Packaged microchip with premolded-type package

A MEMS inertial sensor is secured within a premolded-type package formed, at least in part, from a low moisture permeable molding material. Consequently, such a motion detector should be capable of being produced more economically than those using ceramic packages. To those ends, the package has at least one wall (having a low moisture permeability) extending from a leadframe to form a cavity, and an isolator (with a top surface) within the cavity. The MEMS inertial sensor has a movable structure suspended above a substrate having a bottom surface. The substrate bottom surface is secured to the isolator top surface at a contact area. In illustrative embodiments, the contact area is less than the surface area of the bottom surface of the substrate. Accordingly, the isolator forms a space between at least a portion of the bottom substrate surface and the package. This space thus is free of the isolator. Moreover, due to the low moisture permeability of the package, further production steps can be avoided while ensuring that moisture does not adversely affect the MEMS inertial sensor within the cavity.

FIELD OF THE INVENTION

The invention generally relates microchips and, more particularly, the invention relates packaging techniques for microchips.

BACKGROUND OF THE INVENTION

Microelectromechanical systems (“MEMS”) are used in a growing number of applications. For example, MEMS currently are implemented as gyroscopes to detect pitch angles of airplanes, and as accelerometers to selectively deploy air bags in automobiles. In simplified terms, such MEMS devices typically have structure suspended above a substrate, and associated electronics that both senses movement of the suspended structure and delivers the sensed movement data to one or more external devices (e.g., an external computer). The external device processes the sensed data to calculate the property being measured (e.g., pitch angle or acceleration).

The associated electronics, substrate, and movable structure typically are formed on one or more dies (referred to herein simply as a “die”) that are secured within a package. The package includes interconnects that permit the electronics to transmit the movement data to the external devices. To secure the die to the package interior18, the bottom surface of the die commonly is bonded (e.g., with an adhesive or solder) to an internal surface of the package. Accordingly, in such case, substantially all of the area of the die bottom surface is bonded to the internal surface of the package.

MEMS inertial sensors/die are sensitive to environmental factors, such as disparate material expansion between the die and its package. Specifically, this disparate expansion commonly is caused by mismatched coefficients of thermal expansion (“CTE”) between the materials forming the die bottom surface and the internal package surface that secures the die. In fact, these CTE mismatches can cause the sensor to deliver incorrect motion measurements. For example, when implemented as an accelerometer within an automobile airbag system or as a gyroscope in an automobile traction control system, CTE mismatches can produce results that can cause the automobile to operate erratically. Consequently, such incorrect measurements can lead to bodily injury or death for drivers, their passengers, or others near the moving automobile (e.g., people in other automobiles).

To reduce this problem, MEMS inertial sensors commonly are secured within ceramic packages rather than within two other well known, less costly, but widely used package types; namely, “transfer molded” packages and “premolded” packages. Specifically, both transfer molded packages and premolded packages have a copper leadframe to which the die is secured. The CTE difference between copper and silicon (i.e., the material forming the die), however, is much greater than the CTE difference between ceramic and silicon.

Moreover, transfer molded and premolded packages also typically cannot provide hermeticity. Accordingly, if such types of packages are used with certain MEMS sensors, additional processes are required to ensure that moisture does not affect the sensor itself.

For these and other reasons, those in the art are motivated to use ceramic packages for MEMS inertial sensors rather than the other two types of packages. Undesirably, ceramic packages typically are more expensive than the other two types of packages. In addition, securing a MEMS sensor die within a ceramic package requires a larger number of process steps (when compared to the other noted types of packages), thus further increasing production costs. In fact, in many MEMS sensor applications using ceramic packages, the packaging cost far exceeds the cost of producing the MEMS sensor itself.

SUMMARY OF THE INVENTION

In accordance with one aspect of the invention, a MEMS inertial sensor is secured within a premolded-type package formed, at least in part, from a low moisture permeable molding material. Consequently, such a motion detector should be capable of being produced more economically than those using ceramic packages. To those ends, the package has at least one wall (having a low moisture permeability) extending from a leadframe to form a cavity, and an isolator (with a top surface) within the cavity. The MEMS inertial sensor has a movable structure suspended above a substrate having a bottom surface. The substrate bottom surface is secured to the isolator top surface at a contact area. In illustrative embodiments, the contact area is less than the surface area of the bottom surface of the substrate. Accordingly, the isolator forms a space between at least a portion of the bottom substrate surface and the package. This space thus is free of the isolator. Moreover, due to the low moisture permeability of the package, further production steps can be avoided while ensuring that moisture does not adversely affect the MEMS inertial sensor within the cavity.

The wall may be formed from any low moisture permeability material, such as a liquid crystal polymer. To ensure hermeticity, the motion detector also has a low moisture permeability lid secured to the at least one wall.

Different types of isolators may be used. For example, the isolator may be formed from the leadframe. Additionally or alternatively, the isolator may have a plurality of protrusions extending from the base of the cavity. In some embodiments, the isolator is formed from a low moisture permeability molding material. In still other embodiments, the isolator is at least in part formed from a silicone material. Such a soft moldable material should mitigate stresses. Other embodiments form the isolator, at least in part, from one or more different moldable materials.

In accordance with another aspect of the invention, a motion detector has a premolded-type package that contains an inertial sensor die. The package has an isolator, a leadframe, and at least one wall extending from the leadframe to at least in part form a cavity. The cavity contains the isolator and is formed from at least one material having a low moisture permeability. The bottom surface of the die is secured to the isolator top surface at a contact area that is less than the bottom surface area of the die. In a manner similar to other aspects, the isolator forms a space between at least a portion of the die bottom surface and the package. This space is free of the isolator.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

Illustrative embodiments of the invention substantially reduce chip stresses associated with conventional premolded-type packages so they can be successfully used to package stress sensitive microchips. Accordingly, because of the lower cost of premolded packages (when compared to ceramic packages), such embodiments can significantly reduce production costs while maintaining desired performance. Moreover, illustrative embodiments further enable premolded-type packages to provide hermetically sealed environments, thus enabling use with a variety of microchips requiring hermeticity. Details of illustrative embodiments are discussed below.

FIG. 1schematically shows a partially cut-away isometric view of a packaged microchip10that can implement various embodiments of the invention. In illustrative embodiments, the packaged microchip10is a MEMS device implemented as an angular rate sensor. Accordingly, for illustrative purposes, various embodiments are discussed herein as a MEMS angular rate sensor. The MEMS devices shown inFIGS. 1–4thus may be generally identified herein as angular rate sensors10or motion detectors. It should be noted, however, that discussion of various embodiments as a MEMS angular rate sensor is exemplary only and thus, not intended to limit all embodiments of the invention. Accordingly, some embodiments may apply to other types of microchip devices, such as integrated circuits. In addition, embodiments of the invention can be applied to other types of MEMS devices, such as MEMS-based optical switching devices and MEMS-based accelerometers.

The angular rate sensor10shown inFIG. 1includes a conventional premolded-type package12having walls13extending from a base21, a lid14secured to the walls13for sealing the package12, and a conventional angular rate sensor die16(referred to herein as “die16,” also referred to as a “microchip”) secured within the hermetically sealed interior18. The die16includes the well known mechanical structure and electronics (discussed below) that measure angular motion about a given axis. A plurality of pins20extending from the package12electrically connect with the die16to permit electrical communication between the angular rate sensor electronics and an exterior device (e.g., a computer).

In accordance with illustrative embodiments of the invention, the die16is bonded to a stress reducing isolator22(shown inFIGS. 2–4) extending inwardly from the base21of the package interior18. The isolator22illustratively has a top surface25with a surface area that is less than that of the die bottom surface26. Accordingly, because less than the entire die bottom surface26couples with the package12, stresses from the package12should be minimized. Moreover, in illustrative embodiments, the package12is formed from a moldable material having a low moisture permeability (i.e., a moldable material that is capable of providing hermeticity). Combining these features should enable use of a pre-molded type of package12with a stress sensitive die16requiring hermeticity. Details of various embodiments follow.

The isolator22preferably is formed from a material that substantially further minimizes stresses between the remainder of the package12and the sensor die16. In one embodiment of the invention, the isolator22is integrally formed from the moldable material making up the remainder of the package. Among other ways, such a package12may be formed by using conventional injection molding processes, or dispensing processes. In another embodiment, the isolator22may be formed from the die paddle38portion of the package leadframe24. In yet another embodiment, the isolator22may be a non-moldable material (e.g., a silicon stud) secured to molding material forming much of the remainder of the package12.

FIGS. 2–4schematically show three embodiments of the packaged microchip10along line A—A ofFIG. 1. In particular,FIG. 2schematically shows a first embodiment of the invention, in which a moldable material substantially integrally mates with a leadframe24(not shown inFIG. 1) to form the noted isolator22. A moldable material having a low moisture permeability and adequate adhesion property with the copper leadframe24should be used. Among others, such a moldable material may include a liquid crystal polymer, which provides hermeticity.

The package12is considered to be “hermetic” when it complies with the well known “MIL Standard.” Specifically, the package12is considered to be hermetic when it complies with MIL-STD 883D, method 1014.9, entitled, “Seal,” which is incorporated herein, in its entirety, by reference.

As noted above, the die16includes conventional silicon MEMS structure28to mechanically sense angular rotation, and accompanying electronics30. Such structure28and electronics30(both shown schematically inFIGS. 2–4) illustratively are formed on a silicon-on-insulator wafer, which has an oxide layer between a pair of silicon layers. Alternatively, the structure28and electronics30may be formed by conventional surface deposition techniques, or some other conventional means known in the art. As an example, among other things, the MEMS structure28may include one or more vibrating masses suspended above a silicon substrate by a plurality of flexures. The structure28also may include a comb drive and sensing apparatus to both drive the vibrating masses and sense their motion.

Accordingly, the electronics30may include, among other things, the driving and sensing electronics that couple with the comb drive and sensing apparatus, and signal transmission circuitry. Wires23electrically connect the accompanying electronics30with the pins20. Exemplary MEMS inertial sensors/motion detectors are discussed in greater detail in U.S. Pat. Nos. 5,939,633 and 6,505,511, which are assigned to Analog Devices, Inc. of Norwood, Mass. The disclosures of both of the noted patents are incorporated herein, in their entireties, by reference.

In alternative embodiments, the MEMS structure28and accompanying electronics30are on different dies16. For example, the die16having the MEMS structure28may be mounted to the package12at a first isolator22, while the die16having the accompanying electronics30may be mounted to the package12at a second isolator22. Alternatively, both dies16may be mounted to the same isolator22. In some cases, one of the dies16(e.g., a stress sensitive die16) may be mounted to an isolator22having stress reduction properties, while the other die16(e.g., a non-stress sensitive die16) may be mounted directly to the package12.

The die16, which is a microchip/integrated circuit, is sensitive to both linear and torsional stress. In this context, the term “sensitive” generally means that the operation of the structure28and/or electronics30on the die16can be compromised when subjected to such stress. For example, as suggested above, stress applied to the die16can cause the flexures suspending the mass to bend or compress to some extent. As a consequence, the mass may not vibrate at a prescribed rate and angle, thus producing a quadrature problem. As a further example, the comb drive may become misaligned, or the electronics30may become damaged. Any of these exemplary problems undesirably can corrupt the resulting data produced by the MEMS die16. Accordingly, for these reasons, the die16or other microchip may be referred to as being “stress sensitive.”

To solve these stress related problems, as noted above, the isolator22illustratively is formed to have a top surface25with a smaller area than that of the bottom surface of the die16. In other words, the top surface25of the isolator22contacts the bottom surface of the die16at a contact area. This contact area is smaller than the area of the die bottom surface26. As a result, the isolator22forms a space between the die bottom surface26and the base21of the package12. Contacting less than the total die bottom surface26should reduce stress transmitted from the package12to the die16(when compared to contacting the entire die bottom surface26).

Although the isolator22in this embodiment is shown as extending above all other portions of the base21, in alternative embodiments, its top surface25may be flush with or below some other portions of the base21. In either case, the isolator22still contacts less than the entire surface area of the die bottom surface26. Accordingly, the isolator top surface25illustratively is not the lowest point of the package base21.

To further minimize stress, the isolator22may be formed from a material having a CTE that more closely matches that of the die16than that of the molding material forming the remainder of the package12. In other words, the CTE of the isolator22contacting the die16may be closer to that of the die16than that of the molding material forming much of the remainder of the package12. To that end, the isolator22may be formed from silicon, which has a CTE that matches that of a silicon substrate. Of course, other materials may be used, depending upon the substrate type.

Rather than (or in addition to) matching the CTEs of the die16and isolator22, some other embodiments form the isolator22from a material having a modulus of elasticity that substantially negates CTE mismatches. More specifically, in such embodiments, the package12is formed from at least two materials; namely, a first (moldable) material that makes up much of the package12(i.e., the portion referred to above as the remainder of the package12), and a second (moldable) material that makes up the isolator22. In illustrative embodiments, the second material is much softer than the first material. If the softness of the second material is sufficiently lower than that of the first material, CTE mismatches should not affect die performance. To that end, the second material, which forms the isolator portion contacting the die16, is selected (relative to the first material) to ensure that no more than a negligible thermal stress is transmitted to the die16. In other words, the ratio of the two moduli is selected so that no more than a negligible amount of stress is transmitted from the package12to the die16(via the isolator22). For example, the first material may be a liquid crystal polymer, while the second material, which contacts the die16, may be silicone. Such negligible amount of stress should have a negligible impact on die performance.

A negligible impact on performance means that the die16produces a signal that is satisfactory for its intended purpose. For example, a negligible error may be considered to have occurred when the output results can be used (for their intended purpose) without the need for additional corrective circuitry to correct a stress-induced error. As known by those skilled in the art, such results depend upon the application for which the die16is produced. If the die16is a roll-over angular rate sensor, for example, a negligible error may be considered to occur if the die results are within about fifteen percent of the results it would produce in a completely unstressed condition. In other applications, however, to be a negligible error, the results must be much closer to the unstressed results.

See the above incorporated concurrently filed U.S. patent application entitled, “STRESS SENSITIVE MICROCHIP WITH PREMOLDED-TYPE PACKAGE,” for additional details relating to this and related embodiments.

In the embodiment shown inFIG. 2, the leadframe24does not have a die paddle38for supporting the substrate. Instead, the moldable material (either the first material or both the first and second materials, depending upon the implementation) supports the die16. There may be instances, however, when a die paddle38could be used to support the isolator22. To those ends,FIG. 3schematically shows another embodiment of the invention, in which the isolator22is formed from the die paddle38. In other embodiments, the isolator22is formed over the die paddle38. In such case, the isolator22also may include electrically conductive properties for, among other things, grounding the die16to the die paddle38. Moreover, the isolator22also may include thermally conductive properties.

The top surface25of the isolator22shown inFIGS. 2 and 3is a substantially flat, rectangular shape. In other embodiments, however, the isolator22may be formed in a variety of shapes that minimize surface contact. For example, the isolator22inFIG. 4, which is formed from the die paddle38, has a top surface25interrupted by a plurality of trenches40. The trenches40effectively form a plurality of protrusions extending from the base21of the package12. The cumulative surface area of the top surfaces25of the protrusions still should be less than that of the die bottom surface26. Alternatively, the die paddle38could be etched to form an effective single projection in a predetermined shape (e.g., a cross or rectangular shape). Among other ways, such shapes could be molded (e.g., if the isolator22is moldable material) or etched (e.g., if the isolator22is part of the die paddle38, such as shown inFIGS. 3 and 4).

FIG. 5shows a process of forming the packaged sensors10shown inFIGS. 1–4. The process begins at step500, in which conventional processes pattern and otherwise process a sheet of copper in accordance with conventional processes to form the leadframe24. In the embodiments shown inFIGS. 3 and 4, the die paddle38of the leadframe24is etched to have the desired shape/form. In particular, for the embodiment shown inFIG. 3, the die paddle38is etched to form a trench/space40around its center portion. This trench40effectively reduces the surface area of the paddle surface that secures to the die bottom surface26. In a similar manner, conventional etching processes form a plurality of trenches40in the die paddle38of the embodiment shown inFIG. 4to effectively form the noted plurality of protrusions.

After the leadframe24is substantially formed, the moldable material is added (step502). In illustrative embodiments, the moldable material is a low moisture permeability plastic material that adheres well to the leadframe24. For example, the moldable material may be a liquid crystal polymer. Accordingly, forming the package12with such a polymer should provide hermeticity.

In the embodiments that do not use a die paddle38, conventional injection molding processes may form the isolator22from the moldable material. In a manner similar to the embodiments ofFIGS. 3 and 4, the isolator22can be molded to a desired shape that effectively reduces contact with the die16. As also noted above, the isolator22may be molded with a second material (e.g., having a lower modulus of elasticity than that of the moldable material), or with both the moldable material and a non-moldable material (e.g., a silicon stud). Details are discussed in the noted incorporated patent application filed on even date herewith.

After the moldable material cures, the leadframe24carries an array of open packages12having walls13that form cavities. Accordingly, a die16may be secured to the isolator22in the base21of the cavity of each package12(step504) in the array. Among other ways, each die16may be secured by conventional means, such as with an adhesive.

The process continues to step506, in a lid14is secured to each package12. In some embodiments, the packages12and die16may be inserted into a gas chamber, which saturates the interiors18with a gas. In that case, the lids14may be secured when within the gas chamber. Finally, the leadframe24and lid14may be diced (step508) to produce a plurality of packaged microchips.

Accordingly, among other benefits, various embodiments discussed above enable stress sensitive microchips to be packaged in high yields while significantly reducing the stresses associated with currently available low cost packaging techniques. Stress sensitive microchips thus can receive the low cost benefits of premolded packages while avoiding both the higher costs and stress associated with ceramic packages. Moreover, illustrative premolded packages provide hermetic environments, thus enabling use of both capped and uncapped dies16within such premolded packages.