Surface Science 599 (2005) 107–112

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A scanning photoelectron microscopy studyof AlN/SixNy insulating stripes

Chien-Hsun Chen a, Shih-Chieh Wang a, Chung-Ming Yeh a,Jennchang Hwang a,*, Ruth Klauser b

a Department of Materials Science and Engineering, National Tsing Hua University, 101, Sec. 2, Kuang-Fu Road,

Hsinchu 30043, Taiwan, ROCb National Synchrotron Radiation Research Center, Hsinchu 30043, Taiwan, ROC

Received 1 July 2005; accepted for publication 21 September 2005Available online 27 October 2005

Abstract

Scanning photoelectron microscopy (SPEM) has been applied to measure a series of Al 2p images from a 900 nmthick AlN insulating overlayer covering a SixNy mesh on the Si substrate. The Al 2p core level exhibits an energy shiftthat is induced by the local charging. This energy shift depends on the thickness of the AlN/SixNy stripe, which is deter-mined to be �0.12 eV/nm. The Al 2p SPEM images from the AlN/SixNy stripes change with different kinetic energies ofthe photoelectrons. The line width of the AlN/SixNy stripe varies from 4.5 to 11 lm. A ‘‘spatial-charging’’ model is pro-posed to explain those changes in the images. The present study shows that small local variations in insulating thicknesscan be monitored by the SPEM non-destructive technique.� 2005 Elsevier B.V. All rights reserved.

Keywords: Photoelectron spectroscopy; Scanning photoelectron microscopy

1. Introduction

Scanning photoelectron microscopy (SPEM)[1–3] is a surface imaging technique, which is ableto characterize the chemical information of a sur-face with typical spatial resolution of �90 nm.

0039-6028/$ - see front matter � 2005 Elsevier B.V. All rights reserv

doi:10.1016/j.susc.2005.09.041

* Corresponding author. Tel.: +886 3 5722577; fax: +886 35722366.

E-mail address: jch@mx.nthu.edu.tw (J. Hwang).

With its non-destructive character, SPEM hasdrawn more and more attention in the fields ofsemiconductor devices [4–6] and self-assembledmonolayer based molecular electronics [7,8].SPEM, similar to the traditional photoemissionspectroscopy [9], exhibits photoelectron energyshift in characterizing insulating surfaces.Recently, Shin and Lee [10] have applied SPEMto probe the microstructure of multilayer metalinterconnects passivated with SiO2 insulating layers.

ed.

108 C.-H. Chen et al. / Surface Science 599 (2005) 107–112

The Si 2p core level exhibits energy shift due tolocal charging in the SiO2 insulating layer. Theenergy shift of Si 2p increases with the insulatinglayer thickness, which provides useful depth infor-mation. Shin et al. [11] also have performed SPEMstudies of photoresist/Si, photoresist/Au/Si, andSiO2/Si structures. The photoelectron energy shifthas been confirmed to depend on the thickness ofthe insulating layer due to local charging.

The SPEM image is expected to be influencedby the energy shift due to local charging. In thisarticle, we will demonstrate the variations ofSPEM images of AlN/SixNy insulating stripesdue to local charging. Al 2p images have beentaken at different kinetic energies of the photoelec-trons for a spatially defined SixNy mesh embeddedin an AlN coverlayer. The magnitude of the Al 2penergy shift is indeed related to the thickness of theAlN/SixNy stripe and results in the change of theline width of the stripe in the SPEM images taken.A spatial-charging model is proposed to explainthese variations of line widths.

Fig. 1. (a) Sketch of the SixNy mesh pattern on the Si(111)substrate. (b) FESEM image of an AlN/SixNy stripe in cross-sectional view.

2. Experimental details

The SixNy mesh pattern is sketched in Fig. 1(a).A layer of SixNy of 1000 A thick was deposited onthe Si(111) substrate in a plasma enhanced chem-ical vapor deposition (PECVD) system at 180 �C.A mesh pattern of SixNy was then producedthrough a photolithographic process using wetetching. The mesh pattern contains ‘‘window’’squares of 80 · 80 lm2 in size, surrounded bySixNy stripes of 10 lm in width. The Si(111) sam-ples with SixNy mesh pattern were rinsed in alco-hol and de-ionized water before being introducedinto a reactive RF induced couple plasma (ICP)sputtering system to deposit an aluminum nitride(AlN) layer at a pressure of 2 mTorr. The N2/Armixed gas used in the AlN deposition was keptat a flow rate ratio of 6/2 and the substrate temper-ature was 350 �C. The coil power and the RF gunpower were fixed at 180 W and 450 W, respec-tively. After the AlN deposition, the film thicknessof the AlN layer of about 900 nm was measured bya JEOL JSM-6330F Field emission scanning elec-tron microscope (FESEM). The AlN layer with

the embedded SixNy stripes is called AlN/SixNy

stripe-pattern and the FESEM image of an AlN/SixNy stripe is displayed in Fig. 1(b). It is obviousthat the etched SixNy stripe has no sharp edges anda rather trapezoidal shape. The Si(111) samplespatterned with AlN/SixNy stripes were then trans-ferred into the SPEM station for imaging. This sta-tion is located at the U5-SGM undulator beamlineof National Synchrotron Radiation Research Cen-ter in Hsinchu, Taiwan. The selected photon beamof an energy of 480 eV was focused by a Fresnelamplitude zone plate with a theoretical spatial res-olution of about 70 nm. A hemispherical electronenergy analyzer with a 16-channel detectionscheme was employed to collect Al 2p photoelec-trons, which resulted in the simultaneous acquisi-tion of 16 SPEM images with an image size of40 · 40 lm2 (100 pixel · 100 pixel) and a dwellingtime per pixel of 50 ms. Note that all the imagesand spectra were stabilized during data taking.

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3. Results and discussion

Fig. 2 shows a set of 16 simultaneously acquiredAl 2p SPEM images of an AlN/SixNy ‘‘cross-stripe’’ that is a part of the AlN/SixNy stripe-pat-tern. The SPEM images were taken at a kineticenergy range between 383.4 eV and 395.4 eV, wherethe Al 2p emission is located. Two interesting find-ings can be extracted from the SPEM images. First,the Al 2p SPEM images vary drastically withkinetic energy. When the energy increases from383.4 to 395.4 eV, the images change from the

Fig. 2. A set of 16 simultaneously acquired Al 2p SPEM images from40 · 40 lm2 (100 pixel · 100 pixel).

‘‘stripe-lighted’’ (>384.2 eV) to the ‘‘stripe edge-lighted’’ (385.8–392.2 eV) and the ‘‘window-lighted’’ (<393 eV). Second, the observed line widthof the AlN/SixNy stripe gradually increases whenthe kinetic energy increases. The narrowest linewidth is 4.5 lm at 383.4 eV (see Fig. 3). As thekinetic energy increases to 394.6 eV, the line widthbecomes wider and is estimated from the intensityprofile to be 11 lm (see Fig. 4).

As mentioned earlier, in the work of Shin et al.,the photoelectron energy shift increases as theinsulating layer thickness increases, due to local

AlN/SixNy ‘‘cross-stripe’’ on Si(111). The size of the images is

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charging in the insulating layer [10,11]. Similar tothis, the changes in emission intensity of the Al2p SPEM images in Fig. 2 are considered to resultfrom local charging on the oblique edge area of theAlN/SixNy cross-stripe because of its insulatingcharacter with resistivities of AlN (�1014 X cm)and SixNy (�1015 X cm) [12,13]. In order to char-acterize the extent of local charging on the AlN/SixNy cross-stripe, C 1s core level emission fromresidual carbon contamination has been utilizedsince no chemical reaction between residual car-bon and the AlN layer is expected. The charging-induced energy shifts of C 1s core levels are shownin Fig. 5 where the kinetic energies of C1s photo-electrons are measured to be 174 eV and 185 eVat positions A and B, respectively marked inFig. 1(a). This indicates that the residual charge

Fig. 3. Al 2p SPEM image from AlN/SixNy ‘‘cross-stripe’’ at akinetic energy of 383.4 eV and the corresponding intensityprofile along the dashed line.

Fig. 4. Al 2p SPEM image from AlN/SixNy ‘‘cross-stripe’’ at akinetic energy of 394.6 eV and the corresponding intensityprofile along the dashed line.

density at point A is higher than that at point Bduring photoexcitation, implying that the localresistance at point A is higher. The local resistancedepends theoretically on the resistivity and thethickness of the insulating layer at different spatialpositions. At point A, the insulating layer is thecombined AlN overlayer with the SixNy stripe,whereas at point B the insulating layer only con-sists of AlN. It is expected that the local resistanceat point A is higher since it has an additionalembedded SixNy stripe.

A ‘‘spatial-charging’’ model is proposed in orderto explain the intensity variations in the Al 2pSPEM images. Nine spatial points were selectedalong the AlN/SixNy sample, schematically indi-

Fig. 6. (a) An exaggerated sketch of an AlN/SixNy stripe in across-sectional view. (b) Energy distribution curves (EDC) ofthe Al 2p core level taken at the nine selected points in Fig. 6(a).

Fig. 5. C1s spectra at positions A and B in Fig. 1(a).

C.-H. Chen et al. / Surface Science 599 (2005) 107–112 111

cated in Fig. 6(a), to demonstrate the variation ofcharging at these points. The energy distributioncurves (EDC) of Al 2p core level at these nine spa-tial points can be extracted from the SPEM imagesand are stacked together in Fig. 6(b). The Al 2penergy position remains about the same for thepoints a–c on top of the stripe, then it shifts towardhigher kinetic energy for the edge area where theSixNy embedded layer is reduced in thickness (spa-tial points d–f), and finally reaches at a value whichremains unchanged for the points from g to i onareas without the SixNy embedded layer. Overall,the Al 2p core level shifts from 383.4 eV at pointc to 395.4 eV at point g, which is induced by theresidual charge at these spatial points. The SPEMimages changing from ‘‘light’’ to ‘‘dark‘‘ cross-stripes in Fig. 2 is related to the energy shifts ofAl 2p and can be easily realized by moving the ver-tical dash-line to different kinetic energy inFig. 6(b). First, look at the vertical dash-line at383.4 eV. The maximum intensity of Al 2p occursat points between a and c, which results in thewidth of the ‘‘light’’ cross-stripe taken at 383.4 eVin Fig. 2. When the dash-line moves toward385 eV, three major changes occur. First, the‘‘light’’ cross-stripe becomes darker due to reduc-tion of the Al 2p intensity at points a–c. Second,the stripe edge becomes brighter since the maxi-mum intensity of Al 2p occurs at the stripe edge(point d). Third, the line width of the stripebecomes wider since the bright edge moves frompoint c–d. When the dash-line moves furthertoward 389 eV, the center part of the stripe

becomes very dark that is due to negligible intensityof Al 2p between a–d. The bright stripe edges havelarger spacing, which is due to the shift of Al 2pfrom point d to e. The image thus changes from‘‘light’’ cross-stripe to ‘‘edge-lighted’’ one. Finally,the ‘‘light’’ stripe edge disappears at 394.6 eV dueto the negligible intensity of Al 2p at point f. TheAl 2p SPEM image becomes a ‘‘dark’’ cross-stripe.

The intensity patternof the SPEMimages reflectsthe trapezoidal shape of the SixNy stripe. The linewidth of the AlN/SixNy stripe measured from theintensity profiles confirms this picture. The imagetaken at 394.6 eV shows the largest stripe line widthof about 11 lm that corresponds to points g–i,. Thisis essentially the width of the larger base line of thetrapezoid at the interface between SixNy and the Sisubstrate. The image taken at 383.4 eV has thesmallest stripe linewidth of 4.5 lmthat correspondsto points a–c. This is the width of the top line of thetrapezoid at the interface between SixNy and AlN.

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The Al 2p images and energy shifts at points d–freflect the gradual changes in combined thicknessof the insulating AlN/SixNy stripe. The energy shiftof Al 2p per unit thickness is accordingly deter-mined to be �0.12 eV/nm.

4. Conclusions

The SPEM microscope has been applied to col-lect the Al 2p images of an AlN insulating layercovering a SixNy mesh. The Al 2p core level showsenergy shifts at different spatial positions resultingfrom the local charging on the combined AlN/SixNy insulating layers during SPEM measure-ments. The Al 2p SPEM image varies with thekinetic energy of the photoelectrons. The observedchange in line width of the AlN/SixNy stripe in theSPEM images at different kinetic energies can becorrelated with local charging on the geometricalshape of the combined AlN/SixNy insulatingstripe. It could be demonstrated that small localvariations in insulating thickness can be monitoredby the SPEM non-destructive technique.

Acknowledgement

The work is supported by the National ScienceCouncil, ROC through project No. NSC 93-2212-M-007-007.

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