Ge 2 Sb 2 Te 5 Film Deposition and Properties
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Ge2Sb2Te5 Film Deposition and Properties Mengqi Ye, Rong Tao, Peijun Ding, and Abner Bello Applied Materials, Inc., 3050 Bowers Avenue, P.O. Box 58039, Santa Clara, CA, 95054 ABSTRACT Magnetron sputter deposition of Ge2Sb2Te5 film is presented. Good thermal control of the wafer is found critical for maintaining process repeatability. In-situ wafer heating conditions have strong impact on as-deposited Ge2Sb2Te5 film properties such as resistivity, density, stress, composition, and microstructure. The effects of wafer bias and sputtering pressure are also discussed.
INTRODUCTION Chalcogenide based phase change memory (PCM), considered the most promising alternative non-volatile memory (NVM) technology for the next decade, has demonstrated the best scalability among the few NVM candidates, and is being actively investigated in semiconductor industry [1-3]. Ge2Sb2Te5 is the most widely used material for PCM, and has many unique properties, including strongly temperature-dependent film properties, low thermal conductivity, and high electrical resistivity. The unique film properties pose many challenges for deposition equipment, processes and film characterization. In this work, we studied the effects of various process parameters on film properties including sheet resistance, thickness, resistivity, density, stress, composition, and structure.
EXPERIMENTAL DETAILS Ge2Sb2Te5 films of 50 nm to 250 nm thick were deposited on 200 mm diameter Si/SiO2 substrates by magnetron sputtering. An electrostatic chuck (ESC) was used to provide good thermal control and biasing of the wafer during deposition. The film sheet resistance was measured by four point probe method. The film density and thickness were measured by x-ray reflectivity (XRR). The film composition was measured by x-ray fluorescence (XRF).
RESULTS AND DISCUSSION It has been found that in situ control of wafer temperature is critical for achieving consistent film properties. Figure 1 shows that the film sheet resistance and uniformity fluctuated if wafer chucking was not used, whereas good repeatability was achieved when chucking was used to maintain good ESC/wafer thermal coupling during the deposition process. XRD scans of the films, processed with pedestal temperature set at 25∞C but without chucking, varied from wafer to wafer, and many of the samples were crystalline as-deposited due to plasma heating. On the contrary, repeatable amorphous and crystalline phases can be obtained by chucking the wafers at
25∞C and 235∞C, respectively, as shown in Figure 2. The XRD spectra match well with those reported in the literature [4].
Rs Unif w /o ESC Rs Unif w ith ESC
10000
235C, #1 235C, #2 235C, #4 235C, #5 25C, #1 25C, #2 25C, #3 25C, #4 25C, #5 235C, #3
80%
Rs w ith ESC
100000 Rs (Ohm/sq)
90%
Rs w /o ESC
70% 60% 50% 40%
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Rs Unif (%)
1000000
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10% 10
0% 0
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25
30 10
Wafer #
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2theta (deg)
Figure 1. Wafer repeatability test shows that Figure 2. Repeatable amorphous and crystalline
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