HIGH RESOLUTION TEXTURE ANALYSIS OF THIN BLANKET FILMS AND DISCREET TEST STRUCTURES IN SEMICONDUCTOR DEVICES

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1 Centre for Diffraction Data 2001,Advances in X-ray Analysis,Vol HIGH RESOLUTION TEXTURE ANALYSIS OF THIN BLANKET FILMS AND DISCREET TEST STRUCTURES IN SEMICONDUCTOR DEVICES K. J. Kozaczek, R. I. Martin, D. S. Kurtz, P. R. Moran, S. P. O Leary, R. L. Martin HyperNex, Inc., 3006 Research Drive, State College, PA ABSTRACT Traditional texture analysis by XRD has two drawbacks when applied to semiconductor test structures on a full size wafer: it lacks precision in positioning of a small diameter x-ray beam with respect to small, discreet test structures (hundreds of microns or less) on a large wafer, and it lacks appropriate algorithms for calculating the orientation distribution function in the case of very sharp textures. We present a method that overcomes these two drawbacks. This particular measurement protocol eliminates the sample chi rotation thus enabling texture analysis on a wafer with in-plane motion only. The wafer positioning is controlled by high precision motion stages and a high magnification video camera. Such an arrangement allows one to measure texture anywhere on a full size wafer with a spatial resolution of approximately 100 microns. Several incomplete pole figures are collected simultaneously from one or more phases present in the sample and the orientation distribution function is calculated with a resolution as high as 1 degree. Examples of quantitative texture analysis in blanket films and interconnects are presented. INTRODUCTION The performance of advanced logic and memory devices is often linked to preferred crystallographic orientation, or texture of discreet structures. The most common materials used for interconnects, such as aluminum and copper, as well as typical barrier layers materials, such as Ti, TiN, Ta, and TaN have a strong preferred crystallographic orientation, or texture. The commercially available methods of quantitative texture analysis use a 5-degree resolution in the orientation distribution function (ODF) space and a matching resolution in the experimental pole figure space [e.g. 1, 21. Such a resolution is not sufficient for analyzing textures in thin films that have textures sharper then 5 degrees as measured by the angular spread around the ideal orientation. A direct method of the ODF calculation from incomplete pole figures was developed specifically for the thin film applications. Another issue in experimental texture analysis in interconnects is the required high spatial resolution of measurements over the entire wafer. The test structures are very often smaller then several hundred micrometers squared which imposes tight criteria on x-ray beam optics and sample positioning. A method has been developed and implemented for texture mapping on 200 mrn wafers with a spatial resolution of 70x180 microns. FUNDAMENTALS Analysis of back-reflection diffraction patterns The method uses an area detector (Hi-Star by Bruker was used for this work), which captures approximately 18 percent of the reciprocal space. Therefore, several sections of Debye

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3 Centre Centrefor fordiffraction DiffractionData Data2001,Advances 2001,AdvancesininX-ray X-rayAnalysis,Vol.44 Analysis,Vol.44 ISSN rings originating from the thin layers of a film stack are registered simultaneously, as shown in Figure 1. The intensities registered by the detector are processed in the following two ways. 1 (111)CU ewcu mm (220) mm (11O)Ta I I (2CQ)Ta (211)Ta Figure 1. Difhaction pattern of an electroplated copper film (0.5 pm thick) deposited on a a-ta barrier layer (300 A thick). (blue) Raw data, (red) peaks corrected for texture used for phase analysis. Right: detector image. 1. Crvstallogranhic texture. The dfiactometer equipped with an area detector (x-ray collimation-sample stagedetector) has been described by an analytical model [3]. The location of each pixel on the detector face is defined in terms of the sample coordinate system and in particular in terms of the coordinates used commonly in x-ray difi?action: 20 (Bragg angle), and polar coordinates of a pole figure (x-radial, ~-azimuthal). The intensities are corrected for the parallax errors (image distortion) and integrated over A20 and Ax according to required resolution of texture analysis. The x-ray intensity in the detector space is converted to a pole density in the pole figure space through a number of corrections. They include the background subtraction, absorption correction, and corrections for the varying acceptance angle on the detector face. The pole densities are then interpolated in the pole figure space on an equi-distant mesh. The most common one is a 1 by 1 mesh (pole figure resolution) suitable for texture studies in discreet structures. In the case of blanket films with cyclic fiber textures, the pole figure reduces to a onedimensional line plot (x coordinate only). The detector and x-ray collimator are fixed in space, and the wafer is placed on a x-y-z-$ positioning stages, as shown in Figure 2 [4]. Eliminating the chi rotation of the wafer with respect to the detector provides for a large mapping range and eliminates errors associated with sample rotation such as defocusing and varying difhacting volume, and uncertainty of beam positioning on the sample. However, it also reduces the tilt range covered on each pole figure. The diii?actometer geometry has been optimized in such a way that the number of pole figures and their ranges are sufficient to reproduce the ODF with a controllable error (usually in the range of 5 to 10 percent). The basic criterion used for the optimization is the Multiple Pole Density Set [5] that defines the minimal ranges that need to be covered on experimental pole figures in order to determine crystal orientation in an unambiguous way. The difhactometer geometry is optimized for a database of materials used in semiconductor applications. The accuracy of the ODF reproduction from the partial pole figures was determined by using model functions [6] simulating textures typical of polycrystalline thin films

4 Centre for Diffraction Data 2001,Advances in X-ray Analysis,Vol Figure 2. A schematic of the fixed-geometry wafers. diflkactometer for texture mapping on 200 mm The ODF is calculated from several partial pole figures using direct methods such as WIMW [7] or Arbitrary Defined Cell [8] modified for high resolution of l xloxlo in the ODF space. The details about the software for ODF analysis and a demo version are presented in [9]. From the ODF an arbitrary pole figure or line plot may be recalculated. The preferred output from the quantitative texture analysis is in the form of volume fractions of texture components. The orientation distribution function f(g) of the volume is defined as [e.g. lo]: dv 7 = f(b& where dv is the volume of crystallites that have the orientation g within the element of orientation dg, and V is the total sample volume. Therefore, the volume fraction of material having a particular orientation g is calculated directly by integrating the right hand side of equation (1) over the region corresponding to the angular spread around the ideal orientation Ag (and all symmetrical orientations). The integration is done in the ODF space defined by Euler angles $I,@, $2 [lo] where: dg= The finite intervals AcD1, A@, and A@, are determined by evaluating the sharpness of texture. In the case of cyclic fiber textures, typical of thin metallic films, texture components are separated in the ODF space and the integration is straightforward, as shown in Figure 3. The volume fractions of texture components in thin films with cyclic fiber textures are determined with accuracy better than 5% and precision of 0.05% [l Semi-quantitative phase analysis For the phase analysis purpose the intensities are summed along the 20 rings (Figurel). At each interval Ax the integrated intensities are corrected for texture in addition to standard

5 Centre ininx-ray X-ray Analysis,Vol.44 Centrefor fordiffraction DiffractionData Data2001,Advances 2001,Advancesin X-rayAnalysis,Vol.44 Analysis,Vol.44 ISSN =Y (5713)!!!slz 221) 111) oooo Figure 3. A +l=const. section of an experimental ODF for a blanket electroplated copper. The strongest texture component is a (111) fiber accompanied by its first-generation twin (511) fiber and second-generation twin (5 7 13) fiber, (221) is a first generation twin of (100). Lorentz Polarization Absorption corrections. The texture correction factor comes Corn the value of the corresponding pole figure at a given tilt x. In such a way the O-20 spectrum of a textured sample is reduced to a spectrum of an equivalent random sample. The phase analysis for twolayer film stacks is carried out using the direct comparison method [12]: I a ka*c, cg I, -kp (3) c, +cp =l where ci is the volume fraction of i-phase in the stack, Ii is the integrated intensity, and ki is the ratio of the integrated intensity of the i-phase to the integrated intensity of the reference material. For thin films (less than a few microns thick) ci translates directly to film thickness. A database of ki values has been developed using stacks of thin fihns with known thicknesses. Sample positioning Sample motion is intentionally limited to translations and rotation in the sample plane. The position of the x-ray beam in the sample plane is determined by automatically scanning a 100 micron diameter wire in front of the 100 micron x-ray beam (a 50 micron wire and a 50 micron collimator are used for high precision experiments). The position which produces the highest flux of the difhacted beam is taken as location of the incident beam. The center of rotation of the $-stage is then aligned with that point. Finally, a high magnification video camera is centered over the center of rotation. The accuracy of such sample positioning is approximately 20 microns and the precision is 10 microns. For blanket fihns with cyclic fiber textures (symmetry around sample normal) the data is averaged over several +-rotations. For patterned structures a full wafer rotation is carried out with a step of 1 degree.

6 Centre for Diffraction Data 2001,Advances in X-ray Analysis,Vol EXAMPLES OF APPLICATIONS One of the issues in multilayer fihn structures is the orientation dependence of the overlayers on the underlayers. Texture analysis can be done simultaneously for all materials present in the stack, provided that there is no excessive overlap of the diti?action peaks. Figure 4 shows the pole figures for Ta barrier layer and electroplated Cu film, which were determined from sections of the Debye rings shown in Figure 1. In the case of a dual damascene electroplated copper interconnects the geometry of the trench affects the grain growth and grain orientations in the trench. The grain growth occurs at the sidewalls as well as at the bottom of the trench. Using the approach described in the previous sections we analyzed the texture in damascene copper interconnect test structures. Each test structure measured 250 x 360 microns, and was composed of parallel lines that varied from 0.27 microns to 12 microns. Figure 5 shows the effect of line geometry (aspect ratio and spacing) on the strength of the (111) fiber texture. The volume fraction of the (111) fiber texture increases as the line width decreases. For comparison a blanket film test structure (on the same wafer) had a 12.8% volume of (111) fiber. -_ Figure 4. Ta barrier layer (300 A thick) shows a (110) fiber texture (left); electroplated Cu (0.5 micron thick) shows a (111)+(5 11) fiber texture (right). Data collection and processing approximately 1 minute. 1 mm tapered monocapillary collimator, 45kV/35mA sealed tube with graphite monochromator. SUMMARY A method of quantitative texture analysis in thin films and interconnects has been presented. The advantages are the high spatial resolution and the ability to deal with very sharp textures, which makes the method suitable for automated quality control of thin films and patterned structures. The quantitative nature of the ODF lends itself for using the texture data for modeling the physical and mechanical properties of thin films. The potential applications include studies of stabihty of manufacturing processes in terms of microstructure, uniformity of microstructure across the wafer, recrystallization kinetics across the wafer, and variation of film stiffness and hardness across the wafer.

7 Centre for Diffraction Data 2001,Advances in X-ray Analysis,Vol line spacing 0.27 microns 0 / line width (microns) Figure 5. Dependence of texture on line geometry in electroplated copper damascene lines. Error bars are lsigma from measurements at three different locations on the wafer. REFERENCES [l] BEARTEX, Berkeley Texture Package, beartex@seismo.berkeley.edu. [2] TEXTOOLS and TEXVIEWER, [3] Martin R.I, Kozaczek K.J. Principles of Texture Analysis with Area Detector: Theoretical Modeling, Numeriacal Implementation and Experimental Verification, in preparation for J. Appl. Phys. [4] Kozaczek K.J., Martin R.I., Moran P.R., Kurtz, D.S., An Instrument for Rapid Texture Mapping on 200 mm Wafers, in these proceedings. [S] Helming K., Minimal Pole Figure ranges for Quantitative Texture Analysis, Textures and Microstructures 19 (1992), [6] Matthies S., Vine1 G.W.,. Helming K, Standard Distributions in Texture Analysis (Academic Verlag, Berlin Germany, 1987). [7] Matthies, Phys. Stat. SoE. (b) 92 (1979), K135-K138. [S] Pawlik, Phys. Stat. SOL (b) 124 (1986), 477. [9] www. HypemexInc.com. [lo] Bunge H.-J., Texture Analysis in Materials Science (Butterworths, London, 1982). [l l] Kozaczek K.J.,Kurtz D.S., Moran P.R., Gross M.E., Evans-Lutterodt K. Methodology of Quantitative texture Analysis in Thin Films and Interconnects, submitted to Advanced Metallization Conference, October 3-5,2000, San Diego, CA. [ 121 Cullity B.D., Elements of X-ray Diffraction, (Addison-Wesley, 1978), 411.