Polyimide (PI), with its unique thermal and electrical stability, has been widely used as an insulating and passivation layer in the field of microelectronic manufacturing, especially for advanced packaging applications. As the line width and spacing of leads in microelectronic devices continue to shrink, the thickness of the Fan-Out Trace Insert (FTI) is becoming increasingly thinner, imposing higher requirements for the insulation performance of the leads. The FTI is usually composed of SiO₂ or polymers (predominantly PI). However, compared with SiO₂, PI offers lower costs as an FTI material, which has led to its extensive application in microelectronic manufacturing.

In the manufacturing process of microelectronic devices, the processing quality of the PI layer determines the product yield rate. Therefore, it is of great significance to explore effective methods for PI layer processing. At present, plasma etching is the primary method for PI layer processing. This technique boasts excellent selectivity and controllability, and will not cause damage to other materials or structures of the chip.

Basic Principles of PI Plasma Etching

Plasma is mainly composed of ions, electrons, free radicals, and other species. Particles carry high energy under the action of an electric field while maintaining overall electrical neutrality. The characteristics of plasma determine that its interactions with materials occur primarily via two mechanisms: physical bombardment by high-energy particles and chemical reactions by reactive species.

As a hydrocarbon-based polymer, PI consists mainly of carbon and hydrogen. Plasma treatment of PI using different process gases yields varying effects. Ar and N₂ plasmas modify the PI layer primarily through physical bombardment, whereas O₂ and CF₄ plasmas treat and remove the PI layer mainly via chemical reactions. Therefore, selecting the appropriate process gas according to the specific PI treatment objectives will achieve the desired processing outcome.

Plasma Etching Rate of PI Layers

Throughout the entire electronic device packaging process, multiple procedures involve PI removal. Therefore, a higher etching rate can effectively boost the production capacity of electronic devices. It is a practically significant issue to improve the plasma etching rate of PI layers without increasing power output.

Early research has indicated that the etching rate of polymers using pure O₂ plasma is limited, especially at low temperatures (below 30℃), where it is difficult to achieve an etching rate of 1 μm/min. Although increasing the temperature can enhance the etching rate, it will compromise the performance of microelectronic devices.

Thus, in microelectronic device packaging, the conventional method to increase the PI etching rate is to add an appropriate amount of CF₄ to O₂ plasma. The effect of CF₄ concentration in the O₂-CF₄ mixture on the etching rate is illustrated in Figure 1 (with fixed parameters: O₂ flow rate at 1000 sccm, microwave power at 2000 W, pressure at 0.4 Torr, and temperature at 25℃). As shown in Figure 1, the etching rate of pure O₂ plasma is higher than that of pure CF₄ plasma. With the increase of CF₄ concentration in O₂, the PI etching rate increases gradually, peaking at 1.492 μm/min when the CF₄ concentration reaches 20%, after which the etching rate decreases progressively.

Figure 1: Relationship between CF4 content in O2 and plasma etching rate of PI

Based on the above analysis, it can be concluded that adding an appropriate amount of CF4 to O2 can effectively enhance the etching rate of PI. Upon the addition of CF4, F atoms react with PI, extracting H from the PI surface and forming highly reactive sites on the surface. These active sites are prone to combine with O radicals, thereby increasing the etching rate of PI. However, since the number of active sites that can be generated on the PI surface is limited, when the amount of CF4 exceeds a certain threshold, excess F atoms occupy these active sites, hindering the reaction between O radicals and PI, and consequently leading to a decrease in the etching rate.