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Low Warpage Wafer Buffer Coating Material
Keywords: Low Stress, Reduced Warpage, Lower Process Temperature
DMI-3006 is a new, formulated product for low stress wafer buffer coatings. The unique, patented resin which forms the backbone of this coating is an extended polyimide having photoactive maleimide end groups. This UV light cured resin provides exceptional thermal stability, good adhesion and solvent resistance, excellent electrical properties and very low warpage on thin wafers. The presentation will highlight the key process and performance advantages of DMI-3006 over conventional polyimide wafer coating materials. Conventional polyimides are formed on the wafer after spin coating by way of a condensation reaction which requires a “hard bake” step at high temperature. New chemical bonds are formed as the fugitive condensate is driven out of the coating and the material crosslinks into a thermoset polymer. The conventional polyimide loses mass and shrinks significantly during this condensation reaction causing stress resulting in wafer warpage. DMI-3006 formulation contains the polyimide monomer already in the spin coating solution. No “hard bake” is required. Crosslinking occurs when DMI-3006 is exposed to UV light which initiates a chain reaction that propagates through the addition of chemical bonds between the photoactive maleimide end groups. The resulting thermoset has minimal shrinkage and very low stress. DMI-3006 has no detectable vertical shrinkage from thickness after image development to thickness after the final bake step. Warpage for DMI-3006 is 5.7 mm versus 93.6, 99.1 and 82.0 mm, respectively, for three different, commercially available, conventional wafer coating materials (measured on 50 µm thick 300 mm wafers with 10 µm coating thickness). The lower warpage value for DMI-3006 combined with its lower tensile modulus produce significantly lower residual stress on thin wafers than do conventional polyimide wafer coating materials. Processing of DMI-3006 uses standard wafer coating methods. The surface is non-tacky after spin coating and drying. Imaging is negative tone and requires 500 mJ/cm2. Common solvents are used for developing. DMI-3006 is capable of resolving a 0.5 aspect ratio in the image patterns. Final step is a low temperature bake. Excellent adhesion is obtained on silicon and copper before and after 96 hours of 121°C pressure cooker as measured using the cross cut adhesion tests following the ISO 2409 method. Moisture absorption is 0.1% after water immersion. Thermal decomposition temperature of DMI-3006 is 457°C. The dissipation factor and the dielectric constant of DMI-3006 are excellent and DMI-3006 performs very well in 1000 hour 85°C /85%RH electrical migration testing. DMI-3006 has the thermal and electrical performance of conventional polyimide wafer coatings with the advantage of lower processing temperatures, superior stress reduction and low warpage on thin wafers.
James T. Huneke, Ph.D., Consultant
Designer Molecules Inc.
San Diego, CA

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