Development of the hottest ultra high removal rate

2022-08-02
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Development of ultra-high removal rate copper CMP abrasives

high density IC devices involve interconnected multilayer stacks. Chemical mechanical planarization (CMP) process provides a smooth surface on the wafer for lithography, and has become a key process to obtain high yield in semiconductor manufacturing. When the semiconductor industry is moving towards 45nm and smaller nodes, the integration scheme is facing the challenge of balancing the reduction of interconnect feature size and the limit of physical properties of interconnect materials. Silicon via (TSV) is constructed by stacking chips through the interconnection structure of the wafer, which can reduce the Joule heat effect and the area occupied by the chip, and increase the interconnection density at the same time. It can increase the input-output points and minimize the actual cost of the chip

tsv is a conductive path formed vertically through the total wafer thickness, which can reduce the complexity of the interconnection path and enable more input-output channels in the package. Copper is used as interconnection material in TSV fabrication. In this Cu 3D TSV process, the through holes are filled with copper, and a thick uneven copper layer is deposited on the wafer. The thickness of the covered copper layer is m, which needs to be removed by CMP, leaving only Cu in the through hole to establish interconnection through the wafer (Fig. 1). Removing this thick Cu layer requires optimizing the Cu CMP process. One of the main requirements is the ultra-high speed Cu CMP process to obtain reasonable process time and yield

in general, Cu CMP Abrasives contain abrasive particles, oxidizers, corrosion inhibitors, and pH regulators. In order to obtain certain properties, complexing agents, surfactants, rheological regulating components and other special components are sometimes added. CMP requires a balance between removal rate, planarization and surface quality. To achieve this balance, corrosion inhibitors were introduced into the formulation. Its function is to passivate the surface, control the oxidation rate, and prevent surface pits and etching. The Cu removal rate depends on the surface oxidation rate, passivation rate, etching rate, Cu disappearance rate and the polishing efficiency of the passive film by abrasive particles. High removal rate of Cu may cause high surface corrosion, high defect rate and poor surface morphology control. The real challenge of developing 3D TSV Cu CMP abrasives is how to achieve the super long section, open the dialog box, high Cu removal rate, and good planarization, surface morphology, defect rate and surface roughness

this paper describes the development of abrasives for 3D. Based on the csl-904x Cu abrasive platform of planar, a new Cu abrasive with high removal rate was developed. For further improvement, an ultra-high removal rate abrasive er9212 has been developed. The target removal rate for Cu 3D TSV is 4 ~ 5 m/min. It can also be used in Cu MEMS process

experiment

all experiments were conducted on Mirra polisher. 200mm Cu patterned wafers, Cu SL new materials, UG and MIT 854 patterned wafers were used in the study. The removal rate was determined by measuring the weight loss and Cu thickness, and the block resistance was measured with a 4-Probe on resmap. The corrosion response was measured by electrochemical technique, and the defect rate was measured on ait-uiv. The surface morphology was measured by Veeco AFM profiler. In order to obtain reliable and stable process during CMP, multiple process variables must be considered. After optimization, the machine parameters (such as pressure, workpiece table speed, grinding head speed, environmental conditions, etc.) remain unchanged throughout the experimental process of data collection. The abrasive selected in the study was er9212 from planar solutions. Er9212 is developed based on the csl-904x Cu abrasive platform of planar. It has high bulk Cu removal rate and good uniformity and defect rate. It is designed for Cu CMP applications that require high removal rates, such as Cu 3D TSV, Cu MEMS, and top-level Cu planarization. Table 1 shows the relevant characteristics of the abrasive

results and discussion

acidic platform abrasives were initially observed. These Abrasives can be polished at a very high removal rate due to the high oxidation and dissolution rates of Cu under acidic conditions. In the test, most of the acid Abrasives have a high removal rate (Fig. 2), but most of the polished silicon wafers have surface corrosion problems. The contamination of the polishing pad was also observed during CMP, which would affect the stability of the process, reduce the service life of the polishing pad and increase the defect rate

planar Solutions LLC has successfully commercialized a range of csl904x platform Abrasives based on neutral pH. Typical bulk Cu removal rates for these Abrasives range from 8000 to 11000/min。 For 3D applications, such a rate is not enough. These Abrasives contain Cu removal rate enhancers and corrosion inhibitors. If the removal rate enhancer content is simply increased, the removal rate will increase, but the morphology and surface roughness will also increase (Fig. 3). The first attempt was to use a higher removal enhancer. The removal rate of 3 ~ 5 m/min can be achieved, and all Cu in the groove is dissolved in the process of patterned wafer polishing. The surface roughness after Cu CMP is about?, This is unacceptable. This indicates that strong corrosion occurred during CMP. After optimizing the chemical composition of the abrasive, a new type of abrasive with excellent performance, named er9212, was obtained. The practical results show that this component is suitable for 3D TSV Cu CMP process

since much thicker copper layer needs to be removed in Cu 3D TSV, it is necessary to have a high and adjustable removal rate. Factor A is introduced to control the removal rate of er9212. See Figure 4 for the removal rate obtained when the polishing downforce is 4psi. The removal rate of er9212 is adjusted by factor a. The achievable removal rate is 2.5 m/min to 7.7 m/min

data on removal rate and pad temperature were collected at different polishing downpressures (Fig. 5). The results show that the removal rate increases with the increase of polishing pressure. Higher polishing downforce results in higher removal rate. When the downward pressure is above 3 psi, the bulk Cu removal rate of er9212 is higher than 4 m/min. For high downforce Cu CMP, polishing pad temperature is also a key parameter, because excessive temperature (75 C) will cause delamination of polishing pad and failure of CMP equipment. The mechanical friction of CMP and the heat generated by chemical reaction are the main factors affecting the temperature of polishing pad. When the polishing downforce is increased, the polishing pad temperature becomes higher. Decreasing the rotating speed and increasing the flow rate of abrasive can help to reduce the temperature of polishing pad during CMP. Even when the downward pressure is 5psi, the polishing pad temperature of er9212 is about 70 ℃. When using patterned wafers, the temperature of polishing pad is expected to be lower than that of forgings

for high-speed Cu CMP process, high concentration by-products can be seen on the polishing pad. These Cu by-products will contaminate the polishing pad at high downforce and high temperature, which will reduce the Cu removal rate and increase the rate of chemical changes or physical damage defects caused by thermal expansion and contraction of the sample in a short time. The decrease of Cu removal rate will also affect CMP process control, such as end point and over polishing control. No contamination of polishing pad was observed during polishing of er9212. When the downward pressure is 2psi, about 1.5 m of Cu is removed, and less than 10% is removed under the same conditions? TA of. The cu/ta selection ratio is greater than 1500:1, which indicates that er9212 is a highly selective Cu abrasive

mit 854-teos has patterned wafers for morphology research (Fig. 6). The Cu layer covered on this wafer was about 1.0 m and polished to the end point at a downward pressure of 1.2psi. There is no Cu residue on the polished wafer. The depression formed on the 10 m line with high removal rate abrasive is about 1265?, This is acceptable for Cu 3D TSV applications. Since the main bottleneck in the application of Cu CMP in 3D TSV is the polishing step of bulk Cu, a much lower morphology can be obtained if a low sag abrasive is used at the soft landing step. The surface roughness was measured on the Cu blank after the removal of the 0.6 m Cu layer. Good uniformity was observed after polishing. The surface of the processed Cu wafer is very smooth and the surface roughness is low (between 9 and 13 °). The electrochemical test results also show that the corrosion rate of Cu wafer surface is 9.3/min。 This indicates that the abrasive has less corrosion and high planarization efficiency. The results show that Cu complexing and Cu surface passivation are very important in the design of high-speed Cu CMP abrasives. A good balance of these factors can obtain a high Cu removal rate and maintain a fairly good morphology, surface roughness and defect rate, which is important for the application of 3D TSV Cu CMP

conclusion

er9212 abrasive provides high removal rate of bulk Cu, good morphology and low surface roughness. It is an excellent alternative abrasive for high-speed Cu CMP applications such as Cu 3D TSV, Cu MEMS and top-level Cu CMP

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