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Silicon will soon run into its limit - and its replacement has already learned to be processed almost without joints.
In modern chips, billions of transistors work on silicon, but it is becoming increasingly difficult to reduce such elements. Engineers are looking for materials that can be used next to silicon in thinner and denser devices. One of the main candidates is Molybdenum disulfide, or MoS2. This material consists of only three atomic layers: the molybdenum layer is located between two layers of sulfur.
Molybdenum disulfide refers to the dichalcogenids of transition metals. This is a class of ultra-thin materials that are well suited for the electronics of the future: they can be made almost atomic thickness, and electrical properties remain useful for transistors and other elements. The problem is that it is almost impatient to work with such materials. Sometimes the manufacturer needs to remove only the upper layer of sulfur and not damage the layer of molybdenum under it.
For such processing, plasma is usually used. It is an ionized gas that has charged particles. In the production of chips, plasma helps to poison materials, remove individual layers and form the desired structure. When plasma ions hit the MoS2 surface, they can knock out sulfur atoms. But here you need a very precise mode: too weak blow will not remove the top layer, too strong will damage the molybdenum part.
The researchers checked whether this narrow operating range could be expanded. They simulated treatment of molybdenum disulfide and found that pre-coating the surface with oxygen or fluorine helps to remove sulphur atoms softer. In this case, the plasma does not need to literally knock out sulfur only by the force of the impact. Chemistry takes part of the job.
On the untreated surface, according to the calculations of the command, about 30 electron volts are required to remove the sulfur atom. After filling with fluorine, the threshold drops to about 10 electron-volts, after oxygen coating - up to about 14 electron volts. Electron-volt is a unit of energy that is convenient to describe processes at the level of atoms and particles.
The difference is important because of how the plasma itself works. Ions in it do not fly to the surface with absolutely the same energy. Some particles are weaker, others are stronger. If the sulfur removal threshold is too close to the energy at which the molybdenum damage has already begun, some of the ions will inevitably fall into a dangerous range. The treatment will become uneven: somewhere the upper layer will be removed, and somewhere the material under it will receive defects.
Oxygen and fluoride change the situation. When the surface is pre-coated with such atoms, the upper layer of sulfur breaks away from the crystal more easily. For the manufacturer, this means a wider technological corridor: it is possible to choose the energy of the plasma so that the upper sulfur leaves, and the lower layer remains intact. It is this selectivity that is necessary for future transistors, where each atomic layer affects the operation of the device.
When the plasma ion strikes the surface, the oxygen atoms and the sulfur atom are nearby. They can be connected to a sulphur dioxide molecule, SO2. This is a stable gas that is already easier to leave the surface. Instead of a rough rupture of bonds, an intermediate chemical step is obtained in the crystal: the sulfur first binds to oxygen, and then leaves the material in the composition of the new molecule.
The fluorine acts in a similar way, but forms sulfur compounds with fluorine. In both cases, the researchers suggest not to increase the force of the impact, but to prepare the surface in advance so that the desired atom is more easily separated. This approach is useful for ultra-thin materials, where excess energy is rapidly converted to damage to the neighboring layer.
The work is still based on computer modeling, not on the finished production line. But such calculations help to understand in advance what plasma regimens should be checked in experiments. In microproduction, this is important: the selection of parameters by trial and error of roads, and damage to the atomically thin material can completely spoil the future transistor.
The next stage is to assess not only the fact of damage, but also their scale. Scientists need to understand how many defects remain after processing, where exactly they appear and how they affect the electrical properties of MoS2. For chips, it is not enough to gently remove one layer on a separate area. The process should work stably in large areas and give a repeatable result.
The team also wants to check how much the method is applicable to related materials. In the dicholicgenides of transitional metals, you can change metal and chalcogen: for example, replace molybdenum with tungsten or sulfur on selenium. If pretreatment with oxygen or fluorine helps in these cases, manufacturers will have a more general method of accurate plasma treatment of super-thin semiconductors.
For future chips, this task is important no less than new architectures. Silicon is likely to remain the basis of electronics for a long time, but atomically thin materials with other properties may appear next to it. To build them into production, engineers need to learn not just to grow such layers, but to treat them without unnecessary defects. The new calculation shows one of the ways: first weaken the necessary connections chemically, and only then carefully remove the upper atomic layer with plasma.
In modern chips, billions of transistors work on silicon, but it is becoming increasingly difficult to reduce such elements. Engineers are looking for materials that can be used next to silicon in thinner and denser devices. One of the main candidates is Molybdenum disulfide, or MoS2. This material consists of only three atomic layers: the molybdenum layer is located between two layers of sulfur.
Molybdenum disulfide refers to the dichalcogenids of transition metals. This is a class of ultra-thin materials that are well suited for the electronics of the future: they can be made almost atomic thickness, and electrical properties remain useful for transistors and other elements. The problem is that it is almost impatient to work with such materials. Sometimes the manufacturer needs to remove only the upper layer of sulfur and not damage the layer of molybdenum under it.
For such processing, plasma is usually used. It is an ionized gas that has charged particles. In the production of chips, plasma helps to poison materials, remove individual layers and form the desired structure. When plasma ions hit the MoS2 surface, they can knock out sulfur atoms. But here you need a very precise mode: too weak blow will not remove the top layer, too strong will damage the molybdenum part.
The researchers checked whether this narrow operating range could be expanded. They simulated treatment of molybdenum disulfide and found that pre-coating the surface with oxygen or fluorine helps to remove sulphur atoms softer. In this case, the plasma does not need to literally knock out sulfur only by the force of the impact. Chemistry takes part of the job.
On the untreated surface, according to the calculations of the command, about 30 electron volts are required to remove the sulfur atom. After filling with fluorine, the threshold drops to about 10 electron-volts, after oxygen coating - up to about 14 electron volts. Electron-volt is a unit of energy that is convenient to describe processes at the level of atoms and particles.
The difference is important because of how the plasma itself works. Ions in it do not fly to the surface with absolutely the same energy. Some particles are weaker, others are stronger. If the sulfur removal threshold is too close to the energy at which the molybdenum damage has already begun, some of the ions will inevitably fall into a dangerous range. The treatment will become uneven: somewhere the upper layer will be removed, and somewhere the material under it will receive defects.
Oxygen and fluoride change the situation. When the surface is pre-coated with such atoms, the upper layer of sulfur breaks away from the crystal more easily. For the manufacturer, this means a wider technological corridor: it is possible to choose the energy of the plasma so that the upper sulfur leaves, and the lower layer remains intact. It is this selectivity that is necessary for future transistors, where each atomic layer affects the operation of the device.
When the plasma ion strikes the surface, the oxygen atoms and the sulfur atom are nearby. They can be connected to a sulphur dioxide molecule, SO2. This is a stable gas that is already easier to leave the surface. Instead of a rough rupture of bonds, an intermediate chemical step is obtained in the crystal: the sulfur first binds to oxygen, and then leaves the material in the composition of the new molecule.
The fluorine acts in a similar way, but forms sulfur compounds with fluorine. In both cases, the researchers suggest not to increase the force of the impact, but to prepare the surface in advance so that the desired atom is more easily separated. This approach is useful for ultra-thin materials, where excess energy is rapidly converted to damage to the neighboring layer.
The work is still based on computer modeling, not on the finished production line. But such calculations help to understand in advance what plasma regimens should be checked in experiments. In microproduction, this is important: the selection of parameters by trial and error of roads, and damage to the atomically thin material can completely spoil the future transistor.
The next stage is to assess not only the fact of damage, but also their scale. Scientists need to understand how many defects remain after processing, where exactly they appear and how they affect the electrical properties of MoS2. For chips, it is not enough to gently remove one layer on a separate area. The process should work stably in large areas and give a repeatable result.
The team also wants to check how much the method is applicable to related materials. In the dicholicgenides of transitional metals, you can change metal and chalcogen: for example, replace molybdenum with tungsten or sulfur on selenium. If pretreatment with oxygen or fluorine helps in these cases, manufacturers will have a more general method of accurate plasma treatment of super-thin semiconductors.
For future chips, this task is important no less than new architectures. Silicon is likely to remain the basis of electronics for a long time, but atomically thin materials with other properties may appear next to it. To build them into production, engineers need to learn not just to grow such layers, but to treat them without unnecessary defects. The new calculation shows one of the ways: first weaken the necessary connections chemically, and only then carefully remove the upper atomic layer with plasma.
