*Polyimide in Electronics: Applications and Processability Overview DOI: http://dx.doi.org/10.5772/intechopen.92629*

case of polyimide deposition, the thickness variation versus the spin speed shows experimentally different behavior ranging between ω0.5 and ω<sup>1</sup> . It has been proved that the polyimide has a different behavior depending on the residual solvents at the end of the process. The double behavior is emphasized with two different power laws. This proves that the model is verified only if the solvent is completely removed at the end of the process.

Nowadays, the semiconductor manufacturing industry commonly process 300 mm wafers. Thus, polyimides have been developed to be spin-coated and patterned at this scale with very good thickness uniformity accuracy (around 2% of difference in thickness across the wafer) [12].

Conventional, or non-photosensitive, polyimide cannot be directly patterned on wafers due to the absence of photo-active agent (see **Figure 7a**). They require several process steps after the fabrication of the active device. To process non-photosensitive polyimide, a first thick polyimide film is spin coated on the wafer similarly to the photoresist process (see **Figure 7b**). Then, a thin layer of photoresist is applied and exposed using a photolithography tool. A standard development process of the photoresist using a mask is used to define the pattern. This pattern is transferred to the polyimide layer by wet etching through openings during the photoresist lithography step. The wet etch is an isotropic process that causes critical dimension and sidewall control issues. This technical difficulty, combined with the complexity of the process, has limited the non-photosensitive polyimide application.

### **Figure 7.**

*Comparison of precursor PAA monomers between non-photosensitive and photosensitive polyimides (a). Lithography process steps comparison between conventional and photosensitive polyimides (*reproduced from *[13]) (b).*

*Polyimide for Electronic and Electrical Engineering Applications*

### **Figure 8.**

*Lithography process steps comparison between negative and positive photosensitive polyimide resins.*

To overcome such difficulties, photosensitive polyimides have been developed to offer an alternative cost savings to the buffer coat polyimide application (see **Figure 7a**). Photosensitive polyimides can be processed similarly to standard resists using photolithography techniques, as shown in **Figure 7b**. Thus, the eight-step non-photosensitive polyimide process can be reduced into a three-step process using photosensitive polyimide. In addition to providing process simplification, this threestep process offers the significant advantages of superior resolution and improved sidewall profiles. As a consequence, cycle time and chemical consumption are reduced. All of these benefits translate into cost savings, ease of use and better quality.

Photosensitive polyimide, like photoresist, can be divided into two categories: positive and negative tones (see **Figure 8**). In the case of the positive tone, the photosensitive polyimide is degraded by UV light and the developer will dissolve away the regions that were exposed. That will leave behind the coating where the mask was initially placed. In the case of the negative tone, the photosensitive polyimide is cross-linked by UV light and the developer will remove only the unexposed regions, leaving behind the coating in areas where the mask was not placed. Application of positive photosensitive polyimide is limited because of the narrow film thickness range available. This makes the negative photosensitive one the most commonly used in electronic industry with a wide viscosity range.

### *2.4.3 Vapor-deposition process (VDP)*

The vapor-deposition polymerization (VDP) is a method where polyimide is directly deposited and synthesized from its two precursor monomers (dianhydride and diamine), evaporated separately at high temperature in a vacuum chamber and collected on a heated substrate for imidization (see **Figure 9**).

The temperature of the transported vapors and of the substrate is usually between 100 and 200°C. A higher temperature post-annealing (≥300°C) is sometimes carried out to complete the imidization reaction. The synthesis of polyimides (with thickness between a few 1 to 10 μm) has been successfully demonstrated by this method for PMDA/ODA [16–19] and other variants of polyimides. Polyimides synthesized by VDP generally have a low oxygen permeability in comparison with conventional spincoated versions and adhere relatively well to their substrate. However, this method remains difficult to fit with industrial manufacturing processes for electrical and electronic systems because of certain inhomogeneities in terms of thickness of the layers.

*Polyimide in Electronics: Applications and Processability Overview DOI: http://dx.doi.org/10.5772/intechopen.92629*

**Figure 9.**

*Polyimide thin-films deposited by VDP from vapor phase (*reproduced from *[14, 15]).*

## **2.5 Curing process techniques**

### *2.5.1 Thermal curing*

Different techniques for curing polyimides and complete the imidization reaction can be used and are reported in the literature. The curing enables to convert PAA into polyimide and so that to finalize the physical properties of the deposited layer. The most commonly used method is the thermal curing, which is carried out optimally at temperatures of at least 250°C under inert gas, like N2. It is simple to implement and leads to good properties of the material.

During that critical process step, the PAA coating is slowly heated up until a first temperature plateau at 200°C corresponding to the NMP solvent boiling point. It is usually held on for at least 15 minutes in order to fully remove the solvent from the layer (see **Figure 10**). Then, a subsequent temperature rising is performed up to a second plateau at temperature from 250 to 400°C and between 30 minutes up to 2 hours to complete the imidization reaction and obtain the final polyimide. Of course, the final chemical structuration and physical properties strongly depend on the temperature and time duration [20]. Moreover, heating and cooling ramps also need to be controlled to avoid thermomechanical stress storage within the films [21].

### *2.5.2 Variable frequency microwave (VFM) curing*

In addition, other annealing methods have started to emerge, leading to equivalent properties of the deposited polyimide layers. This is the case with variable frequency microwave (VFM) curing [22, 23].
