Laser thermal annealing of lightly doped silicon substrates
View Patent ↗A method for performing laser thermal annealing (LTA) of a substrate using an annealing radiation beam that is not substantially absorbed in the substrate at room temperature. The method takes advantage of the fact that the absorption of long wavelength radiation (1 micron or greater) in some substrates, such as undoped silicon substrates, is a strong function of temperature. The method includes preheating a portion of the substrate to a critical temperature where the absorption of long-wavelength annealing radiation is substantial, and then irradiating the portion of the substrate with the annealing radiation to generate a temperature capable of annealing the portion of the substrate.
1. A method of preheating a portion of a substrate having a surface in order to subsequently perform laser thermal annealing of the portion of the substrate with an annealing radiation beam that is not substantially absorbed by the substrate at room temperature, the method comprising:
irradiating the portion of the substrate with a preheating radiation beam that is substantially absorbed by the substrate at room temperature;
receiving preheating radiation that is reflected from the portion of the substrate; and
directing the received preheating radiation back to the portion of the substrate as a recycled radiation beam.
2. The method of claim 1 , wherein directing the received preheating radiation back to the portion of the substrate includes reflecting the received preheating radiation with a corner cube reflector.
3. The method of claim 1 , wherein directing the received preheating radiation back to the portion of the substrate includes reflecting the received preheating radiation from a roof mirror and a cylindrical mirror.
4. The method of claim 1 , wherein directing the received preheating radiation back to the portion of the substrate includes diffracting the received preheating radiation with a diffraction grating that is tilted with respect to the received preheating radiation so the recycled radiation beam directed back to the substrate is kept in focus across the substrate surface.
5. A method of preheating a portion of a substrate having a surface in order to subsequently perform laser thermal annealing of the portion of the substrate with an annealing radiation beam that is not substantially absorbed by the substrate at room temperature, the method comprising:
irradiating the portion of the substrate with first and second preheating radiation beams each having a wavelength that is substantially absorbed by the substrate at room temperature forming first and second scanned images respectively on the substrate surface; and
maintaining the first and second scanned images ahead of a third scanned image formed on the substrate surface by an annealing radiation beam when the preheating radiation beams and the annealing radiation beam are scanned relative to the substrate surface so the annealing radiation beam is substantially absorbed by the substrate when it encounters the portion heated by the first and second scanned images.
6. The method of claim 5 , wherein the first and second preheating radiation beams have the same wavelength.
7. The method of claim 5 , wherein the annealing radiation beam is incident the substrate at Brewster's angle, and wherein the first and second preheating radiation beams are each incident the substrate over a range of angles that includes a central angle, wherein the central angle for each range of angles is different than Brewster's angle.
8. The method of claim 5 , wherein the annealing radiation beam and the first and second preheating radiation beams are incident the substrate at respective angles that minimize variations in absorption from structures present on the substrate surface.
9. The method of claim 5 , including forming the first and second preheating radiation beams to each have I) a numerical aperture at the substrate between 0.15and 0.5,and ii) an incident angle of about 52°.