n-wells voltage contrast imaging with a Focused Ion Beam
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R10.9.1
n-wells voltage contrast imaging with a Focused Ion Beam Erwan Le Roy and Mark Thompson NPTest, Inc, 150 Baytech Drive San Jose, CA 95134, USA
Introduction Using a focused ion beam (FIB), secondary electron (SE) imaging of n-wells under oxide from the backside of thinned integrated circuits without electrical bias was accomplished. From the backside, the n-wells were initially observed at a remaining silicon thickness ~4.5µm, which correlates to the actual implant depth where n and p carrier concentrations are equal. When the wells were FIB imaged, contrast appeared dark relative to the p substrate. During deposition of the oxide film, the n-well brightness changed from dark relative to the p-substrate, to bright. It appears that initially during this deposition step the interaction volume of the beam reached the silicon/oxide interface to create tunneling electrons. This phenomenon dominated the capacitive effect. Then as the film thickness increased the capacitive effect prevailed. The imaging structure is analogous to a MetalOxide-Semiconductor (MOS) capacitor. The n- and p-MOS capacitive properties yielded a permanent imaging contrast. At an optimized oxide thickness (130nm), the n-wells appear white relative to the p-substrate with a contrast up to 85% {(Ip-substrate – In-wells)/(Ipsubstrate + In-wells)}.
Experimental details A trench (150 µm)2 to ~4.5µm remaining silicon was performed by an IDS OptiFIB (NPTest, Inc). After much experimentation a procedure to observe permanent voltage contrast has been optimized. Trenching was first done with 30keV Ga+ at a XeF2 pressure of 2.5x10-5 Torr. When approaching approximately 10 µm of remaining silicon thickness, the beam energy was switched to 15 keV. It is important to reduce the beam energy to observe the n-wells. A higher energy would create a higher electronic surface state density and reduce the contrast [1]. It will pin the energy level for the exposed p and n materials toward the Fermi level, decrease the ionization energy difference between n and p and reduce the contrast level. The next step was to remove implanted Ga and its induced silicon amorphous layer. Auger Electron Spectroscopy (AES) experiments showed that during 15keV Ga trenching Ga implantation reached ~4.5 nm deep (Figure 1). An 8sec XeF2 treatment without using any ion beam removed completely Ga from the silicon.
Figure 1 – AES analysis of trench floor before and after spontaneous XeF2 treatment .
R10.9.2
To enhance the n-well voltage contrast, silicon monoxide was deposited with 6keV Ga. The film deposition was in two steps. First, a “protective” layer was applied at an ion current density of 0.05pA/µm2 to a thickness of 12nm. As the lower the beam current density the higher the deposition rate, the Gallium used was minimal. (Note: the thickness of this “protective” film equals the maximum 6keV Ga simulated implantation depth, 12nm.) Secondly, an oxide was deposited at 0.1pA/µm2 to the 130nm thickness. Voltage contrast was then observed permanently at 30 keV with collected SEs (
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