On a typical day, most people will interact with about 300 semiconductors. Semiconductors are all around us. They are in cars, radios, stereos, cell phones, pagers, microwave ovens, kitchen appliances, and medical equipment, to name just a few applications. They are everywhere. The question is, why is that? One of the big reasons is that the semiconductor industry has been able to improve the productivity of the integrated circuit (IC) by between 25 and 30 percent every year for more than 30 years. What does that mean for the consumer, for you and me?
Consider the power of computers. My car has more computing power than the first Apollo spacecraft that went to the moon. In 1970, a computer that could execute 1 million instructions per second cost over $1 million. This cost had dropped to well below $100,000 by 1980. Last week, it was less than $2. This exponential growth in performance is why the silicon-based IC is so pervasive. Indeed, the silicon IC is the engine that drives the information age. By concentrating on silicon, I don't want to downplay other important base technologies of the information age. Fibre optics, for example, is another key base technology. If silicon is the engine that drives the information age, then optical fibre is the highway of the information age. There are other types of semiconductors that are important today. Compound semiconductors, for instance, provide the photons that are the vehicles that carry the information along optical-fibre highways. Compound semiconductors also make possible the very-high-frequency devices-cell phones and pagers-used in today's wireless communications.
Other key materials-intensive technologies for the information age include high-speed, high-density storage, both magnetic and optical, and displays, which are the interface between computers and humans. So, it's really the convergence of these technologies that is driving the information age. The semiconductor industry is very conservative when it comes to introducing new materials. The same basic materials used in the 1960s are still in use today. In the 1970s, when the design rule was 3 mm, integrated circuits were made using silicon doped with phosphorus, boron, and arsenic to control the electrical properties.
The insulators were silicon dioxide and silicon nitride, and interconnects were aluminium at 2 mm feature sizes, molybdenum was introduced, at 1.2 mm, titanium silicide was added. At 0.8 mm, the aluminium interconnects were doped with copper to improve resistance to electro migration. When feature sizes decreased to 0.35 mm, tungsten plugs were used to connect the insulating layers. As feature sizes became even smaller, the dielectric constant in the insulator had to be reduced, and some companies have added fluorine to the silicon dioxide. Now, we are facing massive introduction - by this industry's standards - of new materials as we move forward. Silicon dioxide is going to be fluorinated; the industry will introduce families of low-dielectric-constant materials in the interlayers; there will also be high-dielectric-constant materials, deposited copper, barrier materials, and other materials.
Why do we need these new materials? One important reason relates to gate-delay performance. As feature size continues to shrink, switching speed continues to get faster. To take advantage of that speed, one has to connect these transistors in the ICs. If the industry stayed with aluminium, silicon dioxide, circuit performance would be significantly degraded at very small feature sizes. Today’s high-end microprocessor, 0.25 mm technology, has six levels of aluminium conductor separated by silicon dioxide dielectrics and connected tungsten plugs. In the future, say with 0.1 mm devices, the conductors will be copper and the insulators will be some low-dielectric-constant material. This will allow IC manufacture with about eight levels of interconnect.
There are two reasons for making the switch from aluminium to copper. The first is that copper gives superior performance in the circuits. The second is that if the industry were to stay with aluminium and silicon dioxide, we would require up to 14 levels of interconnect, and it is not clear that such circuits would be manufactured at an affordable cost. Another reason for introducing new materials relates to the devices themselves. As the device size gets smaller, the junctions must become shallower. The insulators in the gate must become thinner. Everything has to scale on all dimensions. That means that more and more control will be required in manufacturing. More characterization will be required, leading to in situ monitoring and on-line control.
Lucent Technologies is a leader in research on thin oxides and new materials in this area. The company has been able to apply a layer of oxide that is about 15, or 5 atoms, thick. The oxide has to be put down uniformly, with perfect interfaces across the entire wafer.
One of the big limitations of this technology as it is scaled to smaller dimensions is electron tunnelling current, which travels through the oxide between the substrate and the gate as voltage is applied to the gate. This gate leakage is especially a problem if the thin oxide layers have any no uniformities, pin holes, or surface imperfections. By understanding the properties of the materials, how to clean the surfaces, and how to process and deposit or grow oxides with good uniformity, Lucent has been able to reduce the tunnelling current by a factor of 100 compared with the best previous results.
Another approach that is being looked at is building transistors out of silicon-germanium alloys. A silicon-germanium heterojunction bipolar transistor under development at IBM, for example, is highly planar and operates at a much higher speed than the comparable silicon transistor. The advantage of SOI and silicon-germanium is that they are compatible with conventional silicon processing and manufacturing technologies.
So, silicon ICs and the silicon-germanium ICs can enter the same processing flow, thus taking advantage of the enormous worldwide manufacturing infrastructure that the semiconductor industry has built. An interesting question is; how will these new transistors be manufactured in high volume? Fifty nanometres, after all, is pretty small compared with the wavelength of light that the industry has used to etch its circuits. Optical lithography, using steppers and scanners, has been the approach up to this point. We started with visible light, have now moved to the deep ultraviolet (248 nm lasers), and will eventually move to 193 nm. The practical limit for conventional through-the-lens lithography might be around 100 nm, or 0.1 um.
Several approaches are being studied to develop the industry's next-generation lithography. That's the technology that will print the smaller feature sizes. One of these, pioneered and led by Bell Laboratories, uses electron scattering, or electron printing, of feature sizes. Instead of trying to absorb the electrons in the metal, which would require thick masks, or reflect the electrons as we do with photons, for the mask Bell researchers simply put some heavy-metal scattering features where you do not want features printed. This scatters the electrons. These scattered electrons are stopped by an aperture, while electrons that come through the membrane go through the aperture and print the desired features.
Another approach is called extreme ultraviolet lithography. This technology is being developed by Sandia National Laboratories, Lawrence Livermore National Laboratories, and Lawrence Berkeley National Laboratories. The work is funded by industry in a consortium led by Intel. The idea behind this technology is that a laser pulse is used to vaporize a material, creating a plasma that radiates at all wavelengths, including soft X-rays. Optics collect the 13-nm, 134 light, which then goes to a reflective mask through reduction optics to print the pattern on the wafer. Metal is deposited in the patterned regions.
Semiconductors are enabling the convergence of computing, communication, and consumer electronics. The result is shaping the way we live and work. It is having a huge effect not only in our country, but also around the world. The question is, how far will it go? How long will this technology revolution continue? I don't know the answer to that. But, for it to continue, advances in materials are required. Our ability to understand and control materials, and to manufacture them at the atomic level, will be essential if the computer industry is to continue on its historical productivity growth curve.
