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The future of computing? Pencils
Bye-bye silicon It has become synonymous with microelectronics, but now any number of pretenders are jockeying for its crown. Joerg Heber discovers an unlikely heir apparent
●
IN 1965, a year before the first pocket calculator was invented, a young physicist from Silicon Valley, Gordon Moore, made a daring prediction. He claimed that the number of components squeezed onto a single silicon chip would double about every two years. And double, and double and continue to double. If he had been right, the best silicon chips today would contain an unbelievable 100 million single components. The true figure is more like 2 billion: Moore had underestimated how fast the shrinking trend would take off. Since the mid-1970s, though, his “law” has been a bankable certainty, influencing economic, social and scientific developments in ways that are hard to overstate. Google, genome sequencing, multiplayer video games, the search for the “God particle”: all rely on silicon’s seemingly limitless ability to deliver more computing speed and capacity – for an ever-diminishing price. Moore’s law means that the cellphone or iPod in your pocket today has more bytecrunching power than the mainframes on the Apollo spacecraft, at the planning stage when the prediction was made. Had aviation followed a similar trajectory, a flight from New York to Paris in 2005 would have cost a cent and lasted less than a second, Intel – the chip giant Moore co-founded in 1968 – calculated. Can the trend go on? Reports of the imminent death of Moore’s law have been around almost as long as the law itself, and have always proved exaggerated. But now www.newscientist.com
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there is concrete cause for concern. The smallest features on today’s state-of-the-art chips are just a few nanometres across. At the current rate of shrinking, they will reach the size of a few silicon atoms by about 2020. At this kind of scale, the properties that make silicon the microelectronic material of choice will fail. We must then either abandon silicon and find an alternative, or accept that the ever-increasing computing power we have come to depend on has reached its upper limit – until the unforeseeable time when more exotic computing schemes, such as quantum computing, become commercially viable. And so, even as the last drops of computing power are squeezed out of silicon, the race is on to find its successor. Bit by bit, a startling picture is emerging. Not only might natural processes hold the key to the computer’s further evolution, but the ideal nanoelectronic material might also have been under our noses all along. It could well be nature’s own favoured building block – carbon.
Silicon squeeze It is easy to forget that no natural law says chips equal silicon. The element wasn’t even the first-choice microelectronic material. It is relatively difficult to obtain in the very pure form needed, and was initially passed over in favour of its neighbour in the periodic table, germanium. In the end, though, silicon’s reliability, performance and abundance – the basic material is freely available in
sand from most beaches – won out. Germanium and silicon are neither insulators that refuse to conduct electricity at all, nor conductors that let electrons flow all too freely. They are semiconductors: reluctant to conduct in their natural state, but requiring just a nudge, in the form of a small voltage applied to them, to be persuaded. That sweatfree swapping between conducting and nonconducting states makes semiconductors the ideal materials for transistors, the basic on-off switches that make up electronic logic circuits. The first transistors were rather crude and bulky devices, with large wire contacts pressed against an underlying slab of germanium. Today, lasers are used to etch them in their billions onto “wafers” of ultra-pure silicon just a few micrometres thick. Over the past 20 years, Robert Chau has seen efforts to squeeze this compact construct get ever more challenging. He leads research into advanced transistor designs at Intel, which supplies about 80 per cent of the world’s silicon processors. “Until the 1990s, Moore’s law was continued by relatively simple scaling,” he says. Now his researchers are having to change the materials: almost every part of the transistor that was originally silicon is being replaced by something else. Take the two contacts through which electrons flow into and out of the transistor, the source and the drain. Traditionally, miniaturising these was a question of finding lasers with smaller and smaller wavelengths to etch out ever tinier features on the chip. In 2003, Chau and his Intel colleagues started to use an alloy of silicon and germanium for the contacts. The separation of atoms in the alloy is larger than in pure silicon, and the atomic bonds in the channel become slightly stretched to span the extra space. Electrons can nip more speedily through this strained silicon, allowing more information to be processed by a smaller number of transistors. Or take the gate. This is the transistor’s on-off switch, which sits atop the channel separated by a thin insulating film of silicon dioxide. This film must be as thin as possible so as not to slow down the transistor’s response when a switching voltage is applied to the gate. But at just a few atoms thick, it is now about as thin as it can go. The route to further miniaturisation is again to break silicon’s monopoly on the chip, adding different, sometimes exotic, elements into the transistor brew. The very latest chips use a superior 6 December 2008 | NewScientist | 35
insulator, hafnium dioxide, for their gates. Then there is the general transistor layout. Traditionally, this is a low-rise sprawl that takes up valuable real estate on the wafer. The obvious solution is to do what we do in city centres – build upwards. Intel has made prototypes of what it calls trigate transistors, in which the gate, rather than lying on top of a flat channel, is wrapped around three sides of a raised channel. The larger contact area makes the gate more effective, increasing the switching speed and allowing the same number of transistors to do more work. Chau thinks the trigate is such a breakthrough that it will become a permanent feature of any future transistor. Put it all together, and he is confident that enough life can be squeezed out of silicon for the next two or three generations of Moore’s law. “Beyond that, our view gets more fuzzy,” he says. Wei Lu, an electrical engineer from the University of Michigan in Ann Arbor, thinks we can see a little further – provided we turn established ways of thinking on their head. Currently we take a “top-down” approach to microelectronics, taking a silicon wafer as the raw material, then carving ever smaller components into it. But the burgeoning field of nanotechnology allows us to experiment with the kind of thing nature does all the time: building things from the bottom up by letting atoms self-assemble into tiny structures. “This approach significantly broadens the choice of materials that can be used,” says Lu. Atoms of almost any element, including silicon, can be coerced into forming nanometre-scale structures. In effect, you just throw the required ingredients – molecules with the right shapes or with particularly desirable electronic properties – into the mixing bowl, and let chemistry take its course.
Back to basics There is a catch. Nature does not have the same need for quality control as the electrical engineer: it tends to build as it pleases, and if one result is slightly different from the next, that’s all part of the fun. The uniformity needed for transistors is a lot more tricky to achieve when building them up from scratch than it is when using a template to etch out identical transistors on a silicon wafer. Tricky, but not impossible. Lu champions a technology called the crossbar array, which sidesteps the replication problems. One of 36 | NewScientist | 6 December 2008
the most useful types of array, pioneered by Charles Lieber and Yi Cui of Harvard University, is made up of a series of selfassembled semiconducting nanowires laid over each other at right angles to create a square mesh. In this arrangement, whether individual wires are exactly the same ceases to matter (Science, vol 291, p 851). What matters is whether two wires at a junction have the same voltage signal across them. If they do, the junction conducts, and assumes a logical “on” state. Conversely, if the two signals are of different strengths, or if there is no voltage on either, that particular
junction is switched off. The junction thus reproduces the basic controlled switching function of a transistor. An array of them can be wired up, just as a group of transistors can, to produce all the main types of logic gate needed to make an integrated circuit. A promising brand of crossbar array uses molecules known as rotaxanes for its switching junctions. These consist of two components, looking rather like a dumb-bell with a ring encircling its midriff. The relative positions of these two molecules, easily changed by a small voltage, determine whether the arrangement conducts or not. Rotaxanes have been used to create some of the densest crossbar arrays yet, cramming 9>7CF?D=7JJ>;8?JI in junctions as densely as the electronics J^[i_b_YedjhWdi_ijeh_ij^[XWi_Yc_Yhe[b[Yjhed_Yim_jY^$7jj^[ ceb[YkbWhiYWb["j^[iWc[[\\[YjiYWdX[WY^_[l[Zki_d]]hWf^[d[ industry would like to fit transistors on a I?B?9EDJH7DI?IJEHE<< chip by 2020 (Nature, vol 445, p 414). 7d[]Wj_l[lebjW][Wffb_[Zjej^[]Wj[YedjWYjYh[Wj[iWd[b[Yjh_Y The bottom-up approach to building \_[bZj^WjXbeYaij^[\beme\[b[Yjhedij^hek]^Wi[c_YedZkYj_d] microelectronics has collateral benefits: it Y^Wdd[bX[jm[[dj^[c[jWbb_YiekhY[WdZZhW_dYedjWYji can produce materials that do things silicon IEKH9;#:H7?DLEBJ7=; D[]Wj_l[lebjW][ never could. Semiconducting polymers built from organic molecules can be used to make IEKH9; transparent, flexible electronic circuits that would be impossible with brittle, opaque =7J; silicon. If that sounds frivolous, just consider 9>7DD;B ;B;9JH?9;D;JH7DI?IJEH into chips in the coming years. But they are Fej[dj_WbbockY^icWbb[hj^Wdj^[i_b_YedjhWdi_ijeh"_jki[ij^[ stopgaps: they might maintain Moore’s law iWc[cWj[h_Wbj^hek]^ekjÅbWh][hc[jWbb_Y]hWf^[d[\ehj^[ for a while, but sooner or later, the limitations YedjWYji"icWbb[hi[c_YedZkYj_d]]hWf^[d[\ehj^[Y^Wdd[b of silicon will make a wholesale switch to a different base material unavoidable. What material that will be is less certain. =7J; IEKH9; Andre Geim, a physicist from the University of Manchester, UK, has seen successors come 9>7DD;B and go. “Eventually, they all failed miserably,” :H7?D he says. Now, though, he thinks he might have IEKH9;#:H7?D the answer in a material he and his colleagues LEBJ7=; discovered in 2004. If his hunch is correct, www.newscientist.com
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“We already have a material which allows us to go to molecular-scale electronics” the future of computing could lie in shavings of carbon just one atom thick, a material known as graphene. Other scientists are also coming round to the idea. Lu is one. “It’s a definite contender,” he says. Firstly, its base material is easy to get hold of. Make a pencil sketch on a piece of paper and – if you happen to have an electron microscope to hand – here and there you will see a single-layer sheet of graphite among your scribblings. That is graphene. The microscope will reveal a two-dimensional honeycomb structure of six-atom carbon rings. What it will not reveal is the material’s quite astounding properties. Electrons can travel along its single layer, like bullets from www.newscientist.com
a gun, without hitting the obstacles they would encounter in a 3D structure. A graphene sheet is mechanically extremely stable, so a transistor channel could be just a few interlocking carbon rings carved from it. Graphene’s atomic bonds, like those of that other notoriously hardcore form of carbon, diamond, are also supremely strong, allowing very many electrons to flow across them simultaneously – essential if you want to send useful electric currents through a transistor channel consisting of just a few atoms. Normally, graphene sheets are metallic, making them far too easy on electron movement to allow the kind of subtle switching that transistors depend on.
To encourage the necessary semiconducting behaviour, the electrons must be reined in a little by cutting the graphene sheet into pieces. Large pieces of graphene can only be made semiconducting by lowering their temperature significantly, but the smaller the piece the higher the temperature at which the switch-over happens. At room temperature, the critical scale is about 10 nanometres. Coincidentally, that is just about the scale at which silicon technology becomes unstable. Once the limits of silicon miniaturisation have been reached, graphene could therefore be ideally poised to take on the Moore’s law mantle. Geim and his colleagues have already shown the potential of the technology, creating the smallest graphene transistor yet earlier this year, with contacts made of metallic graphene and a channel of semiconducting graphene just 30 nanometres across (see diagram, opposite). They also achieved room-temperature operation – albeit somewhat erratically – with even smaller pieces (Science, vol 320, p 356). The challenge is clear if carbon is to become the new silicon: mass-production fabrication technologies are needed so that graphene transistors can be reliably and reproducibly made. “We have a material which allows you to go to molecular-scale electronics,” says Geim. “But we don’t yet have the tools.” Change will come, but silicon is so entrenched that the price for abandoning it is huge. For that reason, Lu thinks that the coming years will be more evolution than revolution. “I expect we’ll see some hybrid circuits first, with new materials doing things such as high-performance logic,” he says. In those hybrid chips, silicon will gradually be reduced to a peripheral role, performing housekeeping functions such as controlling data flow and timekeeping. The urgency of efforts to improve and replace silicon underscore our insatiable demand for computing power. Moore’s law has become a self-fulfilling prophecy as chip companies invest huge sums to ensure its promise is maintained. Soon, it will enter a new phase, decoupled from the material with which it has been synonymous for 40 years. No longer a measure of the development of silicon alone, Moore’s law will become a yardstick of our progress as we harness the cunning of nature’s design strategies. ● Joerg Heber is an editor of Nature Materials 6 December 2008 | NewScientist | 37
●
IN 1965, a year before the first pocket calculator was invented, a young physicist from Silicon Valley, Gordon Moore, made a daring prediction. He claimed that the number of components squeezed onto a single silicon chip would double about every two years. And double, and double and continue to double. If he had been right, the best silicon chips today would contain an unbelievable 100 million single components. The true figure is more like 2 billion: Moore had underestimated how fast the shrinking trend would take off. Since the mid-1970s, though, his “law” has been a bankable certainty, influencing economic, social and scientific developments in ways that are hard to overstate. Google, genome sequencing, multiplayer video games, the search for the “God particle”: all rely on silicon’s seemingly limitless ability to deliver more computing speed and capacity – for an ever-diminishing price. Moore’s law means that the cellphone or iPod in your pocket today has more bytecrunching power than the mainframes on the Apollo spacecraft, at the planning stage when the prediction was made. Had aviation followed a similar trajectory, a flight from New York to Paris in 2005 would have cost a cent and lasted less than a second, Intel – the chip giant Moore co-founded in 1968 – calculated. Can the trend go on? Reports of the imminent death of Moore’s law have been around almost as long as the law itself, and have always proved exaggerated. But now www.newscientist.com
Si
14
C
6
there is concrete cause for concern. The smallest features on today’s state-of-the-art chips are just a few nanometres across. At the current rate of shrinking, they will reach the size of a few silicon atoms by about 2020. At this kind of scale, the properties that make silicon the microelectronic material of choice will fail. We must then either abandon silicon and find an alternative, or accept that the ever-increasing computing power we have come to depend on has reached its upper limit – until the unforeseeable time when more exotic computing schemes, such as quantum computing, become commercially viable. And so, even as the last drops of computing power are squeezed out of silicon, the race is on to find its successor. Bit by bit, a startling picture is emerging. Not only might natural processes hold the key to the computer’s further evolution, but the ideal nanoelectronic material might also have been under our noses all along. It could well be nature’s own favoured building block – carbon.
Silicon squeeze It is easy to forget that no natural law says chips equal silicon. The element wasn’t even the first-choice microelectronic material. It is relatively difficult to obtain in the very pure form needed, and was initially passed over in favour of its neighbour in the periodic table, germanium. In the end, though, silicon’s reliability, performance and abundance – the basic material is freely available in
sand from most beaches – won out. Germanium and silicon are neither insulators that refuse to conduct electricity at all, nor conductors that let electrons flow all too freely. They are semiconductors: reluctant to conduct in their natural state, but requiring just a nudge, in the form of a small voltage applied to them, to be persuaded. That sweatfree swapping between conducting and nonconducting states makes semiconductors the ideal materials for transistors, the basic on-off switches that make up electronic logic circuits. The first transistors were rather crude and bulky devices, with large wire contacts pressed against an underlying slab of germanium. Today, lasers are used to etch them in their billions onto “wafers” of ultra-pure silicon just a few micrometres thick. Over the past 20 years, Robert Chau has seen efforts to squeeze this compact construct get ever more challenging. He leads research into advanced transistor designs at Intel, which supplies about 80 per cent of the world’s silicon processors. “Until the 1990s, Moore’s law was continued by relatively simple scaling,” he says. Now his researchers are having to change the materials: almost every part of the transistor that was originally silicon is being replaced by something else. Take the two contacts through which electrons flow into and out of the transistor, the source and the drain. Traditionally, miniaturising these was a question of finding lasers with smaller and smaller wavelengths to etch out ever tinier features on the chip. In 2003, Chau and his Intel colleagues started to use an alloy of silicon and germanium for the contacts. The separation of atoms in the alloy is larger than in pure silicon, and the atomic bonds in the channel become slightly stretched to span the extra space. Electrons can nip more speedily through this strained silicon, allowing more information to be processed by a smaller number of transistors. Or take the gate. This is the transistor’s on-off switch, which sits atop the channel separated by a thin insulating film of silicon dioxide. This film must be as thin as possible so as not to slow down the transistor’s response when a switching voltage is applied to the gate. But at just a few atoms thick, it is now about as thin as it can go. The route to further miniaturisation is again to break silicon’s monopoly on the chip, adding different, sometimes exotic, elements into the transistor brew. The very latest chips use a superior 6 December 2008 | NewScientist | 35
insulator, hafnium dioxide, for their gates. Then there is the general transistor layout. Traditionally, this is a low-rise sprawl that takes up valuable real estate on the wafer. The obvious solution is to do what we do in city centres – build upwards. Intel has made prototypes of what it calls trigate transistors, in which the gate, rather than lying on top of a flat channel, is wrapped around three sides of a raised channel. The larger contact area makes the gate more effective, increasing the switching speed and allowing the same number of transistors to do more work. Chau thinks the trigate is such a breakthrough that it will become a permanent feature of any future transistor. Put it all together, and he is confident that enough life can be squeezed out of silicon for the next two or three generations of Moore’s law. “Beyond that, our view gets more fuzzy,” he says. Wei Lu, an electrical engineer from the University of Michigan in Ann Arbor, thinks we can see a little further – provided we turn established ways of thinking on their head. Currently we take a “top-down” approach to microelectronics, taking a silicon wafer as the raw material, then carving ever smaller components into it. But the burgeoning field of nanotechnology allows us to experiment with the kind of thing nature does all the time: building things from the bottom up by letting atoms self-assemble into tiny structures. “This approach significantly broadens the choice of materials that can be used,” says Lu. Atoms of almost any element, including silicon, can be coerced into forming nanometre-scale structures. In effect, you just throw the required ingredients – molecules with the right shapes or with particularly desirable electronic properties – into the mixing bowl, and let chemistry take its course.
Back to basics There is a catch. Nature does not have the same need for quality control as the electrical engineer: it tends to build as it pleases, and if one result is slightly different from the next, that’s all part of the fun. The uniformity needed for transistors is a lot more tricky to achieve when building them up from scratch than it is when using a template to etch out identical transistors on a silicon wafer. Tricky, but not impossible. Lu champions a technology called the crossbar array, which sidesteps the replication problems. One of 36 | NewScientist | 6 December 2008
the most useful types of array, pioneered by Charles Lieber and Yi Cui of Harvard University, is made up of a series of selfassembled semiconducting nanowires laid over each other at right angles to create a square mesh. In this arrangement, whether individual wires are exactly the same ceases to matter (Science, vol 291, p 851). What matters is whether two wires at a junction have the same voltage signal across them. If they do, the junction conducts, and assumes a logical “on” state. Conversely, if the two signals are of different strengths, or if there is no voltage on either, that particular
junction is switched off. The junction thus reproduces the basic controlled switching function of a transistor. An array of them can be wired up, just as a group of transistors can, to produce all the main types of logic gate needed to make an integrated circuit. A promising brand of crossbar array uses molecules known as rotaxanes for its switching junctions. These consist of two components, looking rather like a dumb-bell with a ring encircling its midriff. The relative positions of these two molecules, easily changed by a small voltage, determine whether the arrangement conducts or not. Rotaxanes have been used to create some of the densest crossbar arrays yet, cramming 9>7CF?D=7JJ>;8?JI in junctions as densely as the electronics J^[i_b_YedjhWdi_ijeh_ij^[XWi_Yc_Yhe[b[Yjhed_Yim_jY^$7jj^[ ceb[YkbWhiYWb["j^[iWc[[\\[YjiYWdX[WY^_[l[Zki_d]]hWf^[d[ industry would like to fit transistors on a I?B?9EDJH7DI?IJEHE<< chip by 2020 (Nature, vol 445, p 414). 7d[]Wj_l[lebjW][Wffb_[Zjej^[]Wj[YedjWYjYh[Wj[iWd[b[Yjh_Y The bottom-up approach to building \_[bZj^WjXbeYaij^[\beme\[b[Yjhedij^hek]^Wi[c_YedZkYj_d] microelectronics has collateral benefits: it Y^Wdd[bX[jm[[dj^[c[jWbb_YiekhY[WdZZhW_dYedjWYji can produce materials that do things silicon IEKH9;#:H7?DLEBJ7=; D[]Wj_l[lebjW][ never could. Semiconducting polymers built from organic molecules can be used to make IEKH9; transparent, flexible electronic circuits that would be impossible with brittle, opaque =7J; silicon. If that sounds frivolous, just consider 9>7DD;B ;B;9JH?9;D;JH7DI?IJEH into chips in the coming years. But they are Fej[dj_WbbockY^icWbb[hj^Wdj^[i_b_YedjhWdi_ijeh"_jki[ij^[ stopgaps: they might maintain Moore’s law iWc[cWj[h_Wbj^hek]^ekjÅbWh][hc[jWbb_Y]hWf^[d[\ehj^[ for a while, but sooner or later, the limitations YedjWYji"icWbb[hi[c_YedZkYj_d]]hWf^[d[\ehj^[Y^Wdd[b of silicon will make a wholesale switch to a different base material unavoidable. What material that will be is less certain. =7J; IEKH9; Andre Geim, a physicist from the University of Manchester, UK, has seen successors come 9>7DD;B and go. “Eventually, they all failed miserably,” :H7?D he says. Now, though, he thinks he might have IEKH9;#:H7?D the answer in a material he and his colleagues LEBJ7=; discovered in 2004. If his hunch is correct, www.newscientist.com
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“We already have a material which allows us to go to molecular-scale electronics” the future of computing could lie in shavings of carbon just one atom thick, a material known as graphene. Other scientists are also coming round to the idea. Lu is one. “It’s a definite contender,” he says. Firstly, its base material is easy to get hold of. Make a pencil sketch on a piece of paper and – if you happen to have an electron microscope to hand – here and there you will see a single-layer sheet of graphite among your scribblings. That is graphene. The microscope will reveal a two-dimensional honeycomb structure of six-atom carbon rings. What it will not reveal is the material’s quite astounding properties. Electrons can travel along its single layer, like bullets from www.newscientist.com
a gun, without hitting the obstacles they would encounter in a 3D structure. A graphene sheet is mechanically extremely stable, so a transistor channel could be just a few interlocking carbon rings carved from it. Graphene’s atomic bonds, like those of that other notoriously hardcore form of carbon, diamond, are also supremely strong, allowing very many electrons to flow across them simultaneously – essential if you want to send useful electric currents through a transistor channel consisting of just a few atoms. Normally, graphene sheets are metallic, making them far too easy on electron movement to allow the kind of subtle switching that transistors depend on.
To encourage the necessary semiconducting behaviour, the electrons must be reined in a little by cutting the graphene sheet into pieces. Large pieces of graphene can only be made semiconducting by lowering their temperature significantly, but the smaller the piece the higher the temperature at which the switch-over happens. At room temperature, the critical scale is about 10 nanometres. Coincidentally, that is just about the scale at which silicon technology becomes unstable. Once the limits of silicon miniaturisation have been reached, graphene could therefore be ideally poised to take on the Moore’s law mantle. Geim and his colleagues have already shown the potential of the technology, creating the smallest graphene transistor yet earlier this year, with contacts made of metallic graphene and a channel of semiconducting graphene just 30 nanometres across (see diagram, opposite). They also achieved room-temperature operation – albeit somewhat erratically – with even smaller pieces (Science, vol 320, p 356). The challenge is clear if carbon is to become the new silicon: mass-production fabrication technologies are needed so that graphene transistors can be reliably and reproducibly made. “We have a material which allows you to go to molecular-scale electronics,” says Geim. “But we don’t yet have the tools.” Change will come, but silicon is so entrenched that the price for abandoning it is huge. For that reason, Lu thinks that the coming years will be more evolution than revolution. “I expect we’ll see some hybrid circuits first, with new materials doing things such as high-performance logic,” he says. In those hybrid chips, silicon will gradually be reduced to a peripheral role, performing housekeeping functions such as controlling data flow and timekeeping. The urgency of efforts to improve and replace silicon underscore our insatiable demand for computing power. Moore’s law has become a self-fulfilling prophecy as chip companies invest huge sums to ensure its promise is maintained. Soon, it will enter a new phase, decoupled from the material with which it has been synonymous for 40 years. No longer a measure of the development of silicon alone, Moore’s law will become a yardstick of our progress as we harness the cunning of nature’s design strategies. ● Joerg Heber is an editor of Nature Materials 6 December 2008 | NewScientist | 37