We present theoretical calculations and experimental measurements of silicon micromachining rates, efficiency of laser pulse utilization, and morphology changes under UV nanosecond pulses with intensities ranging from 0.5 GW/cm2 to 150 GW/cm2. Three distinct irradiance regimes are identified based on laser intensity. At low intensity, proper gas dynamics and ablation vapor plume kinetics are taken into account in our theoretical modeling. At medium high intensity, we incorporate the proper plasma dynamics, and predict the effects of the laser generated vapor plasma and the electron hole plasma on the laser-matter interaction. At even higher intensity, we attribute the observed increased ablation rate to energy re-radiation from the laser heated hot plasma, the strong shock wave, and the accompanied strong shock wave heating effects. Experimentally measured data in these regimes agree well with our calculations, without changing parameters in the calculations used for the three regimes. Our results can be applied toward quantitatively characterize the behavior of ablation results under different laser parameters to achieve optimal results for micromachining of slots and vias on silicon wafers.
The generation of debris is critical in the future application of laser technology in IC, MEMS, MOEMS manufacture. Re-deposition of debris is also critical in optimising throughput of multi-pass laser ablative processes.
In this study, the debris formed in laser micromachining of wafer grade silicon is investigated. Details of the laser workstation, based on a UV DPSS laser, will be presented and the development of real time diagnostic capabilities and off-line techniques will then be described. A real time imaging capability has been used to monitor plasma and shock front propagation with nanosecond resolution. The detection system is also used to monitor spectral emission of debris and micron-sized particulate ejected from the silicon surface. Emission spectroscopy of the laser ablated silicon in the plasma show spectral features that are characteristic of atomic and molecular species on timescales of nanoseconds and microseconds, respectively, after the laser pulse.
Off-line characterisation techniques have focused on investigating the distribution and chemical composition of entrapped particulate. A number of novel experimental configurations for particulate entrapment, both adjacent to and remote from the laser-ablated surface, will be described. EDX results indicate that debris generated in air is composed principally of oxygen and silicon. Additional SEM results indicate that the particulate size grows through aggregation and depends on the environment in which they are generated.
The focussed spot size of industrial laser beams is a critical processing parameter in most laser machining applications as it determines the machined feature size and the irradiance produced by the laser at the material interface. There are a number of standard methods available for accurately measuring and analysing the focussed spot. These methods often require expensive equipment that can be time consuming and difficult to set up in a production environment.
This paper presents an investigation into a cost effective and straightforward method for the measurement of focussed laser spot sizes based on drilling of holes in mylar film. It can be shown that the slope of a plot of the square of the hole diameter versus the natural log of the laser pulse energy is equal to twice the square of the spot radius. A measure of the laser spot size can be calculated by generating laser-drilled holes at number of laser pulse energies. The practicality and accuracy of this method is investigated in this paper for a number of laser types including a diode pumped solid state laser (UV DPSS) operating at the third harmonic (355nm), a femtosecond laser and a flash lamp pumped Nd:YAG laser. A comparison between the measured results and the results generated with other available techniques is also presented.
Process monitoring is a vital part of industrial laser applications that enables intelligent control of processes by observing acoustic, optical, thermal and other emissions. By monitoring these emission during laser processing, it is possible to ascertain characteristics that help diagnose features of the laser processed material and hence to optimize the technique. An experimental set up of observing plasmas during laser spot welding is described here. A pulsed Nd:YAG laser was used to spot-weld a variety of materials of different thickness, the plasmas generated during welding were monitored by a number of techniques, and the data obtained was used to characterize the welds. In the study photodiodes were set at different angles and observed the intensity and generation of the plasmas during the laser spot-welding process thereby giving a weld 'signature.' A portable spectrometer was used off-axis to obtain spectra of the emissions from the plasmas. Post process analysis was performed on the materials by mechanical polishing and chemical etching and observations of weld penetration depth and weld quality were correlated with the data collected on the plasmas. Different cover gases were also used during laser welding and the results of the effects of the various gases on the plasma are shown. The results indicate the relationship between laser weld generated plasma characteristics and weld features such as penetration depth. A direct correlation between the intensities of the photodiode and portable spectrometer signals was observed with weld penetration depth.
The small focused spot size and localized heating possible with laser soldering makes it an attractive alternative technique for bonding surface mount devices with small lead pitch onto a printed wiring board. Using a Nd:YAG laser, we have applied this technique to reflow soldering of a test device-a 224-pin quad ceramic chip (lead spacing 25 mil)-onto pretinned substrates. The soldering step was incorporated into a larger workcell, in which an Adept robot was used to place the component in position and also to scan the laser beam, delivered via a fiber, over the leads to be soldered. The mobile laser head was modified to accept a miniature CCD camera coupled to a Cognex vision system, which allowed coincident viewing of the soldering process and postsoldering inspection of the joints.
Conference Committee Involvement (2)
SPIE Eco-Photonics 2011: Sustainable Design, Manufacturing, and Engineering Workforce Education for a Green Future
28 March 2011 | Strasbourg, France
The Role of Photonics in Sustainable Manufacturing Development and Processes