Process and Apparatus for the Fabrication of Low-Cost Polyimide Sublaminate with Anisotropic Copper Field Vias
Original Publication Date: 1995-Oct-01
Included in the Prior Art Database: 2005-Mar-31
Bartha, J: AUTHOR [+4]
Disclosed is a single-sided, anisotropic plasma etch process at reasonable costs which allows a higher "via-density".
Process and Apparatus for the Fabrication of Low-Cost
with Anisotropic Copper Field Vias
a single-sided, anisotropic plasma etch process at
reasonable costs which allows a higher "via-density".
"Dycostrate process" uses a basic substrate made
of a copper-polyimide-copper foil sandwich. The copper foil is
structured by use of photoresist, double-sided exposure and
subsequent wet etching. The structured copper foil then serves as a
mask in a double-sided plasma process wherein the via hole is etched
at the same time but anisotropically, through the polyimide layer.
The resulting lateral dimensions of the through-hole are at least two
times the thickness of the polyimide foil plus the diameter of the
mask opening. This represents a strong restriction for the increase
of the number of vias per unit area.
sequence of the proposed process is shown in the
Figure. For cost reasons and to avoid lithographic "overlay
problems" between front and back side, a one-sided mask is suggested.
This mask can be made either of copper or of masking photoresist.
So-called "silylated" photoresist (whereby the silylation can occur
either before or after exposure and development) has a masking effect
in an O(2) plasma, i.e., the resist 1 is typically etched at a rate
twenty times more slowly than the polyimide 2 (Fig. 1a). If the
polyimide foil is to be etched at an etch rate as high as possible in
an anisotropic plasma etch process (i.e., at etch rates greater than
1&mu.m/min), it is important to cool the substrate. A silicon wafer,
e.g., can be cooled easily using helium gas, due to its mechanical
properties. An extensive, thin polyimide foil cannot be cooled in
this way. Therefore, the foil is adhered to a smooth surface 4,
e.g., a cathode by means of silicone grease 3 which has a low vapour
pressure. This temporary fastening, however, has to be easy
removable after the plasma etching step. The adhesive 3, therefore,
must also show a good heat conductivity and should have an etching
rate as small as possible, as well as little influence on the species
of the plasma.
By means of
the following gases and plasma parameters an etch
rate of the polyimide of 1.6 &mu.m/min could be achieved: