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edfas.org ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 22 NO. 4 12 The following paragraphs present data demonstrating the significance of electrical property depth profiles and the capabilities of DHEM technique in two specific appli- cation areas—ohmic contact engineering and channel mobility characterization. IMPORTANCE OF ELECTRICAL PROPERTY DEPTH PROFILES As stated earlier, interpreting 4PP and Hall effect measurement data to decide on the success or failure of a processing step may be problematic. These measure- ments only yield the average or effective values of sheet resistance (R s ) and mobility. If these parameters show variability through the film, the results from these bulk measurements may not be meaningful for a region of the film (e.g., surface) that may be of special interest. The fol- lowing example demonstrates this point. Two Si samples were phosphorus-doped under two different conditions (Process A and Process B). 4PP and Hall effect measurements yielded lower R s and effective mobility (µ Heff ) values for Process B compared to process A, as shown in Table 1. Assuming a 12 nm thickness, the carrier concentration value for Process B was also estimated to be higher than the value for Process A. The goal in this study was to improve dopant activation at the surface region of the films for low-resistance contact fabrication. If the bulk data was relied upon for process development or for prediction of device performance, one would wrongly conclude that Process B was better, because it yielded lower sheet resistance and higher effective carrier concentration value. The DHEM depth profiles obtained from the two samples and presented in Fig. 2, however, clearly demonstrated that the sample subjected to Process A conditions actually had a higher near-surface dopant activation compared to Process B, and therefore would yield a lower contact resistance. It should be noted that the SIMS profiles taken from the two samples suggested a higher dopant concentration near the surface region of the sample subjected to Process B. Clearly, this higher dopant concentration did not result in higher dopant activation in this sample. APPLICATION EXAMPLE: N-TYPE GERMANIUM OHMIC CONTACT ENGINEERING Contact engineering for semiconductors is a problem that requires extensive process development for high dopant activation at the near-surface region of the mate- rial. Germanium is a material that shows considerable promise for advanced device manufacturing because of its higher electron and hole mobilities compared to Si. In addition, similar electron and holemobility valuesmakes CMOS technology viable, which simplifiesmanufacturing process significantly. Germanium PMOS has been dem- onstrated to be high performance, but NMOS has lagged Fig. 2 Two phosphorous-doped Si samples subjected to two different process conditions (Process A and Process B) have been characterized by DHEM and SIMS. DHEM depth profiles show a large variation in the carrier concentration and mobility values for Process A, as well as higher degree of dopant activation near the surface. Table 1 Sheet resistance and effective mobility values Process A Process B R s (Ω/ □ ) 835.86 494.09 μ H eff (cm 2 /V-s) 58.15 40.41 n eff (cm -3 ) 1.07x1020 2.6x1020 *Sheet resistance (R s ) and effective mobility (µ Heff ) values measured by 4PP and Hall effect techniques for two phosphorous-doped Si samples processed in two different ways. Effective carrier concentration (n eff ) values were calculated assuming a 12 nm junction depth.