Adoption of Hybrid Dicing Technique to Minimize Sawing-Induced Damage during Semiconductor Wafer Separation
2017 Volume 58 Issue 4 Pages 530-534
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2017 Volume 58 Issue 4 Pages 530-534
This article demonstrates that the chipping damage resulting from silicon wafer separation can be more effectively suppressed by the adoption of hybrid dicing (a duel process that uses laser ablation prior to mechanical sawing) than single dicing, such as sawing. This work shows that the adoption of the hybrid dicing technique induces sacrificial fracture at the laser-induced groove tip to save the regions outside of the groove formed along the scribe region of the wafer. Particularly, this study details how a pre-existing groove (i.e., a laser-induced groove) on the front surface of the wafer interacts with the saw blade penetration-induced trench during the second step of the hybrid dicing process. According to the experimental results, the magnitude of the chipping damage in the chips diced from a wafer including a laser-induced groove with a larger aspect ratio was more effectively reduced at a more rapid sawing velocity. However, since a laser-induced groove with a larger aspect ratio could also have a bigger curvature in its tip, adopting a groove with the optimum shape was necessary for effective prevention of dicing-induced damage during hybrid dicing.
Semiconductor devices are usually formed on a single silicon wafer during a batch processing method that provides great economic benefit. Individual semiconductor devices are separated from the wafer during the wafer dicing process. That is, the wafer with so many circuits is cut into rectangular pieces, called “dies”. After the die cutting of a semicoductor wafer, individual dies can contain defects formed along their edges that often provide potential sites for serious reliability problems. So, in between those functional parts, a very thin non-functional spacing, which is called the scribe region, exists to cut the wafer without damaging the circuits during the wafer dicing process. However, each individually diced device is generally encapsulated within the molding compound to create the plastic package body, which protects the electrical interconnection and device chip from exterior environmental attacks. Then, the dicing-induced defects can cause serious reliability problems due to the thermal displacement mismatch between the device chip and the plastic package body during reliability tests such as thermal-cycling. Furthemore, thinner micro-electronic packages have been developed to accommodate demands for portable communications devices, such as smart telephones or notebook computers. With the development of such compact packaging technology, device chip thickness has been reduced down to micro-scale. Shrinkage of the scribe region to keep step with such compact packages has caused various new reliability problems in the device pattern.1,2) Particularly, wafer dicing-induced device failure has been one of key challenges in continuing to develop more compact packages.1,2) For instance, conventional mechanical sawing is not a dicing process that sufficiently protects device patterns during the dicing of silicon wafers because of chipping damage.1–5) It has already been reported that in mechanical sawing, a diamond particle-embedded saw blade does not completely cut through the silicon wafer by mechanical friction, but instead the wafer can be finally separated by brittle fracture before the blade reaches the bottom.2) Since such a brittle fracture at the final wafer separation stage can be influenced by various factors such as the revolving velocity of the saw blade or the crystallographic orientation of the dicing direction, it is very difficult to characterize such a complicated chipping aspect along the diced chip edges.6–10) Moreover, in recently developed laser ablation dicing techniques, reliability of the diced devices has not been achieved due to laser-induced thermal damage.5) Conclusively, a single dicing process, such as mechanical sawing or laser ablation, cannot prevent dicing-induced degradation of the reliability of the devices. To address this limitation, it was focused on how effectively the combined dicing of mechanical sawing with laser ablation can minimize the dicing-induced pattern damage during the separation of silicon wafers.
Silicon wafers with a thickness of 200 um were used in this work. For the purpose of comparison, 10 wafers were diced by only mechanical sawing or only by laser ablation. The other wafers were diced by the hybrid dicing method (i.e., the combined dicing of mechanical sawing with laser ablation), as shown in Fig. 1. In the hybrid dicing method, a laser-assisted groove was first made along the center line of the scribe region on the wafer surface. The wafer was then turned over to expose its back surface for subsequent mechanical sawing. Finally, the saw blade penetrated into the back surface of the grooved wafer to meet the pre-existing groove tip (i.e., the laser-induced groove tip). The depth of a laser-induced groove for the hybrid dicing technique was mainly controlled by changing the laser power. The laser power used during the first step of hybrid dicing ranged from 50 watts to 200 watts and the aspect ratio of the resulting laser-induced groove had a value from 1 to 4. For the second step of the hybrid dicing process (i.e., mechanical sawing), the silicon wafers with laser-induced grooves were diced with a diamond-pointed saw blade as shown in Fig. 2.2) The revolving velocity of the saw blade was controlled to have a value from 10,000 rpm to 40,000 rpm in order to study the effect of sawing velocity on the hybrid dicing efficiency. A predetermined chip size was 0.5 × 1.1 cm2. When dicing-related defects were found under an optical microscope (OM), the corresponding chips were examined under a scanning electron microscope (SEM) in order to measure precise dimensions of the defects.
A micrograph showing the mechanical dicing method.2)
A schematic illustrating the hybrid dicing method.
This study confirmed that the hybrid dicing method (i.e., the combined dicing of mechanical sawing and laser ablation) is more effective in suppressing dicing-induced damage than a single dicing process such as mechanical sawing or laser ablation. The proposed hybrid dicing method basically consists of two process steps. In the first step of the hybrid dicing process, laser ablation carries only a limited role in making a groove mark along the center line of the scribe region (i.e., the dicing path) on the active surface of the wafer. In the second step, the wafer with the laser-induced groove is turned over, and then a saw blade penetrates into the opposite side of the grooved wafer surface. As a result, the laser-induced groove just provides the role of stress raiser during the subsequent sawing that starts from the back surface of the wafer.
SEM examinations clearly showed that there was a distinct difference in chipping damage between the single dicing process and the hybrid dicing process. Figure 3 shows the edge view of the chip separated from a silicon wafer by the single dicing process (mechanical sawing). This micrograph reveals very rough and broad chipping damage along the chip edge, indicating the probablility that the silicon wafer was brittlely fractured in the last stage of mechanical sawing. Figure 4 shows dicing-induced damage resulting from another single dicing process (laser ablation). Figure 4(a) shows the top surface view resulting from laser ablation on the wafer surface. This micrograph proves that the wafer surface can be exposed to thermally-melted damage during the laser-induced dicing. Figure 4(b) is another micrograph showing the cross-sectional view of the laser ablated wafer. We also see in this micrograph that the laser-cut chip has a diced surface tilted from the (110)-predetermined dicing plane. The reason the laser-diced surface has an angle tilted from the pre-determined dicing plane is probably that the silicon can be exposed to different thermal energy, depending on its depth during the laser ablation process. Thus, if laser ablation is used as the sole dicing method for wafer separation, the wafer must have a broad area just for dicing, which reduces productivity.
A micrograph showing sawing-induced chipping damage.
Micrographs showing a laser ablation diced wafer: (a) top view, and (b) cross-sectional view.
On the other hand, in the proposed hybrid dicing method, the laser ablation was just used to induce a groove mark in a very limited region of the wafer surface, so the ablated region only played the role of a stress raiser during the second step of the hybrid dicing process (i.e., sawing). Thus, in the hybrid dicing process, the laser ablation was done during a very limited time in order to minimize the laser-induced thermal damage on the wafer surface. Figure 5 shows a micrograph of the laser-induced groove formed on the wafer surface for the purpose of hybrid dicing. Figure 5(a) is a top view of the wafer subjected to laser ablation for hybrid dicing. This figure illustrates that there is much less thermal damage on the wafer surface, as compared to the thermal damage shown in Fig. 4(a). Figure 5(b) is a side view of the wafer subjected to the limited laser ablation. This figure also shows that even though the laser-induced groove is not deep enough, it does not cause broad thermal damage at the wafer surface region. Figure 6 is a schematic illustrating the penetration of the saw blade during hybrid dicing. As shown in Fig. 6, the second step process is done to allow the saw blade to approach the laser-induced groove tip instead of the free surface of the wafer to complete hybrid dicing. Therefore, if the stress at the laser-induced groove tip is properly concentrated during sawing, it causes preferential fracture at the tip and saves the regions outside of the groove. Consequently, the reason the proposed hybrid dicing process minimizes chipping damage is the sacrificial breaking at the laser-induced groove tip during the second step of the hybrid dicing process (i.e., sawing).
Micrographs showing a wafer with a laser ablation-induced notch: (a) top view, and (b) cross-sectional view.
A schematic showing the sacrificial fracture at the tip of a laser ablation-induced notch during the penetration of a sawing blade.
The magnitude of the sawing-induced stress concentration at the laser-induced groove tip is affected by various factors, such as the groove shape or sawing velocity. If the groove is deeper, the sawing-induced stress concentration at its tip is larger. In order to elucidate how much the groove depth is effective for reducing the magnitude of chipping damage, I designed the aspect ratio of the laser-induced groove to have a value from 1 to 4. Figure 7 is a 3-dimensional graph showing the magnitude of chipping damage as a function of the aspect ratio of the laser-induced groove and the revolving velocity of the saw blade. This investigation produced several interesting results as described below. First of all, the magnitude of the chipping damage in the chips diced from the wafer including the laser-induced groove with an aspect ratio of 1 did not meaningfully differ from that separated from the single dicing process (i.e., sawing), particularly at a sawing velocity that was faster than 40000 rpm. Thus, we see that to enhance productivity, if the sawing velocity is faster than 40000 rpm, the laser-induced groove should have an aspect ratio larger than 2. Secondly, this study showed that the dependence of the chipping damage on the aspect ratio is stronger at a faster sawing velocity during hybrid dicing. That is, the magnitude of the chipping damage estimated at a sawing velocity of 40000 rpm drops more abruptly than that of 10000 rpm, which indicates that the proposed hybrid dicing is more effective at a faster sawing velocity. Thirdly, although the magnitude of the chipping damage rapidly decreased as the aspect ratio of the laser-induced groove increased from 1 to 3, the reduction rate did not increase so meaningfully with further aspect ratio enhancement. This result suggests that even though a larger aspect ratio is more effective for suppressing dicing-induced damage, an aspect ratio larger than 3 may have a limitation in maximizing the hybrid dicing efficiency for various reasons, such as the complicated interaction between the laser-induced groove and the sawing-induced trench as well as ablation damage. In fact, laser-induced deep grooving to allow a large aspect ratio can result in an unexpected increase in the tip curvature, which reduces the hybrid dicing efficiency, as explained in the discussion. Consequently, based on these experimental results, it is concluded that laser-induced grooving during the first step of hybrid dicing should be done to satisfy both minimum tip curvature as well as an appropriate aspect ratio.
A 3-dimensional graph showing the magnitude of chipping damage as a function of the aspect ratio of laser ablation-induced notches and the revolving velocity of a sawing blade.
If the laser-induced groove is assumed to have a rectangular parallelepiped trench shape, the stress intensity factor (KI) resulting from sawing at its tip would be approximately proportional to the root square of its depth.11) Thus, a trench with an aspect ratio of 4 is theoretically expected to be subjected to a stress concentration higher by more than two times than that of 1. Thus, a deeper laser-induced groove would have more chance to accelerate the sacrificial breaking at a particular region of the silicon wafer. That is, the preferential sacrificial fracture at the deeper laser-induced groove would more effectively protect individually diced chips from chipping damage. This is obviously the reason why the wafer including the laser-induced groove with a larger aspect ratio results in less chipping damage than that with an aspect ratio of 1 during hybrid dicing.
However, deep laser-induced grooving up to an aspect ratio of 4 can also result in an undesirable groove shape due to the enlargement of the groove tip curvature. Figure 8 is a schematic illustration of a laser-induced trench and a saw blade penetration-induced trench. This figure also illustrates the biaxial tensile stress induced at the trench-shaped groove during saw blade penetration (i.e., the second step of hybrid dicing). This drawing reveals that the tip width (WB) as well as the aspect ratio (WH/WL) of the laser-induced trench also plays a critical role in determining the magnitude of dicing damage. That is, deep laser-induced grooving results in an increase in the groove tip width (WB) as well as an increase in the aspect ratio (WH/WL). Then, the sacrificial fracture mechanism at the laser-induced trench tip differs according to the value of WB. If the trench tip width (WB) is smaller than saw blade thickness (T), the sawing-driven biaxial tensile stress induced at the laser-induced trench tip may be large enough to cause a preferential fracture at the laser-induced trench tip of the wafer without causing severe damage to the outside of the trench due to the notch effect. Meanwhile, if the trench tip width (WB) is larger than the blade thickness (T), the sawing-driven tensile stress induced at the trench tip would not be large enough to cause the preferential fracture. The sawing of the wafer with such a wide laser-induced trench tip might cause sawing-induced damage up to a region outside of the trench, which increases chipping damage. Thus, the width of the laser-induced groove tip with respect to the saw blade thickness is one of the critical factors that determine the magnitude of chipping damage in a die separated by the hybrid dicing process. It has been reported that the biaxial tensile stress induced at the notch tip under uniaxial tensile loading is inversely proportional to its curvature radius.12) Therefore, if the laser-induced groove has a V-shape (WB/WL ≈ 0), it contributes to maximization of the hybrid dicing efficiency. However, since the V-shaped notch can also allow thermal damage over a broad area of the wafer surface, it is not desirable to make such a notch only by laser ablation. Thus, the adoption of the laser-induced groove with a proper shape is essential for the effective prevention of dicing-induced damage during the wafer separation utilizing the hybrid dicing technique.
A schematic illustrating a laser-induced trench and a saw blade penetration-induced trench.
The experimental result shown in Fig. 7 also indicates that the magnitude of the chipping damage in a die separated from a wafer including a laser-induced groove (i.e., a pre-existing trench) with a higher aspect ratio is more influenced by the revolving velocity of a saw blade than that with an aspect ratio of 1. This phenomenon can be explained as follows: The magnitude of the sawing-induced stress induced at a pre-existing trench tip (i.e., a laser-induced groove tip) differs according to the sawing velocity. That is, with slower sawing, the saw blade penetrates more deeply into the wafer and leaves a shorter distance between the pre-existing trench tip and the blade-induced trench tip before the final separation of the wafer. Such a shorter region between two trench tips can be subjected to a larger stress during the final step of hybrid dicing. Thus, such a shorter silicon would be more easily fractured to save the region beyond the laser-induced groove, which minimizes the chipping damage. On the other hand, with rapid sawing, the saw blade does not penetrate deeply enough into the wafer before the final fracture of the wafer because of the brittle property of silicon, and so there is a larger inter-distance between two trench tips. In this case, the depth of the pre-existing groove (i.e., the aspect ratio of the laser-induced groove) becomes more important for inducing the sacrificial fracture at the tip. This should be the reason why the laser-induced groove with a larger aspect ratio is more effective in preventing chipping damage than that of 1 at a faster sawing velocity.
This article demonstrated that degradation of individually diced devices resulting from silicon wafer separation can be more effectively suppressed by the adoption of a hybrid dicing technique (the combined dicing of mechanical sawing with laser ablation) than a single dicing technique. This work showed how the sacrificial fracture at a laser-induced groove tip saves the regions outside of the groove during the second step of the hybrid dicing process (i.e., sawing). Particularly, this experimental work confirmed that the dependence of the hybrid dicing efficiency on the aspect ratio of the laser-induced groove is stronger under a faster sawing velocity. The mechanism for this was revealed by in-situ examinations showing that the saw blade does not penetrate deeply enough into the wafer with rapid sawing, so the depth of the laser-induced groove becomes more important for the sacrificial fracture at its tip than saw blade penetration depth. Indeed, the experimental results proved that the effect of hybrid dicing for chipping damage prevention can be waived for wafers with an aspect ratio smaller than 1 and a sawing velocity faster than 40000 rpm. Meanwhile, since the laser-induced groove with too large an aspect ratio could also have an enlarged tip curvature, such a large aspect ratio could rather reduce the hybrid dicing efficiency. Thus, the adoption of the laser-induced groove, which satisfies both a minimum tip curvature and a large aspect ratio, is necessary to allow semiconductor devices to have better reliability margins during the wafer separation by hybrid dicing.
This work was supported by Incheon National University fund for research year, 2016.
