WO2001076818A1

WO2001076818A1 - A polishing apparatus and a method of detecting an end point of polishing

Info

Publication number: WO2001076818A1
Application number: PCT/IB2001/000567
Authority: WO
Inventors: David Weston Haggart
Original assignee: Motorola, Inc.
Priority date: 2000-04-06
Filing date: 2001-04-03
Publication date: 2001-10-18

Abstract

A polishing apparatus (10) comprises a circular-disk type surface plate (12) being driven by a plate motor (14), and a circular-disk type polish head (16) for supporting an polishing object (18). An abrasive cloth (20) which being opposed and contacted to the polishing object is stuck on the rotating surface plate. The polish head is coupled by shaft (22) to head motor (24) driving the polish head. The shaft has mounted a strain sensor (36) to its surface. The strain sensor provides a sensor signal representing strain tilting the shaft by a force to the polish head due to friction between polishing object and polish pad. This allows end point detection of the polishing process.

Description

A POLISHING APPARATUS AND A METHOD OF DETECTING AN END

POINT OF POLISHING

FIELD OF THE INVENTION

The present invention pertains to polishing, and especially to the planarization of semiconductor wafers and the like thin, flat workpieces. BACKGROUND OF THE INVENTION

As is known in the art, many types of semiconductor devices are made by stacking multiple thin layers one on top of the other using metalization, sputtering, ion implantation and other techniques. The thicknesses of such layers are very small, typically on the order of several molecular dimensions. These techniques allow integrated circuits to be made up of multiple millions of circuit devices which are typically formed in a semiconductor substrate which is relatively thin and therefore fragile.

In chemical/mechanical polishing (CMP) wafers, typically of semiconductor material such as silicon, the wafer is typicallly placed face down on a polish pad carried on a rotating, driven table. A chemically active media, frequently referred to as a "slurry" and often- times containing abrasive particles, is introduced between the wafer and the polishing pad. A polishing force is applied to the back side of the wafer, pressing the wafer against the polish pad. Polishing force is typically applied by a relatively massive polish head, with a backing pad interposed between the polish head and the back side of the wafer.

Typically, semiconductor wafers are polished many times during the course of semiconductor device fabrication. As multiple layers of conductors and dielectrics are built up on the surface of a wafer, polishing is usually required after the deposition of each layer to restore any deviation from highly demanding local and global flatness tolerances. Because so-called "out-of-flatness" tolerances must be related to the total, finished construction, it is critical that the polishing process be held to extremely close tolerances such that finished densely packed structures do not interfere with one another.

It is important, during the course of preparing the semiconductor surface, that proper amounts of polishing are applied to assure that the desired degree of flatness is attained without undesirable intrusion into the deposited layers, which might compromise their intended electronic operation. While it is possible to periodically remove the wafer being processed from the polishing apparatus in order to inspect or measure the wafer surface, such practices are undesirable in that they subject the wafer to additional handling with an attendant risk of damage to the wafer. In order to overcome these drawbacks, attention has been directed to so-called in- situ end point detection. A variety of techniques have been developed over the years. In general, such techniques rely upon an indirect detection of the wafer surface characteristics. Most techniques apply factional or vibration techniques or optical inspection of the wafer surface or the polish pad. A polishing equipment disclosed in JP 10256209 shows a polish head rotated by a shaft via torque drive pins. Each torque drive pins has a stain detection gage mounted thereon. The assembly shall detect frictional force between a wafer and the polish pad indirectly. This assembly needs a torque between polish head and shaft. This is disadvantageous when using a rotating polish pad because then it is frequently desired that the rotating polish head and the rotating polish pad have the same rotation frequency and therefore no torque on the polish head shaftln the practice, the polish head rotation frequency is driven by a separate motor. For a matched process the power required is small and sometimes irregular. In General, there is a need for improved end point detection for CMP processes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified schematic diagram of a polishing apparatus according to the present invention;

FIG. 2 is a simplified schematic diagram of part of a polishing apparatus according to another embodiment of the present invention;

FIG. 3 is a block diagram of a method according to the present invention;

FIG. 4 is a block diagram of a method according to another embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

It is an advantage of the present invention to provide in-situ monitoring of polishing process characteristics during a polishing operation. A further advantage of the present invention is to provide a polishing apparatus giving improved end point detection.

According to the present invention, a polishing apparatus comprises a polish head for receiving an object to be polished and a surface plate opposite to the polish head having an abrasive cloth. The polish head is movable relative to the surface plate. A head driving mechanism for driving the polish head has a shaft which is coupled to the polish head. The shaft is supported in a housing by bearings spaced from each other. The polishing apparatus comprises a strain sensor for measuring tangential frictional force applied to the shaft when the polish head is moving relative to the surface plate. The present invention makes use of indirectly monitoring the friction between the abrasive cloth and the object to be polished. This friction is dependant on the operating condition of the polishing machine including the material and condition of the object polished. Thus, if the other conditions are kept constant, the friction is characteristic of the material and condition of the object polished. Situations where the other conditions are kept sufficiently constant are realized e.g. in massproduction, when many objects of the same size and surface material are polished (like in the semiconductor production). Due to the friction, a lateral force is applied to the polish head by the moving surface plate, which tries to carry the polish head, via the abrasive cloth and the object to be polished. Mounted to some external support structure the polish head is fixed in space during polishing. Therefore the lateral force of interest in carry-direction is substantially tangential to the rotation of the surface plate and substantially independent of a rotation of the polish head. This leads to a tilting force to the shaft substantially perpendicular to its axis. According to the invention, the tilting of the shaft is used to indirectly monitor the material and condition of the object polished. When polishing a single material, the monitoring of the friction between the abrasive cloth and the object to be polished shows that the measured force and thus the friction at the beginning of the polish process' can be significantly different from that at a final state of the polishing. From the strain signal as it develops after the start of polishing it can be determined for each material polished how far the final state of the polishing process is away or when this state is reached. It can be useful to define a signal reference value which is equal to the final state value, i.e. the sensor signal value characteristic for the final state. This signal reference value can be a given value, a measure of the rate of change of the signal or any of several other signal processing techniques well known in th einductry. Then the end point of polishing is defined as the event that the sensor signal value equals the defined signal reference value.

Due to tolerances it can be advantageous to define a signal reference value which is different from the final state value but for which the polish time left to reach the final can be calculated. Then the end point of polishing is defined as after the polish time left from the event that the sensor signal value equals the defined signal reference value. Thus according to the invention, the tilting of the shaft is used to determine the end point of polishing.

When polishing in order to remove a layer of a layer material upon buried material, the monitoring of the friction between the abrasive cloth and the object to be polished shows that the measured force characteristic for polishing the covering layer material is frequently significantly different from that characteristic for polishing the buried material. It is desired to detect when the buried material is reached or when the covering layer material is assumed to be completely removed. Then, further polishing would remove additional buried material but without changing the buried surface quality. Especially if the buried is a thin layer it is not desirable to thin it more than necessary, i.e. not to over-polish. From the strain signal as it develops after the start of polishing it can be determined for each layer material and each buried material polished when the final state of polishing for this material is reached. Thus it can be determined when the bottom of the covering layer to be removed is reached and also when the final state of polishing the buried layer is reached. This state when the sensor signal value equals a signal reference value characteristic for polishing the buried material is assumed the state where no covering layer material is left and thus the covering layer has been completely removed. Tolerance behavior can be considered. Thus according to the invention, the tilting of the shaft is used to determine the end point of polishing in order to remove a layer of a covering layer material upon an buried material. FIG. 1 shows pertinent portions of a polishing apparatus according to a first embodiment of the present invention.

Polishing apparatus 10 comprises a circular-disk type surface plate 12 being driven by plate motor 14, and a circular-disk type polish head 16 for supporting an polishing object. Abrasive cloth 20 which being opposed and contacted to polishing object 18 is adheres to the surface plate 12. Polish head 16 is coupled by shaft 22 to head motor 24 driving polish head 16. Bearing housing 26 bears shaft 22 by lower bearing 28 and upper bearing 30. Polish head 16, bearing housing 26 and head motor 24 are supported by structure 32 and form liftable group 34 being liftable from surface plate 12 as indicated by arrow 35. Shaft 22 has mounted strain sensor 36 to its surface located below and close to lower bearing 28.

In operation, surface plate 12 being driven by plate motor 14 rotates. Liftable group 34 supporting polishing object 18 is let down such that polishing object 18 is in contact to abrasive cloth 20 upon surface plate 12. Polish head 16 and polishing object 18 fixed thereto rotate with about the same frequency as surface plate 12. Polishing object 18 is polished as it in motion relative to abrasive cloth 20 fixed to surface plate 12. A slurry which can be chemically active is applied to improve the polishing (not shown).

Due to the friction between rotating surface plate 12 and the carried/ rotating polish head 16 a first lateral force is applied to polishing object 18 and thus to polish head 16. With respect to the lever principle second and third lateral forces are consequently applying to lower bearing 28 and upper bearing 30, respectively. These forces depend on the friction and thus the material being polished and on the quality of the surface. According to the invention, these forces are measured to determine the condition or state of the polishing process.

There are several ways to measure the strain applying to the shaft. Among these are optical detection of the shaft deformation with a nonrotating sensor remote to the shaft, e.g. a position sensor sensing a light beam reflected on the shaft surface. Also possible is a strain sensor mounted on the shaft surface like sensor 36 in HG. 1 sensing mechanical stress directly. Such a sensor rotates with the shaft and experiences periodic strain fluctuation with the rotation period. Then, the amplitude of the periodic signal is a measure for the strain. It can be advantageous to provide a strain sensor comprising two detector heads observing rectangular to each other.

Strain sensor 36 is electrically coupled to an evaluation device (not shown) receiving a sensor signal from the strain sensor for detecting a change in the sensor signal. Those skilled in the art know how to transfer the sensor signal from the strain sensor to the evaluation device, in case the sensor rotates with the shaft by contacts or wireless. The evaluation device monitors the development of the sensor signal and compares its value to a signal reference value which is characteristic for the final state of the current polish process. The evaluation device provides a polish end signal when the sensor signal strain to the shaft and thus the sensor signal change predefined ways characteristic for the current polish process. Then the evaluation device provides polish end signal. Preferably, the polishing apparatus has means for stopping polishing when or after a calculated time after the sensor signal changes in a predefined way characteristic for the current polish process.

A polishing apparatus according the present invention can be a multi-head machine comprising additional polish heads on a single pad each having an additional strain sensor. Advantageously, this invention provides a means for stopping polishing for each polish head individually.

Advantageously, a polishing apparatus according the present invention is a multi- polish pad machine comprising additional polish pads on the surface plate. Then, the friction between head and pads is monitored for each pad separately. This allows to monitor the wear of each polish pad separately.

FIG. 2 shows a liftable group 134 which is a part of a polishing apparatus according to another embodiment of the present invention. Liftable group 134 is similar to liftable group 34 of FIG. 1 and comprises polish head 116, bearing housing 126 and head motor 124 are supported by structure 132. Polish head 116 supports an polishing object 118. Polish head 116 is coupled by shaft 122 to head motor 124 driving polish head 116. Bearing housing 126 bears shaft 122 by lower bearing 128 and upper bearing 130. Shaft 122 has thickness extension 140 located below and close to lower bearing

128. Thickness extension 140 has lower extension portion 142 and upper extension portion 144 both fixed to shaft 122. Strain sensor 146 is mounted to measure the train between lower extension portion 142 and upper extension portion 144. This assembly allows to a larger dimension to be observed. Those skilled in the art know that this principle allows variations such as to couple both extension portions by a material softer than the shaft with the sensor mounted on this material, or to locate lower extension portion 142 and upper extension portion 144 distant from each other, or to use an optical distance sensor measuring the portion distance and thus indirectly measuring the strain.

FIG. 3 shows method 50 of monitoring strain applying to a shaft coupled to a polish head of a polishing apparatus according to the present invention. The polishing apparatus has a strain sensor monitoring strain applying to the shaft when the polish head is moving relative to a surface plate. Method 50 has the following steps. In step 52 polishing the object to be polished is started. In step 54 a sensor signal of the strain sensor is provided. In step 56 the sensor signal is monitored.

In step 58 polishing the object to be polished is stopped.

Method 50 makes use of monitoring a lateral force applied to the polish head during polishing which leads to a tilting force to the shaft.

FIG. 4 shows method 60 of monitoring strain applying to a shaft coupled to a polish head of a polishing apparatus according to another embodiment of the present invention. The polishing apparatus has a strain sensor monitoring strain applying to the shaft when the polish head is moving relative to a surface plate. Method 60 has the following steps.

Method 60 begins with preparation, determining a first reference sensor signal relating to a first material being polished in step 62; determining a second reference sensor signal relating to a second material being polished, step 64; determining a threshold signal from the first reference sensor signal and the second reference sensor signal in step 66. Then, after this preparation, polishing the object to be polished is started in step 68. A sensor signal of the strain sensor is provided in step 70. Then the sensor signal is monitored which includes displaying the sensor signal in step 72. The monitoring further includes comparing a value of the strain signal to the threshold value in step 74. An alarm signal is provided when the difference between the sensor signal and the threshold signal changes its sign in step 76. The alarm signal initiates the next step, step 78 stopping polishing. This method provides end point detection and automatical stopping the polishing process. Those skilled in the art will know from this description that some steps of method 60 can be omitted or put in different order under special circumstances. For example displaying the sensor signal is sufficient for a process controlled by a human operator stopps the pohshing manually. Displaying the sensor signal can be omitted in a fully automated process. The preparational steps can be replaced by determining the threshold value during the polishing from an asymptotic curve form of the sensor signal.

REFERENCE NUMBERS

Polishing apparatus 10 surface plate 12 plate motor 14 polish head 16 polishing object 18

Abrasive cloth 20 shaft 22 head motor 24 Bearing housing 26 lower bearing 28 upper bearing 30 structure 32 liftable group 34 Strain sensor 36 method 50

52 polishing the object to be polished is started.

54 a sensor signal of the strain sensor is provided 56 the sensor signal is monitored

58 polishing the object to be polished is stopped method 60 determining a first reference sensor signal in step 62 determining a second reference sensor signal, step 64 determining a threshold signal in step 66 polishing the object to be polished is started in step 68. sensor signal of the strain sensor is provided in step 70 displaying the sensor signal in step 72 comparing a value of the strain signal to the threshold value in step 74 alarm signal is provided in step 76 step 78 stopping polishing polish head 116 polishing object 118 shaft 122 head motor 124 bearing housing 126 lower bearing 128 upper bearing 130 structure 132 liftable group 134 thickness extension 140 lower extension portion 142 upper extension portion 144 Strain sensor 146

Claims

CLAIMS:

1. A polishing apparatus comprising: a polish head for receiving an object to be polished; a surface plate opposite to the polish head, the surface plate having an abrasive cloth, the polish head being movable relative to the surface plate; a head driving mechanism for driving the polish head having a shaft and being coupled to the polish head and beared in a housing by a first and a second bearing spaced from each other; a strain sensor for measuring strain applying to the shaft when the polish head is moving relative to the surface plate.

2. The apparatus of claim 1 wherein and the strain sensor is adapted to sense a bending of the shaft.

3. The apparatus of claim 1 wherein the strain sensor is located upon the shaft.

4. The apparatus of claim 1 wherein and the strain sensor is located close to the bearing next to the polish head.

5. The apparatus of claim 1 wherein the strain sensor is a stress sensor sensing stress in axial direction of the shaft.

6. The apparatus of claim 1 wherein the shaft has a radial extension and the strain sensor is mounted distant from an axis of the shaft.

7. The apparatus of claim 1 wherein the strain sensor is an optical sensor sensing light reflected by the shaft.

8. The apparatus of claim 1 wherein the strain sensor is an optical sensor sensing a distance.

9. The apparatus of claim 1 wherein the strain sensor comprises two detector heads observing rectangular to each other.

10. The apparatus of claim 1 comprising additional polish pads on the surface plate.

11. The apparatus of claim 1 having an evaluation device receiving a strain signal from the strain sensor for detecting a change in the strain signal.

12. The apparatus of claim 11 wherein the evaluation device is adapted to provide a polish end signal when the strain to the shaft changes in a predefined way.

13. The apparatus of claim 12 having a means for stopping polishing when the strain to the polish head changes in a predefined way.

14. The apparatus of claim 13 comprising additional polish heads and additional strain sensors, the means for stopping polishing being adapted to stop polishing for each polish head individually.

15. A method of monitoring strain applying to a shaft coupled to a polish head of a polishing apparatus when the polish head is moving relative to a surface plate, the polishing apparatus having a strain sensor, comprising the steps of: a. starting polishing the object to be polished; b. providing a sensor signal of the strain sensor; c. monitoring the sensor signal; d. stopping polishing the object to be polished.

16. The method of claim 15 comprising the additional step of: displaying the sensor signal.

17. The method of claim 15 comprising the additional step of: aa. determining a threshold value relating to a predefined polishing condition; bb. comparing a value of the sensor signal to the threshold value; cc. providing an alarm signal when the difference between the sensor signal and the threshold signal changes its sign.

18. The method of claim 17 comprising the additional step of: stopping polishing when when the difference between the sensor signal and the threshold signal changes its sign.

19. The method of claim 17, further comprising the steps aaa. determining a first reference sensor signal relating to a first material being polished; bbb. determining a second reference sensor signal relating to a second material being polished; ccc. determining the threshold signal from the first reference sensor signal and the second reference sensor signal.