Electricity: measuring and testing – Magnetic – With means to create magnetic field to test material
Reexamination Certificate
2002-12-23
2004-09-07
Le, N. (Department: 2862)
Electricity: measuring and testing
Magnetic
With means to create magnetic field to test material
C324S071500, C324S226000, C324S227000, C702S170000
Reexamination Certificate
active
06788050
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to systems and methods for determining thin-film thickness in the semiconductor process and more particularly to systems and methods for differentiating an eddy current sensor signal induced by a substrate from a signal induced by a thin-film on the substrate.
2. Description of the Related Art
An ECS (eddy current sensor) detects a conductive material by generating and projecting an electromagnetic field (EMF) and detecting a change in the EMF when the conductive material (e.g., a wire, a conductive film, etc.) is placed into the space where the EMF is generated. When the conductive material is placed in the EMF, an eddy current is induced in the conductive material to compensate for the electrical field penetrating into the volume of the conductive material. The eddy current generates its own EMF, which interacts with the primary EMF resulting in a compensative change of the EMF. The change in the EMF is detected by the ECS. Through various calibration techniques an ECS a known distance from a conductor can determine certain aspects of the conductor from the effect the conductor exerts on the EMF. The amplitude of the EMF change depends on the resistance of the conductive material and the proximity of the conductive material to the ECS. By making constant some variables combined with various calibration techniques, this property of the ECS can be used to determine various aspects of the properties of the conductive material as well as its proximity to the ECS,
By way of example, an ECS produces an EMF consisting of a 1 megahertz (MHz) signal. A conductive film (e.g., copper, aluminum, etc.) on a silicon substrate is passed through the EMF. The EMF induces an eddy current into the conductive film and the induced eddy current interferes with the EMF. The ECS detects an ECS signal that is a result of the interference caused by the eddy current.
FIG. 1
shows a typical ECS
110
. A substrate
120
has a conductive film
130
or layer thereon and an EMF
112
emitted from the ECS
110
(not drawn to scale). Typically, the EMF
112
is considered to effectively penetrate through a conductor a depth quantity referred to as “a skin depth.” A skin depth is the distance into a target (e.g., a conductor), which an EMF wave will decay to about 1/e (about 37%) of the initial value of the EMF wave. Skin depth is a function of the frequency of the EMF
112
and the conductor material type and other factors. If the conductor is the conductive film
130
and the conductive film is copper, the skin depth is about 220,000 angstrom, at 1 MHz. If the copper film
130
is thinner than the skin depth (e.g. about 5000 angstrom), then the EMF
112
will induce an eddy current in both the copper film
130
and the substrate
120
.
The resulting signal that is detected by the ECS
110
includes components attributable to both an eddy current induced in the substrate
120
and an eddy current induced in the copper film
130
. However, even if the conductor
130
were thicker than skin depth, the EMF does not actually stop penetrating at skin depth as at least part (e.g., about 37%) of the EMF penetrates further beyond the conductor
130
(e.g., into the substrate
120
and the environment beyond the substrate
120
). In the present example, where the conductor
130
is thinner than skin depth, a large portion of the EMF penetrates into and even through the substrate
120
a penetration distance
114
.
However, because the substrate
120
offers significant resistivity, a very small eddy current is induced in the substrate
120
. As a result, the majority (e.g., about 90-95%) of the detected ECS signal is due to the eddy current induced into the conductor
130
. Only about 5-10% of the detected ECS signal is due to the eddy current induced in the substrate
120
.
Unfortunately, if the substrate
120
is a silicon substrate, the resistivity of the silicon substrate
120
can vary from edge to center due to the varying physical characteristics (e.g., crystalline structure, dopant concentration, and other physical characteristics) of the crystal from which the substrate was cut. Because the resistivity varies, the eddy current in the silicon substrate
120
can also vary a proportional amount between the center and the edge of the substrate
120
.
A typical silicon substrate is identified as having an “average resistivity” value. The average resistivity value indicates that it is possible for the resistivity at the edge of the substrate
120
to be half the resistivity at the center of the substrate
120
, resulting in a 100% or more variation in resistivity. By way of example, if a wafer can be labeled as having an average resistivity of 1.0 ohm/cm. A resistivity of 1.0 ohm/cm could allow a resistivity of 0.5 ohm/cm on the edge of the wafer and a resistivity of 1.5 ohm/cm or more at the center of the wafer, resulting in a variation of 300% or more between the edge and the center. Silicon substrates can also be labeled with a range of resistivity (e.g., 0.008-0.020 ohm/cm) indicating that the resistivity anywhere on the wafer will fall within the stated range. A range of 0.008-0.020 ohm/cm allows for a 250% variation in resistivity. Therefore, even if only about 5-10% of the detected ECS signal is due to the eddy current induced in the substrate
120
, the 5-10% can vary widely. By way of example, between about 2% and about 6% or between about 4% and about 10%.
This variation in the detected ECS signal due to the substrate
120
makes it difficult to accurately detect the component of the detected ECS signal that is attributable to the thin conductive film
130
. What is needed is a system and method for minimizing or eliminating the component of detected ECS signal resulting from the eddy current induced in the substrate.
SUMMARY OF THE INVENTION
Broadly speaking, the present invention fills these needs by providing an improved system and method of measuring an ECS signal. It should be appreciated that the present invention can be implemented in numerous ways, including as a process, an apparatus, a system, computer readable media, or a device. Several inventive embodiments of the present invention are described below.
One embodiment includes a method for determining a component of an eddy current sensor (ECS) signal attributable to a substrate. The method includes placing a substrate in a first position relative to an ECS at a first distance from the ECS. The substrate can include a conductive film on a first surface of the substrate. A first ECS signal can be detected with the substrate in the first position. The substrate can then be inverted relative to the ECS such that the substrate is in a second position relative to the ECS at a second distance from the ECS. The second distance is equal to the first distance less about a thickness of the substrate. A second ECS signal is detected with the substrate in the second position. A difference signal is determined. The difference signal is equal to a difference between a first signal level on a calibration graph for the ECS and the second signal level. The second signal level being shifted a distance about equal to the thickness of the substrate. A first substrate component of the first ECS signal is calculated. The first substrate component of the first ECS signal is equal to a product of the first distance and the difference signal, divided by the thickness of the substrate.
The conductive film has a thickness of between about 10 and about 20,000 angstroms. The conductive film is a film residue.
The ECS can be aligned with a first point in both the first position and the second position, the first point being on the first surface of the substrate.
The conductive film is juxtaposed between the substrate and the ECS in the first position.
Inverting the substrate can include moving the ECS.
Inverting the substrate can include moving the substrate.
Inverting the substrate can include adjusting the substrate an amount equal to about a thickness of the conductive film
Kinder Darrell
Lam Research Corp.
Le N.
Martine & Penilla LLP
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