Device and method for testing magnetic switches at wafer-level stage of manufacture

A testing mechanism for testing magnetically operated microelectromechanical system (MEMS) switches at a wafer level stage of manufacture includes an electromagnetic fixture configured to be received in a standard probe ring. The electromagnetic fixture is rotatable, relative to the probe ring, to permit adjustment of orientation of a generated magnetic field relative to the MEMS devices of a subject wafer. The testing mechanism also includes a probe card with probes positioned to contact test pads on the subject wafer. During operation, the probe card is positioned over the wafer to be tested, with the test probes in electrical contact with respective contact pads of the wafer, and the electromagnetic fixture is positioned above the probe card. An electrical potential is applied across the switches on the subject wafer, and the electromagnetic fixture is energized at selected levels of power and duration. Current flow across each switch is measured to determine one or more of: open circuit contact resistance, closed circuit contact resistance, response time, response to switching magnetic field, frequency response, current capacity, critical dimensions, critical angles of magnetic field orientation, etc. Wafer level testing enables rejection of non-compliant switches before the cutting and packaging levels of manufacture.

BACKGROUND

1. Technical Field

The principles of the disclosed invention are related to the testing of magnetically actuated switches formed on semiconductor substrates, and in particular, to the testing o such switches at the wafer level of manufacture, before singulation and packaging.

2. Description of the Related Art

A magnetic switch is an electrical switch that is activated by magnetic attraction or repulsion.FIG. 1Ais a schematic view of a well known prior art magnetic switch100that includes first and second contact plates102,104made from a ferromagnetic material such as, for example, nickel-iron. The first and second contact plates each lie parallel to an X axis, and are offset with respect to each other so that only their respective free ends108,110overlap, and are spaced a small distance apart. Each of the first and second contact plates102,104has a contact terminal106, by which the switch100is coupled to an electrical circuit.

The magnetic switch100is a normally-open type switch that closes when exposed to a magnetic force of sufficient strength.FIG. 1Bshows the switch100in proximity to a magnet112, with the magnetic force of the magnet depicted as lines114that arc from the north pole to the south pole of the magnet. The magnetic north and south poles of the magnet112define a polar axis P of the magnet. The magnet112is shown positioned near the switch100with its polar axis P lying substantially parallel to the X axis, and thus also parallel to the first and second contact plates102,104. When the magnet112and the switch100are brought into close proximity, the ferromagnetic material of the first and second contact plates102,104is exposed to the magnetic force, which induces a magnetic polarity in the first and second contact plates102,104that is opposite the polarity of the magnet112. Thus, when the magnet112has a north pole on the left and a south pole on the right, each of the first and second contact plates102,104has a north pole on the right and a south pole on the left. Because of the relative positions of the first and second contact plates102,104, the left-hand end108of the first contact plate102is adjacent to the right-hand end110of the second contact plate104. Under the influence of the magnet112, the end108of the first contact plate102is polarized as a south pole, while the end110of the second contact plate104is polarized as a north pole. Accordingly, as the first and second contact plates102,104flex slightly, the ends108,110of the first and second contact plates are drawn together by magnetic attraction, thereby closing the switch100.

If the magnet112is positioned so that its polar axis P is perpendicular to the X axis of the switch100, as shown inFIG. 2A, the direction or polarity of the magnetic force will be balanced across the contact plates102,104, so that the contact plates will not become polarized as described with reference toFIG. 1A. Thus, in the position shown inFIG. 2A, the switch will be in the open position. Rotation of the magnet112around a Y axis that lies perpendicular to the X axis brings the magnet toward the parallel position shown inFIG. 1B. The Y axis is perpendicular to the page, and not shown, but can be understood from the axis markings onFIG. 1A. As the magnet114is rotated away from the perpendicular position, as shown inFIG. 2B, at some angle of rotation, sufficient polarity will be induced in the contact plates102,104to cause the switch100to close.

FIGS. 3A and 3Bshow a normally-closed type magnetic switch120that includes first and second contact plates122,124, each having a contact terminal126. The first and second contact plates122,124lie parallel to each other and are substantially coextensive. As shown inFIG. 3A, ends128,130of the first and second contact plates122,124are in electrical contact with each other under normal conditions.FIG. 3Bshows the magnetic switch120in an actuated condition. As described above with reference to the magnetic switch100, when the first and second contact plates122,124are exposed to the magnetic energy of a magnet oriented as shown inFIG. 1B, they become magnetically polarized in a similar fashion. However, because the first and second plates122,124are coextensive, their respective north and south poles are directly opposite each other. The magnetic repulsion between the ends128,130causes the first and second contact plates to flex away from each other, opening the switch120.

Turning now toFIGS. 4 and 5, a magnetic switch140is shown, which is one of a large plurality of switches formed on a semiconductor wafer142using methods that are well known in the art.FIG. 4shows a perspective view of a portion of the wafer142, whileFIG. 5is a cross-sectional view of the switch140, taken along lines5-5ofFIG. 4. For the sake of clarity, it will be assumed that any magnetic switches discussed hereafter are positioned so that their longitudinal axes lie parallel to the X axis, and that the substrate surfaces on which they are positioned lie parallel to a plane defined by the X and Y axes, with the Z axis being perpendicular to that plane.

The switch140is one of a broad class of devices that are commonly referred to as microelectromechanical systems (MEMS) devices. The particular structure of the switch140is merely exemplary, inasmuch as there are a number of different configurations for MEMS type magnetic switches. The switch140includes a cavity144formed in the upper surface of the wafer142, over which a dielectric layer146is formed. A conductive layer148is positioned over the dielectric layer146and a channel150is provided in the conductive layer148to electrically isolate the two sides of the switch140. A first contact plate152of ferromagnetic material is positioned in the cavity144, with a layer of conductive material154positioned on an upper surface thereof. A second ferromagnetic-material contact plate156is suspended over the surface of the substrate142by a pair of springs158extending from the second contact plate156to respective anchors160positioned on the surface of the substrate142. Finally, a segment of a conductive layer162is positioned on an underside of an end164of the second contact plate156, where it will touch the upper surface of the first contact plate152when the switch140is activated.

The ferromagnetic material of the first and second contact plates152,156behaves substantially as described with reference to the first and second contact plates102,104ofFIGS. 1A and 1B. When the switch140is activated, the second contact plate156rotates around an axis defined by the springs158to bring its end164into contact with the upper surface of the first contact plate152. The material of the conductive layers148,154, and162is selected to resist formation of oxides that could interfere with a good electrical contact upon closing, such as, e.g., gold. There may be as many as 6,000 to 8,000 switches formed on a single wafer.

During the manufacturing process, as shown inFIG. 6, following the formation of the switches on the semiconductor material wafer142, a second wafer170is positioned above the first wafer142and bonded to the surface thereof, to form a composite wafer171. The second wafer170includes a first plurality of cavities172in positions that correspond to each of the switches140so that each of the switches is hermetically sealed within an enclosed chamber. A second plurality of cavities174is formed in positions corresponding to contact terminals176on the first wafer142. After the second wafer170is bonded to the first wafer142, the second wafer is thinned, by removing a portion of the upper surface, at least far enough to open the second cavities174, as indicated by dotted line T. Thereafter, the composite wafer171is cut into individual dice180, which removes material between the kerf lines K ofFIG. 6. Each die180now contains a single, hermetically sealed, magnetically operated switch, which is thereafter packaged according to requirements of a particular application.

InFIG. 7, the exemplary die180is mounted to a paddle186of a lead frame and electrically coupled via wire bonding to leads184, all of which is encapsulated within a flat-pack type package182. Following the packaging step, each switch140is tested for conformance to a specific set of performance parameters.

BRIEF SUMMARY

According to one embodiment, a testing mechanism is provided, for testing magnetically operated micro-electro-mechanical system (MEMS) switches at a wafer level stage of manufacture. The mechanism includes an fixture configured to be supported in a probe ring of a wafer prober. The fixture supports an electromagnetic field generator that can be rotated, relative to the probe ring, to permit adjustment of orientation of a generated magnetic field relative to the MEMS devices of a subject wafer. The testing mechanism also includes a probe card with probes positioned to contact test pads on the subject wafer. During operation, the probe card is positioned over the wafer to be tested, with the test probes in electrical contact with respective contact pads of the wafer, and the electromagnetic fixture is positioned above the probe card. An electrical potential is applied across the switches on the subject wafer, and the electromagnetic fixture is energized at selected levels of power and duration. Current flow across each switch is measured to determine one or more of: open circuit contact resistance, closed circuit contact resistance, response time, response to switching magnetic field, frequency response, current capacity, critical dimensions, critical angles of magnetic field orientation, etc. Wafer level testing enables rejection of non-compliant switches before the cutting and packaging levels of manufacture.

According to another embodiment, a method is provided for testing various parameters of one or more magnetically operated MEMS devices at the wafer stage.

DETAILED DESCRIPTION

Most semiconductor devices undergo wafer-level testing prior to singulation and packaging of the individual chips. Wafer-level testing typically involves testing the electronic function of each of the individual devices on a wafer, either one at a time, or in larger groups of devices. For example, on a memory chip, each address location on the chip is tested, as well as the logic circuits, power regulators, buffering circuits, etc. At this level, many electronic faults in such a device can be detected and repaired prior to packaging, and if repair is not possible, that particular device can be noted, so that after the wafer is cut, the defective devices can be discarded before being packaged. In a similar fashion, many, but not all, MEMS devices also undergo wafer-level testing prior to packaging. There are currently no known wafer-level test systems or methods for testing magnetically operated MEMS devices, including switches such as those described with reference toFIGS. 4-7. According to various embodiments of the invention, a device and a method are provided for testing magnetic switches at the wafer level.

FIGS. 8 and 9show a testing device200according to an embodiment of the invention.FIG. 8shows a perspective view of the device200(shown partially cut away to more clearly show other elements), andFIG. 9shows the testing device in a partial cross section.

Referring jointly toFIGS. 8 and 9, a wafer222is to be tested by the testing device200. The testing device200comprises a fixture202and a probe card204, The fixture202includes an electromagnetic field generator206coupled thereto. The field generator206includes a electromagnet that has a polar axis P, and is configured to be selectively rotatable around an axis R that lies perpendicular to the axis P and parallel to the Y axis. The fixture202is configured to be received by a probe ring208of a wafer prober, as shown inFIG. 9. The electromagnetic field generator206is supported by brackets210affixed to the fixture202. A stepper motor212controls the angular position of the electromagnet field generator206with respect to the probe card204.

The probe card204is supported below the fixture202by a pair of probe card brackets214so as to be held in a fixed position relative to the fixture. The probe card204and brackets214are configured to be separable so that a variety of different probe cards can be selectively coupled to the fixture, according to the configuration of the particular wafer to be tested. The probe card204includes a plurality of probe pins216coupled thereto, in positions selected to contact individual test probe contact points on a semiconductor wafer. The exact number of probe pins is determined, at least in part, by the number of switches to be tested simultaneously, as will be discussed later.

FIG. 9also has a wafer chuck220with the semiconductor wafer222positioned thereon. The wafer chuck220and the probe ring208are components of a wafer prober machine such as is commonly used for wafer-level testing of semiconductor devices. Such a machine is typically configured to receive and interface with a wide variety of specialized probe cards, so as to be capable of testing many different semiconductor devices merely by coupling the required probe card to the probe ring, and providing the wafer prober with the appropriate programming.

The wafer chuck220is movable in the X and Y axes, and rotatable around an axis that lies perpendicular to the horizontal plane defined by the X and Y axes. The probe ring208is movable in the Z axis. The wafer prober includes a control unit230which controls movement of the chuck220and probe ring208, and also includes electrical terminals240for electrically coupling the prober to a probe card. The testing device200includes a plurality of coupling terminals242configured to mate with the coupling terminals240of the wafer prober. The plurality of coupling terminals242includes: a pair of terminals that are coupled to the electromagnetic coil of the electromagnetic field generator206, and by which the field generator is energized; leads of the stepper motor212, by which the angular position of the electromagnet field generator206is controlled; and a plurality of leads coupled to respective ones of the probe pins216, by which the testing device200and the control unit230can be coupled to individual switches on a semiconductor wafer.

FIG. 10shows a portion of the wafer222in plan view, showing magnetic switches234formed thereon, arranged in columns of switches having common longitudinal axes, and rows of switches lying in parallel columns. The magnetic switches234are structured similar to those described with reference toFIGS. 4-8, except that test probe contact points236are provided, to enable a secure contact by the probe pins216during wafer level testing. In the embodiment shown, the test probe contact points236are positioned between the scribe lines S of the wafer222, which define the kerf that will be removed when the wafer is sawn into individual dice180. Thus, the additional area of the test probe contact points236will require little or no net increase in the footprint of each device, and so may not reduce the number of devices that can be made on the wafer222.

Magnetic switches are designed to actuate when exposed to a magnetic field of a specific strength, which is selected according to the requirements of a particular application. The field strength is a function of the strength of the magnet, the distance of the magnet from the switch, and the angle of the magnet's polar axis with respect to the longitudinal axis of the switch.

FIG. 11shows a graph that depicts the response of an exemplary magnetic switch to a field generated by a magnet as the angle of the polar axis of the magnet changes, where the switch has a nominal switching angle of 14 degrees. While the magnet is positioned so that the switch is outside the outer lines O, the switch will not actuate; while the switch is inside the inner lines I, the switch will actuate; and while the switch is between the inner lines I and the outer lines O, the switch will remain unchanged from a previous condition. Thus—assuming the magnet is about 20-25 mm from the switch—as the magnet rotates from 0 degrees (i.e., perpendicular) through about 14 degrees, the switch will actuate, as it crosses an inner line I. As the magnet rotates back toward zero degrees, the switch will release at around 12 degrees, as it crosses an outer line O.

It will be recognized that if a magnet is positioned above a wafer with many switches formed thereon, the angle of the magnet relative to each of the switches will be different, according to the position of the particular switch on the wafer. the number of switches that can be accurately tested simultaneously will be limited to a small number of switches that are close together. Additionally, that number may vary, depending upon the permissible tolerances for a given design. Thus, if the separation between devices on the wafer is equivalent to 4 degrees of arc, with respect to the rotational axis R of the electromagnetic field generator206, and if the device under test must switch within one degree of its rated value, no more than one switch along the X axis can be tested at a time. On the other hand, if deviations from nominal of greater than two degrees are acceptable, it may be possible to test more than one switch simultaneously.

Operation of the testing device will be described hereafter with reference to an exemplary magnetic switch having a specific set of design parameters, e.g., magnetic switch234a, as shown inFIG. 10. It will be recognized that, in practice, magnetic switches are designed and manufactured to meet a very wide range of applications, each of which has a particular set of parameters, and that those parameters will vary as widely as the applications. Thus, the parameters listed hereafter in describing the test procedures are merely illustrative. The testing device200can be adapted to accommodate many different design requirements. Usually, this accommodation is only a matter of modifying the program executed by the control unit.

In operation, the fixture202of the testing device200is coupled to the probe ring208of a wafer prober, with electrical leads from the electromagnetic field generator206and the probe card204coupled to the control unit230. A semiconductor material wafer222is placed on the chuck220, where it is held in place by suction. Using an optical alignment system, which is well known in the art, the control unit230adjusts the position of the chuck220in the X and Y axes and the angle of rotation θ, to align the wafer222under the probe card so that the probe pins216are positioned over the test probe contact points236of a first one of the switches234, as shown inFIG. 10. The probe ring208is lowered until the probe pins216contact the test probe contact points236to electrically couple the switch234with the control unit230.

FIG. 10shows two test probes216, which are in contact with the test probe contact points236of switch234a. Alternatively, two probe pins216can be provided and positioned to contact each contact points236. This increases the likelihood of a secure contact with the contact points and so reduces the occurrence of false error detection, or the need for retesting.

With the probe pins216positioned as shown inFIG. 10, the test of the magnetic switch234ais executed. First, with the polar angle of the electromagnetic field generator206at an angle that is at least equal to, and preferably greater than, the minimum effective switching angle for the particular switch design, the field generator is energized in a series of pulses. This causes the switch to open and close several times, which scrubs away debris or material that may have been left on the conductive layers between the contact plates of the switch234during the manufacturing process. After this first step, the remaining test procedures are not limited to a specific order, although there may be some benefit in performing some tests before, during, or after other tests.

In a first parametric test, the field generator206is positioned at the rated switching angle of the switch, e.g., 13 degrees, and provided with a current of 250 mA, to produce a magnetic field equal to the rated field strength. The current is switched on and off at a pulse rate of 1 Hz, and a potential of 20 mV is applied across the contact points of the switch236a. A resulting pulsed current through the switch is detected, indicating that the switch actuates at the rated field strength and angle, and at the rated frequency. Current flow across the contact points is measured while the switch is closed to determine its closed circuit contact resistance, and while the switch is open to determine its open circuit contact resistance. Time lags between formation of the magnetic field and switch actuation, and between collapse of the field and switch release, are measured to determine response time to open and close.

Next, a ramping current is applied to the field generator206, rising from zero to 250 mA, while a potential is maintaining across the contact points. As current rises, resistance across the contact points is monitored. From the current level at which resistance drops below the rated closed circuit contact resistance, the switch's minimum field strength for switching can be derived. After the switch closes, the ramp is reversed, so that the current is reduced back to zero. From the current level at which the resistance rises above the rated open circuit contact resistance, the switch's hysteresis value can be derived.

Because field strength and magnet angle are correlated, the ramping current procedure discussed above can in some cases also be used to determine the minimum effective switching angle of the switch. Alternatively, the field generator206is moved to a zero angle, then, while producing the appropriate field strength, rotated away from zero until the switch closes, to determine the minimum effective switching angle.

After the test is complete, the probe ring208is raised, which lifts the probe pins216from the surface of the wafer222. Contact by probe pins216with contact points236leaves distinct marks when the probe pins are removed. Not only can the precise positions of contact be determined from the marks, but also whether sufficient pressure was applied to establish a full connection. Therefore, a final test is to examine the contact points236to confirm that the probe pins were correctly coupled while testing was performed. In particular with respect to switches that have failed to meet nominal parameters, a final inspection of the contact terminals may show that the failure was a probe pin positioning failure, rather than a manufacturing defect of the device.

According to one embodiment, if a switch fails to meet one of the test parameters, the remaining tests for that switch are omitted, and the position of the failed switch is noted, so that the switch can be discarded once the wafer is singulated. As testing of the switches of one or a number of wafers proceeds, it may be determined that the switches of a particular production run tend to fail to meet a required value for one design parameter more frequently than the remaining parameters. In such a case, the order in which the tests are performed on the remaining switches may be modified so that that one parameter is tested early in the series. Thus, a bad switch will be more likely to fail early in the process, reducing the time spent testing bad switches.

According to another embodiment, the test is continued, even if a switch fails to meet certain ones of the parameters. For example, if a switch fails to close at a rated angle of 13 degrees, the angle of the field generator is increased until the switch closes, or it becomes clear it will not close. If the switch closes at 20 degrees, this value is logged, and when the switch is singulated and packaged, it is re-rated to close at 20 degrees. The manufacturer can thereafter sell that switch to a customer that requires a switch rated at 20 degrees. Other parameters, such as field strength and open and closed circuit contact resistance can likewise be measured, and the switch re-rated, where the switch fails to meet the original rated values.

Preferably, most or all of the operations are performed automatically by a wafer prober, as directed by its programming. This is especially true with respect to production runs in which large numbers of switches are manufactured and tested. Such programming is within the abilities of one having ordinary skill in the art. On the other hand, any of the procedures can be performed manually or under direct control of a testing machine by an operator. In particular, this may be preferable in research and development applications.

According to an embodiment, the electromagnetic field generator206is gimbaled, so as to be rotatable around the axis R, as described above, and also around a second axis, perpendicular to the axis R and parallel to the X axis. Additionally, the probe card204is configured to couple with a plurality of switches that lie in a common row, e.g., switches234b-234f. The field generator206is positioned for testing one of the switches in the row, e.g., switch234b, and the switch is tested as described above. Following completion of the testing, the field generator206is rotated around the second axis until its polar axis P is substantially aligned a longitudinal axis of the next switch in the row, whereupon the test is repeated. This procedure is repeated for each of the plurality of switches. In this way, a larger number of switches is tested each time the probe card204is positioned, and the probe card is repositioned fewer times during testing of all of the switches of the wafer. Alternatively, if groups of more than one switch in a given row are tested simultaneously, the field generator206is rotated around the second axis until its polar axis P is substantially aligned midway between the longitudinal axes of the first and last switches of the next group of switches that are to be tested.

Turning now toFIG. 12, a testing fixture250is shown, in accordance with another embodiment. The fixture250is similar in most respects to the fixture202described with reference toFIGS. 8 and 9. However, the electromagnetic field generator252is coupled to the fixture250by a hinge254and a bracket256. The bracket256includes a slot258which is traversed by a pin260that is coupled to the fixture250. Rotation of the field generator252on the hinge254is constrained to a range by travel of the pin260in the slot258. As shown inFIG. 12, rotation is limited to a range between zero degrees and 13 degrees, shown in dotted lines. According to other embodiments, the bracket256is configured to permit rotation to larger angles, and is provided with detents at selected angles so the field generator can be rotated to one of the selected angles and will hold itself at that angle until it is moved to a different angle.

In operation, a user moves the field generator to a selected angle, where it remains for the duration of a wafer test series. It is not always necessary to measure the minimum effective switching angle of a switch, but only to confirm actuation at the rated angle. Thus, it is not essential that the magnet be rotatable during a test.

Testing of a magnetic switch has been described with reference to a particular series of examples of test procedures. The specific parameters are exemplary, as are the procedures themselves. In practice, a wafer level test of one or a plurality of magnetic switches can include any, all, or none of the procedures described above, and can also include procedures not described above. Accordingly, the claims are not limited by the procedures described, except where specifically recited.

Embodiments have been described with reference to a wafer prober, a probe ring, and a wafer chuck. These devices are among a wide range of devices that are commercially available for examining and testing wafers. In many cases, substantially identical devices are known by different names, and in other cases, devices having distinct appearances and operation overlap in function to the extent that they can perform some or all of the functions described above with reference to a wafer prober, a probe ring, and a wafer chuck. Accordingly, these terms are to be considered generic, and are to be construed broadly to refer to any device or system that operates in the manner described. In particular, any device, or combination of devices intended to be used together, that incorporate means for supporting a wafer, means for interchangeably receiving and supporting a variety of specialized equipment related to testing or examining the wafer, and means for precisely positioning the wafer and the specialized equipment relative to each other are considered equivalent to the wafer prober, the wafer chuck, and the probe ring described below.

Where a claim limitation recites a structure as an object of the limitation, that structure itself is not an element of the claim, but is a modifier of the subject. For example, in a limitation that recites “a plurality of probe pins configured to make contact with respective terminals of a wafer of semiconductor material,” the wafer is not an element of the claim, but instead serves to define the scope of the term probe pins. Additionally, subsequent limitations or claims that recite or characterize additional elements relative to the wafer do not render the wafer an element of the claim.

The abstract of the present disclosure is provided as a brief outline of some of the principles of the invention according to one embodiment, and is not intended as a complete or definitive description of any embodiment thereof, nor should it be relied upon to define terms used in the specification or claims. The abstract does not limit the scope of the claims.