With the advancement of VLSI (Very Large Scale Integration) technology, humanity has entered the ultra-deep sub-micron era. This progress has enabled the integration of various components—processors, memory, analog circuits, interface logic, and even RF circuits—onto a single large-scale chip, known as an SoC (System on Chip). This integration has revolutionized the way electronic systems are designed and manufactured, offering higher performance, lower power consumption, and reduced size.
Embedded memory plays a crucial role in SoCs and is gradually increasing in proportion within these chips. Let’s explore this topic with a closer look at embedded memory and its significance.
What is embedded memory?
Embedded memory is not a new concept. Unlike off-chip memory, it refers to memory that is integrated directly into the chip or combined with IP blocks and mixed-signal components to form a single system-on-chip. Today, it is a fundamental component of SoCs, with each SoC typically containing some amount of embedded memory.
Based on whether data is lost after power failure, embedded memory can be categorized into two types: volatile and non-volatile. Volatile memory, such as SRAM and DRAM, loses its data when power is turned off. Non-volatile memory, including eFlash, EEPROM, and emerging technologies like eMRAM, eRRAM, and ePRAM, retains data even without power.
Although all are forms of memory, they differ significantly. The key distinction between embedded memory and discrete memory lies in their process compatibility. Embedded memory is closely tied to the process characteristics of the application IC. For example, chips made using 90nm and 45nm processes may have vastly different internal embedded memory sizes. In contrast, discrete memory devices are optimized for memory-specific processes.
As information technology continues to evolve, the proportion of embedded memory in SoCs has grown substantially. According to available data, it increased from an average of 20% in 1999 to 60–70% by 2007 and reached up to 90% by 2014. This trend highlights the growing impact of embedded memory on overall chip performance.
The development of embedded memory began in the 1960s and 1970s when the semiconductor industry was dominated by IDM (Integrated Device Manufacturer) models. Companies handled everything from design to manufacturing and packaging independently. At that time, system requirements were not very demanding, so discrete memory was the norm.
In the 1980s and 1990s, the fabless and foundry models emerged, along with third-party IP vendors like ARM. As chip integration increased, discrete memory faced challenges. One major issue was the growing disparity between MPU (Microprocessor Unit) speed and memory speed. While MPUs grew at around 60% per year, memory speeds only improved by about 10%. This gap led to the rise of on-chip memory, which offered faster access, lower latency, and better integration.
By the mid-1990s, Intel introduced on-chip caches, marking a turning point for embedded memory. This innovation led to the decline of many discrete cache vendors and set the stage for the dominance of embedded memory.
Today, over 90% of mobile processors consist of embedded SRAMs, such as register files, level 1, and even level 3 caches. SRAM has become a key metric for evaluating foundry processes. Its six-transistor structure gives it a clear area advantage over DRAM, which uses one transistor and one capacitor.
Despite its advantages, DRAM integration has been challenging due to differences in process compatibility. Although companies like TSMC are re-evaluating the feasibility of eDRAM, it has not yet become a mainstream choice.
Meanwhile, the growth of consumer electronics has driven the need for more storage, leading to the development of embedded flash (eFlash). From early ROMs to OTP, EEPROM, and now high-density eFlash, embedded memory has proven essential for storing code and data reliably, even after power loss.
However, existing storage technologies like SRAM and DRAM suffer from volatility, making them unsuitable for critical applications such as aerospace and defense. Flash and EEPROM also face limitations in speed, complexity, and power consumption, which restrict their use in real-time systems.
To address these challenges, new non-volatile memory technologies are emerging, including FRAM, MRAM, PRAM, and RRAM. These offer promising alternatives with features like high speed, low power, and non-volatility.
Ferroelectric RAM (FRAM) uses ferroelectric materials to store data, offering fast read/write speeds and radiation resistance. Magnetic RAM (MRAM) leverages magnetic fields for storage, providing high reliability and low power consumption. Phase Change Memory (PRAM) stores data through phase changes in chalcogenide materials, while Resistive RAM (RRAM) uses resistance changes in specific films.
Each of these technologies has its own strengths and challenges. For example, FRAM faces issues with material integration, RRAM is still largely experimental, MRAM has high manufacturing costs, and PRAM struggles with heat management and cost.
Looking ahead, the future of embedded memory will depend on factors such as CMOS compatibility, scalability, and bandwidth requirements. Companies that can overcome these challenges and deliver reliable, cost-effective solutions will likely lead the market.
In conclusion, the development of embedded memory is a dynamic and evolving field, full of opportunities and challenges. Those who embrace innovation and adapt to changing demands will ultimately succeed in this competitive landscape.
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