Dual Lens Electron Holography, Scanning Capacitance Microscopy (SCM), Scanning Spreading Resistance Microscopy (SSRM) Comparison for Semiconductor 2-D Junction Characterization
Y.Y. Wang,1† * J. Nxumalo,1,2† *
charlie444wang@gmail.com
Abstract: 2-D junction characterization by dual lens electron hologra- phy, scanning capacitance microscopy (SCM), and scanning spread- ing resistance microscopy (SSRM) on a variety of semiconductor devices is reported, including optical modulators, regular comple- mentary metal-oxide-semiconductor
(CMOS) devices, and SiGe
hetero-junction bipolar transistors. In most cases these techniques provide comparable results, while in some instances one technique has advantages over the other and vice versa. Advantages and disad- vantages of each technique are discussed.
Keywords: Dual lens electron holography, scanning capacitance
microscopy (SCM), scanning spreading resistance microscopy (SSRM), 2-D junction profile, SiGe hetero-junction bipolar transistor (HBT)
Introduction Junction profiling at the microscopic scale is a critical
part of characterization for semiconductor development and manufacturing. Over the years, the accuracy and repeatabil- ity of semiconductor junction profiling techniques have been improved significantly, especially electron holography, scan- ning capacitance microscopy (SCM), and scanning spreading resistance microscopy (SSRM). However, the potential of these junction profiling techniques has not yet been fully realized and well publicized in the semiconductor industry. In this paper, we provide a review of a few examples
illustrating how these techniques compare in the junction characterization of semiconductor devices and discuss their common features and differences. Although the techniques address two-dimensional (2-D) junction characterization in semiconductor devices, they measure different types of physical properties of the semiconductor material. Holog- raphy measures electrostatic potential due to carrier con- centration variation and is a bulk measurement technique; SCM measures carrier type and concentration and is a near surface technique; and SSRM measures local spreading resis- tance and is also a near surface technique. Tese techniques provide complementary results of active dopant distribution in a semiconductor sample. In general, electron holography can achieve higher spatial resolution compared to SCM and SSRM. Electron holography requires thick samples for sensi- tivity, whereas SCM is more sensitive to surface active dopant and does not require thick samples. Overall, these techniques provide comparable results with minor differences. Depend- ing on the application, one technique has an advantage over the other.
36 doi:10.1017/S1551929521000675
Theoretical Background Electron holography. Electron holography measure-
ments are performed using a transmission electron microscope (TEM). Here, electrostatic potential is determined by measuring the phase difference between electron beams passing through n-doped and p-doped regions of the semiconductor devices [1–14]. A schematic of an electron holography setup is shown in Figure 1a, where an electron beam passing through the sample interferes with the reference beam passing through a nearby vacuum and forms interference fringes over the image of the sample. Figure 1b shows a phase shiſt of the fringe where holog- raphy fringes are overlaid on top of different types of semicon- ductor: n-type or p-type. Fourier transform on the holographic image is used to obtain an image in reciprocal space where one of the two sidebands is selected, and the main beam is masked off. An inverse Fourier transform on one of the selected side- bands is performed to obtain phase and amplitude maps [9,10]. Te amplitude image is similar to a regular TEM image, while the phase image can only be measured through the interference imaging method. Te phase shiſt is proportional to the electro- static potential. Based on the theoretical calculations for Si at room temperature shown in Figure 2, the electrostatic potential varies linearly with the active dopant concentration [15]. Dual lens electron holography. In general, meaning-
ful semiconductor junction mapping by electron holography requires the following: (1) a fringe width (fringe overlap) in the range of about 100 to 800 nm for an adequate field of view (FOV); (2) fringe spacing between 0.2 and 10 nm for meaningful spatial resolution; (3) visibility of the fringe contrast (10–30%) for useful signal-to-noise ratio; and (4) adjustability of both the field of view and the fringe spacing relative to the sample. In previous papers and a patent disclosure, we reported
a dual lens electron holography method that meets the above requirements [2–6]. Te dual lens operation allows electron holography to be performed from low to high magnification and provides the field of view and fringe spacing necessary for 2-D junction profiling in devices with various sizes. Figure 3 summarizes the results for fringe width and
fringe spacing relative to the objective lens excitation from a FEI Titan TEM on which dual lens electron holography was implemented. Te results show the achieved range of values for FOV and fringe spacing necessary for semiconductor device characterization. Te fringe spacing decreases from 4nm
www.microscopy-today.com • 2021 May W. Zhao,2 K. Bandy,1 and K. Nummy1
1Globalfoundries Inc., 2070 Route 52, Hopewell Junction, NY 12533 2Globalfoundries Inc., 400 Stonebreak Road Extension, Malta, NY 12020 †Current address: Micron Technology, 8000 S. Federal Way, Boise, ID 83716
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