Channeling effects during focused-ion-beam micromachining of copperJ. R. Phillips, D. P. Griffis, and P. E. Russell Citation: Journal of Vacuum Science & Technology A 18, 1061 (2000); doi: 10.1116/1.582300 View online: http://dx.doi.org/10.1116/1.582300 View Table of Contents: http://scitation.aip.org/content/avs/journal/jvsta/18/4?ver=pdfcov Published by the AVS: Science & Technology of Materials, Interfaces, and Processing Articles you may be interested in Correlation of stress and texture evolution during self- and thermal annealing of electroplated Cu films J. Appl. Phys. 93, 3796 (2003); 10.1063/1.1555274 Copper device editing: Strategy for focused ion beam milling of copper J. Vac. Sci. Technol. B 20, 2682 (2002); 10.1116/1.1521736 Chemically enhanced focused ion beam micromachining of copper J. Vac. Sci. Technol. B 19, 2539 (2001); 10.1116/1.1418406 GaN focused ion beam micromachining with gas-assisted etching J. Vac. Sci. Technol. B 19, 2547 (2001); 10.1116/1.1417550 Ion channeling effects on the focused ion beam milling of Cu J. Vac. Sci. Technol. B 19, 749 (2001); 10.1116/1.1368670

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Channeling effects during focused-ion-beam micromachining of copperJ. R. Phillips, D. P. Griffis, and P. E. Russella)

Analytical Instrumentation Facility, North Carolina State University, Raleigh, North Carolina 27695-7531

~Received 10 February 2000; accepted 17 April 2000!

The rapid introduction of copper metallization for semiconductor devices has prompted increasedresearch into focused-ion-beam micromachining of copper. Studies with the aim of increasing thematerial removal rate of Cu by focused-ion-beam micromachining have been complicated byvariable micromachining behavior apparently resulting from differing Cu film morphologiesproduced by the various Cu deposition procedures. This work examined the micromachiningbehavior of thin copper films produced by physical-vapor deposition~PVD! and electroplating, aswell as single-crystal copper samples. PVD copper films were found to be preferentially texturedalong^111&, with a columnar grain structure. Channeling effects within this type of grain structureprovide a geometric enhancement of the material removal rate of 30% when the sample normal istilted 12° from the incident ion beam, regardless of sample rotation. Single-crystal~111! copper wasfound to exhibit similar material removal rate enhancement~averaged over 360° rotation! whentilted 12°, verifying that the etching enhancement observed in the PVD films is directly related totheir ^111& texture. Compared to the PVD film, electroplated~EP! copper thin films contained asignificantly more random grain orientation. Consequently, the EP films did not exhibit anyappreciable variation in material removal rate beyond the expected cosine dependence when tiltedwith respect to the incident Ga1 beam normal. Micromachining of the electroplated films, whichhave larger randomly oriented grains, results in grain decoration due to preferential etchingproducing severe micromachining-induced topography. ©2000 American Vacuum Society.@S0734-2101~00!15904-4#

I. INTRODUCTION

The ability to quickly and precisely micromachine copperis rapidly growing in importance for the semiconductor in-dustry due to the transition toward copper and away fromaluminum metallization for integrated circuits. Microma-chining of copper by mechanical or by chemical means suchas reactive ion etching remains impractical despite havingbeen studied for some time. However, new fabrication tech-niques such as dual-damascene processing1–3 allow devicefabrication by patterning the dielectric and filling trenchesand vias in one step with metal, as compared to previoustechnologies that first filled the vias and then patterned themetal lines. Although copper devices are currently in produc-tion, a practical technique for micromachining Cu using Ga1

focused-ion-beam micromachining~FIBM!, commonly usedfor failure analysis and design debug4–6 of devices, has notbeen fully developed. Not only is it necessary to increase theCu material removal rate, but it is also necessary to minimizeFIBM-induced surface roughness such as that resulting fromdifferential sputtering of regions containing different crystalorientations, to develop end-point detection techniques andto avoid redeposition problems.

It has been previously shown7 that tilting a sample rela-tive to an incident ion beam results in an increased materialremoval rate usually following a 1/cos2(u) behavior. In thiswork, the effect of sample tilt on the FIBM material removalrate of Cu from two Cu films has been studied. Cu filmsdeposited using two processes currently used by the semi-

conductor industry, physical-vapor deposition~PVD! andelectroplating, were examined and aspects of material re-moval related to their morphologies are discussed.

II. EXPERIMENT

All FIBM was performed using an FEI model 610focused-ion-beam workstation operated in the raster scanmode ~Fig. 1!. This system includes a precision five-axiscomputer-controlled stage providing control of rotation andtilt to a precision of approximately 1°. All tilts were refer-enced to the stage 0° position. Rotational positions were ref-erenced to a fiducial mark placed on the sample. A 25 keVGa1 ion beam focused to a nominal spot size of 110 nm wasused to micromachine the Cu samples. All micromachiningwas performed by rastering the ion beam over a nominal 15by 15 mm multipass raster with 50% beam overlap and 1.0ms pixel dwell time. Micromachined depths and area dimen-sions were measured using a Digital Instruments Dimension3000 atomic-force microscope~AFM! operated inTAPPING-

MODE™ and equipped with a high aspect ratio Olympus tip.The crystallography of the Cu samples was observed usingtransmission electron microscopy~TEM! and orientation wasmeasured using x-ray diffraction~XRD! @Fig. 2~a!#.

Based on information from the literature with regard tothe effect of the ion incident angle on material removalrates,7–10 material removal rate data for the PVD Cu filmwere gathered for sample tilts of 0°–50° from normal in 10°increments. Data for this range of tilts were obtained at 0°,30°, 70°, and 90° rotational positions. Due to an unexpectedCu material removal rate increase at approximately 12° tilta!Electronic mail: prussell@ncsu.edu

1061 1061J. Vac. Sci. Technol. A 18 „4…, Jul ÕAug 2000 0734-2101 Õ2000Õ18„4…Õ1061Õ5Õ$17.00 ©2000 American Vacuum Society

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for the PVD film, a more detailed study of the effect ofsample tilt was carried out on the PVD film and a single-crystal ~111! Cu sample was added to the study~single-crystal experimental details given below!. The additionalPVD data were acquired at additional tilts ranging from 0° to28° stepped in 4° increments; and also from 0° to 20°stepped in 1° increments. All data for the PVD film wereacquired using a 250 pA beam rastered over a nominal 15 by15 mm raster for 90 s.

Single-crystal~111! copper was FIB micromachined us-ing a 500 pA beam rastered over a nominal 15 by 15mmraster for 180 s. Micromachined depths for the single-crystalsamples were measured using AFM. Craters were milled toapproximately 300 nm to reduce the effects of surface, andthus crater bottom roughness effects on the AFM depth mea-surements. The angle and rotational dependence of the mi-cromachining material removal rate for the single crystal was

investigated by tilting this sample from 0° to 28° with 4°steps at rotational positions ranging from290° to 90° at 10°steps with the remaining 180° of rotation covered in 30°steps.

Electrodeposited~EP! Cu film samples were FIB micro-machined using a 250 pA beam rastered over a nominal 15by 15 mm raster for 120 s. The angle and rotational depen-dence of the micromachining material removal rate for theelectrodeposited Cu film was investigated by tilting thissample from 0° to 28° with 4° steps at rotational positions of0°, 45°, and 90°.

III. RESULTS AND DISCUSSION

A. PVD copper thin film

FIBM material removal rates obtained for the PVD Cufilm are presented in Fig. 3. The base-line~0° tilt! FIBMmaterial rate for PVD Cu films was 0.76mm3/nC ~less than 1ML/pass!. Although the well-documented cosine-dependentincrease in material removal rate might have been expected,the observed material removal rate for the PVD samplepeaked at 1.1mm3/nC at approximately 12° tilt~Fig. 3!. Be-yond 12° tilt, the material removal rate declined until a tilt ofabout 30° was reached. Increased tilt resulted in a materialremoval rate which followed the predicted cosine depen-dence.

Although the maximum observed material removal rateobtained was observed at 50°~the maximum tilt angle usedin this experiment; data not shown!, this extreme tilt angle isnot practical for integrated circuit debugging and/or modifi-cation due to the inability to sufficiently control the micro-machined volume with respect to device dimensions and dueto the micromachining-induced topography~21.5 nm rms!.At the optimum tilt angle of 12°~i.e., the angle at which thehighest sputter rates were obtained!, the PVD Cu film, whichhad an as-received surface roughness value of 2.3 nm rms,had an after-micromachining roughness of 12.3 nm rms~compared to the 0° tilt after-micromachining roughness of5.6 nm rms!. Micromachining of the PVD Cu film at differ-

FIG. 1. Schematic of raster scan mode showing the ion beam following aserpentine pattern. Pattern refresh times on the order of milliseconds pro-duce a periodic dose for each pixel. Ion-beam spot size and pixel overlap areexaggerated for simplicity.

FIG. 2. XRD data of thin-film samples showing the copper film preferen-tially textured along$111% for PVD ~a!, while EP~b! shows copper crystalorientations in addition to$111%.

FIG. 3. Sputter rate of PVD thin-film copper at various tilt angles withrespect to FIBM normal.

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ent rotational angles resulted in no measurable change in thematerial removal rates.

In an effort to document the channeling-inducedeffect8,11–13 on the FIBM material removal rate with tiltangle, the PVD Cu thin-film sample was examined usingx-ray diffraction and TEM. Examination of a PVD film crosssection by TEM showed approximately 100 nm columnargrains and x-ray diffraction results indicated a strong prefer-ential orientation in the copper film along^111& @Fig. 2~a!#.Since channeling is known to affect the sputtering, and thusthe micromachining rate of many materials, it was hypoth-esized that channeling of the Ga1 ions in the preferentially^111& oriented PVD film affects the material removal ratewith the maximum rate occurring where the minimum chan-neling, averaged over all grain rotational orientations, oc-curs, i.e., at 12° tilt.

B. Single-crystal „111… copper

To understand the behavior of the FIBM material removalrate for the^111& textured PVD Cu film, the FIBM materialremoval rate of~111! single-crystal copper was examinedwith respect to tilt and rotation. Material removal rate mea-surements were performed for tilts of 0°–28° in 4° incre-ments and repeated for 360° sample rotation. The resultingFIBM crater volumes were then measured by AFM. Asexpected,8 FIBM of the single-crystal Cu over the range oftilt and rotational angles resulted in significant variations ofthe material removal rate. For tilts of 0°–8°, rotation of thesingle-crystal Cu sample produced minimal changes in ma-terial removal rate. The most dramatic changes in materialremoval rate with sample rotation were observed for 16° and28° tilts ~see Fig. 4!. These variations can be explained interms of channeling, or lack thereof, through the Cu crystallattice.8,11–13 Plotting FIBM micromachining rate versussample rotation angle for these two tilts~16° and 28°! illus-trates the threefold symmetry of$111% fcc structures~Fig. 4!with minimum ~maximum! material removal rates occurringwhen the Ga1 beam impinges on the sample in a direction of

maximum ~minimum! channeling.8 The material removalrate for the single crystal ranges from a constant 0.70mm3/nC ~0° tilt! to sinusoidal variations of 0.50–1.70mm3/nC ~28° tilt! ~Fig. 4!.

The material removal rate behaviors of the PVD andsingle-crystal copper systems are related as follows. ThePVD Cu sample is composed of a large number of single-crystal columnar grains having111& orientation with respectto the sample normal but which are rotated with respect toeach other. Thus, the material removal rate obtained from theCu PVD sample is equivalent to the average material re-moval rate~averaged over 360° of rotation! obtained fromthe ~111! Cu single crystal~Table I!. To verify this conjec-ture, the material removal rates obtained from the Cu singlecrystal over 360° rotation were averaged for each tilt angle.Figure 5 compares the average single-crystal material re-moval rates determined in the above manner at the varioustilt angles with the corresponding tilt angle data obtainedfrom the PVD sample. Both the functional form and themagnitudes of the variation of the material removal rateswith tilt angle match within experimental error. The closematch of these data provides strong evidence that the in-crease in the Cu PVD film material removal rate at 12° tilt isthe direct result of the PVD film’s111& orientation.

FIG. 4. Sputter rate of~111! single-crystal copper. Channeling effects causeperiodic fluctuations for the two higher tilts shown~16° and 28°! as com-pared to 0°. Rotation is relative to an arbitrary fiducial mark made on thesample.

TABLE I. Averaged sputter rate of~111! single-crystal copper for differenttilts.

Tilt angle ~deg! Sputter rate (mm3/nC)

0 0.704 0.818 1.07

12 1.1316 0.9220 0.7424 0.8128 1.15

FIG. 5. Sputter rate vs tilt for the~111! single crystal~averaged! and thePVD thin film.

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C. Electrodeposited copper thin film

The FIBM material removal rate for the electrodepositedCu film was measured over a tilt range of 0°–28° by 4°increments. Contrary to either the PVD Cu film or the single-crystal Cu~see Fig. 6!, the FIBM material removal rate ofthe electrodeposited Cu shows only the predicted cosinevariation with sample tilt. No variation in the removal ratewith rotation was detected. The lack of variation in the ma-terial removal rate of the EP Cu film with tilt and/or rotationangle can be explained by the absence of preferential textur-ing. XRD examination of this film showed copper peaks for^111&, ^110&, and ^100& @Fig. 2~b!#, verifying this lack ofpreferential texturing as compared to the PVD sample. Thisobserved texture behavior is in agreement with timestudies14,15 of EP Cu films evaluating recrystallization of thegrains~orientation and size! after deposition; however, theseresults should not be considered to be a general property ofall EP films.

FIBM of the EP copper resulted in strong FIBM-inducedtopography in the micromachined craters~Fig. 7!. The 4.4

nm as-received rms surface roughness of the EP Cu film wasincreased to 21 nm rms after micromachining. This behavior,which differs significantly from the PVD Cu films~5.6 nmrms after micromachining!, appears to result from differen-tial sputtering of the EP Cu film. The differential sputteringresults from the differing crystalline alignments of the Cugrains composing the EP film with respect to the FIBMbeam. Figure 7, an AFM micrograph of a representative mi-cromachined EP crater, illustrates the extent of differentialsputtering and the resulting ion-beam-induced topography.Crystal lattice steps representing the^111& threefold symme-try for fcc crystal systems are clearly visible. The magnitudeof the micromachining-induced roughness makes practicalend-point detection during device modifications by FIBMdifficult or impossible. Use of FIBM for EP films requiresdevelopment of new Cu FIBM techniques which do not re-sult in the differential material removal seen in Fig. 7.

IV. CONCLUSION

In this work, the effect of sample tilt on the FIBM mate-rial removal rate of Cu from two Cu films currently in use bythe semiconductor industry was investigated. The columnargrains forming the PVD Cu films were found to be preferen-tially textured in the 111& direction. The change in the ma-terial removal rate for the PVD Cu film as the sample wastilted with respect to FIB normal did not follow a cosinedependence, but exhibited a local maximum at approxi-mately 12° tilt. Based on data obtained by micromachining a$111% Cu single crystal, this behavior was determined to re-sult from the highly textured nature of the PVD Cu film.When the PVD film was micromachined at 12° tilt, a 30%increase in micromachining rate versus the nontilted casewas achieved and little micromachining-induced roughnesswas observed. Due to the significantly more random orienta-tion of the grains of the EP film as compared to the PVDfilm, the micromachining material removal rate for the EPsample did not appear to deviate from a cosine dependence.Due to the random orientation of the grains, micromachiningof the EP film resulted in significant differential sputtering,which produced roughness sufficient to make end-point de-tection difficult. Additional work is required to develop aFIBM technique for the EP film which will reduce or preventthe formation of this roughness.

ACKNOWLEDGMENTS

The authors thank R. D. Day~Los Alamos National Labo-ratory! for providing the single-crystal Cu samples and C. B.Vartuli ~Lucent Technology! for providing the EP thin Cufilms.

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FIG. 6. Sputter rate of the EP thin-film copper vs sample tilt.

FIG. 7. AFM topographic image~with height scale! of the EP thin filmwithin a FIBM crater micromachined at 12° tilt showing lattice steps andgrain decoration resulting from differential sputtering.

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