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Intrinsic-Thermocouple Process Monitor

IP.com Disclosure Number: IPCOM000039057D
Original Publication Date: 1987-Apr-01
Included in the Prior Art Database: 2005-Feb-01
Document File: 3 page(s) / 39K

Publishing Venue

IBM

Related People

Chalco, PA: AUTHOR [+6]

Abstract

In order to replace the ultrasonic wire bonding technique which requires the coating of heavy gold on EC pads, laser welding using fiber optics has been developed. The figure shows this technique in which a continuous laser beam 10 of short duration (<1 sec) is delivered via an optical fiber 12 to heat a tungsten tip 14 which, in turn, melts the gold-coated copper wire 16 (60 microns in diameter) and bonds it to the EC pads 18 on the alumina or glass ceramic substrate 20. To minimize oxidation of tungsten tip 14 at elevated temperatures, it is highly desirable to heat the tip with an amount of laser energy which is just sufficient to achieve good bonding between EC pad 18 and copper wire 16.

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Intrinsic-Thermocouple Process Monitor

In order to replace the ultrasonic wire bonding technique which requires the coating of heavy gold on EC pads, laser welding using fiber optics has been developed. The figure shows this technique in which a continuous laser beam 10 of short duration (<1 sec) is delivered via an optical fiber 12 to heat a tungsten tip 14 which, in turn, melts the gold-coated copper wire 16 (60 microns in diameter) and bonds it to the EC pads 18 on the alumina or glass ceramic substrate 20. To minimize oxidation of tungsten tip 14 at elevated temperatures, it is highly desirable to heat the tip with an amount of laser energy which is just sufficient to achieve good bonding between EC pad 18 and copper wire 16. This can be conveniently achieved by turning off the laser power when the wire temperature exceeds the melting point of gold (1062oC) or when a temperature (>1062oC) required for good bonding is reached in the wire. By supplying only the necessary amount of heat to the wire during bonding, one can also minimize heat flow into the substrate and consequently the thermal stress produced in the substrate. High thermal stresses can cause cracking of the ceramic substrates during very rapid laser heating. In order to determine the optimum laser duration which depends largely on the wire temperature, a means of monitoring the wire temperature history during bonding is required. Even in the absence of substrate cracking and tip oxidation (e.g., when the bonding experiment proceeds in an inert atmosphere), knowledge of the wire temperature history during bonding is still very important from the viewpoint of process control. Because of the requirement of fast time response (<<duration of laser beam <1 sec), the small dimensions (several mils) of the space between the tip and the EC pad, and the difficulties involved in calibration, indirect techniques such as Raman scattering, X-ray diffraction and electron diffraction are not suitable for wire temperature measurements. In the present tehcnique, the thermally generated emf between the copper wire and the tungsten tip is used as an in-situ thermocouple to measure directly the wire temperature d...