Thermal Stability of SiO2 Doped Ge2Sb2Te5 for Application in ... - JSTS

http://dx.doi.org/10.5573/JSTS.2011.11.3.146

JOURNAL OF SEMICONDUCTOR TECHNOLOGY AND SCIENCE, VOL.11, NO.3, SEPTEMBER, 2011

Thermal Stability of SiO2 Doped Ge2Sb2Te5 for Application in Phase Change Random Access Memory Seung Wook Ryu*, Young Bae Ahn**, Jong Ho Lee**, and Hyeong Joon Kim**

Abstract—Thermal stability of Ge2Sb2Te5 (GST) and SiO2 doped GST (SGST) films for phase change random access memory applications was investigated by observing the change of surface roughness, layer density and composition of both films after isothermal annealing. After both GST and SGST films were annealed at 325 oC for 20 min, root mean square (RMS) surface roughness of GST was increased from 1.9 to 35.9 nm but that of SGST was almost unchanged. Layer density of GST also steeply decreased from 72.48 to 68.98 g/cm2 and composition was largely varied from Ge : Sb : Te = 22.3 : 22.1 : 55.6 to 24.2 : 22.7 : 53.1, while those of SGST were almost unchanged. It was confirmed that the addition of a small amount of SiO2 into GST film restricted the deterioration of physical and chemical properties of GST film, resulting in the better thermal stability after isothermal annealing.

memory devices not only because it satisfies various demands for non-volatile memory devices, but also because its fabrication process is relatively simple [1-3]. PCRAM is operated by reversibly changing the phase between the crystalline and amorphous state of chalcogenide materials, such as Ge2Sb2Te5 (GST), brought by joule heating. Crystalline GST has a low resistivity while amorphous GST has a high resistivity, which corresponds to the “0” and “1” states in the memory devices, respectively. In PCRAM cells, the reversible switching between these two states can be achieved by applying a short (~ 50 ns) and high current pulse (Ires ~ 1 mA) for the transition from the crystalline to the amorphous state (reset process) and a relatively long (Iset ~ 100 ns) and low current pulse (~ 0.2 mA) for switching from the amorphous to the crystalline state (set process).[3, 4] However, the high level of reset current (Ires) has been a major obstacle to the further scaling of PCRAM, because of the limited on-current

Index Terms—Thermal stability, Ge2Sb2Te5, SiO2, SiO2 doped Ge2Sb2Te5, decoposition, reliability, phase change, PcRAM, next generation non-volatile memory (NG-NVM)

drive capability of the cell transistor (< 0.5 mA/μm). There have been various investigations on the improvement of the switching performance of GST [512]. Although many improvements have been made in reducing Ires, there still remain several issues to be resolved and one of them is the device reliability during the repeated switching cycles; degradation or failure of PCRAM devices, such as reset and set stuck, and compositional variation of phase change material, have been reported [13-16]. It have been reported that SiO2 doped Ge2Sb2Te5 (SGST) leads to the reduction of Ires by the improvement of thermal efficiency as well as the enhancement of the reliability by the increment of crystallization temperature [13, 17]. However, there are

I. INTRODUCTION Phase change random access memory (PCRAM) is a promising candidate for next generation non-volatile Manuscript received Jun. 25, 2011; revised Aug. 18, 2011. * Department of Electrical Engineering, Stanford University, USA. ** Department of Materials Science and Engineering, and Inter-university Semiconductor Research Center, Seoul National University, Korea. E-mail : [email protected]

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no reports in the aspect of its thermal stability. Thermal stability is very important because PcRAM is operated by joule heating. Repetitive operation can introduce a degradation of phase changing properties. The research on thermal stability in PcRAM is necessary. In this study, physical and chemical variations of GST and SGST films with isothermal annealing were investigated from the point of view of thermal stability.

II. EXPERIMENTAL PROCEDURE 150 nm-thick SGST thin films were deposited by cosputtering of SiO2 and GST on Si substrates using a multi-gun sputtering system at room temperature. The rf-plasma power (Prf) of 150 W and a dc power of 40 W were applied for SiO2 and GST, respectively. The stoichiometry (Ge:Sb:Te ratio) of the deposited films was measured by Rutherford backscattering spectroscopy (RBS), proton induced x-ray emission and X-ray fluorescence spectroscopy (XRF). The SiO2 concentration in SGST thin films was determined by RBS; the Si concentration is approximately 23 atomic % at the Prf of 150 W. Isothermal annealing was performed at 325 oC in the sputtering system without vacuum break. The morphological change of surface was observed by scanning electron spectroscope (SEM) and optical microscope (OM). Root mean square (RMS) surface roughness was measured by atomic force microscopy (AFM). The distribution of elements (Ge, Sb, and Te) after isothermal annealing was mapped by electron probe microanalysis (EPMA) and the variation of layer density was determined by XRF. High resolution transmission electron microscopy (HRTEM) and x-ray diffractometer (XRD) were performed to observe the crystallinity and phases of GST and SGST films.

III. EXPERIMENTAL RESULTS Fig. 1 shows the cross-sectional SEM and plan-view OM (the inset one) images of the GST and SGST films before and after annealing at 325 oC for 20 min. In Fig. 1(a) and (c), surfaces of both as-deposited GST and SGST films appear very smooth with a RMS roughness of approximately 1.9 and 1.7 nm, respectively. However, after annealing, GST film shrinks in thickness with the

Fig. 1. SEM images of GST and SGST. (a) as-deposited GST, (b) 325 oC-20 min annealed GST, (c) as-deposited SGST, and (d) 325 oC-20 min annealed SGST (inset : OM image).

formation of hillocks, as shown in Fig. 1(b). The surface of GST becomes rugged with high RMS roughness of approximately 35.9 nm and the size of formed hillocks is measured to be approximately 2 μm, while SGST film looks the same with no sensible change. Hillock formation of GST films seems to be different from the reported agglomeration phenomena because hillock formation of GST films is generated at the conditions of approximately a half of melting point (Tm) and 150 nm, which are lower temperature and larger thickness than the reported agglomeration conditions of above 0.65 Tm and below 100 nm [19]. To find out the reasons for the formation of hillocks in GST film with annealing, the variation of layer density and composition of both films with annealing time was investigated by XRF analysis, as shown in Fig. 2(a). Layer density of as-deposited GST is 72.48 μg/cm2, from which the density of as-deposited GST film can be calculated to be 4.83 g/cm3. It is approximately 17% lower than the previously reported one by X-ray reflectance (XRR) [20]. This discrepancy might be originated from a difference in measuring methods : XRF and XRR. Layer density of GST steeply decreases from 72.48 to 70.01 μg/cm2 after annealing for 1 min and then followed by a slight decrease to 68.98 μg/cm2 in the 20min annealing, suggesting that some elements composed of GST film vaporized away during isothermal annealing. On the other hand, as-deposited SGST film with the layer density of 72.16 μg/cm2, which is the

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Fig. 2. (a) The variation of layer density, (b) The compositional variation of GST and SGST with isochronal annealing time at 325 oC, respectively.

similar value of that of as-deposited GST, is almost unchanged (0.3% decrease) compared to that of GST (4.8% decrease) after the 20-min annealing. In order to deeply understand the vaporization of GST films, the compositional variation of GST and SGST films after isothermal annealing was also measured by XRF, as shown in Fig. 2(b). The as-deposited GST film has the composition of Ge : Sb : Te=22.3 : 22.1 : 55.6. After annealing for 1 min, Te composition steeply decreases from 55.6% to 53.5% and finally reaches 53.1% in 20 min. The compositions of Ge, Sb, and Te after annealing for 20 min are 24.2%, 22.7%, and 53.1%, respectively. The decrement of layer density of GST film might be due mainly to a loss of Te in the GST, because Te is known as a very volatile element [20]. On the other hand, the composition of as-deposited SGST was Ge : Sb : Te=22.1 : 22.2 : 55.7, which got unchanged after annealing for 20 as well as for 1 min. Fig. 3 shows the compositional distribution mapping of GST and SGST films observed by EPMA. Fig. 3(a) shows that all of Ge, Sb, and Te are uniformly distributed in as-deposited GST film. However, after annealing for 20 min, the elements in GST film become non-uniformly distributed as shown in Fig. 3(b). This discrepancy is generated by the formation of hillocks, which seemes to possess different composition from the

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Fig. 3. The elemental distribution of Ge, Sb, and Te in the GST and SGST films. (a) As-deposited GST, (b) 325 oC-20 min annealed GST, (c) Aas-deposited SGST, (d) 325 oC-20 min annealed SGST.

matrix. All of Ge, Sb, and Te have a higher concentration in the hillocks than that in the matrix and the magnitude of elemental deviation between hillock and matrix has the order of Ge > Te > Sb after annealing for 20 min, as shown in the Fig. 3(b). This order is unchanged even considering the difference of thickness between hillock and matrix. It is believed that Te bounded with Ge is preferentially volatilized than that bounded with Sb because formation enthalpy and Gibbs free energy of GeTe are higher than those of Sb2Te3. The remained Ge atoms come to be agglomerated after volatilizing of Te because a clustering of Ge atoms is more stable than dispersed atoms. This result is a good agreement with those in Fig. 1 and Fig. 2. Elements of Ge, Sb, and Te in SGST film are uniformly distributed regardless of annealing as shown in Fig. 3(c) and (d). The authors already reported the microstructure of SGST, which has inhomogeneous distribution and it is

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already known that SiO2 does not mix up with GST [18] Besides, SiO2 have a very low vapor pressure at 325 oC. Therefore, SiO2 in SGST film might be piled up in the surface of SGST film as the elements of Te is vaporizing away. SiO2 pile-up on the surface prevents Te from vaporizing away, resulting in a slight compositional change compared to that of GST as shown in Fig. 2(b). The reason, for that SGST film has a smooth surface after annealing as shown in Fig. 3(d), might be because piled-up SiO2 suppressed the clustering of the elemental Ge, Sb, and Te. Fig. 4 shows cross-sectional bright field HRTEM images and the patterns of XRD measured after severely annealing at 400 and 500 oC for 1hr to confirm a thermal stability of GST and SGST films. The thicknesses of asdeposited GST and SGST films were approximately 145 and 140 nm, respectively. After annealing at 400 oC for 20 min, GST film consisted of large grains and its thickness also decreased from 145 to 115 nm by approximately 20.7%, while SGST film consisted of smaller grains with approximately 15 nm diameter and its thickness decreased from 140 to 125 nm by approximately 10.7%, which is originated from shrinkage of thickness during crystallization [19]. Although the GST film annealed at 400 oC for 1 hr has high intense XRD peaks in Fig. 4(c), those are partially coincident with those of relevant elements and compounds such as Ge, Sb, Te, Sb2O3, Sb2O4 and etc., as shown at table I. It is believed that the change of composition, originated from volatilizing of Te, leads to the formation of various phases. The pattern of SGST film, annealed at 400 oC for 1hr, shows that SGST film has a meta-stable fcc phase in spite of high thermal budget of 400 oC for 1 hr, as shown in Fig. 4(f). After annealing at 500 oC for 1 hr, most of GST film was volatilized away, while SGST film sustained the thickness of approximately 125 nm, as shown in Fig. 4(b) and (d). SGST film has a lamellar structure (LS) in Fig. 4(d), but after annealing at 500 oC for 1hr, it disappears, becoming homogeneous structure in Fig. 4(e). It is already reported that the formation of LS in SGST results from heterogeneous distribution of SiO2 [17]. After annealing 500 oC, the disappearance of LS in SGST began from surface. Stress in LS is relatively nonuniform. Therefore, when there is a driving force

Intensity (arb. unit)

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Fig. 4. Cross-sectional bright field HR-TEM images and XRD data of GST and SGST after additional 1hr-annealing : GST annealed at (a) 400 oC, (b) 500 oC and (c) X-ray diffraction patterns for those, and SGST annealed at (d) 400 oC, (e) 500 oC and (f) Xray diffraction patterns for those.

enough to relax it, it can disappear as shown at Fig. 4(e). The patterns of XRD annealed at 500 oC in Fig. 4(c) and (f) show that GST film doesn’t have any peaks except the peaks of Si substrate while SGST film still remains its thickness of approximately 125 nm and fcc phase with higher intensity compared to that of SGST film annealed at 400 oC for 1hr. Phase change material in real device is covered by inter-metal dielectric. Therefore, the volatilization of Te is hard to occur because intermetal dielectric suppresses the volatilization of Te. Even

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REFERENCES

Table 1. Indexes for probable materials in Fig. 4(c) [21]

[1]

[2]

[3]

[4]

[5] if inter-metal dielectric restrains the volatilization of Te, these results imply that GST has a relatively poor reliability, i.e., low endurance, and SGST film has a better device performance because SGST film withstands high thermal budget and possesses high stability of fcc structure.

IV. CONCLUSIONS In this work, SGST films were deposited by the cosputtering of SiO2 and GST targets at room temperature. SEM and OM showed that the surface of SGST film remained smooth, while that of GST was roughened with formation of hillocks after annealing. XRF analysis confirmed that layer density and Te content of GST film decreased with increasing the isothermal annealing time due to volatilization of Te. On the other hand, layer density and composition of SGST films were almost unchanged. Therefore, the doping of SiO2 in the GST film improves the thermal stability of GST films, possibly resulting in enhancing the performance of PCRAM.

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Seung Wook Ryu received Ph.D. degree in materials science and engineering from department of Materials Science and Engineering (MSE), Seoul National University (SNU), Seoul, Korea, in 2010. He worked as a teaching assistant for semiconductor process education from 2006 to 2010 at Inter-university Semiconductor Research Center (ISRC) in SNU. Also, he worked as a postdoctoral researcher at MSE, SNU from March to August 2010. He worked as a visiting scholar at the Department of Electrical Engineering, Stanford University, CA, USA from September 2011, where he is currently working as a postdoctoral researcher. He has research interests in new emerging nonvolatile memories including phase change RAM (PcRAM) and resistive switching RAM (ReRAM). He received “Best Graduate Student award” for outstanding research papers from MSE, SNU, in 2009.

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Young Bae Ahn received the B.S. and M.S degrees in 2006 and 2008 from Seoul National University (SNU), Seoul, Korea, respectively. He is currently working toward the Ph.D. degree in MSE at the same university. He worked as a teaching assistant for semiconductor process education at ISRC in SNU. His current main research interest is new emerging nonvolatile memories including PcRAM and ReRAM.

Jong Ho Lee received the B.S. degree in 2005 from Seoul National University (SNU), Seoul, Korea, where he is currently working toward the Ph.D. degree in MSE. He is working as a teaching assistant for semiconductor process education at ISRC in SNU. His interests include cross-bar array and new emerging nonvolatile memories.

Hyeong Joon Kim received his B.S. and M.S. degrees in inorganic materials engineering and materials science from SNU and Korea Advanced Institute of Science (KAIST) in 1976 and 1978, respectively, and his Ph.D. degree in materials engineering from North Carolina State University in 1985. From 1978 to 1981, he worked at Agency for Defense Development (ADD) as a researcher. From 1981 to 1985, he was with North Carolina State University as a research associate. In 1986, he joined SNU as an assistant professor in the Department of Inorganic Materials Engineering, where he is currently a professor. His current research interests include SiC devices for high power and high frequency applications and new materials for new emerging nonvolatile memory and display. He has authored and co-authored over 500 research papers in journals and conferences. He was leading the Inter-university Semiconductor Research Center (ISRC) at SNU as the director from June 1997 to June 1999. He served as the director of General Affairs and the Korean Vacuum Society from 1999 to 2001. He has been holding Vice President of the Materials Research Society of Korea since 2001 and Chairman of the Advisory Council on Next Generation Semiconductors Division of the Korean Ministry of Commerce, Industry and Energy since 2003.