Dynamical Effects in High Resolution Topographic. Imaging of Electronic Devices

Transcription

1 203 Dynamical Effects in High Resolution Topographic Imaging of Electronic Devices W.T. Beard Laboratory for Physical Sciences College Park, Md K.G. Lipetzky Johns Hopkins University, Baltimore, Md and R.W. Armstrong University of Maryland, College Park, Md Abstract Improved resolution of diffraction topographic imaging has been pursued with the application of Line Modified - Asymmetric Crystal Topography (LM-ACT) technique to partially processed or completed Si crystal-based parametric test chip devices. This apparatus provides beam divergences comparable to that obtained in a synchrotron beam, while still being a compact affordable laboratory instrument. Adaptation of our system for topography of the full width of 3-inch GaAs wafers, employing { 224) reflections with Cu radiation, required changing the asymmetric beam conditioning Si crystal from being cut 11 deg off [ 11 l] towards [ 1 lo] to 5.5 deg off [ 11 l] towards [ 1 lo]. This change lead to a comparison of dynamical theory based predictions for diffraction contrast and spatial resolution in images produced with the differently cut beam conditioner cases and a specimen crystal of parametric test chip devices. Predicted improvement of images at micrometer-scale dimensions occurred for images obtained when using the 5.5 deg cut beam conditioner which has a smaller perfect crystal reflecting width. Otherwise both crystal conditioners are shown to give high spatial resolution of such device features as implantation and component strains, geometrical step height influences and effective diffraction verses absorption penetration depth influences. At this stage of development, limits on further improvements in spatial resolution are attributed to effects of the x-ray penetration depth and the practical characteristics of the nuclear emulsion films employed to record the images. Introduction Conventional Asymmetric Crystal Topography (ACT) systems employ a standard spot source on the order of lmm (H)x lmm (W) with an asymmetric cut beam conditioning crystal in the beam expansion orientation [ 11. The spatial resolution of a horizontal line on the sample is determined by the geometric vertical divergence of the probe beam. With a normal source/conditioner distance of -300 mm and conditioner/sample distance of -430 mm, the vertical divergence of the beam at the sample in an ACT system is arcsec. Assuming a sample/film distance of 20 mm, as is convenient in the ACT geometry without interfering with

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3 204 the incident beam [ 11, this vertical beam divergence results in an effective horizontal line spatial resolution of a=( 1/73O)x20mm - 27 microns. This system derives its high spatial resolution of vertically oriented lines and contrast control from the narrow diffraction peak profile of the beam conditioner (typically 6-12 arcsec). The corresponding resolution of vertical lines is 1.2 microns. From these simple calculations, we see that the standard ACT system works well to image vertically oriented features, analogous to angular resolution with a diffractometer, but will be severely limited on highly perfect crystals with variably oriented strains induced via device processing where typical device geometries are < 3 microns. For imaging small device geometries in highly perfect crystals, the as yet unconventional Line Modified Asymmetric Crystal Topography (LM-ACT) system uses the line source in a horizontal (not vertical) orientation [2], see Fig. 2a and later explanation. The horizontal line spatial resolution is now set by a projected source height which is now 0.4mm x sin(6 deg)=42 microns. Maintaining the same source/conditioner and conditioner/sample distances as in ACT, the corresponding horizontal line spatial resolution using LM-ACT with sample/film distance of 2Omm is a=(o.o42/730)x20mm microns. The vertical line spatial resolution is still controlled by the beam conditioning crystal Bragg diffraction. In addition, the asymmetric cut beam conditioning crystal is oriented in LM-ACT for geometric beam compression, so as to give a reconstituted shorter line image of consequently greater intensity. In this geometry each individual point of the horizontal line source corresponds to a unique Bragg arc across the beam conditioning crystal, allowing us to more effectively use the full intensity of the line source without loss of spatial resolution. The intensity of the beam from the conditioning crystal is also affected in two ways by the asymmetric diffraction geometry. There is the obvious increase in beam intensity associated with the smaller beam width (ie., asymmetric beam compression). However, the dynamically diffracted intensity from any point on the crystal is reduced proportionally by the square root of the same asymmetry factor. This paper demonstrates the dynamical diffraction effects on contrast, spatial resolution and beam intensity which are associated with the change in asymmetric cut of the beam conditioning crystal in the LM-ACT system. Results Sample x-ray topographic images obtained from a silicon crystal based parametric test chip device, examined with beam-conditioned Cu Kal radiation, are shown in Figs. l(a) - (c ) at increasing magnifications of -5X, -50X and -500X. The individual images trace the development of a topographic experiment from (a) recording of a diffraction spot having optimized the sample angular position for maximum image brightness, shown here at -5X, to (b) printing a normal topograph, frequently done at a magnification of <50X, to (c ) pushing current resolution of contrast details in any diffraction imaging experiment to a limiting nuclear film magnification of -200X or greater. In Fig. 1, resolution of image details at the highest magnification of -500X is achieved with Line Modified - Asymmetric Crystal Topography (LM-ACT) employing a horizontal line source of x-rays, with low vertical divergence, reconstituted with a Si crystal beam conditioner surface normal at 11 deg to the [ 11 l] diffracting plane vector, to give a well-defined beam probe of reasonable intensity at the investigated specimen crystal device [2,3]. The image contrast details of interest in Figs. l(b), (c ) are not in general associated with dislocations because modem Si crystal based advanced devices are fabricated on essentially dislocation-free material. These image details are correlated with shorter range elastic strains associated with edge effects at solute implantation zones or from accompanying diffusion

4 Copyright (C) JCPDS-International Centre for Diffraction Data 1999 treatments as well as compatibility strains between component mismatches that may also produce very localized dislocation dipole configurations with corresponding short range strain fields. Figure 1. (422) reflection topograph of a parametric test device using CuKctt radiation and 11 deg off cut beam conditioner, after 20hrs exposure recorded on Ilford L4 1Oumthick nuclear emulsion and developed with Kodak D19 for 1 hr at 0 deg C; x-ray probe beam incident from the left: (4 total chip image (-5X) showing width limited by the beam compression (b) conventional enlargement (-50X) of device elements including sections of lmm length orthogonal graticules cc> grain size limited enlargement (-500X) showing penetration depth influence on edge

5 206 resolution of test pattern, and two graticule marks indicating 10 micrometer spacing. cc 1 By comparison, individual dislocation strain fields are generally taken to scale with the x-ray extinction distance, 5, that is of the order of tens of microns in x-ray transmission topography experiments and somewhat less for surface reflection. Diffraction contrast in electron transmission micrographs is frequently employed to derive important details in these kinds of regions [4]. There are the advantages in the x-ray case, however, of precise angular characteristics being measurable and clear diffracted intensity differences are visible between adjacent device areas depending on subtle lattice parameter changes [5]. Previous studies of a sub-unit, QUAD U, in the parametric test device section shown in Fig. lb provided for evaluation of the various diffraction parameters involved in obtaining LM- ACT images. Of particular interest is the computation of the angular ranges of acceptance and reflection for total Bragg diffraction of the x-ray beam, the so-called perfect crystal angular range for total reflection, both at the Si crystal beam conditioner and at the device specimen [6]. The relative values of the angular beam width from the beam conditioner as compared with the specimen crystal determines the type and degree of contrast for strain fields in the specimen crystal. In the present case the angular comparison is associated with an enhanced diffracted intensity because of reduction in primary extinction of the x-ray beam [7]. The current investigation, while aimed primarily at obtaining a diffraction spot from a larger crystal area by adjusting the beam probe width at the crystal conditioner, provided also for investigating the dependence of the horizontal, Bragg angle determined, divergences in the beam widths on effecting spatial resolutions in the recorded topographs. The dependence on asymmetric diffraction conditions of the angular range for total reflection has also been pointed to for synchrotron experiments [8]. The experimental LM-ACT system is shown schematically in Fig. 2 (a) on a physical basis. The angular range of total reflection from the beam conditioner relative to the angular range of acceptance at the specimen crystal is illustrated in Fig. 2(b) using a dynamical dispersion surface construction. Also shown is the increasing collimation of the diffracted x-ray beam exiting the specimen crystal surface. The actual diffraction conditions for each of the crystals are described in Table 1, including: Bragg angles, angle between diffracting plane and crystal surface normals, extinction depth, angular beam widths, and integrated intensities on a primary extinction basis. The relative impact of these parameters will be shown in the discussion section to connect directly with the topographic results presented here. Material t&, (deg) x (deg) &, (urn) ADA (arcsec) At& (arcsec) PP 11 deg beam conditioner unpolar (cr+n)- 2.2E-5 pol. 5.5 deg beam conditioner unpolar (o+ri)- 4.OE-5 pol. Si (100) specimen o-polar. 1.2 o-polar. 5.OE-5 GaAs (100) specimen o-pal. 2.4 o-polar. 14.7E-5

6 207 TOPOGRAPHIC R [1001 [4El AL 8. \f ASYMMETRICALLY CUT SILICON BEAM CONDITIONER INCIDENT X-RAY SPECIMEN CRYSTAL ROTATION AXIS FOR BOTH CRYATALS h Figure 2a. Schematic illustration of LM-ACT experimental configuration; the SEMI specification is employed for the beam conditioner and specimen crystallographic orientations. ASYMMETRICALLY CUT SILICON BEAM CONDITIONER -,, (100) crystal surface SPECIMEN CRYSTAL INCIDENT X-RAY BEAM Figure 2b. Dispersion surface construction illustration of angular resolution and basis for extinction contrast in LM-ACT topographs.

7 208 Of primary importance to our original experimental objective, Fig. 3 shows in comparative reflections the larger image width able to be obtained with the 5.5 deg cut conditioner. Figure 3. Beam width limited Bragg reflection spots from a parametric test chip device: a) conditioner cut 5.5 deg off [ 11 l] toward [ 1 lo], b) conditioner cut 11 deg off [ 11 l] toward [ 1 lo]. (b) Figs. 4(a, b) shows topographs at -250X, obtained with the 5.5 and 11 deg cut conditioned beams, in this case of the same local region containing vertical metal strips. The topographs were obtained using an incompletely processed parametric chip device [6] having been pulled for inspection after process step #9120. Attention is directed to the vertical fine line structure in Fig. 4(a) that is clearly resolved in comparison with the smeared or absent intensities for the same line structure in Fig. 4(b). The comparison of vertical line visibility may be referenced also against the appearances in the two figures of the horizontal narrow band edges intersecting the left side boundary of the vertical line structure, near to the center of the figures. These horizontal band edges as well as other horizontal features are resolved approximately equally in the two figures, as expected on the basis of the geometrically-determined vertical divergence of 12 arcsec being the same in both cases and despite an apparent difference in diffracted intensity being received in this locale for the two cases. A value of At3~ of 7.9 arc set applies for the perfect crystal reflecting width of the specimen as compared with AOR being received from the 5.5 deg cut crystal of 13.7 arc set and 25.4 arc set for the 11 deg cut crystal. The greater spread of intensity in the latter case is associated with the reduction in resolution.

8 209 (a) = 13.7 arcsec (b) AOR(KI1 t) = 25.4 arcsec Figure 4. { 422) reflection topographs at -250X to show improved resolution of vertical lines, attributed to the smaller dynamically determined angular divergence from the 5.5 deg cut beam conditioner. The probe beam is incident from the right and A@~(Krtt) = 7.9 arcsec for the specimen crystal in each case. Figs. 5(a, b) show, at a further enlargement, another region from the same pair of topographs presented in Fig. 4. This time a pattern of vertical lines is shown above and below a horizontal line band used for reference in the two figures. Here attention is directed in Fig. 5(a) to the three groups of fine vertical lines in the lower half of the topograph spaced between thicker lines occurring at intervals across the horizontal distance of the figure. The lines are essentially unresolved in Fig. 5(b), illustrating at a higher magnification the effect of the difference in A& s described for Figs. 4(a,b). Also of special interest in Figs. 5(a, b) is the significantly enhanced intensity shown to be spread over the full height of the horizontal line band at the right side of each figure. Such extinction contrast spread over larger distances, say, of order of 20 micrometers, is associated with the larger elastic or plastic strains introduced within a device by incompatibility strains between component elements or contacts. This is comparable to the scale at which dislocation influences are frequently studied in less perfect materials.

9 Copyright (C) JCPDS International Centre for Diffraction Data (a) AOa(Kttt) = 13.7 arcsec (b) AOR(KIll) = 25.4 arcsec Figure 5. (422) reflection topographs shown at -500X to show improved resolution of vertical lines, attributed to the smaller dynamically determined angular divergence from the 5.5 deg cut beam conditioner. The incident beam is incident from the right and A@~(Ktrt) = 7.9 arcsec for the specimen crystal in each case. After careful examination of the diffraction images of the parametric test device for any evidence of dislocation strain fields demonstrating the larger spatial extent of such fields, one relatively faint image of such a defect was found and is shown in Figs. 6(a, b) obtained for both 5.5 and 11 deg cut cases. In this case, the somewhat crescent curved arc of enhanced contrast is suggested to mark a single dislocation segment partially buried in the perfect crystal substrate. The observation of the segment length running largely parallel to the vertical divergence of the beam probe is in agreement with greater dislocation visibility associated with such line orientations in conventional topographic studies [9].

10 211 (a) AC&(Krtr) = 13.7 arcsec (b) ACSR(KIII) = 25.4 arcsec Figure 6. Demonstration of faint defect contrast (crescent shape) at -500X in both topographic images obtained on otherwise essentially perfect crystal device. Discussion From CIB and x given in Table 1 one can easily calculate the geometric compression ratio for each of the beam conditioners. In fact, the beam leaving the 5.5 deg off cut crystal is 3.4 times wider, and therefore less intense on an areal basis, than the beam reflecting from the 11 deg cut crystal. One would expect a corresponding increase in exposure time. However, Table 1 also shows that there is a (4/2.2) times increase in the dynamical diffraction integrated intensity from the 5.5 deg off cut crystal. The net effect is that one should expect to have a 1.85 times longer exposure time when using the 5.5 deg off cut crystal as compared to the 11 deg off cut crystal. In practice, the topographs were take with exposure times of 18 hrs (11 deg) and 24 hrs (5.5 deg), showing an increase of 1.33 times in exposure for the 5.5 deg off cut crystal. The shorter exposure time for the 5.5 deg off cut crystal relative to that calculated above can be attributed to more careful alignment of the crystals relative to their diffraction peak profiles. Of course, one sees from A@ s given in Table 1 that alignment of the crystals must be on the order of 10 arcsec and that precise alignments relative to the diffraction peak profile settings are difficult to reproducibly duplicate.

11 212 The comparison of topographic results in Figs. 4 through 6 shows that improved horizontal resolution is obtained using the 5.5 deg cut beam conditioning crystal, in agreement with the narrower divergence computed from the dynamical theory of x-ray diffraction. This has been demonstrated by examining topographic images at much greater magnification than normally employed in crystal dislocation studies with imperfect crystals. It is significant that the improved resolution has been achieved for the condition of imaging a larger portion of the crystal specimen, i.e., with less compression of the beam probe as described above. The different resolution of dynamically effected details in the two beam conditioned cases is reasonably clear when compared to the equivalent resolution of horizontal line details which is dependent only on the geometrical vertical divergence that is the same in both topographic experiments. The comparisons of image resolution made here are given for topographs where significant effort has been made to ensure the recorded diffracted intensities are comparable. Consideration of the foregoing discussion of dynamical effects on controlling beam divergences for high resolution leads to the suggestion that similar high resolution topographic images might be obtained in less time by positioning tandem orthogonal, or nearly orthogonal, beam conditioners close to the x-ray target source. The idea is that satisfying the Bragg condition ideally in orthogonal planes for a perfect crystal beam conditioner would produce diffraction-controlled horizontal and vertical beam with low divergences at a close distance from the x-ray source while providing for capture of a greater amount of the generated x-rays. Such orthogonally set diffracting beam conditioners placed near to the source would obviate the need for the small source height and large sample/source distances currently used to achieve a vertical divergence comparable to that obtained in the orthogonal plane with the Bragg diffraction. The proposed orthogonal diffraction based double crystal system relates to the description of an x-ray magnifier demonstrated for enlargement of radiographic images [lo]. There, a transmitted radiographic image using CuK oil radiation was shown to be enlarged at a ten micrometer scale, first in one dimension by employing a beam conditioner in the expansion mode, then by a second expansion mode crystal orthogonally oriented relative to the first. Attention was drawn to the effect of a narrow AOR for the diffraction at each crystal on the resulting high spatial resolution. Here we propose that the relatively small angular divergences, both horizontally and vertically, can be achieved near to an x-ray source, thus giving an enhanced intensity probe beam without loss of beam divergence in either dimension of the probe beam. Topographs generated with such a conditioned probe beam close to the exit aperture of the x-ray source should optimize use of the x-ray source intensity while providing a beam of extremely low horizontal and vertical divergences for the production of high resolution images. Finally, we have demonstrated an x-ray imaging technique which can be easily implemented in a compact affordable laboratory apparatus with beam divergences comparable to synchrotron sources. In addition, information in these images is complementary to that found in the omnipresent scanning electron microscope (SEM) technique. Available SEM techniques working on capture of secondary electron yield provide for quantitative descriptions of: (1) surface elevations at the sub-micron scale, (2) sub-surface electron scattering observations at micrometer dimensions and (3) relatively sensitive electron channeling measurements and perfections at the ten nanometer scale. By comparison, the LM-ACT system technique provides image contrast based on x-ray diffraction where the resolution is dictated by the diffracted beam divergences. Beam divergences in the LM-ACT system are controlled by the geometrical and Lauekagg perfect crystal diffraction angles, which are comparable that obtained with x-ray topography fixtures at dedicated synchrotron beam lines (SBL). The LM-ACT technique, as demonstrated here, has been shown to provide excellent complementary information to SEM images and, with image resolution similar to SBL diffraction, can be used in convenient laboratory optimization of experiments which can be carried forward to SBL facilities.

12 213 References [l] W.J. Boettinger, H.E. Burdette, M. Kuriyama, and R.E. Green Jr., Asymmetric Crystal Topography Camera, Rev. Sci. Instrum., 47, No. 8 (1976) 906. [2] W.T. Beard, Jr., and R.W. Armstrong, LM-ACTfor Imaging RAM Devices in X-ray Difluction Topogruphs, Adv. X-ray Anal. 32 (1989) 659. [3] W.T. Beard, Jr., K.A. Green, X.-J. Zhang, and R.W. Armstrong, High resolution imaging of electronic devices via x-ray difsraction topography, App. Phys. Lett., 69 (22 July 1996) 488. [4] B.Y. Tsui, Y.F. Hsieh and C.H. Chang, Impact of Structure Enhanced Defects Multiplication on Junction Leakage, in 1994 IEEE International Reliability Physics Proceedings, IEEE Catalog No. 94CH3332-4,1994, p. 383 [5] W.T. Beard, Jr., W.G. Hutchison, R.W. Armstrong, X.J. Zhang, J.L. Fitz and J.K. Whisnant, Characterization of Semiconductor Devices via Line-Modijied Asymmetric Crystal Topography, in Physics of Semiconductor Devices, ed. Krishan La1 (Narosa Publ., New Delhi, 1993) p [6] K.A. Green, W.T. Beard, Jr., X.J. Zhang, and R.W. Armstrong, Application of Line Modi$ed-Asymmetric Crystal Topography for Qualitative and Quantitative Evaluation of Integrated Circuits, Adv. X-ray Anal. 38 (1995) 227. [7] B. Roessler and R.W. Armstrong, A Dynamical Theory Approach to the Berg-Barrett Technique, Adv. X-ray Anal. 12 (1969) 139. [8] M. Hart, T. Koga and Y. Takano, Mixing Symmetric and Oblique Bragg RejZections in Rigid Channel-Cut Crystals, J. Appl. Cryst. 28 (1995) 568. [9] J.M. Schultz and R.W. Armstrong, Seeing Dislocations in Zn, Philos. Mag. 10 (1964) 497. [lo] W.J. Boettinger, H.E. Burdette and M. Kuriyama, X-ray Magnifier, Rev. Sci. Instrum. 50, No. 1, (1979) 27.