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MultiMetrixs Resonance Sensor Technology                Technical Overview

A team of MultiMetrixs´ scientists and engineers has developed a novel metrology method that in our believes will not only replace majority of in-line wafer measurement and inspection tools for metallic and dielectric layers on the market today, but will allow for smooth transition to 65nm node and below. This new technology represents new methodology of wafer layers and thin film parameter measurements.  Unlike the majority of existing electrical methods, this method is not based on the probe-generated small excitation electro-magnetic field.  Our method is based on the phenomena of resonance in the system consisting of the sensor and measured object.  MultiMetrixs has developed a measurement technology based on this observation and coined it - Resonance Sensor Technology (RST).  By observing the resonance pattern, RST instrument can extract electro-physical film parameters, such as thickness, uniformity, resistivity, dielectric constant, and other properties.

Procedures for measuring parameters of the conducting films with the help of inductive sensors have been known for decades.  In general these sensors are referred to as Eddy Current sensors.  They are widely used for inspection and metrology in numerous industries from metallurgy and aerospace to semiconductor in particular.  Eddy Current measurements along with a popular four-point probe method are used to characterize conductive films and layers.  Measurements performed by using Eddy Current sensors have advantage over 4-point probe by being non-contact and non-distractive. One can measure and/or extract the following parameters using Eddy Current technology: resistance, thickness, thickness uniformity and many others.  In general, inductive sensors may be used for analyzing multi-layer coatings and patterned wafers. However, capabilities of traditional Eddy Current sensor are limited by its sensitivity and relatively large spot size.

The semiconductor industry trend towards decrease of CDs dictate the decrease in the layer thickness.  Lately thicknesses of some films used in the industry became as thin as tens of nanometers, while the new processes for making of interconnects, such as ELD and EPD have appeared, requiring generation of "auxiliary" layers (i.e. barrier layers).  Thickness of such an auxiliary layer can be less than hundred angstroms.  Reliable diagnostics of their parameters could be a challenge for traditional metrology procedures, such as Eddy Current, four-point probe and others, due to their physical limitations and insufficient sensitivity.  Therefore, one of the important tasks for today´s technologists is development of electrical methods of measurement of conductive films corresponding to the thicknesses dictated by the modern industrial design rules.

In the simplest case (like the Eddy Current sensor) inductive resonance sensor represents a multiple-loop miniature wire coil located in the vicinity of the object (e.g. thin conducting film) parameters of which (conductivity, thickness, etc.) need to be measured at the distance comparable with the size of the coil.  Such a sensor can be considered an antenna producing the electromagnetic waves of particular wavelength.  This antenna is a part of the oscillating contour, which is activated by an external generator.  Spatial distribution of the field produced by the sensor is determined by its geometry. Varying this geometry also could optimize the field distribution to make the sensor suitable for particular process.
Figure 1: RST Sensor Circuit Representation in the condition of full complex resonance
     where
     R, L and C tank parameters
     L´ and R´ inductance and resistance of vortex current
     C" and R" capacitance and resistance of capacitive coupling current
     M=κ√(L*Lec) mutual inductance
The sensor working in the electrical resonance of the oscillating contour where the sensor being a part of this contour we call RST sensor.  It is not a mandatory requirement for proper functioning of a sensor.  In fact, majority of the sensors used in the industry (Eddy Current sensors too) are not designed to work under resonance conditions.  However, sensitivity of the sensors used in the resonance mode is orders of magnitude higher than that of conventional sensors.

The inductive sensor could be seen as an antenna that consists of a wire coil of several tens of loops and anywhere between 100 micron and a few millimeters in diameter.  If the circuit is energized, it emits electromagnetic waves.  If the circuit represents a resonance contour, and current in the contour oscillates at the resonance frequency, then the sensor - antenna emits the waves at the resonance frequency.  Assuming that the parameters of the contour are chosen for resonance frequency to be between 10 MHz and 100 MHz, any conducting object in the vicinity of a sensor will change the resonance frequency of the oscillating contour.  This effect can be well illustrated by analogy with the car radio receiver tuned for the station broadcasted at 80 - 100 MHz.  If you will touch the antenna with your hand, you will detune the radio and move away from the frequency of the broadcasted station.  It is exactly this effect that is allowing to use oscillating contour to measure parameters of thin films, both conductive and dielectric.

In the first case (conductive) the antenna (sensor) is a coil, in the second (dielectric) - it is an electrical capacitance designed and tuned so that its electric field could interact with the measured film.  More opportunities are provided by inductive sensors with the distributed capacitance, allowing measuring both insulators and conductors.

When the inductive coil can be represented as a superposition of the magnetic dipoles, electrical capacitance can be viewed as that of electrical dipoles.  Having this concept in mind allows better understanding the structure of the field interacting with the measured film.
Figure 2: Experimentally Detectable Parameters of RST
     where
     Ir OC Current Amplitude in the OC Resonance
     fres1 resonance frequency of the OC
     fres2 resonance frequency after certain condition change
     Δf=fres2 - fres1 resonance deviation change
     ΔS resonance power
     Q = (f2 - f1) / fres Q-factor
     Is = ΣIi sum of the Fourier current amplitudes corresponding to the resonance curve
Since currently we are working mostly with conductive films, let us limit ourselves to the inductive sensors only.  If the inductive coil is oriented so that its axis is perpendicular to the film surface, and the distance between the film and the coil is significantly smaller than the size of the coil itself, the field emitted by the coil will be axially symmetrical with respect to the point of intersection of the coil axis and the surface of the film (both components, electrical and magnetic).  These fields can be calculated as a superposition of the fields produced by individual loops of the coil (magnetic dipoles). This field is called the near field.  The condition for the near field is that the distance from the source of the field to the observation point is much less than the wavelength.  It can be shown that the field (both components, electrical and magnetic) in the conductive film is subject to exponential extinction.
If the subject of our interest is a metallic film with thickness less than 100 nm, which is typical for current design rules in today´s semiconductor technologies, it is obvious that films thicknesses will be much smaller than the penetration depth for electromagnetic field into the metals of the film.  Hence, we may consider that for wavelengths of tens of centimeters to hundreds of meters, our electrical field is homogenous.  The field produced by the antenna sensor is axially symmetrical with axis perpendicular to the film plane.  It generates closed loop electrical current in the examined film.  It is obvious that this current distribution should also be axially symmetrical.  Topologically the only adequate shape for the current is a ring.  Area of the film where this current flows could be considered as a unit enclosed loop and or a virtual loop, since it appears only when the sensor is energized.  Like an antenna sensor it can be described in terms of its inductance, capacitance and resistivity.

While proposed methodology is still in its infancy and not all capabilities were discovered and only few applications were reviewed, RST allows real-time extremely high accuracy measurement of the thickness of conductive films below 10nm in real-time (up to 10,000+ measurements per second). Lateral resolution of the method is in the nanoscale range and probe sizes are in the range from 4mm down to microns.  The method allows examination of the multi-layer structures and even submersed layers.  Among applications currently reviewed are measurement thickness and uniformity of thin conductive film (like cobalt cap layer) on top of other conductive film (cooper), wafer edge exclusion zone measurement, electrical adhesion of bumps to die, etc.  Furthermore, RST-based sensors will give eyes to manufacturers to control in-situ numerous production processes such as CMP, PVD, CVD, ion implantation, and others.
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