US20070082582A1

US20070082582A1 - Apparatus for endpoint detection during polishing

Info

Publication number: US20070082582A1
Application number: US11/338,667
Authority: US
Inventors: Tsung-Ju Gwo; Ming-Wei Chang; Chung Yang
Original assignee: Industrial Technology Research Institute ITRI
Current assignee: Industrial Technology Research Institute ITRI
Priority date: 2005-10-06
Filing date: 2006-01-25
Publication date: 2007-04-12
Also published as: TWI289091B; TW200714408A

Abstract

An apparatus for endpoint detection during polish, being used to monitor the surface polishing status of a polishing surface of a workpiece, is disclosed, which comprises: a probe, being positioned above the workpiece by a height; and at least a sensing element, being disposed on the probe at a position corresponding to the polishing surface, capable of sensing the thickness variation of the workpiece during a polishing process as it is performing a surface dynamic scan upon the workpiece in rotating while it is being driven to proceed a linear motion of displacement. Preferably, the sensing element can be a device selected from the group consisting of an optical sensor, an eddy current sensor and the combination of the two for carrying out optical detection and magnetic flux detection. It is noted that the apparatus for endpoint detection of the invention not only is advantaged in its all-zone detection ability, but also it has enhanced detection accuracy with respect to different metal layer of different thickness.

Description

FIELD OF THE INVENTION

The present invention relates to an apparatus capable of monitoring the thickness of a workpiece, and more particularly, to an apparatus having a sensing element disposed on a probe thereof while enabling the sensing element to perform a linear movement, which is capable of monitoring and measuring the surface polishing status of a workpiece during a polishing process.

BACKGROUND OF THE INVENTION

It is known that miniaturization is the strategy to success, which requires small, high performance, versatile, highly reliable and inexpensive microstructured components, the structure of microstructured component has becoming more and more complex. As manufacturers reduce the size of integrated circuit (IC) components to fit more components on each IC chip and as the substrate of IC manufacturing is getting larger, there is more going on in the IC fabrication process itself than a simple reduction in scale. That is, the demanding for higher inspection precision and better accuracy of each manufacturing process are required. Especially in the process of surface stacking, the yield of process can be enhanced if the substrate thickness and the surface flatness can be measured and inspected with high precision.
It is well known in the art that the creation of semiconductor devices frequently requires the creation of these devices in a number of overlaying layers of material, which further uses and depend on crating surfaces of near ideal planarity or flatness by using a polishing process. Several methods are available for determining the endpoint of the polishing process, the most common of which is by using an end point detector (EPD). By using the EPD to monitor a target layer optically, data can be detected by a photo detector of the intensity of a reflected light beam to produce a trace curve of which is then used to determine the endpoint of the polishing process.
Please refer to FIG. 1, which is a schematic diagram of an end point detector for determining the endpoint of a chemical mechanical polishing (CMP) process according to the prior art. As seen in FIG. 1, an unpolished semiconductor wafer 11 is positioned within a holder 13 of a wafer head 15 while enabling the target surface of the wafer 11 to face toward a polishing pad 12 supported by a polishing platen 16, whereas a window penetrates both the pad 12 and the platen 16 to the target surface of the semiconductor wafer 11. An optical detecting device 17 generates a light beam and direct the light beam to pass through the hole of the polishing pad 12 onto the target surface of the semiconductor wafer 11 at a predetermined angle, whereas the intensity of the reflected light beam can be continually detected by the optical detecting device 17. Then, the data is transmitted to the controller 18 where the detected intensity I of the reflected light can be used to determine the CMP endpoint.
There is another endpoint detection device shown in U.S. Pat. No. 6,517,413, entitled “METHOD FOR A COPPER CMP ENDPOINT DETECTION SYSTEM”, by which the amount of copper dioxide removed from a target surface that is being polished is being monitored by means of a laser beam, whereas the laser beam is reflected off the target surface while the reflected light beam is being analyzed by a computer to determine at what time no more copper dioxide is present on the target surface which is the time that the polishing process is completed. Further, in U.S. Pat. No. 6,336,841, patented by Macronix International Co. Ltd., entitled “METHOD OF CMP ENDPOINT DETECTION”, an infrared spectroscopic method of endpoint detection is disclosed, by which once the CMP process is performed, a change in the IR absorptivity is detected whereas the IR absorptivity of the target layer progressively decreases for a length of time until significant change in the absorbance curve is no longer detected that is the endpoint of the CMP process. In addition, in U.S. Pat. No. 6,586,337, entitled “METHOD AND APPARATUS FOR ENDPOINT DETECTION DURING CHEMICAL MECHANICAL POLISHING”, an endpoint detection apparatus comprising a probe capable of emitting and receiving light is disclosed, that by positioning the probe underneath a workpiece, it can detect and monitor the variation of the reflected light and thus determine the endpoint of a CMP process.
However the aforesaid methods can be used for detecting thickness of the target layer, there are still several shortcomings listed as following:

- (1) The thickness of the target layer is required to be greater than 3000 angstroms so that the signal-to-noise ratio generated by the reflecting light beam can be sufficient enough for the endpoint detection.
- (2) The EPDs used in the prior methods are all being fixed to a specific position that the area of detection on a large-sized wafer are limited and thus they can only be used as local detectors so that the accuracy of detection acquired thereby is not preferred since the overall thickness variation of the large-sized wafer can not be detected and measured by a local EPD.
- (3) As seen in FIG. 2A to FIG. 2C, the metal layer 21 is first being deposited on the substrate 20 and then is covered by the depositing of the dielectric layer 22 thereon. It is noted that the planarity of the device surface seen in FIG. 2B is not ideal enough for the next processing step to be performed thereon, so that a polishing process is required to create a surface of enough planarity of flatness as seen in FIG. 2C. However, since the prior-art EPD can only detect a portion of the substrate wafer while it is being used for determining the endpoint of a polishing process, a portion of the wafer might be over-polished while another portion thereof might be under-polished such that the thickness of the target layer is not uniform.

Furthermore, It is known that all the prior-art polishing processes are dependent on the calculation of total thickness variation (TTV) which used a designated base level as reference level that can cause error while the planarity of a workpiece is not sufficient or the workpiece is not being positioned properly. As seen in FIG. 3, the workpiece 80 is being positioned on an inclined platen 81 that causes errors to occur while the reference level 82 is used by an EPD for monitoring the thickness and planarity of a target layer.
For all of these aspects, it is required to provide an apparatus of endpoint detection during polishing that is cost effective and can be applied without having to alter existing process equipments.

SUMMARY OF THE INVENTION

It is the primary object of the present invention to provide an apparatus of endpoint detection during polishing, which comprises at least a sensing element, capable of sensing the thickness variation an d planarity of a workpiece during a polishing process while it is being driven to proceed a linear motion of displacement, whereas each sensing element can be a device selected from the group consisting of an optical sensor, an eddy current sensor and the combination of the two, so that the apparatus of endpoint detection of the invention not only is advantaged in its all-zone detection ability, but also it has enhanced detection accuracy with respect to different metal layer of different thickness.
To achieve the above object, the present invention provide an apparatus of endpoint detection during polishing, being used to monitor the surface polishing status of a polishing surface of a workpiece, which comprises: a probe, being positioned above the workpiece by a height; and at least a sensing element, being disposed on the probe at a position corresponding to the polishing surface, capable of sensing the thickness variation of the workpiece during the polishing process while it is being driven to proceed a linear motion of displacement.
In a preferred aspect of the invention, the sensing element further comprises an inductor, capable of generating a magnetic filed on the workpiece for inducing eddy current on the workpiece.
In a preferred aspect of the invention, the sensing element further comprises an eddy current sensor for detecting the intensity of magnetic field generated on the workpiece.
In a preferred aspect of the invention, the sensing element further comprises a light source for emitting a light beam onto the workpiece.
In a preferred aspect of the invention, the apparatus of the invention further comprises at least an optic sensor, for receiving light beams reflected from the workpiece, each being disposed on a linear rail enabling the same to be driven to proceed a linear motion of displacement so as to enable the same to receive the plural light beams reflected from the workpiece.
In a preferred aspect of the invention, the apparatus of the invention further comprises a signal integrating unit, electrically connected to the sensing element, for integrating signals received from the sensing element to output a sensing signal. Moreover, the signal integrating unit is further coupled to a control unit, which can evaluate the surface polishing status of the workpiece basing on the sensing signal.
Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an end point detector for determining the endpoint of a chemical mechanical polishing (CMP) process according to the prior art.
FIG. 2A to FIG. 2C are consecutive views of metal layer deposition.
FIG. 3 is a schematic diagram showing a workpiece being positioned on an inclined platen.
FIG. 4 is a schematic diagram of a first preferred embodiment according to the present invention.
FIG. 5 is a schematic diagram of a second preferred embodiment according to the present invention.
FIG. 6 is a schematic diagram showing the assembly of a polishing device and an endpoint detection apparatus of the invention.
FIG. 7A is a schematic diagram showing an eddy current sensor according to a preferred embodiment of the invention.
FIG. 7B is a schematic diagram showing an eddy current sensor according to another preferred embodiment of the invention.
FIG. 8A is a schematic diagram showing a light source and an optic sensor according to the invention.
FIG. 8B is a schematic diagram showing the use of the light source and the optic sensor for thickness detection during a polishing process.
FIG. 8C shows a phase difference between a light incident onto a workpiece and it reflected light, being detected by an optic sensor.

DESCRIPTION OF THE PREFERRED EMBODIMENT

For your esteemed members of reviewing committee to further understand and recognize the fulfilled functions and structural characteristics of the invention, several preferable embodiments cooperating with detailed description are presented as the follows.
Please refer to FIG. 4, which is a schematic diagram of a first preferred embodiment according to the present invention. As seen in FIG. 4, an endpoint detection apparatus 3, being used to monitor the surface polishing status of a polishing surface of a workpiece, comprises: a probe 30, being positioned above the workpiece by a height; and a sensing element 31, being disposed on the probe 30 at a position corresponding to the polishing surface, capable of sensing the thickness variation of the workpiece during the polishing process while it is being driven to proceed a linear motion of displacement 91. Moreover, the sensing element 31 further comprises an optic sensor 311, an eddy current sensor 312, and the like. It is noted that since the sensing element 31 can be driven to proceed a linear motion of displacement 91 along the axis of the probe, the endpoint detection apparatus 3 is equipped with all-zone detection ability that it can monitor the overall thickness variation and surface planarity of the whole workpiece.
The configuration of the sensing element 31 are selected basing on actual detection requirement, which can be an optical sensor 311, an eddy current sensor 312, or the combination of the two, as shown in the first embodiment of FIG. 4, or other sensing devices. As the first preferred embodiment shown in FIG. 4, the sensing element is the combination of the optic sensor 311 and the eddy current sensor 312 so as to enhance the accuracy and quality of the detection since the workpiece is made of metallic material. That is, any sensor suitable for detection polishing status of a workpiece can be used as the sensing element.
The linear motion of displacement 91 is realized by a linear moving mechanism, such as a linear rail 32 shown in FIG. 4. The sensing element 31 is being disposed on a linear rail 32 while the linear rail 32 is arranged in the probe 30 such that the sensing element 31 can be driven to proceed a linear motion of displacement 91 along the linear rail 32 while enabling the same to monitor the polishing status of any location as required.
Please refer to FIG. 5, which is a schematic diagram of a second preferred embodiment according to the present invention. As seen in FIG. 5, there are a plurality of sensing elements 61 being arranged on the probe 60 of the endpoint detection apparatus 6, and the plural sensing elements 61 are being separated from each other by a specific interval while each is capable of being driven to proceed a linear motion of displacement 91 along a linear rail 64 arranged on the probe 60. The second embodiment of the invention shown in FIG. 5, which allows a larger detection range, is suitable to be used for detecting a large-sized workpiece since the displacement of each sensing element 61 is small and thus the accuracy of detection inversely affected thereby is reduced.
Please refer to FIG. 6, which is a schematic diagram showing the assembly of a polishing device and an endpoint detection apparatus of the invention. The polishing device 7, which can be a mechanical polishing device but is not limited thereby, comprises a platen 71, a polishing pad 72, a wafer head 74 and a holder 73, wherein the platen 71 and the wafer head 74 are driven to perform a rotating movement 92 while a workpiece 5 is fixed in the wafer holder by means of placing the polishing pad 72 on the platen 71, and arranging the holder 73 on the wafer head 74, and moreover, the polishing surface of the workpiece 5 is being arranged facing toward the polishing pad 72.
Furthermore, an accommodating space 711 is arranged inside the platen 71 for receiving the endpoint detection apparatus, which can be the device shown in the first or the second embodiment of the invention. The device of the second embodiment of the invention is adopted as the endpoint detection apparatus 6 used in FIG. 6. The signal lines of the endpoint detection apparatus 6 pass through the rotating axis of the platen 71 and are electrically connected to a signal integrating unit 62. The signal integrating unit 62 is capable of integrating signals received from the endpoint detecting apparatus 6 to output a sensing signal. Moreover, the signal integrating unit 62 is further coupled to a control unit 63, which can evaluate the surface polishing status of the workpiece 5 basing on the sensing signal. The workpiece 5 hold by the wafer head 74 is a wafer 5 undergoing a process of overlaying layers of material.
As seen in FIG. 5 and FIG. 6, the eddy current sensor 612 can evaluate the magnetic flux density of a target area, which is a kind of general area detection, and on the other hand, the optic sensor 611 can pick up reflected light beams while moving along the linear rail 64 so that it can evaluate the thickness of the workpiece 5 at each area where the corresponding light beam was reflected and then the result of the evaluation is transmitted to the control unit 63 for further processing. Since the endpoint detection apparatus 6 has a plurality of sensing elements 61, at the beginning of a polishing process, a plurality of thickness values can be acquired by the plural sensing elements 61 at a plural positions corresponding thereto in respective and are used as a reference, and during the polishing process, similarly, a plurality of current thickness values are acquired by the plural sensing elements 61 at the corresponding positions while being compared to the reference so as to evaluate the current polishing status of the workpiece during the polishing process.
Please refer to FIG. 7A, which is a schematic diagram showing an eddy current sensor according to a preferred embodiment of the invention. As seen in FIG. 7A, the eddy current sensor 612 further comprises a core 6120, which has an inductor 6121 arranged in the middle thereof. In a preferred embodiment of the invention, the inductor 6121 is the driver coil wrapping around the middle of the core 6120. Moreover, there are two eddy detectors 6122 respectively located at the two end areas of the core 6120, whereas each can be a pickup coil wrapping around an end of the core 6120.
By the characteristic of eddy current, the density of eddy current is decreasing with the increasing of depth, which follows the following formula: $δ = \frac{1}{\sqrt{π f μ σ}}$
where δ represents depth of penetration (m)

- f represents frequency (Hz)
- σ represents electrical conductivity (mho/m)
- μ represents Magnetic Permeability (Henry/m)

Furthermore, as the increase of depth, a regular phase lay will occur between the phase angles of corresponding eddy currents. With respect to the AC magnetic filed caused by the lagging eddy currents, the phase of the induced current caused by the pickup coil of the eddy current detector 6122 will be varied. Since it is known that the thickness variation of the workpiece 5 during a polishing process will have affect on the penetration depth of eddy current, the thickness variation will have affect on the phase of the induced current. Therefore, the phase-varying induced current detected by the eddy current detector 6122 can be used for evaluating the change of penetration depth with respect to a predefined standard, and further the thickness variation of the workpiece 5 can be measured.
Since the workpiece 5 is a structure of overlaying layers of metal, eddy currents can be caused on the workpiece 5 by the AC magnetic field of the inductor 6121 when the workpiece 5 is approached by the inductor 6121 conducting an AC current, and the eddy currents will all generated AC magnetic fields opposite to the AC magnetic field of inductor 6121 which can cause the pickup coil of the eddy current detector 61222 to generate an induced current. As the resistance of the workpiece 5 will vary along the thickness variation of the same, the eddy currents and the AC magnetic fields caused thereby will also vary according which further cause the phase variation or the variation of voltage magnitude to be detected by the eddy current detector 6122.
It is noted that the eddy current sensor shown on FIG. 7A is only one of the embodiment and is not limited thereby. The eddy current sensor 4 shown in FIG. 7B is another embodiment, also comprising an inductor 40 and an eddy current detector 41, that the working principle is similar to that of FIG. 7A and thus is not further described herein.
Please refer to FIG. 8A, which is a schematic diagram showing a light source and an optic sensor according to the invention. The optic sensor 611 further comprises: a light source, for emitting a light beam onto the workpiece; and an optic detector 6112; wherein the light beam incident to the workpiece by a specific angle where it is being reflected to be received by the optic detector 6112.
As seen in FIG. 8B and FIG. 8C, the light path of the reflected light beam received by the optic detector 6112 will vary with the thickness variation of the workpiece 5 as the progress of the polishing process that signals of phase difference can be generated. As the reflected light beam 90 received by the optic detector 6112 is reflected by a first thickness 50, the generated optical signal 94 will be the one seen in FIG. 8C. When the polishing process keeps progressing and as the workpiece 5 is of a second thickness 51, the reflected light beam 90 received by the optic detector 6112 will cause the same to generate another optic signal 94′. As seen in FIG. 8C, the phase difference between the optic signals 94, and 94′ is represented by Δ φ that the phase difference Δ φ can be used by the control unit for evaluating the thickness of the workpiece 5.
To sum up, the apparatus for endpoint detection of the present invention not only is advantaged in its all-zone detection ability, but also it has enhanced detection accuracy with respect to different metal layer of different thickness, so that it can be used for evaluating the thickness of various target layers and also for monitoring the planarity while removing unwanted particles from the surface.
While the preferred embodiment of the invention has been set forth for the purpose of disclosure, modifications of the disclosed embodiment of the invention as well as other embodiments thereof may occur to those skilled in the art. Accordingly, the appended claims are intended to cover all embodiments which do not depart from the spirit and scope of the invention.

Claims

1. An apparatus of endpoint detection during a polishing process, being used to monitor the surface polishing status of a polishing surface of a workpiece, comprising:

a probe, being positioned above the workpiece by a height; and

at least a sensing element, being disposed on the probe at a position corresponding to the polishing surface, capable of sensing the thickness variation of the workpiece during the polishing process while it is being driven to proceed a linear motion of displacement.

2. The apparatus of claim 1, wherein the sensing element further comprises an inductor, capable of generating a magnetic filed on the workpiece for inducing eddy currents to be generated on the workpiece.

3. The apparatus of claim 2, wherein the sensing element further comprises an eddy current sensor for detecting the intensity of the magnetic field generated on the workpiece.

4. The apparatus of claim 3, wherein the sensing element further comprises a light source for emitting a light beam onto the workpiece.

5. The apparatus of claim 4, further comprising at least an optic sensor, for receiving light beams reflected from the workpiece.

6. The apparatus of claim 5, wherein each optic sensor is being disposed on a linear rail for enabling the same to be driven to proceed a linear motion of displacement so as to receive the plural light beams reflected from the workpiece.

7. The apparatus of claim 2, wherein the workpiece is made of a metal.

8. The apparatus of claim 1, wherein the sensing element further comprises a light source for emitting a light beam onto the workpiece.

9. The apparatus of claim 8, wherein further comprising at least an optic sensor, for receiving light beams reflected from the workpiece

10. The apparatus of claim 9, wherein each optic sensor is being disposed on a linear rail for enabling the same to be driven to proceed a linear motion of displacement so as to receive the plural light beams reflected from the workpiece.

11. The apparatus of claim 1, wherein the sensing element is being disposed on a linear rail while the linear rail is arranged in the probe.

12. The apparatus of claim 1, further comprising a signal integrating unit, electrically connected to the sensing element, for integrating signals received from the sensing element to output a sensing signal.

13. The apparatus of claim 12, wherein the signal integrating unit is further coupled to a control unit for enabling the control unit to evaluate the surface polishing status of the workpiece basing on the sensing signal.