A paper published this week in Nature by Arm Research and Cambridge UK-based PragmatIC revealed details of PlasticArm, a flexible Arm Cortex-M0 based system-on-chip (SoC) fabricated using thin-film transistors (TFT) on a flexible substrate.

While this is not yet a commercial solution, it has significant potential for embedding microprocessors, and hence intelligence, into many more everyday products. A co-author of the paper, John Biggs, a distinguished engineer at Arm Research, said of the development, “As ultra-low-cost microprocessors become commercially viable, all sort of markets will open with interesting use cases such as smart sensors, smart labels and intelligent packaging. Products using these devices could help with sustainability by reducing food waste and promote the circular economy with smart life-cycle tracking. Personally, I think that the biggest impact could be in healthcare — this technology really lends itself to building intelligent disposable health monitoring systems that can be applied directly to the skin.”

Flexible electronic devices, unlike conventional semiconductor devices, are built on alternative substrates such as paper, plastic, or metal foil. Using thin film semiconductor materials such as organics, metal oxides or amorphous silicon, they offer properties not available from silicon, including thinness, conformability, and low manufacturing costs. TFTs can be fabricated on flexible substrates at a significantly lower processing cost than metal–oxide semiconductor field-effect transistors (MOSFETs) fabricated on crystalline silicon wafers.

EE Times spoke to one of the other co-authors of the paper, Catherine Ramsdale, who is senior vice president of technology at PragmatIC. She explained,

“This is essentially a proof of concept, showing what you can do and the complexity that can be achieved. We’ve been working with Arm since 2013, and now we can say the technology is at a certain maturity level where the gate count is achievable, and the ecosystem is in place.”

Arm Research and PragmatIC began exploring the feasibility of an Arm-based flexible processor back in 2013, starting off by building prototype circuits, including ring oscillators, counters, and shift register arrays. While some flexible components, such as sensors, memories, light-emitting diodes, and more have been prototyped, until now a flexible microprocessor has been a major blocker to the realization of fully flexible electronics.

When the PragmatIC FlexLogIC manufacturing system became available a few years later, together with the advances made in the PlasticArmPit project, which uses the same cell library, tool flow and process technology, all the pieces of the puzzle fell into place, according to Arm. The teams from both companies decided the time was right to give it another try, and on 27 October 2020, they declared that the world’s first fully functional non-silicon Arm processor, PlasticArm, was produced.

Although PlasticArm is an ultra-minimalist Cortex-M0-based SoC, with just 128 bytes of RAM and 456 bytes of ROM, it is twelve times more complex than the previous state-of-the-art flexible electronics. Details of this development are featured in the paper, “A Natively Flexible 32-bit Arm Microprocessor“.

We asked Ramsdale about some of the challenges in the development of PlasticArm. “A lot of the early work was to make sure we were talking the same language: for example, Arm would talk about larger circuits while we would be referencing smaller circuits. We needed to make sure the design process co-optimization was successful. We had to ensure that the technology utilized standard design tools that a silicon designer would use.”

“Hence, we provided Arm with a PDK, which they implemented in their tools. Our part of this puzzle is in enabling and ensuring accurate capture of all the design rules for our process and ensure repeatability with the FlexLogIC box.” She added, “Where we are now today is validation of how far we have got with FlexLogIC: to produce something at this scale, and not just for RFID chips.”

PlaticArm elements

There are three major components of the natively flexible microprocessor: a 32-bit CPU, a 32-bit processor containing a CPU and CPU peripherals, and a system-on-a-chip (SoC) containing the processor, memories and bus interfaces—all fabricated with metal-oxide TFTs on a flexible substrate. The natively flexible 32-bit processor is derived from the Arm Cortex-M0+ processor supporting the Armv6-M architecture (a rich set of 80+ instructions) and existing toolchain for software development (compilers, debuggers, linkers, integrated development environments).

The entire natively flexible SoC, called PlasticARM, is capable of running programs from its internal memory. It contains 18,334 NAND equivalent gates, which makes it the most complex FlexIC (at least 12× more complex than previous integrated circuits) that has been ever built with metal-oxide TFTs on a flexible substrate.

This processor fully supports the Armv6-M instruction set architecture, which means that the code generated for a Cortex-M0+ processor will also run on the processor derived from it. The processor comprises the CPU and a nested vector interrupt controller (NVIC) tightly coupled to the CPU, handling interrupts from external devices.

The rest of the SoC consists of memories (ROM/RAM), the AHB-LITE interconnect fabric (a subset of the advanced high-performance bus (AHB) specification) and interface logic to connect the memories to the processor, and finally an external bus interface that is used to control two general-purpose input-output (GPIO) pins to communicate off-chip. The ROM contains 456 bytes of system code and test programs, and has been implemented as combinational logic. The 128 bytes of RAM has been implemented as a latch-based register file and is mainly used as a stack.

PlasticARM is implemented with PragmatIC’s 0.8-μm process using industry-standard chip implementation tools, plus a process design kit (PDK), standard cell library and device/circuit simulations in order to implement the PlasticARM FlexIC. The flexible SoC is fabricated using the commercial ‘fab-in-a-box’ manufacturing line, FlexLogIC.

The process uses an n-type metal-oxide TFT technology based on indium−gallium−zinc oxide (IGZO) and generates the FlexIC design on a 200-mm-diameter polyimide wafer. The IGZO TFT circuits are made using conventional semiconductor processing equipment adapted and configured to produce devices on a flexible (polyimide) substrate with a thickness of less than 30 μm. They have a channel length of 0.8 μm, and a minimum supply voltage of 3 V.