Surface Metrology for In-Situ Pad Monitoring in Chemical Mechanical Planarization (CMP)

Introduction to the CMP Process

One of the most important processes in the manufacture of semiconductor hard disk and LED wafers is chemical mechanical planarization (CMP) (Figure 1).

This technique is used to provide the required planarity of the substrate wafer and also re-planarization at certain intermediate points post deposition and for the lithographic processing of structures that are built on the wafer. The principal need for planarization is to assure the functionality of the multilevel interconnects present in the structure.

In addition to this, planarization may also be used to minimise the wafer thickness whilst maintaining uniformity. The wafer is fitted onto a rotating fixture and then pressed against a rotating polishing pad during the CMP process. Simultaneously, an abrasive chemical liquid known as a slurry is distributed between the pad and the wafer.

Figure 1. CMP process schematic diagram

The pad surface is provided with pad rotation and concentric grooves that facilitate the transportation of the slurry across the pad- wafer interface. The purpose of the slurry is to loosen the surface of the wafer for the removal of material by the rough surface of the pad.

The surface properties of the polishing pad influence the amount of material removed from the wafer and the overall quality of the process. Continuous reconditioning of the pad surface by abrasion is required as it gets degraded during polishing.

A rotating abrasive or a conditioning disk made up of stainless steel or electroplated diamond is used for reconditioning the pad surface. The schematic diagram of the CMP process is presented in Figure 1 and the wafer manufacturing processes with and without CMP is shown in Figure 2.

Figure 2. Wafer manufacturing without CMP (a) and with CMP (b)

Surface Metrology for the CMP

The pad surface is provided with pad rotation and concentric grooves that facilitate the transportation of the slurry across the pad- wafer interface.

The purpose of the slurry is to loosen the surface of the wafer for the removal of material by the rough surface of the pad. The surface properties of the polishing pad influence the amount of material removed from the wafer and the overall quality of the process. Continuous reconditioning of the pad surface by abrasion is required as it gets degraded during polishing.

A rotating abrasive or a conditioning disk made up of stainless steel or electroplated diamond is used for reconditioning the pad surface. The schematic diagram of the CMP process is presented in Figure 1 and the wafer manufacturing processes with and without CMP is shown in Figure 2.

Figure 3. 3D topography of conditioning disk surface

Groove Occlusion

The material that is removed from the wafer during the polishing process is deposited in the grooves of the pad, causing occlusion in the grooves. Groove occlusion hampers the uniform distribution of slurry across the water, resulting in the non-uniform removal of material between the center and the edge of the wafer.

Groove occlusion needs to be monitored continuously so that the pad grooves can be cleaned at the right time. Timely cleaning of the grooves helps extend the lifetime of the pad by up to 20%

Pad Glazing

Pad glazing is a more complicated phenomenon that brings down the polishing ability of the pad because of degradation of the surface. As a result of glazing, the wear between the wafer and pad increases, which, in turn, increases the process temperature. The increase in the process temperature may end up in material selectivity during polishing.

Unlike groove occlusion, glazing is not easy to predict and needs continuous monitoring for ensuring ideal performance of the CMP process. For effective in-situ pad surface monitoring, the adopted metrology method should work under wet conditions. The only method that meets these requirements is immersion metrology (Figure 4).

Figure 4. Immersion objective

The principal advantage of this method is that for characterization of the pad, it does not need to be removed from the polisher, which enables in-situ monitoring of pad glazing and groove occlusion at different points throughout the pad's lifecycle. The lifetime of the pad can be extended and the operators can utilize the pad all through its lifecycle by following in-situ metrology.​

A New Immersion Metrology System

In collaboration with field experts, Sensofar has come up with an appropriate solution for solving this issue. This solution enables an increase in yield per pad and minimizes the downtime of the polishing systems by changing the polishing pads only when there is an absolute need to change.

The S mart CMP is a non-destructive, in-situ metrology system. The sensor can be easily placed on the pad with the help of a portable stand. This arrangement eliminates the need for pad removal from the polishing system and also provides instant updates on the condition of the pad.

The laptop computer provided with the S mart CMP system improves its portability and is capable of functioning as a stand-alone sensor and also as an automatic metrology solution that can be integrated into the production line. Irrespective of the way on which the S mart CMP is used, it is capable of acquiring and analyzing data quickly for monitoring the characteristics of the pad.

Pad glazing and groove occlusion have been successfully monitored by this new solution. Further, the underutilization of the CMP pads and their premature discarding has also been highlighted by this solution.

The S mart CMP and a 3D optical sensor are shown in Figure 5. The optical diagram of the new solution and the pixel axial responser are shown in Figure 6.

Figure 5. S mart CMP, 3D optical sensor

Figure 6. (a) Optical diagram of S mart sensor; (b) Pixel axial responser

Basic Operation of the S mart CMP

The patented microdisplay technology from Sensofar and a high intensity blue LED light are incorporated in the S mart CMP. These components provide the capabilities of Focus Variation technology and confocal microscopy in a single sensor. The tools and analysis required for the functioning of the CMP application are provided by the software plug-in that is present at the S mart CMP control interface. A confocal image of the surface is generated by the microdisplay that projects various fringe patterns on the surface being monitored.

The confocal image appears like a grayscale map of the surface in which pixels on the focus plane appear brighter, while those not in focus appear black. A vertical scan of the surface provides a series of confocal images and also the axial response of the focus for each pixel. A 3D reconstruction of the surface can be obtained by processing the axial response.

The S mart sensor combines a suitable immersion objective and this metrology technique for measuring the roughness of the pad whilst it is still on the polisher. This immersion objective can also be integrated with the Focus Variation technology, to measure the width and depth of the groove to monitor groove occlusion.

Groove Occlusion Monitoring

Armed with the Focus Variation technology, the S mart CMP is capable of measuring the pad groove almost instantly (Figure 7).

Based on the measured values, the groove width and depth can be characterized (Figure 8), which, in turn, enables effective monitoring of groove occlusion monitoring. The software plug-in within the CMP, which measures both the depth and width of the groove, irrespective of groove orientation, enables automatic analysis in production environments.

Figure 7. 3D measurement of a new pad groove

Figure 8. Evolution of pad groove width and depth along 12 hour CMP processing

Monitoring of Pad Glazing

The S mart CMP is capable of characterizing pad roughness after conditioning and polishing with the help of the microdisplay confocal microscopy that is designed to work even when immersed under water. This feature enables effective monitoring and determination of the right time for conditions, and also the characterization of the optimal conditioning time for regenerating the pad surface.

The usable lifetime of the pad can be extended and the need for control wafers can be minimized based on all these aspects. The additional benefits are optimization of the process by avoiding the re-processing of the wafers that have been completed otherwise.

Figure 9a shows the way in which the height of the pad asperity changes during the polishing process. A glazing peak in the surface distribution is observed after several hours of polishing. Effective conditioning of the pad can be done by monitoring its surface at regular intervals during polishing so that it can be reverted to the surface condition before polishing.

Figure 9b shows the comparison for the pad prior to intervention (black) and after intervention (red). The left side image shows the pad after 6 hours of polishing and the right hand side image shows its condition after 12 hours.

Figure 9. (a) New pad; (b) Good pad after 10h use; (c) Bad pad after 3h use

Automatic Analysis

Automated analysis (Figure 10 and Table 1) of the recorded data simplifies the control of the surface parameters. All that the operator has to do is place the sensor over the surface of the pad and focus on it, when the data acquisition starts, the software will display the target parameter value automatically.

Automatic detection of the width and depth of the groove can be carried out with the help of special analysis algorithms that have been exclusively developed for this application. The analysis of the critical parameters of the pad asperity for detecting pad glazing is carried out by a separate algorithm.

Table 1. Technical Specifications of the Sensofar S mart CMP

SENSOR OBJECTIVE CONFOCAL FOCUS VARIATION
Dimensions Weight Magnification Working Distance (mm) Field of View (µm) Optical Resolution (µm) Vertical Resolution (nm) Measurement Time (s) Vertical Resolution (nm) Measurement Time (s)
26x28x37cm
10.2x11x14.6“
9 Kg
19,8 Ibs
20X
(Immersion)
2.00 877x660 0.31 20 20 15 5

This information has been sourced, reviewed and adapted from materials provided by Sensofar.

For more information on this source, please visit Sensofar.

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