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Novel design for eliminating paths of surface conductivity due to surface metallization on insulators

IP.com Disclosure Number: IPCOM000124560D
Publication Date: 2005-Apr-27
Document File: 4 page(s) / 96K

Publishing Venue

The IP.com Prior Art Database

Abstract

During the operation of Liquid Metal Ion Sources (LMIS), electrically conductive material may be deposited onto the emitter-facing surface of the electrically insulating base. If enough of this material coats the base, an electrical short can develop between the base pins of the LMI source, causing failure. An angled (non-normal) trench dividing the insulator surface into discrete regions is presented here as a novel solution to this problem. It is tested for general LMIS operating conditions and is shown to be a successful design. Applicability to other uses is discussed.

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Novel design for eliminating paths of surface conductivity due to surface metallization on insulators

ABSTRACT:

During the operation of Liquid Metal Ion Sources (LMIS), electrically conductive material may be deposited onto the emitter-facing surface of the electrically insulating base.  If enough of this material coats the base, an electrical short can develop between the base pins of the LMI source, causing failure.  An angled (non-normal) trench dividing the insulator surface into discrete regions is presented here as a novel solution to this problem.  It is tested for general LMIS operating conditions and is shown to be a successful design.  Applicability to other uses is discussed.

INTRODUCTION

Surface deposition of a conducting material will increase electron mobility along the coated surface.  In the case of the Liquid Metal Ion Source (LMIS), metallization can occur on the support insulator which isolates the two base pins  of the structure to which is attached the heating filament.  An electrically conductive path along the surface of this insulator results in a shorting of the heating filament and hence end of life for the LMIS.  The metallization occurs from the evaporation of metal from the LMIS reservoir.  Means must be taken to either eliminate the source of the deposition or add some device which will break the offending paths of surface conduction.  Since removing the sources of metallization may be impossible, a simple device for breaking surface conductivity is reported here.  This concept can be used in any situation where metallization of insulator by evaporation is a problem.

Surface conductivity can be broken by the use of a trench cut into the insulating surface with at least one surface of this trench is overhung.  This results in a shadow area where evaporated metal from the LMIS structure can not land.  If this overhung surface extends the entire path across the insulator, it can become a continuous isolating boundary which electrically separates the base posts.  Note that this device does not stop the metallization of the insulator from occurring, rather it simply allows one to avoid any electrical conductivity problems which come along with metallization.

DESIGN GUIDELINES

When placing this angled trench into an insulating surface, consideration must be paid to the following two criteria: 1) trench layout—how well does the trench or series of trenches serve to isolate the points which need isolation from surface conductivity; and 2) trench dig angle—how well does the trench angle accommodate the directional onset of new metallization.

Criteria 1 can be successfully satisfied by making sure that, for any points which need electrical isolation from each other, all surface paths connecting any pair of points are bisected by at least one trench.  Proper layout will ensure that surface conductivity paths will never form between isolated points.

Criteria 2 can be satisfied only with consideration of al...