Kaidong D. Ye

Removal of submicron particles from nickel-phosphorus surfaces by pulsed laser irradiation

Abstract Pulsed laser cleaning was demonstrated to be an efficient way for removing submicron particles from the nickel-phosphorus (NiP) surface both experimentally and theoretically. Experimentally, it is found that using KrF excimer laser with a pulse width of 23 ns the cleaning threshold is about 20 mJ / cm 2 for removing quartz particles from the NiP surface and laser cleaning efficiency increases rapidly with increasing laser fluence. The theoretical analysis shows that the peak cleaning force (per unit area) is larger than the adhesion force (per unit area) for submicron quartz particles on the NiP surface when it is irradiated by excimer laser with a fluence above 10 mJ / cm 2 . Therefore, it is possible to remove submicron quartz particles from NiP surfaces by laser irradiation. The difference between the cleaning force (per unit area) and the adhesion force (per unit area) increases with increasing laser fluence, leading to a higher cleaning efficency for quartz particles on the NiP surface.

Laser applications in integrated circuit packaging

Laser processing has large potential in the packaging of integrated circuits (IC). It can be used in many applications such as laser cleaning of IC mold tools, laser deflash to remove mold flash form heat sinks and lead wires of IC packages, laser singulation of BGA and CSP, laser reflow of solder ball on GBA, laser marking on packages and on SI wafers. During the implementation of all these applications, laser parameters, material issues, throughput, yield, reliability and monitoring techniques have to b taken into account. Monitoring of laser-induced plasma and laser induced acoustic wave has been used to understand and to control the processes involved in these applications.

Cleaning force in laser cleaning of silicon substrates

A laser cleaning model was established for removal of non- absorbing particles from an absorbing solid surface by taking adhesion force and cleaning force into account. The cleaning force per unit area due to laser-induced thermal expansion of a substrate surface is (gamma) E (Delta) T(0, t), where (gamma) , E, and (Delta) T(0, t) are the linear thermal expansion coefficient, the elastic modulus and temperature rise at the substrate surface, respectively. The cleaning condition and threshold fluence can be obtained by comparing the cleaning force and the adhesion force. The theoretical analysis shows that cleaning force increases with increasing laser fluence, deducing the pulse duration, or decreasing laser wavelength, which leads to a higher cleaning efficiency at higher laser fluence, smaller pulse duration or shorter laser wavelength. The experimental results show that the cleaning threshold fluence for laser removal of quartz particles from silicon surfaces is about 135 mJ/cm2, which is in good consistency with the theoretical threshold fluence of 120 mJ/cm2. With increasing laser fluence, the cleaning efficiency increases, which has been predicted by our theoretical analysis.

Laser induced nanobump array on magnetic glass disk for low flying height application

Laser processing has found an important application in the hard disk drive (HDD) industry to fabricate bumps for screen testing of the manufactured magnetic disk media prior to HDD assembly. The flying height of the head slider needs to be calibrated by a specifically designed bump array. With areal density of HDD already exceeds 100Gbits/in2, the head slider flies at 6 nm flying height and will become even lower. Sub-10nm bump height, as low as 3.5nm, is required to match future slider flying height. In this study, bump arrays with sub- 10nm bump height, good height uniformity and precise distribution was fabricated on glass disk substrate using an integrative control system consisting of a pulsed CO2 laser and a high precision stage. The bump disk requirements were demonstrated successfully and compared with industrial specifications.

Low resistivity glass metallization by laser induced plasma-assisted ablation

A novel direct-write glass metallization based on laser induced plasma assisted ablation (LIPAA) was investigated. Laser is passed through a glass substrate and irradiated onto a metal target placed beneath the substrate. By tuning laser fluence above target ablation threshold, target ablation and plasma generation occur. The plasma flies towards the glass at a high speed and deposits metal materials onto glass backside surface. Metal films were fabricated and their sheet resistances were measured by a four-point probe. It was found that sheet resistances of the metal films vary with processing parameters. Experimental results reveal that low resistivity metal film (< 0.3 Ω/∠) can be obtained at an optimal laser scanning speed and pulse repetition rate. When target-to-substrate distance increases, film resistance also increases. Optimal design of overlap schemes among metal tracks provides a lower film resistance. Meanwhile, thin film and bulk metal targets were used to study their difference on film resistance. It was discovered that deposition using thin film target is more efficient. Laser annealing technique was also applied to activate the deposited metal materials to get higher quality glass surface metallization.

Laser ablation for MEMS microfabrication on Si and Kapton substrates

Laser-ablation-based microfabrication technology is applied to fabricate micro-electro-mechanical-systems (MEMS) devices on polymer substrates. A micromachining apparatus is designed and developed which includes a 355 nm laser, an uncoated focusing lens, computer-controlled precision x-y-z stages and in-situ process monitoring systems. Concentric rings microstructures are formed by efficiently changing the laser intensity distribution. Tiny via holes and micro-nozzles with different diameters have been obtained by low power laser direct drilling. Optical microscopy and scanning electron microscopy (SEM) are used to evaluate the processing results at different laser processing parameters. This method has the advantages of low-cost and time-saving in circle via holes fabrications. Potential applications of this novel MEMS fabrication technique are also briefed.

Laser singulation of IC packages

The separation of IC packages from a BGA board is realized by means of laser multi-scan method. The laser used in the study is a double frequency Nd-YAG laser with wavelength of 532 nm. The big problem in the laser processing approach mainly arises from the multi-layer materials of BGA board with copper, polyethylene and epoxy glass fiber, because of their different absorption coefficient to the laser beam and their different absorption coefficient to the laser beam and their different heat conductivity. In the experiment approach, the effects of laser parameters, such as wavelength, on the dicing efficiency has been investigated for choosing laser. The influence of sample side for laser incidence on cut profile and, the influence of the focused extent of laser beam on singulation speed are discussed. The experimental results show that laser singulation of IC packages is efficient and reliable.

Laser marking on IC package by micro-encapsulated tapes

In this paper, we are reporting a new way to do marking on IC package. In this way, white ink is wrapped in microcapsules that are coated on a transparent tape. Laser is irradiated on the tape surface, the microcapsules are broken and the ink is released onto the IC package surface. After an UV light treatment, the ink will stick on the IC surface, forming a high contrast marking. It is found that the quality of the marking depends on tape configuration, tape-IC distance, laser peak power, scan speed of laser irradiation and other laser parameters.

Laser creating precise three-dimensional image of object inside glass

Fabrication of an object image inside glass material can be realized by laser direct writing with a programmable motion system. In the focus depth direction, namely Z-direction, the really formed dimension will have some difference from the distance moved by the motion system because of light refraction effect, which causes a deformation of the image from the object. Therefore, it is necessary to modify the object dimension in Z-direction in the executive program. The required amount of the dimension modification depends on the refractive index of the glass and the numerical aperture of the focus lens used. A formula for the dependence of the amount of dimension modification on the refractive index and the numerical aperture is presented based on Snell¡¯s law. Experimental results show that the image created inside glass really reflects the dimension feature of the object with a programming dimension modification in Z-direction in terms of the formula presented. A comparison of a sphere and a cub images created inside glass with the dimension modification with that without the modification is produced for making sure the importance of the dimension modification.

Wafer dicing by laser induced thermal shock process

In this report, a new way of wafer dicing is carried out by laser induced thermal shock process. This system consists of the use of a Nd:YAG laser to heat up the wafer surface following by a cooling fluid along the scanned line. The temperature gradient created by the laser heating and the gas cooling will cause a micro-crack on the wafer surface along the scanned line and the resulting crack propagation finally separate the silicon wafer into two pieces. As there is no material loss and removal during the separation process, the wafer dicing line width can be as small as sub-micron. The cross section of the wafer is smooth comparing with other separation methods and a high separation speed of 70 mm/s is achieved.