Browse Prior Art Database

Liquid Crystals for Package Parts Testing

IP.com Disclosure Number: IPCOM000039554D
Original Publication Date: 1987-Jun-01
Included in the Prior Art Database: 2005-Feb-01
Document File: 3 page(s) / 16K

Publishing Venue

IBM

Related People

Chiu, GL: AUTHOR [+3]

Abstract

One known method of package parts testing employs conventional mechanical probing where the probe is mechanically stepped to various pads for testing of opens and shorts. This mechanical approach is time consuming and hence an expensive way of testing. Another method is to use an electron beam which is steered to the metal lines under test which offers some advantage in testing time, but suffers the disadvantage of requiring a vacuum environment. This publication describes an improved scheme which uses various electro-optic properties of liquid crystals for package parts testing. Electro-optic crystalline solids (e.g., gallium arsenide, lithium niobate, or lithium tantalate) can be used to sense the electric field between the metal lines.

This text was extracted from a PDF file.
This is the abbreviated version, containing approximately 46% of the total text.

Page 1 of 3

Liquid Crystals for Package Parts Testing

One known method of package parts testing employs conventional mechanical probing where the probe is mechanically stepped to various pads for testing of opens and shorts. This mechanical approach is time consuming and hence an expensive way of testing. Another method is to use an electron beam which is steered to the metal lines under test which offers some advantage in testing time, but suffers the disadvantage of requiring a vacuum environment. This publication describes an improved scheme which uses various electro-optic properties of liquid crystals for package parts testing.

Electro-optic crystalline solids (e.g., gallium arsenide, lithium niobate, or lithium tantalate) can be used to sense the electric field between the metal lines. It is well known that the electro-optic effect in solids has a very fast response time (femtosecond) to an electric field. This is very useful for high-speed characterization of transmission lines, for instance.

However, such a fast response is not needed for testing of opens and shorts. Instead, liquid crystals have far stronger electro-optic effects which are easier to detect. The electro-optic response time (submillisecond to 1 second) of liquid crystal is adequate for this application (faster than that of mechanical stepping in any case).

There is no need for vacuum with liquid crystals (in contrast to the electron beam approach). Also, being a viscous liquid, it is possible to sense the electric field for a metal line lying underneath the top layer through a via-hole. It is not necessary to fill the vias with metal studs; rather, the liquid crystal can fill the via holes for electric-field sensing during testing. In contrast, with electro-optic solids, an unfilled via hole lowers the sensitivity for electric-field detection (because of the larger distance between the metal lines and the electro-optic solid). There are numerous electro-optic effects in liquid crystals (see, e.g., [1,
2]). Of particular interest to package parts testing are the following effects: (1) Field-induced birefringence of nematic liquid crystals: For a nematic liquid with e < e , (where e and e are the dielectric constants parallel and perpendicular to the optic axis), the liquid crystal is arranged in the homeotropic state (director is perpendicular to the electrodes) in the absence of an electric field. For a nematic material with e > e , the liquid crystal is arranged in the homogeneous orientation (director is parallel to the electrodes). Electric field distorts the fluid director when voltage exceeds the threshold voltage and, thereby, alters the polarization of the light. The changes of the polarization signal the presence of voltage between the electrodes under test. (2) Twisted nematic: Director is parallel to the electrodes (i.e., homogeneously aligned), but with a twist of 90 degrees between the two electrodes with no applied voltage. However, with an applied voltage...