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Introduction to Electron Beam Lithography Philip Coane Louisiana Tech University, Institute for Micromanufacturing Introduction The merits of electron beam lithography are widely recognized. It provides better resolution and greater accuracy than optical lithography. Due to its well-known accuracy advantages over optical lithography, electron beam lithography has been used for many years in the research environment for advanced semiconductor device fabrication. Fine resolution is provided by the small size of the focused electron beam [1], while accurate site-by-site pattern registration, and the ability to electronically adjust field size provides unequalled overlay accuracy. Most often, the lithography tools utilize round electron beams similar to those used in scanning electron microscopes (SEM) [2]. In these systems the electron beam is focused to the smallest size possible for a given set of electron optics and operating conditions. The beam serially exposes individual pattern pixels and offers the highest degree of resolution and patterning flexibility possible. The current distribution within the beam is nearly Gaussian and spots are overlapped to provide the smoothest features. Serial exposure techniques have the inherent disadvantage of low writing speed (throughput) for most patterns. While electrons would seem to be ideal for imaging patterns, they are not always ideal for exposing the resists used as the recording media for patterns. The low mass of the electron leads to relatively short penetration with significant lateral scattering. Resist sensitivity to electron beam exposure is not always efficient which leads to high dose levels and heating of the resist. Writing Strategies Raster Scan is the method whereby the beam is swept across the entire field, pixel element by pixel element, with the beam being turned off and on (blanked and unblanked) as needed to expose the desired pattern (fig. 1). This strategy is based on a relatively simple architecture that is easy to calibrate. The disadvantage is that because the beam is scanned across the entire writing field, sparse patterns take just as long to write as dense patterns. An additional drawback is that dose adjustment within the pattern, for purposes of correcting for proximity effect, is inherently difficult. Vector Scan utilizes the method of jumping from one patterned area to the next skipping over all non-patterned areas (fig. 1). This method of not visiting every point in the pattern makes the vector scan approach faster than raster scan for sparse patterns. There is little difference in writing time for dense patterns. Adjustments to dose can be accomplished on the fly. A disadvantage is that longer beam settling time is required which equates to increased difficulty in maintaining placement accuracy. Coane 1 One means of increasing the throughput is to introduce some degree of parallelism into the exposure process. An early tool of this type was developed, in which a square of 5x5 pixels (representing the minimum pattern feature) could be exposed simultaneously [3]. RASTER SCAN VECTOR SCAN ROUND BEAM SHAPED BEAM Fields STATIONARY STAGE Stripes MOVING STAGE Fig. 1. Typical electron beam writing strategies (Courtesy SPIE) This was soon followed by the variable shaped beam concept, which increased the maximum number of pixels per flash to several hundred, and also allowed fine variations in the size of the spot [4,5]. The hardware and software that is required for this type of system is inherently more complex than round beam systems. Shaping apertures are used to define squares, rectangles and, in some systems, triangles. Typical spot sizes range up to 2x2 um. Focus and resolution depend on shape size (coulomb effects). The writing strategy used is typically vector scan. An extension of the shaped beam approach is cell projection. Etched silicon masks that contain elemental shapes are placed on the electron optical axis so that the beam can be deflected to illuminate shapes or parts of shapes thus projecting a cell onto the substrate. Coane 2 An alternative approach is to use a broad beam to illuminate a mask and project the resulting image onto the substrate. The mask and wafer are scanned simultaneously. The field size, however, is limited to about 1mm and the brightness by coulomb interactions. System Details A typical electron beam lithography system that is shown schematically in figure 2 consists of an electron gun, electron optical column, and a vacuum chamber containing a laser controlled x/y stage for accurately positioning the substrate under the beam. Gun Assembly Blanking Plates Electron Optical Column Final Lens Deflection Plates Electron Detector Load Lock x/y Stage Chamber Vacuum System Fig. 2. Schematic representation showing the basic components of a typical electron beam lithography system (excluding electronic components). The electron optical column is used to form and direct a focused beam of electrons onto the surface of the substrate, which is mounted in a holder and clamped to the stage. The electrons are produced in the uppermost section, which is called the electron source or gun. After the beam of electrons emerges from the gun it passes through several additional stages in the electron optical column that perform specific beam modification processes to produce a beam having the required current and spot size and correctly focused onto the substrate. Coane 3 Beam current and current density are critical parameters in the optimization of pattern writing time. The electron optical system is limited by the brightness of its electron gun, the imaging limitations of its lenses, and beam interactions along the beam path. The relative importance of these limitations depends upon the writing strategy used. Electron Gun The electron gun or cathode can use a poly-crystalline tungsten wire filament, a tungsten single crystal cold field emitter, a lanthanum hexaboride crystal (LaB6), or a thermal field emitter (tungsten needle coated with zirconium oxide) as the electron source. The latter two types are the more commonly used sources in modern electron beam lithography systems. Of these two, the thermal field emitter providing the highest brightness and smallest source size Cathode Tip Grid (Wehnelt) Crossover at T3 Crossover at T2 Anode Limiting Aperture Crossover at T1 Fig. 3. Schematic representation of a typical gun assembly showing electron emission crossover points for a Lanthanum Hexaboride cathode as a function of temperature. (Courtesy Leica lithography Systems Ltd.) Except for the cold field emitter, the cathode is heated sufficiently to produce free electrons from the surface. A high potential voltage is applied between the cathode and the anode, which accelerates the electrons towards and through an aperture in the anode. The value of the applied potential determines the energy level of the electrons that reach the substrate. An electrode called the grid or Wehnelt surrounds the cathode and is negatively biased. This electrode is used to control the emission current from the cathode. By varying the ratio of the voltages applied to the gun components, electrostatic fields are Coane 4 generated that influence the trajectories of the electrons traveling between the cathode, Wehnelt and anode (fig. 3). The resultant beam of electrons leaving the anode is a diverging beam having a “virtual” source at the crossover formed within the vicinity of the grid. The beam current is set by the source brightness and convergence angle. The beam voltage determines the brightness, current density and energy spread of the cathode. The beam is then aligned and focused by the electron optics contained within the column sections located below the gun. Electron Optics The magnetic lens acts on electrons much in the same fashion as a conventional glass lens acts on light. This type of lens is fabricated by wrapping a circularly symmetric iron core with turns of copper wire. Passing a current through the wire produces a magnetic field. The flux path is rotationally symmetric except for the air gap in the center of the lens. In this air gap the flux “leaks out” to form a flux gradient, which acts as a lens for the electron beam (fig. 4). Electron Optical Axis Copper windings Iron shell Magnetic field Lines Pole Pieces Fig. 4. Schematic representation of a magnetic lens The strength of the axial field determines the strength of the lens. Due to the interaction of the electrons with the magnetic field lines, the electrons pass through the lens in a spiral path. A magnetic lens typically exhibits reduced aberrations as compared to an electrostatic lens. The diverging electrons are focused into a converging beam, which produces an image of the object plane below the lens. Coane 5 In the electron beam lithography system, the magnetic lenses are operated in demagnification mode that is, the image of the electron source is demagnified and then further demagnified in successive stages as the beam traverses the column (fig. 5). Gun (Cathode) 1st Condenser Lens Blanking Plates 2nd Condenser Lens Beam Limiting Aperture Beam Deflector Deflection Angle a Final Lens Substrate Fig. 5. Schematic Representation of the Electron Optical Column Electron beam deflection is typically accomplished by the application of electrostatic or electromagnetic fields to steer the beam in the desired direction. For the electrostatic case, a different voltage applied to parallel plates results in a perpendicular field gradient, which deflects the beam off axis. Thus, by varying the electric field, the degree of deflection of the beam can be varied. Similarly, varying the current through a set of deflection coils can also be used to pull the electron beam off axis. The angular spread Da in the deflection angle a can be expressed by: Coane 6 Da = 1 DV a 2 V (1) where: a is the deflection angle (fig. 5) and DV is the energy spread. A - B C e Deflection Coils Fig. 5. Transverse chromatic aberration due to the energy spread of source electrons. Fig. 6. Statistical effects of electron – electron interactions. Statistical coulomb interactions in a gun were originally known as the Boersch effect. The lateral effects, called trajectory displacement, resulted in larger electron probe sizes and, as a consequence, reduced brightness. The longitudinal displacement, or Boersch effect is illustrated in figure 6. In beam A, all electrons experience the same force from neighboring electrons. In beam B, where electrons are randomly distributed along the beam path, electrons are accelerated and retarded. The effect on individual electrons is based on their proximity to adjacent electrons, which leads to having different velocities. Chromatic aberration of a lens occurs when the velocities of the electrons passing through a lens or a variation in the focusing magnetic field change the point at which the electrons are focused. The energy spread of electrons from tungsten is about 2 eV. For LaB6 the spread is about 1 eV and from 0.2 to 0.5 eV for a field emission cathode [6]. This influences the spread of electron arrival velocities and kinetic energies. Higher energy (faster) electrons are focused less strongly. Therefore, accelerating electrons across higher voltages permits sharper focusing. The coefficient of chromatic aberration is expressed by the relationship: Coane 7 dc = C c a DV V (2) where: Cc is the chromatic aberration of the lens. The coefficient is roughly equal to the focal length of the lens. In beam C, electrons repel each other, widening the beam. Offaxis electrons are focused more strongly (spherical aberration). The net effect is that the spot becomes larger. The coefficient of spherical aberration is expressed by the relationship: ds = 1 Cs a 2 3 (3) The coefficient of spherical aberration is approximately equal to the focal length of the lens. Electrons have a finite quantum mechanical wavelength, l =1.2 Vb-1/2 which gives a diffraction limited beam size expressed as: dd = 0.6 l a (4) The diameter of the circle of confusion (the blurred circular image of a point object, which is formed by a lens, even with the best focusing) due to these effects is given by: dCc = 1 DV Ccq 2 V (5) where: q is the lens aperture angle. For electron field emitters, the tungsten single crystal cold field emitter and the zirconiated tungsten Schottky emitter both have relatively low energy spread (DV) around 0.3 eV and high brightness (greater than 1000 A/cm2/sr). The aberrations of electron lenses are considerable. A numerical aperture is chosen that optimizes the beam resolution at a chosen beam current. In this way, the spherical aberration, chromatic aberration, and blurring caused by Coulomb repulsion are balanced [6]. In the absence of beam interactions, an electron optical system with an aperture half angle b (radians) can provide a current density Jmax=pb2/B (Amps/cm2), where B is the gun brightness (Amps/cm2/sr). Including beam interactions, the exposure edge slope db may be approximated from the expression [7]: db~[{CS b3}2 +{CCDV/V}2 +{PI2/3L2/3/V4/3b4/3}2]1/2 (6) where: CS and CC are spherical and chromatic aberrations coefficients (cm) of the final lens, DV/V is the energy spread in the beam, including Boersch effects. The last term is an expression for transverse beam interactions where P is proportionality constant, I is the beam current, and L is the length of the imaging optics. Coane 8 Electron Beam Tool and Process Characteristics A thorough understanding of all the parameters that influence the accuracy of the pattern definition and transfer process becomes increasingly critical as minimum feature size is reduced. Sources of error, which affect the accuracy of electron beam lithography, are extensive with some being specific to electron beam systems while others are of a more general nature. These error sources must be thoroughly understood and controlled. The more common sources of error are attributed to 1) electron beam / electron optics, 2) overall system thermal stability, 3) magnetic environment interaction, 4) electron beam / resist / substrate interaction, and 5) resist system / process control. Each of these parameters is important, and each one could be the limiting factor determining ultimate system performance. Not all of these source errors are completely independent of the others. Compromises in system performance may be necessary. For example, when the beam energy increases, the resolution of the system will generally improve; the electron beam becomes “stiffer”, reducing the potential for perturbation of the beam by environmental conditions. Both of these factors contribute to an improvement in resolution. However, accurate pattern element definition in resist, especially at high pattern density, is limited by forward scattering (df = 0.9(Rt/Vb)1.5) of the electron beam in the resist and backscattered electron contributions from the substrate (fig. 7). Electron Beam Resist Backscattering Substrate 20 keV df = Forward Scattering Distribution 50 keV 3 mm db = backscattered distribution Fig. 7. Both the incident electrons and the electrons scattered back from the substrate contribute to resist exposure. Coane 9 This limitation is the well-known proximity effect [8-10]; i.e., adjacent areas not directly exposed to the incident electron beam receive partial exposure from backscattered electrons. The amount of backscattered electrons primarily depends on the incident beam energy, the substrate material, and the resist thickness. The proximity effect is particularly severe for dense patterns with dimensions and spacing of 1 mm or less. Proximity effect correction methods, such as modifying the pattern data or compensating the exposure dose, have been developed and applied efficiently in submicron pattern exposure. Numerical approximations of this effect are typically included in the software programs of the more advanced electron beam lithography systems, which controls the exposure and assigns the required dose to all shapes. In electron beam lithography, an important quantity that characterizes the magnitude of the proximity effect is the effective backscatter coefficient h. This parameter is defined as the ratio of the forward scatter resist exposure dose to the backscatter exposure dose. Proximity effect correction techniques require precise knowledge of this parameter in order to properly adjust doses to shapes such that minimum feature size dimensional tolerances are maintained within acceptable limits. The backscatter coefficient has been well characterized for beam energies in the 10-50 keV range on silicon substrates [1113]. Figure 8 shows that as beam energy is increased, the backscatter range increases. This means, for example, that a pattern element would receive a relatively lower dose contribution from backscatter at 50 keV as compared to 20 keV. Resist Silicon Fig. 8. Plot of backscattered electron contribution to resist exposure as a function of two beam energies. Coane 10 The backscatter dose profile, Db (r) in units of area –1, is measured relative to the incident exposure dose of 1.0 located at the position r=0 and is approximately Gaussian in shape. Db (r) can be determined from the relationship: Db(r) = h p × s b2 e - r2/s b2 (7) Where: sb is the characteristic width parameter (sb µ V1.75), which is about 30 mm at 100 keV on a silicon substrate. The value for h is reported as 0.75 [14]. Figure 9 illustrates the effect of forward scattered and backscattered electrons on resist exposure. The resulting width of the exposed feature as well as the edge profile is a result of an integration of these dose contributions with the effects of electron beam size and the exposure / development characteristics of the electron sensitive resist. Proximity correction is essential at higher beam energies to resolve complex patterns where the size and shape of pattern elements are distributed over a wide range (dense, nested, isolated). RESIST SUBSTRATE Fig. 9. Schematic diagram illustrating the proximity effect (after Greeneich) Figure 10 shows cross sections of exposed / developed resist patterns, which were exposed with and without proximity correction. The images were taken on a scanning electron microscope (SEM) with the sample tilted at a viewing angle of 30O. The base Coane 11 dose was adjusted such that the isolated lines developed to a nominal linewidth of 0.25 mm for both non-proximity corrected and proximity corrected exposures. The same dose was applied to the non-proximity corrected line-space array and isolated space. Since less overall area is exposed (compared to the isolated line) for these patterns, the backscatter contribution is less, and the overall exposure dose is insufficient to completely develop the line-space array. For the isolated space almost no development occurred. No Proximity Correction With Proximity Correction Isolated Line Base Dose = 1 Equal LineSpace Array 1.58 x Base Dose Isolated Space 1.98 x Base Dose Fig. 10. Proximity test patterns exposed with a 50 keV electron beam Proximity correction algorithms, which are derived for exposure control so that each pattern receives the correct exposure dose regardless of geometrical differences, are Coane 12 resident in the software of the electron beam control system. As can be seen in figure 10, with these algorithms it is possible to develop all three pattern types to their nominal dimensions. As previously noted, the energy of the electron beam also has a profound effect on upon the range of the proximity effect [9]. Decreasing Beam Diameter Similar to the proximity effect is the effect of electron beam probe size (measured as the full width at half maximum of an electron beam with a Gaussian distribution of beam current density and as the edge slope definition [15] for a shaped beam) on exposed resist profiles. As shown in the SEM micrographs of figure 11 for single-line exposures, the beam size has a distinct effect upon the development of the exposures [16]. The lines shown are nominally 0.3 mm wide. The electron beam size is indicated in the top of each micrograph. 0.156 mm 0.171 mm 0.197 mm 0.214 mm 0.273 mm 0.295 mm Increasing Beam Diameter Fig. 11. The effect of electron beam diameter on exposed / developed resist profile The micrographs demonstrate conclusively the degradation in the image quality of the exposed and developed patterns with increased electron beam size. A change of about Coane 13 0.02 mm in beam diameter alters the exposure profiles significantly, as is clearly evident in the center set of micrographs. Summary From the preceding discussion, it is evident that electron beam exposure tools satisfy all of the requirements, i.e., flexibility, resolution, linewidth control, pattern overlay, etc., for patterning submicron structures. With scanned electron beams no mask is required and the ability to write a variety of pattern geometries is a significant advantage over other lithographic techniques. However, the electron beam tool cannot be viewed as an independent entity, but must be considered as an integral part of a lithographic system in which the resist material and development process are equally important. The resist and process development must be coupled closely with the functional dependencies of the electron beam tool. Electron beam lithography tools are inherently slow, vector scan being one or more orders of magnitude slower than optical lithography systems due to the serial pattern writing method used. However, advances in electron beam lithography technology, such as variable shaped electron beam and the use of arrays of microcolumns [17], throughput can be significantly improved. Further Reading For more in-depth information on electron beam lithography, the following sources are suggested to the reader. 1.) P. Rai-Choudhury, editor, Handbook of Microlithography, Micromachining, and Micrfabrication Vol. 1 Chapter 2, SPIE (1997). 2.) G. Brewer, editor, Electron-Beam Technology in Microelectronic Fabrication, Academic Press (1980). 3.) Proceedings of the Electron, Ion, and Photon Beam Symposium, Journal of Vacuum Science and Technology B. References [1] A.N. Broers, W.W. Molzen, J.J. Cuomo, and N.D. Wittels, Appl. Phys. Lett. 29, 596 (1976). [2] D.R, Herriott, R.J. Collier, D.S. Alles, and J.W. Staffford, IEEE Trans. On Electron Devices ED-22 (7), 385 (1975). [3] H.C. Pfeiffer, J. Vac. Sci.Technol., 12, 1170 (1975). [4] H.C. Pfeiffer, J. Vac. Sci.Technol., 15 (3), 887 (1978). [5] R.D. Moore, Electronics 54, 138 (1981). [6] T.R. Groves, H.C. Pfeiffer, T.H. Newman, and F.J. Hohn, J. Vac. Sci. Technol. B, 6(6), 2028 (1988). [7] W.B. Glendinning and J.N. Helbert, editors, Handbook of VLSI Microlithography, (1991). [8] M. Parikh, “Corrections to Proximity Effects in Electron Beam Lithography,” Research Report RC-2254 (30447), IBM T.J. Watson Research Center, Yorktown Heights, NY, (1978). [9] A.N. Broers, “Resolution Limits for Electron Beam Lithography”, IBM J. RES. Develop. 32, 502 (1988) Coane 14 [10] T.H.P. Chang, et. al., “Nanostructure Technology,” IBM J. RES. Develop. 32, 462 (1988). [11] T.H.P. Chang, J. Vac. Sci. Technol, 12, 1271 (1975). [12] M. Parikh and D.F. Kyser, J. Appl. Phys. 50, 1104 (1979). [13] S.A. Rishton and D.P. Kern, J. Vac. Sci. Technol. B 5, 135 (1987). [14] L.D. Jackel, et. Al., Appl. Phys. Lett. 57, 153 (1990). [15] H.C. Pfeiffer, T.R. Groves, and T.H. Newman, “High-Throughput, High-Resolution Electron Beam Lithography,” IBM J. RES. Develop. 32, 494 (1988). [16] M.G. Rosenfield, et. al., “Submicron Electron Beam Lithography Using a Beam Size Comparable to the Linewidth Tolerance,” J. Vac. Sci. Technol. B 5, 114 (1987). [17] T.H. P. Chang, et. al., “Electron Beam Microcolumns for Lithography and Related Applications,” J. Vac. Sci. Technol. B 6, 774 (1996). Coane 15