BACKGROUND OF THE INVENTION
Microelectromechanical systems (MEMS) devices or computer chips are often mounted on dies. Stresses due to die mounting can affect the performance of MEMS devices. The stresses change with temperature because of the differing coefficients of thermal expansion (CTE) of the package and the die; dies made of silicon or borosilicate glass typically have a lower CTE than that of the package, so as temperature changes, the package stresses the die. Thus, as shown in FIGS. 1A and 1B, a die 10 with a lower CTE than a package 12 are heated when attaching the die 10 to the package 12. When the assembly 14 cools, the package experiences more shrinkage than the die, and creates stress at attachment points 16. The stress affects the performance of the MEMS device 9.
The stresses also change due to shock when wire bonds change shape, which causes problems for capacitive sensors that require stable geometry for stable output. Additionally, making wire bonds at the package level makes the parts more expensive because each part must be handled separately, as well as increasing the chance of damage to the assembly.
SUMMARY OF THE INVENTION
The present invention provides a system and method of reducing the stress associated with a die mount, as well as eliminating the wire bonding operation.
In an example embodiment, a die is prepared with traces and pads as desired for the intended use of the die. A MEMS device is mounted to the die. The die is then mounted to a substrate of the same material as the die. [MEMS TO DIE ATTACH STEP.]The substrate is then mounted to a package.
BRIEF DESCRIPTIONS OF THE DRAWINGS
Preferred and alternative embodiments of the present invention are described in detail below with reference to the following drawings:
FIG. 1A is a cross-section of a package-mounted die according to the prior art at a temperature sufficient to attach the die to the package;
FIG. 1B is a cross-section of the package-mounted die of FIG. 1A after it has cooled;
FIG. 2A is a perspective view of a package-mounted die according to the present invention;
FIG. 2B is an exploded perspective view of the package-mounted die of FIG. 2A;
FIG. 2C is a cross-sectional view of the package-mounted die of FIG. 2A through the plane AA′;
FIG. 3 is an alternate embodiment of a cover plate according to the present invention; and
FIG. 4 is a flow diagram of a method according to the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
With reference to FIGS. 2A-2C, a device 20 according to an embodiment of the present invention is shown. A MEMS device 22, which may be an accelerometer, a gyroscope, or any other suitable MEMS device, is mounted to a rectangular block die 24 during the MEMS build. The die 24 is mounted to a rectangular block cover plate 26 via gold bumps 30 (which can be approximately 0.001″ to 0.005″ in diameter [0.001 TO 0.005 INCHES IN DIAMETER?], and the die 24 can be flip-chip mounted to the cover plate 26. The gold bumps 30 provide all of the structural strength necessary to prevent dislodging the die 24 during use. The MEMS device 22 can be protected from contact during flip-chip mounting using etched features or process controls. The cover plate 26 is mounted to a package 28, a rectangular block including a recess 23 (or wirebond shelf) with a step 25, sized and shaped to receive the MEMS device 22, the die 24, and the cover plate 26. The cover plate 26 is mounted to the step 25 of the package 28 via gold bumps 32, and the cover plate 26 can be flip-chip mounted to the package 28. The package 28 can be a Leadless Ceramic Chip Carrier (LCCC). Electrically conductive traces 34 from the MEMS device 22 to the gold bumps 30, and conductive traces 36 from the gold bumps 30 to the gold bumps 32 provide a connection between the MEMS device 22 and the package 28 so that the MEMS device 22 can be in electrical contact with devices or systems (not shown) outside the package as necessary depending on the intended use of the device 22.
The die 24 has a preselected CTE, and the package 28 has a preselected CTE that is higher than the CTE of the die 24. The cover plate 26 is preferably made of the same material as the die 24 with the same CTE as the die 24. Alternatively, the cover plate 26 is made of a different material than the die 24, as long as the CTE of the cover plate 26 has an intermediate CTE higher than the CTE of the die 24 and lower than the CTE of the package 28, and preferably the CTE of the cover plate 26 is closer to the CTE of the die 24 than the package 28.
FIG. 3 shows a portion of an alternate embodiment of the present invention. The cover plate 26 includes a composite of first and second materials 27,29. The die has a first CTE. The second material 29 has a CTE greater than the CTE of the die 24 and less than the CTE of the first material 29. The first material 27 has a CTE less than the CTE of the package 24. The package 24 is attached to the first material 27, and the die is attached to the second material 29.
FIG. 4 shows a process flow diagram of a method 40 for creating an embodiment of the present invention. At a block 42, a MEMS device is mounted to a die having a CTE equal or greater than the CTE of the MEMS device. At a block 44, the die is mounted to a cover plate having a CTE greater than the CTE of the die and less than the CTE of a package. Finally, at a block 46, the die is mounted to the package.
Note that the electrically conductive traces 34, 36 electrically connect the MEMS device to the package. The traces are mechanically more stable under shock than wirebonds, because they are printed on the package and cover plate.
While the preferred embodiment of the invention has been illustrated and described, as noted above, many changes can be made without departing from the spirit and scope of the invention. For example, the embodiments disclosed use gold bumps, but solder balls, conductive polymers, or directional conductors may be used instead. Accordingly, the scope of the invention is not limited by the disclosure of the preferred embodiment. Instead, the invention should be determined entirely by reference to the claims that follow.
Patent Application
20080315397
