the signal from the different pixels on one output line.

FIGURE 1 The graphene sensor is comprised of a layer of colloidal quantum dots, a graphene layer and a CMOS circuitry layer.

FIGURE 1
The graphene sensor is comprised of a layer of colloidal quantum dots, a graphene layer and a CMOS circuitry layer.

HETTINGA: What hurdles did you have to overcome in the development?

GOOSSENS: You always encounter difficulties during the course of a research study and sometimes you’re close to giving up. However, we knew it would work. And with the right team, the right technologies and the lab at ICFO we have shown it is indeed possible. The biggest problem was the mismatch we faced between the graphene layer and the CMOS layer. When there’s a mismatch, that means there’s a lack of an efficient resistance read-out of the graphene—but we were able to solve that problem.

HETTINGA: What is the next step in the research?

GOOSSENS: Together with the European Graphene Flagship project, we are developing a production machine that will allow us to start a more automated production process for these graphene sensors.

HETTINGA: Where will we see graphene-based cameras?

GOOSSENS: One of the most interesting applications will be related to self-driving cars. A self-driving car needs a clear vision to function efficiently. If you want to be able to drive a car through a foggy night or under extreme weather conditions, you’ll definitely need an infrared camera to see what’s ahead of you. Today’s infrared cameras are expensive. With our newly-developed image sensor, you will have a very effective, low-cost solution. Another application will be in the food inspection area. When fruit ripens, the infrared light absorption changes. With our camera, you can measure this change in absorption, which will allow you to identify which fruits to buy in the supermarket. We expect this technology to be integrated in smartphone cameras in the near future.

ICFO | www.icfo.eu

This article appeared in the September 326 issue of Circuit Cellar

RISC-V and Moore’s Law : An Interview with Krste Asanovic

During his busy sabbatical, Krste Asanovic took time to share his thoughts on developments n the world of processors and the open sourcing of processor architecture.

Moore’s Law and the Chip Industry’s Perfect Storm

By Wisse Hettinga

With the end of Moore’s Law in sight and a silicon manufacturing world that is struggling to protect their investments, the RISC-V foundation is throwing its nets out on the other side of the boat. How? By creating an opensource platform for future new silicon development.

“There is a lot of friction in the market,” Asanovic explains. Being a professor at Berkeley University in Computer Architecture, he knows what he is talking about. “With RISC-V we want to reduce this friction in the industry. One of the problems is the IP protection and business involvement in the industry. With SiFive you don’t have to deal with complicated contracts. Users can just come and take the material that’s all published and open source and use it in their future chip design.”

Lead Opening Image

Krste Asanovic is a SiFive founder and Professor of Computer Architecture at Berkeley University.

“The Barcelona Computer Centre is showing great interest in what we are doing with RISC-V. And the UPC computer architecture department is one of the strongest architecture group in Europe. Here—and also in the rest of Europe—there is a lot of interest in RISC-V for research projects and also for possible industrial use,” says Asanovic.

HETTINGA: Can you explain what RISC-V is?

ASANOVIC: RISC-V is an instruction set architecture (ISA). An ISA is what you use to encode software to run on hardware. In the industry there are common standards like the x86 from Intel and AMD. There’s also the ARM architecture we all know from our mobile phones and tablets. RISC-V uses different encoding which is meant to be free and open so that everyone can use without paying license fee—which is unlike the existing proprietary standards. Our goal is to have an open standard anybody can use.

HETTINGA: And what’s the level of interest from the market today for RISC-V?

ASANOVIC: If you look at the market, the x86 architecture is dominant in desktops and servers. ARM is dominant in mobile phones and tablets—and it will probably remain so. But what is interesting is that there are always new markets coming along: IoT (Internet of Things) and automotive are big markets. At the high-end of the market we see storage controllers and machine learning accelerators. These are all new greenfield areas where people are looking at new chip designs. They don’t have a large legacy of software and they are open to a new instruction set—particularly ones that are free of all sorts of legal and financial strings and give them flexibility to bring new things into the controller architecture.

HETTINGA: Give us a little history of RISC and of RISC versus CISC.

ASANOVIC: The RISC architecture goes a long way back and it’s still alive. I trace the roots of RISC way back to Seymour Cray’s early machines—like the CDC 6600—from 1964. RISC machines are register rich and have a load/store architecture. They have a lot of general registers and all operations are between registers except for the memory operations. That style of machine has remained popular for over 50 years and has outlived Moore’s Law.

Meanwhile, CISC has also been around for some time. CISC was a product of the time before integrated silicon started replacing the vacuum tubes and magnetic core memory systems. It is interesting to see that over the last couple decades there has been very little new development in the CISC architecture arena. I think everyone will agree that if you start from scratch, CISC is not a great idea.

RISC-V follows the heritage of the earlier RISC processor designs developed at Berkeley University. “RISC-V” means it is the fifth generation. We started on the project in 2010 and we were tired of using commercial ISAs for research. They were sometimes too complicated for what we wanted to do, and with the IP entanglements it is very difficult to share that research with others.

As academics, we like to share our work with others. We realized we did not want to invest in proprietary architectures. Also, a lot of commercial products are not that good. There was a quality problem and we thought we could do a lot better.

The response was overwhelming and very quickly it was getting too big for Berkeley and we started the RISC-V foundation. The goal of the foundation is to maintain the RISC-V ISA standard and we have grown to over 60 companies—including the biggest names like Qualcomm, Samsung, Microsoft, Western Digital, IBM and Google.

HETTINGA: From there, how did RISC-V lead to the creation of the SiFive organization?

ASANOVIC: At Berkeley we’ve done a great deal of research into RISC architecture, involving teams and activities. They did implementations, ported the compilers and Linux and got other operating systems up and running. Having a ‘critical mass’ of graduate students working on this project allowed people from outside to pick it up and do real work with it. It started off as an idea to have a consultancy activity around RISC-V. The co-founders—Andrew Waterman and Yunsup Lee—soon realized the opportunity and that’s why I also decided to get involved as a founder.

HETTINGA: This seems to be a very significant time in the semiconductor industry. How would you characterize where we’re at today?

ASANOVIC: The semiconductor industry is in this perfect storm where we see that Moore’s Law is ending and that new technologies and developments are getting more and more expensive. There are fewer and fewer companies capable of pulling off a new design and making money out of it. At the same time there is a growth in demand for custom chips. Everybody is talking about the Internet of Things and all those devices will need a processor—and that cannot be the same processor for all solutions. There will be a growth in silicon products, but that growth will be in many fragmented markets. The old semiconductor business model—having one design and selling many millions of it— doesn’t work anymore. That has worked with the traditional computer and mobile phone markets, but the future will see perhaps hundreds of designs in lower volumes.

With SiFive we try to figure out how this works. The traditional users of the chips are now becoming the new manufacturers. Google. Microsoft, Amazon and a lot of other companies will design and make their own chips—not to sell to others, but to use them in their own products. It will allow them to add capabilities that are not available in standard off-the-shelf chips.

Our mission is to find out if we can help smaller companies and startups to do custom silicon design and invent new products with new capabilities. We believe there is a lot of untapped innovation there. But the problem is the barrier to enter custom silicon design is too high and those great ideas do not become a product. Solving that problem is the goal of SiFive.

Photo 1

SiFive’s RISC-V Arduino board makes it easy for small companies to get started with new designs.

HETTINGA: Tell us about SiFive’s RISC-V Arduino reference design board.

ASANOVIC: Our business model is to do quick developments of new chipsets and help the client to get into production at very low costs. To enable that, we made an Arduino board (at the time of the interview the new Arduino Cinque was introduced / WH) that runs very fast. And by putting it into the Arduino format a lot of small design companies will see it and can use it for new designs. The interesting thing about this product is that it will take the focus from the board to the chip level. Not only the board is open source but the chip design is too. That can open up completely new perspectives for makers, start-up companies and medium-sized businesses. All the design files of the chip are open source are on Github. This is unique in the semiconductor business. With SiFive we want to get rid of the friction in the industry. We don’t have a costly structure with NDAs and lawyers. A lower cost structure will also mean lower costs for our clients. You can come and take the designs as they are and use them.

SiFive | www.sifive.com