Scanning control for a pulsed laser beam in ablation

Abstract

A fast and smooth scanning is described for achieving uniform ablated surface without relying on any synchronization between the laser pulses and the scanner mirror positions. The scanning takes a series of close loops and the scanning speed on each loop is fine-tuned according to the perimeter of the loop. A uniform and close-packed pulse disposition along each loop can be achieved by multiple successive scans along the loop, while the consecutive pulses of a scan can be well separated. The scanning pattern is such designed that the energy distribution is uniform for every layer and the smoothness of the ablated surface remains substantially unchanged as the number of the layer increases.

What is claimed is:  
     
         1 . A method for scanning a pulsed laser beam for uniform surface ablation, comprising: 
 projecting and scanning a pulsed laser beam on a target surface at a scanning rate;    controlling the scanning of the laser beam in a programmable fashion so as to produce a scanning pattern of a series of smooth, close circular loops without synchronization between laser pulses and scanner positions and without predetermining pulse positions on the target surface; and    controlling a scanning speed of the laser beam along each close circular loop so that consecutive laser pulses are uniformly separated in each scan on the target surface and that a plurality of successive scans are carried out on each close circular loop.    
     
     
         2 . The method as in    claim 1   , wherein the scanning speed for a circular loop is dependent on a perimeter of the circular loop, a given pulse repetition rate, a number of laser pulses in each scan, and a number of the plurality of successive scans on each close circular loop.  
     
     
         3 . The method as in    claim 2   , wherein the scanning speed for a circular loop of a perimeter of P for a given pulse repetition rate K is mPK/(nm+1), where n is a number of laser pulses in each scan and m is a number of the plurality of successive scans on each close circular loop.  
     
     
         4 . The method as in    claim 1   , wherein the laser beam has a pulse repetition rate from about 0.2 kHz to about 10 kHz.  
     
     
         5 . The method as in    claim 1   , wherein the laser beam is the ultraviolet spectral range.  
     
     
         6 . The method as in    claim 1   , wherein the laser beam is the visible spectral range.  
     
     
         7 . The method as in    claim 1   , wherein the laser beam is the infrared spectral range.  
     
     
         8 . The method as in    claim 1   , wherein the laser beam is in the spectrum of deep UV ranging from about 220 nm to about 180 nm.  
     
     
         9 . The method as in    claim 1   , wherein the laser beam is at a wavelength of about 3 microns.  
     
     
         10 . The method as in    claim 1   , further comprising controlling the laser beam to have a pulse energy in the range approximately from 40 uJ to 1000 uJ.  
     
     
         11 . The method as in    claim 1   , wherein the laser beam has a pulse duration in the range approximately from 0.1 ns to 10 ns.  
     
     
         12 . The method as in    claim 1   , wherein the laser beam has a spot size ranging from 50 to 1000 microns on the target surface.  
     
     
         13 . The method as in    claim 1    wherein the selecting a scanning pattern includes selecting a scanning pattern that consists of a series of concentric rings on each layer.  
     
     
         14 . The method as in    claim 1    wherein two consecutive pulses are spaced from each other on the target by a distance to reduce an effect of a plume produced by one pulse on the ablation of the target by another pulse.  
     
     
         15 . The method as in    claim 1   , wherein scanning of the laser beam from loop to loop is spirally inward and outward alternately.  
     
     
         16 . The method as in    claim 1    wherein scanning of the laser beam layer by layer displaces peaks and valleys ablated on the target from one layer to another successive layer so as to control the radial average over the layers.  
     
     
         17 . The method as in    claim 1   , wherein the scanning rate in scanning the laser beam is higher than a pulse repetition rate of the laser beam.  
     
     
         18 . A method for scanning a pulsed laser beam for uniform surface ablation, comprising: 
 projecting and scanning a pulsed laser beam on a target surface;    controlling the scanning of the laser beam to produce a scanning pattern of a series of smooth, close circular loops without synchronization between laser pulses and scanner positions and without relying on predetermining pulse positions on the target surface;    controlling a scanning speed of the laser beam along each close circular loop so that consecutive laser pulses are uniformly separated in each scan on the target surface;    maintaining a scanning speed at a constant for scanning in different loops; and    adjusting a pulse repetition rate of the laser beam in response to a change in the loop perimeter when scanning the laser beam from one loop to another loop.    
     
     
         19 . The method as in    claim 18   , further comprising scanning the laser beam smoothly from loop to loop and then from layer to layer according to the scanning pattern so that the smoothness of the ablated surface remains substantially unchanged as the number of layers increases.  
     
     
         20 . The method as in    claim 18   , wherein the scanning speed for a circular loop is dependent on a perimeter of the circular loop, a given pulse repetition rate, a number of laser pulses in each scan, and a number of the plurality of successive scans on each close circular loop.  
     
     
         21 . The method as in    claim 20   , wherein the scanning speed for a circular loop of a perimeter of P for a given pulse repetition rate K is mPK/(nm+1), where n is a number of laser pulses in each scan and m is a number of the plurality of successive scans on each close circular loop.  
     
     
         22 . The method as in    claim 18   , wherein the scanning rate in scanning the laser beam is higher than a pulse repetition rate of the laser beam.