Original Date: 01/23/1995
Revision Date: 01/18/2007
Information : Microporous Polyimide Films for Reduced Dielectric Applications
Strong demand for fast, highly-condensed microcircuits and the unavailability of low dielectric-constant insulators have led Sandia National Laboratories' researchers to look for alternative methods to insulate multiple layers of patterned conductors in multi-chip modules. Device dimensions and processing speed are largely limited by the thickness and dielectric-constant value of the insulating material. Therefore, Sandia researchers developed a technique to deposit a sponge- like microporous polyimide to insulate conductor layers.
Researchers found that the dielectric constant of the applied polyimide decreased in value proportionally with pore size and film thickness The technique developed by Sandia consisted of spin-coating the polymer solution on a wafer, and then immediately submersing the still 'wet' wafer in a non-solvent. The non-solvent caused the polymer to phase separate out of solution on a micron-size scale, creating the microporous structure. Pore size and film thickness are controlled by the spin rate and time, and higher RPMs yield smaller pores and thinner films. In 1993, researchers were able to deposit a polyimide film with a pore size of 1-1.5 micrometers and dielectric constant of 1.88 at 1 Khz.
Porosity = 68% Pore Size =1 -1.5µm
Thickness =22 µ
Dielectric Constant = 1.88
(1 KHz) Dielectric Loss = 0.002
Stress from Solid Film on Wafer = 20 Mpa
Stress from Porous Film on Wafer = 2.0 Mpa
These values compare favorably considering that silicon dioxide and polyimide have a dielectric value of 4.5 and 3.4 respectively. This decrease in the dielectric constant results in an increase in pulse propagation velocity. In addition, thinner insulation layers minimize cross-talk between adjacent conductors. This enables chip designers to place conducting lines closer and obtain denser microcircuits.
Sandia researchers have shown that porous polyimide films with low dielectric values can be made and applied to multi-chip modules. Film thickness and pore size can be controlled through spin rate and length of spin time. This microporous film can contribute to the manufacturing of faster, denser multi-chip modules.
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