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Non-Silylated Lift-Off Stencil Definition for P-Metal Deposition in GaAs Laser Process

IP.com Disclosure Number: IPCOM000101058D
Original Publication Date: 1990-Jun-01
Included in the Prior Art Database: 2005-Mar-16
Document File: 3 page(s) / 147K

Publishing Venue

IBM

Related People

Chakravarti, A: AUTHOR [+3]

Abstract

A new multilayered lift-off stencil has been developed in order to pattern the top side ohmic contact for GaAs/AlGaAs laser devices. The new stencil serves as an alternative to the current silylation process. In addition to the difficulty of controlling the silylation conditions, a primary disadvantage of the silylation process is the use of xylene, which is both toxic and flammable. Xylene's threshold limit value (TLV) is 100 ppm, and its regular use at 40oC is above its flash point.

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Non-Silylated Lift-Off Stencil Definition for P-Metal Deposition in GaAs Laser Process

       A new multilayered lift-off stencil has been developed in
order to pattern the top side ohmic contact for GaAs/AlGaAs laser
devices.  The new stencil serves as an alternative to the current
silylation process.  In addition to the difficulty of controlling the
silylation conditions, a primary disadvantage of the silylation
process is the use of xylene, which is both toxic and flammable.
Xylene's threshold limit value (TLV) is 100 ppm, and its regular use
at 40oC is above its flash point.

      The new lift-off stencil requires three layers.  The top layer
is photoresist which merely serves as a mask to pattern the middle
layer of evaporated titanium.  The Ti layer then serves as a mask to
etch and undercut the bottom polyimide layer.  This stencil satisfies
two major requirements for the laser's ohmic contact.  First, the
polyimide undercut is approximately 0.5 micron which is needed to
provide lift-off of the angled evaporated TiPtAu contact.  This
metalization is evaporated away from normal incidence so that
sufficient coverage of the laser ridge is insured.  The second
requirement is to provide a residue-free stencil such that a buffered
hydrofluoric (BHF) preclean is unnecessary.  A BHF preclean is not
permitted because it would erode the thin p+GaAs contact layer and
the Si3N4 passivation layer.  The titanium middle layer is chosen
because it does not cause silicon-based residues that hexamethyl
disilazane (HMDS), Si3N4, and Si02 have been known to create.

      The non-silylated lift-off stencil is described using the
diagrams in Fig. 1.  A 0.7-micron layer of XU-285 polyimide is
applied and partially cured.  The polyimide thickness is kept thin
because the required undercut for lift-off is directly proportional
to its thickness.  Next, a 1000Ao blanket layer of Ti is E-beam
evaporated.  The final layer is...