edfas.org 13 ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 25 NO. 4 The CupraSelect chemical reaction is a disproportionation reaction as shown in Fig. 2. This reaction mechanism is well supported in the literature and through experimental results.[18-20] The main component of the reaction involves a rearrangement of the precursor when two precursor molecules are adsorbed on the surface. With the addition of heat, the attached CuHfac molecule slides over the adjacent adsorbed precursor molecule leaving a copper atom on the target surface. The resulting conductivity measurements support pure copper deposition with the lack of silicon and carbon incorporation into the deposited material support. Interestingly, the reaction has been found to be completely reversible. To date FIB based copper deposition has been accomplished with improved resistivity values, but with limited success in achieving bulk resistivity.[21-23] Laser-based systems have also been used in conjunction with FIBs to deposit material that lowers the resistance of the FIB connections deposited on the IC.[24,25] The challenges to FIB implementation include the limited precursor volatility (vapor pressure), which limits the amount of copper precursor that can be delivered to the target surface location and the room temperature deposition ion beam process. Remes has demonstrated this by improving the resistance of FIB deposited lines.[25] This work included deposition for expanding the debug capabilities for integrated circuits. Copper deposition can be performed with an existing VarioEdit Platform configured with a selection of optical objectives 5x, 10x, and 20x. These different objectives focus the laser spot on the sample surface and control the lateral dimensions of deposition by confining the thermal spreading of the heat. The system can be equipped with three lasers: a 532 nm continuous wave (CW) laser, a pulsed 532 nm laser, and 355 nm UV pulsed laser. These lasers enable a range of laser power delivery typically from 30 milliwatt (mW) for glass and ~1 W for thermally conductive sample surfaces such as silicon and metals. The laser system is equipped with a dedicated vacuum system capable of reaching a base pressure of 50 millitorr (mTorr). In general, a predominant component molecule in vacuum chambers at the base pressure is residual water. Previous work has indicated that the water can have beneficial effects on surface adhesion of the copper film, removing requirements to thermally bake out the chamber before deposition work is attempted. Pulsed valves for chemical delivery in the vacuum chamber take advantage of the use of excess TMVS to increase vapor pressure, stabilize the precursor, and prevent clogging of valves.[26] ANALYSIS OF LASER-BASED COPPER DEPOSITION The morphology, growth rate, and chemical composition of laser deposited thin films have been widely studied.[14] The deposited material is composed of 50 to 100 nm grain sizes and has been widely reported in the published literature.[14,18-19] The majority of previous copper laser chemical vapor deposition (LCVD) based results used carrier gases such as hydrogen,[14,19,25] helium,[25] or nitrogen[19] with a bubbler to entrain the liquid precursor and deliver it to the work chamber with flow rates of 1-2 standard cubic centimeter (sccm). Carrier gas methodologies are not efficient in their use of the precursor gas since the reaction area is minimal in comparison to the chamber and sample geometries. Hydrogen carrier gas can also present safety concerns, and helium can be difficult to resource. CupraSelect in an excess of TMVS and a direct delivery provides an efficient chemical delivery mechanism while reducing practical and reliability challenges such as clogged valves and precursor storage. Figure 3 shows the deposition of a 20 µm x 20 µm copper pad with a 20x objective using a 532 nm laser set to 0.3 W power. The initial deposition is shown in Fig. 3a, the copper films start thin and optically bright and shiny with direct illumination but become dark due to light scattering as the thickness increases due to additional Fig. 2 The CupraSelect chemical reaction is a disproportionation reaction.
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