Copper-based chip attach for chip-scale semiconductor packages
A semiconductor device comprising a semiconductor chip having an active and a passive surface, the passive surface adhesively attached to a substrate film by means of a multilayer composite; this composite comprising a metal foil having first and second surfaces and an adhesive layer attached on each of these surfaces. The multilayer composite has an average modulus larger than the modulus of the encapsulating molding compound used in the semiconductor device. By applying the composite to assembling face-up chip-scale devices, stress in solder joints is reduced and solder fatigue life enhanced.
The present invention is related in general to the field of electronic systems and semiconductor devices, and more specifically to structure and fabrication methods of chip-scale packages which use a copper-based chip-attach material to reduce stresses in solder joints.
DESCRIPTION OF THE RELATED ARTOne of the major trends in semiconductor packaging is the effort to shrink the package outline so that the package consumes less area and less height when it is mounted onto the circuit board. Another powerful trend is the effort to achieve the outline reduction with minimum cost (both material and manufacturing cost) . One of the most successful approaches has been the development of so-called “chip-scale packages”. This expression is commonly used for packages which have an outline adding less than 20% to the chip area. A chip-scale package which has only the size of the chip itself is often referred to as “chip-size package”.
For assembling an integrated circuit (I/C) chip in a chip-scale package, there are two options: In the so-called “face-up” assembly, the chip is assembled so that the “passive” surface is attached, by some adhesive, to a substrate while the “active” surface, embedded with the (IC) and its plurality of input/output (I/O) contact pads, is facing away from the substrate. In contrast, in the “face-down” assembly, the chip is assembled so that the “active” surface faces the substrate and is attached, usually by solder balls, to this substrate.
The “face-up” assembly requires an adhesive between chip and substrate which should exhibit a number of characteristics, which are in part difficult to combine:
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- The chip attach material, preferably an epoxy, should show tightly controllable “bleed-out” during chip attachment and little release of solvents (“outgassing”) during polymerization (hardening).
- After polymerization, the attach material should be hardened to a degree that it guarantees an almost immovable positioning of the chip during the wire bonding process at elevated temperatures.
- The polymerized attach material should not change further during the subsequent transfer molding process and the molding material polymerization process at elevated temperatures and extended times.
- Since the finished semiconductor device represents a system combining materials with different coefficients of thermal expansion (CTE), the attach material has to withstand the thermomechanical stresses, which naturally arise in this multi-material system during significant temperature variations.
- After the semiconductor device is attached to an outside part such as a wiring board (usually by solder bumps), the chip attach material has to withstand the thermomechanical stresses between the device, the board attach material (solder bumps) and the board itself in temperature variations. These stresses are a natural consequence of the different CTE's of this assembled system. In reverse, the chip attach material should help to reduce thermomechanical stresses in solder joints in order to increase the solder fatigue life and thus increase the reliability of the device.
- The attach material and the attach process should be low cost.
A number of different approaches for chip-scale device and package design, material selection and process conditions have been chosen by different semiconductor manufacturers for different market segments. A good overview of these approaches is presented in the book entitled “Chip Scale Package” by J. H. Lau and S. R. Lee (McGraw-Hill 1999). This overview shows that the latter two requirements listed above are particularly hard to fulfill jointly.
An urgent need has, therefore, arisen for a coherent, low-cost concept and method of attaching IC chips to substrates. The concept and the fabrication method should further provide increased solder joint fatigue life expectancy. The fabrication method should be simple, yet flexible enough for different semiconductor product families and a wide spectrum of design and process variations. Preferably, these innovations should be accomplished without extending production cycle time, and using the installed equipment, so that no investment in new manufacturing machines is needed.
SUMMARY OF THE INVENTIONThe present invention describes a semiconductor device comprising a semiconductor chip having an active and a passive surface, the passive surface adhesively attached to a substrate film by means of a multilayer composite; this composite comprising a metal foil having first and second surfaces and an adhesive layer attached on each of these surfaces. By applying this composite to assembling face-up chip-scale devices, stress in solder joints is reduced and solder fatigue life enhanced.
In a preferred embodiment, the multilayer composite consists of a copper foil (thickness range 30 to 150 μm) with a layer of epoxy resin/acrylic resin blend on each of its surfaces. The composite has an average modulus larger than the modulus of the encapsulating molding compound.
For a simple manufacturing process flow of the I/C chip attachment to the substrate film, the epoxy resin/acrylic resin layer facing the wafer support film (a wafer carrier) is ultra-violet sensitive (thickness range 20 to 50 μm). However, the epoxy resin/acrylic resin layer facing the passive chip surface is non-ultraviolet sensitive (thickness range 10 to 30 μm).
Solder joint fatigue studies based on Finite Element Modeling (FEM) showed a pronounced influence of the chip attach copper foil thickness on the board level reliability of device solder balls. As an example, for certain chip-scale devices, such as the Texas Instruments MicroStar™ Ball-Grid Array (BGA), a copper foil thickness of about 50 μm relieves the thermomechanical stress on solder balls so much that 800 temperature cycles of −40° C. to 125° C. can be passed with 1.0% failure rate. This result is equivalent to the one obtained for a hardened, epoxy-based chip attach material of about 100 μm thickness. Increasing the copper foil thickness to 75 μm pushes the board level reliability 1000 temperature cycles.
The technical advances represented by the invention, as well as the aspects thereof, 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.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 8 to 15 are schematic and simplified cross sections of the parts and successive stages in the assembly of a chip-scale device, when attach materials and methods are used according to the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
A preferred choice for metal foil 101 is copper in the thickness range from about 30 to 150 μm. When rolled copper is used, its elastic modulus in this thickness range is approximately 200 GPa; it has a fracture strength of about 44 MPa and an elongation capability of about 25%.
The adhesive layers 102 and 103 are epoxy resin and acrylic resin blends with a modulus of about 1 GPa. Layer 102 is to be attached to the semiconductor chip and has a thickness preferably between 10 and 30 μm. For the assembly process flow of the present invention, layer 102 should be non-ultraviolet (non-UV) curable. Layer 103 is to be attached to a substrate film and has a thickness preferably between 20 and 50 μm. For the assembly process flow of the present invention, layer 103 has to be ultraviolet (UV) curable. The UV-curable epoxy resin and acrylic resin blend includes urethane resin, polyester, and ketonic resin. As usual, for semiconductor applications stringent purity requirements apply. Hot water extraction ion density values should indicate values for:
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- K: 0.12 ppm or less
- Na: 0.40 ppm or less
- NH4: 10.90 ppm or less
- F: 1.70 ppm or less
- Cl: 8.20 ppm or less
The UV-adhesive layer has to serve two purposes: First, it needs to provide a secure positioning of the semiconductor wafer on a UV-transparent support film for the process step of wafer dicing (singulating discreet chips) with a saw. In an initial phase, the multilayer composite 100 is placed on the support film 301 with the UV-adhesive 103 resting on support film 103, as illustrated in
The second purpose of the UV-adhesive is to provide permanent adhesion of the singulated chip to the substrate film, after the chip has been positioned onto the substrate. In order to stabilize this positioning, the adhesive has to be hardened by polymerization (“curing”) For a typical epoxy/acrylic blend resin,
After this curing, a shear test may be performed to evaluate the shear adhesion strength between the chip and the substrate. In order to select unambiguous test conditions, it is practical to perform the test with a copper substrate, 300 μm thick. Examples of useful test conditions are: 100 g force applied to chip at 250° C. temperature, 30 s hold time (chip shear adhesion is measured after epoxy/acrylic blend resin was cured at 160° C. for 30 min).
Examples of acceptable results are displayed in
The modulus of the epoxy/acrylic blend resin is a strong function of temperature. As an example for a specific blend material, the modulus-temperature relation is shown in
The average modulus Eav (in GPa) can be expressed by the following equation:
Eav=(Enon-uv·Anon-uv+Ecu·Acu+Euv·Auv)/Atot;
where
- Enon-uv is the modulus of the non-UV adhesive (in GPa);
- Euv is the modulus of the UV adhesive (in GPa);
- Ecu is the modulus of the copper foil (in GPa);
- Anon-uv is the area (=length·width) of the non-UV adhesive (in mm2)
- Auv is the area (=length·width) of the UV adhesive (in mm2);
- Acu is the area (=length·width) of the copper foil (in mm2);
- Atot is the area (additive) of the multilayer composite (in mm2)
As an example, for the modulus data listed above for the copper foil (0.03 mm thickness, 200 GPa) and the two epoxy/acrylic blend resin layers (0.01 mm and 0.02 mm thickness respectively, 1 GPa) at a 6×6 mm area size, the average modulus is calculated as 100.5 GPA. This value is significantly higher than the 20 GPa determined for the molding compound used in the example chip-size package.
For a given area size, various combinations of copper foil thickness and adhesive thickness can be used in finite element modeling to study the tensile and shear strains in solder balls at various positions in the device, when the device is attached to a board and subjected to temperature cycling from −40 to 125° C. In order to determine “board level reliability”, the number of temperature cycles is monitored, at which the solder balls reach 1.0% creep (relaxation) failures, as a function of copper foil thickness. Of particular interest. is a comparison of solder balls located under the chip and at the device package corners. As an example, a summary of these results is displayed in
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- Molded chip scale ball grid array device such as TI Japan MicroStar BGA having a solder ball pitch of 0.5 mm
- Chip size 6×6 mm
- Copper foil of composite chip attach 6×6 mm size, thickness at various values from 25 to 100 μm
- Chip attach material thickness 10 μm
- Substrate thickness 50 μm, substrate via for solder ball 280 μm
- Printed circuit board thickness 0.8 mm
In
The curves of
FIGS. 8 to 15 illustrate significant steps of the manufacturing process flow of the multilayer composite, the wafer assembly and the chip assembly to create a semiconductor device having a chip-scale package.
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FIG. 8 : Providing a metal foil 801 having first and second surfaces 801a and 801b, respectively; - attaching an adhesive layer on each of said surfaces; preferred method is attaching a non-UV curable adhesive layer 802 on metal surface 801a, and attaching an UV-curable adhesive layer 803 on metal surface 801b; thereby creating a multilayer composite 804 having an average modulus greater than the modulus of a polymerized device encapsulation material;
- placing the composite film 804 with the UV-curable adhesive layer 803 onto a transparent support film 805;
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FIG. 9 : providing a semiconductor wafer 901 having an active surface 901a and a passive surface 901b; - attaching passive surface 901b of semiconductor wafer 901 onto the UV-curable adhesive layer 802 of the composite film 804; preferred chip attachment conditions are temperature 100° C. and higher, and force 100 g/chip;
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FIG. 10 : Shining ultraviolet light 1001 through the transparent support film 805 on the UV-curable adhesive film 803 in order to reduce the adhesive strength between the composite film 804 and the support film 805; preferred UV illumination intensity is about 120 mW/cm2 and higher, UV luminous intensity about 70 to 200 mJ/cm2 -
FIG. 11 : Dicing wafer 901 and attached composite film 804 into singulated chips 1101; -
FIG. 12 : Lifting singulated chips 1101, one by one, from support film 805; -
FIG. 13 : Providing an insulating substrate film 1301, integral with electrically conductive routing lines, a first plurality 1302 of terminals on one surface of this substrate 1301, and a second plurality 1303 of terminals on the opposite surface of the substrate; - picking one singulated chip 1101 at a time from support film 805 and attaching the UV-cured surface 803b of each singulated chip (see also
FIG. 12 ) to the substrate film 1301 (notice enlarged lateral scale inFIG. 13 compared toFIG. 12 ); -
FIG. 14 : Polymerizing (“curing”) the adhesive layers 802 and 803, creating hardened layers; -
FIG. 15 : Wire bonding 1501. the active surface 901a of each chip to the first plurality 1302 of terminals on the substrate 1301, respectively; - encapsulating each chip in molding compound 1502 so that the active chip surface 901a, the bonding wires 1501, and portions of the substrate film 1301 are protected;
- attaching solder balls 1503 to the second plurality 1303 of terminals on the substrate film 1301; and
- singulating the substrate film 1301 to create individual devices with outlines of chip-scale packages.
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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 semiconductor chip may be made from a material selected from a group consisting of silicon, silicon germanium, gallium arsenide, or any other semiconductor material used in integrated circuit fabrication.
As another example, the choice of the metal foil in the composite chip attach film may be any metal with a modulus so that the composite modulus is greater than the modulus of the selected molding compound. Examples include, but are not limited to, nickel, zinc and aluminum.
It is therefore intended that the appended claims encompass any such modifications or embodiments.
Claims
1-18. (canceled)
19: A method for assembling a semiconductor device, comprising the steps of:
- providing a multilayer composite attachment film comprising a metal foil having first and second surfaces, a non-ultraviolet-curable adhesive layer attached on said first surface, and an ultraviolet-curable adhesive layer attached on said second surface;
- providing a semiconductor wafer having an active and a passive surface;
- placing said multilayer composite attachment film with said ultraviolet-curable adhesive layer onto a transparent support film;
- attaching said passive surface of said semiconductor wafer onto said non-ultraviolet-curable adhesive layer of said multilayer composite attachment film; and
- shining ultraviolet light through said transparent support film on said ultraviolet-curable adhesive layer in order to reduce the adhesive strength between said multilayer composite attachment film and said transparent support film.
20: The method according to claim 19 further comprising the steps of:
- dicing said wafer and said multilayer composite attachment film into singulated chips;
- providing an insulating substrate film, integral with electrically conductive routing lines, a first plurality of terminals on one surface of said insulating substrate film, and a second plurality of terminals on an opposite surface of said insulating substrate film;
- picking one singulated chip at a time from said transparent support film and attaching said ultraviolet-curable adhesive layer on each singulated chip to said insulating substrate film;
- curing said adhesive layers, creating hardened layers;
- wire bonding said active surface of each chip to said first plurality of terminals on said insulating substrate film with bonding wires;
- encapsulating each chip in molding compound so that said active surface of each chip, said bonding wires, and portions of said insulating substrate film are protected;
- attaching solder balls to said second plurality of terminals on said insulating substrate film; and
- singulating said insulating substrate film to create individual devices.
21: The method according to claim 20 wherein said step of singulating said insulating substrate film creates devices with outlines of chip-scale packages.
Type: Application
Filed: Feb 3, 2005
Publication Date: Jun 16, 2005
Inventors: Masazumi Amagai (Oita-shi), Akira Karashima (Oita)
Application Number: 11/050,234