The invention relates to the testing of items of sheet material for mechanical stability as the items pass along a line carrying the items, while the line is in continuous movement in a direction parallel to the plane of the items of sheet material. In particular (but not exclusively), the invention relates to the testing of wafers of silicon (or silicon based material such as silicon carbide) suitable for use in photovoltaic cells.

Background of the Invention.

The term 'sheet material' as used herein could relate to brittle sheets such as wafers of silicon, or panes of glass, or thin sheets of metal in a brittle condition, or other items of similar properties. The sheet material is substantially shaped to resemble a four sided polygon. Examples of suited shapes of the sheet material include, but are not limited to; square, rectangle, and pseudo-square (an originally circular sheet material where the peripheral portions are cut-off to form a square with rounded corners).

Three methods of testing for items of sheet material such as wafers of silicon are widely used to determine the acceptability of these items. These methods are Three/Four Line bending, Twist bending and Ring on Ring bending. All three methods are used in off-line measurement of acceptability of silicon wafers. Offline measurement takes a significant time because of the need to align the wafers manually for the test. As a result, only a few sample wafers can be tested from each batch. In Off-line testing, it is necessary to determine the load under which the items break. The items which accept a certain degree of bending without breaking or cracking can be designated Ά-wafers' indicating 'acceptable wafers' . These which do break or crack are unacceptable. Parameters of acceptability depend upon the end use to which the items may be put.

However, if Twist bending is used, it is particularly susceptible to camera resolution and is also dependent on crack detections capability. For this reason, wafers have to be moved to another instrument for more detailed investigation to check cracks and other defects. This extends the duration of the test, and so is impractical to use for more than a few wafers in a real production line.

Bending about specific axes - such as Three line bending or Four line bending - is carried out by successive steps of bending an item of sheet material about an axis in one direction (in the case of a rectangular sheet for instance in a direction parallel to one pair of sides), and then relaxing the bending forces and bending the item about an axis in a direction at a specific angle to the axis of the first bending operation. Three/Four line bending has the advantage in off-line testing that it examines a larger surface area than the other two methods used off-line. However, Four line bending is more susceptible to dependency on saw-mark direction.

By contrast, Twist bending consists of bending about one, or alternatively sequentially about both, of the diagonal axes of the sheet material. The term

'diagonal' is not intended to mean diagonal in the strict mathematical sense of the term, where the diagonals are two linear segments where each of them is linking one of the two sets of non-adjacent vertices of the four sided polygon. Thus, as used herein, the term "diagonal" means a bending axis imposed on the sheet material which is more or less in parallel with one of its diagonals (now in the mathematical sense of the term) and which may be somewhat misaligned from the diagonal (in the mathematical sense of the term) such that the bending axis does not necessarily run over two non-adjacent vertices (corners) of the sheet material. That is, the diagonal (now in the meaning as defined above) may be allowed to deviate a certain extent such that it run from a position on the periphery of the sheet material a distance apart from one vertex to a position adjacent on the periphery of the sheet material a distance apart from the non-adjacent vertex.

Private experimental work has been carried out on the in-line and off-line application of Three and Four Line bending. However, this has shown that in-line application of Twist bending (again on a private experimental basis) gives better discrimination between acceptable and unacceptable wafers. Early experimental results showed that in-line Twist bending showed results with discrimination approaching twice that shown by in-line Four Line bending.

Summary of the Invention

The invention provides a module for in-line identification of weak items of sheet material such as wafers as they pass along a moving line of such items, including provision to apply twist to the items as they pass along the line by applying bending in one, or alternatively sequentially above each of its two generally diagonal directions, in which there is means whereby the applied degree of twist can be selected, such that the applied degree of twist will not crack items with sufficient strength to provide acceptable products, so that after satisfactorily resisting the applied twist, acceptable wafers continue along the moving line, and after breakage or cracking unacceptable wafers can be removed from the line.

It is preferred that the twist is applied by rotatable discs above and below the moving line of items, and in which the discs are mounted for rotation about axes parallel to the plane of the items and perpendicular to the direction of movement of the sheets or wafers. It is further preferred that the rotatable discs are mounted on rollers with axes parallel to the axes of rotation of the discs, and there is means to move the rollers and with the rollers the rotatable discs, so that the disposition of the discs will apply local point loads to the items to create the bending about the generally diagonal directions and so to twist the items.

It is still further preferred that adjacent rollers are mounted for coordinated operation to bring the discs into contact with the items on the line. Advantageously, the module can be configured to accept generally square items of silicon or silicon based material such as silicon carbide, with thicknesses of between 50 and 400 μιη, preferably in a range from 100 to 250 μιη and more specifically in a range from 150 to 210 μιη. In a form in which the twist is applied by discs, it is preferred that discs are arranged to move from the edges of the items towards the middles of the items, but the discs may alternatively be arranged to move from the middles of the items towards the edges of the items.

If the twist is applied by discs it is preferred that the discs have tyres of suitable material not to damage the surfaces of the items e.g. by scratching or other contamination

Advantageously, the displacement of discs to create twist bending is between 1 to 10 mm, and more specifically between 3 and 4 mm.

Advantageously, the items are normally carried by spaced apart belt conveyers with the edges of the items parallel to the direction of travel of the in-line motion.

The module as described above may be used in combination with one or more other modules arranged in series along an in-line production line, in which the other modules are set up to apply different degrees of twist.

Brief description of the Drawings

A specific embodiment of the invention will now be described by way of example with reference to and as shown in the accompanying drawings, in which:-

Fig la shows a module for testing items of sheet material in a production line,

Fig lb shows the same module from a different perspective,

Fig lc is a side view of the module illustrated in Figs l a and lb,

Fig 2 is a side view corresponding to Fig lc with an item at the start of a test, Fig 3a is a side view corresponding to Fig l c and Fig 2 while the test is underway, but with the items removed for clarity,

Fig 3b is an end view on the arrow 'A' of the situation in Fig 3a.

Fig 4a shows the configuration of the module components after a test, and

Fig 4b shows the same configuration as a narrow angle perspective view.

Detailed description of the Specific Embodiment

This embodiment of the invention deals with a module for testing wafers of silicon intended for use in photovoltaic cells. These crystalline wafers may be referred to as 'items', and could, with other parameters, be panes of glass or thin sheets of brittle metal or of brittle composites or of ceramics.

As shown in Fig la and Fig lb, a testing module 10 - to be set in a production line for silicon wafers - has a two spaced apart belt conveyors 1 1 and 12 set in the direction of movement of the in-line path of the wafers. One wafer is shown as 14. This wafer is shown as square in the illustrations. In practice, such wafers are predominately generally square, but in some cases may have the corners cut back to make an eight sided shape, with four long sides and four short sides. The square wafer has plan dimensions (between the long sides) of 156x156 mm, and has a thickness of 0.18 mm. The wafers pass down the line at a rate of one wafer/sec. Conveniently the testing module 10 is set directly downstream of a dryer (not shown). Use of the invention is not limited to a thickness of 0.18 mm. The thickness could be from 50 - 400 μιη with PV wafers as described in this specific embodiment. However more specific ranges of wafer thickness may advantageously be used such as 100 - 250 μιη or particularly 150 - 210 μιη. Greater thicknesses might be accommodated with other materials and item sizes.

The testing module 10 has four rollers TL, TR, BL and BR. Each roller has a contact disc close to one of its ends. The contact discs have axes parallel to the axes of the associated rollers and have their axes offset from the axes of the rollers.

Roller TL has disc 15 at its far end as seen in Fig l a and Fig lb, roller TR has a disc 16 at its near end, and rollers BL and BR have contact discs 17 and 18, at their near and far ends respectively. The top discs 15 and 16 can be fixed, and have no contact with the wafers as they pass through the module.

Normally, discs 17 and 18 are located below the belt conveyors 1 1 and 12, and have no contact with passing wafers. In operation, discs 17 and 18 push the wafers 14 upwards against the discs 15 and 16, as described below. As an example, the discs 15 to 18 for this particular module may be 25 mm diameter. The number of rollers and discs is not critical, and rollers and discs in excess of four may be added if appropriate. It is emphasised that use of the invention is not limited to four rollers. By keeping a wafer in contact with the belt conveyers 1 1 and 12 throughout the entire passage of the wafer through the module, Twist bending can be effected with fewer than 4 discs.

The discs 15 to 18 are in this case made with their contact surfaces added as tyres, so that a material which will not damage the silicon wafers can be used as contact surfaces for the discs. The tyres have arcuate surfaces to contact the wafers in order to avoid scratching the wafers, particularly at their edges. For discs which will contact silicon wafers, a suitable tyre material is 'PEEK' . PEEK is an acronym for Polyetheretherketone, and has a number of different grades.

As may be seen particularly from Fig lc, the discs 15 to 18 are inset into the surfaces of the rollers TL to BR so that the discs can rotate about axes parallel to but spaced from the axes of the rollers.

Fig lc shows a first position of the rollers and discs. (This is the configuration of the module 10 when out of use.) Here the discs 15 and 16 are in position below the rollers TL and TR respectively. The discs 15 and 16 will remain in these positions throughout the testing process. Discs 17 and 18 are in position below the rollers BL and BR respectively. (However, in general, the discs 15 and 16 can be in any position, but when the bottom discs 17 and 18 lift the wafers from the conveyor, the upper discs 15 and 16 should be ready in the position shown in Fig lc.) As the test proceeds, the rollers BL and BR will be rotated through 180deg, so that the discs 17 and 18 are above the rollers. For convenience, but not essential to the invention, the rollers BL and BR are linked by a crank 19. The crank enables one driven roller BR to rotate the other roller BL so that both discs 17 and 18 reach the path of the wafer 14 at the same time. It is possible that both rollers are driven independently, but a single drive to BR with the crank 19 is preferred.

The module 10 has opposed pairs of closely spaced external side plates 21 (of which only one is shown). These side plates have slots 22 to mount individually the roller support shafts such as shaft 23 for roller TL. One slot maintains the upper rollers and the second slot maintains the lower rollers. The slots allow for repositioning of the rollers relative to the path of the wafers 14. This may be done by the use of two pairs of micrometers installed between the two parts of the closely spaced plates 21. One pair of micrometers one can be arranged to change the distance between front and back rollers (horizontal distances). This will have an indirect effect on the bending displacement. The other pair of micrometers can change the distance between top and bottom rollers and thus directly change the bending displacement. So the distances between the rollers' axes can be varied in both x and z directions.

The module 10 also has a sensor (not shown) some 10cm before roller TL, which activates the testing mode of the module as described below. In use, the roller BR is rotated anti clockwise (and so roller BL is rotated clockwise) to bring the discs 17 and 18 into the direct path of the wafer 14. Fig 2 illustrates a configuration in which the discs have just been brought into the path of the wafer. The weight of the wafer 14 is still on the belt conveyors 1 1 and 12. Optionally, the top rollers TL and TR can rotate in the same direction as the bottom rollers BL and BR, but it is preferred that they are fixed in place without rotation.

Preferably the bottom discs move from the edges of the wafer 14 towards it's middle in order to produce a better stress regime in the wafer, but it is possible for the opposite movement to be made.

Fig 3a and Fig 3b are drawn without the wafer 14 for clarity. In this case further rotation of the rollers has brought the discs 17 and 18 into positions to exert maximum diagonal bending or 'twist' on the wafer 14. The discs 15 and 16 on the upper rollers remain in the same positions. As seen particularly in Fig 3b, discs 15 and 17 impinge on the path of the wafer 14 (not shown) in a direction

preponderantly perpendicular to the plane of the wafers, so that bending can be imposed about axes joining discs 16 and 17, and 15 and 18.

If roller BL is rotated clockwise and BR is rotated anticlockwise, and with the same distance between discs 17 and 18 and discs 15 to 16, for most of the time the bending axis is the line joining the fixed discs 15 and 16. However, if the distance 17-18 is less than distance 15-16, then for most of the time the bending axis will be 17-18.

So in this example, the distance 17-18 need not necessarily be the same as distance 15-16. Different distances give differently stressed areas. It may be appropriate to have two modules 10 in the production testing line to produce differently stressed areas. By having a combination of different modules one after another along an inline production line, and using different relative positions of the discs, different stress regimes can be set up. Thus by having different modules, the scope of testing can be extended.

This bending about one approximately diagonal axis creates the twist bending as the wafer passes along the line. As this twist is maximised, the weight of the wafer is reduced or completely removed on or from the belt conveyers 1 1 and 12, because of the loads introduced by changing the positions of the discs into the path of the wafer. It is possible that in some other configurations the wafer may be in touch with the belt conveyers throughout the twist process.

With the dimensions quoted in this specific embodiment, the preferred distance between the diagonal points of contact when the planes of axes of the rollers and associated discs are perpendicular to the plane of the wafers, is 120 mm. The amount of displacement of the discs to create twist bending (here 3-4mm) can be changed to suit the acceptability criteria for the units being produced and tested. The specific embodiment described above is suited to silicon wafers to be used for photovoltaic cells, but other displacements may be used for other items. In such cases the top rollers (TL and TR) can be moved by adjusting micrometers (not shown in this embodiment). With other materials, the displacement of the discs could vary from 1 mm to 10 mm.

Unacceptable wafers are broken or cracked by the application of twist bending.

Fig 4a and Fig 4b show the configuration of the module when roller BR is rotated further anticlockwise to remove the discs 17 and 18 from the path of the wafer 14.

The lower discs move towards the middle of the wafer so that the wafers are carried again by the belt conveyers 1 1 and 12. Twist bending of the wafer 14 has been effected as the wafer moves in-line through the module, and so the continuous progression of the wafer along the production line is uninterrupted.

After the test of one wafer, the rollers BL and BR can be rapidly rotated to their original positions to accept the next wafer on the in-line path.

The use of four rollers and four discs is very suitable to check the acceptability of silicon wafers with square dimensions of up to 6" for PV cells in an in-line module as described above. However, for larger items, it is possible to install more than one disc on a single roller, and also to use more rollers, so to create a different mode of bending. For bigger wafers, (e.g. 8" wafers), more discs can be used.

The alternative bending over each of the general diagonal axes of the sheet material may i.e. be obtained by having two modules in series along the conveyor belts 1 1 and 12, and where the location of the discs (15, 16, 17, 18) on the rollers (TL, TR, BL, BR, respectively) of the second module is changed as would result from mirroring the rollers with discs over a plane which is normal to the rotational axis of the rollers (TL, TR, BL, BR).

Advantages of the Invention

The embodiment described above allows the use of Twist bending in a continuously operating production line, so speeding up the manufacture and testing of wafers.

It also reduces the number of defective wafers which are undetected by current methods, because of the increased sensitivity of the test.

All wafers passing along a production line can be tested, whereas with off-line testing only a few samples can be tested from each batch.