Quantitative thickness measurement of dual layer materials using X-ray absorption-based technique

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1 SIMTech technical reports Volume 8 Number 1 Jan - Mar 7 Quantitative thickness measurement of dual layer materials using X-ray absorption-based technique L. M. Sim and A. C. Spowage Abstract Gray levels data were used to provide information on the material type or thickness of hidden features and materials within devices. Metallic materials essential to semiconductor technology were arranged accordingly to simulate different combination of dual layer features and thickness. A specific X-ray tube accelerating voltage and target current were set with the aim of optimising the measurement conditions. The results demonstrated the capability of the method to determine the material or thickness of one of the layers in the dual layer system. As a second layer in the dual-layered material system, aluminium and copper achieved a thickness resolution of an order of 5 mm and 5 mm respectively. The results were repeatable and provided a platform for the development of a novel absorption-based thickness measurement system for multi-layered components that is not restricted to this example and can be optimised for a range of industrial applications. Keywords: X-ray, Absorption-based, Thickness measurement, Thickness resolution, Dual layered 1 BACKGROUND The use of solder interconnections is expected to significantly increase with the number of input/output interconnects in integrated circuits. Two-dimensional (D) area array of solder interconnects that are commonly used as flip-chip interconnects are used to meet this requirement. Solders of lead-tin are most commonly used but due to health hazard and international legislative ruling to ban the used of lead, electronic industry are developing lead-free solders. Common alloys for lead-free solders are tin-rich with alloying elements of Ag, Cu, Zn in binary or ternary composition. The solder in form of bumps or balls is applied to the device e.g. wafer to facilitate the connection between the device and circuit board. As shown in Fig. 1, the solder bump structure on a wafer consists of many different layers of materials. The reliability of the interconnection relies on the physical dimension of these materials as it influences the solder properties. Optical methods have been used to obtain this information. The difference in brightness of the bump surface however may affect the accuracy of the measurements as the optical reflection rate would not be consistent. Acknowledging the need for an alternative method, this work aims to provide information on the thickness of these layers by initially simulating different combination of dual layer features and imaging them using the X-ray. Although this arrangement does not represent the actual solder bump structure, it provides a stepping-stone for the development of a novel absorption-based thickness measurement system for multi-layered structure. Fig. 1. Schematic diagram of a solder joint showing multilayer structure. X-ray imaging provides information on the hidden features in an object based on the difference in absorption characteristics. The degree of X-ray absorption through a material is largely governed by a number of interrelated factors [1,]. The most significant of which are the characteristics of the incident X-rays and the nature of the material. The characteristic of the incident X-rays are influenced by three primary factors: the target material, the energy distribution, and the number of incident electrons. The spectral characteristics of the emitted X-rays are tailored with the manipulation of these three factors. The target material affects the efficiency of the X-ray tube in producing X-rays. A material of high atomic number would produce more X-rays for the same electron beam [3]. Varying the tube voltage primarily changes the X-ray energy distribution and therefore the penetrating power. As the accelerating voltage increases, the energy distribution shifts to a higher level and therefore increasing the extent of transmission. However, there are a wide range of energy dependent interactions between the sample and the X-ray photons. As a consequence, the ability to optimise quantitative absorption-based measurements for a specific material type and sample geometry requires knowledge of the X-ray spectral distribution. The current emitted to the target primarily affects the number of electrons that are incident onto the target. This in turn results in a higher number of X-ray photons. The accelerating voltage and applied tube current are the easiest of the three variables to modify and will be used to optimise the system resolution in this work [4-6]. 38

2 Quantitative thickness measurement of dual layer materials using X-ray absorption-based technique The principal material factors include the specific absorption characteristics of the constituent elements, sample thickness and density. A thick and dense material would provide larger X-ray absorption. OBJECTIVE The objectives of this work were as follows: To correlate the gray level data from the X-ray detector against material thickness for dual-layered materials To measure the material thickness of one layer of the dual-layered combination To determine the material of one layer of the dual-layered combination To measure the thickness resolution for each dual-layered material 3 METHODOLOGY A nanofocus Fein Focus FOX 16.5 X-ray imaging system equipped with a 16-bit Varian Direct Digital Detector (DDD), as shown in Fig., was used in this study. Thin and pinhole free 99.99% pure foils of tin, aluminium and copper of different thickness were used to simulate the dual-layered material configuration. Table 1 provides a summary of the experimental configuration. A 5 μm thick silicon wafer, acting as a substrate was also used to simulate the dual-layered material configuration. and Sn-Al combination, and at 1 kv and 1 μa for Si-Cu and Cu-Si combination so as to optimise the instrumental system. The 16-bit clear images were captured after integrating 4 frames and the gray levels statistics were extracted from a cm by cm area of the images using the Image Analysis Software. The second layered material was also analysed as a single material for comparison. The experimental procedures were repeated three times to ensure repeatability of the results. Table 1. Summary of the dual-layered combination at different thickness and materials. Dual layered configuration Mat l Substrate Thick (mm) Mat l nd layer Thick (mm) Si-Al Si.5 Al Sn-Al Sn 5 Al Si-Cu Si.5 Cu Cu- Si Cu Si.5 Fig. 3. Positioning of the foils at the centre of a light cone at an equidistance position between the tube and the detector. In measuring the thickness of a layer in the dual-layered material configuration, the reduction in the intensity of the monochromatic X-rays of the silicon wafer or the material with a fixed thickness representing the substrate is first measured by using an exponential relation which is known as Beer s Law, is given by Eq. (1) [4]: Fig.. A nanofocus Fein Focus FOX 16.5 X-ray imaging system. These foils were positioned with first the substrate and followed by the second layer on it, at the centre of a light cone, at an equidistant position between the tube and detector, as shown in Fig. 3, to limit the effect of the geometric unsharpness and difference in path length associated with the cone beam. The accelerating voltage and applied tube current of the x-ray were set at 54 kv and 54 μa for Si-Al I ln = μ1x I o (1) where I is the intensity of transmitted beam I o after passing through a thickness, x, as shown in Fig. 4(a) and μ 1 is the linear absorption coefficient which is dependent on the material considered, its density and the wavelength. When a dual-layered configuration is formed as shown in Fig. 4(b), on the other hand, Eq. (1) is modified and can be represented by [7]: 39

3 L. M. Sim and A. C. Spowage I1 = I1 = I1 ln I { Io exp( μ1x) }.exp( μh) I exp( μ h) = μh () where I 1 is the intensity of transmitted beam I o after passing through a thickness, x, of the substrate and a thickness, h, of the second layer as shown in Fig. 4(b), and μ is the linear absorption coefficient of the second layer. The function on the left hand side of Eqs. (1) and () were correlated against the actual thickness to optimise the system. I o 4 RESULTS AND DISCUSSION At a respective fixed accelerating voltage and applied current, the four dual-layered combinations showed a similar trend. Based on the ln (I/Io) and ln (I 1 /I) versus thickness plots in Figs. 5 to 7, the absorption of the X-rays observed an increased as the material thickness of the second layer increases. Similarly, the corresponding gray values decreases exponentially with increase in thickness as given by Fig. 8. This is an important observation and indicates that the use of gray values combined with Beer s Law can be used as a traceable technique for quantifiable absorption based measurements for dual layered components. Using the ln (I 1 /I) versus thickness plots in Figs. 5 to 7 for each dual-layered material system, the linear absorption coefficient can be obtained from the slope of the graph. The linear absorption coefficient observed is within ±1% of the linear absorption coefficient of the second material when analysed as a single material at the same X-ray settings. These results served as a method of determining one of the material used in the dual layered system. The effectiveness of this method however relies on knowing at least one of the materials beforehand. The results here also seemed to suggest that the thickness resolution measured for the second layered material were not affected as thickness of that material increased within the limits of the experiments carried out, as shown in Fig. 9. The variation of the results is generally within experimental limits although the Si-Al combination in Fig. 5 was comparatively higher. The inconsistency of the repeatability results maybe cause by the instability of the machine during the tests. Substrate / Silicon wafer x -.1 I Fig. 4(a). Schematic illustrates the method of measuring thickness of a single layered system. I o Second layer Substrate / Silicon wafer h x ln (I/Io), ln(i 1/I) y = x R =.9833 y = -.353x R = Al Thickness (mm) I 1 Al Si-Al Si Fig. 4(b). Schematic illustrates the method of measuring the thickness of the second layer in a dual-layered system. Fig. 5. Variation of X-ray absorption with different thickness of aluminium in Si-Al dual-layered system. ln (I/Io), ln (I 1/I) y = x R =.9853 y = -.356x R = Al Thickness (mm) Al Sn-Al Sn Fig. 6. Variation of X-ray absorption with different thickness of aluminium in Sn-Al dual-layered system. 4

4 Quantitative thickness measurement of dual layer materials using X-ray absorption-based technique y =.67x - 67 R = ln (I/Io), ln(i 1/I) y = x R =.9941 y = R = y = -.945x R =.9957 Material thickness used (mm) Cu Thickness, mm Cu Si-Cu Cu-Si Si Fig. 7. The variation of X-ray absorption with different thickness of copper in Si-Cu and Cu-Si dual-layered system. Gray values Al Thickness, mm Si-Al Fig. 8. The variation of X-ray absorption with different thickness of copper in Si-Cu and Cu-Si. Al 1 3 Minimum detectable thickness (mm) Si-Al Sn-Al Si-Cu Fig. 9. The minimum detectable thickness of the second layered for each configuration. 5 CONCLUSION The following conclusions were made: The use of gray values combined with Beer s Law can be used as a quantifiable absorption-based measurement for dual layer materials Minimum thickness resolution measured was 5 mm for copper at 54 kv and 5 mm aluminium respectively at 1 kv Gray level data from the DDD of the X-ray imaging system can be used for quantitative absorption-based measurement in dual-layered systems A method of defining the material of the second layer in the dual layered component based on gray level data has been established provided the first material is made known 6 INDUSTRIAL SIGNIFICANCE The work provides part of the basis for understanding the interaction between X-rays and sample materials under investigation. This understanding is being used to develop novel absorption-based thickness measurement tools for industrial applications. The materials chosen reflect the use of such materials primarily in the semiconductor and MEMS technologies. 41

5 L. M. Sim and A. C. Spowage REFERENCES [1] Y. Chu, B. Douglas, and V. Marconcini, Real time X-ray systems for non-destructive inspection, Soc. Manuf. Engineers, Technical paper MS87-37, [] J.A. Seibert, X-Ray Imaging Physics for Nuclear Medicine Technologists. Part 1: Basic Principles of X-Ray Production, J. Nuclear Medicine Technol., vol. 3(3), pp , 4. [3] B.D. Cullity, Properties of X-rays, in Elements of X-ray Diffraction, edited by B.D. Cullity, Addison-Wesley Publishing Company, [4] R. Halmshaw, Radiographic Absorption, British J. NDT, vol. 7(1), pp. 61-6, 199. [5] J.A. Seibert, X-Ray Imaging Physics for Nuclear Medicine Technologists. Part : X-Ray Interactions and Image Formation, J. Nuclear Medicine Technol., vol. 33, pp. 3-18, 5. [6] L.M. Sim, B.S. Wong, and A.C. Spowage, Quantitative materials analysis of micro devices using absorption-based thickness measurements, J. Physics: Conference Series, vol. 8, pp , 6. [7] K. Muruyama and M. Higashi, US Patent 6,5,719B, 3. 4