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Recently, a surface energy reduction process (SERP) for stiction-free drying is used for a high aspect pattern structure like shallow trench isolation (STI). The key technology of SERP is trimethylsilyl (TMS) groups termination of the device surface. To explain the electrical influence of TMS groups was found acceptable by measuring the planar capacitor flat-band voltage shift, and the threshold voltage characteristics of 1x-nm node MOS transistors.

Introduction Ceaseless scaling of semiconductor devices has caused nano-structure stiction issues during drying step of wafer cleaning. The following mechanism is considered to explain nano-structure stiction. Capillary force of remaining liquid in one trench where neighboring trenches are empty causes nano-structure bending, and an adhesive force generated by neighboring nano-structures contact area tends to keep stiction [1][2]. Within the elastic limit, an elastic force overwhelms the adhesive force, so the contacted nano-structures are forced detached again [3][4]. However, if the adhesive force defeats the elastic restoring force, the nano-structures may stay adherent even after drying [5]. To reduce the capillary forces, a low surface tension liquid such as isopropyl alcohol (IPA) has been used rinsing liquid. Furthermore, zero surface tension with super critical fluid (SCF) is beginning to be employed for semiconductor mass production. To suppress the adhesive forces, we have proposed the surface energy reduction process [6]. Moreover, we have already demonstrated this process is more effective for preventing nano-structure stiction than conventional IPA drying. The ideal surface termination is to minimize the number of high energy molecule sites. In case of silicon oxide surfaces, it means to replace highly reactive hydroxyl groups with inactive molecules. In this paper, we will present the proper molecular structure for the surface energy reduction agents and its evaluation method. Experimental Protocol We used two types of silylation agent for the surface energy reduction. The carbon number of straight-chain alkyl group of the agents is 1 and 8, so named as C1 and C8 respectively. We dipped thermally oxidized Si pieces in these agents, and measured the water contact angles. We also estimated the surface free energy from the critical surface tension obtained by Zisman plot. We treated 15nm half pitch SiO2 high aspect ratio line and space patterns with a single wafer cleaning tool, and checked the presence or absence of its pattern stiction with scanning electron microscopy (SEM). Results and Discussion As shown in Fig.1, the water contact angles of thermal SiO2 with C1 and C8 treatment are 85º and 101º. Supposedly, longer straight-chain alkyl groups enables more highly hydrophobic surface. As shown in Figure 2 Zisman plot, critical surface tension of the C8-treated SiO2 surface is 25 mN/m. Whereas the critical surface tension of the C1-treated surface is 22.5 mN/m in spite of its lower water contact angle than that of C8 treated surface. It is supposed that hydrogen bonds influence should be considered in use of a water probe since hydrogen bonds in water generate higher liquid surface tension. As explained by Cassie-Baxter theory, complex surfaces water pinning effect enhances the apparent water contact angle. Namely, in order to suppress nano-structure stiction,we should reduce the surface free energy at the interface between contacting nano-structures [6]. To obtain the surface free energy, we had to eliminate the influence of hydrogen bonds. So, we have focused a nonpolar hydrocarbon probe used for Zisman plot method which is free from hydrogen bonds influence. Eventually, we have obtained the ideal surface free energy of the nano-structure surface. Figure 3 shows the SEM images of the 15nm half pitch line and space samples treated with C1 and C8 respectively. The only C1-treated sample shows no stiction. This result indicates that it is not so important to obtain high water repellency, but is very important to obtain lower surface free energy. Summary We evaluated two types of silylation agent (C1,C8) for suppression of nano-structure stiction. Although C1-treated surface enables lower water contact angle than that of C8, its surface free energy becomes lower. For 15nm half pitch line and space, C8-treatment cannot suppress the line stiction since C8-treatment is insufficient to reduce the surface free energy as C1. We have confirmed that surface free energy reduction is important for the suppression of stiction, and have attained stiction free drying of 15nm half pitch high aspect ratio line & space using C1-treatment. References [1] T.Tanaka, M.Morigami, N.Atoda, Jpn. J. Appl. Phys. 1993,32,6059-6064 [2]Cao, Nealey and Domke, J. Vac. Sci. Technol.B 2000, vol.18 No.6., 3303 [3] C.H. Mastrangelo, J. Microelectromech. Syst. Vol2, No.1 44 (1993) [4] Boar and Michalske, J. Appl. Phys. Vol.86, 817 (1999) [5] M. Sankarapandian, B. Peethala etc, Solid State Phenomena, Vol.195, 107 (2013) [6] T. Koide, S. Kimura, H. Iimori, T. Sugita, K. Sato, Y. Sato and Y. Ogawa, ECS Trans., Vol. 69, No. 8, 131 (2015) Figure 1

Solar cells with a structure of p−n heterojunction, “ITO/p-CuI/n-Si” (ITO = indium tin oxide), were fabricated by depositing p-CuI on hydrogen (H)-terminated or methyl (CH 3)-terminated single-crystal n-Si (111) followed by the sputter deposition of ITO or by the press-contact of commercial conductive oxide glass with p-CuI. The solar cells generated very high open-circuit photovoltages ( V oc), reaching 0.617 V without surface texturing and back surface field treatments. Experiments have shown that a morphologically soft property of p-CuI as well as a low density of surface states at H-terminated or CH 3-terminated n-Si (111) is responsible for the generation of high V oc. Theoretical investigation has also shown that the p-CuI/n-Si contact forms an ideal minority-carrier controlled junction suitable for the generation of high V oc.

Introduction Nano-structure stiction during drying has often occurred due to scaling of semiconductor devices. Nano-structure contact caused by capillary forces is considered to cause the stiction [1][2]. The nano-structure stiction tends to be reduced by decreasing the surface tension of the rinsing liquid such as isopropyl alcohol (IPA), or using zero surface tension super critical fluid. Micro-electromechanical systems (MEMS) devices also have been faced on the stiction issue which is termed of “static friction”. Supposedly, it is caused by not only capillary forces but also adhesion forces, electrostatic forces, Van Der Walls forces and hydrogen bonding [3][4]. It is supposed that the release of the stiction is influenced by the correlation between the elastic restoring forces and the adhesion forces [5]. We have focused on the influence of the surface energy in order to release the stiction. Within the elastic limit, even if the nano-structures are deflected by capillary forces, they may be restored to their undeformed state after drying. We have illustrated the effect of the surface energy reduction process for high aspect-ratio nano-structures by both calculation and experimental data. Calculation The adhering nano-structure is imitated as S-shaped beam (Figure1). The elastic restoring energy is (equation 1) Where E is Young modulus, I is moment of inertia of area, s is half of space, h is detachment length, t is thickness and w is width (not shown). The adhesive energy is determined by the surface tension of the solid material (g s ) and the area of contact (w(L-h)) in the opposite direction against the restoring energy. (equation 2) To minimize the total energy (UT ) given from UE+US indicates that the nano-structures are stable. If UT is larger than zero, the stiction is released. We calculated these energies for nano-structures of a single material such as silicon oxide. Figure 2 shows the calculated energy curves in condition A and B, respectively, given the parameters from Table I. When the surface tension is 70 mN/m [6] on condition A, the total energy minimizes at about a 220 nm detachment length. This means that the nano-structures with about an 80nm contact length remain adhered. When the surface tension is brought down to 22 mN/m (condition B), the total energy does not have a minimal point below a 300nm detachment length. This indicates that even the adhering nano-structures will be released. Experimental We fabricated a 31nm half pitch line patterned structure of silicon oxide which is about a 320nm height. We obtained various line thickness samples by changing the etching time of diluted hydro-fluoride (DHF) treatment. As etched, the samples have a high surface energy (HSE). The state of the low surface energy (LSE) is achieved by using a surface modification agent. As shown in Figure 3, we plot the experimental data in STICTION as cross symbols, and in NO STICTION as open symbols, respectively, in each condition A (HSE) and B (LSE). Also the dotted line is a calculated curve in condition A. The solid line is calculated in condition B. The value of the surface energy is determined 22mN/m by a Zisman plot (not shown). In the left-hand area “Stiction” of the solid line, all the samples adhered. In the area ”Release Stiction” between the solid and dotted lines, only the samples with LSE did not adhere. This indicates that once the lines come in contact with each other, the elastic restoring force will release the stiction. In the area “Separation” under the dotted line, all the samples were not found not to have adhered. This means that the samples were kept separated over the whole process period. The experimental data correspond to the calculated lines very well. Summary We have evaluated the effect of the surface energy on the stiction with both calculation and experimental data, and have successfully demonstrated that the surface energy reduction process can suppress high aspect-ratio nano-structure stiction. Figure 1