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Unformatted text preview: PROBLEM 1.41 KNOWN: Hot plate-type wafer thermal processing tool based upon heat transfer modes by conduction through gas within the gap and by radiation exchange across gap. FIND: (a) Radiative and conduction heat fluxes across gap for specified hot plate and wafer temperatures and gap separation; initial time rate of change in wafer temperature for each mode, and (b) heat fluxes and initial temperature-time change for gap separations of 0.2, 0.5 and 1.0 mm for hot plate temperatures 300 < T h < 1300 C. Comment on the relative importance of the modes and the influence of the gap distance. Under what conditions could a wafer be heated to 900 C in less than 10 seconds? SCHEMATIC: ASSUMPTIONS: (1) Steady-state conditions for flux calculations, (2) Diameter of hot plate and wafer much larger than gap spacing, approximating plane, infinite planes, (3) One-dimensional conduction through gas, (4) Hot plate and wafer are blackbodies, (5) Negligible heat losses from wafer backside, and (6) Wafer temperature is uniform at the onset of heating. PROPERTIES: Wafer: = 2700 kg/m 3 , c = 875 J/kg K; Gas in gap: k = 0.0436 W/m K. ANALYSIS: (a) The radiative heat flux between the hot plate and wafer for T h = 600 C and T w = 20 C follows from the rate equation, ( 29 ( 29 ( 29 ( 29 4 4 4 4 8 2 4 4 2 rad h w q T T 5.67 10 W / m K 600 273 20 273 K 32.5kW / m - =- +- + = = < The conduction heat flux through the gas in the gap with L = 0.2 mm follows from Fouriers law, ( 29 2 h w cond 600 20 K T T q k 0.0436W / m K 126 kW / m L 0.0002 m-- = = = < The initial time rate of change of the wafer can be determined from an energy balance on the wafer at the instant of time the heating process begins, w in out st st i dT E E E E c d dt - = = where out E = and in rad cond E q o r q . = Substituting numerical values, find 3 2 w r a d 3 i,rad dT q 32.5 10 W / m 17.6 K /s dt cd 2700kg / m 875 J / kg K 0.00078 m = = = < w cond i,cond dT q 68.4 K /s dt cd = = < Continued .. PROBLEM 1.41 (Cont.) (b) Using the foregoing equations, the heat fluxes and initial rate of temperature change for each mode can be calculated for selected gap separations L and range of hot plate temperatures T h with T w = 20 C. In the left-hand graph, the conduction heat flux increases linearly with T h and inversely with L as expected. The radiative heat flux is independent of L and highly non-linear with T h , but does not approach that for the highest conduction heat rate until T h approaches 1200 C. The general trends for the initial temperature-time change, (dT w /dt) i , follow those for the heat fluxes....
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- Fall '07