Laser Micro-Machining of Spherical and Elliptical 3-D Objects using Hole Area Modulation Method

Transcription

1 Laser Micro-Machining of Spherical and Elliptical 3-D Objects using Hole Area Modulation Method Wen-Hui Lee Dept. of Industrial and Systems Engineering Rutgers University Piscataway, NJ USA Tuğrul Özel Dept. of Industrial and Systems Engineering Rutgers University Piscataway, NJ USA Abstract Laser micromachining has the capability to fabricate very small and basic.5 D geometric features on a range of materials in the form of laser ablation or irradiation. Short pulsed lasers that can achieve wide range of wavelengths in the form of harmonics of infrared laser beam at 1064 nm wavelength have been a very effective micro-machining tool used for hole drilling, cutting, scribing, trimg and marking. A review of laser processing of materials is given in this paper. Direct laser ablation can be performed by controlling laser beam properties such as laser energy, intensity, pulse duration and wavelength in micro-machining 3-D geometric features. However, this method requires additional capabilities for a typical laser beam generating and delivery system. When the laser beam properties cannot be altered, Hole Area Modulation (HAM) method becomes alternative solution by controlling the density of the holes and the step size in a mask to improve the accuracy of the 3-D geometric feature. In this paper, we perform modeling, planning and simulation of laser micromachining with hole area modulation method to produce spherical and elliptical objects 3-D geometrical features that are typical for aspheric and or refractive micro-lenses. A computational methodology is developed to design a mask with varying density and diameter of the holes. The masks are created using micro-drilling and utilized in laser micromachining of 3-D objects on polycarbonate polymer substrates. 1. Introduction Micro-engineering of products, devices and systems has been a growing field of research demanding new, innovative fabrication processes for nano/micro manufacturing of a wide range of materials [1]. Laser ablation or irradiation based micro-machining of basic geometric features on a variety of solid materials has been investigated by many researchers [-0]. Lasers are usually categorized as two groups: continuous wave (c.w.) and pulsed lasers. C.W. and pulsed lasers commonly used for micro-machining include: (1) Excimer lasers with ultraviolet (UV) wavelengths at 351, 308, 48, and 193 nm, () Oscillator-amplified Ti:Sapphire lasers based on chirped-pulsed-amplification (CPA) technique with extreme peak powers usually in the order of 10 6 Watts with nm wavelengths (3) Neodymium doped YAG (Nd: YAG) lasers with near infrared, visible and UV wavelengths at 1064, 533, 355, and 66 nm, and (4) CO lasers with deep infrared wavelength at 10.6 nm. In continuous wave and long pulse regimes the doant process involved is the heating of the target material through the liquid phase to the vapor phase, resulting in expansion and expulsion of the desired target material [0]. This is accompanied by heating and collateral damage to the surrounding area, the degree of which is detered by the rate of energy absorption and the rate of energy loss through thermal conduction in the material [1]. This collateral damage is often detrimental and is a limiting factor when high precision ablation is required or when it may present a hazard, e.g. laser surgery.

2 Among pulsed lasers, short-pulsed (nanosecond to femtosecond) lasers are of the current research interest. Short pulsed lasers produce a small heat affected zone and a small recast layer on the machined surface by causing material vaporization through high peak power and shorter interaction time [9] Laser Radiation, Absorption and Thermal Effects Laser radiation and thermal material softening is utilized in material processing for many years. The advent of short pulsed lasers opens up a new area for research and applications in material processing. In laser micro-machining, short-pulsed lasers produce a very small heat affected zone when compared to c.w. or long-pulsed lasers enabling extremely localized laser heating in the microscale (see Fig. 1). Fig.1 Illustration of heat-affected zones subjected to different laser types. Localized microscale heating reduces thermal damage to the bulk material. This damage is often detrimental and is a limiting factor when high precision is required. The major processes in the laser-material thermal interaction include radiation absorption and the associated thermal effects. 1. Laser Ablation and Micro-Machining Pulsed laser micro-machining has been a great interest to the many researchers because of its potential to wide range of applications. Pronko et al. [] first studied using 00 fs and 800 nm pulses from a Ti: sapphire laser, focused to a spot size of 3000 nm. Femtosecond pulses offer an advantage over nanosecond pulses because of the reduced effects of thermal diffusion. Chichkov et al. [3] also reported experimental results on femtosecond, picosecond and nanosecond laser ablation of metals. Selection of the optimum laser wavelengths is influenced by the imum feature size and the optical properties of the work material. The characteristics in absorption, reflectivity and thermal diffusion of different kinds of materials are shown in Fig. and can be summarized as follows. Fig. The relationship of wavelength and transmission for some metals, polymers, glass and ceramics Metals and Alloys: When radiation interacts with metals, the energy absorbed raises the temperature level. However, if much of the radiation is reflected due to surface characteristics, the energy absorbed may not be enough to achieve the softening of the material to substantially affect the material removal process. The use of short-pulsed lasers with a proper choice of laser parameters may still achieve thermal softening in highly reflective metals. As shown in Fig., aluum is highly reflective. However, copper and steel have better absorption at UV wavelengths. Zhang et al. [3] studied laser micro-machining of copper using a frequency tripled pulsed single mode Nd:YAG laser of 50 ns pulse duration and reported favorable experimental results. The reflectivity of metals often decreases with temperature, and they are effectively to be machined by Nd:YAG and CO lasers. Zhao et al. [8] reported favorable results on micro machining of aluum using a fs Ti:Sapphire laser. 1.. Polymers and Composites: Polymers exhibit strong absorption in the UV and deep infrared wavelengths, but with weak absorption at visible wavelengths. However, the reaction to lasers is somewhat different in polymers, as compared to that of metals. It is believed that UV produces a cooler excitation in polymers. On the other hand, infrared causes the most molecular vibration and material change through thermal process. However, the properties of most polymers and composites are very strong functions of temperature. This implies that even slight changes in temperature can have a strong effect on machining and localized laser heating can be used effectively for increasing the productivity and the product characteristics Glasses and Silica: Micro-machining of hard and brittle glasses finds applications in biochemistry, biomedicine, lab-on-chip devices, sensors, and Bio-MEMS devices. Many crystals and glasses exhibit strong optical absorption at deep UV and infrared

3 wavelengths, with much weaker absorption in the visible and near infrared. The absorption of light by some glasses can have a very non-linear behavior. Such an example is Pyrex, the transmission wavelengths of which ranging from 300nm to 3 µ m, but in.5 µ m the material has the best absorption. However, since glass is non-opaque, absorption occurs largely within the volume of the material, rather than at the surface. Zhang et al. [5] studied micro-machining of glass materials by laser-induced plasma-assisted ablation using a 53 nm laser and reported experimental results. Zhao et al. [8] applied a fs Ti:Sapphire laser for micro machining of fused silica successfully Ceramics and Silicon: Synthetic CVD diamond is an attractive material since it has various applications such as in infrared (IR) optical applications, detectors, sensors, and thermal management systems. Among all materials, diamond is the most difficult one to be machined due to its hardness and inertness, and, like glass, it is highly transparent over a broad range of the optical spectrum. Laser etching of silicon permits a wide variety of structures to be made since it is independent of the crystal plane orientation unlike wet etching. Various pulse lasers such as infrared Nd: YAG [11] and fs (Ti:Sapphire) lasers [10,15,16] have been used for machining of diamond and silicon [19] for a number of years. Similarly, crystal growth from the melt in the manufacture of silicon chips is followed by several processes of laser machining to obtain desired shape, size and other characteristics of silicon wafers. The focus in these processes is on melting and ablation caused by lasers. The use of lasers with micro-machining techniques have not received much attention, though this approach to lead to a better control of the product characteristics and lower costs. 1.3 Laser Processing Parameters There are several key parameters influencing laser ablation and directly affecting the energy working on material. Larger reduction in laser power or increases in cutting speed will result in incomplete penetration of the cut zone, or poor quality in laser ablation. Chen and Yao [6] investigated pulsed laser micromachining and its influence on dross attachment, burn and recast layer thickness by using designed experiments and statistical analysis. They identified the significant factors affecting dross attachment are average power, orifice size and tracking speed. They found that when the average power increases, the dross attachment rating number decreases. Bordatchev and Nikumb [1] investigated the relationship of energy vs. crater diameter, depth, and volume in pulsed laser micro-machining by using designed experiments and statistical analysis. They considered only pulse energy as a major controlled parameter. Shallow craters were created in copper foil with a thickness of 70 µm by applying a single pulse to one location and then moving the part to a new position for the subsequence pulse. They obtained approximate relations between geometric parameters and pulse energy. Many other research findings indicate that the followings are the main parameters in laser processing: Laser spot size and beam quality: Beam quality is measured by energy, the focus ability, and the homogeneity. If the beam is not of a controlled size, the laser-affected region may be larger than desired size with excessive slope in the sidewalls. Ho and Ngoi [17] reported that a sub-spot size micro-machining technique utilizing the phenomenon of short pulsed laser interference Peak power: The peak power must be able to soften the workpiece, but not strong enough to cause direct ablation. There exist optimum values of laser beam intensity such that the extremely localized material softening will occur Pulse duration: Theoretically, the pulse duration should not be longer than the thermal relaxation time for thermal diffusion. The short pulse duration can maximize peak power and imize thermal diffusion to the surrounding bulk work material, leading to localized heating. The advantages of short-pulsed lasers such as negligible heat conduction and liquid phase absence are promising for the future applications in precision thermal processing of materials with imal damage Pulse repetition rate: When the energy is sufficient, every pulse makes an effect on the workpiece. It is necessary for successive spots to overlap a series of drill operations. If the rate were too low, the energy would leave the thermal zone and become no use. If the residual heat would be retained by rapid repetition rate (limiting the time for thermal conduction), the thermal effect would be more efficient. On the other hand, a pulsed laser has an upper limit in pulse repetition. Direct laser ablation can be performed by controlling laser beam properties such as laser energy, intensity, pulse duration and wavelength. However, this method requires additional capabilities for atypical laser beam generating and delivery system. Laser beam interference technique can create a capability for micro-machining of even smaller features that are typically not achievable. Ho and Ngoi [17] reported a sub-spot size micromachining technique by utilizing the phenomenon of ultra-fast pulse laser interference. The results show much more reduction in machined featured size compared to a non-interfered laser beam. 300nm holes were successfully drilled on a 1000Å thick gold using the interfered laser beam. Most of these methods are for fabrication of -D or ½ D features such as holes and channels. Micro-machining of 3-D geometrical features such as spherical, conical and cylindrical surfaces remains a challenge. Malshe and Deshpande [15] studied femtosecond pulsed laser micro-machining on optoelectronic material with -D 3

4 and 3-D periodic patterns in the form of ripples, clusters and combination of features. They found that the amorphous and defective areas in the nano laser region allowing selective light trapping and surface passivation without contaating the surface. Choi et al. [18] proposed a 3-D micro-machining method called Hole Area Modulation (HAM). They reported the laser ablation depth was influenced by hole diameter on the mask, pitch, transferring velocity, transferring distance and the number of pulses. The machined cavity could be converted to -D distribution with depth information, and then 3-D cavity will be generated. When the laser beam properties cannot be altered, HAM method becomes alternative solution by controlling the density of the holes and the step size to improve the accuracy of the 3-D geometry. In this paper, we perform modeling, simulation and experimentation of laser micromachining with HAM method to produce objects with 3-D geometries that are typical for aspheric and or refractive micro-lenses.. Laser-Micro Machining Experimental Set-Up Pulsed laser system used in this research produces a pulsed Nd:YAG laser beam with a wavelength of 1064 nm, pulse energy of 485 mj, peak power of 4.85 Watts, pulse duration of 5-7 ns, repetition rate of 10 Hz and a focused laser spot area of µm. This laser ablation system has many attributes similar to conventional a CNC machine tool as shown in Fig. 3. The system has a central computer control, which controls the movement of stages for translating the work under the focused laser spot. The video microscope system is necessary for proper location and to in-situ monitoring of the laser ablation operation. The power monitor is used to adjust optical attenuation to reduce or increase the power in conditioned optical beam. Isolation chamber Nd:YAG Laser circular polarizer monitor beam expander beam CCD camera energy detector splitter 3. Process Modeling for Hole Area Modulation 3.1. Laser beam intensity A systematic method can be used to micro-machine even smaller features by first interfering with the laser light, and then using the central periphery of the interfered beam for ablation. In non-interfered conventional laser beam, the Gaussian beam intensity distributions is seen in Fig. 4 as given with: ( ) I = I exp r / ω (1) 0 0 where I 0 is intensity of the incident beam, r is radius of the beam, w 0 is Gaussian beam radius. When a common optical path interferometer is used, the ideal interference intensity 4I0 cos ( δ / ) is convoluted with the Gaussian beam function and the resultant beam intensity function becomes ( ω ) 0 δ 0 I = 4I cos ( /) exp r / () Fig.4 Gaussian beam intensity 3.. Laser irradiation In laser ablation, the work material is irradiated by the laser pulse with extremely high intensity, and experience fast melting and intensive evaporation. The phenomenon of laser ablation by irradiation is illustrated in Fig. 5. Laser intensity workpiece Z Y X Plasma vibration-free table Laser power supply Laser power meter Position stage power supply Vaporization front Absorption region Melting front Fig.3 Laser-micromachining experimental set-up Substrate material Fig.5 Illustration of laser irradiation and ablation. 4

5 3.3. Hole area modulation method Hole area modulation method provides a convenient way without the information on the ablation depth. However, it demands a lot of energy loss and more machining time without depth information because the uniform laser beam intensity causes waste. For hole area modulation method, the laser beam intensity is fixed i.e. it does not change with the ablation depth. The factors to control ablation depth are hole size, mask movement velocity, and step size. Those parameters are used to control the laser exposure time and then decide the ablation depth. Tracking velocity and the step size are found to imize the difference between actual and desired ablated shape. Velocity does not influence a lot on average difference, but there is also a pattern that the average difference goes slightly down with the increase of velocity. The step size influences more on difference than velocity does. In this paper, we simulate for experimentation of the hole area modulation method. If the laser power density on the machining surface is fixed in laser machining, machining depth is proportioned to the exposure time to laser beam. Hole area modulation method provides a convenience way without the information on the ablation depth. However, it results in energy loss and more machining time without depth information because the uniform laser beam intensity causes waste. According to hole area modulation, the ablation depth is proportional to each corresponding hole area on the mask as depth 1 4 π D (3) The hole diameter on the mask can be decided based on the proportion of difference of the desired cavity depth. Machining depth at any location can be calculated by the hole diameter on the mask as depth x y (, ) kd ( x, y) = (4) The mask size is scaled down to 1/15, which means the micro-lens shape ablated by laser beam through mask could be as small as 1/15 of the mask size. The first step of experiment is simulation, which is used to see the mask design and how it works on micro-lens shape. The mask size is scaled down to 1/15 and irradiated by the laser beam on the surface of the material, the pitch size, the step size, and the transfer length on the surface of the material are also scaled down to 1/15. The coefficient k is obtained from the ablation depth created by the smallest hole diameter on the mask as given in Eq. (5). depth k = *15 (5) D The laser ablation depth is corresponded to the imum hole diameter on the mask, the number of pulse per second (f), etching depth per pulse (e), mask moving velocity, and step size ( s ). n D ( ) i, j m s D i, j depth( i, j) = fe + (6) m 1 V V By combining Eqs. (5) and (6), k can be derived from the imum hole diameter as, k = fe n m 1 ( ) D m s D + V V D (7) The hole diameters on the mask can be calculated by ablation depth, which is variant with the center point of the hole (i,j). Using this formulation, laser ablation depth for a given diameter hole can be estimated as given in Fig. 6. (, ) (, ) D i j = depth ic jc (8) k Fig Hole width Simulation of laser ablation through a single hole Simulation of laser micro-machining The desired 3-D feature is an elliptical shaped micro-lens having R x =000 µm, R y =1000 µm, R z =00 µm. The imum hole diameter on the mask is 500 µm, and the maximum one is 100 µm. Since the pitch size should be larger than the maximum hole diameter, it could be set 000 µm as optimal value. The mask size is about 6000 µm x 3000 µm, and 496 holes could be 5

6 created. The ablation depth of elliptical shaped micro-lens is calculated as given in Eq. (9). h( x, y) = Z + R 1 x 1 y + 0 Z Rx Ry h(x,y) Z 0 (9) Volume to be machined Matlab is used to develop the mask design and to simulate ablation micro-lens shape. Micro-drilling of the mask is performed in a CNC milling machine using NSK high speed spindle at 40,000 rpm by using two polycarbonate plates to sandwich the aluum sheet metal as fixture. Input Parameter: Rx=000 µ m, Ry=1000 µ m, Z0=75 µ m, Rz=00 µ m, f =11, e =0.1 µ m/pulse, scaling factor=1/15, D=500 µ m Ry R z Rx Calculate desired depth of the curved surface according to each ablated position on the material surface. x y depth( x, y) = Z0 + R Z Rx R y Fig.7 Elliptical micro-lens diagram Once getting the mask with hole pattern, the laser can be projected through the holes and ablate the material surface. The holes form an elliptical shape on the mask and outside area. The bigger the holes, the more the material removed and the larger the depth due to the larger summation of exposure time. The mask moves within a cell, which is an area corresponding within the pitch. It starts from a corner, then move as much as moving distance, and then goes perpendicular direction as the distance of the step size, then turn opposite direction as the first step, and then moves back and forward again and again until the summation of total step size reach to pitch size as illustrated in Fig. 8. End position Assumptions V, s depth = Z0 k = depth / D *15 Calculate the desired diameter or the holes on the mask 1 ( ) x y D x, y = 15 * Z + 0 RZ k Rx R y Calculate the accurate laser ablation depth ha(i,j). n 1 D ( ) i, j m s D i, j depth( i, j) = fe + m= 0 V V Minimize ( ha ( x, y) hd ( x, y) If the difference < Actual ablation depth x Step Size Pitch Complete the mask design Fig.9 Simulation flow chart Start position Fig.8 Mask moving path First the mask size should be decided and the mask machining limit has to be considered. The mask size in the paper is about 6000 µm x 3000 µm, and aluum sheet is the material chosen to make the mask. Since micro-drilling is the machining process used to make mask holes, each hole size should be the same as drill size, which means grouping some ranges of diameter sizes into some specific drill sizes is necessary. The drill size ranges from 300 µm to 1600 µm. Before experiment, 4. Results and Discussions There are two parameters being controlled in this paper: velocity (V) and step size ( s ), and the differences between actual and desired ablation depth is observed to find the optimal control parameters. Matlab is used to simulate the models. The results represent that the average difference decreases with the increase of step size. As the step size is around 10, 11, 1 µm, the average differences are much smaller and those values are close to each other. The velocity does not influence a lot, but it also has trend that the average difference becomes smaller with the increase of the velocity. Especially when step size is less than 9 µm, the trend is more obvious. Theoretically, high velocity can retain more heat in ablation zone; however, if the speed is too high, the exposure time to laser beam could not be 6

7 enough so that the ablation can not be completed. Relatively, too low speed can not keep the heat and make energy less effective. The following plot shows how velocity and step size influence the differences between actual and desired ablation depth. The simulation results show that when the step size is µm and velocity is around 1 µm/s the average difference between actual and desired micro-lens shape is the smallest (see Fig.10). Average difference (mm) Velocity (mm / s) Fig.10 Average difference between actual and desired micro-lens shape for various step size The mask design is created with Matlab is showed in Fig. 11, and the simulation result of ablation is and elliptical shaped micro-lens is show in Fig Fig.11 Holes design on the mask for ellipsoidal micro-lens Fig.1 Simulation on ablation result--elliptical shaped micro-lens For hole area modulation method, the laser beam intensity is fixed (It does not change with the ablation depth). The factors to control ablation depth are the hole size, mask movement velocity, and step size. Those parameters are used to control the laser exposure time and then decide the ablation depth. Some other laser ablation method is to control laser beam properties such as laser energy, pulse duration, and wavelength. Laser energy is directly related to ablated crater volume, and pulse duration corresponds to laser energy and peak power. The ablation properties, such as ablation depth, crater volume are detered by the above laser properties, such as laser energy or pulse repetition rate. When the laser beam properties can not be changed, hole area modulation method could be an eligible solution, and we can control the density of the holes and step size to improve the accuracy of the geometry. The following table is the comparison for general laser machining and hole area modulation method. 5. Conclusions This paper introduces the properties of lasers and the optical characteristics when lasers interact with several kinds of material. Laser energy is influenced by laser spot size, pulse duration, and peak power and could detere the ablation depth. Hole modulation method is experimented. The ablation depth is proportioned to the area of hole diameter on the mask, since the exposure time increases with the increase of hole area. Frequency and laser energy are fix in the ablation process, and what could be controlled are transfer velocity and step size. Optimal velocity and step size are found to imize the average difference between actual and desired micro-lens shape. Velocity does not influence a lot on average difference, but there is also a pattern that the average difference goes slightly down with the increase of velocity. The step size influences more on difference than velocity does. The simulation results show that when the step size is around µm and velocity is around 1 µm/s the average differences between actual and desired micro-lens shape is the smallest. 6. References 1. Alting, L., Kimura, F., Hansen, H.N. and Bissacco, G. (003), Micro Engineering, Annals of the CIRP, 5/, Pronko, P.P. Dutta, S.K., Squier, J., Rudd, J.V., Du, D. and Mourou, G. (1995), Machining of sub-micron holes using a femtosecond laser at 800nm, Optics Communications, 114, Chichkov, B.N., Momma, C., Nolte, S., von Alvensleben, F., and Tunnermann, A. (1996), Femtosecond, picosecond and nanosecond laser ablation of solids, Applied Physics, A 63, Momma, C., Nolte, S., Chichkov, B.N., von 7

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