Thermal simulation of UV laser ablation of polyimide
Introduction
Laser processing of polymers has been subject to extensive scientific research now for more than two decades. The work has, however, still far not been completed. The interaction of the laser beam and the polymer material is rather complex, while modeling always comes with simplification of reality. Papers published on this discipline usually describe only some specific aspects of the ablation process or investigate e.g. etch rate in the function of a processing parameter or side-effect. At the same time models incorporate laser sources with completely different beam characteristics, as well as material properties also change type by type.
Our modeling and simulation work basically originates from industry-initiated process optimization projects. As feature sizes decrease in today’s high-density flexible substrates and there is constant need to increase the production speed, process windows of laser micromachining get critically narrow. Especially pad and pitch sizes of the patterned copper layer and focal spot diameters are within the same order of magnitude. This also means that the laser beam exposes inhomogeneous areas and volumes, thus parameters of the process need to be adjusted in accordance with the specific structure. The final goal of our basic research is to determine an adequate model for laser-material interaction, and to integrate it into the process control software of industrial laser machining appliances.
This paper deals with the 355 nm UV Nd:YAG laser light interaction with flexible polyimide substrates. The main outcome expected from our model is etch rate in the function of the beam parameters as well as material properties and structure of the patterned or processed layers. The model is supposed to give answer for our application: industry needs to introduce lasers for direct processing of polyimide foils to achieve high resolution patterning and controlled ablation of the substrate [1]. At the same time the flexible substrate is a multilayer structure: laminated and patterned copper areas increase the number of factors that influence the interaction by making the substrate inhomogeneous as far as its thermal conductivity is concerned. We have described in our previous work, that the underlying copper pattern influences the ablation of the polymer layer so its effect has to be compensated [2]. Such compensation requires a simulation tool that cannot only calculate the temperature distribution induced by laser and affected by the multilayer structure, but it also has to be able to reckon with the temperature dependency of ablation.
Former studies on UV beam—polymer interaction were carried out with excimer lasers, recent publications also deal with frequency tripled Nd:YAG lasers as these have been commercially available now for almost a decade. Models established with excimer lasers are now subject to validation experimentally outside the operation ranges of excimer lasers and new models are composed specifically concentrating on the beam properties of solid state lasers. Even though the wavelength and pulse duration of an excimer can be close to those of a frequency tripled Nd:YAG laser, their pulse repetition frequency (PRF) is limited to a few hundreds per second and the energy distribution in the beam is completely different, too.
Frequency tripled Nd:YAG lasers only appeared commercially in the last decade. Their advantages over excimer lasers are obvious as far as our application is concerned. Direct patterning of polyimides with this laser source is, however still a new field of application and needs its background to be explored. So far only Yung et al. published their theoretical and experimental results on the thermal aspects of this interaction [3], [4], [5]. The Gaussian beam shape, the effect of other layers of the structure are just some of the factors that were not considered in their work. As features of the structures to be produced are comparable with the spot size of the beam the above issues cannot be neglected. Thus our model does respect the Gaussian beam shape, as well as the cumulative heat effect of consecutive shots as result of high (up to 100 kHz) PRF.
One can experience either photo-thermal or photo-chemical effects or even both at the same time [6], while many other side-effects could be considered, like plume, plasma formation [7], [8] resulting complex shielding, acoustic waves due to explosion-like processes causing mechanical impact and stress [9], different heat transfer methods, different material properties in function of temperature and state of matter. Our model, however, does not consider these factors at this state of the work. Laser shots are treated as one-time direct energy transfers into the material, no molecular dynamics (MD) level considerations [10] are implemented.
Section snippets
Basic theory for modeling
The purpose of our research work is to obtain a model and its constants that could be built into the control software of laser machining systems. The following chapters will describe how we established and verified the basic equations of our model and what our methods were to set up and verify the constants of the model for single-pulse ablation. A properly determined model is expected to result in similar characteristics to what we experience in our real experiments. A properly determined set
Description of light energy transformation to heat
The energy density, fluence distribution of a Gaussian laser beam can be written aswhere α is the absorption coefficient, x, y, z are the space coordinates, so that the origin is at the centre of the beam and axis z is perpendicular to the surface of the material. The first term in the exponent describes the Gaussian distribution of the laser beam and the second comes from the Lambert–Beer Law of absorption. The parameter of width (σ) comes from the minimal spot
Experimental
The experiments were carried out by a Coherent AVIA 355-4500 frequency tripled, Q-switched Nd:YAG laser. The laser has a built in thermal lens compensation called ThermaTrack. The length of the resonator is optimized at each pumping diode current—PRF pair to obtain the highest output power and best beam quality. This means that the beam diameter and the energy distribution of the beam can be considered independent from the pumping current i.e. the pulse energy.
Positioning of the samples was
Results
The etch rate dependence on pulse energy was measured by the above described method. The PRF was 50 kHz and holes were drilled by pulse energies ranging from 5 to 61 μJ. In Fig. 2 the results of an experiment carried out at room temperature can be seen. Fig. 3, Fig. 4 show how the etch rate increases if the polyimide foil is heated up to 110 °C and 186 °C. Please note that the pulse energy is scaled logarithmically on all graphs. The error bars growing larger at higher pulse energies because of the
Modeling the temperature distribution between consecutive shots
In the basic theory section, the mode of energy conversion to heat was described. After the ablation is over the residual heat will be dissipated by three modes (conduction, radiation, and transfer). This is how cells in our finite element simulation communicate with each other.
Thermal conduction is described by the Fourier law:where q is the heat flow, λ is the thermal diffusivity, A is the area.
Radiation, according to the Stefan–Boltzmann law is proportional with the fourth
Discussion
The experiments have proved that the higher the initial temperature of the substrate the lower the ablation threshold energy is. If we substitute the 1.8, 1.5 and 1.3 μJ threshold energies in (8) we get the highest temperature rise in the material. In order to obtain the same ablation rates by simulation as we observed in the experiments Tth has to be chosen in a way that the value of this maximal temperature rise has to be added to the initial temperature of the substrate (see Table 1).
If ETF
Conclusions
The etch rate of UV Nd:YAG lasers can be described by the expression (12), which is similar to the equation used for the description of excimer laser ablation (11). The difference is that since fluence is hardly applicable for Gaussian beams, in the argument of the logarithm quotient of pulse energy and energy threshold is written instead of fluence and fluence threshold. Our experiments proved that (12) describes the ablation process precisely if the pulse energy is below 49 μJ, however the
References (14)
Laser processing for microelectronics packaging applications
Microelectron Reliab
(2001)- et al.
355 nm Nd:YAG laser ablation of polyimide and its thermal effect
J Mater Process Technol
(2000) - et al.
XPS investigation of Upilex-S polyimide ablated by 355 nm Nd:YAG laser irradiation
Appl Surf Sci
(2001) - et al.
High repetition rate effect on the chemical characteristics and composition of Upilex-S polyimide ablated by a UV Nd:YAG laser
Surf Coat Technol
(2002) - et al.
Laser ablation for analytical sampling: what can we learn from modeling
Spectrochim Acta Part B
(2003) - Balogh B, Gordon P, Berényi R, Illyefalvi-Vitéz Zs. Effect of patterned copper layer on selective polymer removal by...
- et al.
Enhanced polymer ablation rates using high-repetition-rate ultraviolet lasers
IEEE J Select Topics Quant Electron
(1999)
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