Slicing is the process of splitting a semiconductor wafer into a single chip, generally on the basis of the wafer has completed the previous process and electrical performance tests. At the same time, as the first step of semiconductor packaging, the quality of scribing will directly affect the final reliability of the packaged product. Grinding wheel cutting is one of the most widely used cutting methods at present. The blade composed of diamond particles and adhesive is rotated at high speed through the spindle linkage, grinding each other with the material to be processed, and feeding at a certain speed to divide the wafer into independent chips. The defects such as cracking caused by residual stress and mechanical damage in the process are the main problems restricting the development of grinding wheel.
With the rapid development of semiconductor integrated circuits, the new requirements of scribing efficiency and quality have been combined with the characteristics of different processing materials, from the scribing tool selection, process parameter optimization and scribing method improvement, and so on, the cutting technology has a certain development. In the face of the development trend of device diversification, especially in the development of non-contact scribing requirements such as ultra-thin wafers and MEMS wafers containing movable structures, grinding wheel scribing can not be fully satisfied. Laser cutting can effectively avoid the problem of grinding wheel cracking, and at the same time in small size and MEMS chips, highlights the increasingly important advantages, this paper will mainly discuss the main application of laser invisible cutting and laser ablation cutting two kinds of cutting methods.
Laser invisible cutting technology
Invisible cutting technology is a technology that focuses a translucent wavelength laser beam through an optical focusing lens on the inside of the wafer, forming a starting point for segmentation inside the wafer, that is, the modified layer, and then applying external force to the wafer to divide it into an independent chip. Therefore, invisible cutting technology generally includes two processes: laser cutting and chip separation. The laser cutting process, according to the different processing materials, chooses the corresponding wavelength laser through a specific optical path, focuses on the inside of the wafer to form a modified layer, the modified layer in the formation of the same time, will form a crack pointing to the positive and negative surface of the wafer. For a certain thickness of the wafer, the laser needs to focus several times at different focal depths to scan the inside of the wafer, so that the modified layers are connected to each other, and finally form a whole modified layer suitable for segmentation, which is an important step to promote chip separation.
On the basis of the formation of the modified layer, the chip separation process is to make the modified layer run through the surface and bottom of the wafer, and then separate into an independent chip by external force such as splitting pressure, or directly by spreading and splitting.
Laser ablation cutting technology
Laser ablation cutting is the use of high-energy pulsed laser, after the optical system collimation and focusing, the formation of high energy density, beam spot size of only micron laser beam, acting on the surface of the workpiece, so that the irradiated area local melting, gasification, so that the scribing interchannel material removal, and finally achieve grooving or direct scribing.
Laser ablation cutting takes high temperature as the mechanism of action, and at the ablation edge will form a heat-affected zone with the phenomenon of frequent recasting of the processed material. How to control the size of the heat-affected zone is the main way to realize the development of laser cutting in the semiconductor industry. The corresponding laser and optical system are selected and configured according to the absorption characteristics of the processed material to different wavelengths of laser. Among them, the pulse width is an important parameter affecting the cutting quality, actually refers to the duration of each laser single pulse, in the case of the same power and frequency, the smaller the pulse width, the shorter the action time between the laser and the processed material, the smaller the heat affected zone, which can reduce the adverse impact of the ablation process on the edge. In addition, the selection of lasers also need to take into account the efficiency and quality, generally the shorter the laser wavelength, the smaller the processing heat affected zone, but the laser speed is slow and the efficiency is low.
At present, there are two main types of laser cutting technologies used in semiconductor wafer cutting, namely laser stealth cutting and laser ablation cutting. The two technologies have their own characteristics, and both show the advantages that the traditional grinding wheel can not match. With the continuous development of laser technology and the maturity of related equipment, laser cutting will occupy a more dominant position in the field of semiconductor wafer cutting.
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