Apparatus and method for detecting a work piece in an automatic processing apparatus

Abstract

A method and an apparatus for detecting the presence of a work piece in an automatic processing apparatus are described. Previously known detection devices cannot make a clear distinction between the presence and absence of the work piece under all conditions. An apparatus is therefore provided which contains an ultrasound transmitter, an ultrasound receiver and a detection device (controller). The detection as to whether a work piece is held in a holder in the processing apparatus is carried out by irradiating the holder with ultrasound waves, receiving the reflected ultrasound waves and detecting on the basis of the reflected ultrasound waves. The method can be used advantageously in particular in polishing machines for wafers, where the polishing cloth is distinguished considerably, in terms of its acoustic reflection capacity, from the wafer to be polished.

Description

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] In modern production systems, automatic processing apparatus is increasingly used, in which a work piece is processed without the intervention of a human operator. In pieces of apparatus which receive the work piece to be processed from a unit connected upstream, it must be ensured that the transfer has been carried out, that is to say the work piece is also actually located in a holder provided for the purpose. In pieces of processing apparatus that automatically pick a work piece from a pool of the same, the same task arises. Adding to this is the fact that, to some extent even during the processing of a work piece, the latter can be lost, for example by its slipping out of its holder. This state must also be detected, in order that the apparatus can interrupt the processing and react in a suitable manner, for example by informing the operator or halting further processing.

[0004] Various methods have been developed in order to establish the presence of a work piece, such as optical detectors (cameras, light barriers) or electrical contacts. An optical method is described, for example, in U.S. Pat. No. 5,823,853.

[0005] By using the example of a wafer polishing device, the intention is to show in the following text how such detection mechanisms have been implemented in the prior art.

[0006] Wafer polishing devices are used for the polishing of wafers, monocrystalline, flat silicon disks which can be used for the production of integrated circuits. The wafer is placed, in this case, on a polishing cloth and held by a polishing tool (polishing head, “carrier”). A polishing agent (“slurry”) is applied to the polishing cloth, and a relative movement is produced between the polishing cloth and the wafer.

[0007] During the polishing process of the wafer, in rare cases, wafer losses on the so-called polishing platen may occur as a result of various irregularities, such as tool problems, process instabilities, faults in the transport or in the provision of the wafers. In order to minimize the resulting damage to the wafer that has been lost and to the polishing tool, and also to the substrate, or in the best case to prevent it entirely, rapid detection of a wafer loss is important in order to be able to interrupt the polishing process.

[0008] Polishing machines of the prior art therefore to some extent have optical sensors which, using the different optical reflection capacity of the wafer, on the one hand, and the substrate (polishing cloth) on the other hand, are able to detect a loss of the wafer. Depending on the surface composition of the wafer or the color of the polishing cloth used, however, these sensors are suitable to only a limited extent for the reliable detection of the presence of a wafer, since they cannot always distinguish between the wafer and the polishing cloth. They can therefore not be employed universally and, from time to time, lead to erroneous detection. Furthermore, many of the polishing agents normally used lead to a coloration of the polishing cloth, which results in a change in the reflection behavior. This effect occurs above all in the case of polishing agents for metal polishing. As a result of these problems, the proportion of erroneous detections is increased, which runs counter to the trouble-free and automatic processing of the wafers in the polishing machine.

[0009] International Patent Disclosure WO-A-98/42605 discloses a method of detecting a work piece in which a surface of a piece of material is irradiated with ultrasound. In this case, by use of the reflected sound it is determined whether the material is located correctly or the wrong way round.

[0010] A further method of detecting the presence or absence of a wafer has been proposed in the prior art. In this method, the vacuum on the polishing head is measured, in order in this way to detect a possible loss of the wafer by a dip in vacuum that occurs.

[0011] Although it is possible, with this type of detection, to monitor the wafer transport which is effected by the polishing head, detection during the actual polishing process is impossible in many cases, since it is often the case that, in order to achieve uniform removal, a gas cushion is applied to the wafer from behind, between the wafer and the polishing head, so that during the polishing operation, no measurable vacuum is available.

[0012] In summary, it is possible to say that at the present time there is no method available for detecting the presence or absence of a wafer in a polishing machine that satisfies all the requirements.

[0013] In addition to the wafer polishing systems described above, there are also, in other processing devices for work pieces, problems with specific, previously known detection methods, which restrict their universal applicability.

SUMMARY OF THE INVENTION

[0014] It is accordingly an object of the invention to provide an apparatus and a method for detecting a work piece in an automatic processing apparatus which overcomes the above-mentioned disadvantages of the prior art devices and methods of this general type, which provides a physically different type of approach to distinguishing between the work piece and holder or processing apparatus, and therefore permits a distinction to be made in accordance with other physical variables.

[0015] With the foregoing and other objects in view there is provided, in accordance with the invention, a method of detecting a presence of a work piece in an automatic processing apparatus. The method includes the steps of providing a holder for the work piece in the automatic processing apparatus; irradiating the holder and/or at least part of a surroundings of the holder with ultrasound waves; receiving reflected ultrasound waves; and using the reflected ultrasound waves to determine if the work piece is held in the holder or if the work piece is located outside the holder.

[0016] In one aspect, the invention makes use of the different acoustic reflection capacity of a work piece as compared with its surroundings, for example a gripping arm or a substrate.

[0017] In a further aspect, the invention is used in particular to provide a wafer loss detection system that operates reliably.

[0018] In yet another aspect, the invention is intended to include the application of sound waves to the detection of the presence and absence of a work piece to be processed.

[0019] In a further aspect, the invention provides work piece detection mechanisms precisely for very flat work pieces.

[0020] The detection is preferably carried out by using the ratio of emitted to received ultrasound waves. Processing of a work piece is usually not started if its absence is detected. However, in programming terms, it may be expedient instead to determine the presence of the work piece in the surroundings of the holder. However, these alternatives correspond in terms of their result, since absence from the holder results in a presence in the surroundings.

[0021] In an added mode of the invention, there is the step of processing the work piece if its presence has been detected.

[0022] The irradiating step, the receiving step and the using step can be carried out repetitively during the entire processing of the work piece. The processing is preferably interrupted if the absence of a work piece from the holder is detected.

[0023] The invention is in particular directed to applications in which the work piece is flat, for example a wafer. The holder has a support plane for holding the wafer.

[0024] The invention provides that the holder has a polishing head and a polishing cloth within a polishing apparatus.

[0025] The invention is likewise directed to an apparatus for detecting the presence of a work piece in an automatic processing apparatus, provision being made for an ultrasound transmitter for irradiating the area of a work piece holder in the processing apparatus, at least one ultrasound receiver and a detection device for detecting the presence of a work piece.

[0026] The ultrasound receiver preferably contains a means or is programmed for converting the intensity of the received ultrasound waves into an electrical signal. The detection device preferably contains means (or programmed) for determining the intensity of the ultrasound waves received by the ultrasound receiver, means (or programmed) for comparing the intensity with the intensity of the ultrasound waves emitted by the ultrasound transmitter, and means (or programmed) for deciding about the presence or absence of a work piece.

[0027] The detection device can contain a control device for starting and interrupting the processing of a work piece. A plurality of ultrasound receivers can be present in one apparatus. Likewise, the apparatus can be equipped in such a way that the ultrasound transmitter can output directed ultrasound waves.

[0028] Other features which are considered as characteristic for the invention are set forth in the appended claims.

[0029] Although the invention is illustrated and described herein as embodied in an apparatus and a method for detecting a work piece in an automatic processing apparatus, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.

[0030] The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0031] FIG. 1 a is a block diagram of a detection apparatus in a processing apparatus having a gripping arm for holding a work piece according to the invention;

[0032] FIG. 1 b is a block diagram of the processing apparatus without the work piece;

[0033] FIG. 2 a is a block diagram of a detection apparatus in a wafer polishing system with a wafer; and

[0034] FIG. 2 b is a block diagram of the detection apparatus in the wafer polishing system after the loss of the wafer.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0035] The present invention makes use of an acoustic reflection capacity of work pieces, which always differs from the reflection capacity of the background, for example a polishing cloth in a wafer polishing system, or from the reflection capacity of an empty space, which exists in the absence of a work piece between the holders of a gripping arm.

[0036] From an ultrasonic transmitter, ultrasound waves of a defined intensity are emitted and strike the work piece, if present, and/or strike the surroundings, for example a polishing cloth, of the work piece. From there, a portion of the ultrasound waves is reflected and received by an ultrasound receiver. A detection device, that is to say a controller, for example a process computer, then carries out a detection method in which a decision is made as to whether there is a work piece at the proper point or not, use being made of the different reflection capacity of the work piece as compared with the holder or the background.

[0037] To this end, a ratio of the intensities of the emitted and the received ultrasound waves is preferably determined.

[0038] If no work piece can be detected, no processing step is initiated. If the presence of the work piece could be detected, the controller can bring about or indicate the processing of the work piece.

[0039] If it cannot safely be assumed that no loss of the work piece during the treatment can occur, a repetitive measurement is preferably carried out, which ensures continuous monitoring of the presence of the work piece.

[0040] Referring now to the figures of the drawing in detail and first, particularly, to FIG. 1a thereof, there is shown in schematic form a configuration having a machine tool with a gripping arm 1, which holds a work piece 2. A spindle drive 3 with a milling head 4 is ready to process the work piece 2. An ultrasound transmitter 5 emits ultrasound waves (arrow), which are reflected by the work piece and pass to the ultrasound receiver 6. They are converted by the receiver 6 into electrical signals, which are received by a controller 7 via a line 8 and are evaluated there. The work piece 2 reflects a high proportion of the ultrasound waves that strike it, and therefore indicates its presence.

[0041] FIG. 1 b shows the same machine tool with identical reference symbols, in which no work piece 2 is held by the gripping arm 1. In this case, only a small proportion of the ultrasound is reflected by the gripping arm 1, while the major part passes through its holder. Therefore, the ultrasound receiver 6 receives only a few ultrasound waves, so that the absence of the work piece 2 is detected.

[0042] The present invention can be used particularly beneficially in the case of flat work pieces 2, such as the wafers described above. In the case of the wafers, the additional difficulty arises that, in the case of flat work pieces, detection by light barriers or the like, for example, is difficult.

[0043] The wafers are polished on polishing cloths that, in terms of their compressibility (elasticity), differ considerably from a wafer lying on them. The compressibility of the polishing cloth is comparatively high, which, on account of the associated sound absorption coefficient, leads to the sound waves being returned with a considerably lower intensity than in the case of the wafer alone. The polishing cloth and the wafer can therefore be distinguished clearly and simply.

[0044] FIG. 2 a shows schematically a polishing system for a wafer 12, having a polishing cloth 11 as the background and a wafer 12 disposed thereon. The detection device 7 has the same structure as in FIGS. 1a and 1b, so that the same reference symbols are used. The wafer 12 reflects a large proportion of the ultrasound radiation striking it, so that the received intensity at the ultrasound receiver 6 is high.

[0045] FIG. 2 b shows the polishing system without the presence of the wafer 12. The polishing cloth 11 reflects less ultrasound than the wafer 12, so that correspondingly fewer ultrasound waves are received by the ultrasound receiver 6. By using the intensity of the reflected ultrasound, given a known emission intensity, a wafer loss can therefore be detected in a simple way, quickly, independently of the process (e.g. polishing paste, etc.) and independently of the hardware.

[0046] Commercially used wafer holders, carriers or polishing heads, as they are known, often cover the wafer 12 completely from above. Ultrasound irradiation of the wafer 12 for the purpose of monitoring is therefore not possible. In such a case, it may be advantageous to irradiate the surroundings of the carrier, that is to say for example the polishing cloth 11. If the wafer 12 becomes detached from its holder in the carrier, it will slide out of the carrier as a result of the friction between itself and the background, so that it then comes to lie on the polishing cloth 11 outside the carrier. In this case, the reflection capacity of the background at the point where the wafer is located changes. This difference can be detected and, during the evaluation, can be judged to be a wafer loss, so that the processing apparatus can be stopped.

[0047] The controller 7 is used for the detection of the presence or absence of the work piece at the envisaged position. It is equipped with a device to receive the electrical signals generated by the ultrasound receiver 6 via the line 8. In addition, it has a means that decides whether a work piece is present or not. The controller can also has a means that causes the ultrasound transmitter 5 to emit ultrasound. This can be done via a further line 8′, which connects the controller 7 and ultrasound transmitter 5 to each other. In this way, synchronization of emission and measurement is possible, which reduces the influence of possible interference waves. Furthermore, the intensity of the emission can also be controlled and therefore defined clearly.

[0048] In a preferred embodiment, the controller 7 compares the intensity of the emitted ultrasound waves with the intensity of the waves received by the ultrasound receiver 6, and can carry out the detection of the work piece from their ratio. The various means used for the control may be accommodated, for example, in a process computer.

[0049] The controller 7 can advantageously intervene directly in the processing of the work piece, by starting the latter and, if necessary, interrupting it. This can be achieved via conventional control lines between the controller 7 and the processing apparatus 3, 4.

[0050] In addition, it is possible to provide a plurality of ultrasound receivers 6 instead of one such receiver. This is advantageous in the case of complexly shaped work pieces or if a plurality of work pieces, for example a plurality of wafers, are to be processed at the same time.

[0051] In addition, it is possible to operate with directed ultrasound, in order to permit still more accurate distinction between presence and absence of a work piece. Directed ultrasound, which is emitted by a specifically aimed ultrasound transmitter, permits more accurate echo location of the location in the apparatus at which the work piece should be found. In this way, the influence of other elements in the processing apparatus is minimized, which further improves the precision of the decision process of the controller.

Claims

1. A method of detecting a presence of a work piece in an automatic processing apparatus, which comprises the steps of: providing a holder for the work piece in the automatic processing apparatus; irradiating at least one of the holder and at least part of a surroundings of the holder with ultrasound waves; receiving reflected ultrasound waves; and using the reflected ultrasound waves to determine if the work piece is held in the holder or if the work piece is located outside the holder.

2. The method according to claim 1, which comprises using a ratio of emitted to received ultrasound waves for determining the presence of the work piece.

3. The method according to claim 1, which comprises preventing a processing of the work piece if an absence of the work piece is detected.

4. The method according to claim 1, which comprises processing the work piece if the presence of the work piece has been detected.

5. The method according to claim 4, which comprises carrying out the irradiating step, the receiving step and the using step repetitively during the processing of the work piece.

6. The method according to claim 5, which comprises interrupting the processing if an absence of the work piece from the holder is detected.

7. The method according to claim 1, which comprises processing a flat work piece as the work piece.

8. The method according to claim 7, which comprises processing a wafer as the work piece.

9. The method according to claim 1, which comprises providing the holder with a support plane.

10. The method according to claim 1, which comprises providing the holder with a polishing head and a polishing cloth in a polishing apparatus.

11. The method according to claim 1, which comprises providing the holder with a gripping arm.

12. An apparatus for detecting a presence of a work piece in an automatic processing apparatus, comprising: an ultrasound transmitter for irradiating ultrasound waves over an area of a work piece holder of the automatic processing apparatus; at least one ultrasound receiver for receiving reflected ultrasound waves; and a detection device for detecting a presence of the work piece and connected to said ultrasound receiver.

13. The apparatus according to claim 12, wherein said ultrasound receiver converts an intensity of the reflected ultrasound waves received into an electrical signal.

14. The apparatus according to claim 12, wherein said detection device includes first means for determining an intensity of the reflected ultrasound waves received by said ultrasound receiver, second means for comparing the intensity with an intensity of the ultrasound waves emitted by said ultrasound transmitter, and third means for deciding about a presence and an absence of the work piece.

15. The apparatus according to claim 12, wherein said detection device includes a control device for starting and interrupting a processing of the work piece.

16. The apparatus according to claim 12, wherein said ultrasonic receiver is one of a plurality of ultrasonic receivers.

17. The apparatus according to claim 1, wherein said ultrasonic transmitter can output directed ultrasound waves.

18. The apparatus according to claim 12, wherein said detection device is programmed to: determine an intensity of the reflected ultrasound waves received by said ultrasound receiver, compare the intensity of the reflected ultrasound waves with an intensity of the ultrasound waves emitted by said ultrasound transmitter, and make a determination about a presence or an absence of the work piece.