THERMAL STRESS-RELIEVED COMPOSITE MICROELECTRONIC DEVICE

Abstract: A microelectronic package comprising a rigid substrate (1) having mounted thereon a component (3) having a plurality of sides and an electrically functional layer (9) adjacent to at least two sides of the component (3), the component (3) and the electrically functional layer (9) being spatially separated by a guard band (7), the volume of the substrate (1) underlying the guard band (7) having at least one region (13d) of reduced rigidity which permits the substrate (1) to dissipate mechanical stresses generated therein when the device is subjected to heating.

Description: THERMAL STRESS-RELIEVED COMPOSITE MICROELECTRONIC DEVICE Field of Invention The invention relates to composite microelectronic devices which are configured to reduce the thermally induced stresses in the device generated by ordinary operation. Background of Invention To accomplish high functional density, high speed and highly reliable operation, microelectronic devices, such as VLSI (very large scale integration) silicon chips, are increasingly being combined with an interconnected multilayer structure on a substrate to form a compact composite microelectronic package. A typical composite package is comprised of a ceramic substrate on which is mounted firmly an integrated circuit (IC) chip. The IC chip is electrically interconnected to a plurality of conductive and dielectric layers surrounding the chip which are also firmly attached to the substrate, but spatially separated from the chip. Figures 1 and 2a show a typical composite package with a chip wire-bonded across a narrow channel to interconnected multiple conductive and dielectric layers surrounding the chip, and both the chip and the multilayer structure are attached firmly to a common base substrate. This type of package involves several materials with different coefficients of thermal expansion and moduli of elasticity. A difference between operating temperatures and the temperature at which the package was manufactured results in thermally induced stresses and deformations that affect the reliability. They are very difficult to eliminate due to the inherent mismatch in the coefficients of thermal expansion between the multilayer component, usually made of copper and polyimide, and the silicon chip. The conventional approach in this situation is to select a substrate material that produces the lowest stresses in both the chip and the multilayer component. But a complex thermomechanical interaction still exists through the substrate, since it is impossible to match the coefficients of thermal expansion for the three key package components, i.e., the chip, the multilayer and the substrate, all at once. The present invention provides an improved substrate design that reduces the mechanical interaction among the three. Summary of the Invention The invention is directed to a microelectronic device comprising a rigid substrate having a component mounted thereon, the component having a plurality of sides and an electrically functional layer adjacent to at least two sides of the component and the electrically functional layer being separated by a guard band, the volume of the substrate underlying the guard band having one or more regions of reduced rigidity which permit the substrate to flex, thereby to dissipate mechanical stresses generated therein when the device is subjected to heating. Definition: As used herein, the term "guard band" refers to the unoccupied surface area of a substrate which by virtue of physical spacing serves to isolate adjacent electrically functional elements mounted on the substrate. Brief Description of Drawings The drawing consists of six figures. Figure 1 is an orthographic projection of a conventional composite microelectronic package. Figure 2a is a schematic section of a conventional composite microelectronic package and figures 2b through 2e are schematic sections of microelectronic packages which have been configured in accordance with the invention. Figure 3a is a graphical representation of the in- plane normal stresses of a conventional microelectronic package and 3b is a graphical representation of such stresses in a like microelectronic package which has been configured in accordance with the invention. Figure 4 is a bar chart which compares the in-plane thermal stresses of ungrooved and grooved substrates. Figure 5 is a bar chart which compares the in-plane thermal stresses of copper layers on both ungrooved and grooved substrates. Figure 6 is a bar chart which compares the in-plane thermal stresses of a silicon die mounted on both ungrooved and grooved substrates. Prior Art U,S, 3.325,882 and U.S. 3,428,86$, CtøPti ?t al These divisional patents are directed to a packaging arrangement and method for interconnecting metal lands to a solid state device which is bonded to the floor of a cavity in the substrate so as to form a gap around the device to separate the device from the cavity walls. This is in contrast to the invention which uses a circumferential groove to decouple the stresses. U,S. 4.374.457, tech gt a l In this patent, iech provides a series of conductive filled grooves around the periphery of the cavity containing one or more semiconductor chips to form an electrical bus structure. In contrast with the invention multiple grooves are fabricated around the periphery of the cavity in the substrate and metallized. U.S. 4.495.025 Haskell The patent is directed to a photoresist process for forming grooves in semiconductor materials for the purpose of isolation of integrated circuits. The patent does not disclose or suggest the function of grooves in substrates for any reason. Bar-Cohen "Bonding Relations for Natural Convection Heat Transfer from Vertical Printed Circuit Boards" - Proceedings of the IEEE, Vol. 73, No. 9, September 1985. This article suggests that vertical and horizontal grooves that are machined in the surface of plates with component-carrying printed wiring boards enhance the heat transfer characteristics of the assembly when the plates are spaced closely together. However, there is no mention made of thermal-mechanical coupling effects and how these can be neutralized by grooving substrates around each component bonded to the substrate. Petaifcd Descri tion of the Invention The invention is based upon fabricating the substrate in such a manner that the attached chip expands and contracts at a rate relatively independently of the expansion and contraction rates of the remaining interconnected package components that are also attached to the substrate. The substrate in accordance with the present invention includes a small flexible substrate region of reduced rigidity having a finite surface area wherein the area occupies a substantial space beneath the guard band that separates the chip and the surrounding package components. This region of reduced rigidity is connected to two rigid substrate sections wherein one rigid section supports the chip and the other supports the remaining interconnected package components. The flexible section provides a forgivable mechanical linkage between the two rigid sections wherein the chip attached to one rigid section can expand and contract more or less independently, irrespective of the expansion and contraction of other package components attached to the other rigid section. As a feature of the present invention, the small flexible section can be a flexible material (Fig. 2b) or a thin section (Fig. 2c-e) formed by continuous or discontinuous grooves. In the complete package, the thermally-induced stress and deformation in the chip caused by other package components, or vice versus, are extremely small so that the changes for chip or interconnects failures are reduced. Yet the complete substrate or package has the traditional exterior dimensional appearance and overall is still very rigid structurally so that it can be handled with traditional package assembly methods. -. Turning now to the Drawing, Figures 1 and 2a illustrate the structure of a conventional composite microelectronic package consisting of an inert ceramic substrate 1 on which is firmly mounted an integrated circuit chip 3. The substrate can be made of such materials as A1N, SiC, AI 2 O3, Si, quartz, mullite, cordierite and galium arsenide. The chip 3 is adhered to substrate 1 by means of an adhesive layer 5 which may be either inorganic and/or organic in nature. Typically, the adhesive layer, also known as the die- attach layer, is an organic thermoplastic or thermoset polymer such as those which are disclosed in EP 88104940.7, which is incorporated herein by reference. Surrounding the chip 3 is a dielectric layer 7 on which is mounted a series of conductive signal/power planes 9. The chip and dielectric layer 7 are separated by a guard plane and the chip 3 is connected electrically to the signal/power planes 9 by a series of fine conductive wires 11 which are typically made of gold or copper metal. The substrate 1 of Figure 1 has not been grooved or otherwise stress-relieved in accordance with the invention. Figures 2b through 2e illustrate devices similar to that of Figures 1 and 2a, but differ in that each has a region of reduced rigidity to effect relief of the thermally induced stresses. In Figure 2b, the reduced rigidity region is comprised of a solid material 13b which is a more flexible solid than the rigid material of the remainder of the substrate. Figure 2c illustrates a package in accordance with the invention in which the region of reduced rigidity consists of a groove 13c cut into the top of the substrate and extending through about 80% of the substrate thickness. The groove can be continuous around the outer edges of the chip or it can be intermittent or discontinuous so long as the degree of stress relief is sufficient. Figure 2d illustrates a package in accordance with the invention in which the region of reduced rigidity is comprised of two grooves 13d - one extending into the substrate from the top of the substrate and the other from the bottom of the substrate. Figure 2e then illustrates a similar package in which the region of reduced rigidity is a single groove 13e extending into the substrate from the bottom of the substrate. As mentioned above, the region or regions of reduced rigidity may extend continuously or discontinuously around the edges of the component so long as the region of reduced rigidity is sufficient to provide the desired degree of stress relief. In particular, it is preferred that the ratio of the modulus of elasticity of the substrate containing a region of reduced rigidity to the modulus of elasticity of the remainder of the substrate (Modulus Ratio) be less than 0.3. However, in order not to weaken the mechanical strength of the substrate excessively and make it too susceptible to breakage, the grooves or other configuration which constitute the regions of reduced rigidity should not extend through more than 80% of the thickness of the substrate. As used herein, the term "rigid substrate" refers to substrates having a modulus of elasticity before modification in accordance with the invention of at least lOGpa. Various configurations can be used for the regions of reduced rigidity. For example they can be round-bottomed or square-bottomed, they can have vertical or sloped sides. In general, the sides of the grooves need not correspond with the edge of the guard level and the outer edge of the chip. Nevertheless, such configurations are preferred. The over¬ riding consideration is whether the reduced rigidity regions are of sufficient volume to render the Modulus Ratio below 0.3 and preferably below 0.1. Compliance with this criterion of Modulus Ratio can readily be determined by direct measurement of the moduli or it can be determined by computer modeling and analysis of the system. The substrate can be made simply of a single base material such as those mentioned above or it can be a composite material consisting of a dispersion of particles or fibers in a matrix of a base material or base material mixture. Fibers can be used as a substrate filler in order to increase substrate strength. On the other hand, heat-conductive materials may be added as well in order to increase the ability of the substrate to conduct heat away from the microelectronic component. In general, substrates will be on the order of 40-80 mils in thickness of which 60 mils is typical. Thermally induced stresses on the substrate tend to be directly related to thickness, whereas stresses in the surrounding dielectric layers tend to be inversely related to substrate thickness. Because the microelectronic chip is the source of heat which gives rise to the thermally induced mechanical stresses, it is preferred that the regions of reduced rigidity, which are most frequently grooves, be as close as possible to the chip but not underlying the chip. Typically, the grooves surrounding a component will not exceed about 10 mils in width, which is the usual maximum width of the guard band in most composite packages. In addition the groove depth should not exceed about 80% of the substrate thickness. A substrate thickness of at least 10 mils should remain to avoid excessively weakening the mechanical strength of the substrate. When grooves are used to form the reduced rigidity region, they can be left open or they can be filled with a flexible material such as an elastomeric polymer or other non-rigid filler material. EXAMPLES In Table 1 below, data are given which show the maximum in-plane thermal stresses which are incurred in cooling from 170°C to 20°C. A single chip composite package comprising a copper ground layer and an IC chip mounted by means of polymeric die attach adhesive to an aluminum substrate. Because of the technical difficulty in measuring actual in-plane stresses of such composite devices, the data were derived from finite element modeling of the deformation of each element based on the known TCE characteristics of each component. The dimensions of the components on which the modeling was based are as follows: IC chip (1.27x1.27x0.051 cm) Substrate (3.81x3.81x0.153cm) Laminant (2 planes with 1.53x1.53 cm cavity) Copper Layer (3.81x3.81x0.0036 cm) Dielectric Layer (3.81x3.81x0.0025 cm) Adhesive (3.81x3.81x0.0025 cm) Groove dimensions are presented with the calculated maximum stresses shown in Table 1. In Table 1, data are presented showing the maximum in-plane thermal stresses incurred by the copper layer, the die attach adhesive and the die itself when they are heated to 170°C and allowed to cool to 20°C under ambient temperature conditions. In particular, maximum stress data are given for these components on an ungrooved substrate and substrates having three groove configurations and several groove sizes. These data show that all three groove configurations - single groove up, single groove down and double grooves up - were effective to reduce residual in-plane thermal stresses in all three components. However, the double groove up configuration was most effective and the single groove up configuration was significantly more effective than the single groove down configuration. Table 1 In-Plane Residual Stresses of Grooved and Ungrooved Substrates - Effect of Groove Size and Configuration (1) Width/depth in mils The data given in the second column of the above Table with respect to thermal stresses in the copper layer can be observed graphically in Figure 4.

Claims: CLAIMS 1. A microelectronic device comprising a rigid substrate having a component mounted thereon, the component having a plurality of sides and an electrically functional layer adjacent to at least two sides of the component, at least two sides of the component and the electrically functional layer being separated by a guard band, the volume of the substrate underlying the guard band having at least one region of reduced rigidity which permits the substrate to flex, thereby to dissipate mechanical stresses generated therein when the device is subjected to heating. 2. The device of claim 1 in which the region of reduced rigidity is continuous. 3. The device of claim 1 in which the ratio of the modulus of elasticity of the volume of the substrate containing a region of reduced rigidity to the modulus of elasticity of the remainder of the substrate (Modulus Ratio) is less than 0.3. 4. The device of claim 3 in which the Modulus Ratio is less than 0.1. 5. The device of claim 1 in which the substrate has a predetermined thickness and the region of reduced rigidity has a predetermined depth which does not exceed 80% of the substrate thickness. 6. The device of claim 1 in which the modulus of elasticity of the unmodified substrate is at least 10 Gpa. 7. The device of claim 1 in which the region of reduced rigidity has a predetermined width which corresponds with the width of the guard band. 8. The device of claim 7 in which the region of reduced rigidity extends from the upper surface into the volume of the substrate. 9. The device of claim 7 in which the region of reduced rigidity extends from the lower surface into the volume of the substrate. 10. The device of claim 1 having a plurality of reduced rigidity regions. 1 1. The device of claim 10 in which the regions of reduced rigidity extend from the upper surface into the volume of the substrate. 12. The device of claim 10 in which the regions of reduced rigidity alternately extend from the upper and lower surfaces into the volume of the substrate. 13. The device of claim 12 in which a first region of reduced rigidity extends from the upper surface into the volume of the substrate adjacent to the edge of the component. 14. The device of claim 1 in which the region of reduced rigidity is a groove. 15. The device of claim 14 in which the groove is continuous. 16. The device of claim 1 in which the region of reduced rigidity is comprised of a discontinuous series of grooves. 17. The device of claims 14 or 15 in which the grooves are filled with an inert non-rigid solid material. 18. The device of claim 1 in which the region of reduced rigidity is comprised of non-rigid material which is an integral part of the substrate. 19. The device of claim 1 in which the substrate is a composite layer of particles dispersed in a rigid solid matrix. 20. The device of claim 1 in which the substrate is a composite layer of fibers dispersed in a rigid solid matrix. 21. The device of claim 1 in which the substrate is comprised of ceramic solid material selected from A1N, SiC, AI2O3, silicon, quartz, mullite, cordierite and galium arsenide. 22. The device of claim 1 in which the substrate is comprised of rigid polymeric material. 23. The device of claim 1 in which the component is an integrated circuit chip. 24. The device of claim 1 in which the electrically functional layer is a dielectric layer. 25. The device of claim 1 in which the substrate has a plurality of components mounted thereon.

Inventors: YOUNG JAMES C, YOUNG, JAMES, C.
Applicants: DU PONT, E.I. DU PONT DE NEMOURS AND COMPANY
Application year: 1991
Main classification: H01L23
IPC codes: H01L23, H05K1