Feb_March_AMP_Digital

A D V A N C E D M A T E R I A L S & P R O C E S S E S | F E B R U A R Y / M A R C H 2 0 1 8 6 4 14 FEATURE in a solution of Almco 2510 (citric acid solution) to remove surface oxidation and contaminants. They were rinsed in distilled water, methanol, and dried with compressed nitro- gen gas. Each sample was weighed using an analytical bal- ance that records up to four decimal points in grams. Upon removal from the heat treat furnace, they were immediately reweighed using the same scale. The measured weight loss indicates the capability of the gas/partial pressure combina- tion to suppress copper evaporation. A graphite-insulated vacuum furnace with a cylindrical, vertical hot zone, 10 in. diameter × 18 in. high, was used for this study. The three coupons were hung on separate stain- less steel wires attached to the furnace lid. All cycles began after an initial pump down to 1 x 10 -4 Torr (1.3 × 10 -2 Pa) to en- sure vacuum integrity. Three temperatures were chosen for testing: 1575°F (857°C), 1700°F (927°C), and 1825°F (996°C), to cover the common LPC temperature range. Four atmo- sphere types were used for processing: vacuum at <1 × 10 -3 Torr (<13.3 × 10 -2 Pa), partial pressurenitrogen, argon, andhy- drogen, at 0.25 Torr (33.3 Pa), 2.5 Torr (333.3 Pa), and 10 Torr (1.3 kPa). The partial pressure was set prior to heating and maintained during the six-hour hold time at temperature. All samples were cooled in nitrogen at 625 Torr (83.3 kPa). RESULTS The average rate of copper evaporation for each tem- perature, gas species, and pressure is listed in Table 1. The evaporation rates versus pressure curves in Fig. 3 are re- presentative of all three gases and <1 × 10 -3 Torr (<13.3 × 10 -2 Pa) studied for the 1700°F (927°C) test. Copper evapora- tion drops off sharply in nitrogen and argon, even at pres- sures as low as 0.25 Torr (33.3 Pa). Argon is slightlymore sup- pressive than nitrogen, and hydrogen is considerably less effective at reducing metal vapor losses at the temperatures studied. The data reveals that copper evaporation decreases with a decrease in temperature and an increase in pressure. Figure 3 also shows a flattening of the curve as pressure in- creases, indicating that suppression slows down with a con- tinued increase in pressure. CONCLUSIONS Stringent pressure control and gas species type both play an important role in minimizing the evaporation rate of not only copper, but other elements susceptible to evap- oration in vacuum systems. Nitrogen at 2.5 Torr is the most economic and effective gas for minimizing copper evapo- ration. Argon was marginally more effective at suppressing evaporation compared to nitrogen. The use of nitrogen gas is increasingly important during longer time diffusion steps and as the LPC temperature increases in order to minimize copper evaporation and copper contamination of furnace components. ~HTPro For more information: Trevor Jones is CEO of Solar Manufacturing Inc., 1983 Clearview Rd., Souderton, PA 18964, 267.384.5040 ext. 1501, trevor@solarmfg.com, www .solarmfg.com . References 1. V. Heuer, Low Pressure Carburizing, ASM Handbook, Volume 4A: Steel Heat Treating Fundamentals and Process- es, ASM International, Materials Park, Ohio, 2013. 2. D. Herring, Using Partial Pressure in Vacuum, Industrial Heating, November 2005. 3. W. Jones, Partial Pressure Vacuum Processing – Part I and II, Industrial Heating, September and October 1997. TABLE 1 — EVAPORATION RATE (GRAM/CM 2 MINUTE) VS. PRESSURE Pressure 1575°F 1700°F 1825°F Torr N 2 Ar H 2 N 2 Ar H 2 N 2 Ar H 2 < 0.001 4.95E-07 4.95E-07 4.95E-07 3.79E-06 3.79E-06 3.79E-06 1.93E-05 1.93E-05 1.93E-05 0.250 1.47E-07 1.31E-07 2.28E-07 1.14E-06 8.68E-07 3.06E-06 5.26E-06 4.43E-06 1.70E-05 2.5 3.58E-08 4.30E-08 1.22E-07 1.36E-07 1.51E-07 8.29E-07 7.31E-07 6.93E-07 4.70E-06 10 2.51E-08 1.43E-08 5.70E-08 2.50E-08 5.38E-08 3.19E-07 2.79E-07 1.99E-07 1.61E-06 Fig. 3 — Evaporation rate of copper vs. pressure at 1700°F.