Femtosecond laser-induced formation of submicrometer spikes on a semiconductor substrate
The present invention generally provides semiconductor substrates having submicron-sized surface features generated by irradiating the surface with ultra short laser pulses. In one aspect, a method of processing a semiconductor substrate is disclosed that includes placing at least a portion of a surface of the substrate in contact with a fluid, and exposing that surface portion to one or more femtosecond pulses so as to modify the topography of that portion. The modification can include, e.g., generating a plurality of submicron-sized spikes in an upper layer of the surface.
1. A system for fabricating a radiation-absorbing semiconductor substrate, comprising:
a processing chamber having an inlet port for introducing a gas into the chamber,
a movable substrate holder disposed in said chamber and configured for holding a semiconductor substrate,
a motion controller coupled to said substrate holder for moving the holder,
a window coupled to the chamber for allowing passage of radiation into the chamber for processing said semiconductor substrate,
a laser radiation source for generating laser pulses, said source being optically coupled to said window such that the laser pulses impinge on a surface of the semiconductor substrate,
a lens for focusing said laser radiation pulses onto the semiconductor substrate surface,
a camera positioned outside the chamber,
a mirror configured to redirect the laser radiation after its passage through the lens onto the camera when the mirror is positioned in the path of the laser radiation,
wherein said camera is positioned at a distance from the lens that is substantially equal to a distance of the lens from the substrate surface, thereby providing a measure of a spotsize of the radiation pulses on the substrate surface, and
wherein said controller is capable of moving the substrate while the surface of the substrate is irradiated by said radiation pulses in presence of the gas in the chamber.
2. The system of claim 1 , wherein said substrate holder is configured for movement in at least two orthogonal directions.
3. The system of claim 1 , wherein said laser radiation source generates radiation pulses having a duration of about 50 femtoseconds to about a few ns.
4. The system of claim 3 , wherein said laser radiation source generates radiation pulses having duration of about 50 femtoseconds to about 50 picoseconds.
5. The system of claim 1 , wherein said focused radiation pulses exhibit a fluence in a range of about 1 kJ/cm 2 to about 8 kJ/cm 2 at said semiconductor surface.
6. The system of claim 1 , wherein said laser radiation source comprises an amplified, Ti:Sapphire laser system.
7. The system of claim 1 , wherein said motion controller provides a micrometer precision in moving the substrate holder.
8. The system of claim 1 , further comprising a roughing pump coupled to the chamber for evacuating the chamber.
9. The system of claim 1 , wherein a repetition rate of said laser pulses and a speed by which said controller moves the substrate holder are configured such that each location of the irradiated surface is exposed to a number of laser pulses in a range of about 2 to about 2000.
10. The system of claim 1 , further comprising a viewport for viewing the substrate in the chamber.
11. The system of claim 1 , further comprising a single axis translation stage onto which the lens is mounted for moving the lens relative to the substrate surface so as to vary a spotsize of the laser radiation pulses on the substrate surface.
12. The system of claim 2 , wherein said substrate holder is configured for movement along a third direction orthogonal to said at least two directions.
13. The system of claim 1 , wherein said mirror comprises a flipper mounted mirror.
14. The system of claim 1 , wherein said camera comprises a CCD camera.
15. The system of claim 1 , further comprising
a source of visible radiation optically coupled to said window of the chamber for illuminating the substrate surface.
16. The system of claim 15 , wherein said camera comprises a CCD camera.