Patent Description:
The wide variety of products collectively called microelectromechanical system (MEMS) devices are small, low weight devices on the micrometer to millimeter scale, which may have sensors or mechanically moving parts, and often movable electrical power supplies and controls, or they may have parts sensitive to mechanical, thermal, acoustic, or optical energy. MEMS have been developed to sense mechanical, thermal, chemical, radiant, magnetic, and biological quantities and inputs, and produce signals as outputs. Because of the sensitive parts and moving parts, MEMS have a need for physical and atmospheric protection. Consequently, MEMS are placed on or in a substrate and have to be surrounded by a housing or package, which has to shield the MEMS against ambient and electrical disturbances, and against mechanical and thermal stress.

A MEMS device integrates mechanical elements, sensors, actuators, and electronics on a common substrate. The manufacturing approach of a MEMS aims at using batch fabrication techniques similar to those used for microelectronics devices. MEMS can thus benefit from mass production and minimized material consumption to lower the manufacturing cost, while trying to exploit the well-controlled integrated circuit technology. The mechanically moving parts and the electrically active parts of a MEMS are fabricated together with the process flow of the electronic integrated circuit (IC) on a semiconductor chip.

Following the technology trends of miniaturization, integration and cost reduction, recently developed substrates and boards can embed and interconnect chips and packages to reduce board space, thickness and footprint while increasing power management, electrical performance and fields of application. Examples include penetration of integrated boards into the automotive market, wireless products, and industrial applications.

As examples, integration boards have been successfully applied to embed wafer level packages, passives, power chips, stacked and bonded chips, wireless modules, power modules, generally active and passive devices for applications requiring miniaturized areas and shrinking thickness. <CIT> (A3) describes a MEMS chip electrically connected to a lead frame by metallic wires. A control chip is electrically connected to the MEMS chip and to the lead frame by the wires and provides a control signal for the MEMS chip. A protective resin, covers the MEMS chip, the control chip, the wires and a part of the lead frame. An encapsulant, covers the resin, the MEMS chip and the part of the lead.

In described examples, a packaged microelectromechanical system (MEMS) device comprises a circuitry chip attached to a pad of a substrate with leads, and a MEMS vertically attached to the chip surface by a layer of low modulus silicone compound. On the chip surface, the MEMS device is surrounded by a polyimide ring with a surface phobic to silicone compounds. A dome-shaped glob of cured low modulus silicone material covers the MEMS and MEMS terminal bonding wire spans. The glob is restricted to the chip surface area inside the polyimide ring and has a surface non-adhesive to epoxy-based molding compounds. A package of polymeric molding compound encapsulates a vertical assembly of the glob embedding the MEMS, the chip, and portions of the substrate; the molding compound is non-adhering to the glob surface yet adhering to all other surfaces.

A general trend of the electronic industry requires fewer and smaller components in a system. In this trend, a technical and market advantage will be awarded when the number of components in a system is reduced and the product consumes less space and operating power yet offers improved electrical characteristics and higher reliability. For semiconductor devices, a particular advantage can be gained, when a low cost packaging technology can be realized so that it promotes miniaturization by integrating or eliminating parts, and protection by absorbing or shielding disturbances.

As for microelectromechanical systems (MEMS), a particular performance and market advantage can be obtained when a MEMS device can be stacked on an existing electronic circuit device already in production, and the same device package can be used. The problem to be solved in each case is the requirement to perform the processes of stacking and packaging so that the specific parameter-to-be-monitored by the MEMS under consideration remains unaffected and the package is shielding the new device effectively against disturbances of the parameter-to-be-monitored.

For a MEMS of high sensitivity to mechanical and thermal stresses, as a solution to the problem of device integration and protection against mechanical and thermal stress disturbances, a process flow has a unique application of low modulus material and a unique set of process steps to protect the stress-sensitive MEMS device while still using standard, low cost package assembly material and assembly methods practiced in production.

The integration of unique materials and process steps into the material set and process flow used to assemble standard molded packages, such as quad flat no-lead (QFN) packages, includes a polyimide ring with phobic surface characteristics deposited on a circuitry chip; low modulus silicone compounds and methods to dispense these compounds under controlled conditions to surround hexahedron-shaped MEMS devices on all sides; and controlled component adhesion to polymeric packaging compounds.

<FIG> illustrates an example of a device <NUM>, which includes a semiconductor chip <NUM> with terminals <NUM>. A portion of the surface of chip <NUM> is suitable for the attachment of a MEMS device <NUM>. In this example, the MEMS is stress sensitive. An example chip <NUM> may square shaped, have a side length of several millimeters, such as <NUM>, and include a fully functional integrated circuit. In <FIG>, chip <NUM> is attached to the pad <NUM> of a leadframe, preferably using an adhesive epoxy-based polymeric compound <NUM>. The terminals <NUM> of chip <NUM> are connected by bonding wires <NUM> to respective leads <NUM> of a metallic leadframe; the preferred wire metal is a copper alloy, but alternatively gold or aluminum may be used. The vertical assembly of MEMS, chip, and portions of the leadframe chip <NUM> is embedded in an insulating package <NUM>, preferably made of an epoxy-based molding compound.

In the example device of <FIG>, the leadframe belongs to the quad flat no-lead (QFN) or in Small Outline No-Lead (SON) families. These leadframes are preferably made from a flat sheet of a base metal, which is selected from a group including copper, copper alloys, aluminum, aluminum alloys, iron-nickel alloys, and Kovar. For many devices, the parallel surfaces of the leadframe base metal are treated to create strong affinity for adhesion to plastic compound, especially molding compounds. As an example, the surfaces of copper leadframes may be oxidized, because copper oxide surfaces are known to exhibit good adhesion to molding compounds. Other methods include plasma treatment (described hereinbelow) of the surfaces, or deposition of thin layers of other metals on the base metal surface. As an example for copper leadframes, plated layers of tin have been used, or a layer of nickel (about <NUM> to <NUM> thick) followed by a layer of palladium (about <NUM> to <NUM> thick) optionally followed by an outermost layer of gold (<NUM> to <NUM> thick).

Other embodiments may use other types of leadframes with a chip attachment pad, such as leadframes with elongated leads, with cantilevered leads, or with frames having one or more pads in a plane offset from the plane of the leads. Still other embodiments may use laminated substrates made of insulating material alternating with conductive layers. These substrates may have an area suitable for attaching one or more chips, and conductive connections suitable for stitch bonding wires.

As <FIG> illustrates, one surface of chip <NUM> is attached to leadframe pad <NUM> by epoxy-based compound <NUM>, the opposite chip surface includes an area of diameter <NUM> suitable for vertically attaching MEMS device <NUM> using compound <NUM>. However, in contrast to the polymeric attach compound <NUM> with a modulus of > 2GPa, the compound <NUM> is made of a silicone compound with a comparatively very low modulus of < <NUM> MPa.

As an example, compound <NUM> may be a silicone compound commercially available from Dow Corning Corporation (Corporate Center, Midland, Michigan, USA). The compound may be further characterized by low viscosity and thixotropic behavior so that it exhibits weakened constitution when disturbed and strengthened behavior when left standing. Because the modulus of a material characterizes its strain response to an applied stress (or pressure), compound <NUM> has very compliant mechanical characteristics. This feature is essential to protect the stress-sensitive MEMS, because low modulus material does not transmit stress but rather distributes and absorbs stress. Consequently, low modulus materials, when applied to a side of a stress-sensitive device such as a MEMS, can protect this device against external stress from the covered side. The stress-protecting characteristics of material <NUM> must be preserved through the silicone polymerization cycle occurring during the elevated temperature required for the wire-bonding the MEMS terminals (see hereinbelow).

To protect a stress sensitive hexahedron-shaped MEMS on all sides against external stress in conjunction with semiconductor devices, the surrounding cocoon of stress-absorbing and stress-dispersing material has to have a thickness suitable to perform functions in conjunction with semiconductor products. As an example, the material has to have a thickness permitting the attachment of the MEMS to portions of the semiconductor device. As another example, the material has to have a thickness to allow the incorporation of arching spans of bonding wires.

<FIG> shows that the diameter <NUM> of the silicone attachment layer is positioned within the inner diameter <NUM> of a ring <NUM> of polyimide material. However, the terminals <NUM> of chip <NUM> remain outside ring <NUM>. Ring <NUM> is made of a polyimide compound. As described hereinbelow, the configuration of ring <NUM> as a circle, a rectangle, or any other closed structure, is patterned from a polyimide layer deposited on a semiconductor (preferably silicon) wafer, which includes multiple chips with integrated circuits (ICs). After patterning, the polyimide ring is made non-wettable and repellent to silicone materials by reducing the polyimide surface energy in a process cycle, which includes a first curing cycle followed by an ashing process and then in turn by a second curing cycle. In examples not forming part of the invention materials other than polyimide may be used if they have silicone-repellent surface characteristics.

After the polyimide ring is made repellent, the IC chips are singulated from the wafer. Each chip is then attached to the pad of a substrate, such as to the pad <NUM> of a metallic leadframe as shown in <FIG>. The adhesive used for the attachment layer <NUM> is preferably an epoxy-based polymeric formulation.

In the example of <FIG>, the stress-sensitive MEMS <NUM> is stacked vertically on IC chip <NUM> by low modulus silicone layer <NUM> so that the attachment is inside diameter <NUM> of the silicone ring <NUM>; layer <NUM> protects MEMS <NUM> against stress from the direction of chip <NUM>. After the MEMS attachment, terminals of MEMS <NUM> can be connected to contact pads <NUM> of chip <NUM> using bonding wires <NUM>. Pads <NUM> are located inside diameter <NUM> of silicone ring <NUM>. Thereafter, chip terminals <NUM> are connected to substrate connectors (such as leadframe leads <NUM>) using bonding wires <NUM>.

As <FIG> illustrates, MEMS <NUM> is surrounded by a glob <NUM>, which also covers bonding wires <NUM> and pads <NUM> and therefore has a dome-shaped configuration. Glob <NUM> is made of low modulus silicone material, preferably the same silicone material as layer <NUM>. As an example, the silicone of glob <NUM> may be a silicone commercially available from Dow Corning Corporation. The silicone material is selected so that is has hydrophobic and non-adhesive characteristics towards the polymeric molding formulation, which has been selected as encapsulation compound. The viscosity, modulus, and thixotropic index of the silicone material is selected so that glob <NUM> can be dispensed in a precision volume and glob <NUM> is restricted to the surface area inside the polyimide ring <NUM> and does not bleed out across the ring.

After the dispensing process, the silicone compound of glob <NUM> is polymerized in a curing process. Thereafter, glob <NUM> surrounds the non-attached five sides of the hexahedron-shaped MEMS device and its connecting wire bonds. Glob <NUM> may have approximately the shape of a hemisphere. Together with the low modulus silicone compound of the attach material, all six sides of the MEMS devices are thus surrounded by low modulus material operable to protect the MEMS against any stress by blunting, dispersing and absorbing external stress arriving at the glob surface.

The device including MEMS <NUM> stacked on circuit chip <NUM>, which in turn is mounted on substrate <NUM>, must be in a rigid and strong package. The embodiment of <FIG> depicts an encapsulation material <NUM> forming the package for a device <NUM> belonging to the QFN/SON product families. Other embodiments may belong to different product families. Independent of the actual configuration of the package and the leadframe, package material <NUM> must not affect the operation of MEMS <NUM>. Consequently, for the stress-sensitive MEMS of <FIG>, this requirement means that package material <NUM> must adhere strongly to all parts packaged inside (to prevent delamination) except to blob <NUM> surrounding MEMS <NUM>. Two methods achieve this goal, and the different process flows are discussed hereinbelow.

<FIG> indicates a surface 160a of glob <NUM> with a layer <NUM> on the surface. Based on the first process flow (described in <FIG> and <FIG>), surface 160a has been activated by plasma etching and thereafter covered by layer <NUM> comprising cured low modulus silicone compound. The packaging compound <NUM> will not adhere to layer <NUM> made of cured (and thus de-activated) silicone compound. Based on the second process flow (described in <FIG> and <FIG>), surface 160a constitutes cured (and thus de-activated) low modulus silicone, which will not adhere to the packaging compound <NUM>, and is covered by layer <NUM> comprising cured epoxy compound. The packaging compound <NUM>, also an epoxy compound, will adhere to and eventually even merge into layer <NUM> and face the non-adhesive cured silicone surface 160a.

In summary, by either one of these methods, packaging compound <NUM> (preferably a molding compound) will not adhere to the cured glob <NUM> and thus cannot transmit external stress into the glob <NUM> and to the stress-sensitive MEMS <NUM>. Consequently, MEMS <NUM> can operate undisturbed by external stress.

Another embodiment is a method to fabricate a common package for a stress-sensitive microelectromechanical system (MEMS) vertically stacked onto a semiconductor circuitry chip. The method is described as a process flow, which includes processes (<FIG>) performed in a semiconductor wafer factory and processes (either <FIG> and <FIG>, or <FIG> and <FIG>) performed in a semiconductor assembly factory.

The method starts in a wafer factory by providing a silicon wafer, which contains multiple integrated circuit (IC) chips and has completed the front-end processes for fabricating the ICs. The intention is to couple a discrete IC chip with a stress-sensitive MEMS and unite them in a common package executed so that the IC and the MEMS can operate undisturbed. The process flow begins with the processes listed in <FIG>.

In the first step <NUM> of the process flow, a semiconductor wafer is provided, which includes multiple chips with integrated circuits (ICs). In process <NUM>, a layer of polyimide material is coated over the surface of the chips. Then, the polyimide layer is patterned to form a polyimide ring on each chip, wherein the rings have an inner diameter greater than the largest linear dimension of the MEMS device-to-be-assembled. For the patterning process, a photolithographic method is used which employs sequentially the process of spinning-on a photoresist layer, masking the photoresist, exposing the mask-protected layer to irradiation, and developing the layer.

After the patterning, the surface energy of the polyimide rings is reduced in process <NUM> by subjecting the polyimide material consecutively to the processes of curing the polyimide compound for a first time, then ashing the polyimide compound, and finally curing the polyimide compound for a second time. The ashing process involves an oxygen plasma and elevated temperatures; alternatively, a hydrogen plasma could be used. The reduced surface energy renders the polyimide rings hydrophobic and repellent to low viscosity and low modulus silicone compounds; silicone material deposited inside the polyimide ring will not bleed out across the polyimide rings. The IC chips are singulated from the wafer in process <NUM>.

In process <NUM>, the discrete chips can be attached to respective assembly pads of a substrate in an assembly factory. The assembly pad may be the chip pad of a leadframe, or alternatively the assembly pad of a laminated substrate. Preferably, the chip is attached to the pad using an epoxy-based compound, which may be cured in a later process at elevated temperature (such as during wire bonding).

To attach a MEMS device to the circuitry chip and use only a single and preferably small package for the IC chip and MEMS combination, the MEMS must be vertically attached to the IC chip. In the process <NUM> of assembling a MEMS device vertically on an IC chip, the MEMS is attached to the chip surface inside the respective polyimide ring using a layer of low modulus silicone material. Such materials are commercially available, such as from Dow Corning Corporation (Midland, Michigan). For the low modulus material, an attachment layer thickness between about <NUM> and <NUM> may be satisfactory for protection against external stress. Preferably, the silicone of the attachment layer is polymerized before advancing to the wire bonding process.

For the process of wire bonding the terminals of the MEMS device to respective contact pads on the chip surface inside the ring, wires including gold or copper may be used. During the bonding operation, the wires are usually spanned with an arch from the MEMS terminals to the chip contact pads.

In conjunction with the wire bonding process for the MEMS, the process of wire bonding the terminals <NUM> of the circuitry chip to respective contact pads of the substrate may be performed. Preferably, the bonding wires are made of copper. In <FIG>, the chip bonding wires are designated <NUM>.

In process <NUM>, a glob is formed by dispensing low-modulus silicone material over the MEMS device and the MEMS bonding wires, while restricting the glob to the surface area inside the polyimide ring. Due to the arches of the bonding wires, the glob has a dome-shaped configuration. The silicone material of the glob is hydrophobic and non-adhesive to thermoset molding compounds. Due to its low viscosity and thixotropic index, it can be dispensed in precision volume. In process <NUM>, the silicone glob is cured.

Because the MEMS of the example embodiment of <FIG> is stress sensitive and has to be protected against transmission of stress, and because the MEMS shares the same overall package with the circuitry chip, the packaging compound encapsulating the assembly must not adhere to the surface of the glob covering the MEMS, but must adhere reliably to all other surfaces inside the package. This dichotomy of requirements can be achieved by two methodologies.

The first methodology is summarized in <FIG>. To achieve good adhesion to molding compounds, process <NUM> subjects all surfaces to a plasma etch. The plasma, involving its gas mixture and power for a prescribed time, preferably operates on cooled surfaces. The plasma accomplishes thorough cleaning of the surfaces from adsorbed films, especially water monolayers, thereby freeing up electrical bonds. Also, the plasma induces some roughening of the surfaces. These effects enhance the adhesion to polymeric filler-filled molding compounds.

Because the cured silicone of the glob has been affected by the plasma of process <NUM>, the glob needs an additional process so that it effectively returns to its original low modulus characteristic. To that end, a layer of the low-modulus silicone material is dispensed onto the surface of the glob in process <NUM>. The nozzle of a syringe releases a controlled drop of the silicone material, which hits the glob and spreads out into a layer of not quite uniform thickness. While a major amount of the drop may remain on the impact location of the glob, some material is bleeding out, resulting in a somewhat non-uniform layer as exemplified by layer <NUM> in <FIG>. For the dimensions of the example in <FIG>, a drop may have a diameter of approximately <NUM> and create a layer of non-uniform thickness with a height of about <NUM> height at the maximum.

In process <NUM>, the silicone layer is cured. Then, in process <NUM>, the assembly comprising the stress-sensitive MEMS embedded in the stress-blocking glob and vertically attached to the semiconductor circuitry chip is encapsulated in a polymeric molding compound, together with the portion of the substrate, onto which the chip is attached and wire bonded. The integrated device is thus packaged.

The second methodology is summarized in <FIG>. After the processes of forming a dome-shaped glob by dispensing a low-modulus silicone material over the MEMS and the wire spans, and curing the silicone glob, described hereinabove in processes <NUM> and <NUM>, a layer of epoxy compound is dispensed onto the surface of the glob in process <NUM>. The epoxy may be the same as the molding compound, or it may be a formulation such as CRP-<NUM>, commercially available from the Sumitomo Corporation, Japan. The layer may be uniform, or it may look similar to layer <NUM> in <FIG>. The layer's nature enables it to merge with, or be absorbed into, the polymeric encapsulation compound.

In process <NUM>, the epoxy layer is cured. Then, in process <NUM>, all surfaces are subjected to a plasma etch in order to achieve reliable adhesion to molding compounds. The plasma, involving its gas mixture and power for a prescribed time, preferably operates on cooled surfaces. The plasma accomplishes thorough cleaning of the surfaces from adsorbed films, especially water monolayers, thereby freeing up electrical bonds. Also, the plasma induces some roughening of the surfaces. These effects enhance the adhesion to polymeric filler-filled molding compounds.

In process <NUM>, the assembly comprising the stress-sensitive MEMS embedded in the stress-blocking glob and vertically attached to the semiconductor circuitry chip is encapsulated in a polymeric molding compound, together with the portion of the substrate, onto which the chip is attached and wire bonded. The integrated device is thus packaged.

For example, embodiments are applicable to products using any type of semiconductor chip, discrete or integrated circuit, and the material of the semiconductor chip may comprise silicon, silicon germanium, gallium arsenide, gallium nitride, or any other semiconductor or compound material used in integrated circuit manufacturing.

As another example, embodiments are applicable to MEMS having parts moving mechanically under the influence of an energy flow (acoustic, thermal, or optical), a pressure, temperature or voltage difference, or an external force or torque. Certain MEMS with a membrane, plate or beam are useful as a pressure sensor (such as a microphone or speaker), inertial sensor (such as an accelerometer), or capacitive sensor (such as a strain gauge or RF switch). Other MEMS operate as movement sensors for displacement or tilt. Bimetal membranes work as temperature sensors.

Claim 1:
A method for packaging an integrated microelectromechanical system, MEMS, device (<NUM>) comprising:
providing a semiconductor chip (<NUM>) including circuitry with first and second terminals (<NUM>), the chip surface having a polyimide ring (<NUM>) surrounding an area greater than the footprint of the MEMS and including the first terminals, the surface of the ring having reduced energy rendering the polyimide ring hydrophobic and repellent to silicone compounds;
attaching the chip (<NUM>) on an assembly pad (<NUM>) of a substrate having leads (<NUM>);
attaching the MEMS device (<NUM>) on the chip area inside the polyimide ring (<NUM>) using a layer (<NUM>) of low modulus silicone material;
spanning bonding wires (<NUM>) from the terminals of the MEMS to the first circuitry terminals inside the ring;
forming a dome-shaped glob (<NUM>) by dispensing a low-modulus silicone material over the MEMS and the wire spans, restricting the glob to the chip surface area inside the polyimide ring, the silicone material hydrophobic and non-adhesive to epoxy-based molding compounds;
curing the silicone glob, thereby completing the integration of the MEMS and the chip;
subjecting all surfaces to a plasma etch for enhancing adhesion to molding compounds;
dispensing a layer (<NUM>) of the low-modulus silicone material onto the surface of the glob;
curing the silicone layer (<NUM>) and encapsulating the integrated device including the MEMS (<NUM>) embedded by the glob (<NUM>), the circuitry chip (<NUM>) and portions of the substrate in a polymeric molding compound (<NUM>), thereby packaging the integrated device, wherein the molding compound is non adhesive to silicone and non-adhering to the glob surface yet adhering to all other surfaces.