extreme ultraviolet lithography full report
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Silicon has been the heart of the world's technology boom for nearly half a century, but microprocessor manufacturers have all but squeezed the life out of it. The current technology used to make microprocessors will begin to reach its limit around 2006. At that time, chipmakers will have to look to other technologies to cram more transistors onto silicon to create more 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.
Potential successors to optical project and implimentationion lithography are being aggressively developed. These are known as "Next-Generation Lithographies" (NGL's). EUV lithography (EUVL) is one of the leading NGL technologies; others include x-ray lithography, ion-beam project and implimentationion lithography, and electron-beam project and implimentationion lithography. 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.
Extreme ultraviolet lithography (EUVL) is an advanced technology for making microprocessors a hundred times more powerful than those made today.
EUVL is one technology vying to replace the optical lithography used to make today's microcircuits. It works by burning 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 onto the wafer. However, extreme ultraviolet light, operating at a different wavelength, has different properties and must be reflected from mirrors rather than refracted through lenses. The challenge is to build mirrors perfect enough to reflect the light with sufficient precision
2.1 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 radiation. Current lithography techniques have been pushed just about as far as they can go. They use light in the 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, manufacturers plan to make chips with features measuring from 100 to 70 nanometers, using deep ultraviolet light of 193- and 157-nanometer wavelengths. Beyond that point, smaller features require wavelengths in the extreme ultraviolet (EUV) range. Light at these wavelengths is absorbed instead of transmitted by conventional lenses
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 of stencil of an integrated circuit pattern-and the image of that pattern is then project and implimentationed onto a semiconductor wafer covered with light-sensitive photoresist. Creating circuits with smaller and smaller features has required using shorter and shorter wavelengths of light.
3. 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. Manufacturers are concerned that this 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 etched 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.
Processors built using EUV technology are expected to reach speeds of up to 10 GHz in 2005-2006. By comparison, the fastest Pentium 4 processor today is 1.5 GHz.
3.1 MOOREâ„¢S LAW
Each year, manufacturers 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. 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 became 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 chipmaking 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 10-gigahertz (GHz) microprocessor by 2007. By comparison, the fastest 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 sizes that are small enough to support 10 GHz clock speed. It doesn't necessarily make it happen," Don Sweeney, 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.
3.2 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 to 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 transistors than the chip of 1971.
A microprocessor -- also known as a CPU or central processing unit -- is a complete computation engine that is fabricated on a single chip. The first microprocessor was the Intel 4004, introduced in 1971. The 4004 was not very powerful -- all it could do was add and subtract, and it could only do that 4 bits 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 80486 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!
A microprocessor sometimes called a logic chip. It is the "engine" that goes into motion when you turn your computer on. A microprocessor is designed to perform arithmetic and logic operations that make use of small number-holding areas called registers. Typical microprocessor operations include adding, subtracting, comparing two numbers, and fetching numbers from one area to another. These operations are the result of a set of instructions that are part of the microprocessor design. When the computer is turned on, the microprocessor is designed to get the first instruction from the basic input/output system (BIOS) that comes with the computer as part of its memory. After that, either the BIOS, or the operating system that BIOS loads into computer memory, or an application program is "driving" the microprocessor, giving it instructions to perform. The following table helps you to understand the differences between the different processors that Intel has introduced over the years.
Name Date Transistors Microns Clock speed Data width MIPS
8080 1974 6,000 6 2 MHz 8 bits 0.64
8088 1979 29,000 3 5 MHz 16 bits
8-bit bus 0.33
80286 1982 134,000 1.5 6 MHz 16 bits 1
80386 1985 275,000 1.5 16 MHz 32 bits 5
80486 1989 1,200,000 1 25 MHz 32 bits 20
Pentium 1993 3,100,000 0.8 60 MHz 32 bits
64-bit bus 100
Pentium II 1997 7,500,000 0.35 233 MHz 32 bits
64-bit bus ~300
Pentium III 1999 9,500,000 0.25 450 MHz 32 bits
64-bit bus ~510
Pentium 4 2000 42,000,000 0.18 1.5 GHz 32 bits
64-bit bus ~1,700
4. 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 project and implimentationion lithography. Nonetheless, in other 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 differences 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 reflectivityâ„¢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 photoresists 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 investments in Extreme Ultraviolet (EUV) research at our Hillsboro, Oregon, and Santa Clara, California, sites.
The completion of the prototype machine (Engineering Test Stand) marks a major milestone for the program, since we have proven that EUV lithography works," said Chuck Gwyn, program manager of the EUV Limited Liability Company. "Our next step is to transfer the technology to lithography equipment manufacturers to develop beta and production tools."
Processors built using EUV technology are expected to reach speeds of up to 10 GHz in 2005-2006. By comparison, the fastest Pentium 4 processor today is 1.5 GHz. The prototype machine, called the Engineering Test Stand, was developed by industry-government collaboration among three U.S. Department of Energy national laboratories and a consortium of semiconductor companies called the EUV LLC. The consortium includes Intel Corporation, Motorola Inc., Advanced Micro Devices Inc., Micron Technology Inc., Infineon Technologies, and International The ETS was assembled at Sandia in Livermore, Calif. It will be used by LLC partners and lithography tool suppliers during the next year to refine the technology and get it ready to create a prototype commercial machine that meets industry requirements for high-volume chip production. The EUV LLC has developed relationships with more than 40 U.S.-based infrastructure companies to ensure that all of the key components can be attained for commercialization Business Machines
5. HOW EUV CHIPMAKING WORK
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, manufacturers 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 physicists 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
Lithography is 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 used 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 project and implimentationed onto a silicon, or semiconductor, wafer.
The wafer is covered with a 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 equals a more powerful, faster microprocessor. That's the big reason why an Intel Pentium 4 processor, which has 42 million transistors, is faster than the 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 as the light's wavelengths 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 a 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.
But such 'photolithography' is expensive and complex, and the resolution of the technique is fast approaching the diffraction limit. This 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, this 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.
Now Moore's Law could be back on track. Chou and co-workers say that their technique - known as laser-assisted direct imprint - can create features as small as 10 nm on silicon wafers. The new process also eliminates the need for the resist and washing steps
5.2 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 a 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.
Engineering Test Stand
3. The light travels into a condenser, which gathers in the light so that it is directed onto the 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 project and implimentationion 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 Veeco'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 ions 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, semiconductors, 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 optics 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 like 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 optics design team from Lawrence Livermore and Sandia designed a condenser optics system that collects and transports a significant fraction of the EUV light from the source to the reflective mask.
Once the image is reflected from the mask, it travels through the project and implimentationion optics system. According to Sweeney, the project and implimentationion optics 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 project and implimentationion optics 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.
The optics teams are now working on advanced designs for the project and implimentationion optics. They have a six-mirror design that promises to extend EUVL systems so that they can print features as small as 30 nanometers- a significant jump from the 70-nanometer limit of the ETS. According to Sweeney, extendability to smaller features is an important requirement for whatever lithographic technology the semiconductor industry finally decides
This wafer was patterned on a prototype device using extreme-ultraviolet lithography (EUVL).
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 Advanced 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 manufacturers 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 project and implimentationed onto a 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 the 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.
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 project and implimentationion 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.
I would like to place on record my deep sense of gratitude to Mr.PURUSHOTHAMAN Head of Department of Electronics & communication, Vimal Jyothi Engineering College for his valuable help and guidance in carrying out the seminar and presentation.
I also thank all the staff of The Department Electronics & Communication for their assistance and encouragement through out the course of the seminar and presentation.
Last, but not the least I would like to thank my parents and friends who encouraged me and gave me the motivation to complete the seminar and presentation.
Above all I would like to thank God for His abundant grace upon my seminar and presentation.
Silicon has been the heart of the world's technology boom for nearly half a century. Each year, manufacturers 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 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. Here we discusses the basic concepts 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 project and implimentationion imaging. EUVL is one technology vying to become the successor to optical lithography.
2. EUVL DEFINITION
2.1 EUV RADIATION
3. WHY EUVL?
3.1 MOOREâ„¢S LAW
3.2 INCREDIBLE SHRINKING CHIPS
4. EUVL TECHNOLOGY
5. HOW EUV CHIPMAKING WORK
5.1 MAKING CHIPS
5.2 THE EUVL PROCESS
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extreme ultraviolet lithography Presentation1.ppt (Size: 3.01 MB / Downloads: 298)
Lithography is akin to photography in that it uses light to transfer images onto a substrate.
Silicon is the traditional substrate used in chip making.
The key to create more powerful Microprocessors is the size of the lights wave lengths.
Shorter the wavelength the more transistors can be etched on to the Silicon wafer.
Lithography uses a wavelength of 240nm.
EUVL glass lenses will be replaced by mirrors.
Use smaller wavelength.
To reduce the wavelength of light used for imaging.
To design imaging system with ever layer numerical aperture.
Reason for changes
a) Resolution (RES)
b) Depth of focus (DOF)
Better resolution achieved by reducing and NA.
Different technology from what is familiar.
Properties of materials in EUVL are different from properties in visible and UV rays.
EUVL imaging are entirely reflective.
Reflectivity in near normal incidence is very low .
A laser is directed as a jet of xenon gas. When laser hits the Xenon gas it heats up and creates a plasma.
Once is created , electron begins to come off and radiates light at 13nm.
Light travels to condenser and is directed to the mask.
Pattern on the mask is reflected on to the series of four to six curved mirrors , reducing the size of the image and focusing the image onto the silicon wafer.
Inorder to achive reflectivities the reflecting surface are coatd with multi layer thin films
Resonance reflectivity approximately /2
2. EUV Cameras
Composed of 4mirrors
2types of euv cameras are1. small field micro setter like desidn known â„¢10x camers 2.proto type lithography camera
3. EUV masks
NO defects are ever allowed in a completed mask
Extremely flat and defect-free substrate, perfected by smoothing layer
All defects in multilayer reflecting stack must be completely repaired
No defects allowed in absorber layer
All defects in final absorber pattern must be completely repaired
(No wonder mask sets are so expensive!)
SOURCES OF EUV RADIATION
Laser produced plasmas.
The undulators associated with synchrotrons.
Development of discharging sources.
Strong absorption of EUV radiation.
Printing occurs in a very thin imaging layer.
Resist types are:
Silylated single-layer resist.
Refractory bilayer resists.
Successful resist must possesses excellent etch resistance.
EUVL leverages much of the learning and supplier infrastructure established for conventional lithography.
EUVL technology achieves good depth of focus and linearity for both dense and isolated lines with low NA systems without OPC.
The robust4X masks are patterned using standard mask writing and repair tools and similar inspection methods can be used as for conventional optical masks.
The low thermal expansion substrates provide good critical dimension control and image placement.
Experiments have shown that existing DUV can be extended for use with EUV.
FUTURE OF EUVL
Projection photolithography to remain the semiconductor industry's.
Demonstrated in a full-field proof of Concept .
There are no known showstoppers that will prevent EUVL from
becoming a manufacturing reality.
Projection photolithography to remain the semiconductor industry's.
Work to determine EUVL will ever be ready for the production line.
Consensus is that 193 nm lithography will have to do the job.
Became crucial for any potential NGL to be able to address the printing of feature.
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This paper discusses the basic concepts and current state of development of EUV lithography (EUVL), a relatively new form lithography that uses extreme ultraviolet (EUV) radiation with a wavelength in the range of 10 to 14 nanometer (nm) to carry out project and implimentationion imaging. Currently, and for the last several decades, optical project and implimentationion lithography has been the lithographic technique used in the high-volume manufacture of integrated circuits. It is widely anticipated that improvements in this technology will allow it to remain the semiconductor industryâ„¢s workhorse through the 100 nm generation of devices. However, some time around the year 2005, so-called Next-Generation Lithographies will be required. EUVL is one such technology vying to become the successor to optical lithography. This paper provides an over view of the capabilities of EUVL, and explains how EUVL might be implemented. The challenges that must be overcome in order for EUVL to qualify for high-volume manufacture are also discussed.
Optical project and implimentationion lithography is the technology used to print intricate patterns that define integrated circuits onto semiconductor wafers. Typically, a pattern on a mask is imaged, with a reduction of 4:1, by a highly accurate camera onto a silicon wafer coated with photo resist. Continued improvements in optical project and implimentationion lithography have enabled the printing of ever finer features, the smallest feature size decreasing by about 30% every two years. This, in turn, has allowed the integrated circuit industry to produce ever more powerful and cost-effective semiconductor devices. On average, the number of transistors in a state-of-the â€œart integrated circuit has doubled every 18 months.
Currently, the most advanced lithographic tools used in high-volume manufacture employ deep-ultraviolet (DUV) radiation with a wavelength of 248 nm to print features that have line widths as small as 200 nm. It is believed that new DUV tools, presently in advanced development, that employ radiation that has a wavelength of 193 nm, will enable optical lithography to print features as small as 100 nm, but only with very great difficulty for high volume manufacture. Over the next several years it will be necessary for the semiconductor industry to identify a new lithographic technology that will carry it into the future, eventually enabling the printing of lines as small as 30 nm. Potential successors to optical project and implimentationion lithography are being aggressively developed. These are known as Next-Generation Lithographies (NGLâ„¢s). EUV lithography (EUVL) is one of the leading NGL technologies; others include X-Ray lithography, ion beam project and implimentationion lithography, and electron-beam project and implimentationion lithography.
In many respects, EUVL may be viewed as a natural extension of optical project and implimentationion lithography since it uses short wavelength radiation (light) to carry out project and implimentationion imaging. In spite of this similarity, there are major differences between the two technologies. Most of these differences occur because the properties of materials in the EUV portion of the electromagnetic spectrum are very different from those in the visible and UV wavelength ranges. The purpose of this paper is to explain what EUVL is and why it is of interest, to describe the current status of its development, and to provide the reader with an understanding of the challenges that must be overcome if EUVL is to fulfill its promise in high volume manufacture.
In order to keep pace with the demand for the printing of ever smaller features, lithography tool manufacturers have found it necessary to gradually reduce the wavelength of the light used for imaging and to design imaging systems with ever larger numerical apertures. The reasons for these changes can be understood from the following equations that describe two of the most fundamental characteristics of an imaging system: its resolution (RES) and depth of focus (DOF). These equations are usually expressed as
RES = k1 / NA (1a)
DOF = k2 / (NA)2 (1b)
Where is the wavelength of the radiation used to carry out the imaging, and NA is the numerical aperture of the imaging system (or camera). These equations show that better resolution can be achieved by reducing and increasing NA. The penalty for doing this, however, is that the DOF is decreased. Until recently, the DOF used in manufacturing exceeded 0.5 m, which provided for sufficient process control.
The case k1 = k2 = Ã‚Â½ corresponds to the usual definition of diffraction limited imaging. In practice, however, the acceptable values for k1 and k2 are determined experimentally and are those values which yield the desired control of critical dimensions (CDâ„¢s) within a tolerable process window. Camera performance has a major impact on determining these values; other factors that have nothing to do with the camera also play a role. Such factors include the contrast of the resist being used and the characteristics of any etching processes used. Historically, values for k1 and k2 greater than 0.6 have been used comfortably in high - volume manufacture. Recently, however, it has been necessary to extend imaging technologies to ever better resolution by using smaller values for k1 and k2 and by accepting the need for tighter process control. This scenario is schematically diagrammed in Figure 1, where the values for k1 and DOF associated with lithography using light at 248 nm and 193 nm to print past, present, and future CDâ„¢s ranging from 350 nm to 100 nm are shown. The Comfort Zone for Manufacture corresponds to the region for which k1 > 0.6 and DOF > 0.5 m. Also shown are the k1 and DOF values currently associated with the EUVL printing of 100 nm features, which will be explained later. As shown in the figure, in the very near future it will be necessary to utilize k1 values that are considerably less than 0.5. Problems associated with small k1 values include a large iso / dense bias (different conditions needed for the proper printing of isolated and dense features), poor CD control, nonlinear printing (different conditions needed for the proper printing of large and small features), and magnification of mask CD errors. Figure 1 also shows that the DOF values associated with future lithography will be uncomfortably small. Of course, resolution enhancement techniques such as phase â€œ shift masks, modified illumination schemes, and optical proximity correction can be used to enhance resolution while increasing the effective DOF. However, these techniques are not generally applicable to all feature geometries and are difficult to implement in manufacturing. The degree to which these techniques can be employed in
manufacturing will determine how far optical lithography can be extended before an NGL is needed.
Figure 1: The k1 and DOF values associated with 248 nm and 193 nm lithographies for the printing of CD values ranging from 350 nm down to 100 nm assuming that k2 = k1 and NA = 0.6
EUVL alleviates the foregoing problems by drastically decreasing the wavelength used to carry out imaging. Consider Figure 2. The dashed black line shows the locus of points corresponding to a resolution of 100 nm; the region to the right of the line corresponds to even better resolution.
Figure 2: The region between the lines shows the wavelength and numerical aperture of cameras simultaneously having a resolution of 100 nm or better and a DOF of 0.5 m or better
The solid red line shows the locus of points for which the DOF is 0.5 m; in the region to the left of that line the DOF values are larger. Points in the region between the two lines corresponds to situations in which the resolution is 100 nm or better, and the DOF is 0.5 m or longer. As shown to be in this favorable region, the wavelength of the light used for imaging must be less than 40 nm, and the NA of the imaging system must be less than 0.2. The solid circle shows the parameters used in current imaging experiments. Light having the parameters used in current imaging experiments. Light having wavelengths in the spectral region from 40 nm to 1 nm is variously referred to as extreme uv, vaccum uv, or soft x-ray radiation. Projection lithography carried out with light in this region has come to be known as EUV lithography (EUVL). Early in the development of EUVL, the technology was called x-ray project and implimentationion lithography (SXPL), but that name
was dropped in order to avoid confusion with x-ray lithography, which is a 1: 1, near contact printing technology.
As explained above, EUVL is capable of printing features of 100 nm and smaller while achieving a DOF of 0.5 m and larger. Currently, most EUVL work is carried out in a wavelength region around 13 nm using cameras that have an NA of about 0.1, which places the technology well within the Comfort Zone for Manufacture as shown in Figure 1 by the data point farthest to the right.
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 project and implimentationion lithography. Nonetheless, in other respects optical 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 differences 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 reflectivities near normal incidence, surfaces must be coated with multi layer, thin film coatings known as distributed Bragg reflectors. The best of these function in the region between 11 and 14 nm. EUV absorption in standard optical photoresists is very high, and new resist and processing techniques will be required for application in EUVL.
Because EUVL utilizes short wavelength radiation for imaging, the mirrors that comprise the camera will be required to exhibit an unprecedented degree of perfection in surface figure and surface finish in order to achieve diffraction-limited imaging. Fabrication of mirrors exhibiting such perfection will require new and more accurate polishing and metrology techniques.
Clearly, then, there are a number of new technology problems that arise specifically because of the use of EUV radiation. Intel has formed a consortium called the EUV, LLC (The LLC), which currently also includes AMD and Motorola, to support development of these EUV-specific technologies. The bulk of this development work is carried out by 3 national laboratories functioning as a single entity called the Virtual National Laboratory (VNL). Participants in the VNL are Lawrence Livermore National laboratory, Sandia National laboratories and Lawrence Berkeley National Laboratory. Development work is also carried out by LLC members, primarily on mask fabrication and photoresist development. Recently, additional support for some of this work has come from Sematech. The work described in the following sections was carried out with in this program, primarily by workers within the VNL.
In order to achieve reasonable reflectivities, the reflecting surfaces in EUVL imaging systems are coated with multilayer thin films (MLâ„¢s). These coatings consist of a large number of alternating layers of materials having dissimilar EUV optical constants, and they provide a resonant reflectivity when the period of the layers is approximately Without such reflectors, EUVL would not be possible. On the other hand, the resonant behaviour of MLâ„¢s complicates the design, analysis, and fabrication of EUV cameras. The most developed and best understood EUV
multilayers are made of alternating layers of Mo and Si, and they function best for wavelengths of about 13 nm. Figure 3 shows the reflectivity and phase change upon reflection for an Mo:Si ML that has been optimized for peak reflectivity at 13.4 nm at normal incidence; similar resonance behaviour is seen as a function of angle of incidence for a fixed wavelength. While the curve shown is theoretical, peak reflectivities of 68% can now be routinely attained for Mo:Si MLâ„¢s deposited by magnetron sputtering.
Figure 3: Curve showing the normal incidence reflectivity and phase upon reflection of an Mo : Si ML as a function of wavelength; the coating was designed to have peak reflectivity at 13.4nm.
This resonance behaviour has important implications for EUVL. A typical EUVL camera is composed of at least four mirrors, and light falls onto the various mirrors over different angular ranges. As a consequence, the periods of the MLâ„¢s applied to the various mirrors must be different so that all the mirrors are tuned to reflect the same wavelength. Proper matching of the peak wavelengths is crucial for achieving high radiation through put and good imaging performance. The range of angles of incidence over a single mirror surface must also be considered. For some optical designs, the angular ranges are small enough that MLâ„¢s with a uniform period over the surface can be used. In other designs, the angular ranges are so large that the ML
period must be accurately varied over the surface in order to achieve uniform reflectivity. There are optical designs in which the angular ranges are so large that ML reflectors cannot be utilized.
The effects of imaging performance due to the variations of ML reflectivity and phase with wavelength and angle have been extensively modeled. The effects have been shown to be minimal for cameras of interest to us. The primary perturbations of the wave front transmitted by the camera are described as a simple tilt and defocus.
In our work we are fabricating two types of EUV cameras. The first is a small field, micro stepper â€œ like design that utilizes two mirrors and that images with a reduction factor of 10. We call it the 10X camera. This camera has been used extensively in our early investigations of EUV imaging. One of the mirrors in this camera requires a strongly graded ML coating. Three of these cameras have been fabricated and have been shown to perform well. (Examples of the imaging performance of these cameras are shown later in this paper). The second camera, currently being fabricated, is a prototype lithography camera with a ring field of 26 mm X 1.5mm. This camera was designed so that it will perform well with uniform ML coatings. The VNL has demonstrated the ability to achieve the ML matching, uniformity, and grading requirements of EUVL cameras currently of interest.
Designing an all â€œ reflective camera that achieves lithographic quality imaging is more difficult than designing refractive imaging system because mirrors have fewer degrees of freedom to vary than to lenses. As a result, most of the mirrors in a EUVL camera will have aspheric surfaces.
The detailed reasoning that leads to this conclusion was first discussed in 1990.
A schematic of a four mirror camera that the VNL is in the process of fabricating is shown in figure 4. The mirror segments shown in blue are the pieces actually being fabricated, while the full, on â€œ axis parent mirrors are shown in red. This camera will become part of an engineering test stand, so it is called the ETS camera.
Figure 4: Schematic diagram of the 4 mirror ETS camera
It has an NA = 0.1 and is designed to be used with Mo:Si MLâ„¢s at a wavelength of 13.4 nm. Mirror 3 is spherical, and the other three mirrors are aspheres. Some of the most important features of this camera are as follows:
Â¢ Its resolution is better than 100 nm over a 26 mm x 1.5 mm, ring shaped field.
Â¢ Its images with a reduction factor of 4
Â¢ The departures of the aspheres from a best fit sphere are less than 10 m
The camera is intended for use in a step and scan lithography system. In actual operation, the mask and wafer are simultaneously scanned in opposite directions, with the mask moving four times faster than the wafer, as done in current DUV step-and-scan systems. The design of this camera has been optimized so that the effective distortion when scanning (about 1 nm) is considerably less than the distortion obtained for static printing (15 nm).
Because short wavelength radiation is used to carry out the imaging, the surfaces of the mirrors are required to exhibit unprecedented perfection. Ion order to achieve diffraction-limited imaging at 13.4 nm, the root-mean-square (rms) wave front error of type camera must be less than 1 nm. Assuming that the surface errors on the mirrors are randomly distributed, this means that the surface figure (basic shape) of each mirror must be accurate to 0.25 nm (2.5 angstroms ) rms, or better. Until recently, achieving this kind of surface figure accuracy was out of the question, even for spheres. Furthermore, aspheres are much more difficult to fabricate than spheres.
The figure of a surface refers to its basic shape. Stringent requirements must also be placed on the roughness of the surfaces. For our purposes, we define surface figure errors as those errors that have a spatial wavelength scale of 1 mm or longer; such errors are typically measured deterministically using instruments such as interferometers. We define surface roughness as surface errors with a spatial wavelength scale shorter than 1 mm. Typically such surface errors are described and measured statistically. We define roughness with wavelengths in the range of 1 mm through 1 m as mid-spatial frequency roughness (MSFR). Roughness in this frequency range causes small-angle scattering of light off the mirror surfaces. This scattering causes a reduction in the contrast of images because it scatters light from bright regions of the image plane onto regions intended to be dark. This scattering is often called flare. Because the effects of scatter scale as 1/2, the deleterious effects of flare are becoming more evident as the
wavelengths used for lithography continue to be reduced. For a given surface roughness, the amount of scattering at 13.4 nm is approximately 340 times larger than that at 248 nm. In order to keep flare to manageable levels in EUVL, the MSFR must be 0.2 nm rms, or less. Until recently, even the best surfaces exhibited MSFR of 0.7 nm rms. Roughness with spatial wavelengths less than 1 m is called high-spatial-frequency roughness (HSFR), and it causes large angle scattering off the mirrors. Light scattered at such angles is typically scattered out of the image field and represents a loss mechanism for light. We require HSFR to be less than 0.1 nm rms. Optical fabricators have for some time been able to use super polishing techniques to produce surfaces with HSFR even better than this. A well-polished silicon wafer also exhibits such HSFR.
The challenge for a fabricator of optics for EUVL is to achieve the desired levels of figure accuracy and surface roughness simultaneously. As a measure of the progress that has been made, the first copy of Mirror 3 has been completed, and its surface has been measured and found to have the following characteristics.
Â¢ Surface figure: 0.44 nm rms
Â¢ MSFR: 0.31 nm rms
Â¢ HSFR: 0.14 nm rms
This result demonstrates excellent progress towards the surface specifications that we need to achieve.
The progress made in optics fabrication described above could not have been achieved without access to appropriate metrology tools. Some of the required tools were recently developed by workers within the VNL.
Two very significant advances have been made in the measurement of figure. Previous to these advances, no tools existed that could measure figure to the accuracy we require. The first of these innovations is the Sommargren interferometer, which uses visible light to achieve unprecedented accuracy. In this version of a point-diffraction interferometer, the wave front to be measured is compared with a highly accurate spherical wave generated by an optical fiber or by an accurate, small pinhole. Interferogram stitching algorithms have been developed that allow aspheric surfaces to be measured without the need for null optics, which are typically the weak link in such measurements. An accuracy of 0.25 nm rms has already been demonstrated, and an engineering path exists for improvements down to one half that value. Four versions of the interferometer have been supplied to the optics manufacturer for use in the fabrication of the four individual mirrors of the ETS camera. The interferometer can also be configured to measure the wave front quality of an assembled camera. However, visible light does not interact with ML reflectors in the same manner as EUV light. Thus it is of great importance to be able to characterize a EUV camera using light at the wavelength of intended operation. To this end, and EUV interferometer has been developed which will be used to characterize the wave front quality of assembled EUV cameras and to guide final adjustments of the camera alignment. This system has been shown to have an innate rms accuracy of better than 0.003 waves at the EUV wavelength Its accuracy is far better than needed to qualify an EUV camera as diffraction â€œ limited.
Several commercial instruments have been used to measure surface roughness. An interference microscope was used to measure MSFR, and an atomic force microscope (AFM) was used to measure HSFR. The relevance of these measurements was verified by making detailed precision measurements of the magnitude and angular dependence of EUV scattering off of surfaces characterized with the other instruments. Excellent agreement
has been obtained between the direct scattering measurements and the predictions based on the measurements of MSFR and HSFR.
EUVL masks are reflective, not transmissive. They consist of a patterned absorber of EUV radiation placed on top of an ML reflector deposited on a robust and solid substrate, such as a silicon wafer. Membrane masks are not required. The reflectance spectrum of the mask must be matched to that of the ML-coated mirrors in the camera. It is anticipated that EUVL masks will be fabricated using processing techniques that are standard in semiconductor production. Because a 4:1 reduction is used in the imaging. The size and placement accuracy of the features on the mask are achieved relatively easily.
Nonetheless, there are a number of serious concerns about mask development. The foremost is the fact that there is no known method for repairing defects in an ML coating. Since masks must be free of defects, a technique must be developed for depositing defect-free ML reflectors. Defect densities in coatings produced by magnetron sputtering have been found to be adequate for camera mirrors, but far too high for mask blanks. As for camera mirrors, but far too high for mask blanks. As a result, a much cleaner deposition system that uses ion-beam sputtering has been constructed. A reduction of about 1000 in the density of defects larger than 130 nm, to a level of better than 0.1/cm2, has been obtained with this system, but further improvement will certainly be required. Present defect detection techniques use visible light, and it is all but certain that the density of defects printable with EUV light is higher. Defects can take the form of amplitude or phase perturbations, and the proper tools for detecting EUV printable defects are currently being developed. Initially it will be necessary to inspect the mask
blanks using EUV radiation. In the long run, it is hoped that experience will show that adequate inspection can be carried out with commercially available visible-light and e-beam inspection tools. Finally, in current practice, pellicles are used to protect masks from contamination. The use of pellicles in EUVL will not be possible because of the undesirable absorption that would be encountered. Other methods for protecting EUV masks are under development.
Source of EUV Radiation
A number of sources of EUV radiation have been used to date in the development of EUVL. Radiation has been obtained from a variety of laser-produced plasmas and from the bending magnets and the undulators associated with synchrotrons. The work has used a succession of continually improved laser-produced plasma sources. Work is also being done on the development of discharge sources that might be able to provide adequate power in the desired wavelength range. Eventually a source will be required that reliably provides sufficient power to yield adequate wafer throughput in a manufacturing tool.
The main problem to be confronted in developing a satisfactory photoresist for EUV radiation is its absorption by all materials. The absorption depth in standard organic resists used today is less than 100 nm. EUV resists will most likely be structured so that printing occurs in a very thin imaging layer at the surface of the resist. Resist types being actively worked on include silylated single-layer resists, refractory bi-layer resists, and tri-layer resists. A resist acceptable for high volume manufacture must
exhibit high contrast for printing in combination with a sensitivity that will yield an acceptable throughput.A sensitivity of 10 mJ/cm2 is our goal since it represents a good compromise between the need for high throughput and the desire to minimize the statistical fluctuations due to photon shot noise. Of course, a successful resist must also possess excellent etch resistance. As the features printed in resist have continued to shrink, the roughness at the edges of resist lines has begun to a serious problem for all lithographies. While not strictly a EUVL problem, a successful EUV resist will be required to solve the line edge roughness (LER) problem.
The imaging experiments to date have been carried out using the 10X EUVL micro stepper. These experiments have allowed us to evaluate the EUV imaging performance of the camera and to relate it to the measured surface figure and surface roughness of its mirrors. The imaging performance also correlated well with the camera wave front as measured directly with the EUV interferometer. Additionally, these experiments have been used to investigate various resists and masks and to help us understand a number of system issues. Three cameras have been built for this system, all of which image with a 10X reduction. The camera itself is a simple Schwarzschild design and is comprised of two spherical mirrors. A schematic diagram of this camera is shown in Figure 5. As shown in the lower part of the figure, we used off-axis portions of the full mirrors to avoid obscuration of the light by the mirrors; the NA used was 0.07 or 0.08.
The cameras were originally aligned using visible interferometry. Subsequent EUV interferometry revealed that the at-wavelength measurements yielded nearly identical results. Not all camera designs allow for alignment with visible light
Figure 6 shows the cross-sectioned profiles of dense lines and spaces printed in resist with the 10X camera. The figure snows resist profiles of lines and spaces with widths of 200 nm, 150 nm, and 100 nm. As can be seen, the resist profiles are well defined. From a series of measurements like this it is possible to demonstrate the excellent linearity of the printing.
That is, the width of the resist image is equal to the intended size as written on the mask. Figure 7 demonstrates excellent linearity for dense lines and spaces from a line width of 250 nm down to 80 nm.
Exposures such as the above can also be used to demonstrate the large DOF inherent in EUVL. Figure 8 presents the data from such a series of exposures: it shows how the line width of a 130 nm line (the remaining resist) varies as the camera image is defocused on the wafer. As seen, the line width only changes by about 5% as the wafer is moved from best focus to a position 2 m away from best focus. This observation is in reasonable agreement with the behavior predicted by Equation 1. In manufacturing of high â€œperformance ICâ„¢s, it is desired to control the critical line widths to +/- 10% or better.
Finally, in figure 9, we show cross-sectioned resist image of 80 nm lines and spaces (with a line space ratio of 1:2). This demonstrates the resolving power of the 10X camera and our ability to print such fine features in resist.
While the 10X camera has been of great use in our program, we look forward to the completion of the ETS camera so that we can explore EUV imaging with a camera of the kind needed for production-type lithography.
Successful implementation of EUVL would enable project and implimentationion photography to remain the semiconductor industryâ„¢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 and any NGL technology, has to prove itself is somewhat uncertain. Several years ago, it was assumed that an NGL would be needed by around 2006 in order to implement the 0.1 m generation of chips. Currently, industry consensus is that 193 nm 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.
The battle to develop the technology that will become the successor to 193 nm lithography is heating up, and it should be interesting to watch.
For readers interested in digging deeper, I recommend the following sources:
1. For a compilation of papers on EUVL see OSA Trends in Optics and Photonics Vol.4, Extreme Ultraviolet Lithography, G.D. Kubiak and D.R. Kania, eds. (Optical Society of America, Washington, Dc 1996).
2. For recent papers on the various NGLâ„¢s and on optical lithography see J. Vac. Sci. Technol. B15, Nov./Dec 1997.
3. T.E. Jewell , J. M. Rodgers, and K.P. Thompson, J. Vac. Sci. Technol. B8, 1509 (1990).
4. G.E Sommargren, Laser Focus World 32, 61 (1996).
5. E. Tejnmil, et al., J Vac. Sci. Technol. B15, 2455 (1997).
Â¢ Introduction 2
Â¢ Why EUVL 3
Â¢ EUVL Technology 7
Â¢ Multilayer Reflectors 8
Â¢ EUV Cameras 10
Â¢ Metrology 14
Â¢ Masks 15
Â¢ Source of EUV Radiation 16
Â¢ Resists 17
Â¢ Experimental Results 17
Â¢ Conclusion 22
Â¢ References 23
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