EXTREME ULTRAVIOLET LITHOGRAPHY
CHAPTER 1
INTRODUCTION
Microprocessors, also called computer chips, are made using a process called lithography. Specifically, deep-ultraviolet lithography is used to make the current breed of microchips and was most likely used to make the chip that is inside your computer.
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. 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 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 equal a more powerful, faster microprocessor.
Deep-ultraviolet lithography uses a wavelength of 240 nanometers As chipmakers reduce to smaller wavelengths, they will need a new chip making 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(Extreme Ultraviolet Lithogrphy) will take over. In EUVL, glass lenses will be replaced by mirrors to focus light and thus EUV lithography can make use of smaller wave lengths. Hence more and more transistors can be packed into the chip. The result is that using EUV lithography, we can make chips that are upto 100 times faster than today’s chips with similar increase in storage capacity.
Lithography is the most challenging technology in the semiconductor industry. The most promising next generation lithography technology is extreme ultraviolet lithography (EUVL). EUVL was proposed long ago, in 1988, but its implementation has been postponed several times. Presently, most “showstoppers” are gone, but there are still several challenges that need to be addressed. The semiconductor industry is now getting ready to use EUVL in a pre-production phase, and EUVL might be implemented for 32 nm and 22 nm technological nodes. High volume manufacturing EUVL printers will be delivered to multiple end-users from 2010..
In many respects, EUVL may be viewed as a natural extension of optical projection lithography since it uses short wavelength radiation (light) to carry out projection 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.
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 projection 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, ionbeam projection lithography, and electron-beam projection lithography.
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EXTREME ULTRAVIOLET LITHOGRAPHY
CHAPTER 2
HISTORICAL BACKGROUND
Nearly all of today’s electronic devices rely on key internal semiconductor components, known as integrated circuits (ICs). ICs are manufactured through a critical process known as lithography, which is the determining factor in keeping pace with the quest of the electronics industry to shrink ICs and other related products even more.
Lithography is a patterning method that creates an IC layout on a resist layer of a silicon wafer or other semiconducting substrate. It mainly consists of three parts: a) the pattern printer, b) photoresist technology, and c) the mask fabrication.
Lithography technology was introduced to the semiconductor industry when ICs were invented in 1958. The original lithography used light of the visible g-line (436 nm) and the ultraviolet i-line (365 nm), which was easily produced with a mercury arc lamp. With the progress of technology and the reduction of the feature size, the wavelength of the exposure light had to be reduced several times. When the IC feature size was reduced to about half a micron (500 nm), the g-line and the i-line could no longer be used, and therefore deep ultraviolet 248 nm KrF and 193 nm ArF excimer lasers were introduced. Currently, the 193 nm lithography combined with immersion and double patterning technology is the state of the art.
Shorter wavelength lithography, known as next generation lithography (NGL), has been studied in order to produce IC with even smaller features. NGL uses shorter ultraviolet light (157 nm), extreme ultraviolet (EUV) light (e.g. 13.5 nm), X-ray (0.4 nm), and the even shorter wavelengths of electron and ion beams. Back in 1988 a technology named soft X-ray projection lithography was proposed. However, since the wavelength range of EUV and soft X-ray is not sharply defined (the former lays approximately between 50 nm and 5 nm, and the latter between 5 nm and 0.2 nm), this technology in 1994 came to be known worldwide as EUVL.
Compared with other NGL methods — e.g. proximity X-ray lithography (PXL), electron projection lithography (EPL), and ion projection lithography (IPL) — EUVL is a relatively new member of the NGL league. Due to its remarkable optical convenience — it is accepted as the natural extension of optical lithography — the development of EUVL technology has been relatively fast and since 1999 it has been the most promising NGL technology.
To this day, research and development of EUV technology has cost several billion US dollars worldwide. In order to understand this, we must keep in mind that a single EUV exposure tool is very costly, e.g. about US$ 70 million.
Figure 2.1: ASML Alpha Demo Tool. The sketch re-presents a developmental full-field EUVL scanner re-cently developed by ASML. The UV light source (based on a discharge-produced plasma DPP) is placed on the left.
This can only be supportedbecause global lithography production itself is a large-scale industry, measured on an annual revenue basis of several billion US dollars.
While most other NGLs require one-fold image reduction membrane masks, EUVL uses masks with four-fold image reduction, which makes mask fabrication feasible with current technology. However, in abandoning 157 nm lithography, the industry has created
a technological jump from 193 nm to 13.5 nm wavelength, creating complex challenges across the board. Therefore, EUVL technology includes EUV resist technology, EUV aligners or printers, and EUV masks, as well as metrology, inspection, and defectivity
controls.One important aspect to bear in mind is the fact that all available materials are strong absorbers of EUV light and no material is transparent enough to make use of refractive optics (e.g. lenses).
To date, the Mo/Si multilayer for 13.5 nm EUV is the leading candidate. Theoretically, the thickness of a pair of layers should be about half the wavelength: for 13.5 nm EUV light, the Mo/Si thickness is approximately 6.75 nm (e.g., Mo 2.7 nm and Si 4.1 nm); for 11.4 nm EUV, Mo/Be thickness is 5.7 nm (Mo 2.3 nm and Be 3.4 nm). The map of worldwide research Since 1988, many studies on EUVL have been conducted in North America, Europe, and Asia, through state sponsored programs, industrial consortiums, and individual companies. In the early and mid-1990s, systematic research was mainly performed by the Lawrence Livermore National Laboratory (LLNL), Sandia National Laboratory (SNL), and Lawrence Berkeley National Laboratory (LBNL), as well as AT&T Bell Laboratories and several universities. In 1997, an industrial consortium, the EUV LLC, was formed by Intel, Motorola, and Advanced Micro Device (AMD), to continue work on EUVL. At the same time, the Virtual National Laboratories (VNL) was also formed by LLNL, SNL, and LBNL to conduct a program sponsored by EUV LLC. In Europe, an industrial consortium, the Extreme Ultraviolet Concept Lithography Development System (EUCLIDES), was formed in 1998 by ASM Lithography (ASML), Carl Zeiss, and Oxford Instruments. Since then, EUVL studies in Europe have made significant progress, with ASML leading.
Figure 2.2: The Moore Law of lithography.
In Japan, original studies in EUVL were performed in NTT LSI Laboratories, and publications were found dating from 1989. Other EUVL pioneer work was carried out by Nikon and Hitachi around 1990. The Association of Super-Advanced Electronics Technologies (ASET) was established in 1996, launching its EUVL program in 1998. The Extreme Ultraviolet Lithography System Development Association (EUVA) was established in 2002. Today, EUVL studies are conducted mainly by industrial consortiums and companies, including SEMATECH in US, IMEC in Europe, Selete in Japan, as well as Globalfoundry, Intel, Samsung, TSMC, Toshiba, Hynix, and IBM.
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EXTREME ULTRAVIOLET LITHOGRAPHY
CHAPTER 3
WORKING PRINCIPLE
3.1 WHY EUVL?
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 λ / NA (1a)
and
DOF = k2 λ / (NA)2, (1b)
where λ 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 λ 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 um, which provided for sufficient process control.
The case k1 = k2 = ½ 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 (CD'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
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EXTREME ULTRAVIOLET LITHOGRAPHY
and k2 and by accepting the need for tighter process control. This scenario is schematically diagrammed in Figure 2.1, where the values for k1 and DOF associated with lithography using light at 248 nm and 193 nm to print past, present, and future CD 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 um. 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 2.1 also shows that the DOF values associated with future lithography will be uncomfortably small. Of course, resolution enhancement techniques such as phase-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 3.1: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 3.2: 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 um or better.
The solid red line shows the locus of points for which the DOF is 0.5 um; in the region to the left of that line the DOF values are larger. Points in the region between the two lines correspond to situations in which the resolution is 100 nm or better, and the DOF is 0.5 um 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.
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EXTREME ULTRAVIOLET LITHOGRAPHY
Light having wavelengths in the spectral region from 40 nm to 1 nm is variously referred to as extreme UV, vacuum 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 soft x-ray projection lithography (SXPL), but that name was dropped in order to avoid confusion with x-ray lithography, which is a 1:1, near-contact printing technologyAs explained above, EUVL is capable of printing features of 100 nm and smaller while achieving a DOF of 0.5 um 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 2.1 by the data point farthest to the right.
3.2 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 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 reflectivities near normal incidence, surfaces must be coated with multilayer, 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.
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EXTREME ULTRAVIOLET LITHOGRAPHY
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 three 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 within this program, primarily by workers within the VNL.
3.3 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 a plasma.
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.