Electrical resistors – Resistance value responsive to a condition – Ambient temperature
Reexamination Certificate
2001-09-14
2003-02-11
Easthom, Karl D. (Department: 2832)
Electrical resistors
Resistance value responsive to a condition
Ambient temperature
C338S028000, C338S014000, C338S002000, C338S006000
Reexamination Certificate
active
06518872
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to scanning thermal microscopy; and more particularly, to a resistance-based probe which is used for mapping spatial variation of the thermal properties of a surface, such as temperature, thermal conductivity, and thermal diffusivity as well as being used for detecting various chemical reactions and phase transformations taking place within the studied sample.
More particularly, the present invention relates to a high resolution scanning thermal probe which includes a nanometer sized filament structure formed at the end of an AFM-type cantilever where the force is detected by either optically measuring the deflection or by means of an integrated piezoresistive element.
Further, the present invention relates to a free-standing nanometer sized probe for thermal measurements having decreased thermal conductivity and thermal capacitance thus insuring faster response time, higher frequencies in the active measurement mode (when the probe is heated), and improved spatial
Still further the present invention relates to a four legged thermal probe and a method for producing the four legged thermal probe as well as to a four points resistance measurement technique which results in the elimination of contact potential and contact resistivity thus increasing both the temperature sensitivity and the signal-to-noise ratio of the thermal measurements.
BACKGROUND OF THE INVENTION
Scanning thermal microscopy is a near field technique which permits mapping of spatial variations of thermal properties of a sample, such as temperature, thermal conductivity and diffusivity with sub-micrometer resolution. This type of microscopy has been applied to the study of thermal properties of polymers and pharmaceuticals, locally induced phase transformations, and spatially resolved photothermal spectroscopy as well as other scientific areas. With the continued reduction of the size of integrated circuits, temperature mapping of electronic and optoelectronic devices has become increasingly more important to optimize heat dissipation in the circuits and to identify phase modes caused by local “hot spots”.
Various types of probes with different heat sensitive elements, including thermocouples, contact potentials, Joule expansion elements, Schottky diodes, and resistance based transducers have been developed over the last few years. Prior art systems include thermal probes which may be a resistive probe consisting of a wire making point contact with a sample for scanning the sample surface. In a passive mode of the measurement when no heat is applied to the probe, the temperature of the sample is measured by monitoring the change in the resistivity of the wire. While in the active mode, the sample is locally heated by applying alternating electric current to the wire, thus allowing measurement of thermal conductivity and thermal diffusivity of the sample. Additionally induced local changes such as phase transformations or chemical reactions in the sample may be measured. Due to the fact that the measurement involves heat flow from the sample to the probe, a large thermal resistance of the probe is required in order to improve the accuracy of the thermal measurement. Disadvantageously, prior art thermal probes such as, for example, TM Microscopes Cantilever conventionally used in resistance based transducers, use a sensitive element consisting of a five micron diameter Platinum Rhodium wire which, due to its large dimensions, is unable to provide a high spatial resolution measurement. Additionally, such a wire has undesirably low thermal resistance and high thermal capacitance which decreases the accuracy of the measurements and deteriorates the sensitivity as well as the response time of the temperature measurement.
Building or fabricating a freestanding nanometer sized probe would advantageously decrease both the thermal conductance and the capacitance, thus insuring faster response time and higher frequencies in the active mode of measurements, as well as providing an improvement in the spatial resolution of the probe.
As described in U.S. Pat. No. 5,171,992, nanometer scale probes for magnetic measurement are produced by an electron beam chemical vapor deposition (CVD) process in which a substrate is placed in an evacuated chamber within an electron beam unit, and a volatile organometallic compound gas stream is introduced into the sub-chamber at the same time an electron beam is initiated.
The electron beam impinges upon an upper surface of the substrate and causes decomposition and preferential deposition of the decomposed product of the organometallic gas onto the surface of the substrate. Such deposition occurs within the region irradiated by electron beam. Some deposition also occurs outside the region irradiated by the electron beam due to electron scattering from the surface of the substrate. As the process continues, additional layers of the deposited decomposed components of the organometallic gas continue to build up thereby creating a needle like structure. A conical tip shape for the created needle and its shank diameter are achieved by control of the primary beam voltage and the beam's Gaussian profile. The fabricated needle is covered by a magnetic metal layer to allow the intended magnetic measurements. By manipulating the electron beam, two and three dimensional needle tip structures may be fabricated.
Although the technique described in U.S. Pat. 5,171,992 permits production of nanometer scale probes, the resulting probes are not suited for thermal measurements and are not applicable to four point thermal measurement techniques.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a mechanically stable nanometer scale thermal probe adapted for thermal measurements, having a high spatial resolution, fast response time, high thermal resistance and high signal-to-noise ratio.
It is another object of the present invention to provide a technique for producing a multi-leg nanometer scale thermal probe enabling highly accurate measurements of temperature, thermal conductivity and thermal diffusivity of a sample, as well as inducing local changes such as phase transformations and/or chemical reactions.
It is still a further object of the present invention to provide a four point thermal measurement technique employing a four leg nanometer scale probe through which a current (AC or DC) is applied to two legs of the probe and the voltage drop indicative of a temperature value is measured by contacting the opposite two legs, thus eliminating contact resistance, improving temperature dependence of the resistance and eliminating the error in the temperature readings introduced by temperature gradients along the filament wire.
According to the teachings of the present invention, a thermal nanometer scale probe for thermal measurements of a sample includes an AFM-type cantilever where the force is detected by either optically measuring the deflection or by means of an integrated piezoresistive element. The AFM-type cantilever includes a conductive patterned layer (preferably Au) formed on the surface, and a multi-leg nanometer scale filament structure deposited on the electrically isolated segments of the conductive patterned layer of the AFM-type cantilever. The multi-leg nanometer scale filament structure includes a plurality of legs, a bridge portion, and a contact tip positioned in the center of the bridge portion. Each leg of the multi-leg filament structure has a bottom end contiguously engaging a respective one of the plurality of electrically isolated segments of the conductive patterned layer on the AFM-type cantilever. A top end of each leg of the multi-leg filament is joined with the top ends of other legs of the filament structure by the bridge portion from which the contact tip extends into point contact with the measured sample.
Although the multi-leg nanometer scale filament structure may include two and three legs, a three-dimensional four legged structure is preferred due to its mechanical rigidity,
Edinger Klaus
Rangelow Ivaylo
Easthom Karl D.
Rosenberg , Klein & Lee
University of Maryland
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