The present invention is related in general to the field of semiconductor devices and processes, and more specifically to the structure and fabrication method of chip attachment layers with traverse-aligned conductive filler particles.
When semiconductor chips have to be attached to substrates or leadframes, it is common practice to use a layer of adhesive compound, such as an epoxy-based polymeric formulation, as a coupler between the chip and the substrate. The polymeric compound is usually a thermoset resin, applied to the chip attach pad of the substrate as a low-viscosity precursor to allow spreading of the compound over the attach pad. After the precursor resin is distributed, the chip is pressed onto the layer with a force sufficient to partially redistribute the adhesive by flowing and thus to ensure a uniform layer thickness across the whole chip area. Thereafter, the layer, together with the chip and the substrate, is subjected to elevated temperatures for a certain amount of time to activate a resin polymerization process, which hardens the compound and thus irreversibly couples chip and substrate together.
For electrical circuit operation as well as for removal of the operational heat, it is common practice to add to the adhesive compound filler particles, which are electrically and thermally conductive. The most frequently used filler particles are elongated silver flakes with a length between 1 and 10 μm and an approximately uniform distribution across the attach layer. To achieve good electrical and thermal conductivity, the filler loadings typically have to be high, usually more than 80 weight % of the attach compound.
Applicant detected in microscopic analysis that during the phase of pressuring the chip onto the attach layer, the flowing adhesive resin causes the conductive filler particles throughout the layer to become horizontally oriented with respect to the chip/layer and substrate/layer interfaces. Applicant further found that the particles are wetted on all surfaces by the low-viscosity resin, inhibiting metal-to-metal contact by surface tension and thus decreasing the electrical conductivity. In addition, continuous resin-rich films are formed on both chip/layer and substrate/layer interfaces, further lowering the electrical conductivity of the attach layer. The drop in conductivity becomes particularly dominant with decreasing layer thickness even when the layers include more than 80 weight % filler loadings.
Applicant saw that the problem of mediocre electrical and thermal conductivity of adhesive resin layers can be solved by aligning the electrically and thermally conductive filler particles in chains normal to the chip/layer and substrate/layer interfaces and piercing the chains through the resin-rich films to achieve contact both with the chip and the substrate. The electrical and thermal conductivity can be dramatically improved even at filler fillings significantly lower than 80 weight %; the lower filler loading, in turn, improves the mechanical adhesion.
Applicant discovered that the alignment in the layer of the conductive filler particles can be achieved by a method wherein the suspended conductive particles are intermixed with a second kind of suspended filler particles comprising a ferromagnetic core coated by surfactants at less than 10 weight % loading.
After spreading the resin layer of sufficiently low viscosity over the substrate, an external magnetic field normal to the layer is applied, which arrays the suspended ferromagnetic particles normal to the layer with enough force to simultaneously steer and align the conductive particles in chains normal to the layer surfaces. The chip is then pressed onto the resin layer, piercing the tips of the conductive chains through the resin-rich films on the layer surfaces and achieving contact both with the chip and the substrate. Finally, the resin with the aligned conductive chains is hardened by polymerization.
It is a technical advantage that dependent on the viscosity of the resin and the strength of the magnetic field, the external magnetic field may be applied continuously for the duration of the step of pressing the chip onto the layer, or the field may be cycled. The magnetic field can be created by permanent magnets or by electromagnets; the permanent magnets may be mounted on the assembly transport, or may applied through the polymerization step.
It is another technical advantage that the chips may be provided with a backside metallization including nickel; the magnetic field of the nickel will increase the chip press-down force, further improving the filler alignment and pierce-through performance.
The preferred conductive filler particles include elongated silver flakes; alternatively, they may be carbon nano-tubes, or particles comprising an elongated magnetic metal core (for example, iron) surrounded by a film of high electrical and thermal conductivity (for example, silver).
A second effective filler type contains a magnetic core coated by surfactants. The preferred filler particles of the second kind have a core selected from a group including iron, magnetite, nickel, cobalt, and compounds thereof, and a surfactant selected from a group including tetramethylammonium hydroxide, phosphoric acid ester, and ethoxylated aliphatic acid. The second filler type need not be intrinsically conductive, but must have magnetic susceptibility so as to transfer the force of magnetic attraction to other particles which are then oriented in a favorable direction.
A third filler type comprises elongated magnetic particles, such as iron particles, which are effective in changing orientation.
In a schematic cross section,
Workpiece 110 may be a semiconductor chip or any other piece part to be assembled on substrate 101. In either case, workpiece 110 may have a metal layer 111; for reasons of the invention to be discussed later (ferromagnetism), a preferred metal for layer 111 is nickel. Workpiece 110 has a second surface 110a. If workpiece 110 has metal layer 111, second surface 110a is actually the surface of the metal layer. Second surface 110a is parallel to first surface 101a and is spaced from the first surface by gap 120. Dependent on device 100, the width of gap 120 may vary from 100 μm or more to 4 μm or less.
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The second particles 141 include a ferromagnetic core coated by surfactants so that the second particles can be magnetized and suspended in compound 130. The second particles have an outer diameter of about 20 to 30 nm and an inner core of about 10 to 15 nm diameter. The core may be selected from a group including iron, magnetite, nickel, cobalt, and compounds thereof. The surfactant may be selected from a group including tetramethylammonium hydroxide, phosphoric acid ester, and ethoxylated aliphatic acid. The second filler type need not be intrinsically conductive, but must have magnetic susceptibility so as to transfer the force of magnetic attraction to other particles which are then oriented in a favorable direction. When the second particles are magnetized, neighboring second particles are arrayed in lines approximately normal to the first surface 101a and second surface 110a.
In order to describe an example of a surfactant,
Since ferromagnetic materials respond to external magnetic fields by aligning their unpaired electron spins with the external vector fields, dominating the forces of surface tension and gravity, the magnetite nanoparticles align, or spike, in the direction of the magnetic field lines; the stronger the vector field lines (and the lower the viscosity of the compound), the more forceful the alignment and larger the spikes, provided that the viscosity of the polymer compound is sufficiently low to facilitate the alignment.
Another embodiment of the invention is a method of attaching a workpiece 110 onto a substrate 101 using an adhesive compound 130 with conductive filler particles. Certain conditions of the method are illustrated in
In the first step of the method, a predetermined amount of an adhesive polymeric compound is deposited on surface 101a. The compound is preferably an epoxy-based thermoset low-viscosity precursor. A preferred method is by letting a certain amount of the compound drop onto surface 101a from the orifice of a syringe. The compound may spread by surface tension over at least a portion of surface 101a to form an approximate layer, potentially with irregular outline and non-uniform thickness. The compound includes intermixed suspensions of two kinds of filler particles: The first particles have good electrical and thermal conductivity and are preferably made of silver flakes between about 1 and 10 μm length. In the suspension of the first particles, the length of the particles is oriented in random fashion, and the concentration of the first particles is preferably less than 90 weight % of the compound. Alternative to pure silver, the first particles may include a core of ferromagnetic metal such as iron or nickel, surrounded by a film of high-conductivity metal such as silver. As yet another alternative, the first particles may be carbon nanotubes.
The second particles are ferromagnetic and preferably made of a core of about 10 nm diameter of a ferromagnetic compound, where the core is coated with a surfactant to prevent the second particles from agglomerating; the outer diameter of the second particles is preferably between about 20 and 30 nm. In the suspension of the second particles, the concentration of the second particles is preferably less than 10 weight percent of the compound. The ferromagnetic core is selected from a group including iron, nickel, cobalt, and compounds thereof such as magnetite Fe3O4, and the surfactant coating is selected from a group including tetramethylammonium hydroxide, phosphoric acid ester, and ethoxylated aliphatic acid.
In the next step, an external magnetic vector force field is applied to the layer of polymeric compound with the suspensions of particles. As
In addition, the strength of the magnetic field is powerful enough to cause, along with the alignment of the magnetic second particles 141, the concurrent steering and aligning of the conductive first particles 140 in chains so that the chains of the first particles become oriented normal, i.e., vertical, to the first surface 101a. As
In the next process step, a workpiece 110 with a second surface 110a is provided; as an example, workpiece 110 may be a semiconductor integrated circuit chip and the second surface 110a may be the chip surface remote from the integrated circuit; surface 110a may have a layer of ferromagnetic metal such as nickel (not shown in
While the magnetic field is continuously applied and first particles 140 are oriented normal (i.e., vertical) to substrate surface 101a, a mechanical force in the direction towards substrate 101, indicated by arrow 410 in
As has been mentioned above, the strength of the magnetic field can be enhanced by providing a ferromagnetic metal layer over surface 110a of the workpiece (such layer 111 is shown in
It is advantageous for most devices 100 to harden compound 130 by polymerizing the thermoset precursor, preferably while the external magnetic field remains applied. The orientation of the conductive chains of particles 130 is thus frozen in the direction normal to the workpiece and to the substrate.
While this invention has been described in reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. As an example, the viscosity of the polymeric precursor is variable over a wide range; further the strength of the magnetic field is variable over a wide range. Consequently, the time of applying the external magnetic field may for some combinations of precursor and field strength be shortened so that the field is no longer applied during polymerization.
As an another example, since a relatively small percent of magnetite fillers is sufficient to align other, high-conductivity filler particles in the preferred orientation, carbon nanotubes may be used instead of the silver flakes as the high-conductivity fillers. Because of the high electrical and thermal conductivity of carbon nanotubes, the filler percentage may then be reduced to values substantially below 80 weight %. In turn, based on the lower filler loadings, more attachment area becomes available for improved mechanical adhesion of the polymeric compound to the substrate and the workpiece.
It is therefore intended that the appended claims encompass any such modifications or embodiments.