Backscattered Electron Imaging in the Scanning Electron Microscope: the Use of Either: (a) High Incident Energy or (b) an Array Detector

Abstract

When high incident electron beam energies (E o ) are used in a scanning electron microscope (SEM), electrons penetrate far below the sample surface and a backscattered electron (BSE) detector can be used to image sub-surface structures. For semiconductors, bulk Cu interconnects encapsulated in low dielectric constant SiCOH dielectrics that are located at depths of ≤ 3 µm below the surface can be imaged with BSE's at E o ≥ 100 keV in a transmission electron microscope equipped with a BSE detector [1]. Sub-surface Cu interconnects are ideal structures for high energy BSE imaging because the BSE yield is atomic number (Z) dependent and the higher Z Cu/Ta interconnects stand out against the low Z SiCOH background. BSE imaging can be used to identify defects in sub-surface Cu lines without having to delayer or cross-section the structure. In Fig. 1a, a 400 keV BSE image of the cathode end of a 3-level Cu electromigration (EM) test structure is shown. The 70 nm wide, 2 nd metal level (M2) EM test Cu line is connected to a 0.4 µm wide, 3 rd Cu level (M3) strap through a Cu V2 via. There are 1 st Cu level (M1) fill squares below the M2/V2/M3 structure. The structure blew out once current was passed through this structure and the BSE image shows missing M2 line near the V2 via region. The TEM cross-section image in Fig. 1b shows a severely voided M2 EM test line of a similar test structure with the same electrical signature. In Fig. 2, 400 keV images of 2 µm wide Cu M1 EM test lines are shown connected below to a W via bar. There are M2 filled squares above this structure. Fig. 2a is a BSE image of an untested line, Fig. 2b is a BSE image of a tested line and Fig. 2c is a secondary electron (SE) image of the Fig. 2b line. The W via bar is more prominently seen in 400 keV BSE images than the SE image but the void shape better seen in the SE image. These images were used to measure void volume and determine Cu EM diffusion kinetics.Another BSE imaging technique that we are developing is the use of a BSE diode array detector to simulate a moveable BSE detector [2]. Many SEM's are equipped with an electron backscatter patterning (EBSP) array detector to determine the orientation of crystalline materials. However, instead of collecting only the high frequency crystalline signal component on the detector, the full image on the detector can be collected at each position that the electron beam scans on a sample, and, with the use of computer programming, BSE images can be formed by summing the intensity values on various regions on the detector. An early example of this technique is shown in Fig. 3. The sample studied consists of Cu squares in SiCOH. The diagram in Fig. 3a shows an electron beam impinging on a tilted sample and BSE's emanating from the surface at various angles. The electrons are sensed with an EBSP array detector and images are formed when the intensities on the top (Fig. 3b) and bottom (Fig. 3c) quarters of the images are summed for each point scanned on the sample. The top image shows more Z contrast detail wh...

Past, present, and future of backscatter electron (BSE) imaging

“…Use of large solid angle detector arrays The idea of using a large-area segmented backscattered (pixilated) detector has already been proposed and couple of initial experiments performed. [38][39][40][41] The detector array was developed for collecting images in an EBSP system. The advantage of using an array of backscatter detectors in a SEM is obvious: (1) information from each detector could be analyzed offline, (2) different detector elements could be combined to capture the backscattered electrons efficiently with large effective solid angle, (3) some detector elements could be combined in the "high" mode for large angle scattering where atomic contrast dominates, and others in the "low" mode for topological contrast sensitivity and (4) the detectors could be combined to form images synchronized with the scan of the incident electron beam.…”