WO2000014774A1

WO2000014774A1 - Method and device for automatically counting chips on semiconductor wafers

Info

Publication number: WO2000014774A1
Application number: PCT/FR1999/002097
Authority: WO
Inventors: Jean-Luc Corre; Thierry Daniel; Vincent Gauthier; Bernard Cadet
Original assignee: Stmicroelectronics Sa; Timeat
Priority date: 1998-09-03
Filing date: 1999-09-02
Publication date: 2000-03-16
Also published as: FR2783070A1; FR2783070B1

Abstract

The invention concerns a method for counting chips on a batch of semiconductor wafers, in particular for counting good chips, comprising the following steps: a) transferring a specific wafer (30) from a storage cassette (22) to a zone for analysis under a count camera (26) lens; b) storing the wafer (30) image as delivered by the count camera (26); c) analysing the stored image to count the number of good chips while the wafer (30) is being transferred from the analysis zone towards the storage cassette (22) and/or while the next wafer in the batch is being transferred from the storage cassette (22) towards the analysis zone. The invention enables fast automatic counting of good chips on a batch of wafers.

Description

METHOD AND DEVICE FOR AUTOMATIC COUNTING OF CHIPS ON WAFERS OF SEMICONDUCTOR MATERIAL

The present invention relates to a method and to an automatic device for counting chips on wafers of semiconductor material.

It applies in the field of industrial manufacturing of chips for integrated circuits.

It is known that electronic chips are manufactured according to successive stages of doping a wafer of semiconductor substrate (of polysilicon) using appropriate masks. On such a slice (Wafer, in English) in fact several chips are produced, generally identical, the number of which depends on the size of a chip and on the size of a slice expressed in inches (Inches, in English). In the following, the term "wafer" will be used to denote a wafer of semiconductor material on which chips have been produced.

In Figure 1 there is shown a wafer 10 having chips 11. Some chips, such as the chip 13, are marked with an ink dot 15 during a test step, when they have a defect. They are said to be "bad" chips. Such a defect can for example be a sawing defect in the chips. Other chips, such as chip 12, do not have such an ink dot, because they do not have a defect. They are said to be "good" chips.

The wafers are generally manufactured by a specialized industrialist, called a "smelter", and sold to a manufacturer of integrated circuits which carries out the packaging of the chips in the form usable on electronic circuits. Both for reasons of statistical control of the quality of production and in order to invoice its customers for the exact number of good chips supplied, the founder needs to know with precision how many good chips are in each wafer he manufactures .

This is why, an example of counting chips according to the invention consists in counting the good chips on a wafer, that is to say those which are not marked with an ink dot. However, other applications of the counting principle according to the invention are possible.

Of course, this counting is carried out by automatic means, at the end of the production line, that is to say after the test of the chips having led to their possible marking with an ink point.

In the state of the art, a method of automatically counting good chips on a batch of wafers consists, for each wafer of the batch, in moving the wafer to position the center of a chip under the lens of a camera; means for analyzing the image delivered by the camera then have the function of detecting the possible presence of an ink point on the chip during the immobilization of the wafer in this position; if an ink dot is detected around the center of the chip, it is not counted as good; the above operations are repeated successively for each chip in the wafer, and for each wafer in the batch. With this counting process, a wafer must be moved and immobilized a number of times equal to the number of chips it contains.

After each of these movements, the positioning of the plate must be very precise so that it is immobilized in a position such that the chip to be examined is centered under the lens of the camera. As a result, the movement of the wafer between two image analysis steps must be slow enough to allow such precision positioning.

In addition to the time taken to analyze the image of each of the chips in each wafer, the time taken to move the wafer between two image analysis steps and the time to transfer the wafers from a storage cassette to the area under the camera and vice versa.

Given the constraints set out above, the implementation of such a counting process takes a long time. In practice, it is necessary to allow two and a half hours (2:30) for counting the chips on a batch of 25 plates.

In addition, the precise positioning of the wafer is increasingly difficult to achieve, given the current trend towards greater miniaturization of the chips. Indeed, the smaller the chips, the higher the density of chips on the wafer, and the smaller the amplitude of the movements of the wafer between two image analysis steps. However, improper positioning of the plate under the camera lens implies a risk of erroneous counting since the ink point of a chip having a defect may not be detected if it is far from the center of the camera lens. In practice, an automatic counting method as described above is not reliable for chips whose size is less than 3 mm × 3.5 mm.

According to other known methods, it is not the wafer which is moved but the camera. The The problems mentioned above then arise in the same way.

The object of the present invention is to overcome the aforementioned drawbacks of the automatic chip counting methods known in the prior art.

To this end, the invention provides a method of counting chips on a batch of wafers of semiconductor material, in particular of counting good chips. This process is implemented by an automatic counting device. It includes the following steps:

- transfer of a specific plate from a storage cassette to an analysis area, under the lens of a camera;

- memorization of the image of the wafer as delivered by the camera;

- analysis of the stored image to count the number of good chips during the transfer of the wafer from the analysis zone to the storage cassette and / or during the transfer of a next wafer of the batch from the storage cassette to the analysis area.

Thus, the counting of the good chips of a given wafer is carried out during the steps of transferring this wafer from the analysis zone to the storage cassette and / or during the steps of transferring another wafer from the storage towards the analysis area. The chip counting time of a wafer therefore does not penalize the rate of the device.

In addition, the method no longer comprises a single image analysis step and the small successive displacements of the wafer to bring each chip under the lens of the camera are eliminated. The invention also provides a device for implementing the above method. This device includes:

- a transfer table; an automatic wafer loader comprising means for loading / unloading and means for centering and orienting a wafer on the transfer table, as well as means for keeping the wafer integral with the table; - a counting camera;

a control unit which comprises a memory and image analysis means, which receives the image of the wafer delivered by the counting camera to store it in said memory, and which delivers control signals for the charger automatic and for a transfer table drive motor. Other characteristics and advantages of the invention will become apparent on reading the description which follows. This is purely illustrative and should be read in conjunction with the accompanying drawings, in which:

- in Figure 1, already partially analyzed: a wafer of semiconductor material comprising chips;

- in Figure 2: a diagram of a device for implementing the automatic counting method according to the invention.

In Figure 2, there is shown a diagram of a device for implementing the automatic chip counting method according to the invention.

The device comprises an automatic charger 21 and a transfer table 23. A storage cassette 22 such as those used for transporting the wafers can be received by the automatic charger 21.

This comprises means for loading / unloading the wafers contained in the storage cassette 22. These means include in particular a double fork for depositing a wafer on the transfer table 23 and for picking it up later, another wafer then being , if necessary, simultaneously placed on the table.

It also includes means ensuring the centering and the correct orientation of the wafer on the transfer table 23. For this purpose, these means use as reference a large flat 16, as well as possibly a small flat 18 formed at the periphery of the plate 1 (figure 1).

In an exemplary embodiment, the automatic charger used is the AL100L6 charger from OLYMPUS. This charger is suitable for 4, 5 or 6 inch inserts, and for cassettes with 25 inserts. For this charger, the precision of the orientation means of the wafer is +/- 1 degree.

The transfer table 23 is movable in translation and is driven by a motor 24.

The loading and unloading of a wafer 30 on the transfer table 23 takes place in a first extreme position of the table, located on the left in FIG. 2. Between the loading of a wafer on the transfer table and its unloading , the wafer is held integral with the transfer table 23 by suction means not shown.

The device further comprises a first camera 25 as well as a second camera 26. This the latter is called a counting camera because it is used for counting the chips themselves.

In a first embodiment, the transfer table 23 can be stopped in a first rest position such that the plate 30 is under the camera 25, and in a second rest position such that the plate 30 is under the camera counting 26.

In the second rest position, the counting camera 26 delivers the overall image of the wafer 30. In the following, the terms "analysis zone" and "rest position (s)" may be used with reference to the transfer table 23 or to the plate 30, insofar as these are integral during the process steps where this zone and / or these positions are involved.

Preferably, a lamp 27 uniformly illuminates the wafer when it is in the second rest position. A control unit 40 receives the images delivered by the cameras 25 and 26 and controls the supply of the lamp 27. In addition, it delivers control signals for the automatic charger 21 and for the motor 24. This control unit 40 is for example the central unit of a computer. It has a memory with several memory zones. It is connected to a screen 41 and to a keyboard 42 forming a man / machine interface. This screen and this keyboard are for example arranged on a console for controlling and operating the device, in front of which an operator can stand.

The control unit 40 is controlled by essentially software means for operating the device so as to implement the automatic chip counting method according to the invention. The operation of this device will now be described.

A storage cassette 22 comprising a batch of wafers to be examined is inserted into the automatic charger 21. On these wafers, chips have an ink dot and others do not according to the results of a test phase which is not the subject of this description. The device which is the subject of the invention makes it possible to count the number of good chips on each wafer, and more generally, the number of good chips in the batch of wafers contained in the cassette 22.

The fork of the loader 21 is controlled by the control unit 40 to load the first plate 30 of the batch onto the transfer table 23. Once the plate 30 is correctly centered and oriented, it is made integral with the table 23 by the means above.

The motor 24 is then controlled by the control unit 40 to move the table 23 into the first rest position, under the first camera 25.

In this position, the camera lens 25 points to an area 17 of the wafer (FIG. 1) on which a reference number of the wafer has been written, for example according to a method using a laser. The area 17 on the wafer 1 is for example opposite the large flat 16.

Character recognition means, for example included in the control unit 40, allow the control unit 40 to read the reference number of the wafer 30. This includes in particular the batch number, the number of the label in the batch, the type of chips in the label and possibly other useful information. Based on this information, the control unit 40 loads, in a first memory area, the mapping of the wafer 30 from a database. The expression cartography is understood here to mean the drawing of the chips, that is to say their shape and their arrangement on the wafer.

The motor 24 is then controlled by the control unit 40 to move the transfer table 23 into the second rest position, where it is immobilized. It is recalled that this second rest position is in the analysis zone.

In this position, the plate 30 is then uniformly illuminated by the lamp 27. In addition, the objective of the counting camera 26 points on the plate 30 so that it delivers the overall image of the plate.

Once the transfer table has been immobilized in the second rest position, the image delivered by the counting camera 26 is stored in a second memory area of the control unit 40. The time of immobilization of the wafer 30 in the second rest position is of the order of 100 milliseconds. The transfer table 23 is then in a second extreme position (on the right in FIG. 2). The motor 24 is then controlled by the control unit 40 so that the transfer table 23 is brought back to its first extreme position, or the wafer 30 is unloaded by the fork of the loader 21 controlled for this purpose by the control unit, and stored in its initial place in the cassette 22. The next plate in the batch is in turn loaded on the transfer table 23.

The use of a double fork makes it possible simultaneously to unload the wafer 30 and to load the following brochure, which saves time.

The above operations are then repeated for this new brochure. According to an advantageous characteristic of the invention, the analysis of the image stored in the second memory area of the control unit 40 to count the number of good chips is carried out during the transfer of the wafer 30 from the second position rest in the analysis zone to the storage cassette 22 and / or during the transfer of the next wafer in the batch from the storage cassette 22 to said second rest position.

In a first embodiment, the analysis of the image stored in the second memory area of the unit 40 consists in comparing parts of this image each corresponding to a chip with the equivalent parts of the image of a wafer of which all the chips are good, which is contained in the first memory area of the unit 40. The mapping of the wafer which is stored in the first memory area of the unit 40 is indeed, the image of a wafer of the same type of which all the chips are good, that is to say on which there is no ink point. This is why the comparison of these 2 images, stored respectively in the first and in the second memory areas of the unit 40, is carried out chip by chip in such a way that if the corresponding image parts are identical, then the chip is counted as good whereas, otherwise, it is not counted as good.

In a second embodiment, the analysis of the image stored in the second memory area of the unit 40 consists in comparing parts of this image each corresponding to a chip at: on the one hand the equivalent part of the image of a wafer of which all the chips are good, which is contained in the first memory area of the unit 40; on the other hand, the equivalent part of the image of a wafer of which all the chips are bad, which is contained in a third memory area of the memory of the unit 40.

The mapping of such a wafer is for this purpose loaded into the third memory area at the same time as the mapping of a wafer of which all the chips are good is loaded into the second memory area. This is the image of a plate of the same type as plate 30 on which each chip is marked with an ink dot. Thus, as in the first embodiment, the comparison of the image of the wafer 30 on the one hand with that of a wafer of which all the chips are good, and, on the other hand, the comparison of the image of the wafer 30 with that of a wafer of which all the chips are bad, are produced chip by chip with respectively a first comparison step and a second comparison step for each chip. If, at the first comparison step, the corresponding image parts are identical, the chip is counted as good. If in the second comparison step the corresponding images are identical, the chip is counted as bad. If neither of the above two conditions are met, the chip is counted as "unknown". This corresponds to the case of a chip which could not be identified either as good or as bad. At the end of the analysis, there are therefore three count values: the number of good chips, the number of bad chips and the number of unknown chips. The sum of these three values normally corresponds to the total number of chips on the wafer. This second embodiment allows a finer interpretation of the results of chip counting.

The count results are available 10 seconds later. This is roughly the time required to return the wafer 30 to the storage cassette 22 and transfer a next wafer in the batch to the analysis area.

The advantage of the method and of the device according to the invention lies in the fact that the analysis of the image of the wafer is carried out during the steps of transferring the wafers. In addition, the repetition of very precise positioning of the plate under the lens of the camera 26 so as to analyze only the image of a chip, is avoided thanks to the invention. This is why the method and the device according to the invention can be used for very small chips, for example 1 mm × 1.2 mm.

In another embodiment, the camera 26 is a linear camera with 3456 sensitive elements (pixels), of which only 2048 are used for a wafer six inches in diameter (1 inch =

2.54 centimeters). However, more or all of the sensitive elements will be used for larger platelets. In this embodiment, the transfer table 23 does not have a second rest position but is moved at low continuous speed so that the wafer 30 travels slowly and at constant speed in the analysis zone under the counting camera 26 .

The digital information delivered by the camera 26 is then stored in registers of the second memory area of the unit 40, in stages successive. At each of these stages, part of the image of the wafer, as delivered by the counting camera 26, is stored in one of the aforementioned registers. In other words, the table 23, the camera 26 and the unit 40 then operate in the manner of a scanner. The lamp 27 is then preferably of linear shape oriented parallel to the axis of the camera 26. It may for example be a cylindrical tube containing neon. In this way, the lamp 27 uniformly illuminates the analysis area where the wafer 30 travels.

This second embodiment is advantageous because it allows greater precision of the image of the wafer which is stored in the second memory area.

In addition, immobilization of the plate 30 in the first rest position, under the camera 25, is not essential. However, it is advantageous because it allows the unit 40 to read the reference number of the wafer so that the operation of the device can be fully automatic. Indeed, from this reference number, the control unit can know the map of the wafer and therefore perform an appropriate image analysis. The operator only needs to put the cassette 22 in position in the magazine 21 and start the operation of the device. About ten minutes later, the results of the chip count on the platelets in the batch are available on screen 41.

Finally, a color filter 28 can be placed between the lamp 27 and the analysis area to improve the contrast of the drawing of the chips on the wafer 30. In this way, the analysis of the image of the wafer is easier and More reliable.

Claims

1. Method for counting chips on a batch of semiconductor material wafers, in particular for counting "good" chips characterized in that it comprises the following steps: a) transfer of a determined wafer (30) from a cassette of storage (22) towards an analysis area under the lens of a counting camera (26); b) storing the image of the wafer as delivered by the counting camera (26); c) analysis of the stored image to count the number of good chips during the transfer of the wafer (30) from the analysis zone to the storage cassette (22) and / or during the transfer of a next wafer from the batch from the storage cassette (22) to the analysis area.

2. Method according to claim 1, characterized in that, between step a) and step b), the wafer (30) is immobilized in an analysis area under the objective of the counting camera (26 ) so that it delivers the overall image of the plate (30).

3. Method according to claim 1, characterized in that, in step b), the wafer scrolls slowly and at constant speed in the analysis zone under the counting camera (26) so that the image of the wafer is memorized in successive stages, at each of which a part of the image of the wafer as delivered by the counting camera (26) is memorized.

4. Method according to one of claims 1 to 3, characterized in that the analysis of the image of the wafer consists in comparing parts of this image each corresponding to a chip with the equivalent part of the image of a wafer of which all the chips are good.

5. Method according to one of claims 1 to 3, characterized in that the analysis of the image of the wafer consists in comparing parts of this image corresponding to each of the chips with on the one hand the equivalent part of the image of a wafer of which all the chips are good, - on the other hand the equivalent part of the image of a wafer of which all the chips are "bad".

6. Method according to claim 5, characterized in that at the end of the analysis there are three count values: the number of good chips, the number of bad chips and the number of chips that could not be identified nor as good nor as bad.

7. Method according to one of claims 1 to 6, characterized in that, during step a), the wafer is immobilized in another analysis zone under the lens of another camera (25) , so as to read the reference of the plate to know the cartography of this plate.

8. Device for implementing a method according to one of claims 1 to 7, characterized in that it comprises:

- a transfer table (23);

an automatic charger (21) for wafers comprising means for loading / unloading and means for centering and orienting a wafer (30) on the transfer table (23), as well as means for holding the wafer ( 30) secured to the table (23); - a counting camera (26);

- a control unit (40) which comprises a memory and image analysis means, which receives the image of the wafer (30) delivered by the camera (26) to store it in said memory, and which delivers control signals for the automatic loader (21) and for a motor (24) for driving the transfer table (23).

9. Device according to claim 8, characterized in that it comprises means for immobilizing the transfer table in a first analysis area in which the objective of a camera (25) points to an area (17) of the plate on which a reference number is written, character recognition means for reading this reference number, and means for loading into the memory of the control unit (40) the map of the plate (30) from a database based on the reference number.

10. Device according to one of claims 8 or 9, characterized in that it comprises means for immobilizing the wafer in a second rest position, in the analysis zone, in which the objective of the counting camera (26) delivers the overall image of the wafer (30).

11. Device according to claim 10, characterized in that it comprises a lamp (27) which uniformly illuminates the surface of the wafer when it is immobilized in the second rest position.

12. Device according to one of claims 8 or 9, characterized in that, the camera (26) being a linear camera, it comprises means for moving the plate (30) slowly and at speed constant in the analysis area under the counting camera (26).

13. Device according to claim 12, characterized in that it comprises a lamp (27) of linear shape oriented parallel to the axis of the camera (26), which uniformly illuminates the analysis area.

14. Device according to claim 11 or according to claim 13, characterized in that it comprises a colored filter (28) interposed between the lamp (27) and the analysis area to improve the contrast of the drawing of the chips (11,12 , 13) on the plate (30).