Semiconductor Characterization
sample surface. For silicon samples, high pressure (>8 GPa) causes the Si crystal structure to undergo a local phase transformation from diamond to a beta-tin phase in which the band gap collapses to zero, therefore forming an ohmic contact between the tip and sample. Te measured current is used to calculate the local spreading resistance at the tip- sample contact interface. Te spreading resistance (Rsp) var- ies with the local active doping concentration according to equation (1):
R qN a sp
= 1 µ 4 ⋅⋅ (1)
where N is the active dopant concentration in the sample near the tip contact, a is the tip contact radius, q is electron charge, and µ is the majority carrier mobility [20].
Experimental Results Electron holography and SCM junction profiles of an
Figure 8: Line profile of junction from left to right in Si of Figure 7b.
vacuum environment to prevent surface oxidation and a hard material (diamond) tip to allow high-pressure contact and thereby measure spreading resistance that directly correlates to active dopant concentration. By conducting these measure- ments in a high-vacuum environment (10−5
Torr), the sample
surface oxidation and water vapor are minimized. Vacuum SSRM allows reduction of the minimum pressure required to achieve spreading resistance mode imaging and, therefore, improved spatial resolution and good repeatability of results. Figure 6 also shows the required back contact to all regions to be imaged. Sample preparation methods have been developed to ensure that low-resistance ohmic back contact is formed for all regions of interest. Te typical spatial resolution of SSRM is ∼2 nm. To perform SSRM measurements, high-contact pressure is applied between a degenerately doped diamond tip and a
optical modulator. An optical modulator is a critical part of Si photonics circuitry, where properties of light propagating through a waveguide are modified to convert from a contin- uous beam to pockets of optical signals [21,22]. One example of the modulator design is a lightly doped p/n junction that connects with highly doped n+ and p+ on the leſt and right side for contact, shown in Figure 7a. Since this device has n+, n-, p-, and p+ dopant at close vicinity, it is an ideal device to study the variation of electrostatic potential and differential capacitance with active dopant concentration using electron holography and SCM. Figure 7b is an electrostatic potential map (phase map) measured by electron holography: the right side is the p+ dopant contact with approximately a one-rad phase shiſt, and the leſt side is the n+ dopant contact with a five-rad phase shiſt, shown as a line profile in Figure 8. Te middle section, which is indicated by the circled region in Figure 7b, is the optical modulator p/n junction with a smaller phase shiſt difference between n- and p-. Te phase shiſt steps down from leſt to right with n+, n-, p-, and p+ as dopant concentration changes, which is consistent with the plot of the elec- trostatic potential versus dopant con- centration shown in Figure 2. Ideally, if calibrated, the active dopant concentra- tion can be measured. Figure 7c is the differential capaci-
tance (dC/dV) map (amplitude and phase) by SCM: the extreme right is the p+ doped contact, and the extreme leſt is the n+ dopant contact. Te middle portion is an optical waveguide formed with a lower doped p/n junction, which forms an opti- cal modulator. A 1-D profile through the modulator structure in Figure 9 shows opposite polarity signals in n versus p doped Si,
low-signal
Figure 9: Lateral line profiles of carrier type and concentration taken through STI (shallow trench isolation) and Si in Figure 7c.
40 intensity in highly
doped n+ and p+ regions, and high inten- sity in n- and p- regions. Te p/n junction is observed as a signal swing between pos- itive and negative.
www.microscopy-today.com • 2021 May
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