Method and apparatus for via drilling and selective material removal using an ultrafast pulse laser

Abstract

A method and apparatus for selective material removal and via drilling for semiconductor applications using an ultrafast laser pulse directly from an ultrafast pulse laser oscillator without amplification are disclosed. The method and apparatus includes techniques to avoid/reduce the cumulative heating effect and to avoid machine quality degrading in multi shot ablation. Also the disclosed method and apparatus provide a technique to change the polarization state of the laser beam to reduce the focused spot size, and to improve the machining efficiency and quality. The disclosed method and apparatus provide a cost effective and stable system for high volume manufacturing and inspection applications. The disclosed method and apparatus have particular applications in, but not limited to, drilling vias for interconnect formation, selective material removal for application specific integrated circuits, selective material removal for flash memory applications, exposing layers for further semiconductor processing such as wire bonding etc. The ultrafast laser oscillator can be a called a femtosecond laser oscillator or a picosecond laser oscillator depending on the pulse width of the laser beam generated.

1 . A laser machining method for via drilling and selective material removal in a work piece or a semiconductor wafer using an ultrafast laser pulse directly from an ultrafast laser oscillator without an amplifier, comprising:
 emitting a pulsed laser beam from a diode pumped or CW laser pumped solid state ultrafast laser oscillator without an amplifier;   controlling the laser pulse, to minimize the cumulative heating effect and to improve the machining quality;   varying the diameter of the laser beam in at least one axis;   scanning the laser beam in two axes; and   focusing the pulsed laser beam on to a work piece;   wherein a via is drilled or material is selectively removed from the semiconductor wafer.   
   
   
       2 . The method according to  claim 1 , further comprising changing the polarization of the laser beam. 
   
   
       3 . The method according to  claim 1 , further comprising moving the wafer in three dimensions. 
   
   
       4 . The method according to  claim 1 , further comprising injecting a liquid or gas to assist in reducing the cumulative heating effect. 
   
   
       5 . The method according to  claim 1 , further comprising controlling the scanning speed. 
   
   
       6 . The method according to  claim 1 , further comprising imaging the laser beam in order to align the laser beam with the wafer and to monitor the machining process. 
   
   
       7 . The method according to  claim 1 , further comprising using a longer wavelength laser beam. 
   
   
       8 . The method according to  claim 1 , further comprising controlling the laser pulse energy and the pulse number. 
   
   
       9 . The method according to  claim 1 , further comprising changing the shape of the laser beam to improve the machining efficiency and quality. 
   
   
       10 . The method according to  claim 1 , further comprising reducing the ablated feature size below the focused spot size by controlling the laser threshold fluence. 
   
   
       11 . The method according to  claim 1 , wherein the work piece is a semiconductor wafer. 
   
   
       12 . The method according to  claim 1 , for use in selective remove material or a layer in semiconductor wafer by pulses from ultrafast laser oscillator wherein:
 a layer of material can be selectively removed without ablating the underlying material by precisely controlling the pulsed laser fluence;   the laser fluence of the material depends on the material, the number of pulses at each scan point, scanning speed, focused spot size, repletion rate of the laser pulse, laser wavelength and the pulse width.   
   
   
       13 . The method according to  claim 12 , wherein the selectively ablated area can be round, square or of any desired shape depending on the applications and further processing. 
   
   
       14 . The method according to  claim 12 , wherein:
 overlying layers can be removed layer by layer or a few layers together by controlling the laser fluence; and   each layer can vary in thickness from a few micrometers to a few nanometers.   
   
   
       15 . The method according to  claim 1  for use in formation of interconnect via in semiconductor wafer or multilayer printed circuit board wherein:
 blind via holes are drilled through an insulator layer and an conductive plate/layer causing minimal or no damage to the underlying conductive layer;   via interconnects are then formed by filling the via holes formed between conductive layers/plane with conductive material by metallization; and   the insulating layers are made of dielectric, glass or any other insulating material and the conductive layer are made of metals.   
   
   
       16 . The method according to  claim 15 , wherein:
 the vias have a smaller diameter at the lower portion of via compared to the upper portion;   via sidewall angles may range from 89 degrees to 1 degree depending on the depth and diameter of the via; and   the number of layers though which via hole is drilled and the thickness of each layer can vary depending on the application.   
   
   
       17 . The method according to  claim 15 , wherein:
 the shape of via hole can be round and slotted of single and multiple depths depending on application; and   the ablated feature size can be reduced below the focused spot size by controlling the laser threshold fluence.   
   
   
       18 . The method of  claim 1 , wherein a thin film material to be removed is a metal, dielectric, semiconductor, insulating material, polymer, glass, silicon. 
   
   
       19 . The method of  claim 1 , wherein debris is loosely bound to the surface of the work piece and can be removed while machining using pressurized gas assist and hence the process may not require post processing. 
   
   
       20 . The method of  claim 1 , where the via holes can be a through hole or a blind hole in the work piece. 
   
   
       21 . The method of  claim 1 ;
 wherein micro cracks are minimized or eliminated during the ablation process;   wherein a recast layer along the via side walls is minimized or eliminated to avoid formation of voids during metallization of the via holes; and   wherein the via holes have a high aspect ratio and near vertical sidewalls.