Measuring and testing – Surface and cutting edge testing – Roughness
Reexamination Certificate
1998-02-17
2001-02-13
Larkin, Daniel S. (Department: 2856)
Measuring and testing
Surface and cutting edge testing
Roughness
C250S306000, C250S307000
Reexamination Certificate
active
06185991
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to the measurement of the local properties of a sample surface, and specifically, to the measurement of the mechanical and electrical characteristics of a surface using electrostatic force modulation microscopy which operates in contact mode.
BACKGROUND OF THE INVENTION
The atomic force microscope (AFM) has matured and developed into an instrument for the routine inspection of the surface topography of both insulating and conducting samples at an atomic resolution. Recent efforts have revealed a much broader potential for the measurement of local mechanical and electrical surface properties such as the hardness, surface potential, surface charge, and capacitance of a sample.
Force modulation microscopy (FMM), in which a probe tip or sample is mechanically modulated in contact mode, typically by a piezoelectric transducer, has been used to measure the local mechanical elasticity of a surface. Electrostatic force microscopy (EFM), which operates in noncontact mode, has been developed to investigate the electrical properties of a sample such as the dielectric constant, surface charge, and surface potential. Scanning capacitance microscopy (SCM) has been used to measure the dopant profile on a semiconductor surface.
Unfortunately, the earlier FMM techniques using mechanical modulation measure only the mechanical properties of the sample, such as hardness and viscosity. Furthermore, the mechanical modulation excites other normal modes of the microscope system, which not only causes uncertainty and noise in the measurement but also limits the maximum modulation frequency to a level below the resonant frequency of the mechanical transducer and supporting structure.
Conventional EFM is implemented in a noncontact mode AFM, where the force gradient (which includes the electrostatic force gradient as well as the van der Waals force gradient) is used to control the tip-sample distance. Therefore, the probe of the conventional EFM fails to follow the true topography of a sample where a large electrostatic force gradient is present. For example, abrupt changes in the surface potential or surface charge on the sample surface produce large electrostatic force gradients. In such cases, the AFM probe does not follow the true topography but follows the constant force gradient contour.
Such errors in following topography cause successive errors in measuring the electrical properties of the sample. In addition, noncontact mode operation is complicated and its spatial resolution is significantly lower than in contact mode operation since the tip is separated, approximately 10 nm, from the sample.
Therefore, what is desired is a system and method which (1) performs both mechanical and electrical measurements, (2) does not require the probe tip or sample to be mechanically modulated, and (3) operates in contact mode.
SUMMARY OF THE INVENTION
The present invention provides a new measurement technique called “electrostatic force modulation microscopy” (“EFMM”), which measures the mechanical and electrical properties of insulating, semiconducting, and conducting samples with a more precise spatial resolution than that provided by conventional microscopy. In one embodiment, spatial resolution is provided on the order of nanometers.
In accordance with the invention, a sample is provided in an AFM in which the probe tip is maintained in contact with the sample and a voltage having an ac component is applied between the tip and the sample. In one embodiment, the applied voltage also has a dc component.
The present invention also provides an improved structure that implements EFMM. In accordance with the invention, a microscope such as an AFM is provided in which the probe tip contacts a sample. A voltage source is coupled between the tip and sample for providing a bias voltage between the tip and sample. In one embodiment, this bias voltage has both ac and dc components.
Since only the probe tip is vibrated by electrostatic force, mechanical noise from other modes of vibration is eliminated and the force modulation can be performed at a much higher frequency compared to the conventional structures and methods (e.g. FMM and EFM). Furthermore, while conventional electrostatic force microscopy (EFM) is operated in noncontact mode, the present invention operates in contact mode. Another advantage is that the present invention measures both mechanical and electrical characteristics of a sample.
The method and system measure a variety of properties such as the tip-sample capacitive gradient, sample insulation thickness, tip-sample capacitance, sample mechanical hardness, sample surface potential, sample surface charge density, and sample topography and simultaneous combinations thereof.
The tip-sample capacitive gradient, the sample insulation thickness, and the tip-sample capacitance at a given location is measured by monitoring the vibration of the probe either at the frequency or at twice the frequency of the applied ac voltage while keeping the applied ac voltage constant.
The relative sample mechanical hardness is measured by monitoring the amplitude of the probe tip vibration at the frequency, or at twice the frequency, of the applied ac voltage. During this time, the ac and dc voltage component amplitude are maintained constant. A frequency spectrum for a local area is also obtained by, for example, changing the frequency of the ac applied voltage and monitoring the result on the amplitude of the vibration of the probe.
The polarity of the surface potential of the sample is measured by comparing the phase of the probe tip vibration to the phase of an ac voltage applied to the tip. A relative magnitude of surface potential is determined by holding the ac voltage component constant while maintaining the dc voltage component at zero and comparing the amplitude of the probe vibration for each location taking into account the hardness data to the corresponding amplitude for other locations. The absolute magnitude of the surface potential of the sample is measured by using a feedback loop to alter the dc voltage until a detected vibration is eliminated. The dc voltage at which this occurs is then measured and is determined to be equal to the absolute value of the surface potential.
The polarity of the surface charge density of the sample is measured by comparing the phase of the vibration of the probe tip at the frequency of the ac applied voltage to the phase of the ac applied voltage. The magnitude of the surface charge density of the sample is measured by adjusting the dc voltage applied to the probe tip until the system detects that a component of the tip vibration is eliminated.
The topography of the sample is measured by tracking the static deflection of the probe tip and is derived from the same signal that is used to manipulate the position of the tip in constant force mode.
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Hong Jaewan
Khim Zheong-Gu
Park Sang-il
Larkin Daniel S.
PSIA Corporation
Skjerven, Morrill, Macpherson, Franklin & Friel
Steuber David E.
Suryadevara Omkar K.
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