A breakthrough announced by IBM last year has paved the way for development of new, low-cost, and faster copper chips. Apart from IBM, Texas Instruments is also evincing a keen interest in this new technology which may bring about a paradigm shift in chip-making.
A mid the stupefying growth of the software industry, the semiconductor industry has taken a nosedive, plagued as it is with growth stagnation, erosion in demand, pricing pressures, paucity of funds, and high fabrication costs. But a recent breakthrough by IBM has now made it possible to develop low-cost, faster copper chips. Apart from IBM, Texas Instruments, Applied Materials, Motorola, and Advanced Micro Devices among others are evincing keen interest in this new copper-based microprocessor technology. If miniaturisation of integrated circuit (IC) has been made possible by lithography, finesse in device design and means of fabrication have made it possible to manufacture a new generation of ICs. The industry took a major leap forward last year when the performance of an IC was significantly enhanced through copper interconnects.
Advantage of Copper Technology :
The traditional use of aluminium in microprocessor circuitry has, of late, become a problem with the an increasing emphasis on reducing space occupied by transistors. Aluminium’s low conductivity and the need for greater space in wiring account for this problem. However, this problem can be solved by using copper which enables the reduction of the drip feature size from current 0.25 microns to 0.18 microns. This is impossible to achieve using aluminium wires. For years scientists have struggled with the limitations of aluminium interconnects which have fairly high resistance, vulnerability to electromigration, and stress-induced voids in the alloy, aggravated by the ever-shrinking widths of advanced lithographics. Copper, despite having lower resistance, was kept out of the mainstream production of transistors owing to difficulties in fabricating copper wiring for small-dimension chips (0.2 microns or lower) and contamination it causes to tools used for producing transistors. But the recent breakthrough in chip-manufacturing process, involving six layers of copper wiring, has made copper a better replacement than aluminium. Apart from enabling a reduction in size, copper, being a better conductor of electricity than aluminium, would ensure less emission of heat. Designers can now increase the clock speed or decrease the operating voltage. This technology also solves the problem arising due to the circuit molecules being driven into the surrounding wafer, destroying the interconnecting lines. It allows shrinkage at interconnecting lines, reducing the delay caused by Miller capacitance and improves the circuit speed as well. Aluminium technology’s saturation point of 400 MHz can be taken up to 1 GHz through this copper technology. The manufacture of copper-based ICs has been made possible by the innovations in methods of patterning, depositing, and containing the copper from diffusing into surrounding space.
These hurdles were overcome by a two-step process, called dual-damascene, in which copper is electroplated into trenches etched in a wafer’s surface and then chemically and mechanically polished. IBM contains the copper by lining the trenches with a special material initially, and later capping the polished copper lines with silicon nitride. Motorola uses titanium nitride to line the trenches, to contain the diffusion of copper, and to help it adhere to the surrounding insulation. Measurements show that the resistivity of copper interconnects is about 40-to-45 per cent lower than the resistivity of aluminium. The lower resistivity not only enhances performance but also makes it possible to scale down the width and the thickness of wires to reduce their capacitance. The process developed by IBM enables the use of a thinner copper wire instead of an aluminium wire. The use of copper reduces cross-talk and results in better performance, density, cost, and reliability. During testing, it was found that these copper chips were resistant to electromigration—the drift of metal atoms under an applied voltage. Their electromigration was much better than that of aluminium. In fact, today’s standard aluminium interconnects already contain some copper to guard against electromigration.
Electromigration performance improves through the use of copper by two orders and, importantly, there is no stress migration (that is, formation of voids in fine lines under tensile stress). This breakthrough is significant since in conventional interconnects the voids increase considerably. IBM’s fabrication technology involves six layers of copper metallisation which is a part of CMOS7s. It also includes transistors with an effective channel length of 0.12 micrometre, 1.8V operation, and integration levels of up to 200 million transistors on a single silicon chip. Motorola’s process closely resembles the IBM technology which involves six layers of copper interconnects formed with a dual damascene process of effective channel lengths of 0.15 micrometre. Both these companies are set to use the copper interconnects technology to manufacture microprocessors. In fact, CMOS7S technology is already available to IBM’s application-specific IC customers. Motorola is on the verge of implementing the dual electromigration process to develop smaller interconnects. And this is just the beginning. What lies ahead are faster interconnect systems combining copper wiring with inter-layer insulations with lower dielectric constraints to reduce wiring capacitance and cross-talk. Early-generation ICs used sputtered quartz or silicon nitride with dielectric constraints near four. Insulating materials with dielectric constraints down to 1.5 are under development. Efforts are being made presently to develop low dielectric materials mechanically and thermally compatible with semiconductor fabrication process, and partially with the damascene technique used to make copper wire. Texas Instruments has reported that it has been able to integrate ‘Xerogel’ insulator with a tunable dielectric constraint into the copper damascene process. Xerogel is porous silicon dioxide. Since air has a dielectric constraint of one, the more porous the silicon dioxide (SiO2), the lower is the dielectric constraint. The porosity also controls Xerogel’s mechanical properties. This technology can cram 500 million transistors on a single chip as compared to 5.5-to-7.5 million transistors in today’s desktop processors. The process developed by Texas Instruments employed Xerogel with a porosity of 75 per cent and a dielectric constraint of 1.8 between adjacent copper wires fabricated with damascene process. Through this process the resistance of copper wire can be decreased by 30 per cent (compared to aluminium wire) and the capacitance by 14 per cent. These improvements of electrical characteristics, together with compatibility with the damascene process, are encouraging signs for the future of Xerogel in future generations of integrated circuits. Chip-manufacturing company Advanced Micro Devices (AMD) makes use of the ion metal plasma (IMP) technology to make copper chips. It uses a physical vapour deposition technique to sputter atoms from a target, ionise them in a plasma, and send them to the wafer. The ions are attracted towards the wafer surface by electrical charges. This results in a layer that forms the base for copper fills. Applications IBM has shipped its first copper-based microprocessor, Power PC 740/750, which operates at 400 MHz. IBM now plans to incorporate the copper-chip technology into S/390, RS/6000, and AS/400 server families as well. Motorola’s quad-integrated communications controller quick chip is yet another copper-based product in the pipeline. The above steps elucidate processors where copper is electrolytically plated into trenches etched in a surface and then chemically polished.
These hurdles were overcome by a two-step process, called dual-damascene, in which copper is electroplated into trenches etched in a wafer’s surface and then chemically and mechanically polished. IBM contains the copper by lining the trenches with a special material initially, and later capping the polished copper lines with silicon nitride. Motorola uses titanium nitride to line the trenches, to contain the diffusion of copper, and to help it adhere to the surrounding insulation. Measurements show that the resistivity of copper interconnects is about 40-to-45 per cent lower than the resistivity of aluminium. The lower resistivity not only enhances performance but also makes it possible to scale down the width and the thickness of wires to reduce their capacitance. The process developed by IBM enables the use of a thinner copper wire instead of an aluminium wire. The use of copper reduces cross-talk and results in better performance, density, cost, and reliability. During testing, it was found that these copper chips were resistant to electromigration—the drift of metal atoms under an applied voltage. Their electromigration was much better than that of aluminium. In fact, today’s standard aluminium interconnects already contain some copper to guard against electromigration.
Electromigration performance improves through the use of copper by two orders and, importantly, there is no stress migration (that is, formation of voids in fine lines under tensile stress). This breakthrough is significant since in conventional interconnects the voids increase considerably. IBM’s fabrication technology involves six layers of copper metallisation which is a part of CMOS7s. It also includes transistors with an effective channel length of 0.12 micrometre, 1.8V operation, and integration levels of up to 200 million transistors on a single silicon chip. Motorola’s process closely resembles the IBM technology which involves six layers of copper interconnects formed with a dual damascene process of effective channel lengths of 0.15 micrometre. Both these companies are set to use the copper interconnects technology to manufacture microprocessors. In fact, CMOS7S technology is already available to IBM’s application-specific IC customers. Motorola is on the verge of implementing the dual electromigration process to develop smaller interconnects. And this is just the beginning. What lies ahead are faster interconnect systems combining copper wiring with inter-layer insulations with lower dielectric constraints to reduce wiring capacitance and cross-talk. Early-generation ICs used sputtered quartz or silicon nitride with dielectric constraints near four. Insulating materials with dielectric constraints down to 1.5 are under development. Efforts are being made presently to develop low dielectric materials mechanically and thermally compatible with semiconductor fabrication process, and partially with the damascene technique used to make copper wire. Texas Instruments has reported that it has been able to integrate ‘Xerogel’ insulator with a tunable dielectric constraint into the copper damascene process. Xerogel is porous silicon dioxide. Since air has a dielectric constraint of one, the more porous the silicon dioxide (SiO2), the lower is the dielectric constraint. The porosity also controls Xerogel’s mechanical properties. This technology can cram 500 million transistors on a single chip as compared to 5.5-to-7.5 million transistors in today’s desktop processors. The process developed by Texas Instruments employed Xerogel with a porosity of 75 per cent and a dielectric constraint of 1.8 between adjacent copper wires fabricated with damascene process. Through this process the resistance of copper wire can be decreased by 30 per cent (compared to aluminium wire) and the capacitance by 14 per cent. These improvements of electrical characteristics, together with compatibility with the damascene process, are encouraging signs for the future of Xerogel in future generations of integrated circuits. Chip-manufacturing company Advanced Micro Devices (AMD) makes use of the ion metal plasma (IMP) technology to make copper chips. It uses a physical vapour deposition technique to sputter atoms from a target, ionise them in a plasma, and send them to the wafer. The ions are attracted towards the wafer surface by electrical charges. This results in a layer that forms the base for copper fills. Applications IBM has shipped its first copper-based microprocessor, Power PC 740/750, which operates at 400 MHz. IBM now plans to incorporate the copper-chip technology into S/390, RS/6000, and AS/400 server families as well. Motorola’s quad-integrated communications controller quick chip is yet another copper-based product in the pipeline. The above steps elucidate processors where copper is electrolytically plated into trenches etched in a surface and then chemically polished.
The Road Ahead :
The pace at which new-generation technology is being introduced is growing sharply. The introduction of new technologies has been accelerated by leading semiconductor companies from the traditional three-year cycle to an approximate two-year cycle. The technology has defined the size of smallest feature on an IC in the semiconductor industry. The standard to describe the minimum feature size is no longer micrometric: it is nanometric. 250nm technology is already available for logic chips. ICs with 180nm features are now expected to emerge as performance microprocessors with more than a billion transistors, size of less than 50nm, and clock speed as high as 10 GHz.
A major concern, however, is the development of new lithographic tools to create lines matching super-small features and dense chips-steppers operating at wavelengths of 248nm in production lines at state-of-the-art chip fabrication plants. Such steps should take ICs down to 0.18 micrometric generation. Using some advanced techniques, such as proximity correction and phase-shift masks, these could go down to 0.15 micrometre. Most technologists believe that these machines will be capable of producing ICs with 130nm features and could even go down to 100nm ground rules.
The new technology will not only help in the improvement of the semiconductor industry’s bottomline but also provide a strong foothold. These emerging technologies will make it possible to manufacture cheaper computer systems.
A major concern, however, is the development of new lithographic tools to create lines matching super-small features and dense chips-steppers operating at wavelengths of 248nm in production lines at state-of-the-art chip fabrication plants. Such steps should take ICs down to 0.18 micrometric generation. Using some advanced techniques, such as proximity correction and phase-shift masks, these could go down to 0.15 micrometre. Most technologists believe that these machines will be capable of producing ICs with 130nm features and could even go down to 100nm ground rules.
The new technology will not only help in the improvement of the semiconductor industry’s bottomline but also provide a strong foothold. These emerging technologies will make it possible to manufacture cheaper computer systems.
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