Thickness sensor for conductive features

Various embodiments provide a thickness sensor and method for measuring a thickness of discrete conductive features, such as conductive lines and plugs. In one embodiment, the thickness sensor generates an Eddy current in a plurality of discrete conductive features, and measures the generated Eddy current generated in the discrete conductive features. The thickness sensor has a small sensor spot size, and amplifies peaks and valleys of the measured Eddy current. The thickness sensor determines a thickness of the discrete conductive features based on a difference between a minimum amplitude value and a maximum amplitude value of the measured Eddy current.

BACKGROUND

Semiconductor devices are increasingly including smaller device features. In order to fabricate such device features with desired dimensions, it is important to accurately control a thickness or depth of the device features during fabrication.

Eddy current sensors are commonly used to measure a thickness of a conductive or metal film. Current Eddy current sensors are capable of measuring thicknesses of continuous conductive films (e.g., a continuous conductive film covering a whole wafer after a metal deposition application) with acceptable accuracy. However, when such Eddy current sensors are used to measure thicknesses of discrete conductive features (e.g., metal patterns for conductive lines or plugs), the signals received by current Eddy current sensors are often weak and/or noisy. Consequently, the accuracy of measuring thicknesses of discrete conductive features with current Eddy current sensors is compromised.

DETAILED DESCRIPTION

The present disclosure is directed to a thickness sensor and method for measuring a thickness of conductive features on a substrate, such as conductive lines and plugs.

FIG. 1is a diagram of a thickness sensor10in accordance with some embodiments. The thickness sensor10includes a driver12, a drive coil14, an Eddy current reader16, a sensor coil18, and a processor20.

The driver12generates and provides a driving signal to the drive coil14. In some embodiments the driving signal is an alternating current (AC) signal. In response to receiving the driving signal, the drive coil14generates a magnetic field22.

When the thickness sensor10is disposed above or near a plurality of discrete conductive features substantially at a same level on a substrate which are separated by one or more regions of non-conductive material at substantially the same level as the discrete conductive features, the magnetic field22generates Eddy currents in the plurality of conductive features. For example, in the embodiment shown inFIG. 1, when the thickness sensor10is disposed above or near a discrete conductive feature24on a substrate26, the magnetic field22generates an Eddy current28in the discrete conductive feature24. The Eddy current28, in turn, generates a secondary magnetic field30. In various embodiments, the conductive features are formed from a conductive film or layer. For example, the discrete conductive feature24may be part of a conductive film23.

The Eddy current reader16measures the Eddy current28. In some embodiments described herein, the Eddy current reader16receives the secondary magnetic field30via the sensor coil18, measures the Eddy current28based on the secondary magnetic field30, and generates an Eddy current signal. In some embodiments, the Eddy current signal is the measured Eddy current of the Eddy current28. The Eddy current reader16provides the Eddy current signal to the processor20.

In various embodiments described herein, the drive coil14is a cylindrical coil having a diameter d1. In various embodiments, the sensor coil18is a cylindrical coil having a diameter d2. In one embodiment, the drive coil14and the sensor coil18have the same diameters (i.e., the diameter d1is substantially equal to the diameter d2). In other embodiments, the diameter d1of the drive coil14is larger than the diameter d2of the sensor coil18. In other embodiments, the diameter d1of the drive coil14is smaller than the diameter d2of the sensor coil18. As will be discussed in further detail below, the diameter d2of the sensor coil18determines a sensor spot size of the thickness sensor10.

The processor20receives the Eddy current signal from the Eddy current reader16and determines various parameters of the Eddy current signal. In one embodiment, as will discussed in further detail below with respect toFIG. 4, the processor20determines a maximum amplitude of the Eddy current signal and a minimum amplitude of the Eddy current signal and uses these amplitudes to calculate a thickness value representing a thickness of a plurality of conductive features. In addition, in various embodiments the processor20controls the driver12; however, driver12could be controlled by a processor (not shown) different from processor20. In one embodiment, the processor20instructs the driver12to generate and provide the driving signal to the drive coil14. The processor20may be any type of controller, microprocessor, application specific integrated circuit (ASIC), or the like that communicates with the driver12and the Eddy current reader16.

It is noted that, although the driver12, the Eddy current reader16, and the processor20are shown inFIG. 1as separate modules, the various modules may be combined with each other in to a single module. For example, in one embodiment, the functions of the Eddy current reader16and the processor20are performed by a single module. In addition, although the driver12, the Eddy current reader16, and the processor20, are included within the thickness sensor10, one or more of the driver12, the Eddy current reader16, and the processor20may be external to the thickness sensor10. For example, in one embodiment, the processor20is external to the thickness sensor10.

In accordance with embodiments described herein, the thickness sensor10has a small sensor spot size. The sensor spot size is the size of the area in which the Eddy current reader16is able to measure an Eddy current. For example, in one embodiment, the thickness sensor10has a sensor spot size having a diameter less than 3 millimeters. In this embodiment, the Eddy current reader16is able to measure an Eddy current within a circular area having a diameter less than 3 millimeters. The present description is not limited to thickness sensors having a sensor spot size diameter less than 3 millimeters, for example, thickness sensor10can have a sensor spot size diameter greater than 3 millimeters.

The sensor spot size is determined by the size of the sensor coil18. A sensor spot size increases when the diameter d2is increased, and the sensor spot size decreases when the diameter d2is decreased. Current Eddy current sensors typically include sensor coils with large diameters, and, thus, have large sensor spot sizes. For example, current Eddy current sensors include sensor coils with diameters between 15 millimeters and 20 millimeters, and have sensor spots with diameters between 5 millimeters and 7 millimeters. In one embodiment of the sensor coils described herein, the diameter d2of the sensor coil18is less than 10 millimeters. In other embodiments described herein, the diameter d2can be 10 millimeters or greater than 10 millimeters.

By having a small sensor spot size, the spatial resolution of the thickness sensor10is increased. By having a higher spatial resolution, the thickness sensor10, more specifically the Eddy current reader16, is able to generate an Eddy current signal with less averaging between an area with a large amount of conductive material and an area with a small amount of conductive material. Stated differently, the thickness sensor10is able to amplify and exaggerate measurements of an Eddy current. As a result, peaks of the Eddy current signal, which correspond to areas with large amounts of conductive material, will have increased amplitudes, and valleys of the Eddy current signal, which correspond to areas with small amounts of conductive material, will have decreased amplitudes.FIG. 2is a diagram of the thickness sensor10scanning along a sensor sweep path32in accordance with some embodiments.FIG. 3is a diagram of an Eddy current signal34measured by the thickness sensor10in accordance with some embodiments and an Eddy current signal36generated by an Eddy current sensor having a large sensor spot size (e.g., an Eddy current sensor including a sensor coil with a diameter between 15 millimeters and 20 millimeters). The Eddy current signal34is in response to the thickness sensor10scanning along the sensor sweep path32, and the Eddy current signal36is in response to the Eddy current sensor having a large sensor spot size scanning along the sensor sweep path32. It is beneficial to reviewFIGS. 2 and 3together.

In accordance with various embodiments for calculating a thickness value representing a thickness of conductive features at substantially the same level on a substrate described herein, the thickness sensor10scans along the sensor sweep path32(e.g., from left to right inFIG. 2) above a plurality of discrete conductive features39in a substrate40. The plurality of discrete conductive features39are substantially at a same level on the substrate40which are separated by one or more regions of non-conductive material at substantially the same level as the discrete conductive features. The discrete conductive features39may be any type of conductive feature. For example, each of the discrete conductive features may be a conductive line, a conductive plug or a similar conductive feature. In various embodiments, the discrete conductive features39are electrically isolated from each other. In various embodiments, the discrete conductive features39are part of a conductive film38. As the thickness sensor10moves along the sensor sweep path32, the amplitude of the Eddy current signal34fluctuates depending on a number of factors, including one or more of whether the thickness sensor10directly overlies conductive material and the density of the conductive material which the thickness sensor directly overlies.

When the thickness sensor10directly overlies conductive material (e.g., a discrete conductive feature), an Eddy current is generated in the conductive material and the Eddy current signal34peaks. For example, as shown inFIG. 3, as the thickness sensor10moves along the sensor sweep path32, the Eddy current signal34includes six peaks as the thickness sensor passes over six discrete conductive features which are at substantially the same level on substrate40. When the thickness sensor10is in a first position42and directly overlies the largest discrete conductive feature44, the Eddy current signal34has its largest peak45.

When the thickness sensor10does not directly overlie conductive material (i.e., overlies an area that includes a non-conductive material), very little or no Eddy current is generated and the Eddy current signal34dips. For example, as shown inFIG. 3, when the thickness sensor10is in a second position46and directly overlies an area47which is free of conductive material, the Eddy current signal34has a valley48. The amplitude of the valleys of the Eddy current signal34is dependent on the amount of conductive material, or lack of conductive material, that underlies the thickness sensor10. For example, as shown inFIG. 3, when the thickness sensor10is in the second position46, the Eddy current signal34has its lowest valley48as the area47is the largest area without conductive material.

Comparing the Eddy current signal34to the Eddy current signal36, which was generated by the Eddy current sensor having a large sensor spot size (e.g., an Eddy current sensor including a sensor coil with a diameter between 15 millimeters and 20 millimeters), it can be seen that the thickness sensor10amplifies the peaks and valleys of the Eddy current signal generated by the Eddy current reader16. For example, the amplitude of a peak50of the Eddy current signal36, which corresponds to the first position42, is much smaller than the amplitude of the peak45. Similarly, the amplitude of a valley52of the Eddy current signal36, which corresponds to the second position46, is much greater than the amplitude of the valley48. Accordingly, the fluctuations between the peaks and valleys of the Eddy current signal34are much more exaggerated compared to the Eddy current signal36.

The thickness sensor10takes advantage of the exaggerated fluctuations (i.e., peaks and valleys) of the Eddy current signal generated by the Eddy current reader16to interpret a thickness of discrete conductive features.FIG. 4is a flow diagram of a method54of operating the thickness sensor10in accordance with some embodiments.

In block56, the thickness sensor10scans along a sensor sweep path over a plurality of target conductive features, and generates an Eddy current signal. For example, in the embodiment shown inFIGS. 2 and 3, the thickness sensor10scans along the sensor sweep path32over the conductive features39and generates the Eddy current signal34. As previously discussed with respect toFIG. 1, in various embodiments, the Eddy current signal is a measured Eddy current of an Eddy current generated in discrete conductive features by the drive coil14. As previously discussed, in various embodiments, the plurality of target conductive features is formed from a conductive film or layer. For instance, in one or more embodiments, the conductive features39are part of the conductive film38.

In block58, the thickness sensor10determines a reference amplitude value based on the Eddy current signal. In one embodiment, the reference amplitude value is determined by the processor20. As will be discussed in further detail with respect to block60, the reference amplitude value is used as a calibration point to determine a thickness value representing a thickness of a plurality of conductive features.

In one embodiment, the reference amplitude value is set to be equal to the minimum amplitude value of the Eddy current signal. For example, in the embodiment shown inFIG. 3, the reference amplitude value for the Eddy current signal34is set to be equal to the amplitude of the valley48. In one embodiment, the minimum amplitude value is a non-zero value. As previously discussed, the amplitude of peaks of an Eddy current signal is dependent on the amount of conductive material that underlies the thickness sensor10. Accordingly, by using the minimum amplitude value of the Eddy current signal for the reference amplitude value, the reference amplitude value represents an amplitude value corresponding to a small thickness of conductive material or an area with a lower density of conductive material (i.e., an area with very little, if any, conductive material). For example, in the embodiment shown inFIG. 3, when the reference amplitude value is set to be equal to the amplitude of the valley48, the reference amplitude value represents an amplitude value corresponding to the area47, which has no conductive material.

In block60, the thickness sensor10determines or calculates a thickness value representing a thickness of the plurality of target conductive features. In one embodiment, the thickness value of the plurality of target conductive features is determined by the processor20.

In one embodiment, the thickness sensor10determines the thickness value based on a difference between the reference amplitude value determined in block58and a maximum amplitude value of the Eddy current signal. For example, in the embodiment shown inFIGS. 2 and 3, when the reference amplitude value is set to be equal to the amplitude of the valley48, a thickness t1of the conductive features39is determined based on a difference s1between the amplitude value of the valley48and the amplitude value of the peak45.

As the reference amplitude value represents an amplitude value corresponding to a small thickness, the difference between the reference amplitude value and a maximum amplitude value of the Eddy current signal is proportional to the thickness of the conductive features within the scan area (i.e., along the sensor sweep path). That is, a large difference between the reference amplitude value and a maximum amplitude value indicates a large thickness, and a small difference between the reference amplitude value and a maximum amplitude value indicates a small thickness. In one embodiment, the difference between the reference amplitude value and a maximum amplitude value is compared to a look up table to determine a corresponding thickness of the conductive features.

In another embodiment, in block60, the thickness sensor10determines or calculates a density value representing a density of the plurality of target conductive features. In one embodiment, the density value of the plurality of target conductive features is determined by the processor20.

Similar to the determining of a thickness value, in one embodiment, the thickness sensor10determines the density value based on a difference between the reference amplitude value determined in block58and a maximum amplitude value of the Eddy current signal. For example, in the embodiment shown inFIGS. 2 and 3, when the reference amplitude value is set to be equal to the amplitude of the valley48, a density of the conductive features39is determined based on a difference s1between the amplitude value of the valley48and the amplitude value of the peak45.

As the reference amplitude value also represents an amplitude value corresponding to an area with a lower density of conductive material, the difference between the reference amplitude value and a maximum amplitude value of the Eddy current signal is proportional to the density of the conductive features within the scan area (i.e., along the sensor sweep path). That is, a large difference between the reference amplitude value and a maximum amplitude value indicates a high density of conductive material, and a small difference between the reference amplitude value and a maximum amplitude value indicates a low density of conductive material. In one embodiment, the difference between the reference amplitude value and a maximum amplitude value is compared to a look up table to determine a corresponding density of the conductive features.

In block62, the thickness sensor10determines if there is another scan area. In one embodiment, the processor20determines if there is another scan area. The other scan area may be on the same wafer as the first scan in block56, or on another separate wafer. If there is not another scan area, the method54moves to block64and the method54ends. If there is another scan area, the method54returns to block56where blocks58,60, and62are repeated.

In one embodiment, the thickness sensor10is used for in-situ monitoring of thickness during a polishing process, such as chemical mechanical polishing (CMP), of conductive features formed on a wafer. By performing the method54simultaneously with a polishing process, a thickness of the conductive features may be monitored in real time. For example, in one embodiment, blocks56,58, and60are performed simultaneously with a CMP process, and a current thickness determined in block60is used to adjust the CMP process (e.g., stop the CMP process when a desired thickness is reached, continue the CMP process if a desired thickness is not reached, etc.) in real time. Using the thickness sensor10during a polishing process will be discussed in further detail with respect toFIG. 6.

It is noted that in accordance with various embodiments described herein, the reference amplitude value is recalculated for each scan of the method54. That is, for every sensor sweep path, a new reference amplitude value is determined in block58. By updating the reference amplitude value, the thickness sensor10is recalibrated scan-to-scan. As a result, variations in Eddy current signals between scans caused by noise sources, such as temperature changes, mechanical vibration, etc., are minimized, and signal-to-noise ratio is improved.FIG. 5is a diagram of two Eddy current signals66,68measured by the thickness sensor10for two scans over the same sensor sweep path32in accordance with some embodiments. The Eddy current signal66is generated by a first scan over the sensor sweep path32, and the Eddy current signal68is generated by a second scan over the sensor sweep path32.

Although the Eddy current signals66,68were obtained by scanning over the same sensor sweep path, the amplitudes of the Eddy current signal68is lower than the amplitudes of the Eddy current signal66. The lowering of the amplitudes of the Eddy current signal68may be caused by, for example, noise sources, such as temperature changes, mechanical vibration, etc. However, as shown inFIG. 5, the peak-to-valley value remains substantially the same. For example, a difference s2between the minimum amplitude value (i.e., the reference amplitude value) and the maximum amplitude value of the Eddy current signal66is substantially equal to the difference s3between the minimum amplitude value and the maximum amplitude value of the Eddy current signal68. Thus, although the overall amplitude of the Eddy current signal may change from scan-to-scan, the difference between the minimum amplitude value and the maximum amplitude value of the Eddy current signal remains substantially the same. As a result, the determination of a thickness in block60remains consistent between two scans by the thickness sensor10. Accordingly, a thickness of discrete conductive features may be accurately measured repeatedly.

As previously discussed, signals received by existing Eddy current sensors are often weak and/or noisy when the existing Eddy current sensors are used to measure thicknesses of discrete conductive features. Consequently, the accuracy of measuring thicknesses of discrete conductive features with existing Eddy current sensors is compromised. In contrast, the thickness sensor10is capable of providing accurate thickness measurements of discrete conductive features. Accordingly, the thickness sensor10is well suited for in-situ monitoring of thickness during a polishing process of discrete conductive features formed on a substrate.FIG. 6is a flow diagram of a method69of performing a polishing process in accordance with some embodiments.

In block70, a polishing module performs a polishing process on a target conductive film or layer on a substrate. The polishing module may be any type of device that is used to planarize or smooth a surface. For example, in one embodiment, the polishing module is a CMP module.

In one embodiment, the target conductive film is polished to form a plurality of discrete conductive features, such as conductive lines or conductive plugs. For example, referring toFIG. 2, the target conductive film may be the conductive film38, and is polished to form the discrete conductive features39.

In one embodiment, the polishing process is performed as part of backend of line (BEOL) processing of a semiconductor device. For example, the target conductive film may be a metallization layer, and the polishing module polishes the target conductive film to form wiring to interconnect a plurality of electrical components (e.g., transistors, resistors, capacitors, etc.).

In block72, the thickness sensor10determines a current thickness of the target conductive film. In one embodiment, the thickness sensor10determines a current thickness using the method54described with respect toFIG. 4. For example, the thickness sensor10scans along a sensor sweep path over the target conductive film and generates an Eddy current signal (block56), determines a reference amplitude value based on the Eddy current signal (block58), and determines or calculates a thickness value representing a thickness of the target conductive film.

In one embodiment, the thickness sensor10determines the current thickness simultaneously with the polishing process performed in block70. Stated differently, the thickness sensor10performs the method54(e.g., blocks56,58, and60) and measures the current thickness concurrently with the target conductive film being polished.

In one embodiment, the thickness sensor10determines the current thickness subsequent to the polishing process performed in block70. In this embodiment, the polishing module performs the polishing process for a predetermined period of time and is stopped. Once the polishing module has stopped, the thickness sensor10then determines the current thickness of the target conductive film.

In one embodiment, the thickness sensor10is incorporated in to the polishing module. Stated differently, the thickness sensor10and the polishing module are combined in a single module or apparatus that is configured to perform polishing and thickness measurement. In one embodiment, the polishing module and the thickness sensor10are separate modules.

In block74, it is determined whether the current thickness is at a target thickness. In one embodiment, the processor20of the thickness sensor10determines whether the current thickness is at a target thickness. In one embodiment, the thickness sensor10transmits the current thickness to the polishing module, and the polishing module determines whether the current thickness is at a target thickness. In one embodiment, the thickness sensor10transmits the current thickness to an external processor or controller that is separate from the thickness sensor10and the polishing module, and the external processor determines whether the current thickness is at a target thickness.

If the current thickness is not substantially equal to the target thickness, the method69returns to block70. Upon returning to block70, the polishing process is continued.

If the current thickness is substantially equal to the target thickness, the method69moves to block76. In block76, the polishing module adjusts the polishing process. In one embodiment, the polishing module stops the polishing process. In one embodiment, the polishing module increases the speed of the polishing process. In one embodiment, the polishing module decreases the speed of the polishing process.

The method69provides closed-loop control for the polishing module. Namely, as the thickness sensor10is able to measure the thickness of the target conductive film in conjunction with the polishing process, the thickness of the target conductive film may be used to as feedback loop to adjust the polishing process. As a result, discrete conductive features with accurate thicknesses may be obtained. Accordingly, the need for post thickness measurement and polishing reprocessing due to thickness variation is reduced. In addition, repeatability of a polishing process for a desired thickness for both “wafer-to-wafer” and “with-in-wafer” control is improved.

The various embodiments provide a thickness sensor and method for measuring a thickness of discrete conductive features. The thickness sensor may be used for real time, in-situ monitoring of a thickness of discrete conductive features during a polishing process, such as CMP.

According to one embodiment disclosed herein, a method includes generating an Eddy current signal based on Eddy currents induced in a plurality of conductive features on a substrate, and calculating a thickness value representing a thickness of the conductive features. The calculating of the thickness value includes determining a maximum amplitude value of the Eddy current signal; determining a minimum amplitude value of the Eddy current signal; and determining a difference between the maximum amplitude value and the minimum amplitude value.

According to one embodiment disclosed herein a method includes measuring Eddy currents in a plurality of conductive features on a substrate; generating an Eddy current signal based on the measured Eddy currents; determining a maximum amplitude value of the Eddy current signal; determining a minimum amplitude value of the Eddy current signal; and calculating a thickness value representing a thickness of the conductive features based on the maximum amplitude value and the minimum amplitude value.

According to one embodiment disclosed herein a method includes scanning a first plurality of conductive features on a substrate. The scanning of the first plurality of conductive features includes inducing first Eddy currents in the first plurality of conductive features; measuring the first Eddy currents in the first plurality of conductive features; and generating a first Eddy current signal based on the measured first Eddy currents. The method further includes determining a reference amplitude value based on a minimum amplitude value of the first Eddy current signal; and calculating a thickness value representing a thickness of the first plurality of conductive features based on the reference amplitude value.