Testing integrated circuits on a wafer using a cartridge with pneumatic locking of the wafer on a probe card

An embodiment of a cartridge is proposed for testing integrated circuits on a wafer with the wafer that has a wafer front surface with a plurality of terminals of the integrated circuits. The cartridge includes a probe card, which has a card front surface with a plurality of probes for contacting the terminals of the integrated circuits electrically, and a card back surface opposite the card front surface. Locking means is provided for locking the wafer on the probe card. The locking means includes one or more through-holes that cross the probe card from the card front surface to the card back surface; sealing means is arranged on the card front surface around the probes and the through-holes. In this way, a substantially airtight chamber is defined by the probe card, the sealing means and the wafer when the wafer front surface abuts against the sealing means. Coupling means is arranged on the card back surface. The coupling means is used to couple the cartridge with pneumatic means for creating a depression in the chamber, by removing air from the chamber through the through-holes; the same coupling means is also used to seal the airtight chamber when the cartridge is decoupled from the pneumatic means.

PRIORITY CLAIM

The present application is a national phase application filed pursuant to 35 USC §371 of International Patent Application Serial No. PCT/EP2008/051790, filed Feb. 14, 2008; which further claims the benefit of European Patent Application 07102587.8, filed Feb. 16, 2007; all of the foregoing applications are incorporated herein by reference in their entireties.

RELATED APPLICATION DATA

This application is related to U.S. patent application Ser. No. 12/527,418 entitled TEST OF ELECTRONIC DEVICES AT PACKAGE LEVEL USING TEST BOARDS WITHOUT SOCKETS, filed on Aug. 14, 2009, and which is incorporated herein by reference in its entirety.

TECHNICAL FIELD

An embodiment of the present invention relates to the test field. More specifically, an embodiment of the present invention relates to the test of integrated circuits on a wafer.

BACKGROUND

Integrated circuits are generally subjected to a test process in order to verify their correct operation; this is important to ensure high quality of a manufacturing cycle of the integrated circuits. The test process may be aimed at identifying defects that are either evident or potential (i.e., which might occur after a short life of the integrated circuits). In the latter case, the integrated circuits are commonly tested under stress conditions; a typical example is the burn-in test, which consists of making the integrated circuits work for some hours at very high or very low temperature (such as ranging from −50° C. to +150° C.), in order to simulate a long period of operation of the same integrated circuits at room temperature (i.e., 25° C.-50° C.).

The test process may be performed when the integrated circuits are still on a wafer of semiconductor material (where a high number of identical integrated circuits are formed in different areas thereof). In this way, it is possible to save the costs of any next manufacturing steps (such as for the packaging) when the integrated circuits are defective; moreover, a high number of integrated circuits included in each wafer may be tested concurrently. In some cases, it is also possible to fix certain defects of the integrated circuits (so as to avoid their rejection).

Typically, the test process is implemented by means of a prober. The prober interfaces with the wafer by means of a probe card, which is provided with multiple probes for contacting corresponding terminals of the integrated circuits electrically. For this purpose, the wafer is locked on a chuck plate (for example, by means of vacuum grooves). The prober moves the chuck plate until the wafer is aligned with the probe card (so that each terminal of the integrated circuits is coupled with a corresponding probe); the prober then applies stimulus signals to the integrated circuits and receives corresponding response signals (through the probes). Particularly, availability of a full wafer probe card allows testing all the integrated circuits of the wafer at the same time. However, once the wafer has been aligned with and contacted by the (full wafer) probe card, the prober remains idle for the duration of the entire test process of the wafer (typically several hours), but at the same time completely unavailable for any other operation.

In order to solve this problem, WO-A-01/04643, which is incorporated by reference, proposes the use of a test cartridge. Particularly, the probe card is mounted in a movable manner on a corresponding plate. A mechanical system locks the chuck plate and the probe plate together (for example, by means of a pair of jaws). The position of the probe card with respect to the chuck plate (and then to the wafer) is controlled by means of a pneumatic system. For this purpose, in a proposed implementation an O-ring defines a sealed chamber between the chuck plate and the probe card; holes are formed in the probe card for controlling an air pressure in the chamber, so as to advance or retract the probe card with respect to the chuck plate. In this way, the combined actions of the mechanical system and the pneumatic system allow obtaining the desired pressure between the probe card and the wafer.

In operation, the prober aligns the wafer with the probe card and then locks them in the correct position (by means of the above-described mechanical and pneumatic systems). The cartridge is then removed from the prober so at to make it available for assembling another cartridge. A batch of cartridges so obtained is then provided to a distinct test machine. Particularly, the cartridges are placed into a burn-in chamber, and they are connected electrically to a test circuit (outside the burn-in chamber) by means of corresponding connectors arranged on the probe plates. During the test process, the temperature of the wafers is controlled by means of a heating or cooling fluid that circulates through channels formed in the chuck plates. This allows exploiting the (very expansive) mechanical section of the prober at its best—since it is used only for the assembling of the cartridges; moreover, it is now possible to test more wafers concurrently in the same test machine.

However, the structure of the cartridge disclosed in WO-A-01/04643 (and especially its mechanical system for locking the chuck plate and the probe plate together) is relatively complex and difficult to actuate. Moreover, each cartridge includes a dedicated chuck plate (required to couple the wafer with the probe card by means of the above-described mechanical and pneumatic systems); this has a detrimental effect on the cost of the cartridge.

An alternative solution is disclosed in US-A-2002/0003432, which is incorporated by reference. In this case as well, a plurality of cartridges are assembled off-line in a prober, and then provided to a distinct test machine. However, each cartridge is now obtained by locking the probe card and the wafer with a vacuum clamp. Particularly, the clamp is formed by a first mechanical member that is closed around a back surface of the probe card (opposite the one with the probes) through a first seal, and a second mechanical member that is closed around a back surface of the wafer (opposite the one with the terminals) through a second seal; the two mechanical members are coupled by means of a third seal. The clamp has a port, which is used to create the vacuum in a region defined by the probe card, the wafer and the clamp (so as to obtain the desired pressure between the probe card and the wafer). A mechanical latch (such as a wing nut) maintains the clamp tightened (even if vacuum is lost during transport of the cartridge). Springs may be associated with the probes to remove heat from a predefined internal region of the wafer where the integrated circuits are formed (thereby directing the cooling away from its peripheral region, which generates little or no heat); moreover, this structure leaves access to most of the back surface of the wafer for cooling (with the exception of the part covered by the clamp).

However, the above-described cartridge may generate unwanted bowing at an edge of the wafer; indeed, as pointed out in the same document, in order to avoid this problem the probes should extend to an edge of the probe card or the seal (between the clamp and the wafer) should extend to a last row of probes. Moreover, the structure of this cartridge remains relatively complex and difficult to actuate; particularly, its assembling requires a number of mechanical actions to mount the clamp and to tighten the latch. In any case, the solution disclosed in US-A-2002/0003432 cannot be implemented in standard probers (wherein the wafer is aligned with the probe card by acting on the chuck plate); indeed, in this case an optical alignment system based on glass reticles provided in the probe card is required.

All of the above maintains the overall cost of the test process relatively high; this drawback limits the use of the test process, and accordingly lowers the level of quality and reliability in the production of the integrated circuits.

SUMMARY

In its general terms, an embodiment of the present invention is based on the idea of coupling the wafer with the probe card directly.

More specifically, an embodiment of the invention provides a cartridge for testing integrated circuits on a wafer—with the wafer that has a wafer front surface with a plurality of terminals of the integrated circuits. The cartridge includes a probe card, which has a card front surface with a plurality of probes for contacting the terminals of the integrated circuits electrically, and a card back surface opposite the card front surface. Locking means is provided for locking the wafer on the probe card. The locking means includes one or more through-holes that cross the probe card from the card front surface to the card back surface; sealing means is arranged on the card front surface around the probes and the through-holes. In this way, a substantially airtight chamber is defined by the probe card, the sealing means and the wafer—when the wafer front surface abuts against the sealing means. Coupling means is arranged on the card back surface. The coupling means is used to couple the cartridge with pneumatic means for creating a depression in the chamber, by removing air from the chamber through the through-holes; the same coupling means is also used to seal the airtight chamber when the cartridge is decoupled from the pneumatic means.

As a further improvement, spacing means may be used to maintain a minimum distance between the wafer and the probe card.

For this purpose, a plurality of spacers (distributed substantially uniformly throughout the probes) may be provided.

A further improvement may be obtained by providing means for restoring the depression in the chamber (in response to a leakage of air into it when the cartridge is decoupled from the pneumatic means).

This result may be achieved by means of a flexible cap that defines a further chamber on the back surface of the probe card.

Moreover, it is possible to add removable protective means (such as a cap) for enclosing the wafer.

Another embodiment of the invention proposes a prober for assembling those cartridges.

A further embodiment of the invention proposes a test machine for testing the wafers included in the cartridges (wherein the wafers are conditioned thermally by acting on their exposed surfaces).

In an embodiment, this result is achieved by means of a conditioning fluid.

A further improvement may be obtained by controlling the conditioning of different areas of the exposed surfaces of the wafers selectively.

The local temperature of the exposed surfaces may be monitored so as to control their conditioning accordingly.

For example, this result may be achieved by means of an infrared camera.

In an embodiment, the conditioning fluid is supplied to different areas of the exposed surfaces of the wafers by means of corresponding nozzles (which can be driven individually).

A different embodiment of the invention proposes a corresponding method.

DETAILED DESCRIPTION

With reference in particular toFIG. 1, a prober100is illustrated. The prober100includes an alignment station105. The alignment station105is formed by a recess having a platform on its bottom, wherein a chuck plate110is mounted. The chuck plate110is used to lock a wafer of semiconductor material115, wherein a high number of identical or substantially identical integrated circuits to be tested are formed. For this purpose, the chuck plate110is provided with grooves on its upper surface (where the wafer115is placed); those grooves are used to create the vacuum between the chuck plate110and the wafer115, so as to hold the wafer115firmly on the chuck plate110.

The test of the integrated circuits of the wafer115involves coupling it with a probe card120, which is provided with multiple probes for contacting corresponding terminals of the integrated circuits electrically. The probe card120may be mounted on a cartridge125to facilitate its handling. The prober100aligns the wafer115with the probe card120. Particularly, the prober110moves the chuck plate115(and then the wafer110held on it) laterally, until the terminals of the integrated circuits of the wafer115are coupled with the probes of the probe card120. Typically, this process is carried out in two steps. At first, the wafer115is aligned with the probe card120coarsely—for example, by exploiting reference terminals of the wafer115(with the chuck plate115that is moved until dedicated probes of the probe card120contact the reference terminals, as detected by the receipt of a predefined response signal by the reference probes). The wafer115may then be aligned with the probe card120finely—for example, by means of optical systems (such as of the laser or infrared type).

The prober100is also provided with an external pneumatic system (not shown in the figure), which is used to implement the locking of the wafer115on the chuck plate110; the same pneumatic system is also used to lock the wafer115on the probe card120after their alignment (as described in detail in the following). Operation of the prober100is controlled by means of a console135.

Considering nowFIGS. 2A and 2Btogether, the probe card120is formed by a Printed Circuit Board (PCB)205, which consists of an insulating substrate with (single or multiple layer) conductive tracks for routing the desired signals; typically, the insulating substrate is made of a material with a Coefficient of Thermal Expansion (CTE) similar to the one of the wafer. A high number of probes210(for example, from a few hundreds to several thousands) project downwards from a lower (front) surface of the board205; when the probe card120is aligned and coupled with the wafer115, the probes210electrically contact corresponding terminals212of the integrated circuits formed on an upper (front) surface of the wafer115(for example, consisting of pads or bumps). For this purpose, a compliant interposer (not shown in the figure) is generally provided between the board205and the probes210, so as to compensate any warp of the wafer115. Moreover, each probe210may be formed with an elastic core that is covered by a metal layer in the form of a pyramid (for the pads) or of a frusto-pyramid (for the bumps), as described in WO-A-2006/066620 (the entire disclosure of which is herein incorporated by reference); in this way, the probes210may also compensate any non-homogeneity of the terminals212individually.

One or more through-holes213cross the board205from its lower surface to its upper (back) surface; the through holes213(for example, from some tens to a few hundreds, each one with a diameter ranging from 0.1 mm to 2 mm) may be concentrated in a central area of the board205. A seal215(for example, consisting of an O-ring—i.e., a toroidal element made of elastomer) is arranged on the lower surface of the board205; the seal215surrounds all the probes210and the through holes213(in plant view), and it has a height (defined by a diameter of its cross-section) substantially the same as the one of the probes210(such as from some tens of μm to a few mm). The board205may also be provided with spacers220that project downwards from its lower surface (within the seal215in plant view). For example, the spacers220consist of some tens of pins, which are uniformly distributed throughout the probes210; the spacers220have a height slightly lower than the one of the probes210(such as about 70-90% thereof). When the wafer115is placed with its upper surface abutting against the seal215, a substantially airtight chamber225is formed between the probe card120(above), the wafer115(below) and the seal215(laterally).

The probe card120is mounted on a frame235of the cartridge125. For example, the frame235includes an inner ring and an outer ring that are connected through a series of spokes; the probe card120is fastened to the inner ring (for example, by means of a few bolts), while the outer ring is coupled with a bearing240. The bearing240in turn includes a ring matching the outer ring of the frame235, which rings are fastened together (for example, by means of a few tap screws); the bearing240also includes two lateral handles, which facilitate the picking up of the cartridge125.

A flexible cap245is arranged on the upper surface of the board205(within the inner ring of the frame235); the flexible cap245encloses all the through-holes213, so as to define a compensation chamber250for the airtight chamber225. A suction duct255extends from the flexible cap245to the bearing240; the duct255ends with a nipple (on the ring of the bearing240) for coupling the compensation chamber250—and then the airtight chamber225—with the pneumatic system of the prober; the nipple is provided with a one-way valve that prevents any leakage of air through the duct255(when the nipple is decoupled from the pneumatic system).

A protective cap260closes the ring of the bearing240at its bottom, so as to enclose the wafer115completely. A snap fitting mechanism265is used to couple the protective cap260with the bearing240in a removable manner.

In operation, the cartridge125is placed against the wafer115and it is connected to the pneumatic system of the prober (through the nipple of the duct255). The wafer115is then aligned with the probe card120as usual (by moving the chuck plate wherein it is held).

In an embodiment of the invention, once the alignment of the wafer115has been completed, the pneumatic system of the prober removes air from the airtight chamber225(through the duct255, the compensation chamber250and the through-holes213). In this way, a depression is formed in the airtight chamber225(and in the compensation chamber250as well). The difference between the (internal) pressure of the airtight chamber225and the (external) atmospheric pressure generates a force that presses the wafer115against the probe card120, in opposition to the elastic force due of the compression of the seal215(with the same pressure difference that also makes the flexible cap245collapse against the upper surface of the board205). More precisely, the force applied to the wafer115is:
F=(Pe−Pi)*A
where F is the force, Pe is the external pressure, Pi is the internal pressure, and A is the area of the wafer115within the airtight chamber225. Typically, the pneumatic system of the prober controls the internal pressure of the airtight chamber225to obtain a force of the order of 0.5-2 kN (for example, by means of an internal pressure of about 20-80 kPa for an area of some hundreds of cm2). The cartridge125is then disconnected from the pneumatic system of the prober (with the valve of the duct255that automatically seals the chambers225and250). The cartridge125is lifted, so as to carry away the wafer115now fastened to the probe card120(being previously released from the chuck plate of the prober).

An embodiment provides an optimal locking of the wafer115on the probe card120. This result is achieved in a very simple manner; particularly, the wafer115is locked on the probe card120directly, so that no additional mechanical member is required. This strongly simplifies the structure of the cartridge125, and then its cost.

Moreover, the chuck plate110now remains in the prober100. Therefore, a single chuck plate110may be used for whatever cartridges125—since it is only used to lock the wafer115during its alignment with the probe card120; this further contributes to reduce the cost of the cartridge125. At the same time, the cartridge125leaves a lower (back) surface of the wafer115entirely exposed; this facilitates the control of its temperature during the test process (as it will be apparent in the following).

At the same time, the operations for assembling the cartridge125in the prober are strongly simplified; indeed, no mechanical action is required, apart from coupling/decoupling the cartridge125with the pneumatic system. Moreover, the cartridge125may be assembled in standard probers (wherein the wafer is aligned with the probe card by acting on the chuck plate), without requiring any special alignment system.

All of the above substantially lowers the overall cost of the test process; this fosters the widespread diffusion of the test process, and accordingly increases the level of quality and reliability in the production of the integrated circuits.

In the above-described embodiment of the invention, when the wafer115is pressed against the probe card120, it abuts against the spacers220(after compressing the higher seal215). The spacers220then maintain a minimum distance between the wafer115and the probe card120, so as to avoid any damage to the probes210. Moreover, the distribution of the spacers220improves the planarity of the wafer115; this provides the correct electrical contact with the probe card120.

Once the cartridge125has been removed from the prober, the protective cap260can be mounted on the bearing240, so as to close the region around the wafer115. In this way, the protective cap260shields the wafer115during its moving (before the actual test process). This prevents any damage to the wafer115(for example, due to atmospheric contaminations, shocks, and the like).

In this condition (with the chambers225and250that are disconnected from the pneumatic system of the prober) any leakage of air into the airtight chamber225would increase its internal pressure, which is not controlled by the pneumatic system any longer; for example, this leakage may be due to a non-perfect tightness of the seal215. However, the increase of the internal pressure expands the flexible cap245so as to enlarge the volume of the compensation chamber250; as a result, the internal pressure in the compensation chamber250decreases accordingly, thereby restoring the desired internal pressure in the airtight chamber225as well. This provides the application of the correct force to the wafer115, so as to prevent any risk of loosening its coupling with the probe card120. In should be noted that the desired result may be achieved automatically without any mechanical member; therefore, no additional mechanical action is required during the assembling of the cartridge125.

Moving toFIG. 3, the above-described cartridge125(with the wafer including the integrated circuits to be tested) is provided to a test machine300; typically, a high number of cartridges125(for corresponding wafers) are assembled in succession in the prober and then provided to the same test machine300for their concurrent test.

For this purpose, all the cartridges125(only one shown in the figure) are placed into an operative region305of the test machine300—after removing their protective caps. As described in detail in the following, the operative region305controls the temperature of the wafers in the cartridges125during the test process; for example, the operative region305is an oven, which provides the required very high or very low temperature (such as ranging from −50° C. to +150° C.) for a test process of the burn-in type. During the test process, the cartridges125are connected electrically to a test circuit (not shown in the figure), which provides a power supply and stimulus signals to and that receives corresponding result signals from the wafers in the cartridges125; the test circuit is arranged in a control region307of the test machine300at room temperature (with the two regions305and307that communicate through a slot provided with an insulating seal). The cartridges125may also be connected to a pneumatic system (not shown in the figure) arranged in the control region307as well; this allows maintaining the pressure in their airtight chambers substantially constant when the cartridges125are warmed or cooled during the test process (even in the presence of any leakage of air).

More in detail, the operative region305includes a conditioning chamber310. The conditioning chamber310has a top cover with a matrix of windows. Each window is formed by an opening with a seal arranged around it above the cover, which opening has a size comprised between the one of the probe card and the one of the wafer of each cartridge. In this way, when the cartridge125is placed on the cover and fastened to it (for example, by means of a clamp not shown in the figure), the probe card is pressed against the seal so as to close the window; at the same time, the wafer directly faces the conditioning chamber310through the window.

An inlet conduit320(for example, consisting of a series of intakes ending into a funnel) connects the conditioning chamber310to a pump325. The pump325removes water from the conditioning chamber310and forces it towards a heat exchanger330. The heat exchanger330warms or cools the water according to the temperature that is desired in the conditioning chamber310. An outlet conduit335connects the heat exchanger330to a plurality of nozzles340, which open in the conditioning chamber310(for example, some hundreds facing upwards and being uniformly distributed). In this way, the heat exchanger330returns the warmed/cooled water to the conditioning chamber310.

An infrared camera345monitors the local temperature of the wafers in the cartridges125. For this purpose, the camera345takes a snapshot (from below) of the cover of the conditioning chamber310periodically (for example, every 30-120 s); this snapshot then provides a temperature map of the exposed surfaces of the wafers in the cartridges125facing the conditioning chamber310(through its windows).

A controller350(in the control region307) manages operation of the whole test machine300. Particularly, the controller350receives the temperature map from the camera345; moreover, the controller350drives the heat exchanger330and it regulates the flow of water for each nozzle340individually (setting it between completely open and completely close).

During the test process, the water continuously flows through the circuit defined by the inlet conduit320, the pump325, the heat exchanger330, the outlet conduit335, and the nozzles340. As a result, the water in the conditioning chamber310is maintained at the desired temperature, so as to warm or cool the wafers in the cartridges125accordingly.

The proposed structure allows controlling the temperature of the wafers in the cartridges125in a very effective way (since the warm/cool water now acts on the exposed surfaces of the wafers in the cartridges125directly); therefore, it is possible to improve the uniformity of the temperature of the integrated circuits under test. Moreover, the terminals of the wafers are sealed into the airtight chambers of the corresponding cartridges, while their probe cards remain outside the conditioning chamber210(and then they are not reached by the water); in this way, it is possible to use the water (or any other conductive liquid) to warm/cool the wafers without any insulation problems.

At the same time, the temperature map taken by the camera345provides an accurate representation of the actual distribution of the local temperature throughout the wafers in the cartridges125. The controller350drives the nozzles340accordingly; particularly, the nozzles340facing a too cold area are open while the nozzles340facing a too warm area are closed—when the wafers in the cartridges125are to be warmed (or vice-versa when they are to be cooled). In this way, it is possible to regulate the temperature of the wafers in the cartridges125dynamically in a selective way; for example, this allows reducing the warming of an area including power components (which already dissipate a high amount of heat by themselves), or to cool an area including a defective component (causing an anomalous local warming). All of the above makes it possible to obtain a substantially uniform distribution of the temperature throughout the integrated circuits under test in substantially any condition.

Naturally, in order to satisfy local and specific requirements, a person skilled in the art may apply to the solution described above many logical and/or physical modifications and alterations. More specifically, although one or more embodiments of the present invention have been described with a certain degree of particularity, it should be understood that various omissions, substitutions and changes in the form and details as well as other embodiments are possible. Particularly, an embodiment may even be practiced without the specific details (such as the numerical examples) set forth in the preceding description to provide a more thorough understanding thereof; conversely, well-known features may have been omitted or simplified in order not to obscure the description with unnecessary particulars. Moreover, it is expressly intended that specific elements and/or method steps described in connection with any disclosed embodiment of the invention may be incorporated in any other embodiment as a matter of general design choice.

For example, similar considerations apply if the cartridge has a different structure or includes equivalent components; for example, the probe card may be mounted on a similar frame and/or bearing (or it may even be left free), the probes may be of other types (such as based on cantilever blades or micro-springs), and the like. Moreover, it is emphasized that an embodiment may be applied to MEMS structures, to optical devices, or more generally to any type of circuits integrated on a generic wafer.

Alternatively, the through-holes crossing the probe card may be in a different number and/or they may have a different section. Moreover, it is possible to replace the O-ring with an equivalent seal (for example, with a different shape and/or section—even arranged on the wafer instead of on the probe card), or to couple the cartridge with the pneumatic system of the prober in another way.

In a different embodiment, the spacers are implemented with equivalent elements (such as small balls).

Moreover, nothing prevents arranging them in a different way; in any case, an implementation without any spacers is possible (especially for very small wafers).

Alternatively, the compensation chamber may be implemented with a piston that is biased by a spring, or with other similar mechanisms.

In any case, this feature is not strictly necessary and it may be omitted in a simplified implementation.

It is also possible to protect the wafer (once the cartridge has been assembled) with any other device (for example, based on an a pivoting cap); however, nothing prevents leaving the wafer always exposed (without any protection).

Similar considerations apply if the prober has a different structure or includes equivalent components; for example, the chuck plate may be of an equivalent type (such has capable of locking the wafer temporarily with an electromagnetic or mechanical action), the probe card may be aligned with the wafer by means of another procedure (even in a single step), and the like. In any case, the assembling of the proposed cartridge in a prober with a non-standard alignment system is contemplated.

Likewise, the cartridges may be used in a similar test machine; for example, it is possible to have more conditioning chambers, to open the windows in a generic covering thereof (even laterally and/on on its bottom), to work at different temperatures, and so on. Moreover, although the devised solution has been specifically designed for a test process of the burn-in type, this is not to be interpreted in a limitative manner, with the same solution that is also suitable to be used in different test processes (for example, of the functional type). In any case, nothing prevents the use of the proposed cartridge in standard test machines (for example, with a rack structure for housing the cartridges).

Alternatively, a different fluid (for example, air or nitrogen) may be used to condition the wafers thermally (either to warm or to cool them). Nevertheless, the use of any other system capable of acting directly on the exposed surfaces of the wafers is within the scope of this disclosure; for example, a different implementation of the test machine may be based on an infrared lamp, on a laser, or on any equivalent heating source.

The above-described selective control of the temperature of different areas of the wafers may also be implemented with other techniques; for example, it is possible to have a (static) shielding mask interposed between the heating source and the wafers.

Similar considerations apply if the local temperature is controlled in a different way; for example, it is possible to measure the local temperature continuously, to update the conditioning of the wafers only when predefined threshold temperatures are reached, to filter sharp changes of the local temperature, and the like.

Without departing from the principles of the disclosure, the local temperature may be monitored with sensors arranged on the probe cards, or with any other equivalent means.

In a different embodiment of the invention, it is possible to have a glass sheet with a printed mask (arranged between the heating source and the wafers). A roll is used to delete the mask, while an inkjet head is used to print a new mask, with a pattern based on the measured local temperature of the wafers—for example, with dots that span from completely dark to completely opaque (with a scale of gray levels therebetween).

At the end, the proposed solution lends itself to be implemented with an equivalent method (by using similar steps, removing some steps non-essential steps, or adding further optional steps); moreover, the steps may be performed in a different order, concurrently or in an interleaved way (at least in part).