Thermal laser separation for wafer dicing
05/01/2009
Hans-Ulrich Zühlke, Jenoptik Automatisierungstechnik GmbH, Jena, Germany
Thermal laser separation (TLS) is used to separate brittle materials using well known basic principles. The range of applications vary from cutting display glass (including laminated glass), cutting the edges in float glass production lines, and the scribing of Al2O3 ceramics. Additionally, TLS wafer dicing is an interesting alternative to the established mechanical dicing saws and has many advantages, compared to other laser technologies. In this paper, the application of the TLS-technology for the separation of semiconductor wafers will be described.
The thermal laser separation (TLS) process uses very simple techniques to separate brittle materials; it is a cleaving process that uses thermal-induced mechanical stress. The process can be explained in two steps. To initiate the process, a very small initial scribe (defect) is required. This defect could be the result of former process steps, e.g., grinding or wire sawing, or — in most cases — it must be made by a tool like a diamond tool tip or a special scribing laser (step 1). This initial scribe gives the cleaving process a well-defined starting point; otherwise, the cleaving process might start at the nearest defect site.
In the cleaving step, the material is heated up by a laser with a well-defined quantum of energy (step 2). The heated material expands and radial pressure forces occur in the heated zone. Around this heated zone, tangential tensile stress (TS I) is induced.
 Figure 1. SEM photo of the diced edge. |
The laser focus is directly followed by cooling; as the material cools, it shrinks. Within the cooling zone, additional tensile forces (TF II) are induced (step 3). Within the overlaying zone of both tensile forces (TF I and TF II) — and only at this point in the process — a crack can be opened. So the cleaving process can be done, starting from the initial defect. The result is excellent edge quality for the separated dies (Fig. 1).
For crystalline materials such as silicon, the separation is complete (step 4). The local maximum of tensile stress is located on the axis between focus of heat and cooling, so the system of forces and strength is a self-centering one. If the crack tends to run faster than the combination of heat and cooling, it runs into the zone of pressure forces and is delayed.
 Figure 2. Cleaving runs independent of the lattice plane. |
Although TLS works nearly independent of the orientation of the lattice plane (Fig. 2) of the crack front determines properties of the cut edges, as well as for classical mechanical scribe and break technology [1]. A more vertical crack front is less dependent on the orientation of the crystal planes and shows the best results. The key to gain this vertical crack front is the usage a laser wavelength, where the laser light is absorbed in the depth.
Process conditions/properties
Let us discuss some influencing factors and basic conditions of the process.
The TLS process requires a significant gradient of thermal expansion in the material. It is not possible to compensate an insufficient small value by higher laser energy, because of the effect of thermal destruction especially at the surface of the wafer. Due to this fact, the separation of fused silica was not previously possible.
The technology is based on thermally induced mechanical stress in the material. In the case of very thin wafers, the volumes that are heated and cooled, respectively, could be extremely small, and the required forces to guide and draw the break will not be generated. Up to now, the separation was successfully tested for wafers with a minimum thickness of 50µm. The lower limits need to be determined by further testing.
The cleaving process comes about as a result of induced forces in combination with inherent forces within the material. For this reason, the separation of multicrystalline (photovoltaic) wafers is more difficult, but possible. Also in the border area of the wafer, where the separating lines meet the edge of the wafer tangentially at small angles, special actions are required to stabilize the process.
 Figure 3. Separation of back side metal layers. |
As usual, for all laser applications, absorption and reflection are important values. To gain a balanced heating of the surface and lower regions of the silicon material, a laser wavelength in the near infrared spectrum (NIR, 1030???1080nm) is optimal. It is necessary to consider the changes of modified optical properties caused by functional layers, e.g., surface passivation layers, or process control monitor (PCM) fields.
Heat conduction plays a significant role for the process. Higher heat conduction means that the heated zone will dissipate faster. Because of this, a 200W laser system can cleave 250µm Si wafers, and 675µm GaAs wafers. Heat conduction is also the transport mechanism for the cooling process, because the cooling media cannot take effect directly at depth of the material as the laser is able to do.
 Figure 4. Achieved bending strength (3-point bending test). |
Additional layers, such as back-side metallization, have been evaluated for their influence on the separation process. For wafers of ~100µm thickness, experiments have shown that typical metal layers could be separated without any problem. The damage of the metallization edge is in the range of 1???2µm (Fig. 4).
Utilization of TLS dicing
In the back end, the processed wafers are separated into dies. Because of the high value inherent in these nearly completed dies, demands on the dicing process are high (throughput, yield, edge quality and process costs, etc.) For TLS dicing, the wafer is mounted on a standard tape that is clamped on a frame for dicing.
Technologies in use
Mechanical sawing. The classical dicing technology is mechanical sawing, whereby a rotating saw blade cuts the wafer completely. In doing so, a large volume of cooling and cleaning media is required, which must be removed in a subsequent cleaning step. Thin wafers can only be treated at reduced speeds (e.g., 30???50mm/sec. for a wafer thickness of 100µm) because thin wafers are very sensitive and fragile, therefore, the saw blade must be moved very carefully and slowly along the street. the mechanical contact between saw blade and wafer. Sawing generates chipping at the edges and a limited bending strength is the result. Additionally, particles are generated and mechanical vibrations could damage parts of MEMS devices.
Ablating laser. This technique removes material from the dicing street by thermal processes. Because each laser ablates a maximum volume per time, the thinner the wafer, the higher the process speed. A disadvantage of using laser ablation is the generation of residues from the evaporated or melted material. Most suppliers of ablating laser systems recommend temporarily protecting the wafer by using a protection layer, such as PVA. Due to the substantial thermal modification, the edges are very rough and the bending strength is low compared to mechanical sawing and especially TLS.
In some other processes (e.g., dicing by grinding), only a trench will be made on the surface. Grinding of the wafer is then done until the trenches are open from the back side and the dies are separated.
To accommodate low-k layer materials in particular, pre-dicing or low-k grooving is implemented along with very fast placement of the saw blade. The laser ablates the critical layers, followed by a conventional sawing step.
Laser micro jet (LMJ). This technology uses a water beam as light guide. An additional benefit from the process is that the water cools and cleans the working zone. Because of the reduced heat affected zone (HAZ), the achieveable bending strength is slightly beyond the strength of that obtained using a mechanical saw.
Stealth dicing. This technology separates the wafer — as the name implies — by a modification of inner layers of the wafer material. The technology has the most similarities to TLS dicing. Because the modification of the material is limited to the inner layers, the surface is not influenced and there is no chipping at the edges; however, the cross-sectional area is modified or damaged. Unfortunately, the position and properties of the modified amorphous separation zone depend on the varying relationship between laser wavelength and dopants. The process requires a street width (the zone between the dies) that is half of the wafer thickness, and the resulting speed depends on the number of required passes (approx. one pass for every 50µm of the thickness of the wafer). The separation is completed by a subsequent singulation by stretching step.
Mechanical properties
The cleaving-like principle of TLS dicing results in very smooth edges without chipping and a measurable higher bending strength for the separated dies. Measurement of typical break strength of TLS samples was done by a three-point bending test [2] (Fig. 4). The benefits of the increased bending strength are reduced breakage and more yield in the subsequent packaging steps, as well as higher reliability for flexible products (smart cards, etc.) and in case of thermal stress (when a chip is used).
Electrical and optical properties
Due to the very smooth edges and plain cut faces of the die — resulting from the cleaving technology — the inverse voltage is increasing. This has been measured for TLS-diced samples in an earlier phase of the project. The first results are good, but they must be verified by comprehensive tests with statistical relevance for real products.
The optical properties of the cut face are comparable to the excellent quality of cleaved edges obtained by using mechanical scribe and break processes (especially used for separating LEDs and laser diodes on GaAs). Both outcomes are also interesting for CPV (concentrator photovoltaic) dies on GaAs or Ge wafers.
Improved purity or dry process
TLS is a very clean dicing technology. An investigation of particle generation during mechanical scribing (for initial starting point generation) was performed and the measurements have not shown significant contamination ??? the nature and number of counted particles were comparable with an untreated reference wafer. For the standard process a deionized (DI-) water-aerosol spray is used (10???20ml/min). In this case, during the dicing process, a combination of compressed air jets and a vacuum nozzle is used to remove the droplets.
Alternatively, in the lab, a complete dry cooling by dry ice (CO2) has been tested, although the heat capacity of CO2 is lower, the results are promising. In particular, the complete dry cooling will be an excellent technology for water-sensitive products such as MEMS, optical devices, and others.
Increased yield
A significant improvement in the yield can be attained by reducing the width of the dicing streets. Wafers for mechanical sawing have a width of 50???100µm (the kerf width is typically 30µm or more). For TLS, the street width can be minimized to 30µm (calculated by the minimal suggested width of the laser’s focal spot). The cut itself is a zero-gap and the passive safety zone for mechanical-damaged edges can be reduced as well. For a 1mm edge length of the die, a reduction of the street width from 70µm down to 30µm means 10.8% more yield for the same wafer. As opposed to other laser-dicing technologies, the required street width is independent of the thickness of the wafer. When new designs and masks are prepared, it is especially important to consider a change in the dicing technology.
As described before, TLS dicing is done in two steps (I-Scribe followed by the cleaving step). The resulting speed of both steps is 200???300mm/sec. — nearly independent of the thickness of the wafer.
Prototype system
The TLS process has been qualified on a lab system. After reaching a stable running process, the prototype system was designed and built. The heart of this system is the technology module, consisting of: laser and optical system, process cooling, and the I-Scribe module. The laser is a cw-mode, 200W fiber-coupled fiber laser with a wavelength of 1070nm. For maximum throughput, there are two cooling nozzles to do the cleaving step bidirectionally. The I-scribe is done mechanically by a diamond tool tip in the standard version; or a higher throughput can be obtained using a scanned, pulsed green (λ = 532nm) laser.
The handling, the alignment, and other standard-modules are comparable with other dicing tools. A special chuck with patent pending features was developed. One of these features is an integrated micro-stretching that protects the edges of the dies immediately after dicing against scratches or any other mechanical damages that could occur because of the zero kerf width. An alternative to the typical kerf check is provided by an integrated microscope camera system for process control.
The prototype system is equipped with an automatic handling system. Wafers of 150mm and 200mm can be processed within the first prototype system. This prototype will be tested in an evaluation program under industrial conditions. In spring 2009, the 300mm version will be finished.
Conclusion
TLS suits applications that have demanding requirements for edge quality — such as optical devices or power devices with vertical current flow. The process is clean and free of vibrations or mechanical shock, so it can be used for MEMS products as well.
The process is particularly suited for standard products, especially thin wafers, but it also works for most Si-wafer-based PV cells.
In 2009, there will be a comprehensive evaluation phase for the prototype. In the next step, complete water-free cooling media will by evaluated — important for projects with optical surfaces or humidity-sensitive materials (e.g., low-k dielectrics).
Acknowledgments
The development of the prototype system was funded by the European Commission (SEA NET, IST 027982).
References
- M.S. Acker, “The Back-end Process: Step 11 — Scribe and Break,” Advanced Packaging, November 2001.
- S. Schoenfelder, M. Ebert, C. Landesberger, K. Bock, J. Bagdahn, “Investigations of the Influence of Dicing Techniques on the Strength Properties of Thin Silicon,” Microelectronics Reliability, Vol. 47, pp. 168-178, 2007.
Hans-Ulrich Zühlke received his doctor of natural sciences at Friedrich-Schiller-University of Jena and is business development manager at Jenoptik, Konrad.-Zuse-Stra??e 6, 07743, Jena, Germany; ph.: +49 3641 65 3890; email [email protected].