Active solid-state devices (e.g. – transistors – solid-state diode – Housing or package – With window means
Reexamination Certificate
2002-03-09
2003-11-25
Quach, T. N. (Department: 2814)
Active solid-state devices (e.g., transistors, solid-state diode
Housing or package
With window means
C257S735000, C382S124000
Reexamination Certificate
active
06653723
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to semiconductor devices, and more particularly, to a system for providing a fingerprint sensor with increased sensor accessibility.
BACKGROUND OF THE INVENTION
Semiconductor devices are increasingly being used as input devices for digital systems. For example, in identification and security applications, semiconductor devices are used to provide user identification information. One such device is a semiconductor fingerprint sensor.
FIG. 1
shows a portion of a typical semiconductor fingerprint sensor
100
. Generally, such a sensor is provided as an integrated circuit (IC). The sensor
100
includes a die (or wafer)
102
attached to a substrate
104
via an adhesive or epoxy bond
106
. A sensor surface
108
of the die
102
has a conductive grid, shown in detail at
110
, that is used to form a capacitive circuit to detect characteristics of a person's finger when the sensor surface is touched. The grid is coupled to a plurality of die contact members
112
at the surface of the die.
A technique known as wire bonding is used to couple the die contact members
112
to substrate contacts
114
located on the substrate material, which is normally made up of metallic lead frame or build up layers of substrate. Typically, wire bonding involves attaching small wires (gold or aluminum) between two contact members. A capillary device, shown at
116
, is typically used to bond the wire between the contacts. When bonding the wire, the capillary device first forms a ball
118
at the end of a wire
120
by using an electronic flame-off (EFO) technique. Once the ball is formed, the capillary device attaches the ball
118
to a die contact pad
112
by a thermal-sonic process. In this process, the contact is heated and ultrasonic power is used to agitate the ball onto contact to flatten out the ball to form an inter-metallic weld between the ball and the contact, as shown at
122
.
After the first weld is made, the capillary device
116
extends the wire
120
over to a substrate contact
114
to form a weld with that contact. To bond the wire to the substrate contact
114
, a stitch weld is formed. The stitch weld bonds the wire to the substrate contact and cuts the wire at the same time, so that the capillary device may form a new ball on a next portion of the wire and proceed to the next die contact. For example, a stitch weld is shown at
124
.
The wire
126
shows the result of the wire bonding process described above. Because the wire extends in generally a vertical direction from the weld of the ball to the die contact, a wire loop is formed when the wire is extended to the substrate contact. The wire loop has a height above the surface of the die is shown at
128
. For standard wire bonding processes, this loop height is between six to ten thousandths (mils) of an inch high. As described in the following text, the loop height has an effect on the operation of finger sensor
100
.
Once the wire bonding is completed and all bonding wires are installed, the device is protected by an encapsulation process in which a material, such as plastic, completely covers the bonded wires. For example, a molding process may be used where a material is molded around the device. Another process that may be used is referred to as “glob-top” dispensing, where material is dispensed onto the top of the device and allowed to flow around the sides and bottom of the device.
FIG. 2
shows the finger sensor
100
after an encapsulation process is completed so that the bonding wires are completely protected by an encapsulation material
202
. However, for the finger sensor to operate, the sensor surface
108
is exposed by a cavity
204
in the encapsulation material to allow a person finger to come in contact with the sensor surface.
To cover the bonding wires and still provide access to the sensor surface
108
, the cavity in the encapsulation material includes cavity walls
206
that are at least as high as the loop height of the bonding wires. The cavity walls form what is referred to as a pedestal that has a pedestal height, shown at
208
. Unfortunately, as a result of the pedestal height, portions of the sensor surface
108
may not be reachable by a person's finger. For example, the sensor surface regions shown at
210
and
212
may be inaccessible to a person's finger because it is not possible squeeze the finger into the corner formed by the sensor surface and the cavity wall.
Finger sensors typically provide their best operation when a maximum number of grid points can be touched. However, due to the effects of the pedestal height, portions of the sensor grid are unreachable, and so, the performance of the sensor may be degraded. Another problem associated with convention fingerprint sensors is the package size. Typical fingerprint sensors have die contacts on either side of the sensor surface. This results in a very large package that may be unsuitable for use in portable applications.
One way to overcome the above problems is to provide a larger cavity to account for the unreachable portions of the sensor surface. However, due to the geometry of the die, it may not be possible to provide a larger cavity without exposing portions of the die. Furthermore, even if a larger cavity were possible, the overall height of the encapsulation is undesirable because typical applications for finger sensors include portable devices, such as cell phones, that require the smallest possible size. For example, one conventional fingerprint sensor has approximate dimensions of 22×12×0.4 millimeters, which is a relative large package that is unsuitable for use in portable applications.
Therefore, what is needed is a way to provide maximum access to a finger sensor surface while providing the smallest possible size to allow the device to be used in a variety of portable applications.
SUMMARY OF THE INVENTION
The present invention includes a system for wire bonding a finger sensor die to an external circuit to provide maximum access to the finger sensor surface while providing the smallest possible size to allow the device to be used in a variety of portable applications. The system reduces the height of wire loops formed by bonding wires so that the pedestal height of the encapsulation is reduced. The reduced height of the pedestal provides greater access to the sensor surface. Thus, by providing greater access to the sensor surface, more sensor grid points are used to produce a sensor read-out, which results in more accurate sensor operation. The system is equally applicable to stationary finger sensors and sweep finger sensors. Furthermore, by reducing the encapsulation height, the overall device package is reduced in size. This results in cost savings, as well as, allowing the device to be integrated into a variety of small portable devices.
In one embodiment of the present invention, a method for wire bonding a finger sensor die to an external circuit is provided. The finger sensor die includes a sensor array having one or more die contacts that are wire bonded to one or more external contacts of the external circuit so that a usable portion of the sensor array is maximized. The method comprises steps of forming a ball at a first end of a bonding wire, forming an electrically conductive connection between the ball and a selected external contact of the external circuit, extending the bonding wire to a selected die contact so as to form a wire loop having a low loop height, forming an electrically conductive stitch connection between a second end of the bonding wire and the selected die contact, and repeating the above steps until the one or more die contacts are wire bonded to the one or more external contacts of the external circuit.
In another embodiment of the present invention, a portable fingerprint sensor device is provided. The device includes a finger sensor die that includes a sensor array having one or more die contacts that are wire bonded to one or more external contacts of an external circuit so that a usable portion
Quach T. N.
Sheppard Mullin Richter & Hampton LLP
LandOfFree
System for providing an open-cavity low profile encapsulated... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with System for providing an open-cavity low profile encapsulated..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and System for providing an open-cavity low profile encapsulated... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3184181