Method for fabricating flip-attached and underfilled semiconductor devices

A semiconductor device (1700), which comprises a workpiece (1201) with an outline (1711) and a plurality of contact pads (1205) and further an external part (1701) with a plurality of terminal pads (1702). This part is spaced from the workpiece, and the terminal pads are aligned with the workpiece contact pads, respectively. A reflow element (1203) interconnects each of the contact pads with its respective terminal pad. Thermoplastic material (1204) fills the space between the workpiece and the part; this material adheres to the workpiece, the part and the reflow elements. Further, the material has an outline (1711) substantially in line with the outline of the workpiece, and fills the space (1707) substantially without voids. Due to the thermoplastic character of the filling material, the finished device can be reworked, when the temperature range for reflowing the reflow elements is reached.

FIELD OF THE INVENTION

The present invention is related in general to the field of electronic systems and semiconductor devices and more specifically to methods for fabricating flip-assembled and underfilled semiconductor devices.

DESCRIPTION OF THE RELATED ART

When an integrated circuit (IC) chip is assembled on an insulating substrate with conducting lines, such as a printed circuit motherboard, by solder bump connections, the chip is spaced apart from the substrate by a gap; the solder bump interconnections extend across the gap. The IC chip is typically a semiconductor such as silicon, silicon germanium, or gallium arsenide, the substrate is usually made of ceramic or polymer-based materials such as FR-4. Consequently, there is a significant difference between the coefficients of thermal expansion (CTE) of the chip and the substrate; for instance, with silicon (about 2.5 ppm/° C.) as the semiconductor material and plastic FR-4 (about 25 ppm/° C.) as substrate material, the difference in CTE is about an order of magnitude. As a consequence of this CTE difference, thermomechanical stresses are created on the solder interconnections, especially in the regions of the joints, when the assembly is subjected to temperature cycling during device usage or reliability testing. These stresses tend to fatigue the joints and the bumps, resulting in cracks and eventual failure of the assembly.

In order to distribute the mechanical stress and to strengthen the solder joints without affecting the electrical connection, the gap between the semiconductor chip and the substrate is customarily filled with a polymeric material, which encapsulates the bumps and fills any space in the gap. For example, in the well-known “C-4” process developed by the International Business Machines Corporation, polymeric material is used to fill any space in the gap between the silicon chip and the ceramic substrate.

The encapsulant is typically applied after the solder bumps have undergone the reflow process and formed the metallic joints for electrical contact between the IC chip and the substrate. A viscous polymeric, thermoset precursor, sometimes referred to as the “underfill”, is dispensed onto the substrate adjacent to the chip and is pulled into the gap by capillary forces. The precursor is then heated, polymerized and “cured” to form the encapsulant; after the curing process, the encapsulant is hard and cannot be softened again.

It is well known in the industry that the temperature cycling needed for the underfill curing process can create thermomechanical stress on its own, which may be detrimental to the chip and/or the solder interconnections. Additional stress is created when the assembly is cooled from the reflow temperature to ambient temperature. The stress created by these process steps may delaminate the solder joint, crack the passivation of the chip, or propagate fractures into the circuit structures. In general, the sensitivity to cracking of the layered structures of integrated circuits is increasing strongly with decreasing thickness of the various layers and increasing mechanical weakness of low dielectric constant insulators.

SUMMARY OF THE INVENTION

Consequently, a need has arisen for an assembly methodology in which the stress-distributing benefits of the underfill material can be enjoyed without the deleterious side-effects of the underfilling process, resulting in enhanced device reliability. It is a technical advantage if the methodology provides an opportunity for device repair or re-working. The methodology should be coherent, low-cost, and flexible enough to be applied to different semiconductor product families and a wide spectrum of design and process variations. It is another technical advantage, if these innovations are accomplished while shortening production cycle time and increasing throughput.

One embodiment of the invention is a tape for use as a carrier, which comprises a base sheet of polymeric, preferably thermoplastic, material having first and second surfaces. A first polymeric adhesive film and a first foil of different material are attached to the base sheet on both the first and second surface sides; they thus provide a partial thickness to the tape. Further, a second polymeric adhesive film and a second foil of different material are attached to the first foil on the second surface side. A plurality of holes is formed through the partial thickness of the tape; and a reflow metal element is placed in each of the holes; the element adheres to the second adhesive film, and has preferably a diameter about equal to the partial thickness.

Another embodiment of the invention is a semiconductor device, which comprises a workpiece with an outline and plurality of contact pads and further an external part with a plurality of terminal pads. This part is spaced from the workpiece, and the terminal pads are aligned with the workpiece contact pads, respectively. A reflow element interconnects each of the contact pads with its respective terminal pad. Thermoplastic material fills the space between the workpiece and the part; this material adheres to the workpiece, the part and the reflow elements. Further, the material has an outline substantially in line with the outline of the workpiece, and fills the space substantially without voids.

When the workpiece is a semiconductor chip, the external part is a substrate suitable for flip-assembly of the chip. When the workpiece is a semiconductor package encapsulating an assembled semiconductor chip, the external part is board suitable for flip-attachment of the package. Due to the thermoplastic character of the filling material, the finished device can be reworked, when the temperature range for reflowing the reflow elements is reached.

Another embodiment of the invention is a method for assembling a semiconductor device, in which a workpiece with an outline and a plurality of contact pads is provided, further a tape as described above; the location of the holes, and thus the reflow metal elements in the holes, match the locations the contact pads. The first foil is removed from the first tape surface side, whereby the first polymeric adhesive film on the first tape side is exposed. The reflow elements of the tape are then placed in contact with the contact pads of the workpiece such that the first polymeric adhesive film on the first tape side holds the workpiece in place. Thermal energy is supplied to the workpiece and the tape sufficient to reflow the reflow elements and liquefy the thermoplastic base sheet. After cooling to ambient temperature, the tape is attached to the workpiece substantially without leaving voids.

The process steps of the method may continue by providing an external part with a plurality of terminal pads in locations matching the locations of the reflow elements in the tape holes. The second foil is removed, together with the second polymeric adhesive film and the first foil, from the second surface side, whereby the first polymeric adhesive film on the second tape side is exposed. The reflow elements of the tape are then placed in contact with the terminal pads of the external part such that the first polymeric adhesive film on the second tape side holds the external part in place. Thermal energy is supplied to the workpiece, the tape, and the external part sufficient to reflow the reflow elements and liquefy the thermoplastic base sheet. After cooling to ambient temperature, the tape is attached to the external part, while the workpiece is spaced apart from the external part and the space is filled substantially without leaving voids.

When the workpiece is a semiconductor chip, the external part is a substrate suitable for flip-assembly of the chip. When the workpiece is a semiconductor wafer containing a plurality of semiconductor devices, the external part is a substrate suitable for flip-assembly of the wafer. When the workpiece is a semiconductor package, which encapsulates an assembled semiconductor chip, the external part is a board suitable for flip-attachment of the package. When the workpiece is a stack of semiconductor packages, the external part is a board suitable for flip-attachment of the stack.

Embodiments of the present invention are related to flip-chip assemblies, ball grid array packages, chip-scale and chip-size packages, and other devices intended for reflow attachment to substrates and other external parts. It is a technical advantage that the invention offers a methodology to reduce the thermomechanical stress between the semiconductor part of a device and a substrate of dissimilar thermal expansion coefficient while concurrently controlling essential assembly parameters such as spacing between the semiconductor part and the substrate, adhesion between the parts, and selection of the temperature ranges needed in the assembly process. Additional technical advantages derive from the fact that the devices made with the thermoplastic tape are reworkable. Further, the process flow is simplified since the conventional underfill process after the flip-assembly is eliminated.

The technical advantages represented by certain embodiments of the invention will become apparent from the following description of the preferred embodiments of the invention, when considered in conjunction with the accompanying drawings and the novel features set forth in the appended claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

One embodiment of the invention is depicted in the schematic cross section ofFIG. 1as a tape, generally designated100, for use as a carrier and specifically in semiconductor device assembly. Tape100comprises a base sheet101of polymeric, preferably thermoplastic material in the thickness range from about 25 to 450 μm; for some devices, the thickness may reach approximately 800 μm. Preferred thermoplastic base sheet materials include long-chain polyimides with acrylic resin or silicone resin, long-chain polyethylenes with acrylic resin, and long-chain polypropylenes with acrylic resin. The base sheet material is preferably selected so that it softens and enters the low viscosity or liquid phase in the same temperature range, which is needed for reflowing the reflow element embedded in the tape (see below). This temperature range includes, for example, the melting temperature of the solder selected for assembling the device. It is a technical advantage, when the base sheet is selected from thermoplastic materials, since the processes of liquefying and solidifying the thermoplastic material may be repeated numerous times without difficulty. Preferably, the coefficient of thermal expansion is selected between about 8 and 120 ppm, and the elasticity modulus between about 100 and 10000 MPa.

Base sheet101has a first surface101aand a second surface101b. Attached to the first surface101aare a first polymeric adhesive film102followed by a first foil103of different material. In similar fashion, attached to the second surface101bare a first polymeric adhesive film104followed by a first foil105of different material. The adhesive films102and104preferably include polymer materials such as epoxy, polyimide, or silicone, which have not only adhesive properties, but can also easily be peeled off; the adhesive films have a preferred thickness range from about 25 to 100 μm. The foils103and105comprise inert materials such as PVC and PET, and have a preferred thickness range from about 25 to 50 μm.

The combination of the base sheet101, the polymeric adhesive films102and104, and the foils103and105provides a partial thickness110to tape100. It is this partial thickness110, which is penetrated by a plurality of holes in tape100in order to provide space for reflow elements such as solder balls (seeFIGS. 2 and 4).

AsFIG. 1shows, tape100further comprises a second polymeric adhesive film106attached to the first foil105on the second surface side of the base sheet, followed by a second foil107. The second polymeric adhesive film106is preferably selected from materials such as epoxy, polyimide, and silicone in the thickness range from about 25 to 100 μm. The second foil107is preferably an inert material such as PVC and PET in the thickness range from about 10 to 50 μm. Laminated tapes such as tape100are commercially available and can be made to custom specification, for instance by the company Lintec, Japan.

AsFIG. 3schematically illustrates, a plurality of holes301,302, . . . ,30nis formed in tape100. The position of these holes can be selected in any predetermined pattern.FIG. 2shows one specific hole of diameter201in more detail. The hole penetrates the laminated tape100to the depth110, which is defined inFIG. 1. Depth110reaches to the second polymeric adhesive film106, but does not penetrate it fully. Among the techniques available for the opening processes are laser, mechanical drill, and mechanical punching. Experience has shown that the laser technique is superior to the drilling or punching techniques. The preferred laser method is excimer laser, because excimer laser has an accuracy of +/−5 μm for defining the depth110and the diameter201. The hole may be round or may have any other predetermined outline; the hole diameter may be same for all holes, or it may be different.

The hole illustrated inFIG. 2is shown to have approximately vertical walls. However, for certain applications such as stable fitting of solder balls, tapered walls as illustrated inFIG. 4may be preferable. The tapered walls form an angle401with second adhesive film106. The preferred angle401is between about 70° and 80°.

FIG. 6illustrates how one reflow metal element is placed in each of the holes in tape100. As an example, the reflow elements may be solder balls601,602, . . . ,60n.FIG. 5shows one specific reflow metal element501in more detail in a hole of depth110. Reflow element501has preferably a diameter502equal to or slightly less than the hole diameter201. In area503, reflow element502is in contact with second polymeric adhesive film106of tape100; in this fashion, reflow element501is securely held in place in the hole and cannot be dislodged or fall out, even when the tape is positioned upside down relative to the position illustrated inFIG. 5so that the hole opening with the reflow element faces downward.

In order to highlight the technically superior features of tape100,FIGS. 7 through 20describe various process steps of assembly and device fabrication employing a workpiece, which has an outline and a plurality of contact pads. The tape is provided with the plurality of holes and inserted reflow elements in locations, which match the locations of the contact pads of the workpiece. In embodiments for the semiconductor industry, the workpiece is either a semiconductor wafer containing a plurality of semiconductor devices, or a semiconductor chip, or a semiconductor package, which encapsulates an assembled semiconductor chip on a substrate.

The process flow starts withFIG. 7, wherein the first foil103has been removed and the position of the hole with the inserted reflow element is inverted relative to the starting position inFIG. 5. First polymeric adhesive film102is now exposed. Reflow element501remains firmly in place, since it is in contact with polymeric adhesive film106in area503. For many applications, the size of element501and the hole have been selected so that element501is slightly protruding from the hole at this stage of the process flow.

As a specific workpiece, the schematicFIG. 8shows in perspective view a semiconductor wafer801with the plurality of semiconductor devices facing upward. Each device has a plurality of contact pads, facing upward. Tape802is positioned upside down as shown in the portion ofFIG. 7; the locations of the plurality of reflow elements in the tape holes match the locations of the contact pads of the semiconductor devices on the wafer. As arrow803indicates, each reflow element of tape802is brought into contact with its corresponding contact pad of wafer801. For this embodiment, tape802has preferably the same outline as the semiconductor wafer801.

The simplified cross section ofFIG. 10illustrates tape1001contacting workpiece1002; as stated above, workpiece1002may be specifically a semiconductor wafer. At this stage, the assembly is ready for the next process step of heating (see below).

As another specific workpiece, the schematicFIG. 9shows a molded entity901containing a plurality of semiconductor chips assembled on a substrate and encapsulated by molding compound. The substrate has a plurality of contact pads for each assembled chip, facing upward. Tape902is positioned upside down as shown in the portion ofFIG. 7; the locations of the plurality of reflow elements in the tape holes match the locations of the contact pads of the substrate of the molded entity901. As arrow903indicates, each reflow element of tape902is brought into contact with its corresponding contact pad of molded entity901. For this embodiment, tape902has preferably the same outline as the molded entity901.

The simplified cross section ofFIG. 11illustrates tape1101contacting workpiece1102; as stated above, workpiece1102may be specifically a molded semiconductor entity containing a plurality of assembled semiconductor chips1103on a substrate1104; the chips1103are connected to substrate1104by bonding wires1105and encapsulated by molding compound1106. At this stage, the assembly is ready for the next process step of heating (see below).

The schematic cross section ofFIG. 12illustrates the next step of the fabrication process. Each reflow element1203of the tape is brought into contact with the respective contact pad1205of the workpiece; for example, the workpiece may be a semiconductor chip or a semiconductor package. This step may be facilitated by the first polymeric adhesive film102holding workpiece1201in place. Thermal energy is then supplied to workpiece1201and tape1202sufficient to reflow the reflow element1203and liquefy the thermoplastic base sheet1204(designated101inFIG. 1before liquefying), whereby tape1202is attached to workpiece1201. InFIG. 12, the effect of the heating cycle is schematically indicated by two results: The reflow element (for example, solder ball) has formed a joint1206across the whole length of pad1205, while the remaining surface of the element has been pulled by surface tension into an approximately spherical shape. The softened thermoplastic material1204has filled the available space1207around joint1206and the reflowed metal neck1208. By selecting the appropriate heating temperature and time, the surrounding thermoplastic material is filling space1207substantially without leaving voids.

When those embodiments, in which the workpiece is an individual chip cr an individual package, have been cooled to ambient temperature, the thermoplastic material has formed an outline, which is substantially in line with the outline of the workpiece. As defined herein, “in line” does not only include straight line, continuing the outline of the workpiece; it also includes minor concave or convex contours. However, “in line” excludes the well-known meniscus, which is typically formed in conventional technology by dispensing thermoset underfill material. In the conventional fabrication process, the low-viscosity thermoset material is driven by surface tension to protrude somewhat outside the workpiece contours to form the well-known meniscus.

In the next process step, the second foil107and the second polymeric adhesive film106are removed, exposing the approximately spherical shape of the reflow element1203. The result is displayed inFIG. 13. In the next process step, the first foil105from the second tape surface side is removed, exposing the first polymeric adhesive film104on the second side of tape1204. The result is displayed inFIG. 14.

When workpiece1201is not an individual semiconductor chip, but a whole semiconductor wafer containing a plurality of semiconductor devices, the next process step after the stage shown inFIG. 14comprises the separation of the wafer, assembled with the tape, into discrete assembled devices. The preferred method of separation is sawing. The schematic top view ofFIG. 15illustrates the result of this step.

When workpiece1201is not an individual semiconductor package, but a whole molded entity containing a plurality of assembled and encapsulated semiconductor chips, the next process step after the stage shown in FIG.14comprises the separation of the entity, assembled with the tape, into discrete assembled devices. The preferred method of separation is sawing. The schematic perspective view ofFIG. 16illustrates the result of this step.

For the next process step, an external part is provided, which has a plurality of terminal pads in locations matching the locations of the reflow elements. As an example, the external part may be a substrate suitable for flip-assembly of the semiconductor chip, which has previously been attached to the tape. As another example, the external part ray be a substrate suitable for flip-assembly of a whole semiconductor wafer. As yet another example, the external part may be a board suitable for flip-assembly of the semiconductor package, which has previously been attached to the tape.

InFIG. 17, the external part is designated1701, and one of the plurality of terminal pads is designated1702. The workpiece1201with its contact pad1205together with the attached remainder1720of the tape and the reflow element form unit1710. Notice that the side contours of unit1710are shown as substantially straight contours1711; the straight contours are a consequence either of the singulation steps described above, or of the assembly using the tape with the thermoplastic base sheet.

The reflow element1203of the tape, soldered to workpiece contact pad1205, is placed in contact with the terminal pad1702of the external part. In addition, the first polymeric adhesive film104on the second tape side may hold the external part1701in place. Thermal energy is then supplied to the workpiece1201, the tape1720, and the external part1701sufficient to reflow the reflow element1203and to liquefy the thermoplastic base sheet1204of the tape1720. InFIG. 17, the effect of the heating cycle is schematically indicated by two results: The reflow element1203has formed a joint1706across the whole length of terminal pad1202; and the softened thermoplastic material1204has filled the available space1707around joint1706and the reflowed metal neck1708. By selecting the appropriate heating temperature and time, the surrounding thermoplastic material is filling space1707substantially without leaving voids. Further, after cooling to ambient temperature, the thermoplastic material1204has approximately retained its outline1711, which is substantially in line with the outline1711of the workpiece.

As a result of the assembly process, the tape1720and the workpiece1201are attached to the external part1701, while the workpiece1201is spaced apart form the external part1701. The thermoplastic “underfill” material is in place to mitigate thermo-mechanical stress at the reflow interconnection and the solder joints due to its insignificant thermal shrinkage compared to conventional thermoset underfill materials. The finished product is generally designated1700inFIG. 17.

For the assembly process steps described above, the materials for the polymeric adhesive films102,104, and106are preferably selected so that they remain sticky in the temperature range from ambient temperature to about 300° C. and even higher, do not require a specific curing process, and have a decomposition temperature above about 300° C.

It is evident from the above description of the material selection and process flow that no flux is required for the metal reflow and soldering action, and any process-related stress on the metal reflow ball during the temperature cycles is minimized due to the continued presence of the thermoplastic polymer. Further, the thermoplastic material fills any available space substantially void-free. Experience has further shown that the choice of thermoplastic material and its continued presence during the fabrication process provides the semiconductor products with characteristics of reliability performances under use conditions as well as tests of temperature cycling, moisture sensitivity, and drop examinations, which are three to ten times higher than products manufactured using prior art fabrication technologies.

The schematicFIG. 18is an example of an embodiment, in which the workpiece is a semiconductor chip1801flip-attached by means of tape1820onto an external board1802. In the reflow process step, the solder joint formation and the substantially void-free underfilling are performed concurrently. Notice that the tape1820has an outline1821substantially in line with the outline1801aof the chip1801. This approximately straight outline is a consequence of the thermoplastic nature of the tape base material; for a chip singulated from a wafer it may also be created by the chip separation process.

The schematicFIG. 19is an example of an embodiment, in which the workpiece is a semiconductor package1901having a substrate1902with terminal pads, which are attached by means of tape1920onto an external board1902. In the reflow process step, the solder joint formation and the substantially void-free underfilling are performed concurrently. Notice that the tape1920has an outline1921substantially in line with the outline1901aof the package1901. This approximately straight outline is a consequence of the thermoplastic nature of the tape base material; for a package singulated from a molded entity it may also be created by the package separation process.

Another embodiment, a semiconductor product generally designated2000, is displayed in the schematicFIG. 20. A first package2001having an extended substrate2002is attached by means of tape2010to a second package2020, also having an extended substrate2021. The stack of two packages is attached by means of tape2030to an external part such as a board2040. The use of thermoplastic material in the base sheet of tapes2010and2030enables a substantially straight outline2011and2031. Stacks of packages are generally known to be sensitive to thermo-mechanical stress due to the distributed components of widely different coefficients of thermal expansion (silicon, metals, polymers, etc.). It is, therefore, a particular technical advantage of the invention to offer a stack structure and fabrication method based on thermoplastic underfill material, which reduces thermo-mechanical stress significantly by having a much smaller thermal shrinking than the thermoset materials of conventional art. With this advantage, it is easy for someone skilled in the art to construct composite devices in view ofFIG. 20, which can be realized by the concept and method of the invention.

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, for assemblies having interconnection elements with significantly higher or lower reflow temperatures, suitable base sheet thermoplastics and adhesives can be formulated by modifying the polymer chains of their materials. As another example, underfill materials of lower coefficients of thermal expansion can be formulated by adding inert (inorganic) fillers to the polymer base material. It is therefore intended that the appended claims encompass any such modifications and embodiments.