Micromachine package fabrication method

Details Micromachine package fabrication method Micromachine package fabrication method

1999-11-15

2002-07-09

Arbes, Carl J. (Department: 3729)

Metal working

Method of mechanical manufacture

Electrical device making

C029S825000, C029S829000, C029S840000

Reexamination Certificate

active

06415505

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates generally to the packaging of electronic components. More particularly, the present invention relates to a method of fabricating a micromachine package.
BACKGROUND OF THE INVENTION
Micromachine sensing elements (hereinafter micromachines) are well known. A micromachine includes a miniature moveable structure, such as a bridge, cantilevered beam, suspended mass, membrane or capacitive element, which is supported over a cavity formed in a silicon wafer. Since the operation of the micromachine depends upon the moveability of the miniature moveable structure, it is critical that the package, which includes the micromachine, does not contact the miniature moveable structure in any manner.
FIG. 1
is a cross-sectional view of a structure
8
during the formation of a plurality of micromachine packages in accordance with the prior art. As shown in
FIG. 1
, a silicon wafer
10
includes a plurality of micromachine chips
12
. Micromachine chips
12
include micromachine areas
14
formed in an upper surface
10
U of wafer
10
. Micromachine areas
14
include the miniature moveable structure of the micromachine. Micromachine chips
12
further include bond pads
16
on upper surface
10
U of wafer
10
. Bond pads
16
are connected to the internal circuitry of micromachine chips
12
.
Micromachine chips
12
are integrally connected together in an array format. Each of micromachine chips
12
is delineated by a singulation street
20
which is located between adjacent micromachine chips
12
.
A silicon lid
30
formed from a silicon wafer is positioned above wafer
10
. Lid
30
includes a plurality of caps
42
integrally connected to one another. Each cap
42
includes a micromachine cavity
32
. Each micromachine cavity
32
is positioned over a corresponding micromachine area
14
. Generally, micromachine cavities
32
are wider than micromachine areas
14
.
Each cap
42
further includes a bond pad cavity
34
. Each bond pad cavity
34
is positioned over a corresponding set of bond pads
16
on a micromachine chip
12
. Generally, bond pad cavities
34
are wider than bond pads
16
, and are at least as deep as bond pads
16
are tall.
FIG. 2A
is a cross-sectional view of structure
8
of
FIG. 1
at a further stage in fabrication in accordance with the prior art. As shown in
FIG. 2A
, lid
30
is attached to wafer
10
. Micromachine cavities
32
are positioned above corresponding micromachine areas
14
. Further, bond pad cavities
34
are positioned above corresponding sets of bond pads
16
.
FIG. 2B
is a cross-sectional view of structure
8
of
FIG. 2A
at a further stage of fabrication in accordance with the prior art. Referring to
FIG. 2B
, a series of shallow cuts are made to remove a portion of each cap
42
to expose bond pads
16
. Micromachine chips
12
are electrically tested by connecting test probes to bond pads
16
. Should testing of a micromachine chip
12
indicate that the micromachine chip
12
is defective, the micromachine chip
12
and/or corresponding cap
42
is marked. For example, micromachine chip
12
A is marked as being defective. Wafer
10
is then singulated along singulation streets
20
. Micromachine chips
12
which are marked as defective are discarded.
Disadvantageously, a cap
42
is attached to a micromachine chip
12
even if the micromachine chip
12
is defective. The cap
42
and defective micromachine chip
12
are discarded. However, since a cap
42
is attached to the defective micromachine chip
12
, the cost associated with the defective micromachine chip
12
is increased compared to the cost associated with the defective micromachine chip
12
alone. This increase is the cost of fabricating each batch of micromachine packages. This, in turn, increases the cost of fabricating each individual micromachine package which passes testing.
After singulation of wafer
10
, each good micromachine chip
12
with cap
42
is further package
FIG. 3
is a cross-sectional view of a single micromachine package
40
in accordance with the prior art. As shown in
FIG. 3
, micromachine chip
12
and cap
42
are attached to a substrate
52
. Bond pads
16
are electrically connected to traces
44
by bond wires
46
. To prevent accumulation of static charge on cap
42
which would render micromachine chip
12
inoperable, cap
42
is electrically connected to a ground trace
48
by a bond wire
50
. Ground trace
48
is grounded during use. Although effective at prevent accumulation of static charge on cap
42
, grounding cap
42
by electrically connecting cap
42
to ground through bond wire
50
and ground trace
48
is relatively labor intensive and complex which increases the cost of fabricating package
40
.
SUMMARY OF THE INVENTION
In accordance with the present invention, a micromachine package includes a micromachine chip having a micromachine area in an upper surface of the micromachine chip. A bead attached to the upper surface of the micromachine chip and a coupon attached to the bead form an enclosure. The enclosure defines a cavity, i.e., a free space, above the micromachine area. The cavity allows a moveable structure of the micromachine area to freely move.
Of importance, of importance, the coupon has an area substantially less than an area of the upper surface of the micromachine chip. More particularly, the coupon is sized to extend only slightly beyond the micromachine area. In this manner, the amount of material used for the coupon is minimized. By minimizing the amount of material used for the coupon, the costs associated with the micromachine package are also minimized.
Recall that in the prior art, a single lid was attached to a plurality of micromachine chips while still in wafer form. Thus, in the prior art, the cap which covered the micromachine area had an area before trimming equal to the area of the upper surface of the micromachine chip. Accordingly, more material was used for the cap of the prior art than for the coupon in accordance with the present invention. As a result, the micromachine package in accordance with the present invention is fabricated at a lower cost than the micromachine package of the prior art.
To further reduce the costs associated with each micromachine package, in one embodiment, a plurality of coupons are attached to a plurality of micromachine chips while the micromachine chips are still in wafer form. Of importance, the plurality of micromachine chips are tested for validity (i.e., to determine whether each micromachine chip is good or defective) before the coupons are attached and while the plurality of micromachine chips are integrally connected to one another. Any defective micromachine chips are marked or otherwise identified.
Advantageously, the coupons are attached only to the micromachine chips which have been tested and found to be good. The coupons are not attached to defective micromachine chips. In this manner, waste of coupons is avoided and labor associated with attaching the coupons to defective micromachine chips is saved. This, in turn, minimizes the cost associated with the fabrication of each batch of micromachine packages and, more particularly, with each micromachine package.
Recall that in the prior art, a single lid was attached to a plurality of micromachine chips while still in wafer form. Thus, in the prior art, a cap was attached to each micromachine chip even if the micromachine chip was defective. Accordingly, the caps and labor associated with attaching the caps to the defective micromachine chips were wasted in the prior art. As a result, a micromachine package in accordance with the present invention is less expensive to manufacture than a micromachine package of the prior art.
In one embodiment, a method includes attaching a bead to an inner surface of a coupon. The method further includes attaching the bead to an upper surface of a micromachine chip. The bead and the coupon form an enclosure which defines a cavity above a micromachine area in the upper surface of the micromachine chip.
A lower sur

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