Abstracts
The application space for severely scaled microsystems is rapidly expanding; especially for the biological and medical sciences. The focus of the text is on the feasibility of realizing micron-scale, autonomous systems using the best projected technologies for electronic technologies. A “nanomorphic cell” whose dimensions are in the micron range is introduced in Chapter 1 and the various subsystems that comprise the cell are described, for example, sensors and actuators, communications systems, memory and logic units, and energy sources. The physical scaling of each of these subsystems is addressed in the ensuing chapters and their integration is considered in the final chapter. The goal throughout the text is to use fundamental scientific principles to elucidate the effects of scaling on the subsystems in such a way as to bring these topics within the grasp of the nonspecialist. Throughout the book, the theoretical limits that are obtained are compared with data obtained from existing applications.
The operational properties and parameters of living cells are also outlined in Chapter 1, and typical properties such as their mass, volume, size, and power consumption are given. It is remarkable that so much functionality is achieved by the living cells whose dimensions are typically a few microns. Also included in this chapter is a brief survey of the size of various current medical devices, including pacemakers, endoscopic systems, and intraocular pressure monitors. These systems are typically a few centimeters in size and therein arises the challenge of designing micron-scale systems that can perform their biological/medical functions as efficiently as the living cell.
1.1. Introduction
Nanoelectronics is now a reality since the critical feature sizes of semiconductor components, both logic transistors and memory, are below 22 nm. This march to the far-sub-nanometer regime has enabled an array of new applications for information and communication technologies (ICT). However, the overall sizes of practical electronic ICT systems that utilize these nanoscale components remain relatively large, typically on the order of centimeters or larger. Back in 1959, Richard Feynman
[1] gave a visionary presentation in which he suggested the possibility of building computers whose dimensions were “submicroscopic.” Although the progress of semiconductor technology has been extraordinary, submicroscopic and even microscopic computers remain outside of our grasp. Moreover, it is not known what minimum system size could be achieved with existing and/or projected semiconductor technologies. This book seeks to address this question by offering a physics-based analysis of the limits of physical scaling for computers and other functional ICT systems.
In order to comprehend scaling limits for systems, scaling and energy limits for many electronic components are needed, including logic and memory devices, input/output (I/O) components, communication subsystems, sensors, etc. For a system-level analysis of extremely scaled ICT, several hypothetical applications will be considered. A silicon computer whose size is on the order of a cube 1–100 µm must contain logic circuitry and nonvolatile memory for program and data storage and it must be able to process the data. It also needs I/O components, an energy source, and perhaps, sensors.
An area experiencing substantial growth is that of utilizing integrated intelligent sensor systems for the ubiquitous collection of data. Applications for future integrated sensor systems include environmental monitoring, energy management, well-being, security, and safety, integrated into a broader smart city concept. Currently, sensor technologies are experiencing exponential growth, and a wide range of promising applications for electronic sensing have emerged, for example, chemical hazard detection, food storage/processing control and safety, seismic geo-imaging, agriculture, defense and security, etc. The technological challenges that must be addressed to develop new generations of integrated sensor systems are daunting and encompass almost every facet of integrated system technology, including information processing, energetics, communication, packaging, etc. New materials and disruptive architectures, heterogeneous three-dimensional (3D) integration, and other technologies will need to be introduced to make intelligent integrated sensor systems possible. Once again, an understanding of scaling-performance projections and tradeoffs to achieve maximum performance at minimum energy and limited size is needed.
As another example where extreme system scaling is important in an autonomous ICT system embedded in the human body whose mission is to analyze the health of cells that it encounters and to report its findings to an external agent. The living cell, which is an organic autonomous system, provides an existence proof that functional and autonomous systems are possible at the scale of a few microns. This text investigates the feasibility of the design of a functional inorganic system on the same physical scale as the living cell, that is, with overall dimensions of several microns. One reason to believe that such a design might be possible is the remarkable progress that has been made in technologies for semiconductor chips, where some of the devices on the chip already have dimensions on the order of a few nanometers, and dimensional scaling is anticipated to continue for a few more generations. In addition, there is a trend to incorporate more functionality onto a single chip by including devices whose domains of operation are not only electrical but also mechanical, thermal, chemical, etc. These “System-on-a-Chip” designs may point the way to integrated chips with increasing degrees of functionality. The term “nanomorphic cell” is used herein to reflect the fact that emphasis is on inorganic integrated systems whose inspiration is derived from their biological counterparts. (The term “morphic” literally means “in the shape of.”)
To help fix ideas, imagine that the nanomorphic cell is to be injected into the body to interact with the living cells and to support certain diagnostic and/or therapeutic actions. In order to do this, it is stipulated that the nanomorphic cell must acquire data indicative of the health of the living cells that it contacts, analyze the sensed data, and communicate its findings to an external agent. Since the nanomorphic cell is untethered, it must either harvest energy from its surroundings or carry an embedded energy source.
Subjectively, it seems reasonable to postulate that a micron-sized embedded system would contain only minute and harmless amounts of materials that in larger quantities might be harmful to the body and, furthermore, that the normal body waste disposal processes might manage the removal of nanomorphic cells when they have reached the end of their useful lives. The nanomorphic cell would need to employ some sort of triggering mechanism to signal its elimination from the body. Of course, this is all hypothetical and would need to be verified, for example, by careful toxicology studies. The in vivo functional nanomorphic cell is used as an example throughout the text as a vehicle to motivate the study of the impact of extreme scaling on system component performance limits.
1.2. Electronic Scaling
Electronic circuits and systems are constructed from a number of components, the most basic of which is the semiconductor transistor (see
Chapter 4) that is used in digital applications as a binary switch. Tremendous progress has been achieved in reducing the physical size of semiconductor transistors—within the last 40 years, the number of transistors in a ~1 cm2 integrated circuit (IC) chip increased from several thousand in the 1970s to several billion in 2014. The long-term trend of transistor scaling is known as Moore’s law: The number of transistors in an IC chip approximately doubles every 2 years (see...
