Vol.34,No.5Journal of Semiconductors May2013 Reactive ion etching of Si2Sb2Te5in CF4/Ar plasma for a nonvolatile phase-change memory deviceLi Juntao(李俊焘)1;2; ,Liu Bo(刘波)1; ,Song Zhitang(宋志棠)1,Yao Dongning(姚栋宁)1,Feng Gaoming(冯高明)3,He Aodong(何敖东)1;2,Peng Cheng(彭程)1;2,and Feng Songlin(封松林)11State Key Laboratory of Functional Materials for Informatics,Shanghai Institute of Microsystem and Information Technology,Chinese Academy of Sciences,Shanghai200050,China2University of Chinese Academy of Sciences,Beijing100049,China3United Laboratory,Semiconductor Manufacturing International Corporation,Shanghai201203,ChinaAbstract:Phase change random access memory(PCRAM)is one of the best candidates for next generation non-volatile memory,and phase change Si2Sb2Te5material is expected to be a promising material for PCRAM.In thefabrication of phase change random access memories,the etching process is a critical step.In this paper,the etchingcharacteristics of Si2Sb2Te5films were studied with a CF4/Ar gas mixture using a reactive ion etching system.Weobserved a monotonic decrease in etch rate with decreasing CF4concentration,meanwhile,Ar concentration wentup and smoother etched surfaces were obtained.It proves that CF4determines the etch rate while Ar plays an im-portant role in defining the smoothness of the etched surface and sidewall edge pared with Ge2Sb2Te5,it is found that Si2Sb2Te5has a greater etch rate.Etching characteristics of Si2Sb2Te5as a function of power andpressure were also studied.The smoothest surfaces and most vertical sidewalls were achieved using a CF4/Ar gasmixture ratio of10/40,a background pressure of40mTorr,and power of200W.Key words:reactive ion etching;phase-change material;Si2Sb2Te5DOI:10.1088/1674-4926/34/5/056001PACC:7360F;81601.IntroductionNowadays,phase change random access memory (PCRAM)has been regarded as one of the most promising non-volatile memories,and has received more and more attention because of its superior performance and other mer-itsŒ1;2 .It was devised by Ovshinsky in1968Œ3 based on the rapid reversible phase change effect in some materials under the influence of an electric current pulse,and the different resistances between crystalline and amorphous states define the logic state of an individual bit.Phase change Si2Sb2Te5material,expected as a promising material for PCRAM,possesses a wider band-gap comparing to Ge2Sb2Te5.The band-gap width of amorphous and poly-crystalline Si2Sb2Te5are determined to be0.89and0.62eV by means of Fourier transform infrared spectroscopyŒ4 .The mate-rial possesses a low threshold current from amorphous to poly-crystalline state in voltage–current measurement,and shows a good data retention.These properties prove Si2Sb2Te5is a po-tential materialŒ4;5 .In this paper,the reactive ion etching(RIE)process of Si2Sb2Te5films in CF4/Ar plasma is described.The etch rate and surface roughness were examined systematically as a func-tion of pressure,power,and Ar concentration in the CF4/Ar mixture gas.A smooth surface was successfully obtained us-ing the optimization approach described below.2.ExperimentIn this study,Si2Sb2Te5films were deposited with the ra-dio frequency(RF)-magnetron sputtering method using singleelement targets at room temperature.The thickness of the filmswas about400nm measured by a cross-sectional scanning elec-tron microscope(SEM,Hitachi S-4700).The compositions offilms were determined by means of energy dispersive spec-troscopy(EDS).Shipley6809photo-resist was used for patterndefinition.An Oxford80plus RIE system with a maximum RFpower of600W was used to etch the Si2Sb2Te5films.Theetch gas ratio was controlled by mass flow controllers,and thegas pressure in the chamber was adjusted by a clapper valve.The temperature of the sample holder was controlled by heattransfer fluid(Hexid)and held at30ıC.The experimental con-trol parameters were the gas flow rate,chamber backgroundpressure,CF4/Ar ratio and the incident RF power applied tothe lower electrode.A total flow rate of CF4C Ar was50sccm throughout the experiment,while the CF4/Ar ratio was varied as an optimization parameter.Etching depths were measured using a surface profile-*Project supported by National Key Basic Research Program of China(Nos.2010CB934300,2011CBA00607,2011CB9328004),the Na-tional Integrate Circuit Research Program of China(No.2009ZX02023-003),the National Natural Science Foundation of China(Nos. 60906004,60906003,61006087,61076121,61176122,61106001),the Science and Technology Council of Shanghai(Nos.11DZ2261000, 11QA1407800),and the Chinese Academy of Sciences(No.20110490761).†Corresponding author.Email:jet_lee@;liubo@Received25August2012,revised manuscript received3December2012©2013Chinese Institute of ElectronicsFig.1.Etch rate of the Si2Sb2Te5and RMS roughness as a function of CF4/Ar gas mixture ratio.meter.The surface morphology and patterning of the mesa structure were examined using SEM,and the surface rough-ness was examined using atomic force microscopy(AFM). 3.Results and discussionFigure1shows the etch rate as a function of the CF4/Ar gas mixture ratio.The etchings were carried out at a constant pres-sure of50mTorr and an application of200W.The etch rate de-creased monotonically with decreasing CF4concentration in-dicating its importance in defining the material removal rate. In the plasma system,when an energetic electron strikes a neu-tral gas molecule,it can excite the molecule to a higher energy state.These energetic F containing molecules,known as free radicals,cause most of the chemical etching of Si2Sb2Te5Œ6 .In this experiment,as the content of CF4went down,the concen-tration of energetic F decreased,so the etch rate of Si2Sb2Te5 decreased in turn.The phenomenon observed is consistent with this pared with Ge2Sb2Te5,we found that the etch rate of Si2Sb2Te5was faster,this should be related with the different boiling points of the etch products,such as SiF4 [boiling point(bp): 65ıC]and GeF4[bp: 36:5ıC].The lower boiling point of SiF4makes it easier to be removed from the chamber,as the volatile product of Si2Sb2Te5,this property leads to a faster etch rate of Si2Sb2Te5Œ7 .The quality of etched surfaces is very important for the device fabrication processŒ8 .Many short-circuit defects are due to RIE pillars caused by micro-masks.The smoother the etched surface is the better contact between Si2Sb2Te5and the top/bottom electrode is obtained,which can result in a low contact resistance.The sidewall is also important for the de-vice fabrication process,particularly for the nanoscale etching of Si2Sb2Te5films.Therefore,etched surfaces with a smooth surface,vertical sidewall,and low sidewall roughness are pre-ferred to meet the requirements of the high-density memory devices.Etch residues are not observed on the sidewalls or the film surfaces for all conditions.As the Ar concentration is in-creased,both the etch slopes and the root-mean-square(RMS) roughness of the etched surfaces shows improvement.When the CF4/Ar ratio is40/10,the etched surface is very rough and pillars formed because of micro masks can be observed.As the CF4/Ar ratio was decreased to10/40,an almost vertical etch slope was obtained,and the etched surface showed signif-icantly improved smoothness.In order to validate these results,AFM images of the etched surfaces under different gas mixture are shown in Fig.2. When the CF4/Ar ratio is40/10,the RMS value of this etched surface is3.38nm.It reduces to1.67nm when the CF4/Ar ra-tio increases to10/40.This phenomenon should be attributed to the increase in Ar concentration.The role of Ar is to re-move the non-volatile etch products deposited on the substrate by physical bombardment.It is evident that ion bombardment contributes positively to improving the smoothness in the etch-ing processŒ9 .For the fabrication of PCRAM devices,etch selectivity (the ratio of etch rates)of Si2Sb2Te5films to insulating ma-terials is a key parameter in the etching process.In this experi-ment,SiO2films were prepared by plasma enhanced chemical vapour deposition.Etchings were carried out at a constant pres-sure of50mTorr and applying power of200W.As shown in Fig.3,the selectivity decreases along with the Ar concentra-tion which indicates that the concentration of F has a greater impact on Si2Sb2Te5.The etch rate of the Si2Sb2Te5film as a function of power is shown in Fig.4.The etch rate decreases linearly with RF power.The decreasing etch rate with increasing power may be related to the plasma sheath layer that exists in the cham-ber and the influence of non-volatile etch products.When the thickness of the plasma sheath increases,the distance crossed by the radicals to reach the substrate also increased.On the other hand,more fluorine radicals lead to the polymer form-ing species that eventually deposit as unwanted masking ma-terials on the etched surface.Thus,the etch rate appeared to decreaseŒ10 .From Fig.5,it is found that the surface of higher power etched Si2Sb2Te5films is much smoother than that of lower power etched ones.In lower power conditions,the ki-netic energy of Ar radicals is too low to remove the chemical products in time which causes a rough surface.However,this problem is resolved in high power conditions.The etch rate and RMS roughness of the Si2Sb2Te5films as a function of pressure are shown in Fig.6.The experiments were carried out when the CF4/Ar ratio was40/10and the PF power kept at200W.The etch rate increased with cham-ber pressure and then decreased.The highest etch rate hap-pened under60mTorr.The pressure dependence of the etch rate should be dominated by the active abundance of neutral etchant species.In general,the ion energy and the direction of physical bombardment to the specimens are determined by the direct current(DC)bias voltage which is strongly influ-enced by the chamber pressure.When the pressure is lower than60mTorr,the etch rate is mainly dominated by the ac-tive abundance of neutral etchant species,the concentration of radicals increases with the gas pressure,which leads to the in-crease of etch rate.On the other hand,when the gas pressure is higher than60mTorr,according to the collisional plasma sheath modelŒ11 ,a collisional effect should be considered. When the pressure increases,the mean free path of the charged particles decreases and hence the dc bias will be lower.As a result,the physical bombardment to the substrate by positive ions becomes lower which leads to a decrease of etch rate.The effect of pressure on the surface roughness was also examined using AFM,and the corresponding RMS roughnessFig.2.AFM images of the etched Si 2Sb 2Te 5surface with a CF 4/Ar ratio of (a)40/10,(b)30/20,(c)20/30,and (d)10/40.Fig.3.Etch selectivity of Si 2Sb 2Te 5to SiO 2as a function of CF 4/Ar ratio.Fig.4.Etch rate of the Si 2Sb 2Te 5film as a function of power.Fig.5.SEM images of the Si 2Sb 2Te 5surface after etching under dif-ferent powers.(a)100W.(b)150W.(c)200W.(d)250W.Fig.6.Etch rate and RMS roughness of the Si 2Sb 2Te 5film as a func-tion of pressure.Fig.7.AFM images of the etched Si2Sb2Te5surface at different pressures.(a)30mTorr.(b)40mTorr.(c)50mTorr.(d)60mTorr.(e)70mTorr.values are shown in Fig.7.The etched surface is rough undera background pressure of30mTorr(RMS roughness measured6.11nm),and it becomes smoother as the pressure increases. As stated above,as the DC bias decreases,the physical bom-bardment by positive ions decreases and enhances the chemi-cal activity.At a pressure of40mTorr,the surface is smoothest with an RMS roughness value of1.00nm.However,etching at higher pressure(>40mTorr)created a rough surface.This is probably due to the re-deposition of etch products.The ion bombarding energy is too low to remove the re-depositions which act as micro-masks resulting in a rough surfaceŒ12 14 .4.ConclusionReactive ion etching of Si2Sb2Te5thin films with a photo-resist mask was studied using a CF4/Ar gas mixture in in-ductively coupled plasma.The etch rate of Si2Sb2Te5films in a CF4/Ar plasma decreased with the decrease of CF4con-centration at constant background pressure and power.Ar helped to promote the etching process as it removed the non-volatile products by physical bombardment resulting in a smooth surface.Meanwhile,the selectivity of Si2Sb2Te5 films to SiO2decreased.Etched features of Si2Sb2Te5films in CF4/Ar gas mixture were best when the CF4/Ar ratio is10/40, and a smooth surface and a vertical sidewall were obtained. 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