Multi Beam Laser Grooving Process Parameter Development and Die Strength Characterization for 40nm Node Low-K/ULK Wafer

Transcription

1 Multi Beam Laser Grooving Process Parameter Development and Die Strength Characterization for 40nm Node Low-K/ULK Wafer Koh Wen Shi 1,3, K. Y. Yow 1, Calvin Lo 1, Dr. Yap Boon Kar 2, Dr. Halina Misran 3 1 Freescale Semiconductor Sdn. Bhd., No. 2 Jalan SS 8/2, Free Industrial Zone Sungai Way, Petaling Jaya, Selangor. 2 Center of Microelectronic and Nanotechnology Engineering (CeMNE), College of Engineering, Universiti Tenaga Nasional, Kajang, Selangor. 3 Department of Mechanical Engineering, College of Engineering, Universiti Tenaga Nasional, Kajang, Selangor. wenshi.koh@freescale.com, Tel: ABSTRACT This paper describes the development work of enabling a multi beam laser grooving technology for 40nm node lowk/ulk semiconductor device. A Nd:YAG ultraviolet (UV) laser diode operating at a wavelength of 355 nm was used in the study. The effects of multi beam laser micromachining parameters, i.e. laser power, laser frequency, feed speed, and defocus amount were investigated. The laser processed die samples were thoroughly inspected and characterized, which included the die edge and die sidewall grooving quality, the grooving shape/profile and the laser grooving depth examination. Die strength is important and critical. Die damage from thermal and ablation caused by the laser around the die peripheral weakens the mechanical strength within the die, causing a reduction in die strength. The strength of a laser grooved die was improved by optimizing the laser process parameter. High power optical microscopy, scanning electron microscopy (SEM), and focused ion beam (FIB) are the inspection tools/methods used in this study. Package reliability and stressing were carried out to confirm the robustness of the multi beam laser grooving process parameter and condition in a mass production environment. The dicing defects caused by the laser were validated by using failure analysis. The advantages and limitations of conventional single beam compared to multi beam laser grooving process were also discussed. It is shown that, multi beam laser grooving is possibly one of the best solutions to choose for dicing quality and throughput improvements for low-k/ulk wafer dicing. The multi beam laser process is a feasible, efficient, and cost effective process compared to the conventional single beam laser ablation process. 1.0 INTRODUCTION Classic electronic packaging presents significant problems. The integrated circuit (IC) package which provides I/O connections from the chip to the rest of the system is typically bulky and costly, limiting the performance and the reliability of the IC. To address these concerns, form factor packaging is the current trend in electronic packaging technology. Form factor packaging allows the silicon chip to become thinner, smaller, and faster [1]. The smallest feature size on today s IC s are already reduced to 40nm node and lower, and the increasingly used of low-k dielectric materials provide more function capacity and higher performance to the chip, makes the chip capable of surviving daily use. The aluminium metal interconnects are replaced by copper (being a better electrical conductor), and the low-k dielectric material (being a better insulator). The low-k materials are typically more brittle and have lower adhesion than silicone dioxide [2]. As wafers become thinner and larger in diameter, the street widths between the dies become narrower. Conventional diamond blade dicing is not capable of meeting the dicing process efficiency, quality and yield. The diamond blade sawing process may induce subsurface damages which included delamination of low-k layers, die chipping and microcracks. This happens due to the brittleness and poor adhesion of low-k materials and silicon. Reduction in sawing feed speed reduction doesn t seem to help to resolve these dicing problems, instead results to high dicing wheels breakage, high blade consumption costs and low dicing productivity [3-5]. The use of laser ablation in the semiconductor industry is a well known technology in recent years. Laser dicing is a powerful tool, as it presents some benefits over traditional diamond blade sawing, such as high flexibility, high accuracy, high speed, and excellent cut quality for semiconductor low-k/ulk wafer processing. There are various established laser dicing techniques currently available in the market included wet laser dicing (water jetguided laser), dry laser dicing and stealth dicing. However, conventional dry laser dicing produces relatively large amount of debris (the heated/molted material expelled during laser) that deposited on the wafer surface, it is difficult to remove and thus, may have damaged the active circuitry of the die. Also, laser dicing with very high pulses generates significant heat affected zone which may have decreased the die fracture strength. The multi beam laser dicing technology has been applied in a wide range of application areas including micro-lithography in the wafer fabrication process as well as in the assembly of electronic components. However, it was found that there are limited attention and visibility put on multi beam laser dicng technology, and the effects of the critical laser process parameters. This includes laser power, frequency, defocus, and feed speed optimization to improve the grooving die edge quality as well as the chip strength performance. This paper is intended to investigate a novel high quality multi beam laser dicing technique for low-k semiconductor wafer, which creates an opportunity to resolve the low-k dicing defects and challenges encountered by the traditional diamond blade dicing. The top low-k/ulk dielectric layer is ablated by the multi beam laser follwed by full singulation of the die. This is accomplished by using a standard blade saw to cut through the rest of the silicon wafer. The experimental details on the test vehicle information, the multi beam laser machining process parameter setup and optimization; and the chip strength characterization and examination procedures will be presented.

2 2.0 EXPERIMENTAL WORK 2.1 Test Vehicle Information A 8.1 x 8.1 mm dicing-specific test vehicle die was used in this study. It was fabricated in 40nm CMOS copper technology with 6 to 7 copper layers with 2.8um aluminium pad thickness. The wafer diameter and die thickness used in these experiments are 300mm and 178um respectively. The die design was placed on a pizza mask reticle along with other logic blocks. The saw street width evaluated in the test vehicle was 60um which is much narrower than the common saw street width of 100um to 120um currently used in mass production. The cross sectional view of the 40nm metal stack structure is shown in Figure 1. The 60um saw street outside the test vehicle, has various scribe test structures in place. Scribe structures with high metal density and non nitride exposed alumimum pads were placed on the sides of the test vehicle. Scribe structures called Scribe Grid Production Control (SGPC) with high metal density and nitride defined aluminum probe pads were strategically placed flanking the die corners as these corners are stress concentration points. In order to isolate and extract the test vehicle, the laser would cut through the scribe test structures. Fig. 1. Cross sectional view of 40nm low-k metal stack structure 2.2 Setup of Multi Beam Laser System Figure 2 depicts the setup of the UV laser system, which has a pulsed Nd: YAG laser diode with a 355nm wavelength, pulse width from 110ns to 200ns, repetition rate ranging from 40kHz to 150kHz and a maximum power of 12W at 40kHz. The laser specifications and process parameters are listed in Table I. 355nm UV laser diode Mechanical shutter Beam splitter Mirror (X-Y scanning) Wafer stage (Z positioning) Optic lens Wafer Fig. 2. Setup of multi beam laser dicing system The concept of the multi beam laser dicing principle is to feed the main laser beam (laser source) through a beam splitting device which splits the high power laser beam into a plurality of laser beams. Each laser beam carries an equal amount of power with a fixed distance between them. The number of beams generated and the distance between them are dependent on the design of the beam splitting device. The semiconductor wafer was fixed on the stage and was driven by stepping motors (X- and Y- axis) for long travel and fine positioning. The laser beam was focused on the wafer. The laser power was measured with a power meter and the processing atmosphere was under room temperature and air condition. Table I: Laser system specification and process parameter Parameters Specifications Maximum output power 12W at 40kHz Wavelength 355nm Repetition rate 40kHz 150kHz Pulse width 110ns 120ns Feed speed 0 500mm/s Focus -100um 100um Number of beams 1 50 beams Figure 3 shows the schematic diagram of a typical multi beam laser dicing. Successive grooves were irradiated with constant spacing within consecutive scanning of the wafer surface by multi beam laser. The process involves applying a polyvinyl alcohol (PVA) coating on the wafer prior to laser dicing in order to prevent contamination of laser debris. After laser dicing, the PVA coating is removed by washing with de-ionized (DI) wafer. Final singulation of the die is accomplished by utilizing a standard saw blade to cut through the remaining silicon. 1 1 pass grooving with double beam groove blade kerf Dicing tape 2 1 pass grooving with 3x3 matrix beams Si wafer Fig. 3. Schematic diagram of multi beam laser dicing The laser responses are governed by 4 main input parameters, included laser power, repetition rate, feed speed and focus. The critical laser parameters and its respective operating range are shown in Table I. Experiments were carried out for all possible combinations of laser parameters in order to determine the optimal parameters with the lowest defect and most efficient laser dicing on the 40nm CMOS Cu low-k wafer. The cutting edge quality, groove depth and geometry are characterized under optical visual inspection, scanning electron micrography (SEM) and focused ion beam milling (FIB). The characterization results will be discussed in detail in Section 3.0.

3 2.3 Evaluation of Die Strength Characteristics To characterize the strength of Si die, a four-point bending test was employed. Bending test is a common technique for brittle material testing, thereby providing a better understanding of the stress accumulated in the die before failure. As illustrated in Figure 4, the die strength can be computed from the breaking load (W) applied to the Si die for a given die thickness (h) and width (b), and length of the supporting span (a), equation (1). In this test setup (see Figure 4) the die is rested on two fixed points and loaded by double loading rods from the top at a constant rate. Bending moment is constant and high along the bottom side of the test. W = breaking load (N) a = supporting span (mm) b = width of the die (mm) h = thickness of the die (mm) from a non-optimized laser process parameters. It was seen from the optical photos, (refer Figure 5(a) and 5(b)), peeling of the metal layers and Si debris were produced as the results of using the extremely high pulse energy above 90uJ and low pulse overlap of 25% and below. The occurrences of these defects are fairly high, and significantly affects the dicing quality. It was found that higher laser pulse energy is undesirable, resulting in poor and rough dicing edge quality. Another round of assessment was conducted by using a much lower pulse energy and higher pulse overlap resulting in zero metal layers peeling and Si debris. However, hairline cracks and dark markings were observed, see Figure 5(c) and 5(d). About 45% of die samples were affected with the dark marking defects. Further process parameter optimization was required to eliminate the dicing defects caused by the laser. FIB cross section confirmed that the dark mark did not show damage to the die edge seal (see Figure 6) but metal peeling was observed instead. Pulse energy above 90uJ, Pulse overlap of 25% and below W/2 W/2 metal layers peeling Si debris Die b h (a) metal layers peeling (b) Si debris Lower pulse energy, Higher pulse overlap Die Active Side (in tension) a Fig. 4. Schematic diagram of four-point bending test The fracture strength of Si die was studied with emphasis on the effects of 3 different dicing technologies, included single beam laser dicing (baseline process), multi beam laser dicing and conventional blade dicing. Die samples (8.1 x 8.1mm) were prepared from a 178um thick low-k Si wafer, the die strength was determined by a four-point bending test. For every dicing condition, 25 die samples were tested. The influences of laser parameter, such as the pulse energy and repetition rate on the die strength improvement will be discussed in next section. 3.0 RESULTS AND DISCUSSION 3.1 Laser Diced Edge Quality Laser pulse energy and pulse overlap are critical factors to dicing quality. Laser pulse energy and pulse overlap equations are defined as below: During the laser process parameter development phase (by tuning the pulse energy and pulse overlap setting), the laser induced damages on the die edge were studied. Figure 5 shows the typical laser dicing defects found at the die edge hairline crack (c) hairline crack Fig. 5. Laser dicing defects produced by using not optimized process parameters SEM image of burn mark on Al pad FIB cut Active circuitry dark markings (b) dark markings defects Edge seal Laser burn mark on Al pad Saw street Fig. 6. FIB cross section view of dark marking defect With respect to the literature and the basic principle of multi beam laser dicing technology, the impact on the edge quality is expected to be smaller by using lower laser pulse energy and higher pulse overlap [6-10]. In this study, our multi beam laser process is using a UV diode with 355nm wavelength and 115ns pulse, which is considerably a shorter pulse width process. Thus, a high repetition rate (ranging from 150kHz - 200kHz) shall realize a rather lower laser pulse energy, which is possible to effectively groove off the low-k metallization. Based on all the experimental results, the best combination of laser pulse energy and pulse overlap Al pad

4 Laser Groove Depth / Laser Groove Width (um) for grooving low-k wafers were with lower laser pulse energy, higher repetition rate and higher pulse overlap. The edge quality from using the best optimized laser parameter was verified passable without any significant anomaly caused by the laser, see Figure 7. Lower pulse energy, Higher pulse overlap width, its decreasing / shrinking rate is much lesser compared to the increasing rate of laser pulse energy. Thus, the laser groove depth is significantly affected by the laser pulse energy. And, for the laser groove width development, the laser pulse energy was observed to be an insignificant factor. Figure 10 shows the cross section view of the produced laser groove profile with varying laser pulse energy. Laser Groove Geometry (Width and Depth) By Laser Pulse Energy y = x R² = Fig. 7. Dicing defect free process at lower pulse energy and higher pulse overlap y = x R² = Die edge cleanliness after laser processing is crucial. It is important to spin coat a layer of protective film on the wafer before laser dicing. The typical coating material used is polyvinyl alcohol (PVA), which is water soluble. Coating process parameter optimization was conducted by tuning the rotation speed, spinning time as well as the total coat volume dispense in one cycle in order to obtain an uniform coating layer across the wafer diameter. Insufficient coating coverage due to non-optimized coating recipe, will result in stains/contamination on the die / die periphery after laser dicing. Refer illustration in Figure 8, this defect can be reduced to a minimum level, either by optimizing the coating parameter or changing the coating material, else it may pose a serious risk on device characteristic and reliability Laser Pulse Energy (uj) kerf width, um Fig. 9. Laser groove width and laser groove depth measurements with varying the laser pulse energy width = 43um depth = 5.64um 3uJ width = 39um ablation depth, um width = 43um depth = 10.55um 4.7uJ width = 37um depth = 17.7um depth = 24um Clean Stains on the grooving edge 6uJ 7.36uJ Fig. 10. Cross sectioning view of laser groove geometry with varying the laser pulse energy Fig. 8. Stains found along the die edge after laser (after wafer cleaning) 3.2 Laser Groove Geometry The effect of laser pulse energy in the range from 3uJ to 8uJ on the width and depth of the grooved geometry was determined under optical and SEM imaging. The process was operated at a fixed repetition rate, a fixed pulse overlap, and a 2-3 pass multi beam process. Figure 9 summarizes the relationship between the input laser pulse energy and the produced laser groove depth and width on a 40nm low-k wafer. It can clearly be seen that the laser groove depth increased as the laser pulse energy increased. The laser groove width, however, responded in the opposite direction, where the groove width decreased as the laser pulse energy increased. It was observed that the laser pulse energy has greater impact on the development of the laser groove depth over the laser groove width. The laser groove depth increases with higher laser pulse energy. Unlike the laser groove Laser groove profile is an important physical characteristic that must be inspected. It reflects the quality of the grooving surface. A comparison of two laser grooving methods was performed, i.e. multi beam laser grooving and single beam laser grooving. Figure 11 shows the 2- dimensional laser groove profile for the two evaluated laser dicing techniques. It is clearly seen that the multi beam laser dicing has exhibited quite an even and uniform grooving profile, unlike the single beam laser dicing which produced an irregular grooving shape that is similar to a cone-like cavity. The 2-dimensional grooving profile produced by the single laser beam running with 5-pass has a fairly sharp profile. The deepest point is found at the center and looks much rougher. It also has an uneven depth across the laser kerf width. It s believed that the uneven groove profile may have resulted from the uneven laser intensity distribution across the beam spot size. It is crucial to note that the single beam laser dicing always removes the highest amount in the

5 center of the kerf width, because of the highest laser intensity. The irregular groove shape from single beam laser dicing is undesirable as it has a risk of causing blade breakage, and poor blade placement accuracy. These are some issues found in the final stage of full die singulation. It is suggested to use multi beam laser dicing, a total 2- pass laser process to improve the uniformity of the groove shape. The basic principle of multi beam laser technology is to split the main laser beam (high laser power) into multiple laser beams which carries relatively lower in power during laser processing. The material removal rate/mechanism is found to be far more gentle, as compared to the single beam laser dicing, see Figure 11. It is found that by using multi beam laser dicing, an uniform groove shape is attainable and favorable for low-k wafer dicing. The sidewall surface produced by multi beam laser dicing was found to have a certain level of roughness however, no pit holes was found in the Si. The heat impact on the sidewall surface by using the multi beam laser is considerably smaller than single beam laser process. It is also partly due to the multi beam laser concept, which is using a few/multiple lower pulse laser beams to remove the low-k metallization on the saw street. The lower pulse laser process produces a relatively lower thermal stress on the sidewall of the die and thus improved/retained the mechanical die strength. metal peeling Excess Si recast Si recast Multi Beam Laser Grooving Single Beam Laser Grooving pit-holes on the die side-walls Groove depth, um Groove depth, um Fig dimension laser groove profile comparison between multi beam laser grooving and single beam laser grooving 3.3 Laser Induced Die Sidewall Surface Damage Die sidewall quality influences die strength. It is important to keep the die surface damage to the minimum level, to maintain a good mechanical die strength. Three types of dicing techniques were studied to characterize the sidewall surface damages. This includes classic blade dicing, single beam laser dicing and multi beam laser dicing. The sidewall surface of the die processed by various dicing methods was examined using SEM. Figure 12 shows the sidewall surface quality across the three different dicing techniques. It is clearly seen that the traditional blade dicing produced a relatively flat and smooth side wall. Thus, the blade diced sample is expected to have less stress induced on the sidewall and has greater die strength. The sidewall surface damages are clearly visible when using both single and multi beam laser dicing. The sidewall surface roughness for the single beam laser dicing looks obviously greater than the multi beam laser dicing. The formation of pitting holes (porosity) and excessive Si debris/recast were noticeable at the sidewall surface of the single beam laser dicing. The presence of voids in the Si is not favourable and needs to be removed/resolved. As these sidewall defects were caused from the thermal stress induced during the laser dicing process, thus the mechanical die strength is expected to be lower than the blade dicing process. Blade dicing Single beam laser dicing Multi beam laser dicing Fig. 12. Die sidewall quality comparison by using traditional blade dicing, single beam laser dicing and multi beam laser dicing 3.4 Die Strength Performance The influences of the three different dicing techniques as well as the variation of sidewall condition/quality on the die strength characteristic were investigated by four-point bending test. The three evaluated dicing methods in this study are traditional blade dicing, single beam (SB) laser dicing and multi beam (MB) laser dicing. It was found that the sidewall surface and edge damages produced by various dicing approaches should not be neglected. The sidewall condition does influence the die strength performances. Figure 13 shows the SEM images focused on the die sidewall condition, which processed by using various dicing methods. Both single and multi beam laser dicing resulted rougher sidewall condition, unlike the conventional blade dicing process which created a smoother (less roughness) edge and sidewall. The blade dicing process is expected to have higher and better die strength performance over the laser dicing process. It can be clearly seen that single beam laser dicing produces several micro cracks along the sidewall of the die as well as the micro void in the Si (See Figure 13). Such defects found at the die edge and surface are not favourable and can lead to die fracture. Blade Dicing Single Beam Laser (High Energy Intensity) Multi Beam Laser (Low Energy Intensity) Fig. 13. SEM micrographs reveal the die sidewall condition by various dicing methods There are two types of fracture modes observed when four-point bending test was conducted (See Figure 14). Blade dicing samples always failed under fracture mode 1, and laser grooved samples mostly fail under fracture mode 2. Dies that fail under the mode 1 catergory are the samples

6 that breaks into many pieces with different sizes, due to the high energy required to fracture the die. This corresponds to higher die strength. Failure mode 2 dies are samples that breaks into 2 pieces of similar size. This correlates to lower die strength. Mode 1: Blade Saw Singulation Mode 2: Laser Groove Singulation Fig. 14. Die fracture modes from four-point bending test formation in the Si from the single beam laser dicing process results lower die strength performance. The authors also believe that as this mass of materials are being ablate away, the molten metals and Si do not completely fall out of the groove, but a percentage of it falls back into the groove. As this material falls back into the groove, it traps some air in the process and creates these pockets of voids. Therefore, the shallower the groove, the easier it is for the out-gassing, and less recast material to inhibit the formation of voids. Figure 11 and Figure 13 show shallow groove depth with minimum voids in the Si from the optimized multi beam laser dicing process produces higher die strength performance. Aside from the die sidewall surface condition, the produced laser groove depth was found significant and has great impact on the die strength performance. Figure 15 summarizes the relationship between the produced laser groove depth and the measured die strength data. The die strength performance has high correlation and was found strongly related to the laser groove depth. The shallower the groove depth, the higher the die strength revealed. Fig. 15. Relationship between the laser groove depth and the produced die strength measurement It was found that the higher the laser power (energy intensity) and pulse overlap have resulted higher groove depth and lower die strength. Figure 16 illustrated the effects of different energy intensity on the groove depth and die strength. It was also noted that the shallower the groove depth (lower energy intensity from optimize the laser power, frequency, defocus and feed speed), the higher the die strength performance was observed. When a laser beam strikes the wafer surface, the light is converted into electrical, thermal, photochemical, and mechanical energy. During this process of energy release, plasma is also created. The deeper the laser goes into the Si, the harder it is for the plasma to be fully released as there would be a higher mass of molten metals and Si at the top surface which hinders plasma out-gassing. Therefore, it was found that increasing the energy intensity and increasing the pulse overlap, more heat will be generated and a deeper grooving depth was created, thereby creating more voids in the process. Figure 11 and Figure 13 show excess void Fig. 16. Laser groove depth and die strength measurements with varying energy intensity The die strength characteristic comparison by various dicing methods is completed by using Weibull distribution/analysis, see Figure 17. It clearly shows that the blade dicing process has resulted the highest die strength over the laser dicing process. The multi beam laser dicing with the deepest groove depth and the highest energy intensity produced the lowest die strength. If the single beam laser dicing process (SB Laser: moderate groove depth, moderate energy intensity) is used as a reference, it was found that the die strength performance for multi beam laser dicing (MB Laser: shallow groove depth, low energy intensity) increased by nearly 20%. However, the die strength performance for multi beam laser dicing (MB Laser: deeper groove depth, high energy intensity) decreased by about 19%, weaker over the standard single beam laser dicing process (SB Laser: moderate groove depth, moderate energy intensity). It is found that the dicing process parameter variation and its dicing response outcomes, specifically on the die sidewall condition and groove depth have significant impact on the die strength performance. Therefore, laser process

7 parameter optimization is critical and crucial so that one must not neglect in order to realize better dicing quality and enhance the total die strength performance and reliability of the chip. Notes: MB Laser (Hi) = Multi beam laser created the deepest laser groove depth and utilized the highest energy intensity MB Laser (Lo) = Multi beam laser created the shallowest laser groove depth and utilized the lowest energy intensity SB Laser = Single beam laser created moderate laser groove depth and utilized moderate energy intensity Blade Saw = Standard blade dicing Fig. 17. Weibull distribution for die strength measurement 3.5 Package Stressing and Reliability Testing Mechanical stressing and package reliability assessments were conducted on the assembled dies which were processed with the optimized 2-pass multi beam laser parameters and diced with the single pass blade dicing. Post assembly CSAM and time zero electrical test were performed on the assembled parts prior to MSL3 testing, in which after that the parts were subjected to 3 times reflow at 260 degree C as per IPC/JEDEC J-STD-020D. The electrical test results were recorded at time zero, after MSL3/260 degree C and after 96 hour uhast (100 degree C, 85%RH) at room temperature condition. Table II summarizes the CSAM and electrical test results for the die samples assembled in 17x17mm 208 MAP BGA. The package stress and reliability result has passed MSL level 3 at 260 degree C preconditioning, and uhast 96 hours stress and electrical test at room temperature condition with no dicing related failures. Therefore, the dicing quality produced by the established multi beam laser grooving process parameter has been validated good and robust. It is shown that, the evaluated new laser dicing technique, i.e. multi beam laser grooving is possibly one of the best solutions to select for dicing quality and throughput improvements for low-k/ulk wafer dicing. Table II: Package stressing and reliability results Wafer technology 40nm CMOS Cu Technology Low-k metallization 7 metal layers 2.8um MSL3/260C + uhast MSL3/260C (110C 85% RH) Al Laser dicing Time Zero After MSL3/260C 96 hrs Laser Pass thickness method Qty In CSAM Test CSAM Test Multi beam 2 60 Pass Pass Pass Pass laser grooving 4.0 CONCLUSIONS The characteristics and responses of 40nm CMOS Cu low-k wafer grooved by using a multi beam laser dicing technique (UV laser diode with 355nm wavelength and 115ns pulse width) have been studied and investigated successfully. The following results were obtained: Laser diced edge quality: The laser power, laser frequency, feed speed, and defocus amount are the critical process parameters influence the dicing edge quality. The best combination of laser parameters for grooving off the 40nm CMOS Cu low-k metallization on the saw street included: a high repetition rate, a low laser pulse energy and a high pulse overlap. Die edge cleanliness: Insufficient coating coverage due to non-optimized coating recipe used, may result in stains/contamination on the die/die periphery after laser dicing. Stained and contaminated die may pose a serious risk on device characteristic and reliability. Laser groove geometry: The laser groove depth increased as the laser pulse energy increased. The laser groove width (kerf width) responds in the opposite direction, i.e. the laser groove width decreases as the laser pulse energy increases. The laser pulse energy has greater impact on the development of the laser groove depth over the laser groove width. Laser groove profile: The multi beam laser dicing has exhibited a relatively even and uniform grooving profile. The single beam laser dicing produced a grooving shape that was irregular, similar to a cone-like cavity. The irregular groove shape from the single beam laser dicing is undesirable as it poses a risk of causing blade breakage, and poor blade placement accuracy issues in the final stage of full die singulation. Laser induced die sidewall surface damage: It is important to keep the die surface damage to a minimum level in order to maintain the mechanical die strength. The traditional blade dicing produced a relatively smoother side wall. The sidewall surface damages are clearly visible by using either the single or multi beam laser dicing. The sidewall surface processed by multi beam laser dicing was found to have a certain level of roughness, with minimal pit holes found in the Si. The formation of pitting holes (porosity) and excessive Si debris/recast were more noticeable at the sidewall surface of the single beam laser dicing. Die strength performance: The die strength performance has a high correlation to the laser groove depth. The shallower the groove depth, the higher the die strength observed. It was found that the die strength performance for multi beam laser dicing (shallow groove depth, low energy intensity) increased by nearly 20% over the single beam laser dicing process. Package stressing and reliability testing: The die samples grooved with the optimized multi beam parameters and assembled in 17x17mm 208 MAP BGA have passed MSL level 3 at 260 degree C preconditioning, uhast 96 hours stress and electrical test at room temperature conditions. No dicing related failures was observed.

8 In summary, the multi beam laser dicing process as presented above has been developed and enabled for 40nm CMOS Cu low-k wafer. It is shown that, the multi beam laser grooving is possibly one of the best solutions to select for dicing quality and throughput improvements for lowk/ulk wafer dicing. The multi beam laser process is a feasible, efficient, and cost effective process compared to the conventional single beam laser ablation process. 5.0 ACKNOWLEDGEMENTS The authors would like to thank LC Tan from Freescale (Malaysia) Sdn. Bhd. Packaging Solutions Development (PSD-TSO) for the management support in this project. Special thanks to Jeroen van Borkulo, SS Fong, Mathieu Brand, Marvin Aquino, Won Chul Jong, and Peter Dijkstra (ASM Laser Separation International) for the technical and analytical supports, Betty Yeung and Derek Morgan for the die strength charaterization; and Burton Carpenter for the packaging reliability stressing and electrical test supports. 6.0 REFERENCES 1. Kwak, H., Hubing, T., An Overview of Advanced Electronic Packaging Technology, May. 2007, pp Driel, W. D., Facing the Challenge of Designing for Cu/Low-k Reliability, Microelectronics Reliability, Vol. 47, (2007), pp Lee, K.S., Laser as a Furture Direction for Wafer Dicing: Parametric Study and Quality Assessment, 31 st International Conference on Electronic Manufacturing and Technology, Nov. 2007, pp Gatzen, H. H., Dicing Challenges in Microelectronics and Micro Electro-Mechanical Systems (MEMS), Microsystem Technologies, Vol. 7, (2001), pp Li, J. et al, Laser Dicing and Subsequent Die Strength Enhancement Technologies for Ultra-thin Wafer, Proc 57 th Electronic Components and Technology Conference, May. 2007, pp Richard, V.D.S. et al, Multi Beam Low-k Grooving Evaluation of Various Removal Principals, 46 th International Symposium on Microelectronics, Sep. 2013, pp Jeroen, V.B. et al, Multi Beam Grooving and Full Cut Laser Dicing of IC Wafers, 10 th International Conference Semiconductor Technology China, Mar. 2011, pp Nitin, S. et al, Laser Singulation of Thin Wafer: Die Strength and Surface Roughness Analysis of 80um Silicon Dice, Optics and Lasers in Engineering, Vol. 47, (2009), pp Daisuke, E. et al, Effect of Mechanical Surface Damage on Silicon Wafer Strength, Procedia Engineering, Vol. 10, (2011), pp