Acoustically Measuring Polycrystalline Diamond Thickness
Wafer manufacturers can use acoustic imaging to measure the thickness of PCD inserts without sectioning the wafers.
Various types of polycrystalline diamond (PCD) cutting tool inserts are made from a wafer in which tungsten carbide (WC) serves as a substrate. The PCD provides the hardness and durability that makes cutting possible, while the WC provides a strong base.
The tool inserts are cut from the wafers. It is desirable that the top layer of PCD and the lower layer of WC each maintain a consistent thickness across the whole area of the wafer. The sintering process is not easy to control, however, and variations in thickness frequently occur. The acoustic methods described here permit the wafer manufacturer to measure the thickness of the PCD without sectioning the wafer, and to map that thickness over the area of the wafer.
Ultrasound used by acoustic microscopes* travels at very high velocities through both PCD (17,500 m/s) and WC (6,655 m/s). This makes it possible for a transducer scanning at a speed in excess of 1 m/s to pulse ultrasound into a wafer and to receive the return echoes in a few millionths of a second, and therefore to receive echo data from thousands of x-y locations per second.
The echoes are returned only from material interfaces; homogeneous materials with no internal anomalies will send back no echoes, and the acoustic image will be black (no signal). A material interface may be the boundary between two different solid materials (gray) or the boundary between a solid material and a gap such as a void or crack (bright white). The echoes from these interfaces report the amplitude of the reflection; amplitude is vital in analyzing anomalies and defects. The echoes also, by their travel time, report the depths of interfaces. In the case of PCD wafers, measuring the internal interface between the PCD and the WC tells the thickness of the PCD.
This method** was developed in order to measure the precise thickness of material layers within a sample. In scanning a sample, the transducer is coupled to the surface of the sample by water or another liquid because ultrasound does not travel well through air. What is measured during scanning is the arrival time of the echoes from the relevant interfaces—in the case of a PCD wafer, the interface between the top surface of the PCD and the water couplant, and the interface between the PCD and the WC. From the arrival time of the echoes from these two interfaces at each of the thousands or millions of scanned coordinates, the thickness of the PCD across the wafer can be mapped. (Because the echoes also contain amplitude data, internal gap-type defects can be imaged during the same scan.)
Figure 1 is the acoustic map of a PCD/WC wafer 52 mm in diameter. The scale was designed to accommodate thicknesses ranging from 0.70-1.20 mm. Each color in the scale represents a thickness of 0.05 mm, or 50 microns. Placing dissimilar colors side by side makes it easier to identify good and bad regions of the wafer. There are typically about 10 colors in a scale. In this image, near the left edge, is a small black anomaly.
The thickest region of the PCD material is at the center of the wafer, and is identified by its pale blue color, indicating a thickness of 1.00 mm or more. At the lower left edge of the wafer the thickness is less than 0.75 mm. In fact, from the center of the wafer to the lower left, seven thickness ranges are represented by their respective colors. What is desirable is a wafer in which a single color or a few colors at the middle of the scale are displayed.
Figure 2 is the acoustic image of four of the wafers that were imaged at the same time on a tray. Note that the thickness gradations differ from those used in Figure 1. The optimal thickness for these 52 mm wafers is 0.60 mm, represented by brown and light blue. Much of the wafer at the upper left is blue, red, or brown—the three adjacent colors that best meet the desired thickness. But the wafer at the lower right has a large region that is very thin—between 0.30 and 0.35 mm, although it has areas in the desired range. Later, the wafers are cut to make tool inserts. If a wafer has areas of PCD that are too thin or too thick to be used, other areas of the wafer may still yield acceptable inserts.
*such as Sonoscan’s C-SAM® series
**Time Difference Method, developed by Sonoscan
Another method† produces a non-destructive, cross-sectional view of the sample. The image shows the same details as a destructive physical section would, and the vertical and horizontal distances in the image are accurate.
Because what is imaged acoustically is a single vertical plane within the sample, that plane is first identified for scanning by a straight line drawn on the surface of the planar acoustic image of the sample. Later, the transducer will scan along this line. This method is nondestructive, so it permits the user to make cross-sectional images of the sample in as many places as desired. If the sample is square or rectangular in top view, but the most desirable section would be described by a diagonal line of the surface, the sample is simply rotated to permit the transducer to scan the diagonal line. If the data acquired is more important than the physical sample, the user may, after imaging, section the sample physically along the most significant plane found acoustically in order to be able to see the same internal features optically.
An ultrasound system making a cross-sectional view of a PCD wafer scans along the single line defined by the user. During the first pass of the transducer across the sample, only those echoes whose arrival time indicates that they are from a narrow zone (known as the gate) at the bottom of the wafer are used for imaging. Next, the transducer scans the narrow gate that lies just above the first gate. The acoustic cross-sectional image is built up by successive scans that eventually reach the top surface of the sample.
Figure 3 is the image made through a 52-mm PCD wafer along a surface line that passes through the center of the wafer. If the wafer were physically cut along the same line, the features seen acoustically would be seen visually in the same locations. The three dark lines identify the three interfaces from which echoes were received:
• at the top, the interface between the water couplant and the PCD
• the interface between the PCD and the WC
• the bottom surface of the WC
What went wrong in the sintering process is immediately evident: at the left side of the image, the thickness of the PCD is, roughly, within acceptable limits. But the thickness of the PCD layer narrows as one moves across the wafer. At the right side of the image, the two lines representing the interfaces essentially merge, indicating that the PCD is very thin or perhaps even absent.
†Q-BAM™ method, developed by Sonoscan
These two methods both nondestructively provide information about the thickness of the PCD layer. In other applications, they are used to measure the thickness of buried layers. When used to image PCD wafers, they show which wafers, or which regions of a single wafer, can be used to make a particular tool. The acoustic images may also be employed as process control tools to produce more uniform wafers.
For more information, visit www.sonoscan.com.