(Invited) Characterization of Buried Interfaces with Scanning Probe Microscopes
Authors: Kopanski Joseph J. , You Lin , Li Jennifer , Ahn Jung Joon , Obeng Yaw S. 
Published: February 2020

Buried interfaces in electronics determine device properties. Characterization and control of these interfaces is the key to reliably manufacturing devices with high performance. We see an opportunity for electrical scanning probe microscope (eSPM) based techniques for characterizing the through silicon vias (TSVs) and interposers for three-dimensional integrated circuits (3D-ICs); fault detection in back end of the line (BEOL) multi-level metallization processes, and for the characterization of 2D materials, stacked 2D materials, and other nano-structured materials [1]. The most common electrical eSPMs are the scanning spreading resistance (SSRM), scanning capacitance (SCM), scanning microwave (SMM), electrostatic force (EFM) and scanning Kelvin force (SKFM), and tunneling atomic force (TUNA) microscopes. For a review of many of these techniques see [2]. Since these eSPMs gather information from such a wide volume, it is possible to isolate electrical information from buried interfaces using proper data acquisition techniques.
We have been systemically developing the metrology techniques to make eSPMs practical and quantitative techniques to non-destructively interrogate buried interfaces in semiconductors and other structures found in integrated circuits. An important set of tools for understanding the operation and limitations of eSPMs are well-defined test structures. We have built dopant gradient test structures, transistor like test structures, and most recently an electro-magnetic field test chip [3]. Precise and accurate models of the electrical behavior of these test structures are required for comparison to the eSPM measured behavior. We have employed the COMSOL*Multi-Physics simulation software to develop models of the SKFM, EFM, and the resonant behavior of the SMM [1]. Through comparison of models and measurements on well-known structures an understanding of eSPM behavior and techniques to extract quantitative electrical properties from images can be developed. An important parameter in both model and measurement is the actual electrical shape of the measurement electrode consisting of the tip-shank-cantilever assembly.
We have characterized the ability of the SMM to image metal lines embedded in a dielectric film [4]. Metal lines under 2 micrometers of surface oxide could be resolved with the SMM. The contrast mechanism was found to vary complexly with the data acquisition parameters and the tip-to-sample impedance, with the buried lines sometimes increasing and sometimes decreasing the image contrast compared to regions without buried metal. This is due to two competing effects: changes in the SMM resonant Q-factor (which changes the width and peak height of the resonant peak) and shifts in the resonant frequency (which changes the peak location). The SMM can serve as a platform for broad-band microwave-based metrology, which we are developing as a technique for characterizing TSVs and other integrated circuit structures [5]. Significant improvements in the spatial resolution of the details of subsurface structures are expected as the SMM technique is refined.
SKFM and EFM have also proven very sensitive to changes in surface potential arising from sub-surface structure. A SKFM image of an interdigitated electrical field test structures buried beneath 800 nm of oxide is shown in Figure 1. Here, alternating lines are biased at either +1V or -1V and show up distinctly in an SKFM image of surface potential. An EFM phase image of a large metal structure (in this case a NIST logo) buried beneath 800 nm oxide is shown in Figure 2. EFM phase is extremely sensitive to surface potential variation. The buried metal, though electrically floating, still substantially alters the EFM phase signal detected at the surface. Details of the buried metal-oxide interface topography are visible in the EFM image.
*Certain commercial equipment, instruments, or materials are identified in this paper in order to specify the experimental procedure adequately. Such identification is not intended to imply recommendation or endorsement by NIST, nor is it intended to imply that the materials or equipment identified are necessarily the best available for the purpose.

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Figure 1