Silicon Non-Volatile Memories

Paths of Innovation

Barbara de Salvo, Barbara De Salvo

• Publisher: Wiley-ISTE
• Year: 2013
• Print ISBN:
  9781848211056
• eBook ISBN:
  9781118617809
• Edition: 1
• Topic: Tecnologia e ingegneria
• Subtopic: Ingegneria elettronica e telecomunicazioni

Semiconductor flash memory is an indispensable component of modern electronic systems which has gained a strategic position in recent decades due to the progressive shift from computing to consumer (and particularly mobile) products as revenue drivers for Integrated Circuits (IC) companies. This book provides a comprehensive overview of the different technological approaches currently being studied to fulfill future memory requirements. Two main research paths are identified and discussed. Different "evolutionary paths" based on the use of new materials (such as silicon nanocrystals for storage nodes and high-k insulators for active dielectrics) and of new transistor structures (such as multi-gate devices) are investigated in order to extend classical floating gate technology to the 32 nm node. "Disruptive paths" based on new storage mechanisms or new technologies (such as phase-change devices, polymer or molecular cross-bar memories) are also covered in order to address 22 nm and smaller IC generations. Finally, the main factors at the origin of these phenomena are identified and analyzed, providing pointers on future research activities and developments in this area.

Chapter 1

Introduction

Semiconductor Flash memory is an indispensable component of modern electronic systems which has gained a strategic position in the last decades due to the progressive shift from computing to consumer (and particularly mobile) products, as revenue drivers for IC (integrated circuits) companies. Today, memories are used in personal computers (PCs), cellular phones, MP3 players, digital cameras, televisions, wireless handsets, smart-media, networks, automotive systems and global positioning systems. At the core of the successful development of memories has been the steady increase of capacity, strictly linked to the shrink ability of the IC technologies.

For decades now, semiconductor scientists and engineers have been looking for a universal memory, combining low-cost, speed and capacity, non-volatility, compatibility with current complementary-metal-oxide-semiconductor (CMOS) manufacturing techniques, and scalability. Today, it has been finally recognized that different applications have different requirements, and no one solution could be optimal for a wide range of conflicting demands. Nevertheless, in the past few years, the research and development into new types of memory have dramatically accelerated. There are several reasons for this: first, the “end” of Moore's Law. Traditional semiconductor technology will not easily scale much further. How far it can be pushed is a matter of conjecture (32 nm node or some smaller nodes), but it is clear that we are close to the end of CMOS scaling. With 45 nm-32 nm semiconductor devices in volume production and 22 nm devices in the design stage, alternative technologies are the subject of a great deal of attention worldwide. This consideration obviously also applies to microprocessors and logic. Second, there is the need for better memories. Conventional memory technology has inherent limitations. DRAM (Dynamic RAM) is cheap but volatile and requires continual refresh. SRAM (Static RAM) is faster but expensive and prone to soft errors – and it is still volatile. Flash is non-volatile but slow. Both magnetic and optical storage are high capacity, but are running up against the limits of current technology. The growing role of mobile computing and communications has led both semiconductor firms, such as Intel, and OEMs (Original Equipment Manufacturers), such as Motorola, to speculate about a future ubiquitous computing paradigm in which most of the customer equipment is untethered. There is a burgeoning demand for low cost, low power consuming nonvolatile memory technologies, with a considerable improvement on today's Flash in terms of capacity and speed, since the new mobile paradigm for computing and communications will require that these new memories will need to provide rapid write and read capabilities for very large amounts of data (including video data). The third element will be memory for “disposable” electronics products, such as smart cards, RFIDs (radio-frequency identification), electronic tickets, toys, electronic greeting cards, etc., which are beginning to proliferate, as a result of the commercialization of printable and organic electronics. While capacity, speed and most of the other requirements for memory in such applications are not essential requirements, a significant cost reduction would be welcome in this market.

In this book, based on the knowledge that we have acquired over several years of experimental and theoretical research in the field of non-volatile memories, we will try to give a general overview of the different technological approaches currently studied worldwide to fulfill the requirements of future memory applications.

Although a critical economic analysis of the semiconductor industry is beyond the scope of this dissertation, as scientists working in this field, we cannot ignore the fact that we are currently living in a period of strong changes. All people working today in microelectronics (in companies, research laboratories or universities) live in a climate of challenging contradictions and constant urgency. In Chapter 2, based on historical data, we will show that semiconductor manufacturing is a highly competitive industry, where high technology, device cost and speed are the major economic driving forces. Rapid obsolescence of products (becoming obsolete in less than a year) and dramatic price decline are the main characteristics. We will talk about the volatility of the semiconductor markets, the constant price wars among IC companies and the impact of accelerated device scaling on technology costs. Particular attention will be given to the evolution of CMOS memory technologies over the last few decades. Using this analysis, we will try to understand the main economic factors at the basis of the current consolidation of the IC industry. In fact, the semiconductor industry is currently continuing its transformation from a relatively young, high-growth industry to a more mature one, facing many of the same issues that other established industries have already faced, as their markets became more saturated and geographical patterns of supply and demand shifted. Chip-makers are redefining business strategies to compensate for a variety of difficult economic pressures, including the increasing costs of developing new technology and adding new high yield manufacturing capacities. In addition to these fundamental changes, there is an unmistakable shift of capital and research and development (R&D) investments away from Europe (and the USA) to regions which are increasingly important as an end-market for semiconductors, i.e.: South Korea, Taiwan, China, Singapore and Malaysia. In the longer term, the migration of manufacturing and R&D could mean that regions which initiated the industry in terms of innovation and capital investment risk losing their status as centers of the semiconductor sector, along with the current economic leverage that status commands. This is the reality of the new global economy facing high technology-based companies, as well as applied research laboratories, in Europe today. We are crossing a threshold where the pace of disruptive shifts is no longer inter-generational with a meaningful impact over the span of careers and future of our students. In this context, in Chapter 2, we will try to identify and understand the main factors at the origin of these phenomena, looking at the semiconductor industry and market trends from an economic perspective.

In Chapter 3, we will shortly introduce the main features and scaling limits of current Flash memory technologies. Then, the main strategy of the innovative research in this field will be presented. Today, two main research paths can be identified. To extend the classical floating gate technology to the 22 nm and possibly lower nodes, different “evolutionary paths”, essentially based on the use of new materials and of new transistor structures, can be investigated. On the other hand, to address smaller IC generations, “disruptive paths”, based on new storage mechanisms and new technologies, are envisaged. In this chapter, we will focus on the “evolutionary approaches”, paying particular attention to the results obtained in our laboratory (LETI, CEA-Grenoble/France) in recent years, in the framework of research funded by internal projects, the French government, European institutions and industrial partnerships. Note that a crucial point in the definition of the research plans has always been maintaining a good equilibrium between short-term (made in collaboration with IC companies) and long-term (developed in collaboration with fundamental research laboratories, universities) solutions. Given the large variety of technologies currently invoked as potential replacements for conventional Flash, one of the hardest tasks for a scientist working in this field, at least concerning near-middle term research (i.e. time to be in production < 10 years), is to identify the right framework of study (for example, embedded or stand-alone environment) for the different technologies, in order to be able to assess the main advantages and disadvantages, and thus to prospect future applications. The “evolutionary approaches” include new modules (i.e. discrete trap memories, and more specifically silicon nanocrystal memories), new materials (high-k materials for the interpoly layer of Flash) and innovative architectures (as FinFlash memories). Moreover, obviously targeting a longer term application, hybrid approaches, which make use of organic molecules – grafted on silicon substrates – as storage sites, have been developed. Finally, the main theoretical limits of charge storage memories (i.e. reliability issues linked to certain few electron phenomena) have been identified, opening up the path to the introduction of disruptive memory technologies based on new storage mechanisms.

In Chapter 4, we will try to define the perspective of the accomplished and main paths of research for the next few years. As discussed previously, in order to reduce bit cost and increase bit density, the shrinkage of Flash memories has been aggressively driven by reducing the cell size and introducing multi level technologies. Nevertheless, the linear scaling down of MLC NAND Flash memories is approaching its critical physical, electrical and reliability limits. Conventional cost-reduction approaches, notably smaller design rules, are having less effect, and new approaches should be considered. According to the main IC companies, the short-term way to circumvent these barriers is stacking memory cells on a single Si wafer. In this context, innovative integration paths, suitable for three-dimensional integration of IC memory circuits, and new design/system solutions are introduced. On the other hand, “new breakthrough memory technologies”, such as phase-change memories, resistive RAMs, insulator and organic polymer cross-bar memories, are also considered as possible candidates for future memory applications. In the last part of this chapter, we will give a general overview of their main advantages and disadvantages in view of future memory applications.

In the conclusion, we will summarize the main findings of the previous chapters and make some general considerations on innovation paths in the field of memories and on the future of microelectronics technologies as a whole.

Chapter 2

Semiconductor Industry Overview

In this chapter, we will give a general overview of the semiconductor industry from an economic perspective. Based on historical data, we will show that the integrated circuits (IC) industry is a highly competitive sector, where high technology, device cost and speed are the major economic driving forces. Rapid obsolescence of products (becoming obsolete in less than a year) and dramatic fall in price are the main characteristics. We will talk about the volatility of the semiconductor market, the constant price wars among IC companies, the impact of accelerated device scaling on technology costs. Particular attention will be given to the evolution of semiconductor memory technologies over the last few decades. Using this analysis, we will try to understand the main economic factors at the root of the current transformation and consolidation of the IC industry, from a relatively young, high-growth industry to a more mature and established industry, with an ever smaller number of competitors.

2.1. The cyclical semiconductor market

Semiconductor microelectronics is the enabling technology at the heart of all information society technologies (IST) applications, such as personal computers (PCs), workstations, servers and other computer equipment, communication and Internet infrastructures (networking electronic systems and services), and consumer electronics (Internet appliances, mobiles).

Figure 2.1. A semiconductor-centric view of the value chain (2007 values)

Other applications of semiconductor integrated circuits (ICs) can be found in the automotive, energy, transportation, medical and pharmaceutical sectors. The semiconductor industry is part of a value chain with mature links (see Figure 2.1).

This industry has played a particularly significant role in world economic and productivity growth over the last few decades. According to the Semiconductor Industry Association, in 2007, more than 60 years after the invention of the first transistor at Bell Labs (USA) in 1947, the worldwide IC market revenue totaled nearly $256 billion (~0.5% of world GDP).

The semiconductor industry is highly cyclical. IC companies face constant booms and busts in demand for products. In fact, IC demand is an indirect demand, being driven by end-products created with ICs, such as PCs, cell phones and other electronic equipment. Generally rising demand drives up utilization in existing facilities and therefore profits, construction of a large number of new facilities begins, the new facilities come on-line and prices drop rapidly as the market moves into over-supply and companies try to keep their new facilities full.

This cycle is driven by the long time required to bring up a new facility and by the high fixed costs of facilities. Changes in the worldwide economy and over-inventory can also trigger downturns. In fact, during times of strong market demand, it is not unusual for companies to double order in trying to get parts, artificially overstating demand. Figure 2.2 shows the global industry sales trend with possible new waves of applications relaunching the market every 5/7 years (medical and robotics applications are foreseen as future market drivers).

Figure 2.2. Semiconductor market applications waves (source: Semico Research Corporation) and the IC industry cycle [ICI 07]

Figure 2.3. Growth of GDP, the electronics industry and sales of semiconductors [ICK 06]

As shown in Figure 2.3, IC industry revenues have grown at an average annual rate of 17% between 1970 and 2002. However, net growth over the last ten years was much less (i.e. 10% to 6%) [ICK 06, UCLA 03, SIA 05]. 1999 and 2000 were two strong revenue growth years, when revenue grew 22% and 34%, respectively. 2001 represented the most severe crisis in the semiconductor industry history (sales declined 32.5% from the previous year, dropping from $204 billion in 2000 to $139 billion in 2001). ICs placed in PCs and other computers accounted for 47% of IC revenues in 2001, down from 55% in 1998. The most significant factor in the slower growth in IC demand in 2001 was found in the maturing of the PC business or saturation of the market. The PC boom in previous years had been powered by the IC. Revenues growth in PCs and servers during 1997-2002 had been essentially zero, down from 16% per annum during the previous five years. Then, some applications, like desktop publishing or the Internet, generally created the need for more complex software and hardware, spurring new purchases in the PC industry. In telecommunication equipment, the 1999-2000 wave of expansion had fallen sharply in 2001-2002. A recovery in this sector happened in 2003.

Concerning 2008, total IC sales were $248.6 billion compared to $255.6 billion in 2007, a decrease of 2.8% [SIA 09]. According to several analysts [JEB 09], the global economic recession severely dampened semiconductor sales in the fourth quarter of 2008 (semiconductor sales in December declined by 16.6% from November 2008), historically a strong quarter for the industry. The worldwide financial turmoil is expected to continue to have an impact on demand for semiconductors in 2009. Never before has the semiconductor industry experienced revenue declines in back-to-back years, but it is forecast that this will occur in 2009, as worldwide semiconductor revenue is predicted to total $219.2 billion, a 16% decline from 2008 revenue. Nevertheless, several analysts suggest that this downturn is different in many ways from the 2001 case, being due to the combination of an overdue industry down-cycle with a global economic crisis of unprecedented magnitude. The 2008 downturn is broad-based, not only limited to technology, has a much different growth profile before the downturn and has far less inventory buildup. The financial meltdown, which began in the summer of 2007, has finally pushed the USA and the whole world into a recession. Consequently, it has spread fear an...

Table of contents

• Cover
• Title Page
• Copyright
• Preface
• Dedication
• Chapter 1: Introduction
• Chapter 2: Semiconductor Industry Overview
• Chapter 3: Research on Advanced Charge Storage Memories
• Chapter 4: Future Paths of Innovation
• Chapter 5: Conclusions
• Index