Method and system for precisely positioning a waist of a material-processing laser beam to process microstructures within a laser-processing site

Abstract

A high-speed method and system for precisely positioning a waist of a material-processing laser beam to dynamically compensate for local variations in height of microstructures located on a plurality of objects spaced apart within a laser-processing site are provided. In the preferred embodiment, the microstructures are a plurality of conductive lines formed on a plurality of memory dice of a semiconductor wafer. The system includes a focusing lens subsystem for focusing a laser beam along an optical axis substantially orthogonal to a plane, an x-y stage for moving the wafer in the plane, and a first air bearing sled for moving the focusing lens subsystem along the optical axis. The system also includes a first controller for controlling the x-y stage based on reference data which represents 3-D locations of microstructures to be processed within the site, a second controller, and a first voice coil coupled to the second controller for positioning the first air bearing sled along the optical axis also based on the reference data. The reference data is generated by the system which includes a modulator for reducing power of the material-processing laser beam to obtain a probe laser beam to measure height of the semiconductor wafer at a plurality of locations about the site to obtain reference height data. A computer computes a reference surface based on the reference height data. A trajectory planner generates trajectories for the wafer and the waist of the laser beam based on the reference surface. The x-y stage and the first air bearing sled controllably move the wafer and the focusing lens subsystem, respectively, to precisely position the waist of the laser beam so that the waist substantially coincides with the 3-D locations of the microstructures within the site. The system also includes a spot size lens subsystem for controlling size of the waist of the laser beam, a second air bearing sled for moving the spot size lens subsystem along the optical axis, a third controller for controlling the second air bearing sled, and a second voice coil coupled to the third controller for positioning the second air bearing sled along the optical axis.

What is claimed is:  
     
         1 . A method for precisely positioning a waist of a material-processing laser beam to dynamically compensate for local variations in height of microstructures located on a plurality of objects spaced apart within a laser-processing site, the method comprising: 
 providing reference data which represents 3-D locations of microstructures to be processed within the site;    positioning the waist of the laser beam along an optical axis based on the reference data; and    positioning the objects in a plane based on the reference data so that the waist of the laser beam substantially coincides with the 3-D locations of the microstructures within the site.    
     
     
         2 . The method as claimed in  claim 1  wherein the objects are dice of a semiconductor wafer.  
     
     
         3 . The method as claimed in  claim 2  wherein the microstructures are conductive lines of the dice.  
     
     
         4 . The method as claimed in  claim 3  wherein the conductive lines are metal lines.  
     
     
         5 . The method as claimed in  claim 1  wherein the objects are semiconductor devices.  
     
     
         6 . The method as claimed in  claim 5  wherein the semiconductor devices are semiconductor memory devices.  
     
     
         7 . The method as claimed in  claim 2  wherein the dice are semiconductor dice.  
     
     
         8 . The method as claimed in  claim 2  wherein the step of providing includes the step of measuring height of the semiconductor wafer at a plurality of locations about the site to obtain reference height data.  
     
     
         9 . The method as claimed in  claim 8  wherein the step of providing further includes the steps of computing a reference surface based on the reference height data and generating trajectories for the wafer and the waist of the laser beam based on the reference surface.  
     
     
         10 . The method as claimed in  claim 9  wherein the reference surface is non-planar.  
     
     
         11 . The method as claimed in  claim 1  further comprising varying size of the waist of the laser beam along the optical axis.  
     
     
         12 . The method as claimed in  claim 8  wherein the step of providing includes the steps of reducing power of the material-processing laser beam to obtain a probe laser beam and utilizing the probe laser beam to perform the step of measuring.  
     
     
         13 . A system for precisely positioning a waist of a material-processing laser beam to dynamically compensate for local variations in height of microstructures located on a plurality of objects spaced apart within a laser-processing site, the system comprising: 
 a focusing lens subsystem for focusing a laser beam along an optical axis;    a first actuator for moving the objects in a plane;    a second actuator for moving the focusing lens subsystem along the optical axis;    a first controller for controlling the first actuator based on reference data which represents 3-D locations of microstructures to be processed within the site; and    a second controller for controlling of the second actuator also based on the reference data wherein the first and second actuators controllably move the objects and the focusing lens subsystem, respectively, to precisely position the waist of the laser beam and the objects so that the waist substantially coincides with the 3-D locations of the microstructures within the site.    
     
     
         14 . The system as claimed in  claim 13  further comprising a support for supporting the second actuator and the focusing lens subsystem for movement along the optical axis.  
     
     
         15 . The system as claimed in  claim 14  further comprising; 
 a spot size lens subsystem for controlling size of the waist of the laser beam;  
 a third actuator for moving the spot size lens subsystem wherein the support supports the spot lens subsystem and the third actuator for movement along the optical axis; and  
 a third controller for controlling the third actuator.  
 
     
     
         16 . The system as claimed in  claim 13  wherein the first actuator is an x-y stage.  
     
     
         17 . The system as claimed in  claim 14  wherein the second actuator is an air bearing sled for supporting the focusing lens subsystem and mounted for sliding movement on the support.  
     
     
         18 . The system as claimed in  claim 15  wherein the third actuator is an air bearing sled for supporting the spot size lens subsystem and mounted for sliding movement on the support.  
     
     
         19 . The system as claimed in  claim 17  further comprising a voice coil coupled to the second controller for positioning the air bearing sled along the optical axis.  
     
     
         20 . The system as claimed in  claim 13  further comprising a position sensor for sensing position of the focusing lens subsystem and providing a position feedback signal to the second controller.  
     
     
         21 . The system as claimed in  claim 20  wherein the position sensor is a capacitive feedback sensor.  
     
     
         22 . The system as claimed in  claim 13  wherein the laser beam is a Gaussian laser beam.  
     
     
         23 . The system as claimed in  claim 13  wherein the objects are dice of a semiconductor wafer.  
     
     
         24 . The system as claimed in  claim 23  further comprising a trajectory planner coupled to the first and second controllers for generating trajectories for the wafer and the waist of the laser beam.  
     
     
         25 . The system as claimed in  claim 24  wherein at least one of the trajectories has an acceleration/deceleration profile.  
     
     
         26 . The system as claimed in  claim 24  further comprising a modulator for reducing power of the material-processing laser beam to obtain a probe laser beam to measure height of the semiconductor wafer at a plurality of locations about the site to obtain reference height data.  
     
     
         27 . The system as claimed in  claim 26  further comprising a computer for computing a reference surface based on the reference height data and wherein the trajectory planner generates the trajectories based on the reference surface.  
     
     
         28 . The system as claimed in  claim 27  wherein the reference surface is non-planar.