Application of Domestic Substrates in Fabrication of Bipolar ICs
Abstract: With the continuous improvement of the production process and control ability of domestic substrates, the application of domestic substrates has also become more and more widespread. The authors made a comprehensive assessment of the issues commonly considered in the manufacture of bipolar ICs for domestic substrates, including physical parameters, electrical parameters, wafer pass rates, and large-scale production engineering capability indices. The evaluation results showed that the quality of domestically produced substrates is comparable to that of imported substrates and meets the requirements of large-scale production.
Keywords: integrated circuit fabrication; silicon substrate; bipolar integrated circuit; substrate physical parameters; substrate warpage; out-of-focus; electrical parameters; engineering capability index.
1 Introduction
Domestic silicon substrates have been widely used in the production of discrete devices such as TVS, Schottky diodes, and VDMOS, but they are rarely used in the manufacture of bipolar ICs. The main reason is that bipolar ICs In the production process, there are a large number of high-temperature and high-temperature processes, such as epitaxial growth processes, DN deep phosphorus and DP-isolated propulsion processes. These process temperatures are all above 1 150 °C, and the DP or DN propulsion process takes 24 hours. Above, the stress and warping deformation of the substrate are relatively large. A slight carelessness can cause stress debris or lithography to defocus, resulting in scrap or low yield. Therefore, on the large-scale use of domestically produced substrates, we have been conservative and cannot produce scale production.
The quality of home-made substrates has made great progress in recent years. Repeatability and consistency have also been greatly improved. In addition to the recent tight supply of substrates, the localization of substrates has become increasingly urgent. The author of this paper compares the performance of the substrate itself, the electrical parameters of the product (PCM parameters), and the pass rate of the wafer by comparing the substrate of a domestic substrate with that of a major foreign manufacturer. Whether the wafer is warped on a scale production track observation. The process capability index of electrical parameters has also been calculated statistically. The results are very satisfactory, laying the foundation for the application of domestic substrates in large-scale production.
2 experiments
2.1 Subjects
Typical products in bipolar ICs, power management and audio amplifier products were selected as evaluation objects. The process includes 1 150 °C epitaxial process, and 1 150 °C, 24 hours DN or DP advance process.
2.2 Comparison of Experimental Methods and Results
For the comparison of the imported substrate and the homemade substrate, the physical properties of the silicon substrate are compared first, and then the characteristics of the electrical parameters and the wafer pass rates of different products are compared.
Finally, the actual performance of domestic substrates in large-scale production was tracked and evaluated. The main assessment is whether wafer breakage, defocusing of the lithography, and slippage on the e-chuck stage occur during production.
At the same time, the engineering capability index (Cpk) of the main PCM data is evaluated.
3 packet comparison data
3.1 Comparison of Physical Parameters of Silicon Substrate
Table 1, the main measurement parameters compared. Figure 1, under the front mirror comparison. Figure 2, contrast under the microscope. conclusion as below.
(1) The geometric dimensions of the silicon wafer, the resistivity radial gradient, the oxygen gradient, the oxygen content, the carbon content, the TIR, the STIR, the OISF, the dislocation density, and other parameters are comparable for the domestic substrate wafer and the inlet substrate wafer, and there is no significant difference.
(2) The microscopic examination results on the surface (front and back) of the silicon wafer showed that the domestic substrate wafer and the inlet substrate wafer were equivalent and there was no significant difference.
(3) WARP and TTV parameters. The value of domestic substrate is slightly worse than that of imported substrate. Although they are all within specifications, the difference does exist.
3.2 Packet Electrical Parameters (PCM) Comparison
Due to the relatively large number of PCM measurement parameters, we have selected the main parameters that affect the device characteristics, focusing on the following six parameters. LPNP_BETA, NPN__BETA, R_CHAIN_SP/CO/IN, R_CHAIN_SN/CO/IN, BVISO, and R_PINCH (BASE).
The power factor of the PNP transistor and the NPN transistor, BETA, is an important parameter of the bipolar integrated circuit and has a significant influence on the characteristics of the final product.
BVISO is the withstand voltage of the isolation zone and reflects the level of isolation. The leakage current of devices with low withstand voltage will increase significantly.
R_PINCH(BASE) is the emitter and base crush resistance and reflects the control of junction depth and junction concentration.
R_CHAIN_SP/CO/IN, R_CHAIN_SN/CO/IN are the contact chain resistances of the SP area and SN area. Since they are in direct contact with the silicon substrate, they are also considered as the object of inspection.
The comparison of data is shown in Figure 3 to Figure 8.
These PCM data are very close to match the conditions. The domestic substrate performed slightly better than the inlet substrate on the LPNP-BETA project. In terms of electrical parameters, there is no significant difference between domestic substrates and imported substrates, and small differences are also within a fully acceptable range. Some parameters of domestic substrates perform better.
3.3 Comparison of Qualification Rate of Grouped Wafers
Table 2 shows the comparison results of the qualification rates of the grouped wafers for audio products and power products. There is no significant difference in the pass rate of wafers between domestic substrates and imported substrates, which has reached a relatively high level.
4 Performance of electrical parameters in large-scale production
We made Cpk statistics for the six key electrical parameters in Section 3.2, which included 2,000 wafers and 10 000 raw data sources. The process capability index CpK is shown in Table 3. Our requirement is that CpK is greater than or equal to 1.66, and all CpK indexes are in compliance with the requirements of large-scale production.
5 The stability of the silicon substrate itself
In the production of silicon substrates, there will be various stresses in the silicon substrate, and at the same time, the silicon substrate wafer will have a certain degree of warpage. If the control is not good, it will cause various problems in the subsequent integrated circuit production, mainly as follows: The following aspects.
(1) Stress debris. Due to the bipolar IC production process, a long period of heat and high temperature process is more. It is particularly easy to cause stress debris, which is manifested in the fact that a piece of silicon wafer in the boat will split along the crystallographic direction or somehow broken.
(2) Slides. Due to the warpage of the silicon substrate, the e-chuck can not absorb the silicon wafer well. The silicon wafer slides on the plate of the device, causing the device to alarm. In severe cases, it will cause debris and require the on-site disposal of the engineer. The equipment downtime.
(3) Lithography defocus phenomenon. Still due to the warpage of the silicon substrate, the lithography process cannot be perfectly focused, and the partial area cannot be focused. The pattern of these areas cannot be perfectly formed, resulting in a low yield of the wafer.
We tracked hundreds of thousands of pieces of domestic substrate films online, and counted the number of adverse events that occurred in 10,000 tablets. Table 4 shows the actual statistical results of 100,000 tablets.
These results show that: Although the warpage degree is slightly inferior to the inlet sheet, the substrate manufacturer's production process control capability is still very strong, and the repeatability consistency is very good. The substrate sheet can adapt to the subsequent so many hot and high temperature processes. The challenges are maintained in a good state, there is no obvious deformation warping, and the number of adverse events is almost zero. This has a crucial role in the formation and improvement of large-scale production capacity.
6 Conclusion
Through the comparison of the main physical parameters and electrical parameters, it is shown that the performance of domestically produced substrates is comparable to that of imported substrates. No difference can be seen in the qualification rate data. As for the deficiency of WARP and TTV, it can be improved by optimizing mortar dosing [1], wire cutting process [2], optimizing the etching process and optimizing the polishing process [3-5].
Mass production data shows that the quality of domestic substrates is very stable, and its process capability data is also very good, so domestic substrates can completely and perfectly replace imported substrates. Inspired by this, we are also actively verifying 200 mm domestic substrates, and the preliminary results are also in line with expectations. The era of home-made substrates really came.
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