Kenshi Fukumitsu

Advanced Dicing Technology for Semiconductor Wafer—Stealth Dicing

ldquoStealth dicing (SD)rdquo was developed to solve inherent problems of a dicing process such as debris contaminants and unnecessary thermal damages on a work wafer. A completely dry process is another big advantage over other dicing methods. In SD, the laser beam power of transmissible wavelength is absorbed only around focal point in the wafer by utilizing the temperature dependence of the absorption coefficient of the wafer. The absorbed power forms a modified layer in the wafer, which functions as the origin of separation in the separation process. In this paper, we applied this method for an ultra-thin wafer. The reliability of devices that is diced by SD was confirmed.

Development of PET Detectors Using Monolithic Scintillation Crystals Processed With Sub-Surface Laser Engraving Technique

New monolithic scintillation detectors for PET have been developed, where the crystals are processed using internal focused laser processing technique, which is called subsurface laser engraving (SSLE) technique. When high intensity light pulses of short duration from a laser are focused into a scintillation crystal, they induce multi-photon absorption at the focal point and result in refractive index changes or micro-cracks inside the crystal. By applying the SSLE technique to a monolithic scintillation block, fine segmentation in the crystal can be formed without inter-pixel gaps. We have fabricated 2D segmented arrays engraved various patterns of micro-cracks inside monolithic LYSO crystal blocks by using a Nd:YAG laser. The processed crystal array segmented to 12 × 12 with 1.67 mm pitch have been evaluated by coupling to a position-sensitive photomultiplier tube (PS-PMT). The 2D position histograms were measured for uniform irradiation of gamma-rays and each crystal segment was clearly separated. The average energy resolution was 9.7%, similar to that of the conventional arrays, so that the laser processed LYSO crystals have kept their primary scintillation properties. We have also evaluated the laser processed crystals by using multi-pixel photon counters (MPPCs) to investigate the possibilities as a future PET detector. These results suggest that it is possible to fabricate high performance PET detectors using the SSLE technique.

Fabrication of finely pitched LYSO arrays using sub-surface laser engraving technique with picosecond and nanosecond pulse lasers

We propose to adopt the sub-surface laser engraving (SSLE) technique for efficient and precise fabrication of finely pitched scintillation crystal arrays. However, its application to thicker crystals is still challenging. It is hard to focus the laser beam tightly at a point far from the crystals surface because of the large refractive index of the scintillator. Therefore, a higher laser energy is needed to create microcracks at deep positions in the crystal. Because this would cause excessive damage to the scintillation crystal during laser scans, the process yield of SSLE is reduced. We found that this issue could be overcome by a novel SSLE technique using both picosecond (ps) and nanosecond (ns) pulse lasers. The experimental results indicated that the total laser energy required for creating microcrack walls in a LYSO crystal can be reduced compared to that of conventional SSLE using only a ns pulse laser. The SSLE technique using both ps and ns pulse lasers would enable the fabrication of finely pitched LYSO arrays with a higher process yield.

Analysis of internal crack propagation in silicon due to permeable pulse laser irradiation: study on processing mechanism of stealth dicing

Stealth dicing (SD) is an innovative dicing method developed by Hamamatsu Photonics K.K. In the SD method, a permeable nanosecond laser is focused inside a silicon wafer and scanned horizontally. A thermal shock wave propagates every pulse toward the side to which the laser is irradiated, then a high dislocation density layer is formed inside the wafer after the thermal shock wave propagation. In our previous study, it was concluded that an internal crack whose initiation is a dislocation is propagated when the thermal shock wave by the next pulse overlaps with this layer partially. In the experimental result, the trace that a crack is progressed gradually step by step was observed. In this study, the possibility of internal crack propagation by laser pulses was investigated. A two-dimensional thermal stress analysis based on the linear fracture mechanics was conducted using the stress distribution obtained by the axisymmetric thermal stress analysis. As a result, the validity of the hypothesis based on a heat transfer analysis result previously presented was supported. Also it was concluded that the internal crack is propagated by at least two pulses.

Analysis of modified layer formation into silicon wafer by permeable nanosecond laser

When a permeable nanosecond pulse laser which is focused into the inside of a silicon wafer is scanned in the horizontal direction, a belt-shaped high dislocation density layer including partially polycrystalline region is formed at an arbitrary depth in the wafer. Applying tensile stress perpendicularly to this belt-shaped modified-layer, silicon wafer can be separated easily into individual chip without creating any damage to the wafer surface comparing with the conventional blade dicing method, because the cracks that spread from the modified layer up and down progress to the surface. This technology is called “stealth dicing” (SD), and attracts attentions as a novel dicing technology in semiconductor industries. The purpose of this study is to clarify the formation mechanism of modified layer. We paid attention to an experimental result that the absorption coefficient varies with temperature. We analyzed a coupling problem composed of focused laser propagation in a silicon single crystal, laser absorption, temperature rise, and heat conduction. Simple thermal stress analysis was also conducted based on those results. As a result, formation mechanism of the modified layer could be explained clearly. Temperature dependence of absorption coefficient is the most important factor of the modified layer formation. The present analysis can be applied to find the optimum laser irradiation condition for SD method, and it is a future subject to confirm it experimentally. It was supported by the present analysis that the problem of thermal effect on the active region can be solved by the SD method. SD method for wafer dicing is original firstly and it is valuable that formation mechanism of the modified layer in SD method was clarified theoretically.When a permeable nanosecond pulse laser which is focused into the inside of a silicon wafer is scanned in the horizontal direction, a belt-shaped high dislocation density layer including partially polycrystalline region is formed at an arbitrary depth in the wafer. Applying tensile stress perpendicularly to this belt-shaped modified-layer, silicon wafer can be separated easily into individual chip without creating any damage to the wafer surface comparing with the conventional blade dicing method, because the cracks that spread from the modified layer up and down progress to the surface. This technology is called “stealth dicing” (SD), and attracts attentions as a novel dicing technology in semiconductor industries. The purpose of this study is to clarify the formation mechanism of modified layer. We paid attention to an experimental result that the absorption coefficient varies with temperature. We analyzed a coupling problem composed of focused laser propagation in a silicon single crystal, laser absorp...

Modified-layer formation mechanism into silicon with permeable nanosecond laser

The purpose of this study is to clarify the formation mechanism of modified layer. A coupling problem composed of focused laser propagation in a silicon single crystal is examined, considering laser absorption, temperature rise and heat conduction, with particular attention to an experimental result that the absorption coefficient varies with temperature. Simple thermal stress analysis was also conducted based on those results. As a result, the formation mechanism of the modified layer could be explained clearly. It was seen that the temperature dependence of absorption coefficient is the most important factor of the modified layer formation. This present analysis can be applied to find the optimum laser irradiation condition for Stealth Dicing (SD) method, and it is a future subject to confirm it experimentally. It was supported by this present analysis that the problem of thermal effect on the device region can be solved by the SD method. [Received 7 May 2006; Accepted 2 February 2007]

Compact, Rigid, and High-Power Ultrafast Laser System Applying a Glass-Block Cavity

We developed a compact Yb:YAG ceramic regenerative amplification system. A rectangular glass block is used to elongate the cavity. A pulse to be amplified is propagated in a long distance in the glass block by being reflected repetitively at the end faces of the glass under a condition of total internal reflection. Furthermore, we produced transmission gratings with a diffraction efficiency of more than 95%. The floor area of the entire amplification system is reduced to less than 2,000 cm2. In 20-kHz operation, the system generates 1.0-ps compressed pulses of 4.5-W average power, i.e., 0.225-mJ energy.

Laser microprocessing unit and its application

This is the report for compact laser micro processing unit excimer laser employed featuring a very fine process with high accuracy. This unit consists of objective lens, of which magnification is 10 to 80, used for both processing and observation. It makes possible high energy density resulting 0.5micrometers resolution at 248nm, accurate positioning and compact size. Applications 1) Removing upper metal layer of LSI in order to inspect pattern of the bottom layer. 2) Creating fine geometrical pattern on PET fiber cloth in order to apply new function such as better dyeing and adhesiveness. 3) Creating 100micrometers dia. Hole to artificial blood vessel made of polyurethane tube with 2mm inner dia. In order to have similar mechanical property to real blood vessel.