INTRODUCTION
Lithography is one of the fundamental technologies by which nanoscale patterns required for the fabrication and integration of nanodevices are generated. Lithographic requirements are becoming increasingly severe. In the semiconductor industry, the past three decades have seen the critical length-scales of component devices decrease by several orders of magnitude, from 15 mm in the case of the first integrated circuit, to less than 130 nm routinely obtained today. The phenomenal rate of decrease in size seems to reach the limitation of conventional optical lithography. The exposure wavelength, photoresist performance, and equipment determine the lithography limitation. The size of the constituent atoms imposes a fundamental limit on the minimum length scale that can be ultimately attained. This article first provides an overview of the fundamentals of optical lithography. It is shown how minimum attainable device dimensions are intimately related to the wavelength of light used. Then, several techniques under investigation for further enhancing the resolution of this workhorse of the microelectronics industry are described. As the options available to industry are not all ”optical,” the discussions cover the various nonoptical lithographic techniques currently being explored.
TECHNOLOGY OVERVIEW OF LITHOGRAPHY
Fig. 1 illustrates how the lithographic options vary as the critical device dimension decreases, and provides estimates of the timescales on which decisions may need to be made regarding which options to adopt.[1] Optical lithography technology has been traditionally used by reducing wavelengths of light sources from mercury lamps to ex-cimer lasers such as KrF and ArF excimer lasers, and probably F2 and Ar2 lasers in the future. Phase shift masks (PSM) and immersion configuration can extend the resolution limit for each optical lithography. Extreme ultraviolet (EUV) of wavelengths ranging from 11 to 14 nm is used as extension of the optical method. Nonoptical lithography with proximity X-ray (PXL), electron beam (EBL), and ion beam (IPL) have a potential for finer patterns (as opposed to those produced via optical methods) because of their shorter wavelengths. Electron-beam projection lithography (EPL), which uses mask projection technique, projects to be most promising. However, electron beam direct writing (EBDW) has a drawback on its throughput. Nanoimprint is a simple technique just like paper printing. A shift by the microelectronics industry to any nonoptical lithographic technique will require the introduction of a new infrastructure of tools, materials, and processing technologies, resulting in huge research and development costs.
PROJECTION OPTICAL LITHOGRAPHY
Key elements of a practical lithographic system are essentially the same for all technologies—optical and nonoptical. As shown in Fig. 2, they include the following: 1) A set of ”masks” containing the patterns of components to be fabricated in and/or on the substrate, the tools for making the masks, and the metrology for ensuring precise dimensions and pattern overlay. 2) An energy source (e.g., a light source) for transferring the pattern from a mask to the substrate. 3) A medium—known as a ”photoresist” or ”resist”—for recording the pattern on the substrate following exposure to the source, and which allows subsequent processing of material in and/or on the underlying substrate. 4) Procedures for reliable detection of pattern defects, which clearly becomes more challenging as the critical dimensions decrease. The lithographic process— especially projection optical lithography—is closely related to the developing process in print photography, where the photographic negative plays the role of the mask, and the photographic emulsion on the print is the resist.
Resolution Limits
The resolution limit in conventional projection optical lithography is largely determined by the well-known Rayleigh’s equation. The resolution (minimum resolvable feature) R and the corresponding depth of focus (DOF) are given by the following:[2]
Fig. 1 Technology options of lithography.
Here l is the exposure wavelength, NA is the numerical aperture of the optical system, and k1 and k2 are constants that depend on the specific resist material, process technology, and image-formation technique used. Fig. 3 shows the evolution of projection optical lithography. It compares the required minimum feature size and the wavelength of the exposure light. At the beginning of the introduction of the projection system, the required minimum feature size was relatively large compared to the wavelength of the exposure light. Then low-NA lens system was used. However, as the miniaturization requirement of the semiconductor devices is faster than the reduction rate of the wavelength of exposure light, higher resolution was required. Therefore, to obtain higher resolutions, shorter-wavelength light and lens systems with larger numerical apertures are required. In general, the minimum feature size that can be obtained is almost the same as (or slightly smaller than) the wavelength of light used for the exposure, for which one needs a relatively large numerical aperture (typically > 0.5). In such high-NA lens systems, the depth of focus becomes very small, and so the exposure process becomes sensitive to slight variations in the thickness and absolute position of the resist layer.[3] The smaller the depth of focus, the more rapidly a focused beam diverges on moving away from the focal point. With the recent introduction of a ”chemical mechanical polishing” technology, the topographic variations of substrate surfaces have been reduced, making it possible to use extremely large NA systems; however, the margin for error becomes extremely small under such high-NA exposure conditions.
Fig. 2 Principle of lithography.
Fig. 3 Trend minimum feature size and exposure wavelength for optical lithography. Practical resolution limit is reduced to approximately half of wavelength with extremely higher NA and resolution enhancement techniques. Miniaturization exceeds the wavelength reduction trend for ULSI manufacturing.
Resolution Enhancement
To improve the resolution of an optical lithography system without introducing other impractical constraints (on, e.g., wafer smoothness), several resolution-enhancement strategies were proposed.[4-6] Fig. 4 shows a schematic view of a typical projection optical-exposure system. Such a system consists of several subsystems, and resolution-enhancement ideas can be applied at various points. These may take the form of modified illumination at the light source, phase shifting of the wave front in the mask plane, and/or filtering with an aperture (pupil) at the projection lens. Some basic ideas of resolution enhancement are illustrated in Fig. 5, which compares image formation in: a) the conventional setup with b) phase shifting and c) modified illumination.
Fig. 4 Schematic view of exposure optical system and various resolution enhancement technologies for each stage of the system.
Fig. 5 Grating pattern image formation.
Consider the case when the image on the mask is a simple grating. In the conventional system (a), several diffracted light rays are generated by the grating. Zero-th order light proceeds straight through the system, and several rays of higher order are generated with diffraction angles of 0 (first order), 20 (second order), 30 (third order) and so on, with 0 ~ l/a for small 0 (where a is the grating periodicity). Near the resolution limit, only the zero-th order and first-order rays pass through the lens pupil. Now consider case (b), where the phase of the light passing through the grating mask is modified to have a periodicity twice that of the grating itself. (Phase shifting also requires a higher degree of coherency in the source light compared to the conventional case.) The transmitted wave front is in turn modified, such that the zero-th order light is canceled out and the diffraction angle of the first-order rays halved to 0.50. As a result, the spatial frequency of the patterns that can be imaged is doubled (or, put in another way, the resolvable grating periodicity is halved). Although both the opaque pattern and the phase modifications on a real mask will be considerably more complex than a simple grating, this example nevertheless serves to illustrate the substantial improvement in resolution that can be achieved by this approach.
Fig. 6 shows an example of resist patterns formed by using KrF (248 nm) light and a phase shift mask: structures having half the wavelength of the exposure light are clearly visible.
Next, we consider oblique (or off-axis) illumination, as shown in (c). Zero-th order light no longer passes through the center of the pupil, but at an angle to the vertical. The first-order rays again emerge at angles of ±0 with respect to the zero-th order ray. And at the resolution limit, one of these passes through the side of the lens pupil opposite to that of the zero-th order ray, while the other diffracted ray is blocked. Therefore the result is geometrically equivalent to the phase-shifted case (b), and again leads to a doubling of the spatial frequency of the images that can be resolved. At present, the most advanced mass-produced LSI circuits have a critical dimension of 90 nm. This is achieved by using partially coherent light from ArF excimer laser with a wavelength of 193 nm, in combination with some resolution-enhancement techniques such as off-axis illumination, phase shift mask, and optical proximity correction, as well as advanced resist systems. To further reduce the critical dimension, wavelength reduction is being actively pursued using F2 and Ar2 excimer lasers (wavelengths of 157 and 126 nm, respectively). The combination of shorter-wavelength light and resolution-enhancement techniques may ensure optical lithography’s position as the most productive lithographic technology for nanofabrication of length-scale of even less than 50 nm.
Fig. 6 An example of resolution enhancement technology and the effect of phase shifting technology.
Fig. 7 Various electron beam writing systems; simple point electron beam, shaped electron beam, patterned cell projection, and chip pattern projection.
