Chiang Mai J. Sci. 2007; 34(1) 35

FESEM Analysis of BPSG Films After ReflowUda Hashim*, Nik H.N. Hamat, and Ramzan M. AyubMicro Fabrication Cleanroom, School of Microelectronic Engineering, Northern Malaysia University College ofEngineering, 02000 Kuala Perlis, Perlis, Malasia.*Author for correspondence; e-mail : uda@kukum.edu.my

Chiang Mai J. Sci. 2007; 34(1) : 35-46www.science.cmu.ac.th/journal-science/josci.htmlContributed Paper

Received: 11 March 2006Accepted: 11 June 2006.

ABSTRACTThe reflow behavior of BPSG films deposited by plasma enhanced chemical vapor

deposition has been studied in several annealing ambient atmospheres. A new recipe for annealingprocess has been created and performed to improve the planarization for BPSG film afterreflow. This improvement is for 0.35µm technology using steam annealing method at differenttemperatures. Reflow process was carried out by furnace heating. Reflow was studied overthe temperature range of 850°C to 900°C. Cross sectional scanning electron microscopyusing FESEM has been employed to study the angles of BPSG films slope after imagesanalysis. The samples have been separated into two groups of samples: staining and withoutstaining with HF. The experiments lead to the conclusion that the staining samples with HFprovides clearer images for analysis. By using steam annealing method, the better surfaceplanarization of BPSG film has been achieved.

Keywords: FEM, BPSG films, reflow process, annealing.

1. INTRODUCTIONAll multilayer metallization schemes now

are almost required some planarization ofthe dielectric material which is deposited overthe poly or metal lines. This planarizationis needed because of poor metal stepcoverage, or to minimize the amount of overetch required for blank metal deposition andetch back processes. The three most widelyused processes are BPSG reflow, spin on glass,and photoresist [1]. In this research paper,BPSG reflow has been used. Reflow ofdielectric films has been effective to a degreein reducing local variations in topography ofmultilevel interconnections [2]. As a premetaldielectric, BPSG has been widely used fordevice planarization.

Phosphosilicate-doped silicon dioxide,

commonly referred to as phosphosilicate glass(PSG), has been used widely as an interlayerin multilevel metallization. It is an expansivelyuseful as a passivation layer because it inhibitsthe diffusion of impurities, and it softens andflows at 950°C to 1100°C to create a smoothtopography that is useful for depositingmetals. PSG films with 4-7 wt% phosphorusconcentration require temperature in excessof 1000°C to demonstrate substantial flow.Borophosphosilicate glass (BPSG) reveals abetter performance in this respect as it reflowsat temperatures below 900°C [3, 4].

BPSG reflow process has beencommonly used to obtain insulating films withreasonably flat surfaces. These films exhibitprofiles over steps that get progressively

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smoother with higher phosphorus and boronconcentration, reflecting the correspondingenhancement in viscous flow. Post-depositionfilm reflow can be performed using eitherconventional furnace or rapid thermalannealing; the reflowed profile depends onannealing temperature as well as ambient gases[5]. In this experiment, reflow process wascarried out by furnace heating. A low thermalbudget can be maintained using steam anneal.Earlier studies from Thakur et al. [6] revealedthat a BPSG thermal budget can be reducedthrough both steam annealing and rapidthermal processing. The effect of annealingambient reveals that using steam yields the bestreflow [5]. The purpose of this experiment isto improve the planarization of BPSG filmfor 0.35 µm technology after reflow by usingsteam annealing method. Steam annealingmethod was done at different temperatures.

2. EXPERIMENTSThe topography substrates understudy

were fabricated on <100> oriented P-type

silicon wafers with resistivity of 16-25 -cm.Six wafers were used in this experiments.

In this experiment, the angles of theBPSG films slope were the response observedafter annealing process. The angles must be aslow as possible in order to get a goodtopography of the BPSG films. The thicknessof the BPSG film is 3500Å while theuniformity has to be maintained as low as 5%.The Boron and Phosphorus contents of theBPSG layer are 4.7% and 5.3% respectively.

A new recipe was created and performedfor annealing process using diffusion furnacesystem. This new recipe used steam annealingat 850°C and 900°C while there was onlynitrogen gas used for current annealing recipeduring the reflow process. A standard 0.35µm CMOS technology is utilized. Six wafersare following accordingly to the 0.35µmCMOS process sequences until reached theBPSG deposition steps whereby theexperiment is begun. These wafers weremarked as number 1 until 6.

All wafers were loaded into PECVD

chamber to deposit BPSG film for 3500Åthickness. Details of the PECVD systemdesign and process conditions used indepositing the BPSG films are discussedelsewhere [7-8] . The first wafer was used forpre annealing process while the rest proceededto the next step. After BPSG film deposition,they were flowed in furnace. The sampleswere split into several conditions for reflowingthe BPSG films in furnace after have gonethrough the scrubbing process. Fourconditions of reflow process are comparedand illustrated in Table 1. The steam generated

Table 1. BPSG reflow process at different conditions.

Wafer Number Recipes for BPSG annealing1 Pre annealing2 N2 annealing at 850oC3 steam annealing at 850oC4 steam annealing at 850oC5 steam annealing at 900oC6 steam annealing at 900oC

by the pyrogenic combustion of hydrogenand oxygen was readily available in annealfurnace. In this process, the wafers wereseparated for three different annealing processrecipes. Wafer number 2 annealed in nitrogengas for current BPSG annealing process at850°C. Wafers number 3 and 4 were usedsteam annealing process at temperature of850°C and as 900°C for wafers number 5and 6. Each wafer was run individually inorder to get repeatable results. The processflow is shown in Figure 1.

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Figure 1. Simple process flow for BPSG flow and reflow conditions.

The wafers were finally sent to the FailureAnalysis Laboratory for physical analysis ofBPSG film topography cross section viewusing Field Emission Scanning ElectronMicroscope (FESEM). The FESEM providesa high current density into a small spot size[5]. The analysis of the images has been donelocally at the edge and the center of thewafers. The wafers were cut into two foranalyzing the effect of HF staining andwithout HF staining. For staining effectanalyzing, the wafers were cleaved, polishedand stained with HF before physical cross-

sectional analysis using FESEM. For each area,two micrograph photos were taken at activeand LOCOS area. Based on the photos, theangles of the BPSG films slope weremeasured.

3. RESULTS AND DISCUSSIONFor each experimental run, the cross-

sections of the film profiles were taken withFESEM. The after-reflow angles were alsomeasured. Figure 2 shows how the after-reflow angle is measured.

Figure 2. Measurement of an after-reflow angle.

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Active area LOCOS areaFigure 4. FESEM micrograph of BPSG for wafer number 1 at the center area.

Figure 3 shows the topography taken atthe edge area of the wafer for active andLOCOS area. The angles of BPSG film slopesare 73º for active area and 72º for LOCOSarea. These images are for the samplesunderwent the pre-annealing process. Figure 4

illustrates the topography taken at the activeand LOCOS area for the center of the waferfor the same conditions. The angles for theseslopes are 70º and 76º. It is clearly seen thatthe sample is not well planarized.

Active area LOCOS areaFigure 3. FESEM micrograph of BPSG for wafer number 1 at edge area.

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Active area LOCOS areaFigure 5. FESEM analysis for wafer number 1 at edge area without staining with HF.

Active area LOCOS areaFigure 6. FESEM micrograph for wafer number 1 at center area without staining with HF.

Figures 5 and 6 indicates the images takenat active and LOCOS area at the edge andthe center of the wafer number 1 withoutunderwent the staining with HF process. Theimages are not very clear compared to the

images of the staining wafer for FESEManalysis in Figures 3 and 4. The angles of theBPSG film slopes are 59º, 49º, 60º and 64º.These angles are smaller than the anglesobtained from the staining sample.

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Active area LOCOS areaFigure 8. FESEM cross section of BPSG for wafer number 2 at center area.

Active area LOCOS areaFigure 7. FESEM cross section of BPSG for wafer number 2 at edge area.

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Active area LOCOS areaFigure 9. FESEM cross section of BPSG for wafer number 2 at edge area without stainingwith HF.

Active area LOCOS areaFigure 10. FESEM micrograph of BPSG at center area for wafer number 2 without HFstaining.

Illustrated images in Figures 7, 8, 9 and 10are for the wafer that has gone through thecurrent annealing process with nitrogen gasduring reflow process at 850ºC. Accordingto the literature, the use of nitrogen anneals inreflow studies of BPSG result in superiorreflow properties. The slopes of BPSG filmfor this wafer produced the angles of 21º, 50º, 49º, 65º, 40º, 53º, 31º and 58º. The angles ofslopes extracted from the images of wafernumber 1 and 2 are quite bigger. This meansthe BPSG film was not planarized well. The

HF staining samples are shown clear imagesfor analysis. Then, the experiment has beencreated using steam annealing method in orderto improve the planarization of BPSG film.This experiment has been performed attemperatures of 850ºC and 900ºC.

Steam often used in PSG and BPSG flow,was the next choice for anneals, although therewas no reason to expect that the compositionof the films would be altered following steamanneals, since compositional change appearedto be associated with the presence of H2.

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Active area LOCOS areaFigure 12. FESEM micrograph of BPSG at center area for wafer number 3.

Active area LOCOS areaFigure 11. FESEM micrograph of BPSG at edge area for wafer number 3.

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Active area LOCOS areaFigure 13. FESEM micrograph at edge area for wafer number 3 without HF staining.

Active area LOCOS areaFigure 14. FESEM micrograph at center area for wafer number 3 without HF staining.

Figures 11, 12, 13 and 14 show the imagesof the wafer underwent steam annealingmethod at 850ºC at both active and LOCOSarea for the edge and center of wafernumber 3. The angles obtained for this wafer

are 24º, 19º, 27º, and 42º for stained sampleand 14º, 20º, 22º and 20º for without HFstained sample. These angles indicate bettersurface planarization of BPSG film has beenobtained.

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Active area LOCOS areaFigure 16. FESEM cross section at the center area for wafer number 6.

Figures 15 and 16 illustrate FESEMtopography of wafer number 6 at active andLOCOS area for the edge and center area.

The BPSG slope angles of these images are7º, 7º, 6º and 9º. This wafer was gone throughthe steam annealing process at 900ºC.

Active area LOCOS areaFigure 15. FESEM cross section at edge area for wafer number 6.

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Active area LOCOS areaFigure 17. FESEM micrograph at edge area of wafer number 6 without HF staining.

Active area LOCOS areaFigure 18. FESEM micrograph at the center area of wafer number 6 without HF staining.

Figures 17 and 18 are the images of thewafer number 6 without HF staining at theedge and center area. This wafer used steamannealing process at 900ºC. The producedangles of BPSG for this condition are 6º, 7º,

7º and 9º respectively for each image. Thistemperature produced the smallest anglesamong the others. Steam annealing at 900ºCprovides BPSG surface that almost planarized.

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4. CONCLUSIONSFrom the experiment, we concluded that

by using steam annealing method at 900°Cduring reflow process produces better surfaceplanarization of BPSG compared to conven-tional method and current annealing methodusing nitrogen gas. The angles obtained fromsteam annealing method at 900°C are less than11° that indicate the BPSG surface is almostplanarized. The results also show that thesamples stained with HF are clearer forFESEM analysis contrast with without HFstaining samples. This steam annealing processis proven such a promising method to improvethe planarization of BPSG film after reflow.The effect of annealing ambient reveals thatusing steam annealing yields the best reflowfor planarization.

[3] French P.J., and WolffenbuttelR.F., Reflow of BPSG For SensorApplications, J. Micromac. Microeng. 1993;3: 135-137.

[4] Oberemerok O., and Lytvyn P.,Borophos-phosilicate Glass ComponentAnalysis Using Secondary Neutral MassSpectro-metry, Semiconductor Physics,Quantum Electronics & Optoelectronics 2000;5: 101-105 .

[5] Xia L.Q., Conti R., Galiano M., CampanaF., Chandran S., Cote D., Restino D., andYieh E., High aspect Trench Filling UsingTwo-Step Subat-mospheric ChemicalVapor Deposited BorophospholicateGlass for 0.18 µm Device Application,J. Electrochem. Soc., 1999; 146 (5): 1884-1888.

[6] Thakur R.P.S., Gonzalez F., HawthorneR., Ward V., and Jeng N., Material ResearchSociety Symposium Proceeding, 1993; 303:283.

[7] Hashim U., and Ayub R.M., FieldEmission Scanning Electron MicroscopyAnalysis of Metal Step CoverageImprovement by Reflow, Journal of theInstitute of Materials Malaysia, 2004; 5(2),: 105-113.

[8] Banerjee I., Tracy B., Davies P., andMcDonald B., Use of Advanced Analy-tical Techniques for VLSI Failure Analysis,Proceeding of IEEE, 1990; 61-68.

REFERENCES[1] Marks J., Law K., and Wang D., In Situ

Planarization of Dielectric SurfacesUsing Boron Oxide VMIC Conference1989; 89-95.

[2] Simpson D.L., Croswell R.T., ReismanA., Temple D., and Williams C.K.,Planarization Processes and Applications:I Undoped GeO2- SiO2 Glasses, J.Electrochem. Soc., 1999; 146 (10): 3860-3871.