Logic Non-Volatile Memory by Charles Ching-Hsiang Hsu, Yuan-Tai Lin, Evans Ching-Sung Yang, Rick Shih-Jye Shen Chapter 1 Introduction List of Sections Section 1.1 What Are Logic NVMs 3 Section 1.2 When to Use Logic NVM 5 Section 1.3 Why Use Logic NVM 6 Section 1.4 Which Logic NVM is Better to Use 8 Section 1.5 Where to Get the Logic NVM Platform 9 Section 1.6 How to Use Logic NVM 11 References 13 List of Figures Fig. 1.1 Key functions of embedded Logic NVM. 5 Fig. 1.2 Illustration of NeoBit OTP memory structure or memory cell. 8 Fig. 1.3 Typical business model of Logic NVM. 10 Fig. 1.4 Application model of embedded NVM for different functions. 13
Chapter 1 Introduction 1 Chapter 1 Introduction This monogram is intended to serve as an introductory text and reference book for readers to learn what is Logic Non-Volatile Memory (LNVM), when to use it, how to use it, and where to get it. This book also helps IC designers to use Logic NVM to improve their product performance and production yield, hence cost. Non-Volatile Memory (NVM) is a silicon semiconductor basic building block (BBB) circuit containing a memory transistor and some logic transistors and RC circuits to read, write and erase the stored one-bit of data [1]. It has been widely used by silicon integrated circuit (IC) chip designers for data storage. Designers of electronic products usually use NVM embedded in the IC chip to store program instruction, identification and encryption data for security and protection considerations. In traditional embedded NVM s, many additional processing steps are required to fabricate the NVM transistors using the logic circuit fabrication process, which significantly raise the costs for producing the embedded NVMs by Foundries and the costs of the products using embedded NVMs, severely limiting the widespread applications of the NVMs. In 2002, our company, the ememory Technology, introduced its Logic NVM solutions which add no extra fabrication mask or very few masks to the existing logic circuit fabrication processes. Consequently, the development cost of NVM embedded into the logic IC, using the fabrication processes of the logic IC, is greatly
2 Logic Non-Volatile Memory by Charles Ching-Hsiang Hsu, Yuan-Tai Lin, Evans Ching-Sung Yang, Rick Shih-Jye Shen reduced. Therefore, increasing number of Foundries are able to offer logic circuit fabrication processes with NVM embedded inside the IC chip, As a result, the utilization of NVM has increased greatly in logic circuit designs. In addition to the logic circuits, high voltage, mixed-mode, and radio-frequency (RF) circuits can also be easily fabricated with our Logic NVM technology. Due to the wide and easy adoption of the Logic NVM, new applications of NVM have been developed, such as trimming to improve the performance and yield of IC products. Our products are grouped into the following four categories to help the readers to recognize their memory transistor and circuit operation principles and the terminologies. The historical and subsequent engineering usages of the descriptive were discussed in the Preface and the review literature [2]. For example, the 1971 flash for erase of the entire memory chip by flashes of ultraviolet (UV) light, has been extended to block erase by electrical means, such as tunneling, without light. The following categorizations provide the descriptions and definitions to correlate the trade usage with the device and circuit operation principles. 1. EPROM: Hot electron byte programming, UV erase; NeoBit is in this category. 2. Flash EPROM: Hot electron byte programming, FN tunneling block erase, NOR Flash, NeoFlash, and NeoMTP are in this category. 3. EEPROM: FN tunneling byte programming, FN tunneling byte erase; NeoEE is in this category. 4. Flash EEPROM: FN tunneling page programming, FN tunneling block erase, NAND Flash is in this category.
Chapter 1 Introduction 3 Section 1.1 What Are Logic NVMs In this book, the memory transistor device type and its circuits, as well as its reliability and qualification, will be discussed in great details for three different Logic NVMs produced by ememory: One Time Programmable (OTP) in Chapter 2, Flash in Chapter 3, and Electrically Erasable Programmable Read Only Memory (EEPROM) in Chapter 4. Chapter 5 describes the implementation procedures of using the embedded Logic NVM (elnvm) both in chip fabrication at Foundries and in the design of fabrication processes at our company, the ememory Technology. 1.1 What Are Logic NVMs Logic NVM is the Non-Volatile Memory that can be fabricated concurrently with logic circuit fabrication, with no extra fabrication mask or very few additional masks. Therefore, indeed the Logic NVM is simply embedded into logic circuit fabrication process. Hence, the logic circuit fabrication process is able to fabricate both the logic circuit and the NVM at the same time. Thus, it is an embedded NVM or envm logic circuit fabrication process. NVM is widely used in various electronic devices for consumer electronics and industrial applications. Many types of product designs use NVM to store: program codes, security or identification (ID) codes, and various parameters for calibration or function settings. Standalone and embedded NVM are often used by chip designers to provide the function of non-volatile storage in their chip designs. Standalone flash memory is one choice for storing these types of data, which are frequently updated. However, when compact system is a priority, chip designers prefer to reduce pin count, board space and system cost, while retaining data storage and high performance, by eliminating inter-chip communication. Embedded NVM meets these requirements.
4 Logic Non-Volatile Memory by Charles Ching-Hsiang Hsu, Yuan-Tai Lin, Evans Ching-Sung Yang, Rick Shih-Jye Shen Two kinds of embedded NVM technology are available for chip designers to use: a) the commodity NVM technology, which may be embedded into the generic CMOS logic circuit fabrication process with about ten additional fabrication steps and masks, or b) the Logic NVM, which employs a single-poly NVM process which is a part of the generic CMOS logic circuit fabrication process. The traditional embedding commodity NVM memory in the logic circuit fabrication process introduces extra process steps and mask layers to produce the memory transistor, which increases the complicity of the manufacturing process. The complication impacts production yield stability, and increases manufacturing cost. Logic NVM uses a single-poly NVM process, which is fully compatible with the generic CMOS logic circuit fabrication process, and which does not require any process modification. Therefore, Logic NVM is easier to implement and simpler to transfer across standard CMOS process platforms. Low manufacturing cost and widespread availability of Logic NVM technology speed up the product design cycle and increase product competitiveness. Logic NVM is currently the most costeffective embedded NVM solution for chip designers. Several kinds of Logic NVM technology are available for embedded applications. NeoBit, NeoFlash and NeoEE technologies, provided by ememory Technology Incorporated, are the most well-known and widely used Logic NVM technologies. Based on storage features, Logic NVM can be further categorized as: One Time Programmable (OTP) memory, Multiple Times Programmable (MTP) memory, Electrically Erasable Programmable Read Only Memory (EEPROM). The following chapters present NeoBit, NeoFlash, and NeoEE technologies, respectively for OTP, MTP, and EEPROM memories.
Chapter 1 Introduction 5 Section 1.2 When to Use Logic NVM Fig. 1.1 Key functions of embedded Logic NVM. 1.2 When to Use Logic NVM Demands of diversified markets and applications drive embedded Logic NVM for distinct functions. Figure 1.1 shows key functions employed widely in chip designs using embedded Logic NVM. The functions include identification setting, encryption, trimming, parameter setting, function selection and code storage. Identification setting is the most common usage of embedded Logic NVM for chip designers. It includes storage of system serial number, product ID, manufacturer ID, and user identification. Storing identification settings improves production management and logistics tracking. Embedded Logic NVM has also been seen in security-sensitive applications, such as digital rights protection applications and smart cards. The encryption function is indispensable for these sensitive applications, and setting security
6 Logic Non-Volatile Memory by Charles Ching-Hsiang Hsu, Yuan-Tai Lin, Evans Ching-Sung Yang, Rick Shih-Jye Shen keys for content protection algorithms is of significant importance. In analog or mixed-mode circuit design, process variation typically leads to circuit mismatch or signal bias, and electrical correction options are needed in the wafer test flow. Trimming is frequently adopted for this purpose in analog circuit design to provide design options and signal fine tuning. Another similar use of embedded Logic NVM is parameter setting for system function calibration. For example, in LCD displays, embedded Logic NVM is used to calibrate the gamma curve or gray level settings. Through parameter adjustment, designers can calibrate performance discrepancies to match system specifications. When a chip or system is designed to perform multiple functions, embedded Logic NVM can play the role of function selection to choose the appropriate function as each application requires. Embedded Logic NVM is also indispensable in microcontroller units (MCU), which are widely used in electronic products, such as automobiles. In light of potentially diverse applications, product designers have to provide diversified firmware, program code or instructions to convert the MCU based on different market requirements. Embedded Logic NVM gives designers high flexibility in reprogramming and in-circuit programming, allowing rapid product customization through program code settings. 1.3 Why Use Logic NVM With the demand for diversified functions, chip designers are adopting embedded NVM in chip designs. Two kinds of embedded NVM are available to chip designers, as mentioned in section 1.1. Embedding commodity NVM in the generic CMOS logic circuit fabrication process requires complicated process modification and development to bring flash memory into the CMOS logic circuit fabrication process while keeping CMOS device characteristics
Chapter 1 Introduction 7 Section 1.3 Why Use Logic NVM unchanged. Typically, 10 extra mask layers and complicated process steps are added to fabricate commodity NVM in the CMOS logic circuit fabrication process. These increase manufacturing cost, and also lower production yield and yield stability. Logic NVM, however, is a single-poly NVM technology, which is fully compatible with generic CMOS processes without any process modification and without extra mask layers. These make Logic NVM easy to implement in standard CMOS processes, and easily transferable across the standard CMOS process platforms of different Foundries. Logic NVM gives chip designers advantages of low manufacturing costs and widespread availability for production. Beyond manufacturing considerations, Logic NVM also helps designers integrate distinct functions into their designs. For security-sensitive applications, designers may be required to design some protections into the product itself, so the product may contain a security ID, encryption keys, or/and code for digital rights management. Designers can easily integrate product designs and Logic NVM with a self-defined security interface in a single chip. Data storage and access through the self-defined interface requires internal authority, which greatly improves data security. As opposed to two-chip solutions, e.g. product design with standalone NVM, chip designers are afforded high-level protections with Logic NVM and do not need to worry about potential security leaks through exposed, inter-chip connections. Using Logic NVM makes sense not only from the standpoint of manufacturing, but also as a chip-integration solution available to chip designers, to design products for diversified markets and applications.
8 Logic Non-Volatile Memory by Charles Ching-Hsiang Hsu, Yuan-Tai Lin, Evans Ching-Sung Yang, Rick Shih-Jye Shen 1.4 Which Logic NVM is Better to Use Many different kinds of Logic NVM technologies are available; each has one or more different technical features. Designers can choose the Logic NVM solution appropriate to their chip design according to the required design specifications, e.g. high-speed data programming or low-power access, or they can choose based on production control or manufacturing considerations. Fig. 1.2 Illustration of NeoBit OTP memory structure or memory cell. One well-known and widely used floating gate-based (FGbased) Logic NVM architecture is described here as a reference for designers. Figure 1.2 illustrates the FG-based NeoBit OTP memory cell. Floating gate-based memory technology uses an insulated floating gate to store electrical charges (electrons-soliddots in the figure, or holes) for data storage. NeoBit OTP memory adopts this floating gate storage concept, and utilizes a structure consisting of two p-channel MOSFETs (pmosts) in serial
Chapter 1 Introduction 9 Section 1.4 Which Logic NVM is Better to Use; Section 1.5 Where to Get the Logic NVM Platform connection, with the gate of one of the pmosts (the right one in Figure 1.2) floating for data storage. Data is written to the NeoBit OTP memory by injecting hot electrons (solid dots in Figure 1.2) into the floating gate. These hot or energetic electrons are generated by the channel hot holes via interband generation of electron-hole pairs, commonly known as the hot carrier effects (but among many hot carrier effects), which alters the conduction characteristics of the pmost. Erase is performed by recovering the current-voltage characteristics of the written floating-gate pmost via the removal of the stored charges (electrons) from the insulated floating gate either through electrical tunneling or UV light exposure. Being both programmable and erasable makes NeoBit OTP memory testable for efficient production monitoring. Key technical features of NeoBit OTP memory include: 1) full compatibility with generic CMOS process, 2) structural simplicity, 3) low manufacturing cost, 4) testability for manufacturing monitoring, and 5) flexible and customizable design for customer products. Such features help chip designers to easily implement the Logic NVM in product designs, and to increase product competitiveness. 1.5 Where to Get the Logic NVM Platform Logic NVM represents one type of Intellectual Property (IP) design service. Logic NVM IP is provided by the NVM IP supplier, and certified by the Foundry for high quality, in order to be used by chip designers. Figure 1.3 illustrates the Logic NVM platform and the relationships between IP users (chip designers), the Foundry and the NVM IP supplier (IP Vendor). In our Logic NVM platform, to meet the demands of chip designers for diversified markets, the Foundry develops and provides process technologies, and the
10 Logic Non-Volatile Memory by Charles Ching-Hsiang Hsu, Yuan-Tai Lin, Evans Ching-Sung Yang, Rick Shih-Jye Shen NVM IP supplier (us from ememory) develops and provides a Logic NVM portfolio according to the process technologies provided by the Foundry. In Logic NVM technology development, the NVM IP supplier also provides macro design services to IP users according to their custom specifications. Customized design helps chip designers integrate product design and Logic NVM more easily, and provides greater flexibility for diversified applications. The NVM IP supplier provides IP users with a Logic NVM design database according to specifications for database merging and mask making. After wafer manufacturing, the NVM IP supplier also provides NVM technical support. IP users can get more information about Logic NVM IP readiness from the Foundry, or contact the NVM IP supplier directly for more technical information and discussion, especially for custom macro designs. Fig.1.3 Typical business model of Logic NVM.
Chapter 1 Introduction 11 Section 1.6 How to Use Logic NVM 1.6 How to Use Logic NVM As mentioned in section 1.1, Logic NVM can be further categorized into OTP, MTP, and EEPROM/flash memories according to how often data will be updated. Chip designers, for their product design, can decide a suitable type of Logic NVM depending on the required data update frequency. NeoBit OTP is a favorable choice for applications which need customized Read Only Memory (ROM) code. Designers can write data to the OTP memory, electrically, after the manufacturing process, according to customer demands. Key functions of the OTP memory for such applications are recording chip ID for IC products, encryption keys for e-payment or set-top box systems, and anti-piracy codes for digital rights management. Designers also use OTP memory to adjust data for analog design trimming, to set parameters for product functions, and to store boot code and firmware for microcontrollers. Key benefits of using OTP memory are security, field update flexibility, product customizability, and low cost. Some applications require infrequent data updates, e.g. 10 updates over the product lifetime. Function selection, parameter setting, and calibration for production verification may each require multiple-time write functionality. Designers can adopt MTP memory, or use OTP memory with multi-bank configuration, such as NeoBit pseudo MTP, for this purpose. Other applications require frequent data updates, e.g. more than 1000 updates over the product lifetime. In this case, flash memory, such as NeoFlash, or EEPROM, such as NeoEE, is the standard choice. The key difference between flash memory and
12 Logic Non-Volatile Memory by Charles Ching-Hsiang Hsu, Yuan-Tai Lin, Evans Ching-Sung Yang, Rick Shih-Jye Shen EERPOM is the memory configuration for erase operations. Flash memory adopts sector or block erasing for fast volume erasing, whereas EEPROM adopts byte erasing for byte rewriting. As mentioned in section 1.3, embedding flash memory into logic circuit fabrication processes adds extra mask layers (approximately 10 additional layers) and complicated process steps. This impacts manufacturing cost and product yield, but generally provides the advantage of small memory chip size. Therefore, embedded flash memory is mainly used in products requiring high storage density. From the standpoint of cost and foundry availability, single-poly flash memory, which is fully compatible with CMOS logic circuit fabrication processes, is recommended. NeoEE memory is fully compatible with generic CMOS processes, and designed for EEPROM configuration and operation. It is specifically developed for high-endurance. Figure 1.4 summarizes the application model of embedded NVM for different functions. For real-life applications, embedded memory may serve not just one, but a combination of several functions. For example, in MCU applications for image capture or speech recording, embedded NVM is used for setting device ID or serial number, trimming analog signals, setting user parameters, storing ROM codes or updating user data. Of the required functions, identification, trimming, and ROM code storage may only require OTP or MTP memory. However, storage of user parameter settings and data updates require flash memory capable of handling frequent updates (more than 1K or 10K rewrite cycles). To integrate all of the required functions into a single chip design, chip designers are essentially forced into using embedded flash memory technology based on the most frequent data update requirement, which increases the manufacturing cost. Costs issues and potential yield loss, due to the complicated process required to embed flash technology, significantly impact
Chapter 1 Introduction 13 Section 1.6 How to Use Logic NVM product profitability. Demand is increasing for a comprehensive, embedded NVM IP platform that provides chip designers with a total solution for embedded NVM IP. NeoBit, NeoEE and NeoFlash technologies are process compatible, and can be embedded in the same CMOS process platform to provide OTP/MTP storage, high-reliability EEPROM, and high-density storage simultaneously. Customized, multi-function IP can be designed easily by using a combination of embedded NVM technologies, while keeping product cost competitive. Fig. 1.4 Application model of embedded NVM for different functions. References [1] Chih-Tang Sah, Fundamentals of Solid-State Electronics, pp. 596-643, World Scientific Publishing Co. Pte. Ltd., 1991. [2] Chih-Tang Sah, Evolution of the MOS Transistor From Conception to VLSI, Proc. IEEE 76(10), 1280-1326, October 1988. Section IV. P and Q, on p.1301-1302. Table 3 on p.1303.