Electricity: measuring and testing – Magnetic – With means to create magnetic field to test material
Reexamination Certificate
2000-04-07
2002-06-18
Patidar, Jay (Department: 2862)
Electricity: measuring and testing
Magnetic
With means to create magnetic field to test material
C324S202000, C324S226000, C324S716000, C324S071100
Reexamination Certificate
active
06407546
ABSTRACT:
FIELD OF THE INVENTION
The present invention is directed to a method and apparatus for measuring the thickness and sheet resistance of metal coatings disposed on semiconductor wafer products.
BACKGROUND OF THE INVENTION
As semiconductor wafers increase in size, the costs involved in the production of these wafers also increase. Additionally, the semiconductor industry continues to demand higher yield outputs from manufactured semiconductor wafers, as well as having an ongoing demand for quickly produced, high quality, semiconductor products. As such, there is a continual need for nondestructive testing, conducted either inline during the semiconductor manufacturing process or by way of a standalone unit, to measure and monitor the thickness of metal deposition on semiconductors wafers. Additional need exists for nondestructive testing for semiconductor wafers that have undergone a chemical machining process.
It is well known that an eddy current can be used to measure a material's thickness as well as its conductance, as illustrated in U.S. Pat. No. 4,849,69. In the '694 Patent, a microscope is used to maintain the eddy current probe at repeatable and precise distances above a measured sample. However, this approach is slow and cumbersome for inline production monitoring of metal film thickness of wafers. For example, the focal point location of the microscope is different than the location of the detecting eddy current sensor. As such, even if the microscope can maintain a constant distance above the sample (e.g., wafer), this technique does not provide an eddy current sensor at a constant distance above the measured sample.
Another well known technique for determining the thickness of a semiconductor wafer is illustrated in U.S. Pat. No. 4,727,322. In the '322 Patent, a predetermined value of one component is set and acts as a gate trigger. The wafer's thickness, which is in the range of a calibration curve, can be determined by the value measure at some predetermine value.
Other typical application of an eddy current measurement is described in U.S. Pat. No. 5,552,704. In this Patent, a system is described as being capable of measuring the conductance (e.g., conductivity, resistance, or resistivity) on a sample using an eddy current probe, without the need to measure the separation between the probe and sample. However, in the '704 system, a minimum of 25 data points are needed to generate the lift-off curve of all of the known conductance wafers. The system also generates a calibration curve by pre-selecting a curve to intersect all of the life-off curves. The unknown sample can then be measured by finding the intersecting point between the calibration curve and the unknown sample curve.
Because of the '704 Patent utilizes a pre-selected curve to intersect the known conductance lift-off curves, it does not accurately represent the conductance as a function of conductance. The '704 Patent's method only provides an estimation of unknown conductance when the intersected value is plugged into the conductance function. As such, the pre-selected curve does not represent a true conductance function versus intersecting point.
It is to be further noted that although traditional systems are able to obtain accurate calculations of metal coating thicknesses; however, these systems often utilize methods that destroy the inspected sample. In these types of systems, a standard or electron microscope is utilized to measure the thickness of a wafer's coating after a cross-section has been cut through the coating.
SUMMARY OF THE INVENTION
The present invention is capable of measuring the thickness of metal coatings disposed upon semiconductor wafer products, as well as calculating sheet resistance from a known resistivity constant.
The terms calibration sample and inspection sample will be repeatedly used throughout the specification. The calibration sample term denotes a material sample having a known thickness and resistivity. The calibration sample is utilized during a calibration session to obtain a variety of baseline measurements. The inspection sample term denotes a sample having a material layer where the thickness and sheet resistance are unknown.
The present invention preferably obtains several different thicknesses measurements from a calibration sample that cover the possible range of thicknesses of the inspection sample. Preferably, the present invention includes a single absolute eddy current probe comprised of a probe housing and a spring load. In one embodiment, the eddy current probe housing is mounted in a vertical position, perpendicular to the surface of the measuring surface (i.e., wafer surface). However, the present invention is not so limited and other configurations are possible. For example, in another embodiment, the eddy current probe is mounted in a horizontal position, parallel to the wafer surface.
The present invention utilizes an instrument, such as an eddy current personal computer (PC) card that is configured to operate with a PC having a hard drive and CPU. The PC typically will include the necessary software to support the eddy current PC card and as well as perform the necessary data collection.
During a system calibration session, a calibration sample is measured to produce a set of data values associated with the known thickness of the sample. It is to be understood that the calibration sample (i.e., a sample having a known thickness and resistivity), and the unknown thickness sample (i.e., the inspection sample) comprise identical materials. During the calibration session, the measurement frequency generated is at 10 MHz or higher.
The measuring starting point of the calibration sample is denoted by locus (0,0), which is typically a null point or reference point, and indicates the starting locus of the eddy current signal. In other words, locus (0,0) defines the starting point of data collection of the known (i.e., calibration sample) thickness metal coating on the semiconductor wafer.
At the beginning of the calibration session measuring process, the eddy current probe is placed into contact with the calibration sample. More particularly, the probe is positioned so that an eddy current sense coil contacts the top surface metal coating of the calibration sample.
The present invention utilizes a spring load inside the eddy current probe housing to ensure that the calibration sample and eddy current probe (i.e., eddy current sense coil) remain in contact during the calibration process. This spring load ensures that the eddy current signal readings of the calibration sample are obtained from an absolute fixed distance.
Data obtained from the calibration sample consists of an X voltage value and an Y voltage value for each of the series of data samples taken during the calibration session. The X voltage value represents resistance, while the Y voltage represents reactance. Accordingly, the calibration sample provides a series of voltage data values, (X, Y), which are each associated with a known thickness.
For example, after the calibration session (i.e., a measurement of the known thickness sample) is performed, a series of data points will have been measured (e.g., data points A, B, C, D and E). Each data point (A, B, C, D and E) has an associated (X, Y) thickness value denoting their respective thickness (e.g., 500, 1000, 1500, 1700, 2000 angstroms). That is, data point A corresponds to a thickness of 500 angstroms, while data point E has a corresponding thickness of 2000 angstroms.
Since each data point has a voltage value (i.e., X voltage value representing resistance, Y voltage value representing reactance), as well as an associated thickness value (e.g., 500 angstroms, 1000, angstroms, etc.), one of ordinary skill will recognize that each data point will have associated voltage values as well as identifiable thicknesses. As such, each of the identified thicknesses (e.g., 500, 1000, 1500, 1700, 2000 angstroms) may be associated with a particular voltage value (X, Y).
Once the data points have been measu
Le Cuong Duy
Ngo Anh The
Eastman Gary L.
Lotspeich Jeffrey J.
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