Optics and Photonics Letters Vol. 4, No. 2 (2011) 75 81 c World Scientific Publishing Company DOI: 10.1142/S1793528811000226 UV EXCIMER LASER BEAM HOMOGENIZATION FOR MICROMACHINING APPLICATIONS ANDREW F. ZHOU Department of Physics, Indiana University of Pennsylvania 167 Northpointe Blvd., Freeport, Pennsylvania 16229, USA fzhou@iup.edu Received 12 September 2011 A UV excimer laser beam delivery system is designed with conventional microlens arrays and diffractive diffuse. Ultraflat and ultrasmooth ablated surfaces with sharp edges are obtained on fused silica. Ablated surfaces of a depth of 5.9 µm with less than 10 nm surface roughness are obtained on fused silica, as measured by an atomic force microscope. Keywords: Excimer laser; beam homogenization; diffractive diffuser; micromachining. 1. Introduction Recent advances in high power, short pulse UV excimer laser technology have made the excimer laser a useful tool for producing fine features and structures in micromachining, electronic packaging, and medical applications. 1 In the laser ablation process, high energy UV photons from an excimer laser directly break the molecular bonds holding the material together. The unilluminated surrounding material is virtually unaffected by this nonthermal process, resulting in a sharply defined feature and a minimal heat-affected zone. In order to obtain a flat and smooth ablated surface, various laser beam shaping and homogenizing schemes have been proposed to obtain flat-top beam profiles. 2 7 The key element used in a conventional excimer laser homogenizing system is the microlens array. The microlens array splits a nonuniform incident beam into different beamlets traveling along different paths, and then overlaps them on the same plane. Spatial energy nonuniformities are then statistically averaged out, which leads to a uniform flat-top beam profile. However, the homogenizing system based on microlens arrays works well for incoherent beams only. A coherent or partial coherent laser beam will generate interference modulations when the different beamlets from each individual microlens are recombined. In order to get rid of the interference modulations, rotating or moving diffuser has been suggested to smooth out the nonuniform profiles. 8 Unfortunately, this approach is not applicable to applications that require a single shot from a pulse laser. Hence, there is a need to develop a beam shaping and homogenization system for either single or multiple laser pulse ablations. 75
76 A. F. Zhou In this paper, the design of a beam shaping and homogenizing system based on microlens array with a diffractive diffuser is given, as well as the laser ablation results on fused silica analyzed by an atomic force microscope and white light interferometer. Such a homogenizer has potential applications in micromachining with a submicrometer accuracy in ablation thickness, surface flatness and surface roughness. 2. Beam Shaping and Homogenizing System Design The spatial distribution of the laser pulse from an excimer laser at 193 nm wavelength (TuiLaser AG, Model COMPexPro) is measured to have a quasi-rectangular cross section. The measured beam divergence half angles are 1.4 mrad 0.4 mrad, respectively, corresponding to a Gaussian intensity distribution along the short axis, and a super-gaussian profile along the long axis, at 150 mj output energy per pulse. Due to the beam divergence,thespotsizeis28mm 8 mm when it reaches the beam expander, which is located 1.35 m away from the laser output coupler. Because the homogenizer has a clear aperture of 31.5mm 31.5 mm in square, the beam is expanded to cover the entire usable area of the homogenizer. Hence a 3.5 expansion ratio is needed for the beam along the short axis, and is accomplished by using two AR coated fused silica cylindrical lenses. After the beam expander, the beam is close to 31.5 mm 31.5 mm when it reaches the homogenizer. The typical layout of the homogenization is shown in Fig. 1. This imaging homogenizer configuration, consisting of two crossed cylindrical lenslet arrays on front and back surfaces and one spherical focusing lens (condenser), is chosen for the required flat-top intensity distribution. When a focusing lens is used after the two microlens arrays, the formed square beam of a width D at the focal plane of the condenser is determined by 8 : D = p f eff (2f h s)/f 2 h (1) where p is the pitch of microlenses; f eff is the effective focal length of the condenser lens; f h is the focal length of the microlens, and s is the separation between two microlens arrays. The two microlens arrays used in the system are identical with 9 cylindrical lens elements cross-oriented on both surfaces. The microlens has a focal length, f h, of 120 mm, and a pitch, p, of 3.5 mm. With such a microlens array, the incident beam is cut into 81 segments and then overlaid in the plane of integration by the second microlens array and the focusing Fig. 1. The typical homogenizer layout. The laser beam is cut into segments by the first microlens array and overlaid onto the illumination field by the second microlens array and focusing lens.
UV EL Beam Homogenization 77 lens. Although more segments lead to better averaging result, the closely placed grooves between cylindrical microlenses cause more unwanted scattering and grating effects. According to Eq. (1), the spot size of the square beam depends on the separation distance s between the two microlens arrays, after the microlens arrays of focal length f h are chosen. When a separation distance of 210 mm and an effective focal length of 618 mm are used, the calculated spot size is 4.5mm 4.5 mm. Figure 2 shows the optical system layout and the simulated beam profile at the focal plane of the condenser. In principle, when each segment is overlaid, only the intensity variation with random phase information will be smoothed out. Hence such a homogenizer system suffers problems such as coherence, hotlines, hotspots, and speckle in the incident laser beams. As a result, the ablated surface flatness and roughness are severely affected. Since the excimer laser output has different divergence half angles, θ s and θ l, along its short and long axes, the spatial coherences, x s and x l, are calculated according to their intensity distribution profiles 9 : x l θ l =1.55λ(top-hat type), (2) x s θ s =1.97λ(Gaussian type). (3) According to Eqs. (2) and (3), if we use the measured beam divergence half angles, the estimated spatial coherence is quite short, in the order of 300 to 800 micrometers along the long and short axes. However, when a beam expander is used, the beam divergence is reduced and the spatial coherence is increased. To further improve the optical system, a diffractive diffuser is designed using LightTrans VirtualLab TM software, 10 which introduces a distributed time delay longer than the coherence time given by Eqs. (2) and (3). This component effectively eliminates the intensity Fig. 2. The optical simulation of the beam expander and homogenizer system. The optical system layout; and the output beam profile.
78 A. F. Zhou ArF Excimer Laser Homogenizer Microlens Arrays Expander Condenser Diffuser Aperture Objective Sample Tube lens Window Fig. 3. Schematic of the excimer laser beam shaping and homogenizing system (not drawn to scale). modulation caused by beam coherence. In addition, the diffuser converts a square beam of 4.5mm 4.5 mm in size into a circular beam of 5.5 mm in diameter with flat-top profile at the aperture wheel (see Fig. 3). The complete optical system, as shown in Fig. 3, consists of the beam expander, microlens arrays, condenser, diffuser, aperture wheel, tube lens, UV objective lens, and protection window. 3. Experimental Method Great care has to be taken to align the two microlens arrays. The second microlens array needs to be adjusted both vertically and horizontally to make sure that each beamlet formed by the first array will pass through the center of each cylindrical element of the second array. Otherwise, any mismatch between the first and second cylindrical lens arrays will lead to straight lines of large separation across the whole ablated surface. In addition, the second microlens array should not be placed close to the focal plane of the first one, because it may lead to possible damage to the second microlens array over time. Once the homogenizers are properly aligned, a uniform square beam profile with well-defined clean edges is expected on the focal plane. The diffractive diffuse designed and fabricated for this system, combined with the condenser lens, forms a telescope system to extend the laser beam to the plane where the aperture wheel is located. For micromachining applications, the mask containing the pattern to be machined is now illuminated at that plane by the homogenized laser beam. The tube lens and objective lens then image this pattern onto the work surface. Other issues such as the control of optical component surface quality and geometrical aberrations are critical for a flat and smooth surface to be ablated by UV laser beam. Each individual pulse ablates only a very thin layer of material. Often the workpiece is exposed to numerous laser pulses to achieve the desired depth. It is fundamental to have a uniform flat-top beam profile so that the fluence is constant as much as possible over the whole area to be treated on a workpiece.
February 23, 2012 13:7 WSPC/241-OPL 00022 UV EL Beam Homogenization 79 The micromachining test of the beam shaping and homogenizing system is performed with optically polished UV grade fused silica [National Institute of Standards and Technology (NIST) 612], the standard test material for 193 nm laser ablation. The obtained surface is clean with no thermal damage to the material such as micro-cracking. Only a small amount of material is redeposited at the edge. Figure 4 shows the atomic force microscope (AFM) analysis of the ablated surface with an energy density of 14.5 J/cm2 per pulse. The crater generated has a depth of 5.9 µm with a RMS surface roughness of 8.5 nm within a measured area of 36 µm 36 µm. Figure 5 shows the laser ablation under 3.5 J/cm2, 100 Fig. 4. The AFM test results of the crater generated under 14.5 J/cm2 on fused silica (NIST 612). The ablation depth is 5.9 micrometer; and the RMS surface roughness is 8.5 nm. Fig. 5. The crater generated under 3.5 J/cm2, 100 shots on fused silica (NIST 612). The CCD image. The Zygo white light interferometer shows the 11.3 µm deep ablated surface is quite flat with a peak to valley variation less than 2 µm across the entire ablated surface of 170 µm in diameter.
80 A. F. Zhou Fig. 6. White light interferometer measurement of the ablated surfaces, when the incident beam spot size is larger than the value used in diffuser design; and when the optical axis of the diffuser is offset in vertical direction. shots on fused silica analyzed by a Zygo white light interferometer. The 11.3 µm deep ablation is quite flat, with a peak to valley variation less than 2 µm across its entire surface of 170 µm indiameter. When the beam spot size on the diffractive diffuser changes or there is a displacement between the optic axis of the incident laser beam and the center of the diffuser, a detrimental effect on the ablated surface occurs. Figure 6 shows the white light interferometer analysis of the ablated surface, when the beam spot size is larger than the value used for the design of the diffractive diffuser. As shown in Fig. 6, the crater edge is deeper than the center, and the ablated surface becomes convex. Similarly, the ablated surface is concave when the beam spot size is smaller than the value used for the design of the diffuser. As shown in Fig. 6, the diffuser is over filled with the laser beam. In addition, an offset of the diffuser optical axis in the vertical direction leads to the measured slope of the ablated surface, while there is no offset of the diffuser optical axis in the horizontal direction. 4. Conclusion A beam shaping and homogenizing system for 193 nm ArF lasers has been designed with a diffractive diffuser. This system converts the excimer laser output into a circular beam with a uniform flat-top profile. The ablated surfaces on fused silica and other materials are highly flat with sharp edges. The micromachining performed on UV grade fused silica shows ablated holes of a few micrometer in depth with less than 10 nm surface roughness. After the interference modulation and other imperfections have been carefully removed from the homogenized beam, it is possible to perform laser ablations with submicrometer accuracy in ablation depth, surface flatness and surface roughness. Such a system will be a useful tool in both micro- and nano-machining applications.
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