Lock in thermal laser stimulation through one side of the device while acquiring lock-in thermal emission images on the opposite side

Abstract

Controlled amount of heat is injected into a stacked die using a light beam, and the propagated heat is measuring with LIT camera from the other side of the die. The thermal image obtained can be characterized so that it can be used to calibrate the phase shift from a given stack layer, or can be used to identify defects in the stacked die. The process can be repeated for each die in the stack to generate a reference for future testing. The thermal image can be investigated to detect faults, such as voids in vias, e.g., TSV.

1 . A system for analysis of a device under test (DUT), comprising:
 a test bench to support the DUT;   a heat source position to deliver a prescribed amount of energy to a localized spot on the DUT from one side thereof;   a thermal imaging system positioned so as to image the other side of DUT; and,   a controller activating said heat source to deliver the prescribed amount of energy and receiving output signal from said thermal imaging system.   
     
     
         2 . The system of  claim 1 , wherein said heat source comprises a light source. 
     
     
         3 . The system of  claim 2 , wherein said light source is a laser light source. 
     
     
         4 . The system of  claim 2 , wherein said controller activates the light source at a lock-in frequency f1, and activates the thermal imaging system at a lock-in frequency f2. 
     
     
         5 . The system of  claim 4 , wherein lock-in frequency f2 is at least four times higher than lock-in frequency f1. 
     
     
         6 . The system of  claim 5 , wherein said thermal imaging system comprises an infrared camera. 
     
     
         7 . The system of  claim 6 , wherein said heat source further comprises:
 a movable stage;   an optical scanner coupled to the movable stage, said optical scanner receiving output of said light source and directing the output to a specified spot on the DUT; and,   an objective lens focusing the output onto a the specified spot on the DUT.   
     
     
         8 . A system for analysis of a device under test (DUT), comprising:
 a test bench to support the DUT;   a lock-in laser light source operating at a lock-in frequency f1 and position to deliver a pulsed laser light beam onto the DUT from one side thereof;   a lock-in thermal imaging system operating at a lock-in frequency f2 and positioned so as to image the other side of DUT; and,   a controller activating said heat source at the lock-in frequency f1 and activating said thermal imaging system at the lock-in frequency f2.   
     
     
         9 . The system of  claim 8 , wherein the controller outputs a lock-in frequency f1 trigger signal to said lock-in laser light source and outputs a lock-in frequency f2 trigger signal to said lock-in thermal imaging system. 
     
     
         10 . The system of  claim 9 , wherein lock-in frequency f2 is at least four times larger than lock-in frequency f1. 
     
     
         11 . The system of  claim 8 , wherein the lock-in laser light source comprises:
 an excitation source receiving said lock-in frequency f1 trigger signal and generating pulsed laser light beam;   a scan unit receiving and directing the pulsed laser light beam onto a defined location on the DUT; and,   an objective lens focusing the pulsed laser light beam onto the defined location.   
     
     
         12 . The system of  claim 11 , wherein the lock-in thermal imaging system comprises an infrared camera. 
     
     
         13 . A method for testing a device under test (DUT), comprising:
 situating the DUT onto a test bench;   illuminating a defined spot on one side of the DUT;   obtaining a thermal image of the other side of the DUT; and,   analyzing the thermal image to thereby characterize the thermal propagation of heat within the DUT.   
     
     
         14 . The method of  claim 13 , wherein illuminating a defined spot comprises delivering a series of laser light pulses at a frequency f1. 
     
     
         15 . The method of  claim 14 , wherein obtaining a thermal image comprises obtaining a series of images at a frequency f2. 
     
     
         16 . The method of  claim 15 , further comprising synchronizing frequencies f1 and f2. 
     
     
         17 . The method of  claim 16 , wherein frequency f2 is at least four times higher than frequency f1. 
     
     
         18 . The method of  claim 14 , further comprising focusing the series of pulses into a predefined depth within the DUT. 
     
     
         19 . The method of  claim 14 , wherein the defined spot includes at least one through-silicon via (TSV), and further comprising comparing the thermal image to a reference thermal image to thereby investigate faults in the TSV. 
     
     
         20 . The method of  claim 14 , further comprising assembling a reference database of thermal propagation of heat within various layers of the DUT.