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- Herausgeber: John Wiley & Sons
- Kategorie: Fachliteratur
- Sprache: Englisch
Silicon is the material of the digital revolution, of solar energy and of digital photography, which has revolutionized both astronomy and medical imaging. It is also the material of microelectromechanical systems (MEMS), indispensable components of smart objects. The discovery of the electronic and optoelectronic properties of germanium and silicon during the Second World War, followed by the invention of the transistor, ushered in the digital age. Although the first transistors were made from germanium, silicon eventually became the preferred material for these technologies. Silicon, From Sand to Chips 1 traces the history of the discoveries, inventions and developments in basic components and chips that these two materials enabled one after the other. The book is divided into two volumes and this first volume is devoted to basic microelectronic components.
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Veröffentlichungsjahr: 2024
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Silicon, From Sand to Chips 1
Microelectronic Components
Alain Vignes
First published 2024 in Great Britain and the United States by ISTE Ltd and John Wiley & Sons, Inc.
Apart from any fair dealing for the purposes of research or private study, or criticism or review, as permitted under the Copyright, Designs and Patents Act 1988, this publication may only be reproduced, stored or transmitted, in any form or by any means, with the prior permission in writing of the publishers, or in the case of reprographic reproduction in accordance with the terms and licenses issued by the CLA. Enquiries concerning reproduction outside these terms should be sent to the publishers at the undermentioned address:
ISTE Ltd27-37 St George’s RoadLondon SW19 4EUUK
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John Wiley & Sons, Inc.111 River StreetHoboken, NJ 07030USA
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© ISTE Ltd 2024The rights of Alain Vignes to be identified as the author of this work have been asserted by him in accordance with the Copyright, Designs and Patents Act 1988.
Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s), contributor(s) or editor(s) and do not necessarily reflect the views of ISTE Group.
Library of Congress Control Number: 2023950040
British Library Cataloguing-in-Publication DataA CIP record for this book is available from the British LibraryISBN 978-1-78630-921-1
Preface
At the beginning of the 20st century, silicon “metal” was used as an alloying element for steels with electrical properties. The year 1906 saw the first application of crystalline silicon as a component of electromagnetic wave detection circuits in radio receivers, competing with galena.
Research carried out during the Second World War on silicon and germanium, the materials used in the components (point-contact diodes) of radar receiver circuits for aircraft detection and tracking, revealed that these materials are semiconductors whose basic characteristic is the control of electrical conductivity through doping. This characteristic prompted the search, after the Second World War, for solid components to replace “triodes” (vacuum tubes). This quickly led to the invention of the transistor.
The invention of the transistor is the founding act of the digital revolution (of the information society in which we live).
Germanium then silicon are the first two materials that enabled the invention of the transistor and the initial development of computers, while silicon dethroned germanium to produce the “MOSFET” (metal–oxide–semiconductor field-effect transistor), the basic component of integrated circuits: microprocessors and memories, the building blocks of computers.
But these components require materials (germanium and silicon) of extraordinary purity and perfect crystallinity. The purification of basic materials to purities of up to 11N, the production of single crystals of germanium, then silicon, the manufacture of components (based on transistors) and their miniaturization have posed problems of a complexity rarely encountered in the development of manufactured products.
These are the same properties and characteristics that have made silicon the material of choice for converting solar energy into electricity and for photographic sensors.
Silicon’s exceptional mechanical properties, combined with its electrical properties, make it the material of micro-electro-mechanical systems (MEMS), the key components of “intelligent objects”.
In 2018, there were no materials on the horizon that were likely to dethrone silicon as the material of choice for microelectronics and optoelectronics alike. According to Gérard Berry: “Silicon is not dead, far from it”.
This book is aimed at readers who want to know and understand how it was possible to go from the ENIAC computer, built during the Second World War, to calculate shell trajectories, 30 m long and 2 m high, with 17,468 triodes (vacuum tubes) and capable of executing 5,000 additions and subtractions in 1 s, to centimetric microprocessors with 20 billion transistors, processing power (number of instructions processed per second) of several gigahertz, making up the basic components of the individual computer, which is the size of a thin book.
To this end, this book, by tracing the history of discoveries, inventions, innovations and technological developments in materials, components, integrated circuits and memories, presenting the physical bases of their operation, and focusing on the materials and technologies used to make these components, attempts to answer the following questions:
What specific properties (characteristics) – electrical, physicochemical, mechanical – are behind the successive dominance of silicon, then germanium, then silicon again in the development of microelectronics, the dominance of silicon in the conversion of solar energy into electricity, the dominance of silicon as the basic material for electromechanical microsystems?
What properties (purity, crystallinity, doping) had to be imparted to the material, and how were they obtained to achieve the performance achieved by these components today?
What processes had to be developed to produce these components, and then to meet the demands of miniaturization, enabling the high-speed data processing performance we are seeing today, efficient conversion of solar energy into electricity, etc.?
Who were the architects of this epic? According to Gérard Berry
1
, “its extraordinary success (that of silicon) is clearly to the credit of semiconductor materials physicists, who made technological advances that required enormous imagination and skill to overcome all the obstacles”.
Until 1942, silicon extracted from silica (SiO2) and germanium extracted from sulfide (GeS2) were considered as metals. The semiconductors known at the time were chemical compounds: oxides (Cu2O) and sulfides (galena PbS), composed of a metal and a metalloid (oxygen or sulfur), whose basic characteristic was the increasing variation of their conductivity with temperature, whereas the conductivity of metals decreases with increasing temperature. It was not until the summer of 1942 that it was recognized that purified silicon and germanium were not metals, but semiconductors.
This book is divided into two volumes. Volume 1 is devoted to basic components (diodes and transistors).
Chapter 1 presents (1) the work that led to the extraction of silicon from silica and its purification and the discovery, extraction and purification of germanium; (2) the basic physical characteristics of semiconductors made from these two materials, knowledge of which is essential for understanding how components work.
Chapters 2–6 of Volume 1 present the basic components (diodes, transistors) in the chronological order of their discovery/invention, and the technological developments required for their realization.
Each chapter includes a presentation of the component, how it works and its basic functions, followed by the history of the research and development that led to its invention and production. The physical basis of its operation is presented in the appendicies of each chapter. The technologies used to satisfy the requirements of purity and crystalline perfection of the base material are presented chronologically, as are the technologies used to produce the components and the evolutions required by their miniaturization. The industrial development of the first components is presented according to their importance for subsequent developments.
Volume 2 is devoted to “chips, optoelectronic components and MEMS”.
Chapters 1 and 2 present microcomputer integrated circuits and memories.
Chapter 3 presents the silicon thin film transistor TFT, which led to the development of flat-panel liquid crystal displays.
Chapters 4 and 5 present silicon optoelectronic components. These include solar cells for converting solar energy into electricity and photoelectric image sensors for digital cameras, which have revolutionized astronomy and medical imaging.
Chapter 6 presents microelectromechanical systems (MEMS), the exceptional mechanical properties of silicon that have enabled their development, and the specific technologies developed for building structures with moving parts.
Many English and American books present the “history of semiconductors”. Compared with the reference works cited in the reference lists, this book presents not only the historical aspects, but also the recent technological developments that have enabled the current performance of microprocessors, memories, solar cells and electromechanical microsystems. The book is based on numerous works by historians and original publications.
The author would particularly like to thank Professors Jean Philibert and André Pineau.
December 2023
References
Burgess, P.D. (n.d.). Transistor history [Online]. Available at:
https://sites.google.com/site/transistorhistory
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Computer History Museum (n.d.). The silicon engine timeline [Online]. Available at:
www.computerhistory.org
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Hu, C. (2009).
Modern Semiconductor Devices for Integrated Circuits.
Pearson, London.
Krakowiak, S. (2017). Éléments d’histoire de l’informatique. Working document, Université Grenoble Alpes & Aconit, CC-BY-NC-SA 3.0 FR.
Lazard, E. and Mounier-Kuhn, P. (2022).
Histoire illustrée de l’informatique
. EDP Sciences, Les Ulis.
Lilen, H. (2019).
La belle histoire des révolutions numériques.
De Boeck Supérieur, Louvain-la-Neuve.
Lojek, B. (2007).
History of Semiconductor Engineering
. Springer, New York.
Mathieu. H. (2009).
Physique des semi-conducteurs et des composants électroniques
, 6th edition. Dunod, Paris.
Nouet, P. (2015). Introduction to microelectronics technology. Working document, Polytech Montpellier, ERII4 M2 EEA Systèmes Microelectronics.
Orton, J.W. (2004).
The Story of Semiconductors
. Oxford University Press, Oxford.
Orton, J.W. (2009).
Semiconductors and the Information Revolution: Magic Crystals that made IT Happen
. Elsevier, Amsterdam.
Riordan, M. and Hoddeson, L. (1997).
Crystal Fire: The Invention of the Transistor and the Birth of the Information Age
. W.W. Norton & Company, New York.
Seitz, F. and Einspruch, N.G. (1998).
Electronic Genie: The Tangled History of Silicon
. University of Illinois Press, Illinois.
Sze, S.M. (2002).
Semiconductor Devices: Physics and Technology
. Wiley, New York.
Verroust, G. (1997). Histoire, épistémologie de l’informatique et révolution technologiques. Course summary, Université Paris VIII, Paris.
Ward, J. (n.d.). Transistor museum [Online]. Available at: transistormuseum.com.
Note
1
Berry, G. (2017).
L’Hyperpuissance de l’informatique
. Odile Jacob, Paris, p. 88 and 401.
IntroductionThe Digital Revolution
The “digital revolution” is also known as the “computing or IT revolution”. These expressions reflect “a radical transformation of the world we are witnessing today”.
The first term refers to the binary digitization of texts and numbers, as well as images, sounds and videos, using sequences of symbols. This makes it possible to store images, sounds, etc., and transmit them, replicate them, analyze them and transform them using digital computers (Abiteboul and Dowek 2017, p. 29).
The second expression, “the computing revolution”, refers to the science and technique of processing digitized information using algorithms. According to Berry (2017, p. 25), “Computing is the conceptual and technical engine of the digital world. The computer is the physical engine”.
The “birth certificate of the digital revolution” is Claude Shannon’s 1937 master’s thesis, A symbolic analysis of relay and switching circuits (1938). This thesis relied on the theory of the Englishman George Boole (An Investigation of the Laws of Thought, 1847), which established the link between calculus and logic and where the basic logical functions “AND”, “OR” and “NOT” were treated as arithmetic operations, taking the value 0 or 1, depending on whether the proposition was true or false.
The master’s thesis of Claude Shannon1 was the result of an internship at Bell Labs2, where he observed the power of telephone exchange circuits that used electromechanical relays (switches)3 to route calls and imagined that electrical circuits could perform these logical operations using an on-off switch configuration.
The first demonstration of the feasibility of executing logic functions using a device made up of two electromechanical relays was carried out in 1937 by George Stibitz of Bell Labs; this led to the construction in 1939 of the first CNC (complex number calculator) (400 electromechanical relays), capable of opening and closing 20 times a second, executing complex number multiplication and division operations. This was followed by five other models. “Stibitz’s calculator demonstrated the potential of a relay circuit to do mathematics in binary, process information, and manipulate logical procedures” (Isaacson 2015, p. 93).
The “digital” revolution is the third major revolution in human history. The first was the agricultural revolution 8,000 years ago. The second was the “industrial revolution” of the 19th century.
The technology at the heart of this third revolution, also known as the “second industrial revolution”, is microelectronics4. In 1979, the US National Academy of Sciences published a report5 entitled “Microstructure, Science, Engineering and Technology”, which stated: “The modern era of electronics has ushered in a ‘second’ industrial revolution, the consequences of which may be even more profound than those of the first”. According to Ian Ross, President of Bell Labs from 1979 to 1991: “The semiconductor odyssey produced a revolution in our society at least as profound as the total industrial revolution. Today electronics pervades our lives and affects everything” (Ross 1997).
I.1. Microelectronics components
In 1903, Arthur Fleming invented the diode (vacuum tube), a current rectifier, and in 1906, Lee de Forest invented the triode (vacuum tube) by adding a grid between the diode’s cathode and anode. As well as rectifying the current, this allowed weak currents induced by electromagnetic waves to be amplified, hence the development of radio receivers: a small variation in the signal on the grid resulted in an amplification of the cathode-anode current. In addition, a sudden variation in the signal applied to the grid switched the triode on or off, enabling it to function as a switch. The triode is also capable of self-oscillation, hence its use in radio transmitters.
The invention of the bipolar transistor in 1948 by William Shockley (Nobel Prize winner), a solid-state device capable of performing the same functions (amplification of weak currents and switching), but much faster, ushered in the era of the digital revolution.
Like transistors, triodes work by controlling a current of electrons, which can either be amplified or interrupted and reignited. These components function like a switch that can be set to 0 or 1 on command, thus performing logic functions. But with triodes, switching times are much longer and the permissible frequencies much lower than in solid-state components, because these variables are linked to the time taken for the electrons to cross the distance between the cathode and the anode (around 1 mm); whereas, in a transistor, the distance traveled by the electrons between the emitter and the collector is less than 1 μm, down to around 20 nm.
Before the invention of the transistor, prototype “computers” had been built with triodes, the ENIAC during the Second World War, then with solid diodes (made of germanium) combined with triodes. Diodes can only be used to create logic circuits (OR and AND gates). They cannot restore the signal at the output of a gate, hence the presence of triodes to restore the signal, enabling cascades of gates to be created, and hence logic circuits.
The discovery of silicon N and silicon P6, at the beginning of the Second World War, in other words of the effect of doping on the conductivity of silicon and therefore its control, and the discoveries of the rectifier effect and the photoelectric effect presented by the solid-state PN diode7 by Rüssel Ohl, led to the invention of the bipolar transistor (with PN junctions) in 1949 by William Shockley. The development of circuits made up of solid-state diodes and transistors producing NAND and NOR logic gates and all the universal logic functions by combining one or the other, with the added feature of restoring the signal at the output of each gate, thus enabling cascades of logic gates, led to the development of integrated circuits, invented in 1958–1959 by Jack Kilby (Nobel Prize winner) and Robert Noyce.
The development of the silicon-based field-effect MOSFET transistor, designed by William Shockley in 1945 and by Dawon Kahng and Martin Atalla in 1960, because of its miniaturization capacity, enabled the development of integrated circuits: memories and microprocessors. Microprocessors were the ultimate innovation in the digital revolution, enabling the development of the personal computer. According to Reid (1984), “A new era in electronics had begun”.
The miniaturization of components down to the nanometer scale is delivering high performance in terms of information processing speed and substantial savings in power consumption. The number of transistors has risen from 2,400 for the Intel 4004, the first integrated microprocessor, to around 20 billion for today’s largest graphics processors (2017). The processing power of a microprocessor (the number of instructions a microprocessor is capable of processing per second) rose from a few megahertz in the early 1980s to several gigahertz in the early 2000s. This clock frequency (as it is known) is directly linked to the switching speed of the microprocessor transistors. We can only marvel that an astronomical set of phenomenally fast electronic components as simple as switches could be the basis of humanity’s third revolution.
I.2. Microelectronics materials
These “components” require materials of extraordinary purity and perfect crystallinity to obtain very specific electronic characteristics, as well as completely new technologies for manufacturing transistors and integrated circuits (a list of which is given in the Appendix).
It was the availability and technological mastery of two materials, germanium and silicon, which were virtually unknown at the beginning of the 20th century, with the appropriate electronic characteristics, that enabled the invention of the transistor and conversion of solar energy into electricity.
The purification of base materials to purities of up to 11N (99.999999999), the production of perfectly crystalline single crystals of germanium and then silicon, enabling the conductivity of these materials to be controlled by doping, and the manufacture of components and their miniaturization have posed problems of a complexity rarely encountered in the development of manufactured products (Queisser 1998).
It was with germanium (on purified, coarse-grained (quasi monocrystalline) wafers that were available) that power amplification was first observed in December 1947 on a device made by John Bardeen and Walter Brattain (Nobel Prize winners), which was named the “point contact transistor” (Bardeen and Brattain 1948). This invention led to the development of a process for obtaining single crystals of germanium by Teal (1976). The successful purification and manufacture of germanium single crystals and the development of the bipolar transistor established germanium as the basic material for transistors. In 1952, Ralph Hunter, in a speech as President of the Electrochemical Society of the United States, predicted: “A revolution in the electronics industry as a result of the development of germanium”. Germanium transistors were manufactured until 1961. The CDC 1604 and IBM 1401 computers marketed in 1960 were made using germanium transistors.
In 1952, following the successful manufacture of silicon single crystals and of a PN junction in a single crystal, again by Gordon Teal, whose properties were superior to those of the germanium PN junction, “silicon immediately became a rival to germanium” (Leamy and Wernick 1997). Given the difficulties in obtaining “electronic” silicon, silicon very gradually became the preeminent material for transistors, under pressure from the military, who were virtually the only customers at the time – particularly for the temperature resistance of silicon diodes and transistors up to around 150°C.
When the first silicon MOSFET transistor was produced in 1959, silicon’s supremacy became total, thanks to the qualities of its oxide and its high thermal dissipation. Since the 1970s, silicon MOSFETs have been the basic components of integrated circuits and computer memories.
In 1951, Heinrich Welker (Nobel Prize) began studies on compounds with the same structure as silicon and germanium, such as gallium arsenide GaAs, revealing their semiconductor characteristics. It was not until 1978 that it was shown that a gallium arsenide component was twice as fast as the same silicon component under the same conditions (Welker 1976). Nevertheless, this factor of 2 did not convince manufacturers to abandon silicon, thanks to its two advantages: its high heat dissipation and its mastered technology (Bols and Rosencher 1988).
I.3. The driving forces behind the development of microelectronics and computer components
Research studies carried out in England and the United States from the start of the Second World War on the reception of radar electromagnetic waves by the “point contact diode” were the first driving force behind the development of silicon and germanium, and marked the first victory of this solid component over vacuum tubes.
The second driving force behind the development of microelectronics was the desire of Bell Labs8, from the end of the war, to find a solid substitute for the triode lamps used as amplifiers along telephone transmission lines and for the electromechanical relays in their ATT telephone exchanges9.
It was the discoveries of silicon N and silicon P, of the property of rectifying an electric current through a unidirectionally solidified silicon ingot, constituting a PN diode, and of the photovoltaic effect presented by this ingot in 1940, that were at the origin of Bell Labs’ adventure in microelectronics. When these remarkable properties of a silicon ingot were brought to the attention of the Bell Labs director, Mervin Kelly considered this discovery of great value to the electronics industry, and decided that absolute secrecy should be preserved until in-depth studies revealed its full power: “It was too important a breakthrough to bruit about”.10 The studies were resumed in 1945.
In the summer of 1945, as reported by Ian Ross, Kelly set up a research group with the following objectives: the fundamental study of semiconductors, concentrating on germanium and silicon, materials which were beginning to be well known, and, in the long term, the creation of a solid-state component constituting an amplifier “to replace triodes (vacuum tubes) and constituting a switch to replace the electromechanical relays of telephone exchanges”.
This research, in 1947 and 1949, led to the invention of the point contact transistor and the bipolar transistor with PN junctions.
In 1950, according to Ian Ross, Bell Labs researchers realized that, given the characteristics of transistors, their size and low energy consumption, it was not the replacement of vacuum tubes that should be sought, but their use as components of logic circuits11.
As soon as the reproducible manufacture of transistors became possible, in the mid-1950s, “replacing vacuum tubes in as many applications as possible became the objective”.
Transistor specimens were entrusted to various Bell Labs engineers with the task of developing applications. John H. Felker was one of them. Felker (1951) showed that the transistor could be used as a component of logic circuits. This potential use of the transistor was presented by Felker to the companies that had acquired the “Western Electric” license. According to McMahon12, “none of us imagined the revolution that would take place over the next forty years”, “even at IBM” according to Rick Dill13.
Following this presentation, in 1951, the Air Force asked Bell Labs to develop a computer, the TRADIC (transistorized airborne digital computer), which was entrusted to Felker. This resulted in the successive production of four TRADIC computers, of which the Leprechaum version, operational in 1956, was the first fully transistorized computer based on logic circuits made up of bipolar germanium transistors (Irvine 2001).
Most of the discoveries, inventions and technological developments relating to transistors, solar cells and digital photography between 1947 and 1970 were made by Bell Labs researchers (see Table I.1 in the Appendix in this chapter).
Nevertheless, as we shall see, the inventions and technological developments of Bell Labs were not always followed by industrial development and production by Western Electric. There was a good reason for this: ATT, which had a monopoly over telephone and telegraph transmissions in the United States by court order under the anti-trust laws, was only authorized to produce electronic components for its own needs and had to inform the entire electronics industry of any discoveries that might be of interest to it. Therefore, after the first bipolar transistor was produced in 1950, Western Electric began to grant manufacturing licenses to companies producing diodes, triodes (vacuum tubes), etc., “licensing the rights to manufacture transistors for a $25,000 fee”, and for these licensees, Bell Labs organized a Transistor Technology Symposium in April 1951.
The third driving force was the interest shown by a number of industrial companies who foresaw the importance of these inventions. This was as early as 1948, with the publication of the discovery of the point contact transistor, companies that had been heavily involved in the development and production of germanium diodes during the Second World War: General Electric, Sylvania, RCA, CBS and IBM. Subsequently, other vacuum tube manufacturers who had acquired the Bell license, such as Raytheon, Philco, Telefunken and Siemens, and companies set up by researchers or engineers, who moved from one company to another, produced components and then integrated circuits. In 1958, there were 70 diode and transistor manufacturers in the world, the vast majority of these in the United States (Morton and Pietenpol 1958).
The first commercial computers to use bipolar transistors as logic circuit components appeared in 1956 with the Philco S-2000 and 2600 computers.
Three companies – Texas Instruments (TI), founded in 1952, Fairchild Semiconductor, founded in 1957, and Intel, founded in 1968 – took over from Bell Labs, both in the development and industrial production of the ultimate components.
The inventions and achievements of the integrated circuit in 1958–1959 were due to Jack Kilby of Texas Instruments and Robert Noyce of Fairchild Semiconductor and their collaborators. This invention paved the way for the creation of the “microprocessor” by Intel, a company founded by Fairchild Semiconductor defectors Noyce, Grove and Moore (author of “Moore’s Law” in 1969). This was a universal integrated circuit that integrated all the functions of a computer’s central processing unit, capable of following programming instructions. In November 1971, Intel presented the Intel 4004 microprocessor.
At the same time, another major driving force behind the development of microelectronics, as with many other major innovations, was the needs of the military or prestige of the state.
The civil space program and the military program to build balistic missiles boosted demand for transistors. The state organizations responsible for these programs financed the companies mentioned above.
The Polaris sea-to-ground ballistic missile program in 1956, then the Minuteman ground-to-ground missile program at the end of the 1950s, for their on-board guidance system, the Vanguard and Explorer earth satellites, launched in 1958. For their transmissions, the Apollo program, at the beginning of 1960, endowed with 25 billion dollars, gave a real boost to research into integrated circuits and computers14.
I.4. The material of solar energy
Diodes made of silicon, germanium and other semiconductors convert photons into electrons.
The photovoltaic effect, presented by a silicon ingot forming a PN diode, was discovered by Russell Ohl of Bell Labs in 1940 (Ohl 1946).
The solar cell development program began in 1952. On April 26, 1954, Bell Labs announced the manufacture of silicon solar cells using the diffusion doping process. It was the development of this doping process for solar cells that ensured the development of transistors (Chapin et al. 1954).
The space program was the driving force behind the development of solar cells; the first use of solar cells was on the Vanguard 1 satellite, launched on March 17, 1958, to power a radio transmitter. The system operated for 8 years. The space program stimulated (financed) a great deal of research and a veritable cell production industry.
The energy crisis of 1974–1975 sparked renewed interest in solar cells. Silicon is the material of a major energy source (solar energy): 99.4% of solar panels are based on silicon, and 0.4% on CdTe and GaAs.
Solar Impulse 2, the fragile aircraft with its huge wings covered with solar panels (11,628 ultra-fine monocrystalline silicon photovoltaic cells (each 135 μm thick)), is the symbol of the progress made in just a few years in the field of materials and renewable energies.
I.5. The material of digital image sensors
Silicon photoelectric image sensors, whose invention by W.S. Boyle and G.E. Smith, also of Bell Labs, in 1970 won them a Nobel Prize, have made digital photography possible and revolutionized astronomy. They are crucial components of fax machines, cameras, scanners and medical imaging (Boyle and Smith 1970).
I.6. The material of micro-electro-mechanical components (MEMS)
A MEMS “sensor” or “actuator” is an essential part of what we call intelligent objects, since it is thanks to them that we can obtain information linked to our environment and vice versa. A series of technological breakthroughs and industrial bets have helped to explode a market that continues to evolve (Vigna 2013).
The development of these microsystems has been made possible by the availability of a material, silicon, with its exceptional electrical and mechanical properties, and by the development of specific miniaturization technologies for this material.
