Non-Destructive Evaluation of Strains and Voiding in Passivated Copper Metallizations
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NON-DESTRUCTIVE EVALUATION OF STRAINS AND VOIDING IN PASSIVATED COPPER METALLIZATIONS Richard P. Vinci, Thomas N. Marieb, and John C. Bravman Department of Materials Science and Engineering, Stanford University, Stanford, CA 94305 ABSTRACT Stress induced voiding in passivated Cu lines was investigated by x-ray strain analysis and in-situ high voltage scanning electron microscope (HVSEM) techniques. Cu lines on a Ta underlayer and Cu lines on an Al underlayer were patterned by a trilayer liftoff technique and passivated with Si3N4 . For direct observation of stress voiding, specimens were heated to 350'C in the HVSEM and then cooled and held at 150'C. Identical samples were subjected to the same thermal cycle for strain state determination using x-ray techniques. The hydrostatic stress state at each temperature was calculated from the measured strains. Few initial voids were observed after passivation in either sample. After heating to 350'C and cooling to the dwell temperature, no new voiding was seen in the Ta/Cu lines. Measured hydrostatic strains were half those measured in the Al/Cu lines. Heavy voiding was observed in the Al/Cu lines after cooling to the dwell temperature. INTRODUCTION The primary interconnect metallizations used in integrated circuits are based on Al alloys. As interconnect pitch decreases, electrical performance and mechanical reliability of interconnects is of increasing concern. A potential replacement material for Al in critical applications is Cu, due to its low bulk electrical resistance and potential for limited electromigration voidingl, 2 . Stress induced voids have been reported for Cu lines stripped of their passivation,3 indicating that stress induced voiding may be a problem with Cu metallizations as it is for Al. Stress voiding is the result of large triaxial stresses that develop in passivated metal lines. The difference in thermal expansion coefficient between the metal and the substrate, coupled with the constraint applied by a stiff passivation, can lead to the development of large strains in the metal during a temperature change 4 . The corresponding hydrostatic stresses can be very large compared to the yield stress of the metallization. A volumetric relaxation mechanism, such as voiding, is required to relieve these stresses. Stress induced voiding in passivated copper lines was investigated by x-ray strain analysis and in-situ high voltage scanning electron microscope (HVSEM) techniques. Copper lines, patterned by a liftoff technique and passivated with Si3N4 , were subjected to a thermal cycle which consisted of heating from room temperature to 350'C and then cooling to 150*C where they dwelled for 1-22 hours. For direct observation of stress relaxation, specimens were heated in the HVSEM. Identical samples were heated and cooled to the same temperatures for strain state determination using x-ray techniques. The hydrostatic stress state at each temperature was calculated. NON-DESTRUCTIVE EVALUATION OF STRAINS AND VOIDING X-ray techniques for measuring the strain state o
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