Optics: measuring and testing – Inspection of flaws or impurities
Reexamination Certificate
2001-03-05
2004-02-03
Lee, Diane I. (Department: 2876)
Optics: measuring and testing
Inspection of flaws or impurities
Reexamination Certificate
active
06686993
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to a probe card for testing optical micro electromechanical system (MEMS) devices at wafer level. More particularly, the present invention relates to providing an optical test device on a probe card to enable optical and electrical testing of the electrical and optical components of an optical MEMS wafer and characterization thereof at the wafer level.
BACKGROUND OF THE INVENTION
Conventional probe cards have been used to test the electrical properties and functionality of electrical components on a wafer at the wafer level. Conventional testing at the wafer level allows the fabricator to test the wafer to find the defective dies. The identified defective die can then be discarded prior to package, thus increasing the yield rate of the electronic packages. This is desirable because of the significant cost difference between a die and a package, the package costs being the greater of the two.
An example of the top view of a conventional probe card is illustrated in FIG.
1
. As illustrated in
FIG. 1
, a probe card
1
includes an opening
3
through which a plurality of electrical probes
5
extends to contact a wafer under test. In this example, the wafer under test includes optical MEMS devices in the form of rotatable mirrors
7
. The electrical probes
5
are aligned to contact pads on the wafer under test. Moreover, the electrical probes
5
are electrically coupled to a circuit for providing electrical signals to the wafer under test as well as for receiving electrical signals from the wafer under test. The circuit on the probe card
1
uses the signals to determine if the components under test are defective or not. If the components are determined defective, the die associated with the defective components can be discarded. The testing circuit and electrical probes
5
could also be located on the side of the probe card
1
that is adjacent to the wafer under test, thereby eliminating the need for the opening
3
.
FIG. 2
illustrates a side view of the probe card
1
of FIG.
1
. As illustrated in
FIG. 2
, a probe card
1
includes an opening
3
through which a plurality of electrical probes
5
extends to contact a wafer under test
9
. In this example, the wafer under test
9
includes optical MEMS devices in the form of rotatable mirrors
7
. The electrical probes
5
are aligned to contact pads on the wafer under test
9
. Moreover, the electrical probes
5
are electrically coupled to a circuit for providing electrical signals to the wafer under test
9
as well as for receiving electrical signals from the wafer under test. The circuit on the probe card
1
uses the signals to determine if the components under test are defective or not. If the components are determined defective, the die associated with the defective components can be discarded.
As described above, conventional testing of integrated circuits and optical MEMS devices utilize a probe card that tests these circuits at the wafer level. The testing is accomplished by having the electrical probes of the probe card make temporary contact with the device. From the testing, the device can be characterized before it is singulated and assembled into a package, thereby increasing the yield rate of the assembled packages. However, conventional probe cards do not test for the optical performance of the optical MEMS device at the wafer level.
To determine the characterization of a die having optical MEMS devices thereon, both the electrical and optical performance criteria need to be tested. For example, for optical MEMS devices having movable mirrors, both the electrical components controlling the angle or rotation of the mirrors and the optical performance of the mirrors as the mirrors are placed into position need to be tested. Conventionally, the optical performance of optical MEMS devices are tested and characterized after the die having the optical MEMS devices thereon has been singulated from the wafer and assembled into a package. This decreases the yield rates of the assembled packages because defective optical MEMS devices are no being detected until after packaging.
Testing of a MEMS mirror at an individual component level could be done in a manner very similar to what is described above. However, the drawback is that in order to test each mirror chip, the chip would need to be mounted on a suitable fixture that provides a means to supply the mirror's actuation signal. This fixture would then mounted such that a deflectable surface of the mirror was aligned correctly with respect to the incoming laser beam. The mirror would then be tested and if it met specifications it could be assembled and if not acceptable it would be discarded. Testing in this manner would be slow, tedious, and would subject the mirror chips to considerable handling risk that would be compounded by the fragile nature of the mirror's deflectable plate and the very small size of the chip itself (as for example 0.7 mm×1.5 mm).
Probe control stations equipped with device-specific probe cards are used in the semiconductor industry for the wafer level testing of integrated circuits. These machines allow for the rapid and automated testing of a plurality of circuits patterned onto a wafer. Devices tested in this manner can be rejected and appropriately marked at the wafer level if they don't perform to specifications, thus, simplifying the sorting of good from bad parts. Connection to a computer allows for the transfer and storage of the test results for later retrieval and analysis. Such systems have been typically applied to electrical testing of discrete semiconductor devices or semiconductor integrated circuits. What is needed is a method for testing the electrical-optical-mechanical performance of MEMS devices at the wafer level in a manner that is compatible with the high volume manufacturing procedures.
An example of a method of testing the electrical-optical-mechanical performance of MEMS devices at the wafer level in a manner that is compatible with the high volume manufacturing procedures is disclosed in U.S. Pat. No. 6,052,197. U.S. Pat. No. 6,052,197 discloses an electro-optic probe assembly that is used to perform a wafer level test of a torsional micro-machined mirror. In operation, the electro-optic probe assembly is connected to a control station. The electro-optic probe assembly is oriented horizontally and parallel to the surface of a control station wafer support stage.
A wafer is placed on the support stage and held in place with vacuum. The control station provides for vertical translation such that probes can be raised out of contact with the wafer as needed. Prior to testing, the probes are aligned to the wafer surface such that the probes electrically connect to a set of mirror drive pads. Once the probes are in contact with the drive pads, computer-controlled testing of the mirror device proceeds. A laser beam is directed and focused onto a reflective rotatable plate of an individual mirror on the wafer and the mirror plate vectored by a changing electrical charge applied to the mirror drive pads. The tests involve electro-optical-mechanical characterization of return laser light deflection as the mirror plate is vectored, with overall results sent to a computer for storage and data reduction. At the end of the test, the probes and the drive pads are separated.
Although U.S. Pat. No. 6,052,197 discloses the testing of optical MEMS devices at the wafer level, the testing device has many disadvantages. For example, the testing device uses a complex optical path, requiring special beam splitters. The use of such beam splitters increases the cost of a probe card. Also, the inclusion of these optical components in the probe card introduces possible distortion detecting the light. Lastly, the conventional probe cards are not able to provide a test light beam that is parallel to the wafer for testing pop-up mirrors or pop-up optical MEMS devices.
Thus, it is desirable to provide a compact probe card that is capable of electrically and
Karpman Maurice S.
Pizzi, Jr. Nicholas
Analog Devices Inc.
Franklin Jamara A.
Lee Diane I.
Samuels , Gauthier & Stevens, LLP
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