Analysis of Microelectronic MaterialsAndreas Stadler

HDU Hochschule DeggendorfUniversity of Applied Science

andreas.stadler@stud.hdu-deggendorf.de

Günther Benstetterand Werner FrammelsbergerHDU Hochschule DeggendorfUniversity of Applied Science

Abstract—Successful microelectronic manufacturing is closelylinked to appropriate tools for material analysis. In recent yearscopper has been increasingly used for metallization and wirebonding in microelectronic manufacturing. Scanning electronmicroscopy, energy dispersive x-ray spectroscopy and Fouriertransform infrared spectroscopy have been used to analysecopper oxide films. It is shown that different stages of copperoxidation may be monitored. In addition, attenuated total reflec-tion Fourier transform infrared spectroscopy was used to analysepolyethylene films. Utilizing the interference pattern thicknessassessment was not possible. However, by use of the raw dateconclusions may be drawn with respect to the film thickness.

I. INTRODUCTION

The scanning electron microscopy (SEM) is used for mag-nification which can not be archieved by a light microscope.If the size of details is about the wavelength of visiblelight an optical microscope can not be used any more. Anelectron microscope may be employed in this cases. The firstelectron microscopes were transmission electron microscopes(TEM). Some years later, in 1942, the SEM was invented[6]. The ongoing miniaturisation asks for microscopy methodsshowing structures smaller than 1 μm. Electron microscopy isan appropriate technique.

The use of copper increases in semiconductor industry.Unintentional oxidation causes problems during the manufac-turing. Hence there is a necessity to investigate the oxidationprocess of copper.

Infrared spectrometer have been used for decades. In the70s powerful computer initialised the spread of Fourier trans-form infrared spectrometer (FTIR). They are faster and thecorrection of atmospheric disturbances is easier. The collectedspectra can be compared automatically, this speeds up theanalysis [7].

The infrared spectroscopy can be used to measure thethickness of epitaxial layers [3] and monomolecular layers [5]which is from interest in wafer processing. A special reflectionmode and a suitable signal processing are necessary. The lightbeam is reflected twice, on the upper surface and also onthe boundary surface between the upper layer and the layerbeyond. The two parts of the beam interfere afterwards andbecause of different optical path length there is a phase shift.An interference pattern is formed (figure 11). Usually a veryacute angle between light beam and surface is used [3].

II. EXPERIMENTAL

A. Instrumentation

Analyses were carried out with FTIR and SEM.1) Fourier transform infrared spectroscopy: For the infra-

red spectroscopy a Thermo Scientific Nicolet Nexus 470 wasused. It collects the spectrum from 4000 to 400 cm−1 . Theapparatus has a glow bar source emitting infrared light at thetemperature of 1300 K. The power of the emitted infrared lightis 10 W. In a Michelson interferometer a interference patternis formed which is collected by a DTGS1 sensor. The mirrorsare made of polished metal and the beam splitter consistsof potassium bromide (KBr)2 which avoids influence on theinfrared light.

2) Attenuated total reflection Fourier transform infraredspectroscopy: This is a special way for archieving a spectrum.The infrared beam is transmitted through a crystal by totalreflection at the boundary surface several times. The sampleis pressed on this surface. Due to the effect of transcendentwaves the light interacts with the sample. The analysis of theinfrared light is performed by an ordinary FTIR as describedearlier. Advantages of the Attenuated total reflection (ATR)measurement are less atmospheric effects and an improvedspectral pattern. The measurements were carried out by aPerkin Elmer Spectrum 100 FTIR with an ATR unit.

3) Scanning electron microscopy: Analysis were carriedout with a Zeiss Ultra 55 SEM. Electrons are emitted by azirconium oxide cathode, are accelerated by an electric fieldand focused by a magnetic field. The electron beam interactswith the surface and is detected. The operator can choosebetween a detector beside the sample and a further one onthe top next to the magnetic lens, called in-lens-detector. Thefocal depth varies on both detectors.

By use of a computer an image is created that can enlargethe sample’s view up to 1 million times [6]. Inside the SEM ahigh vacuum is needed due to avoid scattering of the electronbeam by atmospherical molecules. The scattering would leadto decreased image contrast.

4) Energy dispersive x-ray spectroscopy: For analysing thecomposition of elements in a compound energy dispersive x-ray spectroscopy (EDX) was employed. It is combined withthe SEM. The SEM gun directs accelerated electrons onto thesample. The atoms are stimulated and emit x-radiation. The

1Deuterated triglycine sulfate2KBr is a salt getting transparent when extruded

EDX detector detects the x-ray quanta which have a specificamount of energy.

The electron beam exites electrons from the lower orbits.These are replaced by electrons from higher orders. The energydifference of the electrons between higher and lower orbit isemitted by an single x-ray quantum. The magnitude of theenergy released is typical for a specific element. The amountof energy depends on the nuclear charge and is associatedwith one specific element. Thus every element can be detectedclearly.

For further details please refer to Flegler et al. [6].

B. Measuring of copper oxide

Measurements were carried out on the 5 μm copper coatingon top of a silicon wafer. The copper film was analysed before,during and after the oxidation process. The oxidation tookplace in four steps. It was logged by SEM images, done withan acceleration voltage of 5 kV.

The copper oxides were analysed by FTIR in reflectionmode and ATR-FTIR. Ardelean et al. [10] identified thecharacteristic absorbtion bands for CuO at about 410, 510 and610 cm−1 .

1) IR spectrum of copper oxide: The FTIR spectrum ofcopper oxide was collected in reflection mode. Due to thesmooth surface a reflection factor of nearby 100 % couldbe archieved. In figure 1 at 1050 cm−1 the spectrum showsan absorption line. From interest is that the absorption is nodecrease of the basic line, which would be expected. It is anincrease. The phenomenon is due to the Christiansen effect asdescribed by Liu et al. [1].

2) SEM image of copper oxide: Grown oxides on a copperlayer on α-Si wafers were imaged by use of a SEM. Thelateral size of larger crystallites was about 2 μm. The waferwas broken into pieces which were heated in an oven atsubsequent temperature steps up to about 270◦ C. The pieceswere analysed by a SEM. The copper oxide was also analysedby EDX.

The oxidation of copper is associated with a color change.At every optically recognizable change a measurement wasprocessed.

C. Measuring of synthetics films

1) Thickness assessment: The FTIR was employed to as-sess the thickness of synthetic layers on top of metal sub-strates. Polypropylene thin films were put on aluminium andmeasured in reflection mode. Use has been made of the raredata before Fourier transform processing and of the Fouriertransformed data.

III. RESULTS AND DISCUSSION

A. Copper and copper oxides

Figure 3 shows SEM image of the copper surface beforethe oxidation. There was only native oxide on the surface. Atoxidation step 1 (Fig. 4) the surface turned reddish-brown. Itseemed to developed Cu2O. The oxidation started at the grainboundaries.

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Fig. 1. The figure shows the FTIR spectrum of the copper surface, collectedin reflection mode. At 1050 cm−1 the impact of the christiansen effect canbe seen. The bands at 2380 cm−1 results from the influence of athmosphericcarbon dioxide.

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Fig. 2. The figure shows four separately collected FTIR spectra. The oneon the top is the background which was taken for reference, the disturbingbands have to be attended in the spectra below. The second spectrum shows acopper surface at a early oxidation status, the third one a completely oxidatedcopper. The spectrum below is from unoxidated copper.

Then the sample was heated at the next temperature step.The surface changed, according to literature it is probable thatCu3O2 was formed. This second step is illustrated in Figure5. Subsequent it was oxidized again. The surface turned darkwhich was reported to be an indication for CuO (Fig. 6).At step 4 the metallic lustre vanished. FTIR measurementindicated a band at 600 cm−1 (Fig. 8), it seemed, CuO hadbeen grown on the surface. The SEM image refering to thefinal step 4 is shown in Figure 7 [8] [9].

From interest is that the surface topology changed with theongoing oxidation process. Figure 3 shows the typical surfaceof copper, the grain boundaries can be seen clearly. After theoxidation process (Fig. 7) the grain boundaries have nearlyvanished.

Fig. 3. SEM image of copper

Fig. 4. SEM image of copper at beginning of oxidation, step 1

The copper film was analysed by EDX in addition. Figure9 was made at the early oxidation process, there is alreadyoxygen in the copper film and on its surface. After theoxidation process Figure 10 was collected, it shows abouttwice as much oxygen than Figure 9 collected before theoxidation. Thus the amount of oxygen was growing duringthe oxidation process. This seems reasonable because theoxidation process includes oxygen during the time.

B. Synthetic films thickness

The FTIR interferogramme (Fig. 12) shows a single fluc-tuation on both sides of the central peak. The thicker thefilm is, the larger is the distance between the swinging andthe central peak. It indicates that the number of datapointsbetween the swinging and the central peak increases. Figure12 views the right half side of the complete interferogrammes.The abscissa shows the data points measured while the mirrorwas moving. Thus on the abscissa the mirror distance inside

Fig. 5. SEM image of a copper surface at step 2

Fig. 6. SEM image of a copper surface at step 3

Fig. 7. SEM image of a copper oxide surface at step 4

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Fig. 8. IR sprectrum of CuO, collected by an ATR-FTIR equipment,corresponding to Fig. 7

Fig. 9. EDX analysis of the copper film refering to Figure 4

Fig. 10. EDX analysis of the copper film refering to Figure 7

Fig. 11. The measuring of film thickness results in a interference pattern inthe IR spectrum. A thicker film causes a oscillation with shorter periods [3].

the Michelson interferometer is outlined. The ordinate showsthe signal strength.

There could also be seen an effect in the Fourier transformeddata. The spectrum of the thicker film shows broader bands(Fig. 13).

Walder and Boyle [3] discovered that the FTIR measuringof film thickness results in an interference pattern (Fig. 11).The period of this pattern depends on film thickness. Thisresult could not be repeated. Though the oscillations could berepeated in further measurements, there was no decrease ofthe period for thicker films as it was expected. One reasonmay be that the angle of the beam was not optimized becausethe equipment had no adjustable beam incidence angle.

IV. CONCLUSION

The oxidation process of copper was analysed at differentstages. SEM, EDX, FTIR and ATR-FTIR were used foranalysing purposes. SEM showed the change of the surfacetopology during the oxidation process. The growth of copperoxide could be seen clearly. EDX proved the composition.There was a increase of oxygen during the heating process,no further elements were bound in. FTIR in reflection modeworks on CuO films after the completed surface oxidation foridentifing CuO.

The measurement of film thickness can be done, but forquantitative and wide-range measurements special equipmentis needed to adjust the beam incidence angle [3] [4]. Theequipment used for this work can identify thickness in therange from 100 to about 250 μm.

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Fig. 12. The figure shows the raw data points collected by the FTIR. Onlythe right half side is printed. The measuring was done at a polyethylene filmon aluminium. The thicknesses are top down 0, 40 μm, 100 μm and 200 μm.

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Fig. 13. The spectrum on the top results from a thin synthetics film, the onebelow from a thicker one.

REFERENCES

[1] Xiao-dong Liu, Lan-tian Hou, Hui-tian Wang, Investigations into the mid-infrared Christiansen effect of the dispersive materials. Infrared Physics& Technology, Vol. 43, 2002, pp. 401 – 405.

[2] Jan Albers, Grundlagen integrierter Schaltungen, Bauelemente undMikrostrukturierungen Hanser Fachbuchverlag, Leibzig, 2007.

[3] Fred Walder and Ross Boyle, FT-IR Measurement of Epitaxial Film Thick-ness Applications, Application Note: 50639 Thermo Fisher Scientific,Madison, WI, USA, 2008.

[4] K. Molt, Chemical Characterization of Thin Films on Metal Surfaces withthe Aid of Infrared Spectroscopy Bodenseewerk Perkin-Elmer & Co.GmbH, berlingen, Germany, 1984.

[5] Yuhei Shimoyama, Growth process of poly (3-dodecyl thiophene) self-assembled monolayers: FTIR-RAS and gravimetric studies, Thin SolidFilms, 464 – 465, 2004, pp. 403 – 407.

[6] Stanley L. Flegler, John W. Heckman Jr., Karen L. Klomparens, Elektro-nenmikroskopie Spektrum Akademischer Verlag Heidelberg.

[7] Helmut Günzler, H. Michael Heise, IR-Spektroskopie. Eine Einführung.VCH Verlagsgesellschaft mbH, Weinheim, 1996.

[8] D. L. Cocke, R. Schennach, M. A. Hossain, D. E. Mencer, H. McWhinney,J. R. Parga, M. Kesmez, J. A. G. Gomes, M. Y. A. Mollah, The low-

temperature thermal oxidation of copper, Cu3O2, and its influence onpast and future studies. Vacuum, Vol. 79, 2005, pp. 71 – 83.

[9] D. L. Cocke, D. E. Mencer, M. A. Hossain, R. Schennach, M. Kesmez, J.R. Parga, D. G. Naugle, Investigation of the metal-oxide buried interfacialzone with linear sweep voltammetry. Journal of Applied Electrochemistry,34, 2004, pp. 919 – 927.

[10] I. Ardelean, Simona Cora, Dorina Rusu, EPR and FT-IR spectroscopicstudies of Bi2O3-B2O3-CuO glasses. Physica B, 403, 2008, pp. 3682 –3685.