METHOD AND A SYSTEM FOR EVALUATING THE PHYSICAL CONSUMPTION OF A POLISHING PAD OF A CMP APPARATUS, AND CMP APPARATUS

Abstract

A method for evaluating the physical consumption of a polishing pad of a CMP apparatus provided with an eddy current sensor that is configured to sense a distance with a substrate on which a layer to be processed extends. The method includes: generating a magnetic field adapted to induce eddy currents in the layer; acquiring an eddy current signal generated by the layer in response to the magnetic field; calculating an average value of the eddy current signal in the initial time frame; comparing the average value with a pre-set threshold value; and determining a maintenance condition of the polishing pad based on a result of the comparison.

Description

PRIORITY CLAIM

This application claims the priority benefit of Italian Application for Patent No. 102020000015790, filed on Jun. 30, 2020, the content of which is hereby incorporated by reference in its entirety to the maximum extent allowable by law.

TECHNICAL FIELD

This disclosure relates to a method and a system for evaluating the physical consumption of a polishing pad of a Chemical Mechanical Polishing (CMP), apparatus. This disclosure further relates to a CMP apparatus including this system.

BACKGROUND

Integrated circuits are manufactured by sequential steps of deposition and partial removal of layers on a substrate of a silicon wafer; such layers may either be conductive, semiconductive or insulating. One manufacturing step involves depositing a conductive (typically, metallic) filler layer and then planarizing the filler layer. Chemical mechanical polishing (CMP) is a key step in the modern semiconductor manufacturing processes to achieve planarization of layers. For example, to form vias or plugs, one technique is to deposit a conductive filler layer (typically, made of Copper) over a pre-patterned barrier layer (e.g., an insulating film), which presents trenches or holes, extending over the substrate, with the aim of filling the trenches/holes by the conductive filler layer. With a CMP process, the conductive filler layer is removed until the barrier layer is reached and exposed. After the CMP, the portions of the conductive layer remaining within the trenches in the barrier layer effectively form conductive vias, or plugs that provide vertical conductive paths for signal routing on the substrate or other uses. The CMP can be used for planarization steps that may be utilized to planarize the wafer for, for example, further photolithography steps.

During CMP, the wafer is mounted on a carrier or polishing head. The surface of the wafer to be planarized is placed against a rotating polishing pad having a roughened surface. The carrier head rotates the wafer while pushing it against the polishing pad. Usually, a polishing slurry (including at least one chemically-reactive agent and abrasive particles) is supplied to the surface of the polishing pad.

The endpoint condition of the polishing process can be achieved in different ways, depending on the material that is removed or the stating condition.

The typical ones are optical endpoints, where there is a measure of the reflected light intensity (used when a metal film is removed until an oxide layer is reached) or an interferometric measurements in the case of solely oxide layers, or Eddy current method, which is a non-destructive method typically used in CMP of Copper layers (detection of the end point is achieved by measuring the variations of the current induced the thin metallic layer that is on top on the wafer prior CMP). Specifically, the eddy current sensor operates based on the inductive eddy-current principle. It measures the distance based on the extraction of energy from an oscillating circuit, which is used to generate eddy current in an electrically-conductive materials. When the sensing coil is supplied with an alternating current, it causes a magnetic field to form around the coil. If an electrically conducting material is placed in this field, an eddy current field is induced according to the Faraday's induction law. When the object moves, it causes the change in the impedance of the coil, which is proportional to the change in the distance between the sensor and the target.

FIG. 1 is a simplified schematic diagram of the principle upon which an eddy current sensor operates. An alternating current iac flows through a coil 1 in close proximity to a conductive layer 2 extending on a substrate 5 of a wafer 3. In a CMP apparatus, the coil 1 is fixed or otherwise coupled to the polishing pad (not shown). The alternating current iac is made to flow through the coil 1, thereby causing the generation of an electromagnetic field. In FIG. 1, the conductive layer 2 represents the above-mentioned conductive layer (Copper film), the thickness of which has to be reduced (and measured) during CMP. The electromagnetic field generated by the coil 1 induces eddy currents 4 in conductive layer 2. The eddy currents, in turn, have an impact on the impedance of the coil 1. Since, during the CMP, the polishing pad is pressed against the conductive layer 2, the effect of eddy currents 4 on coil 1 is a function of a distance 6 between the conductive layer 2 and the coil 1. Therefore, the reduction of the distance 6 caused by a thickness reduction of the conductive layer 2 can be measured by measuring the impedance change of the coil 1.

One major issue of the CMP process is the evaluation of the actual consumption of the polishing pad of the CMP apparatus, which is of interest because the overconsumption of the pad can lead to overpolishing or underpolishing. Currently, the lifetime of each polishing pad is estimated based on experience/working hours, so that the polishing pad is replaced after a predetermined number of working hours, regardless of the actual consumption of the polishing pad.

That means that a polishing pad may be replaced well before complete consumption (thus increasing the costs), and another polishing pad may be replaced after complete consumption (thus compromising the polishing step). Both cases should be avoided.

A procedure to automatically check the consumption of the pad is therefore desirable.

SUMMARY

The aim of the present disclosure is to provide a method and a system for evaluating the physical consumption of a polishing pad of a CMP apparatus, and a CMP apparatus including such system, that overcome the drawbacks mentioned above.

According to this disclosure, a method and a system for evaluating the physical consumption of a polishing pad of a CMP apparatus, and a CMP apparatus including such system.

For example, disclosed herein is a method for evaluating physical consumption of a polishing pad of a Chemical Mechanical Polishing (CMP) apparatus, where the CMP apparatus is provided with an eddy current sensor that is configured to sense a distance using a substrate on which a layer to be processed by the CMP apparatus extends. This method includes: generating, using the eddy current sensor during an initial time frame of CMP processing of the layer, a magnetic field configured to induce eddy currents in the layer; acquiring, using the eddy current sensor during the initial time frame, an eddy current signal generated by the layer in response to the magnetic field; calculating an average value of the eddy current signal in the initial time frame; comparing the average value with a pre-set threshold value; and determining a maintenance condition of the polishing pad based on a result of the comparison.

The maintenance condition may be desire for replacement of the polishing pad.

The initial time frame may be such that the eddy current signal generated by the layer during the initial time frame is saturated and/or substantially time-invariant.

The initial time frame may be between 0 and 70 seconds of the CMP processing of the layer.

The initial time frame may be between 0 and 20 seconds of the CMP processing of the layer.

The eddy current sensor may be arranged with respect to the polishing pad in such a way that the eddy current signal is a function of both a thickness of the layer and a thickness of the polishing pad.

The eddy current sensor may be supported by, and move with, a supporting platen over which the polishing pad is coupled.

The method may further include arranging the layer against the polishing pad before the initial time frame of processing of the layer by the CMP apparatus.

Also disclosed herein is a system for evaluating physical consumption of a polishing pad of a Chemical Mechanical Polishing (CMP) apparatus. The system includes: an eddy current sensor configured to sense a distance using a substrate on which a layer to be processed by the CMP apparatus extends, wherein the eddy current sensor is configured to generate, during an initial time frame of CMP processing of the layer, a magnetic field configured to induce eddy currents in the layer and to acquire, during the initial time frame, an eddy current signal generated by the layer in response to the magnetic field; and processing circuitry coupled to the eddy current sensor. The processing circuitry may configured to: calculate an average value of the eddy current signal in the initial time frame, compare the average value with a pre-set threshold value, and determine a maintenance condition of the polishing pad based on a result of the comparison.

The maintenance condition may be a replacement of the polishing pad.

The initial time frame may be such that the eddy current signal generated by the layer during the initial time frame is saturated and/or substantially time-invariant.

The initial time frame may be between 0 and 70 seconds of said CMP processing of the layer.

The initial time frame may be between 0 and 20 seconds of said CMP processing of the layer.

The eddy current sensor may be arranged with respect to the polishing pad in such a way that the eddy current signal is a function of a thickness of the layer plus a thickness of the polishing pad.

The eddy current sensor may be supported by, and move with, a supporting plate over which the polishing pad is coupled.

Also disclosed herein is a chemical machine polishing (CMP) apparatus, including: a wafer carrier facing a polishing pad, the polishing pad being disposed over a supporting plate; a wafer carried by the wafer carrier, the wafer having a top surface over which a conductive layer extends; and an eddy current sensing device configured to induce an eddy current field in the conductive layer, and detect a change in impedance of a coil of the eddy current sensing device; wherein the eddy current sensing device is further configured to generate, from the change in impedance, an output signal based upon a thickness of the conductive layer and a distance between the eddy current sensing device and the conductive layer, the output signal corresponding to a thickness of the polishing pad.

The eddy current sensing device may be supported by, and move with, the supporting plate.

The eddy current sensing device may induce the eddy current field in the conductive layer by an alternating current flowing through the coil.

The change in the impedance may indicate that a thickness of the polishing pad has reduced below a threshold level.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding, some embodiments thereof will now be described, purely by way of non-limiting example and with reference to the attached drawings, wherein:

FIG. 1 shows a schematic implementation of an eddy current sensor operatively coupled to a wafer, according to the prior art;

FIG. 2 shows a schematic implementation of a CMP apparatus provided with an eddy current sensor operatively coupled to a wafer, in an embodiment described herein;

FIG. 3 shows an eddy current signal, represented in arbitrary units as a function of polishing time; and

FIG. 4 shows a collection of measures representing a variation of the mean current collected at the very beginning of the CMP process, each point being referred to the processing of a respective wafer by the same polishing pad.

DETAILED DESCRIPTION

In an embodiment, schematically shown in FIG. 2, a CMP apparatus 10 includes control electronics configured to calculate a thickness of a conductive layer 21 belonging to a wafer 20 during a planarization or polishing or thickness-reduction process (in general, a chemical-mechanical polishing “CMP” process). The CMP apparatus 10 can also be employed to remove a bulk film and then stop, as in damascene copper and tungsten polishing.

The CMP apparatus 10 includes a wafer carrier 14 facing a polishing pad 16 that is arranged over a supporting plate 17. Both the wafer carrier 14 and the plate 17 are controllable in rotation, as exemplified by arrows 19a, 19b, for performing the CMP process. The polishing pad 16 is, for example, of polyurethane. A slurry delivery system (not shown), configured to provide a substantially uniform slurry layer onto the polishing pad 16, may also be provided.

During a CMP process, the wafer 20 supported by the wafer carrier 14 is pressed against polishing pad 16 to process a surface of the wafer 20. In particular, the wafer 20 includes a substrate 13 having a top surface 13a on which the conductive film or layer (e.g., of Cu) 21 extends. The conductive film or layer 21 is the target to be processed (e.g., planarized) by the polishing pad 16.

The wafer carrier 14 is configured to support the wafer 20 at a bottom surface 13b of the substrate 13, during the CMP process. A coil-based eddy current sensor 18 is configured to induce an eddy current field in the conductive layer 21, according to Faraday's induction law. In furtherance of this aim, a sensing coil 18a of the eddy current sensor 18 is supplied with an alternating current (AC) generated by an AC current generator (see, e.g., FIG. 1), thus causing a magnetic field to form around the coil 18a. The eddy current sensor 18, which is arranged in (or otherwise coupled to) the plate 17, is configured to detect a signal i_EC that is related to, or a function of, an actual thickness 24 of the film 21. The signal i_EC is, in particular, the eddy current field that is induced in the layer 21 and that can be sensed by the sensor 18 as a change in the impedance of the coil 18a of the sensor 18; it is to be noted that the change of impedance is proportional to the actual distance 25 between the sensor 18 and the top surface 13a of the substrate 13 (not only to the thickness 24 of the layer 21). In the embodiment exemplified in FIG. 2, distance 25 is the sum, along a same axis Z, of the thickness 24 of layer 21 plus the thickness of the polishing pad 16.

The eddy current sensor 18 generates at output a signal S_EC, which is based on signal i_EC and is a function of the thickness of the layer 21 and of the thickness of the pad 16. As better explained later, in an initial time window of the CMP process, the thickness of the layer 21 of each wafer being processed can be considered the same for each wafer (i.e., the thickness is such that the current is in saturation, so minor variation the film thickness do not influence the eddy current signal). In that time window, the signal can be seen as a function of the pad thickness only.

Eddy current sensors (ECS) allow for the contactless measuring of a metal film thickness in the full range of thicknesses normally utilized in semiconductor manufacturing. The embodiment of FIG. 1 is therefore merely exemplificative of the functioning of an eddy current sensor 18; other eddy current sensors may be employed.

A computer or processor, or any other processing circuitry 22 is in communication with the sensor 18 and may be part of the CMP apparatus 10, or operatively coupled to it. Alternatively, sensor 18 includes or integrates the processor or the other processing circuitry 22, to perform certain computation on signal S_EC, with the aim of evaluating the pad's consumption.

The CMP apparatus 10 is configured such that the signal S_EC is a function of the thickness 24 of the film 21 and of the distance between eddy current sensor 18 and the film 21, which substantially corresponds to the thickness of the polishing pad 16. Both thicknesses of the film 21 and of the polishing pad 16 are considered along a same axis (here, the Z axis). That is to say, a variation, during use, of the magnetic field sensed by the coil 18a derives from both the thickness variation of the polishing pad 16 and the thickness variation of layer 21. In an aspect, information related to the actual thickness of the polishing pad 16 is calculated or otherwise acquired by processing the signal S_EC. In fact, the polishing pad 16 wears or erodes over time, causing variation in the distance between the plate 17 and the wafer 20, which influences the appropriated contribution to the total eddy current signal i_EC.

During operation of the eddy current sensor 18, when an alternating current is caused to flow through the coil 18a, which is set in close proximity to the film 21, the electromagnetic field of the coil 18a induces eddy currents in the film 21. The phase of the eddy currents in turn affect the loading on the coil 18a. Thus, the impedance of the coil 18a is impacted by the eddy currents. This impact is measured to sense the proximity of the film 21 to the sensor 18, as well as a thickness of the film 21.

FIG. 3 represents an elaborated eddy current signal as function of the process time. With reference to region R1 of FIG. 3, at the beginning of the CMP process (in particular, from the start at 0 s until about 70 s, more in particular in the range 0-20 s), the variation of distance 25 between eddy current sensor 18 and the substrate 13, as sensed by the eddy current sensor 18, is substantially constant. In fact, even though film 21 begins to be polished, the film 21 remains sufficiently thick (i.e., up to 3-6 micrometers) that the eddy current signal i_EC deriving from the film 21 is saturated for at least 70 seconds from the beginning of processing. In other words, in region R1, the film 21 is thick, continuous and has a low resistivity, such that relatively strong eddy currents are generated in it and thus the elaborated current signal is in saturation. It has been found that the eddy current signals (fields) coming from the film 21 are saturated until the film 21 has a thickness above about 2-2.5 μm. It follows that, in this situation, variation of the eddy current signal i_EC mainly derives from a thickness reduction of the polishing pad 16.

As the film 21 erodes, the bulk portion of film 21 is thinned. As the film 21 thins, its sheet resistivity increases, and the eddy currents become dampened and their contribution increases (region R2 in FIG. 3).

Eventually, the bulk portion of film 21 is removed (e.g., leaving conductive interconnects in the trenches between the patterned insulating layer). At this point, the coupling between the conductive portions in the substrate 13 (which are generally small and generally non-continuous) and the sensing circuitry of sensor 18 reaches a minimum.

In region R3, a polishing endpoint is reached.

The above observations can be exploited during a plurality of consecutive CMP processes, wherein each process relates to the CMP treatment of a respective wafer. Therefore, when a plurality of wafers are consecutively processed with the CMP apparatus 10, the signal S_EC during an initial timeframe, which corresponds to region R1 of FIG. 3 (e.g., in the time window 0-70 s, as exemplified above) of each CMP process is monitored; potential drift of the signal S_EC from the expected value is only (or at least mainly) caused by a variation of the polishing pad's thickness.

FIG. 4 shows a collection of points, each one related to one respective wafer 20 processed by the CMP apparatus 10. Each point represents a current signal value measured by the eddy current sensor 18 during said initial time frame (e.g., 0-70 s) of processing of one respective wafer 20. Each point represents, therefore, a value of thickness 25 of FIG. 2 (the ordinate axis of FIG. 4 is an arbitrary unit representing pad thickness, while the abscissae axis is time). It is noted that, after wafer processing with the same polishing pad 16, the value of thickness 25, which is calculated from the signal provided by the eddy current sensor 18, decreases. It is also noted that each newly processed wafer 20 has a film 21 having, at the beginning of the respective CMP process, a similar thickness 24. Since the eddy current i_EC generated in the conductive film 21 is substantially constant (saturated) in the considered initial time frame, even minor variations in the metallic film thickness do not influence the output signal, and such variation of the current signal 25 is due to a variation of thickness of the polishing pad 16.

By fitting the points of FIG. 4, one obtains a curve that drifts with time (in particular, a descending curve); this drift depends by the variation of the distance 25 (lift off) caused by reduction of the thickness of pad 16, which is consumed wafer after wafer.

Even though the signal S_EC (provided by the eddy current sensor 18 in the initial time frame of processing of a wafer 20) is theoretically constant during the initial time frame, small variations may anyway be observed, caused by noise or other disturbing sources. Therefore, in an aspect, each point of FIG. 4 is the mean (average) value of the respective signal S_EC acquired for the respective wafer 20 in the initial time frame of processing of such wafer 20.

It has been verified that, when the value of thickness 25 acquired during the above-mentioned time window (e.g., 0-70 s or 0-20 s) of initial processing of a certain wafer 20 is below a pre-set threshold, the polishing pad 16 it so undergo maintenance, in particular it is to be replaced with a new polishing pad 16.

Such threshold can be different for different CMP apparatuses, subject to the specific implementation and construction details of the CMP apparatus. Given a CMP apparatus, one can inspect the consumption status of the polishing pad 16 and identify the value conveyed by signal S_EC outputted by the sensor 18 when the polishing pad 16 is completely consumed or erased (i.e., it is to be replaced with a new one); the value is identified during the time window of initial processing of the wafer, when the eddy current signal from layer 21 is saturated. In this way, it is possible to pre-set a threshold that is specific for each CMP apparatus.

When the pre-set threshold is reached, a warning signal may be optionally generated, signaling that the pad 16 should be replaced with a new one. In a non-limiting embodiment, in order not to risk a total consumption of the pad 16 during CMP processing, the warning signal can be issued before the pre-set threshold has been reached; in another non-limiting embodiment, the pre-set threshold can be set at a value higher than the minimum value that corresponds to a complete warn or complete erase of the pad 16.

The advantages achieved by the disclosures herein are apparent from the above description.

In particular, the above can be implemented in currently available equipment, without the need for need for further (e.g., external) systems or devices to be used for checking the polishing pad's status and consumption.

Finally, it is evident that modifications and variations may be made to the present disclosure, without departing from the scope thereof, as defined in the annexed claims.

For example, the system including the eddy current sensor 18 and the processor or processing circuitry 22 can be integral part of the CMP apparatus 10 or can be external to the CMP apparatus 10 and operatively coupled to the CMP apparatus 10.

Moreover, the system can be employed in a variety of polishing systems. Either the polishing pad, or the carrier head, or both can move to provide relative motion between the polishing surface and the substrate. The polishing pad can be a circular (or some other shape) pad secured to the plate, a tape extending between supply and take-up rollers, or a continuous belt. The polishing pad can be affixed on a plate, incrementally advanced over a plate between polishing operations, or driven continuously over the plate during polishing. The pad can be secured to the plate during polishing, or there could be a fluid bearing between the plate and polishing pad during polishing. The polishing pad can be a standard (e.g., polyurethane with or without fillers) rough pad, a soft pad, or a fixed-abrasive pad.

Claims

1. A method for evaluating physical consumption of a polishing pad of a Chemical Mechanical Polishing (CMP) apparatus, where the CMP apparatus is provided with an eddy current sensor that is configured to sense a distance using a substrate on which a layer to be processed by the CMP apparatus extends, the method comprising:

generating, using the eddy current sensor, a magnetic field configured to induce eddy currents in the layer during an initial time frame of CMP processing of the layer;

acquiring, using the eddy current sensor during the initial time frame, an eddy current signal generated by the layer in response to the magnetic field;

calculating an average value of the eddy current signal in the initial time frame;

comparing the average value with a pre-set threshold value; and

determining a maintenance condition of the polishing pad based on a result of the comparison.

2. The method of claim 1, wherein the maintenance condition is a desire for replacement of the polishing pad.

3. The method of claim 1, wherein the initial time frame is such that the eddy current signal generated by the layer during the initial time frame is saturated.

4. The method of claim 1, wherein the initial time frame is such that the eddy current signal generated by the layer during the initial time frame is saturated and substantially time-invariant.

5. The method of claim 1, wherein the initial time frame is such that the eddy current signal generated by the layer during the initial time frame is substantially time-invariant.

6. The method of claim 1, wherein the initial time frame is between 0 and 70 seconds of the CMP processing of the layer.

7. The method of claim 1, wherein the initial time frame is between 0 and 20 seconds of the CMP processing of the layer.

8. The method of claim 1, wherein the eddy current sensor is arranged with respect to the polishing pad in such a way that the eddy current signal is a function of both a thickness of the layer and a thickness of the polishing pad.

9. The method of claim 8, wherein the eddy current sensor is supported by, and moves with, a supporting platen over which the polishing pad is coupled.

10. The method of claim 1, further comprising arranging the layer against the polishing pad before the initial time frame of processing of the layer by the CMP apparatus.

11. A system for evaluating physical consumption of a polishing pad of a Chemical Mechanical Polishing (CMP) apparatus, the system comprising:

an eddy current sensor configured to sense a distance using a substrate on which a layer to be processed by the CMP apparatus extends, wherein the eddy current sensor is configured to generate, during an initial time frame of CMP processing of the layer, a magnetic field configured to induce eddy currents in the layer and to acquire, during the initial time frame, an eddy current signal generated by the layer in response to the magnetic field; and

processing circuitry coupled to the eddy current sensor and configured to: calculate an average value of the eddy current signal in the initial time frame, compare the average value with a pre-set threshold value, and determine a maintenance condition of the polishing pad based on a result of the comparison.

12. The system of claim 11, wherein the maintenance condition is a replacement of the polishing pad.

13. The system of claim 11, wherein the initial time frame is such that the eddy current signal generated by the layer during the initial time frame is one of saturated and/or substantially time-invariant.

14. The system of claim 11, wherein the initial time frame is between 0 and 70 seconds of said CMP processing of the layer.

15. The system of claim 11, wherein the initial time frame is between 0 and 20 seconds of said CMP processing of the layer.

16. The system of claim 11, wherein the eddy current sensor is arranged with respect to the polishing pad in such a way that the eddy current signal is a function of a thickness of the layer plus a thickness of the polishing pad.

17. The system of claim 11, wherein the eddy current sensor is supported by, and moves with, a supporting plate over which the polishing pad is coupled.

18. A CMP apparatus comprising a system according to claim 11.

19. A chemical machine polishing (CMP) apparatus, comprising:

a wafer carrier facing a polishing pad, the polishing pad being disposed over a supporting plate;

a wafer carried by the wafer carrier, the wafer having a top surface over which a conductive layer extends; and

an eddy current sensing device configured to induce an eddy current field in the conductive layer, and detect a change in impedance of a coil of the eddy current sensing device;

wherein the eddy current sensing device is further configured to generate, from the change in impedance, an output signal based upon a thickness of the conductive layer and a distance between the eddy current sensing device and the conductive layer, the output signal corresponding to a thickness of the polishing pad.

20. The CMP apparatus of claim 19, wherein the eddy current sensing device is supported by, and moves with, the supporting plate.

21. The CMP apparatus of claim 19, wherein the eddy current sensing device induces the eddy current field in the conductive layer by an alternating current flowing through the coil.

22. The CMP apparatus of claim 19, wherein the change in the impedance indicates that a thickness of the polishing pad has reduced below a threshold level.