## 3 Results and Discussion
### Thermal Properties of Polymers  
The thermal performance of photoresist polymers plays a critical role in the lithography process. Firstly, the thermal decomposition temperature of the copolymer PTBM is 200.1 \({}^{\circ}\)C at 5% heat loss by TGA analysis (Figure 7A), which meets the temperature requirement in the photolithography process. Then the glass transition temperature (T\({}_{\text{g}}\)) of copolymer PTBM was analyzed by DSC. The DSC curves of PTBM samples with different contents of POSS unit were shown in Figure 7B, revealing the glass transition temperature (T\({}_{\text{g}}\)) of the samples. It can be observed that with the increase of POSS units, the T\({}_{\text{g}}\) of the copolymers gradually increased. The T\({}_{\text{g}}\) values of PTBM-0, PTBM-1, PTBM-2, PTBM-3, PTBM-4 are 102 \({}^{\circ}\)C, 108 \({}^{\circ}\)C, 113 \({}^{\circ}\)C, 117 \({}^{\circ}\)C and 122 \({}^{\circ}\)C, respectively. It was speculated that the increase of T\({}_{\text{g}}\) was attributed to the rigid cage-like Si-O-Si structure in the POSS units, as well as the significant steric hindrance of the POSS units, which hindered the mobility of polymer chain segments. Furthermore, the comparison of the thermal stability of PTBM with other photoresist polymers reported indicate the promising potential of PTBM in photolithography. For example, [PERSON] et al. [39] constructed a poly (glycidyl methacrylate)-based copolymer with T\({}_{\text{g}}\) of 75 \({}^{\circ}\)C. [PERSON] et al. [3] proposed a methacrylate-based quaternary copolymer with T\({}_{\text{g}}\) of 105 \({}^{\circ}\)C. [PERSON] and co-workers [40] constructed an acrylate-based photoresist copolymer, by introducing _p_-chlorostyrene, with T\({}_{\text{g}}\) of 101.5 \({}^{\circ}\)C. In addition, IBM developed a _p_-hydroxystyrene-based copolymer with T\({}_{\text{g}}\) at 85-110 \({}^{\circ}\)C [11]. Compared with these copolymers, PTBM-4 possesses a higher T\({}_{\text{g}}\) of 122 \({}^{\circ}\)C, which has the advantage of thermal stability. Chemically amplification photoresists require sufficient temperature (80-120 \({}^{\circ}\)C) during the PEB stage to maintain the energy required for the deprotection reaction, so photoresist resins with high T\({}_{\text{g}}\) can maintain thermal stability during PEB to avoid pattern collapse. Figure 6: UV–vis absorption curves of PTBM-0, PTBM-4, PBM, and TPS-ST (dissolved in ethyl lactate, 0.1 mg/mL). The thermal expansion coefficient \(\alpha\) of the copolymer was measured using static thermal mechanical analysis (TMA). The calculation method is as shown in Equation (1):  
\[\alpha=\frac{\Delta L}{L\times\Delta T} \tag{1}\]  
where, \(\Delta L\) represents the deformation, \(L\) is the original size, and \(\Delta T\) is the temperature change. The calculation yielded thermal expansion coefficients \(\alpha\)0 to \(\alpha\)4 for the copolymers PTBM-0-PTBM-4 as \(1.56\times 10^{-4}\), \(1.26\times 10^{-4}\), \(9.17\times 10^{-5}\), \(5.96\times 10^{-5}\), and \(4.67\times 10^{-5}\) (\({}^{\circ}\)C), respectively. The results indicated that with an increase in the content of POSS units, the thermal expansion coefficient of the copolymers decreased (Figure 8). A low thermal expansion coefficient is advantageous for maintaining the original morphology of photolithography patterns during pre-baking and post-baking, ultimately improving the resolution of photoresist. The thermal expansion coefficient of methyl methacrylate-based copolymers commonly used as photoresist resins [41] is typically between \(8\times 10^{-5}\) and \(12\times 10^{-5}\)\({}^{\circ}\)C\({}^{-1}\). However, [PERSON] [11] and co-workers pointed out that the PEB temperature stability is desired to be <5 \(\times\) 10\({}^{-5}\)\({}^{\circ}\)C\({}^{-1}\). By introducing POSS in the photoresist matrix, the expansion coefficient of the methyl methacrylate-based copolymer was reduced to meet the process requirements. Figure 8: TMA curves of PTBM-0, PTBM-1, PTBM-2, PTBM-3, and PTBM-4.