Electrical AFM Measurements for Evaluation of Nitride Erosion in Shallow Trench Isolation Chemical Mechanical Planarizat
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Electrical AFM Measurements for Evaluation of Nitride Erosion in Shallow Trench Isolation Chemical Mechanical Planarization Yordan Stefanov, Tino Ruland, Udo Schwalke Institute of Semiconductor Technology, Darmstadt University of Technology Schlossgartenstr. 8, 64289 Darmstadt, Germany ABSTRACT This article proposes a new application of tunneling current measurements Atomic Force Microscopy (AFM) for evaluation of silicon nitride stop-layer erosion in Shallow Trench Isolation (STI) Chemical Mechanical Planarization (CMP). Simultaneous topographical and electrical AFM measurements allow a clear identification of ‘open’ silicon surfaces on nanometer scale by enhanced tunneling currents in those areas. The measurement technique is non-destructive and can be successfully implemented for process control. INTRODUCTION STI CMP is one of the hardest tasks in CMOS front-end processing. Polish pad compressibility combined with a small process window and pattern density variations leads to oxide dishing and nitride erosion after the CMP step [1-3]. Excessive nitride erosion results in exposed and damaged active area surfaces, deteriorating device characteristics [4] and decreasing yield [5]. Standard measurement techniques such as ellipsometry and profilometry, and even the powerful Scanning Electron Microscopy, Scanning Tunneling Microscopy (STM) and topographical AFM do not provide a simple, non-destructive and efficient means of detecting fully eroded areas on the device level. In this work the application of simultaneous tunneling current and topographical AFM measurements for such a means is described and its advantages over other measurement methods are discussed. EXPERIMENTAL DETAILS Measurement setup The AFM device used in this work is a Dimension 3100 Scanning Probe Microscope (SPM) by Digital Instruments. The basic operating principle of the microscope is the following: a sharp probe is dragged along a sample surface and the deflection of a laser beam, reflected by the elastic probe holder, is detected by an optical system, yielding topographical information. An AFM is a much more precise tool than a profilometer, capable of nanometrer scale topographical measurements, such as surface roughness measurements. Among the numerous applications of AFM, the possibility for post-CMP dishing metrology has been demonstrated [6]. An additional tunneling current (TUNA) application module on the D3100 SPM is capable of performing electrical measurements simultaneously with the topographical ones. A certain bias is applied to the sample and the resulting tunneling current between the sample and an electrically conductive metal coated or diamond-like-carbon (DLC) coated probe is measured. In this work, DLC probes supplied by NanoAndMore were used. Probe tip radius of around 10 nm and current sensitivity in the fA range allow high resolution electrical analysis. Current suggested
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Fig 1. Test wafer surface after Chemical Mechanical Planarization
Fig. 2 Optical microscope picture of a part of an STI test wafer be
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