TEMPERATURE FEEDBACK METHOD FOR DIFFUSION PROFILE CONTROL
Original Publication Date: 1991-Dec-01
Included in the Prior Art Database: 2001-Dec-27
Assuming an error function or gaussian impurity dis- tribution in a semiconductor, the relationship D, t,=D, t, can be shown to hold for a given diffusion impurity con- centration profile. In the equation, D,=D, exp(-Ev/kT,), D, is the diffusion coefficient for an impurity with activa- tion energy Ev, at an absolute temperature T, for a time t,. Similarly, D2 is the ditfusion coefficient at a diflerent tem- perature T2 and a different time, t> The integral "D t" can be measured with an in situ thermocouple. "D" is com- puted from the average temperature sampled in small increments of time, At. When the computed summation of X.(D At), equals a standard D2 t2 product (which is a constant for a given process), the concentration profiles are equal. Thus, compensations for temperature fluctuation can be easily made. For example, when the measured temperature becomes higher than the desired temperature T2, a suitable control circuit would cause the furnace to ramp down the temperature at a time t,= IAt c t2 until 1 D At=D, t2. This ensures equal "D t" products resulting in equal junction protiles which decreases the spread in junction bre'akdown voltages. Process simulations with SUPREM show this to be a viable procedure for dii- sion parameter ControL The feedback system can be easily implemented using a simple computer program along with multiplexing hardware for thermocouple inputs and feedback signals to the furnaces.