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Biasing Means for Contactless Laser Inspection Testing of Circuit Packages

IP.com Disclosure Number: IPCOM000035911D
Original Publication Date: 1989-Aug-01
Included in the Prior Art Database: 2005-Jan-28
Document File: 4 page(s) / 54K

Publishing Venue

IBM

Related People

Beha, H: AUTHOR [+4]

Abstract

A technique is described whereby limitations of laser photoemission testing of wiring are removed by contacting and properly biasing the wiring to be tested. By contacting and biasing the wiring, using techniques such as metal-clad flexible sheeting, problems associated with wire charging and materials variations are removed, for laser photoemission inspection testing of the wired packages.

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Biasing Means for Contactless Laser Inspection Testing of Circuit Packages

A technique is described whereby limitations of laser photoemission testing of wiring are removed by contacting and properly biasing the wiring to be tested. By contacting and biasing the wiring, using techniques such as metal-clad flexible sheeting, problems associated with wire charging and materials variations are removed, for laser photoemission inspection testing of the wired packages.

Contactless laser testing, for testing of logic states and switching transient waveforms on high speed semiconductor chips, has proved to be quite feasible. A critical application in packaging is the inspection of wiring for shorts and opens as well as for imperfections which may lead to shorts or opens during the functional operation of the circuit package. The concept described herein extends the laser testing to carry out this type of inspection in both single- point scanning and in full-chip modes of operation.

The concept takes advantage of the fact that the photoelectron threshold energy for exciting emission from the metal wiring, typically 4 to 5 eV, is considerably below that for causing emission from the surrounding insulating material of the package. For example, Al2O3 has a band gap of 7.3 eV, so the threshold must be at least this large and polymer materials should be similar. With laser photon energies just above the threshold for the metal but below that of the insulator, high contrast inspection testing by laser-induced photoelectron emission can be expected.

(Image Omitted)

The band alignments for such an ideal situation in the insulating material of the package is depicted in Fig. 1(a). With a perfect host material, the Fermi level EF would be at the middle of the energy gap EG which separates the valence band maximum Eval and the conduction band minimum Econd . The level for vacuum emission is the vacuum level Evac, which is normally just above Econd . Since the material is assumed ideal, there are no electronic states in the energy gap, so the highest energy occupied electron states lie at Eval, and the minimum energy required to raise electrons above Evac is the photoelectric threshold Ephoto = Evac - Eval . If any low concentration of electrons were available within the band gap, electron emission would be possible at threshold excitation energies starting at Ethermal = Evac - EF, because the highest such occupied electron states in the gap would be at EF .

There are two contrasting situations which limit this approach: non-ideal band alignments due to defect states and ideal band alignments with electronically isolated wiring. Defects and non-ideal band alignment

Band alignment may be different either at the surface or in the bulk of the insulating packaging material, as shown in Fig. 1(b). Here, a sufficient concentration of defect states exist within the band gap to shift the Fermi level away from its midgap position. With enough defects, EF move...