Probes with high current carrying capability and laser machining methods

The present invention is a probe having a distal end made of one material, a tip and a portion disposed between the distal end and the tip that is a different second material. The probe is laser machined manufactured using a nanosecond or picosecond laser.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to electrical probes for automated circuit and device testing.

2. Description of Related Art

In the field of electronic circuitry testing, scrubbing and contact force is an important factor in establishing a low resistance electrical contact between a probe tip and the test contact. During scrubbing, an eventual insulating oxide layer is removed in the interface between the contact tip and the test contact. Scrubbing is a microscopic shear movement of the probe tip along the test contact surface while a certain pressure is exerted from the probe tip onto the test contact. As size and pitch of test contacts decrease, it becomes increasingly difficult to tune the scrub motion irrespective of friction influences in the tip/contact interface. Also, as the integrated circuit (IC) manufacturers incorporate designs with IC pads and bumps placed over chip's active circuitry it becomes important that the scrub of the probe does not cause damage to the underlying circuitry. The size of the window of acceptable probe operation therefore, is restrained from one side by the contact resistance requirements calling for a sizable scrub, smaller scrub size required by smaller targets that need to be probed as pitches decrease, and smaller scrub (including depth) to avoid damage to the underlying circuitry.

The new generation of IC chips has pads that are placed over active circuitry in order to maximize use of the real estate. These types of chips are commonly referred in the industry as chips with “low-K dielectric”. The low-K dielectric refers to the fragile polymer-based insulator now placed between the pads and the underlying circuits for electrical purposes. It is not acceptable to damage the low-K dielectric during probing operations either.

Well known buckling beam probes have been utilized to provide a combined resilient deflection and scrubbing. In order for a buckling beam probe to operate properly with a well defined scrub motion it needs to be rigidly held on its peripheral shaft and additionally guided close to the contact tip. This makes the buckling beam probe's assembly increasingly challenging with ever decreasing scale. Therefore, there exists a need for a probe that may be easily assembled in large numbers and small scale while providing a well definable scrub motion. The present invention addresses this need.

The contact resistance issue has also been addressed by probes having separate parts for scrubbing and for making electrical contact. For example, US 2004/0239352 considers dual probes having a contact probe and a separate scrub probe, where the scrub probe moves in such a way as to clean the part of the contact pad that will end up under the contact probe during test. In some cases (e.g., copper deposition manufacturing), circuit fabrication processes provide contact pads which are covered with a protective dielectric film (e.g., a silicon dioxide film). U.S. Pat. No. 6,727,719 considers a probe having an inner contact needle and an outer hard layer, where the hard outer layer is adapted for penetrating such a protective film.

An important consequence of decreasing probe and contact pad dimensions is that the current density at the probe-pad contact increases. This increased current density also raises issues which have not come up before in connection with large probes on large pads. More specifically, the current density can be high enough to form micro-welds between the probe and the pad due to local heating. Breaking these micro-welds as the probe is removed from the contact pad can lead to degradation of the probe tip (e.g., by accumulation of non-conductive material), thereby reducing probe reliability and/or lifetime.

Testing of electrical devices and circuits has been an important component of electronic manufacturing processes for some time. Such testing typically entails probing a circuit with a fixture including multiple flexible probes, each probe making electrical contact to a contact pad on the circuit chip. Various practical issues that have arisen in this context have been addressed in the prior art, especially in connection with providing reliable, low-resistance electrical contact.

Electrical contact between the probe and the contact pad can also be hampered by the presence of non-conductive material on the pad and/or the probe (e.g., a thin oxide film). Accordingly, considerable attention has been devoted to details of how the tip of the probe moves relative to the contact pad in order to improve the resulting electrical connection. This relative motion is usually referred to as a scrub motion. For example, U.S. Pat. No. 5,923,178 considers a probe having a shape which provides a scrub motion which is primarily a rocking motion without appreciable sliding. U.S. Pat. No. 5,952,843 considers a probe having a canted tip portion to facilitate penetration of the passivation layer. U.S. Pat. No. 6,529,021 considers a probe arrangement where the probe tip can be made to move in a reciprocating manner to reduce contact resistance.

As circuit manufacturing technology continues to evolve to smaller critical dimensions, new practical issues relating to probing tend to arise which are not fully addressed by prior art approaches. For example, the decreasing size of contact pads as critical dimensions are reduced leads to increasingly demanding requirements on the ability to precisely control the probe scrub motion. Excessive scrub motion can cause loss of electrical contact, if the probe moves off the contact pad.

Accordingly, it would be an advance in the art to provide greater control of probe scrub motion.

A normally operating IC typically draws a current on the order of 100 mA through its probes. Providing this level of current carrying capacity is not problematic with present day technology. However, when probing ICs, it is possible to encounter a defective IC that draws far more probe current than normal (e.g., 1 A or more). In such situations, it is important to ensure that the probe is not permanently damaged by this over-current condition. The probe failure mode of most concern is typically inelastic probe deformation that causes the probe to go out of spec (e.g., to have an out-of-spec contact force), as opposed to catastrophic fusing of the probe. This inelastic deformation failure mode is thermally driven, and only arises as a current driven failure mode because of resistive heating of the probe by current passing through the probe. This problem has only become apparent recently, because earlier probe technology had probe dimensions such that probes were not damaged by this kind of over-current condition.

In embodiments of the present invention, this problem is addressed by providing forced gas cooling of the probe array and/or employing two different types of probes for probing an IC, namely power/ground probes and signal probes.

BRIEF SUMMARY OF EMBODIMENTS OF THE PRESENT INVENTION

One embodiment of the present invention comprises a probe for testing a device. The probe of this embodiment comprises a probe body comprising a first high strength material and a portion of the probe body comprising a coating of a different second material, the different second material comprising a conductive material. The portion can be a distal end of a probe. The distal end is preferably shortened and is between about 0.1 mm to about 0.5 mm. The portion can be a flexing portion, a straight portion, a curved portion or a rigid portion. The first material can comprise tungsten and/or molybdenum and/or a mixed-phase composite. The second material can comprise copper, aluminum, gold, and/or silver. A tip end of the probe body can comprise the first material, the second material or a different third material. The different third material can comprises rhodium, rhenium, ruthenium, a metal alloy, a metal from the platinum element group, combinations thereof or the like. The distal end of the probe of this embodiment can comprise a trench. The trench preferably comprises a metal, for example, copper. The tip of the probe can also comprise a trench. The trench in the tip of the probe preferably comprises a metal, for example, copper. A material can also be sintered onto the probe. The tip of the probe preferably comprises a skate. The skate can be comprised of the first material, the different second material or the different third material.

Another embodiment of the present invention comprises a method of laser machining a probe. The method of this embodiment comprises fixturing a probe material on a surface such that the probe material is elevated above the surface, cutting the probe material to a depth via a laser following a predetermined probe path, and repeating the laser cutting step until the probe material is cut through. The laser can also cut a step in the tip of a probe, reduce the width of the tip of the probe to create a skate, cut a trench in the probe and/or form a chisel at a tip of the probe. The laser is preferably a picosecond or nanosecond laser.

DETAILED DESCRIPTION OF THE INVENTION

Columnar Structure

Referring toFIG. 1, a probe1in accordance with a preferred embodiment of the invention features a rigid columnar structure2having a peripheral end21, a connect end22, a knee opposing face23, a connect face24, a front face25and a back face26. The columnar structure2is preferably symmetric with respect to a central column axis CA. At the connect end22, a suspension knee3is laterally connecting via its base arm32, which propagates away from the column axis CA substantially up to a lateral knee extension PK. A reverse arm34continues from the base arm32. The reverse arm34propagates away from the lateral knee extension PK in direction towards the column axis CA with a reverse length RL. At the end of the reverse arm34is a contacting tip35. The contacting tip35has a contacting face36with a tip axis TA central with respect to the contacting face36. The tip axis TA is offset from the column axis CA in a tip offset TO. The tip offset TO is smaller than the lateral knee extension PK such that during application of a contacting force preferably along the tip axis TA a first deflection of the base arm32and a second deflection of the reverse arm34counteract, resulting in a predetermined scrub motion of the contacting tip35. The suspension knee3is connected to the rigid columnar structure2via a suspension connect31.

The probe1is preferably symmetric with respect to a symmetry plane SP that coincides with the column axis CA and the tip axis TA. As a preferred result, the scrub motion is substantially in plane with the symmetry plane SP. The probe1may have a continuous profile in direction perpendicular with respect to the symmetry plane SP such that the columnar structure2as well as the elements of the suspension knee3have substantially rectangular cross sections.

The columnar structure2has a first pair of adjacent faces and a second pair of adjacent faces, the first pair opposing the second pair. A first pair may be for example faces24,26and a second pair may be faces23,25. The probe1may be fabricated in a layered fabrication technique such as well known electroplating in combination with negative shaped mask. Relevant dimensions of the probe1include probe thickness TH, total probe width WT, column width CW, column height CH, tip offset TO, lateral knee extension BL and reverse arm length RL. In the preferred case of substantially linearly protruding base arm32and/or reverse arm34, relevant dimensions include also a base arm angle AB and reverse arm angle AR between a knee axis KA and their respective arms32,34. The knee axis KA is a geometric element coinciding with a center of a knee bent33referencing the orientation of the knee bent33with respect to the column axis CA. The knee axis may be utilized to characterize the displacement behavior of the suspension knee3as depicted in the spectral displacement plots ofFIGS. 10, 12, 14, 16, 17, 19.

In theFIGS. 1-8, the arms32,34as well as the knee bent33and contacting tip35are depicted as having constant cross sections. Nevertheless, arms32,34, knee bent33and contacting tip35may have tuned configurations to provide a scrub motion predetermined in direction and magnitude in response to a contacting force exerted onto the contacting face36during operational contacting of the probe1with a test contact as is well known in the art. Such tuned configurations and their influence on the scrub motion are described in more detail underFIGS. 9-18.

Referring toFIGS. 2, 3, multiple representations of probe1may be arrayed with a first pitch PX that is substantially smaller than the total width WT. Base and reverse angles AB, AR are selected such that for a given first pitch PX sufficient base arm clearance BC and reverse arm clearance RC is established for an unimpeded deflection of each suspension knee3within the array. The first pitch PX may be selected in conjunction with the column width CW such that a first gap GX remains at a minimum required for an assembly for the arrayed probes1.

Multiple representations of probe1may be arrayed in a two dimensional probe array10with the first pitch PX in a preferred direction parallel to the probes'1knee axes KA and a second pitch PY preferably perpendicular to the first pitch PX. The second pitch PY may be selected in conjunction with the probe thickness TH such that a second gap GY remains at a minimum required for an assembly for the arrayed probes1. Providing the probes1in a configuration for a sole assembly via their rigid columnar structures2and for a scrub motion predetermined in direction and magnitude is highly advantageous for a tight interlaced array of the probes1. For example, probes1having a probe thickness TH of about 2 mils, a total width WT of about 8 mils and a column width CW of about 2 mils may be assembled with a first pitch PX of about 4 mils and a second pitch of about 3 mils.

Referring toFIGS. 4-7, the probes1may be fixedly held in a probe assembly100including fixture plates4that may be combined and/or part of a well known probe apparatus for testing electronic circuitry. Each fixture plate4has a number of fixing cutouts41with a contour larger than the rectangular cross section of the columnar structure2. Each fixing cutout41has two fixing faces411,412that correspond to the first pair of adjacent faces24,25. The probe assembly further includes a clamping plate5having a number of clamping cutouts51also with a contour larger than the rectangular cross section of the columnar structure2. Each clamping cutout51has two clamping faces511,512that correspond to the second pair of adjacent faces23,26. Fixing cutouts41and clamping cutouts51are fabricated into their respective plates4,5with pitches PX and PY.

The clamping plate may be held with respect to the fixture plates4in an assembly position as seen inFIGS. 4, 5and a clamping position as seen inFIGS. 6, 7. In the assembly position, the clamping cutouts51align with the fixing cutouts41such that a columnar structure2may be inserted in the fixing cutouts41and the clamping cutouts51. In the clamping position, the clamping plate51is offset in a clamp direction DC relative to its assembly position. The clamp direction DC is in a clamp angle AC which preferably corresponds approximately with a diagonal between the enclosed edges of the first pair of adjacent faces24,25and the second pair of adjacent faces23,26. As a result of the angled clamping offset, the first pair of adjacent faces24,25is forced into snuggle contact with the fixing faces411,412and the second pair of adjacent faces23,26is forced into snuggle contact with the clamping faces511,512such that each probe is fixedly held in a predetermined pitch and orientation with respect to the fixture plates4and with respect to each other.

Plates4,5may be fabricated from ceramic with the cutouts41,51being deep trench etched as may be well appreciated by anyone skilled in the art. The clamping plate5may be forced into the clamping offset via any well known mechanical feature such as a screw pressing against a clamping access feature55. The clamping direction DC is self adjusting as long as the clamping force is applied in direction approximately complying with the predetermined clamping direction DC as may be well appreciated by anyone skilled in the art. The clamping plate5may be actuated without particular guides. Assembly position stoppers may be provided for the clamping plate to warrant alignment of the clamping cutouts51with the fixing cutouts41in assembly position. Positioning of the probes1in direction along the column height CH may be provided via an auxiliary stop plate (not shown) temporarily placed adjacent opposite an insertion side of the plate assembly such that the peripheral ends21contact the stop plate once fully inserted into the cutouts41,51. After clamping, the stop plate may be removed. The probes1may be bonded in clamped position by an epoxy or other well known techniques. The cutouts41,51may also be configured as conductively coated via holes conductively connected to peripheral terminals on the plates41, and/or51. The probes1may also be conductively accessed via well known wire bonding techniques bonding wires to the peripheral ends21as may be well appreciated by anyone skilled in the art. The fully fabricated probe assembly100may be inserted and/or assembled in a well known probe apparatus.

To facilitate the assembly of large numbers of probes1, a number of probes1may be simultaneously fabricated as a probe comb11as is exemplarily illustrated inFIG. 8. The probe comb11is held together by a probe bridge6connected to each of the arrayed probes'1peripheral end21. A number of probe combs11may be stacked with second pitch PY in protrusion direction of the probe comb11, forming large two dimensional probe arrays. Individual probe combs11may be spaced apart by spacers that provide second gaps GY. The probe combs11may be held in alignment along second pitch PY direction by surrounding frame structures and/or by form features on both sides of the spacers. The form features may fit into the gaps GX. The probe combs11may alternately be assembled by inserting them with there probe bridges6in correspondingly shaped grooves of a template plate (not shown).

After the probe combs11are positioned with respect to each other, they may be fixed by use of a resin filled into the gaps between the probes1. After curing of the resin, the probe bridges6may be removed and the individual probes1conductively accessed as described above.

Suspension connect31, base arm32, knee bent33, reverse arm34, and contacting tip35may have various tuned configurations resulting in varying scrub motions. Referring toFIGS. 9-13a first tuned configuration is described in which a lateral scrub motion with respect to the tip axis TA is substantially zero. InFIGS. 9-19, numerals pertaining to the deflected elements of the suspension knee3have a suffix letter D, whereas numerals pertaining to non deflected elements of the suspension knee3have a suffix letter N. A contacting force resulting from the operative approach of the contacting tip35on a test contact210(seeFIGS. 20, 21) may act upon the contacting face36N/36D along the tip axis TA. Where the tip axis TA crosses the base arm32N/32D, the base arm32N/32D has its local bending stresses at a minimum as can be seen in the spectral stress plots ofFIGS. 11, 13 and 15, 18. At these low stress regions LS, LSN/LSD, the central base arm portion321D has its maximum angular central base arm deflection DAB1with respect to the central base arm portion's321N natural orientation and the peripheral base arm portion322D has its maximum angular peripheral base arm deflection DAB2with respect to the peripheral base arm portion's322N natural orientation. This is, because a first bending momentum acting on the central base arm portion321N/321D is opposing a second bending momentum acting on the peripheral base arm portion322N/322D. According toFIG. 9, the first bending momentum and the second bending momentum act counter clock wise or generally speaking in a direction away from the upper portion of the column axis CA. The first bending momentum hinges thereby on the suspension connect31and the second bending momentum hinges on the knee bent33.

A third bending momentum acts on the reverse arm34N/34D hinging on the knee bent33generally in direction opposite the second bending momentum. According toFIGS. 10, 11, the third bending momentum acts clock wise. First, second and third bending momentums result from the contacting force as may be well appreciated by anyone skilled in the art. The third bending momentum results in a maximum angular reverse arm deflection DAR with respect to the reverse arm's34N natural orientation.

The first tuned configuration includes dimensional and structural configurations of suspension connect31, central base arm portion321, peripheral base arm portion322, knee bent33and reverse arm34such that maximum local angular deflections DAB1, DAB2and DAR are substantially equal. An indication for the first tuned configuration is that the natural knee axis KAN of the non deflected suspension knee3is substantially parallel to the deflected knee axis KAD of the operationally deflected suspension knee3.

During deflection of the central base arm portion321N/321D a lateral offset NOF may be introduced to the remainder of the suspension knee3due to the geometric conditions and geometric relations of the deflected and non deflected central base arm portion321N/321D as may be well appreciated by anyone skilled in the art. The contacting tip35may be configured in length and deflection behavior such that the lateral offset NOF may be substantially compensated for. At the contacting face36D, the contacting tip35D may consequently have a maximum angular tip deflection DAT contributing to the scrub motion. Hence, in the first tuned configuration, the scrub motion includes substantially only angular movement of the contacting face36.

For a required contacting force, the operational deflection of the suspension knee3may be adjusted by configuring the elements of the suspension knee3for a leveled stress maxima as can be seen in theFIGS. 12, 13. There, the cross sections are adjusted with continuous thickness TH such that stress maxima propagate highly continuous along suspension connect31, central and peripheral base arm portions321,322, knee bent33, reverse arm34and contacting tip35. Optimizing the suspension knee3with constant thickness TH is particularly preferred in combination with continuous profile of probe1and fabrication techniques layered in profile direction such as well known electroplating in combination with a negative mask corresponding to the contour of the probe's1continuous profile. Nevertheless, the suspension knee3may also be optimized by varying the thickness TH as may be well appreciated by anyone skilled in the art.

Referring toFIGS. 14-16, a second tuned configuration of the suspension knee3provides a scrub motion in direction towards the column axis CA. According toFIGS. 14 and 15, the second tuned configuration may be provided for a continuously shaped base arm32by extending the reverse arm34such that the tip axis TA divides the base arm into a central base arm portion321that is shorter than the peripheral base arm portion322. Consequently, the maximum angular deflection DAB1of central arm portion321D is smaller than the maximum angular deflection DAB2of the peripheral arm portion322D. Since base arm32and reverse arm34have substantially equal and continuous cross sections, DAB2is equal DAR. The summary of DAB1, DAB2and DAR results generally in a tilt of the displaced knee axis KAD in direction away from the upper portion of the column axis CA. With respect toFIGS. 14 and 16, the displaced knee axis KAD is tilted in clockwise direction with respect to the natural non deflected knee axis KAN. The resulting lateral scrub motion is in direction towards the central axis CA.FIG. 15depicts the corresponding stresses.

The same condition of DAB1being smaller than DAM with DAB2being equal DAR is depicted inFIG. 16. There, the central base arm portion321is configured with larger bending stiffness than the peripheral base arm portion322. Even though the tip axis TA is at a distance to CA equal to the above described first tuned condition ofFIGS. 9-13, the dissimilar structural configuration of both base arm portions321,322is the prevailing condition determining the direction and magnitude of the scrub motion.

The teachings ofFIGS. 14-16may be inverted to obtain a third tuned configuration in which the scrub motion is in a direction away from the central axis CA as may be well appreciated by anyone skilled in the art. Accordingly and as shown inFIGS. 17, 18, the suspension knee3is configured with the tip axis TA dividing the base arm32in a central base arm portion321that is longer than the peripheral base arm portion322. Despite continuous cross sections of base arm32and reverse arm34, DAB1being larger than DAB2results in a scrub motion away from the central axis CA irrespective of DAB2being equal DAR, which is illustrated inFIG. 17by the deflected knee axis KAD being rotated in counter clockwise direction with respect to the natural knee axis KAN or generally speaking, in the third tuned configuration the deflected knee axis KAD is rotated with respect to the natural knee axis KAN in direction towards the upper portion of the column axis CA.

Second or third tuned configuration may be obtained also by adjusting the reverse arm's34deflection behavior in conjunction with the peripheral base arm portion's322deflection behavior as illustrated inFIG. 19. There, the base arm portions321,322are configured with equal deflection behavior such that DAB1equals DAB2. The reverse arm34on the other hand is stiffer than the peripheral arm portion322resulting in DAR being smaller than DAB2and consequently a third tuned configuration with a linear scrub motion away from the central axis CA. In case, the reverse arm34would be less stiff than the peripheral base arm portion322, the second tuned configuration would be established with the linear scrub motion towards the central axis CA.

As may be well appreciated by anyone skilled in the art, the teachings presented under theFIGS. 9-19may be well applied to configure various shapes of the suspension knee's3elements. Further more, the contacting force represented in the Figures by the tip axis TA may be adjusted in angle with respect to the column axis CA. Consequently, for a given geometry of the suspension knee3, first, second or third tuned configuration may be provided by assembling the probe1with its column axis CA in predetermined angle with respect to the contacting force defined by the probe apparatus in conjunction with the test contact210(seeFIGS. 20, 21) as may be well appreciated by anyone skilled in the art. For example, the probe1may be provided with a first tuned configuration in case of the tip axis TA being parallel to the column axis CA. Tilting such probe1in direction towards its knee33may result in a second tuned configuration whereas a tilting of such probe1in direction away from its knee33may result in a third tuned configuration. Tilting the probe1may be a convenient technique of fine tuning the linear scrub motion in direction and magnitude without need to remanufacture the probe1.

As taught underFIGS. 9-19, scrub motion may be adjusted for its lateral movement component in direction and, magnitude and for its angular movement component in magnitude as may be well appreciated by anyone skilled in the art. The advantageous combination of angular and lateral scrub motion adjustability may be combined with a multi-radius contacting face38as illustrated inFIGS. 20, 21. The multi-radius contacting face38may have at least a first contacting radius R381at the initial contacting region381where the multi-radius face38initially contacts the test contact210of a tested electronic device210. An initial tip axis TA1may origin in the initial contacting region381.

As the probe1is brought into operational deflection with respect to the test contact210, the multi-radius face38may be rotated with maximum tip deflection angle DAT such that an operational contacting region382comes into contact with the test contact210. An operational tip axis TA2may origin from the central interface between operational contacting region382and the test contact210. Between initial contacting at scrub start location SS and operational contacting, the multi-radius face38prescribes a lateral scrub SL and an angular scrub equal DAT. Orientation of TA1and TA2may be affected by friction in the tip/contact interface CI as may be well appreciated by anyone skilled in the art.

The operational contacting region382has second contacting radius R382substantially larger than first contacting radius R381. The multi-radius face38hence features at least two radii R381, R382that contribute to a smooth and continuously curvature of the multi-radius face38. The two radii R381, R382may be selected in conjunction with the change of contacting force as a function of angular tip displacement such that contacting pressure in the tip/contact interface CI remains within a predetermined limit.

Multiple Layered Probes

Referring toFIG. 22, area of and pressure in the tip/contact interface CI may also be adjusted by varying the contacting face thickness FT to levels less than the probe thickness TH. Also, the contacting tip35may be split into tip segments351,352,353of which one or more may provide contacting face(s)36or38. For that purpose, the probe1may be fabricated from a number of layers L1, L2, L3deposited in multiple steps for example by electroplating in combination with multiple masks as may be well appreciated by anyone skilled in the art. The layers L1, L2, L3may partially and/or fully extend across the probe's1profile contour and may be made of materials suitable for their particular task. For example, the layer L2illustrated inFIG. 22with the contacting face36may be fabricated from a material specifically suitable for probe tips such as rhodium. A single contacting face36or38may be placed centrally as shown inFIG. 22. Alternatively, dual contacting faces36or38may be provided by tip segments351,353, one adjacent the front face25and the other adjacent the back face26. This may also assist in stabilizing the suspension knee's3deflection behavior within the symmetry plane SP and to reduce the risk of inadvertent lateral scrub motion deviations.

The contacting tips351,352,353may be arranged in a tripod like fashion with each contacting segment having a contacting face36or38for providing a self centering contacting on a test contact in the well known spherical configuration. The suspension knee3may be layered in direction along the symmetry plane SP. The layer configuration may also be adjusted in view of low surface resistance for high frequency current flow from the contacting tip36or38to the peripheral end21or the column2. Tip segments351,352and353may also be fabricated from same material resulting in a monolithic structure.

The spectral plots ofFIGS. 10-19are generated with a commercially available FEA software.

Probes with a Shank and a Knee

Referring toFIG. 23a, a “first contact” probe configuration is illustrated according to an embodiment of the invention.FIG. 23bshows a corresponding operational probe configuration. Here “first contact” refers to the situation where a probe is in contact with a contact pad, but no contact force is applied. In contrast, an operational probe configuration makes contact with the contact pad with a predetermined contact force. Since the probe deforms in response to the contact force, the shape of the probe differs in the two cases. In particular, how the probe moves from the first contact configuration to the operational configuration is a key aspect of the invention.

A probe2300includes a shank2302, a knee section2304and a probe tip2306as parts of a single structure, as shown. Shank2302is straight and does not deflect appreciably during contact, so it is convenient to regard shank2302as defining a probe axis2310with which it is aligned. Knee section2304extends from shank2302to probe tip2306, and includes two parts. A first part of knee section2304starts at shank2302and extends outward from probe axis2310to reach a knee point2312. Knee point2312is a point of maximum separation from probe axis2310. A second part of knee section2304starts at knee point2312and extends to a tip location2314, such that probe axis2310is between knee point2312and tip location2314. A lateral tip offset2316is thereby defined between the probe tip and the probe axis. Probe tip2306is in contact with a contact pad2320defining a contact point2324.

Thus knee section2304can be regarded as extending outward for a certain distance D (the first part) and then curving back for a distance greater than D (the second part), thereby establishing the lateral offset2316. The present inventor has found that this probe configuration can provide improved probing performance. For comparison, U.S. patent application Ser. No. 10/850,921 by the present inventor considers a knee probe having a knee which curves back by a distance less than D (i.e., it does not overshoot the probe axis).

FIG. 23bshows the corresponding operational probe configuration for the example ofFIG. 23a. Here contact pad2320is moved toward probe shank2302by a vertical displacement2330. Equivalently, a predetermined contact force is applied to the probe shank. For any particular probe design, there is a one to one relation (i.e., this relation is a mathematical function, which can be linear or nonlinear) between vertical displacement and contact force, as is well known in the art, so both ways of describing the operational configuration are employed interchangeably in the following description. Probe2300deforms under the contact force, andFIG. 23bshows key parameters of this deformation. More specifically, contact point2324′ onFIG. 23bis farther from probe axis2310than the corresponding contact point2324onFIG. 23a. Thus the probe tip slides along the contact pad for a certain distance (i.e., the difference between2316′ and2316onFIGS. 23a-b). In addition to this sliding motion, the probe tip also “rocks” relative to the contact pad. This rocking motion can be more clearly appreciated by defining a “tip axis”2318onFIG. 23awhich is required to be parallel to probe axis2310and which passes through the contact point2324. In the operational configuration ofFIG. 23b, tip axis2318is no longer parallel to probe axis2310. The angle between tip axis2318and probe axis2310onFIG. 23bis a measure of the amount of rocking motion provided.

Thus the scrub motion provided in this example includes both a sliding motion of the probe tip relative to the contact pad, and a rocking motion of the probe tip relative to the contact pad. A key aspect of the invention is that parameters of the scrub motion (e.g., slide length and rocking angle) can be predetermined, in part, by geometrical parameters of the probe and by the predetermined contact force (or equivalently, predetermined vertical displacement). More explicitly, a probing method according to the invention includes: providing a probe having the general configuration ofFIG. 23a(i.e., having a knee section with an overshoot), making contact between the probe tip and a device under test, and applying a predetermined contact force to the probe shank, thereby providing a predetermined scrub motion of the probe tip on the contact pad. The scrub motion is predetermined in part by the contact force and by geometrical parameters of the probe.

The friction provided by the contact pad is also a relevant factor for determining the scrub motion, so probe designs and/or methods will typically need to account for variations in contact pad friction. The speed with which contact is made has also been found to be relevant. More specifically, the sliding motion length on the contact pad (also referred to as scrub length) tends to decrease as the relative contact velocity between probe tip and contact pad increases. Another method of further controlling the scrub length is by laterally moving the probe as contact is made. Lateral probe motion in the direction of the tip offset will increase the scrub length, and lateral probe motion in the opposite direction will decrease the scrub length. Such lateral probe motion can be provided by appropriate motion control of a chuck holding the probe (or probes), or by appropriate motion control of a stage holding the device under test. Further scrub length control can be provided by controlling relative velocity and/or lateral probe motion. Scrub length can be measured after probing has occurred by measuring the length of the mark left by the probe on the contact pad. Such measurements are important for verifying proper probe performance.

A scrub motion including both a sliding motion and a rocking motion has provided improved results in practice. Investigations indicate that the sliding motion acts to scrape non-conductive material from the contact pad to create an exposed area of the contact pad, and the rocking motion acts to bring a clean part of the probe tip into contact with the freshly exposed area of the contact pad. FromFIGS. 23a-b, it is apparent that the rocking motion causes a different point of the probe tip to be in contact with the contact pad in the operational configuration than in the “first contact” configuration. Providing a scrub motion including both of these motions is therefore preferred.

Suitable materials for probe2300and probe tip2306are well known in the art, and any such materials can be employed in practicing the invention. Suitable tip materials are electrically conductive and wear-resistant, and include Rh and Cr.

Detailed design work in accordance with the above-identified principles of the invention has led to a point design as well as identification of some preferred parameter ranges. A point design for making contact to Cu or Al contact pads (or flat topped columns) has a tip offset (2316onFIG. 23a) of approximately 0 to 0.3 mm and preferably approximately 0.1 to 0.2 mm and more preferably approximately 0.18 mm, a knee offset (distance between knee point2312and probe axis2310onFIG. 23a) of approximately 0 to 0.7 mm and preferably approximately 0.1 to 0.5 mm and more preferably approximately 0.31 mm, and a probe length (combined length of probe tip and knee section in Z direction onFIG. 23a) of approximately 0 to 5 mm and more preferably approximately 1 to 3 mm and more preferably approximately 1.95 mm. In this point design, the probe width is approximately 0 to 0.2 mm and more preferably approximately 0.05 to 0.1 mm and more preferably approximately 0.076 mm, and the probe material is Nickel-Cobalt alloy. The tip offset is preferably in a range from about 0.05 mm to about 0.25 mm. The knee offset is preferably in a range from about 0.05 mm to about 0.5 mm. The probe length is preferably between about 0.5 mm and about 3.0 mm.

For a configuration with a small knee offset and large tip offset one can expect a longer scrub length. For a configuration with large knee offset and small tip offset, a shorter scrub length is expected. Large contact friction requires a probe design that generates larger horizontal reaction force typically produced with larger scrub length. Smoother, less frictional contact pad surfaces require a probe design producing a shorter scrub length.

As indicated above, for any particular probe, there is a predetermined relation between contact force and vertical deflection. As the probe stiffness increases, the amount of contact force required for a given vertical deflection increases. A typical vertical deflection in practice is about 75 μm (i.e. about 3 mils), and for this amount of deflection, the contact force is preferably between about 0.025 N and about 0.15 N (i.e., about 2.5 to 15 grams force), and is more preferably between about 0.08 N and about 0.10 N (i.e., about 8 to 10 grams force). The vertical deflection during contact is preferably between about 12 μm and about 125 μm and is more preferably between about 50 μm and about 75 μm.

Another way to describe probing according to the invention is in terms of parameters of the scrub mark left on the contact pad by the probe. The scrub mark width is preferably between about 3.0 μm and about 15.0 μm and is more preferably about 7 μm wide. The scrub mark depth is preferably between about 0.1 μm and about 2.0 μm and is more preferably about 0.6 μm.FIG. 29shows an example of a measured scrub mark depth profile. The scrub mark length is preferably between about 3.0 μm and about 44.0 μm and is more preferably about 10 μm. This description of scrub marks assumes Al or Cu contact pads.

FIG. 24shows a close up view of a probe tip in contact with a contact pad. More specifically, probe tip2306makes contact with contact pad2320on a circuit (or device)2402. Note that the view ofFIG. 24(and ofFIGS. 25 and 26) differs from the view ofFIGS. 23a-bby a 90 degree rotation about the Z-axis. Thus moving left or right onFIGS. 24-26corresponds to moving into or out of the page ofFIG. 23.

Probes with a Skate

FIG. 25shows an alternative embodiment of the invention, where probe tip2306includes multiple layers (one of which is shown as2502and another of which is shown as2504). Such a multilayer probe configuration provides several advantages. First, as shown onFIG. 25, one of the layers (2504in this example) can extend past the others, thereby defining a “skate” having a width (i.e., y extent) substantially less than the width of probe tip2306. Reducing the probe contact area can enhance scrub motion performance, because the force per unit area is thereby increased. However, decreasing the width of the entire probe can undesirably allow the probe to deflect in the y direction. A probe tip with a skate, as shown inFIG. 25, allows most of the probe to have a y-thickness sufficient to render y-deflection negligible, while also desirably reducing the contact area.

A further advantage of the multi-layer skate configuration ofFIG. 25is that only the skate layer (i.e., layer2504) should be a material selected for suitability as a tip contacting material. The remaining layers (e.g.,2502) can be selected to optimize the overall probe performance without regard for their suitability as tip materials, since they never actually make contact with contact pad2320.

FIG. 26shows a dual-skate configuration, where probe tip2306includes two skates2602and2604. This dual skate configuration is suitable for probing a contact pad2320having a dimple2606at its center. Such a dimple is characteristic of contact pads formed by metal plating (e.g. as in flip-chip wafers). Typical dimple dimensions are about 10 μm diameter on a contact pad having a 110 μm diameter, with the size of the dimple depending on the pitch of the contact pads. A single skate configuration as inFIG. 25will undesirably require a choice between probing at the dimple location (which can degrade the electrical contact made by the probe), or off-center probing (which can be difficult to align). Probing at the dimple can also cause high mechanical stress on the probe if the probe tip gets caught by the dimple. In contrast, the dual-skate approach ofFIG. 26avoids probing the dimple, but still has the probe tip centered on the contact, thereby simplifying automatic probe alignment.

FIGS. 27a-bshow results from an embodiment of the invention. More specifically,FIG. 27ais a photograph of a probe tip andFIG. 27bis a picture of the probe tip ofFIG. 27aafter 1,000,000 probing cycles according to the invention. The probe of this example is a multi-layer single-skate configuration, as inFIG. 25.FIG. 27bshows no significant degradation of the probe tip, either by wear or by accumulation of debris.

FIGS. 28a-dare photographs of a probe array suitable for practicing the invention. Such arrays are often required in practice, since many circuits being tested have a large number of contact pads which must be probed. For probe arrays, it is important that each probe deform in a uniform and predictable manner when the contact force is applied, to prevent probe-to-probe contact resulting from probe deflection. Thus it is preferred for the probe configuration ofFIGS. 23a-bto only deform in the X-Z plane responsive to the contact force, as also indicated above in connection with probe tip skates.

Probes with Variable Thickness

FIG. 30shows a probe3000according to an embodiment of the invention. A shank3002defines a probe axis3016. A curved knee section3004is connected to shank3002and includes an upper knee section3006and a lower knee section3008. A probe tip3012is connected to an end of knee section3004opposite from the shank. Upper knee section3006extends outward from shank3002and reaches a knee point3010of maximum separation from probe axis3016, thereby defining a lateral knee offset3018from the probe axis. Lower knee section3008extends from knee point3010toward probe axis3016and to a tip location3014, thereby defining a lateral tip offset3020from the probe axis.

A probe plane includes and is thereby defined by probe axis3016and knee point3010. In this example, the plane ofFIG. 30is the probe plane. A thickness of upper knee section3006in the probe plane varies along the length of the upper knee section according to a thickness function h(z), where z is position along the probe. The upper knee section includes a reduced thickness section, as described above. More specifically, h(z) has a local minimum located between shank3002and knee point3010, as shown in the example ofFIG. 30.

Suitable materials for shank3002, knee section3004and probe tip3012are well known in the art, and any such materials can be employed in practicing the invention. Suitable tip materials are electrically conductive and wear-resistant, and include Rh and Cr. Known probe fabrication methods are applicable for fabricating embodiments of the invention. These methods include, but are not limited to, standard multi-layer metal deposition techniques such as plating, sputtering, photolithographic techniques and microelectromechanical systems (MEMS) techniques. No unusual fabrication difficulties due to the reduced thickness section arise in fabricating probes according to the invention.

Preferably, h(z) varies smoothly (i.e., h(z) is preferably continuous with a continuous first derivative) to avoid stress concentration at discontinuities and/or sharp corners of h(z). It is also preferred for the minimum probe thickness (i.e., the local minimum of h(z)) to have a value between about 0.5 hnomand about 0.95 hnom, where hnomis a nominal in-plane probe thickness. In some cases (e.g., as onFIG. 30), lower probe section3008has a roughly constant thickness h1, and in such cases, hnomcan equal h1. In other cases, the nominal probe thickness hnomcan be taken to be the maximum value of h(z) (i.e., the maximum thickness of the upper knee section). In either of these two cases, the nominal in-plane probe thickness hnomis typically between about 25 μm and about 55 μm, although the invention can also be practiced outside of this thickness range.

Preferably, h(z) varies smoothly along the entire length of upper knee section3006, in order to minimize stress concentration for a given minimum thickness. It is also preferred for the probe thickness perpendicular to the probe plane to be somewhat higher than the nominal in-plane probe thickness, so that deformation of the probe is easiest in the probe plane. More specifically, the out of plane thickness is preferably between about 1.1 hnomand about 1.5 hnom.

For a configuration with a small knee offset and large tip offset one can expect a longer scrub length. For a configuration with large knee offset and small tip offset, a shorter scrub length is expected. Preferred probe design approaches depend on the friction between probe and contact pad. For large contact friction, probe designs that generate larger horizontal reaction force typically produced with larger scrub length are preferred. For smoother, less frictional contact pad surfaces, probe designs producing a shorter scrub length are preferred.

Preferably, the reduced thickness section is in the upper knee section as shown and described above, although the invention can also be practiced by having the reduced thickness section anywhere along the length of knee section3004. Placing the reduced thickness section in the upper knee section tends to decrease scrub motion without appreciably decreasing the contact force, while placing the reduced thickness section in the lower knee section (e.g., as shown onFIG. 38) tends to decrease both scrub motion and contact force. More specifically, a negative tip offset probe having an upper knee section reduced thickness section tends to rotate toward the knee during deflection, thereby decreasing scrub motion. A probe having a lower knee section reduced thickness section tends to have increased flexibility (which reduces contact force). This reduced horizontal scrubbing force decreases the scrub motion. Probes having multiple reduced thickness sections can also be employed (e.g., one being in the upper knee section and the other being in the lower knee section) in practicing the invention.

As described above, the invention is applicable to probes having a positive tip offset, a negative tip offset, or no tip offset. The example ofFIG. 30shows a probe having a negative tip offset. Here probe axis3016is between knee point3010and tip location3014.FIG. 31shows an embodiment of the invention having no tip offset. Here tip location3014is substantially on probe axis3016.FIG. 32shows an embodiment of the invention having a positive tip offset. Here tip location3014is between probe axis3016and knee point3010. For the probe ofFIG. 32, the knee section does not cross the probe axis.

Reduced thickness sections of probes according to the invention can be regarded as resulting from removing material from the left and/or right sides of a smooth, constant-thickness probe profile. For example,FIG. 33shows an embodiment of the invention where the reduced thickness section is formed by variation of a right probe boundary f2(z).FIG. 34shows an embodiment of the invention where the reduced thickness section is formed by variation of a left probe boundary f1(z).FIG. 30shows an embodiment of the invention where the reduced thickness section is formed by variation of both a left probe boundary f1(z) and a right probe boundary f2(z).

In the preceding examples, lower knee section3008has a roughly constant in-plane thickness. The detailed shape of lower knee section3008is not critical in practicing the invention, and any other lower knee section shape can also be employed in practicing the invention. For example,FIG. 35shows an embodiment of the invention having a tapered lower knee section3008. More specifically, the in-plane thickness of lower knee section3008decreases monotonically along the length of the lower knee section from knee point3010to the tip location3014.

Operation of the invention can be appreciated in connection withFIGS. 36-37, which show an embodiment of the invention in an initial contact configuration and an operating contact configuration respectively. OnFIG. 36a probe according to the invention makes initial contact with a contact pad3604. It is convenient to describe the initial contact point between the probe and contact pad3604in terms of an initial contact offset3608defined with respect to probe axis3016. In operation, the arrangement ofFIG. 36is vertically compressed (e.g., by moving contact pad3604up by a vertical deflection3606). Under this compression, the probe elastically deforms as schematically shown onFIG. 37. As a result of this deformation, the probe tip moves relative to contact pad3604. Typically this relative motion includes a translation (i.e., operating contact offset3608′ being different from initial contact offset3608) and a rocking motion of the probe tip relative to the contact pad surface. The rocking motion can be appreciated by noting the different orientations of a tip axis3602onFIGS. 36 and 37, The scrub mark length is the difference between offset3608and offset3608′.

In one example, a reference probe (probe A) having a nominal in-plane probe thickness of 52 μm and a negative tip offset (as shown onFIG. 30), provided a 25 μm scrub length on an Al surface for 75 μm vertical deflection. A probe according to the invention (probe B) had the same shape as the reference probe, except that the upper knee section of probe B smoothly varied to provide a local minimum thickness of 33 μm in the upper knee section. This local minimum was located about halfway between the knee point and the shank. The thickness variation of the upper knee section was distributed over the entire length of the upper knee section. Probe B provided a 10 μm scrub length on the same Al surface used for testing probe A. For both probes A and B, the contact force was about the same (2 grams per 25 μm vertical deflection).

FIG. 39illustrates another embodiment of the present invention comprising layered probe3800. Layered probe3800of this embodiment preferably comprises at least two inner layers3806,3808and two outer layers3802,3804. The layers can be the same or different materials. For example, layer3806can comprise Cu and layers3802,3804, and3808can comprise Rd or Cr. Segments along the length of the probe inFIG. 39can be different material structures. One of the inner layers3806is preferably shaped like a C, although other shapes may be employed wherein one inner layer partially or fully surrounds the other inner layer. This embodiment of the present invention allows at least one of the inner layers to comprise a more conductive material than the outer layers.

Layered Probes with Core

Arrays of closely-spaced probes have been extensively used for IC testing for many years. As integrated circuit technology progresses, circuit dimensions decrease, and it becomes desirable to probe at ever-decreasing spacings between probe pads. To keep up with this evolution, probe dimensions also tend to decrease as technology progresses. However, new technical problems can arise as probe dimensions are reduced.

One such new problem relates to probe current carrying capacity (CCC). Present day and projected probe dimensions are sufficient to provide normal device operating currents (on the order of 100 mamps) without compromising probe mechanical integrity. However, integrated circuits that are probed do not always operate normally, and probing a faulty circuit (e.g., a short) can lead to a current flow of about 2 amps or more through a probe.

Accordingly, this behavior can be characterized by measuring contact force as a function of current, and looking for irreversible changes in contact force as a result of high current operation. The probe current carrying capacity (CCC) can be defined as the maximum current at which no irreversible deformation of the probe occurs during normal mechanical over-travel.

Probes suitable for probing contact pads having relatively large spacing (e.g., earlier stage IC technology) naturally tended to have sufficiently large dimensions that probing a faulty circuit would not lead to probe damage. However, present day and projected IC contact pad spacings will require small probe dimensions and thus insufficient probe current carrying capacity becomes a pressing problem.

In embodiments of the present invention, two approaches alleviate this problem. Both approaches rely on introducing materials into the probes having a higher degree of electrical conductivity than typical probe materials, without compromising the mechanical performance of the probes. Increased electrical conductivity reduces resistive heating of the probes, thereby increasing the probe CCC.

Referring toFIG. 40, one embodiment of the present invention comprises multi-layer probe structure4000where layers4002are preferably in the plane of probe deformation. Layers4002are preferably planar. Layers4002preferably comprise NiCo. Central or core layer4004of probe structure4000preferably comprises a high conductivity material (including, but not limited to, Cu), and at tip4006of probe structure4000, a harder material (including, but not limited to Rd and/or Cr) is preferably used to make electrical contact to devices being probed. Methods for fabricating multi-layer probe structure4000are employed for making these probes. For example, central or core layer4004is deposited as the high conductivity, low strength layer (e.g., Cu). Patterning and etching opens up space for tip4006regions of core layer4004, and tip4006material is then deposited into these opened up spaces. Core layer4004and tip4006can be either the same thickness as illustrated inFIG. 40or different thicknesses. Core layer4004and tip4006can be approximately 1 to 30 μm thick, and more preferably approximately 5 to 20 μm thick and most preferably approximately 8-12 (preferably about 10 μm) thick. Layers4002can be different thicknesses or the same thickness as illustrated inFIG. 40. Layers4002are preferably approximately 10 to 80 μm thick, and more preferably approximately 20 to 50 μm thick and most preferably approximately 30-35 (preferably about 33) μm thick.

Three probe embodiments of the present invention were evaluated for increased CCC. The three embodiments are illustrated inFIG. 41. Probe B comprises highly conductive core4100, e.g. an approximately 10 micron Cu core. Probe A is the same as Probe B except that core layer4102is made of NiCo (i.e., probe A is all NiCo). Probe C is the same as Probe A, except that the outer surface comprises a highly conductive material, e.g. an approximately 3 micron thick Cu layer deposited on the outer surface of Probe C.

The Probe C embodiment showed unsatisfactory results because the Cu plating layer tended to crack in operational cycling tests, as seen inFIG. 42. Probe C cracked after 7 k/75 μm cycling. This cracking result can be better understood by considering the stress distribution in probe designs A, B, and C as illustrated in section view inFIG. 43. Regions4300on the plots ofFIG. 43show regions of high stress, and it is apparent that the Cu plating of probe C experiences high stress (see regions4300on outer edges of probe C), which when combined with the poor mechanical properties of Cu gives rise to the above identified cracking problem. Regions4302are low to no stress regions of probe designs A (region4302in center of probe), B (regions4302in center and left side of probe), and C (region4302in center of probe).

Probe embodiments were further investigated by considering temperature and stress distributions within the probes.FIG. 44illustrates an example of such investigations. Region4402illustrates the high-temperature and high-stress region of probe4400. Regions4404illustrate low stress and low temperature regions of probe4400. Frequently, as in the example ofFIG. 44, the high-temperature and high-stress regions of a probe coincide and determine the CCC of the probe. In this case, region4402of probe4400determined the CCC of probe4400.

In a preferred embodiment of the present invention, the high-conductivity core in probe B ofFIG. 41is trimmed to remove some of its highest-stress material, as illustrated inFIG. 45. To compensate for the loss in electrical conductivity caused by this trimming, core layer4500can be made somewhat thicker.FIGS. 46-47illustrate two probe embodiments that were compared. In probe B1, core4600is, for example, approximately 10 microns of Cu, and no trimming of core4600is performed. In probe B2, core4602is, for example, approximately 20 microns of Cu, and the high-stress regions of core4602are trimmed away. The CCC of probe B2(about 1.25 amps) is higher than the CCC of probe B1(about 1.0 amp). Finite element analysis modeling, as employed above, is used to guide the configuration of further embodiments of the present invention comprising probes with highly conductive material. For example, the trimming of the high conductivity core can eliminate the regions of the core which are modeled as having stress that exceeds the yield strength of the conductivity core (at the relevant temperature).

In the examples above, two approaches for improving probe CCC were considered. The preceding description relates to a first approach of including a high-conductivity layer in a multi-layer probe. In a second approach, a high conductivity layer is also included in the probe, but the geometry is significantly different. More specifically, as illustrated inFIG. 48, high conductivity core4802(e.g., Cu) is vertical with respect to the overall layer structure of probe4800, as opposed to being horizontal as it was in the first two examples.

The significance of this difference is shown when comparing the orientation of the cores inFIGS. 45-47to the orientation of the core inFIG. 48. More specifically, the plane of probe deformation is horizontal in this geometry. When probe deformation occurs in this plane, the left side of the probe is in a state of compression and the right side of the probe is under tension (or vice versa). As a result of this stress pattern, the center of the probe is a neutral axis for stress, where stress is relatively low compared to other regions of the probe. This stress pattern is clearly visible in the calculated results shown inFIG. 43. The point of core4802geometry shown inFIG. 48is to align core4802with the neutral axis of probe stress. Thus the mechanically weak core material is placed in regions of the probe that experience the least stress, thereby improving probe CCC. In the example ofFIG. 48, a CCC of about 1.3 amps was obtained, and the mechanical stress in the Cu core was about 2 times less than it was in the probe A example considered above. Another preferred feature of the example ofFIG. 48is that core4802is completely surrounded by the mechanically stronger material.

Surrounding core4802is preferred, although not required, for chemical isolation from environment as well as for fabrication purposes. It is preferable to etch a shallow trench in a Ni—Co layer and fill it with Cu, than creating a separate “mold” in which Cu is plated. Also, encapsulating Cu with stronger metal/material adds mechanical robustness, particularly in cases when “slender” probes are employed.

Preferred fabrication processes can entail formation of separate vertical cores in two or more metal layers, thereby creating multi-core geometries as illustrated inFIGS. 49-51. Such multi-core approaches can also be employed in connection with the horizontal geometry ofFIGS. 46-47. Referring toFIG. 49, cores4901and4903are a part of metal layer4902and4904. Metal layer4905preferably caps core4903. Layer4906comprises the tip of probe4900. Referring toFIG. 50, cores5001and5003are part of layers5002and5004. Layers5005and5006preferably cap cores5001and5003. Layer5007comprises the tip of probe5000. Referring toFIG. 51, cores5101,5103, and5105are part of layers5102,5104and5106. Layer5107preferably caps core5105. Layer5103preferably comprises the tip of probe5100.

The use of a vertical probe core requires different processing steps than for probes that have only horizontal layers. Various approaches can be employed to provide a vertical probe core. For example, the central horizontal layer can be fabricated first; then patterning followed by etching can be used to open up space for the vertical core material. Selective deposition of the core material into this opened up space, followed by deposition of the top probe horizontal probe layer, can complete the structure.

Power/Ground Probes and Signal Probes

Embodiments of the present invention solve probe CCC issues as seen in practice. Contact pads on a device under test (DUT) (and the corresponding probes) can be divided into at least two categories, for example, power/ground pads and signal pads. Power/ground pads are for providing external power and ground connections to the IC, while the signal pads are for the IC inputs and outputs (I/O). Device failures that lead to high-current conditions at a pad tends to occur at power/ground pads, and not at signal pads. Power/ground pads are typically provided as several separate pads that are electrically connected to each other on-chip, and are contacted by separate probes (one for each pad) that are electrically connected to the relevant external power/ground. When a high-current device failure occurs for a set of power/ground pads, parasitic on-chip and off-chip resistances tend to force the high current to pass through a single one of the corresponding probes, which leads to a high risk of damage to the affected probe.

Based on these considerations, embodiments of the present invention address the probe CCC issues as follows:

1) At least two different kinds of probes are preferably employed in probing an IC—power/ground probes and signal probes. The power/ground probes are preferably larger in cross section than the signal probes. Preferably, the dimensions and shape of the power/ground probes are the same as for the signal probes, except that the width of the power/ground probes is a multiple of the width of the signal probes (e.g., 2×, 3×, etc.). An example of this geometry is shown inFIGS. 52A and 52B.FIG. 52Aillustrates a power/ground probe having a wide body as compared to the signal probe illustrated inFIG. 52B. Certain features of such probes (e.g. the skates at the tips) preferably have a relatively small cross section, thus the current path through such sections is also short. In one embodiment of the present invention, the electrical resistance of the long current path that runs the length of the probe body is decreased by increasing the cross section area of the probe body.

2) A power/ground probe can make contact to multiple power/ground pads on a device under test. The wide body of the power/ground probe enables high CCC for power/ground probes. Since power/ground probes are preferably bussed, two or more bumps or skates can be bridged with one probe. Making such contact to multiple pads is a significant and surprising departure from the normally assumed requirement that each probe only contact a single pad. However, no problems arise by making such multiple pad contact if a power/ground probe makes contact to multiple pads that are all connected to the same IC power or ground line. Single probe to multiple pad connections as discussed herein are preferably all of this type (i.e., all of the pads contacted by any one probe are also electrically connected on-chip). The result of allowing for multiple-pad contact is that probe cross section dimensions are no longer under a hard constraint determined by the IC contact pad pitch (i.e. separation). This allows for the use of large probes as in the power/ground probe ofFIG. 52Aeven for small contact pad pitches.

In an embodiment of the present invention, a probe array comprises probes that have different cross sections (e.g. seeFIG. 55). In this embodiment, it is possible for this non-uniformity to cause undesirable non-uniformity in probe life and/or performance. However, this problem can be avoided by providing a power/ground probe comprising a width that is a multiple of the width of a signal probe. The basic factor to consider is that probe elastic force scales linearly with probe width (e.g., if a power/ground probe has 3× the width of an otherwise similar signal probe, then the force it exerts in response to a deformation will be 3× that of the corresponding signal probe).

One situation where non-uniformity can be troublesome is when cleaning the tips of a probe array. Tip wear due to cleaning can be greater for the larger probes because of their greater contact force.FIG. 53illustrates an embodiment of the present invention that minimizes tip wear. In this embodiment, probe tip5300preferably comprises sacrificial structure5302in addition to skates5304and5306at which contact on a contact pad is made. The width and/or materials of the sacrificial structure can be designed as needed to even out and/or slow probe wear due to cleaning. As seen inFIG. 54, sacrificial structure5402is in the center of tip5400and does not make contact to IC contact pads5404. However, sacrificial structure5402can optionally make contact with IC contact pads5404. Sacrificial structure5402can be located anywhere on the probe tip, but is preferably located in the center.

Another factor to consider is the desirability of having the same local contact force between probe and pad for each probe of a non-uniform probe array. One example for providing such contact uniformity is illustrated inFIGS. 55A and 55B. In this example, power/ground probe5500is about three times the width of signal probe5502width, and has triple skate5504on tip5506as opposed to single skate5508on signal probe5502. With this example, the force per skate is about the same for both kinds of probes5500and5502. Having the force per skate be about the same for all probes of a non-uniform probe array is beneficial because details of probe scrub motion tend to depend on the force per skate more than on the total force for the probe. Since contact pads of ICs tend to all be the same, it is preferred for each probe making contact to have the same scrub motion as contact is made. In the example ofFIGS. 55A and 55B, power/ground probe5500is in mechanical contact with single contact pad5510, and can be re-flow soldered to make electrical contact with other contact pads of the same IC power/ground line (e.g., the neighboring contact pads on the left and right of the wide probe). Also in the example ofFIGS. 55A and 55B, only about one-third of the power/ground pads on the IC are probed. This provides sufficient physical room for the larger power/ground probes, and also results in about the same total force between probe array and wafer during testing (each power/ground probe is about three times stiffer than normal, but there are about three times less of these probes). Although triple skate5504is shown inFIGS. 55A and 55B, any multiple of skates may be utilized in accordance with embodiments of the present invention. The multiple skates preferably control probe-to-pad alignment and reduce wear during cleaning. Distal end5512of power/ground probe5500can be attached to space transformer pads using one or more attachment points. For example,FIG. 55Cshows probe5500with attachment points5514and5516.FIG. 55Dshows probe5500with single attachment point5518.

FIGS. 56 and 57show examples of a top view of contact uniformity probing. Referring toFIG. 56, a set of contact pads having a horizontal pitch of about 110 μm is shown. The pads are arranged as a staggered set of rows, and the vertical separation between rows is also about 110 μm.FIG. 57shows an example of a probing strategy for this array, where the signal probes have a width of about 55 μm and the power/ground probes have a width of about 165 μm. Every third pad of the power/ground lines is probed, as indicated with the darker disks. Embodiments of the present invention are not limited to these pitches, separations, and widths—the numbers above are for illustrative purposes.

Although the preceding discussion has focused on the geometry at the probe tips, these and similar ideas are also applicable to a space transformer which is typically present as the base of a probe array. Thus, a space transformer can use multiple contact points to make contact to each of the power/ground probes. Alternatively, the space transformer contact points can be larger for power/ground probes than for signal probes.FIGS. 55C and 55Dshow some examples of probe distal end configurations, specifically attachment points5514,5516, and5518. Any combination of these approaches can also be employed.

Although some of the FIGS. illustrate rectangular or substantially rectangular or rectangular with curved end, skate geometries, other possible configurations are useful, including curved configurations.

Embodiments of the present invention can also be practiced in combination with any other way of increasing probe CCC. In particular, in one embodiment of the present invention a multiple contact point probe can be combined with the use of electrically conductive cores and/or with the use of forced gas cooling. As an example, an I/O probe of NiCo gives a CCC of about 0.55 Amps, and when this probe is used as a base for the about 3× wide design, the CCC equals about 1.65 Amps. By adding a Cu-core to the about 3× probe design, the CCC equals about 2.0 Amps. Even higher CCC can be reached by further applying forced air convection including forced chilled air convection.

Gas Cooling

The probe failure mode that is of most concern today is inelastic probe deformation that causes a probe to go out of spec (e.g., to have an out-of-spec contact force). This inelastic deformation failure mode is thermally driven, and only arises as a current driven failure mode because of resistive heating of the probe by current passing through the probe. In embodiments of the present invention, this problem is addressed by providing forced gas cooling of a probe array.

In one embodiment of the present invention, gas preferably flows in a predetermined gas flow configuration. In another embodiment of the present invention, gas flows in a single direction through an array of probes, removing heat from the probes and thus lowering the temperature of the probes, which in turn prevents the inelastic probe deformation. In another embodiment of the present invention and shown inFIG. 58, vertically separated gas flows5800and5802comprising different flow directions. Here, the vertical direction is taken to be perpendicular to wafer5804under test. Vertical probes5806are preferably employed, so each probe is in the path of both gas flows. In one preferred embodiment (FIG. 58), gas flows5800and5802are in opposite directions. In another preferred embodiment and as illustrated inFIG. 59, gas flows5900and5902are in perpendicular directions. Gas flows5900and5902of the example ofFIG. 59are at different vertical positions (e.g., as shown onFIG. 58).

The use of two gas flows at different vertical positions increases the turbulence of the gas flow at probes of a probe array, and it is preferred for most or all of the probes of the probe array to be surrounded by gas in unsteady or turbulent flow. Heat transfer away from the probes is significantly enhanced by having unsteady or turbulent gas flow surrounding the probes. By increasing heat transfer away from the probes in this manner, the probe current carrying capacity can be significantly increased in situations where the current capacity of a probe is thermally limited, (e.g., as described above).

In alternative embodiments of the present invention, several features can be added to this basic scheme, individually or in any combination. A first such feature is cooling the gas flow to substantially below room temperature (e.g., −25° C., −15° C. etc.), preferably using Peltier coolers disposed in the gas flows at locations upstream of the probes being cooled. A second such feature is the use of a gas flow apparatus that provides substantially the same gas flow rate at the center and edges of the probe array being cooled. One way to accomplish this is to provide the gas flow with a suitable array of gas nozzles (e.g., having nozzles more closely spaced at the edges of the nozzle array than at the center of the nozzle array). Control of gas flow can be provided by altering nozzle shape and/or spacing (as in the examples ofFIGS. 60-62). This is especially helpful in situations where the probe hot spots (where inelastic deformation/failure most easily occurs) are near the probe tips, which often occurs in practice.

A third such feature is to have some nozzles operating at positive pressures (i.e., blowing) and other nozzles operating at negative pressures (i.e., vacuuming). In this case, it is preferable that corresponding nozzles at opposite sides of the probe array have opposite operating pressures. A fourth such feature is the use of acoustic excitation to increase gas flow turbulence. A fifth such feature is to employ pulsed gas flow as opposed to a continuous steady state gas flow. These optional features preferably increases gas flow unsteadiness or turbulence, thereby improving heat transfer away from the probes.

Referring toFIGS. 60 and 61, gas nozzles preferably comprises openings that can be in the shape of holes (as inFIG. 60) and/or slots (as inFIG. 61) and/or any other shape (as inFIG. 62), including but not limited to squares, rectangles, ovals, parallelograms, hexagons, pentagons, combinations thereof and the like. The nozzle arrays for gas flows can have the same nozzle shapes or they can have different nozzle shapes.FIG. 60illustrates a cross-section of nozzle array6000. Holes6002can either be on a regular grid or an irregular grid and can be either aligned or misaligned with the “opposite” nozzle. Holes can be any shape, including but not limited to, circular, rectangular, oval, elliptical, square, combinations thereof and the like. Probe6004illustrates an example of how a probe is aligned against nozzle array6000.

FIG. 61illustrates a cross-section of nozzle array6100. Slots6102can either be on a regular grid or an irregular grid and can be aligned or misaligned with the “opposite” nozzle. Slots6102can be any shape, including but not limited to rectangular, oval, elliptical, combinations thereof and the like. Probe6104illustrates an example of how a probe is aligned against array6100.

FIGS. 63-66illustrate examples of gas flow being circulated around and through a probe array. The gas flow is preferably circulated in a predetermined gas flow configuration.FIGS. 63-66also show the velocity profiles of the gas flows.FIG. 63illustrates an embodiment of the present invention comprising two opposite gas flows without offset being circulated around and through a probe array.FIG. 64illustrates an embodiment of the present invention comprising two opposite gas flows with offset. As illustrated in the drawings, the two opposite gas flows with offset has improved flow circulation around and through the probe array.FIGS. 65 and 66illustrate an embodiment of the present invention comprising one main gas flow and two lateral reinforced gas flows. In this embodiment, the gas flow is improved, particularly around the center of the probe array. The gas flow around and through the probe array preferably comprises one main gas flow and two lateral reinforced gas flows, however that particular configuration is not required.

In one embodiment of the present invention, the gas flow is preferably moving at a velocity of approximately 0.5 to 10 ft/s. Referring toFIG. 63, the velocity of the gas flow is approximately 0.5 to 3 ft/s around the probe array and within the probe array.FIG. 64shows a slightly increased velocity of gas flow of approximately 1 to 6 ft/s around the probe array.FIGS. 65 and 66also show a velocity of gas flow of about 1 to 6 ft/s, as the region within the probe array has higher gas velocities than those shown inFIGS. 63 and 64.FIGS. 65 and 66have more gas flow flowing through and around the center of the probe array.

Fine Pitch Probes Having High Current Carrying Capability and Probe Laser Machining Methods

As integrated circuit (IC) technology evolves, the spacing between chip contact pads decreases. This trend causes various difficulties for automatic chip testing using probes that make electrical contact with the chip contact pads. Generally, as the chip contact pad spacing decreases, the probes must have smaller lateral dimensions, and must be more closely spaced to match the contact pad pitch. However, it is often required for the probes to have a substantial current carrying capability, so that the probe is not destroyed if a faulty chip that draws excess current is probed. For example, a power supply probe or ground probe to a faulty chip can experience a current flow on the order of about 1 Amp, so it is highly desirable for probes to be able to withstand this level of current flow without failing destructively. In this context, it is important to note that plastic deformation of a probe is a destructive failure mode. This problem becomes increasingly acute as probe lateral dimensions decrease in order to keep pace with evolving IC technology. For example, conventional NiCo probes cannot provide a 1 Amp current carrying capability for a 90 micron probe pitch. These alloys tend to have high resistivity and low mechanical strength at elevated temperatures, which detrimentally reduces the current carrying capability.

Embodiments of the present invention provide several approaches for increasing probe current carrying capability (CCC). For example, tungsten (W) as a probe material enables high CCC, preferably about 0.9 to about 1.5 Amps and more preferably about 1 Amp, of wafer probes. This capability is especially desirable as probe pitches get smaller. Tungsten is a very good probe material because it has a very high mechanical strength at elevated temperatures while its resistivity is not too high. Therefore, as current flows through the probe, the Joule heating is relatively low while the strength of tungsten exceeds the bending stresses. It is possible to achieve 1.0 Amp CCC for about 90 μm pitch full grid array with tungsten, particularly if the distal end of the probe is short and plated or otherwise coated with about 1 μm to about 5 μm, and preferably about 1 μm to about 3 μm and more preferably about 2.5 μm of a very conductive material, such as, for example, copper (Cu), aluminum (Al), silver (Ag) and/or gold (Au) thereby creating a heat sink.

Referring toFIG. 67, one embodiment of the present invention comprises probe6700, which preferably comprises at least two different materials and a shortened tip as compared to probe6702. Material6704is different from second material6706on distal end of probe6700. Probe6700is preferably an approximately 1 Amp probe, while probe6702is about 0.65 Amp probe. As can be seen fromFIG. 67, probe6700is shorter then probe6702. Thus, a probe array comprising a plurality of probe6700can include more probes with smaller pitches.

Traditionally, tungsten (W) probes were made out of drawn wire in the shape of a cantilever beam. In order to make a probe for a full grid array, the shape of the probe is preferably substantially vertical (as opposed to a cantilever beam) and is preferably not made out of wire due to required geometrical accuracy. Also, the dimension of a probe for an area array of about 90 μm is about 40 μm×40 μm—a very small dimension. Thus, traditional tungsten wire technology is incapable of providing W probes as considered herein. Further increase of CCC above about 1.0 Amp at about 90 μm pitch can be accomplished by creating copper-tungsten (Cu—W) mixtures.

In one embodiment of the present invention, a probe comprises a shortened distal end and/or a different second material such as, for example, copper, deposited on the distal end. The shortened distal end has a preferably length of between about 0.1 mm to about 0.5 mm and more preferably about 0.4 mm. The different second material can also optionally be deposited on a tip end. The probe of this embodiment is preferably laser-machined out of tungsten, preferably tungsten foil and is more preferably laser-machined out of about 40 μm tungsten foil. In order to increase CCC of the probe, electrical resistivity is lowered by sintering about 1 micron to about 5 microns of a second different material, such as, for example, copper, on the outside of the tungsten probe. The copper on the outside of the probe lowers the Joule heating and keeps the probe material cool. In an alternative embodiment, the different second material is sintered to a distal end and a tip end but not the “flexing” belly of the probe. This avoids delamination under cyclic probe deflection.

As can be seen inFIG. 68, a probe comprising tungsten and having a short copper distal end (about 0.1 to about 0.5 mm) coated with a different second and highly conductive material, such as copper, increases the CCC of the probe.FIG. 68shows a graph of distal end (DE) length v. CCC@10% CF, Amps. On the other hand, a tungsten probe with a long distal end results in a lower CCC (about 0.65-0.7 Amp).

Referring toFIGS. 69A-69C, one embodiment of the present invention includes laser cut probe6900comprising tungsten and/or molybdenum and/or other suitable materials providing high strength at elevated temperatures with resistivity's that are not too high (seeFIG. 69A). In another embodiment and shown inFIG. 69B, probe6902comprises the same material as probe6900but the distal end of probe6902is preferably plated or otherwise coated with different second material6904. Alternatively,FIG. 69Cshows probe6910which comprises probe material6907, which can be the same or different material from probe6900. Probe6910is coated with different second material6906at distal end of probe6910and/or different third material6908at tip end of probe6910. Different second material6906and different third material6908can be the same or different materials. Different second material6906and different third material6908can be the same or different from probe material6907. In one embodiment of the present invention, probe6912comprises tip coated or plated with different second material6914. Probe6912can comprise the same or different material as probe6900with different second material6914coated over the probe material. Different second material6904,6906and6914can be, but is not limited to, Cu, Al, Ag, Au, rhodium (Rh), rhenium (Re), and/or ruthenium (Ru), and/or an alloy, for example, copper beryllium and/or a member of the platinum element group or other suitable materials that are either high in conductivity or are corrosion-resistive and strong. Different third material6908can be but is not limited to, Cu, Al, Ag, Au, Rh, Re, and/or Ru, and/or an alloy, for example, copper beryllium and/or a member of the platinum element group. The flexing portion of the probe of this embodiment preferably has no material plated to avoid peeling due to probe flexing. Alternatively, the flexing portion of the probe comprises a thinner layer of a different second material than the distal end and/or tip end to allow for flexing. The different second material can comprise a highly conductive material, for example, Cu, Ag, Au and/or Al. The different second material can also optionally comprise a highly conductive alloy, for example, a copper beryllium alloy. In one embodiment of the present invention, the tip of probe6910comprises a different third material, for example, a member of the platinum element group, rhodium (Rh), rhenium (Re), and/or ruthenium (Ru). Different third material6908is preferably corrosion-resistive and strong so it can withstand the repeated contact with multiple devices under test.

In one embodiment, a probe comprises a skate disposed on a probe tip. The skate is preferably coated with a very hard and non-oxidizing material so that it is very strong and can take mechanical stresses when contacting multiple devices under test over the life of the probe. In this embodiment of the present invention, the material coating the skate is preferably rhodium (Rh), rhenium (Re) and/or ruthenium (Ru) or metal alloys or a member of the platinum element group or other similar materials. The body of the probe is then preferably springy and strong and can take mechanical stresses. The distal end of this embodiment preferably does not flex and is coated with a very conductive material to improve conductivity, CCC, thermal and electrical properties and allow the probe to carry more current. The distal end is optionally coated with a copper beryllium alloy.

In one embodiment of the present invention and referring toFIG. 75, trench7500is laser cut into a distal end of probe7502. Trench7500can extend the entire probe length or be stopped where mechanical stresses are large. Trench7500can be shallow or deep. Laser cut trenches are optionally disposed with conductive metals, alloys, or other conductive materials, for example, copper, to increase the CCC of the probe. The conductive material is preferably disposed in trench7500through electroplating, electrodepositing or sputtering. Copper core7504in trench7500at the distal end of probe7506enables solder reflow to probe7506. Probe7506with copper filled trench7500can optionally be over-plated with copper and sintered. Probe7506can optionally include trench7508at tip7510. Although copper is described herein as the conductive material disposed in the trench, other metals, alloys or other conductive materials can also be used to fill trenches7500and7508including but not limited to Al, Ag, Au, Re, Ru, Rd, and/or a member of the platinum element group and/or metal alloys. The conductive materials preferably decrease overall probe resistivity and increase CCC.

One embodiment of the present invention comprises a probe having a composition of about 40% Cu 60% W metal. Other similar compositions, such as CuxW1-x, with 0.1≦x≦0.4 are also included in this embodiment. The probes of this embodiment are preferably made using powder metallurgy, and the resulting structure is preferably a mixed-phase composite (e.g., regions of copper (Cu) mixed with regions of W), as opposed to a single-phase alloy. The matrix of this embodiment provides a strong mechanical lattice with high strength at elevated temperature, while the conductive material lowers the resistivity by about 35% at elevated temperatures. This way, the Joule heating is minimized while the mechanical strength at temperature remains very high. Another benefit of CuW probes are the low contact resistance between the probe tip and the chip, because of the conductive Cu being at the contact point between the probe and the chip contact bump/pad. However, CuW probes cannot be etched. Each component requires very different chemistry. For example, what etches Cu does not etch W and vice versa. Cu is preferably sintered onto the W probe.

One embodiment of the present invention comprises a probe having a tungsten copper mix of about 10% Cu to about 90% W. A molybdenum copper mix is also contemplated. Other additives are optionally used to lower resistivity and to manage thermal expansion of the mixed chemistry. The probes of this embodiment are preferably plated or coated with conductive metals such as, but not limited to, Cu, Al, Ag, or Au. To avoid peeling in the flexing area, the thickness of the deposited metal is preferably reduced or alternatively, the deposited metal is eliminated in the flexing area of the probe.

Another embodiment of the present invention comprises a probe made from a high CCC material, such as, but not limited to tungsten and molybdenum. An alloy, preferably a copper beryllium alloy, is then coated or plated over the probe and is preferably coated or plated over at least the distal end of the probe. When a probe comprising a copper beryllium coating on the distal end is used during testing, if the current becomes too high during testing, the probe disengages or deforms mechanically and does not carry more current that it is capable of and therefore does not have the potential to cause damage to the device under test by exploding and/or causing dust.

Laser machining is a preferable way to fabricate probes. Laser micromachining is enabled by utilizing a nanosecond or picosecond laser as well as special fixturing that enables proper holding of the probe material. The probe material is preferably a metal plate or metal foil. The fixturing elevates the material over a surface so the laser does not penetrate the surface while cutting the probe material. The fixturing enables the laser beam to go into open space as it passes through to cut the probe material. This technique eliminates “back-splash”, which is the re-deposition of molten metal onto itself that causes geometrical inaccuracies. The metal plate or metal foil probe material is mounted such that there is nothing directly underneath and touching the metal plate or foil probe material. Thus, if there is splashing from a surface on which the metal plate or metal foil probe material is disposed above, it does not redeposit on the probe material. The probe material is preferably held by its edges and supported by small columns. There is preferably nothing that the laser can penetrate underneath the metal plate or metal foil probe material and thus there is no bounce back molten material. The metal plate or metal foil probe material is preferably fixed above a surface (preferably fixed above a surface about 1 mm). A nitrogen and argon gas flush is alternatively used to avoid oxidation.FIGS. 70A and 70Bprovide further details relating to laser machining in this context.

Referring toFIG. 70A, one embodiment of the present invention comprises a method of laser machine manufacturing a probe.FIG. 70Ais a flow chart illustrating how probe design and probe material come together. The method comprises producing a CAD drawing of desired probe design7001. The CAD drawing is then input into a computer and software converts the drawing into X and Y dimension measurements for a laser to follow when manufacturing a probe. The computer then communicates the X and Y coordinates to the laser. The laser then follows the contour of the measurements to manufacture a laser probe shape. The other side of the flowchart illustrates how the probe material is chosen and tested. Desired material7002is preferably a metal foil material and more preferably a tungsten foil material. Material7002is tested at7004to insure it meets specified requirements such as, but not limited to, thickness, flatness, surface composition and material composition. If7002meets the desired specifications, particularly a thickness parameter, it is then cleaned and prepped at7006for laser machining. CAD drawing7001, in the meantime, provides laser machining parameters7008, such as, but not limited to, probe material, material thickness, and desired taper to the laser. Material7002is then laser machined7010into a probe. The probe is then tested7012and, if passes, is harvested7013and goes to probe assembly7014.FIG. 70Billustrates examples of laser machined cross-sections and laser machined probes. These examples illustrate the high quality of the laser machined cuts.

FIG. 70Billustrates laser beam7016from the tip down. Laser beam preferably penetrates and cuts through the metal plate or metal foil probe material. The thickness of the probe material is preferably about 20 microns to about 100 microns, and is preferably between about 30 microns and about 80 microns. The laser beam preferably cuts out the contour of the probe as created from a CAD drawing. Probes7018are shown still attached to the metal plate or metal foil probe material. “Rabbit ears”7020are created so that the laser when cutting can avoid 90 degree turns, which causes increased acceleration and decreased accuracy when cutting. Rabbit ears7020are smooth to allow the laser beam to go through the probe material and make a smooth turn. When laser machine manufacturing a probe, the laser preferably penetrates and cuts the probe material about 2 microns deep all the way around the contour of the probe. The laser then makes several rounds of cuts of the contour until the metal plate or metal foil probe material is cut through. In other words, the laser does not cut through the metal foil or metal plate probe material in one pass. It takes several passes of the laser to cut through the metal foil or metal plate probe material. A nano- or picosecond laser is suitable for machining probes and preferably cuts a few microns deep all the way around the perimeter of a probe and then cuts another few microns deep all the way around. It may take, for example, twenty passes with the laser to create a 40 micron thickness probe. The final resulting probe is shown as probe7028and probe7030. As a result of this laser machining method, the side surface of a probe is very smooth. Compare probes7022and7024ofFIG. 70BwithFIG. 71vertical sides to see the accuracy of the laser cuts. Openings7026allow a probe to be plated with a different second material and/or different third material on a distal end and/or tip end. Once a probe is completed being laser machined, it goes into a probe assembly.

Referring toFIG. 71, etching of probes produces non-uniform walls and cross-sections. A pitch of about 90 μm or less, in full grid array layout, requires tight control of the probe dimension.FIG. 72Ashows an etched probe width distribution with a standard deviation of approximately 6.1.FIG. 72Bshows a laser micromachined probe width distribution that results in a much narrower width distribution with a standard deviation of about 1. Etched probes have a poor yield of about 25%, as seen inFIGS. 72A and 72B. There is about a 90% critical dimension (CD) yield by laser micromachining. Thus, laser machined probes in particular, W probes and CuW probes result in a uniform cross-section and tight control of dimensions, as seen in the photographs of laser machined probes inFIG. 73. Another benefit of using laser micromachining is the ability to “sculpt” the surface of the probe material. This technique allows for creating 3-dimensional features such as narrower contact tips and skates needed to increase force/area and thus decrease contact resistance—seeFIGS. 74A and 74B. For example, normal laser beam cuts sides7400. In this embodiment, a laser beam is used to create step7402in the thickness of the probe. This technique can be regarded as analogous to milling with a laser, where each pass of the laser removes a small amount of material (e.g., about 3-5 μm). Such laser milling can be used to define various probe features, such as reduced-width probe tips. Referring toFIG. 74B, skate7404disposed on probe tip7406can be created using laser micromachining. Skate7404contacts a wafer during testing. Step7402is created by a laser removing a thickness of the probe material to result in an offset skate.FIGS. 76A and 76Billustrate probes having offset skate7606. To create centrally located skate7404, the laser removes a thickness from both sides of a probe.FIG. 74Billustrates probe material being removed to create a skate. Skate7404is centrally located which requires the probe to be flipped in order to remove thickness from both sides of the probe. As shown inFIGS. 76A and 76B, skate7606is off center and thus does not require a flip during laser machining for producing the skate.

Referring to the probe shown inFIG. 76A, distal end7602is preferably thinner than body7604as distal end7602is attached to a substrate via solder reflow to allow for testing. When a pitch of a probe is very small, it is desirable for the end of the distal portion to be smaller in diameter. Solder is attached to the substrate part, the distal end goes into a solder bump, and if distal end has a cross section that it too large, there can be electrical shorting. The taper on distal end7602is manufactured using laser machining and prevents electrical shorting when connecting to a substrate part. The same method as describe above for step7402and skate7404is also used to taper distal end7602. The laser machining method can cut thicknesses of probes as well as widths of probes when contouring.

FIGS. 77A and 77Billustrate probe7700having chisel7702. In this embodiment, when probe7700is being manufactured by laser machining from a metal plate or metal foil material, chisel7702is part of the contour shape being cut. In this embodiment, chisel tip7702is made during the manufacturing of probe7700. Chisel tip7702is preferably located in the approximate center of the end of probe7700.

One embodiment of the present invention comprises a method of laser machining probes. This method preferably comprises producing non-linear optical absorption pulse with a laser, preferably a nanosecond or a picosecond laser. The pulse results in a peak power that make it possible to produce high quality and accurate laser drilled features. In one embodiment of the present invention, nanosecond end/or picosecond pulses are combined with a small spot size (small spot size is the size of the laser beam light that is used to cut through the material when making a probe) to produce high accurate cuts. The width of the cut through material is approximately equal to size of spot. Thus, spot size is preferably small to cut small parts, preferably a spot size less than approximately 20 microns, produces high irradiances (preferably greater than about 1 terrawatt (TW)/cm2). Small spot size allows for enhanced absorption of the incident laser light. The enhanced absorption improves the laser micromachining quality. A short wavelength [nano-(10−9) and pico wavelength (10−12)] does not cut through a probe material in one pass, but gently removes a thin layer of probe material during each pass. Each cut is precise without causing stress to the probe material so that individual photons have enough photon energy so that fewer photons are required to excite individual electrons across a bandgap. It is preferable to have localized removal of probe material. In one embodiment, about 2 microns of probe material is removed per laser pass in order to prevent overheating the probe material and thereby damaging the material. Picosecond lasers are preferable due to the increased optical absorption of shorter pulse durations which produce high edge quality. See, for example,FIG. 73.

Referring toFIG. 78, probe7800is shown attached to frame7802after being laser machine manufactured. InFIG. 78, distal end7804is attached to frame7802on a side of distal end7804, a vertical side of distal end7804. Probe7800is then snipped from frame7802without creating inaccuracy by creating a variance in dimension. In an alternative embodiment, probe7800can be cut from frame7802with a laser.

In an embodiment of the present invention, tooth7806of frame7802is attached to probe7800on a vertical side of distal end7804of probe7800instead of the very tip of distal end7804. The tip of the distal end of the probe is cut to precise dimensions. If a metal is deposited at the very tip of the distal end, snipping at that location and exposing the metal under the coating can cause errors. Snipping probe7800off frame7802at tooth7806prevents errors since this portion of probe7800is not critical. The base metal that is exposed in that area does not cause errors.

Another embodiment of the present invention comprises an ultrashort laser pulse ablation method that emits particles up to several hundred nanoseconds after a laser pulse. A thin surface layer on the order of the optical penetration depth is ablated from a probe material by electron emission, sublimation and transition to the plasma state. This ablation proceeds on a nanosecond time scale or faster. The remaining heat diffuses into the probe material and leads to the emission of particles and droplets after several nanoseconds, preferably between 1 and 100 nanoseconds, and more preferably between 5 and 50 nanoseconds and most preferably approximately 10 nanoseconds due to a thermal boiling process. Note that the material ejected from the probe in the initial ablation step preferably carries away much of the deposited laser energy, especially when working close to the ablation threshold, thereby minimizing the amount of thermal energy that later diffuses into the probe material. Only for femtosecond (the SI unit of time equal to about 10−15of a second) pulses does the thermal energy diffuse into the probe material, and it preferably leads to reduced collateral damage to the probe material surrounding the ablation zone.