Very low pressure VLPCVD growth of high quality r Al2O3 films on silicon by multi step process

Journal of Crystal Growth 236(2002)261–266Very low-pressure VLP-CVD growth of high quality g -Al 2O 3films on silicon by multi-step processLiwen Tan a,*,Yude Zan a ,Jun Wang a ,Qiyuan Wang a ,Yuanhuan Yu a ,ShuruiWang b ,ZhongliLi u b ,Lanying Lin aaNovel Semiconductor Materials Laboratory,Institute of Semiconductors,Chinese Academy of Sciences,Beijing 100083,People’s Republic of ChinabMicroelectronics R&D Center,Institute of Semiconductors,Chinese Academy of Sciences,Beijing 100083,People’s Republic of ChinaReceived 7November 2001;accepted 15November 2001Communicated by M.SchieberAbstractg -Al 2O 3films were grown on Si(100)substrates using the sources of TMA (Al(CH 3)3)and O 2by very low-pressure chemical vapor deposition.The effects of temperature control on the crystalline quality,surface morphology,uniformity and dielectricity were investigated.It has been found that the g -Al 2O 3film prepared at a temperature of 10001C has a good crystalline quality,but the surface morphology,uniformity and dielectricity were poor due to the etching reaction between O 2and Si substrate in the initial growth stage.However,under a temperature-varied multi-step process the properties of g -Al 2O 3film were improved.The films have a mirror-like surface and the dielectricity was superior to that grown under a single-step process.The uniformity of g -Al 2O 3films for 2-in epi-wafer was o 5%,it is better than that disclosed elsewhere.In order to improve the crystalline quality,the g -Al 2O 3films were annealed for 1h n O 2atmosphere.r 2002Elsevier Science B.V.All rights reserved.PACS:68.55;81.15Keywords:A3.Metalorganic chemical vapor deposition1.IntroductionSilicon on insulator (SOI)structures have attractive features for use in very fast logic devices,environment-hardened devices such as high-tem-perature-operated and radiation-resistant devicesand three-dimensional integrated circuits [1,2].Recently,SOI structures are mainly fabricated by means of silicon direct bonding (SDB),separa-tion by implanted oxygen (SIMOX)and smart cutting.With these methods,however,it is difficult to obtain multi-stacked SOI structures,to realize a fundamental structure of new devices,as well as three-dimensional integrated circuits (3D-ICS).SOI structure of double-heteroepitaxial*Corresponding author.E-mail address:lwtan@ (L.Tan).0022-0248/02/$-see front matter r 2002Elsevier Science B.V.All rights reserved.PII:S 0022-0248(01)02159-5Si/g-Al2O3/Si is the candidate to realize multi-stacked SOI structure[1].Single crystalline Al2O3 layer has been grown on Sisubstrates by the low-pressure chemical vapor deposition(LP-CVD)[3]. Metal-oxide semiconductorfield-effect transistors (MOSFETS)have been fabricated using Si(100)/ g-Al2O3(100)/Si(100)[4].The multi-stacked SOI structure of epitaxially grown Si(100)/g-Al2O3(100)/Si(100)/g-Al2O3(100)/Si(100)have been prepared and using the structure,the high-temperature sensors were fabricated[5].Moreover, it is hopeful to provide a new substrate of g-Al2O3/ Si instead of sapphire for growth of GaN,SiC and other semiconductor materials.Epitaxial single crystalline GaNfilm have been successfully grown on g-Al2O3/Sisubstrates on our lab[6]. However,the quality of the oxidefilms such as the crystalline perfection,dielectricity and thick-ness uniformity should be improved for the device application of Si/g-Al2O3/SiSOI structures.The key to successfully prepare the device quality SOI structure Si/g-Al2O3/Si is to grow single crystalline g-Al2O3films on Sisubstrates at relati vely low temperature.TakayukiKi nura et al.[7]have reported that the uniformity offilm thickness was improved by UHV-CVD method.It is proven that UHV-CVD method is an effective way to grow uniform g-Al2O3films.But it was still far from the appropriate uniformity for practical use. And problems also remain to be resolved,such as surface morphology and dielectric properties.In the initial process of growth at high temperature by UHV-CVD the Sisurface would be etched by O2,and the quality of thefilms would be degraded. While at lower temperature,the crystalline quality of g-Al2O3films would be poor.In this work,the g-Al2O3film was grown on Si(100)by very low-pressure chemical vapor deposition(VLP-CVD) method with the use of Al(CH3)3and O2.We used a temperature-varied multi-step process not only to improve the uniformity of thefilm but also to improve the surface morphology and dielectric properties of thefilms.Thefilm thickness was measured by ellipsometer,and the thickness for f200epi-wafer was ranged from500to1000(A.The films were also characterized by X-ray diffraction, reflect high-energy electron diffraction(RHEED) and scan electron mirror(SEM).2.Experimental procedureRecently,fabrication of SOI structure consisting of epitaxial Al2O3and Siwas carri ed out usi ng the UHV-CVD[7,8]and MBE method[9,10].How-ever,those systems are very complex and at a high price and are hard to operate and maintain.The VLP-CVD system used in our lab is very similar to LP-CVD system,which is operated at moderate (10À6Pa)vacuum levels.The system is simple, relatively less costly and can be easily operated. Fig.1shows the cross-sectional view of the VLP-CVD growth system.The chamber consists of a RF-heated vertical reaction tube.The upper part of the reaction chamber is water-cooled.Si wafers (in2-in diameter)were put on a rotating SiC-coated graphite susceptor after chemical wet cleaning and etching the native oxide with the diluted HF.The graphite susceptor can move up or down by magnetic coupling transport.The reaction chamber was evacuated to5Â10À6Pa at room temperature through a turbo molecular pump.The pressure for pre-cleaning silicon wafer was 3.0Â10À5Pa at10001C before growth.In Fig.1.The schematic diagram of the VLP-CVD system for growth of g-Al2O3films.L.Tan et al./Journal of Crystal Growth236(2002)261–266 262UHV-CVD the partial pressure of water and oxygen was o 10À7Pa [7].Therefore,without any pre-clean step,the hydrogen passivation of Si surface is enough to guarantee the good epitaxial material.However,in our VLP-CVD system with moderate (10À6–10À5Pa)vacuum levels,the hy-drogen passivation and thermal pre-clean are both performed.The difficulty in applying the lower temperature epitaxial growth technique to CVD at the moderate vacuum levels lies in maintaining the oxide and carbon free surface during the wafer heat up cycle and subsequent anneal.Therefore,the growth temperature and time as well as ramping process that the Si wafer is subjected are critical,so as to keep the Si substrate surface from contamination and etching.In our experiment,when the growth chamber was evacuated to 5Â10À6Pa,the sample temperature can be heated up at high rates by induction heating.And the Si substrate was pre-cleaned at o 10001C under the pressure of 3Â10À5Pa for 5min,then the substrate temperature was decreased to 9301C.The source gas was introduced to the growth chamber at the pressure of 10À2–10À1Pa,avoidingthe possibility of grown discharge during the growth period from RF induction heating.The epitaxial growth proceeds first for 30min to cover the Sisubstrate at a lower temperature and the thickness of g -Al 2O 3film was then about 15–20nm.Afterwards the g -Al 2O 3film was grown at a high temperature of 10001C for 60min.Sequentially,the g -Al 2O 3/Siwas annealed at 10001C for 1h in O 2atmosphere in a furnace.The schematic diagram of growth process is shown in Fig.2and the growth conditions are summar-ized in Table 1.3.Results and discussionIn order to compare the uniformity of samples,we prepared samples using multi-step and single-step processes respectively,and characterize the samples.The single-step samples were grown at a temperature of 10001C through the process.Fig.3shows SEM images of the surface morphology of the g -Al 2O 3films,(a)grown under a single-step and (b)under multi-step process.It shows that the sample under the single-step temperature process has a rough surface,while the sample under multi-step process shows a mirror-like surface.The result arises from the parasitic reactions between the Si surface and O 2in the initial growth stage [11].In the initial growth stage of g -Al 2O 3,the reactions between the Si surface and O 2occur as following:Si þO 2¼SiO 2;ð1ÞSi þSiO 2¼2SiO :ð2ÞReaction (1)is the oxidizing reaction of Si substrate by O 2,and reaction (2)is the etchingT E M P E R A T U R EFig.2.The schematic diagram of growth process.Table 1Epitaxial conditions of g -Al 2O 3filmCleaningLow-temperature epitaxial layer High-temperature epitaxial layer Pressure3Â10À5Pa 3.8Â10À2Pa 3.8Â10À2Pa N 2bubble TMA (sccm)02525O 2(sccm)7070Substrate temperature10001C9301C10001CL.Tan et al./Journal of Crystal Growth 236(2002)261–266263reaction of SiO 2.In order to grow mirror-like g -Al 2O 3film,the reaction (2)must be prevented in the initial growth stage of g -Al 2O 3film [11–13].But the higher the temperature of Si substrate,the easier reaction (2)took place.So the mirror-like g -Al 2O 3film must be grown at a lower tempera-ture.But if the substrate temperature were too low,the crystalline quality would decrease.In order to prevent the reaction (2)from taking place,the g -Al 2O 3film was grown at a lower temperature in the initial growth stage,and then at higher temperature to improve the crystalline quality.Fig.4(1)shows the RHEED patterns of g -Al 2O 3grown at 9301C,10001C and multi-step respec-tively.From Fig.4,we can see ring pattern in image (a),but in (b)and (c)there are only spot patterns.Meanwhile,there are some twin crystal-line spot patterns in (b).In order to improve the crystalline quality the samples were further annealed in O 2atmosphere at 10001C for 1h.Fig.4(2)shows the results.It indicates that twin crystalline patterns disappear in (b),but the ring pattern in (a)still exists.The results show that the g -Al 2O 3films have good crystalline quality under multi-step process with annealing in O 2atmo-sphere.The film thickness was measured by ellips-ometer,and the thickness of a sample grown under multi-step process (2in)was ranged in 72.5%,while under single-step temperature the process was 77–8%.The thickness uniformity is calculated by equation D t =t ¼7ðt max Àt min Þ=ðt max þt min ÞÂ100%:Table 2shows the data of different locations.Location1is at center of the g -Al 2O 3/Siwafer,and 2–5near the edge of the wafer (about 2mm)at differentlocations.Fig.3.SEM images of the g -Al 2O 3films on silicon:(a)single-step,(b)multi-step (both amplified 20,000times).Fig.4.RHEED patterns of (1)as-grown g -Al 2O 3films:(a)single-step at 9301C,(b)multi-step,(c)single-step at 10001C (2)annealed g -Al 2O 3films:(a)single-step at 9301C,(b)multi-step growth.L.Tan et al./Journal of Crystal Growth 236(2002)261–266264Fig.5shows the XRD result of g -Al 2O 3film under multi-step process (after annealing).A strong diffraction peak is observed at 2y ¼45:81:The peak belongs to the (400)diffraction of g -Al 2O 3[11].The result indicated that the g -Al 2O 3epi-layer on Si substrate was single crystalline,and the orientation relationship was g -Al 2O 3(100)/Si(100).In order to characterized the dielectric proper-ties,we measured the leakage current of g -Al 2O 3film.Fig.6shows the leakage current of the 65-nm-thick g -Al 2O 3film grown by multi-step and single-step (10001C)process respectively (both annealed in O 2atmosphere for 1h).It is shown from Fig.6that the sample grown under single-step process has poor dielectric properties,the breakdown voltage was about 15V.While the sample grown under multi-step process has a better dielectric properties,the breakdown voltage increases to 53V.The increase of breakdownvoltage may be due to the improvement of the surface morphology and the uniformity of the g -Al 2O 3film.4.ConclusionWe have grown g -Al 2O 3films on silicon under a temperature-varied multi-step process by VLP-CVD.And the g -Al 2O 3films were also character-ized by X-ray diffraction,RHEED,ellipsometer and SEM.The results indicate that the etching reaction between O 2and Sisubstrate can be prevented in the initial growth of g -Al 2O 3film under multi-step process.The surface morphology,uniformity and dielectricity were all improved compared to the single-step process.The film grown under multi-step process has mirror-like surface,and the breakdown voltage for 65-nm-thick g -Al 2O 3film on silicon increases from 15to1020304050601E-81E-71E-61E-51E-41E-30.010.1110Single-stepMulti-stepJ (A /c m -2)Gate Voltage(V)Fig.6.Leakage current for a 65-nm-thick g -Al 2O 3film on silicon.Table 2The thickness and uniformity of g -Al 2O 3epi-layer (2-in)LocationThickness ((A)Uniformity (%)12345Multi-step temperature processSample no.40370370570769871171Sample no.40995495594194492471.65Single-step temperature process (10001C)74085576573477277–8203040506070802θ (deg)I n t e n s i t y (a . u .)γ-Al 2O 3(400)Si(400) Fig.5.The XRD of g -Al 2O 3film under multi-step process.L.Tan et al./Journal of Crystal Growth 236(2002)261–26626553V.The uniformity for2-in diameter epi-wafer g-Al2O3film was o5%,it is better than that has been disclosed elsewhere.AcknowledgementsThis work is subsidized by the Special Funds for Major State Basic Research Projects G2*******. And the authors would like to thank Ms.Y.P. 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