Preparation of Ge-doped SbTe Material Thin Film for Phase Change Random Access Memory by Magnetron Sputtering on Small Hole Patterns

Fig.2. Cross section SEM image of GST filling profile prepared by a) conventional type sputtering and b) Long Throw Sputtering (LTS). Phase Change Random Access Memory (PCRAM) is one of the candidates for next generation memory due to its non-volatility, high speed, high density and compatibility with Si-based semiconductor process. Reduction of reset current is considered to pose a major technical challenge, a future memory device may need a new phase change material featuring a low melting point. Ge doped SbTe material (Ge:SbTe=SGT) has a melting point (540 ) of about 100 lower than that of Ge2Sb2Te5 (GST). [1] In the other hand, in order to integrate PCRAM to beyond 512Mbit, thermal interference between small memory cells become a problem. To resolve this problem, Confined Cell structure PRAM was suggested. [2] However, it was difficult to fill GST layer in a small hole with a conventional sputtering tool because a big overhang occurred. In this work, we prepared SGT films on the small hole patterned wafer by a new concept sputtering tool which designed developed a new concept sputtering tool. The structure of SGT film was observed with cross section SEM and the film composition was measured with XRF. It was observed an overhang was suppressed and a SGT film was filled in a small hole with a new concept tool. In addition, the uniformity of the SGT film composition was good at less than 3% in 200mm substrate. A multi-chamber sputtering system ULVAC ETRONTM-EX was used for SGT deposition on the small hole patterns substrates. SGT films were deposited at R.T. by magnetron sputtering with a 300mm sintered target. A scanning electron microscope (SEM) was used to observe the filling profile of SGT. The Sheet resistances of the SGT films are measured by Omnimap RS-100 (KLA tencore). The process flow of test device is shown in Figure 1. The fabricated device was reversibly switched between crystalline (set) and amorphous (reset) phases using a pulse generator. The voltage and the width of set and reset pulse were 500nsec and 100nsec respectively. Figure 2 shows cross section SEM images of SGT filling profile prepared by a) conventional type and b) LTS. An overhang was formed by the conventional type sputtering tool because the sputtering atoms were scattered. On the other hand the mean free path was long with low pressure in LTS tool [3]. The sputtering atoms reached to the bottom in a hole directly. Therefore an overhang was suppressed. Figure 3. shows the relation between the voltage of the reset pulse and an electric current of GST and SGT in a test device. The reset voltage of SGT was small compared with GST. The reset current of SGT was also lower than GST. It is considered as the small electric power with a reset step because the melting point of SGT is low. Reference [1] J.Jeong., et al, 2007 MRS Spring Meeting, I11.3. [2] S.L.Cho., et al, 2005 Symposium on VLSI Technology Digest of Technical Papers,6B-1. [3] N.Motegi, et al, Journal of Vacuum Science & Technology B: Microelectronics and Nanometer Structures, 13(1995), pp. 1906-1909.