Structural MEMS Testing

Figure 1: Diffractometer used for HRXRD applications.

Alex Dommann and Antonia Neels*

EMPA, Lerchenfeldstrasse 5, 9014 St. Gallen, *CSEM, Microsystems Technology Division, 2002 Neuchâtel

 

In single crystal silicon (SCSi) based devices, stress and loading in operation introduces defects during the MicroElectroMechanical Systems (MEMS) life time and increases the risk of failure. Reliability studies on potential failure sources have an impact on MEMS design and are essential to assure the long term functioning of the device. Defects introduced by Deep Reactive-Ion Etching (DRIE), thermal annealing, dicing and bonding and also the device environment (radiations, temperature) influence the crystalline perfection and have a direct impact on the mechanical properties of MEMS and their aging behavior. Defects and deformations are analyzed using High Resolution X-ray Diffraction Methods (HRXRD) such as Reciprocal Space Maps (RSM). Micro systems technology can be highly reliable, but can be different from those of solid-state electronics. Therefore testing techniques must be developed to accelerate MEMS-specific failures [1 - 4].

 

Fig 2: Diffractometer setup for RSM’s (detector position 1) and RC’s (detector position 2)
Figure 3: Schematic HRXRD measurement setup and silicon crystal orientation (top) and measured rocking curves RC’s from the bonding interface 1 of the device layer to the bonding interface 2 (bottom) resulting in a stress profiling.

HRXRD allows measuring the strain of a crystal with high resolution (Fig. 1). We use HRXRD to assess the strain in DRIE etched processed silicon beams. Strain deforms the silicon beam leading to an appreciable sample curvature which is detected via the broadening of the X-ray peak in a "rocking-curve" (RC) measurement. Analysis is performed with high resolution X-ray diffraction. The set-up is composed by curved multilayer x-ray mirrors named after Herbert Göbel (Göbel mirror) in front of the X-ray tube, fol-lowed by a monochromator for monochromatization and collimation of X-ray beams by using a 4-Crystal monochromator with two channel-cut Ge [220] called Bartels monochromator (Fig. 2). Two configurations are possible for the diffracted beam side, depending on the methods applied: Rocking curve (RC) or Reciprocal space map (RSM). Both methods allow measuring strain and defects concentration in a crystal. The instrument used for HRXRD is shown (Figure 1).

New MEMS fabrication processes and packaging concepts find applications in areas where a high reliability is needed such as in aerospace, automotive or watch industry. This creates a strong demand in quality control and failure analysis and also brings new challenges, particularly in the fields of testing and qualification. Non-destructive HRXRD methods are applied to monitor the mobility of defects and strain and along the MEMS fabrication and packaging processes.
RSM is a powerful tool for evaluating the strain state of the entire structure. Due to the much more limited vertical divergence and the very small horizontal divergence of the RSM settings, it is possible to get a 'γ-function like' reciprocal-space probe which is almost invariant over the Ewald sphere. Figure 2 shows a setup used for measuring such RSMs.

HRXRD is a very sensitive and non-destructive technique for determining the strain in MEMS devices.

An example of such a test structure is a silicon based piezoelectric resonator (Fig. 3), developed at CSEM, targeting vacuum hermetic wafer-level packaging technology [5]. The monitoring of quality factors (Q) for the resonators permits to evaluate the pressure level (hermeticity) of the device cavity and the leakage rate.

As the Q-factor is not only dependent on the vacuum level of the MEMS cavity but also on the strain state of the device, the simultaneous data collection for the Q factor determination and the strain state is evident. The packaging induced strain created at the important interfaces such as the interfaces close to the bonding material and more importantly to the device layer has been analyzed by means of X-ray Rocking Curves (Fig. 3). The stress profile can be determined. Especially the strain close to the functional device is important as the strain state influences the application relevant physical parameters such as the resonance frequency and the Q factor in resonator.
The combination of functional testing with state-of-the-art X-ray methods for the evaluation of defect and strain gradients will serve as a useful tool for setting up a fundamental understanding of the reliability and also aging problems of MEMS.

 

A rocking curve (RC) is obtained as angular distribution of the reflected X-ray beam, when the detector is set at a specific Bragg angle and when the sample is rotated about small angles normal to the Bragg plane axis. The rocking curve is broadened by disruptions of the plane parallelity and by crystal defects like those introduced by mechanical stress. Reciprocal space mapping (RSM) adds, by the restriction of the angular acceptance of the detector, another dimension to the information available from the HRXRD experiment. Strain and tilt elements being present in a sample are identified separately. An excellent introduction to both analytical methods can be found at http://prism.mit.edu/xray/tutorials.htm. See 'Basics of High Resolution X-Ray Diffraction for Studying Epitaxial Thin Films'.

 

References

[1] A. Dommann, A. Neels, “Reliability of MEMS” (2011), Proceedings of SPIE - The International Society for Optical Engineering, 7928, art. no. 79280B.
[2] A. Neels, A. Dommann, A. Schifferle, O. Papes, E. Mazza, Reliability and Failure in Single Crystal Silicon MEMS Devices, Microelectronics Reliability, 48, 1245-1247, 2008.
[3] Herbert R. Shea, Reliability of MEMS for space applications, Proc. SPIE Int. Soc. Opt. Eng. 6111, 61110A (2006).
[4] Schweitz, J.-Å, Mechanical Characterization of Thin Films by Micromechanical Techniques, MRS Bulletin, XVII, 7, 1992, pp.34-45.
[5] J. Baborowski, et al. "Wafer level packaging technology for silicon resonators", Procedia Chemistry 1, 1535-1538 (July 2009)

 

Alex Dommann's research concentrates on the structuring, coating and characterization of thin films, MEMS and interfaces. In July 2013 he was appointed Head of Departement "Materials meet Life" at Empa, Swiss Federal Laboratories for Materials Science and Technology. He is member of different national and international committees.

Antonia Neels is heading the XRD Application Lab of CSEM’s Microsystems Technology Division. She has a broad experience in the application of X-ray diffraction methods for microsystems (MEMS) and thin films with respect to quality control and failure mode analysis.

 

 

[Released: July 2013]