Dual-loop control for laser annealing of semiconductor wafers
View Patent ↗Systems and methods for performing semiconductor laser annealing using dual loop control are disclosed. The first control loop operates at a first frequency and controls the output of the laser and controls the 1/f laser noise. The second control loop also controls the amount of output power in the laser and operates at second frequency lower than the first frequency. The second control loop measures the thermal emission of the wafer over an area the size of one or more die so that within-die emissivity variations are average out when determining the measured annealing temperature. The measured annealing temperature and an annealing temperature set point are used to generate the control signal for the second control loop.
1. A method of annealing with a laser a wafer having a melt temperature and a surface that supports an array of dies with each die having a varying emissivity, comprising:
scanning an annealing laser beam from the laser over the array of dies, wherein the laser has laser noise and is adjustable to control an amount of power in the annealing laser beam, wherein the annealing laser beam creates an annealing temperature at the wafer surface that does not exceed the wafer melt temperature;
measuring and controlling the amount of power in the annealing laser beam using a first control loop that measures the amount of power and operates at a first frequency f 1 to control the laser noise in the laser; and
controlling the amount of power in the annealing laser beam using a second control loop that operates at a second frequency f 2 <f 1 by measuring thermal emission radiation from the wafer, including averaging the thermal emission radiation over at least one die and determining therefrom a corresponding average measured temperature, and using the average measured temperature and an annealing temperature set point to adjust the laser to control the amount of power in the annealing laser beam.
2. The method of claim 1 , wherein the first frequency f 1 is in a range from 1 kHz to 100 kHz and the second frequency f 2 is in the range from 1 Hz to 100 Hz.
3. The method of claim 1 , wherein f 1 is about (100)·f 2 .
4. The method of claim 1 , wherein measuring the amount of power in the laser annealing beam includes deflecting a portion of the laser annealing beam to a photodetector system.
5. The method of claim 4 , wherein deflecting a portion of the laser annealing beam includes diffracting a portion of the laser annealing beam using a grating formed on a reflective surface from which the laser annealing beam otherwise reflects.
6. The method of claim 1 , wherein the wafer has a Brewster's angle, and wherein the measuring thermal emission radiation from the wafer is performed at Brewster's angle.
7. The method of claim 1 , wherein the wafer is either a logic wafer or a memory wafer.
8. The method of claim 1 , wherein the first and second control loops respectively employ first and second proportional-integral-derivative controllers that are coupled to one another.
9. The method of claim 1 , wherein the laser annealing beam creates an annealing temperature at the wafer surface in the range from 400° C. to 1350° C.