Relaxation and Thermal Transition of Nanoscale Polyimide Ultrathin Films★

doi:10.6023/A25050197

Abstract

Polyimide (PI) films are widely used in semiconductor packaging due to their excellent properties, including low dielectric constant, high temperature resistance, and enhanced thermal and chemical stability. In recent decades, the development of modern nanotechnology has led to devices being downsized to the nanoscale, and packaging these nanodevices would require ultrathin films of PI. Understanding the thermal transition and molecular relaxation of the PI ultrathin films is essential for high-quality microelectronic packaging. Although the dynamics of bulk PI have been extensively studied using techniques such as broadband dielectric spectroscopy (BDS) and dynamic mechanical analysis (DMA), the impact of reduced film thickness on these dynamics remains unclear. In this study, we used temperature-variable spectroscopic ellipsometry to examine the thermal expansion and molecular relaxation of PI films as thin as 6 nm, and concurrently, the bulk dynamics of PI were investigated using BDS and DMA for comparison purposes. The PI films were prepared by the heat imidization of poly(amide acid) films, which were polymerized using biphenyltetracarboxylic diandhydride (BPDA) and 4,4'-oxydianiline (ODA). In 300-nm-thick films, we observed the α-relaxation arising from the segmental cooperative rearrangement, and the β-relaxation originating from phenyl ring motion at approximately 260 ℃ (T_α) and 99 ℃ (T_β), respectively. As the film thickness decreased below 100 nm, T_α decreased. Specifically, T_α decreased by 30 ℃ for 6-nm PI films. In contrast, T_β remains unchanged in the ultrathin films, indicating a thickness-independent β-relaxation. Furthermore, the coefficients of thermal expansion increased remarkably with a reduction in film thickness across various temperature ranges (i.e., T<T_β, T_β<T<T_α, and T>T_α), indicating that ultrathin PI films are more sensitive to temperature variations than bulk samples. Such observation of the thickness-dependent thermal expansivity and T_α of ultrathin PI films provide a deep understanding of the effects of nanoconfinement on the dynamics of polymers with rigid chain backbones, which is also meaningful for designing and fabricating stable nano-devices based on ultrathin PI films.

Key words: polyimide, relaxation, ellipsometry, thermal expansion, nanoconfinement

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Quanyin Xu, Xinyang Shi, Jintian Luo, Biao Zuo. Relaxation and Thermal Transition of Nanoscale Polyimide Ultrathin Films^★[J]. Acta Chimica Sinica, 2025, 83(9): 1006-1012.

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Fig. & Tab. 9

Figure 1. (a) Synthesis route for the BPDA-ODA polyimide (PI) films, (b) 1H NMR spectrum of poly(amido acid) (PAA) and (c) FT-IR spectra of PAA and PI films

Figure 2. (a) Dielectric loss of polyimide film at different temperatures, (b) Dielectric loss at T=310 ℃ and the dashed line shows the HN function fitting. Thickness of the film is 80 μm

Figure 3. α and β relaxation time plotted as a function of the inverse temperature for polyimide film. The dotted lines (red and black) are the fitting of the data to the VFT and Arrhenius equation, respectively

Figure 4. Temperature dependence of storage modulus (E') and loss factor (tan δ) of polyimide film. Thickness of the film is 80 μm

Figure 5. (a) Temperature dependence of the thickness and (b) the thermal expansion coefficient of the 300 nm thick polyimide film

Figure 6. (a) Normalized thickness (h/h0, h0 is the thickness of films at 330 ℃) and (b) thermal expansion coefficient as a function of temperature for polyimide ultrathin films. The curves in panel (b) have been vertically shifted for better presentation of their difference

Figure 7. Film thickness dependence of (a) Tα, Tβ and (b) k for polyimide ultrathin films

Figure 8. Temperature protocol for preparing PI films by imidization of PAA films at high temperature

Figure 9. AFM morphological images of (a) 20 nm PAA film and (b) the corresponding PI films by imidization

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