X-Ray energy is widely used as an inspection tool during the fabrication of semiconductor devices; but, until now, no one has used X-Ray energy to build semiconductor assemblies and subassemblies. Immunolight has a technology set consisting of specialty materials that are activated by X-Ray energy for the purposes of bonding and encapsulating advanced semiconductor assemblies and subassemblies.
Polymers in Semiconductors
The miniaturization of semiconductors at the wafer level and the subsequent fabrication of devices containing advanced Integrated Circuits (ICs) require the use of a special category of polymeric materials. These special polymeric materials are used in different stages of assembly throughout the packaging of the IC.
Thermosetting polymers and adhesives are well known and are used for a wide variety of applications. One particularly important application domain is in the field of microelectronics assembly, where thermoset adhesives are used to bond bare die to substrate, establish conductive contacts, and perform various roles in packaging and sealing structures such as glob-top and die-underfill structures.
As modern electronic components evolve to smaller sizes, and integrated circuits include ever-smaller features such as ultra-shallow junctions, heat becomes a more and more important factor in the manufacturing process. When the devices are heated to cure the polymers, different parts expand at different rates which leads to misalignments between critical parts of the assembly. To address these issues, many light-curing polymer systems have been developed. However light-curable polymers need a direct line of sight, and even so, can result in “shadowing”, which leads to an inefficient cure. The solution path offered by Immunolight’s technology overcomes the shadowing effects encountered by the standard photo curable chemistries by using deeply penetrating X-Ray energy.
An important consideration is the ability to cure polymeric materials on demand (the cure reaction in this case is controllable). The initiation of the curing reaction is gated by the delivery of X-Ray energy to the polymeric materials used in the semiconductor device – therefore the cure does not proceed until the X-Ray initiation energy is initiated.
Encapsulation Of Semiconductors
A dam and fill encapsulation is a potting application where two polymeric materials are used. One material is used to create a dam and the other material is used for the encapsulation purpose. In dam and fill, under the same X-Ray energy intensity, one bead would cure faster than the other. In some Dam and Fill applications, such as in RF-ID tags, one could apply a dam, cure it and then fill and cure it. However, one could also co-cure the two adhesive beads at the same time. The polymeric materials used for both cases (possibly the same chemical formulation) are different only in viscosity or in the content of the energy modulators.
Schematic Representation of a Dam and Fill application
In another type of encapsulation, the Adhesive is dispensed and wicked under the chip using capillary forces. The novel Immunolight adhesives enables curing through the chip followed by inspection under the chip. This process could be particularly important for microprocessors and high performance ICs.
Schematic representation of an underfill application
Bonding Of Semiconductors
The increase in density of semiconductor ICs can be achieved through junction miniaturization or through the stacking of multiple devices. The stacking of semiconductors can be achieved using wafer bonding, die to wafer bonding, die on die bonding, package on package assembly at room temperature, etc. This is a particularly useful area for anisotropically conductive adhesives. The bonding takes place via the correct alignment and subsequently bonding of devices.
The X-Ray polymeric technology is under development for wafer-to-wafer and bonding using an X-ray activated, UV or visible light curable, Anisotropic Conductive Adhesive (ACA) adhesive. Of importance, it is a room temperature process. There is no need to apply external heat to the wafer surfaces being joined.
Elements of the Anisotropic Conductive Adhesive connecting one IC with it electrical pads to a another semiconductor device
An X-ray Exposure Device is used to generate a field of high energy photons that stimulate the fluorescent coating of the down-converting spheres which then spontaneously emit UV light of the correct wavelength and luminosity to cause the UV resin to rapidly cure by photo initiation. As the high energy photons pass through the materials they may also advantageously be detected on the surface of an X-ray Imaging Detector, positioned directly across from the X-ray exposure.