Takahiko Takahashi

Cutting of conductors on VLSI chips — A comparison between laser and focused ion beam removal

Abstract A comparison was made between laser evaporation-explosion and focused ion beam (FIB) sputter etching for cutting of conductors on VLSI chips. In laser cutting, thin film layers underlying the conductors and the neighboring area are liable to be damaged because of its evaporation-explosion mechanism. FIB etching has not only a submicron focusing ability but also little material dependence in its etching characteristics. Therefore, FIB cutting is more advantageous for applications to fine pattern and multilayer conductor systems in VLSIs. Under these considerations, FIB conductor cutting was successfully carried out for VLSI circuits debugging, and the effectiveness and usefulness of FIB cutting were demonstrated.

Evaluation of Molybdenum Conductors Formed by Laser-induced CVD.

This paper describes the evaluation results of molybdenum conductors formed by laser-induced CVD using thermal decomposition of molybdenum hexacarbonyl as a material gas. In order to apply this technique to LSI products, it is very important that the conductors deposited on LSI chips have sufficient reliability. From the fusing test, the applicable current density was found to be about 3 × 106 A/cm2 which is sufficiently high compared with the maximum current density, 1-3 × 105 A/cm2, for practical use. Molybdenum conductors, after laser annealing. endured a heat cycle test 70 times without a change of resistance. And, these conductors did not change resistance for 1 000 hours when a current density of 1.3 × 106 A/cm2was applied in a 125°C atmosphere. These results suggest the possibility of application in LSI products.

Focused Ion Beam Milling Technology for On-chip Wiring Modification System for LSI.

During debugging logic LSIs, a period to reproduce LSI in order to change the logic design is becoming longer. To reduce the period from several weeks down to one day, an on-chip direct wiring modification system, using the focused ion beam (FIB) milling and the laser CVD, has been developed.This paper describes a precise FIB milling technique for cutting the wirings and making the via holes to the wirings of the LSI. Milling depth control by monitoring the ion induced photo-emissions and the milling strategy to overcome the surface steps of the LSI resulted in the milling depth accuracy of ±0.25μm. The system, consist of the FIB milling described in this paper and the laser CVD, has been applied to the logic modification of the LSIs of Hitachi M880 mainframe computer. Several tens of cuts, vias on an LSI chip were made by FIB, and the several jumper wirings were made by laser CVD. The average yield of modified LSI chips of 91.8% was achieved.