ABSTRACT
Silicon has been the heart of the world’s technology boom for nearly half a century. Each year, manufactures bring out the next great computer chip that boosts computing power and allows our Personal Computers to do more than we imagined just a decade ago. The current technology used to make microprocessors, deep ultraviolet lithography will begin to reach its limit around 2005. At that time, chipmakers will have to look to other technologies to cram more transistors onto silicon to create powerful chips. Many are already looking at extreme-ultraviolet lithography (EUVL) as a way to extend the life of silicon at least until the end of the decade.
Akin to photography, lithography is used to print circuits onto microchips Extreme Ultra violet Lithography (EUVL) will open a new chapter in semiconductor technology. In the race to provide the Next Generation Lithography (NGL) for faster, more efficient computer chips, EUV Lithography is the clear frontrunner. Here we discusses the basic concept and current state of development of EUV lithography (EUVL), a relatively new form of lithography that uses extreme ultraviolet (EUV) radiation with a wavelength in the range of 10 to 14 nanometers (nm) to carry out projection imaging. EUVL is one technology vying to become the successor to optical lithography.
INTRODUCTION
The current technology used to make microprocessors will begin to reach its limit around 2005. At that time, chipmakers will have to look to other technologies to cram more transistors onto silicon to create more powerful chips.
Potential successors to optical projection lithography are being aggressively developed. These are known as “Next Generation Lithography” (NGL’s). EUV lithography (EUVL) is one of the leading NGL technologies. Using extreme-ultraviolet (EUV) light to carve transistors in silicon wafers will lead to microprocessors that are up to 100 times faster than today’s most powerful chips, and to memory chips with similar increases in storage capacity.
EUVL DEFINITION
Extreme ultraviolet lithography (EUVL) is an advanced technology for making microprocessors a hundred times more than powerful than those today.
EUVL is one technology vying to replace the optical lithography used to make today’s microcircuits. It works by during intense beams of ultraviolet light that are reflected from a circuit design pattern into a silicon wafer. EUVL is similar to optical lithography in which light is refracted through camera lenses on to the wafer.
EUV RADIATION
We know that Ultraviolet radiations are very shortwave (very low wavelength) with high energy. If we further reduce the wavelength it becomes Extreme Ultraviolet radiations. Current lithography techniques have been pushed just about as far as they can go. They use light in deep ultraviolet range- at about 248-nanometer wavelengths-to print 150- to 120- nanometer-size features on a chip. (A nanometer is a billionth of a meter). In the next half dozen years, manufactures plan to make chips with features measuring from 100 to 70 nanometers, using deep ultraviolet light of 193- and 157- nanometer wavelengths.
LITHOGRAPHY
Computers have become much more compact and increasingly powerful largely because of lithography, a basically photographic process that allows more and more features to be crammed onto a computer chip.
Lithography is akin to photography in that it uses light to transfer images onto a substrate. Light is directed onto a mask-a sort stencil of an integrated circuit pattern-and the image of that pattern is then projected onto a semiconductor wafer covered with light-sensitive photo resist. Creating circuits with smaller and smaller features has required using shorter and shorter wavelengths of light.
WHY EUVL?
The current process used to pack more and more transistors onto a chip is called deep-ultraviolet lithography, which is a photography-like technique that focuses light through lenses to carve circuit patterns on silicon wafers. Manufactures are concerned that technique might soon be problematic as the laws of physics intervene.
Intel, AMD, and Motorola have joined with the U.S. department of Energy in a three-year venture to develop a microchip with ached circuit lines smaller than 0.1 micron in width. (Today’s circuits are generally .18 micron or greater.) A microprocessor made with the EUVL technology would be a hundred times more powerful than today’s. Memory chips would be able to store 1,000 times more information than they can today. The aim is to have a commercial manufacturing process ready before 2005.
MOORE’S LAW
Each year, manufacturing bring out the next great computer that boosts computing power and allows our personal computers to do more than we imagined just a decade ago. Intel founder Gordon Moore predicted this technology phenomenon more than 35 years ago, when he said that the number of transistors on a microprocessor would double every 18 months. This becomes known as Moore’s Law.
Industry experts believe that deep-ultraviolet lithography will reach its limits around 2004 and 2005, which means that Moore’s law would also come to an end without a new chip making technology. But once deep-ultraviolet hits its ceiling, we will see chipmakers move to a new lithography process that will enable them to produce the industry’s first Intel Pentium 4 Processor (as of May 2001) is 2.4 GHz. EUVL could add another 10 years to Moore’s Law.
“EUV lithography allows us to make chips with feature size that are smaller that are small enough to support 10 GHz clock speed. It doesn’t necessarily make it happen,” Don Sweeny, EUV Lithography program manager at Lawrence Livermore National Laboratory (LLNL), said. “The first thing we need to do is to make integrated circuits down to 30 nanometers, and EUV lithography will clearly do that. “By comparison, the smallest circuit that can be created by deep-ultraviolet lithography is 100 nanometers.
THE INCREDIBLE SHRINKING CHIPS
Twenty five years ago, the computing equivalent of today’s laptop was a room full of computer hardware and a cartload of punch cards. Since then, computers have become much more compact and increasingly powerful largely because of lithography.
Why are smaller computer chips better and faster? It might seem a paradox, but as the size decreases, the chips become more powerful. It’s as simple as getting grandma’s house faster if she lives next door rather than across town: the electronic signals zipping around the circuitry to solve computing problems have less distance to travel. Today’s chip contains about 3,260 times more than the chip of 1971.
A microprocessor – also known as a CPU or central processing unit – is a complete computating engine that is fabricated on a single chip. The first microprocessor was the Intel 4004, introduced in 1971. The 4004 was not very powerful – could do was add substrate, and it could only do that 4 bites at a time. But it was amazing that everything was on one chip.
The first microprocessor to make it into a home computer was the Intel 8080, a complete 8- bit computer on one chip, introduced in 1974. The PC market moved from the 8088 to the 80286 to the 80386 to the 80846 to the Pentium to the Pentium II to the Pentium III to the Pentium 4. All of these microprocessors are made by Intel and all of them are improvements on the basic design of the 8088. The Pentium 4 can execute any piece of code that ran on the original 8088, but it does it about 5,000 times faster!
EUVL TECHNOLOGY
In many respects, EUVL retains the look and feel of optical lithography as practiced today. For example, the basic optical design tools that are used for EUV imaging system design and for EUV image simulations are also used today for optical projection lithography. Nonetheless, in order respects EUVL technology is very different from what the industry is familiar with. Most of these differences arise because the properties of materials in the EUV are very different from their properties in the visible and UV ranges.
Foremost among those difference is the fact that EUV radiation is strongly absorbed in virtually all materials, even gases.EUV imaging must be carried out in a near vacuum. Absorption also rules out the use of refractive optical elements, such as lenses and transmission masks. Thus EUVL imaging systems are entirely reflective. Ironically, the EUV reflectivity of individual materials at near-normal incidence is very low. In order to achieve reasonable reflectivity’s near normal incidence, surfaces must be coated with multilayer, thin-film coatings known as distributed Bragg reflectors. The best of these functions in the region between 11 and 14 nm. EUV absorption in standard optical photo resists is very high, and new resist and processing techniques will be required for application in EUVL.
Lithography is one of the key technologies that enable Intel to meet the challenge of Moore’s Law by allowing a 30% decrease in the size of printed dimensions every two years. Intel has been an industry leader in advanced lithography with the early introduction of 248 nm and 193 nm lithography tools into high volume manufacturing. Intel is continuing this trend with strong investment in Extreme Ultraviolet (EUV) research at our Hillsboro, Oregon, and Santa Clara, California, sites.
HOW EUV CHIPMAKING WORKS
For describing the EUV chipmaking process we should have a clear idea of chipmaking process. Both are described in the following sections.
Ultraviolet lithography can produce lines for integrated circuits as small as 39 nm in one recent test. To help sustain Moore’s law and cram more and more gates and memory units into a given space, manufactures of microchips must make the lines in their circuitry ever smaller. This usually means working with a shorter-wavelength light beam for creating the patterns used for inscribing fine features on silicon or metal surfaces. The form of lithography currently in mass production now can produce a half-pitch size (equal lines and spaces in between) of 90 nm and isolated line widths of 65 nm. To produce a later generation after that you would need even shorter wavelengths.
Silicon chips could be made more quickly and cheaply using a new technique developed by physicist in the US. Stephen Chou and colleagues at Princeton University have successfully imprinted patterns onto silicon using quartz moulds instead of the usual combination of lithography and etching. With a resolution of just 10 nm and an ‘imprint time’ of 250 ns, the new process could revolutionize the semiconductor industry – and keep ‘Moore’s Law’ on track for another 25 years.
CHIPMAKING
Lithography’s akin to photography in that it uses light to transfer images onto a substrate. In the case of a camera, the substrate is film. Silicon is the traditional substrate in chipmaking. To create the integrated circuit design that’s on a microprocessor, light is directed onto a mask. A mask is like a stencil of the circuit pattern. The light shines through the mask and then through a series of optical lenses that shrink the image down, this small image is then projected onto a silicon, or semiconductor, wafer.
The wafer is covered with light-sensitive, liquid plastic called photoresist. The mask is placed over the wafer, and when light shines through the mask and hits the silicon wafer, it hardens the photoresist that isn’t covered by the mask. The photoresist that is not exposed to light remains somewhat gooey and is chemically washed away, leaving only the hardened photoresist and exposed silicon wafer.
The key to creating more powerful microprocessors is the size of the light’s wavelength. The shorter the wavelength, the more transistors can be etched onto the silicon wafer. More transistors equal a more powerful, faster microprocessor. That’s the big reason why Intel Pentium 4 processor, which has 42 million transistors, is faster than Pentium 3, which has 28 million transistors.
As of 2001, deep-ultraviolet lithography uses a wavelength of 240 nanometers. A nanometer is one-billionth of a meter. As chipmakers reduce to 100-nanometer wavelengths, they will need a new chipmaking technology. The problem posed by using deep-ultraviolet lithography is that the light’s wavelength get smaller, the light gets absorbed by the glass lenses that are intended to focus it. The result is that the light doesn’t make it to the silicon, so no circuit pattern is created on the wafer.
This is where EUVL will take over. In EUVL, glass lenses will be replaced by mirrors to focus light. In the next section, you will learn just how EUVL will be used to produce chips that are at least five times more powerful than the most powerful chips made in 2001.
The components on a microchip are made by carving patterns into layers of doped and undoped silicon. In the standard technique, light is shone through a stencil onto a silicon wafer that is coated with a light-sensitive polymer known as resist. Chemical etching then removes the regions of silicon coated with either the unexposed or the exposed polymer, until the desired structure is achieved. Finally, the remaining polymer is washed off.
Put such ‘photolithography’ is expensive and complex, and the resolution of the technique is fast approaching the diffraction limit. Thus means that it will not be able to make features much smaller than the current minimum size of about 130 nm – and that the semiconductor industry could soon violate one of its guiding principles. The Moore’s Law, Coined in 1965, the law described how the density of components on a chip doubled every 18 months, and was soon adopted by the semiconductor industry as a target.
THE EUVL PROCESS
Here’s how EUVL works:
1. A laser is directed at a jet of xenon gas. When the laser hits the xenon gas, it heats the gas up and creates plasma.
This source of extreme ultraviolet light is based on plasma created when a laser is focused on a beam of xenon gas clusters expanding at supersonic speeds. (Besides invisible-to-the-eye extreme ultraviolet light, some visible light is also created, as seen in the blue glow in the photo.)
2. Once the plasma is created, electrons begin to come off of it and it radiates light at 13 nanometers, which is too short for the human eye to see.
3. The lights into a condenser, which gathers in the light so that it is directed onto a mask.
4. A representation of one level of a computer chip is patterned onto a mirror by applying an absorber to some parts of the mirror but not to others. This creates the mask.
5. The pattern on the mask is reflected onto a series of four to six curved mirrors, reducing the size of the image and focusing the image onto the silicon wafer. Each mirror bends the light slightly to form the image that will be transferred onto the wafer. This is just like how the lenses in your camera bend light to form an image on film.
The ETS (Engineering Test Stand, also called prototype machine) includes a condenser optics box and a projection optics box. Both boxes house complex optical trains of precision concave and convex spherical mirrors.
The conventional method for making the reflective masks for EUV lithography is called magnetron sputtering. But the defect rate for the process is about 10,000 defects per square centimeter, far too many for successful EUV lithography. The new process, embodied in Veecco’s IBSD-350, produces precise, uniform, highly reflective masks with 81 alternating layers of molybdenum and silicon, each 3 to 4 nanometers thick. As the machine directs a beam of ions at the masks, the ion physically collide with each mask and form a vapor, which is precisely deposited on it at a defect density of less than 0.1 per square centimeter – a 100,000-fold improvement over conventional methods. This process also holds great promise for a number of other applications using virtually any material or combination of materials including metals, semiconductor, and insulators. A near-term possibility is making very-low-defect-density films for ultrahigh-density heads for the magnetic recording industry.
The main role of the condenser optic box is to bring light to the reflective pattern on the mask. “We want to bring as much light to the mask and, ultimately, the wafer, as possible,” explains Sweeney. “The more light we deliver, the shorter the exposure time. It’s life taking a picture with a camera. A picture taken in bright noonday sun requires a shorter exposure time than does a picture of the same scene taken at twilight.”
For the semiconductor industry, brighter EUV images mean shorter exposure times, which translate to manufacturing more chips at a faster rate. The optic design team from Lawrence Livemore and Sandia designed a condenser optic system that collects and transports a significant fration of the EUV light from the source to the reflective mask. Once the image is reflected from the mask, it travels through the projection optic system. According to Sweeney, the projection optic box is the optical heart of the lithographic exposure system. “It is to the system what an engine is to a car,” he explains. The four mirrors of the ETS projection optic system reduce the image and form it onto the wafer. “Again, imagine using a pocket camera. The camera lens transmits an image to the film, which-like the wafer-has a light-sensitive surface,” says Sweeney.
This wafer was patterned on an integrated laboratory research system capable of printing proof-of-principle, functioning microelectronic devices using extreme ultraviolet lithography (EUVL). The EUV lithography research tool was assembled at Sandia National Laboratories in Livermore, Calif., which has joined with two other Department of Energy laboratories – Lawrence Livermore National Laboratory and Lawrence Berkeley National Laboratory – creating a Virtual National Laboratory to help develop EUV lithography for commercial use.
According to Sweeney, Deputy Program leader for extreme Ultraviolet Lithography and Advance Optics. In Lawrence Livermore National laboratory, California, the entire process relies on wavelength. If you make the wavelength short, you get a better image. He says to think in terms of taking a still photo with a camera.
“When you take a photograph of something, the quality of the image depends on a lot of things,” he said. “And the first thing it depends on is the wavelength of the light that you’re using to make the photograph. The shorter the wavelength, the better the image can be. That’s just a law of nature.”
As of 2001, microchips being made with deep-ultraviolet lithography are made with 248-nanometer light. As of May 2001, some manufactures are transitioning over to 193-nanometer light. With EUVL, chips will be made with 13-nanometer light. Based on the law that smaller wavelengths create a better image, 13-nanometer light will increase the quality of the pattern projected on to silicon wafer, thus improving microprocessor speeds. This entire process has to take place in a vacuum because these wavelengths of light are so short that even air would absorb them. Additionally, EUVL uses concave and convex mirrors coated with multiple layers of molybdenum and silicon – this coating can reflect nearly 70 percent of EUV light at a wavelength of 13.4 nanometers. The other 30 percent is absorbed by the mirror. Without the coating, the light would be almost totally absorbed before reaching wafer. The mirror surfaces have to be nearly perfect; even small defects in coatings can destroy the shape of the optics and distort the printed circuit pattern, causing problems in chip function. Hence Before new lithography tools are even built, Chip makers must develop and demonstrate the necessary mask making capabilities.
CONCLUSION
Extreme Ultraviolet Lithography (EUVL) will open a new chapter in semiconductor technology. In the race to provide the Next Generation Lithography (NGL) for faster, more efficient computer chips, EUV Lithography is the clear frontrunner. At EUV Technology,
Successful implementation of EUVL would enable projection photolithography to remain the semiconductor industry’s patterning technology of choice for years to come. However, much work remains to be done in order to determine whether or not EUVL will ever be ready for the production line. Furthermore, the time scale during which EUVL, and in fact any NGL technology, has to prove itself is somewhat uncertain.
Several years ago, it was assumed that an NGL would be needed by around 2005 in order to implement the 0.1 um generation of chips. Currently, industry consensus is that 193nm lithography will have to do the job, even though it will be difficult to do so. There has recently emerged talk of using light at 157 nm to push the current optical technology even further, which would further postpone the entry point for an NGL technology. It thus becomes crucial for any potential NGL to be able to address the printing of feature sizes of 50 nm and smaller! EUVL does have that capability.
