# Additively Manufactured Polymers for Electronic Components
## 4 Applications in the Electronic Field of Additively Manufactured Polymers
### Wafer Bonding  
Wafer bonding is an important process in semiconductor manufacturing in which two or more semiconductor wafers are bonded together to create complex integrated circuits and MEMS. This critical technique plays a fundamental role in achieving enhanced device performance, the integration of heterogeneous materials, and the development of new electronic and optical devices. In the last few years, the combination of 3D printing and wafer bonding technologies has been an exciting direction for transforming semiconductor fabrication. The use of 3D printing in wafer bonding enables the development of intricate three-dimensional structures and customised device architectures. It also reduces manufacturing flows. One of the AM technologies investigated for this purpose is AJP, as presented in a recent study, which used BCB-based ink and assessed the material bonding quality after a thorough characterisation of the AJP process. A bonding test was performed using two silicon (Si) wafers, with a thermally grown Si oxide layer, bonded by a BCB layer deposited using AJP. The process, illustrated in Figure 15a, involved an initial heat of 150 \({}^{\circ}\)C for 30 min, above the 23 \({}^{\circ}\)C glass transition temperature of the BCB ink. This allowed the BCB to flow and evenly coat the surfaces of the wafers under a compressive force of 2.4 kN. After curing and bonding, gel content analysis revealed over 99% cross-linked material, indicating successful cross-linking. It was, nevertheless, noted that BCB is prone to oxidation upon curing. To assess potential oxidation, Fourier transform infrared spectroscopy (FTIR) analysis was performed, comparing BCB cured under nitrogen and air atmospheres (Figure 15b). In the FTIR spectra, BCB cured in the air showed a higher peak at 3000-3200 cm\({}^{-1}\), attributed to OH group stretching, along with oxidised benzylic CH\({}_{2}\) groups detected at 1700-1850 cm\({}^{-1}\). This indicated a higher concentration of oxidised species in BCB cured under an air atmosphere.  
Die shear tests were also conducted to measure the shear strength of wafers bonded with a BCB layer. These tests were performed on a pad with a constant cross-section to calculate shear strength accurately and to compare coatings from AJP and spin-coating. Testing complex patterns with a high aspect ratio would not yield precise results and would not be feasible with spin-coating. Figure 15e presents the die shear test setup, where a tip applies a displacement to the top silicon wafer while the bottom wafer is secured by a physical barrier. The test is concluded when the bonding layer breaks, and the force at which this happens is recorded [131]. Figure 15f shows the die shear test results for wafers bonded with a BCB layer deposited using AJP and spin-coating. The shear strengths were 38.74 \(\pm\) 1.88 MPa for AJP and 36.25 \(\pm\) 2.33 MPa for spin-coating. These comparable values indicated that AJP did not negatively impact bonding quality compared to spin-coating, and these results were consistent with the literature results for spin-coated BCB [171; 172; 173].  
Table 2 displays a summary of all of the abovementioned applications with the materials and the technologies employed.  
Figure 15: **(a)** Plot of temperature and piston force measured during the bonding process. **(b)** FTIR spectra of BCB cured under nitrogen and air atmospheres. **(c)** Representative SEM image of bonded silicon (Si) wafers. **(d)** Representative scanning acoustic microscopy image of 3 cm \(\times\) 3 cm bonded Si wafers. **(e)** Schematic illustration of the die shear test performed on bonded Si wafers. A tip applies a displacement on the top Si wafer in the direction of the arrow, while the bottom wafer is forced not to move due to the presence of a physical barrier. **(f)** Shear strength measurements of bonded Si wafers, comparing BCB deposition via aerosol jet printing (AJP) and spin-coating. Adapted and reprinted with permission from ref. [131].