Direct injection molded solder process for forming solder bumps on wafers
Solder bumps are provided on round wafers through the use of injection molded solder. Copper pillars or ball limiting metallurgy are formed over I/O pads within the channels of a patterned mask layer. Solder is injected over the pillars or BLM, filling the channels. Molten solder can be injected in cavities formed in round wafers without leakage using a carrier assembly that accommodates wafers that have been previously subjected to mask layer deposition and patterning. One such carrier assembly includes an elastomeric body portion having a round recess, the walls of the recess forming a tight seal with the round wafer. Other carrier assemblies employ adhesives applied around the peripheral edges of the wafers to ensure sealing between the carrier assemblies and wafers.
1. A method comprising:
providing a round wafer having a circular peripheral edge and a plurality of electrically conductive contact pads;
depositing a photoresist layer on the wafer;
patterning the photoresist layer such that a plurality of channels are formed over the contact pads, each of the channels having substantially the same height;
depositing a layer of metal over the contact pads;
providing a carrier including a recess having a circular configuration, wherein the carrier includes an elastomeric body portion defining the recess, wherein the carrier includes a rigid ring mounted to and extending through the elastomeric body portion of the carrier and communicating with the recess;
subsequent to depositing and patterning the photoresist layer and depositing the layer of metal over the contact pads, positioning the round wafer in the recess of the carrier such that a top surface of the carrier is coplanar with a top surface of the photoresist layer, further including causing the elastomeric body portion to engage the peripheral edge of the wafer, thereby forming a compliant seal between the wafer and elastomeric body portion that prevents ingress of molten solder between the wafer and the carrier;
passing a fill head containing molten solder over the channels while the wafer is in a low oxygen environment;
injecting molten solder containing no flux into the channels from the fill head;
allowing the solder to solidify, whereby said solder adheres to the metal and forms bumps, further including exerting a force on the wafer through the ring, thereby detaching the wafer from the carrier, and
removing the photoresist layer from the wafer.
2. The method of claim 1 , wherein the step of depositing the photoresist layer includes spin coating the photoresist layer.
3. The method of claim 1 , wherein the step of depositing metal includes electroplating metal over the contact pads to form metal pillars within the channels.
4. The method of claim 1 , further including depositing an adhesive within the recess adjacent to the peripheral edge of the wafer, curing the adhesive, and flattening a surface of the adhesive such that the adhesive surface is coplanar with the top surface of the photoresist layer.
5. The method of claim 4 , further including the step of removing the adhesive.
6. The method of claim 1 , further including placing a spacer in the recess, depositing an adhesive about the periphery of the spacer, and placing the wafer on the spacer and over the adhesive.
7. The method of claim 6 , wherein the spacer comprises a disc-shaped body having a diameter smaller than the diameter of the wafer, further including depositing additional adhesive within the recess about the periphery of the wafer, and flattening a surface of the additional adhesive such that the surface of the additional adhesive is coplanar with the top surface of the photoresist layer.
8. The method of claim 4 , wherein the step of depositing the photoresist layer includes spin coating the photoresist layer.