EVALUATION OF MICROCRACKS IN MICROELECTRONIC COMPONENTS

Transcription

1 EVALUATION OF MICROCRACKS IN MICROELECTRONIC COMPONENTS Bernd Michel and Jürgen Keller Franhofer Micro Materials Center Berlin Gstav-Meer-Allee 25, D Berlin Abstract Advanced applications of modern micro- and nanotechnologies more and more reqire improved ver local stress and strain analsis of micro- and nanocomponents to meet the increasing reqirements for reliabilit and lifetime. Digital Image Correlation tools (DIC) have become a ver powerfl means to reach this goal. The athors deal with special DICtechniqes the microdac and nanodac deformation analsis (DAC Deformation Analsis b Correlation Method) which enable to determine deformation fields in ver small dimensions. The method is applied to describe the local deformation fields arond microcracks in varios applications. NanoDAC techniqe has been shown to become an important tool for measrements of local stress fields investigated b scanning probe microscop (AFM, AFAM etc.). The method has been performed as well on blk materials, thin films and on devices, i.e. microelectronic components, sensors, and MEMS/NEMS as well. Ths, modern fractre mechanics can be sed in the immediate transition range between micro- and nanotechnolog. 1. Introdction Proceeding progress in miniatrization of electronic packages and the development of nanomaterials reslt in the need for experimental methods on the nanoscale. The evalation of local strain fields at material interfaces, microcracks and defects is the ke for the sccessfl design of highl integrated sstems and nanostrctred materials. Frthermore, the qalit of electronic sstems and nanomaterials have to be garanteed for end sers. To flfill this crack and fractre avoidance in the micro and sbmicron regions have become essential. In most cases experimental procedres are complemented b simlations to assess, evalate and then predict the mechanical integrit of the strctre in qestion. As cracks and delaminations contine to be a worring problem in the field of micro- and nano-electronics, it is vital to have a tool at hand to make conclsive statements abot the reliabilit of components, which, following the roadmaps, show the tendenc towards smaller featres and strctres down into the nano-region. Of special interest in the field of electronic packaging is the emploment of micro- and nano-filled polmers and thin laers. As far as material characterization is concerned recent research showed that deformation measrement on the nanoscale is the ke for mechanical evalation of micro- and nanomaterials. The application of SPM-based imaging techniqes combined with digital image correlation (DIC) methods is a powerfl method for the determination of displacement fields with the accrac in the range of a few nanometers.

2 Two examples of this so-called nanodac method (nano-deformation Analsis b Correlation) are given in the experimental part of this paper followed b the simlative approach. The aim is the development of a combined experimental and simlative procedre towards an improved nanomechanical material analsis. The final reslt of the approach are fractre parameters (e.g. K Ic ) and related field qantities in the immediate vicinit of ver tine micro- and nanocracks. FIGURE 1. AFM topograph image of a crack in a thermoset polmer material for different crack opening displacements in different load stages of the crack specimen. 2. Crack Tip Analsis b NanoDAC Method in AFM The first example for a nanomechanical analsis is performed at a crack tip of a neat thermoset polmer. Therefore a compact tension (CT) specimen is loaded in-sit nder the AFM. Topographic non-contact AFM scans are carried ot before and after loading and digital image correlation is applied to the derived images. Figre 2 illstrates the evalated crack tip opening field.

3 FIGURE 2. AFM topograph image with overlaid displacement reslts in the (vertical) -direction (crack opening),. The derived crack tip field is an inevitable data sorce for fractre mechanical analsis of the materials so that concepts sch as crack opening displacement (COD) or J-integral are applicable. The presented displacement measrement at a polmer material emphasize the analsis of all kinds of composite materials. Especiall the interface between micro- or nanoparticles and the polmer matrix will be of interest for the design of new materials. A straightforward approach for crack evalation in the AFM is the techniqe of crack opening displacement (COD) determination. In order to extract the mode I stress intensit l factor K I crack opening displacements, and, are measred along both the pper and lower crack bondaries. If determined b linear elastic fractre mechanics the mst eqal to, l K I χ = ± ( κ + 1) χ 0 (1) 2µ 2π = 0 χ > 0 (2) = l where µ is the shear modls and κ is a fnction of Poisson s ratio, ν; κ = (3 4ν) for plane strain and κ = (3 ν)/(1 + ν) for plane stress. Taking the sqare of the difference of pper and lower displacements, we obtain a linear fnction of the χ-coordinate or 0, depending on the position relative to the crack tip:

4 2 l 2 = Cχ χ 0 (3) 0 χ > 0 (4) The expression of Eqn. 3 does not change if specimen rotation de to inaccrate loading is inclded into the considerations. In this case, eqal rotational terms on both sides of the crack bondar are sbtracted from each other. For the eqation above, the crack tip is set at location χ = 0. The crack tip location on the real specimen can be fond at the interception of a linear fit of the crve Cχ with the χ-axis. The slope C allows to estimate the stress intensit factor K I, which is a measre of the crack tip load. It is given b: K I = E 1 2πC 1 + υ κ + 1 (5) where E is the Yong s modls. The discssed analsis is applied to the displacement field measrements presented in Figre 3. From (5) the K I can be obtained immediatel from the experimental data via eqation (3). (x) crack l (x) θ r x r (x) FIGURE 3. Crack opening displacement, mode I crack opening. A similar procedre has been developed to derive critical vales K Ic (fractre toghness). The athors are going to generalize the method for more general fractre concepts. Generalized integral concepts e.g. C*, βt, Ĵ etc. can be sed to get more realistic reslts taking into accont the exact material behavior. Another kind of applications of microdac and nanodac methods is to std the thermomechanical behavior of varios microcomponents, microsensors, MEMS, NEMS etc. In the following example nanodac deformation analsis has been carried ot at the membrane of a gas sensor. 3. Application of DIC to Micromachined Gas Sensor Figre 4 shows a topographic AFM scan of a Pt-laer on top of a SiO 2 membrane where the platinm represents the heater of the gas sensor. Delaminations at the edge of the Pt-laer are cased b mismatch of material properties (CTEs; Pt: 9 ppm K -1 SiO 2 : 0.65 ppm K -1 ) in the presence of thermal loading p to 450 C.

5 FIGURE 4. Micromachined gas sensor. AFM topograph scan of membrane laers. Ptelectrode destrction at edge laer. The qestion whether these delaminations are cased b large scale deformation dring thermal loading is answered b nanodac analsis. In-sit non-contact AFM scans in top of the gas sensor membrane are carried ot at room temperatre and at 100 C. The derived topograph images of these load states are compared b digital image correlation and inplane displacement fields are determined. The Pt-laer reveals an inherent expansion towards the edge of the laer. Therefore, heating ccles of p to 450 C as carried ot at the membrane of Figre 1 ma case large scale deformation leading to delaminations or catastrophic failre in form of membrane cracking. With in-sit AFM measrements on this microsstem the capabilit of the nanodac approach is demonstrated. Measring of material deformation reslting from mismatch of material properties are detectable in nanoscale resoltion. B means of Digital Image Correlation method (DIC) based on AFM images the nanodac techniqes enables to determine the local displacement fields and from these it is possible to derive stresses and strains as well.

6 FIGURE 5. Displacement fields determined b nanodac techniqe from AFM image. References 1. Michel, B.; Keller, J.: NanoDAC Nano Scale Deformation Measrements for Reliabilit Assessment of Microsstems and Micro Materials, Proc. Microsstem Technologies Conf., Mnich, October 7-8, Keller, J.; Vogel, D.; Schbert, A.; Michel, B.: Displacement and strain field measrements from SPM images, in: Applied Scanning Probe Methods, B. Bhshan, H. Fchs and S. Hosaka (eds.), Springer 2003, pp Michel, B. (ed.): Micromaterials and Nanomaterials, No. 1 (2002), Franhofer IZM Berlin, pp. 7ff. 4. Michel, B. et al.: Risse in Packagingafbaten von der Mikromechanik zr Nanomechanik, in: K.-J. Wolter nd S. Wiese (eds.), Interdisziplinäre Methoden in der Afba- nd Verbindngstechnik, Verlag ddp goldenbogen, Dresden 2003, S Michel, B.; Vogel, D.: MicroDAC and nanodac Powerfl Techniqes for Nondestrctive Microcrack Evalation, Int. Conf. SPIE, Nondestrctive Evalation of Micro- and Nanomaterial Sstems, San Diego, USA, March 18-20, 2002, proc. p Wnderle, B.; Keller, J.; Vogel, D.; Michel, B.: Fractre mechanical characterization of micro- and nanofilled polmers b a combined experimental and simlative procedre, IEEE Nano 04, Mnich, 2004 (im Drck).