Method of fabricating high accuracy embedded resistors in flex substrates

An integrated circuit stack comprising:

• a polyimide layer;
• a thin film resistor formed on the polyimide layer;
• a metallization layer having metal contacts formed on opposing ends of the thin film resistor and that leave an exposed surface of the polyimide layer;
• and
• a hydrophobic moisture barrier layer formed on the exposed surface of the polyimide layer and that prevents absorption of water into the polyimide layer to avoid blistering of the thin film resistor when the thin film resistor is laser trimmed,
• wherein the thin film resistor is laser trimmed to a target resistance between the metal contacts.

A method comprising:

• forming a polyimide layer;
• forming multiple thin film resistors on the polyimide layer;
• forming, on the multiple thin film resistors and the polyimide layer, a metallization layer that includes metal contacts on opposing ends of each thin film resistor but leaves an exposed surface of the polyimide layer;
• baking the polyimide layer, the multiple thin film resistors, and the metallization layer to remove water from the polyimide layer;
• forming, on the exposed surface of the polyimide layer, a hydrophobic moisture barrier layer that prevents absorption of water into the polyimide layer to avoid blistering of the multiple thin film resistors during subsequent laser trimming of the multiple thin film resistors;
• and
• laser trimming a resistance of each thin film resistor between the metal contacts for each thin film resistor.