Hermetic device package for microelectronic frequency selection components

Abstract: A package (200) for a frequency selection component (70) includes a base (10), a thermoplastic die attachment (60), and either a solder (260) or low-melting-temperature glass (110) lid attachment. These techniques can be employed in either a leadless chip carrier or a single layer ceramic base configuration. A package (200) for a surface acoustic wave device (70) in accordance with one embodiment of the invention is suitable for enclosing the device (70) in a hermetically sealed environment such that the resulting structure takes up a minimum volume of space. The package (200) facilitates automated circuit assembly techniques, allowing compact, low cost products to incorporate frequency selection components (70) when these are made in accordance with the present invention.

Description: Background of the Invention This invention relates to packages for microelectronic frequency selection components such as surface acoustic wave (SAW) devices and more specifically to housings suitable for mass assembly of packaged hermetically sealed devices. At the present time, much effort and expense is devoted to research and development relating to microelectronic frequency selection devices. Examples of devices which fall into this category are film bulk acoustic wave resonators (FBARs) (see, for example, "SBAR filter monolithically integrated with HBT amplifier," by D. Cushman et al., presented at the 1990 IEEE Ultrason. Symp., Honolulu, HI, Dec. 1990) and surface acoustic wave devices. Inasmuch as many such devices derive their electrical properties from mechanical motion of some portion of the device, events which affect that motion can in turn affect the frequency selection characteristics of such devices. Circuits including this type of device tend to provide fairly compact frequency selection components while taking up minimal space. However, the volume reduction achieved by improved packaging techniques can be as much as 100:1 as compared to other device packaging methods. These devices are not usable for practical applications unless they can be enclosed in and protected by proper hermetic packages or housings. Prior art packages for such device chips tend to be expensive and tend to be much larger than the device enclosed within. While some prior art packages allow mounting of die within the package in a small area, the packages themselves require excessive amounts of space. This is because some such packages utilize leads that extend beyond the boundaries of the package. These leads often must be tied down to conductors on a substrate surface and then brought from the conductors on the substrate to an adjacent package. This requires that sufficient space be maintained between the prior art packages to facilitate the required electrical and mechanical connections. Prior art die bonding by use of thermosetting polymers can result in trapping of contaminants in the polymer or in substantial outgassing of reaction products from the polymer on heating. These contaminants result in greatly impaired device performance when they condense on the frequency selection component surface, by promoting attenuation of the mechanical vibrations and thereby greatly diminishing the magnitude of the desired output signals. For example, epoxies outgas large proportions of their mass during cure. Three leading brand adhesives tested by John Hopkins University scientists (Proc. IEEE Elec. Comp. Conf., IEEE Catalogue No. 89CH2775-5, pp. 301-308), outgassed 9 to 36% of their mass during cure. Such significant outgassing can result in void formation, diminishing the strength of the die bond, and can also result in redeposition of heavier organic chemicals on the frequency selection component surface. An additional problem occurs when automated machine placement and assembly of such components is attempted. This is due to bending of the leads during insertion into appropriately treated holes in the substrate, causing unreliable connections of the part to the surrounding circuitry. Moreover, frequency selection components require hermetically sealed environments in order to work reliably. This has led to use of metallic packages, such as TO-5 and TO-8 containers, and various types of dual-in-line packages, which are individually resistance or seam welded. Due to the piezoelectric nature of the materials required for many types of frequency selection devices, such as surface wave filters, surface skimming bulk wave filters, shallow bulk acoustic wave filters, and the like, these devices must not be exposed to temperatures above the substrate Curie temperature. Such welding techniques produce adequately sealed hermetic packages without exposing the die to excessive heat. However, these manufacturing techniques are not well suited to batch processing, resulting in high costs which are prohibitive for high volume consumer product manufacturing. Further, SAW die require mounting techniques and materials which do not result in stresses in the mounted die. Such stresses cause random phase distortion and frequency response distortion in the finished part. Patent Abstracts of Japan, Vol. 011, No. 244 (E-530) & JP-A-62 054 931 discloses use of a thermoplastic resin, for example, acrylic or polyolefin mixed with metal such as silver to impart conductivity to the resin, as a die bonding material for semiconductor elements to a substrate of a hybrid integrated circuit. Patent Abstracts of Japan, Vol. 01, No. 352 (E-458) & JP-A-61 152 112 discloses hermetic sealing of the joint of a surface acoustic wave element part comprising a hermetically sealed box-shaped body and a plastic cap. The elastic wave chip is fixed with an adhesive on an insulating plastic substrate equipped with I/O and grounding leads. A plastic cap is put on the substrate and the joint therebetween sealed with an adhesive so as to provide a sufficient moisture proof closure and the ends of the leads folded along the sides of the cap. The sealing material is applied to the outside of the substrate-cap joint. U. S. Patent 4,622,433 describes a ceramic package system using a combination of vitreous and devitrifying low temperature sealing glasses to attach a lead frame to a ceramic substrate and seal a lid to the combined lead frame and substrate after a semiconductor die has been bonded to a pad on the substrate. Patent Abstracts of Japan, Vol. 015, No. 054 (E-1031) & JP-A-02 283 112 describes a surface acoustic wave element mounted on a single layer dielectric substrate and enclosed by a metal cap soldered to an inorganic conductor film near the perimeter of the substrate to provide a hermetic enclosure for the surface acoustic wave element. The metal cap is electrically connected to one of a ground lead of the finished device to provide electrical shielding. Summary of the Invention In accordance with a first aspect of the present invention there is provided a method for mounting and encapsulating a frequency selective component die using a thermoplastic material for mounting the die on a base and a sealing material for hermetically attaching a lid to the base, wherein the thermoplastic material is disposed between the die and the base and the die-base combination heated to effectuate a bond therebetween and a lid is placed on the base with the sealing material disposed between the lid and the base and the die-base-lid combination heated to hermetically seal the lid to the base, the method CHARACTERIZED BY, during the first heating step, using a thermoplastic material having a minimum bonding temperature T₁ at or above which it softens and bonds and heating the die-base combination to at least T₁, and during the second heating step using a sealing material having a sealing temperature T₂ ≧ T₁ and heating the die-base-lid combination at least to T₂. In accordance with a second aspect of the present invention there is provided a frequency selection assembly comprising a frequency selective component die, a base for supporting the die, a lid, a thermoplastic material for bonding the die to the base and a sealing material for hermetically sealing the lid to the base, CHARACTERIZED BY the thermoplastic material having a characteristic temperature T₁ at or above which it becomes plastic and bonds, and the sealing material having a characteristic temperature T₂ at or above which it softens and seals, wherein T₂ is equal or greater than T₁ so that the thermoplastic material is plastic during lid sealing. Accordingly, one advantage of the present invention is to provide packages for frequency selection devices which enable compact final product configurations by allowing the packages to be mounted side-by-side with substantially no wasted space between them. Another advantage of the present invention is to provide packages for frequency selection devices which provide a hermetically sealed environment and yet which occupy as little volume as is possible. Still another advantage of the present invention is to provide low cost, easily manufactured packages for SAWs which can be easily handled by automated circuit assembly apparatus to enable efficient mass production of products incorporating such devices. This invention discloses a method for frequency selection component die or semiconductor circuit die packaging. The frequency selection component die is bonded to a package base which has conductors disposed thereon with thermoplastic die attachment material. Electrical interconnections are formed from the frequency selection component to the package base conductors, and a lid is hermetically sealed to the package base. The thermoplastic material preferably includes polyetheretherketone, polysulfone, polyethersulfone, polyetherimide, polybutlyeneterepthalate or polycarbonate. Brief Description of the Drawings FIG. 1 is a plan view of the bottom surface of the package base, in accordance with the present invention. FIG. 2 is a plan view of the top surface of the package base, in accordance with the present invention. FIG. 3 is a side view of the package base with a SAW mounted on the package base, in accordance with the present invention. FIG. 4A is a top view of a surface acoustic wave device. FIG. 4B is a top view of a surface acoustic wave device mounted on a package base, in accordance with the present invention. FIG. 5A is a bottom view of the package lid, in accordance with the present invention. FIG. 5B is a cross-sectional view of the package lid, in accordance with the present invention. FIG. 6A is a plan view of an alternative package embodiment, in accordance with the present invention. FIG. 6B is a cross-sectional view of the alternative package embodiment, in accordance with the present invention. FIG. 7 is a diagram of a measured SAW filter transfer function from a SAW device mounted in accordance with the present invention. Detailed Description of the Preferred Embodiment FIG. 1 illustrates a bottom view of the package base 10, showing thick-film interconnection pads 20 and through-plated vias 25. FIG. 2 depicts a top view of the package base 10, with thick-film conductors 30, 35, 40, 45, 50, and 55. The conductors 30, 35, 40, 45, 50, and 55 make contact with through-plated vias 25 to effect electrical connection to the interconnection pads 20 on the lower surface of the package base 10. Fig. 3 illustrates a side view of the package base 10 with a thermoplastic die attach material 60 and a frequency selection component die, for example a surface acoustic wave (SAW) die 70, attached thereto. The thermoplastic die attachment adhesive 60 may be any of several such materials, depending on the melting point desired for a particular application. Table 1 provides several illustrative examples of thermoplastic die attachment materials 60 together with the temperatures needed for forming bonds with these examples. Fig. 4A shows a top view of a SAW device die 70, with bond pad areas 72, 74, 76, 78, 80, and 82 illustrated. These bond pad areas 72, 74, 76, 78, 80, and 82 are interconnected by means of wire bonds 27 to portions of conductor regions 32, 35, 42, 45, 52, and 55, respectively, of Figs. 2 and 4B. The wire bonds 27 are made to those portions of the conductor regions 32, 42, and 52 as shown in Fig. 4B which are not covered by the thermoplastic die attach material 60 beneath the SAW die 70 and which are also not located where the low-melting-temperature glass 110 attaches the lid 100 to the package base 10 as shown in Fig. 5A. The portions of conductor regions 32, 42, and 52 which are covered by thermoplastic die attach material 60 serve as die attach pads, however, die attach pads are not necessary to this concept for die attachment. One example of a low-melting-temperature glass 110 is a mixture of PbO and V₂O₅. This can be used with a two minute bake at 3600° C to seal the lid 100 to the package base 10. Fig. 5A provides a plan view of the bottom of the ceramic lid 100, with the central region 105 hollowed out, as is shown in Fig.5B. Fig. 5B is a cross-sectional view along line A-A in Fig. 5A of the ceramic lid 100 and low-melting-temperature glass 110. Fig. 6A depicts a plan view of an alternative embodiment of the package 200. In this embodiment, the package base 200 is shown in Fig 6B to be composed of a layer of ceramic material 210, which has gold-plated Kovar leads 230 in a gold-plated Kovar lead frame 240 sealed to the ceramic material 210 by fused glass 220. The glass-free area 201 of the gold-plated Kovar lead 230 inside the package base 200 is used for bonding of interconnection wires 27 from the bond pads 72, 74, 76, 78, 80 and 82 of the SAW die 70, as was shown in Fig. 4B. As shown in Fig. 6B, the SAW die 70 is attached to the die bonding pad 250 by means of a thermoplastic die attach material 60 and electrical connection to the SAW die 70 is made by means of wire-bonds 27 as described and illustrated previously in Fig. 4B. Subsequent to the wirebonding operation, a solder preform 260 is placed on top of the upper metallized ring 240. A lid 270 is placed on top of the solder preform 260 and the lid 270 is soldered to the upper metallization 240, hermetically sealing the lid 270 to the package base 200. An example of a measured SAW transmission frequency response 300 from a SAW device mounted in accordance with the present invention is provided in Fig. 7. The data shown in Fig. 7 illustrate that the device response 300 is in accordance with the SAW device design, showing that the packaging steps in accordance with the present invention does not inhibit the operation of the SAW device 70. A package 200 for a microelectronic device such as a circuit surface acoustic wave device 70 or a semiconductor circuit die in accordance with one embodiment of the invention is suitable for enclosing the SAW device 70 in a hermetically sealed environment such that the resulting structure 200 takes up a minimum volume of space. The package 200 may include a die pad 250 for the surface acoustic wave die 70 and does include a thermoplastic adhesive 60 for affixing the SAW die 70 to the package base 210 without causing the top surface of the SAW die 70 to be in excessive compression or tension, either of which causes device malfunction. The thermoplastic die attach adhesive 60 allows the SAW device 70 to be exposed to greater temperatures than prior art enabled without suffering SAW die 70 contamination from outgassing of the die attach adhesive 60. This in turn allows the cover 270 to be affixed to the package base 200 by means of solder 260 to hermetically seal the SAW die 70 from the surrounding environment. The base 210, thermoplastic die attach adhesive 60, cover 270, and cover attach solder 260 cooperate to provide a package which encloses and protects the SAW die 70.

Claims: A method for mounting and encapsulating a frequency selective component die (70) using a thermoplastic material (60) for mounting the die on a base (10) and a sealing material (110) for hermetically attaching a lid (100) to the base (10), wherein the thermoplastic material (60) is disposed between the die (70) and the base (10) and the die-base combination heated to effectuate a bond therebetween and a lid (100) is placed on the base with the sealing material (110) disposed between the lid and the base and the die-base-lid combination heated to hermetically seal the lid to the base, the method CHARACTERIZED BY, during the first heating step, using a thermoplastic material (60) having a minimum bonding temperature T₁ at or above which it softens and bonds and heating the die-base combination to at least T₁, and during the second heating step using a sealing material (110) having a sealing temperature T₂ ≧ T₁ and heating the die-base-lid combination at least to T₂. The method of claim 1 wherein the method includes using a thermoplastic material (60) comprising polyetheretherketone, polysulfone, polyethersulfone, polyetherimide, polybutlyeneterepthalate or polycarbonate. The method of claim 1 or 2 wherein the die comprises a surface acoustic wave device (70) and the step of heating the die-base-lid combination includes heating so that the thermoplastic material supporting the surface acoustic wave device is plastic during lid sealing. The method of claim 1 or 2 wherein the die comprises a surface acoustic wave device and the using and heating steps comprise using a thermoplastic material (60) having T₁ < T₂ so that, after lid sealing at T₂ and during a subsequent further cooling down step, the die bonding material remains plastic at least during the temperature interval from T₂ to T₁ so that device characteristic altering stresses are avoided. A frequency selection assembly comprising a frequency selective component die (70), a base (10) for supporting the die (70), a lid (100), a thermoplastic material (60) for bonding the die to the base and a sealing material (110) for hermetically sealing the lid to the base, CHARACTERIZED BY the thermoplastic material (60) having a characteristic temperature T₁ at or above which it becomes plastic and bonds, and the sealing material (110) having a characteristic temperature T₂ at or above which it softens and seals, wherein T₂ is equal or greater than T₁ so that the thermoplastic material is plastic during lid sealing. The assembly of claim 5 wherein the thermoplastic material (60) comprises polyetheretherketone, polysulfone, polyethersulfone, polyetherimide, polybutlyeneterepthalate or polycarbonate. The assembly of claim 5 or 6 wherein the T₂ > T₁ so that the thermoplastic material (60) remains plastic in the temperature interval from T₂ to T₁ after lid sealing for avoiding device characteristic altering stresses.

Inventors: FALKNER ROBERT FRANK JR, FIORENZO RUSSELL THOMAS, FALKNER, ROBERT FRANK, JR., FIORENZO, RUSSELL THOMAS
Applicants: MOTOROLA INC, MOTOROLA, INC.
Application year: 1992
Main classification: H03H9
IPC codes: H01L21, H03H9, H03H3