Testing semiconductor wafers requires extreme precision and speed in order to keep quality up and costs down. In the case of test equipment manufacturer Andrew NDT, measuring the thickness of the thin-film metallization layer on wafers also requires exacting motion control. The problem is, what if you don't have any mechanical motion in order to close the loop? President Cuong Le and his engineering team came up with a clever way to achieve positioning accuracies on the order of ±0.5 microns at a high bandwidth, using the analog signal from a capacitance sensor. Here's how they did it.
Look, Ma, No Hands!
In metrology machines, non-contact sensing is a necessity, since touching metallization layers only a few Angstroms thick can damage them. The measurements are typically made before the wafers are polished. "If the surfaces aren't uniform, polishing to remove extra copper will create problems with erosion or undercutting. In the same wafer, you'll have different performance levels," says Le.
Andrew NDT Engineering has developed a patented, non-contact testing process that involves the use of eddy current sensing to measure the thin-film deposits on 300-mm wafers quickly and accurately. The company's Thin Film Electro-Magnetic (TFEM) system is faster than lasers and X-rays, two other types of non-destructive testers now on the market, Le says.
The system checks each wafer in two passes. First, it uses the planarity of the wafer, using a capacitive sensor to determine the variations in the wafer. This sensor was developed in-house by the engineering team, and it consists of two sensing elements. One measures to an accuracy of 0.100 inches, the other to an accuracy of 0.050 inches.
Eddy Current Role
Though testing while the wafer is spinning is more difficult than when it's stable, the approach for doing rotational tests is the same. The capacitive sensor determines the height at any given point and adjusts the read head accordingly, moving to that level when the wafer is tested.
Once the setup is done, the eddy current sensors are used to measure the thickness of the metal layers. The eddy current signal needed to measure down to the Angstrom level has to be very clean, so even cable selection matters. "The cabling system has to be perfectly matched, and the algorithm has to maintain accuracy and repeatability," Le says. Capacitance can change if cable lengths aren't the same, and bending cables can also alter capacitance.
During this reading pass, the wafer moves under the heads. The table-top that holds the wafer moves in three axes, each with its own motor. "The advantage of using three axes is that we can go from one end of the wafer in a straight line, then rotate it to another angle for another pass, or we can measure the circumference and go inside to outside," Le says.
The same basic platform can be configured in different ways. The simplest is to use a single sensor head. It can be positioned anywhere over the wafer. This technique offers the most flexibility, but it's quite slow. It's also less expensive than multi-head techniques, so it's a good match for R&D and other areas where testing isn't at production volumes. It takes about four seconds to measure 40 points on a wafer, with much of that time needed to move to the desired location.
More demanding applications typically use systems with more heads. Up to 40 eddy cur-rent sensors can be used. These sensors read data sequentially.
The most critical aspect of this non-contact testing technique is to maintain a consistent (and very small) gap width between the sensor and the wafer surface in the z-axis orientation, since eddy currents are very sensitive to distance. "If you're calibrated to be 100 microns above the wafer, you have to remain exactly at 100 microns above the surface," Le says. Adding to the challenge is the fact that the surface topology will vary up to 30 microns across the width of the wafer. The wafer surface can also be bowed or convex.
Motion-Free Feedbank: Voltage signals from the capacitive proximity sensor used to measure gap width between the eddy current sensor and wafer is fed to a PCI bus controller that adjusts the z-axis position of the wafers on the fly.
To keep the wafer positioned to within such tight requirements (from a constant 0.1 to 0.001 inches away from the sensor) essentially on the fly, Andrew engineers came up with the creative idea of using the data coming from the capacitive proximity sensor that measures the gap width. This analog voltage is fed to the controller (a DMC-1840 PCI bus controller from Galil) that commands the motor driving z-axis of the multi-axis system. With such a closed loop system, engineers get the accuracy and response time they require.
Besides controlling X-Y-Z and Theta axes, the controller has eight I/O lines. One is used to control the vacuum on the chuck system that holds the wafer in place for testing. The Galil controller uses the PCI interface, marking a significant change from earlier models. On previous iterations, communications from this vacuum system were transmitted to a controller that communicated over a serial port.
Getting the right motion controller took a few tries. "We tested several other motion controllers. They were not as easy to use, and not as precise with the closed loop controls," Le says.
Some of the boards couldn't provide the constant feedback needed to maintain height accurately over a number of passes, failing in the critical job of repeatedly making exactly the same measurements.
Another factor was writing code. There are a number of different techniques for testing wafers, sometimes checking them using linear motions, other times examining spinning wafers. Le didn't want software to become a bottleneck, so controllers that were difficult to program were eliminated early on.
Engineers saved a significant amount of time by using macros. "The macros let me do something and it's done. I wrote software for a PCI board, then switched to Ethernet, and all I had to do was change the cable. I didn't have to change software," says Anh Ngo, senior software engineer at Andrew NDT. Besides the macros, most of the code is written in C, with some C++ code.
Patented: Andrew NDT Engineering has developed a patented, non-contact testing process that involves the use of eddy current sensing to measure the think-film deposits on 300-mm wafers quickly and accurately. A capacitance sensor ensures the correct gap width between wafer and eddy current sensor is maintained.
Spinning Faster Poses Challenges
Andrew NDT recently made a significant leap forward in speed, devising a system that can measure wafers while they're spinning. As compared to the conventional method of measuring a point, moving the wafer to another position, then taking another reading, this approach doubles the throughput from 200 to 400 wafers per hour. Making the switch to servo motors improved speed and smoothness-giving Andrews NDT another leg up in the competitive market of semiconductor testing.