Design of a Rotating Sensor for Stress Measurement in Metallisation
Sorin Soare
Due to the miniaturisation of electronic devices, operating conditions have become more demanding and residual stress is playing an increasing role in electronic device failure. In particular, the reliability of back-end metallization is critically dependent on stress in the conductor and how this is changed by its surrounding passivation layers. Thus early detection of high levels of stress in metal lines is critical from the reliability point of view. Current stress measurement techniques, such as x-ray diffraction, are able to measure stress averaged over a large number of lines but there is a need to determine the stress in a single line and how this varies during processing of the device. This work involves the design and development of a sensor able to measure the stress in a single sub-micron metallic line and its demonstration for aluminium metallisation. The sensor consists of a pointer connected to two arms which are offset. When the sensor is released from the substrate the pointer rotates in response to the release of stress in the arms. The rotation angle is therefore a direct measure of stress in the arms if a suitable calibration constant is available. The calibration constant depends critically on the design of the sensor and thus design can be used to optimise sensor performance.
The research on
this sensor was conducted in two stages: sensor design and analysis by finite
element (FE) methods, which is the main subject of this thesis, and
experimental verification which was undertaken in conjunction with partners
skilled in processing at
Modelling potential sensor designs was undertaken using the commercially available finite element software package, ANSYS. This program was used to assess the sensor response to changes in stress and to optimise its structure for maximum sensitivity [2, 4-6]. Final sensor designs for fabrication were produced where the sensor rotation is measurable by reflected light microscopy and the whole structure occupies minimum space on the chip, a requirement for industrial acceptance of the design. For the optimised sensor structure elastic, elastic-plastic and elastic plastic simulations incorporating creep were undertaken. Plastic deformation reduces the sensor rotation, resulting in geometry changes in the pointer-arm hinge region but creep has a negligible effect on the sensor rotation during release, even if it does lead to relaxation of stress in the arms before release.
The correct mechanical properties of the metal are essential to achieve a reasonable model of sensor performance and for a thin coating these are not the same as those of bulk material. Nanoindentation testing in combination with FE modelling of the load-displacement curves produced was used to extract suitable material parameters which were then used as input for sensor modelling in this study [1, 3, 10, 11].
The modelled structures were then fabricated in aluminium at 1mm and 2mm feature size and released from the substrate using specially-developed etching techniques [7, 8]. The sensor rotation results obtained from modelling and experiment were comparable and have the correct trends. Rotation was calibrated using x-ray diffraction stress measurements to allow for any discrepancies [12-15]. Slight differences between the experimental and modelled results have been attributed to problems during processing, chiefly incomplete underetching of the aluminium and problems with pattern transfer from the mask to the metal leading to rounding of corners. However, despite this the sensor design has significant potential for use as an in-line stress monitoring tool.
[1] Assessment of aluminium thin films by nanoindentation, S. Soare, S. J. Bull, A. Horsfall, J. dos Santos, A.G. O’Neill and N.G. Wright, Mat. Res. Soc. Symp. Proc., 750 (2003) 83-88.
[2] A sensor for the direct measurement of process-induced residual stress in sub-micron interconnects, A.B. Horsfall, J.M.M. dos Santos, S.M. Soare, N.G. Wright, A.G. O'Neill, S.J. Bull, A.J. Walton, A.M. Gundlach and J.T.M. Stevenson, Semiconductor Science and Technology, 18 (2003) 1-5.
[3] Nanoindentation assessment of aluminium metallisation; the effect of creep and pile-up, S. Soare, S. J. Bull, A. O’Neill, N. Wright, A. Horsfall and J.M.M. dos Santos, Proc. Int. Conf. Metallurgical Coatings and Thin Films, San Diego, April 2003, Surf. Coat. Technol., 177-178 (2004) 497-503.
[4] A novel sensor for the direct
measurement of process induced residual stress in interconnects, A.B. Horsfall, J.M.M. dos Santos, S.M. Soare,
N.G. Wright, A.G. O’Neill, S.J. Bull, A.J. Walton, A.M. Gundlach,
J.T.M. Stevenson, Proc. ESSDERC 2003, 16-18 September, 2003,
[5] Direct measurement of residual stress in
integrated circuit interconnect features, A.B. Horsfall,
J.M.M. dos Santos, S.M. Soare, N.G. Wright, A.G.
O’Neill, S.J. Bull, A.J. Walton, A.M. Gundlach,
J.T.M. Stevenson, Proc. European symposium on the reliability of electron
devices (ESREF), 6-10 October 2003, Bordeaux; Microelectronics
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43
(2003)
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[6] Residual stress sensor for the
microelectronics industry, J.M.M. dos Santos, N.G. Wright, A.B. Horsfall, S.M. Soare, A.G.
O’Neill, S.J. Bull, A.J. Walton, A.M. Gundlach,
J.T.M. Stevenson, Proc Eurosensors XV11, University
of Minho, Portugal, 21-24 September 2003, pp956-968.
[7] Test chip for the development and evaluation of test structures for measuring stress in metal interconnects, J.G. Terry, A.J. Walton, A.M. Gundlach, J.T.M. Stevenson, A.B. Horsfall, K. Wang, J.M.M. dos Santos, S.M. Soare, N.G. Wright, A.G. O’Neill and S.J. Bull, Proc. IEEE Int. Conf. Microelectronics Test Structures, Awaji Yumebutai, Hyogo, Japan, March 23-25, 2004 pp. 69-73.
[8] Sensitivity of a rotating beam sensor for
stress evaluation in aluminium thin films, J.M.M. dos
Santos, J.C.P. Pina, A.C. Batista, A.B. Horsfall, K. Wang, N.G. Wright, S.M. Soare,
S.J. Bull, A.G. O’Neill, J.G. Terry, A.J. Walton, A.M. Gundlach,
and J.T.M.Stevenson, Proc. 7th Int. Conf.
on Residual Stress (ICRS-7), Materials Science Forum, 490-491 (2005) 649-654.
[9] Obtaining mechanical parameters for metallisation stress sensor design using nanoindentation, S. Soare, S. J. Bull, A. Oila, A.G. O’Neill, N.G. Wright, A. Horsfall and J.M.M. dos Santos, Z. Metall., 96 (2005) 1262-1266.
[10] Calibration and optimisation of interconnect-based MEMS test structures for predicting thermo-mechanical stress in metallisation, J.MM. dos Santos, A.B. Horsfall, J.C.P. Pina, N.G. Wright, A.G. O’Neill, K. Wang, S.M. Soare, S.J. Bull, J.G. Terry, A.J. Walton, A.M. Gundlach and J.T.M. Stevenson, Proc. IEEE Int. Reliability Physics Symposium, Phoenix, April 25-29, 2004, pp255-258.
[11] Determination
of mechanical parameters for rotating MEMS structures as a function of
deposition method, S. Soare, S. J. Bull, A. Oila, A. O’Neill, N. Wright, A. Horsfall
and J. dos Santos, Proc. Mat. Res. Soc. Fall Meeting,
[12] High sensitivity in a micro-rotating
structure for predicting induced thermo-mechanical stress in integrated circuit
metal interconnects, J.M.M. dos
[13] Dependence of process parameters on stress generation in aluminium thin films, A.B. Horsfall, K. Wang, J.MM. dos Santos, S.M. Soare, S.J. Bull, N.G. Wright, A.G. O’Neill, J.G. Terry, A.J. Walton, A.M. Gundlach and J.T.M. Stevenson,. IEEE Trans. Device and Material Reliability, 4 (2004) 482-487.
[14] Test chip for the development and evaluation of test structures for measuring stress in metal interconnects, J.G. Terry, S. Smith, A.J. Walton, A.M. Gundlach, J.T.M. Stevenson, A.B. Horsfall, K. Wang, J.M.M. dos Santos, S.M. Soare, N.G. Wright, A.G. O’Neill and S.J. Bull, IEEE Trans. Semiconductor Manufacturing, 18 (2005) 255-261.
[15] Calibration of MEMS based test structures for predicting thermo-mechanical stress in integrated circuit interconnect structures, J.M.M. dos-Santos, K. Wang A.B. Horsfall, J.C. Prata Pina, N.G. Wright, A.G. O’Neill, S.M. Soare, S.J. Bull, J.G. Terry, A.J. Walton, A.M. Gundlach, J.T.M. Stevenson, IEEE Trans Device and Materials Reliability 5 (2005) 713-719.