ADDRESS：The intersection of future road and Qinlong line in Qinyang City, Jiaozuo City, Henan Province
Abstract: in the process of wafer back grinding, mechanical grinding causes damage to the back of wafer, resulting in surface roughness and warpage. The 15.24cm (6-inch) silicon wafers were thinned by four different back thinning methods: rough grinding, fine grinding, polishing after fine grinding and wet etching after fine grinding. The surface and cross-section morphology of the thinned silicon wafers were characterized by scanning electron microscope. The surface roughness of the silicon wafers was measured by atomic force microscope, and the warpage of the silicon wafers was measured by warpage tester. The results show that there is mechanical damage on the silicon wafer after rough grinding and fine grinding, the surface roughness is 0.15 μ m and 0.016 μ m, and the warpage is 147 μ m and 109 μ m, respectively; after polishing and wet etching, there is no surface damage, the surface roughness is less than 0.01 μ m, and the warpage is less than 60 μ M.
Wafer back thinning is an important wafer manufacturing process. The purpose is to remove the excess material on the back of the wafer, so as to effectively reduce the packaging volume of the wafer, reduce the thermal resistance, improve the heat dissipation performance of the device, reduce the risk of chip cracking due to uneven heating after packaging, and improve the reliability of the product. At the same time, the mechanical and electrical properties of the thinned chip are also significantly improved. There are many kinds of wafer back thinning technologies, such as grinding, polishing, dry polishing, electrochemical etching, wet etching, plasma assisted chemical etching and atmospheric pressure plasma etching. Silicon wafer grinding thinning technology is a kind of high efficiency and low cost thinning technology, which has been widely used. In this technology, the grinding wheel is rotated on the surface of the silicon wafer to apply pressure, damage, fracture and remove, so as to realize the silicon wafer thinning. Damage is inevitable in the process, which reduces the reliability and stability of the device. The polishing process uses the relative movement between the silicon wafer and polishing head to flatten the surface of the silicon wafer, and there are Abrasives between the silicon wafer and polishing head; wet chemical etching is a process technology to realize the thinning of the silicon wafer through the chemical reaction between the etching solution and the silicon wafer, and the commonly used etching solutions are acid etching solution (such as nitric acid, glacial acetic acid and hydrofluoric acid) and alkaline etching solution (such as KOH solution ).
Taking 15.24cm (6-inch) silicon wafer back thinning as an example, this paper analyzes the damage, surface roughness and warpage of the silicon wafer after grinding, polishing after grinding and wet etching after grinding, so as to optimize the silicon wafer back thinning technology.
Four 6-inch silicon wafers with a thickness of 675 μ m and a protective film with a thickness of 140 μ m were selected. The silicon wafers were thinned with a 325 # - Disco grinding wheel with a grain size of 40-60 μ m by using Fangda technology single-chip grinding machine. The four 6-inch silicon wafers were thinned by 140 μ m and 535 μ m, and one sample was taken out with the number of 1 #; Then, the remaining three samples were ground by Fangda single-sided grinding machine. 2000 # - Disco grinding wheel was selected. The grinding wheel particle size was 4-6 μ m, and the thickness of silicon wafer removed was 30 μ M. one of the samples was taken out, and the number was 2 #; For the remaining two samples, one was chemically mechanical polished by Fangda technology single-sided polishing machine for 40 min, No. 3, and the other was thinned by wet chemical corrosion method, with corrosion time of 5 min, No. 4. The mixture of nitric acid, glacial acetic acid and hydrofluoric acid is used for wet etching. The volume ratio of nitric acid, glacial acetic acid and hydrofluoric acid is 5 ∶ 4 ∶ 1. The reaction principle is as follows :
After finishing the above thinning process, the front protective film of the sample is removed and cleaned. Then, the thickness of the sample is measured by mitotuyo thickness tester, the warpage is measured by warpage tester, the surface and cross-section morphology of the sample are observed by feg450 scanning electron microscope, and the roughness of the sample is measured by atomic force microscope.
2. Results and discussion
2.1 surface morphology analysis
Fig. 2 is the scanning electron micrograph (SEM) of the sample surface after thinning. It can be seen from the figure that the surface of 1 sample is rough, undulating, with bumps and pits; 2 sample surface has a large number of small scratches. The surface roughness Ra of 1 sample and 2 sample are 0.15 and 0.016 μ m, respectively.
The atoms of monocrystalline silicon are arranged periodically according to the diamond structure in the crystal. The back grinding of silicon wafer is a kind of physical damage process. The grinding of grinding wheel will destroy the periodic arrangement of silicon atoms and cause mechanical damage on the surface of silicon wafer. Comparing Fig. 2 (a) and Fig. 2 (b), it can be seen that the surface morphology and surface roughness of the sample are related to the grinding wheel used for grinding. The smaller the grinding wheel particle size is, the smoother the silicon wafer surface is, and the smaller the roughness is.
Fig. 2 (c) and Fig. 2 (d) are the surface morphologies of 3 and 4 samples respectively. Comparing the two figures, it can be seen that the surface of the sample after chemical mechanical polishing is smooth without grinding scratch, and the scratch left by grinding wheel can still be seen on the surface of the sample after wet etching. The surface roughness Ra of 3 # and 4 # samples are 0.001 and 0.008 μ m, respectively. Chemical mechanical polishing process is a global planarization process. From the above results, it can be seen that the polishing process can effectively improve the surface morphology of silicon wafer; the surface roughness of the sample decreases after the etching process. Combined with the acid wet etching process, it is an isotropic etching process , and the surface damage and convex parts of the sample are preferentially corroded. It can be seen that wet etching can improve the surface morphology of silicon wafer. The surface roughness Ra of four samples is shown in Table 2.
2.2 section morphology analysis
Figure 3 is the cross-section morphology (SEM) of the sample. It can be seen that the cross-section of 1 sample is rough with cracks, and the cross-section of 2 sample is flat with microcracks. It can be seen from the figure that the thickness of the damage layer of the sample section is 5.382 μ m and 1.115 μ m respectively.
Silicon wafer grinding thinning is a physical damage process. According to hadamovsky's damage layer model , after mechanical grinding thinning process, there is a damage layer on the silicon substrate, and the damage layer is divided into multi-layer structure from top to bottom. According to the above experiments, the thickness of the damage layer is related to the grain size of the grinding wheel. The larger the grain size of the grinding wheel, the larger the damage layer and the rougher the surface of the silicon wafer. The cross sections of 3 # and 4 # samples are shown in Fig. 3 (c) and Fig. 3 (d). There is no damage layer on the sample cross section. According to the above, the mechanical damage on the sample surface is removed by polishing process and wet etching process. The thickness of the thinned sample and the thickness of the surface damage layer are shown in Table 3. The warpage of the sample is also listed in Table 3. It can be seen from the table that the warpage of the sample is related to the thickness of the surface damage layer. As shown in Figure 4, the greater the thickness of the damage layer, the greater the warpage. The mechanical damage on the surface of the silicon wafer destroys the crystal structure of the silicon wafer, reduces the strength of the silicon wafer, increases the fragmentation rate of the silicon wafer in the transfer process, affects the reliability and stability of the subsequent process, and reduces the yield of the product. Polishing process and wet etching can not only remove the surface damage layer of silicon wafer, but also reduce the warpage of silicon wafer, and effectively improve the quality of thinned silicon wafer. It is of great significance to improve the yield of production line, broaden the service conditions of components, improve the performance of components, and extend the service life of components.
Mechanical grinding thinning on the back of silicon wafer is a kind of physical damage process, which leads to mechanical damage on the surface of silicon wafer. In this paper, the surface and cross-section morphology of silicon wafers after rough grinding, fine grinding, polishing and wet etching are compared and analyzed, and the thickness, roughness and warpage of silicon wafers are tested. Combined with theoretical analysis, the conclusions are as follows: the surface morphology and damage layer thickness of silicon wafers in Mechanical grinding thinning process are related to the grinding wheel grain size. The larger the grinding wheel grain size is, the more uneven and rough the silicon wafer surface is The thickness of the damaged layer increases with the increase of the damage degree. The results show that the combination of rough grinding and fine grinding can improve the surface morphology of silicon wafer and reduce the thickness of damage layer; the wet etching and polishing process can improve the surface morphology of silicon wafer and remove the mechanical damage in the process of silicon wafer thinning; the polishing process can obtain the silicon wafer surface with lower roughness, slow rate, high efficiency of wet etching process and rough surface; The warpage of the thinned wafer is related to the thickness of the damaged layer. The greater the thickness of the damaged layer, the greater the warpage. In the actual process, combined with the device requirements, we can use the method of grinding, polishing or wet etching to get high quality thinned silicon wafer.