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Electroetching Process for Chrome Barrier Layer

IP.com Disclosure Number: IPCOM000112398D
Original Publication Date: 1994-May-01
Included in the Prior Art Database: 2005-Mar-27
Document File: 2 page(s) / 85K

Publishing Venue

IBM

Related People

Imken, RL: AUTHOR

Abstract

Disclosed is an enhancement to processing of reference plane foil used in the fabrication of a high density electronic carrier [1]. As described in [2], the creation of blind holes in the reference plane foil permits the construction of very small vias which can be differentiated in processing to produce groundplane and isolated connections. To achieve this structure, the reference plane foil construction requires an etch barrier layer to prevent etching of through holes. Typically, Copper/Invar/Copper (CIC) foil is sputtered with chrome and copper, and then copper electroplated on the sputtered copper surface to create the foil structure shown in Fig. 1. After etching from the CIC side through an etch mask, blind holes such as those shown in Fig. 2 are formed.

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Electroetching Process for Chrome Barrier Layer

      Disclosed is an enhancement to processing of reference plane
foil used in the fabrication of a high density electronic carrier
[1].  As described in [2], the creation of blind holes in the
reference plane foil permits the construction of very small vias
which can be differentiated in processing to produce groundplane and
isolated connections.  To achieve this structure, the reference plane
foil construction requires an etch barrier layer to prevent etching
of through holes.  Typically, Copper/Invar/Copper (CIC) foil is
sputtered with chrome and copper, and then copper electroplated on
the sputtered copper surface to create the foil structure shown in
Fig. 1.  After etching from the CIC side through an etch mask, blind
holes such as those shown in Fig. 2 are formed.

      Removal of the chrome layer is typically accomplished by
chemical etching in a basic potassium permanganate solution.  This
removal process requires fixturing of the foil, and exposure to the
solution which is a strong oxidizer.  Use of a permanganate solution
is problematic in that it is chemically unstable, and will lose
oxidizing activity both when being used or stored.  It also requires
thorough rinsing to remove manganate residue prior to a subsequent
copper plating step.

      The disclosed concept substitutes electrolytic removal of the
chrome barrier layer in a sulfuric acid solution in place of chemical
removal in permanganate solution.

      As documented in [3], chrome can be removed in the presence of
copper through galvanic interaction with a counterelectrode which is
more electronegative than chrome.  However, in the presence of invar
(nickel/iron alloy), this reaction does not take place spontaneously.
By externally applying a voltage to the part, however, the chrome
will be oxidized from metallic Cr(0) to soluble Cr(+3) without
dissolving the invar or copper components.

      To accomplish this, the foil is placed in a dilute sulfuric
acid solution (10-25% by weight) and electrically connected to a
power supply as the anode.  The counterelectrode (titanium, stainless
steel, or a similar metal which is stable to dilute sulfuric acid
solutions) is made the cathode.  An external voltage is applied
between the foil and counterelectrode to induce dissolution of the
exposed chrome layer.  For an external voltage of 5-15 volts, removal
of a 3000 angstrom...