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Japanese Journal of Applied Physics 52 (2013) 05FF01 http://dx.doi.org/10.7567/JJAP.52.05FF01
Superlattice Phase Change Memory Fabrication Process for Back End of Line Devices Takasumi Ohyanagi1 , Norikatsu Takaura1 , Masahito Kitamura1 , Mitsuharu Tai1 , Masaharu Kinoshita1 , Kenichi Akita1 , Takahiro Morikawa1 , and Junji Tominaga2 1
Low-Power Electronics Association and Projects (LEAP), Tsukuba, Ibaraki 305-8569, Japan National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki 305-8562, Japan E-mail:
[email protected] 2
Received October 30, 2012; revised December 27, 2012; accepted January 11, 2013; published online May 20, 2013 The superlattice film with the periodical thin film layers of Sb2 Te3 /GeTe used as a phase change memory was studied for deposition in the crystal phase. We successfully fabricated the superlattice structure with the sputtering temperature of 200 C. Moreover, the pillar structure with the size of 70 nm was dry-etched using a HBr/Ar gas mixture. # 2013 The Japan Society of Applied Physics
1. Introduction
6-fold structure
Phase change memory (PCM) is a promising candidate for next-generation nonvolatile memory devices.1–5) One of the most attractive candidates for PCM materials has been Ge2 Sb2 Te5 (GST) because its phase changes rapidly and reversibly between amorphous and crystalline states. Basically, phase change devices take advantage of Joule heating generated by electric current in phase change materials. The temperature in the phase change material during RESET operation should be higher than the melting point. This thermal energy is quite high and it can cross-erase the data recored in the neighboring cells. Recently, a new mechanism of phase change was proposed where the electric resistivity and the optical indices are greatly modified only by the switching of the Ge atoms in GST.6) Furthermore, using this model, the superlattice phase change memory was proposed, where the power consumed during recording was demonstrated to be dramatically reduced. This method utilizes periodical thinfilm layers (superlattice) of Sb2 Te3 /GeTe.7–9) Moreover, the superlattice PCM has attracted much attention as a topological insulator.10,11) Moreover, many PCM developers are trying to realize a cross-point memory12–14) that rewrites data with twoterminal devices (diodes) by exploiting the fact that data stored in a PCM can be rewritten just by flowing a unidirectional current in its memory cells. The cross-point structure of the PCM is embedded in the back end of line (BEOL) of CMOS processes and also makes it easy to increase the density of memory cells and realize a capacity equivalent to that of NAND flash memory.15) To replace the NAND flash memory, the PCM cell is scaled down to tensof-nanometer size. We studied fabrication processes of the superlattice PCM to embed in the BEOL. The key processes were sputtering and dry etching. The superlattice PCM has a special feature that the phase change occurs between 4-fold structures and 6-fold structures of the Ge atoms, as shown in Fig. 1. The 4fold structure has a high resistivity and the 6-fold structure has a low resistivity. These are the crystal-crystal transitions. The superlattice film must be crystalline and the sputtering temperature is important in the BEOL of the CMOS processes. Moreover, dry etching is an essential tool for the fabrication of the superlattice PCM, where the size of the PCM cell is scaled down to tens-of-nanometer size. The
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Fig. 1. (Color online) Schematic diagram of the superlattice phase change memory.
scale down leads to the reduction of the required current and immunity to device failure. 2. Experimental Methods
In this study, we used a DC magnetron sputtering system with 300 mm wafers to fabricate the superlattice films. Single Sb2 Te3 films, single GeTe films, and the [Sb2 Te3 / GeTe] superlattice films were deposited at various sputtering temperatures on Si substrates. The films were examined by powder X-ray diffraction (XRD) using Cu K radiation and transmission electron microscopy (TEM) with the acceleration voltage of 200 kV. A microwave plasma etching system with 300 mm wafers was used to etch the single Sb2 Te3 films, single GeTe films, and the superlattice films with HBr/Ar gas mixtures. The etching rates of Sb2 Te3 and GeTe were studied in comparison to those of GST with various HBr/Ar ratios. Sb2 Te3 , GeTe, and GST were prepared on SiO2 /Si substrates. Sb2 Te3 and GeTe were deposited at 200 C and GST was deposited at room temperature. The flow rate of HBr and microwave power were fixed at 50 sccm and 900 W, respectively. The substrate temperature and the substrate RF bias were set to 20 C by a cooling unit and 150 W, respectively. To define the patterns, an immersion ArF lithography was employed. The etching profile, etching selectivity, and etching rate were studied by scanning electron microscopy (SEM).
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Fig. 3. XRD spectra of the superlattice films (black) with the periodical thin film layers of Sb2 Te3 /GeTe and the GeTe (gray).
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Fig. 2. (a) XRD spectra of the Sb2 Te3 films on the Si substrate with the sputtering temperatures from 170 to 230 C. (b) XRD spectra of the Ge films on the Si substrate with the sputtering temperatures from 170 to 230 C.
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3. Results and Discussion
Fig. 4. TEM image of the superlattice film.
3.1 Sputtering
Figure 2(a) shows the XRD spectra of single Sb2 Te3 films deposited on a Si substrate and Fig. 2(b) shows the XRD spectra of single GeTe films on a Si substrate with the temperatures ranging from 170 to 230 C. The film thicknesses were all 50 nm. At 170 C, the Sb2 Te3 is already crystalline, and up to 210 C, the Sb2 Te3 continues to be crystalline. However, at 220 C, the XRD peaks were slightly weak, and at 230 C, no peaks were observed. Hence, the GeTe was crystalline between 190 and 210 C. The superlattice PCM must be deposited in the temperature range between 190 and 210 C. These temperatures do not affect the CMOS. Figure 3 shows the XRD spectra of the superlattice film, which were obtained 8 times repeating of [Sb2 Te3 = 4 nm/GeTe = 1 nm] deposited at 200 C on a Si substrate. This film was crystalline, and we found that almost all the peaks were originated from Sb2 Te3 . In this spectrum, no peaks originating from the GeTe were observed. We considered that these findings showed that the Ge atoms were in other positions compared with the positions on the Si substrate, as shown in Fig. 1. However, we did not clarify whether the Ge atoms had 4- or 6-fold structures. Moreover, some unknown peaks were also observed, as shown by the arrows in Fig. 3. These peaks were slightly shifted by the original positions of Sb2 Te3 on the Si substrate. This showed that these peaks were considered to be the strained Sb2 Te3 due to the moving of the Ge atoms. Figure 4 shows a TEM image of the
superlattice film deposited at 200 C. The interference fringes were observed and this showed that the atoms were arranged in an ordered structure. We also measured XRD spectra of the superlattice deposited on SiO2 . The intensities of individual peaks were different from each other, but the peak positions were the same as on the Si substrate. It is reported that the underlying Sb2 Te3 plays an important role in forming the superlattice,16) and we are optimizing the sputtering conditions to deposit superlattice films on metals such as tungsten (W) or titanium nitride (TiN). 3.2 Dry etching
Figure 5(a) shows the etching rates of Sb2 Te3 and GeTe compared with that of GST. The etching rates of Sb2 Te3 and GeTe were low compared with that of GST, and the etching rate of Sb2 Te3 saturated with increasing Ar ratio. The etching products suppressed the etching of Sb2 Te3 . The etching rate of the superlattice is also shown in Fig. 5(a). The rate was similar to that of Sb2 Te3 , and this showed that the etching of the superlattice is also suppressed by the etching products from Sb2 Te3 . Figure 5(b) shows the selectivity of the Sb2 Te3 , GeTe, and superlattice to the mask of TiN. TiN is widely used as a top electrode and hard masks in the dry etching of GST and related materials.17–20) TiN was stable and no diffusion occurred in the GST films. Moreover, TiN was not easily dry
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the Ge atoms of the superlattice film were moving compared with the Ge atoms of the GeTe film on the Si substrate. Moreover, the pillar structure of the superlattice film with the size of 70 nm was fabricated by dry etching with the HBr/Ar gas mixture.
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Acknowledgments 2
This work is supported by the Ministry of Economy, Trade and Industry (METI) and the New Energy and Industrial Technology Development Organization (NEDO) of Japan. A part of this study was conducted by the Innovation Center for Advanced Nanodevices (ICAN), National Institute of Advanced Industrial Science and Technology (AIST), Japan.
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40 60 HBr/(HBr+Ar) [%]
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100 1) H. S. P. Wong, S. B. Kim, J. Lang, J. P. Reifenberg, B. Rajendran, M.
Asheghi, and K. E. Goodson: Proc. IEEE 98 (2010) 2201.
Fig. 5. (a) Dry etching rates of GeTe, Sb2 Te3 , Ge2 Sb2 Te5 , and the
2) M. H. R. Lankhorst, B. W. S. M. M. Ketellaars, and R. A. M. Wolters: Nat.
superlattice. (b) Selectivity to the TiN mask of GeTe, Sb2 Te3 , and the superlattice.
3) N. Matsuzaki, K. Kurotsuchi, Y. Matsui, O. Tonomura, N. Yamamoto, Y.
Mater. 4 (2005) 347.
etched by the gas mixture of HBr/Ar. We selected TiN hard masks for the dry etching of the superlattice films and TiN was used as the top electrode to be embedded in the CMOS processes. We found that the selectivity was over 1 for Sb2 Te3 , and for GeTe with all ranges of HBr/Ar ratios; however, the selectivity of the superlattice was over 1 with the HBr/Ar ratio below 40%. A small amount of bromine compound from 1 to 2% was detected by X-ray photoelectron spectroscopy (XPS) measurements from the surface of the dry etched sample, but this was removed by pre-cleaning such as hydrogen fluoride (HF) or HF + H2 O2 (FPM) treatment in the next process. Figure 6 shows a cross-sectional SEM photograph of the superlattice with the TiN mask. The superlattice film thickness was 50 nm. The etching condition was that HBr/ Ar at 200 sccm/50 sccm was used. We successfully fabricated the pillar structure of the superlattice with the size of 70 nm.
4) 5) 6) 7) 8) 9) 10) 11) 12) 13)
14) 15) 16) 17)
4. Conclusions
18)
We studied a superlattice film with periodical layers of Sb2 Te3 /GeTe used as a phase change memory to be embedded in the BEOL of the CMOS processes. The sputtering temperature ranged from 190 to 210 C to deposit the crystal. From the XRD measurement, it is suggested that
19) 20)
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