Dependence of Process Parameters on Stress Generation in Aluminum Thin Films

482 IEEE TRANSACTIONS ON DEVICE AND MATERIALS RELIABILITY, VOL. 4, NO. 3, SEPTEMBER 2004

Dependence of Process Parameters on StressGeneration in Aluminum Thin Films

Alton B. Horsfall, Kai Wang, Jorge M. M. dos-Santos, Sorin M. Soare, Steve J. Bull, Nick G. Wright, Member, IEEE,Anthony G. O’Neill, Member, IEEE, Johnathan G. Terry, Anthony J. Walton, Alan M. Gundlach, and

J. Tom M. Stevenson

Abstract—The dependence of residual stress on the processparameters for aluminum metallization has been studied usinga rotating beam sensor. This shows increasing tensile stress withboth the target power and ambient pressure used during thesputter deposition of the aluminum layer. The bulk resistivity ofthe deposited aluminum has been measured using a Van der Pauwtechnique on test structures fabricated alongside the sensors andthis shows different trends with respect to the target power andambient pressure. This indicates that the stress in an interconnectfeature is dominated by extrinsic components, which result fromthe mismatch in thermal expansion coefficient between the con-stituent layers, rather than the defects formed during the sputterdeposition of the metallization. This indicates the suitability of thestress sensor technique to the monitoring of interconnect featuresin a production line environment.

Index Terms—Integrated circuit reliability, interconnect, metal-lization, reliability, stress.

I. INTRODUCTION

THE demand for higher operating frequencies and greatercircuit complexity has forced the drive toward deep-sub-

micron silicon technology. As gate lengths for commercial pro-cesses approach 50 nm and below [1], there is a requirement forinterconnect track widths to decrease and the number of levelsof interconnect to increase. This trend has led to higher currentdensities in interconnect tracks with the attendant increase inpower dissipated by the chip, placing unprecedented demandsfor ever higher quality metallization. The resulting geometricconstraints placed on such narrow lines reduces their ability torelax stress by plastic deformation. These stresses in turn in-crease the diffusion of metal ions, leading to premature trackfailure. Moreover, the shrinking lateral dimensions of all inte-grated circuit components means that less diffusion needs tooccur to initiate line failure.

The residual stress in aluminum films used for interconnectfeatures has been shown previously to depend upon the process

Manuscript received October 31, 2003; revised March 22, 2004. This workwas supported by the Engineering and Physical Science Research Council, U.K.,under Grant GR/N32945/01. The work of J. M. M. dos-Santos was supportedby the Portugese Government through the “Fundação para a Ciência e a Tec-nologia” scheme.

A. B. Horsfall, K. Wang, J. M. M. dos-Santos, N. G. Wright, and A. G. O’Neillare with the School of Electrical, Electronic and Computer Engineering, Univer-sity of Newcastle, Newcastle NE1 7RU, U.K. (e-mail: [email protected]).

S. M. Soare and S. J. Bull are with the School of Chemical Engineering andAdvanced Materials, University of Newcastle, Newcastle NE1 7RU, U.K.

J. G. Terry, A. J. Walton, A. M. Gundlach, and J. T. M. Stevenson are withthe Institute of Integrated Micro and Nano Systems, Scottish MicroelectronicsCentre, University of Edinburgh, Edinburgh EH9 3JF, U.K.

Digital Object Identifier 10.1109/TDMR.2004.829389

conditions experienced during the deposition process. Films de-posited by RF magnetron sputtering have demonstrated a depen-dence on both the substrate bias and the working pressure withinthe reactor [2]. At low pressure and substrate bias, the films areheavily tensile, the value of which decreases with increases inboth bias and pressure. In particular, the use of high substratebias results in a compressive stress in the film, evenat pressures as low as 1 mTorr.

While spectroscopic analysis shows evidence of both inertgas species and oxygen in sputtered films [3], this is not theonly mechanism responsible for compressive stress in films [4].Bombardment of the deposited film by energetic particles pro-duces lattice damage by direct and recoil atomic displacementas well as localized heating, and this serves to densify the layerby forward sputtering, a process referred to as atomic peening[5]. As the reactor pressure decreases, the amount of scatteringbetween the reflected neutral particles (Ar) and sputtered atoms(Al) decreases, thereby increasing both the energy of the parti-cles and their bombardment effect on the growing surface of thefilm [6]. Hence, films deposited at lower pressure have a morecompressive stress.

The measurement of stress in the metallization for submi-cron tracks required for modern integrated circuits presents amajor challenge. The most commonly utilized techniques in-volve x-ray diffraction [7], where the stress of a thin film can beevaluated, but the lateral dimensions of the film must be greaterthan the spot size of the beam. The smallest spot sizes achievabletoday are of the order of 1 m at synchrotron facilities. Anothertechnique to quantify stress is by measuring the bow across awafer [8]. This technique cannot be applied to a single metalstrip, but would have to be averaged over a wafer patterned withmany similar strips.

Use of these techniques for the routine monitoring of residualstress in a production line is not realistic, and this raises theissue of how to examine the influence of process conditions onthe residual stress in individual interconnect features. There is,therefore, a need for new methods of measuring stress in metalinterconnect structures. One solution to this problem is the de-velopment of a suitable test structure which can be fabricatedon the wafer along with integrated circuits or other devices andused to monitor stress generation and relaxation as a functionof process conditions, including post pattern sintering. Such asensor would continue to work as the critical feature dimensionsreduce and would not require expensive specialist measuring ap-paratus. The rotating beam sensor design [9] has potential forfulfilling this function for both current and future integrated cir-cuit metallization technologies.

1530-4388/04$20.00 © 2004 IEEE

HORSFALL et al.: DEPENDENCE OF PROCESS PARAMETERS ON STRESS GENERATION IN ALUMINUM THIN FILMS 483

Fig. 1. Schematic representation of the sensor structure, showing the positionof the rotating pointer before and after release.

A further possibility for the measurement of process-inducedstress in a production environment is the measurement of sheetresistance, a commonly employed technique for the monitoringof implants. Previous work has demonstrated that the bulk resis-tivity of an aluminum layer varies monotonically with stress, inthe region of to MPa [2] for variations in both sub-strate bias and working pressure. The resistivity values quotedare in the region of 4 to 7.5 cm, which is far higher than thebulk value of 2.74 cm, and the authors attribute this to the in-clusion of process gas species within the layer [2]. In this paper,the suitability of this technique for the observation of stress ininterconnect features will be evaluated by comparison with theangle of rotation extracted from the rotating beam sensor.

In Section II, the design of the sensor structure is detailed andthe dependence of the theoretical angle of rotation on the phys-ical dimensions discussed. Section III outlines the fabrication ofthe samples used in the study, the results of which are given inSection IV.

II. SENSOR DESIGN

It can be observed that when the fixed beams (labeled as arms)are freed from the underlying dielectric layer, shown as SiO inFig. 1, they contract or extend in order to relieve any residualtensile or compressive stress. As these two arms are offset onopposing sides of the rotating beam (pointer), any deviation inthe length of the arms exerts a torque about the centre of thesensor structure, causing the pointer arm to rotate.

Finite element software (ANSYS) has been used to modelthis rotation as a function of the process conditions and ma-terials properties of the metal, which is aluminum in this case[10], [11]. As the plasticity and creep properties of thin filmsare known not to be the same as those observed for bulk mate-rials, input parameters for the modeling have been determinedby nanoindentation testing in conjunction with finite elementmodeling [11].

Previous work has shown the suitability of the sensor struc-ture for the measurement of compressive and tensile stress inaluminum films [12] by the observation of pointer rotation inboth the clockwise and anticlockwise directions, depending onthe thermal history of the layer.

III. FABRICATION

The sensors were fabricated n-type silicon wafers ofthickness 380 m, using the process outlined schematically inFig. 2. After cleaning, 0.1 m of TiN was sputter deposited toact as an etch stop layer in the regions where the sensors are

Fig. 2. Process summary of the fabricated devices.

TABLE IPROCESS CONDITIONS USED FOR THE PREPARATION

OF THE EXPERIMENTAL WAFERS

to be fabricated. This was followed by the deposition of 2 mof PECVD SiO after which 1 m of aluminum was sputteredusing a Balzers BAS 450 under a variety of operating condi-tions as specified in Table I. All depositions were undertakenwith a floating substrate potential and an uncooled target. Afterperforming the photolithography using a 5 wafer stepper andShipley SPR2 photoresist, the aluminum was patterned using achlorine-based plasma in an STS reactive ion etch system.

The final process step is an isotropic oxide etch, which isrequired to release the structures from the sacrificial silicondioxide underneath. Initial work [12] on the sensor structureused a dry etch process based on a SF O plasma to avoid theproblem of stiction [13] that often accompanies wet etching. Inaddition, wet chemical etches generally result in a much lowerselectivity between SiO and Al unless highly concentrated HFis used. However, highly concentrated HF gives an etch rate inexcess of 2 m per minute, making accurate control of the etchdepth extremely difficult.

When the sensors were characterized, it was suspected thatdespite optimization of the plasma conditions, some siliconoxide remained under the sensor structure dominating theobserved rotation. By considering the sensor to be formed bysilicon dioxide and aluminum, the in-plane stress, , of thecomposite beam can be expressed as

(1)

where and are the stress in the aluminum and silicondioxide layers, respectively, and and are their thicknesses.


Fig. 3. Scanning electron micrograph of a sensor showing the existence ofresidual oxide under the structure. A silicon flouride based polymer can also beseen as the regions in the lower right of the micrograph.

Equation (1) provides a relationship between the composite filmstress and that of the individual layers.

The PECVD oxide used in the sensor is deposited at a tem-perature of around 300 C. It has a thermal expansion coefficientlower than the silicon substrate and exhibits a highly compres-sive stress at room temperature. The aluminum, with a larger ex-pansion coefficient than silicon is tensile, but with a lower mag-nitude. Hence, the observed rotation of the sensor structure mayindicate a compressive stress, when the stress in the aluminumlayer is actually tensile. Fig. 3 shows evidence of the residualoxide which is protected by a silicon flouride based polymerwhich is deposited during the SF O plasma etch process.

In order to alleviate this problem, a solution based on wetchemistry has been used to release the sensor from the dielectric.The samples are mounted in a closed volume with concentrated(60%) HF vapor. The highly concentrated HF does not attack thealuminum and using the vapor rather than the liquid itself givesa controllable etch time. After etching, the samples are rinsed inDI water to remove any residual HF prior to handling.

Sensors released in this manner are affixed to the substrateby stiction, but we have found that because the rotation occurswhist the device is in the vapor etch stage of the process, anystiction which occurs during the rinse does not affect the trendsin observed angles of rotation observed [13].

After processing, the rotation of the devices is measured usinga reflected light microscope with a digitizing frame grabber. Re-sistivity measurements are performed on 100 m 100 m Vander Pauw structures using a Keithley 2400 Source Measure Unit.

IV. RESULTS AND DISCUSSION

Fig. 4 shows reflected light micrographs of a sensor with anarm separation of 4 m, arm length of 140 m, and a featurewidth of 2 m for the five wafers having process conditions de-tailed in Table I. From this, it can be observed that the difference

Fig. 4. Reflected light micrographs of sensor structures after release, showingthe variation in stress from tensile to compressive.

Fig. 5. Variation in the angle of rotation with target power and reactor pressure.

in rotation between the different samples is small, and the anglesquoted in the following section are extracted using image pro-cessing software, based on MatLab. This custom software hasan accuracy of better than 0.1 .

The angle of rotation for these samples and the dependence ontarget power and reactor pressure is presented in Fig. 5 wheretensile stress is indicated by a positive angle of rotation. Thelow power and low pressure sputter deposition of aluminum(wafer 1) resulted in a released layer with a small tensile stress.The stress can be increased by raising either the target power orprocess pressure during the deposition. It should be noted thatthe process pressure cannot be reduced below 2 mTorr as theplasma will not sustain during the deposition, which limits theavailable process space.

The increase in tensile stress with target power dependson two factors, the generation of compressive stress by thebombardment of the growing film with energetic particles (ionsand fast neutrals—the atomic peening mechanism) and tensilestresses generated by the mismatch in thermal expansion co-efficient between the aluminum and the silicon substrate. The


Fig. 6. Variation of bulk resistivity with angle of rotation of the sensorstructure.

total stress, , in the film measured at room temperatureafter deposition is given by

(2)

where is the stress introduced into the coating duringthe deposition process (by atomic peening) and is thestress induced on cooling after deposition by thermal expansionmismatch with the substrate.

In this case, the latter dominates since creep relaxation ofgrowth stresses during deposition is likely. As target power in-creases, this givies a higher flux of energetic atoms at the sur-face of the wafer and defects are created by atomic peening gen-erating a higher compressive growth stress. However, this alsoraises the wafer temperature and once this rises to above 150 C,creep mechanisms can relax the growth stress, reducing it closeto zero at the highest power. On cooling, this compressivelystressed aluminum layer is forced into tension by the thermalstress. The higher target power increases the maximum temper-ature attained during the deposition relaxing more of the growthstress and so a higher tensile stress results.

Increasing the process pressure during deposition reduces themean free path of the sputtered atoms during their transfer to thewafer surface. With a higher number of collisions, the averagekinetic energy of the atoms is reduced and this limits the atomicpeening, which reduces the compressive growth stress in thealuminum, giving the appearance of a more tensile layer.

The measured bulk resistivity of the aluminum layers isshown in Fig. 6 as a function of their angle of rotation. Thelinear fits to the data are not based on an underlying physicalprinciple, but are for the purposes of comparing sensitivity ofthe techniques. The measured resistivity of a metal film canbe affected by a number of parameters including grain size,crystal orientation and uncertainty in the thickness but it is therole of point defects, which scatter electrons, which is oftenkey. Although the resistivity is affected by mechanisms whichalso affect the stress, other mechanisms unrelated to the stresswill also play a role. Hence, the use of resistivity to directlymeasure the stress in aluminum films is not conclusive.

It can be observed from Fig. 6 that the resistivity measuredis far lower than that observed in previous work in the literature

[2], where the values ranged from 4.2 to 7.5 m cm, suggestinga reduction in the concentration of point defects in our films. Italso shows that the change in resistivity for a given angle of ro-tation is greater for samples with a variation in target power. Theincrease of the resistivity with target power suggests the increasein defects in the aluminum caused by the inclusion of voidsand neutral gas species, such as argon, from the sputter process.These inclusions limit the mean free path of the electrons andhence increase the resistivity. However, as previously discussed,relaxation of the compressive stress has taken place for samplesdeposited with high target power and the residual stress is nolonger simply related to the defects introduced during growth.

The resistivity of the aluminum is more weakly controlledby variation of the reactor pressure, as shown by the lower gra-dient of that trend line. This indicates that varying the reactorpressure again changes the concentration of defects in the layerbut to a lesser extent. As the reactor pressure is increased, theenergy of neutral species from the target that are implantedinto the surface of the aluminum is reduced, which reduces thenumber of defects created in the layer. The resistivity mightbe expected, therefore, to decrease with reactor pressure, butthe opposite trend is observed. However, one other function ofatomic peening processes is to resputter impurity atoms such asargon or oxygen from the growing film and the concentrationof such defects (and hence resistivity) is, therefore, expected toincrease with reactor pressure, resulting in the observed trend.

It is clear from these observations that whereas resistivitymeasurements can give an indication of the stress in a film wheregrowth stresses dominate, once stress relaxation processes occurthe direct link between defect density and stress no longer ap-plies and resistivity measurements no longer give a true indica-tion of the stress state of the film.

V. CONCLUSION

We have shown that the rotating beam structure is suitablefor the observation of stress variation with sputter conditionsin a process environment. The stress in the aluminum metal-lization shows distinct variation with both the target power andambient pressure used during the sputtering process. While theatomic peening mechanism is partially responsible for the ob-served change in stress, the extrinsic stress, which originatesfrom the mismatch in thermal expansion coefficient betweenthe aluminum and the silicon substrate, dominates the observedstress. Measurement of the bulk resistivity of the sputtered alu-minum layers supports this hypothesis, as the trends observed inthe resistivity are different to those of angle of rotation. The ro-tating beam stress sensor used in this study is more suitable forthe in-line monitoring of stress in interconnects than the com-monly used four-point probe technique, due to the insensitivityof the latter to residual stress.

REFERENCES

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[5] F. M. D’Heurle, Metall Trans, vol. 1, pp. 725–725, 1970.[6] D. W. Hoffman and J. A. Thornton, “The compressive stress transition

in Al, V, Zr, Nb and W metal films sputtered at low working pressures,”Thin Solid Films, vol. 45, pp. 387–396, 1977.

[7] P. Besser, “Mechanical strains and stresses in aluminum and copper in-terconnect lines for 0.18�m logic technologies,” in 5th Int. Workshop onStress Induced Phenomena in Metallization, Stuttgart, AIP Conf. Proc.,vol. 491, 1999, pp. 229–239.

[8] C. M. Kottel, J. A. Sprague, and F. A. Smidt, 1994 ASM Hand-book. Materials Park, OH: ASM Int., vol. 5, Surface Engineering, pp.647–647.

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[10] S. M. Soare, S. J. Bull, A. G. O’Neill, N. G. Wright, A. B. Horsfall, and J.M. M. dos Santos, “Nanoindentation assessment of aluminummetalliza-tion: The effect of creep and pile up,” Surface and Coatings Technology,vol. 177–178, pp. 497–503, 2004.

[11] X. Zhang, T. Y. Zhang, and Y. Zohar, “Measurements of residual stressesin thin films using micro-rotating-structures,” Thin Solid Films, vol. 335,pp. 97–105, 1998.

[12] A. B. Horsfall, J. M. M. dos Santos, S. M. Soare, N. G. Wright, A. G.O’Neill, A. J. Walton, A. M. Gundlach, and J. T. M. Stevenson, “Directmeasurement of residual stress in submicron interconnects,” Semicon-ductor Science and Technology, vol. 18, pp. 992–996, 2003.

[13] J. F. L. Goosen, “Design and fabrication of a surface micromachinedpositioning device,” Ph.D. dissertation, Delft University of Technology,The Netherlands, 1996.

Alton B. Horsfall received the undergraduate andPh.D. degrees in the field of semiconductor charac-terization at the University of Durham before joiningthe University in August 1998.

He is currently a Lecturer in the MicroelectronicTechnology Research Group at the University ofNewcastle, U.K. His research interest are in thereliability of submicron interconnect structures andthe dependence on process conditions as well as thedevelopment of chemical gas sensors for applica-tion in extreme environments, such as automotive

exhausts and outer space.

Kai Wang is currently pursuing the M.Eng. degreein the School of Electrical, Electronic and ComputerEngineering, University of Newcastle, U.K. He willbe pursuing the Ph.D. degree at Newcastle on the re-liability and residual stress of copper-low k dielectricsystems.

He was awarded a vacation scholarship to studythe stress in submicron interconnect features, andthis was followed by a placement at Filtonic Com-pound Semiconductors at Newton Aycliffe, CountyDurham.

Jorge M. M. dos-Santos received the Masters degreein engineering physics from the Faculty of Sciencesand Technology of the University of Coimbra, Por-tugal, in 1999. Currently, he is pursuing the Ph.D.degree in the Semiconductor Technology Group ofthe University of Newcastle, U.K. His subject of re-search is in the area of metallization reliability in theintegrated circuits

Sorin M. Soare graduated from the Department ofScience of Materials, University “Politehnica” ofBucharest, Romania, in 1999. Since January 2001he has been pursuing the Ph.D. degree in the Schoolof Chemical Engineering and Advanced Materialsof the University of Newcastle, U.K. His subject ofresearch is in the area of metallization reliability insubmicron integrated circuits.

Steve J. Bull is a Professor of surface engineering inthe School of Chemical Engineering and AdvancedMaterials, University of Newcastle, U.K. He has over16 years experience in surface engineering research,including eight years at Harwell Laboratory in whichhe rose to become head of the section conducting re-search in all coating technologies and providing thecoating service activities. His main research interestsare related to understanding the mechanical proper-ties of layered systems, developing advanced coat-ings and surface treatments for applications where tri-

bological performance or corrosion resistance are essential and understandingthe mechanical response and failure mechanisms of microelectronic and opticscoatings. He has published over 100 scientific papers in refereed journals andover 50 commercially funded reports with restricted distribution, as well as nu-merous conference presentations and papers.

Nicholas G. Wright received the undergraduate andPh.D. degrees at Edinburgh University, working onthe characterization of a wide range of semicon-ducting materials using x-ray techniques.

He is currently a Senior Lecturer at NewcastleUniversity, U.K. He joined Newcastle Universityin 1994. He has wide experience in semiconductorprocessing with particular strengths in implantation,SiC technology, silicide contacts, and advancedinterconnects. Recently, he has concentrated onstudying the processing of the novel semiconductor

material SiC and has led several major projects in this area.

Anthony G. O’Neill was born in Leicester, U.K., in1959. He received the B.Sc. degree from the Univer-sity of Nottingham in 1980 and the Ph.D. degree fromthe University of St. Andrews in 1983.

From 1983 to 1986, he worked for Plessey Re-search (Caswell) Ltd., before taking up his post at theUniversity of Newcastle upon Tyne. He has workedon a wide range of topics in the field of semicon-ductor device and process technology and publishedmany papers. In 1994, he was a Visiting Scientist atMIT Microsystems Technology Laboratories, and in

2002 he became a Royal Society Industry Fellow with Atmel. He is SiemensProfessor of Microelectronics and currently has interests in strained Si/SiGeand SiC device technology, interconnect reliability, and carbon nanotubes.


Johnathon G. Terry received the B.Eng. (hons.)degree in electronic engineering in 1993, the M.Sc.degree in microelectronic material and device tech-nology in 1995, and the Ph.D. degree in 1999, fromthe University of Manchester Institute of Scienceand Technology.

He joined the Institute for Integrated Micro andNano Systems, University of Edinburgh, in 1999 asa Research Fellow. His previous work included thestudy of the properties and gettering of impurities insilicon. Presently, his main area of interest is in the

area of MEMS structures, including involvement in a variety of projects con-cerned with bioMEMS, optical MEMS, microfluidics, and MEMS process de-velopment. He is also a part of the Edinburgh team funded by the DTI BEACONinitiative “Understanding Genomics.”

Anthony J. Walton is Professor of microelectronicmanufacturing in the School of Engineering andElectronics at the University of Edinburgh, U.K. Hehas been actively involved with the semiconductorindustry in a number of areas which include siliconprocessing (both IC technology and microsystems),microelectronic test structures, yield improvement,design for manufacturability (DFM) and technologycomputer aided design (TCAD). His present interestsencompass a number of microelectromechanicalsystem (MEMS) fields (RF, bio, optical, fluidics,

and structural), and also include the optimization of semiconductor processesthrough the integration of experimental design and TCAD simulation tools.

Alan M. Gundlach received the B.Sc. degree inphysics in 1959 and the M.Sc. degree in 1965, bothfrom the University of London.

He was employed in the semiconductor industryfor 19 years from 1959, initially with A.E.I. Radioand Electronics Components Division and withTexas Instruments Ltd. working on the manu-facture of bipolar discrete devices and integratedcircuits and, finally, with Elliott Automation Mi-croelectronics Ltd. and with General InstrumentMicroelectronics Ltd. as MOS Process Development

Manager. In 1978, he was appointed to a Senior Research Fellowship at theUniversity of Edinburgh in the role of Operations Manager of the EdinburghMicrofabrication Facility, in which capacity he has been responsible fordesigning and installing processes ranging from silicon machining to completeCMOS fabrication sequences. He is currently engaged in research in the areaof interfacing biological materials with silicon-based microstructures.

J. Tom M. Stevenson received the B.Sc. degree inphysics in 1967, the M.Sc. degree in instrument de-sign in 1969 from the University of Aberdeen, U.K.,and the Ph.D. degree from the University of Edin-burgh, U.K., in 1988.

He spent five years at Ferranti Ltd, Dalkeith, as aDevelopment Engineer on moire measuring systems.In 1974, he joined the Wolfson Microelectronics In-stitute to work on the design of a pattern generatorfor the production of integrated circuits. In 2003, hewas appointed operation director at the Scottish Mi-

croelectronics Centre, University of Edinburgh. His main research interests arein optical lithography, optical measurement techniques, and chemical mechan-ical polishing (CMP).

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Dependence of Process Parameters on Stress Generation in Aluminum Thin Films