Embedded non-volatile memory (NVM) intellectual property (IP) is a requirement for storing data that must be preserved when power to the chip is removed. NVM is found in almost every system on chip (SoC) design today, especially those targeting connected devices accessing content protected by digital rights management and sensitive financial or personal data. As these SoC designs migrate toward 28 nm and lower processes, engineering teams are re-examining the available commercial options. This reappraisal is occurring because of challenges presented by these smaller geometry processes. Suddenly, what was once an insignificant commodity is threatening to become a technology bottleneck.
NVMs can be classified in two groups: those that are programmed once and those that can be reprogrammed. In the former category, the most fundamental form of NVM is the masked ROM, which is programmed during the chip fabrication process. Next, comes the e-Fuse, which gets programmed during final test, followed by, anti-fuse technology, which can be programmed at wafer sort or final test or in the field at later point in time. In the latter category of reprogrammable memory are commercially available embedded Flash and floating gate alternatives. In the lab, new resistive RAM alternatives are being developed to compete in the reprogrammable arena.
Before entering a discussion of the strengths and weaknesses of these various NVMs, consider the application of NVM in high volume consumer electronics SoCs and why engineering teams are giving NVM a more critical examination. In multimedia SoCs, NVMs store sensitive encryption key data and enable digital rights management of audio and video content being accessed by the SoC for display or for transmission. Storage requirements range from 8 kbits to 4 Mbits. They are also used in portable devices to store program code for the baseband SoC, and to contain data for configuration, trimming, and calibration of mixed signal functions such as power amplifiers and RF receivers. For SoCs targeting the industrial and automotive markets, NVMs store program code for microcontrollers, and trim and calibration data for the many analog and mixed-signal circuits found in these applications.
What follows is a discussion of each of these memory types pointing out their various strengths and weaknesses (see table 1). Beyond the technical considerations for each of these memories, another characteristic that is becoming important in the selection of embedded NVM is its susceptibility to reverse engineering or hacking. This importance is growing as NVM is being used to store encryption key data for digital rights management and now increasingly for personal identification data for digital wallets. In addition, with the increase in smart connected devices and consumer mobile devices, as well as autonomous connected machines, the demand for secure program code storage in NVM is rising to protect effectively against hacking.
Click image to enlarge Table 1: Comparison of commercially available NVM technologiesRead-only memory
Semiconductor read-only memory (ROM) is best known as the storage medium of video game cartridges, though the application also leverages other media such as DVDs. Still found on portable handheld games, ROM is the lowest cost form of non-volatile memory since data is stored in the metal vias in the routing layer of a chip. In embedded SoC designs, ROM is used to store program code for microcontrollers. ROM can also be used to store look-up tables for configuration and feature control. This NVM has the advantage of being process agnostic. It scales with each process shrink without requiring any technical changes and it will operate with any future generation of technology.
ROM is the lowest cost, smallest, and fastest NVM available on the market. It requires no license fee or per-chip royalty, and no additional processing steps or mask layers. It features a small footprint and operates at high speeds with read access of a few nanoseconds.
Most common is the so-called VIA ROM, which is generated by a ROM compiler utilizing metal vias in the metal stack of the ROM array. As such it needs to be completed and included in the GDSII of the SoCprior to mask making. This presents the one major drawback of this memory type: It is inflexible in that any change to the ROMís content requires a via mask re-spin, which entails time and expense. A typical 65 nm via re-spin will cost around $50K, but the real expense is in the time it takes the foundry to perform the re-spin and begin producing new chips. Lost time for chip integration, testing and validation will delay the time-to-market for the end product, which is likely to present an even bigger cost impact.
ROM is not an ideal medium for storing secure data, as any video game supplier will testify. Each new generation of video game has been hacked, with bootleg versions of the software made available on-line and downloaded by the millions. The same vulnerabilities suffered by ROM in video game cartridges apply to ROM in embedded chip designs.