BACKGROUND OF THE INVENTION
Current Microelectromechanical (MEMS) Leadless Chip Carriers (LCCs) have a ±5 mil (125 micron) tolerance on dimensions and fiducials. The bump pattern alignment to the MEMS die can be no better than the tolerance of the LCC package. The MEMS die to bump alignment is important to the performance of the packaged MEMS sensor.
SUMMARY OF THE INVENTION
The present invention provides an alignment plate attached to the die bond pad attached to an LCC package. The alignment plate includes fiducials fabricated into top and bottom metal layers, thus producing a tolerance that will be an order of magnitude better than the tolerance of fiducials included in the LCC. A bump pattern and/or MEMS die are attached to the alignment plate based on the alignment plate fiducials giving a much improved die to bump alignment.
BRIEF DESCRIPTION OF THE DRAWINGS
Preferred and alternative embodiments of the present invention are described in detail below with reference to the following drawings:
FIG. 1 illustrates a cross-sectional view of an LCC with a secured MEMS device formed in accordance with an embodiment of the present invention;
FIGS. 2-1 through 2-4 illustrate steps in an example process for attaching a MEMS device within the LCC package in accordance with an embodiment of the present invention;
FIG. 3 illustrates a top-down view of an alignment plate mounted within an LCC package; and
FIG. 4 illustrates a side cross-sectional view of an alignment plate formed in accordance with an embodiment of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
As shown in FIG. 1, an example LCC package 40 includes an alignment plate 68 and MEMS die 74 secured within a cavity formed by a base section 60 (die bond pad) and a cover 62. The alignment plate 68 is die bonded into the bottom of the base section 60. The alignment plate 68 includes one or more fiducials that are applied to a top and bottom surfaces of the alignment plate 68 using wafer fabrication processing. The tolerance or accuracy of the alignment plate fiducials are an order of magnitude better than that of fiducials that may be formed onto the base of the base section 60. A bump pattern 70 is formed onto the alignment plate 68 based on the alignment plate fiducials. Then, the MEMS die 74 is attached to the bump pattern 70, thus improving MEMS die to bump alignment.
In one embodiment, the alignment plate 68 is formed of glass. The top and bottom sides of the alignment plate 68 include a deposited metallization layer (e.g. Ti/Pt/Au). The thickness of the glass plate is between 0.01″ (250 μm) and 0.03″ (760 μm). Other thicknesses can be used. On both sides of the glass plate (wafer) saw streets and fiducials are patterned into the metallization layer using typical water fab photolithography and etching techniques.
As shown in FIG. 2-1, the alignment plate 68 will be bumped with a first bump pattern 64 in wafer form to ensure alignment of a second bump pattern 70. After the alignment plate 68 in wafer form is sawed with prepatterned bumps (the first bump pattern 64), the alignment plate 68 is die bonded into the LCC (the base section 60) using flip chip thermal compression die bonding (FIG. 2-2). As shown in FIG. 2-3, the second bump pattern 70 is applied by a wirebonder device onto the alignment plate 68 based on the previously fabricated fiducials on the alignment plate 68. Then, as shown in FIG. 2-4, the MEMS die 74 is attached on the alignment plate 68 (the second bump pattern 70) based on the same fiducials used to apply the second bump pattern 70.
FIG. 3 illustrates a top view of an alignment plate 68-1 mounted inside the base section 60 of the LCC. In this embodiment, the alignment plate 68-1 includes 2 sets of fiducials 78 and 80 (i.e., visual cues). The fiducials 78 of the first set are located at opposing corners of the bottom surface of the alignment plate 68-1. The fiducials 80 of the second set are located at opposing corners of the top surface of the alignment plate 68-1. FIG. 4 illustrates a partial cross-sectional view of the alignment plate 68-1. The fiducials 78 and 80 are etched into a respective metal layer but not into the glass of the alignment plate 68-1.
A bump application device (not shown) (e.g., an automatic (robotic) wirebonder device) visually acquires the fiducials 80 using an imaging device. The bump application device determines where to apply the bumps (the bump pattern 70) to the alignment plate 68-1 based on the acquired fiducials 80. The MEMS die 74 is then attached (robotically) to the second bump pattern 70 using automatic visual acquisition of the fiducials 80.
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. 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
20100020517
