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Abstract: Laser essentially is a coherent, monochromatic beam of electromagnetic radiation that can propagate in a straight line with negligible divergence and occur in a wide range of wavelength (ranging from ultra-violet to infrared). Lasers are widely used in manufacturing, communication, measurement and medical. Energy density of the laser beam can be altered by varying the wavelength. This property has made the lasers proficient for removing extremely small amount of material and has led to the use of lasers to manufacture very small features in workpiece materials. The production of miniature features (dimensions from 1 µm to 999 µm) in sheet materials using laser machining is termed as laser micro-machining. In the present study, the fundamental understanding of short and ultra short laser ablation process has been explained. The critical analysis of various theoretical and experimental studies is used to describe the performance of laser beam micromachining (LBMM) on some of the advanced engineering materials.
1. INTRODUCTION
Laser, an acronym for light amplification by stimulated emission of radiation, is surely one of the greatest and considerable innovations of 20th century. Its continued advancement has been an exciting chapter in the history of science, engineering and technology. Emergence of advanced engineering materials, intricate shape and unusual size of workpiece restrict the use of conventional machining methods .Laser beam machining is one of the most widely used thermal energy based non-contact type advance machining process which can be used for almost all range of materials. The foundation of laser was laid by Einstein in 1917 when he first introduced the concept of photon emission, where a photon interacts with an excited molecule or atom and causes the emission of a second photon having the same frequency, phase and direction [References].
The laser beam is generated by providing energy to lasing medium from an external source. The electrons at ground state of lasing medium gets excited, which causes electron to move from a lower energy level to a higher energy level and due to very high unstability at higher energy level it comes back to its ground state within a very small time by emitting a photon. The frequency of this emission is then amplified by using two mirrors one with 100% reflective surface and the other with partially reflective one. Laser beam machining is used to perform various operations such as drilling, cutting, turning, milling etc. [References].
Although material removal by pulsed light sources has been studied since the invention of the laser [1, 2], reports in 1982 of polymers etched by excimer lasers [3,4] stimulated widespread investigations aimed at using the process for micromachining. In the interceding years scientific and industrial researches in this field has proliferated to a staggering extent, probably aroused by the remarkably small features that can be etched with little damage to surrounding region of material.
In recent years, manufacturing industry has observed a rapid increase in demand for micro-products and micro-components in many industrial sectors including the electronics, optics, medical, biotechnology and automotive sectors. These micro-system-based products are an important contributor to a sustainable economy. The laser pulses used in micromachining processes are divided in two groups, one is short (nanosecond) laser pulse and other is ultra short (picoseconds, femtosecond) laser pulse. Due to the short pulse duration, peak powers of more than 15GW can be reached, which gives access to further ablation mechanisms, like multi-photon ionisation. Due to the short interaction time, only the electrons within the material are heated during the pulse duration and heat affected zones are negligible [References].
Laser micromachining or ablation phenomenon can occur during laser–polymer interaction in two distinct mechanisms: one of them is photo thermal and the other one is photochemical ablation. Since polymers exhibit strong absorption in ultraviolet and deep infrared wavelengths, but weak absorption at visible and near-infrared spectra, thus the ablation mechanism is a combination of photochemical and photo thermal processes. The chemical bonds of the polymer material decompose by the photon energy of the laser light, whereas in photo thermal mechanism, polymer ablation takes place by rapid melting and vaporizing. For photochemical ablation to occur, energy of the photons at that wavelength should exceed the intermolecular bond energies of the polymer. The photon energy decreases as the wavelength increases. Thus, the high energy of the UV photon breaks the molecular bonds and results in direct photochemical ablation [References].
The excimer laser has wavelengths available at 308 and 248 nm when using gas mixes of XeCl and KrF respectively. The frequency converted Nd:YAG lasers with a fundamental wavelength of 1,064 nm have wavelengths of 355 nm and 266 nm for the third and fourth harmonics, respectively. Pulsed Nd: YAG laser beam was used by Lau et al. [5] for experimentation to see the effect of HAZ on 2.5 mm thick carbon fibre composite plate with some parameters. They found that HAZ increases with increase in pulse width, pulse frequency, pulse energy and decreasing with the feed rate. They also observed that heat affected zone will be more when compressed air used as assist gas while argon used as assist gas have smoother cut surface and less HAZ.
In contrast to the UV laser, a CO2 laser emits infrared radiation at a wavelength of 10.6 μm which means that the laser beam always ablates the underlying material photo thermally. The area on which focused laser beam meets the workpiece surface, temperature of the irradiated spot rises so rapidly that the material first melts and then decomposes, leaving a void in the workpiece. Laser power, pulse length, cutting speed was used as main parameters by Olsen et al. [6] for cutting mild steel cutting using pulsed Nd: YAG and CO2 laser to perform experiments. It was observed by their study that the CO2 laser can cut faster than Nd: YAG but Nd: YAG laser serves better surface roughness. It has also been observed that the two parameters i.e. pulse length and cutting speed also affects the surface roughness.
This research work includes some comments made about the developments in this technology and it also deals with the various applications using short and ultrashort pulsed lasers.
2. LASER BEAM MICROMACHINING (LBMM)
2.1 TYPES OF LASERS USED IN LBMM
There is range of industrial lasers available in a present scenario for micromachining applications (Figure 1). Generally two types of laser are used for micromachining metals—short pulse laser which emits short pulses of light, of the order of picoseconds to nanoseconds and ultrashort pulse laser which emits ultrashort pulses of light, of the order of femtoseconds to ten picoseconds [7]. These lasers are named based on the duration of their beam pulses. For example, the pulse emitted by a femtosecond laser only lasts femtoseconds (a femtosecond is one millionth of a nanosecond or 10-15 of a second). Similarly each pulse emitted by a picoseconds laser lasts picoseconds and each pulse emitted by nanosecond laser lasts nanoseconds. The short pulse means the energy is localized at small depth.
2.2 INTERACTION OF SHORT AND ULTRASHORT-LASERS WITH DIFFERENT MATERIALS
The laser beam can be modelled as a radiant energy source with an arbitrary spatial and temporal intensity distribution. The fraction of total beam energy absorbed at the erosion front depends on the radiation properties of the workpiece and the geometry of erosion front. The intensity of the incident beam is expressed by Io. The decrement in the laser intensity with the depth is given by Ix = Io e−αx where α is optical absorptivity of the material and x is depth into the material. The optical absorptivity (α) of the material accounts for the decay of laser intensity with depth inside the material. The absorption coefficient depends on the temperature and wavelength but at constant α, decay of laser intensity with depth is given by Beer–Lambert Law as, I(z)=Ioeαz where Io is the intensity just inside the surface after considering reflection losses. Lambert's law states that absorption of a particular material sample is directly proportional to its thickness (path length). The depth at which the intensity of the laser drops to 1/e value of its initial value at the interface is its optical penetration or absorption depth (δ) given by δ = 1/α [8].
2.3 MECHANISM OF LASER ABLATION
Laser ablation is the process of removal of material from a solid surface by irradiating it with a laser beam. At very low energy density, there is no material removal until a point is reached where material removal starts to occur; this is called the ablation threshold [9]. A significant removal of materials occurs above a certain threshold power density, and the ejected material forms a luminous ablation plume. This threshold power density required to form plasma depends on the absorption properties of the given material, the wavelength of laser employed, and pulse duration [10].
The removed material is directed towards a substrate where it re-condense to form a film. The total mass ablated from the target material per laser pulse is called as ablation rate. To enhance the reactivity of the background gas with the ablated species, either a RF-plasma source or a gas pulse configuration are used [11].
2.4 GENERATION OF SHORT LASER PULSES
Short or ultrashort optical pulses can be generated with the help of continuous light source and fast modulator, which lets the light pass only for a short period of time. However, this method is not competent, as most of the light gets lost at the modulator, and also the duration of pulse is limited by the speed (bandwidth) of the modulator[13]. The most commonly used methods are:
2.4.1 Q-Switching - Photons that evolve as a result of spontaneous emission in directions other than that of the laser axis are amplified. These photons, however, are not reflected back into the cavity and are lost to the environment. The total loss of photons because of travelling off-axis is called as amplified spontaneous emission (ASE). “Q-Switching” is a pulse energy enhancement technique and is used in many solid state lasers to minimize the negative effects of ASE [14]. The Q switch (figure 3) device is an electronic shutter, sometimes called a Pockels cell, which is triggered open and shut by an electrical signal.
3. APPLICATIONS OF LBMM
The pulsed lasers are being used for micro processing materials in several manufacturing industries. Microvia, ink jet printer head and biomedical catheter hole drilling, thin-film scribing and micro electro-mechanical system (MEMS) are some of the important applications of LASER micromachining.
3.1 HOLE DRILLING
The ability of drilling smaller holes up to 1µm diameter is a key enabling technology to manufacture high-tech products. Laser micromachining provides solutions to vital problems in manufacturing integrated circuits, hard disks,
4. MAJOR AREAS OF RESEARCH IN LBMM
4.1 NUMERICAL ANALYSIS OF LASER MICRO DRILLING
Laser drilling is an extensively used process for fabrication of microvias and micro nozzles as discussed above. After being exposed to short pulses, material is removed by ablation, leaving a crater. By this means, well-shaped micro-hole can be drilled into the material bulk. It is experimentally observed that time delay between pulses has great influence on the shape and machining quality [25]. There is a threshold time at which abrupt shape degradation occurs. To achieve good machining quality and desired shape, longer time delay is preferred. The effect of the time delay can be explained by coupling of residual heat between successive pulses. In this section, our focus will be on quantitative investigation of accumulation of residual heat.
1. The heat dissipation after laser irradiation will be described by a mathematical model.
2. FEM method will be used for the analysis of temperature at the bottom of the crater ablated by a train of pulses
The approximate solutions to boundary value problems for partial differential equations can be found by a numerical technique called Finite element method (FEM). In FEM the whole problem domain is subdivided into simpler parts, called finite elements, and it minimizes an associated error function by using vibrational methods from the calculus of variations to solve the problem [24]. Unlike the idea that joining several small straight lines can approximate a larger circle, FEM includes methods for connecting several simple element equations over many small sub domains, named finite elements to approximate a more complex equation over a larger domain.
The threshold-like time delay has been observed for various lasers, including Nd-YAG laser and femtosecond laser. Through this research optical specifications for lasers, especially ultrashort laser, used for micromachining can be found.
4.2 LASER ASSISTED BONDING FOR MEMS ASSEMBLY
Wafer bonding has an important role in the assembly of Microsystems used for space application, such as microtrusters. For providing excellent mechanical strength, chemical durability and optical transparency combinations of silicon-glass couple and silicon-silicon couple is used. The two most common techniques used for silicon-glass and silicon-silicon joining are anodic bonding and fusion bonding, respectively [27]. Since both the method operates at very high temperature (over 1000K) thus it tends to damage the prefabricated microelectronics and hermetic sealing. Laser beam with pulse width in the order of 10ns has recently been proposed for wafer bonding. The major advantage of laser assisted bonding is that it operates at room temperature. Moreover, the very short laser pulse limits the heat conduction to a very small area of micron scale. This highly localized heating results in small thermal damage to the active area. Till now this technique hasn’t been developed much and in depth research work is required. Investigations needs to be focused on the effects of pulse energy and pulse width on melting time and penetration depth and on exploring other materials and material combinations that might be fused by laser assisted bonding.
4.3 LASER DIRECT WRITING
Laser direct writing technique is used for the formation of computer controlled two and three-dimensional pattern in a serial fashion. The versatility offered by laser-based direct-write methods is unique as it provides freedom to add, remove, and customize different types of materials without physical contact between a tool or nozzle and the material [23]. Laser pulses used to generate the patterns can be manipulated to control the composition, structure and properties of three-dimensional volumes of materials across length scales spanning six orders of magnitude, from nanometres to millimetres. The key elements of any LDW system can be divided into three subsystems: (1) Source of laser (2) beam delivery system and (3) target mounting system. Figure 7 is schematic representation of a laser direct-write system. The main components of an LDW system are (left to right) a substrate mounting system, a beam delivery system, and a laser source. Motion of either beam delivery system or the substrate mounting system is controlled by using computer-aided design and manufacturing (CAD/CAM) combined with the laser source.
5. CONCLUSIONS
1. LBMM is a powerful machining method for cutting complex profiles and drilling holes at micro level in wide range of workpiece materials. It uses short and ultra-short pulse lasers for processing of materials. Picosecond lasers are preferred over nanosecond and femtosecond lasers.
2. Laser Micromachining enables us to machine advanced engineering materials using lasers of different wavelength. For short pulse beams the energy is localized at small depth. With ultrashort laser pulses the ablated spot size may be smaller than the laser focus spot size. Beam pulse duration and repetition rate are two factors that influence laser usefulness for industrial micromachining applications.
3. The most distinct characteristic of ultrashort laser micromachining is that the ablation threshold is clearly defined. Therefore, feature size smaller than the laser spot-size can be achieved by carefully control the fluency of the laser pulses.
4. The energy contained in the photons is imparted as heat into the material and it potentially changes the surface structure of the target material. The laser beam induces a non-equilibrium electronic distribution that thermalizes via electron–electron and electron–phonon interactions.
5. Laser-based direct-write methods is unique, given their ability to add, remove, and modify different types of materials without physical contact between a tool or nozzle and the material of interest. Thin films are preferably patterned by mask projection using flat top beams of excimer lasers.