This paper 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 projection imaging. Currently, and for the last several decades, optical projection 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 industry'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 overview 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.
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.
EXTREME ULTRAVIOLET LITHOGRAPHY
2.1 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)
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 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'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 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.