Introduction to DBC

DBC is a new type of process developed with the rise of chip-on-board (COB) packaging technology, which is mainly made of metallic copper directly bonded to glass ceramics (e.g., alumina Al2O3, aluminium nitride AlN, etc.), the dielectric material therein. With the development of modern microelectronics technology and changes in application requirements, direct bonded copper (DBC) ceramic substrate gradually replaced the traditional PCB substrate, widely used in multi-chip power semiconductor module electronic circuits.

DBC substrate has excellent thermal conductivity and electrical insulation, which makes it has the outstanding advantages in practical application process, especially suitable for power device packaging and heat dissipation, some high power and high temperature state of electronic components such as IGBT (insulation gate bipolar transistor) module, MOSFET (metal oxide semiconductor field effect transistor) module, laser diode (LD) in the process of its figure.

With the power semiconductor module is a large number and widely used in electronic automotive, aerospace, military defence and many other fields, but also greatly driven by the rapid growth of the demand for the use of DBC substrate. Then, these applications require to ensure the high reliability and performance of the final chip products, the quality of the module and DBC packaging naturally becomes a key factor that can not be ignored.

In the process of DBC encapsulation, on the one hand, it is necessary to ensure that the electrodes are well insulated to avoid circuit failures between the electrodes or between the electrodes and other metal parts; on the other hand, the encapsulation should have sufficient mechanical strength and be able to withstand the vibration, impact and other forces from the external environment, so as to ensure the reliability and long-term stability of the components.

As we all know, DBC substrate is used in IGBT module packaging is very common, or even essential. In the IGBT module packaging process steps, the first step is to prepare the DBC substrate, to choose the appropriate substrate surface treatment method, which is very critical.

The general process flow of IGBT module packaging is as follows.

First of all, through the DBC substrate surface treatment, to remove the dirt, oxides and impurities on the surface of the material, to improve the level of surface cleanliness, in order to ensure that the subsequent soldering, interfacial bonding, wiring and other process steps are well carried out, and ultimately have a positive impact on the reliability of the IGBT module package.  The traditional cleaning method is mainly wet cleaning, although the cost is low, but it is easy to cause damage to the substrate, and there are cleaning agent residues and other issues. In contrast, dry cleaning is safer and does not have the problem of cleaning agent residue.

In dry cleaning, it is necessary to mention the important technology of microwave plasma cleaning. There are also many kinds of plasma cleaning methods, among which microwave plasma excitation frequency is the highest, reaching 2.45GHz, with excellent cleaning effect. Compared with radio frequency plasma and medium frequency plasma, the use of microwave plasma for material surface cleaning treatment has two outstanding advantages.

1. No damage, no secondary contamination

Radio frequency, medium frequency plasma in the cleaning process to the strong physical cleaning, easier to cause physical damage to the workpiece and material; and microwave plasma is different, it takes into account the gentle physical cleaning and high efficiency of chemical cleaning, and even can be done in a purely chemical reaction to clean, will not cause damage to the workpiece and material.

In the case of chemical cleaning, the free radicals inside the plasma have a very high chemical reactivity and will react with organic pollutants or oxides to produce gaseous products that can be discharged. Therefore, there is no electrode sputtering contamination due to strong bombardment as in the case of physical cleaning.

2. Cleans small crevices and holes

Chemical cleaning is isotropic and diffusive, meaning that the plasma can diffuse uniformly in all directions across the substrate surface, which, combined with the high density of microwave plasma, makes it possible to effectively clean hard-to-reach areas such as internal holes and crevices. This makes microwave plasma cleaning technology an effective method for processing substrates with complex three-dimensional structures.

All in all, plasma cleaning technology is a highly efficient and effective means of surface treatment. So, what kind of effect can be achieved in the practical application of surface treatment of DBC substrate with it?

Example of DBC Substrate Cleaning Test

[Test Objective] Removal of surface oxides and activation degree of DBC substrate after plasma cleaning

[Test Material] DBC Substrate

[Test Equipment] Microwave Plasma Cleaning System MWD-80

For this experiment, we used a full-featured cabinet plasma cleaning system manufactured by Opto-intel, with a chamber volume of 80L, and an optimised integrated PLC process control system and industrial computer control system to choose from. Whether it is for R&D or mass production, it can meet the demand, and the consumption of gas in the process is not high, which has outstanding advantages in terms of both performance and cost.

[Test conditions]

[Test procedure]

Following the execution of the above test procedure, we end up with the results:

Comparison before and after microwave plasma cleaning, DBC substrate surface improvement effect is very obvious, the substrate surface copper oxide removal effect is obvious and easy to see. The contact angle of the water droplet also changed from 85.574° to 16.821°, which means the substrate surface cleanliness and the activation degree both have been good to improve.

Conclusion

In the practical application of microwave plasma cleaning case, we saw it after DBC substrate cleaning surface good improvement effect. The application scope of cleaning process is very wide, involving chip manufacturing and packaging, the seemingly humble "basic skills", in fact to improve the packaging reliability and overall quality are critical. Only by first ensuring the performance of the material surface, the subsequent process steps can be carried out smoothly, and the quality and yield of the final product can be guaranteed. If the quality of the package is poor, resulting in impaired thermal performance, electrical insulation failure or mechanical strength problems, not only may affect the reliability, stability and life of the power device, and even lead to component failure or damage.

In recent years, with the development of China's semiconductor industry, a variety of high-end semiconductor manufacturing equipment localisation projects emerged. And plasma cleaning equipment is also being gradually localised alternative, more and more like Opto-intel Vacuum Microwave Plasma System such as good quality and full cost-effective domestic equipment to the market one after another.

We look forward to seeing the day when high-end equipment for semiconductor manufacturing is fully localised! Opto-intel Intelligence itself will also precipitate with heart, adhere to the R & D, for the development of the industry to assume responsibility and pay full attention.