Testing head for microstructures

Abstract: The invention relates to a testing head for microstructures. The testing head (1) includes a top guide plate (2) and bottom guide plate (3), separated by an air gap (9). Each of the plates (2, 3) include respective guide holes (4, 5) for accommodating a contact probe (6) having a contact tip (7) that is arranged to mechanically and electrically contact a contact pads (8) on a device under test. The contacting tip (7) of the testing head (6) has a non-zero pitch angle (α OUT ) relative to the contact pad (8), and "scrubs" the pad as the device under test is drawn against the contacting tip (7), causing the contact probe (6) to bend within the air gap (9). The invention further relates to a method for creating an electro/mechanical connection between a testing head for microstructures and a device to be tested.

Description: Technical Field The present invention relates to a testing head for microstructures. More particularly, the invention relates to a testing head comprising: a top guide plate having a top guide hole formed therethrough; a bottom guide plate having a bottom guide hole formed therethrough, said bottom guide plate being separated from said top guide plate by an air gap; and a contact probe having a contacting tip arranged to mechanically and electrically contact a contact pad of a test device. The invention further relates to a method for creating an electrical and mechanical connection between a testing head for microstructures and a device to be tested. The invention relates, particularly but not exclusively, to a testing head for use on semiconductor integrated devices and the following description is made with reference to this field of application for convenience of explanation. Prior Art A testing head is essentially a device adapted to electrically interconnect a number of contact pads of a microstructure with corresponding paths of a measuring machine employed to carry out the testing. The circuit testing procedure is used to detect any faulty integrated circuits directly at the manufacturing stage of the circuits. Testing heads are generally employed to electrically test the integrated circuits on the wafer itself before the circuits are separated and inserted into a chip package. A testing head comprises one or more pairs of parallel guide plates placed a given distance apart (to leave an air space therebetween), and a set of special movable contact elements. The pair of guide plates consists of a top guide plate and a bottom guide plate, both of which are formed with guide holes for the movable contact elements to be passed therethrough. The individual contact elements are typically small wires made of special alloys with good electrical and mechanical properties. These contact elements will be referred to as "probes" or "contact probes" through the remainder of this specification, to highlight the function that they serve. A good contact between the probes and the contact pads of a device under test is achieved by keeping the testing head pressed against the device, with the probes bending or flexing in the air gap between the guide plates. Testing heads of this type are commonly known as "vertical probes". The amount of bending undergone by the probes, and the force required to produce the bending, is related to a number of factors, including the physical characteristics of the alloy used to make the probes, and an amount of offset between the guide holes in the top plate and the corresponding guide holes in the bottom plate, as well as other factors. Excessive bending of the probes should be avoided, however, because a probe may be flexed too much and not return to its original shape, or may otherwise become stuck in the guide holes. It should be noted that, for the testing head to perform satisfactorily, the probes must be allowed an amount of axial play in the guide holes. Thus, if a single probe breaks, the broken probe can be removed and replaced, without the need to replace the entire testing head. These factors should be taken into account when manufacturing the testing head, because a good electric connection between the probes and the device under test is mandatory. In some cases, the contact probes are affixed to the top guide plate of the testing head in a permanent manner. This is known as a clamped probe testing head. However, testing heads with loose-mounted probes are more frequently used, where the probes are electrically connected to a "board" by a microcontact interface called the "space transformer". This is known as a loose probe testing head. In the latter case, each contact probe has a second contacting tip opposite the one used to contact the device under test. This second contacting tip is aimed at one of the contact pads on the space transformer. A good electric contact is established between the probes and the space transformer in a similar way as the contact to the device being tested, i.e., by pressing the probes against the contact pads on the space transformer. One advantage of a loose probe testing head is that one or more faulty probes, or the whole set of probes, can be replaced with greater ease than if clamped probe testing heads are used. However, the top and bottom guide plates must be designed to hold the contact probes in place, even when no device is abutting their contacting tips for testing, or when the whole set of probes is moved during a replacement operation. From the French patent application No. 2 535 064 filed on 21.10.1983 is known a testing head comprising a plurality of probes, such probes having respective bent portions. Also known from the US Patent No. 5,952,843 to Vihn is a variable contact pressure probe having a spring formed in a central portion thereof to provide a contact force control area. Moreover, the US patent No. 3,217,283 describes a miniature printed circuit pinboard comprising a plurality of pinholes including a plurality of straight line bores able to receive flexible conductive wire members in a substantially straight manner. On insertion, the wire members flex to conform to the longitudinal tortuous shape of the pinholes. Summary of the Invention Embodiments of the invention provide a testing head for microstructures that makes firm electric contact with a device under test, holds the probes securely in their guides, and minimizes the likelihood of bent probes becoming stuck in their guides. Presented is a device that has contact probes whose contacting tips meet the contact pads under a non-zero pitch angle and scrub their surfaces the moment a device to be tested is drawn against the contacting tips, thereby causing the contact probes to bend within an air gap. Additionally presented is a method of creating an electrical and mechanical connection between a microstructure testing head and a device to be tested. The features and advantages of a testing head according to the invention will be apparent from the following description of embodiments thereof, given by way of non-limiting examples with reference to the accompanying drawings. Brief description of the drawings Figure 1 is a cross-sectional view of a testing head according to an embodiment of the invention. Figure 2 is a diagram illustrating mechanical aspects of the operation of the testing head of Figure 1. Figure 3 is a cross-sectional view of a testing head according to an example useful for understanding the invention of the invention. Figures 4A and 4B are a diagrams illustrating mechanical aspects of the operation of the testing head of Figure 3. Figure 5 is a cross-sectional detailed view of a testing head according an embodiment of the invention. Figure 6 is a diagram of a clamped-probe testing head. Figures 7 and 8 are diagrams of loose-probe testing heads. Figure 9 is a diagram illustrating a replacement operation in a testing head like that shown in Figures 7 and 8. Figure 10 is a cross-sectional view of a guide for a testing head according to an embodiment of the invention. Detailed description With reference to Figure 1, a testing head 1 according to an embodiment of the invention is shown. The testing head 1 includes a top guide plate 2 and a bottom guide plate 3. A top guide hole 4 is formed in the top guide plate 2, and a bottom guide hole 5 is formed in the bottom guide plate 3. Although for clarity only one set of guide holes 4, 5 and one contact probe 6 is shown, typically a testing head includes several contact probes, for example between 10 and 3000. The invention is equally applicable to testing heads having any number of contact probes. A contact probe 6 passes through the top guide hole 4 and the bottom guide hole 5. The contact probe 6 has a first section 6a near the top guide hole 4, a second section 6b near the bottom guide hole 5, and at least one contacting end or tip 7. The tip 7 of the contact probe 6 is used to contact the device being tested. The other end of the contact probe is coupled to the testing device, in order to read signals from the device being tested through the contact probe, as will be discussed below. In operation of the testing head 1, the contacting tip 7 is brought to touch a contact pad 8 on a device under test, thus establishing a mechanical and electrical connection between the device and a testing apparatus (not shown). The testing head 1 represents the working end of the testing apparatus. When the contact tip 7 of the probe 6 properly touches the device, enough force is placed on the probe to cause it to flex or bend in an area of the probe located between the first section 6a and the second section 6b. Providing enough pressure to the probe 6 to cause it to bend ensures that a good mechanical and electrical connection is made between the tip 7 and the contact pad 8. The top and bottom guide plates, 2 and 3, are suitably separated by an air gap 9 to accommodate the bent portions of the contact probes 6. The top and bottom guide holes, 4 and 5, are adequately sized to receive the contact probes 6. Advantageously, the contacting tip 7 of a contact probe 6 is set to meet the contact pad 8 at a pitch angle α OUT . This pitch angle α OUT causes the contacting tip 7 to "scrub" the contact pad 8 as pressure is applied to the probe 6, so as to improve the electric contact established between the tip 7 and the pad 8. Even if, for example, the pad 8 is soiled, oxidized, or otherwise inhibiting good electrical contact. Of course, the contacting tip pitch angle α OUT between the probe tip 7 and the contact pad 8 should be determined for optimum scrubbing effect and minimum resistance of the tip-to-pad contact, as well as to avoid the risk of the tip 7 skidding off of the contact pad area and possibly causing damage to the device under test or to the testing head 1 itself. To provide a testing head 1 with contact probes 6 pitched at a suitable angle α OUT to the contact pad 8, different embodiments of the invention use a number of variations in the top and bottom guide plates of the testing head. For convenience of explanation, the same reference numerals will be used through the remainder of this description to designate substantially similar elements in the various embodiments of the invention. By way of example, the terms "horizontal" and "vertical" will be used in relation to the drawings, although complementary or other arrangements are possible. In a first embodiment, the testing head 1 has at least one guide plate formed with a crooked guide hole that is effective to deform the contact probe 6 and set the contacting tip 7 at a predetermined pitch angle α OUT . This arrangement will be hereinafter referred to as "constant scrub angle" because the contacting tip 7 is always held at a constant angle relative to the device being tested. In a preferred embodiment of the invention shown in Figure 1, the bottom guide plate 3 has an S-like shaped crooked bottom guide hole 5. The S-like shaped pattern is effective to set the contact probe 6 into a predetermined position by frictional engagement at a plurality of support points A, B, C and D on the interior of the bottom guide hole 5. Embodiments of the invention can also include a top guide hole 4 in the testing head 1 that holds the probe 6 at an angle somewhat slanted from vertical. This arrangement can facilitate a smoother bending of the probe 6 than if the probe 6 is held perpendicular relative to the top guide plate 2. Preferably, the angle of the guide hole 4 through the top guide plate 2 may be offset from perpendicular between about 0 to 10°, but may be any angle that facilitates the proper bending of the probe 6. In some instances the best perpendicular offset angle may need to be determined by trial and error. The probe 6 may be attached to the guide plate 2 in a firmly held or a sliding fit. Alternatively, the top guide plate 2 could be formed with an S-like shaped crooked top guide hole 4, and not the bottom guide hole 5, or both the top and bottom guide plates 2, 3 could be formed with S-like shaped crooked guide holes 4, 5. Furthermore, differently shaped crooked guide holes could be provided, such as curvilinear holes, still providing a number of points of support. As used in this description, "crooked" is meant to describe any form of guide hole that is non-straight, including those holes with one bend, such as a "C" shape, two bends, such as an "S" shape, or any number of bends, such as a "stairstep" profile, as well as others. Additionally, the guide holes 4, 5 in the guide plates 2, 3, may have smooth shaped bends (referred to herein as "curvelinear", no matter how many bends the hole includes), such as that shown in Figure 1, or they could be rather abrupt, such as those possible to be made by having a guide plate formed by multiple layers, shown in Figure 10 and discussed below. The mechanical aspect of the operation of a constant scrub angle type of the testing head 1, having top 2 and bottom 3 guide plates formed with crooked guide holes, is as described herein below. A device under test properly drawn against the testing head 1 will produce "overtravel" of the contact probe 6 in the air gap 9, meaning that downward force is applied to the top of the contact probe 6 while keeping the device steady, or upward force is applied to the bottom tip 7 of the contact probe 6 by the device under test while keeping the top guide plate 2 steady, in excess of that merely necessary to make an initial contact between the contact pad 8 and the probe tip 7. This overtravel will cause the probe 6 to bend. But the support points A, B, C and D bind the movement of the contacting tip 7 to a constant pitch angle α OUT to the contact pad 8. Thus, an upward movement or overtravel of the contact probe 6 is accompanied by a horizontal or scrubbing movement of the contacting tip 7, over the contact pad 8, with the pitch angle α OUT being held constant, as shown schematically in Figure 2. In other words, a desired scrubbing action can be achieved by suitably adjusting the pitch angle α OUT and the amount of overtravel of the contacting tip 7. In another modification, the testing head 1 has at least one guide plate 2, 3 formed with non-vertical guide holes 4, 5 that deflects the contact probe 6 within the air gap 9 and shifts the other guide plate in the horizontal direction in order to force a bend in the contact probe 6 and set its contacting tip 7 at a predetermined pitch angle α OUT . This arrangement will be referred to as "variable scrub angle" hereinafter, because the angle at which the contacting tip meets the contact pad 8 will change as the contact probe 6 flexes in the air gap 9. In an example useful for understanding the invention of the invention shown in Figure 3, the top guide plate 2 has a straight non-vertical top guide hole 4, and the bottom guide hole 5 in the bottom guide plate 3 does not have the same angle deflection. Additionally, the guide holes 4 and 5 may be horizontally shifted relative to one another, for example, between 1 um and 4 mm, and, more preferably, between 1 um and 1.5 mm. This arrangement forces a bend in the contact probe 6 and pitches its contacting tip 7 at an angle α OUT relative to the contact pad 8. The contact probe 6 has at least two support points E, F in the bottom guide hole 5, as shown in Figure 3. The probe 6 can either be a sliding fit or held firmly in the top guide hole 4. Having the top guide hole 4 lie non-perpendicular to the top guide plate 2 encourages the contact probe 6 to bend, and keeps the bent section of the probe 6 constrained within the air gap 9. The mechanical aspects of the operation of a variable scrub angle type testing head 1, having a top guide plate 2 formed with non-vertical holes and a bottom guide plate 3 adapted to be shifted relative to the top guide plate 2 in the horizontal direction, is as described herein below. A device under test, drawn against the testing head 1, produces "overtravel" of the contact probe 6 in the air gap 9, so that the probe is bent. This is similar to the constant scrub angle type testing head 1, described above. In this case, where the testing head 1 has a variable scrub angle, however, the movement of the contacting tip 7 of the probe 6 is not required to maintain a constant pitch angle α OUT relative to the contact pad 8. The tip 7 moves progressively into a vertically upright position over the pad 8, and in so doing, the tip 7 scrubs the pad 8 as shown in Figures 4A and 4B. Once the bending force of the probe 6 is released (for instance by removing the device under test), the probe 6 can drop smoothly through the bottom guide hole 5 into its initial position. In particular, as shown in Figure 4A, the contact pad 8 of the device under test is brought in contact with the probe 6. As the probe 6 flexes as indicated in Figure 4A, the friction arrangement of the support points E and F in the bottom guide hole 5 also causes the probe tip 7 to scrape the contact pad 8, as the probe 6 continues to flex into the shape as shown in Figure 4B. The ending position of the probe 6, because the probe is nearly vertically aligned in the bottom hole 5, effectively prevents the probe 6 from becoming stuck in the bottom guide hole 5. In a further modification of the guide plates, which is useful with both of the above arrangements, a film 10 of an elastic material is provided, as shown in Figure 5, on either the top or bottom guide plates, or both, to hold the probes more positively inside their corresponding top and bottom guide holes, yet still maintaining a sliding fit arrangement. In addition, the elastic material film 10 contributes to prevent the probe from sticking in the guide holes by virtue of the elastic bias that it applies to the probes as these are bent against the film. In particular, the elastic bias from the film facilitates the probe 6 retraction from the holes 4 or 5 as the bending force is removed. As discussed previously, the contact probes 6 may be fixedly mounted on the testing head 1. For that purpose, the top guide plate 2 would securely hold a plurality of long wires in the top guide holes 4, e.g. bonded therein with a resin, or some other suitable material. The long wires continue through the top guide holes 4 and are conventionally soldered at a bond 20 to a board 11 of the testing head 1, as shown in Figure 6. This arrangement forms a clamped probe testing head. It should be noted that the foregoing arrangements could be used with loose-mounted probes as well, for example, as shown in Figure 7. In that Figure, a microcontact interface 12, known as a "space transformer," is itself electrically connected to the board 11. In this case, the contact probes 6 are attached neither to the bottom guide plate 3 nor to the top plate 2, and would be formed with additional contacting tips 7a aimed at a plurality of contact pads 8a on the space transformer 12. The electric contact of the probes with the space transformer may be established in the same way as the contact to the device under test, by urging the probes 6 onto the contact pads 8a of the space transformer, the latter being held away from the device under test by side spacers 13a and 13b, as shown in Figure 8. In particular, the contact pads 8a on the space transformer 12 are aligned with micrometric accuracy to the contacting tips 7A that jut out from the top guide plate 2. This yields a loose probe type testing head. The testing head 1 with loose-mounted contact probes 6 allows the set of probes 6, sometimes referred to as a probe block 14, to be easily replaced. As shown in Figure 9, one or more faulty probes 6 within the probe block 14 can be easily replaced, with no need for replacing soldered joints as would be required if the clamped probe testing head 1 of Figure 6 were instead used. In particular, with respect to Figure 9, the contact probes 6 in a probe block 14 may slide within their top 2 and bottom 3 guide plates, but not completely out of the plates, on the occasion of a faulty contact probe 6 or the whole probe block 14 being replaced, or when no device is abutting the testing head 1 for testing purposes. Advantageously, either of the previously discussed arrangements, i.e. the constant scrub angle or the variable scrub angle arrangements, could be applied to a testing head 1 with loose mounted contact probes 6. In this case, in addition to an improved electric contact between the probes 6 and the device under test from the scrubbing action of the contacting tips 7, a frictional resistance is created between the guides 4, 5 and the probes 6, effective to stop the contact probes 6 from slipping out while the probes 6 or the probe block 14 are being replaced, or when no device is abutted against the testing head 1 for testing. Accordingly, guide plates formed with crooked guide holes, in particular holes having an S-like pattern, can be used, and the guide plates be suitably offset in the horizontal direction to force a deformation in the contact probe, thereby ensuring that the probe will stay in place even while probes or probe blocks are being replaced, or no device is abutting the head for testing. This effect can also be achieved using a film of an elastic material 10, to be applied preferably on the inner face of the top guide plate. With the elastic material 10 so applied, the film is formed with contact probe clearance holes having a smaller diameter than the corresponding guide holes, so that the probes 6 will be retained within the guide holes 4, 5 by the slight frictional drag created between the film 10 and the probes. A further embodiment of a portion of this invention is shown in Figure 10. Guide plates having crooked guide holes could be implemented in the form of stacks of thinner guide plates 22, laid in mutual contact to define a guide hole having a desired pattern, as shown schematically in Figure 10. In other words, according to the invention, an electro/mechanical connection between the testing head 1 and a device to be tested is created, by means of the following steps: holding the contact probe 6 within the testing head 1 with the contacting tip 7 at a pitch angle α OUT non-perpendicular to the contact pad 8; causing the contacting tip 7 to scrape against the contact pad 8 as the device to be tested is pressed against the contacting tip 7; and causing the contact probe 6 to bend as the device to be tested is further pressed against the contacting tip 7. In particular, the contact probe 6 is held in a frictional relationship with a plurality of support points located in the guide holes of the guide plates. It should be noted that while the device to be tested is pressed against the contacting tip 7, which scrapes against the contact pad 8, the pitch angle α OUT of the contact tip 7 changes.

Claims: A testing head (1) for microstructures comprising: a top guide plate (2) having a top guide hole (4) formed therethrough; a bottom guide plate (3) having a bottom guide hole (5) formed therethrough, said bottom guide plate (3) being separated from said top guide plate (2) by an air gap (9); and a contact probe (6) having a contacting tip (7) arranged to mechanically and electrically contact a contact pad (8) of a test device, in such a way that, as said test device is drawn against said contacting tip (7), said contacting tip (7) has a non-zero pitch angle (α OUT ) relative to said contact pad (8) and is structured to scrub said contact pad (8) against said contacting tip (7), causing said contact probe (6) to bend within said air gap (9) characterized in that at least one of said guide plates (2, 3) has guide holes (4, 5) formed with a crooked shape that is structured to force said contact probe (6) to bend within said air gap (9), and said contacting tip (7) to deflect according to said pitch angle (α OUT ) relative to said contact pad (8) of said test device. Testing head (1) for microstructures according to claim 1, characterized in that at least one of said guide holes (4, 5) has a curvelinear pattern to hold said contact probe (6) firmly in a predetermined position by friction between said contact probe (6) and said guide holes (4, 5) at a plurality of support points. Testing head (1) for microstructures according to claim 1, wherein at least one of said guide holes (4, 5) is shaped in an S-like pattern to hold said contact probe (6) firmly in a predetermined position by friction between said contact probe (6) and said guide holes (4, 5) at a plurality of support points (A, B, C, D). Testing head (1) for microstructures according to claim 1, characterized in that at least the other one of said guide plates (2, 3) has guide holes (4, 5) formed with substantially straight shapes non-perpendicular to said other guide plate (2, 3), and structured to deflect said contact probe (6) within said air gap (9) in order to facilitate said contact probe (6) to bend. Testing head (1) for microstructures according to claim 1, characterized in that at least the other one of said guide plates (2, 3) has guide holes (4, 5) formed with a crooked shape. Testing head (1) for microstructures according to claim 1, characterized in that at least the other one of said guide plates (2, 3) has guide holes (4, 5) shaped in a curvilinear pattern. Testing head (1) for microstructures according to claim 1, characterized in that at least the other one of said guide plates (2, 3) has guide holes (4, 5) shaped in an S-like pattern. Testing head (1) for microstructures according to claim 1, characterized in that said top guide plate (2) and said bottom guide plate (3) are horizontally shifted relative to each other and have corresponding guide holes (4, 5) which are not aligned relative to each other to force said contact probe (6) to bend, and said contacting tip (7) to deflect according to said pitch angle (α OUT ). Testing head for microstructures according to claim 8, characterized in that at least one of said guide plates (2, 3) have guide holes (4,5) formed with substantially straight shapes non-perpendicular to said another guide plate (2, 3), and structured to deflect said contact probe (6) within said air gap (9) in order to facilitate said contact probe (6) to bend. Testing head for microstructures according to claim 8, characterized in that at least one of said guide plates (2, 3) has guide holes (4, 5) formed with a crooked shape. Testing head (1) for microstructures according to claim 8, characterized in that at least another one of said guide plates (2, 3) has guide holes (4, 5) shaped in a curvilinear pattern. Testing head (1) for microstructures according to claim 8, characterized in that at least another one of said guide plates (2, 3) has guide holes (4, 5) shaped in an S-like pattern. Testing head (1) for microstructures according to claim 10, characterized in that at least another one of said guide plates (2, 3) has guide holes (4, 5) formed with substantially straight shapes non-perpendicular to said another guide plate (2, 3). Testing head (1) for microstructures according to claim 10, characterized in that at least another one of said guide plates (2, 3) has guide holes (4, 5) formed with a crooked shape. Testing head for microstructures according to any of the claims 1 to 14, characterized in that it further comprises a film of an elastic material (10), to be applied on at least one of said guide plates (2, 3) to hold said contact probes (6) more positively inside corresponding guide holes (4, 5), yet still maintaining a sliding fit arrangement. A method for creating an electrical and mechanical connection between a testing head (1) for microstructures and a device to be tested, the method comprising the following steps: holding a contact probe (6) within said testing head (1) with a contacting tip (7) thereof at a pitch angle (α OUT ) non-perpendicular to a contact pad (8) on said device to be tested, prior to a time when said contact pad (8) touches said contacting tip (7) by holding a portion of said contact probe (6) in a frictional relationship with a plurality of support points (A, B, C, D) located in a crooked guide hole (4, 5) in a guide plate (2, 3); causing said contacting tip (7) to scrape against said contact pad (8) as said device to be tested is pressed against said contacting tip (7); and causing said contact probe (6) to bend as said device to be tested is further pressed against said contacting tip (7). Method according to claim 16, characterized in that it further comprises: while said device to be tested is pressed against said contacting tip (7) and said contacting tip (7) is scraping against said contact pad (8), allowing said contacting tip (7) to change said pitch angle (α OUT ). Method according to claim 16, characterized in that it further comprises, before said contact pad (8) touches said contacting tip (7): inserting said contact probe (6) through a top guide hole (4) in a top guide plate (2) and a bottom guide hole (5) in a bottom guide plate (3) wherein said top guide hole (4) and said bottom guide hole (5) are not vertically aligned with one another. Method according to claim 18, characterized in that it further comprises applying a layer of elastic material (10) to at least one of said top and bottom guide plates (2, 3).

Inventors: CRIPPA GIUSEPPE, FELICI STEFANO, CRIPPA, GIUSEPPE, FELICI, STEFANO
Applicants: TECHNOPROBE S P A, TECHNOPROBE S.P.A
Application year: 2001
Main classification: G01R1
IPC codes: G01R1