Laser-trimmable digital resistor

Abstract

A laser system and method for cleanly trimming or severing resistive links fabricated on an undoped gallium arsenide substrate without damaging or affecting adjacent circuit structures or the underlying or surrounding substrate is disclosed. The system comprises a laser source adapted to generate an output at a wavelength within the range of 0.9 to 1.5 μm, a resistive film structure formed on an undoped gallium arsenide substrate, and a beam positioner and alignment system to align the laser source with the target structure. The method comprises generating a laser output at a wavelength in a range of about 0.9 to 1.5 μm and directing the laser output to illuminate a resistive thin-film structure fabricated on a gallium arsenide substrate. The resistive film structure comprises a first layer of protective dielectric and a layer of resistive thin-film material. Preferably, a second layer of protective dielectric lies upon the layer of resistive thin-film material. Further, there is disclosed a resistive trim network suitable for use with a bias circuit for a power amplifier that requires a quiescent current of 130 mA. The trim network comprises eleven thin-film resistors arranged in an asymmetrical array of series resistors, parallel resistors, and tying resistors.

What is claimed is:  
     
         1 . A laser system for processing a gallium arsenide integrated circuit, comprising: 
 a. a substrate of undoped gallium arsenide    b. a resistor formed on said substrate; and    c. a laser adapted to generate an output at a wavelength within the range of about 0.9 to about 1.5 μm and at a power sufficient to modify the resistance of said resistor; and    d. a beam positioning and alignment system;    e. wherein said beam positioning and alignment system causes said laser to target and trim said resistor.    
     
     
         2 . The laser system of  claim 1 , wherein said laser source produces an output within a wavelength range of about 0.9 to 1.19 μm.  
     
     
         3 . The laser system of  claim 1 , wherein said laser source produces an output having a wavelength of about 1.047 μm.  
     
     
         4 . The laser system of  claim 1 , wherein said laser source produces an output having a power of about 0.25 μJ to about 0.45 μJ.  
     
     
         5 . The laser system of  claim 1 , wherein said laser source produces an output having a spot size of about 4 μm to about 12 μm.  
     
     
         6 . The laser system of  claim 1 , wherein said laser source has a Q-rate of about 8,000 pulses per second to 12,000 pulses per second.  
     
     
         7 . The laser system of  claim 1 , wherein said resistive film is one of nichrome and nickel.  
     
     
         8 . The laser system of  claim 1 , wherein said resistive film has an optical absorption coefficient that is of about the same magnitude as that of nichrome.  
     
     
         9 . A method for processing an integrated circuit, comprising: 
 a. generating a laser output at a wavelength in a range of about 0.9 to 1.5 μm and at a power sufficient to vaporize portions of a resistive film fabricated on a gallium arsenide substrate; and    b. directing the laser output at the resistive film.    
     
     
         10 . The laser system of  claim 9  wherein said laser output is generated within a wavelength range of 0.9 to 1.19 μm.  
     
     
         11 . The laser system of  claim 9 , wherein said laser output is generated with a wavelength of about 1.047 μm.  
     
     
         12 . In a semiconductive integrated circuit, a laser-trimmable resistive network, comprising: 
 a. first and second input terminals;    b. a first thin-film resistor and a second thin-film resistor connected in series between said first and second input terminals;    c. a third thin-film resistor and a fourth thin-film resistor connected in series between said first and second input terminals;    d. a fifth thin-film resistor connected at one end to the node formed by the junction of said first and second thin-film resistors and connected at the other end to the node formed by the junction of said third and fourth thin-film resistors.    
     
     
         13 . The laser-trimmable resistive network of  claim 12 , further comprising: 
 a. a sixth thin-film resistor and a seventh thin-film resistor connected in series between said first and second input terminals; and    b. an eighth thin-film resistor connected at one end to the node formed by the junction of said third and fourth thin-film resistors and connected at the other end to the node formed by the junction of said sixth and seventh thin-film resistors.    
     
     
         14 . The laser-trimmable resistive network of  claim 12 , wherein one of said first and second resistive-film resistors has a resistance which is less than the resistance of one of said fourth and fifth resistive-film resistors.  
     
     
         15 . The laser-trimmable resistive network of  claim 12 , wherein said first resistive-film resistor has a resistance which is no more than half of the resistance of one of said fourth resistive-film resistor and said fifth resistive-film resistor.