Problems with The Concept of Thermal Budget: Experimental Demonstrations
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technological requirements. The total processing time lies on the order of 10-100 seconds, rather than the 10 minutes typifying conventional furnace processing. This significant reduction in cycle time can increase throughputs. Experiments often suggest reduced dopant diffusion, as well [3,4]. Over the last several years, RTP has appeared in a wide range of processing steps, including annealing, thermal oxide growth, silicidation, glass reflow, and chemical vapor deposition (CVD). Despite the widespread application, the kinetic modeling often remains quite crude and qualitative. Quantitative analysis of RTP usually employs the concept of "thermal budget," which is commonly defined as the area under the time-temperature curve characterizing the processing sequence. Efforts focus on minimizing the thermal budget in an attempt to reduce unwanted solid-phase diffusion or interface degradation. However, no rigorous kinetic demonstration exists to validate the utility of this approach. In this work we demonstrate experimentally that minimizing the budget suggests inferior heating programs in some cases. By accounting explicitly for rate selectivity, we lay out a superior framework that relies principally on comparing the activation energies for the desired and undesired physical phenomena. This framework permits rapid qualitative comparison of the kinetic consequences of various heating programs. While these ideas apply to many physical phenomena that can occur in RTP, they do not attempt to balance kinetic effects against considerations of wafer warpage, heating uniformity, particle control, cost-of-ownership, and the like. 313
Mat. Res. Soc. Symp. Proc. Vol. 470 01997 Materials Research Society
SELECTIVITY CONSIDERATION: A BRIEF PARABLE Although "thermal budget" has come to represent the area under a t-T curve, actual usage is often neither as precise nor as quantitative as this definition suggests. See Refs. [5] and [6] for a further discussion of this subject. Nevertheless, "thermal budget" presents an appealing metaphor for process design. In an industry heavily driven by cost constraints, "thermal budget" evokes images of fiscal budgets. However, this mental picture is misleading because it focuses on the integrated amount of some quantity as measured in the initial and final states. Fiscal budgets employ dollars, while thermal budgets employ time-temperature units. Unfortunately, in actual process steps multiple phenomena occur in parallel. For example, during silicidation there is dopant diffusion. The accurate depiction of a process step requires consideration of both phenomena throughout the entire step, rather than just at the initial and final states. An analogy may help clarify this idea. Consider a quaint situation drawn from the lives of children. It is winter, and Johnny wants to drill his much-younger sister Katie with a snowball in retaliation for all the names she calls him. He cannot accomplish this nefarious act at home because his mother will see, so he decides to lure Katie about 1 km away to a
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